the dwelling. The committee has examined health-risk reduction only for an average dwelling within the community.

It is necessary to consider that each home contains the average number of individuals and the same fraction of smokers. If 113 homes were found with high concentrations of airborne radon (concentrations in excess of 150 Bq m-3, the EPA guidance concentration) and these were mitigated so that the average long-term 222Rn reduction in each home were 100 Bq m-3, then the mitigation of these dwellings would provide the same level of risk reduction as reducing the radon in the drinking water to the MCL (100 Bq m-3 per dwelling × 113 dwellings = 11,300 Bq m-3). If a typical home mitigation costs $1,500, the mitigation of the 113 homes costs about $170,000 plus the estimated cost of home testing (1,000 homes in the community × $75 per home = $75,000), for a total of at least $245,000, which includes the cost of distributing detectors, collecting them, and analyzing the resulting data.

Because of the nature of water-quality regulations, there would be a requirement for continued monitoring to ensure continuing compliance with the equivalent health-risk reduction. Thus, there are O&M costs which would involve annual measurements in the mitigated homes and replacement of fans that fail. The typical mean time to failure for the fans is estimated to be about 10 years. Thus, some fans would probably have to be replaced each year. In this scenario, all the homes with concentrations above 150 Bq m-3 are to be found and mitigated. To obtain a high level of participation, it would be necessary to attempt to measure the activity concentration in each home with a long-term detector that leads to the high estimated costs to perform the radon survey. The cost to mitigate the water in a community of about 3,000 individuals is estimated by EPA (1991b) to be $78,000 plus annual O&M costs of $3,000. Mitigation of radon in indoor air in the 113 homes is substantially more than the cost of buying and operating the system to aerate the water to remove the radon. Thus, the water-supply utility would not choose to adopt the multimedia approach to risk reduction rather than fully mitigate the water to the MCL. However, the American Water Works Association estimate (Kennedy/Jenks 1991a) for the acquisition of an appropriate water-treatment system is $275,000 plus annual O&M costs of $23,000, so multimedia mitigation might be considered as a cost-effective alternative.

Scenarios 2–4: Effects of Distribution of Radon in Indoor Air

On the basis of previously described scenario, an important consideration in deciding on the feasibility of the multimedia approach relative to the water-treatment approaches is whether a subpopulation of dwellings can be identified that would provide the needed equivalent health-risk reduction when their airborne radon concentrations were reduced sufficiently. Rather than mitigate all the homes that exceed 150 Bq m-3, it would be more cost-effective to mitigate only enough homes to achieve the target level of risk reduction. The prevalence of

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