Exposure to air pollution and risk of respiratory tract infections in the adult Danish population—a nationwide study

Objectives: The association between air pollution and risk of respiratory tract infection (RTI) in adults needs to be clarified in settings with low to moderate levels of air pollution. We investigated this in the Danish population between 2004 and 2016. Methods: We included 3 653 490 persons aged 18–64 years in a nested case-control study. Exposure was defined as the average daily concentration at the individual's residential address of CO, NO_X, NO₂, O₃, SO₂, NH₃, PPM_2.5, black carbon, organic carbon, mineral dust, sea salt, secondary inorganic aerosols, SO₄^2-, NO₃^-, NH₄⁺, secondary organic aerosols, PM_2.5, and PM₁₀ during a 3-month exposure window. RTIs were defined by hospitalization for RTIs. Incidence rate ratios (IRRs) and 95% CIs were estimated comparing highest with lowest decile of exposure using conditional logistic regression models. Results: In total, 188 439 incident cases of RTI were identified. Exposure to most air pollutants was positively associated with risk of RTI. For example, NO₂ showed an IRR of 1.52 (CI: 1.48–1.55), and PM_2.5 showed an IRR of 1.45 (CI: 1.40–1.50). In contrast, exposure to sea salt, PM₁₀, NH₃, and O₃ was negatively associated with a risk of RTIs. Discussion: In this nationwide study comprising adults, exposure to air pollution was associated with risk of RTIs and subgroups hereof. Sea salt, PM₁₀, NH₃, and O₃ may be proxies for rural areas, as the levels of these species in Denmark are higher near the western coastlines and/or in rural areas with fewer combustion sources.