Thoron in the Environment

Thoron (²²⁰Rn), an isotope of the radon family, is produced in the earth’s crust at a rate comparable to that of common radon (²²²Rn). Thoron’s average activity concentration in soil gas and ground-level outside air is comparable to that of radon. Recent data from Europe and the United States indicate that in terms of the energy of the alpha particle decays of thoron’s progeny, its concentration in indoor air is significant, typically about half that due to radon progeny. This paper reviews current knowledge about thoron and its progeny in the outdoor and indoor environments and discusses issues involved in assessing whether or not it is a significant indoor pollutant.