Air pollution is major risk factor for chronic cardiorespiratory diseases, while the evidence on respiratory infections remains sparse. We examined the association between long-term exposure to air pollution and incidence and episodes of acute lower respiratory infections (ALRIs).

We followed 23,912 female nurses in Danish Nurse Cohort (aged > 44) from baseline (1993 or 1999) to 2018. Using primary diagnosis, we defined ALRIs incidence as the first-ever hospital contact and ALRIs episodes as all hospital contacts after baseline. The residential annual mean concentrations of particulate matter with diameters ? 2.5 µm (PM2.5), nitrogen dioxide (NO2) and black carbon (BC), since 1979 were modelled by the Danish DEHM/UBM/OPSM modeling system. Marginal Cox models with time-varying exposures were used to assess the association between 3-year mean air pollution and ALRIs incidence and episodes. Concentration-response functions (CRF) were assessed using natural cubic splines with 3 degrees of freedom.

During mean 21.3 years? follow-up, 4,746 ALRIs were observed, of which 2,553 were incident (first-ever) events. We observed strong associations of all three pollutants with ALRIs incidence, with hazard ratios and 95% confidence intervals of 1.19 (1.08-1.31) per 2.5 µg/m3 for PM2.5, 1.17 (1.11-1.24) per 8.0 µg/m3 for NO2, and 1.09 (1.05-1.12) per 0.3 µg/m3 for BC. We detected even stronger associations with ALRIs episodes. We found supra-linear CRFs for all pollutants, indicating a stronger association at lower concentrations.

Long-term exposure to air pollution, even at low levels, was associated with the risk of both new and subsequent ALRIs.