Background Exposure to air pollution in early life contributes to the burden of childhood asthma, but it is not clear whether long-term exposure to air pollution can lead to asthma onset or progression in adulthood.

Objectives The authors studied the effect of exposure to traffic-related air pollution over 35 years on the risk for asthma hospitalisation in older people.

Methods 57 053 participants in the Danish Diet, Cancer and Health cohort, aged 50–65 years at baseline (1993–1997), were followed up for first hospital admission for asthma until 2006, and the annual nitrogen dioxide (NO₂) levels were estimated as a proxy of the exposure to traffic-related air pollution at the residential addresses of the participants since 1971. The association between NO₂ and hospitalisation for asthma was modelled using Cox regression, for the full cohort and in people with and without previous hospitalisations for asthma, and the effect modification by comorbid conditions was assessed.

Results During 10.2 years' median follow-up, 977 (1.9%) of 53 695 eligible people were admitted to hospital for asthma: 821 were first-ever admissions and 176 were readmissions. NO₂ levels were associated with risk for asthma hospitalisation in the full cohort (HR and 95% CI per IQR, 5.8 μg/m³: 1.12; 1.04–1.22), and for first-ever admissions (1.10; 1.01–1.20), with the highest risk in people with a history of asthma (1.41; 1.15–2.07) or chronic obstructive pulmonary disease (COPD) (1.30; 1.07–1.52) hospitalisation.

Conclusions Long-term exposure to traffic-related air pollution increases the risk for asthma hospitalisation in older people. People with previous asthma or COPD hospitalisations are most susceptible.