radon in water from public water supplies indicate that elevated concentrations of radon in water occur primarily in the New England states, the Appalachian states, the Rocky Mountain states, and small areas of the Southwest and the Great Plains.

Because radon is easily released by agitation in water, many uses of water release radon into the indoor air, which contributes to the total indoor airborne radon concentration. Ingestion of radon in water is also thought to pose a direct health risk through irradiation of sensitive cells in the gastrointestinal tract and in other organs once it is absorbed into the bloodstream. Thus, radon in drinking water could potentially produce adverse health effects in addition to lung cancer.

Drinking-water quality in the United States is regulated by the Environmental Protection Agency (EPA) under the Safe Drinking Water Act originally passed in 1974. In the 1986 amendments to the act, EPA was specifically directed to promulgate a standard for radon as one of several radionuclides to be regulated in drinking water. Because of delays in implementing the regulation of radionuclides in drinking water, EPA was sued. In a consent decree, EPA agreed to publish final rules for radionuclides in drinking water, including radon, by April 1993.

EPA proposed national primary drinking water regulations for radionuclides in 1991. Because radon is a known carcinogen, its maximum contaminant level goal (MCLG) was automatically set at zero. A maximum contaminant level (MCL) of 11,000 Bq m-3 was subsequently proposed as the level protective of public health and feasible to implement taking costs into account. Public comments on the proposed regulations suggested that the MCL for radon be set somewhere from less than 1,000 Bq m -3 to 740,000 Bq m-3; a large majority favored setting the MCL at value higher than 11,000 Bq m-3.

In 1992, Congress directed the Office of Technology Assessment to analyze the EPA health risk assessment and outline actions that could address regulation of radon, considering both air and water. Also in 1992, the Chaffee-Lautenberg amendment to the EPA appropriation bill for FY 1993 directed the agency to seek an extension of the deadline for publishing a final rule until October 1993 and to submit a report, reviewed by EPA's Science Advisory Board (SAB), to Congress by July 1993. That report was to address the risks posed by human exposure to radon and consider both air and water sources, the costs of controlling or mitigating exposure to waterborne radon, and the risks posed by treating water to remove radon. The SAB review of the report questioned EPA's estimates of the number of community water supplies affected, the extrapolation of the risk of lung cancer associated with the high radon exposures of uranium miners to the low levels of exposure experienced in domestic environments and the magnitude of risk associated with ingestion. The SAB report also emphasized that the risk of cancer from radon in domestic settings was a multimedia issue and that the risk for radon in water must be considered within the context of the total risk from radon, which is dominated by radon in indoor air. The Office of Management and

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