RADON

Radon-222 and radon-220 (thoron) are invisible, inert, and odorless radioactive gases formed in the decay of uranium-238 and thorium-232, respectively. Uranium-238 and thorium-232 are radionuclides that are widely distributed in the earth's crust. The half-life of radon-222 is long enough (3.82 days) to enable appreciable quantities of this element to accumulate in the environment, whereas the half-life of radon-220 is so short (55 seconds) that it does not attain environmental concentrations that produce demonstrable biological effects. Radon-222, seeping out of the soil, is ubiquitous in outdoor air, where its concentration averages about 15 becquerels per cubic meter (5 Bqm^-3 or 0.4 pCi/L). (The becquerel [Bq] and the curie [Ci] are units of radioactivity; 1 Bq = 1 disintegration per second, and 1 Ci = 3.7 × 10¹⁰disintegrations per second. Radon is measured in picocuries per liter of air [pCi/L] or becquerels per cubic meter [Bqm^-3].) In indoor air, the concentration of radon tends to be much higher than in outdoor air, especially in poorly ventilated basements and underground mines, where it may exceed 1,000 Bqm^-3 (20 pCi/L). Indoor levels may be increased substantially by the use of groundwater or well water containing elevated concentrations of radon.

The alpha particles emitted by radon outside the body do not penetrate the skin, and radon itself, like other inert gases, is breathed in and out of the lungs without interacting significantly with the surrounding tissues. Hence the biological effects of radon result from inhalation of its solid, short-lived, alpha-emitting decay products (principally polonium-218 and polonium-214), which deposit on the lining of the bronchial airway. The dose to internal organs from radon that is ingested in drinking water, even at high concentrations, is extremely low.

In humans and laboratory animals, the risk of lung cancer increases with increasing exposure to inhaled radon and its short-lived decay products. In underground miners the risk appears to increase in proportion to the total cumulative dose to cells lining the airway, and to be about two times higher in smokers than in nonsmokers. The risk from exposure to residential indoor radon at a given concentration, although yet to be defined precisely, is generally estimated to be comparable to the corresponding risk in miners. As a result, radon is thought to be the single most important cause of lung cancer in nonsmokers and to cause 10 to 15 percent of all lung cancers, or 15,000 to 20,000 lung cancer deaths each year in the United States. Hence, the U.S. Environmental Protection Agency has recommended that indoor radon concentrations not be allowed to exceed 4 pCi/L, a concentration that might be expected to double the risk of lung cancer if inhaled throughout an average lifespan.

Methods for reducing the concentration of radon and its decay products in indoor air include ventilation; air filtration; sealing of cracks in basement floors and walls; installation of a subslab exhaust system beneath the basement floor; and remediation of heavily contaminated groundwater or well water that is used for drinking, bathing, or showering. Radon can be measured in the home with a number of relatively inexpensive devices, which are available from some state and local governments as well as private firms. Pertinent information can generally be obtained from the local state radiation or the Environmental Protection Agency office.

ARTHUR C. UPTON

BIBLIOGRAPHY

Eisenbud, M., and Gesell, T. (1997). Environmental Radioactivity: From Natural, Industrial, and Military Sources, 4th edition. San Diego, CA: Academic Press.

Harley, N. (2000). "Radon and Daughters." In Environmental Toxicants, 2nd edition, ed. M. Lippmann. New York: John Wiley and Sons.

National Academy of Sciences/National Research Council (1998). Health Effects of Exposure to Radon. Washington, DC: National Academy Press.

U.S. Geological Survey. The Geology of Radon. Available at http://energy.ct.us.gov/radonhome.html.