In order to evaluate the risk from thoron progeny indoors in China, measurements of thoron progeny have been performed by 24 h integrating measurement in Beijing (13 dwellings) and Zhuhai (54 dwellings) areas, whereas in Guangdong (220 dwellings) and Fujian (204 dwellings) provinces 10 min filter sampling was adopted. The highest average EEC_(Tn) was 4.0 ± 2.3 Bqm~(-3), which was found in Zhuhai city, the thorium-rich area, some 3 times higher than that of Beijing city. The range of mean EEC_(Tn)/EEC_(Rn) ratio was 0.07-0.10 in all the areas, the dose ratio of thoron progeny to radon progeny being 0.31-0.47 accordingly. It is suggested that attention should be paid to the exposure from the inhalation of thoron progeny in thorium-rich areas in China. Elevated indoor radon progeny levels were also found in Zhuhai areas.
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