Evaluation of Indoor Radon Risk for Discussion on National Action in Japan

International organizations such as the World Health Organization and the International Commission on Radiological Protection have recommended reference levels in the range of 100–300 Bq m–3 for residential radon exposure. In Japan, indoor radon activity concentrations are generally low, and no national plan or reference level has been established. This study evaluated the indoor radon risks in the Japanese population to determine the effectiveness of an internationally recommended reference level in reducing lung cancer deaths. The US Environmental Protection Agency model was used to calculate lifetime risks and population attributable fractions for ever-smokers and never-smokers using recent Japan-specific information such as demographics and smoking prevalence. The results indicated that the number of lung cancer deaths in the population exposed to > 100 Bq m–3 was very low (approximately 10 in 2020). This means that even if a reference level of 100 Bq m–3 is set, no statistically significant reduction in lung cancer deaths can be observed. Based on our risk evaluation, an expected national action in Japan is to disseminate basic information about radon to the public and to support individuals who are considering any action and are concerned about the measurement results, rather than to introduce regulations. Comprehensive arguments and judgments are desired for better public health based on information not only about radon but also about other common modifiable risk factors.