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The term pollution is derived from the Latin pollutus, which means to be made foul, unclean, or dirty. Anything that corrupts, degrades, or makes something less valuable or desirable can be considered pollution. There is, however, a good deal of ambiguity and contention about what constitutes a pollutant. Many reserve the term for harmful physical changes in our environment caused by human actions. Others argue that any unpleasant or unwanted environmental changes whether natural or human-caused constitute pollution. This broad definition could include smoke from lightening-ignited forest fires, ash and toxic fumes from volcanoes, or bad-tasting algae growing naturally in a lake. Some people include social issues in their definition of pollution, such as noise from a freeway, visual blight from intrusive billboards, or cultural pollution when the worst aspects of modern society invade a traditional culture. As you can see, these definitions depend on the observer's perspective. What is considered unwanted change by one person might seem like a welcome progress to someone else. A chemical that is toxic to one organism can be an key nutrient for another.

The seven types of air pollution considered the greatest threat to human health in the United States, and the first regulated by the 1970 United States Clean Air Act, include sulfur dioxide, particulates (dust, smoke, etc.), carbon monoxide, volatile organic compounds, nitrogen oxides, ozone, and lead. In 1990, another 189 volatile chemical compounds from more than 250 sources were added to the list of regulated air pollutants in the United States. Air contaminants are divided into two broad categories: primary pollutants are those released directly into the air. Some examples include dust, smoke, and a variety of toxic chemicals such as lead, mercury, vinyl chloride, and carbon monoxide. In contrast, secondary pollutants are created or modified into a deleterious form after being released into the air.

A variety of chemical or photochemical reactions (catalyzed by light) produce a toxic mix of secondary pollutants in urban air. A prime example is the formation of ozone in urban smog. A complex series of chemical reactions involving volatile organic compounds, nitrogen oxides, sunlight, and molecular oxygen create highly reactive ozone molecules containing three oxygen atoms. Stratospheric ozone in the upper atmosphere provides an important shield against harmful ultraviolet radiation in sunlight. Stratospheric ozone depletion—destruction by chlorofluorocarbons (CFCs) and other anthropogenic (human-generated) chemicals—is of great concern because it exposes living organisms to dangerous ultraviolet radiation. Ozone in ambient air (that surrounding us), on the other hand, is highly damaging to both living organisms and building materials. Recent regulations that have reduced releases of smog-forming ozone in ambient air have significantly improved air quality in many American cities.

Among the most important types ofwater pollution are sediment, infectious agents, toxins, oxygen-demanding wastes, plant nutrients, and thermal changes. Sediment (dirt, soil, insoluble solids) and trash make up the largest volume and most visible type of water pollution in most rivers and lakes. Worldwide, erosion from crop-lands, forests, grazing lands, and construction sites is estimated to add some 75 billion tons of sediment each year to rivers and lakes. This sediment smothers gravel beds in which fish lay their eggs. It fills lakes and reservoirs, obstructs shipping channels, clogs hydroelectric turbines, and makes drinking water purification more costly. Piles of plastic waste, oil slicks, tar blobs, and other flotsam and jetsam of modern society now defile even the most remote ocean beaches.

Pollution control regulations usually distinguish between point and nonpoint sources. Factory smoke stacks, sewage outfalls, leaking underground mines, and burning dumps, for example, are point sources that release contaminants from individual, easily identifiable sources that are relatively easy to monitor and regulate. In contrast, nonpoint pollution sources are scattered or diffuse, having no specific location where they originate or discharge into our air or water. Some nonpoint sources include automobile exhaust, runoff from farm fields, urban streets, lawns, and construction sites. Whereas point sources often are fairly uniform and predictable, nonpoint runoff often is highly irregular. The first heavy rainfall after a dry period may flush high concentrations of oil, gasoline, rubber, and trash off city streets, for instance. The irregular timing of these events, as well as their multiple sources, variable location, and lack of specific ownership make them much more difficult to monitor, regulate, and treat than point sources.

In recent years, the United States and most of the more developed countries have made encouraging progress in air and water pollution control. While urban air and water quality anywhere in the world rarely matches that of pristine wilderness areas, pollution levels in most of the more prosperous regions of North America, Western Europe, Japan, Australia, and New Zealand have generally been dramatically reduced. In the United States, for example, the number of days on which urban air is considered hazardous in the largest cities has decreased 93% over the past 20 years. Of the 97 metropolitan areas that failed to meet clean air standards in the 1980s, nearly half had reached compliance by the early 1990s. Perhaps the most striking success in controlling air pollution is airborne lead. Banning of leaded gasoline in the United States in 1970 resulted in a 98% decrease in atmospheric concentrations of this toxic metal. Similarly, particulate materials have decreased in urban air nearly 80% since the passage of the U.S. Clean Air Act, while sulfur dioxides, carbon monoxide, and ozone are down by nearly one-third.

Unfortunately, the situation often is not so encouraging in other countries. The major metropolitan areas of developing countries often have appalling levels of air pollution, which rapid population growth, unregulated industrialization, lack of enforcement, and corrupt national and local politics only make worse. Mexico City, for example, is notorious for bad air. Pollution levels exceed World Health Organization (WHO) standards 350 days per year. More than half of all children in the city have lead levels in their blood sufficient to lower intelligence and retard development. The 130,000 industries and 2.5 million motor vehicles spew out more than 5,500 metric tons of air pollutants every day, which are trapped by mountains ringing the city.

Although the United States has not yet met its national goal of making all surface waters "fishable and swimmable," investments in sewage treatment, regulation of toxic waste disposal and factory effluents and other forms of pollution control have resulted in significant water quality increases in many areas. Nearly 90% of all the river miles and lake acres that are assessed for water quality in the United States fully or partly support their designed uses. Lake Erie, for instance, which was widely described in the 1970s as being "dead," now has much cleaner water and more healthy fish populations than would ever have been thought possible 25 years ago. Unfortunately, surface waters in some developing countries have not experienced similar progress in pollution control. In most developing countries, only a tiny fraction of human wastes are treated before being dumped into rivers, lakes, or the ocean. In consequence, water pollution levels often are appalling. In India, for example, two-thirds of all surface waters are considered dangerous to human health. Hopefully, as development occurs, these countries will be able to take advantage of pollution control equipment and knowledge already available in already developed countries.

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