EPA's analyses indicated that only 86 deaths each year may result from inhaling radon emitted from household water supplies, whereas the committee estimated 160 lung cancer deaths per year. The committee's risk estimates were different because it developed new models with updated biological data on the cancer-causing effects of ingesting radon. The committee also drew upon findings of a recent Research Council report on health risks posed by radon in air. Radon is a gas produced from the radioactive decay of uranium that occurs naturally in rocks and soil. Although radon is chemically inert and electrically uncharged, it also is radioactive, which means that radon atoms can spontaneously decay and might damage cells when inhaled or ingested.
Outside air contains very low levels of radon, but indoors the gas builds to higher concentrations.
Radon is also found in ground water tapped by wells, which supply about half the drinking water in the United States. Ground water moves through rock containing natural uranium that releases radon into the water. Water from wells usually has higher concentrations of radon than does surface water such as lakes and streams. National data on radon distribution across the United States indicates that the northern United States and some areas in southern states tend to have higher than average indoor radon, while New England states and some areas in the Southwest have higher concentrations of radon in water. The Appalachian and Rocky Mountain states and some areas in the Great Plains have higher than average radon in both water and indoor air.