Characterization of Japanese wooden houses with enhanced radon and thoron concentrations.

Indoor thoron and radon concentrations were surveyed in the 42 rooms of 21 houses in Hiroshima Prefecture with a radon-thoron discriminative dosimeter in 1992. The survey estimated the average indoor thoron concentration to be 84.7 +/- 15.6 Bq m-3 and that of radon to be 25.6 +/- 1.1 Bq m-3 at 20 cm from the interior wall. The results indicated the potential risk of indoor thoron in the Japanese style wooden houses. Multivariate regression analysis among some factors which characterize each of the houses examined and concentrations of radon and thoron suggested that enhanced concentration of indoor thoron was often observed in a typical Japanese style room with interior soil wall and tatami flooring. Characterization of the rooms with enhanced levels of thoron showed that mud-based plaster wall material was probably the source.