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Abstract: Numerous epidemiological studies have been carried out to clarify the health effects caused by indoor radon exposure. Some of passive monitors for radon are sensitive to thoron, and thus the results of measurements might be possibly overestimated as radon concentration. Recently, we developed a simple radon–thoron discriminative monitor by using two passive detectors and a special method for concentration measurements of thoron decay products, and applied them to field measurements in conducted in Chinese loess plateau where cave dwellings are widely distributed. Our new monitors revealed the presence of high thoron concentration there. The highest concentration of 1471 Bq/m3 was observed and the mean was 240 Bq/m3. The mean concentration of thoron decay products was 2.2 Bq/m3. It was very low, compared with the expected concentration from the equilibrium factor of 0.1 adopted by UNSCEAR 2000. On the other hand, the mean radon concentration was 73 Bq/m3. The effective dose estimated from our discriminative measurements was 2.4 mSv/year, which is below a half of 5.9 mSv/year estimated from the results of the nondiscriminative measurements. This means the significance of radon and thoron discriminative measurement and the direct measurement of thoron decay products. [Copyright &y& Elsevier]