United States 

Environmental Protection 

Agency 

Latest Findings on National 

Air Quality 

2001 ST A TUS AND TRENDS 
Cover Photo of Mount McKinley, Denali National Park,Alaska, by Kim Ferguson,Waynesville, North Carolina 
EPA 454/K-02-001

September 2002

Latest Findings on National Air Quality

2001 STATUS AND TRENDS

Contract No. GS-10F-0283K

Task Order No. TO 1307

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Emissions, Monitoring, and Analysis Division

Research Triangle Park, North Carolina
Contents

National Air Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Six Principal Pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . 3

Acid Rain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Toxic Air Pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Stratospheric Ozone. . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

More detailed information on air pollution trends is available at 

www.epa.gov/airtrends.

Information on global warming and global climate change is available at

www.epa.gov/globalwarming/publications/emissions and 

www.epa.gov/globalwarming/publications/car.
EPA tracks air pollution 

in two ways: 

• Air quality measured 

from over 3,000 

locations (over 5,200 

monitors) across the 

nation operated 

primarily by state, 

local, and tribal 

agencies. 

• Emissions from all 

sources going back 

30 years. 

1 

■ EPA, states, and tribes have only recently 

begun to measure fine particles (known as 

PM2.5) in the air on a broad national basis. 

In many locations, EPA now has 3 years of air 

quality monitoring data to use in comparing 

to the health-based standards for PM2.5. Based 

on those data, areas across the Southeast, 

Mid-Atlantic, and Midwest regions and 

California have air quality that is unhealthy 

due to fine particles. High PM concentrations 

in the eastern United States are due to 

regional emissions from power plants and 

motor vehicles in combination with local 

emissions from transportation and other 

sources. In California, high PM concentrations 

tend to be due to mobile source emissions. 

■ Of the six tracked pollutants, progress has been 

slowest for ground-level ozone.The Northeast 

and West exhibited the greatest improvement, 

while the South and North Central regions 

experienced slower progress in lowering ozone 

concentrations. Despite this progress in most 

regions of the country, the average ozone 

(8-hour) levels in 33 of our national parks have 

increased over the past 10 years. 

■ Ground-level ozone is not emitted directly 

into the air but is formed in the atmosphere 

by the reaction of volatile organic compounds 

(VOCs) and nitrogen oxides (NOx) in the 

presence of heat and sunlight. Although 

emissions of VOCs have decreased 16 percent 

over the past 20 years, the lack of significant 

reductions in regional-scale emissions of NOx, 

a family of chemicals that can contribute to 

the formation of ozone hundreds of miles 

downwind, has slowed progress in reducing 

ozone levels. Between 1982 and 2001, NOx 

emissions in the United States increased 

National Air Quality 

This summary report highlights the U.S. Environmental 

Protection Agency’s (EPA’s) most recent evaluation of the 

status and trends in our nation’s air quality. 

Highlights 

■ Since 1970, aggregate emissions of the six 

principal pollutants tracked nationally have 

been cut 25 percent. During that same time 

period, U.S. gross domestic product increased 

161 percent, energy consumption increased 

42 percent, and vehicle miles traveled increased 

149 percent. 

■ National air quality levels measured at thousands of monitoring stations across the country 

have shown improvements over the past 

20 years for all six principal pollutants. 

■ Despite this progress, almost 170 million tons 

of pollution are emitted into the air each year 

in the United States, and approximately 

133 million people live in counties where 

monitored air in 2001 was unhealthy at times 

because of high levels of at least one of the six 

principal air pollutants. 

■ The vast majority of areas that experienced 

unhealthy air did so because of one or both 

of two pollutants—ozone and particulate 

matter (PM). EPA is focusing its efforts to 

control these pollutants by implementing 

more stringent National Ambient Air Quality 

Standards for ozone and PM and rules 

reducing emissions from on-road transportation and stationary combustion sources.These 

rules will bring reductions in emissions over 

the next few years. Additional reductions will 

be needed to provide clean air in the future. 

EPA has submitted to Congress Clear Skies 

legislation that, if enacted, would mandate 

reductions of particle- and ozone- forming 

compounds from power generators by 70 

percent from current levels through a nationwide cap and trade program. EPA also expects 

to propose nonroad vehicle regulations that 

would help improve ozone and PM air quality. 
2 

National Air Quality 

9 percent (with a 3 percent decrease in the 

past 10 years).The majority of this increase 

is attributed to growth in emissions from 

nonroad engines (like construction and recreation equipment) and diesel vehicles. Emissions 

of NOx also contribute to acid rain, haze, 

particulate matter, and damage to waterbodies 

like the Chesapeake Bay. 

■ Sulfates formed primarily from sulfur dioxide 

(SO2) emissions from coal-fired power plants 

are a major component of fine particles in the 

eastern United States. SO2 emissions decreased 

25 percent from 1981 to 2001 and 24 percent 

from 1992 to 2001. Nationally, average SO2 

ambient concentrations have been cut 52 percent from 1982 to 2001 and 35 percent over 

the more recent 10-year period from 1992 to 

2001. Reductions in SO2 concentrations and 

emissions since 1990 are primarily due to 

controls implemented under EPA’s Acid 

Rain Program. 

■ Based on EPA’s recent National-Scale Air 

Toxics Assessment for 1996, 3 of 32 urban air 

toxics (chromium, a metallic compound used 

in industrial processes such as plating; benzene, 

primarily emitted by mobile sources such as 

0 

20 

40 

60 

80 

100 

120 

140 

CO 

(-19%) 

NOx (+15%) 

VOC 

(-38%) 

SO2 

(-44%) 

PM10 

(-76%) 

0 

50 

100 

200 

250 

150 

Pb 

(-98%) 

1970 

2001 

Million tons 

Comparison of 1970 and 2001 Emissions Thousand tons 

Six Principal Air Pollutants Tracked Nationally 

■ Nitrogen Dioxide (NO2) 

■ Ozone (O3) - formed by volatile organic 

compounds (VOCs) and nitrogen oxides 

(NOx) 

■ Sulfur Dioxide (SO2) 

■ Particulate Matter (PM) - formed by SO2, 

NOx, ammonia, VOCs, and direct particle 

emissions 

■ Carbon Monoxide (CO) 

■ Lead (Pb) 

cars and trucks; and formaldehyde, emitted 

by mobile sources and formed when other 

compounds chemically react in sunlight) 

appear to pose the greatest nationwide cancer 

risk. One air toxic, acrolein — a by product of 

combustion in mobile and industrial sources 

— is estimated to pose the highest potential 

on a nationwide basis for significant chronic 

adverse effects other than cancer. 

■	 Scientific evidence shows that the Montreal 

Protocol has been effective in reducing stratospheric ozone depletion. Measurements have 

shown that atmospheric concentrations of 

methyl chloroform are falling, indicating that 

emissions have been greatly reduced. Concentrations of other ozone-depleting substances 

in the upper layers of the atmosphere, like 

chlorofluorocarbons (CFCs), are also beginning 

to decrease. 

Air Pollution 

The Concern 

The average person breathes 3,400 gallons of air 

each day. Exposure to air pollution is associated 

with numerous effects on human health, including 

respiratory problems, hospitalization for heart 

or lung diseases, and even premature death. 

Children are at greater risk because they are 

generally more active outdoors and their lungs 

are still developing.The elderly and people with 

heart or lung diseases are also more sensitive to 

some types of air pollution. 

Air pollution can also significantly affect ecosystems. For example, ground-level ozone has been 

estimated to cause over $500 million in annual 

reductions of agricultural and commercial forest 

yields, and airborne releases of NOx are one of 

the largest sources of nitrogen pollution in certain 

water-bodies such as the Chesapeake Bay. 

The Causes 

Air pollution comes from many different sources. 

These include stationary sources such as factories, 

power plants, and smelters; smaller sources such 

as dry cleaners and degreasing operations; mobile 

sources such as cars, buses, planes, trucks, and 

trains; and natural sources such as windblown 

dust and wildfires. 
3 

Six Principal Pollutants 

sulfur dioxide (SO2), particulate matter (PM), 

carbon monoxide (CO), and lead (Pb). Four of 

these pollutants (CO, Pb, NO2, and SO2) result 

primarily from direct emissions from a variety of 

sources. PM results from direct emissions, but is 

also commonly formed when emissions of nitrogen oxides (NOx), sulfur oxides (SOx), ammonia, 

organic compounds, and other gases react in the 

atmosphere. Ozone is not directly emitted but is 

formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight. 

Each year EPA examines changes in levels of 

these ambient pollutants and their precursor 

emissions over time and summarizes the current 

air pollution status. 

Summary of Air Quality and Emissions 

Trends 

EPA tracks trends in air quality based on actual 

measurements of pollutant concentrations in the 

ambient (outside) air at monitoring sites across the 

country. Monitoring stations are operated by state, 

tribal, and local government agencies as well as 

some federal agencies, including EPA.Trends are 

derived by averaging direct measurements from 

these monitoring stations on a yearly basis.The 

tables to the left show that the air quality based 

on concentrations of the principal pollutants has 

improved nationally over the past 20 years 

(1982–2001). 

EPA estimates nationwide emissions of ambient 

pollutants and their precursors based on actual 

monitored readings or engineering calculations 

of the amounts and types of pollutants emitted 

by vehicles, factories, and other sources. Emission 

estimates are based on many factors, including 

the level of industrial activity, technology developments, fuel consumption, vehicle miles traveled, 

and other activities that cause air pollution. 

Under the Clean Air Act, EPA establishes air quality standards to protect public health, 

including the health of “sensitive” populations 

such as people with asthma, children, and the 

elderly. EPA also sets limits to protect public 

welfare, including protection against decreased 

visibility and damage to animals, crops, vegetation, 

and buildings. 

EPA has set national air quality standards for six 

principal air pollutants (also referred to as criteria 

pollutants): nitrogen dioxide (NO2), ozone (O3), 

NO2 O3 SO2 

PM CO Pb 

Percent Change in Air Quality 

1982-2001 1992-2001 

NO2 -24 -11 

O3 1-h -18 -3 

8-h -11 0 

SO2 -52 -35 

PM10 — -14 

PM2.5 Trend data not available 

CO -62 -38 

Pb -94 -25 

Percent Change in Emissions 

1982-2001 1992-2001 

NOx +9 -3 

VOC -16 -8 

SO2 -25 -24 

PM10 * -51 -13 

PM2.5 * -10 

CO 0 +6 

Pb -93 -5 

*Includes only directly emitted particles. 

Negative numbers indicate improvements in 

air quality or reductions in emissions. Positive 

numbers show where emissions have increased. 

Air quality concentrations do not always track nationwide emissions.There 

are several reasons for this. First, most monitors are located in urban areas 

so air quality is most likely to track changes in urban air emissions rather 

than in total emissions. Second, not all of the principal pollutants are 

emitted directly to the air. Ozone and many particles are formed after 

directly emitted gases react chemically to form them.Third, the amount of 

some pollutants measured at monitoring locations depends on the chemical 

reactions that occur in the atmosphere during the time it takes the pollutant 

to travel from its source to the monitoring station. Finally, weather conditions often contribute to the formation and buildup of pollutants in the 

ambient air. For example, peak ozone concentrations typically occur during 

hot, dry stagnant summertime conditions. 

— 
4 

Six Principal Pollutants 

-50% 

0% 

50% 

100% 

150% 

200% 

70 01 

161% 

149% 

42% 

39% 

-25% 

Comparison of Growth Areas and Emissions 

80 90 95 96 97 98 99 00 

Aggregate Emissions 

(Six Principal Pollutants) 

Gross Domestic Product 

Vehicle Miles Traveled 

Energy Consumption 

U.S. Population 

Between 1970 and 2001, gross domestic product increased 161 percent, vehicle miles traveled increased 149 percent, energy 

consumption increased 42 percent, and U.S. population increased 39 percent.At the same time, total emissions of the six principal 

air pollutants decreased 25 percent. 

Emission estimates also reflect changes in air 

pollution regulations and installation of emission 

controls.The 2001 emissions reported in this 

summary are projected numbers based on available 

2000 information and historical trends. Emission 

estimation methods continue to evolve and 

improve over time. Methods have changed for 

many significant categories beginning with the 

years 1985, 1990, and 1996, and, consequently, 

the estimates are not consistently developed across 

all years in this trend period. Because emissions 

estimation methods for many significant categories 

have changed over time, comparisons of the 

estimates for a given year in this report to the same 

year in a previous report may not be appropriate. 

Check www.epa.gov/ttn/chief for updated 

emissions information. 
5 

Emissions of all principal pollutants except NOx 

have decreased or remained essentially unchanged 

over the past 20 years (1982–2001), while all air 

quality measures for the six principal pollutants 

have gone down. Although NOx emissions have 

increased, air quality measurements for NO2 across 

the country are below the national air quality 

standards. NOx plays an important role in a 

number of air pollution issues.These compounds 

contribute to the formation of ozone and particles 

as well as the deposition of acids and nutrients and 

visibility impairment. 

The improvements are a result of effective 

implementation of clean air laws and regulations, 

as well as improvements in the efficiency of 

industrial technologies. 

Despite great progress in air quality improvement, 

approximately 133 million people nationwide still 

lived in counties with pollution levels above the 

National Ambient Air Quality Standards (NAAQS) 

in 2001.This annual “snapshot” view of the 

nation’s air quality can be used to show levels that 

people might currently be experiencing across the 

county.There are still 130 nonattainment areas out 

of the 230 originally resulting from the 1990 

Clean Air Act Amendments designation process. 

The Clean Air Act 

The Clean Air Act provides the principal framework for national, state, tribal, and local efforts to 

protect air quality. Under the Clean Air Act, EPA 

has a number of responsibilities, including 

■ Setting NAAQS for the six principal pollutants 

that are considered harmful to public health 

and the environment. 

■ Ensuring that these air quality standards are 

met (in cooperation with the state, tribal, and 

local governments) through national standards 

and strategies to control air pollutant emissions 

from vehicles, factories, and other sources. 

■ Reducing emissions of SO2 and NOx that 

cause acid rain. 

■ Reducing air pollutants such as PM, SOx, 

and NOx that can cause visibility impairment 

across large regional areas, including many of 

the nation’s most treasured parks and wilderness areas. 

■ Ensuring that sources of toxic air pollutants 

that may cause cancer and other adverse 

human health and environmental effects are 

well controlled and that the risks to public 

health and the environment are substantially 

reduced. 

■ Limiting the use of chemicals that damage 

the stratospheric ozone layer in order to 

prevent increased levels of harmful ultraviolet 

radiation. 

Six Principal Pollutants 

110.3 (8-hour) 

0 0 100 150 

0.007 

11.1 

72.7 

0.7 

2.7 

133.1 

40.2 (1-hour) 

0 

Millions of People 

Number of People Living in Counties 

with Air Quality Concentrations above 

the Level of the NAAQS in 2001 

Multiple years of data are generally used to determine if an area 

attains the NAAQS. 

5

NO2 

O3 

SO2


PM10


PM2.5


CO


Pb


Any NAAQS

6 

Six Principal Pollutants 

NITROGEN DIOXIDE (NO 2 )

Thousand Short Tons Concentration, ppm 

Nature and Sources of the Pollutant 

Nitrogen dioxide is a reddish brown, highly reactive gas that is formed in the ambient air through 

the oxidation of nitric oxide (NO). Nitrogen 

oxides (NOx), the term used to describe the sum 

of NO, NO2, and other oxides of nitrogen, play a 

major role in the formation of ozone, particulate 

matter, haze,and acid rain.While EPA tracks 

national emissions of NOx, the national monitoring network measures ambient concentrations of 

NO2 for comparison to national air quality standards.The major sources of man-made NOx emissions are high-temperature combustion processes, 

such as those that occur in automobiles and power 

NO2 Air Quality, 1982–2001 

Based on Annual Arithmetic Average 

0.06 

0.05 

0.04 

0.03 

0.02 

0.01 

0.00 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

NAAQS 

121 Sites 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

1982–01: 24% decrease 

1992–01: 11% decrease 

Air quality concentrations do not always track nationwide 

emissions. For a detailed explanation, see page 3. 

NOx Emissions, 1982–2001 

Fuel Combustion Industrial Processes 

Transportation Miscellaneous 30,000 

25,000 

20,000 

15,000 

10,000 

5,000 

In 1985, EPA refined its methods for estimating emissions. 

0 

82 85 92 93 94 95 96 97 98 99 00 01 

1982–01: 9% increase 

1992–01: 3% decrease 

plants. Home heaters and gas stoves can also 

produce substantial amounts of NO2 in indoor 

settings. 

Health and Environmental Effects 

Short-term exposures (e.g., less than 3 hours) to 

low levels of NO2 may lead to changes in airway 

responsiveness and lung function in individuals 

with preexisting respiratory illnesses and increases 

in respiratory illnesses in children. Long-term 

exposures to NO2 may lead to increased susceptibility to respiratory infection and may cause irreversible alterations in lung structure. NOx reacts 

in the air to form ground-level ozone and fine 

particle pollution, which are both associated with 

adverse health effects. 

NOx contributes to a wide range of environmental 

effects directly and/or when combined with other 

precursors in acid rain and ozone (see environmental discussion under Ozone and Acid Rain). 

Nitrogen inputs to terrestrial and wetland systems 

can alter existing competitive relationships among 

plant species, leading to changes in community 

composition and diversity. Similarly, direct nitrogen 

inputs to aquatic ecosystems such as those found 

in estuarine and coastal waters (e.g., Chesapeake 

Bay) can lead to eutrophication (a condition that 

promotes excessive algae growth, which can lead 

to a severe depletion of dissolved oxygen and 

increased levels of toxins harmful to fish and other 

aquatic life). Nitrogen, alone or in acid rain, also 

can acidify soils and surface waters.Acidification of 

soils causes the loss of essential plant nutrients and 

increased levels of soluble aluminum that are toxic 

to plants.Acidification of surface waters creates 

conditions of low pH and levels of aluminum that 

are toxic to fish and other aquatic organisms. 

Finally, NOx is a contributor to visibility impairment. 

Trends in NO2 Levels and NOx Emissions 

Over the past 20 years, monitored levels of NO2 

have decreased 24 percent.All areas of the country 

that once violated the NAAQS for NO2 now 

meet that standard.While air quality levels of NO2 

around urban monitors have fallen, national emissions of NOx have actually increased over the past 

20 years by 9 percent.This increase is the result of 

a number of factors, the most significant being an 

increase in NOx emissions from nonroad engines 

and diesel vehicles.This increase is of concern 

because NOx emissions contribute to the formation of ground-level ozone (smog), but also to 

other environmental problems like acid rain and 

nitrogen loadings to waterbodies. 
7 

Six Principal Pollutants 

Nature and Sources of the Pollutant 

Ground-level ozone (the primary constituent 

of smog) continues to be a pollution problem 

throughout many areas of the United States. 

Ozone is not emitted directly into the air but is 

formed by the reaction of VOCs and NOx in the 

presence of heat and sunlight. Ground-level ozone 

forms readily in the atmosphere, usually during hot 

summer weather.VOCs are emitted from a variety 

of sources, including motor vehicles, chemical 

plants, refineries, factories, consumer and commercial products, and other industrial sources. NOx is 

emitted from motor vehicles, power plants, and 

other sources of combustion. Changing weather 

patterns contribute to yearly differences in ozone 

concentrations from region to region. Ozone and 

the pollutants that form ozone also can be transported into an area from pollution sources found 

hundreds of miles upwind. 

Health and Environmental Effects 

Short-term (1- to 3-hour) and prolonged 

(6- to 8-hour) exposures to ambient ozone have 

been linked to a number of health effects of 

concern. For example, health effects attributed to 

ozone exposure include significant decreases in 

lung function and increased respiratory symptoms 

such as chest pain and cough. Exposures to ozone 

can make people more susceptible to respiratory 

infection, result in lung inflammation, and aggravate preexisting respiratory diseases such as asthma. 

Also, increased hospital admissions and emergency 

room visits for respiratory problems have been 

associated with ambient ozone exposures.These 

effects generally occur while individuals are 

actively exercising, working, or playing outdoors. 

Children, active outdoors during the summer 

when ozone levels are at their highest, are most 

at risk of experiencing such effects. Other at-risk 

groups include adults who are active outdoors 

(e.g., some outdoor workers) and individuals with 

preexisting respiratory disease such as asthma and 

chronic obstructive pulmonary disease. In addition, 

longer-term exposures to moderate levels of ozone 

present the possibility of irreversible changes in the 

lung structure, which could lead to premature 

aging of the lungs and worsening of chronic 

respiratory illnesses. 

Ozone also affects vegetation and ecosystems, 

leading to reductions in agricultural crop and commercial forest yields, reduced growth and survivability of tree seedlings, and increased plant susceptibility to disease, pests, and other environmental 

stresses (e.g., harsh weather). In long-lived species, 

these effects may become evident only after several 

years or even decades, thus having the potential for 

long-term effects on forest ecosystems. Groundlevel ozone damage to the foliage of trees and 

other plants can also decrease the aesthetic value 

of ornamental species as well as the natural beauty 

of our national parks and recreation areas. 

GROUND-LEVEL OZONE (O 3 ) 

Air quality concentrations do not always track nationwide emissions. For detailed explanation, see page 3. 

82 85 92 93 94 95 96 97 98 99 00 01 

VOC Emissions, 1982–2001 

In 1985, EPA refined its methods for estimating emissions. 

1982–01: 16% decrease 

1992–01: 8% decrease 

Fuel Combustion 

Transportation 

Industrial Processes 

Miscellaneous 

Ozone occurs naturally in the stratosphere and provides a protective layer 

high above the Earth. See page 23 for 

more information on the stratospheric 

ozone layer. 

30,000 

25,000 

Thousand Short Tons 

20,000 

15,000 

10,000 

5,000 

0 
8 

Six Principal Pollutants 

Concentration, ppm Concentration, ppm 

0.20 

0.15 

0.10 

0.05 

0.00 

Trends in Ozone Levels and Related 

Emissions 

In 1997, EPA revised the NAAQS for ozone by 

setting new 8-hour 0.08-ppm standards. Currently, 

EPA is tracking trends based on both 1-hour and 

8-hour data. 

Over the past 20 years, national ambient ozone 

levels decreased 18 percent based on 1-hour data 

and 11 percent based on 8-hour data. Between 

Ozone Air Quality, 1982–2001 

Based on Annual 4th Maximum 8-Hour Average 

NAAQS 

10% of sites have concentrations below this line 

Average 

379 Sites 

90% of sites have concentrations below this line 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

1982–01: 11% decrease 

1992–01: 0% change 

Air quality concentrations do not always track nationwide 

emissions. For a detailed explanation, see page 3. 

Ozone Air Quality, 1982–2001 

Based on Annual 2nd Maximum 1-Hour Average 

0.20 

0.15 

0.10 

0.05 

0.00 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

379 Sites 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

NAAQS 

1982 and 2001, emissions of VOCs decreased 

16 percent. During that same time period, 

emissions of NOx increased 9 percent. 

For the period 1982 to 2001, the downward trend 

in 1-hour ozone levels seen nationally is reflected 

in every broad geographic area in the country. 

The Northeast and West exhibited the most 

substantial improvement, while the South and 

North Central regions experienced the least 

rapid progress in lowering ozone concentrations. 

Similar to the 1-hour ozone trends, all regions 

experienced improvements in 8-hour ozone levels 

between 1982 and 2001 except the North Central 

region, which showed little change during this 

period.Again, the West and Northeast have exhibited the most substantial reductions in 8-hour 

ozone levels for the past 20 years. 

Across the country, the highest ambient 1-hour 

ozone concentrations are typically found at suburban sites, consistent with the downwind transport 

of emissions from urban centers. During the past 

20 years, ozone concentrations decreased approximately 20 percent at urban and suburban sites. 

In the past 10 years, urban sites show declines of 

approximately 5 percent and suburban sites show 

a 6 percent decrease. However, at rural monitoring 

locations, national improvements have slowed. 

One-hour ozone levels for 2001 are 11 percent 

lower than those for 1982 but less than 1 percent 

below 1992 levels. In 2001, for the sixth consecutive year, rural 1-hour ozone levels, on average, are 

greater than the levels observed for the urban sites, 

but they are still generally lower than levels 

observed at suburban sites. 

Over the past 10 years, the average 8-hour ozone 

level in 33 of our national parks increased almost 

4 percent. Six monitoring sites in five of these 

parks experienced statistically significant upward 

trends in 8-hour ozone levels: Great Smoky 

Mountains (Tennessee), Craters of the Moon 

(Idaho), Mesa Verde (Colorado), Mammoth Cave 

(Kentucky),and Yellowstone (Wyoming). Monitoring data for two parks showed statistically significant improvements over the same time period: 

Saguaro (Arizona) and Sequoia (California). For 

the remaining 26 parks where ozone monitoring 

takes place, the 8-hour ozone levels at 18 increased 

only slightly between 1992 and 2001, while 5 

showed decreasing levels and 3 were unchanged. 

1982–01: 18% decrease 

1992–01: 3% decrease 
9 

Six Principal Pollutants 

Trend in 1-Hour Ozone Levels, 1982–2001, Averaged across EPA Regions 

Based on Annual 2nd Highest Daily Maximum 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

.156 .119 

24% 

17% 

.126 

10% 

7% 

.105 .098 

The National Trend 

14% 

.115 .099 

6% 

.099 .093

16% 

.128 .108

15% 

.115 .098

29% 

.111 

32% 

.154 

18% 

.104 .127 

.133 .111 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

1982 2001 

.079 

.104 .113 

Rural 

Suburban 

Urban 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

0.12 

0.14 

0.16 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01

Concentration, ppm 

NAAQS 

Trend in 1-hour Ozone Levels, 

1982–2001, by Location of Site 

Based on Annual 2nd Highest 

Daily Maximum 

Concentrations are in parts per million (ppm). 

Trend in 8-Hour Ozone Levels, 1982–2001, Averaged across EPA Regions 

Based on Annual 4th Maximum 8-Hour Average 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

20% 

8% 

4% 

2% 

The National Trend 

9% 

0% 

9% 

9% 

21% 

24% 

1982 2001 

.112 .090

1982 2001 

.101 .093

1982 2001 

.096 .092

1982 2001 

.089 .081

1982 2001 

.082 .080

1982 2001 

.090 .082

1982 2001 

.074 .074

1982 2001 

.081 .074

1982 2001 

.102 .078

1982 2001 

.072 .057

1982 2001 

.092 .082

11% 

*EPA Regional Office contacts can be found at www.epa.gov/epahome/locate2.htm. 

* 
10 Six Principal Pollutants 

Thousand Short Tons Concentration, ppm 

SULFUR DIOXIDE (SO 2 ) 

Nature and Sources of the Pollutant 

Sulfur dioxide belongs to the family of SOx gases. 

These gases are formed when fuel containing 

sulfur (mainly coal and oil) is burned and during 

metal smelting and other industrial processes. Most 

SO2 monitoring stations are located in urban areas. 

The highest monitored concentrations of SO2 

are recorded in the vicinity of large industrial 

facilities. Fuel combustion, largely from coal-fired 

power plants, accounts for most of the total 

SO2 emissions. 

SO2 Air Quality, 1982–2001 

Based on Annual Arithmetic Average 

0.04 

0.03 

0.02 

0.01 

0.00 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

NAAQS 

253 Sites 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

1982–01: 52% decrease 

1992–01: 35% decrease 

Air quality concentrations do not always track nationwide 

emissions. For a detailed explanation, see page 3. 

SO2 Emissions, 1982–2001 

Fuel Combustion Industrial Processes 

Transportation 30,000 

25,000 

20,000 

15,000 

10,000 

5,000 

0 

82 85 92 93 94 95 96 97 98 99 00 01 

In 1985, EPA refined its methods for estimating emissions. 

Health and Environmental Effects 

High concentrations of SO2 can result in temporary breathing impairment for asthmatic children 

and adults who are active outdoors. Short-term 

exposures of asthmatic individuals to elevated 

SO2 levels during moderate activity may result 

in breathing difficulties that can be accompanied 

by symptoms such as wheezing, chest tightness, 

or shortness of breath. Other effects that have 

been associated with longer-term exposures to 

high concentrations of SO2, in conjunction with 

high levels of PM, include aggravation of existing 

cardiovascular disease, respiratory illness, and alterations in the lungs’ defenses.The subgroups of 

the population that may be affected under these 

conditions include individuals with heart or lung 

disease, as well as the elderly and children. 

Together, SO2 and NOx are the major precursors 

to acidic deposition (acid rain), which is associated 

with the acidification of soils, lakes, and streams 

and accelerated corrosion of buildings and monuments. SO2 also is a major precursor to PM2.5, 

which is a significant health concern as well as a 

main contributor to poor visibility. (See Acid Rain 

section, page 16, for a more detailed discussion.) 

Trends in SO2 Levels and Emissions 

Nationally, average SO2 ambient concentrations 

have decreased 52 percent from 1982 to 2001 and 

35 percent over the more recent 10-year period 

1992 to 2001. SO2 emissions decreased 25 percent 

from 1982 to 2001 and 24 percent from 1992 

to 2001. Reductions in SO2 concentrations and 

emissions since 1990 are due, in large part, to 

controls implemented under EPA’s Acid Rain 

Program beginning in 1995. 

1982–01: 25% decrease 

1992–01: 24% decrease 
11 Six Principal Pollutants 

Nature and Sources of the Pollutant 

Particulate matter is the general term used for a 

mixture of solid particles and liquid droplets found 

in the air. Some particles are large enough to be 

seen as dust or dirt. Others are so small they can be 

detected only with an electron microscope. PM2.5 

describes the “fine” particles that are less than or 

equal to 2.5 μm in diameter. “Coarse fraction” particles are greater than 2.5 μm, but less than or equal 

to 10 μm in diameter. PM10 refers to all particles 

less than or equal to 10 μm in diameter. A particle 

10 μm in diameter is about one-seventh the diameter of a human hair. PM can be emitted directly 

PA R TICULA TE MA TTER 

or form in the atmosphere.“Primary” particles, 

such as dust from roads or elemental carbon (soot) 

from wood combustion, are emitted directly into 

the atmosphere. “Secondary” particles are formed 

in the atmosphere from primary gaseous emissions. 

Examples include sulfates, formed from SO2 emissions from power plants and industrial facilities, and 

nitrates, formed from NOx emissions from power 

plants, automobiles, and other types of combustion 

sources. The chemical composition of particles 

depends on location, time of year, and weather. 

Generally, coarse PM is composed largely of 

primary particles and fine PM contains many more 

secondary particles. 

Health and Environmental Effects 

Particles that are small enough to get into the 

lungs (those less than or equal to 10 μm in diameter) can cause numerous health problems and 

have been linked with illnesses and deaths from 

heart and lung diseases. Various health problems 

have been associated with long-term (e.g., multiyear) exposures as well as daily and even, potentially, peak (e.g., 1-hour) exposures to particles. 

Particles can aggravate respiratory conditions such 

as asthma and bronchitis and have been associated 

with cardiac arrhythmias (heartbeat irregularities) 

and heart attacks. Particles of concern can include 

both fine and coarse-fraction particles, although 

fine particles have been more clearly linked to the 

most serious health effects. People with heart or 

lung disease, the elderly, and children are at highest 

risk from exposure to particles. In addition to 

health problems, PM is the major cause of reduced 

visibility in many parts of the United States. 

Airborne particles also can impact vegetation and 

ecosystems and can cause damage to paints and 

building materials. (See Acid Rain, NO2, and SO2 

sections.) 

Trends in PM10 Levels and Direct Emissions 

Between 1992 and 2001, average PM10 concentrations decreased 14 percent, while direct PM10 

emissions decreased 13 percent. 

0 

10 

20 

30 

40 

50 

60 

92 93 94 95 96 97 98 99 00 01 

Concentration, μg/m3 

NAAQS 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

PM10 Air Quality, 1992–2001 

Based on Seasonally Weighted Annual Average 

770 Sites 

1992–01: 14% decrease 

82 85 92 93 94 95 96 97 98 99 00 01 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

7,000 

Thousand Short Tons 

PM10 Emissions, 1982–2001 

6,000 

In 1985, EPA refined its methods for estimating emissions. 

Fuel Combustion 

Transportation 

Industrial Processes 

1992–01: 13% decrease 

Emission 

trends 

data not 

available. 

Air quality concentrations do not always track nationwide emissions. 

For a detailed explanation, see page 3. 
12Six Principal Pollutants

15 μg/m3

20 μg/m3

10 μg/m3

Sulfate

Ammonium

Nitrate

Total Carbon

Crustal Material

Annual Average PM2.5 Concentrations (μg/m3) 

and Particle Type in Rural Areas,1999

Source: Interagency Monitoring of Protected Visual Environments

Network, 1999.

Trends in PM2.5 Levels and Direct Emissions

The chart at right shows that direct PM2.5

emissions from man-made sources decreased 10

percent nationally between 1992 and 2001.This

chart tracks only directly emitted particles and does 

not account for secondary particles formed when

emissions of NOx, SO2,ammonia,and other gases

react in the atmosphere.The principal types of secondary particles are sulfates and nitrates, which are

formed when SO2 and NOx react with ammonia.

The maps at right show how sulfates and nitrates,

along with other components, contribute to PM2.5

concentrations.The first map represents the most

recent year of data available from the Interagency

Monitoring of Protected Visual Environments

(IMPROVE) network, which was established in

1987 to track trends in pollutants, such as PM2.5,

that contribute to visibility impairment. Because

the monitoring sites are located in rural areas

throughout the country, the network is a good

source for assessing regional differences in PM2.5.

The second map represents the most recent year of

data from EPA’s urban speciation network, which

was established in 1999.All of these sites are located in urban areas.

Sites in the rural East typically have higher annual

average PM2.5 concentrations than those in the

rural West,as shown by the larger circles in the

East. Most of this regional difference is attributable

to higher sulfate concentrations in the eastern

United States. Sulfate concentrations in the East

largely result from SO2 emissions from coal-fired

power plants.

Sites in urban areas,as shown in the circles in the

map at right, generally have higher annual average

PM2.5 concentrations than nearby rural areas.

Carbon from soot and organic compounds

accounts for much of the difference, while sulfate

concentrations are about the same. Sites in central

California show that nitrates, in addition to carbon,

are responsible for higher urban concentrations in

that region.

Trends in rural PM2.5 concentrations can be examined with data from the IMPROVE network.At

the time of this report, 2000 and 2001 data were

not available. However, 36 sites have enough data

to assess trends from 1992 to 1999. In the East,

where sulfates contribute most to rural PM2.5, the

Sulfate

Ammonium

Nitrate

Total Carbon

Crustal Material

15 μg/m3

20 μg/m3

10 μg/m3

Annual Average PM2.5 Concentrations (μg/m3) 

and Particle Type in Urban Areas, 2001

Source: EPA Speciation Network, 2001.

0

500

1,000

1,500

2,000

2,500

Thousand Short Tons

Direct PM2.5 Emissions, 1992–2001

92 93 94 95 96 97 98 99 00 01

1992–01: 10% decrease

Fuel Combustion

Transportation

Industrial Processes

Note: Direct comparisons of the information in these two maps should

take into consideration: the fact that they represent different years; that one

is an urban network and the other is a rural network; and that there are

also differences in instruments and measurement methods.

If enacted, President Bush's Clear Skies Initiative would

decrease PM concentrations by dramatically reducing

emissions of SO2 and NOx. This initiative would also

reduce mercury emissions (www.epa.gov/clearskies).
13Six Principal Pollutants

annual average across the 10 sites decreased 5 percent from 1992 to 1999.The peak in 1998 is associated with increases in sulfates and carbon from

soot and organic compounds.Average PM2.5 concentrations across the 26 sites in the West were less

than one-half of the levels measured at eastern sites

from 1992 to 1999.

In 1999, EPA and its state, tribal,and local air 

pollution control partners deployed a monitoring

network to begin measuring PM2.5 concentrations

nationwide.The map at left shows annual average

PM2.5 concentrations by county. This map also

indicates that PM2.5 concentrations vary regionally.

Based on the monitoring data, parts of California

and much of the eastern United States have annual

average PM2.5 concentrations above the level of the

annual PM2.5 standard,as indicated by the orange

and red on the map.With few exceptions, the rest

of the country generally has annual average concentrations below the level of the annual PM2.5

health standard.

Now that there are 3 years of monitoring data

available, we have begun to examine trends at the

national level.Annual average PM2.5 concentrations

decreased 5 percent nationally from 1999 to 2001,

with much of that decrease occurring between

2000 and 2001.This decrease may or may not

represent a trend, given the few years of data available at this time.The Southeast was responsible for

most of that reduction, where the monitored levels

of PM2.5 decreased 10 percent from 2000 to 2001.

Lower 2001 annual average concentrations in the

Southeast are due, in part, to less demand on utilities during a very warm winter.This is illustrated

by the reduction in direct emissions of SO2 and

PM2.5 from fuel combustion in 2001.

Note: The NAAQS for PM2.5 is 

15 μg/m3 but is based on the average of

3 years of monitoring data. In addition,

PM2.5 concentration measurements from

the new nationwide monitoring network

are not directly comparable to the

measurements from the IMPROVE

network due to differences in instruments

and measurement methods.

Concentration (μg/m3)

> 20

15 - 20

12 - 15

< 12

Do not meet minimum

data completeness

Source: U.S. EPA AIRS databases as of 7/8/02.

Minimum 11 samples per calendar quarter required.

0

5

10

15

20

25

30

486 Sites

NAAQS

PM2.5 Air Quality, 1999–2001

Based on Seasonally Weighted Annual Average

1999–01: 5% decrease

Concentration, μg/m3

92 93 94 95 96 97 98 99 00 01

Average

90% of sites have concentrations below this line

10% of sites have concentrations below this line

Trends monitoring data for

PM2.5 not available.

Source: Interagency Monitoring of Protected Visual

Environments Network, 1999.

Annual Average PM2.5 Concentrations (μg/m3), 2001

14

12

10

8

6

4

2

0

Annual Average PM2.5 Concentrations

in Rural Areas

Sulfate (Eastern U.S.–10 sites)

Measured PM2.5 (Western U.S.–26 sites)

Measured PM2.5 (Eastern U.S.–10 sites)

Carbon (Eastern U.S.–10 sites)

92 93 94 95 96 97 98 99

Concentration, μg/m3
CARBON MONOXIDE (CO)


14 Six Principal Pollutants 

Concentration, ppm 

Nature and Sources of the Pollutant 

Carbon monoxide is a colorless and odorless gas, 

formed when carbon in fuel is not burned completely. It is a component of motor vehicle exhaust, 

which contributes about 60 percent of all CO 

emissions nationwide. Nonroad vehicles account 

for the remaining CO emissions from transportation sources. High concentrations of CO generally 

occur in areas with heavy traffic congestion. In 

cities, as much as 95 percent of all CO emissions 

may come from automobile exhaust. Other sources 

of CO emissions include industrial processes, 

Air quality concentrations do not always track nationwide 

emissions. For a detailed explanation, see page 3. 

82 85 92 93 94 95 96 97 98 99 00 01 

0 

20,000 

40,000 

60,000 

80,000 

100,000 

140,000 

Thousand Short Tons 

CO Emissions, 1982–2001 

120,000 

In 1985, EPA refined its methods for estimating emissions. 

Fuel Combustion 

Transportation 

Industrial Processes 

Miscellaneous 

2 

4 

6 

8 

10 

12 

14 

0 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

CO Air Quality, 1982–2001 

Based on Annual 2nd Maximum 8-hour Average 

NAAQS 

218 Sites 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

1982–01: 62% decrease 

1992–01: 38% decrease 

nontransportation fuel combustion, and natural 

sources such as wildfires. Peak CO concentrations 

typically occur during the colder months of the 

year when CO automotive emissions are greater 

and nighttime inversion conditions (where air 

pollutants are trapped near the ground beneath a 

layer of warm air) are more frequent. 

Health and Environmental Effects 

CO enters the bloodstream through the lungs and 

reduces oxygen delivery to the body’s organs and 

tissues.The health threat from levels of CO sometimes found in the ambient air is most serious for 

those who suffer from cardiovascular disease, such 

as angina pectoris.At much higher levels of exposure not commonly found in ambient air, CO can 

be poisonous, and even healthy individuals may be 

affected.Visual impairment, reduced work capacity, 

reduced manual dexterity, poor learning ability, and 

difficulty in performing complex tasks are all 

associated with exposure to elevated CO levels. 

Trends in CO Levels and Emissions 

Nationally, the 2001 ambient average CO concentration is almost 62 percent lower than that for 

1982 and is the lowest level recorded during the 

past 20 years. CO emissions from transportation 

sources, the major contributor to ambient CO 

concentration, have decreased slightly during this 

period. Between 1992 and 2001, ambient CO 

concentrations decreased 38 percent.This air 

quality improvement occurred despite an approximately 35 percent increase in vehicle miles traveled 

in the United States during this 10-year period 

and an increase in total CO emissions of 6 percent. 

The recent increase in CO emissions was caused 

by an extremely serious wildfire season in 2000. 

Nearly twice the number of U.S. acres burned in 

2000 compared to the average year since 1982. 

1982–01: 0% change 

1992–01: 6% increase 
15 Six Principal Pollutants 

LEAD (Pb) 

Nature and Sources of the Pollutant 

In the past, automotive sources were the major 

contributor of lead emissions to the atmosphere. 

As a result of EPA’s regulatory efforts to reduce the 

content of lead in gasoline, however, the contribution of air emissions of lead from the transportation 

sector, and particularly the automotive sector, has 

greatly declined over the past two decades.Today, 

industrial processes, primarily metals processing, are 

the major source of lead emissions to the atmosphere.The highest air concentrations of lead are 

usually found in the vicinity of smelters and 

battery manufacturers. 

Health and Environmental Effects 

Exposure to lead occurs mainly through inhalation 

of air and ingestion of lead in food, water, soil, or 

dust. It accumulates in the blood, bones, and soft 

tissues and can adversely affect the kidneys, liver, 

nervous system, and other organs. Excessive exposure to lead may cause neurological impairments 

such as seizures, mental retardation, and behavioral 

disorders. Even at low doses, lead exposure is 

associated with damage to the nervous systems of 

fetuses and young children, resulting in learning 

deficits and lowered IQ. Recent studies also show 

that lead may be a factor in high blood pressure 

and subsequent heart disease. Lead can also be 

deposited on the leaves of plants, presenting a 

hazard to grazing animals and humans through 

ingestion. 

Trends in Lead Levels and Emissions 

Because of the phaseout of leaded gasoline, lead 

emissions and concentrations decreased sharply 

during the 1980s and early 1990s. The 2001 

average air quality concentration for lead is 94 

percent lower than in 1982. Emissions of lead 

decreased 93 percent over that same 20-year 

period.Today, the only violations of the lead 

NAAQS occur near large industrial sources such 

as lead smelters and battery manufacturers. Various 

enforcement and regulatory actions are being 

actively pursued by EPA and the states for cleaning 

up these sources. 

Air quality concentrations do not always track nationwide 

emissions. For a detailed explanation, see page 3. 

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 

0.0 

Lead Air Quality, 1982–2001 

Based on Annual Maximum Quarterly Average 

NAAQS 

1.0 

1.5 

2.0 

0.5 

Concentration, μg/m3 

39 Sites 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

1982–01: 94% decrease 

1992–01: 25% decrease 

82 85 92 93 94 95 96 97 98 99 00 01

0 

Short Tons 

Lead Emissions, 1982–2001 

In 1985, EPA refined its methods for estimating emissions. 

1982–01: 93% decrease 

20,000 

40,000 

60,000 

80,000 

Fuel Combustion 

Transportation 

Industrial Processes 
16 

Acid Rain 

Nature and Sources of the Problem 

Acidic deposition or “acid rain” occurs when 

emissions of sulfur dioxide and nitrogen oxides 

in the atmosphere react with water, oxygen, and 

oxidants to form acidic compounds.These 

Acid Rain 

SO2 

NOx 

Acid Rain Formation 

Coal-fired electric utilities and other sources that burn 

fossil fuels emit SO2 and NOx. 

SO2 Emissions Covered under 

the Acid Rain Program 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

1980 1985 1990 1995 1996 1997 1998 1999 2000

Phase II units not affected for NOx 

Phase II NOx affected units (1,046) 

Projected NOx emissions without the Acid Rain Program 

1990 1995 1996 1997 1998 1999 2000

4.48 4.82 

5.29 5.53 5.54 5.44 5.49 5.11 5.49 

6.09 5.91 6.04 5.97

6.66 

8.1 

6.7 

NOx Emissions Covered under 

the Acid Rain Program 

compounds fall to the Earth in either dry form 

(gas and particles) or wet form (rain, snow, and 

fog). Some are carried by the wind, sometimes 

hundreds of miles, across state and national borders. 

In the United States, about 64 percent of annual 

SO2 emissions and 26 percent of NOx emissions 

are produced by electric utility plants that burn 

fossil fuels. 

Health and Environmental Effects 

In the environment, acid deposition causes soils 

and waterbodies to acidify (making the water 

unsuitable for some fish and other wildlife) and 

damages some trees, particularly at high elevations. 

It also speeds the decay of buildings, statues, and 

sculptures that are part of our national heritage. 

The nitrogen portion of acid deposition contributes to eutrophication in coastal ecosystems, 

the symptoms of which include algal blooms 

(some of which may be toxic), fish kills, and loss 

of plant and animal diversity. Finally, acidification 

of lakes and streams can increase the amount 

of methyl mercury available in aquatic systems. 

Most exposure to mercury comes from eating 

contaminated fish. Reductions in SO2 and 

NOx have begun to reduce some of these 

negative environmental effects and are leading 

to significant improvements in public health 

(described previously). 

Program Structure 

The goal of EPA’s Acid Rain Program, established 

by the Clean Air Act, is to improve public health 

and the environment by reducing emissions of SO2 

and NOx.The program was implemented in two 

phases: Phase I for SO2 began in 1995 and targeted 

the largest and highest-emitting coal-fired power 

plants. Phase I for NOx began in 1996. Phase II for 

both pollutants began in 2000 and sets restrictions 

on Phase I plants as well as smaller coal-, gas-, 

and oil-fired plants. Over 2,000 sources are now 

affected by the Acid Rain Program. 

The Acid Rain Program will reduce annual SO2 

emissions by 10 million tons from 1980 levels 

by 2010.The program sets a permanent cap of 

8.95 million tons on the total amount of SO2 that 

may be emitted by power plants nationwide, about 

half the amount emitted in 1980. It employs an 

emissions trading program to reach that emissions 

cap more efficiently and cost-effectively. Sources 

are allocated allowances efficiently each year (one 

allowance equals 1 ton of SO2 emissions), which 

can be bought or sold or banked for future use. 

Million Tons NOx Million Tons SO2 

0 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 
This approach gives sources the flexibility and

incentive to reduce emissions at the lowest cost

while ensuring that the emission cap is met.

The NOx component of the Acid Rain Program

limits the emission rate for all affected utilities,

resulting in a 2 million ton NOx reduction from

1980 levels by 2000.There is no cap on total NOx

emissions, but under this program a source can

choose to over-control at units where it is technically easier to control emissions,average these

emissions with those at their other units,and

thereby achieve overall emissions reductions at

lower cost.

Emissions and Atmospheric Trends

SO2 emissions reductions have been significant in

the first 6 years of EPA’s Acid Rain Program.The

first year of compliance with Phase II of the Acid

Rain Program was 2000. Sources in the Acid Rain

Program emitted 11.2 million tons in 2000, down

from 16 million tons in 1990. Emissions of SO2

dropped 1 million tons between 1999 and 2000.

Sources began drawing down the bank of unused

allowances in 2000, resulting in emissions levels

greater than the allowances allocated in 2000 but

still lower than emissions during any previous year.

Actual NOx emissions,as shown in the graph 

on the bottom left of page 16, have also declined

since 1990. NOx emissions decreased steadily from 

6 tons in 1997 to just over 5 tons in 2000.The

more than 1,000 sources affected by Phase II

emitted 4.5 tons in 2000, over 1 million tons

(almost 20 percent) less than they did in 1990.

NOx emissions in 2000 were somewhat lower 

(7 percent) than in 1999 and almost half of what

emissions were projected to have been in 2000

without the Acid Rain Program.

For all years from 1995 through 2000, sulfate

deposition exhibited dramatic and unprecedented

reductions over a large area of the eastern United

States.Average sulfate deposition in 1996–2000

compared to 1990-1994 was 10 percent lower

nationwide and 15 percent lower in the East.

Similarly, sulfate air concentrations, which contribute to human health and visibility problems,

were reduced significantly, especially in the East.

Nitrate deposition decreased slightly in some places

but increased in others, causing an overall average

increase in nitrate deposition between 1990–1994

and 1996–2000 of 3 percent.

These reductions in acid precipitation are directly

related to the large regional decreases in SO2

and NOx emissions resulting from the Acid Rain

Program.The largest reductions in sulfate concentrations occurred along the Ohio River Valley 

and in states immediately downwind.The largest

reductions in wet sulfate deposition occurred across

the Mid-Appalachian and Northeast regions of 

the country. Reductions in the East in hydrogen

ion concentrations, the primary indicator of 

precipitation acidity, were similar to those of 

sulfate concentrations, both in magnitude and 

location.The largest reductions in wet nitrate

deposition were in the northeastern United States,

Michigan,and Texas.The Midwest, the Southeast,

and California showed the highest increases in

deposition even though emissions from acid rain

sources have not increased substantially there.Acid

rain sources account for only one-third of nationwide nitrogen emissions, so emissions trends in

other source categories, especially agriculture and

mobile sources,affect air concentrations and

deposition.

17Acid Rain

Change in Deposition from Precipitation

1990–1994 to 1996–2000

Sulfate

Nitrate

Source: U.S. EPA analysis of National Atmospheric

Deposition Program data.
18 

Visibility


In 2000, the IMPROVE Monitoring Network, used to track visibility trends at national parks and wilderness areas, started an 

expansion from 30 to 110 monitoring sites. The expansion 

work was completed in the fall of 2001. However, due to the 

level of resources required to complete the network expansion, reporting of 2000 IMPROVE data will occur while this 

brochure is being finalized. Therefore, no update is provided 

for visibility trends reported in the 1999 Status and Trends 

Brochure. Reporting of 2000 and 2001 visibility trends are 

scheduled to be included in U.S. EPA’s 2001 National Air 

Quality and Emission Trends Report. 

Nature and Sources of the Problem 

Visibility impairment is one of the most obvious 

effects of air pollution and occurs at many of 

the best known and most treasured natural parks 

and wilderness areas, such as the Grand Canyon, 

Yosemite,Yellowstone, Mount Rainier, Shenandoah, and the Great Smoky Mountain National 

Park, as well as in urban areas. 

Visibility impairment results from the scattering 

and absorption of light by air pollution, including 

particles and gases.The scattering and absorption 

by air pollution limits the distance we can see and 

can also degrade the color, clarity, and contrast of 

scenes.The same fine particles that are linked to 

serious health effects and premature death can 

also significantly affect our ability to see. 

Some particles that contribute to visibility impairment are emitted directly into the atmosphere 

from their sources, such as dust from roads or 

elemental carbon (soot) from wood combustion. 

In other cases, particles are formed in the atmosphere from primary gaseous emissions such as 

SO2 emissions from power plants and other industrial facilities and nitrates formed from NOx emissions from power plants, automobiles, and other 

types of combustion sources.These types of particles are referred to as secondarily formed particles. 

In the eastern United States, reduced visibility is 

mainly attributable to secondarily formed sulfates. 

Although these secondarily formed particles still 

account for a significant amount of particulate 

loading in the West, primary emissions from 

sources like wood smoke contribute a larger percentage of the total particulate loading than in the 

East. 

Also, humidity can significantly increase the effect 

of pollution on visibility, causing some particles 

to become more efficient at scattering light and 

causing visibility impairment.Annual average 

relative humidity levels are 70 to 80 percent in the 

East as compared to 50 to 60 percent in the West. 

Poor summer visibility in the eastern United States 

is primarily the result of high sulfate concentrations combined with high humidity levels. 

East West 

Sulfates 60–86% 25–50% 

Organic Carbon 10–18% 25–40% 

Nitrates 7–16% 5–45% 

Elemental Carbon (soot) 5–8% 5–15% 

Crustal Material (soil dust) 5–15% 5–25% 

This table shows pollutants that contribute to visibility impairment in the eastern and western parts 

of the United States. Sulfates are generally the 

largest contributor in both the East and the West. 

Program Structure 

The Clean Air Act provides for the protection 

of visibility in national parks and wilderness areas, 

also known as Class I areas. The Clean Air Act’s 

national goal calls for remedying existing visibility 

impairment and preventing future impairment in 

these 156 Class I areas across the country. 

Class I Areas 

The Clean Air Act provides for the protection of visibility 

in our national parks and wilderness areas, also known as 

Class I areas.There are 156 Class I areas across the 

United States as shown. 

In 1987, the IMPROVE visibility network was 

established as a cooperative effort among EPA, 

states, National Park Service, U.S. Forest Service, 

Bureau of Land Management, and U.S. Fish and 

Wildlife Service. Data are collected and analyzed 

from this network to determine the type of pollutants primarily responsible for reduced visibility 
and to track progress toward the Clean Air Act’s 

national goal. 

In April 1999, EPA initiated a new regional haze 

program. The program addresses visibility impairment in national parks and wilderness areas caused 

by numerous sources located over broad regions. 

The program sets a framework for states to develop 

goals for improving visibility on the worst visibility 

days each year and to adopt emission strategies 

to meet these goals. Because fine particles are 

frequently transported hundreds of miles, pollution 

that occurs in one state may contribute to the 

visibility impairment in another state. For this 

reason, EPA encourages states to coordinate 

through regional planning organizations to develop 

regional strategies to improve visibility and to 

reduce pollutants that contribute to fine particles 

and ground-level ozone. States are also required 

to review progress every 5 years and revise any 

strategies as necessary. 

Visibility Trends 

Without the effects of pollution, a natural visual 

range in the United States is approximately 75 to 

150 km (45 to 90 miles) in the East and 200 to 

300 km (120 to 180 miles) in the West. 

Data collected by the IMPROVE network show 

visibility impairment for the worst visibility in 

the West is similar to days with the best visibility 

in the East. In 1999, mean visual range for the 

19 Visibility 

Shenandoah 

National Park 

under bad and 

good visibility 

conditions.The 

visual range in 

the top photo is 

25 km while the 

visual range in 

the bottom photo 

is 180 km. 

Yosemite 

National Park 

under bad and 

good visibility 

conditions.The 

visual range in 

the top photo is 

111 km while 

the visual range 

in the bottom 

photo is greater 

than 208 km. 

50 

100 

150 

200 

250 

0 

Year 

Visibility Trends for Eastern 

U.S. Class I Areas, 1992–1999 

Visual Range (km) 

92 93 94 95 96 97 98 99 

Best visibility 

range is 79-90 km 

Mid-range visibility 

is 42-48 km 

Worst visibility 

range is 20-23 km 

Worst Visibility 

Mid-Range 

Best Visibility 

Visibility Metrics. Comparisons of extinction 

(Mm-1 ), deciviews (dv), and visual range (km). 

Notice the difference in the three scales: 10 

Mm-1 corresponds to about 400 km visual 

range and 0.0 dv, while 1,000 Mm-1 is about 

4 km visual range and 46 dv. 

worst days in the East was only 24 km (14.4 miles) 

compared to 84 km (50.4 miles) for the best 

visibility. In the West, visibility impairment for 

the worst days remained relatively unchanged over 

the 1990s, with the mean visual range for 1999 

(80 km) nearly the same as the 1990 level (86 km). 

50 

100 

150 

200 

250 

0 

Year 

Visibility Trends for Western 

U.S. Class I Areas, 1990–1999 

Visual Range (km) 

90 91 92 93 94 95 96 97 98 99 

Best visibility 

range is 177-208 km 

Mid-range visibility 

is 118-133 km 

Worst visibility 

range is 73-85 km 

Worst Visibility 

Mid-Range 

Best Visibility 
20 

Toxic Air Pollutants 

Nature and Sources of the Problem 

Toxic air pollutants, or air toxics, are those 

pollutants that cause or may cause cancer or other 

serious health effects, such as reproductive effects 

or birth defects.Air toxics may also cause adverse 

environmental and ecological effects. Examples 

of toxic air pollutants include benzene, found in 

gasoline; perchloroethylene, emitted from some dry 

cleaning facilities; and methylene chloride, used as 

a solvent by a number of industries. Most air toxics 

originate from man-made sources, including 

mobile sources (e.g., cars, trucks, construction 

equipment) and stationary sources (e.g., factories, 

refineries, power plants), as well as indoor sources 

(e.g., some building materials and cleaning 

solvents). Some air toxics are also released from 

natural sources such as volcanic eruptions and 

forest fires.The Clean Air Act identifies 188 

air toxics from industrial sources. EPA has identified 20 of these pollutants that are associated with 

mobile sources and one additional mobile source 

air toxic designated “diesel particulate matter and 

diesel exhaust organic gases.” 

Health and Environmental Effects 

People exposed to toxic air pollutants at sufficient 

concentrations may experience various health 

effects, including cancer, damage to the immune 

system, as well as neurological, reproductive 

(e.g., reduced fertility), developmental, respiratory, 

and other health problems. In addition to exposure 

from breathing air toxics, risks also are associated 

with the deposition of toxic pollutants onto soils 

or surface waters, where they are taken up by 

plants and ingested by animals and eventually 

magnified up through the food chain. Like 

humans, animals may experience health problems 

due to air toxics exposure. 

Trends in Toxic Air Pollutants 

EPA and states do not maintain an extensive 

nationwide monitoring network for air toxics as 

they do for many of the other pollutants discussed 

in this report.While EPA, states, tribes,and local air 

regulatory agencies collect monitoring data for a 

number of toxic air pollutants, both the chemicals 

monitored and the geographic coverage of the 

monitors vary from state to state. EPA is working 

with these regulatory partners to build upon the 

existing monitoring sites to create a national 

monitoring network for a number of toxic air 

pollutants.The goal is to ensure that those 

compounds that pose the greatest risk are measured.The available monitoring data help air pollution control agencies track trends in toxic air pollutants in various locations around the country. 

EPA began a pilot city monitoring project in 2001 

and is scheduled to include at least 12 months 

of sampling in four urban areas and six small 

city/rural areas (see map below).This program is 

intended to help answer several important national 

network design questions (e.g., sampling and 

analysis precision, sources of variability, and minimal detection levels). In addition, an initial 11-city 

trends network is being established that will help 

develop national trends for several pollutants of 

concern. For the latest information on national 

air toxics monitoring, see www.epa.gov/ttn/ 

amtic/airtxfil.html. 

EPA also compiles an air toxics inventory as 

part of the National Emissions Inventory (NEI, 

formerly the National Toxics Inventory) to estimate and track national emissions trends for the 

188 toxic air pollutants regulated under the Clean 

Air Act. In the NEI, EPA divides emissions into 

four types of sectors: 

San J

T

Detroit Cedar Rapids 

San J

Gr

Seattle 

Char

uan 

ampa 

acinto 

and J

Providence 

Rio Rancho 

unction 

leston 

Map of 10 cities in monitoring pilot project 
Risk Assessment 

EPA has developed a National-Scale Air Toxics 

Assessment, which is a nationwide analysis of air 

toxics. It uses computer modeling of the 1996 NEI 

air toxics data as the basis for developing health 

risk estimates for 33 toxic air pollutants (a subset of 

the Clean Air Act’s list of 188 air toxics plus diesel 

PM).The national-scale assessment is intended to 

provide state, local, and tribal agencies and others 

with a better understanding of the risks from 

inhalation exposure to toxic air pollutants from 

outdoor sources. It will help EPA and states prioritize data and research needs to better assess risk in 

the future and will provide a baseline to help 

measure future trends in estimated health risks.The 

next national-scale analysis will focus on 1999 data 

and will be released by the end of 2003. 

21 

Trends for individual air toxics vary from pollutant 

to pollutant. Benzene, which is the most widely 

monitored toxic air pollutant, is emitted from cars, 

trucks, oil refineries, and chemical processes.The 

graph below shows measurements of benzene 

taken from 95 urban monitoring sites around the 

country.These urban areas generally have higher 

levels of benzene than other areas of the country. 

Measurements taken at these sites show, on average, 

a 47 percent drop in benzene levels from 1994 

to 2000. During this period, EPA phased in new 

(so-called “tier 1”) car emission standards; required 

many cities to begin using cleaner burning gasoline; and set standards that required significant 

reductions in benzene and other pollutants emitted 

from oil refineries and chemical processes. EPA 

estimates that, nationwide, benzene emissions from 

all sources dropped 20 percent from 1990 to 1996. 

Toxic Air 

Pollutants 

(1) major (large industrial) sources; (2) area and 

other sources, which include smaller industrial 

sources like small dry cleaners and gasoline 

stations, as well as natural sources like wildfires; 

(3) onroad mobile sources, including highway 

vehicles; and (4) nonroad mobile sources like aircraft, locomotives, and construction equipment. 

As shown in this pie chart, based on 1996 

estimates, the most recent year of available data, 

the emissions of toxic air pollutants are relatively 

equally divided between the four types of sources. 

However, this distribution varies from city to city. 

Ambient Benzene, Annual Average Urban 

Concentrations, Nationwide, 1994–2000 

0 

1 

2 

3 

4 

5 

6 

7 

94 95 96 97 98 99 00 

10% of sites have concentrations below this line 

90% of sites have concentrations below this line 

Average 

1994–00: 47% decrease 

Concentration, μg/m3 

National Air Toxics Emissions, 1996 

4.7M Tons 

Onroad 

31% 

Area/Other 

25% 

Major 

24% 

Nonroad 

20% 

Baseline 

(1990–1993) 

Million Tons 

National Air Toxics Emissions 

Total for 188 Toxic Air Pollutants 

1996 

0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

7.0 

33 urban 

air toxics 

155 other 

air toxics 

Based on the data in the NEI, estimates of nationwide air toxics emissions have dropped approximately 24 percent between baseline (1990–1993) 

and 1996.Thirty-three of these air toxics, which 

pose the greatest threat to public health in urban 

areas, have similarly dropped 31 percent. Although 

changes in how EPA compiled the national inventory over time may account for some differences, 

EPA and state regulations, as well as voluntary 

reductions by industry, have clearly achieved 

large reductions in overall air toxic emissions. 
22Toxic Air 

Pollutants

Programs to Reduce Air Toxics

Since 1990, EPA’s technology-based emission

standards for industrial sources (e.g., chemical

plants, oil refineries,and dry cleaners) have proven

extremely successful in reducing emissions of air

toxics. Once fully implemented, these standards 

will cut annual emissions of toxic air pollutants 

by nearly 1.5 million tons from 1990 levels.

EPA has also put into place important controls 

for motor vehicles and their fuels, including 

introduction of reformulated gasoline and low 

sulfur diesel fuel,and is continuing to take additional steps to reduce air toxics from vehicles.

Furthermore,air toxics emissions will further

decline as the motor vehicle fleet turns over, with

newer vehicles replacing older higher emitting

vehicles. By the year 2020, these requirements are

expected to reduce emissions of a number of air

toxics (benzene, formaldehyde,acetaldehyde,and

1,3-butadiene) from highway motor vehicles by

about 75 percent and diesel PM by over 90 percent from 1990 levels.

EPA has begun to look at the risk remaining 

(i.e., the residual risk) after emission reductions 

for industrial sources take effect and is also 

investigating new standards for nonroad engines

such as construction equipment.

In addition to national regulatory efforts, EPA’s

program includes work with communities on

comprehensive local assessments,as well as federal

and regional activities associated with protecting

waterbodies from air toxics deposition (e.g., the

Great Waters program, which includes the Great

Lakes, Lake Champlain, Chesapeake Bay,and many

coastal estuaries) and EPA initiatives concerning

mercury and other persistent and bioaccumulative

toxics. For indoor air toxics, EPA’s program has

relied on education and outreach to achieve

reductions. Information about indoor air activities

is available at www.epa.gov/iaq/.

For more information about EPA’s air toxics

program, visit the Agency’s Web site at 

www.epa.gov/ttn/atw.

The following map shows a pattern of the 

distribution of relative cancer risk across the 

continental United States as estimated by the

national-scale assessment.The highest ranking 20

percent of counties in terms of risk (622 counties)

contain almost three-fourths of the U.S. population.Three air toxics (chromium, benzene,and

formaldehyde) appear to pose the greatest nationwide carcinogenic risk.This map does not include

the potential risk from diesel exhaust emissions.

This is because existing health data are not 

sufficient to develop a numerical estimate of cancer

risk for this pollutant. However, exposure to diesel

exhaust is widespread and EPA has concluded that

diesel exhaust is a likely human carcinogen and

ranks with the other substances that the nationalscale assessment suggests pose the greatest relative

risk. One air toxic,acrolein, is estimated to pose

the highest potential nationwide for significant

chronic adverse effects other than cancer. For more

information, visit www.epa.gov/ttn/atw/nata.

This technical assessment represents an important

step toward characterizing air toxics nationwide.

It is designed to help identify general patterns in

air toxics exposure and risk across the country and

is not recommended as a tool to characterize or

compare risk at local levels (e.g., to compare risks

from one part of a city to another). More localized

assessments, including monitoring and modeling,

are under way to help characterize local-level risk.

0.0E+000 to 2.2E-005

2.2E-005 to 2.5E-005

2.5E-005 to 2.8E-005

2.8E-005 to 3.5E-005

3.5E-005 to 1.9E-005

Median County Risk

County Risk Comparison

Estimated by National-Scale Assessment

Highest

Lowest
23 

Stratospheric Ozone 

through the lower part of the atmosphere, called 

the troposphere, and into the stratosphere.There, 

strong solar radiation releases chlorine and bromine 

atoms that attack protective ozone molecules. 

Scientists estimate that one chlorine atom can 

destroy 100,000 ozone molecules. 

Health and Environmental Effects 

Some UV-b radiation reaches the Earth’s surface 

even with normal ozone levels. However, because 

the ozone layer normally absorbs most UV-b 

radiation from the sun, ozone depletion is expected 

to lead to increases in harmful effects associated 

with UV-b radiation. In humans, UV-b radiation 

is linked to skin cancer, including melanoma, the 

form of skin cancer with the highest fatality rate. 

It also causes cataracts and suppression of the 

immune system. 

The effects of UV-b radiation on plant and aquatic 

ecosystems are not well understood. However, the 

growth of certain food plants can be slowed by 

excessive UV-b radiation. In addition, some scientists suggest that marine phytoplankton, which are 

the base of the ocean food chain, are already under 

stress from UV-b radiation.This stress could have 

adverse consequences for human food supplies 

from the oceans.

Programs to Restore the Stratospheric 

Ozone Layer 

In 1987, 27 countries, including the United States, 

signed the Montreal Protocol, a treaty that recognized the international nature of ozone depletion 

and committed the world to limiting the production of ozone-depleting substances.Today, more 

than 180 nations have signed the Protocol, which 

has been strengthened five times and now calls for 

the elimination of those chemicals that deplete 

stratospheric ozone. 

The 1990 Clean Air Act Amendments established 

a U.S. regulatory program to protect the stratospheric ozone layer. In January 1996, U.S. production of many ozone-depleting substances virtually 

ended, including CFCs, carbon tetrachloride, and 

methyl chloroform. Production of halons ended 

in January 1994. Many new products that either 

Monthly average total ozone measured in Dobson units 

(DU) at four mid-latitude stations across the United 

States from 1979 to 1997.The trend line shows a 3.4 

percent decrease in average total ozone over mid-latitudes 

in the United States since 1979.The large annual 

variation shown in each of the four cities is a result of 

ozone transport processes that cause increased levels 

in the winter and spring and lower ozone levels in 

the summer and fall at these latitudes. 

Nature and Sources of the Problem 

The stratosphere, located about 6 to 30 miles 

above the Earth, contains a layer of ozone gas that 

protects living organisms from harmful ultraviolet 

radiation (UV-b) from the sun. Over the past three 

decades, however, it has become clear that this 

protective shield has been damaged. Each year, an 

“ozone hole” forms over the Antarctic, and ozone 

levels there can fall to 60 percent below normal. 

Even over the United States, ozone levels are 

about 3 percent below normal in the summer 

and 5 percent below normal in the winter. 

As the ozone layer thins, more UV-b radiation 

reaches the Earth.The 1998 Scientific Assessment 

of Stratospheric Ozone firmly established the link 

between decreased ozone and increased UV-b 

radiation. In the 1970s, scientists had linked 

several substances associated with human activities 

to ozone depletion, including the use of chlorofluorocarbons (CFCs), halons, carbon tetrachloride, 

methyl bromide, and methyl chloroform.These 

chemicals are emitted from commercial air conditioners, refrigerators, insulating foam, and some 

industrial processes. Strong winds carry them 

78 80 82 84 86 88 90 92 96

Total Ozone (DU) 

275 

300 

325 

350 

375 

400 

225 

94 98

Trend: 

3.4% 

decrease 

Year 

Boulder, CO Wallops

Is. Fresno, CA 

Nashville, TN 

Source: National Oceanic and Atmospheric Administration (NOAA), 1998. 
24Stratospheric Ozone

do not affect or are less damaging to the ozone

layer are now gaining popularity. For example,

computer makers are using ozone-safe solvents to

clean circuit boards, and automobile manufacturers

are using HFC-134a, an ozone-safe refrigerant,

in new motor vehicle air conditioners. In some

industries, the transition away from ozone-depleting substances has already been completed. EPA is

also emphasizing new efforts like the UV Index, a

daily forecast of the strength of UV radiation to

which people may be exposed outdoors, to educate

the public about the health risks of overexposure to

UV radiation and the steps they can take to reduce

those risks.

Trends in Stratospheric Ozone Depletion

Scientific evidence shows that the approach taken

under the Montreal Protocol has been effective to

date. Measurements have shown that atmospheric

concentrations of methyl chloroform are falling,

indicating that emissions have been greatly

reduced. Concentrations of other ozone-depleting

substances in the upper layers of the atmosphere,

like CFCs, are also beginning to decrease. It takes

several years for these substances to reach the

stratosphere and release chlorine and bromine.

For this reason, stratospheric chlorine levels are

currently peaking and are expected to slowly

decline in the years to come. Because of the

stability of most ozone-depleting substances,

chlorine will be released into the stratosphere 

for many years, and the ozone layer will not fully

recover until the second half of this century.All

nations that signed the Protocol must complete

implementation of ozone protection programs 

if full repair of the ozone layer is to happen.

A 1996 study using 

satellite-based analyses of

UV-b trends demonstrated

that UV-b level had

increased at ground level.

This figure shows the 

percent increases in average

annual UV-b reaching the

surface from 1986 to

1996. UV-b incidence 

is strongly dependent on

latitude.At latitudes that

cover the United States,

UV-b levels are 4 to 5 

percent higher than they

were in 1986.

UV-b Radiation Increases by Latitude

65N: 6.8%

55N: 7.3%

45N: 5.0%

35N: 3.9%

25N: 1.2%

15N: 0.1%

Equator

15S: 2.3%

25S: 2.6%

35S: 2.9%

45S: 5.5%

55S: 9.9%

65S: 11.0%

DC

Edmonton

Buenos Aires

Johannesburg

London

Moscow

Melbourne
25

Conclusions

The Clean Air Act has resulted in many improvements in the quality of the air in the United States.

Scientific and international developments continue

to have an effect on the air pollution programs 

that are implemented by the U.S. Environmental

Protection Agency and state, local, and tribal agencies. New data help identify sources of pollutants

and the properties of these pollutants.Although

much progress has been made to clean up our 

air, work must continue to ensure steady improvements in air quality, especially because our lifestyles

create more pollution sources. Many of the strategies for air quality improvement will continue 

to be developed through coordinated efforts with

EPA, state, local and tribal governments, as well as

industry and other environmental organizations.
26

Acronyms

CFCs chlorofluorocarbons

CO2 carbon dioxide

CO carbon monoxide

DU Dobson units

EPA U.S. Environmental Protection Agency

FCCC Framework Convention on Climate Change

IMPROVE Interagency Monitoring of Protected Visual Environments

IPCC Intergovernmental Panel on Climate Change

km kilometers

NAAQS National Ambient Air Quality Standards

NAS National Academy of Sciences

NCDC National Climatic Data Center

NEI National Emissions Inventory

NESDIS National Environmental Satellite Data and Information Service

NO2, NOx nitrogen dioxide, nitrogen oxides

NO nitric oxide

NOAA National Oceanic and Atmospheric Administration

O3 ozone

Pb lead

PM10, PM2.5 particulate matter (10 μm or less, 2.5 μm or less in diameter)

ppm parts per million

SO2, SOx sulfur dioxide, sulfur oxides

VOCs volatile organic compounds

UV ultraviolet
27

For Further information

Detailed information on Air Pollution Trends: www.epa.gov/airtrends

Real-Time Air Quality Maps and Forecasts: www.epa.gov/airnow

On-line Air Quality Data: www.epa.gov/air/data/index.html

Office of Air and Radiation: www.epa.gov/oar

Office of Air Toxics: www.epa.gov/ttn/atw

Office of Air Quality Planning and Standards: www.epa.gov/oar/oaqps

Office of Transportation and Air Quality: www.epa.gov/otaq

Office of Atmospheric Programs: www.epa.gov/air/oap.html

Office of Radiation and Indoor Air: www.epa.gov/air/oria.html

Global Warming Emissions Information: www.epa.gov/globalwarming/index.html

Acid Rain Web site: www.epa.gov/airmarkets/arplindex.html

Acid Rain Hotline: (202) 564-9620

Energy Star (Climate Change) Hotline: (888) STAR-YES

Mobile Sources National Vehicles and Fuel Emissions Lab: (734) 214-4200

Ozone Depletion Web site: www.epa.gov/ozone/
Agency

Environmental ProtectionUnited States

Research Triangle Park, NCAir Quality Strategies and Standards DivisionOffice of Air Quality and Standards EPA Publication No. EPA 454/K-02-001