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Radon in drinking water - study finds health risk is small.

Radon is a gas produced by the radioactive decay of uranium that occurs naturally in rocks and soil. Although radon is chemically inert and electrically uncharged, it is still radioactive, which means that radon atoms can spontaneously decay and might damage cells when inhaled or ingested. Radon levels are very low in outdoor air, but indoors the gas builds to higher concentrations. Radon is also found in groundwater tapped by wells, which supply about half the drinking water in the United States. Radon enters groundwater when the water moves through rock that contains natural uranium.

National data on radon distribution across the United States indicate that the northern United States and some areas in southern states tend to have higher-than-average amounts of radon in indoor air, while the 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.

In Risk Assessment of Radon in Drinking Water, a congressionally requested study funded by U.S. EPA, an NRC committee found that it is much less of a health risk to drink water that contains radon than to inhale radon. The risk of stomach cancer - the most likely threat to health from ingestion - is extremely small. The committee estimated that about 20 of the 13,000 stomach cancer deaths that occur each year may result from consuming water that contains radon. No evidence suggests that radon causes any reproductive problems or birth defects, regardless of whether it is ingested or inhaled.

Small amounts of radon in water can escape into the air whenever the water is used - for example, when one is showering or washing dishes. A relatively small volume of water is used in homes, however; because of the large volume of air into which the radon is emitted and the exchange of indoor air with outside air, radon in water typically adds only a small increment to overall indoor air concentrations of the gas.

Estimating Risk

About 160,000 people - mostly smokers - die from lung cancer each year in the United States. Some 19,000 of these deaths are attributable to a combination of indoor radon and smoking. Of those 19,000 deaths, the committee estimated that about 160 result from inhaling radon that is emitted from household water.

Because radon can diffuse into the stomach wall and damage sensitive cells, it could cause stomach cancer in rare cases. In 1991 and 1994, U.S. EPA analyzed the risks posed by radon in drinking water and found the risks to be higher than those estimated by the NRC committee. U.S. EPA calculated that about 100 stomach, colon, and liver cancer deaths would result annually from the ingestion of radon - compared with 20 cancer deaths estimated by the NRC committee. Once radon enters the bloodstream through the stomach or small intestine, according to the NRC committee, it is typically eliminated from the body through the lungs and does not target other organs.

Conversely, the NRC committee estimated the risks posed by inhalation of radon released from water to be higher than those estimated by U.S. EPA. The committee estimated 160 lung cancer deaths per year, while U.S. EPA estimated only 86. The estimates differed because the NRC committee was using new models with updated biological data on the cancer-causing effects of ingesting radon. The committee also drew upon the findings of a recent NRC report on health risks posed by radon in air.

Reducing Risk and Setting Standards

To lessen the health risks, mitigation efforts should focus on the use of ventilation systems to remove radon from indoor air, according to the committee. Except in rare situations in which concentrations of radon in water are very high, bringing levels of radon down in water alone generally will not significantly reduce radon-related health risks for most individuals.

Based on its own risk estimates, U.S. EPA proposed in 1991 that the maximum contaminant level for radon in drinking water be set at 11 becquerel per liter (Bq/L). (A becquerel is a unit by which radiation is measured). U.S. EPA is required to propose a new standard for radon in water next year, to be based in part on the findings of the NRC report.

In addition, U.S. EPA must set an alternative maximum-contaminant-level standard that provides options for mitigation in communities that have water with radon levels above the current standard. Radon will be regulated as a radionuclide in public water supplies, but a major portion of the associated risk occurs because of its contribution to the airborne radon concentration - which is not regulated. The purpose of the alternative standard is to provide methods for reducing health risks by ensuring that radon expelled from household water uses will not raise levels of radon in indoor air above those found naturally outdoors. To meet that goal, the NRC study recommends that the alternative standard be set at 150 Bq/L.

Under the law, communities whose water supplies have concentrations of radon above the alternative standard would have to bring those levels down. States whose water supplies contain levels of radon that fall between the two standards could reduce health risk to their populations by using a combination of strategies - called a "multimedia approach" - to lower radon levels in water, in indoor air, or in both.

States that choose multimedia programs will need to develop plans for reducing public health risks to levels no greater than they would be if the radon in water supplies were below U.S. EPA's current standard. To meet that requirement, state plans would have to identify and mitigate homes with high concentrations of indoor radon. On their own, education and outreach programs designed to entice homeowners to reduce indoor radon probably would not be effective. Trained staff would be required to regularly evaluate the performance of ventilation equipment and other systems to ensure that multimedia programs meet federal requirements. Reducing high concentrations of radon in a few homes instead of mitigating the water supply might meet public health standards, but only residents in those homes would receive the health benefits. The cost of radon reduction to homeowners, water utilities, and state governments should be considered as well, the committee said.

Drinking Water Project Proposals Sought by AWWA

The American Water Works Association Research Foundation (AWWARF) seeks proposals for research projects to protect drinking water. All proposals will be evaluated by project advisory committees; those meeting AWWARF's criteria will be awarded contracts for funding.

Broad topic areas requested include: protecting the drinking water consumer from microbial risk; protecting the drinking water consumer from adverse health effects due to chemicals; improving utility management to obtain optimum water quality and system reliability; improving utility infrastructure for the reliable delivery of high-quality water to the customer's tap; and ensuring access to and wise use of water resources and protection of the environment.

For information about submitting proposals (including deadlines and specific topics requested), please visit AWWARF's website at <> or contact an AWWARF representative by phone at (303) 347-6211 or (303) 347-6117.
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Publication:Journal of Environmental Health
Date:Apr 1, 1999
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