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The purpose of this study was to obtain more information about health effects among the dust-exposed male miners of Bayun Obo Rare-Earth and Iron Mine, China. From 2,390 male miners from the seven dust-generating workshops of the mine, 136 dust-exposed miners were randomly selected for study. Of these, 64 men were from the high-dust-generating workshop and 72 from the lower-dust-generating workshops; the latter group was used as an internal control. Physical measurements and medical examinations were carried out on each of these 136 men. The average measured thorium lung burden for the high-dust-exposure miners was significantly greater than that for the group of lower-exposure miners, and the incidence of severe breathlessness and pneumoconiosis of stage 0(+) was also significantly raised in the high-exposure group relative to the low-exposure group. An epidemiological study of lung cancer mortality among all the miners and staff of this mine was also carried out. This showed significantly raised levels of lung cancer mortality in both exposed miners and unexposed workers when compared with the Chinese population, and the level in exposed miners was significantly higher than that in unexposed men. The general high rate of lung cancer mortality in the workers of the mine is attributed to high levels of cigarette smoking, and the raised rate in the exposed miners relative to the unexposed workers to inhalation of silica- and thorium-bearing dusts and thoron progeny.

No abstract

The results of an investigation (1982-91) on the natural thorium lung burden and its health effects on miners in the Baiyan Obo mine are reported. An electrostatic collection system was used to measure the exhaled thoron activity. Measurements of exhaled thoron activity were carried out on 588 miners (985 measurements) who had inhaled thorium-containing dusts. Exhaled thoron activity was also determined on 124 persons (124 measurements), who had not inhaled thorium-containing dusts. The average thorium lung burdens of the 588 miners and the 124 controls were 0.85 Bq and 0.15 Bq, respectively. Of the 588 miners, 7 had a thorium lung burden higher than 3.7 Bq, 5 had a thorium lung burden higher than 4.44 Bq and 1 miner had the highest thorium lung burden of 5.56 Bq. In about 20% of these miners the thorium lung burden was 0.74 Bq. There was no definite increase in the incidence of lung cancers in more than 2000 miners who had inhaled thorium dusts and thoron daughters in the period 1958-90. Of the miners who had inhaled thorium dusts, there was a significant increase in the incidence of respiratory symptoms, lung function disorders and pneumoconiosis stage 0+ and stage 1. Based on the results of animal experiments and the low percentage of thorium in the dust, the authors suggest that disorders of the respiratory tract were due mainly to other chemical components in the dust, such as SiO2 and other chemical elements. The role of thorium is negligible. The average air thorium dust concentrations in the main workshops and the average thorium lung burden of the thorium dust-inhaling miners were lower by a factor of 20 and 3 respectively in 1991 compared with 1983.