Air Quality

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According to the U.S. Environmental Protection Agency (EPA), in "Why Should You Be Concerned about Air Pollution?" (June 25, 2007, /, the average person breathes in more than three thousand gallons of air each day. Because air is so essential to life, it is important that it be free of pollutants. Throughout the world poor air quality contributes to hundreds of thousands of deaths and diseases each year, as well as to dying forests and lakes and the corrosion of stone buildings and monuments. Air quality is also important to quality of life and recreation because air pollution causes haze that decreases visibility during outdoor activities.

Air pollutants are generated by natural and anthropogenic (human-related) sources. Fossil fuels and chemicals have played a major role in society's pursuit of economic growth and higher standards of living. However, burning fossil fuels and releasing toxic chemicals into the air alter Earth's chemistry and can threaten the very air on which life depends.

Air quality plays a major and complex role in public health. Among the factors that must be considered are the levels of pollutants in the air, the levels of individual exposure to these pollutants, individual susceptibility to toxic substances, and the time of exposure related to ill effects from certain substances. Blaming health effects on specific pollutants is also complicated by the health impact of nonenvironmental causes (such as heredity or poor diet).

Scientists do know that air pollution is related to a number of respiratory diseases, including bronchitis, pulmonary emphysema, lung cancer, bronchial asthma, eye irritation, weakened immune system, and premature lung tissue aging. In addition, lead contamination causes neurological and kidney disease and can be responsible for impaired fetal and mental development. The American Lung Association estimates in "Clean Air Is up to You!" (2007, that the annual health cost of exposure to the most serious air pollutants is at $40 billion to $50 billion.


Air pollution from the burning of fossil fuels was a problem in urban areas of England as early as the fourteenth century. In 1307 King Edward I (12391307) banned the burning of coal in London "to avoid the sulfurous smoke" and commanded Londoners to burn wood instead. The ban was short lived, however, as a wood shortage forced the city to switch back to coal. Historians record that future British monarchs also tried unsuccessfully to curtail the use of coal to reduce air pollution.

The onset of the Industrial Revolution in the late 1700s was accompanied by a tremendous increase in the use of fossil fuels and air pollution in England and the United States. Major U.S. cities began passing smoke ordinances during the late 1800s. Air pollution control remained a local issue for several more decades.

By the late 1940s smog had become a serious problem in many urban areas. Extensive industrial growth during World War II (19391945), a boom in car ownership, and unregulated outdoor burning were the primary culprits. Los Angeles and other large U.S. cities suffered from smog during hot summer months. In 1952 London experienced an episode of smog so severe that thousands of people prematurely died from respiratory illnesses aggravated by poor air quality. The incident was a wake-up call for many governments. Air pollution legislation was quickly passed in England and across Europe.

U.S. Air Pollution Legislation

In the United States concerns about smog led to the passage of the Air Pollution Control Act of 1955. It provided grants to public health agencies to research the threats posed to human health by air pollution. In 1963 the first Clean Air Act (CAA) was passed. It set aside even more grant money for research and data collection and encouraged the development of emissions standards for major sources of pollution. The act was amended several times through the remainder of the decade to expand research priorities and local air pollution control agencies and set national emission standards for some sources.

the caa of 1970

the caa of 1970. In 1970 the CAA received a major overhaul. It required the newly established EPA to set the National Ambient Air Quality Standards (NAAQS) for major pollutants. These standards are divided into two classes:

  • Primary standards are designed to protect public health, with special focus on so-called sensitive populations, including children, the elderly, and people with chronic respiratory problems, such as asthma.
  • Secondary standards are designed to protect the overall welfare of the public by reducing air pollution that impairs visibility and damages resources, such as crops, forests, animals, monuments, and buildings.

State environmental agencies have to prepare state implementation plans to show how they plan to achieve compliance with the NAAQS. Counties that meet the NAAQS for a particular pollutant are called attainment areas for that pollutant; counties that do not meet the NAAQS are called nonattainment areas.

The revised CAA also required the setting of National Emissions Standards for Hazardous Air Pollutants and resulted in the New Source Performance Standards (NSPS). These are technology-based standards that apply when certain types of facilities are first constructed or undergo major modifications. Even though the NSPS are set by the EPA, state governments are responsible for enforcing them.

In 1977 the CAA was amended again. The major change was expansion of a program called the Prevention of Significant Deterioration (PSD). The PSD program is designed to ensure that new facilities built in attainment areas do not significantly degrade the air quality.

the clean air act amendments of 1990

the clean air act amendments of 1990. In 1990 the CAA was substantially revised to better address three issues of growing concern: acid rain, urban air pollution (particularly smog), and emissions of toxic air pollutants. In addition, a national permits program was established, and enforcement and compliance procedures were strengthened. The Clean Air Act Amendments (CAAA) of 1990 included new and innovative approaches to air pollution legislation. Market-based programs allow businesses more choices in how they achieve pollution control goals. Economic incentives were also included to reduce the reliance on regulations to obtain certain goals. An outline of the major sections of the CAAA is:

  • Title IAir Pollution Prevention and Control
  • Title IIEmission Standards for Moving Sources
  • Title IIIGeneral
  • Title IVAcid Deposition Control
  • Title VPermits
  • Title VIStratospheric Ozone Protection


The CAAA of 1990 established the NAAQS for six major air pollutants:

  • Carbon monoxide
  • Lead
  • Nitrogen oxides
  • Ozone
  • Particulate matter
  • Sulfur oxides

These are called the priority or criteria pollutants and are identified as serious threats to human health. The CAAA 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. In addition, regulations developed under the CAAA cover nearly two hundred chemical substances classified as hazardous air pollutants, which are also called air toxics.

The EPA has documented air pollution trends in the United States since 1970. Two kinds of trends are tracked 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. The most recent comprehensive report on priority pollutant emission sources is the National Air Quality and Emissions Trends Report, 2003 (September 2003,, which is based on emissions data from 2001 and 2002.

Table 2.1 compares emissions of the principal air pollutants for various years between 1970 and 2006. The table shows that emissions have declined for each pollutant. This occurred even as the United States experienced massive increases in the gross domestic product (the total value of goods and services produced by a nation) and vehicle miles traveled and moderate increases in overall energy consumption and population. However, there is still much work to be done to clear the air. The EPA estimates that during 2006 nearly 137 million tons of air pollutants were emitted in the United States.


Trends in national air pollutants, selected years 19702006
Millions of tons per year
1. In 1985 and 1996 EPA refined its methods for estimating emissions. Between 1970 and 1975, EPA revised its methods for estimating PM emissions.
2. The estimates for 2002 are from 2002 National Emissions Inventory v2; the estimates for 2003 and beyond are preliminary and based on 2002 NEI v2.
3. For CO, NOx, SO2 and VOC emissions, fires are excluded because they are highly variable; for direct PM emissions both fires and dust are excluded.
4. PM estimates do not include condensable PM.
5. EPA has not estimated PM2.5 emissions prior to 1990.
6. The 1999 estimate for lead is used for 2000, and the 2002 estimate for lead is used for 2005 and 2006.
7. PM2.5 emissions are not added when calculating the total because they are included in the PM10 estimate.
8. Fires and dust are excluded.
SOURCE: "National Air Pollutant Emissions Estimates (fires and dust excluded) For Major Pollutants," in Air Quality and EmissionsProgress Continues in 2006, U.S. Environmental Protection Agency, June 12, 2007, (accessed July 19, 2007)
Carbon monoxide (CO)1971841781701441201029188
Nitrogen oxides (NOx)272627262525221918
Particulate matter (PM)
Sulfur dioxide (SO2)312826232319161514
Volatile organic compounds (VOC)343030272322171515

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.

emissions and sources

emissions and sources. The EPA reports in Air Quality and EmissionsProgress Continues in 2006 (June 12, 2007, that in 2006 approximately eighty-eight million tons of CO were emittedintothe air. As shownin Table 2.2, CO emissions have decreased by 74% since 1980. Transportation has historically been the largest source of CO emissions. Figure 2.1 shows the primary sources of CO emissions in 2002. Transportation accounted for the vast majority (77%) of CO emissions that year. Miscellaneous processes accounted for 15% of CO emissions. Stationary source fuel combustion (for example, in power plants), industrial processes, and waste disposal and recycling each contributed 4% or less to the total.

air quality

air quality. Air quality concentrations of CO from 1990 to 2006 are shown in Figure 2.2 based on monitoring data from 243 sites around the country. Over this time period, CO concentrations decreased by 62%. In 2006 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 these improvements, there are still areas of the country with air quality concentrations of CO that are consistently above the NAAQS. These nonattainment areas are designated as having "serious" or "moderate" CO pollution, depending on the air quality concentrations. In August


Percent change in air quality concentrations, 1980 vs. 2006 and 1990 vs. 2006
1980 vs 20061990 vs 2006
In the table above for PM (particulate matter) these are percent changes between 1999 vs, 2006.
NO2=Nitrogen dioxide. O3=Ozone. SO2=Sulfur dioxide. PM10=Particulate matter less than 10 microns. PM2.5=Particulate matter less than 2.5 microns. CO=Carbon monoxide. Pb=Lead.
SOURCE: "Percent Change in Air Quality," in Air Quality and EmissionsProgress Continues in 2006, U.S. Environmental Protection Agency, June 12, 2007, (accessed July 19, 2007)

2007 the EPA ( classified Las Vegas, Nevada, as a serious CO nonattainment area, and El Paso, Texas, Missoula, Montana, and Reno, Nevada, were moderate CO nonattainment areas.

adverse health effects

adverse health effects. 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.



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

emissions and sources

emissions and sources. The EPA reports that lead emissions declined from 740,000 tons in 1980 to 2,000 tons in 2006, a decrease of 95%. (See Table 2.1 and Table 2.2.) Before 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 a result, ground transportation has virtually been eliminated as a source of lead emissions. As shown in Figure 2.3, industrial processes (chiefly metals smelting and battery manufacturing) are responsible for 78% of lead emissions. The contribution by the transportation sector (12%) is largely because of airplane traffic.

air quality

air quality. Air quality concentrations of lead based on monitoring data from forty-two sites from 1980 to 2006 are shown in Figure 2.4. Over this time period, lead concentrations decreased by 54%. Despite great progress in lead reduction, the EPA ( noted in August 2007 that there were still two lead nonattainment areas in the country: East Helena, Montana, and Jefferson County, Missouri.



adverse health effects

adverse health effects. Lead is a particularly dangerous pollutant because it accumulates in the blood, bones, and soft tissues of the body. It can adversely affect the nervous system, kidneys, liver, and other organs. Excessive concen- FIGURE 2.4 trations are associated with neurological impairments, mental retardation, and behavioral disorders. Even low doses of lead can damage the brain and nervous system 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. 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.

The chemical formula NOx is used collectively to describe NO, NO2, and other nitrogen oxides.

emissions and sources

emissions and sources. As shown in Table 2.1, there were eighteen million tons of NOx emitted during 2006. Emissions have decreased by 41% since 1980. (See Table 2.2.) Most of this improvement occurred during the late 1990s and early 2000s.

NO2 primarily comes from burning fuels such as gasoline, natural gas, coal, and oil. The exhaust from transportation vehicles is the major source of NOx, accounting for 54% of emissions during 2002. (See Figure 2.5.) Fuel combustion in power plants, homes, and businesses accounted for 39% of NOx emissions. Minor contributors



include industrial processes, miscellaneous sources, and waste disposal and recycling.

air quality

air quality. NO2 is a major precursor of smog and contributes to acid rain and haze. It can also undergo reactions in the air that lead to the formation of particulate matter and ozone. Figure 2.6 illustrates the air quality concentrations of NO2 basedon monitoringdata from 170 sites around the country from 1990 to 2006. Over this time period NO2 concentrations decreased by 30%. In 2006 the average measured NO2 concentration at the monitoring sites was less than 0.02 parts per million. As of August 2007 the EPA ( reported that all U.S. counties attained the EPA standards for NO2 air quality.

adverse health effects

adverse health effects. NOx reacts with ammonia and water droplets in the atmosphere to form nitric acid and other chemicals that are potentially harmful to human health. Inhalation of these particles can interfere with respiratory processes and damage lung tissue. Particles inhaled deeply into the lungs can cause or aggravate respiratory conditions such as bronchitis and emphysema.


Ozone is a gas naturally present in Earth's upper atmosphere. The National Safety Council's Environmental Health Center indicates in Reporting and Climate Change: Understanding the Science (June 2000,


guidebks/climtoc.htm) that approximately 90% of Earth's ozone lies in the stratosphere at altitudes greater than about twenty miles. Ozone molecules at this level absorb ultra-violet radiation from the sun and prevent it from reaching the ground. Thus, stratospheric ozone (the ozone layer) is good for the environment. By contrast, tropospheric (ground-level) ozone is a potent air pollutant with serious health consequences. It is the most complex, pervasive, and difficult to control of the six priority pollutants.

emissions and sources

emissions and sources. Unlike other air pollutants, ground-level ozone is not emitted directly into the air. It forms mostly on sunny, hot days because of complex chemical reactions that take place when the atmosphere contains other pollutants, primarily volatile organic compounds (VOCs) and NOx. 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.

As shown in Table 2.1, VOC emissions dropped from thirty-four million tons per year in 1970 to fifteen million tons per year in 2006. Figure 2.7 provides a breakdown of sources for VOC emissions in 2002. Transportation accounted for nearly half of the emissions (44%), followed closely by industrial processes (42%). Minor contributors included stationary source fuel combustion, miscellaneous sources, and waste disposal and recycling.

air quality

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 2.8, the average national ozone concentration, based on an eight-hour average, decreased by 9% between 1990 and 2006. Between 1980 and 2006 the average one-hour concentration decreased by 29%. (See Figure 2.9.)

In August 2007 the EPA ( reported that there were dozens of locations around the country classified as nonattainment for ozone air quality. One area was classified as "severe" nonattainment for the eight-hour ozone standard: the Los Angeles basin in California. Locations classified as "serious" nonattainment for the eight-hour standard included three other California areas: Riverside County, Sacramento, and the San Joaquin Valley. Many


other areas in and around major cities were deemed "moderate" or "marginal" nonattainment.

ozone contributes to smog

ozone contributes to smog. Ground-level ozone is the primary component in smog. Smog, a word made up by combining the words 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 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 urban areas such as Los Angelesa city known for its air quality problems. In a major industrial nation such 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 mountains' 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. In Latest Finds on National Air Quality: 2002 Status and Trends (August 2003,, the EPA notes that between 1993 and 2002 it monitored ozone levels based on eight-hour average concentrations at twenty-eight national parks around the country. The results indicate that ozone levels increased at eighteen of the parks, remained unchanged at four other parks, and decreased at six parks.

adverse health effects

adverse health effects. 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 because of respiratory problems such 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 Lara J. Akinbami of the Centers for Disease Control and Prevention, in "The State of Childhood Asthma, United States, 19802005" (Advance Data from Vital and Health Statistics, no. 381, December 12, 2006), the percentage of American children with asthma more than doubled between 1980 and 2005. In 1980, 3.6% of all children aged seventeen and younger suffered from asthma. By 2005 this figure had climbed to 8.9%. In general, asthma levels are greater among children who live in inner cities, areas that are 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 because of premature aging of the tissues.

The EPA maintains the Air Quality Index (AQI) as a means for warning the public when air pollutants exceed unhealthy levels. The AQI values range from zero to five hundred. Higher values correspond to greater levels of air pollution and increased risk to human health. An AQI value of one hundred is assigned to the concentration of an air pollutant equal to its NAAQS. For example, the average eight-hour ozone level considered unhealthy is 0.08 parts per million (ppm). Table 2.3 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: 2007 (May 1, 2007,, the American Lung Association assesses the quality of air in U.S. communities. The organization ranks metropolitan areas and counties in terms of their air pollutant levels. The ten metropolitan areas with the worst ozone pollution in 2007 were:

  • Los Angeles, Long Beach, and Riverside, California
  • Bakersfield, California


Air Quality Index (AQI): Ozone
Index valuesLevel of health concernCautionary statements
*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 IndexA Guide to Air Quality and Your Health, U.S. Environmental Protection Agency, Office of Air and Radiation, June 2000, (accessed July 27,2007)
51100*ModerateUnusually sensitive people should consider limiting prolonged outdoor exertion.
101150Unhealthy for sensitive groupsActive children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
151200UnhealthyActive children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit outdoor exertion.
201300Very unhealthyActive children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
301500HazardousEveryone should avoid all outdoor exertion.
  • Visala and Porterville, California
  • Fresno and Madera, California
  • Houston, Baytown, and Huntsville, Texas
  • Merced, California
  • Dallas and Fort Worth, Texas
  • Sacramento and Arden-Arcade, California; and Truckee, Nevada
  • Baton Rouge and Pierre Part, Louisiana
  • New York City; Newark, New Jersey; and Bridgeport, Connecticut

Particulate Matter

Particulate matter (PM) is the general term for the mixture of solid particles and/or liquid droplets found in the air. The primary particles are those emitted directly to the atmospherefor example, dust, dirt, and soot (black carbon). Secondary particles form in the atmosphere because of 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 PM: PM10and PM2.5. PM10particles are those less than or equal to ten micro-meters in diameter. This is roughly one-seventh the diameter of a human hair and small enough to be breathed into the lungs. PM2.5are the smallest of these particles (less than or equal to 2.5 micrometers in diameter). PM2.5 is also called fine PM. The particles ranging in size between 2.5 and 10 micrometers in diameter are known as coarse PM. Most coarse PM is primary particles, whereas most fine PM is secondary particles.

emissions and sources

emissions and sources. The EPA tracks trends in direct PM emissions from certain anthropogenic sources, mainly fuel combustion at power plants and in homes and businesses, industrial processes, and transportation exhaust. These are called traditionally inventoried sources. As shown in Table 2.1 and Table 2.2, direct PM emissions of both sizes declined dramatically between 1990 and 2006.

The EPA believes that the bulk of PM10in the atmosphere comes from fugitive dust and agricultural and forestry practices that stir up soil. Fugitive dust is dust thrown up into the air when vehicles travel over unpaved roads and during land-disturbing construction activities such as bulldozing. In 2002 these sources were estimated to account for 85% of all PM10 emissions as shown in Figure 2.10. 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 far into the atmosphere.

Most PM2.5is 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 make up most of the secondary particles. On a national level secondary PM2.5 concentrations are generally higher in urban areas than in rural areas.

air quality

air quality. 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.

Figure 2.11 shows the historical trend in PM10 air quality based on data collected by the EPA from 381 monitoring sites. Between 1990 and 2006 PM10 concentrations decreased by 30%. The EPA ( reported in August 2007 that dozens of areas around the country were non-attainment for PM10 concentrations. Areas classified "serious" nonattainment were in Southern California and parts of Nevada and Arizona.

In 1999 the EPA began nationwide tracking of PM2.5 air quality concentrations. Between 1999 and 2006 these


concentrations decreased by 15% based on data collected from 750 monitoring sites. (See Figure 2.12.) In August 2007 the EPA ( noted that there were thirty-nine locations around the country deemed nonattainment for PM2.5, mostly major- and medium-sized metropolitan areas. Under the CAA, states with nonattainment areas must submit to the EPA by 2008 a plan for reducing air pollutant emissions that lead to the formation of PM2.5 particles in the atmosphere. The plan must list the enforceable measures to be taken and provide a schedule to become attainment as quickly as possible.

adverse health effects

adverse health effects. 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 particles are the most damaging, because their small size allows them access to deeper regions of the lungs. These small particles have been linked with the most serious health effects in humans. Particulates pose the greatest health risk to those with heart or lung problems, the elderly, and especially children, who are particularly susceptible because of 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 SOx is used collectively to describe sulfur oxide, SO2, and other sulfur oxides.

emissions and sources

emissions and sources. One of the primary sources of SO2 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. Between 1970 and 2006 SO2 emissions declined by more than half as shown in Table 2.1. A 66% decrease was obtained between 1980 and 2006. (See Table 2.2.)

Fuel combustion in stationary sources (for example, in power plants) has traditionally produced most SO2 emissions. In 2002 this source accounted for 85% of SO2 emissions. (See Figure 2.13.) Industrial processes contributed another 9%. Transportation, miscellaneous sources, and waste disposal and recycling were minor contributors.

air quality

air quality. Trends in air quality concentrations of SO2 are shown in Figure 2.14. The average concentration fell by 53% between 1990 and 2006. The EPA ( reported in August 2007 that there were ten nonattainment areas around the country for SO2. These included locations in Montana, Utah, New Jersey, Pennsylvania, Arizona, and Guam.

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 (which stands for potential hydrogen and means the level of acidity; a lower value indicates more acid) of surface waters; impairs the growth of forests; causes the 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 Congress established the Acid Rain Program under Title IV of the 1990 CAAA. The program called for major reductions in SO2 and NOx emissions from certain coal-fired power plants and other combustion units generating


electricity around the country. The EPA notes in Acid Rain Program: 2005 Progress Report (October 2006, that the program set two emissions goals:

  • Reduce SO2 emissions by half (in other words, by ten million tons) by 2010 compared with the emissions released in 1980 and maintain a cap after 2010.
  • Achieve a two-million-ton reduction in NOx emissions compared with the NOx emissions projected for 2000 if the program had not been implemented.

The program expects to meet its goals by tightening annual emission limits on thousands of power plants around the country.

adverse health effects

adverse health effects. Inhaling SO2 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 preexisting respiratory and cardiovascular diseases and conditions are the most susceptible to adverse effects from breathing this gas.

The Clear Skies Controversy

In 2002 President George W. Bush (1946) proposed his Clear Skies initiative to set nationwide caps on emissions


of sulfur dioxide, nitrogen oxides, and mercury from power plants. Even though Clear Skies legislation has been introduced in Congress several times, as of September 2007 it had not been passed. According to the EPA, in "Clear Skies: Basic Information" (March 2, 2006,, the program would reduce sulfur dioxide emissions by 73%, mercury emissions by 69%, and nitrogen dioxide emissions by 67% from 2000 levels when fully implemented by 2018.

However, environmental groups are opposed to the proposed Clear Skies Act because it would institute a cap-and-trade system. This would set overall caps on emissions but allow utility companies operating below emission limits to sell credits to other companies having trouble meeting the limits. Even though a similar system is used to control emissions that cause acid rain, critics believe this approach is not appropriate for more potent air pollutants, such as mercury. They fear that the capand-trade system would allow utility plants in some areas to release unacceptably high levels of these pollutants.

Nevertheless, some of the objectives of the Clear Skies Act have been implemented by the EPA through other programs, including the Clean Air Interstate Rule and the Clean Air Mercury Rule.

clean air interstate rule

clean air interstate rule. In 2005 the EPA issued the Clean Air Interstate Rule (CAIR; April 5, 2007, to tackle problems in the eastern United States with air pollutants that move across state boundaries. CAIR puts permanent caps on emissions of NOx and SO2 in twenty-eight eastern states and the District of Columbia. The rule is projected to reduce SO2 emissions by more than 70% and reduce NOx emissions by more than 60% compared with 2003 levels. Control of these pollutants is expected to reduce the formation of fine particulate matter, acid rain, and ground-level ozone across the country.

The program is to be carried out using the cap-and-trade system. The EPA predicts that full implementation of CAIR in 2015 will provide between $85 billion and $100 billion in annual health benefits and substantially reduce premature deaths in the eastern United States. Improvements are expected in visibility within southeastern national parks, which have been plagued by smog in recent years.

clean air mercury rule

clean air mercury rule. Mercury is a hazardous air pollutant that can fall out of the atmosphere into water supplies, where it is absorbed by fish and shellfish. The consumption of contaminated fish and shellfish is the primary source of mercury exposure to humans. Sources of mercury emissions to the air include gold mines, institutional boilers, chlorine production, waste incinerators, and coal-fired power plant boilers. The EPA (April 5, 2007, indicates that approximately forty-eight tons of mercury were emitted to the air by power plants in 1999, accounting for 43% of total U.S. mercury emissions.

In 2005 the EPA issued the Clean Air Mercury Rule (CAMR) to limit and reduce mercury emissions nationwide from coal-fired power plants. The EPA reports in "Clear Skies: Basic Information" that the CAMR two-phase program sets up an optional cap-and-trade system that will be fully implemented by 2018 and should reduce mercury emissions by nearly 70% from 1999 levels. The first-phase cap under the program takes effect in 2010 and is expected to reduce mercury emissions to around twenty-six tons per year. The last-phase cap implemented by 2018 will reduce mercury emissions to around fifteen tons per year.

The CAMR was immediately unpopular with some state governments, particularly those in the eastern United States, where many coal-fired power plants operate. The EPA chose 2018 for the final cap, because it claims that the control technologies needed to properly restrict mercury emissions will not be commercially available until after 2010. However, in Mercury Emissions from Electric Power Plants: States Are Setting Stricter Limits (February 22, 2007,, James E. McCarthy of the Congressional Research Service notes that eighteen states have established more stringent emissions limits for mercury that go into effect sooner than those set in the CAMR program. Even though the state programs rely on different mechanisms for achieving reductions, McCarthy indicates that many of them prohibit interstate and in-state trading of mercury credits, because they fear the creation of mercury "hot spots" in their jurisdictions. The high incidence of states opting out of the federal CAMR program casts doubts on its reliance on national emission trading as an effective means to reduce U.S. mercury emissions.

Air Toxics

Hazardous air pollutants (HAPs), also referred to as air toxics, are pollutants that can cause severe health effects and/or ecosystem damage. Serious health risks linked to HAPs include cancer, immune system disorders, neurological problems, reproductive effects, and birth defects. The 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.

The major sources of HAP emissions include transportation vehicles, construction equipment, power plants, factories, and refineries. Some air toxics come from common 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 three hundred sites nationwide.

In 2003 the EPA launched the National Air Toxic Trend Site network. It is designed to follow trends in high-risk air toxics such as benzene, chromium, and formaldehyde. The EPA has also awarded grants to state and local environmental agencies to conduct short-term monitoring of air toxics.

national-scale air toxics assessments

national-scale air toxics assessments. In May 2002 the EPA released the 1996 National-Scale Air Toxics Assessment ( During 1996 approximately 4.6 million tons of air toxics were released into the air, down from a baseline value of 6 million tons between 1990 and 1993. Air toxics were emitted from many sources, including industrial and mobile (vehicles and nonroad 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.

In February 2006 the EPA released the 1999 National-Scale Air Toxics Assessment ( It evaluates 177 HAPs plus particulate matter from the burning of diesel fuel. The assessment inventoried air toxics emissions, estimated ambient concentrations and population exposures, and characterized the potential public health risks, including cancer and noncancer effects. Table 2.4 shows that toluene, xylenes, hydrochloric acid, benzene, and formalde-hyde were the five most commonly emitted air toxics in 2002. Overall, the EPA finds that most Americans face a lifetime cancer risk because of exposure to outdoor air toxics of between one and twenty-five in a million. In other words, out of one million people, between one and twenty-five of them face an increased chance of developing cancer because of breathing outdoor air toxics. The upper range applies to people in urban areas. The risk is even greater (more than fifty in a million) for people living in certain high-risk areas, such as transportation corridors (areas where major roads and/or other modes of transport are clustered).

the toxics release inventory

the toxics release inventory. 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.

Accordingtothe EPA, inthe 2005 Toxics Release Inventory (TRI) Public Data Release eReport (March 2007,, in 2005 there were nearly 4.3 billion pounds of chemical releases reported by covered facilities. The vast majority (88%) of the releases were on-site releases to air, land,


The five most commonly emitted air toxics, 2002
PollutantPercentage of total air toxics emissionsPrimary sources of emissionsHealth effects
Note: Health effects are dependent upon the concentration of the air toxic and the length of exposure.
SOURCE: "Table 1. The Five Most Commonly Emitted Air Toxics, 2002," in Clean Air Act: EPA Should Improve the Management of Its Air Toxics Program U.S. Government Accountability Office, June 2006, (accessed June 19, 2007)
Toluene18Mobile sourcesImpairment of the nervous system with symptoms including tiredness, dizziness, sleepiness, confusion, weakness, memory loss, nausea, loss of appetite, and hearing and color vision loss; kidney problems; unconsciousness; and death.
Xylenes13Mobile sources, asphalt pavingIrritation of the skin, eyes, nose, and throat; headaches, dizziness, memory loss, and changes in sense of balance; lung problems; stomach discomfort; possible effects on the liver and kidneys; unconsciousness; and death.
Hydrochloric acid12Coal-fired utility and industrial boilersEye, nose, and respiratory tract irritation; corrosion of the skin, eyes, mucous membranes, esophagus, and stomach; severe burns; ulceration; scarring; inflammation of the stomach lining; chronic bronchitis; and inflammation of the skin.
Benzene9Mobile sources, open burning, pesticide applicationDrowsiness, dizziness, vomiting, irritation of the stomach, sleepiness, convulsions, rapid heart rate, headaches, tremors, confusion, unconsciousness, anemia, excessive bleeding, weakened immune system, increased incidence of cancer (leukemia), and death.
Formaldehyde7Mobile sources, open burningIrritation of the eyes, nose, throat, and skin; severe pain; vomiting; coma; limited evidence of cancer; and death.

and water. The remainder were off-site releases (when a facility sends toxic chemicals to another facility where they are then released). On-site air emissions amounted to 1.5 billion pounds and accounted for 35% of the total.

epa's air toxics program under fire

epa's air toxics program under fire. Since the passage of the original CAA in 1970, the EPA has concentrated on reducing emissions of the priority pollutants described earlier. Critics complain that the nation's air toxics program has been slow to be implemented effectively. This controversy is described by the U.S. Governmental Accountability Office (GAO), in Clean Air Act: EPA Should Improve the Management of Its Air Toxics Program (June 2006, The GAO states that the 1970 CAA required EPA to list HAPs and develop regulations for their control. The agency moved slowly in this area, developing regulations for only seven HAPs over the following two decades. As a result, Congress made dramatic changes to the air toxics program in the 1990 CAA.

The amended law listed a number of air toxics and directed the EPA to develop technology-based emissions limits for them applicable to eighty-four thousand "major stationary sources," such as chemical plants and incinerators that emit at least ten tons per year of a single HAP or at least twenty-five tons per year of combined HAPs. These standards are called maximum achievable control technology (MACT) limits. The EPA was supposed to follow up eight years after issuing the MACT limits to ensure that the limits were still effectively protecting human health and environmental quality from residual risks posed by the emissions. In addition, the EPA was to regulate HAP emissions from small stationary sources, such as dry cleaners and small manufacturers, and evaluate the feasibility of regulating mobile sources, such as automobiles. The GAO finds that the EPA has met less than half of the CAA requirements and has been late meeting the vast majority of those it has completed.


For several decades following the passage of the original CAA in 1970, air pollution from industrial sources was the primary focus of lawmakers and the public. As dramatic achievements in air quality were obtained in this sector, more attention was focused on air pollutants associated with transportation. Table 2.5 shows that in 2002 highway vehicles were major sources of U.S. emissions of CO (55.5%), NOx (34.9%), and VOCs (27.5%), and minor sources of other criteria air pollutants.

Exhaust Emission Limits

The 1990 CAAA included a program to control air pollution from new motor vehicles. The so-called EPA Tier 1 emission standards were issued in 1991 and took effect in the mid-1990s. They applied to all new light-duty vehicles weighing less than eighty-five hundred pounds. This included cars, pick-up trucks, and sports utility vehicles (SUVs). There were different emissions standards by weight class within Tier 1. In 1997 the EPA issued regulations for the National Low Emission Vehicle (NLEV) program, a voluntary program modeled after California standards for emission reductions from motor vehicles.

Because of California's extreme air pollution problems, the CAAA allowed states to set stricter emission


Total national emissions of the criteria air pollutants by sector, 2002
[Millions of short tons/percentage]
SectorCONOxVOCPM-10PM-2.5SO 2NH 3
Note: CO=Carbon monoxide. NOx=Nitrogen oxides. PM-10=Particulate matter less than 10 microns. PM-2.5=Particulate matter less than 2.5 microns. SO2=Sulfur dioxide.
VOC=Volatile organic compounds. NH3=Ammonia.
SOURCE: Adapted from Stacy C. Davis and Susan W. Diegel, "Table 12.1. Total National Emissions of the Criteria Air Pollutants by Sector, 2002," in Transportation Energy Data Book: Edition 26, U.S. Department of Energy, Oak Ridge National Laboratory, May 2007, (accessed June 29, 2007)
Highway vehicles55.5%34.9%27.5%0.9%2.2%1.8%8.0%
Other off-highway21.4%10.0%15.8%1.1%3.3%1.3%0.8%
Transportation total86.6111.457.230.520.430.700.29

standards than those required by the amendments, which California did. These included strict new laws on automobile pollution. The California low-emission vehicle (LEV) regulations were originally adopted in 1991 and became applicable in 1994. LEV II regulations were passed in 1998 and became applicable with 2004 model year vehicles. The LEV I and II programs classify vehicles into the following emissions categories:

  • TLEVtransitional low-emission vehicle (this category was phased out in 2004)
  • LEVlow-emission vehicle
  • ULEVultra low-emission vehicle
  • SULEVsuper ultra low-emission vehicle
  • ZEVzero-emission vehicle
  • PZEVpartial zero-emission vehicle (meets SULEV limits, has zero evaporative emissions, and a 15-year/150,000-mile warranty on emissions equipment)
  • AT PZEVadvanced technology partial zero-emission vehicle (meets PZEV limits and uses additional clean technology such as alternative fuel, electric drive, or other advanced technology system)

The remaining forty-nine states have the option of choosing either the standards of California or the federal CAAA. Some states have tougher tests for auto emissions than others. In most major metropolitan areas owners of cars and light trucks are required to pay for exhaust emission tests. For those that do not pass, repairs must be made to bring them into compliance.

epa tier 2 limits

epa tier 2 limits. In 1999 the EPA introduced its Tier 2 federal emission limits for new vehicles. They took effect in 2004 and include a five-year schedule for complete implementation. Emission limits apply to CO, NOx,PM, formaldehyde (HCHO), and nonmethane organic gases (NMOG). The latter are carbon-containing compounds that combine with NOx in sunlight to produce smog. Table 2.6 lists the emissions standards that will be in effect in 2009 when the Tier 2 standards are fully implemented. Manufacturers are allowed the flexibility to certify new vehicles to different sets of exhaust emissions standards called "bins." The manufacturers must choose bins for their vehicles that ensure that their corporate sales fleet emits an average of no more than 0.07 grams of NOx per mile. Table 2.6 also shows California LEV II standards for comparison with EPA's Tier 2 standards.

Gasoline Formulations

During the 1980s lead was phased out of gasoline to provide substantial improvements in air quality. A variety of other federal and state standards have gone into effect that dictate particular properties of gasoline, such as volatility (tendency to evaporate) and levels of NOx, heavy metals, toxic compounds, sulfur, and oxygen. The CAA requires the use of specially blended gasoline in areas of the country deemed nonattainment for ozone or CO levels. Attainment areas can choose to opt-in to these requirements. All the varying standards have resulted in the creation of many so-called boutique gasolines that greatly complicate the distribution dynamics for gasoline in the country. Critics complain that a gasoline shortage in one area can often not be relieved by shipping in gasoline from another part of the country because of the highly varying standards. The Energy Policy Act of 2005 limits the number of boutique fuels that can be designated by the states and requires a study of possible methods for harmonizing the nation's fuel system requirements. In 2006 the EPA formed a task group to examine issues associated with boutique fuels and published the findings in Report to the President: Task Force on Boutique Fuels (June 2006,


Light vehicle exhaust emission standards proposed to go into effect in 2009
[Grams/mile. Gasoline and diesel unless noted otherwise. Vehicle size up to 8,500 pounds. Gross vehicle weight (GVW) unless oth erwise noted.]
12,000 miles
Useful life:Bins, category, sizeNMOGCONO xPMHCHO
aIncludes medium-duty passenger vehicles which are also required to meet bin standards.
bA LEV option 1 with higher NOx levels also exists for up to 4% of LDTs above 3,750 lbs.
cOnly apply to cars and LDTs 0-3750 lbs LVW.
NMOG=Non-methane organic gases.
CO=Carbon monoxide.
NOxNitrogen oxides.
PM=Particulate matter.
LEV=Low emission vehicle.
ULEV=Ultra low emission vehicle.
SULEV=Super ultra low emission vehicle.
ZEV=Zero emission vehicle.
LDT=Light duty truck.
SOURCE: Stacy C. Davis and Susan W. Diegel, "Table 12.13. Light Vehicle Exhaust Emission Standards in Effect in 2009 when U.S. Tier 2 Standards are Final(grams/mile)," in Transportation Energy Data Book: Edition 26, U.S. Department of Energy, Oak Ridge National Laboratory, May 2007, (accessed June 29, 2007)
U.S emission standardsBins
California LEV II emission standards(Diesel only)

reformulated gasoline

reformulated gasoline. Reformulated gasoline (RFG) is a boutique fuel specially blended to diminish ground-level ozone formation through limits on volatility and on benzene, NOx, and toxic emissions. These limits vary by season, with stricter limits imposed during the summer months, when ozone formation is more common. Historically, RFG has required the addition of oxygen. This process produces a lower-octane fuel and usually an increase in price. Low-octane fuels can cause more engine knocking and pinging, making the fuels less desirable to consumers. Oxygenation of fuel makes combustion more complete. Incomplete fuel combustion is a major cause of CO emissions. Even though RFG combustion results in lower CO emissions, higher carbon dioxide emissions result because of the presence of additional oxygen.

The most frequently used oxygenates in RFG have been ethanol and methyl tertiary-butyl ether (MTBE). Fuel ethanol is derived from fermented agricultural products such as corn. MTBE is a chemical compound made from methanol and isobutylene and is soluble in water.

The CAA standards that went into effect in 1995 required those areas with the worst polluted air to sell RFG. By the early 2000s RFG accounted for more than one-third of all gasoline sold. However, increasing problems with MTBE found in water bodies led to state bans against the chemical. The passage of the 2005 Energy Policy Act provided for the removal of the RFG oxygen requirement and effectively eliminated the use of MTBE in RFG.

Corporate Average Fuel Economy Standards

In 1973 the Organization of Petroleum Exporting Countries (OPEC) imposed an oil embargo that provided a painful reminder to Americans of how dependent the country had become on foreign sources of fuel. Congress passed the 1975 Automobile Fuel Efficiency Act, which set the initial Corporate Average Fuel Economy (CAFE) standards.

CAFE standards required each domestic automaker to increase the average mileage of the new cars sold to 27.5 miles per gallon (mpg) by 1985. Under CAFE rules automakers could still sell the big, less efficient cars with powerful eight-cylinder engines, but to meet average fuel efficiency rates they also had to sell smaller, more efficient cars. Automakers that failed to meet each year's CAFE standards were required to pay fines. Those who managed to surpass the rates earned credits that they could use in years when they fell below CAFE requirements.

In Light-Duty Automotive Technology and Fuel Economy Trends: 1975 through 2005 (July 2005,, Robert M. Heavenrich of the EPA states that the CAFE standard was lowered during the mid to late 1980s and then raised back to 27.5 mpg for 1990 model automobiles. In 1996 a standard of 20.7 mpg was established for light trucks. This category includes pickup trucks, minivans, and SUVs. In 2003 the National Highway Traffic Safety Administration issued a rule setting new CAFE standards for light trucks produced in model years 2005 to 2007. The standard increased to 21 mpg for 2005, to 21.7 mpg for 2006, and to 22.2 mpg for 2007. The CAFE standard for passenger cars for these model years remained 27.5 mpg. In June 2007 the U.S. Senate passed an energy bill increasing CAFE standards to 35 mpg for cars and light trucks by 2020. As of October 2007, these new standards had not received passage by the U.S. House of Representatives.

CAFE standards are important to air quality, because higher standards have emissions benefits. Better fuel efficiency lowers tailpipe exhaust emissions of greenhouse gases (for example, carbon dioxide). Also, decreased demand for fuel reduces air pollutant emissions from the gasoline production and distribution industries.

Alternative Fuels

Early pollution-reducing efforts by vehicle manufacturers focused on reducing tailpipe emissions instead of eliminating their formation in the first place. Automakers introduced lighter engines, fuel injection systems, catalytic converters, and other technological improvements. In recent decades concerns about U.S. dependence on foreign oil supplies and environmental issues have focused attention on the development of alternative fuels for transportation vehicles. The Energy Policy Act of 1992 defines alternative fuels as those that are "substantially non-petroleum and yield energy security and environmental benefits." Under the act the following are designated as alternative fuels:

  • Coal-derived liquid fuels
  • Liquefied petroleum gas (propane)
  • Natural gas and liquid fuels domestically produced from natural gas
  • Methanol, ethanol, and other alcohols
  • Blends of 85% or more of alcohol with gasoline
  • Biodiesel and other fuels derived from biological materials
  • Electricity
  • Hydrogen
  • P-series fuels (blends of natural gas liquids, ethanol, and the biomass-derived co-solvent methyltetrahydrofuran)

Table 2.7 summarizes information for some of the major alternative fuels related to their physical state, sources, and environmental impacts. Even though these fuels offer advantages, their use may substitute one problem for another. For example, the alcohol fuel methanol reduces ozone formation but increases formaldehyde, a human carcinogen, and is twice as toxic as gasoline if it comes in contact with the skin. Engines require twice as much methanol as gasoline to travel a similar distance. Natural gas reduces hydrocarbons and CO but increases NOx.

In the Transportation Energy Data Book: Edition 26 (May 2007,, Stacy C. Davis and Susan W. Diegel of the Oak Ridge National Laboratory report that in 2004 nearly 548,000 alternative fuel vehicles (AFVs) were in use in the United States. Over 194,000 relied on liquefied petroleum gas, another 146,000 depended on alcohol fuel containing at least 85% ethanol, nearly 144,000 used compressed natural gas, and close to 56,000 used electricity.

alternative fuels and the marketplace

alternative fuels and the marketplace. AFVs cannot become a viable transportation option unless a fuel supply is readily available. Ideally, the infrastructure for supplying alternative fuels will be developed simultaneously with the vehicles. According to the Alternative Fuels Data Center of the U.S. Department of Energy (DOE; October 10, 2006,, which tabulates the number of alternative fuel stations by state and fuel type, in 2006 there were 5,627 of these stations around the country. Nearly half (2,423 stations) provide liquefied petroleum gas. Together, California (880 stations) and Texas (658 stations) accounted for 27% of all the stations.

Market success of alternative fuels and AFVs depends on public acceptance. People are accustomed to using gasoline as their main transportation fuel, and it is readily available. As federal and state requirements for alternative fuels increase, so should the availability of such fuels as well as their acceptance by the general public. In the long run, electricity and hydrogen seem the most promising of the alternative fuels for vehicles.

electric vehicles: promise and reality

electric vehicles: promise and reality. The electric vehicle (EV) is not a new invention. Popular during the 1890s, the quiet, clean, and simple vehicle was expected to dominate the automotive market of the twentieth century. Instead, it quietly disappeared as automakers chose to


Characteristics of major alternative fuels
GasolineNo. 2 dieselBiodieselCompressed natural gas (CNG)ElectricityEthanol (E85)HydrogenLiquified natural gas (LNG)Liquefied petroleum gas (LPG)Methanol (M85)
SOURCE: Adapted from Alternative Fuels Comparison Chart, U.S. Department of Energy, National Renewable Energy Laboratory, Alternative Fuels Data Center, 2007,
Main fuel sourceCrude oilCrude oilSoy bean oil, waste cooking oil, animal fats, and rapeseed oilUnderground reservesCoal; however, nuclear, natural gas, hydroelectric, and renewable resources can also be usedCorn, grains, or agricultural wasteNatural gas, methanol, and other energy sourcesUnderground reservesA by-product of petroleum refining or natural gas processingNatural gas, coal, or, woody biomass
Physical stateLiquidLiquidLiquidCompressed gasN/ALiquidCompressed gas or liquidLiquidLiquidLiquid
Types of vehicles available todayAll types of vehicle classesMany types of vehicle classesAny vehicle that runs on diesel today-no modifications are needed for up to 5% blends. Many engines also compatible with up to 20% blendsMany types of vehicle classesNeighborhood electric vehicles, bicycles, light-duty vehicles, medium and heavy duty trucks and busesLight-duty vehicles, medium and heavy-duty trucks and busesthese vehicles are flexible fuel vehicles that can be fueled with E85 (ethanol), gasoline, or any combination of the two fuelsNo vehicles are available for commercial sale yet, but some vehicles are being leased for demonstration purposesMedium and heavy-duty trucks and busesLight-duty vehicles, which can be fueled with propane or gasoline, medium and heavy-duty trucks and buses that run on propaneMostly heavy-duty buses are available
Environmental impacts of burning the fuelProduces harmful emissions; however, gasoline and gasoline vehicles are rapidly improving and emissions are being reducedProduces harmful emissions; however, diesel and diesel vehicles are rapidly improving and emissions are being reduced especially with after-treatment devicesReduces particulate matter and global warming gas emissions compared to conventional diesel; however, NOx emissions may be increasedCNG vehicles can demonstrate a reduction in ozone forming emissions compared to some conventional fuels, however, HC emissions may be increasedEVs have zero tailpipe emissions; however, some amount of emissions can be contributed to power generationE-85 vehicles can demonstrate a 25% reduction in ozone-forming emissions compared to reformulated gasolineZero regulated emissions for fuel cell-powered vehicles, and only NOx emissions possible for internal combustion engines operating on hydrogenLNG vehicles can demonstrate a reduction in ozone forming emissions compared to some conventional fuels, however, HC emissions may be increasedLPG vehicles can demonstrate a 60% reduction in ozone-forming emissions compared to reformulated gasolineM-85 vehicles can demonstrate a 40% reduction in ozone-forming emissions compared to reformulated gasoline
Fuel availabilityAvailable at all fueling stationsAvailable at select fueling stationsAvailable in bulk from an increasing number of suppliers. There are 22 states that have some biodiesel stations available to the publicMore than 1,100 CNG stations can be found across the country California has the highest concentration of CNG stations. Home fueling will be available in the fall of 2005.Most homes, government facilities, fleet garages, and businesses have adequate electrical capacity for charging, but, special hookup or upgrades may be required. More than 600 electric charging stations are available in California and Arizona.Most of the E-85 fueling stations are located in the Midwest, but in all, approximately 150 stations are available in 23 statesThere are only a small number of hydrogen stations across the country Most are available for private use onlyPublic LNG stations are limited (only 35 nationally), LNG is available through several suppliers of cryogenic liquidsLPG is the most accessible alternative fuel in the U.S. There are more than 3,300 stations nation wideMethanol remains a qualified alternative fuel as defined by EPAct, but it is not commonly used

invest billions of dollars in the internal combustion engine. It has taken a century, but the EV has returned.

The primary difficulty with EVs lies in inadequate battery power. The cars have a range of seventy to one hundred miles on a single charge and must be recharged often. In addition, EVs are expensive. Despite their high price, EVs have many advantages, including low noise, simple design and operation, and low service and maintenance costs. Over time the cost gap between cars that pollute and EVs that do not will narrow. With advances in battery development, the gap could close entirely.

hydrogen-fueled vehicles on the horizon

hydrogen-fueled vehicles on the horizon. Hydrogen is the simplest naturally occurring element and can be found in materials such as water, natural gas, and coal. For decades advocates of hydrogen have promoted it as the fuel of the future because it is abundant, clean, and cheap. Hydrogen researchers from universities, laboratories, and private companies claim that their industry has already produced vehicles that could be ready for consumers if problems of fuel supply and distribution could be solved. Other experts contend that economics and safety concerns will limit hydrogen's wider use for decades.

In 2002 the DOE formed a government-industry partnership called Freedom Cooperative Automotive Research (FreedomCAR). The goal of FreedomCAR is to develop highly fuel-efficient vehicles that operate using hydrogen produced from renewable energy sources. Industrial partners in the venture include Ford, General Motors, and Chrysler. FreedomCAR research takes place at facilities operated by the DOE's National Renewable Energy Laboratory in Golden, Colorado.

In his 2003 State of the Union Address (January 28, 2003,, President Bush announced the creation of the Hydrogen Fuel Initiative (HFI). This $1.2 billion program is designed to develop the technology needed for commercially viable hydrogen-powered fuel cells by 2020. Fuel cells designed for transportation vehicles and home/business use are to be developed. The HFI has three primary missions as part of its goals:

  • Lower the cost of hydrogen production to make it cost effective with gasoline production by 2010
  • Develop hydrogen fuel cells that provide the same vehicle range (at least three hundred miles of travel) as conventional gasoline fuel tanks
  • Lower the cost of hydrogen fuel cells to be comparable in cost with internal combustion engines

The DOE predicts that hydrogen fuel cell vehicles will reach the mass consumer market by 2020.

advanced technology vehicles (hybrids)

advanced technology vehicles (hybrids). Many experts believe that the most feasible solution in the near future is to produce vehicles that use a combination of gasoline and one of the alternative fuel sources. These are called advanced technology vehicles or hybrid vehicles. Figure 2.15 depicts a hybrid automobile that relies on a small internal combustion engine and electricity (from batteries).

Table 2.8 provides information about 2007 model year hybrid vehicles for sale in the United States as of June 2007. Manufacturers continue research on hybrid cars, which they hope will eventually satisfy American tastes and pocketbooks and provide even greater fuel efficiency.

The EPA's "Green Vehicle Guide"

The EPA, in the "Green Vehicle Guide" (May 24, 2007,, provides a database of emission information about thousands of vehicles (cars, light trucks, SUVs, and minivans) from domestic and foreign manufacturers for model years dating back to 2000. For each model users can obtain two scores compiled by the EPA: an air pollution score and a greenhouse gas score. The air pollution score reflects the presence in exhaust emissions of pollutants that cause health problems and smog formation. Scores range from zero to ten, where ten is the best score, having zero emissions. The air pollution scores are tied to the EPA Tier II and California LEV II emissions standards. The greenhouse gas score provides a relative rating of the exhaust emissions of carbon dioxide, a primary contributor to the enhanced greenhouse effect associated with global warming. The score ranges from zero to ten, where ten is the best score. The score is calculated based on a vehicle's fuel economy (miles per gallon of fuel) and fuel type (gasoline, diesel, ethanol blend, etc.). Models that



Hybrid electric vehicles available for sale to the public, model year 2007
ManufacturerModelVehicle bodyEmission certification standardFuel economy (city)Fuel economy (highway)
Notes: SULEV=Super ultra low emission vehicle. ULEV=Ultra low emission vehicle. ZEV=Zero emission vehicle. PZEV=Partial zero emission vechicle. AT PZEV=Advanced technology partial zero emission vechicle.
SOURCE: Adapted from "Model Year 2007: Alternative Fuel Vehicles and Advanced Technology Vehicles," in Current Model Listing, U.S. Department of Energy, Alternative Fuels Data Center, August 24, 2006, (accessed June 19, 2007)
ToyotaPriusSedanSULEV and AT PZEV
and Tier 2, Bin 360 mpg51 mpg
HondaCivicSedanSULEV and AT PZEV49 mpg51 mpg
ToyotaCamrySedanAT PZEV40 mpg38 mpg
Ford Motor CompanyEscape HybridSUVSULEV and AT PZEV36 mpg31 mpg
Ford Motor CompanyMercury MarinerSUVSULEV and ATPZEV33 mpg29 mpg
ToyotaHighlanderSUVSULEV and AT PZEV31 mpg27 mpg
ToyotaLexus RX 400hSUVSULEV31 mpg27 mpg
HondaAccordSedanSULEV and AT PZEV28 mpg35 mpg
General Motors-SaturnVUE Green LineSUVULEV and Tier 2, Bin 527 mpg32 mpg
ToyotaLexus GS 450hSedanSULEV25 mpg28 mpg
NissanAltimaSedanAT PZEVN/AN/A

achieve scores of at least six in both tests receive the EPA's Smartway rating. Those that score nine or above in both tests are deemed Smartway Elite vehicles. According to the EPA (May 18, 2007,, only six 2007 models achieved Smartway Elite status: Toyota Prius Hybrid, Honda Civic Hybrid, Nissan Altima Hybrid, Toyota Camry Hybrid, Ford Escape Hybrid, and Honda Civic (compressed natural gas fuel).


In 1970 Congress passed the landmark CAA, proclaiming that it would restore urban air quality. It was no coincidence that the law was passed during a fourteen-day Washington, D.C., smog alert. The act was amended several times over the following decades, including a massive overhaul in 1990 resulting in the CAAA. Even though the act has had mixed results, and many goals remain to be met, most experts credit it with making great strides toward cleaning up the air.

In The Benefits and Costs of the Clean Air Act, 1970 to 1990 (October 1997,, the first report mandated by the CAA on the monetary costs and benefits of controlling pollution, the EPA concludes that the economic value of clean air programs is forty-two times greater than the total costs of air pollution control during the twenty-year period. The study finds that many positive consequences occurred in the U.S. economy because of CAA programs and regulations. The CAA affected industrial production, investment, productivity, consumption, employment, and economic growth. In fact, the study estimates that total agricultural benefits from the CAA were almost $10 billion. The EPA compares benefits with direct costs or expenditures. The total costs of the CAA were $523 billion for the twenty-year period; total benefits equaled $22.2 trilliona net benefit of approximately $21.7 trillion.

According to The Benefits and Costs of the Clean Air Act Amendments of 1990 (November 1999,, the second mandated review of the CAA and the most comprehensive and thorough review ever conducted, the act produced major reductions in pollution that causes illness and disease, smog, acid rain, haze, and damage to the environment. Using a sophisticated array of computer models and the latest cost data, the EPA finds that by 2010 the act will have prevented twenty-three thousand Americans from dying prematurely and averted more than 1.7 million asthma attacks. The CAA will prevent forty-seven thousand episodes of acute bronchitis, ninety-one thousand occurrences of shortness of breath, 4.1 million lost work days, and thirty-one million days in which Americans would have had to restrict activity because of illness. Another twenty-two thousand respiratory-related hospital admissions will be averted, as well as forty-two thousand admissions for heart disease and forty-eight hundred emergency room visits for asthma.

The EPA estimates that the benefits of CAA programs in the reduction of illness and premature death alone will total about $110 billion. By contrast, the study finds that the cost of achieving these benefits is only about $27 billion, which is a fraction of the value of the benefits. In addition, the study reports that there are other benefits that scientists and economists cannot quantify and express in monetary terms, such as controlling cancer-causing air toxins and bringing benefits to crops and ecosystems by reducing pollutants.

At the same time, many cities are still not in compliance with the law. One reason efforts to clean the air have been only partly successful is that they have focused on specific measures to combat individual pollutants rather than addressing the underlying social and economic structures that create the problemfor example, the distance between many Americans' residences and their places of work.


Every year the Gallup Organization conducts a poll on the environment around the time of the nation's celebration of Earth Day. In Environment (2007,, poll participants were asked about their level ¼ of concern ¼ related to particular environmental problems. (See Table 2.9.) The results show that in March 2004, 46% of those asked expressed a great deal of concern about air pollution, compared with 33% who expressed a fair amount of concern. Another 15% indicated a little concern, and 5% expressed no concern.

The percentage of poll respondents indicating a great deal of concern about air pollution has dropped dramatically in recent years from a high of 63% in 1989. During the 2007 poll Gallup also asked people their opinion about some specific environmental proposals. The results indicated that 86% of those asked favored government spending to develop alternative fuel sources for automobiles


Public concern about air pollution, 19892007
Great dealFair amountOnly a littleNot at allNo opinion
*Less than 0.5%.
SOURCE: "I'm going to read you a list of environmental problems. As I read each one, please tell me if you personally worry about this problem a great deal, a fair amount, only a little, or not at all. First, how much do you personally worry aboutAir Pollution," in Environment, The Gallup Organization, 2007, (accessed June 19, 2007). Copyright © 2007 by The Gallup Organization. Reproduced by permission of The Gallup Organization.
2007 Mar 11144633155*
2006 Mar 13164434157*
2004 Mar 8113930238*
2003 Mar 354232206*
2002 Mar 474533184*
2001 Mar 574834144*
2000 Apr 39592993*
1999 Apr 13145235103*
1999 Mar 12144733164*
1997 Oct 272842341851
1991 Apr 11145928104*
1990 Apr 58582994*
1989 May 47632484*

biles and 79% favored setting higher emissions standards for automobiles.