radon A radioactive gaseous element emitted naturally from rocks and minerals where radioactive elements are present. It is released in non-coal mines, e.g. tin, iron, fluorspar, uranium. Radon is an alpha particle emitter as are its decay products and has been indicted as a cause of excessive occurrence of lung cancer in uranium miners. Concern has been expressed at radon levels in some housing usually adjacent to granite rocks or old tin mining regions