Long-term exposure to air pollution has been linked with adult-onset asthma and chronic obstructive pulmonary diseases (COPD), but the literature is still mixed, and the definition of the incidence of these outcomes differs between studies.

We explored associations of long-term exposure to air pollution with asthma and COPD incidence in adults and a combined outcome defined by medication use for obstructive airway disease.

We followed all Danish residents aged ? 30 years (3,074,599) for first-ever asthma or COPD hospital contact and the redeemed prescription of medication for obstructive airway diseases from 2000 until 2018. We assessed annual mean levels of fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and black carbon (BC) at baseline residential addresses with hybrid land-use regression models. We estimated associations by Cox proportional hazard models, adjusting for potential confounders.

During 15.6 years of mean follow-up, 52,648 and 146,269 individuals were identified with incident asthma and COPD, respectively, and 419,203 individuals redeemed the medication. A 5 µg/m³, 10 µg/m³, and 0.5×10^-5/m increase in PM_2.5, NO₂, and BC, respectively, were associated with higher risks of asthma incidence [hazard ratio: 1.30 (95% confidence interval: 1.23-1.38); 1.17 (1.14-1.19); 1.18 (1.15-1.21)], COPD incidence [1.11 (1.07-1.16); 1.06 (1.05-1.08); 1.08 (1.06-1.09)], and medication use [1.04, (1.01-1.06); 1.05 (1.04-1.06); 1.03 (1.02-1.05)].

In this nationwide analysis, we found strong associations between long-term exposure to air pollution and asthma and COPD incidence defined by hospital contact and weaker for the combined outcome defined by medication use.