From AMS Glossary
(Symbol Rn.) A radioactive gas, atomic number 86, atomic weight 222; an inert gaseous element with the property of emitting highly ionizing alpha particles during its radioactive disintegration, which makes it important in the atmospheric ion economy.
Radon is a member of the uranium–radium family of radioactive elements. It forms by alpha emission from radium, which has a half-life of 1590 years. Radon itself decays by alpha emission in a half-life of only 3.82 days. Short as this radon half-life is, it is much longer than that of its two radioactive isotopes, thoron and actinon, so radon has a much greater opportunity to be carried by turbulence and convection to considerable heights and contribute to the ionization of the atmosphere before decaying. The oceans contain such slight concentrations of dissolved radium salts that radon production is very small over the oceans. Thus, its presence in air samples is often used to indicate the time since the air was over land. Its effects on human health are currently cause for concern since it can accumulate in inadequately ventilated underground areas where it decomposes into radioactive polonium that forms particles.
Israël, H. 1951. Compendium of Meteorology. 155–157.