Host: The Japan Radiation Research Society
Radon (222Rn) has been recognized as one of the most important contributors to exposure from natural radiation sources over the past 30 years. Recent studies also revealed a positive relationship between indoor radon concentration and lung cancer risk even at a low exposure level below 200 Bq m-3. In the WHO International Radon Project launched in January, 2005, radon is regarded as a global cause of disease and the second leading cause of lung cancer next to tobacco smoking. Many countries world wide are about to take measures against radon. In general, residential radon is regulated by the action level at 200-600 Bqm-3 based on the ICRP recommendation. However, WHO is planning to recommend a new guideline for radon exposure. The action level will be set at a lower level (100-400 Bq m-3) than current one. The new guideline would necessitate the survey of indoor radon level; the measurement data have to be sufficiently assured from the viewpoint of their reliability.For radon measurements, there are many measuring devices: alpha track detectors, charcoal canisters, electrets and so on. In particular, alpha track detectors and electrets are suitable for large-scale and long-term surveys so as to obtain annual radon concentrations. Those detectors are also often used in epidemiological studies. They are generally calibrated in a well-controlled environment such as a radon chamber. However, Tokonami (2005) has pointed out that some of them are sensitive to thoron (220Rn). This finding implies that radon readings will be overestimated and consequently may lead to incorrect estimates of lung cancer risk. The present study describes thoron interferences on radon measurements in some of typical detectors, overviews of thoron concentrations in Japan, China, Korea and Hungary, and some related issues from the epidemiological viewpoint.
