A cohort study of cancer mortality on workers exposed to thorium-containing dust in Baotou Iron and Steel Company

A cohort study was performed in workers exposed to thorium-containing dust in Baotou iron and Steel Company during a period of 17 years, 111286 man-years including retired workers were observed, of which 74827 man-years were of exposed group. The average lung cumulative dose was 21.7 mSv for inhaled thorium-containing dust, and 0.01 to 75 mSv for γ-rays. The first cause of all deaths was malignant tumor. The RR for leukemia and lung cancer were 6.13 and 1.24, respectively and the SMR for the two diseases were 3.06 and 1.55, respectively. The analysis did not show any dose-response relationship between malignant tumor and exposure to thorium-containing dust and γ-rays. Besides, the relationship between the increase of lung cancer and cigarette smoking was significant. The increase of the mortality rate of leukemia may be attributed to factors other than radiation which are worthy of further research