Long-Term Exposure to Ambient Air Pollution and Lower Respiratory Tract Infection in Adults: Danish Nationwide Evidence

Rationale: Long-term exposure to air pollution has been linked with acute lower respiratory infections (ALRIs) in children, but the evidence in adults is still mixed and sparse. Objective: We aimed to examine the association between long-term exposure to air pollution and incident ALRIs in adults. Methods: We followed all Danish residents aged >30 years (N = 3,083,227) for the first-ever hospital contact (inpatient, outpatient, or emergency) for ALRIs (and pneumonia or influenza separately) from 2000 to 2018. Long-term exposure to air pollution was defined as annual mean concentrations of fine particulate matter (i.e., aerodynamic diameter <2.5 μm [PM_2.5]), nitrogen dioxide (NO₂), and black carbon (BC), which were modeled using hybrid land-use regression models and assigned to baseline residential addresses. Cox regression models were used to assess the association between air pollution and the incidences of ALRIs in total, pneumonia, and influenza. Results: During a mean of 16 years of follow-up, there were 322,035, 309,092, and 11,977 incident cases of ALRIs, pneumonia, and influenza, respectively. We detected positive associations of air pollution and incident ALRIs, with hazard ratios (95% confidence interval) of 1.08 (1.07–1.09) per 10 μg/m³ for NO₂, 1.07 (1.06–1.08) per 0.5 10^-5‧m^-1 for BC, and 1.04 (1.03–1.04) per 2 μg/m³ for PM_2.5. We detected similar associations with pneumonia but detected no association with influenza. Conclusions: Long-term exposure to air pollution may contribute to an increased risk of contracting ALRIs, particularly pneumonia, that require hospital care. Associations persisted for all major pollutants (PM_2.5,NO₂, and BC) even at exposure lower than the current European Union limits and World Health Organization guideline.