Cohort Mortality Study of Roofing Granule Mine and Mill Workers. Part II. Epidemiologic Analysis, 1945–2004

Abstract

The mortality of 2650 employees (93.4% males) in the mine and mill production of roofing granules at four plants was examined between 1945 and 2004. Hypotheses focused on diseases associated with exposure to silica: nonmalignant respiratory disease, lung cancer, and nonmalignant renal disease. Study eligibility required ≥ 1 year of employment by 2000. Work history and vital status were followed through 2004 with < 1% lost to follow-up. Industrial hygiene sampling data (1871 sampling measurements over a 32-year period) and professional judgment were used to construct 15 respirable crystalline silica exposure categories. A category was assigned to all plant-, department-, and time-dependent standard job titles. Cumulative respirable crystalline silica exposure (mg/m(3)-years) was calculated as the sum of the product of time spent and the average exposure for each plant-, department-, job-, and calendar-year combination. The cohort geometric mean was 0.17 mg/m(3)-years (geometric standard deviation 4.01) and differed by plant. Expected deaths were calculated using U.S. (entire cohort) and regional (each plant) mortality rates. Poisson regression was used for internal comparisons. For the entire cohort, 772 deaths (97.4% males) were identified (standardized mortality ratio 0.95, 95% CI 0.88-1.02). There were 50 deaths from nonmalignant respiratory diseases (1.14, 95% CI 0.85-1.51). Lagging exposure 15 years among the male cohort, the relative risks for nonmalignant respiratory disease were 1.00 (reference), 0.80, 1.94, and 2.03 (p value trend = 0.03) when cumulative exposure was categorized < 0.1, 0.1- < 0.5, 0.5- < 1.0, and ≥ 1.0 mg/m(3)-years, respectively. There was a total of 77 lung cancer deaths (1.11, 95% CI 0.88-1.39). Lagging exposure 15 years, the relative risks for males were 1.00 (reference), 1.83, 1.83, and 1.05 (p value trend = 0.9). There were 16 deaths from nonmalignant renal disease (1.76, 95% CI 1.01-2.86). This exposure-response trend was suggestive but imprecise. The study results are consistent with other cohorts with similar levels of exposure to respirable crystalline silica.