A nation-wide survey of radon indoors and outdoors in Japan

A nation-wide survey of radon concentration was conducted to estimate contributions of radon and its daughters to the population dose in Japan. A total of 500 monitoring apparatuses were distributed in 13 carefully selected prefectures, taking into account geographical configurations to Pacific Ocean and Asian Continent that would be a significant source of radon, as well as areas of heavy snowfall, basin, and seagirt sites. Two hunderd and fifty houses, made of wood or concrete, were measured indoors and outdoors. The monitoring apparatuses were settled mainly in bed rooms and living rooms where inhabitants would stay for a long time. Cellulose nitrate (CN) films along with drying agent (P2O5) were exchanged every two months. The etch tracks on the films represented the integrated radon concentration over two months. Such a procedure was repeated for more than 1.5 years, and a total of 5,000 films or more were finally obtained. Analyses of the CN films revealed: (1) radon concentration was generally high between fall and spring, and low in summer; (2) it tended to be higher in northern than southern areas; (3) the indoor concentration of radon was generally double that the outdoor concentration (10 Bq/m3 vs 5 Bq/m3); (4) both indoor and outdoor radon concentrations did not differ largely from those obtained in the United Kingdom. (N.K.)