CHAPTER 2 

AIR 

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AIR CHAPTER CONTENTS 

2.1 INTRODUCTION ........................................................................................................................2-5

2.1.1 Overview of the Data .......................................................................................................2-6

2.1.2 Organization of This Chapter...........................................................................................2-7

2.2 WHAT ARE THE TRENDS IN OUTDOOR AIR QUALITY AND THEIR EFFECTS ON 

HUMAN HEALTH AND THE ENVIRONMENT? ....................................................................2-9

2.2.1 Introduction......................................................................................................................2-9

2.2.2 ROE Indicators ..............................................................................................................2-11

INDICATOR: Ozone and Particulate Matter Concentrations for U.S. Counties in

INDICATOR: Ambient Concentrations of Manganese Compounds in EPA Region 

INDICATOR: Carbon Monoxide Emissions...............................................................2-13

INDICATOR: Ambient Carbon Monoxide Concentrations........................................2-16

INDICATOR: Lead Emissions....................................................................................2-19

INDICATOR: Ambient Concentrations of Lead.........................................................2-21

INDICATOR: Nitrogen Oxides Emissions .................................................................2-23

INDICATOR: Ambient Concentrations of Nitrogen Dioxide.....................................2-26

INDICATOR: Volatile Organic Compounds Emissions.............................................2-29

INDICATOR: Ambient Concentrations of Ozone.......................................................2-32

INDICATOR: Ozone Injury to Forest Plants ..............................................................2-37

INDICATOR: Particulate Matter Emissions...............................................................2-40

INDICATOR: Ambient Concentrations of Particulate Matter ....................................2-44

INDICATOR: Regional Haze......................................................................................2-50

INDICATOR: Sulfur Dioxide Emissions....................................................................2-53

INDICATOR: Acid Deposition ...................................................................................2-56

INDICATOR: Lake and Stream Acidity .....................................................................2-62

INDICATOR: Percent of Days with Air Quality Index Values Greater Than 100 .....2-65

INDICATOR: Mercury Emissions..............................................................................2-68

INDICATOR: Air Toxics Emissions...........................................................................2-70

INDICATOR: Ambient Concentrations of Benzene ...................................................2-74

INDICATOR: Concentrations of Ozone-Depleting Substances..................................2-76

INDICATOR: Ozone Levels over North America ......................................................2-79

the U.S./Mexico Border Region.....................................................................................2-82

5 .....................................................................................................................................2-86

2.2.3 Discussion......................................................................................................................2-89

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What These Indicators Say About Trends in Outdoor Air Quality and Their Effects 

on Human Health and the Environment.........................................................................2-89

Limitations, Gaps, and Challenges ................................................................................2-91

2.3 WHAT ARE THE TRENDS IN GREENHOUSE GAS EMISSIONS AND 

CONCENTRATIONS?...............................................................................................................2-93

2.3.1 Introduction....................................................................................................................2-93

2.3.2 ROE Indicators ..............................................................................................................2-95

INDICATOR: U.S. Greenhouse Gas Emissions..........................................................2-96

INDICATOR: Atmospheric Concentrations of Greenhouse Gases...........................2-100

2.3.3 Discussion....................................................................................................................2-106

What These Indicators Say About Trends in Greenhouse Gas Emissions and 

Concentrations .............................................................................................................2-106

Limitations, Gaps, and Challenges ..............................................................................2-106

2.4 WHAT ARE THE TRENDS IN INDOOR AIR QUALITY AND THEIR EFFECTS ON 

HUMAN HEALTH?.................................................................................................................2-108

2.4.1 Introduction..................................................................................................................2-108

2.4.2 ROE Indicators ............................................................................................................2-110

INDICATOR: U.S. Homes Above EPA’s Radon Action Level................................2-111

INDICATOR: Blood Cotinine Level.........................................................................2-114

2.4.3 Discussion....................................................................................................................2-118

What These Indicators Say About Trends in Indoor Air Quality and Their Effects on 

Human Health..............................................................................................................2-118

Limitations, Gaps, and Challenges ..............................................................................2-118

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2.1 INTRODUCTION 

Air provides the oxygen and carbon dioxide needed to sustain human, animal, and plant life on Earth, and 

the composition of trace gases in the atmosphere plays an important role for the climate. Air pollution can 

adversely affect these critical functions of the atmosphere in many ways. High levels of air pollution, 

whether indoors or outdoors, can harm human health by triggering asthma attacks, aggravating allergies, 

and contributing to or potentially causing various diseases. Certain types of outdoor air pollution can 

impair visibility and damage other valued resources, such as forests, lakes and streams, and building 

surfaces. On a global scale, air pollution released worldwide can eventually change the atmosphere’s 

composition with important consequences, including depletion of the Earth’s ozone layer and climate 

change. 

An important component of EPA’s mission is to protect and improve air quality in order to avoid or 

mitigate the consequences of air pollution’s harmful effects. State and tribal air pollution control agencies 

help fulfill this mission by implementing many of the air pollution control requirements that EPA sets at 

the federal level. Other federal partners, the academic community, industry and trade associations, and 

non-government organizations all conduct important research that contributes to the current 

understanding of regional, national, and global air quality issues. 

Efforts to maintain good air quality are complicated by population increase, energy consumption, motor 

vehicle use, and other factors that can lessen air quality. Outdoor air is polluted by emissions from a broad 

array of industrial and mobile sources, as well as everyday activities like dry cleaning, painting, and 

refueling vehicles. Emissions from natural sources, such as wildfires, also contribute to outdoor air 

pollution. Similarly, indoor air quality is affected not only by these outdoor sources, but also by sources 

found within buildings, such as home heating devices, tobacco smoke, consumer products, and building 

materials. In this chapter, EPA assesses national trends in the condition of air, stressors that influence air 

quality, and associated exposures and effects among humans and ecological systems. ROE indicators are 

presented to address three fundamental questions about the state of the nation’s air: 

• What are the trends in outdoor air quality and their effects on human health and the 

environment? This question examines a broad spectrum of outdoor air quality issues, 

including polluted air that people breathe at ground level, deposition of air pollutants to land 

and water, and depletion of the Earth’s ozone layer. For each issue, information is provided 

both on the main stressors (emission sources) and potential health and environmental effects. 

• What are the trends in greenhouse gas emissions and concentrations? This question 

focuses on releases and atmospheric concentrations of certain so-called “greenhouse gases,” 

or gases in the atmosphere that help regulate Earth’s temperature and thus contribute to 

climate change—a topic introduced in this chapter and revisited in Chapter 6, Ecological 

Condition. 

• What are the trends in indoor air quality and their effects on human health? This question 

considers air quality in indoor settings, such as homes, offices, and schools, and how poor 

indoor air quality can affect human health, whether through causing adverse health effects or 

by impairing productivity. 

These ROE questions are posed without regard to whether indicators are available to answer them. This 

chapter presents the indicators available to answer these questions, and also points out important gaps 

where nationally representative data are lacking. 

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While this chapter focuses on air quality, readers should not infer that air quality trends are completely 

independent of the other themes in ROE: Water, Land, Human Health, and Ecological Condition. High 

levels of air pollution are linked to many broader environmental concerns. Because air interfaces directly 

with water and land, air pollutants can enter these media through various fate and transport mechanisms, 

such as wet deposition onto surface waters and gravitational settling onto soils. Conversely, chemicals in 

surface water and soil can enter outdoor air through processes like evaporation and resuspension of windblown dust. Thus, in a very general sense, air quality is related to selected topics covered in the Water 

Chapter and the Land Chapter. Further, nearly every topic addressed in this chapter is primarily motivated 

by some specific concern regarding human health or ecological effects. Therefore, air quality and climate 

change are conceptually linked to many topics addressed in the Human Health and Ecological Condition 

Chapters. Air quality issues that are connected with other ROE themes are introduced and examined in 

this chapter, and addressed further in later sections of ROE as appropriate. 

2.1.1 Overview of the Data 

When developing the 27 ROE indicators in this chapter, EPA accessed and compiled data collected by 

many parties. The individual data sources that were evaluated can be classified into four general 

categories: 

• National emission inventories. Emissions data were queried from databases known as 

emission inventories. These inventories are composites of measured and estimated emission 

rates for industrial sources, mobile sources, and natural sources. Industry and state, tribal, and 

local agencies provide most of the data compiled in these inventories. 

• Ground-level ambient air monitoring data. Ambient air concentrations measured at ground 

level primarily come from measurements collected in a nationwide network of ambient air 

monitoring stations (i.e., State and Local Air Monitoring Stations, National Air Monitoring 

Stations). State, tribal, and local agencies operate most of these stations and submit their 

validated measurement results to a centralized database. 

• Deposition measurements. Representative data on deposition of outdoor air pollutants come 

from samples collected and analyzed at fixed locations throughout the country as part of the 

National Atmospheric Deposition Program and the Clean Air Status and Trends Network. 

• Other data sources. The remaining ROE indicators in this chapter draw from various other 

data sources, including satellite measurements of stratospheric ozone depletion, an evaluation 

of pollution-related injury to forest plants, surveys on radon in homes and evidence of 

exposure to environmental tobacco smoke, an inter-agency assessment of regional haze, and 

articles in the peer-reviewed literature on historical concentrations of greenhouse gases 

estimated from ice core samples. 

Tracking the country’s air quality is a complicated endeavor and cannot be done with any single indicator. 

Multiple indicators are needed to characterize indoor air quality separately from outdoor air quality, air 

quality trends at ground level separately from changing atmospheric conditions aloft, and air pollution 

levels for the many different pollutants of potential concern. Regardless of the issue of interest, a 

particular challenge in developing this chapter’s indicators is that air quality can vary considerably with 

location and time. Consequently, all underlying data sources must be sufficiently representative, both 

spatially and temporally. 

Spatial resolution is a critical consideration due to associated spatial variations in population density, 

industrial emissions sources, traffic patterns, and meteorological conditions that dictate relevant 

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atmospheric fate and transport processes. Temporal resolution also must be considered because ambient 

air concentrations of certain pollutants vary considerably with time of day (due to sunlight’s contribution 

to photochemical reactions), day of week (due to commuting patterns), and season (due to changes in 

meteorological conditions). Temporal resolution is particularly important when interpreting air quality 

trends, because sufficiently long time frames often must be considered to ensure that trends reflect actual 

changes in air quality, rather than natural fluctuations in atmospheric conditions. 

This chapter presents only data that meet the ROE indicator definition and criteria (see Chapter 1, 

Introduction). Note that non-scientific indicators, such as administrative and economic indicators, are not 

included in this definition. Thorough documentation of the indicator data sources and metadata can be 

found online at [INSERT URL]. All indicators were peer-reviewed during an independent peer review 

process (see [INSERT URL] for more information). Readers should not infer that the indicators included 

reflect the complete state of knowledge on the nation’s air. Many other data sources, publications, and 

site-specific research projects have contributed substantially to the current understanding of air quality 

trends, but are not used in this report because they did not meet some aspect of the ROE indicator criteria. 

2.1.2 Organization of This Chapter 

This chapter’s remaining three sections are framed around the three overarching questions that EPA seeks 

to answer about trends in air. Each section introduces the question and its importance, presents the 

National Indicators that help answer the question, and discusses what these indicators, taken together, say 

about the question. The chapter also presents two Regional Indicators that meet the ROE indicator 

definition and criteria and help to answer a question at a smaller geographic scale. Each section concludes 

by listing major challenges to answering the questions and identifying important data gaps. 

The table on the next page lists the indicators used to answer the three questions in this chapter and shows 

the locations where the indicators are presented. 

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1 Table 2.1.1. Air—ROE Questions and Indicators 

Question Indicator Name Section Page # 

What are the trends in 

outdoor air quality and 

their effects on human 

health and the 

environment? 

Carbon Monoxide Emissions (N/R) 

Ambient Concentrations of Carbon Monoxide 

(N/R) 

Lead Emissions (N) 

Ambient Concentrations of Lead (N) 

Nitrogen Oxides Emissions (N/R) 

Ambient Concentrations of Nitrogen Dioxide 

(N/R) 

Volatile Organic Compounds Emissions (N/R) 

Ambient Concentrations of Ozone (N/R) 

Ozone Injury to Forest Plants (N/R) 

Particulate Matter Emissions (N/R) 

Ambient Concentrations of Particulate Matter 

(N/R) 

Regional Haze (N) 

Sulfur Dioxide Emissions (N/R) 

Acid Deposition (N) 

Lake and Stream Acidity (N) 

Percent of Days with Air Quality Index Values 

Greater Than 100 (N/R) 

Mercury Emissions (N) 

Air Toxics Emissions (N/R) 

Ambient Concentrations Benzene (N) 

Concentrations of Ozone-Depleting Substances 

(N) 

Ozone Levels over North America (N) 

Ozone and Particulate Matter Concentrations for 

U.S. Counties in the U.S./Mexico Border 

Region (R) 

Ambient Concentrations of Manganese 

Compounds in EPA Region 5 (R) 

2.2.2 2-13 

2.2.2 2-16 

2.2.2 2-19 

2.2.2 2-21 

2.2.2 2-23 

2.2.2 2-26 

2.2.2 2-29 

2.2.2 2-32 

2.2.2 2-37 

2.2.2 2-40 

2.2.2 2-44 

2.2.2 2-50 

2.2.2 2-53 

2.2.2 2-56 

2.2.2 2-62 

2.2.2 2-65 

2.2.2 2-68 

2.2.2 2-70 

2.2.2 2-74 

2.2.2 2-76 

2.2.2 2-79 

2.2.2 2-82 

2.2.2 2-86 

What are the trends in 

greenhouse gas 

emissions and 

concentrations? 

U.S. Greenhouse Gas Emissions (N) 

Atmospheric Concentrations of Greenhouse 

Gases (N) 

2.3.2 2-96 

2.3.2 2-100 

What are the trends in 

indoor air quality and 

their effects on human 

health? 

U.S. Homes Above EPA’s Radon Action Level 

(N) 

Blood Cotinine Level (N) 

2.4.2 2-111 

2.4.2 2-114 

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N = National Indicator 

R = Regional Indicator 

N/R = National Indicator displayed at EPA Regional scale 

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2.2 WHAT ARE THE TRENDS IN OUTDOOR AIR QUALITY AND THEIR EFFECTS 

ON HUMAN HEALTH AND THE ENVIRONMENT? 

2.2.1 Introduction 

Outdoor air—the air outside of buildings, from ground level to several miles above the Earth’s surface— 

is a valuable resource for current and future generations because it provides essential gases to sustain life 

and it shields the Earth from harmful radiation. Air pollution can compromise outdoor air quality in many 

ways. Outdoor air pollution, for instance, is associated with various adverse health effects including 

asthma attacks and cancer; and outdoor air pollution can contribute to “smog” and “acid rain,” damage 

crops and surfaces of treasured buildings and monuments, and diminish the protective ozone layer in the 

upper atmosphere. Maintaining clean air is a challenging task, especially considering the growing 

stressors on outdoor air quality such as increased population growth, increased use of motor vehicles, and 

increased energy consumption. 

Outdoor air pollution contains numerous substances of both natural and anthropogenic origin. While 

natural sources release some potentially harmful substances into the air (e.g., pollen, mold spores, dust), 

emissions sources of anthropogenic origin are of particular interest because regulatory and voluntary 

reductions can lead to decreased emissions and associated air quality improvements. Accordingly, this 

section focuses on outdoor air quality issues caused at least in part by human activity and acknowledges 

and quantifies contributions from natural sources, as appropriate. 

Most outdoor air pollution issues can be traced back to emissions sources that release pollutants into the 

air. Emissions sources are typically classified into different categories, such as point sources (e.g., power 

plants, industrial facilities), area sources (e.g., air pollution sources over a diffuse area, such as gasoline 

stations, dry cleaners, and waste treatment facilities), mobile sources (e.g., cars, trucks, airplanes, off-road 

vehicles), and natural sources (e.g., wildfires, wind-blown dust, volcanoes). Once pollutants are airborne, 

prevailing wind patterns carry them from their sources to downwind locations, and pollutants disperse in 

air as they mix in the atmosphere. Depending on their chemical and physical properties, some pollutants 

deposit to the Earth’s surface near their corresponding emissions sources, while others remain airborne 

for hours, days, or years. Deposition of air pollutants, especially those that are persistent and 

bioaccumulative, can lead to accumulation of contaminants in other media. The levels of air pollution at a 

given location and at a given time are influenced by emissions from nearby and distant sources as well as 

by atmospheric factors, such as meteorology. 

Human exposure to outdoor air pollution is a function of the composition and magnitude of air pollution, 

combined with human activity patterns. Whether people are harmed by poor air quality depends on 

exposure doses and durations, individuals’ susceptibilities to diseases, and other factors. Similarly, air 

pollutants’ interactions with ecosystems determine whether air pollution causes harmful environmental 

effects. For a complete understanding of a given air pollution issue, information is therefore typically 

sought on emissions sources, ambient air concentrations, exposures, and effects. 

Outdoor air pollution can contain hundreds of different pollutants, which are typically grouped into 

various categories based on shared attributes. Some categories are defined by pollutants’ physical 

attributes (e.g., gases, aerosols, particulates), while others by regulatory terminology (e.g., criteria 

pollutants, air toxics). The indicators used to answer the question regarding outdoor air quality are 

organized into the following three categories, which were selected based on the different parts of the 

atmosphere to which they pertain and the different types of information available to support indicator 

development: 

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• Criteria Pollutants. The following six common pollutants are referred to as criteria 

pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter of different 

size fractions, and sulfur dioxide. For each criteria pollutant, EPA has established National 

Ambient Air Quality Standards (NAAQS) to protect public health and the environment. 

Extensive data are available on criteria pollutants’ emissions (or emissions of the pollutants’ 

precursors) and ambient concentrations. 

• Air Toxics and Other Air Pollutants. Air toxics, also known as hazardous air pollutants, are 

known or suspected to cause cancer and are associated with other serious health effects, such 

as reproductive effects or birth defects, or adverse environmental effects. The Clean Air Act 

specifically identifies 188 air toxics. Numerous other air pollutants exhibit toxicity even 

though they are not classified as air toxics; included among these other pollutants are several 

hundred additional chemicals whose emissions are tracked in EPA’s Toxics Release 

Inventory. 

• Stratospheric Ozone Issues. The ozone layer occurs in the stratosphere between 6 and 20 

miles above the Earth’s surface and protects the Earth’s biota from harmful effects of the 

sun’s ultraviolet radiation. Past and ongoing releases of a number of synthetic chemicals from 

throughout the world have depleted the ozone layer, allowing more ultraviolet radiation to 

reach the Earth’s surface. This can lead to increased incidence of skin cancer, cataracts, and 

other health problems.1

 Further, high levels of ultraviolet radiation can cause detrimental 

ecological effects, such as stressing productivity of marine phytoplankton, which are essential 

components of the oceanic food web.2 

Air pollution is manifest over a range of spatial and temporal domains—an important factor to consider 

when evaluating trends for the three categories considered in this section. The spatial domains of air 

pollution issues vary widely. Air pollution can be local in nature. For instance, ambient concentrations of 

benzene tend to be greatest in the proximity of major sources (e.g., oil refineries, chemical production 

facilities) and in high-traffic areas; long-range transport is relatively unimportant due to benzene’s high 

photochemical reactivity. Air pollution can also extend over regional and national scales. For example, 

emissions sources hundreds of miles away can contribute to airborne fine particulate matter at a given 

location.3

 Finally, a few air pollution issues are global in nature. Stratospheric ozone depletion, for 

example, is affected by releases of ozone-depleting substances from countries worldwide. The spatial 

domains ultimately determine the minimum spatial resolution of monitors needed to adequately 

characterize trends. 

Temporal scales also vary among pollutants and typically reflect some combination of changes in 

emissions and fluctuations in weather. Ambient air concentrations of some air pollutants, like groundlevel ozone, have considerable diurnal and seasonal variations.4

 However, temporal variations are far less 

1 World Meteorological Organization, et al. 2003. Scientific assessment of ozone depletion: 2002. Geneva, 

Switzerland. 

2

 DeMora, S., S. Demers, and M. Vernet. 2000. The effects of UV radiation in the marine environment. Cambridge, 

United Kingdom: Cambridge University Press. 

3

 U.S. EPA. 2004. The particle pollution report: current understanding of air quality and emissions through 2003. 

EPA/454/R-04/002. Research Triangle Park, NC. 

4

 U.S. EPA. 2004. The ozone report: measuring progress through 2003. EPA/454/K-04/001. Research Triangle Park, 

NC. 

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pronounced for pollutants that are long-lived in the atmosphere, including many ozone-depleting 

substances. Temporal variations largely determine the appropriate monitoring frequency for quantifying 

trends and the most meaningful statistic (or averaging time) used to report ambient air concentrations. 

When quantifying and interpreting long-term trends in outdoor air quality, attention also must be paid to 

changes in emissions estimation techniques and advances in ambient air monitoring technologies. Unless 

otherwise noted, the outdoor air quality indicators only come from data sets generated using consistent 

methodologies over the entire time frame of interest. 

The nationwide air quality trends in this section are generally consistent with those documented in other 

EPA publications, though readers should not expect to find perfect concordance among individual data 

points. This is because some publications address different spatial domains or time frames and may use 

less rigorous selection criteria when identifying and compiling data sets. 

2.2.2 ROE Indicators 

The 23 outdoor air quality indicators track emissions, ambient concentrations, and pollution-related 

effects over varying spatial domains and time spans, depending on the availability of underlying data. The 

indicators include 21 National Indicators (12 of which break national data down into the 10 EPA Regions) 

and 2 Regional Indicators. The most extensive temporal coverage of these indicators tracks trends from 

1965 to the present. 

Indicators were developed using data compiled from multiple sources. Emissions indicators are based on 

EPA’s National Emissions Inventory (NEI), a database of measured and estimated emissions for 

numerous pollutants and source categories. At the writing of this report, NEI data were available from 

1990 to 2002, but the indicators only present data for those inventory years that are fully updated and are 

developed using consistent methodologies. Ground-level ambient air concentration indicators were 

developed from data in EPA’s Air Quality System (AQS), a clearinghouse of validated ambient air 

monitoring results submitted largely by tribal, state, and local environmental agencies. The ambient 

concentration indicators present data through calendar year 2004, which is the most recent calendar year 

having a complete, validated set of monitoring data available from AQS when this report was prepared. 

Remaining indicators draw from different monitoring programs, including regional haze data from the 

Interagency Monitoring of Protected Visual Environments, acid deposition measurements from the multiagency National Atmospheric Deposition Program and Clean Air Status and Trends Network, ozone 

injury observations from the U.S. Forest Service’s Forest Health Monitoring Program, and monitoring of 

stratospheric ozone levels and concentrations of ozone-depleting substances conducted by the National 

Oceanic and Atmospheric Administration. 

Table 2.2.1 shows how indicators are classified into three general categories (criteria pollutants, air toxics 

and other pollutants, stratospheric ozone issues) and then further organized by pollutant. For each 

pollutant and to the extent supported by ROE indicators, relevant emissions indicators are presented first, 

immediately followed by ambient concentration indicators, and next by effects indicators. With this 

organization, readers can readily compare relevant trends in emissions, ambient concentrations, and 

effects for the same pollutant. 

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Table 2.2.1. ROE Indicators of Trends in Outdoor Air Quality and Their Effects on Human Health 

and the Environment 

NATIONAL INDICATORS LOCATION 

Criteria Pollutants and Their Precursors 

Carbon Monoxide Emissions (N/R) 2.2.2 – p. 2-13 

Ambient Concentrations of Carbon Monoxide (N/R) 2.2.2 – p. 2-16 

Lead Emissions 2.2.2 – p. 2-19 

Ambient Concentrations of Lead 2.2.2 – p. 2-21 

Nitrogen Oxides Emissions (N/R) 2.2.2 – p. 2-23 

Ambient Concentrations of Nitrogen Dioxide (N/R) 2.2.2 – p. 2-26 

Volatile Organic Compounds Emissions (N/R) 2.2.2 – p. 2-29 

Ambient Concentrations of Ozone (N/R) 2.2.2 – p. 2-32 

Ozone Injury to Forest Plants (N/R) 2.2.2 – p. 2-37 

Particulate Matter Emissions (N/R) 2.2.2 – p. 2-40 

Ambient Concentrations of Particulate Matter (N/R) 2.2.2 – p. 2-44 

Regional Haze 2.2.2 – p. 2-50 

Sulfur Dioxide Emissions (N/R) 2.2.2 – p. 2-53 

Acid Deposition 2.2.2 – p. 2-56 

Lake and Stream Acidity 2.2.2 – p. 2-62 

Percent of Days with Air Quality Index Values Greater Than 100 

(N/R) 

2.2.2 – p. 2-65 

Air Toxics and Other Air Pollutants 

Mercury Emissions 2.2.2 – p. 2-68 

Air Toxics Emissions (N/R) 2.2.2 – p. 2-70 

Ambient Concentrations of Benzene 2.2.2 – p. 2-74 

Stratospheric Ozone Issues 

Concentrations of Ozone-Depleting Substances 2.2.2 – p. 2-76 

Ozone Levels over North America 2.2.2 – p. 2-79 

REGIONAL INDICATORS LOCATION 

Ozone and Particulate Matter Concentrations for U.S. Counties in the 

U.S./Mexico Border Region 

2.2.2 – p. 2-82 

Ambient Concentrations of Manganese Compounds in EPA Region 5 2.2.2 – p. 2-86 

N/R = National Indicator displayed at EPA Regional scale 

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1 INDICATOR: Carbon Monoxide Emissions

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Carbon monoxide (CO) gas forms primarily when carbon fuels are not burned completely. Mobile 

sources account for the majority of CO emissions (U.S. EPA, 2003). These sources include both on-road 

vehicles (e.g., cars, trucks, motorcycles) and nonroad vehicles and engines (e.g., farm equipment, 

construction equipment, aircraft, and marine vessels). Consequently, 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 (U.S. EPA, 2003). Other sources of CO emissions include industrial 

processes, non-transportation fuel combustion, and natural sources, such as wildfires. Fuel-burning 

appliances also are a large source of CO releases in indoor environments. Undetected releases of carbon 

monoxide in indoor settings can present serious health risks to building occupants. The CO 

Concentrations indicator (p. 2-16) describes health hazards associated with inhaling CO. 

This indicator presents CO emissions from traditionally inventoried anthropogenic source categories: 

1) “Fuel combustion,” which includes emissions from coal, gas and oil-fired power plants, industrial, 

commercial, and institutional sources, as well as residential heaters (e.g., wood-burning stoves) and 

boilers; 2) “Industrial and other processes,” which includes chemical production, petroleum refining, and 

metals production; 3) “On-road vehicles” which includes cars, trucks, buses, and motorcycles; and 

4) “Nonroad vehicles and engines,” such as farm and construction equipment, lawnmowers, chainsaws, 

boats, ships, snowmobiles, aircraft, and others. The indicator also includes estimates of biogenic CO 

emissions in 2002 which were obtained using the Biogenic Emissions Inventory System (BEIS) Model, 

Version 3.12.

CO emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of 

data from many different data sources, including industry and numerous state, tribal, and local agencies. 

Different data sources use different data collection methods, and many of the emissions data are based on 

estimates rather than actual measurements. For most fuel combustion sources and industrial sources, 

emissions are estimated using emission factors. Emissions from on-road and nonroad sources were 

estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and Virgin Islands, and some of the territories of federally-recognized American 

Indian nations. Data are presented for 1990 and from 1996 to 2002; prior to 1996, only the 1990 data have 

been updated to be comparable to the more recent inventories. 

What the Data Show

Only CO emissions from anthropogenic sources are included in the NEI. However, CO emissions from 

biogenic sources were estimated for 2002 to provide a sense of the relative contributions of natural versus 

anthropogenic emissions (Exhibit 2-1, panel B). Nationally, biogenic emissions were estimated to 

contribute approximately 5 percent to the CO emissions from all sources during 2002. 

Nationwide estimated anthropogenic CO emissions have decreased 33 percent between 1990 and 2002, 

the most recent year for which aggregate NEI emissions estimates are available (Exhibit 2-1, panel A). 

Almost the entire emissions reduction is attributed to decreased emissions from on-road mobile sources. 

In 2002, mobile sources (both on-road and nonroad sources combined) accounted for 91 percent of the 

nation’s total anthropogenic CO emissions. The CO emissions reductions are reflected in corresponding 

reductions in ambient concentrations (the CO Concentrations indicator, p. 2-16). 
Net estimated anthropogenic CO emissions declined in all EPA Regions between 1990 and 2002 (Exhibit 

2-2). The largest decrease (8.95 million pounds) occurred in Region 5, and the smallest decrease (1.74 

million pounds) occurred in Region 8. 

Indicator Limitations 

• Comparable CO emissions estimates through the NEI are available only for 1990 and 1996­

2002. Data for 1991-1995 are not provided due to differences in emission estimation 

methodologies from other inventory years which could lead to improper trend assessments. 

• CO emissions from “miscellaneous sources,” including wildfires, are not included in the total 

emissions. Yearly fluctuations in wildfire emissions have the potential to mask trends in 

anthropogenic emissions and therefore have been excluded from the trends graphics. Details 

on emissions from miscellaneous sources can be found at 

http://www.epa.gov/ttn/chief/eiinformation.html. 

• The emissions data for CO are largely based on estimates that employ emission factors 

generated from empirical and engineering studies, rather than on actual measurements of CO 

emissions. Although these estimates are generated using well-established approaches, the 

estimates have uncertainties inherent in the emission factors and emission models used to 

represent sources for which emissions have not been directly measured. 

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• The methodology for estimating emissions is continually reviewed and is subject to revision. 

Trend data prior to any revisions must be considered in the context of those changes. 

• Not all states and local agencies provide the same data or level of detail for a given year. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw CO emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by

source type (anthropogenic or biogenic), source category, and EPA Region. 

References 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa

d.pdf> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

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INDICATOR: Ambient Carbon Monoxide Concentrations 

Carbon monoxide (CO) gas forms primarily when carbon fuels are not burned completely. Elevated 

ambient air concentrations of CO are hazardous because inhaled CO enters the bloodstream and reduces 

the amount of oxygen that the blood can deliver to the body’s organs and tissues. If exposure 

concentrations are high enough, potentially serious cardiovascular and neurological effects can result. 

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 (U.S. EPA, 2000). 

Motor vehicle exhaust currently accounts for the majority of CO emissions nationwide, and as much as 95 

percent of CO emissions in cities with high traffic congestion. Other anthropogenic emissions sources of 

CO include fossil fuel combustion for heating and power generation, metals processing, and chemical 

manufacturing. The highest ambient air concentrations of CO often occur during nighttime inversion 

conditions, which trap pollutants near ground level. These conditions are most frequently observed during 

the cold winter months (U.S. EPA, 2003). 

This indicator presents ambient CO concentrations in parts per million (ppm) from 1980 to 2004, based 

on continuous measurements averaged over 8-hour time frames. The 8-hour standard is indicative of 

exposures occurring over a sustained period of time, for example, an outdoor worker’s exposure over the 

course of a work day. This indicator displays trends in the second highest annual 8-hour average CO 

concentrations for 160 sites that have consistent data for the period of record in the National Air 

Monitoring Stations (NAMS), State and Local Air Monitoring Stations (SLAMS) network, and other 

special purpose monitors. It also shows trends in the average 8-hour measurements in each EPA Region. 

This indicator’s exhibits display the National Ambient Air Quality Standard (NAAQS) for CO as a point 

of reference, but the fact that the national or any regional 8-hour values fall below the standard does not 

mean that all monitoring sites nationally or in the EPA Region also are below the standard. The indicator 

displays trends in the number of the 160 sites nationwide at which CO concentrations exceeded the 8-hour 

standard, but this statistic is not displayed for each EPA Region. 

What the Data Show

The 2004 CO concentration averaged across 160 monitoring sites nationwide was 72 percent lower than 

that for 1980, and is the lowest level recorded during the past 25 years (Exhibit 2-3, panel A). The 

downward trend in CO concentrations in the 1990s parallels the downward trend observed in CO 

emissions, which has been attributed largely to decreased emissions from mobile sources (the CO 

Emissions indicator, p. 2-13). In addition, of the 160 sites comprising this trend (out of 425 total 

monitoring sites that were operating in 2004), the number reporting CO concentrations above the CO 

standard declined to zero over the same period (Exhibit 2-3, panel B). 
Also shown on Exhibit 2-3 (panel A) are the 90th 

and 10th percentiles based on the distribution of 

annual statistics at the monitoring sites. This 

provides additional graphical representation of the 

distribution of measured concentrations across the 

monitoring sites for a given year. Thus, the graphic 

displays the concentration range where 80 percent 

of measured values occurred for that year. 

Consistent with the nationwide trend, CO levels in all ten EPA Regions have steadily decreased since 

1980, with percent reductions over this period ranging from 64 percent (Region 4) to 80 percent (Region 

1) (Exhibit 2-4). 

Indicator Limitations 

• Because most CO monitoring sites are located in high-traffic urban areas, the nationwide 

trends presented in this indicator might not accurately reflect conditions outside the 

immediate urban monitoring areas. 

• Because of the relatively small number of trend sites in some EPA Regions, the regional 

trends are subject to greater uncertainty than the national trends. Some EPA Regions with low 

average concentrations may include areas with high local concentrations, and vice versa. 

• To ensure that long-term trends are based on a consistent set of monitoring sites, selection 

criteria were applied to identify the subset of CO monitoring sites with sufficient data to 

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assess trends since 1980. Monitoring sites without sufficient data are not included in the trend 

analysis. Some excluded monitoring sites reported CO concentrations above the CO standard 

over the time frame covered by this indicator. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw CO ambient air monitoring data in EPA’s Air Quality System (AQS) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the 

subset of CO monitoring stations that have sufficient data to assess trends since 1980. 

References 

U.S. EPA. 2006. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 2000. Air quality criteria for carbon monoxide, 2000. EPA/600/P-99/001F. Research Triangle 

Park, NC. 

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INDICATOR: Lead Emissions

Lead is a naturally occurring metal found in small amounts in rock and soil. Lead has been used 

industrially in the production of gasoline, ceramic products, paints, metal alloys, batteries, and solder. In 

the past, automotive sources were the major contributors of lead emissions to the atmosphere. After 

leaded motor vehicle fuels were phased out during the 1970s and 1980s, the contribution of air emissions 

of lead from the transportation sector, and particularly the automotive sector, greatly declined. Today, 

industrial processes, primarily metals processing, account for a large portion of lead emissions to the 

atmosphere and the highest levels of airborne lead are usually found near industrial operations that 

process materials containing lead, such as smelters (U.S. EPA, 2003). Exposure to lead occurs mainly 

through inhalation of air and ingestion of lead in food, water, soil, or dust. The Lead Concentrations 

indicator (p. 2-21) describes health hazards associated with lead exposures. 

This indicator presents lead emissions from traditionally inventoried anthropogenic source categories: 

1) “Fuel combustion,” which includes emissions from coal, gas and oil-fired power plants, industrial, 

commercial, and institutional sources, as well as residential heaters and boilers; 2) “Other sources,” which 

include chemical production and petroleum refining; 3) “On-road vehicles,” which includes cars, trucks, 

buses, and motorcycles; 4) “Nonroad vehicles and engines,” such as farm and construction equipment, 

lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and others; and 5) “Metals industrial 

processing.” Since metals processing is one of the largest sources of lead emissions, the indicator includes 

a metals source category in addition to the four categories presented in the other emissions indicators. 

For the years 1970 through 1985, the primary source for lead emissions data was the National Emissions 

Data System (NEDS) archives. Since 1990, lead emissions data have been tracked by the National 

Emissions Inventory (NEI). The NEI is a composite of data from many different data sources, including 

industry and numerous state, tribal, and local agencies. Different data sources use different data collection 

methods, and many of the emissions data are based on estimates rather than actual measurements. For 

most industrial processes and fuel combustion sources, emissions are estimated using emission factors. 

Emissions from on-road and nonroad sources were estimated using EPA-approved modeling approaches 

(U.S. EPA, 2005).

Data for lead emissions cover all 50 states and their counties, D.C., the U.S. territories of Puerto Rico and 

Virgin Islands, and some of the territories of federally-recognized American Indian nations. 

What the Data Show

Between 1970 and 2002, estimated nationwide lead emissions decreased by 99 percent (219,350 tons), 

mostly due to reductions from on-road vehicle sources after lead was removed from gasoline (Exhibit 2­

5). Since 1990, further declines in lead emissions occurred, mostly due to reductions from on-road 

vehicles, nonroad vehicles and engines, and metals industrial processing . Sharp declines in nationwide 

air concentrations of lead between 1980 and 1990 paralleled the emissions reductions (the Lead 

Concentrations indicator, p. 2-21). 
Indicator Limitations 

• Although lead emissions trends have 

been generated using well-established 

estimation methods, the data reflect 

estimates based on empirical and 

engineering models and not actual 

measurement of lead emissions. These 

estimates have uncertainties inherent 

in the emission factors and emission 

models used to represent sources for 

which emissions have not been 

directly measured. 

• The method for estimating lead 

emissions for fuel combustion and 

industrial sources changed in 1999 to 

reduce uncertainties inherent in the 

previous method (U.S. EPA, 2003). 

Despite the change in methodology, 

the long-term trend is still reliable. 

• Not all states and local agencies 

provide the same data or level of 

detail for a given year. 

Data Sources 

Summary data in this indicator were provided by 

EPA’s Office of Air Quality Planning and 

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Standards, based on raw lead emissions data from two sources. Raw emissions data from 1970 to 1985 

are from EPA’s National Emissions Data System (NEDS) archives, and data summaries for this time 

frame can be found in various EPA publications (e.g., U.S. EPA, 2001). Raw emissions data for 1990­

1999 and 2002 are available from EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw emissions data 

by source category. 

References 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 2001. National air quality and emissions trends report, 1999. EPA/454/R-01-004. Research 

Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd99/> 

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1 INDICATOR: Ambient Concentrations of Lead 

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Lead is a naturally occurring metal found in small amounts in rock and soil. Despite steep decreases in 

emissions since 1970 (the Lead Emissions indicator, p. 2-19), lead remains an important environmental 

health issue because exposure to high levels has been associated with serious health effects, including 

neurological impairments such as seizures, mental retardation, and behavioral disorders (CDC, 2005). 

Even at low doses, lead exposure can have adverse effects to the nervous systems of fetuses and young 

children (the Blood Lead indicator, p. 5-76) (U.S. EPA, 2006a). Lead exposure can occur by inhalation of 

airborne particles that contain lead, drinking contaminated water, eating contaminated food items, or by 

ingesting non-food items that contain lead, such as dust and paint chips. 

Lead has been used industrially in the production of gasoline, ceramic products, paints, metal alloys, 

batteries, and solder. Some chemicals containing lead were previously added to gasoline to enhance 

vehicle performance, but that practice was phased out during the 1970s and 1980s. As a result, air 

emissions of lead from the transportation sector decreased dramatically during that period (the Lead 

Emissions indicator, p. 2-19). Today, the highest levels of airborne lead are usually found near industrial 

operations that process materials containing lead, such as smelters (U.S. EPA, 2003). 

This indicator reflects ambient lead concentrations in micrograms per cubic meter (μg/m3

) from 1980 to 

2004. Trends for this indicator are based on measurements made at the 18 monitoring stations in the 

National Air Monitoring Stations (NAMS) and State and Local Air Monitoring Stations (SLAMS) 

network that have consistently measured ambient air concentrations of lead over the entire period of 

interest. Reported values are annual maximum quarterly averages. This indicator’s exhibit displays the 

lead National Ambient Air Quality Standard (NAAQS) as a point of reference, but the fact that the 

national lead concentrations fall below the standard does not mean that all monitoring sites also are below 

the standard.

What the Data Show

Between 1980 and 2004, average lead concentrations decreased 97 percent nationally (Exhibit 2-6, panel 

A). This decrease, which occurred mostly during the 1980s and early 1990s, is largely attributed to 

reduced lead content in gasoline (U.S. EPA, 2003). In addition, of the 18 sites comprising this trend (out 

of 185 total monitoring sites that were operating in 2004), the number reporting lead concentrations above 

the NAAQS declined to zero over the same period (Exhibit 2-6, panel B). 

Also shown in Exhibit 2-6 (panel A) are the 90th and 10th percentiles based on the distribution of annual 

statistics at the monitoring sites. This provides additional graphical representation of the distribution of 

measured concentrations across the monitoring sites for a given year. Thus, the exhibit displays the 

concentration range where 80 percent of measured values occurred for each year. 
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Indicator Limitations 

• Because most lead monitoring sites are 

located in urban areas, the nationwide 

trends might not accurately reflect 

conditions outside the immediate urban 

monitoring areas. 

• To ensure that long-term trends are 

based on a consistent set of monitoring 

sites, selection criteria were applied to 

identify the subset of lead monitoring 

sites with sufficient data to assess trends 

since 1980. Monitoring sites without 

sufficient data are not included in the 

trend analysis. Some excluded 

monitoring sites reported lead 

concentrations above the lead standard 

over the time frame covered by this 

indicator. 

Data Sources 

Summary data in this indicator were provided by 

EPA’s Office of Air Quality Planning and 

Standards, based on raw lead ambient air monitoring 

data in EPA’s Air Quality System (AQS) (U.S. 

EPA, 2006b) (http://www.epa.gov/ttn/airs/airsaqs/). 

National trends in this indicator are based on the 

subset of lead monitoring stations that have 

sufficient data to assess trends since 1980. 

References 

CDC (Centers for Disease Control and Prevention, 

National Center for Environmental Health). 2005. Third national report on human exposure to

environmental chemicals. NCEH Pub. No. 05-0570. July 2005. (Accessed September 9, 2005) 

<http://www.cdc.gov/exposurereport/3rd/results_01.htm> 

U.S. EPA. 2006a. Air quality criteria for lead. EPA/600/R-5/144aF. 

U.S. EPA. 2006b. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

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1 INDICATOR: Nitrogen Oxides Emissions 

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Nitrogen oxides (NOx) is the term used to describe the sum of nitric oxide (NO), nitrogen dioxide (NO2),

and other oxides of nitrogen. Most airborne NOx come from combustion-related emissions sources of 

human origin, primarily fossil fuel combustion in electrical utilities, high temperature operations at other 

industrial sources, and operation of motor vehicles. However, natural sources, like biological decay 

processes and lightning, also contribute to airborne NOx. Fuel-burning appliances, like home heaters and 

gas stoves, produce substantial amounts of NOx in indoor settings (U.S. EPA, 2003). 

NOx plays a major role in several important environmental and human health issues. Short-term and longterm exposures to elevated air concentrations of NO2 are associated with various acute and chronic 

respiratory effects (U.S. EPA, 1993). NOx and volatile organic compounds (VOCs) react in the presence 

of sunlight to form ozone, which also is associated with human health and ecological effects (the Ozone 

Concentrations indicator, p. 2-32). NOx and other pollutants react in the air to form compounds that 

contribute to acid deposition, which can damage forests and cause lakes and streams to acidify (the Acid 

Deposition indicator, p. 2-56). Deposition of NOx also affects nitrogen cycles and can contribute to 

nuisance growth of algae that can disrupt the chemical balance of nutrients in water bodies, especially in 

coastal estuaries (the Lake and Stream Acidity indicator, p. 2-62; the Trophic State of Coastal Waters 

indicator, p. 3-62). NOx also plays a role in several other environmental issues, including formation of 

particulate matter (the PM Concentrations indicator, p. 2-44), decreased visibility (the Regional Haze 

indicator, p. 2-50), and global climate change (the U.S. Greenhouse Gas Emissions indicator, p. 2-96; the 

Greenhouse Gas Concentrations indicator, p. 2-100). 

This indicator presents NOx emissions from traditionally inventoried anthropogenic source categories: 

1) “Fuel combustion: selected power generators,” which includes emissions from coal, gas and oil-fired 

power plants that are required to use continuous emissions monitors (CEMs) to report emissions as part of 

the Acid Rain Program (ARP); 2) “Fuel combustion: other sources,” which includes industrial, 

commercial, and institutional sources, as well as residential heaters and boilers not required to use CEMs; 

3) “Other industrial processes,” which include chemical production and petroleum refining; 4) “On-road 

vehicles,” which includes cars, trucks, buses, and motorcycles; 5) “Nonroad vehicles and engines,” such 

as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and 

others. Since a substantial portion of airborne NOx comes from fossil fuel combustion in electric utilities, 

this indicator includes the separate category for “selected power generators” in addition to the four 

categories presented in the other emissions indicators. The indicator also includes estimates of biogenic 

NOx emissions in 2002 which were obtained using the Biogenic Emissions Inventory System (BEIS) 

Model, Version 3.12. 

NOx emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of 

data from many different data sources, including industry and numerous state, tribal, and local agencies. 

Different data sources use different data collection methods, and many of the emissions data are based on 

estimates rather than actual measurements. For major electric generating units, most data come from 

continuous emissions monitors that measure actual emissions. For other fuel combustion sources and 

industrial processes, data are estimated using emission factors. Emissions from on-road and nonroad 

sources were estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and Virgin Islands, and some of the territories of federally-recognized American 
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Indian nations. Data are presented only for 1990 and the years from 1996 to 2002; prior to 1996, only the 

1990 data have been updated to be comparable to the more recent inventories. 

What the Data Show 

Only NOx emissions from anthropogenic sources are included in the NEI. However, NOx emissions from 

biogenic sources were estimated for 2002 to provide a sense of the relative contributions of natural versus 

anthropogenic emissions. Nationally, biogenic emissions were estimated to contribute approximately 5 

percent to NOx emissions from all sources during 2002 (Exhibit 2-7, panel B). 

According to the NEI data, estimated nationwide anthropogenic emissions of NOx decreased by 18 

percent between 1990 and 2002 (from 25,160,000 to 20,746,000 tons) (Exhibit 2-7, panel A). This 

downward trend results primarily from emissions reductions at electrical utilities and among on-road 

mobile sources. Although total nationwide anthropogenic NOx emissions decreased during this period, 

emissions from some sources (such as nonroad vehicles and engines) have increased since 1990. 

Estimated anthropogenic NOx emissions in all ten EPA Regions decreased between 1990 and 2002 

(Exhibit 2-8). The percent reductions in emissions over this time frame ranged from 3 percent (in Region 

6) to 35 percent (in Region 1), and the largest absolute reduction (1,020,000 pounds) occurred in Region 

5. 

Indicator Limitations 

• Comparable NOx emissions estimates 

through the NEI are available only for 

1990 and 1996-2002. Data for 1991-1995 

are not provided due to differences in 

emission estimation methodologies from 

other inventory years which could lead to 

improper trend assessments. 

• NOx emissions from miscellaneous 

sources are not included in the total 

emissions. 

• Though NOx emissions from most electric 

utilities are measured directly using 

continuous monitoring devices, NOx 

emissions data for most other source types 

are estimates. These estimates are 

generated using well-established 

approaches, but still have uncertainties 

inherent in the emission factors and 

emission models used to represent sources 

for which emissions have not been directly 

measured. 

• The methodology for estimating emissions 

is continually reviewed and is subject to 

revision. Trend data prior to any revisions 

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must be considered in the context of 

those changes. 

• Not all states and local agencies 

provide the same data or level of 

detail for a given year. 

Data Sources 

Summary data in this indicator were provided by 

EPA’s Office of Air Quality Planning and 

Standards, based on raw NOx emissions data in 

EPA’s National Emissions Inventory (NEI) (U.S. 

EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.h 

tml). This indicator aggregates the raw NEI data 

by source type (anthropogenic or biogenic), 

source category, and EPA Region. 

References 

U.S. EPA. 2006. Data from the National 

Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory. 

html> 

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U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 1993. Air quality criteria for oxides of nitrogen. EPA/600/8-91/049aF-cF. Research Triangle 

Park, NC. 

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1 INDICATOR: Ambient Concentrations of Nitrogen Dioxide 

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Nitrogen dioxide (NO2) is a reddish-brown, highly reactive gas that is formed in the ambient air through 

the oxidation of nitric oxide (NO). Nitrogen dioxide is one in a group of highly reactive gases generically 

referred to as “nitrogen oxides” (NOX), all of which contain nitrogen and oxygen in varying amounts. 

Nitrogen oxides play a major role in the formation of ozone in the atmosphere through a complex series 

of reactions with volatile organic compounds (VOCs). Nitrogen dioxide is the most widespread and 

commonly found nitrogen oxide (U.S. EPA, 2003). 

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. These exposures 

may also increase 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 (U.S. EPA, 

1995).

Atmospheric transformation of NOX can lead to the formation of ozone and nitrogen-bearing particles 

(e.g., nitrates and nitric acid). Deposition of nitrogen can lead to fertilization, eutrophication, or 

acidification of terrestrial, wetland, and aquatic (e.g., fresh water bodies, estuaries, and coastal water) 

systems. These effects can alter competition among existing species, leading to changes in species 

abundance and distribution within communities. For example, eutrophic conditions in aquatic systems can 

produce explosive growth of algae leading to hypoxia or an increase in levels of toxins harmful to fish 

and other aquatic life (U.S. EPA, 1993). 

This indicator presents ambient NO2 concentrations in parts per million (ppm) from 1980 to 2004, based 

on the annual arithmetic average. The indicator displays trends averaged over 91 sites that have consistent 

data for the period of record in the National Air Monitoring Stations (NAMS), State and Local Air 

Monitoring Stations (SLAMS) network, and other special purpose monitors. It also shows trends in the 

annual average NO2 measurements in each EPA Region. This indicator’s exhibits display the NO2

National Ambient Air Quality Standard (NAAQS) as a point of reference, but the fact that the national or 

any regional mean values fall below the standard does not mean that all monitoring sites nationally or in 

the EPA Region also are below the standard. This indicator displays trends in the number of the 91 sites 

nationwide at which NO2 concentrations exceeded the annual average standard over the period of record, 

but this statistic is not displayed for each EPA Region. 

What the Data Show

The national annual mean NO2 concentration in 2004 was 37 percent lower than that recorded in 1980 

(Exhibit 2-9, panel A). Also shown on this graph are the 90th and 10th percentiles of NO2 concentrations 

based on the distribution of annual statistics at the monitoring sites. This provides additional graphical 

representation of the distribution of measured concentrations across the monitoring sites for a given year. 

Thus, for each year, the graphic displays the concentration range where 80 percent of measured values 

occurred. The highest annual mean NO2 concentrations are typically found in urban areas. In addition, of 

the 91 sites comprising this trend (out of 373 total monitoring sites that were operating in 2004), the 

number reporting NO2 concentrations above the NO2 standard declined from seven sites in 1981 to zero 

sites since 1992 (Exhibit 2-9, panel B). 

NO2 levels in all ten EPA Regions have steadily decreased since 1980, with percent reductions over this 

time ranging from 25 percent in Region 8 to 44 percent in Region 9 (Exhibit 2-10). 
The decrease in NO2 concentrations in this indicator 

is consistent with decreasing NOx emissions 

observed over the past decade (the Nitrogen Oxides 

Emissions indicator, p. 2-23). 

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Indicator Limitations 

• Because ambient monitoring for NO2 occurs almost exclusively in high traffic urban areas, 

the average concentrations presented in this indicator likely are not representative of NO2 

levels in rural areas. Also, in rural areas, air mass aging could foster greater relative levels of 

peroxyacetyl nitrate (PAN) and nitric acid which can cause a positive interference in NO2 

measurements. 

• The measurement of NO2 is based on the conversion of NO2 to NO and the subsequent 

detection of NO using the chemiluminescence technique. Because there are other nitrogencontaining compounds, such as PAN and nitric acid that can be converted to NO, the 

chemiluminescence technique may overestimate NO2 due to these interferences. 

Measurement devices with ultraviolet photolytic converters are less prone to interferences 

than devices with heated surfaces (or catalysts) upstream of the chemiluminescence detector. 

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• Because of the relatively small number of trend sites in some EPA Regions, the regional 

trends are subject to greater uncertainty than the national trends. Some EPA Regions with low 

average concentrations may include areas with high local concentrations, and vice versa. 

• To ensure that long-term trends are based on a consistent set of monitoring sites, selection 

criteria were applied to identify the subset of NO2 monitoring sites with sufficient data to 

assess trends since 1980. Monitoring sites without sufficient data are not included in the trend 

analysis. Some excluded monitoring sites reported NO2 concentrations above the NO2 

standard over the time frame covered by this indicator. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw NO2 ambient air monitoring data in EPA’s Air Quality System (AQS) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the 

subset of NO2 monitoring stations that have sufficient data to assess trends since 1980. 

References 

U.S. EPA. 2006. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 1995. Review of the national ambient air quality standards for nitrogen oxides: assessment of 

scientific and technical information. EPA/452/R-95/005. Research Triangle Park, NC. 

U.S. EPA. 1993. Air quality criteria for oxides of nitrogen. EPA/600/8-91/049aF-cF. Research Triangle 

Park, NC. 

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1 INDICATOR: Volatile Organic Compounds Emissions 

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Volatile organic compounds (VOCs) are a large group of organic chemicals that include any compound of 

carbon (excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and 

ammonium carbonate) and that participate in atmospheric photochemical reactions. VOCs are of interest 

because they contribute to ozone formation (U.S. EPA, 2003a) and because many individual VOCs are 

known to be harmful to human health (the Benzene Concentrations indicator, p. 2-74; the Air Toxics 

Emissions indicator, p. 2-70). Health effects vary by pollutant. Emissions of all VOCs combined are also 

considered an important environmental and human health issue because ozone, another air pollutant (the 

Ozone Concentrations indicator, p. 2-32), is formed from chemical reactions involving airborne VOCs, 

airborne nitrogen oxides, and sunlight. VOCs are emitted from a variety of sources, including motor 

vehicles, chemical manufacturing facilities, refineries, factories, consumer and commercial products, and 

natural (biogenic) sources (mainly trees) (U.S. EPA, 2003b). 

This indicator presents VOC emissions from traditionally inventoried anthropogenic source categories: 

1) “Fuel combustion,” which includes emissions from coal, gas and oil-fired power plants, industrial, 

commercial, and institutional sources, as well as residential heaters and boilers; 2) “Industrial and other 

processes,” which includes chemical production, petroleum refining, and metals production; 3) “On-road 

vehicles,” which includes cars, trucks, buses, and motorcycles; and 4) “Nonroad vehicles and engines,” 

such as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, 

and others. The indicator also includes estimates of biogenic VOC emissions in 2002 which were 

obtained using the Biogenic Emissions Inventory System (BEIS) Model, Version 3.12. 

VOC emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of 

data from many different data sources, including industry and numerous state, tribal, and local agencies. 

Different data sources use different data collection methods, and many of the emissions data are based on 

estimates rather than actual measurements. For most fuel combustion sources and industrial sources, 

emissions are estimated using emission factors. Emissions from on-road and nonroad sources were 

estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and Virgin Islands, and some of the territories of federally-recognized American 

Indian nations. Data are presented only for 1990 and the years from 1996 to 2002; prior to 1996, only the 

1990 data have been updated to be comparable to the more recent inventories. 

What the Data Show

Only VOC emissions from anthropogenic sources are included in the NEI. However, VOC emissions 

from biogenic sources were estimated for 2002 to provide a sense of the relative contributions of natural 

versus anthropogenic emissions. Nationally, biogenic emissions were estimated to contribute 

approximately 72 percent to VOC emissions from all sources during 2002 (Exhibit 2-11, panel B). Thus, 

VOC emissions from biogenic sources are larger than the VOC emissions from all anthropogenic sources 

combined. 

According to NEI data, national total estimated VOC emissions from anthropogenic sources, excluding 

wildfires and prescribed burns, decreased by 32 percent between 1990 and 2002 (from 23,048,000 to 

15,662,000 tons) (Exhibit 2-11, panel A). The overwhelming majority of anthropogenic emissions 

reductions were observed among industrial processes and on-road mobile sources. Combined, these two 
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source categories accounted for 84 percent of the total nationwide estimated anthropogenic VOC 

emissions in 1990 (excluding wildfires and prescribed burns), but accounted for only 76 percent of the 

nationwide anthropogenic emissions in 2002. 

Trends in estimated anthropogenic VOC emissions among the ten EPA Regions were consistent with the 

overall decline seen nationally from 1990 to 2002 (Exhibit 2-12). VOC emissions decreased in every EPA 

Region, with the percent decrease ranging from 12 percent (Region 10) to 44 percent (Region 9). 

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Indicator Limitations 

• Comparable VOC emissions estimates through the NEI are available only for 1990 and 1996­

2002. Data for 1991-1995 are not provided due to differences in emission estimation 

methodologies from other inventory years which could lead to improper trend assessments. 

• VOC emissions from “miscellaneous sources” are not included in the total emissions. Details 

on emissions from miscellaneous sources can be found at 

http://www.epa.gov/ttn/chief/eiinformation.html. 

• VOC emissions data are largely based on estimates that employ emission factors generated 

from empirical and engineering studies, rather than on actual measurements of VOC 

emissions. Although these estimates are generated using well-established approaches, the 

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estimates have uncertainties inherent in the emission factors and emission models used to 

represent sources for which emissions have not been directly measured. 

• The methodology for estimating emissions is continually reviewed and is subject to revision. 

Trend data prior to any revisions must be considered in the context of those changes. 

• Not all states and local agencies provide the same data or level of detail for a given year. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw VOC emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by 

source type (anthropogenic or biogenic), source category, and EPA Region. 

References 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2003a. Requirements for preparation, adoption, and submittal of implementation plans: 

definitions. Code of Federal Regulations 40CFR51.100(s). 

U.S. EPA. 2003b. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

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1 INDICATOR: Ambient Concentrations of Ozone 

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Ozone is a gas found in different parts of the atmosphere. Ozone in the upper atmosphere, or stratosphere, 

helps protect the Earth from the sun’s harmful rays. (The Ozone Levels over North America indicator, on 

page 2-79, describes trends in stratospheric ozone levels over the U.S.) In the lowest level of the 

atmosphere, or troposphere, ozone is harmful to both human health and the environment. For this reason, 

ozone is often described as being “good up high and bad nearby” (U.S. EPA, 2003a). Although some 

industrial sources release ozone directly into the environment, most ground-level ozone forms in the air 

from chemical reactions involving nitrogen oxides (NOX), volatile organic compounds (VOCs), and 

sunlight. Ozone levels are typically highest during the afternoon hours of the summer months when the 

influence of direct sunlight is the greatest. These highest levels occur during what is known as the “ozone 

season” which typically occurs from May 1 to September 30, but the time frame of the ozone season 

varies by state (U.S. EPA, 2003b). 

Variations in weather conditions play an important role in determining ozone levels. Daily temperatures, 

relative humidity, and wind speed can affect ozone levels. In general, warm dry weather is more 

conducive to ozone formation than cool wet weather. Wind can affect both the location and concentration 

of ozone pollution. NOX and VOC emissions can travel hundreds of miles on air currents, forming ozone 

far from the original emission sources. Ozone also can travel long distances, affecting areas far 

downwind. High winds tend to disperse pollutants and can dilute ozone concentrations. However, 

stagnant conditions or light winds allow pollution levels to build up and become more concentrated. 

Inhalation exposure to ozone has been linked to numerous respiratory health effects, including acute 

reversible decrements in lung function, airway inflammation, cough, and pain when taking a deep a 

breath. Ozone exposure can aggravate lung diseases such as asthma, leading to increased medication use 

and increased hospital admission and visits to emergency rooms. In addition, evidence is highly 

suggestive that ozone directly or indirectly contributes to non-accidental and cardiopulmonary-related 

mortality, but the underlying mechanisms by which such effects occur have not been fully established 

(U.S. EPA, 2006a). Although people with lung disease are most susceptible to the effects of ozone, even 

healthy people who are active outdoors can suffer from ozone-related health effects. Further, evidence 

suggests that older adults (greater than 65 years old) appear to be at excess risk of ozone-related mortality 

or hospitalization (U.S. EPA, 2006a). Elevated concentrations of ozone can also affect vegetation and 

ecosystems, as the Ozone Injury to Forest Plants indicator (p. 2-37) describes further (U.S. EPA, 1996). 

This indicator presents ambient ground-level ozone concentrations in parts per million (ppm) from 1978 

to 2004. Data are shown for 1-hour and 8-hour averaging times, based on continuous ozone monitoring 

data and consistent with this pollutant’s National Ambient Air Quality Standards (NAAQS). The 1-hour 

standard is useful in indicating potential effects during short-term “spikes” in concentrations. The longer 

8-hour standard is indicative of exposures occurring over a more sustained period of time (e.g., an 

outdoor worker’s exposure over the course of a work day). Trends for this indicator represent 213 sites 

that have data for the period of record in the National Air Monitoring Stations (NAMS), the State and 

Local Air Monitoring Stations (SLAMS) network, and other special purpose monitors. The indicator also 

displays trends in ozone measurements in each EPA Region. This indicator’s exhibits display the 

corresponding NAAQS as points of reference, but the fact that the national or regional concentrations fall 

below the standards does not mean that all monitoring sites nationally or in any EPA Region also are 

below the standard. The indicator displays trends in the number of the 213 sites nationwide at which 

ozone concentrations exceeded standards, but these statistics are not displayed for each EPA Region. 
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Trends in ozone concentrations can be difficult to discern because of the year-to-year variations in the 

concentrations. By presenting data on a rolling 3-year time period, this indicator smooths out the “peaks” 

and “valleys” in the trend, making it easier to read without changing the overall trend statistic. Three 

years is consistent with the 3-year period used to assess compliance with the ozone standards. For the 1­

hour trends in this report, the second highest daily maximum over a 3-year period is used to be consistent 

with the 1-hour ozone standard. For the 8-hour trends in this report, a 3-year average of the fourth highest 

daily maximum in each year is used to be consistent with the 8-hour ozone standard. The 3-year statistic 

is assigned to the last year in each 3-year period. For example, 1980 is based on 1978-1980, and 2004 is 

based on 2002-2004. Thus, when endpoint comparisons are used in this report to describe long-term 

changes (i.e., 1978-2004), they are based on the first 3-year period and the last 3-year period. 

What the Data Show 

Between the 1978-1980 and 2002-2004 averaging periods, nationwide 1-hour average ambient ozone 

levels decreased by 26 percent (Exhibit 2-13, panel A). In addition, among the 213 monitoring sites used 

to estimate the 1978-2004 trend, the number measuring ozone concentrations above the 1-hour NAAQS 

(National Ambient Air Quality Standard) declined 77 percent over the same period (Exhibit 2-13, panel 

B). 

All ten EPA Regions also experienced some decline in 1-hour average ozone levels dating back to the 

1978-1980 averaging period (Exhibit 2-14). The greatest percent reductions in 1-hour ozone 

concentrations were observed in the two EPA Regions (Regions 1 and 9) that had the highest ozone levels 

in 1980, although the South Coast area in EPA Region 9 continues to have some of the highest ozone 

levels in the nation. EPA Region 10 showed the lowest ambient ozone concentrations over most of the 

period of record. 

Between the 1978-1980 and 2002-2004 averaging periods, nationwide 8-hour average ambient ozone 

concentrations decreased by 21 percent (Exhibit 2-15, panel A). Although the 8-hour ozone levels in 2004 

ranked among the lowest on record and the number of trend sites measuring ozone concentrations above 

the 8-hour NAAQS decreased by 84 percent over the time frame covered in this indicator (Exhibit 2-15, 

panel B), ambient air monitoring data collected in 2005 and reported to EPA’s Air Quality System 

indicates that approximately 118 million people live in 47 metropolitan areas where 8-hour average ozone 

concentrations do not meet EPA’s health-based air quality standards.Among the ten EPA Regions, the 

most substantial declines in 8-hour levels were again observed in EPA Regions that originally had the 

highest ozone concentrations (EPA Regions 1 and 9) (Exhibit 2-16). Most other Regions showed less 

pronounced declines (e.g., EPA Regions 6 to 10). Region 10 again consistently showed the lowest 

Regional ozone levels. 

Also shown in Exhibits 2-13 and 2-15 (panel A, in both cases) are the 90th and 10th percentiles based on 

the distribution of annual statistics at the monitoring sites. This provides additional graphical 

representation of the variability of measured concentrations across the monitoring sites for a given year. 

Thus, the graphic displays the concentration range where 80 percent of measured values occurred for that 

year. 

In summary, despite reductions in ambient concentrations of ozone over the past quarter century and 

decreases in the emissions of ozone precursors since 1990 (the Nitrogen Oxides Emissions indicator, p. 2­

23; the VOC Emissions indicator, p. 2-29), ozone remains one of the most persistent and ubiquitous air 

pollution issues in the U.S. 

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Indicator Limitations 

• Short-term trends in ozone 

concentrations are often highly 

dependent on meteorological conditions. 

This complicates efforts to interpret data 

for any given year. Air quality trends 

over the longer term are far less likely to 

be influenced by unusual meteorological 

conditions. 

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• Because most of the monitoring sites are located in urban areas, the trends might not 

accurately reflect conditions outside the immediate urban monitoring areas. 

• Because of the relatively small number of trend sites in some EPA Regions, the regional 

trends are subject to greater uncertainty than the national trends. Some EPA Regions with low 

average concentrations may include areas with high local concentrations, and vice versa. 

• To ensure that long-term trends are based on a consistent set of monitoring sites, selection 

criteria were applied to identify the subset of ozone monitoring sites with sufficient data to 

assess trends since 1978. Monitoring sites without sufficient data are not included in the trend 

analysis. Some excluded monitoring sites reported ozone concentrations above the ozone 

standard over the time frame covered by this indicator. 

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Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw ozone ambient air monitoring data in EPA’s Air Quality System (AQS) (U.S. EPA, 2006b) 

(http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the 

subset of ozone monitoring stations that have sufficient data to assess trends since 1978. 

References 

U.S. EPA. 2006a. Air quality criteria for ozone and related photochemical oxidants. EPA/600/R05/004aF-cF. Research Triangle Park, NC. 

U.S. EPA. 2006b. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003a. Ozone: good up high, bad nearby. EPA/451/K-03/001. Washington, DC. 

U.S. EPA. 2003b. Latest findings on national air quality—2002 status and trends. EPA/454/K-03/001. 

Research Triangle Park, NC. 

U.S. EPA. 1996. Air quality criteria for ozone and related photochemical oxidants. EPA/600/P-93/004FcF. Research Triangle Park, NC. 

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INDICATOR: Ozone Injury to Forest Plants 

Air pollution can have noteworthy cumulative impacts on forested ecosystems by affecting regeneration, 

productivity, and species composition (U.S. EPA, 1996). In the U.S., ozone in the lower atmosphere is 

one of the pollutants of primary concern. Ozone injury to forest plants can be diagnosed by examination 

of plant leaves. Foliar injury is usually the first visible sign of injury to plants from ozone exposure and 

indicates impaired physiological processes in the leaves (Grulke, 2003). 

This indicator is based on data from the U.S. Department of Agriculture Forest Service Forest Health 

Monitoring (FHM) Program (USDA, 2004). The FHM Program, now known as Phase 3 of the Forest 

Inventory Analysis, examines ozone injury to ozone-sensitive plant species at ground monitoring sites in 

forest land across the country. For this indicator, forest land does not include woodlots and urban trees. 

Sites are selected using a systematic sampling grid, based on a global sampling design (White et al., 1992; 

Smith et al., 2003). At each site that has at least 30 individual plants of at least three ozone-sensitive 

species and enough open space to ensure that sensitive plants are not protected from exposure by the 

forest canopy, FHM looks for damage on the foliage of ozone-sensitive forest plant species. Because 

ozone injury is cumulative over the course of the growing season, examinations are conducted in July and 

August, when ozone injury is typically highest. 

Monitoring of ozone injury to plants in the FHM program has expanded over the last 10 years from 

monitoring sites in ten states in 1994 to nearly 1,000 monitoring sites in 41 states in 2002. The data 

underlying this indicator are based on averages of all observations collected in 2002, the latest year for 

which data are publicly available, and are broken down by EPA Region. The ozone damage to forest 

plants is classified using a subjective five-category biosite index based on expert opinion, but designed to 

be equivalent from site to site. Ranges of biosite values translate to no injury, low or moderate foliar 

injury (visible foliar injury to highly sensitive or moderately sensitive plants, respectively), and high or 

severe foliar injury, which would be expected to result in tree-level or ecosystem-level responses, 

respectively (Coulston et al., 2004; U.S. EPA, 1996). 

What the Data Show

There is considerable regional variation in ozone injury to sensitive plants (Exhibit 2-17). The highest 

percentages of observed high and severe foliar injury, which are most likely to be associated with tree or 

ecosystem-level responses, are primarily found in the Mid-Atlantic and Southeast Regions. In EPA 

Region 3, 12 percent of ozone-sensitive plants showed signs of high or severe foliar damage, and in 

Regions 2 and 4, the values were 10 percent and 7 percent, respectively. High and severe ozone injury 

ranged from 2 percent to 4 percent in EPA Regions 1, 7, and 9, and no high or severe foliar damage was 

observed in EPA Regions 5, 6, 8, and 10. The percentage of sites showing no damage was greater than 50 

percent in every EPA Region, and no ozone-related foliar damage was observed at any of the 129 biosites 

in EPA Regions 8 and 10. 
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Indicator Limitations 

• Field and laboratory studies were 

reviewed to identify the forest plant 

species in each region that are highly 

sensitive to ozone air pollution. Other 

forest plant species, or even genetic 

variants of the same species, may not 

be harmed at ozone levels that cause 

effects on the selected ozone-sensitive 

species. 

• Because species distributions vary 

regionally, different ozone-sensitive 

plant species were examined in 

different parts of the country. These 

target species could vary with respect 

to ozone sensitivity, which might 

account for some of the apparent 

differences in ozone injury among 

EPA Regions. 

• Ozone damage to foliage is 

considerably reduced under conditions 

of low soil moisture, but most of the 

variability in the index (70 percent) 

was explained by ozone concentration 

(Smith et al., 2003). 

• Ozone may have other adverse 

impacts on plants (e.g., reduced 

productivity) that do not show signs of 

visible foliar injury (U.S. EPA, 1996). 

• Though the FHM has extensive spatial 

coverage based on a robust sample 

design, not all forested areas in the 

U.S. are monitored for ozone injury. 

• Even though the biosite data have 

been collected over a seven year 

period, most biosites were not 

monitored over the entire period, so 

these data cannot provide more than a 

baseline for future trends. 

Data Sources 

Data were provided by the USDA Forest Service’s Ozone Biomonitoring Program, which maintains a 

database of plant injury statistics by state (USDA Forest Service, 2006) 

(http://www.fiaozone.net/tabular.html). This indicator aggregates the state data by EPA Region. 

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References 

Coulston, J.W., K.H. Riitters, and G.C. Smith. 2004. A preliminary assessment of the Montréal process 

indicators of air pollution for the United States. Environ. Monit. Assess. 95:57-74. 

Grulke, N.E. 2003. The physiological basis of ozone injury assessment attributes in Sierran conifers. In: 

Bytnerowicz, A., M.J. Arbaugh, and R. Alonso, eds. Ozone air pollution in the Sierra Nevada: 

distribution and effects on forests. New York, NY: Elsevier Science, Ltd. pp. 55-81. 

Smith, G., J. Coulston, E. Jepsen, and T. Prichard. 2003. A national ozone biomonitoring program— 

results from field surveys of ozone sensitive plants in Northeastern forests (1994-2000). Environ. Monit. 

Assess. 87:271-291. 

USDA Forest Service. 2006. Ozone bioindicator data. Accessed 2006. 

<http://www.fiaozone.net/tabular.html> 

USDA Forest Service. 2004. Data report: a supplement to the national report on sustainable forests— 

2003. Washington, DC. 

U.S. EPA. 1996. Air quality criteria for ozone and related photochemical oxidants. EPA/600/AP93/004aF-cF (NTIS PB96-185582, PB96-185590, PB96-185608). Research Triangle Park, NC. 

<http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_cd.html> 

White, D., A.J. Kimerling, and W.S. Overton. 1992. Cartographic and geometric component of a global 

sampling design for environmental monitoring. Cartogr. Geograph. Info. Sys. 19:5-22. 

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1 INDICATOR: Particulate Matter Emissions

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Particulate matter (PM) is the general term used to describe solid particles and liquid droplets found in the 

air. The composition and size of these airborne particles and droplets vary. Some particles are large 

enough to be seen as dust or dirt, while others are so small they can only be seen using a powerful 

microscope. Two size ranges, known as PM10 and PM2.5, are widely monitored, both at major emissions 

sources and in ambient air. PM10 includes particles that have aerodynamic diameters less than or equal to 

10 microns (μm), approximately equal to one-seventh the diameter of human hair. PM2.5 is the subset of

PM10 particles that have aerodynamic diameters less than or equal to 2.5 μm.

Particles within the two size ranges behave differently in the atmosphere. PM2.5, or fine particles, can 

remain airborne for long periods and travel hundreds of miles. Coarse particles, or the subset of PM10 that

is larger than 2.5 μm, do not remain airborne as long and their spatial impact is typically limited because 

they tend to deposit on the ground downwind of emissions sources. Larger coarse particles are not readily 

transported across urban or broader areas because they are generally too large to follow air streams and 

they tend to be removed easily on contact with surfaces. In short, as the particle size increases, the amount 

of time the particles remain airborne decreases. The PM Concentrations indicator (p. 2-44) describes the 

various ways PM can harm human health and the environment (U.S. EPA, 2004). 

PM can be emitted directly or formed in the atmosphere. “Primary” particles refer to those released 

directly to the atmosphere. These include dust from roads and soot from combustion sources. In general, 

coarse PM is composed largely of primary particles. “Secondary” particles, on the other hand, are formed 

in the atmosphere from chemical reactions involving primary gaseous emissions. Thus, these particles can 

form at locations distant from the sources that release the precursor gases. Examples include sulfates 

formed from sulfur dioxide emissions from power plants and industrial facilities and nitrates formed from 

nitrogen oxides released from power plants, mobile sources, and other combustion sources. Unlike coarse 

PM, a much greater portion of fine PM (PM2.5) contains secondary particles (U.S. EPA, 2004). 

This indicator presents trends in annual average primary PM emissions data tracked by the National 

Emissions Inventory (NEI). The NEI tracks emission rate data, both measured and estimated, for primary 

particles only. Because secondary particles are not released directly from stacks, the NEI instead tracks 

the precursors that contribute to formation of secondary particles. These precursors include nitrogen 

oxides, sulfur dioxide, ammonia, and other gases (e.g., particle-producing organic gases), some of which 

are addressed in separate indicators (the Nitrogen Oxides Emissions indicator, p. 2-23; the Sulfur Dioxide 

Emissions indicator, p. 2-53). Particles formed through secondary processes are not included in this 

indicator.

Primary emissions of particulate matter can exist as solid or liquid matter (the “filterable” portion) or as 

gases (the “condensable” portion). Data for the condensable portion exist only for the years 1999 to 2002. 

To allow for a valid comparison of emission trends from 1990 to 2002, only data for the filterable portion 

of PM10 and PM2.5 are included in the trend graphs. Condensables are, however, included in the 

anthropogenic sources displayed in the inset pie charts in Exhibits 2-18 and 2-20. 

NEI data are presented for the traditionally inventoried anthropogenic source categories including: 

1) “Fuel combustion,” which includes emissions from coal, gas, and oil-fired power plants, industrial, 

commercial, and institutional sources, as well as residential heaters and boilers; 2) “Industrial and other 

processes,” which includes chemical production, petroleum refining, and metals production; 3) “On-road 

vehicles” which includes cars, trucks, buses, and motorcycles; and 4) “Nonroad vehicles and engines,” 
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such as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, 

and others. This indicator includes a comparison of these anthropogenic sources with emissions from 

miscellaneous and natural sources, such as agriculture and forestry, wildfires and managed burning, and 

fugitive dust from paved and unpaved roads. The indicator also includes estimates of miscellaneous and 

natural sources in 2002 which were obtained using the Biogenic Emissions Inventory System (BEIS) 

Model, Version 3.12. 

The NEI is a composite of data from many different data sources, including industry and numerous state, 

tribal, and local agencies. Different data sources use different data collection methods, and many of the 

emissions data are based on estimates rather than actual measurements. For most fuel combustion sources 

and industrial sources, emissions are estimated using emission factors. Emissions from on-road and 

nonroad sources were estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and Virgin Islands, and some of the territories of federally-recognized American 

Indian nations. Data are presented for 1990 and the years from 1996 to 2002; prior to 1996, only the 1990 

data have been updated to be comparable to the more 

recent inventories. 

What the Data Show 

PM10 Emissions Trends 

Estimated PM10 emissions from anthropogenic 

sources decreased 29 percent nationally between 

1990 and 2002 (Exhibit 2-18, panel A). Of these 

sources, the fuel combustion category saw the largest 

absolute decrease in emissions (422,000 tons; 35 

percent), and the on-road vehicle sector showed the 

largest relative decrease (47 percent; 183,000 tons). 

PM10 emissions from the group of sources including 

miscellaneous and natural sources and fugitive dust 

were estimated to account for 86 percent of total 

PM10 emissions (including condensables from 

stationary and mobile sources) in 2002, the majority 

of which was attributable to fugitive dust from roads 

(Exhibit 2-18, panel B). 

Changes in estimated anthropogenic PM10 emissions 

from 1990 to 2002 varied widely among EPA 

Regions, ranging from an increase of 56 percent 

(Region 8) to a decrease of 64 percent (Region 1) 

(Exhibit 2-19). 

PM2.5 Emissions Trends 

Estimated PM2.5 emissions from anthropogenic 

sources decreased 27 percent nationally between 

1990 and 2002 (Exhibit 2-20, panel A). Like PM10, 

the largest absolute decline in PM2.5 was seen in the 

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fuel combustion source category (347,000 tons; 38 

percent), and the largest relative decline was in the 

on-road vehicle category (54 percent; 175,000 tons). 

Emissions from the group of miscellaneous and natural sources were estimated to account for 64 percent 

of the total PM2.5 emissions (including condensables from stationary and mobile sources) nationally in 

2002 (Exhibit 2-20, panel B). 

Anthropogenic PM2.5 emissions decreased in all ten EPA Regions from 1990 to 2002, with percent 

reductions ranging from 6 percent (Region 8) to 71 percent (Region 1) (Exhibit 2-21). 

Indicator Limitations 

• Comparable PM emissions estimates through the NEI are available only for 1990 and 1996­

2002. Data for 1991-1995 are not provided due to differences in emission estimation 

methodologies from other inventory years which could lead to improper trend assessments. 

• Because the emissions indicators focus on sources of anthropogenic origin, PM emissions 

from miscellaneous sources (e.g., wildfires) are not included in the trend line. Details on 

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emissions from these sources can be found 

at 

http://www.epa.gov/ttn/chief/eiinformation 

.html. 

• The emissions data for PM are largely 

based on estimates that employ emission 

factors generated from empirical and 

engineering studies, rather than on actual 

measurements of PM emissions. Although 

these estimates are generated using wellestablished approaches, the estimates have 

uncertainties inherent in the emission 

factors and emission models used to 

represent sources for which emissions have 

not been directly measured. 

• The methodology for estimating emissions 

is continually reviewed and is subject to 

revision. Trend data prior to these revisions 

must be considered in the context of those 

changes. 

• The indicator tracks primary PM 

emissions. Particles that form in the air 

through secondary processes are not 

included in this indicator, but are 

considered in the PM Concentrations 

indicator (p. 2-44). 

• Not all states and local agencies provide 

the same data or level of detail for a given 

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Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw PM emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by 

source type (anthropogenic or biogenic), source category, and EPA Region. 

References 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2004. Air quality criteria for particulate matter (October 2004). EPA 600/P-99/002aF-bF. 

Research Triangle Park, NC. 

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1 INDICATOR: Ambient Concentrations of Particulate Matter 

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Particulate matter (PM) is the general term used for a mixture of solid particles and liquid droplets found 

in the air. Airborne PM comes from many different sources. Primary particles are released directly into 

the atmosphere from sources such as cars, trucks, heavy equipment, forest fires, and burning waste. 

Primary particles also consist of crustal material from sources such as unpaved roads, stone crushing, 

construction sites, and metallurgical operations. Secondary particles are formed in the air from reactions 

involving precursor chemicals such as sulfates (which are formed from sulfur dioxide emissions from 

power plants and industrial facilities), nitrates (which are formed from nitrogen dioxide emissions from 

cars, trucks, and power plants) and carbon (which comes from reactive organic gas emissions from cars, 

trucks, industrial facilities, forest fires, and biogenic sources such as trees). 

Ambient air monitoring stations throughout the country measure air concentrations of two size ranges of 

particles: PM2.5 and PM10. PM2.5 consists of “fine particles” with aerodynamic diameters less than or 

equal to 2.5 μm. PM10 includes both fine particles (PM2.5) and “coarse particles,” which is the subset of 

PM10 that is larger than 2.5 μm,. The chemical makeup of particles varies across the U.S. For example, 

fine particles in the eastern half of the U.S contain more sulfates than those in the West, while fine 

particles in southern California, for example, contain more nitrates than those in other areas of the U.S. 

Carbon is a substantial component of fine particles everywhere (U.S. EPA, 2004a). 

Fine particles also have a seasonal pattern. PM2.5 values in the eastern half of the U.S. are typically higher 

in the third calendar quarter (July-September) when sulfates are more commonly formed from sulfur 

dioxide emissions from power plants in that part of the country. Fine particle concentrations tend to be 

higher in the fourth calendar quarter in many areas of the West, in part because fine particle nitrates are 

more readily formed in cooler weather, and wood stove and fireplace use produces more carbon. 

Many recent epidemiologic studies show statistically significant associations of various ambient PM 

indicators (e.g., coarse or fine particulate, short-term or long-term concentrations) with a variety of 

cardiovascular and respiratory health endpoints, including mortality, hospital admissions, emergency 

department visits, other medical visits, respiratory illness and symptoms, and physiologic changes in 

pulmonary function (U.S. EPA 2004b). Sensitive groups that appear to be at greatest risk to such PM 

effects include older adults, individuals with cardiopulmonary disease such as asthma or congestive heart 

disease, and children (U.S. EPA, 2004b). Unlike other criteria pollutants, PM is not a single specific 

chemical entity, but rather a mixture of particles from different sources with different sizes and chemical 

compositions. Toxicological studies suggest that some airborne particles are more toxic than others, due 

to differences in their chemical composition—a topic that is thoroughly reviewed in other publications 

(e.g., U.S. EPA, 2004b). 

PM also can cause adverse impacts to the environment. Fine particles are the major cause of reduced 

visibility in parts of the U.S., including many National Parks and Wilderness Areas (the Regional Haze 

indicator, p. 2-50). PM deposition affects vegetation and ecosystems by altering nutrient and chemical 

cycles in soils and surface water. For example, deposition of particles containing nitrogen and sulfur may 

change the nutrient balance and acidity of aquatic environments so that species composition and buffering 

capacity change (the Lake and Stream Acidity indicator, p. 2-62). Some particles that deposit onto plant 

leaves can corrode leaf surfaces or interfere with plant metabolism. PM also causes soiling and erosion 

damage to materials, including monuments, statues, and other objects of cultural importance (U.S. EPA, 

2004b). 
This indicator presents trends in PM10 and PM2.5 concentrations, using averaging times consistent with the 

pollutants’ corresponding National Ambient Air Quality Standards (NAAQS). For PM10, trend data from 

1990 to 2004 are presented for the second maximum 24-hour average concentrations measured at the 

trend sites during each calendar year. For PM2.5, trend data from 1999 to 2004 are presented for 

seasonally-weighted annual average concentrations and for the 98th percentiles of 24-hour average 

concentrations measured at the trend sites during each calendar year. Trend data are based on 

measurements from two nationwide networks of monitoring stations: the National Air Monitoring 

Stations (NAMS) and the State and Local Air Monitoring Stations (SLAMS). For both PM10 and PM2.5,

the indicator displays trends for the entire nation and for the ten EPA Regions. 

The indicator’s exhibits display the pollutants’ NAAQS as points of reference. However, the fact that the 

national values or those shown for EPA Regions fall below the standards does not mean that all 

monitoring sites nationally or in any particular EPA Region also are below the standards. The indicator 

displays trends in the number of the 489 PM10 monitoring sites and 707 PM2.5 monitoring sites nationwide 

that exceed the standards, but these statistics are not displayed for each EPA Region. 

What the Data Show

PM10 Concentration Trends 

(1988-1989 PM10 data will be added)

In 2004, the national 24-hour PM10 concentration was 31 percent lower than the average 1990 level 

(Exhibit 2-22, panel A). Additionally, of the 489 sites comprising this trend (out of 808 total monitoring 

sites that were operating in 2004), the number reporting PM10 concentrations above the 24-hour standard 

declined 88 percent between 1990 and 2004 (Exhibit 2-22, panel B). All EPA Regions experienced a 

steady decrease in 24-hour PM10 levels over this period (Exhibit 2-23). EPA Regions 9 and 10 showed the 

greatest decreases since 1990. 

Also shown in Exhibit 2-22 (panel A) are the 90th and 10th percentiles based on the distribution of annual

statistics at the monitoring sites. This provides additional graphical representation of the distribution of 

measured concentrations across the monitoring sites for a given year. Thus, the graphic displays the 

concentration range where 80 percent of measured values occurred for that year. (Note, this presentation 

style also applies to panel A in Exhibits 2-24 and 2-26, discussed below.) 

PM2.5 Concentration Trends 

Annual average PM2.5 concentrations in 2004 were the lowest since nationwide monitoring began in 1999 

(Exhibit 2-24, panel A). The trend is based on measurements collected at 707 monitoring stations that 

have sufficient data to assess trends over that period (U.S. EPA, 2003). The seasonally-weighted annual 

average concentrations decreased 14 percent between 1999 and 2004. The number of monitoring sites in 

this trend (707 out of 781 total sites that were operating in 2004) reporting violations of the annual 

average PM2.5 standard declined 69 percent over this period (Exhibit 2-24, panel B). 

Regional declines were greatest in portions of the West (EPA Region 9) and the Southeast (EPA Region 

4), where annual average PM2.5 levels in 2004 were 21 percent and 18 percent lower than those in 1999, 

respectively (Exhibit 2-25). However, annual average PM2.5 levels in the Rocky Mountains (EPA Region 

8) were essentially unchanged over the period. 

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In 2004, the average of 98th percentiles of 24-hour PM2.5 concentrations at the 707 monitoring sites used 

for the trend was 11 percent lower than the average 1999 level (Exhibit 2-26, panel A). The number of 

monitoring sites in this trend (707 out of 781 total sites that were operating in 2004) reporting violations 

of the 24-hour PM2.5 standard declined 64 percent over this period (Exhibit 2-26, panel B). All EPA 

Regions except Region 8 experienced decreasing 24-hour PM2.5 levels, and Regions 9 and 4 again showed 

the largest declines (25 percent and 17 percent, respectively) (Exhibit 2-27). 

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1 Indicator Limitations 

• Because there are far more PM10 and PM2.5 monitors in urban areas than in rural areas, the 

trends might not accurately reflect conditions outside the immediate urban monitoring areas. 

• Potential biases may exist for some PM2.5 ambient concentration measurements due to losses 

from volatilization of nitrates and other semi-volatile materials and retention of particlebound water associated with hygroscopic species. 

• Due to the relatively small number of monitoring sites in some EPA Regions, the regional 

trends are subject to greater uncertainty than the national trends. Some EPA Regions with low 

average concentrations may include areas with high local concentrations, and vice versa. 

• To ensure that long-term trends are based on a consistent set of monitoring sites, selection 

criteria were applied to identify the subset of PM monitoring sites with sufficient data to 

assess trends over the time frames covered by this indicator. Monitoring sites without 

sufficient data are not included in the trend analysis. Some excluded monitoring sites reported 

PM concentrations above the PM standard during the years covered by this indicator. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw PM ambient air monitoring data in EPA’s Air Quality System (AQS) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the 

subset of PM monitoring stations that have sufficient data to assess trends over the period of record (i.e., 

since 1990 for PM10 and since 1999 for PM2.5). 

References 

U.S. EPA. 2006. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2004a. The particulate pollution report: current understanding of air quality and emissions 

through 2003. EPA 454/R-04/002. Research Triangle Park, NC. 

U.S. EPA. 2004b. Air quality criteria for particulate matter (October 2004). EPA 600/P-99/002aF-bF. 

Research Triangle Park, NC. 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

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1 INDICATOR: Regional Haze

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Regional haze is the term describing visibility impairment that results from fine particles and their 

precursors emitted by a variety of sources and activities (U.S. EPA, 2004a). Visibility impairment is 

observed not only in urban areas, but also in many of the best known and most treasured National Parks 

and Wilderness Areas, such as the Grand Canyon, Great Smoky Mountains, Mount Rainier, Shenandoah, 

Yellowstone, and Yosemite National Parks (U.S. EPA, 2003). Visibility impairment occurs when air 

pollution, both particles and gases, scatter and absorb light. Haze not only limits the distance one can see, 

but also degrades the color, clarity, and contrast of scenes. As the PM Concentrations indicator (p. 2-44) 

describes further, the same pollutants that impair visibility are also linked to serious health effects. 

The particles that impair visibility include both primary and secondary pollutants. The primary pollutants 

of concern are particles that are emitted directly into the atmosphere, such as dust from roads or soot 

(elemental carbon) from combustion sources, such as wood combustion. Secondary pollutants of concern 

are particles that form in the atmosphere from chemical reactions and physical processes, such as sulfates 

(formed from sulfur dioxide emissions from power plants and other industrial facilities) and nitrates 

(formed from nitrogen oxides emitted from power plants, automobiles, and other types of combustion 

sources).

Humidity can increase the effect of pollution on visibility, causing some particles to become more 

efficient at scattering light and impairing visibility (U.S. EPA, 2003). In the eastern U.S., where annual 

average relative humidity levels are between 70 percent and 80 percent, reduced visibility mainly results 

from secondarily formed sulfates and high humidity, along with a somewhat lower contribution from 

organic carbon and nitrates (U.S. EPA, 2004b). The effect of humidity is particularly strong in summer. 

Humidity is less of a factor in the West, as average values are generally between 50 percent and 60 

percent. In western states, primary emissions from sources like wood smoke and nitrates contribute a 

large percentage of the total particulate loading, though secondarily formed sulfates also contribute to 

visibility impairment. Without the effects of anthropogenic sources of pollution, the natural visual range 

in the U.S. would vary with location, and is estimated to range from 75 to 150 km (45 to 90 miles) in the 

East and from 200 to 300 km (120 to 180 miles) in the West (U.S. EPA, 2003). 

This indicator reports visibility data calculated from measurements of particulate matter constituents 

collected at 38 monitoring sites between 1992 and 2004 at National Parks, Wilderness Areas, and other 

protected sites under the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. 

Data are presented for 10 Eastern (east of 100o

 W longitude) sites and 28 Western (west of 100o W

longitude) sites. Visibility, or visual range, is calculated from the measured levels of different components 

within airborne particles and these components’ light extinction efficiencies. The algorithm includes an 

adjustment for ammonium sulfate and ammonium nitrate to account for their adsorption of water vapor 

from the atmosphere under elevated relative humidity conditions. The IMPROVE particle data are 

generated by laboratory analysis of 24-hour duration filter samples collected at each site on a one-day-inthree schedule. The indicators track visibility in three categories: worst visibility conditions refer to the 

mean of the 20 percent worst visibility days; best visibility conditions refer to the mean of the 20 percent 

best visibility days; and mid-range visibility conditions refer to the mean of the remaining 60 percent of 

days. 

What the Data Show

On average, the best visibility in the East, as calculated from the measured concentrations of components 

of PM, is only slightly better than the worst visibility in the West (Exhibit 2-28). In 2004, the mean visual 
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range for the worst days in the East was 31 km (19.3 miles), compared to 137 km (85.1 miles) for the best 

visibility days. In the West, the mean visual range extended from 109 km (67.7 miles) on the worst days 

to 260 km (161.6 miles) on the best days. In both regions, the average visual range increased since 1992 

for worst, mid-range, and best visibility days. The increased visual ranges between 1992 and 2004 for 

mid-range visibility days were 46 percent in the East and 14 percent in the West. 

Indicator Limitations 

• These data represent visibility in a sampling of selected National Parks and Wilderness Areas 

and are not representative of other rural or urban areas. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw ambient air monitoring data collected as part of the Interagency Monitoring of Protected 

Visual Environments (IMPROVE) network (IMPROVE, 2006) 

(http://vista.cira.colostate.edu/improve/Data/IMPROVE/summary_data.htm). Visibility trends in this 

indicator are derived from the subset of IMPROVE monitoring stations outside of urban areas that have 

sufficient data to assess trends between 1992 and 2004. 

References 

IMPROVE. 2006. Data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) 

network. Accessed 2006. <http://vista.cira.colostate.edu/improve/Data/IMPROVE/summary_data.htm> 

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U.S. EPA. 2004a. The particulate pollution report: current understanding of air quality and emissions 

through 2003. EPA/454/R-04/002. Research Triangle Park, NC. 

U.S. EPA. 2004b. The ozone report: measuring progress through 2003. EPA/454/K-04/001. Research 

Triangle Park, NC. 

U.S. EPA. 2003. Latest findings on national air quality—2002 status and trends. EPA/454/K-03/001. 

Research Triangle Park, NC. 

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1 INDICATOR: Sulfur Dioxide Emissions 

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Sulfur dioxide (SO2) belongs to the family of sulfur oxide (SOx) gases. These gases are formed when fuel 

containing sulfur (mainly coal and oil) is burned (e.g., for electricity generation) and during metal 

smelting and other industrial processes. High concentrations of SO2 are associated with multiple health 

and environmental effects (U.S. EPA, 2003). The highest concentrations of SO2 have been recorded in the 

vicinity of large industrial facilities. Although relatively few people live in areas where SO2

concentrations exceed the National Ambient Air Quality Standards (NAAQS), a much larger number of 

people live in PM2.5 non-attainment areas, and SO2 is a major precursor to PM2.5 which also has human 

health and ecological effects (the PM Concentrations indicator, p. 2-44). 

Health effects associated with SO2 depend on the exposure concentrations and durations, and on the 

susceptibility of exposed populations. Asthmatics are much more susceptible to SO2 exposure than people 

who do not have asthma (U.S. EPA, 1986). Effects associated with longer-term exposures to high 

concentrations of SO2, in conjunction with high levels of PM, include respiratory illness, alterations in the 

lungs’ defenses, and aggravation of existing heart or lung disease. The most susceptible populations under 

these conditions include individuals with cardiovascular disease or chronic lung disease, children, and 

older adults (U.S. EPA, 1982). 

Many other environmental concerns are associated with high concentrations of SO2. For example,

airborne SO2, along with NOx, contribute to acidic deposition (the Acid Deposition indicator, p. 2-56); 

SO2 is a major precursor to PM2.5 (the PM Concentrations indicator, p. 2-44); and SO2 contributes to

impaired visibility (the Regional Haze indicator, p. 2-50). SO2 exposure also can harm vegetation by 

increasing foliar injury, decreasing plant growth and yield, and decreasing the number and variety of plant 

species in a given community. Finally, SO2 can accelerate the corrosion of natural and man-made 

materials (e.g., concrete and limestone) that are used in buildings, statues, and monuments that are part of 

the nation’s cultural heritage (U.S. EPA, 1982).

This indicator presents SO2 emissions from traditionally inventoried anthropogenic source categories: 

1) “Fuel combustion: selected power generators,” which includes emissions from coal, gas, and oil-fired 

power plants that are required to use continuous emissions monitors (CEMs) to report emissions as part of 

the Acid Rain Program (ARP); 2) “Fuel combustion: other sources,” which includes industrial, 

commercial, and institutional sources, as well as residential heaters and boilers not required to use CEMs; 

3) “Other industrial processes,” which include chemical production and petroleum refining; 4) “On-road 

vehicles,” which includes cars, trucks, buses, and motorcycles; 5) “Nonroad vehicles and engines,” such 

as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and 

others. Since a substantial portion of airborne SO2 comes from fossil fuel combustion in electric utilities, 

this indicator includes the separate “Fuel combustion: selected power generators” category in addition to 

the four categories presented in the other emissions indicators. 

SO2 emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite of 

data from many different data sources, including industry and numerous state, tribal, and local agencies. 

Different data sources use different data collection methods, and many of the emissions data are based on 

estimates rather than actual measurements. For major electric generating units, most data come from 

continuous emissions monitors that measure actual emissions. For other fuel combustion sources and 

industrial processes, data are estimated using emission factors. Emissions from on-road and nonroad 

sources were estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 
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NEI data have been collected since 1990 and cover all 

50 states and their counties, D.C., the U.S. territories 

of Puerto Rico and Virgin Islands, and some of the 

territories of federally-recognized American Indian 

nations. Data are presented only for 1990 and the years 

from 1996 to 2002; prior to 1996, only the 1990 data 

have been updated to be comparable to the more recent 

inventories. 

What the Data Show 

National estimated SO2 emissions decreased 34 

percent between 1990 and 2002 (from 23,060,000 to 

15,260,000 tons) (Exhibit 2-29). This downward trend 

resulted primarily from emissions reductions at 

electrical utilities. Between 1990 and 2002, air 

emissions from electric utilities have consistently 

accounted for roughly two-thirds of the nationwide 

SO2 emissions. 

Net SO2 emissions declined in all EPA Regions 

between 1990 and 2002 except in Region 6 where 

there was a slight increase (Exhibit 2-30). Since 1996, 

the largest percent reductions in SO2 emissions 

were seen in Regions 9 (33 percent), 10 (30 

percent), 1 (29 percent), and 5 (25 percent), and the 

smallest reductions were observed in Regions 6 (7 

percent) and 3 (10 percent). 

Indicator Limitations 

• Though emissions from most electric 

utilities are measured directly using 

continuous monitoring devices, SO2 

emissions data for other source types 

are based on estimates that employ 

emission factors generated from 

empirical and engineering studies. 

Although these estimates are generated 

using well-established approaches, the 

estimates have uncertainties inherent in 

the emission factors and emission 

models used to represent sources for 

which emissions have not been directly 

measured. 

• Comparable SO2 emissions estimates 

through the NEI are available only for 

1990 and 1996-2002. Data for 1991­

1995 are not provided due to 

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differences in emission estimation methodologies from other inventory years which could 

lead to improper trend assessments. 

• SO2 emissions from “miscellaneous sources” are not included in the total emissions. Details 

on emissions from miscellaneous sources can be found at 

http://www.epa.gov/ttn/chief/eiinformation.html. 

• The methodology for estimating emissions is continually reviewed and is subject to revision. 

Trend data prior to these revisions must be considered in the context of those changes. 

• Not all states and local agencies provide the same data or level of detail for a given year. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw SO2 emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by 

source category and EPA Region. 

References 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 1986. Second addendum to the air quality criteria for particulate matter and sulfur oxides 

(1982): assessment of newly available health effects information. EPA/450/S-86/012. Research Triangle 

Park, NC. 

U.S. EPA. 1982. Air quality criteria for particulate matter and sulfur oxides. EPA/600/P-82/020a-c. 

Research Triangle Park, NC. 

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1 INDICATOR: Acid Deposition 

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Every year, millions of tons of sulfur dioxide and nitrogen oxides are emitted to the atmosphere as a result 

of the burning of fossil fuels and from other high temperature sources (the Sulfur Dioxide Emissions 

indicator, p. 2-53; the Nitrogen Oxides Emissions indicator, p. 2-23). These gases react with water, 

oxygen, and oxidants to form acidic compounds, which may be carried hundreds of miles by the wind— 

even across state or national borders. Acid deposition occurs when these compounds fall to the Earth in 

one of two forms: wet (dissolved in rain, snow, and fog) or dry (solid and gaseous particles deposited on 

surfaces during periods of no precipitation). While wet deposition is the more widely recognized form 

(more commonly referred to as “acid rain”), dry deposition can account for 20 percent to 80 percent of 

total acid deposition depending on location and climate (MACTEC Engineering and Consulting, 2005). In 

the environment, acid deposition causes soils and water bodies to acidify, which can make the water 

unsuitable for some fish and other wildlife. Some types of ecosystems, those with less “buffering” 

capacity, are more sensitive to acid deposition than others. 

Scientists often use acid neutralizing capacity (ANC), a measure of the amount of anions, protons, and 

non-proton cations in the water, as an indicator of which lakes and streams are most sensitive to 

acidification (NAPAP, 1991). Most surface waters in the West do not exhibit many symptoms of 

acidification, because relatively small amounts of acid deposition occur in acid-sensitive regions. In the 

Northeast and along the Appalachian Mountains, however, relatively high levels of acid deposition occur 

in acid-sensitive regions, or regions without enough geochemical buffering capacity to prevent 

acidification of surface waters by acid deposition (the Lake and Stream Acidity indicator, p. 2-62). 

Therefore, reductions in acid deposition have the largest impact on acidification of lakes and streams in 

those areas.

Acid deposition damages some trees, particularly at high elevations, and speeds the decay of buildings, 

statues, and sculptures that are part of our national heritage (U.S. EPA, 2003). The nitrogen portion of 

acid deposition also contributes to eutrophication in coastal ecosystems, the symptoms of which include 

potentially toxic algal blooms, fish kills, and loss of plant and animal diversity. Acidification of lakes and 

streams can increase the amount of methylmercury available in aquatic systems (Winfrey and Rudd, 

1990). Finally, increased levels of sulfate in ground-level air, a phenomenon related to dry deposition, can 

contribute to decreased visibility as well as a variety of human health problems (U.S. EPA, 2003). 

Total acid deposition in this indicator is determined using wet deposition measurements and dry 

deposition calculated from ambient air concentration measurements. Wet deposition is measured through 

chemical analysis of rainwater collected at sites across the U.S. The primary source of wet deposition 

information comes from the National Atmospheric Deposition Program/National Trends Network 

(NADP/NTN). The chemical components of wet deposition include sulfate, nitrate, and ammonium. Dry 

deposition is not measured directly. EPA’s Clean Air Status and Trends Network (CASTNET) determines 

dry deposition inferentially by measuring ambient air concentrations of acidic compounds and then 

calculating deposition rates using a multi-layer model that depends on meteorological data collected at the 

sites as well as local vegetative conditions (http://www.epa.gov/castnet/). Chemicals measured include 

components of particulate matter (sulfate (SO4) and nitrate (NO3)), gaseous nitric acid (HNO3), sulfur

dioxide (SO2), and ammonium (NH4). This indicator uses the three-year average from 1989-1991 as a 

baseline, as this period immediately predates controls on sulfur and nitrogen oxide emissions mandated 

by the 1990 Clean Air Act Amendments. Use of three-year average data helps ensure that trends reflect 

actual changes in acid deposition, instead of shorter-term fluctuations in meteorological conditions. 
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What the Data Show 

Analyses of long-term monitoring data from the National Atmospheric Deposition Program (NADP) 

show that wet deposition of both sulfur and nitrogen compounds has decreased over the last 15 years 

(Exhibits 2-31 and 2-32). 

Wet sulfate deposition decreased across much of the U.S. during the 1990s (Exhibit 2-31). The greatest 

reductions in wet sulfate deposition occurred in the Mid-Appalachian region (Maryland, New York, West 

Virginia, Virginia, and most of Pennsylvania) and the Ohio River Valley. Less dramatic reductions were 

observed across much of New England and portions of the Southern Appalachians. Average regional 

decreases in wet deposition of sulfate between the periods 1989-1991 (panel A) and 2002-2004 (panel B) 

were approximately 36 percent in the Northeast, 32 percent in the Midwest, 24 percent in the MidAtlantic and 19 percent in the Southeast. 

Wet nitrate deposition decreased approximately 16 percent across the Northeast and 8 percent in the MidAtlantic between the periods 1989-1991 (Exhibit 2-32, panel A) and 2002-2004 (panel B). Wet deposition 

of inorganic nitrogen has not changed substantially in the rest of the country over this period. 

As with wet deposition, total deposition (the sum of wet and dry deposition) decreased between 1989­

1991 and 2002-2004, and reductions were more substantial for sulfur compounds than for nitrogen 

compounds (Exhibits 2-33 and 2-34). Note that total nitrogen deposition in this indicator does not include 

nitrogen components, such as ammonia, which can be a significant portion of the dry deposition. 

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Indicator Limitations 

• Geographic coverage is limited, particularly for dry deposition (and thus total deposition as 

well), but the concentration of sites in the Midwest and Northeast is justified by the fact that 

acid rain is much more of a problem in those regions than it is in the West, Great Plains, or 

Southeast. 

• Measurement techniques for dry deposition have improved substantially, but characterization 

of dry deposition still requires a combination of measurements and modeling, which has 

inherent uncertainties. Further, dry deposition presented in this indicator does not include 

contributions from deposition of gaseous ammonia. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Atmospheric Programs, based on raw 

deposition data from two sources. Raw wet deposition data are from the National Atmospheric Deposition 

Program/National Trends Network (NADP, 2006) (http://nadp.sws.uiuc.edu/), and raw dry deposition 

data are from the Clean Air Status and Trends Network (U.S. EPA, 2006) (http://www.epa.gov/castnet). 

This indicator aggregates raw data across 3-year periods to avoid influences from short-term fluctuations 

in meteorological conditions, and wet deposition data were interpolated among monitoring stations to 

generate the maps shown in Exhibits 2-31 and 2-32. 

References 

MACTEC Engineering and Consulting, Inc. 2005. Clean Air Status and Trends Network (CASTNET): 

2004 annual report. Prepared for U.S. EPA, Office of Air and Radiation. 

NADP. 2006. Data from the National Atmospheric Deposition Program/National Trends Network. 

Accessed 2006. <http://nadp.sws.uiuc.edu> 

NAPAP (National Acid Precipitation Assessment Program). 1991. 1990 integrated assessment report. 

Washington, DC. 

U.S. EPA. 2003. Latest findings on national air quality: 2002 status and trends. EPA/454/K-03/001. 

Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd02/2002_airtrends_final.pdf> 

U.S. EPA. 2006. Data from the Clean Air Status and Trends Network. Accessed 2006. 

<http://www.epa.gov/castnet/> 

Winfrey, M.R., and J.W.M. Rudd. 1990. Environmental factors affecting the formation of methyl mercury 

in low pH lakes. Environ. Toxicol. Chem. 9(7):853-869. 

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1 INDICATOR: Lake and Stream Acidity

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Acid deposition can have serious effects on aquatic ecosystems. For example, aquatic organisms in 

acidified waters can develop calcium deficiencies that weaken bones and exoskeletons and cause eggs to 

be weak or brittle. Acidified waters can impair the ability of fish gills to extract oxygen from water and 

change the mobility of certain trace metals (e.g., aluminum, cadmium, manganese, iron, arsenic, 

mercury), which in turn can place fish and other species sensitive to these metals at risk (NAPAP, 1991). 

The Acid Deposition indicator (p. 2-56) explains the factors that contribute to acid deposition and 

describes how acid deposition patterns have changed over the last 15 years. 

The susceptibility of a water body to acidification depends on the ability of the water and watershed soils 

to neutralize the acid deposition it receives. The best measure of this ability is acid neutralizing capacity 

(ANC), which characterizes the amount of dissolved compounds that will counteract acidity. All bodies of 

water have a measurable acid neutralizing capacity. ANC depends largely on the surrounding watershed’s 

physical characteristics, such as geology, soils, and size. The ANC of a body of water reflects the relative 

proportions of positive and negative ions entering the water from sources such as atmospheric inputs and 

the soil and bedrock surrounding and underlying the water body. The higher the ANC, the more acid a 

water body can neutralize and the less susceptible it is to acidification. As ANC approaches zero, the 

ability to neutralize acidity decreases. Surface water with an ANC greater than 200 micro equivalents per 

liter (μeq/L) is usually considered non-acidic; surface water with an ANC less than 50 μeq/L is 

considered highly sensitive to acidification (is often seasonally acidic); and surface water with an ANC 

less than 0 μeq/L is considered chronically acidic, meaning the watershed no longer has the capacity to 

neutralize further acid deposition (U.S. EPA, 2003). ANC can be negative when anions exceed nonproton cations (i.e., when there are free protons [H+

 ions] in solution). 

The National Acid Precipitation Assessment Program identified several regions in the U.S. as containing 

many of the surface waters sensitive to acidification (Exhibit 2-35). Where soil buffering capacity is poor, 

lakes and streams may be vulnerable to acidification (NAPAP, 1991). 

This indicator is derived from ANC measurements 

on probability survey samples representing 5,617 

lakes and 72,000 stream miles in the five 

geographic regions shown in Exhibit 2-36. These 

measurements were collected as part of the 

Temporally Integrated Monitoring of Ecosystems 

(TIME) project and on 120 additional acidsensitive lakes and 78 acid-sensitive streams in the 

Long-Term Monitoring (LTM) project, for which 

data were available between 1990 and 2000 (U.S. 

EPA, 2003). The lakes sampled include only those 

in areas potentially sensitive to acidification with 

areas greater than 4 hectares. Smaller lakes 

generally are not used in this type of assessment 

because they are more likely to be naturally acidic, 

although acid deposition can cause them to 

become further acidified. This indicator focuses 

only on the northeastern U.S.; because monitoring 
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is not ongoing for western and southeastern water bodies, trend data for those parts of the country are not 

available. 

What the Data Show 

Between 1990 and 2000, ANC in lakes in the Upper Midwest (northeastern Minnesota, northern 

Wisconsin, and northern Michigan), in lakes in the Adirondack Mountains, and in streams in the Northern 

Appalachians (southern New York, west-central Pennsylvania, and eastern West Virginia) increased to a 

degree where many water bodies that were considered “chronically acidic” in 1990 were no longer 

classified as such in 2000 (Exhibit 2-36, panels A, B, and D). Specifically, between 1990 and 2000, the 

percent of chronically acidic water bodies decreased in the Adirondack Mountains (from 13.0 percent to 

8.1 percent), in the Upper Midwest (from 2.9 percent to 0.9 percent), and in the Northern Appalachian 

Plateau (from 11.8 percent to 8.5 percent). This trend suggests that surface waters in these areas are 

beginning to recover from acidification, though acidic surface waters are still found in these regions. 

The trend of increasing ANC in the Adirondack Mountains, the Upper Midwest, and the Northern 

Appalachian Plateau during the 1990s corresponds with a decrease in acid deposition in each of these 

regions (the Acid Deposition indicator, p. 2-56) and reduced air emissions of the main precursors to acid 

deposition, which are sulfur dioxide (the Sulfur Dioxide Emissions indicator, p. 2-53) and nitrogen oxides 

(the Nitrogen Oxides Emissions indicator, p. 2-23). 

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The ANC in lakes in New England and streams in the Ridge and Blue Ridge Region (east-central 

Pennsylvania, western Maryland, and western Virginia) have not risen from their 1990 levels (Exhibit 2­

36, panels C and E). Therefore, the number of water bodies classified as “chronically acidic” in these 

regions remained essentially unchanged between 1990 and 2000. 

Indicator Limitations 

• ANC sampling is limited to five regions, concentrated in the Northeast. There is no coverage 

in the Southeast, West, or much of the Midwest. These regions were chosen for sampling 

because previous research has shown that they are among the most sensitive to acid 

deposition due to the soils and other watershed characteristics. In addition, as the Acid 

Deposition indicator (p. 2-56) shows, many of these regions receive the highest rates of acid 

deposition in the U.S. For these two reasons the waters sampled are likely to be at the greatest 

risk of becoming acidified. 

• Interpreting trends for this indicator is complicated because multiple factors contribute to 

changes in ANC levels. For example, in areas where watershed soil characteristics are 

changing (e.g., decreases in concentrations of base cations in the soil), even dramatic 

reductions in acid deposition will not necessarily result in large rebounds in ANC levels. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Atmospheric Programs and are taken 

from a publication documenting how surface waters have responded to reduced air emissions of acid rain 

precursors (U.S. EPA, 2003). Trends are based on data collected in two networks: the Temporally 

Integrated Monitoring of Ecosystems (TIME) project and the Long-Term Monitoring project. Because 

both networks are operated by numerous collaborators in state agencies, academic institutions, and other 

federal agencies, the raw monitoring data are not available in a single publication or database. The trend 

data in this indicator are based on observations documented in several publications (see pages 15-17 of 

U.S. EPA, 2003). 

References 

NAPAP (National Acid Precipitation Assessment Program). 1991. Acid deposition: state of science and 

technology, volume II, aquatic processes and effects. Washington, DC. 

U.S. EPA. 2003. Response of surface water chemistry to the Clean Air Act Amendments of 1990. 

EPA/620/R-03/001. Research Triangle Park, NC. 

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1 INDICATOR: Percent of Days with Air Quality Index Values Greater Than 100 

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The Air Quality Index (AQI) provides information on pollutant concentrations of ground level ozone, 

particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Formerly known as the 

Pollutant Standard Index (PSI), the nationally uniform AQI is used by state and local agencies for 

reporting daily air quality and air quality related health advisories to the public. 

In 1999, the AQI was updated to reflect the latest science on air pollution health effects and to make it 

more appropriate for use in contemporary news media (U.S. EPA, 2003a). It also serves as a basis for 

community-based programs that encourage the public to take action to reduce air pollution on days when 

levels are projected to be of concern. The index has been adopted by many other countries (e.g., Mexico, 

Singapore, and Taiwan) to provide the public with information on air quality. 

The AQI is based on pollutant concentration data measured by the State and Local Air Monitoring 

Stations (SLAMS). The AQI is monitored in city groupings known as metropolitan statistical areas 

(MSAs) which are defined by the Office of Management and Budget. For most pollutants in the index, the 

concentration is converted into index values between 0 and 500, “normalized” so that an index value of 

100 represents the short term, health-based standard for that pollutant as established by EPA (U.S. EPA, 

1999). The higher the index value, the greater the level of air pollution and health risk. An index value of 

500 reflects a risk of imminent and substantial endangerment to public health. The level of the pollutant 

with the highest index value is reported as the AQI level for that day. An AQI value greater than 100 

means that at least one criteria pollutant has reached levels at which people in sensitive groups may 

experience health effects. A complete description of how AQI values are calculated and what they 

represent is documented in many publications (e.g., U.S. EPA, 2003b). 

This indicator is based on the percent of days across 93 large MSAs (500,000 people or more) during the 

year that recorded an AQI greater than 100 at one or more monitoring sites in the MSA. While the AQI 

indicator is calculated from ambient concentration data for criteria pollutants, this indicator’s trends 

should not be expected to mirror the trends in the other ambient concentration indicators, due to the 

differing spatial coverage of monitoring stations across the various indicators. The percent of days with 

AQI greater than 100 was calculated in two steps: (1) For each year, the total number of days with 

AQI>100 in each of the 93 MSAs was summed in order to get a national total. (2) The national total was 

then divided by the total number of days in the annual sample (365 x 93) to obtain the percentage of days 

with AQI>100 in a year. Data are presented from 1990 to 2005. However, because meteorology can 

strongly influence AQI values in a given year, the change in AQI over time is evaluated by comparing the 

3-year average observation at the beginning of the period of record (i.e., 1990-1992) to the 3-year average 

at the end (i.e., 2003-2005). Comparing 3-year averages reduces the potential for biases introduced by 

years with unique meteorological conditions. The air quality data that go into the index consist of daily 

(24-hour) measurements for PM10 and PM2.5 and continuous (1-hour) measurements for CO, NO2, ozone,

and SO2. However, of these pollutants, only four (CO, ozone, PM, and SO2) usually exhibit AQI values 

greater than 100. The data come from a subset of ambient monitoring sites that meet the trends 

requirements in Appendix B of the 2003 National Air Quality and Emissions Trends Report (U.S. EPA, 

2003a).

What the Data Show

The percent of days with AQI greater than 100 in 93 large MSAs based on all criteria pollutants decreased 

from 4.5 percent over the 1990-1992 time frame to 3.2 percent over the 2003-2005 time frame (Exhibit 2­

37, panel A). The AQI data based on all criteria pollutants are not directly comparable over this time 
frame, because PM2.5 measurements started to 

factor into the index in 1999. For this reason, the 

indicator also presents AQI trends based strictly 

on ozone and PM2.5 measurements. 

For the same subset of MSAs and time frame, the 

percent of days with AQI values greater than 100 

decreased from 4.4 percent to 2.1 percent 

(Exhibit 2-37, panel B). Before PM2.5 became 

part of the index in 1999, ozone typically 

accounted for more than 90 percent of the days 

with AQI greater than 100. 

Between 1999 and 2005, PM2.5 accounted for 

some of the days with AQI greater than 100. 

From 1999 to 2001, 2.1 percent of days with AQI 

greater than 100 occurred due to PM2.5 

concentrations, with this contribution decreasing 

to 1.4 percent from 2003 to 2005. 

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Trends in AQI between 1990 and 2003 varied across the ten EPA Regions (Exhibit 2-38). In EPA 

Regions 2, 6, and 9, the percent of total days with AQI greater than 100 averaged over 2001-2003 was 

lower than the percent of total days with AQI greater than 100 averaged over the 1990-1992 time frame. 

In the other seven EPA Regions, the percent of total days with AQI greater than 100 increased between 

these two time frames. However, as noted above, the AQI values for 1990-1992 and 2001-2003 are not 

directly comparable, because PM2.5 measurements did not factor into AQI prior to 1999. 

Indicator Limitations 

• The AQI does not address hazardous air pollutants (HAPs). 

• Air quality may vary across a single MSA. In assigning a single number for each pollutant in 

each MSA, the AQI does not reflect this potential variation. 

• The data for this indicator are limited to MSAs comprising urban and suburban areas with 

populations greater than 500,000. Thus, this indicator does not reflect MSAs smaller than 

500,000 or rural areas. 

• The AQI does not show which pollutants are causing the days with an AQI of more than 100, 

or distinguish between days >100 and days with much higher AQIs. 

• This composite AQI indicator does not show which specific MSAs, or how many MSAs, 

have problems—a specific number of days could reflect a few areas with persistent problems 

or many areas with occasional problems. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on AQI values computed from raw ambient air monitoring data for criteria pollutants found in 

EPA’s Air Quality System (U.S. EPA, 2006). Spreadsheets with the processed AQI data for the 93 MSAs 

considered in this indicator are publicly available (http://epa.gov/air/airtrends/factbook.html). This 

indicator aggregates the processed AQI data nationally and by EPA Region. 

References 

U.S. EPA. 2006. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003a. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

U.S. EPA. 2003b. Air Quality Index: a guide to air quality and your health. EPA-454/K-03-002. 

<http://www.epa.gov/airnow//aqibroch/AQI_2003_9-3.pdf> 

U.S. EPA. 1999. Air quality index reporting, 40 CFR part 58. 

<http://www.epa.gov/ttn/oarpg/t1/fr_notices/airqual.pdf> 

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1 INDICATOR: Mercury Emissions 

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Mercury is an element that occurs naturally in the environment. However, many industrial processes, such 

as coal combustion, medical and hazardous waste incineration, municipal waste combustion, gold mining, 

and certain chemical manufacturing operations have increased the amount of mercury released to the air. 

What happens to mercury after it is emitted depends on several factors: the form of mercury emitted, the 

location of the emission sources, how high above the landscape the mercury is released (e.g., the height of 

the stack), the surrounding terrain, and the weather. Depending on these factors, atmospheric mercury can 

be transported over a range of distances before it is deposited, potentially resulting in deposition on a 

local, regional, continental, or global scale. While some domestic anthropogenic mercury emissions are 

deposited within the contiguous U.S., the majority of such emissions combine with anthropogenic 

emissions from other countries and natural emissions worldwide to form a pool of mercury that circulates 

globally (Seigneur et al., 2004; U.S. EPA, 1996). 

Because it does not degrade in the environment, most mercury emitted to the atmosphere eventually 

deposits onto land or water bodies. Through a series of chemical transformations and environmental 

transport processes, airborne mercury that deposits to the Earth’s surface can eventually accumulate in the 

food web (the Lake Fish Tissue indicator, p. 3-107), most profoundly in those species near the top of the 

food web (e.g., shark, swordfish). The Blood Mercury indicator (p. 5-79) describes the human health 

effects associated with mercury exposure. 

This indicator presents mercury emissions from the following categories: 1) “Industrial processes: gold 

mining”; 2) “Industrial processes: hazardous waste incineration”; 3) “Industrial processes: electric arc 

furnaces”; 4) “Industrial processes: chlorine production”; 5) “Industrial processes: medical waste 

combustors”; 6) “Industrial processes: municipal waste combustors”; 7) “Industrial processes: other 

sources,” which includes chemical production and other miscellaneous industrial processes; 8) “Fuel 

combustion: industrial, commercial, and institutional boilers”; and 9) “Fuel combustion: utility coal 

boilers.” In order to better characterize mercury emissions, this indicator presents different source 

categories than other emissions indicators in the Report on the Environment, including separate categories 

for utility coal boilers and various industrial processes that release mercury (e.g., medical waste 

incineration, municipal waste combustion, hazardous waste incineration, gold mining). 

Mercury emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite 

of data from many different data sources, including industry and numerous state, tribal, and local 

agencies. Different data sources use different data collection methods, and many of the emissions data are 

based on estimates rather than actual measurements. For most fuel combustion sources and industrial 

processes emissions are estimated using emission factors. 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and Virgin Islands, and some of the territories of federally-recognized American 

Indian nations. Data are presented for the baseline year (1990-1993) and the latest period for which data 

are available (2002). The baseline period 1990 to 1993 represents a mix of years depending on data 

availability for various source types. While NEI data for air toxics (including mercury) were also 

compiled for 1996 and 1999, the methodology used in those years for air toxics differs considerably from 

the methodology used in 1990-1993 and 2002 and therefore cannot be compared directly to those data. 
1 What the Data Show 

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Between 1990-1993 and 2002, annual 

nationwide air emissions of mercury 

decreased from 245 tons per year to 

114 tons per year, a decrease of 54 

percent (Exhibit 2-39). The decline in 

mercury emissions is attributed 

primarily to decreased emissions from 

medical waste incinerators and 

municipal waste combustors. In 2002, 

coal-burning power plants were the 

largest anthropogenic source of 

mercury emissions to the air in the 

U.S., accounting for 44 percent of all 

domestic anthropogenic mercury 

emissions. 

Indicator Limitations 

• The emissions data in this 

indicator are primarily 

based on estimates, not 

direct measurements. Although these estimates have inherent uncertainties, the data have 

been generated using well-established estimation methods. 

• The trend shown is based on nationwide aggregate data. Regional and state trends may be 

different. 

• Not all states and local agencies provide the same data or level of detail for a given year. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw mercury emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by 

source category. 

References 

Seigneur, C., K. Jayaraghavan, K. Lohman, P. Karamchandani, and C. Scott. 2004. Global source 

attribution for mercury deposition in the United States. Environ. Sci. Technol. 38:555-569. 

U.S. EPA. 2006. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 1996. Mercury study report to Congress, volumes I to VII. EPA/452/R-96/001b. Washington, 

DC. <http://www.epa.gov/mercury/report.htm> 

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INDICATOR: Air Toxics Emissions

Toxic air pollutants, also known as air toxics or hazardous air pollutants (HAPs), are those pollutants that 

are known or suspected to cause cancer or are associated with other serious health (e.g., reproductive 

problems or birth defects) or ecological effects. Examples of air toxics 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 anthropogenic sources, including mobile 

sources (e.g., cars, trucks, construction equipment), stationary sources (e.g., factories, refineries, power 

plants), and indoor sources (e.g., building materials, cleaning solvents). Some air toxics are also released 

from natural sources such as volcanic eruptions and forest fires. Secondary formation of certain air toxics, 

such as acetaldehyde and formaldehyde, can also occur when precursor chemicals react in the 

atmosphere. The Clean Air Act identifies 188 air toxics associated with industrial sources. Twenty of 

these air toxics also are associated with mobile sources (U.S. EPA, 2003). 

People who inhale certain air toxics at sufficient concentrations may experience various health effects, 

including cancer, damage to the immune system, and neurological, reproductive (e.g., reduced fertility), 

developmental, or respiratory health problems (CDC, 2005). Air toxics also can present risks through 

other exposure pathways. For example, air toxics may deposit onto soils or surface waters, where they can 

then enter the food web and may eventually be ingested by humans. Plants and animals also may be 

harmed by exposures to air toxics (U.S. EPA, 2003). 

Air toxics emissions data are tracked by the National Emissions Inventory (NEI). The NEI is a composite 

of data from many different data sources, including industry and numerous state, tribal, and local 

agencies. Different data sources use different data collection methods, and many of the emissions data are 

based on estimates rather than actual measurements. For most fuel combustion sources and industrial 

sources, emissions are estimated using emission factors. Emissions from on-road and nonroad sources 

were estimated using EPA-approved modeling approaches (U.S. EPA, 2005). 

NEI data have been collected since 1990 and cover all 50 states and their counties, D.C., the U.S. 

territories of Puerto Rico and the Virgin Islands, and some of the territories of federally-recognized 

American Indian nations. The NEI includes baseline air toxics data for the period 1990-1993 and since 

then has been updated every 3 years. The baseline period 1990 to 1993 represents a mix of years 

depending on data availability for various source types. While NEI data for air toxics were also compiled 

for 1996 and 1999, the methodology used in those years for air toxics differed considerably from the 

methodology that was used in 2002. Therefore, the 1996 and 1999 data are not presented because 

comparing the two inventories might lead to invalid conclusions. 

This indicator first presents emissions data for all air toxics combined, both at the national level and 

broken down into the ten EPA Regions. Consistent with the other emissions indicators, the national data 

are organized into the following source categories: 1) “Stationary sources,” which include fuel 

combustion sources (coal, gas and oil-fired power plants, industrial, commercial, and institutional 

sources, as well as residential heaters and boilers) and industrial processes (chemical production, 

petroleum refining, and metals production) categories; 2) “Fires: prescribed burns and wildfires,” for 

insights on contributions from some natural sources; 3) “On-road vehicles” which include cars, trucks, 

buses, and motorcycles; and 4) “Nonroad vehicles and engines,” such as farm and construction 

equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, aircraft, and others. 

In addition to presenting emissions data aggregated across all 188 air toxics, the indicator presents 

emissions trends for five individual air toxics: acrolein, benzene, 1,3-butadiene, ethylene dibromide, and 
hydrazine. These compounds were selected for 

display because EPA’s 2002 National Air Toxics 

Assessment estimates that these air toxics present 

the greatest nationwide health risks (whether for 

cancer or non-cancer endpoints) among the subset 

of air toxics for which available emissions and 

toxicity data supported an evaluation (U.S. EPA, 

2006a). This indicator breaks the emissions data for 

these five air toxics into multiple source categories, 

with the most appropriate categories for display 

purposes differing from one air toxic to the next. 

What the Data Show 

Trends Aggregated Across All 188 Air Toxics 

According to NEI data, estimated annual emissions 

for the 188 air toxics combined decreased 29 

percent from 7.2 million tons per year in the 

baseline period of 1990-1993 to 4.5 million tons 

per year in 2002 (Exhibit 2-40). This downward 

trend resulted primarily from reduced emissions 

from stationary sources and on-road mobile 

sources. 

In 2002, air toxics emissions in the ten EPA 

Regions ranged from 153,000 tons per year in 

Region 1 to 1,150,000 tons per year in Region 4 

(Exhibit 2-41). Regional trends cannot be

quantified, because a complete set of state and 

local air toxics emissions data are not available for 

the 1990-1993 baseline period. 

Trends for Selected Air Toxics 

Exhibit 2-42 shows emissions trends for five 

compounds believed to account for the greatest 

health risks that are attributed to air toxics, 

according to a recent modeling study (U.S. EPA, 

2006a). The five plots in this exhibit show how 

emissions trends vary from compound to 

compound. Estimated emissions decreased 

between the baseline period (1990-1993) and 2002 

for all five selected air toxics: acrolein (55 percent 

decrease; see panel A), benzene (24 percent; panel 

B), 1,3-butadiene (43 percent; panel C), ethylene 

dibromide (62 percent; panel D), and hydrazine (82 

percent; panel E). 

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Indicator Limitations 

• The emissions data are largely based on estimates. Although these estimates are generated 

using well-established approaches, the estimates have inherent uncertainties. The 

methodology for estimating emissions is continually reviewed and is subject to revision. 

Trend data prior to any revisions must be considered in the context of those changes. 

• The indicator is an aggregate number that represents contributions from 188 different 

chemicals with widely varying toxicities and human exposures. Therefore, the nationwide 

trend for total air toxics and the resulting health effects likely differs from emissions trends 

for specific chemicals. Similarly, because the indicator is a nationwide aggregate statistic, the 

trend may not reflect emissions trends for specific locations. 

• Not all states and local agencies provide the same data or level of detail for a given year. 

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• There is uncertainty associated with identifying which air toxics account for the greatest 

health risk nationwide. Toxicity information is not available for every compound, and 

emissions and exposure estimates used to characterize risk have inherent uncertainties. 

Additional limitations associated with the National Air Toxics Assessment are well 

documented (U.S. EPA, 2006a). 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw air toxics emissions data in EPA’s National Emissions Inventory (NEI) (U.S. EPA, 2006b) 

(http://www.epa.gov/ttn/chief/net/2002inventory.html). This indicator aggregates the raw NEI data by 

source category, EPA Region, and selected air toxics. 

References 

CDC (Centers for Disease Control and Prevention). 2005. Third national report on human exposure to 

environmental chemicals. NCEH Pub. No. 05-0570. Accessed September 9, 2005. 

<http://www.cdc.gov/exposurereport/3rd/results_01.htm> 

U.S. EPA. 2006a. 1999 national-scale air toxics assessment. <http://www.epa.gov/ttn/atw/nata1999> 

February. 

U.S. EPA. 2006b. Data from the National Emissions Inventory. Accessed 2006. 

<http://www.epa.gov/ttn/chief/net/2002inventory.html> 

U.S. EPA. 2005. Documentation for the final 2002 mobile National Emissions Inventory. 

<ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_mobile_onroadandnonroa 

d.pdf> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/> 

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1 INDICATOR: Ambient Concentrations of Benzene 

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Benzene is an air toxic emitted from gasoline service stations, motor vehicle exhaust, and the burning of 

coal and oil. In addition to being a common air pollutant, benzene may also contaminate water. Urban 

areas generally have higher concentrations of benzene than other areas. 

People exposed to benzene 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. Plants and animals may also be harmed by 

exposures to benzene (U.S. EPA, 2003). 

Benzene is the most widely monitored air toxic. Data from the National Air Toxics Trends Sites 

(NATTS) network is expected to provide trends information for other air toxics pollutants in the next 

Report on the Environment. 

This indicator reflects ambient concentrations in micrograms per cubic meter (μg/m3

) of benzene from

1994 to 2004, based on the annual arithmetic average. This indicator displays trends averaged over 35 

urban monitoring sites that have consistent data for the period of record in Photochemical Assessment 

Monitoring Stations (PAMS), Urban Air Toxics Monitoring (UATMP) Stations, and Non-Methane 

Organic Compound (NMOC) Monitoring Stations. 

What the Data Show

Benzene concentrations declined 61 percent from 

1994 to 2004 (Exhibit 2-43). 

Indicator Limitations

• Benzene data represent only 35 urban 

sites in the U.S.

• Because of the limited number of sites 

that are primarily located in urban 

areas, Exhibit 2-43 does not 

necessarily represent an overall 

national trend in benzene

concentrations.

• Benzene, while an important air toxic, 

represents only one of many air toxics 

that may occur in air. 

Data Sources

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

based on raw benzene ambient air monitoring data in EPA’s Air Quality System (AQS) (U.S. EPA, 2006) 

(http://www.epa.gov/ttn/airs/airsaqs/). National trends in this indicator are based on the subset of benzene 

monitoring stations that have sufficient data to assess trends since 1994. 
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U.S. EPA. 2006. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2003. National air quality and emissions trends report—2003 special studies edition. 

EPA/454/R-03/005. Research Triangle Park, NC. <http://www.epa.gov/air/airtrends/aqtrnd03/>

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1 INDICATOR: Concentrations of Ozone-Depleting Substances 

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Ozone, a gas present throughout the Earth’s atmosphere, is a pollutant at the Earth’s surface but forms a 

protective layer in the stratosphere, helping shield the Earth from the sun’s ultraviolet (UV) radiation. 

Exposure to UV-rays is associated with skin cancer, cataracts, and other human health and ecological 

problems (U.S. EPA, 1996). 

Starting in the late 1970s, stratospheric ozone levels were observed to be declining due to worldwide 

releases of various human-produced chemicals referred to as ozone-depleting substances (ODS), 

particularly halocarbons such as the long-lived chlorofluorocarbons (CFCs), bromine-containing halons, 

and methyl bromide. Through rapid catalytic reactions with ozone, the chlorine and bromine from these 

chemicals have depleted the protective ozone layer (the Ozone Levels over North America indicator, p. 2­

79).

Worldwide production and consumption of ODS is being progressively eliminated under the provisions of 

the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. Over time, reducing the 

atmospheric loading of ODS is expected to result in global increases in stratospheric ozone. However, 

because some ODS gases have long atmospheric lifetimes, and because of pre-phaseout ODS stockpiling 

for post-phaseout use, ambient concentrations of ODS have only recently begun to stabilize and in some 

cases begun to decline. While some gases, like methyl chloroform, decay quickly in the atmosphere, other 

gases, like CFCs and halons, have atmospheric lifetimes on the order of hundreds or thousands of years. 

Measures of effective equivalent troposphere chlorine (EECl) and effective equivalent stratospheric 

chlorine (EESC) are commonly used to represent atmospheric concentrations of ODS. Both represent 

ODS concentrations weighted by their potential to catalyze the destruction of stratospheric ozone relative 

to the ability of chlorine to do so. EECl is the equivalent effective chlorine in the troposphere. (EESC is 

typically derived by adding a 3-year time lag to EECl to account for the time it takes for emissions of 

ODS at the Earth’s surface to migrate to the stratosphere and cause stratospheric ozone depletion.) 

This indicator presents trends in concentrations of tropospheric ODS as EECl. The EECl trend is based on 

measurements from the National Oceanic and Atmospheric Association (NOAA) Climate Monitoring and 

Diagnostics Laboratory and estimates of halocarbon emissions from industrial and international sources 

from 1995 to 2004. Concentrations of EECl are presented as weighted averages based on ground-based 

measurements of mixing ratios5

 since 1995 at the following remote locations: Alert, North West 

Territories, Canada; Barrow, Alaska; Niwot Ridge, Colorado; Mauna Loa, Hawaii; American Samoa; 

Cape Grim, Tasmania, Australia; and the South Pole (NOAA CMDL, 2003). Data on total EECl are also 

available for 1992 and 1994, but these years’ of monitoring are only presented in the chemical-specific 

graphs because the monitoring did not include methyl bromide, a quantitatively important ODS. Because 

most ODS have long atmospheric half-lives, the ODS concentrations shown in this indicator reflect past 

and recent contributions from emissions sources within the U.S. and worldwide. 

5

 The mixing ratio is the ratio of the partial pressure of a gas to the total atmospheric pressure. 
What the Data Show 

Total effective equivalent chlorine (EECl) resulting 

from ODS emissions reached its peak concentration in 

the mid-1990s at slightly over 2,700 parts per trillion 

of air by volume (pptv) and has slowly declined by 

approximately 11 percent since then (Exhibit 2-44). 

Although concentrations of CFCs and several other 

individual ODS compounds have begun to decline, 

concentrations of halons and 

hydrochlorofluorocarbons (HCFCs) have not yet 

stabilized. 

Declines in EECl abundances of several ODSs in the 

troposphere between 1992 and 2005 have contributed 

to the decline in total EECl (Exhibit 2-45). EECl 

attributed to methyl chloroform has decreased more 

than 80 percent over this period due to decreased 

emissions as well as its short atmospheric lifetime. 

EECl associated with CFCs has decreased more 

slowly—more than 3 percent from its peak 

tropospheric concentration in 1995-1996. The slow 

decay of CFCs is a result of continued emissions of 

CFCs from stockpiles in developed countries, 

continued use in developing countries, and their 

longer atmospheric lifetimes. EECl from methyl 

bromide (CH3Br) has decreased nearly 20 percent 

from its peak in 1998; however, continued use of 

methyl bromide in developing countries and in 

developed countries through critical use exemptions 

slows the decrease in EECl associated with this 

compound. EECl from methyl bromide exhibits 

seasonal variations, which likely results from the 

seasonal use of this chemical as a soil fumigant. 

Despite declines in concentrations of some 

tropospheric ODS, others, including halons and 

HCFCs, continue to increase (Exhibit 2-45). EECl 

estimated from halon emissions has increased by more 

than 50 percent from 1992 to 2005, and EECl 

attributed to HCFCs in 2005 is more than 2.5 times 

higher than that from 1992. These trends reflect 

continued emissions of these ODS from stockpiles in 

developed countries and continued production and 

consumption in developing countries (and developed 

countries for HCFCs), as well as the longer 

atmospheric lifetimes of halons. 

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• The calculation of EECl depends on the understanding of the interactions and atmospheric 

residence times of many different gases; incorrect knowledge about these factors could affect 

trends in the EECl. 

• EECl is calculated by weighing individual ODS concentrations by the substances’ abilities to 

catalyze destruction of stratospheric ozone, or the ozone destruction potential. The ozone 

destruction potentials used to transform the data have inherent uncertainties, which can affect 

the trend analyses. 

• Factors additional to trends in halocarbons affect trends in stratospheric ozone, including 

changes in climate (e.g., temperature, winds), changes in emissions and concentrations of 

trace gases like nitrous oxide and methane, and changes in aerosol loading such as occurs 

after an explosive volcanic eruption. 

Data Sources 

Tropospheric concentrations of ODSs presented in this indicator are based on measurements made by 

NOAA’s Global Monitoring Division and summarized at an online data repository (NOAA, 2006) 

(ftp://ftp.cmdl.noaa.gov/hats/Total_Cl_Br/). The trend in this indicator was developed from a 2006 data 

file available from the repository, which updates tropospheric ODS concentrations previously reported in 

the peer-reviewed literature (Montzka et al. 1999, 2003). 

References 

Montzka, S.A., J.H. Butler, B.D. Hall, D.J. Mondeel, and J.W. Elkins. 2003. A decline in tropospheric 

organic bromine. Geophys. Res.Lett. 30(15):1826. 

Montzka, S.A., J.H. Butler, J.W. Elkins, T.M. Thompson, A.D. Clarke, and L.T. Lock. 1999. Present and 

future trends in the atmospheric burden of ozone-depleting halogens. Nature 398(6729):690-694. 

NOAA. 2006. Online repository of global tropospheric mixing ratios of ozone-depleting gases. Accessed 

2006. <ftp://ftp.cmdl.noaa.gov/hats/Total_Cl_Br/> 

NOAA CMDL (National Oceanographic and Atmospheric Administration, Climate Monitoring and 

Diagnostics Laboratory). 2003. Summary report no. 27. Boulder, CO. 

<http://www.cmdl.noaa.gov/publications/annrpt27/> 

U.S. EPA. 1996. Air quality criteria for ozone and related photochemical oxidants. EPA/600/P-93/004FcF. Research Triangle Park, NC. 

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1 INDICATOR: Ozone Levels over North America

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Ozone (O3) is a gas present throughout the Earth’s atmosphere; 90 percent resides in the stratosphere, the 

layer of the atmosphere that starts about 6 to 9 miles above the Earth’s surface at mid-latitudes, and the 

rest is located in the troposphere, the atmospheric layer that lies between the stratosphere and the Earth’s 

surface. The environmental and human health implications of ground-level ozone are very different from 

those of ozone higher in the atmosphere, leading to the maxim: “Good up high, bad nearby” (U.S. EPA, 

2003). In the troposphere, ozone poses both health and ecological risks, but the natural layer of ozone in 

the stratosphere shields and protects the Earth’s surface from the sun’s harmful ultraviolet (UV) rays 

which can in turn lead to more cases of skin cancer, cataracts, and other health problems (U.S. EPA, 

1996).

Increases in surface UV radiation have been associated with reductions in total column ozone levels based 

on spectral measurements at a number of sites in Europe, North America, South America, Antarctica, and 

New Zealand (Kerr and McElroy, 1993; Booth and Madronich, 1994; WMO et al., 2003). For example, 

measurements between 1989 and 1993 over Toronto indicated that for every 1 percent decrease in total 

column ozone, after accounting for seasonal and daily variables not related to ozone, there was a 

corresponding increase between 1.1 percent and 1.3 percent in erythemally active UV-B radiation (Kerr 

and McElroy, 1993). 

Ozone in the stratosphere is constantly being produced naturally from dissociation of oxygen molecules 

by highly energetic UV solar radiation. While this ozone is being transported poleward and downward 

through the natural motions of air in the stratosphere, it also is being naturally destroyed through catalytic 

reactions involving primarily nitrogen and hydrogen oxides. 

Releases of various human-produced chemicals, such as the long-lived chlorofluorocarbons, brominecontaining halons, and methyl bromide (the Concentrations of Ozone-Depleting Substances indicator, p. 

2-76) have depleted the levels of protective stratospheric ozone starting in the late 1970s, particularly at 

mid- to high latitudes. The U.S. has been a major contributor to the global emissions of these halocarbons, 

accounting for about a quarter of total worldwide emissions before the major ozone-depleting substances 

(ODS) were banned in the 1990s. It takes about 3 years for emissions of ODS at the Earth’s surface to 

migrate to the stratosphere and cause stratospheric ozone depletion (WMO et al., 2003). 

This indicator tracks trends in the deviation from pre-1980 levels in total annually-averaged ozone values 

integrated over the 35° to 60°N latitude belt (the latitudes roughly corresponding to North America) from 

1965 to 2005. The estimates are based on data from several different sources including ground-based and 

satellite measurements. The data on total ozone from ground-based measurements are from a network of 

surface stations, which are equipped with spectrophotometers. These instruments measure how thick the 

ozone layer would be if compressed in the Earth’s atmosphere (at sea level and at 0°C), where one 

Dobson Unit (DU) is defined to be 0.01 mm thickness at standard temperature and pressure. Reliable data 

from regular measurements at these stations are available extending back to the 1960s, although 

geographical coverage is limited before the 1970s (Fioletov et al., 2002; WMO et al., 2003). 

Near-continuous global total ozone data are available from satellite measurements beginning in 1979. The 

first set of satellite data are obtained from the Total Ozone Mapping Spectrometer (TOMS). The TOMS 

instrument measures total ozone using the “backscattered albedo,” which is the ratio of the radiance 

backscattered from the Earth’s atmosphere to the extraterrestrial solar irradiance. The second dataset 

consists of total ozone measurements from the Solar Backscatter Ultraviolet (SBUV) spectrometer 

(Fioletov et al., 2002; WMO et al., 2003). 
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In addition to the ground-based and satellite data, two merged datasets are presented for this indicator. 

The Goddard merged ozone dataset and the National Institute of Water and Atmospheric Research 

(NIWA) ozone dataset were derived using measurements from a number of satellite instruments (Fioletov 

et al., 2002; WMO et al., 2003). 

What the Data Show 

There was little ozone change (beyond natural 

variations such as those resulting from the 11-year 

solar sunspot cycle) before the late 1970s, but 

decreases in stratospheric ozone began to occur after 

1979 (Exhibit 2-46). The ground-based data and 

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have similar ozone variations, with differences 

typically less than 0.5 percent. The mid-latitude 

decline of approximately 6 percent between 1979 

and 1995 is in general agreement with previous 

profile trend estimates from satellite and groundbased records. For the mid-latitudes of the Northern 

Hemisphere, the average of the total ozone levels for 

the 3-year period from 1998 to 2001 is about 3 

percent lower than the average for the 3-year period 

from 1977 to 1980, and average total ozone levels 

have not changed considerably between 2001 and 

2005. While this indicator covers the entire 35° to 

60°N latitude belt, ozone varies little by longitude 

and the estimated 3 percent change in total ozone 

levels can be taken to apply to North America. 

This 3 percent change over North America is very similar to the statistically significant globally-averaged 

decrease in total ozone over the 1979 to 2001 period (WMO et al., 2003). The decrease in the midlatitudes of the Southern Hemisphere, by contrast, has been twice as high, associated with the springtime 

“ozone hole” over Antarctica. The trends in this indicator are consistent with well understood seasonal 

variations in ozone, and with natural variations such as those due to the 11-year solar cycle and the effects 

of volcanic eruptions, suggesting that the long-term trends are those resulting from the emissions of ODS. 

Indicator Limitations 

• Fioletov et al. (2002) used estimates of ozone changes from several different, independent 

sources to derive the figure used for this indicator. Differences in the calibration of 

instruments used to obtain the ground-based and satellite datasets together with interruptions 

in the observational records produce datasets with measurement errors typically around a few 

percent (WMO et al., 2003). The figure presented does, however, show good overall 

agreement among the different data sources for changes in total ozone. 

Data Sources 

Summary data for this indicator were provided by the World Meteorological Organization (WMO). The 

1965-2003 data in this indicator are taken from WMO’s 2002 Scientific Assessment of Ozone Depletion 

(WMO et al. 2003), which presents ozone data based on multiple sets of measurements (e.g., Fioletov et 

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al., 2002). The 2004-2005 data in this indicator were not publicly available at the time this report went to 

press, but they will be published in WMO’s upcoming 2006 Scientific Assessment of Ozone Depletion. 

References 

Booth, R.C., and S. Madronich. 1994. Radiation amplification factors—improved formulation accounts 

for large increases in ultraviolet radiation associated with Antarctic ozone depletion. In: Weiler, C.S., and 

P.A. Penhale, eds. Ultraviolet radiation and biological research in Antarctica. Antarctic Research Series. 

Washington, DC: American Geophysical Union. pp. 39-42. 

Fioletov, V.E., G.E. Bodeker, J.B. Kerr, A.J. Miller, R.D. McPeters, and R. Stolarski. 2002. The global 

ozone and zonal total ozone variations estimated from ground-based and satellite measurements: 1978­

2000. J. Geophys. Res. 107(D22). 

Kerr, J.B., and C.T. McElroy. 1993. Evidence for large upward trends of ultraviolet-B radiation linked to 

ozone depletion. Science 262:1032-1034. 

U.S. EPA. 2003. Ozone: good up high, bad nearby. EPA/451/K-03/001. Washington, DC. 

U.S. EPA. 1996. Air quality criteria for ozone and related photochemical oxidants. EPA/600/P-93/004FcF. Research Triangle Park, NC. 

WMO (World Meteorological Organization), et al. 2003. Scientific assessment of ozone depletion: 2002. 

Geneva, Switzerland. 

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INDICATOR: Ozone and Particulate Matter Concentrations for U.S. Counties in 

the U.S./Mexico Border Region

The border between the U.S. and Mexico spans approximately 2,000 miles, from the Pacific Ocean to the 

Gulf of Mexico. The area is subjected to a unique blend of increased industrial development (especially 

on the Mexico side of the border), intense pressures because of the shifting and growing population 

related to this development, and an arid climate that can exacerbate many air quality problems. Ozone and 

particulate matter are air pollutants of particular concern. Rapid population growth in urban areas of the 

(U.S./Mexico) border has resulted in unplanned development, greater demand for land and energy, traffic 

congestion, increased waste generation, overburdened or unavailable waste treatment and disposal 

facilities, increased frequency of chemical emergencies, and had had an adverse impact on air quality 

(U.S. EPA, 2003).

Ground-level ozone is harmful to both human health and the environment (the Ozone Concentrations 

indicator, p. 2-32). Although some industrial sources release ozone directly into the environment, most 

ground-level ozone forms from chemical reactions involving nitrogen oxides (NOx), volatile organic 

compounds (VOCs), and sunlight. Ozone levels are typically highest during the afternoon hours of the 

summer months, when the influence of direct sunlight is the greatest (U.S. EPA, 2006a). 

Particulate matter (PM) is the general term used for a mixture of solid particles and liquid droplets found 

in the air. Primary PM is released directly from emissions sources into the atmosphere, while secondary 

PM is formed in the air from reactions involving precursor chemicals (e.g., nitrogen oxides, sulfur 

dioxide, particle-producing organic gases). Ambient air monitoring stations measure air concentrations of 

two size ranges of particles: PM2.5 (fine particles with aerodynamic diameter less than or equal to 2.5 

micrometers (μm)) and PM10 (both fine particles (PM2.5) and coarse particles with aerodynamic diameters 

between 2.5 and 10 μm). Exposure to coarse particles can aggravate respiratory conditions such as 

asthma, and exposure to fine particles is associated with various additional human health effects (the PM 

Concentrations indicator, p. 2-44) (U.S. EPA, 2004). 

This indicator shows trends in the design values for ozone and particulate matter in the U.S. counties at 

the U.S./Mexico border area in comparison to U.S. national trends. These trends are shown for the longest 

duration of time supported by the underlying monitoring data. The ozone design value is defined as a 3­

year average of the fourth highest daily maximum in each year. The design value for PM10 is the 3-year 

average of the second maximum 24-hour concentrations, and the design value for PM2.5 is the 3-year 

average of the seasonally-weighted annual mean concentration. This indicator establishes a baseline for 

measuring against future air quality levels and is based on all monitoring stations that operated on the 

U.S. side of the border during this time period. 

In EPA Region 6, ozone monitoring data from border locations were collected in Dona Ana County in 

New Mexico, and El Paso, Brewster, Webb, Hidalgo, and Cameron Counties in Texas. In EPA Region 9, 

ozone monitoring data from border locations were collected in the Counties of Cochise, Pima, and Yuma 

in Arizona and Imperial and San Diego in California. PM10 sampling data for EPA Region 6 are from 

Cameron, Hidalgo, Webb and El Paso Counties in Texas and Dona Ana, Luna, and Grant Counties in 

New Mexico. PM2.5 data were available for all of the above counties except for Luna County, New 

Mexico. For EPA Region 9, PM10 monitoring data were collected in the Counties of Cochise, Pima, Santa 

Cruz, and Yuma in Arizona and Imperial and San Diego in California. For EPA Region 9, PM2.5 

monitoring data were collected in the Counties of Cochise, Pima, and Santa Cruz in Arizona and Imperial 

and San Diego in California. 

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What the Data Show 

Trends for 8-Hour Ozone Concentrations 

In EPA Region 6, average border ozone concentrations decreased by 11 percent between the 1986-1988 

and 1992-1994 time periods and by 3 percent between the 1993-1995 and 2002-2004 periods, which was 

similar to the national decreases of 9 percent and 3 percent, respectively (Exhibit 2-47, panel A). In EPA 

Region 9, however, border ozone concentrations increased by 2 percent between the 1986-1988 and 1992­

1994 time periods but decreased by 11 percent between the 1993-1995 and 2002-2004 periods. 

Trends for 24-Hour PM10 Concentrations 

In EPA Region 6, the average second maximum 24-hour PM10 concentrations at border monitoring sites 

varied considerably from year to year, most likely due to variation in meteorological conditions (e.g., 

rainfall, wind speed) and soil erosion (Exhibit 2-47, panel B). In EPA Region 9, on the other hand, 

corresponding PM10 concentrations at border monitoring sites did not exhibit such strong temporal 

variations, and the average second maximum 24-hour concentration at border monitoring sites for the 

2002-2004 time frame was only slightly lower than that for the 1990-1992 time frame. 

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Trends for Annual Average PM2.5 Concentrations 

Mean annual ambient PM2.5 concentrations increased by 16 percent in the border counties of EPA Region 

6 between 1999-2001 and 2002-2004, but decreased by 16 percent in the border counties of EPA Region 

9 (Exhibit 2-47, panel C). Mean annual ambient PM2.5 concentrations decreased 9 percent nationwide 

over the same period. 

Indicator Limitations 

• Many counties along the U.S./Mexico border do not have ambient air quality monitors; these 

counties are not characterized by this indicator. 

• This indicator does not include data from the Mexican side of the border. When the technical 

review concludes the quality of this data is appropriate for the intended use, the indicator will 

be updated. 

• Short-term trends in PM10 concentrations are often highly dependent on meteorological 

conditions. The maximum concentration for a given site can be influenced by wind-blown 

dust and will exhibit considerable variations from day to day. Trends over the longer term are 

far less likely to be influenced by unusual meteorological conditions. 

• The long-term ozone trends are derived from an increasing number of monitors over the 

course of time from 1986-2004, but an analysis of the limited number of border sites that 

have full periods of record show that the slopes of the trends are similar to those in this 

indicator. 

• Mean air pollutant concentrations may mask higher values in some areas along the border and 

in the nation. 

• Because most of the monitoring sites are located in urban areas, the trends might not 

accurately reflect conditions outside the immediate urban monitoring areas. 

Data Sources 

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, 

Region 6, and Region 9. These summaries were based on raw ozone and PM ambient air monitoring data 

in EPA’s Air Quality System (U.S. EPA, 2006b) (http://www.epa.gov/ttn/airs/airsaqs/). Trends in this 

indicator are based on the subset of ozone and PM monitoring stations located in counties along the 

U.S./Mexico border that have sufficient data to assess trends over the period of record. 

References 

U.S. EPA. 2006a. Air quality criteria for ozone and related photochemical oxidants. EPA/600/R05/004aF-cF. Research Triangle Park, NC. 

U.S. EPA. 2006b. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2004. Air quality criteria for particulate matter (October 2004). EPA 600/P-99/002aF-bF. 

Research Triangle Park, NC. 

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U.S. EPA. 2003. Border 2012: U.S.-Mexico environmental program. EPA/160/R-03/001. Washington, 

DC. 

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INDICATOR: Ambient Concentrations of Manganese Compounds in EPA Region 

5

Manganese is a naturally occurring metal that is ubiquitous in the environment. Exposure to low levels of 

manganese in the diet is considered to be nutritionally essential for people and animals (ATSDR, 1997). 

However, exposures to elevated concentrations of manganese are harmful to human health and have been 

associated with subtle neurological effects, such as slowed eye-hand coordination. Manganese 

compounds are hazardous air pollutants emitted by iron and steel production plants, power plants, coke 

ovens, and many smaller metal processing facilities. Manganese also may be contributed in border 

communities by vehicles using Canadian fuel with the additive methylcyclopentadienyl manganese 

tricarbonyl (MMT). 

Although manganese compounds are air pollutants of concern nationwide, they are of special concern in 

EPA Region 5. The 1999 National Emissions Inventory showed that Region 5 had the highest manganese 

emissions of all EPA Regions, contributing 36.6 percent of all manganese compounds emitted nationwide 

(U.S. EPA, 2005a). Emissions from industrial sources in Region 5 occurred from various facilities, such 

as those that manufacture steel or process iron ores and alloys for steelmaking. Between 1988 and 2003, 

manganese emissions from point sources declined both nationally (26.2 percent) and in EPA Region 5 

(36.7 percent). Year-to-year variability in manganese emissions is high, however, and recent emissions 

data (1996-2003) suggest a weaker trend: emissions dropped 7.6 percent and 12.4 percent nationwide and 

in EPA Region 5, respectively (U.S. EPA, 2005b). 

EPA’s National-Scale Air Toxics Assessment (NATA) is intended to provide a better understanding of 

the health risks resulting from inhalation exposure to air toxics. Based on 1999 emission inventories, the 

most recent NATA results (U.S. EPA, 2006a) identify manganese compounds as the largest contributor to 

neurological non-cancer health risk in the U.S. Modeled estimates of ambient manganese compounds in 

all 3,222 U.S. counties show that among the 50 counties with the highest concentrations nationwide, 20 

are located in EPA Region 5. 

This indicator presents ambient concentrations of manganese compounds measured as total suspended 

particulates (TSP) by direct monitoring. This indicator addresses manganese in the TSP fraction (not 

PM10 or PM2.5) because it is the most complete dataset in EPA Region 5 in terms of geographic and 

temporal coverage. TSP metals data have been commonly used in human health risk assessments. EPA 

recently has begun to recommend PM10 as the most appropriate fraction for evaluating people’s exposure 

to toxic metals (U.S. EPA, 2002), but PM10 metals data are sparse at this time, both nationally and in EPA 

Region 5. Data from a limited number of sites in AQS with collocated PM10 and TSP speciation monitors 

suggest that the proportion of manganese in PM10 versus TSP is about 50 percent at most sites and can be 

as high as 75 percent. TSP manganese data therefore should be considered a conservative estimate of 

PM10 manganese exposures. PM2.5 metals data are plentiful since the establishment of the Speciation 

Trends Network (STN) in 2000, but this size fraction is believed to underestimate human exposures. 

Data were considered for 53 monitoring sites in EPA Region 5 that had a complete year of data reported 

to the Air Quality System (AQS) national database in 2004. Average manganese concentrations were 

calculated for each monitoring site. A concentration trend was determined using a subset of 21 of the 

monitoring sites with four or more complete years of data between 2000 and 2004. As annual average 

concentrations are representative of long-term inhalation exposures, the ambient monitoring data are 

displayed in comparison with the manganese Reference Concentration (RfC). The RfC is an estimate of a 

chronic inhalation exposure that is likely to be without appreciable risk of adverse non-cancer effects 

during a lifetime. The RfC for manganese is 0.05 micrograms per cubic meter (ug/m3

), based on

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impairment of neurobehavioral function in people. At exposures increasingly greater than the RfC, the 

potential for harmful effects increases (ATSDR, 1997; U.S. EPA, 1999). Monitoring sites were classified 

into different categories based on land use as defined in AQS. 

What the Data Show 

In 2004 the median average annual ambient concentrations of manganese as TSP in EPA Region 5 were: 

0.035 μg/m3

 at the 16 residential sites, 0.036 μg/m3 at the 14 sites in commercial or high-traffic areas, and 

0.049 μg/m3

 at the 19 industrial sites (Exhibit 2-48). The median average annual ambient concentration of 

manganese at 4 predominantly agricultural and forest sites in EPA Region 5 was 0.02 μg/m3

, but this is 

not depicted in the figure due to the limited number of monitoring sites to characterize a distribution. 

Greater concentration differences were observed in the 90th percentile values, with average concentrations 

ranging from 0.10 μg/m3

 at the residential sites to 0.33 μg/m3 at the predominantly industrial sites. 

Eighteen of the 53 sites had average manganese concentrations higher than the RfC; 10 of these sites were 

categorized as industrial, 4 commercial or mobile, and 4 residential. 

The median annual manganese concentration averaged across 21 trend sites showed a 15 percent decline 

between 2000 and 2004 (Exhibit 2-49). Additional years of data will be needed to confirm this apparent 

trend. The trend sites had the following land use designations: commercial and mobile (6 sites), industrial 

(9 sites), residential (6 sites), and agricultural and forest (no sites). 

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Indicator Limitations 

• AQS data represent several sites per state, but do not have full geographic or temporal 

coverage. Some emissions “hotspots” are included, while others may exist that have not been 

monitored. 

• The land use categories are only generally indicative of the area represented by an ambient air 

monitor. For example, a site categorized as “industrial” may adjoin a densely populated 

community where many residents are exposed to ambient pollution. 

Data Sources 

Summary data in this indicator were provided by EPA Region 5, based on raw ambient air monitoring 

data for manganese compounds reported in EPA’s Air Quality System (U.S. EPA, 2006b) 

(http://www.epa.gov/ttn/airs/airsaqs/). Trends in this indicator are based on the subset of monitoring 

stations located in EPA Region 5 that have sufficient manganese concentration data to assess trends over 

the period of record. 

References 

ASTDR (Agency for Toxic Substances and Disease Registry). 1997. Toxicological profile for manganese 

(update). Draft for public comment. Atlanta, GA: U.S. Department of Health and Human Services. 

U.S. EPA. 2006a. 1999 national-scale air toxics assessment. <http://www.epa.gov/ttn/atw/nata1999> 

February. 

U.S. EPA. 2006b. Data from the Air Quality System. Accessed 2006. 

<http://www.epa.gov/ttn/airs/airsaqs/> 

U.S. EPA. 2005a. 1999 National Emissions Inventory data. <http://www.epa.gov/air/data> August. 

U.S. EPA. 2005b. 1999 Toxics Release Inventory data. <http://www.epa.gov/tri> December. 

U.S. EPA. 2002. Quality assurance guidance document—model quality assurance project plan for the 

national air toxics trends stations. Washington, DC. 

U.S. EPA. 1999. Integrated Risk Information System (IRIS) on manganese. Washington, DC. 

<http://www.epa.gov/iris/subst/0373.htm> 

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2.2.3 Discussion 

What These Indicators Say About Trends in Outdoor Air Quality and Their 

Effects on Human Health and the Environment 

Criteria Pollutants and Their Precursors 

Because of statutory monitoring and reporting requirements, criteria pollutants have some of the most 

extensive data available to support National Indicators for emissions and ambient air concentrations. 

Nationwide, air emissions of every criteria pollutant (or their corresponding precursors) have decreased 

between 1990 and 2002—the period of record covered by the National Emissions Inventory. During that 

time frame, substantial decreases in air emissions were observed for carbon monoxide, nitrogen oxides, 

particulate matter, sulfur dioxide, and volatile organic compounds. Even more pronounced emissions 

reductions occurred for lead, but this decrease extends back to the 1970s. With few exceptions, downward 

trends in criteria pollutant emissions were observed in the 10 EPA Regions, similar to the corresponding 

national trends. 

Consistent with the emissions trends, every criteria pollutant showed decreasing ambient air 

concentrations based on aggregate measurements from the nation’s ambient air monitoring system, which 

measures levels of air pollution primarily in urban and suburban areas. The magnitude of air quality 

improvements, observed both nationally and in all 10 EPA Regions, varies across pollutants. Carbon 

monoxide, lead, and nitrogen dioxide concentrations decreased considerably between 1980 and the 

present, and ambient concentrations of these three pollutants in most or all areas in the U.S. are now 

below the corresponding air quality standards that protect human health and the environment. In contrast, 

air quality improvements for ozone and particulate matter (particularly PM2.5) were less pronounced; and, 

based on monitoring data collected in 2005 and reported in EPA’s Air Quality System, ambient 

concentrations in 68 metropolitan statistical areas where approximately 128 million people live did not 

meet the health-based NAAQS for ozone, particulate matter, or both pollutants. In short, every criteria 

pollutant has showed improving air quality over the past one or two decades, but the progress has been 

slowest for the two pollutants—ozone and PM2.5—most influenced by meteorology and secondary 

formation processes. 

The nationwide trends and those presented for the 10 EPA Regions are based on aggregate statistics 

across numerous monitoring stations and may not reflect air quality trends at finer scales or for different 

subsets of monitoring stations. For example, the significant downward trend in ozone in EPA Region 9 is 

largely influenced by air quality improvements in Los Angeles and other metropolitan areas in southern 

California. In other urban areas in EPA Region 9, ozone improvements have been more modest or even 

different directionally.

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monitoring stations near the border between U.S. and Mexico (the Ozone and PM Concentrations Along 

U.S./Mexico Border indicator, p. 2-82), even though the national trend for this pollutant is downward. 

The ROE indicators on criteria pollutants’ environmental effects are limited to three issues. First, longterm monitoring data show that wet deposition of acidic sulfates and nitrates decreased between 1989 and 

2004, consistent with the decreased emissions for sulfur dioxide and nitrogen oxides over roughly the 

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NC. 

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same time frame. As a result of the decreased acid deposition, many surface waters throughout the Upper 

Midwest, the Adirondack Mountains, and the Northern Appalachian region have begun to recover from 

past acidification (the Lake and Stream Acidity indicator, p. 2-62). Second, data on ozone injury to forest 

plants are sufficient for establishing national and regional baseline conditions against which future data 

can be compared. These baseline conditions show considerable variation in ozone damage across EPA 

Regions (the Ozone Injury to Forest Plants indicator, p. 2-37). Third, visibility in protected areas (e.g., 

National Parks, Wilderness Areas) has remained relatively unchanged between 1992 and 2004 (the 

Regional Haze indicator, p. 2-50). 

Overall, for criteria pollutants, the ROE indicators provide fairly complete information on outdoor air 

quality trends, but limited insights on associated health and environmental effects. As expected, emissions 

trends are generally consistent with trends observed among corresponding ambient concentrations and, 

where data are available, effects. 

Air Toxics and Other Air Pollutants 

Between 1990 and 2002, nationwide emissions aggregated across 188 air toxics (hazardous air pollutants) 

decreased (the Air Toxics Emissions indicator, p. 2-70), and decreased emissions were also observed for 

two air toxics of particular interest: benzene and mercury (the Mercury Emissions indicator, p. 2-68). 

However, sufficiently complete and consistent monitoring data currently cannot support ROE indicators 

for ambient concentration of air toxics, with two exceptions. First, ambient air concentrations of benzene 

at 35 monitoring sites across the nation decreased 61 percent between 1994 and 2004—a decrease 

reasonably consistent with corresponding emissions reductions. Second, ambient air concentrations of 

manganese compounds measured at 21 monitoring sites in EPA Region 5 decreased between 2000 and 

2004, but the period of record evaluated may be too short to consider this decrease an actual air quality 

trend. 

Stratospheric Ozone Issues 

Since 1990, the U.S. phased out most production and import of ozone-depleting substances. 

Consequently, consumption of ozone-depleting substances in the U.S. decreased during this last decade,7 

along with globally representative ambient air concentrations of ozone-depleting substances in the lower 

atmosphere (the Concentrations of Ozone-Depleting Substances indicator, p. 2-76). While such decreases 

are expected to help restore the stratospheric ozone layer, stratospheric ozone levels over North America 

actually decreased slightly in the last decade (the Ozone Levels over North America indicator, p. 2-79) 

due to various factors, including ongoing use of ozone-depleting substances worldwide and the fact that 

ozone-depleting substances are extremely long-lived in the atmosphere. 

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Limitations, Gaps, and Challenges8 

The 23 ROE indicators in this section characterize trends for numerous important outdoor air quality 

issues, but also have notable limitations. All emissions indicators, for instance, are partly based on 

estimates. Although these estimates have inherent uncertainties, the emissions inventory data are believed 

to be of high quality and are periodically updated to remain consistent with the current scientific 

understanding of emissions from different source categories. The main limitation of the ambient 

concentration indicators is the monitoring sites’ limited spatial coverage. These indicators are composites 

of air quality measurements taken across the country, but primarily in populated areas and may not totally 

characterize trends for rural settings. While the national trends for criteria pollutants and benzene are 

toward improved air quality, ambient concentrations for these pollutants can vary greatly on a local scale. 

In some areas, such as those experiencing rapid population growth or near newly constructed point 

sources, ambient air concentrations of selected pollutants are increasing, contrary to the national trends; 

conversely, ambient air concentration in other parts of the country are decreasing more rapidly than the 

national trends depict. 

Though the emissions and ambient concentration indicators are reasonably complete for the criteria 

pollutants, gaps in nationally representative indicators remain for most air toxics and other air pollutants. 

However, a large number of these air toxics and other air pollutants are released by a small number of 

sources nationwide, and these pollutants’ emissions and ambient concentrations are more appropriately 

tracked at the local level, rather than with National Indicators. Another gap in National Indicators is for 

air toxics and other air pollutants that are ubiquitous in the nation’s outdoor air (e.g., mobile source air 

toxics). Although nationwide trends in air toxics concentrations have been estimated with models, 

nationally representative ambient air monitoring data on air toxics would provide EPA a more direct 

measure of important outdoor air quality trends. Many local-scale monitoring networks have tracked 

trends for some of these pollutants, but nationwide indicators could not be developed for pollutants other 

than benzene due to limited spatial coverage of monitoring sites, use of differing sampling and analytical 

methods over the years, inconsistent application of quality assurance and quality control practices, and 

other factors. 

ROE indicators for ambient concentrations of some common air toxics are expected to be developed in 

coming years, based on measurements currently being collected at National Air Toxics Trends Stations. 

This network of monitoring sites is specifically designed to characterize long-term trends in several air 

toxics believed to account for the greatest health risks nationwide.9

 Additionally, ongoing operation of the 

Mercury Deposition Network (part of the National Atmospheric Deposition Program) is gathering data to 

support trends analysis on atmospheric deposition of mercury—an issue of particular significance when 

evaluating contamination levels in fish and shellfish. 

National-level exposure and effects indicators can help EPA better characterize nationwide trends in 

outdoor air quality and their effects, but key challenges complicate efforts to develop these. For example, 

ambient concentration data do not quantify exposures, because ambient air monitoring equipment 

8 While the ROE indicators provide valuable information about trends in outdoor air quality, the indicators are more 

limited in their ability to describe trends in associated effects on human health and the environment. As described in 

Chapter 1, it is difficult to establish causal relationships between specific stressors and outcomes. In the case of 

outdoor air, there are few “effects” indicators with clear causal linkages. 

9

 U.S. EPA. 2004. National monitoring strategy: air toxics component. Final draft. July. 

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measures air quality at fixed outdoor locations, while people breathe air in multiple indoor and outdoor 

settings during a typical day. Actual human exposure to air pollution can be measured through use of 

personal monitoring devices, which sample the air that people breathe as they move through different 

microenvironments. Some researchers have used such devices to quantify exposures to specific pollutants 

in some locations.10 However, conducting such studies on a national scale over an extended time frame 

would be an extremely resource-intensive task. Consequently, no nationally representative studies 

currently support ROE indicators that characterize exposure to outdoor air pollutants. Another gap 

pertaining to effects attributed to outdoor air quality is that the scientific understanding of how all air 

pollutants, whether acting alone or in combination, can affect human health and the environment is 

incomplete and continues to evolve. 

While the indicators document what is currently known about selected outdoor air quality issues, ongoing 

scientific research continues to broaden the knowledge base on many important topics, ranging from 

designing innovative emission control technologies to enhancing atmospheric fate and transport modeling 

to developing metrics that better connect air quality to public health and ecological outcomes. 

10 Jantunen, M., O. Hanninen, K. Koistinen, and J.H. Hashim. 2002. PM measurements: personal and indoor air 

monitoring. Chemosphere 49:993-1007. 

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2.3 WHAT ARE THE TRENDS IN GREENHOUSE GAS EMISSIONS AND 

CONCENTRATIONS?

2.3.1 Introduction 

Greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and certain synthetic chemicals, trap 

some of the Earth’s outgoing energy, thus retaining heat in the atmosphere.11 Changes in the radiative 

balance of the Earth—the balance between energy received from the sun and emitted from Earth—as a 

result of this heat trapping alter weather patterns and climates at global and regional scales.

12 Natural 

factors, such as variations in the sun’s output, volcanic activity, the Earth’s orbit, the carbon cycle, and 

others also affect the radiative balance.13 However, increasing concentrations of greenhouse gases due to 

human activity are likely affecting various aspects of climate, such as surface air temperature and 

subsurface ocean temperature; while these and other changes observed over the past few decades are 

likely mostly due to human activities, scientists cannot rule out the possibility that some significant part 

of these changes is also a reflection of natural variability.14 Human health, agriculture, water resources, 

forests, wildlife, and coastal areas all are vulnerable to climate change.15 The purpose of this section is to 

evaluate long-term trends in air emissions and ambient concentrations of greenhouse gases that are 

contributing to climate change, but not to evaluate the effects that these emissions and concentrations 

cause.16 

Though the focus of this question is on greenhouse gases, related factors can also alter the Earth’s 

climate. Certain radiatively important substances, like black carbon (soot), are technically not greenhouse 

gases due to their physical state, but they nonetheless affect the flow of energy through the atmosphere. 

Some of these substances, such as sulfate aerosols, have negative radiative forcings that can lead to 

cooling effects. Another related factor is albedo (the reflectivity of the Earth’s surface), which affects the 

portions of absorbed and outgoing energy. Natural and human factors can affect albedo on a global scale 

through changes in large-scale features like the polar ice caps or on a local or regional scale (e.g., by 

increased amounts of dark paved surfaces that absorb energy). Although this question does not address 

radiatively important substances that are not greenhouse gases or non-chemical factors like albedo, these 

11 National Research Council, Radiative Forcing of Climate Change: Expanding the Concept and Addressing 

Uncertainties, March 2005. p. 1, 9, vii, and others. 

12 Op. cit. p. 11. 

13 Op cit. p. 13. 

14 National Research Council, Climate Change Science: An Analysis of Some Key Questions, 2001. p. 1. 

15 NRC op. cit., p. 4 and p. 19-20. 

16 In a general sense, climate change is conceptually connected to every other theme in this report. The broadest 

discussion of potential effects associated with greenhouse gases in this report is in Section 6.5, which discusses 

critical physical and chemical attributes of ecosystems, including ROE indicators that track changes in air 

temperature, precipitation, sea-surface temperature, and sea level—all of which affect ecosystems. 

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influences are also important to understanding the planet’s energy balance and the ways human activities 

may affect that balance.17 

Some greenhouse gases are emitted exclusively from human activities (e.g., synthetic halocarbons). 

Others occur naturally but are found at elevated levels due to human inputs (e.g., carbon dioxide). The 

anthropogenic sources result from energy-related activities (e.g., combustion of fossil fuels in the 

electrical utility and transportation sectors), agriculture, land-use change, waste management and 

treatment activities, and various industrial processes. Major greenhouse gases and emission sources 

include: 

• Carbon dioxide (CO2), which occurs naturally as part of the global carbon cycle, but human 

activities have increased atmospheric loadings through combustion of fossil fuels and other 

emissions sources.18 Natural sinks that remove CO2 from the atmosphere (e.g., oceans, plants) 

help regulate CO2 concentrations, but human activities can disturb these processes (e.g., 

deforestation) or enhance them. 

• Methane (CH4), which comes from many sources, including human activities such as coal 

mining, natural gas distribution, waste decomposition in landfills, and digestive processes in 

livestock and agriculture.19 Natural sources include wetlands and termite mounds. 

• Nitrous oxide (N2O), which is emitted during agricultural and industrial activities, as well as 

during combustion of solid waste and fossil fuels. 

• Various synthetic chemicals, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), 

sulfur hexafluoride (SF6), and other synthetic gases, which are released as a result of 

commercial, industrial, or household uses. 

• Many other gases that are known to trap heat in the atmosphere. Examples include water 

vapor, which occurs naturally as part of the global water cycle, and ozone, which occurs 

naturally in the stratosphere and is found in the troposphere largely due to human activities. 

Each gas has a different ability to absorb heat in the atmosphere, due to differences in its atmospheric 

half-life and the amount and type of energy that it absorbs. For example, it would take thousands of 

molecules of CO2 to equal the warming effect of a single molecule of sulfur hexafluoride—the most 

potent greenhouse gas in terms of ability to absorb heat evaluated by the Intergovernmental Panel on 

Climate Change (IPCC).20 To facilitate comparisons between gases that have substantially different 

17 Detailed information on these related factors may be found in various scientific publications, such as those 

prepared by the Intergovernmental Panel on Climate Change, a panel formed by the Worldwide Meteorological 

Organization to compile and synthesize the growing body of scientific literature on climate change. 

18 U.S. EPA. 2006. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2004. EPA/430/R-06/002. 

Washington, DC. 

19 National Research Council. 2001. Climate change science: an analysis of some key questions. Washington, DC: 

National Academy Press. 

20 Intergovernmental Panel on Climate Change. 2001. Climate change 2001: the scientific basis (third assessment 

report). Cambridge, UK: Cambridge University Press. 

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properties, IPCC has developed a set of scaling factors called “global warming potentials” (GWPs), as 

discussed further in the indicator write-ups. 

2.3.2 ROE Indicators 

To characterize trends in greenhouse gases, this chapter presents two indicators—one describing 

emissions from U.S. sources and the other describing concentrations (Table 2.3.1). 

The U.S. Greenhouse Gas Emissions indicator covers the period 1990-2003, with data from EPA’s 

Inventory of U.S. Greenhouse Gas Emissions and Sinks. This inventory is a database that tracks both 

greenhouse gas emissions directly attributable to human activities and greenhouse gas sinks (e.g., 

sequestration of carbon in forests). The indicator stratifies emissions into trends for different gases and 

source categories. Emissions are weighted by “global warming potentials” to facilitate comparison among 

the gases. 

The Atmospheric Concentrations of Greenhouse Gases indicator summarizes both direct measurements of 

ambient air concentrations from the last half-century and observations for earlier time frames based on 

chemical analyses of air bubbles found in ice core samples. The gases in these bubbles represent the 

outdoor air that was trapped in ice at the time the ice was formed. Combined, these two measurements 

provide extensive historical coverage for the atmospheric concentrations of greenhouse gases. 

Because many greenhouse gases are extremely long-lived in the atmosphere, with some remaining 

airborne for tens to hundreds of years after being released, these long-lived greenhouse gases become 

globally mixed in the atmosphere and their concentrations reflect past and recent contributions from 

emissions sources worldwide. This context is an important backdrop for the two greenhouse gas 

indicators in this section: increasing atmospheric concentrations of greenhouse gases is a global issue, 

resulting from emissions from sources in the U.S. combined with emissions from sources in other 

countries. 

Table 2.3.1. ROE Indicators of Trends in Greenhouse Gas Emissions and Concentrations 

NATIONAL INDICATORS LOCATION 

U.S. Greenhouse Gas Emissions 2.3.2 – p. 2-96 

Atmospheric Concentrations of Greenhouse Gases 2.3.2 – p. 2-100 

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INDICATOR: U.S. Greenhouse Gas Emissions 

The Earth’s climate is determined by the balance between energy received from the sun and emitted back 

to space from the Earth and its atmosphere. Certain gases in the atmosphere, such as carbon dioxide 

(CO2), methane (CH4) and nitrous oxide (N2O), water vapor and others, trap some of the outgoing energy, 

retaining heat in the Earth’s atmosphere. These are the so-called “greenhouse gases” (GHG). The best 

understood greenhouse gases emitted by human activities are CO2, CH4, N2O, and certain fluorinated 

compounds. 

Changes in GHG emissions are influenced by many long-term factors, including population and economic 

growth, land use, energy prices, technological changes, and inter-annual temperatures. On an annual 

basis, consumption of fossil fuels, which accounts for most GHG emissions in the U.S., generally 

fluctuates in response to changes in general economic conditions, energy prices, weather, and the 

availability of non-fossil alternatives (U.S. EPA, 2005). 

This indicator uses data and analysis from the Inventory of U.S. Greenhouse Gas Emissions and Sinks 

(U.S. EPA, 2005), an assessment of the anthropogenic sources and sinks of greenhouse gas emissions for 

the U.S. and its Territories for the period 1990 through 2003. The inventory constitutes estimates derived 

from direct measurements, aggregated national statistics, and validated models in most source categories. 

An extensive discussion of the methods for each source type and gas and the uncertainties inherent in the 

calculations is available in EPA (2005) and its Annex 7. 

The indicator is expressed in terms of CO2 equivalents, meaning that emissions of different gases are 

weighted by their “global warming potential” (GWP). A GWP is a measure of how much a given mass of 

GHG is estimated to contribute to radiative forcing that contributes to global warming over a selected 

period of time, compared to the same mass of carbon dioxide, for which the GWP is 1.0. EPA is 

mandated to use the GWPs documented in the Intergovernmental Panel on Climate Change’s Second 

Assessment Report (IPCC, 1996) and these characterize GWP for a 100 year time horizon—the effect of 

the gas on radiative forcing over 100 years. Annex 6 of the U.S. GHG Inventory includes extensive 

information on GWPs and how they relate to emissions estimates (U.S. EPA, 2005). 

This indicator focuses on the six types of compounds currently covered by agreements under the United 

Nations Framework Convention on Climate Change. These compounds are CO2, CH4, N2O,

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). This indicator does 

not include emissions estimates for substances such as chlorofluorocarbons (CFCs), methyl bromide, 

sulfates, black carbon, and organic carbon. These substances are excluded primarily because either their 

emissions have not been quantified in the U.S. GHG Inventory or they have different types of effects on 

climate than those of the six GHG included in the U.S. inventory and, therefore, most cannot be compared 

directly to the GHG. Combined, these excluded substances may account for a considerable portion of 

climate change, but their omission cannot be scientifically quantified in comparable terms. 

This indicator presents emissions data in units of teragrams of CO2 equivalents (Tg CO2 Eq). These units 

are conventionally used in GHG inventories prepared worldwide. For reference, one teragram (Tg) is 

equal to one million metric tons. 
What the Data Show 

In 2003, total U.S. greenhouse gas emissions were 6,900 Tg CO2 Eq, up 13 percent from 1990 (Exhibit 2­

50). CO2 is the primary greenhouse gas emitted by human activities, representing approximately 85 

percent of greenhouse gas emissions in 2003. From 1990 to 2003, total emissions of CO2 increased by 

832 Tg CO2 Eq (17 percent). Methane (CH4) represents the second largest U.S. GHG emission, 

accounting for 8 percent of net emissions in 2003. Methane emissions declined about 10 percent from 

1990 to 2003, due largely to reduced emissions from landfills and coal mining operations. The primary 

sources of methane emissions include decomposition of wastes in landfills, coal mine seepage, natural gas 

systems, and enteric fermentation in domestic livestock. Nitrous oxide (N2O) constituted about 5 percent 

of net U.S. GHG emissions in 2003; these emissions declined by about 1 percent from 1990 to 2003. The 

main anthropogenic activities producing N2O are agricultural soil management, fuel combustion in motor 

vehicles, manure management, nitric acid production, human sewage, and stationary fuel combustion. 

Emissions of HFCs, PFCs, and SF6 accounted for the remaining 2 percent of GHG emissions in 2003, and 

the aggregate weighted emissions of this group of gases rose by 46 Tg CO2 Eq (50 percent) since 1990. 

Despite being emitted in smaller quantities relative to the other principal greenhouse gases, emissions of 

HFCs, PFCs, and SF6 are important because many of them have extremely high global warming 

potentials and, in the cases of PFCs and SF6, atmospheric lifetimes of 700 to 50,000 years. The emissions 

in this indicator are a continuation of the trend of increasing GHG emissions observed over many 

decades, with total CO2-equivalent emissions increasing by about one fifth since 1970 (U.S. EPA, 2005; 

RIVM/TNO, 2003). CO2 has constituted a slightly growing portion, while CH4 has been a declining 

component of the total. 

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Looking at GHG emissions by source shows that energy use (i.e., fuel combustion and gas leakage) has 

accounted for about 84 percent of net U.S. emissions since 1990 (Exhibit 2-51). Emissions due to energy 

use have increased 16 percent between 1990 and 2003. Agriculture is the second largest source of GHG 

emissions, accounting for 6 percent of the total in 2003. U.S. GHG emissions are partly offset by uptake 

of carbon and “sequestration” in forests, trees in urban areas, agricultural soils, and landfilled yard 

trimmings and food scraps. In aggregate, these removals of CO2 from the atmosphere offset about 12 

percent of U.S. emissions in 2003 (Exhibit 2-51). 

With one-twentieth of the world’s population (U.S. Bureau of the Census, 2006), the U.S. currently emits 

about one-fifth of global greenhouse gases, CO2, CH4, N2O, HFCs, PFCs, and SF6 (Baumert et al., 2005). 

Indicator Limitations 

• This indicator does not yet include emissions of greenhouse gases or other radiatively 

important substances that are not explicitly covered by the United Nations Framework 

Convention on Climate Change and its subsidiary protocol. Thus, it excludes such gases as 

those controlled by the Montreal Protocol and its Amendments, including CFCs and 

hydrochlorofluorocarbons (HCFCs). Although the U.S. reports the emissions of these 

substances as part of the U.S. GHG inventory (see Annex 6.2 of the U.S. GHG Inventory), 

the origin of the estimates is fundamentally different from those of the other GHG and 

therefore cannot be compared directly with the other emissions discussed in this indicator. 

• This indicator does not include aerosols and other emissions that do affect radiative forcing 

and that are not well-mixed in the atmosphere, such as sulfate, ammonia, black carbon and 

organic carbon. Emissions of these compounds are highly uncertain and have qualitatively 

different effects than the six types of emissions in this indicator. 

• This indicator does not include emissions of other compounds such as CO, NOx, nonmethane 

volatile organic compounds (VOCs), and substances that deplete the stratospheric ozone 

layer, which indirectly affect the Earth’s radiative balance, for example, by altering 

greenhouse gas concentrations, changing the reflectivity of clouds, or changing the 

distribution of heat fluxes. 

• The U.S. GHG Inventory does not account for “natural” emissions of greenhouse gases, such 

as from wetlands, tundra soils, termites, and volcanoes. These excluded sources are discussed 

in Annex 5 of the U.S. GHG Inventory (U.S. EPA, 2005). The U.S. GHG Inventory does 

include, in its Land Use and Land Use Change and Forestry categories, emissions from 

changes in the forest inventory due to fires, harvesting, and other activities, and from 

agricultural soils. 

Data Sources 

The data used for this indicator were published in EPA’s inventory of greenhouse gas emissions and sinks 

for years 1990-2003 (U.S. EPA, 2005). Specifically, emissions by GHG shown in Exhibit 2-50 are taken 

from Table ES-2 of that reference, and emissions by industrial sector are taken from Table ES-4. 

References 

Baumert, K., T. Herzog, and J. Pershing. 2005. Navigating the numbers: greenhouse gas data and 

international climate policy. Washington DC: World Resources Institute. 

<http://pdf.wri.org/navigating_numbers_chapter2.pdf> 

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IPCC (Intergovenmental Panel on Climate Change). 1996. Climate change 1995: the science of climate 

change. Cambridge, United Kingdom: Cambridge University Press. 

RIVM/TNO. 2003. Emission database for global atmospheric research. Version 3.2. 2003. (Olivier, 

J.G.J., and J.J.M. Berdowski. 2001. Global emission sources and sinks.) <http://www.mnp.nl/edgar/> 

U.S. Bureau of the Census. 2006. World POPClock; USPOPClock. 

<http://www.census.gov/ipc/www/popclockworld.html> 

U.S. EPA. 2005. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2003. 

<http://www.epa.gov/climatechange/emissions/usgginv_archive.html> 

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INDICATOR: Atmospheric Concentrations of Greenhouse Gases 

The Earth’s temperature depends mainly on the amount of energy received from the sun, the portion 

reflected back into space, and the extent to which the atmosphere retains heat. Natural forces (e.g., 

volcanoes, changes in the Earth’s orbit) and human activities (e.g., emissions of so-called “greenhouse 

gases,” land use change) affect the amount of energy held in the Earth-atmosphere system and therefore 

affect the Earth’s climate. Human activities in all countries have altered the chemical composition of the 

atmosphere by the emissions and accumulation in the atmosphere of greenhouse gases. The primary gases 

that retain heat in the atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and certain 

manufactured gases such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and sulfur 

hexafluoride (SF6).

Once emitted, gases remain in the atmosphere for varying amounts of time. Very “short-lived” 

compounds, such as particulate matter, may remain airborne on average for only hours or days. Methane 

also has a relatively short average lifetime, though much longer than PM, remaining in the atmosphere for 

roughly 12 years. The half-life of CO2 emissions is roughly 100 years (5 to 200 years: IPCC, 2001), but 

about a quarter of emissions today will still be in the atmosphere after hundreds of years and about onetenth for hundreds of thousands of years (Archer, 2005; Archer et al., 1998). Finally, many of the 

synthetic gases such as halocarbons are extremely long-lived, remaining in the atmospheric for hundreds 

or even tens of thousands of years. When emissions—from the U.S. (the U.S. Greenhouse Gas Emissions 

indicator, p. 2-96) as well as other countries—remain in the atmosphere over long periods, they 

accumulate and are measured as atmospheric concentrations. U.S. GHG emissions from 1890 to 2000 are 

estimated to have contributed about one-fifth of the increase in global GHG concentrations (den Elzen et 

al., 2005). 

This indicator shows trends in the accumulation of the following principal greenhouse gases in the 

atmosphere: CO2, CH4, N2O, and selected halocarbons. Recent data are from global networks that monitor 

the concentrations of these gases in the atmosphere. Geologic data result from gas measurements made of 

air trapped in ice cores at the time the ice was formed. Because the gases shown in this indicator remain 

in the atmosphere for long periods, they are well-mixed, so that measurements at individual locations are 

globally representative. This indicator summarizes greenhouse gas concentration measurements reported 

in a collection of studies published in the peer-reviewed literature. In order to provide the most extensive 

temporal coverage, this indicator aggregates comparable, high quality data from individual studies that 

each focused on different time frames. None of the data in this indicator is based on modeled 

concentrations.

What the Data Show

Exhibits 2-52 through 2-54 show the evolution of concentrations of three principal greenhouse gases in 

the atmosphere over three intervals: geologic time (hundreds of thousands of years), the past 11,000 

years, and in recent decades. The exhibits represent data sets covering a wide range of latitudes, showing 

some latitudinal differences in concentrations, but also showing a high level of consistency—indicating 

that the gases are well-mixed and that the sampling can be considered spatially representative. The graphs 

show patterns of large cycles of concentrations over geologic time, but also increases in concentrations 

since the Industrial Revolution that exceed concentrations over the past hundreds of thousands of years. 

The concentration of CO2 has varied considerably over geological time (Exhibit 2-52). Over the past 

420,000 years, CO2 concentrations have cycled over several thousand year periods from highs around 

285-300 ppm to lows around 180-185 ppm. From at least 900 A.D. to 1800 A.D., CO2 concentrations
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stayed relatively constant at about 270-290 ppm (panel B). Over the past 150 years, CO2 concentrations 

have increased by 31 percent; from 1958 to 2002, they have increased by about 18 percent (plus or minus 

1 percent) to 374 ppm (panels B and C). 

Methane (CH4) concentrations also cycled widely over the past 420,000 years but peaks remained below 

800 ppb until after 1800 A.D. (Exhibit 2-53). Essentially no increase occurred from 1000 A.D. to 1730 

A.D. (panel B). It then took approximately 175 years (c. 1905) to add 200 ppb to atmospheric CH4 

concentrations, 40 years (c. 1945) to add the next 200 ppb, 20 years to add the next 200 ppb (c. 1965), and 

10 years (c. 1975) to add the next 200 ppb (panels B and C). The rates of methane increase began to slow 

by the late 1970s, with less than 200 ppb added to atmospheric concentrations between 1978 and the 

present (panel C). Overall, global methane concentrations more than doubled in the past 150 years. The 

most recent data show that there is a significant difference in methane concentrations across latitudes, 

with a pattern of peak concentrations in the most northern latitudes decreasing toward the southern 

latitudes, which suggests net sources of methane in northern latitudes. Yet, despite the latitudinal 

differences in concentrations, the pattern over the past two centuries shows a common trend in all 

locations. 

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Nitrous oxide (N2O) concentrations (Exhibit 2-54) vacillated widely through geologic time, with ice 

sample measurements ranging from as low as 180 ppb to above 280 ppb. Despite considerable interdecadal variability, N2O stayed mostly below 280 ppb from 1756 A.D. until the 1920s (panel B), from 

which point levels began to rise rapidly to approximately 318 ppb in 2004 (panels B and C), the highest 

level ever recorded. 

Concentrations of the halocarbons (or gases that contain the halogens chlorine, fluorine, bromine, or 

iodine) were essentially zero a few decades ago, but have increased rapidly as they were incorporated into 

industrial products and processes (Exhibit 2-55). Concentrations of hydrochlorofluorocarbons HCFC141b and HCFC-142b increased sharply through 2005 (panel A), but are expected to gradually stabilize 

over this decade as they are phased out in industrialized countries as part of the Montreal Protocol on 

Substances the Deplete the Ozone Layer. The concentration of HFC-23, which is a by-product of HCFC22 production, has increased more than five-fold between 1978 and 2005 (panel B). HFCs that are not 

controlled by the Protocol (because they do not contribute to stratospheric ozone losses) mostly continue 

to increase because of their widespread use as substitutes for the Montreal Protocol gases. 

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Indicator Limitations 

• Water vapor is not tracked in this indicator as it is generally accepted that human activities 

have not increased the concentration of water vapor in the atmosphere. 

• Some radiatively important atmospheric constituents that are substantially affected by human 

activities such as tropospheric ozone, black carbon, aerosols, and sulfates are not included in 

this indicator because of their spatial and temporal variability and the inadequacy of available 

data to characterize long-term averages or trends. 

• Concentration data are not available for all the halogenated compounds that are potentially 

important greenhouse gases. For instance, global concentration data are not sufficient yet to 

track trends in concentrations of SF6 and PFCs. 

• Ice core measurements are not taken in real time, which introduces some error into the date of 

the sample. Dating accuracy for the ice cores ranged up to +20 years (often less), depending 

on the method used and the time period of the sample. Diffusion of gases from the samples, 

which would tend to reduce the measured values, may also add a small amount of 

uncertainty. More information on the accuracy of measurements of ice samples and other 

measurement methods can be found at: 

http://cdiac.esd.ornl.gov/by_new/bysubjec.html#atmospheric. 

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Data Sources 

The data in this indicator come from multiple 

sources. Summary global atmospheric concentration 

data for CO2 (Exhibit 2-52), CH4 (Exhibit 2-53), and 

N2O (Exhibit 2-54) were provided by EPA’s Office 

of Atmospheric Programs, based on greenhouse gas 

concentration measurements reported in a collection 

of studies published in the peer-reviewed literature. 

Citations for these studies and links to the 

corresponding underlying data can be found online 

at: 

http://www.epa.gov/climatechange/science/recentac 

_majorghg.html. 

Summary global atmospheric concentration data for 

selected halocarbons (Exhibit 2-55) were provided 

by the Australia Commonwealth Scientific and 

Industrial Research Organisation (CSIRO). The 

1975-2001 data in this exhibit are taken from 

IPCC’s third assessment report on climate change 

(IPCC, 2001). The 2002-2005 data in this exhibit 

were not publicly available at the time this report 

went to press, but they will be published in IPCC’s 

upcoming fourth assessment report on climate 

change. 

References 

Exhibits 2-52, 2-53, and 2-54 refer to 16 data 

sources. Complete citations for these references and 

links to the corresponding underlying data can be 

found online at: 

http://www.epa.gov/climatechange/science/recentac 

_majorghg.html. 

Archer, D., and A. Ganopolski. 2005. A movable 

trigger: fossil fuel CO2 and the onset of the next 

glaciation. Geochem Geophys Geosys 6(Q05003) 

<http://geosci.uchicago.edu/~archer/reprints/archer. 

ms.next_ice_age.pdf> 

Archer, D., H. Kheshgi, and E. Maier-Reimer. 1998. Dynamics of fossil fuel neutralization by Marine 

CaCO3. Global Biogeochem Cycles 12:259-276. 

<http://geosci.uchicago.edu/~archer/reprints/gbc98/neutral_gbc.pdf> 

den Elzen, M., J. Fuglestvedt, N. Höhne, C. Trudinger, J. Lowe, B. Matthewso, B. Romstadv, C. Pires de 

Campos, and N. Andronova. 2005. Analysing countries’ contribution to climate change: scientific and 

policy-related choices. Env Sci percent Policy June 05. 

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IPCC (Intergovernmental Panel on Climate Change). 2001. Climate change 2001: the scientific basis 

(third assessment report). Cambridge, UK: Cambridge University Press. 

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2.3.3 Discussion 

What These Indicators Say About Trends in Greenhouse Gas Emissions 

and Concentrations 

The nation’s estimated combined emissions of several greenhouse gases that are directly attributable to 

human activity have increased 13 percent between 1990 and 2003 (the U.S. Greenhouse Gas Emissions 

indicator, p. 2-96). Emissions sources occur from several sectors of the U.S. economy, with the highest 

contribution—and the greatest recent growth—attributed to energy use, which primarily includes 

electricity generation and transportation sources. This indicator also compares contributions of different 

greenhouse gases by normalizing for each gas’s ability to affect the Earth’s energy balance. The results 

show that CO2 makes up the bulk of the nation’s anthropogenic greenhouse gas emissions. Both 

observations demonstrate that fossil fuel combustion is clearly the country’s major source of 

anthropogenic greenhouse gas emissions. 

Data on atmospheric concentrations of greenhouse gases have extraordinary temporal coverage (the 

Greenhouse Gas Concentrations indicator, p. 2-100). For CO2, methane, and N2O, concentration data span 

several hundred thousand years; and for selected halocarbons, concentration data span virtually the entire 

time frame that these synthetic gases were widely used. Thus, these concentration data provide an 

excellent basis for answering the question regarding trends in greenhouse gas concentrations. The 

historical data for CO2, methane, and N2O show considerable temporal variability in these gases’ 

concentrations; however, concentrations observed in the past 50 years are higher than those over the 

entire period of record evaluated—even when considering natural fluctuations. In short, the historical 

context provided by ice cores shows that present concentrations of these three greenhouse gases are 

unprecedented over the last 420,000 years, and strongly suggest that the recently increasing levels mostly 

reflect the influence of human activity. For the various halocarbons considered, concentrations have 

increased between 1980 and 2000, with some increases spanning more than two orders of magnitude, but 

the rate at which these concentrations is increasing has slowed in recent years. 

Taken together, the well-documented long-term trends in concentrations of greenhouse gases, along with 

corresponding increases in emissions from anthropogenic sources, show that human activity is causing 

increased concentrations of greenhouse gases in the Earth’s atmosphere—a finding echoed in many 

prominent reviews on the science of climate change.21, 22 

Limitations, Gaps, and Challenges 

Although they provide extensive insights into greenhouse gas emissions and concentrations, the two 

greenhouse gas indicators have limitations and gaps that should be acknowledged. The emissions trends, 

for instance, are based largely on estimates, which have uncertainties inherent in the engineering 

calculations and estimation methodologies developed for the U.S. greenhouse gas emissions inventory. 

Uncertainty of the magnitude of the emissions vary among the gases and sources, though estimated 

emissions from some of the largest sources (e.g., CO2 emissions from fossil fuel combustion) are 

21 National Research Council. 2001. Climate change science: an analysis of some key questions. Washington, DC: 

National Academy Press. 

22 Intergovernmental Panel on Climate Change. 2001. Climate change 2001: the scientific basis (third assessment 

report). Cambridge, United Kingdom: Cambridge University Press. 

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considered highly accurate.23 One gap in the emissions indicator is that EPA’s GHG inventory does not 

track every greenhouse gas or every emissions source. Examples of greenhouse gases not included in the 

inventory are ozone and selected chlorofluorcarbons. The most notable sources not tracked in the 

inventory are natural sources, such as CH4 from wetlands, CO2 and CH4 from thawing permafrost, and 

multiple emissions from volcanoes. Although worldwide emissions of greenhouse gases from natural 

sources actually far outweigh inputs attributed to human origin,24 the incremental increases of 

anthropogenic emissions are environmentally noteworthy because they upset the balance of natural 

sources and sinks of greenhouse gases. Though not necessarily a limitation or a gap, it is important to 

note that EPA’s Greenhouse Gas Inventory, by design, tracks only this nation’s anthropogenic emissions 

of greenhouse gases. For perspective on how the nation’s emissions compare to those from other 

countries, recent data estimate that the U.S. emits approximately 20 percent of the total worldwide 

amounts of selected greenhouse gases.25 Having national emissions indicators on a more complete set of 

greenhouse gases and emissions sources would further improve EPA’s ability to track pressures that 

affect climate change. 

The Greenhouse Gas Concentrations indicator (p. 2-100) tracks trends in measured airborne levels of 

greenhouse gases regardless of the anthropogenic or natural sources that released them, which helps 

account for some of the inherent limitations and uncertainties in the emissions indicator. However, the 

concentration data have limitations and gaps of their own. Historical concentrations from ice core samples 

are not measured in real time, which introduces some minor uncertainty into the data set; consistency 

among measurements made by multiple laboratories at different locations suggests this uncertainty is 

relatively low.

26 A gap in the concentration data, as with the emissions data, is that not all greenhouse 

gases have been monitored. Long-term trend data for ozone, for instance, are currently not available. 

Measuring globally representative trends in tropospheric ozone concentrations presents technical 

challenges, because ozone is a short-lived gas (which does not lend well to ice core measurements) with 

concentrations that exhibit tremendous spatial variations (which would require extensive monitoring to 

characterize worldwide trends). Another gap is the lack of ROE indicators for radiatively important 

substances, such as soot and aerosols. Though these substances technically are not greenhouse gases, 

tracking trends in these substances’ concentrations is important due to their ability to alter the Earth’s 

energy balance. 

23 U.S. EPA. 2003. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2003. EPA/430/R-05/003. 

Washington, DC. 

24 U.S. EPA. 2000. Global warming and our changing climate: answers to frequently asked questions. EPA/430/F00/011. Washington, DC. 

25 den Elzen, M., J. Fuglestvedt, N. Höhne, C. Trudinger, J. Lowe, B. Matthews, B. Romstad, C. Pires de Campos, 

and N. Andronova. 2005. Analysing countries’ contribution to climate change: scientific and policy-related choices. 

Env. Sci. Policy 8(6):614-636. 

26 Barnola, J., D. Raynaud, C. Lorius, and N.I. Barkov. 2003. Historical CO2 record from the Vostok ice core. In: 

Trends: a compendium of data on global change. Oak Ridge, TN: U.S. Department of Energy. 

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2.4 WHAT ARE THE TRENDS IN INDOOR AIR QUALITY AND THEIR EFFECTS 

ON HUMAN HEALTH? 

2.4.1 Introduction 

Indoor air quality refers to the quality of the air in a home, school, office, or other building environment. 

Most pollutants affecting indoor air quality come from sources inside buildings, although some originate 

outdoors. Typical pollutants of concern include combustion products such as carbon monoxide, 

particulate matter, and environmental tobacco smoke; substances of natural origin such as radon; 

biological agents such as molds; pesticides; lead; asbestos; ozone (from some air cleaners); and various 

volatile organic compounds (VOCs) from a variety of products and materials. Indoor concentrations of 

some pollutants have increased in recent decades due to such factors as energy-efficient building 

construction and increased use of synthetic building materials, furnishings, personal care products, 

pesticides, and household cleaners. 

The potential impact of indoor air quality on human health nationally is considerable, for several reasons. 

Americans, on average, spend approximately 90 percent of their time indoors,27 where the concentrations 

of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.28 Moreover, people 

who are often most susceptible to the adverse effects of pollution (e.g., the very young, older adults, and 

people with cardiovascular or respiratory disease) tend to spend even more time indoors.29 Health effects 

that have been associated with indoor air pollutants include irritation of the eyes, nose, and throat; 

headaches, dizziness, and fatigue; respiratory diseases; heart disease; and cancer. 

Indoor air pollutants originate from many sources. These sources can be classified into two general 

categories: 

• Indoor sources (sources within buildings themselves). Combustion sources in indoor 

settings, including tobacco, heating and cooking appliances, and fireplaces, can release 

harmful combustion by-products such as carbon monoxide and particulate matter directly into 

the indoor environment. Cleaning supplies, paints, insecticides, and other commonly used 

products introduce many different chemicals, including volatile organic compounds, directly 

into the indoor air. Building materials are also potential sources, whether through degrading 

materials (e.g., asbestos fibers released from building insulation) or from new materials (e.g., 

chemical off-gassing from pressed wood products). Other substances in indoor air are of 

natural origin, such as mold and pet dander. 

27 U.S. EPA. 1989. Report to Congress on indoor air quality: volume 2. EPA/400/1-89/001C. Washington, DC. 

28 U.S. EPA. 1987. The total exposure assessment methodology (TEAM) study: summary and analysis. EPA/600/6­

87/002a. Washington, DC. 

29 U.S. EPA. 1997. Exposure factors handbook: volume 3—activity factors. EPA/600/P-95/002Fa. Washington, DC. 

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• Outdoor sources. Outdoor air pollutants can enter buildings through open doors, open 

windows, ventilation systems, and cracks in structures. Some pollutants come indoors 

through building foundations. For instance, radon forms in the ground as naturally occurring 

uranium in rocks and soils decays. The radon can then enter buildings through cracks or gaps 

in structures. In areas with contaminated ground water or soils, volatile chemicals can enter 

buildings through this same process. Finally, when people enter buildings, they can 

inadvertently bring in soils and dusts on their shoes and clothing from the outdoors, along 

with pollutants that adhere to those particles. 

In addition to pollutant sources, the air exchange rate with the outdoors is an important factor in 

determining indoor air pollutant concentrations. The air exchange rate is affected by the design, 

construction, and operating parameters of buildings and is ultimately a function of infiltration (air that 

flows into structures through openings, joints, and cracks in walls, floors, and ceilings and around 

windows and doors), natural ventilation (air that flows through opened windows and doors), and 

mechanical ventilation (air that is forced indoors or vented outdoors by ventilation devices, such as fans 

or air handling systems). Outdoor climate and weather conditions combined with occupant behavior can 

also affect indoor air quality. Weather conditions influence whether building occupants keep windows 

open or closed, or whether occupants operate air conditioners, humidifiers, or heaters, all of which can 

impact indoor air quality. Weather also has a large effect on infiltration. Certain climatic conditions can 

increase the potential for indoor moisture and mold growth if not controlled by adequate ventilation or air 

conditioning. 

The link between some common indoor air pollutants and health effects is very well established. Radon is 

a known human carcinogen and is the second leading cause of lung cancer.30,31 Carbon monoxide is toxic 

and short-term exposure to elevated carbon monoxide levels in indoor settings can be lethal.32 Episodes of 

“Legionnaires’ Disease,” a form of pneumonia caused by exposure to the Legionella bacterium, have 

been associated with buildings with poorly maintained air conditioning or heating systems.33,34 In 

addition, numerous indoor air pollutants—dust mites, mold, pet dander, environmental tobacco smoke, 

cockroach allergens, and others—are “asthma triggers,” meaning that some asthmatics might experience 

asthma attacks following exposure.35 

30 U.S. EPA. 2003. EPA assessment of risks from radon in homes. EPA/402/R-03/003. Washington, DC. 

31 National Research Council. 1999. Health effects of exposure to indoor radon: biological effects of ionizing 

radiation (BEIR), report VI. Washington, DC: National Academy Press. 

<http://www.epa.gov/iaq/radon/beirvi.html> 

32 Raub, J.A., M. Mathieu-Nolf, N.B. Hampson, S.R. Thom. 2000. Carbon monoxide poisoning—a public health 

perspective. Toxicology 145:1-14. 

33 Allan, T., et al. 2001. Outbreak of legionnaires’ disease among automotive plant workers—Ohio, 2001. MMWR 

50(18):357-359. 

34 Fields, B.S., R.F. Benson, and R.E. Besser. 2002. Legionella and legionnaires’ disease: 25 years of investigation. 

Clin. Microbiol. Rev. 15(3):506-526. 

35 U.S. Institute of Medicine. 2000. Clearing the air: asthma and indoor air exposures. Washington, DC: National 

Academy Press. 

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While these and other adverse health effects have been attributed to specific pollutants, the scientific 

understanding of some indoor air quality issues continues to evolve. One example is “sick building 

syndrome,” which occurs when building occupants experience similar symptoms after entering a 

particular building, with symptoms diminishing or disappearing after leaving the building, and these 

symptoms are increasingly being attributed to a variety of building indoor air attributes. 

Researchers also have been investigating the relationship between indoor air quality and important issues 

not necessarily related to health, such as student performance in the classroom and productivity in 

occupational settings.36 Another evolving area is research in “green building” design, construction, 

operation, and maintenance that achieves energy efficiency and enhances indoor air quality. 

2.4.2 ROE Indicators 

Two National Indicators that fully meet the indicator criteria are discussed in this section. These 

indicators address two specific issues and do not cover the wide range of issues associated with indoor air 

quality. The two indicators focus on radon and environmental tobacco smoke. The indicator on radon 

levels in homes is based on EPA’s 1992 National Residential Radon Survey, 2000 U.S. Census data, and 

production and sales statistics for radon mitigation systems. Environmental tobacco smoke is addressed 

using biomarker data for cotinine, a metabolite linked exclusively to chemicals found in tobacco smoke. 

The Centers for Disease Control and Prevention (CDC) have reported these biomarker data in multiple 

releases of its National Health and Nutrition Examination Survey. 

Table 2.4.1. ROE Indicators of Trends in Indoor Air Quality and Their Effects on Human Health 

NATIONAL INDICATORS LOCATION 

U.S. Homes Above EPA’s Radon Action Level 2.4.2 – p. 2-111 

Blood Cotinine Level 2.4.2 – p. 2-114 

36 U.S. EPA. 2003. Indoor air quality and student performance. EPA/402/K-03/006. Washington, DC. 

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INDICATOR: U.S. Homes Above EPA’s Radon Action Level 

Radon is a radioactive gas. It comes from the decay of uranium that is naturally occurring and commonly 

present in rock and soils. It typically moves up through the ground to the air above and into a home 

through pathways in ground contact floors and walls. Picocuries per liter of air (pCi/L) is the unit of 

measure for radon in air (the metric equivalent is becquerels per cubic meter of air, or Bq/m3

).

Each year radon is associated with an estimated 21,100 lung cancer deaths in the U.S. with smokers at an 

increased risk; radon is the second leading cause of lung cancer after smoking, and 14.4 percent of lung 

cancer deaths in the U.S. are believed to be radon-related (U.S. EPA, 2003). To reduce the risk of lung 

cancer, EPA has set a recommended “action level” of 4 pCi/L for homes. At that level, it is cost-effective 

for occupants to reduce their exposure by implementing preventive measures in their homes. 

This indicator presents: (1) the number of U.S. homes estimated to be at or above the EPA recommended 

radon action level of 4 pCi/L; and (2) the number of homes with an operating radon mitigation system. 

The gap between the homes in (1) and in (2) is the number of homes that have not yet been mitigated 

(generally, homes are only mitigated if the EPA recommended radon action level of 4 pCi/L or more is 

measured). The data for this indicator were extracted from the National Residential Radon Survey (U.S. 

EPA, 1992a) which estimated radon levels in the U.S. housing stock. The number of homes at or above 4 

pCi/L was estimated by applying: (1) the results of the National Residential Radon Survey (U.S. EPA, 

1992a) to (2) 2000 U.S. Census data on the number of U.S. households. The 1992 National Residential 

Radon Survey was based on the housing stock that would be covered by EPA’s radon testing policy (i.e., 

homes that should test). This included only homes intended for regular (year-round) use but covers singlefamily homes, mobile homes, multi-unit and group quarters (U.S. EPA, 1992b). The 1992 residential 

survey estimated that about one in fifteen homes in EPA’s “should test” category would have a radon 

level of 4 pCi/L or more. The measure of the number of homes with an operating mitigation system 

(HOMS) was developed from radon vent fan (RVF) sales data provided voluntarily by fan manufacturers. 

What the Data Show

There was a 272 percent increase in the number of homes with an operating mitigation system from 1990 

to 2004, going from 155,000 to 577,000 homes over 15 years; but during the same period, there has been 

a 26 percent increase in the estimated number of homes with radon levels greater than 4 pCi/L, from 

about 5 million to 6.3 million (Exhibit 2-56, panel A). Panel B of Exhibit 2-56 shows radon potential in 

homes at the county level based on indoor radon measurements, geology, aerial radioactivity, soil 

permeability, and foundation type. Zone 1 is the highest radon potential area, followed by Zone 2 

(medium), and Zone 3 (low). 

It has been reported anecdotally that radon vent fans and mitigation systems are also being used to control 

for soil gases/intrusion in homes in the vicinity of RCRA, Superfund, UST/AST and similar sites as an 

element of corrective action plans. While fans used in this way may provide a radon reduction benefit, 

they could be considered a subtraction from the number of homes with an operating mitigation system, 

thus reducing slightly the slope of the trend line. 
Indicator Limitations 

• The indicator presumes that radon 

vent fans are used for their intended 

purpose; the available information 

supports this premise. 

• Homes with an operating mitigation 

system are presumed to have a vent 

fan with an average useful life of 10 

years. Each year the total of HOMS is 

adjusted to reflect new additions and 

subtractions (i.e., fans added 11 years 

earlier). 

• The number of homes with radon 

levels at or greater than 4 pCi/L is an 

estimate based on one year of 

measurement data extrapolated for 

subsequent years based on population 

data, rather than on continuing 

measurements. 

• This indicator does not track the 

number of homes designed and built 

with radon-resistant new construction 

features, which can prevent radon 

from entering homes. Thus, more 

people are likely being protected from 

elevated indoor air exposures to radon 

than suggested by the trends in radon 

mitigation systems alone. 

Data Sources 

Summary data in this indicator were provided by 

EPA’s Office of Radiation and Indoor Air, based 

on two types of information. The number of homes 

with estimated radon indoor air concentrations 

above EPA’s radon action level was derived from 

the National Residential Radon Survey (EPA, 

1992a) and U.S. Census data; and the number of 

homes with radon mitigation systems was 

developed from unpublished sales data provided 

by radon vent fan manufacturers. 

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References 

U.S. EPA. 2003. EPA assessment of risks from radon in homes. EPA/402/R-03/003. Washington, DC. 

<http://www.epa.gov/radiation/docs/assessment/402-r-03-003.pdf> 

U.S. EPA. 1992a. National residential radon survey: summary report. EPA/402/R-92/011. Washington, 

DC. October.

U.S. EPA. 1992b. Technical support document. EPA/400/R-92/011.

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1 INDICATOR: Blood Cotinine Level

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Environmental tobacco smoke (ETS) contains a mixture of toxic chemicals, including known human 

carcinogens. Persistent exposure to ETS is associated with numerous health related disorders or 

symptoms, such as coughing, chest discomfort, reduced lung function, acute and chronic coronary heart 

disease, and lung cancer (IARC, 2004; NTP, 2002; U.S. EPA, 1992; CDC, 2005). Children are at 

particular risk from exposure to ETS, which may exacerbate existing asthma among susceptible children 

and also greatly increase the risk for lower respiratory tract illness, such as bronchitis and pneumonia, 

among younger children (CDC, 2005). Younger children appear to be more susceptible to the effects of 

ETS than are older children (U.S. EPA, 1992).

Household ETS exposure is an important issue because many people, especially young children, spend 

much time inside their homes. Based on data reported from the 1994 National Health Interview Survey, 

the Department of Health and Human Services (DHHS) estimates that 27 percent of children aged 6 years 

and younger are exposed to ETS in the home (U.S. DHHS, 2000). 

Exposure to ETS leaves traces of specific chemicals in people’s blood, urine, saliva, and hair. Cotinine is 

a chemical that forms inside the body following exposure to nicotine, an ingredient in all tobacco 

products and a component of ETS. Following nicotine exposures, cotinine can usually be detected in 

blood for at least 1 or 2 days (Pirkle et al., 1996). Active smokers almost always have blood cotinine 

levels higher than 10 nanograms per milliliter (ng/mL), while non-smokers exposed to low levels of ETS 

typically have blood concentrations less than 1 ng/mL (CDC, 2005). Following heavy exposure to ETS, 

non-smokers can have blood cotinine levels between 1 and 10 ng/mL. 

This indicator reflects blood cotinine concentrations in ng/mL among non-smokers for the U.S. 

population, aged 3 years and older, as measured in the 1999-2000 and 2001-2002 National Health and 

Nutrition Examination Survey (NHANES). NHANES is a series of surveys conducted by CDC’s National 

Center for Health Statistics (NCHS) that is designed to collect data on the health and nutritional status of 

the civilian, non-institutionalized U.S. population using a complex, stratified, multistage, probabilitycluster design. Blood cotinine also was monitored in non-smokers aged 4 years and older as part of 

NHANES III, between 1988 and 1991. CDC’s National Center for Environmental Health (NCEH) 

conducted the laboratory analyses for the biomonitoring samples. Beginning in 1999, NHANES became a 

continuous and annual national survey. 

What the Data Show

As part of NHANES III (1988-1991), CDC estimated that the median blood serum level (50th percentile)

of cotinine among non-smokers in the general U.S. population was 0.20 ng/mL. In NHANES 1999-2000, 

the estimated median serum level among non-smokers nationwide was 0.06 ng/mL. During the 2001­

2002 survey, the estimated blood cotinine levels for the U.S. population were very similar to 1999-2000, 

with the median concentration actually below the limit of detection, and the geometric mean 0.06 ng/mL 

(see Exhibit 2-57). This marks a 70 percent decrease from levels measured in the 1988-1991 NHANES III 

survey—a reduction that suggests a marked decrease in exposure to ETS. 
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Exhibit 2-57 also shows the results of the NHANES 1999-2000 and 2001-2002 survey, for different 

subpopulations. Similar decreasing trends in blood cotinine levels between NHANES III (1988-1991) and 

the most recent 2001-2002 survey were observed in each of the population groups defined by age, sex, 

and race/ethnicity (CDC, 2005). These data reveal three additional observations: (1) non-smoking males 

have higher cotinine levels than non-smoking females; (2) of the ethnic groups presented, non-Hispanic 

blacks had the highest cotinine levels; and (3) on average, people below age 20 have higher levels of 

blood cotinine than people aged 20 years and older. 

Exhibit 2-58 shows the percentage of children between the ages of 4 and 17 with specified blood cotinine 

levels, for the total age group and by selected race and ethnicity breakdowns within the specified age 

group. Among the three subgroup populations presented, Mexican American children had the lowest 

percentage of blood cotinine levels greater than 1.0 ng/mL and this was evident for both 1988-1994 and 

1999-2002 time periods (10.7 percent and 5.2 percent, respectively). Black, non-Hispanic children had the 

largest decline of the three subgroups in the percentage of blood cotinine levels greater than 1.0 ng/mL, 

however, this population also started off with the highest percentage above 1.0 ng/mL (36.6 percent) 

(Federal Interagency Forum on Child and Family Statistics, 2005). 

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Indicator Limitations 

• The measurement of an 

environmental chemical in 

a person’s blood or urine 

does not by itself mean that 

the chemical has caused or 

will cause harmful effects. 

• Because the data from 

NHANES 1999-2000 and 

2001-2002 represent only 

two survey periods, 

changes in estimates 

between the two time 

periods do not necessarily 

reflect a trend. Earlier data 

sets are available (e.g., 

NHANES III), but the data 

are not directly comparable 

to NHANES 1999-2002. 

When CDC releases 

additional survey results 

(e.g., 2003-2004) it will 

become possible to more 

fully evaluate trends (CDC, 

2002; 2004). 

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Data Sources 

Data used for this indicator were extracted from two reports that present results of the ongoing National 

Health and Nutrition Examination Survey (NHANES): the data shown in Exhibit 2-57 were obtained 

from a CDC report (2005), and the data shown in Exhibit 2-58 were obtained from a report by the Federal 

Interagency Forum on Child and Family Statistics (2005). The underlying laboratory data supporting both 

reports are available online in SAS® transport file formats at: 

http://www.cdc.gov/nchs/about/major/nhanes/datalink.htm. 

References 

CDC (Centers for Disease Control and Prevention). 2005. Third national report on human exposure to 

environmental chemicals. NCEH Pub. No. 05-0570. July. Accessed September 9, 2005. 

<http://www.cdc.gov/exposurereport/report.htm> 

CDC (Centers for Disease Control and Prevention). 2004. NHANES analytic guidelines. June 2004 

version. Accessed October 21, 2005. 

<http://www.cdc.gov/nchs/data/nhanes/nhanes_general_guidelines_june_04.pdf> 

CDC (Centers for Disease Control and Prevention). 2002. NHANES 1999-2000 addendum to the 

NHANES III analytic guidelines. Last update August 30, 2002. Accessed October 11, 2005. 

<http://www.cdc.gov/nchs/data/nhanes/guidelines1.pdf> 

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Federal Interagency Forum on Child and Family Statistics. 2005. America’s children: key national 

indicators of well-being, 2005. Washington, DC: U.S. Government Printing Office. Accessed December 

20, 2005. <http://www.childstats.gov/americaschildren/pop9.asp> 

IARC (International Agency for Research on Cancer). 2004. IARC working group on the evaluation of 

carcinogenic risks to humans. Evaluation of carcinogenic risks to humans, volume 83: tobacco smoke and 

involuntary smoking. Lyon, France. 

NTP (National Toxicology Program) 2002. Report on carcinogens, 10th Edition. 

Pirkle JL, Flegal KM, Bernert JT, Brody DJ, Etzel RA, Maurer KR. 1996. Exposure of the U.S. 

population to environmental tobacco smoke: the third national health and nutrition examination survey, 

1988 to 1991. J. Amer. Med. Assoc. 275:1233-1240. 

U.S. DHHS (Department of Health and Human Services). 2000. Healthy people 2010. 2nd ed. With 

understanding and improving health and objectives for improving health. 2 vols. Washington, DC: U.S. 

Government Printing Office 

<http://www.healthypeople.gov/Document/HTML/Volume2/27Tobacco.htm#_Toc489766224> 

U.S. EPA. 1992. Respiratory health effects of passive smoking: lung cancer and other disorders. 

Washington, DC. 

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2.4.3 Discussion 

What These Indicators Say About Trends in Indoor Air Quality and Their 

Effects on Human Health 

The two indoor air quality indicators provide insights into issues of very different origin: radon is a 

substance from a natural source, while environmental tobacco smoke (as evaluated by the presence of 

blood cotinine) is linked entirely to human behavior. 

The Homes Above EPA’s Radon Action Level indicator (p. 2-111) tracks two statistics: the number of 

homes estimated to be at or above EPA’s action level (4 pCi/L) for radon, and the number of homes with 

operating radon mitigation systems. Evaluating trends in radon mitigation systems is relevant because 

properly operated systems are expected to reduce radon to levels below the action level, and therefore 

also are expected to reduce radon-related health risks among building occupants. Between 1990 and 2004, 

the number of homes with radon mitigation systems increased nearly four-fold, but these homes account 

for less than 10 percent of the nation’s homes currently believed to have radon levels greater than EPA’s 

action level. Some residents are being protected against radon exposures through radon-resistant new 

construction techniques used when a home is built. Estimates of radon-resistant new construction 

practices are not included in the indicator, however, because while they substantially reduce radon levels 

in homes with high radon potential, they do not always reduce the levels below the action level. 

The Blood Cotinine indicator (p. 2-114) tracks blood concentrations of cotinine, a metabolite of nicotine, 

and shows that exposure to environmental tobacco smoke among non-smokers decreased considerably in 

the last decade. This decrease was observed for all population groups, defined by age (including children), 

sex, and ethnicity. However, children’s blood cotinine levels, on average, are still more than twice the 

levels observed in adults. A logical explanation for the downward trend in blood cotinine levels is 

modified behavior, whether reduced smoking prevalence or more widespread practices of restricting areas 

in which individuals are allowed to smoke. 

Taken together, these indicators show an increasing number of Americans have reduced indoor air 

exposures to two known carcinogens. Though these improvements are encouraging, both radon and 

environmental tobacco smoke remain important indoor air quality issues. 

Limitations, Gaps, and Challenges 

The two indicators in this section provide extensive, but not comprehensive, information on the 

corresponding indoor air quality issues that they characterize. An important limitation of the Homes 

Above EPA’s Radon Action Level indicator, for instance, is that it does not track the number of homes 

designed and built with radon-resistant new construction features, which can prevent radon from entering 

homes and therefore reduce radon exposures and associated health risks. Thus, more people are likely 

being protected from elevated indoor air exposures to radon than suggested by trends in radon mitigation 

systems alone. 

The Blood Cotinine indicator also has certain limitations. For example, exposure to environmental 

tobacco smoke does not occur exclusively indoors: some proportion of blood cotinine levels measured in 

non-smokers reflects exposures that occurred outdoors. In addition, nationally representative blood 

cotinine data are not available for children under 3 years old—an age group with documented 

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susceptibilities to environmental tobacco smoke.37 Nonetheless, none of these limitations call into 

question this indicator’s main finding: nationwide, exposures to environmental tobacco smoke among 

non-smokers are decreasing. Moreover, reliable survey data on smoking behavior corroborates this 

downward trend.38

The two indoor air quality indicators provide useful insights into trends for radon and environmental 

tobacco smoke, but they leave some gaps that EPA would like to fill to better answer the overarching 

question on nationwide indoor air quality trends and associated health effects. For example, ROE 

indicators could not be developed for indoor air quality trends for molds, some combustion products, 

chemicals found in common household cleaners and building materials, and certain persistent pollutants 

and endocrine disruptors that have been identified in household dust. 39,40 There is no quantitative, 

nationally representative inventory of emission sources in indoor environments, nor is there a nationwide 

monitoring network that routinely measures air quality inside homes, schools, and office buildings. These 

gaps in nationwide indicators do not mean that nothing is known about the broad range of indoor air 

quality issues and associated health effects. Rather, information on these issues can be gleaned from 

numerous publications by governmental agencies and in the scientific literature. Data from these other 

publications and information resources, though valuable in understanding indoor air quality, are not 

presented in this report as indicators because they are not sufficiently representative on a national scale or 

because they do not track an issue over time. 

The challenges associated with filling these indicator gaps are well known. Although methods to monitor 

indoor air quality are available, there is considerable variability among building types, occupants’ 

behaviors, climate conditions, and ventilation systems for indoor environments in the U.S. This 

variability, combined with access issues and the number of different pollutants to address, makes a 

statistically based evaluation of our nation’s millions of residences, thousands of office buildings, and 

more than 100,000 schools a challenging and resource-intensive task. Further, it is difficult to directly 

measure how changes in indoor air quality translate into corresponding changes in human health effects, 

because many health outcomes attributed to poor indoor air quality (e.g., asthma attacks) have numerous 

environmental and non-environmental risk factors. 

37 U.S. EPA. 1992. Respiratory health effects of passive smoking: lung cancer and other disorders. EPA/600/6­

90/006F. Washington, DC. 

38 U.S. Department of Health and Human Services. 2000. Healthy people 2010: understanding and improving health. 

2nd ed. Washington, DC: U.S. Government Printing Office. November. <http://www.health.gov/healthypeople/> 

39 Rudel, R.A., D.E. Camann, J.D. Spengler, L.R. Korn, and J.G. Brody. 2003. Phthalates, alkylphenols, pesticides, 

polybrominated diphenyl ethers, and other endocrine-disrupting compounds in indoor air and dust. Env. Sci. Tech. 

37(20):4543-4553. 

40 Stapleton, H.M., N.G. Dodder, J.H. Offenberg, M.M. Schantz, and S.A. Wise. 2005. Polybrominated diphenyl 

ethers in house dust and clothes dryer lint. Env. Sci. Tech. 39(4):925-931. 

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