Occupational airborne exposures and asthma mortality - examining asthma as the underlying and contributing cause of death

Background: The aim was to elucidate whether occupational airborne exposures increases asthma mortality. Methods: The study comprised men in the Swedish construction industry who participated in health controls in 1971–1993. Exposure was assessed using a job-exposure matrix with focus on exposures in the mid-1970s. Mortality from asthma in 1987–2015 was compared between 147,101 workers exposed to occupational airborne exposures and 26,879 foremen, using underlying and contributory cause of death from the Swedish Cause of Death Register. Mortality was assessed as relative risk with 95% confidence intervals using Poisson regression models adjusting for age, smoking, body mass index, and calendar time. Results: Among exposed workers, there were 82 deaths with asthma as the underlying cause and 212 deaths with asthma as the contributory cause vs. ten and 21 deaths in the controls. The asthma mortality based on the underlying and contributory cause was 1.92 (1.31–2.83) in relation to inorganic dust, 2.17 (1.47–3.20) in relation to fumes, 1.60 (1.04–2.47) in relation to gases, and 1.79 (1.09–2.96) in relation to wood dust. Using only the underlying cause of death showed similar mortality estimates, but with wider confidence intervals including unity. Conclusions: Occupational airborne exposures increased the asthma mortality, underscoring the need for further reduction of the airborne occupational exposures. Workers with asthma should be given information about the effects of exposure and support to decrease exposure. The study shows the importance of using both contributing and underlying cause of death in studies assessing asthma mortality risk in relation to air pollutants.