Visits to the Emergency Room for Asthma Attacks and Short-Term Variations in Air Pollution

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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, O₃) 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 O₃ levels, on the day of admission and also on D –2 and D –3. For an increase of 10 µg/m³, 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.