Health effects and risks from 222Rn in drinking water.

This paper presents an evaluation of the inhalation and ingestion doses from exposure to Rn and Rn progeny; an overview of the human and animal health-effects data; estimations of the cancer risks from Rn and Rn-progeny exposures; and suggested limits for Rn concentrations in drinking water and indoor air. We suggest that a rounded Rn-in-water concentration limit of 10,000 pCi/l can be supported by health-effects considerations alone, based on the conservative "tolerance dose" concept and other conservative assumptions regarding lung dose. A practical concentration limit (or action level) of 20,000 pCi/l has been derived by estimations of exposure distributions in the United States and in relation to current U.S. Environmental Protection Agency (EPA) standards for U-tailings-contaminated buildings. Research needed for resolution of the uncertainties in these estimates is suggested. We conclude that before a maximum contaminant level (MCL) for Rn in water can be firmly established, the broader issue of setting the MCL for Rn in indoor air must be addressed.