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BACKGROUND: Evidence on the association between road traffic noise and diabetes risk is sparse and inconsistent with respect to how confounding by air pollution was treated.

OBJECTIVES: In this study, we aimed to examine whether long-term exposure to road traffic noise over 25 years is associated with incidence of diabetes, independent of air pollution.

METHODS: A total of 28,731 female nurses from the Danish Nurse cohort (>44 years old at recruitment in 1993 or 1999) were linked to the Danish National Diabetes Register with information on incidence of diabetes from 1995 until 2013. The annual mean weighted levels of 24-h average road traffic noise (L-den) at nurses' residences from 1970 until 2013 were estimated with the Nord2000 method and annual mean levels of particulate matter (PM) with diameter <2.5 and 10 nm (PM2.5 and PM10), nitrogen dioxide (NO2), and nitrogen oxide (NOx) with the Danish AirGIS modeling system. Cox proportional hazards regression models were used to examine the association between residential Edcr, in four different exposure windows (1-, 5-, 10-, and 25 -years) and the incidence of diabetes, adjusted for lifestyle factors and air pollutants.

RESULTS: Of 23,762 nurses free of diabetes at the cohort baseline, 1,158 developed diabetes during a mean follow-up of 15.2 years. We found weak positive associations between 5-y mean exposure to L-den (per 10dB increase) and diabetes incidence in a crude model [hazard ratio (HR): 1.07; 95% confidence interval (CI): 0.99, 1.12], which attenuated in a model adjusted for lifestyle factors (HR:1.04; 95% CI: 0.97, 1.12), and reached unity after additional adjustment for PM2.5 (HR: 0.99; 0.91, 1.08). In analyses by level of urbanization, we found a positive association between noise and diabetes in urban areas (HR:1.27; 95% CC 0.98, 1.63) that was unchanged after adjusting for PM2.5 (HR: 1.25; 95% CI: 0.97, 1.62), but we found no apparent association in provincial (HR: 1.02; 95% CI: 0.88, 1.18) or rural areas (HR: 0.97; 95% CI: 0.87, 1.08).

CONCLUSION: In the nationwide cohort of Danish nurses 44 years of age and older, we found no association between long-term exposure to road traffic noise and diabetes incidence after adjustment for PM2.5 but found suggestive evidence of an association limited to urban areas.