Radon (Rn-222) is well known as the second cause of lung cancer after smoking. As radon is everywhere in the environment, WHO encourages general public to mitigate radon in their homes. Recently its radioisotope called thoron (Rn-220) is drawing attention as an additional risk on lung cancer from scientific communities on radiation protection because thoron can be detected by the advanced measurement technology.

Indoor radon surveys have been carried out in many countries. They provided an annual radon concentration in each country and are summarized in an international report such as UNSCEAR. Passive radon monitors were commonly used in these surveys but they gave only radon concentrations. The passive radon monitor is originally designed to detect only radon effectively but several recent studies have revealed that some of them are sensitive to thoron though they are radon monitors. If the sensitivity is large than expected, thoron signals will be mixed into radon signals. As such studies have not sufficiently been done yet, radon concentrations might be overestimated even in major epidemiological studies supporting the recent radon risk assessment. This may imply underestimation of the radon risk and result in somehow exclusion of thoron risk.

This presentation addresses a review of passive radon monitors widely used in the world from the viewpoint of their performance in the environment such as thoron interference on radon measurements, and its resulting issues related to the radon risk assessment as well as the thoron risk itself.