link to page 1 
Updated December 8, 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. In December 2020, EPA completed the
2015 Ozone
70 ppb (8-hour)
122 mil ion
particulate matter (PM) NAAQS review and retained the
2012 Fine PM
12.0 µg/m3 (Annual)
21 mil ion
standards. This In Focus discusses the recently completed
1987 Coarse PM 150 µg/m3 (24-hour)
6 mil ion
PM NAAQS review and the ongoing ozone review.
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
Notes: Units of measure are parts per bil ion (ppb) and micrograms
to as “criteria pollutants” for which EPA has promulgated
per cubic meter of air (µg/m3). See 40 C.F.R. Part 50 for detailed
NAAQS under the CAA (42 U.S.C. §7408(a)(1)).
NAAQS. Table presents the most recent PM and ozone NAAQS.
Ground-level ozone, the primary component of smog, is
NAAQS Statutory Requirements
formed when nitrogen oxides (NOx) react with volatile
NAAQS do not directly limit emissions. Rather, NAAQS
organic compounds (VOCs) in sunlight. Ground-level
are concentration-based standards for ambient (outdoor)
ozone is associated with health effects, such as aggravated
pollution. Under the CAA, Congress mandated that EPA
asthma, chronic bronchitis, heart attacks, and premature
establish two types of NAAQS for each criteria pollutant—
death. EPA has identified natural and anthropogenic
a primary NAAQS, which must protect public health with
sources of ozone and ozone precursors (e.g., NOx and
an “adequate margin of safety,” and a secondary NAAQS,
VOCs), including factories, lightning, power plants,
which must “protect public welfare from any known or
vegetation, vehicles, volatile chemical products (e.g., paints
anticipated adverse effects” (42 U.S.C. §7409(b)). Public
and solvents), and wildfires.
welfare includes damage to crops, vegetation, property,
building materials, and climate (42 U.S.C. §7602(h)).
PM refers to a mixture of solid particles and liquid droplets
in the atmosphere. PM components may include acids,
The CAA establishes a framework for EPA to set NAAQS
organic chemicals, metals, and soil or dust particles. The
based on the “latest scientific knowledge” through a notice-
size of PM varies, ranging from tiny particles that can be
and-comment rulemaking process (42 U.S.C. §§7408,
seen only through a high-power microscope to larger
7409). The CAA requires EPA to review the NAAQS and
particles (e.g., soot). Exposure to PM has been associated
the science upon which they are based every five years and
with adverse health effects (e.g., aggravated asthma,
then revise the NAAQS if necessary. The CAA also
chronic bronchitis, and premature death). PM has also been
requires EPA to appoint an independent scientific review
linked with haze formation and other ecological effects.
committee composed of seven members, which has become
the Clean Air Scientific Advisory Committee (CASAC).
Typical sources of fine PM (PM2.5)—measured at 2.5
The act directs CASAC to review the NAAQS every five
micrometers or less in diameter—include emissions from
years and recommend to the EPA Administrator “any new
vehicles, smokestacks, and fires. Coarse PM (PM10)—
national ambient air quality standards and revisions … as
generally measuring 10 micrometers or less in diameter—is
may be appropriate” (42 U.S.C. §7409(d)(2)).
often associated with dust from paved and unpaved roads,
construction and demolition operations, certain industrial
EPA’s Review of the NAAQS
processes and agriculture operations, and biomass burning.
Beyond the aforementioned CAA requirements, procedural
In addition, precursor emissions (e.g., sulfur oxides, NOx,
and VOCs) contribute to the formation of “secondary PM.”
aspects of the NAAQS review are generally at the
discretion of the EPA Administrator. Historically, the
PM2.5 contains a much greater portion of secondary
agency has undertaken a multi-step process to review each
particles than PM10 does.
NAAQS. Each NAAQS review typically begins with a
planning phase in which EPA seeks public input and
develops an Integrated Review Plan (IRP). The IRP maps
https://crsreports.congress.gov
Ozone and Particulate Matter Air Standards: EPA Review
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). After consideration of the scientific
level (concentration) of the NAAQS. EPA solicits comment
reviews, and information from five public meetings and a
on the PA from CASAC and the public, then finalizes a
reported 60,000 comments, the EPA Administrator decided
decision on the NAAQS standard through the rulemaking
to retain the existing PM standards. The final decision, the
process. The agency proposes a decision—to retain or to
same as proposed, was based partly on the Administrator’s
revise the standard—after considering information in the
conclusion that there are “important uncertainties in the
ISA, REA, and PA and the advice of CASAC.
evidence for adverse health effects below the current” PM2.5
standards (85 Federal Register 24094, April 30, 2020).
EPA Restructuring of the NAAQS Reviews
The NAAQS review process has evolved over time, with
EPA began the current ozone review in 2018 and structured
multiple Administrations introducing procedural
it to last roughly two-and-a-half years. The previous ozone
modifications intended to streamline the process, improve
review lasted about seven years. EPA compressed the
transparency, or strengthen the scientific basis. In 2018,
current review schedule partly by releasing the draft ISA
EPA announced plans to streamline NAAQS reviews by,
and draft PA nearly concurrently requesting simultaneous
for example, releasing some documents for CASAC review
review by the CASAC. This approach differs from
concurrently and folding REA-related analyses into the PA
previously completed reviews in which EPA considered
rather than developing a new REA. EPA also planned to
CASAC input and public comments on the ISA as EPA
seek CASAC advice about background pollution and
developed the PA. CASAC found that the draft ISA did not
potential adverse effects from NAAQS compliance
provide a “comprehensive, systematic assessment” and
strategies and changed the CASAC subcommittees.
recommended that EPA “consider restoring a traditional
interactive discussion process in which the CASAC can
Under its CASAC charter, EPA may form subcommittees
interact directly with external expert panels” (CASAC,
or workgroups, such as pollutant-specific panels, to serve
EPA-CASAC-20-002). CASAC did not reach consensus
under CASAC. Past panels, which included individuals
regarding the draft PA’s recommendation that the
with expertise in specific pollutants, assisted with the
Administrator consider retaining the primary ozone
NAAQS reviews. In 2018, EPA disbanded the Particulate
standard. EPA has since finalized the ISA and the PA,
Matter Review Panel formed in 2015, directing the seven-
which recommends retaining the primary ozone standard.
member CASAC to assist EPA with reviews for the 2012
PM and 2015 ozone NAAQS on an expedited timeline.
Issues for Consideration
Some have expressed concerns about the lack of pollutant
Congress may consider if EPA’s revised approach meets
specific panels, and in its review of PM, CASAC
the CAA objectives to review the NAAQS and the science
recommended EPA either reappoint the CASAC PM panel
upon which they are based in a timely manner. EPA’s
or appoint a new panel with similar expertise. CASAC
modifications to the NAAQS review process underscore the
stated that the “breadth and diversity of evidence to be
tension between competing concerns. Some stakeholders,
considered exceeds the expertise of the statutory CASAC
interest groups, and Members of Congress have criticized
members” (letter from CASAC to EPA, April 11, 2019).
the timeliness of past NAAQS reviews, which routinely
have not been completed within the five-year review cycle.
CASAC also recommended “substantial revisions” to the
Others question whether expedited NAAQS decisions are
draft PM ISA, finding that it did “not provide a sufficiently
able to reflect the latest science and if the scientific basis is
comprehensive, systematic assessment of the available
rigorous and unbiased.
science.” CASAC members did not reach consensus as to
“whether there is robust and convincing evidence to support
Kate C. Shouse, Analyst in Environmental Policy
https://crsreports.congress.gov
Ozone and Particulate Matter Air Standards: EPA Review
IF11288
Robert Esworthy, Specialist in Environmental Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to
congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress.
Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
been provided by CRS to Members of Congress in connection with CRS’s institutional role. CRS Reports, as a work of the
United States Government, are not subject to copyright protection in the United States. Any CRS Report may be
reproduced and distributed in its entirety without permission from CRS. However, as a CRS Report may include
copyrighted images or material from a third party, you may need to obtain the permission of the copyright holder if you
wish to copy or otherwise use copyrighted material.
https://crsreports.congress.gov | IF11288 · VERSION 7 · UPDATED