and information provide support for specific standard levels within the
ranges identified by CASAC and the Policy Assessment as appropriate for
consideration, the Administrator initially considers standard levels
within the range of 13 to 11 [mu]g/m\3\ for the annual standard. In so
doing, the Administrator first considers the long-term mean
PM2.5 concentrations reported in studies of effects
classified as having evidence of a causal or likely causal
relationship, as summarized in Figure 4 and discussed more broadly
above. She notes that a level at the upper end of this range would be
below most but not all the overall study mean concentrations from the
multi-city studies of long- and short-term exposures, whereas somewhat
lower levels within this range would be below all such overall study
mean concentrations. In considering the appropriate weight to place on
this information, the Administrator again notes that the evidence of an
association in any such study is strongest at and around the long-term
average where the data in the study are most concentrated, and that
long-term mean PM2.5 concentrations are available for each
study considered and, therefore, represent the most robust data set to
inform her decisions on appropriate annual standard levels. Further,
she is mindful that this approach does not provide a bright line for
reaching decisions about appropriate standard levels.
    In considering the long-term mean PM2.5 concentrations
reported in studies of effects classified as having evidence suggestive
of a causal relationship, as summarized in Figure 4 for reproductive
and developmental effects, the Administrator notes that a level at the
upper end of this range would be below the overall study mean
concentration in one of the three studies, while levels in the mid- to
lower part of this range would be below the overall study mean
concentrations in two or three of these studies. In considering the
appropriate weight to place on this information, the Administrator
notes the very limited nature of this evidence of such effects and the
additional uncertainties in these epidemiological studies relative to
the studies that provide evidence of causal or likely causal
relationships.
    The Administrator also considers additional distributional analyses
of population-level information that were available from four of the
epidemiological studies that provide evidence of effects identified as
having a causal relationship with long- or short-term PM2.5
concentrations for annual standard levels within the same range of 13
to 11 [mu]g/m\3\. In so doing, the Administrator first notes that a
level in the mid-part of this range generally corresponds with
approximately the 25th percentile of the distributions of health events
data available in three of

these studies. The Administrator also notes that standard levels toward
the upper part of this range would reflect placing substantially less
weight on this information, whereas standard levels toward the lower
part of this range would reflect placing substantially more weight on
this information. In considering this information, the Administrator
notes that there is no bright line that delineates the part of the
distribution of PM2.5 concentrations within which the data
become appreciably more sparse and, thus, where her confidence in the
associations observed in epidemiological studies becomes appreciably
less.
    In considering mean PM2.5 concentrations and
distributional analyses from the various sets of epidemiological
studies noted above, the Administrator is mindful, as noted above, that
such studies typically report concentrations based on composite monitor
distributions, in which concentrations may be averaged across multiple
ambient monitors that may be present within each area included in a
given study. Thus, a policy approach that uses data based on composite
monitors to identify potential alternative standard levels would
inherently build in a margin of safety of some degree relative to an
alternative standard level based on measurements at the monitor within
an area that records the highest concentration, or the maximum monitor,
since once a standard is set, concentrations at appropriate maximum
monitors within an area are generally used to determine if an area
meets a given standard.
    The Administrator also recognizes that judgments about the
appropriate weight to place on any of the factors discussed above
should reflect consideration not only of the relative strength of the
evidence but also on the important uncertainties that remain in the
evidence and information being considered in this review. The
Administrator notes that the extent to which these uncertainties
influence judgments about appropriate annual standard levels within the
range of 13 to 11 [mu]g/m\3\ would likely be greater for standard
levels in the lower part of this range which would necessarily be based
on fewer available studies than would higher levels within this range.
    Based on the above considerations, the Administrator concludes that
it is appropriate to propose to set a level for the primary annual
PM2.5 standard within the range of 12 to 13 [mu]g/m\3\. The
Administrator provisionally concludes that a standard set within this
range would reflect alternative approaches to appropriately placing the
most weight on the strongest available evidence, while placing less
weight on much more limited evidence and on more uncertain analyses of
information available from a relatively small number of studies.
Further, she provisionally concludes that a standard level within this
range would reflect alternative approaches to appropriately providing
an adequate margin of safety for the populations at risk for the
serious health effects classified as having evidence of a causal or
likely causal relationship, depending in part on the emphasis placed on
margin of safety considerations. The Administrator recognizes that
setting an annual standard level at the lower end of this range would
reflect an approach that places more emphasis on the entire body of the
evidence, including the analysis of the distribution of air quality
concentrations most influential in generating health effect estimates
in the studies, and on margin of safety considerations, than would
setting a level at the upper end of the range. Conversely, an approach
that would support a level at the upper end of this range would place
more emphasis on the remaining uncertainties in the evidence to avoid
potentially overestimating public health improvements, and would
generally support a view that the uncertainties remaining in the
evidence are too great to warrant setting a lower annual standard
level.
    While the Administrator recognizes that CASAC advised, and the
Policy Assessment concluded, that the available scientific information
provides support for considering a range that extended down to 11
[mu]g/m\3\, she concludes that proposing such an extended range would
reflect a public health policy approach that places more weight on
relatively limited evidence and more uncertain information and analyses
than she considers appropriate at this time. Nonetheless, the
Administrator solicits comment on a level down to 11 [mu]g/m\3\ as well
as on approaches for translating scientific evidence and rationales
that would support such a level. Such an approach might reflect a view
that the uncertainties associated with the available scientific
information warrant a highly precautionary public health policy
response that would incorporate a large margin of safety.
    The Administrator recognizes that potential air quality changes
associated with meeting an annual standard set at a level within the
range of 12 to 13 [mu]g/m\3\ will result in lowering risks associated
with both long- and short-term PM2.5 exposures. However, the
Administrator recognizes that such an annual standard intended to serve
as the primary means for providing protection from effects associated
with both long- and short-term PM2.5 exposures would not by
itself be expected to offer requisite protection with an adequate
margin of safety against the effects of all short-term PM2.5
exposures. As a result, in conjunction with proposing an annual
standard level in the range of 12 to 13 [mu]g/m\3\, the Administrator
provisionally 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, or for
PM2.5-related effects that may be associated with shorter-
than-daily exposure periods.
    Based on the approach discussed in section III.A.3 above, the
Administrator has relied upon evidence from the short-term exposure
studies as the principal basis for selecting the level of the 24-hour
standard. In considering these studies as a basis for the level of a
24-hour standard, and having selected a 98th percentile form for the
standard, the Administrator agrees with the focus in the Policy
Assessment of looking at the 98th percentile values, as well as at the
long-term mean PM2.5 concentrations in these studies.
    In considering the information provided by the short-term exposure
studies, the Administrator recognizes that to the extent these studies
were conducted in areas that likely did not meet one or both of the
current standards, such studies do not help inform the characterization
of the potential public health improvements of alternative standards
set at lower levels. By reducing the PM2.5 concentrations in
such areas to just meet the current standards, the Administrator
anticipates that additional public health protection will occur.
Therefore, the Administrator has focused on studies that reported
positive and statistically significant associations in areas that would
likely have met both the current 24-hour and annual standards. She has
also considered whether or not these studies were conducted in areas
that would likely have met an annual standard level of 12 to 13 [mu]g/
m\3\ to inform her decision regarding an appropriate 24-hour standard
level. As discussed in section III.E.4.a, 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.
The Administrator views the single-city, short-term exposure studies as
a much

more limited data set providing mixed results and, therefore, she has
less confidence in using these studies as a basis for setting the level
of a 24-hour standard. 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 values averaged across
study areas in 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 anticipated 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 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 proposing to revise the level of the annual standard to within
the range of 12 to 13 [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 in conjunction with
an annual standard level of 12 [mu]g/m\3\, and in two of these three
studies in conjunction with an annual standard level of 13 [mu]g/m\3\.
She notes that the study-wide mean concentrations are based on
averaging across monitors within study areas and that compliance with
the standard would be based on concentrations measured at the monitor
reporting the highest concentration within each area. The Administrator
believes it would be reasonable to conclude that revision to the 24-
hour standard would not be warranted in conjunction with an annual
standard within this range. Based on the above considerations related
to the epidemiological evidence, the Administrator provisionally
concludes 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 in the proposed range of 12 to 13 [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 retaining the 24-hour standard level at 35 [mu]g/m\3\
in conjunction with an annual standard level within the proposed range
of 12 to 13 [mu]g/m\3\. She has considered whether this suite of
standards would meet a public health policy goal which includes setting
the annual standard to be the ``generally controlling'' standard in
conjunction with setting the 24-hour standard to provide supplemental
protection to the extent that additional protection is warranted. As
discussed above, the Administrator provisionally concludes that this
approach is the most effective and efficient way to reduce total
population risk associated with both long- and short-term
PM2.5 exposures, resulting in more uniform protection across
the U.S. than the alternative of setting the 24-hour standard to be the
controlling standard.
    In considering the air quality information, the Administrator first
recognizes that there is no annual standard within the proposed range
of levels, when combined with a 24-hour standard at the proposed level
of 35 [mu]g/m\3\, for which the annual standard would be the generally
controlling standard in all areas of the country. She further observes
that such a suite of PM2.5 standards with an annual standard
level of 12 [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).
Although not explicitly delineated on Figure 2-10 in the Policy
Assessment, an annual standard of 13 [mu]g/m\3\ would be somewhat less
likely to be the generally controlling standard in some regions of the
U.S. outside the Northwest in conjunction with a 24-hour standard level
of 35 [mu]g/m\3\.
    Taking the above considerations into account, the Administrator
proposes to revise the level of the primary annual PM2.5
standard from 15.0 [mu]g/m\3\ to within the range of 12.0 to 13.0
[mu]g/m\3\ and to retain the 24-hour standard level at 35 [mu]g/m\3\.
In the Administrator's judgment, such a suite of primary
PM2.5 standards and the rationale supporting such levels
could reasonably be judged to reflect alternative approaches to the
appropriate consideration of the strength of the available evidence and
other information and their associated uncertainties and the advice of
CASAC.
    The Administrator recognizes that the final suite of standards
selected from within the proposed range of annual standard levels, or
the broader range of annual standard levels on which public comment is
solicited, must be clearly responsive to the issues raised by the D.C.
Circuit's remand of the 2006 primary annual PM2.5 standard.
Furthermore, the final suite of standards will reflect the
Administrator's ultimate judgment in the final rulemaking as to the
suite of primary PM2.5 standards that would be requisite to
protect the public health with an adequate margin of safety from
effects associated with fine particle exposures. The final judgment to
be made by the Administrator will appropriately consider the
requirement for a standard that is neither more nor less stringent than
necessary and will recognize 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 with an
adequate margin of safety.
    Having reached her provisional judgment to propose revising the
annual standard level from 15.0 to within a range of 12.0 to 13.0
[mu]g/m\3\ and to propose retaining the 24-hour standard level at 35
[mu]g/m\3\, the Administrator solicits public comment on this range of
levels and on approaches to considering the available evidence and
information that would support the choice of levels within this range.
The Administrator also solicits public comment on alternative annual
standard levels down to 11 [mu]g/m\3\ and on the combination of annual
and 24-hour standards that commenters may believe is appropriate, along
with the approaches and rationales used to support such levels. In
addition, given the importance the evidence from epidemiologic studies
plays in considering the appropriate annual and 24-hour levels, the
Administrator solicits public comment on issues related to translating
epidemiological evidence into standards, including approaches for
addressing the uncertainties and

limitations associated with this evidence.

F. Administrator's Proposed 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 proposes to revise the current primary PM2.5
standards. Specifically, the Administrator proposes to revise: (1) The
level of the primary annual PM2.5 standard to a level within
the range of 12.0 to 13.0 [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 allowance 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
proposes to retain the level and form of the primary 24-hour
PM2.5 standard to provide supplemental protection for areas
with high peak PM2.5 concentrations. The Administrator
provisionally concludes that such a revised suite of standards,
including a revised annual standard together with the current 24-hour
standard, could provide requisite protection against health effects
potentially associated with long- and short-term PM2.5
exposures. The Administrator is not proposing any revisions to the
current PM2.5 indicator and the annual and 24-hour averaging
times for the primary PM2.5 standards. Data handling
conventions are specified in proposed revisions to appendix N, as
discussed in section VII below. The Administrator solicits comment on
all aspects of this proposed decision.

IV. Rationale for Proposed Decision on Primary PM10 Standard

    This section presents the rationale for the Administrator's
proposed decision to retain the current 24-hour PM10
standard to continue to provide public health protection against short-
term exposures to thoracic coarse particles, that is inhalable
particles which can penetrate into the trachea, bronchi, and deep lungs
and which are in the size range of 2.5 to 10 [mu]m
(PM10-2.5). As discussed more fully below, this rationale is
based on a thorough review, in the Integrated Science Assessment, of
the latest scientific information, published through mid-2009, on human
health effects associated with long- and short-term exposures to
thoracic coarse particles in the ambient air. This proposal also takes
into account: (1) Staff assessments of the most policy-relevant
information presented and assessed in the Integrated Science Assessment
and staff analyses of air quality and health evidence presented in the
Policy Assessment, upon which staff conclusions regarding appropriate
considerations in this review are based; (2) CASAC advice and
recommendations, as reflected in discussions of drafts of the
Integrated Science Assessment and Policy Assessment at public meetings,
in separate written comments, and in CASAC's letters to the
Administrator; and (3) public comments received during the development
of these documents, either in connection with CASAC meetings or
separately. The EPA notes that the final decision for retaining or
revising the current primary PM10 standard is a public
health policy judgment made by the Administrator. The Administrator's
final decision will draw upon scientific information and analyses
related to health effects; judgments about uncertainties that are
inherent in the scientific evidence and analyses; CASAC advice; and
comments received in response to this proposal.
    In presenting the rationale for the proposed decision to retain the
current primary PM10 standard, this section begins with
background information on EPA's past reviews of the PM NAAQS and the
general approach taken to review the current PM10 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), and the Administrator's proposed
conclusions regarding the adequacy of the current primary
PM10 standard (section IV.E). Section IV.F summarizes the
Administrator's proposed decision with regard to the primary
PM10 NAAQS.

A. Background

    The following sections discuss previous reviews of the PM NAAQS
(section IV.A.1), the litigation of the 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). 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). Thus, the
EPA concluded that a PM10 standard could provide requisite
protection against effects associated with particles in the size range
of 2.5 to 10 [mu]m.\87\ 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, it was judged 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 newly adopted PM2.5
standards.
---------------------------------------------------------------------------

    \87\ 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 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 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.\88\ 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).
---------------------------------------------------------------------------

    \88\ 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 decision that it was requisite to provide
protection from exposure to all thoracic coarse PM, regardless of its
origin, rejecting 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 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
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 to 96, October 17, 2006). 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 PM10
standard afforded requisite protection with an adequate margin of
safety, the Agency retained the level and form of the 24-hour
PM10 standard.\89\
---------------------------------------------------------------------------

    \89\ 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.
---------------------------------------------------------------------------

    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.
    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).
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 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 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 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 accorded 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 EPA's decision to revoke the annual
PM10 standard. Specifically, the court stated the following:

    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 Policy Assessment for 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 basic elements of the NAAQS: Indicator, averaging time, form, and
level.
    In contrast to previous reviews, where PM10 studies
conducted in locations where PM10 is comprised predominantly
of PM10-2.5 were considered (U.S. EPA, 2005, pp. 5-49 to 5-
50), the focus in the current review is on PM10-2.5 studies.
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). In light of the
considerable uncertainty in the extent to which PM10 effect
estimates reflect associations with PM10-2.5 versus
PM2.5, together with the availability in this review of a
number of studies that evaluated associations with PM10-2.5
and the fact that the Integrated Science Assessment weight of evidence
conclusions for thoracic coarse particles were based on studies of
PM10-2.5, the EPA focuses in this review on studies that
have specifically evaluated PM10-2.5.
    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, section 2.4.3).
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.
    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).

B. Health Effects Related to Exposure to Thoracic Coarse Particles

    The following sections discuss available information on the health
effects associated with exposures to PM10-2.5, including the
nature of such health effects (section IV.B.1), the impacts of sources
and composition on particle toxicity (section IV.B.2), ambient
PM10 concentrations in PM10-2.5 study locations
(section IV.B.3), at-risk populations (section IV.B.4), and limitations
and uncertainties (section IV.B.5).
1. Nature of Effects
    Since the conclusion of the last review, the Agency has developed a
more formal framework for reaching causal inferences from the body of
scientific evidence. As discussed above in section III.B.1, this
framework uses a five-level hierarchy that classifies the overall
weight of evidence using the following categorizations: Causal
relationship, likely to be a 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, section 1.5).
Applying this framework to thoracic coarse particles, the Integrated
Science Assessment concludes 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).\90\ In
contrast, the Integrated Science Assessment concludes that available
evidence is ``inadequate'' to infer a causal relationship between long-
term PM10-2.5 exposures and various health effects (U.S.
EPA, 2009a, sections 7.2 to 7.6). Similar to the judgment made in the
2004 AQCD regarding long-term exposures (U.S. EPA, 2004, p. 9-79), the
Integrated Science Assessment states, ``To date, a sufficient amount of
evidence does not exist in order to draw conclusions regarding the
health effects and outcomes associated with long-term exposure to
PM10-2.5'' (U.S. EPA, 2009a, section 2.3.4). Given these
weight of evidence conclusions in the Integrated Science Assessment,
EPA's consideration of the scientific evidence for PM10-2.5
focuses on effects that have been linked with short-term exposures. The
evidence supporting a link between short-term thoracic coarse particle
exposures and adverse health effects is discussed in detail in the
Integrated Science Assessment (U.S. EPA, 2009a, Chapter 6) and is
summarized briefly below for mortality (section IV.B.1.a),
cardiovascular effects (section IV.B.1.b), and respiratory effects
(section IV.B.1.c).
---------------------------------------------------------------------------

    \90\ The Integrated Science Assessment discusses the framework
for causality determinations (U.S. EPA, 2009a, section 1.5). 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).
---------------------------------------------------------------------------

a. Short-Term PM10-2.5 Exposure and Mortality
    The Integrated Science Assessment assesses a number of multi-city
and single-city epidemiological studies that have evaluated
associations between mortality and short-term PM10-2.5
concentrations (U.S. EPA, 2009a, Figure 6-30 presents
PM10-2.5 mortality studies assessed in the last review and
the current review). Different studies have used different approaches
to estimate ambient PM10-2.5. Some studies have used the
difference between PM10 and PM2.5 mass, either
measured at co-located monitors (e.g., Lipfert et al., 2000; Mar et
al., 2003; Ostro et al., 2003; Sheppard et al., 2003; Wilson et al.,
2007) or as the difference in county-wide average concentrations
(Zanobetti and Schwartz, 2009), while other studies have measured
PM10-2.5 directly with dichotomous samplers (e.g., Burnett
and Goldberg, 2003; Fairley et al., 2003; Burnett et al., 2004; Klemm
et al., 2004). Despite differences in the approaches used to estimate
ambient PM10-2.5 concentrations, the majority of multi- and
single-city studies have reported positive associations between
PM10-2.5 and mortality, though most of these associations
were not statistically significant (U.S. EPA, 2009a, Figure 6-30).
    One important PM10-2.5 study conducted since the last
review of the PM NAAQS is the U.S. multi-city study by Zanobetti and
Schwartz (2009), which reported positive and statistically significant
associations with PM10-2.5 for all-cause, cardiovascular-
related, and respiratory-related mortality (U.S. EPA, 2009a, section
<IP_ADDRESS>). In this study, effect estimates for all-cause and
respiratory-related mortality remained statistically significant in co-
pollutant models that included PM2.5, while the effect
estimate for cardiovascular-related mortality remained positive but not
statistically significant. Several other multi-city studies have
reported positive, but not statistically significant,
PM10-2.5 effect estimates for mortality (U.S. EPA, 2009a,
Figure 6-30).
    When risk estimates in the study by Zanobetti and Schwartz (2009)
were evaluated by climatic region (U.S. EPA, 2009a, Figure 6-28), a mix
of positive and negative PM10-2.5 effect estimates were
reported in the regions that typically have the highest ambient
PM10-2.5 concentrations (i.e., regions corresponding to the
western and southwestern U.S.). Regional effect estimates from western
regions of the United States were generally not statistically
significant. Positive and statistically significant effect estimates
were more often reported in regions that typically have lower
PM10-2.5 concentrations (i.e., regions generally
corresponding to the eastern half of the U.S.) (Schmidt and Jenkins,
2010 for PM10-2.5 concentrations). In addition, single-city
empirical Bayes-adjusted effect estimates (calculated using the methods
discussed in Le Tertre et al., 2005) for the 47 cities evaluated by
Zanobetti and Schwartz (2009) were generally positive, though typically
not statistically significant (U.S. EPA, 2009a, Figure 6-29).
    Of the available single-city PM10-2.5 mortality studies,
most reported positive, but not statistically significant,
PM10-2.5 effect estimates (U.S. EPA, 2009a, Figure 6-30). Of
the three studies that did report statistically significant effect
estimates (Mar et al., 2003; Ostro et al., 2003; Wilson et al., 2007),
Ostro et al. (2003) reported that PM10-2.5 effect estimates
remained statistically significant in co-pollutant models that included
either ozone or NO2. The single-city studies by Mar et al.
(2003) and Wilson et al. (2007) did not utilize co-pollutant models.
b. Short-Term PM10-2.5 Exposure and Cardiovascular Effects
    The Integrated Science Assessment assesses a number of studies that
have evaluated the link between short-term ambient concentrations of
thoracic coarse particles and cardiovascular effects. Single- and
multi-city epidemiological studies generally report positive
associations between short-term PM10-2.5 concentrations and
hospital admissions or emergency department visits for cardiovascular
causes (U.S. EPA, 2009a, sections 2.3.3 and <IP_ADDRESS>). However, as is
the case for the mortality studies, most of these positive associations
are not statistically significant. In addition, most
PM10-2.5 effect estimates remained positive, but not
statistically significant, in co-pollutant models that included either

gaseous or particulate co-pollutants (U.S. EPA, 2009a, Figure 6-5).
    An important cardiovascular morbidity study published since the
last review of the PM NAAQS is the U.S. multi-city study by Peng et al.
(2008). This study evaluates hospital admissions and emergency
department visits for cardiovascular disease in Medicare patients
(MCAPS, Peng et al., 2008). The authors report a positive and
statistically significant association between 24-hour
PM10-2.5 concentrations and cardiovascular disease
hospitalizations in a single pollutant model using air quality data for
108 U.S. counties with co-located PM10 and PM2.5
monitors. The magnitude of this effect estimate was larger in counties
with higher degrees of urbanization and larger in the eastern U.S. than
the western U.S., though this regional difference was not statistically
significant (Peng et al., 2008). The PM10-2.5 effect
estimate was reduced only slightly in a co-pollutant model that
included PM2.5, but it was no longer statistically
significant (U.S. EPA, 2009a, sections 2.3.3, <IP_ADDRESS>).
    In addition to this U.S. multi-city study, positive associations
reported for short-term PM10-2.5 exposures and
cardiovascular-related morbidity reached statistical significance in a
multi-city study in France (Host et al., 2007) and single-city studies
in Detroit (Ito, 2003) and Toronto (Burnett et al., 1999) (U.S. EPA,
2009a, Figures 6-2 and 6-3). In contrast, associations were positive
but not statistically significant in single-city studies conducted in
Atlanta (Metzger et al., 2004; Tolbert et al., 2007) and Boston (Peters
et al., 2001) (and for some endpoints in Detroit (Ito, 2003)) (U.S.
EPA, 2009a, Figures 6-1 to 6-3, and 6-5).
    The plausibility of the positive associations reported for
PM10-2.5 and cardiovascular-related hospital admissions and
emergency department visits receives some measure of support from a
small number of controlled human exposure studies that have reported
alterations in heart rate variability following short-term exposure to
PM10-2.5 (Gong et al., 2004; Graff et al., 2009); by short-
term PM10-2.5 epidemiological studies reporting positive
associations with cardiovascular-related mortality; by a small number
of recent epidemiological studies that have examined dust storm events
and reported increases in cardiovascular-related emergency department
visits and hospital admissions (see below); and by associations with
other cardiovascular effects including heart rhythm disturbances and
changes in heart rate variability (U.S. EPA, 2009a, sections 2.3.3 and
<IP_ADDRESS>). The few toxicological studies that examined the effect of
PM10-2.5 on cardiovascular health effects used intratracheal
instillation and, as a result, provide only limited evidence on the
biological plausibility of PM10-2.5 induced cardiovascular
effects (U.S. EPA, 2009a, sections 2.3.3 and <IP_ADDRESS>).
c. Short-Term PM10-2.5 Exposure and Respiratory Effects
    The Integrated Science Assessment also assesses a number of studies
that have evaluated the link between short-term ambient concentrations
of thoracic coarse particles and respiratory effects. This includes
recent studies conducted in the U.S., Canada, and France (U.S. EPA,
2009a, section 6.3.8), including the U.S. multi-city study of Medicare
patients by Peng et al. (2008). As discussed above, Peng estimated
PM10-2.5 concentrations as the difference between
PM10 and PM2.5 concentrations measured by co-
located monitors. The authors reported a positive, but not
statistically significant, PM10-2.5 effect estimate for
respiratory-related hospital admissions. Single-city studies have
reported positive, and in some cases statistically significant,
PM10-2.5 effect estimates for respiratory-related hospital
admissions and emergency department visits (U.S. EPA, 2009a, Figures 6-
10 to 6-15). Some of these PM10-2.5 respiratory morbidity
studies have reported positive and statistically significant
PM10-2.5 effect estimates in co-pollutant models that
included gaseous pollutants while others reported that
PM10-2.5 effect estimates remain positive, but not
statistically significant, in such co-pollutant models (U.S. EPA,
2009a, Figure 6-15).
    A limited number of epidemiological studies have focused on
specific respiratory morbidity outcomes and reported both positive and
negative, but generally not statistically significant, associations
between PM10-2.5 and lower respiratory symptoms, wheeze, and
medication use (U.S. EPA, 2009a, sections <IP_ADDRESS> and <IP_ADDRESS>; Figures
6-7 to 6-9). Although controlled human exposure studies have not
observed an effect on lung function or respiratory symptoms in healthy
or asthmatic adults in response to short-term exposure to
PM10-2.5, healthy volunteers have exhibited increases in
markers of pulmonary inflammation.\91\ Toxicological studies using
inhalation exposures are still lacking, but pulmonary injury and
inflammation has been reported in animals after intratracheal
instillation exposure (U.S. EPA, 2009a, section <IP_ADDRESS>) and, in some
cases, PM10-2.5 was found to be more potent than
PM2.5.
---------------------------------------------------------------------------

    \91\ PM10-2.5 controlled human exposure studies have
not been conducted in children.
---------------------------------------------------------------------------

2. Potential Impacts of Sources and Composition on PM10-2.5
Toxicity
    In the absence of a systematic national effort to characterize
PM10-2.5 components, relatively little information (e.g.,
compared to fine particles) is available in the current review to
inform consideration of the potential for composition to impact
PM10-2.5 toxicity. Given this, the Integrated Science
Assessment concludes that currently available evidence is insufficient
to draw distinctions in 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 is associated with
adverse health effects (U.S. EPA, 2009a, section 2.4.4).
    As discussed above, positive associations between short-term
PM10-2.5 concentrations and mortality and morbidity have
been reported in a number of urban locations in the U.S., Canada, and
Europe. While little is known about how PM10-2.5 composition
varies across these locations or about how that variation could affect
particle toxicity (U.S. EPA, 2009a, sections 2.3.3, 2.3.4, 2.4.4), a
number of trace elements (e.g., chromium, cobalt, nickel, copper, zinc,
arsenic, selenium, and lead) have been detected in PM10-2.5
from urban locations (U.S. EPA, 2004, section 3.2.4).
    An indication of the sources of some of these trace elements (e.g.,
metals such as lead, copper, and zinc) in ambient PM10-2.5
samples has been obtained by examining urban runoff (U.S. EPA, 2004,
section 3.2.4). Wind-abrasion on building siding and roofs (coatings
such as lead paint and building material such as brick, metal, and wood
siding); brake wear (brake pads contain significant quantities of
copper and zinc); tire wear (zinc is used as a filler in tire
production); and burning engine oil could all produce particles
containing metals (U.S. EPA, 2004, section 3.2.4). Once deposited on
the ground, these elements can be resuspended with other material as
PM10-2.5. In addition, resuspended crustal particles may
become contaminated with trace elements and other components from
previously deposited fine PM (e.g., metals from smelters or steel
mills, PAHs from automobile exhaust, pesticides from agricultural
lands) (U.S. EPA, 2004, section 8.5, p. 8-344).
    In considering the potential for PM10-2.5 composition to
impact toxicity,

it is useful to consider studies conducted in locations where
PM10-2.5 composition is expected to be very different from
that in typical urban locations. Specifically, a small number of
studies have examined the health impacts of dust storm events (U.S.
EPA, 2009a, sections <IP_ADDRESS> and <IP_ADDRESS>). Although these studies do
not link specific particle constituents to health effects, they do
provide some information on the toxicity of particles of non-urban
crustal origin. Several of these studies have reported positive and
statistically significant associations between dust storm events and
morbidity or mortality, including the following:

    (1) Middleton et al. (2008) reported that dust storms in Cyprus
were associated with a statistically significant increase in risk of
hospitalization for all causes and a non-significant increase in
hospitalizations for cardiovascular disease.
    (2) Chan et al. (2008) studied the effects of Asian dust storms
on cardiovascular-related hospital admissions in Taipei, Taiwan and
reported a statistically significant increase associated with 39
Asian dust events. Evaluating the same data, Bell et al. (2008) also
reported positive and statistically significant associations between
hospitalization for ischemic heart disease and PM10-2.5.
    (3) Perez et al. (2008) tested the hypothesis that outbreaks of
Saharan dust exacerbate the effects of PM10-2.5 on daily
mortality in Spain. During Saharan dust days, the
PM10-2.5 effect estimate was larger than on non-dust days
and it became statistically significant, whereas it was not
statistically significant on non-dust days.

In addition, a study in Coachella Valley by Ostro et al. (2003)
reported statistically significant associations in a location where
thoracic coarse particles are expected to be largely due to windblown
dust.
    In contrast to the studies noted above, some dust storm studies
have reported associations that were not statistically significant.
Specifically, Bennett et al. (2006) reported on a dust storm in the
Gobi desert that transported PM across the Pacific Ocean, reaching
western North America in the spring of 1998. The authors reported no
excess risk of cardiovascular-related or respiratory-related hospital
admissions associated with the dust storm in the population of British
Columbia's Lower Fraser Valley (Bennett et al., 2006). In addition,
Yang et al. (2009) reported that hospitalizations for congestive heart
failure were elevated during or immediately following 54 Asian dust
storm events, though effect estimates were not statistically
significant.
3. Ambient PM10 Concentrations in PM10-2.5 Study
Locations
    As discussed above, a 24-hour PM10 standard is in place
to protect public health against exposures to PM10-2.5.
Given this, the EPA considers ambient PM10 concentrations in
locations where PM10-2.5 health studies have been conducted
(U.S. EPA, 2011a, section 3.2.1). Specifically, the Agency considers
study locations for which ambient PM10 data are available
for comparison to the current standard,\92\ including study locations
evaluated in single-city U.S. studies, in Bayes-adjusted single-city
analyses of the U.S. locations assessed by Zanobetti and Schwartz
(2009), in single-city studies conducted outside the U.S., and in
recent U.S. multi-city studies (Peng et al., 2008; Zanobetti and
Schwartz, 2009).
---------------------------------------------------------------------------

    \92\ As discussed in more detail in the Policy Assessment (U.S.
EPA, 2011a), these analyses are based on comparison of the one-
expected-exceedance concentration-equivalent design values in study
locations to the level of the current standard. The one-expected-
exceedance concentration-equivalent design value is used as a
surrogate concentration for comparison to the standard level in
order to gain insight into whether a particular area would likely
have met or violated the current PM10 standard.
Therefore, locations with one-expected-exceedance concentration-
equivalent design values below the level of the current
PM10 standard (i.e., 150 [mu]g/m\3\) would likely meet
that standard (U.S. EPA, 2011a, section 3.2.1).
---------------------------------------------------------------------------

    In considering 24-hour PM10 concentrations in locations
of specific PM10-2.5 epidemiological studies, the EPA has
focused primarily on U.S. study locations where single-city analyses
have been conducted (U.S. EPA, 2011a, sections 3.2.1 and 3.3.4). While
multi-city studies are particularly important when drawing conclusions
about health effect associations,\93\ it can be difficult to use these
studies to link air quality in a given location to health effects in
that same location. Multi-city studies often present overall effect
estimates rather than single-city effect estimates, while short-term
air quality can vary considerably across cities. Therefore, the extent
to which effects reported in multi-city studies are associated with the
short-term air quality in any particular location is uncertain,
especially when considering short-term concentrations at the upper end
of the distribution of daily concentrations for pollutants with
relatively heterogeneous spatial distributions such as
PM10-2.5 and PM10 (U.S. EPA, 2009a, section
<IP_ADDRESS>). In contrast, single-city studies are more limited in terms of
power and geographic coverage but the link between reported health
effects and the short-term air quality in a given city is more
straightforward to establish. As a result, in considering 24-hour
PM10 concentrations in locations of epidemiological studies,
the EPA has focused primarily on single-city studies and single-city
analyses of the locations evaluated in the multi-city study by
Zanobetti and Schwartz (2009) (U.S. EPA, 2011a, sections 3.2.1 and
3.3.4).
---------------------------------------------------------------------------

    \93\ Multi-city studies assess PM10-2.5-associated
health effects among large study populations and provide enhanced
power to detect PM10-2.5-associated health effects. In
addition, multi-city studies often provide spatial coverage for
different regions across the country, reflecting differences in
PM10-2.5 sources, composition, and potentially other
factors that could impact PM10-2.5-related effects. These
factors make multi-city studies particularly important when drawing
conclusions about health effect associations.
---------------------------------------------------------------------------

    Of the single-city mortality studies conducted in the United States
where ambient PM10 concentration data were available for
comparison to the current standard, positive and statistically
significant PM10-2.5 effect estimates were only reported in
study locations that would likely have violated the current
PM10 standard during the study period (U.S. EPA, 2011a,
Figure 3-2).\94\ In U.S. study locations that would likely have met the
current standard, PM10-2.5 effect estimates for mortality
were positive, but not statistically significant (U.S. EPA, 2011a,
Figure 3-2). Amongst U.S. study locations where single-city morbidity
studies were conducted, and which would likely have met the current
PM10 standard during the study period, PM10-2.5
effect estimates were both positive and negative, with most not
statistically significant (U.S. EPA, 2011a, Figure 3-3).
---------------------------------------------------------------------------

    \94\ See a previous footnote above and the Policy Assessment
(U.S. EPA, 2011a, section 3.2.1) for an explanation of how
PM10 air quality in study locations was compared to the
current PM10 standard.
---------------------------------------------------------------------------

    As discussed above, PM10-2.5 effect estimates for
mortality were generally positive but not statistically significant in
Bayes-adjusted single-city analyses in the locations evaluated by
Zanobetti and Schwartz (U.S. EPA, 2009a, Figure 6-30). These effect
estimates were generally similar in magnitude and precision,
particularly for cardiovascular-related mortality, across a wide range
of estimated PM10-2.5 concentrations (U.S. EPA, 2009a,
Figure 6-29). In most of the cities evaluated (37 of the 45 for which
appropriate PM10 air quality data were available for
comparison to the current standard, as described in Schmidt and Jenkins
(2010) and Jenkins (2011), PM10 concentrations were below
those that would have been allowed by the current PM10
standard (U.S. EPA, 2011a, section 3.2.1). Of these 37 cities that
would likely have met the current PM10 standard during

the study period, positive and statistically significant
PM10-2.5 effect estimates were reported in three locations
(Chicago, Pittsburgh, Birmingham). Of the eight cities likely to have
violated the current PM10 standard during the study period,
PM10-2.5 effect estimates were positive and statistically
significant in three (Detroit, St. Louis, Salt Lake City).
    In considering PM10-2.5 epidemiological studies
conducted in Canada and elsewhere outside the U.S., the EPA notes that
PM10 air quality information beyond that published by the
study authors is generally not available. The available PM10
concentration data for these study areas is typically not appropriate
for comparison to the current PM10 standard (i.e.,
concentrations are averaged across monitors, rather than from the
highest monitor in the study area, and/or concentrations are reported
as means or medians). However, in a small number of cases it is
possible to draw conclusions based on available air quality information
about whether a study area would likely have met or violated the
current PM10 standard.
    For example, Lin et al. (2002) reported positive and statistically
significant associations between PM10-2.5 and asthma
hospital admissions in children in Toronto (U.S. EPA, 2009a; Figures 6-
12 and 6-15). The authors reported a maximum PM10
concentration measured at a single monitor in the study area of 116
[mu]g/m\3\, indicating that the PM10 air quality in Toronto
during this study would have been allowed by the current 24-hour
PM10 standard.\95\
---------------------------------------------------------------------------

    \95\ This is the case because the maximum monitored 24-hour
PM10 concentration (116 [mu]g/m\3\) was below the level
of the current PM10 standard (150 [mu]g/m\3\).
---------------------------------------------------------------------------

    In contrast Middleton et al. (2008), who reported that dust storms
in Cyprus were associated with a statistically significant increase in
risk of hospitalization for all causes and a non-significant increase
in hospitalizations for cardiovascular diseases, reported a maximum 24-
hour PM10 concentration of 1,371 [mu]g/m\3\. Thus, the dust
storm-associated increases in hospitalizations reported in this study
occurred in an area with PM10 concentrations that were
likely well above those allowed by the current standard. Other dust
storm studies did not report maximum 24-hour PM10
concentrations from individual monitors, though the studies by Chan et
al. (2008) and Bell et al. (2008), which reported positive and
statistically significant associations between dust storm metrics and
cardiovascular-related hospital admissions, reported that 24-hour
PM10 concentrations, averaged across monitors, exceeded 200
[mu]g/m\3\. It is likely that peak concentrations measured at
individual monitors in these studies were much higher and, therefore,
24-hour PM10 concentrations in these study areas were likely
above those allowed by the current standard.
    In addition to the single-city studies discussed above, multi-city
averages of PM10 one-expected-exceedance concentration-
equivalent design values \96\ for recent U.S. multi-city studies were
110 [mu]g/m\3\, for the locations evaluated by Zanobetti and Schwartz
(2009), and 100 [mu]g/m\3\, for the locations evaluated by Peng et al.
(2008) (U.S. EPA, 2011a, section 3.2.1). As discussed above, the extent
to which multi-city PM10-2.5 effect estimates are associated
with the air quality in any particular location is uncertain.
---------------------------------------------------------------------------

    \96\ The one-expected-exceedance concentration-equivalent design
value is used as a surrogate concentration for comparison to the
standard level in order to gain insight into whether a particular
area would likely have met or violated the current PM10
standard. Therefore, locations with one-expected-exceedance
concentration-equivalent design values below the level of the
current PM10 standard (i.e., 150 [mu]g/m\3\) would likely
meet that standard (U.S. EPA, 2011a, section 3.2.1).
---------------------------------------------------------------------------

4. At-Risk Populations
    Specific groups within the general population are likely at
increased risk for suffering adverse effects following
PM10-2.5 exposures. As discussed in section III.B.3 above,
in this proposal, the term ``at-risk'' is the all encompassing term
used for groups with specific factors that increase the risk of PM-
related health effects in a population.
    Although studies have primarily used exposures to PM10
or PM2.5 to investigate potential at-risk populations, the
available evidence suggests that the identified factors also increase
risk from PM10-2.5 \97\ (U.S. EPA, 2009a, section 8.1.8). As
discussed in section III.B.3 above, at-risk populations include those
with preexisting heart and lung diseases (e.g., asthma), 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 (U.S. EPA, 2009a, section 2.4.1 and Table 8-2).
---------------------------------------------------------------------------

    \97\ Although the Integrated Science Assessment notes that in
PM10-2.5 studies of respiratory-related hospital
admissions and emergency department visits, ``the strongest
relationships were observed among children'' (U.S. EPA, 2009a,
section <IP_ADDRESS>). As discussed above (section III.B.3), children may
be more at increased risk for effects associated with ambient PM
exposures because, compared to adults, children typically spend more
time outdoors and at higher activity levels; they have exposures
that result in higher doses per body weight and lung surface area;
and there is the potential for irreversible effects on the
developing lung (U.S. EPA, 2009a, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    Given the range of at-risk groups, the population potentially
affected by PM10-2.5 is large. In the United States,
approximately 7 percent of adults (approximately 16 million adults) and
9 percent of children (approximately 7 million children) have asthma
(U.S. EPA, 2009a, Table 8-3; CDC, 2008 \98\). In addition,
approximately 4 percent of adults have been diagnosed with chronic
bronchitis and approximately 2 percent with emphysema (U.S. EPA, 2009a,
Table 8-3). Approximately 11 percent of adults have been diagnosed with
heart disease, 6 percent with coronary heart disease, 23 percent with
hypertension, and 8 percent with diabetes (U.S. EPA, 2009a, Table 8-3).
In addition, approximately 3 percent of the U.S. adult population has
suffered a stroke (U.S. EPA, 2009a, Table 8-3). Therefore, although
exposures to ambient PM10-2.5 have not been well
characterized on a national scale, the size of the potentially at-risk
population suggests that ambient PM10-2.5 could have a
significant impact on public health in the United States.
---------------------------------------------------------------------------

    \98\ For percentages, see http://www.cdc.gov/ASTHMA/nhis/06/table4-1.htm. For population estimates, see http://www.cdc.gov/ASTHMA/nhis/06/table3-1.htm.
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5. Limitations and Uncertainties Associated With the Currently
Available Evidence
    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, are discussed below.
    The Integrated Science Assessment concludes that an important
uncertainty in interpreting PM10-2.5 epidemiological studies
is the potential for confounding by co-occurring pollutants,
particularly PM2.5. This issue 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
toxicology studies) available to support the plausibility of
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.
    Another important uncertainty is related to exposure error. The
Integrated Science Assessment concludes that ``there is greater spatial
variability in PM10-2.5 concentrations than PM2.5
concentrations, resulting in increased exposure error for the larger
size fraction'' (U.S. EPA, 2009a, p. 2-8) and that 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).
    In addition, there is uncertainty in the air quality estimates used
in PM10-2.5 epidemiological studies (U.S. EPA, 2009a,
sections 2.3.3, 2.3.4) and, therefore, in the ambient
PM10-2.5 concentrations that are associated with mortality
and morbidity. 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.\99\ Given these limitations in routine monitoring,
epidemiological studies have employed different approaches for
estimating PM10-2.5 concentrations. For example, several of
the studies discussed above, including the multi-city study by Peng et
al. (2008), estimated PM10-2.5 by taking the difference
between mass measured at co-located PM10 and
PM2.5 monitors while the study by Zanobetti and Schwartz
(2009) used the difference between county-wide average PM10
and PM2.5 concentrations. In addition, a small number of
studies have directly measured PM10-2.5 concentrations with
dichotomous samplers (e.g., Burnett et al., 2004; Villeneuve et al.,
2003; Klemm et al., 2004). It is not clear how computed
PM10-2.5 measurements, such as those used by Zanobetti and
Schwartz (2009), compare with the PM10-2.5 concentrations
obtained in other studies either by direct measurement with a
dichotomous sampler or by calculating the difference using co-located
samplers (U.S. EPA, 2009a, section <IP_ADDRESS>).\100\ 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 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>).
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    \99\ The EPA has required PM10-2.5 mass monitoring,
as part of the NCore network, beginning January 1, 2011 at
approximately 80 stations. The NCore network is a multi-pollutant
network that includes measurements of particles, gases, and
meteorology (71 FR 61236, October 17, 2006). NCore monitoring
stations are located away from direct emissions sources that could
substantially impact the detection of area-wide concentrations. The
network is comprised of stations in both urban and rural areas.
Urban NCore stations are generally to be located at an urban or
neighborhood scale to provide exposure concentrations that are
expected to be representative of the metropolitan area. Rural NCore
stations are to be located, to the maximum extent practicable, at a
regional or larger scale away from any large local emission source,
so that they represent ambient concentrations over an extensive area
(U.S. EPA, 2011a, Appendix B, section B.4).
    \100\ In addition, several sources of uncertainty can be
specifically associated with PM10-2.5 concentrations that
are estimated based on co-located monitors. For example, the
potential for differences among operational flow rates and
temperatures for PM10 and PM2.5 monitors add
to the potential for exposure misclassification. As discussed in
Appendix B of the Policy Assessment (U.S. EPA, 2011a, sections B.2
and B.3), PM10 data are often reported at standard
temperature and pressure (STP) while PM2.5 is reported at
local conditions (LC). In these cases, the PM10 data
should be adjusted to LC when estimating PM10-2.5
concentrations. In many of the epidemiological studies that
estimated PM10-2.5 concentrations based on co-located
monitors, it is not made explicitly clear whether this adjustment
was made, adding to the overall uncertainty in the
PM10-2.5 concentrations that are associated with health
effects.
---------------------------------------------------------------------------

    Another uncertainty results from the relative lack of 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). As discussed above, a few recent studies have evaluated
associations between health effects and particles of non-urban, crustal
origin by evaluating the health impacts of dust storm events. Though
these studies provide some information on the health effects of ambient
particles that likely differ in composition from the particles of urban
origin that are typically studied, 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>).
    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 (U.S. EPA, 2009b, Appendix H). The
decision not to conduct a PM10-2.5 risk assessment for the
current review was based on consideration of several key uncertainties,
including the following:

    (1) Concerns that monitoring data that would be used in a
PM10-2.5 risk assessment (i.e., for the period 2005 to
2007) would not match ambient monitoring data used in the underlying
epidemiological studies providing concentration-response functions.
    (2) Uncertainty in the prediction of ambient levels under
current and alternative standard levels.
    (3) Concerns that locations used in the risk assessment may not
be representative of areas experiencing the most significant 24-hour
peak PM10-2.5 concentrations (and consequently, may not
capture locations with the highest risk).
    (4) Concerns about the relatively small (i.e., compared to
PM2.5) health effects database that supplies the
concentration-response relationships.

    When considered together, the limitations outlined above resulted
in the conclusion that a quantitative PM10-2.5 risk
assessment would not significantly enhance the review of the NAAQS for
coarse-fraction PM. Specifically, these limitations would likely result
in sufficient uncertainty in the resulting risk estimates to
significantly limit their utility to inform policy-related questions,
including the assessment of whether the current standard is protective
of public health and characterization of the degree of additional
public health protection potentially afforded by alternative standards.
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.

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

    The following sections discuss EPA's consideration of whether to
revise the current PM10 standard, as well as our
consideration of potential alternative

standards, drawing from such considerations in the Policy Assessment
(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 (section IV.C.2.a), averaging
time (section IV.C.2.b), form (section IV.C.2.c), and level (section
IV.C.2.d).
1. Consideration of the Current Standard in the Policy Assessment
    As discussed above, a 24-hour PM10 standard is in place
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 EPA considers 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 EPA notes 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 notes 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 3.2.1):

    (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
in 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 characterize.
    (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 concludes 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. Although such an approach to considering the
adequacy of the current standard would recognize the positive, and in
some cases statistically significant, associations between
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 notes that, when considering the
uncertainties and limitations in the PM10-2.5 health
evidence and air quality information, the EPA judged that it would not
be 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 notes an alternative approach to
considering the evidence and its uncertainties would place emphasis on
the following:

    (1) Several multi-city epidemiological studies conducted in the
U.S., Canada, and Europe, as well as a number of single-city
studies, have reported generally positive, and in some cases
statistically significant, associations between short-term
PM10-2.5 concentrations and adverse health endpoints
including mortality and cardiovascular-related and respiratory-
related hospital admissions and emergency department visits.
    (2) Both single-city and multi-city analyses, using different
approaches to estimate ambient PM10-2.5 concentrations,
have reported positive PM10-2.5 effect estimates in
locations that would likely have met the current 24-hour
PM10 standard. In a few cases, these PM10-2.5
effect estimates were statistically significant.
    (3) While limited in number, studies that have evaluated co-
pollutant models have generally reported that PM10-2.5
effect estimates remain positive, and in a few cases statistically
significant, when these models include gaseous pollutants or fine
particles.
    (4) Support for the plausibility of the associations reported in
epidemiological studies is provided by a small number of controlled
human exposure studies reporting that short-term (i.e., 2-hour)
exposures to PM10-2.5 decrease heart rate variability and
increase markers of pulmonary inflammation.

    This approach to considering the health evidence, air quality
information, and the associated uncertainties would place substantial
weight on the generally positive PM10-2.5 effect estimates
that have been reported for mortality and morbidity, even those effect
estimates that are not statistically significant. The Policy Assessment
concludes that this could be judged appropriate given that consistent
results have been reported across multiple studies using different
approaches to estimate ambient PM10-2.5 concentrations and
that exposure measurement error, which is likely to be larger for
PM10-2.5 than for PM2.5, tends to bias the
results of epidemiological studies toward the null hypothesis, making
it less likely that associations will be detected. Such an approach
would place less weight on the uncertainties and limitations in the
evidence that resulted in the Integrated Science Assessment conclusions
that the evidence is only suggestive of a causal relationship.
    Given all of the above, the Policy Assessment concludes that it
would be appropriate to consider either retaining or revising the
current 24-hour PM10 standard, depending on the approach
taken to considering the available

evidence, air quality information, and the uncertainties and
limitations associated with that evidence and information.
2. Consideration of Potential Alternative Standards in the Policy
Assessment
    Given the conclusion that it would be appropriate to consider
either retaining or revising the current PM10 standard, the
Policy Assessment also considered what potential alternative standards,
if any, could be supported by the available scientific evidence in
order to increase public health protection against exposures to
PM10-2.5. These considerations are discussed below in terms
of indicator, averaging time, form, and level.
a. Indicator
    As noted above, PM10 includes both PM10-2.5
and PM2.5, with the relative contribution of each to
PM10 mass varying across locations and over time. In the
most recent review completed in 2006, the EPA concluded that the
PM10 indicator remained appropriate in large part because a
PM10 standard would provide some measure of protection
against exposures to all PM10-2.5 regardless of source or
location, while also targeting protection to urban areas, where the
evidence of effects from exposure to coarse PM is the strongest (71 FR
at 61196, October 17, 2006). As noted above, the court explicitly
endorsed this reasoning. 559 F. 3d at 535-36.
    In considering the indicator in the current review, the Policy
Assessment evaluated the extent to which PM10 is comprised
of PM10-2.5 across locations and over time. Based on the air
quality analyses in the Integrated Science Assessment (U.S. EPA, 2009a,
section <IP_ADDRESS>) and Schmidt and Jenkins (2010), and based on the
concentration estimates of Zanobetti and Schwartz (2009), the Policy
Assessment notes that PM10-2.5 typically makes up a larger
portion of PM10 mass in the western United States, with the
southwest region having the highest ratios of PM10-2.5 to
PM10. In addition, the ratios of PM10-2.5 to
PM10 across the U.S. tended to be higher on days with
relatively high PM10 concentrations than on days with more
typical PM10 concentrations (i.e., comparing days with
concentrations at or above the 95th percentile to all days) (U.S. EPA,
2011a, section 3.3.1, Figure 3-4). Given this, the Policy Assessment
concludes that high daily PM10 concentrations are driven, at
least in part, by elevated PM10-2.5 mass and that a
PM10 standard focusing on the upper end of the distribution
of daily PM10 concentrations could effectively control
ambient PM10-2.5 concentrations (U.S. EPA, 2011a, p. 3-28).
    The Policy Assessment also considered the appropriateness of a
PM10 standard, given that such a standard allows lower
PM10-2.5 concentrations in areas with higher fine particle
concentrations (urban areas) than areas with lower fine particle
concentrations (rural areas) (U.S. EPA, 2011a, section 3.3.1). In
considering this issue, the Policy Assessment notes that most of the
evidence for positive associations between PM10-2.5 and
morbidity and mortality, particularly evidence for these associations
at relatively low concentrations of PM10-2.5, comes from a
number of studies conducted in locations where the PM10-2.5
is expected to be largely of urban origin (U.S. EPA, 2009a, Chapter 6).
Although some studies have reported positive associations between
relatively high concentrations of particles of non-urban origin (i.e.,
crustal material from windblown dust in non-urban areas, see above) and
mortality and morbidity, the Policy Assessment notes that the extent to
which these associations would remain at the lower particle
concentrations more typical of U.S. and Canadian urban study locations
remains uncertain.\101\
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    \101\ Other than the dust storm studies, we note that the study
in Coachella Valley by Ostro et al. (2003) reported statistically
significant associations in a location where thoracic coarse
particles are expected to be largely due to windblown dust.
Specifically, we note the CASAC conclusion in the last review that
``studies from Ostro et al. showed significant adverse health
effects, primarily involving exposures to coarse-mode particles
arising from crustal sources'' (Henderson, 2005b). In considering
this study, we also note the relatively high PM10
concentrations in the study area (U.S. EPA, 2011a, Figure 3-2),
which would not have met the current PM10 standard.
---------------------------------------------------------------------------

    Given these considerations, and given the increased potential for
coarse particles in urban areas to become contaminated by toxic
components of fine particles from urban/industrial sources (U.S. EPA,
2004 at 8-344; 71 FR 61196, October 17, 2006), the Policy Assessment
concludes that it is reasonable to consider an indicator that targets
control to areas with the types of ambient mixes generally present in
urban areas. The Policy Assessment notes that such an indicator would
focus control on areas with ambient mixes known with greater certainty
to be associated with adverse health effects and, therefore, would
provide public health benefits with the greatest degree of certainty.
Therefore, as in the last review, the Policy Assessment reaches the
conclusion that a PM10 indicator would appropriately target
protection to those locations where the evidence is strongest for
associations between adverse health effects and exposures to thoracic
coarse particles (U.S. EPA, 2011a, p. 3-29).
    In contrast, the Policy Assessment notes that a PM10-2.5
indicator, for a standard set at a single unvarying level, would not
achieve this targeting, given that allowable thoracic coarse particle
concentrations would be the same regardless of the location or the
likely sources of PM. Therefore, given the currently available
evidence, one possible result of using a PM10-2.5 indicator
would be a standard that is overprotective in rural areas and/or
underprotective in urban areas (Id.).
    Given all of the above considerations, the Policy Assessment
concludes that the available evidence supports consideration in the
current review of a PM10 indicator for a standard that
protects against exposures to thoracic coarse particles. The Policy
Assessment further concludes that consideration of alternative
indicators (e.g., PM10-2.5) in future reviews is desirable
and could be informed by additional research (U.S. EPA, 2011a, section
3.5).
b. Averaging Time
    Based primarily on epidemiological studies that reported positive
associations between short-term (24-hour) PM10-2.5
concentrations and mortality and morbidity, the Administrator concluded
in the last review that the available evidence supported a 24-hour
averaging time for a standard intended to protect against exposures to
thoracic coarse particles. In contrast, given the relative lack of
studies supporting a link between long-term exposures to thoracic
coarse particles and morbidity or mortality (U.S. EPA, 2004, Chapter
9), the Administrator further concluded that an annual coarse particle
standard was not warranted at that time (71 FR 61198-61199, October 17,
2006).
    In the current review, the Policy Assessment notes the conclusions
from the Integrated Science Assessment regarding the weight of evidence
for short-term and long-term PM10-2.5 exposures as well as
the studies on which those conclusions are based. Specifically, as
discussed above, the Integrated Science Assessment concludes 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). This
conclusion is based largely on epidemiological studies which have
primarily evaluated associations between 24-hour PM10-2.5
concentrations and morbidity and

mortality (e.g., U.S. EPA, 2009a, Figure 2-3), though a small number of
controlled human exposure studies have reported effects following
shorter exposures (i.e., 2-hours) to PM10-2.5 (U.S. EPA,
2009a, sections <IP_ADDRESS> and <IP_ADDRESS>). In contrast, with respect to
long-term exposures, the Integrated Science Assessment concludes that
available evidence is inadequate to infer a causal relationship with
all health outcomes evaluated (U.S. EPA, 2009a, section 2.3).
Specifically, the Integrated Science Assessment states, ``To date, a
sufficient amount of evidence does not exist in order to draw
conclusions regarding the health effects and outcomes associated with
long-term exposure to PM10-2.5'' (U.S. EPA, 2009a, section
2.3.4).
    In considering these weight-of-evidence determinations, the Policy
Assessment concludes that, at a minimum, they suggest the importance of
maintaining a standard that protects against short-term exposures to
thoracic coarse particles. Given that the majority of the evidence
supporting the link between short-term PM10-2.5 and
morbidity and mortality is based on 24-hour average thoracic coarse
particle concentrations, the Policy Assessment concludes that the
evidence available in this review continues to support consideration of
a 24-hour averaging time for a PM10 standard meant to
protect against effects associated with short-term exposures to
PM10-2.5 (U.S. EPA, 2011a, p. 3-31).
    The Policy Assessment further concludes that the available evidence
does not support consideration of an annual thoracic coarse particle
standard at this time. In reaching this conclusion, the Policy
Assessment also notes that, to the extent a short-term standard
requires areas to reduce their 24-hour ambient particle concentrations,
long-term concentrations would also be expected to decrease (Id.).
Therefore, a 24-hour standard meant to protect against short-term
exposures to thoracic coarse particles would also be expected to
provide some protection against potential effects associated with long-
term exposures to ambient concentrations.
c. Form
    The ``form'' of a standard defines the air quality statistic that
is to be compared to the level of the standard in determining whether
an area attains that standard. As discussed above, in the last review
the Administrator retained the one-expected exceedance form of the
primary 24-hour PM10 standard. This decision was linked to
the overall conclusion that ``the level of protection from coarse
particles provided by the current 24-hour PM10 standard
remains requisite to protect public health with an adequate margin of
safety'' (71 FR 61202, October 17, 2006). Because revising either the
level or the form of the standard would have altered the protection
provided, the Administrator concluded that such changes ``would not be
appropriate based on the scientific evidence available at this time''
(71 FR 61202). Therefore, the decision in the last review to retain the
one-expected-exceedance form was part of the broader decision that the
existing 24-hour standard provided requisite public health protection.
    In the current review, the Policy Assessment considers the form of
the standard within the context of the overall decision on whether, and
if so how, to revise the current 24-hour PM10 standard.
Given the conclusions above regarding the appropriate indicator and
averaging time for consideration for potential alternative standards,
the Policy Assessment considers potential alternative forms for a 24-
hour PM10 standard.
    Although the selection of a specific form must be made within the
context of decisions on the other elements of the standard, the Policy
Assessment notes that the EPA generally favors concentration-based
forms for short-term standards. In 1997, the EPA established a 98th
percentile form for the 24-hour PM2.5 standard and, in 2010,
the EPA established a 98th percentile form for the primary 1-hour
NO2 standard (62 FR 38671, July 18, 1997; 75 FR 6474,
February 9, 2010) and a 99th percentile form for the primary 1-hour
SO2 standard (75 FR 35541, June 22, 2010).\102\ In making
these decisions, the EPA noted that, compared to an exceedance-based
form, a concentration-based form is more reflective of the health risks
posed by elevated pollutant concentrations because such a form gives
proportionally greater weight to days when concentrations are well
above the level of the standard than to days when the concentrations
are just above the level of the standard. In addition, when averaged
over three years, these concentration-based forms were judged to
provide an appropriate balance between limiting peak pollutant
concentrations and providing a stable regulatory target, facilitating
the development of stable implementation programs.
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    \102\ As noted above (section IV.A.1.a), in the 1997 review the
EPA revised the form of the 24-hour PM10 standard to the
99th percentile. However, the D.C. Circuit Court vacated the revised
rule, based on EPA's retention of the PM10 indicator, and
the 1987 standards remained in place (including the one-expected-
exceedance form for the 24-hour standard).
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    These considerations are also relevant in the current review of the
24-hour PM10 standard. Specifically, the Policy Assessment
concludes that it is appropriate to consider concentration-based forms
that would provide a balance between limiting peak pollutant
concentrations and providing a stable regulatory target. To accomplish
this, it would be appropriate to consider forms from the upper end of
the annual distribution of 24-hour PM10 concentrations.\103\
However, given the potential for local sources to have important
impacts on monitored PM10 concentrations (U.S. EPA, 2009a,
section <IP_ADDRESS>), the Policy Assessment also notes that it would be
appropriate to consider forms that, when averaged over three years,
would be expected to promote the stability of local implementation
programs.\104\ In considering these issues in the most recent review of
the primary NO2 NAAQS, the Policy Assessment notes that a
98th percentile form was adopted, rather than a 99th percentile form,
due to the potential for ``instability in the higher percentile
concentrations'' near local sources (75 FR 6493, February 9,
2010).105 106
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    \103\ With regard to this conclusion, the Policy Assessment also
notes that PM10-2.5 is likely to make a larger
contribution to PM10 mass on days with relatively high
PM10 concentrations than on days with more typical
PM10 concentrations (see above).
    \104\ As noted in section III.E.3.b above, stability of
implementation programs has been held to be a legitimate
consideration in determining a NAAQS (American Trucking Associations
v. EPA, 283 F. 3d at 374 to 75).
    \105\ See also, ATA III, 283 F. 3d at 374-75 (upholding 98th
percentile form since ``otherwise States would have to design their
pollution control programs around single high exposure events that
may be due to unusual meteorological conditions alone, rendering the
programs less stable--and hence, we assume, less effective--than
programs designed to address longer-term average conditions.''). In
contrast, in the recently completed review of the primary
SO2 NAAQS, a 99th percentile form was adopted. However,
in the case of SO2, the standard was intended to limit 5-
minute exposures and a 99th percentile form was markedly more
effective at doing so than a 98th percentile form (75 FR 35540 to
41, June 22, 2010).
    \106\ Similar considerations are noted in section III.E.3.b
above, with regard to the form of the primary 24-hour
PM2.5 standard.
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    In considering the potential appropriateness of a 98th percentile
form in the current review, the Policy Assessment notes that, compared
to the current PM10 standard, attainment status for a
PM10 standard with a 98th percentile form would be based on
a more stable air quality statistic and would be expected to be less
influenced by relatively rare events that can cause elevations in
PM10 concentrations over short periods of time (Schmidt,
2011b).

Specifically, the Policy Assessment notes that in areas that monitor
PM10 every six days, every three days, or every day the
PM10 concentrations that are comparable to the current
standard level are, respectively, the highest, 2nd highest, or 4th
highest 24-hour PM10 concentrations measured during a three
year period. In contrast, for the same monitoring frequencies, the
PM10 concentrations that would be comparable to the level of
a standard with a 98th percentile form would be the three-year average
of the 2nd highest, 3rd highest, or 7th/8th highest 24-hour
PM10 concentrations measured during a single year (U.S. EPA,
2011a, p. 3-33).
    In further considering this issue the Policy Assessment notes that,
compared to the current one-expected-exceedance form, a concentration-
based form specified as a percentile of the annual distribution of
PM10 concentrations (e.g., such as a 98th percentile form)
would be expected to better compensate for missing data and less-than-
daily monitoring. This is a particularly important consideration in the
case of PM10 because, depending largely on ambient
concentrations, the frequency of PM10 monitoring differs
across locations (i.e., either daily, 1 in 2 days, 1 in 3 days, or 1 in
6 days) (U.S. EPA, 2011a, section 1.3 and Appendix B). As discussed in
earlier reviews of the PM NAAQS (e.g., 62 FR 38671, July 18, 1997), an
area's attainment status for a standard with a 98th percentile form
would be based directly on monitoring data rather than on a calculated
value adjusted for missing data or less-than-every-day monitoring, as
is the case with the current one-expected-exceedance form.
    In light of all of the above considerations, the Policy Assessment
concludes that, to the extent it is judged appropriate to revise the
current 24-hour PM10 standard, it would be appropriate to
consider revising the form to the 3-year average of the 98th percentile
of the annual distribution of 24-hour PM10 concentrations
(U.S. EPA, 2011a, p. 3-34).\107\
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    \107\ As noted above, local sources can have important impacts
on monitored PM10 concentrations. In the recent review of
the NO2 primary NAAQS, where this was also an important
consideration, a 98th percentile form was adopted, rather than a
99th percentile form, due to the potential for ``instability in the
higher percentile concentrations'' near local sources (75 FR 6493,
February 9, 2010). A similar conclusion in the current review led
the Policy Assessment to focus on the 98th percentile rather than
the 99th percentile, in considering potential alternative forms for
a PM10 standard.
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    In their review of the second draft Policy Assessment, CASAC noted
that such a change in form ``will lead to changes in levels of
stringency across the country'' and recommended that this issue be
explored further (Samet, 2010d). In considering this issue, the Policy
Assessment acknowledges that, given differences in PM10 air
quality distributions across locations (U.S. EPA, 2009a, Table 3-10), a
revised standard with a 98th percentile form would likely target public
health protection to some different locations than does the current
standard with its one-expected-exceedance form (U.S. EPA, 2011a, p. 3-
34). The final Policy Assessment notes that a further consideration
with regard to the appropriateness of revising the form of the current
PM10 standard is the extent to which, when compared with the
current standard, a revised standard with a 98th percentile form would
be expected to target public health protection to areas where we have
more confidence that ambient PM10-2.5 is associated with
adverse health effects (Id., p. 3-34 to 3-35).
    In giving initial consideration to this issue, the Policy
Assessment used recent PM10 air quality concentrations
(i.e., from 2007-2009) to identify counties that would meet, and
counties that would violate, the current PM10 standard as
well as potential alternative standards with 98th percentile forms
(Schmidt, 2011b).108 109 In some cases, counties that would
violate the current standard do so because of a small number of
``outlier'' days (e.g., as few as one such day in three years) with
PM10 concentrations well-above more typical concentrations
(Schmidt, 2011b). Mean and 98th percentile PM10 and
PM10-2.5 concentrations were higher in counties that would
have violated a revised standard with a 98th percentile form but met
the current standard \110\ than in counties that violated the current
standard, but would have met a revised standard with a 98th percentile
form (Schmidt, 2011b). This analysis suggests that, to the extent a
revised PM10 standard with a 98th percentile form could
target public health protection to different areas than the current
standard, those areas preferentially targeted by a revised standard
generally have higher ambient concentrations of thoracic coarse
particles. The issue of targeting public health protection is
considered further in section 3.3.4 of the Policy Assessment (U.S. EPA,
2011a) and below, within the context of considering specific potential
alternative standard levels for a 24-hour PM10 standard with
a 98th percentile form.
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    \108\ Section 3.3.4 of the Policy Assessment (U.S. EPA, 2011a)
discusses potential alternative standard levels that would be
appropriate to consider in conjunction with a revised standard with
a 98th percentile form.
    \109\ The memo by Schmidt (2011b) identifies specific counties
that are expected to meet, and counties that are not likely to meet
the current standard and potential alternative standards with 98th
percentile forms.
    \110\ This analysis considered a revised PM10
standard with a 98th percentile form and a level from the middle of
the range discussed in section 3.3.4 of the Policy Assessment (i.e.,
75 [micro]g/m\3\) (U.S. EPA, 2011a).
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d. Level
    As noted above, the Policy Assessment concluded that, to the extent
it is judged in the current review that the 24-hour PM10
standard does not provide adequate public health protection against
exposures to thoracic coarse particles, potential alternative standards
could be considered. The Policy Assessment considers potential
alternative levels for a 24-hour PM10 standard with a 98th
percentile form. To inform consideration of this issue, the Policy
Assessment considers the available scientific evidence and air quality
information (U.S. EPA, 2011a, section 3.3.4).
i. Evidence-Based Considerations in the Policy Assessment
    As discussed above, in considering the evidence as it relates to
potential alternative standard levels, the Policy Assessment first
considers the relative weight to place on specific epidemiological
studies, including the weight to place on the uncertainties associated
with those studies. The Policy Assessment considers several factors in
placing weight on specific epidemiological studies including the extent
to which studies report statistically significant associations with
PM10-2.5 and the extent to which the reported associations
are robust to co-pollutant confounding, in particular confounding by
PM2.5. In addition, the Policy Assessment considers the
extent to which associations with PM10-2.5 can be linked to
the air quality in a specific location. With regard to this, as noted
above, the Policy Assessment places the greatest weight on information
from single-city analyses.
    In considering PM air quality in study locations, the Policy
Assessment also notes that the available evidence does not support the
existence of thresholds, or lowest-observed-effects levels, in terms of
24-hour average concentrations (U.S. EPA, 2009a, section 2.4.3).\111\
In the absence of an apparent threshold, for purposes of identifying a
range of

standard levels potentially supported by the health evidence, the
Policy Assessment focuses on the range of PM10
concentrations that have been measured in locations where U.S.
epidemiological studies have reported associations with
PM10-2.5 (U.S. EPA, 2009a, Figures 6-1 to 6-30 for studies).
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    \111\ Most studies that have evaluated the potential for
thresholds have focused on PM10 or PM2.5.
However, there is no scientific basis for drawing different
conclusions for PM10-2.5.
---------------------------------------------------------------------------

    In single-city mortality studies, as well as the single-city
analyses of the locations evaluated by Zanobetti and Schwartz (2009),
positive and statistically significant PM10-2.5 effect
estimates were reported in some locations with 98th percentile
PM10 concentrations ranging from 200 [mu]g/m\3\ to 91 [mu]g/
m\3\ (U.S. EPA, 2011a, section 3.3.4). Lower PM10
concentrations were present in locations where positive, but not
statistically significant, effect estimates were reported and when
averaged across locations evaluated in the multi-city study by
Zanobetti and Schwartz (2009) (U.S. EPA, 2011a, section 3.3.4).
    Among U.S. morbidity studies, Ito (2003) reported a positive and
statistically significant PM10-2.5 effect estimate for
hospital admissions for ischemic heart disease in Detroit, where the
98th percentile PM10 concentration (102 [mu]g/m\3\) was also
within this range (U.S. EPA, 2011a, section 3.3.4 and Figure 3-6).
PM10-2.5 effect estimates in this study remained positive,
and in some cases statistically significant, in co-pollutant models
with gaseous pollutants (U.S. EPA, 2009a, Figures 6-5 and 6-15). Lower
PM10 concentrations were present in locations where
positive, but not statistically significant, effect estimates were
reported and when averaged across locations evaluated in the multi-city
study by Peng et al. (2008) (U.S. EPA, 2011a, section 3.3.4).
ii. Air Quality-based Considerations in the Policy Assessment
    In addition to the evidence-based considerations described above,
the Policy Assessment estimated the level of a 24-hour PM10
standard with a 98th percentile form that would approximate the degree
of protection, on average across the country, provided by the current
24-hour PM10 standard with its one-expected-exceedance form.
The initial approach to estimating this ``generally equivalent'' 98th
percentile PM10 concentration was to use EPA's Air Quality
System (AQS)\112\ as the basis for evaluating correlations between 98th
percentile PM10 concentrations and one-expected-exceedance
concentration equivalent design values (Schmidt and Jenkins,
2010).\113\ Based on these correlations, using monitoring data from
1988 to 2008, a 98th percentile PM10 concentration of 87
[mu]g/m\3\ is, on average, generally equivalent to the current standard
level (U.S. EPA, 2011a, Figure 3-7). However, given the variability in
the distributions of PM10 concentrations across locations
(U.S. EPA, 2009a, Table 3-10; Schmidt and Jenkins, 2010), the range of
equivalent concentrations varies considerably (95 percent confidence
interval ranges from 63 to 111 [mu]g/m\3\) (Schmidt and Jenkins, 2010).
As a consequence, the Policy Assessment notes that in some locations a
98th percentile standard with a level of 87 [mu]g/m\3\ would likely be
more protective than the current standard while in other locations it
would likely be less protective than the current standard.\114\
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    \112\ See http://www.epa.gov/ttn/airs/airsaqs/.
    \113\ As discussed above, the one-expected-exceedance
concentration-equivalent design value is used as a surrogate
concentration for comparison to the standard level in order to gain
insight into whether a particular area would likely have met or
violated the current PM10 standard. Therefore, locations
with one-expected-exceedance concentration-equivalent design values
below the level of the current PM10 standard (i.e., 150
[mu]g/m\3\) would likely meet that standard (U.S. EPA, 2011a,
section 3.2.1).
    \114\ The ``generally equivalent'' concentration also differs
depending on the years of monitoring data used. For example, when
this analysis was restricted to only the most recent years available
(i.e., 2007 to 2009), the ``generally equivalent'' 98th percentile
PM10 concentration was 78 [mu]g/m\3\. Given the temporal
variability in the relationship between the current standard level
and 98th percentile PM10 concentrations, and the
potential for the ``generally equivalent'' 98th percentile
concentration to vary year-to-year, staff concluded that it remains
appropriate to consider the correlation analyses that use the
broader range of available monitoring years (i.e., 1998-2008), as
these analyses are likely to be more robust than analyses based on a
shorter period of time.
---------------------------------------------------------------------------

    The Policy Assessment also evaluates regional differences in the
relationship between 98th percentile PM10 concentrations and
one-expected-exceedance concentration equivalent design values (U.S.
EPA, 2011a, Figure 3-8), based on air quality data from 1988 to 2008.
The 98th percentile PM10 concentrations that are, on
average, generally equivalent to the current standard level ranged from
just below 87 [mu]g/m\3\ in the Southeast, Southwest, upper Midwest,
and outlying areas (i.e., generally equivalent 98th percentile
PM10 concentrations ranged from 82 to 85 [mu]g/m\3\ in these
regions) to just above 87 [mu]g/m\3\ in the Northeast, industrial
Midwest, and southern California (i.e., generally equivalent 98th
percentile PM10 concentrations ranged from 88 to 93 [mu]g/
m\3\ in these regions) (Schmidt, 2011b). However, within each of these
regions there is considerable variability in the ``generally
equivalent'' 98th percentile PM10 concentration across
monitoring sites (U.S. EPA, 2011a, Figure 3-8).
    To provide a broader perspective on the relationship between the
current standard and potential alternative standards with 98th
percentile forms, the Policy Assessment also compares the size of the
populations living in counties with PM10 one-expected-
exceedance concentration-equivalent design values greater than the
current standard level to the size of the populations living in
counties with 98th percentile PM10 concentrations above
different potential alternative standard levels (based on air quality
data from 2007 to 2009 \115\). Such comparisons can be considered as
surrogates for comparisons of the breadth of public health protection
provided by the current and potential alternative standards. Based on
these comparisons, a 98th percentile PM10 standard with a
level between 75 and 80 [mu]g/m\3\ would be most closely equivalent to
the current standard. That is, compared to the number of people living
in counties that would violate the current PM10 standard, a
similar number live in counties that would violate a revised 24-hour
PM10 standard with a 98th percentile form and a level
between 75 and 80 [mu]g/m\3\ (U.S. EPA, 2011a, Table 3-2). However,
there is considerably more variability across regions in the potential
alternative standard that, based on this analysis, would be generally
equivalent to the current PM10 standard (U.S. EPA, 2011a,
section 3.3.4).
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    \115\ These analyses are based on three years of air quality
data in order to simulate the requirements for determining whether
areas attain or violate the current PM10 standard, which
requires consideration of 3 years of air quality data.
---------------------------------------------------------------------------

    Given the variability in the relationship between the current
standard and potential alternative standards with 98th percentile
forms, the Policy Assessment concludes that no single potential
alternative standard level, for a revised standard with a 98th
percentile form, would provide public health protection equivalent to
that provided by the current standard, consistently over time and
across locations.
    One consequence of this variability, as noted above in the
discussion of the form of the standard, would be that a 24-hour
PM10 standard with a 98th percentile form and a revised
level would likely target public health protection to some different
locations than does the current standard. Therefore, in further
considering the appropriateness of revising the form and level of the
current PM10 standard, the

Policy Assessment considered the extent to which, when compared with
the current standard, a revised PM10 standard would be
expected to target public health protection to areas where we have more
confidence that PM10-2.5 is associated with adverse health
effects. To address this question, the Policy Assessment considered the
potential impact of revising the form and level of the PM10
standard in locations where health studies have reported associations
with PM10-2.5.
    The Policy Assessment initially considers U.S. study locations that
would likely have met the current PM10 standard during the
study period and where positive and statistically significant
associations with PM10-2.5 were reported. Only Birmingham,
Chicago, Pittsburgh, and Detroit \116\ met these criteria. During study
periods, none of these areas would likely have met a 98th percentile
24-hour PM10 standard with a level at or below 87 [mu]g/m\3\
(U.S. EPA, 2011a, section 3.3.4 and Table 3-3).
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    \116\ Positive and statistically significant PM10-2.5
effect estimates for Birmingham, Chicago, and Pittsburgh are
reported in the Integrated Science Assessment (U.S. EPA, 2009a,
Figure 6-29; from cities evaluated by Zanobetti and Schwartz, 2009).
Effect estimates for Detroit are reported by Ito et al. (2003).
---------------------------------------------------------------------------

    The Policy Assessment also considered U.S. locations where health
studies have reported positive associations (both statistically
significant and non-significant) between PM10-2.5 and
mortality or morbidity. Such positive associations were reported in 47
locations that would likely have met the current PM10
standard during the study period.\117\ Of these 47 locations, 13 would
likely not have met a 98th percentile 24-hour PM10 standard
with a level at 87 [mu]g/m\3\, 20 would likely not have met a 98th
percentile 24-hour PM10 standard with a level of 75 [mu]g/
m\3\, and 31 would likely not have met a 98th percentile 24-hour
PM10 standard with a level of 65 [mu]g/m\3\ (U.S. EPA,
2011a, section 3.3.4).
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    \117\ Philadelphia (Lipfert et al., 2000), Detroit (Ito et al.,
2003), Santa Clara (CA) (Fairley et al., 2003), Seattle (Sheppard et
al., 2003), Atlanta (Klemm et al., 2004), Spokane (Slaughter et al.,