from water into air increases the health risk due to radon already in the air in homes.

Approximately half of the drinking water in the United States comes from ground water that is tapped by wells. Underground, this water often moves through rock containing natural uranium that releases radon to the water. Water from wells normally has much higher concentrations of radon than does surface water such as lakes and streams.

Radon concentrations can be measured either in terms of a volume of air (becquerel of radon per cubic meter) or a volume of water (becquerel of radon per liter). The average concentration of radon in public water supplies derived from ground water sources is about 20 becquerel per liter (540 pCi). Some wells have been identified with high concentrations, up to 400 times the average. Surface water, such as in lakes and streams, has the lowest concentrations, about one-tenth that of most wells.

Drinking-water quality in the United States is regulated by the Environmental Protection Agency (EPA) under the Safe Drinking Water Act (SDWA). Since radon is acknowledged as a cancer-causing substance, the law directs EPA to set a maximum contaminant level (MCL) for radon to restrict the exposure of the public to the extent that is possible, that is, as close to zero as is feasible.

In 1991, EPA proposed an MCL for radon of 11 becquerel per liter (about 300 pCi per liter) for radon in drinking water. In 2000, the agency is required to set a new MCL based in part on this report. The law also directed EPA to set an alternative MCL (AMCL); an AMCL is the concentration of radon in water that would cause an increase of radon in indoor air that is no greater than the level of radon naturally present in outdoor air. Limiting public risk from radon by treating the water alone is not feasible because radon is also naturally present in the air. Thus, the AMCL is the tool that allows EPA to limit exposure to radon in water to a practical level, that is, allowing no more risk from the radon in water than is posed by the level of radon naturally present in outdoor air.

The 1996 amendments to the Safe Drinking Water Act required EPA to fund the National Academy of Sciences (NAS) to determine the risk from radon in drinking water and also to determine the public-health benefits of various methods of removing radon from indoor air.

In response to that agreement, the NAS established through its principal operating agency, the National Research Council, a committee which has evaluated various issues related to the risk from radon in drinking water and provides here the information needed by EPA to set the AMCL. The primary conclusion from the committee's investigation into the risk of inhaling radon as compared to drinking water containing dissolved radon is as follows:

Most of the cancer risk resulting from radon in the household water supply is due to inhalation of the radioactive by-products that are produced from radon that has been released from the water into the air, rather than from drinking the water. (The risk from radon is higher among smokers because the combination

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