This appendix examines the epidemiologic evidence of an association between indoor radon-progeny exposure and lung-cancer. Although data from indoor-radon studies are not yet sufficient to develop a general risk-assessment model or to estimate precisely the magnitude of risk posed by radon in houses, the data do support a small increase in lung-cancer risk due to indoor radon exposure and are consistent both with the extrapolation of lung-cancer risk using miner-based models and with relative risks among miners with cumulative exposures similar to exposures that might be experienced by long-term residents in houses that exceed the Environmental Protection Agency (EPA) action level. However, there are sufficient uncertainties in current epidemiologic studies that the residential data alone do not conclusively support a definable excess lung-cancer risk associated with radon-progeny exposure.

Ecologic studies and analytic case-control studies are the 2 types of epidemiologic studies that have considered the issue. In an ecologic study, regional rates of lung-cancer are related to a measure of regional radon concentration. The measures of radon concentration are regional mean radon concentrations obtained from direct measurement in a small number of houses and purported correlates of indoor-radon concentration, such as geologic formations and housing characteristics. In an analytic case-control study, data are obtained directly from lung-cancer cases and controls, or their surrogates, through personal interviews. Radon-progeny exposure is estimated for each person and is based on either direct data from indoor-radon measurement or surrogate measures, such as housing type.

The committee concludes that only analytic case-control studies that rely on