responds to each of these specific comments in the Response to
Comments document.
    \58\ As noted above, however, Janes et al. (2007) and Greven et
al. (2011) focused on temporal variability and other studies of
long-term exposure to PM2.5 and mortality focus on
spatial variability.
---------------------------------------------------------------------------

    Furthermore, the EPA disagrees with commenters that studies by
Janes et al. (2007) and Greven et al. (2011) provide evidence that
other studies of long-term exposure to PM2.5 and mortality
are affected by unmeasured confounding. As noted above, the design of
the studies conducted by Janes et al. (2007) and Greven et al. (2011)
are fundamentally different than those used in other studies of long-
term exposure to PM2.5 and mortality, including the ACS
cohort and the Harvard Six Cities study. Studies, such as the ACS and
Harvard Six Cities studies, used the spatial variation between cities
to measure the effect of long-term (annual) exposures to
PM2.5 on mortality risk, and did not conduct any analyses
relying on the temporal variation in PM2.5. The opposite is
true of the Janes et al. (2007) and Greven et al. (2011) studies which
first removed the spatial variability in PM2.5 and then
examined the temporal variation at both the national and local scale to
measure the effects of temporal differences in PM2.5 on
mortality risk. Janes et al. (2007) and Greven et al. (2011) focus on
changes in PM2.5 concentrations over time and, therefore,
control for confounders would be based on including variables that vary
over time rather than over space. As a result, any evidence of
potential confounding of the PM2.5-mortality risk
relationship derived from Janes et al. (2007) and Greven et al. (2011)
cannot be extrapolated to draw conclusions related to potential spatial
confounding in studies based on the spatial variation in
PM2.5 concentrations.
    As detailed in the Integrated Science Assessment (U.S. EPA, 2009a,
section 7.6), and recognized by the authors of Janes et al. (2007) and
Greven et al. (2011), the cohort studies that informed the causality
determination for long-term PM2.5 exposure and mortality
``have developed approaches to adjust for measured and unmeasured
confounders'' (Dominici et al., 2012, p. 2). These approaches were
specifically designed to adjust for spatial confounding. The hypothesis
that the authors of Janes et al. (2007) and Greven et al. (2011) chose
to examine was that differences in the local and national effects
indicated unmeasured temporal confounding in either the local or
national effect estimate. This hypothesis was specific to these two
studies that examined temporal variability in exposure to air pollution
and did not include known potential confounders at either the national
or local scale as time-varying covariates in the statistical model. The
authors acknowledged that the interpretation of either the national or
local estimates needs to occur with an appreciation of the potential
confounding effects of national and local scale covariates that were
omitted from the model (Dominici et al., 2012).
    It is important to recognize that because Janes et al. (2007) and
Greven et al. (2011) focused on variations in PM2.5 over
time and not space, the results from these two studies do not provide
any indication that other studies of long-term exposure to
PM2.5 and mortality exhibit spatial confounding, or that
PM2.5 does not cause mortality.\59\ The authors of Janes et
al. (2007) and Greven et al. (2011) recognized that ``it is entirely
possible that these papers are looking for an association at a
timescale for which no association truly exists'' (Dominici et al.,
2012, p. 3). Furthermore, as highlighted in the Integrated Science
Assessment and discussed by Pope and Burnett (2007), the conclusions of
Janes et al. (2007) ``are overstated * * * [T]heir analysis tells us
little or nothing about unmeasured confounding in those and related
studies because the methodology of Janes et al. largely excludes the
sources of variability that are exploited in those other studies. By
using monthly mortality counts and lagged 12-month average pollution
concentrations, the authors eliminate the opportunity to exploit short-
term or day-to-day variability.''
---------------------------------------------------------------------------

    \59\ Further, the EPA notes that Janes et al. (2007) and Greven
et al. (2011) provide no information relevant to examining
confounding in studies of short-term exposure to PM2.5.
---------------------------------------------------------------------------

    In conclusion, the EPA interprets the results of the analyses
conducted by Janes et al. (2007) and Greven et al. (2011) as being
consistent with prior knowledge of examining associations with long-
term exposure to PM2.5 at the national scale using long-term
average PM2.5 concentrations. For the reasons presented
above and discussed in more detail in the Response to Comments
document, the Agency disagrees with the commenters' assumption that the
results of Janes et al. (2007) and Greven et al. (2011) indicate
unmeasured confounding in the results of other cohort studies of long-
term exposure to PM2.5 and mortality. Therefore, the EPA
concludes that these studies do not invalidate the large body of
epidemiological evidence that supports the EPA's determination that a
causal relationship exists between long-term PM2.5 exposure
and mortality.\60\
---------------------------------------------------------------------------

    \60\ The EPA notes that the EPA's conclusion with regard to
interpretation of the results from Janes et al. (2007) and Greven et
al. (2012) is supported by the study authors' conclusion that
``[o]ur results do not invalidate previous epidemiological studies''
(Dominici, 2012, p. 1 (emphasis original)).
---------------------------------------------------------------------------

(b) Heterogeneity in Risk Estimates
    Some commenters argued that the heterogeneity in risk estimates
observed in multi-city epidemiological studies and the lack of
statistical significance in many regional or seasonal estimates
highlights a potential bias associated with combined multi-city
epidemiological study results (e.g., API, 2012, Attachment 1, pp. 15 to
19). These commenters further argued that more refined intra-urban
exposure estimates conducted for two of the largest cities included in
the ACS study, Los Angeles and New York City, based on land-use
regression models and/or kriging methods (Krewski et al., 2009)
``underscore the importance of considering city-specific health
estimates, which may account for heterogeneity in PM2.5
concentrations or other differences among cities, rather than relying
on pooled nationwide results from multi-city studies'' (API, 2012,
Attachment 1, p. 17).
    With respect to understanding the nature and magnitude of
PM2.5-related risks, the EPA agrees that epidemiological
studies evaluating health effects associated with long- and short-term
PM2.5 exposures have reported heterogeneity in responses
between cities and effect estimates across geographic regions of the
U.S. (U.S. EPA, 2009a, sections <IP_ADDRESS>, <IP_ADDRESS>, 6.5.2, and 7.6.1;
U.S. EPA, 2011a, p. 2-25). For example, when focusing on short-term
PM2.5 exposure, the Integrated Science Assessment found that
multi-city studies that examined associations with mortality and
cardiovascular and respiratory hospital admissions and emergency
department visits demonstrated greater cardiovascular effects in the
eastern versus the western U.S. (Dominici, et al., 2006a; Bell et al.,
2008; Franklin et al. (2007, 2008)).
    In addition, the Integrated Science Assessment evaluated studies
that provided some evidence for seasonal differences in
PM2.5 risk estimates, specifically in the northeast. The
Integrated Science Assessment found evidence indicating that
individuals may be at greater risk of dying from higher exposures to
PM2.5 in the warmer months, and at greater risk of
PM2.5 associated hospitalization for

cardiovascular and respiratory diseases during colder months of the
year. The limited influence of seasonality on PM risk estimates in
other regions of the U.S. may be due to a number of factors including
varying PM composition by season, exposure misclassification due to
regional tendencies to spend more or less time outdoors and air
conditioning usage, and the prevalence of infectious diseases during
the winter months (U.S. EPA, 2009a, p. 3-182).
    Overall, the EPA took note in the proposal that uncertainties still
remain regarding various factors that contribute to heterogeneity
observed in epidemiological studies (77 FR 38909/3). Nonetheless, the
EPA recognizes that this heterogeneity could be attributed, at least in
part, to differences in PM2.5 composition across the U.S.,
as well as to exposure differences that vary regionally such as
personal activity patterns, microenvironmental characteristics, and the
spatial variability of PM2.5 concentrations in urban areas
(U.S. EPA, 2009a, section 2.3.2; 77 FR 38910).
    As recognized in the Policy Assessment, the current epidemiological
evidence and the limited amount of city-specific speciated
PM2.5 data do not allow conclusions to be drawn that
specifically differentiate effects of PM2.5 in different
locations (U.S. EPA, 2011a, p. 2-25). Furthermore, the Integrated
Science Assessment concluded ``that many constituents of
PM2.5 can be linked with multiple health effects, and the
evidence is not yet sufficient to allow differentiation of those
constituents or sources that are more closely related to specific
health outcomes'' (U.S. EPA, 2009a, p. 2-17). CASAC thoroughly reviewed
the EPA's presentation of the scientific evidence indicating
heterogeneity in PM2.5 effect estimates in epidemiological
studies and concurred with the overall conclusions presented in the
Integrated Science Assessment.
(c) Exposure Measurement Error
    Some commenters argued that the EPA did not adequately consider
exposure measurement error, which they asserted is an important source
of bias in epidemiological studies that can bias effect estimates in
either direction (e.g., API, 2012, Attachment 1, pp. 19 to 20).
    The EPA agrees that exposure measurement error is an important
source of uncertainty and that the variability in risk estimates
observed in multi-city studies could be attributed, in part, to
exposure error due to measurement-related issues (77 FR 38910).
However, the Agency disagrees with the commenters' assertion that
exposure measurement error was not adequately considered in this
review. The Integrated Science Assessment included an extensive
discussion that addresses issues of exposure measurement error (U.S.
EPA, 2009a, sections 2.3.2 and 3.8.6). Exposure measurement error may
lead to bias in effect estimates in epidemiological studies. A number
of studies evaluated in the last review (U.S. EPA, 2004, section 8.4.5)
and in the current review (U.S. EPA, 2009a, section 3.8.6) have
discussed the direction and magnitude of bias resulting from specified
patterns of exposure measurement error (Armstrong 1998; Thomas et al.
1993; Carroll et al. 1995) and have generally concluded ``classical''
(i.e., random, within-person) exposure measurement error can bias
effect estimates towards the null. Therefore, consistent with
conclusions reached in the last review, the Integrated Science
Assessment concluded ``in most circumstances, exposure error tends to
bias a health effect estimate downward'' (U.S. EPA, 2009a, sections
2.3.2 and 3.8.6) (emphasis added). Thus, the EPA has both considered
and accounted for the possibility of exposure measurement error, and
the possible bias would make it more difficult to detect true
associations, not less difficult.
(d) Model Specification
    Commenters contended that the EPA did not account for the fact that
``selecting an appropriate statistical model for epidemiologic studies
of air pollution involves several choices that involve much ambiguity,
scant biological evidence, and a profound impact on analytic results,
given that many estimated associations are weak'' (ACC, 2012, p. 5).
For short-term exposure studies, the EPA recognizes, as summarized in
the HEI review panel commentary that selecting a level of control to
adjust for time-varying factors, such as temperature, in time-series
epidemiological studies involves a trade-off (HEI, 2003). For example,
if the model does not sufficiently adjust for the relationship between
the health outcome and temperature, some effects of temperature could
be falsely ascribed to the pollution variable. Conversely, if an overly
aggressive approach is used to control for temperature, the result
would possibly underestimate the pollution-related effect and
compromise the ability to detect a small but true pollution effect
(U.S. EPA, 2004, p. 8-236; HEI, 2003, p. 266). The selection of
approaches to address such variables depends in part on prior knowledge
and judgments made by the investigators, for example, about weather
patterns in the study area and expected relationships between weather
and other time-varying factors and health outcomes considered in the
study. As demonstrated in section 6.5 of the Integrated Science
Assessment, the EPA thoroughly considered each of these issues and the
overall effect of different model specifications on the association
between short-term PM2.5 exposure and mortality. Regardless
of the model employed, consistent positive associations were observed
across studies that controlled for the potential confounding effects of
time and weather using different approaches (U.S. EPA 2009a, Figure 6-
27). The EPA also considered the influence of model specification in
the examination of long-term PM2.5 exposure studies. For
example, in section 7.6 of the Integrated Science Assessment, Figures
7-6 and 7-7 summarize the collective evidence that evaluated the
association between long-term PM2.5 exposure and mortality.
Regardless of the model used, these studies collectively found evidence
of consistent positive associations between long-term PM2.5
exposure and mortality.
    The EPA, therefore, disagrees with commenters that model
specification was not considered when evaluating the epidemiological
evidence used to form causality determinations. The EPA specifically
points out that the process of assessing the scientific quality and
relevance of epidemiological studies includes examining ``important
methodological issues (e.g., lag or time period between exposure and
effects, model specifications, thresholds, mortality displacement)
related to interpretation of the health evidence (U.S. EPA, 2009, p. 1-
9).'' Consistent with the conclusions of the 2004 PM Air Quality
Criteria Document, the EPA recognizes that there is still no clear
consensus at this time as to what constitutes appropriate control of
weather and temporal trends in short-term exposure studies, and that no
single statistical modeling approach is likely to be most appropriate
in all cases (U.S. EPA, 2004, p. 8-238). However, the EPA believes that
the available evidence interpreted in light of these remaining
uncertainties does provide increased confidence relative to the last
review in the reported associations between short- and long-term
PM2.5 exposures and mortality and morbidity effects, alone
and in combination with other pollutants.
(e) Concentration-Response Relationship
    Additionally, commenters questioned the interpretation of the shape
of the

concentration-response relationship, specifically stating that multiple
studies have demonstrated that there is a threshold in the PM-health
effect relationship and that the log-linear model is not biologically
plausible (API, 2012, Attachment 9; ACC, 2012, Appendix A, pp. 7 to 8).
The EPA disagrees with this assertion due to the number of studies
evaluated in the Integrated Science Assessment that continue to support
the use of a no-threshold, log-linear model to most appropriately
represent the PM concentration-response relationship (U.S. EPA, 2009a,
section 2.4.3). While recognizing that uncertainties remain, the EPA
believes that our understanding of this issue for both long- and short-
term exposure studies has advanced since the last review. As discussed
in the Integrated Science Assessment, both long- and short-term
exposure studies have employed a variety of statistical approaches to
examine the shape of the concentration-response function and whether a
threshold exists. While the EPA recognizes that there likely are
individual biological thresholds for specific health responses, the
Integrated Science Assessment concluded the overall evidence from
existing epidemiological studies does not support the existence of
thresholds at the population level, for effects associated with either
long-term or short-term PM exposures within the ranges of air quality
observed in these studies (U.S. EPA, 2009a, section 2.4.3).\61\ The
Integrated Science Assessment concluded that this evidence collectively
supported the conclusion that a no-threshold, log-linear model is most
appropriate (U.S. EPA, 2009a, sections <IP_ADDRESS>, <IP_ADDRESS>, and 7.6.4).
CASAC likewise advised that ``[a]lthough there is increasing
uncertainty at lower levels, there is no evidence of a threshold''
(Samet, 2010d, p. ii).
---------------------------------------------------------------------------

    \61\ While epidemiological analyses have not identified a
population threshold in the range of air quality concentrations
evaluated in these studies, the EPA recognizes that it is possible
that such thresholds exist towards the lower end of these ranges (or
below these ranges).
---------------------------------------------------------------------------

    The EPA recognizes that some short-term exposure studies have
examined the PM2.5 concentration-response relationship in
individual cities or on a city-to-city basis and observed heterogeneity
in the shape of the concentration-response curve across cities. As
discussed in (b) above, these findings are a source of uncertainty that
the EPA agrees requires further investigation. Nonetheless, the
Integrated Science Assessment concluded that ``the studies evaluated
further support the use of a no-threshold, log-linear model, but
additional issues such as the influence of heterogeneity in estimates
between cities and the effects of seasonal and regional differences in
PM on the concentration-response-relationship still require further
investigation'' (U.S. EPA, 2009a, p. 2-25).
(f) Relative Toxicity of PM2.5 Components
    Some commenters highlighted uncertainties in understanding the role
of individual constituents within the mix of fine particles. These
commenters asserted that a mass-based standard may not be appropriate
due to the growing body of evidence indicating that certain
PM2.5 components may be more closely related to specific
health outcomes (e.g., EC and OC) (EPRI, 2012, p. 2).
    With regard to questions about the role of individual constituents
within the mix of fine particles, as a general matter, the EPA
recognizes that although new research directed toward this question has
been conducted since the last review, important questions remain and
the issue remains an important element in the Agency's ongoing research
program. At the time of the last review, the Agency determined that it
was appropriate to continue to control fine particles as a group, as
opposed to singling out any particular component or class of fine
particles (71 FR 61162 to 61164, October 17, 2006). This distinction
was based largely on epidemiological evidence of health effects using
various indicators of fine particles in a large number of areas that
had significant contributions of differing components or sources of
fine particles, together with some limited experimental studies that
provided some evidence suggestive of health effects associated with
high concentrations of numerous fine particle components.
    In this review, as discussed in the proposal (77 FR 38922 to 38923)
and in section III.E.1 below, while most epidemiological studies
continue to be indexed by PM2.5 mass, several recent
epidemiological studies included in the Integrated Science Assessment
have used PM2.5 speciation data to evaluate health effects
associated with fine particle exposures. In the Integrated Science
Assessment, the EPA thoroughly evaluated the scientific evidence that
examined the effect of different PM2.5 components and
sources on a variety of health outcomes (U.S. EPA, 2009a, section 6.6)
and observed that the available information continues to suggest that
many different chemical components of fine particles and a variety of
different types of source categories are all associated with, and
probably contribute to, effects associated with PM2.5. The
Integrated Science Assessment concluded that the current body of
scientific evidence indicated that ``many constituents of PM can be
linked with differing health effects and the evidence is not yet
sufficient to allow differentiation of those constituents or sources
that are more closely related to specific health outcomes'' (U.S. EPA,
2009a, p. 2-26 and 6-212). Furthermore, the Policy Assessment concluded
that the evidence is not sufficient to support eliminating any
component or group of components associated with any specific source
categories from the mix of fine particles included in the
PM2.5 indicator (U.S. EPA, 2009a, p. 2-56). CASAC agreed
that it was reasonable to retain PM2.5 as an indicator for
fine particles in this review as ``[t]here was insufficient peer-
reviewed literature to support any other indicator at this time''
(Samet, 2010c, p. 12).
    This information is relevant to the Agency's decision to retain
PM2.5 as the indicator for fine particles as discussed in
section III.E.1 below. The EPA also believes that it is relevant to the
Agency's conclusion as to whether revision of the suite of primary
PM2.5 standards is appropriate. While there remain
uncertainties about the role and relative toxicity of various
components of fine PM, the current evidence continues to support the
view that fine particles should be addressed as a group for purposes of
public health protection.
    In summary, in considering the above issues related to
uncertainties in the underlying health science, on balance, the EPA
believes that the available evidence interpreted in light of these
remaining uncertainties does provide increased confidence relative to
the last review in the reported associations between long- and short-
term PM2.5 exposures and mortality and morbidity effects,
alone and in combination with other pollutants, and supports stronger
inferences as to the causal nature of the associations. The EPA also
believes that this increased confidence, when taken in context of the
entire body of available health effects evidence and in light of the
evidence from epidemiological studies of associations observed in areas
meeting the current primary PM2.5 standards, specifically in
areas meeting the current primary annual PM2.5 standard,
adds support to its conclusion that the current suite of
PM2.5 standards needs to be revised to provide increased
public health protection.
    (4) In asserting that there is no evidence of greater risk since
the 2006

review to justify lowering the current annual PM2.5
standard, some commenters argued that, ``if the current primary
PM2.5 annual standard of 15 [mu]g/m\3\ was considered to be
adequately protective of public health in 2006, given relative risk
estimates that EPA was using at that time, then that standard would
surely still be adequately protective of the public health if relative
risk estimates remain at the same level (or lower)'' (UARG, 2012,
Attachment 1, p. 24). These commenters compared risk coefficients used
for mortality in the EPA's risk assessment done in the last review with
those from the Agency's core risk assessment done as part of this
review, and they concluded that ``the entire range of the core relative
risk for long-term mortality is lower now than it was in the prior
review'' (UARG, 2012, Attachment 1, p. 24). These commenters used this
conclusion as the basis for a claim that there is no reason to revise
the current annual PM2.5 standard.
    The EPA believes that this claim is fundamentally flawed. In
comparing the scientific understanding of the risk presented by
exposure to PM2.5 between the last and current reviews, one
must examine not only the quantitative estimate of risk from those
exposures (e.g., the numbers of premature deaths or increased hospital
admissions at various concentrations), but also the degree of
confidence that the Agency has that the observed health effects are
causally linked to PM2.5 exposure at those concentrations.
As documented in the Integrated Science Assessment and in the
recommendations and conclusions of CASAC, the EPA recognizes
significant advances in our understanding of the health effects of
PM2.5, based on evidence that is stronger than in the last
review. As a result of these advances, the EPA is now more certain that
fine particles, alone or in combination with other pollutants, present
a significant risk to public health at concentrations allowed by the
current primary PM2.5 standards. From this more
comprehensive perspective, since the risks presented by
PM2.5 are more certain, similar or even somewhat lower
relative risk estimates would not be a basis to conclude that no
revision to the suite of PM2.5 standards is ``requisite'' to
protect public health with an adequate margin of safety. This also
ignores that the relative risk estimate is only one factor considered
by the Administrator, e.g. it ignores that epidemiological studies
since the last review indicate associations between PM2.5
and mortality and morbidity in areas meeting the current annual
standard.
    In any case, the commenters' reliance on the flawed 2006 review is
misplaced. As discussed in section III.A.2 above, the D.C. Circuit
remanded Administrator Johnson's 2006 decision to retain the primary
annual PM2.5 standard because the Agency failed to
adequately explain why the annual standard provided the requisite
protection from both short- and long-term exposure to fine particles
including protection for at-risk populations. The 2006 standard was
also at sharp odds with CASAC advice and recommendations as to the
requisite level of protection (Henderson, 2006a,b). In other words, the
2006 primary annual PM2.5 standard is not an appropriate
benchmark for comparison.
    (5) Some of these commenters also identified ``new'' as well as
older studies that had been included in prior reviews as providing
additional evidence that the causality determinations presented in the
Integrated Science Assessment did not consider the totality of the
scientific literature, further supporting their view that a revision of
the PM2.5 is unwarranted. As discussed in section II.B.3
above, the EPA notes that, as in past NAAQS reviews, the Agency is
basing the final decisions in this review on the studies and related
information included in the Integrated Science Assessment that have
undergone CASAC and public review, and will consider newly published
studies for purposes of decisionmaking in the next PM NAAQS review. In
provisionally evaluating commenters' arguments (see Response to
Comments document), the EPA notes that its provisional assessment of
``new'' science found that such studies did not materially change the
conclusions reached in the Integrated Science Assessment (U.S. EPA,
2012b).
3. Administrator's Final Conclusions Concerning the Adequacy of the
Current Primary PM2.5 Standards
    Having carefully considered the public comments, as discussed
above, the Administrator believes the fundamental scientific
conclusions on the effects of PM2.5 reached in the
Integrated Science Assessment, and discussed in the Policy Assessment,
are valid. In considering whether the suite of primary PM2.5
standards should be revised, the Administrator places primary
consideration on the evidence obtained from the epidemiological
studies. The Administrator believes that this literature, combined with
the other scientific evidence discussed in the Integrated Science
Assessment, collectively represents a strong and generally robust body
of evidence of serious health effects associated with both long- and
short-term exposures to PM2.5. As discussed in the
Integrated Science Assessment and Policy Assessment, the EPA believes
that much progress has been made since the last review in reducing some
of the major uncertainties that were important considerations in
establishing the current suite of PM2.5 standards. In that
context, the Administrator finds the evidence of serious health effects
reported in exposure studies conducted in areas with long-term mean
concentrations ranging from approximately at or above the level of the
annual standard to long-term mean concentrations significantly below
the level of the annual standard to be compelling, especially in light
of the extent to which such studies are part of an overall pattern of
positive and frequently statistically significant associations across a
broad range of studies. The information in the quantitative risk
assessment lends support to this conclusion.
    There has been extensive critical review of this body of evidence,
the quantitative risk assessment, and related uncertainties, including
review by CASAC and the public. The public comments on the basis for
the EPA's proposed decision to revise the suite of primary
PM2.5 standards have identified a number of issues about
which different parties disagree including issues for which additional
research is warranted. Having weighed all comments and the advice of
CASAC, the Administrator believes that since the last review the
overall uncertainty about the public health risks associated with both
long- and short-term exposure to PM2.5 has been diminished
to an important degree. The remaining uncertainties in the available
evidence do not diminish confidence in the associations between
exposure to fine particles and mortality and serious morbidity effects.
Based on her increased confidence in the association between exposure
to PM2.5 and serious public health effects, combined with
evidence of such an association in areas that would meet the current
standards, the Administrator agrees with CASAC that revision of the
current suite of PM2.5 standards to provide increased public
health protection is necessary. Based on these considerations, the
Administrator concludes that the current suite of primary
PM2.5 standards is not sufficient, and thus not requisite,
to protect public health with an

adequate margin of safety, and that revision is needed to increase
public health protection.
    It is important to note that this conclusion, and the reasoning on
which it is based, do not resolve the question of what specific
revisions are appropriate. That requires looking specifically at the
current 24-hour and annual PM2.5 standards, including their
indicator, averaging times, forms, and levels, and evaluating the
scientific evidence and other information relevant to determining the
appropriate revision of the suite of standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator
    In initially setting standards for fine particles in 1997, the EPA
concluded it was appropriate to control fine particles as a group,
rather than singling out any particular component or class of fine
particles. The EPA noted that community health studies had found
significant associations between various indicators of fine particles,
and that health effects in a large number of areas had significant mass
contributions of differing components or sources of fine particles. In
addition, a number of toxicological and controlled human exposure
studies had reported health effects associations with high
concentrations of numerous fine particle components. It was also not
possible to rule out any component within the mix of fine particles as
not contributing to the fine particle effects found in the
epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a
size-based indicator in 1977 to distinguish fine particles from
particles in the coarse mode, the EPA noted that the available
epidemiological studies of fine particles were based largely on
PM2.5 and also considered monitoring technology that was
generally available. The selection of a 2.5 [micro]m size cut reflected
the regulatory importance of defining an indicator that would more
completely capture fine particles under all conditions likely to be
encountered across the U.S., especially when fine particle
concentrations and humidity are likely to be high, while recognizing
that some small coarse particles would also be captured by current
methods to monitor PM2.5 (62 FR 38666 to 38668, July 18,
1997). In the last review, based on the same considerations, the EPA
again recognized that the available information supported retaining the
PM2.5 indicator and remained too limited to support a
distinct standard for any specific PM2.5 component or group
of components associated with any source categories of fine particles
(71 FR 61162 to 61164, October 17, 2006).
    In this current review, the same considerations continue to apply
for selection of an appropriate indicator for fine particles. As an
initial matter, the Policy Assessment recognizes that the available
epidemiological studies linking mortality and morbidity effects with
long- and short-term exposures to fine particles continue to be largely
indexed by PM2.5. For the same reasons discussed in the last
two reviews, the Policy Assessment concluded that it was appropriate to
consider retaining a PM2.5 indicator to provide protection
from effects associated with long- and short-term fine particle
exposures (U.S. EPA, 2011a, p. 2-50).
    The Policy Assessment also considered the expanded body of evidence
available in this review to consider whether there was sufficient
evidence to support a separate standard for ultrafine particles \62\ or
whether there was sufficient evidence to establish distinct standards
focused on regulating specific PM2.5 components or a group
of components associated with any source categories of fine particles
(U.S. EPA, 2011a, section 2.3.1).
---------------------------------------------------------------------------

    \62\ Ultrafine particles, generally including particles with a
mobility diameter less than or equal to 0.1 [micro]m, are emitted
directly to the atmosphere or are formed by nucleation of gaseous
constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
---------------------------------------------------------------------------

    A number of studies available in this review have evaluated
potential health effects associated with short-term exposures to
ultrafine particles. As noted in the Integrated Science Assessment, the
enormous number and larger, collective surface area of ultrafine
particles are important considerations for focusing on this particle
size fraction in assessing potential public health impacts (U.S. EPA,
2009a, p. 6-83). Per unit mass, ultrafine particles may have more
opportunity to interact with cell surfaces due to their greater surface
area and their greater particle number compared with larger particles
(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the
potential for soluble components (e.g., transition metals, organics) to
adsorb to ultrafine particles and potentially cross cell membranes and
epithelial barriers (U.S. EPA, 2009a, p. 6-83). In addition, evidence
available in this review suggests that the ability of particles to
enhance allergic sensitization is associated more strongly with
particle number and surface area than with particle mass (U.S. EPA,
2009a, p. 6-127).
    New evidence, primarily from controlled human exposure and
toxicological studies, expands our understanding of cardiovascular and
respiratory effects related to short-term ultrafine particle exposures.
However, the Policy Assessment concluded that this evidence was still
very limited and largely focused on exposure to diesel exhaust, for
which the Integrated Science Assessment concluded it was unclear
whether the effects observed are due to ultrafine particles, larger
particles within the PM2.5 mixture, or the gaseous
components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition,
the Integrated Science Assessment noted uncertainties associated with
the controlled human exposure studies using concentrated ambient
particle systems which have been shown to modify the composition of
ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 1.5.3).
    The Policy Assessment recognized that there are relatively few
epidemiological studies that have examined potential cardiovascular and
respiratory effects associated with short-term exposures to ultrafine
particles (U.S. EPA, 2011a, p. 2-51). These studies have reported
inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).
    Collectively, in considering the body of scientific evidence
available in this review, the Integrated Science Assessment concluded
that the currently available evidence was suggestive of a causal
relationship between short-term exposures to ultrafine particles and
cardiovascular and respiratory effects. Furthermore, the Integrated
Science Assessment concluded that evidence was inadequate to infer a
causal relationship between short-term exposure to ultrafine particles
and mortality as well as long-term exposure to ultrafine particles and
all outcomes evaluated (U.S. EPA, 2009a, sections 2.3.5, <IP_ADDRESS>,
<IP_ADDRESS>, <IP_ADDRESS>, <IP_ADDRESS>, 7.3.9, <IP_ADDRESS>, <IP_ADDRESS>, and <IP_ADDRESS>;
Table 2-6).
    With respect to our understanding of ambient ultrafine particle
concentrations, at present, there is no national network of ultrafine
particle samplers; thus, only episodic and/or site-specific data sets
exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy Assessment
recognized a national characterization of concentrations, temporal and
spatial patterns, and trends was not possible at this time, and the
availability of ambient ultrafine measurements to support health
studies was extremely limited (U.S. EPA, 2011a, p. 2-51). In general,
measurements of ultrafine particles are highly dependent on monitor
location and, therefore, more subject to exposure error than

accumulation mode particles (U.S. EPA, 2009a, p. 2-22). Furthermore,
the number of ultrafine particles generally decreases sharply downwind
from sources, as ultrafine particles may grow into the accumulation
mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited
studies of ambient ultrafine particle measurements have suggested that
these particles exhibit a high degree of spatial and temporal
heterogeneity driven primarily by differences in nearby source
characteristics (U.S. EPA, 2009a, p. 3-84). Internal combustion engines
and, therefore, roadways are a notable source of ultrafine particles,
so concentrations of these particles near roadways are generally
expected to be elevated (U.S. EPA, 2009a, p. 2-3). Concentrations of
ultrafine particles have been reported to drop off much more quickly
with distance from roadways than fine particles (U.S. EPA, 2009a, p. 3-
84).
    In considering both the currently available health effects evidence
and the air quality data, the Policy Assessment concluded that this
information was still too limited to provide support for consideration
of a distinct PM standard for ultrafine particles (U.S. EPA, 2011a, p.
2-52).
    In addressing the issue of particle composition, the Integrated
Science Assessment concluded that, ``[f]rom a mechanistic perspective,
it is highly plausible that the chemical composition of PM would be a
better predictor of health effects than particle size'' (U.S. EPA,
2009a, p. 6-202). Heterogeneity of ambient concentrations of
PM2.5 constituents (e.g., elemental carbon, organic carbon,
sulfates, nitrates) observed in different geographical regions as well
as regional heterogeneity in PM2.5-related health effects
reported in a number of epidemiological studies are consistent with
this hypothesis (U.S. EPA, 2009a, section 6.6).
    With respect to the availability of ambient measurement data for
fine particle components in this review, the Policy Assessment noted
that there were now more extensive ambient PM2.5 speciation
measurement data available through the Chemical Speciation Network
(CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and
Appendix B, section B.1.3). The Integrated Science Assessment observed
that data from the CSN provided further evidence of spatial and
seasonal variation in both PM2.5 mass and composition among
cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-60;
Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be
related to regional differences in meteorology, sources, and topography
(U.S. EPA, 2009a, p. 2-3).
    The currently available epidemiological, toxicological, and
controlled human exposure studies evaluated in the Integrated Science
Assessment on the health effects associated with ambient
PM2.5 constituents and categories of fine particle sources
used a variety of quantitative methods applied to a broad set of
PM2.5 constituents, rather than selecting a few constituents
a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies have used
measured ambient PM2.5 speciation data, including monitoring
data from the CSN, while all of the controlled human exposure and most
of the toxicological studies have used concentrated ambient particles
and analyzed the constituents therein (U.S. EPA, 2009a, p. 6-203).\63\
The CSN provides PM2.5 speciation measurements generally on
a one-in-three or one-in-six day sampling schedule and, thus, does not
capture data every day at most sites.\64\
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    \63\ Most studies considered between 7 to 20 ambient
PM2.5 constituents, with elemental carbon, organic
carbon, sulfates, nitrates, and metals most commonly measured. Many
of the studies grouped the constituents with various factorization
or source apportionment techniques to examine the relationship
between the grouped constituents and various health effects.
However, not all studies labeled the constituent groupings according
to their presumed source and a small number of controlled human
exposure and toxicological studies did not use any constituent
grouping. These differences across studies substantially limit any
integrative interpretation of these studies (U.S. EPA, 2009a, p. 6-
203).
    \64\ To expand our understanding of the role of specific
PM2.5 components and sources with respect to the observed
health effects, researchers have expressed a strong interest in
having access to PM2.5 speciation measurements collected
more frequently (U.S. EPA, 2011a, p. 2-53, including footnote 47).
---------------------------------------------------------------------------

    The Policy Assessment recognized that several new multi-city
studies evaluating short-term exposures to fine particle constituents
are now available. These studies continued to show an association
between mortality and cardiovascular and/or respiratory morbidity
effects and short-term exposures to various PM2.5 components
including nickel, vanadium, elemental carbon, organic carbon, nitrates,
and sulfates (U.S. EPA, 2011a, section 2.3.1; U.S. EPA, 2009a, sections
<IP_ADDRESS> and 6.6).
    Limited evidence is available to evaluate the health effects
associated with long-term exposures to PM2.5 components
(U.S. EPA, 2009a, section 7.6.2). The Policy Assessment noted the most
significant new evidence was provided by a study that evaluated
multiple PM2.5 components and an indicator of traffic
density in an assessment of health effects related to long-term
exposure to PM2.5 (Lipfert et al., 2006a). Using health data
from a cohort of U.S. military veterans and PM2.5
measurement data from the CSN, Lipfert et al. (2006a) reported positive
associations between mortality and long-term exposures to nitrates,
elemental carbon, nickel, and vanadium as well as traffic density and
peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a,
pp. 7-89 to 7-90).
    With respect to source categories of fine particles potentially
associated with a range of health endpoints, the Integrated Science
Assessment reported that the currently available evidence suggests
associations between cardiovascular effects and a number of specific
PM2.5-related source categories, including oil combustion,
wood or biomass burning, motor vehicle emissions, and crustal or road
dust sources (U.S. EPA, 2009a, section 6.6; Table 6-18). In addition, a
few studies have evaluated associations between PM2.5-
related source categories and mortality. For example, one study
reported an association between mortality and a PM2.5 coal
combustion factor (Laden et al., 2000), while other studies linked
mortality to a secondary sulfate long-range transport PM2.5
source (Ito et al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section
<IP_ADDRESS>). Other studies have looked at different components of
particulate matter. There was less consistency in associations observed
between selected sources of fine particles and respiratory health
endpoints, which may be partially due to the fact that fewer studies
have evaluated respiratory-related outcomes and measures. However,
there was some evidence for PM2.5-related associations with
secondary sulfate and decrements in lung function in asthmatic and
healthy adults (U.S. EPA, 2009a, p. 6-211; Gong et al., 2005; Lanki et
al., 2006). A couple of studies have observed an association between
respiratory endpoints in children and adults with asthma and surrogates
for the crustal/soil/road dust and traffic sources of PM (U.S. EPA,
2009a, p. 6-205; Gent et al., 2009; Penttinen et al., 2006).
    Recent studies have shown that source apportionment methods have
the potential to add useful insights into which sources and/or PM
constituents may contribute to different health effects. Of particular
interest are several epidemiological studies that compared source
apportionment methods and reported consistent results across research
groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al.,
2006; Mar et al., 2006; Thurston et al., 2005).

These studies reported associations between total mortality and
secondary sulfate in two cities for two different lag times. The
sulfate effect was stronger for total mortality in Washington, DC and
for cardiovascular-related mortality in Phoenix (U.S. EPA, 2009a, p. 6-
204). These studies also found some evidence for associations with
mortality and a number of source categories (e.g., biomass/wood
combustion, traffic, copper smelter, coal combustion, sea salt) at
various lag times (U.S. EPA, 2009a, p. 6-204). Sarnat et al. (2008)
compared three different source apportionment methods and reported
consistent associations between emergency department visits for
cardiovascular diseases with mobile sources and biomass combustion as
well as increased respiratory-related emergency department visits
associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-
211).
    Collectively, in considering the currently available evidence for
health effects associated with specific PM2.5 components or
groups of components associated with any source categories of fine
particles as presented in the Integrated Science Assessment, the Policy
Assessment concluded that additional information available in this
review continues to provide evidence that many different constituents
of the fine particle mixture as well as groups of components associated
with specific source categories of fine particles are linked to adverse
health effects (U.S. EPA, 2011a, p. 2-55). However, as noted in the
Integrated Science Assessment, while ``[t]here is some evidence for
trends and patterns that link particular ambient PM constituents or
sources with specific health outcomes * * * there is insufficient
evidence to determine whether these patterns are consistent or robust''
(U.S. EPA, 2009a, p. 6-210). Assessing this information, the Integrated
Science Assessment concluded that ``the evidence is not yet sufficient
to allow differentiation of those constituents or sources that are more
closely related to specific health outcomes'' (U.S. EPA, 2009a, pp. 2-
26 and 6-212). Therefore, the Policy Assessment concluded that the
currently available evidence is not sufficient to support consideration
of a separate indicator for a specific PM2.5 component or
group of components associated with any source category of fine
particles. Furthermore, the Policy Assessment concluded that the
evidence is not sufficient to support eliminating any component or
group of components associated with any source categories of fine
particles from the mix of fine particles included in the
PM2.5 indicator (U.S. EPA, 2011a, p. 2-56).
    The CASAC agreed with the EPA staff conclusions presented in the
Policy Assessment and concluded that it is appropriate to consider
retaining PM2.5 as the indicator for fine particles and
further asserted, ``There [is] insufficient peer-reviewed literature to
support any other indicator at this time'' (Samet, 2010c, p. 12). CASAC
expressed a strong desire for the EPA to ``look ahead to future review
cycles and reinvigorate support for the development of evidence that
might lead to newer indicators that may correlate better with the
health effects associated with ambient air concentrations of PM * * *''
(Samet, 2010c, p 2).
    Consistent with the staff conclusions presented in the Policy
Assessment and CASAC advice, the Administrator proposed to retain
PM2.5 as the indicator for fine particles. Further, the
Administrator provisionally concluded that currently available
scientific information does not provide a sufficient basis for
supplementing mass-based, primary fine particle standards with
standards using a separate indicator for ultrafine particles or a
separate indicator for a specific PM2.5 component or group
of components associated with any source categories of fine particles.
In addition, the Administrator also provisionally concluded that the
currently available scientific information did not provide a sufficient
basis for eliminating any individual component or group of components
associated with any source categories from the mix of fine particles
included in the PM2.5 mass-based indicator.
    The EPA received comparatively few public comments on issues
related to the indicator for fine particles.\65\ Some commenters
emphasized the need to conduct additional research to more fully
understand the effect of specific PM2.5 components and/or
sources on public health. These commenters expressed views about the
importance of evaluating health effect associations with various fine
particle components and types of source categories as a basis for
focusing ongoing and future research to reduce uncertainties in this
area and for considering whether alternative indicator(s) may be
appropriate to consider in future PM NAAQS reviews for standards
intended to protect against the array of health effects that have been
associated with fine particles as indexed by PM2.5. For
example, the PSR encouraged more research and monitoring related to
PM2.5 components and noted the importance of components
associated with coal combustion (PSR, 2012, pp. 5 to 6). EPRI asserted
that ``new'' studies support focusing on EC and OC and encouraged the
EPA to seriously consider the mass-based approach (EPRI, 2012, p. 2).
Likewise, Georgia Mining Association supported additional monitoring
and research efforts related to PM2.5 composition and
specifically encouraged the evaluation of using particle number (e.g.,
particle count) (GMA, 2012, pp. 2 to 3).
---------------------------------------------------------------------------

    \65\ No public comments were submitted regarding the use of a
different size cut for fine particles.
---------------------------------------------------------------------------

    The Administrator agrees with CASAC as well as these commenters
that the results of additional research and monitoring efforts will be
helpful for informing future PM NAAQS reviews. Information from such
studies could also help inform the development of strategies that
emphasize control of specific types of emission sources so as to
address particles of greatest concern to public health. However, based
upon the scientific information considered in the Integrated Science
Assessment as well as the public comments summarized above, the
Administrator continues to take note there is evidence that many
different constituents of the fine particle mixture as well as groups
of components associated with specific sources of fine particles are
linked to adverse health effects. Furthermore, she recognizes that the
evidence is not yet sufficient to differentiate those constituents or
sources that are most closely related to specific health outcomes nor
to exclude any PM2.5 components or sources of fine particles
from the mix of particles included in the PM2.5 indicator.
    Having considered the public comments on this issue, the
Administrator concurs with the Policy Assessment conclusions and CASAC
recommendations and concludes that it is appropriate to retain
PM2.5 as the indicator for fine particles.
2. Averaging Time
    In 1997, the EPA initially set both an annual standard, to provide
protection from health effects associated with both long- and short-
term exposures to PM2.5, and a 24-hour standard to
supplement the protection afforded by the annual standard (62 FR 38667
to 38668, July, 18, 1997). In the last review, the EPA retained both
annual and 24-hour averaging times (71 FR 61164, October 17, 2006).
These decisions were based, in part, on evidence of health effects
related to both long-term (from a year to several years) and short-term
(from less than one day to up to several days) measures of
PM2.5.

    The overwhelming majority of studies conducted since the last
review continue to utilize annual (or multi-year) and 24-hour averaging
times, reflecting the averaging times of the current PM2.5
standards. These studies continue to provide evidence that health
effects are associated with annual and 24-hour averaging times.
Therefore, the Policy Assessment concluded it is appropriate to retain
the current annual and 24-hour averaging times to provide protection
from effects associated with both long- and short-term PM2.5
exposures (U.S. EPA, 2011a, p. 2-57).
    In considering whether the information available in this review
supports consideration of different averaging times for
PM2.5 standards specifically with regard to considering a
standard with an averaging time less than 24 hours to address health
effects associated with sub-daily PM2.5 exposures, the
Policy Assessment noted there continues to be a growing body of studies
that provide additional evidence of effects associated with exposure
periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to
information available in the last review, recent studies provide
additional evidence for cardiovascular effects associated with sub-
daily (e.g., one to several hours) exposure to PM, especially effects
related to cardiac ischemia, vasomotor function, and more subtle
changes in markers of systemic inflammation, hemostasis, thrombosis and
coagulation (U.S. EPA, 2009a, section 6.2). Because these studies have
used different indicators (e.g., PM2.5, PM10,
PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2,
and 4 hours), and health outcomes, it is difficult to draw conclusions
about cardiovascular effects associated specifically with sub-daily
exposures to PM2.5.
    With regard to respiratory effects associated with sub-daily
PM2.5 exposures, the currently available evidence was much
sparser than for cardiovascular effects and continues to be very
limited. The Integrated Science Assessment concluded that for several
studies of hospital admissions or medical visits for respiratory
diseases, the strongest associations were observed with 24-hour average
or longer exposures, not with less than 24-hour exposures (U.S. EPA,
2009a, section 6.3).
    Collectively, the Policy Assessment concluded that this
information, when viewed as a whole, is too unclear, with respect to
the indicator, averaging time and health outcome, to serve as a basis
for consideration of establishing a primary PM2.5 standard
with an averaging time shorter than 24-hours at this time (U.S. EPA,
2011a, p. 2-57).
    With regard to health effects associated with PM2.5
exposure across varying seasons in this review, Bell et al. (2008)
reported higher PM2.5 risk estimates for hospitalization for
cardiovascular and respiratory diseases in the winter compared to other
seasons. In comparison to the winter season, smaller statistically
significant associations were also reported between PM2.5
and cardiovascular morbidity for spring and autumn, and a positive, but
statistically non-significant association was observed for the summer
months. In the case of mortality, Zanobetti and Schwartz (2009)
reported a 4-fold higher effect estimate for PM2.5-
associated mortality for the spring as compared to the winter. Taken
together, these results provided emerging but limited evidence that
individuals may be at greater risk of dying from higher exposures to
PM2.5 in the warmer months and may be at greater risk of
PM2.5-associated hospitalization for cardiovascular and
respiratory diseases during colder months of the year (U.S. EPA, 2011a,
p. 2-58).
    Overall, the Policy Assessment observed that there are few studies
presently available to deduce a general pattern in PM2.5-
related risk across seasons. In addition, these studies utilized 24-
hour exposure periods within each season to assess the
PM2.5-associated health effects and do not provide
information on health effects associated with a season-long exposure to
PM2.5. Due to these limitations in the currently available
evidence, the Policy Assessment concluded that there was no basis to
consider a seasonal averaging time separate from a 24-hour averaging
time.
    Based on the above considerations, the Policy Assessment concluded
that the currently available information provided strong support for
consideration of retaining the current annual and 24-hour averaging
times but does not provide support for considering alternative
averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC
considered it appropriate to retain the current annual and 24-hour
averaging times for the primary PM2.5 standards (Samet,
2010c, pp. 2 to 3). At the time of the proposal, the Administrator
concurred with the staff conclusions and CASAC advice and proposed that
the averaging times for the primary PM2.5 standards should
continue to include annual and 24-hour averages to protect against
health effects associated with long- and short-term exposures.
Furthermore, the Administrator provisionally concluded, consistent with
conclusions reached in the Policy Assessment and by CASAC, that the
currently available information was too limited to support
consideration of alternative averaging times to establish a national
standard with a shorter-than 24-hour averaging time or with a seasonal
averaging time.
    The EPA received no significant public comments on the issue of
averaging time for the PM2.5 primary standards. The
Administrator concurs with recommendations made by CASAC and the staff
conclusions presented in the Policy Assessment and concludes, as
proposed, that it is appropriate to retain the current annual and 24-
hour averaging times for the primary PM2.5 standards to
protect against health effects associated with long- and short-term
exposure periods.
3. 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 the standard. In this review, the EPA considers whether
currently available information supports retaining or revising the
forms for the annual or 24-hour PM2.5 standards.
a. Annual Standard
    In 1997, the EPA established the form of the annual
PM2.5 standard as an annual arithmetic mean, averaged over 3
years, from single or multiple community-oriented monitors. This form
was intended to represent a relatively stable measure of air quality
and to characterize longer-term area-wide PM2.5
concentrations, in conjunction with a 24-hour standard designed to
provide adequate protection against localized peak or seasonal
PM2.5 concentrations. The level of the standard was to be
compared to measurements made at each community-oriented monitoring
site, or, if specific criteria were met, measurements from multiple
community-oriented monitoring sites could be averaged (i.e., spatial
averaging) \66\ (62 FR 38671 to 38672, July 18, 1997). The constraints
were intended to ensure that spatial averaging would not result in
inequities in the level of protection provided by the standard (62 FR
38672, July 18, 1997). This approach was consistent with the
epidemiological studies on which the PM2.5 standard was
primarily based, in which air quality data were generally averaged
across multiple monitors in an

area or were taken from a single monitor that was selected to represent
community-wide exposures.
---------------------------------------------------------------------------

    \66\ Spatial averaging as part of the form of the annual
PM2.5 standard is unique to this standard and is not used
with other PM standards nor with other NAAQS.
---------------------------------------------------------------------------

    In the last review, the EPA tightened the criteria for use of
spatial averaging to provide increased protection for vulnerable
populations exposed to PM2.5. This change was based in part
on an analysis of the potential for disproportionate impacts on
potentially at-risk populations, which found that the highest
concentrations in an area tend to be measured at monitors located in
areas where the surrounding population is more likely to have lower
education and income levels and higher percentages of minority
populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section
<IP_ADDRESS>).
    In this review, as outlined in section III.B above and discussed
more fully in section III.B.3 of the proposal, there now exist more
health data such that the Integrated Science Assessment has identified
persons from lower socioeconomic strata as an at-risk population (U.S.
EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover,
there now exist more years of PM2.5 air quality data than
were available in the last review. Consideration in the Policy
Assessment of the spatial variability across urban areas that was
revealed by this expanded data base has raised questions as to whether
an annual standard that allows for spatial averaging, even within
specified constraints as narrowed in 2006 (71 FR 61165 to 61167,
October 17, 2006), would provide appropriate public health protection.
    In considering the potential for disproportionate impacts on at-
risk populations, the Policy Assessment considered an update of an air
quality analysis conducted for the last review (U.S. EPA, 2011a, pp. 2-
59 to 60; Schmidt, 2011, Analysis A). This analysis focused on
determining whether the spatial averaging provisions, as modified in
2006, could introduce inequities in protection for at-risk populations
exposed to PM2.5. Specifically, the Policy Assessment
considered whether persons of lower socioeconomic status, minority
groups, or different age groups (i.e., children or older adults) are
more likely than the general population to live in areas in which the
monitors recording the highest air quality values in an area are
located. Data used in this analysis included demographic parameters
measured at the Census Block or Census Block Group level, including
percent minority population, percent minority subgroup population,
percent of persons living below the poverty level, percent of persons
18 years of age or older, and percent of persons 65 years of age and
older. In each candidate geographic area, data from the Census Block(s)
or Census Block Group(s) surrounding the location of the monitoring
site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles)
in which the highest air quality value was monitored were compared to
the average of monitored values in the area. This analysis looked
beyond areas that would meet the current spatial averaging criteria and
considered all urban areas (i.e., Core Based Statistical Areas or
CBSAs) with at least two valid annual design value monitors (Schmidt,
2011, Analysis A). Recognizing the limitations of such cross-sectional
analyses, the Policy Assessment observed that the highest
concentrations in an area tend to be measured at monitors located in
areas where the surrounding populations are more likely to live below
the poverty line and to have higher percentage of minorities (U.S. EPA,
2011a, p. 2-60).
    Based upon the analysis described above, the Policy Assessment
concluded that the existing constraints on spatial averaging, as
modified in 2006, may be inadequate to avoid substantially greater
exposures in some areas, potentially resulting in disproportionate
impacts on at-risk populations of persons with lower SES levels as well
as minorities. Therefore, the Policy Assessment concluded that it was
appropriate to consider revising the form of the annual
PM2.5 standard such that it did not allow for the use of
spatial averaging across monitors. In doing so, the level of the annual
PM2.5 standard would be compared to measurements made at the
monitoring site that represents area-wide air quality recording the
highest PM2.5 concentrations \67\ (U.S. EPA, 2011a, p. 2-
60).
---------------------------------------------------------------------------

    \67\ As discussed in section VIII.B.1 below, the EPA is revising
several terms associated with PM2.5 monitor placement.
Specifically, the EPA is revoking the term ``community-oriented''
and replacing it with the term ``area-wide'' monitoring.
---------------------------------------------------------------------------

    The CASAC agreed with staff conclusions that it was ``reasonable''
for the EPA to eliminate the spatial averaging provisions (Samet,
2010d, p. 2). Further, in CASAC's comments on the first draft Policy
Assessment, it noted, ``Given mounting evidence showing that persons
with lower SES levels are a susceptible group for PM-related health
risks, CASAC recommends that the provisions that allow for spatial
averaging across monitors be eliminated for the reasons cited in the
(first draft) Policy Assessment'' (Samet, 2010c, p. 13). In its review
of the second draft Policy Assessment, CASAC recognized ``although much
of the epidemiological research has been conducted using community-wide
averages, several key studies reference the nearest measurement site,
so that some risk estimates are not necessarily biased by the averaging
process. Further, the number of such studies is likely to expand in the
future'' (Samet, 2010d, pp. 1 to 2).
    Only two areas in the country used the initial spatial averaging
provisions for demonstrating attainment with the primary annual
PM2.5 standard set in 1997 (70 FR 19847, April 14, 2005;
U.S. EPA, 2006c). Since these provisions were tightened in 2006, no
area has used spatial averaging to demonstrate attainment. No areas in
the country are currently using the spatial averaging provisions to
demonstrate attainment with the current primary annual PM2.5
standard.
    In considering the Policy Assessment's conclusions based on the
results of the analysis discussed above and concern over the evidence
of potential disproportionate impacts on at-risk populations as well as
CASAC advice, the Administrator proposed to revise the form of the
annual PM2.5 standard to eliminate the use of spatial
averaging. Thus, the Administrator proposed revising the form of the
annual PM2.5 standard to compare the level of the standard
with measurements from each ``appropriate'' monitor in an area \68\
with no allowance for spatial averaging. Thus, for an area with
multiple monitors, the appropriate reporting monitor with the highest
design value would determine the attainment status for that area.
---------------------------------------------------------------------------

    \68\ As discussed in section VIII.B.2.b below, the EPA concludes
that PM2.5 monitoring sites at micro- and middle-scale
locations are comparable to the annual standard if the monitoring
site has been approved by the Regional Administrator as representing
an area-wide location.
---------------------------------------------------------------------------

    Of the commenters noted in section III.D.2 above who supported a
more stringent annual PM2.5 standard, those who commented on
the form of the annual PM2.5 standard supported the EPA's
proposal to eliminate the spatial averaging provisions. These
commenters contended that the EPA's analyses of the potential impacts
of spatial averaging, discussed above and in the proposal (77 FR
38924), demonstrated that the current form results in uneven public
health protection leading to disproportionate impacts on at-risk
populations. Specifically, the ALA and other environmental and public
health commenters contended that ``spatial averaging allows exposure of
people to unhealthy levels of pollution at specific locales even within
an area meeting the standard'' (ALA et al., 2012, p. 23).

These commenters particularly focused on the importance for low-income
and minority populations of eliminating the spatial averaging
provisions. They concluded that spatial averaging ``is an environmental
justice concern because poor people are more likely to live near roads,
depots, factories, ports, and other pollution sources.'' Id. p. 24.
    Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the
elimination of spatial averaging in order to ``avoid potential
disproportionate impacts on at-risk populations'' and to maximize ``the
benefits to public health of reducing the annual PM2.5
standard.'' However, these groups expressed concern that the
elimination of spatial averaging, in combination with the requirement
for near road monitors (as discussed in section VIII.B.3.b.i of the
proposal), would effectively and inappropriately increase the
stringency of the annual PM2.5 standard.
    This concern was also shared by other commenters who disagreed with
the elimination of spatial averaging. For example, the Class of '85 RRG
emphasized concerns about increasing the stringency of the standard
while providing few health benefits if spatial averaging is eliminated,
particularly in combination with the requirement for near-road
monitors. These commenters contended that ``[b]ecause EPA proposes to
use the readings from the highest single worst case monitor (rather
than the average of all community area monitors), and since roadway
monitoring locations will likely be worst case monitors, the proposed
NAAQS will become more stringent without targeting the PM2.5
species most harmful to human health'' (Class of '85 RRG, 2012, p. 6).
    Several commenters also maintained that because spatial averaging
is consistent with how air quality data are considered in the
underlying epidemiological studies, such averaging should not be
eliminated. Specifically, commenters including NAM et al., AFPM, and
ACC pointed out that PM2.5 epidemiological studies use
spatially averaged multi-monitor concentrations, rather than the single
highest monitor, when evaluating health effects. Therefore, these
commenters contended that allowing spatial averaging would make the
PM2.5 standard more consistent with the approaches used in
the epidemiological studies upon which the standard is based. In
addition, some commenters also contended that the EPA failed to
consider whether modifying, rather than eliminating, the constraints on
spatial averaging would have been sufficient to protect the public
health. If so, these commenters argued that ``elimination of spatial
averaging would go beyond what is requisite to protect the public
health'' (NAM et al., 2012, p. 20).
    In considering the public comments on the form of the annual
standard, the EPA recognizes a number of commenters agreed with the
basis for the EPA's proposal to eliminate spatial averaging. While
other commenters expressed disagreement or concern with the proposed
decision to eliminate the spatial averaging provisions, the Agency
notes that these commenters did not challenge the analyses or
considerations that provided the fundamental basis for the
Administrator's proposed decision. Rather, these commenters generally
raised concerns that eliminating the option for spatial averaging would
increase the stringency of the standard, especially in light of
additional monitoring sites in near-road environments (as discussed in
section VIII.B.3.b.1 below).
    The EPA does not agree with the comment that siting some monitors
in near roadway environments makes the standard more stringent or
impermissibly more stringent. As discussed in section VIII.B.3.b.i
below, a significant fraction of the population lives in proximity to
major roads, and these exposures occur in locations that represent
ambient air. Monitoring in such areas does not make the standard more
stringent than warranted, but rather affords the intended protection to
the exposed populations, among them at-risk populations, exposed to
fine particles in these areas. Thus, in cases where monitors in near
roadway environments are deemed to be representative of area-wide air
quality they would be compared to the annual standard (as discussed
more fully in section VIII below). The 24-hour and annual NAAQS are
designed to protect the public with an adequate margin of safety, and
this siting provision is fully consistent with providing the protection
the standard is designed to provide and does not make the standard more
stringent or more stringent than necessary.
    Monitors that are representative of area-wide air quality may be
compared to the annual standard. This is consistent with the use of
monitoring data in the epidemiological studies that provide the primary
basis for determining the level of the annual standard. In addition,
the EPA notes that the annual standard is designed to protect against
both long- and short-term exposures through controlling the broad
distribution of air quality across an area over time.\69\ It is fully
consistent with the protection the standard is designed to provide for
near road monitors to be compared to the annual standard if the monitor
is representative of area-wide air quality. This does not make the
standard either more stringent or impermissibly more stringent.
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    \69\ This is in contrast to the 24-hour standard which is
designed to provide supplemental protection, addressing peak
exposures that might not otherwise be addressed by the annual
standard. Consistent with this, monitors are not required to be
representative of area-wide air quality to be compared to the 24-
hour standard.
---------------------------------------------------------------------------

    In further considering these comments, the EPA notes that the
stringency or level of protection provided by each NAAQS is not based
solely on the form of the standard; rather, the four elements of the
standard that together serve to define each standard (i.e., indicator,
averaging time, form, and level) must be considered collectively in
evaluating the protection afforded by each standard. Therefore, the EPA
considers these comments are also appropriate to discuss collectively
with other issues related to the appropriate level for annual standard,
and are discussed below in sections III.E.4.c-d.
    In reaching a final decision on the form of the annual standard,
the Administrator considers the available analyses, CASAC advice, and
public comments on form as discussed above. She also considers related
issues in the public comments on the level of the annual standard as
discussed in section III.E.4.c below. She notes that even when the
annual PM2.5 standard was first set in 1997, the spatial
averaging provisions included constraints intended to ensure that
inequities in the level of protection would not result. These
constraints on spatial averaging were tightened in the last review,
based on an analysis showing the potential for spatial averaging to
allow higher PM2.5 concentrations in locations where
subgroups within the general population were potentially
disproportionately exposed and hence, at disproportionate risk (e.g.,
low income and minority communities). The Administrator notes that in
proposing to eliminate spatial averaging altogether in this review, she
has relied on further analyses in the current review (Schmidt, 2011,
Analysis A). As discussed above and in the proposal (77 FR 38924),
these analyses showed that the current constraints on spatial averaging
may be inadequate in some areas to avoid substantially greater
exposures for people living near monitors recording the highest
PM2.5 concentrations. Such exposures could result in

disproportionate impacts to at-risk populations, including low-income
populations as well as minority groups.
    On this basis, the Administrator concludes that public health would
not be protected with an adequate margin of safety in all locations, as
required by law, if disproportionately higher exposure concentrations
in at-risk populations such as low income communities as well as
minority communities were averaged together with lower concentrations
measured at other sites in a large urban area. See ALA v. EPA, 134 F.
3d 388, 389 (D.C. Cir., 1998) (``this court has held that `NAAQS must
protect not only average healthy individuals, but also sensitive
citizens such as children,' and `if a pollutant adversely affects the
health of these sensitive individuals, EPA must strengthen the entire
national standard''') and Coalition of Battery Recyclers Association v.
EPA, 604 F 3d. 613, 617 (D.C. Cir., 2010) (``Petitioners' assertion
that the revised lead NAAQS is overprotective because it is more
stringent than necessary to protect the entire population of young U.S.
children ignores that the Clean Air Act allows protection of sensitive
subpopulations.'') In reaching this conclusion, the Administrator
further notes that her concern over possible disproportionate
PM2.5-related health impacts in at-risk populations extends
to populations living near important sources of PM2.5,
including the large populations that live near major roadways.\70\
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    \70\ Section VIII.B.3.b.i below discusses public comments
specifically related to the proposed requirement for near-road
monitors.
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    In light of all of the above considerations, including
consideration of available analyses, CASAC advice, and public comments,
the Administrator concludes that the current form of the annual
PM2.5 standard should be revised to eliminate spatial
averaging provisions. Thus, the level of the revised annual
PM2.5 standard established with this rule will be compared
with measurements from each appropriate monitor in an area, with no
allowance for spatial averaging. The Administrator's conclusions with
regard to the appropriate level of the annual PM2.5 standard
to set in conjunction with this form are discussed below in section
III.E.4.d.
b. 24-Hour Standard
    In 1997, the EPA established the form of the 24-hour
PM2.5 standard as the 98th percentile of 24-hour
concentrations at each population-oriented monitor within an area,
averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The
Agency selected the 98th percentile as an appropriate balance between
adequately limiting the occurrence of peak concentrations and providing
increased stability which, when averaged over 3 years, facilitated
effective health protection through the development of more stable
implementation programs. By basing the form of the standard on
concentrations measured at population-oriented monitoring sites, the
EPA intended to provide protection for people residing in or near
localized areas of elevated concentrations. In the last review, in
conjunction with lowering the level of the 24-hour standard, the EPA
retained this form based in part on a comparison with the 99th
percentile form.\71\
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    \71\ In reaching this final decision, the EPA recognized a
technical problem associated with a potential bias in the method
used to calculate the 98th percentile concentration for this form.
The EPA adjusted the sampling frequency requirement in order to
reduce this bias. Accordingly, the Agency modified the final
monitoring requirements such that areas that are within 5 percent of
the standards are required to increase the sampling frequency to
every day (71 FR 61164 to 61165, October 17, 2006).
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    In revisiting the stability of a 98th versus 99th percentile form
for a 24-hour standard intended to provide supplemental protection for
a generally controlling annual standard, an analysis presented in the
Policy Assessment considered air quality data reported in 2000 to 2008
to update our understanding of the ratio between peak-to-mean
PM2.5 concentrations. This analysis provided evidence that
the 98th percentile value was a more stable metric than the 99th
percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62).
    At the time of the proposal, the Agency recognized that the
selection of the appropriate form of the 24-hour standard includes
maintaining adequate protection against peak 24-hour concentrations
while also providing a stable target for risk management programs,
which serves to provide for the most effective public health protection
in the long run.\72\ As in previous reviews, the EPA recognized that a
concentration-based form, compared to an exceedance-based form, was
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. Further, the Agency provisionally concluded that a
concentration-based form, when averaged over three years, provided an
appropriate balance between limiting peak pollutant concentrations and
providing a stable regulatory target, thus facilitating the development
of more stable implementation programs.
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    \72\ See ATA III, 283 F.3d at 374-376 which concludes that it is
legitimate for the EPA to consider overall stability of the standard
and its resulting promotion of overall effectiveness of NAAQS
control programs in setting a standard that is requisite to protect
the public health.
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    In considering the information provided in the Policy Assessment
and recognizing that the degree of public health protection likely to
be afforded by a standard is a result of the combination of the form
and the level of the standard, the Administrator proposed to retain the
98th percentile form of the 24-hour standard. The Administrator
provisionally concluded that the 98th percentile form represents an
appropriate balance between adequately limiting the occurrence of peak
concentrations and providing increased stability relative to an
alternative 99th percentile form.
    Few public commenters commented specifically on the form of the 24-
hour standard. None of the public commenters raised objections to
continuing the use of a concentration-based form for the 24-hour
standard. Many of the individuals and groups who supported a more
stringent 24-hour PM2.5 standard noted in section III.D.2
above, however, recommended a more restrictive concentration-based
percentile form, specifically a 99th percentile form. The limited
number of these commenters who provided a specific rationale for this
recommendation generally expressed their concern that the 98th
percentile form could allow too many days where concentrations exceeded
the level of the standard, and thus fail to adequately protect public
health. Other public commenters representing state and local air
agencies and industry groups generally supported retaining the current
98th percentile form. In most cases, these groups expressed the overall
view that the current 24-hour PM2.5 standard, including the
form of the current standard, should be retained.
    The EPA notes that the viewpoints represented in this review are
similar to comments submitted in the last review and through various
NAAQS reviews. The EPA recognizes that the selection of the appropriate
form includes maintaining adequate protection against peak 24-hour
values while also providing a stable target for risk management
programs, which serves to provide for the most effective public

health protection in the long run.\73\ Nothing in the commenters' views
has provided a reason to change the Administrator's previous conclusion
regarding the appropriate balance represented in the proposed form of
the 24-hour PM2.5 standard. Therefore, the Administrator
concurs with staff conclusions presented in the Policy Assessment and
CASAC recommendations and concludes that it is appropriate to retain
the 98th percentile form for the 24-hour PM2.5 standard.
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    \73\ As just noted above, it is legitimate for the EPA to
consider promotion of overall effectiveness of risk management
programs designed to attain the NAAQS, including their overall
stability, in setting a standard that is requisite to protect the
public health. The context for the court's discussion in ATA III is
identical to that here; whether to adopt a 98th percentile form for
a 24-hour standard intended to provide supplemental protection for a
generally controlling annual standard.
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4. Level
    In the last review, the EPA selected levels for the annual and the
24-hour PM2.5 standards using evidence of effects associated
with periods of exposure that were most closely matched to the
averaging time of each standard. Thus, as discussed in section III.A.1,
the EPA relied upon evidence from long-term exposure studies as the
principal basis for selecting the level of the annual PM2.5
standard that would protect against effects associated with long-term
exposures. The EPA relied upon evidence from the short-term exposure
studies as the principal basis for selecting the level of the 24-hour
PM2.5 standard that would protect against effects associated
with short-term exposures. As summarized in section III.A.2 above, the
2006 decision to retain the level of the annual PM2.5
standard at 15 [mu]g/m\3\ \74\ was challenged and on judicial review,
the DC Circuit remanded the primary annual PM2.5 standard to
the EPA, finding that EPA's explanation for its approach to setting the
level of the annual standard was inadequate.
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    \74\ Throughout this section, the annual standard levels are
denoted as integer values for simplicity, although, as noted above
in section II.B.1, Table 1, the annual standard level is defined to
one decimal place, such that the current annual standard level is
15.0 [mu]g/m\3\. Alternative annual standard levels discussed in
this section are similarly defined to one decimal place.
---------------------------------------------------------------------------

a. General Approach for Considering Standard Levels
    Building upon the lessons learned in the previous PM NAAQS reviews,
in considering alternative standard levels supported by the currently
available scientific information, the Policy Assessment used an
approach that integrated evidence-based and risk-based considerations,
took into account CASAC advice, and considered the issues raised by the
court in remanding the primary annual PM2.5 standard.
Following the general approach outlined in section III.A.3 above, for
the reasons discussed below, the Policy Assessment concluded it was
appropriate to consider the protection afforded by the annual and 24-
hour standards taken together against mortality and morbidity effects
associated with both long- and short-term PM2.5 exposures.
This was consistent with the approach taken in the review completed in
1997 rather than considering each standard separately, as was done in
the review completed in 2006.
    Beyond looking directly at the relevant epidemiologic evidence, the
Policy Assessment considered the extent to which specific alternative
PM2.5 standard levels were likely to reduce the nature and
magnitude of both long-term exposure-related mortality risk and short-
term exposure-related mortality and morbidity risk (U.S. EPA, 2011a,
section <IP_ADDRESS>; U.S.EPA, 2010a, section 4.2.2). As noted in section
III.C above, patterns of increasing estimated risk reductions were
generally observed as either the annual or 24-hour standard, or both,
were reduced below the level of the current standards (U.S. 2011a,
Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and
5.2.3).
    Based on the quantitative risk assessment, the Policy Assessment
observed, as discussed in section III.A.3, that analyses conducted for
this and previous reviews demonstrated that much, if not most, of the
aggregate risk associated with short-term exposures results from the
large number of days during which the 24-hour average concentrations
are in the low-to mid-range, below the peak 24-hour concentrations
(U.S. EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C
above and in section III.C.3 of the proposal, the Risk Assessment
observed that alternative annual standard levels, when controlling,
resulted in more consistent risk reductions across urban study areas,
thereby potentially providing a more consistent degree of public health
protection (U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk
Assessment noted that the results of simulating alternative suites of
PM2.5 standards including different combinations of
alternative annual and 24-hour standard levels suggested that an
alternative 24-hour standard level can produce additional estimated
risk reductions beyond that provided by an alternative annual standard
alone. However, the degree of estimated risk reduction provided by
alternative 24-hour standard levels was highly variable, in part due to
the choice of rollback approached used (U.S. EPA, 2010a, p. 5-17).
    Based on its review of the second draft Policy Assessment, CASAC
agreed with the EPA staff's general approach for translating the
available epidemiological evidence, risk information, and air quality
information into the basis for reaching conclusions on alternative
standards for consideration. Furthermore, CASAC agreed ``that it is
appropriate to return to the strategy used in 1997 that considers the
annual and the short-term standards together, with the annual standard
as the controlling standard, and the short-term standard supplementing
the protection afforded by the annual standard'' and ``considers it
appropriate to place the greatest emphasis'' on health effects judged
to have evidence supportive of a causal or likely causal relationship
as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).
    Therefore, the Policy Assessment concluded, consistent with
specific CASAC advice, that it was appropriate to set a ``generally
controlling'' annual standard that will lower a wide range of ambient
24-hour concentrations. The Policy Assessment concluded this approach
would likely reduce aggregate risks associated with both long- and
short-term exposures with more consistency than a generally controlling
24-hour standard and would be the most effective and efficient way to
reduce total PM2.5-related population risk and so provide
appropriate protection. The staff believed this approach, in contrast
to one focusing on a generally controlling 24-hour standard, would
likely reduce aggregate risks associated with both long- and short-term
exposures with more consistency and would likely avoid setting national
standards that could result in relatively uneven protection across the
country due to setting standards that were either more or less
stringent than necessary in different geographical areas.
    The Policy Assessment recognized that an annual standard intended
to serve as the primary means for providing protection against effects
associated with both long- and short-term PM2.5 exposures
cannot be expected to offer an adequate margin of safety against the
effects of all short-term PM2.5 exposures. As a result, in
conjunction with a generally controlling annual standard, the Policy
Assessment concluded it was appropriate to

consider setting a 24-hour standard to provide supplemental protection,
particularly for areas with high peak-to-mean ratios possibly
associated with strong local or seasonal sources, or PM2.5-
related effects that may be associated with shorter-than-daily exposure
periods.
    At the time of the proposal, the Administrator agreed with the
approach discussed in the Policy Assessment as summarized in section
III.A.3 above, and supported by CASAC, of considering the protection
afforded by the annual and 24-hour standards taken together for
mortality and morbidity effects associated with both long- and short-
term exposures to PM2.5. Furthermore, based on the evidence
and quantitative risk assessment, the Administrator provisionally
concluded it was appropriate to set a ``generally controlling'' annual
standard that will lower a wide range of ambient 24-hour
concentrations, with a 24-hour standard focused on providing
supplemental protection, particularly for areas with high peak-to-mean
ratios possibly associated with strong local or seasonal sources, or
PM2.5-related effects that may be associated with shorter-
than daily exposure periods. The Administrator provisionally concluded
this approach would likely reduce aggregate risks associated with both
long- and short-term exposures more consistently than a generally
controlling 24-hour standard and would be the most effective and
efficient way to reduce total PM2-5-related population risk.
    The Administrator is mindful that considering what standards are
requisite to protect public health with an adequate margin of safety
requires public health policy judgments that neither overstate nor
understate the strength and limitations of the evidence or the
appropriate inferences to be drawn from the evidence. At the time of
the proposal, in considering how to translate the available information
into appropriate standard levels, the Administrator weighed the
available scientific information and associated uncertainties and
limitations. For the purpose of determining what standard levels were
appropriate to propose, the Administrator recognized, as did the EPA
staff in the Policy Assessment, that there was no single factor or
criterion that comprised the ``correct'' approach to weighing the
various types of available evidence and information, but rather there
were various approaches that were appropriate to consider. The
Administrator further recognized that different evaluations of the
evidence and other information before the Administrator could reflect
placing different weight on the relative strengths and limitations of
the scientific information, and different judgments could be made as to
how such information should appropriately be used in making public
health policy decisions on standard levels. This recognition led the
Administrator to consider various approaches to weighing the evidence
so as to identify appropriate standard levels to propose. In so doing,
the Administrator encouraged extensive public comment on alternative
approaches to weighing the evidence and other information so as to
inform her public health policy judgments before reaching final
decisions on appropriate standard levels.
b. Proposed Decisions on Standard Levels
i. Consideration of the Alternative Standard Levels in the Policy
Assessment
    In recognizing the absence of a discernible population threshold
below which effects would not occur, the Policy Assessment's general
approach for identifying alternative annual standard levels that were
appropriate to consider focused on characterizing the part of the
distribution of PM2.5 concentrations in which we had the
most confidence in the associations reported in the epidemiological
studies and conversely where our confidence in the association became
appreciably lower. The most direct approach to address this issue,
consistent with CASAC advice (Samet, 2010c, p. 10), was to consider
epidemiological studies reporting confidence intervals around
concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based
on a thorough search of the available evidence, the Policy Assessment
identified only one study (Schwartz et al., 2008) that conducted a
multi-model analysis to characterize confidence intervals around the
estimated concentration-response relationship. The Policy Assessment
concluded that this single relevant analysis was too limited to serve
as the principal basis for identifying alternative standard levels in
this review (U.S. EPA, 2011a, p. 2-70).
    The Policy Assessment explored other approaches to characterize the
part of the distributions of long-term mean PM2.5
concentrations that were most influential in generating health effect
estimates in long- and short-term epidemiological studies, and placed
greatest weight on those studies that reported positive and
statistically significant associations (U.S. EPA, 2011a, p. 2-63).
First, as discussed in section III.A.3 above, the Policy Assessment
considered the statistical metric used in previous reviews. This
approach recognized the EPA's views that the strongest evidence of
associations occurs at concentrations around the long-term mean
concentration. Thus, in earlier reviews, the EPA focused on identifying
standard levels that were somewhat below the long-term mean
concentrations reported in PM2.5 epidemiological studies.
The long-term mean concentrations represented air quality data
typically used in epidemiological analyses and provided a direct link
between PM2.5 concentrations and the observed health
effects. Further, these data were available for all long- and short-
term exposure studies analyzed and, therefore, represented the data set
available for the broadest set of epidemiological studies.

    However, consistent with CASAC's comments on the second draft
Policy Assessment \75\ (Samet, 2010d, p. 2), in preparing the final
Policy Assessment, the EPA staff explored ways to take into account
additional information from epidemiological studies, when available
(Rajan et al., 2011). These analyses focused on evaluating different
statistical metrics, beyond the long-term mean concentration, to
characterize the part of the distribution of PM2.5
concentrations in which staff continued to have confidence in the
associations observed in epidemiological studies and below which there
was a comparative lack of data such that the staff's confidence in the
relationship was appreciably less. This would also be the part of the
distribution of PM2.5 concentrations which had the most
influence on generating the health effect estimates reported in
epidemiological studies. As discussed in section III.A.3 above, the
Policy Assessment recognized there was no one percentile value within a
given distribution that was the most appropriate or ``correct'' way to
characterize where our confidence in the associations becomes
appreciably lower. The Policy Assessment concluded that focusing on
concentrations within the lower quartile of a distribution, such as the
range from the 25th to the 10th percentile, was reasonable to consider
as a region within which we begin to have appreciably less confidence
in the associations observed in epidemiological studies.\76\ In the EPA
staff's view, considering lower PM2.5 concentrations, down
to the lowest concentration observed in a study, would be a highly
uncertain basis for selecting alternative standard levels (U.S. EPA,
2009a, p. 2-71).
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    \75\ While CASAC expressed the view that it would be most
desirable to have information on concentration-response
relationships, they recognized that it would also be ``preferable to
have information on the concentrations that were most influential in
generating the health effect estimates in individual studies''
(Samet, 2010d, p. 2).
    \76\ In the last review, staff believed it was appropriate to
consider a level for an annual PM2.5 standard that was
somewhat below the averages of the long-term concentrations across
the cities in each of the key long-term exposures studies,
recognizing that the evidence of an association in any such study
was strongest at and around the long-term average where the data in
the study are most concentrated. For example, the interquartile
range of long-term average concentrations within a study and a range
within one standard deviation around the study mean were considered
reasonable approaches for characterizing the range over which the
evidence of association is strongest (U.S. EPA, 2005, pp. 5-22 to 5-
23). In this review, the Policy Assessment noted the
interrelatedness of the distributional statistics and a range of one
standard deviation around the mean which contains approximately 68
percent of normally distributed data, in that one standard deviation
below the mean falls between the 25th and 10th percentiles (U.S.
EPA, 2011a, p. 2-71).
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    As outlined in section III.A.3 above, the Policy Assessment
recognized that there were two types of population-level information to
consider in identifying the range of PM2.5 concentrations
which have the most influence on generating the health effect estimates
reported in epidemiological studies. The most relevant information to
consider was the number of health events (e.g., deaths,
hospitalizations) occurring within a study population in relation to
the distribution of PM2.5 concentrations likely experienced
by study participants. However, in recognizing that access to health
event data may be restricted, and consistent with advice from CASAC
(Samet 2010d, p. 2), EPA staff also considered the number of
participants within each study area, in relation to the distribution of
PM2.5 concentrations (i.e., study population data), as a
surrogate for health event data.
    In applying this approach, the Policy Assessment focused on
identifying the part of the distribution of PM2.5
concentrations which had the most influence on generating health effect
estimates in epidemiological studies, as discussed in section III.A.3
above. As discussed below, in working with study investigators, the EPA
staff was able to obtain health event data for three large multi-city
studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et
al., 2008) and population data for the same three studies and one
additional long-term exposure study (Miller et al., 2007), as
documented in a staff memorandum (Rajan et al., 2011).\77\ For the
three studies for which both health event and study population data
were available, the EPA staff analyzed the reliability of using study
population data as a surrogate for health event data. Based on these
analyses, the EPA staff recognized that the 10th and 25th percentiles
of the health event and study population distributions are nearly
identical and concluded that the distribution of population data can be
a useful surrogate for event data, providing support for consideration
of the study population data for Miller et al. (2007), for which health
event data were not available (Rajan et al., 2011, Analysis 1 and
Analysis 2, in particular, Table 1 and Figures 1 and 2).
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    \77\ The distributional statistical analysis of population-level
data built upon an earlier analysis that evaluated the distributions
of air quality and associated population data for three long-term
exposure studies and three short-term exposure studies (Schmidt et
al., 2010, Analysis 2).
---------------------------------------------------------------------------

    With regard to the long-term mean PM2.5 concentrations
which are relevant to the first approach, Figures 1 through 3 (U.S.
EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) summarize data available
for multi-city, long- and short-term exposure studies that evaluated
endpoints classified in the Integrated Science Assessment as having
evidence of a causal or likely causal relationship or evidence
suggestive of a causal relationship, showing the studies with long-term
mean PM2.5 concentrations below 17 [mu]g/m\3\.\78\ As
discussed in more detail in section III.E.4.b of the proposal, Figures
1 and 3 summarize the health outcomes evaluated, relative risk
estimates, air quality data, and geographic scope for long- and short-
term exposure studies, respectively, that evaluated mortality (evidence
of a causal relationship); cardiovascular effects (evidence of a causal
relationship); and respiratory effects (evidence of a likely causal
relationship) in the general population, as well as in older adults, an
at-risk population. Figure 2 provides this same summary information for
long-term exposure studies that evaluated respiratory effects (evidence
of a likely causal relationship) in children, an at-risk population, as
well as developmental effects (evidence suggestive of a causal
relationship).
---------------------------------------------------------------------------

    \78\ Additional studies presented and assessed in the Integrated
Science Assessment report effects at higher long-term mean
PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-1,
2-2, 7-6, and 7-7).
    \79\ The long-term mean PM2.5 concentrations reported
by the study authors for the Miller et al. (2007) and Lipfert et al.
(2006a) studies are discussed more fully in the Response to Comments
document (U.S. EPA, 2012a).

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    With regard to consideration of additional information from
epidemiological studies which was relevant to the second approach, the
EPA staff compiled a summary of the range of PM2.5
concentrations

corresponding with the 25th to 10th percentiles of health event or
study population data from the four multi-city studies, for which
distributional statistics are available \80\ (U.S. EPA, 2011a, Figure
2-7; Rajan et al., 2011, Table 1). By considering this approach, one
could focus on the range of PM2.5 concentrations below the
long-term mean ambient concentrations over which we continue to have
confidence in the associations observed in epidemiological studies
(e.g., above the 25th percentile) where commensurate public health
protection could be obtained for PM2.5-related effects and,
conversely, identify the range in the distribution below which our
confidence in the associations is appreciably less, to identify
alternative annual standard levels.
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    \80\ The EPA staff obtained health event data (e.g., number of
deaths, hospitalizations) occurring in a study population for three
multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz,
2009; Bell et al., 2008) and study population data were obtained for
the same three studies and one additional study (Miller et al.,
2007) (U.S. EPA, 2011a, p. 2-71). If health event or study
population data were available for additional studies, the EPA could
employ distributional statistics to identify the broader range of
PM2.5 concentrations that were most influential in
generating health effect estimates in those studies.
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    The mean PM2.5 concentrations associated with the
studies summarized in Figures 1, 2, and 3 and with the distributional
statistics analyses (Rajan et al., 2011) are based on concentrations
averaged across ambient monitors within each area included in a given
study and then averaged across study areas to calculate an overall
study mean concentration, as discussed above. Figure 4, discussed in
more detail in section III.E.4.a of the proposal, summarizes
statistical metrics for those key studies \81\ included in Figures 1,
2, and 3 that provide evidence of positive and generally statistically
significant PM2.5-related effects, which are relevant to the
two approaches for translating epidemiological evidence into potential
standard levels as discussed above. The top of Figure 4 includes
information for long-term exposure studies evaluating health outcomes
classified as having evidence of a causal or likely causal relationship
with PM2.5 exposures (long-term mean PM2.5
concentrations indicated by diamond symbols). The middle of Figure 4
includes information for short-term exposure studies evaluating health
outcomes classified as having evidence of a causal or likely causal
relationship with PM2.5 exposures (long-term mean
PM2.5 concentrations indicated by triangle symbols). The
bottom of Figure 4 includes information for long-term exposures studies
evaluating health outcomes classified as having evidence suggestive of
a causal relationship (long-term mean PM2.5 concentrations
indicated by square symbols). Figure 4 also summarizes the range of
PM2.5 concentrations corresponding with the 25th (indicated
by solid circles) to 10th (indicated by open circles) percentiles of
the health event or study population data from the four multi-city
studies (highlighted in bold text) for which distributional statistics
are available.
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    \81\ Long- and short-term exposure studies considered ``key''
studies for consideration are summarized in Figure 4 and include
those studies observing effects for which the evidence supported a
causal or likely causal association. This figure represents the
subset of multi-city studies included in Figures 1 through 3 that
provided evidence of positive and generally statistically
significant effects associated in whole or in part with more recent
air quality data, generally representing health effects associated
with lower PM2.5 concentrations than had previously been
considered in the last review. The EPA notes that many of these
studies evaluated multiple health endpoints, and not all of the
effects evaluated provided evidence of positive and statistically
significant effects. For purposes of informing the Administrator's
decision on the appropriate standard levels, the Agency considers
the full body of scientific evidence and focuses on those aspects of
the key studies that provided evidence of positive and generally
statistically significant effects.
    \82\ The long-term mean PM2.5 concentrations reported
by the study authors for the Miller et al. (2007) and Lipfert et al.
(2006a) studies are discussed more fully in the Response to Comments
document (U.S. EPA, 2012a).

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[GRAPHIC] [TIFF OMITTED] TR15JA13.003

    In considering the evidence, the Policy Assessment recognized that
NAAQS are standards set so as to provide requisite protection, neither
more nor less stringent than necessary to protect public health, with
an adequate margin of safety. This judgment, ultimately made by the
Administrator, involves weighing the strength of the evidence and the
inherent uncertainties and limitations of that evidence. Therefore,
depending on

the weight placed on different aspects of the evidence and inherent
uncertainties, consideration of different alternative standard levels
could be supported.
    Given the currently available evidence discussed in more detail in
section III.E.4.b of the proposal and considering the various
approaches discussed above, the Policy Assessment concluded it was
appropriate to focus on an annual standard level within a range of
about 12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-
106). As illustrated in Figure 4, the Policy Assessment recognized that
a standard level of 12 [mu]g/m\3\, at the upper end of this range, was
somewhat below the long-term mean PM2.5 concentrations
reported in all the multi-city, long- and short-term exposure studies
that provided evidence of positive and statistically significant
associations with health effects classified as having evidence of a
causal or likely causal relationship, including premature mortality and
hospitalizations and emergency department visits for cardiovascular and
respiratory effects as well as respiratory effects in children.
Further, a level of 12 [mu]g/m\3\ would reflect consideration of
additional population-level information from such epidemiological
studies in that it generally corresponded with approximately the 25th
percentile of the available distributions of health events data in the
studies for which population-level information was available. In
addition, a level of 12 [mu]g/m\3\ would reflect some consideration of
studies that provided more limited evidence of reproductive and
developmental effects, which were suggestive of a causal relationship,
in that it was about at the same level as the lowest long-term mean
PM2.5 concentrations reported in such studies (see Figure
4).
    Alternatively the Policy Assessment recognized that an annual
standard level of 11 [mu]g/m\3\, at the lower end of this range, was
well below the lowest long-term mean PM2.5 concentrations
reported in all multi-city long- and short-term exposure studies that
provide evidence of positive and statistically significant associations
with health effects classified as having evidence of a causal or likely
causal relationship. A level of 11 [mu]g/m\3\ would reflect placing
more weight on the distributions of health event and population data,
in that this level was within the range of PM2.5
concentrations corresponding to the 25th and 10th percentiles of all
the available distributions of such data. In addition, a level of 11
[mu]g/m\3\ was somewhat below the lowest long-term mean
PM2.5 concentrations reported in reproductive and
developmental effects studies that are suggestive of a causal
relationship. Thus, a level of 11 [mu]g/m\3\ would reflect an approach
to translating the available evidence that places relatively more
emphasis on margin of safety considerations and less certain causal
relationships than would a standard set at a higher level. Such a
policy approach would tend to weigh uncertainties in the evidence in
such a way as to avoid potentially underestimating PM2.5-
related risks to public health. Further, recognizing the uncertainties
inherent in identifying any particular point at which our confidence in
reported associations becomes appreciably less, the Policy Assessment
concluded that the available evidence did not provide a sufficient
basis to consider alternative annual standard levels below 11 [mu]g/
m\3\ (U.S. EPA, 2011a, p. 2-81).
    The Policy Assessment also considered the extent to which the
available evidence provided a basis for considering alternative annual
standard levels above 12 [mu]g/m\3\. As discussed below, the Policy
Assessment concluded that it could be reasonable to consider a standard
level up to 13 [mu]g/m\3\ based on a policy approach that weighed
uncertainties in the evidence in such a way as to avoid potentially
overestimating PM2.5-related risks to public health,
especially to the extent that primary emphasis was placed on long-term
exposure studies as a basis for an annual standard level. A level of 13
[mu]g/m\3\ was somewhat below the long-term mean PM2.5
concentrations reported in all but one of the long-term exposure
studies providing evidence of positive and statistically significant
associations with PM2.5-related health effects classified as
having a causal or likely causal relationship. As shown in Figure 4,
the one long-term exposure study with a long-term mean PM2.5
concentration just below 13 [mu]g/m\3\ was the Miller et al., (2007)
study. However, as noted in section III.D.1.a of the proposal and
discussed in more detail in the Response to Comments document, the
Policy Assessment observed that in comparison to other long-term
exposure studies, the Miller et al. study was more limited in that it
was based on only one year of air quality data and the one year was
after the health outcomes were reported (U.S. EPA, 2011a, pp. 2-81 to
2-82). Thus, to the extent that less weight was placed on the Miller et
al. study than on other long-term exposure studies with more robust air
quality data, a level of 13 [mu]g/m\3\ could be considered as being
protective of long-term exposure related effects classified as having a
causal or likely causal relationship. In also considering short-term
exposure studies, however, the Policy Assessment noted that a level of
13 [mu]g/m\3\ was below the long-term mean PM2.5
concentrations reported in most but not all such studies. In
particular, two studies--Burnett et al. (2004) and Bell et al. (2008)--
reported long-term mean PM2.5 concentrations of 12.8 and
12.9 [mu]g/m\3\, respectively. In considering these studies, the Policy
Assessment found no basis to conclude that these two studies were any
more limited or uncertain than the other short-term exposure studies
shown in Figures 3 and 4 (U.S. EPA, 2011a, p. 2-82). On this basis, as
discussed below, the Policy Assessment concluded that consideration of
an annual standard level of 13 [mu]g/m\3\ would have implications for
the degree of protection that would need to be provided by the 24-hour
standard, in order that the suite of PM2.5 standards, taken
together, would provide appropriate protection from effects on public
health related to short-term exposure to PM2.5 (U.S. EPA,
2011a, p. 2-82).
    The Policy Assessment also noted that a standard level of 13 [mu]g/
m\3\ would reflect a judgment that the uncertainties in the
epidemiological evidence as summarized in section III.B above and
discussed in more detail in section III.B.2 of the proposal, including
uncertainties related to the heterogeneity observed in the
epidemiological studies in the eastern versus western parts of the
U.S., the relative toxicity of PM2.5 components, and the
potential role of co-pollutants, are too great to warrant placing any
weight on the distributions of health event and population data that
extend down below the long-term mean concentrations into the lower
quartile of the data. This level would also reflect a judgment that the
evidence from reproductive and developmental effects studies that is
suggestive of a causal relationship was too uncertain to support
consideration of any lower level.
    Beyond evidence-based considerations, the Policy Assessment also
considered the extent to which the quantitative risk assessment
supported consideration of these alternative standard levels or
provided support for lower levels. In considering simulations of just
meeting alternative annual standard levels within the range of 13 to 11
[mu]g/m\3\ (in conjunction with the current 24-hour standard level of
35 [mu]g/m\3\), the Policy Assessment concluded that important public
health improvements are associated with risk

reductions estimated for standard levels of 13 and 12 [mu]g/m\3\ and
noted that the level of 11 [mu]g/m\3\ was not included in the
quantitative risk assessment. The Policy Assessment noted that the
overall confidence in the quantitative risk estimates varied for the
different alternative standard levels evaluated and was stronger for
the higher levels and substantially lower for the lowest level
evaluated (i.e., 10 [mu]g/m\3\). Based on the above considerations, the
Policy Assessment concluded that the quantitative risk assessment
provided support for considering alternative annual standard levels
within a range of 13 to 11 [mu]g/m\3\, but did not provide strong
support for considering lower alternative standard levels (U.S. EPA,
2011a, pp. 2-102 to 2-103).
    Taken together, the Policy Assessment concluded that consideration
of alternative annual standard levels in the range of 13 to 11 [mu]g/
m\3\ may be appropriate. Furthermore, the Policy Assessment concluded
that the currently available evidence most strongly supported
consideration of an alternative annual standard level in the range of
12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-82). The Policy Assessment
concluded that an alternative level within the range of 12 to 11 [mu]g/
m\3\ would more fully take into consideration the available information
from all long- and short-term PM2.5 exposure studies,
including studies of at-risk populations, than would a higher level.
This range also reflected placing weight on information from studies
that helped to characterize the range of PM2.5
concentrations over which we continue to have confidence in the
associations observed in epidemiological studies, as well as the extent
to which our confidence in the associations was appreciably less at
lower concentrations.
    As recognized in sections III.A.3 and III.E.4.a above, 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 is not expected to provide appropriate
protection against the effects of all short-term PM2.5
exposures (unless established at a level so low as to undoubtedly
provide more protection than necessary for long-term exposures). Of
particular concern are areas with high peak-to-mean ratios possibly
associated with strong local or seasonal sources, or PM2.5-
related effects that may be associated with shorter-than-daily exposure
periods. As a result, the Policy Assessment concluded that it was
appropriate to consider alternative 24-hour PM2.5 standard
levels that would supplement the protection provided by an annual
standard.
    As outlined in section III.A.3 above, the Policy Assessment
considered the available evidence from short-term PM2.5
exposure studies, as well as the uncertainties and limitations in that
evidence, to assess the degree to which alternative annual and 24-hour
PM2.5 standards can be expected to reduce the estimated
risks attributed to short-term fine particle exposures. In considering
the available epidemiological evidence, the Policy Assessment took into
account information from multi-city studies as well as single-city
studies. The Policy Assessment considered the distributions of 24-hour
PM2.5 concentrations reported in short-term exposure
studies, focusing on the 98th percentile concentrations to match the
form of the 24-hour standard as discussed in section III.E.3.b above.
In recognizing that the annual and 24-hour standards work together to
provide protection from effects associated with short-term
PM2.5 exposures, the Policy Assessment also considered
information on the long-term mean PM2.5 concentrations from
these studies.
    In addition to considering the epidemiological evidence, the Policy
Assessment considered air quality information, specifically peak-to-
mean ratios using county-level 24-hour and annual design values, to
characterize air quality patterns in areas possibly associated with
strong local or seasonal sources. These patterns helped in
understanding the extent to which different combinations of annual and
24-hour standards would be consistent with the policy goal of setting a
generally controlling annual standard with a 24-hour standard that
provides supplemental protection especially for areas with high peak-
to-mean ratios (U.S. EPA, 2011a, p. 2-14).
    In considering the information provided by the short-term exposure
studies, the Policy Assessment recognized that to the extent these
studies were conducted in areas that likely did not meet one or both of
the current standards, such studies did not help inform the
characterization of the potential public health improvements of
alternative standards set at lower levels. Therefore, in considering
the short-term exposure studies to inform staff conclusions regarding
levels of the 24-hour standard that are appropriate to consider, the
Policy Assessment placed greatest weight on studies conducted in areas
that likely met both the current annual and 24-hour standards.
    With regard to multi-city studies that evaluated effects associated
with short-term PM2.5 exposures, as summarized in Figure 3
above and discussed in more detail in section III.E.4.c of the
proposal, the Policy Assessment noted that, to the extent air quality
distributions were reduced to reflect just meeting the current 24-hour
standard, additional protection would be anticipated for the effects