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Ozone Air Quality Standards:
EPA’s 2015 Revision

James E. McCarthy
Specialist in Environmental Policy
February 26, 2015
Congressional Research Service
7-5700
www.crs.gov
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Ozone Air Quality Standards: EPA’s 2015 Revision

Summary
On November 26, 2014, the Environmental Protection Agency (EPA) announced proposed
revisions to the National Ambient Air Quality Standards (NAAQS) for ground-level ozone. If
finalized, the proposal would set more stringent standards, lowering both the primary (health-
based) and secondary (welfare-based) standards from the current 75 parts per billion (ppb) to
somewhere in a range of 65 to 70 ppb. This report discusses the standard-setting process, the
specifics of the current and past reviews, and issues raised by the proposal.
NAAQS are standards for outdoor (ambient) air that are intended to protect public health and
welfare from harmful concentrations of pollution. If EPA changes the primary standard for ozone
to a lower level, it would be concluding that protecting public health requires lower
concentrations of ozone pollution than were previously judged to be safe. In high enough
concentrations, ozone aggravates heart and lung diseases and may contribute to premature death.
Ozone also can have negative effects on forests and crop yields, which the secondary (welfare-
based) NAAQS is intended to protect.
NAAQS do not directly limit emissions of a pollutant; rather, they set in motion a long process in
which states and EPA identify areas that do not meet the standards, and states prepare
implementation plans to demonstrate how emissions will be lowered sufficiently to reach
attainment.
Ground-level ozone, or “smog,” is a widespread pollutant: as of July 2014, 123 million people
(40% of the U.S. population) lived in areas classified “nonattainment” for the current 75 ppb
ozone NAAQS. A more stringent standard might affect more areas. If the nonattainment
designations were made using current data, 358 counties would be in nonattainment with a 70 ppb
NAAQS (rather than 155 counties at 75 ppb); at 65 ppb, 558 counties would have monitors
showing nonattainment. Emission sources in these areas might have to adopt more stringent
controls.
EPA maintains that most areas will be able to reach attainment of the new standards—whether at
65 or 70 ppb—as a result of already promulgated regulations for gasoline, autos, power plants,
and other sources of emissions. Thus, the agency’s estimates of the cost of NAAQS compliance
are substantially lower than many earlier estimates. EPA estimates the cost of meeting a 70 ppb
standard in all states except California at $3.9 billion annually in 2025; the cost of meeting a 65
ppb standard in the same states is estimated at $15 billion annually. Because most areas in
California would have until the 2030s to reach attainment, EPA provided separate cost estimates
for California ($0.80 billion to $1.6 billion annually in 2038). EPA’s cost estimates are
substantially less than one from the National Association of Manufacturers that was widely
circulated before the release of EPA’s proposal.
Members of Congress have shown particular interest in whether the expected benefits of the
proposed standards would justify their costs—a perennial issue raised by stakeholders when EPA
considers revising NAAQS. Both nationwide and in California, the agency expects the benefits of
attainment to exceed the costs, but there is controversy over the methods used to estimate both.
More importantly, as the Clean Air Act is currently written, the agency is prohibited from
weighing costs against benefits in setting the standards. The statute simply states that the
Administrator is to set the primary standard at a level requisite to protect public health, allowing
an adequate margin of safety.
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Because of the potential cost, various interest groups have lobbied against strengthening the
standards. In the 113th Congress, H.R. 4947/S. 2514, H.R. 5505/S. 2833, and H.R. 5665 were
introduced to delay the promulgation of a revised NAAQS or to change EPA’s authority to revise
the standards. No action was taken on these bills.
EPA’s November 26 proposal is not a final action. Publication of the proposal in the Federal
Register
, December 17, began a 90-day public comment period. EPA held three public hearings
on January 29 and February 2. The agency must address significant public comments when it
publishes a final standard, currently scheduled for October 1, 2015.
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Contents
Introduction ...................................................................................................................................... 1
What Are NAAQS? ......................................................................................................................... 5
Reviewing the Ozone NAAQS ........................................................................................................ 6
The NAAQS Review Process .................................................................................................... 6
Recent Reviews of the Ozone Standard..................................................................................... 8
The Primary Standard .......................................................................................................... 9
The Secondary Standard .................................................................................................... 10
Controlling Ozone Pollution .......................................................................................................... 12
Wintertime Ozone .......................................................................................................................... 13
Costs and Benefits of Control ........................................................................................................ 14
EPA’s Cost Estimates ......................................................................................................... 14
EPA’s Benefit Estimates .................................................................................................... 17
California and Other Regional Costs and Benefits............................................................ 18
Industry Estimates ............................................................................................................. 18
Issues.............................................................................................................................................. 19
The Role of Cost ...................................................................................................................... 20
Background Ozone Levels ...................................................................................................... 20
Monitoring ............................................................................................................................... 22
The Role of Federal Versus State and Local Pollution Control Measures ............................... 23
Next Steps ...................................................................................................................................... 24

Figures
Figure 1. Current Ozone Nonattainment Areas, 2014 (2008 Standard, 0.075 ppm) ........................ 2
Figure 2. Counties Where Measured Ozone Is Above the Proposed Range of Standards,
Based on 2011-2013 Monitoring Data .......................................................................................... 4
Figure 3. EPA Projects That Most Counties Would Meet the Proposed Ozone Standards
in 2025 Without Promulgation of Additional Emission Controls ............................................... 16

Tables
Table 1. History of the National Ambient Air Quality Standards for Ozone ................................... 7
Table 2. Estimated Annual Costs and Health Benefits of Ozone NAAQS Options,
Nationwide Except California, in 2025 ...................................................................................... 14
Table 3.Estimated Annual Costs and Health Benefits of Ozone NAAQS Options in
California, Post-2025 .................................................................................................................. 18

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Contacts
Author Contact Information........................................................................................................... 25

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Introduction
The Environmental Protection Agency (EPA), nearing completion of a statutorily required review,
announced, on November 26, 2014, a proposal to reduce the National Ambient Air Quality
Standards (NAAQS) for ground-level ozone1 from the current level of 75 parts per billion (ppb)
to somewhere in the range of 65 to 70 ppb. The formal proposal appeared in the Federal Register
on December 17.2 The proposal is based on a review of 2,300 scientific studies by EPA staff,
which itself was reviewed by a panel of 20 outside scientific experts, a group known as the Clean
Air Scientific Advisory Committee (CASAC).3 The EPA Administrator was under a court order to
sign any proposed changes to the standards by December 1, 2014. A comment period, public
hearings, and interagency review follow publication of the proposal in the Federal Register. The
standards are to be finalized by October 1, 2015.
Ground-level ozone (often referred to as “smog”) is associated with potentially serious health
effects when present in high enough concentrations. These health effects include aggravated
asthma, chronic bronchitis, and heart attacks, and in some cases premature death. In the
Regulatory Impact Analysis accompanying the proposed standards, EPA states that reducing
concentrations to 65 parts per billion (ppb) versus the current standard of 75 ppb4 would avoid
710 to 4,300 premature deaths annually in 2025 nationwide (excluding California).5 [Note:
California was excluded from EPA’s estimate of both costs and benefits of the nationwide
standard, because most areas of the state will have until the 2030s to reach attainment of the
NAAQS.6 California costs and benefits are presented separately in the “Costs and Benefits”
section of this report.]

1 Ground-level (tropospheric) ozone can be a lung irritant with serious adverse health effects. In the stratosphere,
however, ozone protects the Earth from harmful ultraviolet rays of the sun. For more information, see U.S. EPA,
“Ozone—Good Up High Bad Nearby,” at http://www.epa.gov/airquality/gooduphigh/.
2 79 Federal Register 75234, available at http://www.gpo.gov/fdsys/pkg/FR-2014-12-17/pdf/2014-28674.pdf. As of
this writing, the link to the proposal on EPA’s website still connects the reader to a pre-publication copy of the
proposal. Footnotes citing the proposal in this report will give both citations.
3 See “EPA Assessment Underscores Relationship Between Ozone Exposure, Respiratory Effects,” Daily Environment
Report
, February 19, 2013. The Integrated Science Assessment is 1,251 pages long and is available at
http://www.epa.gov/ncea/isa/ozone.htm. CASAC’s review of the ISA, dated November 14, 2012, can be found at
http://yosemite.epa.gov/sab/sabproduct.nsf/60C2732674A5EEF385257AB6007274B9/$File/EPA-CASAC-13-
001+unsigned.pdf.
4 The ozone NAAQS is also frequently expressed in terms of parts per million rather than parts per billion. A standard
of 0.075 parts per million (ppm) would be the same as a standard of 75 parts per billion (ppb), and is often referred to
as such. This report uses parts per billion in most cases, as does EPA in most of the explanatory materials
accompanying the proposed standard. The proposed standard itself, as seen in the pre-publication Federal Register
notice, is expressed in ppm, however.
Until 2008, the standard was expressed in parts per million with two decimal places (e.g., 0.08 ppm from 1997 to
2008). Without a third decimal place, concentrations as high as 0.084 ppm (or 84 ppb) could be rounded to 0.08 and
considered to be in attainment of the standard. The 2008 standard added greater precision to the standard by adding a
third decimal place (0.075 ppm).
5 U.S. EPA, Regulatory Impact Analysis of the Proposed Revisions to the National Ambient Air Quality Standards for
Ground-Level Ozone
, November 2014, pp. ES-13 to ES-17, at http://www.epa.gov/air/ozonepollution/pdfs/
20141125ria.pdf. Hereafter, “2014 RIA.” A separate estimate for California shows a 65 ppb NAAQS resulting in 789-
990 premature deaths avoided annually post-2025.
6 Most of California’s population lives in areas classified as Severe or Extreme for their nonattainment, under the Clean
Air Act’s statutory categorization scheme. These areas have more stringent emission control requirements and more
time to reach attainment, so EPA provided separate cost-benefit estimates for the state.
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Figure 1. Current Ozone Nonattainment Areas, 2014 (2008 Standard, 0.075 ppm)

Source: U.S. EPA Green Book, http://www.epa.gov/airquality/greenbk/map8hr_2008.html. Map shows areas designated nonattainment by EPA as of July 2, 2014.
Notes: Nonattainment designations were based on 2008-2010 monitoring data in most cases. Some areas that attain the 2008 standard are also designated
nonattainment for the less stringent 1997 standard. These areas may have more recent monitoring data that demonstrate attainment with the 1997 (and the 2008)
standards, but they must have an approved maintenance plan in place before they will be removed from the list of nonattainment areas.
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High ozone concentrations also affect the growth of plants, causing damage to both forests and
field crops. In 2006, the U.S. Forest Service examined 380 monitoring sites in the Mid-Atlantic
and Southeastern states and found visible injury to forest plants from ozone at 121 (32%) of them.
At 20 of the sites, the damage was described as “severe.”7 In addition, EPA found that “several
economically important crop species are sensitive to ozone levels typical of those found in the
United States,” and estimated that crop losses could be reduced by $400 million to $620 million
annually by implementation of a more stringent ozone standard.8
While EPA’s analysis has found that there would be substantial health and welfare benefits to
reducing ozone, the agency also has concluded that there could be substantial costs. EPA
estimates these costs (for areas other than California) at $3.9 billion annually in 2025 to achieve a
70 ppb NAAQS, or $15 billion if the standard is set at 65 ppb.9
The ozone NAAQS are among EPA’s most far-reaching standards. At the current level, set in
March 2008 (75 ppb), 123 million people live in areas that have not attained the standards (see
Figure 1).10 These 46 areas (referred to as “nonattainment areas”) generally coincide with
metropolitan areas, but may be larger or smaller.
The agency states that if the most recent available monitoring data (for the years 2011-2013) were
used to identify nonattainment areas for the proposed standards, the number of counties showing
nonattainment would increase from 155 for the current 75 ppb standard to 358 if the standard
were set at 70 ppb, and to 558 if it were set at 65 ppb11 (see Figure 2).
Nonattainment designations will not be made until 2017 at the earliest, however, and EPA notes
that the decisions will most likely be based on 2014-2016 monitoring data. In the intervening
years, the emissions that contribute to ozone formation are likely to decline, in response to
already promulgated standards for motor vehicles, gasoline, power plants, and other sources of
emissions. Thus, EPA expects the number of counties showing nonattainment to be less than the
estimate shown in Figure 2.12
The potential economic, health, and environmental impacts of a change in the ozone NAAQS
have led to great interest in EPA’s ongoing review of the standards. To assist Members and staff in
evaluating EPA’s review, this report provides background on NAAQS, the process used to
establish them, the current ozone standards, the remaining controversy over the most recent
revision, and issues that may be raised as EPA brings the current review to completion.

7 U.S. EPA, Regulatory Impact Analysis, Final National Ambient Air Quality Standard for Ozone, July 2011, p. 64, at
http://www.epa.gov/glo/pdfs/201107_OMBdraft-OzoneRIA.pdf.
8 Ibid., p. 74.
9 2014 RIA, p. ES-14. Most of California’s population lives in areas classified as Severe or Extreme for their
nonattainment, under the Clean Air Act’s statutory categorization scheme. These areas have more stringent emission
control requirements and more time to reach attainment, so EPA provided separate cost-benefit estimates for the state.
10 For specifics, see EPA’s “Green Book,” at http://www.epa.gov/oaqps001/greenbk/hntc.html.
11 U.S. EPA, “Ozone by the Numbers: EPA’s Proposal to Update the Air Quality Standards for Ground-Level Ozone,”
p. 2, at http://www.epa.gov/air/ozonepollution/pdfs/20141125fs-numbers.pdf.
12 When the agency last promulgated a revision to the ozone NAAQS in March 2008, it identified 345 counties with
monitors showing nonattainment of the new standard. By the time the agency designated nonattainment areas in May
2012, the number of nonattaining counties had declined to 221.
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Figure 2. Counties Where Measured Ozone Is Above the Proposed Range of Standards, Based on 2011-2013 Monitoring Data

Source: U.S. EPA, http://epa.maps.arcgis.com/apps/StorytellingSwipe/index.html?appid=a3c9f378699045749a85e9c04728fc79&webmap=
3b3e0960060141c7828fc93b14e3d4d2#.
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What Are NAAQS?
NAAQS are standards that apply to ambient (outdoor) air. Section 109 of the Clean Air Act
directs EPA to set both primary NAAQS, which are standards, “the attainment and maintenance
of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health,”
with “an adequate margin of safety,” and secondary NAAQS, which are standards necessary to
protect public welfare, a broad term that includes damage to crops, vegetation, property, building
materials, climate, etc.13
The pollutants to which NAAQS apply are generally referred to as “criteria” pollutants. The act
defines them as pollutants that “endanger public health or welfare,” and whose presence in
ambient air “results from numerous or diverse mobile or stationary sources.”14 Six pollutants are
currently identified as criteria pollutants: ozone, particulates, carbon monoxide, sulfur dioxide,
nitrogen oxides, and lead. The EPA Administrator can add to this list if she determines that
additional pollutants meet the definition, or delete pollutants from the list if they no longer meet
the definition. There have been no changes to the list, however, since the late 1970s.
NAAQS are at the core of the Clean Air Act, even though they do not directly regulate emissions.
In essence, they are standards that define what EPA considers to be clean air for the pollutant in
question. Once a NAAQS has been set, the agency, using monitoring data and other information
submitted by the states, identifies areas that exceed the standards and must reduce pollutant
concentrations. This designation process is often delayed by litigation over the standards, by
EPA’s agreement to reconsider aspects of them, or by consultations with the states over the
specifics of the areas to be designated. Designation of nonattainment areas for the 1997 NAAQS,
for example, took seven years. Designations under the 2008 standards took four years.
After nonattainment areas are designated, state and local governments have up to three years to
produce State Implementation Plans (SIPs), which outline the measures that will reduce emission
levels and attain the standards. Finalizing SIPs, through EPA review and approval, often takes
longer. Under the statute, actual attainment of the standards is allowed to stretch over a 3-year to
20-year period, depending on the severity of the area’s pollution. Ozone nonattainment areas are
designated as Marginal, Moderate, Serious, Severe, or Extreme, depending on the level of
pollution. Each of these classifications comes with required pollution control measures: the more
severe the pollution, the more stringent are the required controls, and the longer the area is
allowed before it must demonstrate attainment.15
Thus, establishment or revision of a NAAQS is not an event that requires immediate compliance
with an air quality standard; rather, it sets in motion a long and complicated implementation

13 The Clean Air Act’s definition of welfare is found in Section 302(h) of the act (42 U.S.C. 7602(h)).
14 Authority to establish NAAQS comes from both Sections 108 and 109 of the act; this definition of criteria pollutants
is found in Section 108. The authority and procedures for controlling the sources of criteria pollutants are found
throughout Titles I, II, and IV of the act. Many pollutants that are less widely emitted are classified as “hazardous air
pollutants” and are regulated under a different section of the act (Section 112). That section lists 187 pollutants or
groups of pollutants as hazardous and establishes different authorities and requirements for controlling their emissions.
15 For a more detailed discussion, see CRS Report RL30853, Clean Air Act: A Summary of the Act and Its Major
Requirements
.
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process. That process may ultimately have far-reaching impacts for public health and welfare, for
sources of pollution in numerous economic sectors, and for states and local governments.
EPA has several tools available to get areas to comply with a NAAQS. The most frequently
mentioned of these is the potential for highway-fund sanctions: failure to submit or implement a
SIP adequate to attain or maintain compliance with the NAAQS can lead to the temporary
suspension of federal highway funds for non-safety-related projects. Ultimately, EPA can impose
a federal implementation or maintenance plan (a FIP) in an area that does not have an approved
SIP. Imposition of sanctions or FIPs is relatively rare, however: generally the states avoid
sanctions by submitting plans that require sufficient emission reductions to be deemed adequate
by EPA.
In addition to state and local actions to address ambient concentrations of NAAQS pollutants,
EPA itself acts to control emissions and concentrations of criteria pollutants, through national
standards for products that contribute to ozone pollution (particularly mobile sources, such as
automobiles) and standards for new stationary sources (such as power plants). These standards
lead to emission reductions that states can factor into their implementation plans, reducing the
need for local air pollution control measures.
Reviewing the Ozone NAAQS
Section 109(d) of the Clean Air Act requires the agency to review each NAAQS every five years.
That schedule is rarely met, but it often triggers lawsuits that force the agency to undertake a
review.16 In June 2013, the Sierra Club and three other groups filed suit over EPA’s failure to
complete the current ozone review by the March 2013 deadline, and a court subsequently ordered
EPA to propose any changes to the standards by December 1, 2014, and complete the review, with
promulgation of any revisions by October 1, 2015.17
An historical review of the ozone NAAQS and their revisions is presented in Table 1.
The NAAQS Review Process
Reviewing an existing NAAQS is generally a long process. To begin the process, EPA scientists
compile the scientific literature published since the last NAAQS revision, and summarize it in a
report known as a Criteria Document or Integrated Science Assessment (ISA). The ISA for ozone,
completed in February 2013,18 reviewed 2,300 scientific studies.

16 CRS Report R41563, Clean Air Issues in the 112th Congress, summarized EPA’s recent efforts to review the
NAAQS and implement revisions, including the next steps for each of the six criteria pollutants. Reviews of all six
pollutants (ozone, PM, lead, NO2, carbon monoxide, and SO2) have been completed since 2006, with the standards
being made more stringent for five of the six.
17 Sierra Club v. EPA, No. 13-2809 (N.D. Cal., Apr. 30, 2014). (unpublished)
18 U.S. EPA, Office of Research and Development, Integrated Science Assessment of Ozone and Related
Photochemical Oxidants
, Final Report, February 2013, at http://www.epa.gov/ncea/isa/ozone.htm.
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Table 1. History of the National Ambient Air Quality Standards for Ozone
1971-2014
Final
Primary/
Averaging
Year
Rule/Decision
Secondary
Indicator
Time
Level
Form
1971 36
FR 8186
Primary and
Total
1-hour
0.08 ppm
Not to be exceeded
Secondary
photochemical
more than one hour
Apr 30, 1971
oxidants
per year.
1979 44
FR 8202
Primary and
Ozone
1-hour
0.12 ppm
Attainment is
Secondary
defined when the
Feb 8, 1979
expected number of
days per calendar
year, with maximum
hourly average
concentration
greater than 0.12
ppm, is equal to or
less than one.
1993 58
FR 13008
EPA decided




that revisions to
Mar 9, 1993
the standards
were not
warranted at
the time.
1997 62
FR 38856
Primary and
Ozone
8-hour
0.08 ppm
Annual fourth-
Secondary
highest daily
Jul 18, 1997
maximum 8-hr
concentration,
averaged over 3
years.
2008 73
FR 16483
Primary and
Ozone 8-hour

0.075
Annual fourth-
Secondary
ppm
highest daily
Mar 27, 2008
maximum 8-hr
concentration,
averaged over 3
years.
2014
not final;
Primary and
Ozone 8-hour
0.065-
Annual fourth-
proposal is at
Secondary
0.070
highest daily

ppm
maximum 8-hr
79 FR 75234
concentration,
Dec 17, 2014
averaged over 3
years.
Source: U.S. Environmental Protection Agency, http://epa.gov/ttn/naaqs/standards/ozone/s_o3_history.html. For
2014 proposal, http://www.epa.gov/air/ozonepol ution/actions.html.

Ozone ISAs cover topics as wide-ranging as the physics and chemistry of ozone in the
atmosphere; environmental concentrations, patterns, and exposure; dosimetry; animal-to-human
extrapolation; toxicology; interactions with co-occurring pollutants; controlled human exposure
studies; epidemiology; effects on vegetation and ecosystems; effects on UVB (ultraviolet light)
exposures and climate; and effects on man-made materials.
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Following completion of the ISA, EPA prepares a Risk and Exposure Assessment to identify
exposure pathways, at-risk populations, and health endpoints. This document was completed for
the current review in August 2014.19
A final document prepared by EPA staff, the Staff Paper or Policy Assessment, summarizes the
information compiled in the ISA and Risk Assessment and provides the Administrator with
options regarding the indicators, averaging times, statistical form, and numerical level
(concentration) of the NAAQS. A Policy Assessment was completed and publicly released on
August 29, 2014.20
To ensure that NAAQS reviews meet the highest scientific standards, the 1977 amendments to the
Clean Air Act required the Administrator to appoint an independent Clean Air Scientific Advisory
Committee (CASAC). CASAC has seven members, largely from academia and from private
research institutions. In conducting NAAQS reviews, their expertise is supplemented by panels of
the nation’s leading experts on the health and environmental effects of the specific pollutants that
are under review. These panels can be rather large. The panel for the current ozone review, for
example, has 20 members. CASAC and the public make suggestions regarding the membership
of the panels on specific pollutants, with the final selections made by EPA. The panels evaluate
the agency’s work during NAAQS-setting and NAAQS-revision, rather than conducting their
own independent review of the standards.
Recent Reviews of the Ozone Standard
EPA last changed the NAAQS for ozone in March 2008, from 0.08 ppm (effectively 84 ppb) to
0.075 ppm (75 ppb). Although the standard was strengthened, the level chosen at that time was
subject to controversy. A 23-member CASAC Review Panel unanimously recommended a range
of standards more stringent than what the Administrator chose.21
In September 2009, EPA agreed to reconsider the 2008 standard. As a result, a more stringent
primary standard and a different version of the secondary standard were proposed in January
2010. After a year and a half of public comment and review, EPA sent what it considered a final
set of standards to the Office of Management and Budget (OMB) for interagency review. The
process was short-circuited, however, by a presidential decision to await conclusion of the next
regular review—the review now nearing completion—before promulgating any change.

19 U.S. EPA, Ozone (O3) Standards - Documents from Current Review - Risk and Exposure Assessments, at
http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_rea.html.
20 U.S. EPA, Office of Air and Radiation, Policy Assessment for the Review of the Ozone National Ambient Air Quality
Standards
, August 2014, at http://www.epa.gov/ttn/naaqs/standards/ozone/data/20140829pa.pdf. Hereinafter “U.S.
EPA, 2014 Policy Assessment.”
21 In a letter to the Administrator sent after promulgation of the NAAQS, the panel’s chair stated, “Nevertheless, the
members of the CASAC Ozone Review Panel do not endorse the new primary ozone standard as being sufficiently
protective of public health. The CASAC—as the Agency’s statutorily-established science advisory committee for
advising you on the national ambient air quality standards—unanimously recommended decreasing the primary
standard to within the range of 0.060–0.070 ppm. It is the Committee’s consensus scientific opinion that your decision
to set the primary ozone standard above this range fails to satisfy the explicit stipulations of the Clean Air Act that you
ensure an adequate margin of safety for all individuals, including sensitive populations.” Letter of Rogene F.
Henderson, Chair, Clean Air Scientific Advisory Committee, to EPA Administrator Stephen L. Johnson, April 7, 2008,
at http://yosemite.epa.gov/sab/sabproduct.nsf/4AF8764324331288852574250069E494/$File/EPA-CASAC-08-009-
unsigned.pdf.
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The Primary Standard
The review completed in 2008 found evidence of health effects, including mortality, at levels of
exposure below the then-current 0.08 ppm standard. As a result, both EPA staff and CASAC
recommended strengthening the standard. CASAC stated: “There is no scientific justification for
retaining the current [0.08 ppm] primary 8-hr NAAQS.”22 The panel unanimously recommended
a range of 0.060 to 0.070 ppm (60 to 70 parts per billion) for the primary (health-based) 8-hour
standard.
EPA staff also recommended strengthening the primary standard. They recommended
“considering a standard level within the range of somewhat below 0.080 parts per million (ppm)
to 0.060 ppm.”23
Based on these recommendations, and his own judgment regarding the strength of the science,
Stephen Johnson, the Bush Administration’s last EPA Administrator, chose to finalize the standard
at 0.075 ppm (75 parts per billion).24 That revision led to designation of nonattainment areas in
April and May 2012, as shown above in Figure 1.25
The Regulatory Impact Analysis that accompanied the final 2008 standard identified 345 counties
in 36 states in exceedance of the 0.075 ppm standard, using data for 2004-2006 (the most recent
available at the time). By May 2012, when the nonattainment areas were actually designated, the
number of counties in nonattainment had fallen to 232 in 26 states and the District of Columbia,
based mostly on data for 2008-2010.26 In the intervening years, emissions declined in most areas
as more stringent standards for both mobile and stationary sources took effect. The recession and
other economic factors also contributed to the lower numbers. When the economy is operating
well below capacity, emissions generally decline; and changes in the economy (e.g., fewer
vehicle miles traveled and a switch to cleaner fuels) have resulted in lower emissions.
Reconsideration of the 2008 Standard
As noted, EPA began a process to reconsider the 2008 ozone NAAQS in September 2009, and
proposed a more stringent primary NAAQS in January 2010. The reconsideration process, which
generally relied on the same data as that used to set the 2008 standard, led EPA to recommend a
primary NAAQS of 0.070 ppm (70 ppb), within the range recommended by the CASAC Ozone

22 Letter of Rogene Henderson, Chair, Clean Air Scientific Advisory Committee, to Hon. Stephen L. Johnson, EPA
Administrator, October 24, 2006, available at http://yosemite.epa.gov/sab/sabproduct.nsf/
AB290E0DB8B72A33852572120055858F/$File/casac-07-001.pdf.
23 “Review of National Ambient Air Quality Standards for Ozone Final Staff Paper, Human Exposure and Risk
Assessments and Environmental Report,” Fact Sheet, at http://www.epa.gov/ttn/naaqs/standards/ozone/data/
2007_01_finalsp_factsheet.pdf.
24 All of EPA’s references to the 2008 standard are expressed as parts per million (e.g., 0.075 ppm), but many
references in the press convert this to a more readable parts per billion (i.e., 75 parts per billion).
25 Detailed information on the designations, including links to Federal Register notices, can be found on EPA’s website
at http://www.epa.gov/airquality/ozonepollution/designations/2008standards/regs.htm#may12.
26 U.S. EPA, “Nonattainment Designations for the 2008 Ozone Standards—Counties by State, April 30, 2012 and May
31, 2012,” at http://www.epa.gov/air/ozonepollution/designations/2008standards/final/finaldes.htm. A few states had
certified monitoring data for 2009-2011, and their designations were based on that three-year period. As of January
2015, the number is 227 counties in 26 states and the District of Columbia. For a listing, see U.S. EPA Green Book, “8-
Hr Ozone (2008) Nonattainment Areas,” at http://www.epa.gov/airquality/greenbook/hntc.html.
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Review Panel in 2008. A draft final standard was prepared and sent to OMB for final interagency
review in the summer of 2011, but was withdrawn at the President’s request in September 2011.
The Current Review
The current review, after assessing hundreds of new studies, has reached conclusions similar to
those of the 2008 process:
The available scientific evidence and exposure/risk information provide strong support for
considering a primary O3 [ozone] standard with a revised level in order to increase public
health protection, including for at-risk populations and lifestages. Staff concludes that it is
appropriate in this review to consider a revised primary O3 standard level within the range of
70 ppb to 60 ppb. A standard set within this range would result in important improvements in
public protection, compared to the current standard, and could reasonably be judged to
provide an appropriate degree of public health protection, including for at-risk populations
and lifestages. In its advice to the Administrator, CASAC also concluded that the scientific
evidence and exposure/risk information support consideration of standard levels from 70 to
60 ppb. Within this range, CASAC concluded that a level of 70 ppb would provide little
margin of safety and, therefore, provided the policy advice that the level of the O3 standard
should be set below 70 ppb.27
Based on these recommendations, the EPA Administrator proposed revising the primary NAAQS
on November 26, 2014, to a level somewhere in the range of 65 to 70 ppb. She also asked for
comments on retaining the current 75 ppb standard or promulgating a standard of 60 ppb.
The Secondary Standard
As part of the review completed in 2008, the 2009-2011 reconsideration process, and the current
review, EPA has also assessed the secondary NAAQS for ozone. As explained above, secondary
NAAQS are standards necessary to protect public welfare, a broad term that includes damage to
crops, vegetation, property, building materials, climate, etc.28 Prior to 2008, the secondary
standard was identical to the primary standard—0.08 ppm beginning in 1997.
Ozone affects both tree growth and crop yields, and the damage from exposure is cumulative over
the growing season. In order to address this damage, EPA staff recommended in the 2008 review
that the Administrator establish a new form for the secondary standard: a seasonal (three-month)
average that would cumulate hourly ozone exposures for the daily 12-hour daylight window
(termed a “W126 index”).29 The staff initially recommended a standard in a range of 7-21 parts
per million-hours (ppm-hours). CASAC’s ozone panel agreed unanimously that the form of the
secondary standard should be changed as the staff suggested, but it did not agree that the upper
bound of the range should be as high as 21 ppm-hours, suggesting that the upper bound be no
higher than 15 ppm-hours.30

27 U.S. EPA, 2014 Policy Assessment, p. ES-5.
28 The Clean Air Act’s definition of welfare is found in Section 302(h) of the act (42 U.S.C. 7602(h)).
29 The index gives greater weight to higher concentrations of ozone. For a detailed explanation of how it is calculated,
see U.S. EPA, “June 2007 Proposal to Revise the National Ambient Air Quality Standards for Ground-level Ozone,
General Overview,” pp. 14-15, at http://www.epa.gov/groundlevelozone/pdfs/20070627slides.pdf.
30 Letter of Rogene Henderson, Chair, Clean Air Scientific Advisory Committee, to Hon. Stephen L. Johnson, EPA
(continued...)
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The Administrator’s June 2007 proposal was in line with the staff recommendation, 7-21 ppm-
hours, but his final March 2008 choice was to duplicate the primary standard he promulgated at
that time. He set a secondary standard at 0.075 ppm averaged over 8 hours, rejecting the advice of
both CASAC and his staff.
The secondary standard carries no deadline for attainment and has never been the subject of
penalties or sanctions for areas that failed to meet it, but there was substantial disagreement
between the Bush Administration EPA and the Office of Management and Budget over the form
in which this standard should be set. OMB maintained that EPA had failed to consider or evaluate
the effects of a W126 standard on “economic values, personal comfort, and well-being”—terms
that are also included in the Clean Air Act’s definition of welfare—and thus did not provide a
balanced consideration of welfare effects, as required by the act. OMB also maintained that EPA
had not adequately demonstrated that the proposed secondary standard would be more protective
than one set equal to the primary standard.31 Ultimately, OMB prevailed.32
Upon reconsideration, the 2011 draft final standards would also have adopted the W126 index
and would have set the secondary standard at 13 ppm-hours, in line with CASAC’s
recommendations. With the President’s request to withdraw the draft standard and await
completion of the current five-year review, a seasonal standard has yet to be implemented.
EPA’s August 2014 Policy Assessment renewed this debate, this time with an additional thumb on
the scale. The agency’s staff again recommended that the Administrator set a secondary standard
using the W126 index. The staff recommended a standard somewhere in the range of 7 to 17
ppm-hours, similar to CASAC’s recommended range of 7 to 15 ppm-hours.33 In the interim, the
D.C. Circuit Court of Appeals also weighed in. In a July 23, 2013, decision, Mississipi v. EPA, the
court remanded the 2008 secondary standard to EPA for further explanation or reconsideration:
the court found that “EPA must expressly ‘determine what level of ... protection is requisite to
protect the public welfare,’ [citation omitted] and explain why this is so.”34
The November 2014 proposal is something of a hybrid: it describes the protection offered by the
proposed secondary standard in terms of a cumulative seasonal approach, but retains the practice
of making the standard identical to the primary standard. The overview of the agency’s proposal
states:

(...continued)
Administrator, March 26, 2007, p. 3, available at http://yosemite.epa.gov/sab/sabproduct.nsf/
FE915E916333D776852572AC007397B5/$File/casac-07-002.pdf.
31 See “Ozone Secondary Standard,” Memorandum of Marcus Peacock, EPA Deputy Administrator, to Susan Dudley,
OMB, March 7, 2008, p. 2, at http://oversight-archive.waxman.house.gov/documents/20080520091448.pdf. The House
Oversight and Government Reform Committee made this and other relevant documents from the 2008 review
available, two months after the review’s completion. See House Oversight and Government Reform Committee,
“Supplemental Information on the Ozone NAAQS,” Memorandum from Majority Staff to Members of the committee,
May 20, 2008, in White House Overruled EPA Administrator on Ozone Regulation, at http://oversight-
archive.waxman.house.gov/story.asp?id=1958. For additional discussion, also see CRS Report RL34057, Ozone Air
Quality Standards: EPA’s March 2008 Revision
, by James E. McCarthy.
32 Letter of Susan E. Dudley, Administrator, Office of Information and Regulatory Affairs, OMB, to Stephen L.
Johnson, EPA Administrator, March 13, 2008, at http://oversight-archive.waxman.house.gov/documents/
20080520092019.pdf.
33 See U.S. EPA, 2014 Policy Assessment, pp. ES-9 to ES-12.
34 Mississippi v. EPA, 723 F.3d 246, 272, 273 (D.C. Cir. 2013) (ellipses in original).
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• EPA is proposing that the secondary standard should provide protection against
the cumulative exposures that can damage plants and trees during the consecutive
three months in the growing season when daytime ozone concentrations are the
highest and plant growth is most affected.
• The Agency is proposing to define this necessary protection in terms of a “W126
index” in a range of 13 to 17 parts per million-hours (ppm-hours), averaged over
three years. A “W126 index,” named for the formula used to calculate it, is a
seasonal index often used to assess the impact of ozone on ecosystems and
vegetation.
• To achieve a level of protection equivalent to 13 to 17 ppm-hours based on the
W126 metric, EPA is proposing to set an 8-hour secondary standard at a level
within the range of 65 to 70 ppb. EPA analyzed data from air quality monitors
and found that setting a standard in a W126 form would not provide additional
protection beyond an 8-hour standard.35
Whether the last bullet is sufficient to address the D.C. Circuit decision in the Mississippi case is
an open question, and may depend on the final level chosen for the standard. CASAC, in its
review of the EPA staff recommendations, made two points that might argue against it being
sufficient. First, they stated:
The CASAC does not support a level higher than 15 ppm-hrs. For example, at 17 ppm-hrs,
the median tree species has 6% relative biomass loss, and the median crop species has over
5% yield loss. These levels are unacceptably high. ... Furthermore, there are specific
economically significant crops, such as soybeans, that may not be protected at 15 ppm-hrs
but would be protected at lower levels. A level below 10 ppm-hrs is required to reduce foliar
injury. A level of 7 ppm-hrs is protective of relative biomass loss for trees and offers
additional protection against crop yield loss and foliar injury.36
Second, CASAC specifically did not recommend a three-year average for the secondary standard,
stating that if the agency chose a three-year average, “then the level of the standard should be
revised downward such that the level for the highest three-month summation in any given year of
the three-year period would not exceed the scientifically recommended range of 7 ppm-hrs to 15
ppm-hrs.”37
Controlling Ozone Pollution
Controlling ozone pollution is more complicated than controlling many other pollutants, because
ozone is not generally emitted directly by pollution sources. Rather, it forms in the atmosphere
when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in the presence of

35 U.S. EPA, “Overview of EPA’s Proposal to Update the Air Quality Standards for Ground-Level Ozone,” December
1, 2014, p. 3, at http://www.epa.gov/air/ozonepollution/pdfs/20141125fs-overview.pdf. The preamble to the proposed
rule, on pages 413-428 (or 79 Federal Register 75347-75351), provides a similar, but more detailed explanation, at
http://www.epa.gov/air/ozonepollution/pdfs/20141125proposal.pdf.
36 Letter of Dr. H. Christopher Frey, Chair, Clean Air Scientific Advisory Committee, to Hon. Gina McCarthy,
Administrator, U.S. EPA, June 26, 2014, p. iii, at http://yosemite.epa.gov/sab/sabproduct.nsf/
5EFA320CCAD326E885257D030071531C/$File/EPA-CASAC-14-004+unsigned.pdf.
37 Ibid., pp. iii-iv.
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sunlight. The ozone concentration is as dependent on the temperature and amount of sunshine as
it is on the presence of the precursor gases.
In general, ozone is a summertime pollutant. Other factors being equal, a cool, cloudy summer
will produce fewer high ozone readings than a warm, sunny summer.
There are also complicated reactions that affect ozone formation. In general, lower emissions of
precursor gases (particularly lower emissions of VOCs) lead to less ozone. But under some
conditions, higher emissions of NOx lead to lower ozone readings. This makes modeling ozone
air quality and predicting attainment more difficult and contentious than the modeling of other air
pollutants, and can affect consultations between EPA and the states to determine the boundaries of
nonattainment areas and the adequacy of SIPs.
Most stationary and mobile sources are considered to be contributors to ozone pollution. Thus,
there are literally hundreds of millions of sources of the pollutants of concern, and control
strategies require implementation of a wide array of measures. Among the sources of VOCs are
motor vehicles (about one-fourth of total emissions), industrial processes, particularly the
chemical and petroleum industries, and any use of paints, coatings, and solvents (about 40% for
these sources combined). Service stations, pesticide application, dry cleaning, fuel combustion,
and open burning are other significant sources of VOCs. Nitrogen oxides come overwhelmingly
from motor vehicles and fuel combustion by electric utilities and other industrial sources.
Wintertime Ozone
An emerging set of issues has arisen in regard to wintertime ozone pollution in rural areas of the
Western United States. Ozone is generally considered a summertime pollutant, but winter
exceedances of the ozone NAAQS have recently been found to occur near oil and gas fields in
rural areas of Wyoming, Utah, and Colorado.38 At times, ozone concentrations as high as those in
Los Angeles, the nation’s smoggiest city, have been found in these areas—principally the Upper
Green River Basin of Wyoming, the Uintah Basin of Utah, and a nearby area of Colorado. Thus
far, only one of these areas, the Upper Green River Basin area of Wyoming, has been designated
nonattainment.
The mechanism of ozone formation in the areas is still being studied, but recent studies have
found that the thousands of oil and gas wells in the two basins release volatile organic compounds
(VOCs) that react with nitrogen oxides (NOx) from oil and gas operations and coal-fired power
plants to create ozone. A study of the Uintah Basin by the National Oceanic and Atmospheric
Administration, EPA, the Bureau of Land Management, the Western Energy Alliance, the Utah
Department of Environmental Quality, and seven universities found that sources external to the
basin are not major sources of the ozone found within it, and that among inventoried sources
within the basin, 98% to 99% of the VOCs and 57% to 61% of the NOx come from oil and gas
operations.39 The sunlight necessary for ozone to be created is magnified when it is reflected off

38 Wintertime ozone levels are occasionally elevated in urban areas, as well, when concentrations of pollution become
trapped in cold stagnant air near the Earth’s surface. What is unusual about the rural areas discussed in this section of
the report is that they do not experience high ozone concentrations in warm weather. In addition, elevated ozone in
Western rural areas is a newly observed phenomenon, whereas pollution episodes associated with temperature
inversions in urban areas have been observed for decades.
39 2012 Uintah Basin Winter Ozone & Air Quality Study, Final Report, February 1, 2013, p. 2, at http://rd.usu.edu/files/
(continued...)
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of heavy snow cover. Snow cover also helps create temperature inversions that trap polluted air in
the basins. In winters with little snow, there have been few exceedances of the standards.40
EPA has recently promulgated standards requiring the reduction of VOC emissions from oil and
gas production and transmission operations, including a requirement to use “green completions”
on hydraulically fractured onshore natural gas wells. (For a discussion, see CRS Report R42986,
An Overview of Air Quality Issues in Natural Gas Systems, by Richard K. Lattanzio.) The impact
of these regulations on wintertime ozone concentrations is yet to be determined.
Costs and Benefits of Control
As noted elsewhere in this report (“The Role of Cost,” below), EPA is prohibited by statute from
taking cost into account in setting NAAQS. Despite that prohibition, in order to comply with an
executive order (E.O. 12866) and guidance from the Office of Management and Budget, the
agency produces a Regulatory Impact Analysis (RIA) analyzing in detail the costs and benefits of
new or revised NAAQS standards.41
EPA’s Cost Estimates
The RIA for the 2014 proposed NAAQS shows a range of estimates for three possible standards:
70 ppb (the high end of the range for the proposed NAAQS); 65 ppb (the low end of the range for
the proposed NAAQS); and 60 ppb (which EPA did not propose, but on which EPA seeks
comment). EPA’s estimates of the nationwide benefits and costs of the three options for all areas
except California are summarized in Table 2. The table shows projected annual costs and benefits
for the year 2025. The RIA states: “We selected 2025 as the primary year of analysis because
most areas of the U.S. will likely be required to meet a revised ozone standard by 2025.... ”42
Table 2. Estimated Annual Costs and Health Benefits of Ozone NAAQS Options,
Nationwide Except California, in 2025
(in billions of 2011 dollars)
Option Costs
Health
Benefits
70 ppb
$3.9
$6.4 to $13.0
65 ppb
$15
$19 to $38
60 ppb*
$39
$34 to $70
Source: U.S. EPA, RIA of the Proposed Revisions to the NAAQS for Ground-Level Ozone, November 2014.

(...continued)
uploads/ubos_2011-12_final_report.pdf. The remaining NOx comes primarily from the Bonanza Power Plant.
40 Ibid. In the Uintah Basin, for example, winter 2011-2012 measurements indicated no exceedance of the 75 ppb
NAAQS for ozone: the highest 8-hour average reading was 63 ppb. In 2010-2011, by contrast, there were 25 winter
days with ozone levels exceeding the 75 ppb standard, with the highest 8-hour reading being 139 ppb.
41 http://www.archives.gov/federal-register/executive-orders/pdf/12866.pdf.
42 2014 RIA, p. ES-2, at http://www.epa.gov/air/ozonepollution/pdfs/20141125ria.pdf.
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* EPA did not propose a 60 ppb standard, but sought comment on that option and provided cost and benefit
estimates for it.
Notes: The data reflect annualized costs and annual monetized benefits of achieving the standard in 2025 for all
areas of the United States except California. Because of more severe pollution, the Clean Air Act will give most
areas of California until the 2030s to reach attainment. The estimates assume a 7% discount rate.
The estimated annual nationwide costs are $3.9 billion in 2025 for a 70 ppb standard or $15
billion for a 65 ppb standard. Although these are large sums, they are substantially less than cost
estimates EPA provided for the same range of standards in 2008 and 2011. At that time, EPA
projected costs of $19 billion to $25 billion to attain a 70 ppb standard, or $32 billion to $44
billion to attain a 65 ppb standard. Three factors account for the reduction in cost:
1. The baseline from which additional costs are projected is now set at 75 ppb (the
current standard). In 2011, EPA projected $7.6 billion to $8.8 billion in costs to
reach what is now the baseline.
2. Other rules promulgated since 2011 (notably the Tier 3 auto emission and
gasoline standards and two rules affecting power plants) are expected to reduce
ozone precursors whether or not EPA revises the NAAQS. In fact, as shown in
Figure 3, the RIA projects that, by 2025, these other (already promulgated) rules
will bring monitored ozone levels to 70 ppb or below in all but 9 of the 358
counties outside California currently showing nonattainment with the proposed
70 ppb level. Similarly, the agency states that all but 68 of the 558 counties
currently showing nonattainment with a 65 ppb standard would reach attainment
by 2025 as a result of already promulgated EPA and state standards.43
3. As a result of factors 1 and 2, most areas can reach attainment through the
application of “known” controls, the average cost of which in EPA’s estimation is
$2,300 to $3,000 per ton of emission reduction.44 In the 2008-2011 analysis, on
the other hand, EPA had to assume “unknown” controls, costing at least $15,000
per ton of emission reduction, would be needed in many more areas. So, the
average cost of assumed emission controls is far lower in the current RIA.

43 See “Ozone by the Numbers,” p. 2, at http://www.epa.gov/air/ozonepollution/pdfs/20141125fs-numbers.pdf.
44 2014 RIA, p. 7-7.
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Figure 3. EPA Projects That Most Counties Would Meet the Proposed Ozone Standards
in 2025 Without Promulgation of Additional Emission Controls


Source: U.S. EPA, http://epa.maps.arcgis.com/apps/StorytellingSwipe/index.html?appid=a3c9f378699045749a85e9c04728fc79&webmap=
3b3e0960060141c7828fc93b14e3d4d2#.
Notes: EPA did not include areas in California. Because of more severe pollution, the Clean Air Act will give many areas in California until the 2030s to reach
attainment.

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EPA’s Benefit Estimates
EPA’s estimate of monetized nationwide benefits (excluding those in California) range from $6.4
billion to $13.0 billion annually in 2025 for a 70 ppb standard to a range of $19 billion to $38
billion for a 65 ppb standard. The public health benefits of setting a more stringent ozone
standard are the monetized value of such effects as fewer premature deaths, fewer hospital
admissions, fewer emergency room visits, fewer asthma attacks, less time lost at work and school,
and fewer restricted activity days.45 The RIA for the proposed standard states that the nationwide
benefits of a revised standard (excluding California) would include the avoidance of 710 to 1,400
premature deaths annually in 2025 (at 70 ppb) or 2,000 to 4,300 deaths annually (at 65 ppb).46
The dollar value of the avoided premature deaths accounts for 94% to 98% of EPA’s total
monetized benefits.47
The benefit estimates include benefits of reduced fine particle (PM2.5) concentrations associated
with ozone controls, in addition to the benefits of reduced ozone itself. The RIA states that:
“PM2.5 co-benefits account for approximately two-thirds to three-quarters of the estimated
benefits, depending on the standard analyzed and on the choice of ozone and PM mortality
functions used.”48 Including these co-benefits is consistent with the methodology EPA has used in
valuing benefits of many other proposed and promulgated standards, but some observers are
critical of this approach, noting that including these co-benefits results in a net benefit, whereas
an analysis that considered the costs and benefits of ozone reductions in isolation would not. The
control technologies used to capture ozone precursors do capture particles and their precursors,
however. Since they do so at no additional cost, EPA considers this a benefit of the controls.
Other stated benefits of a 65-70 ppb standard in 2025 would include preventing the following,
annually:
• 64-1,700 nonfatal heart attacks,
• 2,090-6,330 hospital admissions and emergency room visits,
• 790-2,300 cases of acute bronchitis,
• 44,000 cases of upper and lower respiratory symptoms,
• 320,000-960,000 cases of exacerbated asthma,
• 395,000-1,180,000 days when people miss work or school, and
• 1.3 million-4.0 million minor restricted activity days.49

45 For a full discussion of these variables and their monetized values, see Chapter 6 of the 2008 RIA at
http://www.epa.gov/ttn/ecas/regdata/RIAs/452_R_08_003.pdf.
46 2014 RIA, p. ES-14.
47 EPA’s estimate uses an approach called the Value of a Statistical Life (VSL). See 2014 RIA, p. 5-87 and pp. 5-56 to
5-66. For additional information on VSL, see archived CRS Report R41140, How Agencies Monetize “Statistical
Lives” Expected to Be Saved By Regulations
, by Curtis W. Copeland. (The author of that report is no longer with CRS.
Questions on it should be directed to Maeve Carey, Analyst in Government Organization and Management.)
48 2014 RIA, p. ES-13, at http://www.epa.gov/air/ozonepollution/pdfs/20141125ria.pdf.
49 2014 RIA, p. ES-15. Implementing emissions controls to reach some of the alternative ozone standard levels would
also reduce other ambient pollutants, according to EPA. However, because the methods used in the RIA to simulate
attainment did not account for changes in ambient concentrations of other pollutants, the RIA does not to quantify the
(continued...)
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California and Other Regional Costs and Benefits
EPA provided separate cost and benefit estimates for California. Because of the statutory
classification scheme for ozone nonattainment areas, under which more polluted areas have more
stringent emission control requirements and more time to reach attainment, most areas of
California will have until the 2030s to reach attainment. EPA’s California estimates use
projections for 2038 (see Table 3). The agency concluded: “Because of these differences in
timing related to California attaining a revised standard, the separate costs and benefits estimates
for post-2025 should not be added to the primary estimates for 2025.”50
EPA also estimated the costs of the proposed NAAQS separately for the eastern United States
(defined as Texas and the 36 states to its north and east) and the western United States (Montana,
Wyoming, Colorado, New Mexico, and other states to the west, excluding California). These
regional estimates show that all of the $3.9 billion estimated cost to achieve a 70 ppb standard is
incurred by emission sources in the eastern United States. At 65 ppb, about 3% of the $15 billion
cost would be incurred in the west, with 97% incurred in the east.
Table 3.Estimated Annual Costs and Health Benefits of Ozone NAAQS Options in
California, Post-2025
(in billions of 2011 dollars)
Option Costs
Health
Benefits
70 ppb
$0.80
$1.1 to $2
65 ppb
$1.6
$2.2 to $4.1
60 ppb*
$2.2
$3.2 to $5.9
Source: U.S. EPA, RIA of the Proposed Revisions to the NAAQS for Ground-Level Ozone, November 2014.
* EPA did not propose a 60 ppb standard, but sought comment on that option and provided cost and benefit
estimates for it.
Notes: The data reflect annualized costs and annual monetized benefits of achieving the standard post-2025 in
California. Because of more severe pollution, the Clean Air Act will give most areas of California until the 2030s
to reach attainment. The agency states that “estimates of costs and benefits for California in a post-2025 time
frame are likely to be relatively more uncertain than the national attainment estimates for 2025.” In particular,
the agency did not project emissions and air quality for any year other than 2025, although it assumed that
emission controls and associated costs would occur through the beginning of 2038; benefits for California were
modeled using population demographics in 2038. The estimates assume a 7% discount rate for future costs and
benefits.
Industry Estimates
Industry sources have generally estimated the future cost of emission controls necessary to attain
a revised ozone NAAQS to be greater than does EPA. A recent study by the National Association
of Manufacturers, for example, which was published four months before EPA’s proposal,

(...continued)
co-benefits of reduced exposure to these pollutants. See 2014 RIA, p. ES-11.
50 2014 RIA, p. ES-3.
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projected the cost of attaining a more stringent ozone NAAQS, as measured in reduced Gross
Domestic Product, at up to $270 billion annually from 2017 to 2040.51
In reaching these conclusions, the NAM study made a number of assumptions different from
those in EPA’s Regulatory Impact Analysis:
• The study looked only at the most stringent option under consideration in the
review process (60 ppb). As noted earlier, EPA has proposed a standard in the
range of 65 to 70 ppb.
• The study used data and assumptions generated by EPA in its 2008-2011 RIAs
for its analysis, although it stressed the need for EPA to provide updated
information. In proposing the standard, the agency has now provided updated
information in the 2014 RIA.
• The study’s baseline didn’t account for some recently promulgated EPA
regulations that will reduce NOx emissions (e.g., the Cross State Air Pollution
Rule).
• A combination of these factors resulted in the NAM study concluding that two-
thirds of the emission reductions needed to reach attainment would have to come
from “unknown controls,” which it estimated could cost as much as $500,000 per
ton of emissions reduced.52 EPA’s modeling of the rule’s costs found that
unknown controls would play a much smaller role: both the number of tons to
which such controls would apply and the cost per ton would be substantially less.
• NAM’s analysis focused exclusively on emissions of NOx, without any
consideration of VOC controls. VOC emissions from petroleum and related
industries have more than quadrupled since 2005, according to EPA, while
emissions from most other sources have declined.53 Recent EPA analyses suggest
that there are low cost emission control options in the oil and gas sector.54
Issues
The current ozone NAAQS review is likely to raise issues regarding the cost of attainment,
background ozone levels, the adequacy of the ozone monitoring network, and the role of federal
versus state and local pollution control measures.

51 NERA Economic Consulting for the National Association of Manufacturers, Assessing Economic Impacts of a
Stricter National Ambient Air Quality Standard for Ozone
, July 2014, p. S-1, at http://www.nam.org/Special/Media-
Campaign/EPA-Overregulation/Ozone-Regulations.aspx. Hereinafter, “NAM Report.”
52 NAM Report, pp. 12-19.
53 See U.S. EPA, National Emissions Inventory (NEI) Air Pollutant Emission Trends Data, at http://www.epa.gov/ttn/
chief/trends/.
54 See, for example, U.S. EPA, Regulatory Impact Analysis, Final New Source Performance Standards and
Amendments to the National Emissions Standards for Hazardous Air Pollutants for the Oil and Natural Gas Industry,
April 2012, at http://www.epa.gov/ttnecas1/regdata/RIAs/oil_natural_gas_final_neshap_nsps_ria.pdf.
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The Role of Cost
Because of its wide reach and potential cost, a proposed revision to the ozone NAAQS is likely to
be among the most controversial rules EPA will consider in the coming year. In past reviews,
industries that emit ozone precursors and broadly-based business groups have complained
forcefully that a more stringent standard would be too costly to attain.
The issue of cost is a perennial one in NAAQS decisions, even though EPA is prohibited by the
Clean Air Act from considering costs in setting the standards. The Clean Air Act’s §109 has been
so interpreted since the NAAQS provisions were added to the act in 1970; in 2001, this
interpretation was affirmed in a unanimous Supreme Court decision, Whitman v. American
Trucking Associations
.55 The Court pointed to numerous other CAA sections where Congress had
explicitly allowed consideration of economic factors, concluding that if Congress had intended to
allow such factors in the setting of a primary NAAQS, it would have been more forthright—
particularly given the centrality of the NAAQS concept to the CAA’s regulatory scheme. The
court concluded that §109(b)(1) “unambiguously bars cost considerations from the NAAQS-
setting process.”56
This is not to say that cost considerations play no role in Clean Air Act decisions, including in
implementation of a NAAQS. Cost-effectiveness is considered extensively by EPA and the states
in selecting emission control options to meet the standards. Also, as discussed above, the agency
prepares cost and benefit estimates at the time it proposes or promulgates a NAAQS, both for
information purposes, and in order to comply with Executive Order 12866, under which the OMB
requires cost-benefit analysis of economically significant rules. But in deciding what level of
ambient pollution poses a health threat, the statute bars consideration of costs.
Many in Congress would like to change this, by revising the CAA to require consideration of cost
in NAAQS decisions. In the 112th Congress, the House twice passed legislation that would have
done so: H.R. 2401 and H.R. 3409 would have required the EPA Administrator to take feasibility
and costs into consideration in setting National Ambient Air Quality Standards. The Senate did
not pass either bill. In the 113th Congress, H.R. 5505/S. 2833 and H.R. 5665 would have required
EPA to take into consideration feasibility and cost in setting ozone NAAQS, as well as
establishing several other conditions on the Administrator’s ozone-NAAQS-setting authority.
Background Ozone Levels
A number of states, particularly in the inter-mountain west, experience what are termed high
“background” levels of ozone. As EPA explains:
Any ozone formed by processes other than the chemical conversion of local or regional
ozone precursor emissions is generically referred to as “background” ozone. Background
ozone can originate from natural sources of ozone and ozone precursors, as well as from
manmade international emissions of ozone precursors. Natural sources of ozone precursor
emissions such as wildfires, lightning, and vegetation can lead to ozone formation by

55 531 U.S. 457 (2001).
56 For further discussion of the American Trucking case, see CRS Report RS20860, The Supreme Court Upholds EPA
Standard- Setting Under the Clean Air Act: Whitman v. American Trucking Ass'ns
, by Robert Meltz and James E.
McCarthy.
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chemical reactions with other natural sources. Another important component of background
is ozone that is naturally formed in the stratosphere through interactions of ultraviolet light
with molecular oxygen. Stratospheric ozone can mix down to the surface at high
concentrations in discrete events called intrusions, especially at higher-altitude locations. The
manmade portion of the background includes any ozone formed due to anthropogenic
sources of ozone precursors emitted far away from the local area (e.g., international
emissions). Finally, both biogenic and international anthropogenic emissions of methane,
which can be chemically converted to ozone over relatively long time scales, can also
contribute to global background ozone levels.57
In its RIA, EPA identifies three definitions of background ozone:
• natural background (ozone that would exist in the absence of any manmade
emissions of ozone precursors);
• North American background (ozone that would exist in the absence of any
manmade emissions of ozone precursors in North America); and
• United States background (ozone that would exist in the absence of any
manmade emissions of ozone precursors within the United States).
What the definitions have in common is that they identify ozone levels that cannot be influenced
by actions within the jurisdiction of concern.
EPA modeling of natural background ozone at high elevation levels in the western United States
found a median value of 24 ppb, with a maximum 8-hour value of 34 ppb.58 U.S. background
ozone had higher values: since it includes more sources of ozone precursors in the definition of
“background,” it had median values in the 30-35 ppb range, with concentrations greater than 40
ppb in Colorado, Nevada, Utah, Wyoming, northern Arizona, eastern California, and parts of New
Mexico.59
EPA noted in the preamble to the proposed rule that, “A number of commenters [on the agency’s
Integrated Science Assessment] expressed the view that the EPA should not lower the level of the
standard because a lower level would be closer to background O3 [ozone] concentrations.”60 The
agency did not agree.
In response to the comments, the proposed rule’s preamble states that “there can be events where
O3 levels approach or exceed the concentration levels being proposed in this notice.” When this
happens, according to agency modeling, it is typically the result of stratospheric intrusions of
ozone, wildfire plumes, or long-range transport of ozone from sources outside the United States.
“In most locations in the U.S., these events are relatively infrequent and the CAA contains
provisions that can be used to help deal with certain events, including providing varying degrees
of regulatory relief.”61

57 2014 RIA, pp. 2-10 and 2-11.
58 2014 RIA, p. 2-13.
59 2014 RIA, pp. 2-13 and 2-14.
60 Proposed Rule, pre-publication copy, p. 196 (79 Federal Register 75287).
61 Proposed Rule, pre-publication copy, pp. 537-538 (79 Federal Register 75382).
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Regulatory relief can include the exclusion of data affected by exceptional events, relief from
more stringent requirements in areas designated as “rural transport areas,” and the application of
CAA Section 179B, which allows EPA to approve state SIP submissions that demonstrate an area
would have met the ozone NAAQS by the attainment date if not for international emissions
contributing to the area.62
The invocation of these regulatory relief measures, as even EPA admits, is not burden-free: each
would require some level of assessment or demonstration by a state and/or EPA.63 States that have
tried to invoke the “exceptional event” exceptions have expressed frustration with the lack of
clarity and the burden involved in meeting EPA’s data requirements and, thus, may not be
confident in the agency’s offer of regulatory relief.64 But, as noted earlier (in Figure 3), EPA does
not believe it will be necessary in most cases to invoke such measures. EPA modeling shows only
nine counties outside of California exceeding a 70 ppb standard without any emission control
measures additional to those already promulgated as of this date. A 65 ppb standard imposes a
somewhat greater burden, but in that case, too, the modeling shows most areas reaching
attainment without additional controls and without invocation of statutory relief provisions.
EPA continues to seek comment on background ozone and related implementation issues. For a
more detailed discussion, see “EPA’s Proposal to Update the Air Quality Standards for Ground-
Level Ozone: Additional Information for States Regarding Background Ozone.”65
Monitoring
The existing network of ozone monitors is concentrated in urban areas, because of the larger
population potentially affected, and because most of the sources of ozone precursor emissions are
located in such areas. But, as noted earlier, ozone is not generally emitted directly by pollution
sources. It forms in the atmosphere, often downwind of emission sources. Thus, rural areas can
have high ozone concentrations. The new form of the secondary NAAQS discussed by EPA
suggested a need for additional monitoring in rural areas to detect impacts of ozone on forests and
agricultural production. Both EPA and state monitoring budgets are constrained, however, raising
questions as to how any additional monitoring requirement would be funded.
The agency, in a 2009 rulemaking separate from the NAAQS, proposed changing the minimum
ozone monitoring requirements for both urban and non-urban areas.66 That proposal would have
required that each state operate at least three ozone monitors in non-urban areas. It would also
have required at least one ozone monitor in each urban area with a population between 50,000
and 350,000. The requirements were not finalized. Although EPA has proposed to change some of
the monitoring requirements as part of the current ozone NAAQS proposal (in particular,
extending the number of months during which ozone must be monitored in 31 states and the

62 See, U.S. EPA, “EPA’s Proposal to Update the Air Quality Standards for Ground-Level Ozone: Additional
Information for States Regarding Background Ozone,” at http://www.epa.gov/groundlevelozone/pdfs/20141203-
background-ozone-states.pdf.
63 Proposed Rule, pre-publication copy, p. 538 (79 Federal Register 75383).
64 See, for example, the comments of state and local officials on EPA’s proposed 2012 Exceptional Events guidance, at
http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OAR-2011-0887-0051.
65 http://www.epa.gov/groundlevelozone/pdfs/20141203-background-ozone-states.pdf.
66 74 Federal Register 34525, July 16, 2009.
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District of Columbia), the agency’s current proposal would not require additional monitors in
smaller urban or rural areas.
The absence of ozone monitors in rural areas is another factor that should help mitigate concerns
of western states that background levels of ozone will cause rural areas in their states to be
designated nonattainment. EPA’s policy in designating ozone nonattainment areas has always
relied on the submission of EPA-certified monitoring data. Areas without monitors (unless they
are contributing to an area with monitored nonattainment) have generally been termed
“unclassifiable.” When non-regulatory monitors indicate exceedance of the ozone NAAQS,
designation as nonattainment does not automatically follow. Rather, the state must first establish
regulatory monitors67 and collect three years of data before submitting a proposed designation to
EPA.68
The Role of Federal Versus State and Local Pollution Control
Measures

EPA has promulgated more stringent standards for most of the major sources of ozone precursors,
including Tier 3 auto emission and fuel standards that will begin to take effect in 2017 and more
stringent standards for power plants that begin to take effect in 2015. (For additional information
on these standards, see CRS Report R43497, Tier 3 Motor Vehicle Emission and Fuel Standards
and CRS Report R42144, EPA’s Utility MACT: Will the Lights Go Out?) These standards should
make the task of demonstrating attainment with a more stringent ozone NAAQS substantially
easier. As noted earlier, EPA estimates that nationwide (except in California) already promulgated
EPA and state regulations would bring all but nine counties into attainment of a 70 ppb ozone
NAAQS by 2025. Many in Congress objected to the standards for motor vehicles, fuels, power
plants, and other sources when they were under consideration, but at this date, the net effect of
repealing them would be to shift the burden of attaining the ozone NAAQS more squarely in the
direction of state and local governments.
A tighter ozone NAAQS has frequently raised the issue of how to control emission sources that
contribute to pollution downwind, in other states. Ozone, which forms in the atmosphere from
chemical reactions of precursor emissions, is the prime example of such downwind pollution.
Under both the Bush and Obama Administrations, EPA has addressed this interstate air pollution
issue—through the Clean Air Interstate Rule (CAIR, 2005) and the Cross State Air Pollution Rule
(CSAPR, 2011). The D.C. Circuit Court of Appeals found fault with both rules, although it
allowed CAIR to take effect pending promulgation of an acceptable replacement.69

67 Ambient air monitors used to determine whether an area is in attainment of a NAAQS (a “regulatory monitor”) must
meet three criteria: it must use methods specified in 40 C.F.R. Part 58, Appendix C; it must meet siting criteria
specified in 40 C.F.R. Part 58, Appendix E; and it must meet quality assurance criteria specified in 40 C.F.R. Part 58,
Appendix A.
68 The Uintah Basin in Utah provides an example. Non-regulatory monitors indicated ozone levels that exceeded the
NAAQS in December 2009, January-March 2010, and January-March 2011. The state began collecting regulatory
monitoring data in April 2011. As of December 2014, the state has not submitted to EPA data with which to designate
the area nonattainment, and the area has not been designated nonattainment.
69 For a discussion of the Bush and Obama Administration regulations and the D.C. Circuit decisions remanding them,
see CRS Report R42895, Clean Air Issues in the 113th Congress: An Overview.
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Whether EPA had correctly interpreted its authority to control emissions leading to downwind
pollution ultimately reached the Supreme Court. On April 29, 2014, in a 6-2 decision (EPA v.
EME Homer City Generation, LLP
), the Court upheld the methodology at the heart of EPA’s
CSAPR standard-setting process.70 The rule was remanded to the D.C. Circuit for consideration
of additional issues, however, and has still not been implemented.
The complexity of establishing controls on air pollution that crosses state lines has suggested to
many stakeholders that Congress should revisit the Clean Air Act sections that authorize or
require such controls, and numerous bills have been introduced on the subject over the years.
Since the last major CAA revision in 1990, however, none of those bills has made it to the floor
of the House or Senate. The Supreme Court’s decision in the Homer City case may have further
reduced the odds of congressional action on the subject. With pollution transport issues still to be
resolved by the D.C. Circuit, though, and states expressing continued interest, the issue should
perhaps not be considered dead yet.
Next Steps
The ozone NAAQS proposal appeared in the Federal Register on December 17, 2014. The
publication of the proposal began a public comment period that is scheduled to run until March
17, 2015. Public comment periods are frequently extended beyond the original time period in the
case of controversial or complicated rules, such as the ozone NAAQS; but the agency does face a
time constraint (a court order to complete the review by October 1, 2015), which argues against a
longer comment period.
EPA held three public hearings: in Arlington, TX, and Washington, DC, on January 29; and in
Sacramento, CA, on February 2.
Congress had already taken an interest in the ozone proposal before it was released. Three bills
were introduced in the 113th Congress that would have affected the timing and/or EPA’s authority
to promulgate an ozone NAAQS:
• H.R. 4947/S. 2514 would have delayed the review and revision of the standard
for three years and required future reviews at 10-year rather than 5-year intervals;
• H.R. 5505/S. 2833 would have prohibited a more stringent standard until at least
85% of the counties in nonattainment areas as of January 1, 2014, attained the
current standard, and would have required EPA to consider feasibility and cost in
setting an ozone NAAQS, among other provisions; and
• H.R. 5665 would also have required EPA to consider feasibility and cost in
setting a new or revised ozone NAAQS, and would have required a detailed
report to Congress at least 180 days before a new or revised standard could be
proposed and congressional approval before a final standard could take effect.
No action was taken on these bills, but it is likely that these or other bills to address the ozone
NAAQS will be introduced in the 114th Congress.

70 EPA v. EME Homer City Generation, L.P., 134 S. Ct. 1584 (2014).
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The Senate Environment and Public Works Committee Subcommittee on Clean Air and Nuclear
Safety held a hearing on the proposed ozone NAAQS on December 17, 2014.


Author Contact Information

James E. McCarthy

Specialist in Environmental Policy
jmccarthy@crs.loc.gov, 7-7225


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