We don't usually think about it, but radioactivity is part of the natural world. We are all exposed to background radiation which includes cosmic rays, radon, and radioactive materials (like uranium) in rocks and soil. Background varies with location. Radiation exposure in Denver, for instance, is generally higher than in Seattle because of differences in altitude and natural radioactivity in the environment. There is no indication, however, that Denver cancer risks are any higher as a result.