Concerns about environmental contamination began long ago. As we saw in Chapter 1, Benjamin Franklin and other residents of 18th-century Philadelphia tried to reduce water pollution by tanneries (see page 15). However, the sources of environmental pollution have grown far beyond what occurred in centuries past. While each form of pollution has some unique characteristics and impacts human health or the environment, pollutants can be categorized according to a few overlapping features, such as how they are released into the environment.

Pollutants are released into the environment through one of two avenues: from point sources or from nonpoint sources (Figure 13.2). The smokestack of a factory or a sewer pipe discharging pollutants into the environment from a clearly definable place is called a point source of pollution. Nonpoint sources of pollution introduce pollutants from scattered locations or may move around in the environment. Examples of nonpoint sources include automobile exhaust or polluted water draining from city streets or pesticide-laden farmland. In general, it is much easier to identify and manage pollution from point sources than from nonpoint sources.

389

Some pollutants break down rapidly when released into the environment, whereas others persist for months, years, centuries, or millennia. Bacteria and fungi will quickly metabolize simple organic compounds, such as sugars and amino acids. More complex organic compounds, such as the cellulose component of wood, though biodegradable (see Chapter 12, page 366), will decompose much more slowly. Decomposition of such compounds will occur over a period of months or years, depending on environmental conditions, such as oxygen concentration or temperature. In contrast, atmospheric scientists estimate that some fraction of the carbon dioxide released by fossil fuel combustion will remain in the atmosphere for thousands of years before being taken up by natural processes, such as photosynthesis. Meanwhile, heavy metal pollutants do not break down; they simply persist.

Substances that are harmful when released into the environment are called primary pollutants. Primary pollutants in air include carbon monoxide, lead, nitrogen oxides, particulate matter, and sulfur dioxide. Meanwhile, some primary air pollutants react chemically to form secondary pollutants. For example, ozone (O₃) is formed through the reaction of oxides of nitrogen (NO_x) with volatile organic compounds, or VOCs—such as those in the fumes from gasoline, paints, and other substances rich in organic chemicals. Note that while ozone in the upper atmosphere acts as a protective shield for the biosphere, ozone in the lower atmosphere is a serious pollutant that can damage both animal and plant tissues.

Another notable example of the formation of a secondary pollutant occurs when SO₂ and NO_x undergo reactions in the atmosphere that produce strong acids: SO₂ reacts to form sulfuric acid and NO_x reacts to form nitric acid (Figure 13.3). An acid is a substance that releases hydrogen ions (H⁺) upon dissociation when dissolved in water, resulting in reduced pH, an indicator of relative hydrogen ion concentration, of the solution. A pH of 7 indicates a neutral solution, while a pH of less than 7 is acidic (elevated hydrogen ion concentration), and a pH greater than 7 is alkaline (reduced hydrogen ion concentration). An important feature of the pH scale is that the units are base-10 logarithms, which means that a pH difference of 1, for example, pH = 6 versus pH = 7, indicates a 10-fold difference in hydrogen ion concentration. One of the major environmental consequences of the formation of these secondary pollutants is acid rain, which can have major impacts on soils and aquatic ecosystems (discussed later).