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.