Thorax 67:6-11 doi:10.1136/thoraxjnl-2011-200711
  • Asthma and the environment
  • Original article

Long-term exposure to air pollution and asthma hospitalisations in older adults: a cohort study

  1. Ole Raaschou-Nielsen1
  1. 1Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark
  2. 2Copenhagen Prospective Studies on Asthma in Childhood, Health Sciences, University of Copenhagen, Copenhagen University Hospital, Gentofte, Copenhagen, Denmark
  3. 3National Environmental Research Institute, Aarhus University, Roskilde, Denmark
  4. 4Department of Environmental Health, University of Copenhagen, Copenhagen, Denmark
  5. 5School of Public Health, Aarhus University, Aarhus, Denmark
  6. 6Centre for Cardiovascular Research, Aalborg Hospital, Aarhus University Hospital, Aalborg, Denmark
  1. Correspondence to Dr Zorana J Andersen, Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, Copenhagen 2100, Denmark; zorana{at}
  • Received 21 January 2011
  • Accepted 19 July 2011
  • Published Online First 2 September 2011


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.


  • See Editorial, p 2

  • Funding The study was funded by the Danish Research Council and Danish Cancer Society.

  • Competing interests None.

  • Patient consent Obtained.

  • Ethics approval Relevant Danish ethical committees and data protection agencies approved the study.

  • Provenance and peer review Not commissioned; externally peer reviewed.