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Updated October 30, 2019
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 the standards for
PM concentrations exceed the NAAQS in some areas
national ambient air quality every five years. In 2018, EPA
(“nonattainment areas”). Table 1 lists these NAAQS and
announced strategies to expedite the National Ambient Air
the estimated population in nonattainment areas.
Quality Standard (NAAQS) review process while
concurrently disbanding a pollutant-specific scientific
Table 1. Selected NAAQS and the Estimated U.S.
review panel that has historically advised agency staff
Population in Corresponding Nonattainment Areas
during their reviews. Although the CAA allows the EPA
Administrator to specify the procedures for review of the
Estimated U.S.
NAAQS, past EPA reviews and revisions have garnered
Primary
Population in
considerable congressional oversight. This In Focus
NAAQS
Standard
Nonattainment Areas
discusses the status of EPA’s current NAAQS reviews for
2015 Ozone
70 ppb (8-hour)
124 mil ion
ozone and particulate matter (PM), which EPA intends to
complete in 2020, and issues of potential interest to
2012 Fine PM
12.0 µg/m3 (Annual)
22 mil ion
Congress.
1987 Coarse PM 150 µg/m3 (24-hour)
9 mil ion
Background on Ozone and Particulate Matter
Source: CRS, as adapted from EPA, Green Book,
Ozone and PM are two of six principal pollutants referred
https://www.epa.gov/green-book. Estimated population based on
to as “criteria pollutants” for which EPA has promulgated
2010, rounded to nearest mil ion. Data as of May 31, 2019.
NAAQS under the CAA (42 U.S.C §7408(a)(1)).
Notes: Units of measure are parts per bil ion (ppb) and micrograms
Ground-level ozone, the primary component of smog, is
per cubic meter of air (µg/m3). See 40 C.F.R. Part 50 for detailed
formed when nitrogen oxides (NOx) react with volatile
NAAQS. Table presents the most recent PM and ozone NAAQS. For
organic compounds (VOCs) in sunlight. Ground-level
other NAAQS nonattainment areas, see EPA’s Green Book.
ozone is associated with health effects, such as aggravated
asthma, chronic bronchitis, heart attacks, and premature
NAAQS Statutory Requirements
death. EPA has identified natural and anthropogenic
NAAQS do not directly limit emissions. Rather, NAAQS
sources of ozone precursors (e.g., NOx and VOCs) and
are concentration-based standards for ambient (outdoor)
ozone, including factories, lightning, power plants,
pollution. Under the CAA, Congress mandated that EPA
vegetation, vehicles, volatile chemical products (e.g., paints
establish two types of NAAQS for each criteria pollutant—
and solvents) and wildfires.
a primary NAAQS, which must protect public health with
an “adequate margin of safety,” and a secondary NAAQS,
PM refers to a mixture of solid particles and liquid droplets
which must “protect public welfare from any known or
in the atmosphere. PM components may include acids,
anticipated adverse effects” (42 U.S.C. §7409(b)). Public
organic chemicals, metals, and soil or dust particles. The
welfare includes damage to crops, vegetation, property,
size of PM varies, ranging from tiny particles that can be
building materials, and climate (42 U.S.C. §7602(h)).
seen only through a high-power microscope to larger
particles (e.g., soot or smoke). Exposure to PM has been
The CAA establishes a framework for EPA to set NAAQS
associated with adverse health effects, haze formation, and
based on the “latest scientific knowledge” through a notice-
environmental impacts. The potential health effects include
and-comment rulemaking process (42 U.S.C. §§7408,
aggravated asthma, chronic bronchitis, decreased lung
7409). It requires EPA to review the NAAQS and the
function, and premature death.
science upon which they are based every five years and
then revise the NAAQS if necessary. The CAA also
Typical sources of fine PM (PM2.5)—measured at 2.5
requires EPA to appoint an independent scientific review
micrometers or less in diameter—include direct emissions
committee composed of seven members, which has become
from vehicles, smokestacks, and fires. Coarse PM (PM10)—
the Clean Air Scientific Advisory Committee (CASAC).
generally measuring 10 micrometers or less in diameter—is
The act directs CASAC to review the NAAQS every five
often associated with dust from paved and unpaved roads,
years and recommend to the EPA Administrator “any new
construction and demolition operations, certain industrial
national ambient air quality standards and revisions … as
processes and agriculture operations, and biomass burning.
may be appropriate” (42 U.S.C. §7409(d)(2)).
In addition, precursor emissions (e.g., sulfur oxides, NOx,
and VOCs) contribute to the formation of “secondary PM.”
EPA’s Review of the NAAQS
PM2.5 contains a much greater portion of secondary particles
Beyond the aforementioned CAA requirements, procedural
than PM10 does.
aspects of the NAAQS review are generally at the
https://crsreports.congress.gov
Ozone and Particulate Matter Air Standards: EPA Review
discretion of the EPA Administrator. Historically, the
inform the PM review. CASAC stated that the “breadth and
agency has undertaken a multi-step process to review each
diversity of evidence to be considered exceeds the expertise
NAAQS. Each NAAQS review typically begins with a
of the statutory CASAC members” (letter from CASAC to
planning phase in which EPA seeks public input and
EPA, April 11, 2019).
develops an Integrated Review Plan (IRP). The IRP maps
out the schedule and process for the review and identifies
CASAC also recommended “substantial revisions” to the
policy-relevant science issues to guide the review.
Draft PM ISA, finding that it did “not provide a sufficiently
comprehensive, systematic assessment of the available
EPA then reviews the relevant scientific literature published
science.” CASAC members did not reach consensus as to
since the last NAAQS revision, summarizing it in a report
“whether there is robust and convincing evidence to support
currently known as the Integrated Science Assessment
the EPA’s conclusion that there is a causal relationship
(ISA). The ISA compiles information about sources of the
between PM2.5 exposure and mortality” (CASAC letter).
pollutant, exposure pathways, empirical evidence regarding
the causality link between exposure and adverse health
EPA replied that it would make “necessary adjustments” to
effects, and other topics. The ISA is intended as the
the PM ISA while finishing the PA and reaffirmed its goal
scientific foundation for the EPA Administrator’s
to complete the PM review by 2020 (letter from EPA to
assessment of whether the NAAQS sufficiently protect
CASAC, July 25, 2019). EPA has not formed a new PM-
public health and welfare. In the past, EPA solicited public
panel or convened an Ozone panel. In September 2019,
comment and multiple CASAC reviews before finalizing.
EPA announced the availability of 12 subject-matter
experts to assist CASAC with technical questions (press
The final ISA informs EPA’s preparation of the Risk and
release, EPA, Sept. 13, 2019). EPA published a draft PM
Exposure Assessment (REA), which presents quantitative
PA, which reached the preliminary conclusion that
estimates of exposures and health risks under defined air
available scientific evidence, air quality analyses, and risk
quality scenarios. As with the IRP and the ISA, EPA has
assessments call “into question the adequacy of the public
sought CASAC and public comment on the REA.
health protection afforded” by the current PM2.5 standards
(Draft PM PA, September 2019, EPA-452/P-19-001).
Subsequently, EPA prepares a Policy Assessment (PA),
Options discussed range from tightening the PM2.5 standard
which summarizes information from the ISA and REA and
to retaining the current level.
provides the Administrator with options regarding the
indicators, averaging times, statistical form, and numerical
The current ozone review began in 2018, marking the first
level (concentration) of the NAAQS. EPA solicits comment
NAAQS review initiated in the current Administration.
on the PA from CASAC and the public, then finalizes a
EPA projected that the review will last a little over two-
decision on the NAAQS standard through the rulemaking
and-a-half years. The previous ozone review lasted about
process. The agency proposes a decision—to retain or to
seven years. EPA will not develop a new REA in the
revise the standard—after considering information in the
current review. Instead, EPA plans to fold “REA-related
ISA, REA, and PA and the advice of CASAC.
analyses” into the PA (EPA, IRP for Review of the Ozone
NAAQS). EPA released the draft ISA in September 2019
EPA Restructuring of the NAAQS Reviews
and plans to issue the draft PA in October 2019 for
The NAAQS review process has evolved over time, with
“simultaneous review by the CASAC” (EPA letter). This
multiple Administrations introducing procedural
approach differs from previously completed reviews, in
modifications intended to streamline the process, improve
which EPA has considered CASAC input and public
transparency, or strengthen the scientific basis. In 2018,
comments on the ISA as EPA developed other milestone
EPA announced plans to streamline NAAQS reviews and
documents—for example, the PA.
obtain CASAC advice regarding background pollution and
potential adverse effects from NAAQS compliance
Issues for Consideration
strategies. Historically, EPA has not requested CASAC to
Congress, in its oversight capacity, may consider whether
advise the agency with respect to adverse effects from
or not the EPA’s current approach meets the CAA
NAAQS compliance strategies, although it is among the
objectives to review the NAAQS and the science upon
topics listed in CAA Section 109(d)(2)(C).
which they are based in a timely manner.
Under its CASAC charter, EPA may form subcommittees
EPA’s proposed modifications to the NAAQS review
or workgroups, such as pollutant-specific panels, to serve
process underscore the tension between competing
under CASAC. Past panels, which included individuals
concerns. Some stakeholders, interest groups, and Members
with expertise in specific pollutants, assisted with the
of Congress have criticized the timeliness of past NAAQS
NAAQS reviews. In 2018, EPA disbanded the Particulate
reviews, which routinely have not been completed within
Matter Review Panel formed in 2015, directing the seven-
the five-year review cycle. Others have raised concerns
member CASAC to assist EPA with reviews for the 2012
about whether EPA’s NAAQS decisions have been based
PM and 2015 ozone NAAQS on an expedited timeline.
on research that reflect the latest science and whether the
scientific basis is rigorous and unbiased.
Some stakeholders and interest groups have raised concerns
about the lack of pollutant-specific panels. CASAC
Kate C. Shouse, Analyst in Environmental Policy
recommended that EPA either reappoint the CASAC PM
Robert Esworthy, Specialist in Environmental Policy
panel or appoint a new panel with similar expertise to
https://crsreports.congress.gov
Ozone and Particulate Matter Air Standards: EPA Review
IF11288
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
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https://crsreports.congress.gov | IF11288 · VERSION 3 · UPDATED