"Radon is a naturally-occurring radioactive gas that emits ionizing radiation. National and international scientific organizations have concluded that radon causes lung cancer in humans. Most of the radon in indoor air comes from the breakdown of uranium in soil beneath homes. Breathing radon from the indoor air in homes is the primary public health risk from radon, contributing to about 20,000 lung cancer deaths each year in the United States, according to a 1999 landmark report by the National Academy of Sciences (NAS) on radon in indoor air. The U.S. Surgeon General has warned that radon in indoor air is the second leading cause of lung cancer. EPA and the U.S. Surgeon General recommend testing all homes and apartments located below the third floor for radon in indoor air. If you smoke and your home has high indoor radon levels, your risk of lung cancer is especially high.

Radon from tap water is a smaller source of radon in indoor air. Only about 1-2 percent of radon in indoor air comes from drinking water. However breathing radon released to air from household water uses increases the risk of lung cancer over the course of your lifetime. Ingestion of drinking water containing radon also presents a risk of internal organ cancers, primarily stomach cancer. This risk is smaller than the risk of developing lung cancer from radon released to air from tap water. Based on a second 1999 NAS report on radon in drinking water, EPA estimates that radon in drinking water causes about 168 cancer deaths per year, 89 percent from lung cancer caused by breathing in radon released from water, and 11 percent from stomach cancer caused by drinking radon-containing water."

Read more at the U.S. Environmental Protection agency's web site: http://www.epa.gov/safewater/radon/fact.html