Radon gas is the number one environmental hazard in the U.S. It can be inhaled into the lungs, where a radioactive decay process causes the release of alpha particles. These particles can harm lung tissues by damaging the DNA, and the damaged DNA can lead to lung cancer. In fact, radon exposure is second only to cigarette smoking as a cause of lung cancer deaths in the U.S., causing an estimated 21,000 lung cancer deaths annually.
There are considerable biological and epidemiological evidence and data showing the connection between exposure to radon and lung cancer in humans. For this reason, the U.S. Environmental Protection Agency (EPA), the World Health Organization, the National Academy of Sciences, and the US Department of Health and Human Services have classified radon as a Class A human carcinogen.