[en] A fourteen-year follow-up study was carried out at Baiyan Obo Rare-earth Iron Mine in China. It has been operating for forty one years. It's ore contains ThO2 at a percentage of 0.04. The purpose of this study is to investigate the possible health effects of 2903 dust-inhaled miners following long-term exposure to thorium dusts. An electrostatic collection system was used to measure the exhaled thoron activity of each examinee, through simple calculation, each examinee's thorium lung burden obtained. Health status was ascertained through questionnaire, physical examination and clinical laboratory tests. An epidemiological study on the mortality rates of lung cancers of the dust-inhaled miners and internal controls was also carried out. Results showed that the highest thorium lung burden for 1158 measurements of 638 miners was at a value of 11.11 Bq, one tenth of the maximum thorium lung burden. In a ten-year investigation on the four haematological parameters and four hepatic parameters of 638 exposed miners, no adverse effects were observed. The incidence of severe breathlessness, lung function tests, pneumoconiosis of stage O⁺ and the concentration of ceruloplasmin in the serum of high thorium lung burden group are much higher than that of low thorium lung burden group. However, owing to the concentration of SiO2 (10%) are much higher than that of the ThO2 (0.04%) in the dust, the disorders of respiratory tract are mainly due to the fibrogenic effects of SiO2, Epidemiology study showed that both the SMRs of the dust-inhaled miners and the dust-free miners are all greater than one (5.15 vs. 2.30). The SMR of the dust miners was much higher than that of the controls. However, there was no statistic significant difference between these two SMRs (X²=3.75 P>0.05). This means that the high SMR of the dust-inhaled miners was not solely due to the long-term inhaled thorium and short-lived thoron daughters. Because of very high percentage of smokers (- 80%) among both the dust-inhaled miners and dust-free miners, cigarette smoking might be one of the important factors in inducing the high SMRs of lung cancers. As a conclusion, whenever the individual thorium lung burden of the miners not higher than 11.11 Bq. no adverse health effects could be detected due to the inhaled thorium. Based on the results from animal experiment. Authors suggested that the possible practical threshold for thorium lung burden to induce the lung cancer might be higher than 11.11 Bq. (author)