Abstract
Epidemiology investigation of the residents in Misasa, which is famous for radioactive hot spring, was carried out so far. Because dose from natural radiation would be relatively low, accuracy in the dose estimation should be important in the case of investigation for effects of exposure to natural radiation. It was found from the environmental investigations that there are many places where thoron concentration is very high as well as radon concentration. From these points of view, provisional study for measurements of the thoron concentrations in residence and the building for hot spring in Misasa were carried out. >From the result of the measurements, the thoron concentrations at the position of 10cm or 20cm from the surface of the wall ranged from about 0 to more than 1000 Bq/m3. It was found that there were locations of residence or hot spring where equilibrium equivalent thoron concentration (EETC) exceeds several Bq/m3. In the room where EETC are high, contribution of thoron to effective dose could be higher than that of radon. It was suggested that further investigation of measurements and dose estimation from thoron is necessary.
