We previously reported the results of a 25-year follow-up observational study of a fixed cohort of 515 Chinese male workers in an asbestos textile plant. We found excessive lung cancer mortality associated with exposure to chrysotile asbestos in these workers.8 In the current nested case–control study, we further investigated the relationship between lung cancer and different levels of exposure to chrysotile asbestos. The combined effect of smoking and asbestos was also examined.
The study subjects and controls were selected from an open cohort of asbestos workers in an asbestos textile plant in China. Cohort recruitment began in January 1972 and was open to those working in the plant between that time and November 1996. The final cohort consisting of 1139 male workers was followed for the occurrence of lung cancer until 2001. Nearly half of the members entered the cohort in 1972 and thus were followed for 30 years. All of the members were actively working in the plant and had no signs of overt cardiopulmonary disease at the time of cohort entry. According to plant records, only chrysotile asbestos had been used to produce asbestos textiles, cement products, friction materials, rubber products and heat-resistant materials in the plant since 1939. Although we were not able to obtain samples representative of all of the chrysotile historically used in the plant, available samples of chrysotile assessed in 2000 by x-ray diffraction analysis and analytical transmission electron microscopy revealed that amphibole contamination was very low (