Thoron levels in traditional Chinese residential dwellings

A survey on radon (Rn-222), thoron (Rn-220) and its decay products ((RnD)-Rn-220) was conducted in Chinese traditional residential dwellings constructed with loam bricks or soil wall. The activity concentrations in 164 dwellings under investigation were 72.4 +/- 59.2 ( arithmetic mean, AM) and 57.5 +/- 2.0 Bq m(-3) (geometric mean, GM) for Rn-222, and 318 +/- 368 and 162 +/- 3.7 Bq m(-3) for Rn-220, respectively. For (RnD)-Rn-220, 67 dwellings were studied. The AM of the 220 RnD equilibrium equivalent concentration was 3.8 +/- 3.3 Bq m(-3) with a maximum value of 15.8 Bq m(-3). On the basis of these results, the average annual effective doses to the local residents due to radon and thoron exposure were 1.44-4.62 mSv. Thoron contributes 12.9-56.6% to the total doses. Preliminary results show that there is a relation between (RnD)-Rn-220 in air and Th-232 in soil. The correlation factors of outdoor and indoor were 0.88 and 0.40. The Th-232 activity content of Chinese soil is estimated to be about two times the world average. The traditional residential dwellings with soil construction are still common in China. Further investigations on the Rn-220 level in these dwelling with the aim of dose reduction are proposed.