Asthma is a growing problem, and outdoor allergens play a role in exacerbation of many cases. A clearer understanding of this role and its magnitude, and a means of controlling the effects of outdoor allergen exposures is needed. We propose Poisson time-series and conditional panel studies to test these hypotheses: 1) The incidence of acute asthma attacks, as measured by urgent care inhalation treatments and hospitalizations for asthma, has a dose-dependent relationship with exposure to specific outdoor allergens; 2) Specific sensitization to outdoor allergens is a risk factor for having an acute attack; and 3) Exposure conditions that lead to acute asthma attacks can be forecast, creating an opportunity to reduce asthma morbidity and mortality ny targeting pretreatment and/or exposure controls.