below 1 [mu]g/dL. In estimating IQ loss with this function in the risk assessment (section II.A.3) the nonlinear form of the model with varying slope
  was used for all BLL above 1 [mu]g/dL. The slopes shown are the average slopes (IQ points per [mu]g/dL blood Pb) associated with application of the
  LLL functions from zero to the blood Pb levels identified (2 and 3 [mu]g/dL).
\E\ These studies and quantitative relationships are discussed in the Criteria Document (CD, sections 6.2, <IP_ADDRESS> and 8.6.2).
\F\ The BLL for Bellinger and Needleman (2003) are for age 24 months.
\G\ For nonlinear models, this is the estimated average slope for change in IQ with change in blood Pb over the range from the 10th percentile blood Pb
  value in study to 10 [mu]g/dL (CD, p. 6-65). The shape of these models is such that the average slopes from the 10th percentiles to a value lower than
  10 [mu]g/dL are larger negative values than those shown here (e.g., the slopes to 5 [mu]g/dL are 50% larger negative values).
\H\ 69% of children in Al-Saleh et al. (2001) study had BLL<10 [mu]g/dL.

3. Overview of Human Exposure and Health Risk Assessments
    To put judgments about risk associated with exposure to air-related
Pb in a broader public health context, EPA developed and applied models
to estimate human exposures to air-related Pb and associated health
risk for various air quality scenarios and alternative standards. The
design and implementation of the risk assessment needed to address
significant limitations and complexity that go far beyond the situation
for similar assessments typically performed for other criteria
pollutants. The multimedia and persistent nature of Pb and the role of
multiple exposure pathways add significant complexity as compared with
other criteria pollutants that focus only on the inhalation exposure.
Not only was the risk assessment constrained by the timeframe allowed
for this review in the context of the breadth of information to
address, it was also constrained by significant limitations in data and
modeling tools for the assessment, as described in section II.C.2.h of
the proposal.
    The scope and methodology for this assessment were developed over
the last few years with considerable input from the CASAC Pb Panel and
the public, as described in the proposal (section II.C.2.a).\42\ The
following sections provide a brief summary of the quantitative exposure
and risk assessment and key findings. The complete full-scale
assessment, including the associated uncertainties, is more fully
summarized in section II.C of the proposal and described in detail in
the Risk Assessment Report (USEPA, 2007b).
---------------------------------------------------------------------------

    \42\ In their review of the final risk assessment, CASAC
expressed strong support, stating that ``[t]he Final Risk Assessment
report captures the breadth of issues related to assessing the
potential public health risk associated with lead exposures; it
competently documents the universe of knowledge and interpretations
of the literature on lead toxicity, exposures, blood lead modeling
and approaches for conducting risk assessments for lead''
(Henderson, 2008a, p. 4).
---------------------------------------------------------------------------

a. Design Aspects and Associated Uncertainties
    As discussed in section II.C.2 of the proposal, EPA conducted
exposure and risk analyses to estimate blood Pb and associated IQ loss
in children exposed to air-related Pb. As recognized in section II.A.2
above and discussed in the proposal notice and Criteria Document, among
the wide variety of health endpoints associated with Pb exposures,
there is general consensus

that the developing nervous system in children is among, if not, the
most sensitive, and that neurobehavioral effects (specifically
neurocognitive deficits), including IQ decrements, appear to occur at
lower blood Pb levels than previously believed. The selection of
children's IQ for the quantitative risk assessment reflects
consideration of the evidence presented in the Criteria Document as
well as advice received from CASAC (Henderson, 2006, 2007a).\43\
---------------------------------------------------------------------------

    \43\ CASAC advice on the design of the risk assessment is
summarized in section II.C.2.a of the proposal.
---------------------------------------------------------------------------

    The brief summary provided here focuses on blood Pb and risk
estimates for five case studies \44\ that generally represent two types
of population exposures: (1) More highly air-pathway exposed children
(as described below) residing in small neighborhoods or localized
residential areas with air concentrations somewhat near the standard
being evaluated, and (2) location-specific urban populations with a
broader range of air-related exposures.
---------------------------------------------------------------------------

    \44\ A sixth case study (the secondary Pb smelter case study) is
also described in the Risk Assessment Report. However, as discussed
in Section 4.3.1 of that document (USEPA, 2007a), significant
limitations in the approaches have contributed to large
uncertainties in the corresponding estimates.
---------------------------------------------------------------------------

    The case studies representing the more highly air-pathway exposed
children are the general urban case study and the primary Pb smelter
case study. The general urban case study case study is not based on a
specific geographic location and reflects several simplifications in
representing exposure including uniform ambient air Pb levels
associated with the standard of interest across the hypothetical study
area and a uniform study population. Additionally, the method for
simulating temporal variability in air Pb concentrations in this case
study relied on national average estimates of the relationships between
air concentrations in terms of the statistics considered for different
forms of the standard being assessed and the annual ambient air
concentrations required for input to the blood Pb model.\45\ Thus,
while this case study provides characterization of risk to children
that are relatively more highly air pathway exposed (as compared to the
location-specific case studies), this case study is not considered to
represent a high-end scenario with regard to the characterization of
ambient air Pb levels and associated risk. The primary Pb smelter case
study provides risk estimates for children living in a specific area
that is currently not in attainment with the current NAAQS. We have
focused on a subarea within 1.5 km of the facility where airborne Pb
concentrations are closest to the current standard and where children's
air-related exposures are most impacted by emissions associated with
the Pb smelter from which air Pb concentrations were estimated.
---------------------------------------------------------------------------

    \45\ As the blood Pb model used in the risk assessment was
limited in that it did not accept inputs of a temporal time step
shorter than annual average, ratios of relationships in the
available air monitoring data between different statistical forms
being considered for the standard and an annual average were
employed for the urban case studies (that did not rely on dispersion
modeling) as a method of simulating the temporal variability in air
Pb concentrations that occurs as a result of meteorology, source and
emissions characteristics.
---------------------------------------------------------------------------

    The three location-specific urban case studies focus on specific
residential areas within Cleveland, Chicago, and Los Angeles to provide
representations of urban populations with a broader range of air-
related exposures due to spatial gradients in both ambient air Pb
levels and population density. For example, the highest air
concentrations in these case studies (i.e., those closest to the
standard being assessed) are found in very small parts of the study
areas, while a large majority of the case study populations reside in
areas with much lower air concentrations.
    Based on the nature of the population exposures represented by the
two categories of case study, the first category (the general urban and
primary Pb smelter case studies) relates more closely to the air-
related IQ loss evidence-based framework described in the proposal
(sections II.D.2.a.ii and II.E.3.a) with regard to estimates of air-
related IQ loss. As mentioned above, these case studies, as compared to
the other category of case studies, include populations that are
relatively more highly exposed by way of air pathways to air Pb
concentrations somewhat near the standard level evaluated.
    The air quality scenarios assessed include (a) the current NAAQS
(for all five case studies); \46\ (b) current conditions for the
location-specific \47\ and general urban case studies (which are below
the current NAAQS); and (c) a range of alternate standard levels (for
all case studies). The alternative NAAQS scenarios included levels of
0.50, 0.20, 0.05 and 0.02 [mu]g/m\3\, with a form of maximum monthly
average, as well as a level of 0.20 [mu]g/m\3\, with a form of maximum
quarterly average. Details of the assessment scenarios, including the
Pb concentrations for other media are presented in Sections 2.3 and
5.1.1 of the Risk Assessment Report (USEPA, 2007b).
---------------------------------------------------------------------------

    \46\ The current NAAQS scenario for the urban case studies
assumes ambient air Pb concentrations higher than those currently
occurring in nearly all urban areas nationally. While it is
extremely unlikely that Pb concentrations in urban areas would rise
to meet the current NAAQS and there are limitations and
uncertainties associated with the roll-up procedure used for the
location-specific urban case studies (as described in Section
II.C.2.h of the proposal), this scenario was included for those case
studies to provide perspective on potential risks associated with
raising levels to the point that the highest level across the study
area just meets the current NAAQS. This scenario was simulated for
the location-specific urban case studies using a proportional roll-
up procedure. For the general urban case study, the maximum
quarterly average ambient air concentration was set equal to the
current NAAQS.
    \47\ Current conditions for the three location-specific urban
case studies in terms of maximum quarterly average air Pb
concentrations were 0.09, 0.14 and 0.36 [mu]g/m\3\ for the study
areas in Los Angeles, Chicago and Cleveland, respectively.
---------------------------------------------------------------------------

    Exposure and associated blood Pb levels were simulated using the
IEUBK model, as more fully described and presented in the Risk
Assessment Report (USEPA, 2007b). Because of the nonlinear response of
blood Pb to exposure and also the nonlinearity reflected in the C-R
functions for estimation of IQ loss, this assessment first estimated
total blood Pb and risk (air- and nonair-related), and then separated
out those estimates of blood Pb and associated risk associated with the
pathways of interest in this review. We separated out the estimates of
total (all-pathway) blood Pb and IQ loss into a background category and
two air-related categories (referred to as ``recent air'' and ``past
air''). However, significant limitations in our modeling tools and data
resulted in an inability to parse specific risk estimates into specific
pathways, such that we have approximated estimates for the air-related
and background categories.
    Those Pb exposure pathways tied most directly to ambient air, which
consequently have the potential to respond relatively more quickly to
changes in air Pb (i.e., inhalation and ingestion of indoor dust Pb
derived from the infiltration of ambient air Pb indoors), were placed
into the ``recent air'' category. The other air-related Pb exposure
pathways, all of which are associated with atmospheric deposition, were
placed into the ``past air'' category. These include ingestion of Pb in
outdoor dust/soil and ingestion of the portion of Pb in indoor dust
that after deposition from ambient air outdoors is carried indoors with
humans (as noted in section II.A.1 above).
    Among the limitations affecting our estimates for the air-related
and background categories is the apportionment of background (nonair)
pathways. For example, while conceptually indoor Pb paint

contributions to indoor dust Pb would be considered background and
included in the ``background'' category for this assessment, due to
technical limitations related to indoor dust Pb modeling, dust from Pb
paint was included as part of ``other'' indoor dust Pb (i.e., as part
of past air exposure). The inclusion of indoor paint Pb as a component
of ``other'' indoor dust Pb (and consequently as a component of the
``past air'' category) represents a source of potential high bias in
our prediction of exposure and risk associated with the ``past air''
category because conceptually, exposure to indoor paint Pb is
considered part of background exposure. At the same time, Pb in ambient
air does contribute to the exposure pathways included in the
``background'' category (drinking water and diet), and is likely a
substantial contribution to diet (CD, p. 3-48). We could not separate
the air contribution from the nonair contributions, and the total
contribution from both the drinking water and diet pathways are
categorized as ``background'' in this assessment. As a result, our
``background'' risk estimate includes some air-related risk
representing a source of potential low bias in our predictions of air-
related risk.
    Further, we note that in simulating reductions in exposure
associated with reducing ambient air Pb levels through alternative
NAAQS (and increases in exposure if the current NAAQS was reached in
certain case studies) only the exposure pathways categorized as
``recent air'' (inhalation and ingestion of that portion of indoor dust
associated with outdoor ambient air) were varied with changes in air
concentration. The assessment did not simulate decreases in ``past
air'' exposure pathways (e.g., reductions in outdoor soil Pb levels
following reduction in ambient air Pb levels and a subsequent decrease
in exposure through incidental soil ingestion and the contribution of
outdoor soil to indoor dust).\48\ These exposures were held constant
across all air quality scenarios.\49\
---------------------------------------------------------------------------

    \48\ Similarly, since dietary Pb was included within
``background'', reductions in dietary Pb, e.g., as a result of
reduced deposition to crops, were also not simulated.
    \49\ In comparing total risk estimates between alternate NAAQS
scenarios, this aspect of the analysis will tend to underestimate
the reductions in risk associated with alternative NAAQS. However,
this does not mean that overall risk has been underestimated. The
net effect of all sources of uncertainty or bias in the analysis,
which may also tend to under-or overestimate risk, could not be
quantified.
---------------------------------------------------------------------------

    In summary, because of limitations in the assessment design, data
and modeling tools, our risk estimates for the ``past air'' category
include both risks that are truly air-related and potentially, some
background risk. Because we could not sharply separate Pb linked to
ambient air from Pb that is background, some of the three categories of
risk are underestimated and others overestimated. On balance, we
believe this limitation leads to a slight overestimate of the risks in
the ``past air'' category. At the same time, as discussed above, the
``recent air'' category does not fully represent the risk associated
with all air-related pathways. Thus, we consider the risk attributable
to air-related exposure pathways to be bounded on the low end by the
risk estimated for the ``recent air'' category and on the upper end by
the risk estimated for the ``recent air'' plus ``past air'' categories.
    As discussed in the proposal notice and in greater detail in the
Staff Paper and Risk Assessment Report, exposure and risk modeling
conducted for this analysis was complex and subject to significant
uncertainties due to limitations, data, models and time available. Key
assumptions, limitations and uncertainties, which were recognized in
various ways in the assessment and presentation of results, are listed
here, beginning with those related to design of the assessment or case
studies, followed by those related to estimation of Pb concentrations
in ambient air, indoor dust, outdoor soil/dust, and blood, and
estimation of Pb-related IQ loss.
     Temporal Aspects: During the 7-year exposure period, media
concentrations remain fixed and the simulated child remains at the same
residence (while exposure factors and physiological parameters are
adjusted to match the age of the child).
     General Urban Case Study: The design for this case study
employs assumptions regarding uniformity that are reasonable in the
context of a small neighborhood population, but would contribute
significant uncertainty to extrapolation of these estimates to a
specific urban location, particularly a relatively large one. Thus, the
risk estimates for this general urban case study, while generally
representative of an urban residential population exposed to the
specified ambient air Pb levels, cannot be readily related to a
specific large urban population.
     Location-Specific Urban Case Studies: Limitations in the
ambient air monitoring network limit our characterization of spatial
gradients of ambient air Pb levels in these case studies.
     Air Quality Simulation: The proportional roll-up and roll-
down procedures used in some case studies to simulate current NAAQS and
alternate NAAQS levels, respectively, assume proportional changes in
air concentrations across the study area in those scenarios for those
case studies. EPA recognizes that it is extremely unlikely that Pb
concentrations would rise to just meet the current NAAQS in urban areas
nationwide and that there is substantial uncertainty with our
simulation of such conditions in the urban location-specific case
studies. There is also significant uncertainty in simulation conditions
associated with the implementation of emissions reduction actions to
meet a lower standard.
     Outdoor Soil/Dust Pb Concentrations: Uncertainty
regarding soil/dust Pb levels and the inability to simulate the
influence of changing air Pb levels related to lowering the NAAQS
contributes uncertainty to air-related risk estimates.
     Indoor Dust Pb Concentrations: Limitations and
uncertainty in modeling of indoor dust Pb levels, including the impact
of reductions in ambient air Pb levels, contributes uncertainty to air-
related risk estimates.
     Interindividual Variability in Blood Pb Levels:
Uncertainty related to population variability in blood Pb levels and
limitations in modeling of this introduces significant uncertainty into
blood Pb and IQ loss estimates for the 95th percentile of the
population.
     Pathway Apportionment for Higher Percentile Blood Pb and
IQ Loss: Limitations in data, modeling tools and assessment design
introduce uncertainty into estimates of air-related blood Pb and IQ
loss for the upper ends of population distribution.
     IQ Loss Concentration-Response Functions: Specification of
the quantitative relationship between blood Pb level and IQ loss is
subject to significant uncertainty at lower blood Pb levels (e.g.,
below 5 [mu]g/dL concurrent blood Pb).
b. Summary of Blood Pb Estimates
    Key observations regarding the blood Pb estimates from this
analysis are noted here:
     As shown in Table 2 of the proposal (73 FR 29215), median
blood Pb levels for the current conditions air quality scenario in the
urban case studies ranged from 1.7-1.8 [mu]g/dL for the location-
specific case studies up to 1.9 [mu]g/dL for the general urban case
study. These values are slightly larger than the median value from
NHANES for children aged 1-5 years old in 2003-2004 of 1.6 [mu]g/dL
(http://www.epa.gov/envirohealth/children/body_burdens/

b1-table.htm). Blood Pb level estimates for the 90th percentile in the
urban case studies are also higher than the NHANES 90th percentile
blood Pb levels. We note, however, that ambient air Pb levels in the
urban case studies are higher than those at most monitoring sites in
the U.S., as described in section II.C.3.a of the proposal.
     With regard to air-to-blood ratios, estimates for the
general urban case study ranged from 1:2 to 1:9 with the majority of
the estimates ranging from 1:4 to 1:6.\50\ Because the risk assessment
only reflects the impact of reductions on recent air-related pathways
in predicting changes in indoor dust Pb for the general urban case
study (as noted in section II.C.3.a of the proposal), however, the
ratios generated are lower than they would be if they had also
reflected other air-related pathways (e.g., changes in outdoor surface
soil/dust and dietary Pb with changes in ambient air Pb).
---------------------------------------------------------------------------

    \50\ The ratios increase as the level of the alternate standard
decreases. This reflects the nonlinearity in the Pb response, which
is greater on a per-unit basis for lower ambient air Pb levels.
---------------------------------------------------------------------------

     Air-to-blood ratios estimated for the primary Pb smelter
subarea ranged from 1:10 and higher.\51\ One reason for these estimates
being higher than those for the urban case study may be that the dust
Pb model used may somewhat reflect ambient air-related pathways other
than that of ambient air infiltrating a home.
---------------------------------------------------------------------------

    \51\ For the primary Pb smelter (full study area), for which
limitations are noted in section II.C.2.c of the proposal, the air-
to-blood ratio estimates, presented in section <IP_ADDRESS> of the Risk
Assessment Report (USEPA, 2007b), ranged from 1:3 to 1:7. As in the
other case studies, ratios are higher at lower ambient air Pb
levels. It is noted that the underlying changes in both ambient air
Pb and blood Pb across standard levels are extremely small,
introducing uncertainty into ratios derived using these data.
---------------------------------------------------------------------------

c. Summary of IQ Loss Estimates
    As described more fully in the proposal notice and in the Risk
Assessment Report (USEPA, 2007b, section 5.3.1), four sets of IQ loss
estimates were derived from the blood Pb estimates, one for each of
four concentration-response functions derived from the international
pooled analysis by Lanphear and others (2005). Each of these four
functions utilizes a different approach for characterizing low-exposure
IQ loss, thereby providing a range of estimates intended to reflect the
uncertainty in this key aspect of the risk assessment. As described in
section II.C.2.b of the proposal (and in more detail in section 2.1.5
of the Risk Assessment Report), we have placed greater confidence in
the log-linear function with low-exposure linearization (LLL) and
present risk estimates based on that function here.\52\
---------------------------------------------------------------------------

    \52\ As shown in the presentation in the Staff Paper (section
4.4), risk estimates for the LLL function are generally bounded by
estimates based on the other three C-R functions included in the
assessment.
---------------------------------------------------------------------------

    The risk estimates summarized here are those considered most
relevant to the review in considering whether the current NAAQS and
potential alternative NAAQS provide protection of public health with an
adequate margin of safety (i.e., estimates of IQ loss associated with
air-related Pb exposure). In considering these estimates, we note that
IQ loss associated with air-related Pb is bounded on the low end by
risk associated with the recent air category of exposure pathways and
on the upper end by the recent plus past air categories of pathways (as
described above in section II.A.3.a). Key observations regarding the
median estimates \53\ of air-related risk for the current NAAQS and
alternative standards include:
---------------------------------------------------------------------------

    \53\ Because of greater uncertainty in characterizing high-end
population risk, and specifically related to pathway apportionment
of IQ loss estimates for high-end percentiles, results discussed
here focus on those for the population median.
---------------------------------------------------------------------------

     As shown in Table 2 below (Table 3 in the proposal), in
all five case studies, the lower bound of population median air-related
risk associated with the current NAAQS exceeds 2 points IQ loss, and
the upper bound is near or above 4 points.\54\
---------------------------------------------------------------------------

    \54\ As noted in Table 2 below and sections II.C.2.d and
II.C.2.h of the proposal, with regard to associated limitations and
uncertainties, a proportional roll-up procedure was used to estimate
air Pb concentrations in this scenario for the location-specific
case studies.
---------------------------------------------------------------------------

     Alternate standards provide substantial reduction in
estimates of air-related risk across the full set of alternative NAAQS
considered, particularly for the lower bound of air-related risk which
includes only the pathways that were varied with changes in air
concentrations (as shown in Table 2).
     In the general urban case study, the estimated population
median air-related risk falls between 1.9 and 3.6 points IQ loss for an
alternative NAAQS of 0.50 [mu]g/m\3\, maximum monthly average, between
1.2 and 3.2 points IQ loss for an alternative NAAQS of 0.20 [mu]g/m\3\
and between 0.5 and 2.8 points IQ loss for an alternate NAAQS of 0.05
[mu]g/m\3\, maximum monthly average, (as shown in Table 2). Higher risk
estimates are associated with a maximum quarterly averaging time
(USEPA, 2007b).
     At each NAAQS level assessed, the upper bound of
population median air-related risk for the primary Pb smelter subarea,
which due to limitations in modeling is the only air-related risk
estimate for this case study, is generally higher than that for the
general urban case study, likely due to differences in the indoor dust
models used for the two case studies (as discussed in section II.C.3.b
of the proposal).
     Compared to the other case studies, the air-related risk
for the location-specific case studies is smaller because of the
broader range of air-related exposures and the population distribution.
For example, the majority of the populations in each of the location-
specific case studies resides in areas with ambient air Pb levels well
below each standard level assessed, particularly for standard levels
above 0.05 [mu]g/m\3\, maximum monthly average. Consequently, risk
estimates for these case studies indicate little response to
alternative standard levels above 0.05 [mu]g/m\3\ maximum monthly
average (as shown in Table 2).

                        Table 2--Summary of Risk Attributable to Air-related Pb Exposure
----------------------------------------------------------------------------------------------------------------
                                                          Median air-related IQ loss \A\
                                 -------------------------------------------------------------------------------
 NAAQS level simulated  ([mu]g/                                        Location-specific urban case studies
   m\3\ max monthly, except as                      Primary Pb   -----------------------------------------------
          noted below)             General urban      smelter        Cleveland                      Los Angeles
                                    case study    (subarea) case   (0.56 [mu]g/   Chicago  (0.31   (0.17 [mu]g/
                                                    study \B C\        m\3\)        [mu]g/m\3\)        m\3\)
----------------------------------------------------------------------------------------------------------------
1.5 max quarterly \D\...........         3.5-4.8              <6     2.8-3.9 \E\     3.4-4.7 \E\     2.7-4.2 \E\
                                       (1.5-7.7)      <(3.2-9.4)       (0.6-4.6)       (1.4-7.4)       (1.1-6.2)
0.5.............................         1.9-3.6            <4.5         0.6-2.9           (\F\)           (\F\)
                                       (0.7-4.8)      <(2.1-7.7)       (0.2-3.9)
0.2.............................         1.2-3.2            <3.7         0.6-2.8         0.6-2.9     0.7-2.9 \G\
                                       (0.4-4.0)      <(1.2-5.1)       (0.1-3.2)       (0.3-3.6)       (0.2-3.5)

0.05............................         0.5-2.8            <2.8         0.1-2.6         0.2-2.6         0.3-2.7
                                       (0.2-3.3)      <(0.9-3.4)      (<0.1-3.1)       (0.1-3.2)       (0.1-3.2)
0.02............................         0.3-2.6            <2.9        <0.1-2.6         0.1-2.6         0.1-2.6
                                       (0.1-3.1)      <(0.9-3.3)      (<0.1-3.0)      (<0.1-3.1)     (<0.1-3.1)
----------------------------------------------------------------------------------------------------------------
\A\--Air-related risk is bracketed by ``recent air'' (lower bound of presented range) and ``recent'' plus ``past
  air'' (upper bound of presented range). While differences between standard levels are better distinguished by
  differences in the ``recent'' plus ``past air'' estimates (upper bounds shown here), these differences are
  inherently underestimates. The term ``past air'' includes contributions from the outdoor soil/dust
  contribution to indoor dust, historical air contribution to indoor dust, and outdoor soil/dust pathways;
  ``recent air'' refers to contributions from inhalation of ambient air Pb or ingestion of indoor dust Pb
  predicted to be associated with outdoor ambient air Pb levels, with outdoor ambient air also potentially
  including resuspended, previously deposited Pb (see section II.C.2.e of the proposal). Boldface values are
  estimates generated using the log-linear with low-exposure linearization function. Values in parentheses
  reflect the range of estimates associated with all four concentration-response functions.
\B\--In the case of the primary Pb smelter case study, only recent plus past air estimates are available.
\C\--Median air-related IQ loss estimates for the primary Pb smelter (full study area) range from <1.7 to <2.9
  points, with no consistent pattern across simulated NAAQS levels. This lack of a pattern reflects inclusion of
  a large fraction of the study population with relatively low ambient air impacts such that there is lower
  variation (at the population median) across standard levels (see section 4.2 of the Risk Assessment, Volume
  1).
\D\--This corresponds to roughly 0.7-1.0 [mu]g/m\3\ maximum monthly mean, across the urban case studies.
\E\--A ``roll-up'' was performed so that the highest monitor in the study area is increased to just meet this
  level.
\F\--A ``roll-up'' to this level was not performed.
\G\--A ``roll-up'' to this level was not performed; these estimates are based on current conditions in this
  area.

B. Need for Revision of the Current Primary Standard

    The initial issue to be addressed in the current review of the
primary Pb standard is whether, in view of the advances in scientific
knowledge reflected in the Criteria Document and Staff Paper, the
existing standard should be revised. In evaluating whether it is
appropriate to revise the current standard, the Administrator builds on
the general approach used in the initial setting of the standard, as
well as that used in the last review, and reflects the broader body of
evidence and information now available. The approach used is based on
an integration of information on health effects associated with
exposure to ambient Pb; expert judgment on the adversity of such
effects on individuals; and policy judgments as to when the standard is
requisite to protect public health with an adequate margin of safety,
which are informed by air quality and related analyses, quantitative
exposure and risk assessments when possible, and qualitative assessment
of impacts that could not be quantified. The Administrator has taken
into account both evidence-based and quantitative exposure- and risk-
based considerations in developing conclusions on the adequacy of the
current primary Pb standard.
    The Administrator's proposed conclusions on the adequacy of the
current primary standard are summarized below in the Introduction
(section II.B.1), followed by consideration of comments received on the
proposal (section II.B.2) and the Administrator's final decision with
regard to the need for revision of the current primary standard
(II.B.3).
1. Introduction
    As described in section II.D.1.a of the proposal, the current
standard was set in 1978 to provide protection to the public,
especially children as the particularly sensitive population subgroup,
against Pb-induced adverse health effects (43 FR 46246). The standard
was set to provide protection against anemia (as well as effects
associated with higher exposures), with consideration of impacts on the
heme synthesis pathway leading to anemia (43 FR 46252-46253). In
setting the standard, EPA determined that ``the maximum safe level of
blood lead for an individual child'' should be no higher than 30 [mu]g/
dL, and described 15 [mu]g/dL Pb as ``the maximum safe blood lead level
(geometric mean) for a population of young children'' (43 FR 46247,
46253). The basis for the level, averaging time, form and indicator are
described in section II.D.1.a of the proposal.
    As noted in the proposal, the body of available evidence today,
summarized above in section II.A.2 and in section II.B of the proposal,
and discussed in the Criteria Document, is substantially expanded from
that available when the current standard was set three decades ago. The
Criteria Document presents evidence of the occurrence of health effects
at appreciably lower blood Pb levels than those demonstrated by the
evidence at the time the standard was set. Further, subsequent to the
setting of the standard, the Pb NAAQS criteria review during the 1980s
and the current review have provided ``(a) increasingly stronger
evidence that substantiatied still lower fetal and/or postnatal Pb-
exposure levels (indexed by blood-Pb levels extending to as low as 10
to 15 [mu]g/dL or, possibly, below) as being associated with slowed
physical and neurobehavioral development, lower IQ, impaired learning,
and/or other indicators of adverse neurological impacts; and (b) other
pathophysiological effects of Pb on cardiovascular function, immune
system components, calcium and vitamin D metabolism and other selected
health endpoints'' (CD, pp. 8-24 to 8-25). This evidence is discussed
fully in the Criteria Document.
    In the proposal, EPA explained its evidence-based considerations
regarding the adequacy of the current standard. With regard to the
sensitive population, while the sensitivity of the elderly and other
particular subgroups is recognized, as at the time the current standard
was set, young children continue to be recognized as a key sensitive
population for Pb exposures.
    With regard to the exposure levels at which adverse health effects
occur, the proposal noted that the current evidence demonstrates the
occurrence of adverse health effects at appreciably lower blood Pb
levels than those demonstrated by the evidence at the time the standard
was set. This evidence is reflected in

changes over the intervening years in the CDC's identification and
description of their advisory level for Pb in individual children's
blood (as described above in section II.A.2.a). The current evidence
indicates the occurrence of a variety of health effects, including
neurological effects in children, associated with blood Pb levels
extending well below 10 [mu]g/dL (CD, sections 6.2, 8.4 and 8.5). For
example, as noted in the Criteria Document with regard to the
neurocognitive effects in children, the ``weight of overall evidence
strongly substantiates likely occurrence of [this] type of effect in
association with blood-Pb concentrations in range of 5-10 [mu]g/dL, or
possibly lower * * * Although no evident threshold has yet been clearly
established for those effects, the existence of such effects at still
lower blood-Pb levels cannot be ruled out based on available data.''
(CD, p. 8-61). The Criteria Document further notes that any such
threshold may exist ``at levels distinctly lower than the lowest
exposures examined in these epidemiological studies'' (CD, p. 8-67).
    In considering the adequacy of the current standard, the Staff
Paper considered the evidence in the context of the framework used to
determine the standard in 1978, as adapted to reflect the current
evidence. In so doing, the Staff Paper recognized that the health
effects evidence with regard to characterization of a threshold for
adverse effects has changed since the standard was set in 1978, as have
the Agency's views on the characterization of a safe blood Pb level. As
summarized in the proposal (73 FR 29237-38) and described in the Staff
Paper (section 5.4.1), parameters for this framework include estimates
for average nonair blood Pb level, and air-to-blood ratio, as well as a
maximum safe individual and/or geometric mean blood Pb level. For this
last parameter, the Staff Paper for the purposes of this evaluation
considered the lowest population mean blood Pb levels with which some
neurocognitive effects have been associated in the evidence.
    Based on the current evidence, the Staff Paper first concluded that
young children remain the sensitive population of primary focus in this
review and that ``there is now no recognized safe level of Pb in
children's blood and studies appear to show adverse effects at
population mean concurrent blood Pb levels as low as approximately 2
[mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al., 2000)'' (USEPA,
2007c). The Staff Paper further stated that ``while the nonair
contribution to blood Pb has declined, perhaps to a range of 1.0-1.4
[mu]g/dL, the air-to-blood ratio appears to be higher at today's lower
blood Pb levels than the estimates at the time the standard was set,
with current estimates on the order of 1:3 to 1:5 and perhaps up to
1:10'' (USEPA, 2007c). Adapting the framework employed in setting the
standard in 1978, the Staff Paper concluded that ``the more recently
available evidence suggests a level for the standard that is lower by
an order of magnitude or more'' (USEPA, 2007c, p. 5-17).
    Since completion of the Staff Paper and ANPR, the Agency further
considered the evidence with regard to adequacy of the current standard
using an approach other than the adapted 1978 framework considered in
the Staff Paper. This alternative evidence-based \55\ framework,
referred to as the air-related IQ loss framework, shifts focus from
identifying an appropriate target population mean blood lead level and
instead focuses on the magnitude of effects of air-related Pb on
neurocognitive functions. This framework builds on a recommendation by
the CASAC Pb Panel to consider the evidence in a more quantitative
manner, and is discussed in more detail in section II.E.3.a.ii of the
proposal.
---------------------------------------------------------------------------

    \55\ The term ``evidence-based'' as used here refers to the
drawing of information directly from published studies, with
specific attention to those reviewed and described in the Criteria
Document, and is distinct from considerations that draw from the
results of the quantitative exposure and risk assessment.
---------------------------------------------------------------------------

    In this air-related IQ loss framework, EPA draws from the entire
body of evidence as a basis for concluding that there are causal
associations between air-related Pb exposures and population IQ
loss.\56\ We also draw more quantitatively from the evidence by using
evidence-based C-R functions to quantify the association between air Pb
concentrations and air-related population mean IQ loss. Thus, this
framework more fully considers the evidence with regard to the
concentration-response relationship for the effect of Pb on IQ than
does the adapted 1978 framework, and it also draws from estimates for
air-to-blood ratios.
---------------------------------------------------------------------------

    \56\ For example, as stated in the Criteria Document,
``Fortunately, there exists a large database of high quality studies
on which to base inferences regarding the relationship between Pb
exposure and neurodevelopment. In addition, Pb has been extensively
studied in animal models at doses that closely approximate the human
situation. Experimental animal studies are not compromised by the
possibility of confounding by such factors as social class and
correlated environmental factors. The enormous experimental animal
literature that proves that Pb at low levels causes neurobehavioral
deficits and provides insights into mechanisms must be considered
when drawing causal inferences (Bellinger, 2004; Davis et al., 1990;
U.S. Environmental Protection Agency, 1986a, 1990).'' (CD, p. 6-75).
---------------------------------------------------------------------------

    In the proposal, while we noted the evidence of steeper slope for
the C-R relationship for blood Pb concentration and IQ loss at lower
blood Pb levels (described above in sections II.A.2.c), we stated that
for purposes of consideration of the adequacy of the current standard
we were concerned with the C-R relationship for blood Pb levels that
would be associated with exposure to air-related Pb at the level of the
current standard. For this purpose, we focused on a median linear
estimate of the slope of the C-R function from study populations for
which most blood Pb levels were below 10 [mu]g/dL and for which a
linear slope restricted to blood Pb levels below about 10 [mu]g/dL
could be estimated (described in CD, pp. 6-65 to 6-66 and summarized in
section II.B.2.b of the proposal). The median slope estimate is -0.9 IQ
points per [mu]g/dL blood Pb (CD, p. 8-80). Applying estimates of air-
to-blood ratios ranging from 1:3 to 1:5, drawing from the discussion of
air-to-blood ratios in section II.B.1.c of the proposal, to a
population of children exposed at the current level of the standard is
estimated to result in an average air-related blood Pb level above 4
[mu]g/dL.\57\ Multiplying these blood Pb levels by the slope estimate,
identified above, for blood Pb levels extending up to 10 [mu]g/dL (-0.9
IQ points per [mu]g/dL), would imply an average air-related IQ loss for
such a group of children on the order of 4 or more IQ points.
---------------------------------------------------------------------------

    \57\ This is based on the calculation in which 1.5 [mu]g/m\3\ is
multiplied by a ratio of 3 [mu]g blood Pb per 1 [mu]g/m\3\ air Pb to
yield an air-related blood Pb estimates of 4.5 [mu]g/dL; using a 1:5
ratio yields an estimate of 7.5 [mu]g/dL. As with the 1978 framework
considered in the Staff Paper, the context for use of the air-to-
blood ratio here is a population being exposed at the level of the
standard.
---------------------------------------------------------------------------

    In the proposal, EPA also explained its exposure- and risk-based
considerations regarding the adequacy of the current standard. EPA
estimated exposures and health risks associated with air quality that
just meets the current standard (as described in the Risk Assessment
Report) to help inform judgments about whether or not the current
standard provides adequate protection of public health, taking into
account key uncertainties associated with the estimated exposures and
risks (summarized above in section II.C of the proposal and more fully
in the Risk Assessment Report). In considering the adequacy of the
standard, the Staff Paper considered exposure and risk estimates from
the quantitative risk assessment, taking into account associated
uncertainties. The Staff Paper

first considered exposure/risk estimates associated with air-related
risk, which as recognized in section II.A.3 above (and summarized in
section II.C.2.e of the proposal and described more fully in the Risk
Assessment Report) are approximated estimates, provided in terms of
upper and lower bounds. The Staff Paper described the magnitude of
these estimates for the current NAAQS as being indicative of levels of
IQ loss associated with air-related risk that may ``reasonably be
judged to be highly significant from a public health perspective''
(USEPA, 2007c).
    As discussed in section II.D.2.b of the proposal, the Staff Paper
also describes a different risk metric that estimated differences in
the numbers of children with different amounts of Pb-related IQ loss
between air quality scenarios for current conditions and for the
current NAAQS in the three location-specific urban case studies. The
Staff Paper concluded that these estimated differences ``indicate the
potential for significant numbers of children to be negatively affected
if air Pb concentrations increased to levels just meeting the current
standard'' (USEPA, 2007c). Beyond the findings related to quantified IQ
loss, the Staff Paper recognized the potential for other, unquantified
adverse effects that may occur at similarly low exposures as those
quantitatively assessed in the risk assessment. In summary, the Staff
Paper concluded that taken together, ``the quantified IQ effects
associated with the current NAAQS and other, nonquantified effects are
important from a public health perspective, indicating a need for
consideration of revision of the standard to provide an appreciable
increase in public health protection'' (USEPA, 2007c).
    In their letter to the Administrator subsequent to consideration of
the ANPR, the Staff Paper and the Risk Assessment Report, the CASAC Pb
Panel advised the Administrator that they unanimously and fully
supported ``Agency staff's scientific analyses in recommending the need
to substantially lower the level of the primary (public-health based)
Lead NAAQS, to an upper bound of no higher than 0.2 [mu]g/m\3\ with a
monthly averaging time'' (Henderson, 2008a, p. 1). The Panel
additionally advised that the current Pb NAAQS ``are totally inadequate
for assuring the necessary decreases of lead exposures in sensitive
U.S. populations below those current health hazard markers identified
by a wealth of new epidemiological, experimental and mechanistic
studies'', and that ``it is the CASAC Lead Review Panel's considered
judgment that the NAAQS for Lead must be decreased to fully-protect
both the health of children and adult populations'' (Henderson, 2007a,
p. 5). CASAC drew support for their recommendation from the current
evidence, described in the Criteria Document, of health effects
occurring at dramatically lower blood Pb levels than those indicated by
the evidence available when the standard was set and of a recognition
of effects that extend beyond children to adults.
    At the time of proposal, in considering whether the current primary
standard should be revised, the Administrator carefully considered the
conclusions contained in the Criteria Document, the information,
exposure/risk assessments, conclusions and recommendations presented in
the Staff Paper, the advice and recommendations from CASAC, and public
comments received on the ANPR and other documents to date. In so doing,
the Administrator noted the following: (1) A substantially expanded
body of available evidence, described briefly in section II.A above and
more fully in section II.B of the proposal and discussed in the
Criteria Document, from that available when the current standard was
set three decades ago; (2) evidence of the occurrence of health effects
at appreciably lower blood Pb levels than those demonstrated by the
evidence at the time the standard was set in 1978; (3) the currently
available robust evidence of neurotoxic effects of Pb exposure in
children, both with regard to epidemiological and toxicological
studies; (4) associations of effects on the neurodevelopment of
children with blood Pb levels notably decreased from those in the late
1970s; \58\ (5) toxicological evidence including extensive experimental
laboratory animal evidence that substantiates well the plausibility of
the epidemiologic findings observed in human children; (6) current
evidence that suggests a steeper dose-response relationship at recent
lower blood Pb levels than at higher blood Pb levels, indicating the
potential for greater incremental impact associated with exposure at
these lower levels.
---------------------------------------------------------------------------

    \58\ As noted in the proposal (73 FR 29228), while blood Pb
levels in U.S. children have decreased notably since the late 1970s,
newer studies have investigated and reported associations of effects
on the neurodevelopment of children with these more recent blood Pb
levels.
---------------------------------------------------------------------------

    In addition to the evidence of health effects occurring at
significantly lower blood Pb levels, the Administrator recognized in
the proposal that, as at the time the standard was set, the current
health effects evidence together with findings from the exposure and
risk assessments (summarized above in section II.A.3) supports a
finding that air-related Pb exposure pathways contribute to blood Pb
levels in young children by inhalation and ingestion. Furthermore, the
Administrator took note of the information that suggests that the air-
to-blood ratio (i.e., the quantitative relationship between air
concentrations and blood concentrations) is now likely larger, when air
inhalation and ingestion are considered, than that estimated when the
standard was set.
    At the time of proposal, the Administrator first considered the
current evidence in the context of an adaptation of the 1978 framework,
as presented in the Staff Paper, recognizing that the health effects
evidence with regard to characterization of a threshold for adverse
effects has changed dramatically since the standard was set in 1978. As
discussed in the proposal, however, limitations in the application of
that framework to the current situation, where (unlike when the
standard was set in 1978) there is not an evidentiary basis to
determine a safe level for individual children with respect to the
identified health effect, led the Administrator to focus primarily
instead on the air-related IQ loss evidence-based framework, described
in section II.D.2.a.ii of the proposal, in considering the adequacy of
the current standard.
    As discussed in the proposal, the Administrator judged that air-
related IQ loss associated with exposure at the level of the current
standard is large from a public health perspective and that this
evidence-based framework supports a conclusion that the current
standard does not protect public health with an adequate margin of
safety. Further, the Administrator provisionally concluded that the
current evidence indicates the need for a standard level that is
substantially lower than the current level to provide increased public
health protection, especially for at-risk groups, including most
notably children, against an array of effects, most importantly
including effects on the developing nervous system.
    At the time of proposal, the Administrator also considered the
results of the exposure and risk assessments conducted for this review
as providing some further perspective on the potential magnitude of
air-related IQ loss, although, noting uncertainties and limitations in
the assessments, the Administrator did not place primary reliance on
the exposure and risk assessments. Nonetheless, the Administrator
observed that in areas projected to just meet the current standard, the
quantitative estimates of

IQ loss associated with air-related Pb indicate risk of a magnitude
that in his judgment is significant from a public health perspective
and also recognized that, although the current monitoring data indicate
few areas with airborne Pb near or just exceeding the current standard,
there are significant limitations with the current monitoring network
and thus there exists the potential that the prevalence of such Pb
concentrations may be underestimated by currently available data.
    Based on all of these considerations, the Administrator
provisionally concluded that the current Pb standard is not requisite
to protect public health with an adequate margin of safety because it
does not provide sufficient protection, and that the standard should be
revised to provide increased public health protection, especially for
members of at-risk groups.
2. Comments on the Need for Revision
    In considering comments on the need for revision, the Administrator
first notes the advice and recommendations from CASAC with regard to
the adequacy of the current standard. In the four letters that CASAC
has sent the Agency providing advice on the Pb standard, including the
most recent one on the proposal, all have repeated their unanimous view
regarding the need for substantial revision of the Pb NAAQS (Henderson,
2007a, 2007b, 2008a, 2008b). For example, as stated in their letter of
March 2007, the ``unanimous judgment of the Lead Panel is that * * *
both the primary and secondary NAAQS should be substantially lowered''
(Henderson, 2007a).
    General comments based on relevant factors that either support or
oppose any change to the current Pb primary standard are addressed in
this section. Comments on elements of the proposed primary standard and
on studies that relate to consideration of the appropriate indicator,
averaging time and form, and level are addressed below in sections
II.C.1, II.C.2, and II.C.3, respectively. Other specific comments
related to the standard setting, as well as general comments based on
implementation-related factors that are not a permissible basis for
considering the need to revise the current standards are addressed in
the Response to Comments document.
    The vast majority of public comments received on the proposal
generally asserted that, based on the available scientific information,
the current Pb standard is insufficient to protect public health with
an adequate margin of safety and revisions to the standard are
appropriate. Among those calling for revisions to the current standards
are medical groups, including the American Academy of Pediatrics, the
American Medical Association and the American Thoracic Society, as well
as two groups of concerned physicians and scientists, and the Agency's
external Children's Health Protection Advisory Committee (Marty, 2008).
Similar conclusions were also submitted in comments from many national,
state, and local environmental and public health organizations,
including, for example, the Natural Resources Defense Council (NRDC),
the Sierra Club, and the Coalition to End Childhood Lead Poisoning. All
of these medical, public health and environmental commenters stated
that the current Pb standard needs to be revised to a level well below
the current level to protect the health of sensitive population groups.
Many individual commenters also expressed such views. Additionally,
regional organizations of state agencies, including the National
Association of Clean Air Agencies (NACAA), and Northeast States for
Coordinated Air Use Management (NESCAUM) urged that EPA revise the Pb
standard. State and local air pollution control authorities or public
health agencies who commented on the Pb standard also supported
revision of the current Pb standard, including the New York Departments
of Health and Environmental Conservation, Iowa Departments of Natural
Resources and Public Health, the Missouri Departments of Natural
Resources and Health and Senior Services, as well as the Missouri
Office of the Attorney General, among others. All tribal governments
and tribal air and environmental agencies commenting on the standard,
including the InterTribal Council of Arizona, Inc. (an organization of
20 tribal governments in Arizona), the Lone Pine Paiute-Shoshone
Reservation, as well as the Fond du Lac Band of Lake Superior Chippewa,
commented in support of revision of the Pb NAAQS.
    In general, all of these commenters agreed with EPA's proposed
conclusions on the importance of results from the large body of
scientific studies reviewed in the Criteria Document and on the need to
revise the primary Pb standard as articulated in EPA's proposal. Many
commenters cited CASAC advice on this point. The EPA generally agrees
with CASAC and these public commenters' conclusions regarding the need
to revise the primary Pb standard. EPA agrees that the evidence
assessed in the Criteria Document and the Staff Paper provides a basis
for concluding that the current Pb standard does not protect public
health with an adequate margin of safety. Comments on specific aspects
of the level for a revised standard are discussed below in section
II.C.3 below.
    Some of these commenters also identified ``new'' studies that were
not included in the Criteria Document as providing further support for
the need to revise the Pb standards. As noted above in section I.C, as
in past NAAQS reviews, the Agency is basing the final decisions in this
review on the studies and related information included in the Pb air
quality criteria that have undergone CASAC and public review, and will
consider the newly published studies for purposes of decision making in
the next Pb NAAQS review. Nonetheless, in considering these comments
related to these more recent studies (further discussed in the Response
to Comments document), EPA notes that our provisional consideration of
these studies concludes that this new information and findings do not
materially change any of the broad scientific conclusions regarding
neurotoxic and other health effects of lead exposure made in the 2006
Criteria Document. For example, ``new'' studies cited by commenters on
neurocognitive and neurobehavioral effects add to the overall weight of
evidence and focus on findings of such effects beyond IQ in study
groups with some studies including lower blood Pb levels than were
available for review in the Criteria Document.
    Three industry associations (National Association of Manufacturers,
Non-Ferrous Founders' Society, and Wisconsin Manufacturers & Commerce)
commented in support of retaining the current primary Pb standard.
These commenters generally state that most health risks associated with
Pb exposures are more likely to result from past air emissions or
nonair sources of Pb, such as lead-based paint, and that reduction of
the Pb standard will not provide meaningful benefits to public health.
They additionally cite costs to those industries on whose part action
will be required to meet a reduced standard. While EPA recognizes that
nonair sources contribute Pb exposure to today's population, EPA
disagrees with the commenters' premise that Pb exposures associated
with any past air emissions are not relevant to consider in judging the
adequacy of the current standard. Further, EPA disagrees with
commenters, regarding the significance of health risk associated with
air-related Pb exposures allowed by the current standard. As discussed
in summarized in section II.B.1 above and discussed in section II.B.3
below, EPA has concluded

that the health risk associated with air-related Pb exposures allowed
by the current standard is of such a significant magnitude that a
revision to the standard is needed to protect public health with an
adequate margin of safety. EPA further notes that, as discussed above
in section I.B, under the CAA, EPA may not consider the costs of
compliance in determining what standard is requisite to protect public
health with an adequate margin of safety.
3. Conclusions Regarding the Need for Revision
    Having carefully considered the public comments, as discussed
above, the Administrator believes the fundamental scientific
conclusions on the effects of Pb reached in the Criteria Document and
Staff Paper, briefly summarized above in section II.B.1, remain valid.
In considering whether the primary Pb standard should be revised, the
Administrator places primary consideration on the large body of
scientific evidence available in this review concerning the public
health impacts of Pb, including significant new evidence concerning
effects at blood Pb concentrations substantially below those identified
when the current standard was set. As summarized in section II.A.2.b,
Pb has been demonstrated to exert a broad array of adverse effects on
multiple organ systems, with the evidence across this array of effects
much expanded since the standard was set, with the key effects most
pertinent to ambient exposures today including neurological,
hematological and immune effects for children and hematological,
cardiovascular and renal effects for adults. The Administrator
particularly notes the robust evidence of neurotoxic effects of Pb
exposure in children, both with regard to epidemiological and
toxicological studies. While blood Pb levels in U.S. children have
decreased notably since the late 1970s, newer studies have investigated
and reported associations of effects on the neurodevelopment of
children with these more recent blood Pb levels. The toxicological
evidence includes extensive experimental laboratory animal evidence
that substantiates well the plausibility of the epidemiologic findings
observed in human children and expands our understanding of likely
mechanisms underlying the neurotoxic effects. Further, the
Administrator notes the current evidence that suggests a steeper dose-
response relationship at these lower blood Pb levels than at higher
blood Pb levels, indicating the potential for greater incremental
impact associated with exposure at these lower levels.
    In addition to the evidence of health effects occurring at
significantly lower blood Pb levels, the Administrator recognizes that
the current health effects evidence together with findings from the
exposure and risk assessments (summarized above in section II.A.3),
like the information available at the time the standard was set,
supports our finding that air-related Pb exposure pathways contribute
to blood Pb levels in young children, by inhalation and ingestion.
Furthermore, the Administrator takes note of the information that
suggests that the air-to-blood ratio (i.e., the quantitative
relationship between air concentrations and blood concentrations) is
now likely larger, when all air inhalation and ingestion pathways are
considered, than that estimated when the standard was set.
    The Administrator has considered the evidence in the record, and
discussed above, in the context of an adaptation of the 1978 framework,
as presented in the Staff Paper, recognizing that the health effects
evidence with regard to characterization of a threshold for adverse
effects has changed dramatically since the standard was set in 1978. As
discussed in the proposal (73 FR 29229), however, the Administrator
recognizes limitations to this approach and has focused primarily
instead on the air-related IQ loss evidence-based framework described
in section II.B.1 above, in considering the adequacy of the current
standard.
    In considering the application of the air-related IQ loss framework
to the current evidence as discussed above in section II.B.1, the
Administrator concludes that in areas projected to just meet the
current standard, the quantitative estimates of IQ loss associated with
air-related Pb indicate risk of a magnitude that in his judgment is
significant from a public health perspective, and that this evidence-
based framework supports a conclusion that the current standard does
not protect public health with an adequate margin of safety. Further,
the Administrator believes that the current evidence indicates the need
for a standard level that is substantially lower than the current level
to provide increased public health protection, especially for at-risk
groups, including most notably children, against an array of effects,
most importantly including effects on the developing nervous system.
    In addition to the primary consideration given to the available
evidence, the Administrator has also taken into consideration the
Agency's exposure and risk assessments to help inform his evaluation of
the adequacy of the current standard. As at the time of proposal, the
Administrator believes the results of those assessments provide some
further perspective on the potential magnitude of air-related IQ loss
and thus inform his judgment on the adequacy of the current standard to
protect against health effects of concern. While taking into
consideration the uncertainties and limitations in the risk
assessments, the Administrator again observes that in areas projected
to just meet the current standard, the quantitative estimates of IQ
loss associated with air-related Pb indicate risk of a magnitude that
in his judgment is significant from a public health perspective.
Further, although the current monitoring data indicate few areas with
airborne Pb near or just exceeding the current standard, the
Administrator recognizes significant limitations with the current
monitoring network and thus there is the potential that the prevalence
of such Pb concentrations may be underestimated by currently available
data. The Administrator thus finds that the exposure and risk estimates
provide additional support to the evidence-based conclusion, reached
above, that the current standard needs to be revised.
    Based on these considerations, and consistent with the CASAC
Panel's unanimous conclusion that EPA needed to substantially lower the
level of the primary Pb NAAQS to fully protect the health of children
and adult populations, the Administrator agrees with the vast majority
of public commenters that the current standard is not sufficient and
thus not requisite to protect public health with an adequate margin of
safety and that revision is needed to provide increased public health
protection, especially for members of at-risk groups.

C. Conclusions on the Elements of the Standard

    The four elements of the standard--indicator, averaging time, form,
and level--serve to define the standard and must be considered
collectively in evaluating the health and welfare protection afforded
by the standard. In considering comments on the proposed revisions to
the current primary Pb standard, as discussed in the following
sections, EPA considers each of the four elements of the standard as to
how they might be revised to provide a primary standard for Pb that is
requisite to protect public health with an adequate margin of safety.
The basis for the proposed decision, comments on the

proposal, and the Administrator's final decision on indicator are
discussed in section II.C.1, on averaging time and form in section
II.C.2, and on a level for the primary Pb NAAQS in section II.C.3.
1. Indicator
a. Basis for Proposed Decision
    In setting the current standard in 1978, EPA established Pb-TSP as
the indicator.\59\ In comments on the 1977 proposal, EPA received
comments expressing concern that because only a fraction of airborne
particulate matter is respirable, an air standard based on total air Pb
would be unnecessarily stringent and therefore the standard should be
limited to respirable size Pb particulate matter. Such a standard might
have led to a Pb NAAQS with an indicator of Pb in particulate matter
less than or equal to 10 [mu]m in diameter (Pb-PM10) \60\ as
the indicator. The Agency considered this recommendation, but did not
accept it. Rather, EPA reemphasized that larger particles of air-
related Pb contribute to Pb exposure through ingestion pathways, and
that ingestion pathways, including those associated with deposition of
Pb from the air, can be a significant component of Pb exposures. In
addition to these ingestion exposure pathways, nonrespirable Pb that
has been emitted to the ambient air may, at some point, become
respirable through weathering or mechanical action, thus subsequently
contributing to inhalation exposures. EPA concluded that total airborne
Pb, both respirable and nonrespirable fractions, should be addressed by
the air standard (43 FR 46251). The federal reference method (FRM) for
Pb-TSP specifies the use of the high-volume sampler.
---------------------------------------------------------------------------

    \59\ The current standard specifies the measurement of airborne
Pb with a high-volume TSP federal reference method (FRM) sampler
with atomic absorption spectrometry of a nitric acid extract from
the filter for Pb, or with an approved equivalent method (40 CFR
50.12, Appendix G).
    \60\ For simplicity, the discussion in this notice speaks as if
PM10 samplers have a sharp size cut-off. In reality, they
have a size selection behavior in which 50% of particles 10 microns
in size are captured, with a progressively higher capture rate for
smaller particles and a progressively lower capture rate for larger
particles. The ideal capture efficiency curve for PM10
samplers specifies that particles above 15 microns not be captured
at all, although real samplers may capture a very small percentage
of particles above 15 microns. TSP samplers have 50% capture points
in the range of 25 to 50 microns (Wedding et al., 1977), which is
broad enough to include virtually all sizes of particles capable of
being transported any significant distance from their source except
under extreme wind events.
---------------------------------------------------------------------------

    In the 1990 Staff Paper, this issue was again considered in light
of information regarding limitations of the high-volume sampler used
for the Pb-TSP measurements, such as the variability discussed below.
The continued use of Pb-TSP as the indicator was recommended in the
Staff Paper (USEPA, 1990b):

    Given that exposure to lead occurs not only via direct
inhalation, but via ingestion of deposited particles as well,
especially among young children, the hi-vol provides a more complete
measure of the total impact of ambient air lead. * * * Despite its
shortcomings, the staff believes the high-volume sampler will
provide a reasonable indicator for determination of compliance * * *

    As in the past, and discussed in the proposal, the evidence
available today indicates that Pb in all particle size fractions, not
just respirable Pb particles, contributes to Pb in blood and to
associated health effects. Further, the evidence and exposure/risk
estimates in the current review indicate that ingestion pathways
dominate air-related exposure. Lead is unlike other criteria
pollutants, where inhalation of the airborne pollutant is the key
contributor to exposure. For Pb it is the quantity of Pb in ambient
particles with the potential to deposit indoors or outdoors, thereby
leading to a role in ingestion pathways, that is the key contributor to
air-related exposure. The evidence additionally indicates that airborne
Pb particles are transported long or short distances depending on their
size, such that the representation of larger particles is greater at
locations near sources than at sites not directly influenced by
sources.
    In the current review, the Staff Paper evaluated the evidence with
regard to the indicator for a revised primary standard. This evaluation
included consideration of the basis for using Pb-TSP as the current
indicator, information regarding the sampling methodology for the
current indicator, and CASAC advice with regard to indicator (described
below). Based on this evaluation, the Staff Paper recommended retaining
Pb-TSP as the indicator for the primary standard. The Staff Paper also
recommended activities intended to encourage collection and development
of datasets that will improve our understanding of national and site-
specific relationships between Pb-PM10 (collected by low-
volume sampler) \61\ and Pb-TSP to support a more informed
consideration of indicator during the next review. The Staff Paper
suggested that such activities might include describing a federal
equivalence method (FEM) in terms of PM10 and allowing its
use for a TSP-based standard in certain situations, such as where
sufficient data are available to adequately demonstrate a relationship
between Pb-TSP and Pb-PM10 or, in combination with more
limited Pb-TSP monitoring, in areas where Pb-TSP data indicate Pb
levels well below the NAAQS level.
---------------------------------------------------------------------------

    \61\ ``Low-volume PM10 sampling'' refers to sampling
using any of a number of monitor models that draw 16.67 liters/
minute (1 m\3\/hour) of air through the filter, in contrast to
``high-volume'' sampling of either TSP or PM10 in which the monitor
draws 1500 liters/minute (90 m\3\/hour).
---------------------------------------------------------------------------

    The ANPR further identified issues and options associated with
consideration of the potential use of Pb-PM10 data for
judging attainment or nonattainment with a Pb-TSP NAAQS. These issues
included the impact of controlling Pb-PM10 for sources
predominantly emitting Pb in particles larger than those captured by
PM10 monitors (i.e., ultra-coarse) \62\, and the options
included potential application of Pb-PM10 FRM/FEMs at sites
with established relationships between Pb-TSP and Pb-PM10,
and use of Pb-PM10 data, with adjustment, as a surrogate for
Pb-TSP data. The ANPR broadly solicited comment in these areas.
---------------------------------------------------------------------------

    \62\ In this notice, we use ``ultra-coarse'' to refer to
particles collected by a TSP sampler but not by a PM10
sampler. We note that CASAC has variously also referred to these
particles as ``very coarse'' or ``larger coarse-mode'' particles.
This terminology is consistent with the traditional usage of
``fine'' to refer to particles collected by a PM2.5
sampler, and ``coarse'' to refer to particles collected by a
PM10 sampler but not by a PM2.5 sampler,
recognizing that there will be some overlap in the particle sizes in
the three types of collected material.
---------------------------------------------------------------------------

    As noted in the proposal, the Agency in setting the standard and
CASAC in providing their advice (described below) both recognized that
ingestion pathways are important to air-related Pb exposures and that
Pb particles contributing to these pathways include ultra-coarse
particles. Thus, as noted in the proposal, choosing the appropriate
indicator requires consideration of the impact of the indicator on the
protection provided from exposure to air-related Pb of all particle
sizes, including ultra-coarse particles, by both the inhalation and
ingestion pathways.
    As discussed in the proposal (sections II.E.1 and V.A), the Agency
recognizes the body of evidence indicating that the high-volume Pb-TSP
sampling methodology contributes to imprecision in resultant Pb
measurements due to variability in the efficiency of capture of
particles of different sizes and thus, in the mass of Pb measured.
Variability is most substantial in samples with a large portion of Pb
particles greater than 10 microns, such as those samples collected near
sources with emissions of ultra-coarse particles. As noted in the
proposal, this variability contributes to a clear risk of
underestimating the ambient level of total Pb in the air,

especially in areas near sources of ultra-coarse particles, by
underestimating the amount of the ultra-coarse particles. This
variability also contributes to a risk of not consistently identifying
sites that fail to achieve the standard.
    The Agency also recognizes, as discussed in the proposal, that the
low-volume PM10 sampling methodology does not exhibit such
variability \63\ due both to increased precision of the monitor and the
decreased spatial variation of Pb-PM10 concentrations,
associated with both the more widespread distribution of
PM10 sources and aerodynamic characteristics of particles of
this size class which contribute to broader distribution from sources.
Accordingly, there is a lower risk of error in measuring the ambient Pb
in the PM10 size class than there is risk of error in
measuring the ambient Pb in the TSP size class using Pb TSP samplers.
We additionally noted in the proposal that, since Pb-PM10
concentrations have less spatial variability, such monitoring data may
be representative of Pb-PM10 air quality conditions over a
larger geographic area (and larger populations) than would Pb-TSP
measurements. The larger scale of representation for Pb-PM10
would mean that reported measurements of this indicator, and hence
designation outcomes, would be less sensitive to exact monitor siting
than with Pb-TSP as the indicator.
---------------------------------------------------------------------------

    \63\ Low-volume PM10 samplers are equipped with an
omni-directional (cylindrical) inlet, which reduces the effect of
wind direction, and a sharp particle separator which excludes most
of the particles greater than 10-15 microns in diameter whose
collection efficiency is most sensitive to wind speed. Also, in low-
volume samplers, the filter is protected from post-sampling
contamination.
---------------------------------------------------------------------------

    As discussed in the proposal, however, there is a different source
of error associated with the use of Pb-PM10 as the
indicator, in that larger Pb particles not captured by PM10
samplers would not be measured. As noted above, these particles
contribute to the health risks posed by air-related Pb, especially in
areas influenced by sources of ultra-coarse particles. As discussed in
the proposal, there is uncertainty as to the degree to which control
strategies put in place to meet a NAAQS with a Pb-PM10
indicator would be effective in controlling ultra-coarse Pb-containing
particles. Additionally, the fraction of Pb collected with a TSP
sampler that would not be collected by a PM10 sampler varies
depending on proximity to sources of ultra-coarse Pb particles and the
size mix of the particles they emit, as well as the sampling
variability inherent in the method discussed above. Thus, this error is
of most concern in locations in closer proximity to such sources, which
may also be locations with some of the highest ambient air levels.
    Accordingly, we stated in the proposal that it is reasonable to
consider continued use of a Pb-TSP indicator, focusing on the fact that
it specifically includes ultra-coarse Pb particles among the particles
collected, all of which are of concern and need to be addressed in
protecting public health from air-related exposures. We additionally
recognized that some State, local, or tribal monitoring agencies, or
other organizations, for the sake of the advantages noted above, and
described more fully in the proposal, may wish to deploy low-volume Pb-
PM10 samplers rather than Pb-TSP samplers. Thus, we also
considered several approaches that would allow the use of Pb-
PM10 data in conjunction with retaining Pb-TSP as the
indicator. These approaches, discussed more fully in the proposal
(sections II.E.1 and IV), include the development and use of site-
specific scaling factors and the use of default scaling factors for
particular categories of monitoring sites (e.g., source-oriented, non-
source-oriented). Additionally, we solicited comment on changing the
indicator to Pb in PM10, in recognition of the potential
benefits of such a revision discussed above.
    In their advice to the Agency during the current review, the CASAC
Pb Panel provided recommendations to the Agency on the indicator for a
revised standard in conjunction with their recommendations for
revisions to level and averaging time. As noted above in section II.B
and below in section II.C.3, the Panel recommended a significant
lowering of the level for the standard, which they noted would lead to
a requirement for additional monitoring over that currently required,
with distribution of monitors over a much larger area. In consideration
of this, prior to the proposal, the CASAC Pb Panel, as well as the
majority of the CASAC Ambient Air Monitoring and Methods (AAMM)
Subcommittee, recommended that EPA consider a change in the indicator
to PM10, utilizing low-volume PM10 sampling
(Henderson, 2007a, 2007b, 2008a, 2008b; Russell, 2008a). They found
support for their recommendation in a range of areas, as summarized in
the proposal (73 FR 29230). In advising a revision to the indicator,
CASAC also stated that they ``recognize the importance of coarse dust
contributions to total Pb ingestion and acknowledge that TSP sampling
is likely to capture additional very coarse particles which are
excluded by PM10 samplers'' (Henderson 2007b). They
suggested that an adjustment of the NAAQS level would accommodate the
loss of these ultra-coarse Pb particles, and that development of such a
quantitative adjustment might appropriately be based on concurrent Pb-
PM10 and Pb-TSP sampling data \64\ (Henderson, 2007a, 2007b,
2008a).
---------------------------------------------------------------------------

    \64\ In their advice, CASAC recognized the potential for site-
to-site variability in the relationship between Pb-TSP and Pb-
PM10 (Henderson, 2007a, 2007b). They also stated in their
September 2007 letter, ``The Panel urges that PM10
monitors, with appropriate adjustments, be used to supplement the
data. * * * A single quantitative adjustment factor could be
developed from a short period of collocated sampling at multiple
sites; or PM10 Pb/TSP Pb `equivalency ratio' could be
determined on a regional or site-specific basis''.
---------------------------------------------------------------------------

    For reasons discussed in the proposal and recognized above, and
taking into account information and assessments presented in the
Criteria Document, Staff Paper, and ANPR, the advice and
recommendations of CASAC and of members of the CASAC AAMM Subcommittee,
and public comments received prior to proposal, the Administrator
proposed to retain the current indicator of Pb-TSP, measured by the
current FRM, a current FEM, or an FEM approved under the proposed
revisions to 40 CFR part 53. The Administrator also proposed an
expansion of the measurements accepted for determining attainment or
nonattainment of the Pb NAAQS to provide an allowance for use of Pb-
PM10 data, measured by the new low-volume Pb-PM10
FRM specified in the proposed appendix Q to 40 CFR part 50 or by a FEM
approved under the proposed revisions to 40 CFR part 53, with site-
specific scaling factors. The Administrator also solicited comment on
providing States the option of using default scaling factors instead of
conducting the testing that would be needed to develop the site-
specific scaling factors. Additionally, the Administrator invited
comment on an alternative option of revising the indicator to Pb-
PM10.
b. Comments on Indicator
    In considering comments received on the proposal, EPA first notes
the advice provided by CASAC concerning the proposal in a July 2008
letter to the Administrator (Henderson, 2008b). In that advice, CASAC
repeated their prior recommendations regarding the indicator and level
of the revised standard, and emphasized that these recommendations
``were based, in part on an assumption that the level of the primary Pb
NAAQS would be `substantially' lowered to the EPA Staff-

recommended range (with an TSP indicator) of between 0.1 to 0.2 [mu]g/
m\3\ as an upper bound and 0.02 to 0.05 [mu]g/m\3\ as a lower bound
(with the added consideration that the selection be made somewhat
`conservatively' within this range to accommodate the potential loss of
ultra-coarse lead with a PM10 Pb indicator)'' (emphasis in
original) (Henderson, 2008b). They additionally noted that ``at most
population-oriented monitoring sites, levels of PM10 Pb are
essentially the same as TSP Pb, but at source-oriented monitoring sites
with high coarse mode particulate lead emissions, TSP Pb was roughly
twice as high as PM10 Pb'' and that this ``factor-of-two
difference * * * could be readily accommodated by considering a
slightly more conservative upper bound of 0.1 [mu]g/m\3\ rather than
0.2 [mu]g/m\3\ '' (Henderson, 2008b). The CASAC panel concluded that
``a transition to a PM10 indicator would be preferable, but
only at a level conservatively below an upper bound of 0.2 [mu]g/m\3\
or lower'' (Henderson, 2008b). EPA interprets this advice on the whole
to be supportive of Pb-TSP as the indicator for any standard level
greater than 0.10 [mu]g/m\3\, particularly when the level has been
selected with recognition of the inclusion of ultra-coarse particles in
Pb-TSP measurements.
    The EPA received many public comments on issues related to the
indicator for Pb. The large majority of public comments were in support
of EPA's proposal to retain Pb-TSP as the indicator for Pb. Represented
in this group were many state agencies, as well as some Tribes and
tribal environmental agencies, and local environmental agencies. Many
commenters supported Pb-TSP as the indicator regardless of a level for
the standard, variously citing evidence also cited by EPA in the
proposal notice, such as the relevance of all sizes of Pb particles to
exposures, blood Pb levels and effects and the omission of ultra-coarse
particles with PM10 samples. In support of Pb-TSP as the
indicator, a few commenters also stated that air-to-blood ratios used
in the evidence-based framework for considering a level for the
standard are generally based on Pb-TSP data. Some comments, similar to
CASAC, supported Pb-TSP as the indicator for levels above the lower end
of the proposed range (i.e., above 0.10 [mu]g/m\3\), including a level
of 0.15 [mu]g/m\3\. One commenter (NESCAUM) specifically recommended an
indicator of Pb-TSP for a NAAQS with a level of 0.15 [mu]g/m\3\,
recommending a revision to Pb-PM10 only if some other, much
lower, level (0.05 [mu]g/m\3\) was selected.
    EPA generally agrees with CASAC and the large number of public
commenters with regard to the appropriateness of a Pb-TSP indicator for
the level of the standard identified for the revised standard in
section II.C.3 below. This conclusion is supported by the current
scientific evidence, discussed above in section II.C.1.a, recognizing
the range of particle sizes inclusive of ultra-coarse particles which
contribute to Pb exposures, evidence of the presence of ultra-coarse
particles in some areas, particularly near sources, and variation in
the relationship between Pb-TSP and Pb-PM10 at such sites,
which together contribute to uncertainty about the sufficiency of
public health protection associated with a Pb-PM10 standard
at the level of 0.15 [mu]g/m\3\.
    A few commenters (including the National Association of Clean Air
Agencies) recommended transition to a Pb-PM10 indicator for
the standard at levels below 0.2 [mu]g/m\3\. These commenters stated
that low-volume PM10 samplers measure Pb much more
accurately than high-volume TSP samplers, referring to EPA's discussion
in the proposal that recognized the variability of Pb-TSP measurements
associated with wind speed and direction, and also referred to support
among CASAC AAMM members and the July 2008 comments from CASAC on
indicator. These commenters, however, did not provide rationales as to
why a Pb-PM10 indicator might be justified in light of the
health considerations identified by EPA in the proposal. Further, as
noted above, EPA interprets CASAC's July 2008 comments on the whole to
be supportive of Pb-TSP as the indicator for any standard level greater
than 0.10 [mu]g/m\3\.
    A few commenters, including both state and industry commenters,
recommended transition to Pb-PM10 without reference to a
particular level. Some of these commenters, like CASAC, noted concerns
with the high-volume TSP sampling methodology and advantages of the
PM10 monitoring method in reduced variability of the
measurements. Two industry commenters additionally suggested
consideration of an indicator based on Pb-PM2.5, stating as
their rationale that almost all airborne Pb in air is in ``the small
size fraction'', ambient sampling for PM10 and
PM2.5 size fractions is already required, and precision
which might be greater with PM10 monitors is needed for
``lower'' standards. None of this group of commenters provided a
rationale as to why a Pb-PM10 indicator might be justified
in light of the health considerations identified by EPA in the
proposal.
    EPA disagrees with this group of commenters, noting the potential
presence at some sites of particles that would not be captured by
PM10 or PM2.5 samplers yet would contribute to
human exposure to Pb and associated health effects. As discussed below,
EPA believes that, in light of the evidence of all particle sizes of Pb
contributing to blood Pb and health effects by both ingestion and
inhalation pathways, the available data on relationships between Pb-TSP
and Pb-PM10 (discussed in section II.E.1 of the proposal and
in section IV.C below) are inadequate to support development of a Pb-
PM10-based NAAQS that would provide sufficient but not more
than necessary protection of public health, with an adequate margin of
safety, across the wide variety of ambient Pb circumstances affecting
this relationship, and at the level selected by the Administrator.
Although, EPA did not consider relationships between Pb-TSP and Pb-
PM2.5 in the proposal, EPA notes the more restricted
particle size range associated with PM2.5 measurements than
with PM10 measurements, and the associated omission of
substantially more Pb that contributes to blood Pb and associated
health effects.\65\
---------------------------------------------------------------------------

    \65\ Data from collocated TSP and PM2.5 monitors are
generally presented in the Staff Paper (section 2.3.5).
---------------------------------------------------------------------------

    A number of comments were received regarding the potential use of
site-specific or default scaling factors to relate Pb-PM10
data to a Pb-TSP-based standard, with the large majority of these
comments being opposed to these options. With regard to site-specific
scaling factors, commenters note the temporal variability of the
relationship between Pb-TSP and Pb-PM10 at individual sites,
raise concerns about defensibility of attainment and nonattainment
decisions based on the use of scaling factors, and question whether
there are benefits associated with allowance of such scaling factors.
    As discussed below in section IV, EPA generally agrees with these
commenters and has not adopted a provision allowing the use of site-
specific scaling factors. A few commenters supported the use of default
scaling factors that would be developed by EPA, as an approach that
would be most easily implemented. EPA, however, concludes that the
limited available data on relationships between Pb-TSP and Pb-
PM10 are inadequate to support development of

appropriate default scaling factors as described below in section IV.
    Although commenters generally opposed the use of scaling factors
that would relate Pb-PM10 data to specific corresponding
levels of Pb-TSP for all levels of Pb-PM10 and for all
purposes related to implementation of the standard, many commenters
supported some uses of Pb-PM10 monitoring with a Pb-TSP-
based NAAQS. One example of such a use that was suggested by commenters
is at sites well below the standard and in areas without ultra-coarse
particle sources. EPA agrees with these commenters that such a limited
use of Pb-PM10 data in such areas is desirable in light of
the advantages of Pb-PM10 monitoring described in section
II.C.1.a above, and does not raise the concerns discussed above about
sufficiency of public health protection when considering ambient air Pb
concentrations that are closer to the level of the standard. Such uses
allowed by this rulemaking are recognized below in section II.C.1.c and
discussed more fully in sections IV and V below.
    Some States noted agreement with the view expressed by EPA in the
proposal that low-volume TSP sampling offers advantages over high-
volume TSP sampling (the federal reference method for Pb). Issues
regarding the sample collection method for the TSP indicator are
discussed in section V below.
c. Conclusions on Indicator
    Having carefully considered the public comments, as discussed
above, and advice and recommendations from CASAC on this issue, the
Administrator concludes that it is appropriate to retain Pb-TSP as the
indicator for the Pb NAAQS at this time. The Administrator agrees with
CASAC that use of a Pb-TSP indicator is necessary to provide sufficient
public health protection from the range of particle sizes of ambient
air Pb, including ultra-coarse particles, in conjunction with the
selected level (see section II.C.3 below). The Administrator recognizes
that Pb in all particle sizes contributes to Pb in blood and associated
health effects (as discussed in section II.E.1 of the proposal and
II.C.1.a above). The Administrator additionally notes that selection of
the standard level does not include an adjustment or accommodation for
the difference in Pb particles captured by TSP and PM10
monitors which, as discussed elsewhere (section II.E.1 of the proposal,
section II.C.1.a above, and section IV.D below) may be on the order of
a factor of two in some areas. The Administrator also recognizes the
quite limited dataset, particularly for source-oriented sites,\66\ that
is available to the Agency from which to characterize the relationship
between Pb-TSP and Pb-PM10 for purposes of identifying the
appropriate level for a Pb-PM10 based standard. Further, the
Administrator recognizes there is uncertainty with regard to whether a
Pb-PM10-based NAAQS would also effectively control ultra-
coarse Pb particles, which, as noted above, may have a greater presence
in areas near sources where Pb concentrations are highest. In light of
these considerations, the Administrator concludes that it is
appropriate to retain Pb-TSP as the indicator to protect against health
risks from ultra coarse particulate Pb emitted to ambient air.
---------------------------------------------------------------------------

    \66\ As described in the proposal (73 FR29233), collocated data
from source-oriented sites were available from just three locations
near three different types of sources and include data from as long
ago as 1988 (Schmidt and Cavender, 2008). A limited amount of
additional data has been provided in comments on the proposal.
---------------------------------------------------------------------------

    With regard to the use of scaling factors to relate Pb-
PM10 data to a Pb-TSP indicator, the Administrator concludes
that the limited available data on relationships between Pb-TSP and Pb-
PM10 are inadequate to support a use of scaling factors to
relate all valid Pb-PM10 measurements to specific levels of
Pb-TSP concentrations for all purposes of a Pb-TSP-based standard.
    The Administrator concurs with the comments from CASAC and public
commenters that recognize the potential value of providing a role for
Pb-PM10 in the monitoring required for a Pb-TSP standard.
Such comments emphasize the similarity of Pb-TSP and Pb-PM10
measurements at non-source-oriented locations, while recognizing the
potential for differences at sites near sources, and recognize the
sufficiency of public health protection when Pb-PM10 levels
are well below the level of the standard. EPA believes that use of Pb-
PM10 measurements at sites not influenced by sources of
ultra-coarse Pb and where Pb concentrations are well below the standard
would take advantage of the increased precision of these measurements
and decreased spatial variation of Pb-PM10 concentrations,
without raising the same concerns over a lack of protection against
health risks from all particulate Pb emitted to the ambient air that
support retention of Pb-TSP as the indicator. Accordingly, the
Administrator is expanding the types of measurements which may be
considered with regard to implementation of the Pb NAAQS. This
expansion, as discussed more fully in sections IV and V below, provides
a role for Pb-PM10 data under certain limited circumstances
and with certain conditions. The circumstances and conditions under
which such data are allowed, as described in sections IV and V below,
are those in which the Pb concentrations are expected to be
substantially below the standard and ultra-coarse particles are not
expected to be present.
2. Averaging Time and Form
a. Basis for Proposed Decision
    The averaging time and form of the current standard is a not-to-be-
exceeded or maximum value, averaged over a calendar quarter. The basis
for this averaging time and form reflects consideration of the evidence
available when the Pb NAAQS were promulgated in 1978. At that time, the
Agency had concluded that the level of the standard, 1.5 [mu]g/
m3, would be a ``safe ceiling for indefinite exposure of
young children'' (43 FR 46250), and that the slightly greater
possibility of elevated air Pb levels for shorter periods within the
quarterly averaging period, as contrasted to the monthly averaging
period proposed in 1977 (43 FR 63076), was not significant for health.
These conclusions were based in part on the Agency's interpretation of
the health effects evidence as indicating that 30 [mu]g/dL was the
maximum safe level of blood Pb for an individual child, and the
Agency's views that the distribution of air concentrations made it
unlikely there could be sustained periods greatly above the average
value and that the multipathway nature of Pb exposure lessened the
impact of short-term changes in air concentrations of Pb.
    In the 1990 Staff Paper, this issue was again considered in light
of the evidence available at that time. The 1990 Staff Paper concluded
that ``[a] monthly averaging period would better capture short-term
increases in lead exposure and would more fully protect children's
health than the current quarterly average'' (USEPA, 1990b). The 1990
Staff Paper further concluded that ``[t]he most appropriate form of the
standard appears to be the second highest monthly average in a 3-year
span. This form would be nearly as stringent as a form that does not
permit any exceedances and allows for discounting of one `bad' month in
3 years which may be caused, for example, by unusual meteorology.'' In
their review of the 1990 Staff Paper, the CASAC Pb Panel concurred with
the staff recommendation to express the lead NAAQS as a monthly
standard not to be exceeded more than once in three years.
    As summarized in section II.A above and discussed in detail in the
Criteria

Document, the currently available health effects evidence \67\
indicates a wider variety of neurological effects, as well as immune
system and hematological effects, associated with substantially lower
blood Pb levels in children than were recognized when the standard was
set in 1978. Further, the health effects evidence with regard to
characterization of a threshold for adverse effects has changed since
the standard was set in 1978, as have the Agency's views on the
characterization of a safe blood Pb level.\68\
---------------------------------------------------------------------------

    \67\ The differing evidence and associated strength of the
evidence for these different effects is described in the Criteria
Document.
    \68\ For example, EPA recognizes today that ``there is no level
of Pb exposure that can yet be identified, with confidence, as
clearly not being associated with some risk of deleterious health
effects'' (CD, p. 8-63).
---------------------------------------------------------------------------

    In the proposal (section II.E.2), we noted various aspects of the
current evidence that are pertinent to consideration of the averaging
time and form for the Pb standard. We noted those aspects pertaining to
the human physiological response to changes in Pb exposures and also
aspects pertaining to the response of air-related Pb exposure pathways
to changes in airborne Pb. The latter aspects are more complex for Pb
than for other criteria pollutants because the exposure pathways for
air-related Pb include both inhalation pathways and deposition-related
ingestion pathways, which is not the case for other criteria
pollutants. The persistence of Pb in multiple media and in the body
\69\ provides an additional complication in the case of Pb.
---------------------------------------------------------------------------

    \69\ Lead accumulates in the body and is only slowly removed,
with bone Pb serving as a blood PB source for years after exposure
and as a source of fetal Pb exposure during pregnancy (CD, sections
<IP_ADDRESS> and <IP_ADDRESS>).
---------------------------------------------------------------------------

    With regard to the human physiological response to changes in Pb
exposures, as summarized in the Staff Paper and discussed in more
detail in the Criteria Document, the evidence indicates that blood Pb
levels respond quickly to increased Pb exposures, such that an abrupt
increase in Pb uptake results in increased blood Pb levels.
Contributing to this response is the absorption through the lungs and
the gastrointestinal tract (which is both greater and faster in
children as compared to adults), and the rapid distribution (within
days), once absorbed, from plasma to red blood cells and throughout the
body. As noted in the proposal, while the evidence with regard to
sensitive neurological effects is limited in what it indicates
regarding the specific duration of exposures associated with effects,
it indicates both the sensitivity of the first three years of life and
a sustained sensitivity throughout the lifespan as the human central
nervous system continues to mature and be vulnerable to neurotoxicants
(CD, section <IP_ADDRESS>). In general, the evidence indicates the potential
importance of exposures on the order of months (CD, section 5.3). The
evidence also indicates increased vulnerability during some
developmental periods (e.g., prenatal), the length of which indicates a
potential importance of exposures as short as weeks to months.
    As noted in the proposal with regard to the response of human
exposure pathways to changes in airborne Pb, data from NHANES II and an
analysis of the temporal relationship between gasoline consumption and
blood Pb indicate a month lag between changes in Pb emissions from
leaded gasoline and the response of children's blood Pb levels and the
number of children with elevated blood Pb levels (EPA, 1986a, p. 11-39;
Rabinowitz and Needleman, 1983; Schwartz and Pitcher, 1989; USEPA,
1990b). As noted in the proposal with regard to consideration of air-
related Pb exposure pathways, the evidence described in the Criteria
Document and the quantitative risk assessment indicate that today
ingestion of dust can be a predominant exposure pathway for young
children to air-related Pb. Further, the proposal noted that a recent
study of dustfall near an open window in New York City indicates the
potential for a response of indoor dust Pb loading to ambient airborne
Pb on the order of weeks (Caravanos et al., 2006; CD, p. 3-28).
    In the proposal, we additionally noted that the health effects
evidence identifies varying durations in exposure that may be relevant
and important to the selection of averaging time. In light of
uncertainties in aspects such as response times of children's exposure
to airborne Pb, we recognized, as in the past, that this evidence
provides a basis for consideration of both quarterly and monthly
averaging times.
    In considering both averaging time and form in the proposal, EPA
combined the current calendar quarter averaging time with the current
not-to-be exceeded (maximum) form and also combined a monthly averaging
time with a second maximum form, so as to provide an appropriate degree
of year-to-year stability that a maximum monthly form would not
provide. We also observed in the proposal (73 FR 29235) that the second
maximum monthly form provides a roughly comparable degree of protection
on a broad national scale to the current maximum calendar quarter
averaging time and form. This observation was based on an analysis of
the 2003-2005 monitoring data set that found a roughly similar number
of areas not likely to attain alternate levels of the standard for
these two combinations of averaging time and form (although a slightly
greater number of sites would likely exceed the levels based on the
second maximum monthly average). We also noted, however, that the
relative protection provided by these two averaging times and forms may
differ from area to area. Moreover, we noted that control programs to
reduce average Pb concentrations across a calendar quarter may not have
the same protective effect as control programs aimed at reducing
average Pb concentrations on a monthly basis. Given the limited scope
of the current monitoring network, which lacks monitors near many
significant Pb sources, and uncertainty about Pb source emissions and
possible controls, the proposal noted that it is difficult to more
quantitatively compare the protectiveness of standards defined in terms
of the maximum calendar quarter average versus the second maximum
monthly average.
    In their advice to the Agency prior to the proposal, CASAC
recommended that consideration be given to changing from a calendar
quarter to a monthly averaging time (Henderson, 2007a, 2007b, 2008a).
In making that recommendation, CASAC has emphasized support from
studies that suggest that blood Pb concentrations respond at shorter
time scales than would be captured completely by a quarterly average.
With regard to form of the standard, CASAC has stated that one could
``consider having the lead standards based on the second highest
monthly average, a form that appears to correlate well with using the
maximum quarterly value'', while also indicating that ``the most
protective form would be the highest monthly average in a year''
(Henderson, 2007a). Among the public comments the Agency received on
the discussion of averaging time in the ANPR, the majority concurred
with the CASAC recommendation for a revision to a monthly averaging
time.
    On an additional point related to form, the 1990 Staff Paper and
the Staff Paper for this review both recommended that the Administrator
consider specifying that compliance with the NAAQS be evaluated over a
3-year period. As described in the proposal, a monitor would be
considered to be in violation of the NAAQS based on a 3-year period,
if, in any of the three previous calendar years with sufficiently
complete data (as

explained in detail in section IV of the proposal), the value of the
selected averaging time and form statistic (e.g., second maximum
monthly average or maximum quarterly average) exceeded the level of the
NAAQS. Thus, a monitor, initially or after once having violated the
NAAQS, would not be considered to have attained the NAAQS until three
years have passed without the level of the standard being exceeded. In
discussing the merits of this approach in the proposal, we noted that
variations in Pb source emissions and in meteorological conditions
contribute to the potential for a monitor to record an exceedance of a
particular level in one period but not in another, even if no permanent
controls have been applied to the nearby source(s). We further noted
that it would potentially reduce the public health protection afforded
by the standard if areas fluctuated in and out of nonattainment status
so frequently that States do not have opportunity and incentive to
identify sources in need of more emission control and to require those
controls to be put in place. We noted that the 3-year approach would
help ensure that areas initially found to be violating the NAAQS have
effectively controlled the contributing lead emissions before being
redesignated to attainment.
    At the time of proposal, the Administrator considered the
information summarized above (described in more detail in Criteria
Document and Staff Paper), as well as the advice from CASAC and public
comments on the ANPR. The Administrator recognized that there is
support in the evidence for an averaging time as short as monthly
consistent with the following observations: (1) The health evidence
indicates that very short exposures can lead to increases in blood Pb
levels, (2) the time period of response of indoor dust Pb to airborne
Pb can be on the order of weeks, and (3) the health evidence indicates
that adverse effects may occur with exposures during relatively short
windows of susceptibility, such as prenatally and in developing
infants.\70\ The Administrator also recognized limitations and
uncertainties in the evidence including the limited available evidence
specific to the consideration of the particular duration of sustained
airborne Pb levels having the potential to contribute to the adverse
health effects identified as most relevant to this review, as well as
variability in the response time of indoor dust Pb loading to ambient
airborne Pb.
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    \70\ The health evidence with regard to the susceptibility of
the developing fetus and infants is well documented in the evidence
as described in the 1986 Criteria Document, the 1990 Supplement
(e.g. chapter III) and the 2006 Criteria Document. For example,
``[n]eurobehavioral effects of Pb-exposure early in development
(during fetal, neonatal, and later postnatal periods) in young
infants and children <=7 years old) have been observed with
remarkable consistency across numerous studies involving varying
study designs, different developmental assessment protocols, and
diverse populations.'' (CD, p. E-9)
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    Based on these considerations and the air quality analyses
summarized above, the Administrator concluded that this information
provided support for an averaging time no longer than a calendar
quarter. Further, the Administrator recognized that if substantial
weight is given to the evidence of even shorter times for response of
key exposure pathways, blood Pb, and associated effects to airborne Pb,
a monthly averaging time may be appropriate. Accordingly, the
Administrator proposed two options with regard to the form and
averaging time for the standard, and with both he proposed that three
years be the time period evaluated in considering attainment. One
option was to retain the current not-to-be-exceeded form with an
averaging time of a calendar quarter, and the second option was to
revise the averaging time to a calendar month and the form to the
second highest monthly average.
b. Comments on Averaging Time and Form
    In considering comments on averaging time for the revised standard,
the Administrator first notes that the CASAC Pb Panel, in their
comments on the proposal, restated their previous recommendation to
reduce the averaging time from calendar quarter to monthly (Henderson,
2008b). In repeating this recommendation in their July 2008 letter,
CASAC noted that ``adverse effects could result from exposures over as
few as 30 days' duration'' (Henderson, 2008b). Many public commenters
also supported the option of a monthly averaging time, generally
placing great weight on the recommendation of CASAC. Some of these
commenters also provided additional reasons for their support for a
monthly averaging time. These reasons variously included concerns
regarding the lack of a ``safe'' blood Pb level; evidence that
children's blood Pb concentrations respond over time periods shorter
than three months; evidence for very short windows of susceptibility to
some effects during prenatal and infant development; concerns that dust
Pb responds relatively quickly to air Pb; and concerns for large near-
source temporal variability in airborne Pb concentrations and the
exposure and risk contributed by ``high'' months, which, given the
persistence of Pb, may occur for some time subsequent to the ``high''
month.
    Some other commenters supported retaining the current quarterly
averaging time stating that the proposed option of a monthly averaging
time is not well founded in the evidence. In supporting this view, the
commenters variously stated that no evidence has been presented to show
a relationship between a shorter-term air concentration and air-related
blood Pb levels contributing to neurological effects; there is little
known regarding the relationship between neurocognitive effects such as
IQ and a monthly exposure period; there is uncertainty regarding the
time over which indoor dust, a key pathway for air-related Pb, responds
to indoor air; and, the World Health Organization and European
Community air criteria or guidelines for Pb are based on a yearly
average.
    In considering advice from CASAC and comments from the public, EPA
recognizes that the evidence indicates the potential for effects
pertinent to this review to result from Pb exposures (e.g., from
ingestion and inhalation routes) on the order of one to three months,
as summarized in section II.C.2.a and described more fully in the
proposal. EPA additionally notes the greater complexity inherent in
considering the averaging time for the primary Pb standard, as compared
to other criteria pollutants, due to the persistence and multimedia
nature of Pb and its multiple pathways of human exposure. Accordingly,
in considering averaging time in this review, in addition to
considering the evidence with regard to exposure durations related to
blood Pb levels associated with neurological effects, a key
consideration for the Agency is how closely Pb exposures via the major
air-related Pb exposure pathways reflect temporal changes in ambient
air Pb concentrations, recognizing that the averaging period involves
the duration over time of ambient air concentrations, and is not a
direct measure of the duration or degree of exposure.
    With regard to exposure durations related to blood Pb levels
associated with neurocognitive effects, EPA notes that, as described in
section II.A.2.c above, the concurrent blood Pb metric (i.e., blood Pb
measured at the time of IQ test) has been found to have the strongest
association with IQ response. Further, a concurrent blood Pb
measurement is most strongly related to a child's exposure events
within the past few (e.g., one to three) months. This

is supported by multiple aspects of the evidence (e.g., CD, chapter 4;
USEPA, 1986a, chapter 11), including evidence cited by CASAC and
commenters, such as the findings of the significant contribution to
blood Pb of gasoline Pb sales in the past month (e.g., Schwartz and
Pitcher, 1989; Rabinowitz and Needleman, 1983).
    EPA also recognizes, as noted by some commenters and discussed in
the Criteria Document and summarized in the Staff Paper, ANPR and
proposal, that the evidence demonstrates sensitivity of the early years
of life and increased vulnerability of specific types of effects during
some developmental periods (e.g., prenatal) which may be shorter than a
calendar quarter. EPA notes uncertainty, however in some aspects of the
linkages between airborne Pb concentrations and these physiological
responses, including time-related aspects of the exposure pathways
contributing to such effects.
    In considering the evidence regarding how blood Pb levels respond
to changes in ambient air Pb concentrations along the multiple exposure
pathways to blood, EPA recognizes several pertinent aspects of the
evidence. First, the evidence in this area does not specify the
duration of a sustained air concentration associated with a particular
blood Pb contribution. Accordingly, we are uncertain as to the precise
duration of air concentration(s) reflected in any one air-to-blood
ratio and the ways in which an air-to-blood ratio may vary with the
duration of the air Pb concentration. However, as discussed in section
II.C.2.a above, the evidence supports the importance of time periods on
the order of three months or less, and as discussed below, in light of
the prominent role of deposition-related pathways today, EPA concludes
the evidence most strongly supports a time period of approximately
three months.
    Given the varying complexities of the multiple air-related exposure
pathways summarized in section II.A.1 above, exposure durations
pertinent for each pathway may be expected to vary. The most immediate
and direct exposure pathway is the inhalation pathway, while the
ingestion pathways are more indirect and to varying degrees (across the
range of pathways) less immediate. For example, as mentioned above,
when leaded gasoline was a predominant source of air-related exposure
for people in the U.S., the evidence indicates that blood Pb levels
were strongly associated with average sales of leaded gasoline during
the previous month (e.g., Schwartz and Pitcher, 1989). We note that
exposures to the generally fine particles produced by combustion of
leaded gasoline, which remain suspended in the atmosphere for many days
(USEPA, 1986a, p. 5-10), provide a greater role for inhalation pathways
(e.g., as compared to deposition-related ingestion pathways, such as
indoor dust ingestion) than would exposures to generally larger Pb
particles (which tend to more readily deposit). Further, as recognized
in the Staff Paper and the proposal, air-related ingestion pathways are
necessarily slower to respond to changes in air concentrations than the
immediate and direct pathway of inhalation. The ingestion pathways are
affected by a variety of factors that play a lesser, if any, role in
inhalation exposure. For example, human behavior (e.g., activity,
cleaning practices and frequency) and other building characteristics
(e.g., number of windows, presence of screens, air conditioning) would
be expected to modulate the response of indoor dust to changes in
ambient air Pb (Caravanos et al., 2006; CD, p. 3-28).
    As noted previously, the evidence and the results of the
quantitative risk assessment indicate a greater role for ingestion
pathways than inhalation pathways in contributing to the air-related
exposures of children today. Accordingly, the relatively greater focus
today (than at the time of leaded gasoline usage) on deposition-related
pathways of exposure to air-related Pb such as indoor dust ingestion
would tend to support consideration of an averaging time longer than a
month. We additionally note results from dust Pb modeling analyses
performed as part of the quantitative risk assessment. These results
provide an estimate of approximately four months as the time over which
an increase in air Pb will reach 90% of the final steady-state change
in dust Pb (USEPA, 2007b, section G.3.2.2). Additionally, we note that
multiple studies have observed blood Pb levels to exhibit seasonal
patterns, perhaps related to seasonality in exposure variables (e.g.,
Rabinowitz et al., 1985).
    Some commenters who supported a monthly averaging time cited
concern for the potential for the occurrence of single month average
air Pb concentration, within a quarter that met the standard, to be
substantially above the level of the standard. For example, one
commenter suggested that a monthly averaging time would be more likely
to capture exceedances related to periodic activities (such as
industrial activity, construction or demolition). Another commenter
submitted examples of such temporal variability in ambient air
concentrations at specific monitoring sites, one of which indicated a
quarter in which the current standard of 1.5 [mu]g/m3 was
met, while a single month within that quarter was some 30% percent
higher (2.07 [mu]g/m3). In considering this example, we
consider the likelihood of differing blood Pb responses between
children in two different situations: one in which the 3-month average
Pb concentration just met the level of the standard but a single month
within the quarter was 30% higher than that level (with the other two
months below the standard level), and the other in which each of three
consecutive monthly average Pb concentrations just met the level of the
standard. The current evidence is limited with regard to the
consideration of this issue. Given the range of air-related blood Pb
exposure pathways and the processes involved in their relationships
with airborne Pb (e.g., the response of indoor dust Pb to ambient air
Pb), it is highly uncertain, based on the evidence available today,
whether there would be appreciable differences in blood Pb levels
between the children in these two scenarios as a result of these
different 3-month periods. That is, in this example, we consider it
unlikely that a single relatively higher month of air Pb followed by
two months of relatively lower air Pb would translate into a similar
single high month of blood Pb followed by two months of relatively low
blood Pb. Rather, it is expected that the high month would tend to be
modulated into a more extended and less pronounced month-to-month
change in blood Pb levels.
    In considering this issue, however, we recognize that greater
month-to-month variability in air concentrations than that described by
this example is possible, and as such variability increases, it becomes
more likely that a month's air Pb concentration might result in a more
pronounced impact on blood Pb concentrations.
    Another example offered by the commenter described more extreme
month-to-month variability in a quarter in which the current standard
was met. This example indicated a monthly average that was more than 3
times the average for the quarter. The allowance for this seemingly
implausible occurrence results from the current calculation method for
the current quarterly average standard. The current method takes an
average across all valid measurements in a quarter, without according
equal weight to each month's measurements. In situations where a
significantly different number of measurements occur in each month of
the quarter, the current method can have the effect of giving greater
weight

to multiple measurements occurring over a relatively short period. In
the specific example cited by the commenter, the few very high
measurements in a single month were outweighed by a much larger number
of lower measurements occurring in each of the other two months of the
quarter, thus biasing the resulting quarterly average. EPA agrees with
the commenter that the allowance of such significant month-to-month
variability within a 3-month period is inappropriate and may not
provide appropriate protection of public health. In consideration of
this issue, the Agency has identified changes to the method used to
derive the 3-month average that would yield an average that is more
representative of air quality over the 3-month period and lessen the
likelihood and frequency of occurrence of cases where such extremely
high months would be allowed in a 3-month averaging period that met the
standard. More specifically, as discussed below in section IV, the
Agency considers it appropriate to average the measurements within each
month prior to deriving the 3-month average as a way to avoid the
allowance of such large monthly variability as noted by the commenter.
    In considering comments specifically on the current use of a block
calendar quarter average, the Administrator first notes that the CASAC
Pb Panel, in their comments on the proposal, stated that ``there is no
logic for averaging only by `calendar' quarter as there is nothing
unique about effects that may occur exclusively during the four
calendar seasons'' and that a `` `rolling' three-month (or 90-day)
average would be more logical than a `calendar' quarter'' (Henderson,
2008b). Comments from a state environmental agency also recommended use
of a 3-month rolling average, rather than the current block calendar
quarter average.
    EPA agrees with CASAC as to the stronger basis for a ``rolling'' 3-
month average as compared to a block calendar quarter. A 3-month
average not constrained to calendar quarters would consider each of the
twelve 3-month periods associated with a given year, not just the four
calendar years within that year. We agree with CASAC that the averaging
time of calendar quarter inappropriately separates air concentrations
occurring in months such as March and April that span two calendar
quarters. For example, under the calendar quarter approach, two
consecutive ``high'' months that occur in different calendar quarters
(e.g., March and April) may be mitigated by ``low'' months in those
calendar quarters (i.e., January and February for March, May and June
for April). Thus, the same air quality data could cause an exceedance
of the calendar quarter standard if it occurred in February and March
but could meet the calendar quarter standard if it occurred in March
and April. EPA believes there is no evidence-based justification for