Halogenated One-Carbon Compounds

Chlorinated methanes (also ethanes) are among the most widely used and useful chemical compounds. There are potentially 26 compounds with other multiple halogen substitutions. Eleven important or representative compounds were selected to discuss in this chapter. The physical states vary from colorless gases: methyl chloride and methyl bromide; colorless liquids: methyl iodide, methylene chloride, methylene bromide, chloroform, bromoform (heavy), and carbon tetrachloride; and a yellow solid, iodoform, and colorless solid; carbon tetrabromide. The main use of methyl chloride is in the manufacture of silicone while methylene chloride, chloroform, and carbon tetrachloride have been used as solvents, paint removers, degreasers, cleaning compounds, and chemical intermediates. The following table provides toxicity information (reference doses, RfD; reference concentrations, RfC; oral slope factors (carcinogen), OSF; and inhalation unit risks (carcinogen), IUR) from Integrated Risk Information Service (IRIS). It can be seen that the RfDs (oral exposure for lifetime) for the chemicals that have been evaluated are in the order of 10−2 to 10−3 mg/kg/day. The inhalation RfCs (inhalation exposure) for chemicals that have been evaluated are 10−2 to 10−3 mg/m3. For the chemicals indicated as either probable human carcinogens or methylene chloride, chloroform, bromoform, and likely to be a human carcinogen, carbon tetrachloride, the OSF (per mg/kg/day) are in the order of 10−2 to 10−3. The IURs for the chemicals evaluated are of the order of 10−5 to 10−6 per μg/m3. Fluorene compounds are not included in this chapter as they represent a very special case of halogenated compounds. Chemical RfD (mg/kg/day RfC (mg/m3) Cancer Description OSF (Per mg/kg/day) IUR (Per μg/m3) References Methyl chloride None 9E−2 Brain Not classified None None Toxicological Review (2001) Methyl bromide 1.4E−3 5E−3 Nasal Not classified None None No Toxicological Review Methyl iodide None None None None None No Toxicological Review Methylene chloride 6E−2 Liver None Probable human 7.5E−3 4.7E−7 No Toxicological Review Methylene bromide None None None None None None Chloroform 1E−2 Liver None Probable human RfD 1E−2 2.3E−5 Toxicological Review (2001) Bromoform 2E−2 Liver None Probable human 7.9E−3 1.1E−6 No Toxicological Review Iodoform None None None None None No Toxicological Review Carbon tetrachloride 4E−3 Serum SDH 1E−1 Likely human 7E−2 6E−6 Toxicological Review (2010) Carbon tetrabromide None None None None None No Toxicological Review Methylene chlorobromide None None None None None No Toxicological Review Several of the compounds listed in the above table are receiving attention because of their presence as disinfection by-products (DBPs): products formed in reaction with chlorine, ozone, chlorine dioxide, or chloramines with naturally occurring organic matter in drinking water. A comprehensive review was provided in Mutation Research in conjunction with the IARC review. The trihalomethanes (THMs)—chloroform, bromoform, bromodichloromethane, and chlorodibromomethane—are regulated by the U.S. EPA at a level of 80 μg/L (for total halomethanes) and are included in 600 drinking water DBPs. Together, the THMs and haloacetic acids (HHAs) are the two most prevalent classes of DBPs formed in chlorinated drinking water, accounting for approximately 25% of the halogenated DBPs. Of concern is the demonstrated human carcinogenicity of many of the DBPs. The WHO in Trihalomethanes in Drinking-Water (background document for development of WHO Guidelines for Drinking-Water Quality) provides a thorough discussion of four compounds considered the most common forms of halomethanes present in drinking water. These are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. These compounds are liquids at room temperature, relatively to extremely volatile, and only slightly soluble in water. As in the previous editions, this chapter includes relevant information provided in earlier editions along with updated information from several sources including National Toxicology Program (NTP), IRIS, and the Agency for Toxic Substances and Disease Registry (ATSDR) websites. Keywords: Bromoform; carbon tetrachloride; chloroform; cyanosis; fire extinguishing agents; disinfection by-products; iodoform; lacrimator; methyl bromide; methyl chloride; methyl iodide; methylene chloride; refrigerant; solvents; tetrabromomethane