Air pollution over Beijing
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Introduction to Air Pollution

Air Pollution, the contamination of the air by impurities. These impurities, called pollutants, can occur naturally or they can be produced by human activities. Naturally occurring pollutants include dust, pollen, salt particles, smoke from forest fires, and gases from decaying organisms. Most pollution caused by human activities is directly or indirectly the result of burning of fuels in furnaces or engines. This article is primarily concerned with air pollution caused by human activities.

A serious consequence of air pollution is its impact on human health. Air pollution can contribute to respiratory diseases such as chronic bronchitis, asthma, emphysema, and lung cancer. Air pollution also harms or destroys plant and animal life, and damages property.

Types of Air Pollutants

The most significant pollutants include oxides of carbon, sulfur, and nitrogen; hydrocarbons; particulate matter; and chlorofluorocarbons.

By the mid-1980's, scientific studies had shown that the air inside homes and office buildings is often more polluted than the air outdoors. Air pollutants originating indoors include oxides of carbon and nitrogen, particulate matter, radon, and vapors from such household chemicals as insecticides and paint strippers. The construction of well-sealed buildings to lower heating and cooling costs has contributed to the problem of indoor air pollution.

Carbon oxides make up the largest single group of pollutants. Carbon monoxide, a colorless and odorless poison, is a gas produced when fuel is incompletely burned in engines, furnaces, and space heaters. Automobiles and other vehicles are the major source of this pollutant. Another gas, carbon dioxide, is a product of all normal combustion (burning). Although carbon dioxide is not a serious pollutant in itself, some scientists believe that a long-term buildup of this gas in the atmosphere could cause what they call a “greenhouse effect” by reducing the flow of heat from the earth back into space, thus causing a potentially dangerous warming of the earth.

Sulfur oxides—notably sulfur dioxide—are among the most dangerous and irritating of all air pollutants. Factories and electric power plants that use sulfur-containing coal or oil as fuel are major sources of sulfur oxides. In the air, some sulfur dioxide is converted to sulfuric acid, which is then deposited on the earth's surface, usually by rain. Such acid rain has caused serious damage to the environment in many parts of the world.

Nitrogen oxides are produced in automobile engines and other devices where combustion takes place. Indoors, gas ranges are a major source of nitrogen oxides. Chemical reactions between nitrogen oxides and hydrocarbon pollutants in sunlight produce ozone, a major constituent of an irritating mixture of pollutants and fog known as smog. Some nitrogen oxides in the air are converted to nitric acid, which contributes to the formation of acid rain.

Hydrocarbon pollutants are products of unburned fuel, and are emitted mostly by motor vehicles. Like nitrogen oxides, hydrocarbons contribute to smog.

Particulate matter consists of tiny liquid or solid particles in the air. Particulate matter includes smoke, dust, and soot, which may contain such toxic substances as asbestos, fluorides, lead, and mercury. Indoors, tobacco smoke is a major source of particulate matter.

Chlorofluorocarbons (CFC's) are chemically stable synthetic substances that have had a variety of industrial uses. Some scientists believe that CFC's, like carbon dioxide, can contribute to a “greenhouse effect” resulting in a warming of the earth. Some scientists also believe that CFC's are destroying ozone that absorbs potentially harmful ultraviolet radiation from the sun.

Radon is a naturally occurring radioactive gas found in varying concentrations in the ground. The decay products of radon are also radioactive; if they are inhaled and become attached to the lining of the lungs, they can cause cancer. Radon enters buildings from the ground; in areas where the soil contains high concentrations of radon, dangerous levels of the gas may accumulate in a building without adequate ventilation.

Reducing Air Pollution

Although most of the world's cities and industrial regions have been plagued with air pollution for centuries, there was little organized effort to combat the problem until the mid-20th century. In the United States, many cities and all of the states began to set legal standards regarding the amount and types of pollutants that were permitted. The federal government in 1970 established the Environmental Protection Agency (EPA), whose duties include administering the Clean Air Act. This act provides for financial assistance to state pollution-control agencies and sets strict standards for automotive emissions. The EPA sets general standards for air quality and operates an air monitoring network. Pollution levels are reported to the public through the media by means of the Pollution Standards Index. The index indicates pollution levels as numbers on a scale of 0 to 500; values above 100 represent levels known to cause adverse health effects.

Many devices and systems have been developed to reduce or prevent industrial air pollution. Electrostatic precipitators, for example, remove pollutant particles by ionizing them (charging them electrically) and collecting them on electrodes that are oppositely charged. Cyclone separators rotate impure air with a force that hurls particles against the side walls of the separator. In scrubbers, air is passed through water sprays that remove impurities.

Federal regulations for restricting automotive air pollution require the installation of various antipollution devices on automobiles and trucks at the time of manufacture. The most common of these is the catalytic converter.

In the home, most air-conditioning systems contain mechanical filters or electrostatic precipitators to remove dust, smoke, dander, and other particles from indoor air. Mechanical filters and electrostatic precipitators are also used in forced-air furnaces and in portable air cleaners, or air purifiers, which are designed to remove such particles from a relatively small area.