An epidemiology study on lung cancer mortality of the dust-exposed miners and dust-unexposed miners (controls) was performed in 2001. It was found that, during the period from 1977 to March 2001, 27 cases of lung cancers were observed in the dust-exposed miners, while 8 cases of lung cancers were found in controls. The SMRs of them were 6.13 and 1.90 respectively. The SMR of the dust-exposed miners was much higher than that of the controls. The difference between the two SMRs is very significant (χ2 = 9.488, P < 0.005). The probability that 27 or more lung cancers would occur in the dust-exposed group, given the expected numbers and that a total of 35 occurred is 0.0015. The high SMR for lung cancers among dust-exposed miners likely resulted from the inhaled thorium-containing dusts (carcinogens are ThO2 and SiO2) and its short lived thoron progeny. This is the first evidence in humans of the carcinogenicity after long-term inhalation of thorium-containing dusts and thoron progeny. The total person-years of observation of the dust exposed miners and the unexposed miners were 62 712 and 34 672 respectively.

A twenty-year follow-up study was carried out at Baiyun Obo Rare-earth Iron Mine in China, This mine has been mined since 1958. Its ore contains 0.04% of ThO2 and 10% of SiO2. The purpose of this study is to investigate possible health effects in dust-exposed miners following long-term exposure to thorium-containing dusts and thoron progeny. By using the negative high voltage exhaled thoron progeny measurement system to estimate the miner’s thorium lung burden. The highest thorium lung burden among 1 158 measurements of 638 miners was 11.11 Bq. The incidence of stage 0+ pneumoconiosis was increased among dust-exposed miners. An epidemiological study showed that the lung cancer mortality of the dust-exposed miners was significantly (p < 0.005) higher than that of the non-exposed group. It is suggested that the difference results in the long-term exposure to thorium-containing dusts (carcinogens are ThO2 and SiO2) and thoron progeny. This is the first evidence in humans of the carcinogenicity after long-term inhalation of thorium-containing dusts and thoron progeny. The total person-years of observation for the dust-exposed miners and the controls were 62 712 and 34 672 respectively.