lognormal distribution fitted to the low radon potential region data, five sets of values for the hypothetical 1,000 homes were estimated using a random number generator. Based on the average values of the 5 sets of values, only 2.5% of the homes exceed the 150 Bq m-3 indoor-air guidance value suggested by EPA. Thus, only 25 homes would be likely candidates for mitigation of airborne radon. These highest concentration dwellings have an average radon concentration of about 215 Bq m-3. If all were mitigated and the indoor airborne 222Rn concentration were reduced to 75 Bq m-3, a concentration that has been found to be generally achievable with active mitigation systems, it would produce about 3,500 Bq m-3 in total radon-concentration reduction.1 Thus, the multimedia approach could be considered only if the 222Rnconcentration in the water were below 60,000 Bq m-3 [(60,000–25,000)(10-4)(1,000) = 3,500], and it would require that all the high-concentration homes could be found and mitigated. For lower concentrations of radon in the water that still exceed the MCL, it would be possible to identify a subset of dwellings that would provide sufficient reduction in airborne radon in enough dwellings to provide an equivalent or greater health-risk reduction. However, it appears likely that the costs of identifying and mitigating enough of dwellings to provide the equivalent health-risk reduction would exceed the costs of processing the water to reduce the radon concentration to the MCL.
For the medium-and high-potential regions, sets of values were generated in a similar manner. For these regions, the fractions of dwellings with more than 150 Bq m-3 are about 11% and 20%, respectively, on the basis of the NRRS distributions. The average concentration in the dwellings with concentrations of 150 Bq m-3 or more is 250 Bq m-3 in the medium-potential region and 270 Bq m-3 in the high-potential region. Thus, the problems of finding high-concentration dwellings and reducing their indoor airborne 222Rn concentrations are much smaller than in the low-potential region because there will now be 110 and 200 homes, respectively, that exceed the EPA guidance value and are candidates for mitigation. In the medium-regions, there would be over 19,250 Bq m-3 that could be mitigated; in the high-potential area, 39,000 Bq m-3 would be available. Thus, the mitigation of a fraction of the homes that exceed the current EPA guidance level would actually produce a larger health risk reduction than mitigation of the water would provide even if the radon concentration in the water supply approached the AMCL.
The ability to obtain the required health-risk reduction by mitigating fewer homes might make the multimedia approach more financially attractive. For the medium-potential region, mitigating the 38 homes with the highest concentra-