When you breathe in radon, radioactive particles from radon gas can get trapped in your lungs. Over time, these radioactive particles increase the risk of lung cancer. It may take years before health problems appear. Radon is the second leading cause of lung cancer after cigarette smoking. The U.S. Environmental Protection Agency and the Surgeon General's office estimate radon is responsible for more than 20,000 lung cancer deaths each year in the U.S.

Radon is a gas that you cannot smell, taste or see. Radon forms naturally when uranium, thorium, or radium, radioactive metals, breaks down in rocks, soil and groundwater. People can be exposed to radon primarily from breathing radon in air that comes through cracks and gaps in buildings and homes. Because radon comes naturally from the earth, people are always exposed to it.