Thus, in considering the adequacy of the 24-hour standard, the
Administrator notes the importance of considering whether additional
protection is needed against short-term exposures to peak
PM2.5 concentrations. In examining the scientific evidence,
he notes that controlled human exposure studies provide evidence for
health effects following single, short-term PM2.5 exposures
to concentrations. These types of exposures correspond best to those to
ambient exposures that might be experienced in the upper end of the
PM2.5 air quality distribution in the U.S. (i.e., ``peak''
concentrations). However, most of these studies examine exposure
concentrations considerably higher than are typically measured in areas
meeting the current standards (U.S. EPA, 2020, section <IP_ADDRESS>). In
particular, controlled human exposure studies often report
statistically significant effects on one or more indicators of
cardiovascular function following 2-hour exposures to PM2.5
concentrations at and above 120 [mu]g/m\3\ (at and above 149 [mu]g/m\3\
for vascular impairment, the effect shown to be most consistent across
studies). To provide insight into what these studies may indicate
regarding the primary PM2.5 standards, the PA (U.S. EPA,
2020, p.3-49) notes that 2-hour ambient concentrations of
PM2.5 at monitoring sites meeting the current standards
almost never exceed 32 [mu]g/m\3\. In fact, even the extreme upper end
of the distribution of 2-hour PM2.5 concentrations at sites
meeting the current standards remains well-below the PM2.5
exposure concentrations consistently shown in controlled human exposure
studies to elicit effects (i.e., 99.9th percentile of 2-hour
concentrations at these sites is 68 [mu]g/m\3\ during the warm season).
Additionally, the Administrator notes the limited utility of the animal
toxicologic studies in directly informing conclusions on the
appropriate level of the standard given the uncertainty in
extrapolating from effects in animals to those in human populations.
Thus, the available experimental evidence does not indicate the need
for additional protection against exposures to peak PM2.5
concentrations, beyond the protection provided by the combination of
the current 24-hour standard and the current annual standard (U.S. EPA,
2020, section <IP_ADDRESS>).
    With respect to the epidemiological evidence, the Administrator
notes that the available epidemiological studies do not indicate that
associations in those studies are strongly influenced by exposures to
peak concentrations in the air quality distribution and thus do not
indicate the need for additional protection against short-term
exposures to peak PM2.5 concentrations (U.S. EPA 2020,
section 3.5.1). Lastly, the Administrator notes CASAC consensus support
for retaining the current 24-hour standard. Thus, the Administrator
concludes that the 24-hour standard with its level of 35 [micro]g/m\3\
is adequate to provide supplemental protection (i.e., beyond that
provided by the annual standard alone) against short-term exposures to
peak PM2.5 concentrations.
    In reviewing the level of the annual standard, the Administrator
recognizes that the annual standard, with its form based on the
arithmetic mean concentration, is most appropriately meant to limit the
``typical'' daily and annual exposures that are most strongly
associated with the health effects observed in epidemiological studies.
However, the Administrator also recognizes that while epidemiological
studies examine associations between distributions of PM2.5
air quality and health outcomes, they do not identify particular
PM2.5 exposures that cause effects and thus, they cannot
alone identify a specific level at which the standard should be set, as
such a determination necessarily requires the Administrator's judgment.
Thus, any approach that uses epidemiological information in reaching
decisions on what standards are appropriate necessarily requires
judgments about how to translate the information available from the
epidemiological studies into a basis for appropriate standards. This
includes consideration of how to weigh the uncertainties in the
reported associations between daily or annual average PM2.5
exposures and mortality or morbidity in the epidemiological studies.
Such an approach is consistent with setting standards that are neither
more nor less stringent than necessary, recognizing that a zero-risk
standard is not required by the CAA.
    The Administrator recognizes that important uncertainties and
limitations that were present in epidemiological studies in previous
reviews, remain in the current review. As discussed above, these
uncertainties include exposure measurement error; potential confounding
by copollutants; increasing uncertainty of associations at lower
PM2.5 concentrations; and heterogeneity of effects across
different cities or regions. The Administrator also recognizes the
advice given by the CASAC on this matter. As discussed above (section
II.B.1), the CASAC members who support retaining the annual standard
expressed their concerns with available PM2.5
epidemiological studies. They assert that recent epidemiological
studies do not provide a sufficient basis for revising the current
standards. They also identify several key concerns regarding the
associations reported in PM2.5 epidemiological studies and
conclude that ``while the data on associations should certainly be
carefully considered, this data should not be interpreted more strongly
than warranted based on its methodological limitations'' (Cox, 2019a,
p. 8 consensus responses).
    Taking into consideration the views expressed by these CASAC
members, the Administrator recognizes that epidemiological studies
examine associations between distributions of PM2.5 air
quality and health outcomes, and they do not identify particular
PM2.5 exposures that cause effects (U.S. EPA, 2020, section
3.1.2). While the Administrator remains concerned about placing too
much weight on epidemiological studies to inform conclusions on the
adequacy of the current primary standards, he notes that several
commenters advocated for using the epidemiological studies in a manner
they characterized as similar to the last review, to determine the
level of the annual standard. The previous PM NAAQS review completed in
2012 noted that the evidence of an association in any epidemiological
study is ``strongest at and around the long-term average where the data
in the study are most concentrated'' (78 FR 3140, January 15, 2013).
Accordingly, the Administrator notes the characterization of study
reported short-term and long-term mean PM2.5 concentrations
(section II.A.2.c.ii). As discussed in more detail above in section
II.B.3 in responding to comments, when assessing the mean
concentrations of the key short-term and

long-term epidemiological studies in the U.S. that use ground-based
monitoring (i.e., those studies that can provide information most
directly comparable to the current annual standard), the majority of
studies (i.e., 19 out of 21) have mean concentrations at or above the
level of the current annual standard (12.0 [micro]g/m\3\), with the
mean of the study reported means or medians equal to 13.5 [micro]g/
m\3\, a concentration level above the current level of the primary
annual standard of 12 [micro]g/m\3\.\54\ The Administrator further
notes his caution in directly comparing the reported study mean values
to the standard level given that, as discussed in more detail above,
study-reported mean concentrations, by design, are generally lower than
the design value of the highest monitor in an area, which determines
compliance. In fact, analyses of recent air quality in U.S. CBSAs
indicate that maximum annual PM2.5 design values for a given
three-year period are often 10% to 20% higher than average monitored
concentrations (i.e., averaged across multiple monitors in the same
CBSA) (U.S. EPA, 2020, Appendix B, section B.7). He further notes his
concern in placing too much weight on any one epidemiological study but
instead feels that it is more appropriate to focus on the body of
studies together and therefore takes note of the calculation of the
mean of study-reported means (or medians). Thus, in summary, while the
Administrator is cautious about placing too much weight on the
epidemiological evidence on its own, he notes: (1) The reported mean
concentration in the majority of the key U.S. epidemiological studies
using ground-based monitoring data are above the level of the current
annual standard; (2) the mean of the reported study means (or medians)
(i.e., 13.5 [micro]g/m\3\) is above the level of the current standard;
\55\ (3) air quality analyses show the study means to be lower than
their corresponding design values by 10-20%; and (4) that these
analyses must be considered in light of uncertainties inherent in the
epidemiological evidence. When taken together, the Administrator judges
that, even if he were to place greater weight on the epidemiological
evidence, this information would not call into question the adequacy of
the current standards.
---------------------------------------------------------------------------

    \54\ There were two studies, both included in the last review,
for which the mean concentration (11.8 [micro]g/m\3\; Peng et al.,
2009) or median concentration (10.7 [micro]g/m\3\ (Central Region);
Zeger et al., 2008) was somewhat below 12 [micro]g/m\3\.
    \55\ The median of the study reported mean (or median)
PM2.5 concentrations is 13.3 [micro]g/m\3\, which is also
above the level of the current standard.
---------------------------------------------------------------------------

    In addition to the evidence, the Administrator also considers the
potential implications of the risk assessment. He notes that all risk
assessments have limitations and that he remains concerned about the
uncertainties in the underlying epidemiological data used in the risk
assessment. The Administrator also notes that in previous reviews,
these uncertainties and limitations have often resulted in less weight
being placed on quantitative estimates of risk than on the underlying
scientific evidence itself (e.g., 78 FR 3086, 3098-99, January 15,
2013). These uncertainties and limitations have included uncertainty in
the shapes of concentration-response functions, particularly at low
concentrations; uncertainties in the methods used to adjust air
quality; and uncertainty in estimating risks for populations, locations
and air quality distributions different from those examined in the
underlying epidemiological study (U.S. EPA, 2020, section <IP_ADDRESS>).
Additionally, the Administrator notes similar concern expressed by
members of the CASAC who support retaining the current standards; they
highlighted similar uncertainties and limitations in the risk
assessment (Cox, 2019a). In light of all of this, the Administrator
judges it appropriate to place little weight on quantitative estimates
of PM2.5-associated mortality risk in reaching conclusions
about the level of the primary PM2.5 standards.
    The Administrator additionally considers the emerging body of
evidence from accountability studies examining past reductions in
ambient PM2.5, and the degree to which those reductions have
resulted in public health improvements. The Administrator agrees with
public commenters who note that well-designed and conducted
accountability studies can be informative. However, the Administrator
also recognizes that interpreting such studies in the context of the
current primary PM2.5 standards is complicated by the fact
that some of the available studies have not evaluated PM2.5
specifically (e.g., as opposed to PM10 or total suspended
particulates), did not show changes in PM2.5 air quality, or
have not been able to disentangle health impacts of the interventions
from background trends in health (U.S. EPA, 2020, section 3.5.1). He
further recognizes that the small number of available studies that do
report public health improvements following past declines in ambient
PM2.5 have not examined air quality meeting the current
standards (U.S. EPA, 2020, Table 3-3). This includes recent U.S.
studies that report increased life expectancy, decreased mortality, and
decreased respiratory effects following past declines in ambient
PM2.5 concentrations. Such studies have examined
``starting'' annual average PM2.5 concentrations (i.e.,
prior to the reductions being evaluated) ranging from about 13.2 to >20
mg/m\3\ (i.e., U.S. EPA, 2020, Table 3-3). Given the lack of available
accountability studies reporting public health improvements
attributable to reductions in ambient PM2.5 in locations
meeting the current standards, together with his broader concerns
regarding the lack of experimental studies examining PM2.5
exposures typical of areas meeting the current standards (discussed
above), the Administrator judges that there is considerable uncertainty
in the potential for increased public health protection from further
reductions in ambient PM2.5 concentrations beyond those
achieved under the current primary PM2.5 standards.
    When the above considerations are taken together, the Administrator
concludes that the scientific evidence that has become available since
the last review of the PM NAAQS, together with the analyses in the PA
based on that evidence and consideration of CASAC advice and public
comments, does not call into question the adequacy of the public health
protection provided by the current annual and 24-hour PM2.5
standards. In particular, the Administrator judges that there is
considerable uncertainty in the potential for additional public health
improvements from reducing ambient PM2.5 concentrations
below the concentrations achieved under the current primary standards
and, therefore, that standards more stringent than the current
standards (e.g., with lower levels) are not supported. That is, he
judges that such standards would be more than requisite to protect the
public health with an adequate margin of safety. This judgment reflects
the Administrator's consideration of the uncertainties in the potential
implications of the lower end of the air quality distributions from the
epidemiological studies due in part to the lack of supporting evidence
from experimental studies and retrospective accountability studies
conducted at PM2.5 concentrations meeting the current
standards.
    In reaching this conclusion, the Administrator notes that the
current standards provide an adequate margin of safety. With respect to
the annual standard, the level of 12 [micro]g/m\3\ is below the lowest
``starting'' concentration (i.e.,

13.2 [micro]g/m\3\) in the available accountability studies that show
public health improvements attributable to reductions in ambient
PM2.5. In addition, while the Administrator places less
weight on the epidemiological evidence for the purposes of selecting a
standard, he notes that the current level of the annual standard is
below the reported mean (and median) concentrations in the majority of
the key U.S. epidemiological studies using ground-based monitoring data
\56\ (noting that these means tend to be 10-20% lower than their
corresponding area design values which is the more relevant metric when
considering the level of the standard) and below the mean of the
reported means (or medians) of these studies (i.e., 13.5 [micro]g/
m\3\). In addition, the Administrator recognizes that concentrations in
areas meeting the current 24-hour and annual standards remain well-
below the PM2.5 exposure concentrations consistently shown
to elicit effects in human exposure studies.
---------------------------------------------------------------------------

    \56\ As discussed above, the means from these studies are most
relatable to the level of the annual standard. However, because the
reported means in these studies are based on averaging the monitored
concentration across an area, they tend to be lower than the design
value for that same area, since attainment of the standard is based
on the measurements at the highest monitor (and not the average
across multiple monitors.)
---------------------------------------------------------------------------

    In addition, based on the Administrator's review of the science,
including controlled human exposure studies examining effects following
short-term PM2.5 exposures, the epidemiological studies
described above, and accountability studies conducted at levels just
above the current standard, he judges that the degree of public health
protection provided by the current standard is not greater than
warranted. This judgment, together with the fact that no CASAC member
expressed support for a less stringent standard, leads the
Administrator to conclude that standards less stringent than the
current standards (e.g., with higher levels) are also not supported.
    When the above information is taken together, the Administrator
concludes that the available scientific evidence and technical
information continue to support the current annual and 24-hour
PM2.5 standards. This conclusion reflects the fact that
important limitations in the evidence remain. The Administrator
concludes that these limitations lead to considerable uncertainty
regarding the potential public health implications of revising the
existing suite of PM2.5 standards. Given this uncertainty,
and the advice from some CASAC members, he concludes that the current
suite of primary standards, including the current indicators
(PM2.5), averaging times (annual and 24-hour), forms
(arithmetic mean and 98th percentile, averaged over three years) and
levels (12.0 [mu]g/m\3\, 35 [mu]g/m\3\), when taken together, remain
requisite to protect the public health. Therefore, the Administrator
reaches the final conclusion that the current suite of primary
PM2.5 standards is requisite to protect public health from
fine particles with an adequate margin of safety, including the health
of at-risk populations, and is retaining the standards, without
revision.

C. Decision on the Primary PM2.5 Standards

    For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, the advice from the CASAC,
and consideration of public comments, the Administrator concludes that
the current annual and 24-hour primary PM2.5 standards are
requisite to protect public health from fine particles with an adequate
margin of safety, including the health of at-risk populations, and is
retaining the current standards without revision.

III. Rationale for Decisions on the Primary PM10 Standard

    This section presents the rationale for the Administrator's
decision to retain the existing primary PM10 standard. This
decision is based on a thorough review of the latest scientific
information, published through December 2017,\57\ and assessed in the
ISA, on human health effects associated with PM10-2.5 in
ambient air. This decision also accounts for considerations in the PA
of the policy-relevant information, CASAC advice, and consideration of
public comments received on the proposal.
---------------------------------------------------------------------------

    \57\ In addition to the review's opening ``call for
information'' (79 FR 71764, December 3, 2014), ``the current ISA
identified and evaluated studies and reports that have undergone
scientific peer review and were published or accepted for
publication between January 1, 2009 and March 31, 2017. A limited
literature update identified some additional studies that were
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p.
A-3). References that are cited in the ISA, the references that were
considered for inclusion but not cited, and electronic links to
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    Section III.A provides background on the general approach for this
review and the basis for the existing standard, and also presents a
brief summary of key aspects of the currently available health effects
information. Section III.B summarizes the CASAC advice and the
Administrator's proposed decision to retain the existing primary
PM10 standard, addresses public comments received on the
proposal, and presents the Administrator's conclusions on the adequacy
of the current standard, drawing on consideration of information in the
ISA and the PA information, advice from the CASAC, and comments from
the public. Section III.C summarizes the Administrator's decision on
the primary PM10 standard.

A. Introduction

    As in prior reviews, the general approach to reviewing the current
primary PM10 standard is based, most fundamentally, on using
the EPA's assessment of the current scientific evidence to inform the
Administrator's judgment regarding a primary PM10 standard
that protects public health with an adequate margin of safety. In
drawing conclusions with regard to the primary PM10
standard, the final decision on the adequacy of the current standard is
largely a public health policy judgment to be made by the
Administrator. The Administrator's final decision draws upon the
scientific information about health effects, as well as judgments about
how to consider the range and magnitude of uncertainties that are
inherent in the scientific evidence. The approach to informing these
judgments, discussed more fully below, is based on the recognition that
the available health effects evidence generally reflects a continuum,
consisting of levels at which scientists generally agree that health
effects are likely to occur, through lower levels at which the
likelihood and magnitude of the response become increasingly uncertain.
This approach is consistent with the requirements of the NAAQS
provisions in the CAA and with how the EPA and the courts have
interpreted the Act. These provisions require the Administrator to
establish primary standards that, in his judgment, are requisite to
protect public health with an adequate margin of safety. In so doing,
the Administrator seeks to establish standards that are neither more
nor less stringent for this purpose. The Act does not require that
primary standards be set at a zero-risk level, but rather at a level
that avoids unacceptable risks to public health including the health of
sensitive groups. The four basic elements of the NAAQS (indicator,
averaging time, form, and level) are considered collectively in
evaluating the health protection afforded by a standard.
    In evaluating the appropriateness of retaining or revising the
current primary

PM10 standard, the EPA has adopted an approach which is
similar to that used in the last review and which reflects the body of
evidence and information now available. As summarized in section
III.A.1 below, the Administrator's decisions in the prior review were
based on an integration of information on health effects associated
with exposure to PM10-2.5, on the public health significance
of key health effects, on policy judgments as to whether the standard
is requisite to protect public health with an adequate margin of
safety, and on consideration of the CASAC advice and public comments.
    Similarly, in this review, as described in the PA, the proposal,
and elsewhere in this document, we draw on the current evidence
pertaining to the public health risk of PM10-2.5 in ambient
air. The past and current approaches are both based, most
fundamentally, on the EPA's assessment of the current scientific and
technical information. The EPA's assessments are primarily documented
in the ISA and the PA, which have received CASAC review and public
comment (83 FR 53471, October 23, 2018; 84 FR 47944, September 11,
2019). To bridge the gap between the scientific assessment of the ISA
and the judgments required of the Administrator in determining whether
the current standard is requisite to protect public health with an
adequate margin of safety, the PA evaluates the policy implications of
the current evidence in the ISA.
    In considering the scientific and technical information, we
consider both the information available at the time of the last review
and information newly available since the last review, including most
particularly that which has been critically analyzed and characterized
in the current ISA. The evidence-based discussions presented below in
section III.A.2 (and summarized more fully in the proposal) draw upon
evidence from studies evaluating health effects related to exposures to
PM10-2.5, as discussed in the ISA.
1. Background on the Current Standard
    The last review of the PM NAAQS was completed in 2012 (78 FR 3086,
January 15, 2013). In that review, the EPA retained the existing
primary 24-hour PM10 standard, with its level of 150
[micro]g/m\3\ and its one-expected-exceedance form on average over
three years, to continue to provide public health protection against
exposures to PM10-2.5. In support of this decision, the
prior Administrator emphasized her consideration of three issues: (1)
The extent to which it was appropriate to retain a standard that
provides some measure of protection against all PM10-2.5
(regardless of composition or source of origin), (2) the extent to
which a standard with a PM10 indicator can provide
protection against exposures to PM10-2.5, and (3) the degree
of public protection provided by the existing PM10 standard.
    First, the prior Administrator judged that the evidence provided
``ample support for a standard that protects against exposures to all
thoracic coarse particles, regardless of their location or source of
origin'' (78 FR 3176, January 15, 2013). In support of this, she noted
that the epidemiological studies had reported positive associations
between PM10-2.5 and mortality or morbidity in a large
number of cities across North America, Europe, and Asia, encompassing a
variety of environments where PM10-2.5 sources and
composition were expected to vary widely. Though most of the available
studies examined associations in urban areas, the Administrator noted
that some studies had also found associations between mortality and
morbidity and relatively high ambient concentrations of particles of
non-urban crustal origin. In the last review, in considering this body
of evidence, and consistent with the CASAC's advice, the Administrator
concluded that it was appropriate to maintain a standard that provides
some measure of protection against exposures to all thoracic coarse
particles, regardless of their composition, location, or source of
origin (78 FR 3176, January 15, 2013).
    With regard to the appropriateness of retaining a PM10
indicator for a standard meant to protect against exposures to
PM10-2.5 in ambient air, the prior Administrator noted that
PM10 mass included both coarse PM (PM10-2.5) and
fine PM (PM2.5). As a result, the concentration of thoracic
coarse particles (PM10-2.5) allowed by a PM10
standard set at a single level declines as the concentration of
PM2.5 increases. Because PM2.5 concentrations
tend to be higher in urban areas than in rural areas, she observed that
a PM10 standard would generally allow lower
PM10-2.5 concentrations in urban areas than in rural areas.
She judged it appropriate to maintain such a standard given that much
of the evidence for PM10-2.5 toxicity, particularly at
relatively low particle concentrations, came from study locations where
thoracic coarse particles were of urban origin, and given that
contaminants in urban areas would increase PM10-2.5 particle
toxicity. Therefore, in the last review, the Administrator concluded
that it remained appropriate to maintain a standard that requires lower
concentrations of PM10-2.5 in ambient air in urban areas,
where the strongest evidence was for associations between mortality and
morbidity, and allows higher concentrations of PM10-2.5 in
non-urban areas, where the evidence of public health concerns was less
certain. The Administrator concluded that the varying concentrations of
coarse particles that would be permitted in urban versus non-urban
areas under the 24-hour PM10 standard, based the varying
levels of PM2.5 present, appropriately reflected the
differences in the strength of evidence regarding the health effects of
coarse particles.
    With regard to evaluating the degree of public health protection
provided by the current primary PM10 standard, with its
level of 150 [micro]g/m\3\ and its one-expected-exceedance form on
average over three years, the Administrator recognized that the
available scientific evidence and air quality information was much more
limited for PM10-2.5 than for PM2.5. In
particular, the strongest evidence for PM10-2.5-related
health effects was for cardiovascular effects, respiratory effects, and
premature mortality following short-term exposures. For each of these
categories of effects, the 2009 ISA concluded that the evidence was
``suggestive of a causal relationship'' (U.S. EPA, 2009c, section
2.3.3). The Administrator noted the significant uncertainties and
limitations associated with the PM10-2.5 scientific evidence
leading to these causal determinations and questioned whether
additional public health improvements would be achieved by revising the
existing primary PM10 standard. She specifically took note
of several uncertainties and limitations, including the following:
     There were a limited number of epidemiological studies
that employed copollutant models to address the potential for
confounding, particularly by PM2.5, that would further the
understanding of the extent to which PM10-2.5 itself, rather
than copollutants, contributed to the reported health effects.
     The plausibility of the associations between
PM10-2.5 and mortality and morbidity reported in
epidemiological studies was uncertain given the limited number of
experimental studies providing support for these associations.
     Limitations in PM10-2.5 monitoring data (i.e.,
limited data available from FRM/FEM sampling methods) and the different
approaches used to estimate PM10-2.5 concentrations across
epidemiological studies resulted in uncertainties in the ambient
PM10-2.5 concentrations at which the reported

effects occur, increasing uncertainty in estimates of the extent to
which changes in ambient PM10-2.5 concentrations would
likely impact public health.
     While PM10-2.5 effect estimates reported for
mortality and morbidity were generally positive, most were not
statistically significant, even in single pollutant models. This
included effect estimates reported in some study locations where the
ambient PM10 concentrations were above those allowed by the
current 24-hour PM10 standard.
     The composition of PM10-2.5, and the effects
associated with specific components, were also key uncertainties in the
evidence. With a lack of information on the chemical speciation of
PM10-2.5, the apparent variability in associations across
study locations was difficult to characterize.
    In considering these uncertainties and limitations, the prior
Administrator particularly took note of degree of uncertainty
associated with the extent to which health effects reported in the
epidemiological studies are due to PM10-2.5 itself, as
opposed to one or more copollutants, especially PM2.5. This
uncertainty reflects the relatively small number of studies available
for PM10-2.5 in ambient air that had evaluated copollutant
models, and the very limited evidence from controlled human exposure
studies supporting the plausibility of adverse health effects
attributable to PM10-2.5 at ambient concentrations.
    When considering the available evidence overall, the prior
Administrator concluded that the degree of public health protection
provided by the current PM10 standard against exposures to
PM10-2.5 should be maintained (i.e., neither increased nor
decreased). Her judgment that a more stringent standard to provide
additional protection was not necessary was supported by her
consideration of the uncertainties in the overall body of evidence. Her
judgment that a less stringent standard was not needed and that the
degree of public health protection provided by the current standard was
not greater than warranted was supported by the positive and
statistically significant associations with mortality observed in some
single-city study locations that were likely to have violated the
current PM10 standard. Therefore, the prior Administrator
concluded that the existing 24-hour standard, with its one-expected
exceedance form on average over three years and a level of 150
[micro]g/m\3\, was requisite to protect public health with an adequate
margin of safety against effects that have been associated with
PM10-2.5. In light of this conclusion, the EPA retained the
existing primary PM10 standard.
2. Overview of Health Effects Evidence
    In this section, we provide an overview of the policy-relevant
aspects of the PM10-2.5-related health effects evidence
available for consideration in this review. Section III.B of the
proposal provides a detailed summary of key information contained in
the ISA and the PA on the health effects associated with
PM10-2.5 exposures, and the related public health
implications. As described in the proposal, the ISA does not identify
any PM10-2.5-related health outcomes for which the evidence
supports either a ``causal'' or ``likely to be causal relationship''
(85 FR 24122, April 30, 2020). Therefore, for PM10-2.5, we
consider the evidence determined to be ``suggestive of, but not
sufficient to infer, a causal relationship,'' recognizing the greater
uncertainty in such evidence.\58\
---------------------------------------------------------------------------

    \58\ As noted in the Preamble to the ISA, ``suggestive''
evidence is ``limited, and chance, confounding, and other biases
cannot be ruled out'' (U.S. EPA, 2015, Table II).
---------------------------------------------------------------------------

    While studies conducted since the time of the last review have
strengthened support for relationships between PM10-2.5
exposures and some key health outcomes, several key uncertainties from
the last review have, to date, ``still not been addressed'' (U.S. EPA,
2019, section 1.4.2, p. 1-41). For example, in the last review,
epidemiological studies relied on a number of methods to estimate
PM10-2.5 exposures, but the methods had not been
systematically compared to evaluate spatial and temporal correlations
in exposure estimates. Methods employed by these studies included: (1)
Calculating the difference between PM10 and PM2.5
at co-located monitors, (2) calculating the difference between county-
wide averages of monitored PM10 and PM2.5 based
on monitors that are not necessarily co-located, and (3) direct
measurement of PM10-2.5 using a dichotomous sampler (U.S.
EPA, 2019, section 1.4.2). More recent epidemiological studies,
available since the last review, continue to use these approaches to
estimate PM10-2.5 concentrations. Some recent studies
estimate long-term PM10-2.5 exposures as the difference
between PM10 and PM2.5 concentrations based on
information from spatiotemporal or land use regression (LUR) models, in
addition to monitors. As in the last review, the methods used to
estimate PM10-2.5 concentrations have not been
systematically evaluated (U.S. EPA, 2019, section <IP_ADDRESS>),
contributing to the uncertainty regarding spatial and temporal
correlations in PM10-2.5 concentrations across methods and
in PM10-2.5 exposure estimates used in epidemiological
studies (U.S. EPA, 2019, sections <IP_ADDRESS>.3 and <IP_ADDRESS>.3). Given the
greater spatial and temporal variability of PM10-2.5 and
fewer PM10-2.5 monitoring sites compared to
PM2.5, this uncertainty is particularly important for the
coarse size fraction.
    In addition to the uncertainty associated with PM10-2.5
exposure estimates in the epidemiological studies, information in the
current review remains limited with regard to the potential for
confounding by copollutants and provides limited support for the
biological plausibility of serious effects following
PM10-2.5 exposures; both of these limitations continue to
contribute broadly to uncertainty in the PM10-2.5 health
evidence. Uncertainty related to potential confounding is related to
the relatively few epidemiological studies that have evaluated
PM10-2.5 health effect associations in copollutant models
with both gaseous pollutants and other PM size fractions. Uncertainty
related to the biological plausibility of serious effects caused by
PM10-2.5 exposures results from the limited number of
controlled human exposure and animal toxicology \59\ studies that have
evaluated the health effects of experimental PM10-2.5
inhalation exposures. The evidence supporting the ISA's ``suggestive''
causality determinations for PM10-2.5 and health effects,
including the uncertainties in the evidence, are summarized in the
sections below.
---------------------------------------------------------------------------

    \59\ Compared to humans, smaller fractions of inhaled
PM10-2.5 penetrate into the thoracic regions of rats and
mice (U.S. EPA, 2019, section 4.1.6), contributing to the relatively
limited evaluation PM10-2.5 exposures in animal studies.
---------------------------------------------------------------------------

a. Nature of Effects
i. Mortality
    With regard to long-term PM10-2.5 exposure and
mortality, very few studies were available at the time of the last
review. As such, the 2009 ISA concluded that the evidence was
``inadequate to determine if a causal relationship exists'' (U.S. EPA,
2009c). Since the time of the last review, there is limited new
evidence and many of the limitations noted in the 2012 review persist.
In the current review, some recent cohort studies conducted in the U.S.
and Europe reported positive associations between long-term
PM10-2.5 exposure and total (nonaccidental) mortality,
though results are

inconsistent across studies (U.S. EPA, 2019, Table 11-11). The
examination of copollutant models in these studies remains limited, and
when copollutants are included, PM10-2.5 effect estimates
are often attenuated after adjusting for PM2.5 (U.S. EPA,
2019, Table 11-11). These studies employed a number of approaches for
estimating PM10-2.5 exposures, including direct measurements
from dichotomous samplers, calculating the difference between
PM10 and PM2.5 measured at co-located monitors,
and calculating the difference of area-wide PM10 and
PM2.5 concentrations. As discussed above as a limitation in
the last review, temporal and spatial correlations between these
approaches still have not been evaluated, contributing to uncertainty
regarding the potential for exposure measurement error (U.S. EPA, 2019,
section <IP_ADDRESS>, Table 11-11). The 2019 ISA concludes that this
uncertainty ``reduces the confidence in the associations observed
across studies'' (U.S. EPA, 2020, p. 11-125) and that the evidence for
long-term PM10-2.5 exposures and cardiovascular effects,
respiratory morbidity, and metabolic disease provide limited biological
plausibility for PM10-2.5-related mortality (U.S. EPA, 2019,
sections 11.4.1 and 11.4). Taken together, the 2019 ISA concludes that
``this body of evidence is suggestive, but not sufficient to infer,
that a causal relationship exists between long-term PM10-2.5
exposure and total mortality'' (U.S. EPA, 2019, p. 11-125).
    With regard to short-term PM10-2.5 exposures and
mortality, the 2009 ISA concluded that the evidence is ``suggestive of
a causal relationship between short-term exposure to
PM10-2.5 and mortality'' (U.S. EPA, 2009c). Since the last
review, multicity epidemiological studies conducted primarily in Europe
and Asia continue to provide consistent evidence of positive
associations between short-term PM10-2.5 exposure and total
(nonaccidental) mortality (U.S. EPA, 2019, Table 11-9). These studies
contribute to increasing confidence in the relationship between the
short-term PM10-2.5 exposures and mortality, however, the
use of varying approaches to estimate PM10-2.5 exposures
continue to contribute uncertainty to the associations observed.
Additionally, the 2019 ISA notes than an analysis by Adar et al. (2014)
indicates ``possible evidence of publications bias, which was not
observed for PM2.5'' (U.S. EPA, 2019, section 11.3.2, p. 11-
106). Studies newly available in this review expand the assessment of
potential copollutant confounding of the short-term
PM10-2.5-mortality relationship and provide evidence that
PM10-2.5 associations generally remain positive in
copollutant models, although associations are attenuated in some
instances (U.S. EPA, 2019, section <IP_ADDRESS>, Figure 11-28, Table 11-
10). The 2019 ISA concludes that, overall, the assessment of potential
copollutant confounding is limited by a lack of information on the
correlation between PM10-2.5 and gaseous pollutants and the
small number of locations where copollutant analyses have been
conducted. Associations with cause-specific mortality provide some
support for associations with total (nonaccidental) mortality, though
associations with cause-specific mortality, particularly respiratory
mortality, are more uncertain (i.e., wider confidence intervals) and
less consistent (U.S. EPA, 2019, section 11.3.7). As discussed further
below, the ISA concludes that evidence for PM10-2.5-related
cardiovascular and respiratory effects provides only limited support
for the biological plausibility of a relationship between short-term
PM10-2.5 exposure and cause-specific mortality (U.S. EPA,
2019, section 11.3.7). Based on the overall evidence, the 2019 ISA
concludes that ``this body of evidence is suggestive, but not
sufficient to infer, that a causal relationship exists between short-
term PM10-2.5 exposure and total mortality'' (U.S. EPA,
2019, p. 11-120).
ii. Cardiovascular Effects
    With regard to long-term exposures, the evidence available in the
last review describing the relationship between long-term exposure to
PM10-2.5 and cardiovascular effects was characterized in the
2009 ISA as ``inadequate to infer the presence or absence of a causal
relationship.'' The limited number of epidemiological studies available
at that time reported contradictory results and experimental evidence
demonstrating an effect of PM10-2.5 on the cardiovascular
system was lacking (U.S. EPA, 2019, section 6.4).
    The evidence of long-term PM10-2.5 exposures and
cardiovascular mortality remains limited, with no consistent pattern of
associations across studies, and as discussed above, uncertainty from
the use of various approaches for estimating PM10-2.5
concentrations (U.S. EPA, 2019, Table 6-70). The evidence for
associations between PM10-2.5 and cardiovascular morbidity
has grown and, while results across studies are not entirely
consistent, some epidemiological studies report positive associations
with IHD and myocardial infarction (MI) (U.S. EPA, 2019, Figure 6-34);
stroke (U.S. EPA, 2019, Figure 6-35); atherosclerosis (U.S. EPA, 2019,
section 6.4.5); venous thromboembolism (VTE) (U.S. EPA, 2019, section
6.4.7); and blood pressure and hypertension (U.S. EPA, 2019, section
6.4.6). With respect to copollutant confounding, the effect estimates
for PM10-2.5-cardiovascular mortality are often attenuated,
but remain positive, in copollutant models adjusted for
PM2.5. For cardiovascular morbidity outcomes, associations
are inconsistent in copollutant models that adjust for
PM2.5, NO2, and chronic noise pollution (U.S.
EPA, 2019, p. 6-276). The 2019 ISA concluded that ``evidence from
experimental animal studies is of insufficient quantity to establish
biological plausibility'' (U.S. EPA, 2019, p. 6-277). Despite this
substantial data gap in the toxicologic evidence for long-term
PM10-2.5 exposures and based largely on the observation of
positive associations in some high-quality epidemiological studies, the
ISA concludes that ``evidence is suggestive of, but not sufficient to
infer, a causal relationship between long-term PM10-2.5
exposure and cardiovascular effects'' (U.S. EPA, 2019, p. 6-277).
    With regard to short-term PM10-2.5 exposures and
cardiovascular effects, the 2009 ISA found the available evidence was
``suggestive of a causal relationship,'' based primarily on several
epidemiological studies reporting associations between short-term
PM10-2.5 exposure and cardiovascular effects, including IHD
hospitalizations, supraventricular ectopy, and changes in heart rate
variability (HRV). In addition, studies found increases in
cardiovascular disease emergency department visits and hospital
admissions linked to dust storm events resulting in high concentrations
of crustal material. However, the 2009 ISA noted the potential for
exposure measurement error and copollutant confounding in these
studies. Moreover, there was only limited evidence of cardiovascular
effects from a small number of controlled human exposure and animal
toxicologic studies that examined PM10-2.5 exposures (U.S.
EPA, 2009c, section <IP_ADDRESS>). Therefore, the potential for exposure
measurement error and copollutant confounding, along with the limited
evidence of biological plausibility for cardiovascular effects
following inhalation exposure, contributed uncertainty to the
scientific evidence available at the time of the last review (U.S. EPA,
2009c, section 6.3.13).
    The evidence related to short-term PM10-2.5 exposure and
cardiovascular

effects has somewhat expanded since the last review, but a number of
important uncertainties persist. The 2019 ISA notes that there are a
small number of epidemiological studies reporting positive associations
between short-term PM10-2.5 exposures and cardiovascular
morbidity. There continues to be limited evidence, however, to suggest
that these associations are biologically plausible, or independent of
copollutant confounding. Additionally, the ISA concludes that it
remains unclear how the approaches used to estimate PM10-2.5
concentrations in epidemiological studies may impact exposure
measurement error. The 2019 ISA concludes that overall ``the evidence
is suggestive of, but not sufficient to infer, a causal relationship
between short-term PM10-2.5 exposures and cardiovascular
effects'' (U.S. EPA, 2019, p. 6-254).
iii. Respiratory Effects
    With regard to short-term PM10-2.5 exposures and
respiratory effects, the 2009 ISA concluded that, based on a small
number of epidemiological studies observing some respiratory effects
and limited evidence to support biological plausibility, the
relationship is ``suggestive of a causal relationship.''
Epidemiological findings were consistent for respiratory infection and
combined respiratory-related diseases, but not for COPD. Studies were
characterized by overall uncertainty in the exposure assignment
approach and limited information regarding potential copollutant
confounding. Controlled human exposure studies of short-term
PM10-2.5 exposures found no lung function decrements and
inconsistent evidence of pulmonary inflammation. Animal toxicologic
studies were limited to those that used non-inhalation (e.g., intra-
tracheal instillation) routes of PM10-2.5 exposure.
    Recently available epidemiological studies link short-term
PM10-2.5 exposure with asthma exacerbation and respiratory
mortality. Some associations remained positive in copollutant models
including PM2.5 or gaseous pollutants, although associations
were attenuated in some studies of mortality. Limited evidence is
available that observes positive associations with other respiratory
outcomes, including COPD exacerbation, respiratory infection, and
combined respiratory-related diseases (U.S. EPA, 2019, Table 5-36). The
lack of systematic evaluation of the various methods used to estimate
PM10-2.5 concentrations and the resulting spatial and
temporal variability in PM10-2.5 concentrations compared to
PM2.5 continues to be an uncertainty in this evidence (U.S.
EPA, 2019, sections <IP_ADDRESS>.3 and <IP_ADDRESS>). Based on the overall
evidence, the 2019 ISA concludes that the ``evidence is suggestive of,
but not sufficient to infer, a causal relationship between short-term
PM10-2.5 exposure and respiratory effects'' (U.S. EPA, 2019,
p. 5-270).
iv. Cancer
    In the last review, little information was available from studies
of cancer following inhalation exposures to PM10-2.5. Thus,
the 2009 ISA concluded that the evidence was ``inadequate to assess the
relationship between long-term PM10-2.5 exposures and
cancer'' (U.S. EPA, 2009c). Since the last review, the available
studies of long-term PM10-2.5 exposure and cancer remain
limited, with a few recent epidemiological studies that report
positive, but imprecise, associations with lung cancer incidence.
Uncertainty remains in these studies due to exposure measurement error
from the use of PM10-2.5 predictions that have not been
validated by monitored PM10-2.5 concentrations (U.S. EPA,
2019, sections <IP_ADDRESS> and 10.3.4). Very few experimental studies of
PM10-2.5 exposures have been conducted, although the
available studies indicate that PM10-2.5 exhibits
genotoxicity and oxidative stress, two key characteristics of
carcinogens. While limited, these studies provide some evidence of
biological plausibility for the findings in a small number of
epidemiological studies (U.S. EPA, 2019, section 10.3.4). Taken
together, the small number of available epidemiological and
experimental studies, along with uncertainty related to exposure
measurement error, contribute to the 2019 ISA conclusion that ``the
evidence is suggestive of, but not sufficient to infer, a causal
relationship between long-term PM10-2.5 exposure and
cancer'' (U.S. EPA, 2019, p. 10-87).
v. Metabolic Effects
    The 2009 ISA did not make a causality determination for
PM10-2.5-related metabolic effects. Since the last review,
one epidemiological study shows an association between long-term
PM10-2.5 exposure and incident diabetes, while additional
cross-sectional studies report associations with effects on glucose or
insulin homeostasis (U.S. EPA, 2019, section 7.4). Uncertainties with
this evidence include the potential for copollutant confounding and
exposure measurement error (U.S. EPA, 2019, Tables 7-14 7-15). There is
limited evidence to support biological plausibility of metabolic
effects, although a cross-sectional study that investigated biomarkers
of insulin resistance and systemic and peripheral inflammation may
support a pathway leading to type 2 diabetes (U.S. EPA, 2019, sections
7.4.1 and 7.4.3). Based on the somewhat expanded evidence available in
this review, the 2019 ISA concludes that ``the evidence is suggestive
of, but not sufficient to infer, a causal relationship between [long]-
term PM10-2.5 exposures and metabolic effects'' (U.S. EPA,
2019, p. 7-56).
vi. Nervous System Effects
    The 2009 ISA did not make a causal determination for
PM10-2.5 exposures and nervous system effects. Newly
available evidence since that time includes epidemiological studies
that report associations between long-term PM10-2.5
exposures and impaired cognition and anxiety in adults in longitudinal
analyses (U.S. EPA, 2019, Table 8-25, section 8.4.5). Associations of
long-term PM10-2.5 exposure with neurodevelopmental effects
are not consistently reported in children (U.S. EPA, 2019, section
8.4.4 and 8.4.5). Uncertainties in these studies include the potential
for copollutant confounding, given that no studies examined copollutant
models (U.S. EPA, 2019, section 8.4.5), and exposure measurement error
based on the various methods used across studies to estimate
PM10-2.5 concentrations (U.S. EPA, 2019, Table 8-25).
Additionally, there is very limited animal toxicologic evidence to
provide support for biological plausibility of nervous system effects
(U.S. EPA, 2019, sections 8.4.1 and 8.4.5). Considering the available
studies and associated limitations, the 2019 ISA concludes that ``the
evidence is suggestive of, but not sufficient to infer, a causal
relationship between long-term PM10-2.5 exposure and nervous
system effects'' (U.S. EPA, 2019, p. 8-75).

B. Conclusions on the Primary PM10 Standard

    In drawing conclusions on the adequacy of the current primary
PM10 standard, in view of the advances in scientific
knowledge and additional information now available, the Administrator
has considered the evidence base, information, and policy judgments
that were the foundation of the last review and reflects upon the body
of evidence and information newly available in this review. In so
doing, the Administrator has taken into account the evidence-based
considerations, as well as advice from the CASAC and public comments.
Evidence-based considerations draw upon the EPA's assessment and
integrated synthesis of the scientific evidence from animal

toxicologic, controlled human exposure studies, and epidemiological
studies evaluating health effects related to exposures to
PM10-2.5 as presented in the ISA and discussed in section
III.A.2. In addition to the evidence, the Administrator has weighed a
range of policy-relevant considerations as discussed in the PA and
summarized in sections III.B and III.C of the proposal and summarized
in section III.B.2 below. These considerations, along with the advice
from the CASAC (section III.B.1) and public comments (section III.B.3),
are discussed below. A more detailed summary of all significant
comments, along with the EPA's responses (henceforth ``Response to
Comments''), can be found in the docket for this rulemaking (Docket No.
EPA-HQ-OAR-2015-0072). This document is available for review in the
docket for this rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards). The
Administrator's conclusions in this review regarding the adequacy of
the current primary PM10 standard and whether any revisions
are appropriate are described in section III.B.4.
1. CASAC Advice in This Review
    As a part of the review of the draft PA, the CASAC has provided
advice on the adequacy of the public health protection afforded by the
current primary PM10 standard. As for PM2.5
(section II.B.1 above), the CASAC's advice is documented in a letter
sent to the EPA Administrator (Cox, 2019a).
    In its comments on the draft PA, the CASAC concurs with the draft
PA's overall preliminary conclusions that it is appropriate to consider
retaining the current primary PM10 standard without
revision. The CASAC agrees with the draft PA ``that key uncertainties
identified in the last review remain'' (Cox, 2019a, p. 13 of consensus
responses) and that ``none of the identified health outcomes linked to
PM10-2.5'' were judged to be causal or likely causal. (Cox,
2019a, p. 12 of consensus responses). To reduce these uncertainties in
future reviews, the CASAC recommends improvements to
PM10-2.5 exposure assessment, including a more extensive
network for direct monitoring of the PM10-2.5 fraction (Cox,
2019a, p. 13 of consensus responses). The CASAC also recommends
additional controlled human exposure and animal toxicology studies of
the PM10-2.5 fraction to improve the understanding of
biological causal mechanisms and pathway (Cox, 2019a, p. 13 of
consensus responses). Overall, the CASAC agrees with the EPA that ``. .
. the available evidence does not call into question the adequacy of
the public health protection afforded by the current primary
PM10 standard and that evidence supports consideration of
retaining the current standard in this review'' (Cox, 2019a, p. 3 of
letter).
2. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered
the assessment of the current evidence and conclusions reached in the
ISA, considerations and staff conclusions and associated rationales
presented in the PA, and the advice and recommendations of the CASAC
(85 FR 24125, April 30, 2020). In reaching his proposed decision on the
primary PM10 standard, the Administrator first noted the
decision to retain the primary PM10 standard in the last
review recognized that epidemiological studies had reported positive
associations between PM10-2.5 and mortality and morbidity in
cities across North America, Europe, and Asia. The studies encompassed
a variety of environments where PM10-2.5 sources and
composition were expected to vary widely. Although many of the studies
examined associations between PM10-2.5 and health effects in
urban areas, some of the studies also linked mortality and morbidity
with relatively high ambient concentrations of particles of non-urban
crustal origin. Drawing on this information, the EPA judged that it was
appropriate to maintain a standard that provides some measure of
protection against exposures to PM10-2.5, regardless of
location, source of origin, or particle composition (78 FR 3176,
January 15, 2013).
    The Administrator noted that the evidence for several
PM10-2.5-related health effects, particularly for long-term
exposures, has expanded since the time of the last review. Recently
available epidemiological studies conducted in North America, Europe,
and Asia continue to report positive associations with mortality and
morbidity in cities where PM10-2.5 sources and composition
are expected to vary widely, but uncertainties remain with respect to
the methods used to assign exposure in the studies. While the
Administrator recognized that important uncertainties persist in the
scientific evidence, as described below and in section III.A.2 above,
he also recognized that PM10-2.5 exposures may be associated
with a broader range of health effects that have been linked with
PM10-2.5 exposures. These studies provide an important part
of the body of evidence supporting the ISA's revised causality
determinations, including new determinations, for long-term
PM10-2.5 exposures and mortality, cardiovascular effects,
metabolic effects, nervous system effects, and cancer (U.S. EPA, 2019;
U.S. EPA, 2020, section 4.2). Drawing on this information, the
Administrator proposed to conclude that the scientific studies
available since the last review continue to support a primary
PM10 standard that provides some measure of public health
protection against PM10-2.5 exposures, regardless of
location, source of origin, or particle composition.
    With regard to the uncertainties in the scientific evidence, the
Administrator noted that the decision in the last review highlighted
limitations in the estimates of ambient PM10-2.5
concentrations used in epidemiological studies, the limited evaluation
of copollutant models to address potential confounding, and the limited
number of experimental studies to support biologically plausible
pathways for PM10-2.5-related health effects. These and
other limitations raised questions as to whether additional public
health improvements would be achieved by revising the existing
PM10 standard.
    Despite some additional new evidence available in this review, the
Administrator recognized that, similar to the last review,
uncertainties remain in the scientific evidence for
PM10-2.5-related health effects. As summarized above
(section III.A.2), these include uncertainties in the
PM10-2.5 exposure estimates used in epidemiological studies,
in the independence of PM10-2.5 health effect associations,
and in support for the biologic plausibility of PM10-2.5-
related effects from controlled human exposure and animal toxicologic
studies (U.S. EPA, 2020, section 4.2). These uncertainties contributed
to the conclusions in the 2019 ISA that the evidence for key
PM10-2.5 health effects is ``suggestive of, but not
sufficient to infer'' causal relationships (U.S. EPA, 2019). In light
of his emphasis on evidence supporting ``causal'' or ``likely to be
causal'' relationships in the current review, the Administrator judged
that the evidence of health effects associated with PM10-2.5
in ambient air provides an uncertain scientific foundation for making
decisions for standard setting. As such, he further judged that,
consistent with the last review, limitations in the evidence raise
questions as to whether additional public health protections would be
achieved by revising the existing PM10 standard.
    In reaching his proposed conclusions on the primary PM10
standard, the Administrator additionally considered the advice and
recommendations from the CASAC. As described above (section III.B.1),
the CASAC recognized the

uncertainties in the evidence for PM10-2.5-related health
effects, stating that ``key uncertainties identified in the last review
remain'' (Cox, 2019a, p. 13 of consensus responses). Given these
uncertainties, the CASAC agreed with the PA conclusion that the
evidence available in this review ``does not call into question the
adequacy of the public health protection afforded by the current
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). The
CASAC further recommended that this evidence ``supports consideration
of retaining the current standard in this review'' (Cox, 2019a, p. 3 of
letter).
    In considering the information above, the Administrator proposed to
conclude that the available scientific evidence continues to support a
PM10 standard to provide some measure of protection against
PM10-2.5 exposures. This conclusion reflected the expanded
evidence available in this review for health effects from
PM10-2.5 exposures. However, important uncertainties and
limitations in the evidence remain. Consistent with the decision in the
last review, the Administrator proposed to conclude that these
limitations contribute to considerable uncertainty regarding the
potential public health implications of revising the existing
PM10 standard. Given this uncertainty, and consistent with
the advice from the CASAC, the Administrator proposed to conclude that
the available evidence does not call into question the adequacy of the
public health protection afforded by the current primary
PM10 standard. Therefore, he proposed to retain the primary
PM10 standard, without revision.
3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few
commenters provided comments on the primary PM10 standard.
Of those commenters who did provide comments on the primary
PM10 standard, the majority supported the Administrator's
proposed decision to retain the current primary PM10
standard, without revision. This group includes primarily industries
and industry groups. All of these commenters generally note their
agreements with the rationale provided in the proposal and the CASAC
concurrence with the PA conclusion that the current evidence does not
support revision to the standard. Most also cite the EPA and CASAC
statements that the newly available information in this review does not
call into question the adequacy of the current standard. The EPA agrees
with these comments and with the CASAC advice regarding the adequacy of
the current primary standard and the lack of support for revision of
the standard.
    Some commenters disagreed with the Administrator's proposed
conclusion to retain the current primary PM10 standard,
primarily focusing their comments on the need for revisions to the form
of the standard or the level of the standard. With regard to comments
on the form of the standard, some commenters assert that the EPA should
revise the standard by adopting a separate form (or a ``compliance
threshold'' in their words)--the 99th percentile, averaged over three
years--for the primary PM10 standard for continuous
monitors, which provide data every day, while maintaining the current
form of the standard (one exceedance, averaged over three years) for 1-
in-6 samplers, given the widespread use of continuous monitoring and to
ease the burden of demonstrating exceptional events. These commenters,
in support of their comment, contend that the 99th percentile would
effectively change the form from the 2nd high to the 4th high and would
allow no more than three exceedances per year, averaged over three
years. These commenters additionally highlight the EPA's decision in
the 1997 review to adopt a 99th percentile form, averaged over three
years, citing to advantages of a percentile-based form in the
Administrator's rationale in that review. The comments further assert
that a 99th percentile form for the primary PM10 standard is
still more conservative than the form for other short-term NAAQS (e.g.,
PM2.5 and NO2).
    First, the EPA has long recognized that the form is an integral
part of the NAAQS and must be selected together with the other elements
of the NAAQS to ensure the appropriate stringency and requisite degree
of public health protection. Thus, if the EPA were to change the form
according to the monitoring method it would be establishing two
different NAAQS, varying based on the monitoring method. The EPA has
not done this to date, did not propose such an approach, and declines
to adopt it for the final rule, as we believe such a decision in this
final rule is beyond the scope of the proposal, and that each PM
standard should have a single form, indicator, level and averaging
time, chosen by the Administrator as necessary and appropriate. While
certain continuous monitors may be established and approved as a
Federal Equivalent Method (FEM) for PM10, as an alternative
to a Federal Reference Method (FRM), the use of an FEM is intended as
an alternative means of determining compliance with the NAAQS, not as
authorizing a different NAAQS.
    Even if the commenters had asked that the change in form be made
without regard to monitoring method, the EPA does not believe such a
change would be warranted. The change in form for continuous monitors
suggested by the commenters, without also lowering the level of such a
standard, would allow more exceedances and thereby markedly reduce the
public health protection provided against exposures to
PM10-2.5 in ambient air. These commenters have not provided
new evidence or analyses to support their conclusion that an
appropriate degree of public health protection could be achieved by
allowing the use of an alternative form (i.e., 99th percentile), while
retaining the other elements of the standard.
    With regard to the commenters' assertion that an alternate form of
the standard would ease the burden of demonstrating exceptional events,
the EPA first recognizes, consistent with the CAA, that it may be
appropriate to exclude monitoring data influenced by ``exceptional''
events when making certain regulatory determinations. However, the EPA
notes that the cost of implementation of the standards may not be
considered by the EPA in reviewing the standards \60\ and further the
EPA believes it is unnecessary to alter the standard for the purpose of
reducing the burden of demonstrating exceptional events. The EPA
continues to update and develop documentation and tools to facilitate
the implementation of the 2016 Exceptional Events Rule, including new
documents intended to assist air agencies with the development of
demonstrations for specific types of exceptional events. Moreover, with
regard to the commenters' specific concerns for wildfires or high
winds, the EPA released updated guidance documents on the preparation
of exceptional event demonstrations related to wildfires in September
2016, high wind dust events in April 2019, and prescribed fires in
August 2019. These guidance documents outline the regulatory
requirements and provide examples for air agencies preparing
demonstrations for wildfires, high wind dust, and prescribed fire
events.
---------------------------------------------------------------------------

    \60\ See generally Whitman v. American Trucking Associations,
531 U.S. 457, 465-472, 475-76 (2001).
---------------------------------------------------------------------------

    For all of the reasons discussed above, the EPA does not agree with
the commenters that the form of the primary PM10 standard
should be revised to a 99th percentile for continuous monitors.

    Some commenters who disagreed with the proposal to retain the
current standard advocate for revision to the primary PM10
standard to protect public health with an adequate margin of safety. In
their recommendations for revising the standard, some commenters
contend that the current standard, with its indicator of
PM10 to target exposures to PM10-2.5, has become
less protective as ambient concentrations of PM2.5 have been
reduced with revisions to that standard. These commenters assert that
the current primary PM10 standard allows increased exposure
to PM10-2.5 in ambient air because retaining the primary
PM10 would allow proportionately more PM10-2.5
mass as the PM2.5 standard has been revised downward.
Moreover, in support of their recommendations, the commenters note that
the available evidence of PM10-2.5-related health effects
has been expanded and strengthened since the time of the last review.
Taken together, the commenters contend that the primary PM10
standard should be revised and failure to do so would be arbitrary and
capricious.
    We disagree with the commenters that the primary PM10
standard should be revised because reductions in ambient concentrations
of PM2.5 result in a less protective PM10
standard. As an initial matter, we note that overall, ambient
concentrations of both PM10 and PM2.5 have
declined significantly over time. Ambient concentrations of
PM10 have declined by 46% across the U.S. from 2000 to
2019,\61\ while PM2.5 concentrations in ambient air have
declined by 43% during this same time period.\62\ While trends data is
not currently available for PM10-2.5 concentrations in
ambient air, the expanded availability of monitoring data from the
NCore network in this review can provide insight into the relative
contributions of fine and coarse PM to total PM10
concentrations.
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    \61\ PM10 concentrations presented as the annual
second maximum 24-hour concentration (in [micro]g/m\3\) at 262 sites
in the U.S. For more information, see: https://www.epa.gov/air-trends/particulate-matter-pm10-trends.
    \62\ PM2.5 concentrations presented as the
seasonally-weighted annual average concentration (in [micro]g/m\3\)
at 406 sites in the U.S. For more information, see: https://www.epa.gov/air-trends/particulate-matter-pm25-trends.
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    The 2019 ISA provides a comparison of the relative contribution of
PM2.5 and PM10-2.5 to PM10
concentrations by region and season using the more comprehensive
monitoring data from the NCore network available in this review (U.S.
EPA, 2019, section <IP_ADDRESS>.4). The data indicate that, for urban areas,
there are roughly equivalent amounts of PM2.5 and
PM10-2.5 contributing to PM10 in ambient air,
while rural locations have a slightly higher contribution of
PM10-2.5 contributing to PM10 concentrations than
PM2.5 (U.S. EPA, 2019, section <IP_ADDRESS>.4, Table 2-7). There
is generally a greater contribution from the PM2.5 fraction
in the East and a greater contribution from the PM10-2.5
fraction in the West and Midwest. However, as described in the 2019
ISA, PM10 has become considerably coarser across the U.S.
compared to similar observations in the 2009 ISA (U.S. EPA, 2019,
section <IP_ADDRESS>.4; U.S. EPA, 2009c).
    The EPA recognizes that when the primary annual PM2.5
standard was revised from 15 [micro]g/m\3\ to 12 [micro]g/m\3\ while
leaving the 24-hour PM2.5 standards unchanged at 35
[micro]g/m\3\ and the 24-hour PM10 standard unchanged at 150
[micro]g/m\3\, the PM10-2.5 fraction of PM10
could increase in some areas as the PM2.5 fraction
decreases. Moreover, the EPA recognizes that in most areas of the
country PM2.5 and PM10 concentrations have
declined and are well below their respective 24-hour standards, which
may also allow the relative ratio of PM2.5 to
PM10-2.5 to vary. In considering the available health
effects evidence in this review, there continue to be significant
uncertainties and limitations that make it difficult to fully assess
the public health implications of revising the primary PM10
standard even considering the possibility for additional variability in
the relative ratio of PM2.5 to PM10-2.5 in
current PM10 air quality across the U.S. As described in
detail above in section III.A.2 and in the proposal (85 FR 24125, April
30, 2020), these uncertainties contribute to the determinations in the
2019 ISA that the evidence for key PM10-2.5 health effects
is ``suggestive of, but not sufficient to infer, a causal
relationship'' (U.S. EPA, 2019). Beyond these uncertainties, the EPA
also notes that, while the NCore monitoring network has been expanded
since the time of the last review, epidemiological studies available in
this review do not use PM10-2.5 NCore data in evaluating
associations between PM10-2.5 in ambient air and long- or
short-term exposures. In the absence of such evidence, the public
health implications of changes in ambient PM10
concentrations as PM2.5 concentrations decrease remain
unclear. Therefore, the EPA continues to recognize this as an area for
future research, to address the existing uncertainties (U.S. EPA, 2020,
section 4.5), and inform future reviews of the PM NAAQS.
    Taken together, at the time of proposal, the Administrator
concluded that these and other limitations in the PM10-2.5
evidence raised questions as to whether additional public health
improvements would be achieved by revising the existing PM10
standard. Therefore, the EPA does not agree with the commenters that
the currently available air quality information or scientific evidence
support revisions to the primary PM10 standard in this
review.
4. Administrator's Conclusions
    Having carefully considered advice from the CASAC and the public
comments, as discussed above, the Administrator believes that the
fundamental scientific conclusions on health effects of
PM10-2.5 in ambient air that were reached in the ISA and
summarized in the PA remain valid. Additionally, the Administrator
believes the judgments he proposed (85 FR 24125, April 30, 2020) with
regard to the evidence remain appropriate. Further, in considering the
adequacy of the current primary PM10 standard in this
review, the Administrator has carefully considered the policy-relevant
evidence and conclusions contained in the ISA; the rationale and
conclusions presented in the PA; the advice and recommendations from
the CASAC; and public comments, as addressed in section III.B.3 above.
In the discussion below, the Administrator gives weight to the PA
conclusions, with which the CASAC has concurred, as summarized in
section III.D of the proposal, and takes note of the key aspects of the
rationale for those conclusions that contribute to his decision in this
review. After giving careful consideration to all of this information,
the Administrator believes that the conclusions and policy judgments
supporting his proposed decision remain valid, and that the current
primary PM10 standard provides requisite protection of
public health with an adequate margin of safety and should be retained.
    In considering the PA evaluations and conclusions, the
Administrator specifically notes that, while the health effects
evidence is somewhat expanded since the last review, the overall
conclusions are generally consistent with what was considered in the
last review (U.S. EPA, 2020, section 4.4). In so doing, he additionally
notes that the CASAC supports retaining the current standard, agreeing
with the EPA that ``the available evidence does not call into question
the adequacy of the public health protection afforded by the current
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). As
noted below, the newly available evidence for several
PM10-2.5-related health effects has

expanded since the last review, in particular for long-term exposures.
The Administrator recognizes, however, that there are a number of
uncertainties and limitations associated with the available
information, as described in the proposal (85 FR 24125, April 30, 2020)
and below.
    With regard to the current evidence on PM10-2.5-related
health effects, the Administrator takes note of recent epidemiological
studies that continue to report positive associations with mortality
and morbidity in cities across North America, Europe, and Asia, where
PM10-2.5 sources and composition are expected to vary
widely. While significant uncertainties remain, as described below, the
Administrator recognizes that this expanded body of evidence has
broadened the range of effects that have been linked with
PM10-2.5 exposures. These studies provide an important part
of the scientific foundation supporting the ISA's revised causality
determinations (and new determinations) for long-term
PM10-2.5 exposures and mortality, cardiovascular effects,
metabolic effects, nervous system effects, and cancer (U.S. EPA, 2019;
U.S. EPA, 2020, section 4.2). Drawing from his consideration of this
evidence, the Administrator concludes that the scientific information
available since the time of the last review supports a decision to
maintain a primary PM10 standard to provide public health
protection against PM10-2.5 exposures, regardless of
location, source of origin, or particle composition.
    With regard to uncertainties in the evidence, the Administrator
first notes that a number of limitations were identified in the last
review related to: (1) Estimates of ambient PM10-2.5
concentrations used in epidemiological studies; (2) limited evaluation
of copollutant models to address the potential for confounding; and (3)
limited experimental studies supporting biological plausibility for
PM10-2.5-related effects. In the current review, despite the
expanded body of evidence for PM10-2.5 exposures and health
effects, the Administrator recognizes that similar uncertainties
remain. As summarized in section III.B.1 above and in responding to
public comments, uncertainties in the current review continue to
include those associated with the exposure estimates used in
epidemiological studies, the independence of the PM10-2.5
health effect associations, and the biologically plausible pathways for
PM10-2.5 health effects (U.S. EPA, 2020, section 4.2). These
uncertainties contribute to the 2019 ISA determinations that the
evidence is ``suggestive of, but not sufficient to infer'' causal
relationships (U.S. EPA, 2019). In light of his emphasis on evidence
supporting ``causal'' or ``likely to be causal'' relationships
(sections II.A.2 and III.A.2 above), recognizing that the NAAQS should
allow for a margin of safety but finding that there is too much
uncertainty that a more stringent standard would improve public health,
the Administrator judges that the available evidence provides support
for his conclusion that the current standard provides the requisite
level of protection from the effects of PM10-2.5.
    In making this judgment, the Administrator considers whether this
level of protection is more than what is requisite and whether a less
stringent standard would be appropriate to consider. He notes that
there continues to be uncertainty associated with the evidence, for
example exposure measurement error, as reflected by the ``suggestive
of, but not sufficient to infer'' causal determinations. The
Administrator recognizes that the CAA requirement that primary
standards provide an adequate margin of safety, as summarized in
section I.A above, is intended to address uncertainties associated with
inconclusive scientific evidence and technical information, as well as
to provide a reasonable degree of protection against hazards that
research has not yet identified. Based on all of the considerations
noted here, and considering the current body of evidence, including
uncertainties and limitations, the Administrator concludes that a less
stringent standard would not provide the requisite protection of public
health, including an adequate margin of safety.
    The Administrator also considers whether the level of protection
associated with the current standard is less than what is requisite and
whether a more stringent standard would be appropriate to consider. In
so doing, the Administrator considers, as discussed above, the level of
protection offered from exposures for which public health implications
are less clear. In so doing, he again notes the significant
uncertainties and limitations that persist in the scientific evidence
in this review. In particular, he notes limitations in the approaches
used to estimate ambient PM10-2.5 concentrations in
epidemiological studies, limited examination of the potential for
confounding by co-occurring pollutants, and limited support for the
biological plausibility of the serious effects reported in many
epidemiological studies that are reflected by the ``suggestive of, but
not sufficient to infer'' causal determinations. Thus, in light of the
currently available information, including the uncertainties and
limitations of the evidence base available to inform his judgments
regarding protection against PM10-2.5-related effects, the
Administrator does not find it appropriate to increase the stringency
of the standard in order to provide the requisite public health
protection. Rather, he judges it appropriate to maintain the level of
protection provided by the current PM10 standard for
PM10-2.5 exposures and he does not judge the available
information and the associated uncertainties to indicate the need for a
greater level of public health protection.
    In reaching his conclusions on the primary PM10
standard, the Administrator also considers advice from the CASAC,
including that regarding uncertainties that remain in this review
(summarized in section III.B.1 above). In their comments, the CASAC
noted that uncertainties persist in the evidence for
PM10-2.5-related health effects, stating that ``key
uncertainties identified in the last review remain'' (Cox, 2019a, p. 13
of consensus responses). In considering these comments, the
Administrator takes note of the CASAC consideration of the
uncertainties related to the evidence and its conclusion that
``evidence does not call into question the adequacy of the public
health protection afforded by the current primary PM10
standard'' (Cox, 2019a, p. 3 of letter). The Administrator further
notes the CASAC overall conclusion in this review that the current
evidence ``supports consideration of retaining the current standard in
this review'' (Cox, 2019a, p. 3 of letter).
    Thus, in light of the currently available information, including
uncertainties and limitations in the evidence base available to inform
his judgments regarding public health protection, as well as CASAC
advice, the Administrator does not find it appropriate to revise the
standard. Rather, he judges it appropriate to retain the primary
PM10 standard to provide the requisite degree of public
health protection against PM10-2.5 exposures, regardless of
location, source of origin, or particle composition.
    With regard to the uncertainties identified above, the
Administrator notes that his final decision in this review is a public
health policy judgment that draws upon scientific information, as well
as judgments about how to consider the range and magnitude of
uncertainties that are inherent in the information. Accordingly, he
recognizes that his decision requires judgments based on

the interpretation of the evidence that neither overstates nor
understates the strength or limitations of the evidence nor the
appropriate inferences to be drawn. He recognizes, as described in
section I.A above, that the Act does not require that primary standards
be set at a zero-risk level; rather, the NAAQS must be sufficient but
not more stringent than necessary to protect public health, including
the health of sensitive groups with an adequate margin of safety.
    Recognizing and building upon all of the above considerations and
judgments, the Administrator has reached his conclusion in the current
review. As an initial matter, he recognizes the control exerted by the
current primary PM10 standard against exposures to
PM10-2.5 in ambient air. With regard to key aspects of the
specific elements of a standard, the Administrator recognizes continued
support in the current evidence base for PM10 as the
indicator for the standard. In so doing, he notes that such an
indicator provides protection from exposure to all coarse PM,
regardless of location, source of origin, or particle composition.
Similarly, with regard to averaging time, form, and level of the
standard, the Administrator takes note of uncertainties in the
available evidence and information and continues to find that the
current standard, as defined by its current elements, is requisite. He
has additionally considered the public comments regarding revisions to
these elements of the standard and continues to judge that the existing
level and the existing form, in all its aspects, together with the
other elements of the existing standard provide an appropriate level of
public health protection.
    For all of the reasons discussed above, and recognizing the CASAC
conclusion that the current evidence provides support for retaining the
current standard, the Administrator concludes that the current primary
PM10 standard (in all of its elements) is requisite to
protect public health with an adequate margin of safety from effects of
PM10-2.5 in ambient air, and should be retained without
revision.

C. Decision on the Primary PM10 Standard

    For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, the advice from the CASAC,
and consideration of public comments, the Administrator concludes that
the current primary PM10 standard is requisite to protect
public health with an adequate margin of safety, including the health
of at-risk populations, and is retaining the current standard without
revision.

IV. Rationale for the Decision on the Secondary PM Standards

    This section presents the rationale for the Administrator's
decision to retain the current secondary PM standards, without
revision. This decision is based on a thorough review of the latest
scientific information generally published through December 2017,\63\
as presented in the ISA, on non-ecological public welfare effects
associated with PM and pertaining to the presence of PM in ambient air,
specifically visibility, climate, and materials effects. This decision
also accounts for analyses in the PA of policy-relevant information
from the ISA and quantitative analyses of air quality related to
visibility impairment; CASAC advice; and consideration of public
comments received on the proposal.
---------------------------------------------------------------------------

    \63\ In addition to the review's opening ``call for
information'' (79 FR 71764, December 3, 2014), ``the current ISA
identified and evaluated studies and reports that have undergone
scientific peer review and were published or accepted for
publication between January 1, 2009 and March 31, 2017. A limited
literature update identified some additional studies that were
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p.
A-3). References that are cited in the ISA, the references that were
considered for inclusion but not cited, and electronic links to
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    The EPA is separately reviewing the ecological effects associated
with PM in conjunction with reviews of other pollutants that, along
with PM, contribute jointly to atmospheric deposition. As explained in
both the PM IRP (U.S. EPA, 2016, p. 1-17) and the IRP for review of the
secondary NAAQS for oxides of nitrogen, oxides of sulfur and PM (U.S.
EPA, 2017, p. 1-1), and discussed in the proposal for this review (85
FR 24127, April 30, 2020), in recognition of the linkages between
oxides of nitrogen, oxides of sulfur, and PM with respect to
atmospheric deposition, and with respect to the ecological effects, the
reviews of the ecological effects evidence and the secondary standards
for these pollutants are being conducted together. Addressing the
pollutants together enables the EPA to take a comprehensive approach to
considering the nature and interactions of the pollutants, which is
important for ensuring that all scientific information relevant to
ecological effects is thoroughly evaluated. This combined review of the
ecological criteria for oxides of nitrogen, oxides of sulfur, and
particulate matter is ongoing.\64\
---------------------------------------------------------------------------

    \64\ The final ISA was released in October 2020: https://www.epa.gov/isa/integrated-science-assessment-isa-oxides-nitrogen-oxides-sulfur-and-particulate-matter.
---------------------------------------------------------------------------

    Section IV.A provides background on the general approach for this
review and the basis for the existing secondary PM standards, and also
presents brief summaries of key aspects of the currently available
welfare effects evidence and quantitative information. Section IV.B
summarizes the proposed conclusions and CASAC advice, addresses public
comments received on the proposal, and presents the Administrator's
conclusions on the adequacy of the current standards, drawing on
consideration of this information, advice from the CASAC, and comments
from the public. Section IV.C summarizes the Administrator's decision
on the secondary PM standards.

A. Introduction

    As in prior reviews, the general approach to reviewing the current
secondary standards is based, most fundamentally, on using the EPA's
assessment of the current scientific evidence and associated
quantitative analyses to inform the Administrator's judgment regarding
secondary standards for PM that are requisite to protect the public
welfare from known or anticipated adverse effects associated with the
presence of PM in the ambient air. The EPA's assessments are primarily
documented in the ISA and PA, both of which have received CASAC review
and public comment (83 FR 53471, October 23, 2018; 84 FR 47944,
September 11, 2019). To bridge the gap between the scientific
assessments of the ISA and judgments required of the Administrator in
determining whether the current standards provide the requisite welfare
protection, the PA evaluates the policy implications of the assessment
of the current evidence in the ISA and of the quantitative air quality
information documented in the PA. In evaluating the public welfare
protection afforded by the current standards, the four basic elements
of the NAAQS (indicator, averaging time, level, and form) are
considered collectively.
    The secondary standard is to ``specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator . . . is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). The
secondary

standard is not meant to protect against all known or anticipated PM-
related effects, but rather those that are judged to be adverse to the
public welfare, and a bright-line determination of adversity is not
required in judging what is requisite (78 FR 3212, January 15, 2013; 80
FR 65376, October 26, 2015). Thus, the level of protection from known
or anticipated adverse effects to public welfare that is requisite for
the secondary standard is a public welfare policy judgment to be made
by the Administrator. In exercising that judgment, the Administrator
seeks to establish standards that are neither more nor less stringent
than necessary for this purpose. The Act does not require that the
standards be set at a zero-risk level, but rather at a level that
reduces risk to protect the public welfare from known or anticipated
adverse effects. In reaching conclusions on the standards, the
Administrator's final decision draws upon the scientific information
and analyses about welfare effects, environmental exposure and risks,
and associated public welfare significance, as well as judgment about
how to consider the range and magnitude of uncertainties that are
inherent in the scientific evidence and quantitative analyses. The
approach to informing these judgments is based on the recognition that
the available evidence generally reflects a continuum, consisting of
levels at which scientists generally agree that effects are likely to
occur, through lower levels at which the likelihood and magnitude of
the responses become increasingly uncertain. This approach is
consistent with the requirements of the CAA and with how the EPA and
the courts have historically interpreted the Act.
    In considering the scientific and technical information, we
consider both the information available at the time of the last review
and information newly available since the last review, including most
particularly that which has been critically analyzed and characterized
in the current ISA. We additionally consider the quantitative
information described in the PA that estimated visibility impairment
associated with current air quality conditions in areas with monitoring
data that met completeness criteria (U.S. EPA, 2020, chapter 5). The
evidence-based discussions presented below (and summarized more fully
in the proposal) draw upon evidence from studies evaluating visibility,
climate, and materials effects related to PM in ambient air, as
discussed in the ISA. The quantitative-based discussions also presented
below (and summarized more fully in the proposal) have been drawn from
the quantitative analyses for PM-related visibility impairment, as
discussed in the PA.
1. Background on the Current Standards
    In the last review, completed in 2012,\65\ the EPA retained the
secondary 24-hour PM2.5 standard, with its level of 35
[micro]g/m\3\, and the 24-hour PM10 standard, with its level
of 150 [micro]g/m\3\ (78 FR 3228, January 15, 2013). The EPA also
retained the level, set at 15 [micro]g/m\3\, and averaging time of the
secondary annual PM2.5 standard, while revising the form.
With regard to the form of the annual PM2.5 standard, the
EPA removed the option for spatial averaging (78 FR 3228, January 15,
2013). Key aspects of the Administrator's decisions on the secondary PM
standards in the last review for non-visibility effects and visibility
effects are described below. In the previous PM NAAQS review, the prior
Administrator concluded that there was insufficient information
available to base a national ambient air quality standard on climate
impacts associated with ambient air concentrations of PM or its
constituents (78 FR 3225-3226, January 15, 2013; U.S. EPA, 2011,
section 5.2.3). In reaching this decision, the prior Administrator
considered the scientific evidence, noting the 2009 ISA conclusion
``that a causal relationship exists between PM and effects on climate''
and that aerosols \66\ alter climate processes directly through
radiative forcing and by indirect effects on cloud brightness, changes
in precipitation, and possible changes in cloud lifetimes (U.S. EPA,
2009c, section 9.3.10). She also noted that the major aerosol
components with the potential to affect climate processes (i.e., black
carbon (BC), organic carbon (OC), sulfates, nitrates and mineral dusts)
vary in their reflectivity, forcing efficiencies, and direction of
climate forcing (U.S. EPA, 2009c, section 9.3.10). The prior
Administrator recognized the strong evidence indicating that aerosols
affect climate and further considered what the available information
indicated regarding the adequacy of protection provided by the
secondary PM standards. In particular, she noted that a number of
uncertainties in the scientific information (i.e., the spatial and
temporal heterogeneity of PM components that contribute to climate
forcing, uncertainties in the measurement of aerosol components,
inadequate consideration of aerosol impacts in climate modeling,
insufficient data on local and regional microclimate variations and
heterogeneity of cloud formations) affected our ability to conduct a
quantitative analysis to determine a distinct secondary standard based
on climate.
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    \65\ The 2012 decision on the adequacy of the secondary PM
standards was based on consideration of the protection provided by
those standards for visibility and for the non-visibility effects of
materials damage, climate effects and ecological effects. As noted
earlier, the current review of the public welfare protection
provided by the secondary PM standards against ecological effects is
occurring in the separate, on-going review of the secondary NAAQS
for oxides of nitrogen and oxides of sulfur (U.S. EPA, 2016, Chapter
1, section 5.2; U.S. EPA, 2020, Chapter 1, section 5.1.1). Thus, the
consideration of ecological effects in the 2012 review is not
discussed here.
    \66\ In the climate sciences research community, PM is
encompassed by what is typically referred to as aerosol. An aerosol
is defined as a solid or liquid suspended in a gas, but PM refers to
the solid or liquid phase of an aerosol. In this review of the
secondary PM NAAQS the discussion on climate effects of PM uses the
term PM throughout for consistency with the ISA (U.S. EPA, 2019) as
well as to emphasize that the climate processes altered by aerosols
are generally altered by the PM portion of the aerosol. Exceptions
to this practice include the discussion of climate effects in the
last review, when aerosol was used when discussing suspended aerosol
particles, and for certain acronyms that are widely used by the
climate community that include the term aerosol (e.g., aerosol
optical depth, or AOD).
---------------------------------------------------------------------------

    In the last review, the prior Administrator concluded that that it
is generally appropriate to retain the existing secondary standards and
that it is not appropriate to establish any distinct secondary PM
standards to address PM-related materials effects (78 FR 3225-3226,
January 15, 2013; U.S. EPA, 2011, p. 5-29). In reaching this
conclusion, she considered materials effects associated with the
deposition of PM (i.e., dry and wet deposition), including both
physical damage (materials effects) and aesthetic qualities (soiling
effects). She noted the 2009 ISA conclusion that evidence was
``sufficient to conclude that a causal relationship exists between PM
and effects on materials'' (U.S. EPA, 2009c, sections 2.5.4 and 9.5.4),
but also recognized that the 2011 PA noted that quantitative
relationships were lacking between particle size, concentrations, and
frequency of repainting and repair of surfaces and that considerable
uncertainty exists in the contributions of co-occurring pollutants to
materials damage and soiling processes (U.S. EPA, 2011, p. 5-29).
    In considering non-visibility welfare effects in the last review,
as discussed above, the prior Administrator concluded that, while it is
important to maintain an appropriate degree of control of fine and
coarse particles to address non-visibility welfare effects, ``[i]n the
absence of information that would support any different standards . . .
it is appropriate to retain the

existing suite of secondary standards'' (78 FR 3225-3226, January 15,
2013). Her decision was consistent with the CASAC advice related to
non-visibility effects. Specifically, the CASAC agreed with the 2011 PA
conclusions that, while these effects are important, ``there is not
currently a strong technical basis to support revisions of the current
standards to protect against these other welfare effects'' (Samet,
2010a, p. 5). Thus, in considering non-visibility welfare effects, the
prior Administrator concluded that it was appropriate to retain all
aspects of the existing 24-hour PM2.5 and PM10
secondary standards. With regard to the secondary annual
PM2.5 standard, she concluded that it was appropriate to
retain a level of 15.0 [micro]g/m\3\ while revising only the form of
the standard to remove the option for spatial averaging (78 FR 3225-
3226, January 15, 2013).
    Having reached the conclusion it is generally appropriate to retain
the existing secondary standards and that it is not appropriate to
establish any distinct secondary PM standards to address non-visibility
PM-related welfare effects, the prior Administrator next considered the
level of protection that would be requisite to protect public welfare
against PM-related visibility impairment and whether to adopt a
distinct secondary standard to achieve this level of protection. In
reaching her final decision that the existing 24-hour PM2.5
standard provides sufficient protection against PM-related visibility
impairment (78 FR 3228, January 15, 2013), she considered the evidence
assessed in the 2009 ISA (U.S. EPA, 2009c) and the analyses included in
the Urban-Focused Visibility Assessment (2010 UFVA; U.S. EPA, 2010b)
and the 2011 PA (U.S. EPA, 2011). She also considered the degree of
protection for visibility that would be provided by the existing
secondary standard, focusing specifically on the secondary 24-hour
PM2.5 standard with its level of 35 [micro]g/m\3\. These
considerations, and the prior Administrator's conclusions regarding
visibility are summarized below and discussed in more detail in the
proposal (85 FR 24128-24129, April 30, 2020).
    In the last review, the ISA concluded that, ``collectively, the
evidence is sufficient to conclude that a causal relationship exists
between PM and visibility impairment'' (U.S. EPA, 2009c, p. 2-28). In
consideration of the potential public welfare implication of various
degrees of PM-related visibility impairment, the prior Administrator
considered the available visibility preference studies that were part
of the overall body of evidence in the 2009 ISA and reviewed as a part
of the 2010 UFVA. These preference studies provided information about
the potential public welfare implications of visibility impairment from
surveys in which participants were asked questions about their
preferences or the values they placed on various visibility conditions,
as displayed to them in scenic photographs or in images with a range of
known light extinction levels.\67\
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    \67\ Preference studies were available in four urban areas in
the last review. Three western preference studies were available,
including one in Denver, Colorado (Ely et al., 1991), one in the
lower Fraser River valley near Vancouver, British Columbia, Canada
(Pryor, 1996), and one in Phoenix, Arizona (BBC Research &
Consulting, 2003). A pilot focus group study was also conducted for
Washington, DC (Abt Associates, 2001), and a replicate study with 26
participants was also conducted for Washington, DC (Smith and
Howell, 2009). More details about these studies are available in
Appendix D of the PA.
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    In noting the relationship between PM concentrations and PM-related
light extinction, the prior Administrator focused on identifying an
adequate level of protection against visibility-related welfare
effects. She first concluded that a standard in terms of a
PM2.5 visibility index would provide a measure of protection
against PM-related light extinction that directly takes into account
the factors (i.e., PM species composition and relative humidity) that
influence the relationship between PM2.5 in ambient air and
PM-related visibility impairment. A PM2.5 visibility index
standard would afford a relatively high degree of uniformity of visual
air quality protection in areas across the country by directly
incorporating the effects of differences of PM2.5
composition and relative humidity. In defining a target level of
protection in terms of a PM2.5 visibility index, as
discussed below, she considered specific elements of the index,
including the basis for its derivation, as well as an appropriate
averaging time, level, and form.
    The prior Administrator concluded that it was appropriate to use an
adjusted version of the original IMPROVE algorithm,\68\ in conjunction
with monthly average relative humidity data based on long-term
climatological means, as the basis for deriving a visibility index. In
so concluding, she noted the CASAC conclusion on the reasonableness of
reliance on a PM2.5 light extinction indicator calculated
from PM2.5 chemical composition and relative humidity, and
she recognized that the mass monitoring methods available at that time
were unable to measure the full water content of ambient
PM2.5 and did not provide information on the composition of
PM2.5, both of which contribute to visibility impacts (77 FR
38980, June 29, 2012). As noted at the time of the proposal, the prior
Administrator recognized that suitable equipment and performance-based
verification procedures did not then exist for direct measurement of
light extinction and could not be developed within the time frame of
the review (77 FR 38980-38981, June 29, 2012).
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    \68\ The revised IMPROVE algorithm (Pitchford et al., 2007) uses
major PM chemical composition measurements and relative humidity
estimates to calculate light extinction. For more information about
the derivation of and input data required for the original and
revised IMPROVE algorithms, see 78 FR 3168-3177, January 15, 2013.
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    The prior Administrator concluded that a 24-hour averaging time
would be appropriate for a visibility index (78 FR 3226, January 15,
2013). Although she recognized that hourly or sub-daily (4- to 6-hour)
averaging times, within daylight hours and excluding hours with
relatively high humidity, are more directly related to the short-term
nature of the perception of PM-related visibility impairment and
relevant exposure periods for segments of the viewing public than a 24-
hour averaging time, she also noted that there were data quality
uncertainties associated with the instruments used to provide the
hourly PM2.5 mass measurements required for an averaging
time shorter than 24 hours. She also considered the results of analyses
that compared 24-hour and 4-hour averaging times for calculating the
index. These analyses showed good correlation between 24-hour and 4-
hour average PM2.5 light extinction, as evidenced by
reasonably high city-specific and pooled R-squared values, generally in
the range of over 0.6 to over 0.8. Based on these analyses and the 2011
PA conclusions regarding them, the prior Administrator concluded that a
24-hour averaging time would be a reasonable and appropriate surrogate
for a sub-daily averaging time.
    The statistical form of the index, 3-year average of annual 90th
percentile values, was based on the prior Administrator's consideration
of the analyses conducted in the 2011 UFVA of three different
statistics and consistency of this statistical form with the Regional
Haze Program, which targets the 20 percent most impaired days for
improvements in visual air quality in Federal Class I areas. Moreover,
the prior Administrator noted that a 3-year average form provided
stability from the occasional effect of inter-annual meteorological
variability that can result in unusually high pollution levels for a
particular year (78 FR 3198, January 15, 2013; U.S. EPA,

2011, p. 4-58).\69\ The Administrator also noted that the available
studies on people's preferences did not address frequency of occurrence
of different levels of visibility and did not identify a basis for a
different target for urban areas than that for Class I areas (U.S. EPA,
2011, p. 4-59). These considerations led the prior Administrator to
conclude that 90th percentile form was the most appropriate annual
statistic to be averaged across three years (78 FR 3226, January 15,
2013).
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    \69\ The EPA recognized that a percentile form averaged over
multiple years offers greater stability to the air quality
management process by reducing the possibility that statistically
unusual indicator values will lead to transient violations of the
standard, thus reducing the potential for disruption of programs
implementing the standard and reducing the potential for disruption
of the protections provided by those programs.
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    In selecting a level for the index, the prior Administrator
considered the ``candidate protection levels'' (CPLs) \70\ identified
in the 2011 PA based on the visibility preference studies, ranging from
20 to 30 deciviews (dv),\71\ while noting the uncertainties and
limitations in these public preference studies.\72\ She concluded that
that the current substantial degrees of variability and uncertainty
inherent in the public preference studies should be reflected in a
higher target protection level than would be appropriate if the
underlying information were more consistent and certain. Therefore, she
concluded that it was appropriate to set a target level of protection
in terms of a 24-hour PM2.5 visibility index at 30 dv (78 FR
3226-3227, January 15, 2013).
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    \70\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to
64, 112, and 191 megameters (Mm-1), respectively.
    \71\ Deciview (dv) refers to a scale for characterizing
visibility that is defined directly in terms of light extinction.
The deciview scale is frequently used in the scientific and
regulatory literature on visibility.
    \72\ Uncertainties and limitations in the public preference
studies included the small number of stated preference studies
available; the relatively small number of study participants and the
extent to which the study participants may not be representative of
the broader study area population in some of the studies; and the
variations in the specific materials and methods used in each study.
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    Based on her considerations and conclusions summarized above, the
prior Administrator concluded that the protection provided by a
secondary standard based on a 3-year visibility metric, defined in
terms of a PM2.5 visibility index with a 24-hour averaging
time, a 90th percentile form averaged over 3 years, and a level of 30
dv, would be requisite to protect public welfare with regard to visual
air quality (78 FR 3227, January 15, 2013). Having reached this
conclusion, she next determined whether an additional distinct
secondary standard in terms of a visibility index was needed given the
degree of protection from visibility impairment afforded by the
existing secondary standards. Specifically, she noted that the air
quality analyses showed that all areas meeting the existing 24-hour
PM2.5 standard, with its level of 35 [micro]g/m\3\, had
visual air quality at least as good as 30 dv, based on the visibility
index defined above (Kelly et al., 2012b, Kelly et al., 2012a). Thus,
the secondary 24-hour PM2.5 standard would likely be
controlling relative to a 24-hour visibility index set at a level of 30
dv. Additionally, areas would be unlikely to exceed the target level of
protection for visibility of 30 dv without also exceeding the existing
secondary 24-hour standard. Thus, the prior Administrator judged that
the 24-hour PM2.5 standard ``provides sufficient protection
in all areas against the effects of visibility impairment--i.e., that
the existing 24-hour PM2.5 standard would provide at least
the target level of protection for visual air quality of 30 dv which
[she] judges appropriate'' (78 FR 3227, January 15, 2013). She further
judged that ``[s]ince sufficient protection from visibility impairment
would be provided for all areas of the country without adoption of a
distinct secondary standard, and adoption of a distinct secondary
standard will not change the degree of over-protection for some areas
of the country . . . adoption of such a distinct secondary standard is
not needed to provide requisite protection for both visibility and
nonvisibility related welfare effects'' (78 FR 3228, January 15, 2013).
2. Overview of Welfare Effects Evidence
    In this section, we provide an overview of the policy-relevant
aspects of the welfare effects evidence available for consideration in
this review. Sections IV.B and IV.C of the proposal provide a detailed
summary of key information contained in the ISA and in the PA on the
visibility and non-visibility welfare effects associated with PM in
ambient air, and the related public welfare implications (85 FR 24129,
April 30, 2020). The subsections below briefly summarize the nature of
PM-related visibility and non-visibility effects.
a. Nature of Effects
    The evidence base available in the current review includes decades
of research on visibility impairment, climate effects, and materials
effects associated with PM (U.S. EPA, 2004, 2009c, 2019). Visibility
impairment can have implications for people's enjoyment of daily
activities and for their overall sense of well-being (U.S. EPA, 2009c,
section 9.2). The strongest evidence for PM-related visibility
impairment comes from the fundamental relationship between light
extinction and PM mass (U.S. EPA, 2009c), as well as studies of the
public perception of visibility impairment (U.S. EPA, 2010b), which
confirm a well-established ``causal relationship exists between PM and
visibility impairment'' (U.S. EPA, 2009c, p. 2-28). Beyond its effects
on visibility, the 2009 ISA also identified a causal relationship
``between PM and climate effects, including both direct effects of
radiative forcing and indirect effects that involve cloud and feedbacks
that influence precipitation formation and cloud lifetimes'' (U.S. EPA,
2009, p. 2-29). The evidence also supports a causal relationship
between PM and effects on materials, including soiling effects and
materials damage (U.S. EPA, 2009, p. 2-31).
    The evidence newly available in this review is consistent with the
evidence available at the time of the last review and supports the
conclusions of causal relationships between PM and visibility, climate,
and materials effects (U.S. EPA, 2019, chapter 13). Evidence newly
available in this review augments the previously available evidence of
the relationship between PM and visibility impairment (U.S. EPA, 2019,
section 13.2), climate effects (U.S. EPA, 2019, section 13.3), and
materials effects (U.S. EPA, 2019, section 13.4).
i. Visibility
    Visibility refers to the visual quality of a human's view with
respect to color rendition and contrast definition. It is the ability
to perceive landscape form, colors, and textures. Visibility involves
optical and psychophysical properties involving human perception,
judgment, and interpretation. Light between the observer and the object
can be scattered into or out of the sight path and absorbed by PM or
gases in the sight path. Consistent with conclusions of causality in
the last review, the 2019 ISA concludes that ``the evidence is
sufficient to conclude that a causal relationship exists between PM and
visibility impairment'' (U.S. EPA, 2019, section 13.2.6). These
conclusions are based on the strong and consistent evidence that
ambient PM can impair visibility in both urban and remote areas (U.S.
EPA, 2019, section 13.1; U.S. EPA, 2009c, section 9.2.5).
    The fundamental relationship between light extinction and PM mass,
and the EPA's understanding of this relationship, has changed little
since the

2009 ISA (U.S. EPA, 2009c). The combined effect of light scattering and
absorption by particles and gases is characterized as light extinction,
i.e., the fraction of light that is scattered or absorbed per unit of
distance in the atmosphere. Light extinction is measured in units of 1/
distance, which is often expressed in the technical literature as
visibility per megameter (abbreviated Mm-1). Higher values
of light extinction (usually given in units of Mm-1 or dv)
correspond to lower visibility. When PM is present in the air, its
contribution to light extinction is typically much greater than that of
gases (U.S. EPA, 2019, section 13.2.1). The impact of PM on light
scattering depends on particle size and composition, as well as
relative humidity. All particles scatter light, as described by the Mie
theory, which relates light scattering to particle size, shape, and
index of refraction (U.S. EPA, 2019, section 13.2.3; Van de Hulst,
1981; Mie, 1908). Fine particles scatter more light than coarse
particles on a per unit mass basis and include sulfates, nitrates,
organics, light-absorbing carbon, and soil (Malm et al., 1994).
Hygroscopic particles like ammonium sulfate, ammonium nitrate, and sea
salt increase in size as relative humidity increases, leading to
increased light scattering (U.S. EPA, 2019, section 13.2.3).
    As at the time of the last review, direct measurements of PM light
extinction, scattering, and absorption continue to be considered more
accurate for quantifying visibility than PM mass-based estimates
because measurements do not depend on assumptions about particle
characteristics (e.g., size, shape, density, component mixture, etc.)
(U.S. EPA, 2019, section <IP_ADDRESS>). Measurements of light extinction
can be made with high time resolution, allowing for characterization of
sub-daily temporal patterns of visibility impairment. A number of
measurement methods have been used for visibility impairment (e.g.,
transmissometers, integrating nephelometers, teleradiometers,
telephotometers, and photography and photographic modeling), although
each of these methods has its own strengths and limitations (U.S. EPA,
2019, Table 13-1). As recognized in the last review, there are no
common performance-based criteria to evaluate these methods and none
have been deployed broadly across the U.S. for routine measurement of
visibility impairment.
    In the absence of a robust monitoring network for the routine
measurement of light extinction across the U.S., estimation of light
extinction based on existing PM monitoring can be used. The theoretical
relationship between light extinction and PM characteristics, as
derived from Mie theory (U.S. EPA, 2019, Equation 13.5), and can be
used to estimate light extinction by combining mass scattering
efficiencies of particles with particle concentrations (U.S. EPA, 2019,
section 13.2.3; U.S. EPA, 2009c, sections <IP_ADDRESS> and <IP_ADDRESS>). This
estimation of light extinction is consistent with the method used in
the last review. The algorithm used to estimate light extinction, known
as the IMPROVE algorithm,\73\ provides for the estimation of light
extinction (bext), in units of Mm-1, using
routinely monitored components of fine (PM2.5) and coarse
(PM10-2.5) PM. Relative humidity data are also needed to
estimate the contribution by liquid water that is in solution with the
hygroscopic components of PM. To estimate each component's contribution
to light extinction, their concentrations are multiplied by extinction
coefficients and are additionally multiplied by a water growth factor
that accounts for their expansion with moisture. Both the extinction
efficiency coefficients and water growth factors of the IMPROVE
algorithm have been developed by a combination of empirical assessment
and theoretical calculation using particle size distributions
associated with each of the major aerosol components (U.S. EPA, 2019,
section <IP_ADDRESS>, section <IP_ADDRESS>).
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    \73\ The algorithm is referred to as the IMPROVE algorithm as it
was developed specifically to use monitoring data generated at
IMPROVE network sites and with equipment specifically designed to
support the IMPROVE program and was evaluated using IMPROVE optical
measurements at the subset of monitoring sites that make those
measurements (Malm et al., 1994).
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    At the time of the last review, two versions of the IMPROVE
algorithm were available in the literature--the original IMPROVE
algorithm (Malm and Hand, 2007; Ryan et al., 2005; Lowenthal and Kumar,
2004) and the revised IMPROVE algorithm (Pitchford et al., 2007). As
described in detail in the proposal (85 FR 24130, April 30, 2020) and
the ISA (U.S. EPA, 2019, section 13.2.3), the algorithm has been
further evaluated and refined since the time of the last review
(Lowenthal and Kumar, 2016), particularly for PM characteristics and
relative humidity in remote areas. All three versions of the IMPROVE
algorithm were considered in evaluating visibility impairment in this
review.
    Consistent with the evidence available at the time of the last
review, our understanding of public perception of visibility impairment
comes from visibility preference studies conducted in four areas in
North America.\74\ The detailed methodology for these studies are
described in the proposal (85 FR 24131, April 30, 2020), the 2019 ISA
(U.S. EPA, 2019), and the 2009 ISA (U.S. EPA, 2009c). In summary, the
study participants were queried regarding multiple images that were
either photographs of the same location and scenery that had been taken
on different days on which measured extinction data were available or
digitized photographs onto which a uniform ``haze'' had been
superimposed. Results of the studies indicated a wide range of
judgments on what study participants considered to be acceptable
visibility across the different study areas, depending on the setting
depicted in each photograph. Based on the results of the four cities, a
range encompassing the PM2.5 visibility index values from
images that were judged to be acceptable by at least 50 percent of
study participants across all four of the urban preference studies was
identified (U.S. EPA, 2010b, p. 4-24; U.S. EPA, 2020, Figure 5-2). Much
lower visibility (considerably more haze resulting in higher values of
light extinction) was considered acceptable in Washington, DC, than was
in Denver, and 30 dv reflected the level of impairment that was
determined to be ``acceptable'' by at least 50 percent of study
participants (78 FR 3226-3227, January 15, 2013). As noted in the
proposal (85 FR 24131, April 30, 2020), the evidence base for public
preferences of visibility impairment has not been augmented since the
last review. There are no new visibility preference studies that have
been conducted in the U.S. since the time of the last review and there
is very little new information available regarding acceptable levels of
visibility impairment in the U.S.
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    \74\ Preference studies were available in four urban areas in
the last review: Denver, Colorado (Ely et al., 1991), Vancouver,
British Columbia, Canada (Pryor, 1996), Phoenix, Arizona (BBC
Research & Consulting, 2003), and Washington, DC (Abt Associates,
2011; Smith and Howell, 2009).
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ii. Climate
    The current evidence continues to support the conclusion of a
causal relationship between PM and climate effects (U.S. EPA, 2019,
section 13.3.9). Since the last review, climate impacts and been
extensively studied and recent research reinforces and strengthens the
evidence evaluated in the 2009 ISA. New evidence provides greater
specificity about the details of radiative

forcing effects \75\ and increases the understanding of additional
climate impacts driven by PM radiative effects. The Intergovernmental
Panel on Climate Change (IPCC) assesses the role of anthropogenic
activity in past and future climate change, and since the last review,
has issued the Fifth IPCC Assessment Report (AR5; IPCC, 2013) which
summarizes any key scientific advances in understanding the climate
effects of PM since the previous report. As in the last review, the ISA
draws substantially on the IPCC report to summarize climate effects. As
discussed in more detail in the proposal (85 FR 24131, April 30, 2020),
the general conclusions are similar between the IPCC AR4 and AR5
reports with regard to effects of PM on global climate. Consistent with
the evidence available in the last review, the key components,
including sulfate, nitrate, organic carbon (OC), black carbon (BC), and
dust, that contribute to climate processes vary in their reflectivity,
forcing efficiencies, and direction of forcing. Since the last review,
the evidence base has expanded with respect to the mechanisms of
climate responses and feedbacks to PM radiative forcing; however, the
new literature published since the last review does not reduce the
considerable uncertainties that continue to exist related these
mechanisms.
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    \75\ Radiative forcing (RF) for a given atmospheric constituent
is defined as the perturbation in net radiative flux, at the
tropopause (or the top of the atmosphere) caused by that
constituent, in watts per square meter (Wm-2), after
allowing for temperatures in the stratosphere to adjust to the
perturbation but holding all other climate responses constant,
including surface and tropospheric temperatures (Fiore et al., 2015;