Occup Environ Med 67:422-427 doi:10.1136/oem.2009.048702
  • Original article

An approach for estimating the health effects of changes over time in air pollution: an illustration using cardio-respiratory hospital admissions in London

Open Access
  1. Ben Armstrong1
  1. 1London School of Hygiene and Tropical Medicine, London, UK
  2. 2Environmental Research Group, King's College London, London, UK
  3. 3Small Area Health Statistics Unit, Imperial College London, London, UK
  4. 4MRC-HPA Centre for Environment and Health, Imperial College London, London, UK
  1. Correspondence to Cathryn Tonne, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK; cathryn.tonne{at}
  • Accepted 19 October 2009
  • Published Online First 12 November 2009


Objectives First, we present a general analytical approach to estimating the association between medium-term changes in air pollution and health across small areas. As a specific illustration, we then applied the approach to data on London residents from a 4-year period to test whether reductions in traffic-related air pollution were associated with reductions in cardio-respiratory hospital admissions.

Methods A binomial distribution was used to model change in admissions over time in each small area, which was measured as the proportion of admissions in 2003–2004 out of admissions over all study years (2001–2004). Annual average concentrations of nitrogen oxides (NOx) were modelled using an emissions-dispersion model. The association between change in NOx and change in hospital admissions was estimated using logistic regression and an instrumental variable approach.

Results For some diagnostic groups, suggestive associations between reductions in NOx and reductions in admissions were observed, for example, OR=0.97 (95% CI 0.96 to 0.99) for an IQR decrease in NOx (3 μg/m3) and all respiratory admissions. Accounting for spatial dependence attenuated several of the associations; for respiratory admissions, the OR was 1.00 (95% CI 0.98 to 1.02), leaving only that for bronchiolitis significant (OR=0.91; 95% CI 0.84 to 0.99). In this particular illustration, the instrumental variable approach did not appear to add information.

Conclusions In this illustration, there was relatively limited power to detect an association between changes in air pollution and hospital admissions over time. However, the analytical approach could deliver more robust estimates of the health effects of changes in air pollution in settings with greater spatial contrast in changes in air pollution over time.


  • Funding This work was supported by the U.S. National Institute of Environmental Health Sciences (F32ES015432), and the Health Effects Institute.

  • Competing interests None.

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

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited, the use is non commercial and is otherwise in compliance with the license. See: and