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Updated July 6, 2020
Ozone and Particulate Matter Air Standards: EPA Review
The Clean Air Act (CAA) requires the U.S. Environmental
Notwithstanding air quality progress since 1970, ozone and
Protection Agency (EPA) to review standards for national
PM concentrations currently exceed the NAAQS in some
ambient air quality every five years. In 2018, EPA
areas (“nonattainment areas”). Table 1 lists these NAAQS
announced strategies to expedite the National Ambient Air
and the estimated population in nonattainment areas.
Quality Standard (NAAQS) review while concurrently
disbanding a pollutant-specific scientific review panel that
Table 1. Selected NAAQS and the Estimated U.S.
has historically advised agency staff during their reviews.
Population in Corresponding Nonattainment Areas
Although the CAA allows the EPA Administrator to specify
Estimated U.S.
the procedures for review of the NAAQS, past EPA reviews
Primary
Population in
and revisions have garnered considerable congressional
NAAQS
Standard
Nonattainment Areas
oversight. This In Focus discusses EPA’s current NAAQS
2015 Ozone
70 ppb (8-hour)
122 mil ion
reviews for ozone and particulate matter (PM), which EPA
2012 Fine PM
12.0 µg/m3 (Annual)
21 mil ion
had sought to complete in 2020, and issues of potential
1987 Coarse PM 150 µg/m3 (24-hour)
6 mil ion
interest to Congress.
Source: CRS, as adapted from EPA Green Book (May 31, 2020),
which lists nonattainment areas (https://www.epa.gov/green-book).
Background on Ozone and Particulate Matter
Estimated population based on 2010, rounded to nearest mil ion.
Ozone and PM are two of six principal pollutants referred
to as “criteria pollutants” for which EPA has promulgated
Notes: Units of measure are parts per bil ion (ppb) and micrograms
NAAQS under the CAA (42 U.S.C §7408(a)(1)).
per cubic meter of air (µg/m3). See 40 C.F.R. Part 50 for detailed
NAAQS. Table presents the most recent PM and ozone NAAQS.
Ground-level ozone, the primary component of smog, is
formed when nitrogen oxides (NOx) react with volatile
NAAQS Statutory Requirements
organic compounds (VOCs) in sunlight. Ground-level
NAAQS do not directly limit emissions. Rather, NAAQS
ozone is associated with health effects, such as aggravated
are concentration-based standards for ambient (outdoor)
asthma, chronic bronchitis, heart attacks, and premature
pollution. Under the CAA, Congress mandated that EPA
death. EPA has identified natural and anthropogenic
establish two types of NAAQS for each criteria pollutant—
sources of ozone and ozone precursors (e.g., NOx and
a primary NAAQS, which must protect public health with
VOCs), including factories, lightning, power plants,
an “adequate margin of safety,” and a secondary NAAQS,
vegetation, vehicles, volatile chemical products (e.g., paints
which must “protect public welfare from any known or
and solvents), and wildfires.
anticipated adverse effects” (42 U.S.C. §7409(b)). Public
welfare includes damage to crops, vegetation, property,
PM refers to a mixture of solid particles and liquid droplets
building materials, and climate (42 U.S.C. §7602(h)).
in the atmosphere. PM components may include acids,
organic chemicals, metals, and soil or dust particles. The
The CAA establishes a framework for EPA to set NAAQS
size of PM varies, ranging from tiny particles that can be
based on the “latest scientific knowledge” through a notice-
seen only through a high-power microscope to larger
and-comment rulemaking process (42 U.S.C. §§7408,
particles (e.g., soot). Exposure to PM has been associated
7409). The CAA requires EPA to review the NAAQS and
with adverse health effects (e.g., aggravated asthma,
the science upon which they are based every five years and
chronic bronchitis, and premature death). PM has also been
then revise the NAAQS if necessary. The CAA also
linked with haze formation and other ecological effects.
requires EPA to appoint an independent scientific review
committee composed of seven members, which has become
Typical sources of fine PM (PM2.5)—measured at 2.5
the Clean Air Scientific Advisory Committee (CASAC).
micrometers or less in diameter—include emissions from
The act directs CASAC to review the NAAQS every five
vehicles, smokestacks, and fires. Coarse PM (PM10)—
years and recommend to the EPA Administrator “any new
generally measuring 10 micrometers or less in diameter—is
national ambient air quality standards and revisions … as
often associated with dust from paved and unpaved roads,
may be appropriate” (42 U.S.C. §7409(d)(2)).
construction and demolition operations, certain industrial
processes and agriculture operations, and biomass burning.
EPA’s Review of the NAAQS
In addition, precursor emissions (e.g., sulfur oxides, NOx,
Beyond the aforementioned CAA requirements, procedural
and VOCs) contribute to the formation of “secondary PM.”
aspects of the NAAQS review are generally at the
PM2.5 contains a much greater portion of secondary
discretion of the EPA Administrator. Historically, the
particles than PM10 does.
agency has undertaken a multi-step process to review each
NAAQS. Each NAAQS review typically begins with a
planning phase in which EPA seeks public input and
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Ozone and Particulate Matter Air Standards: EPA Review
develops an Integrated Review Plan (IRP). The IRP maps
“whether there is robust and convincing evidence to support
out the schedule and process for the review and identifies
the EPA’s conclusion that there is a causal relationship
policy-relevant science issues to guide the review.
between PM2.5 exposure and mortality” (CASAC letter).
EPA’s causality assessment is consequential, as it factors
EPA reviews the relevant scientific literature published
into the Administrator’s decision about whether to revise
since the last NAAQS revision, summarizing it in a report
the NAAQS.
currently known as the Integrated Science Assessment
(ISA). The ISA compiles information about sources of the
EPA replied that it would make “necessary adjustments” to
pollutant, exposure pathways, empirical evidence regarding
the PM ISA while finishing the PA and reaffirmed its goal
the causality link between exposure and adverse health
to complete the PM review by 2020 (EPA letter to CASAC,
effects, and other topics. The ISA is intended as the
July 25, 2019). EPA did not form a new PM panel or
scientific foundation for the EPA Administrator’s
convene an ozone panel. In September 2019, EPA
assessment of whether the NAAQS sufficiently protect
announced the availability of 12 subject matter experts to
public health and welfare. In the past, EPA solicited public
assist CASAC with technical questions. Incorporating
comment and multiple CASAC reviews before finalizing.
elements of CASAC’s review, EPA finalized its PM PA in
The final ISA informs EPA’s preparation of the Risk and
early 2020, concluding that available scientific evidence, air
Exposure Assessment (REA), which estimates exposures
quality analyses, and risk assessments call “into question
and health risks under defined air quality scenarios.
the adequacy of the public health protection afforded” by
the current PM2.5 standards. The final PA further recognizes
Subsequently, EPA prepares a Policy Assessment (PA),
that contrasting conclusions might be reached dependent on
which summarizes information from the ISA and REA and
judgment of the weight of various types of scientific
provides the Administrator with options regarding the
evidence considered (Final PM PA, January 2020, EPA-
indicators, averaging times, statistical form, and numerical
452/P-19-001). The EPA Administrator proposed to retain
level (concentration) of the NAAQS. EPA solicits comment
the current PM standards, based partly on the proposed
on the PA from CASAC and the public, then finalizes a
conclusion that there are “important uncertainties in the
decision on the NAAQS standard through the rulemaking
evidence for adverse health effects below the current” PM2.5
process. The agency proposes a decision—to retain or to
standards (85 Federal Register 24094, April 30, 2020).
revise the standard—after considering information in the
ISA, REA, and PA and the advice of CASAC.
EPA began the current ozone review in 2018 and structured
it to last roughly two-and-a-half years. The previous ozone
EPA Restructuring of the NAAQS Reviews
review lasted about seven years. EPA compressed the
The NAAQS review process has evolved over time, with
current review schedule partly by releasing the draft ISA
multiple Administrations introducing procedural
and draft PA nearly concurrently requesting simultaneous
modifications intended to streamline the process, improve
review by the CASAC. This approach differs from
transparency, or strengthen the scientific basis. In 2018,
previously completed reviews in which EPA considered
EPA announced plans to streamline NAAQS reviews by,
CASAC input and public comments on the ISA as EPA
for example, releasing some documents for CASAC review
developed the PA. CASAC found that the draft ISA did not
concurrently and folding REA-related analyses into the PA
provide a “comprehensive, systematic assessment” and
rather than developing a new REA. EPA also planned to
recommended that EPA “consider restoring a traditional
seek CASAC advice about background pollution and
interactive discussion process in which the CASAC can
potential adverse effects from NAAQS compliance
interact directly with external expert panels” (CASAC,
strategies and changed the CASAC subcommittees.
EPA-CASAC-20-002). CASAC did not reach consensus
regarding the draft PA’s recommendation that the
Under its CASAC charter, EPA may form subcommittees
Administrator consider retaining the primary ozone
or workgroups, such as pollutant-specific panels, to serve
standard. EPA has since finalized the ISA and the PA,
under CASAC. Past panels, which included individuals
which recommends retaining the primary ozone standard.
with expertise in specific pollutants, assisted with the
NAAQS reviews. In 2018, EPA disbanded the Particulate
Issues for Consideration
Matter Review Panel formed in 2015, directing the seven-
Congress may consider if EPA’s revised approach meets
member CASAC to assist EPA with reviews for the 2012
the CAA objectives to review the NAAQS and the science
PM and 2015 ozone NAAQS on an expedited timeline.
upon which they are based in a timely manner. EPA’s
Some have expressed concerns about the lack of pollutant
modifications to the NAAQS review process underscore the
specific panels, and in its review of PM, CASAC
tension between competing concerns. Some stakeholders,
recommended EPA either reappoint the CASAC PM panel
interest groups, and Members of Congress have criticized
or appoint a new panel with similar expertise. CASAC
the timeliness of past NAAQS reviews, which routinely
stated that the “breadth and diversity of evidence to be
have not been completed within the five-year review cycle.
considered exceeds the expertise of the statutory CASAC
Others question whether expedited NAAQS decisions are
members” (letter from CASAC to EPA, April 11, 2019).
able to reflect the latest science and if the scientific basis is
rigorous and unbiased.
CASAC also recommended “substantial revisions” to the
draft PM ISA, finding that it did “not provide a sufficiently
Kate C. Shouse, Analyst in Environmental Policy
comprehensive, systematic assessment of the available
Robert Esworthy, Specialist in Environmental Policy
science.” CASAC members did not reach consensus as to
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Ozone and Particulate Matter Air Standards: EPA Review
IF11288
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