In this era of increasing interest about environmental contributors to illness, primary care providers may be asked by patients for guidance about air pollution. Some components of outdoor air pollution have improved in the past 30 years, but locally hazardous conditions continue to occur, especially for vulnerable subpopulations. Such groups include people with chronic cardiac and respiratory conditions, the elderly, pregnant women, and newborns. This overview discusses the sources and health effects of the primary pollutant groups-ozone, sulfur dioxide (SO2), nitrogen oxides, carbon monoxide (CO), particulates, and other air toxics. A number of Web links are provided to help clinicians and patients get up-to-date information about pollution in the areas where they live. This section concludes with a discussion of treatment and of behavioral and societal approaches to air pollution.