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Air Pollution

Criteria pollutants

Air pollution is the presence of chemicals in the earth's atmosphere that are not a normal part of the atmosphere. In other words, air pollution is contaminated air.

Air contamination is divided into two broad categories: primary and secondary. 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. The exhaust from vehicles and industrial smokestacks are examples of primary pollution.

A "brown cloud" (pollution) shrouds Denver, Colorado. ©Ted Spiegel/Corbis. Reproduced by permission.

Secondary pollutants are created or modified after being released into the atmosphere. In secondary pollution, a compound is released into the air. This compound is then modified into some other form, either by reaction with another chemical present in the air or by a reaction with sunlight (a photochemical reaction). The altered compound is the secondary pollutant. Smog that gathers above many cities is a prime example of secondary air pollution.

Pollution of the atmosphere occurs in the bulk of the atmosphere that is within 40-50 mi (64.4–80.5 km) of Earth's surface. Nitrogen and oxygen make up 99% of the atmosphere; the remaining components are argon, carbon dioxide, neon, helium, methane, krypton, hydrogen, xenon, and ozone. Ozone is concentrated in a band that is 12-30 mi (19–48 km) above Earth's surface.

Smog can be damaging to human health because of the formation of ozone. 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. The ozone that is present higher up in the atmosphere is beneficial. It provides an important shield against harmful ultraviolet radiation in sunlight. Closer to the ground, however, ozone is highly damaging to both living organisms and building materials.

The 1970 Clean Air Act in the United States recognized seven air pollutants as being in immediate need of regulatory monitoring. These pollutants are sulfur dioxide, particulates (such as dust and smoke), carbon monoxide, volatile organic compounds, nitrogen oxides, ozone, and lead. These pollutants were regarded as the greatest danger to human health. Because criteria were established to limit their emission, these materials are sometimes referred to as "criteria pollutants." Major revisions to the Clean Air Act in 1990 added another 189 volatile chemical compounds from more than 250 sources to the list of regulated air pollutants in the United States.

Some major pollutants are not directly poisonous but can harm the environment over a longer period of time. Excess nitrogen from fertilizer use and burning of fossil fuels is causing widespread damage to both aquatic and terrestrial ecosystems on Earth's surface. For example, over-fertilizing of plants favors the growth of weedy species. Pollutants can also damage the atmosphere above Earth's surface. A well-known example of this damage is that caused by chlorofluorocarbons (CFCs). CFCs were used for many years as coolant in refrigerators and as cleaning agents. While generally chemically inert and non-toxic in these settings, CFCs diffuse into the upper atmosphere where they destroy the ultraviolet-absorbing ozone shield. Ozone depletion is a concern for the health of humans, as increased exposure to the sun's ultraviolet radiation can cause genetic damage that is associated with various cancers, especially skin cancer.

Air pollutants can travel surprisingly far and fast. About half of the fine reddish dust visible in Miami's air during the summer is blown across the Atlantic Ocean from the Sahara Desert. Radioactive fallout from an explosion at the Chernobyl nuclear reactor in the Ukraine was detected many miles away in Sweden within two days after its release and spread around the globe in less than a week.

One of the best-known examples of long-range transport of air pollutants is acid rain. The acids of greatest concern in air are sulfuric and nitric acids, which are formed as secondary pollutants from sulfur dioxide and nitrogen oxides released by burning fossil fuels and industrial processes such as smelting ores. These acids can change the pH (a standard measure of the hydrogen ion concentration or acidity) of rain or snow from its normal, near neutral condition to an acidity that is similar to that of lemon juice. Although this acidity is not directly dangerous to humans, it damages building materials and can be lethal to sensitive aquatic organisms such as salamanders, frogs, and fish. Thousands of lakes in eastern Quebec, New England, and Scandinavia have been acidified to the extent that they no longer support game fish populations. Acid precipitation has also been implicated in forest deaths in northern Europe, eastern North America, and other places where air currents carry urban industrial pollutants.

Air pollution control

Because air pollution is visible and undesirable, most developed countries have had 50 years or more of regulations aimed at controlling this form of environmental degradation. In many cases, these regulations have had encouragingly positive effects. While urban air quality rarely matches that of pristine wilderness areas, air pollution in most of the more prosperous regions of North America, Western Europe, Japan, Australia, and New Zealand has been curtailed in recent years. In the United States, for example, the Environmental Protection Agency (EPA) reports that 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 urban 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.

The situation is not as encouraging in some other countries. The major metropolitan areas of developing countries often have highly elevated levels of air pollution. Rapid population growth, unregulated industrialization, local geography, and lack of enforcement have compounded the air pollution problem in cities such as Mexico City. In this city, pollution levels usually 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 more than 5,500 metric tons of air pollutants released in Mexico City each day from the thousands of industries and millions of motor vehicles are trapped close to the surface by the mountains ringing the city.

Most of the developing world megacities (those with populations greater than 10 million people) have similar problems. Air quality in Cairo, Bangkok, Jakarta, Bombay, Calcutta, New Delhi, Shanghai, Beijing, and Sao Paulo regularly reach levels scientists consider dangerous to human, animal, and plant life.



Colls, J. Air Pollution: An Introduction. London: E & F.N. Spon, 1998.


Environmental Protection Agency, Office of Air Quality, Planning and Standards, Information Transfer Group, Mail Code E143–03, Research Triangle Park, NC 27711. <http://www.epa.gov/airnow/> (October 18, 2002).

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720. <http://www.lbl.gov/Education/ELSI/pollution-main.html> (October 28, 2002).

Brian Hoyle


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—All of the organisms in a biological community interacting with the physical environment.


—A naturally occurring trace gas, having the chemical formula O3. In the stratosphere, it serves to absorb many harmful solar UV rays.


—An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.


—Readily able to form a vapor at a relatively low temperature.

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