e 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 data collected during the National Occupational Hazard Survey (NOHS) conducted by NIOSH during 1972-1974.15
In the United States, methyl chloride is produced primarily by the hydrochlorination of methanol.2,4,16 Although the primary use of methyl chloride is in the manufacture of silicones, this process is a closed system operation with minimal worker exposure. The manufacture of tetramethyl lead and triptane (2,2,3-trimethyl butane), both antiknock fuel additives,2 is the next largest use of methyl chloride. Other uses of methyl chloride include the production of butyl rubber; di-, tri-, and tetra-halogenated methanes; methyl cellulose; quaternary ammonium compounds; methyl mercaptan; methionine; and fungicides and pesticides (primarily methyl arsenate herbicides).16 A former high volume use of methyl chloride as a refrigerant and propellant has declined significantly in recent years with the substitution of chlorofluorinated alkane derivatives (chlorinated fluorocarbons). In 1981, approximately 362 million pounds of methyl chloride were produced in the United States, and domestic consumption is projected to expand by approximately 6.5% per year through the mid-1980's.16 Approximately 41,000 U.S. workers (Table 2) are potentially exposed to methyl chloride.15
Methyl bromide is produced by direct bromination of methane and by the hydrobromination of methanol.2 In the United States, methyl bromide is used primarily as a soil and spore fumigant.17 Methyl bromide is also used as a disinfectant, rodenticide, methylating agent, and wool degreaser and in ionization chambers.2,18,19
Table 2 Number of Workers Potentially Exposed to the Monohalomethanes by Industry15
|Sic* Code||Description||Number of Workers Potentially Exposed|
|Methyl Chloride||Methyl Bromide||Methyl Iodide|
|07||Agriculture Services and Hunting||647||5,922||-|
|13||Oil and Gas Extraction||24||129||-|
|15||General Building Contractors||1,301||934||-|
|16||Heavy Construction Contractors||405||-||-|
|17||Special Trade Contractors||1,143||1,936||-|
|20||Fo