According to a 2003 report EPA's Assessment of Risks from Radon in Homes from the United States Environmental Protection Agency, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon-induced U.S. lung cancer deaths per year—second only to cigarette smoking.[3] Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant.