Exposure to environmental allergens has been analyzed in numerous studies over the past few decades. It seems likely that the level of exposure to allergens such as house-dust mites and cat dander affects a person’s risk for the development of IgE antibodies against these allergens.73 There is, however, more recent evidence to suggest that exposure to house-dust mites early in life is unlikely to be an important risk factor for the onset of asthma. Several prospective birth-cohort studies have found no association between increased levels of exposure to house-dust mites early in life and various phenotypes of childhood asthma. Furthermore, intervention studies have failed to find convincing evidence of a reduction in the risk of asthma among children 1 to 8 years of age after the implementation of allergen- and food-avoidance strategies.Some studies have found that exposure to pets early in life actually decreases the risk of asthma, but consistency among studies is lacking.Indoor exposure to allergens may, however, contribute to the persistence of symptoms among children with allergic asthma.