Outdoor air pollution and environmental tobacco smoke adversely affect health in persons with asthma. Increased levels of ozone, particulate matter, and environmental tobacco smoke have been associated with increased asthma symptoms and health care use and with reduced lung function. These air contaminants have proinflammatory actions that can magnify existing lower airway inflammation in patients with asthma. Exposure to air contaminants can increase the risk of developing asthma in susceptible persons. Outdoor air pollution and environmental tobacco smoke may affect allergen-induced inflammation by initiating TH(2) responses to antigens or by exacerbating such inflammation in persons already sensitized.