Radon, RAY don, is a radioactive chemical element that occurs naturally as a gas. The gas is produced by the radioactive decay (breakdown) of radium, a metallic element found in nearly all soil and rocks. Radon gas is colorless and odorless. It is classified as a noble gas, or an inert gas, because its atoms do not combine readily with other atoms. Radon gas may become a health hazard in certain buildings.