Clinical Epidemiology Unit

Case crossover study of air pollution and asthma

A/Prof Michael Abramson
A/Prof Andrew Forbes

Funded by: Department of Human Services

The aims of the study were to estimate the association between ambient particulate or ozone levels and the risk of death or Emergency Department (ED) presentation for asthma. A case crossover study was conducted utilising 59 confirmed asthma deaths and 315 patients who presented for emergency treatment of acute asthma between 1994 and 1996. Daily maximum levels of the airborne particle index (API) and ozone were obtained for the same period from a network of monitoring stations operated by the Environment Protection Authority.

The case crossover analysis was conducted for a hazard period 1 day and a reference period 28 days before death or ED presentation. Confounders fitted to the conditional logistic regression models included daily temperature, humidity and asthma medications used by subjects. The adjusted odds ratios (OR) showed no significant relationship between API and asthma death or ED presentation. Asthma deaths were not associated with maximum daily 03 concentrations either. However the adjusted ORs suggested a possible protective effect of medium 03 levels upon ED presentations by men with asthma. This may be due to NO2 which was not included in the analysis, but has a well described inverse relationship with 03.

The generally negative results could be due to the levels of air pollution in Melbourne being too low to cause adverse health effects in people with asthma. However we think the most likely explanation is that the study had insufficient statistical power, in particular for the examination of asthma deaths. It is also possible that the results were biased by unmeasured time varying confounders such as activity patterns. Further research with personal exposure measurement is required, but we have demonstrated that the case crossover approach is feasible.