Epidemiologic and controlled exposure studies of human volunteers have shown that exposure to a variety of pollutants induces asthma exacerbations. Interestingly, in the case of ozone, recent evidence suggests that this pollutant acts to enhance the effect of inhaled allergen in persons with asthma. These and other data also suggest that pollutants may influence lung function in persons with asthma by increasing airway inflammation. The interaction of pollutants and inhaled allergens and the effect of pollutant exposure on baseline airway inflammation may be a key mechanism of pollutant-induced exacerbation of asthma. Further study of this interaction, as well as interactions of multiple pollutants, will be crucial for rational development of intervention and regulatory strategies.