Ozone and Particulate Matter Air Standards: EPA Review

Updated December 23, 2020

Ozone and Particulate Matter Air Standards: EPA Review

The Clean Air Act (CAA) requires the U.S. Environmental

Protection Agency (EPA) to review standards for national

ambient air quality every five years. In 2018, EPA

announced strategies to expedite the National Ambient Air

Quality Standard (NAAQS) review while concurrently

disbanding a pollutant-specific scientific review panel that

has historically advised agency staff during their reviews.

Although the CAA allows the EPA Administrator to specify

the procedures for review of the NAAQS, past EPA reviews

and revisions have garnered considerable congressional

oversight. In December 2020, EPA completed the

particulate matter (PM) NAAQS review and retained the

standards. This In Focus discusses the recently completed

PM NAAQS review and the ongoing ozone review.

Background on Ozone and Particulate Matter

Ozone and PM are two of six principal pollutants referred

to as “criteria pollutants” for which EPA has promulgated

NAAQS under the CAA (42 U.S.C. §7408(a)(1)).

Ground-level ozone, the primary component of smog, is

formed when nitrogen oxides (NOx) react with volatile

organic compounds (VOCs) in sunlight. Ground-level

ozone is associated with health effects, such as aggravated

asthma, chronic bronchitis, heart attacks, and premature

death. EPA has identified natural and anthropogenic

sources of ozone and ozone precursors (e.g., NOx and

VOCs), including factories, lightning, power plants,

vegetation, vehicles, volatile chemical products (e.g., paints

and solvents), and wildfires.

PM refers to a mixture of solid particles and liquid droplets

in the atmosphere. PM components may include acids,

organic chemicals, metals, and soil or dust particles. The

size of PM varies, ranging from tiny particles that can be

seen only through a high-power microscope to larger

particles (e.g., soot). Exposure to PM has been associated

with adverse health effects (e.g., aggravated asthma,

chronic bronchitis, and premature death). PM has also been

linked with haze formation and other ecological effects.

Typical sources of fine PM (PM 2.5)—measured at 2.5

micrometers or less in diameter—include emissions from

vehicles, smokestacks, and fires. Coarse PM (PM 10)—

generally measuring 10 micrometers or less in diameter—is

often associated with dust from paved and unpaved roads,

construction and demolition operations, certain industrial

processes and agriculture operations, and biomass burning.

In addition, precursor emissions (e.g., sulfur oxides, NOx,

and VOCs) contribute to the formation of “secondary PM.”

PM 2.5 contains a much greater portion of secondary

particles than PM 10 does.

Notwithstanding air quality progress since 1970, ozone and

PM concentrations currently exceed the NAAQS in some

areas (“nonattainment areas”). Table 1 lists these NAAQS

and the estimated population in nonattainment areas.

Table 1. Selected NAAQS and the Estimated U.S.

Population in Corresponding Nonattainment Areas

Estimated U.S.

Primary

Population in

NAAQS

Standard

Nonattainment Areas

2015 Ozone

70 ppb (8-hour)

122 million

2012 Fine PM

12.0 µg/m3 (Annual)

21 million

1987 Coarse PM 150 µg/m3 (24-hour)

6 million

Source: CRS, as adapted from EPA Green Book (May 31, 2020),

which lists nonattainment areas (https://www.epa.gov/green-book).

Estimated population based on 2010, rounded to nearest million.

Notes: Units of measure are parts per billion (ppb) and micrograms

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.

NAAQS Statutory Requirements

NAAQS do not directly limit emissions. Rather, NAAQS

are concentration-based standards for ambient (outdoor)

pollution. Under the CAA, Congress mandated that EPA

establish two types of NAAQS for each criteria pollutant—

a primary NAAQS, which must protect public health with

an “adequate margin of safety,” and a secondary NAAQS,

which must “protect public welfare from any known or

anticipated adverse effects” (42 U.S.C. §7409(b)). Public

welfare includes damage to crops, vegetation, property,

building materials, and climate (42 U.S.C. §7602(h)).

The CAA establishes a framework for EPA to set NAAQS

based on the “latest scientific knowledge” through a noticeand-comment rulemaking process (42 U.S.C. §§7408,

7409). The CAA requires EPA to review the NAAQS and

the science upon which they are based every five years and

then revise the NAAQS if necessary. The CAA also

requires EPA to appoint an independent scientific review

committee composed of seven members, which has become

the Clean Air Scientific Advisory Committee (CASAC).

The act directs CASAC to review the NAAQS every five

years and recommend to the EPA Administrator “any new

national ambient air quality standards and revisions … as

may be appropriate” (42 U.S.C. §7409(d)(2)).

EPA’s Review of the NAAQS

Beyond the aforementioned CAA requirements, procedural

aspects of the NAAQS review are generally at the

discretion of the EPA Administrator. Historically, the

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

develops an Integrated Review Plan (IRP). The IRP maps

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Ozone and Particulate Matter Air Standards: EPA Review

out the schedule and process for the review and identifies

policy-relevant science issues to guide the review.

EPA’s causality assessment is consequential, as it factors

into the Administrator’s decision about whether to revise

the NAAQS.

EPA reviews the relevant scientific literature published

since the last NAAQS revision, summarizing it in a report

currently known as the Integrated Science Assessment

(ISA). The ISA compiles information about sources of the

pollutant, exposure pathways, empirical evidence regarding

the causality link between exposure and adverse health

effects, and other topics. The ISA is intended as the

scientific foundation for the EPA Administrator’s

assessment of whether the NAAQS sufficiently protect

public health and welfare. In the past, EPA solicited public

comment and multiple CASAC reviews before finalizing.

The final ISA informs EPA’s preparation of the Risk and

Exposure Assessment (REA), which estimates exposures

and health risks under defined air quality scenarios.

EPA replied that it would make “necessary adjustments” to

the PM ISA while finishing the PA and reaffirmed its goal

to complete the PM review by 2020 (EPA letter to CASAC,

July 25, 2019). EPA did not form a new PM panel or

convene an ozone panel. In September 2019, EPA

announced the availability of 12 subject matter experts to

assist CASAC with technical questions. Incorporating

elements of CASAC’s review, EPA finalized its PM PA in

early 2020, concluding that available scientific evidence, air

quality analyses, and risk assessments call “into question

the adequacy of the public health protection afforded” by

the current PM 2.5 standards. The final PA further recognizes

that contrasting conclusions might be reached dependent on

judgment of the weight of various types of scientific

evidence considered (Final PM PA, January 2020, EPA452/P-19-001). After consideration of the scientific

reviews, and information from five public meetings and a

reported 60,000 comments, the EPA Administrator decided

to retain the existing PM standards. The final decision, the

same as proposed, was based partly on the Administrator’s

conclusion that there are “important uncertainties in the

evidence for adverse health effects below the current” PM2.5

standards (85 Federal Register 82685, December 18, 2020).

Subsequently, EPA prepares a Policy Assessment (PA),

which summarizes information from the ISA and REA and

provides the Administrator with options regarding the

indicators, averaging times, statistical form, and numerical

level (concentration) of the NAAQS. EPA solicits comment

on the PA from CASAC and the public, then finalizes a

decision on the NAAQS standard through the rulemaking

process. The agency proposes a decision—to retain or to

revise the standard—after considering information in the

ISA, REA, and PA and the advice of CASAC.

EPA Restructuring of the NAAQS Reviews

The NAAQS review process has evolved over time, with

multiple Administrations introducing procedural

modifications intended to streamline the process, improve

transparency, or strengthen the scientific basis. In 2018,

EPA announced plans to streamline NAAQS reviews by,

for example, releasing some documents for CASAC review

concurrently and folding REA-related analyses into the PA

rather than developing a new REA. EPA also planned to

seek CASAC advice about background pollution and

potential adverse effects from NAAQS compliance

strategies and changed the CASAC subcommittees.

Under its CASAC charter, EPA may form subcommittees

or workgroups, such as pollutant-specific panels, to serve

under CASAC. Past panels, which included individuals

with expertise in specific pollutants, assisted with the

NAAQS reviews. In 2018, EPA disbanded the Particulate

Matter Review Panel formed in 2015, directing the sevenmember CASAC to assist EPA with reviews for the 2012

PM and 2015 ozone NAAQS on an expedited timeline.

Some have expressed concerns about the lack of pollutant

specific panels, and in its review of PM, CASAC

recommended EPA either reappoint the CASAC PM panel

or appoint a new panel with similar expertise. CASAC

stated that the “breadth and diversity of evidence to be

considered exceeds the expertise of the statutory CASAC

members” (letter from CASAC to EPA, April 11, 2019).

CASAC also recommended “substantial revisions” to the

draft PM ISA, finding that it did “not provide a sufficiently

comprehensive, systematic assessment of the available

science.” CASAC members did not reach consensus as to

“whether there is robust and convincing evidence to support

the EPA’s conclusion that there is a causal relationship

between PM 2.5 exposure and mortality” (CASAC letter).

EPA began the current ozone review in 2018 and structured

it to last roughly two-and-a-half years. The previous ozone

review lasted about seven years. EPA compressed the

current review schedule partly by releasing the draft ISA

and draft PA nearly concurrently requesting simultaneous

review by the CASAC. This approach differs from

previously completed reviews in which EPA considered

CASAC input and public comments on the ISA as EPA

developed the PA. CASAC found that the draft ISA did not

provide a “comprehensive, systematic assessment” and

recommended that EPA “consider restoring a traditional

interactive discussion process in which the CASAC can

interact directly with external expert panels” (CASAC,

EPA-CASAC-20-002). CASAC did not reach consensus

regarding the draft PA’s recommendation that the

Administrator consider retaining the primary ozone

standard. EPA has since finalized the ISA and the PA,

which recommends retaining the primary ozone standard.

Issues for Consideration

Congress may consider if EPA’s revised approach meets

the CAA objectives to review the NAAQS and the science

upon which they are based in a timely manner. EPA’s

modifications to the NAAQS review process underscore the

tension between competing concerns. Some stakeholders,

interest groups, and Members of Congress have criticized

the timeliness of past NAAQS reviews, which routinely

have not been completed within the five-year review cycle.

Others question whether expedited NAAQS decisions are

able to reflect the latest science and if the scientific basis is

rigorous and unbiased.

Kate C. Shouse, Analyst in Environmental Policy

https://crsreports.congress.gov

IF11288

Ozone and Particulate Matter Air Standards: EPA Review

Disclaimer

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