Radon is a colorless, odorless and tasteless radioactive gas that occurs naturally in most rocks and soil. It is produced by the breakdown of uranium in soil, rock and water. Radon is harmlessly dispersed in outdoor air, but when trapped in buildings can increase the risk of lung cancer, especially at elevated levels.
"How Radon became a national issue."

What is the Health Risk?

According to the Surgeon General, exposure to radon is the second leading cause of lung cancer in the United States. According to the EPA's 2003 Assessment of Risks from Radon in Homes, radon is estimated to cause about 21,000 lung cancer deaths per year.

Radon is not known to cause other illnesses or problems such as upper respiratory infections, colds or allergic reactions. Its only known health effect is an increased risk of developing lung cancer. However, as with those who smoke, not everyone exposed to high levels of radon will develop lung cancer, and the time between exposure and the onset of cancer may be many years.

Surgeon General - National Health Advisory On Radon