Measurement of nationwide indoor Rn concentration in Japan

Indoor Rn (222Rn) concentrations in Japan were measured to estimate the effective dose to the general public due to Rn and its progeny. Twenty houses were selected in each prefecture throughout Japan. As a total, 940 houses were subjected to the measurement using passive Rn monitors. The Rn monitor was installed in a bedroom or a living room in each house for four successive three-month periods. The annual mean indoor Rn concentration was estimated from the four measurements in each house. The mean, median and geometric means of indoor Rn concentration in Japan were obtained to be 15.5, 11.7 and 12.7 Bq m-3, respectively. The Rn concentration shows approximately a log-normal distribution. Seasonal and structural dependence of Rn concentration was observed. The effective dose to the general public due to Rn and its progeny was estimated to be 0.45 mSv y-1 in Japan