Radon is a naturally occurring radioactive gas that you can't see, taste, or smell. It is produced by the breakdown of uranium in soil, rock, and water.
All rocks contain some uranium, although most contain just a small amount.
Certain types of rock, including granites, dark shales, light-colored volcanic rocks, sedimentary rocks containing phosphate, and metamorphic rocks derived from these rocks, have higher than average uranium contents. High levels of radon have been found in all 50 states. Indoor radon is the second leading cause of lung cancer after smoking. Radon is most concentrated in the lowest level of the home. Radon may also be present in well water and can be released into the air in your home when water is used for showering and other household uses.