Assessment Report). Two factors are described here as contributors to
that uncertainty. Interindividual variability in blood Pb levels for
any study population (as described by the GSD) will reflect, to a
certain extent, spatial variation in media concentrations, including
outdoor ambient air Pb levels and indoor dust Pb levels, as well as
differences in physiological response to Pb exposure. For each case
study, there is significant uncertainty in the specification of spatial
variability in ambient air Pb levels and associated indoor dust Pb
levels, as noted above. In addition, there are a limited number of
datasets for different types of residential child populations from
which a GSD can be derived (e.g., NHANES datasets \94\ for more
heterogeneous populations and individual study datasets for likely more
homogeneous populations near specific industrial Pb sources). This
uncertainty associated with the GSDs introduces significant uncertainty
in exposure and risk estimates for the 95th population percentile.
---------------------------------------------------------------------------

    \94\ The GSD for the urban case studies, in the risk assessment
described in this notice, was derived using NHANES data for the
years 1999-2000.
---------------------------------------------------------------------------

     Exposure pathway apportionment for higher percentile blood
Pb level and IQ loss estimates: Apportionment of blood Pb levels for
higher population percentiles is assumed to be the same as that
estimated using the central tendency estimate of blood Pb in an
exposure zone. This introduces significant uncertainty into projections
of pathway apportionment for higher population percentiles of blood Pb
and IQ loss. In reality, pathway apportionment may differ in higher
exposure percentiles. For example, paint and/or drinking water
exposures may increase in importance, with air-related contributions
decreasing as an overall percentage of blood Pb levels and associated
risk. Because of this uncertainty related to pathway apportionment, as
mentioned earlier, greater confidence is placed in estimates of total
Pb exposure and risk in evaluating the impact of the current NAAQS and
alternative NAAQS relative to current conditions.
     Relating blood Pb levels to IQ loss: Specification of the
quantitative relationship between blood Pb level and IQ loss is subject
to significant uncertainty at lower blood Pb levels (e.g., below 5
[mu]g/dL concurrent blood Pb). As discussed earlier, there are
limitations in the datasets and concentration-response analyses
available for characterizing the concentration-response relationship at
these lower blood Pb levels. For example, the pooled international
dataset analyzed by Lanphear and others (2005) includes relatively few
children with blood Pb levels below 5 [mu]g/dL and no children with
levels below 1 [mu]g/dL. In recognition of the uncertainty in
specifying a quantitative concentration-response relationship at such
levels, our core modeling approach involves the application of four
different functions to generate a range of risk estimates (as described
in Section 4.2.6 and Section 5.3.1 of the Risk Assessment Report,
USEPA, 2007b). The difference in absolute IQ loss estimates for the
four concentration-response functions for a given case study/air
quality scenario combination is typically close to a factor of 3.
Estimates of differences in IQ loss between air quality scenarios (in
terms of percent), however, are more similar across the four functions,
although the function producing higher overall risk estimates (the dual
linear function, stratified at 7.5 [mu]g/dL, peak blood Pb) also
produces larger absolute reductions in IQ loss compared with the other
three functions.
3. Summary of Estimates and Key Observations
    This section presents blood Pb and IQ loss estimates generated in
the exposure and risk assessments. Blood Pb estimates (and air-to-blood
Pb ratios) are presented first, followed by IQ loss estimates.

a. Blood Pb Estimates
    This section presents a summary of blood Pb modeling results for
concurrent blood Pb drawn from the more detailed presentation in the
Staff Paper and the Risk Assessment Report (USEPA, 2007a, 2007b,
2007c).
    Blood Pb level estimates for the current conditions air quality
scenarios for these case studies differ somewhat from the national
values associated with recent NHANES information. For example, median
blood Pb levels for the current conditions scenario for the urban case
studies are somewhat larger than the national median from the NHANES
data for 2003-2004. Specifically, values for the three location-
specific urban case studies range from 1.7 to 1.8 [mu]g/dL with the
general urban case study having a value of 1.9 [mu]g/dL (current-
conditions mean) (presented in Risk Assessment Report, Volume I, Table
5-5), while the median value from NHANES (2003-2004) is 1.6 [mu]g/dL
(http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm).
Additionally, NHANES values for the 90th percentile (for 2003-2004)
were identified and these values can be compared against 90th
percentile estimates generated for the urban case studies (see Risk
Assessment Report, Appendix O, Section O.3.2 for the location-specific
urban case study and Appendix N, Section N.2.1.2 for the general urban
case study). The 90th percentile blood Pb levels for the current
conditions scenario, for the three location-specific urban case studies
range from 4.5 to 4.6 [mu]g/dL, while the estimate for the general
urban case study is 5.0 [mu]g/dL. These 90th percentile values for the
case study populations are larger than the 90th percentile value of 3.9
[mu]g/dL reported by NHANES for all children in 2003-2004. It is noted
that ambient air levels reflected in the urban case studies are likely
to differ from those underlying the NHANES data.\95\
---------------------------------------------------------------------------

    \95\ The maximum quarterly mean Pb concentrations in the
location-specific case studies ranged from 0.09-0.36 [mu]g/m\3\,
which are higher levels than the maximum quarterly mean values in
most monitoring sites in the U.S. The median of the maximum
quarterly mean values across all sites in the 2003-05 national
dataset is 0.03 [mu]g/m\3\ (USEPA, 2007a, appendix A).
---------------------------------------------------------------------------

    Table 2 presents total blood Pb estimates for alternative
standards, focusing on the median in the assessed population, and
associated estimates for the air-related percentage of total blood Pb
(i.e., bounded on the low end by the ``recent air'' contributions and
on the high end by the ``recent'' plus ``past air'' contribution to
total Pb exposure).
    Generally, 95th percentile blood Pb estimates across air quality
scenarios for all case studies (not shown here) are 2-3 times higher
than the median estimates in Table 2. For example, 95th percentile
estimates of total blood Pb for the current NAAQS scenario are 10.6
[mu]g/dL for the general urban case study, 12.3 [mu]g/dL for the
primary Pb smelter subarea, and 7.4 to 10.2 [mu]g/dL for the three
location-specific urban case studies (Staff Paper, Table 4-2). While
the estimates indicate similar fractions of total blood Pb that is air-
related between the 95th percentile and median, there is greater
uncertainty in pathway apportionment among air-related and other
sources for higher percentiles, including the 95th percentile.

                                         Table 2.--Summary of Median Blood Pb Estimates for Concurrent Blood Pb
                                                                         [Total]
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                    Total blood Pb ([mu]g/dL) (air-related percentage) \A\
                                    --------------------------------------------------------------------------------------------------------------------
 NAAQS Level simulated  ([mu]g/m\3\                                                                  Location-specific urban case studies
max monthly, except as noted below)    General urban case      Primary Pb smelter   --------------------------------------------------------------------
                                              study           (subarea) case studyB  Cleveland (0.56 [mu]g/  Chicago (0.31 [mu]g/    Los Angeles (0.17
                                                                        C                    m\3\)                  m\3\)               [mu]g/m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
1.5 max quarterly \D\..............  3.1 (61 to 84%).......  4.6 (up to 87%).......  2.1 \D\ (57 to 86%)..  3.0 \E\ (63 to 83%)..  2.6E (50 to 81%).
0.50...............................  2.2 (41 to 73%).......  3.2 (up to 81%).......  1.8 (39 to 72%)......  (\F\)................  (\F\)
0.20...............................  1.9 (26 to 74%).......  2.3 (up to 78%).......  1.7 (6 to 65%).......  1.8 (17 to 67%)......  1.7 (\G\) (18 to
                                                                                71%).
0.05...............................  1.7 (12 to 65%).......  1.7 (up to 65%).......  1.6 (1 to 63%).......  1.6 (6 to 69%).......  1.6 (13 to 69%).
0.02...............................  1.6 (6 to 69%)........  1.6 (up to 69%).......  1.6 (1 to 63%).......  1.6 (1 to 63%).......  1.6 (6 to 63%).
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ --Blood Pb estimates are rounded to one decimal place. Air-related percentage is bracketed by ``recent air'' (lower bound of presented range) and
  ``recent'' plus ``past air'' (upper bound of presented range). The term ``past air'' includes contributions from the outdoor soil/dust contribution to
  indoor dust, historical air contribution to indoor dust, and outdoor soil/dust pathways; ``recent air'' refers to contributions from inhalation of
  ambient air Pb or ingestion of indoor dust Pb predicted to be associated with outdoor ambient air Pb levels, with outdoor ambient air also potentially
  including resuspended, previously deposited Pb (see Section II.C.2.e).
\B\ --In the case of the primary Pb smelter subarea, only recent plus past air estimates are available.
\C\ --Median blood Pb levels for the primary smelter (full study area) are estimated at 1.5 [mu]g/dL (for the 1.5 [mu]g/m\3\ max quarterly level) and
  1.4 [mu]g/dL for the remaining NAAQS levels simulated. The air-related percentages for these standard levels range from 36% to 79%.
\D\ --This corresponds to roughly 0.7-1.0 [mu]g/m\3\ maximum monthly mean, across the urban case studies.
\E\ --A ``roll-up'' was performed so that the highest monitor in the study area is increased to just meet this level.
\F\ --A ``roll-up'' to this level was not performed.
\G\ --A ``roll-up'' to this level was not performed; these estimates are based on current conditions in this area.

    As described in section II.C.2.f, the risk assessment also
developed estimates for air-to-blood ratios, which are described in
section <IP_ADDRESS> of the Risk Assessment Report (USEPA, 2007b). These
ratios reflect a subset of air-related pathways related to inhalation
and ingestion of indoor dust; inclusion of the remaining pathways would
be expected to yield higher ratios. Additionally, these ratios are
based on blood Pb estimates for the 7th year of exposure (concurrent
blood Pb) which are lower than blood Pb estimates at younger ages (and
than the lifetime-averaged blood Pb metric). Ratios based on other
blood Pb estimates (e.g., lifetime-averaged or peak blood Pb) would be
higher.
     For the general urban case study, estimates of air-to-
blood ratios, presented in section <IP_ADDRESS> of the Risk Assessment
Report (USEPA, 2007b) ranged from 1:2 to 1:9, with the majority of the
estimates ranging from 1:4 to 1:6.\96\ As noted in Section II.C.2.f,

because the risk assessment only reflects the impact of reductions on
recent air-related pathways in predicting changes in indoor dust Pb for
urban case studies, these ratios are lower than they would be if they
had also reflected potential reductions in other air-related pathways
(e.g., changes in outdoor surface soil/dust Pb levels and diet with
changes in ambient air Pb levels). We also note that the median blood
Pb levels associated with exposure pathways that were not varied in
this assessment (and consequently are not reflected in these ratios)
generally range from 1.3 to 1.5 [mu]g/dL for this case study.
---------------------------------------------------------------------------

    \96\ The ratios increase as the level of the alternate standard
decreases. This reflects nonlinearity in the Pb response, which is
greater on a per-unit basis for lower ambient air Pb levels.
---------------------------------------------------------------------------

     For the primary Pb smelter subarea, estimates of air-to-
blood ratios, presented in section <IP_ADDRESS> of the Risk Assessment
Report (USEPA, 2007b) ranged from 1:10 and higher.97 98 One
reason for these estimates being higher than those for the urban case
study is that the dust Pb model used may reflect somewhat ambient air-
related pathways other than that of ambient air infiltrating a home (as
described in Section II.C.2.f above).\99\
---------------------------------------------------------------------------

    \97\ As with such estimates for the urban case study, ratios are
higher at lower ambient air Pb levels, reflecting the nonlinearity
of the dust Pb response with air concentration.
    \98\ For the primary Pb smelter (full study area), for which
limitations are noted above in section II.C.2.c, the air-to-blood
ratio estimates, presented in section <IP_ADDRESS> of the Risk Assessment
Report (USEPA, 2007b), ranged from 1:3 to 1:7. As in the other case
studies, ratios are higher at lower ambient air Pb levels. It is
noted that the underlying changes in both ambient air Pb and blood
Pb across standard levels are extremely small, introducing
uncertainty into ratios derived using these data.
    \99\ Also, as noted above (Section II.C.2.h), there is increased
uncertainty with application of this regression-based model in air
quality scenarios of notably lower air Pb levels than the data set
used in its derivation.
---------------------------------------------------------------------------

b. IQ Loss Estimates
    The risk assessment estimated IQ loss associated with both total Pb
exposure and air-related Pb exposure. This section focuses on findings
in relation to air-related Pb exposure, since this is the category of
risk results considered most relevant to the review in considering
whether the current NAAQS and potential alternative NAAQS provide
protection of public health with an adequate margin of safety
(additional categories of risk results, including IQ loss estimates
based on total Pb exposure and population incidence results, are
presented at the end of the section).\100\
---------------------------------------------------------------------------

    \100\ The detailed results are provided in the Risk Assessment
Report (USEPA, 2007b).
---------------------------------------------------------------------------

    In considering air-related risk results, we note that IQ loss
associated with air-related exposure for each NAAQS scenario is bounded
by recent-air on the low-end and recent plus past air on the high-end
(as described in section II.C.2.e above). In considering differences in
these risk estimates (or in the total risk estimates presented in the
final Risk Assessment Report) for alternative NAAQS, we note that these
comparisons underestimate the true impacts of the alternate NAAQS and
accordingly, the benefit to public health that would result from lower
NAAQS levels. This is due to our inability to simulate in this
assessment reductions in several outdoor air deposition-related
pathways (e.g., diet, ingestion of outdoor surface soil). The magnitude
of this underestimation is unknown.
    As with the discussion of blood Pb results, the IQ loss estimates
are summarized here according to air quality scenario and case study
category (Table 3). In presenting these results, we have focused this
presentation on estimates for the median in each case study population
of children because of the greater confidence associated with estimates
for the median as compared to those for 95th percentile.\101\
Generally, 95th percentile IQ loss estimates for all case studies are
80 to 100% higher than the median results in Table 3. The fraction of
total IQ loss that is air-related for the 95th percentile is generally
similar to that for the median (for a particular combination of case
study and air quality scenario).
---------------------------------------------------------------------------

    \101\ A complete presentation of risk estimates is available in
the final Risk Assessment Report, including a presentation of
estimates for the 95th percentile in Table 5-10 of that report.
---------------------------------------------------------------------------

    The risk estimates presented in boldface in Table 3 are those
derived using the log-linear with low-exposure linearization
concentration-response function, while the range of estimates
associated with all four concentration-response functions is presented
in parentheses. These functions are discussed above in section
II.C.2.b.

                        Table 3.--Summary of Risk Attributable to Air-Related Pb Exposure
----------------------------------------------------------------------------------------------------------------
                                                          Median air-related IQ loss \A\
                                 -------------------------------------------------------------------------------
 NAAQS level simulated  ([mu]g/m                                       Location-specific urban case studies
\3\ max monthly, except as noted                    Primary Pb   -----------------------------------------------
             below)                General urban      smelter        Cleveland                      Los Angeles
                                    case study    (subarea) case   (0.56 [mu]g/m  Chicago  (0.31   (0.17 [mu]g/m
                                                   study \B, C\        \3\)        [mu]g/m \3\)        \3\)
----------------------------------------------------------------------------------------------------------------
1.5 max quarterly \D\...........         3.5-4.8             < 6     2.8-3.9 \E\     3.4-4.7 \E\     2.7-4.2 \E\
                                       (1.5-7.7)      <(3.2-9.4)       (0.6-4.6)       (1.4-7.4)       (1.1-6.2)
0.5.............................         1.9-3.6           < 4.5         0.6-2.9             \F\             \F\
                                       (0.7-4.8)      <(2.1-7.7)       (0.2-3.9)
0.2.............................         1.2-3.2           < 3.7         0.6-2.8         0.6-2.9     0.7-2.9 \G\
                                       (0.4-4.0)      <(1.2-5.1)       (0.1-3.2)       (0.3-3.6)       (0.2-3.5)
0.05............................         0.5-2.8           < 2.8         0.1-2.6         0.2-2.6         0.3-2.7
                                       (0.2-3.3)      <(0.9-3.4)      (<0.1-3.1)       (0.1-3.2)       (0.1-3.2)
0.02............................         0.3-2.6           < 2.9        <0.1-2.6         0.1-2.6         0.1-2.6
                                       (0.1-3.1)      <(0.9-3.3)      (<0.1-3.0)      (<0.1-3.1)     (<0.1-3.1)
----------------------------------------------------------------------------------------------------------------
\A\--Air-related risk is bracketed by ``recent air'' (lower bound of presented range) and ``recent'' plus ``past
  air'' (upper bound of presented range). While differences between standard levels are better distinguished by
  differences in the ``recent'' plus ``past air'' estimates (upper bounds shown here), these differences are
  inherently underestimates. The term ``past air'' includes contributions from the outdoor soil/dust
  contribution to indoor dust, historical air contribution to indoor dust, and outdoor soil/dust pathways;
  ``recent air'' refers to contributions from inhalation of ambient air Pb or ingestion of indoor dust Pb
  predicted to be associated with outdoor ambient air Pb levels, with outdoor ambient air also potentially
  including resuspended, previously deposited Pb (see Section II.C.2.e). Boldface values are estimates generated
  using the log-linear with low-exposure linearization function. Values in parentheses reflect the range of
  estimates associated with all four concentration-response functions.
\B\--In the case of the primary Pb smelter case study, only recent plus past air estimates are available.
\C\--Median air-related IQ loss estimates for the primary Pb smelter (full study area) range from <1.7 to <2.9
  points, with no consistent pattern across simulated NAAQS levels. This lack of a pattern reflects inclusion of
  a large fraction of the study population with relatively low ambient air impacts such that there is lower
  variation (at the population median) across standard levels (see Section 4.2 of the Risk Assessment, Volume
  1).
\D\--This corresponds to roughly 0.7--1.0 [mu]g/m3 maximum monthly mean, across the urban case studies
\E\--A ``roll-up'' was performed so that the highest monitor in the study area is increased to just meet this
  level.
\F\--A ``roll-up'' to this level was not performed.
\G\--A ``roll-up'' to this level was not performed; these estimates are based on current conditions in this
  area.

    Key observations regarding the median estimates of air-related risk
for the current NAAQS and alternative standards presented in Table 3
include:
     For the scenario for the current NAAQS (1.5 [mu]g/m\3\,
maximum quarterly average), air-related risk exceeds 2 points IQ loss
at the median and the upper bound of air-related risk is near or above
4 points IQ loss in all five case studies.\102\
---------------------------------------------------------------------------

    \102\ As noted in Table 3 and section II.C.2.d above, and
discussed further, with regard to associated limitations and
uncertainties, in section II.C.2.h above, a proportional roll-up
procedure was used to estimate air Pb concentrations in this
scenario for the location-specific case studies.
---------------------------------------------------------------------------

     Alternate standards provide substantial reduction in
estimates of air-related risk across the full set of alternative NAAQS
considered in this analysis (i.e., 0.5 to 0.02 [mu]g/m\3\ max monthly).
This is particularly the case for the lower bounds of the air-related
estimates presented in Table 3, which reflect the estimates for
``recent air''-related pathways, which are the pathways that were
varied with changes in air concentrations (as described above in
section II.C.2.e). There is less risk reduction associated with the
upper bounds of these estimates as the upper bound values are inclusive
of the exposure pathways categorized as ``past air'' which were not
varied with changes in air concentrations (as described in section
II.C.2.3). The upper bound estimates for the lowest level assessed
(0.02 [mu]g/m\3\) are 2.6-2.9 points IQ loss.
     In the general urban case study, the lower bound of air-
related risk falls below 2 points IQ loss for an alternative NAAQS of
0.5 [mu]g/m\3\ max monthly, and below 1 point IQ loss somewhere between
an alternative NAAQS of 0.2 and 0.05 [mu]g/m\3\ max monthly.
     The upper-bound of air-related risk for the primary Pb
smelter subarea is generally higher than that for the general urban
case study, likely due to the difference in indoor dust models used for
the two case studies. The indoor dust Pb model used for the primary Pb
smelter considered more completely, the impact of outdoor ambient air
Pb on indoor dust (compared to the hybrid indoor dust Pb model used in
the urban case studies). Specifically, the regression model used for
the primary Pb smelter included consideration for longer-term
relationships between outdoor ambient air and indoor dust (e.g.,
changes in outdoor soil and subsequent tracking in of soil Pb).
     As noted above (section II.C.2.c), the three location-
specific urban case studies provide risk estimates for populations with
a broader range of air-related exposures. Accordingly, because of the
population distribution in these three case studies, the air-related
risk is smaller for them than for the other case studies, particularly
at the population median. Further, the majority of the population in
each case study resides in areas with ambient air Pb levels well below
each standard level assessed, particularly for levels above 0.05 [mu]g/
m\3\ max monthly. Consequently, risk estimates indicate little response
to alternative standard levels above 0.05 [mu]g/m\3\ max monthly.
    In addition to the air-related risk results described above, we
present two additional categories of risk results, including (a)
estimates of median IQ loss based on total Pb exposure for each case
study (Table 4) and (b) IQ loss incidence estimates for each of the
location-specific case studies (Tables 4 and 5).\103\ Each of these
categories of risk results are described in creater detail below:
---------------------------------------------------------------------------

    \103\ As recognized in section II.C.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard.
---------------------------------------------------------------------------

     Estimates of IQ loss for all air quality scenarios (based
on total Pb exposure): Table 4 presents median IQ loss estimates for
total Pb exposure for each of the air quality scenarios simulated for
each case study (as noted earlier in this section, there is greater
uncertainty associated with higher-end risk percentiles and therefore,
they are

not presented in tabular format here--see Table 5-10 of Risk Assessment
Volume 1 for 95th percentile total IQ loss estimates). As with the
incremental risk results presented in Table 3 above, in order to
reflect the variation in estimates derived from the four different
concentration-response functions included in the analysis, three
categories of estimates are presented in Table 4 including (a) IQ loss
estimates generated using the low concentration-response function (the
model that generated the lowest IQ loss estimates), (b) estimates
generated using the log-linear with low-exposure linearization (LLL)
model, and (c) IQ loss estimates generated using the high
concentration-response function (the model that generated the highest
IQ loss estimates). It is important to emphasize, that, as noted in
Section II.C.2.e, because of limitations in modeling methods, we were
only able to simulate reduction in recent air-related exposures in
considering alternate standard levels and could not simulate reduction
in past air-related exposures. This likely results in an underestimate
of the total degree of reduction in exposure and risk associated with
each standard level. Therefore, in comparing total risk estimates
between alternate NAAQS scenarios (i.e., considering incremental risk
reductions), this aspect of the analysis will tend to underestimate the
reductions in risk associated with alternative NAAQS.
     IQ loss incidence estimates for the three location-
specific urban case studies: Estimates of the number of children for
each location-specific urban case study projected to have total Pb-
related IQ loss greater than one point are summarized in Table 5, and
similar estimates for IQ loss greater than 7 points are summarized in
Table 6. Also presented are the changes in incidence of the current
NAAQS and alternative NAAQS scenarios compared to current conditions,
with emphasis placed on estimates generated using the LLL
concentration-response function. Estimates are presented for each of
the four concentration-response functions used in the risk analysis.
This metric illustrates the overall number of children within a given
urban case study location projected to experience various levels of IQ
loss due to Pb exposure and how that distribution of incidence changes
with alternate standard levels. These incidence estimates were only
generated for the location-specific urban case studies, since these
have larger enumerated study populations (additional detail on the
derivation of these incidence estimates is presented in Section <IP_ADDRESS>
of the Risk Assessment Report). The complete set of incidence results
is presented in Risk Assessment Report Appendix O, Section O.3.4.
    Total IQ loss results presented in Table 4 for the primary Pb
smelter case study (full study area) illustrate the reason why these
results were not presented earlier in summarizing air-related IQ loss
estimates for the primary Pb smelter case study in Table 3 (and
instead, results for the subarea were presented). As mentioned earlier
in Section II.C.2.c, the full study area for the primary Pb smelter
case study incorporates a large number of simulated children with
relatively low air-related impacts, which results in little
differentiation between alternate standard levels in terms of total IQ
loss (as well as air-related IQ loss). This can be seen by considering
the results in Table 4 for the primary Pb smelter (full study area).
Those results suggest that total IQ loss varies little across alternate
standard levels for the full study area simulation, with the only
noticeable difference in total IQ loss resulting from analysis of the
current standard (when compared to alternate levels). By contrast,
there are notable differences in total IQ loss between alternative
standard levels for the sub-area of the primary Pb smelter case study.

              Table 4.--Summary of Risk Estimates for Medians of Total-Exposure Risk Distributions
----------------------------------------------------------------------------------------------------------------
                                                                      Points IQ loss  (total Pb exposure) \a\
                                                                 -----------------------------------------------
               Case study and air quality scenario                    Low C-R                        High C-R
                                                                     function         LLL \b\        function
                                                                     estimate                        estimate
----------------------------------------------------------------------------------------------------------------
                                           Location-specific (Chicago)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             2.4             5.6             8.8
Current conditions (0.14 [mu]g/m\3\ max quarterly; 0.31 [mu]g/               1.4             4.2             5.2
 m\3\ max monthly)..............................................
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly).................             1.4             4.2             5.2
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             1.3             4.0             4.8
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             1.3             4.0             4.7
----------------------------------------------------------------------------------------------------------------
                                          Location-specific (Cleveland)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             1.7             4.7             6.3
Current conditions (0.36 [mu]g/m\3\ max quarterly; 0.56 [mu]g/               1.4             4.2             5.2
 m\3\ max monthly)..............................................
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly).................             1.4             4.2             5.2
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)...............             1.4             4.1             5.0
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly).................             1.3             4.1             4.9
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             1.3             4.0             4.7
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             1.2             3.9             4.6
----------------------------------------------------------------------------------------------------------------
                                         Location-specific (Los Angeles)
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             2.1             5.3             7.7
Current conditions (0.09 [mu]g/m\3\ max quarterly; 0.17 [mu]g/               1.4             4.2             5.1
 m\3\ max monthly)..............................................
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             1.3             4.0             4.8
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             1.3             4.0             4.7
----------------------------------------------------------------------------------------------------------------
                                                  General Urban
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             2.5             5.8             9.2

Alternative NAAQS (0.5 [mu]g/m\3\, max monthly).................             1.7             4.8             6.4
Current conditions--high-end (0.87 [mu]g/m\3\ max quarterly)....             1.7             4.7             6.3
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)...............             1.6             4.6             5.9
Current conditions--mean (0.14 [mu]g/m\3\ max quarterly)........             1.5             4.5             5.6
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly).................             1.5             4.4             5.6
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             1.3             4.1             5.0
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             1.3             4.0             4.8
----------------------------------------------------------------------------------------------------------------
                                       Primary Pb smelter--full study area
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             1.2             3.8             4.4
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly).................             1.0             3.7             4.2
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)...............             0.9             3.6             4.2
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly).................             0.9             3.6             4.1
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             0.9             3.6             4.0
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             0.9             3.6             4.1
----------------------------------------------------------------------------------------------------------------
                                        Primary Pb smelter--1.5km subarea
----------------------------------------------------------------------------------------------------------------
Current NAAQS (1.5 [mu]g/m\3\, max quarterly)...................             3.7             6.8            11.2
Alternative NAAQS (0.5 [mu]g/m\3\, max monthly).................             2.6             5.8             9.4
Alternative NAAQS (0.2 [mu]g/m\3\, max quarterly)...............             2.0             5.2             7.4
Alternative NAAQS (0.2 [mu]g/m\3\, max monthly).................             1.9             5.0             6.9
Alternative NAAQS (0.05 [mu]g/m\3\, max monthly)................             1.4             4.2             5.1
Alternative NAAQS (0.02 [mu]g/m\3\, max monthly)................             1.3             4.0            4.8
----------------------------------------------------------------------------------------------------------------
\a\ --These columns present the estimates of total IQ loss resulting from total Pb exposure (policy-relevant
  plus background). Estimates below 1.0 are rounded to one decimal place, all values below 0.05 are presented as
  <0.1 and values between 0.05 and 0.1 as 0.1. All values above 1.0 are rounded to the nearest whole number.
\b\ --Log-linear with low-exposure linearization concentration-response function.

                                             Table 5.--Incidence of Children With >1 Point Pb-Related IQ Loss
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                   Dual linear--stratified       Log-linear with       Dual linear--stratified  Log-linear with cutpoint
                                                   at 7.5 mg/dl peak blood        linearization        at  10 m/dL peak blood  -------------------------
                                                             Pb            --------------------------            Pb
                                                 --------------------------                          --------------------------                 Delta
  Air quality scenario  (for location-specific                    Delta                     Delta                     Delta                   (change in
               urban case studies)                               (change     Incidence    (change in                (change in   Incidence    incidence
                                                   Incidence   inincidence  of >1 point   incidence    Incidence    incidence   of >1 point  compared to
                                                  of >1 point  compared to     IQ loss   compared to  of >1 point  compared to     IQ loss     current
                                                     IQ loss     current                   current       IQ loss     current                 conditions)
                                                               conditions)               conditions)               conditions)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Chicago (total modeled child population:
 396,511):
    Chicago Current Conditions..................      391,602  ...........      389,754  ...........      271,031  ...........      236,257
    Current NAAQS (1.5 mg/m\3\ Maximum                395,797        4,195      395,528        5,773      347,415       76,384      314,053       77,795
     Quarterly).................................
    Alternative NAAQS (0.2 mg/m\3\ Maximum            391,158         -444      389,461         -293      271,444          412      235,559         -698
     Monthly)...................................
    Alternative NAAQS (0.05 mg/m\3\ Maximum           389,572       -2,030      387,407       -2,347      253,775      -17,256      224,394      -11,864
     Monthly)...................................
    Alternative NAAQS (0.02 mg/m\3\ Maximum           389,176       -2,427      386,630       -3,125      249,865      -21,166      219,294      -16,963
     Monthly)...................................
Cleveland (total modeled child population:
 13,990):
    Cleveland Current Conditions................       13,809  ...........       13,745  ...........        9,526  ...........        8,515
    Current NAAQS (1.5 mg/m\3\ Maximum                 13,893           84       13,857          112       10,664        1,137        9,769        1,254
     Quarterly).................................
    Alternative NAAQS (0.2 mg/m\3\ Maximum             13,770          -38       13,703          -42        9,221         -305        8,160         -354
     Quarterly).................................
    Alternative NAAQS (0.5 mg/m\3\ Maximum             13,789          -20       13,720          -25        9,497          -29        8,464          -51
     Monthly)...................................
    Alternative NAAQS (0.2 mg/m\3\ Maximum             13,759          -50       13,694          -51        9,083         -443        8,010         -505
     Monthly)...................................
    Alternative NAAQS (0.05 mg/m\3\ Maximum            13,729          -80       13,642         -103        8,785         -741        7,720         -795
     Monthly)...................................
    Alternative NAAQS (0.02 mg/m\3\ Maximum            13,720          -88       13,628         -117        8,736         -790        7,668         -846
     Monthly)...................................
Los Angeles (total modeled child population:
 372,252):
    Los Angeles Current Conditions..............      282,216  ...........      280,711  ...........      191,675  ...........      170,474  ...........
    Current NAAQS (1.5 mg/m\3\ Maximum,               285,272        3,056      284,945        4,234      240,988       49,313      226,608       56,134
     Quarterly).................................

    Alternative NAAQS (0.05 mg/m\3\ Maximum           281,112       -1,104      279,658       -1,053      183,395       -8,280      161,914       -8,560
     Monthly)...................................
    Alternative NAAQS (0.02 mg/m\3\ Maximum           280,740       -1,476      279,057       -1,654      180,745      -10,929      158,234      -12,240
     Monthly)...................................
--------------------------------------------------------------------------------------------------------------------------------------------------------

                                            Table 6.--Incidence of Children With >7 Points Pb-Related IQ Loss
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                   Dual linear--stratified       Log-linear with       Dual linear--stratified  Log-linear with cutpoint
                                                   at 7.5 ug/dL peak blood        linearization        at  10 ug/dL peak blood -------------------------
                                                             Pb            --------------------------            Pb
                                                 --------------------------                          --------------------------                 Delta
  Air quality scenario (location-specific urban                   Delta                     Delta                     Delta      Incidence    (change in
                  case studies)                    Incidence    (change in   Incidence    (change in   Incidence    (change in     of > 7     incidence
                                                     of > 7     incidence      of > 7     incidence      of > 7     incidence    points IQ   compared to
                                                   points IQ   compared to   points IQ   compared to   points IQ   compared to      loss       current
                                                      loss       current        loss       current        loss       current                 conditions)
                                                               conditions)               conditions)               conditions)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Chicago (total modeled child population:
 396,511):
    Chicago Current Conditions..................      136,709  ...........       33,664  ...........           63  ...........        1,015  ...........
    Current NAAQS (1.5 [mu]g/m\3\ Maximum             244,401      107,692      100,159       66,495          555          492        5,226        4,211
     Quarterly).................................
    Alternative NAAQS (0.2 [mu]g/\3\ Maximum          136,067         -642       32,546       -1,118           48          -16        1,007           -8
     Monthly)...................................
    Alternative NAAQS (0.05 [mu]g/\3\ Maximum         120,706      -16,003       27,367       -6,297           16          -48          864         -151
     Monthly)...................................
    Alternative NAAQS (0.02 [mu]g/\3\ Maximum         117,819      -18,890       26,027       -7,637            8          -56          690         -325
     Monthly)...................................
Cleveland (total modeled child population:
 13,990):
    Cleveland Current Conditions................        4,834  ...........        1,212  ...........            3  ...........           46  ...........
    Current NAAQS (1.5 [mu]g/m\3\ Maximum               6,139        1,305        1,858          647            4            2          105           59
     Quarterly).................................
    Alternative NAAQS (0.2 [mu]g/m\3\ Maximum           4,525         -309        1,073         -139            1           -2           40           -6
     Quarterly).................................
    Alternative NAAQS (0.5 [mu]g/m\3\ Maximum           4,806          -28        1,180          -31            1           -2           43           -3
     Monthly)...................................
    Alternative NAAQS (0.2 [mu]g/m\3\ Maximum           4,424         -410        1,026         -186            1           -2           43           -3
     Monthly)...................................
    Alternative NAAQS (0.05 [mu]g/m\3\ Maximum          4,106         -728          886         -326            0           -3           24          -22
     Monthly)...................................
    Alternative NAAQS (0.02 [mu]g/m\3\ Maximum          4,051         -783          866         -345            0           -3           27          -18
     Monthly)...................................
Los Angeles (total modeled child population:
 372,252):
    Los Angeles Current Conditions..............       94,684  ...........       22,665  ...........           23  ...........          732  ...........
    Current NAAQS (1.5 [mu]g/m\3\ Maximum,            158,171       63,487       57,834       35,168          183          160        3,771        3,038
     Quarterly).................................
    Alternative NAAQS (0.05 [mu]g/m\3\ Maximum,        87,303       -7,382       19,781       -2,884           11          -11          624         -109
     Monthly)...................................
    Alternative NAAQS (0.02 [mu]g/m\3\ Maximum,        83,909      -10,775       17,939       -4,726           17           -6          498         -235
     Monthly)...................................
--------------------------------------------------------------------------------------------------------------------------------------------------------

D. Conclusions on Adequacy of the Current Primary Standard

    The initial issue to be addressed in the current review of the
primary Pb standard is whether, in view of the advances in scientific
knowledge and additional information, the existing standard should be
retained or revised. In evaluating whether it is appropriate to retain
or revise the current standard, the Administrator builds on the general
approach used in the initial setting of the standard, as well as that
used in the last review, and reflects the broader body of evidence and
information now available.
    The approach used is based on an integration of information on
health effects associated with exposure to ambient Pb; expert judgment
on the adversity of such effects on individuals; and policy judgments
as to when the standard is requisite to protect public health with an
adequate margin of safety, which are informed by air quality and
related analyses, quantitative exposure and risk assessments when
possible, and qualitative assessment of impacts that could not be
quantified.
    The Administrator has taken into account both evidence-based \104\
and quantitative exposure- and risk-based considerations in developing
conclusions on the adequacy of the current primary Pb standard.
Evidence-based considerations include the assessment of evidence for a
variety of

Pb-related health endpoints from epidemiological, and animal
toxicological studies. Consideration of quantitative exposure- and
risk-based information draws from the results of the exposure and risk
assessments described above. More specifically, estimates of the
magnitude of Pb-related exposures and risks associated with air quality
levels associated with just meeting the current primary Pb NAAQS have
been considered.\105\
---------------------------------------------------------------------------

    \104\ The term ``evidence-based'' as used here refers to the
drawing of information directly from published studies, with
specific attention to those reviewed and described in the Criteria
Document, and is distinct from considerations that draw from the
results of the quantitative exposure and risk assessement.
    \105\ As described in seciton II.C.2.d above, levels in the
location-specific urban case studies were increased from current
conditions such that the portion of each case study with highest
concentrations would just meet the current NAAQS.
---------------------------------------------------------------------------

    In this review, a series of general questions frames the approach
to reaching a decision on the adequacy of the current standard, such as
the following: (1) To what extent does newly available information
reinforce or call into question evidence of associations of Pb
exposures with effects identified when the standard was set?; (2) to
what extent has evidence of new effects or at-risk populations become
available since the time the standard was set?; (3) to what extent have
important uncertainties identified when the standard was set been
reduced and have new uncertainties emerged?; and (4) to what extent
does newly available information reinforce or call into question any of
the basic elements of the current standard?
    The question of whether the available evidence supports
consideration of a standard that is more protective than the current
standard includes consideration of: (1) Whether there is evidence that
associations with blood Pb in epidemiological studies extend to ambient
Pb concentration levels that are as low as or lower than had previously
been observed, and the important uncertainties associated with that
evidence; (2) the extent to which exposures of potential concern and
health risks are estimated to occur in areas upon meeting the current
standard and the important uncertainties associated with the estimated
exposures and risks; and (3) the extent to which the Pb-related health
effects indicated by the evidence and the exposure and risk assessments
are considered important from a public health perspective, taking into
account the nature and severity of the health effects, the size of the
at-risk populations, and the kind and degree of the uncertainties
associated with these considerations.
    This approach is consistent with the requirements of the NAAQS
provisions of the Act and with how EPA and the courts have historically
interpreted the Act. These provisions require the Administrator to
establish primary standards that, in the Administrator's judgment, are
requisite to protect public health with an adequate margin of safety.
In so doing, the Administrator seeks to establish standards that are
neither more nor less stringent than necessary for this purpose. The
Act does not require that primary standards be set at a zero-risk level
but rather at a level that avoids unacceptable risks to public health,
including the health of sensitive groups.
    The following discussion starts with background information on the
current standard (section II.D.1), including both the basis for
derivation of the current standard and considerations and conclusions
from the 1990 Staff Paper (USEPA, 1990b). This is followed by a
discussion of the Agency's approach in this review for evaluating the
adequacy of the current standard, in section II.D.2, including both
evidence-based and exposure/risk-based considerations (sections
II.D.2.a and b, respectively). CASAC advice and recommendations
concerning adequacy of the current standard are summarized in section
II.D.3. Lastly, the Administrator's proposed conclusions with regard to
the adequacy of the current standard are presented in section II.D.4.
1. Background
a. The Current Standard
    The current primary standard is set at a level of 1.5 [mu]g/m\3\,
measured as Pb-TSP, not to be exceeded by the maximum arithmetic mean
concentration averaged over a calendar quarter. The standard was set in
1978 to provide protection to the public, especially children as the
particularly sensitive population subgroup, against Pb-induced adverse
health effects (43 FR 46246). In setting the standard, EPA relied on
conclusions regarding sources of exposure, air-related exposure
pathways, variability and susceptibility of young children, the most
sensitive health endpoints, blood Pb level thresholds for various
health effects and the stability and distributional characteristics of
Pb (both in the human body and in the environment) (43 FR 46247). The
specific basis for selecting each of the elements of the standard is
described below.
i. Level
    EPA's objective in selecting the level of the current standard was
``to estimate the concentration of Pb in the air to which all groups
within the general population can be exposed for protracted periods
without an unacceptable risk to health'' (43 FR 46252). As stated in
the notice of final rulemaking, ``This estimate was based on EPA's
judgment in four key areas:
    (1) Determining the `sensitive population' as that group within the
general population which has the lowest threshold for adverse effects
or greatest potential for exposure. EPA concludes that young children,
aged 1 to 5, are the sensitive population.
    (2) Determining the safe level of total lead exposure for the
sensitive population, indicated by the concentration of lead in the
blood. EPA concludes that the maximum safe level of blood lead for an
individual child is 30 [mu]g Pb/dl and that population blood lead,
measured as the geometric mean, must be 15 [mu]g Pb/dl in order to
place 99.5 percent of children in the United States below 30 [mu]g Pb/
dl.
    (3) Attributing the contribution to blood lead from nonair
pollution sources. EPA concludes that 12 [mu]g Pb/dl of population
blood lead for children should be attributed to nonair exposure.
    (4) Determining the air lead level which is consistent with
maintaining the mean population blood lead level at 15 [mu]g Pb/dl [the
maximum safe mean level]. Taking into account exposure from other
sources (12 [mu]g Pb/dl), EPA has designed the standard to limit air
contribution after achieving the standard to 3 [mu]g Pb/dl. On the
basis of an estimated relationship of air lead to blood lead of 1 to 2,
EPA concludes that the ambient air standard should be 1.5 [mu]g Pb/
m\3\.'' (43 FR 46252)
    EPA's judgments in these key areas, as well as margin of safety
considerations, are discussed below.
    The assessment of the science that was presented in the 1977
Criteria Document (USEPA, 1977), indicated young children, aged 1 to 5,
as the population group at particular risk from Pb exposure. Children
were recognized to have a greater physiological sensitivity than adults
to the effects of Pb and a greater exposure. In identifying young
children as the sensitive population, EPA also recognized the
occurrence of subgroups with enhanced risk due to genetic factors,
dietary deficiencies or residence in urban areas. Yet information was
not available to estimate a threshold for adverse effects for these
subgroups separate from that of all young children. Additionally, EPA
recognized both a concern regarding potential risk to pregnant women
and fetuses, and a lack of information to establish that these
subgroups are more at risk than young children. Accordingly, young
children, aged 1 to 5, were identified as the group which has the
lowest threshold for adverse

effects of greatest potential for exposure (i.e., the sensitive
population) (43 FR 46252).
    In identifying the maximum safe exposure, EPA relied upon the
measurement of Pb in blood (43 FR 46252-46253). The physiological
effect of Pb that had been identified as occurring at the lowest blood
Pb level was inhibition of an enzyme integral to the pathway by which
heme (the oxygen carrying protein of human blood) is synthesized, i.e.,
delta-aminolevulinic acid dehydratase ([delta]-ALAD). The 1977 Criteria
Document reported a threshold for inhibition of this enzyme in children
at 10 [mu]g Pb/dL. The 1977 Criteria Document also reported a threshold
of 15-20 [mu]g/dL for elevation of erythrocyte protoporphyrin (EP),
which is an indication of some disruption of the heme synthesis
pathway. EPA concluded that this effect on the heme synthesis pathway
(indicated by EP) was potentially adverse. EPA further described a
range of blood levels associated with a progression in detrimental
impact on the heme synthesis pathway. At the low end of the range (15-
20 [mu]g/dL), the initial detection of EP associated with blood Pb was
not concluded to be associated with a significant risk to health. The
upper end of the range (40 [mu]g/dL), the threshold associated with
clear evidence of heme synthesis impairment and other effects
contributing to clinical symptoms of anemia, was regarded by EPA as
clearly adverse to health. EPA also noted that for some children with
blood Pb levels just above those for these effects (e.g., 50 [mu]g/dL),
there was risk for additional adverse effects (e.g., nervous system
deficits). Additionally, in the Agency's statement of factors on which
the conclusion as to the maximum safe blood Pb level for an individual
child was based, EPA stated that the maximum safe blood level should be
``no higher than the blood Pb range characterized as undue exposure by
the Center for Disease Control of the Public Health Service, as
endorsed by the American Academy of Pediatrics, because of elevation of
erythrocyte protoporphyrin (above 30 [mu]g Pb/dL)''.\106\
---------------------------------------------------------------------------

    \106\ The CDC subsequently revised their advisory level for
children's blood Pb to 25 [mu]g/dL in 1985, and to 10 [mu]g/dL in
1991. In 2005, with consideration of a review of the evidence by
their advisory committee, CDC revised their statement on Preventing
Lead Poisoning in Young Children, specifically recognizing the
evidence of adverse health effects in children with blood Pb levels
below 10 [mu]g/dL and the data demonstrating that no ``safe''
threshold for blood Pb in children had been identified, and
emphasizing the importance of preventative measures (CDC, 2005a).
Recently, CDC's Advisory Committee on Childhood Lead Poisoning
Prevention noted the 2005 CDC statements and reported on a review of
the clinical interpretation and management of blood Pb levels below
10 [mu]g/dL (ACCLPP, 2007). More details on this level are provided
in Section II.B.1.
---------------------------------------------------------------------------

    Having identified the maximum safe blood level in individual
children, EPA next made a public health policy judgment regarding the
target mean blood level for the U.S. population of young children (43
FR 46252-46253). With this judgment, EPA identified a target of 99.5
percent of this population to be brought below the maximum safe blood
Pb level. This judgment was based on consideration of the size of the
sensitive subpopulation, and the recognition that there are special
high-risk groups of children within the general population. The
population statistics available at the time (the 1970 U.S. Census)
indicated a total of 20 million children younger than 5 years of age,
with 15 million residing in urban areas and 5 million in center cities
where Pb exposure was thought likely to be ``high''. Concern about
these high-risk groups influenced EPA's determination of 99.5 percent,
deterring EPA from selecting a population percentage lower than 99.5
(43 FR 46253). EPA then used standard statistical techniques to
calculate the population mean blood Pb level that would place 99.5
percent of the population below the maximum safe level. Based on the
then available data, EPA concluded that blood Pb levels in the
population of U.S. children were normally distributed with a GSD of
1.3. Based on standard statistical techniques, EPA determined that a
thus described population in which 99.5 percent of the population has
blood Pb levels below 30 [mu]g/dL would have a geometric mean blood
level of 15 [mu]g/dL. EPA described 15 [mu]g/dL as ``the maximum safe
blood lead level (geometric mean) for a population of young children''
(43 FR 46247).
    When setting the current NAAQS, EPA recognized that the air
standard needed to take into account the contribution to blood Pb
levels from Pb sources unrelated to air pollution. Consequently, the
calculation of the current NAAQS included the subtraction of Pb
contributed to blood Pb from nonair sources, from the estimate of a
safe mean population blood Pb level. Without this subtraction, EPA
recognized that the combined exposure to Pb from air and nonair sources
would result in a blood Pb concentration exceeding the safe level (43
FR 46253). In developing an estimate of this nonair contribution, EPA
recognized the lack of detailed or widespread information about the
relative contribution of various sources to children's blood Pb levels,
such that an estimate could only be made by inference from other
empirical or theoretical studies, often involving adults. Additionally,
EPA recognized the expectation that the contribution to blood Pb levels
from nonair sources would vary widely, was probably not in constant
proportion to air Pb contribution, and in some cases may alone exceed
the target mean population blood Pb level (43 FR 46253-46254). The
amount of blood Pb attributed to nonair sources was selected based
primarily on findings in studies of blood Pb levels in areas where air
Pb levels were low relative to other locations in U.S. The air Pb
levels in these areas ranged from 0.1 to 0.7 [mu]g/m\3\. The average of
the reported blood Pb levels for children of various ages in these
areas was on the order of 12 [mu]g/dL. Thus, 12 [mu]g/dL was identified
as the nonair contribution, and subtracted from the population mean
target level of 15 [mu]g/dL to yield a value of 3 [mu]g/dL as the limit
on the air contribution to blood Pb.
    In determining the air Pb level consistent with an air contribution
of 3 [mu]g Pb/dL, EPA reviewed studies assessed in the 1977 Criteria
Document that reported changes in blood Pb with different air Pb
levels. These studies included a study of children exposed to Pb from a
primary Pb smelter, controlled exposures of adult men to Pb in fine
particulate matter, and a personal exposure study involving several
male cohorts exposed to Pb in a large urban area in the early 1970s (43
FR 46254).\107\ Using all three studies, EPA calculated an average
slope or ratio over the entire range of data. That value was 1.95
(rounded to 2 [mu]g/dL blood Pb concentration to 1 [mu]g/m\3\ air Pb
concentration), and is recognized to fall within the range of values
reported in the 1977 Criteria Document. On the basis of this 2-to-1
relationship, EPA concluded that the ambient air standard should be 1.5
[mu]g Pb/m\3\ (43 FR 46254).
---------------------------------------------------------------------------

    \107\ Mean blood Pb levels in the adult study groups ranged from
10 [mu]g/dL to approximately 30 [mu]g/dL and in the child groups
they ranged from approximately 20 [mu]g/dL up to 65 [mu]g/dL (USEPA,
1986a, section 11.4.1).
---------------------------------------------------------------------------

    In consideration of the appropriate margin of safety during the
development of the current NAAQS, EPA identified the following factors:
(1) The 1977 Criteria Document reported multiple biological effects of
Pb in practically all cell types, tissues and organ systems, of which
the significance for health had not yet been fully studied; (2) no
beneficial effects of Pb at then current environmental levels were
recognized;

(3) data were incomplete as to the extent to which children are
indirectly exposed to air Pb that has moved to other environmental
media, such as water, soil and dirt, and food; (4) Pb is chemically
persistent and with continued uncontrolled emissions would continue to
accumulate in human tissue and the environment; and (5) the possibility
that exposure associated with blood Pb levels previously considered
safe might influence neurological development and learning abilities of
the young child (43 FR 46255). Recognizing that estimating an
appropriate margin of safety for the air Pb standard was complicated by
the multiple sources and media involved in Pb exposure, EPA chose to
use margin of safety considerations principally in establishing a
maximum safe blood Pb level for individual children (30 [mu]g Pb/dL)
and in determining the percentage of children to be placed below this
maximum level (about 99.5 percent). Additionally, in establishing other
factors used in calculating the standard, EPA used margin of safety
considerations in the sense of making careful judgment based on
available data, but these judgments were not considered to be at the
precautionary extreme of the range of data available at the time (43 FR
46251).
    EPA further recognized that, because of the variability between
individuals in a population experiencing a given level of Pb exposure,
it was considered impossible to provide the same margin of safety for
all members in the sensitive population or to define the margin of
safety in the standard as a simple percentage. EPA believed that the
factors it used in designing the standards provided an adequate margin
of safety for a large proportion of the sensitive population. The
Agency did not believe that the margin was excessively large or on the
other hand that the air standard could protect everyone from elevated
blood Pb levels (43 FR 46251).
ii. Averaging Time, Form, and Indicator
    The averaging time for the current standard is a calendar quarter.
In the decision for this aspect of the standard, the Agency also
considered a monthly averaging period, but concluded that ``a
requirement for the averaging of air quality data over calendar quarter
will improve the validity of air quality data gathered without a
significant reduction in the protectiveness of the standards.'' As
described in the notice for this decision (43 FR 46250), this
conclusion was based on several points, including the following:
     An analysis of ambient measurements available at the time
indicated that the distribution of air Pb levels was such that there
was little possibility that there could be sustained periods greatly
above the average value in situations where the quarterly standard was
achieved.
     A recognition that the monitoring network may not actually
represent the exposure situation for young children, such that it
seemed likely that elevated air Pb levels when occurring would be close
to Pb air pollution sources where young children would typically not
encounter them for the full 24-hour period reported by the monitor.
     Medical evidence available at the time indicated that
blood Pb levels re-equilibrate slowly to changes in air exposure, a
finding that would serve to dampen the impact of short-term period of
exposure to elevated air Pb.
     Direct exposure to air is only one of several routes of
total exposure, thus lessening the impact of a change in air Pb on
blood Pb levels.
    The statistical form of the current standard is a not-to-be-
exceeded or maximum value. EPA set the standard as a ceiling value with
the conclusion that this air level would be safe for indefinite
exposure for young children (43 FR 46250).
    The indicator is total airborne Pb collected by a high volume
sampler (43 FR 46258). EPA's selection of Pb-TSP as the indicator for
the standard was based on explicit recognition both of the significance
of ingestion as an exposure pathway for Pb that had deposited from the
air and of the potential for Pb deposited from the air to become re-
suspended in respirable size particles in the air and available for
human inhalation exposure. As stated in the final rule, ``a significant
component of exposure can be ingestion of materials contaminated by
deposition of lead from the air,'' and that, ``in addition to the
indirect route of ingestion and absorption from the gastrointestinal
tract, non-respirable Pb in the environment may, at some point become
respirable through weathering or mechanical action'' (43 FR 46251).
b. Policy Options Considered in the Last Review
    During the 1980s, EPA initiated a review of the air quality
criteria and NAAQS for Pb. CASAC and the public were fully involved in
this review, which led to the publication of a criteria document with
associated addendum and a supplement (USEPA, 1986a, 1986b, 1990a), an
exposure analysis methods document (USEPA, 1989), and a staff paper
(USEPA, 1990b).
    Total emissions to air were estimated to have dropped by 94 percent
between 1978 and 1987, with the vast majority of it attributed to the
reduction of Pb in gasoline. Accordingly, the focus of the last review
was on areas near stationary sources of Pb emissions. Although such
sources were not considered to have made a significant contribution (as
compared to Pb in gasoline) to the overall Pb pollution across large-
urban or regional areas, Pb emissions from such sources were considered
to have the potential for a significant impact on a local scale. Air Pb
concentrations, and especially soil and dust Pb concentrations, had
been associated with elevated levels of Pb absorption in children and
adults in numerous Pb point source community studies. Exceedances of
the current NAAQS were found at that time only in the vicinity of
nonferrous smelters or other point sources of Pb.
    In summarizing and interpreting the health evidence presented in
the 1986 Criteria Document and associated documents, the 1990 Staff
Paper described the collective impact on children of the effects at
blood Pb levels above 15 [mu]g/dL as representing a clear pattern of
adverse effects worthy of avoiding. This is in contrast to EPA's
identification of 30 [mu]g/dL as a safe blood Pb level for individual
children when the NAAQS was set in 1978. The Staff Paper further stated
that at blood Pb levels of 10-15 [mu]g/dL, there was a convergence of
evidence of Pb-induced interference with a diverse set of physiological
functions and processes, particularly evident in several independent
studies showing impaired neurobehavioral function and development.
Further, the available data did not indicate a clear threshold in this
blood Pb range. Rather, it suggested a continuum of health risks down
to the lowest levels measured.\108\
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    \108\ In 1991, the CDC reduced their advisory level for
children's blood Pb from 25 [mu]g/dL to 10 [mu]g/dL.
---------------------------------------------------------------------------

    For the purposes of comparing the relative protectiveness of
alternative Pb NAAQS, the staff conducted analyses to estimate the
percentages of children with blood Pb levels above 10 [mu]g/dL and
above 15 [mu]g/dL for several air quality scenarios developed for a
small set of stationary source exposure case studies. The results of
the analyses of child populations living near two Pb smelters indicated
that substantial reductions in Pb exposure could be achieved through
just meeting the current Pb NAAQS. According to the best estimate
analyses, over 99.5% of children living in areas significantly affected
by the smelters would have blood Pb levels below 15

[mu]g/dL if the current standard was achieved. Progressive changes in
this number were estimated for the alternative monthly Pb NAAQS levels
evaluated in those analyses, which ranged from 1.5 [mu]g/m\3\ to 0.5
[mu]g/m\3\.
    In light of the health effects evidence available at the time, the
1990 Staff Paper presented air quality, exposure, and risk analyses,
and other policy considerations, as well as the following staff
conclusions with regard to the primary Pb NAAQS (USEPA, 1990b, pp. xii
to xiv):
    (1) ``The range of standards * * * should be from 0.5 to 1.5 [mu]g/
m\3\.''
    (2) ``A monthly averaging period would better capture short-term
increases in lead exposure and would more fully protect children's
health than the current quarterly average.''
    (3) ``The most appropriate form of the standard appears to be the
second highest monthly averages {sic{time}  in a 3-year span. This form
would be nearly as stringent as a form that does not permit any
exceedances and allows for discounting of one `bad' month in 3 years
which may be caused, for example, by unusual meteorology.''
    (4) ``With a revision to a monthly averaging time more frequent
sampling is needed, except in areas, like roadways remote from lead
point sources, where the standard is not expected to be violated. In
those situations, the current 1-in-6 day sampling schedule would
sufficiently reflect air quality and trends.''
    (5) ``Because exposure to atmospheric lead particles occurs not
only via direct inhalation, but via ingestion of deposited particles as
well, especially among young children, the hi-volume sampler provides a
reasonable indicator for determining compliance with a monthly standard
and should be retained as the instrument to monitor compliance with the
lead NAAQS until more refined instruments can be developed.''
    Based on its review of a draft Staff Paper, which contained the
above recommendations, the CASAC strongly recommended to the
Administrator that EPA should actively pursue a public health goal of
minimizing the Pb content of blood to the extent possible, and that the
Pb NAAQS is an important component of a multimedia strategy for
achieving that goal (CASAC, 1990, p. 4). In noting the range of levels
recommended by staff, CASAC recommended consideration of a revised
standard that incorporates a ``wide margin of safety, because of the
risk posed by Pb exposures, particularly to the very young whose
developing nervous system may be compromised by even low level
exposures'' (id., p. 3). More specifically, CASAC judged that a
standard within the range of 1.0 to 1.5 [mu]g/m\3\ would have
``relatively little, if any, margin of safety;'' that greater
consideration should be given to a standard set below 1.0 [mu]g/m\3\;
and, to provide perspective in setting the standard, it would be
appropriate to consider the distribution of blood Pb levels associated
with meeting a monthly standard of 0.25 [mu]g/m\3\, a level below the
range considered by staff (id.).
    After consideration of the documents developed during the review,
EPA chose not to propose revision of the NAAQS for Pb. During the same
time period, the Agency published and embarked on the implementation of
a broad, multi-program, multi-media, integrated national strategy to
reduce Pb exposures (USEPA, 1991). As discussed above in section I.C.,
as part of implementing this integrated Pb strategy, the Agency focused
efforts primarily on regulatory and remedial clean-up actions aimed at
reducing Pb exposures from a variety of nonair sources judged to pose
more extensive public health risks to U.S. populations, as well as on
actions to reduce Pb emissions to air, particularly near stationary
sources.\109\
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    \109\ A description of the various programs implemented since
1990 to reduce Pb exposures, including the recent RRP rule, is
provided in section I.C.
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2. Considerations in the Current Review
a. Evidence-Based Considerations
    In considering the broad array of health effects evidence assessed
in the Criteria Document with respect to the adequacy of the current
standard, the discussion here, like that in the Staff Paper and ANPR,
focuses on those health endpoints associated with the Pb exposure and
blood levels most pertinent to ambient exposures. In so doing, EPA
gives particular weight to evidence available today that differs from
that available at the time the standard was set with regard to its
support of the current standard.
    First, with regard to the sensitive population, the susceptibility
of young children to the effects of Pb is well recognized, in addition
to more recent recognition of effects of chronic or cumulative Pb
exposure with advancing age (CD, Sections 5.3.7 and pp. 8-73 to 8-75).
The prenatal period and early childhood are periods of increased
susceptibility to Pb exposures, with evidence of adverse effects on the
developing nervous system that generally appear to persist into later
childhood and adolescence (CD, Section 6.2).\110\ Thus, while the
sensitivity of the elderly and other particular subgroups is
recognized, as at the time the standard was set, young children
continue to be recognized as a key sensitive population for Pb
exposures.
---------------------------------------------------------------------------

    \110\ For example, the following statement is made in the
Criteria Document ``Negative Pb impacts on neurocognitive ability
and other neurobehavioral outcomes are robust in most recent studies
even after adjustment for numerous potentially confounding factors
(including quality of care giving, parental intelligence, and
socioeconomic status). These effects generally appear to persist
into adolescence and young adulthood.'' (CD, p.E-9)
---------------------------------------------------------------------------

    With regard to the exposure levels at which adverse health effects
occur, the current evidence demonstrates the occurrence of adverse
health effects at appreciably lower blood Pb levels than those
demonstrated by the evidence at the time the standard was set, at which
time the Agency identified 30 [mu]g/dL as the maximum safe blood Pb
level for individual children and 15 [mu]g/dL as the maximum safe
geometric mean blood Pb level for a population of children (as
described in section II.D.1.a above). This change in the evidence since
the time the standard was set is reflected in changes made by the CDC
in their advisory level for Pb in children's blood, and changes they
have made in their characterization of that level (as described in
section II.B.1.b). Although CDC recognized a level of 30 [mu]g/dL blood
Pb as warranting individual intervention in 1978 when the Pb NAAQS was
set, in 2005 they recognized the evidence of adverse health effects in
children with blood Pb levels below 10 [mu]g/dL and the data
demonstrating that no ``safe'' threshold for blood Pb had been
identified (CDC, 1991; CDC, 2005).
    As summarized in section II.B above, the Criteria Document
describes current evidence regarding the occurrence of a variety of
health effects, including neurological effects in children associated
with blood Pb levels extending well below 10 [mu]g/dL (CD, Sections
6.2, 8.4 and 8.5).\111\ As stated

in the Criteria Document, ``The overall weight of the available
evidence provides clear substantiation of neurocognitive decrements
being associated in young children with blood-Pb concentrations in the
range of 5-10 [mu]g/dL, and possibly somewhat lower. Some newly
available analyses appear to show Pb effects on the intellectual
attainment of preschool and school age children at population mean
concurrent blood-Pb levels ranging down to as low as 2 to 8 [mu]g/dL''
(CD, p. E-9). With regard to the evidence of neurological effects at
these low levels, EPA notes, in particular (and discusses more
completely in section II.B.2.b above), the international pooled
analysis by Lanphear and others (2005), studies of individual cohorts
such as the Rochester, Boston, and Mexico City cohorts (Canfield et
al., 2003a; Canfield et al., 2003b; Bellinger and Needleman, 2003;
Tellez-Rojo et al., 2006), the study of African-American inner-city
children from Detroit (Chiodo et al., 2004), the cross-sectional study
of young children in three German cities (Walkowiak et al., 1998) and
the cross-sectional analysis of a nationally representative sample from
the NHANES III (collected from 1988-1994) (Lanphear et al., 2000). In
the study by Lanphear et al (2000), the mean blood Pb for the full
study group was 1.9 [mu]g/dL and the mean blood Pb level in the lowest
blood Pb subgroup with which a statistically significant association
with neurocognitive effects was found (individual blood Pb values <5
[mu]g/dL) was 1.7 [mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al.,
2000; Auinger, 2008).\112\ These studies and associated limitations are
discussed above in section II.B.2.b.
---------------------------------------------------------------------------

    \111\ 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'').
    \112\ These findings include significant associations in some of
the study sample subsets of children, namely those with blood Pb
levels less than 10 [mu]g/dL, less than 7.5 [mu]g/dL, and less than
5 [mu]g/dL. The mean blood Pb level in the third subset was 1.7
[mu]g/dL (Auinger, 2008). A positive, but not statistically
significant association, was observed in the less than 2.5 [mu]g/dL
subset (mean blood Pb of 1.2 [mu]g/dL [Auinger, 2008]), although the
effect estimate for this subset was largest among all the subsets
(Lanphear et al., 2000). The lack of statistical significance for
this subset may be due to the smaller sample size of this subset
which would lead to lower statistical power.
---------------------------------------------------------------------------

    As stated in the Criteria Document with regard to the
neurocognitive effects in children, the ``weight of overall evidence
strongly substantiates likely occurrence of type of effect in
association with blood-Pb concentrations in range of 5-10 [mu]g/dL, or
possibly lower, as implied by (???) [in associated Table 8-5 of
Criteria Document]. Although no evident threshold has yet been clearly
established for those effects, the existence of such effects at still
lower blood-Pb levels cannot be ruled out based on available data.''
(CD, p. 8-61). The Criteria Document further notes that any such
threshold may exist ``at levels distinctly lower than the lowest
exposures examined in these epidemiological studies'' (CD, p. 8-67).
i. Evidence-Based Framework Considered in the Staff Paper
    In considering the adequacy of the current standard, the Staff
Paper considered the evidence in the context of the framework used to
determine the standard in 1978, as adapted to reflect the current
evidence. In so doing, the Staff Paper recognized that the health
effects evidence with regard to characterization of a threshold for
adverse effects has changed since the standard was set in 1978, as have
the Agency's views on the characterization of a safe blood Pb level. As
described in section II.D.1.a, parameters for this framework include
estimates for average nonair blood Pb level, and air-to-blood ratio, as
well as a maximum safe individual and/or geometric mean blood Pb level.
For this last parameter, the Staff Paper for the purposes of this
evaluation considered the lowest population mean blood Pb levels with
which some neurocognitive effects have been associated in the evidence.
    As when the standard was set in 1978, there remain today
contributions to blood Pb levels from nonair sources. In 1978, the
Agency estimated the average blood Pb level for young children
associated with nonair sources to be 12 [mu]g/dL (as described in
section II.D.1.a). However, consistent with reductions since that time
in air Pb concentrations \113\ which contribute to blood Pb, nonair
contributions have also been reduced (as described in section II.A.4
above). The Staff Paper noted that the 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 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 1.0-1.4
[mu]g/dL or lower as an estimate of the nonair component of blood Pb
pertinent to average blood Pb levels (as more fully described in
section II.A.4 above; Henderson, 2007b).
---------------------------------------------------------------------------

    \113\ Air Pb concentrations nationally are estimated to have
declined more than 90% since the early 1980s, in locations not known
to be directly influenced by stationary sources (Staff Paper, pp. 2-
22 to 2-23).
---------------------------------------------------------------------------

    As in 1978, the evidence demonstrates that Pb in ambient air
contributes to Pb in blood, with the pertinent exposure routes
including both inhalation and ingestion (CD, Sections 3.1, 3.2, 4.2 and
4.4). In 1978, the evidence indicated a quantitative relationship
between ambient air Pb and blood Pb in terms of an air-to-blood ratio
that ranged from 1:1 to 1:2 (USEPA, 1977). In setting the standard, the
Agency relied on a ratio of 1:2, i.e., 2 [mu]g/dL blood Pb per 1 [mu]g/
m\3\ air Pb (as described in section II.D.1.a above). The Staff Paper
observed that ``[W]hile there is uncertainty and variability in the
absolute value of an air-to-blood relationship, the current evidence
indicates a notably greater ratio * * * e.g., on the order of 1:3 to
1:10'' (USEPA, 2007c).
    Based on the information described above, the Staff Paper concluded
that young children remain the sensitive population of primary focus in
this review, ``there is now no recognized safe level of Pb in
children's blood and studies appear to show adverse effects at
population mean concurrent blood Pb levels as low as approximately 2
[mu]g/dL (CD, pp. 6-31 to 6-32; Lanphear et al., 2000)'' (USEPA,
2007c). The Staff Paper further stated that ``while the nonair
contribution to blood Pb has declined, perhaps to a range of 1.0-1.4
[mu]g/dL, the air-to-blood ratio appears to be higher at today's lower
blood Pb levels than the estimates at the time the standard was set,
with current estimates on the order of 1:3 to 1:5 and perhaps up to
1:10'' (USEPA, 2007c). Adapting the framework employed in setting the
standard in 1978, the Staff Paper concluded that ``the more recently
available evidence suggests a level for the standard that is lower by
an order of magnitude or more'' (USEPA, 2007c).
ii. Air-Related IQ Loss Evidence-Based Framework
    Since completion of the Staff Paper and ANPR, the Agency has
further considered the evidence with regard to adequacy of the current
standard using an approach other than the adapted 1978 framework
considered in the Staff Paper. This alternative evidence-based
framework, referred to as the air-related IQ loss framework, shifts
focus from identifying an appropriate target population mean blood lead
level and instead focuses on the magnitude of effects of air-related Pb
on neurocognitive functions. This framework builds on a recommendation
by the CASAC Pb Panel to consider the evidence in a more quantitative
manner,

and is discussed in more detail below in section II.E.3.a, concerning
the level of the standard.
    In this air-related IQ loss 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.\114\ We have also drawn more quantitatively from the evidence by
using evidence-based C-R functions to quantify the association between
air Pb concentrations and air-related population mean IQ loss. Thus,
this framework more fully considers the evidence with regard to the
concentration-response relationship for the effect of Pb on IQ, and it
also draws from estimates for air-to-blood ratios.
---------------------------------------------------------------------------

    \114\ For example, as stated in the Criteria Document,
``Fortunately, there exists a large database of high quality studies
on which to base inferences regarding the relationship between Pb
exposure and neurodevelopment. In addition, Pb has been extensively
studied in animal models at doses that closely approximate the human
situation. Experimental animal studies are not compromised by the
possibility of confounding by such factors as social class and
correlated environmental factors. The enormous experimental animal
literature that proves that Pb at low levels causes neurobehavioral
deficits and provides insights into mechanisms must be considered
when drawing causal inferences (Bellinger, 2004; Davis et al., 1990;
U.S. Environmental Protection Agency, 1986a, 1990).'' (CD, p. 6-75)
---------------------------------------------------------------------------

    While we note the evidence of steeper slope for the C-R
relationship for blood Pb concentration and IQ loss at lower blood Pb
levels (described in sections II.B.2.b and II.E.3.a), for purposes of
consideration of the adequacy of the current standard we are concerned
with the C-R relationship for blood Pb levels that would be associated
with exposure to air-related Pb at the level of the current standard.
For this purpose, we have focused on a median linear estimate of the
slope of the C-R function for blood Pb levels up to, but no higher
than, 10 [mu]g/dL (described in section II.B.2.b above). The median
slope estimate is -0.9 IQ points per [mu]g/dL blood Pb \115\ (CD, p. 8-
80).
---------------------------------------------------------------------------

    \115\ 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 economic analysis.
---------------------------------------------------------------------------

    Applying estimates of air-to-blood ratios ranging from 1:3 to 1:5,
drawing from the discussion of air-to-blood ratios in section II.B.1.c
above, a population of children exposed at the current level of the
standard might be expected to result in an average air-related blood Pb
level above 4 [mu]g/dL.\116\ Multiplying these blood Pb levels by the
slope estimate, identified above, for blood Pb levels extending up to
10 [mu]g/dL (-0.9 IQ points per [mu]g/dL), would imply an average air-
related IQ loss for such a group of children on the order of 4 or more
IQ points.
---------------------------------------------------------------------------

    \116\ This is based on the calculation in which 1.5 [mu]g/m\3\
is multiplied by a ratio of 3 [mu]g blood Pb per 1 [mu]g/m\3\ air Pb
to yield an air-related blood Pb estimate of 4.5 [mu]g/dL; using a
1:5 ratio yields an estimate of 7.5 [mu]g/dL. As with the 1978
framework considered in the Staff Paper, the context for use of the
air-to-blood ratio here is a population being exposed at the level
of the standard.
---------------------------------------------------------------------------

b. Exposure- and Risk-Based Considerations
    As discussed above in section II.C, we have estimated exposures and
health risks associated with air quality that just meets the current
standard to help inform judgments about whether or not the current
standard provides adequate protection of public health, taking into
account key uncertainties associated with the estimated exposures and
risks (summarized above in section II.C and more fully in the Risk
Assessment Report).
    As discussed above, children are the sensitive population of
primary focus in this review. The exposure and risk assessment
estimates Pb exposure for children (less than 7 years of age), and
associated risk of neurocognitive effects in terms of IQ loss. In
addition to the risks (IQ loss) that were quantitatively estimated, EPA
recognizes that there may be long-term adverse consequences of such
deficits over a lifetime, and there are other, unquantified adverse
neurocognitive effects that may occur at similarly low exposures which
might additionally contribute to reduced academic performance, which
may have adverse consequences over a lifetime (CD, pp. 8-29 to 8-
30).\117\ Other impacts at low levels of childhood exposure that were
not quantified in the risk assessment include: other neurological
effects (sensory, motor, cognitive and behavioral), immune system
effects (including some related to allergic responses and asthma), and
early effects related to anemia. Additionally, as noted in section
II.B.2, other health effects evidence demonstrates associations between
Pb exposure and adverse health effects in adults (e.g., cardiovascular
and renal effects).\118\
---------------------------------------------------------------------------

    \117\ For example, the Criteria Document notes particular
findings with regard to academic achievement as ``suggesting that
Pb-sensitive neuropsychological processing and learning factors not
reflected by global intelligence indices might contribute to reduced
performance on academic tasks'' (CD, pp. 8-29 to 8-30).
    \118\ The weight of the evidence differs for the different
endpoints.
---------------------------------------------------------------------------

    As noted in the Criteria Document, a modest change in the
population mean of a health index, that is quantified for each
individual, can have substantial implications at the population level
(CD, p. 8-77, Sections 8.6.1 and 8.6.2; Bellinger, 2004; Needleman et
al., 1982; Weiss, 1988; Weiss, 1990)). For example, for an individual
functioning in the low range of IQ due to the influence of risk factors
other than Pb, a Pb-associated IQ loss of a few points might be
sufficient to drop that individual into the range associated with
increased risk of educational, vocational, and social handicap (CD, p.
8-77), while such a decline might create less significant impacts for
the individual near the mean of the population. Further, given a
uniform manifestation of Pb-related decrements across the range of IQ
scores in a population, a downward shift in the mean IQ value is
associated not only with a substantial increase in the percentage of
individuals achieving very low scores, but also with substantial
decreases in percentages achieving very high scores (CD, p. 8-81). The
CASAC Pb Panel has advised on this point that ``a population loss of 1-
2 IQ points is highly significant from a public health perspective''
(Henderson, 2007a, p. 6).
    In considering exposure and risk estimates with regard to adequacy
of the current standard, EPA has focused on IQ loss for air-related
exposure pathways. As described in section II.C.2.e above, limitations
in our data and modeling tools have resulted in an inability to develop
specific estimates such that 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.
Thus, the following discussion presents air-related IQ loss estimates
in terms of upper and lower bounds. In addition, as noted above
(section II.C.3.b), this discussion focuses predominantly on risk
estimates derived using the log-linear with low-exposure linearization
(LLL) C-R function, with the range associated with the other three
functions used in the assessment also being noted. Further, air-related
risk estimates are presented for the median and for an upper percentile
(i.e., the 95th percentile of the population assessed).
    EPA and CASAC recognize uncertainties in the risk estimates in the
tails of the distribution and consequently the 95th percentile is
reported as the estimate of the high end of the risk distribution
(Henderson, 2007b, p. 3). In so doing, however, EPA notes that it is
important to consider that there are individuals in the population
expected to have higher risk, particularly in light of the risk
management objectives for the current standard which was set in 1978 to

protect the 99.5th percentile. Further, we note an increased
uncertainty in our estimates of air-related risk for the upper
percentiles, such as the 95th percentile, due to limitations in the
data and tools available to us to estimate pathway contributions to
blood Pb and associated risk for individuals at the upper ends of the
distribution.
    In order to consider exposure and risk associated with the current
standard, EPA developed estimates for a case study based on air quality
projected to just meet the standard in a location of the country where
air concentrations currently do not meet the current standard (the
primary Pb smelter case study). Estimates of median air-related IQ loss
associated with just meeting the current NAAQS in the primary Pb
smelter case study subarea had a lower bound estimate of <3.2 points IQ
loss (``recent air'' category of Pb exposures) and an upper bound
estimate of <9.4 points IQ loss (``recent air'' plus ``past air''
category) for the range of C-R functions (Table 3). This estimate
(recent air plus past air) for the subarea based on the LLL C-R
function is 6.0 points IQ loss for the median and 8.0 points IQ loss
for the 95th percentile, with which we note a greater uncertainty than
for the median estimate (as discussed above).\119\ Modeling limitations
have affected our ability to derive lower bound estimates for this case
study (as described above in section II.C.2.c).
---------------------------------------------------------------------------

    \119\ We note that while we have termed risk estimates derived
for the sum of ``recent air'' plus ``past air'' exposure pathways as
``upper bound'' estimates of air-related risk, the primary Pb
smelter subarea is an area where soil has been remediated and thus
does not reflect any historical deposition. Further, soil Pb
concentrations in this area are not stable and may be increasing,
seeming to indicate ongoing response to current atmospheric
depositon in the area. Thus, for this case study, the ``recent air''
plus ``past air'' estimates are less of an ``upper bound'' for air-
related risk than in other case studies where historical Pb
deposition may have some representation in the ``past air'' soil
ingestion pathway.
---------------------------------------------------------------------------

    Additionally, we developed estimates of blood Pb and associated IQ
loss associated with the current standard for the urban case studies.
We note that we consider it extremely unlikely that air concentrations
in urban areas across the U.S. that are currently well below the
current standard would increase to just meet the standard. However, we
recognize the potential, although not the likelihood, for air Pb
concentrations in some limited areas currently well below the standard
to increase to just meet the standard by way of, for example, expansion
of existing sources (e.g., facilities operating as secondary smelters
may exercise previously used capabilities as primary smelters) or by
the congregation of multiple Pb sources in adjacent locations. We have
simulated this scenario (increased Pb concentrations to just meet the
current standard) in a general urban case study and three location-
specific urban case studies. For the location-specific urban case
studies, we note substantial uncertainty in simulating how the profile
of Pb concentrations might change in the hypothetical case where
concentrations increase to just meet the current standard.
    Turning first to the exposure/risk estimates for the current NAAQS
scenario simulated for the general urban case study, which is a
simplified representation of a location within an urban area (described
in section II.C.2.h above), median estimates of air-related IQ loss
range from 1.5 to 7.7 points (across all four C-R functions), with an
estimate based on the LLL function bounded at the low end by 3.4 points
and at the high end by 4.8 points (Table 3). At the 95th percentile for
total IQ loss (LLL estimate), IQ loss associated with air-related Pb is
estimated to fall somewhere between 5.5 and 7.6 points (Staff Paper,
Table 4-6).
    In considering the estimates for the three location-specific urban
case studies, we first note the extent to which exposures associated
with increased air Pb concentrations that simulate just meeting the
current standard are estimated to increase blood Pb levels in young
children. The magnitude of this for the median total blood Pb ranges
from 0.3 [mu]g/dL (an increase of 20 percent) in the case of the
Cleveland study area (where the highest monitor is estimated to be
approximately one fourth of the current NAAQS), up to approximately 1
[mu]g/dL (an increase of 50 to 70%) for the Chicago and Los Angeles
study areas, where the highest monitor is estimated to be at or below
one tenth of the current NAAQS (Table 1). Median estimates of air-
related risk for these case studies range from 0.6 points IQ loss
(recent air estimate using low-end C-R function) to 7.4 points IQ loss
(recent plus past air estimate using the high-end C-R function). The
corresponding estimates based on the LLL C-R function range from 2.7
points (lowest location-specific recent air estimate) to 4.7 points IQ
loss (highest location-specific recent plus past air estimate). The
comparable estimates of air-related risk for children at the 95th
percentile in these three case studies range from 2.6 to 7.6 points IQ
loss for the LLL C-R function (Staff paper, Table 4-6), although we
note increased uncertainty in the magnitude of these 95th percentile
air-related estimates.
    Another way in which the risk assessment results might be
considered is by comparing current NAAQS scenario estimates to current
conditions, although in so doing, it is important to recognize that, as
stated below and described in section II.C., this will underestimate
air-related impacts associated with the current NAAQS. In making such a
comparison of estimates for the three location-specific urban case
studies, the estimated difference in total Pb-related IQ loss for the
median child is about 0.5 to 1.4 points using the LLL C-R function and
a similar magnitude of difference is estimated for the 95th percentile.
The corresponding comparison for the general urban case study indicates
the current NAAQS scenario median total Pb-related IQ loss is 1.1 to
1.3 points higher than the two current conditions scenarios. As
described in section II.C, such comparisons are underestimates of air-
related impacts brought about as a result of increased air Pb
concentrations, and consequently they are inherently underestimates of
the true impact of an increased NAAQS level on public health.
    In considering the exposure/risk information with regard to
adequacy of the current standard, the Staff Paper first considered the
estimates described above, particularly those associated with air-
related risk.\120\ The Staff Paper described these estimates for the
current NAAQS as being indicative of levels of IQ loss associated with
air-related risk that may ``reasonably be judged to be highly
significant from a public health perspective'' (USEPA, 2007c).
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    \120\ As recognized in section III.B.2.d above, to simulate air
concentrations associated with the current NAAQS, a proportional
roll-up of concentrations from those for current conditions was
performed for the location-specific urban case studies. This was not
necessary for the primary Pb smelter case study in which air
concentrations currently exceed the current standard, nor for the
general urban case study.
---------------------------------------------------------------------------

    The Staff Paper also describes a different risk metric that
estimated differences in the numbers of children with different amounts
of Pb-related IQ loss between air quality scenarios for current
conditions and for the current NAAQS in the three location-specific
urban case studies. For example, estimates of the additional number of
children with IQ loss greater than one point (based on the LLL C-R
function) in these three study areas, for the current NAAQS scenario as
compared to current conditions, range from 100 to 6,000 across the
three locations (as shown above in Table 5). The corresponding
estimates for the additional number of children with IQ

loss greater than seven points, for the current NAAQS as compared to
current conditions, range from 600 to 66,000 (as shown above in Table
6). These latter values for the change in incidence of children with
greater than seven points Pb-related IQ loss represent 5 to 17 percent
of the children (aged less than 7 years of age) in these study areas.
This increase corresponds to approximately a doubling in the number of
children with this magnitude of Pb-related IQ loss in the study area
most affected. The Staff Paper concluded that these estimates indicate
the potential for significant numbers of children to be negatively
affected if air Pb concentrations increased to levels just meeting the
current standard.
    Beyond the findings related to quantified IQ loss, the Staff Paper
recognized the potential for other, unquantified adverse effects that
may occur at similarly low exposures. In summary, the Staff Paper
concluded that taken together, ``the quantified IQ effects associated
with the current NAAQS and other, nonquantified effects are important
from a public health perspective, indicating a need for consideration
of revision of the standard to provide an appreciable increase in
public health protection'' (USEPA, 2007c).
3. CASAC Advice and Recommendations and Public Comment
    CASAC's recommendations in this review builds upon the CASAC
recommendations during the 1990 review, which also advised on
consideration of more health protective NAAQS. In CASAC's review of the
1990 Staff Paper, as discussed in Section II.D.1.b, they generally
recommended consideration of levels below 1.0 [mu]g/m\3\, specifically
recommended analyses of a standard set at 0.25 [mu]g/m\3\, and also
recommended a revision to a monthly averaging time (CASAC, 1990).
    In its letter to the Administrator subsequent to consideration of
the ANPR, the final Staff Paper and the final Risk Assessment Report,
the CASAC Pb Panel unanimously and fully supported ``Agency staff's
scientific analyses in recommending the need to substantially lower the
level of the primary (public-health based) Lead NAAQS, to an upper
bound of no higher than 0.2 [mu]g/m\3\ with a monthly averaging time''
(Henderson, 2008, p. 1). This recommendation is consistent with their
recommendations conveyed in two earlier letters in the course of this
review (Henderson, 2007a, 2007b). Further, in their advice to the
Agency over the course of this review, CASAC has provided rationale for
their conclusions that has included their statement that the current Pb
NAAQS ``are totally inadequate for assuring the necessary decreases of
lead exposures in sensitive U.S. populations below those current health
hazard markers identified by a wealth of new epidemiological,
experimental and mechanistic studies'', and stated that ``Consequently,
it is the CASAC Lead Review Panel's considered judgment that the NAAQS
for Lead must be decreased to fully-protect both the health of children
and adult populations'' (Henderson, 2007a, p. 5). CASAC drew support
for their recommendation from the current evidence, described in the
Criteria Document, of health effects occurring at dramatically lower
blood Pb levels than those indicated by the evidence available when the
standard was set and of a recognition of effects that extend beyond
children to adults.
    The Agency has also received comments from the public on drafts of
the Staff Paper and related technical support document, as well as on
the ANPR.\121\ Public comments received to date that have addressed
adequacy of the current standard overwhelmingly concluded that the
current standard is inadequate and should be substantially revised, in
many cases suggesting specific reductions to a level at or below 0.2
[mu]g/m\3\. Two comments were received from specific industries
expressing the view that the current standard might need little or no
adjustment. One comment received early in the review stated that
current conditions justified revocation of the standard.
---------------------------------------------------------------------------

    \121\ All written comments submitted to the Agency are available
in the docket for this rulemaking, are transcripts of the public
meetings held in conjunction with CASAC's review of the Staff Paper,
the Risk Assessment Report, the Criteria Document and the ANPR.
---------------------------------------------------------------------------

4. Administrator's Proposed Conclusions Concerning Adequacy
    Based on the large body of evidence concerning the public health
impacts of Pb, including significant new evidence concerning effects at
blood Pb concentrations substantially below those identified when the
current standard was set, the Administrator proposes that the current
standard does not protect public health with an adequate margin of
safety and should be revised to provide additional public health
protection.
    In considering the adequacy of the current standard, the
Administrator has carefully considered the conclusions contained in the
Criteria Document, the information, exposure/risk assessments,
conclusions, and recommendations presented in the Staff Paper, the
advice and recommendations from CASAC, and public comments received on
the ANPR and other documents to date.
    The Administrator notes that the body of available evidence,
summarized above in section III.B and discussed in the Criteria
Document, is substantially expanded from that available when the
current standard was set three decades ago. The Criteria Document
presents evidence of the occurrence of health effects at appreciably
lower blood Pb levels than those demonstrated by the evidence at the
time the standard was set. Subsequent to the setting of the standard,
the Pb NAAQS criteria review during the 1980s and the current review
have provided (a) expanded and strengthened evidence of still lower Pb
exposure levels associated with slowed physical and neurobehavioral
development, lower IQ, impaired learning, and other indicators of
adverse neurological impacts; and (b) other effects of Pb on
cardiovascular function, immune system components, calcium and vitamin
D metabolism and other health endpoints (discussed fully in the
Criteria Document).
    The Administrator notes particularly the robust evidence of
neurotoxic effects of Pb exposure in children, both with regard to
epidemiological and toxicological studies. While blood Pb levels in
U.S. children have decreased notably since the late 1970s, newer
studies have investigated and reported associations of effects on the
neurodevelopment of children with these more recent blood Pb levels.
The toxicological evidence includes extensive experimental laboratory
animal evidence that substantiates well the plausibility of the
epidemiologic findings observed in human children and expands our
understanding of likely mechanisms underlying the neurotoxic effects.
Further, the Administrator notes the current evidence that suggests a
steeper dose-response relationship at these lower blood Pb levels than
at higher blood Pb levels, indicating the potential for greater
incremental impact associated with exposure at these lower levels.
    In addition to the evidence of health effects occurring at
significantly lower blood Pb levels, the Administrator recognizes that
the current health effects evidence together with findings from the
exposure and risk assessments (summarized above in section III.B), like
the information available at the time the standard was set, supports
our finding that air-related Pb exposure pathways contribute to blood
Pb levels in young children, by inhalation and ingestion. Furthermore,
the Administrator takes

note of the information that suggests that the air-to-blood ratio
(i.e., the quantitative relationship between air concentrations and
blood concentrations) is now likely larger, when air inhalation and
ingestion are considered, than that estimated when the standard was
set.
    Based on evidence discussed above, the Administrator first
considered the evidence in the context of an adaptation of the 1978
framework, as presented in the Staff Paper, recognizing that the health
effects evidence with regard to characterization of a threshold for
adverse effects has changed dramatically since the standard was set in
1978. As discussed above, however, the 1978 framework was premised on
an evidentiary basis that clearly identified an adverse health effect
and a health-based policy judgment that identified a level that would
be safe for an individual child with respect to this adverse health
effect. The adaptation to the 1978 framework applies this framework to
a situation where there is no longer an evidentiary basis to determine
a safe level for individual children. In addition, this approach does
not address explicitly what magnitude of effect should be considered
adverse. Given these two limitations, the Administrator has focused
primarily instead on the air-related IQ loss evidence-based framework
described above in considering the adequacy of the current standard.
    In considering the application the air-related IQ loss framework to
the current evidence as discussed above in section II.D.2.a, the
Administrator notes that this framework suggests an average air-related
IQ loss for a population of children exposed at the level of the
current standard on the order of 4 or more IQ points. The Administrator
judges that an air-related IQ loss of this magnitude is large from a
public health perspective and that this evidence-based framework
supports a conclusion that the current standard does not protect public
health with an adequate margin of safety. Further, the Administrator
believes that the current evidence indicates the need for a standard
level that is substantially lower than the current level to provide
increased public health protection, especially for at-risk groups,
including most notably children, against an array of effects, most
importantly including effects on the developing nervous system.
    The Administrator has also considered the results of the exposure
and risk assessments conducted for this review, which provides some
further perspective on the potential magnitude of air-related IQ loss.
However, taking into consideration the uncertainties and limitations in
the assessments, notably including questions as to whether the
assessment scenarios that roll up current air quality to simulate just
meeting the current standard are realistic in wide areas across the
U.S., the Administrator has not placed primary reliance on the exposure
and risk assessments. Nonetheless, the Administrator observes that in
areas projected to just meet the current standard, the quantitative
estimates of IQ loss associated with air-related Pb, as summarized
above in section II.D.2.b, indicate risk of a magnitude that in his
judgment is significant from a public health perspective. Further,
although the current monitoring data indicate few areas with airborne
Pb near or just exceeding the current standard, the Administrator
recognizes significant limitations with the current monitoring network
and thus the potential that the prevalence of such levels of Pb
concentrations may be underestimated by currently available data.
    The Administrator believes that the air-related blood Pb and IQ
loss estimates discussed in the Staff Paper and Risk Assessment Report,
summarized above, as well as the estimates of air-related IQ loss
suggested by this evidence-based framework, are important from a public
health perspective and are indicative of potential risks to susceptible
and vulnerable groups. In reaching this proposed judgment, the
Administrator considered the following factors: (1) The estimates of
blood Pb and IQ loss for children from air-related Pb exposures
associated with the current standard, (2) the estimates of numbers of
children with different amounts of increased Pb-related IQ loss
associated with the current standard, (3) the variability within and
among areas in both the exposure and risk estimates, (4) the
uncertainties in these estimates, and (5) the recognition that there is
a broader array of Pb-related adverse health outcomes for which risk
estimates could not be quantified and that the scope of the assessment
was limited to a sample of case studies and to some but not all at-risk
populations, leading to an incomplete estimation of public health
impacts associated with Pb exposures across the country.\122\ In
addition to the evidence-based and risk-based conclusions described
above, the Administrator also notes that it was the unanimous
conclusion of the CASAC Panel that EPA needed to ``substantially
lower'' the level of the primary Pb NAAQS to fully protect the health
of children and adult populations (Henderson, 2007a, 2007b, 2008).
---------------------------------------------------------------------------

    \122\ While recognizing that there are significant uncertainties
associated with the risk estimates from the case studies, EPA places
an appropriate weight on the risk assessment results for purposes of
evaluating the adequacy of the current standard, given the strength
of the evidence of the existence of effects at blood Pb levels
associated with exposures at the level of the current standard, the
magnitude of the IQ losses that are estimated, and the consistency
of these IQ losses with the estimates of IQ loss derived from the
alternative evidence-based framework. The weight to place on the
risk assessment results for purposes of evaluating alterative levels
of the standard is discussed later in the discussion on the level of
the standard.
---------------------------------------------------------------------------

    Based on all of these considerations, the Administrator proposes
that the current Pb standard is not requisite to protect public health
with an adequate margin of safety because it does not provide
sufficient protection, and that the standard should be revised to
provide increased public health protection, especially for members of
at-risk groups.

E. Conclusions on the Elements of the Standard

    The four elements of the standard--indicator, averaging time, form,
and level--serve to define the standard and must be considered
collectively in evaluating the health and welfare protection afforded
by the standard. In considering revisions to the current primary Pb
standard, as discussed in the following sections, EPA considers each of
the four elements of the standard as to how they might be revised to
provide a primary standard for Pb that is requisite to protect public
health with an adequate margin of safety. Considerations and proposed
conclusions on indicator are discussed in section II.E.1, and on
averaging time and form in section II.E.2. Considerations and proposed
conclusions on a level for a Pb NAAQS with a Pb-TSP indicator are
discussed in section II.E.3, and considerations on a level for a Pb
NAAQS with a Pb-PM10 indicator are discussed in section
II.E.4.
1. Indicator
    The indicator for the current standard is Pb-TSP (as described in
section II.D.1.a above).\123\ When the standard was set in 1978, the
Agency proposed Pb-TSP as the indicator, but considered identifying Pb
in particulate matter less than or equal to 10 [mu]m in diameter (Pb-
PM10) as the indicator. EPA had received comments expressing
concern

that because only a fraction of airborne particulate matter is
respirable, an air standard based on total air Pb would be
unnecessarily stringent. The Agency responded that while it agreed that
some Pb particles are too small or too large to be deposited in the
respiratory system, a significant component of exposures can be
ingestion of materials contaminated by deposition of Pb from the air.
In addition to the route of ingestion and absorption from the
gastrointestinal tract, nonrespirable Pb in the environment may, at
some point, become respirable through weathering or mechanical action.
EPA concluded that total airborne Pb, both respirable and nonrespirable
fractions, should be addressed by the air standard (43 FR 46251). The
federal reference method (FRM) for Pb-TSP specifies the use of the
high-volume FRM sampler for TSP.
---------------------------------------------------------------------------

    \123\ The current standard specifies the measurement of airborne
Pb with a high-volume TSP federal reference method (FRM) sampler
with atomic absorption spectrometry of a nitric acid extract from
the filter for Pb, or with an approved equivalent method.
---------------------------------------------------------------------------

    In the 1990 Staff Paper, this issue was reconsidered in light of
information regarding limitations of the high-volume sampler used for
the Pb-TSP measurements, and the continued use of Pb-TSP as the
indicator was recommended in the Staff Paper (USEPA, 1990):

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

    In the current review, the Staff Paper evaluated the evidence with
regard to the indicator for a revised primary standard. This evaluation
included consideration of the basis for using Pb-TSP as the current
indicator, information regarding the sampling methodology for the
current indicator, and CASAC advice with regard to indicator (described
below). Based on this evaluation, the Staff Paper recommended retaining
Pb-TSP as the indicator for the primary standard. The Staff Paper also
recommended activities intended to encourage collection and development
of datasets that will improve our understanding of national and site-
specific relationships between Pb-PM10 (collected by low-
volume sampler) and Pb-TSP to support a more informed consideration of
indicator during the next review. The Staff Paper suggested that such
activities might include describing a federal equivalence method (FEM)
in terms of PM10 and allowing its use for a TSP-based
standard in certain situations, such as where sufficient data are
available to adequately demonstrate a relationship between Pb-TSP and
Pb-PM10 or, in combination with more limited Pb-TSP
monitoring, in areas where Pb-TSP data indicate Pb levels well below
the NAAQS level.
    The ANPR further identified issues and options associated with
consideration of the potential use of Pb-PM10 data for
judging attainment or nonattainment with a Pb-TSP NAAQS. These issues
included the impact of controlling Pb-PM10 for sources
predominantly emitting Pb in particles larger than those captured by
PM10 monitors \124\ (i.e., ultra-coarse), \125\ and the