Radium is found in soil, rocks, and certain building materials.
Its spontaneous decay immediately under or around buildings can result in the accumulation of indoor radon. Drinking water wells can also become contaminated if they come into contact with radon. The highest levels are found in basements and crawl spaces where ventilation is usually poor. Although certain geographic areas are recognized as being at higher risk for radon, considerable differences were found among individual homes in a neighborhood, sometimes as close as the house next door. The U.S. Environmental Protection Agency publishes radon zone maps for the United States .