Radon is a radioactive gaseous element produced in the disintegration of radium, a radioactive metallic element. It cannot be detected by the senses and can be confirmed only by sophisticated instruments and laboratory tests. The gas enters a house through pores and cracks in the concrete or through floorboards of poorly ventilated crawlspaces, especially when wet ground allows the gas to escape easily through the soil and disperse in the atmosphere. Radon is a lung carcinogen: the National Academy of Sciences estimates radon causes some 15,000 to 22,000 lung cancer deaths annually. The U.S. Surgeon General and the EPA recommend all houses be tested for radon. Houses with high radon levels can be fixed.
Excerpts from U.S. Environmental Protection Agency, "Indoor Radon".
Radon is a naturally occurring radioactive gas found in soils, rock, and water throughout the U.S. Radon is a threat to health because it tends to collect in homes, sometimes to very high concentrations. Radon is a cancer-causing natural radioactive gas that you can't see, smell or taste. Its presence in your home can pose a danger to your family's health. Radon is the leading cause of lung cancer among non-smokers. Radon is the second leading cause of lung cancer in America and claims about 20,000 lives annually.
Excerpts from http://www.epa.gov/radon/