considering the evidence and its uncertainties would place emphasis on
the following (U.S. EPA, 2011a, section 3.2.3):

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

    This approach to considering the health evidence, air quality
information, and the associated uncertainties would place substantial
weight on the generally positive PM10-2.5 effect estimates
that have been reported for mortality and morbidity, even those effect
estimates that are not statistically significant. The Policy Assessment
concluded that this could be judged appropriate given that consistent
results have been reported across multiple studies using different
approaches to estimate ambient PM10-2.5 concentrations and
that exposure measurement error, which is likely to be larger for
PM10-2.5 than for PM2.5, tends to bias the
results of epidemiological studies toward the null hypothesis, making
it less likely that associations will be detected. Such an approach
would place less weight on the uncertainties and limitations in the
evidence that resulted in the Integrated Science Assessment conclusions
that the evidence is only suggestive of a causal relationship.
    Given all of the above, the Policy Assessment concluded that it
would be appropriate to consider either retaining or revising the
current 24-hour PM10 standard, depending on the approach
taken to considering the available evidence, air quality information,
and the uncertainties and limitations associated with that evidence and
information (U.S. EPA, 2011a, section 3.2.3).

2. Consideration of Potential Alternative Standards in the Policy
Assessment

    Given the conclusion that it would be appropriate to consider
either retaining or revising the current PM10 standard, the
Policy Assessment also considered what potential alternative standards,
if any, could be supported by the available scientific evidence in
order to increase public health protection against exposures to
PM10-2.5. The Policy Assessment considered such potential
alternative standards defined in terms of the elements of a standard
(i.e., indicator, averaging time, form, and level). Key conclusions
from the Policy Assessment regarding indicator, averaging time, and
form included the following:

    (1) A PM10 indicator would continue to appropriately
target protection against thoracic coarse particle exposures to
those locations where the evidence is strongest for associations
with adverse health effects (i.e., urban areas).
    (2) The available evidence supports the importance of
maintaining a standard that protects against short-term exposures to
all thoracic coarse particles. Given that the majority of this
evidence is based on 24-hour average thoracic coarse particle
concentrations, consideration of a 24-hour averaging time remains
appropriate.
    (3) Given the limited body of evidence supporting
PM10-2.5-related effects following long-term exposures,
which resulted in the Integrated Science Assessment conclusion that
the available evidence is ``inadequate'' to infer a causal
relationship between long-term PM10-2.5 exposures and a
variety of health effects, consideration of an annual thoracic
coarse particle standard is not supported at this time.
    (4) To the extent it is judged appropriate to revise the current
24-hour PM10 standard, it would be appropriate to
consider revising the form to the 3-year average of the 98th
percentile of the annual distribution of 24-hour PM10
concentrations.

    In considering the available evidence and air quality information
within the context of identifying potential alternative standard levels
for consideration (assuming a decision were made that it is appropriate
to amend the standard), the Policy Assessment first noted that a
standard level as high as about 85 [mu]g/m\3\, for a 24-hour
PM10 standard with a 98th percentile form, could be
supported. Based on considering air quality concentrations in study
locations, the Policy Assessment noted that such a standard level would
be expected to maintain PM10 and PM10-2.5
concentrations below those present in U.S. locations of single-city
studies where PM10-2.5 effect estimates have been reported
to be positive and statistically significant and below those present in
some locations where single-city studies reported PM10-2.5
effect estimates that were positive, but not statistically significant.
These include some locations likely to have met the current
PM10 standard during the study periods (U.S. EPA, 2011a,
section 3.3.4).
    The Policy Assessment also noted that, based on analysis of the
number of people living in counties that could violate the current and
potential alternative PM10 standards, a 24-hour
PM10 standard with a 98th percentile form and a level
between 75 and 80 [mu]g/m\3\ would provide a level of public health
protection that is generally equivalent, across the U.S., to that
provided by the current standard. Given this, the Policy Assessment
concluded that it would be appropriate to consider standard levels in
the range of approximately 75 to 80 [mu]g/m\3\ (with a 98th percentile
form), to the extent population counts were emphasized in comparing the
public health protection provided by the current and potential
alternative standards and to the extent it was judged appropriate to
set a revised standard providing at least the level of public health
protection that is provided by the current standard, based on such
population counts (U.S. EPA, 2011a, section 3.3.4).

    The Policy Assessment also concluded that alternative approaches to
considering the evidence could lead to consideration of standard levels
below 75 [mu]g/m\3\ for a standard with a 98th percentile form. For
example, a number of single-city epidemiological studies have reported
positive, though not statistically significant, PM10-2.5
effect estimates in locations with 98th percentile PM10
concentrations below 75 [mu]g/m\3\. Given that exposure error is
particularly important for PM10-2.5 epidemiological studies
and can bias the results of these studies toward the null hypothesis
(see section IV.B above), the Policy Assessment noted that it could be
judged appropriate to place more weight on positive associations
reported in these epidemiological studies, even when those associations
are not statistically significant. In addition, the Policy Assessment
noted that multi-city averages of 98th percentile PM10
concentrations in the locations evaluated by U.S. multi-city studies of
thoracic coarse particles (Zanobetti and Schwartz, 2009; Peng et al.,
2008) were near or below 75 ppb. Despite uncertainties in the extent to
which effects reported in multi-city studies are associated with the
short-term air quality in any particular location, the Policy
Assessment noted that emphasis could be placed on these multi-city
averaged concentrations. The Policy Assessment concluded that, to the
extent more weight is placed on single-city studies reporting positive,
but not statistically significant, PM10-2.5 effect estimates
and on multi-city studies, it could be appropriate to consider standard
levels as low as 65 [mu]g/m\3\ with a 98th percentile form (U.S. EPA,
2011a, section 3.3.4).
    In considering potential alternative standard levels below 65
[mu]g/m\3\, the Policy Assessment noted that the overall body of
PM10-2.5 health evidence is relatively uncertain, with
somewhat stronger support in U.S. studies for associations with
PM10-2.5 in locations with 98th percentile PM10
concentrations above 85 [mu]g/m\3\ than in locations with 98th
percentile PM10 concentrations below 65 [mu]g/m\3\. In light
of the limitations in the evidence for a relationship between
PM10-2.5 and adverse health effects in locations with
relatively low PM10 concentrations, along with the overall
uncertainties in the body of PM10-2.5 health evidence as
described above and in the Integrated Science Assessment, the Policy
Assessment concluded that consideration of standard levels below 65
[mu]g/m\3\ was not appropriate (U.S. EPA, 2011a, section 3.3.4).

D. CASAC Advice

    Following their review of the first and second draft Policy
Assessments, CASAC provided advice and recommendations regarding the
current and potential alternative standards for thoracic coarse
particles (Samet, 2010c,d). With regard to the existing PM10
standard, CASAC concluded that ``the current data, while limited, is
sufficient to call into question the level of protection afforded the
American people by the current standard'' (Samet, 2010d, p. 7). In
drawing this conclusion, CASAC noted the positive associations in
multi-city and single-city studies, including in locations with
PM10 concentrations below those allowed by the current
standard. In addition, CASAC gave ``significant weight to studies that
have generally reported that PM10-2.5 effect estimates
remain positive when evaluated in co-pollutant models'' and concluded
that ``controlled human exposure PM10-2.5 studies showing
decreases in heart rate variability and increases in markers of
pulmonary inflammation are deemed adequate to support the plausibility
of the associations reported in epidemiologic studies'' (Samet, 2010d,
p. 7).\127\ Given all of the above conclusions CASAC recommended that
``the primary standard for PM10 should be revised'' (Samet,
2010d, p. ii and p. 7). In discussing potential revisions, while CASAC
noted that the scientific evidence supports adoption of a standard at
least as stringent as the current standard, they recommended revising
the current standard in order to increase public health protection. In
considering potential alternative standards, CASAC drew conclusions and
made recommendations in terms of the major elements of a standard:
indicator, averaging time, form, and level.
---------------------------------------------------------------------------

    \127\ Nonetheless, CASAC endorsed the Integrated Science
Assessment weight of evidence conclusions for PM10-2.5
(i.e., that the evidence is only ``suggestive'' of a causal
relationship between short-term exposures and mortality, respiratory
effects, and cardiovascular effects) (Samet, 2009e; Samet, 2009f).
---------------------------------------------------------------------------

    The CASAC agreed with the EPA staff's conclusions that the
available evidence supports consideration in the current review of
retaining the current PM10 indicator and the current 24-hour
averaging time (Samet, 2010c, Samet, 2010d). Specifically, with regard
to indicator, CASAC concluded that ``[w]hile it would be preferable to
use an indicator that reflects the coarse PM directly linked to health
risks (PM10-2.5), CASAC recognizes that there is not yet
sufficient data to permit a change in the indicator from
PM10 to one that directly measures thoracic coarse
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously
recommends the implementation of plans for the deployment of a network
of PM10-2.5 sampling systems so that future epidemiological
studies will be able to more thoroughly explore the use of
PM10-2.5 as a more appropriate indicator for thoracic coarse
particles'' (Samet, 2010d, p. 7).
    The CASAC also agreed that the evidence supports consideration of a
potential alternative form. Specifically, CASAC ``felt strongly that it
is appropriate to change the statistical form of the PM10
standard to a 98th percentile'' (Samet, 2010d, p.7). In reaching this
conclusion, CASAC noted that ``[p]ublished work has shown that the
percentile form has greater power to identify non-attainment and a
smaller probability of misclassification relative to the expected
exceedance form of the standard'' (Samet, 2010d. p. 7).
    With regard to standard level, in conjunction with a 98th
percentile form, CASAC concluded that ``alternative standard levels of
85 and 65 [mu]g/m\3\ (based on consideration of 98th percentile
PM10 concentration) could be justified'' (Samet, 2010d,
p.8). However, in considering the evidence and uncertainties, CASAC
recommended a standard level from the lower part of the range discussed
in the Policy Assessment, recommending a level ``somewhere in the range
of 75 to 65 [mu]g/m\3\'' (Samet, 2010d, p. ii).
    In making this recommendation, CASAC noted that the number of
people living in counties with air quality not meeting the current
standard is approximately equal to the number living in counties that
would not meet a 98th percentile standard with a level between 75 and
80 [mu]g/m\3\. CASAC used this information as the basis for their
conclusion that a 98th percentile standard between 75 and 80 [mu]g/m\3\
would be ``comparable to the degree of protection afforded to the
current PM10 standard'' (Samet, 2010d, p. ii). Given this
conclusion regarding the comparability of the current and potential
alternative standards, as well as their conclusion on the public health
protection provided by the current standard (i.e., that available
evidence is sufficient to call it into question), CASAC recommended a
level within a range of 75 to 65 [mu]g/m\3\ in order to increase public
health protection, relative to that provided by the current standard
(Samet 2010d, p. ii).

E. Administrator's Proposed Conclusions Concerning the Adequacy of the
Current Primary PM10 Standard

    In considering the evidence and information as they relate to the
adequacy of the current 24-hour PM10 standard, the
Administrator first noted in the proposal that this standard is meant
to protect the public health against effects associated with short-term
exposures to PM10-2.5. In the last review, it was judged
appropriate to maintain such a standard given the ``growing body of
evidence suggesting causal associations between short-term exposure to
thoracic coarse particles and morbidity effects, such as respiratory
symptoms and hospital admissions for respiratory diseases, and possibly
mortality'' (71 FR 61185, October 17, 2006). Given the continued
expansion in the body of scientific evidence linking short-term
PM10-2.5 to health outcomes such as premature death and
hospital visits, discussed in detail in the Integrated Science
Assessment (U.S. EPA, 2009a, Chapter 6) and summarized in the proposal,
the Administrator provisionally concluded that the available evidence
continued to support the appropriateness of maintaining a standard to
protect the public health against effects associated with short-term
(e.g., 24-hour) exposures to all PM10-2.5. In drawing
provisional conclusions in the proposal as to whether the current
PM10 standard remains requisite (i.e., neither more nor less
stringent than necessary) to protect public health with an adequate
margin of safety against such exposures, the Administrator considered
the following:

    (1) The extent to which it is appropriate to maintain a standard
that provides some measure of protection against all
PM10-2.5, regardless of composition or source of origin;
    (2) The extent to which it is appropriate to retain a
PM10 indicator for a standard meant to protect against
exposures to ambient PM10-2.5; and
    (3) The extent to which the current PM10 standard
provides an appropriate degree of public health protection.

    With regard to the first point, the proposal noted the conclusion
from the last review that dosimetric, toxicological, occupational, and
epidemiological evidence supported retention of a primary standard to
provide some measure of protection against short-term exposures to all
thoracic coarse particles, regardless of their source of origin or
location, consistent with the Act's requirement that primary NAAQS
provide requisite protection with an adequate margin of safety (71 FR
61197). In that review, the EPA concluded that PM from a number of
source types, including motor vehicle emissions, coal combustion, oil
burning, and vegetative burning, are associated with health effects
(U.S. EPA, 2004). This information formed part of the basis for the
D.C. Circuit's holding that it was appropriate for the thoracic coarse
particle standard to provide ``some protection from exposure to
thoracic coarse particles * * * in all areas'' (American Farm Bureau
Federation v. EPA, 559 F. 3d at 532-33).
    In considering this issue in the proposal, the Administrator judged
that the expanded body of scientific evidence in this review provides
even more support for a standard that protects against exposures to all
thoracic coarse particles, regardless of their location or source of
origin. Specifically, the Administrator noted that epidemiological
studies have reported positive associations between PM10-2.5
and mortality or morbidity in a large number of cities across North
America, Europe, and Asia, encompassing a variety of environments where
PM10-2.5 sources and composition are expected to vary
widely. See 77 FR 38959. In considering this evidence, the Integrated
Science Assessment concluded that ``many constituents of PM can be
linked with differing health effects'' (U.S. EPA, 2009a, p. 2-26).
While PM10-2.5 in most of these study areas is of largely
urban origin, the Administrator noted that some recent studies have
also linked mortality and morbidity with relatively high ambient
concentrations of thoracic coarse particles of non-urban crustal
origin. In considering these studies, she noted the Integrated Science
Assessment's conclusion that ``PM (both PM2.5 and
PM10-2.5) from crustal, soil or road dust sources or PM
tracers linked to these sources are associated with cardiovascular
effects'' (U.S. EPA, 2009a, p. 2-26).
    In light of this body of available evidence reporting
PM10-2.5-associated health effects across different
locations with a variety of sources, as well as the Integrated Science
Assessment's conclusions regarding the links between adverse health
effects and PM sources and composition, the Administrator provisionally
concluded in the proposal that it is appropriate to maintain a standard
that provides some measure of protection against exposures to all
thoracic coarse particles, regardless of their location, source of
origin, or composition (77 FR 38959-60).
    With regard to the second point, in considering the appropriateness
of a PM10 indicator for a standard meant to provide such
public health protection, the Administrator noted that the rationale
used in the last review to support the unqualified PM10
indicator (see above) remains relevant in the current review.
Specifically, as an initial consideration, she noted that
PM10 mass includes both coarse PM (PM10-2.5) and
fine PM (PM2.5). As a result, the concentration of
PM10-2.5 allowed by a PM10 standard set at a
single level declines as the concentration of PM2.5
increases. At the same time, the Administrator noted that
PM2.5 concentrations tend to be higher in urban areas than
in rural areas (U.S. EPA, 2005, p. 2-54, and Figures 2-23 and 2-24)
and, therefore, a PM10 standard will generally allow lower
PM10-2.5 concentrations in urban areas than in rural areas.
77 FR 38960.
    In considering the appropriateness of this variation in allowable
PM10-2.5 concentrations, the Administrator considered the
relative strength of the evidence for health effects associated with
PM10-2.5 of urban origin versus non-urban origin. She
specifically noted that, as described above and similar to the
scientific evidence available in the last review, the large majority of
the available evidence for thoracic coarse particle health effects
comes from studies conducted in locations with sources more typical of
urban and industrial areas than of rural areas. Although as just noted,
associations with adverse health effects have been reported in some
study locations where PM10-2.5 is largely non-urban in
origin (i.e., in dust storm studies), particle concentrations in these
study areas are typically much higher than reported in study locations
where the PM10-2.5 is of urban origin. Therefore, the
Administrator noted that the strongest evidence for a link between
PM10-2.5 and adverse health impacts, particularly for such a
link at relatively low particle concentrations, comes from studies
where exposure is to PM10-2.5 of urban or industrial origin.
77 FR 38960.
    The Administrator also noted that chemical constituents present at
higher levels in urban or industrial areas, including byproducts of
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted
as PM2.5 from motor vehicles as well as metals and other
contaminants emitted from anthropogenic sources, can contaminate
PM10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665). While the
Administrator acknowledged the uncertainty expressed in the Integrated
Science Assessment regarding the extent to which, based on available
evidence, particle composition can be linked to health outcomes, she
also considered the possibility that PM10-2.5 contaminants
typical of urban or industrial areas could increase the

toxicity of thoracic coarse particles in urban locations (77 FR 38960).
    Given that the large majority of the evidence for
PM10-2.5 toxicity, particularly at relatively low particle
concentrations, comes from study locations where thoracic coarse
particles are of urban origin, and given the possibility that
PM10-2.5 contaminants in urban areas could increase particle
toxicity, the Administrator provisionally concluded in the proposal
that it remains appropriate to maintain a standard that targets public
health protection to urban locations. Specifically, she concluded at
proposal that it is appropriate to maintain a standard that allows
lower ambient concentrations of PM10-2.5 in urban areas,
where the evidence is strongest that thoracic coarse particles are
linked to mortality and morbidity, and higher concentrations in non-
urban areas, where the public health concerns are less certain. Id.
    Given all of the above considerations and conclusions, the
Administrator judged that the available evidence supported retaining a
PM10 indicator for a standard that is meant to protect
against exposure to thoracic coarse particles. In reaching this initial
judgment, she noted that, to the extent a PM10 indicator
results in lower allowable concentrations of thoracic coarse particles
in some areas compared to others, lower concentrations will be allowed
in those locations (i.e., urban or industrial areas) where the science
has shown the strongest evidence of adverse health effects associated
with exposure to thoracic coarse particles and where we have the most
concern regarding PM10-2.5 toxicity. Therefore, the
Administrator provisionally concluded that the varying amounts of
coarse particles that are allowed in urban vs. non-urban areas under
the 24-hour PM10 standard, based on the varying levels of
PM2.5 present, appropriately reflect the differences in the
strength of evidence regarding coarse particle effects in urban and
non-urban areas (77 FR 38960).
    In reaching this provisional conclusion, the Administrator also
noted that, in their review of the second draft Policy Assessment,
CASAC concluded that ``[w]hile it would be preferable to use an
indicator that reflects the coarse PM directly linked to health risks
(PM10-2.5), CASAC recognizes that there is not yet
sufficient data to permit a change in the indicator from
PM10 to one that directly measures thoracic coarse
particles'' (Samet, 2010d, p. ii). In addition, CASAC ``vigorously
recommends the implementation of plans for the deployment of a network
of PM10-2.5 sampling systems so that future epidemiological
studies will be able to more thoroughly explore the use of
PM10-2.5 as a more appropriate indicator for thoracic coarse
particles'' (Samet, 2010d, p. 7). Given this recommendation, the
Administrator further judged that, although current evidence is not
sufficient to identify a standard based on an alternative indicator
that would be requisite to protect public health with an adequate
margin of safety across the United States, consideration of alternative
indicators (e.g., PM10-2.5) in future reviews is desirable
and could be informed by additional research, as described in the
Policy Assessment (U.S. EPA, 2011a, section 3.5).
    With regard to the third point, in evaluating the degree of public
health protection provided by the current PM10 standard, the
Administrator noted that the Policy Assessment discussed two different
approaches to considering the scientific evidence and air quality
information (U.S. EPA, 2011a, section 3.2.3). These different
approaches, which are described above (section IV.C.1), lead to
different conclusions regarding the appropriateness of the degree of
public health protection provided by the current PM10
standard. The Administrator further noted that the primary difference
between the two approaches lies in the extent to which weight is placed
on the following (U.S. EPA, 2011a, section 3.2.3):

    (1) The PM10-2.5 weight-of-evidence classifications
presented in the Integrated Science Assessment concluding that the
existing evidence is suggestive of a causal relationship between
short-term PM10-2.5 exposures and mortality,
cardiovascular effects, and respiratory effects (a classification
supported by CASAC);
    (2) Individual PM10-2.5 epidemiological studies
reporting associations in locations that meet the current
PM10 standard, including associations that are not
statistically significant;
    (3) The limited number of PM10-2.5 epidemiological
studies that have evaluated co-pollutant models;
    (4) The limited number of PM10-2.5 controlled human
exposure studies;
    (5) Uncertainties in the PM10-2.5 air quality
concentrations reported in epidemiological studies, given
limitations in PM10-2.5 monitoring data and the different
approaches used across studies to estimate ambient
PM10-2.5 concentrations; and
    (6) Uncertainties and limitations in the evidence that tend to
call into question the presence of a causal relationship between
PM10-2.5 exposures and mortality/morbidity.

    In evaluating the different possible approaches to considering the
public health protection provided by the current PM10
standard, the Administrator first noted that when the available
PM10-2.5 scientific evidence and its associated
uncertainties are considered, the Integrated Science Assessment
concluded that the evidence is suggestive of a causal relationship
between short-term PM10-2.5 exposures and mortality,
cardiovascular effects, and respiratory effects. As discussed in
section IV.B.1 above and in more detail in the Integrated Science
Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination
is made when the ``[e]vidence is suggestive of a causal relationship
with relevant pollutant exposures, but is limited because chance, bias
and confounding cannot be ruled out.'' In contrast, the Administrator
noted that she proposed to strengthen the annual fine particle standard
based on a body of scientific evidence judged sufficient to conclude
that a causal relationship exists (i.e., mortality, cardiovascular
effects) or is likely to exist (i.e., respiratory effects) (section
III.B). 77 FR 38961. The suggestive judgment for PM10-2.5
reflects the greater degree of uncertainty associated with this body of
evidence, as discussed above (sections IV.B and IV.C) and summarized
below.
    In the proposal (77 FR 38961), the Administrator noted that the
important uncertainties and limitations associated with the scientific
evidence and air quality information raise questions as to whether
public health benefits would be achieved by revising the existing
PM10 standard. Such uncertainties and limitations include
the following:

    (1) While PM10-2.5 effect estimates reported for
mortality and morbidity were generally positive, most were not
statistically significant, even in single-pollutant models. This
includes effect estimates reported in some study locations with
PM10 concentrations above those allowed by the current
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding,
particularly by PM2.5, remains limited. Therefore, the
extent to which PM10-2.5 itself, rather than one or more
co-pollutants, contributes to reported health effects is less
certain.
    (3) Only a limited number of experimental studies (i.e.,
controlled human exposure and animal toxicological) provide support
for the associations reported in epidemiological studies, resulting
in further uncertainty regarding the plausibility of the
associations between PM10-2.5 and mortality and morbidity
reported in epidemiological studies.
    (4) Limitations in PM10-2.5 monitoring data and the
different approaches used by epidemiological study researchers to
estimate PM10-2.5 concentrations across epidemiological
studies result in uncertainty in the ambient PM10-2.5
concentrations at which the reported effects occur, increasing
uncertainty in estimates of the extent to

which changes in ambient PM10-2.5 concentrations would
likely impact public health.
    (5) The lack of a quantitative PM10-2.5 risk
assessment further contributes to uncertainty regarding the extent
to which any revisions to the current PM10 standard would
be expected to improve the protection of public health, beyond the
protection provided by the current standard (see section III.B.5
above).
    (6) The chemical and biological composition of
PM10-2.5, and the effects associated with the various
components, remains uncertain. Without more information on the
chemical speciation of PM10-2.5, the apparent variability
in associations across locations is difficult to interpret.

    In considering these uncertainties and limitations, the
Administrator noted in particular the considerable degree of
uncertainty in the extent to which health effects reported in
epidemiological studies are due to PM10-2.5 itself, as
opposed to one or more co-occurring pollutants. As discussed above,
this uncertainty reflects the fact that there are a relatively small
number of PM10-2.5 studies that have utilized co-pollutant
models, particularly co-pollutant models that have included
PM2.5, and a very limited body of controlled human exposure
evidence supporting the biological plausibility of a causal
relationship between PM10-2.5 and mortality and morbidity at
ambient concentrations. The Administrator noted that these important
limitations in the overall body of health evidence introduce
uncertainty into the interpretation of individual epidemiological
studies, particularly those studies reporting associations with
PM10-2.5 that are not statistically significant. Given this,
the Administrator reached the provisional conclusion in the proposal
that it is appropriate to place relatively little weight on
epidemiological studies reporting associations with PM10-2.5
that are not statistically significant in single-pollutant and/or co-
pollutant models. Id.
    With regard to this provisional conclusion, the Administrator noted
that, for single-city mortality studies conducted in the United States
where ambient PM10 concentration data were available for
comparison to the current standard, positive and statistically
significant PM10-2.5 effect estimates were only reported in
study locations that would likely have violated the current
PM10 standard during the study period (U.S. EPA, 2011a,
Figure 3-2). In U.S. study locations that would likely have met the
current standard, PM10-2.5 effect estimates for mortality
were positive, but not statistically significant (U.S. EPA, 2011a,
Figure 3-2). In considering U.S. study loc`ations where single-city
morbidity studies were conducted, and which would likely have met the
current PM10 standard during the study period, the
Administrator noted that PM10-2.5 effect estimates were both
positive and negative, with most not statistically significant (U.S.
EPA, 2011a, Figure 3-3).
    In addition, in considering single-city analyses for the locations
evaluated in a large U.S. multi-city mortality study (Zanobetti and
Schwartz, 2009), the Administrator noted that associations in most of
the study locations were not statistically significant and that this
was the only study to estimate ambient PM10-2.5
concentrations as the difference between county-wide PM10
and PM2.5 mass. As discussed in the Policy Assessment and in
the proposal, it is not clear how computed PM10-2.5
measurements, such as those used by Zanobetti and Schwartz (2009),
compare with the PM10-2.5 concentrations obtained in other
studies either by direct measurement or by calculating the difference
using co-located samplers (U.S. EPA, 2009a, section <IP_ADDRESS>). For these
reasons, in the proposal the Administrator noted that ``there is
considerable uncertainty in interpreting the associations in these
single-city analyses'' (77 FR 38961-62).
    The Administrator acknowledged that an approach to considering the
available scientific evidence and air quality information that
emphasizes the above considerations differs from the approach taken by
CASAC. Specifically, in its review of the draft Policy Assessment CASAC
placed a substantial amount of weight on individual studies,
particularly those reporting positive health effects associations for
PM10-2.5 in locations that met the current PM10
standard during the study period. In emphasizing these studies, as well
as the limited number of supporting studies that have evaluated co-
pollutant models and the small number of supporting experimental
studies, CASAC concluded that ``the current data, while limited, is
sufficient to call into question the level of protection afforded the
American people by the current standard'' (Samet, 2010d, p. 7) and
recommended revising the current PM10 standard (Samet,
2010d).
    The Administrator carefully considered CASAC's advice and
recommendations. She noted that in making its recommendation on the
current PM10 standard, CASAC did not discuss its approach to
considering the important uncertainties and limitations in the health
evidence, and did not discuss how these uncertainties and limitations
were reflected in its recommendation. Nor did CASAC discuss
uncertainties in the reported concentrations of PM10-2.5 in
the epidemiological studies, or how reported concentrations in the
various studies relate to one another when differing measurement
methodologies are used. As discussed above, such uncertainties and
limitations contributed to the conclusions in the Integrated Science
Assessment that the PM10-2.5 evidence is only suggestive of
a causal relationship, a conclusion that CASAC endorsed (Samet,
2009e,f). Given the importance of these uncertainties and limitations
to the interpretation of the evidence, as reflected in the weight of
evidence conclusions in the Integrated Science Assessment and as
discussed above, the Administrator judged it appropriate to consider
and account for them when drawing conclusions about the potential
implications of individual PM10-2.5 health studies for the
current standard.
    In light of the above approach to considering the scientific
evidence, air quality information, and associated uncertainties, the
Administrator reached the following provisional conclusions in the
proposal:

    (1) When viewed as a whole the available evidence and
information suggests that the degree of public health protection
provided against short-term exposures to PM10-2.5 does
not need to be increased beyond that provided by the current
PM10 standard. This provisional conclusion noted the
important uncertainties and limitations associated with the overall
body of health evidence and air quality information for
PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions; that
PM10-2.5 effect estimates for the most serious health
effect, mortality, were not statistically significant in U.S.
locations that met the current PM10 standard and where
coarse particle concentrations were either directly measured or
estimated based on co-located samplers; and that PM10-2.5
effect estimates for morbidity endpoints were both positive and
negative in locations that met the current standard, with most not
statistically significant.
    (2) The degree of public health protection provided by the
current standard is not greater than warranted. This provisional
conclusion noted that positive and statistically significant
associations with mortality were reported in single-city U.S. study
locations likely to have violated the current PM10
standard.\128\
---------------------------------------------------------------------------

    \128\ There are similarities with the conclusions drawn by the
Administrator in the last review. There, the Administrator concluded
that there was no basis for concluding that the degree of protection
afforded by the current PM10 standards in urban areas is
greater than warranted, since potential mortality effects have been
associated with air quality levels not allowed by the current 24-
hour standard, but have not been associated with air quality levels
that would generally meet that standard, and morbidity effects have
been associated with air quality levels that exceeded the current
24-hour standard only a few times (71 FR 61202). In addition, the
Administrator concluded that there was a high degree of uncertainty
in the relevant population exposures implied by the morbidity
studies suggesting that there is little basis for concluding that a
greater degree of protection is warranted. Id. The D.C. Circuit in
American Farm Bureau Federation v EPA explicitly endorsed this
reasoning. 559 F. 3d at 534.

---------------------------------------------------------------------------

    In reaching these provisional conclusions, the Administrator noted
that the Policy Assessment also discussed the potential for a revised
PM10 standard (i.e., with a revised form and level) to be
``generally equivalent'' to the current standard, but to better target
public health protection to locations where there is greater concern
regarding PM10-2.5-associated health effects (U.S. EPA,
2011a, sections 3.3.3 and 3.3.4). In considering such a potential
revised standard, the Policy Assessment discussed the large amount of
variability in PM10 air quality correlations across
monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and
the regional variability in the relative degree of public health
protection that could be provided by the current and potential
alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this
variability, the Administrator noted the Policy Assessment conclusion
that no single revised PM10 standard (i.e., with a revised
form and level) would provide public health protection equivalent to
that provided by the current standard, consistently over time and
across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised
standard, even one that is meant to be ``generally equivalent'' to the
current PM10 standard, could increase protection in some
locations while decreasing protection in others (77 FR 38962).
    In considering the appropriateness of revising the current
PM10 standard in this way, the Administrator noted the
following:

    (1) Positive PM10-2.5 effect estimates for mortality
were not statistically significant in U.S. locations that met the
current PM10 standard and where coarse particle
concentrations were either directly measured or estimated based on
co-located samplers, while positive and statistically significant
associations with mortality were reported in locations likely to
have violated the current PM10 standard.
    (2) Effect estimates for morbidity endpoints in locations that
met the current standard were both positive and negative, with most
not statistically significant.
    (3) Important uncertainties and limitations associated with the
overall body of health evidence and air quality information for
PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions, call
into question the extent to which the type of quantified and refined
targeting of public health protection envisioned under a revised
standard could be reliably accomplished.

    Given all of the above considerations, the Administrator noted that
there is a large amount of uncertainty in the extent to which public
health would be improved by changing the locations to which the
PM10 standard targets protection. Therefore, she reached the
provisional conclusion that the current PM10 standard should
not be revised in order to change that targeting of protection.
    In considering all of the above, including the scientific evidence,
the air quality information, the associated uncertainties, and CASAC's
advice, the Administrator reached the provisional conclusion that the
current 24-hour PM10 standard is requisite (i.e., neither
more protective nor less protective than necessary) to protect public
health with an adequate margin of safety against effects that have been
associated with PM10-2.5. In light of this provisional
conclusion, the Administrator proposed to retain the current
PM10 standard in order to protect against health effects
associated with short-term exposures to PM10-2.5 (77 FR
38963).
    The Administrator recognized that her proposed conclusions and
decision to retain the current PM10 standard differed from
CASAC's recommendations, stemming from the differences in how the
Administrator and CASAC considered and accounted for the evidence and
its limitations and uncertainties. In light of CASAC's views and
recommendation to revise the current PM10 standard, the
Administrator welcomed the public's views on these different approaches
to considering and accounting for the evidence and its limitations and
uncertainties, as well as on the appropriateness of revising the
primary PM10 standard, including revising the form and level
of the standard. In doing so, the Administrator solicited comment on
all aspects of the proposed decision, including her rationale for
reaching the provisional conclusion that the current PM10
standard is requisite to protect public health with an adequate margin
of safety and the provisional conclusion that it is not appropriate to
revise the current PM10 standard by setting a ``generally
equivalent'' standard with the goal of better targeting public health
protection.

F. Public Comments on the Administrator's Proposed Decision To Retain
the Primary PM10 Standard

    This section discusses the major public comments received on the
Administrator's proposed decision to retain the primary PM10 standard.
Additional comments are addressed in the Response to Comments Document
(U.S. EPA, 2012a).
    Many public commenters agreed with the Administrator's proposed
decision to retain the current 24-hour primary PM10
standard. Among those expressing a position on this proposed decision,
industry groups and most State and Local commenters endorsed the
Administrator's proposed rationale for retaining the current primary
PM10 standard, including her consideration of the available
scientific evidence and associated uncertainties and her consideration
of CASAC recommendations.
    Although industry commenters generally agreed with the
Administrator's proposed decision to retain the current primary
PM10 standard, some also contended that the current standard
is ``excessively precautionary'' (NMA and NCBA, 2012, p. 4) and a few
expressed support for a less stringent standard for coarse particles
that are comprised largely of crustal material. For example, the Coarse
Particulate Matter Coalition (CPMC) (2012) and several other industry
commenters recommended that the final decision allow application of a
98th percentile form for the current standard (i.e. with its level of
150 [mu]g/m\3\) in cases where coarse particles consist primarily of
crustal material. Such an approach would allow more yearly exceedances
of the existing standard level than are allowed with the current one-
expected-exceedance form. These industry commenters contended that a
98th percentile form applied in this way would provide appropriate
regulatory relief for areas where the evidence for coarse particle-
related health effects is relatively uncertain.
    In reaching her conclusion that the current primary PM10
standard is requisite to protect public health with an adequate margin
of safety, the Administrator considered the degree of public health
protection provided by the current standard as a whole, including all
elements of that standard (i.e., indicator, averaging time, form,
level). As discussed above and in the following section, this
conclusion reflects the Administrator's judgments that (1) the current
standard appropriately provides some measure of protection against
exposures to all thoracic coarse

particles, regardless of their location, source of origin, or
composition and (2) the current standard appropriately allows lower
ambient concentrations of PM10-2.5 in urban areas, where the
evidence is strongest that thoracic coarse particles are linked to
mortality and morbidity, and higher concentrations in non-urban areas,
where the public health concerns are less certain.
    Because the considerations that led to these judgments, and to the
conclusion that the current primary PM10 standard is
requisite to protect public health, took into account the degree of
public health protection provided by the standard as a whole, it would
not be appropriate to consider revising one element of the standard
(e.g., the form, as suggested by commenters in this case) without
considering the extent to which the other elements of the standard
should also be revised. The change in form requested by industry
commenters, without also lowering the level of the standard, would
markedly reduce the public health protection provided against exposures
to thoracic coarse particles.\129\ However, industry commenters have
not presented new evidence or analyses to support their conclusion that
an appropriate degree of public health protection could be achieved by
allowing the use of an alternative form (i.e., 98th percentile) for
some coarse particles, while retaining the other elements of the
current standard. Nor have these commenters presented new evidence or
analyses challenging the basis for the conclusion in the proposal that
the varying amounts of coarse particles allowed in urban versus non-
urban areas under the current 24-hour PM10 standard, based
on the varying levels of PM2.5 present, appropriately
reflect the differences in the strength of evidence regarding coarse
particle effects in urban and non-urban areas. In light of this, EPA
does not believe that a reduction in public health protection, such as
that requested by industry commenters, is warranted.
---------------------------------------------------------------------------

    \129\ Based on regression analyses presented in the PA (U.S.
EPA, 2011a, Figures 3-7 and 3-8), PM10 one-expected-
exceedance concentration-equivalent design values were between
approximately 175 and 300 [mu]g/m\3\ at monitoring locations
recording 3-year averages of 98th percentile 24-hour PM10
concentrations around 150 [mu]g/m\3\ (i.e., the level of the current
standard). This suggests that, depending on the location, a 24-hour
PM10 standard with a 98th percentile form in conjunction
with the current level (i.e., as recommended by these commenters)
could be ``generally equivalent'' to a 24-hour PM10
standard with a one-expected-exceedance form and a level as high as
approximately 300 [mu]g/m\3\. Based on this analysis, a 24-hour
PM10 standard with a 98th percentile form and a level of
150 [mu]g/m\3\ would be markedly less health protective than the
current standard.
---------------------------------------------------------------------------

    In further considering these comments, it is to be remembered that
epidemiologic studies have not demonstrated that coarse particles of
non-urban origin do not cause health effects, and commenters have not
provided additional evidence on this point. While there are fewer
studies of non-urban coarse particles than of urban coarse particles,
several studies have reported positive and statistically significant
associations between coarse particles of crustal, non-urban origin and
mortality or morbidity (Ostro et al., 2003; Bell et al., 2008; Chan et
al., 2008; Middleton et al., 2008; Perez et al., 2008). These studies
formed part of the basis for the PM Integrated Science Assessment
conclusion that ``recent studies have suggested that PM (both
PM2.5 and PM10-2.5) from crustal, soil or road
dust sources or PM tracers linked to these sources are associated with
cardiovascular effects'' (U.S. EPA, 2009a, p. 2-26). Moreover, crustal
coarse particles may be contaminated with toxic trace elements and
other components from previously deposited fine PM from ubiquitous
sources such as mobile source engine exhaust, as well as by toxic
metals from smelters or other industrial activities, animal waste, or
pesticides (U.S. EPA, 2004, p. 8-344). In the proposal, the
Administrator acknowledged the potential for this type of contamination
to increase the toxicity of coarse particles of crustal, non-urban
origin (77 FR 38960; see also 71 FR 61190).
    In suggesting a change in the form of the current standard,
industry commenters also did not address the manifold difficulties
noted above, and in the last review, associated with developing an
indicator that could reliably identify ambient mixes dominated by
particular types of sources of coarse particles. See above and 71 FR
61193. Yet such an indicator would be a prerequisite of the type of
standard these commenters request.
    For all of the reasons discussed above, the EPA does not agree with
industry commenters who recommended allowing the application of a 98th
percentile form for the current standard in cases where coarse
particles consist primarily of crustal material.
    Some industry commenters contended that the uncertainties and
limitations that precluded a quantitative risk assessment also preclude
revising the PM10 standard. Although the EPA agrees that
there are important uncertainties and limitations in the extent to
which the quantitative relationships between ambient
PM10-2.5 and health outcomes can be characterized in risk
models, the Agency does not agree that such limitations alone preclude
the option of revising a NAAQS. As noted above, the lack of a
quantitative PM10-2.5 risk assessment in the current review
adds uncertainty to conclusions about the extent to which revision of
the current PM10 standard would be expected to improve the
protection of public health, beyond the protection provided by the
current standard. However, the EPA does not agree that such
uncertainties necessarily preclude revision of a NAAQS. Indeed, with
respect to thoracic coarse particles, the DC Circuit noted that
``[a]lthough the evidence of danger from coarse PM is, as the EPA
recognizes, `inconclusive', the agency need not wait for conclusive
findings before regulating a pollutant it reasonably believes may pose
a significant risk to public health.'' 559 F. 3d at 533. Thus, the
Administrator's conclusion that the current 24-hour PM10
standard provides requisite protection of public health relies on her
consideration of the broad body of evidence, rather than solely on the
uncertainties that led to the decision not to conduct a quantitative
assessment of PM10-2.5 health risks.
    Commenters representing a number of environmental groups and
medical organizations disagreed with the Administrator's proposal to
retain the current primary PM10 standard. These commenters
generally requested that the EPA revise the PM10 standard to
increase public health protection, consistent with the recommendations
from CASAC.
    As discussed above and in the proposal, in reaching provisional
conclusions in the proposal regarding the current standard, the
Administrator carefully considered CASAC's advice and recommendations.
She specifically noted that in making its recommendation on the current
PM10 standard, CASAC did not discuss its approach to
considering the important uncertainties and limitations in the health
evidence, and did not discuss how these uncertainties and limitations
were reflected in its recommendations. Such uncertainties and
limitations contributed to the conclusions in the Integrated Science
Assessment that the PM10-2.5 evidence is only suggestive of
a causal relationship, a conclusion that CASAC endorsed (Samet,
2009e,f). These commenters also did not address the important
uncertainties in the epidemiologic studies on which their comments are
based. Given the importance of these uncertainties and limitations to
the interpretation of the

evidence, as reflected in the weight of evidence conclusions in the
Integrated Science Assessment and as discussed in the proposal, the
Administrator judges that it is appropriate to consider and account for
them when drawing conclusions about the implications of individual
PM10-2.5 health studies for the current standard. Commenters
have not provided new information that would change the Administrator's
views on the evidence and uncertainties.
    In recommending that the PM10 standard be revised, some
commenters supported their conclusions by referencing studies that
evaluated PM10, rather than PM10-2.5. These
commenters contended that ``[t]he most relevant studies to the setting
of a PM10 standard are the thousands of studies that have
reported adverse effects associated with PM10 pollution''
(ALA et al., 2012).
    As discussed in the Policy Assessment, the proposal, and above,
since the establishment of the primary PM2.5 standards, the
purpose of the primary PM10 standard has been to protect
against health effects associated with exposures to
PM10-2.5. PM10 is the indicator, not the target
pollutant. With regard to the appropriateness of considering
PM10 health studies for the purpose of reaching conclusions
on a standard meant to protect against exposures to
PM10-2.5, the proposal noted that PM10 includes
both fine and coarse particles, even in locations with the highest
concentrations of PM10-2.5. Therefore, the extent to which
PM10 effect estimates reflect associations with
PM10-2.5 versus PM2.5 can be highly uncertain and
it is often unclear how PM10 health studies should be
interpreted when considering a standard meant to protect against
exposures to PM10-2.5. Given this uncertainty and the
availability of a number of PM10-2.5 health studies in this
review, the Integrated Science Assessment considered
PM10-2.5 studies, but not PM10 studies, when
drawing weight-of-evidence conclusions regarding the coarse
fraction.\130\ In light of the uncertainty in ascribing
PM10-related health effects to the coarse or fine fractions,
indicating that the best evidence for effects associated with exposures
to PM10-2.5 comes from studies evaluating
PM10-2.5 itself, and given CASAC's support for the approach
adopted in the Integrated Science Assessment, which draws weight-of-
evidence conclusions for PM2.5 and PM10-2.5 but
not for PM10 (Samet, 2009f), the EPA continues to conclude
that it is appropriate to focus on PM10-2.5 health studies
when considering the degree of public health protection provided by the
current primary PM10 standard, a standard intended
exclusively to provide protection against exposures to
PM10-2.5.
---------------------------------------------------------------------------

    \130\ Although EPA relied in the 1997 review on evidence from
PM10 studies, EPA did so out of necessity (i.e., there
were as yet no reliable studies measuring PM10-2.5). In
the 2006 review, EPA placed primary reliance on epidemiologic
studies measuring or estimating PM10-2.5, although there
were comparatively few such studies. In this review, a larger body
of PM10-2.5 studies are available. EPA regards these
studies as the evidence to be given principal weight in reviewing
the adequacy of the PM10 standard.
---------------------------------------------------------------------------

G. Administrator's Final Decision on the Primary PM10 Standard

    In reaching a final decision on the primary PM10
standard, the Administrator takes into account the available scientific
evidence, and the assessment of that evidence, in the Integrated
Science Assessment; the analyses and staff conclusions presented in the
Policy Assessment; the advice and recommendations of CASAC; and public
comments on the proposal. In particular, as in the proposal, the
Administrator places emphasis on her consideration of the following
issues:

    (1) The extent to which it is appropriate to maintain a standard
that provides some measure of protection against all
PM10-2.5, regardless of composition or source of origin;
    (2) The extent to which it is appropriate to retain a
PM10 indicator for a standard meant to protect against
exposures to ambient PM10-2.5; and
    (3) The extent to which the current PM10 standard
provides an appropriate degree of public health protection.

    Each of these issues is discussed below.
    With regard to the first issue, as in the proposal the
Administrator judges that the expanded body of scientific evidence
available in this review provides ample support for a standard that
protects against exposures to all thoracic coarse particles, regardless
of their location or source of origin. There was already ample evidence
for this position in the previous review,\131\ and that evidence has
since increased. Specifically, the Administrator notes that
epidemiological studies have reported positive associations between
PM10-2.5 and mortality or morbidity in a large number of
cities across North America, Europe, and Asia, encompassing a variety
of environments where PM10-2.5 sources and composition are
expected to vary widely. In considering this evidence, the Integrated
Science Assessment concludes that ``many constituents of PM can be
linked with differing health effects'' (U.S. EPA, 2009a, p. 2-26).
Although PM10-2.5 in most of these study areas is of largely
urban origin, the Administrator notes that some recent studies have
also linked mortality and morbidity with relatively high ambient
concentrations of particles of non-urban crustal origin. In considering
these studies, she notes the Integrated Science Assessment's conclusion
that ``PM (both PM2.5 and PM10-2.5) from crustal,
soil or road dust sources or PM tracers linked to these sources are
associated with cardiovascular effects'' (U.S. EPA, 2009a, p. 2-26).
The Administrator likewise notes CASAC's emphatic advice that a
standard remains needed for all types of thoracic coarse PM.\132\ In
light of this body of available evidence reporting PM10-2.5-
associated health effects across different locations with a variety of
sources, the Integrated Science Assessment's conclusions regarding the
links between adverse health effects and PM sources and composition,
and CASAC's advice, the Administrator concludes in the current review
that it is appropriate to maintain a standard that provides some
measure of protection against exposures to all thoracic coarse
particles, regardless of their location, source of origin, or
composition.
---------------------------------------------------------------------------

    \131\ The D.C. Circuit agreed. See 559 F. 3d at 532-33.
    \132\ Indeed, CASAC recommended making the standard for all
types of thoracic coarse PM more stringent (Samet, 2010d).
---------------------------------------------------------------------------

    With regard to the second issue, in considering the appropriateness
of a PM10 indicator for a standard meant to provide such
public health protection, the Administrator notes that the rationale
used in the last review to support the unqualified PM10
indicator remains relevant in the current review. Specifically, as an
initial consideration, she notes that PM10 mass includes
both coarse PM (PM10-2.5) and fine PM (PM2.5). As
a result, the concentration of PM10-2.5 allowed by a
PM10 standard set at a single level declines as the
concentration of PM2.5 increases. At the same time, the
Administrator notes that PM2.5 concentrations tend to be
higher in urban areas than rural areas (U.S. EPA, 2005, p. 2-54, and
Figures 2-23 and 2-24) and, therefore, a PM10 standard will
generally allow lower PM10-2.5 concentrations in urban areas
than in rural areas.
    In considering the appropriateness of this variation in allowable
PM10-2.5 concentrations, the Administrator considers the
relative strength of the evidence for health effects associated with
PM10-2.5 of urban origin versus non-urban origin. She
specifically notes that, as discussed in the proposal, the large
majority of the available evidence for

thoracic coarse particle health effects comes from studies conducted in
locations with sources more typical of urban and industrial areas than
rural areas. While associations with adverse health effects have been
reported in some study locations where PM10-2.5 is largely
non-urban in origin (i.e., in dust storm studies), particle
concentrations in these study areas are typically much higher than
reported in study locations where the PM is of urban origin. Therefore,
the Administrator notes that the strongest evidence for a link between
PM10-2.5 and adverse health impacts, particularly for such a
link at relatively low particle concentrations, comes from studies of
urban or industrial PM10-2.5.
    The Administrator also notes that chemical constituents present at
higher levels in urban or industrial areas, including byproducts of
incomplete combustion (e.g. polycyclic aromatic hydrocarbons) emitted
as PM2.5 from motor vehicles as well as metals and other
contaminants emitted from anthropogenic sources, can contaminate
PM10-2.5 (U.S. EPA, 2004, p. 8-344; 71 FR 2665, January 17,
2006). While the Administrator acknowledges the uncertainty expressed
in the Integrated Science Assessment regarding the extent to which
particle composition can be linked to health outcomes based on
available evidence, she also considers the possibility that
PM10-2.5 contaminants typical of urban or industrial areas
could increase the toxicity of thoracic coarse particles in urban
locations.
    Given that the large majority of the evidence for
PM10-2.5 toxicity, particularly at relatively low particle
concentrations, comes from study locations where thoracic coarse
particles are of urban origin, and given the possibility that
PM10-2.5 contaminants in urban areas could increase particle
toxicity, the Administrator concludes that it remains appropriate to
maintain a standard that provides some protection in all areas but
targets public health protection to urban locations. Specifically, she
concludes that it is appropriate to maintain a standard that allows
lower ambient concentrations of PM10-2.5 in urban areas,
where the evidence is strongest that thoracic coarse particles are
linked to mortality and morbidity, and higher concentrations in non-
urban areas, where the public health concerns are less certain.
    Given all of the above considerations and conclusions, the
Administrator judges that the available evidence supports retaining a
PM10 indicator for a standard that is meant to protect
against exposures to thoracic coarse particles. In reaching this
judgment, she notes that, to the extent a PM10 indicator
results in lower allowable concentrations of thoracic coarse particles
in some areas compared to others, lower concentrations will be allowed
in those locations (i.e., urban or industrial areas) where the science
has shown the strongest evidence of adverse health effects associated
with exposure to thoracic coarse particles and where we have the most
concern regarding PM10-2.5 toxicity. Therefore, the
Administrator concludes that the varying amounts of coarse particles
that are allowed in urban vs. non-urban areas under the 24-hour
PM10 standard, based on the varying levels of
PM2.5 present, appropriately reflect the differences in the
strength of evidence regarding coarse particle effects in urban and
non-urban areas.133 134
---------------------------------------------------------------------------

    \133\ As discussed in the proposal, the Administrator recognizes
that this relationship is qualitative. That is, the varying coarse
particle concentrations allowed under the PM10 standard
do not precisely correspond to the variable toxicity of thoracic
coarse particles in different areas (insofar as that variability is
understood). Although currently available information does not allow
any more precise adjustment for relative toxicity, the Administrator
believes the standard will generally ensure that the coarse particle
levels allowed will be lower in urban areas and higher in non-urban
areas. Addressing this qualitative relationship, the DC Circuit held
that ``[i]t is true that the EPA relies on a qualitative analysis to
describe the protection the coarse PM NAAQS will provide. But the
fact that the EPA's analysis is qualitative rather than quantitative
does not undermine its validity as an acceptable rationale for the
EPA's decision.'' 559 F. 3d at 535.
    \134\ The D.C. Circuit agreed with similar conclusions in the
last review and held that this rationale reasonably supported use of
an unqualified PM10 indicator for thoracic coarse
particles. American Farm Bureau Federation v. EPA, 559 F. 3d at 535-
36.
---------------------------------------------------------------------------

    In reaching this conclusion, the Administrator also notes that, in
their review of the second draft Policy Assessment, CASAC concluded
that ``[w]hile it would be preferable to use an indicator that reflects
the coarse PM directly linked to health risks (PM10-2.5),
CASAC recognizes that there is not yet sufficient data to permit a
change in the indicator from PM10 to one that directly
measures thoracic coarse particles'' (Samet, 2010d, p. ii). Thus,
consistent the considerations presented above and with CASAC advice,
the Administrator concludes that it is appropriate to retain
PM10 as the indicator for thoracic coarse particles.\135\
---------------------------------------------------------------------------

    \135\ In addition, CASAC ``vigorously recommends the
implementation of plans for the deployment of a network of
PM10-2.5 sampling systems so that future epidemiological
studies will be able to more thoroughly explore the use of
PM10-2.5 as a more appropriate indicator for thoracic
coarse particles'' (Samet, 2010d, p. 7). Consideration of
alternative indicators (e.g., PM10-2.5) in future reviews
could be informed by additional research, as described in the Policy
Assessment (U.S. EPA, 2011a, section 3.5).
---------------------------------------------------------------------------

    With regard to the third issue, in evaluating the degree of public
health protection provided by the current PM10 standard, the
Administrator first notes that when the available PM10-2.5
scientific evidence and its associated uncertainties were considered,
the Integrated Science Assessment concluded that the evidence is
suggestive of a causal relationship between short-term
PM10-2.5 exposures and mortality, cardiovascular effects,
and respiratory effects. As discussed above and in more detail in the
Integrated Science Assessment (U.S. EPA, 2009a, section 1.5), a
suggestive determination is made when the ``[e]vidence is suggestive of
a causal relationship with relevant pollutant exposures, but is limited
because chance, bias and confounding cannot be ruled out.'' In
contrast, the Administrator notes that she is strengthening the annual
fine particle standard based on a body of scientific evidence judged
sufficient to conclude that a causal relationship exists (i.e.,
mortality, cardiovascular effects) or is likely to exist (i.e.,
respiratory effects). The suggestive judgment for PM10-2.5
reflects the greater degree of uncertainty associated with this body of
evidence, as discussed above and in more detail in the proposal, and as
summarized below.
    The Administrator notes that the important uncertainties and
limitations associated with the scientific evidence and air quality
information raise questions as to whether public health benefits would
be achieved by revising the existing PM10 standard. Such
uncertainties and limitations include the following:

    (1) While PM10-2.5 effect estimates reported for
mortality and morbidity were generally positive, most were not
statistically significant, even in single-pollutant models. This
includes effect estimates reported in some study locations with
PM10 concentrations above those allowed by the current
24-hour PM10 standard.
    (2) The number of epidemiological studies that have employed co-
pollutant models to address the potential for confounding,
particularly by PM2.5, remains limited. Therefore, the
extent to which PM10-2.5 itself, rather than one or more
co-pollutants, contributes to reported health effects remains
uncertain.
    (3) Only a limited number of experimental studies provide
support for the associations reported in epidemiological studies,
resulting in further uncertainty regarding the plausibility of the
associations between PM10-2.5 and mortality and morbidity
reported in epidemiological studies.

    (4) Limitations in PM10-2.5 monitoring data and the
different approaches used to estimate PM10-2.5
concentrations across epidemiological studies result in uncertainty
in the ambient PM10-2.5 concentrations at which the
reported effects occur, increasing uncertainty in estimates of the
extent to which changes in ambient PM10-2.5
concentrations would likely impact public health.
    (5) The lack of a quantitative PM10-2.5 risk
assessment further contributes to uncertainty regarding the extent
to which any revisions to the current PM10 standard would
be expected to improve the protection of public health, beyond the
protection provided by the current standard (see section III.B.5
above).
    (6) The chemical and biological composition of
PM10-2.5, and the effects associated with the various
components, remains uncertain. Without more information on the
chemical speciation of PM10-2.5, the apparent variability
in associations across locations is difficult to characterize.

    In considering these uncertainties and limitations, the
Administrator notes in particular the considerable degree of
uncertainty in the extent to which health effects reported in
epidemiological studies are due to PM10-2.5 itself, as
opposed to one or more co-occurring pollutants. As discussed above,
this uncertainty reflects the fact that there are a relatively small
number of PM10-2.5 studies that have evaluated co-pollutant
models, particularly co-pollutant models that have included
PM2.5, and a very limited body of controlled human exposure
evidence supporting the plausibility of a causal relationship between
PM10-2.5 and mortality and morbidity at ambient
concentrations. The Administrator notes that these important
limitations in the overall body of health evidence introduce
uncertainty into the interpretation of individual epidemiological
studies, particularly those studies reporting associations with
PM10-2.5 that are not statistically significant. Given this,
the Administrator reaches the conclusion that it is appropriate to
place relatively little weight on epidemiological studies reporting
associations with PM10-2.5 that are not statistically
significant in single-pollutant and/or co-pollutant models.\136\
---------------------------------------------------------------------------

    \136\ The Administrator acknowledges that this approach to
interpreting the evidence differs in emphasis from the approach she
has adopted for the evidence relating to PM2.5. As
discussed above in section III.E.4, for fine particles the
Administrator has considered not only whether study results are
statistically significant (or remain so after application of co-
pollutant models), but she also places emphasis on the overall
pattern of results across the epidemiological literature. This
includes giving some credence to studies that reported statistically
non-significant associations. This difference in emphasis stems from
the much stronger overall body of evidence available for fine
particles, compared to coarse particles. As discussed above, when
the available PM2.5 scientific evidence and its
associated uncertainties were considered, the Integrated Science
Assessment concluded that the evidence was sufficient to conclude
that causal relationships exist with mortality and cardiovascular
effects, and that a causal relationship is likely to exist with
respiratory effects. In contrast, the Integrated Science Assessment
concluded that the evidence is suggestive of a causal relationship
between short-term PM10-2.5 exposures and mortality,
cardiovascular effects, and respiratory effects. A suggestive
determination is made when the ``[e]vidence is suggestive of a
causal relationship with relevant pollutant exposures, but is
limited because chance, bias and confounding cannot be ruled out''
(U.S. EPA, 2009a, section 1.5). The suggestive judgment for
PM10-2.5 reflects the greater degree of uncertainty
associated with this body of evidence.
---------------------------------------------------------------------------

    With regard to this conclusion, the Administrator notes that, for
single-city mortality studies conducted in the United States where
ambient PM10 concentration data were available for
comparison to the current standard, positive and statistically
significant PM10-2.5 effect estimates were only reported in
study locations that would likely have violated the current
PM10 standard during the study period (U.S. EPA, 2011a,
Figure 3-2). In U.S. study locations that would likely have met the
current standard, PM10-2.5 effect estimates for mortality
were positive, but not statistically significant (U.S. EPA, 2011a,
Figure 3-2). In considering U.S. study locations where single-city
morbidity studies were conducted, and which would likely have met the
current PM10 standard during the study period, the
Administrator notes that PM10-2.5 effect estimates were both
positive and negative, with most not statistically significant (U.S.
EPA, 2011a, Figure 3-3).
    In addition, in considering single-city analyses for the locations
evaluated in a large U.S. multi-city mortality study (Zanobetti and
Schwartz, 2009), the Administrator notes that associations in most of
the study locations were not statistically significant and that this
was the only study to estimate ambient PM10-2.5
concentrations as the difference between county-wide PM10
and PM2.5 mass. As discussed in the proposal, the
Administrator notes that it is not clear how computed
PM10-2.5 measurements, such as those used by Zanobetti and
Schwartz (2009), compare with the PM10-2.5 concentrations
obtained in other studies either by direct measurement by calculating
the difference using co-located samplers (U.S. EPA, 2009a, section
<IP_ADDRESS>). For these reasons, as in the proposal, the Administrator
notes that there is considerable uncertainty in interpreting the
associations, and especially the concentrations at which such
associations may have occurred, in these single-city analyses.
    The Administrator acknowledges that an approach to considering the
available scientific evidence and air quality information that
emphasizes the above considerations differs from the approach taken by
CASAC. Specifically, CASAC placed a substantial amount of weight on
individual studies, particularly those reporting positive health
effects associations in locations that met the current PM10
standard during the study period. In emphasizing these studies, as well
as the limited number of supporting studies that have evaluated co-
pollutant models and the small number of supporting experimental
studies, CASAC concluded that ``the current data, while limited, is
sufficient to call into question the level of protection afforded the
American people by the current standard'' (Samet, 2010d, p. 7) and
recommended revising the current PM10 standard (Samet,
2010d).
    The Administrator has carefully considered CASAC's advice and
recommendations. She notes that in making its recommendation on the
current PM10 standard, CASAC did not discuss its approach to
considering the important uncertainties and limitations in the health
evidence, and did not discuss how these uncertainties and limitations
are reflected in its recommendation. As discussed above, such
uncertainties and limitations contributed to the conclusions in the
Integrated Science Assessment that the PM10-2.5 evidence is
only suggestive of a causal relationship, a conclusion that CASAC
endorsed (Samet, 2009e,f). Given the importance of these uncertainties
and limitations to the interpretation of the evidence, as reflected in
the weight of evidence conclusions in the Integrated Science Assessment
and as discussed above, the Administrator judges that it is appropriate
to consider and account for them when drawing conclusions about the
potential implications of individual PM10-2.5 health studies
for the current standard.
    In light of the above approach to considering the scientific
evidence, air quality information, and associated uncertainties, the
Administrator reaches the following conclusions:

    (1) When viewed as a whole the available evidence and
information suggests that the degree of public health protection
provided against short-term exposures to PM10-2.5 should
be maintained but does not need to be increased beyond that provided
by the current PM10 standard. This conclusion emphasizes
the important uncertainties and limitations associated with the
overall body

of health evidence and air quality information for
PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions; that
PM10-2.5 effect estimates for the most serious health
effect, mortality, were not statistically significant in U.S.
locations that met the current PM10 standard and where
coarse particle concentrations were either directly measured or
estimated based on co-located samplers; and that PM10-2.5
effect estimates for morbidity endpoints were both positive and
negative in locations that met the current standard, with most not
statistically significant.\137\
---------------------------------------------------------------------------

    \137\ This is not to say that the EPA could not adopt or revise
a standard for a pollutant for which the evidence is suggestive of a
causal relationship. Indeed, with respect to thoracic coarse
particles itself, the DC Circuit noted that ``[a]lthough the
evidence of danger from coarse PM is, as the EPA recognizes,
`inconclusive', the agency need not wait for conclusive findings
before regulating a pollutant it reasonably believes may pose a
significant risk to public health.'' American Farm Bureau Federation
v EPA 559 F. 3d at 533. As explained in the text above, it is the
Administrator's judgment that significant uncertainties presented by
the evidence and information before her in this review, both as to
causality and as to concentrations at which effects may be
occurring, best support a decision to retain rather than revise the
current primary 24-hour PM10 standard.
---------------------------------------------------------------------------

    (2) The degree of public health protection provided by the
current standard is not greater than warranted. This conclusion
notes that positive and statistically significant associations with
mortality were reported in single-city U.S. study locations likely
to have violated the current PM10 standard.\138\
---------------------------------------------------------------------------

    \138\ There are similarities with the conclusions drawn by the
Administrator in the last review. There, the Administrator concluded
that there was no basis for concluding that the degree of protection
afforded by the current PM10 standards in urban areas is
greater than warranted, since potential mortality effects have been
associated with air quality levels not allowed by the current 24-
hour standard, but have not been associated with air quality levels
that would generally meet that standard, and morbidity effects have
been associated with air quality levels that exceeded the current
24-hour standard only a few times. 71 FR 61202. In addition, the
Administrator concluded that there was a high degree of uncertainty
in the relevant population exposures implied by the morbidity
studies suggesting that there is little basis for concluding that a
greater degree of protection is warranted. Id. The D.C. Circuit in
American Farm Bureau Federation v EPA explicitly endorsed this
reasoning. 559 F. 3d at 534.

    In reaching these conclusions, the Administrator notes that the
Policy Assessment also discussed the potential for a revised
PM10 standard (i.e., with a revised form and level) to be
``generally equivalent'' to the current standard, but to better target
public health protection to locations where there is greater concern
regarding PM10-2.5-associated health effects (U.S. EPA,
2011a, sections 3.3.3 and 3.3.4).\139\ In considering such a potential
revised standard, the Policy Assessment discusses the large amount of
variability in PM10 air quality correlations across
monitoring locations and over time (U.S. EPA, 2011a, Figure 3-7) and
the regional variability in the relative degree of public health
protection that could be provided by the current and potential
alternative standards (U.S. EPA, 2011a, Table 3-2). In light of this
variability, the Administrator notes the Policy Assessment conclusion
that no single revised PM10 standard (i.e., with a revised
form and level) would provide public health protection equivalent to
that provided by the current standard, consistently over time and
across locations (U.S. EPA, 2011a, section 3.3.4). That is, a revised
standard, even one that is meant to be ``generally equivalent'' to the
current PM10 standard, could increase protection in some
locations while decreasing protection in other locations.
---------------------------------------------------------------------------

    \139\ As discussed in detail above (section IV.C.2.d) and in the
Policy Assessment (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4), a
revised standard that is generally equivalent to the current
PM10 standard could provide a degree of public health
protection that is similar to the degree of protection provided by
the current standard, across the United States as a whole. However,
compared to the current PM10 standard, such a generally
equivalent standard would change the degree of public health
protection provided in some specific areas, providing increased
protection in some locations and decreased protection in other
locations.
---------------------------------------------------------------------------

    In considering the appropriateness of revising the current
PM10 standard in this way, the Administrator notes the
following:

    (1) As discussed above, positive PM10-2.5 effect
estimates for mortality were not statistically significant in U.S.
locations that met the current PM10 standard and where
coarse particle concentrations were either directly measured or
estimated based on co-located samplers, while positive and
statistically significant associations with mortality were reported
in locations likely to have violated the current PM10
standard.
    (2) Also as discussed above, effect estimates for morbidity
endpoints in locations that met the current standard were both
positive and negative, with most not statistically significant.
    (3) Important uncertainties and limitations associated with the
overall body of health evidence and air quality information for
PM10-2.5, as discussed above and as reflected in the
Integrated Science Assessment weight-of-evidence conclusions, call
into question the extent to which the type of quantified and refined
targeting of public health protection envisioned under a revised
standard could be reliably accomplished.

    Given all of the above considerations, the Administrator notes that
there is a large amount of uncertainty in the extent to which public
health would be improved by changing the locations to which the
PM10 standard targets protection. Therefore, she reaches the
conclusion that the current PM10 standard should not be
revised in order to change that targeting of protection.
    In considering all of the above, including the scientific evidence,
the air quality information, the associated uncertainties, CASAC's
advice, and public comments received on the proposed rule, the
Administrator reaches the conclusion in the current review that the
existing 24-hour PM10 standard, with its one-expected
exceedance form and a level of 150 [mu]g/m\3\, is requisite (i.e.,
neither more protective nor less protective than necessary) to protect
public health with an adequate margin of safety against effects that
have been associated with PM10-2.5. In light of this
conclusion, with this rule the Administrator retains the current
PM10 standard.

V. Communication of Public Health Information

    Sections 319(a)(1) and (3) of the CAA require the EPA to establish
a uniform air quality index for reporting of air quality. These
sections specifically direct the Administrator to ``promulgate
regulations establishing an air quality monitoring system throughout
the United States which utilizes uniform air quality monitoring
criteria and methodology and measures such air quality according to a
uniform air quality index'' and ``provides for daily analysis and
reporting of air quality based upon such uniform air quality index * *
*'' In 1979, the EPA established requirements for index reporting (44
FR 27598, May 10, 1979). The requirement for State and local agencies
to report the AQI appears in 40 CFR 58.50, and the specific
requirements (e.g., what to report, how to report, reporting frequency,
calculations) are in appendix G to 40 CFR part 58.
    Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily by AQI reporting through EPA's AIRNow Web site.\140\ The
current AQI has been in use since its inception in 1999.\141\ It
provides accurate, timely, and easily understandable information about
daily levels of pollution (40 CFR 58.50). The AQI establishes a
nationally uniform system of indexing pollution levels for ozone,
carbon monoxide, nitrogen

dioxide, PM, and sulfur dioxide. The AQI is also recognized
internationally as a proven tool to effectively communicate air quality
information to the public.
---------------------------------------------------------------------------

    \140\ See http://www.airnow.gov/.
    \141\ In 1976, the EPA established a nationally uniform air
quality index, then called the Pollutant Standard Index (PSI), for
use by State and local agencies on a voluntary basis (41 FR 37660,
September 7, 1976). In August 1999, the EPA adopted revisions to
this air quality index (64 FR 42530, August 4, 1999) and renamed the
index the AQI.
---------------------------------------------------------------------------

    The AQI converts pollutant concentrations in a community's air to a
number on a scale from 0 to 500. Reported AQI values enable the public
to know whether air pollution levels in a particular location are
characterized as good (0-50), moderate (51-100), unhealthy for
sensitive groups (101- 150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (301-500). The AQI index value of 100 typically
corresponds to the level of the short-term (e.g., daily or hourly
standard) NAAQS for each pollutant. Below an index value of 100, an
intermediate value of 50 was defined either as the level of the annual
standard if an annual standard has been established (e.g.,
PM2.5, nitrogen dioxide), or as a concentration equal to
one-half the value of the short-term standard used to define an index
value of 100 (e.g., carbon monoxide). An AQI value greater than 100
means that a pollutant is in one of the unhealthy categories (i.e.,
unhealthy for sensitive groups, unhealthy, very unhealthy, or
hazardous) on a given day. An AQI value at or below 100 means that a
pollutant concentration is in one of the satisfactory categories (i.e.,
moderate or good). The underlying health information that supports the
NAAQS review also supports the selection of the AQI ``breakpoints''--
the ambient concentrations that delineate the various AQI categories
for each pollutant.
    Historically, state and local agencies have primarily used the AQI
to provide general information to the public about air quality and its
relationship to public health. For more than a decade, many states and
local agencies, as well as the EPA and other Federal agencies, have
been developing new and innovative programs and initiatives to provide
more information to the public in a more timely way. These initiatives,
including air quality forecasting, real-time data reporting through the
AirNow Web site, and state and local air quality action day programs,
can serve to provide useful, up-to-date, and timely information to the
public about air pollution and its effects. Such information will help
individuals take actions to avoid or to reduce exposures to ambient
pollution at levels of concern to them. Thus, these programs have
significantly broadened the ways in which state and local agencies can
meet the nationally uniform AQI reporting requirements and contribute
to state and local efforts to provide community health protection.
    With respect to an AQI value of 50, the historical approach is to
set it at the same level of the annual primary standard, if there is
one. This is consistent with the previous AQI sub-index for
PM2.5, in which the AQI value of 50 was set at 15 [micro]g/
m\3\ in 1999, consistent with the level of the annual PM2.5
standard at that time. In recognition of the proposed change to the
annual PM2.5 standard summarized in section III.F of the
proposal, the EPA proposed a conforming change to the PM2.5
sub-index of the AQI to be consistent with the proposed change to the
annual standard. As discussed below, no state or local agencies, or
their organizations (e.g., NACAA), that commented on the proposed
changes to the AQI disagreed with our proposed approach. Based on these
comments, the EPA continues to see no basis for deviating from this
approach in this review. Thus, the EPA is taking final action to set an
AQI value of 50 at 12.0 [mu]g/m\3\, 24-hour average, consistent with
the final decision on the annual PM2.5 standard level
(section III.F).
    With respect to an AQI value of 100, which is the basis for
advisories to individuals in sensitive groups, in the proposal we
described two general approaches that could be used to select the
associated PM2.5 level. By far the most common approach,
which has been used with all of the other sub-indices, is to set an AQI
value of 100 at the same level as the short-term standard. In the
proposal, the EPA recognized that some state and local air quality
agencies have expressed a strong preference that the Agency set an AQI
value of 100 equal to any short-term standard (77 FR 38964). These
agencies typically express the view that this linkage is useful for the
purpose of communicating with the public about the standard, as well as
providing consistent messages about the health impacts associated with
daily air quality. The EPA proposed to use this approach to set the AQI
value of 100 at 35 [mu]g/m\3\, 24-hour average, consistent with the
proposed decision to retain the current 24-hour PM2.5
standard. Id.
    An alternative approach discussed in the proposal (77 FR 38964),
was to directly evaluate the health effects evidence to select the
level for an AQI value of 100. This was the approach used in the 1999
rulemaking to set the AQI value of 100 at a level of 40 [mu]g/m\3\, 24-
hour average,\142\ when the 24-hour standard level was 65 [mu]g/m\3\.
This alternative approach was used in the case of the PM2.5
sub-index, because the annual and 24-hour PM2.5 standards
set in 1997 were designed to work together, and the intended degree of
health protection against short-term risks was not defined by the 24-
hour standard alone, but rather by the combination of the two standards
working in concert. Indeed, at that time, the 24-hour standard was set
to provide supplemental protection relative to the principal protection
provided by the annual standard. In the proposal, the EPA solicited
comment on this alternative approach in recognition that, as proposed,
the 24-hour PM2.5 standard is intended to continue to
provide supplemental protection against effects associated with short-
term exposures of PM2.5 by working in conjunction with the
annual standard to reduce 24-hour exposures to PM2.5. The
EPA recognized that in the past, some state and local air quality
agencies have expressed support for this alternative approach. Using
this alternative approach could have resulted in consideration of a
lower level for an AQI value of 100, based on the discussion of the
health information pertaining to the level of the 24-hour standard in
section III.E.4 of the proposal. The EPA encouraged state and local air
quality agencies to comment on both the approach and the level at which
to set an AQI value of 100 together with any supporting rationale. Of
the state or local agencies, or their organizations (e.g., NACAA), that
commented on the proposed changes to the AQI, only one organization,
NESCAUM, expressed some support for this approach. In its comments,
NESCAUM expressed support for a 24-hour standard set at 30 [mu]g/m\3\,
24-hour average. NESCAUM also expressed the view that EPA should
carefully consider how to set the breakpoint for an AQI value of 100.
NESCAUM expressed the view that if the EPA were to keep the 24-hour
PM2.5 standard at 35 [mu]g/m\3\, the annual standard would
be controlling, and a 24-hour breakpoint at that level (35 [mu]g/m\3\)
would not be very effective for the purposes of public health
messaging. However, other agencies, such as Georgia Department of
Natural Resources (Georgia DNR), expressed the view that linkage
between the short-term standard and the AQI of 100 is useful for the
purpose of communicating with the public about the standard as well as
providing consistent messages about the health

impacts associated with the daily air quality. Based on these comments,
the EPA sees no basis for deviating from the approach proposed in this
review. Thus, the EPA is taking final action to set an AQI value of 100
at 35 [mu]g/m\3\, 24-hour average, consistent with the final decision
on the 24-hour PM2.5 standard level (section III.F).
---------------------------------------------------------------------------

    \142\ Currently, we are cautioning members of sensitive groups
at the AQI value of 100 at 35 [mu]g/m\3\, 24-hour average,
consistent with more recent guidance from the EPA with regard to the
development of State emergency episode contingency plans (Harnett,
2009, Attachment B).
---------------------------------------------------------------------------

    With respect to an AQI value of 150, this level is based upon the
same health effects information that informs the selection of the level
of the 24-hour standard and the AQI value of 100. The AQI value of 150
was set in the 1999 rulemaking at a level of 65 [mu]g/m\3\, 24-hour
average. In considering what level to propose for an AQI value of 150,
we stated the view that the health effects evidence indicates that the
level of 55 [mu]g/m\3\, 24-hour average, is appropriate to use \143\ in
conjunction with an AQI value of 100 set at the level of 35 [mu]g/m\3\.
The Agency's approach to selecting the levels at which to set the AQI
values of 100 and 150 inherently recognizes that the epidemiological
evidence upon which these decisions are based provides no evidence of
discernible thresholds, below which effects do not occur in either
sensitive groups or in the general population, at which to set these
two breakpoints. Therefore, the EPA concluded the use of a proportional
adjustment would be appropriate. Commenters did not comment on this
proposed approach to revising the AQI value of 150; thus, the EPA is
taking final action to set an AQI value of 150 at 55 [mu]g/m\3\, 24-
hour average.
---------------------------------------------------------------------------

    \143\ We note that this level is consistent with the level
recommended in the more recent EPA guidance (Harnett, 2009,
Attachment B), which is in use by many State and local agencies.
---------------------------------------------------------------------------

    Based on the air quality and health considerations discussed in
section V of the proposal, the EPA concluded that it was appropriate to
propose to retain the current level of 500 [mu]g/m\3\, 24-hour average,
for the AQI value of 500. In addition, the EPA solicited comment on
alternative levels and approaches to setting a level for the AQI value
of 500, as well as supporting information and rationales for such
alternative levels. The EPA also solicited any additional information,
data, research or analyses that may be useful to inform a final
decision on the appropriate level to set the AQI value of 500.
Receiving no information with which to inform alternative approaches to
setting an AQI value of 500, the EPA is taking final action to retain
the current level of 500 [mu]g/m\3\, 24-hour average, for the AQI value
of 500.
    For the intermediate breakpoints in the AQI between the values of
150 and 500, the EPA proposed PM2.5 concentrations that
generally reflected a linear relationship between increasing index
values and increasing PM2.5 values (77 FR 38965). The
available scientific evidence of health effects related to population
exposures to PM2.5 concentrations between the level of the
24-hour standard and an AQI value of 500 suggested a continuum of
effects in this range, with increasing PM2.5 concentrations
being associated with increasingly larger numbers of people likely to
experience such effects. The generally linear relationship between AQI
values and PM2.5 concentrations in this range is consistent
with the health evidence. This also is consistent with the Agency's
practice of setting breakpoints in symmetrical fashion where health
effects information does not suggest particular levels.
    Table 2 below summarizes the finalized breakpoints for the
PM2.5 sub-index.\144\ Table 2 shows the intermediate
breakpoints for AQI values of 200, 300 and 400 based on a linear
interpolation between the proposed levels for AQI values of 150 and
500. If a different level were to be set for an AQI value of 150 or
500, intermediate levels would be calculated based on a linear
relationship between the selected levels for AQI values of 150 and 500.
---------------------------------------------------------------------------

    \144\ As discussed in section VII.C below, the EPA is also
updating the data handling procedures for reporting the AQI and
corresponding updates for other AQI-sub-indices presented in Table 2
of appendix G of 40 CFR part 58.

                                    Table 2--Breakpoints for PM2.5 Sub-Index
----------------------------------------------------------------------------------------------------------------
                                                                                Proposed breakpoints
                            AQI category                                 Index values      ([mu]g/m\3\, 24-hour
                                                                                average)
----------------------------------------------------------------------------------------------------------------
Good................................................................               0-50               0.0-(12.0)
Moderate............................................................             51-100              (12.1)-35.4
Unhealthy for Sensitive Groups......................................            101-150                35.5-55.4
Unhealthy...........................................................            151-200               55.5-150.4
Very Unhealthy......................................................            201-300              150.5-250.4
Hazardous...........................................................            301-400              250.5-350.4
                                                                                401-500              350.5-500.4
----------------------------------------------------------------------------------------------------------------

    In retaining the 500 level for the AQI as described above, we note
that the EPA is not establishing a Significant Harm Level (SHL) for
PM2.5. The SHL is an important part of air pollution
Emergency Episode Plans, which are required for certain areas by CAA
section 110(a)(2)(G) and associated regulations at 40 CFR 51.150, under
the Prevention of Air Pollution Emergency Episodes program. The Agency
believes that air quality responses established through an Emergency
Episode Plan should be developed through a collaborative process
working with State and Tribal air quality, forestry and agricultural
agencies, Federal land management agencies, private land managers and
the public. Therefore, if in future rulemaking the EPA proposes
revisions to the Prevention of Air Pollution Emergency Episodes
program, the proposal will include a SHL for PM2.5 that is
developed in collaboration with these organizations. As discussed in
the 1999 Air Quality Index Reporting Rule (64 FR 42530), if a future
rulemaking results in a SHL that is different from the 500 value of the
AQI for PM2.5, the AQI will be revised accordingly.
    The EPA also received more general comments on AQI reporting,
comments that did not pertain to setting specific breakpoints. One set
of commenters (e.g., API and UARG), expressed the view that changes to
the AQI are not appropriate. They noted that air quality is getting
better, and in fact is better than when EPA established the AQI. These
commenters stated that the proposed changes to the annual standard and
the AQI would mean that the public would hear less often that air
quality is good, and thereby would receive apparently inconsistent or
misleading messages that air quality is

worse. The AQI has been revised several times in conjunction with
revisions to the standards. State and local air quality agencies and
organizations are proficient at communicating with the public about the
reasons for changes to the AQI. Therefore, the EPA strongly disagrees
with these commenters that the public will receive inconsistent or
misleading messages. Recognizing the importance of the AQI as a
communication tool that allows the public to take exposure reduction
measures when air quality may pose health risks, the EPA agrees with
state and local air quality agencies and organizations that favored
revising the AQI at the same time as the primary standard.
    A few state and local air quality agencies and organizations
recommended against using near-roadway PM2.5 monitors for
AQI reporting. In support of this comment, they expressed the following
views, that near-roadway monitors are source-oriented, represent micro-
scale conditions, and the agencies don't have experience using them for
AQI reporting. The EPA disagrees with the comment in that these
monitors will be sited at existing near-road stations sited to be
representative of area-wide PM2.5 concentrations indicative
of general population exposure. Accordingly, data from these near-road
monitors should be included in the AQI since they provide information
about PM2.5 levels that millions of people, who work, live
and go to school near busy roadways, are exposed to. The stations are
representative of somewhat elevated concentrations in near-road
environments, but since these stations represent many such locations
throughout a metropolitan area, they are appropriate for characterizing
exposure in typical portions of major urban areas. The EPA is committed
to helping air quality agencies develop appropriate ways to report
PM2.5 levels from these monitors using the AQI.

VI. Rationale for Final Decisions on the Secondary PM Standards

    This section presents the Administrator's final decisions regarding
the need to revise the current suite of secondary PM2.5 and
PM10 standards to address visibility impairment and other
welfare effects considered in this review. Specifically, this section
describes the Administrator's final decision to retain the current
suite of secondary PM standards to address PM-related visibility
impairment as well as other PM-related welfare effects, including
ecological effects, effects on materials, and climate impacts. This
suite of standards includes an annual PM2.5 standard of 15
[mu]g/m\3\, a 24-hour PM2.5 standard of 35 [mu]g/m\3\, and a
24-hour PM10 standard of 150 [mu]g/m\3\. The Administrator
is revising only the form of the secondary annual PM2.5
standard to remove the option for spatial averaging consistent with
this change to the primary annual PM2.5 standard. Contrary
to what was proposed, the Administrator has decided not to establish a
distinct standard to address PM-related visibility impairment. The
rationale for this decision is presented below.
    The Administrator's final decisions on the secondary standards are
based on a thorough review of the latest scientific information
published through mid-2009 on welfare effects associated with fine and
coarse particles in the ambient air, as presented in the Integrated
Science Assessment. The final decisions also take into account: (1)
Staff assessments of the most policy-relevant information presented and
assessed in the Integrated Science Assessment and staff analyses of air
quality and visibility effects presented in the Visibility Assessment
and the Policy Assessment, upon which staff conclusions regarding
appropriate considerations in this review are based; (2) CASAC advice
and recommendations, as reflected in discussions of drafts of the
Integrated Science Assessment, Visibility Assessment, and Policy
Assessment at public meetings, in separate written comments, and in
CASAC's letters to the Administrator; (3) the multiple rounds of public
comments received during the development of these documents, both in
connection with CASAC meetings and separately; and (4) public comments
received on the proposal.
    In particular, this section presents background information on the
EPA's previous and current reviews of the secondary PM standards
(section VI.A), a summary of the proposed decisions regarding the
secondary PM standards (section VI.B), a discussion of significant
public comments received on those proposed decisions (section VI.C),
and the Administrator's final decisions on the secondary PM standards
(section VI.D).

A. Background

    The current suite of secondary PM standards is identical to the
suite of primary PM standards set in 2006, including 24-hour and annual
PM2.5 standards and a 24-hour PM10 standard. The
current secondary PM2.5 standards are intended to provide
protection from PM-related visibility impairment, whereas the entire
suite of secondary PM standards is intended to provide protection from
other PM-related effects on public welfare, including effects on
sensitive ecosystems, materials damage and soiling, and climatic and
radiative processes.
    The approach used for reviewing the current suite of secondary PM
standards built upon and broadened the approaches used in previous PM
NAAQS reviews. The following discussion focuses particularly on the
current secondary PM2.5 standards related to visibility
impairment and provides a summary of the approaches used to review and
establish secondary PM2.5 standards in the last two reviews
(section VI.A.1); judicial review of the 2006 standards that resulted
in the remand of the secondary annual and 24-hour PM2.5
NAAQS to the EPA (section VI.A.2); and the approach used in this review
for evaluating the secondary PM2.5 standards (section
VI.A.3).
1. Approaches Used in Previous Reviews
    The original secondary PM2.5 standards were established
in 1997, and a revision to the 24-hour standard was made in 2006. The
approaches used in making final decisions on secondary standards in
those reviews, as well as the current review, utilized different ways
to consider the underlying body of scientific evidence. They also
reflected an evolution in EPA's understanding of the nature of the
effect on public welfare from PM-related visibility impairment, from an
approach that focused only on Federal Class I area visibility impacts
to a more multifaceted approach that also considered PM-related impacts
on visibility in non-Federal Class I areas, such as in urban areas.
This evolution occurred in conjunction with the expansion of available
PM data and information from visibility-related studies of public
perception, valuation, and personal comfort and well-being.
    In 1997, the EPA revised the PM NAAQS in part by establishing new
identical primary and secondary PM2.5 standards. In revising
the secondary standards, the EPA recognized that PM produces adverse
effects on visibility and that impairment of visibility was being
experienced throughout the U.S., in multi-state regions, urban areas,
and remote mandatory Federal Class I areas alike. However, in
considering an appropriate level for a secondary standard to address
adverse effects of PM2.5 on visibility, the EPA concluded
that the determination of a single national level was complicated by
important regional differences influenced by factors such as

background and current levels of PM2.5, composition of
PM2.5, and average relative humidity. Variations in these
factors across regions could thus result in situations where attaining
an appropriately protective concentration of fine particles in one
region might or might not provide adequate protection in a different
region. The EPA also determined that there was insufficient information
at that time to establish a level for a national secondary standard
that would represent a threshold above which visibility conditions
would always be adverse and below which visibility conditions would
always be acceptable.
    Based on an assessment of the potential visibility improvements
that would result from reaching attainment with the new primary
standards for PM2.5, the EPA concluded that attainment of
the annual and 24-hour PM2.5 primary standards would lead to
visibility improvements in the eastern U.S. at both urban and regional
scales, but little or no change in the western U.S., except in and near
certain urban areas.
    The EPA also considered the potential effectiveness of a regional
haze program, required by sections 169A and 169B of the CAA \145\ to
address those effects of PM on visibility that would not be addressed
through attainment of the primary PM2.5 standards. The
regional haze program would be designed to address the widespread,
regionally uniform type of haze caused by a multitude of sources. The
structure and requirements of sections 169A and 169B of the CAA provide
for visibility protection programs that can be more responsive to the
factors contributing to regional differences in visibility than can
programs addressing the kinds of nationally applicable secondary NAAQS
considered in the 1997 review. The regional haze visibility goal is
more protective than a secondary NAAQS since the goal is to eliminate
any anthropogenic impairment rather than to provide a level of
protection from visibility impairment that is requisite to protect the
public welfare. Thus, an important factor considered in the 1997 review
was whether a regional haze program, in conjunction with secondary
standards set identical to the suite of PM2.5 primary
standards, would provide appropriate protection for visibility in non-
Federal Class I areas. The EPA concluded that the two programs and
associated control strategies should provide such protection due to the
regional approaches needed to manage emissions of pollutants that
impair visibility in many of these areas.
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    \145\ In 1977, Congress established as a national goal ``the
prevention of any future, and the remedying of any existing,
impairment of visibility in mandatory Federal Class I areas which
impairment results from manmade air pollution,'' section 169A(a)(1)
of the CAA. The EPA is required by section 169A(a)(4) of the CAA to
promulgate regulations to ensure that ``reasonable progress'' is
achieved toward meeting the national goal.
---------------------------------------------------------------------------

    For these reasons, in 1997 the EPA concluded that a national
regional haze program, combined with a nationally applicable level of
protection achieved through secondary PM2.5 standards set
identical to the primary PM2.5 standards, would be more
effective for addressing regional variations in the adverse effects of
PM2.5 on visibility than would be national secondary
standards for PM with levels lower than the primary PM2.5
standards. The EPA further recognized that people living in certain
urban areas may place a high value on unique scenic resources in or
near these areas and as a result might experience visibility problems
attributable to sources that would not necessarily be addressed by the
combined effects of a regional haze program and PM2.5
secondary standards. The EPA concluded that in such cases, state or
local regulatory approaches, such as past action in Colorado to
establish a local visibility standard for the City of Denver, would be
more appropriate and effective in addressing these special situations
because of the localized and unique characteristics of the problems
involved. Visibility in an urban area located near a mandatory Federal
Class I area could also be improved through state implementation of the
then-current visibility regulations, by which emission limitations can
be imposed on a source or group of sources found to be contributing to
``reasonably attributable'' impairment in the mandatory Federal Class I
area.
    Based on these considerations, in 1997 the EPA set secondary
PM2.5 standards identical to the primary PM2.5
standards, that would work in conjunction with the Regional Haze
Program to be established under sections 169A and 169B of the CAA, as
the most appropriate and effective means of addressing the public
welfare effects associated with visibility impairment. Together, the
two programs and associated control strategies were expected to provide
appropriate protection against PM-related visibility impairment and
enable all regions of the country to make reasonable progress toward
the national visibility goal.
    In 2006, the EPA revised the suite of secondary PM2.5
standards to address visibility impairment by making the suite of
secondary standards identical to the revised suite of primary
PM2.5 standards. The EPA's decision regarding the need to
revise the suite of secondary PM2.5 standards reflected a
number of new developments that had occurred and sources of information
that had become available following the 1997 review. First, the EPA
promulgated a Regional Haze Program in 1999 (65 FR 35713, July 1, 1999)
which required states to establish goals for improving visibility in
Federal Class I areas and to adopt control strategies to achieve these
goals. Second, extensive new information from visibility and fine
particle monitoring networks had become available, allowing for updated
characterizations of visibility trends and PM concentrations in urban
areas, as well as Federal Class I areas. These new data allowed the EPA
to better characterize visibility impairment in urban areas and the
relationship between visibility and PM2.5 concentrations.
Finally, additional studies in the U.S. and abroad provided the basis
for the establishment of standards and programs to address specific
visibility concerns in a number of local areas. These studies (Denver,
Phoenix, and British Columbia) utilized photographic representations of
visibility impairment and produced reasonably consistent results in
terms of the visual ranges found to be generally acceptable by study
participants. The EPA considered the information generated by these
studies useful in characterizing the nature of particle-induced haze
and for informing judgments about the acceptability of various levels
of visual air quality in urban areas across the U.S. Based largely on
this information, the Administrator concluded that it was appropriate
to revise the secondary PM2.5 standards to provide increased
protection from visibility impairment principally in urban areas, in
conjunction with the regional haze program for protection of visual air
quality in Federal Class I areas.
    In so doing, the Administrator recognized that PM-related
visibility impairment is principally related to fine particle
concentrations and that perception of visibility impairment is most
directly related to short-term, nearly instantaneous levels of visual
air quality. Thus, in considering whether the then-current suite of
secondary standards would provide the appropriate degree of protection,
he concluded that it was appropriate to focus on just the 24-hour
secondary PM2.5 standard to provide requisite protection.
    The Administrator then considered whether PM2.5 mass
remained the appropriate indicator for a secondary

standard to protect visibility, primarily in urban areas. The
Administrator noted that PM-related visibility impairment is
principally related to fine particle levels. Hygroscopic components of
fine particles, in particular sulfates and nitrates, contribute
disproportionately to visibility impairment under high humidity
conditions. Particles in the coarse mode generally contribute only
marginally to visibility impairment in urban areas. With the
substantial addition to the air quality and visibility data made
possible by the national urban PM2.5 monitoring networks, an
analysis conducted for the 2006 review found that, in urban areas,
visibility levels showed far less difference between eastern and
western regions on a 24-hour or shorter time basis than implied by the
largely non-urban data available in the 1997 review. In analyzing how
well PM2.5 concentrations correlated with visibility in
urban locations across the U.S., the 2005 Staff Paper concluded that
clear correlations existed between 24-hour average PM2.5
concentrations and calculated (i.e., reconstructed) light extinction,
which is directly related to visual range (U.S. EPA, 2005, p. 7-6).
These correlations were similar in the eastern and western regions of
the U.S. These correlations were less influenced by relative humidity
and more consistent across regions when PM2.5 concentrations
were averaged over shorter, daylight time periods (e.g., 4 to 8 hours)
when relative humidity in eastern urban areas was generally lower and
thus more similar to relative humidity in western urban areas. The 2005
Staff Paper noted that a standard set at any specific PM2.5
concentration would necessarily result in visual ranges that vary
somewhat in urban areas across the country, reflecting the variability
in the correlations between PM2.5 concentrations and light
extinction. The 2005 Staff Paper concluded that it was appropriate to
use PM2.5 as an indicator for standards to address
visibility impairment in urban areas, especially when the indicator is
defined for a relatively short period (e.g., 4 to 8 hours) of daylight
hours (U.S. EPA, 2005, p. 7-6). Based on their review of the Staff
Paper, most CASAC Panel members also endorsed such a PM2.5
indicator for a secondary standard to address visibility impairment
(Henderson, 2005a, p. 9). Based on the above considerations, the
Administrator concluded that PM2.5 should be retained as the
indicator for fine particles as part of a secondary standard to address
visibility protection, in conjunction with averaging times from 4 to 24
hours.
    In considering what level of protection against PM-related
visibility impairment would be appropriate, the Administrator took into
account the results of the public perception and attitude surveys
regarding the acceptability of various degrees of visibility impairment
in the U.S. and Canada, state and local visibility standards within the
U.S., and visual inspection of photographic representations of several
urban areas across the U.S. In the Administrator's judgment, these
sources provided useful but still quite limited information on the
range of levels appropriate for consideration in setting a national
visibility standard primarily for urban areas, given the generally
subjective nature of the public welfare effect involved. Based on
photographic representations of varying levels of visual air quality,
public perception studies, and local and state visibility standards,
the 2005 Staff Paper had concluded that 30 to 20 [mu]g/m\3\
PM2.5 represented a reasonable range for a national
visibility standard primarily for urban areas, based on a sub-daily
averaging time (U.S. EPA, 2005, p. 7-13). The upper end of this range
was below the levels at which illustrative scenic views are
significantly obscured, and the lower end was around the level at which
visual air quality generally appeared to be good based on observation
of the illustrative views. This concentration range generally
corresponded to median visual ranges in urban areas within regions
across the U.S. of approximately 25 to 35 km, a range that was bounded
above by the visual range targets selected in specific areas where
state or local agencies placed particular emphasis on protecting visual
air quality. In considering a reasonable range of forms for a
PM2.5 standard within this range of levels, the 2005 Staff
Paper had concluded that a concentration-based percentile form was
appropriate, and that the upper end of the range of concentration
percentiles for consideration should be consistent with the 98th
percentile used for the primary standard and that the lower end of the
range should be the 92nd percentile, which represented the mean of the
distribution of the 20 percent most impaired days, as targeted in the
regional haze program (U.S. EPA, 2005 pp. 7-11 to 7-13). While
recognizing that it was difficult to select any specific level and form
based on then-currently available information (Henderson, 2005a, p. 9),
the CASAC Panel was generally in agreement with the ranges of levels
and forms presented in the 2005 Staff Paper.
    The Administrator also considered the level of protection that
would be afforded by the proposed suite of primary PM2.5
standards (71 FR 2681, January 17, 2006), on the basis that although
significantly more information was available than in the 1997 review
concerning the relationship between fine PM levels and visibility
across the country, there was still little available information for
use in making the relatively subjective value judgment needed in
selecting the appropriate degree of protection to be afforded by such a
standard. In so doing, the Administrator compared the extent to which
the proposed suite of primary standards would require areas across the
country to improve visual air quality with the extent of increased
protection likely to be afforded by a standard based on a sub-daily
averaging time. Based on such an analysis, the Administrator observed
that the predicted percent of counties with monitors not likely to meet
the proposed suite of primary PM2.5 standards was actually
somewhat greater than the predicted percent of counties with monitors
not likely to meet a sub-daily secondary standard with an averaging
time of 4 daylight hours, a level toward the upper end of the range
recommended in the 2005 Staff Paper, and a form within the recommended
range. Based on this comparison, the Administrator tentatively
concluded that revising the secondary 24-hour PM2.5 standard
to be identical to the proposed revised primary PM2.5
standard (and retaining the then-current annual secondary
PM2.5 standard) was a reasonable policy approach to
addressing visibility protection primarily in urban areas. In proposing
this approach, the Administrator also solicited comment on a sub-daily
(4- to 8-hour averaging time) secondary PM2.5 standard (71
FR 2675 to 2781, January 17, 2006).