This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

Concise International Chemical
Assessment Document 28

METHYL CHLORIDE

First draft prepared by
Agneta Löf, National Institute of Working Life, Solna, Sweden
Maria Wallén, National Chemicals Inspectorate (KEMI), Solna, Sweden, and Jonny Bard, Åseda, Sweden

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization
Geneva, 2000

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Methyl chloride.

(Concise international chemical assessment document ; 28)

1.Methyl chloride - toxicity 2.Risk assessment 3.Environmental exposure

I.International Programme on Chemical Safety II.Series

ISBN 92 4 153028 6 (NLM Classification: QV 633)

ISSN 1020-6167

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TABLE OF CONTENTS

FOREWORD

1. EXECUTIVE SUMMARY

2. IDENTITY AND PHYSICAL/CHEMICAL PROPERTIES

3. ANALYTICAL METHODS

4. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

5. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION

6. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

6.1 Environmental levels

6.2 Human exposure

7. COMPARATIVE KINETICS AND METABOLISM IN LABORATORY MAMMALS AND HUMANS

7.1 Absorption

7.2 Distribution

7.3 Metabolism and elimination

7.4 Genetic polymorphism and sex, strain, organ, and species differences

8. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS

8.1 Single exposure

8.2 Irritation and sensitization

8.3 Short-term exposure

8.4 Medium-term exposure

8.5 Long-term exposure and carcinogenicity

8.6 Genotoxicity and related end-points

8.6.1 Studies in vitro

8.6.2 Studies in vivo

8.7 Reproductive and developmental toxicity

8.7.1 Effects on fertility

8.7.2 Developmental toxicity

8.8 Immunological and neurological effects

9. EFFECTS ON HUMANS

9.1 Studies in volunteers

9.2 Case reports

9.3 Epidemiological studies

10. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD

10.1 Aquatic environment

10.2 Terrestrial environment

11. EFFECTS EVALUATION

11.1 Evaluation of health effects

11.1.1 Hazard identification and dose–response assessment

11.1.2 Criteria for setting tolerable intakes or guidance values for methyl chloride

11.1.3 Sample risk characterization

11.2 Evaluation of environmental effects

12. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES

REFERENCES

APPENDIX 1 — SOURCE DOCUMENTS

APPENDIX 2 — CICAD PEER REVIEW

APPENDIX 3 — CICAD FINAL REVIEW BOARD

INTERNATIONAL CHEMICAL SAFETY CARD

RÉSUMÉ D’ORIENTATION

RESUMEN DE ORIENTACIÓN

FOREWORD

Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) — a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their completeness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose–response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all possible exposure situations, but are provided as guidance only. The reader is referred to EHC 1701 for advice on the derivation of health-based tolerable intakes and guidance values.

While every effort is made to ensure that CICADs represent the current status of knowledge, new information is being developed constantly. Unless otherwise stated, CICADs are based on a search of the scientific literature to the date shown in the executive summary. In the event that a reader becomes aware of new information that would change the conclusions drawn in a CICAD, the reader is requested to contact IPCS to inform it of the new information.

Procedures

The flow chart shows the procedures followed to produce a CICAD. These procedures are designed to take advantage of the expertise that exists around the world — expertise that is required to produce the high- quality evaluations of toxicological, exposure, and other data that are necessary for assessing risks to human health and/or the environment.

Flow Chart

The first draft is based on an existing national, regional, or international review. Authors of the first draft are usually, but not necessarily, from the institution that developed the original review. A standard outline has been developed to encourage consistency in form. The first draft undergoes primary review by IPCS to ensure that it meets the specified criteria for CICADs.

The second stage involves international peer review by scientists known for their particular expertise and by scientists selected from an international roster compiled by IPCS through recommendations from IPCS national Contact Points and from IPCS Participating Institutions. Adequate time is allowed for the selected experts to undertake a thorough review. Authors are required to take reviewers’ comments into account and revise their draft, if necessary. The resulting second draft is submitted to a Final Review Board together with the reviewers’ comments.

The CICAD Final Review Board has several important functions:

– to ensure that each CICAD has been subjected to an appropriate and thorough peer review;

– to verify that the peer reviewers’ comments have been addressed appropriately;

– to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and

– to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

1. EXECUTIVE SUMMARY

The assessment of human health aspects in this CICAD on methyl chloride was based primarily on a review prepared by the Nordic Expert Group in collaboration with the Dutch Expert Committee for Occupational Standards (Lundberg, 1992). Relevant databases covering the years 1992–1999 were searched to identify additional data. For the environmental and ecotoxicological aspects of methyl chloride, BUA (1986), ATSDR (1990), WMO (1994), and HSDB (1996) were used as primary sources. ATSDR (1990) was updated in 1998; where ATSDR (1998) provided new information, this has been taken into account. Additional data on environmental issues were identified in relevant databases covering the years 1989–1997. Information concerning the nature and availability of the source documents is presented in Appendix 1. Informa tion on the peer review of this CICAD is presented in Appendix 2. This CICAD was approved as an interna tional assessment at a meeting of the Final Review Board, held in Stockholm, Sweden, on 25–28 May 1999. Participants at the Final Review Board meeting are listed in Appendix 3. The International Chemical Safety Card (ICSC 0419) for methyl chloride, produced by the International Programme on Chemical Safety (IPCS, 1999), has been reproduced in this document.

Methyl chloride (CAS No. 74-87-3) is released mainly to air during its production and use and by incineration of municipal and industrial wastes. How ever, natural sources, primarily oceans and biomass burning, clearly dominate over anthropogenic sources. The total global release of methyl chloride from all sources is estimated to be about 5 × 106 tonnes per year. The contribution from natural sources has been estimated to be well over 90%, and perhaps as much as 99%, of the total release. Methyl chloride is present in the tropo sphere at a concentration of approximately 1.2 µg/m3 (0.6 ppb).

The principal sink for methyl chloride in the tropo sphere is chemical reaction with hydroxyl radicals, and the atmospheric lifetime is estimated to be 1–3 years. A certain amount of methyl chloride reaches the strato sphere; there, photodissociation generates chlorine radi cals, which contribute to ozone depletion. Estimates of the amount of methyl chloride reaching the stratosphere, and thus depleting ozone, vary widely. As estimated from figures presented by the World Meteorological Organization (WMO), methyl chloride contributes approximately 15% of the total equivalent effective stratospheric chlorine. The stratospheric ozone depletion potential (ODP) of methyl chloride has been determined to be 0.02 relative to the reference compound CFC-11, which has an ODP of 1. Methyl chloride is not thought to contribute significantly to either global warming or photochemical air pollution.

The dominant loss mechanism for methyl chloride in water and soil is volatilization. Slow hydrolysis and possibly biotic degradation may contribute to the loss in deeper soil layers and in groundwater. However, little information is available concerning biodegradation.

The most important route of exposure of methyl chloride in humans is via the respiratory pathway. In humans as well as in experimental animals, methyl chloride is readily absorbed through the lungs following inhalation. Following exposure to 14C-radiolabelled methyl chloride, the radioactivity is found throughout the body. Although a large portion of the radiolabelled substance is incorporated into protein through the one- carbon pool, methyl chloride may also bind to protein by direct alkylation. However, if methyl chloride is an alkylating agent, it is so to a very small extent. Methyl chloride is metabolized in mammals either by conjuga tion with glutathione or, to a lesser extent, through oxi dation by cytochrome P-450; the glutathione pathway yields methanethiol, and both pathways yield formalde hyde and formate. Metabolites from methyl chloride are excreted via the urine and by exhalation. Methyl chloride is also exhaled unmetabolized.

In humans, there are large interindividual differ ences in the uptake and metabolism of methyl chloride. These differences depend on the presence or absence of the enzyme glutathione transferase T1 (GSTT1), which displays genetic polymorphism. Humans can be pheno typed as high conjugators, low conjugators, or non- conjugators of GSTT1. However, as it is not evident if high conjugators or non-conjugators incur the highest risk, one must consider all phenotypes as sensitive to methyl chloride exposure.

The acute inhalation toxicity of methyl chloride in rats and mice seems to be fairly low, with an LC50 value above 4128 mg/m3 (2000 ppm). No data on irritation or sensitizing properties were located in the literature.

The main target organs after short-term inhalation exposure to methyl chloride seem to be the nervous system, with functional disturbances and cerebellar degeneration in both rats and mice, as well as testicles, epididymis, and kidneys in rats and kidneys and liver in mice.

In a 2-year inhalation study in mice, axonal swell ing and degeneration of lumbar spinal nerves were observed at 103 mg/m3 (50 ppm) in exposed animals compared with controls, but without an apparent dose–response relationship. At the end of the study, cerebellar degeneration in mice of both sexes and renal adenocarcinomas in male mice were observed at 2064 mg/m3 (1000 ppm). These effects were not observed in the rat at 2064 mg/m3 (1000 ppm).

Methyl chloride is clearly genotoxic in in vitro systems in both bacteria and mammalian cells. Although the positive effects seen in a dominant lethal test most likely were cytotoxic rather than genotoxic, methyl chloride might be considered a very weak mutagen in vivo based on some evidence of DNA–protein cross- linking at higher doses.

Testicular lesions and epididymal granulomas followed by reduced sperm quality lead to reduced fertility in rats at 980 mg/m3 (475 ppm) and to complete infertility at higher doses.

Methyl chloride induced heart defects in mouse fetuses when dams were exposed to 1032 mg/m3 (500 ppm) during the gestation period.

Effects on humans, especially on the central nervous system, can be clearly seen after accidental inhalation exposure. In short-term exposure of volunteers to methyl chloride, no significant effects were seen that could be attributed to the exposure. There are insuffi cient epidemiological data available to assess the risk for humans to develop cancer as a result of methyl chloride exposure.

In conclusion, the critical end-point for methyl chloride inhalation toxicity in humans seems to be neurotoxicity. Guidance values of 0.018 mg/m3 (0.009 ppm) for indirect exposure via the environment and 1.0 mg/m3 (0.5 ppm) for the working environment were derived. Although the nerve lesions were seen at lower exposure levels than those at which infertility in rats (980 mg/m3 [475 ppm]) and renal tumours in male mice (2064 mg/m3 [1000 ppm]) occurred, emphasis should also be laid on these very serious effects in a qualitative risk characterization of methyl chloride.

Few data were found on the short-term toxicity of methyl chloride to both aquatic and terrestrial organisms. No data were found on long-term toxicity. The existing data indicate that methyl chloride has a low acute toxicity to aquatic organisms. The lowest LC50 value for fish is 270 mg/litre. As measured concentrations of methyl chloride in surface waters are generally several orders of magnitude less than those demonstrated to cause effects, it is likely that methyl chloride poses a low risk of acute effects on aquatic organisms. Only very limited data are available on the effects of methyl chloride on terrestrial organisms.

2. IDENTITY AND PHYSICAL/
CHEMICAL PROPERTIES

Methyl chloride (CAS No. 74-87-3; CH3Cl; chloromethane) is a colourless gas at ambient tempera tures. It can be compressed to a liquid, which has a weak ethereal smell. The odour threshold has been estimated to be 21 mg/m3 (10 ppm) (ASTM, 1973). Methyl chloride decomposes in water with a half-time of 4.66 h at 100 °C (IARC, 1986).

Methyl chloride is marketed as a liquefied gas under pressure. The purity of a representative technical grade of methyl chloride is close to 100%. Impurities include water, hydrochloric acid, methyl ether, methanol, and acetone (Holbrook, 1992).

Methyl chloride has a very high vapour pressure and a high solubility in water. The value of its Henry’s law constant is high, which suggests that volatilization of methyl chloride will be significant in surface waters. The calculated octanol/water partition coefficient (log Kow) is low, indicating a low potential for bioaccumulation and low tendency of adsorption to soil and sediment.

Some relevant physical and chemical properties of methyl chloride are listed in Table 1. Additional physical/chemical properties are presented in the International Chemical Safety Card, reproduced in this document.

3. ANALYTICAL METHODS

In air, methyl chloride can be analysed by Method 1001 of the US National Institute for Occupational Safety and Health (NIOSH, 1994). Analysis is performed by gas chromatography (GC), and the sample detection limit is 3.1 µg/m3 (1.5 ppb). Using the method of Oliver et al. (1996), the detection limit is 1.1 µg/m3 (0.53 ppb).

The use of carbon disulfide at dry ice temperature for desorbing the analyte has also been described, as well as a thermal desorption technique as an alternative (Severs & Skory, 1975). A thermally desorbable diffu sional dosimeter for monitoring methyl chloride in the workplace has also been described (Hahne, 1990). Very low concentrations (0.006–0.1 µg/m3 [0.003–0.05 ppb]) of methyl chloride (in ambient air) can be analysed by the use of photoionization, flame ionization, and electron capture detectors in series (Rudolph & Jebsen, 1983).

Table 1: Identity and physical/chemical properties of methyl chloride.

Property

Value

Reference

Relative molecular mass

50.49

 

Melting point

-97.7 °C
-97.1 °C

Holbrook, 1992
Weast, 1988

Boiling point

-23.73 °C
-24.2 °C

Holbrook, 1992
Weast, 1988

Density
liquid at 20/4 °C
gas at 0 °C, 101.3 kPa


0.920 g/ml
2.3045 g/litre


Holbrook, 1992
Holbrook, 1992

Specific gravity

1.74 (air = 1)

Holbrook, 1992

Solubility in water at 25 °C

5.325 g/litre
4.800 g/litre

Horvath, 1982
Holbrook, 1992

Vapour pressure at
5.5 °C
25 °C


3.04 × 105 Pa
5.75 × 105 Pa


BUA, 1986
BUA, 1986

Henry’s law constant at
3 °C (seawater, salinity
30.4‰)a
25 °C


0.1977 ± 1.4%

4.15–6.05 kPa•m3/mol


Moore et al., 1995

BUA, 1986

Log Kow

0.91

Hansch & Leo, 1985

Conversion factor ppm (v/v) to mg/m3 in air at 25 °C

1 ppm = 2.064 mg/m3
1 mg/m3 = 0.4845 ppm

ATSDR, 1990

a Henry’s law constant is defined as the concentration in air divided by the concentration in water at equilibrium; unit of Henry’s law constant: dimensionless (Moore et al., 1995).

Exposure to methyl chloride can also be monitored in air by a direct-reading infrared analyser, at minimum detectable concentrations of 800–3100 µg/m3 (390– 1500 ppb) (IARC, 1985).

Stratospheric air samples are often concentrated by a cryogenic procedure, at liquid nitrogen or argon tem perature, followed by GC analysis employing electron capture detection (Rasmussen et al., 1980; Singh et al., 1983, 1992; Rudolph et al., 1992, 1995; Khalil & Rasmussen, 1993; Fabian et al., 1996) or with GC/mass spectrometry (MS) (Schauffler et al., 1993). The GC may be equipped with a flame ionization detector (Evans et al., 1992) or a mass selective detector (Atlas et al., 1993). Almasi et al. (1993) described a modified version of a method commonly used by the US Environmental Protection Agency (EPA) to analyse low levels of volatile organic compounds in air (EPA Method TO-14). It includes sample concentrations on glass beads at -160 °C, thermal desorption, separation on a GC capil lary column, and detection with ion trap MS. The detec tion limit is about 0.06 µg/m3 (0.03 ppb) for methyl chloride.

In water, methyl chloride can be analysed by EPA Method 502.2 at a detection limit of 0.1 µg/litre (US EPA, 1986b). Other methods for the detection of volatile organic substances in water are EPA Method 502.1, which has a detection limit of 0.01 µg/litre (US EPA, 1986b), and EPA Method 524.2, with a detection limit of 0.05 µg/litre (US EPA, 1986b). Another method involv ing a solid-phase micro-extraction technique has a detec tion limit of <25 µg/litre (Shirey, 1995).

EPA Method 601 (purgeable halocarbons) is suit able for measuring methyl chloride in wastewater. The detection limit is 0.06 µg/litre (US EPA, 1982; CFR, 1990). A similar method is EPA Method 624 (purgeables), with a detection limit of 2.8 µg/litre (US EPA, 1982; CFR, 1991). A third method used for analysis of wastewater is EPA Method 1624, with a minimum detection level of 50 µg/litre (CFR, 1991).

In soil and solid waste, EPA Method 5030 (US EPA, 1986a) may be used to analyse methyl chloride. Analysis is performed by different EPA methods. In Method 8010B, the limit of detection is 12.5 µg/kg for high-concentration soils and sludges (US EPA, 1986a). Gomes et al. (1994) describes another method to collect and analyse methyl chloride, which can also be used to analyse contaminants in groundwater.

4. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

Natural sources of methyl chloride dominate over anthropogenic sources. The major source appears to be the marine/aquatic environment, likely associated with algal growth. Other sources are biomass burning (forest fires), degradation of wood by fungi, and direct and indirect anthropogenic sources.

Methyl chloride is produced industrially by reac tion of methanol and hydrogen chloride or by chlorina tion of methane (Key et al., 1980; Edwards et al., 1982a; Holbrook, 1992). In almost all of the commercial uses, methyl chloride is reacted to form another product (ATSDR, 1998). The current principal uses are in the production of silicones and also as a general methylating agent. The use of methyl chloride in the manufacture of synthetic rubber, its refrigerant and extractant applica tions, and its use as a tetramethyllead intermediate now have secondary importance (Holbrook, 1992).

Indirect sources of methyl chloride are tobacco smoke, turbine exhaust (Wynder & Hoffmann, 1967; Graedel, 1978; Häsänen et al., 1990), incineration of municipal and industrial waste (Graedel & Keene, 1995), chlorination of drinking-water, and sewage effluent (Abrams et al., 1975).

The current production capacity of methyl chloride in the USA has been estimated to be about 0.417 × 106 tonnes per year (CMR, 1995). The production in Japan in 1996 was 0.13 × 106 tonnes (Chemical Daily Co. Ltd., 1998).

It has been concluded that well over 90%, and perhaps as much as 99%, of ambient air concentrations on a global scale appear to originate from natural sources rather than from anthropogenic sources (ATSDR, 1998). Edwards et al. (1982b) estimated the emissions of methyl chloride during production, transport, storage, and use to be approximately 0.02 × 106 tonnes per year, corres ponding to nearly 6% of the amount produced. Accord ing to this estimate, anthropogenic sources would account for 1–2% of the total release, including natural sources. Other estimates of the global yearly release from anthropogenic sources are in the range of 0.024– 0.6 × 106 tonnes (Watson et al., 1980; Gribble, 1992; Dowdell et al., 1994), the higher estimate including indirect anthropogenic sources and possibly also biomass burning.

Methyl chloride, which is the most prevalent halo genated methane in the atmosphere, is present in the troposphere at a concentration of about 1.2 µg/m3 (0.6 ppb) (WMO, 1994). It has been calculated that at a production rate of about 3.5 × 106 tonnes per year, the steady-state mixing ratio of 1.2 µg/m3 (0.6 ppb) is maintained given an atmospheric lifetime in the order of 2 years (WMO, 1994). Estimates of the total global annual release of methyl chloride from all sources are around 5 × 106 tonnes (Rasmussen et al., 1980; Logan et al., 1981; Edwards et al., 1982b; Dowdell et al., 1994; WMO, 1994; Fabian et al., 1996). According to ATSDR (1998), the total release from all sources amounts to approximately 3.2–8.2 × 106 tonnes per year.

Over the Pacific Ocean, the concentration of methyl chloride is higher in the lower troposphere than in the higher layers. However, over the continents, the concentration is independent of the altitude. Thus, the ocean seems to be a source of methyl chloride (Geckeler & Eberhardt, 1995). In the oceans, algae, especially planktonic algae, are considered to be responsible for most of the methyl chloride production. However, this has not been fully proven. Phytoplankton have been shown to produce methyl chloride in laboratory studies (Tait & Moore, 1995). An alternative model is that methyl chloride is formed as a result of exchange processes between methyl iodide and chlorine ions in seawater (Isidorov, 1990). Estimates of the global yearly release of methyl chloride from marine sources are in the range 1–8 × 106 tonnes (Watson et al., 1980; Singh et al., 1983; Isidorov, 1990).

Terrestrial species also produce methyl chloride. The activity of methyltransferases, believed to be responsible for methyl chloride production, has been observed in several herbaceous species (Saini, 1995). According to Harper et al. (1988), 34 species of fungi are also known to biosynthesize methyl chloride.

Estimates of the global annual release of methyl chloride from biomass burning are in the range 0.4–1.8 × 106 tonnes (Watson et al., 1980; Andreae, 1991, 1993; Lobert et al., 1991; Rudolph et al., 1994, 1995). The major part of the methyl chloride released from biomass burning originates from forest fires in the tropics (Andreae et al., 1994). The estimated global release of methyl chloride from temperate and boreal biomass fires has been calculated to be 0.012 × 106 tonnes per year (Laursen et al., 1992). Low-intensity, inefficient combus tion and high chlorine content of the biomass promote methyl chloride formation (Reinhardt & Ward, 1995).

5. ENVIRONMENTAL TRANSPORT, DISTRIBUTION,
AND TRANSFORMATION

Most methyl chloride discharged to the environ ment will be released to air. The principal sink for methyl chloride in the troposphere is chemical reaction with hydroxyl radicals (ASTDR, 1990; Graedel & Keene, 1995; Fabian et al., 1996). The rate constant for this reaction is approximately 4.3 × 10–14 cm3/s per molecule at 25 °C (NASA, 1981; Atkinson, 1985). Atmospheric lifetime estimates range from 1 to 3 years (Atkinson, 1985; BUA, 1986; Warneck, 1988; ATSDR, 1990; WMO, 1990, 1994; Fabian et al., 1996; Houghton et al., 1996). Surface deposition, rainout, and washout are unimportant sinks for methyl chloride (Graedel & Keene, 1995).

Estimates of the amount of methyl chloride reaching the stratosphere vary considerably. Borchers et al. (1994) claim that the contribution of methyl chloride to the stratospheric chlorine budget is significant. Crutzen & Gidel (1983) estimated the flux of methyl chloride to the stratosphere to be about 2 × 106 tonnes per year or 20–25% of the total annual stratospheric chlorine input. According to Fabian et al. (1996), only a fraction (less than 10%) of the amount of methyl chlor ide emitted reaches the stratosphere. Edwards et al. (1982b) claim that about 6% of the methyl chloride released reaches the stratosphere (corresponding to 0.3 × 106 tonnes per year). According to Graedel & Crutzen (1993) and Graedel & Keene (1995), only 0.8% (corres ponding to 0.03 × 106 tonnes chlorine per year) is expected to reach the stratosphere. A global budget for tropospheric methyl chloride is shown in Figure 1.

Figure 1

The ability of the ozone layer to absorb ultraviolet radiation shorter than 290 nm should exclude direct photolysis in the troposphere, because methyl chloride does not absorb any radiation above 290 nm (BUA, 1986). In the stratosphere, photodissociation will occur at a rate approximately equal to its reaction with hydroxyl radicals (Robbins, 1976). The chlorine radicals that are generated contribute to ozone depletion. Methyl chloride has been shown to photochemically decompose at 185 nm. Photooxidation products in the gas phase were carbon dioxide, carbon monoxide, formic acid, formyl chloride, water, and hydrogen chloride (Gürtler & Kleinermanns, 1994).

The stratospheric steady-state ODP of methyl chloride has been determined to be 0.02 relative to CFC- 11 (ODP = 1) (Solomon et al., 1992; WMO, 1994; Fabian et al., 1996). Estimates of the amount of methyl chloride reaching the stratosphere, and thereby also its contribution to ozone depletion, vary considerably. However, as estimated from figures presented by WMO (1994), methyl chloride contributes approximately 15% (0.5 ppb) of the total (3.3 ppb) equivalent effective stratospheric chlorine. The term "equivalent effective stratospheric chlorine" includes both stratospheric chlorine and bromine (alpha1 = 40) and also considers the dissociation rate of each compound involved in ozone depletion.

A radiative forcing value of 0.0053 W/m2 per part per billion has been determined for methyl chloride. This value is about 2% of the forcing of CFC-11 and about 300 times the forcing of carbon dioxide, on a per molecule basis (Grossman et al., 1994). Houghton et al. (1996) gave the radiative forcing value as 0 W/m2 for methyl chloride. The global warming potential (GWP) has been calculated to be about 25, relative to carbon dioxide (GWP = 1), at a time scale of 20 years (Grossman et al., 1994).

As the current concentration of methyl chloride in the atmosphere is relatively low, approximately 1.2 µg/m3 (0.6 ppb), the contribution of this substance to the greenhouse effect will not become a problem unless large releases of this gas occur (Grossman et al., 1994). WMO (1994) also considers that the contribution of methyl chloride to climate forcing is minimal.

The contribution of methyl chloride to the creation of photochemical air pollution is not significant because of its relatively low reactivity and low amounts emitted. The photochemical ozone creation potential (POCP) of methyl chloride has been determined to be 3.5 (integrated ozone formation over 5 days) relative to that of ethylene (POCP = 100) (Derwent et al., 1996).

Reactive chlorine in the lower atmosphere (as distinguished from chlorofluorocarbon-derived chlorine in the stratosphere) is supposed to be important in considerations of precipitation acidity, corrosion of metals and alloys, foliar damage, and chemistry of the marine boundary layer. The tropospheric reactive chlorine burden of approximately 8.3 × 106 tonnes chlorine is dominated by methyl chloride (~45%) and trichloroethane (~25%) (Graedel & Keene, 1995).

If methyl chloride is released into water, it will be lost primarily by volatilization. The volatilization half- life has been calculated to be 2.1 h in a model river (Lyman et al., 1982). The volatilization half-lives of methyl chloride in a pond and in a lake have been estimated to be 25 h and 18 days, respectively, using the model EXAMS (ATSDR, 1990). The low log Kow (0.91) of methyl chloride indicates that the substance does not tend to concentrate in sediments.

The transformation of methyl chloride by hydrol ysis is probably negligible under acid and neutral condi tions. Under basic conditions, slow hydrolysis takes place, yielding methanol as a transformation product (Simon, 1989). Hydrolytic half-lives range from 31 days (pH 11) to 2.5 years (pH not given) at 20–25 °C (Zafiriou, 1975; Mabey & Mill, 1976, 1978; Simon, 1989). The hydrolytic half-life of methyl chloride in seawater varies with temperature (0–30 °C) from 0.5 to 77 years (Elliott & Rowland, 1995). Laboratory data indicate that the photochemical transformation of methyl chloride in water is negligible (Mabey & Mill, 1976).

Methyl chloride was not readily biodegraded in a standardized "closed bottle test" (MITI, 1992). How ever, several isolated bacterial strains have been shown to degrade methyl chloride under both aerobic (Stirling & Dalton, 1979; Hartmans et al., 1986; Bartnicki & Castro, 1994; Chang & Alvarez-Cohen, 1996) and anaerobic conditions (Traunecker et al., 1991; Braus- Stromeyer et al., 1993; Dolfing et al., 1993; Leisinger & Braus-Stromeyer, 1995). A half-life of less than 11 days was estimated for the anaerobic biodegradation of methyl chloride in groundwater, based on laboratory data obtained under conditions favourable for anaerobic biodegradation (Wood et al., 1985).

The very low log Kow (0.91) of methyl chloride indicates that it will not tend to adsorb to soil (Lyman et al., 1982). The adsorption coefficient, Koc, has been calculated to be 5, based on physical/chemical data (ATSDR, 1990). The very high vapour pressure and low adsorption to soil suggest that methyl chloride present near the soil surface will rapidly be lost by volatilization (ATSDR, 1990; HSDB, 1996). As it is not expected to adsorb to soil, methyl chloride present in deeper soil layers may to some extent leach into the groundwater, as well as diffuse to the surface and volatilize (ATSDR, 1990; HSDB, 1996). In groundwater, methyl chloride is expected to biodegrade or hydrolyse very slowly (ATSDR, 1990; HSDB, 1996). The cumulative volatili zation loss of methyl chloride, from a depth of 1 m beneath ground, has been calculated to be at least 70% and 22% in 1 year for a sandy soil and a clay soil, respectively (Jury et al., 1990).

There are no experimental studies on bioaccumu lation. However, only a minor accumulation in biota would be expected on the basis of the low log Kow. A bioconcentration factor of 2.9 has been calculated based on the log Kow (ATSDR, 1990).

6. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

6.1 Environmental levels

Background concentrations of methyl chloride in the troposphere are around 1.2 µg/m3 (0.6 ppb), ranging from about 1.0 to 1.4 µg/m3 (from 0.5 to 0.7 ppb) (Cox et al., 1976; Cronn et al., 1976, 1977; Pierotti & Rasmussen, 1976; Singh et al., 1977, 1979, 1983; Graedel, 1978; Khalil & Rasmussen, 1981, 1993; Guicherit & Schulting, 1985; Gregory et al., 1986; Warneck, 1988; Rudolph et al., 1992; Singh et al., 1992; Atlas et al., 1993; WMO, 1994; Graedel & Keene, 1995; Fabian et al., 1996). In the stratosphere, the concentra tion of methyl chloride decreases with altitude. Concen trations in the Arctic stratosphere in March 1992 ranged from 0.60 to 0.082 µg/m3 (from 0.29 to 0.04 ppb) at altitudes of 11–22 km (von Clarmann et al., 1995). In May 1985, at a latitude of 26–30 °N, Zander et al. (1992) found methyl chloride concentrations ranging from 0.12 to 0.050 µg/m3 (from 0.058 to 0.024 ppb) at altitudes of 12–22 km. Near the tropical tropopause (23.8–25.3 °N, 15–17 km altitude), the mean methyl chloride concentration was measured to be 1.1 µg/m3 (0.531 ppb) during January–March 1992 (Schauffler et al., 1993).

Numerous measurements of methyl chloride levels in air have been performed, especially in the USA. The mean or median concentrations of methyl chloride measured in the air of rural/remote sites in the USA were about 1.0–2.7 µg/m3 (0.5–1.3 ppb), with the majority of the values below 2.1 µg/m3 (1.0 ppb); the maximum concentration measured was 4.3 µg/m3 (2.1 ppb) (Grimsrud & Rasmussen, 1975; Robinson et al., 1977; Singh et al., 1977, 1981b; Brodzinsky & Singh, 1983; Rasmussen & Khalil, 1983; Shah & Singh, 1988). In samples from urban/suburban areas in the USA, the mean/median concentrations were in the range of 0.27– 6.2 µg/m3 (0.13–3.0 ppb), with the majority of the values in the range 1.0–2.3 µg/m3 (0.5–1.1 ppb); the highest concentration found was 25.0 µg/m3 (12.1 ppb) (Singh et al., 1977, 1979, 1981a, 1982, 1992; Brodzinsky & Singh, 1983; Edgerton et al., 1984; Shah & Singh, 1988; Rice et al., 1990; US EPA, 1991a, 1991b; Evans et al., 1992; Kelly et al., 1994; Spicer et al., 1996). Methyl chloride concentrations in three Japanese cities ranged from 4.5 to 35 µg/m3 (from 2.2 to 17 ppb) (Furutani, 1979). In Delft, the Netherlands, and Lisbon, Portugal, concentrations of 6.2 µg/m3 (3.0 ppb) (Guicherit & Schulting, 1985) and 4.5 µg/m3 (2.2 ppb) (Singh et al., 1979), respectively, were found.

From these data, it appears that the concentrations of methyl chloride are slightly higher in the air of urban/ suburban sites than at rural/remote sites. However, a direct comparison is difficult, because samples in urban/ suburban areas were probably often taken at ground level, while several measurements of rural/remote areas were made at higher altitudes.

Methyl chloride has also been occasionally detected in water, soil, and biota. A few studies on measurements of methyl chloride in drinking-water were identified, most of them performed in the USA and Canada (Abrams et al., 1975; Coleman et al., 1976; Burmaster, 1982; Mariich et al., 1982; Otson et al., 1982; Otson, 1987). A maximum concentration of 44 µg/litre was measured in a drinking-water well (Burmaster, 1982).

In measurements of groundwaters in the USA, concentrations of methyl chloride ranged from not detectable up to 100 µg/litre, found at a former waste site of a chemical factory (Page, 1981; Burmaster, 1982; Sabele & Clark, 1984; Lesage et al., 1990; Plumb, 1991; Rosenfeld & Plumb, 1991). The substance was detected in groundwaters at 20 of 479 waste disposal sites in 1991 (Plumb, 1991).

In surface water samples in North America, the concentrations ranged from not detectable up to 224 µg/litre, the highest value reported from New Jersey, USA, in the 1970s (Page, 1981; Otson et al., 1982; Great Lakes Water Quality Board, 1983; Granstrom et al., 1984; Staples et al., 1985; Otson, 1987). In the only European study found (Hendriks & Stouten, 1993), a maximum concentration of 12 µg/litre in the river Rhine was reported. In seawater samples collected near the surface, methyl chloride was mostly found at 0.01– 0.05 µg/litre (Lovelock, 1975; Pearson & McConnell, 1975; NAS, 1978; Singh et al., 1979, 1983; Edwards et al, 1982b); however, a higher concentration of 1.2 µg/li tre was reported from a measurement near the shore of California, USA (Singh et al., 1979).

Methyl chloride was detected in soils at 34 waste sites and in sediments at 13 waste sites in the USA (HazDat, 1998) and in 1 of 345 sampling stations of the US EPA STORET database, at a concentration of <5 µg/kg (Staples et al., 1985). Methyl chloride was also detected in soil at an electronic industrial site in São Paulo, Brazil (Gomes et al., 1994). No data were found on methyl chloride levels in sediment. According to the US EPA STORET database, methyl chloride was detected in 1% of analysed samples of fish and seafood (Staples et al., 1985).

6.2 Human exposure

Data given in section 6.1 suggest that humans are exposed to methyl chloride in ambient air. Background concentrations are around 1.2 µg/m3 (0.6 ppb). In urban areas, mean and median concentrations generally seem to be slightly higher, 1.0–2.3 µg/m3 (0.5–1.1 ppb). How ever, individual measurements may be much higher. The highest value found in the literature was 35 µg/m3 (17 ppb).

Workplace concentrations have been measured in four US chemical plants (NIOSH, 1980). Three of the plants produced methyl chloride. The personal 8-h time- weighted average concentrations in the three plants ranged from 18.4 to 25.6 mg/m3 (from 8.9 to 12.4 ppm), from <0.4 to 15.5 mg/m3 (from <0.2 to 7.5 ppm), and from <0.2 to 26.2 mg/m3 (from <0.1 to 12.7 ppm), respectively. In the fourth plant, where methyl chloride was used as a blowing agent in the production of poly styrene foam, the personal exposures ranged from 6.2 to 44.2 mg/m3 (from 3.0 to 21.4 ppm). In a Dutch methyl chloride production plant, individual 8-h time-weighted averages of methyl chloride exposure in the air ranged from 62 to 186 mg/m3 (from 30 to 90 ppm) (van Doorn et al., 1980).

7. COMPARATIVE KINETICS AND METABOLISM IN
LABORATORY MAMMALS AND HUMANS

The most important route of exposure of methyl chloride in humans is via the respiratory pathway. Data on methyl chloride toxicokinetics cover only inhalation; no relevant information on other routes of administration was located in the literature.

7.1 Absorption

In humans as well as in experimental animals, methyl chloride is readily absorbed through the lungs following inhalation (Andersen et al., 1980; Stewart et al., 1980; Landry et al., 1983; Nolan et al., 1985; Löf et al., 2000). In human volunteers exposed to 21 or 103 mg methyl chloride/m3 (10 or 50 ppm) for 6 h or to 21 mg/m3 (10 ppm) for 2 h, steady state was reached during the first exposure hour (Nolan et al., 1985; Löf et al., 2000). In rats, equilibrium between uptake and elimination was also obtained within 1 h (Landry et al., 1983).

7.2 Distribution

After rats were exposed to 14C-labelled methyl chloride by inhalation, radioactivity was found to the largest extent in the liver, kidneys, and testes and to a smaller extent in the brain and lungs (Redford-Ellis & Gowenlock, 1971; Kornbrust et al., 1982; Landry et al., 1983). The presence of residues was, however, attributed to the metabolism of methyl chloride to formaldehyde and formate and subsequent incorporation of the radio labelled carbon atom into tissue macromolecules through single-carbon anabolic pathways (Kornbrust & Bus, 1982; Kornbrust et al., 1982). Methyl chloride may also bind to macromolecules, especially protein, and to a minimal extent probably also to DNA (Kornbrust et al., 1982; Vaughan et al., 1993).

7.3 Metabolism and elimination

In humans as well as in animals, methyl chloride is mainly metabolized by conjugation with glutathione. S-Methylglutathione can then be further metabolized to S-methylcysteine and methanethiol (Redford-Ellis & Gowenlock, 1971; Bus, 1982; Landry et al., 1983). To a lesser extent, methyl chloride is also metabolized micro somally via cytochrome P-450 in rat liver, resulting in the formation of formaldehyde and formate (Kornbrust & Bus, 1983). Formaldehyde and formate may also be formed via the glutathione pathway (Kornbrust & Bus, 1983).

Inhalation of methyl chloride by male B6C3F1 mice resulted in a concentration-dependent depletion of glutathione in liver, kidney, and brain. The depletion was most pronounced in the liver, where a 6-h inhalation exposure to 206 mg/m3 (100 ppm) decreased the gluta thione level by 45%, and exposure to 5160 mg/m3 (2500 ppm) reduced the glutathione content to 2% of control levels (Kornbrust & Bus, 1984).

Metabolites from methyl chloride are excreted in the urine and in the expired air. S-Methylcysteine has been identified in the urine of occupationally exposed humans and rats (van Doorn et al., 1980; Landry et al., 1983), and formic acid has been found in rat urine (Kornbrust & Bus, 1983). Further, carbon dioxide has been shown to be the major final metabolite of methyl chloride, accounting for nearly 50% of the radiolabelled material recovered after a 6-h exposure of rats to methyl chloride (Kornbrust & Bus, 1983). Methyl chloride is also excreted unmetabolized via the lungs, as seen in studies in volunteers (Stewart et al., 1980; Nolan et al., 1985; Löf et al., 2000).

The plausible metabolic pathways of methyl chloride in mammals are shown in Figure 2.

7.4 Genetic polymorphism and sex, strain, organ, and species differences

In several studies in volunteers, large inter individual differences in concentrations of methyl chloride in breath and blood and amounts of excreted urinary metabolites have been observed (Stewart et al., 1980; van Doorn et al., 1980; Putz-Anderson et al., 1981a; Nolan et al., 1985; Löf et al., 2000).

One explanation for the large interindividual differences in uptake and elimination of methyl chloride in humans is the presence or absence of the enzyme GSTT1 (Coles & Ketterer, 1990). The presence of the GSTT1 gene leads to conjugation between glutathione and methyl chloride (GSTT1+), and the absence of the gene leads to no conjugation (GSTT1-) (Pemble et al., 1994).

About 60% of blood samples from a German population showed a significant metabolic elimination of methyl chloride, whereas 40% did not (Peter et al., 1989). In a Swedish population, Warholm et al. (1994) found a large interindividual variation in the glutathione transferase activity in erythrocytes treated with methyl chloride, as 43% had a high activity, 46% had a medium activity, and 11% lacked activity. Nelson and co-workers (1995) mapped the ethnic differences in the prevalence of the null genotype (GSTT1-) and found the highest prevalence among Chinese (64%), followed by Koreans (60%), African-Americans (22%), and Caucasians (20%), and the lowest among Mexican-Americans (10%). Warholm et al. (1994) concluded that the GSTT1 polymorphism leads to three different phenotypes in humans — namely, non-conjugators (NC), low conjuga tors (LC), and high conjugators (HC). In a comparison involving the three human phenotypes and experimental animals, Thier et al. (1998) established that GSTT1 activity towards methyl chloride in human erythrocytes (HC, LC, or NC) and in liver and kidney cytosol in experimental animals decreased in the following order: female mouse (B6C3F1) > male mouse (B6C3F1) > HC > rat (Fischer 344) > LC > hamster (Syrian golden) > NC. In animals, GSTT1 activity towards methyl chloride was 2–7 times higher in liver cytosol than in kidney cytosol (Thier et al., 1998).

Figure 2

The human GSTT1 polymorphism was illustrated in a study on the toxicokinetics of methyl chloride in volunteers phenotyped for GSTT1 activity (HC, LC, and NC) (Löf et al., 2000). It was seen that conjugators with the fast GSTT1 activity (HC) had the highest respiratory net uptake (respiratory net uptake equals the difference between the amount of methyl chloride in inhaled and exhaled air during exposure) of methyl chloride, whereas subjects with no GSTT1 activity (NC) had a smaller respiratory net uptake. At the end of the exposure, the concentration of methyl chloride in blood declined more rapidly among volunteers with high (HC) and intermedi ate (LC) GSTT1 activity than in those with no activity (NC). The area under the curve for NC was higher than those for HC and LC, and the area under the curve for LC was higher than that for HC. Further, the clearance of methyl chloride by metabolism was high in fast conjuga tors (HC) and close to zero in subjects with no GSTT1 activity (NC).

In an investigation by Dekant et al. (1995), sex-, strain-, and species-specific bioactivation of methyl chloride by cytochrome P-450 2E1 (CYP2E1) was seen in the liver and kidneys of rats and mice. In kidney microsomes, the rate of oxidation of methyl chloride was significantly higher in male mice than in female mice and in rats of both sexes than in mice. It was also observed that the rate of oxidation in kidney microsomes was faster in CD-1 mice and NMRI mice than in C3H/He and C57BL/6J mice. In erythrocytes from other species — rats, mice, cows, pigs, sheep, and rhesus monkeys — no conversion of methyl chloride was seen in erythrocyte cytoplasm (Peter et al., 1989).

8. EFFECTS ON LABORATORY MAMMALS AND
IN VITRO TEST SYSTEMS

8.1 Single exposure

In B6C3F1 mice, the LC50 of methyl chloride via inhalation for 6 h was reported to be 4644 mg/m3 (2250 ppm) in males and 17 544 mg/m3 (8500 ppm) in females (White et al., 1982). The data on lethal doses were obtained from an abstract without any further details. In another experimental series, where no clinical acute toxicity symptoms except for lethality were reported, five male B6C3F1 mice were exposed (whole body) to methyl chloride at concentrations of 1032– 5160 mg/m3 (500–2500 ppm) in increments of 1032 mg/m3 (500 ppm) (Chellman et al., 1986a). The 6-h LC50 value was determined to be 4540 mg/m3 (2200 ppm). In this study, lethality as well as hepato toxicity, renal toxicity, and cerebellar degeneration were prevented in mice exposed to 5160 mg/m3 (2500 ppm) for 6 h by pretreatment with the glutathione synthesis inhibitor L-buthionine-S,R-sulfoximine, indicating that metabolism of methyl chloride by glutathione conjuga tion increases the toxicity.

Although other single-exposure inhalation toxicity studies in small rodents exist, they are very old (published before 1950) and do not meet current standards, and they are therefore not included in the present evaluation on methyl chloride. In any case, the results are similar to those reported here.

No single-exposure studies on methyl chloride toxicity following other routes of administration were located in the literature.

In conclusion, based on scarce data, the acute inhalation toxicity in male mice seems to be fairly low, with an LC50 value above 4128 mg/m3 (2000 ppm). In mice, a sex difference in susceptibility to methyl chloride was indicated.

8.2 Irritation and sensitization

No data on irritation or sensitization were available.

8.3 Short-term exposure

The toxic response to methyl chloride was studied in Fischer 344 rats (10 animals per sex per group) exposed by inhalation to 0, 4128, 7224, or 10 320 mg methyl chloride/m3 (0, 2000, 3500, or 5000 ppm) for 6 h/day for 9 days (5 days of exposure followed by a 2- day break in exposure, then a further 4 days’ exposure) and in C3H, C57BL/6, or B6C3F1 mice (5 animals per strain per sex per group) exposed by inhalation to 0, 1032, 2064, or 4128 mg methyl chloride/m3 (0, 500, 1000, or 2000 ppm) for 6 h/day for 12 days (Morgan et al., 1982). The animals were sacrificed 18 h after their last exposure or immediately after the day’s exposure if they were found to be moribund. Clinical observations and histopathological investigations of the brain, liver, kidneys, and adrenal glands in both species and of testes and epididymis of rats were reported. As a result of high intoxication, some rats from the two highest dose groups were sacrificed in extremis (6 males, 5 females: 10 320 mg/m3 [5000 ppm]; 2 females: 7224 mg/m3 [3500 ppm]). No information was given on whether effects were seen in animals with a fulfilled exposure scheme or with an interrupted scheme.

Clinically, especially in the higher dose groups, the rats were seriously affected by the exposure, and symptoms such as lack of coordination of the forelimbs, paralysis of the hindlimbs, convulsive seizures, perineal urine staining, and diarrhoea were recorded. In the kidneys, concentration-related degeneration and necrosis of the proximal convoluted tubules could be seen (lowest- observed-adverse-effect level or LOAEL [males] = 4128 mg/m3 [2000 ppm]; LOAEL [females] = 7224 mg/m3 [3500 ppm]). Testicular degeneration in the seminiferous tubules (LOAEL = 4128 mg/m3 [2000 ppm]) and adrenal fatty degeneration (LOAEL [males and females] = 7224 mg/m3 [3500 ppm]) were also concentration related. Most animals showed minimal hepatocellular response, including loss of normal areas of cytoplasmic basophilia and variable degeneration. Rats in the 10 320 mg/m3 (5000 ppm) group showed degeneration of the cerebellar granular layer.

All mice in the highest dose group died before or were moribund at exposure day 5. No apparent strain differences could be seen from available mortality data. Prior to death, some of the animals developed moderate to severe ataxia, and all females developed haematurea. In the 2064 mg/m3 (1000 ppm) group, females developed haematurea to a much larger extent than males. Cerebellar degeneration of the same type as in rats was seen at the 2064 and 4128 mg/m3 (1000 and 2000 ppm) concentration levels in female C57BL/6 mice only. The other two mouse strains did not develop brain lesions. On the contrary, in all three mouse strains, degeneration in the kidneys was found at 4128 mg/m3 (2000 ppm), and basophilic renal tubules were observed at 2064 mg/m3 (1000 ppm). Hepatocellular necrosis was confined to the 4128 mg/m3 (2000 ppm) group in male C57BL/6 and B6C3F1 mice. Hepatocellular degeneration was seen in lower dose groups, mainly in the 1032 and 2064 mg/m3 (500 and 1000 ppm) groups of male and female C57BL/6 mice. Liver damage in the low dose groups was considered mild and consisted of, for example, variable degrees of glycogen depletion and cytoplasmic vacuolization.

From these studies, which highlight species and sex differences in methyl chloride-induced toxicity, a rat LOAEL of 4128 mg/m3 (2000 ppm) could be derived from the testicular, epididymal, renal, and to some extent hepatocellular findings, and a mouse LOAEL of 1032 mg/m3 (500 ppm) could be derived from the hepatocellular effects. A no-effect level could not be obtained for either species.

The ultrastructure of the methyl chloride-induced cerebellar lesions observed in mice and rats by Morgan and co-workers (1982) and in guinea-pigs (reported under section 8.4) by von Kolkmann & Volk (1975) was further studied by Jiang et al. (1985) in female C57BL/6 mice. The mice were exposed for 6 h/day, 5 days/week, for 2 weeks to 0 or 3096 mg methyl chloride/m3 (0 or 1500 ppm). In all treated mice, degenerative changes of varying severity were observed in the granular cell layer of the cerebellum. The lesions in the granular cells were characterized by nuclear and cytoplasmic condensation of scattered granule cells and also by watery swelling and disruption of granule cell perikarya. From poorly reported clinical observations, neurological deficiency in motor coordination was seen. Few kidney abnormalities were detected, indicating that the cerebellar degenera tions were not secondary to kidney lesions.

In a study primarily designed to investigate the correlation between neurotoxicity and continuous versus intermittent exposure to methyl chloride, Landry et al. (1985) exposed female C57BL/6 mice for 11 days either continuously (22.5 h/day) to 31, 103, 206, 310, or 413 mg/m3 (15, 50, 100, 150, or 200 ppm) or intermittently (5.5 h/day) to 310, 826, 1651, 3302, or 4954 mg/m3 (150, 400, 800, 1600, or 2400 ppm). A quantitative relationship between neurotoxicity and continuous and intermittent exposure was not observed. The lowest effect level for clinical observations, similar to those reported earlier by Dunn & Smith (1947) and later by von Kolkmann & Volk (1975), Morgan et al. (1982), and Jiang et al. (1985), was 206 mg/m3 (100 ppm) for continuous exposure and 3302 mg/m3 (1600 ppm) for intermittent exposure. Cerebellar lesions were recorded at 206 and 826 mg/m3 (100 and 400 ppm) for continuous and intermittent exposure, respectively. A statistically significant decrease was observed in relative and absolute thymus weights at the 31, 103, and 310 mg/m3 (15, 50, and 150 ppm) exposure levels (con tinuous exposure) and also at the 3302 and 4954 mg/m3 (1600 and 2400 ppm) levels (intermittent exposure). Although the decrease in thymus weight at 31 mg/m3 (15 ppm) suggests that this level might be a LOAEL, the absence of a methyl chloride-induced effect on the thy mus or its function in long-term studies indicates that these weight decreases are of uncertain significance. From the results, a LOAEL of 826 mg/m3 (400 ppm) for intermittent exposure (cerebellar lesions) and of 206 mg/m3 (100 ppm) for continuous exposure can be concluded.

To assess the role of inflammation in the toxicity of methyl chloride, especially to sperm, Chellman et al. (1986b) exposed male Fischer 344 rats for 5 days, 6 h/day, to 0 or 10 320 mg methyl chloride/m3 (0 or 5000 ppm) with and without the presence of the anti- inflammatory agent 3-amino-1-(m-[trifluoromethyl] phenyl)-2-pyrazoline (BW755C), an inhibitor of leuco triene and prostaglandin synthesis. Lesions that were induced by exposure to methyl chloride alone were epididymal sperm granulomas, degeneration of cere bellar granule cells, necrosis of renal proximal tubules, cloudy swelling of hepatocytes, and vacuolization of cell cytoplasm in the outer region of zona fasciculata in the adrenal glands. Virtually none of these effects was seen when BW755C was given in parallel with methyl chlor ide, strongly suggesting an inflammatory response.

The question as to whether methyl chloride- induced renal tumours in male mice are evoked by the metabolic intermediate formaldehyde was studied by Jäger et al. (1988). Fischer 344 rats and B6C3F1 mice of both sexes in groups of five were exposed to 0 or 2064 mg methyl chloride/m3 (0 or 1000 ppm) for 6 days, and DNA lesions (cross-links and single-strand breaks), glutathione transferase (GST) activity, and formaldehyde dehydrogenase (FDH) activity were measured. It was shown that the tumour formation in male mice is not based on any obvious biochemical sex differences in enzymatic transformation with respect to FDH. Neither is the metabolically formed formaldehyde likely to be the effective carcinogen, as the characteristic formaldehyde- induced genetic damage is absent. However, the signifi cant species difference between mice and rats — in that mice, due to higher GST activity, especially in the kid neys, seem to be more susceptible to methyl chloride treatment — could not be ruled out. It was, for example, not shown if toxicity caused by the glutathione conjuga tion pathway was due to a metabolite formed or to gluta thione depletion, as suggested by Jäger et al. (1988).

In conclusion, after short-term exposure, the target organ in both rats and mice is the nervous system, with functional disturbances and cerebellar degeneration. The LOAELs in mice are 206 and 826 mg/m3 (100 and 400 ppm) upon continuous and intermittent exposure, respectively. Higher levels of exposure caused toxicity in the kidney and liver in mice and in the testes, epididy mis, and kidney in rats. A mouse LOAEL of 1032 mg/m3 (500 ppm) could be derived from liver toxicity data. The decrease in thymus weight, unaccompanied by histopath ological changes, in mice exposed to 31 mg/m3 (15 ppm) was not corroborated by either a 90-day (CIIT, 1979) or a 2-year study (CIIT, 1981) (reported in sections 8.4 and 8.5, respectively). Because of this lack of corroboration, the decrease in thymus weight will not be forwarded to the sample risk characterization.

No toxicity data for short-term exposures other than those obtained from administration via the respira tory pathway were located in the literature.

8.4 Medium-term exposure

To investigate methyl chloride-induced neurotox icity, von Kolkmann & Volk (1975) exposed 19 guinea- pigs to 41 280 mg methyl chloride/m3 (20 000 ppm; 2 vol% in a pressurized vessel) by inhalation for 61– 70 days (10 min/day, 6 times/week). Clinically, in approximately half of the animals in the treated group, ataxia, paresis of the hind legs, staggering, atactic moving of the head, and retardation in spontaneous reaction and mobility were observed. No animal died during the exposure period. Histopathologically, necroses were seen in the cerebellar cortex in the granular cell layer. Further, Purkinje’s cell necrosis occurred. Considering the extremely high exposure concentrations, the study can be used for descriptive purposes only.

In a subchronic toxicity study, 80 Fischer 344 rats (40 per sex) and 80 B6C3F1 mice (40 per sex) were exposed to methyl chloride by inhalation at concentra tions of 0, 774, 1548, or 3096 mg/m3 (0, 375, 750, or 1500 ppm) for 90 days (CIIT, 1979). Clinical observa tions and data on food consumption, body and organ weights, haematology, clinical chemistry, urinalysis, ophthalmoscopic examination, gross pathology, and histopathology were recorded.

Female mice in the 3096 mg/m3 (1500 ppm) dose group had significantly depressed total body weight at the end of the exposure period. Absolute and/or relative organ weights were increased for heart, brain, spleen, liver, kidneys, and lungs in female mice (mainly in the highest dose group) and in pancreas in male mice. Cyto plasmic vacuolization of hepatocytes occurred in the two highest dose groups and was considered compound related. In the 1548 mg/m3 (750 ppm) dose group, vacuolization was seen 5 times as frequently in females as in males. Exposure-related fluctuations in haema tology and in clinical chemistry were observed but not considered significant, as they were within the control range. Further, the methyl chloride-exposed mice had a high incidence of a mucopurulent conjunctivitis. How ever, this effect was probably not related to methyl chlor ide exposure, as it was seen mainly in the 774 mg/m3 (375 ppm) dose group. The available results indicate that female mice are more sensitive than male mice to methyl chloride exposure.

Male rats in all dose groups and females in the two highest dose groups showed significantly decreased absolute body weight. In rats (males and/or females; mainly in the highest dose group), absolute and/or rela tive organ weights were increased for heart, brain, testes, ovaries, spleen, liver, kidney, pancreas, and adrenals.

In the 1979 CIIT study (which served as a pilot for the 2-year chronic toxicity/carcinogenicity study by CIIT [1981]), no compound-related lesions were reported from gross pathology or histopathology on kidneys, heart, or testes. The absence of recorded organ lesions in mice could be due to a fairly high mortality, especially in the highest dose group, in combination with the histo logical examination applied. In the histopathological examination, as a first step, the highest dose group was compared with the control group. In the case of positive findings, the control animals were thereafter compared with the 1548 mg/m3 (750 ppm) dose group and then the 375 ppm (774 mg/m3) dose group. This procedure might suffer from a high mortality in the 3096 mg/m3 (1500 ppm) dose group and give rise to false negatives. No similar explanation could be given for rats, as the mortality during the exposure period was low.

8.5 Long-term exposure and carcinogenicity

In a 2-year inhalation study, Fischer 344 rats and B6C3F1 mice (120 animals per sex per group) were exposed to 0, 103, 464, or 2064 mg methyl chloride/m3 (0, 50, 225, or 1000 ppm) for 6 h/day, 5 days/week, with the objective of determining the potential toxicological and oncogenic effects (CIIT, 1981). Planned interim necropsies of the experimental animals were completed at 6, 12, 18, and 24 months following initiation of exposure. As a result of high mortality in the mouse high-dose group, the scheduled 24-month sacrifice was carried out after 21 or 22 months of exposure. After 6 or 12 months, 10 rats per sex per dose group were sched uled to be sacrificed, and after 18 or 24 months, 20 and 80 rats per sex per dose group, respectively. Mice were scheduled for sacrifice in groups of 10 per sex per dose group after 6, 12, or 18 months and in groups of 90 per sex per dose group after 24 (or 21, 22 months). Data on body weights, clinical signs of toxic effects, clinical chemical analyses, gross pathology, and histopathology were recorded.

During the exposure period, rat survival was not affected by methyl chloride exposure. However, mouse survival was low in the 2064 mg/m3 (1000 ppm) dose group compared with the control animals. The high mortality occurred predominantly during the first 6 months and was attributed by CIIT (1981) to fighting for dominance. The number of rats and mice that died during the 2-year study for reasons other than planned sacrifice is given in Table 2.

Total body weight gain was significantly reduced throughout the exposure period for male and female rats in the 2064 mg/m3 (1000 ppm) exposure group. Although female rats in the 464 mg/m3 (225 ppm) group and female mice in the 2064 mg/m3 (1000 ppm) group also had significantly decreased growth rates, these occurred periodically and were not observed at the end of exposure. The relative heart weight was increased in female mice and male and female rats at 2064 mg/m3 (1000 ppm). Otherwise, changes in relative or absolute organ weights were seen at the 2064 mg/m3 (1000 ppm) exposure level for kidney, liver, heart, and brain in both species and in testes in rats. For comparison with the short-term exposure study by Landry et al. (1985), thymus weight was not affected by methyl chloride exposure in the 2-year study.

Clinical observations on toxicity to the central nervous system (hunched posture, tremor, and paralysis) were seen in mice in the highest dose group but not in rats.

In mice, statistically significant hepatocellular changes (vacuolization, karyomegaly, cytomegaly, and degeneration) were seen in male and female mice from the 2064 mg/m3 (1000 ppm) group. In male mice exposed to 2064 mg methyl chloride/m3 (1000 ppm), significantly elevated serum glutamic–pyruvic transami nase (SGPT) values were seen, coupled with histopatho logical findings in the liver. Elevated SGPT values were also observed in the lower dose groups but were not correlated with any histopathological findings.

In the 2064 mg/m3 (1000 ppm) dose group in male mice, a large, statistically significant increase (P > 0.05) in the development of renal tubuloepithelial hyperplasia, hypertrophy, and/or karyomegaly, with onset at 12 months, was observed. Further, in the same group, significant exposure-related increases (P < 0.05) in numbers of observed renal cortical adenomas as well as renal adenocarcinomas (including those designed as renal cortical adenocarcinomas and renal cortical papillary cystadenocarcinomas) were noted in animals sacrificed or dying between 12 and 21 months (inci dences of cortical renal lesions are found in Table 3). Cortical adenomas were also seen in two male mice in the 464 mg/m3 (225 ppm) group. Although this increase was not of statistical significance, the adenomas were similar to those that occurred in the 2064 mg/m3 (1000 ppm) group and were therefore judged to be associated with the methyl chloride exposure. In the 103 mg/m3 (50 ppm) group, there was a slightly increased incidence of renal cortical microcysts in male mice sacrificed at 24 months compared with the control males (6/32 vs. 1/20). Renal microcysts were also observed in the 464 mg/m3 (225 ppm) group; the increase, as compared with the control group, was not, however, significant in either males or females. The incidence of renal cortical microcysts was not reported in animals from the highest dose group. Since the micro cysts appear to be variations of the same lesion observed at higher exposure levels, they should be considered to be related to methyl chloride, although no concentration dependence could be established.

Table 2: Number of rodents that died during the exposure period for reasons other than scheduled sacrifice.

Species

Number of rodents that died

0 ppm

50 ppm

225 ppm

1000 ppm

male

female

male

female

male

female

male

female

Rats

15

23

12

19

12

23

14

19

Mice

75

33

62

34

62

25

93

73

Table 3: Total number of significant cortical renal lesions (malign and benign) observed in male B6C3F1 mice exposed to methyl chloride for 2 years.

Renal lesions

Number of renal lesions/number of animals necropsied

0 ppm

50 ppm

225 ppm

1000 ppm

m

f

m

f

m

f

m

f

Cortex, adenocarcinoma

0/120

0/120

0/118

0/100

0/117

0/123

5/120

0/109

Cortex, papillary cysts, adenocarcinoma

0/120

0/120

0/118

0/100

0/117

0/123

1/120

0/109

Cortex, adenoma

0/120

0/120

0/118

0/100

2/117

0/123

12/120

0/109

Cortex, papillary cysts, adenoma

0/120

0/120

0/118

0/100

0/117

0/123

2/120

0/109

Cortex tubuloepithelium, hypertrophy, hyperplasia, and/or karyomegaly

0/120

0/120

0/118

0/100

0/117

0/123

44/120

0/109

Further, at 2064 mg/m3 (1000 ppm), degeneration and atrophy of the seminiferous tubules were seen, as well as lymphoid depletion and splenic atrophy.

At the 18-month sacrifice, cerebellar lesions (degeneration and atrophy of the cerebellar granular layer) were noted in male and female mice at the 2064 mg/m3 (1000 ppm) level. Mice from the control, low, and intermediate exposure groups did not have lesions in the granular cell layer of the cerebellum. At the 22-month sacrifice, similar but more extensive observations in 17/18 females were reported from the 2064 mg/m3 (1000 ppm) group (the only group examined at this time period).

At the 18-month sacrifice, axonal swelling and degeneration of minor severity were observed in the spinal nerves and cauda equina associated with the lumbar spinal cord. The effects occurred in most treated animals in all dose groups (controls: 1/5 males and 2/10 females; 103 mg/m3 [50 ppm]: 4/5 males and 10/10 females; 464 mg/m3 [225 ppm]: 5/5 males and 5/5 females; 2064 mg/m3 [1000 ppm]: 3/7 males and no data on females). The effects at each exposure level were significantly increased in each dose group as compared with control animals. However, no concentration– response relationship could be established. In the 2064 mg/m3 (1000 ppm) dose group sacrificed at 22 months, minimal to moderate swelling and degener ation of the lumbar spinal nerves were recorded in 13 of 18 female mice. Twelve of 18 females had similar lesions in the thoracic spinal cord, and 6/18 females in the cervical spinal cord. Histopathological examinations of mice in the 2064 mg/m3 (1000 ppm) dose group with unscheduled death showed high incidences of cerebellar lesions and axonal degeneration of lumbar spinal nerves, lesions similar to those found at the 18-month sacrifice.

In rats, exposure to methyl chloride at 2064 mg/m3 (1000 ppm) caused testicular lesions (bilateral, P > 0.05, and unilateral, P > 0.05, diffuse degenerations and atrophies of the seminiferous tubules). These lesions were statistically significant as compared with the control group and were first observed at the 6-month sacrifice. Sperm granulomas were noted in three male rats at 2064 mg/m3 (1000 ppm). No statistically signifi cant changes other than the effects on body weight gain were seen in female rats. This indicates that the maxi mum tolerated dose used in the CIIT (1981) study might have been too low to induce toxicity in female rats.

Significant findings from the long-term studies in mice are disturbances of the nervous system and induction of tumours and microcysts in male mice. Axonal swelling and degeneration of spinal nerves in all exposure groups suggest a LOAEL of 103 mg/m3 (50 ppm). This LOAEL is forwarded to the sample risk characterization. The observation of renal microcysts in the 103 mg/m3 (50 ppm) dose group (although not con centration related) supports the LOAEL of 103 mg/m3 (50 ppm). Further significant findings are the testicular lesions in rats. In male rats, testicular lesions occurred at 2064 mg/m3 (1000 ppm). No toxic effects in females were reported.

No toxicity data from long-term exposure to methyl chloride other than those obtained from adminis tration via the respiratory pathway were located in the literature.

8.6 Genotoxicity and related end-points

For an overview of, and details on, the geno toxicity studies, see Table 4. In all studies referred to below, methyl chloride was administered by inhalation, unless otherwise noted.

8.6.1 Studies in vitro

Using the Ames assay, methyl chloride was shown to induce gene mutations in Salmonella typhimurium TA100 (Simmon et al., 1977) and in S. typhimurium TA1535 (Andrews et al., 1976) in both the presence and absence of metabolic activation. Further, a concentra tion-related increase in the 8-azaguanine-resistant fraction in S. typhimurium was observed (Fostel et al., 1985).

Methyl chloride induced the adaptive response to alkylation damage in Escherichia coli regulated by the ada protein, which suggests that methyl chloride is a direct DNA-alkylating agent (Vaughan et al., 1993).

Methyl chloride caused gene mutations in vitro in TK6 human lymphoblasts, as shown by a dose-related increase in the mutant fraction (Fostel et al., 1985).

In the study by Fostel and co-workers (1985), no increase in DNA strand breaks as measured by alkaline elution was observed. However, the outcome from the positive control (methyl methane sulfonate [MMS]) was questionable, as unexpectedly high doses were needed for a positive result. Thus, the mutagenic lesions pro duced by methyl chloride might be either different from those produced by MMS or formed at a level below the threshold of detection of alkaline solution.

DNA damage following methyl chloride exposure was shown as a statistically significant enhanced transformation of Syrian hamster embryo cells by SA7 adenovirus (Hatch et al., 1983).

In vitro, 1–10% methyl chloride caused induction of unscheduled DNA synthesis (UDS) in rat spermato cytes and hepatocytes (Working et al., 1986).

Methyl chloride was shown to directly bind to bovine serum albumin. No further details were available (Kornbrust et al., 1982).

In TK/6 human lymphoblasts, methyl chloride induced a statistically significant concentration-related induction of sister chromatid exchange (SCE) frequency as well as significantly declined mitotic index and a significant concentration-related increase in second- division metaphases (Fostel et al., 1985).

8.6.2 Studies in vivo

In vivo, methyl chloride did not cause induction of UDS in rat spermatocytes, hepatocytes, or tracheal epithelial cells at exposure concentrations of 6192– 7224 mg/m3 (3000–3500 ppm) for 6 h/day for 5 days. However, exposure to 30 960 mg/m3 (15 000 ppm) for 3 h did cause a marginal increase in UDS in hepatocytes (Working et al., 1986). The doses used were considered below, but close to, the maximum tolerated dose.

In a macromolecular binding study, male rats were exposed to 14C-labelled methyl chloride (specific activity 25–70 dpm/nmol = 11.2–31.4 × 10–3 mCi/mmol), and accumulation of 14C was measured in lipid, RNA, DNA, and protein from isolated liver, kidneys, lungs, and testes (Kornbrust et al., 1982). Radiolabelled carbon was found in all tissues and fractions studied; however, methylation was not found. Pretreatment with the protein synthesis inhibitor cycloheximide or the folic acid antagonist methotrexate, which interferes with single-carbon metabolism, to a large extent inhibited most of the 14C- incorporation in proteins and macromolecules, respectively. Further, the extent of incorporation of methyl chloride into proteins and lipids was consistent with the rates of turnovers in these macromolecules. Therefore, the most likely mechanism for uptake of methyl chloride into macromolecules is via the one-carbon pool. How ever, this does not exclude the possibility that methyl chloride might bind directly to macromolecules to a lesser extent.

In a DNA binding assay, in which Peter et al. (1985) exposed rats and mice to 14C-labelled methyl chloride, no methylation of guanine in DNA at N7 and/or O6 in liver and kidneys by methyl chloride was found. The specific activity in this study (13 mCi/mmol = 2.9 × 104 dpm/nmol) was about 3 orders of magnitude higher than that used by Kornbrust et al. (1982).

Table 4: Genotoxicity of methyl chloride and related end-points.

Species

Protocol

Result

Reference

Gene mutation in vitro; bacteria

S. typhimurium TA100

Ames test
2.5–20%; 8 h; ± S9 mix

positive

Simmon et al., 1977

S. typhimurium TA1535

Ames test
0.5–20.7%; ± S9 mix

positive

Andrews et al., 1976

S. typhimurium TM677
(8-azaguanine resistance)

bacterial forward mutation assay
0, 5, 10, 20, or 30%; 3 h; strain deficient in metabolizing xenobiotics; no metabolizing system was added

positive

Fostel et al., 1985

DNA damage in vitro; bacteria

E. coli B F26

adaptive response to alkylation damage (ada gene)

positive

Vaughan et al., 1993

Gene mutation in vitro; mammalian cells

human lymphoblasts
TK6 (trifluorothymidine resistance)

gene mutation
0, 1, 2, 3, 4, or 5%; 3 h
positive controls: ethyl methane sulfonate (EMS) or methyl methane sulfonate (MMS)

positive

Fostel et al., 1985

DNA damage in vitro; mammalian cells

human lymphoblasts
TK6

alkaline elution
0, 1, 3, or 5%
positive controls: EMS or MMS

negative

Fostel et al., 1985

Syrian hamster embryo cells (primary SHE cells)

DNA damage and repair assay
(DNA adenovirus SA7 transformation)
0, 6000, 12 000, 27 000, 52 000, or 103 000 mg/m3 (0, 3000, 6000, 13 000, 25 000, or 50 000 ppm); 2–20 h

positive

Hatch et al., 1983

rat, F-344
hepatocytes
spermatocytes

unscheduled DNA synthesis
1–10%

positive

Working et al., 1986

bovine serum albumin

protein binding assay

positive

Kornbrust et al., 1982

Chromosomal effects in vitro; mammalian cells

human lymphoblasts
TK6

sister chromatid exchange assay
0, 0.3, 1.0, or 3.0%
positive control: EMS

positive

Fostel et al., 1985

DNA damage in vivo; mammals

 

 

rat, F-344
hepatocytes
spermatocytes
tracheal epithelial cells

unscheduled DNA synthesis
6192–7224 mg/m3 (3000–3500 ppm), 5 days, 6 h/day

negative

Working et al., 1986

unscheduled DNA synthesis
30 960 mg/m3 (15 000 ppm); 3 h

weakly positive

 

rat, F-344, males
3 or 6 animals per group
(no further information on groups was available)

DNA binding study
1032, 3096 mg/m3 (500, 1500 ppm); 6 h + 24 h postexposure
specific radioactivity: 25–70 dpm/nmol = 11.2–31.4 × 10–3 mCi/mmol
liver, kidney, lung, testes

negative

Kornbrust et al., 1982

mouse, B6F3C1, males and females
6 animals per group

DNA–protein cross-links
0 or 2064 mg/m3 (0 or 1000 ppm); 8 h

indications (male mice)

Ristau et al., 1989

rat, F-344, males and females
5 animals per group

DNA binding study
exposure for 4 h of rats in a closed chamber; initial concentration approximately 2064 mg/m3 (1000 ppm) (reading off from graphs)
specific radioactivity: 13 mCi/mmol = 2.9 × 104 dpm/nmol

negative

Peter et al., 1985

mouse, B6C3F1, males and females
25 animals per group

DNA binding study
exposure for 4 h of rats in a closed chamber; initial concentration approximately 2064 mg/m3 (1000 ppm) (reading off from graphs)
specific radioactivity: 13 mCi/mmol = 2.9 × 104 dpm/nmol

negative

Peter et al., 1985

rat, F-344, males and females
5 animals per group
mouse, B6F3C1, males and females
5 animals per group

DNA–protein cross-links
2064 mg/m3 (1000 ppm)
approximately 6 h/day for 6 days
2064 mg/m3 (1000 ppm)
approximately 6 h/day for 6 days
animals sacrificed 6 h postexposure

indication

Jäger et al., 1988

Chromosomal effects in vivo; mammals

rat, Fischer 344,
males
80 animals per group

dominant lethal test
6 h/day for 5 days + 17 exposure-free weeks
0, 2064, 6192 mg/m3 (0, 1000, 3000 ppm) + positive control triethylenemelamine (TEM)

negative (probably a cytotoxic effect)

Working et al., 1985a

Using the alkaline elution technique, no cross-links in male mouse kidneys could be detected after exposure to 2064 mg methyl chloride/m3 (1000 ppm) for 6 days, but some indications of DNA single-strand breaks were obtained (Jäger et al., 1988). However, when mice were exposed to 2064 mg/m3 (1000 ppm) for 8 h only, DNA cross-links were seen in renal tissue of male mice but not in female mice or in hepatic tissues (Ristau et al., 1989). In an attempt to investigate the time-course of the DNA lesions, Ristau et al. (1990) again exposed male mice to 2064 mg methyl chloride/m3 (1000 ppm) for 8 h. In renal tissue, it was observed that DNA–protein cross-links were removed at a fast rate, whereas single-strand breaks appeared to accumulate. At 48 h postexposure, all lesions had disappeared.

In a dominant lethal assay, performed according to Organisation for Economic Co-operation and Develop ment (OECD) test guidelines, male rats were exposed to methyl chloride (Working et al., 1985a). The numbers of live and total implants were decreased, there was an increase in the percentage of preimplantation loss at weeks 2, 4, 6, and 8 postexposure, and there was an increase in the percentage of postimplantation loss at week 1 postexposure. The changes observed were not concentration related. A true dominant lethal effect of genetic origin could be questioned, as the time-courses of the pre- and postimplantation losses after methyl chloride exposure were not the same as those obtained after administration of the positive control, triethylene melamine (TEM). The development of sperm granulo mas in the epididymis, the effects seen in the dominant lethal assay, seems to be cytotoxic rather than genotoxic in origin. However, a genotoxic effect should not be totally excluded.

The role of epididymal inflammation in the induc tion of lethal mutations was studied by Chellman et al. (1986c) in an assay with a test protocol similar to the OECD test guideline for dominant lethal mutations. Rats were exposed to methyl chloride in the presence or absence of the anti-inflammatory agent BW755C. BW755C was effective against the postimplantation losses induced by methyl chloride, but not against pre- implantation losses. The authors’ conclusions, based on unpublished data mentioned in Chellman et al. (1986c), are that the increase in preimplantation losses might be a consequence of testicular lesions caused by methyl chlor ide and that BW755C is effective against epididymal injuries only, thus indicating that epididymal inflamma tion has a role in the induced infertility.

In conclusion, methyl chloride is clearly genotoxic in in vitro systems, in both bacteria and mammalian cells. Methyl chloride binds to protein. Methyl chloride is possibly an alkylating agent; however, the available studies do not allow any quantification. Although the positive effects seen in a dominant lethal test were most likely cytotoxic rather than genotoxic, methyl chloride might be considered a very weak mutagen in vivo based on some evidence of DNA–protein cross-linking at higher doses.

8.7 Reproductive and developmental toxicity

8.7.1 Effects on fertility

In the 1981 CIIT study, referred to in section 8.5, exposure to methyl chloride at 2064 mg/m3 (1000 ppm) caused testicular lesions in rats. The lesions seen were bilateral and consisted of diffuse degeneration and atrophy of the seminiferous tubules.

Chapin et al. (1984) investigated the development of lesions induced in testes and epididymis and effects on reproductive hormones in F-344 rats after exposure to 0 or 6192 mg methyl chloride/m3 (0 or 3000 ppm) for a total of 9 days (6 h/day; approximately 60 exposed ani mals and 16 control animals). Testicular lesions in the form of delay in spermiation, germinal epithelial vacuo lization, and cellular exfoliation as well as bilateral epi didymal granulomas were seen. The effects were observed in most animals, with the onset at day 9 or 11 after the beginning of the exposure. In general, lesions seen in animals at day 19 were of higher severity than those seen earlier. In rats killed 70 days or more after the onset of the exposure, 70–90% of the seminiferous tubules lacked any germinal cells; in 10–30% of the tubules, varying degrees of recovery of spermiation were observed. The LOAEL in this study must be set at 6192 mg/m3 (3000 ppm).

Table 5: Breeding results in rats in the F0 and F2 generations after methyl chloride exposure.

 

Breeding results

 

0 ppm

150 ppm

475 ppm

1500 ppm

F0 generation: number of exposed males proven fertile when mated to exposed females

18/40 (45%)

20/39 (51%)

12/40 (30%)

0/40 (0%)

F0 generation: number of exposed males proven fertile when mated to unexposed females

23/28 (82%)

21/28 (75%)

12/28 (43%)

0/26 (0%)

F1 generation: number of exposed males proven fertile when mated to exposed females

31/40 (78%)

26/40 (65%)

14/23 (61%)

A two-generation inhalation study in Fischer 344 rats was carried out at methyl chloride concentrations of 0, 310, 980, or 3096 mg/m3 (0, 150, 475, or 1500 ppm) (Hamm et al., 1985). The F0 generation (40 males and 80 females per exposure group) was exposed for 10 weeks and during a 2-week mating period (6 h/day, 5 days/week, and 6 h/day, 7 days/week, respectively). A similar exposure schedule was used for the F1 genera tion, with the exclusion of the 3096 mg/m3 (1500 ppm) exposure level. In the high-dose F0-generation males sacrificed immediately after 12 weeks of exposure, treatment-related lesions were found, consisting of minimal to severe atrophy of the seminiferous tubules (10/10 males examined) and granulomas in the epidid ymis (3/10). Severely affected tubules were lined by Sertoli’s cells and by occasional stem cell spermatogo nia. In the less affected tubules, decreased numbers of spermatogonia, primary spermatocytes, and/or secondary spermatocytes were found.

Further, in the F0 generation, no litters were born when high-dose males were mated to exposed or unex posed females, and significantly fewer litters were born to unexposed females mated to the males in the 980 mg/m3 (475 ppm) dose group. No differences in litter size, sex ratio, pup viability, or pup growth were found among the 980 mg/m3 (475 ppm) and 310 mg/m3 (150 ppm) groups compared with the control F0 group. A trend towards decreased fertility was also found in the 980 mg/m3 (475 ppm) dose group in the F1 generation. A LOAEL of 980 mg/m3 (475 ppm) (infertility) was derived from the two-generation study. Breeding results in rats in the F0 and F2 generations after methyl chloride exposure are shown in Table 5.

In a dominant lethal assay in rats exposed to methyl chloride for 5 days, described in section 8.6, visible sperm granulomas in the epididymis were present in the 6192 mg/m3 (3000 ppm) group 17 weeks post exposure but not in the 2064 mg/m3 (1000 ppm) group or in the control group. After exposure to 6192 mg/m3 (3000 ppm), the number of live and total implants was decreased, and there was an increase in postimplantation loss. In both treated groups, there was an increase in preimplantation losses (Working et al., 1985a). The LOAEL for preimplantation loss was 2064 mg/m3 (1000 ppm).

In a subsequent study, Working et al. (1985b) characterized the effect of methyl chloride exposure on sperm quality and histopathology in rats in more detail. Male Fischer 344 rats (80 animals per group) were exposed to 0, 2064, or 6192 mg methyl chloride/m3 (0, 1000, or 3000 ppm) for 5 days, 6 h/day. Besides significantly decreased testis weights in the high-dose group 3–8 weeks postexposure and the findings that more than 50% of the treated animals showed sperm granulomas in the epididymis in the same dose group, observations indicating cytotoxic effects on sperm quality were made. Observations made at the 6192 mg/m3 (3000 ppm) level included significant decreases in testicular spermatid head counts, delay in spermiation, epithelial vacuoliza tion, luminal exfoliation of spermatogenic cells, and multinucleated giant cells. Further, sperm isolated from the vasa deferentia had significantly depressed numbers and an elevated frequency of abnormal sperm head morphology by week 1 postexposure and significantly depressed sperm motility and increased frequency of headless tails by week 3 postexposure. These changes were all within or close to the normal range by week 16 postexposure. A LOAEL of 6192 mg/m3 (3000 ppm) could be derived based on the histopathological findings.

The cause of preimplantation loss induced by methyl chloride was further investigated in rats by Working & Bus (1986). Fischer 344 rats (10–30 animals per group) inhaled 0, 2064, or 6192 mg methyl chlor ide/m3 (0, 1000, or 3000 ppm) 6 h/day for 5 days or received a single injection of TEM as a positive control for genotoxicity. At weeks 1–3 postexposure in the 6192 mg/m3 (3000 ppm) group, preimplantation losses did not exceed unfertilized ova, which was the case for the positive control. From these data, the authors sug gested that preimplantation losses are due to a failure in fertilization rather than to an increase in embryonal deaths.

In conclusion, testicular lesions and epididymal granulomas followed by reduced sperm quality lead to reduced fertility as well as complete infertility in rats. A LOAEL of 980 mg/m3 (475 ppm) and a no-observed- adverse-effect level (NOAEL) of 310 mg/m3 (150 ppm) were identified from the two-generation study of Hamm et al. (1985).

8.7.2 Developmental toxicity

In a study designed to study structural teratogenic ity, pregnant Fischer 344 rats (25 rats per dose group) were exposed to 0, 206, 1032, or 3096 mg methyl chloride/m3 (0, 100, 500, or 1500 ppm) for 6 h/day through gestation days 7–19 (Wolkowski-Tyl et al., 1983a). In the highest dose group, significant reductions in fetal body weight and female crown–rump length were observed. Further, skeletal immaturities such as reduced ossification (metatarsals and phalanges of the anterior limbs, thoracic vertebral centra, pubis of pelvic girdle, and metatarsals of the hindlimbs) were seen. Although these findings were seen in the presence of significantly decreased maternal food consumption, body weight, and weight gain in the same dose group, they should be considered as serious and exposure related. A fetal LOAEL for skeletal immaturities as well as a maternal LOAEL for effects on body weight and food consump tion of 3096 mg/m3 (1500 ppm) were obtained. No other effects, including heart defects, were reported from the 206 and 1032 mg/m3 (100 and 500 ppm) dose groups.

In parallel with the rat study, Wolkowski-Tyl et al. (1983a) also evaluated teratogenicity in pregnant C57BL/6 mice (33 mice per dose group) carrying B6C3F1 fetuses exposed through gestation days 6–17 following the same exposure schedule as the rats. Dams in the 3096 mg/m3 (1500 ppm) group died or were killed in extremis due to very high toxicity (tremor, hunched appearance, difficulty in righting, vaginal bleeding, bloody urine, cerebellar granular cell necrosis and degeneration, etc.). No other maternal toxicity was observed in the other exposure groups. In the 1032 mg/m3 (500 ppm) group, the fetuses (male and female) had a small but significant increase in heart defects (reduction or absence of the atrioventricular valves, chordae tendineae, and papillary muscles). In both the 1032 and 206 mg/m3 (500 and 100 ppm) groups, a significant increase in degree of ossification in the hindlimbs was seen as compared with control animals. A LOAEL for heart defects in fetuses of 1032 mg/m3 (500 ppm) was obtained.

In a subsequent study, Wolkowski-Tyl et al. (1983b) again exposed pregnant C57BL/6 mice carrying B6C3F1 fetuses with the aim of confirming the earlier findings, elucidating the nature of the heart defects more clearly, and establishing a concentration–effect relation ship. Approximately 75 mice per dose group were exposed to 0, 516, 1032, or 1548 mg methyl chloride/m3 (0, 250, 500, or 750 ppm) for 6 h/day during gestation days 6–18. In the 1032 and 1548 mg/m3 (500 and 750 ppm) groups, an exposure-related increase in heart defects (involving effects on the atrioventricular valves, chordae tendineae, and papillary muscles) was observed. Dams were affected at the 1548 mg/m3 (750 ppm) exposure level (decrease in body weight and body weight gain). No maternal toxicity, embryotoxicity, fetotoxicity, or teratogenicity was associated with exposure to methyl chloride at 516 mg/m3 (250 ppm). In this study, the LOAEL for heart defects was 1032 mg/m3 (500 ppm), the NOAEL was 516 mg/m3 (250 ppm), and the maternal LOAEL was 1548 mg/m3 (750 ppm).

In a number of different experiments on small numbers of animals, pregnant C57BL/6 mice carrying B6C3F1 fetuses were exposed to methyl chloride at concentrations of 516, 619, or 2064 mg/m3 (250, 300, or 1000 ppm) for 12–24 h during gestation day 11.5–12.5 (John-Greene et al., 1985). The exposure time was chosen as a critical period in development of cardiac defects. The authors found heart defects when the test was non-blind but not when the technician was unaware of which fetuses were exposed. Further, John-Greene et al. (1985) had concerns regarding the technique used by Wolkowski-Tyl et al. (1983a). However, the investiga tion by John-Greene et al. (1985) is difficult to evaluate, as a small number of animals were used and as the expo sure period was not similar to those in the Wolkowski- Tyl et al. (1983a, 1983b) studies. No LOAEL could be established.

In conclusion, from the studies by Wolkowski-Tyl and co-workers (1983a, 1983b), it seems that methyl chloride could induce heart defects in mice exposed to 1032 mg/m3 (500 ppm) when dams were exposed through gestation days 6–18. A NOAEL of 206 mg/m3 (100 ppm) was established from the developmental toxicity studies.

8.8 Immunological and neurological effects

No specific reports on immunological or neuro logical effects caused by methyl chloride were found in the literature.

9. EFFECTS ON HUMANS

9.1 Studies in volunteers

In order to monitor the physiological response to methyl chloride in healthy volunteers (eight men and nine women) with no previous methyl chloride exposure, Stewart et al. (1980) exposed the volunteers to methyl chloride at concentrations of 0, 41, 206, or (men only) 310 mg/m3 (0, 20, 100, or [men only] 150 ppm), 1, 3, and 7.5 h/day, 5 days/week, for 6 weeks in an exposure chamber. Using a wide battery of behavioural, neuro logical, electromyographic, and clinical tests, no significant decrements were found in the exposed volunteers as compared with controls. For interindividual differences in methyl chloride concentrations in blood and expired air, see section 7.

In volunteers, Putz-Anderson and co-workers (1981a, 1981b) found minimal or no effects on perfor mance after exposure to methyl chloride at 206 or 413 mg/m3 (100 or 200 ppm) for 3 h (n = 56) and at 413 mg/m3 (200 ppm) for 3.5 h (n = 84), respectively.

9.2 Case reports

Available information related to the toxic effects on humans exposed to high concentrations of methyl chloride is mainly derived from accidental exposures in connection with the use of methyl chloride in the produc tion of polystyrene foams and also from refrigerator leakages. Among symptoms described in case reports (see, for example, MacDonald, 1946; McNally, 1946; Hansen et al., 1953; Thordarson et al., 1964; Scharn weber et al., 1974; Spevak et al., 1976; Gudmundsson, 1977; Lanham, 1982) are effects on the nervous system, such as dizziness, weakness, blurred vision, muscular incoordination, drowsiness, sleep disturbances, mental confusion, and paraesthesis. Neurotic and depressive symptoms are also described. Further, gastrointestinal symptoms (nausea, vomiting, abdominal pain, etc.) have been observed, as well as jaundice. In general, the symptoms seem to develop soon after the exposure. However, recovery periods vary to a large extent; for example, in seamen highly exposed to methyl chloride, effects on the nervous system were observed 13 years after the accident (Gudmundsson, 1977).

9.3 Epidemiological studies

Performance and cognitive functions were adversely affected in workers manufacturing foam products. There was also an increase in the magnitude of finger tremors. The workers were exposed for 2 years to approximately 72 mg methyl chloride/m3 (35 ppm), as well as other chemicals (NIOSH, 1976). However, insufficient information was available on exposure to the other chemicals and lifestyle factors, and no relationship could be established between methyl chloride exposure and the various psychological and personality tests employed.

A mortality follow-up study was conducted of 852 male workers employed for at least 1 month between the years 1943 and 1978 in a butyl rubber manufacturing plant using methyl chloride (Holmes et al., 1986). For each cohort member, complete work history and death information were obtained. No information on lifestyle factors was reported. The exposure to methyl chloride and other compounds used in the butyl rubber manufac turing plant was estimated in three categories (high, medium, and low). No detectable excess mortality from any specific cause of death including all cancers was found in the study population after analysis by level and duration of exposure.

In a 32-year follow-up study by Rafnsson & Gudmundsson (1997), indications of elevated mortality from cardiovascular disease after high accidental methyl chloride exposure were seen in Icelandic seamen (deck hands: relative risk [RR] = 3.9, 95% confidence interval [CI] = 1.0–14.4; officers: RR = 1.7, 95% CI = 0.3–6.4). The small number of observed cancers (all cancers and lung cancers) in the exposed group provides an insuffi cient basis for assessing the cancer risk in humans. The reference group used was controlled for age, occupation, social class, and lifestyle factors.

In conclusion, effects on humans, especially on the central nervous system, can clearly be seen after acciden tal (mostly high) exposure or after normal work exposure levels. A rough estimation of the degree of exposure from case reports might be in the order of approximately 200–2000 mg/m3 (100–1000 ppm). In short-term expo sure of volunteers, no significant effects were seen. There are insufficient data available to assess the risk for humans to develop cancer as a result of methyl chloride exposure.

10. EFFECTS ON OTHER ORGANISMS IN
THE LABORATORY AND FIELD

10.1 Aquatic environment

Few data were found on the short-term toxicity of methyl chloride to aquatic organisms, and no data were found on long-term toxicity. The existing data for a cyanobacterium, a green alga, a protozoan, and two fish species indicate a low acute toxicity to aquatic species (see Table 6). The acute toxicity to the two fish species was determined under static conditions, and the concen tration of the test substance was not measured. This means that the toxicity may have been underestimated by the test, if significant amounts of the test substance volatilized during the test. The 96-h LC50 values for the freshwater species, bluegill sunfish (Lepomis macro chirus), and the saltwater species, tidewater silverside (Menidia beryllina), were determined to be 550 and 270 mg/litre, respectively (Dawson et al., 1977).

Table 6: Short-term toxicity to aquatic organisms.

Organism

End-point

Toxicity (mg/litre)

Reference

Cyanobacteria

Microcystis aeruginosa

Toxicity threshold, EC3
(cell multiplication inhibition test)

550

Bringmann & Kühn, 1976

Green algae

Scenedesmus quadricauda

Toxicity threshold, EC3
(cell multiplication inhibition test)

1450

Bringmann & Kühn, 1980

Protozoa

Entosiphon sulcatum

Toxicity threshold, EC5
(cell multiplication inhibition test)

>8000

Bringmann & Kühn, 1980

Fish

Bluegill sunfish (Lepomis macrochirus)

96-h LC50

550

Dawson et al., 1977

Tidewater silverside (Menidia beryllina)

96-h LC50

270

Dawson et al., 1977

Table 7: Short-term toxicity to terrestrial organisms.

Organism

End-point

Toxicity

Reference

Bacteria

Methanogenic bacteria (35 °C,
pH 7, anaerobic conditions)

48-h IC50
(inhibition of gas production)

50 mg/litre

Blum & Speece, 1991

Nitrobacter
(25 °C, pH 9.1)

24-h IC50
(inhibition of NO2-N production)

2010 mg/litre

Tang et al., 1992

Pseudomonas putida

Toxicity threshold, EC3
(cell multiplication inhibition test)

500 mg/litre

Bringmann & Kühn, 1976

Higher plants

Several speciesa
(3-h exposure in gas phase)

Visible symptoms
Photosynthesis
Transpiration

5000–10 000 mg/m3 (2400–4800 ppm)
>5000 mg/m3 (>2400 ppm)
>5000 mg/m3 (>2400 ppm)

Christ, 1996

a Tested plant species were tomatoes (Lycopersicum esculentum Miller), sunflower (Helianthus annuus L.), bush bean (Phaseolus vulgaris L.), nasturtium (Tropaeolum majus L.), sugar-beet (Beta vulgaris L.), soya bean (Glycine maxima (L.) Merill), and wheat (Triticum aestivum L.).

10.2 Terrestrial environment

Data on the short-term toxicity of methyl chloride were found only for three species of bacteria and some higher plants (see Table 7). No chronic toxicity data were found for terrestrial organisms.

11. EFFECTS EVALUATION

11.1 Evaluation of health effects

11.1.1 Hazard identification and dose–response assessment

The database for methyl chloride risk assessment is in general acceptable in terms of toxicity after inhalation exposure. Few data could be located in the literature on methyl chloride toxicity after dermal or oral administra tion. However, as the major route of human exposure to methyl chloride seems to be by the respiratory pathway, the lack of data from other routes of administration is of minor concern. It should be pointed out that there are surprisingly few recent investigations of methyl chloride toxicity for all end-points except genotoxicity. In Table 8, data from inhalation toxicity studies on methyl chlor ide in experimental animals are summarized.

Table 8: Summary of animal inhalation toxicity studies with relevance for risk characterization.

Species

Study duration

End-point

LOAEL,
mg/m3 (ppm)

NOAEL, mg/m3 (ppm)

Reference

Short-term exposure

mouse, C3H, females; C57BL/6, males and females

12 days

hepatocellular necrosis and degeneration

1032 (500)

Morgan et al., 1982

mouse, C57BL/6, females

11 days, continuous exposure

cerebellar lesions

206 (100)

Landry et al., 1985

 

intermittent exposure

 

826 (400)

 

Long-term exposure

rat, F-344

2 years

testicular lesions

2064 (1000)

464 (225)

CIIT, 1981

mouse, B6C3F1

2 years

renal tumours in males; significant increase at 2064 mg/m3 (1000 ppm)

2064 (1000)

464 (225)

CIIT, 1981

 

 

development of renal microcysts in males; also seen at 464 mg/m3 (225 ppm); no dose–response

103 (50)

 

 

 

 

nerve axonal swelling and degeneration; seen in all treated groups; effects significant as compared with control

103 (50)

 

 

Reproductive toxicity — fertility

rat, F-344

9 days

testicular lesions and epididymal granulomas

6192 (3000)

Chapin et al., 1984

rat, F-344

two- generation

infertility; dose dependence or trend

980 (475)

310 (150)

Hamm et al., 1985

rat, F-344

5 days

preimplantation loss

2064 (1000)

Working et al., 1985a

rat, F-344

5 days

effects on sperm quality

6192 (3000)

2064 (1000)

Working et al., 1985b

Reproductive toxicity — development

rat, F-344

gestation days 7–19

skeletal immaturities in the presence of effects of maternal body weight and food consumption

3096 (1500)

1032 (500)

Wolkowski-Tyl et al., 1983a

mouse, B6C3F1

gestation days 6–17

heart defects in fetuses; significant increase as compared with control animals

1032 (500)

206 (100)

Wolkowski-Tyl et al., 1983a

mouse, B6C3F1

gestation days 6–18

heart defects in fetuses; dose dependence

1032 (500)

516 (250)

Wolkowski-Tyl et al., 1983b

Considering the human GSTT1 polymorphism and the suggested metabolic pathways of methyl chloride (Figure 2), it is not possible to conclude whether a rapid metabolic clearance of methyl chloride as in high conjugators (HC) leads to a higher or lower risk than a longer retention of methyl chloride in the body as in low conjugators (LC) or non-conjugators (NC). Further, as the GSTT1 activity decreases in the order mouse liver and kidney cytosol > HC > rat > LC > hamster > NC, it is impossible to select one species of experimental animal as preferable in the risk assessment of methyl chloride when extrapolating animal effects data to humans. Because of these uncertainties, no species of experimental animal can be ruled out in favour of another, and human high conjugators, low conjugators, and non-conjugators must be considered as sensitive to methyl chloride. Consequently, the lowest appropriate LOAEL or NOAEL from any species could be chosen for further risk characterization.

Data on single exposures are poor for methyl chloride, and no firm conclusions can be drawn. However, the acute inhalation toxicity in rats and male mice after single exposure seems to be fairly low, with an LC50 value above 4128 mg/m3 (2000 ppm). In mice, there might be a sex difference in susceptibility to methyl chloride, as the LC50 value obtained for female mice was 17 544 mg/m3 (8500 ppm).

No data on irritation and sensitization were available. However, from short- and long-term exposure data, no indications of respiratory irritation caused by methyl chloride exposure have been reported. Thus, methyl chloride is probably not a strong respiratory irritant.

The principal target in both rats and mice upon short-term exposure is the nervous system, with animals exhibiting functional disturbances and cerebellar degeneration. The LOAEL (based on cerebellar degeneration) in mice is 206 mg/m3 (100 ppm) upon continuous exposure. Higher levels of exposure caused toxicity in the kidney and liver in mice and in the testes, epididymis, and kidney in rats. The decrease in thymus weight, unaccompanied by histopathological changes, in mice exposed to 31 mg/m3 (15 ppm) was not corrobor ated by either a 90-day or a 2-year study.

In the long-term studies, exposure to 103 mg/m3 (50 ppm) caused nerve lesions, such as axonal swelling and degeneration. The effects at each exposure level were significantly increased in each dose group as compared with control animals. However, no concen tration–response relationship could be established. Significant degenerative effects on nerve fibres were also seen at higher doses, although the observations were not concentration related. Further, testicular lesions in rats and renal lesions in male mice were important findings obtained from the 2-year toxicity study. Renal tumours as well as testicular lesions were seen at the 2064 mg/m3 (1000 ppm) exposure level, and, although not of statis tical significance, cortical adenoma was also seen at 464 mg/m3 (225 ppm). Development of renal cortical microcysts in mice was seen in the 103 mg/m3 (50 ppm) dose group and to some extent in the 464 mg/m3 (225 ppm) group (CIIT, 1981); however, no concentra tion–response relationship could be established Although there are indications of low CYP2E1 activity in male human kidney microsomes as compared with male CD-1 mice, as shown by Speerschneider & Dekant (1995), the presence of human renal CYP2E1 cannot be excluded. Further, the presence of CYP2E1 in human tissues other than kidneys might lead to the induction of tumours in other organs. Therefore, the findings of renal tumours in male mice should be considered as relevant for humans.

Methyl chloride is clearly genotoxic in in vitro systems in both bacteria and mammalian cells. Methyl chloride can bind to protein. However, if methyl chloride is an alkylating agent, it is so to a very small extent. Further, methyl chloride might be considered only a very weak mutagen in vivo.

Testicular lesions and epididymal granulomas followed by reduced sperm quality lead to reduced fertility as well as complete infertility in rats. A LOAEL of 980 mg/m3 (475 ppm) could be derived from the reproductive toxicity data where a concentration–response could be established (NOAEL = 310 mg/m3 [150 ppm]).

It seems from the Wolkowski-Tyl et al. (1983a, 1983b) studies that methyl chloride could induce heart defects in mice exposed to 1032 mg/m3 (500 ppm) when dams are exposed through gestation days 6–18. The development of heart defects was concentration depen dent. A NOAEL of 206 mg/m3 (100 ppm) could be estimated from the developmental toxicity studies.

Effects on humans, especially on the central nervous system, can clearly be seen after accidental or normal work exposure levels. A rough estimation of the degree of exposure from case reports might be in the order of approximately 200–2000 mg/m3 (100– 1000 ppm). In short-term exposures of volunteers, no significant effects on the nervous system were seen. There are insufficient data available to assess the risk for humans to develop cancer as a result of methyl chloride exposure.

11.1.2 Criteria for setting tolerable intakes or guidance values for methyl chloride

Human data supplemented by data from short-, medium-, and long-term studies in laboratory animals, principally the mouse, clearly indicate that the nervous system is a particularly vulnerable target of methyl chloride exposure. Histopathological effects in spinal cord nerves were observed in a 2-year exposure of mice (CIIT, 1981) at a LOAEL of 103 mg/m3 (50 ppm), in the absence of renal and hepatic effects. At 2064 mg/m3 (1000 ppm), cerebellar degeneration was noted. This latter lesion, which appears to be of a progressive and persistent nature, has also been observed in mice exposed in other studies (e.g., Landry et al., 1985) for much shorter periods of time. In addition, evidence of functional impairment noted in short-term studies is consistent with the spinal cord and the brain being poten tial sites of injury under chronic exposure conditions.

Consequently, the LOAEL of 103 mg/m3 (50 ppm), derived from the 2-year study by CIIT (1981) for effects on the nervous system, was chosen to be used in the risk characterization.

As no estimation from the available exposure database can be made on the allocation of the tolerable intake from environmental air and from air in the working environment, two separate guidance values were derived.

A guidance value for indirect inhalation exposure to methyl chloride via the environment for the general population was estimated to be:

(103 mg/m3 × 1.0 × 6/24 × 5/7) / 1000

= 0.018 mg/m3 (0.009 ppm)

where:

A guidance value for occupational inhalation exposure was estimated to be:

(103 mg/m3 × 1.0) / 100 = 1.0 mg/m3 (0.5 ppm)

where:

No correction was made for continuous exposure in the working environment.

11.1.3 Sample risk characterization

Based on the sample estimate of exposure, indirect exposure via the ambient air, 0.0012 mg/m3 (0.6 ppb), is 15 times below the guidance value of 0.018 mg/m3 (0.009 ppm). Also, the sample median exposure estimate for urban air, 0.0010–0.0023 mg/m3 (0.5–1.1 ppb), is 18 to 8 times below the guidance value. Finally, the maxi mum value obtained from individual measurements in urban air, 0.035 mg/m3 (17 ppb), is 2 times above the guidance value. From the available exposure data, a risk was identified from exposure in urban air but not ambient air.

The quality of human exposure data in the present report is fairly poor, which will contribute to uncertain ties in the outcome of the sample risk characterization for occupational exposure. However, when more applicable exposure data can be used, the quality of the risk assessment will improve.

The range of the sample estimate of workplace exposure, 0.2–186 mg/m3 (0.1–90 ppm), is 5 times below and 180 times above the guidance value of 1.0 mg/m3 (0.5 ppm), respectively. Thus, a comparison of the available methyl chloride exposure concentrations in the working environment and the guidance value derived from effects on the nervous system leads to the identification of a risk. Although the nerve lesions were seen at lower exposure levels than those at which infertility in rats (980 mg/m3 [475 ppm]) and renal tumours in male mice (2064 mg/m3 [1000 ppm]) occurred, emphasis should also be laid on these very serious effects in a qualitative risk characterization of methyl chloride.

11.2 Evaluation of environmental effects

The troposphere, where methyl chloride reacts with hydroxyl radicals, is the main environmental sink for the chemical. A certain amount of the tropospheric methyl chloride reaches the stratosphere. In the strato sphere, photolysis produces chlorine radicals, which in turn will react with ozone. Estimates of the amount of methyl chloride reaching the stratosphere, and thereby also its contribution to ozone depletion, vary consider ably. However, as estimated from figures presented by the WMO, methyl chloride contributes approximately 15% of the total equivalent effective stratospheric chlorine.3 The relative contribution from methyl chloride to the depletion of the ozone layer will probably increase in the future, as the use of chlorofluorocarbons and hydrochlorofluorocarbons is expected to decrease. The stratospheric ODP of methyl chloride has been deter mined to be 0.02 relative to that of CFC-11 (ODP = 1).

Methyl chloride is not thought to contribute significantly to global warming or to the creation of photochemical air pollution.

The dominant loss mechanism for methyl chloride in water and soil is volatilization. Slow hydrolysis and possibly biotic degradation may contribute at deeper soil depths and in groundwater. However, little information is available concerning biodegradation.

Data on short-term toxicity to both aquatic and terrestrial organisms are sparse. No information was found on long-term toxicity. The available data show that methyl chloride has a low acute toxicity to tested aquatic organisms (e.g., protozoa, green algae, and fish). The lowest LC50 value for fish is 270 mg/litre. As concentrations of methyl chloride in surface waters (maximum 0.22 mg/litre) are generally several orders of magnitude less than those demonstrated to cause effects, it is likely that methyl chloride poses a low risk of acute effects on aquatic organisms. Data on terrestrial organisms are even more sparse than data on aquatic organisms. The only information found shows that methyl chloride causes acute effects (i.e., visible symp toms and effects on photosynthesis and transpiration) on higher plants at concentrations above 5000 mg/m3 (2400 ppm), which is about 105 times higher than the highest concentration found in urban air (i.e., 0.035 mg/m3 [0.017 ppm]).

12. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES

In its series of monographs, the International Agency for Research on Cancer (IARC, 1986) has evaluated methyl chloride. Based on the data available, the Task Group concluded that there is inadequate evidence for the carcinogenicity of methyl chloride to experimental animals and to humans. In the overall assessment of data from short-term tests, the Task Group concluded that there is sufficient evidence for genotoxic activity. In the overall evaluation (IARC, 1987), methyl chloride was placed in group 3 as being not classifiable as to its carcinogenicity to humans.

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Working PK, Doolittle DJ, Smith-Oliver T, White RD, Butterworth BE (1986) Unscheduled DNA synthesis in rat tracheal epithelial cells, hepatocytes and spermatocytes following exposure to methyl chloride in vitro and in vivo. Mutation research, 162:219.

Wynder EL, Hoffmann D, eds. (1967) Tobacco and tobacco smoke: Studies in environmental carcinogenesis. New York, NY, Academic Press, p. 454.

Zafiriou OC (1975) Reaction of methyl halides with seawater and marine aerosols. Journal of marine research, 33(1):75–81 [cited in BUA, 1986; HSDB, 1996].

Zander R, Gunson MR, Farmer CB, Rinsland CP, Irion FW, Mahieu E (1992) The 1985 chlorine and fluorine inventories in the strato sphere based on ATMOS observations at 30° north latitude. Journal of atmospheric chemistry, 15:171–186.

APPENDIX 1 — SOURCE DOCUMENTS

Lundberg P (1992) Methyl chloride. NEG and DECOS basis for an occupational standard. Solna, National Institute of Occupational Health, Nordic Council of Ministers (Arbete och Hälsa 27)

Copies of the Arbete och Hälsa document on methyl chloride (ISSN 0346-7821; ISBN 91-7045-179-6) may be obtained from:

National Institute for Working Life
Publications Department
S-171 84 Solna
Sweden

The document, which is focused on health effects only, was prepared in the series of criteria documents from the Nordic Expert Group for Documentation of Occupational Exposure Limits (NEG) in collaboration with the Dutch Expert Committee for Occupational Standards (DECOS) of the Dutch Directorate-General of Labor. The draft was reviewed by the Dutch Expert Committee as well as by the Nordic Expert Group. The reviewers were industrial and academic specialists who were chosen either because they have an extended knowledge of methyl chloride itself or because they are specialists in the critical effect area of the chemical.

ATSDR (1990) Toxicological profile for chloromethane. Atlanta, GA, US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry (Report No. TP-90-07)

ATSDR (1998) Toxicological profile for chloromethane (update). Atlanta, GA, US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry (Report No. 205-93-0606)

Copies of the ATSDR Toxicological profile for chloromethane may be obtained from:

Agency for Toxic Substances and Disease Registry
Division of Toxicology
1600 Clifton Road NE, E-29
Atlanta, Georgia 30333
USA

A peer review panel was assembled for the Toxicological profile on chloromethane (1990), including Dr Anthony DeCaprio and Dr Nancy Reiches (private consultants), Dr Theodore Mill (SRI International), and Dr Nancy Tooney (Department of Biochemistry, Polytechnic University). A joint panel of scientists from ATSDR and EPA reviewed the peer reviewers’ comments and determined which ones to include in the profile.

For the updated profile (1998), the panel consisted of Dr Herbert Cornish (private consultant), Dr Anthony DeCaprio (Associate Professor, State University of New York at Albany), Dr Theodore Mill (Senior Scientist, SRI International), and Dr Nancy Tooney (Associate Professor, Brooklyn, NY). Scientists from the ATSDR reviewed the peer reviewers’ comments and determined which ones to include in the profile.

The responsibility for the content of the profiles lies with the ATSDR.

BUA (1986) Chloromethane. GDCh-Advisory Committee on Existing Chemicals of Environmental Relevance (BUA). Weinheim, VCH Verlagsgesellschaft mbH; and New York, NY, VCH Publishers, Inc. (BUA Report 7)

Copies of the BUA report on chloromethane (ISBN 3-527- 28558-X [Weinheim] and 1-56081-734-8 [New York]) may be obtained from VCH in Weinheim, Basel, Cambridge, and New York.

BUA reports are written by the largest German producer of the chemical. The draft is examined by BUA (GDCh-Advisory Committee for Existing Chemicals of Environmental Relevance), which consists of representatives from government agencies, industry, and the scientific community. Resulting questions are clarified by the authors as well as by further research (resulting in updated reports). After an average of two readings in the work group and discussions with experts, the BUA plenum debates the report before it is published. More detailed information on the BUA reports is found in Assessment of existing chemicals. A contribution toward improving the environment, a 1993 report by BUA.

HSDB (1996) Hazardous substances data bank. Bethesda, MD, US National Library of Medicine

The version of HSDB used for this CICAD is included in the CD-ROM CHEM-BANK (July 1996), published by:

Silver Platter Information Inc.
100 River Ridge Drive
Norwood, MA 02062-5043
USA

HSDB is also available on CD-ROM from the Canadian Centre for Occupational Health and Safety (CCINFOdisc 2) and on- line by Data-Star, DIMDI, STN International, Toxicology Data Network (TOXNET). HSDB is built, reviewed, and maintained on the National Library of Medicine’s TOXNET. HSDB is a factual data bank, referenced and peer reviewed by a committee of experts (the Scientific Review Panel). All data extracted from HSDB for use in this CICAD were preceded by the symbol denoting the highest level of peer review.

The date for the last revision or modification of the record on methyl chloride was June 1996.

WMO (1994) Montreal protocol on substances that deplete the ozone layer — Scientific assessment of ozone depletion: 1994. Geneva, World Meteorological Organization (Global Ozone Research and Monitoring Project Report No. 37)

Copies of this report (ISBN 92-807-1449-X) for scientific users may be obtained from:

World Meteorological Organization
attn. Dr Rumen Bojkov
P.O. Box 2300
1211-Geneva
Switzerland

The WMO (1994) report was the latest in a series of scientific assessments of ozone depletion, prepared under the auspices of WMO and UNEP, that was available during the preparation of the CICAD on methyl chloride. The genesis of the WMO (1994) report occurred at the 4th meeting of the Conference of the Parties to the Montreal Protocol held in Copenhagen, Denmark, in 1992, at which the scope of the scientific needs was defined. In 1993, an international steering group outlined the report and suggested scientists to serve as authors. The first draft was examined by the authors and a small group of experts, and the second draft was sent to a large number of scientists worldwide for review. At a Panel Review Meeting in July 1994, final changes were discussed and decided upon. The scientists who prepared the report (230) and participated in the peer review process (147) are listed in the report.

APPENDIX 2 — CICAD PEER REVIEW

The draft CICAD on methyl chloride was sent for review to institutions and organizations identified by IPCS after contact with IPCS national Contact Points and Participating Institutions, as well as to identified experts. Comments were received from:

M. Baril, International Programme on Chemical Safety/ Institut de Recherche en Santé et en Sécurité du Travail du Québec, Montreal, Quebec, Canada

R. Benson, US Environmental Protection Agency, Denver, CO, USA

R. Cary, Health and Safety Executive, Bootle, United Kingdom

R. Chhabra, Department of Health and Human Services, National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA

P. Edwards, Department of Health, Protection of Health Division, London, United Kingdom

M. Greenberg, US Environmental Protection Agency, Research Triangle Park, NC, USA

Martin Matisons Environmental Health Service, Health Department of Western Australia

H. Nagy, National Institute for Occupational Safety and Health, Cincinnati, OH, USA

W. Rawson and L. Neuwirth, Methyl Chloride Industry Association, Washington, DC, USA

M. Warholm, Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden

P. Yao, Ministry of Health, Institute of Occupational Medicine, Chinese Academy of Preventive Medicine, Ministry of Health, Beijing, People’s Republic of China

K. Ziegler-Skylakakis, GSF-Forschungszentrum für Umwelt und Gesundheit, Neuherberg, Oberschleissheim, Germany

APPENDIX 3 — CICAD FINAL REVIEW BOARD

Stockholm, Sweden, 25–28 May 1999

Members

Mr H. Abadin, Agency for Toxic Substances and Disease Registry, Centers for Disease Control and Prevention, Atlanta, GA, USA

Dr B. Åkesson, Department of Occupational and Environmental Health, University Hospital, Lund, Sweden

Dr T. Berzins (Chairperson), National Chemicals Inspectorate (KEMI), Solna, Sweden

Mr R. Cary, Health and Safety Executive, Bootle, Merseyside, United Kingdom

Dr R.S. Chhabra, General Toxicology Group, National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA

Dr S. Dobson (Rapporteur), Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, United Kingdom

Dr H. Gibb, National Center for Environmental Assessment, US Environmental Protection Agency, Washington, DC, USA

Dr R.F. Hertel, Federal Institute for Health Protection of Consumers and Veterinary Medicine, Berlin, Germany

Dr G. Koennecker, Chemical Risk Assessment, Fraunhofer Institute for Toxicology and Aerosol Research, Hannover, Germany

Dr A. Nishikawa, National Institute of Health Sciences, Division of Pathology, Tokyo, Japan

Professor K. Savolainen, Finnish Institute of Occupational Health, Helsinki, Finland

Dr J. Sekizawa, Division of Chem-Bio Informatics, National Institute of Health Sciences, Tokyo, Japan

Ms D. Willcocks (Vice-Chairperson), Chemical Assessment Division, National Occupational Health and Safety Commission (Worksafe Australia), Sydney, Australia

Professor P. Yao, Institute of Occupational Medicine, Chinese Academy of Preventive Medicine, Ministry of Health, Beijing, People’s Republic of China

Observers

Dr N. Drouot (representing ECETOC), Elf Atochem, DSE-P Industrial Toxicology Department, Paris, France

Ms S. Karlsson, National Chemicals Inspectorate (KEMI), Solna, Sweden

Dr A. Löf, National Institute of Working Life, Solna, Sweden

Dr A. Poole (representing CEFIC), Dow Europe S.A., Horgen, Switzerland

Dr K. Ziegler-Skylakakis, GSF-Forschungszentrum für Umwelt und Gesundheit, Institut für Toxikologie, Neuherberg, Oberschleissheim, Germany

Secretariat

Dr A. Aitio, Programme for the Promotion of Chemical Safety, World Health Organization, Geneva, Switzerland

Ms M. Godden, Health and Safety Executive, Bootle, United Kingdom

Ms L. Regis, Programme for the Promotion of Chemical Safety, World Health Organization, Geneva, Switzerland

Dr P. Toft, Division of Health and Environment, World Health Organization, Regional Office for the Americas/Pan American Sanitary Bureau, Washington, DC, USA

Dr M. Younes, Programme for the Promotion of Chemical Safety, World Health Organization, Geneva, Switzerland

INTERNATIONAL CHEMICAL SAFETY CARD

METHYL CHLORIDE ICSC:0419

RÉSUMÉ D’ORIENTATION

L’évaluation des effets du chlorure de méthyle sur la santé humaine qui figure dans le présent CICAD repose principalement sur une mise au point rédigée par le Groupe nordique d’experts en collaboration avec le Comité néerlandais pour les normes d’hygiène sur les lieux de travail (Lundberg, 1992). Un dépouillement des banques de données couvrant la période 1992-1999 a été effectué afin de compléter les données disponibles. En ce qui concerne les effets environnementaux et écotoxicol ogiques du chlorure de méthyle, les principales sources d’information utilisées sont les suivantes : BUA (1986), ATDSR (1990) et WMO/OMM (1994). La banque de données ATDSR a été mise à jour en 1998; chaque fois que cette banque de données contenait de nouvelles informations, elles ont été prises en compte. La consul tation des banques de données pertinentes couvrant la période 1989-1997 a permis d’obtenir des données supplémentaires sur les questions d’ordre écologique. On trouvera à l’appendice 1 des renseignements sur la nature des sources documentaires existantes. Des informations sur l’examen par des pairs du présent CICAD sont données à l’appendice 2. Ce CICAD a été approuvé en tant qu’évaluation internationale lors d’une réunion du Comité d’évaluation finale qui s’est tenue à Stockholm (Suède) du 25 au 28 mai 1999. La liste des participants au Comité d’évaluation finale figure à l’appendice 3. La fiche internationale sur la sécurité chimique (ICSC 0419) du chlorure de méthyle, établie par le Programme international sur la sécurité chimique (IPCS, 1999), est également reproduite dans le présent document.

Lorsque du chlorure de méthyle (No CAS 74-87-3) est libéré dans l’atmosphère, c’est surtout au cours de sa production, de son utilisation ou encore lors de l’incinération de déchets municipaux ou industriels. Quoi qu’il en soit, les sources naturelles de chlorure de méthyle (en premier lieu les océans et la combustion de la biomasse) l’emportent largement en importance sur les sources d’origine humaine. On estime que l’ensemble de ces sources libère chaque année quelque 5 × 106 tonnes de chlorure de méthyle. La part des sources naturelles dans ce bilan dépasse largement 90 % selon les esti mations et pourrait même atteindre 99 %. Le composé est présent dans la troposphère à une concentration approximativement égale à 1,2 µg/m3 (0,6 parties par milliard).

Dans la troposphère, le piégeage du chlorure de méthyle s’effectue principalement par sa réaction sur les radicaux hydroxyle et l’on estime que sa demi-vie atmosphérique est de 1 à 3 ans. Une fraction s’échappe dans la stratosphère où le composé subit une photodissociation donnant naissance à des radicaux chlore qui attaquent la couche d’ozone. Les estimations relatives à la proportion de chlorure de méthyle qui atteint la stratosphère et contribue par là à la destruction de la couche d’ozone, sont très variables. Selon les chiffres donnés par l’Organisation météorologique mondiale (OMM), le chlorure de méthyle contribue à hauteur de 15 % à la teneur totale de la stratosphère en chlore actif. Le chlorure de méthyle a un potentiel de destruction de l’ozone stratosphérique de 0,02 par rapport au composé de référence, le CFC-11 dont le potentiel est égal à 1 par définition. On ne pense pas que le chlorure de méthyle ait une influence sensible sur le réchauffement climatique ou sur la pollution de l’air d’origine photochimique.

Le chlorure de méthyle disparaît principalement de l’eau et du sol par évaporation. Dans les couches pro fondes du sol et dans les eaux souterraines, sa disparition pourrait s’expliquer notamment par une lente hydrolyse et éventuellement par une biodégradation. On sait cependant peu de choses sur ce dernier point.

La voie la plus importante d’exposition humaine au chlorure de méthyle est la voie respiratoire. Chez l’Homme comme chez l’animal de laboratoire, il est rapidement absorbé au niveau des poumons après inhalation. Après exposition à du chlorure de méthyle marqué au 14C, la radioactivité se répartit dans l’ensemble de l’organisme. Le produit radiomarqué est incorporé en grande partie aux protéines par le canal du pool des substances à un atome de carbone, mais le chlorure de méthyle peut également se fixer aux protéines par alkylation directe. Toutefois, si le composé se comporte comme un agent alkylant, c’est en très faible proportion. Chez les mammifères, il est métabolisé par conjugaison avec le glutathion et, dans une moindre mesure, par oxydation au niveau du cytochrome P-450. La conjugaison avec le glutathion conduit à la formation de méthanethiol et les deux voies métaboliques ont pour point d’aboutissement le formaldéhyde et le formiate. Les métabolites du chlorure de méthyle sont excrétés dans les urines ainsi que dans l’air expiré, qui contient également une certaine proportion du composé initial.

Chez l’Homme, on observe d’importantes différences individuelles concernant l’absorption et la métabolisation du chlorure de méthyle. Ces différences sont dues à la présence ou à l’absence d’une enzyme, la glutathion-transférase T1 (GSTT1), qui présente un polymorphisme génétique. On distingue en effet 3 phénotypes chez l’Homme qui correspondent à une conjugaison forte, faible ou nulle. De toute manière, comme on voit pas très bien si le risque le plus élevé correspond à une conjugaison forte ou à une conjugaison nulle, il faut considérer que tous les phénotypes ont la même sensibilité au chlorure de méthyle.

La toxicité aiguë du chlorure de méthyle après inhalation semble assez faible pour le rat et la souris, puisque la CL50 est supérieure à 4128 mg/m3 (2000 ppm). On n’a pas trouvé dans la littérature de données qui indiquent que le composé soit irritant ou sensibilisant.

Il semble qu’après inhalation, les principaux organes cibles du chlorure de méthyle soient le système nerveux (avec des troubles fonctionnels ainsi qu’une dégénérescence du cervelet chez le rat et la souris) ainsi que les testicules, l’épididyme et le rein chez le rat ou encore le rein et le foie chez la souris.

Lors d’une étude de 2 ans au cours de laquelle on a fait inhaler du chlorure de méthyle à des souris, on a constaté qu’à la concentration de 103 mg/m3 (50 ppm), les nerfs rachidiens lombaires présentaient un gonfle ment de l’axone et des signes de dégénérescence par comparaison aux animaux témoins, sans qu’on puisse toutefois mettre en évidence une relation dose-réponse. Au terme de cette étude, on a observé chez les souris des deux sexes une dégénérescence du cervelet et chez les mâles, des adénocarcinomes rénaux à la concentration de 2064 mg/m3 (1000 ppm). Ces effets n’ont pas été observés chez le rat à cette concentration.

Le chlorure de méthyle se révèle nettement génotoxique in vitro, tant vis-à-vis des bactéries que des cellules mammaliennes. Même si les effets observés lors d’un test de létalité dominante étaient très vraisemblable ment plutôt cytotoxiques que génotoxiques, on pourrait considérer le chlorure de méthyle comme très faiblement mutagène in vivo, eu égard à certains signes témoignant de la présence de pontages ADN–protéines aux doses élevées.

Chez des rats soumis à une concentration de 980 mg/m3 (475 ppm), on a observé la présence de lésions testiculaires et notamment de granulomes épididymaires, puis une réduction de la vitalité des spermatozoïdes qui a conduit à une baisse de la fertilité aboutissant à une stérilité totale à plus forte dose.

Le chlorure de méthyle a produit des anomalies cardiaques chez des foetus de souris dont la mère avait été exposée à une concentration de 1032 mg/m3 (500 ppm) pendant la gestation.

Après inhalation accidentelle de chlorure de méthyle, les effets sont nets chez l’Homme, notamment au niveau du système nerveux central. Chez des volon taires brièvement exposés à du chlorure de méthyle, on n’a pas constaté d’effets sensibles qui puissent être attribués à ce composé. On n’a pas suffisamment de données épidémiologiques pour évaluer le risque de cancer chez l’Homme par suite d’une exposition au chlorure de méthyle.

En conclusion, on peut dire que chez l’Homme, le point d’aboutissement de l’action toxique du chlorure de méthyle est vraisemblablement le système nerveux central. On a pu en tirer des valeurs-guides de 0,018 mg/m3 (0,009 ppm) pour une exposition indirecte dans l’environnement et de 1,0 mg/m3 (0,5 ppm) pour une exposition sur le lieu de travail. Bien que les lésions nerveuses aient été constatées chez le rat à des doses plus faibles que celles qui provoquaient la stérilité des mâles (980 mg/m3, soit 475 ppm) ou des lésions rénales chez des souris mâles (à la concentration de 2064 mg/m3, soit 1000 ppm), c’est ces très graves effets qui sont à prendre en compte pour toute caractérisation qualitative du risque que comporte une exposition au chlorure de méthyle.

On n’a guère trouvé de données sur la toxicité à court terme du chlorure de méthyle pour les organismes aquatiques ou terrestres. Dans le cas de la toxicité à long terme, c’est même une absence totale. Les données existantes indiquent que le composé n’a qu’une faible toxicité aiguë pour les organismes aquatiques. Chez les poissons, la plus faible valeur de la CL50qui ait été obtenue est de 270 mg/litre. Comme les dosages donnent pour les eaux de surface des concentrations de chlorure de méthyle généralement inférieures de plusieurs ordre de grandeur à celles qui produisent effectivement des effets, il est vraisemblable que ce composé ne présente pas de risque important d’intoxication aiguë pour les organismes aquatiques. On ne possède que des données très limitées concernant les effets du chlorure de méthyle sur les organismes terrestres.

RESUMEN DE ORIENTACIÓN

La evaluación de los aspectos relativos a la salud humana de este CICAD sobre el cloruro de metilo se basó fundamentalmente en un estudio preparado por el Grupo de Expertos Nórdicos en colaboración con el Comité de Expertos Neerlandeses en Normas del Trabajo (Lundberg, 1992). Se realizó una búsqueda en las bases de datos pertinentes para el período de 1992- 1999 con objeto de obtener datos adicionales. Se utilizaron como fuentes principales para los aspectos ambientales y ecotoxicológicos del cloruro de metilo BUA (1986), ATSDR (1990), WMO/OMM (1994) y HSDB (1996). La publicación ATSDR (1990) se actualizó en 1998; cuando esta versión actualizada proporcionaba información, se ha tenido en cuenta. Se obtuvieron datos adicionales sobre cuestiones ambien tales en bases de datos pertinentes para el período 1989- 1997. La información relativa al carácter y a la disponi bilidad de los documentos originales se presenta en el apéndice 1. La información sobre el examen colegiado de este CICAD figura en el apéndice 2. Este CICAD se aprobó como evaluación internacional en una reunión de la Junta de Evaluación Final, celebrada en Estocolmo (Suecia) del 25 al 28 de mayo de 1999. La lista de participantes en esta reunión figura en el apéndice 3. La Ficha internacional de seguridad química (ICSC 0419) para el cloruro de metilo, preparada por el Programa Internacional de Seguridad de las Sustancias Químicas (IPCS, 1999), se reproduce en el presente documento.

El cloruro de metilo (CAS Nş 74-87-3) se libera fundamentalmente en el aire durante su producción y uso y por la incineración de residuos municipales e industriales. Sin embargo, las fuentes naturales, en particular los océanos y la combustión de biomasa, predominan claramente sobre las fuentes antropogénicas. La emisión mundial total de cloruro de metilo de todas las fuentes se estima en alrededor de 5 × 106 toneladas al año. Se ha calculado que la contribución de las fuentes naturales es muy superior al 90%, y tal vez hasta del 99%, de la emisión total. El cloruro de metilo está presente en la troposfera en una concentración aproximada de 1,2 µg/m3 (0,6 ppmm).

El principal medio de absorción del cloruro de metilo en la troposfera es la reacción química con los radicales hidroxilo y su vida en la atmósfera se estima que es de uno a tres años. Cierta cantidad de cloruro de metilo llega a la estratosfera; allí, la fotodisociación genera radicales de cloro, que contribuyen a la destrucción de la capa de ozono. Las estimaciones de la cantidad de cloruro de metilo que llega a la estratosfera y destruye pues la capa de ozono varían ampliamente. A partir de las cifras presentadas por la Organización Meteorológica Mundial (OMM), se estima que el cloruro de metilo contribuye con alrededor del 15% al equivalente total de cloro estratosférico efectivo. Se ha determinado que el cloruro de metilo tiene un potencial de destrucción del ozono estratosférico de 0,02 en relación con el compuesto de referencia, el CFC-11, cuyo potencial es igual a 1. No parece que el cloruro de metilo contribuya de manera significativa al calentami ento mundial o a la contaminación fotoquímica del aire.

El mecanismo predominante de eliminación del cloruro de metilo en el agua y el suelo es la volatiliza ción. La hidrólisis lenta y posiblemente la degradación biótica pueden contribuir a su desaparición en las capas más profundas del suelo y en el agua freática. Sin embar go, hay poca información sobre su biodegradación.

La ruta de exposición más importante del ser humano al cloruro de metilo son las vías respiratorias. En las personas, así como en los animales de experi mentación, el cloruro de metilo se absorbe con rapidez a través de los pulmones después de la inhalación. Tras la exposición a cloruro de metilo marcado con 14C, se detecta radiactividad en todo el organismo. Aunque una gran parte de la sustancia marcada se incorpora a las proteínas a través del "pool" de moléculas de un átomo de carbono, el cloruro de metilo también se puede unir a proteínas por alquilación directa. Sin embargo, si bien es un agente alquilante, lo es en un grado muy pequeño. El cloruro de metilo se metaboliza en los mamíferos por conjugación con el glutatión, y en menor proporción mediante oxidación por el citocromo P-450; la vía del glutatión produce metanotiol y ambas vías dan lugar a formaldehído y formato. Los metabolitos del cloruro de metilo se eliminan con la orina y por exhalación. También se exhala cloruro de metilo sin metabolizar.

En las personas, hay grandes diferencias individuales en la absorción y el metabolismo del cloruro de metilo. Estas diferencias dependen de la presencia o ausencia de la enzima glutatión transferasa T1 (GSTT1), que presenta polimorfismo genético. Se distinguen en el ser humano tres fenotipos, correspondi entes a una conjugación alta, baja o nula de la GSTT1. Sin embargo, como no está claro si el mayor riesgo corresponde a una conjugación alta o a una conjugación nula, hay que considerar que todos los fenotipos son sensibles a la exposición al cloruro de metilo.

La toxicidad aguda por inhalación de cloruro de metilo en ratas y ratones parece ser bastante baja, con un valor de la CL50 superior a 4128 mg/m3 (2000 ppm). No se han encontrado en la bibliografía datos indicativos de que el compuesto sea irritante o sensibilizante.

Los principales órganos destinatarios tras una exposición breve por inhalación al cloruro de metilo parecen ser el sistema nervioso, con trastornos funcionales y degeneración cerebelar tanto en las ratas como en los ratones, así como los testículos, el epidídimo y los riñones en las ratas y los riñones y el hígado en los ratones.

En un estudio por inhalación de dos años con ratones se observaron inflamación y degeneración axonal de los nervios de la médula espinal lumbar con 103 mg/m3 (50 ppm) en los animales expuestos en comparación con los testigos, pero sin una relación dosis-respuesta aparente. Al final del estudio se detectaron degeneración cerebelar en los ratones de ambos sexos y adenocarcinomas renales en los ratones machos con 2064 mg/m3 (1000 ppm). Estos efectos no se observaron en las ratas con 2064 mg/m3 (1000 ppm).

El cloruro de metilo es claramente genotóxico en sistemas in vitro, tanto en bacterias como en células de mamífero. Aunque los efectos positivos observados en una prueba de dominancia letal fueron con toda probabilidad citotóxicos más que genotóxicos, el cloruro de metilo podría considerarse un mutágeno muy débil in vivo sobre la base de algunos signos de entrecruzamiento proteína-ADN a dosis más altas.

Las lesiones testiculares y los granulomas epididi males seguidos de una disminución de la calidad del esperma dieron lugar a una reducción de la fecundidad en las ratas con 980 mg/m3 (475 ppm) y a su pérdida completa a dosis superiores.

El cloruro de metilo indujo afecciones cardíacas en fetos de ratones cuyas madres estuvieron expuestas a 1032 mg/m3 (500 ppm) durante el período de gestación.

Se pueden observar claramente efectos en las personas, especialmente en el sistema nervioso central, tras la exposición accidental por inhalación. En la exposición breve de voluntarios al cloruro de metilo no se observaron efectos significativos que pudieran atribuirse a ella. Hay pocos datos epidemiológicos que permitan evaluar el riesgo de aparición de cáncer para las personas como resultado de la exposición al cloruro de metilo.

En conclusión, el efecto final crítico para la toxicidad por inhalación de cloruro de metilo en las personas parece ser la neurotoxicidad. Se obtuvieron valores guía de 0,018 mg/m3 (0,009 ppm) para la exposición indirecta a través del medio ambiente y de 1,0 mg/m3 (0,5 ppm) para el entorno de trabajo. Aunque las lesiones nerviosas se observaron a niveles de exposición inferiores a los que provocaron la pérdida de fecundidad en las ratas (980 mg/m3 [475 ppm]) y los tumores renales en los ratones macho (2064 mg/m3 [1000 ppm]), en la caracterización del riesgo cualitativo del cloruro de metilo se debe hacer hincapié también en esos efectos muy graves.

Se encontraron pocos datos sobre la toxicidad a corto plazo del cloruro de metilo para los organismos acuáticos o terrestres. No se obtuvo ningún dato sobre la toxicidad a largo plazo. Los datos disponibles indican que el cloruro de metilo tiene una toxicidad aguda baja para los organismos acuáticos. El valor más bajo de la CL50 para los peces es de 270 mg/litro. Teniendo en cuenta que las concentraciones de cloruro de metilo que se han determinado en las aguas superficiales son generalmente varios órdenes de magnitud inferiores a las que se ha demostrado que son causantes de esos efectos, es probable que el cloruro de metilo represente un riesgo bajo de efectos agudos para los organismos acuáticos. Solamente se dispone de datos muy limitados sobre los efectos del cloruro de metilo en los organismos terrestres.

FOOTNOTES

1 International Programme on Chemical Safety (1994) Assessing human health risks of chemicals: derivation of guidance values for health-based exposure limits. Geneva, World Health Organization (Environmental Health Criteria 170).

2 In the stratosphere, each bromine atom is assumed to be 40 times more damaging to ozone than each chlorine atom (WMO, 1994).

3 The term "equivalent effective stratospheric chlorine" includes both stratospheric chlorine and bromine and also considers the dissociation rate of each compound involved in ozone depletion (e.g., chlorofluorocarbons). In the strato sphere, each bromine atom is assumed to be 40 times more damaging to ozone than each chlorine atom.



    See Also:
       Toxicological Abbreviations
       Methyl chloride (ICSC)
       Methyl chloride (WHO Food Additives Series 14)
       Methyl chloride (PIM 339)
       Methyl Chloride (IARC Summary & Evaluation, Volume 71, 1999)