A radioactive inert gas, atomic number 86, the decay product of radium. The most stable isotope, radon-222, has a half-life of 3.825 days. Relatively high concentrations of radon may occur in mines and inside buildings built above certain rocks, such as granite. This is regarded as a health hazard requiring remedial measures, although currently unconfirmed research suggests that the danger in homes may have been overestimated. See also thoron.