Thoron Lung Dosimetry

The lung dose contributed from thoron (Rn) and its progeny was usually neglected in the dose assessment because of the generally low concentrations. Recently, increased concentrations of Rn and its progeny were measured in the traditional residential dwellings in China, in dwellings in India and in other countries. The potential alpha energy concentration contributed from Rn and its decay products to the inhalation exposure was found to be about the same level as radon (Rn). Therefore, a more complete assessment of the contribution of Rn decay products to the lung dose is necessary. The effective and organ equivalent doses of the inhaled Rn progeny of Pb and Bi in the lungs and in other organs were calculated as a function of the activity median thermodynamic diameter (AMTD) of 1 – 1,000 nm, and the activity median aerodynamic diameter (AMAD) of 100 – 10,000 nm, applying the human respiratory tract model and the biokinetic models developed by the International Commission on Radiological Protection (ICRP). Taking into account the unattached fraction of 0.03 for the both progeny and an AMTD of 1.5 nm for unattached particles and an AMAD of 300 nm for attached particles in the indoor air, the dose conversion coefficient was estimated to be 3.8 mSv WML, about 27.5% of the values of 13.8 mSv WML for Rn which was evaluated using the ICRP human respiratory tract model as well. This study provides the dose conversion coefficient for thoron decay products. It can be used to assess the inhalation dose for members of the public who live in areas with a higher thoron concentration in indoor air.