Present state of studies on health effects in the high-level natural radiation areas

This paper describes the significance and global evaluation of the studies in the title, and promoting activities and achievements by Japanese investigators. The high-level natural radiation areas are those exposed to high background area (HBRA) and to high level radon (HLR). The former involves local areas in Brazil, China, India, and Iran where ambient absorption dose rate ranges from 70 to 90,000 nGy/h (cf., the global average level, 59), and the latter, typically, in Brazil, Czech, Iran, China, Indonesia, Spain and England where 222Rn levels are higher than 400 Bq/m3 (cf., 39). Although epidemiological studies on the life span of A-bomb survivors are regarded to be the most reliable for assessment of the low dose radiation risk in carcinogenesis, International Commission of Radiological Protection (ICRP) Pub. 99 has reviewed results of even HBRA exposures as well. Verification of the linear non-threshold hypothesis is one of the present global tasks, where HBRA studies (mainly in China) are referred also in United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). For Rn, its health risk has been evaluated in the cohort of miner workers and in HLR area, in which studies on the relationship between the Rn levels and lung cancer incidence have been mainly performed. The significance of the relationship has been recently elucidated. Actual investigations are described in detail in those areas of Yangjiang (China), Kerala and Tamil (India), Guarapari (Brazil), Ramsar (Iran) and Chinese HLR areas, where Japanese scientists have been joining. Studies in the HBRA and HLR areas can conceivably give more precise, direct findings on health effects of low-dose, long-term exposure than those obtainable in A-bomb survivors and people who encountered the nuclear accidents because they might have concomitant indirect factors like acute exposure and human mental concerns. (R.T.)