the risk of radon in the drinking water. This concept will be illustrated in the following scenarios.

Scenario 1: High Radon Concentrations in Water

In scenario 1, the radon concentration in drinking water exceeds the AMCL. The water utility will be required to install treatment equipment to reduce the concentration of radon in the water to at least the AMCL. The incremental cost of further treating the water so that it achieves the MCL will generally be sufficiently small that the multimedia-mitigation approach would probably not be considered. Some additional considerations arise from the increased quantity of radon being removed from the water, such as increased gamma-ray exposure to the water-treatment workers from a GAC bed or the airborne radon released to the atmosphere by an aeration system at the water-treatment plant. In general these factors would not produce sufficient cost differences between meeting the AMCL and meeting the MCL to constitute an incentive to consider a multimedia mitigation program. Thus, the only cases where it is of practical interest to consider implementation of a multimedia program is for water systems in which the radon concentration in the water is between the MCL and the AMCL.

To provide a perspective on how the risk reductions could be compared, we provide an illustrative calculation. Suppose that a water supply contains radon at 125,000 Bq m-3. If the water is treated to reach the assumed MCL, it would provide an average reduction of 125,000–25,000 = 100,000 Bq m-3. Multiplying this value by the transfer coefficient of 10-4 yields a decrement of 10 Bq m-3 in radon concentration in the air in each dwelling. For a water supply that provides water to 1,000 homes each with the same average number of occupants, the committee assumed that there were to be three persons per home. The total reduction in radon resulting from the mitigation of the water to reach the MCL would be 10 Bq m-3 per dwelling × 1,000 dwellings, or a cumulative reduction within the community of 10,000 Bq m-3. Taking into account the additional ingestion risk, it would require a reduction of 11,300 Bq m-3 in indoor airborne radon to provide the equivalent health-risk reduction.

This risk-reduction analysis could be based on the actual number of occupants in the homes so that the health-risk reductions would be applied to defined populations. However, enumeration of the people in each home presents a potential problem in that the number of individuals in a given dwelling can vary. Homes are sold to new families. Children grow up and move away, and there is the question of the presence or absence of smokers in a home. Because the lung cancer risk posed by radon is significantly higher for smokers than for nonsmokers, a greater health-risk reduction would be obtained by preferentially mitigating the homes of smokers relative to the homes of nonsmokers. Thus, a potential difficulty in demonstrating the continuing benefits of mitigation of homes for health risk reduction is the variability in the number and nature of occupants of



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