The Project on Indoor Radon Measurement in Kazakhstan. What Do We Need to Clarify?

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Exposure to radon is the second leading factor causing lung cancer incidence. According to several epidemiological studies, relative risk of lung cancer was positively correlated with indoor radon concentration higher than 100　Bq m^-3 at statistically significant level and increase rate was 16% per 100 Bq m^-3. Statistical estimate implied that radon-attributable lung cancer deaths for 66 countries totaled 226,057 in 2012 and represent a median of　3.0% of total cancer deaths. The World Health Organizations and the International Commission on Radiological Protection recommend a reference level of indoor concentration as 100-300 Bq m^-3 for member states. Radon　(Rn-222) has a radioisotope of thoron (Rn-220). Both of these radionuclides coexist in the living environment and show similar behavior each other except for different half-lives (3.8 d for radon vs. 56 s for thoron). Thus,　thoron becomes an interfering factor on radon measurements using detectors without appropriate discrimination functions. In 2000s, passive-type radon-thoron discriminative detectors and thoron calibration chambers were　developed, and studies using such equipment and instruments revealed that detectors conventionally used in epidemiological studies overestimated radon concentration due to contribution of exposure to thoron. This finding　raises the question that the reported relationship between lung cancer incidence and exposure to radon may include the effect of exposure to thoron and the risk of radon is higher than that expected. The present authors　launched the project on indoor radon measurements in northern Kazakhstan in 2023 to estimate risk of radon inhalation to lung cancer incidence. In northern Kazakhstan, uranium is actively mined and highest lung cancer　incidence is reported. Preliminary survey performed by the authors revealed that 3-mo average residential radon concentrations are remarkably high, showing 130 to more than 2000 Bq m^-3. Thoron concentrations were lower　than radon by one to two orders of magnitude, which indicates that dose received from exposure to thoron is negligible compared to exposure to radon. The authors plan to expand the scale of surveys to establish the　quantitative relationship between exposure to radon and lung cancer incidence.