Long-term Exposure to Low Concentration of PM2.5 and Mortality: A Danish Nurse Cohort Study

Abstract

Background/Aim. The association between air pollution and mortality is well established, yet studies in areas with low levels of air pollution, below EU limit values, with adjustment for road traffic noise, are sparse. We examined the association of long-term exposure to fine particulate matter (PM2.5) with mortality due to cardiovascular disease (CVD), respiratory disease (RD), and diabetes as well as all-cause natural mortality.Methods. We used data on 24,541 female nurses from the Danish Nurse Cohort who, at recruitment in 1993 or 1999, reported information on risk factors. Data on death were obtained from the Danish Register of Causes of Death until the end of 2013. Annual mean concentrations of PM2.5 in 1990-2013 at the nurses’ residences were estimated using the Danish air pollution dispersion modeling system. We examined associations between the 3-year running mean of PM2.5 with all-cause natural and cause-specific mortality by using time-varying Cox Regression models, adjusting for individual characteristics.Results. During the study time-period, 3,708 nurses died, and of these, 843 were due to CVD, 310 due to RD, and 64 due to diabetes. The mean level of PM2.5 at the baseline was 20.5 µg/m3. In the fully adjusted models, including road traffic noise, we found positive associations of 3-year running mean of PM2.5 with all-cause natural (hazard ratio; 95% confidence interval: 1.06; 1.01-1.11), CVD (1.14; 1.03-1.26), and diabetes mortality (1.41; 1.05-1.90), per interquartile range of 4.39 μg/m3. In subjects exposed to low levels of PM2.5 (< 20 µg/m3), we found even stronger association with all-cause natural (1.19; 1.11-1.27), CVD (1.27; 1.01-1.46), RD (1.27; 1.00-1.60), and diabetes mortality (1.44; 0.83-2.48).Conclusion. In a Danish cohort of female nurses, long-term exposures to low-levels of PM2.5 were associated with all-cause mortality, strongest for diabetes, and CVD, which persisted at the levels below the current EU limit values.