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August 12, 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.
Primary
NAAQS, past EPA reviews and revisions have garnered
NAAQS
Population in
Standard
considerable congressional oversight. This In Focus
Nonattainment Areas
discusses the status of EPA’s current NAAQS reviews for
2015 Ozone
70 ppb
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
22 mil ion
Congress.
1987 Coarse PM
150 µg/m3
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
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Ozone and Particulate Matter Air Standards: EPA Review
discretion of the EPA Administrator. Historically, the
recommended that EPA either reappoint the CASAC PM
agency has undertaken a multi-step process to review each
panel or appoint a new panel with similar expertise to
NAAQS. Each NAAQS review typically begins with a
inform the PM review. CASAC stated that the “breadth and
planning phase in which EPA seeks public input and
diversity of evidence to be considered exceeds the expertise
develops an Integrated Review Plan (IRP). The IRP maps
of the statutory CASAC members” (letter from CASAC to
out the schedule and process for the review and identifies
EPA, April 11, 2019).
policy-relevant science issues to guide the review.
CASAC also recommended “substantial revisions” to the
EPA then reviews the relevant scientific literature published
Draft PM ISA, finding that it did “not provide a sufficiently
since the last NAAQS revision and summarizes it in a
comprehensive, systematic assessment of the available
report currently known as the Integrated Science
science.” CASAC members did not reach consensus as to
Assessment (ISA). The ISA compiles information about
“whether there is robust and convincing evidence to support
sources of the pollutant, exposure pathways, empirical
the EPA’s conclusion that there is a causal relationship
evidence regarding the causality link between exposure and
between PM2.5 exposure and mortality” (CASAC letter).
adverse health effects, and other topics. The ISA is intended
as the scientific foundation for the EPA Administrator’s
EPA replied that it would make “necessary adjustments” to
assessment of whether the NAAQS sufficiently protect
the PM ISA while finishing the PA by fall 2019 and
public health and welfare. EPA solicits public comment
reaffirmed its goal to complete the PM review by 2020.
and, historically, multiple CASAC reviews before finalizing
EPA has not formed a new PM-panel nor convened an
it.
Ozone panel. In its letter to CASAC, EPA stated it plans to
make a “pool of subject matter consultants” available to
The final ISA informs EPA’s preparation of the Risk and
provide feedback on the PM and ozone reviews to the
Exposure Assessment (REA), which presents quantitative
CASAC chair “in a manner consistent with the Federal
estimates of exposures and health risks under defined air
Advisory Committee Act” (letter from EPA to CASAC,
quality scenarios. As with the IRP and the ISA, EPA has
July 25, 2019).
sought CASAC and public comment on the REA.
The current ozone review began in 2018, marking the first
Subsequently, EPA prepares a Policy Assessment (PA),
NAAQS review initiated in the current Administration. To
which summarizes information from the ISA and REA and
date, EPA has released a draft ozone IRP and projected that
provides the Administrator with options regarding the
the review will last a little over two-and-a-half years. The
indicators, averaging times, statistical form, and numerical
previous ozone review lasted about seven years. EPA will
level (concentration) of the NAAQS. EPA solicits comment
not develop a new REA in the current review. Instead, EPA
on the PA from CASAC and the public, then finalizes a
plans to fold “REA-related analyses” into the PA (EPA,
decision on the NAAQS standard through the rulemaking
Draft IRP for Review of the Ozone NAAQS). EPA also
process. The agency proposes a decision—to retain or to
plans to issue drafts of the ozone ISA and PA by October
revise the standard—after considering information in the
2019 for “simultaneous review by the CASAC” and the
ISA, REA, and PA and the advice of CASAC.
public, which “should conclude” by end of 2019 (EPA
letter). This approach differs from previously completed
EPA Restructuring of the NAAQS Reviews
reviews, in which EPA has considered CASAC input and
The NAAQS review process has evolved over time, with
public comments on the ISA as it developed other
multiple Administrations introducing procedural
milestone documents—for example, the PA.
modifications intended to streamline the process, improve
transparency, or strengthen the scientific basis. In 2018,
Issues for Consideration
EPA announced plans to streamline NAAQS reviews and
Congress, in its oversight capacity, may consider whether
obtain CASAC advice regarding background pollution and
or not the EPA’s current approach meets the CAA
potential adverse effects from NAAQS compliance
objectives to review the NAAQS and the science upon
strategies. Historically, EPA has not requested CASAC to
which they are based in a timely manner.
advise the agency with respect to adverse effects from
NAAQS compliance strategies, although it is among the
EPA’s proposed modifications to the NAAQS review
topics listed in CAA Section 109(d)(2)(C).
process underscore the tension between competing
concerns. Some stakeholders, interest groups, and Members
Under its CASAC charter, EPA may form subcommittees
of Congress have criticized the timeliness of past NAAQS
or workgroups, such as pollutant-specific panels, to serve
reviews, which routinely have not been completed within
under CASAC. Past panels, which included individuals
the five-year review cycle. Others have raised concerns
with expertise in specific pollutants, assisted with the
about whether EPA’s NAAQS decisions have been based
NAAQS reviews. In 2018, EPA disbanded the Particulate
on research that reflect the latest science and that the
Matter Review Panel, which was formed in 2015, and
scientific basis is rigorous and unbiased.
directed the seven-member CASAC to assist EPA with the
reviews for the 2012 PM and 2015 ozone NAAQS on an
Kate C. Shouse, Analyst in Environmental Policy
expedited timeline.
Robert Esworthy, Specialist in Environmental Policy
Some stakeholders and interest groups have raised concerns
IF11288
about the lack of pollutant-specific panels. CASAC
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Ozone and Particulate Matter Air Standards: EPA Review
Disclaimer
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Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
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