effects in infants, using various PM indicators. Results from this body
of evidence, though mixed, are suggestive of possible effects; more
research is needed to further elucidate the potential risks of PM
exposure for these health outcomes (EPA, 2004, p. 8-222).
    In summary, there are several population groups that may be
especially susceptible or vulnerable to PM-related effects. These
groups include those with preexisting heart and lung diseases, older
adults and children. Emerging evidence indicates that people from lower
socioeconomic strata or who have particularly elevated exposures may be
more vulnerable to PM-related effects.
5. PM2.5-Related Impacts on Public Health
    As just discussed, there are several population groups that may be
especially susceptible or vulnerable to effects from exposure to PM.
These population subgroups, such as young children or older adults, and
people with pre-existing heart or lung diseases, constitute a large
portion of the U.S. population. For example, approximately 22 million
people, or 11 percent of the U.S. population, have received a diagnosis
of heart disease, about 20 percent of the population has hypertension
and about 9 percent of adults and 11 percent of children in the U.S.
have been diagnosed with asthma. In addition, about 26 percent of the
U.S. population is under 18 years of age,\19\ and about 12 percent is
65 years of age or older (EPA, 2004, Table 9-4). EPA recognizes that
combining fairly small risk estimates and small changes in PM
concentrations with large groups of the U.S. population would result in
large public health impacts.
---------------------------------------------------------------------------

    \19\ Health studies that have suggested that children are
susceptible to PM-related effects include varying age ranges, for
example, for hospital admissions in children up to 18 years of age,
or respiratory symptoms in panels of 4th and 5th grade children.
---------------------------------------------------------------------------

    One issue that is important for interpreting the public health
implications of the associations reported between mortality and short-
term exposure to PM is whether mortality is occurring only in very
frail individuals (sometimes referred to as ``harvesting''), resulting
in loss of just a few days of life expectancy. A number of new analyses
assess the likelihood of such ``harvesting'' occurring in the short-
term exposure studies. Overall, the Criteria Document concludes from
the time-series studies that there appears to be no strong evidence to
suggest that short-term exposure to PM is only shortening life by a few
days (EPA, 2004, Section 8.4.10). In addition to the evidence from
short-term exposure studies discussed above, one new report used the
mortality risk estimates from the ACS prospective cohort study to
estimate potential loss of life expectancy from PM-related mortality in
a population. The authors estimated that the loss of population life
expectancy associated with long-term exposure to PM2.5 was
on the order of a year or so (EPA, 2004, p. 8-334). The Criteria
Document recognizes that these calculations were based on studies in
adult populations, and potential population life shortening would be
increased if the new, albeit limited, evidence from infant mortality
studies was considered (EPA, 2004, p. 8-335). The Criteria Document
also observes that the risk estimates reported for long-term fine
particle exposures and lung cancer mortality are in about the same
range as the risk seen for a nonsmoker living with a smoker (EPA, 2004,
p. 9-94).
    Large subgroups of the U.S. population are included in
subpopulations considered to be potentially sensitive to effects
related to fine particle exposures (EPA, 2004, section <IP_ADDRESS>). While
individual epidemiologic effect estimates may be small in size, the
public health impact of the mortality and morbidity associations can be
quite large. In addition, it appears that mortality risks are not
limited to the very frail. Taken together, these results suggest that
exposure to ambient PM, especially PM2.5, can have
substantial public health impacts (EPA, 2004, p. 9-93).

B. Quantitative Risk Assessment

    This section discusses the approach used to develop quantitative
risk estimates associated with exposures to PM2.5 building
upon a more limited risk assessment that was conducted during the last
review.\20\ At that time, EPA conducted a very limited risk assessment
covering a portion of two cities (i.e., Philadelphia County and
Southeast Los Angeles County) for which ambient PM2.5 data
were available. For short-term exposure mortality and morbidity health
effects, the prior assessment relied on either pooled analyses that
combined the results from several studies of individual cities or
individual single- and multi-city studies, none of which included the
two urban counties for which risks were estimated, to estimate
concentration-response relationships for these two cities. EPA
recognized that the lack of city-specific relative risks introduced
substantial uncertainties in the risk estimates due to inherent
differences (e.g., different population characteristics, PM size
distributions) that might influence the concentration-response
relationships. For long-term exposure mortality, the prior assessment
relied on the concentration-response relationship reported in the
original ACS study (Pope et al., 1995). Additional important
uncertainties noted at the time of that assessment with respect to all
health effects included: (1) The absence of clear evidence regarding
mechanisms of

action for the various effects of interest, (2) uncertainties about the
shape of the concentration-response relationships; and (3) concern
about whether the use of ambient PM2.5 fixed-site monitoring
data adequately reflected the relevant population exposures to PM that
are responsible for the reported health effects (61 FR 65650).
---------------------------------------------------------------------------

    \20\ The methodology, scope, and results from the risk
assessment conducted in the last review are described in Chapter 6
of the 1996 Staff Paper (EPA, 1996b) and in several technical
reports (Abt Associates, 1996; Abt Associates, 1997a,b) and
publications (Post et al., 2000; Deck et al., 2001).
---------------------------------------------------------------------------

    In light of the substantial uncertainties in the prior risk
estimates, EPA placed greater weight on the overall conclusions derived
from the health effect studies--that ambient PM was likely causing or
contributing to significant adverse effects at levels below those
permitted by the then-existing PM10 standards--than on the
specific concentration-response functions and quantitative risk
estimates derived from them. Nevertheless, EPA judged that the
assessment provided reasonable estimates as to the possible extent of
risk for those effects given the available information (62 FR at
38656).
1. Overview
    The updated risk assessment conducted as part of this review
includes estimates of (1) risks of mortality, morbidity, and symptoms
associated with recent ambient PM2.5 levels; (2) risk
reductions and remaining risks associated with just meeting the current
suite of PM2.5 NAAQS; and (3) risk reductions and remaining
risks associated with just meeting various alternative PM2.5
standards in a number of example urban areas. This risk assessment is
more fully described and presented in the Staff Paper (EPA, 2005a,
Chapter 4) and in a technical support document, Particulate Matter
Health Risk Assessment for Selected Urban Areas (Abt Associates,
2005a). The scope and methodology for this risk assessment were
developed over the last few years with considerable input from the
CASAC PM Panel and the public.\21\ The information presented in these
documents included specific criteria for the selection of health
endpoints and studies to include in the assessment. It also addressed
which alternative statistical models (e.g., for control of time-varying
factors such as weather and for various lags) to include in the
assessment, recognizing that some of the health studies presented
results from a large number of alternative models. In an advisory
letter sent by CASAC to the Administrator documenting its advice in May
2002 (Hopke, 2002), CASAC concluded that the general methodology and
framework to be used in the assessment were appropriate.
---------------------------------------------------------------------------

    \21\ In June 2001, OAQPS released a draft document, PM NAAQS
Risk Analysis Scoping Plan (EPA, 2001), for CASAC consultation and
public comment, which described staff's general plan for this
assessment. In January 2002, OAQPS released a more detailed draft
document, Proposed Methodology for Particulate Matter Risk Analyses
for Selected Urban Areas (Abt Associates, 2002), for CASAC review
and public comment, which described staff's plans to assess (a)
PM2.5-related risks for several health endpoints,
including mortality, hospital admissions, and respiratory symptoms
and (b) PM10-2.5-related risks for hospital admissions
and respiratory symptoms (as discussed below in Section III.B).
---------------------------------------------------------------------------

    The goals of the PM2.5 risk assessment were: (1) To
provide estimates of the potential magnitude of mortality and morbidity
effects associated with current PM2.5 levels, and with
meeting the current suite of PM2.5 NAAQS and alternative
PM2.5 standards, in specific urban areas; (2) to develop a
better understanding of the influence of various inputs and assumptions
on the risk estimates; and (3) to gain insights into the distribution
of risks and patterns of risk reductions associated with meeting
alternative suites of PM2.5 standards. EPA recognizes that
there are many sources of uncertainty and variability inherent in the
inputs to this assessment and that there is a high degree of
uncertainty in the resulting PM2.5 risk estimates. While
some of these uncertainties have been addressed quantitatively in the
form of estimated confidence ranges around central risk estimates,
other uncertainties and the variability in key inputs are not reflected
in these confidence ranges, but rather have been addressed through
separate sensitivity analyses or characterized qualitatively.
2. Scope and Key Components
    The risk assessment estimates risks of various health effects
associated with exposure to ambient PM2.5 in nine urban
areas selected to illustrate the public health impacts associated with
a recent year of air quality and potential reductions in risk
associated with just meeting the current suite of PM2.5
standards and alternative suites of standards. The selection of urban
areas was largely determined by identifying areas in the U.S. for which
acceptable epidemiological studies were available that estimated
concentration-response relationships for PM2.5, which were
then used in assessing the risks. Thus, unlike the prior risk
assessment, the current risk assessment for short-term exposure
mortality and morbidity health effects used concentration-response
relationships reported in studies that included the urban areas for
which risks were estimated. Based on a review of the evidence evaluated
in the Criteria Document and Staff Paper, as well as the criteria
discussed in Chapter 4 of the Staff Paper, the following broad
categories of health endpoints were included in the risk assessment for
PM2.5 associated with short-term exposure: Total (non-
accidental), cardiovascular, and respiratory mortality; hospital
admissions for cardiovascular and respiratory causes; and respiratory
symptoms not requiring hospitalization. Also included in the
PM2.5 risk assessment were total, cardiopulmonary, and lung
cancer mortality associated with long-term exposure.
    The available long-term exposure mortality concentration-response
functions are all based on cohort studies, in which a cohort of
individuals is followed over time. Based on the evaluation contained in
the Criteria Document and EPA's assessment of the complete data base
addressing mortality associated with long-term exposure to
PM2.5, studies based on the following two cohorts were
identified as being particularly relevant for the PM2.5 risk
assessment: (1) The Six Cities study cohort (referred to as Krewski et
al. (2000)--Six Cities) and (2) the ACS cohort (referred to Krewski et
al. (2000)--ACS), which includes a much larger number of individuals
from many more cities. In addition, Pope et al. (2002) extended the
follow-up period for the ACS cohort to sixteen years and published
findings on the relation of long-term exposure to PM2.5 and
all-cause mortality as well as cardiopulmonary and lung cancer
mortality (referred to as Pope et al. (2002)--ACS extended).\22\
---------------------------------------------------------------------------

    \22\ The use of these particular cohort studies to estimate
health risks associated with long-term exposure to PM2.5
is consistent with the views expressed in the National Academy of
Sciences (2002) report, ``Estimating the Public Health Benefits of
Proposed Air Pollution Regulations,'' and the Science Advisory Board
Clean Air Act Compliance Council review of the proposed methodology
to estimate the health benefits associated with the Clean Air Act
(SAB, 2004).
---------------------------------------------------------------------------

    The available short-term exposure morbidity and mortality
concentration-response functions used in the risk assessment are all
from time series studies. The risk assessment included only those
health endpoints for which the the Criteria Document concluded that
there is likely to be a causal relationship with short-term exposure to
PM2.5 based on the overall weight of the evidence from the
collective body of available studies. Also, given the large number of
endpoints and studies addressing PM2.5-related effects, the
assessment only included the more severe and better understood (in
terms of health consequences) health effects. As noted above, in
contrast to the prior risk assessment, the concentration-response
functions used in this assessment for each urban area are

based on results of studies for that specific area or from a multi-city
study that included that specific area.
    The concentration-response relationships used in the assessment
were based on findings from human epidemiological studies that have
relied on fixed-site, population-oriented, ambient monitors as a
surrogate for actual ambient PM2.5 exposures. The risk
assessment addresses risks attributable to anthropogenic sources and
activities (i.e., risk associated with concentrations above policy-
relevant background \23\ or above various selected higher cutpoints
intended as surrogates for alternative assumed population thresholds).
This approach of estimating risks in excess of background was judged to
be more relevant to policy decisions regarding ambient air quality
standards than risk estimates that include effects potentially
attributable to uncontrollable background PM concentrations. For the
base case analyses, an estimate of the annual average background level
was used, rather than a maximum 24-hour value, since estimated risks
were aggregated for each day throughout the year.
---------------------------------------------------------------------------

    \23\ Background PM concentrations used in the PM risk assessment
were defined in Chapter 2 of the Staff Paper as the PM
concentrations that would be observed in the U.S. in the absence of
anthropogenic emissions of PM and its precursors in the U.S.,
Canada, and Mexico. For the initial base case risk estimates, the
midpoints of the appropriate ranges of annual average estimates for
PM2.5 background presented in the Staff Paper were used
(i.e., eastern values were used for eastern study locations and
western values were used for western study locations). Estimated
policy-relevant background concentrations are 3.5 [mu]g/m\3\ in
eastern cities, and 2.5 [mu]g/m\3\ in western cities.
---------------------------------------------------------------------------

    In order to estimate the incidence of a particular health effect
associated with recent conditions in a specific county or set of
counties attributable to ambient PM2.5 exposures in excess
of background or various alternative cutpoints, as well as the change
in incidence corresponding to a given change in PM2.5 levels
resulting from just meeting a specified set of alternative
PM2.5 standards, three elements are required. These elements
are: (1) Air quality information (including recent air quality data for
PM2.5 from ambient monitors for the selected location,
estimates of background PM2.5 concentrations appropriate for
that location, and a method for adjusting the recent data to reflect
patterns of air quality estimated to occur when the area just meets a
given set of PM2.5 standards); (2) relative risk-based
concentration-response functions that provide an estimate of the
relationship between the health endpoints of interest and ambient PM
concentrations; and (3) annual or seasonal baseline health effects
incidence rates and population data, which are needed to provide an
estimate of the annual or seasonal baseline incidence of health effects
in an area before any changes in PM air quality.
    The risk assessment for PM2.5 included a series of base
case analyses that characterized the uncertainty associated with the
form of the concentration-response relationship drawn from the studies
used in the assessment--this uncertainty had by far the greatest impact
on estimated risks. Other uncertainties addressed in various
sensitivity analyses (e.g., the use of single-versus multi-pollutant
models, single-versus multi-city models, use of a distributed lag
model, alternative assumptions about the relevant air quality for long-
term exposure mortality, and alternative constant or varying background
levels) all have a more moderate and often variable impact on the risk
estimates in some or all of the cities.
    In estimating health risks remaining upon just meeting the current
and alternative PM2.5 standards, the assessment includes a
series of base cases, while noting that the confidence ranges in the
estimates do not reflect all the identified uncertainties. As discussed
above in section II.A.3, additional uncertainty for short-term exposure
mortality is related to the use of alternative statistical models and
methods to control for time-varying effects, such as weather or season,
and to address alternative lag structures. To provide a consistent
basis for comparison across studies and locations, the risk assessment
used concentration-response functions based on the most common type of
analysis (``generalized additive methods'') and on lag structures
judged to be most appropriate for each specific health endpoint, as
discussed in the Staff Paper (EPA, 2005a, p. 4-24). The risk assessment
included a sensitivity analysis for one location where a wide array of
statistical models and lags was reported in the health study for that
location (Los Angeles, as reported in Moolgavkar, 2003). EPA recognizes
that there is additional uncertainty associated with choices about
appropriate modeling strategy (EPA, 2004, 8.4.2) and that this
uncertainty is not included in the confidence ranges presented for the
risk estimates.
    As noted earlier, EPA recognizes that while there are likely
biological thresholds in individuals for specific health endpoints, the
available epidemiologic studies do not support or refute the existence
of thresholds at the population level for either long-term or short-
term PM2.5 exposures within the range of air quality
observed in the studies (EPA, 2004, <IP_ADDRESS>). Thus, base case risks
were estimated using not only the linear or log-linear concentration-
response functions reported in the studies, but also using a series of
modified linear functions, as discussed below, as surrogates for
assumed non-linear functions that would reflect the possibility that
thresholds may exist in the reported associations within the range of
air quality observed in the studies.
    For short-term exposure mortality and morbidity outcomes associated
with PM2.5, the initial base case includes linear or log-
linear concentration-response models reported in the epidemiology
studies which are applied down to the estimated policy-relevant
background concentration level. Generally, the lowest measured
concentrations in the short-term exposure studies were relatively near
or below the estimated policy-relevant background levels such that
little or no extrapolation was required beyond the range of data in the
studies. In the case of the long-term exposure mortality studies for
PM2.5 that have been included in the risk assessment, the
lowest measured levels were in the range 7.5 to 11 [mu]g/m\3\. For the
initial base case scenario for this endpoint, the reported linear
models were applied down to 7.5 [mu]g/m\3\, which is the lowest
measured level reported in the long-term studies. Going down to an
estimated policy-relevant background level for short-term exposure
studies and to 7.5 [mu]g/m\3\ for long-term studies provides a
consistent framework which facilitates comparison of risk estimates
across urban locations within each group of studies and avoids
significant extrapolation beyond the range of concentrations included
in these studies.
    Additional base case scenarios for both short- and long-term
exposure health endpoints involved the use of alternative
concentration-response functions that incorporated a modified linear
slope with an imposed cutpoint (i.e., an assumed threshold). For
mortality associated with short-term exposure, the base case analyses
included risk estimates associated with cutpoints of 10, 15, and 20
[mu]g/m. For mortality associated with long-term PM2.5
exposure, cutpoints of 10 and 12 [mu]g/m\3\ were included. For the base
case scenarios involving alternative cutpoints, the approach used to
develop alternative functions incorporates a modified linear slope with
an imposed cutpoint (i.e., an assumed population threshold) that is
intended to reflect a

hypothetical inflection point in a typical non-linear, ``hockey-stick''
shaped function, below which there is little or no population response.
More specifically, the slope of the concentration-response relationship
has been adjusted assuming that the upward-sloping portion of the
``hockey stick'' would be the slope estimated in the original
epidemiologic study adjusted by the inverse of the proportion of the
range of PM levels observed in the study that was above the cutpoint.
The Staff Paper concludes that this simple slope adjustment approach
represents a reasonable approach to illustrating the potential impact
of possible non-linear concentration-response relationships. In its
review of the Staff Paper and risk assessment, the CASAC PM Panel
commented that for the purpose of estimating public health impacts, it
``favored the primary use of an assumed threshold of 10 [mu]g/m\3\''
and that ``a major research need is for more work to determine the
existence and level of any thresholds that may exist or the shape of
nonlinear concentration-response curves at low levels of exposure that
may exist'' (Henderson, 2005a).
3. Risk Estimates and Key Observations
    In focusing on the five study areas that do not meet the current
PM2.5 standards based on 2001-2003 air quality data
(Detroit, Los Angeles, Philadelphia, Pittsburgh, and St. Louis), the
total mortality risk estimates associated with simulating air quality
reductions to just meet the current PM2.5 standards (based
on associations with long-term PM2.5 exposure, and using the
lowest cutpoint of 7.5 [mu]g/m\3\) range from several hundred to over
1500 deaths per year, which translate into an incidence rate of
approximately 16 to 35 deaths per year per hundred thousand
population.\24\ These estimated risks associated with long-term
exposure represent approximately 2.6 to 3.2 percent of total mortality
in those areas. Estimated risks associated with long-term exposure
based on an assumed cutpoint of 10 [mu]g/m\3\ are roughly half as large
as the estimates based on a cutpoint of 7.5 [mu]g/m\3\. In the same
five areas, the estimates of mortality risk associated with short-term
PM2.5 exposure, based on a cutpoint equal to policy-relevant
background or 10 [mu]g/m, range from less than 20 percent to over 50
percent of the estimates associated with long-term exposure.\25\
---------------------------------------------------------------------------

    \24\ The full range of quantitative risk estimates associated
with just meeting the current PM2.5 standards are
presented in Tables 4-9, 4-10, 4-12, and 4-13 in Chapter 4 of the
Staff Paper.
    \25\ In some areas, the 95 percent confidence ranges associated
with the risk estimates for short-term exposure (but not long-term
exposure) extend to below zero, reflecting appreciably more
uncertainty in estimates based on positive but not statistically
significant associations.
---------------------------------------------------------------------------

    Reductions in risk associated with simulating air quality to just
meet a range of lower alternative annual and 24-hour PM2.5
standards were also estimated in this assessment. The estimated risk
reductions are depicted graphically in the Staff Paper (EPA, 2005a,
Figures 5-1 and 5-2 and Figures 5A-1 and 5A-2), showing patterns of
estimated risk reductions associated with alternative suites of
standards for all the various assumed cutpoints. As would be expected,
patterns of increasing estimated risk reductions are observed as either
the annual or 24-hour standard, or both, are reduced over the range
considered in this assessment, and the estimated percentage reductions
in risk are strongly influenced by the assumed cutpoint level.
    The discussion below highlights additional observations and
insights from this PM2.5 risk assessment, together with
important caveats and limitations.
    (1) With respect to short-term exposure mortality and morbidity,
this risk assessment provides the basis for greater confidence in the
results as compared to the prior assessment, given that studies are now
available using PM2.5 as the indicator in a much greater
number of locations, and the assessment is able to use city-specific
functions that are matched to the locations for which risks are
estimated. This contrasts with the use of pooled concentration-response
functions in the prior assessment which did not include studies for the
specific cities included in that assessment. However, EPA recognizes
that the confidence ranges, which only reflect uncertainty associated
with the precision of the study (related to the population size and
duration of the study), may be larger for the current risk estimates
due to the use of concentration-response functions from smaller, city-
specific studies now versus the use of concentration-response functions
from pooled sets of studies that have greater statistical precision.
Comparing the risk estimates for the only two specific locations that
were included in both the prior and current assessments, the magnitude
of the estimates associated with just meeting the current annual
standard, in terms of percentage of total incidence, is similar in one
of the locations (Philadelphia) and the current estimate is lower in
the other location (Los Angeles).
    (2) With respect to long-term exposure mortality risk estimates,
the prior risk assessment focused on the estimates based on the
original ACS study (Pope et al., 1995). Since that time additional
cohort analyses have been published and evaluated in the Criteria
Document. EPA has greater confidence in the current risk estimates for
long-term exposure mortality, given the extensive review of these
studies and the extension of the ACS study to additional years of data,
as well as improvements in the statistical approach. However, ACS-based
risk estimates remain sensitive to plausible changes in statistical
model specifications. The choice of studies and concentration-response
functions to use for the base case risk estimates is discussed in the
Staff Paper (EPA, 2005a, p. 4-25) and risk assessment report (Abt
Associates, 2005, pp.49-50) and is consistent with the advice provided
by both the National Academy of Sciences and the Science Advisory board
Clean Air Act Compliance Council (see footnote 22). At the same time,
EPA recognizes that alternative statistical models were examined in the
reanalysis of the ACS and Six-Cities studies, and that the uncertainty
associated with model selection (such as multipollutant models and
different effect estimates associated with different educational
levels) is not reflected in the confidence ranges presented in this
assessment. Thus, for long-term exposure mortality risk estimates there
are additional unquantified uncertainties associated with a lack of
understanding as to which statistical model best represents the actual
concentration-response function. The relative risk estimates used in
the current risk assessment from the ACS extended study are only
slightly smaller (1.06 with 95 percent confidence interval of 1.02-
1.11) compared to the original ACS study (1.07 with 95 percent
confidence interval 1.04-1.10) used in the prior assessment. In terms
of the magnitude of the risk estimates, the estimates in terms of
percentage of total incidence are very similar for the two specific
locations included in both the prior and current assessments.
    (3) A fairly wide range of risk estimates are observed for
PM2.5-related morbidity and mortality risk associated with
recent air quality across the urban areas analyzed. The impact of
adding additional co-pollutants to the models was variable; sometimes
there was relatively little difference, while in other cases there were
larger differences. The wide variability in risk estimates associated
with a recent year of air

quality is to be expected given the wide range of PM2.5
levels across the urban areas analyzed and the variation observed in
the concentration-response relationships obtained from the original
epidemiologic studies. Among other factors, this variability may
reflect differences in the mixture of components or sources of fine
particles, populations, exposure considerations (e.g., degree of air
conditioning use), differences in co-pollutants and/or other stressors,
differences in study design, and differences related to exposure and
monitor measurement error.
    (4) The single most important factor influencing the quantitative
estimates of risk is which of the alternative concentration-response
functions included in this assessment are considered to best represent
the unknown ``true'' concentration-response relationships. In
comparison, the following uncertainties have only a moderate impact on
the risk estimates in some or all of the cities: choice of an
alternative estimated constant background level, use of a distributed
lag model, and alternative assumptions about the relevant air quality
for estimating exposure levels for long-term exposure mortality. Use of
a distribution of daily background concentrations had very little
impact on the risk estimates.
    The overall pattern of risk associated with short-term
PM2.5 exposures across the distribution of PM2.5
air quality, as typically observed in urban areas, is similar to that
observed in the last review. That is, on an annual basis, the very
highest days (which pose the greatest risk in terms of deaths per day)
contribute less to the total annual health risk associated with short-
term exposures than the middle of the distribution, due to the much
greater number of days that occur in this part of the air quality
distribution.
    (5) Risk estimates associated with just meeting the current suite
of PM2.5 standards in five urban areas that do not meet the
current PM2.5 standards showed a wide range of
PM2.5-related risk estimates for short-term exposure
mortality and morbidity. This is likely due, in large part, to
differences in concentration-response relationships among single-
location short-term exposure studies, differences in baseline incidence
rates, and varying population sizes. Results of a sensitivity analysis
which applied one multi-city concentration-response function to all
five urban areas analyzed narrowed considerably the range of risk
estimates when a risk metric was used that normalized for different
population sizes. However, it is still unknown whether the wider range
of estimates observed using single-city concentration-response
functions reflect methodological differences between studies and/or
real city-to-city differences related to exposure, population,
composition of the particles, or other factors.
    (6) For the risk estimates associated with just meeting the current
suite of PM2.5 standards and alternative suites of
standards, the single most important factor influencing the short- and
long-term exposure mortality and morbidity estimates is again which of
the alternative concentration-response functions included in this
assessment are considered to best represent the unknown ``true''
concentration-response relationships. Several additional sources of
uncertainty are introduced into this portion of the risk assessment,
including: (1) Uncertainty in the degree to which the pattern of air
quality concentration reductions estimated for the risk assessment
cities represents the distribution of actual PM concentration changes
that would be observed in a given area (``rollback uncertainty'') and
(2) uncertainty concerning the degree to which current PM risk
coefficients may reflect contributions from other pollutants, or
uncertainty concerning whether all of the constituents of
PM2.5 would be reduced in similar proportion to the
reduction in PM2.5 as a whole, and, if not, what impact this
would have on estimated reductions in risk. For areas where the current
annual standard is the controlling standard, one alternative approach
to rolling back the distribution of daily PM2.5
concentrations, in which the upper end of the distributions of
concentrations was reduced by a greater amount than the rest of the
distribution, had little impact on the risk estimates. This approach or
alternative approaches to rolling back the distribution of daily
concentrations may have a greater impact on the risk estimates in areas
where the daily standard is the controlling standard.
    (7) For the risk estimates associated with just meeting the current
or alternative suites of PM2.5 standards, there is a
significant decrease in the mortality risk estimates based on short-
term PM2.5 exposure remaining as one considers alternative
higher cutpoints. There also is a significant increase observed in the
percent reduction in estimated risk upon just meeting alternative
standards with higher alternative cutpoints. These findings are even
more pronounced for the mortality risk estimates associated with long-
term PM2.5 exposure as higher alternative cutpoint levels
are considered.

C. Need for Revision of the Current Primary PM2.5 Standards

    The initial issue to be addressed in the current review of the
primary PM2.5 standards is whether, in view of the advances
in scientific knowledge reflected in the Criteria Document and Staff
Paper, the existing standards should be revised. Based on the
information and conclusions presented in the Criteria Document,
summarized above in section II.A., the Staff Paper concludes that the
newly available information generally reinforces the associations
between PM2.5 and mortality and morbidity effects observed
in the last review. While important uncertainties and research
questions remain, much progress has been made in reducing some key
uncertainties since the last review. The examination of specific
components, properties, and sources of fine particles that are linked
with health effects remains an important research need. Other important
research needs include better characterizing the shape of
concentration-response functions, including identification of potential
threshold levels, and methodological issues such as those associated
with selecting appropriate statistical models in time-series studies to
address time-varying factors (such as weather) and other factors (such
as other pollution variables), and better characterizing population
exposures. Nonetheless, important progress has been made in advancing
our understanding of potential mechanisms by which ambient
PM2.5, alone and in combination with other pollutants, is
causally linked with cardiovascular, respiratory, and lung cancer
associations observed in epidemiologic studies. In addition, health
effects associations reported in epidemiologic studies have been found
to be generally robust to confounding by co-pollutants, there is now
greater confidence in the results of long-term exposure studies due to
reanalyses and extensions of the critical studies, and there is an
increased understanding of susceptible populations. Based on these
considerations, the Staff Paper finds clear support in the available
evidence for fine particle standards that are at least as protective as
the current PM2.5 standards (EPA, 2005a, p. 5-6).
    Having reached this initial conclusion, the Staff Paper addresses
the question of whether the available evidence supports consideration
of standards that are more protective than the current PM2.5
standards. In so doing, the Staff Paper considers whether there is now
evidence (1) that statistically significant health effects associations

with short-term exposures to fine particles occur in areas that would
likely meet the current PM2.5 standards or (2) that such
associations with long-term exposures to fine particles extend down to
lower air quality levels than had previously been observed.\26\ This
takes into consideration the bases for the decisions made in 1997 in
setting the current PM2.5 standards. In generally
considering what areas would likely meet the current PM2.5
standards, the focus is principally on comparing the long-term average
PM2.5 level in an area with the level of the current annual
PM2.5 standard, since in 1997 that standard was set to be
the ``generally controlling'' standard to provide protection against
health effects related to both short- and long-term exposures to fine
particles. In conjunction with such an annual standard, the current 24-
hour standard was set to provide only supplemental protection against
days with high peak PM2.5 concentrations, localized
``hotspots,'' or risks arising from seasonal emissions that might not
be well controlled by a national annual standard.
---------------------------------------------------------------------------

    \26\ In addressing this question, the Staff Paper first
recognizes, as discussed above in section II.A.3, that although
there are likely biologic threshold levels in individuals for
specific health responses, the available epidemiologic evidence
neither supports nor refutes the existence of thresholds at the
population level for the effects of PM2.5 on mortality
across the range of concentrations in the studies, for either long-
term or short-term PM2.5 exposures (EPA, 2004, section
<IP_ADDRESS>).
---------------------------------------------------------------------------

    In first considering the available epidemiologic evidence related
to short-term exposures, the Staff Paper focuses on specific
epidemiologic studies that show statistically significant associations
between PM2.5 and health effects for which the Criteria
Document judges associations with PM2.5 to be likely causal
(EPA, 2005a, section <IP_ADDRESS>). Many more U.S. and Canadian studies are
now available that provide evidence of associations between short-term
exposure to PM2.5 and serious health effects in areas with
air quality at and above the level of the current annual
PM2.5 standard (15 [mu]g/m3). Moreover, a few newly
available short-term exposure mortality studies provide evidence of
statistically significant associations with PM2.5 in areas
with air quality levels below the levels of the current
PM2.5 standards. In considering these studies, the Staff
Paper focuses on those that include adequate gravimetric
PM2.5 mass measurements, and where the associations are
generally robust to alternative model specification and to the
inclusion of potentially confounding co-pollutants. Three such studies
conducted in Phoenix (Mar et al., 2003), Santa Clara County, CA
(Fairley, 2003) and eight Canadian cities (Burnett and Goldberg, 2003)
report statistically significant associations between short-term
PM2.5 exposure and total and cardiovascular mortality in
areas in which long-term average PM2.5 concentrations ranged
between 13 and 14 [mu]g/m3 and 98th percentile
concentrations ranged between 32 and 59 [mu]g/m3.\27\
---------------------------------------------------------------------------

    \27\ As noted in the Staff Paper, these studies were reanalyzed
to address questions about the application of the statistical
software used in the original analyses, and the study results from
Phoenix and Santa Clara County were little changed in alternative
models (Mar et al., 2003; Fairley, 2003), although Burnett and
Goldberg (2003) reported that their results were sensitive to using
different temporal smoothing methods. Two of these studies also
reported significant associations with gaseous pollutants (Mar et
al., 2003; Fairley, 2003), and the other study included multi-
pollutant model results in reanalyses, reporting that associations
with PM2.5 remained significant with gaseous pollutants
(Fairley, 2003).
---------------------------------------------------------------------------

    In also considering the new epidemiologic evidence available from
U.S. and Canadian studies of long-term exposure to fine particles, the
Criteria Document notes that new studies have built upon studies
available in the last review and concludes that these studies have
confirmed and strengthened the evidence of associations for both
mortality and respiratory morbidity (EPA, 2004, section 9.2.3). For
mortality, the Criteria Document places greatest weight on the
reanalyses and extensions of the Six Cities and ACS studies, finding
that these studies provide strong evidence for associations with fine
particles (EPA, 2004, p. 9-34), notwithstanding the lack of consistent
results in other long-term exposure studies. For morbidity, the
Criteria Document finds that new studies of a cohort of children in
Southern California have built upon earlier limited evidence to provide
fairly strong evidence that long-term exposure to fine particles is
associated with development of chronic respiratory disease and reduced
lung function growth (EPA, 2004, pp. 9-33 to 9-34). In addition to
strengthening the evidence of association, the new extended ACS
mortality study observed statistically significant associations with
cardiorespiratory mortality (including lung cancer mortality) across a
range of long-term mean PM2.5 concentrations that was lower
than was reported in the original ACS study available in the last
review.
    Beyond the epidemiologic studies using PM2.5 as an
indicator of fine particles, a large body of newly available evidence
from studies that used PM10, as well as other indicators or
components of fine particles (e.g., sulfates, combustion-related
components), provides additional support for the conclusions reached in
the last review as to the likely causal role of ambient PM, and the
likely importance of fine particles in contributing to observed health
effects. Such studies notably include new multi-city studies,
intervention studies (that relate reductions in ambient PM to observed
improvements in respiratory or cardiovascular health), and source-
oriented studies (e.g., suggesting associations with combustion- and
vehicle-related sources of fine particles). The Criteria Document also
notes that new epidemiologic studies of asthma-related increased
physicians visits and symptoms, as well as new studies of cardiac-
related risk factors, suggest likely much larger public health impacts
due to ambient fine particles than just those indexed by the mortality
and morbidity effects considered in the last review (EPA, 2004, p. 9-
94).
    In reviewing this information, the Staff Paper recognizes that
important limitations and uncertainties associated with this expanded
body of evidence for PM2.5 and other indicators or
components of fine particles, noted above in section II.A.2, need to be
carefully considered in determining the weight to be placed on the body
of studies available in this review. For example, the Criteria Document
notes that while PM-effects associations continue to be observed across
most new studies, the newer findings do not fully resolve the extent to
which the associations are properly attributed to PM acting alone or in
combination with other gaseous co-pollutants, particularly
SO2, or to the gaseous co-pollutants themselves. The
Criteria Document concludes, however, that overall the various
approaches that have now been used to evaluate this issue substantiate
that associations for various PM indicators with mortality and
morbidity are generally robust to confounding by co-pollutants (EPA,
2004, p. 9-37).
    While the limitations and uncertainties in the available evidence
suggest caution in interpreting the epidemiologic studies at the lower
levels of air quality observed in the studies, the Staff Paper
concludes that the evidence now available provides strong support for
considering fine particle standards that would provide increased
protection beyond that afforded by the current PM2.5
standards. The Staff Paper notes that a more protective suite of
PM2.5 standards would reflect the generally stronger and
broader body of evidence of associations with mortality and morbidity
now available in this review, both at levels

below the current standards and extending to lower levels of air
quality than in earlier studies, as well as increased understanding of
possible underlying mechanisms.
    In addition to this evidence-based evaluation, the Staff Paper also
considers the extent to which health risks estimated to occur upon
attainment of the current PM2.5 standards may be judged to
be important from a public health perspective, taking into account key
uncertainties associated with the quantitative health risk estimates.
In so doing, the Staff Paper first notes that the risk assessment
addresses a number of key uncertainties through various base case
analyses, as well as through several sensitivity analyses, as discussed
above in section II.B. In considering the health risks estimated to
occur upon attainment of the current PM2.5 standards, the
Staff Paper focuses in particular on a series of base case risk
estimates, while recognizing that the confidence ranges in the selected
base case estimates do not reflect all the identified uncertainties.
These risks were estimated using not only the linear or log-linear
concentration-response functions reported in the studies,\28\ but also
using alternative modified linear functions as surrogates for assumed
non-linear functions that would reflect the possibility that thresholds
may exist in the reported associations within the range of air quality
observed in the studies. Regardless of the relative weight placed on
the risk estimates associated with the concentration-response functions
reported in the studies or with the modified functions favored by
CASAC,\29\ the risk assessment indicates the possibility that thousands
of premature deaths per year would occur in urban areas across the U.S.
upon attainment of the current PM2.5 standards.\30\ Beyond
the estimated incidences of premature mortality, the Staff Paper also
recognizes that similarly substantial numbers of incidences of hospital
admissions, emergency room visits, aggravation of asthma and other
respiratory symptoms, and increased cardiac-related risk are also
likely in many urban areas, based on risk assessment results (EPA,
2005a, Chapter 4) and on the discussion related to this pyramid of
effects in the Criteria Document (EPA, 2004, section 9.2.5). Based on
these considerations, the Staff Paper concludes that the estimates of
risks likely to remain upon attainment of the current PM2.5
standards are indicative of risks that can reasonably be judged to be
important from a public health perspective.
---------------------------------------------------------------------------

    \28\ As discussed above in section II.B.2, the reported linear
or log-linear concentration-response functions were applied down to
7.5 [mu]g/m3 in estimating risk associated with long-term
exposure (i.e., the lowest measured level in the extended ACS
study), and down to the estimated policy-relevant background level
in estimating risk associated with short-term exposure (i.e., 3.5
[mu]g/m\3\ for eastern urban areas and 2.5 [mu]g/m\3\ for western
urban areas).
    \29\ The CASAC PM Panel generally favored the primary use of an
assumed threshold of 10 [mu]g/m\3\ for the various concentration-
response functions used in the risk assessment (Henderson, 2005a).
    \30\ The Staff Paper recognizes how highly dependent any
specific risk estimates are on the assumed shape of the underlying
concentration-response functions, noting nonetheless that mortality
risks are not completely eliminated when current PM2.5
standards are met in a number of example urban areas even using the
highest assumed cutpoint levels considered in the risk assessment
(EPA, 2005a, p. 5-15).
---------------------------------------------------------------------------

    In considering available evidence, risk estimates, and related
limitations and uncertainties, the Staff Paper concludes that the
available information clearly calls into question the adequacy of the
current suite of PM2.5 standards and provides strong support
for revising the current PM2.5 standards to provide
increased public health protection. Also taking into account these
considerations, the CASAC advised the Administrator that a majority of
CASAC Panel members were in agreement that the primary 24-hour and
annual PM2.5 standards ``should be modified to provide
increase public health protection'' (Henderson, 2005a). The CASAC
further advised that changes to either the annual standard or the 24-
hour standard, or both, could be recommended, and expressed reasons
that formed the basis for the consensus among the Panel members for
placing more emphasis on lowering the 24-hour standard (Henderson,
2005a).\31\
---------------------------------------------------------------------------

    \31\ Of the individual Panel members who submitted written
comments expressing views on appropriate levels of the
PM2.5 standards, only one did not suppport changes to
either the 24-hour or annual standard to provide additional public
health protection (Henderson, 2005a). In written comments, the
health scientists on the CASAC Panel did not agree on whether the
annual standard should be lowered.
---------------------------------------------------------------------------

    In considering whether the suite of primary PM2.5
standards should be revised to provide requisite public health
protection, the Administrator has carefully considered the rationale
and recommendations contained in the Staff Paper, the advice and
recommendations from CASAC, and public comments to date on this issue.
In so doing, the Administrator places primary consideration on the
evidence obtained from the studies, and provisionally finds the
evidence of serious health effects reported in short-term exposure
studies conducted in areas that would attain the current standards to
be compelling, especially in light of the extent to which such studies
are part of an overall pattern of positive and frequently statistically
significant associations across a broad range of studies that
collectively represent a strong and robust body of evidence. As
discussed in the Criteria Document and Staff Paper, the Administrator
recognizes that much progress has been made since the last review in
addressing some of the key uncertainties that were important
considerations in establishing the current PM2.5 standards.
In considering the risk assessment presented in the Staff Paper, the
Administrator notes that the assessment contained a sensitivity
analysis but not a formal uncertainty analysis, making it difficult to
use the risk assessment to form a judgment of the probability of
various risk estimates. Instead, the Administrator views the risk
assessment in light of his evaluation of the underlying studies. Seen
in this light, the risk assessment informs the determination of the
public health significance of risks to the extent that the evidence is
judged to support an effect at a particular level of air quality. Based
on these considerations, the Administrator provisionally concludes that
the current primary PM2.5 standards, taken together, are not
requisite to protect public health with an adequate margin of safety
and that revision is needed to provide increased public health
protection.

D. Indicator of Fine Particles

    In 1997, EPA established PM2.5 as the indicator for fine
particles. In reaching this decision, the Agency first considered
whether the indicator should be based on the mass of a size-
differentiated sample of fine particles or on one or more components
within the mix of fine particles. Secondly, in establishing a size-
based indicator, a size cut needed to be selected that would
appropriately distinguish fine particles from particles in the coarse
mode.
    In addressing the first question in the last review, EPA determined
that it was appropriate to control fine particles as a group, as
opposed to singling out any particular component or class of fine
particles. Community health studies had found significant associations
between various indicators of fine particles (including
PM2.5 or PM10 in areas dominated by fine
particles) and health effects in a large number of areas that had
significant mass contributions of differing components or sources of
fine particles, including sulfates, wood smoke, nitrates, secondary
organic compounds and acid sulfate aerosols. In addition, a number of
animal

toxicologic and controlled human exposure studies had reported health
effects associations with high concentrations of numerous fine particle
components (e.g., sulfates, nitrates, transition metals, organic
compounds), although such associations were not consistently observed.
It also was not possible to rule out any component within the mix of
fine particles as not contributing to the fine particle effects found
in epidemiologic studies. For these reasons, EPA concluded that total
mass of fine particles was the most appropriate indicator for fine
particle standards rather than an indicator based on PM composition (62
FR 38667, July 18, 1997).
    Having selected a size-based indicator for fine particles, the
Agency then based its selection of a specific size cut on a number of
considerations. In focusing on a size cut within the size range of 1 to
3 [mu]m (i.e., the intermodal range between fine and coarse mode
particles), the Agency noted that the available epidemiologic studies
of fine particles were based largely on PM2.5; only very
limited use of PM1 monitors had been made. While it was
recognized that using PM1 as an indicator of fine particles
would exclude the tail of the coarse mode in some locations, in other
locations it would miss a portion of the fine PM, especially under high
humidity conditions, which would result in falsely low fine PM
measurements on days with some of the highest fine PM concentrations.
The selection of a 2.5 [mu]m size cut reflected the regulatory
importance that was placed on defining an indicator for fine particle
standards that would more completely capture fine particles under all
conditions likely to be encountered across the U.S., especially when
fine particle concentrations are likely to be high, while recognizing
that some small coarse particles would also be captured by
PM2.5 monitoring. Thus, EPA's selection of 2.5 [mu]m as the
size cut for the fine particle indicator was based on considerations of
consistency with the epidemiologic studies, the regulatory importance
of more completely capturing fine particles under all conditions, and
the potential for limited intrusion of coarse particles in some areas;
it also took into account the general availability of monitoring
technology (62 FR 38668).
    In this current review, the same considerations continue to apply
for selection of an appropriate indicator for fine particles. As an
initial matter, the available epidemiologic studies linking mortality
and morbidity effects with short- and long-term exposures to fine
particles continue to be largely indexed by PM2.5. Some
epidemiologic studies also have continued to implicate various
components within the mix of fine particles that have been more
commonly studied (e.g., sulfates, nitrates, carbon, organic compounds,
and metals) as being associated with adverse effects (EPA, 2004, p. 9-
31, Table 9-3). In addition, several recent studies have used
PM2.5 speciation data to evaluate the association between
mortality and particles from different sources (Schwartz, 2003a; Mar et
al., 2003; Tsai et al., 2000; EPA, 2004, section <IP_ADDRESS>). Schwartz
(2003a) reported statistically significant associations for mortality
with factors representing fine particles from traffic and residual oil
combustion that were little changed in reanalysis to address
statistical modeling issues, and also an association between mortality
and coal combustion-related particles that was reduced in size and lost
statistical significance in reanalysis. In Phoenix, significant
associations were reported between mortality and fine particles from
traffic emissions, vegetative burning, and regional sulfate sources
that remained unchanged in reanalysis models (Mar et al., 2003).
Finally, a small study in three New Jersey cities reported significant
associations between mortality and fine particles from industrial, oil
burning, motor vehicle and sulfate aerosol sources, though the results
were somewhat inconsistent between cities (Tsai et al., 2000).\32\ No
significant increase in mortality was reported with a source factor
representing crustal material in fine particles (CD, p. 8-85).
Recognizing that these three studies represent a very preliminary
effort to distinguish effects of fine particles from different sources,
and that the results are not always consistent across the cities, the
Criteria Document found that these studies indicate that exposure to
fine particles from combustion sources, but not crustal material, is
associated with mortality (EPA, 2004, p. 8-77). Animal toxicologic and
controlled human exposure studies have continued to link a variety of
PM components or particle types (e.g., sulfates, notably primary metal
sulfate emissions from residual oil burning, metals, organic
constituents, bioaerosols, diesel particles) with health effects,
though often at high concentrations (EPA, 2004, section 7.10.2). In
addition, some recent studies have suggested that the ultrafine subset
of fine particles (generally including particles with a nominal mean
aerodynamic diameter less than 0.1 [mu]m) may also be associated with
adverse effects (EPA, 2004, pp. 8-67 to 68).
---------------------------------------------------------------------------

    \32\ More specifically, statistically significant associations
were reported with factors representing fine particles from oil
burning, industrial and sulfate aerosol sources in Newark and with
particles from oil burning and motor vehicle sources in Camden, and
no statistically significant associations were reported in
Elizabeth.
---------------------------------------------------------------------------

    The Criteria Document recognizes that, for a given health response,
some fine particle components are likely to be more closely linked with
that response than others. The presumption that different PM
constituents may have differing biological responses is toxicologically
plausible and an important source of uncertainty in interpreting such
epidemiologic evidence. For specific effects there may be stronger
correlation with individual PM components than with aggregate particle
mass. In addition, particles or particle-bound water can act as
carriers to deliver other toxic agents into the respiratory tract,
suggesting that exposure to particles may elicit effects that are
linked with a mixture of components more than with any individual PM
component (EPA, 2004, section <IP_ADDRESS>.3).
    Thus, epidemiologic and toxicologic studies have provided evidence
for effects associated with various fine particle components or size-
differentiated subsets of fine particles. The Criteria Document
concludes: ``These studies suggest that many different chemical
components of fine particles and a variety of different types of source
categories are all associated with, and probably contribute to,
mortality, either independently or in combinations'' (EPA, 2004, p. 9-
31). Conversely, the Criteria Document provides no basis to conclude
that any individual fine particle component cannot be associated with
adverse health effects (EPA, 2005a, p. 5-17). In short, there is not
sufficient evidence that would lead toward the selection of one or more
PM components as being primarily responsible for effects associated
with fine particles, nor is there sufficient evidence to suggest that
any component should be eliminated from the indicator for fine
particles. The Staff Paper continues to recognize the importance of an
indicator that not only captures all of the most harmful components of
fine particles (i.e., an effective indicator), but also emphasizes
control of those constituents or fractions, including sulfates,
transition metals, and organics that have been associated with health
effects in epidemiologic and/or toxicologic studies, and is thus most
likely to result in the largest risk reduction (i.e., an efficient
indicator). Taking into account the above considerations, the Staff
Paper concludes that it remains appropriate to

control fine particles as a group; i.e., that total mass of fine
particles is the most appropriate indicator for fine particle standards
(EPA, 2005a, p. 5-17).
    With regard to an appropriate size cut for a size-based indicator
of total fine particle mass, the Criteria Document concludes that
advances in our understanding of the characteristics of fine particles
continue to support the use of particle size as an appropriate basis
for distinguishing between these subclasses, and that a nominal size
cut of 2.5 [mu]m remains appropriate (EPA, 2004, p. 9-22). This
conclusion follows from a recognition that within the intermodal range
of 1 to 3 [mu]m there is no unambiguous definition of an appropriate
size cut for the separation of the overlapping fine and coarse particle
modes. Within this range, the Staff Paper considered size cuts of both
1 [mu]m and 2.5 [mu]m. Consideration of these two size cuts took into
account that there is generally very little mass in this intermodal
range, although in some circumstances (e.g., windy, dusty areas) the
coarse mode can extend down to and below 1 [mu]m, whereas in other
circumstances (e.g., high humidity conditions, usually associated with
very high fine particle concentrations) the fine mode can extend up to
and above 2.5 [mu]m. The same considerations that led to the selection
of a 2.5 [mu]m size cut in the last review--that the epidemiologic
evidence was largely based on PM2.5 and that it was more
important from a regulatory perspective to capture fine particles more
completely under all conditions likely to be encountered across the
U.S. (especially when fine particle concentrations are likely to be
high) than to avoid some coarse-mode intrusion into the fine fraction
in some areas--led to the same recommendation by the Staff Paper (EPA,
2005a, p. 5-18) and CASAC (Henderson, 2005a) in this review. In
addition, the Staff Paper recognizes that particles can act as carriers
of water, oxidative compounds, and other components into the
respiratory system, which adds to the importance of ensuring that
larger accumulation-mode particles are included in the fine particle
size cut (EPA, 2005a, p. 5-18).
    Consistent with the Staff Paper and CASAC recommendations, the
Administrator proposes to retain PM2.5 as the indicator for
fine particles. Further, the Administrator provisionally concludes that
currently available studies do not provide a sufficient basis for
supplementing mass-based fine particle standards with standards for any
specific fine particle component or subset of fine particles, or for
eliminating any individual component or subset of components from fine
particle mass standards. Addressing the current uncertainties in the
evidence of effects associated with various fine particle components
and types of source categories is an important element in EPA's ongoing
PM research program.
    The Administrator notes that some commenters have expressed views
about the importance of evaluating health effect associations with
various fine particle components and types of source categories as a
basis for focusing ongoing and future research to reduce uncertainties
in this area and for considering whether alternative indicator(s) are
now or may be appropriate for standards intended to protect against the
array of health effects that have been associated with fine particles
as indexed by PM2.5.\33\ Information from such studies could
also help inform the development of strategies that emphasize control
of specific types of emission sources so as to address particles of
greatest concern to public health. While recognizing that the studies
evaluated in the Criteria Document provide some limited evidence of
such associations that is helping to focus research activities, the
Administrator solicits broad public comment on issues related to
studies of fine particle components and types of source categories and
their usefulness as a basis for consideration of alternative
indicator(s) for fine particle standards. In general, comment is
solicited on relevant new published research, recommendations for
studies that would be appropriate for inclusion in future research
activities, and approaches to assessing the available and future
research results to determine whether alternative indicators for fine
particles are warranted to provide effective protection of public
health from effects associated with long- and short-term exposure to
ambient fine particles.
---------------------------------------------------------------------------

    \33\ Such comments have focused in part on newer studies that
have become available since the close of the Criteria Document,
which EPA intends to include in its assessment of potentially
significant new studies discussed above in section I.D.
---------------------------------------------------------------------------

    More specifically, comment is also solicited on a number of related
issues. One such issue is the extent to which reducing particular types
of PM (differentiated by either size or chemistry) might alter the size
and toxicity of remaining particles, and on the extent to which fine
particles in urban and rural areas can be differentiated by size or
chemistry. Another issue deals with assessment of human exposure and
its relationship with pollution measurements at monitors (EPA, 2004,
chapter 5); comment is solicited on the extent to which the latest
scientific information can be used to improve our understanding of the
relationship of monitored pollution levels to human exposure. Comment
is also solicited on studies using concentrated ambient particles
(CAPs) and their use in examining the toxicity of specific mixtures of
pollutants or of particular source categories.

E. Averaging Time of Primary PM2.5 Standards

    In the last review, EPA established two PM2.5 standards,
based on annual and 24-hour averaging times, respectively (62 FR at
38668-70). This decision was based in part on evidence of health
effects related to both short-term (from less than 1 day to up to
several days) and long-term (from a year to several years) measures of
PM. EPA noted that the large majority of community epidemiologic
studies reported associations based on 24-hour averaging times or on
multiple-day averages. Further, EPA noted that a 24-hour standard could
also effectively protect against episodes lasting several days, as well
as providing some degree of protection from potential effects
associated with shorter duration exposures. EPA also recognized that an
annual standard would provide effective protection against both annual
and multi-year, cumulative exposures that had been associated with an
array of health effects, and that a much longer averaging time would
complicate and unnecessarily delay control strategies and attainment
decisions. EPA considered the possibility of seasonal effects, although
the very limited available evidence of such effects and the seasonal
variability of sources of fine particle emissions across the country
did not provide an adequate basis for establishing a seasonal averaging
time.
    In considering whether the information available in this review
supports consideration of different averaging times for
PM2.5 standards, the Staff Paper concludes that the
available information is generally consistent with and supportive of
the conclusions reached in the last review to set PM2.5
standards with both annual and 24-hour averaging times. In considering
the new information, the Staff Paper makes the following observations
(EPA, 2005a, section 5.3.3):
    (1) There is a growing body of studies that provide additional
evidence of effects associated with exposure periods shorter than 24-
hours (e.g., one to several hours) (EPA, 2004, section <IP_ADDRESS>). While
the Staff Paper concludes

that this information remains too limited to serve as a basis for
establishing a shorter-than-24-hour fine particle primary standard at
this time, it also noted that this information gives added weight to
the importance of a standard with a 24-hour averaging time.
    (2) Some recent PM10 studies have used a distributed lag
over several days to weeks preceding the health event, although this
modeling approach has not been extended to studies of fine particles
(EPA, 2004, section 3.5.5). While such studies continue to suggest
consideration of a multiple day averaging time, the Staff Paper notes
that limiting 24-hour concentrations of fine particles will also
protect against effects found to be associated with PM averaged over
many days in health studies. Consistent with the conclusion reached in
the last review, the Staff Paper concludes that a multiple-day
averaging time would add complexity without providing more effective
protection than a 24-hour average.
    (3) While some newer studies have investigated seasonal effects
(EPA, 2004, section <IP_ADDRESS>), the Staff Paper concludes that currently
available evidence of such effects is still too limited to serve as a
basis for considering seasonal standards.
    Based on the above considerations, the Staff Paper and CASAC
(Henderson, 2005a) recommend retaining the current annual and 24-hour
averaging times for PM2.5 primary standards. The
Administrator concurs with the staff and CASAC recommendations and
proposes that averaging times for PM2.5 standards should
continue to include annual and 24-hour averages to protect against
health effects associated with short-term (hours to days) and long-term
(seasons to years) exposure periods.

F. Form of Primary PM2.5 Standards

1. 24-Hour PM2.5 Standard
    In 1997 EPA established the form of the 24-hour PM2.5
standard as the 98th percentile of the annual 24-hour concentrations at
each population-oriented monitor within an area, averaged over three
years (62 FR at 38671-74). EPA selected such a concentration-based form
because of its advantages over the previously used expected-exceedance
form.\34\ A concentration-based form is more reflective of the health
risk posed by elevated PM2.5 concentrations because it 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 standard. Further, a concentration-based form better
compensates for missing data and less-than-every-day monitoring; and,
when averaged over 3 years, it has greater stability and, thus,
facilitates the development of more stable implementation programs.\35\
After considering a range of concentration percentiles from the 95th to
the 99th, EPA selected the 98th percentile as an appropriate balance
between adequately limiting the occurrence of peak concentrations and
providing increased stability and robustness. Further, by basing the
form of the standard on concentrations measured at population-oriented
monitoring sites (as specified in 40 CFR part 58), EPA intended to
provide protection for people residing in or near localized areas of
elevated concentrations.
---------------------------------------------------------------------------

    \34\ The form of the 1987 24-hour PM10 standard is
based on the expected number of days per year (averaged over 3
years) on which the level of the standard is exceeded; thus,
attainment of the one-expected exceedance form is determined by
comparing the fourth-highest concentration in 3 years with the level
of the standard.
    \35\ See American Trucking Associations v. EPA, 283 F. 3d at
374-75 (legitimate for EPA to consider promotion of overall
effectiveness of NAAQS implementation programs, including their
overall stability, in setting a standard that is requisite to
protect the public health).
---------------------------------------------------------------------------

    In this review, the Staff Paper concludes that it is appropriate to
retain a concentration-based form that is defined in terms of a
specific percentile of the distribution of 24-hour PM2.5
concentrations at each population-oriented monitor within an area,
averaged over 3 years. This staff recommendation is based on the same
reasons that were the basis for EPA's selection of this type of form in
the last review. As to the specific percentile value to be considered,
the Staff Paper took into consideration (1) the relative risk reduction
afforded by alternative forms at the same standard level, (2) the
relative year-to-year stability of the air quality statistic to be used
as the basis for the form of a standard, and (3) the implications from
a public health communication perspective of the extent to which either
form allows different numbers of days in a year to be above the level
of the standard in areas that attain the standard. Based on these
considerations, the Staff Paper recommends either retaining the 98th
percentile form or revising it to be based on the 99th percentile form,
and notes that primary consideration should be given to the combination
of form and level, as compared to looking at the form in isolation
(EPA, 2005a, p. 5-44).
    In considering the information provided in the Staff Paper, most
CASAC Panel members favored continued use of the 98th percentile form
because it is more robust than the 99th percentile form, such that it
would provide more stability to prevent areas from bouncing in and out
of attainment from year to year (Henderson 2005a). In recommending
retention of the 98th percentile form, the CASAC Panel recognized that
it is the link between the form and level of a standard that determines
the degree of public health protection afforded by a standard.
    In considering the available information and the Staff Paper and
CASAC recommendations, the Administrator proposes that the form of the
24-hour standard should be based on the 98th percentile form. In so
doing, the Administrator has focused on the relative stability of the
98th and 99th percentile forms as a basis for selecting the 98th
percentile form, while 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.
2. Annual PM2.5 Standard
    In 1997 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 of the annual
standard was intended to represent a relatively stable measure of air
quality and to characterize area-wide PM2.5 concentrations
in conjunction with a 24-hour standard designed to provide adequate
protection against localized peak or seasonal PM2.5 levels.
The current annual PM2.5 standard level is to be compared to
measurements made at the community-oriented monitoring site recording
the highest level, or, if specific constraints are met, measurements
from multiple community-oriented monitoring sites may be averaged (Part
50 App. N section 2.1(a) and (b) and Part 58 App. D at <IP_ADDRESS>.1; 62 FR
38,672, July 18, 1997). Community-oriented monitoring sites were
specified to be consistent with the intent that a spatially averaged
annual standard protect those in smaller communities, as well as those
in larger population centers. The constraints on allowing the use of
spatially averaged measurements were intended to limit averaging across
poorly correlated or widely disparate air quality values.\36\ This
approach was judged to be consistent with the epidemiologic 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, not localized
``hot spots'' (62 FR 38672). These criteria and constraints were
intended to ensure that spatial averaging would not result in
inequities in the level of protection afforded by the PM2.5
standards (Id.).
---------------------------------------------------------------------------

    \36\ The current constraints include the criteria that the
correlation coefficient between monitor pairs to be averaged be at
least 0.6, and that differences in mean air quality values between
monitors to be averaged not exceed 20 percent (Part 58 App. D at
<IP_ADDRESS>.1).
---------------------------------------------------------------------------

    In this review, there now exist much more PM2.5 air
quality data than were available in the last review. Consideration in
the Staff Paper of the spatial variability across urban areas that is
revealed by this new database has raised questions as to whether an
annual standard that allows for spatial averaging, within currently
specified or alternative constraints, would provide appropriate public
health protection. Analyses in the Staff Paper to assess these
questions, as discussed below, have taken into account both aggregate
population risk across an entire urban area and the potential for
disproportionate impacts on potentially vulnerable subpopulations
within an area.
    The effect of allowing the use of spatial averaging on aggregate
population risk was considered in sensitivity analyses included in the
health risk assessment (EPA, 2005a). In particular, analyses were done
in several urban areas that compared estimated mortality risks based on
calculating compliance with alternative standards (1) using air quality
values from the highest community-oriented monitor in an area and (2)
using air quality values averaged across all such monitors within the
constraints allowed by the current standard.\37\ As expected, estimated
risks associated with long-term exposures remaining upon just meeting
the current annual standard are greater when spatial averaging is used
than when the highest monitor is used (i.e., the estimated reductions
in risk associated with just attaining the current or alternative
annual standards are less when spatial averaging is used), as the use
of the highest monitor leads to greater modeled reductions in ambient
PM2.5 concentrations.\38\
---------------------------------------------------------------------------

    \37\ As discussed in the Staff Paper, section 4.2.2, the
monitored air quality values were used to determine the design value
for the annual standard in each area, as applied to a ``composite''
monitor to reflect area-wide exposures. Changing the basis of the
annual standard design value from the concentration at the highest
monitor to the average concentration across all monitors changes the
ambient PM2.5 levels that are needed to just meet the
current or alternative annual standards. With averaging, less
overall reduction in ambient PM2.5 is needed to just meet
the standards.
    \38\ For example, based on analyses conducted in three example
urban areas, estimated mortality incidence associated with long-term
exposure based on the use of spatial averaging is about 10 to over
40 percent higher than estimated incidence based on the use of the
highest monitor (EPA, 2005a, p. 5-41).
---------------------------------------------------------------------------

    In considering the potential for disproportionate impacts on
potentially vulnerable subpopulations, analyses were done to assess
whether any such groups are more likely to live in census tracts in
which the monitors recording the highest air quality values in an area
are located. Data were obtained for demographic parameters measured at
the census tract level, including education level, income level, and
percent minority population. Data from the census tract in each area in
which the highest air quality value was monitored were compared to the
area-wide average value (consistent with the constraints on spatial
averaging provided by the current standard) in each area. (Schmidt et
al., 2005). Recognizing the limitations of such cross-sectional
analyses, the Staff Paper observes that the results suggest 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 percentage minority
levels (EPA, 2005a, p. 5-41).\39\ Noting the intended purposes of the
form of the annual standard, as discussed above, the Staff Paper
concludes that the existing constraints on spatial averaging may not be
adequate to avoid substantially greater exposures in some areas,
potentially resulting in disproportionate impacts on potentially
vulnerable subpopulations.
---------------------------------------------------------------------------

    \39\ As summarized in section II.A.4 above, the Criteria
Document notes that some epidemiologic study results, most notably
the associations between mortality and long-term PM2.5
exposure in the ACS cohort, have shown larger effect estimates in
the cohort subgroup with lower education levels (EPA, 2004, p. 8-
103). The Criteria Document also notes that lower education level
can be a marker for lower socioeconomic status that may be related
to increased vulnerability to the effects of fine particle
exposures, for example, as a result of greater exposure to sources
such as roadways. Lower education level may be associated with other
potential risk factors, such as poorer health status or access to
health care, that may also result in increased susceptibility to the
effects of air pollution exposure (EPA, 2004, section <IP_ADDRESS>)
---------------------------------------------------------------------------

    In considering whether more stringent constraints on the use of
spatial averaging may be appropriate, the Staff Paper presents results
of an analysis of recent air quality data on the correlations and
differences between monitor pairs in metropolitan areas across the
country (Schmidt et al., 2005). For all pairs of PM2.5
monitors, the median correlation coefficient based on annual air
quality data is approximately 0.9, which is substantially higher than
the current criterion for correlation of at least 0.6, which was met by
nearly all monitor pairs. Similarly, the current criterion that
differences in mean air quality values between monitors not exceed 20
percent was met for most monitor pairs, while the annual median and
mean differences for all monitor pairs are 5 percent and 8 percent,
respectively. This analysis also shows that in some areas with highly
seasonal air quality patterns (e.g., due to seasonal wood smoke
emissions), substantially lower seasonal correlations and larger
seasonal differences can occur relative to those observed on an annual
basis. This analysis provides some perspective on the constraints on
spatial averaging that were put in place in the last review, before
data were widely available on spatial distributions of PM2.5
air quality levels, based on the extensive air quality data and related
analyses that have become available since the last review.
    In considering the results of the analyses discussed above, the
Staff Paper concludes that it is appropriate to consider either
eliminating the provision that allows for spatial averaging from the
form of an annual PM2.5 standard or revising the allowance
for spatial averaging to be based on more restrictive criteria. More
specifically, based on the analyses discussed above, the Staff Paper
recommends consideration of revised criteria such that the correlation
coefficient between monitor pairs to be averaged be at least 0.9,
determined on a seasonal basis, with differences between monitor values
not to exceed 10 percent (EPA, 2005a, p. 5-42).
    In considering the Staff Paper recommendations based on the results
of the analyses discussed above, and focusing on a desire to be
consistent with the epidemiologic studies on which the PM2.5
health effects are based and concern over the evidence of potential
disproportionate impact on potentially vulnerable subpopulations, the
Administrator proposes to revise the form of the annual
PM2.5 standard consistent with the Staff Paper
recommendation to change the criteria for use of spatial averaging such
that the correlation coefficient between monitor pairs must be at least
0.9, determined on a seasonal basis, with differences between monitor
values not to exceed 10 percent. The Administrator also solicits
comment on the other Staff Paper-recommended alternative of revising
the form of the annual PM2.5

standard to one based on the highest community-oriented monitor in an
area, with no allowance for spatial averaging.

G. Level of Primary PM2.5 Standards

    In the last review, having concluded that both 24-hour and annual
PM2.5 standards were appropriate, EPA selected a level for
each standard that was appropriate for the function to be served by
such standard (62 FR 38652). As discussed above, EPA concluded at that
time that the suite of PM2.5 standards could most
effectively and efficiently protect public health by treating the
annual standard as the generally controlling standard for lowering both
short- and long-term PM2.5 concentrations.\40\ In
conjunction with such an annual standard, the 24-hour standard was
intended to provide protection against days with high peak
PM2.5 concentrations, localized ``hotspots,'' and risks
arising from seasonal emissions that would not be well controlled by an
annual standard.\41\
---------------------------------------------------------------------------

    \40\ In so doing, EPA noted that an annual standard would focus
control programs on annual average PM2.5 concentrations,
which would generally control the overall distribution of 24-hour
exposure levels, as well as long-term exposure levels, and would
also result in fewer and lower 24-hour peak concentrations.
Alternatively, a 24-hour standard that focused controls on peak
concentrations could also result in lower annual average
concentrations. Thus, EPA recognized that either standard could
provide some degree of protection from both short- and long-term
exposures, with the other standard serving to address situations
where the daily peaks and annual averages are not consistently
correlated (62 FR 38669).
    \41\ See also American Trucking Associations v. EPA, 283 F.3d at
373 (endorsing this reasoning).
---------------------------------------------------------------------------

    In selecting the level for the annual standard in the last review,
EPA used an evidence-based approach that considered the evidence from
both short- and long-term exposure studies. The risk assessment
conducted in the last review, while providing qualitative insights
about the distribution of risks, was considered to be too limited to
serve as a quantitative basis for decisions on the standard levels. In
accordance with Staff Paper and CASAC views on the relative strengths
of the short- and long-term exposure studies, greater emphasis was
placed on the short-term exposure studies. In so doing, EPA first
determined a level for the annual standard based on the short-term
exposure studies, and then considered whether the long-term exposure
studies suggested the need for a lower level. While recognizing that
health effects could occur over the full range of concentrations
observed in the studies, EPA concluded that the strongest evidence for
short-term PM2.5 effects occurs at concentrations near the
long-term (e.g., annual) average in those studies reporting
statistically significant health effects. Thus, in the last review, EPA
selected a level for the annual standard that was below the lowest
long-term average PM2.5 concentration in a short-term
exposure study that reported statistically significant health effects.
Further consideration of the average PM2.5 concentrations
across the cities in the key long-term exposure studies available at
that time did not provide a basis for establishing a lower annual
standard level.
    In this review, the approach used in the Staff Paper as a basis for
staff recommendations on standard levels builds upon and broadens the
general approach used by EPA in the last review. This broader approach
reflects the more extensive and stronger body of evidence now available
on health effects related to both short- and long-term exposure to
PM2.5, together with the availability of much more extensive
PM2.5 air quality data. This newly available information has
been used to conduct a more comprehensive risk assessment for
PM2.5. As a consequence, the broader approach used in the
Staff Paper discusses ways to take into account both evidence-based and
quantitative risk-based considerations and places relatively greater
emphasis on evidence from long-term exposure studies than was done in
the last review.
    Given the extensive body of new evidence based specifically on
PM2.5 that is now available, and the resulting broader
approach presented in the Staff Paper, the Administrator considers it
appropriate to use a different approach from that used in the last
review to select appropriate standard levels. More specifically, the
Administrator's proposal relies on an evidence-based approach that
considers the much expanded body of evidence from short-term exposure
PM2.5 studies as the principal basis for selecting the level
of the 24-hour standard and the stronger and more robust body of
evidence from the long-term exposure PM2.5 studies as the
principal basis for selecting the level of the annual standard. In the
Administrator's view, the very large number of health effect studies
that are now available provide the most reliable basis for standard
setting. With respect to the quantitative risk assessment, the
Administrator recognizes that it rests on a more extensive body of data
and is more comprehensive in scope than the assessment conducted in the
last review, but is mindful that significant uncertainties continue to
underlie the resulting risk estimates. Such uncertainties generally
relate to a lack of clear understanding of a number of important
factors, including for example: The shape of concentration-response
functions, particularly when, as here, effect thresholds can neither be
discerned nor determined not to exist; issues related to selection of
appropriate statistical models for the analysis of the epidemiologic
data; the role of potentially confounding and modifying factors in the
concentration-response relationships; issues related to simulating how
PM2.5 air quality distributions will likely change in any
given area upon attaining a particular standard, since strategies to
reduce emissions are not yet defined; and whether there would be
differential reductions in the many components within PM2.5
and if so whether this would result in differential reductions in risk.
In the case of fine particles, the Administrator recognizes that such
uncertainties are likely to be unusually large due to the complexity in
the composition of the mix of fine particles generally present in the
ambient air. Further, in the Administrator's view, a risk assessment
based on studies that do not resolve the issue of a threshold is
inherently limited as a basis for standard setting, since it will
necessarily predict that ever lower standards result in ever lower
risks, which has the effect of masking the increasing uncertainty
inherent as lower levels are considered. As a result, while the
Administrator views the risk assessment as providing supporting
evidence for the conclusion that there is a need to revise the current
suite of PM2.5 standards, he judges that it does not provide
a reliable basis to determine what specific quantitative revisions are
appropriate.
1. 24-Hour PM2.5 Standard
    Based on the approach discussed above, the Administrator has relied
upon evidence from the short-term exposure PM2.5 studies as
the principal basis for selecting the level of the 24-hour standard. In
considering these studies as a basis for the level of a 24-hour
standard, and having selected a 98th percentile form for the standard,
the Administrator agrees with the focus in the Staff Paper of looking
at the 98th percentile values in these studies. In so doing, the
Administrator recognizes that these studies provide no evidence of
clear effect thresholds or lowest-observed-effects levels. Thus, in
focusing on 98th percentile values in these studies, the Administrator
is seeking to establish a standard level that will require improvements
in air quality generally in areas in which short-term exposure to
PM2.5 can reasonably be expected to be associated with
serious

health effects. While strategies that may be employed in the future to
bring about such improvements in air quality in any particular area are
not yet defined, most such strategies are likely to move the broad
distribution of PM2.5 air quality values in an area lower,
resulting in reductions in risk associated with exposures to
PM2.5 levels across a wide range of concentrations.
    Based on the information in the Staff Paper and a supporting staff
memo,\42\ the Administrator observes an overall pattern of
statistically significant associations reported in studies of short-
term exposure to PM2.5 across a wide range of 98th
percentile values. More specifically, there is a strong predominance of
studies with 98th percentile values down to about 39 [mu]g/m\3\ (in
Burnett and Goldberg, 2003) reporting statistically significant
associations with mortality, hospital admissions, and respiratory
symptoms. For example, within this range of air quality, statistically
significant associations were reported for mortality in the combined
Six City study (and three of the individual cities within that study)
(Klemm and Mason, 2003), the Canadian 8-City Study (Burnett and
Goldberg, 2003), and in studies in Santa Clara County, CA (Fairley,
2003) and Philadelphia (Lipfert, 2000); for hospital admissions and
emergency department visits in Seattle (Sheppard et al., 2003), Toronto
(Burnett et al., 1997; Thurston et al., 1994), Detroit (Ito, 2003, for
ischemic heart disease and pneumonia, but not for other causes), and
Montreal (Delfino et al., 1998, 1997, for some but not all age groups
and years); for respiratory symptoms in panel studies in a combined Six
City study (Schwartz et al., 1994) and in two Pennsylvania cities
(Uniontown in Neas et al., 1995; State College in Neas et al., 1996);
and for lung function in Philadelphia (Neas et al., 1999).\43\ Studies
in this air quality range that reported positive but not statistically
significant associations with mortality include studies in Detroit
(Ito, 2003), Pittsburgh (Chock et al., 2000), and Montreal (Goldberg
and Burnett, 2003).
---------------------------------------------------------------------------

    \42\ As discussed in the Staff Paper (EPA, 2005a, p. 5-30) and
supporting staff memo (Ross and Langstaff, 2005), staff focused on
U.S. and Canadian short-term exposure PM2.5 studies that
had been reanalyzed as appropriate to address statistical modeling
issues and considered the extent to which the reported associations
are robust to co-pollutant confounding and alternative modeling
approaches and the extent to which the studies used relatively
reliable air quality data.
    \43\ Of the studies within this group that evaluated
multipollutant associations, as discussed above in section II.A.3,
the results reported in Fairley (2003), Sheppard et al. (2003), and
Ito (2003) were generally robust to inclusion of gaseous co-
pollutants, whereas the effect estimate in Thurston et al. (1994)
was substantially reduced with the inclusion of O3.
---------------------------------------------------------------------------

    Within the range of 98th percentile PM2.5 concentrations
of about 35 to 30 [mu]g/m\3\, this strong predominance of statistically
significant results is no longer observed. Rather, within this range,
some studies report statistically significant results (Mar et al.,
2003; Ostro et al., 2003), other studies report mixed results in which
some associations reported in the study are statistically significant
and others are not (Delfino et al., 1997; Peters et al., 2000),\44\ and
another study reports associations in two of six cities that are not
statistically significant (Klemm and Mason, 2003). Further, the very
limited number of studies in which the 98th percentile values are below
this range do not provide a basis for reaching conclusions about
associations at such levels (Stieb et al., 2000; Peters et al., 2001).
Thus, in the Administrator's view, this body of evidence provides
confidence that statistically significant associations are occurring
down close to this range, and it provides a clear basis for concluding
that this range represents a range of reasonable values and thus for
selecting a 24-hour standard level from within this range. The
Administrator further notes that focusing on the range of 35 to 30
[mu]g/m\3\ is consistent with the interpretation of the evidence held
by most CASAC Panel members as reflected in their recommendation to
select a 24-hour PM2.5 standard level within this range
(Henderson, 2005a). The Administrator recognizes, however, the separate
point that most CASAC Panel members favored the range of 35 to 30
[mu]g/m\3\ for the 24-hour PM2.5 standard in concert with an
annual standard set in the range of 14 to 13 [mu]g/m3 (Henderson,
2005a), as discussed in section II.G.2 below.
---------------------------------------------------------------------------

    \44\ For example, Delfino et al. (1997) report statistically
significant associations between PM2.5 and respiratory
emergency department visits for elderly people (>64 years old), but
not children (< 2 years old) in one part of the study period (summer
1993) but not the other (summer 1992). Peters et al. (2000) report
new findings of associations between fine particles and cardiac
arrhythmia, but the Criteria Document observes that the strongest
associations were reported for a small subset of the study
population that had experienced 10 or more defibrillator discharges
(EPA, 2004, p. 8-164).
---------------------------------------------------------------------------

    In considering what 24-hour standard is requisite to protect public
health with an adequate margin of safety, the Administrator is mindful
that this choice requires judgment based on an interpretation of the
evidence that neither overstates nor understates the strength and
limitations of the evidence or the appropriate inferences to be drawn
from the evidence. In the absence of evidence of any clear effect
thresholds, the Administrator may select a specific standard level from
within a range of reasonable values. In making this judgment, the
Administrator notes that the general uncertainties related to the shape
of the concentration-response functions and the selection of
appropriate statistical models affect the likelihood that observed
associations are causal down to the lowest concentrations in the
studies. Further, and more specifically, the variation in results found
in the short-term exposure studies in which the 98th percentile values
were below 35 [mu]g/m\3\ indicates an increase in uncertainty as to
whether likely causal associations extend down below this level.
    In considering the extent to which the quantitative risk assessment
inform his selection of a 24-hour PM2.5 standard, the
Administrator recognizes that risk estimates based on simulating the
attainment of standards set at lower levels within this range will
inevitably suggest some additional reductions in risk at each lower
standard level considered. However, these quantitative risk estimates
largely depend upon assumptions made about the lowest level at which
reported associations will likely persist and remain causal in nature.
Thus, the Administrator is hesitant to use such risk estimates as a
basis for proposing a standard level below 35 [mu]g/m\3\, and instead
prefers to rely on inferences that are based directly on the evidence
in the studies themselves.
    Taking the above considerations into account, the Administrator
proposes to set the level of the primary 24-hour PM2.5
standard at 35 [mu]g/m3. In the Administrator's judgment,
based on the currently available evidence, a standard set at this level
would protect public health with an adequate margin of safety from
serious health effects including premature mortality and hospital
admissions for cardiorespiratory causes that are likely causally
associated with short-term exposure to PM2.5. This judgment
by the Administrator appropriately considers the requirement for a
standard that is neither more nor less stringent than necessary for
this purpose and recognizes that the CAA does not require that primary
standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect public health with an
adequate margin of safety. Being mindful that the available evidence
does not provide a basis for identifying a bright line within the range
of 35 to 30 [mu]g/m3 that clearly provides the appropriate
degree of public health protection, the Administrator also

solicits comment on selecting a lower level within this range.
    Having reached this decision to propose a level of 35 [mu]g/
m3 for the 24-hour PM2.5 standard based on the
approach to interpreting the available evidence described above, the
Administrator recognizes that other approaches to selecting a standard
level have been presented to the Agency. These other approaches reflect
alternative views, principally expressed in public comments to date, as
to the appropriate interpretation of the scientific evidence and the
appropriate policy response in light of that interpretation. One such
view focuses very strongly on the uncertainties inherent in the
epidemiologic and toxicologic studies and the quantitative risk
assessment as the basis for concluding that no change to the current
24-hour PM2.5 standard of 65 [mu]g/m3 is
warranted. Such commenters prefer greater weight, for example, on
issues related to the sensitivity in the magnitude and statistical
significance of relative risks reported in studies using different
statistical models, noting that further research is needed to inform
modeling strategies that will appropriately adjust for temporal trends
and weather variables in time-series studies. Additional uncertainties
arise from the potential confounding by co-pollutants, and the
potential differential toxicity of components within the mix of fine
particles. These commenters suggest that the magnitude of risks
associated with fine particle exposures have decreased since the last
review. Some such commenters also focus on considerations such as the
absence of clear evidence from toxicologic studies and from studies
focused on elucidating specific physiologic mechanisms by which
PM2.5 may be causing the observed effects. Such commenters
recognize a need for a 24-hour PM2.5 standard, but consider
the evidence to be too uncertain overall to warrant any tightening of
the standard and instead believe the appropriate policy response in
light of this uncertainty is to retain the current level of the 24-hour
standard.
    Other commenters who also focus strongly on the uncertainties
inherent in the epidemiologic and toxicologic studies and the
quantitative risk assessment reach a somewhat different conclusion as
to the appropriate policy response in light of these uncertainties.
This group of commenters sees a basis for lowering the level of the 24-
hour PM2.5 standard, but does not believe that a level as
low as 35 [mu]g/m3 is warranted. Such commenters note that
while many of the studies within the range of air quality from
approximately 39 [mu]g/m3 up to the level of the current
standard of 65 [mu]g/m3 report statistically significant
results, only a few such studies independently evaluated confounding by
co-pollutants. This lack of a broader assessment of co-pollutants,
together with other types of uncertainties as noted above, leads such
commenters to conclude that a standard level selected from below this
range is not warranted, and that the appropriate policy response is to
select a standard level from within the range of about 40 to 65 [mu]g/
m3.
    In sharp contrast, others view the epidemiologic evidence and other
health studies as strong and robust, and generally place much weight on
the results of the quantitative risk assessment as a basis for
concluding that a much stronger policy response is warranted, generally
consistent with a standard level at or below 25 [mu]g/m3.
While recognizing that important uncertainties are inherently present
in both the evidence and estimated risks, these commenters generally
support a view that such uncertainties warrant a highly precautionary
policy response, particularly in view of the serious nature of the
health effects at issue, and should be addressed by selecting a
standard level that incorporates a large margin of safety.
    The Administrator recognizes that these sharply divergent views on
the appropriate level of the standard are based on very different
interpretations of the science itself including its relative strengths
and limitations and on very different judgments as to how such
scientific evidence should be used in making policy decisions on
proposed standards. Consistent with the goal of soliciting comments on
a wide array of views, the Administrator also solicits broad public
comment on these and other alternative approaches and on the related
standard levels, such as levels from 35 [mu]g/m3 up to 65
[mu]g/m3 or from 30 [mu]g/m3 down to 25 [mu]g/
m3, that commenters may believe are appropriate, along with
the rationale supporting such approaches and levels. In addition, the
Administrator solicits comments on issues related to the interpretation
of relevant epidemiologic and toxicologic studies, including approaches
to addressing uncertainties related to the sensitivity of results to
alternative statistical modeling approaches, co-pollutant confounding,
and the lack of a discernable threshold of effects, as well as
approaches to more fully characterize uncertainties in quantitative
risk assessments based on epidemiologic studies.
2. Annual PM2.5 Standard
    Based on the approach discussed at the beginning of this section,
the Administrator has relied upon evidence from the long-term exposure
PM2.5 studies as the principal basis for selecting the level
of the annual standard. In considering these studies as a basis for the
level of an annual standard, the Administrator agrees with the focus in
the Staff Paper of looking at the long-term mean PM2.5
concentrations across the cities included in such studies. In so doing,
the Administrator recognizes that these studies, like the short-term
exposure studies, provide no evidence of clear effect thresholds or
lowest-observed-effects levels. Thus, in focusing on the cross-city
long-term mean concentrations in these studies, the Administrator is
seeking to establish a standard level that will require improvements in
air quality in areas in which long-term exposure to PM2.5
can reasonably be expected to be associated with serious health
effects.
    Based on the characterization and assessment of the long-term
exposure PM2.5 studies presented in the Criteria Document
and Staff Paper, the Administrator recognizes the importance of the
validation efforts and reanalysis that have been done since the last
review of the original Six Cities and ACS mortality studies. These new
assessments provide evidence of generally robust associations and
provide a basis for greater confidence in the reported associations
than in the last review, for example, in the extent to which they have
made progress in understanding the importance of issues related to co-
pollutant confounding and the specification of statistical models.
Consistent with the information available in the last review, these two
key long-term exposure mortality studies reported long-term mean
PM2.5 concentrations across all the cities included in the
studies of 18 and 21 [mu]g/m3, respectively. The
Administrator also particularly recognizes the importance of the
extended ACS mortality study, published since the last review, which
provides new evidence of mortality related to lung cancer and further
substantiates the statistically significant associations with
cardiorespiratory-related mortality observed in the original studies.
The Administrator notes that the statistically significant associations
reported in the extended ACS study, in a large number of cities across
the U.S., provide evidence of effects at a lower long-term mean
PM2.5 concentration (17.7 [mu]g/m3) than had been
observed in the original study,

although the relative risk estimates are somewhat smaller in magnitude
than those reported in the original study. The assessment in the
Criteria Document of these mortality studies, taking into account study
design, the strength of the study (in terms of statistical significance
and precision of result), and the consistency and robustness of
results, concludes that it would be appropriate to give the greatest
weight to the reanalyses of the Six Cities and ACS studies, and in
particular to the results of the extended ACS study (EPA, 2004, p. 9-
33) in weighing the evidence of mortality effects associated with long-
term exposure to PM2.5. Consistent with that assessment, the
Administrator places greatest weight on these studies as a basis for
selecting the level of the annual PM2.5 standard.
    In addition to these mortality studies, the Administrator also
recognizes the availability of relevant morbidity studies providing
evidence of respiratory morbidity, including decreased lung function
growth, in children with long-term exposure to PM2.5.
Studies conducted in the U.S. and Canada include the 24-city study
considered in the last review and new studies of cohorts of children in
southern California, in which the long-term mean PM2.5
concentrations in all the cities included in the studies are
approximately 14.5 and 15 [mu]g/m3, respectively. As
discussed in section II.A. above, in the 24-city study, statistically
significant associations were reported between long-term fine particle
exposures and lung function measures at a single point in time, whereas
positive but not statistically significant associations were reported
with prevalence of several respiratory conditions. As interpreted in
the last review, the results from the 24-city study are uncertain as to
the extent to which the association extends below a long-term mean
PM2.5 concentration of approximately 15 [mu]g/m3.
The new southern California children's cohort study provides evidence
of important respiratory morbidity effects in children, including
evidence for a new measure of morbidity, decreased growth in lung
function. Reports from this study suggest that long-term
PM2.5 exposure is associated with decreases in lung function
growth, as measured over a four-year follow-up period, although
statistically significant associations are not consistently reported.
The Administrator recognizes that these are important new findings,
indicating that long-term PM2.5 exposure may be associated
with respiratory morbidity in children. However, the Administrator also
observes this is the only study reporting decreased lung function
growth, conducted in just one area of the country, such that further
study of this health endpoint in other areas of the country would be
needed to increase confidence in the reported associations. Thus, at
this time, the Administrator provisionally concludes that this study
provides an uncertain basis for establishing the level of a national
standard.
    As discussed in the Staff Paper (EPA, 2005a, p. 5-22), the
Administrator generally agrees that it is appropriate to consider a
level for an annual PM2.5 standard that is below the
averages of the long-term PM2.5 concentrations across the
cities in the key long-term exposure mortality studies, 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. The Administrator is mindful that considering what
standard is requisite to protect public health with an adequate margin
of safety requires policy judgments that neither overstate nor
understate the strength and limitations of the evidence or the
appropriate inferences to be drawn from the evidence. The Administrator
provisionally concludes that these key mortality studies, together with
the morbidity studies, provide a basis for considering a standard level
no higher than 15 [mu]g/m3. This level is somewhat below the
long-term mean concentrations in the key mortality studies and
consistent with the interpretation of the evidence from the morbidity
studies discussed above. Further, in the Administrator's view, these
studies do not provide a clear basis for selecting a level lower than
the current standard of 15 [mu]g/m3.
    In considering the extent to which the quantitative risk assessment
can help to inform these judgments with regard to the annual
PM2.5 standard, the Administrator again recognizes that risk
estimates based on simulating the attainment of standards set at lower
levels, as expected, continue to suggest some additional reductions in
risk at the lower standard level considered in the assessment, and that
these estimates largely depend upon assumptions made about the lowest
level at which reported associations will likely persist and remain
causal in nature. Thus, the Administrator is again hesitant to use such
risk estimates as a basis for proposing a lower annual standard level
than 15 [mu]g/m3, the level that is based directly on the
evidence in the studies themselves, as discussed above.
    Taking the above considerations into account, the Administrator
proposes to retain the level of the primary annual PM2.5
standard at 15 [mu]g/m3. In the Administrator's judgment,
based on the currently available evidence, a standard set at this level
would be requisite to protect public health with an adequate margin of
safety from serious health effects including premature mortality and
respiratory morbidity that are likely causally associated with long-
term exposure to PM2.5. This judgment by the Administrator
appropriately considers the requirement for a standard that is neither
more nor less stringent than necessary for this purpose and recognizes
that the CAA does not require that primary standards be set at a zero-
risk level, but rather at a level that reduces risk sufficiently so as
to protect public health with an adequate margin of safety.
    In so doing, the Administrator recognizes that the CASAC Panel did
not endorse retaining the annual standard at the current level of 15
[mu]g/m3 (Henderson, 2005a, p. 7). In weighing the
recommendation of the CASAC Panel, the Administrator has carefully
considered the stated reasons for it. In discussing its recommendation
(Henderson, 2005a), the CASAC Panel first noted that changes to either
the annual or 24-hour PM2.5 standard, or both, could be
recommended. Three reasons were then given for placing more emphasis on
lowering the 24-hour standard than the annual standard: (1) The vast
majority of studies indicating effects of short-term PM2.5
exposure were carried out in settings in which PM2.5
concentrations were largely below the current 24-hour standard level of
65 [mu]g/m3; (2) the amount of evidence on short-term
exposure effects, at least as reflected by the number of reported
studies, is greater than for long-term exposure effects; and (3)
toxicologic findings are largely related to the effects of short-term,
rather than long-term, exposures. In not endorsing the option of
retaining the level of the current annual standard in conjunction with
lowering the 24-hour standard, the CASAC Panel observed that some
cities have relatively high annual PM2.5 concentrations
without much day-to-day variation and that such cities would only
rarely exceed a 24-hour standard, even if it were set at a level below
the current standard. In such a city, attaining a 24-hour standard
would likely have minimal if any effect on the long-term mean
PM2.5 concentration and consequently would be less likely to
reduce health effects associated with long-term exposures. These
observations

were taken as an indication of the desirability of lowering the level
of the annual PM2.5 standard as well as that of the 24-hour
standard. Based on these considerations and taking into account the
results of the risk assessment, most CASAC Panel members favored
setting an annual standard in the range of 14 to 13 [mu]g/
m3, along with lowering the 24-hour standard (Henderson,
2005a).
    In considering these views, the Administrator notes that the
appropriateness of setting an annual standard that would lower annual
PM2.5 concentrations in cities across the country depends
upon a policy judgment as to what annual level is required to protect
public health with an adequate margin of safety from long-term
exposures to PM2.5 in light of the available evidence. In
considering the evidence of effects associated with long-term
PM2.5 exposure as a basis for selecting an adequately health
protective annual standard, as discussed above, the Administrator
provisionally concludes that the evidence does not provide a basis for
requiring annual levels below 15 [mu]g/m3. Thus, the
Administrator agrees conceptually with the CASAC Panel that any
particular 24-hour standard may not result in reductions in the level
of long-term exposures to PM2.5 in all areas with relatively
higher than typical annual PM2.5 concentrations and lower
than typical ratios of peak-to-mean values. Further, the Administrator
agrees that this general advice supports relying on the annual
standard, and not the 24-hour standard, to achieve the appropriate
level of protection from long-term exposures to PM2.5.
However, the Administrator does not believe that this advice
necessarily translates into a reason for setting the annual
PM2.5 standard at a level below the current level of 15
[mu]g/m3. As discussed above, the Administrator believes the
principal basis for selecting the appropriate level of an annual
standard should be the evidence provided by the long-term studies, in
conjunction with judgments concerning whether and over what range of
concentrations reported associations are likely causal, and this
evidence reasonably supports retaining the current level of the annual
standard.
    The Administrator places great importance on the advice of CASAC,
and therefore solicits broad public comment on the range of 15 down to
13 [mu]g/m3, the low end of the range recommended by CASAC,
for the level of the annual PM2.5 standard as well as on the
reasoning that formed the basis for that recommendation. A decision to
select a standard from within this range would place greater weight on
the strength of the associations reported in the key epidemiologic
mortality and morbidity long-term exposure studies down to the lower
part of the range of PM2.5 concentrations observed across
all the cities included in these studies. Such a standard could also
reflect greater reliance on the results of the quantitative risk
assessment that suggested increased reductions in risk associated with
meeting an annual standard at such lower levels.
    The Administrator recognizes that an even stronger view of the
appropriate policy response to the currently available evidence has
been expressed by some public commenters. These commenters have focused
principally on the strength of the long-term exposure studies,
including the new children's cohort study conducted in southern
California, as well as on those results from the quantitative risk
assessment that are based on the assumption that there is no threshold
of effects down to the lowest levels observed in those studies. Such
considerations generally have led these commenters to express views
that support a highly precautionary policy response and the selection
of a standard level that incorporates a large margin of safety,
consistent with an annual PM2.5 standard level of 12 [mu]g/
m3. The Administrator recognizes that this view is based on
a different interpretation of the science itself including its relative
strengths and limitations and on different judgments as to how such
scientific evidence should be used in making policy decisions on
proposed standards. Consistent with the goal of soliciting comments on
a wide array of views, the Administrator also solicits broad public
comment on this alternative approach and on the related standard level
of 12 [mu]g/m3.
    The Administrator also recognizes a contrasting view as to the
interpretation of and weight to be accorded to the results from the
ACS-based studies (Pope et al., 1995; Krewski et al., 2000; Pope et
al., 2002). In this view, the ACS-based studies are not sufficiently
robust to support a policy response that would tighten the annual
PM2.5 standard based on the evidence. This view emphasizes
the sensitivity of the results of these studies to plausible changes in
model specification with regard to accounting for the geographical
proximity of cities and the correlation of air pollutant concentrations
within a region, effect modification by education level, and inclusion
of SO2 in the model. In this view, these sensitivities
suggest potential confounding or effect modification that has not been
taken into account. For example, concern has been raised about the
sensitivity of results in the reanalysis of data from the ACS cohort
study (Krewski et al., 2000) to inclusion of SO2 in the
models. As discussed in section II.A.2.b above, the reanalysis found
that PM2.5, sulfates, and SO2 were each
associated with mortality in single-pollutant models. However, in two-
pollutant models with SO2 and PM2.5, the relative
risk for PM2.5 was substantially smaller and no longer
statistically significant, whereas the effect estimates for
SO2 were not sensitive to inclusion of PM2.5 or
sulfates in two-pollutant models. In this view, the ACS-based risk
estimates are more robust for SO2 than for PM2.5
or sulfates. In further extended analyses, Pope et al. (2002) reported
that effect estimates were not highly sensitive to spatial smoothing
approaches intended to address spatial autocorrelation, while findings
of effect modification by education level were reaffirmed. Results of
multi-pollutant models were not reported by Pope et al. (2002). Because
the correlation coefficient between PM2.5 and SO2
was 0.50 in the ACS data, in this view it is plausible to believe that
the independent effects of the two pollutants could be disentangled
with additional study.
    In this view, there is a separate but related concern that
tightening the annual standard now, without a clear understanding of
which specific PM-related pollutants are most toxic, will have very
uncertain public health payoffs. In response to the advice of the
National Research Council (NRC) and other scientists, the Agency is
undertaking, as one of its higher priorities, a substantial research
program to clarify which aspects of PM-related pollution are
responsible for elevated risks of mortality and morbidity. For example,
the Health Effects Institute has issued a request for applications to
analyze the largest database on specific components of PM that has ever
been assembled for public health and medical researchers. The time line
for this multi-million dollar research program is well designed to
inform the Agency's next periodic reevaluation of the primary ambient