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,

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.

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.

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.

Bibliography

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 http://www.epa.gov/air.

Christos Christoforou

AIR POLLUTION

The pernicious effects of air pollution were first documented long ago. As early as 61 c.e., Seneca, a Roman philosopher and noted essayist, wrote, "As soon as I had gotten out of the heavy air of Rome, and from the stink of the chimneys thereof, which being stirred, poured forth whatever pestilential vapors and soot they had enclosed in them, I felt an alteration to my disposition" (Miller and Miller, 1993). With technology being as simple as it was in Rome, however, there was not much that could be done about the problem.

Thirteen hundred years later, controls on the use of coal in London were passed, marking the recorded start of air pollution control. But such controls were not enough to prevent the buildup of pollutants as by-products of industrialization; air pollution was common to all industrialized nations by 1925. Air pollution is still a significant problem in urban centers worldwide. In the United States, pollutant emissions and air pollution concentrations have been falling, for the most part, since the 1970s.

The major constituents of unpolluted air (not including water) at ground level are nitrogen (78.08%) and oxygen (20.95%). The next most abundant constituents are argon (at 0.934%) and carbon dioxide (about 0.034%), followed by the other noble gases: neon (0.002%), helium (0.0005%), krypton (0.0001%), and xenon (0.000009%). A variety of chemicals known as trace gases (some quite toxic) also are found in unpolluted air, but at very low concentrations.

When certain substances in the air rise to the level at which they can harm plant or animal life, damage property, or simply degrade visibility in an area more than it would be in the absence of human action, those substances are considered to be pollutants. Such pollutants enter the atmosphere via both natural processes and human actions.

The pollutants most strongly damaging to human, animal, and sometimes plant health include ozone, fine particulate matter, lead, nitrogen oxides (NO_x), sulfur oxides (SO_x), and carbon monoxide. Many other chemicals found in polluted air can cause lesser health impacts (such as eye irritation). VOC compounds comprise the bulk of such chemicals. Formaldehyde is one commonly mentioned pollutant of this sort, as is PAN (peroxyacyl nitrate). Such chemicals are common components of photochemical smog, a term that refers to the complex mixture of chemicals that forms when certain airborne chemicals given off by plant and human activity react with sunlight to produce a brownish mixture of thousands of different chemical species.

Finally, there are also pollutants that do not cause direct health impacts but that may have the potential to cause harm indirectly, through their actions on the overall ecology, or as they function as precursor chemicals that lead to the production of other harmful chemicals. The major indirect-action pollutants include volatile organic carbon (VOC) compounds that act as precursors to more harmful species; chemicals called halocarbons; and chemicals called greenhouse gases.

ENERGY USE AND POLLUTION

Energy use is the predominant source of global air pollution, though other human actions produce significant amounts of pollution as well. The burning of biomass for agriculture, such as the burning of crop stubble is one such nonenergy source, as is the use of controlled-burn management of forest fires. Natural events such as forest fires as well as volcanic out-gassing and eruption also contribute to air pollution. Finally, even living things produce considerable quantities of emissions considered "pollutants." Animals and insects produce a large share of the world's methane emissions, while plants emit significant quantities of volatile organic carbon compounds—enough, in some areas to produce elevated ozone levels with no other pollutant emissions at all.

Ozone

Ozone, or O₃, a chemical consisting of three atoms of oxygen, is a colorless, odorless gas produced by a variety of chemical reactions involving hydrocarbons and nitrogen oxides in the presence of sunlight. Often there is confusion about ozone's effects because it is found at two different levels of the atmosphere, where it has two very different effects. At ground level, ozone is known to be a respiratory irritant, implicated in causing decreased lung function, respiratory problems, acute lung inflammation, and impairment of the lungs' defense mechanisms. Outdoor workers, elderly people with pre-existing lung diseases, and active children who spend significant amounts of time in areas with elevated ozone levels are thought to be particularly at risk.

Low-altitude ozone forms when sunlight reacts with "precursor" chemicals of both human and nonhuman origin. These precursors include volatile organic carbon compounds, carbon monoxide, and nitrogen oxides. Volatile organic carbon compounds are created both naturally (by plants) and through human activity such as fuel use, biomass burning, and other industrial activities such as painting and coating. Nitrogen oxides are generated by stationary sources of energy use such as power plants and factories, and mobile sources such as cars, trucks, motorcycles, bulldozers, and snowmobiles. Because of the diversity of ozone precursors, sources vary by region.

At high altitudes, ozone forms naturally from the interaction of high-energy solar radiation and normal diatomic oxygen, or O₂. High-altitude ozone is not considered a pollutant, but is actually beneficial to life on Earth, as it screens out solar radiation that can damage plants, or cause skin cancer and cataracts in animals. High-altitude ozone can be destroyed through

the action of chemicals called halocarbons, which are commonly used in refrigeration and air conditioning. Ozone levels across the United States fell, on average, by four percent between 1989 and 1998.

Volatile Organic Carbon Compounds

There are many different chemical species in the VOC compounds group, including such commonly known chemicals as formaldehyde and acetone. The common feature that VOCs share is that they are ring-shaped (organic) molecules consisting principally of carbon, hydrogen, oxygen, and nitrogen. VOCs are released into the environment as a result of human activity, but also because of natural biological processes in plants and animals. Table 1 shows the percentage of global contribution to VOC concentrations from all sources.

VOCs react in the presence of sunlight to produce photochemical smog, a mixture of organic chemicals that can irritate the eyes and other mucous membranes. VOCs also constitute a major precursor chemical leading to ozone production. VOC levels across the United States fell, on average, by 20.4 percent between 1989 and 1998.

Particulate Matter

Particulate matter, generated through a range of natural and manmade processes including combustion and physical abrasion, has been implicated in increased mortality for the elderly, as well as those members of the population with damaged respiratory systems. Studies also have linked particulate matter with aggravation of preexisting respiratory and cardiovascular disease, resulting in more frequent and/or serious attacks of asthma in the elderly or in children.

The particulate matter of most concern consists of particles that are most likely to be trapped in tiny air sacs of the lung (alveoli) after inhalation. Studies suggest that such particles are in the microscopic range, from less than 1 micrometer in diameter to about 2.5 microns in average diameter.

Although it has been shown that exposure to certain air pollutants can aggravate preexisting lung ailments such as asthma, no causal link has been identified between exposure to low-level air pollution and asthma. In fact, while asthma levels have been rising, air pollution levels have been declining, suggesting that there is probably a different cause for the increase in asthma rates. While various indoor air pollutants have been suggested as the cause, no definitive cause for the increase in asthma rates has yet been found.

Particulate matter is generated by stationary sources such as power plants and factories and by mobile sources such as cars, trucks, motorcycles, bulldozers, and snowmobiles. Particulate matter levels across the United States fell, on average, by 25 percent between 1989 and 1998.

Carbon Monoxide

Carbon monoxide, or CO, is a highly toxic chemical that chemically binds to hemoglobin, rendering it incapable of carrying oxygen to the tissues of the body. CO is produced by the incomplete combustion of fossil fuels. Carbon monoxide levels across the United States fell, on average, by 39 percent between 1989 and 1998.

Lead

Lead is an element used in many industrial processes and also has been used in fuels and coatings. Tetraethyl lead was added to gasoline to improve performance as a motor fuel, and elemental lead was extensively used in paints and coatings to improve coverage and durability until the 1970s, when phase-out efforts began to reduce lead emissions to the environment.

Long-term exposure to airborne lead was shown to lead to a variety of health problems, primarily neurological. Atmospheric lead concentrations fell dramatically through the 1970s and continue to fall: Atmospheric lead concentrations fell, on average, by 56 percent between 1989 and 1998. The amount of lead found in the bloodstream of children growing up in urban environments also fell dramatically.

Sulfur Oxides

Sulfur oxide emissions enter the atmosphere from a variety of sources, some of human origin, others of natural origin. The main sulfur oxide is sulfur dioxide, or SO₂.

High concentrations of SO₂ can produce temporary breathing difficulties in asthmatic children and in adults who are active outdoors. Sulfur dioxide also can directly damage plants and has been shown to decrease crop yields. In addition, sulfur oxides can be converted to sulfuric acid and lead to acid rain. Acid rain can harm ecosystems by increasing the acidity of soils as well as surface waters such as rivers, lakes, and streams. Sulfur dioxide levels fell, on average, by 39 percent between 1989 and 1998.

Nitrogen Dioxide

Nitrogen dioxide (NO₂) is a reddish-brown gas that is formed through the oxidation of nitrogen oxide (NO). The term "nitrogen oxides," or NO_x is used to encompass NO₂ as well as NO and the other oxides of nitrogen that lead to NO₂ production.

Nitrogen oxides are generated by both human and nonhuman action, but the major sources of NO_x are high-temperature combustion processes such as those occurring in power plants and automobile engines. Natural sources of NO_x include lightning, chemical processes that occur in soil, and the metabolic activities of plants.

Short-term exposure to elevated levels of NO_x have been shown to cause changes in the function of human lungs, while chronic exposures have been linked to increased susceptibility to lung infections and to lasting changes in lung structure and function. Nitrogen oxides also are of concern because of their roles as ozone precursors, and through their contribution to acid rain in the form of nitric acid.

Nitrogen dioxide concentrations have changed little since 1989, and alterations in measuring techniques make it difficult to accurately assess the trend. Data suggest that NO₂ concentrations may have increased by 2 percent between 1989 and 1998.

AIR POLLUTION REGULATION IN THE UNITED STATES

Air pollution in the United States is regulated at federal, state, and local levels. Allowable concentrations of the major air pollutants are set by the U.S. Environmental Protection Agency (EPA) under the auspices of the Clean Air Act. States and localities implement pollution control plans in accordance with the provisions of the Clean Air Act in regions where air pollutant concentrations exceed the federal standards. Some states and localities have air pollution standards of their own, and in the past, such standards have occasionally been more stringent than those of the EPA.

The EPA sets two kinds of national ambient air quality standards. The primary standard is set at a level intended to protect human health with an adequate margin of safety. The secondary standard, usually less stringent, is set based on protecting the public welfare, which can include factors other than health impacts, such as reduced visibility, and damage to crops.

Table 3 shows the current health-related national ambient air quality standards set by the EPA as of 1999. Regions that violate these standards may be classified as "nonattainment" areas by the EPA, and can face sanctions if they do not promulgate pollution control plans that are acceptable to the agency.

Environmental regulations to curtail air pollution have had a major impact on energy producers, manufacturers of energy-using products, and energy consumers. Before the Clean Air Act of 1970, coal-fired steam turbines were the least expensive means available for utilities to generate electricity. Coal still remains the least expensive fossil fuel, yet the high capital costs of installing the technology to comply with ever more stringent environmental regulations have resulted in many utilities reconsidering their options for new electric-power-generating facilties. The combined cycle natural gas turbine is being favored not only because of the much easier compliance with current air quality regulations but also because of the likelihood of stricter future regulations. A typical coal power plant burns more than 70 lb of coal each second, and considering that these plants number in the hundreds, coal-burning emissions are likely to be a major future target of legislators and regulators eager to curtail emissions further.

Transportation is another sector that is a major contributor to air pollution. To improve air quality, particularly in urban areas, regulations require the use of reformulated gasoline and alternative fuels to reduce emissions of nitrogen oxides and carbon monoxide. Reflecting the higher cost of refining, these regulations have added at least 5 to 10 cents to the price of gasoline at the pump.

Besides cleaner fuels, vehicle makers have developed many emission-reducing technologies—both in "cleaner combustion" and in catalytic converter technologies—to comply with ever stricter tailpipe emission standards. The U.S. EPA stringent standards proposed in 1999 for model year 2004 vehicles will result in new vehicles emitting less than 1 percent of the VOC and NO_x emissions of their 1960s counterparts.

Kenneth Green

See also:Acid Rain; Air Quality, Indoor; Atmosphere; Automobile Performance; Climatic Effects; Emission Control, Vehicle; Emission Control, Power Plant; Environmental Economics; Environmental Problems and Energy Use; Gasoline and Additives; Transportation, Evolution of Energy Use and; Turbines, Gas.

BIBLIOGRAPHY

Finlayson-Pitts, B. J., and Pitts, J. N., Jr. (2000). Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications.San Diego: Academic Press.

Klaassen, C. D. (1996) Casarett & Doull's Toxicology: The Basic Science of Poisons, 5th ed. New York: McGraw-Hill.

Miller, F. W., and Miller, R. M. (1993). Environmental Hazards: Air Pollution—A Reference Handbook. Santa Barbara, CA: ABC-CLIO.

U. S. Environmental Protection Agency. (1998). "National Air Quality and Emissions Trends Report, 1998." <http://www.epa.org/>.

Air pollution

Criteria pollutants

Resources

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

Air pollution is a serious health issue. According to the World Health Organization, nearly five million people die annually from causes directly due to air pollution (examples include aggrevations of asthma, emphysema, bronchitis, lung disease, and heart disease).

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.

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 miles (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 miles (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.

Criteria pollutants

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.

Efforts to curb the use of vehicles in Mexico City during the 1990s have resulted in an improved air quality. This vigilence must be maintained to ensure that the air quality does not return to dangerous levels. As well, in Mexico City and other jurisdictions, the increasing use of bioethanol, biodiesel, solar power, and gasoline-electric hybrid vehicle technologies will curb air pollution.

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,

KEY TERMS

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

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

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

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

Beijing, and Sao Paulo regularly reach levels scientists consider dangerous to human, animal, and plant life.

See also Atmosphere, composition and structure; Global warming; Ozone layer depletion.

Resources

BOOKS

Cooper, C. David and F. C. Alley. Air Pollution Control (3rd Edition). Prospect Heights IL:Waveland Press, 2002.

DuPuis, E. Melanie. Smoke and Mirrors: The Politics and Culture of Air Pollution. New York: New York University Press, 2004.

Somersall, Allan C., and James Marsden. Fresh Air for Life: How to Win Your Unseen War Against Indoor Air Pollution. Mississauga ON: Natural Wellness Group, 2006.

OTHER

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/> (accessed October 29, 2006).

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720. <http://www.lbl.gov/Education/ELSI/pollution-main.html> (accessed October 29, 2006).

Brian Hoyle

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).

cross-references

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

Air Pollution

Introduction

Air pollution refers to the change in the composition of the atmosphere by gases or other material released from Earth’s surface. The pollutants can originate naturally. A well-known example is ash that can be propelled upward during an eruption of a volcano. More commonly, however, air pollution is the result of human activities. Examples include the smoke emitted from the chimneys of factories and wood-burning homes as well as vehicle exhaust.

The released pollutants enter an atmosphere that is only about 250 miles (400 km) thick. Compared to the vastness of the surrounding universe, Earth’s atmosphere is extremely thin.

Air pollution has occurred ever since the formation of the atmosphere. Human-mediated air pollution dates back to prehistoric times, when fire was harnessed as a source of warmth and for food preparation. However, air pollution increased markedly during the Industrial Revolution in the mid-nineteenth century, and accelerated during the latter half of the twentieth century.

A legacy of the twentieth century is the use of compounds such as chlorofluorocarbons (CFCs). Their entry into the atmosphere has caused the depletion of a constituent called ozone, which in turn has led to an increasing amount of potentially harmful ultraviolet radiation reaching Earth’s surface. Other pollutants such as carbon dioxide (CO₂) are fueling global warming—the increasing warming of the atmosphere that has been occurring since the late nineteenth century and which has accelerated since the mid-twentieth century. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) stated that air pollution of human origin is at least partially responsible for global warming.

As well, since the pollutants are in the air we breathe, air pollution can be unhealthy. Respiratory diseases can be aggravated or even caused by breathing polluted air.

Historical Background and Scientific Foundations

Atmospheric pollution has been part of Earth’s history since the atmosphere formed. Early in the planet’s history, outgassing from volcanoes contributed ash and gases to the atmosphere. This form of air pollution continues. For example, Kilauea Volcano—the most recently active volcano on the island of Hawaii, which has been active since 1983—spews out about 2,000 tons of sulfur dioxide every day during eruptions. In another and more spectacular example, a series of huge eruptions of the Krakatoa Volcano on August 27, 1883, sent millions of tons of ash 50 miles (80 km) into the atmosphere. As the ash dispersed throughout the atmosphere, the resulting haze partially blocked the passage of sunlight. Indeed, the average global temperature during 1884 was more than one degree Celsius below normal.

As visually spectacular as volcanic eruptions are, their contribution to air pollution is miniscule compared to the air pollution resulting from human activities, which have occurred for hundreds of thousands of years, ever since the harnessing of fire for cooking and warmth. In these ancient times, human-mediated air pollution was relatively insignificant compared to centuries later. With time and increasingly sophisticated human activities, air pollution began to increase. A historical record of atmospheric change can be found in ice cores—long plugs of compacted snow that are retrieved from the centuries-old accumulation of snow that occurs in glaciers in far northern and southern regions. The analysis of ice cores recovered from the Greenland ice sheet reveals elevated amounts of lead, mercury, and nickel beginning about 5,000 years ago. This corresponds to the beginning of mining and smelting of metal ores in Europe. The smelting byproducts released to the atmosphere dispersed and eventually settled in far-flung regions including Greenland.

Air pollution began to increase near the end of the eighteenth century and early nineteenth century, during the period that is known as the Industrial Revolution. At that time, technological advances including the introduction of the steam engine made it feasible to establish factories and manufacturing plants in or near major cities. The cheap labor pool and ready access to land and water transportation spurred the growth of factories. Their use of coal as fuel increased the amount of concentration of air pollution.

Over the intervening two centuries, the composition of the atmosphere has been affected due to air pollution. Atmospheric data collected since the Industrial Revolution has demonstrated a change in the content of what are termed trace gases. Furthermore, entirely new compounds have appeared and their content has increased. The best example is CFCs, a product of twentieth-century technology.

Air pollutants are classified as being primary or secondary compounds. Primary pollutants are directly given off by a particular process or activity. Emissions from a

WORDS TO KNOW

ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

ANTHROPOGENIC SOURCES: Sources that are due to human activity. An example of anthropogenic air pollution is the burning of wood for fuel.

GREENHOUSE GAS: A gas that accumulates in the atmosphere and absorbs infrared radiation, contributing to the greenhouse effect.

PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.

factory smokestack, the tailpipe of an idling car, or the ash propelled out of an erupting volcano are examples of primary pollutants. Secondary pollutants are not given off directly, but rather form in the air when other compounds interact with one another.

A variety of primary pollutants are associated with human activities (anthropogenic sources). Sulfur oxides are compounds made up of a sulfur atom and varying numbers of oxygen atoms. Sulfur dioxide (SO₂) is given off by the burning of coal and oil. Carbon monoxide (CO) is gas that is given off when natural gas, wood, or coal are inefficiently burned, as for example when wood that is wet or contains resin is used as fuel in home wood stoves, and from automobile exhaust. Particularly in the home, this type of primary pollution is dangerous since the colorless and odorless gas can restrict the amount of oxygen the body receives. Carbon dioxide (CO₂) is also emitted in exhaust. It is one of several compounds that are known as greenhouse gases, since they restrict the reflection of sunlight back out of the atmosphere. Nitrogen oxides are compounds made up of a nitrogen atom and varying numbers of oxygen atoms. Nitrogen dioxide (NO₂), which is emitted by the burning of material at high temperature, is responsible for the brown-colored haze above cities such as Los Angeles, California, and Beijing, China. Smoke, dust, and volatile organic compounds are other primary pollutants.

Examples of secondary pollutants are particles that form when gases in the air interact with another pollutant that is a mixture of smoke and SO₂ (better known as smog), peroxyacetyl nitrate (which is formed by the interaction of nitrogen oxides and volatile organic compounds), and ground-level ozone that forms near Earth’s surface. Ground-level ozone is a particular problem in some major cities including Los Angeles and Mexico City.

Ground-level ozone irritates the respiratory tract and eyes. In cities where it forms frequently, ground-level ozone can be dangerous, causing chest tightness,

IN CONTEXT: TYPES OF AIR POLLUTION

Air pollution is of special concern to the human population. The seven types of air pollution considered the greatest threat to human health in the United States, and the first regulated by the 1970 U.S. Clean Air Act, include sulfur dioxide, particulates (dust, smoke, etc.), carbon monoxide, volatile organic compounds, nitrogen oxides, ozone, and lead. In 1990, another 189 volatile chemical compounds from more than 250 sources were added to the list of regulated air pollutants in the United States.

Air contaminants are divided into two broad categories: 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. In contrast, secondary pollutants are created or modified into a deleterious form after being released into the air.

coughing, and wheezing, especially in those with respiratory and heart problems. Paradoxically, higher in the atmosphere, the problem with ozone is its depletion, which is allowing more harmful ultraviolet sunlight to reach Earth’s surface.

Other air pollutants include heavy metals and both chemical and biological toxins. Some air pollutants are able to persist in the atmosphere for a long time. Known as persistent organic pollutants, compounds such as dioxin and furan may be transported long distances and so have the potential to affect people far from their point of release. Additionally, they can cause adverse health effects at low concentrations.

Impacts and Issues

The adverse health effects of air pollution are well-known. According to the World Health Organization (WHO), nearly 2.5 million people die each year from deaths that are the direct result of air pollution. These include pollution-aggravated bronchitis, asthma, emphysema, lung disease, heart disease, and allergic reactions.

A graphic example of the health threat posed by air pollution occurred in December 1952, when the weeklong formation of smog over London, England, was attributed to the immediate death of more than 4,000 people and the respiratory- and heart-related deaths over the next few months of 8,000 more.

Human industrial activity is overwhelmingly the greatest contributor to atmospheric pollution. According to the U.S. Environmental Protection Agency, in the United States alone more than six billion pounds of toxic compounds are released to the atmosphere every year.

High resolution satellite imaging has documented the influence of industrial development on air quality over developing regions of India and the city of Beijing. Satellite monitoring of China as part of the European Space Agency’s Dragon Programme has revealed that Beijing’s growth into an industrially robust mega-city of over ten million people and millions of cars and other vehicles has produced the planet’s highest levels of nitrogen dioxide. The gas, which is a respiratory irritant, is the main reason why the Chinese government is contemplating banning vehicle traffic in the city during the 2008 Olympics.

Air pollution is also the principal reason for the warming of the atmosphere that has been occurring since the Industrial Revolution, and which has accelerated since the mid-twentieth century.

Efforts to reduce air pollution include implementation of air quality regulations in many countries; use of more environmentally friendly energy sources, such as wind power and solar power; and the use of hybrid vehicles. Also, in the United States, in tandem with a move to reduce dependence on non-U.S. oil resources, the George W. Bush administration committed the nation to an increased use of biofuels.

However, the latter strategy is being re-examined in the light of water availability in the present and the coming century. A U.S. Department of Energy document released in February 2008 chronicled the heavy dependency on water and the large energy requirement of U.S. production of biofuels initiatives.

Efforts to reduce air pollution also require cooperative efforts between different governments. For example, in 1991 the United States and Canada signed an air quality agreement to address the trans-border movement of pollutants. Without such efforts, local pollution efforts would be much less effective.

See Also Acid Rain; Carbon Dioxide (CO₂) Emissions; Chlorofluorocarbons; Light Pollution; Smog

BIBLIOGRAPHY

Books

Ho, Mun S., and Chris P. Nielsen. Clearing the Air: The Health and Economic Damages of Air Pollution in China. Boston: MIT Press, 2007.

Kidd, J. S., and R. A. Kidd. Air Pollution: Problems and Solutions. New York: Facts on File, 2005.

Schwartz, Joel. Air Quality in America: A Dose of Reality on Air Pollution Levels, Trends, and Health Risks. Washington: AEI Press, 2008.

Brian D. Hoyle