Budget also expressed concern about EPA's analysis of the cost of mitigation. In the agency's FY 1994 appropriation bill, Congress ordered EPA to delay publishing a rule for radon in drinking water.

The 1996 amendments to the Safe Drinking Water Act required EPA to contract with the National Academy of Sciences (NAS) to conduct a risk assessment of radon in drinking water and an assessment of the health-risk reduction benefits associated with various measures to reduce radon concentrations in indoor air. EPA is also required to publish an analysis of the health-risk reduction and the costs associated with compliance with any specific MCL before issuing a proposed regulation. The law also directed EPA to promulgate an alternative maximum contaminant level (AMCL) if the proposed MCL is less than the concentration of radon in water ''necessary to reduce the contribution of radon in indoor air from drinking water to a concentration that is equivalent to the national average concentration of radon in outdoor air.'' Under the law, states may develop a multimedia mitigation program which if approved by EPA would allow utilities whose water has radon concentrations higher than the MCL, but lower than the AMCL, to comply with the AMCL. The multimedia programs to mitigate radon in indoor air may include "public education; testing; training; technical assistance; remediation grants, loan or incentive programs; or other regulatory or nonregulatory measures." If a state does not have an EPA-approved multimedia mitigation program, a public water supply in that state may submit such a program to EPA directly. Public water supplies exceeding the AMCL and choosing to institute a multimedia mitigation program to achieve equivalent health risk reductions must, at a minimum, treat their water to reduce radon in water concentrations to less than or equal to the AMCL. The present report was written to address the issues just discussed.

Critical Issues

It has been difficult to set a standard for radon, as opposed to other radionuclides in drinking water, because of the absence of authoritative dosimetric information for radon dissolved in water. Furthermore, radon presents a unique regulatory problem in that its efficient transfer from water into indoor air produces a risk from the inhalation of its decay products. Thus, it is regulated as a radionuclide in water, but a major portion of the associated risk occurs because of its contribution to the airborne radon concentration.

Because of the relatively small volume of water used in homes, the large volume of air into which the radon is emitted, and the exchange of indoor air with the ambient atmosphere, radon in water typically adds only a small increment to the indoor air concentration. Specifically, radon at a given concentration in water adds only about 1/10,000 as much to the air concentration; that is, typical use of water containing radon at 10,000 Bq m-3 will on average increase the air radon concentration by only 1 Bq m-3. There is always radon in indoor air

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