ered are noted here. (a) Long-term radon measurements are needed (after occupancy) in a number of radon-resistant houses in several regions for comparison with measurements on ''control'' houses in the same regions. (b) Success in building effective radon-resistant homes should be correlated with the attention paid to the details of the construction process, as the Florida experience appears to indicate. (c) Soil-gas concentrations should be measured both as a criterion for examining radon resistance and as a means of determining whether there are practical upper limits to the use of radon resistance in limiting indoor concentrations to about 150 Bq m-3.

More specific to the multimedia programs, carefully designed studies are needed on a sufficient number of new homes to provide an adequate basis for quantitatively estimating the health-risk reductions that can be achieved by radon-resistant construction. These studies should be long enough to determine the efficacy and durability of the risk reductions achieved by the package of design features included in a radon-resistant house.

There are several broader research subjects whose study the committee believes would shed additional light on the subjects covered in this report, although they are not likely to alter substantially the overall conclusions presented here. For example, a number of important basic scientific questions regarding the nature of radiation-induced cancers are still unanswered. The most important of these issues is the molecular analysis of the effects of single alpha-particle tracks, including single-cell analysis of DNA damage, DNA repair mechanisms, and the linearity of the dose-response curve under low-dose conditions.

Included in these other research issues are those related to exposure and risk reduction. There is a need for better, more nationally representative data on the water-to-air transfer coefficient, on specific water-use rates in homes, and on home ventilation rates. The interplay between public-health risk perception and the alternative risk-reduction strategies that might be used in a multimedia risk-reduction approach should be better understood.

Furthermore, it would be useful to have better data on the long-term annual average indoor radon concentrations at state or regional levels. A comprehensive, geographically based ambient-radon study that would better incorporate the major populations of the United States and their geologic variability as well as focused regional studies of ambient radon in high radon areas of the country would also be useful. Data from all of these studies would better support the estimation of the national baseline exposure and risk.

Finally, better data on particle size distributions and the resulting indoor exposures is needed. The size of the particles in the indoor aerosol is a key determinant of the deposition of radon decay products in the lungs and of other health risks associated with indoor exposure to particulate matter. Thus, a national, statistically valid assessment of the distribution of human exposures to indoor aerosols would be helpful in many risk-assessment problems, including the effects of the presence of radon decay products in indoor air.

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