The reality of outdoor air pollution is more than the words “ambient air pollution” can convey (see 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 dust of a heavy dust storm, the smoke coming 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 multitudes living in less-developed countries, who are exposed to these and more; see Table 5.1.

In Chapter 5, Section I examines six major air pollutants, which along with volatile organic chemicals (VOCs) account for 98% of US air pollution and similar percentages worldwide. Section II introduces the hazardous air pollutants (HAPs), also called toxic air pollutants. Section III describes massive pollution that can be directly observed or detected from space while sometimes wreaking havoc at ground level – traveling combustion pollutants, major dust storms, and smoke from mammoth fires. Section IV briefly surveys pollution in less-developed countries.