Radon is a radioactive gas that occurs naturally in the earth's rock and soil. It is formed by the natural breakdown of radium, which itself is a decay of uranium. As radon decays it forms radioactive by-products which can damage lung tissue when inhaled and eventually cause lung cancer. Radon is the main source of our exposure to all radiation. In fact, 55% of all radiation to which we are exposed comes from radon gas. Radon enters your home through hollow block walls, cracks in the basement and foundation and openings around the floor drains, pipes, and sump holes and can build to high levels. The Surgeon General warns that indoor radon is second only to smoking as the leading cause of lung cancer in the United States.