Occupational exposure to crystalline silica and the risk of lung cancer in Canadian men

Abstract

Background: Crystalline silica is a highly prevalent occupational exposure and recognized human carcinogen, however its association with lung cancer remains contentious. Aims: We investigated the relationship between occupational silica exposure and lung cancer, and the combined effects of cigarette smoking and silica exposure on lung cancer risk. Methods: This was a population-based case-control study with 1681 incident lung cancer cases and 2053 controls from 8 Canadian provinces between 1994 and 1997. Self-reported questionnaires were used to obtain lifetime occupational history and information on other risk factors. Occupational hygienists, blinded to case-control status, assigned silica exposures to each job on based on concentration (low, medium, high); frequency (<5%, 6-30%, >30% of time in a typical week); and reliability (possible, probable, definite). Logistic regression was used to estimate odds ratios (OR) and their 95% confidence intervals. Four exposure metrics were modeled: ever exposure, concentration, duration, and cumulative exposure. Models were adjusted for cigarette smoking, exposure to second-hand smoke, and occupational exposure to diesel and gasoline. Results: A statistically significant trend in lung cancer risk was observed with increasing duration of exposure to silica at any concentration (OR for ?20 years=1.57, 1.22-2.03; ptrend=0.002), and persisted when exposures were restricted to low concentrations (OR for ?30 years=1.58, 1.15-2.16; ptrend=0.02). The highest tertile of cumulative silica exposure was associated with lung cancer (OR=1.41, 1.10-1.81; ptrend=0.04) relative to the lowest tertile. Men exposed to silica at low concentrations with ?40 cigarette pack-years had the highest risk relative to those unexposed with <10 pack-years (OR=25.40, 17.21-37.50). The joint relationship with smoking was consistent with an additive model. Conclusions: Our findings suggest that occupational exposure to silica is an independent lung cancer risk factor.