Radon is a hazardous, naturally occurring gas that results from the decay of trace amounts of uranium found in the earth’s crust. Odorless and invisible, radon has been classified (by the Environmental Protection Agency) as a carcinogen since 1988, and is the leading cause of cancer among non-smokers in  the U.S. Because radon is an unstable gas, it breaks down and dissipates quickly in the open air. Dangerous levels of radon can accumulate inside a house, however.