What is radon and why is it a concern in real estate?

Radon is a colorless, odorless radioactive gas that occurs naturally from the decay of uranium in soil and rock. It enters buildings through cracks in foundations, gaps around pipes, and other openings, and can accumulate to dangerous levels indoors. The Environmental Protection Agency estimates that radon causes about 21,000 lung cancer deaths annually in the United States, making it the second leading cause of lung cancer after smoking. The EPA recommends mitigation when indoor radon levels reach 4 picocuries per liter of air or higher.