ATSDR Methylene Chloride Tox Profile

TOXICOLOGICAL PROFILE FOR

METHYLENE CHLORIDE

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Agency for Toxic Substances and Disease Registry

September 2000

ii METHYLENE CHLORIDE

DISCLAIMER

The use of company or product name(s) is for identification only and does not imply endorsement by the

Agency for Toxic Substances and Disease Registry.

iii METHYLENE CHLORIDE

UPDATE STATEMENT

Toxicological profiles are revised and republished as necessary, but no less than once every three years.

For information regarding the update status of previously released profiles, contact ATSDR at:

Agency for Toxic Substances and Disease Registry

Division of Toxicology/Toxicology Information Branch

1600 Clifton Road NE, E-29

Atlanta, Georgia 30333

vii METHYLENE CHLORIDE

QUICK REFERENCE FOR HEALTH CARE PROVIDERS

Toxicological Profiles are a unique compilation of toxicological information on a given hazardous

substance. Each profile reflects a comprehensive and extensive evaluation, summary, and interpretation of

available toxicologic and epidemiologic information on a substance. Health care providers treating

patients potentially exposed to hazardous substances will find the following information helpful for fast

answers to often-asked questions.

Primary Chapters/Sections of Interest

Chapter 1: Public Health Statement: The Public Health Statement can be a useful tool for educating

patients about possible exposure to a hazardous substance. It explains a substance’s relevant

toxicologic properties in a nontechnical, question-and-answer format, and it includes a review of

the general health effects observed following exposure.

Chapter 2: Health Effects: Specific health effects of a given hazardous compound are reported by route

of exposure, by type of health effect (death, systemic, immunologic, reproductive), and by length

of exposure (acute, intermediate, and chronic). In addition, both human and animal studies are

reported in this section.

NOTE: Not all health effects reported in this section are necessarily observed in

the clinical setting. Please refer to the Public Health Statement to identify

general health effects observed following exposure.

Pediatrics: Four new sections have been added to each Toxicological Profile to address child health

issues:

Section 1.6 How Can Methylene Chloride Affect Children?

Section 1.7 How Can Families Reduce the Risk of Exposure to Methylene Chloride?

Section 2.7 Children’s Susceptibility

Section 5.6 Exposures of Children

Other Sections of Interest:

Section 2.8 Biomarkers of Exposure and Effect

Section 2.11 Methods for Reducing Toxic Effects

ATSDR Information Center

Phone: 1-888-42-ATSDR or (404) 639-6357 Fax: (404) 639-6359

E-mail: [email protected] Internet: http://www.atsdr.cdc.gov

The following additional material can be ordered through the ATSDR Information Center:

Case Studies in Environmental Medicine: Taking an Exposure History—The importance of taking an

exposure history and how to conduct one are described, and an example of a thorough exposure

history is provided. Other case studies of interest include Reproductive and Developmental

Hazards; Skin Lesions and Environmental Exposures; Cholinesterase-Inhibiting Pesticide

Toxicity; and numerous chemical-specific case studies.

viii METHYLENE CHLORIDE

Managing Hazardous Materials Incidents is a three-volume set of recommendations for on-scene

(prehospital) and hospital medical management of patients exposed during a hazardous materials incident.

Volumes I and II are planning guides to assist first responders and hospital emergency department

personnel in planning for incidents that involve hazardous materials. Volume III—Medical Management

Guidelines for Acute Chemical Exposures—is a guide for health care professionals treating patients

exposed to hazardous materials.

Fact Sheets (ToxFAQs) provide answers to frequently asked questions about toxic substances.

Other Agencies and Organizations

The National Center for Environmental Health (NCEH) focuses on preventing or controlling disease,

injury, and disability related to the interactions between people and their environment outside the

workplace. Contact: NCEH, Mailstop F-29, 4770 Buford Highway, NE, Atlanta, GA 30341-

3724 • Phone: 770-488-7000 • FAX: 770-488-7015.

The National Institute for Occupational Safety and Health (NIOSH) conducts research on occupational

diseases and injuries, responds to requests for assistance by investigating problems of health and

safety in the workplace, recommends standards to the Occupational Safety and Health

Administration (OSHA) and the Mine Safety and Health Administration (MSHA), and trains

professionals in occupational safety and health. Contact: NIOSH, 200 Independence Avenue,

SW, Washington, DC 20201 • Phone: 800-356-4674 or NIOSH Technical Information Branch,

Robert A. Taft Laboratory, Mailstop C-19, 4676 Columbia Parkway, Cincinnati, OH 45226-1998

• Phone: 800-35-NIOSH.

The National Institute of Environmental Health Sciences (NIEHS) is the principal federal agency for

biomedical research on the effects of chemical, physical, and biologic environmental agents on

human health and well-being. Contact: NIEHS, PO Box 12233, 104 T.W. Alexander Drive,

Research Triangle Park, NC 27709 • Phone: 919-541-3212.

Referrals

The Association of Occupational and Environmental Clinics (AOEC) has developed a network of clinics

in the United States to provide expertise in occupational and environmental issues. Contact:

AOEC, 1010 Vermont Avenue, NW, #513, Washington, DC 20005 • Phone: 202-347-4976 •

FAX: 202-347-4950 • e-mail: [email protected] • AOEC Clinic Director: http://occ-env-

med.mc.duke.edu/oem/aoec.htm.

The American College of Occupational and Environmental Medicine (ACOEM) is an association of

physicians and other health care providers specializing in the field of occupational and

environmental medicine. Contact: ACOEM, 55 West Seegers Road, Arlington Heights, IL

60005 • Phone: 847-228-6850 • FAX: 847-228-1856.

ix METHYLENE CHLORIDE

CONTRIBUTORS

CHEMICAL MANAGER(S)/AUTHORS(S):

Jewell D. Wilson, Ph.D.

ATSDR, Division of Toxicology, Atlanta, GA

Margaret E. Fransen, Ph.D.

Syracuse Research Corporation, North Syracuse, NY

Fernando Llados, Ph.D.

Syracuse Research Corporation, North Syracuse, NY

Mona Singh, Ph.D.

Syracuse Research Corporation, North Syracuse, NY

Gary L. Diamond, Ph.D.

Syracuse Research Corporation, North Syracuse, NY

THE PROFILE HAS UNDERGONE THE FOLLOWING ATSDR INTERNAL REVIEWS:

1. Health Effects Review. The Health Effects Review Committee examines the health effects

chapter of each profile for consistency and accuracy in interpreting health effects and classifying

end points.

2. Minimal Risk Level Review. The Minimal Risk Level Workgroup considers issues relevant to

substance-specific minimal risk levels (MRLs), reviews the health effects database of each

profile, and makes recommendations for derivation of MRLs.

3. Data Needs Review. The Research Implementation Branch reviews data needs sections to assure

consistency across profiles and adherence to instructions in the Guidance.

xi METHYLENE CHLORIDE

PEER REVIEW

A peer review panel was assembled for methylene chloride. The panel consisted of the following

members:

1. Harvey Clewell III, M.S., ICF Kaiser International, Ruston, LA.

2. William J. George, Ph.D., Professor of Pharmacology, Director of Toxicology, Tulane University

School of Medicine, New Orleans, LA.

3. Lyman K. Skory, Ph.D., Skory Consulting, Inc., Midland, MI.

These experts collectively have knowledge of methylene chloride's physical and chemical properties,

toxicokinetics, key health end points, mechanisms of action, human and animal exposure, and

quantification of risk to humans. All reviewers were selected in conformity with the conditions for peer

review specified in Section 104(I)(13) of the Comprehensive Environmental Response, Compensation,

and Liability Act, as amended.

Scientists from the Agency for Toxic Substances and Disease Registry (ATSDR) have reviewed the peer

reviewers' comments and determined which comments will be included in the profile. A listing of the

peer reviewers' comments not incorporated in the profile, with a brief explanation of the rationale for their

exclusion, exists as part of the administrative record for this compound. A list of databases reviewed and

a list of unpublished documents cited are also included in the administrative record.

The citation of the peer review panel should not be understood to imply its approval of the profile's final

content. The responsibility for the content of this profile lies with the ATSDR.

xiii METHYLENE CHLORIDE

CONTENTS

FOREWORD ....................................................................... v

QUICK REFERENCE FOR HEALTH CARE PROVIDERS ................................. vii

CONTRIBUTORS ...................................................................ix

PEER REVIEW .....................................................................xi

LIST OF FIGURES ................................................................. xx

LIST OF TABLES................................................................. xxii

1. PUBLIC HEALTH STATEMENT.................................................... 1

1.1 WHAT IS METHYLENE CHLORIDE? .......................................... 1

1.2 WHAT HAPPENS TO METHYLENE CHLORIDE WHEN IT ENTERS THE

ENVIRONMENT? ........................................................... 2

1.3 HOW MIGHT I BE EXPOSED TO METHYLENE CHLORIDE?...................... 3

1.4 HOW CAN METHYLENE CHLORIDE ENTER AND LEAVE MY BODY? ............ 4

1.5 HOW CAN METHYLENE CHLORIDE AFFECT MY HEALTH? ..................... 4

1.6 HOW CAN METHYLENE CHLORIDE AFFECT CHILDREN? ...................... 6

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO METHYLENE

CHLORIDE? ............................................................... 7

1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN EXPOSED

TO METHYLENE CHLORIDE?................................................ 8

1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO

PROTECT HUMAN HEALTH? ................................................ 9

1.10 WHERE CAN I GET MORE INFORMATION? .................................. 10

2. HEALTH EFFECTS .............................................................. 13

2.1 INTRODUCTION .......................................................... 13

2.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE ................. 13

2.2.1 Inhalation Exposure .................................................. 15

2.2.1.1 Death ..................................................... 15

2.2.1.2 Systemic Effects ............................................ 29

2.2.1.3 Immunological and Lymphoreticular Effects ...................... 34

2.2.1.4 Neurological Effects ......................................... 35

2.2.1.5 Reproductive Effects ......................................... 39

2.2.1.6 Developmental Effects ....................................... 40

2.2.1.7 Genotoxic Effects ........................................... 41

2.2.1.8 Cancer .................................................... 44

2.2.2 Oral Exposure ...................................................... 49

2.2.2.1 Death ..................................................... 49

2.2.2.2 Systemic Effects ............................................ 49

2.2.2.3 Immunological and Lymphoreticular Effects ...................... 58

2.2.2.4 Neurological Effects ......................................... 58

2.2.2.5 Reproductive Effects ......................................... 58

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2.2.2.6 Developmental Effects ....................................... 58

2.2.2.7 Genotoxic Effects ........................................... 59

2.2.2.8 Cancer .................................................... 59

2.2.3 Dermal Exposure .................................................... 60

2.2.3.1 Death ..................................................... 60

2.2.3.2 Systemic Effects ............................................ 60

2.2.3.3 Immunological and Lymphoreticular Effects ...................... 62

2.2.3.4 Neurological Effects ......................................... 62

2.2.3.5 Reproductive Effects ......................................... 62

2.2.3.6 Developmental Effects ....................................... 63

2.2.3.7 Genotoxic Effects ........................................... 63

2.2.3.8 Cancer .................................................... 63

2.3 TOXICOKINETICS......................................................... 63

2.3.1 Absorption ......................................................... 65

2.3.1.1 Inhalation Exposure .......................................... 65

2.3.1.2 Oral Exposure .............................................. 66

2.3.1.3 Dermal Exposure ............................................ 68

2.3.2 Distribution ........................................................ 68

2.3.2.1 Inhalation Exposure .......................................... 68

2.3.2.2 Oral Exposure .............................................. 69

2.3.2.3 Dermal Exposure ............................................ 69

2.3.3 Metabolism......................................................... 69

2.3.4 Elimination and Excretion ............................................. 78

2.3.4.1 Inhalation Exposure .......................................... 78

2.3.4.2 Oral Exposure .............................................. 79

2.3.4.3 Dermal Exposure ............................................ 79

2.3.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD)

Models ............................................................ 80

2.3.5.1 Methylene Chloride PBPK Model Comparison ...................... 81

2.3.5.2 Discussion of Methylene Chloride PBPK Models .................... 85

2.4 MECHANISMS OF ACTION ................................................ 107

2.4.1 Pharmacokinetic Mechanisms ......................................... 107

2.4.2 Mechanisms of Toxicity.............................................. 109

2.4.3 Animal-to-Human Extrapolations ......................................

115

2.5 RELEVANCE TO PUBLIC HEALTH ......................................... 117

2.6 ENDOCRINE DISRUPTION ................................................ 133

2.7 CHILDREN’S SUSCEPTIBILITY ............................................ 133

2.8 BIOMARKERS OF EXPOSURE AND EFFECT ................................. 137

2.8.1 Biomarkers Used to Identify or Quantify Exposure to Methylene Chloride ...... 138

2.8.2 Biomarkers Used to Characterize Effects Caused by Methylene Chloride ....... 140

2.9 INTERACTIONS WITH OTHER CHEMICALS ................................. 141

2.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE ...................... 142

2.11 METHODS FOR REDUCING TOXIC EFFECTS ................................ 144

2.11.1 Reducing Peak Absorption Following Exposure ........................... 145

2.11.2 Reducing Body Burden .............................................. 145

2.11.3 Interfering with the Mechanism of Action for Toxic Effects.................. 147

2.12 ADEQUACY OF THE DATABASE ........................................... 147

2.12.1 Existing Information on Health Effects of Methylene Chloride ............... 148

2.12.2 Identification of Data Needs .......................................... 150

2.12.3 Ongoing Studies .................................................... 162

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3. CHEMICAL AND PHYSICAL INFORMATION ...................................... 163

3.1 CHEMICALIDENTITY .................................................... 163

3.2 PHYSICAL AND CHEMICAL PROPERTIES ................................... 163

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL ............................ 167

4.1 PRODUCTION ........................................................... 167

4.2 IMPORT/EXPORT ........................................................ 170

4.3 USE..................................................................... 170

4.4 DISPOSAL ............................................................... 171

5. POTENTIAL FOR HUMAN EXPOSURE ........................................... 173

5.1 OVERVIEW.............................................................. 173

5.2 RELEASES TO THE ENVIRONMENT ........................................ 173

5.2.1 Air .............................................................. 178

5.2.2 Water ............................................................ 179

5.2.3 Soil .............................................................. 180

5.3 ENVIRONMENTAL FATE.................................................. 180

5.3.1 Transport and Partitioning ............................................ 180

5.3.2 Transformation and Degradation ....................................... 181

5.3.2.1 Air ...................................................... 181

5.3.2.2 Water .................................................... 182

5.3.2.3 Sediment and Soil .......................................... 182

5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT ............... 183

5.4.1 Air .............................................................. 183

5.4.2 Water ............................................................ 183

5.4.3 Sediment and Soil .................................................. 185

5.4.4 Other Environmental Media........................................... 185

5.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE .................. 186

5.6 EXPOSURES OF CHILDREN ............................................... 187

5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES...................... 189

5.8 ADEQUACY OF THE DATABASE ........................................... 190

5.8.1 Identification of Data Needs .......................................... 191

5.8.2 Ongoing Studies .................................................... 194

6. ANALYTICAL METHODS ....................................................... 197

6.1 BIOLOGICAL SAMPLES ...................................................

197

6.2 ENVIRONMENTAL SAMPLES .............................................. 197

6.3 ADEQUACY OF THE DATABASE ........................................... 203

6.3.1 Identification of Data Needs .......................................... 204

6.3.2 Ongoing Studies .................................................... 205

7. REGULATIONS AND ADVISORIES .............................................. 207

8. REFERENCES ................................................................. 213

9. GLOSSARY ................................................................... 265

METHYLENE CHLORIDE xvi

APPENDICES

A. ATSDR MINIMAL RISK LEVELS AND WORKSHEETS ......................... A-1

B. USER'S GUIDE ........................................................... B-1

C. ACRONYMS, ABBREVIATIONS, AND SYMBOLS ............................. C-1

xvii METHYLENE CHLORIDE

LIST OF FIGURES

2-1 Levels of Significant Exposure to Methylene Chloride - Inhalation ....................... 26

2-2 Levels of Significant Exposure to Methylene Chloride - Oral ............................ 53

2-3 Proposed Pathways for Methylene Chloride Metabolism ............................... 71

2-4 Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model

for a Hypothetical Chemical Substance ............................................. 82

2-5 Existing Information on Health Effects of Methylene Chloride.......................... 149

5-1 Frequency of NPL Sites with Methylene Chloride Contamination ....................... 177

xix METHYLENE CHLORIDE

LIST OF TABLES

2-1 Levels of Significant Exposure to Methylene Chloride - Inhalation ....................... 17

2-2 Levels of Significant Exposure to Methylene Chloride - Oral ............................ 50

2-3 Genotoxicity of Methylene Chloride In Vitro ........................................ 130

2-4 Genotoxicity of Methylene Chloride In Vivo ........................................ 131

3-1 Chemical Identity of Methylene Chloride .......................................... 164

3-2 Physical and Chemical Properties of Methylene Chloride .............................. 165

4-1 Facilities that Manufacture or Process Methylene Chloride ............................. 168

5-1 Releases to the Environment from Facilities that Manufacture or Process Methylene

Chloride .................................................................... 174

5-2 Summary of Methylene Chloride Levels in Air ...................................... 184

5-3 Ongoing Studies on the Potential for Human Exposure to Methylene Chloride ............. 195

6-1 Analytical Methods for Determining Methylene Chloride in Biological Materials ........... 198

6-2 Analytical Methods for Determining Methylene Chloride in Environmental Samples ........ 199

7-1 Regulations and Guidelines Applicable to Methylene Chloride.......................... 209

1 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

This public health statement tells you about methylene chloride and the effects of exposure.

The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in

the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for

long-term federal cleanup activities. Methylene chloride has been found in at least 882 of the

1,569 current or former NPL sites. However, the total number of NPL sites evaluated for this

substance is not known. As more sites are evaluated, the sites at which methylene chloride is

found may increase. This information is important because exposure to this substance may harm

you and because these sites may be sources of exposure.

When a substance is released from a large area, such as an industrial plant, or from a container,

such as a drum or bottle, it enters the environment. This release does not always lead to

exposure. You are exposed to a substance only when you come in contact with it. You may be

exposed by breathing, eating, or drinking the substance, or by skin contact.

If you are exposed to methylene chloride, many factors determine whether you’ll be harmed.

These factors include the dose (how much), the duration (how long), and how you come in

contact with it. You must also consider the other chemicals you’re exposed to and your age, sex,

diet, family traits, lifestyle, and state of health.

1.1 WHAT IS METHYLENE CHLORIDE?

Methylene chloride, also known as dichloromethane, is a colorless liquid that has a mild sweet

odor, evaporates easily, and does not burn easily. It is widely used as an industrial solvent and

as a paint stripper. It can be found in certain aerosol and pesticide products and is used in the

manufacture of photographic film. The chemical may be found in some spray paints, automotive

cleaners, and other household products. Methylene chloride does not appear to occur naturally

in the environment. It is made from methane gas or wood alcohol. Most of the methylene

2 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

chloride released to the environment results from its use as an end product by various industries

and the use of aerosol products and paint removers in the home.

More information on the properties and uses of methylene chloride may be found in Chapters 3

and 4.

1.2 WHAT HAPPENS TO METHYLENE CHLORIDE WHEN IT ENTERS THE

ENVIRONMENT?

Methylene chloride is mainly released to the environment in air, and to a lesser extent in water

and soil, due to industrial and consumer uses. Many chemical waste sites, including NPL sites,

contain methylene chloride and these might act as additional sources of environmental

contamination through spills, leaks, or evaporation. Because methylene chloride evaporates

readily, most of it is released into the air. In the air, it is broken down by sunlight and by

reaction with other chemicals present in the air. About half of the methylene chloride disappears

from air in 53 to 127 days. Although methylene chloride does not dissolve easily in water, small

amounts may be found in some drinking water. Methylene chloride that is present in water is

broken down slowly by reactions with other chemicals or by bacteria. Over 90% of the

methylene chloride in the environment changes to carbon dioxide (CO

), which is already

present in air. It takes about 1 to 6 days for half the methylene chloride to break down in water.

When methylene chloride is spilled on land, it attaches loosely to nearby surface soil particles. It

moves from the soil into the air. Some may also move into groundwater. We do not know how

long it remains in soil. We do not expect methylene chloride to build up in plants or animals.

More information on what happens to methylene chloride in the environment may be found in

Chapters 4 and 5.

3 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

1.3 HOW MIGHT I BE EXPOSED TO METHYLENE CHLORIDE?

You may be exposed to methylene chloride in air, water, food, or from consumer products.

Because methylene chloride evaporates easily, the greatest potential for exposure is when you

breathe vapors of contaminated air. Background levels in air are usually at less than one part

methylene chloride per billion parts (ppb) of air. Methylene chloride has been found in some

urban air and at some hazardous waste sites at average concentrations of 11 ppb of air. The

average daily intake of methylene chloride from outdoor air in three U.S. cities ranges from 33 to

309 micrograms per day (1 milligram is equivalent to 1,000 micrograms, 1 mg = 1,000 µg.)

Contact with consumer products such as paint strippers or aerosol cans that contain methylene

chloride is another frequent source of exposure. Exposure occurs as a result of breathing the

vapors given off by the product or from direct contact of the liquid material with the skin. The

highest and most frequent exposures to methylene chloride usually occur in workplaces where

the chemical is used; exposure can be dangerously high if methylene chloride is used in an

enclosed space without adequate ventilation. People who work with it can breathe in the

chemical or it may come in contact with their skin. In the past, concentrations ranging from 1 to

1,000 parts of methylene chloride per million parts of air (ppm; 1 ppm is 1,000 times more than

1 ppb) have been detected in general work areas, while higher concentrations (1,400 ppm) have

been detected in samples in the breathing zone of some workers. These exposure levels exceed

the current recommended federal limits. The National Institute for Occupational Safety and

Health (NIOSH) estimated that 1 million workers may be exposed to methylene chloride.

Averages of 68 ppb of methylene chloride in surface water and 98 ppb methylene chloride in

groundwater have been found at some hazardous waste sites. Less than 1 ppb has been found in

most drinking water analyzed. We expect exposure from water and food to be low because very

little methylene chloride has been detected in these sources.

More information on how you might be exposed to methylene chloride is given in Chapter 5.

4 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

1.4 HOW CAN METHYLENE CHLORIDE ENTER AND LEAVE MY BODY?

Methylene chloride may enter your body when you breathe vapors of contaminated air. It may

also enter your body if you drink water from contaminated wells, or it may enter if your skin

comes in contact with it. Since methylene chloride evaporates into air rapidly, exposure by

breathing is the most likely source of exposure at hazardous waste sites, in the home, and in the

workplace. When you breathe in methylene chloride, over 70% of it enters your bloodstream

and quickly spreads throughout your body, with most of it going to the liver, kidney, brain,

lungs, and fatty tissue. Increased physical activity or an increased amount of body fat tends to

increase the amount of methylene chloride that remains or accumulates in your body tissue.

About half of the methylene chloride in the blood leaves within 40 minutes. Some of the

methylene chloride is broken down into other chemicals, including carbon monoxide (CO), a

natural substance in the body occurring from the breakdown of hemoglobin. Unchanged

methylene chloride and its breakdown products are removed from your body mainly in the air

you breathe out. Small amounts leave in your urine. This usually occurs within 48 hours after

exposure. Although the rate of uptake through the stomach has not been measured, uptake is

likely to be fast. Skin absorption is usually small. Trapping the chemical against the skin with

clothing or gloves can lead to greater absorption and possible chemical burns.

More information on how methylene chloride enters and leaves the body is given in Chapter 2.

1.5 HOW CAN METHYLENE CHLORIDE AFFECT MY HEALTH?

To protect the public from the harmful effects of toxic chemicals and to find ways to treat people

who have been harmed, scientists use many tests.

One way to see if a chemical will hurt people is to learn how the chemical is absorbed, used, and

released by the body; for some chemicals, animal testing may be necessary. Animal testing may

also be used to identify health effects such as cancer or birth defects. Without laboratory

animals, scientists would lose a basic method to get information needed to make wise decisions

5 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

to protect public health. Scientists have the responsibility to treat research animals with care and

compassion. Laws today protect the welfare of research animals, and scientists must comply

with strict animal care guidelines.

If you breathe large amounts (800 ppm) of methylene chloride you may not be able to react fast,

remain steady, or perform tasks requiring precise hand movements. You may experience

dizziness, nausea, tingling or numbness of the fingers and toes, and drunkenness if you breathe

methylene chloride for a sufficiently long period of time. In most cases, effects disappear

shortly after the exposure ends. Studies in animals suggest that exposure to higher

concentrations (8,000–20,000 ppm) can lead to unconsciousness and death. There have been

reports of some people becoming unconscious and some people dying after breathing high

concentrations of methylene chloride; accidents of this kind happen more often when methylene

chloride is used without adequate ventilation.

Breathing methylene chloride may cause changes in the liver and kidney in animals, but similar

effects have not been observed in humans. Animal studies indicate that should you be exposed

to high levels of vapors of methylene chloride in air, the vapors may irritate your eyes and affect

your cornea. One study reported these effects at concentrations of 490 ppm; however, the effects

usually disappeared within a few days.

In humans, direct skin contact with large amounts of methylene chloride causes intense burning

and mild redness of the skin. In a workplace accident in which a person was found to have lost

consciousness and partly fallen into an open vat of methylene chloride, extended direct contact

with the liquid caused severe burns of the skin and eyes (cornea); these conditions were

treatable. In rabbits, effects were observed on the eyes (e.g., cornea), but they were reversible

within a few days.

People can smell methylene chloride at about 200 ppm in air. After about 3 hours of exposure at

this level, a person will become less attentive and less accurate in tasks that require hand-eye

6 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

coordination. Because people differ in their ability to smell various chemicals, odors may not be

helpful in avoiding over-exposure to methylene chloride.

There is not clear evidence that methylene chloride causes cancer in humans exposed to vapors

in the workplace. However, breathing high concentrations of methylene chloride for long

periods of time did increase the incidence of cancer in mice. No information was found

regarding the cancer-causing effects of methylene chloride in humans after oral exposure. The

Department of Health and Human Services (DHHS) has determined that methylene chloride may

reasonably be anticipated to be a cancer-causing chemical. The International Agency for

Research on Cancer (IARC) has classified methylene chloride in Group 2B, possibly causing

cancer in humans. The EPA has determined that methylene chloride is a probable cancer-

causing agent in humans.

More information on how methylene chloride can affect your health is given in Chapter 2.

1.6 HOW CAN METHYLENE CHLORIDE AFFECT CHILDREN?

This section discusses potential health effects from exposures during the period from conception

to maturity at 18 years of age in humans.

Children and adults may be exposed to low levels of methylene chloride in drinking water.

Small children who live near factories that produce or use methylene chloride could accidently

eat some of the chemical by putting dirty hands in their mouths, but the amount of methylene

chloride in the soil is thought to be too low to be harmful. Children could breathe in methylene

chloride that is used in a number of household products, since it evaporates easily. Also, since

the vapor of methylene chloride is heavier than air, it will tend to stay close to the ground; as a

result, children, being shorter, would breathe in larger amounts than adults during accidental

exposure.

The effects of methylene chloride have not been studied in children, but they would likely

experience the same health effects seen in adults exposed to the chemical. It is also not known if

7 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

the way in which methylene chloride is absorbed, metabolized, and eliminated from the body is

different in children than it is in adults. Therefore, adverse effects noted in animals and adult

humans (as discussed in Section 1.5) might also occur in children.

There have not been any reports of a connection between methylene chloride exposure during

pregnancy and birth defects in humans. If a pregnant woman is exposed to methylene chloride, a

small amount may cross the placenta, but not enough to harm the fetus. Studies in animals show

that breathing methylene chloride at relatively high levels during pregnancy may lead to bone

variations, none of which are serious and some of which may be outgrown, in newborn pups.

Methylene chloride has been shown to cross the placenta in rats. Methylene chloride has not

been accurately measured in human milk and there are no animal studies testing to what extent it

can pass into milk.

Sections 2.7 and 5.6 contain specific information about the effects of methylene chloride in

children.

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO METHYLENE

CHLORIDE?

If your doctor finds that you have been exposed to significant amounts of methylene chloride,

ask whether your children might also be exposed. Your doctor might need to ask your state

health department to investigate.

Children may be exposed to methylene chloride in consumer household products, such as paint

removers, which contain a large percentage of methylene chloride. In general, the amounts of

methylene chloride in consumer products are low and children are not likely to be harmed unless

large amounts contact the skin or are accidentally swallowed. Using paint removers, especially

in unventilated or poorly ventilated areas, may cause the amount of methylene chloride in the air

to reach potentially dangerous levels. Caution should be used when using paint removers inside

your house; you should follow instructions on the package label for the proper ventilation

8 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

conditions when using these products. It is also advisable to make certain that children do not

remain near indoor paint removal activities.

Household chemicals should be stored out of reach of young children to prevent accidental

poisonings or skin irritation. Always store household chemicals in their original labeled

containers. Never store household chemicals in containers that children would find attractive to

eat or drink from, such as old soda bottles. Keep your Poison Control Center’s number next to

the phone.

Sometimes older children sniff household chemicals in an attempt to get high. Your children

may be exposed to methylene chloride by inhaling products containing it. Talk with your

children about the dangers of sniffing chemicals.

1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN

EXPOSED TO METHYLENE CHLORIDE?

Several tests exist for determining whether you have had measurable exposure to methylene

chloride. The most direct method measures methylene chloride in the air you breathe out. Your

blood can also be analyzed to determine if methylene chloride is present. However, these tests

are only useful for detecting exposures which have occurred within a few days because

methylene chloride remains in the blood for a very short time. Some absorbed methylene

chloride is stored in fat and slowly returns to the bloodstream. A test to measure

carboxyhemoglobin (COHb), a chemical formed in blood as methylene chloride breaks down in

the body, can also be used as an indicator of exposure. However, this test is not specific, since

smoking and exposure to other chemicals may also increase COHb levels. Your urine can also

be tested for methylene chloride itself or for other chemicals (such as formic acid) that are

produced as methylene chloride breaks down in the body. These tests are not routinely available

in a doctor's office, and they require special equipment. Also, the test for formic acid is not

specific for methylene chloride, since other chemicals, such as formaldehyde, are broken down

9 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

to formic acid. The tests may be useful to determine exposure to methylene chloride but do not

by themselves measure or predict health effects.

More information on how methylene chloride can be measured in exposed humans is presented

in Chapters 2 and 6.

1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO

PROTECT HUMAN HEALTH?

The federal government develops regulations and recommendations to protect public health.

Regulations can be enforced by law. Federal agencies that develop regulations for toxic

substances include the Environmental Protection Agency (EPA), the Occupational Safety and

Health Administration (OSHA), and the Food and Drug Administration (FDA).

Recommendations provide valuable guidelines to protect public health but cannot be enforced by

law. Federal organizations that develop recommendations for toxic substances include the

Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for

Occupational Safety and Health (NIOSH).

Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or

food that are usually based on levels that affect animals; then they are adjusted to help protect

people. Sometimes these not-to-exceed levels differ among federal organizations because of

different exposure times (an 8-hour workday or a 24-hour day), the use of different animal

studies, or other factors.

Recommendations and regulations are also periodically updated as more information becomes

available. For the most current information, check with the federal agency or organization that

provides it. Some regulations and recommendations for methylene chloride include the

following:

10 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

The EPA requires that releases of methylene chloride of 1,000 pounds or more be reported to the

federal government. The EPA has provided guidelines on how much methylene chloride you

may be exposed to for certain amounts of time without causing risk to human health. It

recommends that exposure of children to methylene chloride in drinking water should not exceed

10 milligrams/liter (mg/L) for 1 day or 2 mg/L for 10 days.

Because methylene chloride is used in processing spices, hops extract, and decaffeinated coffee,

the FDA has established limits on the amounts of methylene chloride that can remain in these

food products.

The OSHA currently has a “permissible exposure limit” (PEL) of 25 ppm for an 8-hour workday

with 125 ppm as a “short-term exposure limit” (STEL) for 15 minute durations for persons who

work with methylene chloride.

NIOSH no longer has a “recommended exposure limit” (REL) for methylene chloride. Because

methylene chloride causes tumors in some animals, NIOSH currently considers it a possible

cancer-causing substance in the workplace and recommends that exposure be lowered to the

lowest feasible limit.

More information on government recommendations regarding methylene chloride can be found

in Chapter 7.

1.10 WHERE CAN I GET MORE INFORMATION?

If you have any more questions or concerns, please contact your community or state health or

environmental quality department or

Agency for Toxic Substances and Disease Registry

Division of Toxicology

1600 Clifton Road NE, Mailstop E-29

Atlanta, GA 30333

11 METHYLENE CHLORIDE

1. PUBLIC HEALTH STATEMENT

* Information line and technical assistance

Phone: 1-888-42-ATSDR (1-888-422-8737)

Fax: (404) 639-6359

ATSDR can also tell you the location of occupational and environmental health clinics. These

clinics specialize in recognizing, evaluating, and treating illnesses resulting from exposure to

hazardous substances.

* To order toxicological profiles, contact

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Phone: (800) 553-6847 or (703) 605-6000

13 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.1 INTRODUCTION

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and

other interested individuals and groups with an overall perspective on the toxicology of methylene

chloride. It contains descriptions and evaluations of toxicological studies and epidemiological

investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic

data to public health.

A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile.

2.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE

To help public health professionals and others address the needs of persons living or working near

hazardous waste sites, the information in this section is organized first by route of exposure (inhalation,

oral, and dermal) and then by health effect (death, systemic, immunological, neurological, reproductive,

developmental, genotoxic, and carcinogenic effects). These data are discussed in terms of three exposure

periods: acute (14 days or less), intermediate (15–364 days), and chronic (365 days or more).

Levels of significant exposure for each route and duration are presented in tables and illustrated in

figures. The points in the figures showing no-observed-adverse-effect levels (NOAELs) or

lowest-observed-adverse-effect levels (LOAELs) reflect the actual doses (levels of exposure) used in the

studies. LOAELS have been classified into "less serious" or "serious" effects. "Serious" effects are those

that evoke failure in a biological system and can lead to morbidity or mortality (e.g., acute respiratory

distress or death). "Less serious" effects are those that are not expected to cause significant dysfunction

or death, or those whose significance to the organism is not entirely clear. ATSDR acknowledges that a

considerable amount of judgment may be required in establishing whether an end point should be

classified as a NOAEL, "less serious" LOAEL, or "serious" LOAEL, and that in some cases, there will be

insufficient data to decide whether the effect is indicative of significant dysfunction. However, the

Agency has established guidelines and policies that are used to classify these end points. ATSDR

believes that there is sufficient merit in this approach to warrant an attempt at distinguishing between

"less serious" and "serious" effects. The distinction between "less serious" effects and "serious" effects is

14 METHYLENE CHLORIDE

2. HEALTH EFFECTS

considered to be important because it helps the users of the profiles to identify levels of exposure at which

major health effects start to appear. LOAELs or NOAELs should also help in determining whether or not

the effects vary with dose and/or duration, and place into perspective the possible significance of these

effects to human health.

The significance of the exposure levels shown in the Levels of Significant Exposure (LSE) tables and

figures may differ depending on the user's perspective. Public health officials and others concerned with

appropriate actions to take at hazardous waste sites may want information on levels of exposure

associated with more subtle effects in humans or animals (LOAEL) or exposure levels below which no

adverse effects (NOAELs) have been observed. Estimates of levels posing minimal risk to humans

(Minimal Risk Levels or MRLs) may be of interest to health professionals and citizens alike.

Levels of exposure associated with carcinogenic effects (Cancer Effect Levels, CELs) of methylene

chloride are indicated in Tables 2-1 and 2-2 and Figures 2-1 and 2-2. Because cancer effects could occur

at lower exposure levels, Figures 2-1 and 2-2 also show a range for the upper bound of estimated excess

risks, ranging from a risk of 1 in 10,000 to 1 in 10,000,000 (10

-4

to 10

-7

), as developed by EPA.

Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have been

made for methylene chloride. An MRL is defined as an estimate of daily human exposure to a substance

that is likely to be without an appreciable risk of adverse effects (noncarcinogenic) over a specified

duration of exposure. MRLs are derived when reliable and sufficient data exist to identify the target

organ(s) of effect or the most sensitive health effect(s) for a specific duration within a given route of

exposure. MRLs are based on noncancerous health effects only and do not consider carcinogenic effects.

MRLs can be derived for acute, intermediate, and chronic duration exposures for inhalation and oral

routes. Appropriate methodology does not exist to develop MRLs for dermal exposure.

Although methods have been established to derive these levels (Barnes and Dourson 1988; EPA 1990d),

uncertainties are associated with these techniques. Furthermore, ATSDR acknowledges additional

uncertainties inherent in the application of the procedures to derive less than lifetime MRLs. As an

example, acute inhalation MRLs may not be protective for health effects that are delayed in development

or are acquired following repeated acute insults, such as hypersensitivity reactions, asthma, or chronic

bronchitis. As these kinds of health effects data become available and methods to assess levels of

significant human exposure improve, these MRLs will be revised.

15 METHYLENE CHLORIDE

2. HEALTH EFFECTS

A User's Guide has been provided at the end of this profile (see Appendix B). This guide should aid in

the interpretation of the tables and figures for Levels of Significant Exposure and the MRLs.

2.2.1 Inhalation Exposure

2.2.1.1 Death

Case studies of methylene chloride poisoning during paint stripping operations have demonstrated that

inhalation exposure can be fatal to humans (Bonventre et al. 1977; Hall and Rumack 1990; Stewart and

Hake 1976). Although quantitative estimates of exposure levels were not reported for these cases, levels

of methylene chloride in various tissues were reported: liver (14.4 mg/dL), blood (51 mg/dL), serum

(29 µg/mL), and brain (24.8 mg/100 g) (Bonventre et al. 1977; Hall and Rumack 1990). The cause of

death in these cases was uncertain; however, myocardial infarction was reported in one case (Stewart and

Hake 1976). Death also occurred in two workers involved in oleoresin extraction processes and liquid

cleaning operations (Moskowitz and Shapiro 1952; Winek et al. 1981). Exposure reportedly occurred

from less than 1 hour up to 3 hours, but the concentration of methylene chloride was not reported. The

compound was detected in the lung (0.1 mL/500 g wet tissue), brain (0.27 g/L), and blood (29.8 mg%)

(Moskowitz and Shapiro 1952; Winek et al. 1981). Two cases of lethal poisoning following acute

inhalation of extremely high concentrations of methylene chloride in air (estimated as up to 168,000 ppm)

occurred in two workers burying barrels containing mixed solvents and solid chemical waste in a well

about 2 meters below ground level (Manno et al. 1992). Methylene chloride concentrations in blood of

the two workers were 572 and 601 mg/L, respectively. Blood carboxyhemoglobin (COHb) concen-

trations were about 30% higher than normal. Death appears to have been caused by narcosis and

respiratory depression due to the acute effects of high concentration methylene chloride on the central

nervous system. One death in the United Kingdom resulting from acute inhalation occupational exposure

to methylene chloride (concentration not provided) was attributed to acute narcosis resulting in

respiratory depression (Bakinson and Jones 1985); necropsy revealed evidence of liver and spleen

congestion. One case of fatal gassing occurred during paint-stripping of a chemical tank (Tay et al.

1995). The paint stripper contained 74% w/w (weight by weight) of methylene chloride; the

concentration of methylene chloride vapor within the tank was later estimated to have been well above

100,000 ppm. The worker did not wear a respirator and did not engage a forced ventilation system into

the confined space of the tank. He was found unconscious and died 4 days later. His methylene chloride

blood concentration was 281 mg/L.

16 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Another case of fatal poisoning was reported in Korea: the body of the chief executive of a painting and

coating factory was discovered in an underground trichloroethylene tank in which methylene chloride was

being used to removed rust from iron sheets (Kim et al. 1996a). Autopsy revealed pronounced organ

congestion in the brain, kidneys, liver, and lungs; blood tissue samples contained 3% COHb and

252 mg/L of methylene chloride. This methylene chloride blood concentration is similar to that reported

for other fatalities (Manno et al. 1992; Tay et al. 1995). Concentrations of methylene chloride in other

tissues ranged from 26 mg/kg in the lungs to 75 mg/kg in the brain (Kim et al. 1996a). The data were

sufficient to confirm that the death was due to methylene chloride poisoning.

In another study, no increase in deaths in methylene chloride workers, assessed by life table analysis, was

found after exposure to 30–120 ppm (time weighted averages) for over 30 years (Friedlander et al. 1978).

Fiber production workers exposed to methylene chloride (140–475 ppm) for at least 3 months did not

have a significant increase in mortality (Lanes et al. 1993; Ott et al. 1983b).

Studies in animals confirm that methylene chloride may be lethal after inhalation exposure at high

concentrations. Acute exposure to 16,000–19,000 ppm of methylene chloride for 4–8 hours caused death

in rats and mice (NTP 1986; Svirbely et al. 1947). Also, one of four female monkeys died after 10 days

of continuous exposure to 5,000 ppm methylene chloride (MacEwen et al. 1972). Data suggest there is a

narrow margin between concentrations causing anesthesia and death. An LC

of 16,189 ppm was

reported in mice acutely exposed to methylene chloride (Svirbely et al. 1947). No deaths were found in

mice exposed for 4 hours to 16,800 ppm, but 70% of the mice exposed to 17,250 ppm died (NTP 1986).

Repeated exposure from intermediate to lifetime duration at levels ranging from 1,000 to 16,000 ppm can

cause increased deaths in rats, mice, guinea pigs, rabbits, and dogs (Burek et al. 1984; Heppel et al. 1944;

NTP 1986). Results of the different inhalation studies described in the NTP (1986) report illustrate that

with increasing duration, the lethal exposure level decreases. A 19-day intermittent exposure to

6,500 ppm of methylene chloride or a 13-week exposure to 4,200 ppm were not lethal to rats or mice, but

exposure for 2 years at 4,000 ppm reduced survival in female rats and in mice of both sexes (NTP 1986).

Although the same target organs (central nervous system, lungs, liver) are affected in mammals, the

available mortality data suggest differences in sensitivity among species, with dogs being more sensitive

than mice and rats.

An LC

value and all reliable LOAEL values for lethality in each species and duration category are

recorded in Table 2-1 and plotted in Figure 2-1.

29 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.2.1.2 Systemic Effects

No studies were located regarding musculoskeletal or dermal effects in humans or animals after inhalation

exposure to methylene chloride. Effects of methylene chloride on the respiratory, cardiovascular,

gastrointestinal, hepatic, renal, and ocular systems are discussed below.

The highest NOAEL values and all reliable LOAEL values for systemic effects in each species and

duration category are recorded in Table 2-1 and plotted in Figure 2-1.

Respiratory Effects. Asphyxia was determined to be the cause of death in the case of a male worker

who was subjected to acute inhalation exposure (concentration unknown) for 1 hour (Winek et al. 1981);

the autopsy revealed bilateral pulmonary congestion with focal hemorrhage. Respiratory symptoms

(cough, breathlessness, chest tightness) were reported in only 4 of 33 cases of acute inhalation exposure

to methylene chloride that were reported to occupational health authorities in the United Kingdom

between 1961 and 1980 (Bakinson and Jones 1985); no exposure levels were provided in this study. No

pulmonary function abnormalities were found in humans exposed to methylene chloride vapors

(50–500 ppm) for 6 weeks (NIOSH 1974). Irritative symptoms of the respiratory tract were more

prevalent among 12 Swedish male graffiti removers, employed to clean underground stations by using

methylene chloride-based solvent, than those of the general population (Anundi et al. 1993). The 8-hour

time-weighted average (TWA) to which these workers were exposed ranged from 18–1,200 mg/m

Two clinical case studies (Snyder et al. 1992a, 1992b) were reported in which two men who had been

working in confined spaces with a nationally advertised brand of paint remover (consisting of >80% w/w

methylene chloride) presented to the hospital emergency department complaining of dyspnea, cough, and

discomfort in the midchest. In chest x-rays, each of the patients showed alveolar and interstitial

infiltrates. One patient was treated with oxygen and albuterol and his symptoms improved over 48 hours;

a repeat chest x-ray showed complete clearing of the infiltrates. During the next year, the patient

continued to have episodic cough with wheeze and breathlessness which improved with albuterol therapy.

The patient had no prior history of asthma or cough. A methacholine challenge test verified that he had

hyperactive airways. The second patient was treated with oxygen and his symptoms improved during the

next 48 to 72 hours; a repeat chest x-ray taken 3 days later revealed marked, but not complete, resolution

of previously-noted lung infiltrates. Ten days later he was asymptomatic and his chest x-ray was normal.

30 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Pulmonary effects were observed in animals that died following exposure to high concentrations of

methylene chloride (Heppel et al. 1944). Extreme pneumonia was found in 3/14 guinea pigs exposed to

5,000 ppm for up to 6 months, and pulmonary congestion and edema with focal necrosis was found in

3/5 rabbits and 2/16 rats exposed to 10,000 ppm for up to 8 weeks (Heppel et al. 1944). A high incidence

of foreign body pneumonia, involving focal accumulation of mononuclear and multinucleate

inflammatory cells, was observed in 10/20 rats exposed to methylene chloride at 8,400 ppm for 13 weeks

(NTP 1986). The significance of this finding is uncertain since the effect was observed only at the

highest concentration tested. Male B6C3F

mice exposed to 4,000 ppm methylene chloride for

6 hours/day, 5 days/week for 13 weeks showed acute Clara cell damage in the lung after a 1-day exposure

to methylene chloride, which appeared to resolve after 5 consecutive daily exposures (Foster et al. 1992).

The appearance and disappearance of the lesion in Clara cells correlated well with the activity of

cytochrome P-450 monooxygenase in Clara cells, as assessed immunocytochemically in the whole lung,

and biochemically in freshly isolated Clara cells. Nasal cavity squamous metaplasia was observed in rats

exposed intermittently to 1,000 ppm methylene chloride in the NTP (1986) bioassay.

Cardiovascular Effects. Studies in humans exposed to methylene chloride vapors between 50 and

500 ppm have not reported significant electrocardiographic abnormalities (Cherry et al. 1981; Ott et al.

1983c; NIOSH 1974). In cohort studies of methylene chloride workers, no increased ischemic heart

disease mortality was observed with chronic time-weighted average exposures from 26 to 1,700 ppm

(Hearne et al. 1990; Lanes et al. 1993; Ott et al. 1983b). There were no differences in cardiac effects, as

measured by a health history questionnaire relating to heart problems (e.g., chest discomfort with

exercise; racing, skipping, or irregular heartbeat), between 150 workers occupationally exposed for more

than 10 years to relatively high levels of methylene chloride (8-hour TWA of 475 ppm), and a similar,

nonexposed group of employees at a polyester staple plant (Soden 1993). The exposed cohort were also

exposed to mean 8-hour TWA concentrations of 900 and 100 ppm of acetone and methanol, respectively.

Data in animals are limited to one study evaluating cardiac arrhythmia in the mouse (Aviado and Belej

1974). Atrioventricular block was observed following acute exposure to methylene chloride at

concentrations greater than 200,000 ppm. Exposure to high concentrations of this sort is not likely to

occur in the environment under normal conditions.

31 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Gastrointestinal Effects. Nausea and vomiting were reported in 13 out of 33 cases of acute

inhalation exposure to methylene chloride that were registered with occupational health authorities in the

United Kingdom between 1961 and 1980 (Bakinson and Jones 1985); concentration levels were not

provided in this study. Dilatation of the stomach was reported in mice after inhalation exposure to

4,000 ppm methylene chloride for 2 years (NTP 1986).

Hematological Effects. In humans, average blood COHb levels measure less than 1% in an

atmosphere free of carbon monoxide, and less than 4% in a normal atmosphere. Blood COHb

concentrations were about 30% higher than normal in two cases of lethal poisoning following acute

inhalation of extremely high concentrations of methylene chloride in air (estimated ~168,000 ppm) in

workers who were burying barrels containing mixed solvents and solid chemical waste in a well about

2 meters below ground level (Manno et al. 1992). Employees monitored at the end of 1 work day

following exposure to methylene chloride at 7–90 ppm (8-hour TWA) had average COHb concentrations

between 1.7 and 4.0% for nonsmokers, and between 4.95 and 6.35% for smokers (Soden et al. 1996).

Additional daily cumulative exposure to methylene chloride did not produce increased levels of COHb.

In volunteers who were exposed to methylene chloride at 200 ppm for 4 hours, blood COHb levels rose to

approximately 5% (Putz et al. 1979); this was equivalent to the levels seen in volunteers after inhaling

70 ppm of carbon monoxide for 4 hours. In nonsmoking volunteers exposed to 50, 100, 150, or 200 ppm

of methylene chloride for 7.5 hours, blood COHb levels rose to 1.9, 3.4, 5.3, and 6.8%, respectively, and

blood COHb levels declined immediately following exposure (DiVincenzo and Kaplan 1981).

Other studies in humans reported increases in the red cell count, hemoglobin, and hematocrit in women

occupationally exposed to concentrations up to 475 ppm during an 8-hour workday, but no effects were

found in men. These effects were judged by the authors to be suggestive of compensatory hematopoiesis

(Ott et al. 1983d). It may be anticipated that stress polycythemia will occur in the majority of individuals,

especially cigarette smokers, who are chronically exposed to methylene chloride vapor concentrations in

the 500 ppm range.

In animals, no significant hematologic or clinical chemistry alterations were reported in dogs and

monkeys exposed continuously to up to 100 ppm methylene chloride for 100 days (Haun et al. 1972). In

the dogs, COHb increased from 0.5 to about 2% during exposure to 100 ppm methylene chloride, but no

significant increase was seen at 25 ppm. In the monkeys, COHb levels were approximately 0.5, 1.7, and

4.5% in controls, 25 ppm, and 100 ppm exposed groups, respectively. No treatment-related effects on

32 METHYLENE CHLORIDE

2. HEALTH EFFECTS

common hematologic parameters (cell counts, hemoglobin concentration differentials, white cell counts,

etc.) were observed among rats chronically exposed to methylene chloride at concentrations up to

3,500 ppm (Burek et al. 1984; Nitschke et al. 1988a).

Musculoskeletal Effects. No musculoskeletal effects have been reported in either animals or humans

after inhalation exposure to methylene chloride.

Hepatic Effects. There is very little published information on the hepatic effects of methylene chloride

in humans. One autoworker, who was exposed to methylene chloride by inhalation and dermally for

1.5 years, was reported to have an enlarged liver in addition to adverse neurological and reproductive

effects (Kelly 1988). NIOSH found workplace levels of methylene chloride to average 68 ppm (range of

3.3–154.4 ppm), which may be an underestimate given evaporation of the volatile liquid from the

applicator pads and cotton gloves that were used; the worker was also exposed to low levels of styrene

(7.2 ppm, range of 1.5–10.4 ppm). Exposure to methylene chloride was verified by a blood COHb level

of 6.4% in a sample taken more than 24 hours after work. The relative contributions of the inhalation and

dermal exposures to the hepatic effect was not determined. There were no alterations in serum enzyme

activity (alkaline phosphatase, alanine aminotransferase [ALT], or lactic dehydrogenase) or in serum

bilirubin, calcium, and phosphorus in humans exposed to methylene chloride vapors (50–500 ppm) for

6 weeks (NIOSH 1974). In a clinical epidemiologic assessment of methylene chloride workers, an

exposure-related increase (not clinically significant) in serum bilirubin was observed in workers exposed

to methylene chloride (up to 475 ppm) and methanol, but there were no concentration-related changes in

serum enzyme levels that could indicate liver injury (Ott et al. 1983a). In Swedish graffiti removers

employed to clean underground stations using methylene chloride solvent, no exposure-related deviations

in serum concentrations of creatinine, aspartate transaminase (AST), ALT, or gamma-glutamyl

transpeptidase were observed (Anundi et al. 1993). Based on these data, the liver appears to be a less

sensitive target organ in humans than it is in rodents (see below).

In animals, the effects of methylene chloride have been studied more extensively. For the most part,

exposure to methylene chloride has resulted in fatty changes in the liver and elevated plasma enzymes.

These effects were reversible when exposure ceased. No histopathological changes were observed in

guinea pigs following acute exposure to 5,200 ppm; however, there was a 2.5-fold increase in hepatic

triglycerides (Morris et al. 1979). When male guinea pigs were exposed to 5,000 ppm of methylene

chloride for up to 6 months, 3/8 died and exhibited moderate centrilobular fatty degeneration of the liver

33 METHYLENE CHLORIDE

2. HEALTH EFFECTS

(Heppel et al. 1944); no deaths, but similar liver histopathology was observed after exposure to

10,000 ppm for 8 weeks (guinea pigs) or 1 week (dogs). Fatty changes in the liver were noted in

monkeys, mice, and dogs continuously exposed to 5,000 ppm for 4 weeks (MacEwen et al. 1972). In

addition, mice exposed to 1,000 ppm exhibited iron pigmentation, nuclear degeneration, and pyknotic

cells (MacEwen et al. 1972). Hepatic microsomal enzymes were elevated at 500 ppm (p<0.01) following

10 days of exposure, but were not increased significantly over control levels in rats exposed to methylene

chloride at 250 ppm for 28 days (Norpoth et al. 1974). Continuous exposure of mice and rats for

100 days to 25 or 100 ppm caused fatty changes in the liver (Haun et al. 1972; Kjellstrand et al. 1986;

Weinstein and Diamond 1972). No effects were seen in mice continuously exposed at 25 ppm, but

cytoplasmic vacuolization was reported in rats at this exposure level (Haun et al. 1972). No adverse liver

effects were reported in dogs or monkeys exposed to up to 100 ppm methylene chloride in the Haun et al.

(1972) study. Using results from the Haun et al. (1972) study, an intermediate inhalation MRL of

0.3 ppm was derived based on the LOAEL of 25 ppm for liver effects in rats. In 13-week studies,

centrilobular hydropic degeneration was observed in female mice exposed to 4,200 ppm, and in both

sexes at 8,400 ppm (NTP 1986). Repeated exposure of rats to 200–500 ppm or greater for 2 years

resulted in increased incidences of hepatocellular vacuolization and multinucleate hepatocytes (Burek et

al. 1984; Nitschke et al. 1988a; NTP 1986), but not at 50 ppm (Nitschke et al. 1988a). In the 2-year NTP

(1986) study, other liver effects in rats included hemosiderosis, focal necrosis of hepatocytes, basophilic

change (females only), hepatocytomegaly, bile duct fibrosis in males, and granulomatous inflammation in

females. The NOAEL of 50 ppm identified in the Nitschke et al. (1988a) study was used as the basis for

derivation of a chronic inhalation MRL of 0.3 ppm.

Renal Effects. Daily exposures to concentrations up to 500 ppm methylene chloride for 6 weeks did

not alter blood urea nitrogen or urine urobilinogen levels in humans (NIOSH 1974). In Swedish graffiti

removers employed to clean underground stations using methylene chloride solvent, no exposure-related

deviations in urinary concentrations of microglobulins or N-acetyl-beta-glucosaminidase were observed

(Anundi et al. 1993).

Renal tubular vacuolization was observed in dogs following continuous inhalation exposure to 1,000 ppm

for 4 weeks and in rats following exposure at 5,000 ppm for 14 weeks (MacEwen et al. 1972).

Nonspecific renal tubular degenerative and regenerative changes were observed after continuous exposure

in rats at 25 and 100 ppm for 100 days (Haun et al. 1972). No significant gross or histopathologic

alterations in the kidneys were reported in dogs or monkeys exposed continuously to up to 100 ppm

34 METHYLENE CHLORIDE

2. HEALTH EFFECTS

methylene chloride for 100 days (Haun et al. 1972). Inhalation exposure to 2,000 ppm for 2 years

resulted in statistically significant increases in the incidences of kidney degeneration in female rats and of

kidney/tubule casts in mice of both sexes (NTP 1986).

Ocular Effects. One human study reported mild eye irritation in males exposed to 500 ppm methylene

chloride vapors after 1 hour (NIOSH 1974). This was most likely due to direct contact of methylene

chloride vapor with the eyes. Irritative symptoms of the eyes were more prevalent among 12 Swedish

male graffiti removers (employed to clean underground stations using a methylene chloride-based

solvent) compared to the general population (Anundi et al. 1993). The 8-hour TWA to which these

workers were exposed ranged from 5 to 340 ppm.

One study reported transient increases in the thickness of the cornea in rabbits that were acutely exposed

to vapors of methylene chloride at 490 ppm (Ballantyne et al. 1976). It is likely that the effects observed

were due to direct effect of vapors on the cornea (see Section 2.2.3.2).

2.2.1.3 Immunological and Lymphoreticular Effects

No studies were located regarding immunological effects in humans after inhalation exposure to

methylene chloride.

Some animal studies have reported alterations in secondary lymphoid organs following exposure to

methylene chloride. Splenic fibrosis was observed in a chronic rat study (Mennear et al. 1988) and

splenic atrophy in an intermediate dog study (MacEwen et al. 1972) at exposure concentrations of

1,000 ppm or greater. The significance of these effects on the spleen as an indicator of chemical insult to

the immune system is not clear since no information was obtained on alterations in germinal cell

proliferation, the lymphocyte subpopulation, or other immunological parameters. For these reasons, these

studies are not presented in Table 2-1. However, a recent study by the Halogenated Solvent Industry

Alliance, Inc. (2000) in which male and female rats were exposed whole body to 5,187 ppm methylene

chloride 6 hours/day, 5 days/week, for 28 days found no evidence of immunotoxicity as judged by gross

and microscopical examination of lymphoid tissues, hematology, or IgM antibody response to sheep red

blood cells (SRBC). This NOAEL is listed in Table 2-1 and plotted in Figure 2-1.

35 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.2.1.4 Neurological Effects

A number of human studies reveal that the nervous system is perhaps the most important target of acute

methylene chloride toxicity. All 33 cases of acute inhalation exposure to methylene chloride that were

reported to occupational health authorities in the United Kingdom between 1961 and 1980 involved

depression of the central nervous system (Bakinson and Jones 1985). Unconsciousness occurred in 13 of

these cases and other common effects included headache and dizziness; a few instances of confusion,

intoxication, incoordination, and paresthesia were also reported. Acute inhalation exposure to methylene-

chloride-based paint strippers in rooms with inadequate ventilation led to unconsciousness in four cases

and to generalized seizures in one of these (Hall and Rumack 1990); 10/21 respondents to an occupational

health questionnaire reported experiencing dizziness and headache while working in these conditions, but

the symptoms abated when they moved to fresh air. In volunteers, a single 4-hour exposure to 200 ppm

methylene chloride significantly decreased visual and psychomotor performance and auditory function

(Putz et al. 1979). Auditory monitoring, eye-hand coordination, and high-difficulty peripheral brightness

test performances were not degraded until the final hour of exposure, by which time, the level of carbon

monoxide in exhaled breath had risen to 50 ppm and the level of COHb in blood had risen to 5%. A

single 3- to 4-hour exposure to methylene chloride at 300 ppm caused decreased visual and auditory

functions in volunteers, but the adverse effects were reversible once exposure ceased (Fodor and Winneke

1971; Winneke 1974). Winneke (1974) attributed these effects to methylene chloride rather than its

metabolite COHb, since exposure to carbon monoxide at concentrations up to 100 ppm did not cause

similar effects. At the lowest exposure level (300 ppm of methylene chloride), critical flicker fusion

frequency (visual) and auditory vigilance tasks were impaired. These higher-order functions involved

complex visual and central nervous system processes that are assumed to be influenced by the degree of

“cortical alertness” mediated by subcortical structures, especially the reticular formation (Fodor and

Winneke 1971). Similarly, psychomotor performance (reaction time, hand precision, steadiness) was

impaired, but this occurred at higher exposure levels (800 ppm for 4 hours) (Winneke 1974). Since these

parameters are sensitive indicators of overt central nervous system-related depression, drowsiness, or

narcosis, the Winneke (1974) study was selected as an appropriate basis for deriving an MRL for acute

inhalation effects of methylene chloride. Alterations in visual evoked response were observed in humans

exposed to methylene chloride at 515–986 ppm for 1–2 hours (Stewart et al. 1972). In another study,

there were no effects on spontaneous electroencephalogram, visual evoked response, or a battery of

cognitive effects in humans exposed to concentrations of methylene chloride up to 500 ppm (NIOSH

36 METHYLENE CHLORIDE

2. HEALTH EFFECTS

1974). While some changes in tests related to mood have been reported in humans after acute combined

exposure to methylene chloride (28–173 ppm) and methanol (Cherry et al. 1983), no evidence of

neurological or behavioral impairment was observed at exposure levels of 75–100 ppm (Cherry et al.

1981). Dementia and gait impairment were reported in one case of a person exposed to methylene

chloride (500–1,000 ppm) for 3 years (Barrowcliff and Knell 1979). Based on a LOAEL of 300 ppm for

neurological effects (Winneke 1974), an acute inhalation MRL of 0.6 ppm was calculated as described in

Table 2-1 and Section 2.5.

No acute central nervous system effects were observed among 12 Swedish male graffiti removers

employed to clean underground stations using methylene-chloride-based solvent compared to the general

population (Anundi et al. 1993). The 8-hour TWA to which these workers were exposed ranged from

5 to 340 ppm. Two cases of men using a paint remover (>80% methylene chloride by weight) in small

confined spaces were studied by Snyder et al. (1992a, 1992b) in a hospital emergency room. One

reported symptom was severe headache, which disappeared within 24 hours after cessation of exposure.

The authors considered this symptom to be associated with methylene chloride neurotoxicity. No

neurologic effects, as measured by responses to questions relating to neurotoxicity (e.g., recurring severe

headaches, numbness/tingling in hands of feet, loss of memory, dizziness) were reported in a group of

150 employees in a fiber plant occupationally exposed to methylene chloride (mean 8-hour

TWA=475 ppm) for more than 10 years, when compared to a similar, nonexposed cohort (Soden 1993).

In a retrospective epidemiology study, there were no significant associations between potential solvent

exposure and self-reported neurological symptoms (based on a standard battery of medical surveillance

questions) among workers exposed to a variety of solvents, including methylene chloride, at a

pharmaceutical company (Bukowski et al. 1992). However, Bukowski et al. (1992) concluded that

questionnaires were not the most appropriate tool to investigate potential neurobehavioral changes caused

by low-level exposure to solvents, and recommended the use of neurological test batteries. This caveat

would also apply to the study of Soden (1993).

In a group of 34 autoworkers, which included a subgroup of 26 ‘bonders’, complaints of central nervous

system dysfunction were common following occupational exposure to methylene chloride for up to

3 years (Kelly 1988); the precise number of individuals with neurological complaints was not provided,

since the report focused on reproductive effects. The bonding job involved soaking pads from open

buckets of methylene chloride using ungloved hands, so exposures were both by inhalation and by the

dermal route; exposure to methylene chloride was confirmed by analysis of blood COHb levels. In

workplace air samples, NIOSH measured 3.3–154.4 ppm (average=68 ppm) of methylene chloride, but

37 METHYLENE CHLORIDE

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worker exposure is likely to have been sometimes higher from the evaporation of liquid spilled onto

clothing; bonders were also exposed to styrene at 1.5–10.4 ppm (average=7.2 ppm), which is less than the

threshold limit value (TLV) for that chemical (ACGIH 1999). The neurological complaints reported in

this group included dizziness, lightheadedness, memory loss, personality changes, and depression.

Affected individuals were recommended for further neurological testing, but no test results were reported.

The neurotoxicity of occupational exposure to methylene chloride was examined in a cohort study of

retired airline mechanics who had been chronically exposed to methylene chloride at concentrations

ranging from a mean 8-hour TWA of 105 to 336 ppm, with short-term high exposures ranging from

395 to 660 ppm (Lash et al. 1991). Five categories of variables were assessed: demographic and some

potential confounders, health symptoms, and physiological, psychophysical, and psychological variables.

There were three tests of physiological characteristics (each measuring olfactory, visual, and auditory

parameters) and four tests of psychophysical variables (finger tapping, simple reaction time, choice

reaction time, and complex choice reaction time). Six psychological variables were assessed, most by

more than one test: short-term visual memory, retention measure of visual memory, short-term verbal

memory, and retention measures of verbal memory, attention, and spatial ability. None of the measured

variables were statistically different between the exposed and control groups. However, trends in the

effect sizes appeared within clusters of some variables. In the group of psychological variables showing

effects on memory and attention, the exposed group scored higher than the unexposed group on the verbal

memory tasks but lower on the attention tasks. The major potential confounder was exposure

misclassification. Lack of precision, sampling biases, and random measurement errors might also have

affected the results. However, the authors concluded that overall no effects on the central nervous system

were attributable to chronic, low-level exposures to methylene chloride, a finding they reported as being

consistent with that of Cherry et al. (1981).

White et al. (1995) conducted a 2-year prospective study among workers in the screen printing industry to

investigate the association between exposure to mixed solvents with known neurotoxic properties and

neurobehavioral deficits. Thirty subjects participated in the study which involved medical, demographic,

occupational, and neurological screening, completion of a questionnaire on medical history, life style, and

occupational history, and neurobehavioral assessment using a standard battery of neuropsychological

tests. Subjects were evaluated and tested twice. Air monitoring studies identified workplaces within the

plant that were most exposed to the following chemicals: toluene, methyl ethyl ketone (MEK), mineral

38 METHYLENE CHLORIDE

2. HEALTH EFFECTS

spirits, β-ether, methylene chloride, diacetone alcohol, acetic acid, and lead. A measure of total

hydrocarbon content (THC) was also assessed to account for unidentified solvents in the workplaces.

Generally, exposure to each of the major airborne chemical constituents was below the TLV. Among the

12 subjects classified as having high-acute exposure, performance, adjusted for age and education, was

poorer for tasks involving visual memory and manual dexterity. Whether or to what degree exposure to

methylene chloride contributed to these neurological effects cannot be determined.

Acute studies in animals are consistent with findings in humans that methylene chloride affects the central

nervous system. Narcotic effects of methylene chloride (incoordination, reduced activity, somnolence)

were observed in monkeys, rabbits, rats, and guinea pigs exposed to 10,000 ppm for up to 4 hours

(Heppel et al. 1944); reduced activity was measured in rats exposed to 5,000 ppm (Heppel and Neal

1944). Dogs exposed to 10,000 ppm for 4 hours, first became uncoordinated, then excited and

hyperactive to the extent of bruising themselves, but rapidly recovered afterwards (Heppel et al. 1944).

Somatosensory-evoked potentials were altered in rats after 1 hour of exposure to methylene chloride at

concentration levels of 5,000 ppm or greater (Rebert et al. 1989). Decreased levels of succinate

dehydrogenase were measured in the cerebellum of rats exposed to 500 ppm of methylene chloride for

2 weeks (Savolainen et al. 1981).

Changes in neurotransmitter amino acids and brain enzymes were observed in gerbils after continuous

exposure to 210 ppm for 3 months (Briving et al. 1986; Karlsson et al. 1987; Rosengren et al. 1986). The

DNA concentration decreased in the hippocampus and cerebellum in gerbils exposed to $210 ppm of

methylene chloride, indicating decreased cell density in these brain regions, probably due to cell loss

(Karlsson et al. 1987; Rosengren et al. 1986). Methylene chloride (4,500 ppm) did not affect wheel

running activity and avoidance learning in rats born to dams exposed prior to and/or during gestation

(Bornschein et al. 1980). No treatment-related alterations in sensory evoked potentials, reflexes, posture,

or locomotion were observed in rats exposed at 2,000 ppm (Mattsson et al. 1990).

The highest NOAEL values and all reliable LOAEL values for neurological effects for each species and

duration category are recorded in Table 2-1 and plotted in Figure 2-1.

39 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.2.1.5 Reproductive Effects

Exposure to methylene chloride has been reported to result in adverse reproductive effects in humans and

animals.

One group of case studies reported reproductive effects in 8 out of 34 men who complained of central

nervous system dysfunction following occupational exposure to methylene chloride for 0.4–2.9 years

(Kelly 1988). The eight men (ages 20–47) were part of a subgroup of 26 ‘bonders’ who applied

methylene chloride to automobile parts during assembly; this involved soaking pads from open buckets of

methylene chloride using ungloved hands, so exposures were both by inhalation and by the dermal route.

In workplace air samples, NIOSH measured 3.3–154.4 ppm (average=68 ppm) of methylene chloride, but

worker exposure is likely to have been sometimes higher from the evaporation of liquid spilled onto

clothing; bonders were also exposed to styrene at 1.5–10.4 ppm (average=7.2 ppm), which is less than the

TLV for that chemical (ACGIH 1999). Exposure to methylene chloride within the group was suggested

by blood COHb levels of 1.2–11% for six nonsmokers and 7.3 and 17.3% for two smokers. All eight men

had recent histories of infertility and complained of genital pain (testicular, epididymal, and/or prostatic);

the testes were atrophied in two workers (ages 27 and 47). Infectious disease was eliminated as a cause of

these reproductive effects. Four men who submitted to testing had reduced sperm counts and the

proportion of sperm with abnormal morphology ranged from 38 to 50%. Uncertainty regarding this study

involves the small number of subjects, the multiple exposure to other organic chemicals, the lack of blood

or urine samples quantifying exposure to methylene chloride, and the lack of a control group. No other

studies were located that reported similar male reproductive effects from exposure to methylene chloride.

Contrary to Kelly’s (1988) findings, Wells et al. (1989) found no evidence of oligospermia in four

workers who had been exposed to levels of methylene chloride that were twice as high as in the Kelly

study, while involved in furniture stripping for at least 3 months. It is not certain whether the longer

duration of exposure (minimum occupational exposure 1.4 years) or exposure to other chemicals in the

Kelly (1988) study contributed to the different outcomes in the two studies.

In a retrospective study of pregnancy outcomes among Finnish pharmaceutical workers during the late

1970s, female workers at eight factories had a higher rate of spontaneous abortions compared to the

general population (Taskinen et al. 1986). It is likely that the women were exposed to multiple solvents,

including methylene chloride, prior to conception, as well as during pregnancy. In the case-control study

40 METHYLENE CHLORIDE

2. HEALTH EFFECTS

of 44 pharmaceutical workers who had spontaneous abortions, exposure to various solvents, including

methylene chloride, was associated with a slightly higher abortion rate (Taskinen et al. 1986). Exposure

to methylene chloride was associated with an odds ratio (OR) for spontaneous abortion of 2.3, but the

increase was of borderline significance (p=0.06; 95% confidence interval [CI]=1.0–5.7). In a logistic

regression model, the odds ratio of spontaneous abortion increased with increasing frequency of exposure

to methylene chloride, but the sample size was too small for statistical significance. In addition, smoking

and alcohol consumption were not considered. The authors mentioned that improved industrial hygiene

procedures that eliminated solvent vapors in the workplace may have contributed to an overall decline in

rates of spontaneous abortion among pharmaceutical workers during the period in question.

No adverse effects on reproduction were observed in rats exposed to concentrations up to 1,500 ppm of

methylene chloride for two generations (Nitschke et al. 1988b). In dominant lethal tests involving male

mice exposed to #200 ppm methylene chloride for up to 6 weeks, no microscopic lesions were found in

the testes (Raje et al. 1988). Uterine, ovarian, and testicular atrophy was observed in rats and mice

exposed to vapors of methylene chloride (4,000 ppm) for 2 years (NTP 1986), but the authors considered

this effect to be secondary to malignant hepatic and alveolar neoplasms, as described in Section 2.2.1.8

Cancer. Existing data suggest reproductive toxicity may occur following chronic exposure to relatively

high concentrations of methylene chloride.

The highest NOAEL values and all reliable LOAEL values for each study for reproductive effects are

recorded in Table 2-1 and plotted in Figure 2-1.

2.2.1.6 Developmental Effects

The only information reported on potential developmental effects of methylene chloride in humans was

part of retrospective study of pregnancy outcomes among Finnish pharmaceutical workers (Taskinen et al.

1986). In the case-control study of these workers, exposure to methylene chloride (probably both before

and during pregnancy) was associated through an odds-ratio prediction with a higher risk of spontaneous

abortion (Taskinen et al. 1986). (See Section 2.2.1.5 Reproductive Effects for additional details).

In a study examining the relationship between birth weights and environmental exposures to methylene

chloride from Kodak manufacturing processes in Monroe Country, New York, no significant adverse

41 METHYLENE CHLORIDE

2. HEALTH EFFECTS

effects on birth weight were found among 91,302 single births from 1976 through 1987 (Bell et al. 1991).

The highest predicted environmental concentration of methylene chloride was 0.01 ppm, which is

substantially lower than levels found in occupational settings, but significantly higher than the average

ambient background level of 50 parts per trillion.

A study in rats demonstrated that methylene chloride crosses the placental barrier (see Section 2.3.2.1;

Anders and Sunram 1982). No treatment-related visceral abnormalities were reported in fetuses of mice

and rats exposed to 1,250 ppm of methylene chloride during gestation, but an increase in the incidence of

minor skeletal variants (e.g., delayed ossification of sternebra or extra sternebrae) was observed in both

species; rats also exhibit an increased incidence of dilated renal pelvis. A maternal effect of increased

liver weight was observed (Schwetz et al. 1975). When rats were exposed to 4,500 ppm, maternal liver

weights increased and fetal body weights decreased, but teratogenic effects were not observed and

viability and growth were not affected (Bornschein et al. 1980; Hardin and Manson 1980). Wheel

running activity and avoidance learning were not affected in rats born to dams exposed prior to and/or

during gestation to methylene chloride at 4,500 ppm (Bornschein et al. 1980). Longer-term exposure (for

two generations) to concentrations of 1,500 ppm of methylene chloride did not affect neonatal survival or

neonatal growth in rats (Nitschke et al. 1988b). Although fetal body weights were decreased, the absence

of other fetotoxic effects, major skeletal variants, or significant embryolethality suggests that

developmental toxicity is not a major area of concern following exposure to methylene chloride.

The highest NOAEL values and all reliable LOAEL values for developmental effects for each species and

duration category are recorded in Table 2-1 and plotted in Figure 2-1.

2.2.1.7 Genotoxic Effects

No studies were located regarding genotoxic effects in humans after inhalation exposure to methylene

chloride.

Results of in vivo assays following inhalation exposure in animals were mixed. Inhalation exposure of

mice to methylene chloride for 10 days at concentrations of 4,000 ppm or higher resulted in significant

increases in frequencies of sister chromatid exchanges in lung cells and peripheral blood lymphocytes,

chromosomal aberrations in lung and bone marrow cells, and micronuclei in peripheral blood erythrocytes

(Allen et al. 1990). However, no evidence of chromosomal abnormalities was seen in bone marrow cells

42 METHYLENE CHLORIDE

2. HEALTH EFFECTS

of rats exposed by inhalation to concentrations up to 3,500 ppm for 6 months (Burek et al. 1984). The

difference in responses observed may be due, in part, to species differences or test methodology. The

relevance of these two studies to clastogenic mechanisms in humans is not certain.

More recent in vivo genotoxicity studies have attempted to elucidate the mechanism(s) of genotoxic

action of methylene chloride. Casanova et al. (1992) pre-exposed male B6C3F

mice and Syrian Golden

hamsters for 2 days (6 hours/day) to 4,000 ppm of methylene chloride. On the third day, animals were

exposed for 6 hours to a decaying concentration of [

C] methylene chloride and then examined for the

presence of DNA-protein crosslinks. DNA-protein crosslinks were detected in mouse liver, but not in

mouse lung, hamster liver, or hamster lung. Similar results were observed in a second experiment

(Casanova et al. 1996). B6C3F

mice, exposed to methylene chloride for 6 hours/day for 3 days at

concentrations ranging from approximately 500 to 4,000 ppm formed DNA-protein crosslinks in the liver.

The formation of DNA-protein crosslinks was a nonlinear function of airborne concentrations of

methylene chloride. In addition, mice exposed for 6 hours/day for 3 days to concentrations ranging from

approximately 1,500 to 4,000 ppm showed an increased rate of DNA synthesis in the lung, indicating cell

proliferation, but increased cell turnover was not detected in mouse lung at exposure concentrations of

150 or 500 ppm. Hamsters showed no evidence of cell proliferation in the lung at any concentration, nor

did cell proliferation occur in the livers of either species.

Devereux et al. (1993) analyzed liver and lung tumors induced in female B6C3F

female mice by

inhalation of 2,000 ppm of methylene chloride for 6 hours/day, 5 days/week exposure for up to

104 weeks for the presence of activated ras proto-oncogenes. In methylene chloride-induced liver

tumors, mutations, mainly transversions or transitions in base 1 or base 2, were detected, and were similar

to those observed for the H-ras gene in spontaneous liver tumors. Mutations were also identified in the

lung. The K-ras activation profiles in the methylene chloride-induced tumors were not significantly

different from those in spontaneously occurring tumors. No other transforming genes were found in the

nude mouse tumorigenicity assay. The authors were unable to identify any transforming genes other than

ras genes in either mouse liver or lung tumors. Based on liver tumor data, they suggested that methylene

chloride may affect the liver by promoting cells with spontaneous lesions.

Hegi et al. (1994) generated allelotypes of 38 methylene chloride-induced lung carcinomas from female

B6C3F

mice exposed 6 hours/day, 5 days/week for 2 years to 2,000 ppm. The allelotypes were

examined for various genotoxic endpoints, and the results compared to genotoxicity findings in two other

43 METHYLENE CHLORIDE

2. HEALTH EFFECTS

reciprocal-cross mouse strains. Throughout the genome, allelic losses occurred infrequently, except for

markers on chromosome 4, which were lost in approximately half of the carcinomas. In lung adenomas,

chromosome 4 losses were associated with malignant conversion. Methylene chloride-induced liver

tumors did not demonstrate chromosome 4 loss, which indicated that this finding was specific for lung

carcinomas. Preferential loss of the maternal chromosome 4 was also observed in carcinomas in B6C3F

mice. On chromosome 6, an association between K-ras gene activation and allelic imbalances was also

found in B6C3F

mouse lung tumors. When allelotypes of tumors in mice from two reciprocal cross

strains, AC3F

and C3AF

, were examined and compared with the findings in B6C3F

mice, one allele of

the putative chromosome 4 tumor suppressor gene was shown to be inactivated. Whereas the results in

B6C3F

mice suggested that nondisjunction events were responsible for the chromosome 4 losses, tumors

from both reciprocal-cross mouse strains appeared to show small interstitial deletions in a chromosomal

region that is homologous with a region in human chromosomes which is often lost in a variety of tumors,

including lung cancers. In human chromosomes, a candidate tumor suppressor gene, MTS1, is located in

this region.

In another genotoxic analysis with the same cohort (Hegi et al. 1994), loss of heterozygosity at markers

near the p53 gene on chromosome 11 and within the retinoblastoma tumor suppressor gene were

examined in methylene chloride-induced liver and lung tumors and compared to spontaneous tumors in

control mice. The authors concluded that inactivations of p53 and the retinoblastoma tumor suppressor

gene were infrequent events in lung and liver tumorogenesis in mice exposed to methylene chloride.

Replicative DNA synthesis was examined by Kanno et al. (1993) to evaluate the potential role of

treatment-induced lung cell proliferation on pulmonary carcinogenicity in female B6C3F

mice exposed

to 2,000 or 8,000 ppm of methylene chloride for 6 hours/day, 5 days/week for 2 years. By the end of the

study, there was a statistically significant increase in lung tumors in exposed animals when compared to

controls. Cell proliferation was assessed in the lung after 1, 2, 3, or 4 weeks of inhalation exposure to

2,000 or 8,000 ppm, and after 13- and 26-week exposures to 2,000 ppm, as measured by changes in

labeling indices (LI). The LI of both bronchiolar epithelium and terminal bronchioles were substantially

decreased in mice exposed to 2,000 ppm of methylene chloride for 2–26 weeks. Similar findings, but not

as severe, were observed in mice exposed to 8,000 ppm. The decreases in LI were not accompanied by

cytotoxicity. The authors concluded that high-concentration exposure to methylene chloride for up to

26 weeks reduces cell proliferation in lung epithelial cells in female B6C3F

mice.

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2. HEALTH EFFECTS

Maronpot et al. (1995b) assessed replicative DNA synthesis after 13, 26, 52, and 78 weeks of inhalation

exposure of female B6C3F

mice to 2,000 ppm of methylene chloride for 6 hours/day, 5 days/week. A

statistically significant decrease in the hepatocyte LI was only observed at 13 weeks. In lung epithelial

cells, the results were similar to those observed by Kanno et al. (1993). No increases in replicative DNA

synthesis were found in liver foci cells or lung parenchymal cells. K-ras gene activation in liver tumors

and H-ras gene activation in lung tumors did not differ among methylene chloride-induced tumors and

those observed in control animals. The authors concluded that these oncogenes were not involved in

mouse tumorigenesis.

Other genotoxicity studies are discussed in Section 2.5.

2.2.1.8 Cancer

No excess risk of death from malignant neoplasms has been detected in workers exposed to methylene

chloride at levels up to 475 ppm (Friedlander et al. 1978; Hearne et al. 1987, 1990; Lanes et al. 1993; Ott

et al. 1983a). Lanes et al. (1990) reported excess mortality associated with cancer of the buccal cavity

and pharynx (combined), and liver and biliary passages (combined) in workers occupationally exposed to

methylene chloride (#1,700 ppm) in the cellulose fiber production industry for more than 20 years.

Although the actual number of cases was small, the excess mortality for the combined liver/biliary cancer

cases was statistically significant (standard mortality rate [SMR]=5.75; 95% CI=1.82–13.78); since three

of the four deaths were biliary cancer, the SMR was 20 (95% CI=5.2–56.0) for that site alone (Lanes et al.

1990). In a follow-up study of the same cohort (Lanes et al. 1993), no new cancer cases were found, but

there was still an excess mortality for the cohort (SMR=2.98; 95% CI=0.81–7.63). Lanes et al. (1993)

concluded that the excess death in this cohort from liver/biliary cancer was “statistically unstable”, but

warranted further monitoring.

Tomenson et al. (1997) studied mortality due to cancer in a group of workers occupationally exposed to

methylene chloride vapors at a mean exposure concentration of 19 ppm (8-hour TWA); the average

length of employment was 9 years. Compared to national and local rates, the occupationally-exposed

group had lower rates for all cancers, including those of the liver, lung, pancreas, and biliary tract. The

authors suggested that the significant reduction in mortality due to lung cancer was likely associated with

restrictions on smoking in the workplace.

45 METHYLENE CHLORIDE

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Gibbs et al. (1996) examined causes of mortality in an occupationally exposed group of workers similar

to that studied by Lanes et al. (1990, 1993). Exposure was classified categorically, with the airborne

concentrations in the high-exposure group ranging from 350 to 700 ppm, and in the low exposure group

from 50 to 100 ppm. Because there were no jobs in this cohort that did not involve methylene chloride

exposure, a “0” exposure category was created as an internal control. There was no increase in mortality

from cancers of the lung, liver, pancreas, or biliary tract. An unexpected finding was a concentration-

related increase in mortality from prostate cancer among men with 20 or more years of employment.

Another unexpected finding was an increase in mortality from cervical cancer among women with 20 or

more years of employment, although the increase in women was not concentration-related. These results

are not consistent with those from other studies. It should be noted that the confidence intervals were

very large and no potentially confounding variables were measured. The authors concluded that the

results were difficult to interpret biologically and required further investigation.

In a case control study, Heineman et al. (1994) evaluated exposures of men in the petroleum refining and

chemical manufacturing industries to chlorinated aliphatic hydrocarbons (CAHs), including methylene

chloride, as potential risk factors for astrocytic brain tumors. Job-exposure matrices for six individual

CAHs, including methylene chloride, and for total organic solvents, were developed by estimating the

probability of exposure and the frequency and magnitude of exposure to CAH solvents by industry and

by job classification, based on likely solvent usage over 6 decades (1920–1980). An increase in the

incidence of mortality due to astrocytic brain cancer was observed for exposure to four CAHs (carbon

tetrachloride, methylene chloride, tetrachloroethylene, and trichloroethylene); the strongest association

was with methylene chloride. In occupations judged to be associated with methylene chloride exposure,

risk of astrocytic brain cancer increased with increasing exposure, as measured by the job exposure matrix

in conjunction with duration of employment. The authors stated that these trends could not be explained

by exposures to the other solvents.

As first evidence of such an association, these results should be interpreted very cautiously. The principal

limitation of the study was the lack of direct information on exposure to solvents; no quantitative

measurements were made, nor were specific-use records available. Instead, qualitative estimates of

exposure were made by industrial hygienists based on work histories provided by next-of-kin; there were

no workplace records. A lack of quantitative information on the use of specific solvents in various

occupations, the ability of solvents to be used interchangeably for many industrial applications, and the

use of multiple, or mixtures of, solvents also contributed to a high potential for exposure

46 METHYLENE CHLORIDE

2. HEALTH EFFECTS

misclassification. The authors stated that “few individual risks were statistically significant and most

confidence intervals were broad”; interpretation of the results was based on “patterns of trends”

(Heineman et al. 1994). The authors added that “...the trends and consistency of the methylene chloride

and brain cancer association suggest that chance seems unlikely to entirely explain the results.”

In another case-control study, Cocco et al. (1999) examined the occupational risk of central nervous

system cancer among women, using a study design similar to that described for Heineman et al. (1994).

From death certificates in a U.S. 24-state database for the period 1984–1992, the authors identified 12,980

cases of cancers of the brain and other parts of the central nervous system. For each case, four controls

were selected among women who died from nonmalignant diseases (excluding neurological disorders),

frequency-matched by state, race, and 5-year age-group. Job exposure matrices were developed for 11

occupational hazards, including methylene chloride. An estimate of intensity of exposure was developed

for each occupation and industry listed in the U.S. Census code. A final intensity level score and a

probability of exposure score were then developed for each occupation/industry combination appearing in

death certificates of the study subjects. The ORs were calculated with logistic regression for each

workplace exposure, adjusting for marital status, socioeconomic status, and age at death. The authors

found that potential exposure to methylene chloride was associated with a modest, but statistically

significant, 20–30% increase in risk of mortality from central nervous system cancer (OR=1.2; 95%

CI=1.1–1.2). However, the authors characterized the association as “equivocal”, since risk did not show

a clear increase by probability or intensity of exposure. Admitted weaknesses of the study include poor

occupational information in the death certificates, possible diagnostic bias among lower socioeconomic

status cases, and the absence of more detailed information to supplement that provided in the death

certificates.

Cantor et al. (1995) conducted a case control study with 33,509 cases and 117,794 controls, matched for

age, gender, and race, to investigate the association between occupational exposure to workplace

chemicals and the incidence of breast cancer in women. Exposure was indirectly estimated using a job

exposure matrix that ranked the probability and level of 31 workplace exposures, among them methylene

chloride. All workplace chemical exposures were evaluated separately. After adjusting for

socioeconomic status imputed from occupation, the OR in the highest exposure category of methylene

chloride (probability and level of exposure combined) was slightly greater than 1.0, and statistically

significant (OR=1.46, CI=1.2–1.7). The authors caution that this analysis is crude and should be

considered a first-level “hypothesis-generating” evaluation rather than one which is “hypothesis testing.”

47 METHYLENE CHLORIDE

2. HEALTH EFFECTS

They also noted that the study had numerous methodologic limitations, including the use of death

certificates as the primary source of individual information and the use of a job-exposure matrix rather

than quantitative workplace measurements to estimate exposure. Additionally, no adjustments were made

for other risk factors such as smoking, obesity, family history, duration of employment, or other

confounding variables.

In rats exposed to low levels of methylene chloride (100 ppm) for 2 years, there was a nonsignificant

increase in the total incidence of malignant tumors (Maltoni et al. 1988). In mice and rats, inhalation of

very high levels of methylene chloride significantly increased the incidence of liver and lung cancer

(Mennear et al. 1988; NTP 1986) and benign mammary gland tumors (fibroadenomas or adenomas)

(Mennear et al. 1988; Nitschke et al. 1988a; NTP 1986).

In the NTP (1986) study, groups of 50 animals of each sex were exposed to methylene chloride by

inhalation 6 hours/day, 5 days/week for 102 weeks. F344/N rats were exposed to 0, 1,000, 2,000, or

4,000 ppm of methylene chloride and B6C3F

mice were exposed to 0, 2,000, or 4,000 ppm. At or above

2,000 ppm, the incidence of liver tumors (mostly hepatocellular adenomas or carcinomas) in mice was

significantly higher than in chamber and historical control groups (NTP 1986); at 4,000 ppm, the

incidence of liver tumors was highly significant (p#0.001). The incidence of combined benign and

malignant liver tumors was high (67–83%) in the treated animals. There was also a statistically

significant increase in the incidence of lung tumors in mice (p<0.001) exposed at 2,000 ppm or above;

these tumors were primarily alveolar/bronchiolar adenomas or carcinomas. The incidence of combined

benign and malignant lung tumors was 54–85% in the treated animals. The NTP (1986) report concluded

that there was “some evidence of carcinogenicity” in male rats and “clear evidence of carcinogenicity” in

female rats, based on the increased incidence of benign mammary neoplasms following 2 years of

inhalation exposure to methylene chloride. The report concluded that there was “clear evidence for

carcinogenicity” for methylene chloride chronic inhalation exposure, based on the increased incidence of

alveolar/bronchiolar neoplasms and of hepatocellular neoplasms.

In two related studies, Kari et al. (1993) and Maronpot et al. (1995b) examined the progressive

development of lung and liver tumors in B6C3F

mice exposed via chamber inhalation to 2,000 ppm

methylene chloride for 6 hours/day, 5 days/week, for 104 weeks. In addition, a series of stop exposure

experiments were performed to evaluate the effects of differing exposure durations on tumor

development. Kari et al. (1993) examined histology and histopathology of lung and liver tumors, whereas

48 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Maronpot et al. (1995b) evaluated DNA synthesis and oncogene expression during tumor development.

(mechanistic aspects of Maronpot et al. (1995b) are discussed in Sections 2.2.1.7 and 2.4.2). Chronic

high-concentration exposure to methylene chloride resulted in: (1) an 8-fold increase in the incidence of

animals having lung adenomas or carcinomas as compared to controls; (2) a 13-fold increase in the total

number of lung tumors in each animal at risk; (3) a 2.5-fold increase in the incidence of mice having liver

adenomas or carcinomas compared to controls; and (4) a 3-fold increase in the number of liver tumors in

each animal at risk. The development of the first lung tumors in methylene chloride exposed mice

occurred 1 year earlier than in control animals. In contrast, there was no difference in the latency to first

liver tumor period between exposed and control animals. The incidences of tumors in lungs, but not liver,

continued to increase after cessation of exposure. Maronpot et al. (1995b) found that 26 weeks of

exposure was sufficient to significantly and irreversibly increase the incidence of lung tumors at 2 years,

whereas the incidence of hepatic tumors increased with 78 weeks of exposure, but not with 25 or

52 weeks of exposure. Furthermore, vulnerability to methylene chloride may have been age-related, since

no lung tumor increase was observed in mice that were kept under control conditions for 52 weeks prior

to methylene chloride exposure for 52 weeks. Based on these results, Kari et al. (1993) and Maronpot et

al. (1995b) concluded that methylene chloride is a more potent lung than liver carcinogen in female

B6C3F

mice; the differing incidence of lung and liver tumors under various exposure regimes suggests

that the mechanisms of tumorigenesis in these target organs may be different.

The EPA (1985b) reviewed the NTP data on the carcinogenic effects of methylene chloride and

calculated a human potency estimate. The potency factor (q

*) which represents a 95% upper confidence

limit of the extra lifetime human risk, is 1.4x10

-2

(mg/kg/day)

-1

. The unit risk estimate (the excess cancer

risk associated with lifetime exposure to 1 µg/m

) for inhalation exposures is 4.1x10

-6

. The EPA (1987a,

1987b) lowered this risk estimate to 4.7x10

-7

µg/m

on the basis of pharmacokinetics data reported by

Andersen et al. (1987). Based on this value, cancer risk levels of 10

-4

, 10

-5

, 10

-6

, and 10

-7

correspond to

70 years of continuous exposure to 0.06, 0.006, 0.0006, and 0.00006 ppm, respectively. The predicted

cancer risks are considered conservative upper estimates. The actual risk of cancer is unlikely to be

higher and may be substantially lower. These values are recorded in Figure 2-1. EPA is planning to re-

evaluate potential human risks associated with inhalation exposure to methylene chloride based on new

mechanistic data and more recent pharmacokinetic modeling using tissue dosimetry (see Sections 2.3.5

and 2.4).

49 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.2.2 Oral Exposure

2.2.2.1 Death

Hughes and Tracey (1993) reported a case in which a woman ingested 300 mL of Nitromors, a paint

remover solvent containing 75–80% methylene chloride, and died 25 days later. Ingestion of this paint

remover is known to cause severe corrosion of the gastrointestinal tract, and the autopsy revealed that

death was due to the corrosive effects of the paint remover rather than to the metabolic consequences of

methylene chloride ingestion.

Acute oral LD

values of 2,100 (Kimura et al. 1971) and 2,300 mg/kg (Marzotko and Pankow 1987)

were reported for methylene chloride in rats. Ninety-five percent lethality was reported in rats dosed with

4,382 mg/kg of methylene chloride (Ugazio et al. 1973). The cause of death appeared to be respiratory

failure as a result of depression of the central nervous system. Statistically significant increases in

mortality occurred among male rats gavaged with 320 mg/kg/day of methylene chloride and among male

and female mice receiving $64 mg/kg/day for more than 36 weeks (Maltoni et al. 1988).

values and LOAEL values for lethality in each species and duration category are recorded in

Table 2-2 and plotted in Figure 2-2.

2.2.2.2 Systemic Effects

No studies were located regarding respiratory, cardiovascular, musculoskeletal, or dermal effects in

humans or animals following oral exposure to methylene chloride. Gastrointestinal, hematological,

hepatic, renal, endocrine, and metabolic effects after oral exposure to methylene chloride are discussed

below.

The highest NOAEL values and all reliable LOAEL values for systemic effects in each species and

duration category are recorded in Table 2-2 and plotted in Figure 2-2.

Gastrointestinal Effects. A fatal single oral dose of Nitromors, a paint solvent containing 75–80%

methylene chloride resulted in severe corrosion of the gastrointestinal tract, perforation, peritonitis,

septicemia, and death (Hughes and Tracey 1993). A man who ingested 1–2 pints of Nitromors in a

56 METHYLENE CHLORIDE

2. HEALTH EFFECTS

suicide attempt, and started treatment with diuresis and hydrocortisone 1.5 hours later, had episodic

gastrointestinal hemorrhage and duodenojejunal ulceration that developed into diverticula 6 months later

(Roberts and Marshall 1976). No studies were located regarding gastrointestinal effects in animals after

oral exposure to methylene chloride.

Hematological Effects. COHb levels were elevated to 9% in a woman following lethal ingestion of

300 mL of Nitromors, a paint remover solvent containing 75–80% methylene chloride (Hughes and

Tracey 1993). The authors reported that this case study was the first to reveal that ingestion of methylene

chloride results in the formation of COHb, as occurs with methylene chloride inhalation. A man who

ingested 1–2 pints of Nitromors in a suicide attempt exhibited gross hemoglobinuria, a symptom of

intravascular hemolysis (Roberts and Marshall 1976).

In an acute rat study, hemolysis developed within 2 days following a single gavage dose of 1,325 mg/kg

of methylene chloride (Marzotko and Pankow 1987); no such effect was seen at 798 mg/kg. In rats that

were given $420 mg/kg/day of methylene chloride in the drinking water for 3 months, mean hemoglobin

concentrations were elevated in males, and erythrocyte counts were elevated, although mean corpuscular

hemoglobin was reduced, in females (Kirschman et al. 1986); because no quantitative data were

presented, this information is not presented in Table 2-2. In rats of both sexes that were given

55–249 mg/kg/day of methylene chloride in drinking water for 2 years, red blood cell counts and

hematocrit and hemoglobin levels were increased over concurrent control levels (Serota et al. 1986a);

however, Serota et al. (1986a) indicate that half of these increases were statistically significant without

specifying which ones. A dose level of 6 mg/kg/day was a NOAEL. In mice exposed similarly to

60–236 mg/kg/day no significant hematological effects were observed after 104 weeks of treatment

(Serota et al. 1986b).

Hepatic Effects. No studies were located regarding hepatic effects in humans after oral exposure to

methylene chloride.

In rats given two doses (39–1,275 mg/kg each) of methylene chloride by gavage, there were no

significant effects (within 24 hours) in the liver on levels of glutathione, cytochrome P-450, or serum

ALT (Kitchin and Brown 1989). However, at the highest dose level, the activity of ornithine

decarboxylase, an enzyme involved in cell growth, was significantly increased, and DNA damage was

detected in the livers of rats. In another rat study, acute exposure to doses of 1,095 mg/kg/day of

57 METHYLENE CHLORIDE

2. HEALTH EFFECTS

methylene chloride by gavage resulted in liver necrosis (Ugazio et al. 1973). After ingestion of

methylene chloride in drinking water at or above doses of 1,200 mg/kg/day for 3 months, hepatic effects

in rats included centrilobular necrosis, granulomatous foci, accumulation of ceroid or lipofuscin, and

dose-related hepatocytic vacuolation (Kirschman et al. 1986). Liver damage was indicated by increases

in serum ALT in males ($166 mg/kg/day) and females (1,469 mg/kg/day), and by an increase in serum

AST in the latter group. In a parallel 3-month study in mice, there were dose-related hepatic centrilobular

fatty changes over the dose range between 226 and 2,030 mg/kg/day (Kirschman et al. 1986). Chronic

ingestion of methylene chloride in drinking water has been associated with histological alterations of the

liver (cellular foci and areas of cellular alterations) of rats exposed to dose levels of 55 mg/kg/day or

greater (Serota et al. 1986a) and fatty changes in mice exposed to levels of 236 mg/kg/day (Serota et al.

1986b); the NOAELs were 6 and 175 mg/kg/day in rats and mice, respectively. F344 rats and B6C3F

mice were exposed for 104 weeks to methylene chloride in deionized drinking water at target doses of 0,

5, 50, 125, or 250 mg/kg/day (for rats) and 0, 60, 125, 185, or 250 mg/kg/day (for mice) (Serota et al.

1986a, 1986b). Based on the calculated intake of 6 mg/kg/day for rats (Serota et al. 1986a), a chronic oral

MRL of 0.06 mg/kg/day was calculated as described in the footnote of Table 2-2.

Renal Effects. Hemoglobinuria as a result of hemolysis was noted in the case of a suicide attempt by

ingestion of the paint remover Nitromors (Roberts and Marshall 1976); the authors indicated that the renal

damage that could have been a consequence of hemolysis was averted by treatment with diuresis and

hydrocortisone. In an acute animal study, administration of a single oral dose of 1,325 mg/kg of

methylene chloride inhibited diuresis in rats (Marzotko and Pankow 1987). In a 3-month study, the pH of

the urine was lowered in all rats that ingested methylene chloride at levels $166 mg/kg/day, and increased

kidney weights were observed in female rats receiving 1,469 mg/kg/day of methylene chloride, but not

607 mg/kg/day (Kirschman et al. 1986).

Endocrine Effects. No studies were located regarding endocrine effects in humans following oral

exposure to methylene chloride. In the only animal study that mentions endocrine effects, administration

of a single oral dose of methylene chloride ($526 mg/kg) to male rats resulted in angiectasis (dilatation of

capillaries) of the adrenal medulla and statistically significant increases in secretion of catecholamines

(epinephrine and norepinephrine) (Marzotko and Pankow 1987). The authors suggested that these

findings appeared to be stress-related.

58 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Ocular Effects. No studies were located regarding ocular effects in humans after oral exposure to

methylene chloride. No treatment-related ophthalmologic findings in rats administered up to 249 mg

methylene chloride/kg/day in the drinking water for 104 weeks (Serota et al.1986a). No further relevant

information was located.

Metabolic Effects. Metabolic acidosis was detected in a man who attempted suicide by drinking

1–2 pints of Nitromors paint remover that contained methylene chloride as the active ingredient (Roberts

and Marshall 1976); he recovered following treatment with diuresis and hydrocortisone.

2.2.2.3 Immunological and Lymphoreticular Effects

No studies were located regarding immunological or lymphoreticular effects in humans or animals after

oral exposure to methylene chloride.

2.2.2.4 Neurological Effects

One and a half hours after drinking 1–2 pints of paint remover (9,000–18,000 mg/kg) that contained

methylene chloride as the active ingredient, a man was deeply unconscious and unresponsive to painful

stimuli; his pupils were reactive, but tendon jerks were depressed and plantar response was absent

(Roberts and Marshall 1976). He was treated by diuresis and hydrocortisone and regained consciousness

by 14 hours after the initial event; at this time, no apparent cerebral damage was detected. The authors

suggested that recovery would have been unlikely without medical intervention.

2.2.2.5 Reproductive Effects

No studies were located regarding reproductive effects in humans or animals following oral exposure to

methylene chloride.

2.2.2.6 Developmental Effects

No studies were located regarding developmental effects in humans or animals following oral exposure to

methylene chloride.

59 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.2.2.7 Genotoxic Effects

No studies were located regarding genotoxic effects in humans after oral exposure to methylene chloride.

In rats given two doses of 1,275 mg/kg of methylene chloride by gavage (17 hours apart), significant

DNA breakage was detected in the liver 4 hours after the last dose (Kitchin and Brown 1989). Methylene

chloride did not induce a statistically significant increase of micronuclei of polychromatic erythrocytes in

mice when administered at doses up to 4,000 mg/kg (Sheldon et al. 1987). In mice given a single dose of

1,720 mg/kg of methylene chloride, DNA breaks were detected in nuclei from the liver and lung, but not

from stomach, kidney, urinary bladder, brain, or bone marrow; the authors indicated that there was no

evidence of cytotoxicity that might have caused the genetic damage (Sasaki et al. 1998). The variability

in genotoxicity may reflect tissue-specific variation in the metabolism of methylene chloride.

Other genotoxicity studies are discussed in Section 2.5.

2.2.2.8 Cancer

No studies were located regarding carcinogenic effects in humans after oral exposure to methylene

chloride.

Studies in animals provide suggestive evidence that ingestion of methylene chloride may increase the

incidence of liver cancer. In an acute rat study, two doses of 1,275 mg/kg of methylene chloride (given

17 hours apart) caused an increase in the liver activity of ornithine decarboxylase, an enzyme that may

contribute to the promotion of hepatic cancer (Kitchin and Brown 1989). Liver tumors were observed in

female, but not in male rats that ingested methylene chloride (up to 250 mg/kg/day) for 2 years, but the

cancer incidence rates were within historical control ranges (Serota et al. 1986a). Although liver cancer

was observed in male mice, the incidence was not significantly elevated. Female mice did not have

increased liver tumor incidence (Serota et al. 1986b). An increased incidence of mammary tumors was

found in female rats that received methylene chloride by gavage at 500 mg/kg/day for 64 weeks but the

results were not statistically significant (Maltoni et al. 1988). Under the same exposure conditions, an

increased incidence of pulmonary tumors in male mice was statistically significant (p<0.05) when the

increased mortality rate was taken into account (Maltoni et al. 1988). The EPA (1985b, 1987a) reviewed

60 METHYLENE CHLORIDE

2. HEALTH EFFECTS

the available data on the carcinogenic effects of methylene chloride and concluded that there was

borderline evidence for carcinogenicity. The EPA estimated that the upper bound incremental unit

carcinogenic risk for drinking water containing 1 µg/L methylene chloride for a lifetime was

2.1x10

-7

(µg/L)

-1

. This risk estimate derivation was based on the mean of the carcinogenic risk estimates

from the finding of liver tumors in the NTP (1986) inhalation study in female mice; the lung tumor data

from the NTP study were not included in EPA’s analysis. Since the extrapolation model is linear at low

doses, additional lifetime cancer risk is directly proportional to the water concentration of methylene

chloride. Thus, risk levels of 10

-4

, 10

-5

, 10

-6

, and 10

-7

are associated with 0.5, 0.05, 0.005, and

0.0005 mg/L, respectively (0.1, 0.01, 0.001, and 0.0001 mg/kg/day). Because these values are based on

upper bound estimates, the true risk could be lower. These values are shown in Figure 2-2.

2.2.3 Dermal Exposure

2.2.3.1 Death

No studies were located regarding death in humans or animals after dermal exposure to methylene

chloride.

2.2.3.2 Systemic Effects

Respiratory Effects. Shortness of breath was reported among some autoworkers who were exposed to

methylene chloride both dermally and by inhalation for periods of up to 3 years (Kelly 1988). However,

it is likely that inhalation exposure is largely responsible for this effect. No studies were located

regarding respiratory effects in animals following dermal exposure to methylene chloride.

No studies were located regarding the following systemic effects in humans or animals after dermal

exposure to methylene chloride:

Cardiovascular Effects.

Gastrointestinal Effects.

Hematological Effects.

61 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Musculoskeletal Effects.

Renal Effects.

Hepatic Effects. One autoworker who was exposed to methylene chloride by dermal contact and by

inhalation (3.3–154.4 ppm, average of 68 ppm) for 1.5 years was reported to have an enlarged liver in

addition to adverse neurological and reproductive effects (Kelly 1988). This study is described in more

detail under “Inhalation Exposure” in Section 2.1.3.2. The relative contributions of the two exposure

routes to hepatotoxicity were not determined.

No studies were located regarding hepatic effects in animals after dermal exposure to methylene chloride.

Dermal Effects. There are few studies of dermal effects of methylene chloride in humans. In several

cases, workers who were rendered unconscious while stripping furniture using a methylene chloride-

based compound in an open tank became partially immersed in the liquid (Hall and Rumack 1990); first

or second degree chemical burns developed on areas of the body having direct contact with the liquid. In

another case, a worker who was cleaning the interior of a tank with methylene chloride became

unconscious and fell into the solvent when the bucket overturned (Wells and Waldron 1984); during the

30 minutes before he was removed, second and third degree burns developed on the areas of contact. In a

similar workplace accident, in which a man was found dead after 1 hour of exposure to methylene

chloride, chemical burns with excoriation had developed on the areas of contact (Winek et al. 1981).

Ocular Effects. Data are limited regarding ocular effects in humans after dermal exposure to

methylene chloride. Severe corneal burns developed in a worker who was found unconscious and

slumped over with his face partially submerged in an open tank of paint stripper containing methylene

chloride (Hall and Rumack 1990). The duration of exposure was not indicated.

In animals, methylene chloride (0.01–0.1 mL) caused eye irritation and inflammation, increased corneal

thickness, and increased intraocular tension in rabbits following instillation of the liquid solvent into the

conjunctival sac (Ballantyne et al. 1976). Effects were reversible within 3–9 days after treatment. In the

same study, rabbits exposed to vapors of methylene chloride at concentrations of 490 ppm or greater for

10 minutes also showed effects on the eyes. There were small increases in corneal thickness and

intraocular tension. Effects were reversible within 2 days. It is likely that effects observed were due to

62 METHYLENE CHLORIDE

2. HEALTH EFFECTS

direct effect of vapors on the cornea rather than to inhaled methylene chloride or its metabolites acting on

the eyes.

2.2.3.3 Immunological and Lymphoreticular Effects

No studies were located regarding immunological or lymphoreticular effects in humans or animals after

dermal exposure to methylene chloride.

2.2.3.4 Neurological Effects

Neurological effects (loss of memory, loss of concentration, sensory deficit) were reported in a group of

34 male autoworkers, who were exposed to methylene chloride by dermal contact and by inhalation

(3.3–154.4 ppm, average of 68 ppm) for up to 3 years (Kelly 1988). This study is described in more

detail under “Inhalation Exposure” in Section 2.1.3.4. The relative contributions of the two exposure

routes to neurotoxicity were not determined.

No studies were located regarding neurological effects in animals after dermal exposure to methylene

chloride.

2.2.3.5 Reproductive Effects

One group of case studies reported reproductive effects in 8 out of 34 men who complained of central

nervous system dysfunction following occupational exposure to methylene chloride for 0.4–2.9 years

(Kelly 1988). The eight men were part of a subgroup of 26 ‘bonders’ who applied methylene chloride to

automobile parts during assembly; this involved soaking pads from open buckets of methylene chloride

using ungloved hands, so exposures were both by dermal contact and by inhalation (3.3–154.4 ppm,

average of 68 ppm). This study is described in more detail under “Inhalation Exposure” in

Section 2.1.3.5. Infectious disease was eliminated as a cause of the adverse reproductive effects (genital

pain, testicular atrophy, and oligospermia). The relative contributions of the two exposure routes to

reproductive toxicity were not determined. Uncertainty regarding this study involves the small number of

subjects, the multiple exposure to other organic chemicals, the lack of blood or urine samples quantifying

exposure to methylene chloride, and the lack of a control group. No other studies were located that

reported similar male reproductive effects from exposure to methylene chloride. Contrary to Kelly’s

63 METHYLENE CHLORIDE

2. HEALTH EFFECTS

(1988) findings, Wells et al. (1989) found no evidence of oligospermia among four workers who had been

exposed to levels of methylene chloride that were twice as high as in the Kelly study while involved in

furniture stripping for at least 3 months; dermal exposure may have occurred, but this was not specified in

the report. It is not certain whether the longer duration of exposure (minimum occupational exposure 1.4

years) or exposure to other chemicals in the Kelly (1988) study contributed to the different outcomes in

the two studies.

2.2.3.6 Developmental Effects

No studies were located regarding developmental effects in humans or animals after dermal exposure to

methylene chloride.

2.2.3.7 Genotoxic Effects

No studies were located regarding genotoxic effects in humans or animals after dermal exposure to

methylene chloride.

Genotoxicity studies are discussed in Section 2.5.

2.2.3.8 Cancer

No studies were located regarding cancer effects in humans or animals after dermal exposure to

methylene chloride.

2.3 TOXICOKINETICS

Inhalation is the main route of exposure to methylene chloride for humans. Within the first few minutes

of exposure, approximately 70–75% of inhaled vapor is absorbed. However, as the concentration of

methylene chloride in the blood increases, the net uptake is greatly reduced until at steady-state, it is equal

to metabolic clearance, which has a maximum (determined by the fraction of blood flowing to the liver)

of 25% (EPA 1994). Under conditions of continuous exposure to air concentrations of up to

approximately 300 ppm, blood steady state concentrations of methylene chloride are reached in about

4 hours. Pulmonary absorption is influenced by exercise and body fat. In animals, pulmonary absorption

64 METHYLENE CHLORIDE

2. HEALTH EFFECTS

is proportional to magnitude and duration of exposure over a concentration range of 100–8,000 ppm. An

increase of the steady state blood/air concentration ratio at high exposure levels reflects saturation of

metabolic pathways rather than an increased absorption coefficient. There is only qualitative evidence of

oral absorption in humans. In animals, methylene chloride is easily absorbed from the gastrointestinal

tract, particularly from aqueous media. Seventy-five to 98% of an administered dose may be absorbed in

10–20 minutes. There are no quantitative data on dermal absorption of methylene chloride, although it is

known to occur.

Distribution data in humans are lacking, but methylene chloride has been found in human breast milk and

blood. Methylene chloride is widely distributed in animal tissues after inhalation exposure. The highest

concentrations are found in adipose tissue and liver. Methylene chloride has been found in blood from

rats’ fetuses. After acute exposure, methylene chloride disappears rapidly from fat. Distribution of

methylene chloride does not seem to be route-dependent and it does not bioaccumulate in tissues.

There are two main competing metabolic pathways for methylene chloride; one initially catalyzed by

cytochrome P-450 enzymes (CYP2E1) and the other by a theta glutathione-S-transferase (GSSTl-1). The

P-450 pathway (MFO) produces carbon monoxide and carbon dioxide via formyl chloride and the

glutathione pathway (GST) produces carbon dioxide via a postulated glutathione conjugate (S-chloro-

methyl glutathione) and formaldehyde. Both pathways can give rise to toxic metabolites. The oxidative

pathway is preferred at lower exposure concentrations and becomes saturated as exposure levels increase.

Oxidative biotransformation of methylene chloride is similar in rats and humans. In rats, the MFO

pathway is high-affinity low capacity, whereas the GST pathway has lower affinity, but higher capacity.

The GST pathway is more active in mice than in rats and less active in hamsters and humans than in rats.

After inhalation exposure, humans eliminate methylene chloride mainly in expired air, but also in the

urine. In rats, following a single exposure to radioactive methylene chloride, exhaled air had the most

radioactivity, but radioactivity was also found in urine and feces. In exhaled air, the radiolabel was

mostly as carbon monoxide and carbon dioxide. Physiologically based pharmacokinetic (PBPK) models

have been developed to describe disposition of methylene chloride in humans and animals. These models

were designed to distinguish contributions of the two metabolic pathways in lung and liver tissue, to look

for correlations between tumor incidence and various measures of target tissue dose predicted by the

models, and to extrapolate cancer risks from mice to humans.

65 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.3.1 Absorption

There is no information to determine whether absorption of methylene chloride in children is different

than in adults.

2.3.1.1 Inhalation Exposure

The principal route of human exposure to methylene chloride is inhalation. During absorption through

the lungs, the concentration of methylene chloride in alveolar air in equilibrium with pulmonary venous

blood content approaches the concentration in inspired air until a steady state is achieved. After tissue

and total body steady states are reached through the lungs and other routes, uptake is balanced by

metabolism and elimination. Steady state blood methylene chloride concentrations appear to be reached

after 2–4 hours of exposure (DiVincenzo and Kaplan 1981; McKenna et al. 1980).

Evaluation of pulmonary uptake in humans indicated that 70–75% of inhaled methylene chloride vapor

was absorbed initially (DiVincenzo and Kaplan 1981). Initial absorption of methylene chloride was rapid

as indicated by an uptake of methylene chloride into the blood of approximately 0.6 mg/L in the first hour

of exposure to levels of 100–200 ppm. At a concentration of 50 ppm, the increase in blood methylene

chloride concentration was 0.2 mg/L for the first hour (DiVincenzo and Kaplan 1981). There was a direct

correlation between the steady state blood methylene chloride values and the exposure concentration,

with a proportionality constant of approximately 0.008 ppm in blood per ppm in air (DiVincenzo and

Kaplan 1981). The blood concentrations reached steady state values during the 4th through 8th hour of

continuous exposure to the vapor. Once exposure ceased, methylene chloride was rapidly cleared from

the blood. Six hours after the end of exposure, only traces of methylene chloride were present in the

blood in the highest-concentration group, and pre-exposure baseline blood levels were detected in the

other exposure groups.

Similar to other lipophilic organic vapors, methylene chloride absorption appears to be influenced by

factors other than the vapor concentration. Increased physical activity increases the amount of methylene

chloride absorbed by the body due to an increase in ventilation rate and cardiac output (Astrand et al.

1975; DiVincenzo et al. 1972). Uptake also increases with the percent body fat since methylene chloride

dissolves in fat to a greater extent than it dissolves in aqueous media (Engstrom and Bjurstrom 1977).

Therefore, obese subjects will absorb and retain more methylene chloride than lean subjects exposed to

66 METHYLENE CHLORIDE

2. HEALTH EFFECTS

the same vapor concentration. This does not mean obese subjects are more sensitive to toxicity. Under

controlled conditions, there was a 30% greater absorption and retention of methylene chloride by obese

subjects exposed to 75 ppm for 1 hour as compared to lean subjects (Engstrom and Bjurstrom 1977).

Studies of the relationship of inhalation exposures of animals to their blood methylene chloride

concentrations indicate that absorption is proportional to the magnitude and duration of the exposure over

a methylene chloride concentration range of 100–8,000 ppm. This conclusion is based on the monitoring

of blood methylene chloride concentrations following inhalation exposure in dogs and rats (DiVincenzo

et al. 1972; MacEwen et al. 1972; McKenna et al. 1982). As was the case with humans, blood methylene

chloride levels reached a steady state value as the duration of exposure increased (McKenna et al. 1982).

Studies of blood methylene chloride values during 6-hour exposures of rats to between 50 and 1,500 ppm

of methylene chloride suggest that the steady state blood/air concentration ratio increases as the exposure

concentration increases. The ratio of the steady state methylene chloride concentration in the blood to the

exposure concentration increased from 0.001 to 0.005 and 0.007 as the exposure increased from 50 to 500

to 1,500 ppm, respectively (McKenna et al. 1982). It is postulated that the increased ratio at steady state

results from saturation of metabolic pathways as exposure increases rather than from an increased

absorption coefficient.

2.3.1.2 Oral Exposure

No quantitative studies were located regarding absorption in humans after oral exposure to methylene

chloride. There is qualitative evidence that the compound is absorbed when ingested. A male became

deeply unconscious within 1.5 hours after ingestion of 1–2 pints of a paint remover (9,000–18,000 mg/kg)

(Roberts and Marshall 1976).

In animals, the limited available data suggest that methylene chloride is easily absorbed from the

gastrointestinal tract, particularly if exposed via aqueous media. Ten minutes after treatment, 24% of the

administered dose (50 mg/kg in aqueous solution) was recovered from the upper gastrointestinal tract of

mice when the stomach and small intestinal tissues and contents were analyzed (Angelo et al. 1986a).

Only 2.2% of the methylene chloride was in the stomach and small intestines 20 minutes after compound

administration and less than 1% remained after 40 minutes. At 10 minutes the large intestines and

67 METHYLENE CHLORIDE

2. HEALTH EFFECTS

caecum contained 0.08% of the dose; these values were less than those detected after 20 minutes. Thus,

75% of the dose was absorbed within 10 minutes and about 98% of the dose was absorbed within 20 minutes.

On the other hand, after treatment of mice with 10–1,000 mg methylene chloride in corn oil,

approximately 55% of the administered dose was detected in the stomach and small intestines at

20 minutes and remained there at 2 hours (Angelo et al. 1986a). The large intestines and caecum tissues

and contents contained about 5–8% of the dose at 10 minutes. This value declined to 1–2% at the end of

2 hours. This suggests that the absorption processes are slowed when methylene chloride is presented in

a hydrophobic vehicle. During the 2 hours of observation, approximately 40–45% of the 50 mg dose was

absorbed from the oil vehicle as compared to essentially all of a comparable dose in water. These data are

consistent with the slower emptying of chyme from the stomach when lipids are present, the partitioning

of methylene chloride between the corn oil and the aqueous digestive fluids, the necessity that fats be

emulsified by bile prior to digestion and absorption, and delayed mobilization of lipophilic substances

from the gastrointestinal tract.

Staats et al. (1991) developed a two-compartment model of oral absorption, dependent in part on the

vehicle (aqueous or lipid) and on the lipophilic characteristics of the ingested compound. Absorption

along the gastrointestinal tract was predicted to increase in the first compartment (likely the stomach),

when fat-soluble toxicants are administered in water. When administered in corn oil, the toxicants are

likely to adhere to the lipid vehicle in the first compartment and absorption in the stomach is likely to

decrease. The model also predicts that passage of the compound from the first compartment (stomach) to

the second compartment (small intestine) is enhanced when the vehicle is oil rather than water. However,

existing experimental data suggest that absorption from the second compartment (small intestine) is

generally slow for all lipophilic compounds, irrespective of vehicle. Using previously-determined

absorption and transfer constant values, the model developed by Staats et al. (1991) was able to predict

reasonably experimental data for trichloroethylene. The ability of the model to predict the gastrointestinal

absorption of methylene chloride was not determined. However, in general, the model is consistent with

methylene chloride experimental data (Angelo et al. 1986a).

68 METHYLENE CHLORIDE

2. HEALTH EFFECTS

2.3.1.3 Dermal Exposure

No studies were located regarding absorption in humans following dermal exposure to methylene

chloride.

Dermal permeability constants for rats were obtained for 3 concentrations of methylene chloride (30,000,

60,000, and 100,000 ppm) in air for use in developing a pharmacokinetics model for dermal absorption of

vapors (McDougal et al. 1986). The mean permeability constant was 0.28 cm/hr. The total amount

absorbed was determined to be 34.4, 57.5, and 99.4 mg, respectively, for the three concentrations tested.

2.3.2 Distribution

There is no information to ascertain whether distribution of methylene chloride would be different in

children than in adults.

2.3.2.1 Inhalation Exposure

When methylene chloride is absorbed by the lungs it is expected that it will dissolve in the lipoprotein

components of the blood and enter the systemic circulation after passage through the heart. It is

distributed from the systemic circulation to the body organs. However, no quantitative data were located

which showed the distribution of methylene chloride following human inhalation exposure. Some data

are available which relate to the uptake of methylene chloride by human adipose tissues. These data

indicate that the methylene chloride concentrations in the adipose deposits of lean subjects are greater

than those in obese subjects. However, the total methylene chloride adipose tissue load is greater for the

obese subjects due to their greater adipose mass (Engstrom and Bjurstrom 1977).

Distribution studies in rats demonstrate that methylene chloride and/or its metabolites are present in the

liver, kidney, lungs, brain, muscle, and adipose tissues after inhalation exposures (Carlsson and

Hultengren 1975; McKenna et al. 1982). One hour after exposure, the highest concentration of

radioactive material was found in the white adipose tissue, followed by the liver. The concentration in the

kidney, adrenals, and brain were less than half that in the liver (Carlsson and Hultengren 1975).

Radioactivity in the fat deposits declined rapidly during the first 2 hours after exposure (Carlsson and

Hultengren 1975). Concentrations in the other tissues declined more slowly. On the other hand, after

69 METHYLENE CHLORIDE

2. HEALTH EFFECTS

5 days of exposure to 200 ppm, 6 hours per day, the concentration of methylene chloride in the perirenal

fat was 6- to 7-fold greater than that in the blood and liver (Savolainen et al. 1977).

The animal data are accordingly consistent with the human adipose tissue data discussed above. When

pregnant rats were exposed to 500 ppm of methylene chloride by inhalation for 1 hour, the chemical was

found in fetal blood, but at a lower level than in maternal blood (Anders and Sunram 1982).

2.3.2.2 Oral Exposure

No studies were located regarding distribution in humans following oral exposure to methylene chloride.

In animals, radioactivity from labeled methylene chloride was detected in the liver, kidney, lung, brain,

epididymal fat, muscle, and testes after exposure of rats to a single oral gavage dose of 1 or 50 mg/kg

methylene chloride (McKenna and Zempel 1981). The tissue samples were taken 48 hours after dosing.

At that time, the lowest concentration of radioactivity was found in the fat. The highest concentrations

were in the liver and kidney; this was true for both doses. Radioactivity was found in the blood, liver,

and carcass of mice. Similar results were observed in rats administered doses of 50–1,000 mg/kg

methylene chloride for 14 days (Angelo et al. 1986a, 1986b). At each dose tested, and in each tissue, the

label was rapidly cleared during the 240 minutes after each exposure. These data suggest that methylene

chloride and/or its metabolites do not bioaccumulate in any tissues.

2.3.2.3 Dermal Exposure

In humans, direct dermal contact with pure methylene chloride causes an intense burning, mild erythema

and paresthesia (Stewart and Dodd 1964). Absorption is relatively rapid (McDougal et al. 1986).

2.3.3 Metabolism

Methylene chloride metabolism is not known to be qualitatively different in children than adults.

Information regarding the developmental expression of enzymes that metabolize methylene chloride is

discussed in Section 2.7, Children’s Susceptibility.

70 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Inhalation Exposure

Available data suggest that there are two pathways by which methylene chloride is metabolized. One

pathway utilizes the mixed function oxidase (MFO) enzymes and produces carbon monoxide (CO)

(Figure 2-3, Pathway 1). The other pathway involves the glutathione transferase (GST) and produces

carbon dioxide (CO

) (Figure 2-3, Pathway 2). It has been postulated that CO

can also be produced by

the MFO pathway if the reactive intermediate in this pathway (postulated to be formyl chloride) reacts

with a nucleophile prior to elimination of the chloride ion and formation of CO (Figure 2-3, Pathway 3)

(Gargas et al. 1986).

The MFO pathway seems to be the preferred pathway for methylene chloride metabolism following

inhalation exposures. Human subjects exposed by inhalation to 500 ppm or greater for 1 or 2 hours

experienced elevated COHb concentrations indicating that methylene chloride was metabolized to CO by

the MFO pathway (Stewart et al. 1972). The COHb concentrations rose to an average of 10.1%

saturation 1 hour after the exposure of 3 subjects to 986 ppm of methylene chloride for 2 hours. The

mean COHb concentration at 17 hours-post exposure remained elevated (3.9% saturation) above the pre-

exposure baseline value (1–1.5% saturation). The exposure of 8 subjects to 515 ppm of methylene

chloride for 1 hour increased the COHb level, which remained elevated above baseline for more than

21 hours.

In human subjects exposed by inhalation to 50–500 ppm of methylene chloride for up to 5 weeks, COHb

concentrations could be predicted from methylene chloride exposure parameters (Peterson 1978).

However, the exhaled breath concentrations of methylene chloride correlated better with exposure

parameters than did COHb concentrations. No differences in methylene chloride metabolism between

male and female subjects were detectable and there was no induction of metabolism to CO during

5-weeks exposure to concentrations ranging from 100 to 500 ppm of methylene chloride (Peterson 1978).

Metabolism of methylene chloride in animals has been shown to be similar to that in humans in an

experiment by Fodor et al. (1973). Albino rats exposed to methylene chloride showed COHb formation,

confirming the observation that methylene chloride is metabolized to CO following inhalation exposures.

Among rats exposed to methylene chloride for 4 hours, concentrations of 500 or 2,500 ppm resulted in

similar maximal blood levels of COHb (Wirkner et al. 1997). A concentration-response relationship

between inhaled methylene chloride and the maximum changes in COHb values was observed when

CYT P-450

HO X

NADPH

GSH

CTYOSOL

+ Cl

GS-CH

-X

+ Cl

GS-CH

-OH

O + GSH

HCOOH + GSH

GSH

+ Cl

NUCLEOPHILE?

(i.e., GSH)

Source: Gargas et al. 1986

1 Mixed Function Oxidase Pathway

2 Glutathione Transferase Pathway

3 Nucleophile Pathway

Figure 2-3. Proposed Pathways for Methylene Chloride Metabolism

METHYLENE CHLORIDE

2. HEALTH EFFECTS

72 METHYLENE CHLORIDE

2. HEALTH EFFECTS

4 rabbits were exposed to 1,270–11,520 ppm of methylene chloride for a 4-week period (Roth et al.

1975). The larger the exposure concentration, the longer it took for the changes in blood COHb values to

reach their maximum values. The maximum COHb concentration was observed 1.5 hours after exposure

to 1,270 ppm and 3.5 hours after exposure to 11,520 ppm.

In rats, inhalation of 50 ppm for 6 hours resulted in 26 and 27% of the body burden being recovered as

expired CO and CO

, respectively, during the first 48 hours after the exposure period (McKenna et al.

1982). At 1,500 ppm, 14 and 10% of the body burden were recovered as CO and CO

, respectively.

These values are consistent with the concept that the enzymes responsible for the metabolic conversion of

methylene chloride to CO and CO

become saturated at high-exposure concentrations.

Anders and Sunram (1982) monitored the fetal and maternal blood concentrations of methylene chloride

and carbon monoxide following inhalation exposure of pregnant rats to methylene chloride at 500 ppm

for 1 hour. Whereas the level of methylene chloride in fetal blood was significantly lower than in

maternal blood, the levels of carbon monoxide were about the same. The authors suggested that the

maternal liver has higher biotransforming activity than the fetal liver and that the metabolically generated

carbon monoxide equilibrates between the fetal and maternal circulations.

Thier et al. (1991) investigated whether the human metabolism of methylene chloride was similar to

monohalogenated methanes; these compounds are metabolized via a glutathione-dependent pathway in

human erythrocytes in a subgroup of the human population (called “conjugators”), whereas another

subgroup does not exhibit erythrocyte glutathione-mediated metabolism (called “nonconjugators”).

Blood samples were taken from 10 volunteers who had previously been determined to be either

“conjugators” (subgroup B) or “nonconjugators” (subgroup A) of monohalogenated methanes. The

samples were exposed to radiolabeled methylene chloride, incubated, centrifuged to separate blood

plasma from cellular fraction, and the distribution of radioactivity between the different blood

compartments measured. For individuals from subgroup B, radioactivity in blood plasma increased over

time, reaching 30% in the low-molecular weight fraction and 5% in the high-molecular weight fraction

after 9 hours. For individuals in subgroup A, almost no radioactivity was found in either blood plasma

molecular fraction. In all samples from both groups, no radioactivity was found in either erythrocyte

cytoplasm or membranes. Thus, although dihalogenated methylene chloride does not appear to undergo

metabolism in erythrocytes, some metabolic transformation of methylene chloride appears to have

occurred in the plasma of all individuals classified as “conjugators” (subgroup B), whereas this

73 METHYLENE CHLORIDE

2. HEALTH EFFECTS

metabolism did not occur in “nonconjugators” (subgroup A). The authors concluded that these data

provide evidence for enzyme polymorphism in humans with respect to methylene chloride metabolism.

The contributions of the MFO and GST pathways to the metabolism of methylene chloride were studied

in male rats by Gargas et al. (1986). Based on the data from these studies, the MFO pathway is a high

affinity-low capacity pathway with a metabolic rate of 47 µmol/kg/hour. The GST pathway, on the other

hand, has a lower affinity than the MFO pathway but a higher capacity. As part of this study, rats were

pretreated with an inhibitor of the MFO pathway or a glutathione (GSH) depleting agent. Concentrations

of COHb were measured to assess the relative contribution of each pathway to the total metabolism of

methylene chloride. Pretreatment with the MFO inhibitor essentially abolished CO production by the

high-affinity saturable MFO pathway and limited metabolism to the GSH-transferase pathway.

Concentrations of COHb following treatment with the GSH-depleting agent were increased 20–30%

compared to untreated controls, indicating increased activity of the MFO pathway. Some CO

was

produced, indicating that despite some depletion of GSH, some methylene chloride was still metabolized

by the GST pathway or by conversion of CO to CO

via the MFO pathway.

There appear to be species differences in pathway preference. The GST pathway is more active in the

mouse than the rat, based on studies of metabolic end products in both species during and immediately

after 6 hours of exposure to 500, 1,000, 2,000, and 4,000 ppm of methylene chloride (Green et al. 1986c).

The cytochrome MFO pathway leading to CO and COHb was shown to be saturated at the 500 ppm-

exposure level. After saturation of this pathway had occurred, the blood levels of methylene chloride in

the rat increased almost linearly with concentration, indicating little further metabolism in this species. In

contrast, there was evidence for significant metabolism of methylene chloride in the mouse at high-

concentration levels by the GST pathway, which leads to CO

. Comparison of expired CO

levels after

4,000-ppm exposure for 6 hours showed almost an order of magnitude more CO

produced per kilogram

of body weight in the mouse than in the rat. A marked difference was seen in the rate of clearance of

methylene chloride from tissues, as measured by its elimination in expired air. Although cleared from

blood rapidly in both species, the rate of clearance from rat tissues was markedly slower than in the

mouse. This slow release in the rat sustained metabolism for up to 8 hours after the end of exposure and,

consequently, had a marked effect on the overall body burden of metabolites. Methylene chloride was

cleared from tissues in the mouse in less than 2 hours. Overall, saturation of the cytochrome P-450 MFO

pathway occurred at similar levels in both species, but significantly more methylene chloride was

74 METHYLENE CHLORIDE

2. HEALTH EFFECTS

metabolized by the GST pathway in the mouse when assessed either from the blood levels of methylene

chloride or by CO

formation at high-concentration levels.

The isoenzymes involved in the metabolism and biotransformation of methylene chloride have recently

been identified for each major pathway. The MFO pathway involves cytochrome P-450 2E1 and the

glutathione-mediated pathway involves a Θ (theta) class glutathione S-transferase, GSTT1-1 (Blocki et al.

1994; Mainwaring et al. 1996b; Meyer et al. 1991). GSTT1-1 is expressed in a number of human organs

in a tissue-specific manner which is different from the pattern of expression of other glutathione

S-transferases, specifically those from the α, µ, and π classes (Sheratt et al. 1997). A recent in vitro study

by Sheratt et al. (1997) detected very low levels of GSTT1-1 in human lung cells, suggesting that the

human lung is likely to have a low capacity for activating methylene chloride into reactive metabolites.

Although Mainwaring et al. (1996b) found that the overall distribution of mRNA and protein for

GSTT1-1 and GSTT2-2 in the liver and lungs of humans was lower than in mice and rats, they found

localized high concentrations of GSTT2-2 enzyme in human bile-duct epithelial cells. However, since the

enzyme did not localize to the nucleus in these cells, the risk of genotoxic effects from methylene chloride

metabolism would appear to be small. Mainwaring et al . (1996b) also found locally high concentrations

of GSTT1-1 mRNA in a small number of Clara cells and ciliated cells of the alveolar/bronchiolar junction

in one human lung sample out of four. Mainwaring et al. (1996b) also found that rates of metabolism of

methylene chloride were low in human tissue, and lower in the lung than in the liver. Thus, it is possible

that, in some individuals, specific cell types may be vulnerable to genotoxic effects from reactive

intermediates of methylene chloride metabolism, although the overall risk is likely to be low.

The oxidative cytochrome P-450 2E1 is presumed to metabolize methylene chloride to carbon dioxide via

a reactive intermediate, formyl chloride; this metabolic pathway also produces carbon monoxide (Green

1997; Reitz 1990). The glutathione pathway metabolizes methylene chloride to carbon dioxide following

the formation of both formaldehyde and a glutathione conjugate, putatively chloromethyl glutathione. No

carbon monoxide is produced during glutathione-mediated metabolism (Green 1997; Reitz 1990).

Neither the formyl chloride, nor the chloromethyl glutathione, nor any other glutathione conjugate of

methylene chloride, has been isolated and characterized. However, according to Green (1997), the

formation of these reactive intermediates is consistent with the end products formed, and with the

enzymes and cofactors available.

75 METHYLENE CHLORIDE

2. HEALTH EFFECTS

An extremely detailed analysis of these two metabolic pathways was conducted in mice, rats, hamsters,

and humans, both in vivo and in vitro. These studies provide evidence for the concentration-dependent

behaviors of the two pathways and compare the metabolic rates by each pathway in the four species

(Bogaards et al. 1993; Green 1991, 1997; Reitz et al. 1989). In vivo, the cytochrome P-450 MFO

pathway was the major route of metabolism at low-concentration inhalation exposures to methylene

chloride. In both rats and mice, saturation of the MFO pathway occurred at concentration levels of

500 ppm and above, with maximum COHb levels reported to be 12–15%. In vitro studies verified that

this pathway was similar in all four species. In contrast, the glutathione S-transferase pathway was the

major metabolic pathway at exposure concentrations used in the rodent cancer bioassays, and showed a

linear concentration response. Furthermore, metabolic activity in mouse tissue was more than 10-fold

greater than metabolic activity in rat tissue. The glutathione-mediated metabolic rates in hamster and

human tissues were even lower than those observed in the rat (Green 1997). In human tissues, metabolic

rates in the lung were about 10-fold lower than those in the liver (Green 1997). Although the Θ (theta)

class glutathione S-transferase (enzyme 5-5 in rat) has a high specific activity for metabolizing methylene

chloride (Meyer et al. 1991; Sheratt et al.1997), the µ-class GSTs (enzymes 3-3, 3-4, and 4-4) are, as a

group, 800-fold more abundant in rat liver (Blocki et al. 1994); in rat liver cytosol, the Θ-class enzyme

(5-5) and the µ-class group of enzymes (3-3, 3-4, and 4-4) are each responsible for half of the metabolism

of methylene chloride to formaldehyde (Blocki et al. 1994). In addition, another Θ-class enzyme (12-12)

is present in rat liver, but its lability during isolation has prevented analysis of its specific activity (Meyer

et al. 1991). Although Schroder et al. (1996) demonstrated that the human erythrocyte GSTT1-1 is

polymorphic, from N-terminal modification, and differs from liver and lung GSTT1-1, it is not yet known

whether liver and lung human GSTT1-1 are polymorphic. Thus, the enzymatic basis for methylene

chloride metabolism in different tissues in different species is not completely elucidated.

Nelson et al. (1995) examined the distribution of erythrocyte GSTT1-1 polymorphisms among five

different ethnic groups: North American Caucasians, African-Americans, Mexican-Americans, Chinese,

and Koreans. Polymerase chain reaction (PCR)-based genotyping of erythrocyte GSTT1-1 demonstrated

that significant ethnic variations occur. The prevalence of the nonfunctional genotype (i.e., the one

lacking the ability to metabolize methylene chloride) was highest among Chinese (64%), followed by

Koreans (60%), African-Americans (22%), Caucasians (20%), and Mexican-Americans (10%). These

data suggest that there are ethnic differences in metabolizing capacity; additionally, substantial variations

in GSTT1-1 polymorphisms also occur within ethnic groups (Nelson et al. 1995)

76 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Kim et al. (1996b) conducted a series of experiments using the vial equilibration technique on freshly

isolated liver tissue of male Sprague Dawley rats in order to characterize the metabolism of methylene

chloride. Several different hepatic microsomal preparations were used to compare and contrast the MFO-

and GSH-mediated pathways. To produce glutathione depletion in one study, rats were injected

intraperitoneally with 250 mg phorone/kg body weight in corn oil prior to sacrifice and preparation of

liver homogenate for in vitro testing; controls received corn oil only. A glycerol buffer preparation

significantly inhibited methylene chloride metabolism while a sucrose buffer containing EDTA and KCl

did not. The use of substrates for P-450 2E1 (i.e., ethanol, pyrazole) completely inhibited methylene

chloride metabolism, indicating that the methylene chloride was metabolized preferentially by the MFO

pathway under the conditions of this study. Pretreatment with phorone to produce hepatic glutathione

depletion had little effect on the metabolic rate of methylene chloride, demonstrating that the glutathione-

mediated metabolism was not a major pathway under the conditions of the study. The authors concluded

that these results demonstrate that at low exposure levels, little methylene chloride is metabolized by the

GSH pathway, and thus these results confirm the results of other investigators.

Several PBPK models have been developed that can be used to predict tissue-specific exposures to

methylene chloride, taking into account absorption, distribution, and metabolism. A PBPK model was

used to provide quantitative estimates of the levels of methylene chloride in various organs of four

mammalian species (rats, mice, hamsters, and humans) following inhalation exposure (Andersen et al.

1987). The model, which incorporates a variety of variables representing the blood and tissue

concentrations of methylene chloride, exhaled methylene chloride, and instantaneous rates of metabolism

by each pathway, was validated by comparing predictions of concentrations of methylene chloride in

blood with time-course data obtained with Fischer-344 rats, Syrian Golden hamsters, B6C3F

mice, and

human volunteers. The predicted values for each of the four species were in agreement with the

experimental data. The model was also shown to predict the appearance and elimination of methylene

chloride metabolites reasonably well.

Other PBPK models are discussed in Section 2.3.5.

Oral Exposure In a lethal poisoning case, a 56 year-old woman ingested approximately 300 mL of

Nitromors, a paint remover solvent whose major ingredient is 75–80% methylene chloride (Hughes and

Tracey 1993). COHb levels in the patient were increased to about 9% in blood samples taken several

hours later. According to the authors, this case demonstrated that conversion of ingested methylene

77 METHYLENE CHLORIDE

2. HEALTH EFFECTS

chloride to COHb occurs in humans; previously, the conversion of ingested methylene chloride to COHb

had only been reported in rats.

No other studies were located regarding metabolism in humans after oral exposure to methylene chloride.

Animal data on metabolism indicate that the process appears to be similar for inhalation and oral

exposures. When rats were gavaged with a single oral dose of 526 mg/kg of methylene chloride, COHb

levels in blood rose to nearly 10% (Wirkner et al. 1997). When methylene chloride was administered to

rats and mice by gavage at daily doses of 50 and 200 mg/kg in the mice and 50 mg/kg in the rats, there

was a dose-dependent biotransformation of methylene chloride to CO

and CO (Angelo et al. 1986a,

1986b). Pulmonary excretion of methylene chloride, CO, and CO

could be detected within 30 minutes

of administration. Initially, exhaled CO

levels from the methylene chloride subjects exceeded CO levels

in the rats. However, the amount of exhalant did increase with time. A similar profile of metabolism was

apparent in mice based on exhaled methylene chloride, CO, and CO

values except that the exhaled CO

values support studies which indicated that the GST pathway is more important in mice than in rats

(Green et al. 1986c, 1988).

Immediately following administration of doses of 500 or 1,000 mg/kg of methylene chloride in corn oil,

the values for pulmonary excretion of methylene chloride, CO

, and CO were all lower than when

methylene chloride was administered in aqueous solution, reflecting the slower absorption of methylene

chloride from the corn oil vehicle (Angelo et al. 1986a). Three hours after administration the corn oil,

pulmonary excretory patterns were similar, but still had lower values than those for the methylene

chloride in aqueous solution.

Pankow and Jagielki (1993) studied the in vivo metabolism of methylene chloride to carbon monoxide as

measured by the COHb level in blood under the following conditions: pretreatment with methanol;

simultaneous administration of methanol; and pretreatment with glutathione-depleting chemicals. Male

Wistar rats were administered 6.2 mmol (=0.4 mL)/kg body mass methylene chloride via gavage and

>148 mmol/kg of methanol. Six hours following methylene chloride administration, blood samples were

taken for COHb determination. The animals were then sacrificed and glutathione levels were measured in

the liver. The authors concluded that the cytochrome P-450 2E1 oxidative pathway is responsible for the

formation of COHb and the metabolism of methylene chloride to carbon monoxide, and suggested that

the two metabolic pathways of methylene chloride (i.e, oxidation cytochrome P-450 2E1 and conjugation

78 METHYLENE CHLORIDE

2. HEALTH EFFECTS

via glutathione/glutathione-S-transferase) may be independent at study doses in rats. However, the

rationale for this suggestion was unclear.

A comparison of the rates of pulmonary elimination of methylene chloride, CO

, and CO for oral and

intravenous exposures of rats indicated that biotransformation rather than absorption was the rate-limiting

factor that controlled the CO and CO

production (Angelo et al. 1986b). The same situation was true in

mice when the pulmonary elimination patterns of the methylene chloride in water were compared to those

from intravenous injection (Angelo et al. 1986a). This was not the case when the methylene chloride was

given in corn oil.

These data suggest that both the MFO and GST pathways can participate in the metabolism of methylene

chloride. Factors which influence the metabolism of methylene chloride by the oral exposure route are

the rate of absorption and the distribution of the MFO and GST enzyme systems in the various tissues and

the transportation of methylene chloride via the aqueous blood components to the liver or via the

chylomicrons to systemic circulation.

Dermal Exposure No studies were located on the metabolism of methylene chloride after dermal

exposure to humans or animals.

2.3.4 Elimination and Excretion

Elimination and excretion of methylene chloride in children are not expected to be different from adults;

however, this has not been investigated.

2.3.4.1 Inhalation Exposure

Methylene chloride is removed from the body primarily in expired air and urine. In 4 human subjects

exposed to 100 ppm of methylene chloride for 2 hours, an average of 22.6 µg (0.003%) methylene

chloride was excreted in the urine within 24 hours after the exposure; in 7 subjects exposed to 200 ppm of

methylene chloride for 2 hours, the corresponding value was 81.5 µg (0.006%) (DiVincenzo et al. 1972).

The percentage values are based on a respiration rate of 1 mg/m

and the assumption that methylene

chloride is completely absorbed. No data were found on amounts recovered in feces. Methylene chloride

79 METHYLENE CHLORIDE

2. HEALTH EFFECTS

excretion in the expired air was most evident in the first 30 minutes after exposure. Initial postexposure

methylene chloride concentrations in expired breath following 2- and 4-hour exposure periods were about

20 ppm and dropped to about 5 ppm at the end of 30 minutes. Small amounts of methylene chloride

remained in the expired air at 2.5 hours.

In rats, methylene chloride was excreted in the expired air, urine, and feces following a single 6-hour

exposure to 50, 500, or 1,500 ppm of methylene chloride (McKenna et al. 1982). Exhaled air accounted

for 58–79% of the radioactive concentration. At the 50 ppm exposure only 5% of the exhaled label was

found as methylene chloride. The remainder was exhaled as CO and CO

. As the exposures increased, so

did the amount of unmetabolized methylene chloride exhaled. Methylene chloride accounted for 30% of

the label from the 500 ppm concentration and 55% of the label for the 1,500 ppm concentration. Exhaled

methylene chloride, CO

, and CO accounted for 58, 71, and 79% of the inhaled methylene chloride for

the 50, 500, and 1,500 concentrations, respectively. Urinary excretions accounted for 7.2–8.9% of the

absorbed dose and 1.9–2.3% of the absorbed dose was in the feces.

2.3.4.2 Oral Exposure

No studies were located regarding excretion in humans after oral exposure to methylene chloride.

Expired air accounted for 78–90% of the excreted dose in rats in the 48-hour period following a 1 or

50 mg/kg methylene chloride dose in aqueous solution (McKenna and Zemple 1981). The radiolabel was

present in the exhaled air as CO and CO

, as well as in expired methylene chloride. The amount of

methylene chloride in the expired air increased from 12 to 72% as the dose was increased from 1 to

50 mg/kg. Radiolabel in the urine accounted for 2–5% of the dose under the above exposure conditions,

while 1% or less of the dose was found in the feces. These data indicate that the lungs are the major

organ of methylene chloride excretion even under oral exposure conditions.

2.3.4.3 Dermal Exposure

No studies were located regarding excretion in humans or animals after dermal exposure to methylene

chloride.

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2.3.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models

Physiologically based pharmacokinetic (PBPK) models use mathematical descriptions of the uptake and

disposition of chemical substances to quantitatively describe the relationships among critical biological

processes (Krishnan et al. 1994). PBPK models are also called biologically based tissue dosimetry

models. PBPK models are increasingly used in risk assessments, primarily to predict the concentration of

potentially toxic moieties of a chemical that will be delivered to any given target tissue following various

combinations of route, dose level, and test species (Clewell and Andersen 1985). Physiologically based

pharmacodynamic (PBPD) models use mathematical descriptions of the dose-response function to

quantitatively describe the relationship between target tissue dose and toxic end points.

PBPK/PD models refine our understanding of complex quantitative dose behaviors by helping to

delineate and characterize the relationships between: (1) the external/exposure concentration and target

tissue dose of the toxic moiety, and (2) the target tissue dose and observed responses (Andersen et al.

1987; Andersen and Krishnan 1994). These models are biologically and mechanistically based and can

be used to extrapolate the pharmacokinetic behavior of chemical substances from high to low dose, from

route to route, between species, and between subpopulations within a species. The biological basis of

PBPK models results in more meaningful extrapolations than those generated with the more conventional

use of uncertainty factors.

The PBPK model for a chemical substance is developed in four interconnected steps: (1) model

representation, (2) model parametrization, (3) model simulation, and (4) model validation (Krishnan and

Andersen 1994). In the early 1990s, validated PBPK models were developed for a number of

toxicologically important chemical substances, both volatile and nonvolatile (Krishnan and Andersen

1994; Leung 1993). PBPK models for a particular substance require estimates of the chemical substance-

specific physicochemical parameters, and species-specific physiological and biological parameters. The

numerical estimates of these model parameters are incorporated within a set of differential and algebraic

equations that describe the pharmacokinetic processes. Solving these differential and algebraic equations

provides the predictions of tissue dose. Computers then provide process simulations based on these

solutions.

The structure and mathematical expressions used in PBPK models significantly simplify the true

complexities of biological systems. If the uptake and disposition of the chemical substance(s) is

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adequately described, however, this simplification is desirable because data are often unavailable for

many biological processes. A simplified scheme reduces the magnitude of cumulative uncertainty. The

adequacy of the model is, therefore, of great importance, and model validation is essential to the use of

PBPK models in risk assessment.

PBPK models improve the pharmacokinetic extrapolations used in risk assessments that identify the

maximal (i.e., the safe) levels for human exposure to chemical substances (Andersen and Krishnan 1994).

PBPK models provide a scientifically sound means to predict the target tissue dose of chemicals in

humans who are exposed to environmental levels (for example, levels that might occur at hazardous waste

sites) based on the results of studies where doses were higher or were administered in different species.

Figure 2-4 shows a conceptualized representation of a PBPK model.

PBPK models for methylene chloride are discussed in this section in terms of their use in risk assessment,

tissue dosimetry, and dose, route, and species extrapolations.

2.3.5.1 Methylene Chloride PBPK Model Comparison

The first methylene chloride-specific pharmacokinetic model was developed by Andersen et al. (1987) for

use in methylene chloride human risk assessment, formulated by integrating information on mouse

physiology, methylene chloride solubility characteristics, and metabolic rate constants to describe the

disposition of methylene chloride in target tissues. This model was actually a modification of earlier

models developed by Ramsey and Andersen (1984) for generic PBPK analysis of volatile chemicals and

by Gargas et al. (1986) for study of the kinetics and metabolism of dihalomethanes. The model could

predict the time-course of the parent chemical and the production of metabolites by both GSH and MFO

pathways. A more comprehensive model developed by Andersen et al. (1991) is also capable of

describing the production of carbon monoxide during oxidative metabolism by the MFO pathway and the

production of COHb by CO binding to blood hemoglobin. This model (Andersen et al. 1991) provided a

coherent description of experimental data from both rodents and humans. In an earlier draft of this

toxicological profile, the Andersen et al. (1991) model was used to conduct route-to-route extrapolation

of the inhalation data in humans from the Putz et al. (1979) study to develop an equivalent oral dose that

was evaluated as the basis for an acute oral MRL. (However, a different critical study has been selected

and has been modeled as described below). The model calculated the peak level (6.37%) of carbon

Inhaled chemical Exhaled chemical

Ingestion

Skin

Kidney

Richly

perfused

tissues

Slowly

perfused

tissues

Fat

Liver

Lungs

Tract

max

Urine

Chemicals in air

contacting skin

Feces

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Figure 2-4. Conceptual Representation of a Physiologically Based

Pharmacokinetic (PBPK) Model for a Hypothetical Chemical Substance

Source: adapted from Krishnan et al. 1994

Note: This is a conceptual representation of a physiologically based pharmacokinetic (PBPK)

model for a hypothetical chemical substance. The chemical substance is shown to be absorbed

via the skin, by inhalation, or by ingestion, metabolized in the liver, and excreted in the urine or

by exhalation.

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monoxide in the blood that produced a neurological effect (decreased visual and psychomotor

performance and auditory function) following exposure to 200 ppm of methylene chloride (Putz et al.

1979). The equivalent concentration of methylene chloride in drinking water that will produce the same

neurological effect was 865 mg/L. Using a daily drinking water consumption value of 2 L and an average

human body weight of 70 kg, the LOAEL was calculated to be 25 mg/kg/day. Reitz et al. (1988, 1989),

Reitz (1990, 1991), and Andersen and Krishnan (1994) used the Andersen et al. (1987) model to make

predictions about GSH-mediated target tissue doses associated with tumorigenesis, or the lack thereof, in

the long-term inhalation and drinking water bioassays. Using target tissue dosimetry as a concentration

surrogate for atmospheric concentrations of methylene chloride, these investigators calculated a unit

inhalation risk for methylene chloride that was two orders of magnitude lower than EPA’s, using the same

linearized multistage methodology.

Dankovic and Bailer (1994) used the models developed by Andersen et al. (1987) and Reitz et al. (1988,

1989; Reitz 1990, 1991) to examine the effects of exercise and intersubject variability on estimation of

human dose levels of methylene chloride. Casanova et al. (1992) developed an extended PBPK model for

DNA-protein crosslink (DPX) formation in mouse liver, based on the model developed by Andersen et al.

(1987).

Reitz et al. (1997) and DeJongh et al. (1998) have expanded earlier models (Andersen et al. 1987,

DeJongh and Blaauboer 1996, Ramsey and Andersen 1984) to include simulations of brain concentrations

of methylene chloride. Reitz et al. (1997) used their model to make route-to-route dose extrapolations

based on brain methylene chloride concentrations. Winneke (1974) had concluded that neurological

effects (decreased visual and auditory functions) of acute exposure to methylene chloride were based

primarily on the properties of the parent compound, and not on the accumulation of COHb, which had

been the conclusion of Putz et al. (1979). Based on the study of Winneke (1974) in humans, an inhalation

exposure to 300 ppm methylene chloride for 4 hours was predicted to result in a similar peak brain

methylene chloride concentration as an exposure to 565 mg methylene chloride/L drinking water, both

exposures correspond to approximately 4 mg/L of brain tissue. Using a daily drinking water consumption

value of 2 L and an average human body weight of 70 kg, the LOAEL was calculated to be 16 mg/kg/day.

This LOAEL was used as the basis for the acute oral MRL (see Section 2.5). DeJongh et al. (1998) used

a rat PBPK model to estimate brain methylene chloride concentrations in rats corresponding to inhalation

exposures to the rat 15-minute or 6-hour LC

50.

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Earlier models have also been extended to simulate humans during pregnancy and lactation. The Reitz et

al. (1997) model is based on an earlier model of trichloroethylene pharmacokinetics in the pregnant rat

(Fisher et al. 1989) and includes compartments representing mammary tissue, placenta, and the fetus. The

Reitz et al. (1997) model has been used to simulate fetal doses of methylene chloride in rats that were

exposed to methylene chloride by the inhalation route. Exposure to 1,250 ppm methylene chloride

(4,342 mg/m

) for 7 hours/day during gestation days 6–15 was predicted to result in a dose to the fetus of

3.67 mg/L of fetus. The corresponding drinking water intake in the human that would result in the same

fetal dose was estimated from the human PBPK model to be 5,000 mg/L drinking water. Fisher et al.

(1997) described an extension of the PBPK model of Ramsey and Andersen (1984) which simulates

amounts and concentrations of methylene chloride in human breast milk that would result from inhalation

exposures. The human lactation model includes a breast milk compartment and parameters for simulating

breast milk production and intake of breast milk by nursing infants. Simulations of a workday maternal

exposure to methylene chloride at an air concentration of 50 ppm (174 mg/m

) yielded a predicted intake

in the infant of 0.213 mg/24 hours.

Advances in modeling methylene chloride pharmacokinetics have also been achieved by linking

methylene chloride models to other PBPK models or to other types of simulation or prediction models.

For example, OSHA (1997) developed a PBPK model that utilizes a Monte Carlo approach to simulate

variability in methylene chloride pharmacokinetics. The model outputs a probability distribution of the

amount of methylene chloride metabolized in the lung through the glutathione-S-transferase pathway.

Poulin and Krishnan (1999) developed a structure-based PBPK model that uses information on chemical

structure to estimate tissue:blood and tissue:air partition coefficients needed to run the model. This

approach can be used to model volatile organic chemicals for which empirically-based estimates are not

available (Poulin and Krishnan 1999). A PBPK model of methylene chloride has been linked to a PBPK

model of toluene (Pelekis and Krishnan 1997). The composite model has been used to simulate the

pharmacokinetic and toxicodynamic outcomes (e.g., COHb concentrations) that might result from

interactions between methylene chloride and toluene at the level of the mixed function oxidase pathway.

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2.3.5.2 Discussion of Methylene Chloride PBPK Models

The Reitz (1990) Model (Also the Andersen et al. [1987], the Reitz et al. [1988, 1989, 1997],

and Andersen and Krishnan [1994] Models)

Risk assessment. An inhalation to oral route extrapolation was pharmacokinetically modeled using

data from the Haun et al. (1972) study. Reitz (1990) incorporates PBPK principles into estimating excess

lifetime cancer risk for humans continuously exposed to 1 µg/m

. Using the linearized multistage model,

the upper-bound estimate of excess lifetime human cancer risk is 2.8x10

-4

ppm which differs from that of

EPA (4.1x10

-6

) by 2orders of magnitude. It should be noted that EPA used default assumptions of low-

dose linear extrapolation and of surface area adjustment to account for interspecies differences.

Description of the model. Reitz (1990) used the mathematical model developed by Andersen et al.

(1987), which can quantitatively describe the production of metabolites in target tissues by either the

MFO or glutathione (GSH) pathway, to test whether model predictions are consistent with the results

obtained in the rodent inhalation and drinking water cancer bioassays of methylene chloride. The MFO

pathway is oxidative and appears to yield carbon monoxide as well as considerable amounts of carbon

dioxide. The glutathione-dependent pathway produces formaldehyde and carbon dioxide, but no carbon

monoxide. Potentially reactive intermediates are formed in each of the metabolic pathways for methylene

chloride. Distribution of methylene chloride metabolism between these pathways is dose dependent. The

MFO pathway is a high-affinity, limited-capacity pathway which saturates at relatively low airborne

concentrations (about 200–500 ppm). In contrast, the GSH pathway has a lower affinity for methylene

chloride, and does not appear to saturate at experimental concentrations (<5,000 ppm). Thus, at low

concentrations, most of the methylene chloride is metabolized by the MFO pathway. As exposure

concentrations increase and the MFO pathway saturates, metabolism by the secondary GSH pathway is

observed.

Predicted amounts of methylene chloride metabolism produced by each pathway (mg equivalents of

methylene chloride metabolized per L of liver tissue per day) are presented in Reitz’s (1990) analysis.

The predicted amounts of MFO metabolites formed in lung and liver tissue are nearly identical for the 2

concentration levels in the mouse inhalation cancer bioassay (2,000 and 4,000 ppm) because saturation of

the MFO pathway is reached between 200 and 500 ppm of administered methylene chloride. However,

mouse liver and lung tumors are consistently higher in the 4,000 ppm exposure group than in the

86 METHYLENE CHLORIDE

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2,000 ppm group in this study. Therefore, there is no concentration-response relationship between rodent

tumor incidence and MFO-metabolite levels. The PBPK model also predicts that levels of MFO

metabolites produced by drinking water administration of 250 mg/kg/day methylene chloride to B6C3F

mice should be similar to the amounts of MFO metabolites produced in the inhalation study. Because no

statistically significant increases in tumors in the drinking water study were observed, MFO metabolites

would not be predicted to be involved in rodent tumorigenicity. Predicted rates of metabolism of

methylene chloride by the GSH pathway correlate more clearly with the induction of lung and liver

tumors, or lack thereof, in the two chronic studies. In the inhalation study, predicted levels of GSH

metabolites at 4,000 ppm are higher than predicted levels of GSH metabolites at 2,000 ppm. In contrast,

predicted levels of GSH metabolites formed in target tissues during drinking water administration of

methylene chloride are very low. Thus, the pattern of tumor induction, or lack thereof, in both studies

shows a good correlation with the rates of metabolism of methylene chloride by the GSH pathway.

Because the concentration-dependency of GSH-metabolite formation is nonlinear at low concentrations,

the delivered dose arriving at the target sites cannot be directly extrapolated from very high inhalation

concentrations (4,000 ppm) to very low concentrations (<1 ppm) typical of human exposure.

Additionally, with regard to interspecies sensitivity, humans would only be more sensitive than mice

(based on the default surface area interspecies adjustment) if the parent chemical were directly

responsible for observed toxicity. In the case of methylene chloride, metabolism mediated by the GSH

pathway is necessary to activate methylene chloride to reactive intermediates, so this assumption is not

applicable. Reitz et al. (1990) incorporates these PBPK findings and principles into estimating excess

lifetime risk for humans continuously exposed to 1 µg/m

, using the linearized multistage model; his

upper-bound estimate of excess lifetime human cancer risk is 2.8x10

-4

ppm, which differs from that of

EPA (4.1x10

-6

) by two orders of magnitude (EPA used default assumptions of low-dose linear

extrapolation and surface area adjustment for interspecies differences).

Validation of the model. The model was validated by comparing predicted responses with

experimental data.

Target tissues. The target tissues were the liver and the lung.

Species extrapolation. Human physiological parameters and assumptions about interspecies

sensitivity were derived from PBPK modeling and use in extrapolation from mice to humans.

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Interroute extrapolation. The results and conclusions from this model application are limited to

exposure via inhalation.

The Andersen et al. (1991) Model

Risk assessment. A modification of the Andersen et al. (1991) model by Reitz et al. (1997) was used

to derive an acute oral MRL from inhalation data in humans from Winneke (1974). For acute

neurological effects, the associated dose measure was defined as the peak concentration of methylene

chloride in brain tissue of humans exposed to 300 ppm of methylene chloride for 4 hours by inhalation.

The modified PBPK model calculated that the administered inhalation dose was equivalent to 3.95 mg of

methylene chloride per liter of brain tissue. The equivalent administered human concentration in drinking

water that will produce the same neurological effects was 565 mg of methylene chloride/liter. Using a

daily water consumption of 70 kg, the LOAEL was calculated to be 16 mg/kg/day.

Description of the model. Andersen et al. (1991) previously developed a PBPK model expanding the

original PBPK model of Ramsey and Andersen (1984) for the disposition of inhaled volatiles. This

model expands the previously developed Andersen et al. (1987) model by both describing the kinetics of

carbon monoxide (CO), COHb, and parent compound methylene chloride, and by comparing the

inhalation kinetics of CO and methylene chloride in rats and humans. The description of CO and COHb

kinetics was adapted from the Coburn-Forster-Kane equation, which assumes that most heme is bound

with oxygen and that endogenous CO production is constant. Methylene chloride kinetics and

metabolism were originally described by the generic PBPK model for volatile chemicals (Ramsey and

Andersen (1984). Predictions in humans from the model were compared to several experimental data sets

in the literature from human volunteers exposed to CO or to methylene chloride, and to experimental data

collected by Andersen et al. (1991) from six male volunteers exposed to methylene chloride vapors at

concentrations of 100 or 350 ppm for a period of 6 hours.

Physiological and biochemical constants for CO were first estimated by exposing rats to 200 ppm of CO

for 2 hours and examining the time course of COHb after cessation of CO exposure. The CO inhalation

studies provided estimates of CO diffusing capacity under free breathing and for the Haldane coefficient,

i.e., the relative equilibrium distribution ratio for hemoglobin between CO and oxygen. The CO model

was then coupled with the PBPK model for methylene chloride both to predict COHb time-course

concentrations during and after methylene chloride exposures in rats, and to estimate the yield of CO

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produced from oxidation of methylene chloride. In rats, only about 0.7 mol of CO was produced from

1 mol of methylene chloride during oxidation. The combined model predicted COHb and methylene

chloride behavior following 4-hour exposures to 200 or 1,000 ppm of methylene chloride, and COHb

behavior following 30-minutes exposure to 5,160 ppm of methylene chloride. The rat PBPK model was

scaled to predict methylene chloride, COHb, and CO kinetics in humans exposed to either methylene

chloride or CO. Three human data sets from the literature were examined: inhalation of CO at 50, 100,

250, and 500 ppm; seven 30-minute inhalation exposures to 50, 100, 250, and 500 ppm of methylene

chloride; and 2-hour inhalation exposures to 986 ppm of methylene chloride. Additional experimental

data from volunteers exposed to 100 or 350 ppm of methylene chloride were also reported. Endogenous

CO production rates and the initial amount of CO in the blood compartment varied in each study, as

necessary, to provide a baseline value of COHb. The combined PBPK model accurately predicted the

experimental findings in all four human studies.

Validation of the model. In rats, the combined model adequately represented COHb and methylene

chloride behavior following 4-hour exposures to 200 or 1,000 ppm of methylene chloride, and COHb

behavior following ½-hour exposure to 5,160 ppm of methylene chloride. In addition, short-duration

exposures conducted with 5,000 ppm of bromochloromethane, with adjustment of metabolic parameters

and partition coefficients for this different chemical, demonstrated that the PBPK model gave a good

description of COHb levels for up to 6 hours postexposure. In humans, the combined PBPK model

provided a good representation of the experimental data in all four studies examined.

Target tissues. Blood was identified as the target. Both concentrations of COHb and methylene

chloride were considered in the model.

Species extrapolation. The application of the model was validated in both rats and humans. The

authors concluded that this model could be useful for developing biological monitoring strategies for CO

and methylene chloride, based on observed COHb blood concentrations following exposure.

Interroute extrapolation. The model was used to convert inhalation data in humans from Winneke

(1974) into equivalent oral doses that were used as the basis for the acute oral MRL (Reitz 1997).

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The Dankovic and Bailer (1994) Model

Risk assessment. Risk assessment was not addressed directly in this model application. However,

the results indicated that some occupationally-exposed individuals may receive glutathione S-transferase-

metabolized methylene chloride doses several-fold greater than human doses previously estimated by

Reitz et al. (1989), based on differing levels of physical activity, and inter-individual variability in

methylene chloride metabolism. Therefore, the assessment of excess lifetime human risk associated with

methylene chloride exposure would be affected.

Description of the model. Dankovic and Bailer (1994) examined the impact of exercise and inter-

individual variability on human dose estimates to methylene chloride, using the models developed by

Andersen et al. (1987) and used by Reitz et al. (1989; Reitz 1990, 1991). Earlier models used

physiological parameters appropriate for humans at rest and metabolic parameters based on average rates

of methylene chloride metabolism. Dankovic and Bailer (1994) increased model parameters describing

cardiac output, alveolar ventilation, and blood flows to tissues to account for exercise, assuming an

8-hour workday exposed to mean methylene chloride concentrations of 25 ppm. The GSH-mediated

metabolized doses for human liver and lung were increased by a factor of 2.9 and 2.4, respectively, as

compared with the metabolized GSH-mediated dose estimates of Reitz et al. (1989). The model was also

modified to account for inter-individual variability in methylene chloride metabolism. Modeled

metabolized GSH-mediated dose estimates for human liver ranged from 0 to 5.4-fold greater than the

doses estimated by Reitz et al. (1989); for human lung, estimates were 0–3.6-fold greater than those of

Reitz et al. (1989). The authors concluded that their results indicated that some occupationally-exposed

individuals may receive GST-metabolized doses several-fold greater than human doses previously

estimated.

Validation of the model. The model used by Dankovic and Bailer (1994) has been previously

validated (Andersen et al. 1987; Reitz 1990, 1991; Reitz et al. 1988, 1989). The authors merely modified

the model parameters to be consistent with light work conditions, as opposed to the resting condition

parameters used in the earlier models, and to reflect inter-individual variability in methylene chloride

metabolism.

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Target tissues. Target tissues were the human liver and the human lung.

Species extrapolation. The model has been previously validated for use with rodents and humans.

Interroute extrapolation. The results and conclusions from this model application are limited to

exposure via inhalation.

The Casanova et al. (1996) Model

Risk assessment. The use of DNA-protein crosslinks as a tissue dosimeter of methylene chloride

exposure markedly reduced the upper-bound estimate and improved the precision of the low-dose excess

lifetime liver cancer risk estimates (as defined by the ratio of the upper-bound estimate to the maximum

likelihood estimate), while having only a minor effect on the maximum likelihood estimate. The

reduction in excess lifetime cancer risk was 2 orders of magnitude less than those estimated by using

airborne methylene chloride vapor concentrations of 10, 30, and 100 ppm.

Description of the model. Casanova et al. (1996) developed an extended PBPK model for DNA-

protein crosslink (DPX) formation in mouse liver associated with chronic methylene chloride exposure by

modifying the model originally developed by Andersen et al. (1987). This extended PBPK model

estimated area under the curve for methylene chloride in mouse liver as the independent variable.

Formaldehyde, one of at least two reactive intermediates formed during glutathione-mediated metabolism,

was considered to be the proximate metabolite associated with tumorigenicity. Parameter estimates for

formaldehyde disposition in methylene chloride-exposed mouse liver were derived from the published

literature. The amount of DPX formed in the mouse liver was estimated for methylene chloride

concentrations used in rodent cancer inhalation bioassay (i.e., 2,000 and 4,000 ppm). Using the linearized

multistage model, the tumor incidence data in mice were fitted to two alternative measures of exposure:

DPX yields and airborne concentration of methylene chloride. The 2 dose measures gave similar

maximum likelihood estimates for the excess lifetime cancer risk at administered concentrations ranging

from 10 to 100 ppm, but the upper 95% confidence limit on this risk estimate was reduced by 2 orders of

magnitude when DPX yield, as compared with airborne concentrations, was used as the measure of

exposure. These results demonstrate that in the case of methylene chloride, the use of DNA-protein

crosslinks as a dose surrogate markedly reduced the upper-bound estimate and improved the precision of

the low-dose risk estimates (as defined by the ratio of the upper-bound estimate to the maximum-

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likelihood estimate), while having only a slight effect on the maximum-likelihood estimate. However,

Casanova et al. (1996) point out that DNA-protein crosslinks cannot be used directly as a surrogate for

the internal dose in humans because human hepatocytes, unlike mouse hepatocytes, do not appear to form

DNA-protein crosslinks in measurable amounts (Casanova et al. 1996). A surrogate for the internal dose,

RNA adducts of formaldehyde, has been developed and can be detected in human hepatocytes exposed to

methylene chloride (Casanova et al. 1996, 1997). The utility of this measure is under study.

Validation of the model. The model has been previously validated in other applications and for other

endpoints. For this application, predicted concentrations of DPX were compared with actual

concentrations induced by experimental concentrations.

Target tissues. The target tissue effect was the formation of DNA-protein crosslinks in mouse liver.

Species extrapolation. The results and conclusions from this model application are limited to DPX

formation in mouse liver.

Interroute extrapolation. Casanova et al. (1996) point out that DNA-protein crosslinks cannot be

used directly as a surrogate for the internal dose in humans, because human hepatocytes, unlike mouse

hepatocytes, do not appear to form DNA-protein crosslinks in measurable amounts (Casanova et al.

1996). As a surrogate for the internal dose, RNA adducts of formaldehyde have been developed and can

be detected in human hepatocytes exposed to methylene chloride (Casanova et al. 1996). The formation

of this dose surrogate in the hepatocytes of different species is currently being examined.

The Andersen and Krishnan (1994) Model

Risk assessment. The PBPK model-based risk assessment estimated an excess lifetime cancer risk to

humans of 3.7x10

-8

for a lifetime inhalation exposure of 2.8x10

-4

, which is lower by more than two orders

of magnitude than that calculated by the EPA using the linearized multistage model for low-dose

extrapolation and a default body surface correction factor for interspecies scaling.

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Description of the model. The mouse PBPK model developed earlier by Andersen et al. (1987) was

validated by comparing model predictions with observed pharmacokinetic data. To predict the tissue

dosimetry of methylene chloride and its metabolites in humans, the physiological parameters of the model

were scaled and methylene chloride-specific parameters for humans were determined. The metabolic rate

constants for humans were estimated from volunteer human exposure studies (Andersen et al. 1991). The

high-dose to low-dose extrapolation and interspecies extrapolation of methylene chloride induced cancer

risk were conducted with the tissue doses of the glutathione pathway metabolite predicted by the PBPK

model. This was validated by demonstration of a concentration-response association between this dose

metric and the degree of methylene chloride-induced rodent cancers rodent bioassay concentrations

between 2,000 and 4,000 ppm. The cancer risk assessment was conducted using the linearized multistage

model to relate tissue dose of methylene chloride glutathione metabolite to the increases in tumor

incidence in mice exposed to high concentrations of methylene chloride by inhalation. In assessing the

excess lifetime cancer risks associated with human exposure to methylene chloride, the authors assumed

that humans are as sensitive (i.e., not more sensitive) as the most sensitive target species; this assumption

is consistent with what is known about methylene chloride chemistry and metabolism, and about

interspecies variability in carcinogenic response to xenobiotics. The PBPK model-based risk assessment

estimated an excess lifetime human cancer inhalation unit risk 3.7x10

-8

per inhalation exposure of

2.8x10

-4

. This human risk estimate is lower by two orders of magnitude than that calculated by the EPA

using the linearized multistage model for low-dose extrapolation and a default body surface adjustment

factor for interspecies scaling.

Validation of the model. The model has been previously validated by comparing predicted with

actual pharmacokinetic data.

Target tissues. The target tissues were the liver and the lung.

Species extrapolation. The animal PBPK model was used for interspecies extrapolation of

pharmacokinetic behavior of methylene chloride by scaling the physiological parameters and determining

chemical-specific parameters in the species of interest (i.e., humans). Metabolic rates for humans were

developed from experimental literature on human volunteers. It was also assumed that humans are as

sensitive (i.e., not more sensitive than) as the most sensitive target species; this assumption is consistent

with what is known about methylene chloride chemistry and metabolism, and about interspecies

variability in carcinogenic response to xenobiotics.

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Interroute extrapolation. The results and conclusions from this model application are limited to

exposure via inhalation.

The Reitz et al. (1997) Model

1. Inhalation Route-to-Oral Route Extrapolation in Volunteers

Risk assessment. In this toxicological profile, ATSDR (2000) establishes a minimal risk level (MRL)

for acute inhalation using the study reported by Winneke (1974). Volunteers were exposed to

300–800 ppm of methylene chloride for approximately 4 hours and tested for neurobehavioral effects.

Concentration-dependent adverse effects included decreased performance in auditory vigilance-

performance tasks; and three small decrements in the visual critical flicker fusion frequency. Because

similar exposures to 50–100 ppm of carbon monoxide alone did not produce these effects, the author

concluded that they were mediated by methylene chloride directly and not by its oxidative metabolite,

carbon monoxide. A LOAEL of 300 ppm from this study was used to determine an acute oral drinking

water equivalent. Using pharmacokinetic modeling, Reitz et al. (1997) was able to estimate that an

inhalation concentration of 300 ppm of methylene chloride would produce a target organ-specific (brain-

specific) dose equivalent to that produced by a drinking water concentration of 565 mg/L of methylene

chloride. Multiplying the drinking water concentration by the default daily water consumption rate (2 L)

and dividing by the default human body weight yields an acute oral dose of 16 mg/kg/day.

Description of the model. Reitz et al. (1997) modified the basic PBPK methylene chloride model

developed by Andersen et al. (1987), Reitz et al. (1988), and Andersen et al. (1991) in the following

manner: (1) liver weights for rodents were based on the actual organ weights of control animals which

were sacrificed during chronic toxicity studies at 6–18 months of age; (2) partition coefficients for

methylene chloride derived from in vitro experiments performed by Gargas et al. (1986) were used for

liver, fat, muscle, and blood, and (3) a brain compartment was added to the methylene chloride model so

that central nervous system effects could be assessed in female and male rodents and humans. Size of

rodent brains, blood flow rates to the brain, and partition coefficients for brain tissue were obtained from

either published literature (e.g., Stott et al. 1983; Thomas 1975) or personal communication by the

authors. The modified methylene chloride model thus contained six tissue compartments: fat, muscle

(slowly perfused tissue), rapidly perfused tissue, liver tissue, mammary tissue, and brain tissue.

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For acute neurological effects, the associated dose measure was defined as the peak concentration of

methylene chloride in brain tissue (mg/L of brain tissue) of humans exposed to 300 ppm of methylene

chloride for 4 hours, by inhalation. The modified PBPK model calculated that the administered inhalation

concentration was equivalent to 3.95 mg methylene chloride/L of brain tissue. Human exposure patterns

in the PBPK model simulated realistic human drinking water consumption patterns, (i.e., consisting of

bouts of drinking during the day, with and between meals, and little-to-no drinking during the night).

PBPK modeling predicted that peak concentrations of methylene chloride in the brain would increase

rapidly after each episode of drinking water consumption, and then drop sharply, to near-zero, between

bouts of drinking. Additionally, there would be no cumulative effects of repeated exposure. The

equivalent administered human concentration in drinking water was calculated to be 565 mg methylene

chloride/L. Using a daily drinking water consumption value of 2 L and an average human body weight of

70 kg, the LOAEL was calculated to be 16 mg/kg/day.

Validation of the model. Model predictions of brain methylene chloride concentrations in humans or

rats have not been evaluated for comparability with empirical observations. However, this model has

been previously validated with human and animal data.

Target tissues. The target organ of the model was the central nervous system.

Species extrapolation. There was no species extrapolation because the experiment that was

pharmacokinetically-modeled (Winneke 1974) was conducted in human volunteers.

Interroute extrapolation. The PBPK modeling was conducted to extrapolate from the inhalation route

of exposure to an oral route, specifically drinking water concentration of methylene chloride.

2. Inhalation Route-to-Oral Route Extrapolation and Rodent-to-Human Species

Extrapolation Using PBPK Modeling of Subchronic Toxicity Data

Risk assessment. ATSDR established an intermediate inhalation minimal risk level (MRL) using the

study reported by Haun et al. (1972). Rats were exposed continuously to either 25 or 100 ppm of

methylene chloride for 100 days. Data on liver histopathology of rats exposed to 25 or 100 ppm of

methylene chloride were selected as the critical effect. The authors report that liver cytoplasmic

vacuolization and Oil-Red-O staining associated with fatty deposits were observed in rats at both

exposure concentrations. In mice, fatty deposits (but no liver vacuolization) were reported at the higher

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exposure concentration, 100 ppm. Additionally, renal changes were also reported in this study. ATSDR

considered the liver effects at 25 ppm to be adverse and established that concentration as a LOAEL.

Using pharmacokinetic modeling and data on human drinking water consumption patterns, Reitz et al.

(1997) estimated that a drinking water concentration of 6,170 mg/L of methylene chloride produces a

target organ-specific dose equivalent to that produced by an inhalation concentration of 25 ppm.

Multiplying the drinking water concentration by the default daily water consumption rate (2 L) and

dividing by the default human body weight yields an intermediate oral dose of 176 mg/kg/day.

Description of the model. For inhalation-to-oral and rat-to-human extrapolations, Reitz et al. (1997)

modified the basic PBPK methylene chloride model developed by Andersen et al. (1987), Reitz et al.

(1988), and Andersen et al. (1991) in the following manner: (1) liver weights for rodents were based on

the actual organ weights of control animals which were sacrificed during chronic toxicity studies at

6–18 months of age; (2) partition coefficients for methylene chloride derived from in vitro experiments

performed by Gargas et al. (1986), were used for liver, fat, muscle, and blood; and (3) a brain

compartment was added to the methylene chloride model so that central nervous system effects could be

assessed in female and male rodents and humans. Size of rodent brains, blood flow rates to the brain, and

partition coefficients for brain tissue were obtained from either published literature (Stott et al. 1983;

Thomas 1975) or personal communication by the authors. The modified methylene chloride model thus

contained six tissue compartments: fat, muscle (slowly perfused tissue), rapidly perfused tissue, liver

tissue, mammary tissue, and brain tissue.

The PBPK model was utilized to compare the average daily production of methylene chloride metabolites

per L of liver in rats exposed to methylene chloride via inhalation for 24 hours/day with the mean daily

production of methylene chloride metabolites per L of liver in humans drinking water that contained

specific concentrations of methylene chloride. The dose measure was defined as the average daily

concentration of metabolites per L of liver tissue. No distinctions were made between metabolites

produced via the MFO pathway and those produced during glutathione conjugation by the GSH pathway.

Total metabolite production was obtained by integrating the rate of metabolite production during

simulation; the result was then divided by the number of 24-hour exposure periods simulated and the

volume of the liver in each species (rodent and human). At inhalation concentrations by rats of 25 ppm

for 24 hours/day for 100 days, the metabolized tissue-specific dose calculated by the model was 1,259 mg

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metabolites methylene chloride/L liver/day. In humans, the drinking water concentration yielding an

equivalent concentration of metabolized tissue-specific dose was found to be approximately 6,170 mg/L.

Multiplying this value by 2 L/day (human drinking water consumption rate) and dividing by the default

human body weight of 70 kg yields a daily dose (NOAEL) of 142 mg/kg/day.

Validation of the model. Model predictions of the concentrations of methylene chloride or its

metabolites in liver of humans or rats have not been evaluated for comparability with empirical

observations. However, this model has been previously validated with human and animal data.

Target tissue. The target tissue was the liver.

Species extrapolation. Rat-to-human extrapolation was conducted using appropriate partition

coefficients, metabolic rates, and other species-specific PBPK variables.

Interroute extrapolation. The PBPK modeling was conducted to extrapolate from the inhalation route

of exposure to an oral route, described in terms of drinking water concentration of methylene chloride.

3. Inhalation Route-to-Oral Route Extrapolation and Rodent-to-Human Species

Extrapolation Using PBPK Modeling of Developmental Toxicity Data

Risk assessment. The developmental toxicity of methylene chloride in rodents was studied by

Schwetz et al. (1975) who exposed rats to 1,250 ppm of methylene chloride vapors for 7 hours/day from

day 6–15 of gestation. There were slight, statistically significant increases in the incidence of minor

skeletal variants in the offspring of females exposed to 1,250 ppm of methylene chloride during gestation.

Therefore, 1,250 ppm was considered to be a LOAEL. The PBPK model was used to extrapolate from

pregnant rodents to pregnant humans and from inhalation route to oral route of exposure. Using

pharmacokinetic modeling and data on human drinking water consumption patterns, Reitz et al. (1997)

estimated that a maternal drinking water concentration of 5,000 mg/L of methylene chloride produces a

fetal dose equivalent to that produced by a maternal inhalation concentration of 1,250 ppm. Multiplying

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the drinking water concentration by the default daily water consumption rate (2 L) and dividing by the

default human body weight of 70 kg yields an intermediate oral dose of

142 mg/kg/day.

Description of the model. For determination of developmental effects, modifications to the basic

PBPK methylene chloride model (Andersen et al. 1987) were based on procedures reported by Fisher et

al. (1989). The model consisted of the addition of three compartments: mammary tissue; placental tissue;

and fetal tissue. The fetal dose measure was of the parent compound in fetal tissue and was estimated by

integrating the fetal concentration of methylene chloride (expressed as mg/L of fetal tissue) during the

exposure period in order to calculate the total area under the curve (AUC) for the exposure period. The

AUC was then divided by the total number of hours during which gestational exposure occurred to give

the average concentration of methylene chloride during the exposure period. Because it was assumed that

human fetuses would be exposed continuously throughout gestation, the total AUC for the human fetus

was divided by the total gestation period to calculate the fetal concentration of parent compound

methylene chloride (not metabolites), expressed in mg methylene chloride/L of fetal tissue.

Simulation of the exposure paradigm used by Schwetz et al. (1975) with the PBPK model yielded

3.67 mg methylene chloride/L of fetal tissue in the rodent. PBPK simulation of human drinking water

exposures gave equivalent values of mg methylene chloride/L of fetal tissue at methylene chloride

concentrations approximately 5,000 mg/L of water. Multiplying by 2 L/day and dividing by 70 kg body

weight yielded a human LOAEL of 142 mg/kg/day.

Validation of the model. Model predictions of the concentrations of methylene chloride in rat or

human fetal tissues have not been evaluated for comparability with empirical observations. However, this

model has been previously validated with human and animal data.

Target tissues. The target tissue was the developing fetus.

Species extrapolation. Rat-to-human extrapolation was connected using appropriate partition

coefficients, metabolic rates, and other species-specific PBPK variables.

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Interroute extrapolation. The PBPK modeling was conducted to extrapolate from the inhalation route

of exposure to an oral route, described in terms of drinking water concentration of methylene chloride.

The Fisher et al. (1997) Model

Risk assessment. The model provides an approach to estimating rates of intake of methylene

chloride and other volatile chemicals by the nursing infant for a given temporal pattern of maternal

exposure and infant nursing. Simulations of a workday maternal exposure to methylene chloride at an air

concentration of 50 ppm (174 mg/m

) yielded a predicted intake in the infant of 0.213 mg/24 hours

(Fisher et al. 1997).

Description of the model. Fisher et al. (1997) described a PBPK model for estimating amounts and

concentrations of methylene chloride in human breast milk that would result from inhalation exposures to

methylene chloride (other air borne volatile chemicals are also simulated in the model). The human

lactation model is an adaptation of the PBPK model of Ramsey and Andersen (1984), with the addition of

a breast milk compartment and parameters for simulating breast milk production and intake of breast milk

by nursing infants. The model simulates seven tissue compartments: blood, lung, fat, liver, richly

perfused tissues, poorly perfused tissues, and breast milk. Two metabolic pathways for methylene

chloride are assumed to occur exclusively in the liver. A glutathione-S-transferase pathway is represented

by an allometrically scaled first order rate constant, and a mixed function oxidase pathway is represented

by a Michaelis-Menton function with constants V

max

and K

The major innovation in this model is simulation of a breast milk compartment, which allows calculations

of the rates of transfer of chemicals from blood into breast milk and rates of transfer to the infant during

breast feeding. The volume of the breast milk compartment is assumed to decrease from an initial volume

of 0.125 L at the beginning of each nursing session to a residual volume of 0.010 L at the end of each

session. The rate of change in the milk volume is simulated as the difference between a zero order

production rate of 0.06 L/hour and the loss rate from nursing, defined as the product the current milk

volume and a first order loss constant of 20/hour. The amount of methylene chloride in breast milk is

calculated using standard PBPK algorithms for flow-limited transfer from blood, assuming that methylene

chloride partitions from blood directly into breast milk according to empirically derived milk/blood

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partition coefficients (Fisher et al. 1997). The resulting concentrations in breast milk are used to calculate

rates of intake by the nursing infant for a given temporal pattern of maternal exposure and infant nursing.

Validation of the model. The human lactation model, as described in Fisher et al. (1997), has not

been calibrated against an empirical data set or validated for any specific use of the model.

Target tissues. Output from the model described are estimates of the amount and concentration of

methylene chloride in breast milk and the rate of intake of methylene chloride by a nursing infant.

Species extrapolation. The model is designed to predict the transfer of inhaled methylene chloride to

breast milk in humans. Extrapolation to other species would require modification of the model to account

for different tissue masses, blood flows, and possibly other kinetic variables.

Interroute extrapolation. The model is designed to simulate the pharmacokinetics of methylene

chloride when exposure is by inhalation to airborne methylene chloride. The pharmacokinetics of

methylene chloride would be expected to be different for other routes of exposure; therefore, the output of

the model cannot be extrapolated to other exposure pathways (e.g., dietary, drinking water) or routes

(e.g., oral, dermal) without modification of the model.

The DeJongh et al. (1998) Model

Risk assessment. The model provides an approach to estimating the brain concentrations of

methylene chloride associated with acute inhalation exposures in rats. DeJongh et al. (1998) reported the

results of simulations of exposures at the 15-minute and 6-hour LC

in the rat. The predicted methylene

chloride concentrations in brain were as follows: 15-minute LC

exposure, 95,781 ppm (331,770 mg/m

brain 24.3 mM, brain lipid 97.6 mM; and 6-hour LC

exposure, 25,181 ppm (87,469 mg/m

), brain

17.3 mM (1,594 mg/L), brain lipid 69.5 mM (6,403 mg/L).

Description of the model. DeJongh et al. (1998) developed a PBPK model for estimating brain

concentrations of methylene chloride and other airborne volatile chemicals in rats. The model is an

adaptation of PBPK models of toluene (DeJongh and Blaauboer 1996) and styrene (Ramsey and

Andersen 1984), with the addition of a brain compartment. The model simulates seven tissue

compartments: blood, lung, fat, liver, richly perfused tissues, slowly perfused tissues, and brain. Two

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metabolic pathways for methylene chloride are assumed to occur exclusively in the liver. A glutathione-

S-transferase pathway is represented by an allometrically scaled first order rate constant, and a mixed

function oxidase pathway is represented by a Michaelis-Menten function with constants V

max

and K

The major innovation in this model is the simulation of a brain compartment, which allows calculations of

the rates of transfer of methylene chloride from blood into water and lipid compartments of brain. The

amount of methylene chloride in the brain is calculated using standard PBPK algorithms for flow-limited

transfer from blood, assuming that methylene chloride partitions from blood directly into the brain

according to an empirically-derived blood-brain partition coefficient (Gargas et al. 1989). Partitioning of

methylene chloride between aqueous and lipid compartments in the brain is calculated from an octanol-

water partition coefficient (Meylan and Howard 1995).

Validation of the model. The model, as described in DeJongh et al. (1998), has not been calibrated

against an empirical data set or validated for predicting brain methylene chloride concentrations.

Target tissues. Outputs from the model described in DeJongh et al. (1998) are estimates of the amount

and concentration of methylene chloride in whole brain or brain lipid.

Species extrapolation. The model is designed to predict the distribution of inhaled methylene

chloride to the brain in rats. Extrapolation to other species would require modification of the model to

account for different tissue masses, blood flows, and possibly other kinetic variables.

Interroute extrapolation. The model is designed to simulate the pharmacokinetics of methylene

chloride when exposure is by inhalation to airborne methylene chloride. The kinetics would be expected

to be different for other routes of exposure; therefore, the output of the model cannot be extrapolated to

other exposure pathways (e.g., dietary, drinking water) or routes (e.g., oral, dermal) without modification

of the model.

The Poulin and Krishnan (1999) Model

Risk assessment. The model provides an approach to estimating the concentrations of methylene

chloride in venous blood, and possibly other tissues, associated with acute inhalation exposures. Poulin

and Krishnan (1998) reported the results of simulations of a 6-hour inhalation exposure of an adult human

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to 100 ppm methylene chloride (347 mg/m

). The predicted methylene chloride concentrations in venous

blood after 6 hours of exposure were approximately 0.5 and 1.5 mg/L for the bounding assumptions, liver

extraction ratio of one or zero, respectively.

Description of the model. Poulin and Krishnan (1999) described a quantitative structure-toxico-

kinetic relationship (QST

R) model for estimating venous blood concentrations, and tissue:air and

tissue:blood partition coefficients of methylene chloride (and other airborne volatile chemicals) in

humans. The QST

R model is an adaptation of the PBPK model of Andersen et al. (1991) with the

addition of a model for estimating values of the partition coefficients based on molecular structure

fragment and tissue composition information. The model simulates blood, lung, fat, liver, richly perfused

tissues, and slowly perfused tissues. Two innovations in this model are the approach to the simulation of

methylene chloride metabolism in the liver and the inclusion of the structure-based estimation of partition

coefficients.

The rate of metabolism of methylene chloride in the liver is represented as the products of the liver blood

flow rate, the arterial concentration of methylene chloride, and the liver extraction ratio (ratio of hepatic

clearance to hepatic blood flow). This approach does not require specification of values for the kinetic

constants of metabolism (e.g., V

max

, K

), but does require specification of a value, or range of values, for

the liver extraction ratio. In the absence of an empirical basis for any given value for the extraction ratio,

bounding estimates for venous blood concentrations, as affected by hepatic metabolism, can be made by

running simulations in which the extraction ratio is assumed to be either zero or one (Poulin and Krishnan

1999).

The octanol:water partition coefficient, the water:air partition coefficient, and the boiling point are

estimated using molecular structure fragment information (Poulin and Krishnan 1996, 1998). The

estimated values for the above parameters are used with tissue composition information (e.g., water and

lipid content) to estimate values for blood:air and tissue:blood partition coefficients.

Validation of the model. Estimates of blood:air and tissue:blood concentration predicted with the

QST

R compared well with empirical determinations for methylene chloride (and a variety of other

volatile organic compounds). Measured venous blood concentrations of methylene chloride in humans

exposed to 100 ppm methylene chloride (347 mg/m

) for 6 hours (Andersen et al. 1991) were within the

bounding estimates (liver extraction ratio assumed to be zero or one) predicted with the QST

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Target tissues. Output from the model described in Poulin and Krishnan (1998) are estimates of the

concentration of methylene chloride in venous blood.

Species extrapolation. The model is designed to predict venous blood concentration of methylene

chloride in humans. Extrapolation to other species would require modification of the model to account

for different tissue masses, blood flows, and possibly other kinetic variables.

Interroute extrapolation. The model is designed to simulate the pharmacokinetics of methylene

chloride when exposure is by inhalation to airborne methylene chloride. The kinetics would be expected

to be different for other routes of exposure; therefore, the output of the model cannot be extrapolated to

other exposure pathways (e.g., dietary, drinking water) or routes (e.g., oral, dermal) without modification

of the model.

The Pelekis and Krishnan (1997) Model

Risk assessment. The model provides an approach to estimating the COHb levels in blood that

would result from oral or inhalation exposures to mixtures of methylene chloride and toluene if certain

assumptions are made regarding the mechanism of interaction between the two chemicals: (1) the

interaction occurs at the level of the mixed function oxidase-mediated metabolism of methylene chloride

to carbon monoxide in the liver; and (2) the mechanism of inhibition can be simulated by Michaelis-

Menten type models of competitive, noncompetitive or uncompetitive inhibition of the pathway. Pelekis

and Krishnan (1997) extrapolated the rat model to humans by replacing the rat values with human values

for physiological variables and by assuming that partition coefficients and the metabolism kinetic

constants are the same in the rat and human. The resulting human model was used to predict the area

under the COHb-time curve for a human exposure to the ACGIH 8-hour TLV for methylene chloride

(50 ppm, 174 mg/m

), or to a simultaneous exposure to the methylene chloride and toluene at their

respective TLVs (50 ppm, 188 mg/m

). The model predicted a 2–9% decrease in COHb levels in the

mixed exposure compared to the exposure to methylene chloride alone, depending on which mechanism

of inhibition of the mixed function oxidase pathway was assumed.

Description of the model. Pelekis and Krishnan (1997) linked a PBPK model for methylene

chloride (Andersen et al. 1991) with a PBPK model of toluene (Tardif et al. 1993). The composite model

was used to simulate the kinetics of COHb production resulting from exposures to mixtures of the two

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chemicals in rats or humans. The model simulates seven tissue compartments: blood, gastrointestinal

tract, lung, fat, liver, richly perfused tissues, and slowly perfused tissues. Metabolic pathways for

methylene chloride are assumed to occur exclusively in the liver. A glutathione-S-transferase pathway is

represented by a first order rate constant, and a mixed function oxidase pathway, which produces carbon

monoxide as a product, is represented by a Michaelis-Menton function with constants V

max

and K

. The

metabolism of toluene is assumed to occur in the liver through a mixed function oxidase pathway. An

innovation in this model is the simulation of interactions between methylene chloride and toluene at the

level of the mixed function oxidase pathway. This is achieved with Michaelis-Menton equations that

simulate four possible interaction possibilities: (1) no interaction between methylene chloride and

toluene; (2) competitive inhibition of the mixed function oxidase pathway (increase in apparent K

); (3)

noncompetitive inhibition of the pathway (decrease in V

max

); or (4) or uncompetitive inhibition (increase

in K

and decrease in V

max

). The relationship between carbon monoxide production (including

endogenous production) and percent COHb in blood is simulated using the Coburn-Foster-Kane model,

as implemented in the model described by Andersen et al. (1991).

Validation of the model. The results of simulations were compared with observations made in

studies of single or mixed exposures to methylene chloride and toluene in rats (Ciuchta et al. 1979;

Pankow et al. 1991a, 1991b). Pankow et al. (1991a, 1991b) reported the levels of COHb in rats exposed

to either a single oral dose of methylene chloride (6.2 mmol/kg, 527 mg/kg), or combined single oral

doses of 6.2 mmol/kg methylene chloride (527 mg/kg) and 18.8 mmol/kg toluene (1,732 mg/kg) (Pankow

et al. 1991a, 1991b). The observed COHb level 6 hours after dosing with methylene chloride was 9.3%

compared to 1.7% after combined dosing with methylene chloride and toluene. The corresponding COHb

levels simulated with the PBPK model were 8.7% for dosing with methylene chloride alone, and 2.1% for

the combined dosing, assuming competitive inhibition of the mixed function oxidase pathway, or 0.6%,

assuming either noncompetitive or uncompetitive inhibition (Pelekis and Krishnan 1997). The PBPK

model also predicted COHb levels 12 hours after dosing with methylene chloride alone and the

corresponding 12-hour area under the COHb-time curve that agreed well with observations from Pankow

et al. (1991). Observed peak COHb levels following exposure of rats for 1 hour to 5,000 ppm

(17,368 mg/m

) methylene chloride were 10–12% compared to less than 1% when the methylene chloride

exposure occurred 30 minutes after a single intraperitoneal dose of 0.005 mmol/kg (0.46 mg/kg) of

toluene (Ciuchta et al.1979). Corresponding model simulations agreed well with these observations, only

when uncompetitive or noncompetitive inhibition of the mixed function oxidase pathway was assumed.

The competitive inhibition model severely overestimated the peak COHb concentrations observed in the

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combined exposure experiment. These results suggest that, of the three interactions mechanisms

simulated, the noncompetitive and uncompetitive mechanisms more accurately predicted the in vivo

observations.

Target tissues. Output from the model described in Pelekis and Krishnan (1997) are estimates of the

COHb level in arterial blood (expressed in units of percent saturation of hemoglobin).

Species extrapolation. The model was calibrated against observed arterial blood COHb levels in rats.

Although Pelekis and Krishnan (1997) have extrapolated the rat model to simulate human exposures to

methylene chloride and toluene, model outputs have not been compared to empirical observations in

humans; therefore, the accuracy of the extrapolation has not been evaluated. Extrapolation to other

species (other than human) would require additional modifications to the model to account for different

tissue masses, blood flows, and possibly other kinetic variables.

Interroute extrapolation. The model is designed to simulate the pharmacokinetics of methylene

chloride and toluene when exposures are by the inhalation or oral routes. COHb levels predicted by the

model are highly sensitive to the assumed value of the gastrointestinal absorption rate constant of

methylene chloride, and less sensitive to the value of the rate constant for absorption of toluene (Pelekis

and Krishnan 1997). This suggests that potential dose level or exposure medium effects (e.g., diet,

drinking water) or interaction effects on the absorption rate constants should be considered in simulations

of the oral route. The kinetics would be expected to be different for other routes of exposure; therefore,

the output of the models cannot be extrapolated to other exposure pathways (e.g., dermal) without

modification of the model.

The OSHA (1997) Model

Risk assessment. The OSHA model provides an approach to estimating various internal dose

surrogates corresponding to inhalation exposures to methylene chloride. OSHA (1997) used the mouse

PBPK model to translate methylene chloride inhalation exposure levels used in an NTP (1986) mouse

bioassay to equivalent amounts of methylene chloride metabolized by the mouse lung glutathione-

S-transferase pathway. The resulting internal dose estimates were used to establish an internal dose-

response relationship for lung tumors observed in the mouse bioassay. The mouse internal dose-response

relationship was then translated into an equivalent human internal dose-response relationship using the

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human PBPK model. The mean and 95th percentile cancer risks (based on the maximum likelihood of

the dose-response parameters) associated with a human exposure to 25 ppm methylene chloride

(87 mg/m

), 8 hours/day, 5 days/week for 45 years were 1.24x10

-3

and 3.63x10

-3

, respectively.

Description of the model. OSHA (1997) proposed a PBPK model in support of its Final Rule on

limits for occupational exposure to methylene chloride. The OSHA model is based on several more

recent extensions of the Andersen et al. (1991) model as described by Clewell (unpublished report

available as an exhibit in OSHA (1997) and Reitz et al. (1997). The model simulates eight tissue

compartments: blood, gastrointestinal tract, lung, fat, liver, well perfused tissues, poorly perfused tissues,

and bone marrow.

Innovations in the OSHA model include the following. (1) Bone marrow is simulated as a distinct tissue

rather than including it in the well or poorly perfused tissue compartment. (2) Metabolism of methylene

chloride is assumed to occur in the liver and lung tissues. The liver and lung K

values for the mixed

function oxidase pathway are assumed to be identical. The values for the V

max

for the mixed function

oxidase pathway in the lung and the rate constant for the glutathione-S-transferase pathway in the lung

are set as fixed fractions of their respective values in liver. These estimates take into account relative

tissue volumes, in vitro estimates of metabolism rates in the two tissues, and the relative abundance of

microsomal or soluble protein in the two tissues. (3) Alveolar ventilation rates are dependent on cardiac

output. The relationship between the two variables is assumed to be a direct proportionality with a

ventilation-perfusion ratio as the proportionality term. (4) Cardiac output, tissue distribution of cardiac

output, and the ventilation-perfusion ratio are related to work intensity allowing various work-related

exposure scenarios to be simulated. (5) Tissue blood flows, as a fraction of the cardiac output, are

constrained so that either the fractional flow to the well-perfused tissues (mouse model) or poorly

perfused tissues (human model) is set as 1 minus the sum of the fractional flows to all other tissues. This

constraint was imposed as an approach to ensure mass balance of flows when a Monte Carlo sampling

approach was used to select parameter values.

Probability distributions for input parameters in the mouse and human versions of the OSHA model were

developed using a Bayesian analysis of empirical data from gas uptake studies in mice and human open

chamber inhalation studies, respectively. The resulting probability distributions were used in a Monte

Carlo approach to implement the PBPK models. This approach results in probability distributions for

model outputs, rather than single point estimates.

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Validation of the model. The mouse and human models were calibrated using data from gas uptake

studies in mice and human open chamber inhalation studies (OSHA 1997). Comparisons of model

outputs with empirical observations other than those used in the Bayesian analysis were not reported in

OSHA (1997).

Target tissues. The mouse and human models have been used to estimate the amounts of methylene

chloride metabolized through the glutathione-S-transferase pathway in liver and lung (OSHA 1997).

Species extrapolation. Separate models for the mouse and human have been developed, enabling

extrapolations of internal dose surrogates between these two species (OSHA 1997). Extrapolation to

other species would require additional modifications to the models to account for different tissue masses,

blood flows, and possibly other kinetic variables.

Interroute extrapolation. The mouse and human models have been used to simulate the

pharmacokinetics of methylene chloride when exposures are by the inhalation route (OSHA 1997). The

models include gastrointestinal tissue compartment and, therefore, could be applied to oral exposures,

provided that validation studies support such uses of the model. The kinetics would be expected to be

different for other routes of exposure; therefore, the output of the models cannot be extrapolated to other

exposure pathways (e.g., dermal) without modification of the models.

2.4 MECHANISMS OF ACTION

2.4.1 Pharmacokinetic Mechanisms

The physical properties of methylene chloride, particularly its lipophilic nature, high vapor pressure, and

high serum/air partition coefficient, suggest that it is likely to be absorbed across the alveolar membranes

of the lung, mucosal membranes of the gastrointestinal tract, and the skin by passive diffusion. Once in

the body, it is widely distributed, with the greatest amounts accumulating in the more lipophilic tissues;

this probably also occurs by passive diffusion.

Andersen et al. (1987), Reitz (1990, 1991), Reitz et al. (1988), and Andersen and Krishnan (1994) have

used a PBPK model to predict amounts of methylene chloride metabolism produced (mg equivalents of

methylene chloride metabolized per L of liver tissue per day) produced by the two major pathways of

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methylene chloride metabolism: the MFO pathway and the GSH-mediated pathway. This model

identifies the pathway which appears to activate methylene chloride to reactive intermediates but neither

characterizes the putative metabolite nor describes the mechanisms of action. In essence, the MFO

pathway is oxidative and appears to yield carbon monoxide as well as considerable amounts of carbon

dioxide. The glutathione-dependent pathway produces formaldehyde and carbon dioxide, but no carbon

monoxide. Potentially reactive intermediates are formed in each of the metabolic pathways for methylene

chloride: formyl chloride via the MFO pathway and methylchloroglutathione via the GSH pathway.

Distribution of methylene chloride metabolism between these pathways is dose dependent. The MFO

pathway is a high-affinity, limited-capacity pathway which saturates at relatively low airborne

concentrations (about 200–500 ppm). In contrast, the GSH pathway has a lower affinity for methylene

chloride, and does not appear to saturate at experimental concentrations (<5,000 ppm). Thus, at low

concentrations, most of the methylene chloride is metabolized by the MFO pathway. As exposure

concentrations increase and the MFO pathway saturates, metabolism by the secondary GSH pathway is

observed.

Predicted amounts of methylene chloride metabolism produced by each pathway (mg equivalents of

methylene chloride metabolized per L of liver tissue per day) are presented in Reitz’s (1990, 1991)

analyses. The predicted amounts of MFO metabolites formed in lung and liver tissue are nearly identical

for the 2 concentration levels in the mouse inhalation cancer bioassay (2,000 and 4,000 ppm) because

saturation of the MFO pathway occurs at administered airborne concentrations of approximately

200–500 ppm. However, mouse liver and lung tumors are consistently higher in the 4,000 ppm exposure

group than in the 2,000 ppm group in this bioassay. Therefore, there is no concentration-response

relationship between rodent tumor incidence and MFO metabolite levels. The PBPK model also predicts

that levels of MFO metabolites produced by long-term drinking water administration of 250 mg/kg/day

methylene chloride to B6C3F

mice should be similar to the levels of MFO metabolites produced in the

inhalation bioassay. However, no statistically significant increases in tumors in the drinking water

bioassay were observed; therefore, MFO metabolites would not be predicted to be involved in rodent

tumorigenicity by either route of administration. Predicted rates of metabolism of methylene chloride by

the GSH pathway correlate more clearly with the induction of lung and liver tumors, or lack thereof, in

the two chronic studies. In the inhalation study, predicted levels of GSH metabolites at 4,000 ppm are

higher than predicted levels of GSH metabolites at 2,000 ppm. In contrast, predicted levels of GSH

metabolites formed in target tissues during drinking water administration of methylene chloride are very

low. Thus, the pattern of tumor induction, or lack thereof, in both studies shows a good correlation with

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the rates of metabolism of methylene chloride by the GSH pathway. Because the dose dependency of

GSH metabolite formation is nonlinear at low concentrations, the delivered dose arriving at the target

sites cannot be directly extrapolated from very high inhalation concentrations (4,000 ppm) to the very low

concentrations (<1 ppm) typical of human exposure.

Casanova et al. (1996) developed an extended PBPK model for DNA-protein crosslink (DPX) formation

in mouse liver, based on the model originally developed by Andersen et al. (1987). The extended PBPK

model estimated area under the curve for methylene chloride in mouse liver as the independent variable.

Tissue-specific yields of DPX were used as the dose surrogate. Estimates were made of the amount of

DPX formed in the mouse liver at methylene chloride inhalation concentrations used in the bioassay (i.e.,

2,000 and 4,000 ppm) and plotted against liver tumor yields in the mouse. DPX thus served as a

concentration surrogate for airborne methylene chloride concentrations. The model assumes that DPX

formation is associated with methylene chloride mouse liver tumorigenicity. Because formaldehyde

produces DPX, and GSH-mediated metabolism of methylene chloride produces formaldehyde as a

reactive intermediate, the authors suggest that formaldehyde is involved in the genotoxic mechanism(s) of

action associated with mouse liver tumorigenicity. When excess lifetime cancer risk was estimated using

the mouse liver tumor data and two alternate dose measures, DPX and airborne vapor concentrations, the

maximum likelihood estimates were similar, but the upper-bound estimate using DPX was two orders of

magnitude lower than that using airborne concentrations. Thus, DPX appears to be a reasonable dose

surrogate for methylene chloride inhalation exposure. However, most other investigators do not consider

DPX formed by the weakly mutagenic activity of formaldehyde to be the putative mechanism of action of

methylene chloride-induced liver tumorigenicity (see Section 2.4.2, Mechanisms of Toxicity).

Furthermore, human hepatocytes do not appear to form DPX in measurable amounts, as do mouse

hepatocytes (Casanova et al. 1996).

2.4.2 Mechanisms of Toxicity

The lung, the blood system, and the nervous system are the major target organs of toxicity associated with

exposure to methylene chloride.

Non-neoplastic Mechanisms. In humans, Snyder et al. (1992a, 1992b) have reported headache, chest

discomfort, cough, and the presence of alveolar and interstitial infiltrates in the lung as a result of short-

term high-concentration vapor exposure to methylene chloride in confined, unventilated rooms or

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basements. In B6C3F

mice exposed to 4,000 ppm of methylene chloride vapors for 6 hours (Foster et al.

1992), the major initial morphological effect observed in mouse lung was acute Clara cell damage.

However, the damage appeared to resolve after five consecutive daily exposures to methylene chloride.

The appearance and disappearance of the lesion in the Clara cell correlated well with the activity of

cytochrome P-450 monooxygenase in the Clara cell, as assessed immunocytochemically (CYP2B1 and

CYP2B2) in the whole lung and biochemically in the freshly isolated Clara cell (as determined by

ethoxycoumarin O-dealkylation and aldrin epoxidation).

Over 13 weeks (5 days/week) of exposure, the acute Clara cell damage, which developed after a 1-day

exposure but resolved after 5 consecutive exposures, reappeared on reexposure after a 2-day weekly

break. The severity of the lesion diminished as the study progressed. The authors suggest that the reason

for the decrease or disappearance of the lesion was due to an adaptation/tolerance in the Clara cell to

methylene chloride that was linked to a marked decrease of methylene chloride metabolism by

cytochrome P-450 pathways. Glutathione (GST) activity in the Clara cell either remained unchanged or

increased following methylene chloride exposure.

Inhalation and ingestion exposures to methylene chloride result in the production of carbon monoxide

associated mainly with metabolism via the MFO pathway. CO binds to hemoglobin, and can cause

carboxyhemoglobinemia. In two fatal human cases of methylene chloride poisoning, COHb was elevated

to approximately 30% (Manno et al. 1992). Other reports on human and animals show that COHb

increases from baselines of 0–2 to 4–15%, under varying regimes of methylene chloride inhalation

exposure.

Neurotoxicity resulting from exposure to methylene chloride is believed to be associated with the

lipophilic properties of methylene chloride; however, the precise mechanisms of neurotoxicity are not

known. Presumably, the methylene chloride enters cell membranes, which in the case of neurons,

interferes with signal transmission, in a manner similar to general anesthetics (De Jongh et al. 1998;

Sikkema et al. 1995). Neurotoxicity is also assumed to be caused by the hypoxia that results from the

formation of COHb.

Cancer. With regard to tumor induction in the rodent lung and liver, methylene chloride is postulated to

be activated to an unknown reactive intermediate via metabolism. There are two major metabolic

pathways: the MFO pathway, specifically cytochrome P-450 2E1 and glutathione-glutathione

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S-transferase-mediated (GSH-GST) pathway. The isoenzyme involved in the GSH-GST pathway has

been identified as a Θ (theta) class glutathione S-transferase, GSTT1-1, which is present in moderate

quantities in the mouse lung, but has been detected only at very low levels in human lung and liver tissue

samples (Mainwaring et al. 1996b; Sheratt et al. 1997). These findings suggest that in humans the lung

and liver are likely to have little capacity to activate methylene chloride into its reactive metabolites.

However, higher than background levels of GSTT1-1 mRNA were detected in a small number of Clara

cells and alveolar/bronchiolar ciliated epithelial cells of one human lung sample (out of four) and of

GSTT2-2 enzyme in the biliary epithelium of the human liver (Mainwaring et al. 1996b). Thus, it is

possible that, in some individuals, these specific cell types may be vulnerable to genotoxic effects from

reactive intermediates of methylene chloride metabolism, although the overall risk is likely to be low.

The MFO pathway is oxidative and appears to yield carbon monoxide as well as considerable amounts of

carbon dioxide. The glutathione-dependent pathway produces formaldehyde and carbon dioxide, but no

carbon monoxide. Potentially reactive intermediates are formed in each of the metabolic pathways for

methylene chloride: formyl chloride in the oxidative pathway, and formaldehyde and chloromethyl

glutathione in the conjugative pathway. Neither formyl chloride nor the glutathione conjugate of

methylene chloride has been isolated or characterized, although Green (1997) reports that their formation

is entirely consistent with available information on glutathione-mediated metabolism. Distribution of

methylene chloride metabolism between these pathways is dose dependent. The MFO pathway is a high-

affinity, limited-capacity pathway which saturates at relatively low atmospheric concentrations

(approximately 200–500 ppm). The GSH pathway, in contrast, has a lower affinity for methylene

chloride, but does not appear to saturate at experimentally produced concentrations (<5,000 ppm). Thus,

the MFO pathway accounts for most of the metabolized methylene chloride at concentrations less than

500 ppm, but as exposure concentrations increase above the MFO saturation level, increases in the

amount of methylene chloride metabolized by the secondary GSH pathway are seen (Reitz 1990). The

concentration dependency of these two metabolic pathways is consistent with the tumor results obtained

in long-term rodent inhalation and drinking water cancer bioassays of methylene chloride and supports

the assertion that GSH-mediated metabolism is responsible for methylene chloride-induced

tumorigenicity in B6CF

mice.

There is no evidence to suggest that methylene chloride is a direct acting carcinogen; the marked species

differences in carcinogenicity induced by methylene chloride are not typical behavior of direct-acting

compounds. Methylene chloride also does not exhibit the chemical reactivity towards nucleophiles

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normally associated with direct action (Green 1997). Therefore, metabolic activation is required which

interacts in some way with mouse tissues to cause tumors.

A series of bacterial mutagenicity tests has demonstrated that: methylene chloride induction of bacterial

mutagenicity is expressed more strongly in Salmonella typhimurium TA 1535 modified to express a

mammalian GST Θ class enzyme (NM5004 strain) than in the original strain (Oda et al. 1996); meth-

ylene chloride induction of bacterial mutagenicity S. typhimurium strain TA 100 is unaffected by the

presence of GST α or π classes (Simula et al. 1993); methylene chloride is less mutagenic in a S.

typhimurium GSH-deficient strain (TA100/NG11) as compared to TA 100 (Graves et al. 1994a); and

bacterial testing with 3 K12 strains of Escherichia coli showed that methylene chloride (activated by S9

mouse liver fraction) and formaldehyde were mutagenic only in the wild-type E. coli, a characteristic

shared with crosslinking agents; these data initially suggested a mutagenic role for metabolically-derived

formaldehyde in E. coli (Graves et al. 1994a).

These bacterial assays demonstrated that in in vitro tests, methylene chloride was activated by a Θ class

GST enzyme to a bacterial mutagen in S. typhimurium and behaved similarly to formaldehyde in E. coli

tester strains. However, in the Chinese Hamster ovary (CHO) assay involving the hypoxanthine-guanine

phosphoribosyl transferase (HPRT) gene assay, studies of DNA single strand breaks and DNA-protein

crosslinks at mutagenic concentrations of methylene chloride and formaldehyde showed that both these

compounds induced DNA single-strand breaks; only formaldehyde induced significant DNA-protein

crosslinking (Graves et al. 1996). Similar findings were observed in cultured, freshly isolated mouse

hepatocytes (Graves and Green 1996), but not in rat hepatocytes (Graves et al. 1994b, 1995). The authors

concluded that, although formaldehyde might play a role in methylene chloride genotoxicity, its weak

mutagenicity and the absence of methylene chloride-induced DNA-protein crosslinking in the

CHO/HPRT assay suggested that methylene chloride-induced DNA damage and resulting mutations are

likely produced by its glutathione conjugate, putatively chloromethylglutathione. Graves and Green

(1996) also concluded that these results suggested that the mechanism for methylene chloride

tumorigenicity in the mouse liver was likely to be genotoxic and mediated by the GSH pathway.

Observed species differences in liver tumorogenicity between the mouse and the rat might result from

species differences in the amount of GSH-mediated metabolism induced by methylene chloride exposure.

A series of studies have been conducted to elucidate the precise genetic mechanisms of methylene

chloride carcinogenicity. Female B6C3F

mice were exposed to vapor concentrations of

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2,000–8,000 ppm for 2 years and sacrificed at various intervals to evaluate a number of genotoxic

endpoints.

Replicative DNA synthesis in the bronchiolar epithelium, examined by the use of the labeling index (LI),

indicated that mice exposed to 2,000 ppm of methylene chloride for 2–26 weeks decreased to 40–60% of

controls (Kanno et al. 1993). Mice exposed to 8,000 ppm of methylene chloride have a less dramatic

decrease in LI. No pathological changes were found in the exposed lungs. Thus, high-concentration

exposure to methylene chloride for up to 26 weeks reduces the cell turnover of bronchiolar cells in these

mice; therefore, increased cell proliferation does not appear to be involved in mouse lung tumorogenesis.

DNA single-strand breaks were detected in the livers of B6C3F

male mice, but not Syrian Golden male

hamsters, immediately following inhalation exposure to 2,000–8,000 ppm for 6 hours, but not 2 hours

after exposure, suggesting active DNA repair (Graves et al. 1995). The DNA of mouse Clara cells

incubated in vitro with methylene chloride was also damaged at high concentrations. Pretreatment of

mice with a glutathione depletor prior to inhalation exposure caused a decrease in the amount of DNA

damage detected, suggesting a GST-mediated mechanism; similar findings were observed in Clara cells

incubated in vitro with methylene chloride and a glutathione depletor.

Devereux et al. (1993) analyzed liver and lung tumor, induced in female B6C3F

female mice by

inhalation of 2,000 ppm of methylene chloride for 6 hours/day, 5 days/week exposure for up to

104 weeks, for the presence of activated ras proto-oncogenes. In methylene chloride-induced liver

tumors, mutations, mainly transversions or transitions in base 1 or base 2, were detected and were similar

to those observed for the H-ras gene in spontaneous liver tumors. Mutations were also identified in the

lung. The K-ras activation profile in the methylene chloride-induced tumors was not significantly

different from the profile in spontaneously-occurring tumors. No other transforming genes were found in

the nude mouse tumorogenicity assay. The authors concluded that at present, no transforming genes other

than ras genes could be identified in either mouse liver or lung tumors. Based on liver tumor data, they

also suggested that methylene chloride may affect the liver by promoting cells with spontaneous lesions.

Hegi et al. (1993) studied allelotypes of 38 methylene chloride-induced lung carcinomas from female

B6C3F

mice exposed 6 hours/day, 5 days/week for 2 years to 2,000 ppm. The allelotypes were

examined for various genotoxic endpoints, and the results were compared with genotoxicity findings in

two other reciprocal-cross mouse strains. Throughout the genome, allelic losses occurred infrequently,

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except for markers on chromosome 4, which were lost in approximately half of the carcinomas. In lung

adenomas, chromosome 4 losses were associated with malignant conversion. Methylene chloride-induced

liver tumors did not demonstrate chromosome 4 loss, which indicated that this finding was specific for

lung carcinomas. Preferential loss of the maternal chromosome 4 was also observed in carcinomas in

B6C3F

mice. On chromosome 6, an association between K-ras gene activation and allelic imbalances

was also found in B6C3F

mouse lung tumors. When allelotypes of tumors in mice from two reciprocal

cross strains, AC3F

and C3AF

, were examined and compared to the findings in B6C3F

mice, one allele

of the putative chromosome 4 tumor suppressor gene was shown to be inactivated. Whereas the results in

B6C3F

mice suggested that nondisjunction events were responsible for the chromosome 4 losses, tumors

from both reciprocal-cross mouse strains appeared to show small interstitial deletions in a chromosomal

region homologous with a region in human chromosome which is often lost in a variety of tumors,

including lung cancers. In human chromosomes, a candidate tumor suppressor gene, MTS1, is located in

this region.

In another genotoxic analysis with the same cohort (Hegi et al. 1994), loss of heterozygosity at markers

near the p53 gene on chromosome 11 and within the retinoblastoma tumor suppressor gene were

examined in methylene chloride-induced liver and lung tumors and compared to spontaneous tumors in

control mice. The authors concluded that inactivations of p53 and the retinoblastoma tumor suppressor

gene were infrequent events in lung and liver tumorigenesis in mice exposed to methylene chloride.

Replicative DNA synthesis was examined by Kanno et al. (1993) to evaluate the potential role of

treatment-induced lung cell proliferation on pulmonary carcinogenicity in female B6C3F

mice exposed

to 2,000 or 8,000 ppm of methylene chloride for 6 hours/day, 5 days/week for 2 years. By the end of the

study, there was a statistically significant increase in lung tumors in exposed animals when compared to

controls. Cell proliferation was assessed in the lung after 1, 2, 3, or 4 weeks of inhalation exposure to

2,000 or 8,000 ppm, and after 13 and 26 weeks exposure to 2,000 ppm, as measured by changes in

labeling indices (LI). The LI of both bronchiolar epithelium and terminal bronchioles were substantially

decreased in mice exposed to 2,000 ppm of methylene chloride for 2–26 weeks. Similar findings, but not

as severe, were observed in mice exposed to 8,000 ppm. The decreases in LI were not accompanied by

cytotoxicity. The authors concluded that high-concentration exposure to methylene chloride for up to

26 weeks reduces cell proliferation in lung epithelial cells in female B6C3F

mice.

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Maronpot et al. (1995b) assessed replicative DNA synthesis after 13, 26, 52, and 78 weeks of inhalation

exposure by female B6C3F

mice to 2,000 ppm of methylene chloride for 6 hours/day, 5 days/week. A

statistically significant decrease in the hepatocyte LI was only observed at 13 weeks. In lung epithelial

cells, the results were similar to those observed by Kanno et al. (1993). No increases in replicative DNA

synthesis were found in liver foci cells or lung parenchymal cells. K-ras gene activation in liver tumors

and H-ras gene activation in lung tumors did not differ between methylene chloride-induced tumors and

those observed in control animals. The authors concluded that these oncogenes were not involved in

mouse tumorogenesis.

DNA-protein crosslinks (DPX) in lung and liver were examined by Casanova et al. (1992, 1996) in male

B6C3F

mice exposed to 2,000 and 4,000 ppm of methylene chloride for 6 hours/day for 2 days and in

male Syrian Golden hamsters exposed to 3,500 ppm. The authors suggested that formaldehyde derived

from GSH-mediated methylene chloride metabolism might be forming DPX in mouse liver. Although

DPX were detected in mouse liver, there was no evidence of DPX formation in mouse lung, hamster liver,

or hamster lung. Additionally, DPX are not formed in measurable amounts in human liver tissue

(Casanova et al. 1996). Therefore, the induction of DPX by formaldehyde in mouse liver might be a

species-specific, tissue-specific response. A subsequent in vitro study by this laboratory confirmed the

absence of DPX formation in response to methylene chloride in human, rat, and hamster hepatocytes,

whereas a dose-response was observed in mouse hepatocytes (Casanova et al. 1997). In a different

experiment, a dose-response in the formation of RNA-formaldehyde adducts was observed in hepatocytes

of all four species (mouse>human>rat>hamster). RNA adduct production was related to the expression of

the GSTT1-1 enzyme; the human liver sample lacking GSTT1-1 did not produce RNA adducts.

In vitro tests using two different strains of Salmonella, one with and one without GST expression,

revealed that methylene chloride may be mutagenic by at least two pathways (DeMarini et al. 1997). The

bacterial strains employed were TA100 and RSJ100; the latter is a derivative of the strain TA1535 (not

normally mutagenized by methylene chloride) that contains recombinant rat GSTT1-1. In RSJ100,

methylene chloride was mutagenic at moderate doses, and produced a single class of mutation (GC ÷ AT

transversions) only at the middle C of the target CCC. Although methylene chloride was mutagenic in

TA100, it required a much higher dose to match the mutation frequency in RSJ100. Furthermore, it

produced a variety of lesions and mutations (predominantly GC ÷ TA transversions) at the first and

second positions of the CCC target. The implication of these results is that genotoxic effects of

methylene chloride will be different, depending on the GSTT1 phenotype. Those who lack the functional

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gene would only be susceptible to genetic damage from methylene chloride under high exposure levels,

whereas carriers might be vulnerable to genetic damage at much lower exposure levels.

According to Maronpot et al. (1995b), the precise mode of action of methylene chloride-induced mouse

tumorigenicity appears to be elusive and has not yet been confirmed.

2.4.3 Animal-to-Human Extrapolations

The two major difficulties in applying the results of rodent cancer bioassays to humans involve

extrapolation from the high-concentration rodent exposures to the lower concentrations typical of human

exposure conditions; and the interspecies extrapolation. The predominant tumors of interest with regard

to methylene chloride-induced tumorigenicity are mouse lung and mouse liver. However, species

differences among rodent tumorigenic, genotoxic, and morphological responses to methylene chloride, as

well as differences between mice and humans, appear to limit the applicability of mouse tumor data to

humans, according to some authors. Foster et al. (1992) have suggested that the Clara cell may have a

role in mouse lung tumor induction; there is a substantially higher number of Clara cells in the mouse

than in other rodent species or in humans. GSH-mediated activation of the proximate carcinogenic agent

of methylene chloride in mice has been associated with a specific isoenzyme, the Θ (theta) class GSTs;

Sheratt et al. (1997) have shown that this isozyme is expressed at very low levels in human pulmonary

cells in vitro, suggesting that in humans, the lung has little capacity to activate methylene chloride into

biologically reactive intermediates. In vitro studies (e.g., Graves et al. 1995) have shown species

differences in the ability to induce DNA single-strand breaks in mouse, rat, hamster, and human cells that

are compatible with the known rodent carcinogenicity, or lack thereof, in chronic cancer bioassays.

These in vitro studies suggested that humans are unlikely to be more susceptible than rodents to

methylene chloride-induced liver cancer.

In a comparison of rates of methylene chloride metabolism by each of the two major pathways in four

species, the mouse, rat, hamster, and human, Green (1997) presented in vivo and in vitro evidence that the

MFO pathway metabolic rates are similar among all four species and saturated at concentrations of

500 ppm or above. In contrast, the GSH-mediated metabolism is linear over the concentration range

studied; it is also the major metabolic pathway for methylene chloride in mice at concentration levels used

in cancer inhalation bioassays. Furthermore, the activity in mouse tissues is more than an order of

magnitude greater than the activity in rat tissues. Hamster and human tissues show metabolic rates for

this pathway that are even lower than those found in the rat. These findings are corroborated by the

localization study of Mainwaring et al. (1996b), which found higher levels of GSTT1-1 mRNA and

protein in the liver and lung of mice than in rats or humans. In mechanistic terms, Green (1997)

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2. HEALTH EFFECTS

concludes that these studies demonstrate that differences in glutathione-mediated metabolism among

mice, rats, hamsters, and humans are correlated with differences in carcinogenicity of methylene chloride.

This is supported by the levels of glutathione that have been detected in the livers of the different species.

The hepatic concentrations of GSH (in nmol/10

cells) were approximately 129 in B6C3F

mice (Ruch et

al. 1989), 50 in Sprague-Dawley rats (Jones et al. 1978), 22 in Syrian Golden hamsters, and 21 in humans

(Steinmetz et al. 1988). Thus, the mouse is the most sensitive species to metabolic activation of

methylene chloride by glutathione metabolism, whereas humans appear to be the least sensitive.

However, other evidence suggests that methylene chloride may be potentially carcinogenic in humans.

Mainwaring et al. (1996b) analysis of the distribution of mRNA and protein for GSTT1-1 and GSTT2-2

in the analysis of the liver and lungs of mice, rats, and humans corroborated the finding of Sheratt et al.

(1997) that the overall levels in human tissues are much lower than in those of mice. However, the

immunodetection of localized high concentrations of GSTT2-2 enzyme in human bile-duct epithelial cells

is potentially significant, considering the increased incidence of biliary cancer following chronic exposure

to methylene chloride as reported by Lanes et al. (1990). On the other hand, the GSTT2-2 antibody did

not localize to the nucleus of human biliary epithelial cells, which would tend to reduce the potential

genotoxic effect in humans. Although Mainwaring et al. (1996b) found that rates of metabolism of

methylene chloride were very low in human lung, they also detected higher than background amounts of

GSTT1-1 mRNA in a few Clara cells and ciliated cells of the alveolar/ bronchiolar junction of the lung in

one human sample out of four. Therefore, despite the general low level of GSTT1-1and GSTT2-2 in

human tissue, it is possible that in some individuals, specific cell types within the human liver (bile duct)

and lung might produce genotoxic reactive intermediates as a result of methylene chloride metabolism.

Based on GSTT enzyme distributions and concentrations, the carcinogenic risk from methylene chloride

in humans appears to be low as in rats rather than high as in mice.

The mouse model has been employed in assessment of excess lifetime cancer risks in humans from

inhalation exposure to methylene chloride by EPA and others. Recent data, both in vitro and in vivo,

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strongly indicate that the higher sensitivity of the mouse, relative to humans, must be taken into

consideration if mouse data are to be used to estimate potential human health risks.

2.5 RELEVANCE TO PUBLIC HEALTH

Issues relevant to children are explicitly discussed in 2.7, Children’s Susceptibility, and 5.6, Exposures of

Children.

Overview. Methylene chloride has been widely used in industrial processes, food preparation,

agriculture, and consumer products; consequently, there have been numerous studies describing its effects

in a variety of experimental animal species. Humans have not been clinically studied as extensively.

Although methylene chloride uses in agricultural goods and some consumer products have declined in

recent years, there is still potential public health concern due to its continued use in industrial processes,

and continued releases into the environment.

The central nervous system is affected adversely in humans and animals at inhalation exposure levels of

200 ppm or higher (Putz et al. 1979). Effects in animals were also reported on the liver and kidney

following continuous exposure at concentrations of 25 ppm or greater (Haun et al. 1972), and on the

cardiovascular system, but at extremely high exposures (Aviado and Belej 1974). Long-term inhalation

exposure to methylene chloride (500 ppm or greater) increased tumors in some animals (Nitschke et al.

1988a), but did not cause teratogenic or reproductive effects in a two-generation study (Nitschke et al.

1988b). Since inhalation is the principal route of exposure to methylene chloride, most of these effects

have been tested for or observed by this route. Data on effects observed after oral and dermal exposure

are somewhat more limited. Further details are presented below.

Minimal Risk Levels for Methylene Chloride.

Inhalation MRLs.

C An MRL of 0.6 ppm has been derived for acute inhalation exposure (0–14 days) to methylene

chloride. This MRL supersedes the previous MRL of 3 ppm derived in the 1998 draft for public

comment profile. Refer to chapter 7 for additional information.

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METHYLENE CHLORIDE

2. HEALTH EFFECTS

The acute inhalation MRL was derived from the behavioral toxicity study by Winneke (1974) in which a

randomized blind clinical chamber experiment was used to expose 6–20 volunteers to vapors of

methylene chloride (300, 500, or 800 ppm) or filtered air for 3–4 hours. Subjects were tested at

45-minute intervals with standard neurobehavioral tests measuring: (1) critical flicker fusion frequency

(visual); (2) auditory vigilance performance; and (3) performance on psychomotor tasks. The tested

parameters were considered to reflect the status of ‘cortical alertness’ (Fodor and Winneke 1971). A

statistically significant depression in critical flicker fusion (CFF) frequency was observed at all

concentrations. The magnitude of CFF frequency depression was similar at exposure concentrations of

300 and 500 ppm and was larger at 800 ppm. Thus, there was no dose-response at the two lowest

concentrations, and a dose-response was evident at the highest concentration. A decrease in auditory

vigilance performance was observed at 500 ppm and psychomotor task performance was impaired at

800 ppm. Thus, reduced CFF frequency is the most sensitive neurological response to acute inhalation

exposure to methylene chloride. Based on this endpoint, the LOAEL is 300 ppm. A PBPK model for this

experiment was used to adjust the dosage to a 24 hour exposure period, thus resulting in a LOAEL of

60 ppm for the same endpoint (Reitz et al. 1997). The MRL was derived by dividing the LOAEL of

60 ppm by an uncertainty factor of 100 (10 for the use of a LOAEL and 10 for human variability).

An MRL of 0.3 ppm has been derived for intermediate inhalation exposure (15–364 days) to

methylene chloride.

The intermediate inhalation MRL was derived from a study by Haun et al. (1972) in which groups of

mice, rats, dogs, and monkeys were continuously exposed to methylene chloride for 14 weeks at chamber

concentrations of either 0, 25, or 100 ppm. Body weights and clinical signs were monitored throughout

the study. Necropsy was performed and tissues were examined histopathologically and organ-to-body

weight were determined at the end of the exposure. Data on liver histopathology of rats exposed to 25 or

100 ppm of methylene chloride were selected as the critical effect. Liver cytoplasmic vacuolization and

staining associated with fatty deposits were observed in rats at both exposure concentrations; Haun et al.

(1972) did not mention whether there were quantitative differences in the effect observed at the two

exposure levels. The MRL was derived based on a LOAEL of 25 ppm for hepatic effects. Because the

critical effect observed is an extrarespiratory effect (rat liver), a human equivalent concentration (HEC)

was calculated. Since the ratio of the blood:air partition coefficient in the rat to the blood:air partition

coefficient in the human was > 1, the value of 1.0 was used to calculate the LOAEL

[HEC]

(EPA 1994).

This resulted in an MRL of 0.3 ppm by dividing the LOAEL of 25 ppm by an uncertainty factor of 90 (3

for use of a minimal LOAEL, 3 for extrapolation from animals to humans, and 10 for human variability).

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C An MRL of 0.3 ppm has been derived for chronic inhalation exposure ($365 days) to methylene

chloride.

The chronic inhalation MRL was derived from a study by Nitschke et al. (1988a) in which groups of

90 male and 108 female Sprague-Dawley rats were exposed to methylene chloride at 0 (controls), 50,

200, or 500 ppm for 6 hours/day, 5 days/week for 2 years. A number of satellite groups were also

exposed to assess the temporal relationship between methylene chloride exposure and evidence of

toxicity. Subgroups of females in the main study were sacrificed after 6, 12, 15, and 18 months of

exposure. The following end points were evaluated: body weight, food consumption rates, organ weights,

hematology, clinical chemistry, urinalysis, pathology, histopathology, and blood COHb levels. Blood

COHb levels were consistently higher than 10% in animals exposed to $200 ppm. No pathologic or

histopathologic nontumor findings were reported except in the liver. Hepatocellular cytoplasmic

vacuolization consistent with fatty changes, and multinucleate hepatocytes were elevated in female rats

exposed to methylene chloride at 200 and 500 ppm; a slight increase in the incidence of hepatocellular

vacuolization was also observed in male rats exposed to 500 ppm.

The NOAEL of 50 ppm was adjusted for continuous exposure (6 hour/day, 5 day/week) resulting in a

NOAEL

[ADJ]

of 8.92 ppm. Whereas the MRL was derived based on hepatic effects (extrarespiratory), a

human equivalent concentration (HEC) was calculated. Since the ratio of the blood:air partition

coefficient in the rat to the blood:air partition coefficient in the human was > 1, the value of 1.0 was used

to calculate the LOAEL

[HEC]

(EPA 1994). This resulted in an MRL of 0.3 ppm by dividing the LOAEL

of 8.92 ppm by an uncertainty factor of 30 (3 for extrapolation from animals to humans, and 10 for human

variability).

Oral MRLs.

CAn MRL of 0.2 mg/kg/day has been derived for acute oral exposure (0–14 days) to methylene

chloride. This MRL supersedes the previous MRL of 0.5 mg/kg/day derived in the 1998 draft for public

comment profile. Refer to chapter 7 for additional information.

The acute oral MRL was derived by route-to-route extrapolation of the data from Winneke (1974) in

which a randomized blind clinical chamber experiment was used to expose 6–12 volunteers to vapors of

methylene chloride (300, 500, or 800 ppm) or filtered air for 3–4 hours. Subjects were tested at

45-minute intervals with standard neurobehavioral tests measuring: (1) critical flicker fusion frequency

120 METHYLENE CHLORIDE

2. HEALTH EFFECTS

(visual); (2) auditory vigilance performance; and (3) performance on psychomotor tasks. A statistically

significant depression in critical flicker fusion (CFF) frequency was observed at all concentrations. The

magnitude of CFF frequency depression was similar at exposure concentrations of 300 and 500 ppm and

was larger at 800 ppm. Thus, there was no dose-response at the two lowest concentrations, and a dose-

response was evident at the highest concentration. A decrease in auditory vigilance performance was

observed at 500 ppm and psychomotor task performance was impaired at 800 ppm. Thus, reduced CFF

frequency is the most sensitive neurological response to acute inhalation exposure to methylene chloride.

Based on this end point, the LOAEL is 300 ppm.

Reitz et al. (1997) modified the basic PBPK model for methylene chloride that was developed by

Andersen et al. (1987), Reitz et al. (1988), and Andersen et al. (1991) as described in Section 2.3.5.2.

The major modification of the model was the inclusion of a brain compartment so that central nervous

system effects could be assessed. Reitz et al. (1997) modeled the Winneke (1974) data to obtain the

target organ (brain) concentrations of methylene chloride associated with administered inhalation

concentrations, and then calculate the human drinking water concentrations (mg/L) that would result in

the equivalent target organ-specific doses. Human exposure patterns in the PBPK model simulated

realistic human drinking water consumption patterns, (i.e., consisting of bouts of drinking during the day,

with and between meals, and little-to-no drinking during the night). PBPK modeling predicted that peak

concentrations of methylene chloride in the brain would increase rapidly after each episode of drinking

water consumption, and then drop sharply, to near-zero, between bouts of drinking. Additionally, there

would be no cumulative effects from repeated exposure.

For acute neurological effects, the associated dose measure was defined as the peak concentration of

methylene chloride in brain tissue (mg/L of brain tissue) of humans exposed to 300 ppm of methylene

chloride for 4 hours by inhalation. The modified PBPK model calculated that the administered inhalation

dose was equivalent to 3.95 mg of methylene chloride per L of brain tissue. The equivalent administered

human concentration in drinking water that will produce the same neurological effects was 565 mg of

methylene chloride/L. Using a daily drinking water consumption value of 2L and an average human body

weight of 70 kg, the LOAEL was calculated to be 16 mg/kg/day. An acute oral MRL was calculated by

dividing the LOAEL (16 mg/kg/day) by an uncertainty factor of 100 (10 for the use of a LOAEL and

10 for human variability), to yield 0.2 mg/kg/day.

121 METHYLENE CHLORIDE

2. HEALTH EFFECTS

C An MRL of 0.06 mg/kg/day has been derived for chronic oral exposure ($365 days) to methylene

chloride. This MRL supersedes the previous MRL of 0.2 mg/kg/day derived in the 1998 draft for

public comment profile. Refer to chapter 7 for additional information.

The chronic oral MRL was derived from a study by Serota et al. (1986a) in which F344 rats (85/sex/dose)

were exposed to methylene chloride in deionized drinking water at concentrations to provide target doses

of 0, 5, 50, 125, or 250 mg/kg/day for 104 weeks. The nominal mean doses were 0, 6, 55, 131, and

249 mg/kg/day. There were no treatment-related effects on survival or on the incidence of adverse

clinical signs. Organ weights were not significantly affected by treatment. Histopathology was only

observed in the liver, which therefore is the critical target organ. Statistically significant cellular changes

(hepatic foci/areas of cellular alterations and fatty changes) were observed at dose levels $50 mg/kg/day.

Reduced weight gain was observed at 131 and 249 mg/kg/day, but quantitative data were not provided.

Hematological effects (increased mean hematocrit, hemoglobin, and erythrocyte count) were observed at

all dose levels except 6 mg/kg/day. Therefore, the lowest dose in rats was identified as the NOAEL.

Based on measured, mean drinking water consumption rates, this dose was calculated to be 6 mg/kg/day.

The chronic oral MRL was calculated by dividing the NOAEL (6 mg/kg/day) by an uncertainty factor of

100 (10 for extrapolation from animals to humans and 10 for human variability).

No intermediate oral MRL was derived because of an inadequate database.

Death. Acute inhalation exposure to methylene chloride has caused death in humans (Bakinson and

Jones 1985; Bonventre et al. 1977; Hall and Rumack 1990; Stewart and Hake 1976). Although exposure

levels were not measured, estimates suggest that a combination of high exposure levels and/or inadequate

ventilation has contributed to these lethal accidents. Measurements of methylene chloride in tissues or of

COHb in blood have corroborated high exposures in some cases (Manno et al. 1992; Moskowitz and

Shapiro 1952; Winek et al. 1981; Tay et al. 1995). The biologic cause of death was not verified in all

cases, but is thought to have been respiratory depression secondary to narcosis. Asphyxia accompanied

by bilateral pulmonary congestion and focal hemorrhage was reported in one case (Winek et al. 1981) and

myocardial infarction was reported in another (Stewart and Hake 1976). Mortality risk was not increased

in humans exposed occupationally to 30–120 ppm of methylene chloride for over 30 years (Friedlander et

al. 1978) and no excess mortality was found in workers exposed to 140–475 ppm for at least 3 months

(Lanes et al. 1993; Ott et al. 1983b). Inhalation exposure to concentrations of 3,500 ppm for longer

122 METHYLENE CHLORIDE

2. HEALTH EFFECTS

durations (14 weeks to 2 years) was lethal in some animals (Burek et al. 1984; MacEwen et al. 1972). No

mortality increase was noted in chronic inhalation studies at 500 ppm in the rat (Nitschke et al. 1988a).

In one suicide case, ingestion of paint remover containing 75–80% methylene chloride resulted in death

from corrosion of the gastrointestinal tract (Hughes and Tracey 1993). In animals, acute exposure to high

doses (2,100 mg/kg or greater) by gavage caused death (Kimura et al. 1971; Ugazio et al. 1973);

intermediate-duration exposures to 64 or 320 mg/kg/day by gavage significantly increased mortality in

female mice and male rats, respectively (Maltoni et al. 1988). However, exposure to methylene chloride

in drinking water (up to 250 mg/kg/day) did not significantly affect survival (Serota et al. 1986a, 1986b).

Gavage administration may result in more severe effects than ad libitum water ingestion since the

administered dose may temporarily saturate normal metabolic processes, and result in a different

metabolic profile. No data were found on death in humans or animals from dermal exposure.

Systemic Effects

Respiratory Effects. In one workplace accident, acute inhalation exposure to methylene chloride

(probably at a high concentration) resulted in bilateral pulmonary congestion with focal hemorrhage

(Winek et al. 1981). Less severe respiratory symptoms (cough, breathlessness, chest tightness) were

reported in occupational exposure incidents (concentrations unknown; Bakinson and Jones 1985).

Exposure to 18–1,200 mg/m

(5–340 ppm; 8-hour TWA) resulted in irritation of the respiratory tract in

one occupational study (Anundi et al. 1993). However, a study in humans found no effect on pulmonary

function following repeated exposures to methylene chloride vapors (up to 500 ppm) (Stewart et al.

1972). Studies in animals corroborated the severe pulmonary effects (congestion, edema, inflammation)

of acute or intermediate exposures at high concentrations (Heppel et al. 1944; NTP 1986). Resolution of

acute Clara cell damage in mice exposed to 4,000 ppm of methylene chloride was correlated with

cytochrome P-450 activity in the lung (Foster et al. 1992). No information was found on the respiratory

effects of low levels of methylene chloride in humans near hazardous waste sites or industrial urban areas

or in animals.

Cardiovascular Effects. Myocardial infarction occurred in one case of acute inhalation occupational

exposure (Stewart and Hake 1976). However, occupational studies in humans did not find any

association between exposure to methylene chloride at 75–475 ppm and cardiac abnormalities (Cherry et

al. 1981) or excess mortality due to ischemic heart disease (Hearne et al. 1990; Ott et al. 1983b). Further,

123 METHYLENE CHLORIDE

2. HEALTH EFFECTS

a cross-sectional study of workers showed no excess of electrocardiographic abnormalities among those

exposed to methylene chloride (Ott et al. 1983c). Another study in humans did not reveal effects on

cardiovascular functions at concentrations up to 500 ppm (NIOSH 1974). These findings suggest the

cardiovascular system is not a sensitive target for exposure to methylene chloride in humans. One study

in mice reported atrioventricular block following acute inhalation exposure to very high levels of

methylene chloride (>200,000 ppm) (Aviado and Belej 1974). The relevance of this finding is limited

since the exposure level was so high.

In an experiment with rats, using an ischemia-reperfusion model, intravenous methylene chloride infusion

(leading to blood concentrations of <0.1 mg/mL) markedly increased the atrioventricular block during the

reperfusion phase (Scholz et al. 1991). From these results, the authors concluded that the initial coma

resulting from methylene chloride-induced poisoning is likely to result not only from anesthetic effects,

but also from sudden onset of cardiac arrhythmias.

Gastrointestinal Effects. Nausea and vomiting were reported in 13 out of 33 occupational cases of acute

inhalation exposure to methylene chloride in the United Kingdom (Bakinson and Jones 1985); exposure

levels were not reported in these cases. In mice exposed to 4,000 ppm of methylene chloride for 2 years,

dilatation of the stomach was reported (NTP 1986). In humans attempting suicide, ingestion of a single

oral dose of Nitromors, a paint remover solvent containing 75–80% methylene chloride, resulted in severe

corrosion and ulceration of the gastrointestinal tract (Hughes and Tracey 1993), peritonitis and septicemia

occurred in the fatal case, whereas intestinal diverticuli developed during recovery in the another case

(Roberts and Marshall 1976). No other studies were located regarding gastrointestinal effects in humans

or animals after exposure to methylene chloride.

Hematological Effects. Metabolism of methylene chloride results in excess carbon monoxide and

increases in COHb, which contributes to hypoxia (Tomaszewski 1998). Blood COHb concentrations

were about 30% higher than normal in 2 lethal cases in which workers were estimated to be exposed to

extremely high concentrations (up to 168,000 ppm) of methylene chloride in a confined work space

(Manno et al. 1992). Several hours after an adult woman ingested a fatal oral dose of Nitromors, a paint

remover containing 75–80% methylene chloride, her COHb level was 9%. In all three fatal exposures,

cause of death was not associated with elevated COHb. In autoworkers who were exposed to methylene

chloride dermally and by inhalation (3–154 ppm), blood COHb measurements taken within 24 hours of

exposure were 1.2–11% for nonsmokers and 7.3 and 17.3% for two smokers (Kelly 1988). One-day

124 METHYLENE CHLORIDE

2. HEALTH EFFECTS

occupational exposures to methylene chloride at levels below ACGIH standard (50 ppm, 8-hour TWA)

produced small increases in COHb levels in both nonsmoking and smoking adults (Soden et al. 1996);

additional daily cumulative exposure to methylene chloride did not further increase COHb levels.

Increases in blood COHb levels between 5 and 6.8% were measured in nonsmoking volunteers exposed to

methylene chloride at concentrations up to 200 ppm for 4 to 7.5 hours (DiVincenzo and Kaplan 1981;

Putz et al. 1979).

Other studies have reported increases in red cell count, hemoglobin, and hematocrit in women, but not in

men, occupationally exposed to concentrations of up to 475 ppm during an 8-hour workday (Ott et al.

1983d); these effects were judged by the authors to be suggestive of compensatory hematopoietic effects.

Similar findings were not observed in rodents chronically exposed to methylene chloride by inhalation at

concentrations up to 3,500 ppm (Burek et al. 1984), but were observed in rodents exposed orally for

3 months at 480 mg/kg/day (Kirschman et al. 1986), or for 2 years at 55–249 mg/kg/day (Serota et al.

1986a, 1986b). Intravascular hemolysis was reported in the case of a man who attempted suicide by

ingesting the paint remover, Nitromors (Roberts and Marshall 1976) and in a high-dose acute gavage

study in rats (Marzotko and Pankow 1987).

Hepatic Effects. Human data are limited on the effects of methylene chloride on the liver. A slight

exposure-related increase in serum bilirubin (but not at levels of clinical significance) was observed in

workers with exposure up to an average of 475 ppm of methylene chloride, but serum levels of hepatic

enzymes (e.g., aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and alkaline

phosphatase) were not elevated (Ott et al. 1983a); another occupational study found no exposure-related

changes in hepatic enzymes (Anundi et al. 1993). In another study, methylene chloride vapors (up to

500 ppm) did not affect comparable serum enzyme activity in volunteers (Stewart et al. 1972). However,

the liver appears to be a major target organ following methylene chloride exposure in animals,

particularly at high exposure levels (>5,000 ppm; Heppel et al. 1944). Histomorphological and

biochemical changes of the liver occur following acute inhalation (6 hours to 7 days) at high

concentration levels (5,200 ppm) (Morris et al. 1979), while fatty changes and biochemical alterations

(altered cytochrome P-450 levels) were also observed at lower concentrations (100 ppm) for continuous,

24-hour intermediate-duration exposure (100 days) (Haun et al. 1972; Kjellstrand et al. 1986; Weinstein

and Diamond 1972). Cytoplasmic vacuolization was observed in rats at 25 ppm (Haun et al. 1972).

Using these data, ATSDR derived an intermediate inhalation MRL of 0.3 ppm, as calculated in Table 2-1.

Exposure to 1,000–4,000 ppm for 2 years resulted in an increased incidence of hemosiderosis and focal

125 METHYLENE CHLORIDE

2. HEALTH EFFECTS

hepatic necrosis in rats (NTP 1986). An increased incidence of fatty changes occurred following chronic

exposure at 500 ppm, but not at 200 ppm (Nitschke et al. 1988a); fatty changes were reversible when

exposure ceased. Using data from the Nitschke et al. (1988a) study, ATSDR derived a chronic inhalation

MRL of 0.3 ppm, as shown in Table 2-1. Necrosis or fatty changes in the liver have been observed in rats

given high oral doses (>1,000 mg/kg/day) of methylene chloride (Kirschmann et al. 1986; Ugazio et al.

1973). Chronic ingestion of methylene chloride in drinking water has been associated with fatty changes

in rats at 55 mg/kg/day or greater and in mice at 175 mg/kg/day or greater, but not at 6 mg/kg/day (Serota

et al. 1986a, 1986b). Based on this value (6 mg/kg/day), a chronic oral MRL of 0.06 mg/kg/day was

calculated as described in Table 2-2.

Endocrine Effects. No relevant information was located regarding endocrine effects in human or animals

associated with exposure to methylene chloride.

Renal Effects. Kidney function was not altered in humans repeatedly exposed to methylene chloride

vapors (up to 500 ppm) for 6 weeks (Stewart et al. 1972) and no alterations in urinary microglobulins or

N-acetyl-beta-glucosaminidase were detected in workers chronically exposed to methylene chloride

(Anundi et al. 1993). In rats, nonspecific renal tubular and degenerative changes occurred after

continuous intermediate-duration exposure to methylene chloride vapors (100–5,000 ppm) (Haun et al.

1972; MacEwen et al. 1972) or chronic exposure to 4,000 ppm (NTP 1986). Similar renal changes

occurred in dogs exposed to 1,000 ppm for 4 weeks (MacEwen et al. 1972). There were no studies

reporting renal effects following oral exposure to methylene chloride in humans. In rat oral studies using

methylene chloride at doses >1,300 mg/kg/day, a single dose inhibited diuresis (Marzotko and Pankow

1987) and treatment for 3 months increased kidney weights in females (Kirschman et al. 1986). At

$166 mg/kg/day, methylene chloride lowered the pH of urine in rats of both sexes (Kirschman et al.

1986). There are no data that would enable an assessment of the potential for renal effects in humans

living near hazardous waste sites.

Dermal Effects. No studies were located regarding dermal effects in human or animals associated with

inhalation or oral exposure to methylene chloride. However, in some occupational accidents, direct

contact with methylene chloride has resulted in second or third degree chemical burns within 30 minutes

(Hall and Rumack 1990; Wells and Waldron 1984; Winek et al. 1981).

126 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Ocular Effects. No studies were located on ocular effects in humans by oral or dermal exposure.

However, repeated direct exposure to methylene chloride vapors (up to 500 ppm) caused mild irritation to

the eyes of volunteers (Stewart et al. 1972). In one occupational accident, direct contact with the liquid

(duration unspecified) resulted in severe corneal burns (Hall and Rumack 1990). In animals, small

increases in corneal thickness and intraocular tension were reported after exposure to vapors of 490 ppm

of methylene chloride or greater, but effects were reversible within 2 days after exposure ceased

(Ballantyne et al. 1976). Inflammation of the conjunctivas and eyelids as well as increases in corneal

thickness and intraocular tension were observed following direct contact of methylene chloride (0.1 mL)

with the eyes of rabbits. Effects were reversible within 3–9 days (Ballantyne et al. 1976).

Immunological and Lymphoreticular Effects. No studies were located regarding immunological

effects in humans after inhalation, oral, or dermal exposure. Splenic atrophy was evident in dogs that

died following continuous intermediate-duration exposure to vapors of methylene chloride (1,000 ppm)

(MacEwen et al. 1972), but the incidence did not increase over control levels in rats and mice chronically,

but discontinuously, exposed at a level of 4,000 ppm or less (NTP 1986). Splenic fibrosis was observed in

rats after chronic inhalation exposure at $1,000 ppm methylene chloride (Mennear et al. 1988), but was

not observed in a recent intermediate-duration study in rats exposed to 5,187 ppm methylene choride

(Halogenated Solvent Industry Alliance, Inc. (2000). This latter study also found that the IgM antibody

response to SRBC was not significantly altered in exposed rats relative to controls. Due to the very

limited and inconsistent nature of the database, further studies are necessary before conclusions can be

drawn about the relevance of these findings to human health.

Neurological Effects. Studies in humans and animals indicate the central nervous system is an

important target for methylene chloride; in the case of acute exposures, anesthetic responses, which

subsided once exposure ceased, have been reported in humans and animals (Bakinson and Jones 1985;

Hall and Rumack 1990; Heppel et al. 1944; Snyder et al. 1992a, 1992b). Neurological effects in humans

have included headache, dizziness, confusion, memory loss, intoxication, incoordination, paresthesia, and

in severe cases, unconsciousness and seizures (Bakinson and Jones 1985; Hall and Rumack 1990; Kelly

1988). Degraded performance in various psychomotor tasks was reported in humans acutely exposed to

methylene chloride (200 ppm or greater) in experimental studies (Fodor and Winneke 1971; Putz et al.

1979; Stewart et al. 1972; Winneke 1974). Impaired performance during exposure to methylene chloride

was associated with a rise in blood levels COHb to about 5%, which occurred after 3 hours of exposure at

200 ppm (Putz et al. 1979). In more specific measures of cortical function, Winneke (1974) attributed

127 METHYLENE CHLORIDE

2. HEALTH EFFECTS

impaired performance in volunteers following 3–4 hours of exposure to 300 ppm of methylene chloride to

the properties of the parent compound; exposure to 50–100 ppm of carbon monoxide did not generate the

same adverse effects. Based on this value (300 ppm), an acute inhalation MRL of 0.6 ppm was calculated

as described in Table 2-1 (Winneke 1974). Studies in factory workers chronically exposed to methylene

chloride revealed no evidence of neurological or behavioral impairment at exposure levels of 75–100 ppm

(Cherry et al. 1981). There was a reduction in test scores pertaining to mood changes in workers in 1 of

3 rapid rotation shifts exposed to vapors of methylene chloride (28–173 ppm), but no effects were

observed on performance as determined by digit symbol substitution scores (Cherry et al. 1983). Mood

changes are very subjective and these results may reflect other causes.

Neurological effects have been noted in animals following exposure to methylene chloride independent of

the route. Narcotic effects of methylene chloride (incoordination, gait disturbances, reduced activity,

somnolence) were observed in monkeys, rabbits, rats and guinea pigs following acute exposure at

$10,000 ppm (Heppel and Neal 1944; Heppel et al. 1944); dogs exposed at this level became

uncoordinated, then excited and hyperactive (Heppel et al. 1944). These effects wore off when exposure

ceased. Studies in rats and gerbils suggest that methylene chloride induces regional alterations in DNA

concentrations, amino acids levels, and enzyme activities in the brain. There was a decrease in succinic

dehydrogenase activity in the cerebellum in rats exposed to vapors of methylene chloride at concen-

trations of 500 ppm or greater and signs of increased protein breakdown in the cerebrum at 1,000 ppm

(Savolainen et al. 1981). The DNA concentration decreased in the hippocampus and cerebellum in

gerbils exposed to $210 ppm of methylene chloride, indicating decreased cell density in these brain

regions, probably due to cell loss (Karlsson et al. 1987; Rosengren et al. 1986). Levels of aminobutyric

acid increased in the posterior cerebellar vermis of gerbils exposed to 210 ppm of methylene chloride;

however, the significance of this finding is uncertain (Briving et al. 1986). On the other hand, studies in

rats exposed to methylene chloride (2,000 ppm) for 13 weeks revealed that the compound did not cause

clinical, postural, sensory, locomotor, evoked potential, or pathological effects (Mattsson et al. 1990).

However, when rats were given an intraperitoneal injection of methylene chloride, 115 mg/kg was

sufficient to alter the pattern of flash evoked potentials (Herr and Boyes 1997); in this study, the

responses generated in methylene chloride-treated rats were different from those generated by other

solvents, leading the authors to conclude that lipid solubility alone was insufficient to predict the

neurotoxic effects. The mechanism by which methylene chloride exerts its effects on the central nervous

system is not clear. Anaesthetic effects of the parent compound and hypoxic effects of its metabolite

carbon monoxide may both contribute to the observed symptoms.

128 METHYLENE CHLORIDE

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Reproductive Effects. Data on reproductive toxicity in humans are limited; there are no studies

involving oral exposure. One group of cases reported genital pain, testicular atrophy, recent infertility,

and low/abnormal sperm counts in workers who inhaled vapors of methylene chloride and who had direct

contact with the liquid on the job for more than a year (Kelly 1988). Exposure to methylene chloride in

this group was confirmed by blood COHb analysis and the presence of typical neurological deficits. A

retrospective study of pregnancy outcomes among Finnish pharmaceutical workers reported a slightly

increased risk of spontaneous abortion (not quite statistically significant) associated with inhalation

exposure to methylene chloride (Taskinen et al. 1986). No animal reproductive studies were conducted

for oral or dermal exposures to methylene chloride, and the inhalation studies were negative. Methylene

chloride did not adversely affect fertility and litter size in rats that inhaled methylene chloride vapors at

concentrations up to 1,500 ppm or less for two generations (Nitschke et al. 1988b). After intermediate-

duration exposure (6 weeks) to vapors of methylene chloride (200 ppm or less), there were no

microscopic lesions in testes of rats (Raje et al. 1988). Uterine, ovarian, and testicular atrophy has been

noted in rats and mice chronically exposed for 2 years to 4,000 ppm of methylene chloride, but this was

reported to be secondary to malignant neoplasms (NTP 1986). Based on these data, methylene chloride

does not appear to pose a hazard to human reproduction, except at very high exposure levels.

Developmental Effects. There are few studies addressing developmental effects in humans. A

retrospective study of pregnancy outcomes among Finnish pharmaceutical workers reported a slightly

increased risk of spontaneous abortion associated with inhalation exposure to methylene chloride

(Taskinen et al. 1986). A study examining the effect of low environmental concentrations of methylene

chloride (<0.01 ppm) on over 90,000 births found no significant effect on birth weight (Bell et al. 1991).

Studies in rats demonstrated that methylene chloride crosses the placenta and that metabolism of

methylene chloride by the maternal liver elevates the blood levels of carbon monoxide in the fetus

(Anders and Sunram 1982). Animal studies demonstrated that inhalation of methylene chloride vapors at

concentrations of 1,250 ppm produced minor skeletal variants: delayed ossification of sternebrae in rats

and extra center of ossification in the sternum of mice and rats (Schwetz et al. 1975). Fetal weight was

reduced and behavioral changes occurred in rat pups following exposure of dams to 4,500 ppm of

methylene chloride (Bornschein et al. 1980; Hardin and Manson 1980). The significance of these

observations is uncertain since each of the three studies used only one concentration level and the

observed effects occurred at maternally toxic concentrations. Growth (yolk sac blood vessel growth,

body length, protein concentration) and development (somite addition) were significantly retarded in rat

embryos cultured in the presence of methylene chloride at $0.5 mg/mL, but not at 0.2 mg/mL

129 METHYLENE CHLORIDE

2. HEALTH EFFECTS

(Brown-Woodman et al. 1998); the effect on the yolk sac blood vessels may have interfered with the

ability of the embryos to take up nutrients. Methylene chloride had no effect on heart function in these

embryos. Although fetal body weights were decreased in these animal studies, the absence of other

fetotoxic effects, embryolethality, or major malformations, suggests that methylene chloride is not likely

to cause developmental effects or behavioral changes at levels normally encountered in the environment;

with current standards and procedures, workplace exposure of pregnant women is unlikely to be

hazardous to the fetus. The reduction in the rate of spontaneous abortions observed during the later years

of the Finnish study was attributed partly to improved industrial hygiene (Taskinen et al. 1986).

Genotoxic Effects. No studies were located regarding the genotoxic effects of methylene chloride in

humans after inhalation, oral, or dermal exposure. In vitro results were mixed in bacterial assays and in

tests employing mammalian cells (Table 2-3). Methylene chloride has caused chromosomal aberrations

in some studies, but not in others. Given the evidence of in vitro clastogenicity and its negative results in

unscheduled DNA-synthesis and DNA-binding studies, methylene chloride may be a weak mutagen in

mammalian systems. The chemical has been evaluated in several in vivo assay systems in animals to

assess its potential to induce gene mutation and cause chromosomal aberrations or DNA damage and

repair. Many studies were negative and some were positive (Table 2-4). Tissue-specific genetic damage

in mice following exposure to methylene chloride, suggests that the ability of tissues to metabolize

methylene chloride may determine its genotoxicity (Sasaki et al. 1998). Specific polymorphisms in the

metabolizing enzymes are associated with genotoxicity. In vitro, low concentrations of methylene

chloride were more mutagenic when a functional GSTT1-1 gene was present (DeMarini et al. 1997).

Human erythrocytes expressing the GSTT1-null phenotype had a higher incidence of sister chromatid

exchange than normal following exposure to methylene chloride (Hallier et al. 1994). Similarly,

expression of the DraI DD mutant of CYP2E1 was associated with an increase in bleomycin-induced

single-strand breaks in human lymphocyte DNA (El-Zein et al. 1997a); possibly methylene chloride

exposure would induce similar genetic damage in individuals with that genotype. Additional details are

presented in Section 2.10, Populations that are Unusually Susceptible.

130

Table 2-3. Genotoxicity of Methylene Chloride In Vitro

Results

Species (test system) End point

With

activation

Without

activation Reference

Mammalian cells:

Peripheral lymphocytes Chromosomal aberrations + 0 Thilagar et al.

1984a

Mouse/lymphoma L5178Y Chromosomal aberrations + + Thilagar et al. 1984a

Chinese hamster Chromosomal aberrations + + Thilagar et al. 1984a

Human/primary fibroblasts Unscheduled DNA synthesis Not tested – Jongen et al. 1981

Chinese hamster (V79) Unscheduled DNA synthesis Not tested – Jongen et al. 1981

Chinese hamster (V79) Sister chromatid exchanges (+) (+) Jongen et al. 1981

Human/peripheral lymphocytes Unscheduled DNA synthesis – – Perocco and Prodi 1981

Prokaryotic organisms:

Salmonella typhimurium

(TA98, TA100)

Gene mutation + + Gocke et al. 1981

S. typhimurium

(TA1535, TA1538, TA1537)

Gene mutation – – Gocke et al. 1981

DNA = deoxyribonucleic acid; + = positive result; – = negative result; (+) = weakly positive result

METHYLENE CHLORIDE

2. HEALTH EFFECTS

131

Table 2-4. Genotoxicity of Methylene Chloride In Vivo

Species (test system) End point Results Reference

Mammalian cells:

Mouse (bone marrow and lung cells) Chromosomal aberrations + Allen et al. 1990

Mouse (peripheral erythrocytes) Micronuclei + Allen et al. 1990

Mouse (peripheral lymphocytes and lung cells) Sister chromatid exchanges + Allen et al. 1990

Mouse (bone marrow cells) Micronuclei – Sheldon et al. 1987

Mouse Dominant lethality – Raje et al. 1988

Mouse (liver and lung) DNA breakage + Sasaki et al. 1998

Mouse (stomach, urinary bladder, kidney, brain DNA breakage _ Sasaki et al. 1998

and bone marrow)

Rat (liver) DNA breakage + Kitchin and Brown 1989

Rat Unscheduled DNA synthesis – Trueman and Ashby 1987

Mouse Unscheduled DNA synthesis – Trueman and Ashby 1987

Rat (bone marrow cells) Chromosomal aberrations – Burek et al. 1984

Mouse (bone marrow cells) Chromosomal aberrations – Gocke et al. 1981

Rat (liver and lung cells) DNA alkylation – Green et al. 1988

Mouse (liver and lung cells) DNA alkylation – Green et al. 1988

Eukaryotic organisms:

Insect:

Drosphila Gene mutation (sex-linked (+) Gocke et al. 1981

recessive lethal)

+ = positive result; – = negative result; (+) = weakly positive result; DNA = deoxyribonucleic acid

METHYLENE CHLORIDE

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132 METHYLENE CHLORIDE

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Cancer. A significant increase in bile-duct cancer was observed within a cohort of workers who had

been exposed to methylene chloride (at #1,700 ppm, 8-hour TWA) for up to 28 years (Lanes et al. 1990).

Epidemiology studies have not revealed a causal relationship between deaths due to cancer and

occupational exposure to methylene chloride at lower levels (475 ppm or less) (Friedlander et al. 1978;

Hearne et al. 1987, 1990; Ott et al. 1983b). It should be noted that these latter studies had limited power

to detect very small increases in cancer and are not sufficient to rule out a carcinogenic potential of

methylene chloride. Studies in animals exposed via inhalation have demonstrated that methylene chloride

can increase the incidence of naturally-occurring tumors. When administered by inhalation, methylene

chloride (2,000 ppm or greater) increased the incidence of alveolar/bronchiolar neoplasms in

mice of both sexes (NTP 1986). Concentrations of 500 ppm or greater of methylene chloride increased

the incidence of benign mammary gland tumors per animal in females and male rats (Burek et al. 1984;

Nitschke et al. 1988a; NTP 1986). The incidence of liver tumors increased over concurrent control levels

in male mice and female rats administered methylene chloride (50–250 mg/kg/day) in drinking water;

however, the incidence of lesions in treated groups were within the historical range of control values and

showed no dose response (Serota et al. 1986a, 1986b). The results of recent toxicokinetics studies

suggest that the parent compound and/or reactive metabolites produced by the GST pathway are the

source of methylene chloride-induced tumor increases. Based on these findings, the EPA has ranked

methylene chloride as a Group B2 carcinogen (probable human carcinogen). The EPA has calculated that

an upper limit 10

-6

risk level corresponds to 0.0006 ppm and 0.001 mg/kg/day for inhalation and oral

70-year continuous exposures, respectively. OSHA (1997) determined that methylene chloride is a

potential occupational carcinogen, based on rodent inhalation studies and PBPK modeling, and

established an inhalation unit risk of 3.62 x 10

-3

(µg/m

)

-1

for occupational exposures.

The Department of Health and Human Services (NTP 1999) has determined that methylene chloride may

reasonably be anticipated to be a human carcinogen. IARC (1987) has classified methylene chloride in

Group 2B (possibly carcinogenic to humans). EPA (IRIS 1999) has determined that methylene chloride

is a probable human carcinogen. Section 2.10 discusses polymorphisms in genes that metabolize

methylene chloride that may be associated with an increased risk of cancer.

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2.6 ENDOCRINE DISRUPTION

Recently, attention has focused on the potential hazardous effects of certain chemicals on the endocrine

system because of the ability of these chemicals to mimic or block endogenous hormones, or otherwise

interfere with the normal function of the endocrine system. Chemicals with this type of activity are most

commonly referred to as endocrine disruptors. Some scientists believe that chemicals with the ability to

disrupt the endocrine system are a potential threat to the health of humans, aquatic animals, and wildlife.

Others believe that endocrine disrupting chemicals do not pose a significant health risk, particularly in

light of the fact that hormone mimics exist in the natural environment. Examples of natural hormone

mimics are the isoflavinoid phytoestrogens (Adlercreutz 1995; Livingston 1978; Mayr et al. 1992). These

compounds are derived from plants and are similar in structure and action as endogenous estrogen. While

there is some controversy over the public health significance of endocrine disrupting chemicals, it is

agreed that the potential exists for these compounds to affect the synthesis, secretion, transport, binding,

action, or elimination of natural hormones in the body that are responsible for the maintenance of

homeostasis, reproduction, development, and/or behavior (EPA 1997). As a result, endocrine disruptors

may play a role in the disruption of sexual function, immune suppression, and neurobehavioral function.

Endocrine disruption is also thought to be involved in the induction of breast, testicular, and prostate

cancers, as well as endometriosis (Berger 1994; Giwercman et al. 1993; Hoel et al. 1992).

There is no evidence suggesting that methylene chloride is an endocrine disruptor.

2.7 CHILDREN’S SUSCEPTIBILITY

This section discusses potential health effects from exposures during the period from conception to

maturity at 18 years of age in humans, when all biological systems will have fully developed. Potential

effects on offspring resulting from exposures of parental germ cells are considered, as well as any indirect

effects on the fetus and neonate resulting from maternal exposure during gestation and lactation.

Relevant animal and in vitro models are also discussed.

Children are not small adults. They differ from adults in their exposures and may differ in their

susceptibility to hazardous chemicals. Children’s unique physiology and behavior can influence the

extent of their exposure. Exposures of children are discussed in Section 5.6 Exposures of Children.

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Children sometimes differ from adults in their susceptibility to hazardous chemicals, but whether there is

a difference depends on the chemical (Guzelian et al. 1992; NRC 1993). Children may be more or less

susceptible than adults to health effects, and the relationship may change with developmental age

(Guzelian et al. 1992; NRC 1993). Vulnerability often depends on developmental stage. There are

critical periods of structural and functional development during both prenatal and postnatal life and a

particular structure or function will be most sensitive to disruption during its critical period(s). Damage

may not be evident until a later stage of development. There are often differences in pharmacokinetics

and metabolism between children and adults. For example, absorption may be different in neonates

because of the immaturity of their gastrointestinal tract and their larger skin surface area in proportion to

body weight (Morselli et al. 1980; NRC 1993); the gastrointestinal absorption of lead is greatest in infants

and young children (Ziegler et al. 1978). Distribution of xenobiotics may be different; for example,

infants have a larger proportion of their bodies as extracellular water and their brains and livers are

proportionately larger (Altman and Dittmer 1974; Fomon 1966; Fomon et al. 1982; Owen and Brozek

1966; Widdowson and Dickerson 1964). The infant also has an immature blood-brain barrier (Adinolfi

1985; Johanson 1980) and probably an immature blood-testis barrier (Setchell and Waites 1975). Many

xenobiotic metabolizing enzymes have distinctive developmental patterns. At various stages of growth

and development, levels of particular enzymes may be higher or lower than those of adults, and

sometimes unique enzymes may exist at particular developmental stages (Komori et al. 1990; Leeder and

Kearns 1997; NRC 1993; Vieira et al. 1996). Whether differences in xenobiotic metabolism make the

child more or less susceptible also depends on whether the relevant enzymes are involved in activation of

the parent compound to its toxic form or in detoxification. There may also be differences in excretion,

particularly in newborns who all have a low glomerular filtration rate and have not developed efficient

tubular secretion and resorption capacities (Altman and Dittmer 1974; NRC 1993; West et al. 1948).

Children and adults may differ in their capacity to repair damage from chemical insults. Children also

have a longer remaining lifetime in which to express damage from chemicals; this potential is particularly

relevant to cancer.

Certain characteristics of the developing human may increase exposure or susceptibility while others may

decrease susceptibility to the same chemical. For example, although infants breathe more air per

kilogram of body weight than adults breathe, this difference might be somewhat counterbalanced by their

alveoli being less developed, which results in a disproportionately smaller surface area for alveolar

absorption (NRC 1993).

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There is no evidence from human toxicity studies that children are, or are likely to be, more susceptible to

health effects from inhalation, oral, or dermal exposure to methylene chloride than adults. No cases of

accidental poisoning in children due to methylene chloride exposure have been reported. The nervous

system is a sensitive acute target of methylene chloride exposure in adults and the response in children is

likely to be similar. Methylene chloride is neurotoxic at high concentrations, in part because of its

lipophilic characteristics. There are no data in the literature that suggest that children are more

susceptible to the acute or chronic neurotoxic effects of methylene chloride than adults. However, there is

little information available in the literature on the chronic low-concentration neurotoxic effects of

methylene chloride exposure in either children or adults. Methylene chloride did not produce any

toxicologically significant developmental effects in animals in a two-generation reproductive study with

rats (Nitschke et al. 1988b) or in mouse and rat developmental studies, except at very high, maternally

toxic doses (Bornschein et al. 1980; Hardin and Manson 1980; Schwetz et al. 1975).

The only animal study that is suggestive of potentially age-related vulnerability to methylene chloride is

that of Maronpot et al. (1995b), in which B6C3F

mice were exposed to 2,000 ppm of methylene chloride

by inhalation for 26, 52, or 78 weeks. The results of stop-exposure experiments showed that early

exposure to methylene chloride was more effective than late exposure in inducing pulmonary neoplasms.

However, the authors mentioned the possibility that the late-exposed animals were sacrificed too early for

tumors to have developed.

There is no information regarding the pharmacokinetics of methylene chloride in children or regarding

the nutritional factors that may influence the absorption of methylene chloride. A PBPK model has been

developed for estimating the amounts and concentrations of methylene chloride in human breast milk that

would result from inhalation exposures to methylene chloride (Fisher et al. 1997). Animal studies

demonstrate that methylene chloride and/or its metabolites distribute in the liver, kidney, lungs, brain,

muscle, and adipose tissues after inhalation exposures (Carlsson and Hultengren 1975; McKenna et al.

1982) and has also been shown to cross the placenta in rats (Anders and Sunram 1982). Toxicokinetic

data indicate that methylene chloride is rapidly cleared after cessation of exposure. Small amounts of

methylene chloride have been found in breast milk (EPA 1980d; Pellizzari et al. 1982).

It is not known whether the metabolism of methylene chloride in children is different than in adults, but

there are theoretical reasons to suspect it might be. Available data suggest that there are two pathways by

which methylene chloride is metabolized. One pathway utilizes the mixed function oxidase (MFO)

136 METHYLENE CHLORIDE

2. HEALTH EFFECTS

enzymes and produces carbon monoxide (CO). The other pathway involves the glutathione transferase

(GST) and produces carbon dioxide (CO

). No information was located regarding the possibility that the

metabolism of methylene chloride via the GST metabolic pathway is developmentally regulated.

CYP2E1 (the specific MFO pathway for methylene chloride) expression is low in fetal liver but increases

several hours after birth in humans and continues to increase during the first week of life (Carpenter et al.

1996, 1997; Jones et al. 1992; Viera et al. 1996). In the human fetal liver, CYP2E1 transcripts were not

detected at 10 weeks of gestation, but were detected as early as 19 weeks (Carpenter et al. 1996). The

CYP2E1 gene in the liver is modified by cytosine methylation at the 3' region during fetal development,

which contributes to the low expression of the gene during fetal life (Jones et al. 1992; Viera et al. 1996).

CYP2E1 expression in the fetal brain, however, has been detected near baseline levels at 46 days of

gestation, but at significant levels beginning at 58 days (Brzezinski et al. 1999). The early expression of

CYP2E1 in the fetal brain suggests that the brain may be particularly vulnerable to oxidative stress as a

result of xenobiotic metabolism.

In cases of heavy ethanol consumption by the mother, CYP2E1 is induced in the placenta (Rasheed et al.

1997) and in the liver (Carpenter et al. 1997). In vitro tests demonstrated that ethanol treatment of human

fetal hepatocytes upregulated expression of CYP2E1 (Carpenter et al. 1996). This presumably would

increase the rate of metabolism of methylene chloride in the fetus.

Evidence from pregnant rats exposed to methylene chloride indicates that the maternal liver metabolizes

methylene chloride at a higher rate than the fetus, but that the metabolite carbon monoxide equilibrates

between the dam and fetus (Anders and Sunram 1982). The high affinity of fetal hemoglobin for both

carbon monoxide and oxygen suggests that fetuses may be at risk from hypoxia following maternal

exposures to high levels of methylene chloride (Anders and Sunram 1982; Longo 1977). The greater risk

of hypoxia and the potential for greater neurological damage would last until the expression of adult

hemoglobin during the first year of life.

As mentioned above, methylene chloride has been isolated from human breast milk (EPA 1980d;

Pellizzari et al. 1982), so it is possible that maternal exposures could transmit the compound to infants.

However, PBPK modeling suggests that lactating females who breast feed their infants will not deliver

methylene chloride in significant quantities (Fisher et al. 1997). Simulation of a workday maternal

inhalation exposure to methylene chloride at 50 ppm yielded a predicted daily intake in the infant of only

0.213 mg, which is significantly less than the 2.0 mg/day equivalent EPA Health Advisory Intake.

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There are no biomarkers of exposure or effect for methylene chloride that have been validated in children

or in adults exposed as children. There are no biomarkers in adults that identify previous childhood

exposure. No information was located regarding pediatric-specific methods for reducing peak absorption

following exposure to methylene chloride, reducing body burden or interfering with the mechanism of

action for toxic effects. In addition, no data were located regarding whether methods for reducing toxic

effects might be contraindicated in children.

2.8 BIOMARKERS OF EXPOSURE AND EFFECT

Biomarkers are broadly defined as indicators signaling events in biologic systems or samples. They have

been classified as markers of exposure, markers of effect, and markers of susceptibility (NAS/NRC 1989).

Due to a nascent understanding of the use and interpretation of biomarkers, implementation of biomarkers

as tools of exposure in the general population is very limited. A biomarker of exposure is a xenobiotic

substance or its metabolite(s) or the product of an interaction between a xenobiotic agent and some target

molecule(s) or cell(s) that is measured within a compartment of an organism (NAS/NRC 1989). The

preferred biomarkers of exposure are generally the substance itself or substance-specific metabolites in

readily obtainable body fluid(s), or excreta. However, several factors can confound the use and

interpretation of biomarkers of exposure. The body burden of a substance may be the result of exposures

from more than one source. The substance being measured may be a metabolite of another xenobiotic

substance (e.g., high urinary levels of phenol can result from exposure to several different aromatic

compounds). Depending on the properties of the substance (e.g., biologic half-life) and environmental

conditions (e.g., duration and route of exposure), the substance and all of its metabolites may have left the

body by the time samples can be taken. It may be difficult to identify individuals exposed to hazardous

substances that are commonly found in body tissues and fluids (e.g., essential mineral nutrients such as

copper, zinc, and selenium). Biomarkers of exposure to methylene chloride are discussed in

Section 2.8.1.

Biomarkers of effect are defined as any measurable biochemical, physiologic, or other alteration within an

organism that, depending on magnitude, can be recognized as an established or potential health

impairment or disease (NAS/NRC 1989). This definition encompasses biochemical or cellular signals of

tissue dysfunction (e.g., increased liver enzyme activity or pathologic changes in female genital epithelial

138 METHYLENE CHLORIDE

2. HEALTH EFFECTS

cells), as well as physiologic signs of dysfunction such as increased blood pressure or decreased lung

capacity. Note that these markers are not often substance specific. They also may not be directly

adverse, but can indicate potential health impairment (e.g., DNA adducts). Biomarkers of effects caused

by methylene chloride are discussed in Section 2.8.2.

A biomarker of susceptibility is an indicator of an inherent or acquired limitation of an organism's ability

to respond to the challenge of exposure to a specific xenobiotic substance. It can be an intrinsic genetic

or other characteristic or a preexisting disease that results in an increase in absorbed dose, a decrease in

the biologically effective dose, or a target tissue response. If biomarkers of susceptibility exist, they are

discussed in Section 2.10 “Populations That Are Unusually Susceptible”.

2.8.1 Biomarkers Used to Identify or Quantify Exposure to Methylene Chloride

Measurements of parent methylene chloride and its metabolites in expired air, blood, and urine have been

used as indicators of exposure. Elimination of methylene chloride from the body occurs primarily

through pulmonary excretion; approximately 70–75% is excreted unchanged at concentrations from 50 to

200 ppm (DiVincenzo and Kaplan 1981). Pulmonary excretion was rapid during the first hour, then

began to decline as steady state was approached. By 7 hours postexposure, expired air contained less than

1 ppm of methylene chloride for exposures between 50 and 150 ppm. The 7-hour values for an exposure

of 200 ppm were twice that for the other concentrations. At 16 hours, negligible levels of methylene

chloride were detected in all concentration groups. Therefore, measurements of methylene chloride in

expired air as a biomarker are useful only if they occur within 6–8 hours of the most recent exposure

(DiVincenzo and Kaplan 1981).

Methylene chloride can be detected in blood. Because it is cleared from blood very rapidly, this method

is only useful for monitoring recent exposures. A plasma half-life of inhaled methylene chloride in

humans is estimated to be 40 minutes (DiVincenzo et al. 1972).

Methylene chloride has been detected in adipose tissue in humans, but animal studies suggest that

methylene chloride is cleared so rapidly (90% decrease in 2 hours), that it is useful for measuring only

recent exposures (Carlsson and Hultengren 1975). Methylene chloride was detected in the breast milk of

mothers living in urban industrial areas, but the route of exposure was not analyzed (EPA 1980e;

Pellizzari et al. 1982).

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Methylene chloride is also excreted in urine. Humans exposed to 100 ppm of methylene chloride vapor

for 2 hours averaged 26.6 µg methylene chloride in the urine within 24 hours after exposure and 85.5 µg

at exposure levels of 200 ppm (DiVincenzo et al. 1972).

Levels of COHb in the blood may also be used as an indicator of exposure to methylene chloride. Levels

of COHb are concentration- and time-dependent (Stewart et al. 1972). Human subjects exposed to

concentrations of 500 ppm or less for 1 hour experienced elevation in COHb levels (1–4%). These levels

rose to an average of 10% saturation within 1 hour after exposure to higher concentrations (1,000 ppm for

2 hours) (Stewart et al. 1972). Exposure to concentrations as low as 100 ppm for 7.5 hours or 200 ppm

for 4 hours resulted in COHb elevation above 5% in nonsmokers (Putz et al. 1979; Stewart et al. 1972).

Levels of COHb can remain elevated above preexposure levels for more than 21 hours post exposure

(Stewart et al. 1972). DiVincenzo and Kaplan (1981) showed recovery to baseline each day for

5 consecutive days. COHb saturation levels were lower than in Stewart et al. (1972). Different methods

of COHb detection were used by the authors. Although COHb levels can be used as an indicator of

exposure to methylene chloride, this biomarker is not specific. Exposure to other sources of carbon

monoxide, such as tobacco smoke, incomplete organic fuel combustion, and automobile exhaust, will also

increase COHb levels.

Ghittori et al. (1993) examined several methods for conducting biological monitoring of workers exposed

occupationally to methylene chloride. Twenty males (12 smokers and 8 nonsmokers), employed at

different jobs in a pharmaceutical factory where methylene chloride was used to wash gelatin capsules,

wore a personal passive dosimeter in the respiratory zone during half of work days (4 hours) in order to

measure the weighted mean-inspired environmental concentration of methylene chloride.

Tetrachloroethylene was also present in the work environment. Immediately after the end of the

exposure, a urine sample was collected using gas-tight samplers. Carbon monoxide (CO) was determined

at the end of the shift using a portable instrument.

Ambient exposures ranged from 49 to 168 ppm. No significant correlation was observed among the CO

of all subjects and the concentration of methylene chloride in ambient air. When those workers who

smoked were removed from analysis, a correlation between the methylene chloride concentration in air

and the CO concentration in alveolar air was found ® = 0.87). Significant linear correlation was found

between the environmental concentration of methylene chloride in the breathing zone and methylene

chloride concentration in urine ® = 0.9 uncorrected, 0.72 corrected for creatinine). The authors suggest

140 METHYLENE CHLORIDE

2. HEALTH EFFECTS

that measuring methylene chloride in the urine was a more useful measure of exposure, since unlike

COHb in the blood or CO in exhaled breath, it was unaffected by smoking.

Concentrations of CO in exhaled air studied in other groups of volunteers were shown to reach a

maximum at 1–2 hours after exposure and were also directly proportional to the magnitude of exposure

both during and after exposure. These findings are consistent with those studies.

The authors concluded that the results indicated that methylene chloride urinary concentration could be

used as a possible biological index to evaluate methylene chloride air exposure, according to what has

been observed for other solvents. However, at high ambient exposures (i.e., those exceeding 5–7 times

the threshold limit value-time weighted average [TLV-TWA] of 49 ppm), the use of methylene chloride

as a biological index might be problematic because exposure to very high concentrations gives

disproportionately less COHb in the blood and more unmetabolized methylene chloride is produced.

2.8.2 Biomarkers Used to Characterize Effects Caused by Methylene Chloride

As discussed in Section 2.2, the effects that are most often observed in humans exposed to methylene

chloride vapors are central nervous system depression and behavioral effects. Clinical signs and

symptoms which may be monitored include irritability, narcosis, and fatigue. Impairment of visual,

auditory, and psychomotor functions can also be evaluated to detect early effects on the central nervous

system. Since these effects also occur following exposure to numerous other chemicals, they are not

specific for methylene chloride exposure and evaluation is often subjective.

Honma and Suda (1997) have investigated the effects of a single intraperitoneal administration of several

short-chain chlorinated compounds, including methylene chloride, on lipoproteins in plasma and liver in

male Fischer F344 rats. Changes in lipoproteins caused by these solvents were compared with

hepatotoxicity markers such as GPT (ALT). Following administration of methylene chloride in olive oil,

concentrations of lipoproteins (VLDL, LDL, HDL), triglyceride, cholesterol, and GPT in plasma were

determined, as were changes in liver weight and amounts of triglyceride and glutathione in liver tissue.

For methylene chloride, peaks of changes in these endpoints were observed at 8 or 19 hours following

compound administration. The dose dependency of these changes were investigated after intraperitoneal

dosing of 300 and 1,000 mg/kg methylene chloride. HDL decreased significantly at a dose of 300 mg/kg,

whereas a marked increase in LDL occurred at 1,000 mg/kg. GPT also increased significantly at a dose

141 METHYLENE CHLORIDE

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of 1,000 mg/kg. The authors conclude that changes in some lipoproteins in plasma occurred at lower

doses than those causing elevation of GPT activity; therefore, these lipoproteins may be able to serve as

sensitive and simple markers of adverse liver effects (i.e., biomarkers of effects). However, further

investigation is necessary to determine whether these endpoints can serve as biomarkers of effects in

humans when exposure is via inhalation and confounding variables are be numerous.

No other biomarkers have been identified to characterize effects associated with exposure to methylene

chloride.

For more information on biomarkers for renal and hepatic effects of chemicals see ATSDR/CDC

Subcommittee Report on Biological Indicators of Organ Damage (1990) and for information on

biomarkers for neurological effects see OTA (1990).

2.9 INTERACTIONS WITH OTHER CHEMICALS

Limited studies were found in the available literature on the interactions of methylene chloride with other

chemicals. Exposure of adult human subjects to 500 ppm of methylene chloride resulted in levels of

COHb in the blood comparable with those produced by the TLV for CO (50 ppm) (Fodor and Roscovana

1976). A 4-hour exposure of human subjects to 200 ppm of methylene chloride or 70 ppm of CO resulted

in similar blood COHb levels (Putz et al. 1979). This suggests that simultaneous exposure to methylene

chloride and CO has an additive effect on blood COHb levels.

A possible additive effect of methylene chloride and toluene (as well as mineral spirits and methyl ethyl

ketone) may have been operative in an occupational mixed-solvent exposure study in which manual

dexterity, and visual memory task performances were impaired, even though the concentrations of the

specific chemicals were below the recommended their threshold limit values (White et al. 1995); the

reported adverse neurobehavioral effects are similar to those reported for individual exposures of

methylene chloride at higher concentrations (Putz et al. 1979). In a rat study, methylene chloride and

toluene interacted in an additive manner to cause retardation of embryonic growth and development in

vitro (Brown-Woodman et al. 1998). In rats, combined CO and methylene chloride exposure yielded

additive increases in the COHb levels (ACGIH 1986).

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Methylene chloride decreases the nerve conduction velocity in animals when used alone but when the

compound is administered with ethanol there is a more pronounced decrease of nerve conduction velocity

(Glatzel et al. 1987). The additive or synergist effect of ethanol coadministration decreases at higher

doses. Rats maintained on drinking water containing 10% (v/v) ethanol show increased levels of COHb

after a single oral dose or during inhalation exposure to methylene chloride compared to rats that have not

been exposed to ethanol (Wirkner et al. 1997). This interaction has been attributed to the induction of

CYP2E1 by ethanol and a resulting increased production of carbon monoxide from methylene chloride.

Wirkner et al. (1997) indicate that ethanol has no synergistic effect when methylene chloride is

administered at high doses because the cytochrome P-450-dependent monooxygenase pathway becomes

saturated at exposures >500 ppm.

Interactions resulting from the inhibition of metabolism of methylene chloride have been described in

rats. Blood COHb levels in rats, after an oral dose of methylene chloride or during inhalation exposure to

methylene chloride, are decreased by a concurrently administered oral dose of toluene (Ciuchta et al.

1979; Pankow et al. 1991a, 1991b). This interaction has been attributed to inhibition of the metabolism

of methylene chloride to carbon monoxide, possibly as a result of both chemicals being a substrate for the

2E1 isoenzyme of cytochrome P-450 (Pelekis and Krishnan 1997).

In a rat study, methylene chloride (coinjected intraperitoneally) potentiated the hepatotoxic effect of

carbon tetrachloride (Kim 1997). Coadministration reduced the methylene-chloride-induced rise in blood

COHb, but augmented the carbon tetrachloride-induced increase in liver microsomal enzyme activities 3-

to 8-fold. Methylene chloride increased the covalent binding of carbon tetrachloride metabolites to lipids,

which the author suggests may be a cause of enhanced hepatotoxicity of the mixture.

2.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE

A susceptible population will exhibit a different or enhanced response to methylene chloride than will

most persons exposed to the same level of methylene chloride in the environment. Reasons may include

genetic makeup, age, health and nutritional status, and exposure to other toxic substances (e.g., cigarette

smoke). These parameters result in reduced detoxification or excretion of methylene chloride, or

compromised function of organs affected by methylene chloride. Populations who are at greater risk due

to their unusually high exposure to methylene chloride are discussed in Section 5.7, Populations With

Potentially High Exposures.

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There are certain subgroups of the general population that may be more susceptible to methylene chloride

than others. One basis for this concern is the potential effect of COHb, produced from CO, a metabolite

of methylene chloride. The COHb generated from methylene chloride is expected to be additive to COHb

from other sources. Thus, methylene chloride exposure at high concentrations may pose an additional

human health burden. Of particular concern are smokers (who maintain significant constant levels of

COHb), and persons with existing cardiovascular disease. In addition, higher than normal levels of CO

may result when alcoholics are exposed to methylene chloride, since ethanol increases the expression and

activity of CYP2E1 (Carpenter et al. 1996). Similarly, enhanced expression of CY2E1 occurs in the

condition of diabetes, although insulin erases that effect (Thomas et al. 1987).

Varying susceptibility to methylene chloride may be correlated with polymorphism in its metabolizing

enzymes. Genetic polymorphisms have been identified for both GSTT1 and CYP2E1 (Garte and Crosti

1999), and the health consequences of these variants are being explored (d’Errico et al. 1999; Lang and

Pelkonen 1999; Pelkonen et al. 1999; Strange and Fryer 1999; Stubbins and Wolf 1999). In the case of

GSTT1, which is the major metabolic pathway for methylene chloride at concentrations >500 ppm, there

is, in addition to the wild type gene, a nonfunctioning allele with a deletion, so that three phenotypes

exist: “high conjugators” homozygous for the wild type allele; “nonconjugators” homozygous for the

deletion (“null) allele; and “low conjugators”, heterozygotes with one of each (Thier et al. 1998). Thier et

al. (1998) demonstrated that tissue samples representing the three phenotypes varied in the ability to

metabolize methylene chloride. The health implications of the homozygous GSTT1-null genotype for

people exposed to methylene chloride exposure are not clear. A potentially positive effect is that no toxic

reactive GST-intermediates of methylene chloride would be produced, as demonstrated by the absence of

RNA-formaldehyde adduct formation in GSTT1-null human hepatocytes treated with methylene chloride

in vitro (Casanova et al. 1997). However, the overall rate of methylene chloride metabolism and

elimination would be reduced in GSTT1-null individuals, so that narcotic effects of the parent compound

would last longer. Furthermore, increased genotoxic damage (sister chromatid exchange) occurs

following exposure to methylene chloride, in null-phenotype, compared to wild type lymphocytes (Hallier

et al. 1994). Variations in the incidence of GSTT1 polymorphisms have been identified in different

ethnic groups (Strange and Fryer 1999). In one study, the proportion of individuals carrying functional

GSTT1 alleles was 36% among Chinese, 80% among Caucasians, and 90% among Mexican-Americans

(Nelson et al. 1995). In a study of 416 subjects, the GSTT1-null genotype occurred among 24.1% of the

blacks and 15% of the whites (Chen et al. 1996). The GSTT1-null phenotype was detected in 21.1% of

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lung cancer patients with either squamous cell carcinoma or adenocarcinoma (El-Zein et al. 1997a).

Polymorphisms have also been detected in CYP2E1 (Garte and Crosti 1999; Lang and Pelkonen 1999).

One polymorphism involves a mutation in the 5' flanking region of the CYP2E1 gene that leads to a

higher rate of transcription, a higher level of protein, and higher enzyme activity compared to cells

containing the wild type allele (Wan et al. 1998). The frequency of the mutant allele was 16% in a group

of 203 Mexican-Americans (Wan et al. 1998), which is considerably higher than the 1–5% frequency

measured in Caucasians (Stephens et al. 1994). Presumably, this allele would result in higher rates of

metabolism of methylene chloride, and possibly more severe toxic effects, but this has not been assayed.

A number of studies have tried to associate specific polymorphisms with cancer incidence (d’Errico et al.

1999). The CYP2E1 DraI DD genotype was found to be associated with a significantly higher risk of

lung cancer among Mexican-Americans and African-Americans (Wu et al. 1998); expression of this

genotype was also associated with an increase in bleomycin-induced single-strand breaks in lymphocytes

tested in vitro. In another study, a rare CYP2E1 PstI variant (mutated in the transcription regulation

region) was found in 7/57 lung cancer patients and 2/48 controls (El-Zein et al. 1997a); all seven patients

developed adenocarcinoma. In a case control study, combined homozygosity for the CYP2E1 PstI

variant and the GSTT1-null genotype was associated with an increased risk of developing lung cancer

(El-Zein et al. 1997b). Presumably, individuals expressing these mutant genotypes would be susceptible

to adverse effects following exposure to methylene chloride.

2.11 METHODS FOR REDUCING TOXIC EFFECTS

This section will describe clinical practice and research concerning methods for reducing toxic effects of

exposure to methylene chloride. However, because some of the treatments discussed may be

experimental and unproven, this section should not be used as a guide for treatment of exposures to

methylene chloride. When specific exposures have occurred, poison control centers and medical

toxicologists should be consulted for medical advice. The following texts provide specific information

about treatment following exposures to methylene chloride: Ellenhorn 1997; Stewart and Dodd 1964.

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2.11.1 Reducing Peak Absorption Following Exposure

Human exposure to methylene chloride may occur by inhalation, ingestion, or by dermal contact.

Mitigation approaches to reduce absorption of methylene chloride have included general

recommendations of separating contaminated food, water, air, or clothing from the exposed individual.

Externally, exposed eyes and skin are flushed with a clean neutral solution such as water or normal saline.

If the victim is alert and oriented and has an intact gag reflex, water or milk may be administered after

ingestion of small amount of methylene chloride to wash residual chemical through the esophagus and

dilute the contents in the stomach. In cases where persons have ingested more than several swallows,

emesis can be induced with ipecac syrup after it has been determined that the victim is alert and oriented

and precautions have been taken to protect the respiratory tract from aspiration of gastric contents. Once

the individual is placed in the care of a health professional, gastric lavage can be performed within 1 hour

of the exposure. Activated charcoal and cathartics are frequently recommended; however, no data exist to

support their efficacy (Ellenhorn 1997).

Once methylene chloride has been inhaled or ingested, it is readily absorbed through the lungs or

gastrointestinal tract. Dermal exposure also results in absorption, although at a slower rate than the other

exposure routes and some references question whether the amount absorbed would be sufficient to cause

systemic toxicity (Ellenhorn 1997; Stewart and Dodd 1964). Once absorbed, methylene chloride is

rapidly metabolized in the liver in part to carbon monoxide. Because of the affinity of hemoglobin for

carbon monoxide, COHb levels in the blood will increase, but free circulating carbon monoxide will not

increase.

2.11.2 Reducing Body Burden

Inhalation data demonstrate that a portion of the absorbed methylene chloride is stored in fat tissue as

evidenced by continued increase in COHb levels. It is likely to occur only with long-term exposure and

at high concentrations (DiVincenzo et al. 1972). Investigations in humans following oral exposure to

methylene chloride have failed to detect significant retention in fat or other tissue stores (Angelo et al.

1986a, 1986b; Ellenhorn 1997).

Following absorption, methylene chloride is distributed mainly to the liver, brain and subcutaneous

adipose tissue (Carlsson and Hultengren 1975). The liver is the primary site of metabolism, although

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additional transformation occurs in the lungs and kidneys. In the liver, methylene chloride may undergo

metabolism by two pathways. The first pathway produces CO and CO

and is saturable at a few

hundred ppm. The second pathway yields formaldehyde and formic acid and shows no indication of

saturation at inhaled concentrations up to 10,000 ppm. Acute toxic effects (central nervous system

depression) may persist for hours after removal from the source of exposure because of continued

metabolism of methylene chloride released from tissue storage (ATSDR 1990). COHb levels can

continue to rise, peaking 5–6 hours after exposure. Peak levels of 12 (NIOSH 1974) and 50% (Ellenhorn

1997) have been reported. Most of the effects seen following acute high-concentration exposure are due

to the anesthetic properties of the parent compound. Fatalities have occurred with COHb levels less than

10%; thus, it is not likely that carboxyhemoglobinea was the cause of death. A study by Scholz et al.

(1991) suggests that, in addition to anesthetic effects, the sudden onset of cardiac arrhythmia induced by

acute exposure to methylene chloride may contribute to its lethality.

The metabolic contribution of each pathway appears to vary in humans, particularly with the exposure

level, and therefore toxicity extrapolation between high and low doses is complex (Gargas et al. 1986).

Furthermore, recent studies suggest that the second pathway is considerably more active in certain animal

species, particularly mice, a finding that complicates interspecies comparisons (ATSDR 1990; Green et

al. 1986b, 1986c).

The body eliminates methylene chloride primarily through the lungs. A small amount of unchanged

methylene chloride is also eliminated in the urine and feces. At low doses, a large percentage of

methylene chloride is metabolized to form COHb and eliminated as carbon monoxide, while at higher

doses more of the unchanged parent compound is exhaled (ATSDR 1990). The administration of 100%

oxygen is efficacious in reducing the half-life of carbon monoxide and should be continued until COHb is

less than 5%.

The prompt application of hemoperfusion and diuretic therapy eliminated metabolic acidosis and

hemoglobinuria in the case of an acute oral exposure to paint remover containing methylene chloride

(Roberts and Marshall 1976). This treatment was thought to have prevented renal damage, but did not

address the problem of gastrointestinal ulceration.

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2.11.3 Interfering with the Mechanism of Action for Toxic Effects

The primary effects of exposure to methylene chloride appear to be hepatotoxicity and neurotoxicity.

However, mechanisms of action for these toxic effects are not well characterized and it is difficult to

speculate concerning specific methods for preventing these effects.

Because one consequence of metabolism of methylene chloride is production of COHb, methylene

chloride exposure victims are sometimes given supplemental oxygen. The administration of oxygen

increases the dissociation of carbon monoxide from hemoglobin and hastens the reduction of COHb. The

half-life of COHb is normally approximately 5.3 hours but this can be reduced to 60–90 minutes with

inhalation of 100% oxygen. Hyperbaric oxygen as used in carbon monoxide poisoning from direct

carbon monoxide inhalation may be useful when high COHb levels are present (Haddad and Winchester

1990). At high COHb levels, hyperbaric oxygen will reduce the half-life to 20–40 minutes (ATSDR

1990). Because carbon monoxide is generated metabolically, this often necessitates a longer duration of

oxygen therapy after methylene chloride poisoning than with carbon monoxide poisoning.

Specific mechanisms of action relating to neurological effects are not well understood. Steroids and

mannitol have been used to decrease cerebral edema caused by methylene chloride toxicity, but their

value in preventing later neurologic sequelae remains unproven (ATSDR 1990).

Victims with cardiopulmonary disease or workers who are exposed to carbon monoxide sources could

have an elevated risk from methylene chloride metabolism and should be monitored closely. One

reference suggests that physical exercise and smoking be avoided because these activities may have an

additive effect on the COHb level (Ellenhorn 1997).

2.12 ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of methylene chloride is available. Where adequate

information is not available, ATSDR, in conjunction with the National Toxicology Program (NTP), is

required to assure the initiation of a program of research designed to determine the health effects (and

techniques for developing methods to determine such health effects) of methylene chloride.

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The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed.

2.12.1 Existing Information on Health Effects of Methylene Chloride

The existing data on health effects of inhalation, oral, and dermal exposure of humans and animals to

methylene chloride are summarized in Figure 2-5. The purpose of this figure is to illustrate the existing

information concerning the health effects of methylene chloride. Each dot in the figure indicates that one

or more studies provide information associated with that particular effect. The dot does not necessarily

imply anything about the quality of the study or studies, nor should missing information in this figure be

interpreted as a “data need”. A data need, as defined in ATSDR’s Decision Guide for Identifying

Substance-Specific Data Needs Related to Toxicological Profiles (ATSDR 1989), is substance-specific

information necessary to conduct comprehensive public health assessments. Generally, ATSDR defines a

data gap more broadly as any substance-specific information missing from the scientific literature.

As shown in Figure 2-5, studies of humans exposed to methylene chloride by the inhalation route are

available. These have focused mainly on neurological, cancer, and systemic effects (e.g., cardiovascular).

One report focused on possible reproductive effects following inhalation (and possible dermal) exposure.

Other end points (immunological, developmental, and genotoxic effects) have not been evaluated. There

are no studies on the effects of methylene chloride in humans after ingestion. Other than one study

evaluating potential reproductive effects following inhalation and possibly dermal exposure, no other end

points have been evaluated following dermal exposure.

Studies in animals have also focused mainly on inhalation exposure and several end points have been

evaluated. Effects of oral exposure have focused on death, systemic effects after intermediate and chronic

exposure, genotoxicity, and cancer. Other end points have not been evaluated. There are limited reports

on the effects of methylene chloride after direct ocular exposure of animals.

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150 METHYLENE CHLORIDE

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2.12.2 Identification of Data Needs

Acute-Duration Exposure. Available data indicate that the central nervous system is the primary

target of inhaled methylene chloride in humans (Bakinson and Jones 1985; Fodor and Winneke 1971;

Hall and Rumack 1990; Putz et al. 1979; Stewart et al. 1972; Winneke 1974), rats (Heppel and Neal 1944;

Rebert et al. 1989; Savolainen et al. 1981), guinea pigs, rabbits, dogs, and monkeys (Heppel et al. 1944).

An acute inhalation MRL was derived for methylene chloride based on a LOAEL of 300 ppm for

neurological effects in humans (Winneke 1974). Respiratory effects (cough, breathlessness, and in one

fatal case, pulmonary congestion with focal hemorrhage) were observed in several cases of acute

occupational exposure to methylene chloride in humans (Bakinson and Jones 1985; Snyder et al. 1992a,

1992b; Winek et al. 1981). No cardiovascular effects were seen in humans acutely exposed to

concentrations between 100 and 475 ppm (Cherry et al. 1981; Ott et al. 1983c). Gastrointestinal effects

(nausea and vomiting) were reported in several cases of acute occupational exposure to methylene

chloride (Bakinson and Jones 1985). Acute inhalation exposure to methylene chloride increased the

blood COHb level in humans (DiVincenzo and Kaplan 1981; Manno et al. 1992; Putz et al. 1979; Soden

et al. 1996). An acute inhalation study in mice (Aviado and Belej 1974) suggests that the cardiovascular

system may be a target for methylene chloride toxicity; however, the effects were observed at lethal

concentrations. An acute study in guinea pigs demonstrated an increase in hepatic triglycerides, but no

histopathological effects on the liver following exposure to methylene chloride at 5,200 ppm (Morris et

al. 1979). Since the most sensitive acute effects in humans are neurological, additional animal studies

that evaluate changes in analogous functions (visual, auditory discrimination tasks) at low levels of

exposure might be needed. These experiments should monitor CO/COHb levels in the animals to

correlate exposure and effect. Furthermore, the animals should be genotyped with respect to for GSTT1

and CYP2E1 to evaluate the mechanism of toxicity.

The only studies in humans on the effects of acute oral exposure to methylene chloride are reports of

suicide attempts by ingestion of paint removers. One of these studies reported suppression of the central

nervous system and metabolic acidosis (Roberts and Marshall 1976). Corrosion of the gastrointestinal

tract was also reported (Hughes and Tracey 1993; Roberts and Marshall 1976). Hematological effects

included elevated COHb (Hughes and Tracey 1993) and intravascular hemolysis leading to

hemoglobinuria (Roberts and Marshall 1976). No acute respiratory, cardiovascular, hepatic, endocrine, or

immunological effects have been reported in humans following ingestion of methylene chloride, and there

are no acute oral studies on the effect of low doses in humans. Limited studies in animals involved

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exposure of rats to high doses of methylene chloride via gavage, resulting in increased mortality (Kimura

et al. 1971; Ugazio et al. 1973); as in suicide cases, respiratory failure as a result of suppression of the

central nervous system was the cause of death in these animal studies. Hemolysis occurred and diuresis

was inhibited in rats gavaged with 1,325 mg/kg of methylene chloride (Marzotko and Pankow 1987); in

rats given single oral doses $526 mg/kg, the only endocrine effects noted were dilatation of capillaries of

the adrenal medulla and an increase in the secretion of catecholamines (Marzotko and Pankow 1987). In

rats gavaged twice with 1,275 mg/kg, DNA damage and an increase in the activity of ornithine

decarboxylase were detected in the liver (Kitchin and Brown 1989). Genotoxicity (DNA breakage) was

detected in nuclei from selected rat organs following a single dose of methylene chloride (1,720 mg/kg)

(Sasaki et al. 1998). Since there is a potential for oral exposure for humans near hazardous waste sites

(through contaminated groundwater sources where volatization is restricted), additional animal data on

the effects of acute oral exposure to lower doses of methylene chloride would strengthen the database.

An acute oral MRL was derived by using a PBPK model to extrapolate human inhalation data from

Winneke (1974) to estimate the concentration of methylene chloride in drinking water needed to produce

a tissue specific concentration equivalent to that produced by exposure to 300 ppm of methylene chloride

(Reitz et al. 1997).

Case reports indicate that methylene chloride can cause eye and skin damage following direct contact

with the liquid (Hall and Rumack 1990; Wells and Waldron 1984; Winek et al. 1981). An acute study in

rabbits demonstrated adverse ocular effects (Ballantyne et al. 1976). Dermal absorption of methylene

chloride has been demonstrated in animals (McDougal et al. 1986), and therefore, is possible in humans.

Additional dermal studies in animals are needed to provide further insight into the potential risks of living

near hazardous waste sites.

Intermediate-Duration Exposure. Suggestive evidence for liver effects in humans exists, as

workers exposed to methylene chloride (up to 475 ppm) via inhalation have shown increased serum

bilirubin levels, but not at clinically significant levels (Ott et al. 1983a). Other liver injury parameters

were normal. Repeated exposures to methylene chloride vapors (up to 500 ppm) did not alter serum

enzyme activity, pulmonary function, or urine chemistry significantly in exposed and control groups

(NIOSH 1974). However, irritation of the respiratory tract, but no changes in urinary chemistry were

noted after exposure to a TWA of 5–340 ppm in an occupational study (Anundi et al. 1993).

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Available data for rats, dogs, monkeys, rabbits, guinea pigs, gerbils, and mice indicate that repeated

exposure to methylene chloride via inhalation produces central nervous system effects (Briving et al.

1986; Karlsson et al. 1987; Rosengren et al. 1986), respiratory effects (Heppel et al. 1944; NTP 1986)

hepatic effects (Haun et al. 1972; Heppel et al. 1994; Kjellstrand et al. 1986; MacEwen et al. 1972;

Norpoth et al. 1974; NTP 1986; Weinstein and Diamond 1972), and kidney effects (Haun et al. 1972;

MacEwen et al. 1972). An intermediate inhalation MRL was based on a LOAEL of 25 ppm for hepatic

and renal effects in rats (Haun et al. 1972).

No quantitative data are available for oral exposures of intermediate duration in humans. Hepatic damage

and lowered urinary pH in rats was reported following oral exposure $166 mg/kg/day for 3 months

(Kirschman et al. 1986), but the reporting of results was incomplete in this study. Furthermore, since the

lowest dose was a LOAEL, additional intermediate oral studies are needed in animals to evaluate specific

tissue and organ effects at lower doses and to determine threshold levels and dose-response relationships.

Reitz et al. (ATSDR 1997) developed a PBPK model to extrapolate inhalation data for an oral MRL,

based on the inhalation study of Haun et al. (1972), but the reporting of results in this study was

incomplete; a new intermediate oral animal study would be preferable. No intermediate oral MRL was

derived because of lack of adequate data.

There are no quantitative intermediate dermal exposure data available for humans or animals. Given the

potential for human exposure to methylene chloride through paint strippers, adhesives, glues, paint

thinners, wood stain, varnishes, spray paint, and automobile spray primers, information on the effects of

dermal exposures are needed to estimate more accurately the risks to human health from methylene

chloride.

Chronic-Duration Exposure and Cancer. No data are available on the non-neoplastic effects in

humans after chronic exposure to methylene chloride via the oral or dermal routes. Neurological and

reproductive effects have been reported in workers following combined inhalation/dermal exposures for

up to 3 years (Kelly 1988). Studies in animals suggest that the liver is a target organ following chronic

inhalation (Burek et al. 1984; Nitschke et al. 1988a; NTP 1986) and oral (Serota 1986a, 1986b) exposure.

Both a chronic inhalation MRL and a chronic oral MRL were derived using hepatotoxicity as the critical

endpoint (Nitschke et al. 1988a; Serota et al. 1986a). Renal effects were also observed following chronic

inhalation of methylene chloride in rodents (NTP 1986). Dermal data in animals after chronic exposure

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to methylene chloride are not available. Additional dermal studies in animals are needed to enhance our

understanding of potential risk to people living near hazardous waste sites.

Epidemiological studies of human occupational cohorts show no increase in cancer of the lung, liver, or

any other organs from occupational inhalation exposures (Friedlander et al. 1978; Hearne et al. 1987,

1990; Ott et al. 1983a). There are no human oral or dermal exposure data for the cancer endpoint.

Inhalation studies in animals show a concentration-dependent, statistically significant increase in liver and

lung adenomas and carcinomas in mice exposed to high concentration of methylene chloride (Mennear et

al. 1988; NTP 1986) and benign mammary gland tumors in rats (Mennear et al. 1988; NTP 1986)

following 2 years of exposure to methylene chloride. The evidence for carcinogenicity in animals from

oral exposures (Serota et al. 1986a, 1986b) is inconclusive, and there are no dermal data available.

Therefore, additional chronic oral and dermal studies are needed to clarify the cancer risk of ingested

methylene chloride. The carcinogenic mechanism of inhaled methylene chloride in animals is not yet

understood despite an extensive database on toxicokinetics and potential mode(s) of action. Additional

information on the mechanisms of carcinogenesis is needed to provide further insight regarding current

findings that (1) the concentration-response may be nonlinear at low exposure concentrations and there is

a concentration range below which carcinogenicity is unlikely to occur and (2) the mouse is not an

appropriate model for investigation of potential human carcinogenicity because it is much more sensitive

than any other species.

Genotoxicity. Genotoxicity data show mixed results. Generally, mutagenesis assays are negative in

mammalian in vivo studies (Burek et al. 1984; Sheldon et al. 1987). Methylene chloride is mutagenic in

bacteria (Gocke et al. 1981), but results for Drosophila (Gocke et al. 1981) are contradictory. This

compound has shown a dose-response relationship for clastogenesis in mammalian cells in vitro (Thilagar

et al. 1984a) but rats exposed in vivo (Burek et al. 1984) have shown no effects. In mice exposed in vivo,

tissue-specific DNA breaks were observed in mice (Sasaki et al. 1998), which suggests that genotoxicity

of methylene chloride may be dependent on the expression of metabolizing enzymes. In view of the

unknown carcinogenic mechanisms, additional in vivo studies of clastogenesis are needed to provide

useful markers, both for dosimetry and specific mechanisms. A recent in vitro test using strains of

Salmonella with or without recombinant GSTT1 has demonstrated that methylene chloride may have

more than one mechanism of genotoxicity (De Marini et al. 1997). In bacteria expressing mammalian

GSTT1, methylene chloride, at moderate doses, caused a single type of genetic lesion. In bacteria without

GSTT1, higher doses were required to induce genetic lesions and the results were more varied. These

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results may explain the variability observed in previous studies. Therefore, additional genotoxicity

studies should be carried out on mammalian cells for which the expression of GSTT1 and/or CYP2E1

isoenzymes is defined.

Reproductive Toxicity. Data on reproductive toxicity in humans are limited. One case study reported

genital pain, and low sperm counts (testicular atrophy in some cases) in workers who inhaled vapors of

methylene chloride and had direct contact with the liquid on the job for up to 3 years (Kelly 1988); they

were also exposed to styrene at low levels. Exposure to methylene chloride was confirmed by blood

COHb levels. In another study, workers exposed to methylene chloride for a shorter time showed no

reproductive effects (Wells et al. 1989b). A case-control occupational study reported a nearly significant

association of methylene chloride exposure and the incidence of spontaneous abortion (Taskinen et al.

1986). There are no oral or dermal exposure data that pertain to reproductive effects in humans. A two-

generation inhalation study in rats reported no effects on fertility and litter size (Nitschke et al. 1988b). In

addition, no effects on the testes were observed in dominant lethal tests involving male mice that inhaled

concentrations of methylene chloride up to 200 ppm for up to 6 weeks (Raje et al. 1988). Uterine,

ovarian, and testicular atrophy were observed in rodents following chronic exposure to methylene

chloride at 4,000 ppm (NTP 1986). There are no data on reproductive effects in animals following oral or

dermal exposure. Intermediate-duration oral and dermal studies that incorporate histopathological

analysis of the reproductive organs are needed to address this data need. Part of the analysis should

include a determination of whether the reproductive organs express GSTT1 and/or CYP2E1, as a first step

in evaluating their possible role in the reproductive toxicity of methylene chloride.

Developmental Toxicity. No data are available on developmental toxicity of methylene chloride

following inhalation, oral, or dermal exposure in humans, aside from the case-control study relating

exposure and the rate of spontaneous abortion mentioned above (Taskinen et al. 1986). Exposure to

0.01 ppm of methylene chloride had no effect on birth outcome in a study of over 90,000 pregnancies

(Bell et al. 1991). Several inhalation studies in animals indicate that methylene chloride can cross the

placenta (Anders and Sunram 1982). Some of these studies showed statistically nonsignificant

malformations in rats and mice or decreased fetal weight at maternally toxic concentrations (Bornschein

et al. 1980; Hardin and Manson 1980; Schwetz et al. 1975). However, there was a statistically significant

increase in the incidence of delayed ossification of sternebrae (Schwetz et al. 1975). The use of only one

concentration in these studies precludes any evaluation of concentration-response relationships.

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However, Reitz et al. (1997) developed an inhalation route-to-oral route extrapolation and rodent-to-

human species extrapolation using PBPK modeling of the developmental toxicity data in Schwetz et al.

(1975). The resulting LOAEL was an intermediate oral dose of 142 mg/kg/day. Additional studies for

inhalation and dermal exposures in two species would be useful in clarifying the developmental toxicity

potential of this chemical. Conducting studies on animals with known genotypes with respect to

metabolizing enzymes GSTT1 and CYP2E1 are needed to evaluate the risk of exposure to methylene

chloride.

Immunotoxicity. No human data are available on immunotoxicity of methylene chloride. Animal data

are limited on the immunotoxicity of methylene chloride. One study was located that indicated that

methylene chloride caused splenic atrophy in dogs following continuous intermediate-duration inhalation

exposure (MacEwen et al. 1972). Splenic fibrosis was observed in a chronic rat study (Mennear et al.

1988), but occurred only at the highest concentration tested (1,000 ppm) and only in one sex. A recent

intermediate-duration study, also in rats, found no evidence of gross or microscopical alterations in the

spleen at a concentration of 5,187 ppm methylene chloride (Halogenated Solvent Industry Alliance, Inc.

(2000). This study also found no effect on IgM response to SRBC. Additional immunotoxicity studies

including evaluation of humoral- and cell-mediated immunity are needed to determine whether this

system is susceptible to methylene chloride, as some chlorinated hydrocarbons do affect the immune

system.

Neurotoxicity. The central nervous system is a target for both short- and long-term inhalation

exposures in humans. These data are derived from experimental (Fodor and Winneke 1971; Putz et al.

1979; Stewart et al. 1972; Winneke 1974;) and occupational studies (Lash et al.1991; White et al. 1995)

that reported alterations in behavioral performance and various psychomotor tasks following exposure to

methylene chloride. Other neurotoxic effects noted in occupational studies included dizziness, headaches,

nausea, memory loss, paresthesia, tingling in hands and feet, and loss of consciousness (Bakinson and

Jones 1985; Hall and Rumack 1990; Kelly 1988). It should be noted that some of these workers were

exposed to other unspecified solvents at the same time. Winneke (1974) attributed the neurological

effects in volunteers following a 3–4 hour inhalation exposure to 300–800 ppm of methylene chloride to

the anaesthetic properties of the parent compound since exposures to 50–100 ppm of carbon monoxide

alone did not produce these effects. The specific parameters in the Winneke (1974) study, critical flicker

fusion frequency, auditory vigilance, and other psychomotor tasks, were considered to be specific

measures of ‘cortical alertness’ or central nervous system depression. Putz et al. (1979) correlated

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neurological deficits following inhalation exposure to methylene chloride (200 ppm) with the

accumulation of COHb in the blood and carbon monoxide in the exhaled breath; performance did not

deteriorate until 3 hours of exposure had elapsed and blood COHb levels exceeded 5%. Although the

Putz et al. (1979) study seems to implicate the metabolite CO as the neurotoxic agent, there is a need for

studies to determine the relative contributions of parent compound and metabolite to the neurotoxic

effects of methylene chloride. The Winneke (1974) study, which was deemed to have used more specific

measures of central nervous system depression than the Putz et al. (1979) study, was used to derive an

acute inhalation MRL and , by route-to-route extrapolation, also to derive an acute oral MRL (Reitz et al.

1997).

Subtle neurological effects were reported in rats after acute exposure to methylene chloride. Changes in

somatosensory-evoked potentials were observed after 1 hour exposure to 5,000 ppm of methylene

chloride (Rebert et al. 1989), and changes in cerebellar enzymes were detected following 2 weeks of

exposure at 500 ppm (Savolainen et al. 1981). Inhalation studies in rats did not reveal neurobehavioral

effects after intermediate-duration exposure at 4,500 ppm of methylene chloride vapors (Bornschein et al.

1980). In other studies, neurochemical changes were reported in gerbils at 210 ppm (Briving et al. 1986;

Karlsson et al. 1987; Rosengren et al. 1986). There were no treatment-related neurophysiological or

neuropathological effects in rats exposed to concentrations of 2,000 ppm for 13 weeks (Mattsson et al.

1990).

There are no oral or dermal studies in either humans or animals with regard to neurotoxicity of methylene

chloride. Data on electrophysiology, functional observational batteries, and neuropathology from 90-day

oral or dermal exposures in at least two different species would be useful to further evaluate neurotoxic

effects from these routes of exposure, since human exposure can occur from all three routes at hazardous

waste sites.

Epidemiological and Human Dosimetry Studies. Information on the effects of methylene

chloride in humans comes from occupational studies, case reports of acute high exposure, and studies

with volunteers. Asphyxia and eventually death occurred in a subject acutely exposure to a high but

undetermined concentration of methylene chloride in the air (Winek et al. 1981). Exposure to lower

concentrations affects primarily the central nervous system; signs and symptoms observed include

dizziness, incoordination, loss of balance, unconsciousness, and decreased performance in tests of sensory

and motor functions (Bakinson and Jones 1985; Fodor and Winneke 1971; Hall and Rumack 1990; Putz

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et al. 1979; Stewart et al. 1972; Winneke 1974). These effects are likely to be caused by a combination of

the anaesthetic properties of the parent compound and accumulation of COHb which forms as a result of

methylene chloride metabolism. There is a potential for low-level general population exposure from

ambient methylene chloride emissions from paint removal, aerosol use, metal degreasing, electronics and

pharmaceutical manufacturing, and food processing (Callahan 1981; CPSC 1987, 1990; NAS 1978).

Such exposure levels are unlikely to produce adverse neurological effects. Hazardous waste sites release

methylene chloride into the air, groundwater, surface water, and soil. There are very few monitoring data

for methylene chloride outside the occupational setting (Singh et al. 1981). Because methylene chloride

evaporates readily from water and soil, inhalation is the main route of potential exposure. In the unlikely

event that long-term exposure of the general population (in the past or present) to low levels of primarily

methylene chloride is identified, individuals should be monitored for hematological and hepatic effects, as

these have been observed in studies in animals.

Biomarkers of Exposure and Effect.

Exposure. The presence of methylene chloride in the postexposure expired breath is the most commonly

used biomarker of exposure. Methylene chloride is easily detected in expired air for 24 hours following a

vapor exposure (NIOSH 1974). Methylene chloride can be detected in blood, but because clearance is so

rapid, this method is only useful for monitoring recent exposures (DiVincenzo and Kaplan 1980). The

half-life of COHb following methylene chloride exposure is twice that following exposure to carbon

monoxide (NIOSH 1974; Stewart and Hake 1976). COHb in blood may be monitored; however, COHb

is cleared so rapidly (plasma half-life of 40 minutes) that it is unlikely to be useful for monitoring

environmental exposures (DiVincenzo et al. 1972). Urinary excretion of methylene chloride is

measurable for several hours postexposure (DiVincenzo et al. 1972).

Effect. Changes in the nervous system are sensitive, but not specific, biomarkers of methylene chloride

effects. Clinical signs and symptoms which may be monitored include narcosis, fatigue, and analgesia

(Bakinson and Jones 1985; Hall and Rumack 1990; Putz et al. 1979; Winneke 1974). Impairment of

neurobehavioral functions and electromyographic measurements of nerve conduction velocity and

amplitude can be monitored to detect early signs of neurotoxicity in people exposed to methylene chloride

(Glatzel et al. 1987; White et al. 1995). Additional studies that couple measurement of methylene

chloride with tests for determining nervous system effects are needed to correlate exposure with adverse

health effects of this chemical.

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While neurobehavioral tests are not specific for methylene chloride-induced toxicity, they do identify

potential health impairment. Studies to develop more specific biomarkers of methylene chloride-induced

effects are needed to assess the potential health risk of methylene chloride exposure near hazardous waste

sites.

Absorption, Distribution, Metabolism, and Excretion. Methylene chloride is a volatile liquid

with high lipid solubility and modest solubility in water. In humans, it is rapidly absorbed by the

inhalation (DiVincenzo and Kaplan 1981; McKenna et al. 1980) and ingestion routes of exposure

(Roberts and Marshall 1976). Dermal absorption is suspected to occur in humans, but quantitative data

are lacking. Methylene chloride is also readily absorbed by animals following inhalation (DeVincenzo et

al. 1972; MacEwen et al. 1972; McKenna et al. 1982) and oral exposure (Angelo et al. 1986a); dermal

absorption data in animals are also lacking. Once absorbed, methylene chloride is quickly distributed to a

wide range of tissues and body fluids. Absorption and distribution of methylene chloride can be affected

by a number of factors, including dose level, vehicle, physical activity, duration of exposure, and amount

of body fat (Astrand et al. 1975; DiVincenzo et al. 1972; Engstrom and Bjorstrom 1977).

Distribution data in humans are lacking, but it has been found in human breast milk (EPA 1980e;

Pellizzari et al. 1982). Methylene chloride is widely distributed in animal tissues after inhalation

exposure (Carlsson and Hultengren 1975; McKenna et al. 1982). The highest concentrations are found in

adipose tissue and liver. Methylene chloride has been found in blood from rats’ fetuses (Anders and

Sunram 1982). After acute exposure, methylene chloride disappears rapidly from fat (Carlsson and

Hultengren 1975). No studies were located regarding distribution of methylene chloride following

dermal exposure in animals. Distribution of methylene chloride does not seem to be route-dependent and

it does not bioaccumulate in tissues.

Methylene chloride is metabolized via two pathways, the MFO pathway, which produces CO and CO

and the GST pathway which yields only CO

(Gargas et al. 1986). Methylene chloride is metabolized

almost exclusively by the MFO pathway at low exposures. The only data available for humans on the

toxicokinetics parameters are for inhalation exposures. These data show that the GST activity is low in

humans relative to other animal species (Andersen et al. 1987). It has been postulated that the activity of

the GST pathway in rats is less than that of mice and might only become significant when the P-450

pathway (MFO) has been saturated (Green et al. 1986b, 1986c). Toxicokinetic data for oral and dermal

exposures would be useful because some exposures to humans are expected to be from contaminated

159 METHYLENE CHLORIDE

2. HEALTH EFFECTS

drinking water and soils. Toxicokinetic models (Andersen and Krishnan 1994; Andersen et al. 1987,

1991; Dankovic and Bailer 1994; Ramsey and Andersen 1984; Reitz 1990) have been developed that

account for the nonlinearities in the internal dose across exposure levels that arise from concentration-

dependent changes in absorption, distribution, excretion, and saturation of metabolism following

inhalation exposure to methylene chloride. Reitz et al. (1997) have developed an inhalation route-to-oral

route extrapolation for methylene chloride. The metabolism of methylene chloride in humans appears to

be qualitatively similar to that in animals (Fodor et al. 1973; Wirkner et al. 1997).

Methylene chloride is excreted via expired air primarily as CO and CO

at low concentrations (McKenna

et al. 1982). As concentration increases, more unchanged parent compound is exhaled. A small fraction

of absorbed methylene chloride or metabolites has been detected in urine of humans occupationally

exposed (DiVincenzo et al. 1972) and in the urine and feces of experimental animals (McKenna at al.

1982). Additional data on excretion parameters as functions of concentration are needed to understand

the toxicokinetics of methylene chloride.

Comparative Toxicokinetics. Generally, animal data on toxicokinetics parameters substantiate

those from humans. There is rapid absorption through the lung (DiVincenzo and Kaplan 1981;

DiVincenzo et al. 1972; MacEwen et al. 1972; McKenna et al. 1982). The metabolites of methylene

chloride in different species indicate that they are metabolized by the same pathways to CO or to CO

(Fodor et al. 1973; Gargas et al. 1986; Wirnker et al. 1997). The data from both human and animal

studies indicate that the target organs of methylene chloride toxicity are the same across species.

Interesting differences do exist, however, in some metabolic parameters. The rate of clearance from rat

tissues was markedly slower than in the mouse following inhalation exposures. In contrast to findings of

a direct proportional relationship between inspired air concentration of methylene chloride and blood

level in man and other animals, there is a report that the ratio at steady state between blood levels of

methylene chloride and exposure increases as the concentration increases in rats (Green et al. 1986b,

1986c). Distribution of methylene chloride across tissues in rats and humans has been shown to be

lipophilic except for one rat study that showed less of the chemical in adipose tissue (McKenna and

Zempel 1981). Species differences in placental anatomy and physiology may contribute to variations in

fetal exposure to methylene chloride or its metabolites following maternal exposure. Additional data on

the impact of differences in species sensitivity to specific internal doses by different routes of exposure

are needed to resolve questions about extrapolation of predicted effects from different routes in different

species.

160 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Methods for Reducing Toxic Effects. There is no specific treatment or antidote for methylene

chloride intoxication. Available methods and treatments (e.g., gastric lavage with the airway protected, or

the use of oxygen as in carbon monoxide poisoning) for reducing peak absorption have been shown to be

beneficial (Shih 1998; Tomaszewski 1998). There is no evidence that activated charcoal or cathartics are

effective (Shih 1998). Because methylene chloride is metabolized to carbon monoxide, mitigation

strategies (e.g., hyperbaric oxygen) used for carbon monoxide intoxication have been effective in

increasing the oxygen carrying capacity of the blood (Tomaszewski 1998).

Children’s Susceptibility. Data needs relating to both prenatal and childhood exposures, and

developmental effects expressed either prenatally or during childhood, are discussed in detail in the

Developmental Toxicity subsection above.

There are no studies that specifically addressed effects of exposure to methylene chloride in children.

There is no evidence from human toxicity studies that children are, or are likely to be, more susceptible to

health effects of exposure to methylene chloride than adults. No cases of accidental poisoning in children

due to methylene chloride exposure have been reported. The nervous system is a sensitive target for acute

exposure to methylene chloride in adults, and the response in children is likely to be similar.

Although developmental studies in animals indicate that methylene chloride is not a teratogen

(Bornschein et al. 1980; Hardin and Manson1980), there is a need to evaluate neurological/neuro-

behavioral effects in animals exposed in utero. Subtle neurological effects could result from hypoxia

(CO-mediated) or from reactive intermediates of metabolism that would only be revealed by appropriate

behavioral testing of the offspring. It is not clear that the "wheel running activity" and"avoidance

learning" tests that Bornschein et al. (1980) employed in rats exposed in utero were adequate to reveal

neurological deficits. Acute effects observed in an adult human study involved degraded performance on

visual and auditory discrimination tasks (Putz et al.1979). If neurological effects were detected in mice, it

would be useful to conduct additional developmental studies using mice in which functional genes for

GSTT1 and/or CYP2E1 have been knocked-out, to discover which metabolic pathway is implicated in

developmental neurological effects.

There is no direct evidence regarding pharmacokinetics or the mechanism of toxicity of methylene

chloride in children. However, considering what is known about the expression of metabolizing enzymes

(see Section 2.7 Children’s Susceptibility), there is no reason to expect that children over the age of

161 METHYLENE CHLORIDE

2. HEALTH EFFECTS

6 months will be qualitatively different from adults; by 6 months, the last of the fetal hemoglobin has

been replaced by the adult isoforms. At fetal and early postnatal stages, however, children will be more

vulnerable to methylene-chloride-generated CO because of its effect on fetal hemoglobin (Longo 1977).

CO binding to Hb causes oxygen to be held more tightly, so that tissues receive less oxygen; the net effect

is that the CO clears more slowly from the circulation. It is not known to what extent methylene chloride

(or metabolites other than CO or CO

) can cross the placenta in humans or accumulate in breast milk. A

study in animals showed that methylene chloride crosses the placental barrier (Anders and Sunram 1982),

but quantitative data are lacking. There are no animal studies testing whether methylene chloride can

pass into breast milk. There are no studies quantifying maternal exposure and output into breast milk.

The Fisher et al. (1997) PBPK model predicts a relatively low transfer into breast milk following maternal

inhalation exposure.

In adults, the biomarker of exposure to methylene chloride is the parent compound, and the biomarkers of

effect are CO in the exhaled breath or COHb in the blood. There is no reason to expect that biomarkers in

children would be any different.

It is expected that any chemical interactions involving methylene chloride would have the same effects in

children as in adults. The exception would be the fetal period, during which concentrations of

metabolizing enzymes are significantly lower than in the adult. A fetus exposed to methylene chloride

and another chemical that requires CYP2E1 for metabolism, might be at higher risk than an adult.

Treatment with 100% oxygen has been used to enhance the rate of clearance of CO from the bloodstream

after acute methylene chloride exposure. Although there is no specific information regarding pediatric

treatments for methylene chloride exposure, 100% oxygen has been used for treating pregnant women

exposed to CO (Longo 1977). In these cases, it is recommended that the oxygen treatment be extended

3.5–5 times longer than the time required to bring the maternal COHb level down to 3%, to ensure that

the fetus is adequately treated. If an exposed mother has neurological symptoms or a COHb level above

15%, a single hyperbaric oxygen treatment is indicated (Tomaszewski 1998). It does not appear that any

pediatric-specific methods need to be developed.

Since methylene chloride may be genotoxic, depending on the genotype with respect to GSTT1 and

CYP2E1 (see Section 2.10), it is possible that parental exposure could affect the fetus. At issue would be

whether the gonads express alleles that are associated with genotoxicity, since the effect is intracellular.

162 METHYLENE CHLORIDE

2. HEALTH EFFECTS

Child health data needs relating to exposure are discussed in 5.8.1 Identification of Data Needs:

Exposures of Children.

2.12.3 Ongoing Studies

A number of research projects are in progress investigating the health effects and mechanism of action of

methylene chloride. These projects have been identified from FEDRIP (1999).

Dr. J.B. Wheeler, at Vanderbilt University, Nashville, Tennessee, is comparing the ability of rat and

human glutathione S-transferases (GST) to form mutagenic DNA adducts with glutathione in the presence

of different haloalkanes. He will examine the mechanism of activation by GST, the stability of the

glutathione conjugates, and the DNA adducts produced. This research is sponsored by the National

Cancer Institute.

Dr. T.R. Devereux, at the NIEHS, is identifying critical target genes and genetic alterations that may be

important in chemical carcinogenesis. He is characterizing genetic alterations in oncogenes (e.g., ras) and

tumor suppressor genes (e.g., p53 and p16) from rodent tumors and human cancers generated by exposure

to chemical agents. He is also examining polymorphisms in the human p16 gene that may predispose to

lung cancer.

Dr. D.A. Bell, of the Genetic Risk Group at the NIEHS, is conducting case-control studies of

5,000 genotyped individuals to test the impact of cancer susceptibility genes on cancer of the bladder,

lung, liver, colon, stomach, prostate, and breast. He and his colleagues have identified ethnic differences

in the frequency of at-risk genotypes for glutathione transferase (M1, theta 1, and Pi), and

N-acetyltransferase (1 and 2).

163 METHYLENE CHLORIDE

3. CHEMICAL AND PHYSICAL INFORMATION

3.1 CHEMICAL IDENTITY

Information regarding the chemical identity of methylene chloride is located in Table 3-1. Methylene

chloride is a halogenated hydrocarbon. It is also commonly known as dichloromethane.

Table 3-1 lists common synonyms, trade names, and other pertinent identification information for

methylene chloride.

3.2 PHYSICAL AND CHEMICAL PROPERTIES

Information regarding the physical and chemical properties of methylene chloride is located in Table 3-2.

Methylene chloride is a colorless liquid with a sweet, pleasant odor.

164 METHYLENE CHLORIDE

3. CHEMICAL AND PHYSICAL INFORMATION

Table 3-1. Chemical Identity of Methylene Chloride

Characteristic Information Reference

Chemical Name

Synonyms

Registered trade name(s)

Chemical formula

Chemical structure

Identification numbers:

CAS

NIOSH RTECS

EPA hazardous waste

OHM/TADS

DOT/UN/NA/IMCO shipping

HSDB

NCI

Methylene chloride

Dichloromethane

Narkotil; Solaesthin; Solmethine;

and others

H H

75-09-2

PA8050000

U080, F002

7217234

UN1593, IMCO 6.1

C50102

Lide 1994

OHM/TADS 1998

Lide 1994

RTECS 1999

Lewis 1996

OHM/TADS 1998

HSDB 1999

CAS = Chemical Abstracts Services; DOT/UN/NA/IMCO = Department of Transportation/United Nations/North

America/International Maritime Dangerous Goods Code; EPA = Environmental Protection Agency;

HSDB = Hazardous Substances Data Bank; NCI = National Cancer Institute; NIOSH = National Institute for

Occupational Safety and Health; OHM/TADS = Oil and Hazardous Materials/Technical Assistance Data System;

RTECS = Registry of Toxic Effects of Chemical Substances

165 METHYLENE CHLORIDE

3. CHEMICAL AND PHYSICAL INFORMATION

Table 3-2. Physical and Chemical Properties of Methylene Chloride

Property Information Reference

Molecular weight 84.93 Lide 1994

Color Colorless Lewis 1996

Physical state Liquid Lide 1994

Melting point -95.1 EC Weast 1985

Boiling point 40 EC Lide 1994

Density:

at 25 EC 1.3182 g/mL Lide 1994

Vapor density 2.93 (Air =1) Verscheuren 1983

Odor Sweet, pleasant Verschueren 1983

Odor threshold:

Water 9.1 ppm Amoore and Hautala 1983

Air 540–2,160 mg/m

Ruth 1986

(160–620 ppm)

Solubility:

Water at 20 EC 20,000 mg/L Verschueren 1983

at 25 EC 16,700 mg/L Verschueren 1983

Organic solvent(s) Soluble in alcohol, ether, Lewis 1996

acetone, chloroform, and carbon

tetrachloride

Partition coefficients:

Log K

1.3 Hansch and Leo 1979

Log K

1.4 Roy and Griffin 1982

Vapor pressure:

at 20 EC 349 mmHg Verscheuren 1983

at 30 EC 500 mmHg Verscheuren 1983

Henry’s law constant 2.03x10

-3

atm-m

/mol at 25 EC EPA 1982e

Autoignition temperature 1,139 EF (615 EC) Lewis 1996

Flashpoint Nonflammable Sax and Lewis 1987

Flammability limits Nonflammable Sax and Lewis 1987

Coversion factors 1 mg/m

=0.28 ppm WHO 1996

1 ppm=3.53 mg/m

WHO 1996

Explosive limits Not explosive Sax and Lewis 1987

167 METHYLENE CHLORIDE

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

4.1 PRODUCTION

Table 4-1 lists the facilities in each state that manufacture or process methylene chloride, the intended

use, and the range of maximum amounts of methylene chloride that are stored on site. There are currently

461 facilities that reported producing or processing methylene chloride in the United States. The data

listed in Table 4-1 are derived from the Toxics Release Inventory (TRI98 2000). Only certain types of

facilities were required to report (461 facilities reported this type of information out of a total of 714

facilities that reported methylene chloride releases to the environment). Therefore, this is not an

exhaustive list.

Methylene chloride is produced by the chlorination of methane with chlorine or by the chlorination of

methanol with hydrogen chloride followed by chlorination of methyl chloride (Mannsville Chemical

Products Corporation 1988). Production of methylene chloride grew steadily through the 1970s and early

1980s at about 3% each year, with a peak production of about 620 million pounds in 1984. By 1988,

methylene chloride production volume had dropped due to declining demand to about 500 million pounds

(Mannsville Chemical Products Corporation 1988; USITC 1989). This decline in the demand for

methylene chloride was expected to continue at a rate of about 1–2% per year through 1993, as more

manufacturers move toward water-based aerosol systems in anticipation of further regulation of

methylene chloride (HSDB 1999; NTP 1989). In 1994, the latest year for which data are available,

403 million pounds of methylene chloride were produced (C&EN 1996). As of October 1, 1996, the

International Trade Commission ceased to collect or publish annual synthetic organic chemicals data.

The National Petroleum Refiners Association, which currently collects such data, does not include

methylene chloride on its list of organic chemicals.

The Toxics Release Inventory (TRI98 2000) reports that methylene chloride is produced at 23 facilities in

14 states. Table 4-1 summarizes information on the U.S. companies that reported the manufacture and

use of methylene chloride in 1998 (TRI98 2000). The TRI data should be used with caution since only

certain types of facilities are required to report. This is not an exhaustive list.

The Stanford Research Institute (SRI 1999) reports methylene chloride production by two companies at

four U.S. facilities with a total manufacturing capacity of 545 million pounds. These facilities are Dow

168 METHYLENE CHLORIDE

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Table 4-1. Facilities that Manufacture or Process Methylene Chloride

Range of maximum amounts

on site in thousands of

State

Number of facilities pounds

Activities and uses

reporting

AL 12 100-999999 1,4,7,8,10,11,12,13

AR 7 1000-999999 2,3,7,10,11,12,13

AZ 5 100-999999 8,10,11,12,13

CA 23 0-9999999 2,3,4,8,9,10,11,12,13

CO 4 1000-999999 8,10,11,13

CT 8 1000-99999 2,3,10,11,12,13

DE 2 10000-99999 8,11

FL 12 100-999999 8,10,11,12,13

GA 18 0-999999 2,3,8,9,10,11,12,13

IA 10 100-999999 8,10,11,12,13

IL 23 0-9999999 1,2,3,5,7,8,9,10,11,12,13

IN 21 100-9999999 1,2,3,5,8,9,10,10,11,12,13

KS 8 100-999999 1,2,3,4,8,10,11,12,13

KY 9 100-99999 1,5,7,8,10,11,13

LA 8 1000-9999999 1,4,5,6,7,8,10,12,13

MA 10 1000-999999 2,3,8,9,10,11,12,13

MD 5 1000-99999 8,10,12,13

ME 1 1000-9999 13

MI 17 100-999999 1,2,3,4,5,7,8,10,11,12,13

MN 8 100-99999 2,3,8,11,12,13

MO 13 1000-999999 4,8,10,11,12,13

MS 11 100-999999 1,2,3,8,11,12,13

MT 1 1000-9999 12

NC 17 100-999999 1,2,4,5,7,8,10,11,12,13

169 METHYLENE CHLORIDE

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Table 4-1. Facilities that Manufacture or Process

Methylene Chloride (continued)

Range of maximum amounts

on site in thousands of

State

Number of facilities pounds

Activities and uses

NE 3 1000-999999 11,13

NH 3 1000-99999 8,12,13

NJ 19 0-999999 1,2,3,4,7,8,9,10,11,12,13

NM 2 1000-99999 8,12

NV 3 100-9999 1,2,3,8,11,13

NY 14 1000-9999999 2,3,4,7,8,9,10,11,12,13

OH 17 100-9999999 2,3,7,8,10,11,12,13

OK 9 100-999999 8,10,11,13

OR 6 1000-999999 2,4,8,10,11,12,13

PA 17 1000-999999 8,9,10,11,12,13

PR 12 1000-9999999 1,2,5,6,8,9,10,11,12,13

RI 2 10000-99999 10,13

SC 14 100-999999 7,8,10,11,12,13

TN 15 100-9999999 8,9,10,11,12,13

TX 27 0-49999999 1,2,3,4,5,6,7,8,9,10,11,12,

UT 6 1000-999999 8,10,11,12,13

VA 10 1000-999999 8,10,11,12,13

WA 10 0-99999 1,6,7,10,11,12,13

WI 14 1000-999999 1,4,5,8,10,11,12,13

WV 5 0-9999999 8,11,13

TRI98 (2000 )

Post office state abbreviations

Data in TRI are maximum amounts on site at each facility

Activities/Uses:

1. Produce

2. Import

3. For on-site use/processing

4. For sale/distribution

5. As a byproduct

6. As an impurity

7. As a reactant

8. As a formulation component

9. As an article component

10. For repackaging only

11. As a chemical processing aid

12. As a manufacturing aid

13. Ancillary or other use

170 METHYLENE CHLORIDE

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Chemical Company in Freeport, Texas and Plaquemine, Louisiana; and Vulcan Materials Company in

Geismar, Louisiana and Wichita, Kansas.

4.2 IMPORT/EXPORT

Imports of methylene chloride had been decreasing from 60 million pounds in 1985 to 25–27 million

pounds in 1988–1989 (Mannsville Chemical Products Corporation 1988; NTP 1989). During 1992, the

import of methylene chloride had dropped to about 15.5 million pounds, but has since increased to

30 million pounds in 1996 (NTDB 1998). Methylene chloride exports have ranged from about

60–130 million pounds over the period of 1975–1988 (Mannsville Chemical Products Corporation 1988)

and ranged from about 132 to 145 million pounds over the period of 1992–1996 (NTDB 1998).

4.3 USE

Methylene chloride is used as a solvent in paint strippers and removers (25%), as a propellant in aerosols

(25%), as a process solvent in the manufacture of drugs, pharmaceuticals, and film coatings (20%), as a

metal cleaning and finishing solvent (10%), in electronics manufacturing (10%), and as an agent in

urethane foam blowing (10%) (NTP 1989). Aerosol products in which methylene chloride may be found

include paints, automotive products, and insect sprays. However, because of labeling regulations and

concerns over health and environmental issues, the use of methylene chloride in consumer aerosol

products has declined (Mannsville Chemical Products Corporation 1988). Methylene chloride was once

used in hair sprays but this use was banned in 1989 (FDA 1989).

Methylene chloride is also used as an extraction solvent for spice oleoresins, hops, and for the removal of

caffeine from coffee. These uses of methylene chloride are approved by the Food and Drug

Administration (see Chapter 7). However, it has been reported that because of concern over residual

solvent, most decaffeinators no longer use methylene chloride (Mannsville Chemical Products

Corporation 1988). Methylene chloride is also approved for use as a post-harvest fumigant for grains and

strawberries and as a degreening agent for citrus fruit (Hearne et al. 1990; Mannsville Chemical Products

Corporation 1988; Meister 1989; NTP 1989).

171 METHYLENE CHLORIDE

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

4.4 DISPOSAL

Methylene chloride is listed as a toxic substance under Section 313 of the Emergency Planning and

Community Right to Know Act (EPCRA) under Title III of the Superfund Amendments and

Reauthorization Act (SARA) (EPA 1995). Disposal of wastes containing methylene chloride is

controlled by a number of federal regulations (see Chapter 7).

Methylene chloride and wastes containing methylene chloride are considered hazardous wastes and as

such, are subject to handling, transport, treatment, storage, and disposal requirements as mandated by law

(see Chapter 7) (IRPTC 1990; NTP 1989).

Methylene chloride wastes may be disposed of by controlled incineration (liquid injection, rotary kiln, or

fluidized bed) at the appropriate temperatures. No information was found regarding the amount of

methylene chloride disposed of by each incineration method. The percent removal of methylene chloride

by incineration is mandated by law depending upon the quantity of methylene chloride in the waste

(HSDB 1999; IRPTC 1990).

According to the Toxics Release Inventory (TRI98 2000), about 0.37 million pounds of methylene

chloride were transferred to landfills and/or other treatment/disposal facilities including publicly-owned

treatment works in 1998 (see Section 5.2). No information was found on disposal method trends or past

disposal practices.

173 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

5.1 OVERVIEW

Methylene chloride has been identified in at least 882 of the 1,569 hazardous wastes sites that have been

proposed for inclusion on the EPA National Priorities List (NPL) (HazDat 1999). However, the number

of sites evaluated for methylene chloride is not known. The frequency of these sites can be seen in

Figure 5-1. Of these sites, 882 are located in the United States, and none are located in the

Commonwealth of Puerto Rico (not shown).

Methylene chloride is a widely used industrial chemical with reported emissions to air of more than

40 million pounds annually in the United States. Most of the methylene chloride released to the

environment will partition to the atmosphere, where it will degrade with a lifetime of 6 months by

reaction with photochemically produced hydroxyl radicals (WHO 1996). The compound is expected to

be highly mobile in soil and to volatilize rapidly from surface water to the atmosphere. Biodegradation

may be important, but bioconcentration does not appear to be significant.

The principal route of exposure for the general population to methylene chloride is inhalation of ambient

air. Average daily intake of methylene chloride from urban air has been estimated to range from about

33 to 309 µg. Occupational and consumer exposure to methylene chloride in indoor air may be much

higher, especially from spray painting or other aerosol uses.

5.2 RELEASES TO THE ENVIRONMENT

According to the Toxics Release Inventory (TRI), in 1998, a total of about 41 million pounds

(18.6 million kg) of methylene chloride was released to the environment from 714 facilities (714 facilities

reported environmental releases of methylene chloride to TRI, another 113 facilities reported no releases

at all for a total of 827 facilities) (TRI98 2000). This total release volume is approximately 10% of the

total reported for 1997 (TRI97 1999). Table 5-1 lists amounts released from these facilities. In addition,

an estimated 0.37 million pounds (0.17 million kg) were released by manufacturing and processing

facilities to publicly owned treatment works (POTWs) or other off-site facilities (TRI98 2000). The TRI

data should be used with caution because only certain types of facilities are required to report. This is not

an exhaustive list.

174

Table 5-1. Releases to the Environment from Facilities that Manufacture or Process Methylene Chloride

Total reported amounts released in pounds per year

Number Total

State

facilities Air

Water Land

Underground

injection

Total

on-site release

off-site

release

Total on and

Off-site release

AL 14 948907 0 0 0 948907 470 949377

AZ 6 65837 0 0 0 65837 0 65837

AR 12 125869 12 0 360837 486718 269 486987

CA 44 986871 3 0 0 986874 9768 996642

CO 6 37077 0 0 0 37077 0 37077

CT 31 784114 755 3 0 784872 7450 792322

DE 2 4100 0 0 0 4100 0 4100

FL 16 932255 0 0 0 932255 759 933014

GA 31 973595 1160 169543 0 1144298 14935 1159233

IL 45 1745258 57 0 0 1745315 3295 1748610

IN 47 4630842 639 5 0 4631486 1152 4632638

IA 13 1156800 0 5 0 1156805 500 1157305

KS 12 288373 0 0 41323 329696 1799 331495

KY 15 2157130

57 0 0 2157187 11435 2168622

LA 11 41136 1020 0 9954 52110 2236 54346

ME 1 3394 0 0 0 3394 0 3394

MD 5 118361 0 0 0 118361 0 118361

METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

175

Table 5-1. Releases to the Environment from Facilities that Manufacture or Process Methylene Chloride

(continued)

Total reported amounts released in pounds per year

Number

State

facilities Air

Water Land

Underground

injection

Total on-site

release

Total off-site

release

Total on and

Off-site release

MA 29 139895 0 0 0 139895 4772 144667

MI 32 422374 177 0 54800 477351 1954 479305

MN 17 183564 290 0 0 183854 0 183854

MS 15 3355695 0 0 0 3355695 900 3356595

MO 26 302907 110 5 0 303022 0 303022

MT 1 42300 0 0 0 42300 0 42300

NE 3 41152 0 0 0 41152 2085 43237

NV 2 6834 0 0 0 6834 0 6834

NH 3 38696 0 0 0 38696 40 38736

NJ 35 243166 548 3 0 243717 5977 249694

NM 1 105 0 0 0 105 0 105

NY 24 2038566 4305 0 0 2042871 29091 2071962

NC 29 2599415 4271 450 0 2604136 750 2604886

OH 54 1864209 257

0 0 1864466 21070 1885536

OK 12 250064 0 0 0 250064 32 250096

OR 7 79509 0 0 0 79509 2987 82496

METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

176

Table 5-1. Releases to the Environment from Facilities that Manufacture or Process Methylene Chloride

(continued)

Total reported amounts released in pounds per year

Number

State

facilities Air

Water Land

Underground

injection

Total on-site

release

Total off-site

release

Total on and

Off-site release

PA 36 3545433 37 0 1 3545471 47945 3593416

PR 18 2441482 0 0 0 2441482 7555 2449037

RI 2 18677 0 0 0 18677 0 18677

SC 22 1934986 1050 12127 0 1948163 6020 1954183

TN 26 2738055 0 0 0 2738055 17200 2755255

TX 54 822273 567 1990 23749 848579 110891 959470

UT 7 56753 0 0 0 56753 8376 65129

VA 20 1412207 0 5 0 1412212 422 1412634

WA 14 176129 294 176 0 176599 45145 221744

WV 5 67789 147 38 0 67974 0 67974

WI 22 565148 0 0 0 565148 0 565148

Total 827 40387302 15756 184350 490664 41078072 367280 41445352

Source: TRI98 2000

Data in TRI are maximum amounts released by each facility.

Post office state abbreviations are used.

The sum of fugitive and stack releases are included in releases to air by a given facility.

The sum of all releases of the chemical to air, land, water, and underground injection wells.

Total amount of chemical transferred off-site, including to publicly owned treatment works (POTW)

METHYLENE CHLORIDE

POTENTIAL FOR HUMAN EXPOSURE

177 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

1-4

5-8

9-16

21-34

52-72

Frequency of

NPL Sites

Derived from HazDat 2000

Figure 5-1. Frequency of NPL Sites with Methylene Chloride Contamination

178 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Methylene chloride has been identified in a variety of environmental media (air, surface water,

groundwater, soil, and sediment) collected at 882 of the 1,569 NPL hazardous waste sites (HazDat 1999).

5.2.1 Air

According to the Toxics Release Inventory, in 1998, the estimated releases of methylene chloride of

40 million pounds (18.3 million kg) to air from 714 large processing facilities accounted for about 97.4%

of total environmental releases (TRI98 2000). Table 5-1 lists amounts released from these facilities. The

TRI data should be used with caution because only certain types of facilities are required to report. This

is not an exhaustive list.

Methylene chloride has been identified in air samples collected at 108 of the 1,569 NPL hazardous waste

sites where it was detected in some environmental media (HazDat 1999).

Because methylene chloride is a highly volatile substance, most environmental releases are into the

atmosphere. Methylene chloride is released to the atmosphere during its production, storage, and

transport, but most (more than 99%) of the atmospheric releases result from industrial and consumer uses

(EPA 1983c, 1985e). In 1992, 32.5% of the total global emission of methylene chloride was attributed to

North America (McCulloch and Midgley 1996). It has been estimated that 85% of the total amount of

methylene chloride produced in the United States is lost to the environment (EPA 1985e), about 86% of

which is released to the atmosphere (EPA 1982a). Thus, about 73% (370 million pounds) of the U.S.

production volume for 1988 (500 million pounds), of methylene chloride was lost to the atmosphere in

1988. Manufacturers, processors, and users of methylene chloride are required to report the quantities of

methylene chloride released to environmental media or transferred to off-site disposal facilities annually

(EPA 1998j). The data currently available, compiled in the Toxics Release Inventory (TRI98 2000), are

for releases in 1998 and are summarized in Table 5-1. Methylene chloride was the 14

-largest release

and transfer chemical reported in the United States for 1988 (EPA 1990c). The TRI data should be used

with caution because only certain types of facilities are required to report. This is not an exhaustive list.

Industrial methylene chloride emissions to the atmosphere reported to EPA for the 1988 TRI totaled about

127 million pounds (TRI88 1990). Emissions from the use of consumer products and other sources such

as hazardous waste sites may be estimated to be 243 million pounds by subtracting industrial emissions

(127 million pounds) from total atmospheric loss of methylene chloride (370 million pounds). Consumer

179 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

products containing methylene chloride in wide use include paint strippers, aerosols, adhesives and glues,

and cleaning fluids and degreasers (CPSC 1990). Virtually all the methylene chloride in these products is

released to the atmosphere during use. Its use in hair sprays was banned in 1989 by the FDA (1989).

Methylene chloride is formed during water chlorination (NAS 1977) and is emitted to the air from waste

waters in treatment plants (Corsi et al. 1987; Dunovant et al. 1986; Namkung and Rittmann 1987).

Methylene chloride emissions from treatment plants in California are estimated to exceed 400,000 pounds

annually (Corsi et al. 1987). Declining production amounts of methylene chloride will result in a

decrease in the volume emitted to the atmosphere.

5.2.2 Water

According to the Toxics Release Inventory, in 1998, the estimated releases of methylene chloride of about

15,756 pounds (7,090 kg) to water from 714 facilities accounted for < 0.04% of total environmental

releases (TRI98 2000). Table 5-1 lists amounts released from these facilities. The TRI data should be

used with caution because only certain types of facilities are required to report. This is not an exhaustive

list.

Methylene chloride has been identified in surface water and groundwater samples collected at 633 and

218, respectively, of the 1,569 NPL hazardous waste sites where it was detected in some environmental

media (HazDat 1999).

About 2% of environmental releases of methylene chloride are to water (EPA 1982a). Industrial releases

of methylene chloride to surface water and underground injection (potential groundwater release)

reported to the TRI in 1998 totaled 506,420 pounds (TRI98 2000). Methylene chloride has been

identified in industrial and municipal waste waters from several sources at concentrations ranging from

0.08 ppb to 3,400 ppm (Dunovant et al. 1986; EPA 1979a, 1982a; Namkung and Rittmann 1987). It has

also been reported in the leachate from industrial and municipal landfills at concentrations from 0.01 to

184 ppm (Brown and Donnelly 1988; Sawhney 1989). The levels of methylene chloride in surface water

have been reported to vary from nondetectable to 10 ppb (California Environmental Protection Agency

1992).

180 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Methylene chloride has been detected in both surface water and groundwater samples taken at hazardous

waste sites. Data from the Contract Laboratory Program (CLP) Statistical Database indicate methylene

chloride was found at geometric mean concentrations of 68 and 98 ppb in surface water and groundwater

samples, respectively, at about 30% of the sites sampled (CLPSD 1990). Note that the information used

from the CLP Statistical Database includes data from both NPL and non-NPL sites.

5.2.3 Soil

According to the Toxics Release Inventory, in 1998, the estimated releases of methylene chloride of about

184,350 pounds (82,958 kg) to soil from 714 facilities accounted for about <0.44% of total environmental

releases (TRI98 2000). Table 5-1 lists amounts released from these facilities. The TRI data should be

used with caution because only certain types of facilities are required to report. This is not an exhaustive

list.

Methylene chloride has been identified in soil (412 sites) and sediment (179 sites) samples collected at

the 1,569 NPL hazardous waste sites where it was detected in some environmental media (HazDat 1999).

The principal sources of methylene chloride releases to land are disposal of methylene chloride products

and containers to landfills. Substantial reduction to industrial disposal of methylene chloride to the land

is likely since the inception of the Land Disposal Restrictions (EPA 1998c). However, disposal of

containers for consumer products containing residues of methylene chloride may continue to occur. It is

estimated that about 12% of methylene chloride losses to the environment are to land (EPA 1982a).

Methylene chloride has been detected in soil/sediment samples taken at 36% of the hazardous waste sites

included in the CLP Statistical Database at a geometric mean concentration of 104 ppb (CLPSD 1990).

5.3 ENVIRONMENTAL FATE

5.3.1 Transport and Partitioning

Methylene chloride is a volatile liquid, with a boiling point of 40 EC (Lide 1994) and a vapor pressure of

349 mm Hg at 20 EC (Verschueren 1983). Therefore, methylene chloride tends to volatilize to the

atmosphere from water and soil. The half-life of methylene chloride volatilization from water has been

181 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

found to be 21 minutes under experimental conditions (Dilling et al. 1975), but actual volatilization from

natural waters will depend on the rate of mixing, wind speed, temperature, and other factors (Dilling et al.

1975; EPA 1979b). The Henry's law constant value (H) of 0.002 atm/m

/mol (EPA 1982e; Gossett 1987)

indicates that methylene chloride will volatilize rapidly from moist soil and water surfaces (Thomas

1990).

Methylene chloride is not strongly sorbed to soils or sediments (Dilling et al. 1975; Dobbs et al. 1989).

Based on its low soil organic carbon partitioning coefficient (K

) of 25, methylene chloride is likely to be

very highly mobile in soils (Bahnick and Doucette 1988; Roy and Griffin 1985) and may be expected to

leach from soils into groundwater.

Based on a reported log octanol/water partition coefficient (K

) of 1.3 (Hansch and Leo 1979), an

estimated bioconcentration factor (BCF) of 2.3 was derived (EPA 1980a, 1984). There is no evidence of

biomagnification, but because the estimated BCF is low, significant biomagnification of methylene

chloride in aquatic food chains is not expected.

5.3.2 Transformation and Degradation

5.3.2.1 Air

The main degradation pathway for methylene chloride in air is its reaction with photochemically

generated hydroxyl radicals (Cox et al. 1976; Crutzen and Fishman 1977; Davis et al. 1976; EPA 1980f,

1987i). Thus, the atmospheric lifetime of methylene chloride may be predicted from the hydroxyl radical

concentration in air and the rate of reaction. Most reported rates for hydroxyl radical reaction with

methylene chloride range from 1.0x10

-13

to 1.5x10

-13

/mol/sec, and estimates of average atmospheric

hydroxyl radical concentration range from 2.5x10

to 1x10

mol/cm

(Cox et al. 1976; Crutzen and

Fishman 1977; Davis et al. 1976; EPA 1980f, 1985e, 1985g). Using this information, an average

atmospheric lifetime for methylene chloride may be calculated to be 130 days. Other estimates range

from 100 to 500 days (Altshuller 1980; Cox et al. 1976; Davis et al. 1976; EPA 1987i; Sidebottom and

Franklin 1996). Because this degradation pathway is relatively slow, methylene chloride may become

widely dispersed but is not likely to accumulate in the atmosphere. The small amount of methylene

chloride which reaches the stratosphere (about 1%) may undergo direct photolytic degradation; however,

photolysis in the troposphere is not expected (Howard et al. 1990). Reactions of methylene chloride with

182 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

ozone or other common atmospheric species (e.g., oxygen atoms, chlorine atoms, and nitrate radicals) are

not believed to contribute to its breakdown (EPA 1985g, 1987i; WHO 1996).

5.3.2.2 Water

Methylene chloride undergoes slow hydrolysis in water. The experimental half-life reported for the

hydrolysis reaction, at neutral conditions, is approximately 18 months at 25 EC (Dilling et al. 1975).

However, the rate of reaction varies greatly with changes in temperature and pH. A hydrolytic half-life of

14 days was reported for methylene chloride in acidic solutions at 80–150 EC (EPA 1979b, 1985e). This

experimental value, when extrapolated to 25 EC, is about 700 years. Different mechanisms of hydrolyses

may be responsible for these two widely different values.

Both aerobic and anaerobic biodegradation may be an important fate process for methylene chloride in

water (Brunner et al. 1980; Davis et al. 1981; EPA 1985e; Stover and Kincannon 1983; Tabak et al.

1981). In the laboratory, methylene chloride can be biodegraded by aerobic bacteria such as

Methylobacterium sp. and Methylophilus sp. and anaerobic bacteria such as Hyphomicrobium sp. and

Dehalobacterium sp. (Leisinger et al. 1994; Magli et al. 1998). These bacteria are able to efficiently

utilize methylene chloride as a carbon and energy source. The Acinebacter sp. has been shown to use

oxygen or nitrate ion as the electron acceptor (Sheehan and Freedman 1996). Methylene chloride has

been observed to undergo degradation at a rapid rate under aerobic conditions. Reported total methylene

chloride loss was 100% after 7 days in a static culture flask biodegradability screening test (Tabak et al.

1981) and 92% after 6 hours in a mixed microbial system (Davis et al. 1981). Volatilization loss was not

more than 25% (Tabak et al. 1981).

5.3.2.3 Sediment and Soil

Degradation of methylene chloride was found to occur in soils with concentrations ranging from

approximately 0.1 to 5.0 ppm (Davis and Madsen 1991). The rate of biodegradation was found to be

dependent on soil type, substrate concentration, and redox state of the soil. Methylene chloride

biodegradation has been reported to occur under both aerobic conditions and anaerobic conditions (Davis

and Madsen 1991). The biodegradation of methylene chloride appears to be accelerated by the presence

of elevated levels of organic carbon (Davis and Madsen 1991).

183 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Methylene chloride has a low tendency to absorb to soil (Dilling et al. 1975; Dobbs et al. 1989);

therefore, there is a potential for leaching to groundwater. Also, because of the high vapor pressure,

volatilization to air is also a likely fate process from dry soil. Its high Henry’s law constant

(0.002 atm/m

/mol) indicates that volatilization from moist soil is also likely.

5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT

Reliable evaluation of the potential for human exposure to methylene chloride depends in part on the

reliability of supporting analytical data from environmental samples and biological specimens. In

reviewing the data on methylene chloride levels monitored or estimated in the environment, it should also

be noted that the amount of chemical identified analytically is not necessarily equivalent to the amount

that is bioavailable. The analytical methods available for monitoring methylene chloride in a variety of

environmental media are detailed in Chapter 6 (Analytical Methods).

5.4.1 Air

Methylene chloride has been detected in ambient air samples taken from around the world, as shown in

Table 5-2. Background levels are usually at about 50 parts per trillion (0.17 µg/m

) (Singh et al. 1982).

Methylene chloride was among the chemicals monitored in a statewide survey of hazardous air pollutants

by the Arizona Hazardous Air Pollutants Monitoring Program. The average amount of methylene

chloride detected in air ranged from 0.61 ppm on a hillside in Yavapai County to 1.62 ppm in Phoenix

(Zielinska et al. 1998). Concentrations of methylene chloride in urban areas and in the vicinity of

hazardous waste sites are generally one to two orders of magnitude higher. These values are all much

lower than the concentrations which may be encountered inside buildings where products containing

methylene chloride are being used (NAS 1978; Otson et al. 1983).

5.4.2 Water

Methylene chloride has been detected in surface water, groundwater, and finished drinking water

throughout the United States. It was detected in 30% of 8,917 surface water samples recorded in the

STORET database, at a median concentration of 0.1 ppb (Staples et al. 1985). In a New Jersey survey

(Page 1981), methylene chloride was found in 45% of 605 surface water samples, with a maximum

concentration of 743 ppb. Methylene chloride has also been identified in surface waters in Maryland

184 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-2. Summary of Methylene Chloride Levels in Air

Concentration (ppb)

Location Maximum Mean Reference

Background No data 0.05 Singh et al. 1982

Oceanic:

Northern Hemisphere No data 10.37 Koppmann et al. 1993

Southern Hemisphere No data 5.18 Koppmann et al. 1993

Rural/suburban United States No data 0.05

–0.60

EPA 1988d

Urban United States 22–200 0.23–1.93

EPA 1980e, 1988d; Harkov

et al. 1984, 1985; Shikiya

et al. 1984; Singh et al.

1982

Source dominated No data 0.58

EPA 1988d

Hazardous waste sites 10–190 0.09–11.23

Harkov et al. 1985;

La Regina et al. 1986

Indoor (nonresidential) 19,000 0.06–5,472

Harsch 1977; Otson et al.

1983

1 ppb = 3.47 µg/m

Range of values

Median value

Range of individual values

185 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

(Helz and Hsu 1978), in Lakes Erie and Michigan (Konasewich et al. 1978), and at hazardous waste sites

(Hauser and Bromberg 1982). Methylene chloride (0.2 ppm) was also detected in the shallow ground-

water beneath Denver, Colorado (Bruce and McMahon 1996). Seawater also contains small amounts of

methylene chloride; the mean reported concentration is 2.2x10

-3

ppb (Singh et al. 1983).

Since volatilization is restricted in groundwater, concentrations of methylene chloride are often higher

there than in surface water. Occurrence of methylene chloride in groundwater has been reported in

several surveys across the United States, with concentrations ranging from 0 to 3,600 ppb (Dyksen and

Hess 1982; EPA 1985h; Page 1981). Based on CERCLA records compiled during 1987, the compound

was the sixth most frequently detected organic contaminant found in groundwater during hazardous waste

disposal site investigations with a detection frequency of 19% (Plumb 1987).

Methylene chloride has been detected in drinking water supplies in numerous U.S. cities (Coleman et al.

1976; Dowty et al. 1975; EPA 1975b; Kool et al. 1982; Kopfler et al. 1977). Reported mean concentra-

tions are generally less than 1 ppm (EPA 1975b). It was detected in 2.2% of 182 samples of bottled water

surveyed with a mean concentration of positive samples at 0.059 ppb (Page et al. 1993). Water

chlorination in treatment plants appears to increase both the concentration and the frequency of

occurrence of methylene chloride in drinking water supplies (Bellar et al. 1974; EPA 1975b; NAS 1977).

5.4.3 Sediment and Soil

No studies were located on methylene chloride levels in soil. However, methylene chloride was detected

in 20% of 338 sediment samples recorded in the STORET database, at a median concentration of

13 µg/kg (Staples et al. 1985).

5.4.4 Other Environmental Media

Although methylene chloride is used in food processing (solvent extraction of coffee, spices, hops) and as

a post-harvest fumigant for some foods (strawberries, grain), little information was located on levels in

foods. Residual levels of methylene chloride in decaffeinated coffee beans ranging from 0.32 to

0.42 ppm were reported in the United States (IARC 1986), and levels ranging from 0.01 to 0.1 ppm were

reported by a major coffee processor (FDA 1985). These levels are well within the FDA limits of 10 ppm

for methylene chloride in decaffeinated coffee. Although the FDA considers residues at or below the

186 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

FDA limit to be safe (FDA 1985), it has been reported that methylene chloride is no longer used as a

decaffeinating agent by most coffee decaffeinators (Mannsville Chemical Products Corporation 1988).

The FDA has also set a limit of 30 ppm for methylene chloride in spice oleoresins, but EPA has exempted

methylene chloride from the requirement of a tolerance for fumigated food materials (see Chapter 7).

Oysters and clams from Lake Pontchartrain in Louisiana had mean methylene chloride levels of 7.8 and

27 ppb, respectively (Ferrario et al. 1985).

5.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE

Inhalation of methylene chloride from ambient air is the predominant route of exposure for the general

population in the United States. As calculated by Singh et al. (1981), the average daily doses of

methylene chloride from ambient air in three U.S. cities range from 33 to 309 µg/day, based on 1979

monitoring data and a daily air intake of 23 m

by an adult. Since the amount of methylene chloride

manufactured has recently dropped below the production levels of the late 1970s, average daily doses

may currently be somewhat less than these values. Methylene chloride was detected in the blood of fewer

than 10% of the 600 samples obtained from people who participated in the Third National Health and

Nutrition Examination Survey (NHANES III) (Ashley et al. 1994). Inhalation exposure may also occur

through the use of consumer products containing methylene chloride (CPSC 1987) (see Section 5.7).

Since concentrations of methylene chloride in water and food are generally quite low, it appears that

exposure from sources other than air is unlikely to be important. For example, drinking water containing

1 ppb methylene chloride would provide an additional intake of 2 µg per day for an adult drinking 2 L of

water per day.

Methylene chloride has been identified, but not quantified, in eight out of eight samples of human breast

milk collected in four urban areas in the United States (EPA 1980d; Pellizzari et al. 1982).

Occupational exposure to methylene chloride may be important in numerous industries. Workers may be

exposed during the production and processing of methylene chloride. They may also be exposed to

methylene chloride during a variety of industrial activities including spray painting, spray gluing, metal

painting, paint stripping, and aerosol packing (IARC 1986; OSHA 1986; Whitehead et al. 1984). The

NIOSH estimated that the number of workers exposed to methylene chloride increased from about

187 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

1 million in the early 1970s to 1.4 million in the early 1980s (NIOSH 1986; NOES 1990). Since

production and use of methylene chloride has decreased (as have threshold limit values) since the

mid-1980s (see Sections 4.1 and 4.3), the number of workers exposed to methylene chloride has

decreased accordingly.

Monitoring data for methylene chloride in workplace air from 1968 to 1982 indicate that concentrations

in the general work area ranged from 0.086 to 964.8 ppm while samples in the breathing zone of workers

ranged up to 1,411 ppm (IARC 1986). Installation of appropriate ventilation systems has been found to

lower the workers’ exposure to methylene chloride in the breathing zone from 600–1,150 ppm to

28–34 ppm (Estill and Spencer 1996). The current OSHA Permissible Exposure Limit (PEL) for

methylene chloride for an 8-hour workday is 25 ppm (88.25 mg/m

) (OSHA 1997). An estimated

237,496 workers are potentially exposed while involved with methylene chloride manufacturing, paint

manufacturing, metal cleaning, polyurethane foam manufacturing, plastics and adhesives manufacturing,

ink use, pharmaceuticals, construction, and shipyards. The largest numbers of occupationally exposed

individuals occur in areas of metal cleaning, industrial paint stripping, and ink solvent uses (OSHA 1997).

The American Conference of Governmental Industrial Hygienists (ACGIH) reduced their Threshold

Limit Value (TLV) (the recommended 8-hour exposure limit) from 100 to 50 ppm (174 mg/m

) in 1988

(ACGIH 1990), which continues to be the recommended TLV level (ACGIH 1998).

5.6 EXPOSURES OF CHILDREN

This section focuses on exposures from conception to maturity at 18 years in humans. Differences from

adults in susceptibility to hazardous substances are discussed in 2.7 Children's Susceptibility.

Children are not small adults. A child's exposure may differ from an adult's exposure in many ways.

Children drink more fluids, eat more food, breathe more air per kilogram of body weight, and have a

larger skin surface in proportion to their body volume. A child's diet often differs from that of adults.

The developing human's source of nutrition changes with age: from placental nourishment to breast milk

or formula to the diet of older children who eat more of certain types of foods than adults. A child's

behavior and lifestyle also influence exposure. Children crawl on the floor, put things in their mouths,

sometimes eat inappropriate things (such as dirt or paint chips), and spend more time outdoors. Children

also are closer to the ground, and they do not use the judgment of adults in avoiding hazards (NRC

1993).

188 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Young children often play close to the ground and frequently play in dirt, which increases their dermal

exposure to toxicants in dust and soil. They also tend to ingest soil, either intentionally through pica or

unintentionally through hand-to-mouth activity. Children, thus, may be orally and dermally exposed to

methylene chloride present as a contaminant in soil and dust. It has been demonstrated that methylene

chloride is rapidly absorbed by the skin (McDougal et al. 1986). Methylene chloride has a K

of 25,

indicating a very low adsorption to soil (Bahnick and Doucette 1988; Roy and Griffin 1985; Swann et al.

1983). Most of the methylene chloride present in the upper layers of the soil will be rapidly volatilized to

air (vapor pressure=349 mmHg at 20 EC). Loss of methylene chloride from the soil decreases the

potential of dermal and oral exposure to children, but its rapid volatilization results in inhalation being the

most likely route of exposure during play on the ground.

The higher ventilation rate in children compared to adults means that for a brief period, children will be

more vulnerable than adults to acute neurological effects from short-term inhalation exposures. However,

differences between children and adults are eliminated as steady-state concentrations are reached, i.e.,

within 2–4 hours (see Section 2.3.1.1). Thus, children would not be expected to be more vulnerable than

adults in the case of intermediate- or chronic-duration exposures. Young children are closer to the ground

or floor because of their shorter stature. The methylene chloride vapors being heavier than air (vapor

density = 2.93) tend to concentrate near the ground. Children, therefore, are at a greater risk of exposure

than adults during accidental spills or through indoor use of methylene chloride in an unventilated area.

Exposures of the embryo or fetus to volatile organic compounds such as methylene chloride may occur if

the expectant mother is exposed. A newborn infant may be exposed by breathing contaminated air and by

ingestion of mother’s milk, which can contain small amounts of methylene chloride. Children may be

exposed through accidental ingestion of products containing methylene chloride. Older children and

adolescents may be exposed to methylene chloride in their jobs or hobbies, or through deliberate solvent

abuse by “sniffing.” Human epidemiological studies and case reports discussing reproductive and/or

developmental toxicity of methylene chloride in humans have been reviewed. Exposure routes included

occupational duties and sniffing of paint removers. Inhalant abuse during pregnancy poses significant

risks to the pregnancy and endangers both the mother and the fetus. Solvent abuse of methylene chloride

for euphoric effects results in exposure levels that equal or exceed those producing adverse effects in

animals.

189 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

There are no existing studies that have monitored the level of exposure from methylene chloride to

children. Most uses of methylene chloride are associated with occupational purposes, so it is unlikely that

children will receive significant doses. Under extreme conditions where paint thinners or other mixtures

containing high concentrations of methylene chloride are used in the presence of children in an enclosed

area with little or no ventilation, children could receive significant exposure. There are studies that

examine the exposure to children from parents’ work clothes, skin, hair, tools, or other objects removed

from the workplace (NIOSH 1995); however, this type of “take home” or secondary exposure is unlikely

due to the high volatility of methylene chloride. Additional exposure from consumer products can occur,

but is unlikely to be significant, although little data are available at this time.

Methylene chloride has been identified, but not quantified, in eight out of eight samples of human breast

milk collected in four urban areas in the United States (EPA 1980d; Pellizzari et al. 1982).

It is not known whether children differ in their weight-adjusted intake of methylene chloride. However,

children drink more fluids per kilogram of body weight than adults (NRC1993) and methylene chloride

has been detected in drinking water (Section 5.4.2).

5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES

In addition to individuals who are occupationally exposed to methylene chloride (see Section 5.5), there

are several groups within the general population that could have potentially high exposures (higher than

background levels) to methylene chloride. These populations include individuals living in proximity to

sites where methylene chloride was produced or sites where methylene chloride was disposed, and

individuals living near 1 of the 1,569 NPL hazardous waste sites where methylene chloride has been

detected in some environmental media (HazDat 1996).

Individuals using consumer products containing substantial amounts of methylene chloride have the

potential for high exposure to this compound (CPSC 1987). Paint strippers, adhesive removers, spray

shoe polishes, adhesives, glues, paint thinners, and many other household products contain enough

methylene chloride to expose consumers to significant amounts of methylene chloride vapor when the

products are used, especially indoors (CPSC 1987, 1990). Indoor air concentrations resulting from the

use of methylene chloride-containing consumer products have been estimated to range from 0.06 to

5,472 ppb (Callahan 1981; NAS 1978; Otson et al. 1983). Previous estimates indicated that a concentra

190 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

tion of 50 ppm (174 mg/m

) of methylene chloride would have been expected in the breathing zone of

consumers following hair spray use (FDA 1985), resulting in a time-weighted average exposure of

0.174 ppm. Hair care specialists would have been exposed to 10 times this level (FDA 1985). However,

this source of exposure has been virtually eliminated since the FDA (1989) banned the use of methylene

chloride in hairsprays.

Coffee drinkers who consume large amounts of decaffeinated coffee which has been extracted with

methylene chloride may be exposed from this source. Assuming all the methylene chloride is extracted

from the beans during brewing and none is volatilized (FDA 1985), the maximum daily intake for a

person drinking 5 cups of decaffeinated coffee is about 12 µg, a fraction of intake from ambient air.

Since volatilization during brewing is very likely, the actual intake is probably much lower. In addition,

this source of exposure is becoming less important, since most decaffeination processes no longer use

methylene chloride.

People living near industrial or hazardous waste sites with higher than average levels of methylene

chloride in the air or water would have potential above-average exposure. However, the magnitude of

this exposure can only be evaluated on a site-by-site basis.

Currently, workers in the industries identified above (Section 5.5) have the highest potential exposure to

methylene chloride. OSHA has determined that a PEL TWA of 25 ppm substantially reduces significant

risk of cancer from methylene chloride in occupational settings. OSHA also believes that a lower limit

would further reduce risk and has set an action level measured as an 8-hour TWA to 12.5 ppm (OSHA

1997).

5.8 ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of methylene chloride is available. Where adequate

information is not available, ATSDR, in conjunction with the National Toxicology Program (NTP), is

required to assure the initiation of a program of research designed to determine the health effects (and

techniques for developing methods to determine such health effects) of methylene chloride.

191 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed.

5.8.1 Identification of Data Needs

Physical and Chemical Properties. Knowledge of the physical and chemical properties is essential

for estimating the partitioning of the chemical and the fate in the environment. Information about the

physical and chemical properties of methylene chloride is adequate (EPA 1982e; Hansch and Leo 1979;

Lide 1994; Roy and Griffin 1982; Verschueren 1983) and can be used to determine the behaviors of the

chemical. No further investigation is required.

Production, Import/Export, Use, Release, and Disposal. As of October 1, 1996, the Interna-

tional Trade Commission ceased to collect or publish annual synthetic organic chemicals data. The

National Petroleum Refiners Association, which currently collects such data, does not include methylene

chloride on its list of organic chemicals. The available production data for methylene chloride are out of

date. It is essential that these data be updated regularly to allow a more accurate determination of the

potential for human exposure.

Remedial investigations and feasibility studies at NPL sites that contain methylene chloride are needed to

provide further information on environmental concentrations and human exposure levels near these sites.

According to the Emergency Planning and Community Right-to-Know Act of 1986, 42 U.S.C. Section

11023, industries are required to submit chemical release and off-site transfer information to the EPA.

The Toxics Release Inventory (TRI), which contains this information for 1996, became available in May

of 1998. This database will be updated yearly and should provide a list of industrial production facilities

and emissions.

Data on the production and uses of methylene chloride in the United States are available (Mannsville

Chemical Products Corporation 1988; NTP 1989; SRI 1999; TRI98 2000), however the parameters in

which companies are monitored for the production of methylene chloride could be better characterized.

192 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Production of methylene chloride has decreased due to declining demands (HSDB 1990; Mannsville

Chemical Products Corporation 1988; USITC 1989). Import/export data are available (Mannsville

Chemical Products Corporation 1988; NTDB 1998) as well as data on land disposal (TRI98 2000), but

more information is needed on disposal by incineration.

Environmental Fate. Methylene chloride is a highly volatile chemical and tends to volatilize from

water and soil to the atmosphere (Dilling et al. 1975; EPA 1979b; Gossett 1987). The half-life of

methylene chloride volatilization from water has been measured (Dilling et al. 1975). Methylene chloride

does not strongly sorb to soils or sediments and can be expected to leach from soils to ground water

(Dilling et al. 1975; Dobbs et al. 1989). Atmospheric lifetimes have been estimated from the reactions of

methylene chloride with hydroxyl radicals and the concentration of such radicals (Altshuller 1980; Cox et

al. 1976; Davis et al. 1976; EPA 1987i). Half-life values for the hydrolysis of methylene chloride in

water has been reported (Dilling et al. 1975). Degradation of methylene chloride has been found to occur

(Davis and Madsen 1991). Biodegradation of methylene chloride has been found to occur under both

aerobic and anaerobic conditions (Leisinger et al. 1994; Magli et al. 1998; Sheehan and Freedman 1996).

More information and data are needed for this process.

Bioavailability from Environmental Media. Data indicates the predominant route of exposure to

methylene chloride is through inhalation (Angelo et al. 1986a; DiVincenzo et al. 1972). Studies also

indicate the compound is readily absorbed following dermal exposure and ingestion (DiVincenzo and

Kaplan 1981; McDougal et al. 1986; Stewart and Dodd 1964). Little information was found on levels of

methylene chloride in food and no studies were found on the levels of methylene chloride in soil.

Additional data on absorption in humans following ingestion and dermal exposures are needed to

establish the importance of consumption of contaminated drinking water and foodstuffs and dermal

contact with consumer products containing methylene chloride as exposure pathways for the general

population.

Food Chain Bioaccumulation. The bioconcentration factor (2.3) estimated for methylene chloride is

low (EPA 1980a, 1984). More data is needed on the bioaccumulation of methylene chloride by plants,

aquatic organisms, and animals, and the biomagnification of methylene chloride in terrestrial and aquatic

food chains. Data on bioaccumulation and biomagnification would aid in determining if levels of

methylene chloride in the environment affect food chains and potentially impact human exposure.

193 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Exposure Levels in Environmental Media. Studies are available documenting levels of methylene

chloride in air (EPA 1988d; Harkov et al. 1984; Otson et al. 1983; Singh et al. 1982), water (Dyksen and

Hess 1982; EPA 1975b, 1985h; Page 1981; Singh et al. 1983; Staples et al. 1985), and sediments (Staples

et al. 1985). More information is needed regarding the levels of methylene chloride in soils and

sediments since there is little information at this time. Since production and use of methylene chloride

have decreased in recent years and are projected to continue the downward trend, it would be valuable to

have recent data to better estimate current human exposure levels from these media. More information is

needed on methylene chloride levels measured in food.

Reliable monitoring data for the levels of methylene chloride in contaminated media at hazardous waste

sites are needed so that the information obtained on levels of methylene chloride in the environment can

be used in combination with the known body burdens of methylene chloride to assess the potential risk of

adverse health effects in populations living in the vicinity of hazardous waste sites.

Exposure Levels in Humans. Methylene chloride has been identified and quantified in human breast

milk (EPA 1980d; Pellizzari et al. 1982). More data is needed to determine the levels of methylene

chloride in blood, urine, and other tissues in the general population, particularly for populations living in

the vicinity of hazardous waste sites that contain methylene chloride. This information is needed to

establish levels of methylene chloride to which the general population has been exposed through contact

with contaminated air, drinking water, and consumer products. This information is necessary for

assessing the need to conduct health studies on these populations.

Exposures of Children. There are no studies monitoring the level of exposure of children to

methylene chloride. This data gap requires future studies to determine the exposure of methylene

chloride to children and if the significant exposures can be decreased by any means. Additional studies

are needed to examine various exposure pathways that are unique to children. Studies are also needed to

examine children’s weight-adjusted intake of methylene chloride.

Despite their higher ventilation rate per kilogram of body weight compared to adults, children are not at a

greater risk of inhalation exposure to methylene chloride, except, initially for acute exposures. Steady

state considerations tend to eliminate the difference between children and adults after a short time (see

Section 2.3.1.1). They also spend more time closer to ground because of their height. Methylene

chloride vapors, being heavier than air, tend to concentrate closer to the ground, thereby increasing the

194 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

risk for children. No data are available on the exposure of children to methylene chloride present in the

air.

A study on usefulness of intervention methods in cases of inhalant abuse by pregnant women is required.

More research is needed to rule out concomitant risk factors and to identify specific chemicals and

patterns of use associated with adverse effects.

Child health data needs relating to susceptibility are discussed in 2.12.2, Identification of Data Needs:

Children’s Susceptibility.

Exposure Registries. No exposure registries for methylene chloride were located. This substance is

not currently one of the compounds for which a subregistry has been established in the National Exposure

Registry. The substance will be considered in the future when chemical selection is made for

subregistries to be established. The information that is amassed in the National Exposure Registry

facilitates the epidemiological research needed to assess adverse health outcomes that may be related to

exposure to this substance.

5.8.2 Ongoing Studies

As part of the ongoing Third National Health and Nutrition Evaluation Survey (NHANES III), the

Environmental Health Laboratory Sciences Division of the National Center for Environmental Health,

Centers for Disease Control and Prevention, is analyzing human blood samples for methylene chloride

and other volatile organic compounds. These data will give an indication of the frequency of occurrence

and background levels of these compounds in the general population.

A number of ongoing research efforts may provide data regarding the potential for human exposure to

methylene chloride. These projects are summarized in Table 5-3.

195 METHYLENE CHLORIDE

5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-3. Ongoing Studies on the Potential for Human Exposure to

Methylene Chloride

Investigator Affiliation Research Description Sponsor

Argonne National

Laboratory,

Lemont, IL

Destruction/conversion of

hazardous waste chlorocarbons

Smith, G.B. New Mexico State

University,

Las Cruces, NM

Pollutant toxicity reduction through

biodegradation

NIGMS

Anders, M.W. University of

Rochester,

Rochester, NY

Metabolism and toxicity of

halogenated hydrocarbons

NIEHS

Hemingway,

R.W.

Forest Service,

Pineville, LA

Knot VOCs USDA, CRGO

Morra, M.J. University of Idaho,

Moscow, ID

Biotic and abiotic degradation of

glucosinolates and halogenated

hydrocarbons in soil

USDA, CRGO

Scow, K.M. University of California,

Davis, CA

Microbial biodegradation of

organic chemicals in soil

USDA, CRGO

Cook, R. TDA Research, Inc.

Wheat Ridge, CO

Catalysts for the oxidation of

chlorinated hydrocarbons

EPA

Dragan, A. Dragan Engineering,

Encino, CA

Biofilteration of volatile organic

compounds emitted a industrial

waste treatment

Wijmans, J.G. Membrane Technology

and Research,

Menlo Park, NJ

Recovery of liquid hazardous

wastes from carbon adsorption

steam regeneration streams

EPA

Pfefferle, W. Precision Combustion,

New Haven, CT

Catalytically stabilized thermal

incineration

NSF

Bohn, M.S. National Renewable

Energy Laboratory,

Golden, CO

Incineration of hazardous

materials with carbonate salts

USDOE

Strand, S.E. University of

Washington,

Seattle, WA

Using trees to remediate

groundwaters contaminated with

chlorinated hydrocarbons

Bernhard, R.A. University of California,

Davis, CA

Isolation, identification, and

significance of micro-organic

constituents in foods

AF = Air Force; EPA = Environmental Protection Agency; NIEHS = National Institute of Environmental Health

Sciences; NIGMS = National Institute of General Medical Sciences; NSF = National Science Foundation;

USDA, CRGO = U.S. Department of Agriculture, Competitive Research Grant Office; USDOE = U.S. Department of

Energy

197 METHYLENE CHLORIDE

6. ANALYTICAL METHODS

The purpose of this chapter is to describe the analytical methods that are available for detecting,

measuring, and/or monitoring methylene chloride, its metabolites, and other biomarkers of exposure and

effect to methylene chloride. The intent is not to provide an exhaustive list of analytical methods.

Rather, the intention is to identify well-established methods that are used as the standard methods of

analysis. Many of the analytical methods used for environmental samples are the methods approved by

federal agencies and organizations such as EPA and the National Institute for Occupational Safety and

Health (NIOSH). Other methods presented in this chapter are those that are approved by groups such as

the Association of Official Analytical Chemists (AOAC) and the American Public Health Association

(APHA). Additionally, analytical methods are included that modify previously used methods to obtain

lower detection limits and/or to improve accuracy and precision.

6.1 BIOLOGICAL SAMPLES

Analysis of biological materials for methylene chloride is performed most frequently by gas

chromatography with a flame ionization detector (GC/FID) (DiVincenzo et al. 1971; Engstrom and

Bjurstrom 1977). Table 6-1 summarizes the data for specific methods for biological fluids and tissues.

Separation of methylene chloride from biological samples in preparation for GC/FID analysis is most

often achieved by heating the sample in a sealed flask until the analyte concentration is in equilibrium in

the sample matrix and the headspace vapor. The headspace vapor is then drawn off for analysis by gas

chromatography (GC). Headspace sampling eliminates the need for a solvent extraction procedure, thus

simplifying sample preparation and improving sensitivity (DiVincenzo et al. 1971). However,

partitioning of the analyte between the headspace and the sample matrix depends on the nature of the

matrix and must be determined separately for each different kind of matrix (Walters 1986). Acid

hydrolysis of adipose tissue is required prior to headspace sampling (Engstrom and Bjurstrom 1977).

6.2 ENVIRONMENTAL SAMPLES

Several methods are available for the determination of methylene chloride in air, water, soil/sediments,

and food. Table 6-2 summarizes several representative methods appropriate for quantifying methylene In

most analytical methods, methylene chloride is trapped on a solid sorbent such as activated charcoal. Air

198

Table 6-1. Analytical Methods for Determining Methylene Chloride in Biological Materials

Analytical

Sample

detection Percent

Sample matrix Preparation method method limit recovery Reference

Blood Heat sample, collect headspace vapor GC/FID 0.022 ppm 49.8±1.33 DiVincenzo et al. 1971

Urine Heat sample, collect headspace vapor GC/FID No data 59±2.75 DiVincenzo et al. 1971

Urine Heat sample, inject headspace air, loop GC/MS 0.5 ppb 95% Ghittori et al. 1993

Breath Heat sample, inject into gas sample, loop GC/FID 0.2±0.1 ppm No data DiVincenzo et al. 1971

Adipose tissue Hydrolyze with acid, heat sample, collect

headspace vapor

GC/FID 1.6 ppm

No data Engstrom and Bjurstrom 1977

Human milk Purge with helium, trap on sorbent trap,

desorb thermally

GC/MS No data No data Pellizzari et al. 1982

Lowest reported concentration

FID = flame ionization detector; GC = gas chromatography; MS = mass spectrometry

METHYLENE CHLORIDE

6. ANALYTICAL METHODS

199

Table 6-2. Analytical Methods for Determining Methylene Chloride in Environmental Samples

Sample matrix Preparation method

Analytical

method

Sample

detection

limit

Percent

recovery Reference

Air Adsorb on charcoal, desorb with carbon

disulfide

GC/FID 25 ppb

90–100

APHA 1977

Air Adsorb on charcoal, desorb with carbon

disulfide

GC/MS No data No data Ghittori et al. 1993

Air Adsorb on charcoal, desorb with carbon

disulfide

GC/MS No data No data Xiao et al. 1993

Air Adsorb on charcoal, desorb with carbon

disulfide

GC/FID 2,900 ppb

0.01 mg/sample

95.3 NIOSH 1994

[Method 1005]

Air Extract with methylene chloride, purge with

helium, desorb thermally

GC/MS 0.05 ppm -100 Savard et al. 1992

Air Adsorb on charcoal, desorb with benzyl

alcohol

GC/ECD .0.5 ppb No data Woodrow et al. 1988

Air Purge with nitrogen, direct beam with

mirrors to detect signal

RS-FTIR 0.57 ppm-m 99.7–100 Xiao et al. 1993

Air Adsorb on activated charcoal, desorb with

carbon disulfide

Toxic Gas

Vapor Detector

Tube

No data No data EMMI 1999a

Air Adsorb on activated charcoal, desorb with

carbon disulfide

GC/FID No data No data EMMI 1999b

Air Aspirate with a pump through detector tube Toxic Gas

Vapor Detector

Tube

50 ppm No data EMMI 1999c

Stack gas

effluents

Sorption onto Tenax

, thermal desorption GC/MS No data 50–150 EPA 1986e

OSW 5041A

METHYLENE CHLORIDE

6. ANALYTICAL METHODS

200

Table 6-2. Analytical Methods for Determining Methylene Chloride in Environmental Samples(continued)

Analytical

Sample

detection Percent

Sample matrix Preparation method method limit recovery Reference

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with inert gas, trap on sorbent trap,

desorb thermally

Water Purge with helium, trap on sorbent trap,

desorb thermally

Water Purge with helium, trap on sorbent trap,

desorb thermally

Waste water Purge with helium, trap on sorbent trap,

desorb thermally

Waste water Purge with inert gas, trap on sorbent trap,

desorb thermally

Waste water Purge with inert gas, trap on sorbent trap,

desorb thermally

GC/HSD

GC/ELCD

GC/MS

HRGC/MS

HRGC/ELCD

HRGC/MS

GC/MS

GC/MS or

GC/PID or

GC/ECD

GC/MS

No data

0.01 ppb

1.0 ppb

0.03–0.09 ppb

0.01–0.05 ppb

0.02–0.2 ppb

No data

0.099 ppb

0.5 ppb

0.25 ppb

2.8 ppb

97–100

95–97

97±28

95±5

99–105

80–120

25–162

D–221

EPA 1989f

EPA 1989g

EPA 1989c

EPA1989b

APHA 1989a

APHA 1989b

Michael et al. 1988

APHA 1998a

APHA 1998b

EPA 1998h [Method 601]

EPA 1998i [Method 624]

METHYLENE CHLORIDE

6. ANALYTICAL METHODS

201

Table 6-2. Analytical Methods for Determining Methylene Chloride in Environmental Samples(continued)

Sample matrix Preparation method

Analytical

method

Sample

detection

limit

Percent

recovery Reference

Solid/

Solid Waste

Purge with inert gas, trap on sorbent trap,

desorb thermally

GC/MS 0.03 ppb 95 EPA 1996b [Method 8260]

Solid/

Solid Waste

Purge with inert gas, trap on sorbent trap,

desorb thermally; or inject directly into GC

GC/PID/HECD 0.02 ppb 95±2.8 EPA 1996c [Method 8021]

Soil/sediment/

solid waste

Headspace extraction GC/FID or

GC/PID/ELCD

No data No data EPA 1986e

OSW 5021

Soil/sediment Extract with methanol, purge and trap GC/ECD No data 25–162 EPA 1986e

OSW 8010B

Solid waste

matrices

Purge and trap or direct injection GC/ECD/PID 0.02 ppb 97 EPA 1986e

OSW 8021B-PID

Solid waste

matrices

Headspace extraction, purge and trap GC/MS 5 ppb 0–0221 EPA 1986e

OSW 8240B-S

Solid waste

matrices

Purge and trap or direct injection GC/MS 0.03 ppb 95 EPA 1986e

OSW 8260B

Food Equilibrate in heated sodium sulfate

solution, collect headspace vapor

GC/ELCD 0.05 ppm No data Page and Charbonneau

1984

Food Isolate solvent by closed system vacuum

distillation with toluene as carrier solvent

GC/ELCD 7 ng 94 Page and Charbonneau

1984

Food Isolate solvent by closed system vacuum

distillation with toluene as carrier solvent

GC/ECD 7 ng 100 Page and Charbonneau

1984

METHYLENE CHLORIDE

6. ANALYTICAL METHODS

202

Table 6-2. Analytical Methods for Determining Methylene Chloride in Environmental Samples(continued)

Sample matrix Preparation method

Analytical

method

Sample

detection

limit

Percent

recovery Reference

Food Purge with nitrogen, trap on sorbent trap,

elute with hexane

GC/ELCD 1.2 ppb

84–96 Heikes 1987

Food Extract with acetone-water, back extract

with isooctane

GC/ELCD 4 ppb 66 Daft 1987

Lowest value for various compounds reported during collaborative testing of this method.

Estimated accuracy of the method when the personal sampling pump is calibrated with a charcoal tube in the line.

Lowest reported concentration.

D = detected; ECD = electron capture detector; ELCD = electrolytic conductivity detector; FID = flame ionization detector; GC = gas chromatography; HECD =

electrolytic conduction detector; HRGC = high resolution gas chromatography; HSD = halogen specific detector; MS = mass spectrometry; PID = photo ionization

detector; RS-FTIR = remote sensing Fourier transform infrared

METHYLENE CHLORIDE

6. ANALYTICAL METHODS

203 METHYLENE CHLORIDE

6. ANALYTICAL METHODS

samples are drawn directly through the sorbent (APHA 1977; NIOSH 1984). For water, soil, or solid

chloride in each of these environmental media. The EPA and NIOSH methods are standardized to

comply with regulatory requirements.

waste samples, methylene chloride is purged from the sample with an inert gas such as helium, and then

passed through the sorbent (APHA 1989a, 1989b; EPA 1989c, 1989f, 1989g). Desorption is thermal or

by carbon disulfide. Vacuum distillation or solvent extraction are sometimes used for separation of

methylene chloride from food samples (Daft 1987; Page and Charbonneau 1977).

Following separation of the organic compounds by GC, methylene chloride is detected by one of several

types of instruments: a flame ionization detector (FID), electron capture detector (ECD), electrolytic

conductivity detector (ELCD) or halogen specific detector (HSD). A mass spectrometer (MS) may be

used for unequivocal identification. While the ELCD appears to be most sensitive, detection limits for all

these methods are well below levels of health concern, but are not below EPA calculated cancer risk

levels or all MRLs.

Several physical parameters may interfere with analytical accuracy. High sampling flow rates and high

temperature and humidity may cause decreased adsorption of methylene chloride vapor on the solid

sorbent (APHA 1977). In addition, methylene chloride is a common laboratory contaminant and is

frequently found in laboratory blanks and in the environment (e.g., soil and water samples).

6.3 ADEQUACY OF THE DATABASE