The reality of outdoor air pollution is more than the words “ambient air pollution” can convey (Box 5.1). It is the eye-stinging pollution surrounding us in a city crowded with motor vehicles, the odor of ozone on a hot hazy day, the choking of a heavy dust storm, the smoke from wood or coal fires on a winter day, the fumes from an uncontrolled industrial facility, odor from uncontrolled sewage or an open dump. Many living in wealthy countries are spared the worst of these. Not so for the many living in less-developed countries, who are increasingly exposed.

In this chapter, Section I examines six principal air pollutants – the criteria (or priority) pollutants. Section II introduces Volatile Organic Chemicals (VOCs), which together with criteria pollutants account for the large majority of air pollution. Section III introduces HAPS (hazardous air pollutants, also called air toxics). HAPS are not ordinarily as pervasive as criteria pollutants, but can be very important in specific locales. Section IV describes massive movements of criteria air pollutants that can be directly observed from space – traveling combustion pollutants, major dust storms, and smoke from massive fires. Section V briefly examines pollution in less-developed countries.