Occupational exposure to noise and mortality from acute myocardial infarction

Background: Exposure to noise is highly prevalent in the workplace, and an etiologic association with cardiovascular disease has been hypothesized. Although there is evidence of hypertension among noise-exposed workers, evidence of heart disease has been less conclusive. Methods: We identified a cohort of 27,464 blue-collar workers from 14 lumber mills in British Columbia who worked at least 1 year between 1950 and 1995 and who were followed up over the same period. Cumulative noise exposure was quantitatively assessed. Vital status was ascertained from the Canadian Mortality Database. We estimated standardized mortality ratios using the general population as referents, and we estimated relative risks using an internal low-exposure group as controls. To examine acute effects of noise, we assessed relative risks during subjects' working years in lumber mills. Because of the possibility of exposure misclassification as a result of hearing-protector use, we investigated a subgroup that had been employed before widespread use of protectors. Results: During the follow-up period, 2510 circulatory disease deaths occurred. Relative risks for acute myocardial infarction mortality were elevated in the full cohort, with a stronger association in the subgroup without hearing protection. There was an exposure-response trend, with a relative risk in the highest exposed group of 1.5 (95% confidence interval = 1.1-2.2). The highest relative risks (2.0-4.0) were observed during subjects' working years. Smoking did not appear to confound these associations. Conclusions: Chronic exposure to noise levels typical of many workplaces was associated with excess risk for acute myocardial infarction death. Given the very high prevalence of excess noise exposure at work, this association deserves further attention.