The goal of this project is to evaluate the feasibility of an eventual epidemiological test of the linear, non-threshold model of radiation carcinogenesis using health effects data from communities with populations exposed to varying levels of Radon-222 in the water supplies. To accomplish this goal, two sub-projects must be carried out concurrently: 1. The establishment of a data base consisting of radon levels in selected community water supplies; 2. The development of an algorithm which produces a reasonable estimate of the community population dose increment as a function of practical variables such as radon content of the water supply, meteorological data, and local demographic and architectural factors. Data for the first sub-project is being collected by a large-scale systematic sampling program which makes use of mail-out sampling kits and a simplified analysis technique developed in this Department. It is projected that by the end of the project, the water supplies of communities containing up to 15 million inhabitants can be characterized. Laboratory and field experiments are being conducted to determine radon levels, radon progeny equilibrium status, unattached fraction, and other dose modifying factors in the indoor enivronment. Other experiments seek to document the relationship of indoor radon levels to water consumption and radon content of the water.