options included potential application of Pb-PM10 FRM/FEMs
at sites with established relationships between Pb-TSP and Pb-
PM10, and use of Pb-PM10 data, with adjustment,
as a surrogate for Pb-TSP data. The ANPR broadly solicited comment in
these areas.
---------------------------------------------------------------------------

    \124\ For simplicity, the discussion here and below speaks as if
PM10 samplers have a sharp size cut-off. In reality, they
have a size selection behavior in which 50% of particles 10 microns
in size are captured, with a progressively higher capture rate for
smaller particles and a progressively lower capture rate for larger
particles. The ideal capture efficiency curve for PM10
samplers specifies that particles above 15 microns not be captured
at all, although real samplers may capture a very small percentage
of particles above 15 microns. TSP samplers have 50% capture points
in the range of 25 to 50 microns, which is broad enough to include
virtually all particles capable of being transported any significant
distance from their source except under extreme wind events. As
explained below, the capture efficiency of a high-volume TSP sampler
for any given size particle is affected by wind speed and wind
direction.
    \125\ In this notice, we use ``ultra-coarse'' to refer to
particles collected by a TSP sampler but not by a PM10
sampler (we note that CASAC has variously also referred to these
particles as ``very coarse'' or ``larger coarse-mode'' particles),
``fine'' to refer to particles collected by a PM2.5
sampler, and ``coarse'' to refer to particles collected by a
PM10 sampler but not by a PM2.5 sampler,
recognizing that there will be some overlap in the particle sizes in
the three types of collected material.
---------------------------------------------------------------------------

    In the current review, both the CASAC Pb Panel and members of the
CASAC Ambient Air Monitoring and Methods (AAMM) Subcommittee have
recommended that EPA consider a change in the indicator to
PM10, utilizing low-volume PM10 sampling
(Henderson, 2007a, 2007b, 2008; Russell, 2008). \126\ In their January
2008 letter, the CASAC Lead Panel unanimously recommended that EPA
revise the Pb NAAQS indicator to rely on low-volume PM10
sampling (Henderson, 2008). They indicated support for their
recommendation in a range of areas. First, they noted poor precision in
high-volume TSP sampling, wide variation in the upper particle size-cut
as a function of wind speed and direction, and greater difficulties in
capturing the spatial non-homogeneity of ultra-coarse particles with a
national monitoring network. They stated that the low-volume
PM10 collection method is a much more accurate and precise
collection method, and would provide a more representative
characterization on a large spatial scale of monitored particles which
remain airborne longer, thus providing a characterization that is more
broadly representative of ambient exposures over large spatial scales.
They also noted the automated sequential sampling capability of low-
volume PM10 monitors which would be particularly useful if
the averaging time is revised (i.e., to a monthly averaging time, as
recommended by CASAC), which, in CASAC's view would necessitate an
increased monitoring frequency. Further, they noted the potential for
utilization of the more widespread PM10 sampling network
(Henderson, 2007a, 2007b, 2008).\127\ In their advice, CASAC also
stated that they ``recognize the importance of coarse dust
contributions to total Pb ingestion and acknowledge that TSP sampling
is likely to capture additional very coarse particles which are
excluded by PM10 samplers'' (Henderson 2007b). They
suggested that an adjustment of the NAAQS level would accommodate the
loss of these ultra-coarse Pb particles, and that development of such a
quantitative adjustment might appropriately be based on concurrent Pb-
PM10 and Pb-TSP sampling data \128\ (Henderson, 2007a,
2007b, 2008).
---------------------------------------------------------------------------

    \126\ ``Low-volume PM10 sampling'' refers to sampling
using any of a number of monitor models that draw 16.67 liters/
minute (1 m3/hour) of air through the filter, in contrast
to ``high-volume'' sampling of either TSP or PM10 in
which the monitor draws 1500 liters/minute (90 m3/hour).
All commercial TSP FRM samplers at this time are high-volume
samplers; both high-volume and low-volume PM10 FRM
samplers are available. Low-volume sampling is the more recently
introduced method. Low-volume and high-volume samplers differ in
many other ways also, including filter size, accuracy of the flow
control, and degree of computerization.
    \127\ EPA notes that costs, including those of operating a
monitoring network, may not be considered in establishing or
revising the NAAQS.
    \128\ In their advice, CASAC recognized the potential for site-
to-site variability in the relationship between Pb-TSP and Pb-
PM10 (Henderson, 2007a, 2007b). They also stated in their
September 2007 letter, ``The Panel urges that PM10
monitors, with appropriate adjustments, be used to supplement the
data. * * * A single quantitative adjustment factor could be
developed from a short period of collocated sampling at multiple
sites; or a PM10 Pb/TSP Pb 'equivalency ratio' could be
determined on a regional or site-specific basis.''
---------------------------------------------------------------------------

    The Agency received comments on the discussion of the indicator in
the ANPR from several state and local agencies and national/regional
air pollution control organizations, as well as a national
environmental organization. These public comments

were somewhat mixed. Most of these commenters recommended maintaining
Pb-TSP as the indicator to ensure that Pb emitted in larger particles
is not overlooked by the Pb NAAQS. Some of those comments and others
suggested keeping TSP as the indicator but revising the FRM to a low-
volume TSP method \129\ and considering tighter sampling height
criteria to reduce variability.\130\ Others, in considering a potential
PM10-based indicator or the use of PM10 data as a
surrogate for Pb-TSP, noted the need for characterization of the
relationship between Pb-PM10 and Pb-TSP, which varies with
proximity to some sources. One state agency and a national organization
of regulatory air agencies expressed clear support for revising the
indicator to Pb-PM10, predominantly citing advantages
associated with improved technology and efficiency in data collection.
---------------------------------------------------------------------------

    \129\ The Pb-TSP FRM specification, 40 CFR 50 appendix G,
currently explicitly requires the use of the high-volume TSP FRM
sampler which is required by appendix B for the mass of TSP.
Therefore it would require amendments to 40 CFR 50 appendix B and/or
G (or a new dedicated appendix) to establish a low-volume TSP
sampler as the only FRM, or as an alternative FRM, for TSP and/or
Pb-TSP measurement. A number of researchers have utilized both self-
built and commercially available low-volume TSP samplers in ambient
air studies. Typically, these samplers are identical to low-volume
PM10 FRM samplers with the exception that their inlets
and other size separation devices (or lack thereof) are aimed at
collecting TSP. EPA is not aware of any rigorous evaluation of the
performance of these available, non-designated low-volume TSP
samplers or their equivalence to the TSP FRM. No one has applied to
date for designation of a low-volume TSP sampler as a FEM, either
for TSP measurement per se or for purposes of Pb-TSP measurement.
    \130\ Currently, probe heights for Pb-TSP and PM10
sampling are allowed to be between 2 and 15 meters above ground
level for neighborhood-scale monitoring sites (those intended to
represent concentrations over a relatively large area around the
site) and between 2 and 7 meters for microscale sites. Near very
low-height sources of TSP, including fugitive dust sources at ground
level, concentrations of TSP, especially the concentrations of
particles larger than 10 microns, can vary substantially across this
height range with higher concentrations closer to the ground; near-
ground concentrations can also vary more in time than concentrations
higher up.
---------------------------------------------------------------------------

    In considering these issues concerning the appropriate indicator,
EPA takes note of 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.
Further, the evidence and exposure/risk estimates in the current review
indicate that ingestion pathways dominate air-related exposure. Lead is
unlike other criteria pollutants, where inhalation of the airborne
pollutant is the key contributor to exposure. For Pb it is the quantity
of Pb in ambient particles with the potential to deposit indoors or
outdoors, thereby leading to a role in ingestion pathways, that is the
key contributor to air-related exposure. As recognized by the Agency in
setting the standard, and as noted by CASAC in their advice during this
review, these particles include ultra-coarse particles. Thus, choosing
the appropriate indicator requires consideration of the impact of the
indicator on protection from both the inhalation and ingestion pathways
of exposure and Pb in all particle sizes, including ultra-coarse
particles.
    As discussed in section V.A., the Agency recognizes the body of
evidence indicating that the high-volume Pb-TSP sampling methodology
contributes to imprecision in resultant Pb measurements due to
variability in the efficiency of capture of particles of different
sizes and thus, in the mass of Pb measured. For example, the measured
values from a high-volume TSP sampler may differ substantially,
depending on wind speed and direction, for the same actual ambient
concentration of Pb-TSP.\131\ Variability is most substantial in
samples with a large portion of Pb particles greater than 10 microns,
such as those samples collected near sources with emissions of ultra-
coarse particles. The result is a clear risk of error from
underestimating the ambient level of total Pb in the air, especially in
areas near sources of ultra-coarse particles, by underestimating the
amount of the ultra-coarse particles. There is also the potential for
overestimation of individual sampling period measurements associated
with high wind events.\132\
---------------------------------------------------------------------------

    \131\ As noted in section V, the collection efficiency (over the
24-hour collection period) of particles larger than approximately 10
microns in a high-volume TSP FRM sampler varies with wind speed due
to aerodynamic effects, with a lower collection efficiency under
high winds. The collection efficiency also varies with wind
direction due to the non-cylindrical shape of the TSP sampler inlet.
These characteristics tend in the direction of reporting less than
the true TSP concentration over the 24-hour collection period.
    \132\ We note that it is possible for high winds to blow Pb
particles onto a high-volume TSP sampler's filter after the end of
its 24-hour collection period before the filter is retrieved,
causing the reported concentration for the 24-hour period to be
higher than the actual 24-hour concentration.
---------------------------------------------------------------------------

    The low-volume PM10 sampling methodology does not
exhibit such variability \133\ due both to increased precision of the
monitor and decreased spatial variation of Pb-PM10
concentrations. As a result, greater precision is associated with
sample measurements for Pb collected using the PM10 sampling
methodology. The result is a lower risk of error in measuring the
ambient Pb in the PM10 size class than there is risk of
error in measuring the ambient Pb in the TSP size class using Pb TSP
samplers. On the other hand, PM10 samplers do not include
the Pb in particles greater than PM10 that also contributes
to the health risks posed by air-related Pb, especially in areas
influenced by sources of ultra-coarse particles. There are also
concerns over whether control strategies put in place to meet a NAAQS
with a Pb-PM10 indicator will be effective in controlling
ultra-coarse Pb-containing particles. In evaluating these two
indicators, the differences in the nature and degree of these sources
of error between Pb-TSP and Pb-PM10 need to be considered
and weighed, to determine the appropriate way to protect the public
from exposure to air-related Pb.
---------------------------------------------------------------------------

    \133\ Low-volume PM10 samplers are equipped with an
omni-directional (cylindrical) inlet, which reduces the effect of
wind direction, and a sharp particle separator which excludes most
of the particles greater than 10-15 microns in diameter whose
collection efficiency is most sensitive to wind speed. Also, in low-
volume samplers, the filter is protected from post-sampling
contamination.
---------------------------------------------------------------------------

    As noted above, EPA is concerned about the total mass of all Pb
particles emitted into the air and subsequently inhaled or ingested.
Measurements of Pb-TSP address a greater fraction of the particles of
concern from a public health perspective than measurements of Pb-
PM10, but limitations with regard to the sampler mean that
these data are less precise. EPA recognizes substantial variability in
the high-volume Pb-TSP method, meaning there is a risk of not
consistently identifying sites that fail to achieve the standard, both
across sites and across time periods for the same site.
    Alternatively, using low-volume Pb-PM10 as the indicator
would allow the use of a technology that has better precision in
measuring PM10. In addition, since Pb-PM10
concentrations have less spatial variability, such monitoring data may
be representative of Pb-PM10 air quality conditions over a
larger geographic area (and larger populations) than would Pb-TSP
measurements. The larger scale of representation for Pb-PM10
would mean that reported measurements of this indicator, and hence
designation outcomes, would be less sensitive to exact monitor siting
than with Pb-TSP as the indicator.\134\ However, there would be a
different source of error, in that larger Pb particles not captured by
PM10 samplers would not be measured.

The fraction of Pb collected with a TSP sampler that would not be
collected by a PM10 sampler varies depending on proximity to
sources of ultra-coarse Pb particles and the size mix of the particles
they emit (as well as the sampling variability inherent in the method
discussed above). This means that this error is of most concern in
locations in closer proximity to such sources, which may also be
locations with some of the higher ambient air levels. As discussed
below, such variability would be a consideration in determining the
appropriate level for a standard based on a Pb-PM10
indicator.
---------------------------------------------------------------------------

    \134\ The larger scale would also make comparisons between two
or more monitoring sites more indicative of the true comparison
between the areas surrounding the monitoring sites, with regard to
the Pb captured by Pb-PM10 monitors, which could be
informative in studies of Pb uptake and health effects in
populations.
---------------------------------------------------------------------------

    Accordingly, we believe it is reasonable to consider continued use
of a Pb-TSP indicator, focusing on the fact that it specifically
includes the ultra-coarse Pb particles in the air that are of concern
and need to be addressed in protecting public health from air-related
exposures. In considering the option of retaining Pb-TSP as the
indicator, EPA recognizes that high-volume FRM TSP samplers would
continue to be used at many monitoring sites operated by State and
local agencies. In addition, it is possible that one or more low-volume
TSP monitors would be approved as FEM, under the provisions of 40 CFR
53, Ambient Air Monitoring Reference and Equivalent Methods. EPA
believes, along with some commenters as noted above, that low-volume
Pb-TSP sampling would have important advantages over high-volume Pb-TSP
sampling.\135\ To facilitate the ability of monitor vendors and
monitoring agencies to gain FEM status for low-volume Pb-TSP monitors,
EPA is proposing certain revisions to the side-by-side equivalence
testing requirements in 40 CFR 53 regarding the ambient Pb
concentrations required during testing so that testing is more
practical for a monitor vendor to conduct, as described in more detail
in section V below. We note that 40 CFR 53.7, Testing of Methods at the
Initiative of the Administrator, allows EPA itself to conduct the
required equivalence testing for a method and then determine whether
the requirements for equivalence are met. It would also be possible for
EPA to promulgate amendments to 40 CFR 50 establishing one or more
particular designs of a low-volume sampler as a Pb-TSP FRM, or to
establish performance specifications that would facilitate the approval
of low-volume samplers as FRM on a performance basis rather than a
design basis; this could be done as a replacement for the high-volume
TSP and Pb-TSP FRM or as an alternative TSP and/or Pb-TSP FRM. Either
path to FRM status would avoid the need for the side-by-side testing,
prescribed by 40 CFR 53, of low-volume samplers to demonstrate
equivalence to the high-volume FRM sampler, although some amount and
type of new testing in the field or in a wind tunnel may be appropriate
before such changes should be made. EPA invites comments on the low-
volume TSP sampler concept.
---------------------------------------------------------------------------

    \135\ Low-volume Pb-TSP samplers could be assembled by making
low-cost parts substitution to either low-volume PM10 or
low-volume PM2.5 samplers; some models would have the
same sequential sampling ability as CASAC has noted for low-volume
Pb-PM10 samplers; sensitivity to wind direction would be
eliminated; and their flow control and data processing and reporting
abilities would be substantially better than high-volume Pb-TSP
samplers. Low-volume Pb-TSP sampling data would have the same
geographic variability as high-volume Pb-TSP sampling data, however.
The size-specific capture efficiency curves of currently available
commercial low-volume sampling systems are not well characterized,
nor their sensitivity to wind speed. EPA therefore recognizes some
uncertainty about their equivalence to high-volume samplers in terms
of the capture of ultra-coarse particles.
---------------------------------------------------------------------------

    Within the option of continued use of a Pb-TSP indicator, EPA
recognizes that some State, local, or tribal monitoring agencies, or
other organizations, for the sake of the advantages noted above, may
wish to deploy low-volume Pb-PM10 samplers rather than Pb-
TSP samplers. In anticipation of this, we have also considered an
approach within the option of retaining Pb-TSP as the indicator that
would allow the use of Pb-PM10 data (when and if low-volume
Pb-PM10 samplers have been approved by EPA as either FRM or
FEM), with adjustment(s), for monitoring for compliance with the Pb-TSP
NAAQS. This approach would have five components: (1) The establishment
of a FRM specification for low-volume Pb-PM10 monitoring
including both a PM10 sampler specification and a reference
chemical analysis method for determination of Pb in the collected
particulate matter; (2) the establishment of a path to FEM designation
for Pb-PM10 monitoring methods that differ from the FRM in
either the sampler or the analytical method; (3) flexibility for
monitoring agencies to deploy low-volume Pb-PM10 monitors
anywhere that Pb monitoring is required by the revised Pb monitoring
requirements to help implement the revised NAAQS; (4) specific steps
for applying an adjustment to low-volume Pb-PM10 data for
purposes of making comparisons to the level of the NAAQS specified in
terms of Pb-TSP, and (5) a provision in the data interpretation
guidelines that, whenever and wherever Pb-TSP data from a monitoring
site is available and sufficient for determining whether or not the Pb-
TSP standard has been exceeded, any collocated Pb-PM10 data
from that site for the associated time period will not be considered.
The first three and the last components are discussed in depth in
sections IV and V below. Because the issue of adjustment to low-volume
Pb-PM10 data is linked closely to considerations of the
advantages of one indicator option versus another, it is discussed
here.
    In considering how to identify the appropriate adjustment(s) to be
made to Pb-PM10 data for purposes of making comparisons to
the level of the NAAQS specified in terms of Pb-TSP, we recognize the
importance to protecting public health of taking into account the
ultra-coarse particles that are not included in Pb-PM10
measurement. As discussed below, one approach to doing so would be to
adjust or scale Pb-PM10 data upwards before comparison to a
Pb-TSP NAAQS level where the data are collected in an area that can be
expected to have ultra-coarse particles present.
    Pb-PM10/Pb-TSP relationships vary from site to site and
time to time. These Pb-PM10/Pb-TSP relationships have a
systematic variation with distance from emissions sources emitting
particles larger than would be captured by Pb-PM10 samplers,
such that generally there are larger differences between Pb-
PM10 and Pb-TSP near sources. This is due to the faster
deposition of the ultra-coarse particles (as described in section
II.A.1). The exact size mix of particles at the point(s) of emissions
release and the height of the release point(s) also affect the
relationship. Accordingly, EPA is proposing to require the one-time
development and the continued use of site-specific adjustments for Pb-
PM10 data, for those sites for which a State prefers to
conduct Pb-PM10 monitoring rather than Pb-TSP monitoring.
Site-specific studies to establish the relationships between Pb-TSP and
Pb-PM10, conducted using side-by-side paired samplers, would
allow Pb-PM10 monitoring using locally determined factors
based on local study data to determine compliance with a NAAQS based on
Pb-TSP.
    In addition, EPA invites comment on also providing in the final
rule default scaling factor(s) for use of Pb-PM10 data in conjunction
with a Pb-TSP indicator, as an alternative for States which wish to
conduct Pb-PM10 monitoring rather than Pb-TSP monitoring near Pb
sources but prefer not to conduct a site-specific scaling factor study.
EPA has identified and analyzed available collocated Pb-PM10 and Pb-TSP
data from 23 monitoring sites in seven States. (Schmidt and Cavender,
2008). This analysis considered both source-

[[Continued on page 29233]]

]

[[pp. 29233-29282]] National Ambient Air Quality Standards for Lead

[[Continued from page 29232]]

oriented and nonsource-oriented sites. In this analysis, EPA identified
only three of the 23 monitoring sites with collocated data as being
source-oriented. One of these sites was near an operating Pb smelter at
the time of the collocated monitoring; Pb emissions from smelters
typically contain both ultra-coarse particles from materials handling
and resuspension of contaminated dust, and fine and coarse particles
from the high temperature smelting operation itself. However, since
this study was conducted, EPA has promulgated a Maximum Achievable
Control Technology (MACT) standard for primary lead smelting that
controls process and fugitive dust emissions. (64 FR 30194, June 4,
1999). The other two source-oriented sites include one located near a
battery manufacturer, and one located near an automobile plant. The
data for the smelter site was collected in 1988 and indicate an average
Pb-TSP concentration of about 2.5 [mu]g/m\3\. The data for the battery
manufacturer site were collected in the mid-1990s and indicate an
average Pb-TSP concentration of about 0.09 [mu]g/m\3\; data for the
third site, located near an automotive plant, collected within the past
5 years, indicate an average Pb-TSP concentration at that site of about
0.03 [mu]g/m\3\. As discussed in Schmidt and Cavender (2008), ratios
between Pb-TSP and Pb-PM10 concentrations varied somewhat
within the data for each site, but the ratios between the Pb-TSP and
Pb-PM10 concentration averages were 2.0 for the smelter site
(based on 20 data pairs), 1.6 at the site near the battery manufacturer
(based on 107 data pairs), and 1.1 at the site located near an
automotive plant (based on 167 data pairs).
    Collectively, these three monitoring sites suggest that site-
specific scaling factors for source-oriented monitoring sites may vary
between 1.1 and 2.0; the range may also be greater. EPA notes that in
selecting a default factor for source-oriented monitoring sites, if
that approach is taken in the final rule, it may be appropriate to
consider default adjustment factors from within the mid to upper part
of this range rather than the lower end to avoid the possibility of
underestimating the appropriate scaling factor for a large proportion
of the source-oriented sites for which States might choose the default
factor rather than conduct a local study. On this basis, EPA invites
comment on the possibility of providing a default factor(s) for source-
oriented sites and on the selection of a value(s) from within this
range for all source-oriented monitoring sites, as an option to the
proposed requirement for development a site-specific factor through
analysis of paired monitoring data. EPA invites comment on the
selection of a single or multiple default factors for source-oriented
sites from within this range. While the selection of the scaling factor
in concept could depend on a characterization of the particle size mix
emitted by the Pb source, we note that reliable information on the mix
of coarse and ultra-coarse particles may often be unavailable. For
example, EPA could select a default factor that is at or near the upper
end of the range, 2.0, to avoid the risk of underprotection in
situations in which there is as high or nearly as high a proportion of
ultra-coarse Pb as at the smelter site. Alternatively, EPA could
discount the smelter data set on the basis that the 1988 data set does
not reasonably represent any likely current or future smelter
situation. Similarly, EPA could rely on the data taken near the
automotive plant since it is the most recent and largest dataset. EPA
also invites comment on other sets of paired data from near Pb sources
of which we may be unaware, and comment on other approaches of
selecting a default factor for the final rule based on paired data,
including approaches that might use more than one default factor for
source-oriented monitoring sites with the selection of the factor for a
given monitoring site depending on the characteristics of the nearby
sources, the ambient concentration of Pb-PM10, or other
factors.
    EPA also invites comment on whether and what default scaling
factor(s) should be established for monitoring sites which, as far as
is known, are not influenced by nearby emission sources. We have
reviewed paired data from the 20 monitoring sites that appear to fit
this description (Schmidt and Cavender, 2008). Average Pb-TSP
concentrations at nearly all these sites were near to or below the
lowest concentration on which comments are invited as to the NAAQS
level. Judging from ratios at these 20 sites, it appears that site-
specific factors generally range from 1.0 to 1.4 (with the factors for
three sites ranging from 1.8 to 1.9), and the ratios may be influenced
by measurement variability in both samplers as well as by actual air
concentrations. Given the relatively low ambient concentrations that we
believe currently prevail at nonsource-oriented sites, the value of a
default scaling factor selected within the range of 1.0 to 1.4 would
have little effect on the NAAQS compliance determination at such sites.
EPA invites comment on the approach of requiring use of a default
factor(s) for adjusting Pb-PM10 data at nonsource-oriented
sites and on the selection of a value(s) from within the range of 1.0
to 1.4 and also solicits comment on selection of a default scaling
factor from within the broader range of 1.0 to 1.9. We note that
allowing the use of a default scaling factor of 1.0 for nonsource-
oriented sites would in effect allow a State the option of comparing
Pb-PM10 data directly to the level of the Pb-TSP standard at
nonsource-oriented monitoring sites, without conducting a site-specific
study. Below, and in section II.E.4, EPA discusses the possibility of
revising the indicator to Pb-PM10, which would result in
such unadjusted comparisons of Pb-PM10 data to the standard
at all monitoring sites.
    EPA recognizes that the available data from collocated monitoring
of Pb-TSP and Pb-PM10, described above, have limitations
which make their interpretation and use in selecting default scaling
factors subject to considerable uncertainty. All of the Pb-
PM10 measurements at these sites were made with high-volume
PM10 samplers, which are more variable than the low-volume
samplers for which scaling factors would actually be applied after the
final rule; this greater variability no doubt has added to the
variation in ratios discussed above. Only three source-oriented sites
have collocated data; with such a small sample of sites both the range
of ratios and the distribution of ratios among all current and future
source-oriented sites remains uncertain. There were many more
nonsource-oriented sites which tended to show notably lower ratios,
implying lower scaling factors, but all had relatively low
concentrations; these ratios may or may not be representative of
monitoring sites near well controlled Pb sources. In many cases, the
period of collocated testing was only a few months; ratios observed in
such a short period may not be representative of ratios that occur at
other times of the year that may be more critical to attainment status.
Also, EPA has not yet had the benefit of CASAC review of the detailed
compilation of these data, as (Schmidt and Cavender, 2008) was prepared
subsequent to the most recent consultation with CASAC's AAMM
Subcommittee. Because of these uncertainties, EPA is proposing to
require States that wish to use Pb-PM10 data for a Pb-TSP
standard to develop site-specific scaling factors based on their own
collocated monitoring using paired Pb-TSP and low-volume Pb-
PM10 samplers over at least a one-year period, as described
in section IV. EPA intends

to encourage States to consider conducting local studies, even if the
final rule allows the use of default factors. Also, EPA invites comment
on whether to provide for the use of default scaling factors, and the
values of those factors.
    As a possible second option, taking into consideration the advice
of the CASAC Pb Panel and members of the CASAC AAMM Subcommittee, EPA
has also considered potential revision of the indicator to Pb-
PM10. In so doing, we recognize several potential important
benefits of such a revision, as well as the need to reflect such a
revision in the selection of level of the standard.\136\ We recognize
that the low volume PM10 sampler provides better precision
and size selection characteristics which would make the associated data
more comparable across sites.
---------------------------------------------------------------------------

    \136\ EPA recognizes and has specifically considered that such a
decision would affect the selection of the level of the standard,
recognizing that it is the combination of indicator and level (with
averaging and time and form) that determine the degree of protection
afforded by the standard. Section II.E.4 further considers the
impact of adoption of a Pb-PM10 indicator on the
selection of a level for the standard.
---------------------------------------------------------------------------

    In considering a potential revision of the indicator to Pb-
PM10, we recognize that an important issue is whether
regulating concentrations of Pb-PM10 will lead to
appropriate controls on all particle size Pb emissions from sources.
For example, it would be of concern if a NAAQS based on a Pb-
PM10 indicator resulted in different emissions control
decisions at sources with a large percentage of Pb in the size range
not substantially captured by PM10 sampling (e.g., fugitive
dust emissions from Pb smelters) than the emission control decisions
that would be made if the NAAQS was based on Pb-TSP. In that case, a
PM10-based NAAQS might not yield emissions changes by some
Pb sources which under a Pb-TSP indicator would have contributed to
NAAQS exceedances and subsequent emissions changes. Alternatively,
while collocated Pb-TSP and Pb-PM10 data are lacking for a
broad range of source types, there are likely many sources (e.g., high
temperature combustion processes) for which virtually all of the
emitted particles represented in a Pb-TSP measurement would be captured
by a Pb-PM10 measurement. Further, there are likely other
source types with a range of particle sizes extending beyond Pb-
PM10, for which controls adopted to meet a Pb-
PM10 requirement would also achieve a proportional reduction
in ultra-coarse particles. In these situations, one might not expect
any difference in emissions control decisions whether the NAAQS is Pb-
PM10-based or Pb-TSP-based.
    If the indicator were to be revised to Pb-PM10, low-
volume Pb-PM10 samplers would become the required approach
to Pb monitoring at required monitoring sites and would be a logical
choice wherever else NAAQS-oriented Pb monitoring is undertaken.
Nonetheless EPA notes that retaining Pb-TSP monitors at some relatively
small subset of the Pb-PM10 monitoring sites would be
beneficial for purposes of scientific understanding of both ambient
conditions and the performance of the two types of measurement systems.
    For reasons discussed here, and taking into account information and
assessments presented in the Criteria Document, Staff Paper, and ANPR,
the advice and recommendations of CASAC and of members of the CASAC
AAMM Subcommittee, and public comments to date, the Administrator
proposes to retain the current indicator of Pb-TSP, measured by the
current FRM, a current FEM, or an FEM approved under the proposed
revisions to 40 CFR part 53, but with expansion of the measurements
accepted for determining attainment or nonattainment of the Pb NAAQS to
provide an allowance for use of Pb-PM10 data, measured by
the new low-volume Pb-PM10 FRM specified in the proposed
appendix Q to 40 CFR part 50 or by a FEM approved under the proposed
revisions to 40 CFR part 53, with site-specific scaling factors as
described above and more specifically below in section IV. The
Administrator invites comment on also providing States the option of
using default scaling factors instead of conducting the testing that
would be needed to develop the site-specific scaling factors. In
consideration of all of the issues discussed above, the Administrator
also invites comment on a second option, a revision of the current
indicator to Pb-PM10. (Considerations related to the level
of a standard based on a PM10 indicator are discussed below
in section II.E.4.) The Administrator solicits comment on all of the
issues discussed above, and specifically with regard to the potential
for a Pb-PM10 indicator to influence implementation of
controls in ways that would lead to less control associated with larger
particles than might be achieved with a Pb-TSP-based NAAQS, taking into
account the variability noted above for TSP sampling.
2. Averaging Time and Form
    The statistical form of the current standard is a not-to-be-
exceeded or maximum value, averaged over a calendar quarter. This might
also be described as requiring that no average air Pb concentration
representing a time period of duration as long as calendar quarter (or
longer) may exceed the level of the standard. As noted in section
II.D.1.a, EPA set the standard in 1978 as a ceiling value with the
conclusion that this air level would be safe for indefinite exposure
for young children (43 FR 46250).
    The basis for selection of the current standard's averaging time of
calendar quarter reflects consideration of the evidence available when
the Pb NAAQS were promulgated in 1978. At that time, the Agency had
concluded that the level of the standard, 1.5 [mu]g/m\3\, would be a
``safe ceiling for indefinite exposure of young children'' (43 FR
46250), and that the slightly greater possibility of elevated air Pb
levels for shorter periods within the quarterly averaging period as
contrasted to the monthly averaging period proposed in 1977 (43 FR
63076), was not significant for health. These conclusions were based in
part on the Agency's interpretation of the health effects evidence as
indicating that 30 [mu]g/dL was the maximum safe level of blood Pb for
an individual child.
    With regard to averaging time, after consideration of the evidence
available at that time, the 1990 Staff Paper concluded that ``[a]
monthly averaging period would better capture short-term increases in
lead exposure and would more fully protect children's health than the
current quarterly average'' (USEPA, 1990b). The 1990 Staff Paper
further concluded that ``[t]he most appropriate form of the standard
appears to be the second highest monthly average in a 3-year span. This
form would be nearly as stringent as a form that does not permit any
exceedances and allows for discounting of one `bad' month in 3 years
which may be caused, for example, by unusual meteorology.'' In their
review of the 1990 Staff Paper, the CASAC Pb Panel concurred with the
staff recommendation to express the lead NAAQS as a monthly standard
not to be exceeded more than once in three years.
    As summarized in section II.B above and discussed in detail in the
Criteria Document, the currently available health effects evidence
\137\ indicates a wider variety of neurological effects, as well as
immune system and hematological effects, associated with substantially
lower blood Pb levels in children than were recognized when the
standard was set in 1978. Further, the health effects evidence with
regard to characterization of a threshold for

adverse effects has changed since the standard was set in 1978, as have
the Agency's views on the characterization of a safe blood Pb
level.\138\ In consideration of averaging time for the Pb NAAQS, we
note the following aspects of the current health effects evidence.
---------------------------------------------------------------------------

    \137\ The differing evidence and associated strength of the
evidence for these different effects is described in detail in the
Criteria Document.
    \138\ For example, EPA recognizes today that ``there is no level
of Pb exposure that can yet be identified, with confidence, as
clearly not being associated with some risk of deleterious health
effects'' (CD, p. 8-63).
---------------------------------------------------------------------------

     Children are exposed to ambient Pb via inhalation and
ingestion, with Pb that is taken into the body absorbed through the
lungs and through the gastrointestinal tract. Studies on Pb uptake,
elimination, and distribution show that Pb is absorbed into peripheral
tissues in adults within a few days (USEPA 1986a; USEPA 1990b, p. IV-
2). Absorption of Pb from the gastrointestinal tract appears to be
greater and faster in children as compared to adults (CD, Section
4.2.1). Once absorbed, it is quickly distributed from plasma to red
blood cells and throughout the body.
     Lead accumulates in the body and is only slowly removed,
with bone Pb serving as a blood Pb source for years after exposure and
as a source of fetal Pb exposure during pregnancy (CD, Sections <IP_ADDRESS>
and <IP_ADDRESS>).
     Blood Pb levels, including levels of the toxicologically
active fraction, respond quickly to increased Pb exposure, such that an
abrupt increase in Pb uptake rapidly changes blood Pb levels. The
associated time to reach a new quasi-steady state with the total body
burden after such an occurrence is projected to be approximately 75 to
100 days (CD, p. 4-27).
     The elimination half-life, which describes the time for
blood Pb levels to stabilize after a reduction in exposure, for the
dominant phase for blood Pb responses to changes in exposure is on the
order of 20 to 30 days for adults (CD, p. 4-25). Blood elimination
half-lives are influenced by contributions from bone. Given the tighter
coupling in children of bone stores with blood levels, children's blood
Pb is expected to respond more quickly than adults (CD, pp. 4-20 and 4-
27).
     Data from NHANES II and an analysis of the temporal
relationship between gasoline consumption data and blood lead data
generally support the inference of a prompt response of children's
blood Pb levels to changes in exposure. Children's blood Pb levels and
the number of children with elevated blood Pb levels appear to respond
to monthly variations in Pb emissions from Pb in gasoline (EPA, 1986a,
p. 11-39; Rabinowitz and Needleman, 1983; Schwartz and Pitcher, 1989;
USEPA, 1990b).
     The evidence with regard to sensitive neurological effects
is limited in what it indicates regarding the specific duration of
exposure associated with effect, although it indicates both the
sensitivity of the first 3 years of life and a sustained sensitivity
throughout the lifespan as the human central nervous system continues
to mature and be vulnerable to neurotoxicants (CD, Section <IP_ADDRESS>).
The animal evidence supports our understanding of periods of
development with increased vulnerability to specific types of effect
(CD, Section 5.3), and indicates a potential importance of exposures on
the order of months.
     Evidence of a differing sensitivity of the immune system
to Pb across and within different periods of life stages indicates a
potential importance of exposures as short as weeks to months duration.
For example, the animal evidence suggests that the gestation period is
the most sensitive life stage followed by early neonatal stage, and
within these life stages, critical windows of vulnerability are likely
to exist (CD, Section 5.9 and p. 5-245).
    Evidence described in the Criteria Document and the risk assessment
indicate that ingestion of dust can be a predominant exposure pathway
for young children to air-related Pb, and that there is a strong
association between indoor dust Pb levels and children's blood Pb
levels. As stated in the Criteria Document, ``given the large amount of
time people spend indoors, exposure to Pb in dusts and indoor air can
be significant'' (CD, p. 3-27). The Criteria Document further describes
studies that evaluated the influence of dust Pb exposure on children's
blood Pb: ``Using a structural equation model, Lanphear and Roghmann
(1997) also found the exposure pathway most influential on blood Pb was
interior dust Pb loading, directly or through its influence on hand Pb.
Both soil and paint Pb influenced interior dust Pb; with the influence
of paint Pb greater than that of soil Pb. Interior dust Pb loading also
showed the strongest influence on blood Pb in a pooled multivariate
regression analysis (Lanphear et al., 1998).'' (CD, p. 4-134). Further,
a recent study of dustfall near an open window in New York City
indicates the potential for a relatively rapid response of indoor dust
Pb loading to ambient airborne Pb, on the order of weeks (CD, p. 3-28;
Caravanos et al., 2006a).
    We note that the health effects evidence identifies varying length
durations in exposure that may be relevant and important. In light of
uncertainties in aspects such as response times of children's exposure
to airborne Pb, we recognize, as in the past, that this evidence
provides a basis for consideration of both calendar quarter and
calendar month as averaging times.
    In considering averaging time and form, EPA has combined the
current quarterly averaging time with the current not-to-be exceeded
(maximum) form and has also combined a monthly averaging time with a
second maximum form, so as to provide an appropriate degree of year-to-
year stability that a maximum monthly form would not afford. We also
note that, as discussed below, the second maximum monthly form provides
a roughly comparable degree of protection on a broad national scale.
    In this consideration of averaging time and form, EPA has taken
into account analyses using air quality data for 2003-2005 that are
presented in the Staff Paper (chapter 2). These analyses consider both
a period of three calendar years and a period of one calendar year
(with the form of the current standard being the maximum quarterly
mean). These analyses indicate that, with regard to either single-year
or 3-year statistics for the 2003-2005 dataset, a second maximum
monthly mean yields very similar, although just slightly greater,
numbers of sites exceeding various alternative levels as a maximum
quarterly mean, with both yielding fewer exceedances than a maximum
monthly mean.\139\ That is, these two averaging time and form
combinations resulted in roughly the same number of areas that would
not attain a standard at any given level on a broad national scale,
suggesting roughly comparable public health protection. However, the
relative protection provided by these two forms may differ from area to
area. For example, some of the areas meeting a maximum quarterly mean
standard over the 2003-2005 period at a given level did not meet a
second maximum monthly mean standard at the same level because there
were at least two months with high monthly concentrations which were
averaged with a lower concentration month in the same quarter. On the
other hand,

theoretically it is possible for an area to meet a given standard level
with a second maximum monthly mean averaging time and form and not meet
it for a maximum quarterly mean (e.g., the second highest monthly
average may be below the standard level while the quarterly average may
exceed it). Moreover, control programs to reduce quarterly mean
concentrations may not have the same protective effect as control
programs aimed at reducing concentrations in every individual month.
Given the limited scope of the current monitoring network which lacks
monitors near many significant Pb sources and uncertainty about Pb
source emissions and possible controls, it is difficult to more
quantitatively compare the protectiveness of the quarterly mean versus
the second maximum monthly mean approaches.
---------------------------------------------------------------------------

    \139\ For example, 49 sites (of 189) exceed a standard level of
0.10 [mu]g/m\3\ based on a form of maximum quarterly mean while 54
sites exceed based on a form of second maximum monthly mean.
Further, 25 sites exceed a standard level of 0.30 [mu]g/m\3\ based
on a form of maximum quarterly mean while 29 sites exceed based on a
form of second maximum monthly mean (Staff Paper, Table 2-6).
---------------------------------------------------------------------------

    In their advice to the Agency in this review, CASAC has recommended
that consideration be given to changing from a calendar quarter to a
monthly averaging time (Henderson, 2007a, 2007b, 2008). In making that
recommendation, CASAC emphasizes support from studies that suggest that
blood Pb concentrations respond at shorter time scales than would be
captured completely by quarterly values, as indicated by their
description of their recommendation for adoption of a monthly averaging
time as ``more protective of human health in light of the response of
blood lead concentrations that occur at sub-quarterly time scales''
(Henderson, 2007a). With regard to form of the standard, CASAC has
stated that one could ``consider having the lead standards based on the
second highest monthly average, a form that appears to correlate well
with using the maximum quarterly value'', while also indicating that
``the most protective form would be the highest monthly average in a
year'' (Henderson, 2007a).
    Among the public comments the Agency received on the discussion of
averaging time and form in the ANPR, the majority concurred with the
CASAC recommendation for a revision of the averaging time to a calendar
month.
    The 1990 Staff Paper and the Staff Paper for this review both
recommended that the Administrator consider specifying, in the form of
the NAAQS, that compliance with the NAAQS will be evaluated over a 3-
year period. The Administrator has considered this recommendation and
is proposing to adopt it. In the 3-year approach, a monitor would be
considered to be in violation of the NAAQS as of a certain date if in
any of the three previous calendar years with sufficiently complete
data (as explained in detail in section IV below), the value of the
selected form of the indicator (e.g., second maximum monthly average or
maximum quarterly average) exceeded the level of the NAAQS. A monitor,
initially or after once having violated the NAAQS, would not be
considered to have attained the NAAQS until three years have passed
without the form and level of the standard being violated. Many types
of Pb sources have variable emissions from day-to-day and year-to-year
due to market conditions for their products and/or weather variations
that can affect the generation of fugitive dust from contaminated
roadways and grounds. In addition, variations in wind patterns from
year to year can cause a near-source Pb monitor to be exposed to high
concentrations on more days in one year than in another, even if source
emissions are constant, especially if it operates on only some days.
Thus, it is possible for a monitor to indicate a violation of a
hypothetical form and level in one period but not in another, even if
no permanent controls have been applied at nearby source(s). Analysis
of historical Pb air concentration data has confirmed that this pattern
of fluctuating monitoring results can happen at the levels and forms
being proposed. It would potentially reduce the public health
protection afforded by the standard if areas fluctuated in and out of
formal nonattainment status so frequently that states do not have
opportunity and incentive to identify sources in need of more emission
control and to require those controls to be put in place. The 3-year
approach would help ensure that areas initially found to be violating
the NAAQS have effectively controlled the contributing lead emissions
before being redesignated to attainment/maintenance.
    In considering averaging time and form for the standard, the
Administrator has considered the information summarized above
(described in more detail in Criteria Document and Staff Paper), as
well as the advice from CASAC and public comments. The Administrator
recognizes that there is support in the evidence for a monthly
averaging time consistent with the following observations: (1) The
health evidence indicates that very short exposures can lead to
increases in blood Pb levels, (2) the time period of response of indoor
dust Pb to airborne Pb can be on the order of weeks, and (3) the health
evidence indicates that adverse effects may occur with exposures during
relatively short windows of susceptibility, such as prenatally and in
developing infants.\140\ The Administrator also recognizes limitations
and uncertainties in the evidence including the limited available
evidence specific to the consideration of the particular duration of
sustained airborne Pb levels having the potential to contribute to the
adverse health effects identified as most relevant to this review, as
well as variability in the response time of indoor dust Pb loading to
ambient airborne Pb.
---------------------------------------------------------------------------

    \140\ The health evidence with regard to the susceptibility of
the developing fetus and infants is well documented in the evidence
as described in the 1986 Criteria Document, the 1990 Supplement
(e.g., chapter III) and the 2006 Criteria Document. For example,
``[n]eurobehavioral Neurobehavioral effects of Pb-exposure early in
development (during fetal, neonatal, and later postnatal periods) in
young infants and children (7 years old) have been observed
with remarkable consistency across numerous studies involving
varying study designs, different developmental assessment protocols,
and diverse populations.'' (CD, p. E-9)
---------------------------------------------------------------------------

    Based on these considerations and the air quality analyses
summarized above, the Administrator concludes that this information
provides support for an averaging time no longer than a calendar
quarter. Further, the Administrator recognizes that if substantial
weight is given to the evidence of even shorter times for response of
dust Pb, blood Pb, and associated effects to airborne Pb, a monthly
averaging time may be appropriate. Accordingly, the Administrator is
proposing two options with regard to the form and averaging time for
the standard, and with both he proposes making the time period
evaluated in considering attainment be 3 years. One option is to retain
the current not-to-be-exceeded form with an averaging time of a
calendar quarter, such that the form would be maximum quarterly average
across a 3-year span. The second option is to revise the averaging time
to a calendar month and the form to be the second highest monthly
average across a 3-year span. Based on the considerations discussed
above, EPA requests comment on whether a level for a NAAQS with a
monthly averaging time and a second-highest monthly average form should
be based on an adjustment to a higher level than the level for a NAAQS
with a quarterly averaging time and a not-to-be-exceeded form, and, if
so, on the magnitude of the adjustment that would be appropriate.
3. Level for a Pb NAAQS With a Pb-TSP Indicator
    With regard to level of the standard, for a standard using a Pb-TSP
indicator, we first discuss evidence-based and exposure/risk-based
considerations, including considerations and

conclusions of the Staff Paper, in sections II.E.3.a and II.E.3.b
below. This is followed by a summary of CASAC advice and
recommendations and public comments (section II.E.3.c) and the
Administrator's proposed conclusions (section II.E.3.d). In addition,
we discuss considerations and solicit comment with regard to a level of
a standard using a Pb-PM10 indicator in section II.E.4
below.
a. Evidence-Based Considerations
    As a general matter, EPA recognizes that in the case of Pb there
are several aspects to the body of epidemiological evidence that add
complexity to the selection of an appropriate level for the primary
standard. As summarized above and discussed in greater depth in the
Criteria Document (CD, Sections 4.3 and 6.1.3), the epidemiological
evidence that associates Pb exposures with health effects generally
focuses on blood Pb for the dose metric.\141\ In addition, exposure to
Pb comes from various media, only some of which are air-related. This
presents a more complex situation than does evidence of associations
between occurrences of health effects and ambient air concentrations of
an air pollutant, such as is the case for particulate matter and ozone.
Further, for the health effects receiving greatest emphasis in this
review (neurological effects, particularly neurocognitive and
neurobehavioral effects, in children), no threshold levels can be
discerned from the evidence. As was recognized at the time of the last
review, estimating a threshold for toxic effects of Pb on the central
nervous system entails a number of difficulties (CD, pp. 6-10 to 6-11).
The task is made still more complex by support in the evidence for a
nonlinear rather than linear relationship of blood Pb with
neurocognitive decrement, with greater risk of decrement-associated
changes in blood Pb at the lower levels of blood Pb in the exposed
population (Section 3.3.7; CD, Section 6.2.13). In this context EPA
notes that the health effects evidence most useful in determining the
appropriate level of the NAAQS is this large body of epidemiological
studies. Unlike the recent review of the NAAQS for ozone, there are no
clinical studies useful for informing a determination of the
appropriate level for a standard.\142\ The discussion below therefore
focuses on the epidemiological studies, recognizing and taking into
consideration the complexity and resulting uncertainty in using this
body of evidence to determine the appropriate level for the NAAQS.
---------------------------------------------------------------------------

    \141\ Among the studies of Pb health effects, in which blood Pb
level is generally used as an index of exposure, the sources of
exposure vary and are inclusive of air-related sources of Pb such as
smelters (e.g., CD, chapter 6).
    \142\ See, e.g., 72 FR 37878-9 (July 11, 2007) (Ozone NAAQS
Notice of Proposed Rulemaking).
---------------------------------------------------------------------------

    In considering the evidence with regard to selection of the level
of the standard, the Agency has considered the same evidence-based
frameworks discussed above in section II.D.2.a on the adequacy of the
current standard. That is, the Staff Paper considered how to apply an
adapted 1978 framework to the much expanded body of evidence that is
now available, and the Agency has further considered this evidence in
the context of the air-related IQ loss evidence-based framework that
builds on a recommendation by the CASAC Pb Panel. These evidence-based
approaches are discussed below in considering the appropriate standard
levels to propose.
    As noted in section II.D.2.a above, this review focuses on young
children as a key sensitive population for Pb exposures. In this
sensitive population, the current evidence demonstrates the occurrence
of health effects, including neurological effects, associated with
blood Pb levels extending well below 10 [mu]g/dL (CD, sections 6.2, 8.4
and 8.5). As further described in section II.D.2.a above, some studies
indicate Pb effects on intellectual attainment of children for which
population mean blood Pb levels in the analysis ranged from
approximately 2 to 8 [mu]g/dL (CD, Sections 6.2, 8.4.2 and <IP_ADDRESS>).
Further, as noted above, the current evidence does not indicate a
threshold for the more sensitive health endpoints such as neurological
effects in children (CD, pp. 5-71 to 5-74 and Section 6.2.13).\143\
---------------------------------------------------------------------------

    \143\ This differs from the Agency's recognition in the 1978
rulemaking of a threshold of 40 [mu]g/dL blood Pb for an individual
child for effects of Pb considered clearly adverse to health at that
time, i.e., impairment of heme synthesis and other effects which
result in anemia.
---------------------------------------------------------------------------

    As when the standard was set in 1978, there remain today
contributions to blood Pb levels from nonair sources. As discussed
above (section II.D.2), current evidence is limited with regard to
estimates of the aggregate reduction since 1978 of all nonair sources
to blood Pb and with regard to an estimate of current nonair blood Pb
levels (discussed more fully in sections II.A.4) In recognition of
temporal reductions in nonair sources discussed in section II.A.4 and
in the context of estimates pertinent to an application of the 1978
framework, the CASAC Pb Panel recommended consideration of 1.0 to 1.4
[mu]g/dL or lower as an estimate of the nonair component of blood Pb
pertinent to average blood Pb levels in children (as described in
section II.A.4 above; Henderson, 2007a). The Staff Paper considered
this range of 1.0 to 1.4 [mu]g/dL for the nonair component of blood Pb
in its application of the adapted 1978 evidence-based framework.
    As discussed in section II.B.1.c, the current evidence in
conjunction with the results and observations drawn from the exposure
assessment support a focus on air-to-blood ratios for children in the
range of 1:3 to 1:7, based on consideration of both inhalation and
ingestion exposure pathways and on the lower air and blood Pb levels
pertinent to this review. In considerations here, we have described the
value of 1:5 as falling somewhat central within the range supported by
the evidence.
i. Evidence-Based Framework Considered in the Staff Paper
    Recommendations in the Staff Paper on standard levels were based
upon an approach that built upon and adapted the general approach used
by EPA in setting the standard in 1978. In adapting this approach to
the currently available information, the Staff Paper recognized the
more extensive and stronger body of evidence now available on a broader
range of health effects associated with exposure to Pb. For example,
EPA recognizes that today ``there is no level of Pb exposure that can
yet be identified, with confidence, as clearly not being associated
with some risk of deleterious health effects'' (CD, p. 8-63). This is
in contrast to the situation in 1978 when the Agency judged that the
maximum safe individual and geometric mean blood Pb levels for a
population of young children were 30 [mu]g/dL and 15 [mu]g/dL,
respectively.\144\
---------------------------------------------------------------------------

    \144\ More specifically, when the standard was set in 1978, the
Agency stated that the population mean, measured as the geometric
mean, must be 15 [mu]g/dL in order to ensure that 99.5 percent of
children in the United States would have a blood Pb level below 30
[mu]g/dL, which was identified as the maximum safe blood Pb level
for individual children based on the information available at that
time (43 FR 46247-46252).
---------------------------------------------------------------------------

    In the Staff Paper application of an adapted 1978 framework, the
focus shifted away from identifying a safe blood Pb level for an
individual child (and then determining an ambient air level that would
keep a very high percentage of children at or below that safe level),
because information was no longer available to identify such a level.
Rather, the Staff Paper approach focused on identifying an appropriate
population mean blood Pb level, and then identifying an ambient air
level that would keep the mean blood Pb levels of children exposed at
that air level below the target population mean blood Pb level. Based
on the review of

the evidence, the Staff Paper approach substituted a level of 2 [mu]g/
dL for the target population geometric mean blood Pb of 15 [mu]g/dL
used in 1978. In the absence of a demonstrated safe level, at either an
individual or a population level, the Staff Paper used 2 [mu]g/dL as
representative of the lowest population mean level for which there is
evidence of a statistically significant association between blood lead
levels and health effects (e.g., CD, p. E-9; Lanphear et al., 2000).
    This approach does not evaluate the magnitude or degree of health
effects occurring across the population at that mean blood lead level.
In this adaptation of the 1978 approach the focus is solely on the
existence of a relationship between blood lead levels and
neurocognitive effects. The approach takes as the public health goal
the identification of an ambient air lead level that can be expected to
keep the mean blood lead level of an exposed population of children at
or below the lowest level at which a statistically significant
association has been demonstrated between blood lead level and
neurocognitive effects.\145\
---------------------------------------------------------------------------

    \145\ There are some similarities between this approach and the
approach employed in determining the levels for the daily and annual
PM standards in the latest PM review, where EPA determined an
ambient PM level based on the ambient levels in the epidemiology
studies that found statistically significant associations between
changes in ambient PM levels and changes in occurrences of health
effects. See 71 FR 61144 (October 17, 2006). However, there are
several important differences in this adaptation to the 1978
approach for lead. For example, the health effects evaluated in the
PM epidemiological studies were clearly adverse health effects,
ranging from hospital admissions to premature mortality. In
addition, the studies looked directly at the association between
ambient level and occurrences of health effects. Here the
epidemiology studies look at the association between blood lead
level and neurocognitive effect, and there is an additional step to
link the blood lead level to air-related lead. In addition, at a
population level there is a less clear delineation of when the
neurocognitive effect is adverse to public health. This is discussed
below in this section with respect to the impact on public health of
a shift in the mean IQ of a population of children.
---------------------------------------------------------------------------

    Starting with a target population geometric mean blood lead level
of 2 [mu]g/dL for the population of exposed children, then subtracting
1 to 1.4 [mu]g/dL for the nonair component of blood Pb, yields 0.6 to 1
[mu]g/dL as a target for the geometric mean air contribution to blood
Pb. The adapted 1978 approach divides the air-related target by 5, an
air-to-blood ratio somewhat central within the range of air Pb to blood
Pb ratios generally supported by the currently available evidence. This
resulted in a potential standard level of 0.1 to 0.2 [mu]g/m\3\.
    The Staff Paper conclusions on level for the primary Pb standard
built on the staff's conclusion that the overall body of evidence
clearly calls into question the adequacy of the current standard with
regard to health protection afforded to at-risk populations. Based on
consideration of the health effects evidence, as described above, the
Staff Paper concluded that it is reasonable to consider a range for the
level of the standard, for which the upper part is represented by 0.1
to 0.2 [mu]g/m\3\.
ii. Air-related IQ Loss Evidence-Based Framework
    As mentioned above, in analyses subsequent to the Staff Paper and
ANPR, the Agency has primarily considered the evidence in the context
of an alternative evidence-based framework, referred to as the air-
related IQ loss framework. This framework focuses on the contribution
of air-related Pb to neurocognitive effects, with a public health goal
of identifying the appropriate ambient air level of Pb to protect
exposed children from health effects that are considered adverse, and
are associated with their exposure to air-related Pb. This framework
does not focus on overall blood lead levels or on nonair contribution
to blood lead levels. While this avoids some of the limitations noted
above with the adapted 1978 approach, EPA recognizes that looking at
air-related Pb in isolation from other sources of Pb could be
considered a limitation for this framework. The different limitations
of each of these frameworks derive from the limitations in the
underlying body of evidence available for this review.
    In this air-related IQ loss evidence-based framework, we have drawn
from the entire body of evidence as a basis for concluding that there
are causal associations between air-related Pb exposures and population
IQ loss. We have drawn more quantitatively from the evidence by
combining air-to-blood ratios with evidence-based C-R functions from
the epidemiological studies to quantify the association between air Pb
concentrations and air-related population mean IQ loss in exposed
children. This air-related IQ loss framework focuses on selecting a
standard that would prevent air-related IQ loss (and related effects)
of a magnitude judged by the Administrator to be of concern in
populations of children exposed to the level of the standard, taking
into consideration such factors as the uncertainties inherent in such
estimates. In addition to this judgment by the Administrator, this
framework is also based on specifying an air-to-blood ratio (also used
in the adapted 1978 framework) and a C-R function(s) for population
mean IQ response associated with blood Pb level.
    In considering the evidence with regard to C-R functions, and in
recognition of the finding in the evidence of a steeper slope at lower
blood Pb levels (i.e., the nonlinear relationship), we have identified
two sets of C-R functions (discussed more fully above in section
II.B.2.b). The first set focuses on C-R functions reflecting population
mean concurrent blood Pb levels of approximately 3 [mu]g/dL.\146\ The
second set (CD, pp. 8-78 to 8-80) considers functions descriptive of
the C-R relationship from a larger set of studies that include
population mean blood Pb levels ranging from a mean of 3.3 up to a
median of 9.7 [mu]g/dL (see Table 1).\147\
---------------------------------------------------------------------------

    \146\ As noted above in section II.B.2.b, the log-linear C-R
function with low-exposure linearization (LLL) used in the
quantitative risk assessment, based on log-linear model in Lanphear
et al 2005), has a slope that falls intermediate within this first
set of functions at low blood Pb levels. The log-linear model by
Lanphear et al (2005) is derived from the pooled International
dataset for which the median blood Pb is 9.7 [mu]g/dL.
    \147\ 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 discussed above in section II.B.2.b, the C-R functions from
analyses involving the lower mean blood Pb levels, that are closer to
current mean blood Pb levels in U.S. children, provide slopes of IQ
loss with increasing blood Pb that range from -1.71 to -2.94 IQ points
per [mu]g/dL blood Pb. These include C-R function from Lanphear et al.
(2005) recommended for consideration by CASAC, in light of the current
blood Pb levels of U.S. children (Henderson, 2008),\148\ and also the
C-R function

given greatest weight in the risk assessment (discussed above in
section II.C.2.b), the loglinear function with low-exposure
linearization (the LLL function). The function yielding the lowest
slope in this range is from the analysis by Tellez-Rojo and others
(2006) of very young children with blood Pb levels below 5 [mu]g/dL,
with a group mean blood Pb level of 2.9 [mu]g/dL. The function yielding
the highest slope in this range is from the analysis by Lanphear and
others (2005) of children whose blood Pb levels never exceeded 7.5
[mu]g/dL, with a group mean blood Pb level of 3.24 [mu]g/dL. The LLL
function falls within the range of the other two functions at lower
blood Pb levels, with an average slope of -2.29 IQ points per [mu]g/dL
across blood Pb levels extending below 2 [mu]g/dL.
---------------------------------------------------------------------------

    \148\ In their September 2007 letter, the CASAC Pb Panel
``recommends using the two-piece linear function for relating IQ
alterations to current blood lead levels with a slope change or
``hinge'' point closer to 7.5 [mu]g/dL than 10.82 [mu]g/dL as used
by EPA staff in the second draft exposure/risk assessments document.
The higher value used by staff underestimates risk at lower blood Pb
levels, where most of the population will be located. Epidemiologic
data indicate that the slope of the line below 7.5 [mu]g/dL is
approximately minus three (-3) IQ decrements per 1 [mu]g/dL blood
lead and the vast majority of children in the U.S. have maximal
baseline Pb blood levels below 7.5 [mu]g/dL (Lanphear et al., EHP
2005; MMWR 2005). On a population level, the mean increase in blood
lead concentration from airborne lead would generally be up to, but
not exceeding, a blood lead concentration of 7.5 [mu]g/dL. This
approach should also account for sensitive subpopulations of
children.'' In in their January 2008 letter, the Panel also points
to several other studies ``confirming that the relationship of lead
exposure is non-linear and per-sists at blood lead levels
considerably lower than 5 [mu]g/dL (Lanphear, 2000; Wasserman, 2003;
Kordas, 2006; Tellez-Rojo, 2006). In particular, Tellez-Rojo and co-
workers reported that the slope of the association between 24-month
blood lead and the 24-month Mental Development Index (MDI) for 294
children who had peak blood lead levels below 5 [mu]g/dL was
negative (-1.7 points for each 1 [mu]g/dL increase in blood lead
concentration, p=0.01). Collectively, these studies indicate that
there is sufficient evidence to support the use of the dose-response
relationship from the pooled analysis at blood lead levels < 5
[mu]g/dL (Lanphear, 2005), as described in the Final Lead Staff
Paper and previously recommended by CASAC.''
---------------------------------------------------------------------------

    The second set of C-R functions discussed in section II.B.2.b is
drawn from a larger group of studies, although these studies include
groups of children with higher blood Pb levels (CD, pp. 8-78 to 8-80)
such that the population mean levels for these studies include
population mean blood Pb levels ranging from a mean of 3.3 up to a
median of 9.7 [mu]g/dL (see Table 1). This second set of C-R functions
is represented by a median of -0.9 IQ points per [mu]g/dL blood Pb (CD,
p. 8-80).\149\
---------------------------------------------------------------------------

    \149\ As noted above (in section II.B.2.b), this slope is
similar to the slope for the below 10 [mu]g/dL piece of the
piecewise model used in the RRP rule economics analysis.
---------------------------------------------------------------------------

    In applying the air-related IQ loss evidence-based framework, as
with the adapted 1978 framework, we recognize uncertainty in our
estimates for the two input parameters (air-to-blood ratio and C-R
function slope). Accordingly, in associating various standard levels
with the estimated magnitudes of air-related mean IQ loss that would
likely be prevented by keeping exposed populations below such standard
levels, we have considered combinations of parameter estimates that are
potentially supportable within this framework. With regard to the C-R
functions we have drawn estimates from both sets of functions. For the
first set of C-R functions, we have relied on the upper and lower-end
values to provide a range at lower blood Pb levels, and have focused on
the LLL function for blood Pb levels above approximately 2.5 to 3.0
[mu]g/dL, as shown in Table 7.\150\ From the second set of C-R
functions, we have relied on the median estimate across the range of
blood Pb levels considered. For air-to-blood ratios, we have focused on
the estimate of 1:5 as above, and also provided IQ loss estimates using
higher and lower estimates of air-to-blood ratio (i.e., 1:3 and 1:7)
within the range supported by the evidence. These estimates are
presented in Table 7 below.
---------------------------------------------------------------------------

    \150\ We derived estimates of air-related IQ loss using the LLL
(nonlinear) function giving equal weight to all contributions of Pb
to total blood Pb as illustrated by the following example. For a
level of 0.30 [mu]g/m\3\, and an air-to-blood ratio of 1:5, the
resultant estimate of air-related blood Pb is 1.5 [mu]g/dL. Using
estimates for nonair blood Pb levels of 1 and 1.4 [mu]g/dL, the
estimates of total blood Pb are 2.5 and 2.9 [mu]g/dL. The
corresponding total Pb-related IQ loss estimates based on the LLL
function are 5.2 and 5.6 points IQ loss. These estimates are then
multiplied by the fraction of total Pb that is air-related (i.e.,
1.5/2.5 and 1.5/2.9) to derive the estimated range of air-related IQ
loss (2.9-3.1 points).

                      Table 7.--Estimates of Air-Related Population Mean IQ Loss for Children Exposed at the Level of the Standard
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                              Air-related population mean IQ loss (points) for children exposed at level of the standard
                             ---------------------------------------------------------------------------------------------------------------------------
                               Air-to-blood ratio of    Air-to-blood ratio of    Air-to-blood ratio of    Air-to-blood ratio of    Air-to-blood ratio of
Potential level for standard            1:3                      1:4                      1:5                      1:6                      1:7
         ([mu]g/m\3\)        ---------------------------------------------------------------------------------------------------------------------------
                               1st group   2nd group    1st group   2nd group    1st group   2nd group    1st group   2nd group    1st group   2nd group
                                of C-R       of C-R      of C-R       of C-R      of C-R       of C-R      of C-R       of C-R      of C-R      of C-R
                               functions   functions    functions   functions    functions   functions    functions   functions    functions   functions
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.50........................   * 2.9-3.1         1.4    * 3.5-3.8         1.8    * 4.1-4.3         2.3    * 4.6-4.8         2.7    * 5.0-5.3         3.2
0.40........................   * 2.4-2.6         1.1    * 3.0-3.2         1.4    * 3.5-3.8         1.8    * 4.0-4.2         2.2    * 4.4-4.6         2.5
0.30........................     1.5-2.6         0.8    * 2.4-2.6         1.1    * 2.9-3.1         1.4    * 3.3-3.5         1.6    * 3.6-3.9         1.9
0.20........................     1.0-1.8         0.5      1.4-2.4         0.7      1.7-2.9         0.9    * 2.4-2.6         1.1    * 2.7-3.0         1.3
0.10........................     0.5-0.9         0.3      0.7-1.2         0.4      0.9-1.5         0.5      1.0-1.8         0.5      1.2-2.1         0.6
0.05........................     0.3-0.4         0.14     0.3-0.6         0.18     0.4-0.7         0.2      0.5-0.9         0.27     0.6-1.0         0.3
0.02........................     0.1-0.2         0.05     0.1-0.2         0.07     0.2-0.3         0.09     0.2-0.4         0.1      0.2-0.4        0.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
* These estimates were derived using only the nonlinear C-R function from the risk assessment which, given its nonlinearity, EPA considers to better
  assess risk across the range that includes extending into these higher standard levels (and the associated higher blood Pb levels). That is, the upper
  and lower values presented in the asterisked cells are both derived using the LLL function, as described in the text and associated footnote above,
  rather than using the two linear functions of -1.71 from Tellez-Rojo, 2005 (<5 [mu]g/dL subgroup) and -2.94 from Lanphear, 2005 (<7.5 [mu]g/dL peak
  blood Pb subgroup) as is the case in the cells without asterisks.

    Using the air-to-blood ratio of 1:5 with the range of slopes from
the first set of C-R functions indicates an air-related mean IQ loss
estimate of 0.9 to 1.5 points for a population of children exposed at
the standard level of 0.10 [mu]g/m\3\. Similarly, the air-related mean
IQ loss estimate for a standard level of 0.20 [mu]g/m\3\ is 1.7 to 2.9
points. Using the air-to-blood ratio of 1:5 and the slope from the
second set of C-R functions (from blood Pb levels extending up to 10
[mu]g/dL) in the calculation indicates an air-related mean IQ loss of
0.5 points for a population of children exposed at the standard level
of 0.10 [mu]g/m\3\; the corresponding air-related mean IQ loss estimate
for a standard level of 0.20 [mu]g/m\3\ is 0.9 points. Using the 1:5
air-to-blood ratio with first set of C-R functions indicates an air-
related mean IQ loss estimate of approximately 3 points for a
population of children exposed at the standard level of 0.30 [mu]g/
m\3\. Using the slope from the second set of C-R functions indicates an
air-related mean IQ loss estimate of 1.4 points for a population of
children exposed at the standard level of 0.30 [mu]g/m\3\.

    As mentioned above, we recognize uncertainty in the air-to-blood
values, and have accordingly also considered estimates of air-to-blood
ratio that are lower and higher than the 1:5 value used above.
Accordingly, we note that using a lower air-to-blood ratio, such as 1:3
(low end of range from evidence) generally results in lower air-related
IQ loss estimates with either set of C-R functions (approximately 40%
lower than those using a ratio of 1:5). Similarly, use of a higher air-
to-blood ratio, such as 1:7, yields higher air-related mean IQ loss
estimates with either set of C-R functions (approximately 40% higher
than those using a ratio of 1:5).
    In applying this framework, we have also considered higher standard
levels, above 0.30 [mu]g/m\3\ up to the highest alternative level
included in the risk assessment (e.g., up to 0.50 [mu]g/m\3\). Using
the 1:5 air-to-blood ratio with the first set of C-R functions, the
air-related mean IQ loss estimate for a standard level of 0.50 [mu]g/
m\3\ is approximately 4 points. Using the slope from the second set of
C-R functions indicates an air-related mean IQ loss estimate of 2.3
points for a population of children exposed at the standard level of
0.50 [mu]g/m\3\. Using the 1:3 air-to-blood ratio with the first set of
C-R functions indicates an air-related mean IQ loss estimate of
approximately 3 points for a population of children exposed at the
standard level of 0.50 [mu]g/m\3\. Using the 1:3 air-to-blood ratio and
the slope for the second set of C-R functions indicates an air-related
mean IQ loss estimate of 1.4 points for a population of children
exposed at the standard level of 0.50 [mu]g/m\3\.
    Further, we have also considered lower standard levels, down to the
lowest alternative levels included in the risk assessment (e.g., 0.05
to 0.02 [mu]g/m\3\). For example, across both sets of C-R functions and
the range of air-to-blood ratios considered above (1:3 to 1:7), a
standard level of 0.05 [mu]g/m\3\ indicates an air-related mean IQ loss
of approximately 0.1 to 1 point. The estimates for either set of C-R
functions are approximately 50% lower at the standard level of 0.02
[mu]g/m\3\.
b. Exposure- and Risk-Based Considerations
    To inform judgments about a range of levels for the standard that
could provide an appropriate degree of public health protection, in
addition to considering the health effects evidence, EPA also
considered the quantitative estimates of exposure and health risks
attributable to air-related Pb upon meeting specific alternative levels
of alternative Pb standards and the uncertainties in the estimated
exposures and risks, as discussed above in Section III.B. As discussed
above, the risk assessment conducted by EPA is based on exposures that
have been estimated for children of less than 7 years of age in several
case studies. The assessment estimated the risk of adverse
neurocognitive effects in terms of IQ loss associated with total and
air-related Pb exposures, including incidence of different magnitudes
of IQ loss in the three location-specific case studies. In so doing,
EPA is mindful of the important uncertainties and limitations that are
associated with the exposure and risk assessments. For example, with
regard to the risk assessment important uncertainties include those
related to estimation of blood Pb C-R functions, particularly for blood
Pb concentrations at and below the lower end of those represented in
the epidemiological studies characterized in the Criteria Document.
    EPA also recognizes important limitations in the design of, and
data and methods employed in, the exposure and risk analyses. For
example, the available monitoring data for Pb relied upon for
estimating current conditions for the urban case studies are quite
limited, in that monitors are not located near many of the larger known
Pb sources, which results in potential underestimation of current
conditions, and there is uncertainty about the proximity of existing
monitors to other Pb sources potentially influencing exposures, such as
old urban roadways and areas where housing with Pb paint has been
demolished or has undergone extensive exterior renovation. All of these
limitations raise uncertainty as to whether these data adequately
capture the magnitude of ambient Pb concentrations to which the target
population is currently exposed. Additionally, EPA recognizes that
there is not sufficient information available to evaluate all relevant
sensitive groups (e.g., adults with chronic kidney disease) or all Pb-
related health effects (e.g., neurological effects other than IQ loss,
immune system effects, adult cardiovascular or renal effects), and the
scope of our analyses was generally limited to estimating exposures and
risks in case studies intended to illustrate a variety of Pb exposure
situations across the U.S., with three of them focused on specific
areas in three cities. As noted above, however, coordinated intensive
efforts over the last 20 years have yielded a substantial decline in
blood Pb levels in the United States. Recent NHANES data (2003-2004)
yield blood lead level estimates for children age 1 to 5 years of 1.6
[mu]g/dL (median) and 3.9 [mu]g/dL (90th percentile). These median and
90th percentile national-level data are lower than modeled values
generated for the three location-specific urban case studies current
conditions scenarios (described in section II.C.3.a above). As noted in
section II.C.3.a, however, the urban case studies and the NHANES study
are likely to differ with regard to factors related to Pb exposure,
including ambient air levels (e.g., the national median ambient air Pb
concentrations are generally lower than those in the location-specific
case studies).
    As described in section II.C.2.e, we also recognize limitations in
our ability to characterize the contribution of air-related Pb to total
Pb exposure and Pb-related health risk. As a result, we have
approximated estimates for the air-related pathways, bounded on the low
end by exposure/risk estimated for the ``recent air'' category and on
the upper end by the exposure/risk estimated for the ``recent air''
plus ``past air'' categories.\151\
---------------------------------------------------------------------------

    \151\ As noted in section II.C.2.e above, the recent air
category does not include a variety of air-related categories
(including some associated with air deposition to outdoor surfaces
and diet) and both the recent air and past categories may include
some Pb in soil or dust from the historical use of Pb in paint.
---------------------------------------------------------------------------

    We generally focus in this discussion on risk estimates derived
using the LLL (log-linear with low exposure linearization) C-R
function. Further, in considering the risk estimates in light of IQ
loss estimates (described in section II.E.3.a) of the air-related IQ
loss evidence-based framework, we focus here on risk estimates for the
general urban and primary Pb smelter subarea case studies as these
cases studies generally represent population exposures for more highly
air-pathway exposed children residing in small neighborhoods or
lozalized residential areas with air concentrations nearer the standard
level being evaluated than do the location-specific case studies in
which populations have a broader range of air-related exposures
including many well below the standard level being evaluated.
    In considering the results of the risk assessment for the
alternative standard levels assessed, we note that the risk estimates
are roughly consistent with and generally supportive of the evidence-
based mean air-related IQ loss estimates described above (section
II.E.3.a). For example, at a standard level of 0.20 [mu]g/m\3\, the
evidence-based approach indicates estimates of mean air-related IQ loss
ranging from less than

1 to approximately 3 points IQ loss, while the median air-related risk
estimates for this level in the general urban case study are
represented by a lower bound near 1 point IQ loss and an upper bound
near 3 points IQ loss. The corresponding upper bound air-related IQ
loss estimate for the primary Pb smelter case study subarea is 3.7
points. Alternatively, at a standard level of 0.50 [mu]g/m\3\, the
evidence-based approach indicates estimates of mean air-related IQ loss
ranging from approximately 1.5 points to greater than 4 points, while
the median air-related risk estimates for this level for the general
urban case study are represented by a lower bound near 2 points IQ loss
and an upper bound just below 4 points IQ loss (section II.C.3.b). The
corresponding upper bound air-related IQ loss estimate for the primary
Pb smelter case study subarea is 4.5 points. Also, while the risk
assessment did not specifically assess the standard levels of 0.10 and
0.30 [mu]g/m\3\, we note that estimates for these levels based on
interpolation from the estimates described above are also roughly
consistent with and generally supportive of the evidence-based mean
air-related IQ loss estimates described in section II.E.3.a above
(Murphy and Pekar, 2008).
    As mentioned above (section II.E.3.a), the Staff Paper conclusions
on level for the primary Pb standard built on staff 's conclusion that
the overall body of evidence clearly calls into question the adequacy
of the current standard with regard to health protection afforded to
at-risk populations. Drawing from both consideration of the evidence
and consideration of the quantitative risk and exposure information
(described in section II.E.3.b), staff concluded that the available
information provides strong support for consideration of a range of
standard levels that are appreciably below the level of the current
standard in order to provide increased public health protection for
these populations, with support for this conclusion. With regard to the
risk estimates, the Staff Paper recognized that, to the extent one
places weight on risk estimates for the lower standard levels, those
estimates may suggest consideration of a range of levels that extend
down to the lowest levels assessed in the risk assessment, 0.02 to 0.05
[mu]g/m\3\. In summary, the Staff Paper concluded that ``a level for
the standard set in the upper part of [the staff] recommended range
(0.1-0.2 [mu]g/m\3\, particularly with a monthly averaging time) is
well supported by the evidence and also supported by estimates of risk
associated with policy-relevant Pb that overlap with the range of IQ
loss that may reasonably be judged to be highly significant from a
public health perspective, and is judged to be so by CASAC'' (USEPA,
2007c). Further, the Staff Paper concluded that ``a standard set in the
lower part of the range would be more precautionary and would place
weight on the more highly uncertain range of estimates from the risk
assessment'' (USEPA, 2007c).
c. CASAC Advice and Recommendations and Public Comments
    Beyond the evidence- and risk/exposure-based information discussed
above, EPA's consideration of the level for the TSP-based standard also
takes into account the advice and recommendations of CASAC, based on
their review of the Criteria Document, the Staff Paper and the related
technical support document, and the ANPR, as well as comments from the
public on drafts of the Staff Paper and related technical support
document and the ANPR.
    In their advice to the Agency during this review CASAC has
recognized the importance of both the health effects evidence and the
exposure and risk information in selecting the level for the TSP-based
standard (Henderson, 2007a, 2007b, 2008). In two separate letters sent
prior to publication of the ANPR, CASAC stated that it is the unanimous
judgment of the CASAC Lead Panel that the primary NAAQS should be
``substantially lowered'' to ``a level of about 0.2 [mu]g/m\3\ or
less,'' reflecting their view of the health effects evidence
(Henderson, 2007a,b). In their most recent letter, reflecting their
review of the ANPR and Staff Paper, the Panel reiterated their earlier
judgment, stating that ``[t]he Committee unanimously and fully supports
Agency staff's scientific analyses in recommending the need to
substantially lower the level of the primary (public-health based) Lead
NAAQS, to an upper bound of no higher than 0.2 [mu]g/m\3\ with a
monthly averaging time.''
    The CASAC Pb Panel also provided advice regarding how the Agency
should consider IQ loss estimates derived from the risk assessment in
selecting a level for the standard (Henderson, 2007a). The Panel stated
that they consider a population loss of 1-2 IQ points to be ``highly
significant from a public health perspective''.
    Among the many public comments the Agency has received in this
review regarding the level of the standard, the overwhelming majority
recommended appreciable reductions in the level, e.g., setting it at
0.2 [mu]g/m\3\ or less, while only a few recommended that the Agency
make no or only a modest adjustment. Among the comments recommending
appreciable reduction, many noted the importance of considering
exposures and risks to vulnerable and susceptible populations. Some
recognized that blood Pb levels are disproportionately elevated among
minority and low-income children, and recommended more explicit
consideration of issues of environmental justice. And some comments
also noted the need for the standard to provide an adequate margin of
safety, indicating that such a need might provide support for
consideration of much lower levels. The American Academy of Pediatrics
recommended that EPA set the level at 0.2 or lower, and also
recommended that EPA consider the approach developed by the State of
California Environmental Protection Agency (Cal-EPA) for the purposes
of school site assessment, which has at its goal prevention of a rise
in blood Pb level that Cal-EPA has predicted to be associated with an
incremental increase estimated to decrease IQ by 1 point.
d. Administrator's Proposed Conclusion Concerning Level
    For the reasons discussed below, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments to date, the Administrator proposes to revise the existing
primary Pb standard. Specifically, the Administrator proposes to revise
the level of the primary Pb standard, defined in terms of the current
Pb-TSP indicator, to within the range of 0.10 to 0.30 [mu]g/m\3\,
conditional on judgments as to the appropriate values of key parameters
to use in the context of the air-related IQ loss evidence-based
framework discussed below.
    Further, in recognition of alternative views of the science, the
exposure and risk assessments, the uncertainties inherent in the
science and these assessments, and the appropriate public health policy
responses based on the currently available information, the
Administrator also solicits comments on whether to proceed instead with
alternative levels of a primary Pb-TSP standard within ranges from
above 0.30 [mu]g/m\3\ up to 0.50 [mu]g/m\3\ and below 0.10 [mu]g/m\3\.
Based on the comments received and the accompanying rationales, the
Administrator may adopt other standards within the range of the
alternative levels identified above in lieu of the standards he is
proposing today. In addition, as discussed below, the Administrator
also solicits comments on when, if ever, it would be

appropriate to set a NAAQS for Pb at a level of zero.
    The Administrator's consideration of alternative levels of the
primary Pb-TSP standard builds on his proposed conclusion, discussed
above in section II.D.4, that the overall body of evidence indicates
that the current Pb standard is not requisite to protect public health
with an adequate margin of safety and that the standard should be
revised to provide increased public health protection, especially for
members of at-risk groups, notably including children, against an array
of adverse health effects. These effects range from IQ loss, a health
outcome that could be quantified in the risk assessment, to health
outcomes that could not be directly estimated, including decrements in
other neurocognitive functions, other neurological effects and immune
system effects, as well as cardiovascular and renal effects in adults.
In reaching a proposed decision about the level of the Pb primary
standard, the Administrator has considered: the evidence-based
considerations from the Criteria Document and the Staff Paper and those
based on the air-related IQ loss evidence-based framework discussed
above; the results of the exposure and risk assessments discussed above
and in the Staff Paper, giving weight to the exposure and risk
assessments as judged appropriate; CASAC advice and recommendations, as
reflected in discussions of the Criteria Document, Staff Paper, and
ANPR at public meetings, in separate written comments, and in CASAC's
letters to the Administrator; EPA staff recommendations; and public
comments received during the development of these documents, either in
connection with CASAC meetings or separately. In considering what
standard is requisite to protect public health with an adequate margin
of safety, the Administrator is mindful that this choice requires
judgment based on an interpretation of the evidence and other
information that neither overstates nor understates the strength and
limitations of the evidence and information nor the appropriate
inferences to be drawn.
    In reaching a proposed decision on a range of levels for a revised
standard, as in reaching a proposed decision on the adequacy of the
current standard, the Administrator primarily considered the evidence
in the context of the air-related IQ loss evidence-based framework
described above in section II.E.3.a.ii. As a general matter, in
considering this evidence-based framework, the Administrator recognizes
that in the case of Pb there are several aspects to the body of
epidemiological evidence that add complexity to the selection of an
appropriate level for the primary standard. As discussed above, these
complexities include evidence based on blood Pb as the dose metric,
exposure pathways that are both air-related and nonair-related, and the
absence of any discernible threshold levels in the health effects
evidence. Further, the Administrator recognizes that there are a number
of important uncertainties and limitations inherent in the available
health effects evidence and related information, including
uncertainties in the evidence of associations between total blood Pb
and neurocognitive effects in children, especially at the lowest blood
Pb levels evaluated in such studies, as well as uncertainties in key
parameters used in this evidence-based framework, including C-R
functions and air-to-blood ratios. In addition, the Administrator
recognizes that there are currently no commonly accepted guidelines or
criteria within the public health community that would provide a clear
basis for reaching a judgment as to the appropriate degree of public
health protection that should be afforded to neurocognitive effects in
sensitive populations, such as IQ loss in children.
    The air-related IQ loss evidence-based framework considered by the
Administrator focuses on quantitative relationships between air-related
Pb and neurocognitive effects (e.g., IQ loss) in children, building on
recommendations from CASAC to consider the body of evidence in a more
quantitative manner. More specifically, this framework is premised on a
public health goal of selecting a standard level that would prevent
air-related IQ loss (and related effects) of a magnitude judged by the
Administrator to be of concern in populations of children exposed to
the level of the standard, taking into consideration uncertainties
inherent in such estimates. In addition to this public health policy
judgment regarding IQ loss, two other parameters are relevant to this
framework--a C-R function for population IQ response associated with
blood Pb level and an air-to-blood ratio. Based on the discussion of
these parameters in section II.E.3.a above, the Administrator concludes
that, in considering alternative standard levels below the level of the
current standard, it is appropriate to take into account the same two
sets of C-R functions, recognizing uncertainties in the related
evidence, as was done in considering the adequacy of the current
standard (as discussed above in section II.D). He notes that the first
set of C-R functions reflects the evidence indicative of steeper slopes
in relationships between blood Pb and IQ in children, and that the
second set of C-R functions reflects relationships with shallower
slopes between blood Pb and IQ in children. In addition, the
Administrator concludes that it is appropriate to consider various air-
to-blood ratios, again recognizing the uncertainties in the relevant
evidence. He notes that an air-to-blood ratio of 1:5 is within the
reasonable range of values that EPA considers to be generally supported
by the available evidence, which includes ratios of 1:3 up to 1:7.
    With regard to making a public health policy judgment as to the
appropriate level of protection against air-related IQ loss and related
effects, the Administrator first notes that ideally air-related (as
well as other) exposures to environmental Pb would be reduced to the
point that no IQ impact in children would occur. The Administrator
recognizes, however, that in the case of setting a NAAQS, he is
required to make a judgment as to what degree of protection is
requisite to protect public health with an adequate margin of safety.
The NAAQS must be sufficient but not more stringent than necessary to
achieve that result, and does not require a zero-risk standard.
Considering the advice of CASAC and public comments on this issue,
notably including the comments of the American Academy of Pediatrics,
the Administrator proposes to conclude that an air-related population
mean IQ loss within the range of 1 to 2 points could be significant
from a public health perspective, and that a standard level should be
selected to provide protection from air-related population mean IQ loss
in excess of this range.
    The Administrator considered the application of this air-related IQ
loss framework with this target degree of protection in mind, drawing
from the information presented in Table 7 above in section II.E.3.a.ii
that addresses a broad range of standard levels. In so doing, the
Administrator first focused on the estimates associated with the first
set of C-R functions in conjunction with the range of air-to-blood
ratios considered by EPA in this framework. Specifically, using an air-
to-blood ratio of 1:5, the Administrator notes that a standard level of
0.10 [mu]g/m\3\ would limit the estimated degree of impact on
population mean IQ loss from air-related Pb to no more than 1.5 points,
the mid-point of the proposed range of protection. Using the full range
of air-to-blood ratios considered in this framework (1:3 to 1:7), he
notes that a standard set at this level (0.10 [mu]g/m\3\) would limit
the estimated degree of air-

related impact on population mean IQ loss to a range from less than 1
point to around 2 points. Again based on the first set of C-R
functions, the Administrator notes that a standard level of 0.20 [mu]g/
m\3\ would also limit the estimated degree of air-related impact on
population mean IQ loss to within the proposed range of protection
based on using an air-to-blood ratio of 1:3.
    In considering the use of the second set of C-R functions in
conjunction with the range of air-to-blood ratios considered in this
framework (1:3 to 1:7), the Administrator notes for example that a
standard set within the range of 0.10 to 0.30 [mu]g/m\3\ would limit
the estimated degree of air-related impact on population mean IQ loss
to a range from less than one-half point to just under 2 points. More
specifically, based on using an air-to-blood ratio of 1:5 (the
approximately central estimate) in conjunction with the second set of
C-R functions, the Administrator notes that a standard level of 0.30
[mu]g/m\3\ would limit the estimated degree of impact on population
mean IQ loss from air-related Pb to just under 1.5 points, the mid-
point of the proposed range of protection.
    Taking these considerations into account, and based on the full
range of information presented in Table 7 above on estimates of air-
related IQ loss in children over a broad range of alternative standard
levels, the Administrator concludes that it is appropriate to propose a
range of standard levels, and that a range of levels from 0.10 to 0.30
[mu]g/m\3\ is consistent with his target for protection from air-
related IQ loss in children. In recognition of the uncertainties in
these key parameters, the Administrator believes that the selection of
a standard level from within this range is conditional on judgments as
to the most appropriate parameter values to use in the context of this
evidence-based framework. For example, he notes that placing more
weight on the use of a C-R function with a relatively steeper slope
would tend to support a standard level in the lower part of the
proposed range, while placing more weight on a C-R function with a
shallower slope would tend to support a level in the upper part of the
proposed range. Similarly, placing more weight on a higher air-to-blood
ratio would tend to support a standard level in the lower part of the
proposed range, whereas placing more weight on a lower ratio would tend
to support a level in the upper part of the range. In soliciting
comment on a standard level within this proposed range, the
Administrator specifically solicits comment on the appropriate values
to use for these key parameters in the context of this evidence-based
framework, reflecting that his proposal to revise the level of the
primary Pb standard, defined in terms of the current Pb-TSP indicator,
to within the range of 0.10 to 0.30 [mu]g/m\3\ is conditional on
judgments as to the appropriate values of key parameters to use in this
context.
    The Administrator has also considered the results of the exposure
and risk assessments conducted for this review to provide some further
perspective on the potential magnitude of air-related IQ loss. The
Administrator finds that these quantitative assessments provide a
useful perspective on the risk from air-related Pb. However, in light
of the important uncertainties and limitations associated with these
assessments, as discussed above in sections II.C and II.E.3.b, for
purposes of evaluating potential new standards, the Administrator
places less weight on the risk estimates than on the evidence-based
assessments. Nonetheless, the Administrator finds that the risk
estimates are roughly consistent with and generally supportive of the
evidence-based air-related IQ loss estimates described above, as
discussed above in section II.E.3.b. This lends support to the proposed
range based on this evidence-based framework.
    In the Administrator's view, the above considerations, taken
together, provide no evidence- or risk-based bright line that indicates
a single appropriate level. Instead, there is a collection of
scientific evidence and judgments and other information, including
information about the uncertainties inherent in many relevant factors,
which needs to be considered together in making this public health
policy judgment and in selecting a standard level from a range of
reasonable values. Based on consideration of the entire body of
evidence and information available at this time, as well as the
recommendations of CASAC and public comments, the Administrator is
proposing that a standard level within the range of 0.10 to 0.30 [mu]g/
m\3\ would be requisite to protect public health, including the health
of sensitive groups, with an adequate margin of safety. He also
recognizes that selection of a level from within this range is
conditional on judgments as to what C-R function and what air-to-blood
ratio are most appropriate to use within the context of the air-related
IQ loss framework. The Administrator notes that this proposed range
encompasses the specific level of 0.20 [mu]g/m\3\, the upper end of the
range recommended by CASAC and by many public commenters. The
Administrator provisionally concludes that a standard level selected
from within this range would reduce the risk of a variety of health
effects associated with exposure to Pb, including effects indicated in
the epidemiological studies at low blood Pb levels, particularly
including neurological effects in children, and cardiovascular and
renal effects in adults.
    Because there is no bright line clearly directing the choice of
level within this reasonable range, the choice of what is appropriate,
considering the strengths and limitations of the evidence, and the
appropriate inferences to be drawn from the evidence and the exposure
and risk assessments, is a public health policy judgment. To further
inform this judgment, the Administrator solicits comment on the air-
related IQ loss evidence-based framework considered by the Agency and
on appropriate parameter values to be considered in the application of
this framework. More specifically, we solicit comment on the
appropriate C-R function and air-to-blood ratio to be used in the
context of the air-related IQ loss framework. The Administrator also
solicits comment on the degree of impact of air-related Pb on IQ loss
and other related neurocognitive effects in children considered to be
significant from a public health perspective, and on the use of this
framework as a basis for selecting a standard level.
    For the reasons discussed above, the Administrator proposes to
revise the level of the primary Pb standard, defined in terms of the
current Pb-TSP indicator, to within the range of 0.10 to 0.30 [mu]g/
m\3\, conditional on judgments as to the appropriate C-R functions and
air-to-blood ratio to use in the context of the air-related IQ loss
framework.
    The Administrator notes that this framework indicates that for
standard levels above 0.30 [mu]g/m\3\ up to 0.50 [mu]g/m\3\, the
estimated degree of impact on population mean IQ loss from air-related
Pb would range from approximately 2 points to 5 points or more with the
use of the first set of C-R functions and the full range of air-to-
blood ratios considered, and would extend from somewhere within the
proposed range of 1 to 2 points IQ loss to above that range when using
the second set of C-R functions and the full range of air-to-blood
ratios considered. The Administrator proposes to conclude in light of
his consideration of the evidence in the framework discussed above that
the magnitude of air-related Pb effects at the higher blood Pb levels
that would be allowed by standards above 0.30 up to 0.50 [mu]g/m\3\
would be greater than what is requisite to protect

public health with an adequate margin of safety.
    In addition, the Administrator notes that for standard levels below
0.10 [mu]g/m\3\, the estimated degree of impact on population mean IQ
loss from air-related Pb would generally be somewhat to well below the
proposed range of 1 to 2 points air-related population mean IQ loss
regardless of which set of C-R functions or which air-to-blood ratio
within the range of ratios considered are used. The Administrator
proposes to conclude that the degree of public health protection that
standards below 0.10 [mu]g/m\3\ would likely afford would be greater
than what is requisite to protect public health with an adequate margin
of safety.
    Having reached this proposed decision based on the interpretation
of the evidence, the evidence-based frameworks, the exposure/risk
assessment, and the public health policy judgments described above, the
Administrator recognizes that other interpretations, frameworks,
assessments, and judgments are possible. There are also potential
alternative views as to the range of values for relevant parameters
(e.g., C-R function, air-to-blood ratio) in the evidence-based
framework that might be considered supportable and the relative weight
that might appropriately be placed on any specific value for these
parameters within such ranges. In addition, the Administrator
recognizes that there may be other views as to the appropriate degree
of public health protection that should be afforded in terms of air-
related population mean IQ loss in children that would provide support
for alternative standard levels different from the proposed range.
Further, there may be other views as to the appropriate weight and
interpretation to give to the exposure/risk assessment conducted for
this review. Consistent with the goal of soliciting comment on a wide
array of issues, the Administrator solicits comment on these and other
issues.
    In particular, the Administrator solicits comment on alternative
levels of a primary Pb-TSP standard of above 0.30 [mu]g/m\3\ up to 0.50
[mu]g/m\3\. In considering the air-related IQ loss framework and the
case when the second set of C-R functions is used in conjunction with
the lowest air-to-blood ratio considered in this framework (i.e., 1:3),
a standard level as high as 0.50 [mu]g/m\3\ would still limit the
estimated degree of impact on population mean IQ loss from air-related
Pb to no more than 1.5 points, the mid-point of the proposed range of
protection. Comment is solicited on levels within this range and the
associated rationale for selecting such a level in terms of the
appropriate weight to place on relevant parameter values that may
extend to values outside the ranges of values considered by EPA, or in
terms of alternative evidence- or risk-based frameworks that might
support standard levels within this range.
    In addition, the Administrator solicits comment on alternative
levels below 0.10 [mu]g/m\3\. In considering the evidence-based
framework discussed above, a standard level within this range would
likely provide a degree of protection in terms of air-related
population mean IQ loss that is greater than the proposed range based
on the use of any of the relevant parameter values within the ranges
considered by EPA. Comment is solicited on levels within this range and
the associated rationale for selecting such a level in terms of the
appropriate weight to place on relevant parameter values that may
extend to values outside of the ranges considered by EPA, or
alternative public health policy judgments as to the degree of
protection that is warranted, or the appropriate weight to place on the
results of the risk assessment.
    More broadly, as discussed above, the Administrator recognizes that
Pb can be considered a non-threshold pollutant.\152\ In recognizing
that no threshold has been identified below which we are scientifically
confident that there is no risk of harm, EPA's views are consistent
with the views of the CDC, the Federal agency that tracks children's
blood Pb levels nationally and provides guidance on levels at which
medical and environmental case management activities should be
implemented (CDC, 2005a; ACCLPP, 2007). In 2005, CDC revised its
statement on Preventing Lead Poisoning in Young Children, specifically
recognizing the evidence of adverse health effects in children and the
data demonstrating that no ``safe'' threshold for blood Pb had been
identified (CDC, 2005a). EPA's views are also consistent with other
organizations, including, for example, the American Academy of
Pediatrics that recognized in commenting on the ANPR that ``[t]here is
no known ``safe'' level of blood lead in children'' (AAP, 2008). In
addition, the California Environmental Protection Agency, in a recent
risk assessment report, recognizes that ``no safe level has been
definitively established'' for effects of Pb in children (CalEPA, 2007,
p. 1). Given the current state of scientific evidence, which does not
resolve the question of whether or not there is a threshold, we
recognize that there is no level below which we can say with scientific
confidence that there is no risk of harm from exposure to ambient air
related lead.
---------------------------------------------------------------------------

    \152\ Similarly, in the most recent reviews of the NAAQS for
ozone and PM, EPA recognized that the available epidemiological
evidence neither supports nor refutes the existence of thresholds at
the population level, while noting uncertainties and limitations in
studies that make discerning thresholds in populations difficult
(e.g., 73 FR 16444, March 27, 2008; 71 FR 61158, October 17, 2006).
---------------------------------------------------------------------------

    The Administrator also recognizes, as discussed in section I.A
above, that the CAA does not require that NAAQS be established at a
zero-risk level, but rather at a level that reduces risk sufficiently
so as to protect public health with an adequate margin of safety. In
setting primary standards that are ``requisite'' to provide the this
degree of public health protection, the Supreme Court has affirmed that
EPA's task is to establish standards that are neither more nor less
stringent than necessary for this purpose. The question then becomes
how the Agency should reconcile these scientific and legal
understandings in reviewing the Pb NAAQS.
    As discussed above, EPA is proposing a range of levels for the
primary Pb NAAQS, with the range extending down to 0.10 [mu]g/m\3\.
This range reflects the Administrator's proposed conclusion that lower
levels would be more than necessary to protect public health with an
adequate margin of safety. This proposed conclusion is based in large
part on EPA's evaluation of the evidence, recognizing important
uncertainties in the scientific evidence and related assessments, and
reflects the proposed public heath policy judgment of the Administrator
on these issues. As discussed above, these uncertainties stem in part
from the complexities of determining the health impact of air-related
Pb given the multi-media exposure pathways for exposure to lead and the
persistence of Pb in the environment. The major areas of uncertainty
include the appropriate air-to-blood ratio; the apportionment of Pb
between air-related and nonair Pb; the increasing uncertainty at lower
blood Pb levels as to the existence, nature, and degree of health
effects; and the uncertainty over the public health significance of
smaller and smaller impacts on IQ or other similar neurocognitive
metrics from exposure to air-related Pb. In recognition of such
uncertainties, EPA is also soliciting comment on a lower range of
standard levels below 0.10 [mu]g/m\3\.
    In so doing, EPA fully recognizes that a standard set at the lowest
proposed level of 0.10 [mu]g/m\3\, or any non-zero level, would not be
a risk-free standard.

As in numerous prior NAAQS reviews, we recognize that the CAA does not
require that EPA set a risk-free standard. Instead, EPA is to recognize
and take risk into account, and set a standard that is requisite to
protect public health with an adequate margin of safety based on the
currently available information. This calls for a public health policy
judgment informed by many factors, most notably the nature and severity
of the health effects at issue, the size of the population(s) at risk,
and the kind and degree of uncertainties involved. After considering
all of these factors in this review, the Administrator's proposed
judgment is that a standard set below 0.10 [mu]g/m\3\ would not satisfy
this statutory directive.
    The Administrator recognizes that the current state of the
scientific evidence clearly indicates that health effects from Pb occur
at much lower blood Pb levels than we understood in the past, and that
the appropriate level for ambient air Pb is much lower than we thought
in the past. Further the Administrator expects that, as time goes on,
future scientific studies will continue to enhance our understanding of
Pb, and anticipates that such studies might lead to a situation where
there is very little, if any, remaining uncertainty about human health
impacts from even extremely low levels of Pb in the ambient air. As
noted above, this has the potential to raise fundamental questions as
to how the Agency can continue to reconcile such evidence with the
statutory provision calling for the NAAQS to be set at a level that is
requisite to protect public health with an adequate margin of safety.
Faced with scientific evidence that could reasonably be interpreted as
demonstrating that any ambient Pb level above zero contributes to
adverse health effects in at-risk populations, some might conclude that
the only standard requisite to protect public health with an adequate
margin of safety would be a standard set at zero. While EPA's proposed
conclusions on the current scientific evidence and an appropriate
standard based on that evidence and on its interpretation of the
statute clearly differ from such a view, EPA nonetheless believes that
inviting comment in this review on the views described above and the
issues raised by such circumstances is appropriate.
    More specifically, EPA invites comment on when, if ever, it would
be appropriate to set a NAAQS for Pb at a level of zero. Comments on
this question might address issues such as: The level of scientific
certainty that would be needed to support such a decision; the level of
harm, e.g., severity of health effect and size of affected population,
that would be needed to support such a decision; and whether there are
normative or quantitative criteria that could be applied in deciding
whether, and if so, when it would be appropriate to set a standard at
zero. EPA invites comment on how to reconcile the above issues in this
and subsequent NAAQS reviews.
4. Level for a Pb NAAQS with a Pb-PM10 Indicator
    EPA is requesting comment on the option of revising the indicator
for the Pb NAAQS from Pb-TSP to Pb-PM10, based on low-volume
sample collection as discussed above in section II.E.1 and below in
section V.A. In this section, we discuss considerations important to
selection of a level for such a Pb-PM10-based standard
(section II.E.4.a) and CASAC's advice and public comments on this issue
(section II.E.4.b). Approaches for adjusting the level of a Pb NAAQS
with Pb-TSP indicator for a Pb-PM10-based standard, and a
range of levels for a Pb-PM10-based standard, under
consideration and on which EPA is soliciting comment are presented in
II.E.4.c.
a. Considerations With Regard to Particles Not Captured by
PM10
    In the course of deciding to propose the Pb-TSP indicator approach
as described in section II.E.1 above, EPA has noted the important role
of both respirable and non-respirable Pb particles in air-related Pb
exposure of concern and the lesser capture of these particles by
PM10 samplers compared to TSP samplers. We recognize 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. Further, the quantity of Pb in ambient particles with
the potential to deposit (indoors and outdoors, leading to a role in
ingestion pathways) is a key contributor to air-related exposure, and
these particles include ultra-coarse mode particles that are not
captured by PM10 samplers (as discussed in section II.E.1
above). In recognition of these considerations, both of the indicator
options discussed in this notice recognize the need to consider use of
an adjustment related to the use of PM10 measurements,
either when considering the optional use of Pb-PM10 data for
comparison with a Pb-TSP-based NAAQS, or when considering a level for a
NAAQS based on a Pb-PM10 indicator.
    Section II.E.1 above contains extensive discussion of the
relationship between Pb-PM10 and Pb-TSP, including the fact
that Pb-PM10/Pb-TSP relationships vary from site to site and
from time to time, but have a systematic variation with distance from