Order Code RS22682
June 20, 2007
Ozone Air Quality Standards: EPA’s Proposed
James E. McCarthy
Specialist in Environmental Policy
Resources, Science, and Industry Division
EPA is expected to propose changes to the National Ambient Air Quality Standard
for ozone on June 20, 2007. The proposal follows a multi-year review of the science
regarding ozone’s effects on public health and welfare. If the agency decides to
strengthen the standard, it will set in motion a long and complicated implementation
process that has far-reaching impacts for public health, for sources of pollution in
numerous economic sectors, and for states and local governments. This report discusses
the standard-setting process and the specifics of the ozone standard, and describes the
steps that will follow EPA’s proposal.
On June 20, 2007, EPA is expected to propose revisions to the National Ambient Air
Quality Standard (NAAQS) for ozone, with final action expected in March 2008.1 The
proposal will set in motion a public comment period, including public hearings in several
locations around the country. Because it has widespread implications for public health
and for the pollution control measures that will be imposed on sectors of the economy,
the proposal is likely to stir congressional interest.
This report provides background on NAAQS, the process used to establish them, the
existing ozone standard, and EPA’s proposal, as well as information regarding the
potential effects of any revision to the standard.
What Are NAAQS?
As defined in Section 109 of the Clean Air Act, NAAQS are standards that apply to
ambient (outdoor) air. The act directs EPA to set both primary and secondary standards.
Primary NAAQS are standards, “the attainment and maintenance of which in the
The schedule was set by a consent decree that settled a lawsuit filed by the American Lung
Association (American Lung Association v. Leavitt, D.D.C., No. 03-778, modified consent
decree approved 12/16/04). EPA agreed that it would propose whether to retain or revise the
ozone standard by June 20, 2007, and take final action by March 12, 2008.
judgment of the [EPA] Administrator ... are requisite to protect the public health,” with
“an adequate margin of safety.” Secondary NAAQS are standards necessary to protect
public welfare, a broad term that includes damage to crops, vegetation, property, building
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. Once a NAAQS has been set, the agency, using monitoring data and other
information submitted by the states, identifies areas that exceed the standard and must,
therefore, reduce pollutant concentrations to achieve it. After these “nonattainment” areas
are identified, state and local governments have three years to produce State
Implementation Plans which outline the measures they will implement to reduce the
pollution levels and attain the standards. Depending on the severity of the pollution,
ozone nonattainment areas have anywhere from 3 to 20 years to actually attain the
EPA also acts to control many of the NAAQS pollutants wherever they are emitted,
through national standards for products that emit them (particularly mobile sources, such
as automobiles) and emission standards for new stationary sources, such as power plants.
Thus, establishment or revision of a NAAQS sets in motion a long and complicated
implementation process that has far-reaching impacts for public health, for sources of
pollution in numerous economic sectors, and for states and local governments.
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.”3 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 he determines that additional pollutants meet the act’s criteria, or delete
them if he concludes that they no longer do so.
The act requires the agency to review each NAAQS every five years. That schedule
is rarely met, but it often triggers law suits that force the agency to undertake a review.
In the case of ozone, the last review of the NAAQS was completed in 1997. As noted
earlier, the American Lung Association filed suit over EPA’s failure to complete a review
in 2003, and a consent decree established the schedule EPA is following.4
The Clean Air Act’s definition of welfare is found in Section 302(h) of the act.
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. Pollutants that
are less widely emitted are generally classified as “hazardous air pollutants” and are regulated
under a different section of the act (Section 112).
See note 1.
The NAAQS Process
Reviewing an existing NAAQS is a long process that is described elsewhere in more
detail.5 To summarize briefly, EPA scientists review the scientific literature published
since the last NAAQS revision, and summarize it in a report known as a Criteria
Document. The review process frequently identifies more than a thousand scientific
studies on topics as wide-ranging as the physics and chemistry of ozone in the
atmosphere; environmental concentrations, patterns, and exposure; dosimetry and animalto-human extrapolation; toxicology; interactions with co-occurring pollutants; controlled
human exposure studies; epidemiology; effects on vegetation and ecosystems; effects on
UVB exposures and climate; and effects on man-made materials. A second document
that EPA prepares, the Staff Paper, summarizes the information compiled in the Criteria
Document and provides the Administrator with options regarding the indicators,
averaging times, statistical form, and numerical level (concentration) of the NAAQS.
To ensure that these 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 quite large. The current ozone review panel, for example, has 23 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 review the
agency’s work during NAAQS-setting and NAAQS-revision, rather than conducting their
own independent reviews.
The Ozone Standard
The ozone standard affects a larger percentage of the population than any of the other
NAAQS. About half the U.S. population lives in ozone nonattainment areas, 156 million
people in all.6 If the standard is strengthened as a result of the current review, as many
expect, more areas would likely be affected, and those already considered nonattainment
might have to impose more stringent emission controls.
The Primary Standard. The current primary (health-based) standard,
promulgated in 1997, is set at 0.08 parts per million (ppm), averaged over an 8-hour
period. Allowing for rounding, EPA considers areas with readings as high as 0.084 to
have attained the standard.
The current review has found evidence of health effects, including mortality, at
levels of exposure below the current standard. As a result, both EPA staff and the Clean
For a discussion of the process, and of changes to it that EPA is now implementing, see CRS
Report RL33807, Air Quality Standards and Sound Science: What Role for CASAC?, by James
For information on the nonattainment areas, including maps and population data, see EPA’s
“Green Book” at [http://www.epa.gov/oar/oaqps/greenbk/index.html].
Air Scientific Advisory Committee have recommended strengthening the standard.
According to CASAC, “There is no scientific justification for retaining the current
primary 8-hr NAAQS ....”7 The panel unanimously recommended a range of 0.060 to
0.070 ppm for the primary 8-hour standard.
EPA staff also recommended strengthening the standard, in wording not quite so
direct. The staff stated, “The overall body of evidence on ozone health effects clearly
calls into question the adequacy of the current standard.” They recommended
“considering a standard level within the range of somewhat below 0.080 parts per million
(ppm) to 0.060 ppm.”8
Because a strengthening of the standard would result in additional areas being
designated nonattainment, and would mean that current nonattainment areas might have
to adopt additional pollution control measures in order to reach attainment, numerous
industry groups are reported to have challenged the scientific conclusions in meetings
with Administration officials.9
The Secondary Standard. As part of its current review, EPA has also assessed
the secondary NAAQS for ozone, which is currently identical to the primary standard.
Ozone affects both tree growth and crop yields, and the damage from exposure is
cumulative over the growing season. In order to provide protection against ozone’s
adverse impacts, EPA staff recommended a new seasonal (3-month) average for the
standard that would cumulate hourly ozone exposures for the daily 12-hour daylight
window (termed a “W126 index”). The staff recommended a standard in a range of 7 21 parts per million-hours (ppm-hrs). 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. The panel
recommended that the upper bound be no higher than 15 ppm-hours.10
Controlling Ozone Pollution
Controlling ozone pollution is more complicated than controlling many other
pollutants, because ozone is not emitted directly by pollution sources. Rather, it forms
in the atmosphere when volatile organic compounds (VOCs) react with nitrogen oxides
(NOx) in the presence of sunlight. The ozone concentration is as dependent on the
temperature and amount of sunshine as it is on the presence of the precursor gases. Ozone
Letter of Rogene Henderson, Chair , Clean Air Scientific Advisory Committee, to Hon. Stephen
L. Johnson, EPA Administrator, October 24, 2006, available at [http://www.epa.gov/sab/pdf/
“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/
“Activists, Industry Offer Competing Data as EPA Ozone Deadline Nears,” InsideEPA Clean
Air Report, June 14, 2007.
Letter of Rogene Henderson, Chair , Clean Air Scientific Advisory Committee, to Hon.
Stephen L. Johnson, EPA Administrator, March 26, 2007, p. 3, available at [http://www.epa.gov/
is a summertime pollutant, in general. 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 lead to less ozone, particularly lower emissions of VOCs. 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.
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 40% 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.
Costs and Benefits of Control
EPA is prohibited from taking cost into account in setting NAAQS, but to comply
with an executive order, the agency generally produces a Regulatory Impact Analysis
(RIA) analyzing in detail the costs and benefits of new or revised NAAQS standards. In
preparation for proposing the ozone NAAQS, EPA is reported to have analyzed three
alternatives: 0.074 ppm, 0.070 ppm, and 0.064 ppm.11 Until a standard is proposed, it is
unclear how much detail the agency will make available from its analyses.
After the Administrator signs a proposed regulation, the proposal goes to the Federal
Register for publication, a process that may take several weeks. Section 307(d) of the
Clean Air Act sets out the procedures for proposal and promulgation. It requires the
establishment of a rulemaking docket; it requires that the notice of proposed rulemaking
in the Federal Register be accompanied by a statement of the proposal’s basis and
purpose, including a summary of the factual data on which the proposed rule is based, the
methodology used in obtaining and analyzing the data, and the major legal interpretations
and policy considerations underlying the proposed rule. The statement is required to set
forth or summarize and provide a reference to any pertinent findings, recommendations,
and comments by CASAC and the National Academy of Sciences, and, if the proposal
differs in any important respect from any of these recommendations, provide an
explanation of the reasons for such differences. The act also requires that any drafts of
proposed and final rules submitted by the Administrator to the Office of Management and
Budget (OMB) prior to proposal or promulgation, all documents accompanying those
drafts, and all written comments thereon and EPA responses to such comments, be placed
in the docket no later than the date of proposal.
“With June Deadline Looming, EPA Sends Revisions of Ozone Standard to White House,”
Daily Environment Report, May 30, 2007, p. A-11.
Publication in the Federal Register will set in motion a public comment period
(generally 60 or 90 days), which is likely to include public hearings at several locations
around the country. Upon completion of the public comment period, the agency reviews
and summarizes the public comments and the Administrator makes a final choice
regarding the standard. Under the consent agreement between EPA and the American
Lung Association, the Administrator is required to do so by March 12, 2008.