ation, it is recommended that producers and users of methyl chloride, methyl bromide, and methyl iodide disseminate this information to their workers and customers and that professional and trade associations and unions inform their members of otential hazards of working with these compounds.
J. Donald Millar, M.D., D.T.P.H. (Lond.)
Assistant Surgeon General
Director, National Institutue for
Occupational Safety and Health
Centers for Disease Control
The monohalomethanes (methyl chloride, methyl bromide, and methyl iodide) are alkylating agents and thus have generated concern as to their potential for inducing mutations and cancer. All three compounds were found to be direct-acting mutagens in the Ames assay. In experimental studies in either rats or mice using various routes of administration, these three compounds have also demonstrated the ability to produce cancer. Methyl chloride produced a teratogenic effect (heart defects) in the offspring of pregnant mice exposed by inhalation at 500 and 750 ppm.
Based on these data, the National Institute for Occupational Safety and Health (NIOSH) recommends that methyl chloride, methyl bromide, and methyl iodide be considered as potential occupational carcinogens and that methyl chloride be considered a potential occupational teratogen.
Physical and Chemical Properties
The three chemicals addressed in this document are commonly called the monohalogenated derivatives of methane (monohalomethanes): chloromethane, bromomethane, and iodomethane. Fluoromethane is not included since there are no data on the extent of exposure in the workplace and little to no information on toxic effects in animals or humans. These derivatives are represented by the general formula CH3X, where X represents chlorine, bromine, or iodine. Some of the physical and chemical properties of these compounds are summarized in Table 1. In this document, these compounds are referred to by their common names: methyl chloride, methyl bromide, and methyl iodide.
Table 1 Chemical and Physical Properties of the Monohalomethanes1-4
|Chemical Identity||Methyl Chloride||Methyl Bromide||Methyl Iodide|
|CASa Registry No.||74-87-3||74-83-9||74-88-4|
|RTECSb Accession No.||PA6300000||PA4900000||PA9450000|
|Boiling Point, °C||-24.2||3.5||42.5|
|Freezing Point, °C||-97.7||-93.7||-66.1|
|Vapor Pressure||5 atm at 22.0°C||2 atm at 23.3°C||0.5 atm at 25.3.°C|
|Color||Colorless||Colorless||Colorless, turns brown when exposed to light|
|Odor||Faint, sweet odor which is not noticeable at dangerous concentrations||Chloroformlike odor at high concentrations||Pungent|
|Specific Gravity||0.973 (-10°C)||1.736 (-10°C)||2.279 (20°C)|
|Flammability||Flammable, forms explosive mixture with air at 8-17%||Nonflammable in air, burns in oxygen||Nonflammable|
Production, Use, and Potential for Occupational Exposure
Commercially, methyl chloride, methyl bromide, and methyl iodide have been used as methylating agents, laboratory reagents, refrigerants, aerosol propellants, pesticides, fumigants, fire-extinguishing agents, anesthetics, degreasers, blowing agents for plastic foams, and chemical intermediates.
Relatively little data concerning environmental concentrations of these monohalomethanes in the workplace have been reported. Because of their high volatility, they are frequently contained in closed systems. Health Hazard Evaluation surveys and other field studies conducted by the National Institute for Occupational Safety and Health (NIOSH) have found that workplace environmental concentrations of these monohalomethanes were generally quite low. Methyl chloride concentrations ranged from not detectable to 300 parts per million (ppm)5-8; methyl bromide from not detectable to 30 ppm:9-12 and methyl iodide from not detectable to 6 ppm.12,13 Possible exposures during the production of these monohalomethanes may develop from leaks in connecting or flexible joints, pump seals, sight glasses, and quality control sampling sites.14
Approximately 146,000 U.S. workers are potentially exposed to these monohalomethanes (Table 2). This estimate is based on d