Long-term residential road traffic noise and NO2 exposure in relation to risk of incident myocardial infarction - A Danish cohort study.

BACKGROUND: Road traffic is a source of both air pollution and noise; two environmental hazards both found to increase the risk of ischemic heart disease. Given the high correlation between these pollutants, it is important to investigate combined effects, in relation to myocardial infarction (MI).

METHODS: Among 50,744 middle-aged Danes enrolled into the Diet, Cancer and Health cohort from 1993 to 97, we identified 2403 cases of incident MI during a median follow-up of 14.5 years. Present and historical residential addresses from 1987 to 2011 were found in national registries, and traffic noise (Lden) and air pollution (NO2) were modelled for all addresses. Analyses were performed using Cox proportional hazard models.

RESULTS: Road traffic noise and NO2 were both individually associated with a higher risk of MI, with hazard ratios of 1.14 (1.07-1.21) and 1.08 (1.03-1.12) per inter-quartile range higher 10-year mean of road traffic noise and NO2, respectively. Mutual exposure adjustment reduced the association with 10-year NO2 exposure (1.02 (0.96-1.08)), whereas the association with road traffic noise remained: 1.12 (1.03-1.21). For fatal incident MI, the pattern was similar, but the associations for both pollutants were stronger. In analyses of tertiles across both pollutants, the strongest effects were seen for combined medium/high exposure, especially for fatal MI's.

CONCLUSION: Both road traffic noise and NO2 were associated with a higher risk of MI in single-pollutant models. In two-pollutant models, mainly noise was associated with MI. Combined exposure to both pollutants was associated with the highest risk.