The earth's crust contains small amounts of the primordial radionuclides (_^238)U and (_^232)Th which decay through a chain of radioactive nuclides until they produce stable isotopes of lead. Most of the decay products are isotopes of solid elements but two are gases:(_^222)Rn (radon) from the decay of uranium and (_^220)Rn (thoron) from thorium. They can migrate to the earth's surface by transport of radon relative to the gas or liquid (molecular diffusion) and with the gas or liquid (convection or groundwaterflow) (Cothern and Smith, 1987). The half-lives of radon (3.82 d) and thoron (55 s) are different and this difference is important when assessing their release from the ground and their distribution in the open air above the ground as well as in the room air of buildings. Radon and thoron enter the atmosphere mainly by crossing the soil–air, building material–air or water–air interfaces.