\35\ For further discussion of the air quality scenarios and
averaging times included in the risk assessment, see section 2.3.1
of the Risk Assessment Report (USEPA, 2007b)
---------------------------------------------------------------------------

    Current conditions for the three location-specific urban case
studies in terms of maximum quarterly average air Pb concentrations are
0.09, 0.14 and 0.36 [mu]g/m\3\ for the study areas in Los Angeles,
Chicago and Cleveland, respectively. In terms of maximum monthly
average the values are 0.17 [mu]g/m\3\, 0.31 [mu]g/m\3\ and 0.56 [mu]g/
m\3\ for the study areas in Los Angeles, Chicago and Cleveland,
respectively. Two current conditions scenarios were considered for the
general urban case study: One based on the mean value for ambient air
Pb levels in large urban areas (0.14 [mu]g/m\3\ as a maximum quarterly
average) and a high-end ambient air Pb level in large urban areas (0.87
[mu]g/m\3\ as a maximum quarterly average).
    Details of the assessment scenarios, including a description of the
derivation of Pb concentrations for air and other media are presented
in Sections 2.3 (and subsections) and Section 5.1.1 of the Risk
Assessment Report (USEPA, 2007b).
e. Categorization of Policy-Relevant Exposure Pathways
    To inform policy aspects of the Pb NAAQS review, the assessment
estimates for blood Pb and IQ loss were divided into two components:
The fraction associated with policy-relevant pathways, which include
inhalation, outdoor soil/dust ingestion and indoor dust ingestion, and
the fraction associated with background (e.g., diet and drinking
water). The policy-relevant pathways are further divided into two
categories, ``recent air'' and ``past air''. Conceptually, the recent
air category includes those pathways involving Pb that is or has
recently been in the outdoor ambient air, including inhalation and
ingestion of indoor dust Pb derived from recent ambient air (i.e.,

air Pb that has penetrated into the residence recently and loaded
indoor dust). Past air includes exposure contributions from ingestion
of outdoor soil/dust that is contacted on surfaces outdoors, and
ingestion of indoor dust Pb that is derived from past air sources
(i.e., impacts from Pb that was in the ambient air in the past and has
not been recently resuspended into ambient air). In this assessment, as
discussed further below, that portion of indoor dust Pb not associated
with recent air, is classified as ``other'' and, due to technical
limitations includes not only past air impacts, but also contributions
from indoor Pb paint. In the risk assessment, estimates of contribution
to blood Pb and IQ loss were developed for the following pathways or
pathway combinations:
     Inhalation of ambient air Pb (i.e., ``recent air'' Pb):
This is derived using the blood Pb estimate resulting from Pb exposure
limited to the inhalation pathway (and will include exposures to Pb in
ambient air from all sources contributing to the ambient air
concentration estimate).
     Ingestion of ``recent air'' indoor dust Pb: This is
derived using the blood Pb estimate resulting from Pb exposure limited
to ingestion of the Pb in indoor dust that is predicted to be
associated with ambient air concentrations (i.e., via the air
concentration coefficient in the regression-based dust models or via
the mechanistic component of the hybrid blood Pb model (see Section
3.1.4 of the Risk Assessment Report). For the primary Pb smelter case
study, estimates for this pathway are not separated from estimates for
the pathway described in the subsequent bullet due to uncertainty
regarding this categorization with the model used for this case study
(Section <IP_ADDRESS> of the Risk Assessment Report).
     Ingestion of ``other'' indoor dust Pb: This is derived
using the blood Pb estimate resulting from Pb exposure limited to
ingestion of the Pb in indoor dust that is not predicted to be
associated with ambient air concentrations (i.e., that predicted by the
intercept in the dust models plus that predicted by the outdoor soil
concentration coefficient, for models that include an intercept
(Section 3.1.4 of the Risk Assessment Report)). This is interpreted to
represent indoor paint, outdoor soil/dust, and additional sources of Pb
to indoor dust including historical air (see Risk Assessment Report,
Section 2.4.3). As the intercept in regression dust models will be
inclusive of error associated with the model coefficients, this
category also includes some representation of dust Pb associated with
current ambient air concentrations (described in previous bullet). For
the primary Pb smelter case study, estimates for this pathway are not
separated from estimates for the pathway described above due to
uncertainty regarding this categorization with the model used for this
case study (Risk Assessment Report, Section <IP_ADDRESS>). This pathway is
included in the ``past air'' category.
     Ingestion of outdoor soil/dust Pb: This is derived using
the blood Pb estimate resulting from Pb exposure limited to ingestion
of outdoor soil/dust Pb. This pathway is included in the ``past air''
category (and could include contamination from historic Pb emissions
from automobiles and Pb paint).
     Ingestion of drinking water Pb: This is derived using the
blood Pb estimate resulting from Pb exposure limited to ingestion of
drinking water Pb. This pathway is included in the policy-relevant
background category.
     Ingestion of dietary Pb: This is derived using the blood
Pb estimate resulting from Pb exposure limited to ingestion of dietary
Pb. This pathway is included in the policy-relevant background
category.
    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),
modeling for the assessment has only affected the exposure pathways
categorized as recent air (inhalation and ingestion of that portion of
indoor dust associated with outdoor ambient air). The assessment has
not simulated decreases in past air-related 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). 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.
    Additionally, there is uncertainty related to parsing out exposure
and risk between background and policy-relevant exposure pathways (and
subsequent parsing of recent air and past air) resulting from a number
of technical limitations. Key among these is that, while conceptually,
indoor Pb paint contributions to indoor dust Pb would be considered
background and included in modeling background exposures, due to
technical limitations related to indoor dust Pb modeling, ultimately,
Pb paint was included as part of ``other'' indoor dust Pb (i.e., as
part of past air exposure). The inclusion of indoor lead Pb as a
component of ``other'' indoor air (and consequently as a component of
``past air'' exposure) represents a source of potential high bias in
our prediction of total exposure and risk associated with past air
because conceptually, exposure to indoor paint Pb is considered part of
background exposure.
    In summary, because of limitations in the assessment design, data
and modeling tools, the risk attributable to policy-relevant exposure
pathways is 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.
f. Analytical Steps
    The risk assessment includes four analytical steps, briefly
described below and presented in detail in Sections 2.4.4, 3.1, 3.2,
4.1, and 5.1 of the Risk Assessment Report (USEPA, 2007b).
     Characterization of Pb in ambient air: The
characterization of outdoor ambient air Pb levels uses different
approaches depending on the case study (as explained in more detail
below): (a) Source-oriented and non-source oriented monitors are
assumed to represent different exposure zones in the city-specific case
studies, (b) a single exposure level is assumed, based on monitoring
data for various cities, for the general urban case study, and (c)
ambient levels are estimated using air dispersion modeling based on Pb
emissions from a particular facility in the point source case studies.
     Characterization of outdoor soil/dust and indoor dust Pb
concentrations: Outdoor soil Pb levels are estimated using empirical
data and/or fate and transport modeling. Indoor dust Pb levels are
predicted using a combination of (a) regression-based models that
relate indoor dust to ambient air Pb and/or outdoor soil Pb, and (b)
mechanistic models.\36\
---------------------------------------------------------------------------

    \36\ Additional detail on the methods used in characterizing Pb
concentrations in outdoor soil and indoor dust are presented in
Sections 3.1.3 and 3.1.4 of the Risk Assessment, respectively. Data,
methods and assumptions here used in characterizing Pb
concentrations in these exposure media may differ from those in
other analyses that serve different purposes.
---------------------------------------------------------------------------

     Characterization of blood Pb levels: Blood Pb levels for
each exposure zone are derived from central-tendency blood Pb
concentrations estimated using the

Integrated Exposure and Uptake Biokinetic (IEUBK) model, and concurrent
or lifetime average blood Pb is estimated from these outputs as
described in Section <IP_ADDRESS> of the Risk Assessment Report (USEPA,
2007b). For the point source and location-specific urban case studies,
a probabilistic exposure model is used to generate population
distributions of blood Pb concentrations based on: (a) The central
tendency blood Pb levels for each exposure zone, (b) demographic data
for the distribution of children (less than 7 years of age) across
exposure zones in a study area, and (c) a geometric standard deviation
(GSD) intended to characterize interindividual variability in blood Pb
(e.g., reflecting differences in behavior and biokinetics related to
Pb). For the general urban case study, as demographic data for a
specific location are not considered, the GSD is applied directly to
the central tendency blood Pb level to estimate a population
distribution of blood Pb levels. Additional detail on the methods used
to model population blood Pb levels is presented in Sections 3.2.2 and
<IP_ADDRESS> of the Risk Assessment Report (USEPA, 2007b).
     Risk characterization (estimating IQ loss): Concurrent or
lifetime average blood Pb estimates for each simulated child in each
case study population are converted into total Pb-related IQ loss
estimates using the concentration-response functions described above.
    Key limitations and uncertainties associated with the application
of these specific analytical steps are summarized in Section III.B.2.g
below.
g. Generating Multiple Sets of Risk Results
    In the initial analyses for the full-scale assessment (USEPA,
2007a), EPA implemented multiple modeling approaches for each case
study scenario in an effort to characterize the potential impact on
exposure and risk estimates of uncertainty associated with the
limitations in the tools, data and methods available for this risk
assessment and with key analytical steps in the modeling approach.
These multiple modeling approaches are described in Section <IP_ADDRESS> of
the final Risk Assessment Report (USEPA, 2007b). In consideration of
comments provided by CASAC (Henderson, 2007b) on these analyses
regarding which modeling approach they felt had greater scientific
support, a pared down set of modeling combinations was identified as
the core approach for the subsequent analyses. This core modeling
approach includes the following key elements:
     Ambient air Pb estimates (based on monitors or modeling
and proportional rollbacks, as described below),
     Background exposure from food and water (as described
above),
     The hybrid indoor dust model specifically developed for
urban residential applications (which predicts Pb in indoor dust as a
function of ambient air Pb and nonair contribution),
     The IEUBK blood Pb model (which predicts blood Pb in young
children exposed to Pb from multiple exposure pathways),
     The concurrent blood Pb metric,
     A GSD for concurrent blood Pb of 2.1 to characterize
interindividual variability in blood Pb levels for a given ambient
level, and
     four different functions relating concurrent blood Pb to
IQ loss, including two log-linear models (one with a cutpoint and one
with low-exposure linearization) and two dual-linear models with
stratification, one stratified at 7.5 [mu]g/dL peak blood Pb and the
other at 10 [mu]g/dL peak blood Pb.
    For each case study, the core modeling approach employs a single
set of modeling elements to estimate exposure and the four different
concentration-response functions referenced above to derive four sets
of risk results from the single set of exposure estimates. The spread
of estimates resulting from application of all four functions captures
much of the uncertainty associated model choice in this analytical
step. Among these four functions, greater confidence is associated with
estimates derived using the log-linear with low-exposure linearization
concentration-response function as discussed above.
    In addition to employing multiple concentration-response functions,
the assessment includes various sensitivity analyses to characterize
the potential impact of uncertainty in other key analysis steps on
exposure and risk estimates. The sensitivity analyses and uncertainty
characterization completed for the risk analysis are described in
Sections 3.5, 4.3, 5.2.5 and 5.3.3 of the Risk Assessment Report
(USEPA, 2007b).
h. Key Limitations and Uncertainties
    As recognized above, EPA has made simplifying assumptions in
several areas of this assessment due to the limited data, models, and
time available. These assumptions and related limitations and
uncertainties are described in the Risk Assessment Report (USEPA,
2007b). Key assumptions, limitations and uncertainties are briefly
identified below. EPA considers these aspects of the assessment to be
important to the interpretation of the exposure and risk estimates. In
the presentation below, limitations (and associated uncertainty) are
listed, beginning with those regarding design of the assessment or case
studies, followed by those regarding estimation of Pb concentrations in
ambient air indoor dust, outdoor soil/dust, and blood, and lastly
regarding estimation of Pb-related IQ Loss.
     Temporal aspects: Exposure for the simulated child
population begins at birth (including a prenatal maternal contribution)
and continues for 7 years, with Pb concentrations in all exposure media
remaining constant throughout the period, and children residing in the
same exposure zone throughout the period. In characterizing exposure
media concentrations, annual averages are derived and held constant
through the seven year period. Exposure factors and physiological
parameters vary with age of the cohort through the seven year exposure
period, several exposure factors and physiological parameters are
varied on an annual basis within the blood Pb modeling step. These
aspects are a simplification of population exposures that contributes
some uncertainty to our exposure and risk estimates.
     General urban case study: This case study differs from the
others in several ways. It is by definition a general case study and
not based on a specific location. There is a single exposure zone for
the case study within which all media concentrations of Pb are assumed
to be spatially uniform; that is, no spatial variation within the area
is simulated. Additionally, the case study does not rely on any
specific demographic values. Within the single exposure zone a
theoretical population of unspecified size is assumed to be uniformly
distributed. Thus this case study is a simplified representation of
urban areas intended to inform our assessment of the impact of changes
in ambient Pb concentrations on risk, but which carries with it
attendant uncertainties in our interpretation of the associated
exposure and risk estimates. For example, the risk estimates for this
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 urban population. Specific urban
populations are spatially distributed in a nonuniform pattern and
experience ambient air Pb levels that vary through time and space.
Consequently, interpretations of the associated blood Pb and risk
estimates with regard to their relevance to specific urban residential
exposures carry

substantial uncertainty and presumably an upward bias in risk,
particularly for large areas, across which air concentrations may vary
substantially.
     Point source case studies: Dispersion modeling was used to
characterize ambient air Pb levels in the point source case studies.
This approach simulates spatial gradients related to dispersion and
deposition of Pb from emitting sources. The details of this modeling is
presented in the Risk Assessment Report (USEPA, 2007b). In the case of
the point sources modeled, sources were limited to those associated
with the smelter operations, and did not include other sources such as
resuspension of roadside Pb not related to facility operations, and
other stationary sources of Pb within or near the study area. This
means that, with distance from the facility, there is likely
underestimation of ambient air-related Pb exposure because with
increased distance from the facility there would be increasing
influence of other sources relative to that of the facility. This
limitation is likely to have more significant impact on risk estimates
associated with the full study than on those for the subareas (which
are the portions of the study area with 1.5 km from the smelter
facilities), and to perhaps have a more significant impact on risk
estimates associated with the smaller secondary Pb smelter (see below).
As noted above, in their review of the July draft risk assessment
report, the CASAC Pb Panel made several recommendations for additional
exposure and health risk analyses (Henderson, 2007b), including a
recommendation that the general urban case study be augmented by the
inclusion of risk analyses in specific urban areas of the U.S. In this
regard, they specifically stated the following (Henderson, 2007b, p.
3):

The CASAC strongly believes that it is important that EPA staff make
estimates of exposure that will have national implications for, and
relevance to, urban areas; and that, significantly, the case studies
of both primary lead (Pb) smelter sites as well as secondary smelter
sites, while relevant to a few atypical locations, do not meet the
needs of supporting a Lead NAAQS. The Agency should also undertake
case studies of several urban areas with varying lead exposure
concentrations, based on the prototypic urban risk assessment that
OAQPS produced in the 2nd Draft Lead Human Exposure and Health Risk
Assessments.

     Secondary Pb smelter case study: Air Pb concentration
estimates derived from the air dispersion modeling completed for the
secondary Pb smelter case study are subject to appreciably greater
uncertainty than that for those for the primary Pb smelter case study
due to a number of factors, including: (a) A more limited and less
detailed accounting of emissions and emissions sources associated with
the facility (particularly fugitive emissions), (b) a lack of prior air
quality modeling analyses and performance analyses, and (c) a
substantially smaller number of Pb-TSP monitors in the area that could
be used to evaluate and provide confidence in model performance.\37\
Further, as mentioned in the previous bullet, no air sources of Pb
other than those associated with the facility were accounted for in the
modeling. Given the relatively smaller magnitude of emissions from the
secondary Pb smelter, the underestimating potential of this limitation
with regard to air concentrations with distance from the facility has a
greater relative impact on risk estimates for this case study than for
the primary Pb smelter case study. The aggregate uncertainty of all of
these factors results in low confidence in estimates for this case
study. It is observed that exposure and risk estimates are lower than
those for the other case studies. Although this case study was
initially intended to be used as an example of areas near stationary
sources of intermediate size (smaller than the primary Pb smelter),
experience with this analysis indicates that substantially more data
and multiple case studies differing in several aspects would be needed
to broadly characterize risks for such a category of Pb exposure
scenarios.
---------------------------------------------------------------------------

    \37\ The information supporting the air dispersion modeling for
the primary Pb smelter case study provides substantially greater
confidence in estimates for that case study.
---------------------------------------------------------------------------

     Location-specific urban case studies: The Pb-TSP
monitoring network is currently quite limited. The number of monitors
available to represent air concentrations in these case studies ranged
from six for Cleveland to 11 for Chicago. Accordingly, our estimates of
the magnitude of and spatial variation of air Pb concentrations are
subject to uncertainty associated with the limited data. In applying
the available data to each of these case studies, exposure zones, one
corresponding to each monitor, were created and U.S. Census block
groups (and the children within those demographic units) were
distributed among the exposure zones. The details of the approach used
are described in Section 5.1.3 of the Risk Assessment Report (USEPA,
2007b). Although this approach provides a spatial gradient across the
study area due to differences in monitor values for each exposure zone,
this approach assumes a constant concentration within each exposure
zone (i.e., no spatial gradient within a zone). Additionally, the
nearest neighbor approach to assign block groups to exposure zones
assumes that a monitor adequately represents all locations that are
closer to that monitor than to any of the others in the study area. In
reality, across block groups there are more variable spatial gradients
in a study area than those reflected in the approach used here. This
introduces significant uncertainty into the characterization of risk
for the urban case studies. As recognized in Section, III.B.2.a, the
analyses for these case studies were developed in response to CASAC
recommendations on the July 2007 draft Risk Assessment (Henderson,
2007b) and there has not been review of the completed analyses by
CASAC.
     Current NAAQS air quality scenarios: For the location-
specific urban case studies, proportional roll-up procedures were used
to adjust ambient air Pb concentrations up to just meet the current
NAAQS (see Sections 2.3.1 and <IP_ADDRESS> of the Risk Assessment Report,
USEPA, 2007b, for detailed discussion). EPA recognizes that it is
extremely unlikely that Pb concentrations in urban areas would rise to
meet the current NAAQS and that there is substantial uncertainty with
our simulation of such conditions. In these case studies a proportional
roll-up was simulated, such that it is assumed that the current spatial
distribution of air concentrations (as characterized by the current
data) is maintained and increased Pb emissions contribute to increased
Pb concentrations, the highest of which just meets the current
standard. There are many other types of changes within a study area
that could result in a similar outcome such as increases in emissions
from just one specific industrial operation that could lead to air
concentrations in a part of the study area that just meet the current
NAAQS, while the remainder of the study area remained largely unchanged
(at current conditions). For the primary Pb smelter case study, where
current conditions exceed the current NAAQS, attainment of the current
NAAQS was simulated using air quality modeling, emissions and source
parameters used in developing the 2007 proposed revision to the State
Implementation Plan for the area (see Section <IP_ADDRESS> of the Risk
Assessment Report (USEPA, 2007b)).
     Alternative NAAQS air quality scenarios: In all case
studies, proportional roll-down procedures were used to adjust ambient
air Pb concentrations downward to attain

alternative NAAQS (see Sections 2.3.1 and <IP_ADDRESS> of the Risk
Assessment Report, USEPA, 2007b). There is significant uncertainty in
simulating conditions associated with the implementation of emissions
reduction actions to meet a lower standard. There are a variety of
changes other than that represented by a proportional roll-down that
could result in air concentrations that just meet lower alternative
standards. For example, control measures might be targeted only at the
specific area exceeding the standard, resulting in a reduction of air
Pb concentrations to the alternate standard while concentrations in the
rest of the study area remain unchanged (at current conditions).
Consequently, there is significant uncertainty associated with
estimates for the alternate NAAQS scenarios.
     Estimates of outdoor soil/dust Pb concentrations: Outdoor
soil Pb concentration for both the urban case studies and the primary
Pb smelter case study are based on empirical data (see Section 3.1.3 of
the Risk Assessment). To the extent that the underlying sampling data
included areas containing older structures, the impact of Pb paint
weathered from older structures on soil Pb levels will be reflected in
these empirical estimates. In the case of the urban case studies, a
mean value from a sample of houses built between 1940 and 1998 was used
to represent soil Pb levels (see Section <IP_ADDRESS> of the Risk
Assessment). Outdoor soil/dust Pb concentrations in all air quality
scenarios have been set equal to the values for the current conditions
scenarios. An impact of changes in air Pb concentrations on soil
concentrations, and the associated impact on dust concentrations, blood
Pb and risk estimates were not simulated. In areas where air
concentrations have been greater in the past, however, implementation
of a reduced NAAQS might be expected to yield reduced soil Pb levels
over the long term. As described in Section 2.3.3 of the Risk
Assessment Report (USEPA, 2007b), however, there is potentially
significant uncertainty associated with this specification,
particularly with regard to implications for areas in which a Pb source
may locate where one of comparable size had not been previously.
Additionally, it is possible that control measures implemented to meet
alternative NAAQS may result in changes to soil Pb concentrations;
these are not reflected in the assessment.
     Estimates of indoor dust Pb concentrations for the urban
case studies (application of the hybrid model): The hybrid mechanistic-
empirical model for estimating indoor dust Pb for the urban case
studies (see Section <IP_ADDRESS> of the Risk Assessment Report, USEPA,
2007b) has several sources of uncertainty that could significantly
impact its estimates. These include: (a) Failure to consider house-to-
house variability in factors related to infiltration of outdoor ambient
air Pb indoors and subsequent buildup on indoor surfaces, (b)
limitations in data available on the rates and efficiency of indoor
dust cleaning and removal, (c) limitations in the method for converting
model estimates of dust Pb loading to dust Pb concentration needed for
blood Pb modeling, and (d) the approach employed to partition estimates
of dust Pb concentration into ``recent air'' and ``other'' components
(see Section <IP_ADDRESS> of the Risk Assessment Report, USEPA, 2007b).
These last two sources of uncertainty reduce our confidence in
estimates of apportionment of dust Pb between ``recent air'' and
``other''. In recognition of this limitation, in evaluating exposure
and risk reduction trends related to reducing ambient air Pb levels,
focus has been placed on changes in total blood Pb rather than on
estimates of ``recent air'' blood Pb.
     Estimates of indoor dust Pb concentrations for the primary
Pb smelter case study (application of the site-specific regression
model): There is uncertainty associated with the site-specific
regression model applied in the remediation zone (see Section <IP_ADDRESS>
of the Risk Assessment Report), and relatively greater uncertainty
associated with its application to air quality scenarios that simulate
notably lower air Pb levels. Limitations in the dataset from which the
model was derived limited its form to that of a simple regression that
predicts dust Pb concentration as a function of air Pb concentration
plus a constant (intercept). However there may be variables in addition
to air that influence dust Pb concentrations and their absence in the
regression contributes uncertainty to the resulting estimates. To the
extent that these unaccounted-for variables are spatially related to
the smelter facility Pb sources, our estimates could be biased, not
with regard to the absolute dust Pb concentration, but with regard to
differences in dust Pb concentration estimate between different air
quality scenarios. Those differences may be overestimated because of
potential overestimation of the air coefficient and underestimation of
the intercept in the regression model. Examples of such unaccounted-for
variables are roadside dust Pb and historical contributions to current
levels of indoor dust Pb (e.g., Pb that entered a house in the past and
continues to contribute to current dust Pb levels).
     Characterizing interindividual variability using a GSD:
There is uncertainty associated with the GSD specified for each case
study (see Sections 3.2.3 and <IP_ADDRESS> of the Risk Assessment Report).
Two factors are described here as contributors to that uncertainty.
Interindividual variability in blood Pb levels for any study population
(as described by the GSD) will reflect, to a certain extent, spatial
variation in media concentrations, including outdoor ambient air Pb
levels and indoor dust Pb levels. For each case study, there is
significant uncertainty in the specification of spatial variability in
ambient air Pb levels and associated indoor dust Pb levels, as noted
above. In addition, there are a limited number of datasets for
different types of residential child populations from which a GSD can
be derived (e.g., NHANES datasets \38\ for more heterogeneous
populations and individual study datasets for likely more homogeneous
populations near specific industrial Pb sources). This uncertainty
associated with the GSDs introduces significant uncertainty in exposure
and risk estimates for the 95th population percentile.
---------------------------------------------------------------------------

    \38\ For example, the GSD for the urban case studies, in the
risk assessment described in this notice, was derived using NHANES
data for the years 1999-2000.
---------------------------------------------------------------------------

     Exposure pathway apportionment for higher percentile blood
Pb level and IQ loss estimates: Apportionment of blood Pb levels for
higher population percentiles is assumed to be the same as that
estimated using the central tendency estimate of blood Pb in an
exposure zone. This introduces significant uncertainty into projections
of pathway apportionment for higher population percentiles of blood Pb
and IQ loss. In reality, pathway apportionment may differ in higher
exposure percentiles. For example, paint and/or drinking water
exposures may increase in importance, with air-related contributions
decreasing as an overall percentage of blood Pb levels and associated
risk. Because of this uncertainty related to pathway apportionment, as
mentioned earlier, greater confidence is placed in estimates of total
Pb exposure and risk in evaluating the impact of the current NAAQS and
alternative NAAQS relative to current conditions.
     Relating blood Pb levels to IQ loss: 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). As discussed earlier,
there are limitations in the datasets and concentration-response
analyses available for characterizing the concentration-response
relationship at these lower blood Pb levels. For example, the pooled
international dataset analyzed by Lanphear and others (2005) includes
relatively few children with blood Pb levels below 5 [mu]g/dL and no
children with levels below 1 [mu]g/dL. In recognition of the
uncertainty in specifying a quantitative concentration-response
relationship at such levels, our core modeling approach involves the
application of four different functions to generate a range of risk
estimates (see Section 4.2.6 and Section 5.3.1 of the Risk Assessment
Report, USEPA, 2007b). The difference in absolute IQ loss estimates for
the four concentration-response functions for a given case study/air
quality scenario combination is typically close to a factor of 3.
Estimates of differences in IQ loss between air quality scenarios (in
terms of percent), however, are more similar across the four functions,
although the function producing higher overall risk estimates (the dual
linear function, stratified at 7.5 [mu]g/dL, peak blood Pb) also
produces larger absolute reductions in IQ loss compared with the other
three functions.
3. Summary of Results
    This section presents blood Pb and IQ loss estimates generated in
the exposure and risk assessments. Blood Pb estimates are presented
first, followed by IQ loss estimates.
a. Blood Pb Estimates
    This section presents blood Pb estimates for the median (Table 1)
and 95th (Table 2) population percentiles.\39\ Each table presents
estimates of blood Pb levels resulting from total Pb exposure across
all pathways (policy relevant and background), as well as estimates of
the percent contribution from ``recent air'' and ``recent plus past
air'' exposure categories. As noted in Sections 4.2.4 of the Staff
Paper and Section 3.4 of the Risk Assessment Report, given the various
limitations of our modeling tools, the contribution to blood Pb levels
from air-related exposure pathways and current levels of Pb emitted to
the air (including via resuspension) are likely to fall between
contributions attributed to ``recent air'' and those attributed to
``recent plus past air''. Key uncertainties regarding partitioning dust
Pb into ``recent air'' and ``other'' categories are summarized above
(and in Section 4.2.7 of the Staff Paper). The ``recent air'' component
of indoor dust Pb is the projected level associated with outdoor
ambient air Pb levels, with outdoor ambient air potentially including
resuspended, previously deposited Pb which may reflect the resuspension
of historic levels of Pb from gasoline and from exterior house and
building Pb paint. In presenting the 95th population percentile
estimates, it is recognized that 5 percent of the child study
population at each case study are estimated to have blood Pb levels
above these estimates. Due to technical limitations, however, we
believe that it is not possible at this point to reasonably predict the
distribution of blood Pb levels for that top 5 percent. Observations
regarding the blood Pb results presented in Tables 1 and 2 are
presented in Section 4.3 of the Staff Paper.
---------------------------------------------------------------------------

    \39\ Blood Pb level estimates for current conditions for these
cases studies differ from the national values associated with
NHANES. For example, median blood Pb levels presented in Table 1 for
the current conditions scenario for the urban case studies are
somewhat larger than the national median from the NHANES data for
2003-2004. Specifically, values for the three location-specific
urban case studies range from 1.7 to 1.8 [mu]g/dL with the general
urban case study having a value of 1.9 [mu]g/dL (current-conditions
mean) (see Table 1), while the median value from NHANES (2003-2004)
is 1.6 [mu]g/dL (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm
). NHANES values for the 95th percentile were

not available for 2003-2004, precluding a comparison of modeled
estimates presented in Table 2 against NHANES data. We note,
however, that the 95th percentile value in 2001-2002 was 5.8 [mu]g/
dL (see footnote 7). However, NHANES values for the 90th percentile
(for 2003-2004) were identified and these values can be compared
against 90th percentile estimates generated for the urban case
studies (see Risk Assessment Report, Appendix O, Section O.3.2 for
the location-specific urban case study and Appendix N, Section
N.2.1.2 for the general urban case study). The 90th percentile blood
Pb levels for the current conditions scenario, for the three
location-specific urban case studies range from 4.5 to 4.6 [mu]g/dL,
while the estimate for the general urban case study is 5.0 [mu]g/dL.
These 90th percentile values for the case study populations are
larger than the 90th percentile value of 3.9 [mu]g/dL reported by
NHANES for all children in 2003-2004. It is noted that ambient air
levels reflected in the urban case studies are likely to differ from
those underlying the NHANES data.

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    \40\ As noted in footnote 39, median blood Pb levels generated
for the three location-specific urban case studies and the general
urban case study for the current conditions scenario are somewhat
larger than the median value from NHANES for 2003-2004.
    \41\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
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    \42\ As noted in footnote 39, 90th percentile blood Pb levels
generated for the three location specific urban case studies and the
general urban case study for the current conditions scenario are
larger than the 90th percentile value from NHANES for 2003-2004.
Note, 95th percentile values were not available for the NHANES 2003-
2004 dataset, preventing a direct comparison to modeled estimates
presented in Table 2. However, in 2001-2002, the 95th percentile
value was 5.8 [mu]g/dL (see footnote 7).
    \43\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.

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b. IQ Loss Estimates
    This section presents IQ loss estimates in Tables 3 through 6.
These IQ loss estimates need to be understood in the context of the
broader and more comprehensive and detailed presentation provided Risk
Assessment Report (USEPA, 2007b). The tables presented here include
three types of risk estimates:
     Estimates of IQ loss for all air quality scenarios (based
on total Pb exposure): Tables 3 and 4 present IQ loss estimates for
total Pb exposure for each of the air quality scenarios simulated for
each case study. Table 3 presents estimates for the population median
and Table 4 presents results for the 95th population percentile. These
results included both median and 95th population percentile estimates.
To reflect the variation in estimates derived from the four different
concentration-response functions included in the analysis, three
categories of estimates are considered including (a) IQ loss estimates
generated using the low concentration-response function (the model that
generated the lowest IQ loss estimates), (b) estimates generated using
the log-linear with low-exposure linearization (LLL) model, and (c) IQ
loss estimates generated using the high concentration-response function
(the model that generated the highest IQ loss estimates). For reasons
described above, estimates generated using the LLL model have been
given emphasis in the summary below.
     Estimates of IQ loss under the current NAAQS air quality
scenario (with pathway apportionment): Tables 5 and 6 present IQ loss
estimates for total Pb exposure based on simulation of just meeting the
current NAAQS for the case studies to which the core modeling approach
was applied. Specifically, Table 5 presents estimates of the total Pb-
related IQ loss for the population median and Table 6 presents
estimates for the 95th population percentile. Both of these tables
present total IQ loss estimates for (a) total Pb exposure (including
both policy-relevant pathways and background sources) and (b) policy-
relevant exposures alone (bounded by estimates for ``recent air'' and
for ``recent plus past air'').
     IQ loss incidence estimates for the three location-
specific urban case studies: Estimates of the number of children for
each location-specific urban case study projected to have total Pb-
related IQ loss greater than one point are summarized in Table 7, and
similar estimates for IQ loss greater than 7 points are summarized in
Table 8. Also presented are the changes in incidence of the current
NAAQS and alternative NAAQS scenarios compared to current conditions,
with emphasis placed on estimates generated using the LLL
concentration-response function. Estimates are presented for each of
the four concentration-response functions used in the core analysis.
The complete set of incidence results is presented in Risk Assessment
Report Appendix O, Section O.3.4.
    The IQ loss results presented in Tables 3 through 8 need to be
understood in the context of the broader and more comprehensive and
detailed presentation provided in the Risk Assessment Report.
Observations regarding the IQ loss results presented in Tables 3
through 8 are presented in Section 4.4 of the Staff Paper.
    It is important to point out that the range of absolute IQ loss
estimates generated using the four models for a given case study and
air quality scenario is typically around a factor of three. However,
the relative (proportional) change in IQ loss across air quality
scenarios (i.e., the pattern of IQ loss reduction across air quality
scenarios for the same case study) is fairly consistent across all four
models. This suggests uncertainty in estimates of absolute IQ loss for
a median or 95th percentile child with exposures related to a given
ambient air Pb level. Accordingly, we have greater confidence in
predicting incremental changes in IQ loss across air quality scenarios
and this is reflected in the observations presented in Section 4.4 of
the Staff Paper. As with the blood Pb estimates, 5 percent of the child
study population at each case study location is estimated to have IQ
loss above the 95th percentile estimates presented here, however, due
to technical limitations of our modeling tools, it is not feasible at
this point to reasonably predict the distribution of IQ loss levels for
that top 5 percent.
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    \44\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.

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

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    \45\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
[GRAPHIC] [TIFF OMITTED] TP17DE07.003

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    \46\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
    \47\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
[GRAPHIC] [TIFF OMITTED] TP17DE07.004

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    \48\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
[GRAPHIC] [TIFF OMITTED] TP17DE07.005

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    \49\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
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C. Considerations in Review of the Standard

    This section presents an integrative synthesis of information in
the Criteria Document together with EPA analyses and evaluations. EPA
notes that the final decision on retaining or revising the current
primary Pb standard is a public health policy judgment to be made by
the Administrator. The Administrator's final decision will draw upon
scientific information and analyses about health effects, population
exposure and risks, as well as judgments about the appropriate response
to the range of uncertainties that are inherent in the scientific
evidence and analyses. These judgments will be informed by a
recognition that the available health effects evidence generally
reflects a continuum consisting of ambient levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain.
    This approach is consistent with the requirements of the NAAQS
provisions of the Act and with how EPA and the courts have historically
interpreted the Act. These provisions require the Administrator to
establish primary standards that, in the Administrator's judgment, are
requisite to protect public health with an adequate margin of safety.
In so doing, the Administrator seeks to establish standards that are
neither more nor less stringent than necessary for this purpose. The
Act does not require that primary standards be set at a zero-risk level
but rather at a level that avoids unacceptable risks to public health,
including the health of sensitive groups.
    The following discussion starts with background information on the
current standard (section III.C.1), including both the basis for
derivation of the current standard and considerations and conclusions
from the 1990 Staff Paper (USEPA, 1990b). This is followed by a summary
of the general approach for this current review (section III.C.2).
Considerations with regard to the adequacy of the current standard are
discussed in section III.C.3, with evidence and exposure-risk-based
considerations in subsections III.C.3.a and b, respectively, followed
by a summary of CASAC advice and recommendations (section III.C.3.c)
and, lastly, solicitation of comment on the broad range of policy
options (section III.C.3.d). Considerations with regard to elements of
alternative standards--indicator, averaging time and form, and level--
are discussed in sections III.C.4.a., III.C.4.b, and III.C.4.c,
respectively. The discussion with regard to level includes subsections
on evidence and exposure-risk-based considerations (sections III.C.4.a
and b), followed by a summary of CASAC advice and recommendations
(section III.C.4.c) and, lastly, solicitation of comment on the broad
range of policy options (section III.C.4.d).
1. Background on the Current Standard
a. Basis for Setting the Current Standard
    The current primary standard is set at a level of 1.5 [mu]g/
m3, measured as Pb-TSP, not to be exceeded by the maximum
arithmetic mean concentration averaged over a calendar quarter. The
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 basis for
selecting each of the elements of the standard is described below.
i. Level
    EPA's objective in selecting the level of the current standard was
``to estimate the concentration of Pb in the air to which all groups
within the general population can be exposed for protracted periods
without an unacceptable risk to health'' (43 FR 46252). Consistent with
section 109 of the Clean Air Act, the Agency selected a level for the
current standard that was below the concentration that was at that time
identified as a threshold for adverse health effects (i.e., 40 [mu]g/dl
blood Pb), so as to provide an adequate margin of safety. As stated in
the notice of final rulemaking, ``This estimate was based on EPA's
judgment in four key areas:
    (1) Determining the ``sensitive population'' as that group within
the general population which has the lowest threshold for adverse
effects or greatest potential for exposure. EPA concludes that young
children, aged 1 to 5, are the sensitive population.
    (2) Determining the safe level of total lead exposure for the
sensitive population, indicated by the concentration of lead in the
blood. EPA concludes that the maximum safe level of blood lead for an
individual child is 30 [mu]g Pb/dl and that population blood lead,
measured as the geometric mean, must be 15 [mu]g Pb/dl in order to
place 99.5 percent of children in the United States below 30 [mu]g Pb/
dl.
    (3) Attributing the contribution to blood lead from nonair
pollution sources. EPA concludes that 12 [mu]g Pb/dl of population
blood lead for children should be attributed to nonair exposure.
    (4) Determining the air lead level which is consistent with
maintaining the mean population blood lead level at 15 [mu]g Pb/dl [the
maximum safe level]. Taking into account exposure from other sources
(12 [mu]g Pb/dl), EPA has designed the standard to limit air
contribution after achieving the standard to 3 [mu]g Pb/dl. On the
basis of an estimated relationship of air lead to blood lead of 1 to 2,
EPA concludes that the ambient air standard should be 1.5 [mu]g Pb/
m3.'' (43 FR 46252)
    EPA's judgments in these key areas, as well as margin of safety
considerations, are discussed below.
    The assessment of the science that was presented in the 1977
Criteria Document (USEPA, 1977), indicated young children, aged 1 to 5,
as the population group at particular risk from Pb exposure. Children
were recognized to have a greater physiological sensitivity than adults
to the effects of Pb and a greater exposure. In identifying young
children as the sensitive population, EPA also recognized the
occurrence of subgroups with enhanced risk due to genetic factors,
dietary deficiencies or residence in urban areas. Yet information was
not available to estimate a threshold for adverse effects for these
subgroups separate from that of all young children. Additionally, EPA
recognized both a concern regarding potential risk to pregnant women
and fetuses, and a lack of information to establish that these
subgroups are more at risk than young children. Accordingly, young
children, aged 1 to 5, were identified as the group which has the
lowest threshold for adverse effects of greatest potential for exposure
(i.e., the sensitive population) (43 FR 46252).
    In identifying the maximum safe exposure, EPA relied upon the
measurement of Pb in blood (43 FR 46252-46253). The physiological
effect of Pb that had been identified as occurring at the lowest blood
Pb level was inhibition of an enzyme integral to the pathway by which
heme (the oxygen carrying protein of human blood) is synthesized, i.e.,
delta-aminolevulinic acid dehydratase ([delta]-ALAD). The 1977 Criteria
Document reported a threshold for inhibition of this enzyme in children
at 10 [mu]g Pb/dL. The 1977 Criteria Document also reported a threshold
of 15-20 [mu]g/dL for elevation of protoporphyrin (EP), which is an
indication of some disruption of the heme synthesis pathway. EPA
concluded that this effect on the heme synthesis pathway (indicated by
EP) was potentially adverse. EPA further described a range of blood
levels associated with a progression in

detrimental impact on the heme synthesis pathway. At the low end of the
range (15-20 [mu]g/dL), the initial detection of EP associated with
blood Pb was not concluded to be associated with a significant risk to
health. The upper end of the range (40 [mu]g/dL), the threshold
associated with clear evidence of heme synthesis impairment and other
effects contributing to clinical symptoms of anemia, was regarded as
clearly adverse to health. EPA also recognized the existence of
thresholds for additional adverse effects (e.g., nervous system
deficits) occurring for some children at just slightly higher blood Pb
levels (e.g., 50 [mu]g/dL). Additionally, EPA stated that the maximum
safe blood level should not be higher than the blood Pb level
recognized by the CDC as ``elevated'' (and indicative of the need for
intervention). In 1978, that level was 30 [mu]g/dL. \50\
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    \50\ The CDC subsequently revised their advisory level for
children's blood Pb to 25 [mu]g/dL in 1985, and to 10 [mu]g/dL 1991.
In 2005, with consideration of a review of the evidence by their
advisory committee, CDC revised their statement on Preventing Lead
Poisoning in Young Children, specifically recognizing the evidence
of adverse health effects in children with blood Pb levels below 10
[mu]g/dL and the data demonstrating that no ``safe'' threshold for
blood Pb in children had been identified, and emphasizing the
importance of preventative measures (CDC, 2005a). Recently, CDC's
Advisory Committee on Childhood Lead Poisoning Prevention noted the
2005 CDC statements and reported on a review of the clinical
interpretation and management of blood Pb levels below 10 [mu]g/dL
(ACCLPP, 2007). More details on this level are provided in Section
III.A.1.
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    Having identified the maximum safe blood level in individual
children, EPA next made a public health policy judgment regarding the
target mean blood level for the U.S. population of young children (43
FR 46252-46253). With this judgment, EPA identified a target of 99.5
percent of this population to be brought below the maximum safe blood
Pb level. This judgment was based on consideration of the size of the
sensitive subpopulation, and the recognition that there are special
high-risk groups of children within the general population. The
population statistics available at the time (the 1970 U.S. Census)
indicated a total of 20 million children younger than 5 years of age,
with 15 million residing in urban areas and 5 million in center cities
where Pb exposure was thought likely to be ``high''. Concern about
these high-risk groups influenced EPA's determination of 99.5 percent,
deterring EPA from selecting a population percentage lower than 99.5
(43 FR 46253). EPA then used standard statistical techniques to
calculate the population mean blood Pb level that would place 99.5
percent of the population below the maximum safe level. Based on the
then available data, EPA concluded that blood Pb levels in the
population of U.S. children were normally distributed with a GSD of
1.3. Based on standard statistical techniques, EPA determined that a
thus described population in which 99.5 percent of the population has
blood Pb levels below 30 [mu]g/dL would have a geometric mean blood
level of 15 [mu]g/dL. EPA described 15 [mu]g/dL as ``the maximum safe
blood lead level (geometric mean) for a population of young children''
(43 FR 46247).
    When setting the current NAAQS, EPA recognized that the air
standard needed to take into account the contribution to blood Pb
levels from Pb sources unrelated to air pollution. Consequently, the
calculation of the current NAAQS included the subtraction of Pb
contributed to blood Pb from nonair sources from the estimate of a safe
mean population blood Pb level. Without this subtraction, EPA
recognized that the combined exposure to Pb from air and nonair sources
would result in a blood Pb concentration exceeding the safe level (43
FR 46253). In developing an estimate of this nonair contribution, EPA
recognized the lack of detailed or widespread information about the
relative contribution of various sources to children's blood Pb levels,
such that an estimate could only be made by inference from other
empirical or theoretical studies, often involving adults. Additionally,
EPA recognized the expectation that the contribution to blood Pb levels
from nonair sources would vary widely, was probably not in constant
proportion to air Pb contribution, and in some cases may alone exceed
the target mean population blood Pb level (43 FR 46253-46254). The
amount of blood Pb attributed to nonair sources was selected based
primarily on findings in studies of blood Pb levels in areas where air
Pb levels were low relative to other locations in U.S. The air Pb
levels in these areas ranged from 0.1 to 0.7 [mu]g/m\3\. The average of
the reported blood Pb levels for children of various ages in these
areas was on the order of 12 [mu]g/dL. Thus, 12 [mu]g/dL was identified
as the nonair contribution, and subtracted from the population mean
target level of 15 [mu]g/dL to yield a value of 3 [mu]g/dL as the limit
on the air contribution to blood Pb.
    In determining the air Pb level consistent with an air contribution
of 3 [mu]g Pb/dL, EPA reviewed studies assessed in the 1977 Criteria
Document that reported changes in blood Pb with different air Pb
levels. These studies included a study of children exposed to Pb from a
primary Pb smelter, controlled exposures of adult men to Pb in fine
particulate matter, and a personal exposure study involving several
male cohorts exposed to Pb in a large urban area in the early 1970s (43
FR 46254). Using all three studies, EPA calculated an average slope or
ratio over the entire range of data. That value was 1.95 (rounded to 2
[mu]g/dL blood Pb concentration to 1 [mu]g/m\3\ air Pb concentration),
and is recognized to fall within the range of values reported in the
1977 Criteria Document. On the basis of this 2-to-1 relationship, EPA
concluded that the ambient air standard should be 1.5 [mu]g Pb/m\3\ (43
FR 46254).
    In consideration of the appropriate margin of safety during the
development of the current NAAQS, EPA identified the following factors:
(1) The 1977 Criteria Document reported multiple biological effects of
Pb in practically all cell types, tissues and organ systems, of which
the significance for health had not yet been fully studied; (2) no
beneficial effects of Pb at then current environmental levels were
recognized; (3) data were incomplete as to the extent to which children
are indirectly exposed to air Pb that has moved to other environmental
media, such as water, soil and dirt, and food; (4) Pb is chemically
persistent and with continued uncontrolled emissions would continue to
accumulate in human tissue and the environment; and (5) the possibility
that exposure associated with blood Pb levels previously considered
safe might influence neurological development and learning abilities of
the young child (43 FR 46255). Recognizing that estimating an
appropriate margin of safety for the air Pb standard was complicated by
the multiple sources and media involved in Pb exposure, EPA chose to
use margin of safety considerations principally in establishing a
maximum safe blood Pb level for individual children (30 [mu]g Pb/dL)
and in determining the percentage of children to be placed below this
maximum level (about 99.5 percent). Additionally, in establishing other
factors used in calculating the standard, EPA used margin of safety
considerations in the sense of making careful judgment based on
available data, but these judgments were not considered to be at the
precautionary extreme of the range of data available at the time (43 FR
46251).
    EPA further recognized that, because of the variability between
individuals in a population experiencing a given level of Pb exposure,
it was considered impossible to provide the same margin

of safety for all members in the sensitive population or to define the
margin of safety in the standard as a simple percentage. EPA believed
that the factors it used in designing the standards provided an
adequate margin of safety for a large proportion of the sensitive
population. The Agency did not believe that the margin was excessively
large or on the other hand that the air standard could protect everyone
from elevated blood Pb levels (43 FR 46251).
ii. Averaging Time, Form, and Indicator
    The averaging time for the current standard is a calendar quarter.
In the decision for this aspect of the standard, the Agency also
considered a monthly averaging period, but concluded that ``a
requirement for the averaging of air quality data over calendar quarter
will improve the validity of air quality data gathered without a
significant reduction in the protectiveness of the standards.'' As
described in the notice for this decision (43 FR 46250), this
conclusion was based on several points, including the following:
     An analysis of ambient measurements available at the time
indicated that the distribution of air Pb levels was such that there
was little possibility that there could be sustained periods greatly
above the average value in situations where the quarterly standard was
achieved.
     A recognition that the monitoring network may not actually
represent the exposure situation for young children, such that it
seemed likely that elevated air Pb levels when occurring would be close
to Pb air pollution sources where young children would typically not
encounter them for the full 24-hour period reported by the monitor.
     Medical evidence available at the time indicated that
blood Pb levels re-equilibrate slowly to changes in air exposure, a
finding that would serve to dampen the impact of short-term period of
exposure to elevated air Pb.
     Direct exposure to air is only one of several routes of
total exposure, thus lessening the impact of a change in air Pb on
blood Pb levels.
    The statistical form of the current standard is as a not-to-be-
exceeded or maximum value. EPA set the standard as a ceiling value with
the conclusion that this air level would be safe for indefinite
exposure for young children (43 FR 46250).
    The indicator is total airborne Pb collected by a high volume
sampler (43 FR 46258). EPA's selection of Pb-TSP as the indicator for
the standard was based on explicit recognition both of the significance
of ingestion as an exposure pathway for Pb that had deposited from the
air and of the potential for Pb deposited from the air to become re-
suspended in respirable size particles in the air and available for
human inhalation exposure. As stated in the final rule, ``a significant
component of exposure can be ingestion of materials contaminated by
deposition of lead from the air,'' and that, ``in addition to the
indirect route of ingestion and absorption from the gastrointestinal
tract, non-respirable Pb in the environment may, at some point become
respirable through weathering or mechanical action'' (43 FR 46251).
b. Policy Options Considered in the Last Review
    During the 1980s, EPA initiated a review of the air quality
criteria and NAAQS for Pb. CASAC and the public were fully involved in
this review, which led to the publication of a criteria document with
associated addendum and a supplement (USEPA, 1986a, 1986b, 1990a), an
exposure analysis methods document (USEPA, 1989) and a staff paper
(USEPA, 1990b).
    Total emissions to air were estimated to have dropped by 94 percent
between 1978 and 1987, with the vast majority of it attributed to the
reduction of Pb in gasoline. Accordingly, the focus of the last review
was on areas near stationary sources of Pb emissions. Although such
sources were not considered to have made a significant contribution (as
compared to Pb in gasoline) to the overall Pb pollution across large,
urban or regional areas, Pb emissions from such sources were considered
to have the potential for a significant impact on a local scale. Air Pb
concentrations, and especially soil and dust Pb concentrations had been
associated with elevated levels of Pb absorption in children and adults
in numerous Pb point source community studies. Exceedances of the
current NAAQS were found at that time only in the vicinity of
nonferrous smelters or other point sources of Pb.
    In summarizing and interpreting the health evidence presented in
the 1986 Criteria Document and associated documents, the 1990 Staff
Paper described the collective impact on children of the effects at
blood Pb levels above 15 [mu]g/dL as representing a clear pattern of
adverse effects worthy of avoiding. This is in contrast to EPA's
identification of 30 [mu]g/dL as a safe blood Pb level for individual
children when the NAAQS was set in 1978. The Staff Paper further stated
that at blood Pb levels of 10-15 [mu]g/dL, there was a convergence of
evidence of Pb-induced interference with a diverse set of physiological
functions and processes, particularly evident in several independent
studies showing impaired neurobehavioral function and development.
Further, the available data did not indicate a clear threshold in this
blood Pb range. Rather, it suggested a continuum of health risks down
to the lowest levels measured.\51\
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    \51\ In 1991, the CDC reduced their advisory level for
children's blood Pb from 25 [mu]g/dL to 10 [mu]g/dL.
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    For the purposes of comparing the relative protectiveness of
alternative Pb NAAQS, the staff conducted analyses to estimate the
percentages of children with blood Pb levels above 10 [mu]g/dL and
above 15 [mu]g/dL for several air quality scenarios developed for a
small set of stationary source exposure case studies. The results of
the analyses of child populations living near two Pb smelters indicated
that substantial reductions in Pb exposure could be achieved through
just meeting the current Pb NAAQS. According to the best estimate
analyses, over 99.5% of children living in areas significantly affected
by the smelters would have blood Pb levels below 15 [mu]g/dL if the
current standard was achieved. Progressive changes in this number were
estimated for the alternative monthly Pb NAAQS levels evaluated in
those analyses, which ranged from 1.5 [mu]g/m\3\ to 0.5 [mu]g/m\3\.
    In light of the health effects evidence available at the time, the
1990 Staff Paper presented air quality, exposure, and risk analyses,
and other policy considerations, as well as the following staff
conclusions with regard to the primary Pb NAAQS (USEPA, 1990b, pp. xii
to xiv):
    (1) ``The range of standards * * * should be from 0.5 to 1.5 [mu]g/
m\3\.''
    (2) ``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.''
    (3) ``The most appropriate form of the standard appears to be the
second highest monthly averages {sic{time}  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.''
    (4) ``With a revision to a monthly averaging time more frequent
sampling is needed, except in areas, like roadways remote from lead
point sources, where the standard is not expected to be violated. In
those situations, the current 1-in-6 day sampling schedule would
sufficiently reflect air quality and trends.''

    (5) ``Because exposure to atmospheric lead particles occurs not
only via direct inhalation, but via ingestion of deposited particles as
well, especially among young children, the hi-volume sampler provides a
reasonable indicator for determining compliance with a monthly standard
and should be retained as the instrument to monitor compliance with the
lead NAAQS until more refined instruments can be developed.''
    Based on its review of a draft Staff Paper, which contained the
above recommendations, the CASAC strongly recommended to the
Administrator that EPA should actively pursue a public health goal of
minimizing the Pb content of blood to the extent possible, and that the
Pb NAAQS is an important component of a multimedia strategy for
achieving that goal (CASAC, 1990, p. 4). In noting the range of levels
recommended by staff, CASAC recommended consideration of a revised
standard that incorporates a ``wide margin of safety, because of the
risk posed by Pb exposures, particularly to the very young whose
developing nervous system may be compromised by even low level
exposures'' (id., p. 3). More specifically, CASAC judged that a
standard within the range of 1.0 to 1.5 [mu]g/m\3\ would have
``relatively little, if any, margin of safety;'' that greater
consideration should be given to a standard set below 1.0 [mu]g/m\3\;
and, to provide perspective in setting the standard, it would be
appropriate to consider the distribution of blood Pb levels associated
with meeting a monthly standard of 0.25 [mu]g/m\3\, a level below the
range considered by staff (id.).
    After consideration of the documents developed during the review,
EPA chose not to propose revision of the NAAQS for Pb. During the same
time period, the Agency published and embarked on the implementation of
a broad, multi-program, multi-media, integrated national strategy to
reduce Pb exposures (USEPA, 1991). As part of implementing this
integrated Pb strategy, the Agency focused efforts primarily on
regulatory and remedial clean-up actions aimed at reducing Pb exposures
from a variety of non-air sources judged to pose more extensive public
health risks to U.S. populations, as well as on actions to reduce Pb
emissions to air, particularly near stationary sources. This focus
reflected in part the dramatic reduction of Pb in gasoline that
occurred since the standard was set in 1978, which resulted in orders-
of-magnitude reductions in airborne emissions of Pb, and a significant
shift in the types of sources with the greatest Pb emissions. EPA
established standards for Pb-based paint hazards and Pb dust cleanup
levels in most pre-1978 housing and child-occupied facilities.
Additionally, EPA has developed standards for the management of Pb in
solid and hazardous waste, oversees the cleanup of Pb contamination at
Superfund sites, and has issued regulations to reduce Pb in drinking
water (http://www.epa.gov/lead/regulation.htm). Beyond these specific

regulatory actions, the Agency's Lead Awareness Program has continued
to work to protect human health and the environment against the dangers
of Pb by conducting research and designing educational outreach
activities and materials (http://www.epa.gov/lead/). Actions to reduce

Pb emissions to air during the 1990s included enforcement of the NAAQS,
as well as the promulgation of regulations under Section 112 of the
Clean Air Act, including national emissions standards for hazardous air
pollutants at primary and secondary Pb smelters, as well as other Pb
sources.
2. Approach for Current Review
    To evaluate whether it is appropriate to consider retaining the
current primary Pb standard, or whether consideration of revisions is
appropriate, EPA is considering an approach in this review like that
used in the Staff Paper. As discussed below, this approach builds upon
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.
    This approach 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.
    In conducting this assessment, EPA is aware of the dramatic
reductions in air Pb emissions in the U.S. in recent decades.\52\ In
addition to the dramatic reduction of Pb in gasoline, an additional
circumstance that has changed since the standard was set is the
enactment of the Clean Air Act Amendments of 1990, which amended Clean
Air Act Section 112 to list Pb compounds as hazardous air pollutants
(HAP) and to require technology-based and risk-based standards, as
appropriate, for major stationary sources of HAP.\53\ EPA is also aware
that these significantly changed circumstances have raised the question
in this review of whether it is still appropriate to maintain a NAAQS
for Pb or to retain Pb on the list of criteria pollutants. As a result,
this evaluation will consider the status of Pb as a criteria pollutant
and assesses whether revocation of the standard is an appropriate
option for the Administrator to consider.
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    \52\ Detailed information on air Pb emissions, and temporal
trends in emissions since 1980 is provided in Section 2.2 of the
Staff Paper.
    \53\ The use of Pb paint in new houses has declined
substantially over the 20\th\ century. For example ``an estimated
68% of U.S. homes built before 1940 have Pb hazards, as do 43% of
those built during 1940-1959 and 8% of those built during 1960-
1977'' (ACCLPP, 2007). We are uncertain of the implications of these
reductions for ambient air.
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    As discussed below, in conducting this evaluation, EPA will take
into account both evidence-based and quantitative exposure- and risk-
based considerations. To the extent that the available information
suggests that revision of the current standard may be appropriate to
consider, EPA will also evaluate the currently available information to
determine the extent to which it supports consideration of a revised
standard. In this evaluation, EPA will consider the specific elements
of the standard to identify options (in terms of an indicator,
averaging time, level, and form) for consideration in making public
health policy judgments, based on the currently available information,
as to the degree of protection that is requisite to protect public
health with an adequate margin of safety.
    To help inform the Agency's consideration of the quantitative
exposure and risk assessments, summarized above in section III.B, EPA
solicits comment on the appropriate weight to be placed on the results
from these assessments in evaluating the adequacy of the current
primary standard and in considering alternative standards.
Specifically, we solicit comment on a number of aspects of the design
of the assessments and interpretation of the assessment results,
including in particular: (1) The appropriateness of rolling up ambient
Pb concentrations to simulate just meeting the current standard for
areas in which current concentrations are well below the level of the
current standard; \54\ (2) the use of a proportional

method to roll-up and roll-down Pb concentrations to simulate just
meeting the current and alternative standards; \55\ (3) the
categorization and apportionment of policy-relevant exposure pathways
and policy-relevant background, particularly with regard to exposures
related to historically deposited Pb from leaded gasoline and from Pb
paint; and (4) the weight to be given to risk estimates derived using
various concentration-response functions. More broadly, we also solicit
comment on the approach of considering exposures and risks resulting
from the ingestion of historically emitted Pb that may now be present
in indoor dust and outdoor soil (e.g., that associated with past use of
Pb in gasoline or Pb paint) impacted by ambient air Pb as being policy-
relevant for the purpose of setting a NAAQS.
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    \54\ We have not in the past used such an approach in developing
risk assessments for other NAAQS reviews since other risk
assessments (i.e., for ozone and PM) included a number of areas that
did not meet the current NAAQS such that rolling up ambient
pollutant concentrations was not needed to characterize risks
associated with just meeting the current standard.
    \55\ There are other methods that might be used.
---------------------------------------------------------------------------

3. Adequacy of the Current Standard
    In considering the adequacy of the current standard, EPA will first
consider whether it is appropriate to maintain a NAAQS for Pb or to
retain Pb on the list of criteria pollutants. As noted above, this
question has arisen in this review as a result of the dramatic
alteration in the basic patterns of air Pb emissions in the U.S. since
the standard was set, that primarily reflects the dramatic reduction of
Pb in gasoline, which resulted in orders-of-magnitude reductions in
airborne emissions of Pb and a significant shift in the types of
sources with the greatest Pb emissions. In addition, Section 112 of the
Clean Air Act was amended in 1990 to include Pb compounds on the list
of HAP and to require EPA to establish technology-based emission
standards for those listed major source categories emitting Pb
compounds, and to establish risk-based standards, as appropriate, for
those categories of sources.
    EPA notes that CASAC specifically examined several scientific
issues and related public health (and public welfare) policy issues
that the CASAC Lead Review Panel \56\ judged to be essential in
determining whether delisting Pb or revoking the Pb NAAQS would be
appropriate options for the Administrator to consider. In its letter to
the Administrator of March 27, 2007, based on its review of the first
draft Staff Paper (Henderson, 2007a; Attachment A of the Staff Paper),
CASAC's examination of these issues was framed by the following series
of questions:
---------------------------------------------------------------------------

    \56\ This Lead Panel includes the statutorily defined seven-
member CASAC and additional subject-matter experts needed to provide
an appropriate breadth of expertise for this review of the Pb NAAQS.
---------------------------------------------------------------------------

    (1) Does new scientific information accumulated since EPA's
promulgation of the current primary Lead NAAQS of 1.5 [mu]g/m\3\ in
1978 suggest that science previously overstated the toxicity of lead?
    (2) Have past regulatory and other controls on lead decreased PbB
[blood lead] concentrations in human populations so far below levels of
concern as to suggest there is now an adequate margin of safety
inherent in those PbB levels?
    (3) Have the activities that produced emissions and atmospheric
redistribution of lead in the past changed to such an extent that
society can have confidence that emissions will remain low even in the
absence of NAAQS controls?
    (4) Are airborne concentrations and amounts of lead sufficiently
low throughout the United States that future regulation of lead
exposures can be effectively accomplished by regulation of lead-based
products and allowable amounts of lead in soil and/or water?
    (5) If lead were de-listed as a criteria air pollutant, would it be
appropriately regulated under the Agency's Hazardous Air Pollutants
(HAP) program?
    For the reasons presented in its March 2007 letter, the CASAC Lead
Review Panel judged that the answer to each of these questions was
``no,'' leading the Panel to conclude that ``the existing state of
science is consistent with continuing to list ambient lead as a
criteria pollutant for which fully-protection NAAQS are required'' (id,
p. 5). Further, in a subsequent letter to the Administrator of
September 27, 2007, based on its review of the second draft Risk
Assessment Report (Henderson, 2007b; Attachment B of the Staff Paper),
CASAC strongly reiterated its opposition to any considered delisting of
Pb, and expressed its unanimous support for maintaining fully-
protective NAAQS (id., p. 2). The EPA seeks comment and supporting
information on the issue of whether it would be appropriate for EPA to
determine that emissions of Pb no longer contribute to air pollution
that may reasonably be anticipated to endanger public heath. EPA also
solicits comment and supporting information on the extent to which
reductions in the ambient air Pb standard would benefit public health.
    In considering the adequacy of the current standard, EPA will
consider the available evidence and quantitative exposure- and risk-
based information, summarized below.
a. Evidence-Based Considerations
    In considering the broad array of health effects evidence assessed
in the Criteria Document with respect to the adequacy of the current
standard, EPA will focus on those health endpoints associated with the
Pb exposure and blood levels most pertinent to ambient exposures.
Additionally, we will give particular weight to evidence available
today that differs from that available at the time the standard was set
with regard to its support of the current standard.
    First, with regard to the sensitive population, the susceptibility
of young children to the effects of Pb is well recognized, in addition
to more recent recognition of effects of chronic exposure to low level
Pb with advancing age (CD, Sections 5.3.7 and pp. 8-73 to 8-75). The
prenatal period and early childhood are periods of increased
susceptibility to Pb exposures, with evidence of adverse effects on the
developing nervous system that generally appear to persist into later
childhood and adolescence (CD, Section 6.2).\57\ Thus, while the
sensitivity of the elderly and other particular subgroups is
recognized, as at the time the standard was set, young children
continue to be recognized as the key sensitive population for Pb
exposures.
---------------------------------------------------------------------------

    \57\ For example, the following statement is made in the
Criteria Document ``Negative Pb impacts on neurocognitive ability
and other neurobehavioral outcomes are robust in most recent studies
even after adjustment for numerous potentially confounding factors
(including quality of care giving, parental intelligence, and
socioeconomic status). These effects generally appear to persist
into adolescence and young adulthood.'' (CD, p.E-9)
---------------------------------------------------------------------------

    With regard to the exposure levels at which adverse health effects
occur, 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
change in the evidence since the time the standard was set is reflected
in changes made by the CDC in their advisory level for Pb in children's
blood, and changes they have made in their characterization of that
level. Although CDC recognized a level of 30 [mu]g/dL blood Pb as
warranting individual intervention in 1978 when the Pb NAAQS was set,
in 2005 they recognized the evidence of adverse health effects in
children with blood Pb levels below 10 [mu]g/dL and the data
demonstrating that no ``safe'' threshold for blood Pb had been
identified (CDC, 1991; CDC, 2005).
    The Criteria Document describes current evidence regarding the
occurrence of a variety of adverse health

effects, including those on the developing nervous system, associated
with blood Pb levels extending well below 10 [mu]g/dL to 5 [mu]g/dL and
possibly lower (CD, Sections 8.4 and 8.5).\58\ With regard to the
evidence of effects on the developing nervous system at these low
levels, EPA notes, in particular, the international pooled analysis by
Lanphear and others (2005), studies of individual cohorts such as the
Rochester, Boston, and Mexico City cohorts (Canfield et al., 2003a;
Canfield et al., 2003b; Bellinger and Needleman, 2003; Tellez-Rojo et
al., 2006), the study of African-American inner-city children from
Detroit (Chiodo et al., 2004), and the cross-sectional analysis of a
nationally representative sample from the NHANES III (conducted from
1988-1994), in which the mean blood Pb level was 1.9 [mu]g/dL (Lanphear
et al., 2000). Further, current evidence does not indicate a threshold
for the more sensitive health endpoints such as adverse effects on the
developing nervous system (CD, pp. 5-71 to 5-74 and Section 6.2.13).
---------------------------------------------------------------------------

    \58\ For context, it is noted that the 2001-2004 median blood
level for children aged 1-5 of all races and ethnic groups is 1.6
[mu]g/dL, the median for the subset living below the poverty level
is 2.3 [mu]g/dL and 90th percentile values for these two groups are
4.0 [mu]g/dL and 5.4 [mu]g/dL, respectively. Similarly, the 2001-
2004 median blood level for black, non-hispanic children aged 1-5 is
2.5 [mu]g/dL, while the median level for the subset of that group
living below the poverty level is 2.9 [mu]g/dL and the median level
for the subset living in a household with income more than 200% of
the poverty level is 1.9 [mu]g/dL. Associated 90th percentile values
for 2001-2004 are 6.4 [mu]g/dL (for black, non-hispanic children
aged 1-5), 7.7 [mu]g/dL (for the subset of that group living below
the poverty level) and 4.1 [mu]g/dL (for the subset living in a
household with income more than 200% of the poverty level). (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm_then

click on ``Download a universal spreadsheet file of the Body Burdens
data tables'').
---------------------------------------------------------------------------

    As when the standard was set in 1978, EPA recognizes that there
remain today contributions to blood Pb levels from nonair sources.
Estimating contributions from nonair sources are complicated by the
persistent nature of Pb. For example, Pb that is a soil or dust
contaminant today may have been airborne yesterday or many years ago.
The studies currently available and reviewed in the Criteria Document
that evaluate the multiple pathways of Pb exposure do not usually
distinguish between outdoor soil/dust Pb resulting from historical
emissions and outdoor soil/dust Pb resulting from recent emissions.
Further, while indoor dust Pb has been identified as being a
predominant contributor to children's blood Pb, available studies do
not distinguish the different pathways (air-related and other)
contributing to indoor dust Pb. As recognized in Section III.A. above
(including footnote 13), some studies have found that dietary intake of
Pb may be a predominant source of Pb exposure among adults, greater
than consumption of water and beverages or inhalation (CD, p. 3-43).
The exposure assessment for children performed for this review has
employed available data and methods to develop estimates intended to
inform a characterization of these pathways.
    Consistent with reductions in air Pb concentrations \59\ which
contribute to blood Pb, nonair contributions have also been reduced.
For example, the use of Pb paint in new houses has declined
substantially over the 20th century, such that ``an estimated 68% of
U.S. homes built before 1940 have Pb hazards, as do 43% of those built
during 1940-1959 and 8% of those built during 1960-1977'' (ACCLPP,
2007). Additionally, Pb contributions to diet have been reported to
have declined significantly since 1978, perhaps as much as 70% or more
between then and 1990 (WHO, 1995) and the 2006 Criteria Document
identifies a drop in dietary Pb intake by 2 to 5 year olds of 96%
between the early 1980s and mid 1990s. The 1977 Criteria Document
included a dietary Pb intake estimate for the general population of 100
to 350 [mu]g Pb/day (USEPA 1977, p. 1-2) and the 2006 Criteria Document
cites recent studies indicating a dietary intake ranging from 2 to 10
[mu]g Pb/day (CD, Section 3.4 and p. 8-14). Reductions in elevated
blood Pb levels in urban areas indicate that other nonair contributions
to blood Pb (e.g., drinking water distribution systems, and Pb-based
paint) have also been reduced since the late 1970s. In their March 2007
letter to the Administrator, the CASAC Pb Panel recommended that 1.0-
1.4 [mu]g/dL or lower be considered as an estimate of the nonair
component of blood Pb.
---------------------------------------------------------------------------

    \59\ Air Pb concentrations nationally are estimated to have
declined more than 90% since the early 1980s.
---------------------------------------------------------------------------

    As in 1978, the evidence demonstrates that Pb in ambient air
contributes to Pb in blood, with the pertinent exposure routes
including both inhalation and ingestion (CD, Sections <IP_ADDRESS>, 4.2 and
4.4; Hilts et al., 2003). In 1978, the evidence indicated a
quantitative relationship between ambient air Pb and blood Pb--i.e.,
the ratio describing the increase in blood Pb per unit of air Pb--that
ranged from 1:1 to 1:2 (USEPA, 1977). In setting the standard, the
Agency relied on a ratio of 1:2, i.e., 2 [mu]g/dL blood Pb per 1 [mu]g/
m\3\ air Pb (43 FR 46252). The evidence now and in the past on this
relationship is limited by the circumstances in which the data are
collected. Specific measurements of Pb in blood that derived from Pb
that had been in the air are not available. Rather, estimates are
available for the relationship between Pb concentrations in air and Pb
levels in blood, developed from populations in differing Pb exposure
circumstances, which inform this issue. Many of the currently available
reviews of estimates for air-to-blood ratios, which include air
contributions from both inhalation and ingestion exposure pathways,
indicate that such ratios generally fall between 1:3 to 1:5, with some
higher \60\ (USEPA 1986a, pp. 11-99 to 11-100 and 11-106; Brunekreef,
1984). Findings of a recent study of changes in children's blood Pb
levels associated with reduced Pb emissions and associated air
concentrations near a Pb smelter in Canada indicates a ratio on the
order of 1:7 (CD, pp. 3-23 to 3-24; Hilts et al., 2003). In their
advice to the Agency, CASAC identified values of 1:5 as used by the
World Health Organization (2000) and 1:10 as supported by an empirical
analysis of changes in air Pb and changes in blood Pb between 1976 and
the time when the phase-out of Pb from gasoline was completed
(Henderson, 2007a).\61\ While there is uncertainty in the absolute
value of the air-to-blood relationship, the current evidence indicates
a notably greater ratio, with regard to increase in blood Pb, than the
1978 1:2 relationship e.g., on the order of 1:3 to 1:5 with some higher
estimates (see footnote 60) and some lower estimates (down to 1:1).
EPA's consideration of this issue in 1986 indicated that ratios which
consider both inhalation and ingestion pathways are ``necessarily
higher than those estimates for inhaled air lead alone'' (USEPA, 1986a,
p. 11-106). We solicit comment on data or studies that may help inform
our understanding of this important parameter.
---------------------------------------------------------------------------

    \60\ For example, adjusted ratios from Brunekreef (1984, Table
1) ranged up to 1:8.5 and unadjusted ratios extended above 1:10.
    \61\ The CASAC Panel stated ``The Schwartz and Picher analysis
showed that in 1978, the midpoint of the National Health and
Nutrition Examination Survey (NHANES) II, gasoline lead was
responsible for 9.1 [mu]g/dL of blood lead in children. Their
estimate is based on their coefficient of 2.14 [mu]g/dL per 100
metric tons (MT) per day of gasoline use, and usage of 426 MT/day in
1976. Between 1976 and when the phase-out of lead from gasoline was
completed, air lead concentrations in U.S. cities fell a little less
than 1 [mu]g/m\3\ (24). These two facts imply a ratio of 9-10 [mu]g/
dL per [mu]g/m\3\ reduction in air lead, taking all pathways into
account.'' (Henderson, 2007a, page D-2 to D-3).
---------------------------------------------------------------------------

    Based on this information, the Staff Paper concluded that young
children remain the sensitive population of primary focus in this
review, there is now no recognized safe level of Pb in

children's blood, and studies appear to show adverse effects at mean
concurrent blood Pb levels as low as 2 ug/dL (CD, pp. 6-31 to 6-32;
Lanphear et al., 2000). Further, 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. Using the framework
employed in setting the standard in 1978, the more recently available
evidence and more recently available estimates may suggest a level for
the standard that is lower by an order of magnitude or more.
b. Exposure- and Risk-Based Considerations
    In addition to the evidence-based considerations, EPA will also
consider exposures and health risks estimated to occur upon meeting the
current Pb standard to help inform judgments about the extent to which
exposure and risk estimates may be judged to be important from a public
health perspective, taking into account key uncertainties associated
with the estimated exposures and risks.
    As discussed above, young children are the sensitive population of
primary focus in this review. The exposure and risk assessment
estimates Pb exposure for children (less than 7 years of age), and
associated risk of neurocognitive effects in terms of IQ decrements. In
addition to the risks (IQ decrement) that were quantitatively
estimated, EPA recognizes that there may be long-term adverse
consequences of such deficits over a lifetime, that there is evidence
of other health effects occurring at similar or higher exposures for
young children, and that other health evidence demonstrates
associations between Pb exposure and adverse health effects in adults.
As noted in section III.B above, the risk assessment results focus
predominantly on risk estimates derived using the log-linear with low-
exposure linearization (LLL) concentration-response function, with the
range associated with the other three functions also being noted.
    As noted in the Criteria Document, a modest change in the mean for
a health index at the individual level can have substantial
implications at the population level (CD, p. 8-77, Sections 8.6.1 and
8.6.2; Bellinger, 2004; Needleman et al., 1982; Weiss, 1988; Weiss,
1990)). For example, for an individual functioning in the low range of
IQ due to the influence of risk factors other than Pb, a Pb-associated
IQ decline of a few points might be sufficient to drop that individual
into the range associated with increased risk of educational,
vocational, and social handicap (CD, p. 8-77). Further, given a
somewhat uniform manifestation of Pb-related decrements across the
range of IQ scores in a population, a downward shift in the mean IQ
value is not associated only with a substantial increase in the
percentage of individuals achieving very low scores, but also with
substantial decreases in percentages achieving very high scores (CD, p.
8-81). The CASAC Pb Panel has advised on this point that ``a population
loss of 1-2 IQ points is highly significant from a public health
perspective'' (Henderson, 2007a, p. 6).
    In this section, risk estimates for the median and for an upper
percentile, the 95th are discussed. In setting the standard in 1978,
EPA accorded risk management significance to the 99.5th percentile by
selecting a mean blood Pb level intended to bring 99.5 percent of the
population to or below the then described maximum safe blood Pb level.
Similarly, in their advice to EPA in this review, CASAC stated that
``the primary lead standard should be set so as to protect 99.5% of the
population'' (Henderson, 2007a, p. 6). In considering estimates from
the quantitative assessment that will inform conclusions consistent
with this objective, however, EPA and CASAC also recognize
uncertainties in the risk estimates at the edges of the distribution
and consequently the 95th percentile is reported as the estimate of the
high end of the risk distribution (Henderson, 2007b, p. 3). In so
doing, however, EPA notes that there are individuals in the population
expected to have higher risk, the consideration of which is important
given the risk management objectives for the current standard when set
in 1978 with regard to the 99.5th percentile.
    In addition to estimating IQ loss associated with the combined
exposure to Pb from all exposure pathways, EPA estimated IQ loss for
two policy-relevant categories of exposure pathways. These are ``recent
air'', which conceptually is intended to include contributions to blood
Pb associated with Pb that has recently been in the air, and ``past
air'', intended to include contributions to blood Pb associated with Pb
that was in the air in the past but not in the air recently. In the
exposure modeling conducted for the risk assessment, the exposure
pathways assigned to the recent air category were inhalation of ambient
air Pb and ingestion of the component of indoor dust Pb that is
predicted to be associated with ambient air concentrations. The
exposure pathways assigned to the past air category were ingestion of
outdoor soil/dust Pb and ingestion of the component of indoor dust Pb
not assigned to recent air. There are various limitations associated
with our modeling tools that affected the estimates for these two
categories. As a result, blood Pb levels and associated risks of
greatest interest in this review--those associated with exposure
pathways involving ambient air Pb and current levels of Pb emitted to
the air (including via resuspension)--are likely to fall between
estimates for recent air and those for the sum of recent plus past
air.\62\ Accordingly, this notice presents these two sets of estimates
as providing a range of interest, with regard to policy-relevant Pb,
for this review.
---------------------------------------------------------------------------

    \62\ Comparisons of blood Pb levels estimated for individual
case study populations (from all exposure sources in current
conditions scenarios) to national population values from NHANES are
noted in footnote 39 in Section III.B.3.a.
---------------------------------------------------------------------------

    In considering the adequacy of the current standard, it is
important to note that the standard is currently met throughout the
country with very few exceptions. The national composite average
maximum quarterly mean based on 198 active monitoring sites during
2003-2005 is 0.17 [mu]g/m\3\, an order of magnitude below the current
standard, indicating that most of the monitored areas of the country
are well below the standard. Review of the current monitoring network
in light of current information on Pb sources and emissions, however,
indicated that monitors are not located near many of the larger
sources. Therefore, the assessment may be underestimating Pb
concentrations.
    Using the current monitoring data, EPA estimated exposure and risk
associated with current conditions in a general urban case study and in
three location-specific urban case studies in areas where air
concentrations fall significantly below the current standard.\63\ Two
current conditions scenarios were assessed for the general urban case
study, one based on the 95th percentile of levels in large urban areas
(0.87 [mu]g/m\3\, maximum quarterly mean) and one based on mean levels
in such

areas (0.14 [mu]g/m\3\, maximum quarterly. Levels in the three
location-specific case studies ranged from 0.09 to 0.35 [mu]g/m\3\, in
terms of maximum quarterly average. For the general urban case study,
which is a simplified representation of urban areas, median estimates
of total Pb-related IQ loss range from 1.5 to 6.3 points (across all
four concentration-response functions), with estimates based on the LLL
function of 4.5 and 4.7 points, for the mean and high-end current
conditions scenarios, respectively. Associated estimates for exposure
pathway contributions to total IQ loss (LLL estimate) at the population
median in these two scenarios indicate that IQ loss associated with
policy-relevant Pb falls somewhere between 1.3 and 3.6 points. At the
95th percentile for total IQ loss (LLL estimate), IQ loss associated
with policy-relevant Pb is estimated to fall somewhere between 2.2 and
6.0 points (Risk Assessment Report, Table 5-9).
---------------------------------------------------------------------------

    \63\ Comparisons of median and 90th percentile blood Pb levels
estimated for individual case study populations (from all exposure
sources in current conditions scenarios) to national population
values from NHANES are noted in footnote 39 in Section III.B.3.a.
That comparison suggests that modeled estimates generated for the
location-specific urban case studies for both population percentiles
are somewhat larger than values cited in NHANES (for 2003-2004).
However, as mentioned earlier, factors related to Pb exposure,
including ambient air levels, are likely to differ for the urban
case study populations compared with the national population
underlying NHANES.
---------------------------------------------------------------------------

    For the three location-specific areas, median estimates of total
Pb-related IQ loss for current conditions range from 1.4 to 5.2 points
(across all four concentration-response functions), with estimates
based on the LLL function all being 4.2 points.\64\ Median IQ loss
associated with policy-relevant Pb (LLL function) is estimated to fall
between 0.6 to 2.9 points IQ loss. The 95th percentile estimates for
total Pb-related IQ loss across the three location-specific urban case
studies range from 4.1 to 11.4 points (across all four concentration-
response functions), with estimates based on the LLL function ranging
from 7.5 to 7.6 points. At the 95th percentile for the three location-
specific urban case studies, IQ loss associated with policy-relevant Pb
(LLL function) is estimated to fall between 1.2 to 5.2 points IQ loss
(Risk Assessment Report, Tables 5-9 and 5-10).
---------------------------------------------------------------------------

    \64\ Although the maximum quarterly average concentration for
the highest monitor in each study area differs among the three areas
by a factor of 4 (0.09 to 0.36 [mu]g/m\3\), the population weighted
air Pb concentrations for these three study areas are more similar
and differ by approximately a factor of 2, with the study area with
highest maximum quarterly average concentration having a lower
population-weighted air concentration that is more similar to the
other two areas. This similarity in population weighted
concentrations explains the finding of similar total IQ loss across
the three study areas.
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    In order to consider exposure and risk associated with the current
standard, EPA developed estimates for a case study based on air quality
projected to just meet the standard in a location of the country where
air concentrations do not meet the current standard (the primary Pb
smelter case study). In so doing, we consider it extremely unlikely
that air concentrations in urban areas across the U.S. that are
currently well below the current standard would increase to just meet
the standard. However, we recognize the potential for air Pb
concentrations in some areas currently well below the standard to
increase to just meet the standard by way of, for example, expansion of
existing sources (e.g., facilities operating as secondary smelters may
exercise previously used capabilities as primary smelters) or by the
congregation of multiple Pb sources in adjacent locations. We have
simulated this scenario (increased Pb concentrations to just meet the
current standard) in a general urban case study and three location-
specific urban case studies. In this scenario, we note substantial
uncertainty in simulating how the profile of Pb concentrations might
change in the hypothetical case where concentrations increase to just
meet the current standard.
    Turning first to the estimates of total blood Pb for the current
NAAQS scenario simulated for the location-specific urban case studies,
we note the extent to which exposures associated with increased air Pb
concentrations that simulate just meeting the current standard are
estimated to increase blood Pb levels in young children. The magnitude
of this for the median total blood Pb ranges from 0.3 [mu]g/dL (an
increase of 20 percent) in the case of the Cleveland study area for
which current conditions are estimated to be approximately one fourth
of the current NAAQS, up to approximately 1 [mu]g/dL (an increase of 50
to 70%) for the Chicago and Los Angeles study areas for which current
conditions are estimated to be at or below one tenth of the current
NAAQS.
    Estimates of IQ loss (for child with median total IQ loss estimate)
associated with recent air plus past air Pb at exposures allowed by
just meeting the current NAAQS in the primary Pb smelter case study
differ when considering the full study area (10 km radius) or the 1.5
km radius subarea. Estimates for median IQ loss associated with the
recent air plus past air categories of exposure pathways for the full
study area range from 0.6 point to 2.3 points (for the range of
concentration-response functions), while these estimates for the
subarea range from 3.2 points to 9.4 points IQ loss. The estimates
(recent plus past) for the median based on the LLL concentration-
response function are 1.9 points IQ loss for the full study area and
6.0 points for the subarea. The 95th percentile estimates of total IQ
loss in the subarea range from 5.0 to 12.4 points, with an associated
range for the recent air plus past air of 4.2 to 10.4 points.
    For the current NAAQS scenario in the three location-specific case
studies, estimates of IQ loss associated with policy-relevant Pb for
the median total IQ loss range from 0.6 points loss (recent air
estimate using low-end concentration-response function) to 7.4 points
loss (recent plus past air estimate using the high-end concentration-
response function). The corresponding estimates based on the LLL
concentration-response function range from 2.7 points (lowest location-
specific recent air estimate) to 4.7 points IQ loss (highest location-
specific recent plus past air estimate). The comparable estimates of IQ
loss for children at the 95th percentile range from 2.6 to 7.6 points
for the LLL concentration-response function.
    Further, in comparing current NAAQS scenario estimates to current
conditions estimates for the three location-specific urban case
studies, the estimated difference in total Pb-related IQ loss for the
median is about 0.5 to 1.4 points using the LLL concentration-response
function and a similar magnitude of difference is estimated for the
95th percentile. The corresponding estimate for the general urban case
study is 1.1 to 1.3 points higher total Pb-related IQ loss for the
current NAAQS scenario compared to the two current conditions
scenarios.
    Estimates of median and 95th percentile IQ loss associated with
policy-relevant Pb exposure for air quality scenarios under current
conditions (which meet the current NAAQS) and, particularly those
reflecting conditions simulated to just meet the current standard,\65\
indicate levels of IQ loss that some may reasonably consider to be
significant from a public health perspective. Further, for the three
location-specific urban case studies, the estimated differences in
incidences of children with IQ loss greater than one point and with IQ
loss greater than seven points in comparing current conditions to those
associated with the current NAAQS indicate the potential for
significant numbers of children to be negatively affected if air Pb
concentrations increased to levels just meeting the

current standard. Estimates of the additional number of children with
IQ loss greater than one point (based on the LLL concentration-response
function) in these three study areas with the current NAAQS scenario
compared to current conditions range from 100 to 6,000 across the three
locations. The corresponding estimates for the additional number of
children with IQ loss greater than seven points, for the current NAAQS
as compared to the current conditions scenario range from 600 to
35,000. These latter values for the change in incidence of children
with greater than seven points Pb-related IQ loss represent 5 to 17
percent of the children (aged less than 7 years of age) in these study
areas. This increase corresponds to approximately a doubling in the
number of children with this magnitude of Pb-related IQ loss in the
study area most affected.
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    \65\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
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    While the risk assessment has quantified risks associated with IQ
impacts in childhood, there are other, unquantified adverse
neurocognitive effects that may occur at similarly low exposures which
might additionally contribute to reduced academic performance, which
may have adverse consequences over a lifetime (CD, pp. 8-29 to 8-30).
Additional impacts at low levels of childhood exposure that were not
quantified in the risk assessment include: other neurological effects
(sensory, motor, cognitive and behavioral), immune system effects
(including some related to allergic responses and asthma), and early
effects related to anemia.
c. CASAC Advice and Recommendations
    Beyond the evidence- and risk/exposure-based information discussed
above, in considering the adequacy of the current standard, EPA will
also consider the advice and recommendations of CASAC, based on their
review of the Criteria Document and the drafts of the Staff Paper and
the related technical support document, as well as comments from the
public on drafts of the Staff Paper and related technical support
document.\66\ With regard to the public comments, those that addressed
adequacy of the current standard concluded that the current standard is
inadequate and should be revised, suggesting appreciable reductions in
the level. No comments were received expressing the view that the
current standard is adequate. One comment was received arguing not that
the standard was inadequate but rather that conditions justified that
it should be revoked. In both the 1990 review and this review of the
standard set in 1978, CASAC, has recommended consideration of more
health protective NAAQS. In CASAC's review of the 1990 Staff Paper, as
discussed in Section 5.2.2, they generally recommended consideration of
levels below 1.0 [mu]g/m\3\, specifically recommended analyses of a
standard set at 0.25 [mu]g/m\3\, and also recommended a monthly
averaging time (CASAC, 1990). In two letters to the Administrator
during the current review, CASAC has consistently recommended that the
primary NAAQS should be ``substantially lowered'' from the current
level of 1.5 [mu]g/m\3\ to a level of ``0.2 [mu]g/m\3\ or less''
(Henderson, 2007a, b). CASAC drew support for this 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.
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    \66\ All written comments submitted to the Agency will be
available in the docket for this rulemaking, as will be transcripts
of the public meeting held in conjunction with CASAC's review of the
first draft of the Staff Paper and the first draft of the related
technical support document, and of draft and final versions of the
Criteria Document.
---------------------------------------------------------------------------

    CASAC concluded 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 stated that ``Consequently, 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).
d. Policy Options
    In considering the adequacy of the current standard, EPA first
notes the dramatic changes in the basic patterns of air Pb emissions in
the U.S. since the standard was set, reflecting the phase-out of Pb in
gasoline, as well as changes to the CAA related to the inclusion of Pb
compounds on the list of HAPs and associated requirements for
technology- and risk-based standards for major stationary sources. We
are aware that questions have been raised about the appropriateness of
retaining Pb on the list of criteria pollutants and/or maintaining a
NAAQS for Pb in light of these changed circumstances. We take note of
the views of CASAC, summarized above, and the conclusions and
recommendations in the OAQPS Staff Paper on these questions, which do
not support delisting Pb or revoking the Pb NAAQS. We recognize,
however, that there may be differing views on interpreting or weighing
the available information. Thus, EPA solicits comment related to the
questions of delisting and revocation. The EPA also solicits comment on
whether the broad range of current multimedia Federal and State Pb
control programs, summarized above in section II.C, are sufficient to
provide appropriate public health protection in lieu of a Pb NAAQS.
    In further considering the adequacy of the current standard, EPA
will focus on the body of available evidence (summarized above in
section III.A and discussed in the Criteria Document) that is much
expanded from that available when the current standard was set. The
presentation of the evidence in the Criteria Document describes 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. We recognize that the current health effects evidence together
with findings from the exposure and risk assessments (summarized above
in section III.B), like the information available at the time the
standard was set, supports the conclusion that air-related Pb exposure
pathways (by inhalation and ingestion) contribute to blood Pb levels in
young children. Furthermore, we take note of the information that
suggests that the air-to-blood relationship (i.e., the air-to-blood
ratio), is likely larger, with regard to increase in blood Pb per unit
air concentration, when air inhalation and ingestion are considered
than that estimated when the standard was set using only inhalation and
may be several times larger. EPA recognizes there is uncertainty in
estimates of this relationship and solicits comment on on ratios
supported by the current evidence.
    In areas projected to just meet the current standard, the
quantitative estimates of risk (for IQ decrement) associated with
policy-relevant Pb indicate risk of a magnitude that some may consider
to be significant from a public health perspective.\67\ Further,
although the current monitoring data indicate few areas with airborne
Pb near or just exceeding the current standard, we recognize
significant limitations with the current monitoring network and thus
the potential that the prevalence of such levels of Pb

concentrations may be underestimated by currently available data.
---------------------------------------------------------------------------

    \67\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
---------------------------------------------------------------------------

    As summarized above, CASAC conclusions and recommendations and
recommendations presented in the OAQPS Staff Paper reflect the view
that the current standard is not adequate and support consideration of
a revised standard to provide an adequate margin of safety for
sensitive groups. Taking these views into account, we recognize that
one approach is to consider a revised standard. We also recognize that
there may be differing interpretations of the available information.
Thus, EPA solicits comment on delisting, revocation, and the adequacy
of the current standard and the rationale upon which such views are
based.
4. 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 revisions to the current primary Pb
standard, as discussed in the following sections, EPA will consider
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.
a. Indicator
    The indicator for the current standard is Pb-TSP. When the standard
was set, the Agency considered identifying Pb in particles less than or
equal to 10 [mu]m in diameter (Pb-PM10) as the indicator in
response to comments expressing concern that because only a fraction of
airborne particulate matter is respirable, an air standard based on
total air Pb is unnecessarily stringent. The Agency responded that
while it agreed that some Pb particles are too small or too large to be
deposited in the respiratory system, a significant component of
exposures can be ingestion of materials contaminated by deposition of
Pb from the air. In addition to the route of ingestion and absorption
from the gastrointestinal tract, nonrespirable Pb in the environment
may, at some point, become respirable through weathering or mechanical
action. EPA concluded that total airborne Pb, both respirable and
nonrespirable fractions should be addressed by the air standard.
    More recently, in the 1990 Staff Paper, this issue was reconsidered
in light of information regarding limitations of the high-volume
sampler used for the Pb-TSP measurements and the continued use of total
suspended particulate matter (TSP) as the indicator was supported by
OAQPS staff (USEPA, 1990).

    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 * * *
    In the current review, CASAC has recommended that EPA consider a
change in the indicator to utilize low-volume PM10 sampling
(Henderson, 2007a, b). In so doing, CASAC recognized that a scaling of
the NAAQS level would be needed to accommodate the loss of very large
coarse-mode Pb particles and concurrent Pb-PM10 and Pb-TSP
sampling would be needed to inform development of scaling factors. The
September 2007 CASAC letter states that the CASAC Lead Panel ``strongly
encourages the Agency to consider revising the Pb reference method to
allow sample collection by PM10, rather than TSP samplers,
accompanied by analysis with low-cost multi-elemental techniques like
X-Ray Fluorescence (XRF) or Inductively Coupled Plasma-Mass
Spectroscopy (ICP-MS).'' While recognizing the importance of coarse
dust contributions to total Pb exposure via the ingestion route and
acknowledging that TSP sampling is likely to capture additional very
coarse particles which are excluded by PM10 samplers, the
Panel raised some concerns. The concerns were regarding the precision
and variability of TSP samplers, and the inability to efficiently
capture the non-homogeneity of very coarse particles in a national
monitoring network, which the Panel indicated may need to be addressed
in implementing additional monitoring sites and an increased frequency
of sample collection that might be required with the substantial
reduction in the level of the standard and shorter averaging time that
they recommend (Henderson, 2007b).
    In considering the appropriate indicator, EPA takes note of and
solicits comment on previous Agency conclusions that the health
evidence indicates that Pb in all particle size fractions, not just
respirable Pb, contributes to Pb in blood and to associated health
effects. Additionally, the current information does not support the
derivation of a single scaling factor, which might be used to relate a
level for Pb-TSP to a monitoring result using Pb-PM10 on a
national scale. The EPA recognizes, however, that an indicator that
exhibits low spatial variability is desirable such that it facilitates
implementation of an effective monitoring network, i.e., one that
assures identification of areas with the potential to exceed the NAAQS.
    To the extent that Pb-PM10 exhibits less spatial
variability and that a ``crosswalk'' can be developed between a level
in terms of Pb-TSP, EPA recognizes that it is appropriate to consider
moving to a Pb-PM10 indicator in the future. One of the
issues to consider when moving to a Pb-PM10 indicator is
whether regulating concentrations of Pb-PM10 will lead to
appropriate controls on Pb emissions from sources with a large
percentage of Pb in the greater than 10 micron size range (e.g.,
fugitive dust emissions from Pb smelters). It is reasonable to believe
that Pb-PM10/Pb-TSP ratios are sensitive to distance from
emissions sources (due to faster deposition of larger particles). As
such, the use of a Pb-PM10 indicator may have a significant
influence on the degree of Pb controls needed from emission sources.
    The EPA will consider several options that might improve the
available database and facilitate such a move in the future, while
retaining Pb-TSP as the indicator for the NAAQS at this time,
consistent with the recommendations in the Staff Paper. For example, we