The source of radon is the radioactive decay of uranium. Radon moves from its source in rocks and soils through voids and fractures. It can enter buildings as a gas through foundation cracks or dissolve in the ground water and be carried to water-supply wells. The amount of radon in air or water commonly is reported in terms of activity with units of picocuries per liter of air or water. Radon gas commonly enters the air in homes through the basement.
Ground water can carry additional radon into homes and other buildings, creating a health risk.
Dissolved radon is easily released into the air when the water is used for showering, cleaning, and other everyday purposes. Also, in homes built with better insulation and better seals on windows and doors, radon has less chance to be ventilated to the outside and can become concentrated to dangerous levels in indoor air. The radon that escapes from the water adds to the radon that enters the home through the basement, and in some cases the water contributes a large portion of the radon that is present in a home. Water-borne radon is commonly a concern only for those who use wells for their water supply. Because of its short half-life, radon in public water supplies usually decays to low concentrations before the water is delivered to users, especially if the water has been treated.