The Expert Panel Report 2. Guidelines for the Diagnosis and Management of Asthma (1) begins its section on controlling factors that precipitate or worsen asthma with the statement: "For successful long-term asthma management, it is essential to identify and reduce exposures to relevant allergens and irritants and to control other factors that have been shown to increase asthma symptoms and/or precipitate asthma exacerbations." The presence of allergy to indoor allergens and certain seasonal fungal spores has been found to be a risk factor for asthma in epidemiologic studies around the world. Generally between 70% and 85% of asthmatic populations studied have been reported to have positive skin-prick tests. Exposure of allergic patients to inhalant allergens increases airway inflammation, airway hyper-responsiveness, asthma symptoms, need for medication, severe attacks, and even death due to asthma. Environmental tobacco smoke exposure has been shown to increase the prevalence of childhood asthma and to increase asthma symptoms and bronchial hyperresponsiveness while reducing pulmonary function in children chronically exposed. Exposure to other indoor irritants, largely products of unvented combustion, has also been found to increase asthma symptoms. Outdoor air pollution increases asthma symptoms; levels of specific pollutants correlate with emergency room visits and hospitalization for asthma. Rhinitis/sinusitis and gastroesophageal reflux are commonly associated with asthma, and treatment of these conditions has been shown to improve asthma. In patients sensitive to aspirin and nonsteroidal anti-inflammatory drugs or metabisulfites, exposure to these agents can precipitate severe attacks of asthma. Viral infections are common causes for exacerbations of asthma. Infections with Mycoplasma pneumoniae and Chlamydia pneumoniae contribute to acute exacerbations and perhaps to long-term morbidity, as well. This chapter will discuss preventive and therapeutic measures that have been found effective in reducing the impact of aggravating or precipitating factors in patients with asthma.