Pollution
The Gale Encyclopedia of Science, 2014 Updated: February 09, 2015
Pollution can be defined as unwanted or detrimental changes in a natural system. Usually, pollution is associated with the presence of toxic substances in some large quantity, but pollution can also be caused by the presence of excess quantities of heat or by excessive fertilization with nutrients. 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 the 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 lightning-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 or visual blight from intrusive billboards. As can be seen, 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. Pollution also need not be visible to be dangerous. Multiple studies now show that small particulate air pollution, composed of pollutants less than 10 microns in diameter, is a contributing cause of early death among those suffering from lung and heart disease.

Point/nonpoint sources
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
Air pollution
Air pollution is of special concern to the human population. 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. 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 humans), 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 U.S. cities.
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 the 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.