What is it?
A naturally occurring radioactive gas released from uranium in soil and rock. It accumulates indoors, especially in basements and ground-floor rooms, and is invisible and odorless.
Why it matters
Radon is the second-leading cause of lung cancer in the U.S. after smoking, and the leading cause among non-smokers. EPA estimates it causes roughly 21,000 lung cancer deaths per year.
Reference value
EPA recommends mitigation at indoor radon levels of 4 pCi/L (picocuries per liter) or higher. Counties with high underlying geology and a higher share of older housing carry more risk.
Related health outcomes
- Lung cancer
Data source
EPA Radon Zones + state radon programs · View source →