Visits to the emergency room for asthma attacks and short-term variations in air pollution - A case-crossover study

Background: The 'case-crossover' design is a strategy fitted to studying transient effects of intermittent exposure on acute-onset disease occurring shortly after exposure. It has the important advantage of eliminating most confounding variables, allowing a stronger causality inference. Objectives: The objective of the study was to evaluate the relationships between emergency room ( ER) visits for asthma attacks and gaseous air pollution changes. Methods: The study included 549 individuals, 3 - 49 years old, visiting the ER during 1 year. Exposure to gaseous air pollutants (SO2, NO2, O-3) on the same day and up to 4 days before was computed according to the patient's address. The statistical analysis included meteorological data as potential confounding variables. Results: No association could be shown between ER visits and SO2 or NO2 levels. In contrast, there was a statistically significant association between ER visits and mean O-3 levels, on the day of admission and also on D - 2 and D - 3. For an increase of 10 mug/m(3), the risk of requiring an ER admission increased by 6 - 10%. Conclusions: Using this new strategy, we confirmed that ozone changes lead to a moderate increase in risk of requiring an ER admission in asthmatic subjects. Copyright (C) 2004 S. Karger AG, Basel.