protection beyond that afforded by the current PM2.5
standards.

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

1. Indicator
    In initially setting standards for fine particles in 1997, the EPA
concluded it was appropriate to control fine particles as a group,
rather than singling out any particular component or class of fine
particles. The EPA noted that community health studies had found
significant associations between various indicators of fine particles,
and that health effects in a large number of areas had significant mass
contributions of differing components or sources of fine particles. In
addition, a number of toxicological and controlled human exposure
studies had reported health effects associations with high
concentrations of numerous fine particle components. It was also not
possible to rule out any component within the mix of fine particles as
not contributing to the fine particle effects found in the
epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a
size-based indicator in 1977 to distinguish fine particles from
particles in the coarse mode, the EPA noted that the available
epidemiological studies of fine particles were based largely on
PM2.5 and also considered monitoring technology that was
generally available. The selection of a 2.5 [micro]m size cut reflected
the regulatory importance of defining an indicator that would more
completely capture fine particles under all conditions likely to be
encountered across the U.S., especially when fine particle
concentrations and humidity are likely to be high, while recognizing
that some small coarse particles would also be captured by current
methods to monitor PM2.5 (62 FR 38666 to 38668, July 18,
1997). In the last review, based on the same considerations, the EPA
again recognized that the available information supported retaining the
PM2.5 indicator and remained too limited to support a
distinct standard for any specific PM2.5 component or group
of components associated with any source categories of fine particles
(71 FR 61162 to 61164, October 17, 2006).
    In this current review, the same considerations continue to apply
for selection of an appropriate indicator for fine particles. As an
initial matter, the Policy Assessment recognizes that the available
epidemiological studies linking mortality and morbidity effects with
long- and short-term exposures to fine particles continue to be largely
indexed by PM2.5. For the same reasons discussed in the last
two reviews, the Policy Assessment concludes that it is appropriate to
consider retaining a PM2.5 indicator to provide protection
from effects associated with long- and short-term fine particle
exposures (U.S. EPA, 2011, p. 2-50).
    The Policy Assessment also considers the expanded body of evidence
available in this review to consider whether there is sufficient
evidence to support a separate standard for ultrafine particles \68\ or
whether there is sufficient evidence to establish distinct standards
focused on regulating specific PM2.5 components or a group
of components associated with any source categories of fine particles
(U.S. EPA, 2011a, section 2.3.1).
---------------------------------------------------------------------------

    \68\ Ultrafine particles, generally including particles with a
mobility diameter less than or equal to 0.1 [micro]m, are emitted
directly to the atmosphere or are formed by nucleation of gaseous
constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
---------------------------------------------------------------------------

    A number of studies available in this review have evaluated
potential health effects associated with short-term exposures to
ultrafine particles. As noted in the Integrated Science Assessment, the
enormous number and larger, collective surface area of ultrafine
particles are important considerations for focusing on this particle
size fraction in assessing potential public health impacts (U.S. EPA,
2009a, p. 6-83). Per unit mass, ultrafine particles may have more
opportunity to interact with cell surfaces due to their greater surface
area and their greater particle number compared with larger particles
(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the
potential for soluble components (e.g., transition metals, organics) to
adsorb to ultrafine particles and potentially cross cell membranes and
epithelial barriers (U.S. EPA, 2009a, p. 6-83). In addition, evidence
available in this review suggests that the ability of particles to
enhance allergic sensitization is associated more strongly with
particle number and surface area than with particle mass (U.S. EPA,
2009a, p. 6-127).
    New evidence, primarily from controlled human exposure and
toxicological studies, expands our understanding of cardiovascular and
respiratory effects related to short-term ultrafine particle exposures.
However, the Policy Assessment concludes this evidence is still very
limited and largely focused on exposure to diesel exhaust, for which
the Integrated Science Assessment concludes it is unclear if the
effects observed are due to ultrafine particles, larger particles
within the PM2.5 mixture, or the gaseous components of
diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, the Integrated
Science Assessment notes uncertainties associated with the controlled
human exposure studies using concentrated ambient particle systems
which have been shown to modify the composition of ultrafine particles
(U.S. EPA, 2009a, p. 2-22, see also section 1.5.3).
    The Policy Assessment recognizes that there are relatively few
epidemiological studies that have examined potential cardiovascular and
respiratory effects associated with short-term exposures to ultrafine
particles (U.S. EPA, 2011a, p. 2-51). These studies have reported
inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).
    Collectively, in considering the body of scientific evidence
available in this review, the Integrated Science Assessment concludes
that the currently available evidence is suggestive of a causal
relationship between short-term exposures to ultrafine particles and
cardiovascular and respiratory effects. Furthermore, the Integrated
Science Assessment concludes that evidence is inadequate to infer a
causal relationship between short-term exposure to ultrafine particles
and mortality as well as long-term exposure to ultrafine particles and
all outcomes evaluated (U.S. EPA, 2009a, sections 2.3.5, <IP_ADDRESS>,
<IP_ADDRESS>, <IP_ADDRESS>, <IP_ADDRESS>, 7.3.9, <IP_ADDRESS>, <IP_ADDRESS>, and <IP_ADDRESS>;
Table 2-6).
    With respect to our understanding of ambient ultrafine particle
concentrations, at present, there is no national network of ultrafine
particle samplers; thus, only episodic and/or site-specific data sets
exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy Assessment
recognizes a national characterization of concentrations, temporal and
spatial patterns, and trends is not possible at this time, and the
availability of ambient ultrafine measurements to support health
studies is extremely limited (U.S. EPA, 2011a, p. 2-51). In general,
measurements of ultrafine particles are highly dependent on monitor
location and, therefore, more subject to exposure error than
accumulation mode particles (U.S. EPA, 2009a, p. 2-22). Furthermore,
the number of ultrafine particles generally decreases sharply downwind
from sources, as ultrafine particles may grow into the accumulation
mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited
studies of ambient ultrafine particle measurements suggest these
particles exhibit a high degree of spatial and temporal heterogeneity
driven primarily by differences in nearby source characteristics (U.S.
EPA, 2009a, p. 3-84). Internal combustion engines and, therefore,
roadways are a notable source of ultrafine particles, so

concentrations of these particles near roadways are generally expected
to be elevated (U.S. EPA, 2009a, p. 2-3). Concentrations of ultrafine
particles have been reported to drop off much more quickly with
distance from roadways than fine particles (U.S. EPA, 2009a, p. 3-84).
    In considering both the currently available health effects evidence
and the air quality data, the Policy Assessment concludes that this
information is still too limited to provide support for consideration
of a distinct PM standard for ultrafine particles (U.S. EPA, 2011a, p.
2-52).
    In addressing the issue of particle composition, the Integrated
Science Assessment concludes that, ``[f]rom a mechanistic perspective,
it is highly plausible that the chemical composition of PM would be a
better predictor of health effects than particle size'' (U.S. EPA,
2009a, p. 6-202). Heterogeneity of ambient concentrations of
PM2.5 constituents (e.g., elemental carbon, organic carbon,
sulfates, nitrates) observed in different geographical regions as well
as regional heterogeneity in PM2.5-related health effects
reported in a number of epidemiological studies are consistent with
this hypothesis (U.S. EPA, 2009a, section 6.6).
    With respect to the availability of ambient measurement data for
fine particle components in this review, there are now more extensive
ambient PM2.5 speciation measurement data available through
the Chemical Speciation Network (CSN) than in previous reviews (U.S.
EPA, 2011a, section 1.3.2 and Appendix B, section B.1.3). Data from the
CSN provide further evidence of spatial and seasonal variation in both
PM2.5 mass and composition among cities and geographic
regions (U.S. EPA, 2009a, pp. 3-50 to 3-60; Figures 3-12 to 3-18;
Figure 3-47). Some of this variation may be related to regional
differences in meteorology, sources, and topography (U.S. EPA, 2009a,
p. 2-3).
    The currently available epidemiological, toxicological, and
controlled human exposure studies evaluated in the Integrated Science
Assessment on the health effects associated with ambient
PM2.5 constituents and categories of fine particle sources
used a variety of quantitative methods applied to a broad set of
PM2.5 constituents, rather than selecting a few constituents
a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies have used
measured ambient PM2.5 speciation data, including monitoring
data from the CSN, while all of the controlled human exposure and most
of the toxicological studies have used concentrated ambient particles
and analyzed the constituents therein (U.S. EPA, 2009a, p. 6-203).\69\
The CSN provides PM2.5 speciation measurements generally on
a one-in-three or one-in-six day sampling schedule and, thus, do not
capture data every day at most sites.\70\
---------------------------------------------------------------------------

    \69\ Most studies considered between 7 to 20 ambient
PM2.5 constituents, with elemental carbon, organic
carbon, sulfates, nitrates, and metals most commonly measured. Many
of the studies grouped the constituents with various factorization
or source apportionment techniques to examine the relationship
between the grouped constituents and various health effects.
However, not all studies labeled the constituent groupings according
to their presumed source and a small number of controlled human
exposure and toxicological studies did not use any constituent
grouping. These differences across studies substantially limit any
integrative interpretation of these studies (U.S. EPA, 2009a, p. 6-
203).
    \70\ To expand our understanding of the role of specific
PM2.5 components and sources with respect to the observed
health effects, researchers have expressed a strong interest in
having access to PM2.5 speciation measurements collected
more frequently (U.S. EPA, 2011a, p. 2-53, including footnote 47).
---------------------------------------------------------------------------

    The Policy Assessment recognizes that several new multi-city
studies evaluating short-term exposures to fine particle constituents
are now available. These studies continue to show an association
between mortality and cardiovascular and/or respiratory morbidity
effects and short-term exposures to various PM2.5 components
including nickel, vanadium, elemental carbon, organic carbon, nitrates,
and sulfates (U.S. EPA, 2011a, section 2.3.1; U.S. EPA, 2009a, sections
<IP_ADDRESS> and 6.6).
    Limited evidence is available to evaluate the health effects
associated with long-term exposures to PM2.5 components
(U.S. EPA, 2009a, section 7.6.2). The Policy Assessment notes the most
significant new evidence is provided by a study that evaluated multiple
PM2.5 components and an indicator of traffic density in an
assessment of health effects related to long-term exposure to
PM2.5 (Lipfert et al., 2006). Using health data from a
cohort of U.S. military veterans and PM2.5 measurement data
from the CSN, Lipfert et al. (2006) reported positive associations
between mortality and long-term exposures to nitrates, elemental
carbon, nickel, and vanadium as well as traffic density and peak ozone
concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, pp. 7-89 to
7-90).
    With respect to source categories of fine particles associated with
a range of health endpoints, the Integrated Science Assessment reports
that the currently available evidence suggests associations between
cardiovascular effects and a number of specific PM2.5-
related source categories, specifically oil combustion, wood or biomass
burning, motor vehicle emissions, and crustal or road dust sources
(U.S. EPA, 2009a, section 6.6; Table 6-18). In addition, a few studies
have evaluated associations between PM2.5-related source
categories and mortality. These studies include a study that reported
an association between mortality and a PM2.5 coal combustion
factor (Laden et al., 2000), while other studies linked mortality to a
secondary sulfate long-range transport PM2.5 source (Ito et
al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section <IP_ADDRESS>). There
is less consistency in associations observed between sources of fine
particles and respiratory health effects, which may be partially due to
the fact that fewer studies have evaluated respiratory-related outcomes
and measures. However, there is some evidence for PM2.5-
related associations with secondary sulfate and decrements in lung
function in asthmatic and healthy adults (U.S. EPA, 2009a, p. 6-211;
Gong et al., 2005; Lanki et al., 2006). Respiratory effects relating to
the crustal/soil/road dust and traffic sources of PM have been observed
in asthmatic children and adults (U.S. EPA, 2009a, p. 6-205; Gent et
al., 2009; Penttinen et al., 2006).
    Recent studies have shown that source apportionment methods have
the potential to add useful insights into which sources and/or PM
constituents may contribute to different health effects. Of particular
interest are several epidemiological studies that compared source
apportionment methods and reported consistent results across research
groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al.,
2006; Mar et al., 2006; Thurston et al., 2005). These studies reported
associations between total mortality and secondary sulfate in two
cities for two different lag times. The sulfate effect was stronger for
total mortality in Washington, DC and for cardiovascular-related
morality in Phoenix (U.S. EPA, 2009a, p. 6-204). These studies also
found some evidence for associations with mortality and a number of
source categories (e.g., biomass/wood combustion, traffic, copper
smelter, coal combustion, sea salt) at various lag times (U.S. EPA,
2009a, p. 6-204). Sarnat et al. (2008) compared three different source
apportionment methods and reported consistent associations between
emergency department visits for cardiovascular diseases with mobile
sources and biomass combustion as well as increased respiratory-related
emergency department visits associated

with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-211).
    Collectively, in considering the currently available evidence for
health effects associated with specific PM2.5 components or
groups of components associated with any source categories of fine
particles as presented in the Integrated Science Assessment, the Policy
Assessment concludes that additional information available in this
review continues to provide evidence that many different constituents
of the fine particle mixture as well as groups of components associated
with specific source categories of fine particles are linked to adverse
health effects (U.S. EPA, 2011a, p. 2-55). However, as noted in the
Integrated Science Assessment, while ``[t]here is some evidence for
trends and patterns that link particular ambient PM constituents or
sources with specific health outcomes * * * there is insufficient
evidence to determine whether these patterns are consistent or robust''
(U.S. EPA, 2009a, p. 6-210). Assessing this information, the Integrated
Science Assessment concludes that ``the evidence is not yet sufficient
to allow differentiation of those constituents or sources that are more
closely related to specific health outcomes'' (U.S. EPA, 2009a, pp. 2-
26 and 6-212). Therefore, the Policy Assessment concludes that the
currently available evidence is not sufficient to support consideration
of a separate indicator for a specific PM2.5 component or
group of components associated with any source category of fine
particles. Furthermore, the Policy Assessment concludes that the
evidence is not sufficient to support eliminating any component or
group of components associated with any source categories of fine
particles from the mix of fine particles included in the
PM2.5 indicator (U.S. EPA, 2011a, p. 2-56).
    The CASAC concluded that it is appropriate to consider retaining
PM2.5 as the indicator for fine particles and further
asserted, ``There [is] insufficient peer-reviewed literature to support
any other indicator at this time'' (Samet, 2010c, p. 12). CASAC
expressed a strong desire for the EPA to ``look ahead to future review
cycles and reinvigorate support for the development of evidence that
might lead to newer indicators that may correlate better with the
health effects associated with ambient air concentrations of PM * * *''
(Samet, 2010c, p. 2).
    Consistent with the staff conclusions presented in the Policy
Assessment and CASAC advice, the Administrator proposes to retain
PM2.5 as the indicator for fine particles. Further, the
Administrator provisionally concludes that currently available
scientific information does not provide a sufficient basis for
supplementing mass-based, primary fine particle standards with
standards using a separate indicator for ultrafine particles or a
separate indicator for a specific PM2.5 component or group
of components associated with any source categories of fine particles.
Furthermore, the Administrator also provisionally concludes that the
currently available scientific information does not provide a
sufficient basis for eliminating any individual component or group of
components associated with any source categories from the mix of fine
particles included in the PM2.5 mass-based indicator.
2. Averaging Time
    In 1997, the EPA initially set both an annual standard, to provide
protection from health effects associated with both long- and short-
term exposures to PM2.5, and a 24-hour standard to
supplement the protection afforded by the annual standard (62 FR 38667
to 38668, July, 18, 1997). In the last review, the EPA retained both
annual and 24-hour averaging times (71 FR 61164, October 17, 2006).
These decisions were based, in part, on evidence of health effects
related to both long-term (from a year to several years) and short-term
(from less than one day to up to several days) measures of
PM2.5.
    The overwhelming majority of studies conducted since the last
review continue to utilize annual (or multi-year) and 24-hour averaging
times, reflecting the averaging times of the current PM2.5
standards. These studies continue to provide evidence that health
effects are associated with annual and 24-hour averaging times.
Therefore, the Policy Assessment concludes it is appropriate to retain
the current annual and 24-hour averaging times to provide protection
from effects associated with both long- and short-term PM2.5
exposures (U.S. EPA, 2011a, p. 2-57).
    In considering whether the information available in this review
supports consideration of different averaging times for
PM2.5 standards specifically with regard to considering a
standard with an averaging time less than 24 hours to address health
effects associated with sub-daily PM2.5 exposures, the
Policy Assessment notes there continues to be a growing body of studies
that provide additional evidence of effects associated with exposure
periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to
information available in the last review, recent studies provide
additional evidence for cardiovascular effects associated with sub-
daily (e.g., one to several hours) exposure to PM, especially effects
related to cardiac ischemia, vasomotor function, and more subtle
changes in markers of systemic inflammation, hemostasis, thrombosis and
coagulation (U.S. EPA, 2009a, section 6.2). Because these studies have
used different indicators (e.g., PM2.5, PM10,
PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2,
and 4 hours), and health outcomes, it is difficult to draw conclusions
about cardiovascular effects associated specifically with sub-daily
exposures to PM2.5.
    With regard to respiratory effects associated with sub-daily
PM2.5 exposures, the currently available evidence is much
sparser than for cardiovascular effects and continues to be very
limited. The Integrated Science Assessment concludes that for several
studies of hospital admissions or medical visits for respiratory
diseases, the strongest associations were observed with 24-hour average
or longer exposures, not with less than 24-hour exposures (U.S. EPA,
2009a, section 6.3).
    Collectively, the Policy Assessment concludes that this
information, when viewed as a whole, is too unclear, with respect to
the indicator, averaging time and health outcome, to serve as a basis
for consideration of establishing a primary PM2.5 standard
with an averaging time shorter than 24-hours at this time (U.S. EPA,
2011a, p. 2-57).
    With regard to health effects associated with PM2.5
exposure across varying seasons in this review, Bell et al. (2008)
reported higher PM2.5 risk estimates for hospitalization for
cardiovascular and respiratory diseases in the winter compared to other
seasons. In comparison to the winter season, smaller statistically
significant associations were also reported between PM2.5
and cardiovascular morbidity for spring and autumn, and a positive, but
statistically non-significant association was observed for the summer
months. In the case of mortality, Zanobetti and Schwartz (2009)
reported a 4-fold higher effect estimate for PM2.5
associated mortality for the spring as compared to the winter. Taken
together, these results provide emerging but limited evidence that
individuals may be at greater risk of dying from higher exposures to
PM2.5 in the warmer months and may be at greater risk of
PM2.5-associated hospitalization for cardiovascular and
respiratory diseases during colder months of the year (U.S. EPA, 2011a,
p. 2-58).
    Overall, the Policy Assessment observes that there are few studies
presently available to deduce a general

pattern in PM2.5-related risk across seasons. In addition,
these studies utilized 24-hour exposure periods within each season to
assess the PM2.5 associated health effects, and do not
provide information on health effects associated with a season-long
exposure to PM2.5. Due to these limitations in the currently
available evidence, the Policy Assessment concludes that there is no
basis to consider a seasonal averaging time separate from a 24-hour
averaging time.
    Based on the above considerations, the Policy Assessment concludes
that the currently available information provides strong support for
consideration of retaining current annual and 24-hour averaging timers
but does not provide support for considering alternative averaging
times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considers it
appropriate to retain the current annual and 24-hour averaging times
for the primary PM2.5 standards (Samet, 2010c, pp. 2 to 3).
The Administrator concurs with the staff conclusions and CASAC advice
and proposes that the averaging times for the primary PM2.5
standards should continue to include annual and 24-hour averages to
protect against health effects associated with long- and short-term
exposures. Furthermore, the Administrator provisionally concludes,
consistent with conclusions reached in the Policy Assessment and by
CASAC, that the currently available information is too limited to
support consideration of alternative averaging times to establish a
national standard with a shorter-than 24-hour averaging time or with a
seasonal averaging time.
3. Form
    The ``form'' of a standard defines the air quality statistic that
is to be compared to the level of the standard in determining whether
an area attains the standard. In this review, we consider whether
currently available information supports consideration of alternative
forms for the annual or 24-hour PM2.5 standards.
a. Annual Standard
    In 1997, the EPA established the form of the annual
PM2.5 standard as an annual arithmetic mean, averaged over 3
years, from single or multiple community-oriented monitors. This form
was intended to represent a relatively stable measure of air quality
and to characterize longer-term area-wide PM2.5
concentrations, in conjunction with a 24-hour standard designed to
provide adequate protection against localized peak or seasonal
PM2.5 concentrations. The level of the standard was to be
compared to measurements made at each community-oriented monitoring
site, or, if specific criteria were met, measurements from multiple
community-oriented monitoring sites could be averaged (62 FR 38671 to
38672, July 18, 1997). The constraints were intended to ensure that
spatial averaging would not result in inequities in the level of
protection provided by the standard (62 FR 38672, July 18, 1997). This
approach was consistent with the epidemiological studies on which the
PM2.5 standard was primarily based, in which air quality
data were generally averaged across multiple monitors in an area or
were taken from a single monitor that was selected to represent
community-wide exposures.
    In the last review, the EPA tightened the criteria for use of
spatial averaging to provide increased protection for vulnerable
populations exposed to PM2.5. This change was based in part
on an analysis of the potential for disproportionate impacts on
potentially at-risk populations, which found that the highest
concentrations in an area tend to be measured at monitors located in
areas where the surrounding population is more likely to have lower
education and income levels, and higher percentages of minority
populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section
<IP_ADDRESS>).
    In this review, as discussed in section III.B.3, there now exist
more health data such that the Integrated Science Assessment has
identified persons from lower socioeconomic strata as an at-risk
population (U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section
2.2.1). Moreover, there now exist more years of PM2.5 air
quality data than were available in the last review. Consideration in
the Policy Assessment of the spatial variability across urban areas
that is revealed by this expanded data base has raised questions as to
whether an annual standard that allows for spatial averaging, even
within specified constraints as narrowed in 2006, would provide
appropriate public health protection.
    In considering the potential for disproportionate impacts on at-
risk populations, the Policy Assessment recognizes an update of an air
quality analysis conducted for the last review (U.S. EPA, 2011a, pp. 2-
59 to 60; Schmidt, 2011a, Analysis A). This analysis focuses on
determining if the spatial averaging provisions, as modified in 2006,
could introduce inequities in protection for at-risk populations
exposed to PM2.5. Specifically, the Policy Assessment
considers whether persons of lower socioeconomic status are more likely
than the general population to live in areas in which the monitors
recording the highest air quality values in an area are located. Data
used in this analysis included demographic parameters measured at the
Census Block or Census Block Group level, including percent minority
population, percent minority subgroup population, percent of persons
living below the poverty level, percent of persons 18 years of age or
older, and percent of persons 65 years of age and older. In each
candidate geographic area, data from the Census Block(s) or Census
Block Group(s) surrounding the location of the monitoring site (as
delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which
the highest air quality value was monitored were compared to the
average of monitored values in the area. This analysis looked beyond
areas that would meet the current spatial averaging criteria and
considered all urban areas (i.e., Core Based Statistical Areas or
CBSAs) with at least two valid annual design value monitors (Schmidt,
2011a, Analysis A). Recognizing the limitations of such cross-sectional
analyses, the Policy Assessment observes that the highest
concentrations in an area tend to be measured at monitors located in
areas where the surrounding populations are more likely to live below
the poverty line and to have higher percentage of minorities (U.S. EPA,
2011a, p. 2-60).
    Based upon the analysis described above, the Policy Assessment
concludes that the existing constraints on spatial averaging, as
modified in 2006, may be inadequate to avoid substantially greater
exposures in some areas, potentially resulting in disproportionate
impacts on at-risk populations of persons with lower SES levels as well
as minorities. Therefore, the Policy Assessment concludes that it is
appropriate to consider revising the form of the annual
PM2.5 standard such that it does not allow for the use of
spatial averaging across monitors. In doing so, the level of the annual
PM2.5 standard would be compared to measurements made at the
monitoring site that represents area-wide air quality recording the
highest PM2.5 concentrations \71\ (U.S. EPA, 2011a, p. 2-
60).
---------------------------------------------------------------------------

    \71\ As discussed in section VIII.B.1 below, the EPA is
proposing to revise several terms associated with PM2.5
monitor placement. Specifically, the EPA is proposing to revoke the
term ``community-oriented'' and replace it with the term ``area-
wide'' monitoring.
---------------------------------------------------------------------------

    The CASAC agreed with staff conclusions that it is ``reasonable''
for the EPA to eliminate the spatial averaging provisions (Samet,
2010d, p. 2). Further, in CASAC's comments on

the first draft Policy Assessment, they noted, ``Given mounting
evidence showing that persons with lower SES levels are a susceptible
group for PM-related health risks, CASAC recommends that the provisions
that allow for spatial averaging across monitors be eliminated for the
reasons cited in the (first draft) Policy Assessment'' (Samet, 2010c,
p. 13).
    In considering the Policy Assessment's conclusions based on the
results of the analysis discussed above and concern over the evidence
of potential disproportionate impacts on at-risk populations as well as
CASAC advice, the Administrator proposes to revise the form of the
annual PM2.5 standard to eliminate the use of spatial
averaging. Thus, the Administrator proposes revising the form of the
annual PM2.5 standard to compare the level of the standard
with measurements from each ``appropriate'' monitor in an area\72\ with
no allowance for spatial averaging. Thus, for an area with multiple
monitors, the appropriate reporting monitor with the highest design
value would determine the attainment status for that area.
---------------------------------------------------------------------------

    \72\ As discussed in section VIII.B.2.b below, the EPA proposes
that PM2.5 monitoring sites at micro- and middle-scale
locations be comparable to the annual standard unless the monitoring
site has been approved by the Regional Administrator as a
``relatively unique micro-scale, or localized hot-spot, or unique
middle-scale site.''
---------------------------------------------------------------------------

b. 24-Hour Standard
    In 1997, the EPA established the form of the 24-hour
PM2.5 standard as the 98th percentile of 24-hour
concentrations at each population-oriented monitor within an area,
averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The
Agency selected the 98th percentile as an appropriate balance between
adequately limiting the occurrence of peak concentrations and providing
increased stability which, when averaged over 3 years, facilitated
effective health protection through the development of more stable
implementation programs. By basing the form of the standard on
concentrations measured at population-oriented monitoring sites, the
EPA intended to provide protection for people residing in or near
localized areas of elevated concentrations. In the last review, in
conjunction with lowering the level of the 24-hour standard, the EPA
retained this form based in part on a comparison with the 99th
percentile form.\73\
---------------------------------------------------------------------------

    \73\ In reaching this final decision, the EPA recognized a
technical problem associated with a potential bias in the method
used to calculate the 98th percentile concentration for this form.
The EPA adjusted the sampling frequency requirement in order to
reduce this bias. Accordingly, the Agency modified the final
monitoring requirements such that areas that are within 5 percent of
the standards are required to increase the sampling frequency to
every day (71 FR 61164 to 61165, October 17, 2006).
---------------------------------------------------------------------------

    In revisiting the stability of a 98th versus 99th percentile form
for a 24-hour standard intended to provide supplemental protection for
a generally controlling annual standard, an analysis presented in the
Policy Assessment considers air quality data reported in 2000 to 2008
to update our understanding of the ratio between peak-to-mean
PM2.5 concentrations. This analysis provides evidence that
the 98th percentile value is a more stable metric than the 99th
percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62).
    The Agency recognizes that the selection of the appropriate form of
the 24-hour standard includes maintaining adequate protection against
peak 24-hour concentrations while also providing a stable target for
risk management programs, which serves to provide for the most
effective public health protection in the long run.\74\ As in previous
reviews, the EPA recognizes that a concentration-based form, compared
to an exceedance-based form, is more reflective of the health risks
posed by elevated pollutant concentrations because such a form gives
proportionally greater weight to days when concentrations are well
above the level of the standard than to days when the concentrations
are just above the level of the standard. Further, the Agency concludes
that a concentration-based form, when averaged over three years,
provides an appropriate balance between limiting peak pollutant
concentrations and providing a stable regulatory target, thus
facilitating the development of more stable implementation programs.
---------------------------------------------------------------------------

    \74\ See ATA III, 283 F.3d at 374-376 which concludes that it is
legitimate for the EPA to consider overall stability of the standard
and its resulting promotion of overall effectiveness of NAAQS
control programs in setting a standard that is requisite to protect
the public health. The context for the court's discussion is
identical to that here; whether to adopt a 98th percentile form for
a 24-hour primary PM2.5 standard intended to provide
supplemental protection for a generally controlling annual standard.
---------------------------------------------------------------------------

    In considering the information provided in the Policy Assessment
and recognizing that the degree of public health protection likely to
be afforded by a standard is a result of the combination of the form
and the level of the standard, the Administrator proposes to retain the
98th percentile form of the 24-hour standard. The Administrator
provisionally concludes that the 98th percentile form represents an
appropriate balance between adequately limiting the occurrence of peak
concentrations and providing increased stability relative to an
alternative 99th percentile form.
4. Level
    In the last review, the EPA selected levels for the annual and the
24-hour PM2.5 standards using evidence of effects associated
with periods of exposure that were most closely matched to the
averaging time of each standard. Thus, as discussed in section III.A.1,
the EPA relied upon evidence from long-term exposure studies as the
principal basis for selecting the level of the annual PM2.5
standard that would protect against effects associated with long-term
exposures. The EPA relied upon evidence from the short-term exposures
studies as the principal basis for selecting the level of the 24-hour
PM2.5 standard that would protect against effects associated
with short-term exposures. As summarized in section III.A.2 above, the
2006 decision to retain the level of the annual PM2.5
standard at 15 [micro]g/m\3\ \75\ was challenged and on judicial
review, the D.C. Circuit remanded the primary annual PM2.5
standard to the EPA, finding that EPA's explanation for its approach to
setting the level of the annual standard was inadequate.
---------------------------------------------------------------------------

    \75\ Throughout this section, the annual standard level is
denoted as an integer value for simplicity, although, as noted above
in section II.B.1, Table 1, the standard level is defined to one
decimal place, such that the current standard level is 15.0
[micro]g/m\3\. Alternative standard levels discussed in this section
are similarly defined to one decimal place.
---------------------------------------------------------------------------

a. Approach Used in the Policy Assessment
    Building upon the lessons learned in the previous PM NAAQS reviews,
in considering alternative standard levels supported by the currently
available scientific information, the Policy Assessment uses an
approach that integrates evidence-based and risk-based considerations,
takes into account CASAC advice, and considers the issues raised by the
court in remanding the primary annual PM2.5 standard.
Following the general approach outlined in section III.A.3, for the
reasons discussed below, the Policy Assessment concludes it is
appropriate to consider the protection afforded by the annual and 24-
hour standards taken together against mortality and morbidity effects
associated with both long- and short-term PM2.5 exposures.
This is consistent with the approach taken in the review completed in
1997 rather than considering each standard separately, as was done in
the review completed in 2006.

    Beyond looking directly at the relevant epidemiologic evidence, the
Policy Assessment considers the extent to which specific alternative
PM2.5 standard levels are likely to reduce the nature and
magnitude of both long-term exposure-related mortality risk and short-
term exposure-related mortality and morbidity risk (U.S. EPA, 2011a,
section <IP_ADDRESS>; U.S. EPA, 2010a, section 4.2.2). As noted in section
III.C.3 above, patterns of increasing estimated risk reductions are
generally observed as either the annual or 24-hour standard, or both,
are reduced below the level of the current standards (U.S. 2011a,
Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and
5.2.3).
    Based on the quantitative risk assessment, the Policy Assessment
observes, as discussed in section III.A.3, that analyses conducted for
this and previous reviews demonstrate that much, if not most, of the
aggregate risk associated with short-term exposures results from the
large number of days during which the 24-hour average concentrations
are in the low-to mid-range, below the peak 24-hour concentrations
(U.S. EPA, 2011a, p. 2-9). Furthermore, as discussed in section
III.C.3, the Risk Assessment observes that alternative annual standard
levels, when controlling, resulted in more consistent risk reductions
across urban study areas, thereby potentially providing a more
consistent degree of public health protection (U.S. EPA, 2010a, pp. 5-
15 to 5-16). In contrast, the Risk Assessment notes that while the
results of simulating alternative suites of PM2.5 standards
including different combinations of alternative annual and 24-hour
standard levels suggest that an alternative 24-hour standard level can
produce additional estimated risk reductions beyond that provided by an
alternative annual standard alone. However, the degree of estimated
risk reduction provided by alternative 24-hour standard levels is
highly variable, in part due to the choice of rollback approached used
(U.S. EPA, 2010a, p. 5-17).
    Therefore, the Policy Assessment concludes, consistent with CASAC
advice (Samet 2010c, p. 1), that it is appropriate to set a ``generally
controlling'' annual standard that will lower a wide range of ambient
24-hour concentrations. The Policy Assessment concludes this approach
would likely reduce aggregate risks associated with both long- and
short-term exposures with more consistency than a generally controlling
24-hour standard and would be the most effective and efficient way to
reduce total PM2.5-related population risk and so provide
appropriate protection. The staff believes this approach, in contrast
to one focusing on a generally controlling 24-hour standard, would
likely reduce aggregate risks associated with both long- and short-term
exposures with more consistency and would likely avoid setting national
standards that could result in relatively uneven protection across the
country due to setting standards that are either more or less stringent
than necessary in different geographical areas.
    The Policy Assessment recognizes that an annual standard intended
to serve as the primary means for providing protection against effects
associated with both long- and short-term PM2.5 exposures
cannot be expected to offer an adequate margin of safety against the
effects of all short-term PM2.5 exposures. As a result, in
conjunction with a generally controlling annual standard, the Policy
Assessment concludes it is appropriate to consider setting a 24-hour
standard to provide supplemental protection, particularly for areas
with high peak-to-mean ratios possibly associated with strong local or
seasonal sources, or PM2.5-related effects that may be
associated with shorter-than-daily exposure periods.
    Based on the above considerations, the approach used in the Policy
Assessment to identify alternative standard levels that are appropriate
for consideration focuses on translating information from
epidemiological studies into the basis for staff conclusions on levels.
This approach is broader and more integrative than the general approach
used by the EPA in previous reviews (see summary in section III.A.3
above; U.S. EPA, 2011a, sections 2.1.3 and <IP_ADDRESS>) and reflects the
more extensive and stronger body of scientific evidence now available
on health effects related to long- and short-term PM2.5
exposures, a more comprehensive quantitative risk assessment, and more
extensive PM2.5 air quality data. In considering the
currently available information, the Policy Assessment focuses on
identifying levels for an annual standard and a 24-hour standard that,
in combination, provide protection against health effects associated
with both long- and short-term PM2.5 exposures. The Policy
Assessment also considers the extent to which various combinations of
annual and 24-hour standards reflect setting a generally controlling
annual standard with a 24-hour standard providing supplemental
protection (U.S. EPA, 2011a, sections 2.1.3, <IP_ADDRESS>).
    As discussed in the Policy Assessment, EPA staff recognizes that
there is no single factor or criterion that comprises the ``correct''
approach for reaching conclusions on alternative standard levels for
consideration, but rather there are various approaches that are
reasonable to consider (U.S. EPA, 2011a, section <IP_ADDRESS>). In reaching
conclusions in the Policy Assessment on the ranges of standard levels
that are appropriate to consider, staff considered the relative weight
to place on different evidence. The Policy Assessment initially focuses
on long- and short-term PM2.5 exposure studies conducted in
the U.S. and Canada and places the greatest weight on health outcomes
judged in the Integrated Science Assessment as having evidence to
support a causal or likely causal relationship. The Policy Assessment
also considers the evidence for a broader range of health outcomes
judged in the Integrated Science Assessment to have evidence suggestive
of a causal relationship, specifically studies that focus on effects in
susceptible populations, to evaluate whether this evidence provides
support for considering lower alternative standard levels.
    Several factors were taken into account in placing relative weight
on the body of available epidemiological studies, for example, study
characteristics, including study design (e.g., time period of air
quality monitoring, control for potential confounders); strength of the
study (in terms of statistical significance and precision of results);
and availability of population-level and air quality distribution data.
As noted above in section III.A.3, the Policy Assessment places
greatest weight on information from multi-city epidemiological studies
to inform staff conclusions regarding alternative annual standard
levels. These studies have a number of advantages compared to single-
city studies \76\ that include providing representation of ambient
PM2.5 concentrations and potential health impacts across a
range of diverse locations providing spatial coverage for different
regions across the country, reflecting differences in PM2.5
sources, composition, and potentially other exposure-related factors
which might impact PM2.5-related risks; lack of

`publication bias' (U.S. EPA, 2004, p. 8-30); and consideration of
larger study populations that afford the possibility of generalizing to
the broader national population and provide higher statistical power
than single-city studies to detect potentially statistically
significant associations with relatively more precise effect estimates.
---------------------------------------------------------------------------

    \76\ As discussed in section III.B.1 above, the Policy
Assessment recognizes that single-city studies provide ancillary
evidence to multi-city studies in support of calling into question
the adequacy of the current suite of standards. However, in light of
the mixed findings reported in single-city short-term
PM2.5 exposure studies, and the likelihood that these
results are influenced by localized events and not representative of
air quality across the country, the Policy Assessment places
comparatively greater weight on the results from multi-city studies
in considering alternative annual and 24-hour standard levels (U.S.
EPA, 2011a, p. 2-64).
---------------------------------------------------------------------------

    In reaching conclusions in the Policy Assessment regarding
alternative 24-hour standard levels that are appropriate to consider,
staff also considers relevant information from single-city short-term
PM2.5 exposure studies. Although, as discussed above, multi-
city studies have greater power to detect associations and provide
broader geographic coverage in comparison to single-city studies, the
extent to which effects reported in multi-city short-term
PM2.5 exposure studies are associated with the specific
short-term air quality in any particular location is unclear,
especially when considering short-term concentrations at the upper end
of the air quality distribution (i.e., at the 98th percentile value)
for a given study area. In contrast, single-city studies are more
limited in terms of power and geographic coverage but the link between
reported health effects and the air quality in a given study area is
more straightforward. Therefore, the Policy Assessment considers the
results of both multi-city and single-city short-term exposure studies
to inform staff conclusions regarding alternative levels that are
appropriate to consider for a 24-hour standard that is intended to
provide supplemental protection in areas where the annual standard may
not offer appropriate protection against the effects of all short-term
exposures (U.S. EPA, 2011a, pp. 2-62 to 2-65).
b. Consideration of the Annual Standard in the Policy Assessment
    In recognizing the absence of a discernible population threshold
below which effects would not occur, the Policy Assessment's general
approach for identifying alternative annual standard levels that are
appropriate to consider focuses on characterizing the range of
PM2.5 concentrations over which we have the most confidence
in the associations reported in the epidemiological studies, and
conversely where our confidence in the association becomes appreciably
lower. The most direct approach to address this issue, consistent with
CASAC advice (Samet, 2010c, p.10), is to consider epidemiological
studies reporting confidence intervals around concentration-response
relationships (U.S. EPA, 2011a, p. 2-63). Based on a thorough search of
the available evidence, the Policy Assessment identified three long-
term PM2.5 exposure studies reporting confidence intervals
around concentration-response functions (i.e., Schwartz et al., 2008;
Pope et al., 2002; Miller et al., 2007; U.S. EPA, 2011a, pp. 2-65 to 2-
70 and Figure 2-3).\77\ In its assessment of these studies, the Policy
Assessment places greater weight on analyses that averaged across
multiple concentration-response models since this approach represents a
more robust examination of the underlying concentration-response
relationship than analyses considering a single concentration-response
model. Although these analyses of long-term exposure to
PM2.5 provide information on the lack of any discernible
population threshold, only Schwartz et al. (2008) conducted a multi-
model analysis to characterize confidence intervals around the
estimated concentration-response relationship that can help inform at
what PM2.5 concentrations we have appreciably less
confidence in the nature of the underlying concentration-response
relationship. Although analyses of confidence intervals associated with
concentration-response relationships can help inform consideration of
alternative standard levels, the Policy Assessment concludes that the
single relevant analysis now available is too limited to serve as the
principal basis for identifying alternative standard levels in this
review (U.S. EPA, 2011a, p. 2-70).
---------------------------------------------------------------------------

    \77\ The EPA carefully analyzed the published evidence, but was
unable to identify any short-term PM2.5 exposure studies
that characterized confidence intervals around concentration-
response relationships. Nor did CASAC or public comments on this
issue, as addressed in their comments on the second draft Policy
Assessment, identify any additional analyses.
---------------------------------------------------------------------------

    The Policy Assessment explores other approaches that considered
different statistical metrics to identify ranges of long-term mean
PM2.5 concentrations that were most influential in
generating health effect estimates in long- and short-term
epidemiological studies, placing greatest weight on those studies that
reported positive and statistically significant associations (U.S. EPA,
2011a, p. 2-63). First, as discussed in section III.A.3 above, the
Policy Assessment considered the statistical metric used in previous
reviews. This approach recognizes that the strongest evidence of
associations occurs at concentrations around the long-term mean
concentration. Thus, in earlier reviews, the EPA focused on identifying
standard levels that were somewhat below the long-term mean
concentrations reported in PM2.5 exposure studies. The long-
term mean concentrations represent air quality data typically used in
epidemiological analyses and provide a direct link between
PM2.5 concentrations and the observed health effects.
Further, these data are available for all long- and short-term exposure
studies analyzed and, therefore, represent the data set available for
the broadest set of epidemiological studies.
    However, consistent with CASAC's comments on the second draft
Policy Assessment \78\ (Samet, 2010d, p. 2), in preparing the final
Policy Assessment, EPA staff explored ways to take into account
additional information from epidemiological studies, when available
(Rajan et al., 2011). These analyses focused on evaluating different
statistical metrics, beyond the long-term mean concentration, to
characterize the range of PM2.5 concentrations down through
which staff continued to have confidence in the associations observed
in epidemiological studies and below which there is a comparative lack
of data such that the staff's confidence in the relationship was
appreciably less. This would also be the range of PM2.5
concentrations which has the most influence on generating the health
effect estimates reported in epidemiological studies. As discussed in
section III.A.3 above, the Policy Assessment recognizes there is no one
percentile value within a given distribution that is the most
appropriate or ``correct'' way to characterize where our confidence in
the associations becomes appreciably lower. The Policy Assessment
concludes that focusing on concentrations within the lower quartile of
a distribution, such as the range from the 25th to the 10th percentile,
is reasonable to consider as a region within which we begin to have
appreciably less confidence in the associations observed in
epidemiological studies.\79\ In staff's

view, considering lower PM2.5 concentrations, down to the
lowest concentration observed in a study, would be a highly uncertain
basis for selecting alternative standard levels (U.S. EPA, 2009a, p. 2-
71).
---------------------------------------------------------------------------

    \78\ While CASAC expressed the view that it would be most
desirable to have information on concentration-response
relationships, they recognized that it would also be ``preferable to
have information on the concentrations that were most influential in
generating the health effect estimates in individual studies''
(Samet, 2010d, p. 2).
    \79\ In the last review, staff believed it was appropriate to
consider a level for an annual PM2.5 standard that was
somewhat below the averages of the long-term concentrations across
the cities in each of the key long-term exposures studies,
recognizing that the evidence of an association in any such study
was strongest at and around the long-term average where the data in
the study are most concentrated. For example, the interquartile
range of long-term average concentrations within a study and a range
within one standard deviation around the study mean were considered
reasonable approaches for characterizing the range over which the
evidence of association is strongest (U.S. EPA, 2005, pp. 5-22 to 5-
23). In this review, the Policy Assessment noted the
interrelatedness of the distributional statistics and a range of one
standard deviation around the mean which contains approximately 68
percent of normally distributed data, in that one standard deviation
below the mean falls between the 25th and 10th percentiles (U.S.
EPA, 2011a, p. 2-71).
---------------------------------------------------------------------------

    As outlined in section III.A.3 above, the Policy Assessment
recognizes that there are two types of population-level information to
consider in identifying the range of PM2.5 concentrations
which have the most influence on generating the health effect estimates
reported in epidemiological studies. The most relevant information to
consider is the number of health events (e.g., deaths,
hospitalizations) occurring within a study population in relation to
the distribution of PM2.5 concentrations likely experienced
by study participants. However, in recognizing that access to health
event data may be restricted, and consistent with advice from CASAC
(Samet 2010d, p.2), EPA staff also considered the number of
participants within each study area in relation to the distribution of
PM2.5 concentrations (i.e., study population data), as an
appropriate surrogate for health event data.
    In applying this approach, the Policy Assessment focuses on
identifying the broader range of PM2.5 concentrations which
had the most influence on generating health effect estimates in
epidemiological studies, as discussed in section III.A.3 above. As
discussed below, in working with study investigators, EPA staff was
able to obtain health event data for three large multi-city studies
(Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et al., 2008)
and population data for the same three studies and one additional long-
term exposure study (Miller et al., 2007); as documented in a staff
memorandum (Rajan et al., 2011). For the three studies for which both
health event and study population data were available, EPA staff
analyzed the reliability of using study population data as a surrogate
for health event data. Based on these analyses, EPA staff recognized
that the 10th and 25th percentiles of the health event and study
population distributions are nearly identical and concluded that the
distribution of population data can be a useful surrogate for event
data, providing support for consideration of the study population data
for Miller et al. (2007), for which health event data were not
available (Rajan et al., 2011, Analysis 1 and Analysis 2, in
particular, Table 1 and Figures 1 and 2).
    With regard to the long-term mean PM2.5 concentrations
which are relevant to the first approach, Figures 1 through 3 (U.S.
EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) summarize data available
for multi-city, long- and short-term exposure studies that evaluated
endpoints classified in the Integrated Science Assessment as having
evidence of a causal or likely causal relationship or evidence
suggestive of a causal relationship, showing the studies with long-term
mean PM2.5 concentrations below 17 [mu]g/m\3\.\80\ Figures 1
and 3 summarize the health outcomes evaluated, relative risk estimates,
air quality data, and geographic scope for long- and short-term
exposure studies, respectively, that evaluated mortality (evidence of a
causal relationship); cardiovascular effects (evidence of a causal
relationship); and respiratory effects (evidence of a likely causal
relationship) in the general population, as well as in older adults, an
at-risk population. Figure 2 provides this same summary information for
long-term exposure studies that evaluated respiratory effects (evidence
of a likely causal relationship) in children, an at-risk population, as
well as developmental effects (evidence suggestive of a causal
relationship). By following the general approach used in previous PM
NAAQS reviews, one could consider identifying alternative standard
levels that are somewhat below the long-term mean PM2.5
concentrations reported in these epidemiological studies.
---------------------------------------------------------------------------

    \80\ Additional studies presented and assessed in the Integrated
Science Assessment report effects at higher long-term mean
PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-1,
2-2, 7-6, and 7-7).
---------------------------------------------------------------------------

BILLING CODE 6560-50-P

[GRAPHIC] [TIFF OMITTED] TP29JN12.000

[GRAPHIC] [TIFF OMITTED] TP29JN12.001

[GRAPHIC] [TIFF OMITTED] TP29JN12.002

BILLING CODE 6560-50-C

    With regard to consideration of additional information from
epidemiological studies which is relevant to the second approach, EPA
has compiled a summary of the range of PM2.5 concentrations
corresponding with the 25th to 10th percentiles of health event or
study population data from the four multi-city studies, for which
distributional statistics are available \81\ (U.S. EPA, 2011a, Figure
2-7; Rajan et al., 2011, Table 1). By considering this approach, one
could focus on the range of PM2.5 concentrations below the
long-term mean ambient concentrations over which we continue to have
confidence in the associations observed in epidemiological studies
(e.g., above the 25th percentile) where commensurate public health
protection could be obtained for PM2.5-related effects and,
conversely, identify the range in the distribution below which our
confidence in the associations is appreciably less, to identify
alternative annual standard levels.
---------------------------------------------------------------------------

    \81\ Health event data (e.g., number of deaths,
hospitalizations) occurring in a study population were obtained for
three multi-city studies (Krewski et al., 2009; Zanobetti and
Schwartz, 2009; Bell et al., 2008) and study population data were
obtained for the same three studies and one additional study (Miller
et al., 2007) (U.S. EPA, 2011a, p.2-71). If health event or study
population data were available for additional studies, the EPA could
employ distributional statistics to identify the broader range of
PM2.5 concentrations that were most influential in
generating health effect estimates in those studies.
---------------------------------------------------------------------------

    The mean PM2.5 concentrations associated with the
studies summarized in Figures 1, 2, and 3 and with the distributional
statistics analyses (Rajan et al., 2011) are based on concentrations
averaged across ambient monitors within each area included in a given
study and then averaged across study areas to calculate an overall
study mean concentration, as discussed above. As noted above in section
III.A.3 and discussed in the Policy Assessment, a policy approach that
uses data based on composite monitor distributions to identify
alternative standard levels, and then compares those levels to
concentrations at appropriate maximum monitors to determine if an area
meets a given standard, inherently has the potential to build in some
margin of safety (U.S. EPA, 2011a, p. 2-14). In analyses conducted by
EPA staff based on selected long- and short-term exposure studies, the
Policy Assessment notes that the differences between the maximum and
composite distributions were greater for studies with fewer years of
air quality data (i.e., 1 to 3 years) and smaller numbers of study
areas (i.e., 36 to 51 study areas). The differences in the maximum and
composite monitor distribution were much smaller (i.e., generally
within five percent) for studies with more years of air quality data
(i.e., up to 6 years) and larger numbers of study areas (i.e., 112 to
204 study areas) (Hassett-Sipple et al., 2010; U.S. EPA, 2010f, section
<IP_ADDRESS>). Therefore, any margin of safety that may be provided by a
policy approach that uses data based on composite monitor distributions
to identify alternative standard levels, and then compares those levels
to concentrations at appropriate maximum monitors to determine if an
area meets a given standard, will vary depending upon the number of
monitors and air quality distributions within a given area. See also,
section III.A.3 above.
    Figure 4 summarizes statistical metrics for those studies included
in Figures 1, 2, and 3 that provide evidence of statistically
significant PM2.5-related effects, which are relevant to the
two approaches for translating epidemiological evidence into standard
levels discussed above. The top of Figure 4 includes information for
long-term exposure studies evaluating health outcomes classified as
having evidence of a casual or likely casual relationship with
PM2.5 exposures (long-term mean PM2.5
concentrations indicated by diamond symbols). The middle of Figure 4
includes information for short-term exposure studies evaluating health
outcomes classified as having evidence of a casual or likely casual
relationship with PM2.5 exposures (long-term mean
PM2.5 concentrations indicated by triangle symbols). The
bottom of Figure 4 includes information for long-term exposures studies
evaluating health outcomes classified as having evidence suggestive of
a causal relationship (long-term mean PM2.5 concentrations
indicated by square symbols). Figure 4 also summarizes the range of
PM2.5 concentrations corresponding with the 25th (indicated
by solid circles) to 10th (indicated by open circles) percentiles of
the health event or study population data from the four multi-city
studies (highlighted in bold text) for which distributional statistics
are available.
BILLING CODE 6560-50-P

[GRAPHIC] [TIFF OMITTED] TP29JN12.003

BILLING CODE 6560-50-C

    In looking first at the long-term mean PM2.5
concentrations reported in the multi-city long-term exposure studies,
as summarized at the top of Figure 4, the Policy Assessment observes
positive and often statistically significant associations at long-term
mean PM2.5 concentrations ranging from 16.4 to 12.9 [mu]g/
m\3\ \82\ (Laden et al., 2006; Lipfert et al., 2006; Krewski et al.,
2009; Goss et al., 2004; Miller et al.; 2007; Zeger et al., 2008; Eftim
et al., 2008; Dockery et al., 1996; McConnell et al., 2003). In
considering the one long-term PM2.5 exposure study for which
health event data are available (Krewski et al., 2009), the Policy
Assessment observes that the long-term mean PM2.5
concentrations corresponding with study areas contributing to the 25th
and 10th percentiles of the distribution of mortality data are 12.0
[mu]g/m\3\ and 10.2 [mu]g/m\3\, respectively (Figure 4; U.S. EPA,
2011a, Figure 2-7; Rajan et al., 2011, Table 1). As identified above,
although less directly relevant than event data, the number of
participants within each study area can be used as a surrogate for
health event data in relation to the distribution of PM2.5
concentrations. The long-term mean PM2.5 concentrations
corresponding with study areas contributing to the 25th and 10th
percentiles of the distribution of study participants for Miller et al.
(2007) were 11.2 [mu]g/m\3\ and 9.7 [mu]g/m\3\, respectively (Figure 4;
U.S. EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1).
---------------------------------------------------------------------------

    \82\ As discussed in section III.D.1.a above, the lowest long-
term mean PM2.5 concentration reported in the long-term
exposure studies was based on updated air quality data for Miller et
al. (2007). As noted in the Policy Assessment, these air quality
data were based on only one year of ambient measurements (2000) and
in comparison to other long-term exposure studies that considered
multiple years of air quality data, were much more limited (U.S.
EPA, 2011a, pp. 2-81 to 2-82).
---------------------------------------------------------------------------

    In then considering information from multi-city, short-term
exposure studies reporting positive and statistically significant
associations with these same broad health effect categories, as
summarized in the middle of Figure 4, the Policy Assessment observes
positive and statistically significant associations at long-term mean
PM2.5 concentrations in a similar range of 15.6 to 12.8
[mu]g/m\3\ (Franklin et al., 2007, 2008; Klemm and Mason, 2003; Burnett
and Goldberg, 2003; Zanobetti and Schwartz, 2009; Burnett et al., 2004;
Bell et al., 2008; Dominici et al., 2006a; see Figure 3). In
considering the two multi-city, short-term PM2.5 exposure
studies for which health event data are available, the Policy
Assessment observes that the long-term mean PM2.5
concentrations corresponding with study areas contributing to the 25th
and 10th percentiles of the distribution of deaths and cardiovascular-
related hospitalizations are 12.5 [mu]g/m\3\ and 10.3 [mu]g/m\3\,
respectively, for Zanobetti and Schwartz (2009), and 11.5 [mu]g/m\3\
and 9.8 [mu]g/m\3\, respectively, for Bell et al. (2008) (Figure 4;
U.S. EPA, 2011a, Figure 2-7; Rajan et al., 2011, Table 1).
    Taking into consideration additional studies of specific at-risk
populations (i.e., children), the Policy Assessment expands its
evaluation of the long-term exposure studies to include a broader range
of health outcomes judged in the Integrated Science Assessment to have
evidence suggestive of a causal relationship. This evidence was taken
into account to evaluate whether it provides support for considering
lower alternative levels than if weight were only placed on studies for
which health effects have been judged in the Integrated Science
Assessment to have evidence supporting a causal or likely causal
relationship. The Policy Assessment makes note of a limited number of
studies that provide emerging evidence for PM2.5-related low
birth weight and infant mortality, especially related to respiratory
causes during the post-neonatal period. This more limited body of
evidence, as summarized at the bottom of Figure 4, indicates positive
and often statistically significant effects associated with long-term
PM2.5 mean concentrations in the range of 14.9 to 11.9
[mu]g/m\3\ (Woodruff et al., 2008; Liu et al., 2007; Bell et al., 2007;
see Figure 2). As illustrated in Figure 2, although Parker and Woodruff
(2008) did not observe an association between quarterly estimates of
exposure to PM2.5 and low birth weight in a multi-city U.S.
study, other U.S. and Canadian studies did report positive and
statistically significant associations between PM2.5 and low
birth weight at lower ambient concentrations (Bell et al., 2007; Liu et
al., 2007).\83\ There remain significant limitations (e.g., identifying
the etiologically relevant time period) in the evaluation of evidence
on the relationship between PM2.5 exposures and birth
outcomes (U.S. EPA, 2009a, pp. 7-48 and 7-56) which should be taken
into consideration in reaching judgments about how to weigh these
studies of potential impacts on specific susceptible populations in
considering alternative standard levels that provide protection with an
appropriate margin of safety.
---------------------------------------------------------------------------

    \83\ As noted in section 7.4 of the Integrated Science
Assessment, Parker et al. (2005) reported that over a 9-month
exposure period (mean PM2.5 concentration of 15.4 [mu]g/
m\3\) a significant decrease in birth weight was associated with
infants in the highest quartile of PM2.5 exposure as
compared to infants exposed in the lowest quartile.
---------------------------------------------------------------------------

    With respect to carcinogenicity, mutagenicity, and genotoxicity
(evidence suggestive of a causal relationship), the strongest evidence
currently available is from long-term prospective cohort studies that
report positive associations between PM2.5 and lung cancer
mortality. At this time, the PM2.5 concentrations reported
in studies evaluating these effects generally included ambient
concentrations that are equal to or greater than ambient concentrations
observed in studies that reported mortality and cardiovascular and
respiratory effects (U.S. EPA, 2009a, section 7.5). Therefore, in
selecting alternative standard levels appropriate to consider, the
Policy Assessment noted that, in providing protection against mortality
and cardiovascular and respiratory effects it is reasonable to
anticipate that protection will also be provided for carcinogenicity,
mutagenicity, and genotoxicity effects (U.S. EPA, 2011a, p. 2-78).
    In summarizing the currently available evidence and air quality
information within the context of identifying potential alternative
annual standard levels for consideration, the Policy Assessment first
notes that the Integrated Science Assessment concludes there is no
evidence of a discernible population threshold below which effects
would not occur. Thus, health effects may occur over the full range of
concentrations observed in the epidemiological studies. In the absence
of any discernible thresholds, the general approach used in the Policy
Assessment for identifying alternative standard levels that would
provide appropriate protection against effects observed in
epidemiological studies has focused on the central question of
identifying the range of PM2.5 concentrations below the
long-term mean concentrations where we continue to have confidence in
the associations observed in epidemiological studies.
    In considering the evidence, the Policy Assessment recognizes that
NAAQS are standards set so as to provide requisite protection, neither
more nor less stringent than necessary to protect public health with an
adequate margin of safety. This judgment, ultimately made by the
Administrator, involves weighing the strength of the evidence and the
inherent uncertainties and limitations of that evidence. Therefore,
depending on the weight placed on different aspects of the evidence and
inherent uncertainties, considerations of different alternative
standard levels could be supported.

    Given the currently available evidence and considering the various
approaches discussed above, the Policy Assessment concludes it is
appropriate to focus on an annual standard level within a range of
about 12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-
106). As illustrated in Figure 4, a standard level of 12 [mu]g/m\3\, at
the upper end of this range, is somewhat below the long-term mean
PM2.5 concentrations reported in all the multi-city, long-
and short-term exposure studies that provide evidence of positive and
statistically significant associations with health effects classified
as having evidence of a causal or likely causal relationship, including
premature mortality and hospitalizations and emergency department
visits for cardiovascular and respiratory effects as well as
respiratory effects in children. Further, a level of 12 [mu]g/m\3\
would reflect consideration of additional population-level information
from such epidemiological studies in that it generally corresponds with
approximately the 25th percentile of the available distributions of
health events data in the studies for which population-level
information was available.\84\ In addition, a level of 12 [mu]g/m\3\
would reflect some consideration of studies that provide more limited
evidence of reproductive and developmental effects, which are
suggestive of a causal relationship, in that it is about at the same
level as the lowest long-term mean PM2.5 concentrations
reported in such studies (see Figure 4).
---------------------------------------------------------------------------

    \84\ As outlined in section III.A.3, the Policy Assessment
considers the 25th percentile to be the start of the range of
PM2.5 concentrations below the mean within which the data
become appreciably more sparse and, thus, where our confidence in
the associations observed in epidemiological studies begins to
become appreciably less.
---------------------------------------------------------------------------

    Alternatively, an annual standard level of 11 [mu]g/m\3\, at the
lower end of this range, is well below the lowest long-term mean
PM2.5 concentrations reported in all multi-city long- and
short-term exposure studies that provide evidence of positive and
statistically significant associations with health effects classified
as having evidence of a causal or likely causal relationship. A level
of 11 [mu]g/m\3\ would reflect placing more weight on the distributions
of health event and population data, in that this level is within the
range of PM2.5 concentrations corresponding to the 25th and
10th percentiles of all the available distributions of such data.\85\
In addition, a level of 11 [mu]g/m\3\ is somewhat below the lowest
long-term mean PM2.5 concentrations reported in reproductive
and developmental effects studies that are suggestive of a causal
relationship. Thus, a level of 11 [mu]g/m\3\ would reflect an approach
to translating the available evidence that places relatively more
emphasis on margin of safety considerations than would a standard set
at a higher level. Such a policy approach would tend to weigh
uncertainties in the evidence in such a way as to avoid potentially
underestimating PM2.5-related risks to public health.
Further, recognizing the uncertainties inherent in identifying any
particular point at which our confidence in reported associations
becomes appreciably less, the Policy Assessment concludes that the
available evidence does not provide a sufficient basis to consider
alternative annual standard levels below 11 [mu]g/m\3\ (U.S. EPA,
2011a, p. 2-81).
---------------------------------------------------------------------------

    \85\ As discussed in section III.A.3, the Policy Assessment
identifies the range from the 25th to the 10th percentiles as a
reasonable range to consider, in that it is a range where we have
appreciably less confidence in the associations observed in
epidemiological studies (U.S. EPA, 2011a, p. 2-12).
---------------------------------------------------------------------------

    The Policy Assessment also considers the extent to which the
available evidence provides a basis for considering alternative annual
standard levels above 12 [mu]g/m\3\. As discussed below, the Policy
Assessment concludes that it could be reasonable to consider a standard
level up to 13 [mu]g/m\3\ based on a policy approach that tends to
weigh uncertainties in the evidence in such a way as to avoid
potentially overestimating PM2.5-related risks to public
health, especially to the extent that primary emphasis is placed on
long-term exposure studies as a basis for an annual standard level. A
level of 13 [mu]g/m\3\ is somewhat below the long-term mean
PM2.5 concentrations reported in all but one of the long-
term exposure studies providing evidence of positive and statistically
significant associations with PM2.5-related health effects
classified as having a causal or likely causal relationship. As shown
in Figure 4, the one long-term exposure study with a long-term mean
PM2.5 concentration just below 13 [mu]g/m\3\ is the WHI
study (Miller et al., 2007). As noted in section III.D.1.a above, the
Policy Assessment observes that in comparison to other long-term
exposure studies, the WHI study was more limited in that it was based
on only one year of air quality data (U.S. EPA, 2011a, pp. 2-81 to 2-
82). Thus, to the extent that less weight is placed on the WHI study
than on other long-term exposure studies with more robust air quality
data, a level of 13 [mu]g/m\3\ could be considered as being protective
of long-term exposure related effects classified as having a causal or
likely causal relationship. In also considering short-term exposure
studies, the Policy Assessment notes that a level of 13 [mu]g/m\3\ is
below the long-term mean PM2.5 concentrations reported in
most such studies, but is above the long-term means of 12.8 and 12.9
[mu]g/m\3\ reported in Burnett et al. (2004) and Bell et al. (2008),
respectively. In considering these studies, the Policy Assessment finds
no basis to conclude that these two studies are any more limited or
uncertain than the other short-term exposure studies shown in Figures 3
and 4 (U.S. EPA, 2011a, p. 2-82). On this basis, as discussed below,
the Policy Assessment concludes that consideration of an annual
standard level of 13 [mu]g/m\3\ would have implications for the degree
of protection that would need to be provided by the 24-hour standard,
such that taken together the suite of PM2.5 standards would
provide appropriate protection from effects on public health related to
short-term exposure to PM2.5 (U.S. EPA, 2011a, p. 2-82).
    The Policy Assessment also notes that a standard level of 13 [mu]g/
m\3\ would reflect a judgment that the uncertainties in the
epidemiological evidence as summarized in section III.B.2 above,
including uncertainties related to the heterogeneity observed in the
epidemiological studies in the eastern versus western parts of the
U.S., the relative toxicity of PM2.5 components, and the
potential role of co-pollutants, are too great to warrant placing any
weight on the distributions of health event and population data that
extend down below the long-term mean concentrations into the lower
quartile of the data. This level would also reflect a judgment that the
evidence from reproductive and developmental effects studies that is
suggestive of a causal relationship is too uncertain to support
consideration of any lower level.
    Beyond evidence-based considerations, the Policy Assessment also
considered the extent to which quantitative risk assessment supports
consideration of these alternative standard levels or provides support
for lower levels. In considering simulations of just meeting
alternative annual standard levels within the range of 13 to 11 [mu]g/
m\3\ (in conjunction with the current 24-hour standard level of 35
[mu]g/m\3\), the Policy Assessment concluded that important public
health improvements are associated with risk reductions estimated for
standard levels of 13 and 12 [mu]g/m\3\, noting that the level of 11
[mu]g/m\3\ was not included in the quantitative risk assessment. The
Policy Assessment noted that the overall confidence in the quantitative
risk

estimates varied for the different alternative standard levels
evaluated and was stronger for the higher levels and substantially
lower for the lowest level evaluated (i.e., 10 [mu]g/m\3\). Based on
the above considerations, the Policy Assessment concluded that the
quantitative risk assessment provided support for considering
alternative annual standard levels within a range of 13 to 11 [mu]g/
m\3\, but did not provide strong support for considering lower
alternative standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).
    Taken together, the Policy Assessment concludes that consideration
of alternative annual standard levels in the range of 13 to 11 [mu]g/
m\3\ may be appropriate. Furthermore, the Policy Assessment concludes
that the currently available evidence most strongly supports
consideration of an alternative annual standard level in the range of
12 to 11 [mu]g/m\3\ (U.S. EPA, 2011a, p. 2-82). The Policy Assessment
concludes that an alternative level within the range of 12 to 11 [mu]g/
m\3\ would more fully take into consideration the available information
from all long- and short-term PM2.5 exposure studies,
including studies of at-risk populations, than would a higher level.
This range would also reflect placing weight on information from
studies that help to characterize the range of PM2.5
concentrations over which we continue to have confidence in the
associations observed in epidemiological studies, as well as the extent
to which our confidence in the associations is appreciably less at
lower concentrations.
c. Consideration of the 24-Hour Standard in the Policy Assessment
    As recognized in section III.A.3 above, an annual standard intended
to serve as the primary means for providing protection from effects
associated with both long- and short-term PM2.5 exposures is
not expected to provide appropriate protection against the effects of
all short-term PM2.5 exposures (unless established at a
level so low as to undoubtedly provide more protection than necessary
for long-term exposures). Of particular concern are areas with high
peak-to-mean ratios possibly associated with strong local or seasonal
sources, or PM2.5-related effects that may be associated
with shorter-than-daily exposure periods. As a result, the Policy
Assessment concludes that it is appropriate to consider alternative 24-
hour PM2.5 standard levels that would supplement the
protection provided by an annual standard.
    As outlined in section III.A.3 above, the Policy Assessment
considers the available evidence from short-term PM2.5
exposure studies, as well as the uncertainties and limitations in that
evidence, to assess the degree to which alternative annual and 24-hour
PM2.5 standards can be expected to reduce the estimated
risks attributed to short-term fine particle exposures. In considering
the available epidemiological evidence, the Policy Assessment takes
into account information from multi-city studies as well as single-city
studies. The Policy Assessment considers the distributions of 24-hour
PM2.5 concentrations reported in short-term exposure
studies, focusing on the 98th percentile concentrations to match the
form of the 24-hour standard as discussed in section III.E.3.b above.
In recognizing that the annual and 24-hour standards work together to
provide protection from effects associated with short-term
PM2.5 exposures, the Policy Assessment also considers
information on the long-term mean PM2.5 concentrations from
these studies.
    In addition to considering the epidemiological evidence, the Policy
Assessment also considers air quality information, specifically peak-
to-mean ratios using county-level 24-hour and annual design values, to
characterize air quality patterns in areas possibly associated with
strong local or seasonal sources. These patterns help in understanding
the extent to which different combinations of annual and 24-hour
standards would be consistent with the policy goal of setting a
generally controlling annual standard with a 24-hour standard that
provides supplemental protection especially for areas with high peak-
to-mean ratios (U.S. EPA, 2011a, p. 2-14).
    In considering the information provided by the short-term exposure
studies, the Policy Assessment recognizes that to the extent these
studies were conducted in areas that likely did not meet one or both of
the current standards, such studies do not help inform the
characterization of the potential public health improvements of
alternative standards set at lower levels. Therefore, in considering
the short-term exposure studies to inform staff conclusions regarding
levels of the 24-hour standard that are appropriate to consider, the
Policy Assessment places greatest weight on studies conducted in areas
that likely met both the current annual and 24-hour standards.
    With regard to multi-city studies that evaluated effects associated
with short-term PM2.5 exposures, as summarized in Figure 3,
the Policy Assessment observes an overall pattern of positive and
statistically significant associations in studies with 98th percentile
values averaged across study areas in the range of 45.8 to 34.2 [mu]g/
m\3\ (Burnett et al., 2004; Zanobetti and Schwartz, 2009; Bell et al.,
2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; Franklin et
al., 2008). The Policy Assessment notes that, to the extent air quality
distributions were reduced to reflect just meeting the current 24-hour
standard, additional protection would be anticipated for the effects
observed in the three multi-city studies with 98th percentile values
greater than 35 [mu]g/m\3\ (Burnett et al., 2004; Burnett and Goldberg,
2003; Franklin et al., 2008). In the three additional studies with 98th
percentile values below 35 [mu]g/m\3\, specifically 98th percentile
concentrations of 34.2, 34.3, and 34.8 [mu]g/m\3\, the Policy
Assessment notes that these studies reported long-term mean
PM2.5 concentrations of 12.9, 13.2, and 13.4 [mu]g/m\3\,
respectively (Bell et al., 2008; Zanobetti and Schwartz, 2009; Dominici
et al., 2006a). To the extent that consideration is given to revising
the level of the annual standard, as discussed above in section
III.E.4.b, the Policy Assessment recognizes that potential changes
associated with meeting such an alternative annual standard would
result in lowering risks associated with both long- and short-term
PM2.5 exposures. Consequently, in considering a 24-hour
standard that would work in conjunction with an annual standard to
provide appropriate public health protection, the Policy Assessment
notes that to the extent that the level of the annual standard is
revised to within a range of 13 to 11 [mu]g/m\3\, in particular in the
range of 12 to 11 [mu]g/m\3\, additional protection would be provided
for the effects observed in these multi-city studies (U.S. EPA, 2011a,
p. 2-84).
    In summary, the Policy Assessment concludes that the multi-city,
short-term exposure studies generally provide support for retaining the
24-hour standard level at 35 [mu]g/m\3\ in conjunction with an annual
standard level revised to within a range of 12 to 11 [mu]g/m\3\ (U.S.
EPA, 2011a, p. 2-84). Alternatively, in conjunction with an annual
standard level of 13 [mu]g/m\3\, the Policy Assessment concludes that
the multi-city studies provide limited support for revising the 24-hour
standard level somewhat below 35 [mu]g/m\3\, such as down to 30 [mu]g/
m\3\, based on one study (Bell et al., 2008) that reported positive and
statistically significant effects with an overall 98th percentile value
below the level of the current 24-hour standard in conjunction with an
overall long-term mean

concentration slightly less than 13 [mu]g/m\3\ (Figure 3; U.S. EPA,
2011a, p. 2-84).
    In reaching staff conclusions regarding alternative 24-hour
standard levels that are appropriate to consider, the Policy Assessment
also takes into account relevant information from single-city studies
that evaluated effects associated with short-term PM2.5
exposures. The Policy Assessment recognizes that these studies may
provide additional insights regarding impacts on susceptible
populations and/or on areas with isolated peak concentrations.
Although, as discussed in section III.E.4.a above, multi-city studies
have advantages over single-city studies in terms of statistical power
to detect associations and broader geographic coverage as well as other
factors such as less likelihood of publication bias, reflecting
differences in PM2.5 sources, composition, and potentially
other factors that could impact PM2.5-related effects,
multi-city studies often present overall effect estimates rather than
single-city effect estimates. Since short-term air quality can vary
considerably across cities, the extent to which effects reported in
multi-city studies are associated with short-term air quality in any
particular location is uncertain, especially when considering short-
term concentrations at the upper end of the distribution of daily
PM2.5 concentrations (i.e., at the 98th percentile value).
In contrast, single-city studies are more limited in terms of power and
geographic coverage but the link between reported health effects and
the air quality in a given study area is more straightforward to
establish. Therefore, the Policy Assessment also considers evidence
from single-city, short-term exposure studies to inform staff
conclusions regarding alternative levels that are appropriate to
consider for a 24-hour standard that is intended to provide
supplemental protection in areas where the annual standard may not
provide an adequate margin of safety against the effects of all short-
term PM2.5 exposures.
    As discussed above for the multi-city studies, the Policy
Assessment takes into account both the 24-hour PM2.5
concentrations in the single-city studies, focusing on the 98th
percentile air quality values, as well as the long-term mean
PM2.5 concentrations. The Policy Assessment considers
single-city studies conducted in areas that would likely have met the
current suite of PM2.5 standards as most useful for
informing staff conclusions related to the level of the 24-hour
standard (U.S. EPA, 2011a, Figure 2-9). The Policy Assessment notes
that additional single-city studies summarized in that Figure 2-9 were
conducted in areas that would likely have met one but not both of the
current PM2.5 standards. To the extent changes in air
quality designed to just meet the current suite of PM2.5
standards are undertaken, one could reasonably anticipate additional
public health protection will occur in these study areas. Therefore,
the Policy Assessment concludes that these studies are not helpful to
inform staff conclusions regarding alternative standard levels that are
appropriate to consider (U.S. EPA, 2011a, p. 2-87).
    With regard to single-city studies that were conducted in areas
that would likely have met both the current 24-hour and annual
standards, the Policy Assessment first considers studies that reported
positive and statistically significant associations. In considering
this group of studies, the Policy Assessment notes Mar et al. (2003)
reported a positive and statistically significant association for
premature mortality in Phoenix with a long-term mean concentration of
13.5 [mu]g/m\3\ in conjunction with a 98th percentile value of 32.2
[mu]g/m\3\ (U.S. EPA, 2011a, Figure 2-9). To the extent that
consideration is given to revising the level of the annual standard,
within a range of 13 to 11 [mu]g/m\3\, as discussed above, additional
protection would be provided for the effects observed in this study
(U.S. EPA, 2011a, p. 2-87).
    Four additional studies reported positive and statistically
significant associations with 98th percentile values within a range of
31.2 to 25.8 [mu]g/m\3\ and long-term mean concentrations within a
range of 12.1 to 8.5 [mu]g/m\3\ (Delfino et al., 1997; Peters et al.,
2001; Stieb et al., 2000; and Mar et al., 2004; U.S. EPA, 2011a, Figure
2-9). Delfino et al. (1997) reported statistically significant
associations between PM2.5 and respiratory emergency
department visits for older adults (greater than 64 years old) but not
young children (less than 2 years old), in one part of the study period
(summer 1993) but not the other (summer 1992). Peters et al. (2001)
reported a positive and statistically significant association between
short-term exposure to PM2.5 (2-hour and 24-hour averaging
times) and onset of acute myocardial infarction in Boston. Stieb et al.
(2000) reported positive and statistically significant associations
with cardiovascular- and respiratory-related emergency department
visits in Saint John, Canada, in single pollutant models but not in
multi-pollutant models (U.S. EPA, 2004, pp. 8-154 and 8-252 to 8-253).
Mar et al. (2004) reported a positive and statistically significant
association for short-term PM2.5 exposures in relation to
respiratory symptoms among children but not adults in Spokane, however,
this study had very limited statistical power because of the small
number of children and adults evaluated.
    The Policy Assessment also considers short-term single-city
PM2.5 exposure studies that reported positive but
nonstatistically significant associations for cardiovascular and
respiratory endpoints in areas that would likely have met both the
current 24-hour and annual standards. The 98th percentile values
reported in these studies ranged from 31.6 to 17.2 [mu]g/m\3\ and the
long-term mean concentrations ranged from 13.0 to 7.0 [mu]g/m\3\ (U.S.
EPA, 2011a, Figure 2-9). These studies included consideration of
cardiovascular-related mortality effects in Phoenix (Wilson et al.,
2007), asthma medication use in children in Denver (Rabinovitch et al.,
2006), hospital admissions for hemorrhagic and ischemic stroke in
Edmonton, Canada (Villeneuve et al., 2006), and hospital admissions for
ischemic stroke/transient ischemic attack in Nueces County, TX
(Lisabeth et al., 2008).
    Lastly, the Policy Assessment considers single-city studies
conducted in areas that would likely have met both the current 24-hour
and annual standards that reported null findings. The 98th percentile
values reported in these studies ranged from 29.6 to 24.0 [mu]g/m\3\
and the long-term mean concentrations ranged from 10.8 to 8.5 [mu]g/
m\3\ (U.S. EPA, 2011a, Figure 2-9). These studies reported no
associations with short-term PM2.5 exposures and
cardiovascular-related hospital admissions and respiratory-related
emergency department visits (Slaughter et al., 2005) and
cardiovascular-related emergency department visits (Schreuder et al.,
2006) in Spokane; asthma exacerbation in children in Denver
(Rabinovitch et al., 2004); and hospital admissions for transient
ischemic attack in Edmonton, Canada (Villeneuve et al., 2006).
    Viewing the evidence as a whole, the Policy Assessment observes a
limited number of single-city studies that reported positive and
statistically significant associations for a range of health endpoints
related to short-term PM2.5 concentrations in areas that
would likely have met the current suite of PM2.5 standards.
Many of these studies had significant limitations (e.g., limited
statistical power, limited exposure data) or equivocal results (i.e.,
mixed results within the same study area) as briefly identified above
and discussed in more detail in the Policy Assessment (U.S. EPA, 2011a,
p. 2-88). Other studies reported positive but not statistically

significant results or null associations also in areas that would
likely have met the current suite of PM2.5 standards.
Overall, the entire body of results from these single-city studies is
mixed, particularly as 24-hour 98th percentile concentrations go below
35 [mu]g/m\3\.
    Although a number of single-city studies report effects at
appreciably lower PM2.5 concentrations than multi-city
short-term exposure studies, the uncertainties and limitations
associated with the single-city studies were greater and, thus, the
Policy Assessment concludes there is less confidence in using these
studies as a basis for setting the level of a standard. Therefore, the
Policy Assessment concludes that the multi-city short-term exposure
studies provide the strongest evidence to inform decisions on the level
of the 24-hour standard, and the single-city studies do not warrant
consideration of 24-hour standard levels different from those supported
by the multi-city studies (U.S. EPA, 2011a, p. 2-88).
    In addition to considering the epidemiological evidence, the Policy
Assessment takes into account air quality information based on county-
level 24-hour and annual design values to understand the implications
of the alternative standard levels supported by the currently available
scientific evidence, as discussed in section III.E.4.b above. As
discussed in section III.A.3 above, the Policy Assessment concludes
that a policy goal which includes setting the annual standard to be the
``generally controlling'' standard in conjunction with setting the 24-
hour standard to provide supplemental protection, to the extent that
additional protection is warranted, is the most effective and efficient
way to reduce total population risk associated with both long- and
short-term PM2.5 exposures, resulting in more uniform
protection across the U.S than the alternative of setting the 24-hour
standard to be the controlling standard. Therefore, the Policy
Assessment considers the extent to which different combinations of
alternative annual and 24-hour standard levels based on the evidence
would support this policy goal (U.S. EPA, 2011a, pp 2-88 to 2-91,
Figure 2-10).
    Using information on the relationship of the 24-hour and annual
design values, the Policy Assessment examines the implications of three
alternative suites of PM2.5 standards identified as
appropriate to consider based on the currently available scientific
evidence, as discussed above. The Policy Assessment concludes that an
alternative suite of PM2.5 standards that would include an
annual standard level of 11 or 12 [mu]g/m\3\ and a 24-hour standard
with a level of 35 [mu]g/m\3\ (i.e., 11/35 or 12/35) would result in
the annual standard being the generally controlling standard in most
areas although the 24-hour standard would continue to be the generally
controlling standard in the Northwest (U.S. EPA, 2011a, pp. 2-89 to 2-
91 and Figure 2-10). These Northwest counties generally represent areas
where the annual mean PM2.5 concentrations have historically
been low but where relatively high 24-hour concentrations occur, often
related to seasonal wood smoke emissions. Alternatively, combining an
alternative annual standard of 13 [mu]g/m\3\ with a 24-hour standard of
30 [mu]g/m\3\ would result in many more areas across the country in
which the 24-hour standard would likely become the controlling standard
than if an alternative annual standard of 12 or 11 [mu]g/m\3\ were
paired with the current level of the 24-hour standard (i.e., 35 [mu]g/
m\3\).
    The Policy Assessment concludes that consideration of retaining the
24-hour standard level at 35 [mu]g/m\3\ would reflect placing greatest
weight on evidence from multi-city studies that reported positive and
statistically significant associations with health effects classified
as having a causal or likely causal relationship. In conjunction with
lowering the annual standard level, especially within a range of 12 to
11 [mu]g/m\3\, this alternative would recognize additional public
health protection against effects associated with short-term
PM2.5 exposures which would be provided by lowering the
annual standard such that revision to the 24-hour standard would not be
warranted (U.S. EPA, 2011a, p. 2-91).
    The Policy Assessment also recognizes an alternative approach to
considering the evidence that provides some support for revising the
level below 35 [mu]g/m\3\, perhaps as low as 30 [mu]g/m\3\ (U.S. EPA,
2011a, p. 2-92). This alternative 24-hour standard level would be more
compatible with an alternative annual standard of 13 [mu]g/m\3\ based
on placing greater weight on one multi-city short-term exposure study
(Bell et al., 2008) that reported positive and statistically
significant effects at a 98th percentile value less than 35 [mu]g/m\3\
(i.e., 34.2 [mu]g/m\3\) in conjunction with a long-term mean
concentration less than 13 [mu]g/m\3\ (i.e., 12.9 [mu]g/m\3\).
    Beyond evidence-based considerations, the Policy Assessment also
considered the extent to which the quantitative risk assessment
supports consideration of retaining the current 24-hour standard level
or provides support for lower standard levels. In considering
simulations of just meeting the current 24-hour standard level of 35
[mu]g/m\3\ or alternative levels of 30 or 25 [mu]g/m\3\ (in conjunction
with alternative annual standard levels within a range of 13 to 11
[mu]g/m\3\), the Policy Assessment noted that the overall confidence in
the quantitative risk estimates varied for the different standard
levels evaluated and was stronger for the higher levels and
substantially lower for the lowest level evaluated (i.e., 25 [mu]g/
m\3\). Based on this information, the Policy Assessment concluded that
the quantitative risk assessment provides support for considering a 24-
hour standard level of 35 or 30 [mu]g/m\3\ (in conjunction with an
alternative standard level within a range of 13 to 11 [mu]g/m\3\) but
does not provide strong support for considering lower alternative 24-
hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).
    Taken together, the Policy Assessment concludes that while it is
appropriate to consider an alternative 24-hour standard level within a
range of 35 to 30 [mu]g/m\3\, the currently available evidence most
strongly supports consideration for retaining the current 24-hour
standard level at 35 [mu]g/m\3\ in conjunction with lowering the level
of the annual standard within a range of 12 to 11 [mu]g/m\3\ (U.S. EPA,
2011a, p. 2-92).
d. CASAC Advice
    Based on its review of the second draft Policy Assessment, CASAC
agreed with the general approach for translating the available
epidemiological evidence, risk information, and air quality information
into the basis for reaching conclusions on alternative standards for
consideration. Furthermore, CASAC agreed ``that it is appropriate to
return to the strategy used in 1997 that considers the annual and the
short-term standards together, with the annual standard as the
controlling standard, and the short-term standard supplementing the
protection afforded by the annual standard'' and ``considers it
appropriate to place the greatest emphasis'' on health effects judged
to have evidence supportive of a causal or likely causal relationship
as presented in the Integrated Science Assessment (Samet, 2010d, p. 1).
    CASAC concluded that the range of levels presented in the second
draft Policy Assessment (i.e., alternative annual standard levels
within a range of 13 to 11 [mu]g/m\3\ and alternative 24-hour standard
levels within a range of 35 to 30 [mu]g/m\3\) ``are supported by the
epidemiological and toxicological evidence, as well as by the risk and
air quality information compiled'' in the Integrated Science
Assessment, Risk Assessment, and second draft Policy Assessment. CASAC
further noted that

``[a]lthough there is increasing uncertainty at lower levels, there is
no evidence of a threshold (i.e., a level below which there is no risk
for adverse health effects)'' (Samet, 2010d, p. ii).
    Although CASAC supported the alternative standard level ranges
presented in the second draft Policy Assessment, it did not express
support for any specific levels or combinations of standards. Rather,
CASAC encouraged the EPA to develop a clearer rationale in the final
Policy Assessment for staff conclusions regarding annual and 24-hour
standards that are appropriate to consider, including consideration of
the combination of these standards supported by the available
information (Samet, 2010d, p. ii). Specifically, CASAC encouraged staff
to focus on information related to the concentrations that were most
influential in generating the health effect estimates in individual
studies to inform alternative standard levels (Samet, 2010d, p. 2).
CASAC also commented that the approach presented in the second draft
Policy Assessment to identify alternative 24-hour standard levels which
focused on peak-to-mean ratios was not relevant for informing the
actual level (Samet 2010d, p. 4). Further, they expressed the concern
that the combinations of annual and 24-hour standard levels discussed
in the second draft Policy Assessment (i.e., in the range of 13 to 11
[mu]g/m\3\ for the annual standard, in conjunction with retaining the
current 24-hour PM2.5 standard level of 35 [mu]g/m\3\;
alternatively, revising the level of the 24-hour standard to 30 [mu]g/
m\3\ in conjunction with an annual standard level of 11 [mu]g/m\3\)
``may not be adequately inclusive'' and ``[i]t was not clear why, for
example a daily standard of 30 [mu]g/m\3\ should only be considered in
combination with an annual level of 11 [mu]g/m\3\'' (Samet, 2010d, p.
ii). CASAC encouraged the EPA to more clearly explain its rationale for
identifying the 24-hour/annual combinations that are appropriate for
consideration (Samet 2010d, p. ii).
    In considering CASAC's advice as well as public comment on the
second draft Policy Assessment, EPA staff conducted additional analyses
and modified their conclusions regarding alternative standard levels
that are appropriate to consider. The staff conclusions in the final
Policy Assessment (U.S. EPA, 2011a, section <IP_ADDRESS>) differ somewhat
from the alternative standard levels discussed in the second draft
Policy Assessment (U.S. EPA, 2010f, section <IP_ADDRESS>), upon which CASAC
based its advice. Changes made in the final Policy Assessment were
primarily focused on improving and clarifying the approach for
translating the epidemiological evidence into a basis for staff
conclusions on the broadest range of alternative standard levels
supported by the available scientific information and more clearly
articulating the rationale for the staff's conclusions (Wegman, 2011,
pp. 1 to 2). Consistent with CASAC's advice to consider more
information from epidemiological studies, the EPA analyzed additional
population-level data obtained from several study investigators. In
commenting on draft staff conclusions in the second draft Policy
Assessment, CASAC did not have an opportunity to review the staff
analyses of distributional statistics to identify the broader range of
PM2.5 concentrations that were most influential in
generating health effect estimates in epidemiological studies (Rajan et
al., 2011). In addition, CASAC was not aware of the revised long-term
mean PM2.5 concentration in the WHI study as discussed in
section III.D.1.a above or the staff's inclusion of that value in its
evaluation of the evidence (i.e., in Figures 1 and 4 above and related
discussion). The WHI study is the only long-term cohort study that
provides information regarding effects classified as having evidence of
a causal or likely causal relationship associated with a long-term
PM2.5 concentration below 13 [mu]g/m\3\. Furthermore, CASAC
did not have an opportunity to review the staff's revised rationale for
the combinations of alternative standards suggested in the final Policy
Assessment.
e. Administrator's Proposed Conclusions on the Primary PM2.5
Standard Levels
    In reaching her conclusions regarding appropriate alternative
standard levels to consider, the Administrator has considered the
epidemiological and other scientific evidence, estimates of risk
reductions associated with just meeting alternative annual and/or 24-
hour standards, air quality analyses, related limitations and
uncertainties and the advice of CASAC. As an initial matter, the
Administrator agrees with the approach discussed in the Policy
Assessment as summarized in sections III.A.3 and III.E.4.a above, and
supported by CASAC, of considering the protection afforded by the
annual and 24-hour standards taken together for mortality and morbidity
effects associated with both long- and short-term exposures to
PM2.5. This is consistent with the approach taken in the
review completed in 1997, in contrast to considering each standard
separately, as was done in the review completed in 2006. Furthermore,
based on the evidence and quantitative risk assessment, the
Administrator provisionally concludes it is appropriate to set a
``generally controlling'' annual standard that will lower a wide range
of ambient 24-hour concentrations, with a 24-hour standard focused on
providing supplemental protection, particularly for areas with high
peak-to-mean ratios possibly associated with strong local or seasonal
sources, or PM2.5-related effects that may be associated
with shorter-than daily exposure periods. The Administrator
provisionally concludes this approach would likely reduce aggregate
risks associated with both long- and short-term exposures more
consistently than a generally controlling 24-hour standard and would be
the most effective and efficient way to reduce total PM2.5-
related population risk.
    In reaching decisions on alternative standard levels to propose,
the Administrator judges that it is most appropriate to examine where
the evidence of associations observed in the epidemiological studies is
strongest and, conversely, where she has appreciably less confidence in
the associations observed in the epidemiological studies. Based on the
characterization and assessment of the epidemiological and other
studies presented and assessed in the Integrated Science Assessment and
the Policy Assessment, the Administrator recognizes the substantial
increase in the number and diversity of studies available in this
review including extended analyses of the seminal studies of long-term
PM2.5 exposures (i.e., ACS and Harvard Six Cities studies)
as well as important new long-term exposure studies (as summarized in
Figures 1 and 2). Collectively, the Administrator takes note that these
studies, along with evidence available in the last review, provide
consistent and stronger evidence of an association with premature
mortality, with the strongest evidence related to cardiovascular-
related mortality, at lower ambient concentrations than previously
observed. The Administrator also recognizes the availability of
stronger evidence of morbidity effects associated with long-term
PM2.5 exposures, including evidence of cardiovascular
effects from the WHI study and respiratory effects, including decreased
lung function growth, from the extended analyses for the Southern
California Children's Health Study. Furthermore, the Administrator
recognizes new U.S. multi-city studies that greatly expand and
reinforce our understanding of mortality and morbidity effects

associated with short-term PM2.5 exposures, providing
stronger evidence of associations at ambient concentrations similar to
those previously observed (as summarized in Figure 3).
    The newly available scientific evidence builds upon the previous
scientific data base to provide evidence of generally robust
associations and to provide a basis for greater confidence in the
reported associations than in the last review. The Administrator
recognizes that the weight of evidence, as evaluated in the Integrated
Science Assessment, is strongest for health endpoints classified as
having evidence of a causal relationship. These relationships include
those between long- and short-term PM2.5 exposures and
mortality and cardiovascular effects. She recognizes that the weight of
evidence is also strong for health endpoints classified as having
evidence of a likely causal relationship, which include those between
long- and short-term PM2.5 exposures and respiratory
effects. In addition, the Administrator makes note of the much more
limited evidence for health endpoints classified as having evidence
suggestive of a causal relationship, including developmental,
reproductive and carcinogenic effects.
    Based on information discussed and presented in the Integrated
Science Assessment, the Administrator recognizes that health effects
may occur over the full range of concentrations observed in the long-
and short-term epidemiological studies and that no discernible
threshold for any effects can be identified based on the currently
available evidence (U.S. EPA, 2009a, section 2.4.3). She also
recognizes, in taking note of CASAC advice and the distributional
statistics analysis discussed in section III.E.4.b above and in the
Policy Assessment, that there is significantly greater confidence in
observed associations over certain parts of the air quality
distributions in the studies, and conversely, that there is
significantly diminished confidence in ascribing effects to
concentrations toward the lower part of the distributions.
    Consistent with the general approach summarized in section III.A.3
above, and supported by CASAC as discussed in section III.E.4.d above,
the Administrator generally agrees that it is appropriate to consider a
level for an annual standard that is somewhat below the long-term mean
PM2.5 concentrations reported in long- and short-term
exposure studies. In recognizing that the evidence of an association in
any such study is strongest at and around the long-term average where
the data in the study are most concentrated, she understands that this
approach does not provide a bright line for reaching decisions about
appropriate standard levels. The Administrator notes that long-term
mean PM2.5 concentrations are available for each study
considered and, therefore, represent the most robust data set to inform
her decisions on appropriate annual standard levels. She also notes
that the overall study mean PM2.5 concentrations are
generally calculated based on monitored concentrations averaged across
monitors in each study area with multiple monitors, referred to as a
composite monitor concentration, in contrast to the highest
concentration monitored in study area, referred to as a maximum monitor
concentration, which are used to determine whether an area meets a
given standard. In considering such long-term mean concentrations, the
Administrator understands that it is appropriate to consider the weight
of evidence for the health endpoints evaluated in such studies in
giving weight to this information.
    Based on the information summarized in Figure 4 and presented in
more detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) for
effects classified in the Integrated Science Assessment as having a
causal or likely causal relationship with PM2.5 exposures,
the Administrator observes an overall pattern of statistically
significant associations reported in studies of long-term
PM2.5 exposures with long-term mean concentrations ranging
from somewhat above the current standard level of 15 [mu]g/m\3\ down to
the lowest mean concentration in such studies of 12.9 [mu]g/m\3\ (in
Miller et al., 2007). She observes a similar pattern of statistically
significant associations in studies of short-term PM2.5
exposures with long-term mean concentrations ranging from around 15
[mu]g/m\3\ down to 12.8 [mu]g/m\3\ (in Burnett et al., 2004). With
regard to effects classified as providing evidence suggestive of a
causal relationship, the Administrator observes a small number of long-
term exposure studies related to developmental and reproductive effects
that reported statistically significant associations with overall study
mean PM2.5 concentrations down to 11.9 [mu]g/m\3\ (in Bell
et al., 2007).\86\
---------------------------------------------------------------------------

    \86\ With respect to suggestive evidence related to cancer,
mutagenic, and genotoxic effects, the PM2.5
concentrations reported in studies generally included ambient
concentrations that are equal to or greater than ambient
concentrations observed in studies that reported mortality and
cardiovascular and respiratory effects (U.S. EPA, 2009a, section
7.5), such that in selecting alternative standard levels that
provide protection from mortality and cardiovascular and respiratory
effects, it is reasonable to anticipate that protection will also be
provided for carcinogenic effects.
---------------------------------------------------------------------------

    The Administrator also considers additional information from
epidemiological studies, consistent with CASAC advice, to take into
account the broader distribution of PM2.5 concentrations and
the degree of confidence in the observed associations over the broader
air quality distribution. In considering this additional information,
she understands that the Policy Assessment presented information on the
25th and 10th percentiles of the distributions of PM2.5
concentrations available from four multi-city studies to provide a
general frame of reference as to the part of the distribution within
which the data become appreciably more sparse and, thus, where her
confidence in the associations observed in epidemiological studies
would become appreciably less. As discussed in section III.E.4.b above
and summarized in Figure 4, the Administrator takes note of additional
population-level data that are available for four studies (Krewski et
al., 2009; Miller et al., 2007; Bell et al., 2008; Zanobetti and
Schwartz, 2009), each of which report statistically significant
associations with health endpoints classified as having evidence of a
causal relationship. In considering the long-term PM2.5
concentrations associated with the 25th percentile values of the
population-level data for these four studies, she observes that these
values range from somewhat above to somewhat below 12 [mu]g/m\3\
(Figure 4). The Administrator recognizes that these four studies
represent some of the strongest evidence available within the overall
body of scientific evidence and notes that three of these studies
(Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009)
were used as the basis for concentration-response functions used in the
quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). However,
the Administrator also recognizes that additional population-level data
are available for only these four studies and, therefore, she believes
that these studies comprise a more limited data set than one based on
long-term mean PM2.5 concentrations for which data are
available for all studies considered, as discussed above. In
considering this information, the Administrator notes that CASAC
advised that information about the long-term PM2.5
concentrations that were most influential in generating the health
effect estimates in epidemiological

studies can help to inform selection of an appropriate annual standard
level.
    The Administrator recognizes, as summarized in section III.B.2
above, that important uncertainties remain in the evidence and
information considered in this review of the primary fine particle
standards. These uncertainties are generally related to understanding
the relative toxicity of the different components in the fine particle
mixture, the role of PM2.5 in the complex ambient mixture,
exposure measurement errors inherent in epidemiological studies based
on concentrations measured at fixed monitor sites, and the nature,
magnitude, and confidence in estimated risks related to increasingly
lower ambient PM2.5 concentrations. Furthermore, the
Administrator notes that epidemiological studies have reported
heterogeneity in responses both within and between cities and
geographic regions across the U.S. She recognizes that this
heterogeneity may be attributed, in part, to differences in fine
particle composition in different regions and cities. The Administrator
also recognizes that there are additional limitations associated with
evidence for reproductive and developmental effects, identified as
being suggestive of a causal relationship with long-term
PM2.5 exposures, including: the limited number of studies
evaluating such effects; uncertainties related to identifying the
relevant exposure time periods of concern; and limited toxicological
evidence providing little information on the mode of action(s) or
biological plausibility for an association between long-term
PM2.5 exposures and adverse birth outcomes.
    The Administrator is mindful that considering what standards are
requisite to protect public health with an adequate margin of safety
requires public health policy judgments that neither overstate nor
understate the strength and limitations of the evidence or the
appropriate inferences to be drawn from the evidence. In considering
how to translate the available information into appropriate standard
levels, the Administrator weighs the available scientific information
and associated uncertainties and limitations. For the purpose of
determining what standard levels are appropriate to propose, the
Administrator recognizes, as did EPA staff in the Policy Assessment,
that there is no single factor or criterion that comprises the
``correct'' approach to weighing the various types of available
evidence and information, but rather there are various approaches that
are appropriate to consider. The Administrator further recognizes that
different evaluations of the evidence and other information before the
Administrator could reflect placing different weight on the relative
strengths and limitations of the scientific information, and different
judgments could be made as to how such information should appropriately
be used in making public health policy decisions on standard levels.
This recognition leads the Administrator to consider various approaches
to weighing the evidence so as to identify appropriate standard levels
to propose. In so doing, the Administrator encourages extensive public
comment on alternative approaches to weighing the evidence and other
information so as to inform her public health policy judgments before
reaching final decisions on appropriate standard levels.
    In considering the available information, the Administrator notes
the advice of CASAC that the currently available scientific
information, including epidemiological and toxicological evidence as
well as risk and air quality information, provides support for
considering an annual standard level within a range of 13 to 11 [mu]g/
m\3\ and a 24-hour standard level within a range of 35 to 30 [mu]g/
m\3\. In addition, the Administrator recognizes that the Policy
Assessment concludes that the available evidence and risk-based
information support consideration of annual standard levels in the
range of 13 to 11 [mu]g/m\3\, and that the Policy Assessment also
concludes that the evidence most strongly supports consideration of an
annual standard level in the range of 12 to 11 [mu]g/m\3\. In
considering how the annual and 24-hour standards work together to
provide appropriate public health protection, the Administrator
observes that CASAC did not express support for any specific levels or
combinations of standards within in these ranges, although she
recognizes that CASAC did not have an opportunity to review additional
information and analyses presented in the final Policy Assessment
prepared in response to CASAC's recommendations on the second draft
Policy Assessment. Nor did CASAC have an opportunity to review the EPA
staff's revised rationale for the combinations of alternative standards
presented in the final document.
    In considering the extent to which the currently available evidence