2. Materials and methods 2.1. Subjects Sixty subjects who volunteered for the study were divided into three groups: (i) exposure group I consisted of 20 workers exposed to organic BPs from a small fac- tory; (ii) exposure group II was composed of 20 workers exposed to organic BPs from another small factory; (iii) control group included 20 healthy subjects unexposed to harmful agents. The two factories producing organic bentonite were located in Zhe- jiang province of east China. The characteristics of three groups are listed in Table 1. Controls were matched with exposed workers on the basis of age, gender and smok- ing habit. All subjects were male. There were no significant differences in age and smoking habits among three study groups (P > 0.05). The mean year of exposure in group I was 3.95 ± 0.55 years, which was significantly higher than that (2.20 ± 0.17 years) in group II (P < 0.01). The working hours in both group I and group II were the same, ie 8 h per day. All subjects were informed of the objective of this study, and consented to participate in this investigation. The ethical committee of Zhejiang Provincial Center for Disease Control and Prevention (CDC) approved the research schedule used in this investigation