Radon is a naturally occurring, invisible, colorless, odorless, radioactive gas. Elevated levels have been found in homes in every state. Any home can have a problem (new or old homes, well-sealed or drafty homes, and homes with or without basements). The National Academy of Sciences estimates that radon, the second leading cause of lung cancer, causes between 15,400 and 21,800 lung cancer deaths per year in the U.S.