Environmental thoron (220Rn) : A review

Ever since studies on uranium miners established the presence of a positive risk coefficient for the occurrence of lung cancer in miners exposed to elevated levels of 222Rn and its progeny, there was a great upsurge of interest in the measurement of 222Rn in the environment and considerable data is generated on the levels of 222Rn in the environment across the worlds and is periodically reported by UNSCEAR. In contrast to this, data pertaining to 220Rn in indoors and workplace environment is scare due to the general perception that its levels are negligible due to its shorter half life, and subsequently its contribution to the total inhalation dose is ignored, in the presence of other significant sources of natural radiation. Many locations have higher levels of natural background radiation due to elevated levels of primordial radio-nuclides in the soil and their decay products like radon (222Rn), and thoron (220Rn) in the environment. It is estimated inhalation of 222Rn, 220Rn and their short lived progenies contribute more than 54 % of the total natural background radiation dose received by the general population. This component is not adequately estimated for any country so far on a national level. 220Rn problem will also be a problem in industries which uses thorium nitrate. Including India, lamps using throated gas mantles are being still used for indoor and outdoor lighting and hawkers in rural as well as urban areas. Considering the fact that large amount of thorium nitrate is being handled by these industries, contribution to the inhalation dose of workers from 220Rn gas emanated and build up of the progeny in ambient air may also be quite significant. In this article current status of 220Rn levels in the indoor environment and workplaces as well as in other industries where large amount of 232Th is being handled, is being summarized. Iran. J. Radiat. Res., 2010 8 (3): 129-147