Air Pollution

Air Pollution 3563
Photo by: akiyoko

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 SO 2 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,

The New York City skyline on a smoggy day in the 1960s. (Roger Wood/Corbis. Reproduced by permission.)
The New York City skyline on a smoggy day in the 1960s. (
©Roger Wood/Corbis. Reproduced by permission
U.S. National Pollutant Emission Estimates for 1999
U.S. National Pollutant Emission Estimates for 1999 (
Adapted from

(thousand short tons)
Source Category CO NO x VOC SO 2 PM 10 PM 2.5 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.

National Ambient Air Quality Standards
National Ambient Air Quality Standards (
U.S. Environmental Protection Agency

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/m 3 ) Primary
1-hour Average 35 ppm (40 mg/m 3 ) Primary
Nitrogen Dioxide (NO 2 )      
Annual Arithmetic Mean 0.053 ppm (100 μg/m 3 ) Primary & Secondary
Ozone (O 3 )      
1-hour Average 0.12 ppm (235 μg/m 3 ) Primary & Secondary
8-hour Average 0.08 ppm (157 μg/m 3 ) Primary & Secondary
Lead (Pb)      
Quarterly Average 1.5 μg/m 3   Primary & Secondary
Particulate (PM 10) Particles with diameters of 10 micrometers or less
Annual Arithmetic Mean 50 μg/m 3   Primary & Secondary
24-hour Average 150 μg/m 3   Primary & Secondary
Particulate (PM 2.5) Particles with diameters of 2.5 micrometers or less
Annual Arithmetic Mean 15 μg/m 3   Primary & Secondary
24-hour Average 65 μg/m 3   Primary & Secondary
Sulfur Dioxide (SO 2 )      
Annual Arithmetic Mean 0.030 ppm (80 μg/m 3 ) Primary
24-hour Average 0.14 ppm (365 μg/m 3 ) Primary
3-hour Average 0.50 ppm (1300 μg/m 3 ) 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 (NO 2 ), sulfur dioxide (SO 2 ), 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 (O 3 ), and in 1983 the hydrocarbons standard was dropped altogether. In 1987 TSP was changed to PM 10 , and in 1997 PM 2.5 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.

Estimated Pollutant Emissions in the United States in 2000
Estimated Pollutant Emissions in the United States in 2000

(Thousand short tons)
Source Category CO NO x VOC SO 2 PM 10 PM 2.5
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 PM 10 (particulate matter with an aerodynamic diameter of 10 μm or less) standard was set. PM 10 , 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 PM 2.5 (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 (SO 2 ) is colorless, nonflammable, nonexplosive gas. Almost 90 percent of anthropogenic SO 2 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 SO 2 irritates the human upper respiratory tract. The most serious air pollution episodes occurred when there was a synergistic effect of SO 2 with PM and water vapor (fog). Because of this, it has proven difficult to isolate the effects of SO 2 alone.

SO 2 is one of the precursors of acid rain (the term used to describe the deposition of acidic substances from the atmosphere). Also, SO 2 is the precursor of secondary fine sulfate particles, which in turn affect human health and reduce visibility. Prolonged exposure to SO 2 and sulfate PM causes serious damage to materials such as marble, limestone, and mortar. The carbonates (e.g., limestone, CaCO 3 ) in these materials are replaced by sulfates (e.g., gypsum, CaSO 4 ) that are water-soluble and may be washed away easily by rain. This results in an eroded surface.

Nitrogen Dioxide. Nitrogen dioxide (NO 2 ) 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 NO 2 are collectively known as NO x .

Anthropogenic emissions of NO x come from high-temperature combustion processes, such as those occurring in automobiles and power plants. Natural sources of NO 2 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 (O 3 ) is a secondary pollutant and is formed in the atmosphere by the reaction of molecular oxygen, O 2 , and atomic oxygen, O, which comes from the photochemical decomposition of NO 2 . Volatile organic compounds or VOCs (e.g., what one smells when refuelling the car) must also be present if O 3 is to accumulate in the atmosphere.

O 3 occurs naturally in the stratosphere and provides a protective layer from the sun's ultraviolet rays high above the earth. However, at ground level, O 3 is a lung and eye irritant and can cause asthma attacks, especially in young children or other susceptible individuals. O 3 , 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 SO 2 , NO 2 , CO, and Pb, is easier than control of pollutants that are either entirely secondary (O 3 ) or have a significant secondary component (PM 2.5 ). Primary pollutants may be controlled at the source. For example, SO 2 is controlled by the use of scrubbers, which are industrial devices that remove SO 2 from the exhaust gases from power plants. SO 2 emissions are also reduced by the use of low-sulfur coal or other fuels, such as natural gas, that contain lower amounts of sulfur. NO 2 from industrial sources also may be minimized by scrubbing. NO 2 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 PM 2.5 is secondary in nature, its control is achieved by control of SO 2 , NO 2 , and VOC (which are the precursors of sulfates, nitrates, and carbon-containing particulates).



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

User Contributions:

Report this comment as inappropriate
Feb 22, 2009 @ 10:22 pm
..thank you for posting this info in the internet. i hope everyone will have the chance to read and understand it. god bless.
Report this comment as inappropriate
Aug 31, 2011 @ 7:07 am
this can help us in order for us to know what is the importance of our environment especially our atmosphere. an educated site which can give idea not only for me and others but also for the next generation ...

Comment about this article, ask questions, or add new information about this topic:

Air Pollution forum