Each year over a billion metric tons of pollutants enter the atmosphere as a result of human activity. Primary pollutants, which are released directly into the air, include particulate matter (small particles of solid or liquid substances), sulfur oxides, carbon monoxide, nitrogen oxides, and hydrocarbons. These substances make us vulnerable to a range of health problems from minor discomfort to life-threatening disease.

Nearly all industrial processes, as well as the burning of fossil fuels, release particulate matter into the atmosphere--much of it visible as smoke, soot, or dust. Included with the particulates are such materials as airborne asbestos particles and small particles of heavy metals, such as arsenic, copper, lead, and zinc. Among the most significant of the fine particulate pollutants are sulfates and nitrates, which are mainly secondary pollutants; that is, they are produced in the atmosphere through chemical reactions between the primary pollutants--sulfur oxides and nitrogen oxides--and normal atmospheric constituents.

Particulates that enter the lungs may lodge there and have chronic effects on respiration. Children, with their small lungs and relatively rapid respiration, are the most vulnerable to damage--particularly from the effects of lead. Studies reveal that breathing of lead fumes leads to hyperactivity, stunted growth, and anemia. Of greatest concern is the growing body of evidence that suggests that lead exposure can also impair cognitive activity and cause mental retardation. Different studies indicate that the other metals found in air may contribute to the development of lung disease.

Sulfur dioxide (SO₂) is a colorless, odorless gas, under normal conditions found at the earth's surface. Once emitted into the air through the burning of fossil fuels (mostly coal), it may be converted to fine particulate sulfate (SO₄). Sulfur dioxide, particularly in the sulfate form, can cause severe respiratory problems, including chronic bronchitis.

Another colorless, odorless gas is carbon monoxide (CO), a by-product of automobile combustion, certain industrial operations, and cigarette smoke. Hemoglobin in our blood will take up carbon monoxide more than 200 times more rapidly than it will oxygen. Therefore, when a person breathes in air containing high levels of CO, slowly the sites on the hemoglobin molecules that should carry oxygen are taken over by CO, and the individual suffers increasing respiratory insufficiency, general tissue hypoxia, and--in the extreme--death. Additional evidence suggests that long-term low exposure to CO may be involved in depressed neurological response and cardiac disease.

Burning of petroleum and coal releases another group of molecules into the air that we breathe: nitrogen oxides. Scientists have evidence that nitrogen dioxide is capable of causing serious lung impairment in people with asthma. There are other consequences of burning fossil fuels. In addition to the release of nitrogen oxides, a variety of hydrocarbons are added to the air each day. Power plants and automobiles are the two principal sources of the hydrocarbons that may end up in our lungs. This category of compounds, in the presence of sunlight, is capable of combining with nitrogen oxides to produce ozone and photochemical smog.

A colorless, unstable gas with a slightly sweet odor, ozone (O₃) forms a natural layer high in the earth's atmosphere (stratosphere), where it protects us form the harmful ultraviolet light of the sun. At the lower atmospheric levels where we live and breathe, ozone can irritate respiratory passageways and produce an inflammatory response. Overt symptoms range from coughing and chest pain to reduced vital capacity. Some studies even suggest that the bronchial tree may develop scar tissue as a result of exposure to ozone.

In nearly half the cities of the world, tailpipe exhaust is by far the single largest source of air pollution--surpassing wood fires, coal-burning power plants, and chemical manufacturing. In Mexico City, the pollution from the city's 3.2 million cars is so pervasive that young children reach for gray crayons when asked to draw the sky. The city is at an elevation of about 7400 feet in a natural basin surrounded by mountains, but the urban air is so smoggy, the mountains can hardly be seen. In the countries of the former Eastern Bloc, auto exhaust has already replaced much of the industrial smog of the Communist days. In China, as the recent surge of cars has pushed bikes off the streets, health costs are starting to take the their toll. If China attains its dream of one car for every family, the resulting tailpipe emissions could increase carbon concentrations in the air to an extent that would affect the entire world.

Numerous efforts to control air pollution--whatever the source--have been undertaken in recent years. These methods vary from settling chambers for particulates to catalytic converters to remove carbon monoxide and hydrocarbons from automobile exhaust to scrubbers that use lime to remove sulfur before it enters the atmosphere. Although such efforts have achieved a measure of success, urban air quality is still achieved a measure of success, urban air quality is still very poor a good deal of the year in many areas of the United States and other countries throughout the world. For countless numbers of people--whether they are aware of it or not--this air pollution is undermining good health and the quality of their lives.

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