Radon is an odorless, invisible, radioactive gas that naturally occurs in the ground, released from rocks, soil, and water. It can seep into homes and buildings through small cracks or holes in the foundation and build up in the air over time. According to the EPA, radon is the second-leading cause of lung cancer and the most common way people are exposed to radiation. Radon-associated lung cancer can be prevented by limiting exposure to radon in indoor air — starting with testing radon levels in the home and installing a radon reduction system if needed.