The question of what constitutes air pollution should begin with a definition of what constitutes clean air. Even clean air contains many substances other than oxygen and nitrogen, its major constituents, including rare gases such as argon, neon, and helium, as well as ozone, carbon dioxide, radioactive materials from the earth, and various nitrogen and sulfur compounds. The air would also contain variable amounts of water vapor and many suspended solid particles and liquid substances. Suspended materials known as aerosols are defined as dust particles and condensation nuclei, the latter consisting of chloride salts, sulfuric and nitrous acids, phosphorous compounds, and many other chemical substances. These nuclei have an affinity for water and play an important role in the transformation of water vapor into fog, clouds, and precipitation (Miller & Miller, 1989, p. 4).
Some air pollution is natural, deriving from such events as volcanic eruptions, fires, and carbon dioxide eruptions in the form of methane gas. Air pollution consists of the nuclei noted above, which outnumber the number of dust particles in the air many times over, and because of their smaller size they are easily transported by air currents to higher altitudes and greater distances than are dust particles (Miller and Miller, 1989, 4). Natural pollutants may be limited to small areas or may be distributed worldwide on rare occasions (Miller & Miller, 1989, p. 6). When we refer to air pollution, we usually mean air pollutants resulting from human activity and thus something we believe we can control. In the urban and industrial areas of the world, the atmosphere contains a wide variety of pollutants that are caused by human activities, such as chemicals emitted directly into the atmosphere from identifiable sources, and others formed indirectly through photochemical reactions in the air (Miller & Miller, 1989, p. 11). Air pollution can occur indoors as well...