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extrapolation. The report also contains the subcommittee's review and critique of epidemiological studies on skin and internal cancers and other health effects in Taiwanese populations and in other studies in which exposure to arsenic in drinking water has been documented in other parts of the world. Although EPA's Office of Water considers inorganic arsenic the form of concern in drinking water, the subcommittee decided to evaluate whether organic arsenic compounds in drinking water also contribute substantially to health risks. The report contains the subcommittee's review of data on the metabolism, toxicokinetics, and mechanism of action of arsenic in humans and animals and evaluation of the implications of those data for estimating exposure-response relationships below the range of detectable responses in human epidemiological studies. The subcommittee also evaluates the implications of its findings for risks that might be associated with exposure to varying concentrations of arsenic in drinking water and states its assumptions about arsenic intake from food.

The remainder of this chapter is divided into three sections. The first provides a brief background of the current MCL for arsenic, the second outlines the scientific controversies associated with the arsenic MCL, and the third describes the organization of this report.

Background

The current MCL of 50 µg/L has been the standard for arsenic in drinking water in the United States since 1942. In response to the 1974 SDWA, EPA adopted 50 µg/L as the interim standard for total arsenic in drinking water in 1975. Although the SDWA Amendments of 1986 required EPA to finalize its maximum contaminant level goal (MCLG) and enforceable MCL for arsenic by 1989, EPA has not yet finalized the MCLG or MCL for arsenic in part because of the scientific uncertainties and controversies associated with the chronic toxicity of arsenic.

The SDWA Amendments of 1996 required EPA to develop an arsenic research strategy within 180 days of enactment of the amendments. The EPA (1996a,b) strategy was reviewed by its Board of Scientific Counselors (EPA 1997), which encouraged EPA to propose an MCL by the year 2000 that ''balances current scientific information on health risks with costs and other risk management factors," and then "establish a more definitive MCL by the year 2010, or earlier if feasible, based on results available from long-term studies."



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