Radon Gas Exposure in Japan: Public Health Risks, Regulations, and Mitigation Strategies

Radon ( 222 Rn) is a naturally occurring radioactive gas and a significant cause of lung cancer globally, second only to inhaled tobacco smoke. This review examines radon exposure in Japan, its public health risks, current regulations, and mitigation strategies in the context of international standards. Studies show that Japan's indoor radon concentrations average around 15 Bq m -3 , significantly lower than the global mean of approximately 40 Bq m -3 . Furthermore, only a small percentage, estimated at 0.1 to 1%, of Japanese homes exceed the World Health Organization's reference level of 100 Bq m -3 . As a result, radon is responsible for a smaller proportion of lung cancer cases in Japan, around 4%, compared to many Western countries. Nonetheless, radon remains an important public health concern, especially as modern homes become more airtight, potentially elevating indoor radon levels. The review also explores the geographic distribution of radon in Japan, highlighting regions (e.g., parts of western Japan with granitic bedrock) that exhibit higher natural radioactivity. It assesses the absence of a comprehensive national radon regulation in Japan, contrasting it with numerous countries that have established reference levels, building codes, and action plans. This review discusses effective radon mitigation techniques, including active soil depressurization and improved ventilation, applicable to Japan's building context. It argues for a proactive national radon policy to promote routine monitoring, public awareness, and remediation of high-radon homes. Finally, it proposes forming an association of radon scientists and technologists in Japan to enhance research, training, and policy advocacy.