EPA: "Nearly 1 out of every 15 homes in the U.S. is estimated to have elevated radon levels."

Radon-222 is a noble gas produced by radioactive decay of radium-226, which is widely distributed in soils and rocks.Radon-222 decays into a series of short-lived radioisotopes. These Decay products are often referred to as radon progeny or daughters. Because it is chemically inert, most inhaled radon is rapidly exhaled, but the inhaled decay products readily deposit in the lung, where they irradiate sensitive cells in the airways, thereby enhancing the risk of lung cancer.

In 1999, the National Research Council of the National Academy of Sciences published the BEIR VI report, health Effects of the Exposure to Radon (NAS 1999), which assessed the risks to the U.S. population from radon in homes. The authors of this study, sponsored by the EPA, had the benefit of extensive new information not available to the authors of the Academy's previous BEIR IV report on the risks from radon and other alpha emitters (NAS 1988). On the basis of epidemiologic evidence from miners and an understanding of the biologic effects of alpha radiation, the committee concluded that residential exposure to radon is expected to be a cause of lung cancer in the population. Based on a statistical analysis of epidemiologic data on 11 cohorts of occupationally exposed underground miners, the committee developed two preferred risk models from which they projected, respectively, 15,400 or 21,800 excess lung cancer cases in the U.S. each year. An analysis of the uncertainties suggested a range of 3,000 to 33,000 cases per year. The committee concluded that "this indicates a public health problem and makes indoor radon the second leading cause of lung cancer after cigarette smoking."

EPA Radon Publications