Short-term effects of air pollution on daily asthma emergency room admissions

Many time-series studies have shown positive associations between air pollutants and asthma morbidity. However, few studies have included pollen as a potential confounder when examining this relationship. This study analysed the short-term association between air pollutants (sulphur dioxide (SO2), particles measured with a median aerodynamic diameter of <10 microm (PM10), nitrogen dioxide (NO2) and ozone (O3)) and asthma emergency room admissions in Madrid, Spain, in 1995-1998, adjusting for four types of pollen with allergenic potential (Olea europaea, Plantago sp., Poaceae and Urticaceae). Data were analysed using autoregressive Poisson regression and generalised additive models (GAM). The strongest associations were observed at 1 day lag for O3, and 3 days lag for the remaining pollutants. Using Poisson regression, a single-pollutant model showed that a 10-microg x m(-3) rise in pollutant level led to relative risks of: 1.039 for PM10; 1.029 for SO2; 1.033 for NO2; and 1.045 for O3. Adjustment for the different types of pollen led to no substantial variation in these associations. In the multipollutant models for cold-season pollutants (including PM10, SO2 and the four types of pollen) and photochemical pollutants (including NO2, O3 and the four types of pollen) the associations for PM10, NO2 and O3 held, but no relationship with SO2 was evident. GAM analysis yielded the same results, both in terms of lags and of quantification of the effect for all pollutants. In conclusion, the usual air pollution levels in Madrid were associated with an increase in asthma emergency room admissions, and this association remained controlling for the presence of ambient pollen.