Sensitization to indoor allergens has long been recognized as a risk factor for asthma and its associated phenotypes. Immunoglobulin E antibody quantification may be used to confirm that a particular sensitization has a role in the development of respiratory symptoms. However, in order to use quantitative information effectively, it is essential to understand the factors that contribute to the deterioration of symptoms and cause exacerbations. One such risk factor is high allergen exposure. Worsening asthma control is also associated with virus infections, in particular rhinovirus infection. Each of these factors in isolation is unlikely to have a major effect on symptoms. However, the combination of respiratory virus infection, allergen sensitization and concurrent high exposure to allergens markedly increases the risk for hospitalization. Further investigation of this synergism could help to understand the mechanisms of asthma exacerbations and lead to the development of strategies for prevention.