A nationwide survey of outdoor radon concentration in Japan

Nationwide outdoor radon (Rn-222) concentrations in Japan were measured to survey the environmental outdoor Rn-222 level and to estimate the effective dose to the general public from Rn-222 and its progeny. The Rn-222 concentration was measured with a passive-type radon monitor. The Rn-122 monitors were installed at about 700 points throughout Japan from 1997 to 1999. The annual mean Rn-222 concentration in Japan was estimated from four quarters measurements of 47 prefectures in Japan. Nationwide outdoor mean Rn-222 concentration was 6.1 Bq m(-3). This was about 40% of the indoor Rn-222 concentration in Japan. The Rn-222 concentration in Japan ranged from 3.3 Bq m(-3) in the Okinawa region to 9.8 Bq m(-3) in the Chugoku region, reflecting geological characteristics. Seasonal variation of outdoor Rn-222 concentration was also found to be lowest in July to September, and highest in October to December. From the results of this Rn-222 survey and previous indoor Rn-222 survey program, the effective dose to the general public from Rn-222 and its progeny was estimated to be 0.45 mSv y(-1). (C) 2002 Elsevier Science Ltd. All rights reserved.