Long-term exposure to low-level crystalline silica and risk assessment of silicosis: a cohort study

Background High-level exposure to crystalline silica dust is the key factor in silicosis. Long-term exposure to low-level silica dust, for example, lower than that in occupational exposure limits, still needs to be studied for their risk of silicosis. Methods A total of 30 697 workers were included from a cohort in China. Low-level silica dust exposure was defined as those having a lifetime mean silica dust concentration equal to or under permissible exposure limits, including 0.05 mg/m 3 , 0.10 mg/m 3 and 0.35 mg/m 3 . Cumulative respirable silica dust exposure (CDE) for individual workers was assessed by linking a job-exposure matrix to personal work history. Results Among those with average exposure level equal to or lower than 0.05 mg/m 3 , compared with the lowest quartile CDE (Q1), the HRs of silicosis were 1.32 (95% CI 0.82 to 2.10) for Q2, 1.87 (95% CI 1.22 to 2.88) for Q3 and 2.00 (95% CI 1.30 to 3.09) for Q4. Among those exposed to 0.10 mg/m 3 or less exposure level, compared with Q1, the HRs were 2.52 (95% CI 1.88 to 3.38) for Q2, 4.08 (95% CI 3.09 to 5.39) for Q3 and 4.02 (95% CI 3.04 to 5.32) for Q4. Among those exposed to 0.35 mg/m 3 or less exposure level, compared with Q1, the HRs were 2.80 (95% CI 2.38 to 3.28) for Q2, 5.76 (95% CI 4.93 to 6.73) for Q3 and 7.14 (95% CI 6.07 to 8.40) for Q4, respectively. Stratified analysis showed that the results and trends did not change with facilities and smoking status. Conclusion Long-term exposure to low-level silica dust is still associated with a higher risk of silicosis. Control measurements and personal protective equipment should be emphasised to protect the health of workers.