Lung cancer mortality among the miners in a rare-earth iron mine

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