Studies of thoron and thoron progeny: implications for transport of airborne radioactivity from soil to indoor air

Thoron (220Rn) and its progeny are a significant radioactivity component of both indoor and outdoor air. In terms of potential alpha practical energy, the concentration of thoron progeny is typically of the order of one-half of that of radon (222Rn) progeny in the indoor environment. The prevalent view has been that the major source for indoor thoron is building materials. Evidence is presented that soil may be an important source of indoor thoron, particularly in few-story housing such as common in the US. This evidence comes from analysis of a study showing a strong correlation between thoron and radon progeny, reduced concentrations of thoron observed in rooms decoupled from the soil, estimates of transport time from soil to indoor air, and a detailed case study of a one-story house with walls of concrete block. Cracks and joints only a fraction of a millimeter may be sufficient to transport thoron from the soil to indoor air without extensive decay for underpressures common in housing. Since soil is a major source of indoor radon, estimates of transport time should also apply to radon. Comparison of the concentrations of thoron and thoron progeny in indoor air suggests that progeny of both radon and thoron are not typically directly transported through cracks and joints but come from decay of thoron and radon that have entered indoor air. Comprehensive measurements of radon, thoron, and their progeny can provide information on a range of physical processes important to airborne radioactivity in the indoor environment not available from study of radon alone. 17 references, 2 figures, 1 table