The U.S. Environmental Protection Agency (EPA) considers indoor radon to be one of the most important causes of cancer in the U.S. 1 The agency estimates that about 20,000 lung cancer deaths annually are caused by radon exposure in homes. 2 Only cigarette smoking (responsible for about 150,000 lung cancer deaths annually) causes more lung cancer deaths. If the EPA is right, the risk of radon exposure could far exceed hazards associated with typical pollutants in outdoor air, drinking water, and certain foods and would approximate hazards of some more common activities such as automobile travel. 3 Because the entire U.S. population is exposed, radon gas would account for more deaths than almost any other agent the EPA regulates.