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Air Quality - What Are The Major Air Pollutants?

emissions ozone percent figure

Air quality standards established under the Clean Air Act of 1970 (CAA; PL 91-604) are designed to protect public health and welfare. The act was amended by the CAA Amendments of 1990 (CAAA; PL 101-549) with the goal of cleaning up urban air. It established air quality standards called the National Ambient Air Quality Standards (NAAQS) for six major air pollutants:

  • carbon monoxide (CO)
  • lead (Pb)
  • nitrogen oxides (NO5)
  • ozone (O3)
  • particulate matter (PM)
  • sulfur oxides (SO5)

These are called the priority or criteria pollutants and are identified as serious threats to human health. The CAA also required states to develop plans to implement and maintain the NAAQS. The states can have stricter rules than the federal program but not more lenient ones.

The EPA has documented air pollution trends in the United States annually since 1973. Its Latest Findings on National Air Quality: 2002 Status and Trends (2003) reports two kinds of trends for priority pollutants: emissions and air quality concentrations. Emissions are calculated estimates of the total tonnage of these pollutants released into the air annually. Air quality concentrations are based on data collected at thousands of monitoring sites around the country. The EPA maintains a database called the National Emission Inventory that characterizes the emissions of air pollutants in the United States based on data input from state and local agencies.

From 1970 to 2002 total emissions of the six priority pollutants decreased by 48 percent. This occurred even as the United States experienced massive increases in gross domestic product and vehicle miles traveled and moderate increases in overall energy consumption and population. Figure 5.1 compares emissions in 1970 and 2001 for lead and between 1970 and 2002 for all other principal air pollutants. The figure shows that emissions have declined for each pollutant. However, there is still much work to be done to clear the air. The EPA estimates that during 2002 approximately 160 million tons of air pollutants were emitted in the United States.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas created when the carbon in certain fuels is not burned completely. These fuels include coal, natural gas, oil, gasoline, and wood.


In 2002 approximately 100 million tons of CO were emitted into the air. As shown in Figure 5.1, CO emissions have decreased by 48 FIGURE 5.1
Comparison of 1970 and 2002 emissions
percent since 1970. Figure 5.2 shows the primary sources of CO emissions for 1983 to 2002. Transportation has historically been the largest source. In 2002 transportation accounted for 82 percent of known CO emissions. In recent years forest fires (planned and unplanned) have contributed 5–15 percent of the nation's carbon monoxide emissions. Non-transportation fuel combustion, industrial processes, and miscellaneous other sources are minor contributors of CO emissions, each contributing less than 5 percent of the total.

Non-transportation fuel combustion occurs at power plants, homes, and businesses that burn fuel to produce power or heat. In 1940 cars and trucks created about 28 percent of all CO emissions, while homes burning coal and oil made up about 50 percent. From 1940 through 1970 emissions from cars and trucks nearly tripled. By 1970 they accounted for 71 percent of all CO emissions and continued to increase into the 1980s. Their contribution to the total began to fall as vehicle fuel efficiency improved.

The EPA estimates that in 2002 motor vehicle exhaust from road vehicles accounted on average for 60 percent of all CO emissions. The value was as much as 95 percent in cities with heavy traffic congestion. Non-road vehicles account for the remainder of transportation-related CO emissions. Non-road vehicles include tractors, lawn mowers, construction equipment (such as bulldozers), industrial equipment (such as forklifts), snowmobiles, all-terrain vehicles, boats, trains, and airplanes.


Air quality concentrations of CO from 1983 to 2002 are shown in Figure 5.3 based on monitoring FIGURE 5.2
Carbon monoxide emissions, 1983–2002
data from 205 sites around the country. Over this time period CO concentrations decreased by 65 percent. In 2002 the average measured CO concentration at the monitoring sites was just under three parts per million (three parts CO per million parts of air).

Despite this data the EPA reports that in 2002 approximately 700,000 Americans lived in counties with air quality concentrations of CO above the NAAQS. These nonattainment areas, as they are called, are designated as having "serious" or "moderate" CO pollution depending on the air quality concentrations. As of January 2004 serious CO nonattainment areas include the following:

  • Anchorage and Fairbanks, Alaska
  • Las Vegas, Nevada
  • Los Angeles, California
  • Phoenix, Arizona
  • Spokane, Washington

Moderate CO nonattainment areas include Provo, Utah; El Paso, Texas; Missoula, Montana; and Reno, Nevada.


CO is a dangerous gas that enters a person's bloodstream through the lungs. It reduces the ability of blood to carry oxygen to the body's cells, organs, and tissues. The health danger is highest for people suffering from cardiovascular diseases.

Carbon monoxide air quality concentrations, 1983–2002


Lead is a metal that can enter the atmosphere via combustion or industrial processing of lead-containing materials.


Prior to 1985 the major source of lead emissions in the United States was the leaded gasoline used in automobiles. Conversion to unleaded gasoline produced a dramatic reduction in lead emissions. As shown in Figure 5.4, transportation has virtually been eliminated as a source of lead emissions. Non-transportation fuel combustion is also a minor contributor. Industrial processes (chiefly metals smelting and battery manufacturing) are responsible for the bulk of lead emissions. The EPA reports that lead emissions declined by 93 percent between 1982 and 2002.


Air quality concentrations of lead based on monitoring data from 42 sites from 1983 to 2002 are shown in Figure 5.5. Over this time period lead concentrations decreased by 94 percent.

Despite great progress in lead reduction, the EPA reports that in 2002 approximately 200,000 Americans lived in counties with air quality concentrations of lead above the NAAQS. As of January 2004 there are only three lead nonattainment areas in the country: East Helena, Montana, and portions of both Iron County and Jefferson County in Missouri.


Lead is a particularly dangerous pollutant because it accumulates in the blood, FIGURE 5.4
Lead emissions, 1982-2002
bones, and soft tissues of the body. It can adversely affect the nervous system, kidneys, liver, and other organs. Excessive concentrations are associated with neurological impairments, mental retardation, and behavioral disorders. Even low doses of lead can damage the brains and nervous systems of fetuses and young children. Atmospheric lead that falls onto vegetation poses an ingestion hazard to humans and animals.

Nitrogen Dioxide

Nitrogen dioxide (NO2) is a reddish-brown gas that forms in the atmosphere when nitrogen oxide (NO) is oxidized. The chemical formula NO5 is used collectively to describe NO, NO2, and other nitrogen oxides.


NO2 primarily comes from burning fuels such as gasoline, natural gas, coal, and oil. The exhaust from transportation vehicles is the major source of NO2, accounting for 56 percent of emissions during 2002. Fuel combustion in power plants, homes, and businesses accounted for 37 percent of NO5 emissions. Industrial processes and miscellaneous sources were minor contributors.

Overall, emissions of NO5 decreased by 15 percent between 1983 and 2002. (See Figure 5.6.) Most of this improvement occurred during the late 1990s and early 2000s.

The EPA notes that despite the overall decrease in NO5 emissions in this country, emissions from non-road FIGURE 5.5
Lead air quality, 1983–2002
engines increased dramatically between 1983 and 2002. Non-road engines are those in sources such as tractors, lawn mowers, construction and industrial equipment, off-road recreational vehicles, boats, trains, and airplanes. As of April 2004 the EPA has proposed new regulations to control NO5 emissions from non-road diesel engines.


NO2 is a major precursor of smog and also contributes to acid rain and haze. It can also undergo reactions in the air that lead to the formation of particulate matter and ozone.

Air quality concentrations of NO2 based on monitoring data from 125 sites around the country from 1983 to 2002 are shown in Figure 5.7. Over this time period NO2 concentrations decreased by 21 percent. In 2002 the average measured NO2 concentration at the monitoring sites was approximately 0.02 parts per million. That year all U.S. counties attained the EPA standards for NO2 air quality.


Inhalation of even low concentrations of NO2 for short time periods can be harmful to the human body's breathing functions. Longer exposures are considered damaging to the lungs and may cause people to be more susceptible to certain respiratory problems, such as infections.


Ozone is a gas naturally present in Earth's upper atmosphere. Approximately 90 percent of Earth's ozone lies in the stratosphere at altitudes greater than about 20 miles. Ozone molecules at this level absorb ultraviolet FIGURE 5.6
Nitrogen oxides emissions, 1983-2002
(UV) radiation from the sun and prevent it from reaching the ground. Thus, stratospheric ozone (the "ozone layer") is good for the environment. Tropospheric (or ground-level) ozone, on the other hand, is a potent air pollutant with serious health consequences.

Ground-level ozone has become a persistent problem in many parts of the world. Ground-level ozone is the most complex, pervasive, and difficult to control of the six priority pollutants.


Unlike other pollutants, ground-level ozone is not emitted directly into the air. It forms mostly on sunny, hot days due to complex chemical reactions that take place when the atmosphere contains other pollutants, primarily volatile organic compounds (VOCs) and nitrogen oxides (NO5) emitted from industrial chemical processes and fossil fuel combustion. Such pollutants are called ozone precursors because their presence in the atmosphere leads to ozone creation.

VOCs are carbon-containing chemicals that easily become vapors or gases. Paint thinners, degreasers, and other solvents contain a great number of VOCs, which are also released from burning fuels such as coal, natural gas, gasoline, and wood. Cars are a major source of VOCs. From 1940 through 1970 VOC emissions increased about 77 percent, mainly because of the increase in car and truck traffic and industrial production. However, since 1970 national VOC emissions have decreased as a result of emission controls placed on cars and trucks and less open burning of solid waste.

Nitrogen oxides air quality, 1983–2002

VOC emissions dropped by 40 percent from 1983 to 2002. (See Figure 5.8.) Most of the decline occurred in industrial processing and transportation sources. However, these two sources still account for the vast majority of VOC emissions. In 2002 industrial processes and transportation each accounted for approximately 40 percent of the emissions. In recent years forest fires have contributed 5–10 percent of VOC emissions. Fuel combustion and miscellaneous sources are also minor contributors.

VOCs and NO5 are the primary ozone precursors. Although emissions of both decreased during the 1980s and 1990s, the EPA notes that additional reduction in NO5 emissions must take place to achieve any substantial improvements in ozone air quality.


Ozone has different health and environmental effects depending on the time of exposure. The EPA monitors average eight-hour and one-hour ozone levels and sets different standards for each. Ozone concentrations can vary greatly from year to year depending on the emissions of ozone precursors and weather conditions.

As shown in Figure 5.9, the average national ozone concentration, based on an eight-hour average, decreased by 14 percent between 1983 and 2002 but increased by 4 percent between 1993 and 2002. The average one-hour concentration decreased by 22 percent between 1983 and 2002. Between 1993 and 2002, however, the decrease was only 2 percent. (See Figure 5.10.)

In 2002 both the one-hour and eight-hour averages for ozone air quality exceeded the NAAQS. The one-hour FIGURE 5.8
Volatile organic compounds emissions, 1983–2002
exceedance affected 68 million people, while the eight-hour exceedance affected 136.4 million people. This means that ozone has, by far, the largest nonattainment area of any of the six priority pollutants. As of April 15, 2004, the nonattainment area for the eight-hour ozone standard includes more than 400 counties, as shown in Figure 5.11. Most are clustered around major cities throughout the Northeast, Southeast, and Midwest, and in Texas, Colorado, Arizona, Nevada, and California.

Ground-level ozone is the primary component in smog. Smog, a word made up by combining "smoke" and "fog," is probably the most well-known form of air pollution. It retards crop and tree growth, impairs health, and limits visibility. When temperature inversions occur (the warm air stays near the ground instead of rising) and winds are calm, such as during the summer, smog may hang over a huge area for days at a time. As traffic and other pollution sources add more pollutants to the air, the smog gets worse. Wind often blows smog-forming pollutants away from their sources; this is why smog frequently can be more serious miles away from where the pollutants were created.

Most people associate dirty air with cities and the areas around them. There is good reason for this, because some of the worst smog in the country occurs in such urban areas as Los Angeles, California—a city known for its air quality problems. In a major industrial nation such FIGURE 5.9
Ozone air quality based on 8-hour average, 1983–2002
as the United States, however, smog is not limited just to cities. The Great Smoky Mountains, located in western North Carolina and eastern Tennessee, are seeing more air pollution. Harmful emissions from various coal-burning facilities located outside the mountain range, as well as pollution from motor vehicles, are damaging the mountain's environment.

Ground-level ozone is harmful to ecosystems, particularly vegetation. Ozone exposure reduces forest yields by stunting the growth of seedlings and increasing stresses on trees. Such damage can take years to become evident. Between 1993 and 2002 the EPA monitored ozone levels based on eight-hour average concentrations at 28 national parks around the country. The results indicated that ozone levels increased at 18 of the parks, remained unchanged at four other parks, and decreased at six parks. Of the eighteen parks showing increased ozone levels, five of them experienced what the EPA calls "statistically significant" upward trends. These parks are as follows:

  • Great Smoky Mountains (Tennessee)
  • Craters of the Moon (Idaho)
  • Mesa Verde (Colorado)
  • Denali (Alaska)
  • Acadia (Maine)

Only one national park—Saguaro National Park in Arizona—showed significant improvements in ozone levels.

Ozone air quality based on 1-hour average, 1983–2002


Even the smallest amounts of ozone can cause breathing difficulties. Ozone exposure can cause serious problems with lung functions, leading to infections, chest pain, and coughing.

According to the EPA, ozone exposure is linked with increased emergency room visits and hospital admissions due to such respiratory problems as lung inflammation and asthma. Ozone causes or aggravates these problems, particularly in people working outdoors, the elderly, and children. Children are especially susceptible to the harmful effects of ozone because they spend a great deal of time outside and because their lungs are still developing.

According to the Centers for Disease Control and Prevention, the percentage of American children with asthma more than doubled between 1980 and 2001. In 1980 approximately 3.7 percent of all children age 17 and younger suffered from asthma. By 2001 this figure had climbed to 8.7 percent. In general asthma levels are greater among children that live in inner cities, areas also prone to higher concentrations of ozone, smog, and other air pollutants.

Long-term exposure of any age group to moderate levels of ozone is thought to cause irreversible lung damage due to premature aging of the tissues.

The EPA maintains an Air Quality Index (AQI) as a means for warning the public when air pollutants exceed unhealthy levels. AQI values range from 0 to 500. Higher values correspond to greater levels of air pollution and increased risk to human health. An AQI value of 100 is FIGURE 5.11
Attainment and nonattainment areas for 8-hour ozone standard, 2004
assigned to the concentration of air pollutant equal to its NAAQS. For example, the average eight-hour ozone level considered unhealthy is 0.08 parts per million (ppm). Table 5.1 shows the ozone AQI. Index values are commonly reported during summertime radio and television newscasts to warn people about the dangers of ozone exposure.

In State of the Air: 2003 (May 2003), the American Lung Association assessed the quality of air in U.S. communities, giving them grades ranging from "A" through "F" based on how often their air quality exceeds the "unhealthy" limits of the EPA's Air Quality Index. The 2003 report found that 137 million Americans live in areas that received an "F." That is about 69 percent of the nation's population who live in counties where there are ozone monitors.

Living in the monitored counties that received a failing grade are more than seven million adult asthmatics and nearly two million children who had an asthma attack within the previous year, 4.7 million people with chronic bronchitis, and 1.5 million people suffering from emphysema. Just over 16 million older adults (age 65 and up) lived in monitored counties that received a failing grade. The organization found that, although cities in southern California generally scored poorly, such Sunbelt states as Texas, Tennessee, Georgia, North Carolina, and South Carolina also scored low. The American Lung Association's list of the 25 metropolitan areas with the worst ozone pollution in 2003 is shown in Table 5.2.

Particulate Matter

Particulate matter (PM) is the general term for the mixture of solid particles and/or liquid droplets found in the air. Primary particles are those emitted directly to the atmosphere—for example, dust, dirt, and soot (black carbon).

Air Quality Index (AQI): Ozone

Index values Levels of health concern Cautionary statements
0–50 Good None
51–100* Moderate Unusually sensitive people should consider limiting prolonged outdoor exertion.
101–150 Unhealthy for sensitive groups Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
151–200 Unhealthy Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.
151–200 Unhealthy Active children and adults, and people with
201–300 Very unhealthy Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
301–500 Hazardous Everyone should avoid all outdoor exertion.
*Generally, an AQI of 100 for ozone corresponds to an ozone level of 0.08 parts per million (averaged over 8 hours).
SOURCE: "Air Quality Index (AQI): Ozone," in Air Quality Index—A Guide to Air Quality and Your Health, U.S. Environmental Protection Agency, Office of Air and Radiation, Washington, DC, June 2000

Secondary particles form in the atmosphere due to complex chemical reactions among gaseous emissions and include sulfates, nitrates, ammoniums, and organic carbon compounds. For example, sulfate particulates can form when sulfur dioxide emissions from industrial facilities and power plants undergo chemical reactions in the atmosphere.

The EPA tracks two sizes of particulate matter: PM10 and PM2.5. PM10 are all particles less than or equal to 10 micrometers in diameter. This is roughly one-seventh the diameter of a human hair and small enough to be breathed into the lungs. PM2.5 are the smallest of these particles (less than or equal to 2.5 m in diameter). PM2.5 is also called fine PM. The particles ranging in size between 2.5 and 10 m in diameter are known as coarse PM. Most coarse PM is primary particles, while most fine PM is secondary particles.


Sources of particulate matter include unpaved roads, agriculture and forestry, residential wood stoves and fireplaces, and fuel combustion in vehicles, power plants, and industry.

The EPA tracks trends in direct PM emissions from anthropogenic (human-caused) sources. As shown in Figure 5.12, direct PM10 emissions declined by 22 percent between 1993 and 2002. The major sources have historically been fuel combustion, industrial processes, and transportation. In 2002 industrial processes accounted for 46 percent of direct PM10 emissions. Fuel combustion at power plants and in homes and businesses contributed another 33 percent. The exhaust from transportation vehicles contributed 21 percent.

The American Lung Association ranks America's 25 most ozone-polluted cities in 2004

2004 rank Metropolitan statistical areas Total population
1 Los Angeles-Long Beach-Riverside, CA 17,044,188
2 Fresno-Madera, CA 964,897
3 Bakersfield, CA 694,059
4 Visalia-Porterville, CA 381,772
5 Houston-Baytown-Huntsville, TX 5,086,741
6 Merced, CA 225,398
7 Sacramento-Arden-Arcade-Truckee, CA-NV 2,068,427
8 Hanford-Corcoran, CA 135,043
9 Knoxville-Sevierville-La Follette, TN 796,760
10 Dallas-Fort Worth, TX 5,676,171
11 Washington-Baltimore-N. Virginia, DC-MD-VA-WV 7,826,485
12 Philadelphia-Camden-Vineland, PA-NJ-DE-MD 5,899,571
13 New York-Newark-Bridgeport, NY-NJ-CT-PA 21,705,461
14 Charlotte-Gastonia-Salisbury, NC-SC 1,993,372
15 Cleveland-Akron-Elyria, OH 2,950,615
16 Greensboro-Winston-Salem-High Point, NC 1,315,361
17 Pittsburgh-New Castle, PA 2,512,302
18 San Diego-Carlsbad-San Marcos, CA 2,906,660
18 Phoenix-Mesa-Scottsdale, AZ 3,500,151
20 Modesto, CA 482,440
21 Atlanta-Sandy Springs-Gainesville, GA 4,844,726
22 Morristown-Newport, TN 159,648
23 Raleigh-Durham-Cary, NC 1,401,331
23 Lancaster, PA 478,561
25 Sheboygan, WI 112,480
SOURCE: "Table 2b: People at Risk in 25 Most Ozone-Polluted Cities," in State of the Air: 2004, American Lung Association, New York, NY, April 29, 2004. Reprinted with permission © 2004 American Lung Association. For more information on how you can fight lung disease, the third-leading cause of death in the United States, please contact The American Lung Association at 1-800-LUNG-USA (1-800-586-4872) or visit the Web site at

These sources are called "traditionally inventoried" PM sources. As shown in Figure 5.13, they actually contribute only a small portion of total direct emission of PM10. In 2002 the EPA estimates that fugitive dust was the primary culprit, accounting for 63 percent of all emissions. This is dust thrown up into the air when vehicles travel over unpaved roads and during land-disturbing construction activities such as bulldozing. Agriculture and forestry operations also stir up soil; in 2002 they accounted for 22 percent of direct PM10 emissions.

Fugitive dust, agriculture, and forestry combine to contribute 85 percent of all direct PM10 emissions. However, these sources are not as great a concern to air quality as the traditionally inventoried sources. This is because soil, dust, and dirt thrown up into the air does not typically travel far from its original location or climb very far into the atmosphere. The final source category shown in Figure 5.13 is called other combustion. This includes emissions associated with wildfires and managed burning on forests and grasslands. In 2002 these sources contributed 5 percent of all direct PM10 emissions.

Direct emissions of PM2.5 also declined between 1993 and 2002. They dropped from approximately 2,230 tons per year in 1993 to 1,850 tons per year in 2003, a FIGURE 5.12
Emissions of particulate matter less than 10 micrometers in diameter, 1993–2002
decrease of 17 percent. The major sources of direct PM2.5 are the same as those shown for PM10 in Figure 5.12. The historical percentage breakdown of these sources is also very similar.

Most PM2.5 is not comprised of primary particles from direct emissions but of secondary particles that form in the atmosphere. The EPA tracks secondary PM2.5 particle types at monitoring sites around the country. Data collected in 2001 and 2002 indicate that sulfates, ammonium, and carbon are the principal secondary particles found in the eastern part of the nation. These pollutants are largely associated with coal-fired power plants. In western states (particularly California), carbon and nitrates comprise most of the secondary particles. On a national level, secondary PM2.5 concentrations are generally higher in urban areas than in rural areas.


When PM hangs in the air, it creates a haze, limiting visibility. PM is one of the major components of smog and can have adverse effects on vegetation and sensitive ecosystems. Long-term exposure to PM can damage painted surfaces, buildings, and monuments.

From 1940 to 1971, PM in the air generally increased. Pollution-control laws, however, led to a drop in PM, beginning in the 1970s. Figure 5.14 shows the historical trend in PM10 air quality based on data collected by the EPA from 804 monitoring sites. Between 1993 and 2002 PM10 concentrations decreased by 13 percent. The EPA reports that in 2002 there were 15 million people living in FIGURE 5.13
Emissions of particulate matter smaller than 10 micrometers in diameter, 2002
counties with PM10 concentrations greater than the NAAQS. The vast majority of these people lived in southern California. PM10 nonattainment areas were also designated in parts of all other western states, particularly Arizona and Nevada.

In 1999 the EPA began nationwide tracking of PM2.5 air quality concentrations. Between 1999 and 2002 these concentrations decreased by 8 percent based on data collected from 858 monitoring sites. In February 2004 the EPA asked states and tribes to recommend PM2.5 designations for counties within their jurisdictions. Final designations for PM2.5 attainment and nonattainment areas are expected to be published in late 2004 or early 2005. State and local governments with nonattainment areas will be given three years to develop plans for reducing levels of fine particulates.


PM can irritate the nostrils, throat, and lungs and aggravate respiratory conditions such as bronchitis and asthma. PM exposure can also endanger the circulatory system and is linked with cardiac arrhythmias (episodes of irregular heartbeats) and heart attacks. PM2.5 are the most damaging, because their small size allows them access to deeper regions of the lungs. These small particles (less than or equal to 2.5 m in diameter) have been linked with the most serious health effects in humans. Particulates pose the greatest health FIGURE 5.14
Air quality for particulate matter less than 10 micrometers in diameter, 1993–2002
risk to those with heart or lung problems, the elderly, and especially children, who are particularly susceptible due to the greater amount of time they spend outside and the fact that their lungs are not fully developed.

Sulfur Dioxide

Sulfur dioxide (SO2) is a gas composed of sulfur and oxygen. The chemical formula SO5 is used collectively to describe sulfur oxide, SO2, and other sulfur oxides.


One of the primary sources of sulfur dioxide is the combustion of fossil fuels containing sulfur. Coal (particularly high-sulfur coal common to the eastern United States) and oil are the major fuel sources associated with SO2. Power plants have historically been the main source of SO2 emissions. Some industrial processes and metal smelting also cause SO2 to form.

From 1940 to 1970 SO2 emissions increased as a result of the growing use of fossil fuels, especially coal, in industry and power plants. Since 1970 total SO2 emissions have dropped because of greater reliance on cleaner fuels with lower sulfur content and the increased use of pollution control devices, such as scrubbers, to clean emissions. SO2 emissions decreased by 33 percent between 1983 and 2002. (See Figure 5.15.)

In 2002 fuel combustion in power plants accounted for 85 percent of SO2 emissions. The remainder was attributed to industrial processes (9 percent), transportation (5 percent), and miscellaneous sources (1 percent).

Sulfur dioxide (SO2) emissions, 1983–2002


Trends in air quality concentrations of SO2 are shown in Figure 5.16. The average concentration fell by 54 percent between 1983 and 2002. In 2002 the EPA reported that all counties in the United States met NAAQS for sulfur dioxide.

SO2 is a major contributor to acid rain, haze, and particulate matter. Acid rain is of particular concern because acid deposition harms aquatic life by lowering the pH (level of acidity; a lower value indicates more acid) of surface waters, impairs the growth of forests, causes depletion of natural soil nutrients, and corrodes buildings, cars, and monuments. Acid rain is largely associated with the eastern United States because eastern coal tends to be higher in sulfur content than coal mined in the western United States.

In 1990 the U.S. Congress established the Acid Rain Program under Title IV of the 1990 Clean Air Act Amendments. The goal of the program was (and is) to reduce annual emission of SO2 by 10 million tons and of NO5 by two million tons between 1980 and 2010. A permanent national cap of 8.95 million tons of SO2 per year is to be in effect for electric utilities by 2010. The program expects to meets its goals by tightening annual emission limits on thousands of power plants around the country.


Inhaling sulfur dioxide in polluted air can impair breathing in those with asthma or even in healthy adults who are active outdoors. As with other air pollutants, children, the elderly, and those with FIGURE 5.16
Sulfur dioxide (SO2) air quality, 1983–2002
preexisting respiratory and cardiovascular diseases and conditions are most susceptible to adverse effects from breathing this gas.

Summary for Primary Pollutants

In 2002 146.2 million people lived in counties with air quality concentrations greater than the level of the NAAQS. (See Figure 5.17.) Ozone (O3) was, by far, the largest problem, followed by particulate matter (PM), carbon monoxide (CO), and lead (Pb). Note that adding the values for individual pollutants does not equal the nationwide total. This is because some people live in counties that are nonattainment for multiple pollutants. Table 5.3 summarizes the main sources and health risks of the priority pollutants.

Air Toxics

Hazardous air pollutants (HAPs), also referred to as air toxics, are pollutants that may cause severe health effects or ecosystem damage. Serious health risks linked to HAPs include cancer, immune system disorders, neurological problems, reproductive effects, and birth defects. The Clean Air Act (CAA) lists 188 substances as HAPs and targets them for regulation in section 112 (b) (1). The air toxics program complements the NAAQS program. Examples of HAPs are benzene, dioxins, arsenic, beryllium, mercury, and vinyl chloride.

Major sources of HAP emissions include transportation vehicles, construction equipment, power plants, factories, and refineries. Some air toxics come from common FIGURE 5.17
Number of people living in counties with air quality concentrations above the level of the National Ambient Air Quality Standards (NAAQS) in 2002
sources. For example, benzene emissions are associated with gasoline. Air toxics are not subject to intensive national monitoring; the EPA and state environmental agencies monitor air toxic levels at approximately 300 sites nationwide. However, the EPA is working to build a more extensive monitoring network. In 2003 the agency launched the National Air Toxic Trend Site (NATTS) network. It is designed to follow trends in high-risk air toxics such as benzene, chromium, and formaldehyde.

Air pollutant emissions have been linked with increased risk of severe health problems and even death. The results of a study conducted between 1982 and 1998 on 500,000 adults across the United States were reported by C. Pope et al. in "Lung Cancer, Cardiopulmonary Mortality, and Long-Term Exposure to Fine Particulate Air Pollution," (Journal of the American Medical Association, March 6, 2002). The researchers found that high levels of fine particulates and sulfur oxides in the air were linked with higher mortality rates from cancer and cardiopulmonary illnesses. Each increase of 10 micrograms of these pollutants in a cubic meter of air was associated with an 8 percent increase in lung cancer mortality, a 6 percent increase in deaths due to cardiopulmonary diseases, and a 4 percent increase in all cancer deaths.

Air pollutants, health risks, and contributing sources

Pollutants Health risks Contributing sources
Ozone* (O3 Asthma, reduced respiratory function, eye irritation Cars, refineries, dry cleaners
Particulate matter (PM-IO) Bronchitis, cancer, lung damage Dust, pesticides
Carbon monoxide (CO) Blood oxygen carrying capacity reduction, cardiovascular and nervous system impairments Cars, power plants, wood stoves
Sulphur dioxide (SO2) Respiratory tract impairment, destruction of lung tissue Power plants, paper mills
Lead (Pb) Retardation and brain damage, esp. children Cars, nonferrous smelters, battery plants
Nitrogen dioxide (NO2) Lung damage and respiratory illness Power plants, cars, trucks
*Ozone refers to tropospheric ozone, which is hazardous to human health.
SOURCE: Fred Seitz and Christine Plepys, "Table 1. Criteria Air Pollutants, Health Risks and Sources," in Monitoring Air Quality in Healthy People 2000, Healthy People 2000—Statistical Notes, Centers for Disease Control and Prevention, National Center for Health Statistics, Hyattsville, MD, September 1995


In May 2002 the EPA released the latest findings from its National Air Toxic Assessment. The data, which are for 1996, show that approximately 4.6 million tons of air toxics were released into the air, down from a baseline value of six million tons for 1990–1993. Air toxics were emitted from many sources, including industrial and mobile (vehicles and non-road equipment) sources. The known carcinogens posing the greatest risks to human health were benzene and chromium. The suspected carcinogen showing the greatest risk was formaldehyde.


The Toxics Release Inventory (TRI) was established under the Emergency Planning and Community Right-to-Know Act of 1986. The TRI program requires annual reports on the waste management activities and toxic chemical releases of certain industrial facilities using specific toxic chemicals. The TRI list includes more than 650 toxic chemicals.

TRI reports have been required since 1987 from manufacturing facilities (called the "original" industries). In 1998 the reporting requirements were also applied to a second group of industries called the "new" industries. These include metal and coal mining, electric utilities burning coal or oil, chemical wholesale distributors, petroleum terminals, bulk storage facilities, Resource Conservation and Recovery Act subtitle C hazardous water treatment and disposal facilities, solvent recovery services, and federal facilities. However, the requirements FIGURE 5.18
Releases of toxic chemicals, 2001
only apply to facilities with 10 or more full-time employees that use certain thresholds of toxic chemicals.

In July 2003 the 2001 Toxics Release Inventory (TRI) Public Data Release Report was published. There were 6.16 billion pounds of chemical releases reported by covered facilities during 2001. The vast majority of the releases (91 percent) were on-site releases to air, land, and water. The remainder were off-site releases (when a facility sends toxic chemicals to another facility where they are then released). Although most (56.2 percent) of the releases were to landfills or surface impoundments, air emissions did account for just over 27 percent of the total. (See Figure 5.18.)

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