Inert radioactive gas which is produced from the radioactive decay of uranium and thorium. Due to the very long half-lives, the earth’s crust contains since its formation among other substances the radionuclides uranium 238, uranium 235 and thorium 232. These convert via a chain of radioactive intermediate products with quite different half-lives to stable lead as the final product. These intermediate products include three radon nuclides: radon 222 (half-life 3.8 days) is generated as a decay product of radium 226, which results from the radioactive decay of uranium 238. In the decay chain of thorium 232, radon 220 (half-life 54 s) occurs and in the decay chain of U 235, radon 219 (half-life 3.96 s). Radon is released wherever uranium and thorium are present in the ground and enters the atmosphere or houses. The radium concentration of the ground and its permeability for this radioactive noble gas is decisive for the radon concentration in the air. Apart from the regional variations, the radon concentration in the atmosphere close to the ground is also subject to seasonal and climatic variations. In buildings, the radon concentration depends essentially on the structural circumstances. For example, in Germany, the annual average value of the radon concentration in the air close to the ground is 15 Bq/m3 and in buildings 60 Bq/m3. Radon concentrations greater than 200 Bq/m3 in ground floor living rooms are not uncommon. Regarding the radiation exposure of people, it is not the radon itself that is important, but the short-lived decay products. These enter the respiratory tract with breathed in air and may reach radiation-sensitive cells with its energy-rich alpha radiation. The short-lived decay products of radon, with 1.4 millisievert per year, account for more than half the total effective dose by natural radiation sources.