Understanding of high radon concentrations observed in a well-ventilated Japanese wooden house

Two nation-wide indoor radon surveys have been conducted in Japan. There was a significant difference between the two surveys. The first survey covered over 7000 houses using Karlsruhe passive radon detectors developed in Germany. The first survey provided relatively higher radon concentrations than expected though there are many well-ventilated wooden houses in Japan. The arithmetic mean was estimated to be 20.8 Bq m-3. In the second survey, on the other hand, it was 15.5 Bq m-3. In order to understand the inconsistency, passive radon detectors used in the first survey were placed in a traditional room at a Japanese wooden house. The concentration was functioned by the distance from the wall. Although passive radon detectors are designed to detect radon only effectively, a little contamination of thoron was often found in readings obtained from some detectors according to our experimental studies.