Radon is a naturally occurring radioactive gas that may cause cancer, and may be found in drinking water and indoor air. Some people who are exposed to radon in drinking water may have increased risk of getting cancer over the course of their lifetime, especially lung cancer. Radon in soil under homes is the biggest source of radon in indoor air, and presents a greater risk of lung cancer than radon in drinking water.
Radon, in this text, refers to the specific isotope radon-222. Radon is a naturally occurring gas formed from the radioactive decay of uranium-238. Low concentrations of uranium and its other decay products, specifically radium-226, occur widely in the earth's crust, and thus radon is continually being generated, even in soils in which there is no man-made radioactive contamination.
Radon is colorless, odorless, tasteless, chemically inert, and radioactive. A portion of the radon released through radioactive decay moves through air or water-filled pores in the soil to the soil surface and enters the air, while some remains below the surface and dissolves in ground water (water that collects and flows under the ground's surface). Because radon is a gas, when water that contains radon is exposed to the air, the radon will tend to be released into the air. Therefore, radon is usually present in only low amounts in rivers and lakes. If ground water is supplied to a house, radon in the water will tend to be released into the air of the house via various water uses. Thus presence of radon in drinking water supplies leads to exposure via both oral route (ingesting water containing radon) and inhalation route (breathing air containing both radon and radon decay products released from water used in the house such as for cooking and washing).
Radon itself also decays, emitting ionizing radiation in the form of alpha particles, and transforms into decay products, or ``progeny'' radioisotopes. It has a half-life of about four days and decays into short-lived progeny.
Unlike radon, the progeny are not gases, and can easily attach to and be transported by dust and other particles in air. The decay of progeny continues until stable, non-radioactive progeny are formed. At each step in the decay process, radiation is released.