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                Air pollution occurs when wastes dirty the air.  People produce most of the wastes that cause air pollution.  Such wastes can be in the form of gases or particulates (particles of solid or liquid matter).  These substances result chiefly from burning fuel to power motor vehicles and to heat buildings.  Industrial processes and the burning of garbage also contribute to air pollution.  Natural pollutants (impurities) include dust, pollen, soil particles, and naturally occurring gases. 

 The rapid growth of population and industry, and the increased use of automobiles and airplanes, have made air pollution a serious problem.  The air we breathe has become so filled with pollutants that it can cause health problems.  Polluted air also harms plants, animals, building materials, and fabrics.  In addition, it causes damage by altering the earth's atmosphere. 

 The damage caused by air pollution costs the people of the United States billions of dollars each year.  This includes money spent for health care and increased maintenance of buildings.  Air pollution also causes damage to the environment that cannot be reversed. 

 Chief sources of air pollution

 People depend on the atmosphere to dilute and remove pollutants as they are produced.  But weather conditions called thermal inversions can trap the pollutants over a certain area until they build up to dangerous levels.  A thermal inversion occurs when a layer of warm air settles over a layer of cool air that lies near the ground.  This condition traps the impurities and prevents them from rising until rain or wind breaks up the layer of stationary warm air. 

 Forms of transportation, such as automobiles, airplanes, ships, and trains, are the leading source of air pollution in the United States, Canada, and most other industrial nations.  The major pollutants produced by these sources are carbon monoxide, carbon dioxide, hydrocarbons (compounds of carbon and hydrogen), and nitrogen oxides (compounds of nitrogen and oxygen).  Nitrogen oxides can react with hydrocarbons in the presence of sunlight to produce a form of oxygen called ozone.  Ozone is the chief component of photochemical smog, which is a common form of air pollution (see SMOG). 


Fuel combustion for heating and cooling homes, office buildings, and factories contributes significantly to air pollution.  Electric power plants that burn coal or oil also release pollutants into the atmosphere.  The major pollutants from these sources are nitrogen oxides, sulfur oxides (compounds of sulfur and oxygen), particulates, and carbon dioxide. 


Industrial processes produce a wide range of pollutants.  Oil refineries discharge ammonia, hydrocarbons, organic acids, and sulfur oxides.  Metal smelting plants give off large amounts of sulfur oxides and particulates containing lead and other metals.  Plants that make aluminum expel fluoride dust.  Plants that produce plastic foams are a major source of chlorofluorocarbons (CFC's), compounds of chlorine, fluorine, and carbon.  By 1996, most industrialized countries, including the United States, had ended production of CFC's. 


Burning of solid wastes often creates a very visible form of air pollution--thick, black smoke.  The burning of garbage, leaves, and other refuse is banned in most parts of the United States and Canada. 

 Other sources of pollution include chemical sprays and organic chemicals used to start fires on charcoal grills.  Forest fires and structural fires also contribute to air pollution.  In rural areas and in developing countries, the burning of forests and grasslands to clear areas for farming is a major source of air pollution. 

 Natural sources also contribute to air pollution.  Volcanoes emit large amounts of sulfur oxides and particulates.  Microbes in the guts of cattle and in rice paddies break down plant materials and release an odorless gas called methane, a type of hydrocarbon. 

 Indoor air pollution occurs when energy-efficient houses and office buildings trap pollutants inside.  As a result, some pollutants found outdoors are found indoors in even higher concentrations.  Some plastic products, processed wood products, paints, and adhesives can give off hydrocarbons.  Many cleaning products emit poisonous gases such as ammonia and chlorine.  An odorless gas called radon is released into the atmosphere from soil and rocks.  It enters buildings through cracks in the foundation (see RADON). 

 Effects of air pollution

 Health.  When people breathe polluted air, the impurities can irritate their air passages and their lungs.  Particulates often remain in the lungs and can worsen such respiratory ailments as asthma and bronchitis.  Radon can cause lung cancer if inhaled in large quantities.  Certain chemical compounds can cause cancer and birth defects.  Ozone can aggravate emphysema and reduces resistance to colds and pneumonia.  In addition, carbon monoxide interferes with the transfer of oxygen from the lungs to body tissues. 


In London in 1952, about 4,000 people died of respiratory diseases during a "killer smog."  More than 600 people died as a result of thermal inversions that occurred in New York City in 1953 and 1963.  Today, such extreme events are rare because of government emission standards, which limit pollutants released by factories and other sources.  However, air pollution still contributes to a large number of deaths each year. 


Agriculture.  Air pollutants can stunt the growth of crops, harm livestock, and destroy crops as well.  Such damage costs the United States millions of dollars each year.  Forests also have been damaged by air pollution. 


Atmosphere.  Some pollutants are not poisonous but can cause damage by altering the earth's atmosphere.  For example, the amount of carbon dioxide in the atmosphere has been increasing since the early to mid-1800's, chiefly as a result of the burning of coal, oil, and other carbon-containing fuels.  Carbon dioxide allows sunlight to reach the earth and warm its surface, but it prevents some surface heat from escaping out of the atmosphere.  This greenhouse effect may produce significant climatic changes, which could destroy many kinds of plants and animals.

 Chlorofluorocarbons break down the layer of ozone in the earth's upper atmosphere.  This layer protects plants and animals from harmful ultraviolet rays (see OZONE). 

 Other effects.  Most materials deteriorate faster when exposed to the pollutants present in the air.  Concrete and stone are dissolved by air pollutants.  Metals corrode faster than usual.  Plastics, rubber, and fabrics are also damaged by air pollutants. 

 Air pollution is closely related to other forms of pollution.  Sulfur dioxide and nitrogen oxides can react with water droplets in the air to produce acid rain.  Acid rain pollutes lakes and streams and, in high concentrations, can harm soil fertility

 Control of air pollution

 In the United States, all levels of government--federal, state, and local--have passed laws designed to control pollution.  Congress passed the Air Quality Act in 1967.  Under this act, the federal government sets goals called air quality standards for achieving cleaner air.  The states must enforce air pollution controls to meet the goals.  When states fail to enforce the regulations, the federal government can act against the polluters by imposing fines.  However, the lack of funding to enforce these regulations has allowed some polluters to continue releasing harmful pollutants for years. 

States may set stricter air standards than the federal government requires.  Since 1970, California has set the strictest standards for motor-vehicle emissions.  Stricter nationwide standards for emissions have been repeatedly postponed because of opposition from automobile industry groups. 

 Pollution from automobiles has been reduced by changes in motor vehicles.  Since 1975, most American-made cars have been equipped with pollution-control devices called catalytic converters.  Devices called scrubbers have been installed in many electric power plants, factories, and incinerators, to remove sulfur oxides and some other pollutants before they reach the air.  Pollution can also be reduced by increasing energy efficiency and burning less fuel.  In addition, recycling reuses some wastes that otherwise might have been burned.   

Efforts to control air pollution in the United States have had some success.  But many urban areas fail to meet federal air quality standards.  Since 1970, emissions of sulfur oxides, hydrocarbons, and carbon monoxide have decreased by 25 to 40 percent.  But emissions of nitrogen oxides have increased slightly.  Emissions of lead have fallen about 96 percent, mostly because lead has been phased out of gasoline. 

 In 1990, the U.S. Congress amended the Clean Air Act of 1970.  The amendments set stricter standards for air quality and emissions, require the sale of cleaner burning fuels, and call for cuts in sulfur dioxide emissions from power plants to reduce acid rain.  In 1989, a plan took effect to help the Los Angeles area meet federal air quality standards by 2007. 

 In other countries.  The lack of controls on automobile emissions in Western Europe has contributed to extensive damage to forests there.  Countries in Eastern Europe have lacked pollution controls altogether and have suffered enormous environmental damage as a result.  Protecting the environment will require international cooperation. 

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