This study was conducted to investigate the concentrations of thoron (220Rn) progeny and radon (222Rn) progeny and to compare the radiation exposure due to them in different kinds of dwellings. The results indicated that thoron progeny concentrations indoors might strongly depend on the kinds of building material used in each dwelling, whereas the radon progeny concentration did not depend on the kinds of building material. The average thoron progeny concentration in the traditional Japanese dwellings with soil wall was 3.22Bqm-3, and the radon progeny concentration in the dwellings was 8.35Bqm-3. The annual effective dose equivalent to thoron progeny was 1.19mSv, which was higher than that of radon progeny (0.82mSv) in the same dwellings assuming an occupancy factor of 1.
In the new human respiratory tract model of the International Commission on Radiological Protection, ICRP66 1994, the following values are recommended as default parameters characterizing aerosols inhaled; particle density of 3g/cm3, aerodynamic shape factor of 1.5 and AMAD of 5μm for occupational exposure. It seems important to assess the extent of variation of dose depending on the chemical form and the diameter of the aerosols inhaled. The deviations of dose obtained by changing the values of these parameters from the default values are studied. The results are follows. (1) Dose coefficient for 239Pu of type S compounds is 8.6×10-6Sv/Bq for aerosols having the default parameter values, which is one tenth of that in ICRP Publication 30. (2) The dose decreases gradually with the diameter of the aerosols at around 1μm; at 0.2μm it is twice of that for 1μm aerosols and it decreases to half at 8μm. (3) The effects of particle density and particle shape are both small. (4) Type M compounds delivers four times higher doses than type S compounds. The reliability and the precision of dose evaluation depend largely on the availability of data characterizing intake radionuclides, such as diameter of aerosols and chemical form.