Radon
Radon (222Rn) is a colorless, odorless gas that has been linked to increased risk of lung cancer. Radon is formed from the radioactive decay of radium-226 (226Ra) in the uranium-238 decay series. 222Rn has a half-life of 3.8 days; two other radon isotopes (219Rn and 220Rn) are environmentally insignificant because their half-lives are less than 1 minute. Concern over the potential carcinogenic effects of radon gas and its daughter products has led to much research into the sources, distribution, and environmental behavior of radon. Most of the attention has focused on seepage of radon gas into homes from soils and bedrock that are enriched in radon precursors. However, ground-water radon concentrations are commonly several orders of magnitude higher than airborne radon concentrations, and radon in ground waler may contribute significant amounts of airborne radon into homes as it escapes from the water. The relationship between airborne and ground-water radon concentrations in homes is complex , and is affected by residence volumes, air-exchange rates, water use, and other highly variable factors. Most of the risk associated with high radon levels in ground water appears to be due to inhalation of radon after it escapes from the water, rather than by direct ingestion. Radon concentrations tended to be higher in samples from crystalline-rock aquifers than from other lithologies. In general, ground water radon concentrations are highest in granitic and metamorphic rocks, and lower in mafic and carbonate rocks. The Federal drinking-water standard for radon is currently under review by the U.S. Environmental Protection Agency
