Radon is a chemical element (symbol Rn) with an atomic number of 86. Radon is a naturally occurring, odorless, colorless, radioactive gas produced by the radioactive decay of radium-226. Soils, rocks, and particularly granite are the major sources of radium in nature. Radon is generated from the normal radioactive decay of uranium. Uranium has been around since the Earth was formed, and one of its most common isotopes has a very long half-life (4.5 billion years), which is the amount of time required for one-half of uranium to break down. Uranium, radium, and thus radon will continue to exist indefinitely at about the same levels as they do now. A series of tiny radioactive particles are generated as radon gas decays.

Rn is one of the ...