Air Toxic Concentrations: Response

more detailed explanation of the basis for MCIA's position. Woodruff et al. (1) incorrectly suggest that industrial air emissions of methyl chloride present a significant health risk. On the basis of 1990 data for the Toxics Release Inventory (TRI) and the Cumulative Exposure Project (CEP), Woofruff et al. purport to identify listed hazardous air pollutants (HAPs) that are present in the environment above levels of concern based on cancer and noncancer effects. The authors further state that methyl chloride is one of eight pollutants identified as having "mod-eled concentrations exceeding the benchmark concentrations for cancer in 100% of the census tracts" (1). These statements are inaccurate for the following reasons. First, methyl chloride air emissions and resulting concentrations should not be compared to a cancer health benchmark because available data are not sufficient to condude that methyl chloride poses a human cancer hazard. Methyl chloride has been classified by the U.S. Environmental Protection Agency (U.S. EPA) as only a Group C possible human carcinogen (2); this is based on no human data and insufficient animal data. Further, the International Agency for Research on Cancer (IARC) (3) found that the evidence of carcinogenicity of methyl chloride to humans and to animals is inadequate; therefore , IARC classifies methyl chloride in Group 3 (not classifiable). A U.S. EPA Scientific Peer Review Panel (4), convened for a rulemaking proceeding under 1 12(g) of the Clean Air Act, agreed that compounds classified as Group C (possible) carcinogens should not be grouped with "known" and "probable" human carcinogens. The available data simply are not sufficient to justify evaluating or classifying methyl chloride based on a perceived cancer hazard. Second, when background concentrations from natural sources are removed from the analysis, methyl chloride emissions do not exceed benchmark levels in 100% of the census tracts. Perhaps up to 99% of ambient air concentrations of methyl chloride are due to releases from natural sources, rather than releases from manufacturing and use (5). Although in their Table 2 Woodruff et al. (1) acknowledge that the alleged exceedances for methyl chloride are due almost entirely to background concentrations , rather than man-made sources, they nevertheless purport to identify "HAPs representing the highest potential health risks" with the idea that Future regulatory and scientific activities can begin to focus on these pollutants to address and further evaluate their public health significance. Given the almost insignificant amount of methyl chloride emissions …