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Air Pollution

Air Pollution

Air pollution is a phenomenon by which particles (solid or liquid) and gases contaminate the environment. Such contamination can result in health effects on the population, which might be either chronic (arising from long-term exposure), or acute (due to accidents). Other effects of pollution include damage to materials (e.g., the marble statues on the Parthenon are corroded as a result of air pollution in the city of Athens), agricultural damage (such as reduced crop yields and tree growth), impairment of visibility (tiny particles scatter light very efficiently), and even climate change (certain gases absorb energy emitted by the earth, leading to global warming).

Air pollution is certainly not a new phenomenon. Early references to it date back to the Middle Ages, when smoke from burning coal was already such a serious problem that in 1307 King Edward I banned its use in lime kilns in London. More recently, there have been major episodes of air pollution, such as the 1930 catastrophe in the Meuse Valley, Belgium, where SO2 and particulate matter, combined with a high relative humidity, caused sixty-three excess deaths in five days. In 1948 similar conditions in Donora, Pennsylvania, a small industrial city, caused twenty excess deaths in five days,

(thousand short tons)
source category co nox voc so2 pm10 pm2.5 total
source: adapted from
on-road vehicles 49,989 8,590 5,297 363 295 229 64,763
non-road vehicles 25,162 5,515 3,232 936 458 411 35,714
miscellaneous 9,378 320 716 12 20,634 4,454 35,514
fuel combustion 5,322 10,026 904 16,091 1,029 766 34,138
electric utilities 445 5,715 56 12,698 255 128 19,267
industrial 1,178 3,136 178 2,805 236 151 7,684
other 3,699 1,175 670 588 568 487 7,187
waste disposal and recycling 3,792 91 586 37 587 525 5,618
solvent utilization 2 3 4,825 1 6 6 4,843
metals processing 1,678 88 77 401 147 103 2,494
other industrial processes 599 470 449 418 343 191 2,470
chemical manufacturing 1,081 131 395 262 66 40 1,975
storage and transport 72 16 1,240 5 85 31 1,449
petroleum industries 366 143 424 341 29 17 1,320
total 97,441 25,393 18,145 18,867 23,679 6,773 190,298

and in the early 1950s in London, England, two episodes of "killer fogs" claimed the lives of more than 6,000 people.

Classification of Air Pollutants

Not all pollutants are a result of human activity. Natural pollutants are those that are found in nature or are emitted from natural sources. For example, volcanic activity produces sulfur dioxide, and particulate pollution may derive from forest fires or windblown dust. Anthropogenic pollutants are those that are produced by humans or controlled processes. For example, sulfur dioxide is produced by fossil fuel combustion and particulate matter comes from diesel engines.

Air pollutants also are classified as primary or secondary. Primary pollutants are those that are emitted directly into the atmosphere from an identifiable source. Examples include carbon monoxide and sulfur dioxide. Secondary pollutants are those that are produced in the atmosphere by chemical and physical processes from primary pollutants and natural constituents. For example, ozone is produced by hydrocarbons and oxides of nitrogen (both of which may be produced by car emissions) and sunlight. See the table for a listing of estimated pollutant emissions in the United States in 1999.

Air Pollution Control Laws and Regulations

The earliest programs to manage air quality in the United States date to the late 1880s; they attempted to regulate emissions from smokestacks using nuisance law municipal ordinances. Little progress was made in air pollution control during the first half of the twentieth century.

In the 1950s there was a shift away from nuisance law and municipal ordinances as the basis for managing air quality toward increased federal involvement. The Air Pollution Control Act of 1955 established a program for federally funded research grants in the area of air pollution, but the role of the federal government remained a limited one.

pollutant standard value* standard type
*parenthetical value is an approximately equivalent concentration.
source: u.s. environmental protection agency
carbon monoxide (co)      
8-hour average 9 ppm (10 mg/m3) primary
1-hour average 35 ppm (40 mg/m3) primary
nitrogen dioxide (no2)      
annual arithmetic mean 0.053 ppm (100 μg/m3) primary & secondary
ozone (o3)      
1-hour average 0.12 ppm (235 μg/m3) primary & secondary
8-hour average 0.08 ppm (157 μg/m3) primary & secondary
lead (pb)      
quarterly average 1.5 μg/m3   primary & secondary
particulate (pm 10) particles with diameters of 10 micrometers or less
annual arithmetic mean 50 μg/m3   primary & secondary
24-hour average 150 μg/m3   primary & secondary
particulate (pm 2.5) particles with diameters of 2.5 micrometers or less
annual arithmetic mean 15 μg/m3   primary & secondary
24-hour average 65 μg/m3   primary & secondary
sulfur dioxide (so2)      
annual arithmetic mean 0.030 ppm (80 μg/m3) primary
24-hour average 0.14 ppm (365 μg/m3) primary
3-hour average 0.50 ppm (1300 μg/m3) secondary

It was the Clean Air Act (CAA) of 1963 that further extended the federal government's powers in a significant way, allowing direct federal intervention to reduce interstate pollution.

The Clean Air Act Amendments (CAAA) of 1970 continued many of the programs established by prior legislation; however, several aspects of it represented major changes in strategy by expanding the role of the federal government. The 1970 CAAA defined two types of pollutants that were to be regulated: criteria and hazardous pollutants.

Criteria pollutants, regulated to achieve the attainment of the National Ambient Air Quality Standards (NAAQS), including primary standards for the protection of public health, ". . . the attainment and maintenance of which, . . . allowing an adequate margin of safety, are requisite to protect public health," and secondary standards for the protection of public welfare. The first six criteria pollutants were carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), total suspended particulate matter (TSP), hydrocarbons, and photochemical oxidants. Lead was added to the list in 1976. In 1979 the photochemical oxidants standard was replaced by one for ozone (O3), and in 1983 the hydrocarbons standard was dropped altogether. In 1987 TSP was changed to PM10, and in 1997 PM2.5 was added to the official list and the ozone standard revised.

National Emission Standards for Hazardous Air Pollutants (NESHAP) were established. A hazardous air pollutant (HAP) was defined as one "to which no ambient air standard is applicable and that . . . causes, or contributes to, air pollution which may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible or incapacitating reversible illness." Examples include asbestos, mercury, benzene, arsenic, and radionuclides.

(thousand short tons)
source category co nox voc so2 pm10 pm2.5
source: epa data available from
fuel combustion            
electric utilities 445 5,266 64 11,389 270 141
industrial 1,221 3,222 185 2,894 244 157
other 2,924 1,161 957 593 483 458
chemical manufacturing 1,112 134 407 268 67 41
metals processing 1,735 91 79 411 152 107
petroleum industries 369 146 433 346 30 17
other industrial processes 620 487 480 432 355 198
solvent utilization 2 3 4,827 1 7 6
storage and transport 74 17 1,225 5 87 32
waste disposal and recycling 3,609 89 582 35 544 514
on-road vehicles 48,469 8,150 5,035 314 273 209
nonroad vehicles 29,956 5,558 3,404 1,492 436 400
miscellaneous 20,806 576 2,710 21 21,926 5,466
total 109,342 24,899 20,384 18,201 24,875 7,746

Even though the CAAA of 1970 and 1977 placed deadlines on the dates for compliance with the NAAQS, as of 1990 in many areas of the United States, a variety of criteria pollutants existed in concentrations greater than the standards allowed.

As a result, the CAAA of 1990 were passed. They contain eleven major divisions, referred to as titles, the most important of which are the following: Title I: Provisions for Attainment and Maintenance of NAAQS, Title II: Provisions Relating to Mobile Sources, Title III: Hazardous Air Pollutants, Title IV: Acid Deposition Control, Title V: Permits, and Title VI: Stratospheric Ozone Protection, Title VII: Provisions Relating to Enforcement, Title VIII: Miscellaneous Provisions, Title IX: Clean Air Research, Title X: Disadvantaged Business Concerns, and Title XI: Clean Air Employment Transition Assistance.

International Nature of the Problem

Air pollution and the problems it causes are not confined by any geopolitical boundaries. For example, the radioactive cloud resulting from the Chernobyl nuclear accident in 1986 traveled as far as Ireland. A United Nations report warns that haze produced by the burning of wood and fossil fuels is creating a two-mile-thick "Asian browncloud" that covers southeastern Asia and may be responsible for hundreds of thousands of respiratory deaths a year.

In the United States, federal pollution laws and regulations apply to all states, even though some states, such as California, have adopted more stringent standards. Similarly, in the European Union (EU) existing laws apply equally to all members. Countries such as Denmark and Germany, however, have elected to imposed stricter standards than those set by the EU.

International agreements aimed at reducing various pollutants have been signed by various countries. The Montreal Protocol was signed in 1987; its purpose is the reduction of chlorofluorocarbons (CFC), a class of compounds that destroy the stratospheric ozone layer. More recently, in 1997, a conference convened in Kyoto, Japan, to discuss ways of reducing carbon dioxide emissions and other greenhouse gases . The United States has not signed the Kyoto Protocol, arguing that such an agreement would impede its economic progress. It has, however, publicly stated its intention to embark on voluntary reductions of carbon dioxide and other greenhouse gases.

Air Pollutants

In general, air pollutants are divided into two classes: those for which a NAAQS may be set (in other words, the criteria pollutants), and those for which NAAQS are not appropriate (the HAPs). If the ambient concentration of the criteria pollutants is kept below the NAAQS value, then there will be no health damage due to air pollution. The HAP (mostly known or suspected carcinogens), on the other hand, are those that, even in low concentrations, cause significant damage.

Particulate Matter. Particulate matter (PM) is the term used to describe solid or liquid particles that are airborne and dispersed (i.e., scattered, separated). PM originates from a variety of anthropogenic sources, including diesel trucks, power plants, wood stoves, and industrial processes.

The original NAAQS for PM was set in 1970. In 1987, the total suspended particulate matter, TSP, was revised, and a PM10 (particulate matter with an aerodynamic diameter of 10 μm or less) standard was set. PM10, sometimes known as respirable particles, was felt to provide a better correlation of particle concentration with human health.

In 1997 the particulate matter standard was updated, to include the PM2.5 standard. These particles, known as "fine" particles, a significant fraction of which is secondary in nature, are especially detrimental to human health because they can penetrate deep into the lungs. Scientific studies show a link between PM2.5 (alone, or combined with other pollutants in the air) and a series of significant health effects, even death.

Fine particles are the major cause of reduced visibility in parts of the United States, including many of the national parks. Also, soils, plants, water, or materials are affected by PM. For example, particles containing nitrogen and sulfur that are deposited as acid rain on land or water bodies may alter the nutrient balance and acidity of those environments so that species composition and buffering capacity change. PM causes soiling and erosion damage to materials, including culturally important objects such as carved monuments and statues.

Carbon Monoxide. Carbon monoxide (CO) is a colorless, odorless, and at high levels a poisonous gas that is fairly unreactive. It is formed when carbon in fuels is not burned completely. The major source of CO is motor vehicle exhaust. In cities, as much as 95 percent of all CO emissions result from vehicular (automobile) emissions. Other sources of CO emissions include industrial processes, nontransportation-related fuel combustion, and natural sources such as wildfires.

CO has serious health effects on humans. An exposure to 50 ppm of CO for eight hours can cause reduced psychomotor performance, while CO is lethal to humans when concentrations exceed approximately 750 ppm. Hemoglobin, the part of blood that carries oxygen to body parts, has an affinity of CO that is about 240 times higher than that for oxygen, forming carboxyhemoglobin, COHb. Moreover, the release of oxygen by hemoglobin is reduced in the presence of COHb. However, the effects of CO poisoning are reversible once the CO source has been removed.

Sulfur Dioxide. Sulfur dioxide (SO2) is colorless, nonflammable, nonexplosive gas. Almost 90 percent of anthropogenic SO2 emissions are the result of fossil fuel combustion (mostly coal) in power plants and other stationary sources. A natural source of sulfur oxides is volcanic activities.

In general, exposure to SO2 irritates the human upper respiratory tract. The most serious air pollution episodes occurred when there was a synergistic effect of SO2 with PM and water vapor (fog). Because of this, it has proven difficult to isolate the effects of SO2 alone.

SO2 is one of the precursors of acid rain (the term used to describe the deposition of acidic substances from the atmosphere). Also, SO2 is the precursor of secondary fine sulfate particles, which in turn affect human health and reduce visibility. Prolonged exposure to SO2 and sulfate PM causes serious damage to materials such as marble, limestone, and mortar. The carbonates (e.g., limestone, CaCO3) in these materials are replaced by sulfates (e.g., gypsum, CaSO4) that are water-soluble and may be washed away easily by rain. This results in an eroded surface.

Nitrogen Dioxide. Nitrogen dioxide (NO2) is a reddish-brown gas. It is a lung irritant and is present in the highest concentrations among other oxides of nitrogen in ambient air. Nitric oxide (NO) and NO2 are collectively known as NOx.

Anthropogenic emissions of NOx come from high-temperature combustion processes, such as those occurring in automobiles and power plants. Natural sources of NO2 are lightning and various biological processes in soil. The oxides of nitrogen, much like sulfur dioxide, are precursors of acid rain and visibility-reducing fine nitrate particles.

Ozone. Ozone (O3) is a secondary pollutant and is formed in the atmosphere by the reaction of molecular oxygen, O2, and atomic oxygen, O, which comes from the photochemical decomposition of NO2. Volatile organic compounds or VOCs (e.g., what one smells when refuelling the car) must also be present if O3 is to accumulate in the atmosphere.

O3 occurs naturally in the stratosphere and provides a protective layer from the sun's ultraviolet rays high above the earth. However, at ground level, O3 is a lung and eye irritant and can cause asthma attacks, especially in young children or other susceptible individuals. O3, being a powerful oxidant, also attacks materials and has been found to cause reduced crop yields and stunt tree growth.

Lead. The major sources of lead (Pb) in the atmosphere in the United States are industrial processes from metals smelters. Thirty years ago, the major emissions of Pb resulted from cars burning leaded gasoline. In 2002 only aviation fuels contain relatively large amounts of Pb. The United States is currently working with the World Bank to eliminate the use of leaded gasoline in all countries still using such fuel.

Pb is a toxic metal and can accumulate in the blood, bones, and soft tissues. Even low exposure to Pb can cause mental retardation in children.

Hazardous Air Pollutants. Hazardous air pollutants (HAPs), commonly referred to as air toxics or toxic air pollutants, are pollutants known to cause or suspected of causing cancer or other serious human health effects or damage to the ecosystem.

EPA lists 188 HAPs and regulates sources emitting significant amounts of these identified pollutants. Examples of HAPs are heavy metals (e.g., mercury), volatile chemicals (e.g., benzene), combustion by-products (e.g., dioxins), and solvents (e.g., methylene chloride). HAPs are emitted from many sources, including large stationary industrial facilities (e.g., electric power plants), smaller-area sources (e.g., dry cleaners), mobile sources (e.g., cars), indoor sources (e.g., some building materials and cleaning solvents), and other sources (e.g., wildfires).

Potential human health effects of HAPs include headache, dizziness, nausea, birth defects, and cancer. Environmental effects of HAPs include toxicity to aquatic plants and animals as well as the accumulation of pollutants in the food chain.

Because of the potential serious harmful effects of the HAPs, even at very low concentrations, NAAQS are not appropriate. The EPA has set National Emission Standards for Hazardous Air Pollutants, NESHAP, for only eight of the HAP, including asbestos and vinyl chloride. The EPA regulates HAP by requiring each HAP emission source to meet Maximum Achievable Control Technology (MACT) standards. MACT is defined as "not less stringent than the emission control that is achieved in practice by the best controlled similar source."

Control of Air Pollutants

In general, control of pollutants that are primary in nature, such as SO2, NO2, CO, and Pb, is easier than control of pollutants that are either entirely secondary (O3) or have a significant secondary component (PM2.5). Primary pollutants may be controlled at the source. For example, SO2 is controlled by the use of scrubbers, which are industrial devices that remove SO2 from the exhaust gases from power plants. SO2 emissions are also reduced by the use of low-sulfur coal or other fuels, such as natural gas, that contain lower amounts of sulfur. NO2 from industrial sources also may be minimized by scrubbing. NO2 from cars, as well as CO, are controlled by the use of catalytic converters, engine design modifications, and the use of cleaner burning grades of gasoline. Lead emissions have been reduced significantly since the introduction of lead-free gasoline.

Ozone and particulate matter are two of the most difficult pollutants to control. Reduction of oxides of nitrogen emissions, together with a reduction of VOC emissions is the primary control strategy for minimizing ozone concentrations. Because a large portion of PM2.5 is secondary in nature, its control is achieved by control of SO2, NO2, and VOC (which are the precursors of sulfates, nitrates, and carbon-containing particulates).

see also Acid Rain; Carbon Dioxide; Carbon Monoxide; Clean Air Act; Coal; Electric Power; Global Warming; Greenhouse Gases; Lead; Ozone; Petroleum; Toxic Release Inventory; Vehicular Pollution.


boubel, r., fox, d., turner, d., and stern, a. (1994). fundamentals of air pollution, 3rd edition. san diego: academic press.

cooper, c., and alley, f. (2002). air pollution control: a design approach, 3rd edition. prospect heights, il: waveland press.

de nevers, n. (2000). air pollution control engineering, 2nd edition. boston: mcgraw-hill.

heinsohn, r., and kabel, r. (1999). sources and control of air pollution. upper saddle river, nj: prentice hall.

nazaroff, w., and alvarez-cohen, l. environmental engineering science. new york: john wiley & sons.

wark, k., warner, c., and davis, w. (1998). air pollution, its origin and control, 3rd edition. menlo park, ca: addison-wesley.

internet resources

u.s. epa web site. available from

Christos Christoforou

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Air Pollution


Air pollution has plagued communities since the industrial revolution and even before. Airborne pollutants, such as gases, chemicals, smoke particles, and other substances, reduce the value of and ability to enjoy affected property and cause significant health and environmental problems. Despite the long history and significant consequences of this problem, effective legal remedies only began to appear in the late nineteenth and early twentieth centuries. Though some U.S. cities adopted air quality laws as early as 1815, air pollution at that time was seen as a problem best handled by local laws and ordinances. Only as cities continued to grow, and pollution and health concerns with them, did federal standards and a nationwide approach to air quality begin to emerge.

The earliest cases involving air pollution were likely to be brought because of a noxious smell, such as from a slaughterhouse, animal herd, or factory, that interfered with neighboring landowners' ability to enjoy their property. These disputes were handled through the application of the nuisance doctrine, which provides that possessors of land have a duty to make a reasonable use of their property in a manner that does not harm other individuals in the area. A person who polluted the air and caused harm to others was liable for breaching this duty and was required to pay damages or was enjoined (stopped through an injunction issued by a court) from engaging in the activities that created the pollution. In determining whether to enjoin an alleged polluter, courts balanced the damage to the plaintiff landowner's property against the hardship the defendant polluter would incur in trying to eliminate, or abate, the pollution. Courts often denied injunctions because the economic damage suffered by the defendant—and, by extension, the surrounding community if the defendant was essential to the local economy—in trying to eliminate the pollution often outweighed the damage suffered by the plaintiff. Thus, in many cases, the plaintiff was left only with the remedy of money damages—a cash payment equal to the estimated monetary value of the damage caused by the pollution—and the polluting activities were allowed to continue.

Using a nuisance action to control widespread air pollution proved inadequate in other ways as well. At common law, only the attorney general or local prosecutor could sue to abate a public nuisance (one that damages a large number of persons) unless a private individual could show "special" damage that was distinct from and more severe than that suffered by the general public. The private plaintiff with special damages had the necessary standing (legally protectible interest) to seek injunctive relief. In some states, the problem of standing has been corrected through laws that allow a private citizen to sue to abate public nuisances such as air pollution, though these laws are by no means the norm. Moreover, with the nuisance doctrine the plaintiff has the burden of showing that the harm she or he has experienced was caused by a particular defendant. However, since pollutants can derive from many sources, it can be difficult, if not impossible, to prove that a particular polluter is responsible for a particular problem. Last, nuisance law was useful only to combat particular polluters; it did not provide an ongoing and systematic mechanism for the regulation and control of pollution.

Early in the nineteenth century, a few U.S. cities recognized the shortcomings of common-law remedies and enacted local laws that attempted to address the problem of air pollution. Pittsburgh, in 1815, was one of the first to institute air quality laws. Others, like Chicago and Cincinnati, passed smoke control ordinances in 1881, and by 1912, twenty-three U.S. cities with populations of over two hundred thousand had passed smoke abatement laws.

Though the early court cases usually addressed polluted air as an interference with the enjoyment of property, scientists quickly discovered that air pollution also poses significant health and environmental risks. It is believed to contribute to the incidence of chronic diseases such as emphysema, bronchitis, and other respiratory illnesses and has been linked to higher mortality rates from other diseases, including cancer and heart disease.

The shortcomings associated with the common-law remedies to control air pollution and increasing alarm over the problem's long-range effects finally resulted in the development of state and federal legislation. The first significant legislation concerning air quality was the Air Pollution Control Act, enacted in 1955 (42 U.S.C.A. § 7401 et seq. [1955]). Also known as the Clean Air Act, it gave the Secretary of Health, Education, and Welfare the power to undertake and recommend research programs for air pollution control. Amendments passed during the 1960s authorized federal agencies to intervene to help abate interstate pollution in limited circumstances, to control emissions from new motor vehicles, and to provide some supervision and enforcement powers to states trying to control pollution. By the end of the 1960s, when it became clear that states had made little progress in combating air pollution, Congress toughened the Clean Air Act through a series of new laws, which were known as the Clean Air Act Amendments of 1970 (Pub. L. No. 91-604, 84 Stat. 1676 [Dec. 31, 1970]).

The 1970 amendments greatly increased federal authority and responsibility for addressing the problem of air pollution. They provided for, among other things, uniform national emissions standards for the hazardous air pollutants most likely to cause an increase in mortality or serious illness. Under the amendments, each state retained some regulatory authority, having "primary responsibility for assuring air quality within the entire geographic area comprising such state." Thus, states could not "opt out" of air pollution regulation and for the first time were required to attain certain air quality standards within a specified period of time. In addition, the amendments directed the administrator of the environmental protection agency (EPA), which was also established in 1970, to institute national standards regarding ambient air quality for air pollutants endangering public health or welfare, in particular sulfur dioxide, carbon monoxide, and photo-chemical oxidants in the atmosphere. The EPA was also granted the authority to require levels of harmful pollutants to be brought within set standards before further industrial expansion would be permitted.

Despite the ambitious scope of the 1970 legislation, many of its goals were never attained. As a result, the Clean Air Act was extensively revised again in 1977 (Pub. L. No. 95-95, 91 Stat. 685 [Aug. 7, 1977]). One significant component of the 1977 amendments was the formulation of programs designed to inspect, control, and monitor vehicle emissions. The 1977 revisions also sought to regulate parking on the street, discourage automobile use in crowded areas, promote

the use of bicycle lanes, and encourage employer-sponsored carpooling. Unlike the goals of several of the 1970 amendments, many of the 1977 reforms were achieved. Many states, with the help of federal funding, developed programs that require automobiles to be tested regularly for emissions problems before they could be licensed and registered. The 1977 amendments

also directed the EPA to issue regulations to reduce "haze" in national parks and other wilderness areas. Under these regulations the agency sought to improve air quality in a number of areas, including the Grand Canyon in Arizona.

During the 1980s and 1990s, several environmental issues, including acid rain, global climate change, and the depletion of the ozone layer, gave rise to further federal regulation. Acid rain, which has caused significant damage to U.S. and Canadian lakes, is created when the sulfur from fossil fuels, such as coal, combines with oxygen in the air to create sulfur dioxide, a pollutant. The sulfur dioxide then combines with oxygen to form sulfate, which, when washed out of the air by fog, clouds, mist, or rain, becomes acid rain, with potentially catastrophic effects on vegetation and ground water. Amendments to the Clean Air Act in 1990 (Pub. L. No. 101-549, 104 Stat. 2399 [Nov. 15, 1990]) sought to address the challenges posed by acid rain by commissioning a number of federally sponsored studies, including an analysis of Canada's approach to dealing with acid rain and an investigation of the use of buffering and neutralizing agents to restore lakes and streams. The 1990 laws also directed the EPA to prepare a report on the feasibility of developing standards related to acid rain that would "protect sensitive and critically sensitive aquatic and terrestrial resources." In addition, the amendments provided for a controversial system of "marketable allowances," which authorize industries to emit certain amounts of sulfate and which can be transferred to other entities or "banked" for future use.

The problem of global climate change is linked to the accumulation of gases, including carbon dioxide and methane, in the atmosphere. Scientists have disagreed over the net effect of this pollution on the global climate: some have argued that it produces global warming; others have maintained that it gradually cools global temperatures. Scientists do agree that a sustained climate change in either direction could significantly affect the environment.

The 1990 amendments implemented a number of strategies to address changes in the global climate, including the commissioning of studies on options for controlling the emission of methane. The amendments also contained provisions to deal with the depletion of the ozone layer, which shields the earth from the harmful effects of the sun's radiation. Though the long-term consequences were hard to determine in the early 2000s, damage had already been seen in the form of a "hole" in the ozone layer over Antarctica. The destruction of the ozone layer was believed to be caused by the release into the atmosphere of chlorofluorocarbons (CFCs) and other similar substances. The 1990 laws included a ban on "nonessential uses" of ozone-depleting chemicals, and the placement of conspicuous warning labels on certain substances, indicating that their use harms public health and the environment by destroying the ozone in the upper atmosphere.

Regulatory interpretation of the Clean Air Act shifted between the late 1990s and early 2000s. Under President william j. clinton, the Environmental Protection Agency sought to close loopholes in the law's enforcement through the New Source Review (NSR) program. Essentially, these rules used an industrial facility's age to determine when higher pollution emissions would require the facility to go through a permit process and install pollution control equipment. The agency sued some 50 companies in an effort to hold them to the highest pollution control standards. But the EPA shifted direction under President george w. bush, who favored less stringent regulations. Initially, the EPA announced a review of the Clinton-era policy, before issuing proposed rule changes in December 2002 that would relax requirements governing pollution levels and mandatory equipment upgrades. Under its socalled Clear Skies initiative, the Bush administration proposed issuing individual utilities pollution credits; these credits would allow the utility to lawfully generate a fixed amount of pollution, and if unused, any remaining credits could be sold to other utilities exceeding their permitted limit. Environmentalists criticized the proposals for gutting protections, while industry embraced them as flexible cost-savings measures.

In the 1990s, the battle to control air pollution moved indoors, into homes and businesses. Studies showed that people are exposed to higher concentrations of air pollution for longer periods of time inside buildings than out-ofdoors. Furthermore, evidence indicated that this exposure was contributing to a rapidly increasing incidence of illness, thus costing businesses, taxpayers, and the government billions of dollars in healthcare costs and lost work time. The typical U.S. home contains many hazardous chemicals and substances, including radon, which has been linked to lung cancer and other ailments. Congress responded to public concern about indoor air quality by requiring the EPA, with the Superfund Amendments and Reauthorization Act (SARA), to establish a program to study the problem and make appropriate recommendations (Superfund Amendments and Reauthorization Act of 1986, Pub. L. No. 99-499, 100 Stat. 1613 [codified as amended in scattered sections of 10 U.S.C.A., 26 U.S.C.A., 29U.S.C.A., 33 U.S.C.A., and 42 U.S.C.A.]).

One contentious air pollution issue continued to be the effect of smoking in public places, especially as it concerns the rights and health of nonsmokers. Many states have enacted legislation designed to protect nonsmokers in public places, and the battle between smokers and nonsmokers made its way into the courts. An increasing number of restaurants, airlines, and other public facilities dealt with the problem by banning smoking completely.

While the trend has been toward adoption of smoking bans in the 2000s, advocates and opponents have fought pitched battles. Advocates point to successes such as stringent statewide bans in New York, California, and Delaware, along with an estimated 400 bans in cities such as Boston and Dallas, according to the American Nonsmokers' Rights Foundation. They also cited evidence presented at the American College of Cardiology's annual meeting in 2002 showing that the city of Helena, Montana, enjoyed dramatically reduced heart attack rates the year following enactment of its ban. Ironically, enforcement was subsequently halted while a court battle was waged over the ban.

Opposition to indoor smoking bans has come from the bar, restaurant and tobacco industries. Commercial groups argue that bans result in revenue loss, burdensome compliance regulation, and even a diminished labor force. They have achieved some success. Some city councils rejected proposed ordinances after heavy lobbying, such as in Eden Prairie, Minnesota, in 2002, and the city of Pueblo, Colorado, was forced to suspend its ordinances following a successful public signature drive calling for a public referendum in 2003.

further readings

Jackson, Ted. 2003. "Activists Fret President's Plan Hurts Effort on FPL Emissions." Palm Beach Post (February 28).

Menell, Peter S., ed. 2002. Environmental Law. Aldershot, England; Burlington, Vt.: Ashgate/Dartmouth.

Rodgers, William H., Jr. 1986. Environmental Law: Air and Water. Vol. 2. St. Paul, Minn.: West.

Stagg, Michael K. 2001. "The EPA's New Source Review Enforcement Actions: Will They Proceed?" Trends 33 (November-December).


Automobiles; Environmental Law; Environmental Protection Agency; Pollution; Surgeon General; Tobacco.

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Pollution, Air

Pollution, Air





The economy extracts natural resources from the environment to be used as inputs in production processes (the source function of the environment). The output of these production processes may be either produced inputs for yet other production processes or final products to be directly consumed. Yet these produced inputs and final products are not the entirety of the output; there are also residual by-products of these processes (waste).

Just as the economy extracts natural resources from the environment, the economy in turn dumps many residual by-products, or waste, back into the environment (the sink function of the environment). There is waste at each stage of the economic process: waste from extracting and refining natural resources, waste emanating from production processes, waste in the marketing of products, and waste in the sphere of consumption. Wastes may be solid, airborne, or waterborne. Air pollution describes airborne wastes that can harm the environment and human health due to their accumulation in the atmosphere, their concentration geo-spatially, and/or their synergistic effects when combining with other wastes.

There is an interesting relationship between the total natural resources utilized and the total waste produced by the economy. That is, they are ultimately equivalent. This is due to the first law of thermodynamics, which states that matter-energy can neither be created nor destroyed; only the form of matter-energy can change. Of course, it is more complicated than a simple equality. Natural resources are frozen in the form of capital goods during the depreciation process (and capital goods from previous periods are at differing stages in the depreciation process), and there is a time element in the consumption of many final products as well. At a fundamental level, however, the equality holds.


Some wastes are recyclable or reusable and others are not. The fact that all waste is not recyclable or reusable is due to the second law of thermodynamics, which states that any utilization of matter-energy decreases the total available matter-energy. In other words, some of the forms into which matter-energy is transformed can no longer be accessed. This is also known as the entropy law, and put differently means that not all the forms into which matter and energy are transformed are recyclable or reusable. That waste which is not recycled or reused is dumped into the environment.

The environment has an assimilative capacity, which is the ability of the environment to transform waste into harmless (or even beneficial) forms. This assimilative capacity, however, is not infinite. Waste at some level is not only incapable of being assimilated, but will damage or even destroy the assimilative capacity itself.

It is not simply the level of homogeneous waste in relation to the assimilative capacity that needs to be considered, but additionally what specific types of waste are being emitted. Some types of waste (e.g., mercury) are not assimilable in any quantity, and at some stock level can result in various detrimental effects, including damage to the assimilative capacity itself. In addition, it is not sufficient to simply look at each type of waste and the quantity of it emitted in isolation, one must consider also its synergistic effects. The combinations of different forms of waste have effects that are more damaging than the sum of the component waste products independent of one another. A classic case here is sulfur dioxide and nitric oxide resulting in acid precipitation (acid rain, fog, and snow).

The qualities and quantities of waste globally along with spatial considerations concerning the local concentration of wastes are crucial. And it is not simply the case that the assimilative capacity detoxifies or degrades waste instantaneously, or even within some set time period. There are cumulation effects that have to be dealt with. So in assessing the ability of the assimilative capacity to deal with industrial and other waste, combination effects, concentration effects, and cumulation effects all need to be carefully considered.

Furthermore, nothing guarantees that all waste that is capable of being recycled or reused is being recycled or reused. All waste, whether recyclable or not, which is dumped into the environment, may impact on the assimilative capacity. Therefore, when considering the quantities and qualities of wastes confronting the assimilative capacity, only those residuals may be exempted that are actually recycled. Generally speaking, the technologies do not yet exist to capture and recycle airborne emissions.


In the United States, early air pollution laws were enacted locally in Chicago and Cincinnati. These were smoke control laws that addressed only smoke emissions from coal burning. Before 1948, there was almost no real government intervention in the environment, which means that there was, by default, a market approach to natural resource use and environmental protection.

An early recorded disaster resulting from air pollution occurred in Belgium in 1930. A thermal inversion occurred in an area characterized by concentrated industry with substantial amounts of sulfur dioxide emissions and discharges of particulate matter. Air circulation, which requires horizontal or vertical air currents, is one of the keys to the dispersal of air pollution. If there is no horizontal wind movement, then vertical air currents will usually disperse the pollutants due to the fact that atmospheric temperature is inversely related to height. The temperature falls by 5.4 degrees Fahrenheit every thousand feet above the Earths surface. So normally, the warm polluted air, being lighter, will rise and disperse into the cooler air.

However, if the temperature decrease is less than 5.4 degrees Farenheit per thousand feet, warm air, unable to rise because of the existence of even warmer air above it, hovers over the source of the pollution, trapping concentrated pollutants in the lower stratum. This phenomenon is called thermal inversion.

The thermal inversion in Belgium in 1930 resulted in sixty-three deaths and five thousand people becoming seriously ill. A similar episode occurred in Donora, Pennsylvania, a small industrial town thirty miles south of Pittsburgh, in 1948. Twenty people died and six thousand became ill. Thermal inversion combined with pollution and fog killed four thousand and caused numerous respiratory illnesses in London in 1952.

In the United States, the Donora incident led to a greater awareness of the problem of air pollution, and eventually to the Air Pollution Act of 1955. Although this act did little more than authorize and provide limited funding for research, it served as the basis for future amendments to the Act. The Clean Air Act of 1963 authorized the Public Health Service to take corrective action in addressing problems of interstate air pollution, and 1965 amendments gave the federal program the authority to curb auto emissions. The first standards for motor vehicle emissions were applied in 1968.

The Air Quality Act of 1967 strengthened the powers of state and local as well as federal authorities to set and enforce standards on a regional basis. This paved the way for the Clean Air Act of 1970, which was the first legislation to call for uniform air quality standards based on geographic regions.

The newly created Environmental Protection Agency (EPA) was given the authority to enforce two sets of standards: primary and secondary. Primary air quality standards concern the minimum air quality necessary to keep people from getting ill. These standards are based on proven harmful effects of individual pollutants. Secondary standards are intended to promote the general public welfare and prevent damage to plants, animals, and property in general. Within each geographic region, states determine how these standards are to be met.


Direct regulation or standards have been criticized on a number of grounds and have given rise to market approaches. Pollution taxes have been used, which it has been argued gives firms an incentive to reduce their emissions and is a lower cost method than command and control. The problem with such taxes or fees is identifying and calculating the social costs, and even if that is possible, there is no guarantee that they will reduce emissions to levels consistent with assimilative capacity.

These problems resulted in the market permits and emissions trading approach, which entails a market in pollution rights. The government makes some maximum allowable emissions standards, but then auctions off pollution permits to the highest bidders. Firms could purchase in the original market directly from the government or in secondary markets from other firms or individuals who purchased directly from the government, or in secondary markets themselves. Only after having acquired the right to pollute could a firm discharge polluting emissions. Here there is a tax incentive: The firm pays to reduce emissions and to seek ways of producing that pollute less, but the difference is that the total amount of pollution is fixed. In this sense, the market permit approach combines the strengths of both direct regulation and market approaches.

The market permits approach is not without its critics however. Many see the practice as government auctioning off clean air to the highest bidder. These issues are becoming particularly important as scientific evidence about problems such as global climate change becomes more reliable and available.

SEE ALSO Externality; Global Warming; Greenhouse Effects; Pollution; Pollution, Noise; Pollution, Water


Baumol, William. 1972. On Taxation and the Control of Externalities. American Economic Review. 62 (3): 307322.

Georgescu-Roegen, Nicholas. 1971. The Entropy Law and the Economic Process. Cambridge, MA: Harvard University Press.

Heilbroner, Robert. 1950. What Goes Up the Chimney. Harpers (January): 6169.

Intergovernmental Panel on Climate Change (IPCC). 2007. Climate Change 2007: The Physical Science Basis. Geneva:WMO, IPCC Secretariat.

Kapp, Karl W. 1950. The Social Costs of Private Enterprise.Cambridge, MA: Harvard University Press.

Tietenberg, Thomas. 1985. Emissions Trading: An Exercise in Reforming Pollution Policy. Washington, DC: Resources for the Future.

Mathew Forstater

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Air Pollution


AIR POLLUTION became a matter of concern in the United States in the nineteenth century, when population growth and industrialization increased the number of wood and coal furnaces, which generated enough smoke to overwhelm natural air-filtering processes and threaten human health. Coal-burning facilities in industrial centers like Pittsburgh, Pennsylvania, and smelter towns like Butte, Montana, spewed tons of smoke, soot, ash, and gases into the atmosphere. Boosters often applauded the smoke as a sign of prosperity. But by the late nineteenth century the hazards of smoke were better understood. Airborne pollutants became especially dangerous when a so-called thermal inversion occurred, trapping the pollutants and allowing them to build up for days in warm air overlaid by a cold air mass. In these cases, human exposure could and did cause respiratory illnesses and deaths. As early as 1815 some local governments required that manufacturers control emissions, and during the Progressive Era most major cities passed ordinances to control the "smoke nuisance." However, events such as the Donora smog of 1948, a thermal inversion in which twenty residents of Donora, Pennsylvania, died and more than five thousand fell ill, suggested that local efforts to abate air pollution were not sufficiently safeguarding public health.

In 1955, Congress enacted the first federal air-quality legislation, providing research and technical assistance to states. States and localities remained responsible for regulating factory emissions and the brown automobile-induced "photochemical smog" over urban basins, but this act expanded federal authority over air-quality control. With the Clean Air Act of 1963 Congress increased aid to states and for the first time allowed federal control over automobile emissions. In 1965 Congress enacted a law requiring automakers to install emissions-reducing devices on all cars built and sold in the United States after 1967.

The Air Quality Act of 1967 dramatically expanded federal control. It authorized federal regulation of stationary as well as mobile pollution sources, required states to impose air-quality standards in problem regions, and allowed federal controls where states failed to act. The Clean Air Act of 1970, a centerpiece of the burgeoning environmental movement, directed the Environmental Protection Agency (EPA), established the previous year, to set National Ambient Air Quality Standards (NAAQS). Under this law the EPA identified the principal air pollutants (particulates, sulfur dioxide, carbon monoxide, hydrocarbons, nitrogen dioxide, ozone, and, after 1978, lead), set maximum allowable levels for each, and required that states draft plans for meeting the federal standards. The act also required that stationary polluters secure federal permits contingent on their use of "best available" abatement technology and mandated that automakers achieve a 90 percent decrease in vehicle emissions by 1985 (a timetable relaxed somewhat by amendments in 1977).

In general these laws set maximum pollutant levels but left it to the polluters to find ways to meet them. This tactic, called "technology forcing," spurred automakers to adopt catalytic converters in the 1970s and compelled the

"big three" automakers (GM, Ford, and Chrysler) in 1993 to launch their Clean Car Initiative, a joint pledge to develop vehicles averaging ninety miles per gallon by 2003. When California insisted that zero-emission vehicles account for 10 percent of all new cars in the state by 2003, setting a precedent that other large states like New York were likely to follow, automakers stepped up efforts to develop new emissions technology. Much research focused on the "hybrid," an electric car with a small, supplementary fuel-burning motor that could radically cut emissions and reduce gasoline consumption. The first mass-produced hybrid, a Honda, was available in the United States in 1999. Other research focused on hydrogen-fed fuel cells, whose only exhaust is water vapor.

Industrial interests pleaded for less-stringent standards, claiming air-pollution control is expensive and economically damaging. Industries blamed the soaring inflation of the 1970s on environmental-protection legislation. In response the EPA delayed requirements and devised strategies for reducing pollution without placing undue burdens on manufacturers. For example, the "bubble" concept, formally adopted in a 1979 amendment to

the Clean Air Act, placed an imaginary bubble over an entire region and required the air in the bubble to meet NAAQS levels. Firms in the same bubble could trade pollution rights with each other, allowing excess pollution at one source as long as it was offset by lower emissions at another. (The previous approach had forced each individual "stack" to meet national standards.) By defining each factory as part of a larger air shed, the bubble concept was a step toward an ecosystem-oriented approach. Along these lines, the Clean Air Act of 1990 capped the nation's total sulfur oxide emissions and allowed firms to set up a nationwide market in pollution permits.

By the late 1990s, such measures had significantly reduced major air pollutants in most metropolitan areas. However, haze in scenic nonurban areas such as the Grand Canyon caused by nearby urban areas and power plants had emerged as a growing problem. Moreover, as environmentalists adopted an increasingly global perspective, they identified new air pollution issues. Among the issues was acid precipitation, sulfur dioxide and other chemicals that originate in industrial areas, drift across political borders, and wash out of the atmosphere with rain, snow, or fog, causing acid deposits in lakes and forests. Another new issue, especially following the discovery in 1985 of an "ozone hole" over Antarctica, was depletion of the Earth's ozone layer caused by chlorofluorocarbons (CFCs) used in aerosol propellants, foam plastics, refrigerants, and industrial processes. A third issue that entered environmental debates in the 1980s concerned the emission of "greenhouse gases," especially carbon dioxide, that trap heat in the Earth's atmosphere and, according to many scientists, cause global-scale climate changes. These transnational and global air-quality issues stoked the fears of the industrial interests regarding greater government intervention in their affairs. Such reactions reflect the "out of sight, out of mind" axiom that long characterized responses to air pollution. Efforts to control visible pollution, like smoke or smog, traditionally won widespread support. But the less visible and more theoretical problems attracted detractors, who questioned the scientific methods of pollution-control proponents and raised the specter of economic stagnation to forestall stricter regulations.

Along with global air quality, attention focused on indoor air quality. Radon, a naturally occurring radioactive gas that collects in basements across much of the nation, was identified as a significant carcinogen. Secondary tobacco smoke raised substantial alarm in the 1990s, when many businesses, municipalities, and even states (notably California in 1994) banned smoking in indoor workplaces. Mold spores, chemical fumes, and other invisible pollutants that circulate indoors were identified as health hazards in the 1980s, giving rise to the term "sick building syndrome" and forcing businesses to listen more carefully when employees complained of "bad air" in the workplace. Thus despite massive government intervention and the hopes of some environmentalists, air pollution did not disappear. The most visible pollutants generally lessened, but research revealed that air pollution was more complex, widespread, and intimate than previously thought.


Andrews, Richard N. L. Managing the Environment, Managing Ourselves: A History of American Environmental Policy. New Haven, Conn.: Yale University Press, 1999.

Bailey, Christopher J. Congress and Air Pollution: Environmental Policies in the USA. Manchester, U.K., New York: Manchester University Press, 1998.

Grant, Wyn. Autos, Smog, and Pollution Control: The Politics of Air Quality Management in California. Aldershot, U.K., Brook-field, Vt.: Edward Elgar, 1995.

Hays, Samuel P. Beauty, Health, and Permanence: Environmental Politics in the United States, 1955–1985. New York: Cambridge University Press, 1987.

Miller, E. Willard, and Ruby M. Miller. Indoor Pollution: A Reference Handbook. Santa Barbara, Calif.: ABC–CLIO, 1998.

Stradling, David. Smokestacks and Progressives: Environmentalists, Engineers, and Air Quality in America, 1881–1951. Baltimore: Johns Hopkins University Press, 1999.

Switzer, Jacqueline Vaughn. Environmental Politics: Domestic and Global Dimensions. New York: St. Martin's Press, 1994.

DennisWilliams/w. p.

See alsoAcid Rain ; Automobile Industry ; Electric Power and Light Industry ; Energy Industry ; Global Warming ; Ozone Depletion .

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air pollution

air pollution, contamination of the air by noxious gases and minute particles of solid and liquid matter (particulates) in concentrations that endanger health. The major sources of air pollution are transportation engines, power and heat generation, industrial processes, and the burning of solid waste.

Sources of Air Pollution

The combustion of gasoline and other hydrocarbon fuels in automobiles, trucks, and jet airplanes produces several primary pollutants: nitrogen oxides, gaseous hydrocarbons, and carbon monoxide, as well as large quantities of particulates, chiefly lead. In the presence of sunlight, nitrogen oxides combine with hydrocarbons to form a secondary class of pollutants, the photochemical oxidants, among them ozone and the eye-stinging peroxyacetylnitrate (PAN). Nitrogen oxides also react with oxygen in the air to form nitrogen dioxide, a foul-smelling brown gas. In urban areas like Los Angeles where transportation is the main cause of air pollution, nitrogen dioxide tints the air, blending with other contaminants and the atmospheric water vapor to produce brown smog. Although the use of catalytic converters has reduced smog-producing compounds in motor vehicle exhaust emissions, studies have shown that in so doing the converters produce nitrous oxide, which contributes substantially to global warming.

In cities, air may be severely polluted not only by transportation but also by the burning of fossil fuels (oil and coal) in generating stations, factories, office buildings, and homes and by the incineration of garbage. The massive combustion produces tons of ash, soot, and other particulates responsible for the gray smog of cities like New York and Chicago, along with enormous quantities of sulfur oxides (which also may be result from burning coal and oil). These oxides rust iron, damage building stone, decompose nylon, tarnish silver, and kill plants. Air pollution from cities also affects rural areas for many miles downwind.

Every industrial process exhibits its own pattern of air pollution. Petroleum refineries are responsible for extensive hydrocarbon and particulate pollution. Iron and steel mills, metal smelters, pulp and paper mills, chemical plants, cement and asphalt plants—all discharge vast amounts of various particulates. Uninsulated high-voltage power lines ionize the adjacent air, forming ozone and other hazardous pollutants. Airborne pollutants from other sources include insecticides, herbicides, radioactive fallout, and dust from fertilizers, mining operations, and livestock feedlots.

Effects on Health and the Environment

Like photochemical pollutants, sulfur oxides contribute to the incidence of respiratory diseases. Acid rain, a form of precipitation that contains high levels of sulfuric or nitric acids, can contaminate drinking water and vegetation, damage aquatic life, and erode buildings. When a weather condition known as a temperature inversion prevents dispersal of smog, inhabitants of the area, especially children and the elderly and chronically ill, are warned to stay indoors and avoid physical stress. The dramatic and debilitating effects of severe air pollution episodes in cities throughout the world—such as the London smog of 1952 that resulted in 4,000 deaths—have alerted governments to the necessity for crisis procedures. Even everyday levels of air pollution may insidiously affect health and behavior. Indoor air pollution is a problem in developed countries, where efficient insulation keeps pollutants inside the structure. In less developed nations, the lack of running water and indoor sanitation can encourage respiratory infections. Carbon monoxide, for example, by driving oxygen out of the bloodstream, causes apathy, fatigue, headache, disorientation, and decreased muscular coordination and visual acuity.

Air pollution may possibly harm populations in ways so subtle or slow that they have not yet been detected. For that reason research is now under way to assess the long-term effects of chronic exposure to low levels of air pollution—what most people experience—as well as to determine how air pollutants interact with one another in the body and with physical factors such as nutrition, stress, alcohol, cigarette smoking, and common medicines. Another subject of investigation is the relation of air pollution to cancer, birth defects, and genetic mutations.

A relatively recently discovered result of air pollution are seasonal "holes" in the ozone layer in the atmosphere above Antarctica and the Arctic, coupled with growing evidence of global ozone depletion. This can increase the amount of ultraviolet radiation reaching the earth, where it damages crops and plants and can lead to skin cancer and cataracts. This depletion has been caused largely by the emission of chlorofluorocarbons (CFCs) from refrigerators, air conditioners, and aerosols. The Montreal Protocol of 1987 required that developed nations signing the accord not exceed 1986 CFC levels. Several more meetings were held from 1990 to 1997 to adopt agreements to accelerate the phasing out of ozone-depleting substances.

Solutions to Air Pollution

To combat pollution in the United States, the Clean Air Act Amendments of 1970 gave the Environmental Protection Agency (EPA) the authority to establish and enforce air pollution standards and to set emission standards for new factories and extremely hazardous industrial pollutants. The states were required to meet "ambient air quality standards" by regulating the emissions of various pollutants from existing stationary sources, such as power plants and incinerators, in part by the installation of smokestack scrubbers, electrostatic precipitators, and other filters. Auto manufacturers were mandated to install exhaust controls or develop less polluting engines. The Clean Air Act, as amended in 1977, authorized the EPA to impose stricter pollution standards and higher penalties for failure to comply with air quality standards.

In 1990 when the act was reauthorized it required most cities to meet existing smog reduction regulations by the year 2005. The 1990 amendments also expanded the scope and strength of the regulations for controlling industrial pollution. The result has been limited progress in reducing the quantities of sulfur dioxide, carbon monoxide, nitrogen oxide, ozone, particulate matter, and lead in the air. The EPA also regulated hazardous air pollutants, which in 1992 included mercury, beryllium, asbestos, vinylchloride, benzene, radioactive substances, and inorganic arsenic.

The most satisfactory long-term solutions to air pollution may well be the elimination of fossil fuels and the ultimate replacement of the internal-combustion engine. To these ends efforts have begun in the United States, Japan, and Europe to develop alternative energy sources (see energy, sources of), as well as different kinds of transportation engines, such as one powered by electricity. A system of pollution allowances based on trading emission rights has been established in the United States in an attempt to use the free market to reward pollution reductions, and the international sale of surplus emission rights is permitted under the Kyoto Protocol (see below). Other proposed solutions include raising electricity and gasoline rates to better reflect environmental costs and to discourage waste and inefficiency, and mechanical controls on coal-fired utility plants.

In 1992, 150 nations signed a treaty on global warming at the UN-sponsored summit on the environment in Rio de Janeiro. A UN Conference on Climate Change, held in Kyoto, Japan, in 1997, produced an international agreement to combat global warming by sharply reducing emissions of industrial gases produced by industrialized nations. Although the United States abandoned the treaty in 2001, saying it was counter to U.S. interests, most other nations agreed that year on the details necessary to make the protocol a binding international treaty, and the necessary ratifications brought the treaty into force in 2005. Efforts to develop a new, more encompassing binding treaty that would build on the Kyoto Protocol have been unsuccessful, and in 2012 Canada became the first ratifying nation to withdraw. Later in 2012 the Kyoto Protocol was extended to 2020.

See environmentalism; pollution.


See R. G. Bond et al., Air Pollution (1972); U.S. Council on Environmental Quality, Environmental Quality (22d Annual Report, 1991); World Bank, World Development Report (1992).

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Air Pollution

Air Pollution

Air pollution is the presence in the atmosphere of any substance at a concentration great enough to produce an undesirable effect on humans, animals, vegetation, or materials, or to significantly alter the natural balance of any ecosystem. Air pollutants can be solids, liquids, or gases, and can have local, regional, and global impacts.

At urban scales, air pollution is frequently referred to as photochemical smog. "Smog" is a contraction of the words "smoke" and "fog," and was originally used to describe air pollution caused by coal burning in London. Urban smog is photochemical because many of the chemicals found in urban air are formed by chemical reactions driven by sunlight. Among the many air pollutants in urban smog that are produced by photochemical reactions, one of the most abundant is ozone, O3. In contrast to the ozone found in the upper atmosphere (stratospheric ozone), which protects the planet from ultraviolet radiation , ground level or tropospheric ozone is a lung irritant and a danger to human health. It is also responsible for crop damage and is suspected of being a contributor to forest decline in Europe and in parts of the United States. Ground level ozone and other photochemical pollutants are formed in urban atmospheres by the reactions of oxides of nitrogen (mainly NO and NO2) in the presence of hydrocarbons. Oxides of nitrogen are byproducts of combustion processes. At the high temperatures generated during combustion, some of the N2 and O2 in air is converted to oxides of nitrogen and, in general, the higher the combustion temperature, the greater are the amounts of oxides of nitrogen produced. Hydrocarbons are emitted from natural sources and as a result of activities utilizing organic solvents, coatings, or fuel. These hydrocarbons and oxides of nitrogen participate in reactions that yield, not only ozone, but also aldehydes, hydrogen peroxide, peroxyacetyl nitrate (C2H3NO5), nitric acid, and molecular species of low volatility that accumulate in fine particles suspended in the atmosphere. Although many of these constituents of photochemical smog have environmental impacts, fine particulate matter (PM) presents the greatest health endangerment in most urban areas.

Solid and liquid phase material in the atmosphere is variously referred to as particulate matter, particulates, particles, and aerosols. These terms are often used interchangeably, but all refer to particles with diameters between approximately 1 nanometer (3.9 × 108 inches) and 10 micrometers (39.4 × 105 inches) that remain suspended in the atmosphere for long periods. The greatest threats to health are associated with the smallest particles because they have the greatest likelihood of becoming deposited deep within the respiratory system.

Somewhat counterintuitively, particles of about 1 micrometer (39.4 × 106 inches) in size can remain suspended in the atmosphere much longer than gases. Particles much larger than 1 micrometer (39.4 × 106 inches) will, of course, quickly settle out of the atmosphere because of gravity. The smallest particles will coagulate and coalesce quickly, forming larger particles. But particles of approximately 1 micrometer (39.4 × 106 inches) in diameter do not grow as quickly as smaller particles and can remain suspended in the atmosphere for a week or more. It is not unusual, for example, for Saharan dust or particle plumes from Asia to be detected in the United States. Consequently, particulate matter is a continental to global scale air pollution problem.

Also, unlike ozone and other gas phase pollutants that are specific chemical species, particulate matter is a collection of chemical species defined mainly on the basis of particle size. The chemical constituents that make up particulate matter vary with particle size. Windblown dust is a main contributor to particles larger than 10 micrometers (39.4 × 105 inches) in diameter, whereas sulfates, nitrates, and organic compounds are the main constituents of smaller particles that can penetrate deeply into the respiratory system and engender health effects. Organic particles can be emitted directly as soot from combustion processes or can be formed when large hydrocarbon molecules react with oxidants in the atmosphere and form chemicals that condense onto particles. Sulfate particles are formed via a series of reactions that convert sulfur dioxide, SO2, which is released into the atmosphere by the combustion of sulfur containing fuels, into sulfuric acid. Nitrate particles are formed via reactions that convert oxides of nitrogen, which are released into the atmosphere by combustion processes, into nitric acid. If particles containing sulfuric acid, nitric acid, and/or organic compounds

retain their acidity and are washed out of the atmosphere by rainfall, the rainfall becomes acid rain . Figure 1 shows acidity of rainfall averages in the United States and provides a sense of the continental scale of particulate matter air pollution.

The continental and global scale of air pollution problems is not limited to particulate matter. Emissions of greenhouse gases cause global climate change. The presence in the stratosphere of ozone-depleting compounds has created polar ozone holes. Atmospheric releases caused by volcanic eruptions and fires have global effects. Atmospheric particles also influence climate and rainfall. The challenges of reducing air pollution call for a sophisticated understanding of atmospheric chemistry, applied at local, regional, continental, and global scales.

see also Atmospheric Chemistry.

David T. Allen


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National Research Council (2001). Climate Change Science: An Analysis of Some Key Questions. Washington, DC: National Academy Press.

U.S. Environmental Protection Agency (1999). National Air Quality and Emissions Trends Report, 1999. EPA 454/R-01-004. Washington, DC: U.S. Government Printing Office. Also available from <>.

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air pollution

air pollution (atmospheric pollution) The release into the atmosphere of substances that cause a variety of harmful effects to the natural environment. Most air pollutants are gases that are released into the troposphere, which extends about 8 km above the surface of the earth. The burning of fossil fuels, for example in power stations, is a major source of air pollution as this process produces such gases as sulphur dioxide and carbon dioxide. Released into the atmosphere, both these gases, especially carbon dioxide, contribute to the greenhouse effect. Sulphur dioxide and nitrogen oxides, released in car exhaust fumes, are air pollutants that are responsible for the formation of acid rain; nitrogen oxides also contribute to the formation of photochemical smog. See also ozone layer; pollution.

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