Long-Term Exposure to Air Pollution and Incidence of Myocardial Infarction: A Danish Nurse Cohort Study

BACKGROUND: Air pollution exposure has been linked to coronary heart disease, although evidence on PM2.5 and myocardial infarction (MI) incidence is mixed. OBJECTIVES: This prospective cohort study aimed to investigate associations between long-term exposure to air pollution and MI incidence, adjusting for road traffic noise. METHODS: We used data from the nationwide Danish Nurse Cohort on 22,882 female nurses (>44 years of age) who, at recruitment in 1993 or 1999, reported information on cardiovascular disease risk factors. Data on MI incidence was collected from the Danish National Patient Register until the end of 2014. Annual mean concentrations of particulate matter (PM) with a diameter <2.5 mu g/m(3) (PM2.5), PM10, nitrogen dioxide (NO2), and nitrogen oxides (NOx) at the nurses' residences since 1990 (PM10 and PM2.5) or 1970 (NO2 and NOx) were estimated using the Danish Eulerian Hemispheric Model/Urban Background Model/AirGIS (DEHM/UBM/AirGIS) dispersion model. We used time-varying Cox regression models to examine the association between 1- and 3-y running means of these pollutants, as well as 23-y running means of NO2 and NOx, with both overall and fatal incident MI. Associations were explored in three progressively adjusted models: Model 1, adjusted for age and baseline year; Model 2, with further adjustment for potential confounding by lifestyle and cardiovascular disease risk factors; and Model 3, with further adjustment for road traffic noise, modeled as the annual mean of a weighted 24-h average (L-den). RESULTS: Of the 22,882 women, 641 developed MI during a mean follow-up of 18.6 y, 121 (18.9%) of which were fatal. Reported hazard ratios (HRs) were based on interquartile range increases of 5.3, 5.5, 8.1, and 11.5 mu g/m(3) for PM2.5, PM10, NO2, and NOx, respectively. In Model 1, we observed a positive association between a 3-y running mean of PM2.5 and an overall incident MI with an HR = 1.20 (95% CI: 1.07, 1.35), which attenuated to HR = 1.06 (95% CI: 0.92, 1.23) in Model 2. In Model 1 for incident fatal MI, we observed a strong association with a 3-y running mean of PM2.5, with an HR = 1.69 (95% CI: 1.33, 2.13), which attenuated to HR = 1.35 (95% CI: 1.01, 1.81) in Model 2. Similar associations were seen for PM10, with 3-y, Model 2 estimates for overall and fatal incident MI of HR = 1.06 (95% CI: 0.91, 1.23) and HR = 1.35 (95% CI: 1.01, 1.81), respectively. No evidence of an association was observed for NO2 or NOx. For all pollutants, associations in Model 2 were robust to further adjustment for road traffic noise in Model 3 and were similar for a 1-y running mean exposure. CONCLUSIONS: We found no association between long-term exposure to PM2.5, PM10, NO2, or NOx and overall MI incidence, but we observed positive associations for PM2.5 and PM10 with fatal MI. We present novel findings that the association between PM and MI incidence is robust to adjustment for road traffic noise.