standards together, with the annual standard as the controlling
standard, and the short-term standard supplementing the protection
afforded by the annual standard'' (Samet, 2010d, p. 1). For the reasons
discussed above, the EPA disagrees with the comments that this approach
will result in the concerns raised by the commenters; rather the EPA
concludes that this approach will help to address these concerns.
    (2) Many of these commenters asserted that the currently available
scientific information is greatly expanded and stronger compared to the
last review. Some of these commenters highlighted the availability of
multiple, multi-city long- and short-term exposure studies providing
``repeated, consistent evidence of effects below the current annual
standard level'' (ALA et al., 2012, pp. 39 to 49) and, more
specifically, ``significant evidence of harm with strong confidence
well below EPA's proposed annual standard range of 12-13 [mu]g/m\3\''
(AHA et al., 2012, pp. 10 to 12).
    The EPA recognizes that in setting standards that are requisite to
protect public health with an adequate margin of safety, the
Administrator must weigh the various types of available scientific
information in reaching public health policy judgments that neither
overstate nor understate the strength and limitations of this
information or the appropriate inferences to be drawn from the
available science.
    In general, the EPA agrees with these commenters' views that the
currently available scientific evidence is stronger ``because of its
breadth and the substantiation of previously observed health effects''
(77 FR 38906/2) and provides ``greater confidence in the reported
associations than in the last review'' (77 FR 38940/1). The EPA also
agrees with the commenters' position that it is appropriate to consider
the regions within the broader air quality distributions where we have
the strongest confidence in the associations reported in
epidemiological studies in setting the level of the annual standard.
However, as discussed in section III.E.4.d below, in weighing the
available evidence and technical analyses, as well as the associated
uncertainties and limitations in that information, the EPA disagrees
with the commenters' views regarding the extent to which the available
scientific information provides support for considering an annual
standard level below the proposed range (i.e., below 12 to 13 [mu]g/
m\3\). In particular, the EPA disagrees with the degree to which these
commenters place more weight on the relatively more uncertain evidence
that is suggestive of a causal relationship (e.g., low birth weight).
Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency concludes
it is appropriate and reasonable to place the greatest emphasis on
health effects for which the Integrated Science Assessment concluded
there is evidence of a causal or likely causal relationship and to
place less weight on the health effects that provide evidence that is
only suggestive of a causal relationship.
    (3) With regard to using the air quality information from
epidemiological studies to inform decisions on standard levels,
commenters in this group generally supported the EPA's efforts to
explore different statistical metrics from

epidemiological studies to inform the Administrator's decisions. These
commenters argued that by considering different analytic measures--
either concentrations one standard deviation below the long-term means
reported in the epidemiological studies or the EPA's distributional
statistical analysis of population-level data that extends the approach
used in previous PM NAAQS reviews to consider information beyond a
single statistical metric--``the annual standard must be significantly
lower than EPA has proposed'' (ALA et al., 2012, pp. 50 to 61).
Furthermore, with regard to characterizing the PM2.5 air
quality at which associations have been observed, some of these
commenters highlighted CASAC's recommendation that ``[f]urther
consideration should be given to using the 10th percentile as a level
for assessing various scenarios of levels for the PM NAAQS'' (Samet,
2010c, p. 11) (ALA et al., 2012, p. 55). Other commenters urged that
the EPA extend the distributional analysis to include additional
studies. For example, CHPAC urged the EPA to also conduct
distributional analysis for children's health studies to better inform
standards that would protect both children and adults from adverse
health outcomes (CHPAC, 2012, p. 3).
    The EPA agrees with these commenters' views that it is appropriate
to take into account different statistical metrics from epidemiological
studies to inform the decisions on standard levels that are appropriate
to consider in setting a standard that will protect public health with
an adequate margin of safety. In the development of the Policy
Assessment, the EPA staff explored various approaches for using
information from epidemiological studies in setting the standards. The
general approach used in the final Policy Assessment, discussed in
sections III.A.3 and III.E.4.a above, reflects consideration of CASAC
advice (Samet, 2010c, d) and public comments on multiple drafts of the
Policy Assessment.
    With regard to using the distributional statistical analysis to
characterize the confidence in the associations, the EPA emphasizes
that there is no clear dividing line provided by the scientific
evidence, and that choosing how far below the long-term mean
concentrations from the epidemiological studies is appropriate to
identify a standard level that will provide protection for the public
health with an adequate margin of safety is largely a public health
policy judgment. The EPA considers the region from approximately the
25th to 10th percentiles to be a reasonable range for providing a
general frame of reference as to the part of the distribution over
which our confidence in the magnitude and significance of the
associations observed in epidemiological studies is appreciably lower.
Based on these considerations, the EPA concludes that it is not
appropriate to place as much confidence in the magnitude and
significance of the associations over the lower percentiles of the
distributions in each study as at and around the long-term mean
concentrations. Thus, the EPA disagrees with the commenters' views that
this analysis compels placing more emphasis on the lower part of this
range in selecting a level for an annual standard that will protect
public health with an adequate margin of safety. The EPA recognizes
that this information comes primarily from two short-term exposure
studies, a relatively modest data set. In light of the limited nature
of this information, and in recognition of more general uncertainties
inherent in the epidemiological evidence, the Administrator deems it
reasonable not to place more emphasis on concentrations in the lower
part of this range, as discussed below in section III.E.4.d.
    With regard to the scope of the distributional statistical
analysis, the EPA requested additional population-level data from the
study authors for a group of six multi-city studies for which previous
air quality analyses had been conducted (Hassett-Sipple et al., 2010;
Schmidt et al., 2010, Analysis 2). These six studies were originally
selected because they considered multiple locations representing
varying geographic regions across multiple years. Thus, these studies
provided evidence on the influence of different particle mixtures on
health effects associated with long- and short-term PM2.5
exposures. In addition, these multi-city studies considered relatively
more recent health events and air quality conditions (1999 to 2005). As
discussed in section III.E.4.b.i above, the EPA received and analyzed
population-level data for four of the six studies (Rajan et al., 2011).
Three of these four studies (Krewski et al., 2009; Bell et al., 2008;
Zanobetti and Schwartz, 2009) served as the basis for the
concentration-response functions used to develop the core risk
estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees that
it would be useful to have such data from more studies, the Agency
believes that the additional data that was requested and received from
study authors provide useful information to help inform the
Administrator's selection of the annual standard level.
    (4) Many commenters in this group highlighted PM2.5-
related impacts on at-risk populations, including potential impacts on
children, older adults, persons with pre-existing heart and lung
disease, and low-income populations, to support their views that the
annual standard should be revised to a level of 11 [mu]g/m\3\ or lower
(CHPAC, 2012; AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al.,
2012; Air Alliance Houston, et al., 2012). These commenters urged the
EPA to adopt a policy approach that placed less weight on the remaining
uncertainties and limitations in the evidence and placed more emphasis
on margin of safety considerations, including providing protection
against effects for which there is more limited scientific evidence.
For example, CHPAC urged the EPA ``to place the same weight on studies
examining impacts on children's health as that of adult studies. * * *
The fact that there may be stronger evidence from adult studies does
not mean that standards based on adult studies will be protective for
children and consequently will meet the standard requisite to protect
public health with an adequate margin of safety'' (CHPAC, 2012 p. 3).
Furthermore, with regard to the EPA's approach for weighing
uncertainties, some of these commenters stated that ``we find no
justification in the preamble for an annual standard level as high as
13 [mu]g/m\3\, other than the vague assertion that uncertainties
increase at lower concentrations. Further, the final proposal
completely failed to address the Policy Assessment recommendations that
if 13 [mu]g/m\3\ was proposed, the 24-hour standard should be
strengthened as well'' (ALA et al., p. 7).
    The EPA has carefully evaluated and considered evidence of effects
in at-risk populations. With regard to effects classified as having
evidence of a causal or likely causal relationship with long- or short-
term PM2.5 exposures (i.e., premature mortality,
cardiovascular effects, and respiratory effects), the Agency takes note
that it considered the full range of studies evaluating these effects,
including studies of at-risk populations, to inform its review of the
primary PM2.5 standards. Specific multi-city studies
summarized in Figures 1, 2, and 3 above highlight evidence of effects
observed in two different lifestages--children and older adults--that
have been identified as at-risk populations. Thus, the EPA places as
much weight on studies that explored effects in children for which the
evidence is causal or likely causal in

nature as on studies of such effects in adults, including older adults.
As discussed above in responses to commenters supporting the retention
of the current standards, in setting the standard, the EPA has focused
on considering PM2.5 concentrations somewhat below the
lowest long-term mean concentrations from each of the key studies of
both long- and short-term exposures of effects for which the evidence
supports a causal or likely causal relationship (i.e., the first two
sets of studies shown in Figure 4). Absent some reason to ignore or
discount these studies, which the commenter does not provide (and of
which the EPA is unaware), the EPA considers the available evidence of
effects in children as well as other at-risk populations.
    With respect to the EPA's consideration of more limited studies
providing evidence suggestive of a causal relationship (e.g.,
developmental and reproductive effects), as noted above in responding
to comments from the first group of commenters, the Agency agrees that
it is important to place some weight on this body of evidence in
setting standards that provide protection for at-risk populations, as
required by the CAA. However, the Agency does not agree that the same
weight must be placed on this information as on the body of scientific
information for which there is evidence of a causal or likely causal
relationship. To do so would ignore the difference in the breadth and
strength of the evidence supporting the different causality
determinations reached in the Integrated Science Assessment.
    With regard to weighing the uncertainties and limitations remaining
in the evidence and technical analyses, as discussed in section II.A
above, the EPA recognizes that in setting a primary NAAQS that provides
an adequate margin of safety, the Administrator must consider a number
of factors including the nature and severity of the health effects
involved, the size of sensitive population(s) at risk, and the kind and
degree of the uncertainties that remain. As discussed in section
III.E.4.d below, the Agency agrees with these commenters that, in
weighing the available evidence and technical analyses including the
uncertainties and limitations in this scientific information, there is
no justification for setting a primary PM2.5 annual standard
level as high as 13 [mu]g/m\3\.
    Finally, some commenters in both groups also identified ``new''
studies that were not included in the Integrated Science Assessment as
providing further support for their views on the level of the annual
standard. As discussed in section II.B.3 above, the EPA completed a
provisional review and assessment of ``new'' studies published since
the close of the Integrated Science Assessment, including ``new''
studies submitted by commenters (U.S. EPA, 2012b). The provisional
assessment found that the ``new'' studies expand the scientific
information considered in the Integrated Science Assessment and provide
important insights on the relationship between PM2.5
exposure and health effects of PM (U.S. EPA, 2012b). However, the EPA
notes that the provisional assessment found that the ``new'' science
did not materially change the conclusions reached in the Integrated
Science Assessment. The EPA notes that, as in past NAAQS reviews, the
Agency is basing the final decisions in this review on the studies and
related information included in the Integrated Science Assessment that
have undergone CASAC and public review, and will consider newly
published studies for purposes of decision making in the next PM NAAQS
review.
ii. 24-Hour Standard Level
    With respect to the level of the 24-hour standard, the EPA received
comments on the proposal from two distinct groups of commenters. One
group that included virtually all commenters representing industry
associations, businesses, and many States agreed with the Agency's
proposed decision to retain the level of the 24-hour PM2.5
standard. The other group of commenters included many medical groups,
numerous physicians and academic researchers, many public health
organizations, some State and local agencies, five State Attorneys
General, and a large number of individual commenters. These commenters
disagreed with the Agency's proposed decision and argued that EPA
should lower the level of the 24-hour standard to 30 or 25 [mu]g/m\3\.
Comments from these groups on the level of the 24-hour PM2.5
standard are addressed below and in the Response to Comments Document.
    As noted above, of the public commenters who addressed the level of
the 24-hour PM2.5 standard, all industry commenters and most
State and local commenters supported the proposed decision to retain
the current level of 35 [mu]g/m\3\. In many cases, these groups agreed
with the rationale supporting the Administrator's proposed decision to
retain the current 24-hour PM2.5 standard, including her
emphasis on the annual standard as the generally controlling standard
with the 24-hour standard providing supplementary protection, and her
conclusion that multi-city, short-term exposure studies provide the
strongest data set for informing decisions on the appropriate 24-hour
standard level. Many of these commenters agreed with the
Administrator's view that the single-city, short-term studies provided
a much more limited data set (e.g., limited statistical power, limited
exposure data) and more equivocal results (e.g., mixed results within
the same study area), making them an unsuitable basis for setting the
level of the 24-hour standard.
    While these commenters agreed with the EPA's proposed decision to
retain the current 24-hour PM2.5 standard, some did not
agree with the EPA's approach to considering the evidence from short-
term multi-city studies. For example, a commenter representing UARG
pointed out that the 98th percentile concentrations reported in the
proposal for multi-city studies reflect the averages of 98th percentile
concentrations across the cities included in those studies (UARG, 2012;
Attachment 1; p. 25). This commenter contended that such averaged 98th
percentile PM2.5 concentrations do not provide information
that can appropriately inform a decision on the adequacy of the public
health protection provided by the current or alternative 24-hour
standards.
    While the EPA agrees that there is uncertainty in linking effects
reported in multi-city studies to specific air quality concentrations
(U.S. EPA, 2011a, section <IP_ADDRESS>), the EPA disagrees with this
commenter's view that such uncertainty precludes the use of averaged
98th percentile PM2.5 concentrations to inform a decision on
the appropriateness of the protection provided by the 24-hour
PM2.5 standard. In particular, the EPA notes that averaged
98th percentile concentrations do provide information on the extent to
which study cities contributing to reported associations would likely
have met or violated the current 24-hour PM2.5 standard
during the study period. As evidence of this, the EPA notes the three
multi-city studies specifically highlighted by this commenter as having
averaged 98th percentile 24-hour PM2.5 concentrations below
35 [mu]g/m\3\ (Dominici et al., 2006a; Bell et al., 2008; Zanobetti and
Schwartz, 2009). Based on the 98th percentiles of 24-hour
PM2.5 concentrations in the individual cities evaluated in
these studies, the EPA notes that the majority of these study cities
would likely have met the current standard during the study periods
(Hassett-Sipple et al., 2010). Therefore, regardless of whether the
averaged 98th percentile concentrations or the 98th

percentile concentrations in each city are considered, these studies
provide evidence for associations between short-term PM2.5
and mortality or morbidity across a large number of U.S. cities, the
majority of which would likely have met the current 24-hour
PM2.5 standard during study periods. In their review of the
PM Policy Assessment, CASAC endorsed the conclusions drawn from
analyses of averaged 98th percentile 24-hour PM2.5
concentrations, and the EPA continues to conclude that this type of
information can appropriately inform the Administrator's decision on
the current 24-hour PM2.5 standard.\103\
---------------------------------------------------------------------------

    \103\ This is not to say that the EPA's decision on whether to
revise the 24-hour PM2.5 standard should be based on or
only be informed by considerations of whether studies reported
associations with mortality or morbidity in areas with averaged 98th
percentile PM2.5 concentrations less than 35 mg/m\3\. As
discussed below, in reaching a decision in this final notice on the
most appropriate approach to strengthen the suite of
PM2.5 standards, the Administrator considers the degree
of public health protection provided by the combination of the
annual and 24-hour standards together.
---------------------------------------------------------------------------

    Another group of commenters argued that the 24-hour standard level
should be lowered. Many of these commenters supported setting the level
of the 24-hour PM2.5 standard at either 25 or 30 [mu]g/m\3\.
In support of their position, the ALA et. al., AHA et al., five state
Attorneys General, and a number of additional groups pointed to 98th
percentile PM2.5 concentrations in locations of multi-city
and single-city epidemiological studies. For example, the ALA and
others pointed to multi-city studies by Dominici et al. (2006a),
Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al.
(2008) as providing evidence for associations with mortality and
morbidity in study locations with averaged (i.e., averaged across
cities) 98th percentile 24-hour PM2.5 concentrations below
35 [mu]g/m\3\. These commenters also pointed to several single-city and
panel studies reporting associations between short-term
PM2.5 and mortality or morbidity in locations with
relatively low 24-hour PM2.5 concentrations. Because some of
these multi- and single-city studies have reported associations with
health effects in locations with 98th percentile PM2.5
concentrations below 35 [mu]g/m\3\, commenters maintained that the
current 24-hour PM2.5 standard (i.e., with its level of 35
[mu]g/m\3\) does not provide an appropriate degree of protection in all
areas.
    In further support of their position that the level of the current
24-hour standard should be lowered, these commenters pointed out the
variability across the U.S. in ratios of 24-hour to annual
PM2.5 concentrations. They noted that some locations,
including parts of the northwestern U.S., experience relatively low
annual PM2.5 concentrations but can experience relatively
high 24-hour concentrations at certain times of the year. In order to
provide protection against effects associated with short-term
PM2.5 exposures, especially in locations with high ratios of
24-hour to annual PM2.5 concentrations, these commenters
advocated setting a lower level for the 24-hour standard.
    The EPA agrees with these commenters that it is appropriate to
maintain a 24-hour PM2.5 standard in order to supplement the
protection provided by the revised annual standard, particularly in
locations with relatively high ratios of 24-hour to annual
PM2.5 concentrations. However, in highlighting 98th
percentile PM2.5 concentrations in study locations without
also considering the impact of a revised annual standard on short-term
concentrations, these commenters ignore the fact that many areas would
be expected to experience decreasing short- and long-term
PM2.5 concentrations in response to a revised annual
standard.
    In considering the specific multi-city studies highlighted by
public commenters who advocated a more stringent 24-hour standard, the
EPA notes that such studies have reported consistently positive and
statistically significant associations with short-term PM2.5
exposures in locations with averaged 98th percentile PM2.5
concentrations ranging from 45.8 to 34.2 [mu]g/m\3\ and long-term mean
PM2.5 concentrations ranging from 13.4 to 12.9 (Burnett and
Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et
al., 2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009).\104\
The EPA notes that to the extent air quality distributions are reduced
to meet the current 24-hour standard with its level of 35 [mu]g/m\3\
and/or the revised annual standard with its level of 12 [mu]g/m\3\,
additional protection would be anticipated against the effects reported
in these short-term, multi-city studies. Put another way, to attain an
annual standard with a level below the long-term means in the locations
of these short-term studies (as EPA is adopting here), the overall air
quality distributions in the majority of study cities will necessarily
be reduced, resulting in lower daily PM2.5 ambient
concentrations. We therefore expect that the revised annual standard
will result in 98th percentile PM2.5 concentrations in these
cities that are lower than those measured in the studies, and that the
overall distributions of PM2.5 concentrations will be lower
than those reported to be associated with health effects. Thus, even
for effects reported in multi-city studies with averaged 98th
percentile concentrations below 35 [mu]g/m\3\, additional protection
from the risks associated with short-term exposures is anticipated from
the revised annual standard, without revising the 24-hour standard,
because long-term average PM2.5 concentrations in multi-city
study locations were above the level of the revised annual standard
(i.e., 12 [mu]g/m\3\).\105\ As discussed above, reducing the annual
standard is the most efficient way to reduce the risks from short-term
exposures identified in these studies, as the bulk of the risk comes
from the large number of days across the bulk of the air quality
distribution, not the relatively small number of days with peak
concentrations.
---------------------------------------------------------------------------

    \104\ Commenters also highlighted associations with short-term
PM2.5 concentrations reported in sub-analyses restricted
to days with 24-hour concentrations at or below 35 [mu]g/m\3\
(Dominici, 2006b). These sub-analyses were not included in the
original publication by Dominici et al. (2006a). Authors provided
results of sub-analyses for the Administrator's consideration in a
letter to the docket following publication of the proposed rule in
January 2006 (personal communication with Dr. Francesca Dominici,
2006b). As noted in section III.A.3, these sub-analyses are part of
the basis for the conclusion that there is no evidence suggesting
that risks associated with long-term exposures are likely to be
disproportionately driven by peak 24-hour concentrations. Because
the sub-analyses did not present long-term average PM2.5
concentrations, it is not clear whether they reflected
PM2.5 air quality that would have been allowed by the
revised annual PM2.5 standard being established in this
rule.
    \105\ It is also the case that additional protection is
anticipated in locations with 98th percentile 24-hour
PM2.5 concentrations above 35 [mu]g/m\3\, even if long-
term concentrations are below 12 [mu]g/m\3\. As noted in the
proposal and in the Policy Assessment (U.S. EPA, 2011a, Figure 2-
10), parts of the northwestern U.S. are more likely than other parts
of the country to violate the 24-hour standard and meet the revised
annual standard.
---------------------------------------------------------------------------

    In considering the single-city studies highlighted by public
commenters who advocated a more stringent 24-hour standard, the EPA
first notes that, overall, these single-city studies reported mixed
results. Specifically, some studies reported positive and statistically
significant associations with PM2.5, some studies reported
positive but non-significant associations, and several studies reported
negative associations or a mix of positive and negative associations
with PM2.5. In light of these inconsistent results, the
proposal noted that the overall body of evidence from single-city
studies is mixed, particularly in locations with 98th percentiles of
24-hour concentrations below 35 [mu]g/m\3\. Therefore, although some
single-city

studies reported effects at appreciably lower PM2.5
concentrations than short-term multi-city studies, the uncertainties
and limitations associated with the single-city studies were noted to
be greater. In light of these greater uncertainties and limitations,
the Administrator concluded in the proposal that she had less
confidence in using these studies as a basis for setting the level of
the standard (77 FR 38943).
    Given the considerations and conclusions noted above, in the
proposal the Administrator concluded that the short-term multi-city
studies provide the strongest evidence to inform decisions on the level
of the 24-hour standard. Further, she viewed single-city, short-term
exposure studies as a much more limited data set providing mixed
results, and she had less confidence in using these studies as a basis
for setting the level of a 24-hour standard (77 FR 38942). In
highlighting specific single-city studies, public health,
environmental, and State and local commenters appear to have
selectively focused on studies reporting associations with
PM2.5 and to have overlooked studies that reported more
equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004;
Slaughter et al., 2005; Villeneuve et al., 2006) (U.S. EPA, 2011,
Figure 2-9). As such, these commenters have not presented new
information that causes the EPA to reconsider its decision to emphasize
multi-city studies over single-city studies when identifying the
appropriate level of the 24-hour PM2.5 standard.
    In further considering the single-city studies highlighted by
public commenters, the EPA notes that some commenters advocating for a
lower level for the 24-hour PM2.5 standard also discussed
short-term studies that have been published since the close of the
Integrated Science Assessment. These recent studies were conducted in
single cities or in small panels of volunteers. As in prior NAAQS
reviews and as discussed above in more detail (section II.B.3), the EPA
is basing its decisions in this review on studies and related
information assessed in the Integrated Science Assessment. The studies
assessed in the Integrated Science Assessment, and the conclusions
based on those studies, have undergone extensive critical review by the
EPA, CASAC, and the public. The rigor of that review makes the studies
assessed in the Integrated Science Assessment, and the conclusions
based on those studies, the most reliable source of scientific
information on which to base decisions on the NAAQS.
    However, as discussed above (section II.B.3), the EPA recognizes
that ``new studies'' may sometimes be of such significance that it is
appropriate to delay a decision on revision of a NAAQS and to
supplement the pertinent air quality criteria so the studies can be
taken into account. In the present case, the EPA's provisional
consideration of ``new studies'' concludes that, taken in context, the
``new'' information and findings do not materially change any of the
broad scientific conclusions made in the air quality criteria regarding
the health effects of PM2.5 (U.S. EPA, 2012b).
    For this reason, reopening the air quality criteria review would
not be warranted, even if there were time to do so under the court
order governing the schedule for completing this review. Accordingly,
the EPA is basing its final decisions in this review on the studies and
related information included in the PM Integrated Science Assessment
(i.e., the air quality criteria) that has undergone CASAC and public
review. The EPA will consider the ``new studies'' in the next periodic
review of the PM NAAQS, which will provide an opportunity to fully
assess these studies through a more rigorous review process involving
the EPA, CASAC, and the public.
    Some public health, medical, and environmental commenters also
criticized the EPA's interpretation of PM2.5 risk results.
These commenters presented risk estimates for combinations of annual
and 24-hour standards using more recent air quality data than that used
in the EPA's Risk Assessment (U.S. EPA, 2010a). Based on these
additional risk analyses, the ALA and other commenters contended that
public health benefits could continue to increase as annual and 24-hour
standard levels decrease below 13 [mu]g/m\3\ and 35 [mu]g/m\3\,
respectively.
    The EPA agrees with commenters that important public health
benefits are expected as a result of revising the level of the annual
standard to 12 [mu]g/m\3\, as is done in this rule, rather than 13
[mu]g/m\3\. The Agency also acknowledges that estimated
PM2.5-associated health risks continue to decrease with
annual standard levels below 12 [mu]g/m\3\ and/or with 24-hour standard
levels below 35 [mu]g/m\3\. However, the EPA disagrees with the
commenters' views regarding the extent to which risk estimates support
setting standard levels below 12 [mu]g/m\3\ (annual standard) and 35
[mu]g/m\3\ (24-hour standard).\106\
---------------------------------------------------------------------------

    \106\ This section focuses on the 24-hour standard. Section
III.E.4.c.i above also discusses these commenters' recommendations
within the context of the annual PM2.5 standard.
---------------------------------------------------------------------------

    The CAA charges the Administrator with setting NAAQS that are
``requisite'' (i.e., neither more nor less stringent than necessary) to
protect public health with an adequate margin of safety. In setting
such standards the Administrator must weigh the available scientific
evidence and information, including associated uncertainties and
limitations. As described above, in reaching her proposed decisions on
the PM2.5 standards that would provide ``requisite''
protection, the Administrator carefully considered the available
scientific evidence and risk information, making public health policy
judgments that, in her view, neither overstated nor understated the
strengths and limitations of that evidence and information. In
contrast, as discussed more fully above, public health, medical, and
environmental commenters who recommended levels below 35 [mu]g/m\3\ for
the 24-hour PM2.5 standard have not provided new information
or analyses to suggest that such standard levels are appropriate, given
the uncertainties and limitations in the available health evidence,
particularly uncertainties in studies conducted in locations with 98th
percentile 24-hour PM2.5 concentrations below 35 [mu]g/m\3\
and long-term average concentrations below 12 [mu]g/m\3\.
d. Administrator's Final Conclusions on the Primary PM2.5
Standard Levels
    In reaching her conclusions regarding appropriate standard levels,
the Administrator has considered the epidemiological and other
scientific evidence, estimates of risk reductions associated with just
meeting alternative annual and/or 24-hour standards, air quality
analyses, related limitations and uncertainties, the advice of CASAC,
and extensive public comments on the proposal. After careful
consideration of all of these, the Administrator has decided to revise
the level of the primary annual PM2.5 standard from 15.0
[mu]g/m\3\ to 12.0 [mu]g/m\3\ and to retain the level of the primary
24-hour standard at 35 [mu]g/m\3\.
    As an initial matter, the Administrator agrees with the approach
supported by CASAC and discussed in the Policy Assessment as summarized
in sections III.A.3 and III.E.4.a above, of considering the annual and
24-hour standards together in determining the protection afforded
against mortality and morbidity effects associated with both long- and
short-term exposures to PM2.5. This approach is consistent
with the approach taken in the review

completed in 1997, in contrast to the approach used in the review
completed in 2006 where each standard was considered independently of
the other (i.e., only data from long-term exposure studies were used to
inform the level of the annual standard and only data from short-term
exposure studies were used to inform the level of the 24-hour
standard).\107\
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    \107\ See 71 FR 61148 and 61168, October 17, 2006.
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    Based on the evidence and quantitative risk assessment, the
Administrator concludes that it is appropriate to set an annual
standard that is generally controlling, which will lower the broad
distribution of 24-hour average concentrations in an area as well as
the annual average concentration, so as to provide protection from both
long- and short-term PM2.5 exposures. In conjunction with
this, it is appropriate to set a 24-hour standard focused on providing
supplemental protection, particularly for areas with high peak-to-mean
ratios of 24-hour concentrations, possibly associated with strong local
or seasonal sources, and for PM2.5-related effects that may
be associated with shorter-than daily exposure periods. The
Administrator concludes this approach will reduce aggregate risks
associated with both long- and short-term exposures more consistently
than a generally controlling 24-hour standard and is the most effective
and efficient way to reduce total PM2.5-related population
risk and to protect public health with an adequate margin of safety.
    In selecting the level of the annual PM2.5 standard,
based on the characterization and assessment of the epidemiological and
other studies presented and assessed in the Integrated Science
Assessment and the Policy Assessment, the Administrator recognizes the
substantial increase in the number and diversity of studies available
in this review. This expanded body of evidence includes extended
analyses of the seminal studies of long-term PM2.5 exposures
(i.e., ACS and Harvard Six Cities studies) as well as important new
long-term exposure studies (as summarized in Figures 1 and 2).
Collectively, the Administrator notes that these studies, along with
evidence available in the last review, provide consistent and stronger
evidence than previously observed of an association between long-term
PM2.5 exposures and premature mortality in areas with lower
long-term ambient concentrations than previously observed, with the
strongest evidence related to cardiovascular-related mortality. The
Administrator also recognizes the availability of stronger evidence of
morbidity effects associated with long-term PM2.5 exposures,
including evidence of respiratory effects such as decreased lung
function growth, from the extended analyses for the Southern California
Children's Health Study and evidence of cardiovascular effects from the
WHI study. Furthermore, the Administrator recognizes new U.S. multi-
city studies that greatly expand and reinforce our understanding of
mortality and morbidity effects associated with short-term
PM2.5 exposures, providing stronger evidence of associations
in areas with ambient concentrations similar to those previously
observed in short-term exposure studies considered in the previous
review (as summarized in Figure 3).
    The Administrator recognizes the strength of the scientific
evidence for evaluating health effects associated with fine particles,
noting that the newly available scientific evidence builds upon the
previous scientific data base to provide evidence of generally robust
associations and a basis for greater confidence in the reported
associations than in the last review. She notes the conclusion of the
Integrated Science Assessment that this body of evidence supports a
causal relationship between long- and short-term PM2.5
exposures and mortality and cardiovascular effects and a likely causal
relationship between long- and short-term PM2.5 exposures
and respiratory effects. In addition, the Administrator notes
additional, but more limited evidence, for a broader range of health
endpoints including evidence suggestive of a causal relationship for
developmental and reproductive effects as well as for carcinogenic
effects.
    Based on information discussed and presented in the Integrated
Science Assessment, the Administrator recognizes that health effects
may occur over the full range of concentrations observed in the
epidemiological studies of both long-term and short-term exposures,
since no discernible population-level threshold for any such effects
can be identified based on the currently available evidence (U.S. EPA,
2009a, section 2.4.3). To inform her decisions on an appropriate level
for the annual standard that will protect public health with an
adequate margin of safety, in the absence of any discernible
population-level thresholds, the Administrator judges that it is
appropriate to consider the relative degree of confidence in the
magnitude and significance of the associations observed in
epidemiological studies across the range of long-term PM2.5
concentrations in such studies. Further, she recognizes, in taking note
of CASAC advice and the distributional statistics analysis discussed in
the Policy Assessment and in section III.E.4.a above, that there is
significantly greater confidence in the magnitude and significance of
observed associations for the part of the air quality distribution
corresponding to where the bulk of the health events evaluated in each
study have been observed, generally at and around the long-term mean
concentrations. Conversely, she also recognizes that there is
significantly diminished confidence in the magnitude and significance
of observed associations in the lower part of the air quality
distribution corresponding to where a relatively small proportion of
the health events were observed. Further, the Administrator recognizes
that the long-term mean concentrations, or any other specific point in
the air quality distribution of each study, do not represent a ``bright
line'' at and above which effects have been observed and below which
effects have not been observed.
    In considering the long-term mean concentrations reported in
epidemiological studies, the Administrator recognizes that in selecting
a level of the annual standard that will protect public health with an
adequate margin of safety, it is not sufficient to focus on a
concentration generally somewhere within the range of long-term mean
concentrations from the key long-term and short-term exposure studies
that reported lower concentrations than had been observed in earlier
reviews. These key studies provide information for various types of
serious health endpoints (including mortality and morbidity effects),
different study populations (which may include at-risk populations such
as children and older adults), and different air quality distributions
that are specific to each study. A level somewhere within the range of
long-term mean concentrations of the full set of key studies would be
higher than the long-term mean of at least one of the studies being
considered and therefore would not provide a sufficient degree of
protection against the health effects observed in that study. Absent
some reasoned basis to place less weight on the evidence in the
epidemiological study with the lowest long-term mean concentration
among these key studies, this approach would not be consistent with the
requirement to set a standard that will protect public health with an

adequate margin of safety.\108\ Thus, the Administrator recognizes it
is important to protect against the serious effects observed in each of
these studies so as to protect public health with an adequate margin of
safety. In so doing, she looks to identify the study with the lowest
long-term mean concentration within the full set of key studies to help
inform her decision of the appropriate standard level which will
provide protection for the broad array of health outcomes observed in
all of the studies, including effects observed in at-risk populations.
---------------------------------------------------------------------------

    \108\ See American Farm Bureau Federation v. EPA, 559 F. 3d 512,
525-26 (D.C. Cir. 2009).
---------------------------------------------------------------------------

    Further, consistent with the general approach summarized in section
III.E.4.a above and supported by CASAC as discussed in section
III.E.4.b.ii above, the Administrator recognizes that it is appropriate
to consider a level for an annual standard that is not just at but
rather is somewhat below the long-term mean PM2.5
concentrations reported in each of the key long- and short-term
exposure studies. In so doing, she focuses especially on multi-city
studies that evaluated health endpoints for which the associations are
causal or likely causal (i.e., mortality and cardiovascular and
respiratory effects associated with both long- and short-term
PM2.5 exposures). As discussed above, the importance of
considering a level somewhat below the lowest long-term mean
concentrations in this set of key studies is to establish a standard
that would be protective against the observed effects in all of the
studies, and that takes into account the relative degree of confidence
in the magnitude and significance of observed associations across the
air quality distributions in these studies.
    The Administrator recognizes that there is no clear way to identify
how much below the long-term mean concentrations of key studies to set
a standard that would provide requisite protection with an adequate
margin of safety. She therefore must use her judgment to weigh the
available scientific and technical information, and associated
uncertainties, to reach a final decision on the appropriate standard
level. In considering the information in Figures 1-4 for effects
classified as having evidence of a causal or likely causal relationship
with long- or short-term PM2.5 exposures, she observes a
cluster of short-term exposure studies with long-term mean
concentrations within a range of 13.4 [mu]g/m\3\ down to 12.8 [mu]g/
m\3\ (Dominici et al., 2006a; Burnett and Goldberg, 2003; Zanobetti and
Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also
observes a cluster of long-term exposure studies with long-term mean
concentrations within a range of 14.5 [mu]g/m\3\ to 13.6 [mu]g/m\3\
(Dockery et al., 1996; Lipfert et al., 2006a; Zeger et al., 2008;
McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the
reasons discussed in response to public comments in section III.E.4.c
above, the Administrator is less influenced by the long-term mean
PM2.5 concentrations from the Miller et al. (2007) and
Krewski et al. (2009) studies with reported long-term mean
PM2.5 concentrations of 12.9 and 14.0 [mu]g/m\3\,
respectively. In each case, the most relevant exposure periods would
likely have had higher mean PM2.5 concentrations than those
reported in the studies.\109\ Thus, the Administrator considers the
long-term mean PM2.5 concentrations from these two studies
to be a highly uncertain basis for informing her selection of the
annual standard level.\110\
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    \109\ In the case of Miller et al. (2007), the mean
concentration is based on a single year of air quality data which
post-dated by two years the period for which the health events data
were collected. In the case of Krewski et al. (2009), the air
quality data were based on the last two years of the 18-year period
for which the health event data were collected.
    \110\ Nonetheless, as noted above, the EPA notes that the
Krewski et al. (2009) and Miller et al. (2007) studies provide
strong evidence of mortality and cardiovascular-related effects
associated with long-term PM2.5 exposures to inform
causality determinations reached in the Integrated Science
Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).
---------------------------------------------------------------------------

    To help guide her judgment of the appropriate level below the long-
term mean concentrations in the epidemiological studies at which to set
the standard, the Administrator considered additional information from
epidemiological studies concerning the broader distribution of
PM2.5 concentrations which correspond to the health events
observed in these studies (e.g., deaths, hospitalizations). The
Administrator observes that the development and use of this information
in considering standard levels is consistent with CASAC's advice, as
discussed in section III.E.4.b.ii above, to focus on understanding the
concentrations that were most influential in generating the health
effect estimates in individual studies (Samet, 2010d, p. 2).
    In considering this additional population-level information, the
Administrator recognizes that, in general, the confidence in the
magnitude and significance of an association identified in a study is
strongest at and around the long-term mean concentration for the air
quality distribution, as this represents the part of the distribution
in which the data in any given study are generally most concentrated.
She also recognizes that the degree of confidence decreases as one
moves towards the lower part of the distribution. Consistent with the
approach used in the Policy Assessment, the Administrator believes that
the range from approximately the 25th to 10th percentiles is a
reasonable range for providing a general frame of reference as to the
part of the distribution in which her confidence in the associations
observed in epidemiological studies is appreciably lower. However, as
noted above, it is important to emphasize that there is no clear
dividing line or single percentile within a given distribution provided
by the scientific evidence that is most appropriate or `correct' to use
to characterize where the degree of confidence in the associations
warrants setting the annual standard level. The decision of the
appropriate standard level below the long-term mean concentrations of
the key studies, which in conjunction with the other elements of the
standard would protect public health with an adequate margin of safety,
is largely a public health policy judgment, taking into account all of
the evidence and its related uncertainties.
    As discussed in section III.E.4.b, the Administrator takes note of
additional population-level data that were made available to the EPA by
study authors.\111\ In considering this information, the Administrator
particularly focuses on the analysis of the distributions of the health
event data for each area within these studies and the corresponding air
quality data for the two short-term exposure studies (Zanobetti and
Schwartz, 2009; Bell et al., 2008). These short-term exposure studies
evaluate the relationship between daily changes (one or more days) in
PM2.5 concentrations and daily changes in health events
(e.g., deaths, hospitalizations), such that the air quality
concentrations that comprise the most relevant exposure periods in
these

studies are contemporaneous with the health event data. In addition,
these studies considered more recent air quality data, representing
generally lower PM2.5 concentrations, in a large number of
study areas across the U.S. Thus, such studies provide the most useful
evidence for an analysis evaluating the distribution of health event
data and the corresponding long-term mean PM2.5
concentrations across the areas included in each multi-city study.
---------------------------------------------------------------------------

    \111\ As summarized in section III.E.4.a, population-level data
were provided to the EPA for four studies. These four studies
represent some of the strongest evidence showing associations
between health effects and PM2.5 within the overall body
of scientific evidence and include three studies (Krewski et al.,
2009; Bell et al., 2008; and Zanobetti and Schwartz, 2009) that were
used as the basis for concentration-response functions in the
quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). The
Administrator recognizes that the additional population-level data
available for these four multi-city studies represents a more
limited data set compared to the set of long-term mean
PM2.5 concentrations which were available in the
published literature for all studies considered in the Integrated
Science Assessment.
---------------------------------------------------------------------------

    The Administrator also considered the additional population-level
data that were made available to EPA for two long-term exposure studies
(Krewski et al., 2009; Miller et al., 2007). She recognizes that in
long-term exposure studies investigators follow a specific group of
study participants (i.e., cohort) over time and across urban study
areas, and evaluate how PM2.5 concentrations averaged over a
period of years are associated with specific health endpoints (e.g.,
deaths) across cities. As discussed in response to public comments in
section III.E.4.c, disentangling the effects observed in long-term
exposure studies associated with more recent air quality measurements
from effects that may have been associated with earlier, and most
likely higher, PM2.5 exposures introduces some uncertainty
with regard to understanding the appropriate exposure window associated
with the observed effects. This is in contrast to the short-term
exposure studies where the relevant exposure period is contemporaneous
to the period for which the health data were collected. In light of
these considerations, as noted above, the Administrator considers the
analysis of air quality concentrations that correspond to the
distribution of population-level data in these two studies to be a
highly uncertain basis for informing her selection of the annual
standard level.
    Based on the above considerations, the Administrator views the
additional population-level data for the two short-term exposure
studies as appropriate to help inform her judgment of how much below
the long-term mean concentrations to set the level of the annual
standard. The Administrator notes that the long-term mean
PM2.5 concentrations corresponding with study areas
contributing to the 25th percentiles of the distribution of deaths and
cardiovascular-related hospitalizations in these two short-term
exposure studies were 12.5 [micro]g/m\3\ and 11.5 [micro]g/m\3\,
respectively, for Zanobetti and Schwartz (2009) and for Bell et al.
(2008), with the 10th percentiles being lower by approximately 2
[micro]g/m\3\ in each study.
    The Administrator recognizes, as summarized in section III.B above
and discussed more fully in section III.B.2 of the proposal, that
important uncertainties remain in the evidence and information
considered in this review of the primary fine particle standards. These
uncertainties are generally related to understanding the relative
toxicity of the different components in the fine particle mixture, the
role of PM2.5 in the complex ambient mixture, exposure
measurement errors, and the nature and magnitude of estimated risks
related to increasingly lower ambient PM2.5 concentrations.
Furthermore, the Administrator notes that epidemiological studies have
reported heterogeneity in responses both within and between cities and
geographic regions across the U.S. She recognizes that this
heterogeneity may be attributed, in part, to differences in fine
particle composition in different regions and cities.\112\
---------------------------------------------------------------------------

    \112\ Nonetheless, as explained in section III.E.1, the
currently available evidence is not sufficient to support replacing
or supplementing the PM2.5 indicator with any other
indicator defined in terms of a specific fine particle component or
group of components associated with any source categories of fine
particles. Furthermore, the evidence is not sufficient to support
eliminating any component or group of components associated with any
source categories of fine particles from the mix of fine particles
included in the PM2.5 indicator.
---------------------------------------------------------------------------

    With regard to evidence for reproductive and developmental effects
identified as being suggestive of a causal relationship with long-term
PM2.5 exposures, the Administrator recognizes that there are
a number of limitations associated with this body of evidence
including: the limited number of studies evaluating such effects;
uncertainties related to identifying the relevant exposure time periods
of concern; and limited toxicological evidence providing little
information on the mode of action(s) or biological plausibility for an
association between long-term PM2.5 exposures and adverse
birth outcomes. Nonetheless, the Administrator believes that this more
limited body of evidence provides some support for considering that
serious effects may be occurring in a susceptible population at
concentrations lower than those associated with effects classified as
having a causal or likely causal relationship with long-term
PM2.5 exposures (i.e., mortality, cardiovascular, and
respiratory effects).
    Overall, the Administrator believes that the available evidence
interpreted in light of the remaining uncertainties, as summarized
above and discussed more fully in the Integrated Science Assessment and
the Policy Assessment, provides increased confidence relative to
information available in the last review and provides a strong basis
for informing her final decisions in the current review. The
Administrator is mindful that considering what standards are requisite
to protect public health with an adequate margin of safety requires
public health policy judgments that neither overstate nor understate
the strength and limitations of the evidence or the appropriate
inferences to be drawn from the evidence. In considering how to
translate the available information into appropriate standard levels,
the Administrator weighs the available scientific information and
associated uncertainties and limitations. For the purpose of
determining what annual standard level is appropriate the Administrator
recognizes that there is no single factor or criterion that comprises
the ``correct'' approach to weighing the various types of available
evidence and information.
    In considering this information, the Administrator notes the advice
of CASAC that ``there are significant public health consequences at the
current levels of the standards that justify consideration of lowering
the PM2.5 NAAQS further'' (Samet, 2010c, p. 12). In
addition, she recognizes that CASAC concluded, ``although there is
increasing uncertainty at lower levels, there is no evidence of a
threshold (i.e., a level below which there is no risk for adverse
effects)'' (Samet, 2010d, p.ii) and that the final decisions on
standard levels must reflect a judgment of the available scientific
information with respect to her interpretation of the CAA's requirement
to set primary standards that provide requisite protection to public
health with an adequate margin of safety (Samet, 2010d, p. 4). The
Administrator recognizes CASAC's advice that the currently available
scientific information provided support for considering an annual
standard level within a range of 13 to 11 [mu]g/m\3\ and a 24-hour
standard level within a range of 35 to 30 [mu]g/m\3\. In considering
how the annual and 24-hour standards work together to provide
appropriate public health protection, the Administrator observes that
CASAC did not express support for any specific levels or combinations
of standards within these ranges. She also notes that CASAC encouraged
the EPA staff to consider additional data from epidemiological studies
to help quantify the characterization of the PM2.5
concentrations that were most influential in generating the health

effect estimates in these studies (Samet, 2010d, p. 2).
    In response to CASAC's advice, the Administrator recognizes that
the EPA staff acquired additional data from authors of key
epidemiological studies and analyzed these data to characterize the
distribution of PM2.5 concentrations in relation to health
events data to better understand the degree of confidence in the
associations observed in the studies as discussed above. The
Administrator recognizes that the final Policy Assessment included
consideration of these additional analyses in reaching final staff
conclusions with regard to the broadest range of alternative standard
levels supported by the science. She takes note that the final Policy
Assessment concluded that while alternative standard levels within the
range of 13 to 11 [mu]g/m\3\ were appropriate to consider, the evidence
most strongly supported consideration of an annual standard level in
the range of 12 to 11 [mu]g/m\3\. The Administrator is aware that, in
transmitting the final Policy Assessment to CASAC, the Agency notified
CASAC that the final staff conclusions reflected consideration of
CASAC's advice and that those staff conclusions were based, in part, on
the specific distributional analysis that CASAC had urged the EPA to
conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on
the final Policy Assessment, but chose not to provide any additional
comments or advice after receiving it.
    In selecting the annual standard level, the Administrator has
considered many factors including the nature and severity of the health
effects involved, the strength of the overall body of scientific
evidence as considered in reaching causality determinations, the size
of the at-risk populations, and the estimated public health impacts.
She has also considered the kind and degree of the uncertainties that
remain in the available scientific information. She recognizes that the
association between PM2.5 and serious health effects is well
established, including at concentrations below those allowed by the
current standard. Further, she recognizes the CAA requirement that
requires primary standards to provide an adequate margin of safety was
intended to address uncertainties associated with inconclusive
scientific and technical information as well as to provide a reasonable
degree of protection against hazards that research has not yet
identified. In considering the currently available evidence, as
summarized and discussed more broadly above, the information on risk,
CASAC advice, the conclusions of the Policy Assessment, and public
comments on the proposal, the Administrator strongly believes that a
lower annual standard level is needed to protect public health with an
adequate margin of safety.
    In reaching her final decision on the appropriate annual standard
level to set, the Administrator is mindful that the CAA does not
require that primary standards be set at a zero-risk level, but rather
at a level that reduces risk sufficiently so as to protect public
health, including the health of at-risk populations, with an adequate
margin of safety. On balance, the Administrator concludes that an
annual standard level of 12 [mu]g/m\3\ would be requisite to protect
the public health with an adequate margin of safety from effects
associated with long- and short-term PM2.5 exposures, while
still recognizing that uncertainties remain in the scientific
information.
    In the Administrator's judgment, an annual standard of 12 [mu]g/
m\3\ appropriately reflects placing greatest weight on evidence of
effects for which the Integrated Science Assessment determined there is
a causal or likely causal relationship with long- and short-term
PM2.5 exposures. An annual standard level of 12 [mu]g/m\3\
is below the long-term mean PM2.5 concentrations reported in
each of the key multi-city, long- and short-term exposures studies
providing evidence of an array of serious health effects (e.g.,
premature mortality, increased hospitalization for cardiovascular and
respiratory effects). As noted above, the importance of considering a
level somewhat below the lowest long-term mean concentration in the
full set of studies considered is to set a standard that would provide
appropriate protection against the observed effects in all such
studies.
    In reaching her decision, the Administrator has taken into account
that at and around the mean PM2.5 concentration in any given
study represents a part of the air quality distribution in which the
health event data in that study are generally most concentrated.
Furthermore, in identifying an appropriate annual standard level below
the long-term mean concentrations, she recognizes that there is no
evidence to support the existence of any discernible threshold, and,
therefore, she has a high degree of confidence that the observed
effects are associated with concentrations not just at but extending
somewhat below the long-term mean concentration. To further inform her
judgment in setting the annual standard level so as to protect public
health with an adequate margin of safety, the Administrator has placed
weight on additional population-level information available from a
subset of these epidemiological studies, consistent with CASAC advice.
In particular, she has drawn from two short-term exposure studies,
which provide the most relevant information for evaluating the
distribution of health events and corresponding long-term
PM2.5 concentrations. As explained above, this helps inform
her judgment as to the degree of confidence in the observed
associations in the epidemiological studies. In this regard, the
Administrator generally judges the region around the 25th percentile as
a reasonable part of the distribution to help guide her decision on the
appropriate standard level. Since this evidence comes primarily from
two studies, a relatively modest data set, the Administrator deems it
reasonable not to draw further inferences from air quality and health
event data in the lower part of the distribution for the purpose of
setting a standard level. The Administrator notes that the long-term
mean PM2.5 concentrations around the 25th percentile of the
distributions of deaths and cardiovascular-related hospitalizations
were approximately around 12 [mu]g/m\3\ in these two studies. The
Administrator views this information as helpful in guiding her
determination as to where her confidence in the magnitude and
significance of the associations is reduced to such a degree that a
standard set at a lower level would not be warranted to provide
requisite protection that is neither more nor less than needed to
provide an adequate margin of safety.
    The Administrator also recognizes that a level of 12 [mu]g/m\3\
places some weight on studies which provide evidence of reproductive
and developmental effects (e.g., infant mortality, low birth weight).
These studies were identified in the Integrated Science Assessment as
having evidence suggestive of a causal relationship with long-term
PM2.5 concentrations. A level of 12 [mu]g/m\3\ is
approximately the same level as the lowest long-term mean concentration
reported in such studies (Figures 2 and 4; 11.9 [mu]g/m\3\ for Bell et
al., 2007).\113\ While the Administrator

acknowledges that this evidence is limited, she believes it is
appropriate to place some weight on these studies in order to set a
standard that provides protection with an adequate margin of safety,
including providing protection for at-risk populations, as required by
the CAA. Due to the limited nature of this evidence, she has determined
it is not necessary to set a standard below the lowest long-term mean
concentration in these studies.
---------------------------------------------------------------------------

    \113\ With respect to cancer, mutagenic, and genotoxic effects,
the Administrator observes that the PM2.5 concentrations
reported in studies evaluating these effects generally included
ambient concentrations that are equal to or greater than ambient
concentrations observed in studies that reported mortality and
cardiovascular and respiratory effects (U.S. EPA, 2009a, section
7.5). Therefore, the Administrator concludes that in selecting
alternative standard levels that provide protection from mortality
and cardiovascular and respiratory effects, it is reasonable to
anticipate that protection will also be provided for carcinogenic
effects.
---------------------------------------------------------------------------

    In reflecting on extensive public comments received on the proposal
as discussed in section III.E.4.c above, the Administrator recognizes
that some commenters have offered different evaluations of the evidence
and other information available in this review and would make different
judgments about the weight to place on the relative strengths and
limitations of the scientific information and about how such
information could be used in making public health policy decisions on
the annual standard level. One group of such commenters who supported a
higher annual standard level (e.g., above 13 [mu]g/m\3\) would place
greater weight on the remaining uncertainties in the evidence as a
basis for supporting a higher standard level than the Administrator
judges to be appropriate. Such an approach is based on these
commenters' judgment that the uncertainties remaining in the evidence
are too great to warrant setting an annual standard below the current
level. The Administrator does not agree.
    As an initial matter, an annual standard level of 13 [mu]g/m\3\ or
higher would be above the long-term mean concentrations reported in two
well-conducted, multi-city short-term exposure studies reporting
positive and statistically significant associations of serious effects
(Burnett et al., 2004 and Bell et al., 2008). These important studies
are fully consistent with the pattern of evidence presented by the
large body of evidence in this review. As the Administrator recognized
in the proposal, and as advised by CASAC, the appropriate focus for
selecting the level of the annual PM2.5 standard is on
concentrations somewhat below the lowest long-term mean concentrations
from the set of key studies of both long-term and short-term
PM2.5 exposures considered by the EPA (i.e., as shown in
Figure 4). Thus, a standard level set at 13 [mu]g/m\3\ or higher would
clearly not provide protection for the effects observed in the full set
epidemiological studies and, therefore, this standard level could not
be judged to be requisite with an adequate margin of safety.\114\
---------------------------------------------------------------------------

    \114\ The Administrator is mindful that, in reviewing the 2006
final PM NAAQS decisions, the D.C. Circuit Court of Appeals
concluded that the EPA failed to adequately explain why that annual
standard provided requisite protection from effects associated with
both long- and short-term exposures or from morbidity effects in
children and other at-risk populations when long-term means of
important short-term studies were below the level the Administrator
selected for the annual standard. See American Farm Bureau v. EPA.
559 F. 3d 512, 524-26. There is no reasonable basis to discount
these two studies for purposes of setting the level of the annual
standard.
---------------------------------------------------------------------------

    In addition, as noted above, in recognizing that there is no
evidence to support the existence of a discernible threshold below
which an effect would not occur, the Administrator is mindful that
effects occur around and below the long-term mean concentrations
reported in both the short-term and long-term the epidemiological
studies. A standard level of 13 [mu]g/m\3\ or higher would not
appropriately take into account evidence from the two well-conducted,
multi-city, short-term exposure studies reporting serious effects with
long-term mean concentrations below 13 [mu]g/m\3\ noted above (Burnett
et al, 2004; Bell et al., 2008). Such a standard level would also not
appropriately take into account additional population-level data from a
limited number of epidemiological studies. This approach would ignore
CASAC's advice to consider such information in order to better
understand the concentrations over which there is a high degree of
confidence regarding the magnitude and significance of the associations
observed in individual epidemiological studies and where there is
appreciably less confidence.
    Furthermore, a standard level of 13 [mu]g/m\3\ or higher would not
appropriately take into account the more limited evidence of effects in
some at-risk populations (e.g., low birth weight). In the
Administrator's view, a standard set at this level would not provide
protection with an adequate margin of safety, including providing
protection for at-risk populations. The Administrator is mindful that
the CAA requirement that primary standards provide an adequate margin
of safety, discussed in section II.A above, was intended to address
uncertainties associated with inconclusive scientific and technical
information available at the time of standard setting as well as to
provide a reasonable degree of protection against hazards that research
has not yet identified.
    In light of the entire body of evidence as discussed above, the
Administrator judges that an annual standard level set above 12 [mu]g/
m\3\ would not be sufficient to protect public health with an adequate
margin of safety from the serious health effects associated with long-
and short-term exposure to PM2.5.
    The Administrator also recognizes that a second group of commenters
supported a lower annual standard level (e.g., no higher than 11 [mu]g/
m\3\). Such a standard level would reflect placing essentially as much
weight on the relatively more limited data providing evidence
suggestive of a causal relationship for effects observed in some at-
risk populations (e.g., low birth weight) as on more certain evidence
of effects classified as having a causal or likely causal relationship
with PM2.5 exposures. In the Administrator's view, while it
is important to place some weight on such suggestive evidence, it would
not be appropriate to place as much weight on it as the commenters
would do.
    An annual standard level of 11 [mu]g/m\3\ would also reflect these
commenters' judgment that it is appropriate to focus on a lower part of
the distributions of health event data from the small number of
epidemiological studies for which this information was made available
than the Administrator believes is warranted. In the Administrator's
view, using this type of information to set a standard level of 11
[mu]g/m\3\ or below would assume too high a degree of confidence in the
magnitude and significance of the associations observed in the lower
part of the distributions of health events observed in these studies.
Given the uncertainties in the evidence and the limited set of studies
for which the EPA has information on the distribution of health event
data and corresponding air quality data, the Administrator believes it
is not appropriate to focus on the lower part of the distributions of
health events data.
    On balance, the Administrator finds that the available evidence
interpreted in light of the remaining uncertainties does not justify a
standard level set below 12 [mu]g/m\3\ as necessary to protect public
health with an adequate margin of safety.
    After carefully considering the above considerations and the public
comments summarized in section III.E.4.c above, the Administrator has
decided to set the level of the primary annual PM2.5
standard at 12 [mu]g/m\3\. In her judgment, a standard set at this
level provides the requisite degree of public health protection,
including the health of at-risk populations, with an adequate margin of
safety and is neither more nor less stringent than necessary for this
purpose.

    As discussed above, the Administrator concludes that an approach
that focuses on setting a generally controlling annual standard is the
most effective and efficient way to reduce total population risk
associated with both long- and short-term PM2.5 exposures.
Such an approach would result in more uniform protection across the
U.S. than the alternative of setting the levels of the 24-hour and
annual standard such that the 24-hour standard would generally be the
controlling standard in areas across the country (see section III.A.3).
    The Administrator recognizes that potential air quality changes
associated with meeting an annual standard level of 12.[mu]g/m\3\ will
result in lowering risks associated with both long- and short-term
PM2.5 exposures by lowering the overall air quality
distribution. However, the Administrator recognizes that such an annual
standard alone would not be expected to offer sufficient protection
with an adequate margin of safety against the effects of short-term
PM2.5 exposures in all parts of the country. As a result, in
conjunction with an annual standard level of 12 [mu]g/m\3\, the
Administrator concludes that it is appropriate to continue to provide
supplemental protection by means of a 24-hour standard set at the
appropriate level, particularly for areas with high peak-to-mean ratios
possibly associated with strong local or seasonal sources and for areas
with PM2.5-related effects that may be associated with
shorter-than-daily exposure periods.
    In selecting the level of a 24-hour standard meant to provide such
supplemental protection, the Administrator relies upon evidence and air
quality information from key short-term exposure studies. In
considering these studies, the Administrator notes that to the extent
air quality distributions in the study areas considered are reduced to
meet the current 24-hour standard (at a level of 35 [mu]g/m\3\) or to
meet the revised annual standard discussed above (at a level of 12
[mu]g/m\3\), additional protection would be anticipated against the
effects observed in these studies. In light of this, when selecting the
appropriate level for the 24-hour standard, the Administrator considers
both the 98th percentiles of 24-hour PM2.5 concentrations
and the long-term mean PM2.5 concentrations in the locations
of the short-term exposure studies. She notes that such consideration
of both short- and long-term PM2.5 concentrations can inform
her decision on the extent to which a given 24-hour standard, in
combination with the revised annual standard established in this rule,
would provide protection against the health effects reported in short-
term studies.
    As discussed in section III.E.4.a above, the Administrator
concludes that multi-city short-term exposure studies provide the
strongest data set for informing her decisions on appropriate 24-hour
standard levels. With regard to the limited number of single-city
studies that reported positive and statistically significant
associations for a range of health endpoints related to short-term
PM2.5 concentrations in areas that would likely have met the
current suite of PM2.5 standards, the Administrator
recognizes that many of these studies had significant limitations
(e.g., limited statistical power, limited exposure data) or equivocal
results (mixed results within the same study area) that make them
unsuitable to form the basis for setting the level of a 24-hour
standard.
    With regard to multi-city studies that evaluated effects associated
with short-term PM2.5 exposures, the Administrator observes
an overall pattern of positive and statistically significant
associations in studies with 98th percentile 24-hour values averaged
across study areas within the range of 45.8 to 34.2 [mu]g/m\3\ (Burnett
et al., 2004; Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici
et al., 2006a, Burnett and Goldberg, 2003; Franklin et al., 2008). The
Administrator notes that, to the extent air quality distributions are
reduced to reflect just meeting the current 24-hour standard,
additional protection would be provided for the effects observed in the
three multi-city studies with 98th percentile values greater than 35
[mu]g/m\3\ (Burnett et al., 2004; Burnett and Goldberg, 2003; Franklin
et al., 2008). In the three additional multi-city studies with 98th
percentile values below 35 [mu]g/m\3\, specifically 98th percentile
concentrations of 34.2, 34.3, and 34.8 [mu]g/m\3\, the Administrator
notes that these studies reported long-term mean PM2.5
concentrations of 12.9, 13.2, and 13.4 [mu]g/m\3\, respectively (Bell
et al., 2008; Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In
revising the level of the annual standard to 12 [mu]g/m\3\, as
discussed above, the Administrator recognizes that additional
protection would be provided for the short-term effects observed in
these multi-city studies such that revision to the 24-hour standard
would not be warranted. That is, by lowering the level of the annual
standard to 12 [mu]g/m\3\, the 98th percentile of the distribution
would be lowered as well such that additional protection from effects
associated with short-term exposures would be afforded. Therefore, the
epidemiological evidence supports a conclusion that it is appropriate
to retain the level of the 24-hour standard at 35 [mu]g/m\3\, in
conjunction with a revised annual standard level of 12 [mu]g/m\3\.
    In addition to considering the epidemiological evidence, the
Administrator also has taken into account air quality information based
on county-level 24-hour and annual design values to understand the
implications of revising the annual standard level from 15 to 12 [mu]g/
m\3\ in conjunction with retaining the 24-hour standard level at 35
[mu]g/m\3\. She has considered this information to evaluate the public
health protection provided by the two standards in combination and to
evaluate the most appropriate means of developing a suite of standards
providing requisite public health protection with an adequate margin of
safety.
    In considering the air quality information, the Administrator
observes that a suite of PM2.5 standards that includes an
annual standard level of 12 [mu]g/m\3\ and a 24-hour standard level of
35 [mu]g/m\3\ would result in the annual standard as the generally
controlling standard in most regions across the country, except for
certain areas in the Northwest, where the annual mean PM2.5
concentrations have historically been low but where relatively high 24-
hour concentrations occur, often related to seasonal wood smoke
emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact,
these are the type of areas for which the supplemental protection
afforded by the 24-hour standard is intended, such that the two
standards together provide the requisite degree of protection. The
Administrator concludes the current 24-hour standard at a level of 35
[mu]g/m\3\, in conjunction with a revised annual standard level of 12
[mu]g/m\3\, will provide appropriate protection from effects observed
in studies in such areas in which the long-term mean concentrations
were below 12 [mu]g/m\3\ and the 98th percentile 24-hour concentrations
were above 35 [mu]g/m\3\ (e.g., areas in the Northwest U.S.).
    After carefully taking the public comments and above considerations
into account, the Administrator has decided to retain the current level
of the primary PM2.5 24-hour standard at 35 [mu]g/m\3\ in
conjunction with revising the annual standard level from 15.0 [mu]g/
m\3\ to 12.0 [mu]g/m\3\.\115\ In the Administrator's

judgment, this suite of primary PM2.5 standards and the
rationale supporting these levels appropriately reflects consideration
of the strength of the available evidence and other information and its
associated uncertainties as well as the advice of CASAC and
consideration of public comments. In the Administrator's judgment, this
suite of primary PM2.5 standards is sufficient but not more
protective than necessary to protect the public health, including at-
risk populations, with an adequate margin of safety from effects
associated with long- and short-term exposures to fine particles. This
suite of standards will provide significant protection from serious
health effects including premature mortality and cardiovascular and
respiratory morbidity effects that are causally or likely causally
related to long- and short-term PM2.5 exposures. These
standards will also provide an appropriate degree of protection against
other health effects for which there is more limited evidence of
effects and causality, such as reproductive and developmental effects.
This judgment by the Administrator appropriately considers the
requirement for a standard that is requisite to protect public health
but is neither more nor less stringent than necessary.\116\
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    \115\ As noted in section II.B.1, Table 1 and section III.E.4.a
above, the annual standard level is defined to one decimal place.
Throughout this section, the annual standard levels discussed have
been denoted as integer values (e.g., 12 [mu]g/m\3\) for simplicity.
    \116\ The Administrator also judges that this suite of standards
addresses the issues raised by the D.C. Circuit's remand of the 2006
primary annual PM2.5 standard by appropriately revising
that standard.
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D. Administrator's Final Decisions on Primary PM2.5 Standards

    For the reasons discussed above, and taking into account the
information and assessments presented in the Integrated Science
Assessment, Risk Assessment, and Policy Assessment, the advice and
recommendations of CASAC, and public comments to date, the
Administrator revises the current suite of primary PM2.5
standards. Specifically, the Administrator revises: (1) The level of
the primary annual PM2.5 standard to 12.0 [mu]g/m\3\ and (2)
the form of the primary annual PM2.5 standard to one based
on the highest appropriate area-wide monitor in an area, with no option
for spatial averaging. In conjunction with revising the primary annual
PM2.5 standard to provide protection from effects associated
with long- and short-term PM2.5 exposures, the Administrator
retains the level of 35 [mu]g/m\3\ and the 98th percentile form of the
primary 24-hour PM2.5 standard to continue to provide
supplemental protection for areas with high peak PM2.5
concentrations. The Administrator is not revising the current
PM2.5 indicator or the annual and 24-hour averaging times
for the primary PM2.5 standards. The Administrator concludes
that this suite of standards would be requisite to protect public
health with an adequate margin of safety against health effects
potentially associated with long- and short-term PM2.5
exposures.

IV. Rationale for Final Decision on Primary PM10 Standard

    This section presents the rationale for the Administrator's final
decision to retain the current 24-hour primary PM10 standard
in order to continue to provide public health protection against short-
term exposures to inhalable particles in the size range of 2.5 to 10
[mu]m (i.e., PM10-2.5 or thoracic coarse particles). These
are particles capable of reaching the most sensitive areas of the lung,
including the trachea, bronchi, and deep lungs. The current standard
uses PM10 as the indicator for thoracic coarse particles,
and thus is referred to as a PM10 standard.\117\
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    \117\ Throughout this section of the preamble, we are using the
terms ``thoracic coarse particles'', ``inhalable coarse particles'',
and ``PM10-2.5'' synonymously.
---------------------------------------------------------------------------

    As discussed more fully in the proposal and below, this rationale
is based on a thorough review of the latest scientific evidence,
published through mid-2009 and assessed in the Integrated Science
Assessment (U.S. EPA, 2009a), evaluating human health effects
associated with long- and short-term exposures to thoracic coarse
particles. The Administrator's final decision also takes into account:
(1) The EPA staff analyses of air quality information and health
evidence and staff conclusions regarding the current and potential
alternative standards, as presented in the Policy Assessment for the PM
NAAQS (U.S. EPA, 2011a); (2) CASAC advice and recommendations, as
reflected in discussions at public meetings of drafts of the Integrated
Science Assessment and Policy Assessment, and in CASAC's letters to the
Administrator; (3) the multiple rounds of public comments received
during the development of the Integrated Science Assessment and Policy
Assessment, both in connection with CASAC meetings and separately; and
(4) public comments (including testimony at the public hearings)
received on the proposal.
    In presenting the rationale for the final decision to retain the
current primary PM10 standard, this section discusses the
EPA's past reviews of the PM NAAQS and the general approach taken to
review the current standard (section IV.A), the health effects
associated with exposures to ambient PM10-2.5 (section
IV.B), the consideration of the current and potential alternative
standards in the Policy Assessment (section IV.C), CASAC
recommendations regarding the current and potential alternative
standards (section IV.D), the Administrator's proposed decision to
retain the current primary PM10 standard (section IV.E),
public comments received in response to the Administrator's proposed
decision (section IV.F), and the Administrator's final decision to
retain the current primary PM10 standard (section IV.G).

A. Background

    The following sections discuss previous reviews of the PM NAAQS
(section IV.A.1), the litigation of the EPA's 2006 decision on the
PM10 standards (section IV.A.2), and the general approach
taken to review the primary PM10 standard in the current
review (section IV.A.3).
1. Previous Reviews of the PM NAAQS
a. Reviews Completed in 1987 and 1997
    The PM NAAQS have always included some type of a primary standard
to protect against effects associated with exposures to thoracic coarse
particles. In 1987, when the EPA first revised the PM NAAQS, the EPA
changed the indicator for PM from TSP to focus on inhalable particles,
those which can penetrate into the trachea, bronchi, and deep lungs (52
FR 24634, July 1, 1987). In that review, the EPA changed the PM
indicator to PM10 based on evidence that the risk of adverse
health effects associated with particles with a nominal mean
aerodynamic diameter less than or equal to 10 [mu]m was significantly
greater than risks associated with larger particles (52 FR 24639, July
1, 1987).
    In the 1997 review, in conjunction with establishing new fine
particle (i.e., PM2.5) standards (discussed above in
sections II.B.1 and III.A.1), the EPA concluded that continued
protection was warranted against potential effects associated with
thoracic coarse particles in the size range of 2.5 to 10 [mu]m. This
conclusion was based on particle dosimetry, toxicological information,
and on limited epidemiological evidence from studies that measured
PM10 in areas where the coarse fraction was likely to
dominate PM10 mass (62 FR 38677, July 18, 1997). The EPA
concluded there that a PM10 standard could provide requisite
protection against effects associated with particles

in the size range of 2.5 to 10 [mu]m.\118\ Although the EPA considered
a more narrowly defined indicator for thoracic coarse particles in that
review (i.e., PM10-2.5), the EPA concluded that it was more
appropriate, based on existing evidence, to continue to use
PM10 as the indicator. This decision was based, in part, on
the recognition that the only studies of clear quantitative relevance
to health effects most likely associated with thoracic coarse particles
used PM10. These were two studies conducted in areas where
the coarse fraction was the dominant fraction of PM10, and
which substantially exceeded the 24-hour PM10 standard (62
FR 38679). In addition, there were only very limited ambient air
quality data then available specifically for PM10-2.5, in
contrast to the extensive monitoring network already in place for
PM10. Therefore, the EPA considered it more administratively
feasible to use PM10 as an indicator. The EPA also stated
that the PM10 standards would work in conjunction with the
PM2.5 standards by regulating the portion of particulate
pollution not regulated by the then newly adopted PM2.5
standards.
---------------------------------------------------------------------------

    \118\ With regard to the 24-hour PM10 standard, the
EPA retained the indicator, averaging time, and level (150 [mu]g/
m\3\), but revised the form (i.e., from one-expected-exceedance to
the 99th percentile).
---------------------------------------------------------------------------

    In May 1998, a three-judge panel of the U.S. Court of Appeals for
the District of Columbia Circuit found ``ample support'' for the EPA's
decision to regulate coarse particle pollution, but vacated the 1997
PM10 standards, concluding that the EPA had failed to
adequately explain its choice of PM10 as the indicator for
thoracic coarse particles American Trucking Associations v. EPA, 175 F.
3d 1027, 1054-56 (D.C. Cir. 1999). In particular, the court held that
the EPA had not explained the use of an indicator under which the
allowable level of coarse particles varied according to the amount of
PM2.5 present, and which, moreover, potentially double
regulated PM2.5. The court also rejected considerations of
administrative feasibility as justification for use of PM10
as the indicator for thoracic coarse PM, since NAAQS (and their
elements) are to be based exclusively on health and welfare
considerations. Id. at 1054. Pursuant to the court's decision, the EPA
removed the vacated 1997 PM10 standards from the CFR (69 FR
45592, July 30, 2004) and deleted the regulatory provision (at 40 CFR
50.6(d)) that controlled the transition from the pre-existing 1987
PM10 standards to the 1997 PM10 standards (65 FR
80776, December 22, 2000). The pre-existing 1987 PM10
standards thus remained in place. Id. at 80777.
b. Review Completed in 2006
    In the review of the PM NAAQS that concluded in 2006, the EPA
considered the growing, but still limited, body of evidence supporting
associations between health effects and thoracic coarse particles
measured as PM10-2.5.\119\ The new studies available in the
2006 review included epidemiological studies that reported associations
with health effects using direct measurements of PM10-2.5,
as well as dosimetric and toxicological studies. In considering this
growing body of PM10-2.5 evidence, as well as evidence from
studies that measured PM10 in locations where the majority
of PM10 was in the PM10-2.5 fraction (U.S. EPA,
2005, section 5.4.1), staff concluded that the level of protection
afforded by the existing 1987 PM10 standard remained
appropriate (U.S. EPA, 2005, p. 5-67) but recommended that the
indicator for the standard be revised. Specifically, staff recommended
replacing the PM10 indicator with an indicator of urban
thoracic coarse particles in the size range of 10-2.5 [mu]m (U.S. EPA,
2005, pp. 5-70 to 5-71). The agency proposed to retain a standard for a
subset of thoracic coarse particles, proposing a qualified
PM10-2.5 indicator to focus on the mix of thoracic coarse
particles generally present in urban environments. More specifically,
the proposed revised thoracic coarse particle standard would have
applied only to an ambient mix of PM10-2.5 dominated by
resuspended dust from high-density traffic on paved roads and/or by
industrial and construction sources. The proposed revised standard
would not have applied to any ambient mix of PM10-2.5
dominated by rural windblown dust and soils. In addition, agricultural
sources, mining sources, and other similar sources of crustal material
would not have been subject to control in meeting the standard (71 FR
2667 to 2668, January 17, 2006).
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    \119\ The PM Staff Paper (U.S. EPA, 2005) also presented results
of a quantitative assessment of health risks for
PM10-2.5. However, staff concluded that the nature and
magnitude of the uncertainties and concerns associated with this
risk assessment weighed against its use as a basis for recommending
specific levels for a thoracic coarse particle standard (U.S. EPA,
2005, p. 5-69).
---------------------------------------------------------------------------

    The Agency received a large number of comments overwhelmingly and
persuasively opposed to the proposed qualified PM10-2.5
indicator (71 FR 61188 to 61197, October 17, 2006). After careful
consideration of the scientific evidence and the recommendations
contained in the 2005 Staff Paper, the advice and recommendations from
CASAC, and the public comments received regarding the appropriate
indicator for coarse particles, and after extensive evaluation of the
alternatives available to the Agency, the Administrator decided it
would not be appropriate to adopt the proposed qualified
PM10-2.5 indicator, or any qualified indicator. Underlying
this determination was the Administrator's decision that it was
requisite to provide protection from exposure to all thoracic coarse
PM, regardless of its origin. The Administrator thus rejected arguments
that there are no health effects from community-level exposures to
coarse PM in non-urban areas (71 FR 61189). The EPA concluded that
dosimetric, toxicological, occupational and epidemiological evidence
supported retention of a primary standard for short-term exposures that
included all thoracic coarse particles (i.e., particles of both urban
and non-urban origin), consistent with the Act's requirement that
primary NAAQS must be requisite to protect the public health and
provide an adequate margin of safety. At the same time, the Agency
concluded that the standard should target protection toward urban
areas, where the evidence of health effects from exposure to
PM10-2.5 was strongest (71 FR at 61193, 61197). The proposed
indicator was not suitable for that purpose. Not only did it
inappropriately provide no protection at all to many areas, but it
failed to identify many areas where the ambient particle mix was
dominated by coarse particles contaminated with urban/industrial types
of coarse particles for which evidence of health effects was strongest
(71 FR 61193).
    The Agency ultimately concluded that the existing indicator,
PM10, was most consistent with the evidence. Although
PM10 includes both coarse and fine PM, the Agency concluded
that it remained an appropriate indicator for thoracic coarse particles
because, as discussed in the PM Staff Paper (U.S. EPA, 2005, p. 2-54,
Figures 2-23 and 2-24), fine particle levels are generally higher in
urban areas and, therefore, a PM10 standard set at a single
unvarying level will generally result in lower allowable concentrations
of thoracic coarse particles in urban areas than in non-urban areas (71
FR 61195-96). The EPA considered this to be an appropriate targeting of
protection given that the strongest evidence for effects associated
with thoracic coarse particles came from epidemiological studies
conducted in urban areas and that elevated fine particle concentrations
in urban areas could result in increased contamination of coarse
fraction particles by PM2.5,

potentially increasing the toxicity of thoracic coarse particles in
urban areas (id.). Given the evidence that the existing (i.e., 1987)
PM10 standard was established at a level and form which
afforded requisite protection with an adequate margin of safety, the
Agency retained the level and form of the 24-hour PM10
standard.\120\
---------------------------------------------------------------------------

    \120\ Thus, the standard is met when a 24-hour average
PM10 concentration of 150 [mu]g/m\3\ is not exceeded more
than one day per year, on average over a three-year period. As noted
above, the 1987 PM10 standard was not adopted solely to
control thoracic coarse particles. However, when reviewing this
standard in the 2006 review, EPA determined that the level and form
of the standard being reviewed (i.e., the 1987 PM10
standard) provided requisite protection with an adequate margin of
safety from short-term exposures to thoracic coarse particles.
---------------------------------------------------------------------------

    The Agency also revoked the annual PM10 standard, in
light of the conclusion in the PM Criteria Document (U.S. EPA, 2004, p.
9-79) that the available evidence does not suggest an association with
long-term exposure to PM10-2.5 and the conclusion in the
Staff Paper (U.S. EPA, 2005, p. 5-61) that there is no quantitative
evidence that directly supports retention of an annual standard. This
decision was consistent with CASAC advice and recommendations
(Henderson, 2005a,b).
    In the same rulemaking, the EPA also included a new FRM for the
measurement of PM10-2.5 in the ambient air (71 FR 61212 to
61213, October 17, 2006). Although the standard for thoracic coarse
particles does not use a PM10-2.5 indicator, the new FRM for
PM10-2.5 was established to provide a basis for approving
FEMs and to promote the gathering of scientific data to support future
reviews of the PM NAAQS (71 FR 61202/3, October 17, 2006).\121\
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    \121\ As noted below, however, with this rule the EPA is
revoking the requirement for PM10-2.5 speciation at NCore
monitoring sites due to technical issues related to the development
of appropriate monitoring methods (section VIII.B.3.c). The
requirement for PM10-2.5 mass measurements at NCore sites
is being retained.
---------------------------------------------------------------------------

2. Litigation Related to the 2006 Primary PM10 Standards
    A number of groups filed suit in response to the final decisions
made in the 2006 review. See American Farm Bureau Federation v. EPA,
559 F. 3d 512 (D.C. Cir. 2009). Among the petitions for review were
challenges from industry groups on the decision to retain the
PM10 indicator and the level of the PM10 standard
and from environmental and public health groups on the decision to
revoke the annual PM10 standard. The court upheld both the
decision to retain the 24-hour PM10 standard and the
decision to revoke the annual standard.
    First, the court upheld the EPA's decision for a standard to
encompass all thoracic coarse PM, both of urban and non-urban origin.
The court rejected arguments that the evidence showed there are no
risks from exposure to non-urban coarse PM. The court further found
that the EPA had a reasonable basis not to set separate standards for
urban and non-urban coarse PM, namely the inability to reasonably
define what ambient mixes would be included under either `urban' or
`non-urban;' and the evidence in the record that supported the EPA's
appropriately cautious decision to provide ``some protection from
exposure to thoracic coarse particles * * * in all areas.'' 559 F. 3d
at 532-33. Specifically, the court stated,

    Although the evidence of danger from coarse PM is, as EPA
recognizes, ``inconclusive,'' (71 FR 61193, October 17, 2006), the
agency need not wait for conclusive findings before regulating a
pollutant it reasonably believes may pose a significant risk to
public health. The evidence in the record supports the EPA's
cautious decision that ``some protection from exposure to thoracic
coarse particles is warranted in all areas.'' Id. As the court has
consistently reaffirmed, the CAA permits the Administrator to ``err
on the side of caution'' in setting NAAQS. 559 F. 3d at 533.

    The court also upheld the EPA's decision to retain the level of the
standard at 150 [mu]g/m\3\ and to use PM10 as the indicator
for thoracic coarse particles. In upholding the level of the standard,
the court referred to the conclusion in the Staff Paper that there is
``little basis for concluding that the degree of protection afforded by
the current PM10 standards in urban areas is greater than
warranted, since potential mortality effects have been associated with
air quality levels not allowed by the current 24-hour standard, but
have not been associated with air quality levels that would generally
meet that standard, and morbidity effects have been associated with air
quality levels that exceeded the current 24-hour standard only a few
times.'' 559 F. 3d at 534. The court also rejected arguments that a
PM10 standard established at an unvarying level will result
in arbitrarily varying levels of protection given that the level of
coarse PM would vary based on the amount of fine PM present. The court
agreed that the variation in allowable coarse PM was in accord with the
strength of the evidence: Typically less coarse PM would be allowed in
urban areas (where levels of fine PM are typically higher), in accord
with the strongest evidence of health effects from coarse particles.
559 F. 3d at 535-36. In addition, such regulation would not
impermissibly double regulate fine particles, since any additional
control of fine particles (beyond that afforded by the primary
PM2.5 standard) would be for a different purpose: To prevent
contamination of coarse particles by fine particles. 559 F. 3d at 535,
536. These same explanations justified the choice of PM10 as
an indicator and provided the reasoned explanation for that choice
lacking in the record for the 1997 standard. 559 F. 3d at 536.
    With regard to the challenge from environmental and public health
groups, the court upheld the EPA's decision to revoke the annual
PM10 standard. The court rejected the argument that the EPA
is required by law to have an annual PM10 standard, holding
that section 109(d)(1) of the Act allows the EPA to revoke a standard
no longer warranted by the current scientific understanding. 559 F. 3d
at 538. The court further held that the EPA's decision to revoke the
annual standard was supported by the science:

    The EPA reasonably decided that an annual coarse PM standard is
not necessary because, as the Criteria Document and the Staff Paper
make clear, the latest scientific data do not indicate that long-
term exposure to coarse particles poses a health risk. The CASAC
also agreed that an annual coarse PM standard is unnecessary. 559 F.
3d at 538-39.
3. General Approach Used in the Current Review
    The approach taken to considering the existing and potential
alternative primary PM10 standards in the current review
builds upon the approaches used in previous PM NAAQS reviews. This
approach is based most fundamentally on using information from
epidemiological studies and air quality analyses to inform the
identification of a range of policy options for consideration by the
Administrator. The Administrator considers the appropriateness of the
current and potential alternative standards, taking into account the
four elements of the NAAQS: Indicator, averaging time, form, and level.
    Evidence-based approaches to using information from epidemiological
studies to inform decisions on PM standards are complicated by the
recognition that no population threshold, below which it can be
concluded with confidence that PM-related effects do not occur, can be
discerned from the available evidence (U.S. EPA, 2009a, sections 2.4.3
and <IP_ADDRESS>).\122\ As a result, any approach to

reaching decisions on what standards are appropriate requires judgments
about how to translate the information available from the
epidemiological studies into a basis for appropriate standards, which
includes consideration of how to weigh the uncertainties in reported
associations across the distributions of PM concentrations in the
studies. The approach taken to informing these decisions in the current
review recognizes that the available health effects evidence reflects a
continuum consisting of ambient 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. 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.
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    \122\ Studies that have characterized the concentration-response
relationships for PM exposures have evaluated PM10, which
includes both coarse and fine particles, and PM2.5 (U.S.
EPA, 2009a, sections 2.4.3 and <IP_ADDRESS>).
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    Because the purpose of the PM10 standard is to protect
against exposures to PM10-2.5, it is most appropriate to
focus on PM10-2.5 health studies when considering the degree
of public health protection provided by the current PM10
standard. Compared to health studies of PM10, studies that
evaluate associations with PM10-2.5 provide clearer evidence
for health effects following exposures to thoracic coarse particles. In
contrast, it is difficult to interpret PM10 studies within
the context of a standard meant to protect against exposures to
PM10-2.5 because PM10 is comprised of both fine
and coarse particles, even in locations with the highest concentrations
of PM10-2.5 (U.S. EPA, 2011a, Figure 3-4). Therefore, the
extent to which PM10 effect estimates reflect associations
with PM10-2.5 versus PM2.5 can be highly
uncertain. In light of this uncertainty, it is preferable to consider
PM10-2.5 studies when such studies are available. Given the
availability in this review of a number of studies that evaluated
associations with PM10-2.5, and given that the Integrated
Science Assessment weight-of-evidence conclusions for thoracic coarse
particles were based on studies of PM10-2.5, in this review
the EPA focuses primarily on studies that have specifically evaluated
PM10-2.5.\123\
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    \123\ It should also be noted that CASAC endorsed the approach
adopted in the Integrated Science Assessment, which draws weight-of-
evidence conclusions for PM2.5 and PM10-2.5,
but not for PM10 (Samet, 2009f).
---------------------------------------------------------------------------

    As discussed in more detail in the Risk Assessment (U.S. EPA,
2010a, Appendix H), the EPA did not conduct a quantitative assessment
of health risks associated with PM10-2.5. The Risk
Assessment concluded that limitations in the monitoring network and in
the health studies that rely on that monitoring network, which would be
the basis for estimating PM10-2.5 health risks, would
introduce significant uncertainty into a PM10-2.5 risk
assessment such that the risk estimates generated would be of limited
value in informing review of the standard. Therefore, it was judged
that a quantitative assessment of PM10-2.5 risks is not
supportable at this time (U.S. EPA, 2010a, p. 2-6). This decision does
not indicate that health effects are not associated with exposure to
thoracic coarse particles. Rather, as noted above, it reflects the
conclusion that limitations in the available health studies and air
quality information would introduce significant uncertainty into a
quantitative assessment of PM10-2.5 risks such that the risk
estimates generated would be of limited value in informing review of
the standard.

B. Health Effects Related to Exposure to Thoracic Coarse Particles

    This section briefly outlines the key information presented in
section IV.B of the proposal (77 FR 38947 to 38951, June 29, 2012), and
discussed more fully in the Integrated Science Assessment (U.S. EPA,
2009a, Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment (U.S.
EPA, 2011a, Chapter 3), related to health effects associated with
thoracic coarse particle exposures. In looking across the new
scientific evidence available in this review, our overall understanding
of health effects associated with thoracic coarse particle exposures
has been expanded, though important uncertainties remain. Some
highlights of the key policy-relevant scientific evidence available in
this review include the following:

    (1) A number of multi-city and single-city epidemiological
studies have evaluated associations between short-term
PM10-2.5 and mortality, cardiovascular effects (e.g.,
including hospital admissions and emergency department visits), and/
or respiratory effects. Despite differences in the approaches used
to estimate ambient PM10-2.5 concentrations, the majority
of these studies have reported positive, though often not
statistically significant, associations with short-term
PM10-2.5 concentrations. Most PM10-2.5 effect
estimates remained positive in co-pollutant models that included
either gaseous or particulate co-pollutants. In U.S. study locations
likely to have met the current PM10 standard during the
study period, a few PM10-2.5 effect estimates were
statistically significant and remained so in co-pollutant
models.\124\
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    \124\ The statistical significance of effect estimates provides
important information on their statistical precision. However, when
a group of studies report effect estimates that are similar in
direction and magnitude, such a pattern of results warrants
consideration of those studies even if not all reported
statistically significant associations in single- or co-pollutant
models (section III.D.2, above). In considering the
PM10-2.5 epidemiologic studies below, the Administrator
considers both the pattern of results across studies and the
statistical significance of those results.
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    (2) A small number of controlled human exposure studies have
reported alterations in heart rate variability or increased
pulmonary inflammation following short-term exposure to
PM10-2.5, providing some support for the associations
reported in epidemiological studies. Toxicological studies that have
examined the effects of PM10-2.5 have used intratracheal
instillation and, because these studies do not directly mirror any
real-world mode of exposure, they provide only limited evidence for
the biological plausibility of PM10-2.5-induced effects.
    (3) Using a more formal framework for reaching causal
determinations than used in previous reviews, the Integrated Science
Assessment concluded that the existing evidence is ``suggestive'' of
a causal relationship between short-term PM10-2.5
exposures and mortality, cardiovascular effects, and respiratory
effects (U.S. EPA, 2009a, section 2.3.3).\125\ In contrast, the
Integrated Science Assessment concluded that available evidence is
``inadequate'' to infer a causal relationship between long-term
PM10-2.5 exposures and various health effects.
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    \125\ The causal framework draws upon the assessment and
integration of evidence from across epidemiological, controlled
human exposure, and toxicological studies, and the related
uncertainties that ultimately influence our understanding of the
evidence. This framework employs a five-level hierarchy that
classifies the overall weight-of-evidence using the following
categorizations: Causal relationship, likely to be causal
relationship, suggestive of a causal relationship, inadequate to
infer a causal relationship, and not likely to be a causal
relationship (U.S. EPA, 2009a, Table 1-3). In the case of a
``suggestive'' determination, ``the evidence is suggestive of a
causal relationship with relevant pollutant exposures, but is
limited because chance, bias and confounding cannot be ruled out.
For example, at least one high-quality epidemiologic study shows an
association with a given health outcome but the results of other
studies are inconsistent'' (U.S. EPA, 2009a, Table 1-3).
---------------------------------------------------------------------------

    (4) There are several at-risk populations that may be especially
susceptible or vulnerable to PM-related effects, including effects
associated with exposures to coarse particles. These groups include
those with preexisting heart and lung diseases, specific genetic
differences, and lower socioeconomic status as well as the
lifestages of childhood and older adulthood. Evidence for PM-related
effects in these at-risk populations has expanded and is stronger
than previously observed. There is emerging, though still limited,
evidence for additional potentially at-risk populations, such as
those with diabetes, people who are obese, pregnant women, and the
developing fetus.
    (5) The Integrated Science Assessment concludes that currently
available evidence is insufficient to draw distinctions in particle
toxicity based on composition and notes that recent studies have
reported that PM (both PM2.5 and PM10-2.5)
from a variety of sources,

including sources likely to be present in urban and non-urban
locations, is associated with adverse health effects.

    Although new PM10-2.5 scientific studies have become
available since the last review and have expanded our understanding of
the association between PM10-2.5 and adverse health effects
(see above and U.S. EPA, 2009a, Chapter 6), important uncertainties
remain. These uncertainties, and their implications for interpreting
the scientific evidence, include the following:

    (1) The potential for confounding by co-occurring pollutants,
especially PM2.5, has been addressed with co-pollutant
models in only a relatively small number of PM10-2.5
epidemiological studies (U.S. EPA, 2009a, section 2.3.3). This is a
particularly important limitation given the relatively small body of
experimental evidence (i.e., controlled human exposure and animal
toxicological studies) available to support the associations between
PM10-2.5 and adverse health effects. The net impact of
such limitations is to increase uncertainty in characterizations of
the extent to which PM10-2.5 itself, rather than one or
more co-occurring pollutants, is responsible for the mortality and
morbidity effects reported in epidemiological studies.
    (2) There is greater spatial variability in PM10-2.5
concentrations than PM2.5 concentrations, resulting in
increased exposure error for PM10-2.5 (U.S. EPA, 2009a,
p. 2-8). Available measurements do not provide sufficient
information to adequately characterize the spatial distribution of
PM10-2.5 concentrations (U.S. EPA, 2009a, section
<IP_ADDRESS>). The net effect of these uncertainties on
PM10-2.5 epidemiological studies is to bias the results
of such studies toward the null hypothesis. That is, as noted in the
Integrated Science Assessment, these limitations in estimates of
ambient PM10-2.5 concentrations ``would tend to increase
uncertainty and make it more difficult to detect effects of
PM10-2.5 in epidemiologic studies'' (U.S. EPA, 2009a, p.
2-21).
    (3) Only a relatively small number of PM10-2.5
monitoring sites are currently operating and such sites have been in
operation for a relatively short period of time, limiting the
spatial and temporal coverage for routine measurement of
PM10-2.5 concentrations. Given these limitations in
routine monitoring, epidemiological studies have employed different
approaches for estimating PM10-2.5 concentrations. Given
the relatively small number of PM10-2.5 monitoring sites,
the relatively large spatial variability in ambient
PM10-2.5 concentrations (see above), the use of different
approaches to estimating ambient PM10-2.5 concentrations
across epidemiological studies, and the limitations inherent in such
estimates, the distributions of thoracic coarse particle
concentrations over which reported health outcomes occur remain
highly uncertain (U.S. EPA, 2009a, sections 2.2.3, 2.3.3, 2.3.4, and
<IP_ADDRESS>).
    (4) There is relatively little information on the chemical and
biological composition of PM10-2.5 and the effects
associated with the various components (U.S. EPA, 2009a, section
2.3.4). Without more information on the chemical speciation of
PM10-2.5, the apparent variability in associations with
health effects across locations is difficult to characterize (U.S.
EPA, 2009a, section <IP_ADDRESS>).
    (5) One of the implications of the uncertainties and limitations
discussed above is that the Risk Assessment concluded it would not
be appropriate to conduct a quantitative assessment of health risks
associated with PM10-2.5. The lack of a quantitative
PM10-2.5 risk assessment in the current review adds to
the uncertainty in any conclusions about the extent to which
revision of the current PM10 standard would be expected
to improve the protection of public health, beyond the protection
provided by the current standard.\126\
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    \126\ As noted above, the EPA's decision not to conduct a
quantitative risk assessment reflects uncertainty regarding the
value of such an assessment, but does not indicate that health
effects are not associated with exposure to thoracic coarse
particles.
---------------------------------------------------------------------------

C. Consideration of the Current and Potential Alternative Standards in
the Policy Assessment

    The following sections discuss the Policy Assessment's
consideration of the current and potential alternative standards to
protect against exposures to thoracic coarse particles (U.S. EPA,
2011a, chapter 3). Section IV.C.1 discusses the consideration of the
current standard while section IV.C.2 discusses the consideration of
potential alternative standards in terms of the basic elements of a
standard: Indicator, averaging time, form, and level.
1. Consideration of the Current Standard in the Policy Assessment
    As discussed above the 24-hour PM10 standard is meant to
protect the public health against exposures to thoracic coarse
particles (i.e., PM10-2.5). In considering the adequacy of
the current PM10 standard, the Policy Assessment considered
the health effects evidence linking short-term PM10-2.5
exposures with mortality and morbidity (U.S. EPA, 2009a, chapters 2 and
6), the ambient PM10 concentrations in PM10-2.5
study locations (U.S. EPA, 2011a, section 3.2.1), the uncertainties and
limitations associated with this health evidence (U.S. EPA, 2011a,
section 3.2.1), and the consideration of these uncertainties and
limitations as part of the weight of evidence conclusions in the
Integrated Science Assessment (U.S. EPA, 2009a).
    In considering the health evidence, air quality information, and
associated uncertainties as they relate to the current PM10
standard, the Policy Assessment noted that a decision on the adequacy
of the public health protection provided by that standard is a public
health policy judgment in which the Administrator weighs the evidence
and information, as well as its uncertainties. Therefore, depending on
the emphasis placed on different aspects of the evidence, information,
and uncertainties, consideration of different conclusions on the
adequacy of the current standard could be supported. For example, the
Policy Assessment noted that one approach to considering the evidence,
information, and its associated uncertainties would be to place
emphasis on the following (U.S. EPA, 2011a, section 3.2.3):

    (1) While most of PM10-2.5 effect estimates reported
for mortality and morbidity were positive, many were not
statistically significant, even in single-pollutant models. This
includes effect estimates reported in study locations with
PM10 concentrations above those allowed by the current
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding,
particularly by PM2.5, remains limited. Therefore, the
extent to which PM10-2.5 itself, rather than one or more
co-pollutants, contributes to reported health effects remains
uncertain.
    (3) Only a limited number of experimental studies provide
support for the associations reported in epidemiological studies,
resulting in further uncertainty regarding the plausibility of a
causal link between PM10-2.5 and mortality and morbidity.
    (4) Limitations in PM10-2.5 monitoring and the
different approaches used to estimate PM10-2.5
concentrations across epidemiological studies result in uncertainty
as to the ambient PM10-2.5 concentrations at which the
reported effects occur.
    (5) The chemical and biological composition of
PM10-2.5, and the effects associated with the various
components, remains uncertain. Without more information on the
chemical speciation of PM10-2.5, the apparent variability
in associations across locations is difficult to interpret.
    (6) In considering the available evidence and its associated
uncertainties, the Integrated Science Assessment concluded that the
evidence is ``suggestive'' of a causal relationship between short-
term PM10-2.5 exposures and mortality, cardiovascular
effects, and respiratory effects. These weight-of-evidence
conclusions contrast with those for the relationships between
PM2.5 exposures and adverse health effects, which were
judged in the Integrated Science Assessment to be either ``causal''
or ``likely causal'' for mortality, cardiovascular effects, and
respiratory effects.

    The Policy Assessment concluded that, to the extent a decision on
the adequacy of the current 24-hour PM10 standard were to
place emphasis on the considerations noted above, it could be judged
that, although it remains appropriate to maintain a standard to protect
against short-term exposures to

thoracic coarse particles, the available evidence suggests that the
current 24-hour PM10 standard appropriately protects public
health and provides an adequate margin of safety against effects that
have been associated with PM10-2.5 exposures. Although such
an approach to considering the adequacy of the current standard would
recognize the positive, and in some cases statistically significant,
associations between all types of PM10-2.5 and mortality and
morbidity, it would place relatively greater emphasis on the
limitations and uncertainties noted above, which tend to complicate the
interpretation of that evidence.
    In addition, the Policy Assessment noted the judgment that, given
the uncertainties and limitations in the PM10-2.5 health
evidence and air quality information, it would not have been
appropriate to conduct a quantitative assessment of health risks
associated with PM10-2.5 (U.S. EPA, 2011a, p. 3-6; U.S. EPA,
2010a, pp. 2-6 to 2-7, Appendix H). As discussed above, the lack of a
quantitative PM10-2.5 risk assessment adds to the
uncertainty associated with any characterization of potential public
health improvements that would be realized with a revised standard.
    The Policy Assessment also noted an alternative approach to