TOXICOLOGICAL PROFILE FOR
 

METHOXYCHLOR
 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
 

Public Health Service
 

Agency for Toxic Substances and Disease Registry
 

September 2002 
METHOXYCHLOR ii 

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. 
METHOXYCHLOR iii 

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
 

V 

FOREWORD 

This toxicological profile is prepared in accordancewith guidelines* developed by the Agency for 

Toxic Substances and Disease Registry (ATSDR) and the Environmental ProtectionAgency (EPA). The 

original guidelines were published in the Federd Register on April 17, 1987. Each profile will be revised 

and republished as necessary. 

The ATSDR toxicological profile succinctly characterizesthe toxicologic and adverse health 

effects information for the hazardous substance described therein. Each peer-reviewed profile identifies 

and reviews the key literaturethat describes a hazardous substance's toxicologic properties. Other 

pertinent literature is also presented, but is described in less detail than the key studies. The profile is not 

intended to be an exhaustivedocument; however, more comprehensive sources of specialtyinformation 

are referenced. 

The focus of the profiles is on health and toxicologic information; therefore, each toxicological 

profile begins with a public health statement that describes, in nontechnical language, a substance's 

relevant toxicological properties. Following the public health statement is information concerninglevels 

of significant human exposure and, where known, significanthealth effects. The adequacy of 

information to determine a substance's health effects is described in a health effects summary. Data 

needs that are of significanceto protection of public health are identified by ATSDR and EPA. 

Each profile includesthe following: 

The examination, summary,and interpretationof available toxicologic information and 

epidemiologicevaluationson a hazardous substance to ascertain the levels of significant 

human exposure for the substance and the associated acute, subacute, and chronic health 

effects; 

A determination of whether adequate information on the health effects of each substance is 

available or in the process of development to determine levels of exposure that present a 

significantrisk to human health of acute, subacute, and chronic health effects; and 

Where appropriate, identification of toxicologictesting needed to identify the types or levels 

of exposure that may present significant risk of adverse health effects in humans. 

The principal audiences for the toxicologicalprofiles are health professionals at the federal, state, 

and local levels; interested private sector organizationsand groups; and members of the public. 

This profile reflects ATSDR's assessment of all relevant toxicologic testing and information that 

has been peer-reviewed. Staff of the Centers for Disease Control and Prevention and other federal 

scientists have also reviewed the profile. In addition, this profile has been peer-reviewed by a 

nongovernmental panel and was made available for public review. Final responsibility for the contents 

and views expressed in this toxicological profile resides with ATSDR. 

ouise Gerberdingb., M.P& 

Administrator Administrator 

Agency for Toxic Substancesand 

Disease Registry 
METHOXYCHLOR vi 

*Legislative Background 

The toxicological profiles are developed in response to the Superfund Amendments and 

Reauthorization Act (SARA) of 1986 (Public law 99-499) which amended the Comprehensive 

Environmental Response, Compensation, and Liability Act of 1980 (CERCLA or Superfund). This 

public law directed ATSDR to prepared toxicological profiles for hazardous substances most commonly 

found at facilities on the CERCLA National Priorities List and that pose the most significant potential 

threat to human health, as determined by ATSDR and the EPA. The availability of the revised priority 

list of 275 hazardous substances was announced in the Federal Register on November 17, 1997 (62 FR 

61332). For prior versions of the list of substances, see Federal Register notices dated April 29, 1996 (61 

FR 18744); April 17, 1987 (52 FR 12866); October 20, 1988 (53 FR 41280); October 26, 1989 (54 FR 

43619); October 17, 1990 (55 FR 42067); October 17, 1991 (56 FR 52166); October 28, 1992 (57 FR 

48801); and February 28, 1994 (59 FR 9486). Section 104(i)(3) of CERCLA, as amended, directs the 

Administrator of ATSDR to prepare a toxicological profile for each substance on the list. 
 

 

METHOXYCHLOR vii 

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: Relevance to Public Health: The Relevance to Public Health Section evaluates, interprets, 

and assesses the significance of toxicity data to human health. 

Chapter 3: Health Effects: Specific health effects of a given hazardous compound are reported by type 

of health effect (death, systemic, immunologic, reproductive), by route of exposure, 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 Methoxychlor Affect Children?
 

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

Section 3.7 Children’s Susceptibility
 

Section 6.6 Exposures of Children
 

Other Sections of Interest: 

Section 3.8 Biomarkers of Exposure and Effect 

Section 3.11 Methods for Reducing Toxic Effects 

ATSDR Information Center 

Phone: 1-888-42-ATSDR or (404) 498-0110 Fax: (404) 498-0057 

E-mail: atsdric@cdc.gov 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. 
METHOXYCHLOR viii 

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: AOEC@AOEC.ORG • Web Page: http://www.aoec.org/. 

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: 847-818-1800 • FAX: 847-818-9266. 
METHOXYCHLOR ix 

CONTRIBUTORS 

CHEMICAL MANAGER(S)/AUTHORS(S): 

Lori L. Miller, M.P.H.
 

ATSDR, Division of Toxicology, Atlanta, GA
 

M. Olivia Harris, M.A.
 

ATSDR, Division of Toxicology, Atlanta, GA
 

Susan S. Little, Ph.D.
 

Syracuse Research Corporation, Atlanta, GA
 

Peter R. McClure, Ph.D, D.A.B.T.
 

Syracuse Research Corporation, Syracuse, NY
 

William R. Sutton, Ph.D.
 

Syracuse Research Corporation, Arlington, VA
 

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. 

METHOXYCHLOR xi 

PEER REVIEW
 

A peer review panel was assembled for methoxychlor. The panel consisted of the following members: 

1.	 Dr. Martin Alexander, Professor, Department of Crop and Soil Sciences, Cornell University, Ithaca, 

New York. 

2.	 Dr. Neena Schwartz, Professor, Department of Neurobiology and Physiology, Northwestern 

University, Evanston, Illinois. 

3.	 Dr. William Toscano, Professor and Division Head, Environmental and Occupational Health, 

University of Minnesota, Minneapolis, Minnesota. 

These experts collectively have knowledge of methoxychlor'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. 

METHOXYCHLOR xiii 

CONTENTS
 

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

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

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

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

LIST OF FIGURES ................................................................ xvii 
 

LIST OF TABLES .................................................................. xix 
 

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

1.1 WHAT IS METHOXYCHLOR? ................................................ 1 
 

1.2 WHAT HAPPENS TO METHOXYCHLOR WHEN IT ENTERS THE ENVIRONMENT? . 2
 

1.3 HOW MIGHT I BE EXPOSED TO METHOXYCHLOR? ............................ 3 
 

1.4 HOW CAN METHOXYCHLOR ENTER AND LEAVE MY BODY? .................. 5 
 

1.5 HOW CAN METHOXYCHLOR AFFECT MY HEALTH? ........................... 5 
 

1.6	 HOW CAN METHOXYCHLOR AFFECT CHILDREN? ............................ 7 
 

1.7	 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO METHOXYCHLOR? . . 8
 

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

TO METHOXYCHLOR? ..................................................... . 9
 

1.9	 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO
 

PROTECT HUMAN HEALTH? ............................................... 10 
 

1.10 WHERE CAN I GET MORE INFORMATION? .................................. 11 
 

2. RELEVANCE TO PUBLIC HEALTH ............................................... 12
 

2.1	 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO METHOXYCHLOR IN 

THE UNITED STATES ...................................................... 12
 

2.2	 SUMMARY OF HEALTH EFFECTS ........................................... 13
 

2.3	 MINIMAL RISK LEVELS ................................................... 15
 

3. HEALTH EFFECTS .............................................................. 19 
 

3.1 INTRODUCTION .......................................................... 19 
 

3.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE ................. 20 
 

3.2.1 Inhalation Exposure .................................................. 21 
 

3.2.1.1 Death ..................................................... 21 
 

3.2.1.2 Systemic Effects ............................................ 22 
 

3.2.1.3 Immunological and Lymphoreticular Effects ...................... 22 
 

3.2.1.4 Neurological Effects ......................................... 22
 

3.2.1.5 Reproductive Effects ......................................... 23 
 

3.2.1.6 Developmental Effects ....................................... 23 
 

3.2.1.7 Cancer .................................................... 23
 

3.2.2 Oral Exposure ...................................................... 24 
 

3.2.2.1 Death ..................................................... 24 
 

3.2.2.2 Systemic Effects ............................................ 24
 

3.2.2.3 Immunological and Lymphoreticular Effects ...................... 46 
 
 

 

METHOXYCHLOR xiv 

3.2.2.4 Neurological Effects ......................................... 47 
 

3.2.2.5 Reproductive Effects ......................................... 49
 

3.2.2.6 Developmental Effects ....................................... 57 
 

3.2.2.7 Cancer .................................................... 61
 

3.2.3 Dermal Exposure .................................................... 62
 

3.2.3.1 Death ..................................................... 62
 

3.2.3.2 Systemic Effects ............................................ 65 
 

3.2.3.3 Immunological and Lymphoreticular Effects ...................... 65 
 

3.2.3.4 Neurological Effects ......................................... 65 
 

3.2.3.5 Reproductive Effects ......................................... 66 
 

3.2.3.6 Developmental Effects ....................................... 66 
 

3.2.3.7 Cancer .................................................... 66 
 

3.3 GENOTOXICITY .......................................................... 67 
 

3.4 TOXICOKINETICS ......................................................... 71 
 

3.4.1 Absorption ......................................................... 72 
 

3.4.1.1 Inhalation Exposure .......................................... 72 
 

3.4.1.2 Oral Exposure .............................................. 72 
 

3.4.1.3 Dermal Exposure ............................................ 73 
 

3.4.2 	 Distribution ........................................................ 73 
 

3.4.2.1 Inhalation Exposure .......................................... 74 
 

3.4.2.2 Oral Exposure .............................................. 74 
 

3.4.2.3 Dermal Exposure ............................................ 76 
 

3.4.2.4 Other Routes of Exposure ..................................... 76 
 

3.4.3	 Metabolism ......................................................... 77
 

3.4.4	 Elimination and Excretion ............................................. 85
 

3.4.4.1 Inhalation Exposure .......................................... 86 
 

3.4.4.2 Oral Exposure .............................................. 86 
 

3.4.4.3 Dermal Exposure ............................................ 86 
 

3.4.4.4 Other Routes of Exposure ..................................... 87 
 

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

Models ............................................................ 87 
 

3.5 MECHANISMS OF ACTION ................................................. 88 
 

3.5.1 	 Pharmacokinetic Mechanisms .......................................... 90 
 

3.5.2 	 Mechanisms of Toxicity ............................................... 91 
 

3.5.3	 Animal-to-Human Extrapolations ....................................... 96
 

3.6 TOXICITIES MEDIATED THROUGH THE NEUROENDOCRINE AXIS ............. 98 
 

3.7 CHILDREN’S SUSCEPTIBILITY ............................................ 104
 

3.8 BIOMARKERS OF EXPOSURE AND EFFECT ................................. 109
 

3.8.1 Biomarkers Used to Identify or Quantify Exposure to Methoxychlor ........... 110
 

3.8.2 Biomarkers Used to Characterize Effects Caused by Methoxychlor ............ 112 
 

3.9 INTERACTIONS WITH OTHER CHEMICALS ................................. 113 
 

3.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE ...................... 114 
 

3.11 METHODS FOR REDUCING TOXIC EFFECTS ................................ 115 
 

3.11.1 Reducing Peak Absorption Following Exposure ........................... 115
 

3.11.2 Reducing Body Burden .............................................. 116 
 

3.11.3 Interfering with the Mechanism of Action for Toxic Effects .................. 116 
 

3.12 ADEQUACY OF THE DATABASE ........................................... 117 
 

3.12.1	 Existing Information on Health Effects of Methoxychlor .................... 117 
 

3.12.2 	 Identification of Data Needs .......................................... 119 
 

3.12.3	 Ongoing Studies .................................................... 137 
 
 

METHOXYCHLOR xv 

4. CHEMICAL AND PHYSICAL INFORMATION ...................................... 139 
 

4.1 CHEMICAL IDENTITY .................................................... 139 
 

4.2 PHYSICAL AND CHEMICAL PROPERTIES ................................... 141 
 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL ............................ 143 
 

5.1 PRODUCTION ........................................................... 143 
 

5.2 IMPORT/EXPORT ........................................................ 144 
 

5.3 USE ..................................................................... 144 
 

5.4 DISPOSAL ............................................................... 146 
 

6. POTENTIAL FOR HUMAN EXPOSURE ........................................... 147 
 

6.1 OVERVIEW .............................................................. 147 
 

6.2 RELEASES TO THE ENVIRONMENT ........................................ 149 
 

6.2.1 Air .............................................................. 149 
 

6.2.2 Water ............................................................ 151 
 

6.2.3 Soil .............................................................. 151 
 

6.3 ENVIRONMENTAL FATE .................................................. 152 
 

6.3.1 Transport and Partitioning ............................................ 152 
 

6.3.2 Transformation and Degradation ....................................... 154 
 

6.3.2.1 Air ...................................................... 155 
 

6.3.2.2 Water .................................................... 155 
 

6.3.2.3 Sediment and Soil .......................................... 156 
 

6.3.2.4 Other Media ............................................... 157 
 

6.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT ............... 158 
 

6.4.1 Air .............................................................. 158 
 

6.4.2 Water ............................................................ 158 
 

6.4.3 Sediment and Soil .................................................. 159 
 

6.4.4 Other Environmental Media ........................................... 160 
 

6.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE .................. 161 
 

6.6 EXPOSURES OF CHILDREN ............................................... 163 
 

6.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES ...................... 165
 

6.8 ADEQUACY OF THE DATABASE ........................................... 166 
 

6.8.1 Identification of Data Needs .......................................... 166 
 

6.8.2 Ongoing Studies .................................................... 172 
 

7. ANALYTICAL METHODS ....................................................... 173 
 

7.1 BIOLOGICAL MATERIALS ................................................ 173 
 

7.2 ENVIRONMENTAL SAMPLES .............................................. 174 
 

7.3 ADEQUACY OF THE DATABASE ........................................... 176 
 

7.3.1 Identification of Data Needs .......................................... 179 
 

7.3.2 Ongoing Studies .................................................... 181 
 

8. REGULATIONS AND ADVISORIES .............................................. 183 
 

9. REFERENCES ................................................................. 191 
 

10. GLOSSARY .................................................................. 238
 

APPENDICES 
METHOXYCHLOR xvi 

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

E. USER’S GUIDE ........................................................... B-1 
 

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

G. INDEX .................................................................. D-1 
 
METHOXYCHLOR xvii 

LIST OF FIGURES
 

3-1 Levels of Significant Exposure to Methoxychlor - Oral .................................. 38 
 

3-2 Proposed Metabolic Pathways of Methoxychlor ....................................... 79 
 

3-3 Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model for a 

Hypothetical Chemical Substance ................................................. 89 
 

3-4 Existing Information on Health Effects of Methoxychlor ............................... 118 
 

6-1 Frequency of NPL Sites with Methoxychlor Contamination ............................. 148 
 

METHOXYCHLOR xix 

LIST OF TABLES
 

3-1 Levels of Significant Exposure to Methoxychlor - Oral .................................. 26 
 

3-2 Levels of Significant Exposure to Methoxychlor - Dermal ............................... 64 
 

3-3 Genotoxicity of Methoxychlor In Vivo ............................................... 68 
 

3-4 Genotoxicity of Methoxychlor In Vitro .............................................. 69 
 

3-5 Ongoing Studies on the Health Effects of Methoxychlor ................................ 138 
 

4-1 Chemical Identity of Methoxychlor ................................................ 140 
 

4-2 Physical and Chemical Properties of Methoxychlor .................................... 142 
 

5-1 Facilities that Produce, Process, or Use Methoxychlor ................................. 145 
 

6-1 Releases to the Environment from Facilities that Produce, Process, or Use Methoxychlor ...... 150 
 

7-1 Analytical Methods for Determining Methoxychlor in Biological Samples ................. 175 
 

7-2 Analytical Methods for Determining Methoxychlor in Environmental Samples .............. 177 
 

8-1 Regulations and Guidelines Applicable to Methoxychlor ............................... 184 
 

METHOXYCHLOR 1 

1. PUBLIC HEALTH STATEMENT 

This public health statement tells you about methoxychlor 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. Methoxychlor has been found in at least 58 of the 

1,613 current or former NPL sites. However, the total number of NPL sites evaluated for 

methoxychlor is not known. As more sites are evaluated, the sites at which methoxychlor is 

found may increase. This information is important because exposure to methoxychlor 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 methoxychlor, 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 METHOXYCHLOR? 

Methoxychlor, also known as DMDT, Marlate®, or Metox®, is a manufactured chemical now 

used in the United States for controlling insects. Methoxychlor is effective against flies, 

mosquitos, cockroaches, and a wide variety of other insects. This insecticide is used on 

agricultural crops and livestock, and in animal feed, barns, and grain storage bins. Some 

pesticide products that contain methoxychlor are used for controlling insects in gardens or on 

pets. 
METHOXYCHLOR 2 

1. PUBLIC HEALTH STATEMENT 

Pure methoxychlor is a pale-yellow powder that has a slightly fruity or musty odor. It does not 

readily evaporate into air or dissolve in water. Pesticide workers usually dissolve methoxychlor 

in a petroleum-based liquid and apply it as a spray, or they mix it with other chemicals and apply 

it as a dust. Application of methoxychlor as an insect killer accounts for most of the 

methoxychlor that enters the environment. Since the use of methoxychlor is highly seasonal, the 

amount that is released to the environment tends to be greater during periods of insect control 

(spring and summer). Some methoxychlor is released to the environment from chemical 

manufacturing plants that make methoxychlor or from manufacturing sites that formulate 

products containing methoxychlor. A small amount may also be released from hazardous waste 

sites where it has been disposed of. 

More complete information on the sources, properties, and uses of methoxychlor can be found in 

Chapters 4 and 5 of this profile. 

1.2	 WHAT HAPPENS TO METHOXYCHLOR WHEN IT ENTERS THE 

ENVIRONMENT? 

Methoxychlor does not occur naturally in the environment. Most methoxychlor enters the 

environment when it is applied to forests, agricultural crops, and farm animals. Methoxychlor 

can be applied to forests and crops by aerial spraying. This process can contaminate nearby land 

and water. Methoxychlor that is released into the air will eventually settle to the ground, 

although some may travel long distances before settling. Rain and snow cause methoxychlor to 

settle to the ground more quickly. 

Once methoxychlor is deposited on the ground, it becomes bound to the soil. Because of this, 

methoxychlor does not tend to move rapidly from one place to another. However, soil particles 

that contain methoxychlor can be blown by the wind or be carried by rainwater or melted snow 

into rivers or lakes. Most methoxychlor stays in the very top layer of soil, but some of the 

products that it breaks down into may move deeper into the ground. Smaller amounts of 

methoxychlor in air may settle directly into rivers, lakes, and other surface waters. Once 
METHOXYCHLOR 3 

1. PUBLIC HEALTH STATEMENT 

methoxychlor is in water, it usually binds to sediments or organic matter and settles to the 

bottom. 

Methoxychlor is broken down in the environment by several processes. However, these 

processes are slow and may take months. In soil, some methoxychlor is broken down by 

bacteria and other microorganisms, and some is broken down by a reaction with water or 

materials in soil. In air and water, some methoxychlor is broken down by sunlight. 

Methoxychlor is also broken down by reactive chemicals normally present in the air. Some of 

the breakdown products are capable of producing harmful effects similar to those caused by 

exposure to methoxychlor. 

Methoxychlor can accumulate in some living organisms, including algae, bacteria, snails, clams, 

and some fish. However, most fish and animals change methoxychlor into other substances that 

are rapidly released from their bodies, so methoxychlor does not usually build up in the food 

chain. 

More complete information on the environmental fate of methoxychlor can be found in 

Chapter 6 of this profile. 

1.3 HOW MIGHT I BE EXPOSED TO METHOXYCHLOR? 

Most people are not exposed to methoxychlor on a regular basis. Although methoxychlor is not 

usually detected in air, people can be exposed to low levels of methoxychlor by inhaling dusts 

and aerosols in air surrounding areas where methoxychlor is used. Since methoxychlor is not 

usually detected in surface or well water sources, exposure from drinking water is not likely to 

be significant for the general public. However, surface water that has been treated with 

methoxychlor for control of insect larvae should be avoided until methoxychlor residue has 

decreased below the level of concern. Methoxychlor is not usually found in food. However, low 

levels are sometimes detected in foods obtained from areas where methoxychlor has been used. 
METHOXYCHLOR 4 

1. PUBLIC HEALTH STATEMENT 

Fish usually do not contain detectable levels of methoxychlor, but people who eat fish caught in 

water contaminated with methoxychlor may occasionally have above-average intakes of 

methoxychlor. 

People who make or use methoxychlor may be exposed by breathing in the dust or aerosol, or by 

getting it on their skin. For example: 

C	 If you work in a factory that makes methoxychlor or products containing methoxychlor, 

you could be exposed to methoxychlor in air or on your skin during work hours. The 

government has estimated that approximately 3,400 people may be exposed to 

methoxychlor in this way. 

C	 Methoxychlor is present in some pesticides used for home gardening or for spraying 

pets (such as cats and dogs). If you use these products, you could be exposed to aboveaverage levels of methoxychlor in air and on your skin. 

C	 If you live or work on or near a farm where methoxychlor is used on crops or livestock, 

you could be exposed to above-average levels of methoxychlor in air, soil, and possibly 

in water. 

If you live near a hazardous waste site that contains methoxychlor, you could be exposed by 

breathing in methoxychlor from the air, by swallowing contaminated soil or water, or by getting 

contaminated soil or water on your skin. The amount of exposure you receive depends on 

conditions specific to where you live and can only be evaluated on a case-by-case basis. 

More information on how you may be exposed to methoxychlor can be found in Chapter 6. 
METHOXYCHLOR 5 

1. PUBLIC HEALTH STATEMENT 

1.4 HOW CAN METHOXYCHLOR ENTER AND LEAVE MY BODY? 

Scientists do not know how much or how quickly methoxychlor is absorbed into your body if 

you breathe it in or if it contacts your skin. If you get methoxychlor-contaminated soil or water 

on your skin, some of it may pass through your skin and enter your bloodstream. If you breathe 

methoxychlor-containing dust into your lungs, some of the dust will deposit in your lungs. Dust 

that deposits in the upper part of your lungs is likely to be coughed up and swallowed. Dust that 

deposits deep in your lungs is likely to remain long enough for the methoxychlor to pass through 

the lining of your lungs and enter your bloodstream. If you swallow food, water, or soil 

containing methoxychlor, most of it will rapidly pass through the lining of your stomach and 

intestines and enter your bloodstream. 

Once methoxychlor enters your bloodstream, it is distributed to all parts of your body. Animal 

studies suggest that methoxychlor is changed into other substances called metabolites by your 

liver. Most of these metabolites leave your body within 24 hours, primarily in your feces, with 

lesser amounts in your urine. Some methoxychlor can enter the fat in your body, but 

methoxychlor does not accumulate or build up in fat. 

More information on how methoxychlor enters and leaves your body can be found in Chapter 3. 

1.5 HOW CAN METHOXYCHLOR 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 

to protect public health. Scientists have the responsibility to treat research animals with care and 
METHOXYCHLOR 6 

1. PUBLIC HEALTH STATEMENT 

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

with strict animal care guidelines. 

Very few reports exist on the health effects of methoxychlor in humans. In animals, exposure to 

high levels of methoxychlor caused effects on the nervous system. These effects included 

tremors, convulsions, and seizures. These effects are probably caused by methoxychlor itself 

and not by its metabolites. Because methoxychlor is quickly transformed into metabolites by the 

liver, you are not likely to experience nervous system effects unless you are exposed to very high 

levels. 

Some of the breakdown products of methoxychlor cause effects similar to those produced by 

estrogen. Estrogens are naturally occurring hormones that are important in women for the 

development and maintenance of their ovaries, uterus, and breasts, and also play a role in the 

development of the reproductive system in men. Studies in animals show that exposure to 

methoxychlor adversely affects the ovaries, uterus, and mating cycle in females, and the testes 

and prostate in males. Fertility is decreased in both female and male animals. These effects can 

occur both in adult animals and in developing animals exposed prenatally or shortly after birth. 

Effects of methoxychlor on reproduction have been studied mainly in animals given 

methoxychlor in food or water, but it is expected that these effects could occur following 

inhalation and skin exposures as well. Likewise, it is expected that reproductive effects seen in 

animals could occur in humans exposed to methoxychlor, but this has not been reported. 

There is not enough information available to definitely state whether methoxychlor causes 

cancer. However, most of the information that we have indicates that methoxychlor does not 

cause cancer. One very small study in humans indicated a possible link with increased incidence 

of leukemia. However, a definitive connection between a cause of leukemia and exposure to 

methoxychlor cannot be made with so little information. Most animal studies with methoxychlor 

have been negative for cancer. Therefore, the International Agency for Research on Cancer 

(IARC) has determined that methoxychlor is not classifiable as to its carcinogenicity to humans. 

Similarly, the EPA has determined that methoxychlor is not classifiable as to its human 

carcinogenicity. 
METHOXYCHLOR 7 

1. PUBLIC HEALTH STATEMENT 

More information on how methoxychlor can affect your health can be found in Chapters 2 and 3. 

1.6 HOW CAN METHOXYCHLOR 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 are likely to be exposed to methoxychlor in the same way as adults, primarily from 

low-level contamination of food. Other possible sources of methoxychlor exposure for children 

include swallowing soil, crawling on home carpets, breathing in house dust and residues from 

methoxychlor-containing pesticides used for home gardening or pet care, and touching work 

clothes or equipment used to apply products that contain methoxychlor. 

Little information is available regarding the effects of methoxychlor in children. Exposure to 

very high doses of methoxychlor may cause nervous system effects such as tremors or 

convulsions. The reproductive system is likely to be the most sensitive target of methoxychlor in 

both adults and children. Methoxychlor is metabolized in the liver to substances that act like 

estrogen in the body. This probably occurs similarly in children and adults. Estrogens are 

naturally occurring substances that are necessary for the proper development and function of the 

male and female reproductive system. Elevated levels of estrogen, or substances like 

methoxychlor that mimic estrogen, have been shown to disrupt reproductive development and 

function in animals. This has resulted in early puberty in females, delayed puberty in males, 

disruption of the reproductive cycle in females, decreased fertility in males and females, and 

altered hormone levels in the blood. These effects can happen when the exposure occurs before 

birth or between birth and sexual maturity. It is thought that similar effects could occur in 

humans, but this has not been reported. 

There is no evidence in humans that methoxychlor causes birth defects. Methoxychlor does not 

cause structural birth defects in animals, but exposure to high levels of methoxychlor during 

pregnancy caused reduced survival of fetuses. It is unclear whether or at what level of exposure 

this might occur in humans. 
METHOXYCHLOR 8 

1. PUBLIC HEALTH STATEMENT 

Methoxychlor or its metabolites can probably be transferred from a pregnant mother to a 

developing fetus in animals, since abnormal reproductive development has been seen in the 

newborn animals born to mothers exposed during pregnancy. In animals, methoxychlor and 

metabolites of methoxychlor that are estrogenic can be transferred from a nursing mother to her 

newborn babies through breast milk. Methoxychlor and its metabolites can probably cross the 

placenta and have been detected in human breast milk. 

1.7	 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO 

METHOXYCHLOR? 

If your doctor finds that you have been exposed to significant amounts of methoxychlor, ask 

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

department to investigate. 

You can purchase products containing methoxychlor as an over-the-counter pesticide product to 

apply yourself. If you do purchase such a product, be sure that the product is in an unopened 

pesticide container that is labeled and contains an EPA registration number. If you plan to apply 

the pesticide indoors, make sure that the pesticide is approved for indoor use. Be sure to 

carefully follow the instructions on the label and follow any warning statements. Children can 

be exposed to pesticides by entering a room or playing on a lawn too soon after a pesticide has 

been applied. Carefully read and follow the directions on the pesticide label about how long to 

wait before re-entering the treated area. If you or any member of your family feels sick after a 

pesticide has been applied, consult your doctor or local poison control center. Pesticides and 

household chemicals should be stored out of reach of young children to prevent unintentional 

poisonings. Always store pesticides and household chemicals in their original labeled 

containers. Never store pesticides or household chemicals in containers that children would find 

attractive to eat or drink from, such as old soda bottles. Your children may be exposed to 

methoxychlor if an unqualified person applies pesticides containing it around your home. Make 

sure that any person you hire is licensed. Your state licenses each person who is qualified to 

apply pesticides according to EPA standards. Ask to see the license. Also ask for the brand 

name of the pesticide, a Material Safety Data Sheet (MSDS), the name of the product’s active 
METHOXYCHLOR 9 

1. PUBLIC HEALTH STATEMENT 

ingredient, and the EPA registration number. Ask what are the EPA approved uses. This 

information is important if you or your family react to the product. 

Children may be exposed to methoxychlor if they come in contact with family pets or farm 

animals that have been treated with the pesticide. Exposure may occur through skin contact with 

the animal or application devices, or by breathing vapor from animal-dipping solutions and 

baths. Dipping solution that contains methoxychlor should be disposed of according to the 

directions on the product label. Waste methoxychlor should never be discarded in any area 

where children might play. 

Although methoxychlor has been found in some foods, it occurs at very low levels. To reduce 

your family’s risk of exposure, you should thoroughly wash all fruits and vegetables before 

preparing them for consumption. 

Methoxychlor may be released to soil and water, especially near hazardous waste sites. 

Hazardous waste sites are often clearly marked, but children have a tendency to ignore signs that 

are designed to alert us to dangers. Your children should be encouraged not to play at or near 

hazardous waste sites. Low levels of methoxychlor have also been found in carpet and house 

dust. 

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

EXPOSED TO METHOXYCHLOR? 

Specific and sensitive tests have been developed to detect methoxychlor in blood, fat, semen, 

and breast milk of exposed individuals. Because methoxychlor is removed from the body 

relatively quickly, these tests are only useful in detecting recent exposures (within 24 hours) and 

are not useful for detecting past exposures to methoxychlor. These tests currently cannot be used 

to estimate how much methoxychlor you have been exposed to or whether adverse health effects 

will occur. These tests are not usually performed in a doctor's office because special equipment 

is required and samples must be sent to a laboratory for testing. 
METHOXYCHLOR 10 

1. PUBLIC HEALTH STATEMENT 

More information on tests that detect methoxychlor and its metabolites can be found in 

Chapter 7. 

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 methoxychlor include the following: 

The federal government has taken several actions to help protect humans from excess exposure 

to methoxychlor. EPA limits the amount of methoxychlor that may be present in drinking water 

to 0.04 parts of methoxychlor per million parts of water (0.04 ppm). EPA has also set limits of 

1–100 ppm on the amount of methoxychlor that may be present in various agricultural products 

(crops, fruits, vegetables, grains, meats, milk, and food for livestock). FDA limits the amount of 

methoxychlor in bottled water to 0.04 ppm. EPA restricts the amount of methoxychlor that may 
METHOXYCHLOR 11 

1. PUBLIC HEALTH STATEMENT 

be released to the environment during burning or by disposal in landfills. OSHA has set a 

Permissible Exposure Limit (PEL) of 15 milligrams per cubic meter of air (mg/m3) for the 

average amount of methoxychlor that may be present in air during an 8-hour workday. A court 

decision struck down a proposed PEL of 10 mg/m3. The American Conference of Governmental 

Industrial Hygienists (ACGIH) recommends a Threshold Limit Value (TLV) of 10 mg/m3

. 

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
 

Web site: http://www.atsdr.cdc.gov
 

* Information line and technical assistance 

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

Fax: 1-404-498-0057
 

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: 1-800-553-6847 or 1-703-605-6000
 
METHOXYCHLOR 12 

2. RELEVANCE TO PUBLIC HEALTH
 

2.1	 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO METHOXYCHLOR IN THE 

UNITED STATES 

Methoxychlor is produced commercially in the United States, typically as a technical-grade product 

containing 88–90% of the pure chemical and 10–12% of impurities consisting of isomers of 

methoxychlor and other reaction products. Methoxychlor has been used as an insecticide against a wide 

range of pests, including houseflies, mosquitos, cockroaches, chiggers, and various arthropods commonly 

found on field crops, vegetables, fruits, stored grain, livestock, and domestic pets. There are no known 

natural sources of methoxychlor. It is moderately soluble in water and is soluble in a variety of organic 

solvents. Annual usage of methoxychlor in the United States was estimated to range from 500,000 to 

900,000 pounds in 1986. The available data show that use patterns have remained fairly constant since 

1974. 

Methoxychlor is released to the environment mainly as a result of its application to crops and livestock as 

a pesticide. Smaller amounts may be released during its production, formulation, storage, shipment, and 

disposal. Most methoxychlor is probably removed from the air by wet and dry deposition processes in 

less than a month. Methoxychlor binds tightly to soils, but most does not persist due to degradation by 

microorganisms in the soil. Degradation products of methoxychlor are generally detected in lower levels 

of soil, suggesting that they are more mobile than methoxychlor. Methoxychlor is generally not detected 

in surface water or groundwater in the United States, probably due to its degradation and its affinity for 

sediments and organic matter. However, methoxychlor may be detected in waters near release sources. 

Although methoxychlor (a derivative of DDT) is not as persistent as DDT, it does have the potential for 

bioconcentration and has been shown to bioaccumulate to varying degrees in fish, insects, and mammals. 

For the general population, the most likely source of methoxychlor exposure is from low-level 

contamination of food. The FDA’s Total Diet Study program monitors chemical contaminants in the U.S. 

food supply and has calculated average daily intakes of methoxychlor in adults (age 25–65) ranging from 

0.1 to 0.3 ng/kg/day for the period 1986–1991. Exposure to methoxychlor from food may be elevated in 

persons who consume large amounts of fish and seafood from methoxychlor-contaminated waters. 

Because methoxychlor is usually not detected in ground or surface water sources, exposure to 

methoxychlor from drinking water is not expected to be significant for the general population. Based on 

the results of the Non-Occupational Pesticide Exposure Study (conducted between 1986 and 1988), 
METHOXYCHLOR 13 

2. RELEVANCE TO PUBLIC HEALTH 

inhalation exposure to methoxychlor ranged from 0.002 to 0.012 μg/day in one U.S. city. In a monitoring 

study of nonoccupational exposure to pesticides used in and around the home, methoxychlor was detected 

(air concentrations were not provided) in both indoor and outdoor samples. 

Exposures may be greater in individuals who use methoxychlor-containing products for home gardening 

or animal-care purposes. Populations that live or work on or near a farm where methoxychlor has been 

used recently on crops or livestock or that live near a hazardous waste site that contains methoxychlor 

could be exposed to above-average levels of methoxychlor in soil and possibly in water. 

The available data suggest that exposure of children to methoxychlor differs from that of adults. Small 

children may also play close to the ground and are therefore more likely than adults to come in contact 

with dirt and dust found in home carpets, dirt found outside the home, and lawns. Children also may 

intentionally or unintentionally ingest soil that contains low levels of methoxychlor. 

Methoxychlor has been found in at least 58 of the 1,613 current or former NPL sites. However, the total 

number of NPL sites evaluated for methoxychlor is not known. 

2.2 SUMMARY OF HEALTH EFFECTS 

Available data on the toxicity of methoxychlor in humans are limited to a study that found no clinical or 

histopathological changes in four men and four women who ingested 2 mg/kg/day of methoxychlor for 

6 weeks. These data are too limited to allow assessment of the health risks of methoxychlor to humans. 

Oral exposure of animals to methoxychlor has shown that high doses of methoxychlor are capable of 

causing neurological injury (tremors, convulsions), but most studies indicate that the reproductive system 

is the most sensitive target for methoxychlor. The resultant types of reproductive effects are indicative of 

interference with the normal actions of estrogen or androgen. Mechanistic studies have confirmed that 

metabolites of methoxychlor can compete with estrogen for binding to estrogen receptors and can mimic 

some and antagonize other effects of estrogen. Additional studies have shown that methoxychlor or its 

metabolites can interact with the androgen receptor and antagonize androgenic effects. In females, these 

interactions can result in disruption of estrus cyclicity, reduced fertility, and increased pre- and postimplantation losses. Effects in males can include delayed sexual maturation, atrophy of reproductive 

organs and accessory glands, and altered sexual or socio-sexual behavior. Many of these effects may be 

mediated through altered hormone levels. Because methoxychlor and its metabolites are cleared fairly 
 

METHOXYCHLOR 14 

2. RELEVANCE TO PUBLIC HEALTH 

rapidly from the body (approximately 92% in 24 hours in mice), there does not appear to be much 

potential for cumulative toxicity. 

Observable changes in the liver (altered liver weight, altered enzyme and protein levels, pale and mottled 

appearance) and kidneys (cystic tubular nephropathy, elevated blood urea nitrogen [BUN]) of animals, as 

well as weight loss, are caused only by relatively large doses of methoxychlor; these effects are probably 

not mediated by an estrogenic mechanism. 

Most carcinogenicity studies have not shown an increase in the cancer incidence following exposure to 

methoxychlor. Based on a review of all the available data, IARC has classified methoxychlor as a 

Group 3 carcinogen (not classifiable as to its carcinogenicity to humans). Similarly, EPA has classified 

methoxychlor as a Group D carcinogen (not classifiable as to human carcinogenicity). 

Reproductive Effects. Although human data on the reproductive effects of methoxychlor are 

limited, the animal and in vitro data strongly suggest that sufficient exposure to methoxychlor may 

adversely affect the development, histopathology, and function of the human reproductive system. 

Data from oral studies in animals indicate that the reproductive system is the primary and most sensitive 

target of methoxychlor-induced toxicity in both adult and developing animals. Some metabolites and 

contaminants of methoxychlor are estrogenic or anti-androgenic and are capable of producing adverse 

effects on the male and female reproductive system. These effects are thought to be mediated by 

interaction of methoxychlor or its metabolites with the estrogen receptor α, estrogen receptor β, or an as 

yet unknown estrogen receptor, or with the androgen receptor. These interactions can cause disruption of 

reproductive development or can alter reproductive function in adults. Altered serum and pituitary 

hormone levels have frequently been seen in animal studies, which may contribute to the changes in 

reproductive development and function. Developmental reproductive changes include precocious puberty 

and abnormal estrus cyclicity in females, delayed puberty in males, altered weights of reproductive organs 

and accessory glands, and impaired reproductive function in adulthood, including decreased pups/litter 

and increased resorptions. Similar effects have also been seen following exposure of adult animals. 

Additionally, gross and microscopic cellular changes have been observed in the reproductive organs of 

exposed adult females and males. While methoxychlor does have estrogenic properties, it is important to 

note that it is at least several thousand fold less potent than endogenous estrogen. 
 

 

METHOXYCHLOR 15 

2. RELEVANCE TO PUBLIC HEALTH 

There are no human data that report adverse effects on the reproductive system following exposure to 

methoxychlor, but in vitro studies reveal that human liver microsomes are capable of metabolizing 

methoxychlor to estrogenic compounds. Therefore, it is likely that methoxychlor could produce 

reproductive estrogen-like effects in humans. 

Developmental Effects. In animals, signs of fetotoxicity (decreased fetal body weight, increased 

incidence of wavy ribs, resorptions, and death) were noted following exposure to methoxychlor in utero. 

These effects may be due to the maternal toxicity of methoxychlor and may not be true signs of 

teratogenicity. 

Methoxychlor exposure during development can adversely affect the reproductive system of both 

developing and adult animals. These effects were discussed with the reproductive effects above. These 

effects are the result of the disruption by estrogenic methoxychlor metabolites of the normal delicate 

balance of time-sensitive hormone levels during fetal and post-natal development. Taken together, the 

animal data suggest that human exposure to methoxychlor during critical stages of development may 

adversely affect reproductive development, causing effects that may not be detected until after puberty. 

Neurological Effects. Animal studies suggest that exposure to large amounts of methoxychlor can 

produce neurological effects, such as apprehension, nervousness, increased salivation, decreased 

locomotor activity, tremors, convulsions, and death. Methoxychlor has been demonstrated to be a 

neurotoxicant even in the absence of metabolism. This suggests that it is the parent compound that is 

neurotoxic, and that neurotoxicity is of concern only when the metabolic capacity for O-demethylation is 

exceeded. This is supported by the observation that the neurological effects of methoxychlor are similar 

to those associated with exposure of humans and animals to DDT, a structurally similar chemical that is 

very slowly metabolized. The mechanism by which DDT, and therefore possibly methoxychlor, produces 

neurological effects has been proposed to involve the membrane-association of a lipophilic species, which 

alters ion transport across neural membranes. 

2.3 MINIMAL RISK LEVELS 

Inhalation MRLs 

No inhalation MRLs for methoxychlor have been derived because adequate data were not available 

concerning the effects of methoxychlor via this route of exposure. 

Oral MRLs 
16 

C

METHOXYCHLOR 

2. RELEVANCE TO PUBLIC HEALTH 

No acute oral MRL was derived for methoxychlor. A variety of candidate acute-duration MRL studies 

were considered. The acute oral MRL derived in the previous 1994 toxicological profile for 

methoxychlor was based on precocious vaginal opening (early puberty) observed in the study by Gray et 

al. at 25 mg/kg/day. However, since this study did not test doses as low as Chapin et al. (used as the basis 

of the intermediate-duration MRL), which demonstrated the same effect at 5 mg/kg/day administered 

from gestation day 14 to postnatal day 42, it is not known whether the premature puberty would have 

occurred at a lower acute dose than the 25 mg/kg/day observed in the Gray et al. study. There were 

several hypothesis-generating studies at extremely low doses that were not definitive enough to use for 

MRL derivation. The male reproductive parameters observed in these studies included increased prostate 

weight, increased territorial urine marking, increased killing of young mice by adults, and changes in 

aggression. These studies were not used for MRL derivation because the biological significance and 

relevance to human health of extremely low-dose effects has not been definitively established. 

Additionally, prostate weight is normally highly variable, even within a litter, and therefore may not be a 

good indicator of low-dose effects. These studies were all performed by the same laboratory and need to 

be replicated by other laboratories. See Appendix A for further discussion. 

An MRL of 0.005 mg/kg/day has been derived for intermediate-duration oral exposure 

(15–364 days) to methoxychlor. 

This MRL is based on a LOAEL of 5 mg/kg/day from gestation day 14 to postpartum day 42 for 

accelerated onset of puberty (i.e., precocious vaginal opening) in immature female rats exposed to 

methoxychlor in utero, during lactation, and after weaning. Precocious vaginal opening was evident 

(statistically significant) in all methoxychlor-treated groups (vaginal opening occurred on postnatal days 

37.4, 35.2, 30.8, and 33.4, respectively, for groups 0, 5, 50, and 150 mg/kg/day). The LOAEL was 

divided by an uncertainty factor of 1,000 (10 for variation in sensitivity among humans, 10 for 

extrapolation of animal data to humans, and 10 for extrapolation from a LOAEL to a NOAEL). The 

MRL is supported by other observations of reproductive effects associated with intermediate-duration 

exposure including elevated levels of prolactin in the pituitary of male rats exposed to 50 mg/kg/day, 

decreased seminal vesicle weight, caudal epididymal weight, and caudal epididymal sperm count, and 

increased gonadotropin releasing hormone in the mediobasal hypothalamus in male rats exposed to 

50 mg/kg/day. At higher exposure levels, intermediate-duration studies show decreased fertility in male 

rats at doses of 60–400 mg/kg/day and in female rats at doses of 50–150 mg/kg/day. A study in humans 

that identified a NOAEL of 2 mg/kg/day for effects to the testes and menstrual cycle was not chosen as 

the basis for an MRL because reproductive function was not evaluated and the number of subjects was 

small (4/sex/exposure group), and such an MRL may not be protective of reproductive and developmental 

effects in the fetus or child. 
METHOXYCHLOR 17 

2. RELEVANCE TO PUBLIC HEALTH 

No chronic oral MRL was derived for methoxychlor. Studies in rats and mice reported no adverse 

histopathological effects on a number of organ systems, including the reproductive system, following 

chronic exposure to methoxychlor at dose levels of 77–599 mg/kg/day. However, these chronic studies 

did not evaluate sensitive indices of reproductive toxicity. A 3-generation study in rats reported a 

LOAEL of 79 mg/kg/day methoxychlor for decreased fertility, and a NOAEL of 18 mg/kg/day for this 

effect. An MRL was not based on this study because sensitive reproductive end points such as precocious 

vaginal opening were not monitored, and the resultant MRL might not be protective for these types of 

effects. 
METHOXYCHLOR 18 
METHOXYCHLOR 19 

3. HEALTH EFFECTS 

3. HEALTH EFFECTS 

3.1 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 methoxychlor. 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. 

The health effects of methoxychlor have been investigated mainly in studies using technical grade 

methoxychlor, although a few studies have used laboratory grade (98% pure) or recrystallized (>99% 

pure) preparations. Technical grade methoxychlor is the grade used in industrial preparations and 

typically contains about 80–90% methoxychlor, with the remainder being composed of >50 chemicallyrelated compounds (IARC 1979; Lamoureux and Feil 1980). Please refer to Chapter 4 for more detailed 

information on the composition of methoxychlor. Some in vivo studies suggest that technical grade 

methoxychlor is approximately 2–4 times more potent with respect to reproductive and developmental 

effects than is pure methoxychlor (Bitman and Cecil 1970; Tullner 1961). This is because several of the 

contaminants in technical grade methoxychlor are directly estrogenic (Kupfer and Bulger 1987b), 

whereas pure methoxychlor is proestrogenic and requires metabolic activation before exhibiting 

estrogenic activity (Bulger et al. 1978d; Kupfer and Bulger 1979). Some of the estrogenic contaminants 

of technical grade methoxychlor are the same as those formed by metabolism in vivo (Bulger et al. 1985). 

It is important to recognize that because of the biological activity of these contaminants, dose-response 

relationships obtained using technical grade methoxychlor may not be directly applicable to pure 

methoxychlor. For this reason, the chemical purity of the methoxychlor used in key quantitative studies 

is provided. Since most studies employ technical grade methoxychlor, the text refers to purity only when 

other grades are used. 
METHOXYCHLOR 20 

3. HEALTH EFFECTS 

3.2 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 lowestobserved-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 

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 (LOAELs) 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. 
METHOXYCHLOR 21 

3. HEALTH EFFECTS 

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

made for methoxychlor. 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 1990h), 

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. 

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. 

3.2.1 Inhalation Exposure 

In air, methoxychlor is generally associated with particulate matter, and inhalation exposure to 

methoxychlor can occur by inspiration of methoxychlor-laden dust. Although the health effects resulting 

from inhalation exposure to methoxychlor have not been extensively investigated, there are a few reports 

that describe health effects following this type of exposure. 

3.2.1.1 Death 

A single case study reported the death of a 49-year-old male after an acute inhalation exposure to a 

pesticide mixture containing methoxychlor and captan (Ziem 1982). The exposure level was not 

reported. Death occurred 6 months after exposure and was attributed to aplastic anemia. Because only a 

single case was described and because the exposure was to a mixture of pesticides, it is not possible to 

deduce the role of methoxychlor in this outcome. 
 

 

METHOXYCHLOR 22 

3. HEALTH EFFECTS 

No studies were located regarding death in animals after inhalation exposure to methoxychlor. 

3.2.1.2 Systemic Effects 

No studies were located regarding respiratory, cardiovascular, gastrointestinal, musculoskeletal, hepatic, 

renal, endocrine, dermal, or ocular effects in humans or animals after inhalation exposure to 

methoxychlor. 

Hematological Effects. A single case study reported the development of aplastic anemia in a 

49-year-old male after an acute inhalation exposure to a pesticide mixture containing methoxychlor and 

captan (Ziem 1982). Symptoms of fatigue and bruising developed several weeks after exposure. The 

authors stated that although there are published case studies of aplastic anemia in humans following 

exposure to structurally-related pesticides such as dichlorodiphenyltrichloroethane (DDT) and lindane, no 

other reports of methoxychlor-induced aplastic anemia were located. Due to the limited database and the 

exposure to other chemicals (captan) in this study, a causal relationship between methoxychlor and 

aplastic anemia cannot be established. It is possible that this illness had an etiology unrelated to the 

pesticide exposure. 

Body Weight Effects. There is no information regarding body weight effects of methoxychlor in 

humans. One animal study showed a 24% reduction in weight gain in rats intermittently exposed to 

360 or 430 mg/m3

 methoxychlor for 4–5 weeks (Haag et al. 1950). However, the vehicle used to deliver 

the methoxychlor (Pyrax plus 3% Santo-Cel) was toxic and resulted in several deaths in the controls; 

therefore, the results of this study are difficult to interpret. 

3.2.1.3 Immunological and Lymphoreticular Effects 

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

to methoxychlor. 

3.2.1.4 Neurological Effects 

A single case study described neurological effects in a 21-year-old male acutely exposed (15–20 minutes) 

to a pesticide mixture that contained 15% methoxychlor and 7.5% malathion (Harell et al. 1978). The 

subject noted blurred vision and nausea 8–9 hours after exposure. He was admitted to the hospital 
METHOXYCHLOR 23 

3. HEALTH EFFECTS 

36 hours after exposure in a state of dehydration with severe abdominal cramps and diarrhea. 

Approximately 4 days later, he experienced dizziness and complete deafness followed by difficulty 

moving the extremities, hypoesthesias, parasthesias in the limbs, bilateral foot drop, and leg pain. There 

was no improvement in any of these neurological effects 6 years after exposure. The authors noted that 

neither methoxychlor nor malathion typically produced such profound effects, and attributed the special 

susceptibility of this individual to a deficiency in the enzyme responsible for the detoxification of 

malathion. Whether methoxychlor contributed to the effects is not known. 

Only one study was located regarding the neurological effects of inhalation exposure to methoxychlorladen dust in animals. Intermittent exposure (2 hours/day, 5 days/week for 4 weeks) of rabbits to 

430 mg/m3

 methoxychlor produced hind leg paralysis and disseminated nodules in the cerebral cortex in 

one out of two animals (Haag et al. 1950). Neurological effects were not observed in the two control 

animals. The results from this study are limited by the small number of animals tested, and by the 

possibility that the observed paralysis in the affected animal was the result of a disease that is endemic to 

rabbits (Haag et al. 1950). Thus, no firm conclusions can be made on the neurotoxicity of methoxychlor 

in animals after inhalation exposure. 

3.2.1.5 Reproductive Effects 

No studies were located regarding reproductive effects in humans or animals after inhalation exposure to 

methoxychlor. 

3.2.1.6 Developmental Effects 

No studies were located regarding developmental effects in humans or animals after inhalation exposure 

to methoxychlor. 

3.2.1.7 Cancer 

A single epidemiological study was located on the potential association between methoxychlor exposure 

and occurrence of leukemia in farmers (Brown et al. 1990). In this study, 11/578 cases of leukemia and 

16/1,245 controls were found to have been occupationally exposed to methoxychlor. After adjustment for 

vital status, age, state, tobacco use, family history of lymphopoietic cancer, high risk occupations and 

high risk exposures, an odds ratio of 2.2 was calculated. This increase was statistically significant; 
METHOXYCHLOR 24 

3. HEALTH EFFECTS 

however, it is difficult to make firm conclusions on the carcinogenicity of methoxychlor in humans based 

on a single study. 

No studies were located regarding cancer in animals after inhalation exposure to methoxychlor. 

3.2.2 Oral Exposure 

3.2.2.1 Death 

No studies were located regarding death in humans after oral exposure to methoxychlor. 

Reported acute LD50 values for recrystallized and technical grade methoxychlor range from 3,460 to 

7,000 mg/kg in male and female rats (Cannon Laboratories 1976; Hodge et al. 1950; Smith et al. 1946) 

and 2,900 mg/kg in mice (Coulston and Serrone 1969). A more recent study reported an acute LD50 in 

male and female Wistar rats of 2,828 mg/kg for methoxychlor that contained 88% p,p’ isomer and 12% 

o,p’ isomer, and an acute LD50 of 1,782 mg/kg for a methoxychlor formulation that contained only 25% 

methoxychlor (p,p’ isomer) (Dikshith et al. 1990). Mortality following a single exposure to 1,000, 2,000, 

4,000, or 8,000 mg/kg of the mixed isomer methoxychlor was 0, 25, 100, and 100%, respectively, in male 

rats, and 25, 50, 50, and 75%, respectively, in female rats (Dikshith et al. 1990). Two out of 17 pregnant 

rabbits died following exposure to 251 mg/kg/day methoxychlor on days 7–19 of gestation (Kincaid 

Enterprises 1986). Exposure to 790–4,200 mg/kg/day recrystallized and technical grade methoxychlor in 

feed for 4–16 weeks produced significant increases in mortality in rats (Davison and Cox 1976; Haag 

et al. 1950; Hodge et al. 1950). Mortality rates were higher in female Wistar rats (0–25%) than in males 

(0–10%) exposed to 100–1,000 mg/kg/day methoxychlor for 90 days (Dikshith et al. 1990). Mortality 

was low in male and female Wistar rats that received 200–800 mg/kg/day of a formulation containing 

25% methoxychlor for 90 days (Dikshith et al. 1990). Increases in mortality were also reported in dogs 

exposed to 2,000 mg/kg/day for 8–24 weeks (Tegeris et al. 1966). In dogs and rats, death was preceded 

by neurological effects (tremors, convulsions), as discussed below. LOAEL values for the lethal effects 

of methoxychlor are recorded in Table 3-1 and plotted in Figure 3-1. 

3.2.2.2 Systemic Effects 

No studies were located regarding respiratory, musculoskeletal, renal, endocrine, dermal, ocular, body 

weight, or metabolic effects in humans after oral exposure to methoxychlor. Data on the remaining 
 

 

METHOXYCHLOR 25 

3. HEALTH EFFECTS 

systemic effects after methoxychlor ingestion are available from a single study in humans. The highest 

NOAEL values and all LOAEL values from each reliable study for the systemic effects of methoxychlor 

in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1. These studies 

are discussed below. 

Respiratory Effects. A single study reported pulmonary edema in rabbits exposed to 200 mg/kg/day 

methoxychlor by gavage for 1–3 weeks, but the authors attributed this effect to possible gavage error 

resulting in aspiration of the chemical; no control group was included in the study (Smith et al. 1946). 

Chronic exposure of rats to 77 mg/kg/day recrystallized methoxychlor (Deichmann et al. 1967) or 

107 mg/kg/day technical grade methoxychlor (NCI 1978) or of mice to 599 mg/kg/day technical grade 

methoxychlor (NCI 1978) did not produce any significant histopathological effects in the respiratory 

tract. These data are too limited to draw firm conclusions, but suggest that the lung is not especially 

sensitive to ingested methoxychlor. 

Cardiovascular Effects. A single case report documented the ingestion of approximately 125 mL of 

a commercial product that contained methoxychlor (about 15 mg of methoxychlor) by a 62-year-old man 

in an attempted suicide (Thompson and Vorster 2000). Testing of a serum sample collected at the time of 

admission to the hospital showed a methoxychlor level of 0.67 μg/mL serum. His blood pressure was 

very low (58/40) and his pulse rate was high (88 beats per minute). After initiation of treatment, his 

blood pressure recovered to 110/70. 

In animals, one study reported the development of fatty hearts in two of four rabbits administered lethal 

doses of methoxychlor (200 mg/kg/day) for 1–3 weeks (Smith et al. 1946). However, no gross or 

histopathological changes in the heart were noted in rats or mice following chronic oral exposure to 

107 or 599 mg/kg/day methoxychlor, respectively (NCI 1978). Although no firm conclusion can be 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral 

a Key to 

figure 

Species 

(Strain) 

Exposure/ 

Duration/ 

Frequency 

(Specific Route) System 

NOEL 

(mglkglday) 

Less Serious 

(mglkglday) 

LOAEL 

Serious 

(mglkglday) 

Reference 

Chemical Form 

ACUTE EXPOSURE 

1 

Death 

Rat Once 

(GO) 

3460 (LD50) 

Cannon Laboratories 1976 

TG 

2 Rat 

(Wistar) 

2828 (LD50) 

Dikshith et al. 1990 

mixed isomers 

3 

5 

6 

Rat 

Rabbit 

(New 

Zealand) 

Systemic 

Rat 

Once 

(GO) 

Once 

(GI 

Gd 7-19Ixld 

(GW) 

Ixld Gd 6- 15 

(GO) 

Bd Wt 50 (reduced maternal body weight 

gain) 

5000 (LD50) 

2900 (LD50) 

251 (2117deaths) 

Hodge et al. 1950 

RC TG 

Coulston and Serrone 1969 

TG 

Kincaid Enterprises 1986 

TG 

Khera et al. 1978 

TG 

w 

I 

rn 

-I

I 

rn 

n 

n 

m 

F 

2 

v, 

7 Rat Once 

(GO) 

Hepatic 640 

Morgan and Hickenbottorn 1979 

TG 

8 Rabbit Hepatic 5 35.5 (palelmottled appearance of the 

liver) 

Kincaid Enterprises 1986 

TG 

9 

Neurological 

Rat Once 

(Wistar) (GO) 

Bd Wt 5 35.5 

2500 

(anorexia and 14.4% decreased 

body weight gain) 

(decreased locomotor activity) 3000 (tremors) 

Cannon Laboratories 1976 

TG 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Exposure1 LOAEL 

Duration/ 

a NOEL Less Serious Serious Reference Key to Species Frequency 

figure (Strain) (Specific Route) System (mglkglday) (mglkglday) (mglkglday) Chemical Form 

ACUTE EXPOSURE 

Reproductive 

10 Rat Culik and Kaplan 1976 97.3 (dose-related increases in 

post-implantation loss) TG 

11 Rat Cummings 1993 50 F 75 F (induction of uterine 

(Holtzman) decidualization) 98% pure

12 Rat Cummings and Gray 1987 100 200 (decreased uterine receptivity to 

implantation) LG w 

I Cummings and Gray 1989 m 13 Rat 100 (decreased serum 200 (decreased number of F 

progesterone) implantation sites and uterine LG I

-I 

weight: increased number of m 

resorptions) n 

n 

rn 

0 

14 Rat Gd 1-8 Ixld Cummings and Laskey 1993 2 

25 50 (decreased serum 250 (decreased number of 

(GO) progesterone) implantation sites) LG 

15 Rat Gd 1-3 Ix/d Curnmings and Perreault 1990 100 (accelerated embryo transport 200 (decreased number of 

(GO) into uterus) implantations) LG 

16 Rat Ix/d 5-76 d. starting at Gray et al. 1989 

weaning at 25 (precocious vaginal opening 21 ppd and estrus as early as 3 d after TG 

(GO) exposure started) 

17 Mouse lxld 5 dlwk 2 wk Martinez and Swartz 1991 

25 (persistent vaginal cornification) (GO) TG 50% 

18 Rabbit Gd 7-19 IX/d Kincaid Enterprises 1986 5 35.5 (increased frequency of 

(GW) abortion and late resorptions) TG 
Exposure/ 

Duration/ 

a 

Key to Species Frequency

figure (Strain) (Specific Route) 

ACUTE EXPOSURE 

Developmental 

19 Rat Gd 6-15 

(F) 

20 Rat Ixld Gd 14-20 

(GO) 

21 Rat Ixld Gd 6-15 

(GO) 

22 Rabbit Gd 7-19 Ixld 

(GW) 

INTERMEDIATE EXPOSURE 

Death 

23 Rat 16 wk 

(Sherman) (F) 

24 Rat 4 wk 

(F) 

25 Rat 45 d 

(F) 

(continued) Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral 

LOAEL 

NOAEL Less Serious Serious 

System (mglkglday) (mglkglday) (mglkglday) 

40.8 (increased incidence of wavy 

ribs) 

30 

200 (increased percent dead, resorbed, or anomalous 

fetuses) 

5 35.5 (10-1 1% decreased fetal body 

weight and percentage of male 

fetuses) 

1200 (4/12 died) 

917 (6/7 females died) 

4200 (16/20 died) 

Reference 

Chemical Form 

Culik and Kaplan 1976 

TG 

Gellert and Wilson 1979 

NS 

Khera et al. 1978 

TG 0 

I 

rn 

Kincaid Enterprises 1986 5

I TG rn 

n 

n 

rn 

(n 

Davison and Cox 1976 

TG 

Haag et al. 1950 

RC 

Hodge et al. 1950 

TG 
Table 3-1 Levels of Significant Exposure to Methoxvchlor - Oral (continued) 

~~ 

LOAEL Exposure1 

Duration1 Reference a NOAEL Less Serious Serious Key to Species Frequency Chemical Form figure (Strain) (Specific Route) System (mglkglday) (mglkglday) (mglkglday) 

INTERMEDIATE EXPOSURE 

Systemic 

Chapin et al. 1997 26 Rat Gd 14 - ppd 42 Ixld Hemato 50 M 150 M (24% increase in relative spleen (Sprague- (GO) weight) 95% pure 

Dawley) 

Hepatic 50 M 150 M (13% increase in relative liver 

weight) 

Renal 50 M 150 M (16% increase in relative kidney 

weight) 

Endocr 50 M 150 M (36% increase in relative and 

absolute adrenal gland weight) w 

I 

Bd Wt 5 50 (loo/, decrease in body weight rn 

gain) 5

I 

rn 

n 27 Rat 16 wk Davison and Cox 1976 n Hepatic 90 1200 (66% increased relative liver rn (Sprague- (F) weight; decreased total and TG 0 

--I Dawley) relative vitamin A content) cn 

Bd Wt 90 1200 (22% decreased body weight 

gain) 

Gray et al. 1999 28 Rat 309d Ixld 

Hepatic 200 M (37% decrease in absolute liver 

(Long- Evans) (GO) weight) 90% purity (TG) 

Renal 200 M (37% decrease in absolute 

kidney weight) 

Endocr 200 M (17% decrease in absolute 

pituitary weight; 60% increase 

in relative adrenal gland weight) 

Bd Wt 200 M (37% decrease in body weight 

gain) 
~~ 

Exposure/ 

Duration/ 

Key to a Species Frequency 

figure (Strain) (Specific Route) 

INTERMEDIATE EXPOSURE 

29 

lmmunol Lymphoret 

30 Rat 28d; Ixld 

(CD) (GO) 

Neurological 

31 Rat Ixld 55-66 d 

Long-Evans, (GO) 

Sprague-DawC 

F344 

Reproductive 

32 Rat 1xld 70 d for males 5 wk 

for females (Wistar) 

(GO) 

33 Rat Gd 14 - ppd 42; Ixld 

(Sprague- (GO)

Dawley) 

34 Rat 21d Ixld 

(Sprague- (GI Dawley) 

35 Rat Ixld 7 d/wk 8 wk 

(GO) 

Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

LOAEL 

NOAEL Less Serious Serious Reference 

Chemical Form System (mglkglday) (mglkglday) (mgWday) 

Okazaki et al. 2001 

Hepatic 20 F (decreased serum albumin) LG 

Renal 100 500 (basophilic tubules; dilatation of 

renal tubules and casts) 

Okazaki et al. 2001 

100 M 500 M (atrophyof the thymus) LG 

w Gray et al. 1988 

400 F (increased and acyclic running I

wheel activity) TG rn

5

I 

m 

n 

n Bal1984 rn

100 (infertility; degeneration of the 0 

testis and ovaries). TG v, 

-I 

Chapin et al. 1997 

5M !?F (precocious vaginal opening) 50 F (reduced fertility; 68% reduction 

in number of implants; cystic 95% pure 

ovaries, few corpora lutea; 

irregular estrus cycles) 

Cummings and Metcalf 1995b 

250 F (slows the regression of 

endometriosis) NS 

Goldman et al. 1986 

25 M (elevated pituitary prolactin 

levels) TG 

w 

0 
Exposure1 

Duration/ 

Key to a Species Frequency 

figure (Strain) (Specific Route) 

INTERMEDIATE EXPOSURE 

36 Rat Ixld 55-66 d 

(GO) 

37 Rat Ixld 56-66 d 

(GO) 

38 Rat Ixld 59-76 d 

(GO) 

39 Rat Ixld 76-99 d 

(GO) 

40 Rat 309d Ixld 

(Long- Evans) (GO) 

41 Rat 6-9 Wk 

(Sprague- (F)

Dawley) 

Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

LOAEL 

NOAEL Less Serious Serious Reference 

System (mglkglday) (rnglkglday) (mglkglday) Chemical Form 

Gray et al. 1988 

400 (persistent vaginal cornification: 

atrophy of ovary and uterus) TG 

Gray et al. 1989 

100 (80% decreased in fertility when 

treated males and females TG 

mated: decreased number of 

liver pups per litter: decreased 

caudal epididymal sperm count) 

w Gray et al. 1989 

25 M (elevated levels of prolactin in 

the pituitary) TG 

25 F (precocious vaginal opening) 

Gray et al. 1989 m 

100 M (elevated prolactin, FSH and 100 F (40% decreased fertility when 2 TSH in the pituitary) treated females were mated TG 0) 

with untreated males; 

decreased number of live pups 

per litter; ovarian atrophy and 

histopathology) 

Gray et at. 1999 200 M (delayed puberty; decreased 

testes weight, fertility, and 90% purity (TG) 

sperm counts; altered mating 

behavior) 

Harris et al. 1974 

60 (decreased ovary weight) 150 (decreased ovary weight; 

increased uterus weight: TG 

decreased mating frequency 

and fertility in females) 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Exposure/ 

Duration/ 

a Key to Species Frequency 

figure (Strain) (Specific Route) 

INTERMEDIATEEXPOSURE 

System 

NOAEL 

(mglkglday) 

Less Serious 

(mglkglday) 

LOAEL 

Serious 

(mglkglday) 

Reference 

Chemical Form 

I 

c

b 

;D 

42 Rat 12 wk 

(Long- Evans) (F) 

300 

Harris et al. 1974 

(persistent vaginal cornification; 

decreased mating frequency) TG 

43 Rat 6 wk 

(Long- Evans) (F) 

60 

Harris et al. 1974 

(precocious vaginal opening in 

females; decreased mating TG 

frequency and fertility in males 

and females) 

44 Rat 30-45 d 

(F) 

140 1400 (testicular atrophy) Hodge et al. 1950 

TG 

45 Rat 15d Ixld 

(Long- Evans) GO 

20 

50 F (precocious vaginal opening) Laws et al. 2000 

TG 95% 

Okazaki et al. 2001 

LG 

100 M (atrophy of seminiferous tubules 

and Leydig cells in testes; 

decreased sperm; cell debris in 

lumen of epididyrnis; atrophy of 

mammary acinus) 

100 F (abnormal estrous cyclicity) 

-I

I 

rn 

n 

n 

rn 

0 

-I 

(I) 

47 Mouse Ixlday 5 dlwk 4 wk 

(GO) 

50 (persistent vaginal cornification; 

decreased ovary weight) 

Martinez and Swartz 1991 

TG 

48 Mouse Ixld 5 dlwk 4 wk 

(GO) 

100 (increased lipid in interstitial and 

thecal cells of the ovary) 

Martinez and Swab 1992 

TG 50% 
Exposure/ 

Duration/ 

Key to 

a Species Frequency 

figure (Strain) (Specific Route) 

INTERMEDIATE EXPOSURE 

Developmental 

49 Rat Gd 14 - ppd 21 Ixld 

(Sprague- (GO) 

Dawle y) 

50 Rat Ix/d 59-76 d 

(GO) 

51 Rat 6-9 Wk 

(F) 

52 Rat 9 wk 

(Sprague- (F) 

Dawley) 

CHRONIC EXPOSURE 

Systemic 

53 Rat 24-27 mo 

(Osborne- (F) 

Mendel) 

Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (con ti nued) 

LOAEL 

NOAEL Less Serious Serious 

System (mglkglday) (mglkglday) (mgWday) 

5 (decreased click response in M 50 F (decreased fertility; 68% 

and F: decreased approach reduction in number of .. implants)

stimulus in M; no 

dose-response relationship) 

50 (precocious vaginal opening; 

45.3% decreased fertility in 

offspring; acyclic estrus: 

pituitary abnormalities) 

60 (precocious vaginal opening in 

female offspring) 

60 (reduced fertility in males and 

female offspring) 

Resp 77 

Gastro 77 

Hepatic 77 

Renal 77 

Reference 

Chemical Form 

Chipin et al. 1997 

95% pure (LG) 

Gray et al. 1989 

TG 

Harris et al. 1974 

TG 

Harris et al. 1974 

TG 

Deichman et al. 1967 

RC 

w 

m

F

i

I 

rn 

n 

7-l 

m 

0 

-I

cn 

Ww 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Exposure/ 

Duration/ 

Key tt Species Frequency 

figure (Strain) (Specific Route) 

CHRONIC EXPOSURE 

System (mglkglday) 

NOAEL 

(mglkglday) 

Less Serious 

LOAEL 

(mglkglday) 

Serious Reference 

Chemical Form 

54 Rat 

(albino) 

1Yr 

(F) Resp 917 Haag et al. 1950 

RC 

Cardio 917 

Gastro 917 

Musclskel 917 

Hepatic 

Renal 

Dermal 

Ocular 

Bd Wt 

917 

917 

917 

917 

197 M (>lo% decrease in body weight) 

w 

I 

rn

5

I 

m 

n 

n 

rn

9 

vj 

w 

P 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Exposure1 LOAEL 

a 

Key to 

figure 

Species 

(Strain) 

Duration/ 

Frequency 

(Specific Route) System 

NOAEL 

(mglkglday) 

Less Serious 

(mglkglday) 

Serious 

(mglkglday) 

Reference 

Chemical Form 

CHRONIC EXPOSURE 

55 Rat 

(Osborne78 wk 

(F) Resp 107 

NC/ 1978 

TG 

Mendel) 

Cardio 107 

Gastro 107 

Hemato 107 

Musclskel 107 w 

Hepatic 

Renal 

107 

107 

I 

rn

?

--I

I 

Dermal 107 

rn 

n 

n 

Ocular 107 

rn

2 

cn 

0d Wt 107 F (decreased body weight gain) 

W

ul 
Table 3-1 Levels of Significant Exposure to Methoxychlor - Oral (continued) ~~ 

Exposure/ LOAEL 

Duration/

a Serious Reference Key to Species Frequency NOAEL Less Serious 

figure (Strain) (Specific Route) System (mglkglday) (mglkglday) (mglkglday) Chemical Form 

CHRONIC EXPOSURE 

56 Mouse 78 wk NCf 1978 

Resp 599 (B6C3F1) (F) TG 

Cardio 599 

Gastro 599 

Hernato 599 

Musclskel 599 

w 

Hepatic 599 I 

rn 

Renal 599 5

I 

rn Dermal 599 n 

n 

rn 

Ocular 599 2 

0) 

Bd Wt 172 F (12% decrease in body weight) 

lmmunol Lymphoret 

57 Rat 78 wk NCI 1978 

107 (Osborne- (F) TG 

Mendel) 

58 Mouse 78 wk NCI 1978 

599 M (B6C3F1) (F) TG 

Neurological 

59 Rat 78 wk NCI 1978 

107 F

(Osborne- (F) TG 

Mendel) 

60 Mouse 78 wk NCI 1978 

599 M 

(F) TG 
Levels of Significant Exposure to Methoxychlor - Oral (continued) z 

rn ~~ _____~~ ~ ~~ -I

I Exposure/ LOAEL 0 

Duration/ 2 

a NOAEL Less Serious Serious Reference 0 Key to Species Frequency I 

figure (Strain) (Specific Route) System (mglkglday) (mglkglday) (mg/kg/day) Chemical Form 6

A 

CHRONIC EXPOSURE 

Reproductive 

61 Rat 3 generations Haskell Laboratories 1966 

18 92 F (decreased fertility) 

(CD) (F) TG 

a The number corresponds to entries in Figure 3-1 

b Used to derive a minimal risk level (MRL) of 0.005 rng/kg/day; dose divided by an uncertainty factor of 1.000 (10 for use of a LOAEL. 10 for extrapolation from animals to humans, 

and 10 for human variability). 

w 

rn

F

i

I 

rn 

n 

n 

rn 

0 

w 

-4 
Figure 3-1. Levels of Significant Exposure to Methoxychlor - Oral 

Acute (114 days) 

Svstemic -

10000 

1000 

1oc 

.5h 

07r w 

I 

rn

5

I 

rn 

-n 

n 

rn 

0 

-I 

cn 

08h 

06r 

08h 

I( 

c-Cat -Humans f-Ferret n-Mink Cancer Effect Level-Animals V Cancer Effect Level-Humans LD501LC50

&Dog k-Monkey j-Pigeon o-Other LOAEL, More Serious-Animals A LOAEL, More Serious-Humans : Minimal Risk Level 

r-Rat m-Mouse e-Gerbil (B LOAEL, Less Serious-Animals A LOAEL, Less Serious-Humans : for effects 

p-Pig h-Rabbit s-Hamster 0NOAEL -Animals A NOAEL- Humans A other than 

q-Cow a-Sheep g-Guinea Pig Cancer 
Figure 3-1. Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Intermediate (15-364 days) 

10000 

1000 

100 

10 

1 

0.1 

0.01 

w 

c-Cat -Humans f-Ferret n-Mink 4Cancer Effect Level-Animals Cancer Effect Level-Humans LD50/LC50 W d-Dog k-Monkey j-Pigeon o-Other LOAEL, More Serious-Animals A LOAEL, More Serious-Humans : Minimal Risk Level W

r-Rat m-Mouse e-Gerbil 0 LOAEL, Less Serious-Animals A LOAEL, Less Serious-Humans : for effects 

h-Rabbit s-Hamster 0NOAEL -Animals NOAEL - Humans other than 

a-Sheep 9-Guine Cancer

0.001 

a
1000 

Figure 3-1.‘Levels of Significant Exposure to Methoxychlor - Oral (continued) 

Chronic (2365days) 

058m 060m 

Q54r 

056m 

1O( 

-

c-Cat -Humans f-Ferret n-Mink Cancer Effect Level-Animals d-Dog k-Monkey j-Pigeon o-Other LOAEL, More Serious-Animals 

(3LOAEL, Less Serious-Animals e-Gerbil Animals q-Cow a-Sheep g-Guinea Pig 

r-Rat p-Pig m-Mouse h-Rabbit s-Hamster 0NOAEL -

.61r 

061r 

Cancer Effect Level-Humans 

A LOAEL, More Serious-Humans 

A LOAEL, Less Serious-Humans 

A NOAEL - Humans 

w 

I 

F

rn 

-I

I 

rn 

-n 

-n 

7 

rn 

03 

LD501LC50

: Minimal Risk Level 0 

P 

: foreffects other than 

Cancer 

1 
 

 

 

METHOXYCHLOR 41 

3. HEALTH EFFECTS 

drawn regarding cardiovascular effects of methoxychlor, the data suggest that the heart is not a primary 

target organ for methoxychlor. 

Gastrointestinal Effects. No histological evidence of injury to the small intestine was detected in 

biopsy samples from four men and four women who ingested 2 mg/kg/day of methoxychlor for 6 weeks 

(Wills 1969). 

Diarrhea was noted in rats given acutely lethal doses (7,000 mg/kg) of methoxychlor (Smith et al. 1946). 

However, no histopathological changes of the gastrointestinal tract were noted following chronic oral 

exposure of rats to 77 mg/kg/day recrystallized methoxychlor (Deichmann et al. 1967) or 107 mg/kg/day 

technical grade methoxychlor (NCI 1978), or mice to 599 mg/kg/day technical grade methoxychlor (NCI 

1978). These data suggest that the gastrointestinal tract is not a sensitive target organ for methoxychlor. 

Hematological Effects. Only one study was located regarding the hematological effects of 

methoxychlor in humans after oral exposure. Electron microscopy of bone marrow biopsy samples and 

chemical analysis of the blood from four male and four female volunteers exposed to 2 mg/kg/day for 

6 weeks revealed no significant changes (Wills 1969). 

Rabbits administered lethal doses of methoxychlor (200 mg/kg/day) for 1–3 weeks developed 

hemosiderosis of the spleen, but no controls were included in the study (Smith et al. 1946). Rats exposed 

to 150, but not 50, mg/kg/day methoxychlor for 35 days had increased relative spleen weights (Chapin et 

al. 1997). However, no gross or histopathological changes were noted on the hematopoietic system (bone 

marrow, spleen) in rats or mice exposed to 107 or 599 mg/kg/day methoxychlor in the feed for 78 weeks 

(NCI 1978). No changes in red blood cell, white blood cell, or hemoglobin levels were observed in rats 

exposed to up to 1,000 mg/kg/day methoxychlor for 90 days (Dikshith et al. 1990). These data are 

insufficient to evaluate the hematological effects of methoxychlor. 

Musculoskeletal Effects. In animals, no histopathological effects were detected in the skeletal 

muscles or bones of rats or mice following chronic oral exposure to doses of 107 or 599 mg/kg/day 

methoxychlor, respectively (NCI 1978). This suggests the musculoskeletal system is not an important 

target for methoxychlor. 
 

METHOXYCHLOR 42 

3. HEALTH EFFECTS 

Hepatic Effects. Only one study was located regarding the effects of methoxychlor on the liver in 

humans after oral exposure. Electron microscopy of liver biopsy samples from 4 men and 4 women 

exposed for 6 weeks to 2 mg/kg/day revealed no histopathological changes (Wills 1969). In addition, 

there were no changes in serum enzyme levels such as glutamate-oxaloacetate aminotransferase (SGOT; 

also called aspartate aminotransferase or AST), glutamate-pyruvate aminotransferase (SGPT; also called 

alanine aminotransferase or ALT), and alkaline phosphatase (AP) that are indicative of liver injury. 

Based on these limited data, the liver of humans does not appear to be affected by low doses of 

methoxychlor for intermediate durations. The hepatic effects of chronic oral exposure to low doses of 

methoxychlor have not been studied in humans. 

Several studies in animals have described effects on the liver following oral exposure to methoxychlor, 

but the effects were usually mild or moderate, and effects were generally observed only at high doses 

(lethal or near lethal). In male rats given a single oral dose of 640 mg/kg methoxychlor, increased hepatic 

levels of glucose-6-phosphatase and decreased levels of glycogen phosphorylase and lactate were 

detected, but there were no observable histopathological changes (Morgan and Hickenbottom 1979). 

These enzymatic changes suggest that methoxychlor may promote the utilization of liver glycogen 

(Morgan and Hickenbottom 1979). The livers of pregnant rabbits administered 35.5 mg/kg/day 

methoxychlor on days 7–19 of gestation were pale and mottled in appearance (Kincaid Enterprises 1986). 

This effect was not noted in rabbits administered 5.01 mg/kg/day methoxychlor. "Fatty degeneration" of 

the liver (not otherwise described) was noted in two out of four rabbits given lethal doses of 

methoxychlor (200 mg/kg/day) for 1–3 weeks; no controls were included in this study (Smith et al. 1946). 

Levels of serum enzymes (SGOT or AST, SGPT or ALT, AP) were elevated in dogs receiving 2,000, but 

not 1,000, mg/kg/day methoxychlor in feed for 8–24 weeks (Tegeris et al. 1966). Rats that received 

100–1,000 mg/kg/day methoxychlor for 90 days had small, but statistically significant, nondose related 

changes in serum and/or liver enzyme activities (GOT, GPT, and AP) and serum protein levels (Dikshith 

et al. 1990). Significant increases in liver weight were reported in rats administered lethal doses of 

methoxychlor (500–1,200 mg/kg/day for 13–16 weeks) (Davison and Cox 1976; Dikshith et al. 1990). 

Rats fed nonlethal doses of methoxychlor (60 mg/kg/day) for 9 weeks, spanning mating through weaning, 

had enlarged livers and elevated liver vitamin A concentrations prior to mating, but no significant 

differences from controls following weaning (Harris et al. 1974). Although reductions in liver weight 

have also been reported in animals receiving nonlethal doses of methoxychlor for intermediate or chronic 

durations (Cecil et al. 1974; Deichmann et al. 1967; Gray et al. 1989, 1999), these reductions were 

generally associated with decreases in body weight gain and are not considered adverse. No 

histopathological changes were found in the liver of rats or mice chronically exposed to 107 or 
 

METHOXYCHLOR 43 

3. HEALTH EFFECTS 

599 mg/kg/day methoxychlor, respectively (NCI 1978), or rats exposed to 77 mg/kg/day (Deichmann et 

al. 1967). Taken together, these data suggest that high doses of methoxychlor may cause hepatic effects 

in some animals, but that the liver is not a key target organ for methoxychlor. 

Because DDT (a structural analogue of methoxychlor) is known to cause vitamin A depletion in the livers 

of exposed animals, a number of studies have investigated the effects of methoxychlor on the vitamin A 

content of the liver. A significant reduction in total liver vitamin A content was observed in rats receiving 

doses of 9 mg/kg/day or more for 16 weeks (Davison and Cox 1976), and vitamin A concentration 

(μg/100 mg liver), but not total vitamin A, was significantly reduced in rats exposed to 60 mg/kg/day for 

9 weeks (Harris et al. 1974). No significant change in vitamin A content was reported in rats 

administered 5 mg/kg/day methoxychlor for 8–16 weeks (Cecil et al. 1974; Phillips and Hatina 1972). In 

an in vitro assay, methoxychlor did not react with human transthyretin, a carrier protein for vitamin A and 

thyroid hormones (Van den Berg et al. 1991). However, no microsomal activation system was employed 

to determine if methoxychlor metabolites could bind transthyretin. Decreases in the vitamin A content of 

liver are not necessarily adverse by themselves, so the significance of this effect is uncertain. 

Renal Effects. Adverse effects on the kidneys were reported in animals administered lethal or nearlethal doses of methoxychlor. Renal nephrosis was observed in four out of four rabbits administered 

200 mg/kg/day methoxychlor for 1–3 weeks, but the significance is unclear since no controls were 

included (Smith et al. 1946). Basophilic tubules, dilatation of renal tubules, and casts were seen in rats 

exposed to 500, but not 100, mg/kg/day for 28 days; no further description of the lesions was provided 

(Okazaki et al. 2001). Cystic tubular nephropathy was reported in rats exposed to 861 mg/kg/day 

methoxychlor in the feed for 33–55 days (Tullner and Edgcomb 1962). Cystic tubular nephropathy was 

accompanied by elevated blood urea nitrogen (BUN) in pigs exposed to 1,000 mg/kg/day methoxychlor 

in the feed for 24 weeks (Tegeris et al. 1966). No histopathological changes were detected in rats and 

mice exposed to 107 or 599 mg/kg/day, respectively, in the feed for 78 weeks (NCI 1978) or in rats 

exposed to up to 1,000 mg/kg/day for 90 days (Dikshith et al. 1990). One study showed a 37% reduction 

in kidney weight in rats treated with 200 mg/kg/day for 4 weeks, but this was associated with a 37% 

reduction in body weight and therefore, may not be adverse (Gray et al. 1999). Although limited, these 

data suggest that high doses may injure the kidneys, but that renal effects are not of major concern at 

lower doses. 
 

METHOXYCHLOR 44 

3. HEALTH EFFECTS 

Endocrine Effects. Only limited data were located regarding the effects of methoxychlor on 

endocrine glands in animals following oral exposure. A decrease in absolute pituitary weight was seen in 

rats exposed to 200–400 mg/kg/day for 44 weeks or 100–200 mg/kg/day for 11–16 weeks (Gray et al. 

1989, 1999). This was, however, associated with a reduction in body weight and, in most cases, a slight 

increase in relative pituitary weight. In these same studies, Gray et al. (1989) observed an increase in 

absolute and relative adrenal gland weight in male and female rats exposed to 100–200 mg/kg/day for 

11–16 weeks. No treatment-related histopathological effects were seen. No histopathological effects or 

changes in adrenal gland weight were seen in male or female rats that received up to 1,000 mg/kg/day 

methoxychlor for 90 days (Dikshith et al. 1990). Increased relative and absolute adrenal gland weights 

were seen in male rats exposed to 150 mg/kg/day methoxychlor from gestation day 14 through postnatal 

day 42 (Chapin et al. 1997). This effect was not seen in female rats or in male rats at 5 or 50 mg/kg/day. 

No histopathological data were reported. Adrenal gland weight returned to normal (compared to controls) 

with cessation of methoxychlor administration (Chapin et al. 1997). 

In female laboratory animals, exposure to methoxychlor results in a number of effects involving the 

reproductive tract and the major hormone-producing glands that regulate it. Atrophic and degenerative 

changes in the ovaries were seen in female mice and rats exposed to 25–400 mg/kg/day methoxychlor for 

intermediate durations (Bal 1984; Chapin et al. 1997; Gray et al. 1988, 1989; Harris et al. 1974; Martinez 

and Swartz 1991). Decreased serum progesterone levels were observed in female rats acutely exposed to 

50–100 mg/kg/day laboratory grade methoxychlor, but not to 25 mg/kg/day (Cummings and Gray 1989; 

Cummings and Laskey 1993). Ultrastructural changes in ovarian cells of mice exposed to 100 mg/kg/day 

for 4 weeks included an accumulation of lipid in the interstitial and thecal cells (Martinez and Swartz 

1992). It has been postulated that methoxychlor disrupts the feedback regulation of pituitary hormones 

that normally exert an effect on the uterus and other estrogen-sensitive tissues (Martinez and Swartz 

1992). 

Oral exposure to methoxychlor also produces a number of effects in male laboratory animals. A 21% 

decrease in ventral prostate weight was observed in male rats exposed to 154 mg/kg/day for 90 days 

(Shain et al. 1977). Decreased weight of the testes was observed in male rats and mice exposed to 

methoxychlor at 50–1,400 mg/kg/day for intermediate durations (Bal 1984; Gray et al. 1989, 1999; 

Hodge et al. 1950; Tullner and Edgcomb 1962), and regression of the seminiferous epithelium was seen 

in mice exposed to 60 mg/kg/day for 50 days or more (Wenda-Rozewicka 1983). A dose-related 

inhibition of testes development, fewer mature germ cells, and fewer germ cells of each type were 

observed in the testes of male rats exposed to 50 or 150 mg/kg/day methoxychlor from gestation 
 

METHOXYCHLOR 45 

3. HEALTH EFFECTS 

day 14 through postnatal day 42 (Chapin et al. 1997). Decreased caudal epididymal sperm counts were 

observed in rats exposed to 50–400 mg/kg/day for intermediate-duration exposures (Gray et al. 1989, 

1999). Exposure to 25–50 mg/kg/day methoxychlor produced increased levels of prolactin in the 

pituitary of male rats (Goldman et al. 1986; Gray et al. 1989). The hypothalamus of rats exposed to 

50 mg/kg/day developed elevated levels of gonadotropin releasing hormone (GnRH) (Goldman et al. 

1986). Serum levels of thyroid stimulating hormone (TSH) were decreased in rats following exposure to 

100–200 mg/kg/day methoxychlor (Gray et al. 1989). Serum T3 (triiodothyronine) levels were increased 

in female rats at 100 and 500 mg/kg/day, and T3 and TSH levels were increased in male rats exposed to 

500, but not 100, mg/kg/day for 28 days (Okazaki et al. 2001). Serum luteinizing hormone (LH) levels 

were decreased in female rats exposed to 100, but not 20, mg/kg/day methoxychlor, and serum FSH and 

prolactin levels were increased and testosterone levels were decreased in male rats exposed to 500, but 

not 100, mg/kg/day (Okazaki et al. 2001). Serum LH, prolactin, testosterone, and interstitial fluid 

testosterone were unaffected in male rats exposed to 200–400 mg/kg/day for 44 weeks (Gray et al. 1999). 

Effects of methoxychlor on the endocrine system are likely related to methoxychlor-associated 

reproductive disturbances. For example, decreased serum progesterone levels (Chapin et al. 1997; 

Cummings and Gray 1989; Cummings and Laskey 1993) may result from the estrogenic effects of 

methoxychlor on the ovaries that cause decreased follicular and corpus luteum development (Chapin et al. 

1997). The effects on the ovaries may be caused directly by methoxychlor metabolites or may result from 

effects on the pituitary and hypothalamus, which alter the release of regulatory hormones that affect the 

reproductive and accessory sex glands (Chapin et al. 1997; Martinez and Swartz 1992). There is a high 

level of interdependence between the hypothalamus, pituitary, and peripheral endocrine glands; therefore, 

feedback mechanisms between the peripheral endocrine glands and the pituitary and hypothalamus and 

between the pituitary and the hypothalamus are probably also involved in the alteration of hormone levels 

by methoxychlor. Effects of methoxychlor on the reproductive system are discussed in more detail in 

Section 3.2.2.5 Reproductive Effects, and mechanisms for effects on the endocrine and reproductive 

systems are discussed in Section 3.5.2 Mechanisms of Toxicity and Section 3.6 Toxicities Mediated 

Through the Neuroendocrine Axis. 

Dermal Effects. Gross and microscopic examination of the skin did not reveal any treatment related 

effects in rats or mice chronically orally exposed to 107 or 599 mg/kg/day methoxychlor, respectively 

(NCI 1978). These data are too sparse to permit a firm conclusion to be made on the dermal effects of 

methoxychlor, but suggest that the skin is not a target system. 
 

 

 

METHOXYCHLOR 46 

3. HEALTH EFFECTS 

Ocular Effects. No studies were located regarding ocular effects in humans after oral exposure to 

methoxychlor. No ocular effects were seen in rats or mice chronically exposed to 107 or 599 mg/kg/day 

methoxychlor, respectively (NCI 1978). 

Body Weight Effects. Several animal studies suggest that oral exposure to methoxychlor can lead to 

decreased body weight. Acute exposure of rats to 40.8–400 mg/kg/day or intermediate exposure to 

25–400 mg/kg/day resulted in decreased growth (Chapin et al. 1997; Culik and Kaplan 1976; Gray et al. 

1989, 1999; Khera et al. 1978). A 14.4% decrease in body weight gain was noted in pregnant rabbits 

administered 35.5 mg/kg/day methoxychlor, but not 5.01 mg/kg/day methoxychlor (Kincaid Enterprises 

1986). Rats exposed to 100–1,000 mg/kg/day methoxychlor for 90 days had a 5–43% decrease in body 

weight gain (Dikshith et al. 1990). In dogs exposed to 1,000 mg/kg/day for 8–24 weeks, decreased body 

weight was accompanied by an absence of body fat in normal depot areas (Tegeris et al. 1966). Chronic 

exposure of rats to 107 mg/kg/day (NCI 1978) or 229 mg/kg/day (Haag et al. 1950) or mice to 

599 mg/kg/day (NCI 1978) produced a decrease in body weight gain. Since many of these studies also 

reported depressed food intake in methoxychlor-treated animals, it is possible that this is responsible, in 

part, for the observed decreases in body weight gain. However, in studies in which pair-fed controls were 

used, a 22% decreased body weight was observed in rats fed diets containing 861 mg/kg/day for 

33–55 days (Tullner and Edgcomb 1962) or 1,200 mg/kg/day for 16 weeks (Davison and Cox 1976). 

These data indicate that body weight gain may be adversely affected by relatively high doses of 

methoxychlor. 

Metabolic Effects. No studies were located regarding metabolic effects in animals after oral exposure 

to methoxychlor. 

3.2.2.3 Immunological and Lymphoreticular Effects 

No studies were located regarding immunological effects in humans after oral exposure to methoxychlor. 

In animals, no adverse histological effects were detected in the thymus, spleen, or lymph nodes of rats 

chronically exposed to 107 mg/kg/day or mice chronically exposed to 599 mg/kg/day (NCI 1978). 

However, histological examination may not be sensitive enough to detect changes in immune system 

function. In male (but not female) rats exposed to 5 or 50 mg/kg/day methoxychlor from gestation 

day 14 through postnatal day 42, a respective 35 or 42% decrease in plaque-forming cells/spleen was 

observed, indicating a possible attenuation of the primary immune response (Chapin et al. 1997). The 

plaque-forming cell assay quantitates the production of a specific antibody following administration of an 
METHOXYCHLOR 47 

3. HEALTH EFFECTS 

antigen. The physiological functioning of a number of immune system components, including B cells, 

T helper cells, and macrophages, are reflected in this assay. Thymus weights were decreased by about 

15 and 30% in rats exposed to 50 and 150 mg/kg/day, respectively, from gestation day 14 through 

postnatal day 42 (Chapin et al. 1997). No histological data were provided. Spleen weight, splenic natural 

killer cell activity, splenic lymphoproliferative response, and splenic cell-surface phenotypes did not 

differ from controls. The study authors suggested that the observed effects on plaque-forming cell 

numbers in males may have been anomalous because no such effects were observed in females and no 

changes were observed in males in related end points such as splenic lymphoproliferative response 

(Chapin et al. 1997). However, atrophy of the thymus was also observed in male, but not female, rats 

exposed to 500, but not 100, mg/kg/day methoxychlor for 28 days (Okazaki et al. 2001). Relative thymus 

weights were lower in female rats exposed to 500, but not 100, mg/kg/day, but the difference did not 

reach statistical significance (Okazaki et al. 2001). No histological abnormalities were noted. These data 

suggest that high exposure levels of methoxychlor may affect certain immunological parameters in 

rodents and that male rats may be more sensitive to the immunological effects of methoxychlor than 

females. Considering the available data, the immune system does not appear to be a primary target for 

methoxychlor. 

3.2.2.4 Neurological Effects 

A single case report documented the ingestion of approximately 125 mL of a commercial product that 

contained methoxychlor (about 15 mg of methoxychlor) by a 62-year-old man in an attempted suicide 

(Thompson and Vorster 2000). The man showed no response to pain or verbal stimuli and had pale skin 

with profuse sweating. Testing of a serum sample collected at the time of admission to the hospital 

showed a methoxychlor level of 0.67 μg/mL serum. 

Neurological effects have been observed in some animals exposed to methoxychlor. Large acute doses of 

1,000 mg/kg methoxychlor or more administered to rats produced neurological effects such as decreased 

locomotor activity, tremors, lacrimation, salivation, nasal bleeding, dyspnea, diarrhea, convulsions, and 

paralysis (Cannon Laboratories 1976; Dikshith et al. 1990). In dogs, exposure to 1,000–4,000 mg/kg/day 

methoxychlor for 8–24 weeks produced a dose-dependent increase in neurological effects including 

apprehension, nervousness, increased salivation, tremors, convulsions, and death (Tegeris et al. 1966). 

When rats with liver damage (induced by carbon tetrachloride) were exposed to methoxychlor, they 

exhibited tremors similar to those described above for dogs (Lehman 1952). Assuming that one of the 

effects of the liver injury is decreased metabolism of methoxychlor, this observation suggests that the 
METHOXYCHLOR 48 

3. HEALTH EFFECTS 

parent compound (rather than a metabolite) may be mainly responsible for these types of neurological 

effects. This idea is consistent with the observation that DDT (a poorly metabolized analogue of 

methoxychlor) also produces similar neurological signs (Agency for Toxic Substances and Disease 

Registry 1994). An increased incidence (significance not reported) of hunched posture and rough fur was 

reported in rats exposed to 22–107 mg/kg/day methoxychlor in the feed for 78 weeks (NCI 1978). No 

changes in brain weight or histopathology were noted in rats or mice chronically exposed to 107 or 

599 mg/kg/day methoxychlor, respectively (NCI 1978) or in rats exposed to up to 1,000 mg/kg/day for 

90 days (Dikshith et al. 1990). 

Exposure to methoxychlor has also produced behavioral changes in animals. Doses of 200 or 

400 mg/kg/day administered for 55–66 days produced an increased running wheel activity in 

ovariectomized and intact female rats, respectively (Gray et al. 1988). In rats, running wheel activity is 

regulated by estrogen and is synchronous with the 5-day estrus cycle, being greater during proestrus than 

during estrus and diestrus. Ovariectomy reduces running wheel activity and abolishes cyclicity. 

Administration of progesterone to methoxychlor-treated ovariectomized rats significantly lowered 

running wheel activity (Gray et al. 1988). Because progesterone blocks the synthesis of estrogen 

receptors in the central nervous system and reproductive tract but does not lower running wheel activity 

induced by nonestrogenic mechanisms, the authors concluded that the observed inhibition of 

methoxychlor-induced running wheel activity by progesterone was evidence of an estrogenic effect of 

methoxychlor on the central nervous system. 

The administration of 200 mg/kg/day methoxychlor for 2 weeks also increased receptivity to mating in 

ovariectomized hamsters (Gray et al. 1988). Behavioral estrus remained in more than half of the hamsters 

at 1 week post-exposure. Although the magnitude of the effects of methoxychlor on mating receptivity in 

ovariectomized hamsters was not as great as that observed with estradiol (1.0 mg/kg), the effects persisted 

longer following cessation of exposure (Gray et al. 1988). Nine of 14 hamsters exposed to methoxychlor 

for 13 days displayed lordosis behavior, and15/15 hamsters exposed to weekly administration of estrogen 

displayed lordosis behavior. Neurobehavioral effects were also observed following exposure of rats to 5, 

50, or 150 mg/kg/day methoxychlor from gestation day 14 through postnatal 21 (Chapin et al. 1997). 

Male rats treated with 150 mg/kg/day, but not 5 or 50 mg/kg/day, showed an increased handling 

reactivity. Non-dose-related but statistically significant changes were seen for sensorimotor parameters: 

locomotor activity (increased in females in the 50 mg/kg/day group, but not at 5 or 150 mg/kg/day), click 

response (decreased in males at 5 and 150 mg/kg/day, but not 50 mg/kg/day, and in females at 

5 mg/kg/day, but not 50 or 150 mg/kg/day), and approach stimulus (lower in males at 5 mg/kg/day, but 
METHOXYCHLOR 49 

3. HEALTH EFFECTS 

not 50 or 150 mg/kg/day). Temporary but statistically significant changes that occurred at various times 

during the study included increased urination in the open field in females at 50 and 150 mg/kg/day, but 

not 5 mg/kg/day on postpartum day 66, increased defecation in females at 150 mg/kg/day, but not 5 or 

50 mg/kg/day on postpartum day 31, and decreased hindlimb grip strength in females at 50 mg/kg/day, 

but not 5 or 150 mg/kg/day on postpartum days 47 and 66. There were no differences between the 

controls and treatment groups in the passive avoidance/cognitive function tests. 

An increase in urine-marking behavior was observed in adult male offspring of mouse dams exposed to 

0.02 mg/kg/day on gestation days 11–17 (vom Saal et al. 1995). This experiment is also described in 

Parmigiani et al. (1998). Parmigiani et al. (1998) also observed a statistically significantly increased 

incidence of aggressive behavior (infanticide) toward unrelated pups in adult male offspring of mice 

treated with 1.8 mg/kg/day during gestation days 11–17, but not at lower (0.02 or 0.18 mg/kg/day) or 

higher (18.2 or 90.0 mg/kg/day) exposure levels. Exposure to 0.02–182 μg/kg/day of the potent estrogen, 

DES, had no effect on this behavior. However, limitations in the study designs of vom Saal et al. (1995) 

and Parmigiani et al. (1998) make these studies difficult to interpret, (please refer to Appendix A for a 

more detailed discussion of the study limitations). Aggressive grooming behavior was altered in a nonmonotonic fashion in male mice orally exposed in utero to 0.02, 0.2, or 2.0 mg/kg/day methoxychlor 

(Palanza et al. 1999). This behavior was statistically significantly decreased in the 0.02 mg/kg/day group, 

and was increased and decreased in the 0.2 and 2.0 mg/kg/day groups, respectively, but without statistical 

significance. This change in behavior was only seen on day 39, but not on day 54, postpartum. Unlike 

the neurological effects (tremors, convulsions) discussed above, these behavioral effects of methoxychlor 

(increased running wheel activity and receptivity to mating, urine-marking, infanticide), which have been 

shown previously to be influenced by estrogen or testosterone, are most likely attributable to one or more 

of the metabolites or contaminants of methoxychlor that exhibit estrogenic or anti-androgenic activity. 

The highest NOAEL values and all LOAEL values from each reliable study for the neurological effects of 

methoxychlor in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1. 

3.2.2.5 Reproductive Effects 

A single study was located regarding effects of oral exposure to methoxychlor on reproductive tissues in 

humans. Electron microscopic examination of biopsy samples from the testes of men exposed to 

2 mg/kg/day (three men) for 6 weeks revealed no histopathological changes from controls (three men) 
METHOXYCHLOR 50 

3. HEALTH EFFECTS 

(Wills 1969). In addition, no effects on menstrual cycles were noted in similarly dosed women at 0.5, 

1.0, or 2.0 mg/kg/day (four per exposure group). 

The reproductive effects of methoxychlor have been well studied in animals. These studies indicate that 

the reproductive system is a sensitive target of methoxychlor toxicity in both males and females. Effects 

associated with methoxychlor exposure include histopathological changes in the reproductive organs and 

accessory glands, impaired pubertal development and reproductive function, and altered hormone levels. 

These effects are due to the estrogenic activity of both the O-demethylated metabolites of methoxychlor 

and some of the O-demethylated contaminants of technical grade methoxychlor (Bulger et al. 1985). 

Information regarding the reproductive effects of methoxychlor in male and female animals is presented 

separately below. 

Reproductive Effects in Female Animals. Methoxychlor adversely affects the development of the female 

reproductive system. The day of vaginal opening and first estrus was significantly earlier in female rats 

exposed to 25–100 mg/kg/day methoxychlor beginning on postpartum day 21 and extending through 

mating, gestation, and lactation (Gray et al. 1989). In these experiments, vaginal opening occurred at an 

average age of 32 days in controls, but in exposed groups, occurred as early as 3 days after beginning 

exposure. Average age at vaginal opening ranged from 24 to 26 days in the exposed groups. The onset 

of estrus cyclicity was accelerated in animals exposed to 25 mg/kg/day and normal in animals exposed to 

50–100 mg/kg/day. The onset of estrus cyclicity was delayed in animals dosed with 200 mg/kg/day. 

Similarly, precocious vaginal opening and estrus were noted in female rats exposed in utero, during 

lactation and postweaning to 5–150 mg/kg/day (Chapin et al. 1997; Harris et al. 1974). In female rats 

exposed to 5, 50, or 150 mg/kg/day during gestation days 14–21, via lactation during postpartum 

days 1–7, and then directly via oral gavage through postpartum day 42 (dams were no longer treated after 

postpartum day 7 and pups stayed with the dams until postpartum day 21), vaginal opening was 

accelerated by 2–7 days (35.2, 30.8, and 33.4 days postpartum in the 5, 50, and 150 mg/kg/day groups, 

respectively, compared to 37.4 days for the controls) (Chapin et al. 1997). Female rats exposed in utero 

throughout gestation to 0, 60, or 150 mg/kg/day methoxychlor and maintained on the same oral dose 

postnatally had earlier vaginal opening than controls (23 and 19 days postpartum in the 60 and 

150 mg/kg/day groups, respectively, compared to 39 days for the controls) (Harris et al. 1974). Fertility 

was significantly impaired in females exposed to 50 or 150 mg/kg/day, but not to 5 mg/kg/day, between 

gestation day 14 and postnatal day 42 (Chapin et al. 1997). Incidences for females that became pregnant 

after cohabitating with untreated males were 13/15 (control), 11/15 (5 mg/kg/day), 3/15 (50 mg/kg/day), 
METHOXYCHLOR 51 

3. HEALTH EFFECTS 

and 0/15 (150 mg/kg/day) (Chapin et al. 1997). Descriptions of developmental effects can be found in 

greater detail in Section 3.2.2.6 Developmental Effects. 

Methoxychlor produces gross and histopathological changes in the mature female reproductive system 

after oral exposure. An accumulation of lipid was observed in ovarian interstitial and thecal cells of mice 

exposed to 100 mg/kg/day for 4 weeks (Martinez and Swartz 1992). Cellular hypertrophy of the uterine 

epithelial cells was observed in adult female mice exposed to 50 or 100 (but not 2, 10, or 20) mg/kg/day 

methoxychlor (as Marlate, a 50% mixture; contaminants unknown) 5 days/week for 4 weeks; this effect 

was similar to the response to estradiol (Swartz et al. 1994). Electron microscopic examination revealed 

dose-related ultrastructural changes, including one or more of the following at all dose levels: elongated 

microvilli, decreased numbers of microvilli, increased vacuoles, dilated endoplasmic reticulum and Golgi 

complexes, enlarged and disrupted mitochondria, and irregular apical cell surface. A marked uterotrophic 

effect (as indicated by a 2–3-fold increase in uterine weight) was observed in ovariectomized mice 

exposed to 16.7 mg/kg/day and in ovariectomized plus adrenalectomized rats and hypophysectomized rats 

exposed to 95.2 mg/kg/day for 3 days (Tullner 1961). From the time of vaginal opening until mating, 

increased vaginal cornification and a decreased percentage of vaginal smears with leukocytes was 

observed in female rats acutely exposed to 50–200 mg/kg/day pre- and postnatally (Chapin et al. 1997) or 

beginning on postpartum day 21 (Gray et al. 1989) or exposed to 400 mg/kg/day for intermediate 

durations (Gray et al. 1988). An enlarged uterus was observed in rats exposed to 150 mg/kg/day for 

6 weeks (Harris et al. 1974) or 100 or 500 mg/kg/day for 28–31 days (Okazaki et al. 2001) and in pigs 

exposed to 1,000 mg/kg/day for 24 weeks (Tegeris et al. 1966). However, absolute uterine weight was 

reduced by 35% (and relative uterine weight by 27%) in intact virgin female rats exposed to 

150 mg/kg/day from gestation day 14 through postnatal 42 (Chapin et al. 1997). This effect was even 

more pronounced on the pregnant uterus (21–50% reduction in weight of empty uterus at exposures of 

5–50 mg/kg/day) (Chapin et al. 1997). Mice treated initially with ENU (a uterine cancer initiator) and 

then administered 390 mg/kg/day methoxychlor for 26 weeks had no increased incidence of tumors, but 

had increased absolute and relative uterine weights (Mitsumori et al. 2000). Mammary gland hyperplasia 

was also observed in rats exposed to 500, but not 100, mg/kg/day for 28–31 days (Okazaki et al. 2001) or 

pigs exposed to 1,000 mg/kg/day for 24 weeks (Tegeris et al. 1966), and decreased ovarian weight was 

reported in adult rats exposed to 500, but not 100, mg/kg/day methoxychlor for 90 days (Dikshith et al. 

1990). Intermediate-duration exposures to 25–500 mg/kg/day methoxychlor produced atrophic changes 

in the ovaries of exposed female mice and rats (Bal 1984; Chapin et al. 1997; Gray et al. 1988, 1989; 

Harris et al. 1974; Martinez and Swartz 1991; Okazaki et al. 2001). Some of these effects mimic those of 

estrogen, but some differ from estrogenic effects. Methoxychlor (60–300 mg/kg/day) and estradiol 
METHOXYCHLOR 52 

3. HEALTH EFFECTS 

benzoate (0.015–0.075 mg/kg/day) resulted in decreased mating frequency and ovarian weight, increased 

uterine weight, and precocious vaginal opening, whereas only methoxychlor caused increased estrus 

(Harris et al. 1974). In adult ovariectomized female rats in which endometriosis had been surgically 

induced, administration of 250 mg/kg/day methoxychlor for 3 weeks resulted in maintenance of 

endometriotic sites, similar to that seen following administration of estrone (1 μg/rat), and also similar to 

intact rats (endogenous estrogen present) in which endometriosis had been induced (Cummings and 

Metcalf 1995b). Ovariectomized vehicle control rats (no endogenous or exogenous estrogen present) 

showed a substantial regression in size of endometriotic sites. These data indicate that estrogens 

maintain, or prevent regression of, endometriotic sites, and that sufficient levels of methoxychlor produce 

results similar to those of estrogen. 

Oral exposure to methoxychlor adversely affects reproductive function in mature females by interfering 

with estrus cyclicity and decreasing fertility. Female rats exposed to 500, but not 100, mg/kg/day 

methoxychlor for 28–31 days had significantly fewer estrus cycles and remained in estrus longer than 

controls (Okazaki et al. 2001). Rats that received 100 mg/kg/day methoxychlor showed a nonsignificant 

decrease in the number of estrus cycles (Okazaki et al. 2001). Persistent vaginal cornification (sometimes 

referred to as persistent estrus) was observed in female mice acutely exposed to 25 mg/kg/day (Martinez 

and Swartz 1991) and in female rats exposed to 300–400 mg/kg/day for intermediate durations (Gray 

et al. 1988; Harris et al. 1974). Female pigs exposed to 1,000 mg/kg/day for 24 weeks failed to come into 

estrus during the exposure period (Tegeris et al. 1966). Female mice exposed to a wide range of doses of 

methoxychlor (0.02, 0.2, 2, 20, or 100 mg/kg/day) on gestational days 11–17 showed no decrease in 

number of pups/litter, and in fact showed an increase at 0.2 mg/kg/day (Palanza et al. 2001). The study 

authors interpreted the non-monotonic response as an overall lack of effect on number of pups/litter. 

Female rats exposed to 100 mg/kg/day methoxychlor beginning on postpartum day 21 showed a 40% 

decrease in fertility and live pups/litter when mated with untreated males and an 80% decrease when 

mated with similarly treated males (Gray et al. 1989). Higher doses (200 mg/kg/day) produced infertility 

in 100% of the animals. Females that did not become pregnant had no implantation sites indicating that 

the effect occurs prior to implantation. Significant decreases in fertility or complete infertility occurred in 

female rats following intermediate-duration exposure to 100 mg/kg/day (Bal 1984) or to 50 or 

150 mg/kg/day (Chapin et al. 1997). With chronic-duration exposure, decreases in fertility were noted in 

the second and third generation male and female rats exposed to 79 and 92 mg/kg/day methoxychlor, 

respectively, for three generations (Haskell Laboratories 1966); no effects were seen in rats exposed to 

18 mg/kg/day for 3 generations. These decreases were predominantly attributed to effects on female 

animals rather than male animals. No effects on fertility were noted in rats similarly treated with 
METHOXYCHLOR 53 

3. HEALTH EFFECTS 

16 (males) or 18 (females) mg/kg/day methoxychlor. In female rats, a decreased mating frequency and a 

decreased fertility in those that mated were noted following intermediate-duration exposure to 

60–150 mg/kg/day (Harris et al. 1974). An increased number of resorptions have been consistently 

reported in female rats following acute- and intermediate-duration exposure to 35.5–200 mg/kg/day 

laboratory grade and technical grade methoxychlor (Culik and Kaplan 1976; Cummings and Gray 1989; 

Cummings and Perreault 1990; Gray et al. 1989; Harris et al. 1974; Kincaid Enterprises 1986). 

The effects of methoxychlor on uterine decidualization (the buildup of the uterine lining necessary for 

implantation and pregnancy) vary with exposure level. Acute exposures to 100–250 mg/kg/day 

laboratory grade methoxychlor in female rats reportedly inhibit the decidual growth of the uterus 

(Cummings and Gray 1987, 1989; Cummings and Laskey 1993; Cummings and Perreault 1990). Further 

investigation by Cummings (1993) showed that methoxychlor acts as an estrogen to induce uterine 

decidualization only over a narrow exposure range. The artificially induced decidual cell response in the 

rodent mimics the implantation of the blastocyst and induces the development of the uterine decidual 

tissue. This response assay was used by Cummings (1993) to investigate the estrogenicity of 

methoxychlor and its mechanism of reducing fertility in female rats. Estrogen and progesterone are 

required to induce the decidualization of the uterus. In ovariectomized female rats (used because no 

endogenous estrogen or progesterone is present and pseudopregnancy can be initiated with exogenous 

estrone and progesterone administration), exposure to low doses (5 or 50 mg/kg/day) of laboratory grade 

methoxychlor plus 2 mg progesterone for 8 days produced the same degree of decidualization as in 

controls (progesterone alone). Higher doses (75 or 100 mg/kg/day) of methoxychlor plus progesterone 

produced maximal decidualization similar to estrone (0.004 mg/kg/day) plus progesterone. 

Decidualization response decreased with further increased methoxychlor dose (500 mg/kg/day) plus 

progesterone (Cummings 1993). Such a decrease in decidualization might contribute to preimplantation 

loss. Methoxychlor also has been shown to accelerate embryo transport into the uterus, which may 

further contribute to increases in preimplantation loss (Cummings and Laws 2000; Cummings and 

Perreault 1990). These findings provide insight into the possible mechanisms by which methoxychlor 

reduces fertility in female rats. 

Exposure to methoxychlor produces changes in a number of hormone levels in female animals after oral 

exposure. Decreased serum progesterone levels were observed in female rats acutely exposed to 

50–500 mg/kg/day laboratory grade methoxychlor, but not to 25 mg/kg/day (Cummings and Gray 1989; 

Cummings and Laskey 1993). Serum estradiol levels were not altered (Cummings and Laskey 1993). 

When ovaries from methoxychlor-treated rats were removed and incubated with human chorionic 
METHOXYCHLOR 54 

3. HEALTH EFFECTS 

gonadotropin (to stimulate steriod production and release), ovarian progesterone concentration was 

unaffected, but ovarian estradiol and testosterone concentrations were significantly reduced (Cummings 

and Laskey 1993). Pituitary levels of prolactin were decreased in intact female rats but increased in 

ovariectomized rats exposed to 400 mg/kg/day for intermediate-durations (Gray et al. 1988). In female 

rats dosed with 100 mg/kg/day methoxychlor in oil by gavage for 4 weeks, a number of ultrastructural 

changes were noted in the ovarian cells, including the accumulation of lipid (Martinez and Swartz 1992). 

Similar effects were seen in rats exposed to 1 mg/kg/day estradiol-17β. The authors suggested that these 

cells retained their ability to synthesize lipids but have lost the ability to convert lipids to steroid 

hormones. The changes in hormone levels discussed above may also be an important mechanism by 

which methoxychlor can affect female reproduction. Martinez and Swartz (1992) speculated that 

methoxychlor causes a feedback inhibition of pituitary hormone secretions resulting in a lack of 

stimulation of ovarian cells to produce hormones that would have otherwise exerted an effect on the 

uterus and other estrogen-sensitive tissues. 

An intermediate-duration oral MRL of 0.005 mg/kg/day was derived based on the LOAEL of 

5 mg/kg/day for accelerated onset of puberty (i.e., precocious vaginal opening) in immature female rats 

exposed to methoxychlor in utero, during lactation, and after weaning (Chapin et al. 1997), as described 

in the footnote in Table 3-1. 

Reproductive Effects in Male Animals. Methoxychlor adversely affects the development of the male 

reproductive system. Preputial separation was significantly delayed in a dose-dependent manner 

(days 40–42 in controls, day 43.8 in the 100 mg/kg/day methoxychlor group, days 50–53 in the 

200 mg/kg/day, day 68 in the 300 mg/kg/day group, and day 74 in the 400 mg/kg/day group) in male rats 

exposed to 100–400 mg/kg/day beginning on postpartum day 21 for 56 days to 10 months (Gray et al. 

1989, 1999), suggesting that sexual maturity was delayed. Male fertility in rats exposed to methoxychlor 

from gestation day 14 to postpartum day 42 was significantly impaired at 150 mg/kg/day, but not at 5 or 

50 mg/kg/day; only 2/15 150 mg/kg/day rats successfully impregnanted untreated females, compared with 

13/15 in the control group (Chapin et al. 1997). Gray et al. (1989) also observed a slight decrease in 

testes weight and caudal epididymal sperm count in male offspring of rats exposed to 50–200 mg/kg/day 

methoxychlor for 59 days. Welshons et al. (1999) observed a substantial increase in prostate weight in 

adult male mice exposed in utero (gestation days 11–17) to 0.02 and 2.0 mg/kg/day, and seminal vesicle 

weight was increased in a dose-dependent manner. Preputial gland, testes, and adrenal gland weights 

were not significantly affected. Lateral, but not ventral, prostate weight was increased by 44%, and 

inflammation was evident in male rats exposed to 50 mg/kg/day methoxychlor from gestation 
METHOXYCHLOR 55 

3. HEALTH EFFECTS 

day 18 through postpartum day 5 (Stoker et al. 1999). In a simultaneous experiment, lateral prostate 

inflammation was also seen in male rats exposed to estradiol-17β from gestation day 18 through 

postpartum day 5 (dams received 134 μg on gestation day 18–22, then pups received 6.7 μg on 

postpartum days 1–5), but lateral prostate weight was decreased by 59% (Stoker et al. 1999). Testis 

weight was not affected. Mating frequency and fertility in rats were decreased in male offspring of rats 

exposed to 60 mg/kg/day methoxychlor (Harris et al. 1974). In addition to structural and functional 

effects, urine-marking behavior and infanticide behavior toward unrelated pups were altered in male 

offspring of mice exposed to 0.02–91 mg/kg/day methoxychlor in utero (Parmigiani et al. 1998; vom Saal 

et al. 1995). These reproductive/developmental effects are discussed in more detail in Section 3.2.2.6 

Developmental Effects. 

Oral exposure to methoxychlor can produce gross and histopathological changes in the mature male 

reproductive system. Decreased weight of the testes was observed in male rats and mice exposed to 

50–1,400 mg/kg/day for intermediate-durations (Bal 1984; Dikshith et al. 1990; Gray et al. 1989, 1999; 

Hodge et al. 1950; Tullner and Edgcomb 1962; Wenda-Rozewicka 1983). In addition, a decreased 

ventral prostate weight was observed in male rats exposed to 154 mg/kg/day for 90 days (Shain et al. 

1977) or 100 mg/kg/day for 28 days (Okazaki et al. 2001). Atrophy of the dorso-lateral prostate was also 

seen in rats following exposure to 500 mg/kg/day for 28 days (Okazaki et al. 2001). Three studies 

reported a decreased caudal epididymal sperm count in rats exposed to 50–500 mg/kg/day for 

intermediate-duration exposures (Gray et al. 1989, 1999; Okazaki et al. 2001). No gross or 

histopathological changes were noted in the testes of male mice acutely exposed to 60 mg/kg/day 

(Wenda-Rozewicka 1983) or 200–400 mg/kg/day (Gray et al. 1999). A dose-related inhibition of testes 

development, fewer mature germ cells, and fewer germ cells of each type were observed in the testes of 

male rats exposed to 5, 50, or 150 mg/kg/day from gestation day 14 through postnatal day 42 (Chapin et 

al. 1997). Additionally, testis, epididymis, seminal vesicle, and prostate weights were decreased by 

23–85% in males exposed to 50 or 150 mg/kg/day (Chapin et al. 1997) or to 100 or 500 mg/kg/day for 

28 days (Okazaki et al. 2001). Decreases in prostate weight have also been seen following exogenous 

estradiol administration (Rajfer and Coffey 1978; Stoker et al. 1999; vom Saal et al. 1997). 

Exposure to methoxychlor by the oral route adversely affects reproductive function in mature male 

animals. Fertility was decreased by 80% when males exposed to 100 mg/kg/day from postpartum 

day 21 until necropsy (on postpartum days 77–87) were mated with similarly treated females, compared 

to only a 50% decrease when untreated males were mated with treated females (Gray et al. 1989). Since 

pseudopregnancies were not observed in females that were mated with methoxychlor-treated males, the 
METHOXYCHLOR 56 

3. HEALTH EFFECTS 

authors concluded that treated males failed to provide sufficient copulatory stimulation to induce changes 

in the females necessary for pregnancy (Gray et al. 1989). Fertility was decreased by 50–75% when 

males exposed to 200–400 mg/kg/day methoxychlor from postpartum day 21 until necropsy at 11 months 

of age were mated with untreated females (Gray et al. 1999). Decreased fertility was also reported in 

male mice exposed to 60 mg/kg/day for intermediate durations (Wenda-Rozewicka 1983) and in male rats 

exposed to 150 mg/kg/day from gestation day 14 through postnatal day 42 (Chapin et al. 1997). In 

addition, mating frequency and fertility in those that mated were significantly reduced in the male rats 

exposed in utero, during lactation, and postweaning to 60 mg/kg/day (Harris et al. 1974) or 

200–400 mg/kg/day (Gray et al. 1999). These data collectively indicate that methoxychlor can decrease 

fertility in male animals and that the reduced fertility in male animals may be due to impaired mating 

behavior, inadequate stimulation of the female animal, or deficits in other reproductive parameters 

(decreased sperm count, testicular atrophy). 

Methoxychlor also produces changes in hormone levels in male animals after oral exposure. Changes in 

hormone levels often accompany or precede the reproductive effects discussed above. Exposure to 

25–50 mg/kg/day methoxychlor produced increased levels of prolactin, follicle stimulating hormone 

(FSH), and thyroid stimulating hormone (TSH) in the pituitary of male rats (Goldman et al. 1986; Gray 

et al. 1989; Stoker et al. 1999); serum prolactin levels were non-statistically significantly increased at 

200 mg/kg/day and serum FSH levels were not affected (Gray et al. 1989). Prolactin levels in adult male 

rats were not affected by estradiol administration during gestation (gestation days 18–22) and postpartum 

days 1–5 (Stoker et al. 1999). The hypothalamus of rats exposed to 50 mg/kg/day developed elevated 

levels of gonadotropin releasing hormone (GnRH) (Goldman et al. 1986). Serum levels of several 

hormones (TSH, testosterone, progesterone) were decreased in rats as a result of exposure to doses of 

100 mg/kg/day laboratory grade or technical grade methoxychlor (Cummings and Gray 1989; Gray et al. 

1989), although serum LH, prolactin, testosterone, and interstitial fluid testosterone were unaffected in 

male rats exposed to 200–400 mg/kg/day for 44 weeks (Gray et al. 1999). Similarly, reduced levels of 

testosterone were reported in the interstitial fluid and epididymis of male rats exposed to 100 mg/kg/day 

from postpartum day 21 to postpartum day 77 (Gray et al. 1989). Effects of methoxychlor on the 

endocrine system are likely related to some of the histopathological, functional, and behavioral changes 

described above (see Section 3.5.2, Mechanisms of Toxicity for further discussion). 

The highest NOAEL values and all LOAEL values from each reliable study for the reproductive effects of 

methoxychlor in each species and duration category are recorded in Table 3-1 and plotted in Figure 3-1. 
METHOXYCHLOR 57 

3. HEALTH EFFECTS 

3.2.2.6 Developmental Effects 

No studies were located regarding developmental effects in humans after oral exposure to methoxychlor. 

In animals, exposure to methoxychlor during gestation can produce signs of maternal and fetal toxicity. 

The incidence of offspring with wavy ribs was significantly increased in rats exposed to 40.8 mg/kg/day 

or more methoxychlor on days 6–15 of gestation. This effect was also noted in rats administered 

17.8 mg/kg/day, however, the increase (2–3-fold) was not statistically significant (Culik and Kaplan 

1976). Acute doses of 50 mg/kg/day or more caused a decreased body weight in pregnant rats, whereas 

doses of 200 mg/kg/day produced an increased percentage of dead, resorbed, or anomalous fetuses and 

decreased fetal weight (Khera et al. 1978). The anomalies also consisted primarily of wavy or extra ribs. 

The skeletal effects were judged by the authors to be due to delayed development of alkaline phosphatase 

activity and to arrested calcium deposition in the ribs due to the toxicity of methoxychlor, and were not 

considered to be true signs of teratogenicity. Fetal body weights were decreased by 10% when pregnant 

rabbits were administered 35.5 mg/kg/day methoxychlor on days 7–19 of gestation (Kincaid Enterprises 

1986). In addition the percentage of offspring which were male was significantly decreased. These 

effects were not observed in rabbits administered 5.01 mg/kg/day methoxychlor (Kincaid Enterprises 

1986). 

Methoxychlor can adversely affect the reproductive development of rats and mice exposed in utero, 

during lactation, or after weaning. Effects of postweaning exposure on reproductive development are 

discussed in Section 3.2.2.5 Reproductive Effects. No effects were detected on reproductive development 

in male and female rats acutely exposed in utero (gestation days 14–20) to 30 mg/kg/day methoxychlor 

(Gellert and Wilson 1979). Female offspring of rats exposed for intermediate durations to 25 mg/kg/day 

methoxychlor from weaning through gestation and lactation exhibited precocious vaginal opening, and at 

50 mg/kg/day also exhibited abnormal estrus cycling and pituitary abnormalities (Gray et al. 1989). 

Precocious vaginal opening and delayed prepuce separation were also seen in female and male offspring, 

respectively, of rats exposed to 50 or 150 mg/kg/day methoxychlor from gestation day 14 through 

postnatal day 7, followed by direct exposure of the pups through postnatal day 42 (Chapin et al. 1997). 

Male offspring had slightly decreased testes weight and caudal epididymal sperm count compared to 

controls, but these changes were not statistically significant (Gray et al. 1989). 

A substantial and statistically significant increase in prostate weight (61% at 0.02 mg/kg/day and 51% at 

2.0 mg/kg/day) was seen in adult (9.5 months of age) male offspring of CF-1 mice exposed to 0, 0.02, and 
METHOXYCHLOR 58 

3. HEALTH EFFECTS 

2.0 mg/kg/day methoxychlor on gestation days 11–17 (Welshons et al. 1999), and seminal vesicle weight 

was increased by 20% in the 2.0 mg/kg/day group. Although no positive control was included in this 

study, the methoxychlor-induced prostate enlargement was greater than the enlargement observed in 

previous studies by the same investigators examining effects from gestational exposure to other estrogen 

receptor ligands (estradiol or DES) (vom Saal et al. 1997). There were no statistically significant 

exposure-related changes in body weight or weights of preputial glands, testes, or adrenals. These 

findings are in agreement with studies showing increased prostate weight in adult mice exposed to 

slightly elevated levels of estrogen perinatally (Nonneman et al. 1992; Timms et al. 1999; vom Saal 1989; 

vom Saal et al. 1997) and are consistent with the estrogenic activity of some methoxychlor metabolites. 

Only a small number of mice were assessed (one pup/litter from five to six litters) in this study. A more 

detailed analysis of Welshons et al. (1999) is presented in the Acute MRL Worksheet in Appendix A of 

this profile. Additionally, prostate weight in adult mice has been shown to be affected by intrauterine 

position of the fetus. Male mice that are between two female mice during in utero development have 

about a 35% higher serum level of estradiol due to diffusion from the adjacent females and have larger, 

heavier adult prostates than male mice that develop between two male mice (Nonneman et al. 1992; 

Timms et al. 1999; vom Saal 1989). Low levels of exogenous estrogen and estrogenic substances can 

also result in increased adult prostate weights (Rajfer and Coffey 1978; vom Saal et al. 1997). Higher 

levels of these same substances result in decreased adult prostate weights (Rajfer and Coffey 1978; vom 

Saal et al. 1997; Welshons et al. 1999). The exposure-response curve for this effect of estrogens takes on 

an inverted U-shape. This type of dose-response curve has also been reported for prostate weight 

following exposure to DES (vom Saal et al. 1997). U-shaped curves are recognized curvilinear doseresponse relationships in toxicological and epidemiological studies, especially at low doses in the 

threshold region of response (Davis and Svendsgaard 1990). Inflammation, and a corresponding weight 

increase, of the lateral prostate was observed in male offspring of rats exposed to 50 mg/kg/day 

methoxychlor from day 18 of gestation to postpartum day 5 (Stoker et al. 1999). No difference was noted 

in body weight, ventral prostate weight, testes weight, testosterone level, mean serum prolactin, or 

prostate deoxyribonucleic acid (DNA) levels. Reductions in testes, epididymis, seminal vesicle, and 

ventral prostate weights were seen in male offspring of rats exposed to 50 or 150 mg/kg/day from 

gestation day 14 through postnatal day 7, followed by direct exposure of the pups through postpartum 

day 42 (Chapin et al. 1997). Some in vitro data also suggest that methoxychlor and its metabolite, bishydroxy methoxychlor (HPTE), can alter rat embryonic testis development (Cupp and Skinner 2001). 

Breeding of rats (exposed in utero and through lactation to 50 mg/kg/day methoxychlor, but not exposed 

thereafter) resulted in a 30% decrease in fertility and a significant reduction in the number of pups/female 
 

METHOXYCHLOR 59 

3. HEALTH EFFECTS 

and litters/female (Gray et al. 1989). Treated F1 females mated with treated F1 males had 0, 0, 80, and 

100% decreases in fertility in the 25, 50, 100, and 200 mg/kg/day groups, respectively. Treated 

F1 females mated with untreated F1 males had 50 and 0% fertility, respectively, compared to 83% for 

controls. The fertility of treated males was not reduced when they were mated with untreated females. 

These data suggest that the decrements in fertility in the offspring were most likely attributed to effects in 

the female. Similar results were reported by Harris et al. (1974) in which pregnant female rats were 

exposed to 60–300 mg/kg/day for 6 weeks prior to mating and throughout gestation and lactation. After 

weaning, pups were maintained on the same treatment as the dams. The onset of puberty was accelerated 

in female offspring exposed to 60 mg/kg/day. Both males and females exhibited a decreased mating 

frequency and a decreased fertility in those animals that mated. 

A number of reproductive abnormalities were observed in male and female rats that were exposed to 50 or 

150 (but not 5) mg/kg/day from gestation day 14 through postpartum day 42 (Chapin et al. 1997). Female 

rats in both exposure groups had highly irregular or absent estrus cycles, and there was a severe reduction 

in the number of females that conceived when mated to nontreated males. In females that did conceive, a 

severe reduction in the number of implants was seen. This was accompanied by an increased incidence of 

endometrial squamous metaplasia, endometrial hyperplasia, vaginal cornification (an estrogenic 

response), vaginal epithelial hyperplasia, underdeveloped mammary tissue, and atrophied ovaries with 

little or no detectable follicular development and few or no corpora lutea. The serum 

estrogen:progesterone ratio was elevated, which the authors attributed to a lack of progesterone due to 

absence of ovulation and corpora lutea formation, and the FSH level was decreased. Males exposed to 

50 or 150 mg/kg/day from gestation day 14 through postnatal day 42 also displayed reproductive deficits 

including significantly decreased ability to impregnate untreated females. Fewer nontreated females had 

detectable sperm in their vagina when mated with male offspring exposed to 150 mg/kg/day, and sperm 

motility, epididymal sperm count, and testicular spermatid numbers were reduced in these male rats 

(Chapin et al. 1997). 

The offspring of mice (F1a) exposed to 50 mg/kg/day methoxychlor on days 6–15 of gestation and 

through lactation had an increased incidence of atretic follicles (Swartz and Corkern 1992). In addition, a 

second (unexposed) litter (F1b) produced following weaning of the F1a offspring exhibited precocious 

vaginal opening. It is unclear why the F1a exposed litter did not experience precocious vaginal opening 

and by what mechanism this effect was produced in the unexposed F1b offspring. 
METHOXYCHLOR 60 

3. HEALTH EFFECTS 

Effects on neurobehavioral development were also observed following exposure of rats to 5, 50, or 

150 mg/kg/day from gestation day 14 through postnatal 21 (Chapin et al. 1997). Male rats treated with 

150 mg/kg/day showed an increased handling reactivity. Non-dose-related but statistically significant 

changes were seen for sensorimotor parameters: locomotor activity (increased in females in the 

50 mg/kg/day group), click response (decreased in males and females at 5 mg/kg/day and males at 

150 mg/kg/day), and approach stimulus (lower in males at 5 mg/kg/day). Temporary but statistically 

significant changes that occurred at various times during the study included increased urination in the 

open field in females at 50 and 150 mg/kg/day, increased defecation in females at 150 mg/kg/day, and 

decreased hindlimb grip strength in females at 50 mg/kg/day. There were no differences between the 

controls and treatment groups in the passive avoidance/cognitive function tests. Male and female rats 

exposed in utero, via lactation, and then directly in the feed to 98, but not 9.8, mg/kg/day had increased 

intake of a sodium solution (an estrogen-responsive behavior) on postpartum days 69–75 (Ferguson et al. 

2000). 

A series of recent studies examined behavioral effects in adult mice after exposure to low levels of 

methoxychlor or other estrogenic agents during critical in utero developmental periods. A dosedependent increase in urine-marking behavior was observed in adult male offspring of mice exposed to 

0.02–91 mg/kg/day methoxychlor on gestation days 11–17 (vom Saal et al. 1995; also reported in 

Parmigiani et al. 1998). However, only two male pups/litter were tested, and each pup was tested only 

one time. It was also difficult to assess the accuracy of the measurement of the number of urine marks 

and to know how reproducible the results are, and there was no indication of what, if any, statistical 

methods were used to evaluate the results. The low level of exposure in this study was considered by the 

authors to be more reflective of the physiological levels at which hormonal effects from steroid receptor 

binding alone occur, without more wide ranging toxic effects caused by high levels of the (estrogenic) 

chemical. Urine-marking behavior is known to be influenced by testosterone levels and the male mouse’s 

social status, and urine marking also influences the social and reproductive behaviors of other mice, both 

male and female, that come in contact with deposited urine (Parmigiani et al. 1998). Therefore, changes 

in urine-marking behavior may disrupt reproductive behaviors. This group of investigators showed that 

in utero exposure to low levels of other estrogenic chemicals, such as diethylstilbestrol (DES) and o,pDDT, also increased the rate of depositing urine marks by mature male mice when they were placed in 

novel environments (Palanza et al. 1999). An increase in aggressive behavior (infanticide) toward 

unrelated pups was also observed in adult male offspring of mice treated with 1.8 (but not 0.02, 0.18, 

18.2, or 91) mg/kg/day methoxychlor during gestation days 11–17 (Parmigiani et al. 1998). This 

behavior was not affected by 0.02–182 μg/kg/day DES. The significance of these results were difficult to 
METHOXYCHLOR 61 

3. HEALTH EFFECTS 

determine due to the lack of biological information on the strain of mouse used in the study (“house 

mouse”), the small number of pups tested per litter (two males and two females), and the lack of doseresponse (the effect was only seen at one middle dose). These socio-sexual behaviors (urine-marking and 

infanticide) in mice have been proposed to be influenced by similar neuroendocrine and genetic 

mechanisms that may not always exhibit monotonic dose-response relationships (Palanza et al. 1999; 

Parmigiani et al. 1998). 

The highest NOAEL values and all LOAEL values from each reliable study for the developmental effects 

of methoxychlor in each species and duration category are recorded in Table 3-1 and plotted in 

Figure 3-1. 

3.2.2.7 Cancer 

No studies were located regarding carcinogenic effects in humans after oral exposure to methoxychlor. 

Evidence concerning the carcinogenicity of methoxychlor from animal studies is mainly negative, 

although this is somewhat controversial. The National Cancer Institute (NCI) conducted a study in which 

mice and rats were fed up to 599 and 107 mg/kg/day methoxychlor, respectively, for 78 weeks. The types 

and incidences of tumors in methoxychlor-fed animals did not differ significantly from control animals 

(NCI 1978). NCI concluded that methoxychlor was not carcinogenic to mice or rats under the conditions 

of this assay. Recrystallized methoxychlor was not carcinogenic to rats when administered at doses of 

77 mg/kg/day for 24–27 months, either alone or in combination with other chemicals (aldrin, aramite, 

DDT, and thiourea) (Deichmann et al. 1967). Similar results were reported by Radomski et al. (1965) 

with doses of 4 mg/kg/day. Hodge et al. (1952) reported that the tumors observed in methoxychlor-fed 

rats exposed to up to 80 mg/kg/day for 2 years occurred at a similar frequency as controls. Pituitary 

tumors were observed in the female rats exposed in utero to 50 mg/kg/day (Gray et al. 1989), but data 

regarding tumor incidence were not provided in this study. Adult female mice heterozygous for the p53 

allele that received a single intraperitoneal injection of N-ethyl-N-nitrosourea (ENU) to induce uterine 

tumor formation showed no enhancement of endometrial stromal sarcoma or atypical hyperplasia 

induction, whereas ethinylestradiol enhanced the development of ENU-induced uterine tumors 

(Mitsumori et al. 2000). 

The data from the cancer studies discussed above (Deichmann et al. 1967; Hodge et al. 1952; NCI 1978; 

Radomski et al. 1965), in addition to data from a number of unpublished studies by the FDA, were 
METHOXYCHLOR 62 

3. HEALTH EFFECTS 

examined and interpreted in a series of reports by Reuber (1979a, 1979b, 1980). Reuber (1979a) 

re-examined the histological slides from an unpublished FDA study, and reported an increased incidence 

of liver carcinomas (3/9 females and 3/8 males, compared to none observed in controls) in OsborneMendel rats fed 100 mg/kg/day for 2 years. In addition, Reuber reported an incidence of 1/10 and 

5/10 for ovarian tumors in female rats fed 5 and 25 mg/kg/day, respectively, for 2 years, although data 

concerning ovarian tumor incidence in control animals or in animals at higher doses was not provided. 

The incidence of other tumors was similar in both treated and control animals (Reuber 1979a). In a reanalysis of data from another unpublished FDA study, Reuber (1979b) reported an increased incidence of 

testicular tumors (27/51) in male Balb/c mice exposed to 97.5 mg/kg/day methoxychlor in the feed for 

2 years compared to an incidence of 8/71 in controls. In addition, the tumors observed in treated animals 

were reported to be larger, less differentiated and more invasive than those observed in control animals 

(Reuber 1979b). Based on the available data, Reuber concluded that methoxychlor produces liver tumors 

in mice and rats, and possibly in dogs (Reuber 1980). In addition, he concluded that methoxychlor is 

carcinogenic to the testes of male mice, bone of female mice, and the ovaries of female rats. However, 

there is considerable disagreement between Reuber and the original authors in the interpretation of the 

histopathological data. EPA (1987b) concluded that the differences observed between Reuber's 

interpretation and those of the original authors may be due in part to the use of inappropriate control data 

in Reuber's analysis and the difficulty in distinguishing histopathological lesions as regenerative 

hyperplasia, hyperplastic nodules, benign tumors, and malignant tumors. 

Based on a review of all the available data, EPA has classified methoxychlor as a Group D carcinogen, 

not classifiable as to human carcinogenicity (IRIS 2000). Similarly, IARC (1987) has classified 

methoxychlor as a Group 3 carcinogen (not classifiable as to its carcinogenicity to humans) and NCI 

(1978) concluded there was insufficient evidence to classify methoxychlor as a carcinogen. 

3.2.3 Dermal Exposure 

3.2.3.1 Death 

No studies were located regarding death in humans after dermal exposure to methoxychlor. Only two 

studies were located regarding the lethality of methoxychlor in animals after dermal exposure. One out of 

three rabbits died after intermediate-duration exposure to 3 mL/kg/day of a 30% solution (900 mg/kg/day) 

of recrystallized methoxychlor in dimethyl phthalate (Haag et al. 1950). No deaths were observed in 

animals exposed to lower doses. No decrease in survival occurred in mice dermally exposed to 
METHOXYCHLOR 63 

3. HEALTH EFFECTS 

recrystallized methoxychlor at doses of 10 mg/day, 1 day/week for 2 years (Hodge et al. 1966). LOAEL 

values for the lethal effects of methoxychlor are recorded in Table 3-2. 
Table 3-2 Levels of Significant Exposure to Methoxychlor - Dermal 

Exposure/ 

Duration/ LOAEL Reference 

Species Frequency Less Serious Serious Chemical Form (Strain) (Specific Route) System NOEL - INTERMEDIATE EXPOSURE 

Death 

Haag et al. 1950 Rabbit 1 xld 5 dlwk 13 (113 died after 1.4 weeks) wk mUkglday RC 30% 

Systemic Haag et al. 1950 Rabbit Ixld 5 dlwk 13 Hepatic 1 2 (fatty degeneration) wk mUkaldav mUkaldav RC 3G/o ' 

Bd Wt 1 

(decreased body weight mUkglday mUkglday gain) 

30% 30% w 

I CHRONIC EXPOSURE rn

Hodge et al. 1966 5

Mouse lxld 1dlwk 104 Dermal 10 I 

C3HlAnF wk rnglday RC n 

m 

n 

rn 

Bd Wt 10 

mglday 

Bd Wt = body weight; d = day; wk = week(s); x = times@) 

ul 

P 
 

 

METHOXYCHLOR 65 

3. HEALTH EFFECTS 

3.2.3.2 Systemic Effects 

No studies were located regarding respiratory, cardiovascular, gastrointestinal, hematological, 

musculoskeletal, renal, endocrine, ocular, or body weight effects in humans or animals after dermal 

exposure to methoxychlor. Limited data are available on hepatic, dermal/ocular, and other systemic 

effects in animals. The highest NOAEL values and all LOAEL values from each reliable study for the 

systemic effects of methoxychlor in each species and duration category are recorded in Table 3-2. These 

studies are discussed below. 

Hepatic Effects. Fatty degenerative changes in liver were observed in one of three rabbits exposed to 

1 mL/kg/day of a 30% solution (300 mg/kg/day) of recrystallized methoxychlor in dimethyl phthalate, 

and in 4 of 6 rabbits administered higher doses (2–3 mL/kg/day; equivalent to 600–900 mg/kg/day), 

5 days/week for 13 weeks (Haag et al. 1950). The authors state that no apparent effects from dimethyl 

phthalate were observed. 

Dermal Effects. No gross or histopathological changes were observed in the skin of mice exposed to 

dermal doses of up to 10 mg/day recrystallized methoxychlor in acetone, 1 day/week for 2 years (Hodge 

et al. 1966). 

3.2.3.3 Immunological and Lymphoreticular Effects 

No studies were located regarding immunological effects in humans or animals after dermal exposure to 

methoxychlor. 

3.2.3.4 Neurological Effects 

No studies were located regarding neurological effects in humans after dermal exposure to methoxychlor. 

Neurological effects of methoxychlor in animals after dermal exposure were observed in a single study. 

Disseminated nodules and petechial hemorrhages were noted in the brains of rabbits exposed dermally to 

1 mL/kg/day or more of a 30% solution (300 mg/kg/day) of recrystallized methoxychlor in dimethyl 

phthalate, 5 days/week for 13 weeks (Haag et al. 1950). Similar effects were not observed in control 

animals. These lesions were accompanied by foreleg paralysis in two out of six animals exposed to the 

higher doses (2–3 mL/kg/day). The authors suggested the effects may be manifestations of an underlying 
METHOXYCHLOR 66 

3. HEALTH EFFECTS 

disease that is endemic to rabbits and was somehow potentiated by exposure to methoxychlor. The 

foreleg paralysis seen in two of the rabbits may have been related to the lesions of the central nervous 

system. 

3.2.3.5 Reproductive Effects 

No studies were located regarding reproductive effects in humans after dermal exposure to methoxychlor. 

Although no quantitative data were provided, Tullner (1961) reported marked uterine stimulation in 

immature female mice dusted with an insecticide containing methoxychlor. Inhalation and oral exposures 

to the dust were likely to have occurred as well. 

3.2.3.6 Developmental Effects 

No studies were located regarding developmental effects in humans or animals after dermal exposure to 

methoxychlor. 

3.2.3.7 Cancer 

No studies were located regarding carcinogenic effects in humans after dermal exposure to methoxychlor. 

Two studies were located that investigated the carcinogenic potential of methoxychlor in animals after 

dermal exposure. Dermal exposure of mice to 10 mg/day recrystallized methoxychlor, 1 day/week for 

2 years did not produce any treatment-related tumors (Hodge et al. 1966). However, in this study, only 

the skin underwent histopathological examination. Other tissues were only examined grossly. Another 

study examined the effects of topically applied methoxychlor on skin tumor-promotion in mice (Dwivedi 

and Tabbert 1994). Application of 300 nmol methoxychlor in 100 μL acetone to the shaved back skin 

(previously initiated with 7,12-dimethylbenz[a]anthracene (DMBA)) of mice twice a week for 20 weeks 

did not result in papilloma development, while 100% of the mice treated with DMBA followed by 

12-O-tetradecanoyl-phorbol-13-acetate (TPA, a known tumor promoter) developed tumors (Dwivedi and 

Tabbert 1994). Methoxychlor also did not induce ornithine decarboxylase (ODC) activity in mouse skin 

(Dwivedi and Tabbert 1994). 
METHOXYCHLOR 67 

3. HEALTH EFFECTS 

3.3 GENOTOXICITY 

Numerous in vivo and in vitro studies have assessed the genotoxic potential of methoxychlor, and the 

results of these studies are presented in Tables 3-3 and 3-4, respectively. 

The genotoxicity of methoxychlor has been well studied in vitro and to a lesser extent in vivo. 

Methoxychlor does not appear to be genotoxic in prokaryotic test systems. Negative results were 

obtained in mutagenicity tests using many different strains of Salmonella typhimurium (TA98, TA100, 

TA1535, TA1537, TA1538, G46, C3076, and D3052), with or without metabolic activation by rat liver 

microsomal fractions (Grant et al.1976; Mortelmans et al. 1986; Probst et al. 1981; Waters et al. 1980). 

Negative results were also reported for tests of differential toxicity in DNA repair-proficient and repairdeficient strains of Bacillus subtilis or Escherichia coli and for mitotic recombination in Saccharomyces 

cerevisiae (Probst et al. 1981; Waters et al. 1980). When selected contaminants of methoxychlor were 

tested for their mutagenic potential, only 3,6,11,14-tetramethoxydibenzo(g,p)chrysene produced an 

increased mutation frequency (Grant et al. 1976), however, most of the known contaminants of 

methoxychlor were not tested in this study. This compound is a relatively minor contaminant of technical 

methoxychlor comprising only 0.0005% (5 ppm) by weight (Mitchell et al. 1978). 

Mixed results have been obtained in genotoxicity tests using mammalian cells in vitro. Methoxychlor did 

not induce unscheduled DNA synthesis in human lung fibroblasts (Waters et al. 1980) or rat hepatocytes 

(Probst et al. 1981). In cultured bovine uterine epithelial and stromal cells, DNA synthesis (thymidine 

incorporation) was inhibited at high methoxychlor concentrations and stimulated at low methoxychlor 

concentrations (Tiemann et al. 1996). Single-stranded DNA breaks were not induced in human or rat 

testicular cells by methoxychlor (Bjørge et al. 1996b). Methoxychlor did not produce mutations at the 

thymidine kinase (TK) locus in human lymphoma cells, with or without metabolic activation (Caspary 

et al. 1988). However, in mouse lymphoma cells, metabolically activated methoxychlor induced 

increases in mutation frequencies at the TK locus (Caspary et al. 1988; Mitchell et al. 1988; Myhr and 

Caspary 1988). Mixed results were reported on ability of methoxychlor to induce neoplastic 

transformations. A dose-dependent increase in neoplastic transformations were reported in cultured 

mouse fibroblasts (Dunkel et al. 1981), but not in Syrian hamster embryo cells or virally infected rat 

embryo cells (Dunkel et al. 1981; Pienta 1980). The reason for differing results in mouse lymphoma cells 

compared to human lymphoma cells, and in mouse fibroblasts compared to Syrian hamster embryo cells 

and rat embryo cells is uncertain, but may represent important species differences. In the absence of 
METHOXYCHLOR 68 

3. HEALTH EFFECTS 

Table 3-3. Genotoxicity of Methoxychlor In Vivo 

Species (test system) End point Results Reference 

Mammalian cells: 

Mouse hepatic cells Single-stranded DNA breaks – Umegaki et al. 1993 

Mouse bone marrow cells Chromosome aberration – Degraeve and Chollet 1984 

Mouse sperm cells Chromosome aberration – Degraeve and Chollet 1984 

Nonmammalian cells: 

Drosophila melanogaster Sex-linked recessive lethal – Waters et al. 1980 

mutation 

D. melanogaster Sex-linked recessive lethal – Benes and Sram 1969 

mutation 

D. melanogaster Sex-linked recessive lethal – Valencia 1981 

mutation 

– = negative result; + = positive result 
METHOXYCHLOR 

3. HEALTH EFFECTS 

69 

Table 3-4. Genotoxicity of Methoxychlor In Vitro 

Species 

test system) End point 

With 

activation 

Without 

activation Reference 

Prokaryotic organisms: 

Salmonella Reverse mutation – – Mortelmans et al. 1986 

typhimurium 

S. typhimurium	 Reverse mutation – – Grant et al. 1976 

S. typhimurium	 Reverse mutation – – Waters et al. 1980 

S. typhimurium Reverse mutation – – Probst et al. 1981 

Escherichia coli Reverse mutation – – Waters et al. 1980 

E. coli	 Reverse mutation – – Probst et al. 1981 

E. coli Differential toxicitya – No data Waters et al. 1980 

Bacillus subtilis Differential toxicitya – No data Waters et al. 1980 

Eukaryotic organisms: 

Saccharomyces 

cerevisiae Mitotic recombination – – Waters et al. 1980 

Mammalian cells: 

Human testicular cells	 Single-stranded DNA No data – Bjørge et al. 1996b 

breaks 

Human lung Unscheduled DNA – – Waters et al. 1980 

fibroblasts synthesis 

Human lymphoma Mutation of TK locus – – Caspary et al. 1988 

cells 

Mouse lymphoma Mutation at TK locus + – Caspary et al. 1988 

cells 

Mouse lymphoma Mutation at TK locus + – Myhr and Caspary 1988 

cells 

Mouse lymphoma Mutation at TK locus + – Mitchell et al. 1988 

cells 

Chinese hamster cells	 Inhibition of metabolic No data + Kurata et al. 1982 

cooperation 

Syrian hamster Neoplastic – – Pienta 1980 

embryo cells transformation 

Syrian hamster Neoplastic No data – Dunkel et al. 1981 

embryo cells transformation 
METHOXYCHLOR 70 

3. HEALTH EFFECTS 

Table 3-4. Genotoxicity of Methoxychlor In Vitro (continued) 

Species With Without 

(test system) End point activation activation Reference 

Mouse fibroblasts	 Neoplastic No data + Dunkel et al. 1981 

transformation 

Rat embryo cells Neoplastic No data – Dunkel et al. 1981 

(virus-infected) transformation 

Rat hepatocytes	 Unscheduled DNA No data – Probst et al. 1981 

synthesis 

a

 Comparison between DNA repair proficient and repair deficient strains 

– = negative result; + = positive result 
METHOXYCHLOR 71 

3. HEALTH EFFECTS 

a metabolic activation system, methoxychlor inhibited metabolic cooperation in Chinese hamster cells, an 

activity possibly associated with tumor promoters (Kurata et al. 1982). Exposure to methoxychlor did not 

induce morphological transformation in Syrian hamster embryo cells (Pienta 1980). 

In vivo genotoxicity studies generally yielded negative results (Table 3-3). In mice intraperitoneally 

injected with 10 mg/kg methoxychlor, the frequency of chromosomal aberrations was not increased in 

bone marrow cells or sperm cells (Degraeve and Chollet 1984). Single-stranded DNA breaks were not 

increased in hepatic cells of mice injected intraperitoneally with 170 mg/kg/day for 5 days (Umegaki et 

al. 1993). No increased frequency of sex-linked recessive lethal mutations were noted in Drosophila 

melanogaster exposed to methoxychlor (Benes and Sram 1969; Valencia 1981; Waters et al. 1980). 

No studies were located regarding genotoxic effects in humans or animals after inhalation, oral, or dermal 

exposure to methoxychlor. 

3.4 TOXICOKINETICS 

No data were located concerning the toxicokinetics of methoxychlor in humans following any route of 

exposure, or in animals following inhalation exposure. Studies in animals indicate that methoxychlor is 

well absorbed by the gastrointestinal tract and to a lesser extent by the skin. However, some of the data 

from animal studies come from ruminant animals, which may have limited relevance to humans and other 

nonruminant species. Once in the bloodstream, methoxychlor appears to distribute to most tissues of the 

body, with highest levels usually found in fat. Methoxychlor is metabolized rapidly by the liver and 

neither the parent compound nor the metabolites tend to accumulate in fat or other tissue. The 

metabolism of methoxychlor has been fairly well studied in vitro and in vivo in animals and with human 

liver microsomes. Both sets of data indicate that methoxychlor undergoes demethylation to form 

phenolic derivatives, with dechlorination and dehydrochlorination reactions occurring to a lesser extent. 

Most of the ingested dose of methoxychlor is eliminated in the feces via biliary excretion of metabolites. 

Urinary excretion contributes to a lesser extent (approximately 10% of the total administered dose as 

indicated in mouse studies). The toxicokinetics of methoxychlor in humans is expected to be similar to 

the toxicokinetics of methoxychlor observed in animals. 
METHOXYCHLOR 72 

3. HEALTH EFFECTS 

3.4.1 Absorption 

No quantitative studies were located regarding absorption of methoxychlor in humans after exposure by 

any route. There is no information regarding age-related differences in rate or extent of absorption of 

methoxychlor in animals or humans after exposure by any route. Thus, it is unknown whether absorption 

of methoxychlor by children differs from that by adults. 

3.4.1.1 Inhalation Exposure 

No studies were located regarding absorption of methoxychlor in animals after inhalation exposure. 

3.4.1.2 Oral Exposure 

Observations of adverse health effects in animals following oral exposure provide indirect evidence that 

ingested methoxychlor is absorbed by the gastrointestinal tract (see Section 3.2.2 Health Effects: Oral 

Exposure). Data from studies examining fecal and urinary excretion of radioactivity after oral 

administration of radiolabeled methoxychlor to mice and goats indicate that methoxychlor is rapidly and 

efficiently absorbed by the gastrointestinal tract. 

In mice administered single doses of 50 mg/kg recrystallized, radiolabeled methoxychlor in oil by gavage, 

90% of the dose was excreted as metabolites in the feces (85% polar metabolites) and 10% was excreted 

in urine (63.8% polar metabolites) within 48 hours (Kapoor et al. 1970). Assuming that the fecal 

metabolites (primarily demethylated, dechlorinated, and dehydrochlorinated compounds) resulted from 

biliary excretion of absorbed material and not from degradation of unabsorbed parent by enteric bacteria, 

gastrointestinal absorption of methoxychlor appears to exceed 90% in mice. 

In two lactating female goats administered single oral doses of 3.6 or 11.6 mg/kg laboratory grade, 

radiolabeled methoxychlor encapsulated in gelatin, 40.5 and 67.5% of the doses were excreted in the 

feces within 3 days, respectively, and 58.4 and 27.1% were excreted in the urine, respectively (Davison 

et al. 1982). Metabolites (demethylated, dechlorinated, and dehydrochlorinated products of 

methoxychlor) were estimated to represent 70 and 81% of radioactivity in the feces, respectively. 

Assuming that metabolites in feces resulted from biliary excretion of absorbed material (and that the 

remaining radioactivity in feces was not absorbed), the data indicate that at least 87 and 82% of the 

respective administered doses were absorbed. In a bile-cannulated, castrated male goat given an oral dose 
METHOXYCHLOR 73 

3. HEALTH EFFECTS 

of 25.6 mg/kg radiolabeled methoxychlor, 7.8, 35.2, and 44.4% of the radioactivity was excreted within 

3 days in the bile, feces, and urine, respectively (Davison et al. 1983). The profile of metabolites in the 

collected bile was reported to be similar to the metabolite profile in the feces collected from the female 

goats, but Davison et al. (1983) did not report any chemical analysis of the feces collected from the bilecannulated goat. The data provide support that methoxychlor is rapidly and efficiently absorbed by the 

mammalian gastrointestinal tract. However, interpretation of the goat studies is limited, because only one 

animal was tested per dose, bile cannulation may have influenced absorption, and ruminant toxicokinetic 

properties are not always relevant to nonruminant mammals. 

3.4.1.3 Dermal Exposure 

Two studies in animals suggest dermal absorption of methoxychlor may range from 5 to 20%. In one 

study, a single dermal dose of 200 mg laboratory grade methoxychlor in dichloromethane was applied to 

the shaved backs of two goats. Three days later, 5–8% of the dose was recovered in the carcass, urine, 

and feces (Davison et al. 1983). In the second study, four cows were dermally exposed to a single dose of 

5 g methoxychlor in an emulsion (Skaare et al. 1982). The levels of methoxychlor detected in milk were 

comparable to the levels in milk from cows given an intravenous dose of 1 g methoxychlor. The authors 

concluded that approximately 20% of the dermal dose was absorbed. Because of differences in skin, 

dermal absorption by goats and cows may not be a good model for dermal absorption by humans. 

3.4.2 Distribution 

There are no studies regarding distribution of methoxychlor in humans after exposure by any route, nor 

any studies examining possible age-related differences in distribution of methoxychlor in animals or 

humans after exposure by any route. Thus, it is unknown whether distribution of methoxychlor and 

metabolites in children differs from that in adults. 

There are no studies directly examining whether methoxychlor and metabolites cross the placenta in 

humans or animals. Subtle effects on the development of reproductive organs have been observed in 

male and female offspring of rodents exposed to oral doses of methoxychlor during gestation (Chapin et 

al. 1997; Gray et al. 1989; Harris et al. 1974); however, it should be noted that exposure was not 

exclusively during gestation in any of these studies. It is unclear if these effects result directly from 

methoxychlor and metabolites passing through the placenta or indirectly from a disturbance of maternal 

physiological processes. 
METHOXYCHLOR 74 

3. HEALTH EFFECTS 

3.4.2.1 Inhalation Exposure 

No studies were located regarding distribution in animals after inhalation exposure to methoxychlor. 

3.4.2.2 Oral Exposure 

In animals, methoxychlor preferentially distributes to the fat but does not appear to persist. Furthermore, 

metabolic adaptation may enhance elimination. In rats exposed to 1.25, 5, or 25 mg/kg/day methoxychlor 

for 4–18 weeks, fat levels of methoxychlor were reported to be nondetectable, 1–7 mg/kg, and 

14–36 mg/kg, respectively (Kunze et al. 1950). Levels of methoxychlor in fat peaked during the first 

9 weeks of exposure, after which time a gradual decline was noted during the last 9 weeks of exposure 

(Kunze et al. 1950). Methoxychlor was not detected in the fat following the 2-week recovery period. In 

female rats exposed to 50, 125, or 250 mg/kg/day in the feed for 6 weeks, the levels of methoxychlor 

detected in abdominal fat were 21, 68, and 61 mg/kg, respectively (Harris et al. 1974). The levels of 

methoxychlor in fat were 34 and 140 mg/kg in female rats when exposure to 50 and 125 mg/kg/day was 

continued through pregnancy and weaning (exposure duration=12 weeks) (Harris et al. 1974). The 

authors speculated that the higher levels of methoxychlor in fat of female rats after pregnancy and 

weaning was due to a lower fat content in these animals. However, exposure duration (12 vs. 6 weeks) 

may have been a factor as well. Levels of methoxychlor in the fat of sheep exposed to 6 and 

49 mg/kg/day for 18 weeks peaked at 7.8 mg/kg by week 10, and at 24 mg/kg by week 6, respectively 

(Reynolds et al. 1976). A steady decline of methoxychlor in fat was noted after week 10 in the low dose 

group, suggesting that metabolic adaption enhanced elimination (Reynolds et al. 1976). Methoxychlor 

was not detected in fat from either group following a 12–14-week recovery period. In dogs administered 

20 or 100 mg/kg/day methoxychlor for 1 year, the levels of methoxychlor in the fat were 8.9 and 

85 mg/kg, respectively (Hodge et al. 1952). The levels of methoxychlor in the fat of rats exposed to 1.25, 

10, or 80 mg/kg/day for 2 years were 3.7, 2.3–6.8, and 11–22.7 mg/kg, respectively (Hodge et al. 1952). 

In rats receiving 80 mg/kg/day, the highest levels of methoxychlor detected in the kidneys, liver, and 

brain were less than 4.2, 0.5, and 0.2 mg/kg, respectively (Hodge et al. 1952). 

Three days after oral administration of 3.6 or 11.6 mg/kg radiolabeled methoxychlor to lactating goats, 

radioactivity was detected in adrenals, brain, gall bladder, heart, kidneys, and liver (Davison et al. 1982), 

thus indicating that absorbed methoxychlor and metabolites are widely distributed by the blood. 

However, the radioactivity in these tissues represented <1% of the administered dose. These observations 
METHOXYCHLOR 75 

3. HEALTH EFFECTS 

are consistent with other information indicating that methoxychlor is rapidly metabolized and excreted 

from the body. 

Methoxychlor and/or methoxychlor metabolites have been detected in milk following oral exposure of 

animals to methoxychlor during lactation. Accumulation in milk and/or elimination through lactation do 

not appear to be predominant dispositional fates for methoxychlor, but transfer of methoxychlor and 

methoxychlor metabolites to suckling rat pups via milk has been demonstrated. In lactating female goats 

given oral doses of 3.6 or 11.6 mg/kg radiolabeled methoxychlor, radioactivity in milk collected for 

3 days was below limits of detection for one goat and represented only 0.065% of the dose given to the 

other goat (Davison et al. 1982). In female rats fed 5, 50, or 150 mg/kg/day methoxychlor during late 

gestation and early lactation, milk levels of methoxychlor at postnatal day 7 were 25, 128, and 221% of 

plasma methoxychlor levels, respectively (Chapin et al. 1997). Mono-hydroxy methoxychlor and bishydroxy methoxychlor, two major metabolites of methoxychlor, showed similar patterns of concentration 

in the milk with increasing methoxychlor exposure level. The data suggest that methoxychlor and 

metabolites concentrate in milk, relative to maternal plasma levels, after intermediate-duration dose levels 

$50 mg/kg/day. Mean plasma levels of methoxychlor in suckling pups were <5 (below detection limit), 

12.4, 37.8, and 59.9 ng/mL in the 0-, 5-, 50-, and 150-mg/kg/day groups, respectively (Chapin et al. 

1997). Mono-hydroxy methoxychlor was detected only in 150-mg/kg/day pup plasma (6.2 ng/mL), and 

bis-hydroxy methoxychlor was detected in the 50- and 150-mg/kg/day pup plasma (6.4 and 

11.5 mg/kg/day). Pup plasma was not drawn for analysis until 27–30 hours after dams received the last 

dose; therefore, measured methoxychlor and metabolites may not have been indicative of peak body 

burden. 

No studies were located that examined whether preconceptional or pregestational exposure of females to 

methoxychlor would result in exposure to the developing embryo/fetus or neonate. The evidence that 

methoxychlor is rapidly metabolized and eliminated from the body (e.g., all radioactivity from 

radiolabeled oral doses of 50 mg/kg methoxychlor was excreted by mice in feces and urine within 

48 hours [Kapoor et al. 1970]) suggests, however, that it is unlikely that methoxychlor at low background 

exposure levels would be stored in maternal tissues and subsequently mobilized during pregnancy or 

lactation. It is unknown what mechanism produced the results reported by Swartz and Corkern (1992) 

(discussed in Section 3.2.2.6 Developmental Effects); in this experiment, females from a litter produced 

after maternal exposure had ceased exhibited precocious puberty, but earlier offspring of these same dams 

exposed during gestation did not. 
METHOXYCHLOR 76 

3. HEALTH EFFECTS 

3.4.2.3 Dermal Exposure 

A single study was located concerning the distribution of methoxychlor in animals after dermal exposure. 

Three days after two goats were administered a single dermal dose of 200 mg methoxychlor, low levels of 

methoxychlor (<0.3 mg/kg tissue) were detected in the skin, muscle, liver, fat, and kidneys (Davison et al. 

1983). Less than 0.1% of an applied dermal dose of methoxychlor was excreted in the milk of cows after 

30 days (Ivey et al. 1983; Skaare et al. 1982). Interpretation of data from these studies is limited since 

only two goats were tested, and goats and cows may not be good models for dermal exposures in humans. 

3.4.2.4 Other Routes of Exposure 

Further evidence, albeit indirect, that methoxychlor and/or metabolites can be excreted in milk is provided 

by observations of morphological changes in the reproductive tract of 15-day-old female offspring of 

lactating mice given 14 daily intraperitoneal injections of 1, 2, or 5 mg technical-grade methoxychlor in 

sesame oil (30, 60, or 150 mg/kg/day) on postnatal days 1–14 (Appel and Eroschenko 1992). Exposurerelated, statistically significant changes included increased reproductive tract weight, increased thickness 

of epithelia of vagina and uterine horns, and increased incidence of mucified and/or cornified vagina. 

These stimulatory effects on development of the female reproductive tract are consistent with the 

estrogenic activity of methoxychlor metabolites. 

The estrogenic activity of methoxychlor administered intraperitoneally has also been demonstrated in 

adult ovariectomized ND4 Swiss Webster mice (Eroschenko et al. 2000). Three daily intraperitoneal 

doses of 0.125 mg (4 mg/kg/day) methoxychlor resulted in an increase in uterine epithelial height of 

approximately the same magnitude as three intraperitoneal doses of 25 ng (0.9 μg/kg/day) of estradiol 

17-β. However, the potency of methoxychlor was much less than estradiol (approximately 4,400 times 

less) and the range of effects elicited by methoxychlor was different from those elicited by estradiol. 

While estradiol increased reproductive tract weight and uterine luminal fluid albumin content, 

methoxychlor did not, and in fact appeared to reduce the stimulatory response of sensitive uterine cells to 

estradiol. The study authors speculated that these differences may be due to the different affinities of 

methoxychlor for the various forms of ER (ERα and ERβ) compared to estradiol and its possible 

interaction with other receptors that have not yet been elucidated. 

Estrogenic effects have also been observed in female mice following subcutaneous administration. 

Methoxychlor (10 mg/kg/day) administered for 5 days resulted in a significant increase in uterine weight, 
METHOXYCHLOR 77 

3. HEALTH EFFECTS 

similar to the effects of estradiol-17β (Al-Jamal and Dubin 2000). However, this effect was eliminated by 

co-administration with raloxifene (a selective estrogen receptor modulator), whereas the uterine effects of 

estradiol-17β were not. This supports other studies (Eroschenko et al. 2000; Gaido et al. 1999; Ghosh et 

al. 1999) that have suggested that the estrogenic effects of methoxychlor may be mediated via different 

interactions with the estrogen receptors or different mechanisms than estradiol-17β. Increased uterine 

weight was also seen in immature mice administered methoxychlor ($50 mg/kg) or its estrogenic 

metabolite HPTE ($100 mg/kg) subcutaneously for 1 or 3 days (Newbold et al. 2001). Further 

examination of the uterine tissue showed increases in uterine epithelial cell height, uterine gland number, 

cell proliferation, and estrogen-inducible protein production (Newbold et al. 2001). 

Methoxychlor administered subcutaneously to adult male rats resulted in a seemingly complicated 

alteration of serum and hypothalamic hormone levels (Lafuente et al. 2000). Prolactin release from the 

hypothalamus is affected by circadian rhythm. Methoxychlor exposure resulted in increased median 

serum prolactin levels and absolute pulse amplitude and in decreased relative pulse amplitude, but did not 

affect the frequency or duration of prolactin peaks or the half-life of prolactin in serum (Lafuente et al. 

2000). Decreases in serum testosterone and luteinizing hormone (LH) were also noted in this study. 

Testosterone can stimulate prolactin release; therefore, the increase in prolactin does not appear to be 

mediated by testosterone. The study authors speculated that the alteration in prolactin release may be a 

consequence of direct effects of methoxychlor on the hypothalamus (Lafuente et al. 2000). Dopamine, 

which is an inhibitory neuromodulator of prolactin release, was increased in the anterior hypothalamus 

and decreased in the median eminence, which suggests a decrease in dopamine release; this could explain 

the increased prolactin release (Lafuente et al. 2000). 

3.4.3 Metabolism 

Figure 3-2 presents a summary of methoxychlor metabolic pathways. Following Figure 3-2 is a key to 

alternative chemical names for methoxychlor metabolites. There are no data to suggest that metabolism 

of methoxychlor is dependent on the way it enters the body, so the metabolism of methoxychlor is 

discussed below without reference to route of exposure. 

Methoxychlor is metabolized mainly in the liver. In in vivo studies in which rats were exposed to carbon 

tetrachloride to induce liver damage, the metabolism and excretion of methoxychlor were reduced, 

whereas tissue retention and toxicity were increased (Lehman 1952; Weikel and Laug 1958). 
METHOXYCHLOR 78 

3. HEALTH EFFECTS 

The primary pathway by which methoxychlor is metabolized in the liver is sequential demethylation 

reactions to yield mono- and bis-hydroxy methoxychlor. Alternative names for these metabolites are 

2-(p-methoxyphenyl)-2-(p-hydroxyphenyl)-1,1,1-trichloroethane and 2,2-bis (p-hydroxyphenyl)-1,1,1-trichloroethane (sometimes abbreviated as HPTE). Dechlorinated metabolites such as bis-hydroxydiphenyl 

acetic acid and bis-hydroxybenzophenone have also been identified (see Figure 3-2). 

Methoxychlor and 5 metabolites were identified by thin layer chromatography in urine and feces 

collected from mice for up to 11 days after administration of single oral doses of 50 mg/kg/day 

radiolabeled, recrystallized methoxychlor in oil (Kapoor et al. 1970). Mono-hydroxy methoxychlor and 

bis-hydroxy-methoxychlor, the major metabolites, accounted for approximately 30 and 23% of the 

administered dose, respectively. Bis-hydroxy-diphenylacetic acid and bis-hydroxy-benzophenone, 

presumably formed via dechlorination and subsequent oxidation as shown in Figure 3-2, accounted for 

about 11% of the administered radioactivity. Nonmetabolized methoxychlor and methoxy diphenyl 

dichloroethylene (bis-OH-MDDE) accounted for about 8 and 1% of the administered radioactivity, 

respectively (Kapoor et al. 1970). The latter metabolite has been proposed to be formed by dechlorination of bis-hydroxy methoxychlor (i.e., HPTE), or by alternative pathways in which 

dechlorination preceeds demethylation (see Figure 3-2). About 27% of administered radioactivity was 

not recovered in the thin layer chromatography procedure used in this analysis. 

In goats administered single doses of 3.6–25.6 mg/kg, most of the dose was metabolized to demethylated, 

dechlorinated, and dehydrochlorinated derivatives of methoxychlor, although a ring hydroxylated species 
1 

Figure 3-2. Proposed Metabolic Pathways of Methoxychlora 

CH3O 

H COOH 

OCH3 

Dechlorination 

Oxidation 

CH3O 

5 H 

CCl3 

OCH3 CH3O 

Dehydrochlorination 9 

CCl2 

OCH

Methoxy Diphenyl Acetic Acid Methoxychlor (MXC) Methoxy-DDE (MDDE)(g) 

CYP(b) 

Demethylation 

CYP 

Demethylation 

CYP 

Demethylation 

2 

H 10 H 6 Dehydrochlorination Dechlorination 

CH CH3O OH 3O OH CH3O OH Oxidation CCl CCl 2 COOH 3

Hydroxyphenyl Methoxyphenyl Mono-OH-MXC (c,d,e) Mono-OH-MDDE(c,d) Acetic Acid

CYP CYP CYP 

Demethylation Demethylation Demethylation 

3 H 7 H Dehydrochlorination 11 Dechlorination

HO OH HO OH HO OH

Oxidation CCl3 CCl2 COOH 

Bis-OH-MDDE(c,d) Bis-OH-Diphenyl Bis-OH-MXC(c,d,f)

Acetic Acid(d)

CYP RingHydroxylation 

4 8 H 

HO OH HO OH

CCl3 

HO 

O 

Bis-OH-Benzophenone(d) Tris-OH-MXC(d) 

(a) Adapted from Kapoor et al. 1970; Kupfer and Bulger 1987b; Kupfer et al. 1990(b) CYP = cytochrome P-450(c) Estrogenic compound(d) Metabolite identified in excreta(e) Mono-OH-MXC = 2-(p-Methoxyphenyl)-2-(p-hydroxyphenyl)-1,1,1-trichloroethane(f) Bis-OH-MXC = 2,2-Bis(p-hydroxyphenyl)-1,1,1-trichloroethanol (HPTE)(g) MDDE = Methoxydiphenyldichloroethylene 
 

METHOXYCHLOR 80 

3. HEALTH EFFECTS 

Figure 3-2	 Proposed Metabolic Pathways of Methoxychlor 

Key to Metabolite Chemical Names 

1.	 Bis(4-methoxyphenyl)acetic acid 

Methoxy Diphenyl Acetic Acid 

2.	 α-(4-hydroxyphenyl)-α-(4-methoxyphenyl)acetic acid 

Hydroxyphenyl Methoxyphenyl acetic acid 

3.	 Bis(4-hydroxyphenyl)acetic acid 

Bis-OH-Diphenyl acetic acid 

CASRN: 40232-93-7 

4.	 4,4-Dihydroxybenzophenone 

Bis-OH-Benzophenone 

CASRN: 611-99-4 

5.	 1,1,1-Trichloro-2,2-bis(4-methoxyphenyl)ethane 

Methoxychlor (MXC) 

CASRN: 72-43-5 

6.	 1,1,1-Trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane 

Mono-OH-MXC 

2-(p-Methoxyphenyl)-2-(p-hydroxyphenyl)-1,1,1-trichloroethane 

7.	 1,1,1-Trichloro-2,2-bis(4-hydroxyphenyl)ethane 

Bis-OH-MXC 

2,2-Bis(p-hydroxyphenyl)-1,1,1-trichloroethanol (HPTE) 

CASRN: 2971-36-0 

8.	 1,1,1-Trichloro-2-(3,4-dihydroxyphenyl)-2-(4-hydroxyphenyl)ethane 

Tris-OH-MXC 

9.	 1,1-Dichloro-2,2-bis(4-methoxyphenyl)ethene 

Methoxy-DDE (MDDE) 

Methoxydiphenyldichloroethylene 

CASRN: 2132-70-9 

10.	 1,1-Dichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethene 

Mono-OH-MDDE 

11.	 1,1-Dichloro-2,2-bis(4-hydroxyphenyl)ethene 

Bis-OH-MDDE 

CASRN: 14868-03-2 
METHOXYCHLOR 81 

3. HEALTH EFFECTS 

was also identified in the urine as a minor metabolite (Davison et al. 1982, 1983). Most of the urinary 

metabolites were conjugated with glucuronic acid. 

In vitro studies using human and rat liver microsomal preparations confirm the generation of phenolic 

compounds via demethylation by hepatic cytochrome P450 (CYP) enzymes (Bulger et al. 1978d, 1985; 

Dehal and Kupfer 1994; Elsby et al. 2001; Kishimoto and Kurihara 1996; Kupfer et al. 1990; Kurihara 

and Oku 1991; Stresser and Kupfer 1997, 1998; Stresser et al. 1996). Early studies demonstrated that 

methoxychlor produced type I spectral changes when added to CYP enzymes from rats, mice, rabbits, 

sheep, and houseflies (Donovan et al. 1978; Kulkarni et al. 1975; Tsujita and Ichikawa 1993), indicative 

of substrate binding to CYP enzymes. 

Several partially purified CYP isozymes from rat liver microsomes can demethylate methoxychlor: 

CYP2C6 and 2A1 showed lower Km and Vmax values than values for CYP2B1 and CYP2B2, indicating 

that the former have a higher affinity, but lower capacity, for methoxychlor than the latter (Kishimoto et 

al. 1995); thus, which pair of enzymes is likely to be performing most of methoxychlor metabolism will 

depend on the amount of methoxychlor available to the liver. Antibodies to CYP2B1 and CYP2C6 were 

used with rat liver microsomes and purified preparations of CYP2C6 to provide evidence that CYP2C6 

and another unidentified CYP isozyme may represent the most important isozymes in the initial 

demethylation of methoxychlor in rats (Kishimoto and Kurihara 1996). 

Repeated exposure to methoxychlor appears to induce hepatic CYP enzymes involved in its metabolism. 

Intraperitoneal injection of mature female rats with 200 mg/kg/day methoxychlor twice daily for 4 days 

increased western blot-detected hepatic levels of CYP2B1, 2B2, and 3A proteins by 3-, 2.8-, and 1.6-fold 

compared with controls, but did not change levels of CYP2E1 (Li et al. 1995). Injection of immature 

female rats with 300 mg/kg/day for 4 days increased levels of CYP2B1, 2B2, and 3A proteins by 9.0-, 

7.8-, and 5.1-fold without inducing levels of CYP2E1 (Li et al. 1995). In these experiments, 

methoxychlor caused a markedly greater increase in the amount of CYP2B and 3A proteins than in actual 

enzyme activity. A recent in vitro study indicated that induction of the hepatic CYP2B enzyme by 

methoxychlor and its metabolites may be mediated via the constitutive androstane receptor (CAR), which 

initiates transcription of CYP2B RNA and results in increased CYP2B enzyme (Blizard et al. 2001). 

In vitro studies with human liver microsomes and human recombinant CYP isozymes indicate that 

multiple CYP isozymes are involved in the demethylation of methoxychlor in humans (Stresser and 

Kupfer 1998). Incubation of pooled liver microsomes from three human subjects with saturating levels of 
METHOXYCHLOR 82 

3. HEALTH EFFECTS 

methoxychlor (25 μM) and NADPH for up to 125 minutes produced mono-hydroxy methoxychlor, bishydroxy methoxychlor, and tris-hydroxymethoxychlor consistent with the central demethylation and ring 

hydroxylation reactions noted in Figure 3-2 (Stresser and Kupfer 1998). Rates of formation were highest 

for mono-hydroxy methoxychlor followed by bis-hydroxy methoxychlor and tris-hydroxy methoxychlor. 

Rates of mono-demethylation at non-saturating concentrations of 1 μM using liver preparations from 

14 individuals varied 80-fold, whereas rates from preparations from 26 individuals assayed at saturating 

concentrations varied 23-fold. Rates of formation of mono- and bis-hydroxy methoxychlor with human 

liver microsomes were generally lower than rates with rat liver microsomes, but some human liver 

samples displayed higher rates. At nonsaturating methoxychlor concentrations, demethylation by human 

microsomes was strongly inhibited by CYP2C19 inhibitors, tranylcypromine (also inhibits CYP2A6), and 

S-mephenytoin (substrate for CYP2C19). Moderate inhibition was produced by tolbutamide (substrate 

for CYP2C9), furafylline (inhibitor of CYP1A2), sulfaphenazole (inhibitor of CYP2C9), and coumarin 

(substrate of CYP2A6), whereas weak inhibition was produced by quinidine (substrate of CYP3A4) and 

ketoconazole (inhibitor of CYP3A4). The involvement of multiple CYP isozymes was supported by the 

observation of biphasic enzyme kinetics in Eadie-Hofstee plots of methoxychlor demethylation rates with 

human liver microsomes. Recombinant CYP2C19 expressed in lymphoblast cells was more active in 

demethylating methoxychlor than similarly expressed 1A2, and human cDNA-expressed CYP2C19, 

purified from bacterial lysates, catalyzed methoxychlor demethylation at rates 4-fold higher than rates for 

CYP2C9 and CYP2C18. Stresser and Kupfer (1998) proposed that CYP2C19 and CYP1A2 may be the 

major CYP demethylases for methoxychlor, but noted that other forms, including CYP2A6, CYP2C9, and 

CYP2B6, are likely to be major contributors, especially in individuals with low levels of CYP2C19 or 

CYP1A2. 

Ring hydroxylation of methoxychlor or of hydroxy-methoxychlor derivatives (see Figure 3-2 for 

formation of tris-hydroxy methoxychlor from bis-hydroxy methoxychlor) may involve a different set of 

CYP isozymes than CYP-catalyzed demethylation. Studies using nine human cDNA-expressed CYP 

isozymes in microsomes from lymphoblastoid cells (CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2D6, 2E1, and 

3A4) indicated that significantly increased ring hydroxylation of methoxychlor was catalyzed only by 

CYP1A2 or CYP2B6 (Stresser and Kupfer 1997). Ring hydroxylation of mono-hydroxy methoxychlor in 

this system was catalyzed by five isozymes (CYP1A2, 2B6, 2D6, 2E1, and 3A4), with CYP3A4 

displaying the highest rates of ring hydroxylation (Stresser and Kupfer 1997). Ring hydroxylation of 

methoxychlor or bis-hydroxy methoxychlor (ortho to the methoxy or hydroxy moities) was shown to be 

enhanced in liver microsomes from phenobarbital-induced rats compared with microsomes from noninduced rats and to be markedly inhibited by anti-CYP2B monoclonal antibodies (Stresser et al. 1996). 
METHOXYCHLOR 83 

3. HEALTH EFFECTS 

These data suggest that CYP2B isozymes may be most important in catalyzing ring hydroxylation of 

methoxychlor and hydroxy-methoxychlor derivatives in rats. 

Activation of Methoxychlor to Estrogenic Metabolites. The rapid demethylation of methoxychlor 

decreases its neurotoxicity and leads to a rapid elimination from the body (Lehman 1952), making it 

significantly less toxic than its structural analogue, DDT. However, this detoxification pathway also is 

thought to act as an activation pathway for reproductive and developmental effects. Data from in vitro 

and in vivo rat studies indicate that the phenolic metabolites of methoxychlor resulting from 

demethylation (and contaminants in technical grade and laboratory grade methoxychlor) are responsible 

for most of the estrogenic activity rather than methoxychlor itself (Bulger et al. 1978b,1978d; Charles et 

al. 2000; Sumida et al. 2001). Highly purified methoxychlor did not interfere with in vitro binding of 

estradiol-17β to 8S estrogen receptors in rat uterine cytosol, but the demethylated metabolite, 2,2-bis­

(p-hydroxyphenyl)-1,1,1-trichloroethane (i.e., HPTE), was a potent inhibitor of this binding (Bulger et al. 

1978b). In this assay system, technical grade methoxychlor, and even laboratory grade (99% pure), 

inhibited estradiol binding to cytosolic estrogen receptors, indicating that some contaminants in 

methoxychlor preparations have estrogenic activity (Bulger et al. 1978b). In ovariectomized rats, 

intraperitoneal injection of 3 or 10 mg laboratory grade methoxychlor in corn oil significantly increased 

activities of uterine ornithine decarboxylase activity (ODC, a sensitive marker of estrogenic activity) 

within 7 hours by 15- or 136-fold, respectively, and significantly increased relative uterine weight by 

1.5-fold (i.e., 53% increase), at the higher dose only (Bulger et al. 1978b). Injection of lower doses of 

HPTE, 0.1 or 0.5 mg per rat, significantly increased ODC activities by 17- or 607-fold, respectively; the 

0.5 mg dose level also significantly increased relative uterine weight by 1.3-fold (Bulger et al. 1978b). 

Two estrogen receptors, α and β, have recently been identified with overlapping and differential roles in 

mediating estrogenic responses in mammals. The complexity of the mechanism(s) by which 

methoxychlor metabolites induce estrogenic responses has also been recently demonstrated by results 

showing that HPTE acts as an agonist for estrogen receptor α expressed in human hepatoma cells (i.e., 

similarly to estradiol-17β, it induced gene expression mediated by estrogen receptor α) and acts as an 

antagonist for estrogen receptor β (i.e., it abolished estradiol-17β-induced gene expression mediated by 

estrogen receptor β) (Gaido et al. 1999). This was further supported by studies showing differential 

expression of certain ER-related mRNAs in tissues of mice treated with HPTE or estradiol-17β, 

depending on ERα and ERβ content of the tissues (Waters et al. 2001). 
METHOXYCHLOR 84 

3. HEALTH EFFECTS 

The dehydrochlorination of HPTE to methoxydiphenyldichloroethylene (bis-OH-MDDE) (see 

Figure 3-2) also acts as an activation pathway, since bis-OH-MDDE was even more active than HPTE in 

in vitro assays of binding to rat uterine cytosol receptors and in in vivo rat assays for induction of uterine 

ODC activity and increase in relative uterine weight following intraperitoneal exposure (Bulger et al. 

1985). Although the enzyme responsible for catalyzing the dehydrochlorination of methoxychlor and its 

demethylated metabolites has not been studied, it is probably the same glutathione-requiring dehydrochlorinase which catalyzes the conversion of DDT to 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene 

(DDE) (Hodgson and Levi 1987). Data were not located regarding the estrogenic activity of methoxy 

diphenylacetic acid and methoxy benzophenone, so it is not known whether dechlorination and oxidation 

of methoxychlor to these products act as activating or detoxifying pathways. 

Age and Metabolism. Although there is no direct information regarding age-related differences in 

metabolism of methoxychlor in children, multiple CYP enzymes are involved in methoxychlor 

metabolism. It is likely that CYPs 2C19, 1A2, 2B6, 2C9, 2A6, and perhaps 2D6, 2E1, and 3A4 play a 

role in phase I human metabolism (Hong et al. 1987; Lacroix et al. 1997; Leeder and Kearns 1997; Mote 

et al. 1990; Ratenasavanh et al. 1991; Rich et al. 1990; Sonnier and Cresteil 1998; Treluyer et al. 1997; 

Vieira et al. 1996; Yang et al. 1994). Many of these phase I enzymes are likely to have overlapping roles. 

Although nothing is known about phase II metabolism in humans, Davison et al. (1982, 1983) found 

glucuronic acid conjugates of methoxychlor metabolites in goats; these conjugates are a product of 

glucuronosyltransferase and it is possible that this enzyme may be involved in human methoxychlor 

metabolism as well. Although it is unknown whether an overall age-related difference in methoxychlor 

metabolism would be observed in vivo in humans, there is some information that the following enzymes 

may be developmentally regulated: CYP2C19 (Leeder and Kearns 1997), CYP1A2 (Leeder and Kearns 

1997; Rich et al. 1990), CYP2C9 (Ratenasavanh et al. 1991; Treuler et al. 1997), CYP2D6 (Leeder and 

Kearns 1997; Sonnier and Cresteil 1998), CYP2E1 (Hong et al. 1987; Vieira et al. 1996), CYP3A4 

(Lacroix et al. 1997; Leeder and Kearns 1997; Mote et al. 1990; Yang et al. 1994), and glucuronosyltransferase (Leeder and Kearns 1997). In contrast, other researchers have found that CYPs 2A6, 2C9, 

2D6, 2E1, and 3A are expressed at the same levels in perinatal (37 weeks of gestation) to geriatric human 

livers (72 years) (Ratenasavanh et al. 1991; Tateishi et al. 1997). 

Metabolites and Covalent Adducts. In vitro studies indicate that methoxychlor can be metabolized to 

reactive intermediates capable of forming covalent adducts with cellular proteins. Studies with rat liver 

microsomes demonstrated that methoxychlor undergoes CYP mediated activation and the resultant 

reactive metabolites covalently bind to hepatic microsomal proteins (Bulger et al. 1983). Adduct 
METHOXYCHLOR 85 

3. HEALTH EFFECTS 

formation requires the presence of NADPH and oxygen, and is inhibited by metyrapone, SKF-525A, 

hexobarbital and ethylmorphine, indicating that the activation of methoxychlor is catalyzed by the 

cytochrome P450 enzyme system. Liver microsomes from rats pretreated with phenobarbital, an inducer 

of CYP2B1 and 2B2, exhibited enhanced covalent binding to microsomal proteins, and antibodies against 

phenobarbital-induced CYP isozymes inhibited covalent binding, indicating that the major portion of this 

activity was facilitated by catalysis by CYP2B1 and/or CYP2B2 (Bulger and Kupfer 1989, 1990). 

Comparative studies with liver microsomal preparations from humans and rats indicate that the 

mechanism of methoxychlor covalent modification of microsomal proteins is similar in the two species 

(Bulger and Kupfer 1989). Free radical scavengers act to inhibit adduct formation without affecting the 

production of polar (demethylated) metabolites (Bulger et al. 1983). Although the formation of 

methoxychlor adducts has not been well characterized, these adducts contain intact methoxy groups, 

suggesting that this reaction occurs independently from demethylation reactions (Bulger and Kupfer 

1990). The phenolic metabolites of methoxychlor, as well as the dehydrochlorinated product MDDE and 

its phenolic derivatives, are also capable of undergoing CYP mediated covalent binding. It was 

postulated that the formation of a reactive intermediate involves modification of the side chain, possibly 

through homolytic cleavage of the C-H or C-Cl bond (Bulger and Kupfer 1990). The relevance of protein 

adduct formation to mechanisms by which methoxychlor produces health effects is uncertain, but may be 

related to possible inactivation of CYP3A by tris-hydroxy methoxychlor (Li et al. 1993). 

Comparative Toxicokinetics. The available data comparing rat and human liver microsomal metabolism 

of methoxychlor indicate qualitative similarities as well as some indications of quantitative differences. 

Rat liver microsomes were observed to have a higher capacity than human liver microsomes to 

metabolize methoxychlor to covalent binding intermediates (Bulger and Kupfer 1990). In vitro data also 

indicate that covalent binding of methoxychlor to human liver microsomes is similar across age and sex, 

whereas in rats, covalent binding in mature males is much higher than in mature females and immature 

males and females (Bulger and Kupfer 1989). Another comparison found that in vitro rates of 

demethylation of methoxychlor and mono-hydroxy methoxychlor with human liver microsomes were 

generally higher in rat liver microsomes than in human liver microsomes (Stresser and Kupfer 1998). 

3.4.4 Elimination and Excretion 

Direct information regarding possible age-related differences in the rate, extent, or route of elimination of 

methoxychlor and metabolites was not located. 
METHOXYCHLOR 86 

3. HEALTH EFFECTS 

3.4.4.1 Inhalation Exposure 

No studies were located regarding excretion in humans or animals after inhalation exposure to 

methoxychlor. 

3.4.4.2 Oral Exposure 

In comparison to DDT (a pesticide which is relatively persistent and slowly eliminated), animal studies 

indicate that ingested methoxychlor is excreted rapidly, predominantly in the feces and to a lesser extent 

in the urine. Approximately 90% of an oral dose of 50 mg/kg recrystallized methoxychlor was recovered 

in the feces of mice within 48 hours, and 10% was excreted in the urine (Kapoor et al. 1970). Only 7–8% 

of the material in the feces was excreted as the parent compound. In contrast, only 1.02 and 4.3% of the 

administered radioactivity was recovered in feces and urine collected from mice within 24 hours and 

11 days, respectively, of administering single oral doses of 12.5 mg/kg radiolabeled DDT. 

Assuming that the fecal metabolites (primarily demethylated, dechlorinated, and dehydrochlorinated 

compounds) did not result from degradation of unabsorbed parent compound by enteric bacteria, these 

data suggest that biliary excretion of metabolites contributes significantly to methoxychlor clearance. 

Supporting these observations are those that methoxychlor metabolites identified in bile collected from a 

bile-cannulated male goat were similar to those in feces collected from lactating female goats given 3.6 or 

11.6 mg/kg methoxychlor (Davison et al. 1982, 1983). In the female goats, 40.5 and 67.5% of 

administered doses were excreted in the feces within 3 days, respectively, and metabolites accounted for 

70 and 81% of radioactivity in the feces, respectively (Davison et al. 1982). 

3.4.4.3 Dermal Exposure 

No studies were located regarding excretion in humans after dermal exposure to methoxychlor. Three 

days after applying a single dose of 200 mg laboratory grade methoxychlor to the shaved backs of goats, 

0.37–0.91% of the dose was excreted in the feces and 0.53–0.72% of the dose was excreted in the urine 

(Davison et al. 1983). 
METHOXYCHLOR 87 

3. HEALTH EFFECTS 

3.4.4.4 Other Routes of Exposure 

Parenteral exposure studies in animals indicate that biliary excretion of methoxychlor accounts for a 

significant fraction of fecal excretion. In bile-duct cannulated rats given a single intravenous dose of 

radiolabeled methoxychlor, the radiolabel was first detected in the bile within 1 minute (Weikel 1957). 

Fifty percent of the dose was excreted in the bile after 4 hours. The urinary excretion of label in bilecannulated rats was only 0.1–0.2% compared to 5–10% in noncannulated rats, suggesting that the 

appearance of urinary metabolites was largely due to material that was reabsorbed in the gut (i.e., 

enterohepatic circulation). 

3.4.5 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 
METHOXYCHLOR 88 

3. HEALTH EFFECTS 

1994; Leung 1993). PBPK models for a particular substance require estimates of the chemical substancespecific 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 

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 3-3 shows a conceptualized representation of a PBPK model. 

No studies were located regarding PBPK models for methoxychlor. 

3.5 MECHANISMS OF ACTION 

As discussed in Section 3.2, the chief effects of methoxychlor in animals are on the reproductive system. 

Although reproductive effects have mainly been observed in animals following oral exposures, it is likely 

that these types of effects could occur following inhalation and dermal exposures as well, if absorption of 
METHOXYCHLOR 89 

3. HEALTH EFFECTS 

Figure 3-3. 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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equivalent amounts occurred. For this reason, the mechanism of action for methoxychlor is discussed 

below without reference to route of exposure. 

3.5.1 Pharmacokinetic Mechanisms 

Absorption. No studies were located regarding the mechanism of absorption of methoxychlor in 

humans or animals by any route. 

Distribution. No studies were located regarding the mechanism of distribution of methoxychlor in 

humans or animals after exposure by any route. 

Metabolism. Methoxychlor is metabolized to estrogenic compounds in animals and humans by 

hepatic CYP enzymes (Dehal and Kupfer 1994; Kishimoto and Kurihara 1996; Stresser and Kupfer 1997, 

1998; Stresser et al. 1996). In rats, in vivo and in vitro studies have shown that metabolism of 

methoxychlor to its mono-, bis-, tris-, and ring-hydroxy metabolites is mediated by CYP2C6, 2A1, 2B1, 

2B2, and 3A (Kishimoto and Kurihara 1996; Li et al. 1995). The Km values for partially purified 

preparations of CYP2C6, 2A1, 2B1, and 2B2 were 0.36, 0.38, 1.07, and 2.34 μM, respectively, and the 

Vmax values were 0.40, 0.36, 1.20, and 1.22 mol/mol P450/minute, respectively (Kishimoto et al. 1995). 

Studies examining inhibition by antibodies to CYP2B1 and CYP2C6 of methoxychlor demethylation in 

rat liver microsomes and purified preparations of CYP2C6 provided evidence that CYP2C6 and another 

unidentified CYP isozyme make important contributions in the initial demethylation of methoxychlor in 

rats (Kishimoto and Kurihara 1996). Ring hydroxylation of methoxychlor and hydroxy methoxychlor 

derivatives in rats involves CYP2B isozymes, based on observations of inhibition of in vitro activity by 

anti-CYP2B monoclonal antibodies and enhanced ring hydroxylation activity in liver microsomes from 

phenobarbital-pretreated rats (Stresser et al. 1996). 

Results from in vitro studies with human liver microsomes (discussed in Section 3.4.3 Metabolism) have 

led to the proposal that CYP2C19 and CYP1A2 may be the major CYP isozymes catalyzing the 

demethylation of methoxychlor and its mono-hydroxy derivative, and that other CYP forms, including 

CYP2A6, 2C9, and 2B6, may make major contributions, especially in individuals with low levels of 

CYP2C19 or CYP1A2 (Stresser and Kupfer 1998). Nonlinear regression analysis of data from insect 

cells overexpressing human CYP2C19 or 1A2 showed standard Michaelis-Menten kinetics for a oneenzyme model. The Km (μM) values for cytochrome CYP2C19 and 1A2 were similar (0.43 and 0.51), but 

the Vmax (nmol/minute/nmol P450) for 2C19 (26.8) was twice that for 1A2 (12.4), indicating greater 
 

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intrinsic clearance of methoxychlor by CYP2C19 (Stresser and Kupfer 1998). Similar analysis of data 

from human liver microsomes (containing multiple CYPs) showed a fit for a two-enzyme system, even 

though there were wide variations for Vmax (20–576 pmol/minute/mg) and Km (0.35–12 μM) between 

samples (Elsby et al. 2001; Stresser and Kupfer 1998). The discrepancy in kinetic values is probably an 

indication of individual genetic variability in expression of CYP enzymes (Shimada et al. 1994). Studies 

with human cDNA-expressed CYP isozymes suggest that ring hydroxylation of methoxychlor or its 

hydroxy derivatives may involve a different set of CYP isozymes than CYP-catalyzed demethylation, 

including CYP1A2, 2B6, 2D6, 2E1, and 3A4 (Stresser and Kupfer 1997). 

Excretion. No studies were located regarding the mechanism of excretion of methoxychlor in humans 

or animals after exposure by any route. 

3.5.2 Mechanisms of Toxicity 

The primary effects of methoxychlor in animal models involve the reproductive system, and result from 

the interaction of methoxychlor and its metabolites with estrogen receptors, and possibly androgen 

receptors, although the data indicating androgen receptor binding are limited. Many of the reproductive 

effects of methoxychlor are similar to those caused by estrogen, and are believed to be due to the ability 

of the mono- and bis-hydroxy derivatives of methoxychlor and MDDE to act as estrogen analogues 

(Bulger et al. 1978a, 1978b, 1978c, 1978d, 1985; Ousterhout et al. 1981). Androgens and estrogens are 

very lipid soluble and thus, they diffuse easily through the cell membrane into the cytosol and nucleus 

(DeFranco 1999). Once a steroid binds to its receptor and the hormone-receptor complex reaches the 

nucleus, it binds to hormone response elements in the enhancers, silencers, or promoters upstream of the 

genes controlled by the steroid in question. The hormone-receptor complex acts as a transcription factor 

to either stimulate or repress transcription of RNA from the steroid responsive gene. The resulting RNA 

transcript is spliced to form messenger RNA, which in turn directs the synthesis of specific proteins that 

direct the characteristic steroid hormone responses (Fregly and Luttge 1982). Some ancillary models for 

immediate action of steroid hormones have recently been proposed and involve either cross-talk 

interactions with, or possibly direct binding to, other hormone and growth factor cell surface receptors, 

and cell surface receptor mediated uptake of serum carrier proteins bound to steroid hormones (Chen and 

Farese 1999). 

In vitro studies have shown that methoxychlor itself does not bind to the estrogen receptor (Bulger et al. 

1978a, 1978b; Matthews et al. 2000; Ousterhout et al. 1981) but that the mono-hydroxy and bis-hydroxy 
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derivatives of methoxychlor and MDDE do bind to the receptor and cause nuclear translocation with the 

following order of potency: bis-OH-MDDE >> bis-OH-methoxychlor > mono-OH-MDDE > mono-OHmethoxychlor >> MDDE = methoxychlor (inactive) (Bulger et al. 1985). A similar hierarchy (bis-OHMDDE > bis-OH-methoxychlor >> MDDE > methoxychlor) has been observed in vivo for the induction 

of uterine ornithine decarboxylase and increased uterine weight (Bulger et al. 1985). It was long thought 

that estrogen mediated its effects by binding to a single receptor, estrogen receptor α (ERα). Recently, 

however, a second estrogen receptor, ERβ, has been discovered (Kuiper et al. 1996; Mosselman et al. 

1996). Although bis-hydroxy methoxychlor has been shown to bind to ERα and ERβ (Gaido et al. 1999, 

2000), none of the earlier (before 1996) studies examining the difference in estrogen receptor binding 

affinity between methoxychlor and its metabolites differentiated between ERα and ERβ. 

It has not been fully established what effects are mediated by each estrogen receptor. However, there 

appears to be tissue-specific distribution of the two receptors (Kuiper et al. 1997), which may allow for 

tissue-specific effects by estrogens. In tissues where both receptors are expressed, ligand binding to the 

receptors results in heterodimer formation (an ERα receptor pairs up with an ERβ receptor) (Cowley et al. 

1997; Pace et al. 1997; Pettersson et al. 1997), which may result in different patterns of gene regulation 

than seen with homodimeric pairing (an ERα with an ERα or an ERβ with an ERβ). Additionally, each 

different estrogenic compound might act as an estrogen agonist at one receptor type and an estrogen 

antagonist at the other receptor type. This is apparently the case with the bis-hydroxy metabolite of 

methoxychlor, which has been shown to be an ERα agonist and an ERβ antagonist in some in vitro assay 

systems (Gaido et al. 1999, 2000). 

Adding further to the mechanistic complexity, Ghosh et al. (1999) have suggested that methoxychlor acts 

through a third, as yet unelucidated, mechanism, not necessarily via an ER. Ghosh et al. (1999) showed 

that methoxychlor induced an increase in mRNA of two estrogen-responsive genes (lactoferrin, LF, and 

glucose-6-phosphate dehydrogenase, G6PD) in uteri of ovariectomized wild-type and ovariectomized 

ERα-knockout mice. Induction of LF and G6PD mRNA by methoxychlor was slightly greater in wildtype mice than in ERα-knockout mice at 15–30 mg/kg/day. Since no functional ERα receptors were 

present in the ERα knockout mice, the involvement of ERβ was indicated. However, when an estrogen 

inhibitor (ICI 182,780) was added, which should have eliminated methoxychlor interaction with 

ERβ, methoxychlor still induced an increase in LF and G6PD mRNA at a slightly lower level than without 

inhibitor in wild-type mice and at the same level in ERα knockout mice. In the same experiment, 

estradiol-17β resulted in large increases in LF and G6PD mRNA in wild-type mice, but in low levels of LF 

and G6PD mRNA (similar to control mice) in ERα-knockout mice (the authors point out that the 
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concentration of ERβ in uterus of the wild-type and ERα-knockout mice was very low). When an inhibitor 

was added, no increase in LF and G6PD mRNA was induced by estradiol-17β in wild-type mice. Thus, 

the “estrogenic” activity of methoxychlor was not attenuated by the absence of functional ERα receptors or 

the presence of an estrogen inhibitor effective at ERβ, suggesting that an additional mechanism of toxicity 

may exist besides interaction with ERα or ERβ. 

A study that examined the effects of methoxychlor, DDE, and estradiol-17β on steroidogenesis and FSH 

responsiveness in ovarian cells provides additional support for a non-estrogenic mechanism of endocrine 

disruption by methoxychlor (Chedrese and Feyles 2001). In CHO-FSH-R cells (a Chinese hamster ovary 

cell line genetically modified to express the FSH receptor) exposed to DDE in the presence of FSH, DDE 

inhibited FSH-stimulated cAMP synthesis, which likely resulted in the observed decrease in progesterone 

synthesis and in decreased activity or synthesis of steroidogenic enzymes. Methoxychlor did not affect 

FSH-stimulated cAMP synthesis, but did inhibit estradiol-17β-stimulated progesterone synthesis in 

primary culture pig granulosa cells (Chedrese and Feyles 2001). Since progesterone is required for 

normal ovulation and implantation, the study authors speculated that this mechanism may partially 

explain the detrimental effects of methoxychlor on reproduction (Chedrese and Feyles 2001). The 

involvement of an ER is unlikely because no metabolic activation system was included, and 

methoxychlor, unlike its metabolite HPTE, does not interact with the ERs to any great extent (Bulger 

et al. 1978a, 1978b, 1985; Matthews et al. 2000; Ousterhout et al. 1981). 

In contrast, the results of another in vitro study indicated that purified methoxychlor may have weak 

intrinsic estrogenicity (Elsby et al. 2001). A yeast estrogenicity assay was used that did not incorporate a 

microsomal metabolic activation system. The yeast had the DNA sequence of the human ERα integrated 

into the genome and contained transfected expression plasmids with the yeast 3-phosphoglycerate kinase 

promoter, estrogen responsive sequences, and a β-galactosidase reporter gene. Binding of an active 

ligand to the ER initiated transcription of the reporter gene, secretion of β-galactosidase into the medium, 

and ultimately, a color change in the medium from yellow to red. Methoxychlor was at least 

100,000 times less potent than estradiol-17β and about 100 times less potent than the methoxychlor 

metabolite HPTE (Elsby et al. 2001). 

An in vitro assay in human breast cancer MCF-7 cells examined the estrogenicity of methoxychlor and 

other xenoestrogens, as well as estradiol-17β and DES, by quantitatively assaying the induction or 

repression of four endogenous estrogen-regulated marker genes (pS2, transforming growth factor β3, 

monoamine oxidase A, and α1-antichymotrypsin) (Jørgensen et al. 2000). Methoxychlor induced pS and 
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α1-antichymotrypsin and repressed monoamine oxidase A and transforming growth factor β3, as did 

estradiol-17β, but methoxychlor was at least 100,000 times less potent than estradiol-17β (10-13–10-11 M 

for estradiol-17β and 10-6–10-5 M for methoxychlor). These estrogenic activities of methoxychlor were 

apparently mediated through the ERα since no ERβ was detected in MCF-7 cells. 

Methoxychlor, like estradiol-17β, stimulated an increase in uterine peroxidase activity (a marker of 

estrogen action) (Cummings and Metcalf 1994). Also similar to estradiol-17β, methoxychlor stimulation 

of peroxidase was inhibited by actinomycin D (an RNA synthesis inhibitor) and cycloheximide (a protein 

synthesis inhibitor), indicating a similar mechanism of action. Estrogen-induced protein synthesis, an 

early effect of estradiol in the immature rat uterus and indicator of receptor occupancy and mRNA 

synthesis, was stimulated by 99% pure methoxychlor or estrone in an identical manner (Cummings and 

Metcalf 1995a). Estrogen-induced protein synthesis by methoxychlor and estrone was inhibited similarly 

by actinomycin D and cycloheximide. Methoxychlor induced the secretion of proteins in mature 

ovariectomized mice (Rourke et al. 1991) and neonatal mice (Eroschenko and Rourke 1992). Although 

the proteins secreted by methoxychlor were similar to those observed following estradiol treatment, some 

differences were noted, particularly so for neonatal mice. It was postulated by the study authors that 

posttranslational alterations to the estrogen receptors might be responsible for these differences (Rourke 

et al. 1991). These observations strongly support the view that the estrogenic effects of methoxychlor are 

mediated via binding of O-demethylated metabolites to estrogen receptors resulting in protein synthesis. 

Secretion of altered or different proteins into the uterine fluid during implantation and pregnancy could 

alter the essential uterine environment which may ultimately be responsible for decrements in fertility. 

There are a number of physiological mechanisms by which the estrogenic effects of O-demethylated 

metabolites or contaminants of methoxychlor can interfere with reproduction. In female animals, possible 

mechanisms for decreased fertility include decreased mating frequency (Harris et al. 1974) decreased 

decidualization and decreased uterine receptivity to implantation (Cummings and Gray 1987, 1989), 

accelerated tubal transport of fertilized ova (Cummings and Perrault 1990), complete inhibition of 

implantation and altered preimplantation embryonic development and transport in mice (Hall et al. 1997; 

seen after intraperitoneal methoxychlor administration), and atresia of preovulatory follicles with reduced 

corpus luteum formation (Gray et al. 1989). In male animals, possible mechanisms for decreased fertility 

include decreased mating frequency (Gray et al. 1999; Harris et al. 1974), inadequate cervical stimulation 

of female animals to induce events necessary for implantation (Gray et al. 1989), and decreased Leydig 

cell function (Akingbemi et al. 2000; Gray et al. 1989). 
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Many of the effects described above are probably mediated by a direct effect of methoxychlor metabolites 

on estrogen-sensitive tissues. Alternatively, some of these changes may result from the effects of 

methoxychlor metabolites on the endocrine system. For example, methoxychlor has been shown to cause 

increased levels of gonadotropin releasing hormone in the hypothalamus (Goldman et al. 1986), elevated 

levels of prolactin, and thyroid stimulating hormone in the pituitary (Goldman et al. 1986; Gray et al. 

1989), and reduced levels of thyroid-stimulating hormone, testosterone, and progesterone in the serum 

(Cummings and Gray 1989; Gray et al. 1989). These methoxychlor-induced effects on the endocrine 

system may be partially responsible for methoxychlor-induced effects such as uterine and mammary 

gland hyperplasia (Tegeris et al. 1966) and gonadal atrophy (Bal 1984; Gray et al. 1988; Hodge et al. 

1950; Tullner and Edgcomb 1962), and could also be important in impaired reproduction associated with 

methoxychlor exposure (Bal 1984; Gray et al. 1989; Harris et al. 1974). 

Some of the reproductive effects seen in male rodents following methoxychlor exposure are similar to 

those produced by either estrogens or antiandrogens (Gray et al. 1999). Delayed puberty and reduced 

accessory sex gland size can be caused by an estrogen or an antiandrogen. Estrogenic compounds cause a 

decrease in LH secretion from the pituitary and in serum testosterone and an increase in pituitary tumors 

and hyperprolactinemia, whereas antiandrogens enhance LH secretion from the pituitary, cause an 

increase in serum testosterone, and have no effect on prolactin or on pituitary tumorigenesis (Gray et al. 

1999). In male rats dosed with 0, 200, 300, or 400 mg/kg/day methoxychlor from weaning (postpartum 

day 21) through 11 months of age, preputial separation was delayed in a dose-dependent manner, 

pituitary size was decreased at all exposure levels, and there were no statistical differences from controls 

in serum LH, prolactin, and testosterone (Gray et al. 1999). These results suggest that methoxychlor 

might interact with the androgen receptor (AR) as an antagonist; however, it should be noted that in some 

in vitro assays, estradiol-17β can itself act as a weak androgen antagonist (Kelce et al. 1995). Only one 

study has investigated the potential androgen antagonism of methoxychlor and its metabolites; this 

experiment was not a direct receptor binding assay. The methoxychlor metabolite HPTE was a weak AR 

antagonist of dihydrotestosterone in HepG2 human hepatoma cells transiently transfected with the human 

androgen receptor and a reporter gene linked to an androgen responsive promoter; methoxychlor itself 

showed even less androgen antagonism in this experiment (Maness et al. 1998). p,p’-DDE, an isomer of 

the methoxychlor structural analog DDT, is also an AR antagonist of dihydrotestosterone (Danzo 1997; 

Kelce et al. 1995, 1997; Maness et al. 1998). It is thought that androgen antagonism explains some of the 

reproductive and developmental effects seen in male rats exposed to p,p’-DDE, including reduced 

anogenital distance, retention of thoracic nipples, delayed puberty, and reduced accessory sex organ 

weights (Kelce et al. 1997; Loeffler and Peterson 1999; You et al. 1998). Some of these effects (delayed 
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puberty and reduced accessory sex organ weights) are also seen following exposure to methoxychlor (Bal 

1984; Gray et al. 1989, 1999; Hodge et al. 1950; Shain et al. 1977; Tullner and Edgcomb 1962). More 

exhaustive studies regarding methoxychlor metabolite binding to the AR are necessary to be able to 

determine the contribution of androgen antagonism to the reproductive and developmental effects 

resulting from methoxychlor exposure. 

While the reproductive effects of methoxychlor appear to be due to the estrogenic nature of the 

O-demethylated metabolites, parent methoxychlor may be responsible for the neurotoxic effects observed 

at high exposure levels in animals (Cannon Laboratories 1976; Tegeris et al. 1966). The chief reason for 

suspecting this is that methoxychlor is a close structural analogue of DDT (differing in that the methoxy 

groups of methoxychlor are replaced by chlorine groups in DDT), and high levels of DDT are known to 

be neurotoxic in animals (Agency for Toxic Substances and Disease Registry 1994). DDT prevents the 

deactivation of the sodium gate after neuron activation and membrane depolarization (Brown et al. 1981; 

Coats 1990; Wu et al. 1975), resulting in hyperexcitability of the nerve. Other molecular mechanisms 

may contribute to DDT induced hyperexcitability of the nerve; these include prolonging the action 

potential by preventing the full opening of the potassium gates (Narahashi and Haas 1967) and 

interference with two specific neuronal adenosine triphosphatases (ATPases) thought to be involved in 

controlling sodium, potassium, and calcium fluxes through the nerve membrane (Matsumura and Patil 

1969; Matsumura 1985). In situ studies have shown that methoxychlor (and some other DDT structural 

analogs) increase and prolong the depolarizing afterpotential of nerves, similar to DDT (Wu et al. 1975). 

It is important to note that if the neurotoxic effects (tremors, convulsions) of methoxychlor are indeed 

attributable to the parent compound, then O-demethylation to the phenolic metabolites represents a 

detoxification pathway for neurotoxicity. This is consistent with the fact that this type of neurotoxicity 

has only been noted at doses high enough to surpass metabolic capacity (Cannon Laboratories 1976; 

Tegeris et al. 1966) or in animals in which hepatic metabolism has been impaired (Lehman 1952). 

3.5.3 Animal-to-Human Extrapolations 

Some species differences in sensitivity to methoxychlor have been observed. Rabbits appear to be more 

sensitive than rats to short-term exposure to methoxychlor (Kincaid Enterprises 1986; Smith et al. 1946). 

All rats administered a single gavage dose of 5,000 mg/kg methoxychlor survived, whereas 7 of 13 rats 

given 7,000 mg/kg died (Smith et al. 1946). Four of 4 rabbits died following the administration of 

4–15 daily doses of 200 mg/kg/day methoxychlor (Smith et al. 1946). Fatty degeneration of the liver and 

nephrosis were observed in some or all of the rabbits that died following exposure to 200 mg/kg/day. 
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Two of 17 pregnant rabbits died within 10 days of receiving 13 daily oral exposures to 251 mg/kg/day 

(Kincaid Enterprises 1986). 

Recent studies in mice show effects that may be mediated through the estrogen receptor (increased 

prostate weight and altered urine-marking behavior) at low exposure levels (0.02 mg/kg/day) (vom Saal et 

al. 1995; Welshons et al. 1999). No other studies were located that examined similar end points in other 

species exposed to similarly low levels of methoxychlor; therefore, it is unclear whether there are 

significant differences between species in susceptibility to these effects of methoxychlor. 

Several in vitro studies have indicated the presence in human hepatic microsomes of enzymatic activities 

similar to those in rats that are responsible for the metabolism of methoxychlor to its estrogenic 

metabolites (Dehal and Kupfer 1994; Stresser and Kupfer 1998). Therefore, there is some indication that 

rats may be an adequate animal model for the estrogenic effects of methoxychlor in humans. Rates of 

formation of mono- and bis-hydroxy methoxychlor from methoxychlor with human liver microsomes 

were generally lower than rates with rat liver microsomes, but some human liver samples displayed 

higher rates than the rates for rat microsomes (Stresser and Kupfer 1998). 

Another in vitro study examined the relative binding affinities (RBA) of methoxychlor, its metabolite 

HPTE, and other estrogenic substances (compared to estradiol-17β) to bacterially expressed estrogen 

receptor fusion proteins from humans (ERα), mouse (ERα), chicken, green anole (a lizard), and rainbow 

trout (Matthews et al. 2000). The ER fusion proteins contained three of the six domains of the receptor, 

including the ligand binding domain, fused to glutathione-S-transferase. Methoxychlor did not bind to 

the human or mouse receptor proteins (displaced <10% of radiolabeled estradiol-17β), bound only weakly 

to the chicken and green anole receptor proteins (displaced 10–50% of radiolabeled estradiol-17β), and 

had an RBA of 0.95 with the rainbow trout receptor protein. HPTE had RBAs of 1.2 with the human and 

mouse receptor proteins, 4.8 with the chicken and green anole receptor proteins, and 14 with the rainbow 

trout receptor protein. These data indicate some species differences in receptor binding, but support the 

mouse as an adequate model for the study of estrogenic effects of methoxychlor mediated through ER 

binding. 
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3.6 TOXICITIES MEDIATED THROUGH THE NEUROENDOCRINE AXIS 

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. Chemicals 

with this type of activity are most commonly referred to as endocrine disruptors. However, appropriate 

terminology to describe such effects remains controversial. The terminology endocrine disruptors, 

initially used by Colborn and Thomas (1992) and again by Colborn (1993), was also used in 1996 when 

Congress mandated the Environmental Protection Agency (EPA) to develop a screening program for 

“...certain substances [which] may have an effect produced by a naturally occurring estrogen, or other 

such endocrine effect[s]...”. To meet this mandate, EPA convened a panel called the Endocrine 

Disruptors Screening and Testing Advisory Committee (EDSTAC), which in 1998 completed its 

deliberations and made recommendations to EPA concerning endocrine disruptors. In 1999, the National 

Academy of Sciences released a report that referred to these same types of chemicals as hormonally 

active agents. The terminology endocrine modulators has also been used to convey the fact that effects 

caused by such chemicals may not necessarily be adverse. Many scientists agree that chemicals with the 

ability to disrupt or modulate the endocrine system are a potential threat to the health of humans, aquatic 

animals, and wildlife. However, others think that endocrine-active chemicals do not pose a significant 

health risk, particularly in view 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 chemicals are derived from plants and are similar in structure and action 

to endogenous estrogen. Although the public health significance and descriptive terminology of 

substances capable of affecting the endocrine system remains controversial, scientists agree that these 

chemicals may affect the synthesis, secretion, transport, binding, action, or elimination of natural 

hormones in the body responsible for maintaining homeostasis, reproduction, development, and/or 

behavior (EPA 1997). Stated differently, such compounds may cause toxicities that are mediated through 

the neuroendocrine axis. As a result, these chemicals may play a role in altering, for example, metabolic, 

sexual, immune, and neurobehavioral function. Such chemicals are also thought to be involved in 

inducing breast, testicular, and prostate cancers, as well as endometriosis (Berger 1994; Giwercman et al. 

1993; Hoel et al. 1992). 

There is ample evidence, in vitro and in vivo, that methoxychlor has estrogenic properties. Estrogens, 

both endogenous and exogenous, or environmental, mediate many of their effects by binding to estrogen 

receptors (ER) in the cytoplasm of target tissue cells. Estrogens are very lipid soluble and diffuse easily 

through the cell membrane into the cytosol, where they bind to empty estrogen receptors. The occupied 
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receptor is translocated to the nucleus, where the receptor-estrogen complex binds to hormone response 

elements upstream of the gene or genes controlled by the estrogen and induces or inhibits the production 

of specific proteins that then produce a characteristic effect(s). The effect may be seen in the tissue in 

which the protein was synthesized or in a distant tissue following secretion and transport of the newly 

synthesized protein through the bloodstream. For many years, it was thought that estrogens mediated 

their effects by binding to a single receptor, estrogen receptor α (ERα). However, a second estrogen 

receptor, ERβ, has recently been discovered (Kuiper et al. 1996; Mosselman et al. 1996). It has not been 

fully established what effects are mediated by each receptor. However, there appears to be tissue-specific 

distribution of the two receptors (Kuiper et al. 1997), which may allow for tissue-specific effects by 

endogenous and exogenous estrogens. This adds complexity to the effects of estrogenic compounds 

because each compound may, theoretically, interact with each receptor as an estrogen agonist or 

antagonist. The estrogenicity of methoxychlor derives primarily from its metabolites, mono- and bishydroxy methoxychlor. Bis-hydroxy methoxychlor is known to bind to ERα and ERβ and is thought to 

be an ERα agonist and an ERβ antagonist (Gaido et al. 1999, 2000). Recent data by Ghosh et al. (1999) 

indicate that methoxychlor may also act via another, as yet unknown, mechanism not mediated by either 

ERα or ERβ. Estrogenic mechanisms are discussed in more detail in Section 3.5.2 Mechanisms of 

Toxicity. 

There are no data showing estrogenic effects of methoxychlor in humans. However, there are numerous 

in vivo animal studies showing the estrogenic nature of methoxychlor and its metabolites. Below is a 

brief overview of the effects of exposure to methoxychlor on the endocrine and reproductive systems. A 

more detailed discussion of these studies can be found in Section 3.2. 

Exposure to methoxychlor has been shown to affect the reproductive systems of male and female 

mammals in ways that mimic or antagonize the effects of estrogen. Effects have been observed following 

acute, intermediate, and chronic duration exposures and over a wide range of exposure levels. Effects 

have been seen following exposures in utero, during lactation, or post-weaning. There is also a wide 

range of effects, encompassing the structural, functional, and behavioral realms, in a number of different 

species. In female mammalian species, sexual maturity was affected, as demonstrated by precocious 

vaginal opening and delayed onset of estrus cyclicity in rats and mice exposed to 5–200 m/kg/day (Appel 

and Eroschenko 1992; Chapin et al. 1997; Cooke and Eroschenko 1990; Eroschenko 1991; Eroschenko 

and Cooke 1990; Gray et al. 1989; Harris et al. 1974; Swartz and Corkern 1992). A number of structural 

and functional effects were also seen, including mammary gland hyperplasia in pigs exposed to 

1,000 mg/kg/day (Tegeris et al. 1966) and ovarian atrophy in rats and mice exposed to 25–400 mg/kg/day 
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(Bal 1984; Chapin et al. 1997; Gray et al. 1988, 1989; Harris et al. 1974; Martinez and Swartz 1991), 

gross and cellular ultrastructural alterations of the lining of the uterus in mice exposed to 

50–100 mg/kg/day (Swartz et al. 1994), persistent estrus in rats and mice exposed to 25–400 mg/kg/day 

(Gray et al. 1988; Harris et al. 1974; Martinez and Swartz 1991), decreased mating frequency rats 

exposed to 60–160 mg/kg/day (Harris et al. 1974), decreased fertility in rats and mice exposed to 

35.5–200 mg/kg/day (Bal 1984; Chapin et al. 1997; Culik and Kaplan 1976; Gray et al. 1989; Harris et al. 

1974; Haskell Laboratories 1966; Kincaid Enterprises 1986; Wenda-Rozewicka 1983), decreased live 

offspring in rats exposed to 100–150 mg/kg/day (Chapin et al. 1997; Gray et al. 1989), and increased 

number of resorptions of fetuses in rats and rabbits exposed to 17.8–400 mg/kg/day (Culik and Kaplan 

1976; Khera et al. 1978; Kincaid Enterprises 1986). Serum progesterone and pituitary prolactin hormone 

levels were also affected in rats exposed to 400 mg/kg/day (Gray et al. 1988), and cellular biochemistry of 

ovarian cells was altered (an accumulation of lipid in the interstitial and thecal cells) in mice exposed to 

100 mg/kg/day (Martinez and Swartz 1992). 

Similar types of effects are seen in male animals. Delayed preputial separation indicated delayed sexual 

maturity. Structural and functional effects seen included decreased testes, ventral prostate, epididymis, 

and seminal vesicle weights in rats exposed to 50–1,400 mg/kg/day (Bal 1984; Chapin et al. 1997; 

Goldman et al. 1986; Gray et al. 1989, 1999; Hodge et al. 1950; Shain et al. 1977; Tullner and Edgcomb 

1962), increased adrenal gland, seminal vesicle, and prostate weights in rats exposed to 

0.02–400 mg/kg/day (Chapin et al. 1997; Gray et al. 1989, 1999; Stoker et al. 1999; Welshons et al. 

1999), inhibition of testes development in rats exposed to 5–150 mg/kg day (Chapin et al. 1997), 

decreased caudal epididymal sperm count in rats exposed to 50–400 mg/kg/day (Gray et al. 1989, 1999), 

decreased mating frequency in rats exposed to 60–400 mg/kg/day (Gray et al. 1999; Harris et al. 1974), 

decreased copulatory stimulation of females necessary for pregnancy in male rats exposed to 

100 mg/kg/day (Gray et al. 1989), and decreased fertility in rats exposed to 35.5–300 mg/kg/day (Bal 

1984; Chapin et al. 1997; Culik and Kaplan 1976; Gray et al. 1989; Harris et al. 1974; Haskell 

Laboratories 1966; Kincaid Enterprises 1986; Wenda-Rozewicka 1983). Hormone levels were also 

affected in males, with increased levels of pituitary prolactin, FSH, TSH, and hypothalamic GnRH in rats 

exposed to 25–50 mg/kg/day (Goldman et al. 1986; Gray et al. 1989), decreased levels of serum TSH in 

rats exposed to 100–200 mg/kg/day (Cummings and Gray 1989; Gray et al. 1989), decreased serum 

testosterone and progesterone in mice and rats exposed to 33–100 mg/kg/day (Amstislavsky et al. 1999; 

Cummings and Gray 1989; Gray et al. 1989), and decreased interstitial fluid and epididymide testosterone 

in rats exposed to 100 mg/kg/day (Gray et al. 1989). Some hormone alterations occurred at methoxychlor 
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exposure levels below those causing reproductive effects and may have contributed to or caused those 

effects. 

It is thought that most, if not all, of the above endocrine disruptive effects are mediated by the interaction 

of the bis-hydroxy metabolite of methoxychlor (HPTE) with one or both of the estrogen receptors and/or 

the androgen receptor. It is also possible that tissue toxicity or some other indirect route of influence by 

methoxychlor or its metabolites resulted in some of these effects. Numerous in vitro studies have verified 

that methoxychlor binds to ERs, although with much lower affinity than estradiol. Methoxychlor has 

been shown to bind ER (probably both ERα and ERβ) from MCF-7 cells (a human breast cancer cell line) 

(Dodge et al. 1996), rabbit uterine cells (Danzo 1997), bovine uterine endometrial explants (Tiemann et 

al. 1996), GT1 cells (a murine immortal hypothalamic neuron cell line) (Roy et al. 1999), and MtT/E-2 

cells (a estradiol-17β-responsive rat pituitary cell line) (Maruyama et al. 1999). Methoxychlor has also 

been shown to bind to human ERα and ERβ expressed in Sf9 (insect) cells (Kuiper et al. 1998). The 

methoxychlor metabolite bis-OH-methoxychlor (or HPTE) has been shown to bind to human and rat ERα

and ERβ expressed in human hepatoma (HepG2) cells (Gaido et al. 1999, 2000), and to be an ERα

agonist and an ERβ antagonist. Methoxychlor and HPTE can also weakly antagonize dihydrotestosterone 

at the human androgen receptor (AR) (expressed in HepG2 cells) (Maness et al. 1998). In male rats 

exposed to 200 mg/kg/day methoxychlor from weaning (postpartum day 21) through 11 months of age, 

preputial separation was delayed, pituitary size was decreased, and there were no statistical differences 

from controls in serum LH, prolactin, and testosterone (Gray et al. 1999); these effects are typical of 

antiandrogenic, but not estrogenic, compounds. The binding affinity and potency of methoxychlor was 

generally observed to be 1,000–100,000 times less than estradiol (Dodge et al. 1996; Kuiper et al. 1998). 

However, it is thought that estrogen is bound to serum proteins (>95%) in vivo that render it less available 

for migration into cells and receptor binding (Nagel et al. 1999; vom Saal et al. 1995). The fact that 

methoxychlor is not bound by these proteins must be taken into account when using in vitro assays in 

serum-free media to simulate in vivo exposures (Nagel et al. 1999; vom Saal et al. 1995). Receptor 

binding of methoxychlor is discussed in more detail in Section 3.5.2, Mechanisms of Toxicity. 

Effects on Wildlife. Whether methoxychlor causes estrogenic effects in wildlife has been investigated in 

a number of experiments. Although methoxychlor has certainly produced effects in fish, amphibians, and 

sea urchins, only a subset of these effects seem to be due to estrogenic activity. The mechanism for these 

other effects is not known. 
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The estrogenic activity of methoxychlor has been demonstrated in catfish. In catfish, estrogen receptors 

have been found in liver, brain, and testes. The liver is the primary target tissue of estrogen in fish and is 

the site of the greatest ER concentration. Interaction with fish liver ER stimulates the synthesis of 

vitellogenin (the precursor to egg yolk) and also stimulates the synthesis of more ER. Nimrod and 

Benson (1997) tested methoxychlor, the O-demethylated metabolite of methoxychlor and several other 

environmentally relevant compounds for their ability to compete with estradiol for binding to catfish ER 

in vitro. Methoxychlor was only about 1/6,700 as potent and O-demethylated methoxychlor was 

1/1,000 as potent as estradiol. Even though O-demethylated methoxychlor was not a very strong catfish 

ER ligand, it was the most potent environmentally relevant compound tested in this study, including 

o,p’-DDT and o,p’-DDE, which had extremely low binding ability. When injected into catfish 

intraperitoneally, methoxychlor did not induce a detectable estrogenic response, as measured by 

vitellogenin synthesis (Nimrod and Benson 1997). Curiously, after depletion of the liver enzyme thought 

to be responsible for methoxychlor metabolic transformation, serum vitellogenin and estradiol increased 

(Schlenk et al. 1997). 

Methoxychlor causes subtlely different effects from estradiol in trout. Trout exposed every third day to 0, 

0.5, 1, 2, or 4 mg/L methoxychlor (laboratory grade) by immersion for 2 hours followed by rinsing from 

6 days prior to hatching through 24 days post-hatching showed increased mortality. Mortality following 

estradiol exposure was 3 times that of methoxychlor-induced mortality (Krisfalusi et al. 1998a). Further 

experiments showed that methoxychlor caused increased mortality at day 16 post-hatch regardless of 

number of treatments, whereas estradiol caused increased mortality after 10 treatments regardless of 

developmental time. Therefore, methoxychlor does not appear to be acting by the same mechanism as 

estradiol. Also noted in this study was that the skin of methoxychlor-treated fish was lighter than controls 

by 6 days post-hatching, and the skin of estradiol-treated fish was darker than controls between 

days 9 and 12 post-hatching, again indicating different mechanisms. The authors speculated that the 

lighter skin may be due to decreased synthesis and/or release of melanophore stimulating hormone from 

the pituitary and that the darker skin may be due to toxicity. Fish treated with methoxychlor also showed 

a dose-dependent decrease in weight. Estradiol caused weight reduction as well, but in a non-dosedependent manner. A dose-dependent increase in mortality and decrease in growth in trout was also 

observed (Krisfalusi et al. 1998b). Methoxychlor treatment did not disrupt male sex differentiation or 

early testicular development in these trout. 

Methoxychlor did not produce estrogenic effects in Japanese medaka (a teleost, or bony fish) at doses 

comparable to an effective dose of estradiol (Nimrod and Benson 1998). In Japanese medaka, no 
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developmental or reproductive toxicity was seen following exposure to 0, 0.2, 0.6, or 2.3 μg/L laboratory 

grade methoxychlor in the water for the first month after hatching. Parameters monitored included 

ovarian or testicular size, sex ratio, fertility, viability of eggs, and hatchability of eggs. These results 

contrast with the definitive effects that estradiol causes in these fish. Medaka are susceptible to 

phenotypic sex reversal when exposed to estrogens and androgens through the diet during early life 

stages. Exposure to 0.01. 0.12, or 1.66 μg/L estradiol as above resulted in 100% female populations. 

Fish (female) in the 1.66 μg/L estradiol group also had lower fecundity than controls. 

In vitro experiments using cultured carp hepatocytes showed that methoxychlor was 1,000-fold less 

potent than estradiol. The order of estrogenic potency, as measured by vitellogenin induction, of 

compounds tested was methoxychlor > o,p-DDT > chlordecone . bisphenol-A . 4-t-pentylphenol 

(Smeets et al. 1999). When cells were exposed simultaneously to estradiol and methoxychlor, estradiol 

antagonized the effects of methoxychlor. Methoxychlor did not appear to be metabolized to more 

estrogenic metabolites in carp by CYP1A2 (one of the CYP liver enzymes in mammals responsible for 

metabolizing methoxychlor to more estrogenic compounds), as induction of this enzyme did not enhance 

the estrogenicity of methoxychlor. 

In salamanders, methoxychlor sometimes causes subtlely different effects from estradiol and other times 

completely different effects. Salamander eggs exposed to up to 10 mg/L laboratory-grade methoxychlor 

experienced no increase in mortality through post-hatching day 10 (Ingermann et al. 1997). Exposures as 

low as 0.1 mg/L resulted in precocious hatching and reduced startle response. The effects of 

methoxychlor were compared to estradiol in salamanders (Ingermann et al. 1999). Salamander eggs were 

exposed to 0, 0.2, 1.0, or 5.0 μM laboratory grade methoxychlor, recrystallized methoxychlor, HPTE (the 

estrogenic metabolite of methoxychlor in mammals), estradiol, or deoxycorticosterone (DOC, an adrenal 

steroid) until 10 days post-hatching. Laboratory grade and recrystallized methoxychlor were equally 

potent at causing precocious hatching and a blunted startle response, suggesting that contaminants are not 

responsible for these effects. Only the highest exposure level of estradiol resulted in precocious hatching 

and an altered startle response. The startle response in estradiol-treated hatchlings frequently involved 

swimming in circles, whereas the startle response in methoxychlor-treated hatchlings involved a shorter 

straight distance traveled (never circles). Exposure of eggs and hatchlings to HPTE resulted in no effect 

on day of hatch or startle response. DOC had no effect on hatch time and resulted in reduced startle 

response only at the highest exposure level. The effects of laboratory grade and recrystallized 

methoxychlor and HPTE on maturation of frog oocytes in vitro were compared to the effects of estradiol 

(Pickford and Morris 1999). Both grades of methoxychlor caused a highly significant inhibition of 
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progesterone-induced germinal vesicle breakdown (GVBD, necessary for oocyte maturation), while 

estradiol and HPTE had no effect. This indicates that methoxychlor is the compound responsible for this 

effect and that this effect is not estrogenic in nature. Failure of ICI 182,780 (an ER inhibitor) to alter the 

effect of methoxychlor on oocyte maturation indicates that it is not mediated through the ER. The 

mechanism of this effect by methoxychlor also does not appear to involve the progesterone receptor, as 

neither methoxychlor nor HPTE exhibited any competitive binding affinity for this receptor. 

In sea urchins, exposure of sperm to 3 ppm laboratory grade methoxychlor for 10 or 30 minutes caused a 

30 or 100% reduction in fertilization, respectively (Mwatibo and Green 1997). Similar exposure of eggs 

did not result in a decrease in fertilization, but 5.5% of the embryos resulting from these eggs showed 

abnormal, stunted or absent gut, malformed spicules (part of the skeleton), and abnormal shape. Embryos 

from sperm exposed to methoxychlor for 10 minutes showed a slight, but statistically insignificant, 

increase in the same anomalies. The mechanism of the effect was not examined. 

Taken together, these data indicate that methoxychlor has endocrine disruptive effects in fish, amphibian, 

and sea urchin fertility, growth, and development. Sometimes the effects of methoxychlor parallel those 

of estradiol and sometimes there are either subtle or drastic differences from the effects of estradiol. 

Thus, it is likely that not all of the effects of methoxychlor in aquatic wildlife are mediated through 

estrogen receptors. There may be many different mechanisms involved in the varied effects and the 

different species. 

3.7 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 6.6 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, whereas 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 direct information on the toxicity of methoxychlor in children or on effects in adults that were 

exposed as children. Animal data indicate that the primary target of methoxychlor is the reproductive 

system and that methoxychlor can both affect adult animals and influence the development of the 

reproductive system in males and females. Thus, there is concern about whether sufficient exposure to 

methoxychlor might potentially affect the developing reproductive system of fetuses, children, and 

adolescents. Although there is no direct evidence that children are more susceptible to health effects from 

methoxychlor exposure than adults, lower doses of methoxychlor were generally required to produce 

reproductive effects in developing animals than in adults. 

Numerous studies have shown that adult animals, exposed during development or as adults, exhibited 

effects attributed to the “estrogenic” activity of methoxychlor, indicating that metabolism of 

methoxychlor to its estrogenic metabolites occurs in developing and adult animals. These estrogenic 

effects included altered hormone levels, abnormal histology, and decreased fertility (Bal 1984; Chapin et 

al. 1997; Gray et al. 1989, 1999; Harris et al. 1974; Martinez and Swartz 1991). The exact mechanisms 

of disruption of normal reproductive function may differ between developmental and adult exposure. The 

reproductive organs and tissues of adult animals are already developed and capable of responding 

appropriately to normal hormone levels. Methoxychlor disrupts the normal hormone balance by 

interacting with estrogen, and possibly androgen receptors, which causes abnormal reproductive 

responses. Methoxychlor probably also directly affects tissues containing these receptors. Exposure to 

methoxychlor during critical periods of reproductive development may cause abnormal or under- or overdevelopment of certain reproductive organs or tissues, as well as an altered tissue distribution of these 

same receptors, so that the organs/tissues have no capability to respond to hormones in a normal fashion, 

even if hormone levels return to normal following cessation of exposure. 

There is ample information from animal studies to indicate that the developing organism is susceptible to 

the effects of methoxychlor and its metabolites. Wavy ribs have been observed in the offspring of female 

rats exposed to $7.8 mg/kg/day methoxychlor on gestation days 6–15 (Culik and Kaplan 1976). Khera et 

al. (1978) also noted wavy or extra ribs and decreased fetal weight, as well as an increased percentage of 

dead and resorbed fetuses, in offspring of female rats exposed to 200 mg/kg/day during gestation. Body 

weights of fetuses from female rabbits receiving 35.5 (but not 5.01) mg/kg/day methoxychlor on 

days 7–19 of gestation were decreased by 10% and the percentage of male offspring was decreased 

(Kincaid Enterprises 1986). 
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Methoxychlor also causes numerous functional changes in the adult and developing reproductive system 

of animals. Unlike its structural cousin, DDT, methoxychlor is metabolized rapidly to its mono- and bishydroxy derivatives. Therefore, the neurotoxicity associated with DDT is not seen following 

methoxychlor exposure (except at extremely high exposure levels). However, methoxychlor and its 

metabolites, especially bis-hydroxy methoxychlor, or HPTE, are estrogenic and can bind to estrogen 

receptors, enhancing or attenuating the effects of endogenous estrogens (Bulger et al. 1978d; Gaido et al. 

1999, 2000; Kuiper et al. 1998; Kupfer and Bulger 1979, 1987b; Mosselman et al. 1996). Estrogen 

receptors are found in many tissues in males and females, including mammary gland, uterus, vagina, 

ovary, testes, epididymis, prostate, thymus, bone, central nervous system, pituitary, hypothalamus, and 

cardiovascular system (Kuiper et al. 1998). When the delicate balance of endogenous estrogen levels is 

disrupted, transient or permanent functional and/or structural abnormalities may occur in a variety of 

organs or tissues, especially in the reproductive system. Exposure to methoxychlor during critical stages 

of development or in the adult animal has been shown to result in a number of reproductive effects. 

Effects associated with methoxychlor exposure include histopathological changes in the reproductive 

organs and accessory glands (Bal 1984; Chapin et al. 1997; Gray et al. 1988, 1989; Martinez and Swartz 

1991, 1992; Stoker et al. 1999; Swartz et al. 1994; Tegeris et al. 1966), impaired pubertal development 

(Chapin et al. 1997; Gray et al. 1989, 1999; Harris et al. 1974) and reproductive function (Bal 1984; 

Chapin et al. 1997; Gray et al. 1988, 1989; Harris et al. 1974; Haskell Laboratories 1966; (WendaRozewicka 1983), and altered hormone levels (Cummings and Gray 1989; Cummings and Laskey 1993; 

Goldman et al. 1986; Gray et al. 1988, 1989; Martinez and Swartz 1992; Stoker et al. 1999). There is a 

wide range of doses reported in the literature that cause effects in developing animals. The great majority 

of studies report effects from in utero, neonatal, and adult exposure beginning at around 5–150 mg/kg/day 

(Bal 1984; Chapin et al. 1997; Gray et al. 1989, 1999; Harris et al. 1974; Martinez and Swartz 1991). 

Reproductive effects in female mice and rats from in utero and postnatal exposures include precocious 

vaginal opening (Chapin et al. 1997; Gray et al. 1989; Harris et al. 1974), delayed onset of estrus (Gray et 

al. 1989), altered vaginal and uterine histology (Chapin et al. 1997; Gray et al. 1988, 1989), altered 

ovarian histology (Bal 1984; Chapin et al. 1997; Gray et al. 1989, 1999; Harris et al. 1974; Martinez and 

Swartz 1991), and decreased fertility (Chapin et al. 1997; Gray et al. 1989; Harris et al. 1974). In male 

animals, delayed preputial separation, decreased testes, epididymis, seminal vesicle, prostate weight, and 

decreased fertility have been observed following developmental exposure (Chapin et al. 1997; Gray et al. 

1999; Harris et al. 1974). A few studies have shown that in utero exposures as low as 0.02 mg/kg/day 

caused changes in sex accessory organs and changes in neurobehavioral and sociosexual behavioral 

parameters (Palanza et al. 1999; Parmigiani et al. 1999; vom Saal et al. 1999; Welshons et al. 1999). For 
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a more detailed discussion of the reproductive and developmental effects of methoxychlor, see 

Sections 3.2.2.5 Reproductive Effects and 3.2.2.6 Developmental Effects. 

There are no studies directly examining whether methoxychlor and metabolites cross the placenta in 

humans or animals. Subtle effects on the development of reproductive organs have been observed in 

male and female offspring of rodents exposed to oral doses of methoxychlor during gestation (Chapin et 

al. 1997; Gray et al. 1989; Harris et al. 1974); however, it should be noted that exposure was not 

exclusively during gestation in any of these studies. It is unclear if these effects result directly from 

methoxychlor and metabolites passing through the placenta or indirectly from a disturbance of maternal 

physiological processes. Levels of methoxychlor and its metabolites were determined in breast milk and 

plasma of female rats orally administered 5, 50, or 150 mg/kg/day methoxychlor during gestation and 

early lactation (Chapin et al. 1997). Breast milk levels of methoxychlor and mono- and di-hydroxy 

methoxychlor were very high compared to plasma levels (25–215% higher). Levels in pup serum were 

much lower (1.3–5.9% of dam plasma levels), but were still detectable at most exposure levels. Pup 

serum levels may actually reach much higher levels, since more than 24 hours elapsed between 

administration of the last dose of methoxychlor to the dam and collection of pup blood for analysis. 

Therefore, children may be exposed to significant levels of methoxychlor and metabolites via breast milk 

if their mother is orally exposed to sufficient amounts of methoxychlor. One study analyzed human 

breast milk for the presence of methoxychlor and found none (Hooper et al. 1997); however, two other 

studies did detect methoxychlor in human breast milk (Campoy et al. 2001a, 2001b). Unlike its structural 

cousin, DDT, methoxychlor is metabolized and eliminated from the body rapidly. Some methoxychlor is 

distributed to tissues throughout the body, but does not appear to persist, even in fat. Therefore, it is 

unlikely that methoxychlor from pre-conception exposure of the mother would be available for 

mobilization during pregnancy and lactation. However, puzzling, unexplained data suggest that it is 

possible that previous exposure of the mother to methoxychlor results in damage to her reproductive 

system in some way that causes future offspring to experience reproductive anomalies, although the 

evidence of such occurrences in animals is very limited. Swartz and Corkern (1992) noted precocious 

vaginal opening in female offspring of a second unexposed pregnancy (F1b) in mice. It is unclear why 

the F1a exposed litter did not experience precocious vaginal opening and by what mechanism this effect 

was produced in the unexposed F1b offspring. 

The primary pathway by which methoxychlor is metabolized in the liver (the main site of methoxychlor 

metabolism) is sequential demethylation reactions to yield the estrogenic mono- and bis-hydroxy 

methoxychlor. In vitro studies with human liver microsomes and human recombinant cytochrome P450 
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(CYP) isozymes indicate that multiple CYP isozymes are involved in the demethylation of methoxychlor 

in humans (Stresser and Kupfer 1998). This study indicates that CYP2C19 and CYP1A2 may be the 

major CYP demethylases for methoxychlor, but that other forms, including CYP2A6, CYP2C9, and 

CYP2B6, are likely to be major contributors, especially in individuals with low levels of CYP2C19 or 

CYP1A2. Other CYP enzymes that may play a role include CYP2D6, CYP2E1, and CYP3A4. Many of 

these phase I enzymes are likely to have overlapping roles. Although nothing is known about phase II 

metabolism in humans, Davison et al. (1982, 1983) found glucuronic acid conjugates of methoxychlor 

metabolites in goats; these conjugates are a product of glucuronosyltransferase, and it is possible that this 

enzyme may be involved in human methoxychlor metabolism as well. Although it is unknown whether 

an overall age-related difference in methoxychlor metabolism would be observed in vivo in humans, there 

is some information that the following enzymes may be developmentally regulated: CYP2C19 (Leeder 

and Kearns 1997), CYP1A2 (Leeder and Kearns 1997; Rich et al. 1990), CYP2C9 (Ratenasavanh et al. 

1991;Treluyer et al. 1997), CYP2D6 (Leeder and Kearns 1997; Sonnier and Cresteil 1998), CYP2E1 

(Hong et al. 1987; Vieira et al. 1996), CYP3A4 (Lacroix et al. 1997; Leeder and Kearns 1997; Mote et al. 

1990; Yang et al. 1994), and glucuronosyltransferase (Leeder and Kearns 1997). While CYP2C19 and 

CYP1A2 are not present in appreciable levels in human fetal liver, their activities increase to adult levels 

by 4 months to >1 year of age (Leeder and Kearns 1997; Ratenasavanh et al. 1991; Sonnier and Cresteil 

1998). In contrast, other researchers have found that CYPs 2A6, 2C9, 2D6, 2E1, and 3A are expressed at 

the same levels in perinatal (37 weeks of gestation) to geriatric human livers (72 years) (Ratenasavanh et 

al. 1991; Tateishi et al. 1997). 

There are no PBPK models for children, fetuses/pregnant women, infants/lactating women, or humans at 

any other stage of development. 

3.8 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 
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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 methoxychlor are discussed in Section 3.8.1. 

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 

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 methoxychlor are discussed in Section 3.8.2. 

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 3.10 “Populations That Are Unusually Susceptible”. 

3.8.1 Biomarkers Used to Identify or Quantify Exposure to Methoxychlor 

Methods exist for determining the levels of methoxychlor and its metabolites in biological media (tissues, 

fluids, and excreta) (LeBel and Williams 1986; Mes 1981; Steinberg et al. 1989), although data from 

human samples are sparse and detectable levels of methoxychlor have generally not been reported. A 

more detailed discussion of the methods used to detect methoxychlor in biological and environmental 

samples can be found in Chapter 7 Analytical Methods. In a human population exposed to heavy 

agricultural spraying of pesticides (area sprayed 16–30 times per year), serum levels of methoxychlor 

were generally below the level of detection (detection limit of 0.24–4.07 mg/L) (Steinberg et al. 1989). 
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Similar results were also obtained with adipose tissue obtained from human autopsies (LeBel and 

Williams 1986). 

Data from animal studies indicate that blood and tissue (fat) levels of methoxychlor do not remain 

elevated for very long after exposure, owing to its rapid metabolism to more polar, readily excretable 

metabolites (Davison et al. 1982; Hodge et al. 1952; Kapoor et al. 1970; Kunze et al. 1950; Reynolds et 

al. 1976). Because methoxychlor is rapidly metabolized, detection of these metabolites (e.g., the monoand bis-hydroxy derivatives) may serve as a better biomarker of exposure than the parent compound, 

especially in excreta such as feces (Kapoor et al. 1970). Methoxychlor and its metabolites have been 

measured in the milk of lactating rats (exposed to 5, 50, or 150 mg/kg/day from gestation day 14 through 

lactation day 7) and were found to concentrate in the milk with increasing methoxychlor exposure level 

(Chapin et al. 1997). The data also suggest that methoxychlor and metabolites concentrate in milk, 

relative to maternal plasma levels, after intermediate-duration dose levels $50 mg/kg/day. Plasma levels 

of methoxychlor and its metabolites (mono-hydroxy methoxychlor and di-hydroxy methoxychlor) in 

suckling rat pups also increased with increasing dose of methoxychlor (Chapin et al. 1997). Metabolites 

in dam milk and pup plasma were below the level of detection (50 and 5 ng/mL, respectively) at the 

5 mg/kg/day dose level. Pup plasma was not drawn for analysis until 27–30 hours after dams received the 

last dose; therefore, measured methoxychlor and metabolite levels may not have been indicative of peak 

body burden. Because of the relatively rapid clearance of these metabolites, measurements would 

probably only be useful in detecting recent exposures (within the past 24 hours). In fact, in one study that 

tested for methoxychlor in human breast milk, none was detected among regional populations in 

Kazakstan (Hooper et al. 1997). However, two other studies have detected methoxychlor in human milk 

samples (Campoy et al. 2001a, 2001b). 

Only one study was found in which concentrations of methoxychlor and methoxychlor metabolites were 

measured in biological fluids at doses at which health effects were being observed in animals (Chapin et 

al. 1997). Precocious vaginal opening, decreased absolute ovarian weight, decreased absolute and 

relative uterine weight, and decreased serum FSH levels during estrus were observed in female offspring 

of rat dams exposed to $5 mg/kg/day from gestation day 14 to postpartum day 7 (Chapin et al. 1997). 

Exposure of dams was discontinued at postpartum day 7 and pups were dosed directly through 

postpartum day 42. The 5 mg/kg/day dose used in the Chapin et al. (1997) study is one of the lowest 

doses at which health effects have been observed; only a few studies (Palanza et al. 1999; Parmigiani et 

al. 1999; vom Saal et al. 1995; Welshons et al. 1999) have observed health effects at lower doses. 
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3.8.2 Biomarkers Used to Characterize Effects Caused by Methoxychlor 

In animals, the primary target of methoxychlor-induced toxicity is the reproductive system and endocrinerelated end points. Reproductive effects include accelerated development of the female reproductive 

system and delayed development of the male reproductive system in young animals and gonadal atrophy 

or hypertrophy in adult animals (Bal 1984; Chapin et al. 1997; Gray et al. 1988, 1989, 1999; Hodge et al. 

1950; Martinez and Swartz 1991, 1992; Tullner 1961; Tullner and Edgcomb 1962; Welshons et al. 1999). 

These effects may cause a decrease in fertility in either sex. In some cases, the onset of these effects may 

be significantly delayed from the time of exposure. 

Although none of the above reproductive effects have been observed in human studies or case reports, it 

is possible that clinical examination of the reproductive organs of exposed humans might reveal some of 

the changes observed in animals. In exposed human females, a Pap smear may reveal increased 

cornification of the vaginal epithelium, which is seen in response to methoxychlor exposure in female rats 

(Gray et al. 1989). Changes in the menstrual cycle might also occur (Gray et al. 1988, 1989; Martinez 

and Swartz 1991; Tegeris et al. 1966), although this was not observed in a small (n=four females) 

experimental study in humans (Wills 1969). In exposed human males, measurement of sperm count 

might reveal decreases which have been reported in exposed male rats (Bal 1984; Gray et al. 1989), 

although tissue biopsies revealed no adverse effects on the testes of male subjects (n=4) administered 

doses of up to 2 mg/kg/day for 6 weeks (Wills 1969). The serum levels of certain hormones, including 

progesterone, prolactin, and testosterone were decreased in methoxychlor-fed animals, therefore, 

monitoring hormone level changes in the serum of potentially exposed humans might provide a 

convenient biomarker for exposure to methoxychlor (Bal 1984; Goldman et al. 1986; Gray et al. 1989). 

However, none of these potential biomarkers have been validated or are specific for exposures to 

methoxychlor since other estrogenic compounds (other DDT analogs, chlordecone, polychlorinated 

biphenyls (PCBs), 3,9-hydroxybenz[a]anthracene, and cyclosiloxanes) may also produce similar effects 

(Bulger and Kupfer 1985). 

Additional information concerning biomarkers for effects on the reproductive system can be found in the 

National Research Council Report on Biologic Markers in Reproductive Toxicology (NAS/NRC 1989), 

and on the neurological system in the Office of Technology Assessment Report on Identifying and 

Controlling Poisons of the Nervous System (OTA 1990). 
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The neurological system may also be affected by methoxychlor. Neurological effects such as decreased 

locomotor activity, tremors, apprehension, nervousness, and convulsions may be observed following large 

acute doses of methoxychlor. DDT, a poorly metabolized analogue of methoxychlor, has been shown to 

cause similar effects (Agency for Toxic Substances and Disease Registry 1994). In people with 

compromised liver function, neurological signs may occur at lower methoxychlor exposure levels. 

3.9 INTERACTIONS WITH OTHER CHEMICALS 

The joint toxic actions of binary mixtures of methoxychlor and other pesticides (including 

organophosphates and other chlorinated hydrocarbons) on acute lethality were examined in mice 

(Keplinger and Deichmann 1967). After determining oral LD50 values for the individual compounds, 

binary mixtures (with components at equitoxic doses based on LD50 values) were administered to the mice 

at the same dose ranges as the individual compounds. Based on the assumption of joint additive action, 

an expected LD50 value was calculated for each mixture and compared with the observed LD50. The ratio 

of expected:observed LD50 values for the methoxychlor/DDT mixture (0.66) indicated a less than additive 

action (i.e., mutual protection). Ratios for the methoxychlor/aldrin (0.81), methoxychlor/diazinon (0.82), 

methoxychlor/malathion (0.84), methoxychlor/toxaphene (0.92), and methoxychlor/aramite (1.25) 

mixtures were close to one, indicating joint additive action. Ratios for the methoxychlor/parathion (1.51), 

methoxychlor/delnav (1.96), methoxychlor/dieldrin (2.06), and methoxychlor/chlordane (2.26) mixtures 

were suggestive of greater than additive joint action (i.e., potentiation or synergism). In rats fed diets 

containing a mixture of methoxychlor, Aramite, DDT, and thiourea for 2 years, no treatment-related 

synergisms or antagonisms were observed on mortality, food consumption, weight gain, or tumor 

incidence (Radomski et al. 1965). Similar results were reported by Deichmann et al. (1967) in rats fed 

diets containing the same chemical mixture at higher concentrations. 

When methoxychlor was administered orally to rats previously treated with carbon tetrachloride, DDTlike neurological symptoms were observed (Lehman 1952). In addition, methoxychlor was found to 

accumulate in the fat and liver in amounts approximately 15–19 times the levels observed in control 

animals. Carbon tetrachloride is known to inactivate certain hepatic enzymes (CYPs or cytochrome 

P450s) which metabolize xenobiotics, thereby increasing their retention. These data suggest that carbon 

tetrachloride and other chemicals which inhibit the metabolism of methoxychlor may increase the risk of 

neurotoxicity. 
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In mice receiving 25 mg/kg/day methoxychlor along with 12 mg/kg/day bromfenvinphos for 6 weeks, 

inflammatory infiltrations of the liver were larger and denser than observed in animals receiving 

bromfenvinphos alone (Zaleska-Freljan et al. 1983). Small changes were observed in the kidneys at 

similar frequencies and severities in both treatment groups. Although it was not investigated in this 

study, methoxychlor usually does not produce effects of the liver by itself at such low doses. Thus, the 

results of this study suggest that there may be an interaction between methoxychlor and bromfenvinphos 

in producing hepatic effects. 

3.10 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE 

A susceptible population will exhibit a different or enhanced response to methoxychlor than will most 

persons exposed to the same level of methoxychlor 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 methoxychlor, or compromised function 

of organs affected by methoxychlor. Populations who are at greater risk due to their unusually high 

exposure to methoxychlor are discussed in Section 6.7 Populations With Potentially High Exposures. 

As noted earlier, neurotoxic effects were reported following exposure of rats with liver damage to 

methoxychlor (Lehman 1952). This suggests that individuals who have hepatic damage or who otherwise 

have their O-demethylation metabolic pathway compromised may be more susceptible to the DDT-like 

neurotoxic effects of methoxychlor, but this has not been studied in humans. 

There is no information on the effects of methoxychlor in human fetuses, children, or adolescents. By 

extrapolation from animal studies, developing fetuses and young children may be the most susceptible 

human population to the reproductive effects of methoxychlor because the estrogenic and possibly 

antiandrogenic activity of methoxychlor metabolites may interfere with normal development of the 

reproductive tract. The offspring of nursing mothers exposed to methoxychlor may be susceptible since 

methoxychlor has been detected in human milk samples (Campoy et al. 2001a, 2001b) and animal studies 

indicate that methoxychlor and/or its biologically active metabolites can be released in milk (Chapin et al. 

1997; Davison et al. 1982). Acute exposures to methoxychlor during critical periods of development may 

adversely affect the reproductive system. Such effects may not appear until later in life (onset of puberty 

or adulthood/childbearing). 
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A more detailed discussion of children’s susceptibility can be found in Section 3.7 Children’s 

Susceptibility. 

3.11 METHODS FOR REDUCING TOXIC EFFECTS 

This section will describe clinical practice and research concerning methods for reducing toxic effects of 

exposure to methoxychlor. 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 methoxychlor. 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 methoxychlor: 

Ellenhorn MJ, Barceloux DG. 1988. Medical toxicology: Diagnosis and treatment of human poisoning. 

New York, NY: Elsevier, 1078–1080. 

Haddad LM, Winchester JF. 1990. Clinical management of poisoning and drug overdose. Second 

edition. Philadelphia, PA: W.B. Sanders Company, 1084–1085. 

3.11.1 Reducing Peak Absorption Following Exposure 

Data regarding the reduction of methoxychlor absorption in humans after inhalation exposure were not 

located. Oral absorption of methoxychlor can be reduced with gastric lavage, activated charcoal, sodium 

sulfate, and cathartics (Ellenhorn and Barceloux 1988; Haddad and Winchester 1990). Since many 

commercial formulations of organochlorine insecticides contain organic solvents, emesis is not usually 

recommended due to the hazard of solvent aspiration (Ellenhorn and Barceloux 1988). In addition, oils 

should usually not be used as cathartics since they may enhance the absorption of methoxychlor (Haddad 

and Winchester 1990). 

Dermal absorption of methoxychlor can be reduced by removing contaminated clothing and thoroughly 

washing the exposed skin with a mild soap (Ellenhorn and Barceloux 1988; Haddad and Winchester 

1990). Oils should not be used as a cleansing agent since they may facilitate dermal absorption (Haddad 

and Winchester 1990). 
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3.11.2 Reducing Body Burden 

Since animal studies indicate that methoxychlor is rapidly metabolized and cleared from the body, 

methods for reducing body burden are not expected to be especially effective in reducing human 

exposures. Activated charcoal is sometimes administered in serial doses to minimize enterohepatic 

recirculation of persistent chemicals (Ellenhorn and Barceloux 1988). Although a study in rats did 

establish that methoxychlor undergoes enterohepatic recirculation, the extent to which this occurred was 

minimal (5–10% of the dose) (Weikel 1957). Thus, it is not likely that the administration of activated 

charcoal will facilitate the excretion of methoxychlor to any significant extent, but this has not been 

studied. 

3.11.3 Interfering with the Mechanism of Action for Toxic Effects 

Methods for managing the seizures and convulsions which may occur following large exposures to 

methoxychlor or other organochlorine pesticides include the intravenous administration of Diazepam, 

Valium, or phenobarbital. Patients should be monitored for the possibility of cardiopulmonary arrest 

(Ellenhorn and Barceloux 1988; Haddad and Winchester 1990). 

Methods for interfering with the reproductive effects of methoxychlor were not located. These effects are 

presumed to be due to the estrogenic action of the phenolic metabolites of methoxychlor. Therefore, it 

would seem that by blocking the metabolism of methoxychlor to these derivatives, the reproductive 

effects would also be blocked. However, such an approach might actually increase the risk of other 

effects that are due to the parent compound. For example, limited data suggest that the neurotoxicity of 

methoxychlor is most likely due to the parent molecule (Lehman 1952) which may prevent the 

deactivation of the sodium gate after neuron activation and membrane depolarization (Brown et al. 1981; 

Coats 1990; Wu et al. 1975). More information is needed on the mechanism of action of methoxychlor in 

producing effects on the reproductive system (direct or indirect action) before methods for interfering 

with the mechanism can be determined. 
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3.12 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 methoxychlor 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 methoxychlor. 

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. 

3.12.1 Existing Information on Health Effects of Methoxychlor 

The existing data on health effects of inhalation, oral, and dermal exposure of humans and animals to 

methoxychlor are summarized in Figure 3-4. The purpose of this figure is to illustrate the existing 

information concerning the health effects of methoxychlor. 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 
METHOXYCHLOR 118 

3. HEALTH EFFECTS 

Figure 3-4. Existing Information on Health Effects of Methoxychlor 

Inhalation 

Oral 

Dermal 

Systemic

• • •

 • •• •

Human

Inhalation 

Oral 

Dermal 

Systemic 

••

 •

 •

•• • •• • ••

•• •• •

Animal

• Existing Studies 
 

METHOXYCHLOR 119 

3. HEALTH EFFECTS 

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 (Agency for Toxic Substances and 

Disease Registry 1989b), 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 seen in Figure 3-4, information in humans is limited to a single clinical study that investigated 

systemic and reproductive effects following intermediate-duration oral exposure to methoxychlor. A 

single epidemiological study was located regarding cancer and occupational exposure to methoxychlor. 

Oral studies in animals provide information on death, systemic effects for acute, intermediate, and chronic 

exposures, neurological effects, developmental effects, reproductive effects, and cancer. Limited data are 

available from animal studies regarding systemic effects for intermediate and chronic exposures, 

neurological effects, and cancer for dermal exposure to methoxychlor. Limited data are also available for 

intermediate-duration exposures and neurological effects for inhalation exposure to methoxychlor. 

3.12.2 Identification of Data Needs 

Acute-Duration Exposure. No data were located regarding the acute toxicity of methoxychlor in 

humans after inhalation, oral, or dermal exposure. Likewise, no data were located regarding acute 

toxicity in animals following inhalation or dermal exposure. 

Acute oral studies in mice and rats identified LD50 values of 2,900–7,000 mg/kg/day (Coulston and 

Serrone 1969; Hodge et al. 1950; Smith et al. 1946). The cause of death was not determined in these 

studies. Acute oral exposure of animals to sublethal doses of methoxychlor indicate that reproductive 

development is the most sensitive target of methoxychlor (Cummings and Gray 1987, 1989; Cummings 

and Perreault 1990; Gray et al. 1989; Khera et al. 1978; Kincaid Enterprises 1986; Martinez and Swartz 

1991; Parmigiani et al. 1998; Tullner 1961; vom Saal et al. 1995; Welshons et al. 1999). Precocious 

vaginal opening was noted in female rats administered oral doses of 25 mg/kg/day (Gray et al. 1989). 

The data from animal studies are insufficient to derive an acute oral MRL. A number of candidate MRL 

studies were considered. Although Gray et al. (1989) observed precocious vaginal opening (early 

puberty) in rats at 25 mg/kg/day, this study did not test doses as low as the current intermediate-duration 

MRL study (Chapin et al. 1997). The same effect, precocious vaginal opening, was demonstrated at 

5 mg/kg/day administered from gestation day 14 to postnatal day 42 (Chapin et al. 1997), so there is some 

question as to whether the premature puberty would have occurred at a lower acute dose than the 
 

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25 mg/kg/day dose observed in the Gray et al. (1989) study. There were several hypothesis-generating 

studies at extremely low doses that were not definitive enough to use for MRL derivation; they are 

discussed further in both the Developmental Toxicity data needs subsection and Appendix A. These 

acute oral studies identified LOAEL values of 0.02–1.8 mg/kg/day for reproductive/developmental effects 

in mice (Parmigiani et al. 1998; vom Saal et al. 1995; Welshons et al. 1999). Further oral studies that 

better define the threshold dose for reproductive effects in rats, mice, and in other species would be 

useful. 

Studies which identify the inhalation or dermal exposure levels which produce reproductive and 

developmental effects in animals may be valuable for predicting effects in persons acutely exposed to 

methoxychlor. Studies examining the effects of dermal and inhalation exposure to methoxychlor in other 

organ systems would also prove useful. Better pharmacokinetic information on the extent of inhalation 

and dermal absorption would help determine the importance of more acute duration toxicological data 

from these routes. Data from farmers and pesticide workers who have been exposed acutely to 

methoxychlor via these routes would also be useful to determine whether the acute effects observed in 

animals also occur in humans. 

Studies are also needed that examine the effects of exposure to pesticide mixtures that contain 

methoxychlor. 

Intermediate-Duration Exposure. Data regarding the toxicity of methoxychlor in humans exposed 

via the inhalation and dermal routes for intermediate durations are lacking. A very small study in humans 

(four males and four females) reported that oral exposure to 2 mg/kg/day for 6 weeks caused no adverse 

effects on the liver, small intestines, bone marrow, or testes (Wills 1969). A number of oral studies in 

animals indicate that the reproductive system is a target of methoxychlor toxicity. In females, reported 

effects of intermediate-duration oral exposure to methoxychlor include: precocious vaginal opening in 

rats exposed to 25–100 mg/kg/day (Gray et al. 1989; Harris et al. 1974); increased vaginal cornification 

and decreased number of vaginal smears with leukocytes in rats exposed to 50–400 mg/kg/day (Chapin et 

al. 1997; Gray et al. 1988, 1989); effects on uterine weight in rats and pigs exposed to 

150–1,000 mg/kg/day (Chapin et al. 1997; Harris et al. 1974; Tegeris et al. 1966); atrophy and 

degeneration of the ovaries in mice and rats exposed to 25–400 mg/kg/day (Bal 1984; Chapin et al. 1997; 

Gray et al. 1988, 1989; Harris et al. 1974; Martinez and Swartz 1991); persistent vaginal cornification in 

rats and mice, and a lack of estrus in pigs exposed to 25–1,000 mg/kg/day (Gray et al. 1988; Harris et al. 

1974; Martinez and Swartz 1991; Tegeris et al. 1966); decreased mating frequency in rats exposed to 
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60–150 mg/kg/day (Harris et al. 1974); decreased fertility in rats exposed to 35.5–200 mg/kg/day (Bal 

1984; Chapin et al. 1997; Culik and Kaplan 1976; Gray et al. 1989; Harris et al. 1974; Kincaid 

Enterprises 1986); and altered hormone levels in the serum or pituitary of rats exposed to 

100–400 mg/kg/day (Cummings and Gray 1989; Gray et al. 1988). A single multigeneration study 

reported decreased fertility in female rats of the second and third generations exposed to 92 but not to 

18 mg/kg/day methoxychlor (Haskell Laboratories 1966). In males, the effects of intermediate-duration 

oral exposure to methoxychlor include: delayed preputial separation (an indicator of puberty) in rats 

exposed to 100–400 mg/kg/day (Gray et al. 1989, 1999); decreased testes weight, ventral prostate weight, 

and caudal epididymal sperm count in rats exposed to 50–1,400 mg/kg/day (Bal 1984; Gray et al. 1989, 

1999; Hodge et al. 1950; Shain et al. 1977; Tullner and Edgcomb 1962); decreased mating frequency in 

rats exposed to 60 mg/kg/day (Harris et al. 1974) or 200–400 mg/kg/day (Gray et al. 1999); decreased 

fertility in rats and mice exposed to 60–100 mg/kg/day (Gray et al. 1989; Harris et al. 1974; WendaRozewicka 1983); and altered pituitary and serum hormone levels in rats exposed to 25–100 mg/kg/day 

(Goldman et al. 1986; Gray et al. 1989). These types of effects are typical of exposures to estrogen, and 

are consistent with the observation that phenolic metabolites and contaminants of methoxychlor exhibit 

estrogenic activity in vitro and in vivo (Bulger et al. 1985). 

These data are sufficient to derive an MRL for intermediate-duration oral exposures. A LOAEL of 

5 mg/kg/day was identified for accelerated onset of puberty (i.e., precocious vaginal opening) in 

immature female rats exposed to methoxychlor in utero, during lactation, and after weaning (Chapin et al. 

1997). However, a NOAEL was not identified in this study. Further oral studies would be helpful to: 

define the threshold dose for reproductive effects and narrow down the critical developmental window 

during which this effect can be produced. 

Data regarding inhalation and dermal exposures to methoxychlor are limited. They include permanent 

neurological effects (Harell et al. 1978) and death (Ziem 1982) following exposure to pesticide mixtures 

containing methoxychlor and other pesticides; partial paralysis and disseminated nodules in the brains of 

rabbits exposed to methoxychlor dermally or via inhalation (Haag et al. 1950; the authors attributed these 

effects to a disease endemic to rabbits that was potentiated by exposure to methoxychlor); fatty 

degenerative changes of the liver in rabbits (Haag et al. 1950); death in rabbits (Haag et al. 1950); and 

uterine stimulation in mice following exposure to a pesticide mixture containing methoxychlor (Tullner 

1961). No deaths or histopathological changes in the skin were noted in mice receiving intermittent 

dermal exposure to methoxychlor for 2 years (Hodge et al. 1966). Effects on the reproductive system 

have not been adequately investigated following inhalation or dermal exposures to methoxychlor. Studies 
 

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3. HEALTH EFFECTS 

that identify reliable NOAELs and LOAELs for reproductive effects in animals exposed to methoxychlor 

via inhalation for intermediate-duration exposures would be valuable for predicting effect levels in 

humans occupationally exposed to methoxychlor. Information from farmers, pesticide workers, or 

persons who live near pesticide facilities or hazardous waste sites who are exposed to methoxychlor for 

intermediate-durations may serve to indicate whether the effects observed in animals also occur in 

humans at similar response levels. Better pharmacokinetic information on the extent of inhalation and 

dermal absorption would help determine the importance of more intermediate duration toxicological data 

from these routes. 

The results of several studies indicated that methoxychlor exposure might result in alterations of the 

vitamin A content of the liver (Davison and Cox 1976; Harris et al. 1974), while others found no effect 

(Cecil et al. 1974; Phillips and Hatina 1972). An in vitro assay showed that methoxychlor itself does not 

interact with human transthyretin (a carrier protein for vitamin A and thyroid hormones), but did not 

include experiments with a microsomal activation system to determine the interaction of methoxychlor 

metabolites with transthyretin (Van den Berg et al. 1991). 

Chronic-Duration Exposure and Cancer. There are no data regarding the chronic toxicity of 

methoxychlor in humans for inhalation, oral, or dermal exposures. Chronic oral exposure studies have 

been performed in animals (Deichmann et al. 1967; NCI 1978). No histopathological changes were 

detected in the lungs, gastrointestinal tract, liver, kidneys, brain, or pituitary of Osborne-Mendel rats 

chronically exposed to 77 mg/kg/day recrystallized methoxychlor (Deichmann et al. 1967). Similarly, 

histopathological changes were not observed in the lungs, heart, gastrointestinal tract, bone, liver, 

kidneys, skin, lymph nodes, thymus, brain, testes, ovaries, and uterus of B6C3F1 mice chronically 

exposed to 599 mg/kg/day methoxychlor or Osborne-Mendel rats chronically exposed to 107 mg/kg/day 

(NCI 1978). No histopathological changes were noted in the skin of C3H/AnF mice dermally exposed to 

10 mg/day recrystallized methoxychlor, 1 time/week, for 2 years (Hodge et al. 1966). A single 

multigeneration study reported decreased fertility in female rats exposed to 92 mg/kg/day, but not to 

18 mg/kg/day, and not in males exposed to up to 79 mg/kg/day (Haskell Laboratories 1966). These 

studies do not support the derivation of a chronic oral MRL, since most of the studies did not evaluate 

reproductive function, which is likely to be the most sensitive target. Derivation of a chronic oral MRL 

based on the NOAEL of 18 mg/kg/day (Haskell Laboratories 1966), would result in a value greater than 

MRLs derived for acute and intermediate durations, and therefore is not possible. Additional 

multigeneration studies in animals which investigate the chronic toxicity of methoxychlor with particular 

reference to reproductive and developmental effects in both sexes from inhalation, oral, or dermal 
METHOXYCHLOR 123 

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exposures to methoxychlor would provide valuable information. This information would be useful in 

establishing reliable NOAELs and LOAELs for the purposes of estimating risk to humans who live near 

hazardous waste sites or are occupationally exposed to methoxychlor. Epidemiological studies on 

agricultural or pesticide workers exposed to methoxychlor would also serve to indicate whether the 

effects observed in animals also occur in humans. Since animal studies indicate that the offspring of 

females that are exposed to methoxychlor either prior to or during pregnancy may also be affected, 

follow-up studies on the offspring of these exposed human populations may also prove to be valuable. 

One epidemiological study reported that the risk of leukemia was slightly increased in farmers exposed to 

methoxychlor (Brown et al. 1990). Further epidemiological studies on agricultural and pesticide workers 

would be valuable to determine if a relationship between cancer and methoxychlor exposure exist. No 

increased tumor incidence was detected in the lungs, gastrointestinal tract, liver, kidneys, brain, or 

pituitary of Osborne-Mendel rats chronically exposed to 77 mg/kg/day recrystallized methoxychlor 

(Deichmann et al. 1967). Similarly, increased tumor incidences were not observed in the lungs, heart, 

gastrointestinal tract, bone, liver, kidneys, skin, lymph nodes, thymus, brain, testes, ovaries, and uterus of 

B6C3F1 mice chronically exposed to 599 mg/kg/day methoxychlor or Osborne-Mendel rats chronically 

exposed to 107 mg/kg/day (NCI 1978). No increased tumor incidence was noted in the skin of C3H/AnF 

mice dermally exposed to 10 mg/day recrystallized methoxychlor, 1 time/week, for 2 years (Hodge et al. 

1966). Since prolonged exposure to elevated levels of estrogenic compounds have been linked to cancers 

in the reproductive tract of women and their female offspring, multigenerational oral animal studies 

focused on evaluation of tumor incidences in estrogen-sensitive tissues for parental and subsequent 

generations may provide information needed to assess the possible carcinogenicity of methoxychlor in 

humans. 

Data regarding chronic-duration inhalation and dermal exposures are very limited. They include a 

possible link to leukemia seen in a single very small epidemiological study of farmers exposed via 

inhalation (Brown et al. 1990); and a lack of gross or histopathological changes in the skin of mice and 

lack of tumor formation in mice (gross examination) exposed dermally for 2 years (Hodge et al. 1966). 

Effects on the reproductive system have not been adequately investigated following chronic inhalation or 

dermal exposures to methoxychlor. Studies that identify reliable NOAELs and LOAELs for reproductive 

effects in animals exposed to methoxychlor via inhalation for chronic-duration exposures would be 

valuable for predicting effect levels in humans occupationally exposed to methoxychlor. Additional 

information from farmers, pesticide workers, or persons who live near pesticide facilities or hazardous 

waste sites who are exposed to methoxychlor for chronic durations may serve to indicate whether the 
 

 

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effects observed in animals also occur in humans at similar response levels. Better pharmacokinetic 

information on the extent of inhalation and dermal exposure would help determine the importance of 

more chronic-duration toxicological data from these routes. 

Genotoxicity. The mutagenicity of methoxychlor has been well studied in vitro and there are 

sufficient data to indicate that methoxychlor is not mutagenic in prokaryotic systems (Grant et al. 1976; 

Mortelmans et al. 1986; Probst et al. 1981; Waters et al. 1980). However, in vitro studies in mammalian 

cells have yielded conflicting results. DNA synthesis (thymidine incorporation) was inhibited at high 

methoxychlor concentrations and stimulated at low methoxychlor concentrations in cultured bovine 

uterine epithelial and stromal cells (Tiemann et al. 1996). Several studies in mouse lymphoma cells with 

metabolic activation indicate that methoxychlor can induce mutations at the TK locus (Caspary et al. 

1988; Mitchell et al. 1988; Myhr and Caspary 1988). However, mutations of this nature were not noted 

in human lymphoma cells (Caspary et al. 1988). Further in vitro studies using human lymphoma cells 

would confirm whether an important species difference exists between human and mouse lymphoma cells 

for this end point. Single-stranded DNA breaks were not induced in human or rat testicular cells by 

methoxychlor (Bjørge et al. 1996b). Several in vivo studies have shown that intraperitoneal exposure of 

mice does not increase the frequency of chromosomal aberrations (Degraeve and Chollet 1984) or singlestranded DNA breaks (Umegaki et al. 1993), and that methoxychlor does not increase sex-linked 

recessive lethal mutations in Drosophila melanogaster (Benes and Sram 1969; Valencia 1981; Waters 

et al. 1980). Additional in vivo studies that utilize human lymphocytes from workers or farmers exposed 

to methoxychlor would be useful in evaluating whether chromosomal aberrations develop in humans 

exposed to methoxychlor. 

Reproductive Toxicity. There is no information regarding the reproductive effects of methoxychlor 

in humans after inhalation or dermal exposures, or in animals after inhalation or dermal exposures. In a 

single very small human study (16 males and 16 females), tissue biopsies revealed no adverse effects on 

the testes of male subjects (3 males at 2 mg/kg/day) or on the menstrual cycles of female subjects (4 per 

exposure level) administered doses of up to 2 mg/kg/day for 6 weeks (Wills 1969). 

However, oral studies in animals indicate that the reproductive system, particularly the developing 

reproductive system, is the primary target of methoxychlor toxicity. In females, oral exposure to 

methoxychlor caused changes in pituitary and serum hormone levels in rats (Cummings and Gray 1989); 

early onset of puberty in rats (Anderson et al. 1994; Chapin et al. 1997; Gray et al. 1989; Harris et al. 

1974); atrophy of the ovaries in rats and mice (Anderson et al. 1994; Bal 1984; Gray et al. 1988, 1989; 
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Harris et al. 1974; Martinez and Swartz 1991), abnormal estrus in rats, mice, and pigs (Gray et al. 1988; 

Martinez and Swartz 1991; Tegeris et al. 1966); an enlarged uterus in rats and pigs (Cummings 1993; 

Harris et al. 1974; Tegeris et al. 1966); decreased fertility in rats (Bal 1984; Chapin et al. 1997; Culik and 

Kaplan 1976; Gray et al. 1989; Harris et al. 1974; Haskell Laboratories 1966; Kincaid Enterprises 1986); 

and decreased number of corpora lutea, decreased number of live pups, and increased resorptions in mice 

(Swartz and Eroschenko 1998). In addition, dermal exposure to methoxychlor had a uterotrophic effect in 

immature female mice (Tullner 1961) and injection with methoxychlor accelerated the onset of puberty in 

newborn female mice (Martinez and Swartz 1991). 

In males, methoxychlor exposure produced changes in pituitary and serum hormone levels in rats (Fail et 

al. 1994; Goldman et al. 1986; Gray et al. 1989); delayed puberty in rats (Anderson et al. 1994; Chapin et 

al. 1997; Gray et al. 1999); testicular atrophy in rats (Bal 1984; Gray et al. 1999; Hodge et al. 1950; Shain 

et al. 1977; Tullner and Edgcomb 1962) and mice (Wenda-Rozewicka 1983); decreased prostate weight 

in rats (Chapin et al. 1997; Shain et al. 1977; Tullner and Edgcomb 1962); decreased fertility in rats and 

mice (Gray et al. 1989, 1999; Harris et al. 1974; Wenda-Rozewicka 1983); and altered sexual behavior in 

mice and rats (Gray et al. 1999; Parmigiani et al. 1999; vom Saal et al. 1995). In addition, testicular 

atrophy was noted in newborn male mice intraperitoneally injected with methoxychlor (Martinez and 

Swartz 1991). A study reporting early onset of puberty (precocious vaginal opening) in rats (Chapin et al. 

1997) has been utilized to derive an intermediate duration oral MRL value. However, no NOAEL was 

identified in this study. Further oral studies that better define the threshold dose for reproductive effects 

in rats, mice, and in other species, and examine the similarities and differences in response between 

species would be useful. Further data needs relating to the developing reproductive system are discussed 

in the Developmental Toxicity subsection. 

In vitro competitive binding assays have shown that methoxychlor itself does not bind to the estrogen 

receptor (Bulger et al. 1978a, 1978b; Ousterhout et al. 1981), but that the mono-hydroxy and bis-hydroxy 

derivatives of methoxychlor do bind to the receptor and cause nuclear translocation. An additional study 

has shown that bis-hydroxy methoxychlor binds to the ERα and ERβ subtypes of the estrogen receptor 

(Gaido et al. 1999). However, not all effects resulting from methoxychlor exposure mimic those of 

estrogen. Some effects (delayed preputial separation, decreased pituitary size, and a lack of effect on 

serum LH, prolactin, and testosterone) could be explained by androgen antagonism (Gray et al. 1999). 

Only one study has investigated the potential androgen antagonism of methoxychlor and its metabolites; 

this experiment was not a direct receptor binding assay. The methoxychlor metabolite bis-hydroxy 

methoxychlor (HPTE) was a weak AR antagonist of dihydrotestosterone in HepG2 human hepatoma cells 
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transiently transfected with the human androgen receptor and a reporter gene linked to an androgen 

responsive promoter; methoxychlor showed even less antagonism in this experiment (Maness et al. 

1998). While it seems clear that methoxychlor metabolites mediate at least some of their effects via 

estrogen and androgen receptors, it is not entirely clear whether the effects are mediated directly in the 

tissues in which the receptor binding takes place, or if some effects are mediated indirectly through 

feedback mechanisms with the hypothalamus and pituitary and resulting altered hormone levels. Changes 

in pituitary prolactin, TSH, and FSH levels have been noted in rats exposed to methoxychlor (Goldman et 

al. 1986; Gray et al. 1988, 1989). Studies are needed that examine the effects of methoxychlor on the 

hypothalamus-pituitary-gonadal axis and all related hormones and the distribution of the ER and AR 

within the axis to better understand the varied effects of methoxychlor on development and reproduction. 

Induction of two estrogen-responsive genes (lactoferrin [LF] and glucose-6-phosphate dehydrogenase, 

[G6PD]) in uteri of mice was not attenuated by an absence of ERα (in ERα knockout mice) or by an ER 

inhibitor (which would eliminate ERβ availability) (Ghosh et al. 1999). This suggests that not all of the 

effects from methoxychlor are mediated through a known ER. Additional studies are needed 

investigating this effect and incorporating different estrogen-responsive endpoints in mice and rats to 

clarify this alternative mechanism resulting in estrogen-like effects. There is also evidence in a nonmammalian species (frog) that methoxychlor acts via a non-ER mechanism (Pickford and Morris 1999). 

Methoxychlor caused a highly significant inhibition of progesterone-induced germinal vesicle breakdown 

(GVBD, necessary for oocyte maturation), while estradiol and HPTE had no effect. Studies investigating 

the mechanism of this effect of methoxychlor in frogs might also provide important information. 

Animal studies which describe the long-term consequences of methoxychlor exposure during gestation, 

early postnatal life, or adulthood on reproductive parameters and function in both sexes would also be 

useful. Animal studies are also needed to identify reliable NOAELs and LOAELs for the inhalation and 

dermal routes of exposure. Studies in farmers, pesticide workers, or persons living at or near hazardous 

waste sites who are exposed to methoxychlor may serve to indicate whether the effects observed in 

animals also occur in humans. More information is needed regarding which metabolites, contaminants, 

and degradation products of methoxychlor are estrogenic, and therefore, of health concern to the public. 

One study reported a decrease in the percentage of male offspring of rabbit dams exposed orally to 

methoxychlor (Kincaid Enterprises 1986). Other studies have not reported this effect. It is unclear what 

the mechanism for this alteration in sex ratio is. The same study also reported increased incidences of 
 

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dead and resorbed fetuses (Kincaid Enterprises 1986). It would be interesting to know if there is a sexspecific fetotoxic effect of methoxychlor or if sex ratio is really affected by methoxychlor. 

Developmental Toxicity. No data were located regarding the developmental effects of methoxychlor 

in humans for inhalation, oral, or dermal routes of exposure or in animals for inhalation and dermal routes 

of exposure. Although oral is by far the predominant route of exposure, studies focusing on 

developmental effects resulting from inhalation and dermal exposure may be necessary to assess the 

possible impact of such exposures. 

Oral studies in rats and mice indicate that exposure to methoxychlor during gestation can produce 

decreased number of implants, decreased fetal body weight, increased frequency of dead or resorbed 

fetuses, or fetuses with wavy ribs (Chapin et al. 1997; Culik and Kaplan 1976; Khera et al. 1978; Kincaid 

Enterprises 1986; Swartz and Eroschenko 1998). 

Methoxychlor can also cause numerous functional changes in the reproductive system of both adult and 

developing animals. Methoxychlor accelerated the onset of puberty in female rats exposed for 

intermediate durations in utero, during lactation, and/or postweaning (Chapin et al. 1997; Gray et al. 

1989; Harris et al. 1974; Swartz and Corkern 1992). Additional studies examining this effect following 

shorter exposures, during gestation, and beginning at weaning (separate experiments), at doses at or 

below the lowest LOAEL (5 mg/kg/day in Chapin et al. 1997) are necessary to more closely define the 

critical interval and lowest level of exposure at which this occurs. Chapin et al. (1997) observed a 

reduction in testes, epididymis, seminal vesicle, and ventral prostate weights in male offspring of rats 

exposed to 50 or 150, but not 5 mg/kg/day from gestation day 14 through postnatal day 7, followed by 

direct exposure of the pups through postpartum day 42. Studies examining these effects following 

exposures for different durations and during specific life stages, including gestation and pre- and postweaning, are also needed to more closely define the critical interval and lowest level of exposure at which 

this occurs. Decreased performance in neurobehavioral tests in female rats (increased locomotor activity, 

decreased click response, and decreased hindlimb grip strength) and male rats (increased handling 

reactivity, decreased click response, and approach stimulus) was seen following exposure to 

5–150 mg/kg/day for intermediate durations in utero, during lactation, and/or postweaning (Chapin et al. 

1997). However, there was no dose-response relationship for these effects. Methoxychlor caused a slight 

(but not significant) decrease in testes weight and sperm counts of male rats exposed during gestation and 

lactation (Gray et al. 1989). 
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There were several hypothesis-generating studies using extremely low doses that were not definitive 

enough to use for MRL derivation. Adult male mice exposed in utero to 0.02 mg/kg/day methoxychlor 

had increased prostate weights compared to vehicle controls (Welshons et al. 1999). Male mice exposed 

in utero to 0.02 mg/kg/day methoxychlor exhibited increased urine-marking behavior when placed in a 

new territory (vom Saal et al. 1995). A LOAEL of 1.8 mg/kg/day was observed for aggressive behavior 

(infanticide) in male mice exposed in utero toward an unrelated pup (Parmigiani et al. 1998), and a 

LOAEL of 0.02 mg/kg/day was observed for decreased aggression of young male mice exposed in utero 

toward male siblings (Palanza et al. 1999). Further oral studies that better define the threshold dose for 

developmental effects in rats, mice, and in other species, and examine the similarities and differences in 

response between species would be useful. Specifically, additional studies in mice using exposure levels 

encompassing the LOAEL of 0.02 mg/kg/day identified in the Welshons et al. (1999) study, and studies 

in species other than mice, may prove very useful. The Welshons et al. (1999) results need to be 

replicated using a larger number of exposed dams appropriate for a developmental toxicity study 

(Tyl 2000), testing all males in all litters for prostate size (Elswick et al. 2000a, 2000b), including 

estradiol and/or DES as simultaneous positive controls, and perhaps including more doses of 

methoxychlor above and below 0.02 mg/kg/day. A comparative study also needs to be done quantifying 

how much prostate weight in adult mice varies as a function of intrauterine position relative to female 

littermates and the estrogen that diffuses from them (Nonneman et al. 1992; Timms et al. 1999; vom Saal 

1989); the results of such a study would aid in interpreting the biological significance of the Welshons et 

al. (1999) findings. Intrauterine position can be identified by caesarian delivery just prior to natural 

parturition. Additional studies are needed examining the effects of very low doses of methoxychlor on 

neurobehavioral parameters in mice and other species. The behavioral effects seen in male mice 

following in utero exposure need to verified and investigated further to determine their mechanism and 

their significance to human health. 

New studies would be helpful to address the factors which make vom Saal et al. (1995), Parmigiani et al. 

(1998), and Palanza et al. (1999) difficult to interpret. Male mice exposed in utero to 0.02 mg/kg/day 

methoxychlor exhibited increased urine-marking behavior when placed in a new territory (vom Saal et al. 

1995). This study was not used for MRL derivation because only two male pups/litter were tested and 

apparently each was only tested one time; the accuracy of the assessment of the test and how reproducible 

the results are were not determinable; and there was no indication of what, if any, statistical methods were 

used to evaluate the results. A LOAEL of 1.8 mg/kg/day for aggressive behavior (infanticide) in male 

mice exposed in utero toward an unrelated pup (Parmigiani et al. 1998) was not used for MRL derivation 

because only two male pups/litter were tested; there was no dose-response relationship, and no effect was 
 

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seen with DES, a positive estrogenic control. Similarly, a LOAEL of 0.02 mg/kg/day for decreased 

aggression of young male mice exposed in utero toward male siblings at postpartum day 39 but not at 

postpartum day 54 (Palanza et al. 1999) was not a suitable basis for an MRL because there was no doseresponse relationship and the effect was transient. 

There is puzzling, limited evidence that exposure of adult female animals may cause effects in 

subsequent, unexposed offspring. Female offspring (F1a) of mice dams exposed during gestation 

(gestation days 6–15) to 0, 50, or 100 mg/kg/day methoxychlor showed no change in day of vaginal 

opening and no change in ovarian weight (Swartz and Corkern 1992), although methoxychlor-exposed 

female pups had an increase in atretic follicles. Female mice exposed only via lactation and females 

exposed during gestation and lactation showed similar results. However, female offspring (F1b) from a 

second unexposed pregnancy showed a slight, but statistically significant, advance in the day of vaginal 

opening (25.0, 23.9, and 23.2 days for the control, 50–100-mg/kg/day groups, respectively). Since 

methoxychlor is eliminated rapidly from the body (Kapoor et al. 1970), the above data imply that 

methoxychlor might cause a long-term alteration in reproductive physiology that can affect subsequent 

offspring. More studies looking at effects on offspring of previous exposures to dams are needed in order 

to verify this effect. Upon confirmation, subsequent studies would be required to elucidate the 

mechanism(s) by which this effect on the offspring occurred. 

Additional animal studies that identify reliable NOAELs and LOAELs for developmental effects for 

inhalation and dermal exposures would be helpful for extrapolating data to human exposures. 

Information on the offspring of female agricultural workers, pesticide workers, or residents who live at or 

near a hazardous waste site who may be exposed to methoxychlor may indicate whether the 

developmental effects observed in animals also occur in humans. 

Immunotoxicity. No data were located regarding the immunological effects of methoxychlor in 

humans following inhalation, oral, or dermal exposure to methoxychlor, or in animals following 

inhalation or dermal exposure to methoxychlor. The immunotoxicity of methoxychlor has not been well 

investigated in animals following oral exposure to methoxychlor. A single study reported no 

histopathological changes in the thymus, spleen, and lymph nodes of mice and rats following chronic oral 

exposure to methoxychlor (NCI 1978). Male rats, but female rats, exposed to 5 or 50 mg/kg/day during 

gestation and maturation showed a decrease in plaque-forming cells/spleen, indicating a possible 

attenuation of primary immune responses (Chapin et al. 1997). Thymus weights were decreased in rats 

exposed to 50–150 mg/kg/day during prenatal and postnatal development (Chapin et al. 1997) and in 
 

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adult male rats exposed to 500 mg/kg/day (Okazaki et al. 2001). No histological data were provided. 

Spleen weight, splenic natural killer cell activity, splenic lymphoproliferative response, and splenic cellsurface phenotypes did not differ from controls (Chapin et al. 1997). These results provide only limited 

data that methoxychlor may influence immune responses. The lack of any immune effects in females or 

on other immune end points in males that are related to plaque-forming cell numbers led Chapin et al. 

(1997) to suggest that the observed effect may have been anomalous. Additional studies examining more 

sensitive functional immune end points following oral exposure would help to clarify the impact of 

methoxychlor on the immune system. 

Neurotoxicity. Data regarding the neurological effects of methoxychlor in humans after inhalation, 

oral, or dermal exposure are limited to a single case report which described the development of deafness 

and peripheral neuropathies following inhalation exposure to a mixture of pesticides; no causality 

regarding methoxychlor could be determined (Harell et al. 1978). Based on its structural similarity to 

DDT, methoxychlor may be expected to have the potential to produce DDT-like neurological effects 

(tremors, convulsions) in animals and humans at very high acute doses. Effects of this nature have been 

reported following oral exposure to high doses in rats and dogs (Cannon Laboratories 1976; Tegeris et al. 

1966) and in rats whose hepatic metabolic capacity was diminished by coadministration of carbon 

tetrachloride (Lehman 1952), but are not usually associated with lower exposures to methoxychlor. 

Neurological symptoms were also observed in rabbits following inhalation and dermal exposure, but since 

these effects were similar to those produced by a disease which is endemic to rabbits, these effects may 

not have been treatment related (Haag et al. 1950). Since these neurological effects of methoxychlor are 

probably due to the parent compound, additional studies which investigate species differences in the 

capacity to metabolize methoxychlor and differences in sensitivity to the neurotoxic effects of 

methoxychlor may help in evaluating neurotoxic risk to humans. Behavioral effects were noted in male 

and female rats and mice administered methoxychlor (Chapin et al. 1997; Gray et al. 1988; Parmigiani et 

al. 1998; vom Saal et al. 1995), which unlike the neurological effects discussed above, are most likely due 

to one or more metabolites of methoxychlor which exhibit estrogenic activity (Bulger et al. 1985). 

Additional studies that further characterize which neurotoxic effects are caused by methoxychlor and 

which are caused by its metabolites would be useful. Information obtained from humans accidentally 

exposed to large doses of methoxychlor may also serve to reveal whether the neurotoxic effects observed 

in animals also occur in humans. 
 

 

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Epidemiological and Human Dosimetry Studies. A single epidemiological study was located 

which identified a possible association between exposure to methoxychlor and leukemia in farmers 

(Brown et al. 1990). However, the exposed and control groups were small, and the increase only 

marginally significant. Moreover, exposure levels were not quantified and exposure to other chemicals 

probably occurred, so confidence in the study is low. Nevertheless, the possible association detected in 

this study suggests that further investigations should be conducted on agricultural workers, pesticide 

formulators, and pesticide applicators who are frequently exposed to methoxychlor. Other populations 

which could also be studied include residents at or near areas that undergo frequent aerial spraying of 

methoxychlor, areas at or near methoxychlor production facilities, and areas at or near hazardous waste 

sites that contain methoxychlor. Animal studies indicate that reproductive and developmental effects of 

methoxychlor are end points that are of primary concern. Therefore, epidemiological studies which focus 

on reproductive and developmental end points would also be valuable. 

Biomarkers of Exposure and Effect 

Exposure. The presence of methoxychlor and its phenolic metabolites in biological media may be used 

as specific biomarkers for exposure to methoxychlor. Methoxychlor has been detected in human breast 

milk samples from women living in rural and urban areas (Campoy et al. 2001a, 2001b). Methoxychlor 

and its metabolites have also been measured in the milk of lactating rats and were found to concentrate in 

the milk with increasing methoxychlor exposure level (Chapin et al. 1997). Plasma levels of 

methoxychlor and its metabolites (mono-hydroxy methoxychlor and di-hydroxy methoxychlor) in 

suckling rat pups have also been observed to increase with increasing dose of methoxychlor (Chapin et al. 

1997). However, because of the relatively rapid clearance of these metabolites, measurements would 

probably only be useful in detecting recent exposures (within the past 24 hours). Measurements of 

methoxychlor and metabolite levels in relevant biological fluids in animal studies would be helpful in 

establishing a quantitative correlation between exposure level, body burden, and effect level. These 

correlations might prove useful in assessing risk in human exposures. The Chapin et al. (1997) study is 

the only study in which concentrations of methoxychlor and methoxychlor metabolites were measured in 

biological fluids at doses at which health effects were being observed in animals. 

Effect. As discussed earlier, the primary effects of methoxychlor are on reproduction. In exposed female 

rodents, methoxychlor exposure may affect the estrus cycle (Gray et al. 1988; Harris et al. 1974; Martinez 

and Swartz 1992; Okazaki et al. 2001; Tegeris et al. 1966), serum hormone levels (Cummings and Gray 

1987, 1989; Cummings and Laskey 1993; Gray et al. 1988), the uterus and ovaries (Bal 1984; Chapin et 
 

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al. 1997; Cummings and Gray 1987, 1989; Cummings and Laskey 1993; Cummings and Perreault 1990; 

Dikshith et al. 1990; Gray et al. 1988, 1989; Harris et al. 1974; Martinez and Swartz 1992; Mitsumori et 

al. 2000; Okazaki et al. 2001; Swartz et al. 1994; Tegeris et al. 1966; Tullner 1961), and may reduce 

fertility (Bal 1984; Chapin et al. 1997; Gray et al. 1989; Harris et al. 1974; Haskell Laboratories 1966). 

In the female offspring of exposed female rodents, reproductive development, as indicated by age at 

vaginal opening and first estrus, may be accelerated (Chapin et al. 1997; Gray et al 1989; Harris et al. 

1974). In male rodents, methoxychlor exposure may affect sexual maturity (age of preputial separation) 

(Gray et al. 1989, 1999), serum hormone levels (Cummings and Gray 1989; Goldman et al. 1986; Gray et 

al. 1989; Stoker et al. 1999), testes (Bal 1984; Chapin et al. 1997; Dikshith et al. 1990; Gray et al. 1989, 

1999; Hodge et al. 1950; Tullner and Edgcomb 1962; Wenda-Rozewicka 1983), prostate (Okazaki et al. 

2001; Shain et al. 1977; Stoker et al. 1999; Welshons et al. 1999), fertility (Chapin et al. 1997; Gray et al. 

1989, 1999; Harris et al. 1974; Wenda-Rozewicka 1983), and behavior (Parmigiani et al. 1998; vom Saal 

et al. 1995). However, these effects are not specific to exposure to methoxychlor, but could occur 

following exposure to other chemicals with estrogenic activity as well. It is unknown whether any of 

these effects will occur in humans at environmental exposure levels. Some potential end points that 

might be examined in human studies include: increased cornification of vaginal epithelium as measured in 

a pap smear, changes in menstrual cycles or sperm counts, and changes in serum progesterone, prolactin, 

and testosterone levels. Additional studies which investigate the mechanism of action of methoxychlor at 

the molecular level, may be useful in developing a specific biomarker of effect for methoxychlor. 

Development of a specific and sensitive biomarker of effect would be useful to facilitate future medical 

surveillance that can lead to detection and possible treatment of exposed populations. 

Absorption, Distribution, Metabolism, and Excretion. In vivo toxicokinetics data for 

methoxychlor in humans are absent. There are no data on inhalation exposure and only minimal 

information on dermal exposure. As discussed in Section 3.4, studies in which animals were given oral 

doses of radiolabeled methoxychlor indicate that methoxychlor is: 

(1) rapidly and efficiently absorbed by the gastrointestinal tract in mice and goats (Davison et al. 

1982, 1983; Kapoor et al. 1970); 

(2) widely distributed to tissues and organs with some preferential distribution to fatty tissues in 

rats (Harris et al. 1974; Hodge et al. 1952; Kunze et al. 1950), sheep (Reynolds et al. 1976), dogs 

(Hodge et al. 1952), and goats (Davison et al. 1982); and 

(3) very rapidly cleared, predominately as metabolites in feces (via biliary excretion) and urine, in 

mice (Kapoor et al. 1970), goats (Davison et al. 1982), and sheep (Reynolds et al. 1976). 
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Dermal absorption has been estimated to be 5–8% in goats (Davison et al. 1983) and 20% in cows 

(Skaare et al. 1982); however, because of differences in skin, dermal absorption by goats may not be a 

good model for dermal absorption by humans. Methoxychlor and methoxychlor metabolites have been 

detected in milk of lactating goats (Davison et al. 1982) and rats (Chapin et al. 1997) given oral doses of 

methoxychlor. Accumulation in milk and/or elimination through lactation do not appear to be 

predominant dispositional fates for methoxychlor, but transfer of methoxychlor and methoxychlor 

metabolites to suckling rat pups via milk has been demonstrated (Chapin et al 1997). 

Studies examining placental transfer of methoxychlor or methoxychlor metabolites were not located, but 

subtle effects on the development of reproductive organs have been observed in male and female 

offspring of rodents exposed to oral doses of methoxychlor during gestation (Chapin et al. 1997; Gray et 

al. 1989; Harris et al. 1974); however, it should be noted that exposure was not exclusively during 

gestation in any of these studies. Research examining the possibility of placental transport of 

methoxychlor and metabolites and quantifying possible distribution to developing fetuses may help to 

discern if these effects result directly from methoxychlor and metabolites passing through the placenta or 

indirectly from a disturbance of maternal physiological processes. 

As discussed in Section 3.4, the metabolism of methoxychlor has been studied in vivo in orally exposed 

rats (Lehman 1952; Weikel and Laug 1958), mice (Kapoor et al. 1970), and goats (Davison et al. 1982, 

1983). Demethylated, dechlorinated, and dehydrochlorinated metabolites of methoxychlor were 

identified in feces and urine, and minor ring hydroxylated metabolites were also found in urine. Most 

urinary metabolites were conjugated with glucuronic acid (Davison et al. 1982, 1983). In vitro studies 

with human and rat liver microsomes, partially purified rat CYP isozymes, and/or human c-DNAexpressed CYP isozymes indicate that the predominant metabolic pathway involves initial demethylation 

of methoxychlor catalyzed by a number of CYP isozymes in rats (Kishimoto and Kurihara 1996; 

Kishimoto et al. 1995) and in humans (Stresser and Kupfer 1998). In humans, CYP2C19 and 

CYP1A2 may be the major demethylases for methoxychlor, but other forms may play significant roles in 

individuals deficient in these isozymes (Stresser and Kupfer (1998). In rats, CYP2C6 and another 

unidentified CYP isozyme may be the major catalysts for demethylation (Kishimoto and Kurihara 1996). 

Ring hydroxylation of methoxychlor or hydroxy-methoxychlor derivatives appears to be catalyzed by a 

different set of CYP isozymes than those catalyzing demethylation (Stresser and Kupfer 1997; Stresser et 

al. 1996). Studies with immature or mature rats given repeated intraperitoneal injections of methoxychlor 

indicate that methoxychlor can induce hepatic CYP enzymes involved in its metabolism (Li et al. 1995). 
 

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Animal data indicate that methoxychlor and its metabolites are excreted primarily in the bile with a lesser 

amount excreted in the urine (Davison et al. 1982, 1983; Kapoor et al. 1970). This is true regardless of 

the route of exposure. In bile-duct cannulated rats given a single intravenous dose of radiolabeled 

methoxychlor, 50% of the dose was excreted in the bile after 4 hours (Weikel 1957). The urinary 

excretion of label in bile-cannulated rats was only 0.1–0.2% compared to 5–10% in noncannulated rats, 

suggesting that the appearance of urinary metabolites was largely due to material that was reabsorbed in 

the gut (i.e., enterohepatic circulation). Because methoxychlor does not accumulate significantly in 

biological tissues and is rapidly eliminated, additional studies examining the excretion of methoxychlor 

and its metabolites are not warranted. 

Comparative Toxicokinetics. The available data comparing rat and human liver microsomal 

metabolism of methoxychlor indicate qualitative similarities and some indications of quantitative 

differences. Rat liver microsomes were observed to have a higher capacity than human liver microsomes 

to metabolize methoxychlor to covalent binding intermediates (Bulger and Kupfer 1990). In vitro data 

also indicate that covalent binding of methoxychlor to human liver microsomes is similar across age and 

sex, whereas in rats, covalent binding in mature males is much higher than in mature females and 

immature males and females (Bulger and Kupfer 1989). Another comparison found that in vitro rates of 

demethylation of methoxychlor and mono-hydroxy methoxychlor with human liver microsomes were 

generally higher in rat liver microsomes than in human liver microsomes (Stresser and Kupfer 1998). 

Further research comparing quantitative and qualitative aspects of metabolism with mouse, rat, and 

human systems may help in developing PBPK models for methoxychlor in these species. New studies 

comparing species-specific tissue/blood partition coefficients, cross-placental distribution in pregnant 

animals (mice and rats), distribution to milk in lactating animals (mice and rats), elimination kinetics for 

methoxychlor from blood (mice and rats), and/or kinetics of metabolite appearance in urine or feces (rats; 

data are available for mice, Kapoor et al. 1970) would facilitate development of relevant PBPK models 

predicting distribution of methoxychlor and methoxychlor metabolites to the developing fetus and 

suckling infants. Such models may be useful in extrapolating observations of reproductive effects in 

rodents following in utero and/or lactational methoxychlor exposure to relevant human exposure 

scenarios. 
 

 

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Methods for Reducing Toxic Effects. No data were located regarding the mechanism of 

absorption, distribution, or excretion of methoxychlor in humans or animals by any route. Based on the 

chemical and physical properties of methoxychlor, its absorption most likely occurs by passive diffusion. 

However, this has not been investigated. Studies which investigate the mechanism by which 

methoxychlor enters the body may be useful in developing methods for reducing its absorption. Once it 

enters the body, methoxychlor preferentially distributes to tissues with higher fat content; however, 

methoxychlor does not persist in tissues (Harris et al. 1974; Hodge et al. 1952; Kunze et al. 1950; 

Reynolds et al. 1976). Its rapid metabolism to more polar compounds facilitates its rapid clearance from 

these tissues (Davison et al. 1982, 1983; Lehman 1952; Weikel and Laug 1958). Since the majority of 

methoxychlor is cleared from the body within 24 hours (Kapoor et al. 1970), it seems unlikely that 

methods for reducing body burden of methoxychlor would be of much benefit. Therefore, studies 

examining the mechanism of methoxychlor distribution and excretion are not warranted. 

Children’s Susceptibility. There is no information on health effects observed in children after 

methoxychlor exposure via inhalation, oral, or dermal routes, and there is a data need for such 

information, particularly from the predominant oral route of exposure. Animal data indicate that the 

primary target of methoxychlor is the reproductive system, and methoxychlor can affect both adult 

animals and the developing reproductive system in males and females. Thus, there is concern about 

whether exposure to methoxychlor during any stage of development might potentially affect the 

developing reproductive system of fetuses, children, and adolescents. 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. 

There are no studies of methoxychlor metabolism in children; such information would be useful. 

Numerous studies have shown that both adult and developing animals exhibit effects attributed to the 

estrogenic activity of methoxychlor or its metabolites. This suggests that either methoxychlor is 

metabolized by the mother first and the metabolites then cross the placenta, or that methoxychlor crosses 

the placenta and is metabolized by the fetus. However, there are no studies that specifically investigated 

metabolic enzyme activity in fetuses. Although there is no direct information regarding age-related 

differences in metabolism of methoxychlor in children, multiple CYP enzymes are involved in 

methoxychlor metabolism. It is likely that CYPs 2C19, 1A2, 2B6, 2C9, 2A6, and perhaps 2D6, 2E1, and 

3A4 play a role in phase I human metabolism (Hong et al. 1987; Lacroix et al. 1997; Leeder and Kearns 

1997; Mote et al. 1990; Ratenasavanh et al. 1991; Rich et al. 1990; Sonnier and Cresteil 1998; Treuler et 

al. 1997; Vieira et al. 1996; Yang et al. 1994). Many of these phase I enzymes are likely to have 
METHOXYCHLOR 136 

3. HEALTH EFFECTS 

overlapping roles. Although nothing is known about phase II metabolism in humans, Davison et al. 

(1982, 1983) found glucuronic acid conjugates of methoxychlor metabolites in goats; these conjugates are 

a product of glucuronosyltransferase and it is possible that this enzyme may be involved in human 

methoxychlor metabolism as well. Although it is unknown whether an overall age-related difference in 

methoxychlor metabolism would be observed in vivo in humans, there is some information that the 

following enzymes may be developmentally regulated: CYP2C19 (Leeder and Kearns 1997), CYP1A2 

(Leeder and Kearns 1997; Rich et al. 1990), CYP2C9 (Ratenasavanh et al. 1991; Treuler et al. 1997), 

CYP2D6 (Leeder and Kearns 1997; Sonnier and Cresteil 1998), CYP2E1 (Hong et al. 1987; Vieira et al. 

1996), CYP3A4 (Lacroix et al. 1997; Leeder and Kearns 1997; Mote et al. 1990; Yang et al. 1994), and 

glucuronosyltransferase (Leeder and Kearns 1997). In contrast, other researchers have found that CYPs 

2A6, 2C9, 2D6, 2E1, and 3A have been found at the same levels in perinatal (37 weeks of gestation) to 

geriatric human livers (72 years) (Ratenasavanh et al. 1991; Tateishi et al. 1997). 

There are no studies examining placental transfer of methoxychlor or methoxychlor metabolites, but 

subtle effects on the development of reproductive organs have been observed in male and female 

offspring of rodents exposed to oral doses of methoxychlor during gestation (Chapin et al. 1997; Gray et 

al. 1989, 1999; Harris et al. 1974; Swartz and Corkern 1992; Welshons et al. 1999). It is unclear whether 

these effects result directly from methoxychlor and metabolites passing through the placenta or indirectly 

from a disturbance of maternal physiological processes. Research examining the possibility of placental 

transport of methoxychlor and metabolites and quantifying possible distribution to developing fetuses 

may help in elucidating the mechanism by which offspring of an exposed dam are affected. 

Methoxychlor and methoxychlor metabolites have been detected in milk of lactating goats (Davison et al. 

1982) and rats (Chapin et al. 1997) given oral doses of methoxychlor. Accumulation in milk and/or 

elimination through lactation do not appear to be predominant dispositional fates for methoxychlor, but 

transfer of methoxychlor and methoxychlor metabolites to suckling rat pups via milk has been 

demonstrated (Chapin et al 1997). The Chapin et al. (1997) study measured plasma methoxychlor (and 

metabolite) levels 27–30 hours after the dam’s last exposure to methoxychlor; it would be interesting to 

know what pup plasma levels would be shortly after the last exposure of the dams. Additional studies to 

quantitate the plasma levels of methoxychlor in pups exposed only via lactation and to investigate the 

resultant effects through puberty and reproduction would be helpful in understanding the risk to offspring 

of maternal exposure to methoxychlor, even after birth. 
METHOXYCHLOR 137 

3. HEALTH EFFECTS 

Child health data needs relating to exposure are discussed in Section 6.8.1, Identification of Data Needs: 

Exposures of Children. 

3.12.3 Ongoing Studies 

Ongoing studies pertaining to methoxychlor have been identified and are shown in Table 3-5. 
METHOXYCHLOR 138 

3. HEALTH EFFECTS 

Table 3-5. Ongoing Studies on the Health Effects of Methoxychlora 

Investigator Affiliation Research description Sponsor 

Ahmed, SA et al. Virginia Polytechnic 

Institute, College of 

Veterinary Medicine 

In vivo effects of a pesticide on 

the immune system: Age and 

gender factors 

CSREES VA 

Chapin, RE NIEHS, NIH Prenatal/Juvenile exposure to 

pesticides on adult neural, 

immune function 

NIEHS 

Lubahn, DB University of 

Missouri–Columbia 

Environmental estrogen 

response proteins in ER-minus 

mice 

NIEHS 

Chambers, JE Mississippi State 

University, College of 

Veterinary Medicine 

Developmental and comparative 

toxicology of neurotoxic 

insecticides 

U.S. Department of 

Agriculture, 

Cooperative Research 

Service 

Gore, A Mount Sinai School 

of Medicine 

Neuroendocrine mechanisms of 

environmental toxicity during 

development 

NIEHS 

James, MO University of Florida Bioavailability of chlorinated 

compounds 

NIEHS 

Kupfer, D Worcester 

Foundation of 

Experimental Biology 

Effects of chlorinated 

hydrocarbons on mammalian 

systems 

NIEHS 

Poznanski, AA Midwestern 

University 

Methoxychlor effects on 

zebrafish forebrain development 

NIEHS 

Schiffenbauer, J University of Florida Autoimmune toxicity of 

chlorinated compounds 

NIEHS 

Wilson, EM University of North 

Carolina–Chapel Hill 

Mechanisms of action of 

environmental antiandrogens 

NIEHS 

Zacharewski, TR Michigan State 

University 

Comprehensive assessment of 

endocrine active mixtures 

NIEHS 

a

Source: FEDRIP (2001)
 

NIEHS = National Institute of Environmental Health Sciences; NIH = National Institutes of Health
 
METHOXYCHLOR 139 

4. CHEMICAL AND PHYSICAL INFORMATION 

4.1 CHEMICAL IDENTITY 

Information regarding the chemical identity of methoxychlor is located in Table 4-1. 

Methoxychlor is produced commercially in the United States as a technical grade containing 88–90% of 

the pure chemical and 10–12% of impurities consisting of isomers and other reaction products (IARC 

1979; Lamoureux and Feil 1980). Twenty-five of these impurities were characterized in studies 

conducted on technical methoxychlor; evidence for >50 impurities was obtained through gas 

chromatography/mass spectrometry (GC/MS) (Lamoureux and Feil 1980). Purification of technical grade 

(nominally 90%) 1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane (p,p’-methoxychlor; p,p’-DMDT) by 

recrystalization gave 76% p,p’-methoxychlor (99.8% pure by normal phase high performance liquid 

chromatography [HPLC]) and 24% impurities (West et al. 1982). The impurities were found to contain 

approximately 40 components. The major components were identified (HPLC and GC/MS) using 

reference standards. Component identities and percent (w/w) in technical grade methoxychlor were found 

to be as follows: 1,1,1,2-tetrachloro-2-(4-methoxyphenyl)ethane (1.73%), 1,1,1-trichloro-2-(2-methoxyphenyl)-2-(4-methoxyphenyl)ethane (o,p’-methoxychlor; o,p’-DMDT; 4.03%), 

1,1-dichloro-2,2-di(4-methoxyphenyl)ethene (DMDE; 0.39%), a condensation product of p,p’-methoxychlor (p,p’-DMDT; 0.48%), a condensation product of o,p’-methoxychlor (o,p’-DMDT; 0.4%), and 

1,2,2,2-tetrakis(4-methoxybenzyl)ethene (0.5%). The percentages were calculated relative to the quantity 

of methoxychlor contained in technical grade material and were found to vary depending upon 

manufacturing conditions. Another impurity is 1-chloro-1,2,2-tris(4-methoxyphenyl)ethene 

(chlorotrianisene; TACE), a triphenylethylene derivative, that exhibits estrogenic/anti-estrogenic 

characteristics. Other impurities include polycylic hydrocarbons (e.g., 3,6-dimethoxy-9,10-bis (p-methoxyphenyl)phenanthrene, tetrakis(p-methoxyphenyl)ethylene, and 3,6,11,14-tetramethoxydibenzo(g,p)chrysene), which have been studied for mutagenicity and putative carcinogenicity (Grant et 

al. 1976). The substance, 3,6,11,14-tetramethoxydibenzo(g,p)chrysene, was found to be mutagenic. 
METHOXYCHLOR 

4. CHEMICAL AND PHYSICAL INFORMATION 

140 

Table 4-1. Chemical Identity of Methoxychlor 

Characteristic Information Reference 

Chemical name 

Synonym(s) 

Registered trade name(s) 

Chemical formula 

Chemical structure 

Identification numbers: 

CAS registry 

NIOSH RTECS 

EPA hazardous waste 

OHM/TADS 

DOT/UN/NA/IMCO shipping 

HSDB
 

NCI
 

Methoxychlor
 

2,2-bis(p-methoxyphenyl)­

1,1,1-trichoroethane;
 

1,1,1-trichloro-2,2­

bis(4-methoxyphenyl) ethane;
 

methoxy-DDT; 1,1r-(2,2,2­

trichloroethylidene)-bis(4­

methoxybenzene)
 

Marlate®
 

Metox®
 

Prentox®; Methoxcide®
 

C16H15Cl3O2 

H 

CH3O OCH3 

Cl Cl Cl 

1,1,1-Trichloro-2,2-bis(4-meth 

oxyphenyl)ethane (p,p’-methoxy 

chlor; DMDT) 

72-43-5 

KJ 3675000 

U247; D014 

OHM 7216536 

DOT 2761 

UN 2761 

NA 2761 

IMCO 6.1 

1173 

C00497 

Howard 1991 

HSDB 2000 

EPA 1988b; HSDB 2000; 

Sittig 1980 

Howard 1991 

EPA 1988c 

Howard 1991 

Sax and Lewis 1989 

HSDB 2000 

HSDB 2000 

Sax and Lewis 1989 

HSDB 2000 

HSDB 2000 

HSDB 2000 

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 Hazardous Materials/Technical Assistance Data System; 

RTECS = Registry of Toxic Effects of Chemical Substance 
METHOXYCHLOR 141 

4. CHEMICAL AND PHYSICAL INFORMATION 

4.2 PHYSICAL AND CHEMICAL PROPERTIES 

Information regarding the physical and chemical properties of methoxychlor is located in Table 4-2. 
METHOXYCHLOR 142 

4. CHEMICAL AND PHYSICAL INFORMATION 

Table 4-2. Physical and Chemical Properties of Methoxychlor 

Property	 Information Reference 

Molecular weight	 345.65 Howard 1991 

Color	 Pale yellow EPA 1988c 

Physical state	 Crystalline solid EPA 1988c 

Melting point	 89 EC HSDB 2000 

77 EC (technical grade) 

No data (decomposes) Montgomery and Welkom 1990 

Density: 1.41 g/cm3 Montgomery and Welkom 1990 

at 25 EC 

Odor	 Slightly fruity; musty; chlorine-like HSDB 2000; Sigworth 1965 

Odor threshold: 

Water 60 EC 4.7 ppm Sigworth 1965 

Air No data 

Solubility: 

Water at 25 EC 0.045 mg/L
 

at 15 EC 0.02 mg/L
 

at 24 EC 0.04 mg/L
 

at 35 EC 0.095 mg/L
 

at 45 EC 0.185 mg/L
 

Organic solvent(s)	 Soluble in chlorinated aromatic Budavari et al. 1989; EPA 1988a; 

solvents, ketonic solvents, ethanol, HSDB 2000; Montgomery and 

methylene chloride, methylated Welkom 1990 

naphthalene, carbon tetrachloride, 

chloroform, xylene, methanol, 

petroleum ether, benzene 

Partition coefficients: 

Log Kow 4.68–5.08 Howard 1991 

Log Koc 4.9 Montgomery and Welkom 1990 

Vapor pressure at 25 EC 1	 .4x10-6 mmHg (estimated) Howard 1991 

Henry’s law constant: 

at 25 EC 1.6x10-5 atm-m3

/mol (estimated) Howard 1991 

Autoignition temperature	 No data 

Flashpoint	 No data 

Flammability limits	 No data 

Conversion factorsa	 No data 

Explosive limits	 No data 

a

Exists partially in particulate form in air. Conversion factors are only applicable for compounds that are entirely in 

the vapor phase. 
METHOXYCHLOR 143 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

5.1 PRODUCTION 

Methoxychlor was first synthesized in 1893 by the reaction of chloral hydrate with anisole in the presence 

of acetic acid and sulfuric acid. It is produced commercially by the condensation of anisole with chloral 

in the presence of an acidic condensing agent (IARC 1979; Sittig 1980). Commercial production of 

methoxychlor in the United States was first reported in 1946. In 1975, three U.S. companies produced 

approximately 5.5 million pounds of methoxychlor (IARC 1979). U.S. production in 1982 was 3 million 

pounds (EPA 1988c). No data were found for current production volume levels for methoxychlor. 

Methoxychlor is currently registered under the trade names of Prentox®, Methoxcide®, Marlate®, and 

Metox® (EPA 1988b; HSDB 1993). Currently, Kincaid Enterprises Inc. in Nitro, West Virginia is the 

sole producer and distributor of methoxychlor in the United States. Production volumes of the technical 

product in pounds were 631,550 in 1986, 672,000 in 1987, 584,000 in 1988, 476,000 in 1989, 301,235 in 

1990, and 423,832 in 1991 (Kincaid Enterprises 1992). On January 14, 2000, EPA issued a suspension 

order to Kincaid enterprises, Inc. to prevent further manufacture and sale of their methoxychlor products 

(http://www.epa.gov/oppfead1/cb/csb_page/updates/methox.html). The order affects the technical 

product and three products manufactured by Kincaid, but does not directly affect other companies that 

manufacture methoxychlor products. The order was issued because the registrant failed to submit 

overdue (per an agreement signed in September of 1998) environmental fate studies. At the time of the 

writing of this profile (September 2002), EPA is in the process of issuing a notice of intent to suspend to 

all companies that use methoxychlor in their products. 

A number of U.S. facilities use methoxychlor as a component in formulated pesticide products. 

Methoxychlor is produced commercially in the United States as a technical grade containing 88–90% of 

the pure chemical and 10–12% of impurities consisting of isomers and other reaction products (IARC 

1979; Lamoureux and Feil 1980). Twenty-five of these impurities were characterized in studies 

conducted on technical methoxychlor; evidence for more than 50 impurities was obtained through gas 

chromatography/mass spectrometry (GC/MS) (Lamoureux and Feil 1980). Purification of technical grade 

(nominally 90%) 1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane (p,p’-methoxychlor; p,p’-DMDT) by 

recrystalization gave 76% of p,p’-methoxychlor (99.8% pure by normal phase high performance liquid 

chromatography [HPLC]) and 24% impurities (West et al. 1982). The impurities were found to contain 

approximately 40 components. The major components were identified (HPLC and gas 
METHOXYCHLOR 144 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

chromatography/mass spectrometry [GC/MS]) using reference standards. Component identities and 

percent (w/w) in technical grade methoxychlor were found to be as follows: 1,1,1,2-tetrachloro2-(4-methoxyphenyl)ethane (1.73%), 1,1,1-trichloro-2-(2-methoxyphenyl)-2-(4-methoxyphenyl)ethane 

(o,p’-methoxychlor; o,p’-DMDT; 4.03%), 1,1-dichloro-2,2-di(4-methoxyphenyl)ethene (DMDE; 0.39%), 

a condensation product of p,p’-methoxychlor (p,p’-DMDT; 0.48%), a condensation product of 

o,p’-methoxychlor (o,p’-DMDT; 0.4%), and 1,2,2,2-tetrakis(4-methoxybenzyl)ethene (0.5%). These 

percentages were calculated relative to the quantity of methoxychlor contained in technical grade material 

and were found to vary depending upon manufacturing conditions. Formulations of methoxychlor for 

various uses in the United States include wettable powders, dust, granules, emulsifiable concentrates, 

flowable concentrates, liquid soluble concentrates, ready-to-use products (liquids), and pressurized liquids 

(EPA 1988a, 1988b). 

Table 5-1 lists information on U.S. companies that reported the manufacture and use of methoxychlor in 

1999 (TRI99 2001). The TRI data should be used with caution since only certain types of facilities are 

required to report. This is not an exhaustive list. 

5.2 IMPORT/EXPORT 

Data on import and export volumes of methoxychlor are limited. In 1978, 17,700 pounds of 

methoxychlor were imported (HSDB 1993). There is no information on current import volumes (HSDB 

2000). Kincaid Enterprises, Inc. reported information on export volumes of technical grade methoxychlor 

in pounds as 25,750 in 1986, 86,000 in 1987, 22,600 in 1988, 47,150 in 1989, 10,350 in 1990, and 49,750 

in 1991 (Kincaid Enterprises 1992). However, there is no information on current export volumes of 

formulated products containing methoxychlor. 

5.3 USE 

Because of its low toxicity in animals and humans, methoxychlor has been viewed as an attractive 

replacement for DDT (EPA 1988b; IARC 1979). Methoxychlor is registered as an insecticide against a 

wide range of pests, including houseflies and mosquitos, cockroaches, chiggers, and various arthropods 

commonly found on field crops, vegetables, fruits, stored grain, livestock, and domestic pets (EPA 
METHOXYCHLOR 145 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

Table 5-1. Facilities that Produce, Process, or Use Methoxychlor 

Number of Minimum amount Maximum amount 

Statea facilities on site in poundsb on site in poundsb Activities and usesc 

AR 1 10,000 99,999 13
 

OH 1 1,000 9,999 13
 

TX 1 10,000 99,999 13
 

Source: TRI99 2001 

a

Post office state abbreviations used 

b

Amounts on site reported by facilities in each state c

Activities/Uses: 

1. Produce 6. Impurity 10. Repackaging 

2. Import 7. Reactant 11. Chemical Processing Aid 

3. Onsite use/processing 8. Formulation Component 12. Manufacturing Aid 

4. Sale/Distribution 9. Article Component 13. Ancillary/Other Uses 

5. Byproduct 
METHOXYCHLOR 146 

5. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 

1988b; Verschueren 1983). The EPA has approved the use of methoxychlor as a pesticide and fumigant 

on more than 85 crops, including cranberries (EPA 1988a, 1988b). This substance can be applied to large 

areas such as beaches, estuaries, lakes, and marshes for control of fly and mosquito larvae by aerial 

application (EPA 1988b). Other uses include the spray treatment of barns, grain bins, mushroom houses, 

and other agricultural premises and the spraying or fogging of garbage containers, sewer manholes, and 

sewage disposal areas (EPA 1988b). 

Annual usage of methoxychlor in the United States was estimated to range from 500,000 to 

900,000 pounds in 1986 (EPA 1988b). Use patterns for methoxychlor, which is a General Use Pesticide 

(GUP) that is available over the counter, have remained fairly constant since 1974. At that time, home 

and garden applications constituted 30% of usage; livestock and poultry, 15%; alfalfa, 10%; soybeans, 

10%; forests, 10%; ornamental shrubs, 10%; deciduous fruits and nuts, 5%; and vegetables, 5% (IARC 

1979). Likewise, in 1988, EPA reported that predominant use of methoxychlor in the United States was 

as an insecticide for alfalfa, livestock, home orchards, and ornamentals. These applications accounted for 

a total of 37.3% of the total U.S. annual usage (EPA 1988b). More recent information indicates that 

approximately 28% is used for home and garden purposes, 15% for industrial and commercial purposes, 

and 57% for agricultural purposes (Kincaid Enterprises 1992). 

5.4 DISPOSAL 

Methoxychlor and waste containing methoxychlor are classified as hazardous wastes by EPA. Generators 

of waste containing this contaminant must conform to EPA regulations for treatment, storage, and 

disposal (see Chapter 8). The recommended method for disposal of methoxychlor wastes is incineration 

with scrubbing (HSDB 1993; Sittig 1980). Empty containers may be returned to manufacturers for reuse 

or buried in designated landfills away from water supplies. Small quantities of waste methoxychlor may 

be landfilled (HSDB 1993). 

According to the Toxics Release Inventory (TRI99 2001), about 14 pounds of methoxychlor were 

transferred to landfills and/or treatment/disposal facilities in 1999 (see Table 5-1). No information was 

located on disposal trends or past disposal methods. 
METHOXYCHLOR 147 

6. POTENTIAL FOR HUMAN EXPOSURE 

6.1 OVERVIEW 

Methoxychlor has been identified in at least 58 of the 1,613 hazardous waste sites that have been 

proposed for inclusion on the EPA National Priorities List (NPL) (HazDat 2002). However, the number 

of sites evaluated for methoxychlor is not known. The frequency of these sites can be seen in Figure 6-1. 

Methoxychlor is released to the environment mainly as a result of its application to crops and livestock as 

a pesticide. Smaller amounts may be released during its production, formulation, storage, shipment, and 

disposal. There are no known natural sources of methoxychlor. 

In air, methoxychlor exists in both the particulate and, to a small degree, vapor phase. Air levels exhibit 

seasonal variations, paralleling its use for pest control. Levels ranging from 0.002 to 0.6 ng/m3

 have been 

reported in ambient air in Canada and the United States. Methoxychlor may react with photochemicallyproduced hydroxyl radicals in air, but most is probably removed from the atmosphere by wet and dry 

deposition processes. The residence time for methoxychlor in the atmosphere is <1 month. In water, 

methoxychlor preferentially binds to sediments and organic matter, although some methoxychlor may 

remain dissolved in water. Methoxychlor bioconcentrates in a number of aquatic organisms including 

microorganisms, snails, clams, and some fish (see Section 6.3.1), but probably does not accumulate in 

mammalian species due to rapid metabolism and elimination. Methoxychlor in water and sediment is 

degraded to dechlorinated, dehydrochlorinated, and demethylated products by chemical, photochemical, 

and biological processes. Depending on the availability of sunlight, air, plant materials, and 

microorganisms, the half-life of methoxychlor in water may range from 2 to 5 hours to approximately 

1 year. Both its degradation and its affinity for sediments and organic matter may explain, in part, why 

methoxychlor is generally not detected in surface water or groundwater in the United States. However, 

methoxychlor may be detected in waters near release sources. 

Methoxychlor binds tightly to soils, but is not usually detectable in soil except in areas where it has been 

applied as a pesticide. Wind and rain can erode contaminated soils, resulting in the migration of 

methoxychlor-containing particulates. Some methoxychlor can persist in soils for more than a year after 

its application. However, most is degraded to dechlorinated, dehydrochlorinated, and demethylated 

products. The degradation of methoxychlor is mediated by microorganisms and is affected by the level 
Figure 6-1. Frequency of NPL Sites with Methoxychlor Contamination 

1 2 3 4 5 

Frequency of

NPL Sites 

Derived from HazDat 2002 
METHOXYCHLOR 149 

6. POTENTIAL FOR HUMAN EXPOSURE 

of aeration of the soil. Half-lives of <30 days in anaerobic soils (Fogel et al. 1982; Muir and Yarechewski 

1984) and >100 days in aerobic soils were observed (Muir and Yarechewski 1984) for methoxychlor. In 

a study of the effect of methoxychlor on soil microflora, methoxychlor was found to be moderately 

persistent (30 days<t1/2<1 year) (Howard et al. 1991) in soil under anaerobic conditions. Six months after 

the application of methoxychlor to the soil, 42% remained (Polyakova et al. 1984). 

Methoxychlor is generally not detected in foods, although low levels (ranging from 0.001 to 0.004 mg/kg) 

have occasionally been detected in dairy products, grains, fruits, and vegetables. Higher levels (ranging 

from 10 to 120 μg/kg) have been infrequently reported in fish (Camanzo et al. 1987; Devault 1985). 

Most members of the general population have little or no exposure to methoxychlor. People who use 

products containing methoxychlor in farming, home gardening, professional landscaping, or animal care 

are more likely to be exposed to methoxychlor. 

6.2 RELEASES TO THE ENVIRONMENT 

Methoxychlor has been identified in a variety of environmental media (air, surface water, leachate, 

groundwater, soil, and sediment) collected at 58 of 1,613 current or former NPL hazardous waste sites 

(HazDat 2002). 

According to the Toxics Release Inventory (TRI), methoxychlor processing facilities listed for 1998 

(TRI99 2001) report that the major portion of methoxychlor released to the environment is released to the 

land. Table 6-1 lists releases to the environment in 1999 from facilities that manufacture or process 

methoxychlor. One facility that processes methoxychlor reports that 29 pounds were released to the air 

and none was released to land or water. Only certain types of facilities are legally required to report; 

therefore, this is not an exhaustive list. 

The TRI data should be used with caution because only certain types of facilities are required to report. 

This is not an exhaustive list. 

6.2.1 Air 

Release of methoxychlor to the atmosphere occurs mainly as a result of its use as a pesticide. However, 

no data were located on the amount of methoxychlor released to air by this route. Releases to the 
METHOXYCHLOR 150 

6. POTENTIAL FOR HUMAN EXPOSURE 

Table 6-1. Releases to the Environment from Facilities that Produce, Process, or Use
 

Methoxychlor
 

Reported amounts released in pounds per yeara 

Number Total Total Total on and 

Stateb 

of 

facilities Airc Water 

Underground 

injection Land 

on-site 

released 

off-site 

releasee 

off-site 

release 

AR 1 29 No data No data No data 29 14 43 

OH 1 0 0 No data No data 0 0 0 

TX 1 0 No data No data No data 0 No data 0 

Total 3 29 0 0 0 29 14 43 

Source: TRI99 2001 

a

Data in TRI are maximum amounts released by each facility.

b

Post office state abbreviations are used.
 c

The sum of fugitive and stack releases are included in releases to air by a given facility. d

The sum of all releases of the chemical to air, land, water, and underground injection wells
 e

Total amount of chemical transferred off-site, including to publicly owned treatment works (POTW)
 
METHOXYCHLOR 151 

6. POTENTIAL FOR HUMAN EXPOSURE 

atmosphere during production, formulation, and disposal of methoxychlor have been estimated to be 

0.1 kg/metric ton produced (HSDB 2000). Based on the 423,832 pounds of methoxychlor produced in 

the United States in 1991 (Kincaid Enterprises 1992), atmospheric release during production may be 

estimated to be 42 pounds (19 kg). According to TRI (TRI99 2001), three processing facilities in the 

United States reported the release of 29 pounds of methoxychlor to the air in 1999. The TRI data 

(Table 6-1) should be used with caution since only certain types of facilities are required to report; 

therefore, this is not an exhaustive list. Methoxychlor has been identified in 1 air sample collected from 

1,613 current or former NPL hazardous waste sites where it was detected in some environmental media 

(HazDat 2002). 

Methoxychlor has been detected in air samples taken during monitoring studies conducted during the 

years 1986–1988 in the Jacksonville, Florida (EPA 1990e) and seasonal variation was reported. The 

mean outdoor concentration of methoxychlor was highest in winter months (0.1 ng/m3

; detection limits 

not reported); methoxychlor was not present at detectable levels during the spring and summer months. 

6.2.2 Water 

Methoxychlor can be released directly to surface waters on farms when used to control larvae of insects 

(Stoltz and Pollock 1982). Methoxychlor is approved for use on cranberries (EPA 1988b), which are 

grown in bogs, and therefore methoxychlor could be released directly to surface waters where cranberries 

are grown. Methoxychlor may be released to water from runoff from soil containing methoxychlor, 

industrial effluents or from leaks at storage and waste sites. According to TRI (TRI99 2001), three 

processing facilities in the United States reported that no methoxychlor was released to water in 1999. 

The TRI data (Table 6-1) should be used with caution since only certain types of facilities are required to 

report. This is not an exhaustive list. Methoxychlor has been identified in 19 groundwater and 7 surface 

water samples collected from 58 NPL hazardous waste sites, where it was detected in some environmental 

media (HazDat 2002). 

6.2.3 Soil 

Methoxychlor is released to soils primarily through its use as an insecticide for agricultural crops, home 

orchards, and ornamentals. Some methoxychlor may be released to soils through leaks at storage waste 

sites. According to TRI (TRI99 2001), three processing facilities in the United States reported that no 
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6. POTENTIAL FOR HUMAN EXPOSURE 

methoxychlor was released to soil in 1999. The TRI data should be used with caution since only certain 

types of facilities are required to report; therefore, this is not an exhaustive list. 

Methoxychlor has been identified in 46 soil and 11 sediment samples collected from 58 NPL hazardous 

waste sites, where it was detected in some environmental media (HazDat 2002). 

6.3 ENVIRONMENTAL FATE 

This discussion applies primarily to p,p’-methoxychlor, the major component in technical grade 

methoxychlor. Please see Section 4.1 Chemical Identity for a detailed discussion of impurities and 

contaminants in technical grade methoxychlor, which is the primary form used for pesticide formulation. 

Little information is available on the environmental fate of impurities and contaminants in technical grade 

methoxychlor. Methoxychlor can contain as many as 50 contaminants (impurities) that are introduced 

during commercial production. The contaminants include 1-chloro-1,2,2-tris[4-methoxyphenyl]ethene 

(chlorotrianisene; TACE), a triphenylethylene derivative that exhibits estrogenic/anti-estrogenic 

characteristics. Other contaminants include polycylic hydrocarbons (e.g., 3,6-dimethoxy-9,10-bis 

(p-methoxyphenyl)phenanthrene, tetrakis(p-methoxyphenyl)ethylene, and 3,6,11,14-tetramethoxydibenzo(g,p)chrysene), which have been studied for mutagenicity and putative carcinogenicity 

(Grant et al. 1976). 

6.3.1 Transport and Partitioning 

Methoxychlor is expected to exist in both the particulate (bound to particulate matter) and, to a small 

degree, vapor phase in the atmosphere (Kelly et al. 1994). The residence time and dispersion of 

methoxychlor in air is, therefore, a function of particle size, windspeed, and precipitation. Based on 

monitoring data, the majority of methoxychlor is removed by wet or dry deposition processes with a 

residence time of <1 month (Hoff et al. 1992). However, evidence of wide dispersion of methoxychlor in 

the atmosphere by its detection in Canadian arctic snow suggests that some methoxychlor may remain in 

air for extended periods of time (Welch et al. 1991). Methoxychlor has been frequently detected in rain 

(Strachan 1985, 1988). In a 6-year study (1986–1991) conducted in the Great Lakes Region, the mean 

annual concentration of methoxychlor in rain was 2.4 ng/L (Chan et al. 1994). These data suggest that 

wet deposition processes significantly contribute to the removal of methoxychlor from the atmosphere. 

However, wet deposition of methoxychlor will depend upon the amount of precipitation and will vary 

from year to year. Dry deposition due to gravity will also act to remove methoxychlor from air. In the 
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6. POTENTIAL FOR HUMAN EXPOSURE 

Great Lakes region, dry deposition of chlorinated pesticides was estimated to be 1.5–5.0 times as great as 

wet deposition (Eisenreich et al. 1981). 

Methoxychlor is a relatively hydrophobic compound with estimated log octanol/water partition 

coefficient (log Kow) values ranging from 4.68 to 5.08 (Howard 1991). Because of this, methoxychlor in 

water is expected to partition mainly to sediment and organic matter, although a significant fraction may 

remain in solution when the ratio of sediment mass to water volume is low (Wolfe et al. 1977). In 

sediments, the partitioning of methoxychlor was higher for silts and clays than it was for sand (Karickhoff 

et al. 1979). Sorption of methoxychlor to bacteria, algae, and fungi has been reported (Paris and Lewis 

1976). Methoxychlor has an estimated vapor pressure of 1.4x10-6 mmHg (Howard 1991) and an 

estimated Henry's law constant of 1.6x10-5 atm-m3

/mole (Howard 1991). These values indicate that 

volatilization of methoxychlor from water may occur. A half-time of 4.5 days has been estimated for the 

volatilization of methoxychlor from a shallow river (Howard 1991). In addition, methoxychlor has been 

observed to slowly volatilize from foliage (Howard 1991). This process may contribute to the 

environmental cycling of methoxychlor. 

Experimental adsorption coefficients (Koc) that have been reported for methoxychlor are as follows: 

9,700–41,000 in sand, 80,000–100,000 in fine silt, and 73,000–92,000 in clay (Kenaga 1980; Prasad 

1992). Because methoxychlor has a relatively high Koc value, for example, an experimental Koc of 

79,433 was reported by Montgomery and Welkom (1990), methoxychlor has the potential to undergo 

significant adsorption to soils, especially those with high organic carbon content (Karickhoff et al. 1979; 

Richardson and Epstein 1971). Methoxychlor retention is also greater in finer-textured soils (particle size 

<0.08 μm) than in course-textured soils (particle size $0.08 μm) (Richardson and Epstein 1971). The 

mobility of methoxychlor may be higher in sandy soils, since adsorption was significantly less in soil 

with lower organic carbon content and larger particle size (Karickhoff et al. 1979; Richardson and Epstein 

1971). In addition to soil adsorption, methoxychlor may become structurally bound to soil humic 

materials (Mathur and Morley 1978). 

Methoxychlor is generally detected only in the uppermost layer (top 5 cm) of soil on which it was applied 

(Golovleva et al. 1984). Dechlorinated, dehydrochlorinated, and demethylated degradation products of 

methoxychlor were generally detected in lower levels of soil, suggesting that they are more mobile than 

methoxychlor (Golovleva et al. 1984). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

The bioconcentration of methoxychlor has been investigated in microorganisms, lower invertebrates, and 

in fish. Because methoxychlor has a relatively high octanol/water partition coefficient, it is expected to 

have potential for bioconcentration. Reported bioconcentration factors (BCFs) for methoxychlor were 

411–2,758 in Aerobacter aerogenes bacteria, 2,114–8,138 in Bacillus subtilis bacteria (Johnson and 

Kennedy 1973), 348–1,130 in stoneflies, 5,000–8,570 in snails (Anderson and DeFoe 1980), and 1,500 in 

clams (Hawker and Connell 1986). Methoxychlor has been reported to bioaccumulate in phytoplankton, 

and levels up to 80 μg/kg have been reported in samples from Lake Erie, Ontario. In a model ecosystem, 

BCFs of 1,500 for fish and 120,000 for mosquitos were reported (Kapoor et al. 1970). BCFs of 185 for 

fish (flowing water) and 1,550 for trout (static water) have also been reported (Prasad 1992). In 

sheepshead minnows, BCFs were found to be concentration dependent, ranging from 113 at 3 μg/L to 264 

at 23 μg/L (Parrish et al. 1977). In fathead minnows, a BCF of 8,300 was reported (Veith et al. 1979). 

These data suggest that considerable species variation exists in the bioconcentration of methoxychlor in 

fish, perhaps as the result of species differences in the capacity to metabolize and eliminate methoxychlor. 

Methoxychlor has been reported to bioaccumulate in the blubber (0.68 μg/kg wet weight) and liver 

(0.1 μg/kg wet weight) of harp seals (Zitko et al. 1998). The primary source of methoxychlor 

contamination in seals is thought to be their diet, which consists largely of fish. 

The persistence and disappearance (washoff or dryfall) of methoxychlor from mature soybean foliage was 

investigated in a small field plot study under natural rainfall conditions in 1977 and 1978 (Smith et al. 

1981). Methoxychlor washoff rate was 8±4% of the amount on plants (prior to rain) per cm of rainfall, 

regardless of time after application. Total seasonal washoff for 1978 accounted for 33.5% of the applied 

pesticide; however, 30.5% of the total loss was removed by washoff on the second day after application. 

Dryfall or dislodgeable residue accounted for <1% of the amount applied. The amount of dryfall was 

greater in plots entered by workers than in those where entry was avoided. 

6.3.2 Transformation and Degradation 

The structures of many of the methoxychlor degradation products mentioned here are shown in 

Figure 3-2, Proposed Metabolic Pathways of Methoxychlor. 
METHOXYCHLOR 155 

6. POTENTIAL FOR HUMAN EXPOSURE 

6.3.2.1 Air 

Methoxychlor is expected to exist in both the particulate and, to a small degree, vapor phase in the 

atmosphere (Kelly et al. 1994). The rate constant for the vapor-phase reaction of methoxychlor with 

photochemically-produced hydroxyl radicals has been estimated to be 5.2x10-11 cm3

/molecule-second at 

25 EC using a structure estimation method (Meylan and Howard 1993). This corresponds to an 

atmospheric half-life of about 7 hours at an atmospheric concentration of 5x105

 hydroxyl 

radicals/cm3

 (Meylan and Howard 1993). Particulate-phase methoxychlor is not expected to react with 

photochemically produced hydroxyl radicals and, therefore, will not be readily degraded in the 

atmosphere. 

6.3.2.2 Water 

Methoxychlor can be degraded in water by chemical, photochemical, and biological processes. 

Methoxychlor undergoes a spontaneous elimination reaction in aqueous solution to yield 

dehydrochlorinated products, including 1,1-bis(4-methoxyphenyl)-2,2-dichloroethylene (a proestrogenic 

derivative of methoxychlor discussed in Section 3.4.3; also known as methoxy-DDE). The half-life for 

the degradation of methoxychlor by this process was estimated to be approximately 1 year (Wolfe et al. 

1977). Methoxychlor may also be oxidized by hydroxyl radicals or ozone in ozonated waters (Haag and 

Yao 1992; Yao and Haag 1991). The half-life of methoxychlor when reacted with ozone was estimated to 

be 2.1 minutes. 

Methoxychlor is photochemically degraded by sunlight through loss of one chlorine atom to form a 

radical intermediate, which rearranges to the more stable 1,1-bis(4-methoxyphenyl)-2,2-dichloroethylene 

(also known as MDDE) (Zepp et al. 1976). A dramatic difference in half-life was observed for the 

photochemical degradation of methoxychlor in distilled water (4.5 months) and natural water (2–5 hours). 

Methoxychlor was found to form adducts extensively with plant materials via a photochemically-induced 

radical mechanism (Schwack 1988). This observation may explain the dramatic differences in the halflife of methoxychlor in distilled and natural waters. 

DDE and 1,1-bis(4-methoxyphenyl)-2,2-dichloroethylene, respective degradation products of DDT and 

methoxychlor, rapidly undergo an unusual photoisomerization in solution when exposed to sunlight 

(Zepp et al. 1977). The isomerization involves the exchange of a vinyl chlorine and an ortho aromatic 
METHOXYCHLOR 156 

6. POTENTIAL FOR HUMAN EXPOSURE 

hydrogen. Other photodegradation products identified were corresponding benzophenones and 1,1-bis(4­

methoxyphenyl)-2-chloroethylene. 

Methoxychlor is also degraded by bacteria under aerobic conditions to form dechlorinated products 

(Baarschers et al. 1982). Algae are also capable of degrading methoxychlor; however, the degradation 

products were not determined (EPA 1976b). The half-life of methoxychlor by this degradation process 

was reported to be <2 weeks with some organisms. Data regarding the biodegradation of methoxychlor in 

water under anaerobic conditions were not located. However, based on biodegradation of methoxychlor 

in soil under anaerobic conditions (see below), transformation via this process is likely to occur. 

6.3.2.3 Sediment and Soil 

First-order kinetics have been used to model the dissipation of methoxychlor in the environment (under 

anaerobic and aerobic conditions) because a half-life for the chemical can be defined. The half-life 

represents the calculated time for loss of the first 50% of the substance, but the time required for the loss 

of half of that which remains may be substantially longer, and the rate of disappearance may decline 

further as time progresses. The term half-life in this document is used to indicate the estimated time for 

the initial disappearance of 50% of the compound and does not necessarily imply that first-order kinetics 

were observed throughout the experiment unless otherwise noted. 

Half-lives of <30 days in anaerobic soils (Fogel et al. 1982; Muir and Yarechewski 1984) and >100 days 

aerobic soils were observed (Muir and Yarechewski 1984) for methoxychlor. In a study of the effect of 

methoxychlor on soil microflora, methoxychlor was found to be moderately persistent 

(30 days<t1/2<1 year) (Howard et al. 1991) under anaerobic conditions. Six months after the application 

of methoxychlor to the soil, 42% remained (Polyakova et al. 1984). 

In soils and sediments, methoxychlor can be biodegraded under either aerobic or anaerobic conditions. 

Biodegradation of methoxychlor has been reported to be greater under anaerobic conditions than aerobic 

conditions; biodegradation of the compound has been observed to be more rapid as conditions become 

more strongly reducing (i.e., more anaerobic). For anaerobic biodegradation in flooded sediment and 

nitrogen atmosphere (reducing environment), methoxychlor degraded with a half-life of <28 days 

compared with half-lives of 49–50 days in flooded sediment maintained in static aerobic conditions and 

115–206 days in aerobic (artificially aerated, oxidizing environment) flooded sediment (Fogel et al. 1982; 

Muir and Yarechewski 1984). The major degradation products consisted of 1,1-dichloro­
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6. POTENTIAL FOR HUMAN EXPOSURE 

2,2-di(4-methoxyphenyl)ethene (DMDE), 1,1-dichloro-2,2-bis(4-methoxyphenyl)ethane (DMDD), 

1-chloro-2,2-bis(4-methoxyphenyl)ethene (DMDU), 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane, 1,1,1-trichloro-2,2-bis(4-hydroxyphenyl)ethane, 1,1-dichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane, 1,1-dichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethene, and 

1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethene (Muir and Yarechewski 1984). In another study, 

methoxychlor was found to degrade by >95% over 160 days in flooded sediment (anaerobic conditions; 

reducing environment) and by 70% over 185 days in aerated soil (aerobic conditions; oxidizing 

environment) Golovleva et al. (1984). Out of 709 strains of microorganisms found in soils, 17 were able 

to convert methoxychlor to 1,1-dichloro-2,2-bis(4-methoxyphenyl)ethene (DMDE), and 5 were able to 

convert it to nonchlorinated products (Golovleva et al. 1984). Some of the degradation products resulting 

from partial dechlorination of methoxychlor may accumulate in soils (degradation products were 

identified by chromatography/MS analysis; Golovleva et al. 1984). Data regarding the photodegradation 

of methoxychlor in surface soils were not located, but based on the photodegradability of methoxychlor in 

water (see Section 6.3.2.2) and the photodegradation of a structural analogue (ethoxychlor) in soil (Coats 

et al. 1979), this process is likely to occur, but only at the very surface of soil. Because methoxychlor is 

mostly found in the upper layer of soil, the photochemical and aerobic degradative processes would 

probably be more important for methoxychlor applied to crops. 

6.3.2.4 Other Media 

The degradation of methoxychlor has been studied under anaerobic conditions (incubation time = 

48 hours) in microbial suspensions (Van Duck and Van de Voorde 1976). The bacterial strains studied, 

Hydrogenomonas sp., Mycoplana bullata, Mycoplana dimorpha, Pseudomonas aeruginosa, Bacillus 

subtilis, Candida albicans, and Escherichia coli were found to be capable of degrading 85% of the 

methoxychlor at concentration levels of 20,000–160,000 μg/L. The microbial degradation of 

methoxychlor has been studied under anaerobic and aerobic conditions using four cultures: Bacillus 

sphaericus, Rhodoccoccus erythropolis, and Acinetobacter calcoaceticus 5 and 21 (Golovleva et al. 

1984). All four cultures were found to degrade methoxychlor at a concentration of 

100,000–200,000 μg/L. The most extensive degradation was observed to occur under anaerobic 

conditions (incubation time=7 days; degradation=75%) compared to aerobic conditions (incubation 

time=7 days; degradation=18%). In another study of the degradation of methoxychlor by the bacteria, B. 

sphaericus, methoxychlor was shown to be degraded under anaerobic and aerobic conditions (Polyakova 

et al. 1984). The bacteria degraded methoxychlor (at a concentration of 50,000 μg/L) more completely 
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6. POTENTIAL FOR HUMAN EXPOSURE 

under anaerobic conditions (incubation time = 10 days; degradation=83%) compared to aerobic 

conditions (incubation time=10 days; degradation=30%). 

6.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT 

6.4.1 Air 

In a survey (conducted during the years 1987, 1988, and 1989) of pesticide levels in air in two U.S. cities, 

the mean levels of methoxychlor in indoor, outdoor, and personal air samples from Jacksonville, Florida 

were 200–300, 0–100, and 100–600 pg/m3

, respectively (EPA 1990e). Levels of methoxychlor were 

below the level of detection (approximately 36 pg/m3

) in these air samples in Springfield, Massachusetts. 

In a survey of ambient air measurements, atmospheric levels of methoxychlor (from data taken at two 

locations in the United States from 301 samples) ranged from not detected to 7,000 pg/m3

 (Kelly et al. 

1994). In Canada, the yearly mean level of methoxychlor in air was 1.7 pg/m3

 from 1988 to 1989 (Hoff 

et al. 1992). Air levels tended to be higher during insect control periods (up to 27 pg/m3

), whereas levels 

were generally below the detection limit (0.04–0.1 pg/m3

) during non-use periods. In the arctic, mean 

concentrations of methoxychlor in air samples were found to be 0.07–0.72 pg/m3

 for the year 1993 and 

0.18–1.43 pg/m3

 for the year 1994 (Halsall et al. 1998). No data were found for methoxychlor 

degradation products and their levels in air. 

6.4.2 Water 

Methoxychlor is not commonly detected in surface, ground, or drinking waters. In a survey of major 

rivers in the United States, methoxychlor was not detected (detection limit =100 ng/L) at approximately 

180 sites (Gilliom et al. 1985). Methoxychlor was occasionally detected in surface waters from the Great 

Lakes regions at concentrations ranging from 0.032 to 15 ng/L (Biberhofer and Stevens 1987; 

Konasewich et al. 1978; Kuntz and Warry 1983; Maguire et al. 1983). In a survey of 783 rural domestic 

wells and 566 community water systems across the United States, methoxychlor was generally not found 

above the reporting limit (300 ng/L) (EPA 1990f). Methoxychlor was not found above the detection limit 

(5,000 ng/L) in 54 wells in California (Maddy et al. 1982), was not detected above the detection limit 

(5–10 ng/L) in groundwater below irrigated farmland in Nebraska (Spalding et al. 1980), and was not 

detected above the detection limit (150 ng/L) in groundwater tested from 53 residential drinking wells in 

residential areas (non-agricultural past use) of Connecticut where home and garden pesticide use is 
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6. POTENTIAL FOR HUMAN EXPOSURE 

reported by 66% of homeowners (Eitzer and Chevalier 1999). In a survey of the drinking water in 

Jacksonville, Florida and Springfield, Massachusetts, methoxychlor was not detected in any samples 

(EPA 1990e). In the Great Lakes region, methoxychlor has been detected in rain and snow at 

concentrations ranging from 0.43 to 13.1 and 0.1 to 5.8 ng/L, respectively (Strachan 1985, 1988; Strachan 

and Huneault 1979). Methoxychlor has also been detected in snow in the Canadian arctic at levels of 

0.2 ng/L (Welch et al. 1991). In an irrigation canal in which methoxychlor was intentionally added to 

control fly larvae, peak levels of 1,000 ng/L were detected at a site nearly 75 miles downstream, 

45–46 hours after application (Stoltz and Pollock 1982). In another study, conducted during 1981–1986, 

methoxychlor residues were monitored in the Athabasca River in Canada after treatment for control of 

black fly larvae (Kumar and Byrtus 1993). Peak levels of 5,000 ng/L were detected (detection 

limit=0.03 μg/L) 47 miles downstream of the treatment location. Methoxychlor has been detected at high 

concentrations in waters near points of methoxychlor use or application. Shortly after the aerial 

application of methoxychlor to elm trees, levels of 40–160 mg/L were detected downstream in the top 

foot of a nearby river (Wallner et al. 1969). Methoxychlor was not detected (detection limit not reported) 

100 feet downstream after 24 hours. Methoxychlor has also been detected in groundwater at waste 

disposal sites. A review of groundwater monitoring data from 479 site investigations indicates that 

methoxychlor was detected in groundwater at 14 (.3%) of the sites (Plumb 1991). No data were found 

for methoxychlor degradation products and their levels in water. 

6.4.3 Sediment and Soil 

In general, methoxychlor is infrequently detected in soils and sediments in the United States, but higher 

levels may be detected more frequently near release sources. In a survey of soils in the United States, 

methoxychlor was detected in 1 of the 1,729 cropland soil samples at a level of 0.28 μg/kg (IARC 1979). 

Methoxychlor was detected in 1 of the 45 random soil samples in Alabama at a level <100 μg/kg 

(Albright et al. 1974). The detection limit was not reported. At a hazardous waste site in Fresno, 

California, methoxychlor was detected at levels ranging from <150 to 17,000 μg/kg in subsurface soil 

(Agency for Toxic Substances and Disease Registry 1989a). Soil beneath elm trees sprayed with 12–16% 

methoxychlor was reported to contain 2,900–14,600 μg/kg 3 days after application, and 

1,000–8,400 μg/kg 130 days later (Wallner et al. 1969). These data suggest that methoxychlor is 

moderately persistent (30 days<t1/2<1 year) (Howard et al. 1991) in soil. In a survey of major rivers in the 

United States, methoxychlor was detected in sediment samples from only 1 out of 160 sites (detection 

limit =1 μg/kg) (Gilliom et al. 1985). In contrast, 51% of 70 sediments samples from the Niagara River 

contained detectable levels of methoxychlor, with a mean level of 7 μg/kg (standard deviation=14 μg/kg) 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

dry weight (Kuntz and Warry 1983). No data were found for methoxychlor degradation products and 

their levels in soil or sediment. 

6.4.4 Other Environmental Media 

Methoxychlor is generally not detectable or detectable only at very low levels in food. Methoxychlor was 

reported in only 5% of nearly 14,000 composite food samples obtained from 10 states in 1988, and in 

11 of 13,000 samples in 1989 (Minyard and Roberts 1991). However, the levels of methoxychlor 

detected were not provided. In a market basket survey performed in 1980–1982, dairy products and 

cereals/grain products contained levels ranging from a trace to 4 μg/kg (Gartrell et al. 1986a). 

Methoxychlor was infrequently (0.3%) encountered in vegetables in the United States in 1969–1976, with 

a mean level of 1 μg/kg (Duggan et al. 1983). In a study conducted by the Food and Drug Administration 

(FDA) for incidence/level monitoring of domestic and imported fruits and vegetables, methoxychlor was 

not detected in tomatoes or pears (Roy et al. 1995). In Canada from 1980 to 1985, methoxychlor was 

generally not detected in vegetables, fruits, meats, or dairy products (Davies 1988; Frank et al. 1987b). 

However, low levels (4 μg/kg) were detected in strawberries (Frank et al. 1987a). Fish from the Great 

Lakes generally did not contain detectable levels of methoxychlor, but occasionally, some samples 

contained levels ranging from 10 to 120 μg/kg wet weight (Camanzo et al. 1987; Devault 1985). Giesy et 

al. (1994) studied several species of migratory fish in rivers above and below the Great Lakes. 

Methoxychlor was detected at levels ranging from not detectable to 1.4 μg/kg. Fish (bluegill and carp), 

taken from rivers that are downstream from irrigated farmland in California, did not contain detectable 

levels (limit of detection=0.04 mg/kg) of methoxychlor (Saiki and Schmitt 1986). 

Methoxychlor has been detected in the blubber and liver of harp seals (Zitko et al. 1998). Levels detected 

were 0.68 μg/kg (wet weight) in the blubber and 0.1 μg/kg (wet weight) in the liver of male harp seals. 

Seal muscle tissue is used for human consumption, and there is a growing market for seal meat. The 

primary source of methoxychlor contamination in seals is thought to be their diet, which consists largely 

of fish. 

Methoxychlor was detected on grain storage building surfaces in which methoxychlor was used for insect 

control, and this resulted in detectable levels in grain (NIOSH 1986; Watters and Grussendorf 1969). 

In a survey of methoxychlor residues in house dust in 28 homes in an agricultural area of Colorado where 

pesticide use was common, levels (limit of detection for methoxychlor was not reported) ranged from 1.6 
METHOXYCHLOR 161 

6. POTENTIAL FOR HUMAN EXPOSURE 

to 103 mg/kg (mean=14.9 mg/kg) in 8% of the homes of farmers, from 1.9 to 144 mg/kg 

(mean=18.2 mg/kg) in 9% of the homes of pesticide formulators, and from 1.5 to 29 mg/kg in 2% of the 

homes of the control group (Starr et al. 1974). It is not clear if the presence of methoxychlor in the homes 

of the control group was due to migration of methoxychlor from nearby buildings or fields where it was 

applied by farmers, or to in-house use of methoxychlor-containing products by the residents. 

6.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE 

For the general population, the most likely source of methoxychlor exposure is from low-level 

contamination of food. In a study of exposure of the general population to chemical contaminants in 

food, the average daily exposure to methoxychlor was estimated by calculating exposures from individual 

foods and summing across food types (Dougherty et al. 2000). The sources of data for contaminant 

concentrations included the USDA Pesticide Data Program (for years 1992–1993 covering fruits and 

vegetables), FDA Total Diet Study (for years 1988–1993 covering 261 foods), USEPA Dioxin Report and 

USEPA/USDA Dioxin in Beef Study (for years 1989–1991, covering meat, dairy products, pork, chicken, 

milk, and eggs), and National Sediment Inventory (for years 1988–1993, covering fish and shellfish). 

Methoxychlor was detected in 93 of 29,180 samples and exposure to methoxychlor was estimated to be 

5 ng/kg/day (nondetects = 0) and 70 ng/kg/day (nondetects = one half the limit of detection). The 

5 ng/kg/day value is larger than the value calculated in the FDA’s Total Diet Study Program, which may 

be attributed to higher concentrations of methoxychlor in raw fish than in other foods. The FDA’s Total 

Diet Study program monitors chemical contaminants in the U.S. food supply and has calculated average 

daily intakes of methoxychlor (the limit of detection was not reported) in adults (age 25–65) ranging from 

0.1 to 0.3 ng/kg/day for the period 1986–1991 (Gunderson 1995b), 0.6–0.9 ng/kg/day for the period 

1984–1986 (Gunderson 1995a), and 4 ng/kg/day for the period 1980–1982 (Gartrell et al. 1986b). A 

decrease in the average daily intakes of methoxychlor is noted for period 1980–1991. Average daily 

intakes of methoxychlor for infants and children were also monitored (see Section 6.6). Exposure to 

methoxychlor from food may be elevated in persons who consume large amounts of fish and seafood 

from methoxychlor-contaminated waters. Because methoxychlor is usually not detected in ground or 

surface water sources, exposure to methoxychlor from drinking water is not expected to be significant for 

the general population. Based on the results of the Non-occupational Pesticide Exposure Study, 

inhalation exposure to methoxychlor ranged from 0.002 to 0.012 μg/day in one U.S. city (EPA 1990e). 

In a monitoring study of residential and commercial air and dust samples for the occurrence of air born 

endocrine disruptors, methoxychlor was detected in four out of six dust samples at 0.6–3.5 μg/g of dust. 

A methoxychlor metabolite, 2,2-bis (p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE) was also detected at 
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6. POTENTIAL FOR HUMAN EXPOSURE 

0.14 μg/g in three of the four dust samples that contained methoxychlor. Methoxychlor was not reported 

to be detected in collected air samples. Exposures may be greater in individuals who use methoxychlorcontaining products for home gardening or animal-care purposes. In a monitoring study of 

nonoccupational exposure to pesticides used in and around the home, methoxychlor was detected (air 

concentrations were not provided) in both indoor and outdoor samples (Lewis et al. 1988). Inhalation and 

dermal exposures may be greater for workers and farmers exposed to methoxychlor. The National 

Occupational Exposure Survey of 1981–1983 estimated that approximately 3,418 workers (agricultural 

services and personal services) were exposed to methoxychlor in the United States in 1980 (NIOSH 

1992b). However, this number does not include farmers who are exposed by using methoxychlor on their 

crops and livestock. No quantitative data were located regarding blood, tissue, or urine levels of 

methoxychlor following occupational exposure to methoxychlor. 

The occurrence of methoxychlor in blood serum was studied in 39 men of varying occupations and 

similar age and weight, living Southern Honduras (Steinberg et al. 1989). Of the population studied, 

20 men, representing the test group, lived and worked in and near farming cooperatives where pesticide 

use is extensive (16–30 times/year), and 19 of the men, representing a comparison group, lived in an area 

where pesticides are applied only once per season. The blood serum for all participants was analyzed 

using HPLC and GC/MS (EC detection; detection limits not specified). Methoxychlor was detected in 

the serum of one of the comparison group at 5.16 mg/L, but none was detected at or above the detection 

limit in the remaining participants. The absence of detectable levels of methoxychlor in serum of the 

majority of the participants may suggest a low exposure to the pesticide; the detection limit was 

0.24–4.07 mg/L. However, since the majority of the participants were of lean weight (145 pounds; low 

adipose tissue), it may also suggest that in the absence of adipose, methoxychlor is not stored in the body. 

In a study of human adipose tissue (taken from cadavers at autopsy) conducted in Kingston and Ottawa, 

Ontario, Canada, methoxychlor was not detected (using gel permeation chromatography and GC/MS) at 

the minimum level of detection of 13.5 ng/g of adipose (LeBel and Williams 1986). However, it could 

not be determined from the study whether any of the deceased individuals had ever lived in regions of 

high pesticide use or worked with products that contain methoxychlor. 
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6.6 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 Section 3.7 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 to avoid hazards (NRC 1993). 

There are little data about children’s exposure to methoxychlor or its degradation products. Like adults, 

children may be exposed to methoxychlor upon inhalation of contaminated air, ingestion of contaminated 

groundwater that is used as drinking water, ingestion of contaminated food, and dermal contact with 

contaminated soil or products treated with the substance. In addition, since small children may play close 

to the ground they are more likely than adults to come in contact with dirt and dust found in home carpets, 

dirt found outside the home, and lawns. Children and infants exhibit frequent hand-to-mouth activities, 

tend to put foreign objects in their mouths, and sometimes intentionally eat soil. This behavior may result 

in the ingestion of methoxychlor contaminated soil and dust. Playing in dirt may also cause dermal 

exposure and inhalation of airborne soil particles. Children may come in contact with and ingest pesticide 

products that are stored in the home or garage area. If a methoxychlor containing product is used on a 

pet, a child may be exposed through dermal contact or through contact with an applicator or animal 

dipping solution. 

Children are more likely than adults to ignore no trespassing signs and wander into areas that may contain 

hazardous waste. A child playing in such an area may be at risk of exposure to elevated levels of 

methoxychlor from contact with or ingestion of soil since methoxychlor applied at the surface is known to 

adsorb to soil and remain near the surface. No data concerning the risk of exposure to methoxychlor or 

its degradates from ingestion of soil were found. There is no information on the bioavailability of 

methoxychlor from oral ingestion, dermal exposure, or inhalation of soil or dust. 
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6. POTENTIAL FOR HUMAN EXPOSURE 

A pilot study was conducted of nine residential homes to monitor potential exposures of children (age 

6 months to 5 years) to pesticides (Lewis et al. 1994). Methoxychlor was detected in two of the older 

nine households (constructed in 1930 and 1962), but was not detected in newer homes. The mean 

concentration of methoxychlor found in sample test areas that included house dust, child hand rinse, air, 

entryway soil, walkway soil, and play area soil was not reported. In a later study conducted in residential 

areas of North Carolina (the Research Triangle area), house dust was collected, separated into seven size 

fractions (<4–500 μm in diameter), and monitored for pesticide residues (Lewis et al. 1999). The 

concentration of methoxychlor reported by the authors for various dust particle sizes is as follows: 

250–500 μm, 120 ng/g; 150–250 μm, 210 ng/g; 106–150 μm, 310 ng/g; 53–106 μm, 570 ng/g; 

25–53 μm, 740 ng/g; 4–25 μm, 680 ng/g; and <4 μm, 1,000 ng/g. The authors state that ingestion, dermal 

exposure, and inhalation of house dust may represent a substantial portion of a child’s exposure to 

pesticides. 

The FDA tested the occurrence of pesticide residues in domestic and imported adult foods eaten by 

infants/children. The foods were prepared for consumption prior to analysis using the FDA’s regulatory 

monitoring methodology (Yess et al. 1993). Methoxychlor was found in apples (0.16% of samples, 

0.38 ppm) and in plain milk (0.22% of samples, 0.38 ppm). 

In another study, foods that are representative of the diets of eight population groups (ranging from 

infants to elderly adults) were prepared for consumption and analyzed for pesticide residues using the 

methods in the FDA’s revised (April 1982) Total Diet Study (Gunderson 1995a). The mean daily intakes 

for children and adults for the test period 1986–1991 were calculated to be 0.4 ng/kg/day for 6–11-monthold infants, 0.9 ng/kg/day for 2-year-old children; 0.3 ng/kg/day for 14–16-year-old females; 

0.4 ng/kg/day for 14–16-year-old males; and 0.1–0.3 ng/kg/day for 25–65-year-old adults (Gunderson 

1995b). Intakes of methoxychlor for the test period 1984–1986 were calculated to be 1.0 ng/kg/day for 

6–11-month-old infants; 2.4 ng/kg/day for 2-year-old children; 0.8 ng/kg/day for 14–16-year-old females; 

0.6 ng/kg/day for 14–16-year-old males; and 0.6–0.9 ng/kg/day for 25–65-year-old adults (Gunderson 

1995a). Intakes of methoxychlor for the test period 1980–1982 were calculated to be 19 ng/kg/day for 

6–11-month-old infants, 4 ng/kg/day for 2-year-old toddlers, and 4 ng/kg/day for 25–65-year-old adults 

(Gartrell et al. 1986b). A reduction in the estimated daily intake of methoxychlor is noted for the period 

1980–1991 for all population groups. More recent residue analysis from the total diet study 1991–1999 is 

available for various foods, but intakes for various age groups have not been calculated (FDA Residue 

Monitoring 1999). 
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6. POTENTIAL FOR HUMAN EXPOSURE 

The FDA has also collected and analyzed a number of baby foods (12 fruit juices or fruits, 4 fruit 

desserts, 4 grain products, and 1 vegetable) in addition to those covered under the Total Diet Study 

Program (FDA Residue Monitoring 1999). In 1998, methoxychlor was found in 1% of samples at a level 

of 0.001 ppm. In 1999, 20 different food items (7 fruit juices, 5 fruits, 4 fruit deserts, and 4 grain 

products) were analyzed and methoxychlor was not reported to be found at a detectable level (FDA 

Residue Monitoring 1999). 

A possible source of exposure in infants to methoxychlor is breast or formula milk. No data were found 

on the presence of methoxychlor in breast milk in the United States. However, in a recent study 

conducted among regional populations in Kazakstan (represented by large urban centers, an agricultural 

region, a petrochemical region, and rural regions), human milk obtained from 92 donors was analyzed 

according to the World Health Organization Protocol for organochlorine pesticides (Hooper et al. 1997). 

Methoxychlor was not detected (limit of detection=5–100 pg/g of fat) in any of the samples. 

Methoxychlor was also not detected in infant formulas, canned milk, with and without iron (Gunderson 

1995b; Yess et al. 1993). Methoxychlor was not detected in whole milk, infant formula (with or without 

iron), or soy-based infant formula according to the most recent FDA Total Diet Study (FDA Residue 

Monitoring 1999). 

Other possible sources of concern for exposure of methoxychlor to children are parents’ work clothes and 

equipment used to apply products that contain methoxychlor. This mode of exposure would be of special 

concern in agricultural areas and around homes where pesticides are applied for lawn and garden or 

indoor use. However, no data were found regarding increased levels of methoxychlor in children living 

in agricultural areas and no information is available on methoxychlor take-home exposure. See Report to 

Congress on Workers’ Home Contamination Study Conducted Under The Workers’ Family Protection 

Act (NIOSH 1995) for a good review of the literature and examples of other chemicals likely to be taken 

home inadvertently. 

6.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES 

Farmers and pesticide applicators who use methoxychlor are the populations most likely to receive above 

average exposures. In addition, people who live on or near farms, methoxychlor-production plants, 

formulation plants, or hazardous-waste sites may be exposed to above-average levels of methoxychlor. 

Individuals who consume large amounts of seafood obtained from methoxychlor-contaminated waters 

may also be exposed to above-average levels of methoxychlor. 
 

 

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6.8 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 methoxychlor 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 methoxychlor. 

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. 

6.8.1 Identification of Data Needs 

Physical and Chemical Properties. The relevant physical and chemical properties of 

methoxychlor (see Table 4-2) including water solubility (Budavari et al. 1989; EPA 1988a; HSDB 2000; 

Montgomery and Welkom 1990), Kow (Howard 1991), Koc (Montgomery and Welkom 1990), vapor 

pressure (Howard 1991), and Henry’s law constant (Howard 1991) have either been measured 

experimentally or have been estimated accurately enough to permit the evaluation of the environmental 

fate and transport of methoxychlor. Technical grade methoxychlor contains between 10 and 12% 

impurities (IARC 1979; Lamoureux and Feil 1980), and GC/MS studies have identified >50 individual 

impurities at varying percentages (Lamoureux and Feil 1980). Additional data regarding the physical and 

chemical properties of the most abundant impurities in the technical grade product would be useful in 

assessing the environmental fate of methoxychlor. 

Production, Import/Export, Use, Release, and Disposal. According to the Hazardous 

Substances Data Bank (HSDB), 17,700 pounds of methoxychlor were imported in 1978 (HSDB 1993). 

No information on current import volumes are available (HSDB 2000). A manufacturer of methoxychlor, 

Kincaid Enterprises, Inc., reported information on export volumes of technical grade methoxychlor in 

pounds as 25,750 in 1986; 86,000 in 1987; 22,600 in 1988; 47,150 in 1989; 10,350 in 1990; and 

49,750 in 1991 (Kincaid Enterprises 1992). However, there is no information on current export volumes 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

of formulated products containing methoxychlor. No recent data were found for current production
 

volume and import/export levels of methoxychlor. Information regarding current production volumes and
 

import/export amounts are necessary in order to understand the current trends of usage and potential for
 

human exposure in the United States. 

Methoxychlor is a General Use Pesticide (GUP) and is available over the counter. It is predominately
 

used for agriculture (57%) with lesser usage for industrial and commercial purposes (15%) and home and
 

garden use (28%) (Kincaid Enterprises 1992). However, data regarding the widespread use of
 

methoxychlor in the environment, in the workplace, or at home were not found. It would be useful to
 

know the extent to which methoxychlor is used for estimating the percent of the population exposed to
 

methoxychlor.
 

Approximately 25,486 pounds of methoxychlor were released to the environment in 1998, which
 

included 12 pounds to air and 25,474 pounds to land (TRI99 2001). However, only certain types of
 

facilities are legally required to report; therefore, this is not an exhaustive list. Releases of methoxychlor
 

from industrial sources and operations are most likely minimal compared to the release to the
 

environment from its use as a pesticide. Quantitative information on the frequency, extent, and actual
 

application rate of methoxychlor when used as a pesticide would be valuable in estimating potential
 

exposures to people who live or work at or near agricultural areas that use methoxychlor. Currently, the
 

EPA does restrict the amount of methoxychlor that can be disposed of in landfills (EPA 1990d). Data for
 

the quantity of methoxychlor disposed of in landfills would be useful for estimating exposure potentials
 

for persons living near landfills.
 

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 TRI, which contains this information for 1998, became available in May of 2000. This
 

database will be updated yearly and should provide a list of industrial production facilities and emissions.
 

Environmental Fate. In the atmosphere, methoxychlor exists in both the particulate and, to a small
 

degree, vapor phase (Kelly et al. 1994). Vapor-phase methoxychlor is rapidly degraded by
 

photochemically produced hydroxyl radicals with an estimated half-life of 7 hours, calculated from its
 

estimated rate constant (Meylan and Howard 1993). Particulate-phase methoxychlor is removed from the
 

atmosphere by wet and dry deposition (Hoff et al. 1992). Since methoxychlor is a hydrophobic
 

substance, it is expected to partition to sediment and organic matter (Karickhoff et al. 1979; Richardson
 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

and Epstein 1971). Methoxychlor is degraded in water and soil by chemical, photochemical, and 

biological processes, making it less persistent in the environment than its structural analogue, DDT 

(Baarschers et al. 1982; Golovleva et al. 1984; Wolfe et al. 1977; Zepp et al. 1976). Methoxychlor is 

biodegraded more rapidly under anaerobic conditions than under aerobic conditions. Half-lives of 

<28 days were reported in anaerobic flooded soil, compared with a half-lives of 49–50 days in flooded 

sediment (static aerobic conditions) and 115–206 days in aerobic artificially flooded sediment (Fogel 

et al. 1982; Muir and Yarechewski 1984). These half-lives only represent the time that it takes for 50% of 

the initial concentration to degrade, and may not be representative of true first-order kinetics. 

Degradation products of methoxychlor studied under laboratory conditions using lake and pond sediment 

have been identified as: 1,1-dichloro-2,2-di(4-methoxyphenyl)ethene (DMDE), 1,1-dichloro2,2-bis(4-methoxyphenyl)ethane (DMDD), 1-chloro-2,2,-bis(4-methoxyphenyl)ethene (DMDU), 

1,1,1-trichloro-2(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane, 1,1,1-trichloro-2,2-bis(4-hydroxyphenyl)ethane, 1,1-dichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane, 1,1-dichloro2-(4-hydroxyphenyl)2-(4-methoxyphenyl)ethene, and 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethene 

(Muir and Yarechewski 1984). Additional data that not only identifies, but also quantifies degradation 

products of methoxychlor under field conditions would be useful in order to gain a complete 

understanding of the biodegradation process of methoxychlor in the environment. Since many of these 

derivatives are more polar than methoxychlor, they may be more mobile in soils and more water soluble 

than the parent compound. Further studies that investigate the persistence, mobility, and degradation of 

the estrogenic degradation products of methoxychlor (particularly estrogenic ones) in water and soil 

would be valuable in predicting their fate and transport at hazardous waste sites and in assessing exposure 

and hazard potential. Biodegradation data for the degradation products of methoxychlor were not located 

and information regarding the rate of degradation and possible degradation byproducts of these 

compounds would be useful in the determination of the persistence of toxic methoxychlor degradates. 

Bioavailability from Environmental Media. No information was located regarding the 

bioavailability of methoxychlor from environmental media. Toxicokinetic and toxicity studies indicate 

that methoxychlor is absorbed following oral (Bal 1984; Chapin et al. 1997; Davison et al. 1982, 1983; 

Gray et al. 1988, 1989; Harris et al. 1974; Kapoor et al. 1970; Martinez and Swartz 1991; Morgan and 

Hickenbottom 1979) and dermal exposures (Davison et al. 1983; Haag et al. 1950; Skaare et al. 1982). 

No data are available regarding absorption following inhalation exposure. Studies regarding the 

absorption of methoxychlor following inhalation exposure would be useful since inhalation of ambient air 

and house dust may represent a potential route of human exposure (Lewis et al. 1999). The extent to 

which absorption occurs could be affected by the media in which methoxychlor is contained, but there are 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

no data on these subjects. Methoxychlor is strongly adsorbed to soil surfaces and this may ultimately 

limit methoxychlor’s bioavailability from soils. Furthermore, it has been shown that the bioavailability of 

certain pesticides, such as DDT, in soil declines with time as the compound becomes sequestered into the 

soil (Alexander 1995, 1997; Robertson and Alexander 1998). Studies regarding the bioavailability of 

methoxychlor from aged soils would be useful in assessing its potential toxicity from soil surfaces. No 

data were located regarding the bioavailability of methoxychlor’s degradation products. Since these 

compounds can accumulate in soil surfaces, studies regarding the bioavailability of these degradation 

products from soils would be useful. 

Quantitative studies that determine whether methoxychlor contained in soil or food is as well absorbed as 

methoxychlor dissolved in water or oil (form used in most toxicity and pharmacokinetic studies) would be 

useful in estimating the absorbed dose of methoxychlor received by individuals living at or near 

hazardous waste sites. Since young children might be exposed to methoxychlor (or its degradation 

products) orally, dermally, or by inhalation of soil, information on bioavailability from air, water, soil, or 

plant material would be desirable in evaluating risk to the subpopulation. 

Food Chain Bioaccumulation. There are no data regarding food chain biomagnification of 

methoxychlor. BCFs have been measured for microorganisms, lower invertebrates, and fish (Anderson 

and DeFoe 1980; Hawker and Connell 1986; Johnson and Kennedy 1973; Prasad 1992; Veith et al. 1979). 

Although the lower species tend to bioconcentrate methoxychlor, fish generally have low BFCs. There 

are sparse data regarding the concentrations of methoxychlor in aquatic and terrestrial animals. However, 

harp seals had detectable concentrations of methoxychlor in various tissues which was attributed to the 

consumption of fish in the seal’s diet (Zitko 1998). Since some populations use seals as a dietary meat 

source, it is possible that this may be a potential route of exposure in the human food chain. Studies in 

laboratory animals indicate that methoxychlor is rapidly metabolized and eliminated (Kapoor et al. 1970), 

and thus, biomagnification of methoxychlor further up the food chain does not appear to be of concern. 

More monitoring data of animals are required in order to ascertain that methoxychlor is not 

bioaccumulated up the aquatic and terrestrial food chains. There is no information regarding the plant 

uptake of methoxychlor following its application as a pesticide. Such information would be useful in 

assessing human exposure via ingestion of agricultural products, including home grown vegetables 

treated with methoxychlor. 
 

 

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6. POTENTIAL FOR HUMAN EXPOSURE 

Exposure Levels in Environmental Media. Information on the levels of methoxychlor in air 

(EPA 1990e; Kelly et al. 1994), surface water (Biberhofer and Stevens 1987; Konasewich et al. 1978; 

Kuntz and Warry 1983; Maguire et al. 1983), groundwater and drinking water (EPA 1990f; Maddy et al. 

1982; Plumb 1991), and soil (Albright et al. 1974; IARC 1979) are available. In general, methoxychlor is 

detected in the pg/m3

 to the ng/m3 range in the ambient atmosphere of the United States. Methoxychlor is 

not frequently detected in groundwater, drinking water, or surface water, but has been detected at high 

concentrations in waters near points of methoxychlor use or application (Wallner et al. 1969). 

Methoxychlor is infrequently detected in soils and sediments in the United States, but is detected more 

frequently near release sources that use methoxychlor as a pesticide. Common ranges in soils have been 

reported as <150–17,000 μg/kg (Agency for Toxic Substances and Disease Registry 1989a; Wallner et al. 

1969). Methoxychlor is generally found in low levels in foods. Methoxychlor was identified, but not 

quantified, in 5% of nearly 14,000 composite food samples obtained from 10 states in 1998, and in 11 of 

13,000 samples in 1989 (Minyard and Roberts 1991). 

No data exist for methoxychlor degradation products and their levels in environmental media. Since these 

degradation products may accumulate in soils (Golovleva et al. 1984), information regarding the 

frequency and levels detected in soils would be useful. 

For the general population, exposure to methoxychlor is expected to occur primarily from low-level 

contamination of foods. Levels of methoxychlor contamination in a variety of foods and food groups as 

well as estimates of human intake from those foods have been studied (Dougherty et al. 2000; EPA 1992, 

1994; FDA 1994; USDA 1995; Winters et al. 1994). Estimates of the average daily intake of 

methoxychlor in adults (ages 25–65) range from 0.1 to 0.3 ng/kg/day for the period 1986–1991 

(Gunderson 1995b), 0.6–0.9 ng/kg/day for the period 1984–1986 (Gunderson 1995a), and 4 ng/kg/day for 

the period 1980–1982 (Gartrell et al. 1986b). The intake for children and young adults has been 

estimated as 0.4 ng/kg/day for 6–11-month-old infants, 0.9 ng/kg/day for 2-year-old children; 

0.3 ng/kg/day for 14–16-year-old females; 0.4 ng/kg/day for 14–16-year-old males; and 

0.1–0.3 ng/kg/day for 25–65-year-old adults (Gunderson 1995b). 

Exposure Levels in Humans. Methoxychlor has been detected in blood of 1 of 39 males living at 

or near farming cooperatives in southern Honduras where there is extensive use (16–30 times/year) of 

pesticides, but levels are generally below the limit of detection in blood (Steinberg et al. 1989), adipose 

tissue (LeBel and Williams 1986), and milk (Mes 1981). No biomonitoring data for methoxychlor were 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

located for populations living near hazardous waste sites or working at methoxychlor production, 

formulation, or disposal facilities. 

Collecting data on the level of methoxychlor in human blood or tissue along with estimates of 

environmental exposure levels would be useful in estimating recent exposures in these populations. This 

information would aid in assessing the need to conduct health studies on these populations. 

Exposures of Children. Data on exposure levels and body burden measurements of methoxychlor in 

children are needed to determine what extent children are exposed to this compound. Children may be 

exposed to methoxychlor through the ingestion of contaminated media (food or drinking water) and 

inhalation of ambient air or household dust particles since methoxychlor has been detected in household 

dusts in homes where pesticides have been used (Starr et al. 1974). Additionally, children may be 

exposed dermally through play activities from contaminated soils, gardens, or lawns that have recently 

had methoxychlor applied. Since methoxychlor is also commonly used for controlling insects on animals, 

children may also be exposed through contact with family pets or livestock. Exposure may also arise 

when children ingest soil either intentionally through pica or unintentionally through hand-to-mouth 

activity. Methoxychlor has been measured in residential house dust (Lewis et al. 1999); children might 

be exposed to such dust by crawling on the floor. Bioavailability data needs related to dust and soil 

exposure have been discussed above under the subsection for Bioavailability from Environmental Media. 

In the FDA’s Total Diet Study Program for 1986–1991 exposures, daily average intakes for various age 

ranges were calculated; ranges were from 0.9 ng/kg/day for 2-year-old children to 0.1–0.3 ng/kg/day for 

25–65-year-old adults (Gunderson 1995b). More recent data about residue amounts on various food 

products are available for the period of 1991–1999, but the weight adjusted intakes have not been 

published yet (FDA Residue Monitoring 1999). A possible source of exposure in infants to methoxychlor 

is breast or formula milk. No data were found on the presence of methoxychlor in breast milk in the 

United States. However, in a recent study conducted among regional populations in Kazakstan 

(represented by large urban centers, an agricultural region, a petrochemical region, and rural region), 

human milk obtained from 92 donors was analyzed according to the World Health Organization Protocol 

for organochlorine pesticides (Hooper et al. 1997). Methoxychlor was not detected (limit of 

detection=5–100 pg/g of fat) in any of the samples. Methoxychlor was also not detected in infant 

formulas, canned milk, with and without iron (Gunderson 1995b; Yess et al. 1993). Methoxychlor was 

not detected in whole milk, infant formula (with or without iron), or soy-based infant formula according 

to the most recent FDA Total Diet Study (FDA Residue Monitoring, 1999). Monitoring data on levels of 
 

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6. POTENTIAL FOR HUMAN EXPOSURE 

methoxychlor in breast milk in the United States are needed because this is an important potential route of 

exposure for infants. 

Child health data needs relating to susceptibility are discussed in Section 3.12.2, Identification of Data 

Needs: Children’s Susceptibility. 

Exposure Registries. No exposure registries for methoxychlor 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. 

6.8.2 Ongoing Studies 

No ongoing studies were located regarding the environmental fate or potential for human exposure to 

methoxychlor. 
METHOXYCHLOR 173 

7. ANALYTICAL METHODS 

The purpose of this chapter is to describe the analytical methods that are available for detecting, 

measuring, and/ monitoring methoxychlor, its metabolites, and other biomarkers of exposure and effect to 

methoxychlor. 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. 

7.1 BIOLOGICAL MATERIALS 

Gas chromatography (GC) with an electron-capture detector (ECD) has been used to measure 

methoxychlor concentrations in human milk, serum, semen, and adipose tissue (LeBel and Williams 

1986; Mes 1981; Steinberg et al. 1989; Szymczynski and Waliszewski 1982). In GC, samples dissolved 

in a volatile solvent are injected into a heated column with a stationary phase consisting of silica coated 

with a liquid phase. An inert gas carries the sample through the column, and the partitioning of 

methoxychlor between the mobile and stationary phases gives it a characteristic retention time that is used 

to identify it. ECDs use a radioactive source such as 63Ni to generate electrons that are captured by the 

chlorine atoms of methoxychlor. Reduction in electron flow by this capture produces a signal for 

methoxychlor. 

Most methoxychlor in biological samples is dissolved in fat, and thus, samples are prepared for GC/ECD 

by extraction of the fat with organic solvents. The fat extract is generally "cleaned up" by gel permeation 

chromatography (GPC), which separates methoxychlor from higher molecular weight lipids, and/or by 

passage through a Florisil® column which retains lipids and other contaminants (LeBel and Williams 

1986; Mes 1981; Szymczynski and Waliszewski 1982). These methods provide 71–104% recovery of 

methoxychlor (LeBel and Williams 1986; Szymczynski and Waliszewski 1982). Methoxychlor has been 

determined in human serum in women living in agricultural areas of Spain (Frenich et al. 2000). 

Samples of serum were purified using a hexane extraction procedure followed by cleanup using high 

performance liquid chromatography (HPLC) and detection using tandem mass spectrometry 
METHOXYCHLOR 174 

7. ANALYTICAL METHODS 

(GC/MS/MS). Using this method, the analytical limit of detection (LOD) was found to be 2.0 μg/L and 

the and the percent recovery was 113%. The advantage of using GC/MS/MS is a much higher sensitivity 

than GC/ECD. However, the GC/ECD gives greater precision, accuracy, and limit of detection values. In 

serum, the detection limit for methoxychlor was reported to be 0.24–4.07 mg/L (Steinberg et al. 1989). 

No data were located on the specificity or precision of these methods for biological samples. 

As discussed previously, because methoxychlor is metabolized fairly quickly, measurement of 

metabolites may prove to be more useful for assessing exposure than measurement of methoxychlor itself. 

Pure samples of methoxychlor impurities (such as, 1,1,2,2-tetrachloro-2-(4-methoxyphenyl)ethane, 

1,1-dichloro-2,2-(4-methoxyphenyl)ethene, and 1,1,1-trichloro-2-(4-hydroxyphenyl)-2-(4-methoxyphenyl)ethane), that may also be metabolic products of methoxychlor, may be analyzed by GC with a 

flame ionization detector (FID), HPLC with an ultraviolet (UV) detector, or by GC with detection by 

mass spectrometry (MS) (Lamoureux and Feil 1980; West et al. 1982); however, no information was 

located concerning the application of these methods to detecting metabolites in biological samples. 

Table 7-1 summarizes the methods used for sample preparation and analysis of methoxychlor in 

biological samples. 

7.2 ENVIRONMENTAL SAMPLES 

Methoxychlor in environmental samples is also usually measured using GC/ECD (APHA 1992; Ault and 

Spurgeon 1984; EPA 1989b, 1990a, 1990c; Gillespie and Walters 1986; Hopper and King 1991; Hsu et 

al. 1991; Ivey et al. 1983). A halogen electrolytic conductivity detector (HECD) may be used instead of, 

or in conjunction with, an ECD, and GC/MS may be used to confirm the identity of methoxychlor (EPA 

1990b; Hopper and King 1991; NIOSH 1978). Use of MS as the primary detection system can be 

achieved by establishing a library of reference pesticide spectra and inclusion of appropriate standards 

(Liao et al. 1991). Recent work has concentrated on improving methods for extraction of methoxychlor 

from environmental samples. Since methoxychlor in air is usually associated with particulate matter, 

standard methods involve collection of air samples on glass fiber or polyurethane foam and extraction 

with organic solvents prior to GC analysis (EPA 1990a; NIOSH 1978). Methoxychlor has detected in 

dust samples at levels of 0.6–3.5 μg/g using GC/MS selective ion monitoring (Rudel et al. 2001). Reports 

in the development and application of semipermeable membrane devices (SPMDs) describe new 
METHOXYCHLOR 175 

7. ANALYTICAL METHODS 

Table 7-1. Analytical Methods for Determining Methoxychlor in 

Biological Samples
 

Sample 

matrix Preparation method 

Analytical 

method 

Sample 

detection limit 

Percent 

recovery Reference 

Human 

serum 

Extract with hexane/ 

diethyl ether, 

concentrate, acidify 

(H2SO4), centrifuge, 

extract aqueous layer 

with hexane, cleanup 

via HPLC 

GC/MS/MS 500 ng/L 113 Frenich et al. 

2000 

Human 

serum 

No data GC/ECD 

and GC/MS 

0.24–4.07 mg/L No data Steinberg et 

al. 1989 

Adipose 

tissue 

Extract with 

acetone/hexane, 

cleanup with GPC 

and FAC 

GC/ECD 

and GC/MS 

No data 71–98 LeBel and 

Williams 

1986 

Human milk Extract with 

ethanol/hexane/ethyl 

ether, cleanup via 

GPC 

GC/ECD 10 mg/kg No data Hooper et al. 

1997 

Human milk Extract with hexane 

or benzene, cleanup 

with FAC 

GC/ECD No data No data Mes 1981 

Human 

semen 

Extract with 

petroleum ether, 

cleanup with FAC 

GC/ECD No data 104 Szymczynski 

and 

Waliszewski 

1982 

ECD = electron capture detector; FAC = Florisil® adsorption chromatography; GC = gas chromatography; 

GPC = gel permeation chromatography; HPLC = high performance liquid chromatography; MS = mass 

spectrometry. 
METHOXYCHLOR 176 

7. ANALYTICAL METHODS 

approaches to determining bioavailable contaminants in a variety of aquatic systems (Petty et al. 1998). 

The SPMD membrane mimics the diffusion transfer of organic contaminants through the respiratory 

membranes of aquatic organisms. Methoxychlor in water, soil, solid waste, and food is also generally 

extracted with organic solvents, and the extracts are cleaned up by chromatographic methods using GPC, 

Florisil® adsorption, Attagel®, magnesia-silica gel, and/or semi-preparative HPLC (APHA 1992; Ault and 

Spurgeon 1984; EPA 1990c; Gillespie and Walters 1986; Hopper and King 1991; Hsu et al. 1991; Ivey et 

al. 1983). Modifications to extraction and cleanup methods have been proposed, which reduce the 

amount of solvent required or increase the speed of analyses (Kraut-Vass and Thoma 1991; Patterson 

1991). An analytical procedure for the identification of methoxychlor in a drinking water sample matrix 

has been developed that involves the use of a two-step pre-concentration followed by a separation step 

(Fung and Mak 2001). The pre-concentration steps employ a solid-phase extraction (C18 SPE) and 

cleanup followed by stacking with reverse polarity prior to separation using micellar electrokinetic 

capillary chromatography (MEKC). The recovery of methoxychlor using the SPE method is 89%. The 

MEKC procedure was found to give analytical results that are comparable with GC-MS and LC-MS and 

is more sensitive than LC. The detection limit of methoxychlor in drinking water was determined to be 

0.041 μg/L. Supercritical fluid extraction, a process using carbon dioxide liquified above 31 EC at high 

pressure, provides efficient extraction of large samples (Hopper and King 1991). These extraction 

methods typically provide recoveries of 84–100%. Gas chromatographic measurements can detect about 

0.2 μg methoxychlor per L of hexane extract, with actual detection limits for soil and waste dependent 

upon matrix interferences, extraction efficiency, and cleanup procedures (EPA 1990b, 1990c). Although 

detection limits were not reported for analysis of methoxychlor by GC/ECD (Hopper and King 1991; Hsu 

et al. 1991) a detection limit of 0.05 μg/g food was reported for analysis by GC/MS (Liao et al. 1991). 

Pure samples of metabolic products of methoxychlor may be analyzed by GC with a FID, HPLC with an 

UV detector, or by GC with detection by MS (Lamoureux and Feil 1980; West et al. 1982). 

Table 7-2 summarizes representative methods used for sample preparation and analysis of methoxychlor 

in environmental samples. 

7.3 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 methoxychlor is available. Where adequate information is 
METHOXYCHLOR 177 

7. ANALYTICAL METHODS 

Table 7-2. Analytical Methods for Determining Methoxychlor in 

Environmental Samples
 

Sample 

matrix Preparation method 

Analytical 

method 

Sample 

detection 

limit 

Percent 

recovery Reference 

House dust Extract with 

ether/hexane, 

chromatograph on 

florisil with 

GC/MS-SIM No data No data Rudel et al. 

2001 

acetone/hexane 

House dust Extract with diethyl 

ether/hexane, 

cleanup with FAC 

GC/MS 0.02–0.1 μg/g No data Lewis et al. 

1999 

Air Collect on glass fiber 

filter, extract with 

isooctane 

GC/HECD 7.7 mg/m3 103 NIOSH 1978 

Indoor air Collect on 

polyurethane foam, 

extract with diethyl 

ether/hexane 

GC/ECD 0.01 μg/m3 65 EPA 1990a 

Drinking 

Water 

Adjust pH to 3.0 

(HCl), soild phase 

extraction (eluted with 

methanol) 

MEKC 

equipped 

with a 

variable UV 

detector 

0.041 μg/m3 89 Fung and 

Mak 2001 

Water SPMD, extract with 

hexane, and 

fractionate 

Capillary 

column 

GC/ECD 

0.01 μg/­

SPMD 

No data Petty et al. 

1998 

Water Collect analyte on 

SPE cartridge, elute 

with hexane/propanol 

Capillary 

column 

GC/ECD 

0.01–0.018 

μg/L 

No data Kolpin et al. 

1998 

Water Extract with diethyl 

ether/hexane or 

methylene 

chloride/hexane, 

cleanup with 

magnesia-silica gel 

GC/ECD No data No data APHA 1992 

Water Extract with hexane GC/ECD 0.96 μg/L 98–100 EPA 1989b 

Soil/solid 

waste 

Sonicate, extract with 

hexane, cleanup with 

GPC 

GC/ECD or 

GC/HECD 

0.12–18 

mg/kg wet 

weight 

No data EPA 1990b 
METHOXYCHLOR 178 

7. ANALYTICAL METHODS 

Table 7-2. Analytical Methods for Determining Methoxychlor in 

Environmental Samples (continued)
 

Sample 

matrix Preparation method 

Analytical 

method 

Sample 

detection 

limit 

Percent 

recovery Reference 

Soil/solid 

waste 

Extract with 

hexane/acetone or 

methylene chloride/ 

acetone, cleanup with 

silica gel, GPC, 

and/or FAC 

Capillary 

column 

GC/ECD 

5.7–860 

μg/kg wet 

weight 

53–80 EPA 1990c 

Food Extract with SC-CO2 GC/ECD No data 90–105 Lehotay et al. 

1995 

Food Extract with SC-CO2 

and cleanup with 

GPC and FAC 

GC/ECD and 

GC/HECD 

No data 87–112 Hopper and 

King 1991 

Food Extract with 

acetonitrile 

GC/MS 0.05 μg/g 94 Liao et al. 

1991 

Food Extract with 

acetonitrile, cleanup 

with Attagel® (in 

benzene), Florisil® (in 

hexane), or C18 (in 

acetonitrile) 

GC/ECD No data 89–125 Hsu et al. 

1991 

Edible fat Dissolve in hexane, 

filter, HPLC on semipreparative silica 

column 

GC/ECD No data 89–95 Gillespie and 

Walters 1986 

Edible fat Dissolve in methylene 

chloride/cyclohexane, 

cleanup with GPC 

GC/ECD No data 84–86 Ault and 

Spurgeon 

1984 

Human 

milk 

Extract with 

ethanol/hexane/ethyl 

ether, cleanup via 

GPC 

GC/ECD 10 mg/kg No data Hooper et al. 

1997 

Milk Cleanup with Florisil® , 

elute with hexane 

GC/ECD 0.005 ppm 84–96 Ivey et al. 

1983 

C18 = Carbon 18; ECD = electron capture detector; FAC = Florisil® absorption chromatography; GC = gas 

chromatography; GPC = gel permeation chromatography; HECD = halogen electrolytic conductivity detector; 

HPLC = high performance liquid chromatography; MEKC = Micellar electrokinetic capillary chromatography; 

MS = mass spectrometry; SC-CO2 = superficial fluid-carbon dioxide; SIM = selective ion monitoring; SPE = solid 

phase extraction; SPMD = semipermeable membrane device 
 

METHOXYCHLOR 179 

7. ANALYTICAL METHODS 

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 methoxychlor. 

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. 

7.3.1 Identification of Data Needs 

Methods for Determining Biomarkers of Exposure and Effect. Methoxychlor can be 

measured in a variety of human tissues and fluids, including serum (LeBel and Williams 1986), milk 

(Mes 1981), semen (Steinberg et al. 1989), and adipose tissue (Szymczynski and Waliszewski 1982). 

Detection limits are not well characterized, but are probably about 0.2 μg/L in extracts of biological 

samples (EPA 1990b; Steinberg et al. 1989) and 0.05 μg/g in food (Liao et al. 1991). Based on the low 

frequency of methoxychlor detection in biological samples, it would be helpful if suitable sample 

extraction and analysis procedures could be developed to measure background levels of methoxychlor in 

the general population as well as levels at which biological effects occur. These data would be valuable 

in monitoring environmentally exposed populations. Adaptation of methods for measuring pure samples 

of methoxychlor metabolic products (Lamoureux and Feil 1980; West et al. 1982) to biological samples 

could provide additional data on exposure to methoxychlor. Since methoxychlor is rapidly metabolized 

to phenolic derivatives, some of which are estrogenic, additional studies that investigate the applicability 

or adaptability of methods for methoxychlor to metabolites of methoxychlor in biological media would be 

useful in developing other biomarkers of exposure. 

No data were located concerning methods to measure biological markers of methoxychlor effects. 

Methoxychlor produces effects on the reproductive system of exposed animals primarily by the estrogenic 

action of its metabolites (see Chapter 3). In exposed female rodents, methoxychlor exposure may affect 

the estrus cycle (Gray et al. 1988; Harris et al. 1974; Martinez and Swartz 1992; Okazaki et al. 2001; 

Tegeris et al. 1966), serum hormone levels (Cummings and Gray 1987, 1989; Cummings and Laskey 

1993; Gray et al. 1988), and the uterus and ovaries (Bal 1984; Chapin et al. 1997; Cummings and Gray 

1987, 1989; Cummings and Laskey 1993; Cummings and Perreault 1990; Dikshith et al. 1990; Gray et al. 
 

METHOXYCHLOR 180 

7. ANALYTICAL METHODS 

1988, 1989; Harris et al. 1974; Martinez and Swartz 1992; Mitsumori et al. 2000; Okazaki et al. 2001; 

Swartz 1994; Tegeris et al. 1966; Tullner 1961), and may reduce fertility (Bal 1984; Chapin et al. 1997; 

Gray et al. 1989; Harris et al. 1974; Haskell Laboratories 1966). In the female offspring of exposed 

female rodents, reproductive development, as indicated by age at vaginal opening and first estrus, may be 

accelerated (Chapin et al. 1997; Gray et al 1989; Harris et al. 1974). In male rodents, methoxychlor 

exposure may affect sexual maturity (age of preputial separation) (Gray et al. 1989, 1999), serum 

hormone levels (Cummings and Gray 1989; Goldman et al. 1986; Gray et al. 1989; Stoker et al. 1999), 

testes (Bal 1984; Chapin et al. 1997; Dikshith et al. 1990; Gray et al. 1989, 1999; Hodge et al. 1950; 

Tullner and Edgcomb 1962; Wenda-Rozewicka 1983), prostate (Okazaki et al. 2001; Shain et al. 1977; 

Stoker et al. 1999; Welshons et al. 1999), fertility (Chapin et al. 1997; Gray et al. 1989, 1999; Harris et al. 

1974; Wenda-Rozewicka 1983), and behavior (Parmigiani et al. 1998; vom Saal et al. 1995). However, 

these effects are not specific to exposure to methoxychlor, but could occur following exposure to other 

chemicals with estrogenic activity as well. Since several in vitro studies have indicated the presence in 

human hepatic microsomes of enzymatic activities similar to those in rats that are responsible for the 

metabolism of methoxychlor to its estrogenic metabolites (Dehal and Kupfer 1994; Stresser and Kupfer 

1998), it is likely that similar effects may occur in exposed humans. Research into biomarkers for the 

estrogenic effects of methoxychlor would be most useful if performed in conjunction with development of 

sensitive, specific, and reliable methods for measuring these biomarkers. 

Methods for Determining Parent Compounds and Degradation Products in Environmental 

Media. Methods for detection of methoxychlor in air (EPA 1989b), water (EPA 1990a), soil (EPA 

1990b), solid waste (EPA 1990c), and food (Liao et al. 1991) are all based on GC. The media most likely 

to be of concern for human exposure to methoxychlor are soil and food. Developments in GC analysis 

for methoxychlor include development of capillary columns, which provide better resolution than packed 

columns (EPA 1990c; Lopez-Avila et al. 1990; Tessari and Winn 1991), and the use of MS as the primary 

means of detection, which is much more specific than retention time monitored by ECD or halogen 

electrolytic conductivity detector, HECD (Liao et al. 1991; Stan 1989). Immunoassays, which have been 

developed for other pesticides (Stanker et al. 1989), may provide a rapid, inexpensive, and sensitive, 

method for detecting methoxychlor in environmental samples. Another area of interest is the complete 

and selective extraction of methoxychlor from complex environmental samples, such as soil and food; 

supercritical-fluid extraction shows promise in this area (Hopper and King 1991; Lopez-Avila et al. 1990; 

Walters 1990). Supercritical-fluid extraction and GC/MS, if validated in additional studies, appears to 

have the sensitivity, specificity, and reliability to measure background levels of methoxychlor in the 

environment and levels at which health effects occur. 
METHOXYCHLOR 181 

7. ANALYTICAL METHODS 

Methods have been developed to separate methoxychlor-degradation products (West et al. 1982). These 

methods are based on complimentary chromatographic techniques using normal phase HPLC and GC. 

High molecular weight impurities found in technical grade and formulated methoxychlor that are difficult 

to separate by GC are cleanly separated by HPLC. HPLC separation products are then readily analyzed 

by GC/MS. However, it is not known if this technique can be used reliably for the analysis of 

environmental samples. Further studies using HPLC GC/MS are needed to determine if the technique 

could be used in the analysis of air, water, or soil samples contaminated with methoxychlor degradation 

products. 

7.3.2 Ongoing Studies 

The application of microwave assisted solvent extraction, coupled with solid phase extraction cleanup to 

the recovery of methoxychlor from crops (beets, cucumbers, lettuce, peppers, and tomatoes have been 

studied thus far) shows promise in screening foods for pesticide residues, and pesticide recoveries 

compare favorably with standard extraction methods (Pylypiw et al. 1997). Ongoing studies to improve 

methods for analysis of methoxychlor include further research on applications of supercritical-fluid 

extraction procedures to recover methoxychlor and other compounds from solid matrices (AshrafKhorassani et al. 1992; Lopez-Avila et al. 1992). In addition, research to optimize sensitivity, specificity, 

reproducibility, and analysis time using fused silica capillary columns in a dual-column, dual-detector 

arrangement, is providing promising results for analysis of food and solid matrices (Hopper 1991; LopezAvila et al. 1992). 

No information was located concerning on-going studies for improving methods of analysis of 

methoxychlor metabolites, or other biomarkers of exposure and effect for methoxychlor in biological 

materials. 

METHOXYCHLOR 183 

8. REGULATIONS AND ADVISORIES
 

Because of methoxychlor’s potential to cause adverse health effects in exposed people, a number of 

regulations and guidelines have been established by international, federal, and state agencies. The 

international, national, and state regulations and guidelines regarding methoxychlor in air, water, and 

other media are summarized in Table 8-1. 

In addition to these values, ATSDR and EPA have established additional guidelines to protect people 

from the adverse health effects from ingesting methoxychlor. ATSDR has withdrawn the previous MRL 

of 0.02 mg/kg/day for acute-duration exposure derived in the 1994 profile (see Appendix A for further 

discussion). An intermediate-duration oral MRL of 0.005 mg/kg/day was derived based on the LOAEL 

of 5 mg/kg/day for accelerated onset of puberty (i.e., precocious vaginal opening) in immature female rats 

exposed to methoxychlor in utero, during lactation, and after weaning (Chapin et al. 1997). This MRL 

supercedes the previous MRL of 0.02 mg/kg/day for intermediate-duration exposure derived in the 1994 

profile. A chronic-duration oral MRL was not derived. A reference dose (RfD) of 0.005 mg/kg/day was 

derived (in 1991) by EPA based on the NOEL for maternal toxicity in rabbits dosed during gestational 

days 7-19 (IRIS 2001; Kincaid Enterprises 1986). 

IARC (2001) has classified methoxychlor as a Group 3 carcinogen (not classifiable as to its 

carcinogenicity to humans) and NCI (1978) concluded that there was insufficient evidence to classify 

methoxychlor as a carcinogen. Similarly, EPA has classified methoxychlor as a Group D carcinogen, not 

classifiable as to human carcinogenicity (IRIS 2002). 

On January 14, 2000, EPA issued a suspension order to Kincaid Enterprises, Inc. to prevent further 

manufacture and sale of their methoxychlor products. The order affects the technical product and three 

products manufactured by Kincaid, but does not directly affect other companies that manufacture 

methoxychlor products. The order was issued because the registrant failed to submit overdue (per an 

agreement signed in September of 1998) environmental fate studies. At the time of the writing of this 

profile, EPA is in the process of issuing a notice of intent to suspend to all companies that use 

methoxychlor in their products (EPA 2002) (http://www.epa.gov/oppfead1/cb/csb_page/updates/ 

methox.htm). 
METHOXYCHLOR 184 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor 

Agency Description	 Information Reference 

INTERNATIONAL 

Guidelines: 

IARC Carcinogenicity classification Group 3a	 IARC 2001 

NATIONAL 

Regulations and 

Guidelines: 

a. Air: 

ACGIH TLV (8-hour TWA) 10 mg/m3	 ACGIH 2001 

EPA RAC 50 μg/m3	 EPA 2001p
 

40CFR266,
 

Appendix IV
 

NIOSH REL Potential occupational NIOSH 2001
 

carcinogen
 

IDLH 5,000 mg/m3
 

OSHA PEL (8-hour TWA)—total dust 15 mg/m3	 OSHA 2001b 

29CFR1910.1000 

PEL (8-hour TWA) for construction 15 mg/m3 OSHA 2001c 

workers—total dust 29CFR1926.55 

PEL (8-hour TWA) for shipyard 15 mg/m3 OSHA 2001a 

workers—total dust 29CFR1915.1000 

USC Listed as hazardous air pollutant	 USC 2001
 

42 USC 7412
 

b. Water 

EPA	 Designated as hazardous EPA 2001s
 

substance in accordance with 40CFR116.4
 

Section 311(b)(2)(A) of the Clean
 

Water Act
 

Drinking water standard 0.04 ppm	 EPA 2001c 

40CFR141.32 

(e)(43) 

Groundwater monitoring	 EPA 2001d 

Suggested method PQL 40CFR264, 

8080 2 μg/L Appendix IX 

8270 10 μg/L 
METHOXYCHLOR 185 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description Information Reference 

NATIONAL (cont.) 

EPA Health advisories EPA 2000 

10-kg child 

1-day 

10-day 

DWELb 

Lifetime 

0.05 mg/L 

0.05 mg/L 

0.2 mg/L 

0.04 mg/L 

Interim primary drinking water 

standard for owners and operators 

of hazardous waste TSD 

facilities—maximum level 

0.1 mg/L EPA 2001g 

40CFR265, 

Appendix III 

Land disposal restrictions; 

universal treatment standards 

Wastewater concentration 

Non-wastewater concentration 

0.25 mg/L2 

0.18 mg/kg2 

EPA 2001h 

40CFR268.48 

Maximum concentration of 

constituents for groundwater 

protection 

0.1 mg/L EPA 2001i 

40CFR264.94 

MCL—apply to community water 

systems and non-transient, noncommunity water systems 

0.04 mg/L EPA 2001l 

40CFR141.61(c) 

MCL—promulgated under the 

Safe Drinking Water Act 

0.1 mg/L EPA 2001k 

40CFR 257, 

Appendix I 

MCLG 0.04 mg/L EPA 2001m 

40CFR141.50(b) 

National recommended water 

quality criteria 

Freshwater 

Saltwater 

Human health for consumption 

of water and organism 

0.03 μg/L 

0.03 μg/L 

100 μg/L 

EPA 1999j 

Radiation protection—maximum 

concentration for groundwater 

protection 

0.1 mg/L EPA 2001o 

40CFR192, 

Table 1 to 

Subpart A 

Reportable quantity of hazardous 

substance designated pursuant to 

Section 311 of the Clean Water 

1 pound EPA 2001b 

40CFR117.3 

Act 
METHOXYCHLOR 186 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description Information Reference 

NATIONAL (cont.) 

c. Food 

EPA Tolerances for residues 

Alfalfa, clover, cowpeas, grass 

for forage, peanut forage, and 

soybean forage 

Apples, apricots, asparagus, 

beans, beets, blackberries, 

blueberries, boysenberries, 

broccoli, brussel sprouts, 

cabbage, carrots, cauliflower, 

cherries, collards, corn, 

cranberries, cucumbers, 

currants, dewberries, eggplants, 

gooseberries, grapes, kale, 

kohlrabi, lettuce, loganberries, 

melons, mushrooms, nectarines, 

peaches, peanuts, pears, peas, 

peppers, pineapples, plums, 

pumpkins, quinces, radishes, 

raspberries, rutabagas, spinach, 

squash, strawberries, summer 

squash, tomatoes, turnips, 

youngberries 

Sweet potatoes and yams from 

preharvest and postharvest 

application 

Fat of meat from cattle, goats, 

hogs, horses, or sheep 

Barley, corn, oats, rice, rye, 

sorghum grain, and wheat from 

storage-bin treatment 

Milk fat reflecting negligible 

residues in milk 

Potatoes and horseradish 

FDA Beverages—bottled water 

concentration 

USDA Federal seed act regulations—not 

harmful when present at a rate 

less than the number of ppm 

indicated 

100 ppm 

14 ppm 

7 ppm 

3 ppm 

2 ppm 

1.25 ppm 

1 ppm 

0.04 mg/L 

2 ppm 

EPA 2001n 

40CFR180.120 

FDA 2001 

21CFR165.110 

USDA 2001 

7CFR201.31a 
METHOXYCHLOR 187 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description Information Reference 

NATIONAL (cont.) 

d. Other 

ACGIH Carcinogenicity classification A4c ACGIH 2001 

EPA Carcinogenicity classification 

RfD 

RfC 

Group Dd 

5x10-3 mg/kg/day 

Not verifiable 

IRIS 2001 

Health based limits for exclusion 

of waste-derived residues— 

concentration limits for residues 

1x10-1 mg/kg EPA 2001e 

40CFR266, 

Appendix VII 

Identification and listing of 

hazardous waste—hazardous 

waste number 

U247 EPA 2001f 

40CFR261.33(e) 

Maximum concentration of 

contaminants for toxicity 

characteristic—regulatory level 

10 mg/L EPA 2001j 

40CFR261.24 

Organic pesticide active 

ingredient—pesticide code 

34001 EPA 1999l 

40CFR455, 

Subpart E 

Pesticide class Chlorinated organic 

pesticide 

EPA 2001q 

40CFR180.3(e)(4) 

Reportable quantity of hazardous 

substance designated pursuant to 

Section 311(b)(4) of the Clean 

Water Act and Section 112 of the 

Clean Air Act 

1 pound EPA 2001a 

40CFR302.4 

Toxic chemical release reporting; 

community right-to-know— 

effective date for reporting 

01/01/87 EPA 2001r 

40CFR372.65 

STATE 

Regulations and 

Guidelines: 

a. Air 

Arkansas RAC 50 μg/m3 BNA 2001 

Idaho AAC 

EL 

OEL 

0.5 mg/m3 

0.667 pounds/hour 

10 mg/m3 

ID Dept. of Health 

and Welfare 1999 

Montana Occupational air contaminant— 

TLV 

15 mg/m3 BNA 2001 
METHOXYCHLOR 188 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description	 Information Reference 

STATE (cont.) 

New Hampshire Regulated toxic air pollutant— 10 mg/m3 BNA 2001 

OEL
 

New York Dangerous air contaminant—TLV 15 mg/m3 BNA 2001
 

South Carolina RAC 50 μg/m3 BNA 2001
 

Toxic air emissions BNA 2001 

Category 3e 

Maximum allowable category 50 μg/m3 

concentration 

Tennessee RAC 50 μg/m3 BNA 2001
 

Texas Occupational health—TLV 15 mg/m3 BNA 2001
 

Vermont Hazardous air contaminant BNA 2001
 

Washington Acceptable source impact level, 33 μg/m3 WA Dept. of
 

24-hour average Ecology 1998
 

Threshold for hazardous air 0.5 tons/year BNA 2001
 

pollutants—threshold level
 

Wyoming RAC 50 μg/m3 BNA 2001
 

b. Water 

Alaska MCL 0.04 mg/L	 AK Dept. of 

Environ. Conserv. 

1999 

Arizona	 Drinking water standard and 340 ug/L FSTRAC 1999 

guideline 

California MCL 0.04 mg/L	 CA Dept. of 

Health Services 

2000 

Colorado Groundwater organic chemical 40 μg/L CO Dept. of 

standard Public Health and 

Environ. 1999 

Georgia Groundwater criteria concentration 0.04 mg/L	 BNA 2001 

Hawaii	 MCL applying to community and 0.04 mg/L HI Dept. of Health 

non-community, non-transient 1999a 

water systems 
METHOXYCHLOR 189 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description Information Reference 

STATE (cont.) 

Hawaii Toxic pollutant standard 

Freshwater 

Acute 

Chronic 

Saltwater 

Acute 

Chronic 

Fish Consumption 

No standard 

0.03 μg/L 

No standard 

0.03 μg/L 

No standard 

HI Dept. of Health 

1999b 

Illinois Water supply standard 0.1 mg/L IL Environ. 

Protection Agency 

1999 

Kansas Water quality standard 

Aquatic life 

Acute 

Chronic 

Public health 

Food procurement 

Domestic water supply 

Not available 

0.3 μg/L 

Not available 

40 μg/L 

KS Dept. of 

Health and 

Environ. 2001 

Maine Drinking water standard and 

guideline 

100 μg/L FSTRAC 1999 

Missouri Water quality standards 

Aquatic life 

Drinking water supply 

Groundwater 

003 μg/L 

40 μg/L 

40 μg/L 

BNA 2001 

New Jersey Groundwater quality criteria 40 μg/L NJ Dept. of 

Environ. 

Protection 1993 

South Dakota MCL 0.04 mg/L SD Dept. of 

Environ. & Natural 

Resources 1998 

Vermont Groundwater quality standards 

Enforcement standard 

Preventive action limit 

40 μgL 

4 μg/L 

BNA 2001 

Wisconsin Groundwater quality standards 

Enforcement standard 

Preventive action limit 

40 μgL 

4 μg/L 

BNA 2001 

c. Food No data 
METHOXYCHLOR 190 

8. REGULATIONS AND ADVISORIES 

Table 8-1. Regulations and Guidelines Applicable to Methoxychlor (continued) 

Agency Description Information Reference 

STATE (cont.) 

d. Other 

Arizona Soil remediation level 

Residential 

Non-residential 

330 mg/kg 

3,400 mg/kg 

BNA 2001 

California Characteristics of toxicity 

Regulatory level 

STLC 

TTLC 

10 mg/L 

10 mg/L 

100 wet weight mg/kg 

BNA 2001 

a

Group 3: unclassifiable as to its carcinogenicity to humans

b

DWEL: A lifetime exposure concentration protective of adverse, non-cancer health effects, that assumes all of the 

exposure to a contaminant is from drinking water. 

c

A4: not classifiable as a human carcinogen

d

Group D: not classifiable as to human carcinogenicity 

e

Category 3: High toxicity–those pollutants that may cause chronic effects that result in death or permanent injury 

after very short exposure to small quantities. 

AAC = acceptable ambient concentration; ACGIH = American Conference of Governmental Industrial Hygienists; 

BNA = Bureau of National Affairs; CFR = Code of Federal Regulations; DWEL = drinking water equivalent level; 

EL = emissions screening level; EPA = Environmental Protection Agency; FDA = Food and Drug Administration; 

FSTRAC = Federal-State Toxicology and Regulatory Alliance Committee; IARC = International Agency for 

Research on Cancer; IDLH = immediately dangerous to life or health; IRIS = Integrated Risk Information System; 

MCL = maximum contaminant level; MCLG = maximum contaminant level goal; NIOSH = National Institute for 

Occupational Safety and Health; OEL = occupational exposure level; OSHA = Occupational Safety and Health 

Administration; PEL = permissible exposure limit; PQL = practical quantitation limit; RAC = reference air 

concentration; ppm = parts per million; REL = recommended exposure limit; RfC = inhalation reference 

concentration; RfD = oral reference dose; STLC = soluble threshold limit concentration; TLV = threshold limit value; 

TSD = treatment, storage, and disposal; TTLC = total threshold limit concentration; TWA = time-weighted average; 

USC = United States Code; USDA = U.S. Department of Agriculture 
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METHOXYCHLOR 238 

10. GLOSSARY 

Absorption—The taking up of liquids by solids, or of gases by solids or liquids. 

Acute Exposure—Exposure to a chemical for a duration of 14 days or less, as specified in the 

Toxicological Profiles. 

Adsorption—The adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the 

surfaces of solid bodies or liquids with which they are in contact. 

Adsorption Coefficient (Koc)—The ratio of the amount of a chemical adsorbed per unit weight of 

organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium. 

Adsorption Ratio (Kd)—The amount of a chemical adsorbed by a sediment or soil (i.e., the solid phase) 

divided by the amount of chemical in the solution phase, which is in equilibrium with the solid phase, at a 

fixed solid/solution ratio. It is generally expressed in micrograms of chemical sorbed per gram of soil or 

sediment. 

Benchmark Dose (BMD)—Usually defined as the lower confidence limit on the dose that produces a 

specified magnitude of changes in a specified adverse response. For example, a BMD10 would be the 

dose at the 95% lower confidence limit on a 10% response, and the benchmark response (BMR) would be 

10%. The BMD is determined by modeling the dose response curve in the region of the dose response 

relationship where biologically observable data are feasible. 

Benchmark Dose Model—A statistical dose-response model applied to either experimental toxicological 

or epidemiological data to calculate a BMD. 

Bioconcentration Factor (BCF)—The quotient of the concentration of a chemical in aquatic organisms 

at a specific time or during a discrete time period of exposure divided by the concentration in the 

surrounding water at the same time or during the same period. 

Biomarkers—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. 

Cancer Effect Level (CEL)—The lowest dose of chemical in a study, or group of studies, that produces 

significant increases in the incidence of cancer (or tumors) between the exposed population and its 

appropriate control. 

Carcinogen—A chemical capable of inducing cancer. 

Case-Control Study—A type of epidemiological study which examines the relationship between a 

particular outcome (disease or condition) and a variety of potential causative agents (such as toxic 

chemicals). In a case-controlled study, a group of people with a specified and well-defined outcome is 

identified and compared to a similar group of people without outcome. 

Case Report—Describes a single individual with a particular disease or exposure. These may suggest 

some potential topics for scientific research but are not actual research studies. 

Case Series—Describes the experience of a small number of individuals with the same disease or 

exposure. These may suggest potential topics for scientific research but are not actual research studies. 
METHOXYCHLOR 239 

10. GLOSSARY 

Ceiling Value—A concentration of a substance that should not be exceeded, even instantaneously. 

Chronic Exposure—Exposure to a chemical for 365 days or more, as specified in the Toxicological 

Profiles. 

Cohort Study—A type of epidemiological study of a specific group or groups of people who have had a 

common insult (e.g., exposure to an agent suspected of causing disease or a common disease) and are 

followed forward from exposure to outcome. At least one exposed group is compared to one unexposed 

group. 

Cross-sectional Study—A type of epidemiological study of a group or groups which examines the 

relationship between exposure and outcome to a chemical or to chemicals at one point in time. 

Data Needs—Substance-specific informational needs that if met would reduce the uncertainties of human 

health assessment. 

Developmental Toxicity—The occurrence of adverse effects on the developing organism that may result 

from exposure to a chemical prior to conception (either parent), during prenatal development, or 

postnatally to the time of sexual maturation. Adverse developmental effects may be detected at any point 

in the life span of the organism. 

Dose-Response Relationship—The quantitative relationship between the amount of exposure to a 

toxicant and the incidence of the adverse effects. 

Embryotoxicity and Fetotoxicity—Any toxic effect on the conceptus as a result of prenatal exposure to 

a chemical; the distinguishing feature between the two terms is the stage of development during which the 

insult occurs. The terms, as used here, include malformations and variations, altered growth, and in utero 

death. 

Environmental Protection Agency (EPA) Health Advisory—An estimate of acceptable drinking water 

levels for a chemical substance based on health effects information. A health advisory is not a legally 

enforceable federal standard, but serves as technical guidance to assist federal, state, and local officials. 

Epidemiology—Refers to the investigation of factors that determine the frequency and distribution of 

disease or other health-related conditions within a defined human population during a specified period. 

Genotoxicity—A specific adverse effect on the genome of living cells that, upon the duplication of 

affected cells, can be expressed as a mutagenic, clastogenic or carcinogenic event because of specific 

alteration of the molecular structure of the genome. 

Half-life—A measure of rate for the time required to eliminate one half of a quantity of a chemical from 

the body or environmental media. 

Immediately Dangerous to Life or Health (IDLH)—The maximum environmental concentration of a 

contaminant from which one could escape within 30 minutes without any escape-impairing symptoms or 

irreversible health effects. 

Incidence—The ratio of individuals in a population who develop a specified condition to the total 

number of individuals in that population who could have developed that condition in a specified time 

period. 
METHOXYCHLOR 240 

10. GLOSSARY 

Intermediate Exposure—Exposure to a chemical for a duration of 15–364 days, as specified in the 

Toxicological Profiles. 

Immunologic Toxicity—The occurrence of adverse effects on the immune system that may result from 

exposure to environmental agents such as chemicals. 

Immunological Effects—Functional changes in the immune response. 

In Vitro—Isolated from the living organism and artificially maintained, as in a test tube. 

In Vivo—Occurring within the living organism. 

Lethal Concentration(LO) (LCLO)—The lowest concentration of a chemical in air which has been 

reported to have caused death in humans or animals. 

Lethal Concentration(50) (LC50)—A calculated concentration of a chemical in air to which exposure for a 

specific length of time is expected to cause death in 50% of a defined experimental animal population. 

Lethal Dose(LO) (LDLO)—The lowest dose of a chemical introduced by a route other than inhalation that 

has been reported to have caused death in humans or animals. 

Lethal Dose(50) (LD50)—The dose of a chemical which has been calculated to cause death in 50% of a 

defined experimental animal population. 

Lethal Time(50) (LT50)—A calculated period of time within which a specific concentration of a chemical 

is expected to cause death in 50% of a defined experimental animal population. 

Lowest-Observed-Adverse-Effect Level (LOAEL)—The lowest exposure level of chemical in a study, 

or group of studies, that produces statistically or biologically significant increases in frequency or severity 

of adverse effects between the exposed population and its appropriate control. 

Lymphoreticular Effects—Represent morphological effects involving lymphatic tissues such as the 

lymph nodes, spleen, and thymus. 

Malformations—Permanent structural changes that may adversely affect survival, development, or 

function. 

Minimal Risk Level (MRL)—An estimate of daily human exposure to a hazardous substance that is 

likely to be without an appreciable risk of adverse noncancer health effects over a specified route and 

duration of exposure. 

Modifying Factor (MF)—A value (greater than zero) that is applied to the derivation of a minimal risk 

level (MRL) to reflect additional concerns about the database that are not covered by the uncertainty 

factors. The default value for a MF is 1. 

Morbidity—State of being diseased; morbidity rate is the incidence or prevalence of disease in a specific 

population. 

Mortality—Death; mortality rate is a measure of the number of deaths in a population during a specified 

interval of time. 
METHOXYCHLOR 241 

10. GLOSSARY 

Mutagen—A substance that causes mutations. A mutation is a change in the DNA sequence of a cell’s 

DNA. Mutations can lead to birth defects, miscarriages, or cancer. 

Necropsy—The gross examination of the organs and tissues of a dead body to determine the cause of 

death or pathological conditions. 

Neurotoxicity—The occurrence of adverse effects on the nervous system following exposure to a 

chemical. 

No-Observed-Adverse-Effect Level (NOAEL)—The dose of a chemical at which there were no 

statistically or biologically significant increases in frequency or severity of adverse effects seen between 

the exposed population and its appropriate control. Effects may be produced at this dose, but they are not 

considered to be adverse. 

Octanol-Water Partition Coefficient (Kow)—The equilibrium ratio of the concentrations of a chemical 

in n-octanol and water, in dilute solution. 

Odds Ratio (OR)—A means of measuring the association between an exposure (such as toxic substances 

and a disease or condition) which represents the best estimate of relative risk (risk as a ratio of the 

incidence among subjects exposed to a particular risk factor divided by the incidence among subjects who 

were not exposed to the risk factor). An odds ratio of greater than 1 is considered to indicate greater risk 

of disease in the exposed group compared to the unexposed. 

Organophosphate or Organophosphorus Compound—A phosphorus containing organic compound 

and especially a pesticide that acts by inhibiting cholinesterase. 

Permissible Exposure Limit (PEL)—An Occupational Safety and Health Administration (OSHA) 

allowable exposure level in workplace air averaged over an 8-hour shift of a 40-hour workweek. 

Pesticide—General classification of chemicals specifically developed and produced for use in the control 

of agricultural and public health pests. 

Pharmacokinetics—The science of quantitatively predicting the fate (disposition) of an exogenous 

substance in an organism. Utilizing computational techniques, it provides the means of studying the 

absorption, distribution, metabolism and excretion of chemicals by the body. 

Pharmacokinetic Model—A set of equations that can be used to describe the time course of a parent 

chemical or metabolite in an animal system. There are two types of pharmacokinetic models: data-based 

and physiologically-based. A data-based model divides the animal system into a series of compartments 

which, in general, do not represent real, identifiable anatomic regions of the body whereby the 

physiologically-based model compartments represent real anatomic regions of the body. 

Physiologically Based Pharmacodynamic (PBPD) Model—A type of physiologically-based doseresponse model which quantitatively describes the relationship between target tissue dose and toxic end 

points. These models advance the importance of physiologically based models in that they clearly 

describe the biological effect (response) produced by the system following exposure to an exogenous 

substance. 
METHOXYCHLOR 242 

10. GLOSSARY 

Physiologically Based Pharmacokinetic (PBPK) Model—Comprised of a series of compartments 

representing organs or tissue groups with realistic weights and blood flows. These models require a 

variety of physiological information: tissue volumes, blood flow rates to tissues, cardiac output, alveolar 

ventilation rates and, possibly membrane permeabilities. The models also utilize biochemical information 

such as air/blood partition coefficients, and metabolic parameters. PBPK models are also called 

biologically based tissue dosimetry models. 

Prevalence—The number of cases of a disease or condition in a population at one point in time. 

Prospective Study—A type of cohort study in which the pertinent observations are made on events 

occurring after the start of the study. A group is followed over time. 

q1*—The upper-bound estimate of the low-dose slope of the dose-response curve as determined by the 

multistage procedure. The q1* can be used to calculate an estimate of carcinogenic potency, the 

incremental excess cancer risk per unit of exposure (usually μg/L for water, mg/kg/day for food, and 

μg/m3

 for air). 

Recommended Exposure Limit (REL)—A National Institute for Occupational Safety and Health 

(NIOSH) time-weighted average (TWA) concentrations for up to a 10-hour workday during a 40-hour 

workweek. 

Reference Concentration (RfC)—An estimate (with uncertainty spanning perhaps an order of 

magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) 

that is likely to be without an appreciable risk of deleterious noncancer health effects during a lifetime. 

The inhalation reference concentration is for continuous inhalation exposures and is appropriately 

expressed in units of mg/m3 or ppm. 

Reference Dose (RfD)—An estimate (with uncertainty spanning perhaps an order of magnitude) of the 

daily exposure of the human population to a potential hazard that is likely to be without risk of deleterious 

effects during a lifetime. The RfD is operationally derived from the no-observed-adverse-effect level 

(NOAEL-from animal and human studies) by a consistent application of uncertainty factors that reflect 

various types of data used to estimate RfDs and an additional modifying factor, which is based on a 

professional judgment of the entire database on the chemical. The RfDs are not applicable to 

nonthreshold effects such as cancer. 

Reportable Quantity (RQ)—The quantity of a hazardous substance that is considered reportable under 

the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Reportable 

quantities are (1) 1 pound or greater or (2) for selected substances, an amount established by regulation 

either under CERCLA or under Section 311 of the Clean Water Act. Quantities are measured over a 

24-hour period. 

Reproductive Toxicity—The occurrence of adverse effects on the reproductive system that may result 

from exposure to a chemical. The toxicity may be directed to the reproductive organs and/or the related 

endocrine system. The manifestation of such toxicity may be noted as alterations in sexual behavior, 

fertility, pregnancy outcomes, or modifications in other functions that are dependent on the integrity of 

this system. 

Retrospective Study—A type of cohort study based on a group of persons known to have been exposed 

at some time in the past. Data are collected from routinely recorded events, up to the time the study is 
METHOXYCHLOR 243 

10. GLOSSARY 

undertaken. Retrospective studies are limited to causal factors that can be ascertained from existing 

records and/or examining survivors of the cohort. 

Risk—The possibility or chance that some adverse effect will result from a given exposure to a chemical. 

Risk Factor—An aspect of personal behavior or lifestyle, an environmental exposure, or an inborn or 

inherited characteristic, that is associated with an increased occurrence of disease or other health-related 

event or condition. 

Risk Ratio—The ratio of the risk among persons with specific risk factors compared to the risk among 

persons without risk factors. A risk ratio greater than 1 indicates greater risk of disease in the exposed 

group compared to the unexposed. 

Short-Term Exposure Limit (STEL)—The American Conference of Governmental Industrial 

Hygienists (ACGIH) maximum concentration to which workers can be exposed for up to 15 min 

continually. No more than four excursions are allowed per day, and there must be at least 60 min 

between exposure periods. The daily Threshold Limit Value - Time Weighted Average (TLV-TWA) may 

not be exceeded. 

Standardized Mortality Ratio (SMR)—A ratio of the observed number of deaths and the expected 

number of deaths in a specific standard population. 

Target Organ Toxicity—This term covers a broad range of adverse effects on target organs or 

physiological systems (e.g., renal, cardiovascular) extending from those arising through a single limited 

exposure to those assumed over a lifetime of exposure to a chemical. 

Teratogen—A chemical that causes structural defects that affect the development of an organism. 

Threshold Limit Value (TLV)—An American Conference of Governmental Industrial Hygienists 

(ACGIH) concentration of a substance to which most workers can be exposed without adverse effect. 

The TLV may be expressed as a Time Weighted Average (TWA), as a Short-Term Exposure Limit 

(STEL), or as a ceiling limit (CL). 

Time-Weighted Average (TWA)—An allowable exposure concentration averaged over a normal 8-hour 

workday or 40-hour workweek. 

Toxic Dose(50) (TD50)—A calculated dose of a chemical, introduced by a route other than inhalation, 

which is expected to cause a specific toxic effect in 50% of a defined experimental animal population. 

Toxicokinetic—The study of the absorption, distribution and elimination of toxic compounds in the 

living organism. 
METHOXYCHLOR 244 

10. GLOSSARY 

Uncertainty Factor (UF)—A factor used in operationally deriving the Minimal Risk Level (MRL) or 

Reference Dose (RfD) or Reference Concentration (RfC) from experimental data. UFs are intended to 

account for (1) the variation in sensitivity among the members of the human population, (2) the 

uncertainty in extrapolating animal data to the case of human, (3) the uncertainty in extrapolating from 

data obtained in a study that is of less than lifetime exposure, and (4) the uncertainty in using lowestobserved-adverse-effect level (LOAEL) data rather than no-observed-adverse-effect level (NOAEL) data. 

A default for each individual UF is 10; if complete certainty in data exists, a value of one can be used; 

however a reduced UF of three may be used on a case-by-case basis, three being the approximate 

logarithmic average of 10 and 1. 

Xenobiotic—Any chemical that is foreign to the biological system. 

METHOXYCHLOR A-1 

APPENDIX A
 

ATSDR MINIMAL RISK LEVEL AND WORKSHEETS
 

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) [42 U.S.C. 

9601 et seq.], as amended by the Superfund Amendments and Reauthorization Act (SARA) [Pub. L. 

99–499], requires that the Agency for Toxic Substances and Disease Registry (ATSDR) develop jointly 

with the U.S. Environmental Protection Agency (EPA), in order of priority, a list of hazardous substances 

most commonly found at facilities on the CERCLA National Priorities List (NPL); prepare toxicological 

profiles for each substance included on the priority list of hazardous substances; and assure the initiation 

of a research program to fill identified data needs associated with the substances. 

The toxicological profiles include an examination, summary, and interpretation of available toxicological 

information and epidemiologic evaluations of a hazardous substance. During the development of 

toxicological profiles, Minimal Risk Levels (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 for a 

given route of exposure. An MRL is an estimate of the daily human exposure to a hazardous substance 

that is likely to be without appreciable risk of adverse noncancer health effects over a specified duration 

of exposure. MRLs are based on noncancer health effects only and are not based on a consideration of 

cancer effects. These substance-specific estimates, which are intended to serve as screening levels, are 

used by ATSDR health assessors to identify contaminants and potential health effects that may be of 

concern at hazardous waste sites. It is important to note that MRLs are not intended to define clean-up or 

action levels. 

MRLs are derived for hazardous substances using the no-observed-adverse-effect level/uncertainty factor 

approach. They are below levels that might cause adverse health effects in the people most sensitive to 

such chemical-induced effects. MRLs are derived for acute (1–14 days), intermediate (15–364 days), and 

chronic (365 days and longer) durations and for the oral and inhalation routes of exposure. Currently, 

MRLs for the dermal route of exposure are not derived because ATSDR has not yet identified a method 

suitable for this route of exposure. MRLs are generally based on the most sensitive chemical-induced end 

point considered to be of relevance to humans. Serious health effects (such as irreparable damage to the 

liver or kidneys, or birth defects) are not used as a basis for establishing MRLs. Exposure to a level 

above the MRL does not mean that adverse health effects will occur. 
METHOXYCHLOR A-2 

APPENDIX A 

MRLs are intended only to serve as a screening tool to help public health professionals decide where to 

look more closely. They may also be viewed as a mechanism to identify those hazardous waste sites that 

are not expected to cause adverse health effects. Most MRLs contain a degree of uncertainty because of 

the lack of precise toxicological information on the people who might be most sensitive (e.g., infants, 

elderly, nutritionally or immunologically compromised) to the effects of hazardous substances. ATSDR 

uses a conservative (i.e., protective) approach to address this uncertainty consistent with the public health 

principle of prevention. Although human data are preferred, MRLs often must be based on animal studies 

because relevant human studies are lacking. In the absence of evidence to the contrary, ATSDR assumes 

that humans are more sensitive to the effects of hazardous substance than animals and that certain persons 

may be particularly sensitive. Thus, the resulting MRL may be as much as a hundredfold below levels 

that have been shown to be nontoxic in laboratory animals. 

Proposed MRLs undergo a rigorous review process: Health Effects/MRL Workgroup reviews within the 

Division of Toxicology, expert panel peer reviews, and agencywide MRL Workgroup reviews, with 

participation from other federal agencies and comments from the public. They are subject to change as 

new information becomes available concomitant with updating the toxicological profiles. Thus, MRLs in 

the most recent toxicological profiles supersede previously published levels. For additional information 

regarding MRLs, please contact the Division of Toxicology, Agency for Toxic Substances and Disease 

Registry, 1600 Clifton Road, Mailstop E-29, Atlanta, Georgia 30333. 
METHOXYCHLOR A-3 

APPENDIX A 

Rationale Statement 

(Acute-duration oral MRL) 

An acute-duration oral MRL for methoxychlor has not been derived. ATSDR has withdrawn the previous 

MRL of 0.02 mg/kg/day for acute-duration exposure derived in the 1994 Toxicological Profile for 

Methoxychlor. This MRL was based on precocious vaginal opening (early puberty) observed in rats 

exposed to 25 mg/kg/day for 59–104 days (Gray et al., 1989). This study did not test doses as low as the 

current intermediate-duration MRL study (Chapin et al. 1997), which demonstrated the same effect from 

5 mg/kg/day administered from gestation day 14 to postnatal day 42. Thus, there is some question as to 

whether the premature puberty would have occurred at a lower acute dose than the 25 mg/kg/day 

observed in the Gray et al. (1989) study. 

A variety of candidate MRL studies were considered for derivation of an acute-duration oral MRL for 

methoxychlor. There were several hypothesis-generating studies at extremely low doses that were not 

definitive enough to use for MRL derivation. A synopsis of each candidate MRL study and the reasons 

for not using it follow; these two candidate studies probably bracket the upper and lower bounds of where 

the true MRL should lie. 

Upper Bound 

Reference: Gray LE, Otsby J, Ferrell J, et al. 1989. A dose-response analysis of methoxychlor-induced 

alterations of the reproductive development and function in the rat. Fund Appl Toxicol 12:92-109. 

Experimental design: In block 2 of this study, groups of eight immature Long-Evans hooded rats of each 

sex were exposed to either 0, 25, or 50 mg/kg/day technical grade methoxychlor for 59–104 days 

beginning at 21 days of age by gavage in corn oil. Females were monitored for onset of vaginal opening, 

onset of estrus, estrus cyclicity, fertility, litter size, number of implantation sites, organ weights, and 

ovarian and uterine histology. Males were monitored for preputial separation, testis weight, and sperm 

count. 

Effects noted in study and corresponding doses: Female rats exposed to 25 mg/kg/day or more exhibited 

younger age at vaginal estrus and vaginal opening after 1 week of exposure. Vaginal opening occurred at 

an average age of 26 days in rats exposed to 25 mg/kg/day, compared with an average vaginal-opening 

age of 32–33 days in control rats. Atypical vaginal smears (decreased leukocytes, increased 

cornification) were noted in females exposed to 50 mg/kg/day. This study identifies an acute oral 

LOAEL of 25 mg/kg/day for reproductive/developmental effects in female rats. 

Lower Bound 

Reference: Welshons WV, Nagel SC, Thayer KA, et al. 1999. Low-dose bioactivity of xenoestrogens in 

animals: fetal exposure to low doses of methoxychlor and other xenoestrogens increases adult prostate 

size in mice. Toxicol Ind Health 15:12-25. 

Experimental design: Adult female pregnant CF-1 mice were administered 0 (n=9), 0.02 (n=6), or 

2.0 (n=5) mg methoxychlor/kg/day in corn oil on gestation days 11–17. Pups were weaned at postpartum 

day 23 and males were housed together until 8.5 months of age. One male from each litter was housed 

individually for 4 weeks, then killed, and the prostate, seminal vesicles, preputial glands, liver, and 

adrenals were removed and weighed (seminal vesicles and preputial glands were blotted to remove fluid 

before weighing). 
METHOXYCHLOR A-4 

APPENDIX A 

Effects noted in study and corresponding doses: Prostate weight was statistically significantly increased 

by 61 and 51% in the 0.02 and 2.0 mg/kg/day groups, respectively; seminal vesicle weight was increased 

by 20% in the 2.0 mg/kg/day group; and liver weight was decreased by 5% in both treatment groups. 

Although no positive control was done in this study, the methoxychlor-induced prostate enlargement was 

greater than the enlargement observed in previous studies by the same investigators examining effects 

from gestational exposure to other estrogen-receptor ligands (estradiol or DES). There were no 

statistically significant exposure-related changes in body weight or weights of preputial glands, testes, or 

adrenals. No histological examinations were performed. 

Problems with using this study for risk assessment calculations: This is a hypothesis generating study, 

not a definitive one. There are several areas in which its design is less than ideal. EPA 1998 Health 

Effects Test Guidelines OPPTS 870.3700 Prenatal Developmental Toxicity Study (EPA 1998) 

recommends that for a definitive study, each treatment group has 20 pregnant dams yielding offspring; 

Welshons et al.(1999) only used 5–6 pregnant dams in the treatment groups. These guidelines do not 

specify how many offspring from each litter should be measured. In good developmental studies, the 

litter is considered the unit of measurement for statistical calculations (Tyl 2000), but the measurement of 

the litter response still needs to be accurate. In many thorough developmental studies, such as the Chapin 

et al. (1997) study used for intermediate MRL derivation, every animal in the litter is assessed and for sex 

specific end points, each animal of a given sex is measured. The use of only one male from each litter 

raises questions about the representativeness of the measurement of the litter response; some type of 

selection bias might have occurred. In an analysis of a similar study in which only one or two animals out 

of a litter were measured for effects on prostate size, it was found that only measuring one animal per 

litter resulted in incorrect conclusions 50% of the time, when compared to a study in which all male 

offspring in each litter were measured (Elwsick et al. 2000a, 2000b; Janszen et al. 2000). 

Measuring only one male out of each litter for a characteristic known to vary between litter members may 

not be a good experimental design strategy. Prostate size in untreated rodents normally varies between 

males in a litter depending on how they are positioned relative to the females in the litter; males 

positioned between two females are exposed to more estrogen and consequently have larger prostates than 

males positioned between two other males (Timms et al. 1999). It is unknown whether treatment with an 

exogenous estrogen-like compound would decrease the variability of prostate weight within a litter. 

As mentioned above, in utero exposure to estrogens is one factor that influences prostate weight and some 

exposure to estrogen may result from the intrauterine position of male fetuses in relation to females. This 

is a natural consequence of the physiology of multiparous animals. The magnitude of the prostate weight 

differences resulting from intrauterine position has not been precisely measured; data on this topic would 

facilitate comparisons with the magnitude of effects produced by methoxychlor. It would be interesting 

to have data on prostate weights in adult males whose intrauterine position was known via observation 

after caesarian section delivery. Timms et al. (1999) did measure cross-sectional areas of prostate 

histology sections, and lengths of prostate buds and estimated prostate volume in rats with a computer 

model. Data on differences between cross sectional area in prostates from males positioned between two 

other males versus males positioned between two females is presented in Figure 2 of the publication, but 

no direct comparisons are made of volume or weight. It appears from the figure that budding areas of 

certain parts of the prostate can vary by a factor of about 2-fold as a function of intrauterine position. 

Another problem with the Welshons et al. (1999) study is its lack of appropriate positive controls; ideally, 

one of these would have included various doses of estradiol. Vom Saal et al. (1997) includes data on the 

prostate weight effects of in utero exposure to estradiol continuously delivered from implanted silastic 

capsules, but this method differs from the once a day dosing of methoxychlor in the Welshons et al. 

(1999) paper. The prostate effects of the extremely potent synthetic estrogen DES administered by the 
METHOXYCHLOR A-5 

APPENDIX A 

same methods as Welshons et al. (1999) have been reported from experiments done at a different time 

(vom Saal et al. 1997). Although lower doses of DES produced the same increased prostate weight effect 

in these experiments as methoxychlor did in the Welshons et al. (1999) study, big differences in the 

magnitude of the response and less than expected differences in the effectiveness of DES and 

methoxychlor in producing this response raise some questions about how exactly reproducible the results 

of this experimental protocol are. As can be seen in the table below, the maximum percent increase in 

prostate weight produced by methoxychlor is 61.5% while that produced by DES is only 29%. Also, 

there is only a 100-fold difference between the doses of methoxychlor and DES producing the maximal 

prostate weight increase; a greater fold difference would have been expected based on the fact that DES is 

an extremely potent estrogen receptor agonist while methoxychlor is a weak one (Dodge et al. 1996; 

Kuiper et al. 1998; Ousterhout et al. 1981). 

Welshons et al. 1999 vom Saal et al. 1997 

Methoxychlor 

μg/kg/day 

mg prostate weight 

adjusted by ANCOVA 

for body weight (percent 

increase from controls) DES μg/kg/day 

mg prostate weight 

thought to be adjusted by 

ANCOVA for body 

weight (percent increase 

from controls) 

0 40.0 (–) 0 41.5 (–) 

20 64.5 (61%) 0.002 40.0 (-4%) 

2000 60.3 (51%) 0.02 48.0 (20%) 

0.2 55.0 (38%) 

2.0 49.0 (21%) 

20 47.0 (19%) 

200 32.0 (-20%) 

It has been shown that low levels of estradiol (0.32 pg/mL serum, a 50% increase in free-serum estradiol 

over the endogenous level, released continuously from a silastic implant) and DES (0.02, 0.2, and 

2.0 μg/kg/day) administered to pregnant mice during gestation produces increased prostate weight in 

adult male offspring, while higher and lower exposure levels of estradiol (0.21 and 0.56 pg/mL and 

above) and DES (0.002 and 200 μg/kg/day) resulted in a decrease in prostate weight (vom Saal et al. 

1997). This results in an inverted U-shaped dose-response curve. Gestational exposure to low levels of 

methoxychlor (0.02–2.0 mg/kg/day) have also been shown to result in increased prostate weight 

(Welshons et al. 1999), while exposure of weanling to adult rats to high levels (100–1,400 mg/kg/day) of 

methoxychlor have been shown to result in decreased prostate weight (Shain et al. 1977; Tullner and 

Edgcomb 1962), as well as testicular atrophy (Bal 1984; Hodge et al. 1950; Shain et al. 1977; Tullner and 

Edgcomb 1962). Adult mice exposed to 60 mg/kg/day methoxychlor developed testicular degeneration 

(Wenda-Rozewicka 1983). 

Although the Welshons et al. (1999) study is not definitive enough to be the basis of an MRL, the 

suggestion that gestational exposure to methoxychlor, and other estrogenic compounds, can increase 

prostate weight at some dose is worthy of further investigation. This issue has been featured prominently 

in Section 3.12.2, Identification of Data Needs, of this Toxicological Profile. 
METHOXYCHLOR A-6 

APPENDIX A 

A peer review panel was organized by the National Institute of Environmental Health Sciences (NIEHS), 

National Institute of Health (NIH), National Toxicology Program (NTP) to examine low-dose effects of 

endocrine disruptors (NTP 2001). The panel examined a number of studies involving estrogen and 

several estrogenic chemicals (including methoxychlor), androgens, and antiandrogens, as well as 

biological factors and study design, statistics, and dose-response modeling. The panel’s overall 

conclusions included: 

(1)	 While low-dose effects have been observed in some laboratory animals with certain 

endocrine disruptors, they are compound- and end point-specific, and in some cases they 

have not been replicated in studies by other investigators. Additionally, the toxicological 

significance of some of the end points is not known. 

(2)	 The shape of the dose-response curve may be low-dose linear, threshold appearing, or 

non-monotonic. The curve shape varies with the end point and dosing regimen. 

(3)	 Previously reported key low-dose findings need to be replicated, and studies are needed 

to characterize target tissue dosimetry, identify sensitive molecular markers, and 

determine the long-term health consequences of low-dose effects. 

(4)	 The current testing paradigm for reproductive and developmental toxicity should be 

revisited to determine if changes need to be made regarding dose selection, animal model 

selection, age of animals when evaluated, and end points measured. 

Conclusions regarding methoxychlor studies included: 

(1)	 Methoxychlor is a weakly estrogenic chemical that can induce uterotrophism in immature 

rodents. 

(2)	 There is a wide range of changes in estrogen sensitive organs at doses of 5 mg/kg/day and 

higher. 

(3)	 Some immune effects, which need to be further evaluated to determine their toxicological 

significance, were seen following exposure to 1 mg/kg/day. 

(4)	 More data are needed on the differences in toxicology of technical grade and pure 

methoxychlor. 

Therefore, ATSDR has concluded that without additional data to confirm the causal relationship between 

exposure to extremely low doses of methoxychlor and increased prostate weight in adult male offspring 

(and other estrogen-related effects), derivation of an MRL in the nanogram range is not justified. 

However, the current data do suggest that low-dose effects of methoxychlor may be real, and more 

definitive studies are necessary to examine them further. An intermediate oral MRL of 0.005 mg/kg/day 

has been derived based on data that are well supported by the database and based on an end point that is 

well-established for methoxychlor (accelerated onset of puberty). 

A potential acute-duration oral MRL of 0.00002 mg/kg/day can be derived from the LOAEL of 0.02 

mg/kg/day in the Welshons et al. (1999) study dividing by an uncertainty factor of 1,000 (10 for use of a 

LOAEL, 10 for extrapolation from animals to humans, and 10 for human variability). 
METHOXYCHLOR A-7 

APPENDIX A 

Other Studies Considered for Use as the Basis for an Acute Oral MRL: 

(1)	 A LOAEL of 0.02 mg/kg/day for increased urine-marking behavior in mice when placed 

in a new territory (vom Saal et al. 1995). Only two male pups per litter were tested and 

apparently, each was only tested one time; the accuracy of the assessment of the test was 

somewhat questionable; it is unknown how reproducible the results are, and there was no 

indication of what, if any, statistical methods were used to evaluate the results. 

(2)	 A LOAEL of 1.8 mg/kg/day for aggressive behavior (infanticide) in mice toward an 

unrelated pup (Parmigiani et al. 1998). The strain of mouse used was the “house mouse,” 

and no information was provided on the specifics of the strain or whether they were 

inbred; only two male pups per litter were tested; the effect was only seen at one middle 

dose (no dose-response); and no effect was seen with DES, a positive estrogenic control. 

(3)	 A LOAEL of 0.02 mg/kg/day for decreased aggression of young male mice toward male 

siblings at postpartum day 39, but not at postpartum 54 (Palanza et al. 1999). The effect 

was transient and there was no dose-response (no effect was seen at 200 or 

2,000 mg/kg/day). 

(4)	 A LOAEL of 16.7 mg/kg/day methoxychlor for a 2-fold increase in uterine weight in 

ovariectomized mice (Tullner 1961). There was little information about the condition of 

the animals used in the experiments; the estrogenic activity of methoxychlor was 

discovered serendipitously following the dusting of the mice (being used for an 

experiment not related to methoxychlor) for parasite control. 

Agency Contact (Chemical Manager): Lori L. Miller, M.P.H. 
METHOXYCHLOR A-8 

APPENDIX A 

MINIMAL RISK LEVEL WORKSHEET 

Chemical Name: Methoxychlor 

CAS Number: 77-43-5 

Date: September 2002 

Profile Status: Final Post Public 

Route: [ ] Inhalation [ X ] Oral 

Duration: [ ] Acute [ X ] Intermediate [ ] Chronic 

Graph Key: 33 

Species: Mice 

Minimal Risk Level: 0.005 [ X ] mg/kg/day [ ] ppm 

Reference: Chapin RE, Harris MW, Davis BJ, et al. 1997. The effects of perinatal/juvenile methoxychlor 

exposure on adult rat nervous, immune, and reproductive system function. Fundam Appl Toxicol 40:138­

157. 

Experimental design: Pregnant female rats were administered 0, 5, 50, or 150 mg 95% pure 

methoxychlor/kg/day from gestation day 14 to postpartum day 7 by gavage in corn oil, then pups were 

dosed directly in the same manner until postpartum day 42. At birth, pups were weighed, measured for 

anogenital distance, and checked for external malformations. The first day of vaginal opening or 

preputial separation was recorded. At about 12 weeks of age, one male and one female from each litter 

were mated: each treated male to two untreated females and every two treated females to one fertile 

untreated male. Pups were counted, sexed, and weighed at birth and removed from the dam at postpartum 

day 11. The dams were mated again and killed at gestation day 19. Uterine contents were analyzed for 

fetal number, sex, and number of dead implants and resorptions. Blood was drawn from treated males for 

hematology and clinical chemistry. The gonads and associated reproductive organs were examined for 

structural malformations, weighed, and fixed for histology. Testicular spermatid head count and 

epididymide sperm motility and sperm count were performed. Blood was collected from 10 unmated 

treated females for determination of serum estradiol, progesterone, and follicle stimulating hormone 

(FSH). 

Effects noted in study and corresponding doses: Dams exposed to 150 mg/kg/day methoxychlor had 

fewer live pups/litter than controls. Precocious vaginal opening was evident (statistically significant) in 

all methoxychlor-treated groups (postnatal days 37.4, 35.2, 30.8, and 33.4, respectively, for groups 0, 5, 

50, and 150 mg/kg/day). Estrus cycles were highly irregular or absent in the 50 and 150 mg/kg/day 

females. There was a severe reduction in the number of females that conceived in the 50 and 

150 mg/kg/day groups (3/15 and 0/15, respectively, compared to 13/15 in the controls). There were no 

statistically significant effects on litter size or pup mortality or weight gain to postpartum day 10. In the 

second mating, the 50 mg/kg/day litters had only 32% of the implants of the controls. Empty uterine 

weight of pregnant females was reduced by 20 and 51% in the 5 and 50 mg/kg/day groups, respectively. 

Non-pregnant females also showed reduced uterine weight at the high dose. Histologically, uteri from the 

rats in the 50 mg/kg/day group that did not get pregnant showed mild to severe endometrial squamous 

metaplasia and endometrial hyperplasia; vaginas of six of the rats were cornified, and one rat had vaginal 

epithelial hyperplasia. Ovaries in this group were generally polycystic, two females had cystic oviducts, 

there were few corpora lutea, and mammary tissue was underdeveloped. The effects in the 

150 mg/kg/day group were similar. 

In the male mating trials, the number of females with vaginal sperm was statistically significantly reduced 

in the 150 mg/kg/day group. Litter size, postpartum pup death, and pup weight were not affected, and 
 

METHOXYCHLOR A-9 

APPENDIX A 

there was no increase in resorptions or preimplantation losses. Preputial separation was delayed by 8 and 

34 days in the 50 and 150 mg/kg/day groups, respectively. Weights of the testes and right epididymis 

were significantly reduced in the 50 and 150 mg/kg/day group, and weights of the left cauda epididymis, 

seminal vesicles, and ventral prostate were significantly reduced at 150 mg/kg/day. Sperm motility and 

epididymal sperm density were also reduced at 150 mg/kg/day. A dose-related inhibition of testes 

development was observed. Mild epithelial disorganization was seen in the testes of one rat in each of the 

50 and 150 mg/kg/day groups and the testes of one rat in the 150 mg/kg/day group were completely 

atrophic. No treatment-related changes were noted in clinical chemistries and hematology. The serum 

estrogen:progesterone ratio was significantly elevated in the 50 and 150 mg/kg/day groups, and the FSH 

levels were significantly suppressed in all methoxychlor-treated rats verified to be in estrous. 

Dose and end point used for MRL derivation: 5 mg/kg/day from gestation day 14 to postpartum day 42, 

accelerated onset of puberty 

[ ] NOAEL [ X ] LOAEL 

Uncertainty Factors used in MRL derivation: 

[ X ] 10 for use of a LOAEL 

[ X ] 10 for extrapolation from animals to humans 

[ X ] 10 for human variability 

Was a conversion used from ppm in food or water to a mg/body weight dose? 

If so, explain: None needed. 

If an inhalation study in animals, list the conversion factors used in determining human equivalent dose: 

N/A 

Other additional studies or pertinent information which lend support to this MRL: The MRL is supported 

by other observations of reproductive effects associated with intermediate-duration exposure including 

elevated levels of prolactin in the pituitary of male rats exposed to 50 mg/kg/day (Goldman et al. 1986; 

Gray et al. 1989), decreased seminal vesicle weight, caudal epididymal weight, and caudal epididymal 

sperm count (Gray et al. 1989), and increased gonadotropin releasing hormone in the mediobasal 

hypothalamus in male rats exposed to 50 mg/kg/day (Goldman et al. 1986). At higher exposures levels, 

intermediate-duration studies show decreased fertility in male rats at doses of 60–400 mg/kg/day (Chapin 

et al. 1997; Gray et al. 1989, 1999; Harris et al. 1974), and in female rats at doses of 50–150 mg/kg/day 

(Bal 1984; Chapin et al. 1997). A study in humans described by Stein (1968), Coulston and Serrone 

(1969), and Wills (1969) that identified a NOAEL of 2 mg/kg/day for effects to the testes and menstrual 

cycle was not chosen as the basis for an MRL because reproductive function was not evaluated, and such 

an MRL may not be protective of reproductive and developmental effects in the fetus or child. 

Comparison with acute study on which EPA RfD was based. 

An RfD of 0.005 mg/kg/day has been derived by EPA (IRIS 2002; RfD derived 1991) based on a NOEL 

of 5.01 mg/kg/day administered on gestation days 7–19 in New Zealand White rabbits for maternal 

toxicity observed as excessive loss of litters (Kincaid Enterprises 1986) and an uncertainty factor of 

1,000 (10 for interspecies differences; 10 for intraspecies differences; and 10 for the poor quality of the 

critical study and for the incompleteness of the database on chronic toxicity). 

Agency Contact (Chemical Manager): Lori L. Miller, M.P.H. 
METHOXYCHLOR B-1 

APPENDIX B 

USER'S GUIDE 

Chapter 1 

Public Health Statement 

This chapter of the profile is a health effects summary written in non-technical language. Its intended 

audience is the general public especially people living in the vicinity of a hazardous waste site or 

chemical release. If the Public Health Statement were removed from the rest of the document, it would 

still communicate to the lay public essential information about the chemical. 

The major headings in the Public Health Statement are useful to find specific topics of concern. The 

topics are written in a question and answer format. The answer to each question includes a sentence that 

will direct the reader to chapters in the profile that will provide more information on the given topic. 

Chapter 2 

Relevance to Public Health 

This chapter provides a health effects summary based on evaluations of existing toxicologic, 

epidemiologic, and toxicokinetic information. This summary is designed to present interpretive, 

weight-of-evidence discussions for human health end points by addressing the following questions. 

1.	 What effects are known to occur in humans? 

2.	 What effects observed in animals are likely to be of concern to humans? 

3.	 What exposure conditions are likely to be of concern to humans, especially around hazardous 

waste sites? 

The chapter covers end points in the same order they appear within the Discussion of Health Effects by 

Route of Exposure section, by route (inhalation, oral, dermal) and within route by effect. Human data are 

presented first, then animal data. Both are organized by duration (acute, intermediate, chronic). In vitro 

data and data from parenteral routes (intramuscular, intravenous, subcutaneous, etc.) are also considered 

in this chapter. If data are located in the scientific literature, a table of genotoxicity information is 

included. 

The carcinogenic potential of the profiled substance is qualitatively evaluated, when appropriate, using 

existing toxicokinetic, genotoxic, and carcinogenic data. ATSDR does not currently assess cancer 

potency or perform cancer risk assessments. Minimal risk levels (MRLs) for noncancer end points (if 

derived) and the end points from which they were derived are indicated and discussed. 

Limitations to existing scientific literature that prevent a satisfactory evaluation of the relevance to public 

health are identified in the Chapter 3 Data Needs section. 
METHOXYCHLOR B-2 

APPENDIX B 

Interpretation of Minimal Risk Levels 

Where sufficient toxicologic information is available, we have derived minimal risk levels (MRLs) for 

inhalation and oral routes of entry at each duration of exposure (acute, intermediate, and chronic). These 

MRLs are not meant to support regulatory action; but to acquaint health professionals with exposure 

levels at which adverse health effects are not expected to occur in humans. They should help physicians 

and public health officials determine the safety of a community living near a chemical emission, given the 

concentration of a contaminant in air or the estimated daily dose in water. MRLs are based largely on 

toxicological studies in animals and on reports of human occupational exposure. 

MRL users should be familiar with the toxicologic information on which the number is based. Chapter 2, 

"Relevance to Public Health," contains basic information known about the substance. Other sections such 

as Chapter 3 Section 3.9, "Interactions with Other Substances,” and Section 3.10, "Populations that are 

Unusually Susceptible" provide important supplemental information. 

MRL users should also understand the MRL derivation methodology. MRLs are derived using a 

modified version of the risk assessment methodology the Environmental Protection Agency (EPA) 

provides (Barnes and Dourson 1988) to determine reference doses for lifetime exposure (RfDs). 

To derive an MRL, ATSDR generally selects the most sensitive end point which, in its best judgement, 

represents the most sensitive human health effect for a given exposure route and duration. ATSDR 

cannot make this judgement or derive an MRL unless information (quantitative or qualitative) is available 

for all potential systemic, neurological, and developmental effects. If this information and reliable 

quantitative data on the chosen end point are available, ATSDR derives an MRL using the most sensitive 

species (when information from multiple species is available) with the highest NOAEL that does not 

exceed any adverse effect levels. When a NOAEL is not available, a lowest-observed-adverse-effect 

level (LOAEL) can be used to derive an MRL, and an uncertainty factor (UF) of 10 must be employed. 

Additional uncertainty factors of 10 must be used both for human variability to protect sensitive 

subpopulations (people who are most susceptible to the health effects caused by the substance) and for 

interspecies variability (extrapolation from animals to humans). In deriving an MRL, these individual 

uncertainty factors are multiplied together. The product is then divided into the inhalation concentration 

or oral dosage selected from the study. Uncertainty factors used in developing a substance-specific MRL 

are provided in the footnotes of the LSE Tables. 

Chapter 3 

Health Effects 

Tables and Figures for Levels of Significant Exposure (LSE) 

Tables (3-1, 3-2, and 3-3) and figures (3-1 and 3-2) are used to summarize health effects and illustrate 

graphically levels of exposure associated with those effects. These levels cover health effects observed at 

increasing dose concentrations and durations, differences in response by species, minimal risk levels 

(MRLs) to humans for noncancer end points, and EPA's estimated range associated with an upper- bound 

individual lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. Use the LSE tables and figures for a 

quick review of the health effects and to locate data for a specific exposure scenario. The LSE tables and 

figures should always be used in conjunction with the text. All entries in these tables and figures 

represent studies that provide reliable, quantitative estimates of No-Observed-Adverse-Effect Levels 

(NOAELs), Lowest-Observed-Adverse-Effect Levels (LOAELs), or Cancer Effect Levels (CELs). 
METHOXYCHLOR B-3 

APPENDIX B 

The legends presented below demonstrate the application of these tables and figures. Representative 

examples of LSE Table 3-1 and Figure 3-1 are shown. The numbers in the left column of the legends 

correspond to the numbers in the example table and figure. 

LEGEND 

See LSE Table 3-1 

(1)	 Route of Exposure One of the first considerations when reviewing the toxicity of a substance using 

these tables and figures should be the relevant and appropriate route of exposure. When sufficient 

data exists, three LSE tables and two LSE figures are presented in the document. The three LSE 

tables present data on the three principal routes of exposure, i.e., inhalation, oral, and dermal (LSE 

Table 3-1, 3-2, and 3-3, respectively). LSE figures are limited to the inhalation (LSE Figure 3-1) 

and oral (LSE Figure 3-2) routes. Not all substances will have data on each route of exposure and 

will not therefore have all five of the tables and figures. 

(2)	 Exposure Period Three exposure periods - acute (less than 15 days), intermediate (15–364 days), 

and chronic (365 days or more) are presented within each relevant route of exposure. In this 

example, an inhalation study of intermediate exposure duration is reported. For quick reference to 

health effects occurring from a known length of exposure, locate the applicable exposure period 

within the LSE table and figure. 

(3)	 Health Effect The major categories of health effects included in LSE tables and figures are death, 

systemic, immunological, neurological, developmental, reproductive, and cancer. NOAELs and 

LOAELs can be reported in the tables and figures for all effects but cancer. Systemic effects are 

further defined in the "System" column of the LSE table (see key number 18). 

(4)	 Key to Figure Each key number in the LSE table links study information to one or more data points 

using the same key number in the corresponding LSE figure. In this example, the study represented 

by key number 18 has been used to derive a NOAEL and a Less Serious LOAEL (also see the 2 

"18r" data points in Figure 3-1). 

(5)	 Species The test species, whether animal or human, are identified in this column. Chapter 2, 

"Relevance to Public Health," covers the relevance of animal data to human toxicity and 

Section 3.4, "Toxicokinetics," contains any available information on comparative toxicokinetics. 

Although NOAELs and LOAELs are species specific, the levels are extrapolated to equivalent 

human doses to derive an MRL. 

(6)	 Exposure Frequency/Duration The duration of the study and the weekly and daily exposure 

regimen are provided in this column. This permits comparison of NOAELs and LOAELs from 

different studies. In this case (key number 18), rats were exposed to 1,1,2,2-tetrachloroethane via 

inhalation for 6 hours per day, 5 days per week, for 3 weeks. For a more complete review of the 

dosing regimen refer to the appropriate sections of the text or the original reference paper, i.e., 

Nitschke et al. 1981. 

(7)	 System This column further defines the systemic effects. These systems include: respiratory, 

cardiovascular, gastrointestinal, hematological, musculoskeletal, hepatic, renal, and dermal/ocular. 

"Other" refers to any systemic effect (e.g., a decrease in body weight) not covered in these systems. 

In the example of key number 18, 1 systemic effect (respiratory) was investigated. 
METHOXYCHLOR B-4 

APPENDIX B 

(8)	 NOAEL A No-Observed-Adverse-Effect Level (NOAEL) is the highest exposure level at which no 

harmful effects were seen in the organ system studied. Key number 18 reports a NOAEL of 3 ppm 

for the respiratory system which was used to derive an intermediate exposure, inhalation MRL of 

0.005 ppm (see footnote "b"). 

(9)	 LOAEL A Lowest-Observed-Adverse-Effect Level (LOAEL) is the lowest dose used in the study 

that caused a harmful health effect. LOAELs have been classified into "Less Serious" and 

"Serious" effects. These distinctions help readers identify the levels of exposure at which adverse 

health effects first appear and the gradation of effects with increasing dose. A brief description of 

the specific end point used to quantify the adverse effect accompanies the LOAEL. The respiratory 

effect reported in key number 18 (hyperplasia) is a Less serious LOAEL of 10 ppm. MRLs are not 

derived from Serious LOAELs. 

(10)	 Reference The complete reference citation is given in Chapter 9 of the profile. 

(11)	 CEL A Cancer Effect Level (CEL) is the lowest exposure level associated with the onset of 

carcinogenesis in experimental or epidemiologic studies. CELs are always considered serious 

effects. The LSE tables and figures do not contain NOAELs for cancer, but the text may report 

doses not causing measurable cancer increases. 

(12)	 Footnotes Explanations of abbreviations or reference notes for data in the LSE tables are found in 

the footnotes. Footnote "b" indicates the NOAEL of 3 ppm in key number 18 was used to derive an 

MRL of 0.005 ppm. 

LEGEND 

See Figure 3-1 

LSE figures graphically illustrate the data presented in the corresponding LSE tables. Figures help the 

reader quickly compare health effects according to exposure concentrations for particular exposure 

periods. 

(13)	 Exposure Period The same exposure periods appear as in the LSE table. In this example, health 

effects observed within the intermediate and chronic exposure periods are illustrated. 

(14)	 Health Effect These are the categories of health effects for which reliable quantitative data exists. 

The same health effects appear in the LSE table. 

(15)	 Levels of Exposure concentrations or doses for each health effect in the LSE tables are graphically 

displayed in the LSE figures. Exposure concentration or dose is measured on the log scale "y" axis. 

Inhalation exposure is reported in mg/m3

 or ppm and oral exposure is reported in mg/kg/day. 

(16)	 NOAEL In this example, 18r NOAEL is the critical end point for which an intermediate inhalation 

exposure MRL is based. As you can see from the LSE figure key, the open-circle symbol indicates 

to a NOAEL for the test species-rat. The key number 18 corresponds to the entry in the LSE table. 

The dashed descending arrow indicates the extrapolation from the exposure level of 3 ppm (see 

entry 18 in the Table) to the MRL of 0.005 ppm (see footnote "b" in the LSE table). 
METHOXYCHLOR B-5 

APPENDIX B 

(17)	 CEL Key number 38r is 1 of 3 studies for which Cancer Effect Levels were derived. The diamond 

symbol refers to a Cancer Effect Level for the test species-mouse. The number 38 corresponds to 

the entry in the LSE table. 

(18)	 Estimated Upper-Bound Human Cancer Risk Levels This is the range associated with the 

upper-bound for lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. These risk levels are 

derived from the EPA's Human Health Assessment Group's upper-bound estimates of the slope of 

the cancer dose response curve at low dose levels (q1*). 

(19)	 Key to LSE Figure The Key explains the abbreviations and symbols used in the figure. 
B-6 

SAMPLE 

6 Table 3-1. Levels of Significant Exposure to [Chemical x] – Inhalation 1 

Exposure

Key to frequency/figurea Species duration 

2 6 INTERMEDIATE EXPOSURE

5 6

3 6 Systemic 9 9

4 6 18 Rat 13 wk

5 d/wk6 hr/d 

CHRONIC EXPOSURECancer38 Rat 18 mo

5 d/wk7 hr/d 

39 Rat 89–104 wk

5 d/wk6 hr/d 

40 Mouse 79–103 wk

5 d/wk6 hr/d 

NOAEL

System (ppm) 

7 8 

9 9

Resp 3b 

LOAEL (effect) 

Less serious (ppm) Serious (ppm) Reference

9 10 

9 9

10 (hyperplasia) Nitschke et al.

1981 

11

9

20

1010 

(CEL, multiple

organs)

(CEL, lung tumors,

nasal tumors)

(CEL, lung tumors,

hemangiosarcomas) 

Wong et al. 1982 NTP 1982 NTP 1982 

a The number corresponds to entries in Figure 3-1.

6 b Used to derive an intermediate inhalation Minimal Risk Level (MRL) of 5 x 10-3 ppm; dose adjusted for intermittent exposure and divided by 12 an uncertainty factor of 100 (10 for extrapolation from animal to humans, 10 for human variability). 

METHOXYCHLOR 

APPENDIX B 

METHOXYCHLOR C-1 

APPENDIX C 

ACRONYMS, ABBREVIATIONS, AND SYMBOLS 

ACGIH American Conference of Governmental Industrial Hygienists 

ACOEM American College of Occupational and Environmental Medicine 

ADI acceptable daily intake 

ADME absorption, distribution, metabolism, and excretion 

AED atomic emission detection 

AFID alkali flame ionization detector 

AFOSH Air Force Office of Safety and Health 

ALT alanine aminotransferase 

AML acute myeloid leukemia 

AOAC Association of Official Analytical Chemists 

AOEC Association of Occupational and Environmental Clinics 

AP alkaline phosphatase 

APHA American Public Health Association 

AST aspartate aminotranferase 

atm atmosphere 

ATSDR Agency for Toxic Substances and Disease Registry 

AWQC Ambient Water Quality Criteria 

BAT best available technology 

BCF bioconcentration factor 

BEI Biological Exposure Index 

BSC Board of Scientific Counselors 

C centigrade 

CAA Clean Air Act 

CAG Cancer Assessment Group of the U.S. Environmental Protection Agency 

CAS Chemical Abstract Services 

CDC Centers for Disease Control and Prevention 

CEL cancer effect level 

CELDS Computer-Environmental Legislative Data System 

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act 

CFR Code of Federal Regulations 

Ci curie 

CI confidence interval 

CL ceiling limit value 

CLP Contract Laboratory Program 

cm centimeter 

CML chronic myeloid leukemia 

CPSC Consumer Products Safety Commission 

CWA Clean Water Act 

DHEW Department of Health, Education, and Welfare 

DHHS Department of Health and Human Services 

DNA deoxyribonucleic acid 

DOD Department of Defense 

DOE Department of Energy 

DOL Department of Labor 

DOT Department of Transportation 
 

METHOXYCHLOR C-2 

APPENDIX C 

DOT/UN/ Department of Transportation/United Nations/

 NA/IMCO North America/International Maritime Dangerous Goods Code 

DWEL drinking water exposure level 

ECD electron capture detection 

ECG/EKG electrocardiogram 

EEG electroencephalogram 

EEGL Emergency Exposure Guidance Level 

EPA Environmental Protection Agency 

F Fahrenheit 

F1 first-filial generation 

FAO Food and Agricultural Organization of the United Nations 

FDA Food and Drug Administration 

FEMA Federal Emergency Management Agency 

FIFRA Federal Insecticide, Fungicide, and Rodenticide Act 

FPD flame photometric detection 

fpm feet per minute 

FR Federal Register 

FSH follicle stimulating hormone 

g gram 

GC gas chromatography 

gd gestational day 

GLC gas liquid chromatography 

GPC gel permeation chromatography 

HPLC high-performance liquid chromatography 

HRGC high resolution gas chromatography 

HSDB Hazardous Substance Data Bank 

IARC International Agency for Research on Cancer 

IDLH immediately dangerous to life and health 

ILO International Labor Organization 

IARC International Agency for Research on Cancer 

IRIS Integrated Risk Information System 

Kd adsorption ratio 

kg kilogram 

Koc organic carbon partition coefficient 

Kow octanol-water partition coefficient 

L liter 

LC liquid chromatography 

LCLo lethal concentration, low 

LC50 lethal concentration, 50% kill 

LDLo lethal dose, low 

LD50 lethal dose, 50% kill 

LDH lactic dehydrogenase 

LH luteinizing hormone 

LT50 lethal time, 50% kill 

LOAEL lowest-observed-adverse-effect level 

LSE Levels of Significant Exposure 

m meter 

MA trans,trans-muconic acid 

MAL maximum allowable level 

mCi millicurie 
METHOXYCHLOR C-3 

APPENDIX C 

MCL maximum contaminant level 

MCLG maximum contaminant level goal 

MFO mixed function oxidase 

mg milligram 

mL milliliter 

mm millimeter 

mmHg millimeters of mercury 

mmol millimole 

mppcf millions of particles per cubic foot 

MRL Minimal Risk Level 

MS mass spectrometry 

NAAQS National Ambient Air Quality Standard 

NAS National Academy of Science 

NATICH National Air Toxics Information Clearinghouse 

NATO North Atlantic Treaty Organization 

NCE normochromatic erythrocytes 

NCEH National Center for Environmental Health 

NCI National Cancer Institute 

ND not detected 

NFPA National Fire Protection Association 

ng nanogram 

NHANES National Health and Nutrition Examination Survey 

NIEHS National Institute of Environmental Health Sciences 

NIOSH National Institute for Occupational Safety and Health 

NIOSHTIC NIOSH's Computerized Information Retrieval System 

NLM National Library of Medicine 

nm nanometer 

nmol nanomole 

NOAEL no-observed-adverse-effect level 

NOES National Occupational Exposure Survey 

NOHS National Occupational Hazard Survey 

NPD nitrogen phosphorus detection 

NPDES National Pollutant Discharge Elimination System 

NPL National Priorities List 

NR not reported 

NRC National Research Council 

NS not specified 

NSPS New Source Performance Standards 

NTIS National Technical Information Service 

NTP National Toxicology Program 

ODW Office of Drinking Water, EPA 

OERR Office of Emergency and Remedial Response, EPA 

OHM/TADS Oil and Hazardous Materials/Technical Assistance Data System 

OPP Office of Pesticide Programs, EPA 

OPPT Office of Pollution Prevention and Toxics, EPA 

OPPTS Office of Prevention, Pesticides and Toxic Substances, EPA 

OR odds ratio 

OSHA Occupational Safety and Health Administration 

OSW Office of Solid Waste, EPA 

OW Office of Water 
METHOXYCHLOR C-4 

APPENDIX C 

OWRS Office of Water Regulations and Standards, EPA 

PAH polycyclic aromatic hydrocarbon 

PBPD physiologically based pharmacodynamic 

PBPK physiologically based pharmacokinetic 

PCE polychromatic erythrocytes 

PEL permissible exposure limit 

pg picogram 

PHS Public Health Service 

PID photo ionization detector 

pmol picomole 

PMR proportionate mortality ratio 

ppb parts per billion 

ppm parts per million 

ppt parts per trillion 

PSNS pretreatment standards for new sources 

RBC red blood cell 

REL recommended exposure level/limit 

RfC reference concentration 

RfD reference dose 

RNA ribonucleic acid 

RQ reportable quantity 

RTECS Registry of Toxic Effects of Chemical Substances 

SARA Superfund Amendments and Reauthorization Act 

SCE sister chromatid exchange 

SGOT serum glutamic oxaloacetic transaminase 

SGPT serum glutamic pyruvic transaminase 

SIC standard industrial classification 

SIM selected ion monitoring 

SMCL secondary maximum contaminant level 

SMR standardized mortality ratio 

SNARL suggested no adverse response level 

SPEGL Short-Term Public Emergency Guidance Level 

STEL short term exposure limit 

STORET Storage and Retrieval 

TD50 toxic dose, 50% specific toxic effect 

TLV threshold limit value 

TOC total organic carbon 

TPQ threshold planning quantity 

TRI Toxics Release Inventory 

TSCA Toxic Substances Control Act 

TWA time-weighted average 

UF uncertainty factor 

U.S. United States 

USDA United States Department of Agriculture 

USGS United States Geological Survey 

VOC volatile organic compound 

WBC white blood cell 

WHO World Health Organization 

> greater than 
METHOXYCHLOR C-5 

APPENDIX C 

$ greater than or equal to 

= equal to 

< less than # less than or equal to 

% percent α alpha β beta γ gamma δ delta 

μm micrometer 

μg microgram q1

* cancer slope factor 

– negative 

+ positive 

(+) weakly positive result 

(–) weakly negative result 

METHOXYCHLOR D-1 

APPENDIX D
 

INDEX 

acute inhalation exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 22 
 

adipose tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 164, 173, 175, 178, 182
 

adrenal gland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44, 55, 100 
 

adrenals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 75 
 

adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155, 178, 179
 

aerobic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151, 158-160, 170
 

air . . . . . . . . . . . . . . . . . . . . . . . 2-4, 10, 11, 13, 14, 21, 106, 142, 148, 151-155, 157, 160, 164-166, 169, 171-173, 176, 180, 183,
 

184, 186, 187, 190, 191, 193
 

alanine aminotransferase (see ALT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 
 

ALT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 
 

ambient air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148, 160, 171, 173
 

anaerobic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151, 158-160, 170
 

androgen receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 15, 91, 95, 101, 126
 

antiandrogenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101, 115
 

Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154, 160, 161
 

aspartate aminotransferase (see AST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 
 

AST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 
 

average daily intake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 
 

BCF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 
 

bioaccumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 
 

bioavailability ........................................................................... 138, 166, 171, 173
 

bioconcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 156 
 

bioconcentration factors (see BCF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 
 

biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158, 170
 

biomagnification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171, 172
 

biomarker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 111, 113, 132
 

blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 10, 15, 25, 41, 43, 75, 105, 108, 110, 111, 135, 164, 173
 

body weight effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 46, 65 
 

breast cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94, 101 
 

breast milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 10, 108, 112, 131, 165, 167, 174
 

cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 6, 15, 24, 52, 62, 66, 94, 101, 106, 119, 122, 123, 140, 193
 

carcinogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 63, 186, 187, 193
 

carcinogenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 21, 62, 63, 66 
 

carcinogenicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 7, 15, 24, 62, 63, 123, 139, 154, 186, 187, 190, 193
 

cardiovascular effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 41 
 

catfish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 
 

Clean Water Act . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187, 188, 190
 

CYP1A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 
 

CYP1A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82, 84, 90, 91, 103, 109, 134, 136
 

CYP2B1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81, 85, 90 
 

CYP2B2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81, 85 
 

CYP2B6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82, 109 
 

CYP3A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 
 

dechlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71, 78, 84, 159 
 

deoxyribonucleic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 
 

dermal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46, 65 
 

DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 67-71, 93, 110, 124, 134
 

dopamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 
 

endocrine effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 
 

estrogen receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 58, 77, 83, 92, 97, 99, 125 
 

estrogenic . . . . . . . . . . . . . . . . . . . . . 8, 15, 16, 19, 45, 48, 50, 52, 58-61, 76, 77, 83, 84, 90, 92-99, 101-104, 106, 107, 109, 113,
 

115, 117, 121, 123, 127, 129, 131, 132, 136, 139, 154, 170, 182, 183 

FDA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 11, 62, 63, 162, 163, 166, 167, 172, 174, 189, 193 

FEDRIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 

fetus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 18, 59, 75, 105, 135, 136 
METHOXYCHLOR D-2 

APPENDIX D 

fish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 4, 13, 102-104, 148, 151, 156, 162, 163, 171, 172, 192
 

fish consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 
 

flies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 
 

follicle stimulating hormone (see FSH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 
 

Food and Drug Administration (see FDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 162, 193
 

fruits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 11, 13, 145, 147, 151, 162, 163, 167
 

FSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 57, 60, 93, 101, 112, 126
 

gastrointestinal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 
 

general population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 110, 151, 163, 164, 172, 182
 

grass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 
 

groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 148, 151, 153, 161, 165, 172, 187, 188, 191, 192
 

half-life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77, 110, 148, 157, 158, 170
 

hematological effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 41 
 

Henry’s law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142, 168
 

hepatic effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42, 43, 65, 114 
 

immune system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47, 130, 138
 

immunological effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 46, 47, 65, 130 
 

insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 13, 153, 173
 

Integrated Risk Information System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 
 

kidney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 
 

lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156, 170
 

LD50 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 113, 119
 

leukemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 24, 123, 131
 

LH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 57, 77, 95, 101, 126
 

liver . . . . . . . . . . . . . . . . . . . . . . 5-7, 15, 42, 43, 48, 62, 63, 65, 67, 71, 74-76, 78, 81-83, 85, 89-91, 97, 102, 103, 109, 110, 113­

115, 120-123, 134, 135, 156, 162
 

lung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 67, 69, 110 
 

luteinizing hormone (see LH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 77 
 

lymph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46, 122, 123, 130
 

lymphopoietic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 
 

lymphoreticular effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 46, 65 
 

menstrual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 50, 112, 124, 132
 

milk . . . . . . . . . . . . . . . . . . . . . . 8, 10, 11, 73, 75, 76, 108, 111, 112, 115, 131, 133, 135, 137, 163, 165-167, 173-175, 178, 181,
 

182, 189
 

Minimal Risk Levels (see MRL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 20, 21 
 

mosquito . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 
 

MRL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 17, 18, 20, 21, 54, 59, 119-122, 123, 125, 128, 129, 186
 

musculoskeletal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 
 

National Priorities List (see NPL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 148 
 

neoplastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67, 69, 70 
 

neurobehavioral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49, 60, 98, 108, 128
 

NIOSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 140, 163, 164, 167, 175, 176, 180, 187, 193
 

NOAEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 18, 20, 25, 50, 57, 62, 65, 121, 122-124, 123, 125, 126, 129, 186
 

NPL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 14, 148, 150, 151, 153, 154
 

ocular effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 46 
 

odds ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 
 

particulate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 142, 148, 154, 157, 170, 176
 

particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 
 

partition coefficients .............................................................................. 135, 142
 

PBPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 
 

PBPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87-89, 110, 135
 

pharmacodynamic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 
 

pharmacokinetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87-90, 120, 122, 124, 171
 

physiologically based pharmacodynamic (see PBPD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 
 

physiologically based pharmacokinetic (see PBPK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87, 89 
 

precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 
 

public health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 5, 10, 13, 19, 20, 98, 117, 119, 168, 175, 179, 191, 192
 

reference dose (see RfD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186, 193
 

regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 11, 147, 186-188, 190, 193
 

renal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 
 

reportable quantity ............................................................................... 188, 190
 
METHOXYCHLOR D-3 

APPENDIX D 

reproductive system . . . . . . . . . . . . . . . . . . . . 6, 7, 14-16, 18, 45, 50, 51, 55, 88, 91, 106, 107, 109, 112, 113, 115, 117, 120, 122,
 

123, 125, 127, 135, 136, 182
 

RfD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186, 190, 193
 

Safe Drinking Water Act . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 
 

salivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 48 
 

sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 104
 

seal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 
 

sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148, 151, 154, 155, 158, 159, 161-163, 170
 

selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 
 

serum . . . . . . . . . . . . . . . . . . . 15, 25, 42, 44, 45, 47, 54, 57, 59, 60, 77, 91, 95, 100-102, 108, 111-113, 121, 125, 126, 132, 164,
 

175, 176, 178, 182, 183 

soil . . . . . . . . . . . . . . . . . . . . 2-5, 7, 9, 10, 13, 14, 148, 151, 153-155, 158, 159, 161, 162, 165, 166, 170-173, 179-181, 183, 184, 

193
 

solubility ....................................................................................... 142, 168
 

surface water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 13, 148, 151, 153, 164, 172
 

T3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 
 

thyroid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43, 45, 57, 95, 122 
 

thyroid stimulating hormone (see TSH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 57, 95 
 

time-weighted average (see TWA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 
 

toxicokinetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 73, 171 
 

Toxics Release Inventory (see TRI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147, 151
 

transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 54, 94, 98, 99, 133, 137, 154, 168, 170
 

tremors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 7, 14, 16, 25, 48, 49, 96, 113, 130
 

TRI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144, 151-154, 170
 

triiodothyronine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 
 

TSH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 57, 101, 126
 

tumors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52, 62, 63, 66, 95 
 

TWA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187, 193
 

Type I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 
 

U.S. Department of Agriculture (see USDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 193
 

USDA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163, 172, 189, 193
 

vapor phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142, 148, 154, 157, 170
 

vapor pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142, 155, 168
 

volatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 
 

water . . . . . . . . . . . . . . . . . . . . . 2-6, 9-11, 13, 14, 103, 105, 142, 147, 148, 151-153, 155-161, 164, 165, 168, 170-173, 179, 180,
 

183, 184, 186-193