Increased indoor thoron concentrations and implication to inhalation dosimetry.

Increased concentrations of thoron (220Rn) and its progenies were recently measured in traditional residential dwellings and gave rise to concern about thoron dose assessment. A compartment model for the attached and unattached thoron progenies in the human body by inhalation was adapted, applied to individual measurements and examined in regard to model parameters. It was found that the lung dose is the dominant contribution to the thoron effective dose in spite of the transfer of 212Pb to other tissue. The organ equivalent dose and effective dose coefficients may change by about a factor of 2 within the 0.0-0.2 range of the unattached fraction. A decrease of the dissolution half-life of the inhaled particles in the lungs by a factor of 10 results in a decrease of the effective dose by <50%. Individual measurements of total concentration and unattached fraction result in a mean dose conversion factor of 1.3 Sv per Jhm(-3) and a mean annual dose to the residents of 11 mSv for permanent stay.