tration water, it will generally be most cost-effective to mitigate radon in water to the MCL.

For water supplies with radon concentrations between the MCL and the AMCL, the feasibility of implementing a multimedia mitigation program depends on the availability of homes in which the airborne radon concentration is high (greater than 150 Bq m-3). EPA has divided the country into three regions of different potentials for elevated indoor radon concentration. For water supplies in areas of low indoor air radon potential, it will be difficult to identify and mitigate enough homes to achieve an equivalent or better health-risk reduction by treating the air. For such water supplies, it is unlikely that a public water system's multimedia mitigation program will be practical unless the water concentration of radon is only slightly above the MCL.

In areas of medium and high indoor air radon potential, it is more feasible to mitigate a small number of high-indoor-concentration homes to provide an equivalent health-risk reduction at a cost less than the cost of mitigating the water. In this scenario, the public water supply would have to actively recruit high-indoor-air radon concentration homes and mitigate them. Incentives could perhaps be used to get participation of homeowners in these multimedia programs. In addition, the utilities would have to monitor and maintain the air mitigation systems routinely. This scenario would require water utilities to become involved in air mitigation in individual homes, something with which they are likely to have little experience.

Reduction of radon in indoor air can be an alternative means of reducing overall risks associated with radon. One way to achieve this is to install active (mechanical) systems to reduce radon entry into existing or new houses. Adequate testing (long-term measurements in the living space to reflect actual exposures) will be necessary to determine which existing houses should be mitigated. Routine follow-up measurements will be needed, both to determine the risk reduction achieved by the mitigation and to ensure continued successful operation of the mitigation systems. To ensure that health-risk reductions are at least as great as the reductions that would result from reducing the water radon concentration to the MCL, the number of homes with air mitigation systems should be 10–20% greater than the calculated minimum number of homes. Radon-resistant new construction methods could also be used although the technical and practical bases of their implementation are still poorly developed. Evaluation of the baseline radon exposure would require use of radon-monitoring data from existing houses in the community of interest or estimates of average indoor concentrations based on calculated radon potentials for the region. Careful attention to the follow-up monitoring results would be important, both for determining how much radon reduction has resulted (on the basis of aggregate comparisons) and for determining whether radon persists at unacceptable concentrations.

Various educational and outreach programs reviewed by this committee indicate that, in general, public apathy about the potential risks of exposure to



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