Long-term exposure to ambient air pollution and road traffic noise and asthma incidence in adults: The Danish Nurse cohort: The Danish Nurse cohort

BACKGROUND: Ambient air pollution is likely a risk factor for asthma, and recent evidence suggests the possible relevance of road traffic noise.

OBJECTIVES: We examined the associations of long-term exposure to air pollution and road traffic noise with adult-asthma incidence.

METHODS: We followed 28,731 female nurses (age > 44 years) from the Danish Nurse Cohort, recruited in 1993 and 1999, for first hospital contact for asthma from 1977 until 2015. We estimated residential annual mean concentrations of particulate matter with diameter < 2.5 µm (PM 2.5) since 1990 and nitrogen dioxide (NO 2) since 1970 with the Danish DEHM/UBM/AirGIS modeling system, and road traffic noise (L den) since 1970 with the Nord2000 model. Time-varying Cox regression models were used to associate air pollution and road traffic noise exposure with asthma incidence.

RESULTS: During 18.6 years' mean follow-up, 528 out of 23,093 participants had hospital contact for asthma. The hazard ratios (HR) and 95% confidence intervals for asthma incidence associated with 3-year moving average exposures were 1.29 (1.03, 1.61) per 6.3 µg/m 3 for PM 2.5, 1.16 (1.07, 1.27) per 8.2 µg/m 3 for NO 2, and 1.12 (1.00, 1.25) per 10 dB for L den. The HR for NO 2 remained unchanged after adjustment for either PM 2.5 or L den, while the HRs for PM 2.5 and L den attenuated to unity after adjustment for NO 2.

CONCLUSIONS: Long-term exposure to air pollution was associated with adult-asthma incidence independently of road traffic noise, with NO 2 most relevant. Road traffic noise was not independently associated with adult-asthma incidence.