this potential disparity in outcomes. By contrast, with a rolling 3-
month averaging time, each month contributes to three separate 3-month
periods, through separate combinations with three different pairs of
months (e.g. January-March, February-April, and March-June), thus
providing a more complete consideration of air quality during that
month and the periods in which it falls. EPA also notes that analyses
of air quality data for 2005-2007 indicate a greater degree of
protection is afforded by a rolling 3-month average as compared to a
block calendar quarter average (Schmidt, 2008).
    CASAC also provided advice on a form for a monthly average
standard, noting that a ``monthly or `rolling' 30-day averaging time
with a `not to be exceeded' form would be more protective against
adverse short-term effects than a form (such as a `second-highest month
in three years') that periodically allows a month of exposures to much
higher concentrations'' (Henderson, 2008b). Public comments also
included recommendations for a not-to-be-exceeded maximum form for a
monthly average (e.g., NACAA), as well as some recommendations for a
second maximum monthly average (e.g., NESCAUM). While these comments
are instructive on the relative merits of a maximum and a second
maximum form for a monthly averaging time, given the Administrator's
selection of a 3-month averaging time (as described in section II.C.2.c
below), and his reasons for this selection, including his consideration
of the issue of short-term changes in ambient air concentrations over
the 3-month averaging time, EPA believes it is unnecessary to address
comments on the appropriate form for a monthly averaging time further
here.
    EPA notes, however, that a maximum rolling 3-month average would be
expected to provide greater protection from deposition-related pathways
in an area of highly variable air concentrations than the proposed
second maximum monthly average because the former does not allow for
the ``discounting'' or omitting of airborne Pb in any month. While the
averaging time for a maximum rolling 3-month average is longer than the
monthly averaging time recommended by CASAC and several commenters, the
combination of a rolling 3-month averaging time with a maximum form
would be expected to offer greater protection from deposition-related
exposure pathways than the proposed option of a second maximum monthly
average, because each month contributes to three 3-month averages and
no month is omitted from the calculation of averages for comparison to
the standard. Results of analyses of air quality data for 2005-2007 are
consistent with this view, in that a greater percentage of monitors
meeting data completeness criteria are not likely to meet the revised
standard based on a maximum rolling 3-month average as compared to a
second maximum monthly average (Schmidt, 2008).\71\
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    \71\ These analyses incorporate the revised averaging method
identified above and discussed more fully in section IV below.
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    More detailed responses to some of the public comments described
above, as well as responses to other comments related to averaging time
and form not considered here, are provided in the Response to Comments
document.
c. Conclusions on Averaging Time and Form
    Having carefully considered CASAC's advice and the public comments
on the appropriate averaging time and form for the standard, the
Administrator concludes that the fundamental scientific conclusions
pertaining to averaging time described in the Criteria Document and
Staff Paper, briefly summarized above in section II.C.2.a and discussed
more fully in section II.E.2 of the proposal remain valid. In light of
all of the evidence, the Administrator concludes that the appropriate
averaging time for the standard is no longer than a 3-month period.
    In considering the option of a monthly averaging time, the
Administrator recognizes the complexity inherent in considering the
averaging time and form for the primary Pb standard, which is greater
than in the case of the other criteria pollutants, due to the
multimedia nature of Pb and its multiple pathways of human exposure.
Accordingly, while the Administrator recognizes there are some factors
that might support a period as short as a month for the averaging time,
other factors support use of a longer averaging time, as discussed in
section II.C.2.b above. The Administrator believes that in the complex
multimedia, multi-pathway situation for Pb, it is necessary to consider
all of the relevant factors, both those pertaining to the human

physiological response to changes in Pb exposures and those pertaining
to the response of air-related Pb exposure pathways to changes in
airborne Pb, in an integrated manner.
    The Administrator recognizes that the evidence as well as the
results of the quantitative risk assessment for this review indicate a
greater role for ingestion pathways than inhalation pathways in
contributing to children's air-related exposure. He further recognizes
that ingestion pathways are influenced by more factors than inhalation
pathways, and those factors are considered likely to lessen the impact
of month-to-month variations in airborne Pb concentrations on levels of
air-related Pb in children's blood. Accordingly, while the evidence is
limited as to our ability to characterize these impacts, this evidence
suggests that the multiple factors affecting ingestion pathways, such
as ingestion of indoor dust, are likely to lead to response times
(e.g., for the response of blood to air Pb via these pathways)
extending longer than a month. In addition, there remains uncertainty
over the period of time needed for air Pb concentrations to lead to the
health effects most at issue in this review.
    Further, it is important to note, as discussed above, that a
rolling 3-month averaging time is likely to be somewhat more protective
from a broad national perspective than a calendar quarter averaging
time. Over a 3-year time frame, the rolling 3-month averaging time is
also likely to be more protective with regard to air-related Pb
exposures than would be a form that allows one month in three years to
be greater than the level of the standard (i.e., a monthly averaging
time with a second maximum form). In combination with the additional
changes in form discussed below, this means that a rolling 3-month
average can be expected to provide a high degree of control over all of
the months of a three-year period, with few individual months exceeding
the level of the standard. This expectation appears to be generally
supported by analyses of air quality data for 2005-2007 comparing
percentages of monitors not likely to meet a revised standard with
different averaging times and forms (Schmidt, 2008).
    The Administrator further notes that, as discussed in section
II.C.2.b above, the rolling three-month average eliminates the
possibility for two consecutive ``high'' months falling in two separate
calendar quarters to be considered independently (perhaps being
mitigated by ``low'' months falling in each of the same calendar
quarters). Rather, the same month, in the rolling three-month approach,
would contribute to three different 3-month periods through separate
combinations with three different pairs of months, thus providing a
more complete consideration of air quality during that month and the 3-
month periods in which it falls. Taking these considerations into
account, the Administrator concludes that a rolling 3-month averaging
time is appropriate. This conclusion to revise from a block calendar
quarter average to a rolling 3-month average is consistent with the
views of CASAC and some commenters on this issue.
    In recognition of the uncertainty in the information on which the
decision to select a 3-month averaging time is based, the Administrator
further concludes that the month-to-month variability allowed by the
current method by which the 3-month average metric is derived is not
sufficiently protective of public health. Accordingly, he concludes it
is appropriate to modify the method by which the 3-month average metric
is derived, as described in section IV below, to be the average of
three monthly average concentrations, as compared to the current
practice by which the average is derived across the full dataset for a
quarter, without equally weighting each month within the quarter. Thus,
in consideration of the uncertainty associated with the evidence
pertinent to averaging time discussed above, the Administrator notes
that the two changes in form for the standard (to a rolling 3-month
average and to providing equal weighting to each month in deriving the
3-month average) both afford greater weight to each individual month
than does the current form, tending to control both the likelihood that
any month will exceed the level of the standard and the magnitude of
any such exceedance.
    Based on the evidence and air quality considerations discussed
above, EPA concludes that a monthly averaging time is not warranted.
Furthermore, the Administrator concludes that the appropriate averaging
time and form for the revised primary Pb standard is a not-to-be-
exceeded (maximum) 3-month rolling average evaluated over a 3-year
span, derived in accordance with calculation methods described below in
section IV.
3. Level
    As noted in the proposal, EPA recognizes that in the case of Pb
there are several aspects to the body of epidemiological evidence that
add complexity to the selection of an appropriate level for the primary
standard. As summarized above and discussed in greater depth in the
Criteria Document (CD, sections 4.3 and 6.1.3), the epidemiological
evidence that associates Pb exposures with health effects generally
focuses on blood Pb for the dose metric.\72\ In addition, exposure to
Pb comes from various media, only some of which are air-related, and
through both inhalation and ingestion pathways. These complexities are
in contrast to the issues faced in the reviews for other air
pollutants, such as particulate matter and ozone, which involve only
inhalation exposures. Further, for the health effects receiving
greatest emphasis in this review (neurological effects, particularly
neurocognitive and neurobehavioral effects, in children), no threshold
levels can be discerned from the evidence. As was recognized at the
time of the last review, estimating a threshold for toxic effects of Pb
on the central nervous system entails a number of difficulties (CD, pp.
6-10 to 6-11). The task is made still more complex by support in the
evidence for a nonlinear rather than linear relationship between blood
Pb and neurocognitive decrement, with greater risk of decrement-
associated changes per [mu]g/dL of blood Pb at the lower levels of
blood Pb in the exposed population (CD, section 6.2.13). In this
context EPA notes that the health effects evidence most useful in
determining the appropriate level of the NAAQS is the large body of
epidemiological studies discussed in the Criteria Document. The
discussion in the proposal and below therefore focuses on the
epidemiological studies, recognizing and taking into consideration the
complexity and resulting uncertainty in using this body of evidence to
determine the appropriate level for the NAAQS.
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    \72\ Among the studies of Pb health effects, in which blood Pb
level is generally used as an index of exposure, the sources of
exposure vary and are inclusive of air-related sources of Pb such as
smelters (e.g., CD, chapter 6).
---------------------------------------------------------------------------

    The Administrator's proposed conclusions on range of levels for the
primary standard are summarized below in the Introduction (section
II.C.3.a), followed by consideration of comments received on the
proposal (section II.C.3.b) and the Administrator's final decision with
regard to level for the current primary standard (II.C.3.c).
a. Basis for Proposed Range
    For the reasons discussed in the proposal and summarized below, and
taking into account information and assessments presented in the
Criteria Document, Staff Paper, and ANPR, the advice and
recommendations of CASAC, and the public comments received prior to
proposal, the

Administrator proposed to revise the existing primary Pb standard.
Specifically, the Administrator proposed to revise the level of the
primary Pb standard, defined in terms of the current Pb-TSP indicator,
to within the range of 0.10 to 0.30 [mu]g/m \3\, conditional on
judgments as to the appropriate values of key parameters to use in the
context of the air-related IQ loss evidence-based framework summarized
below (and discussed in section II.E.3.a.ii of the proposal). Further,
in recognition of alternative views of the science, the exposure and
risk assessments, the uncertainties inherent in the science and these
assessments, and the appropriate public health policy responses based
on the currently available information, the Administrator solicited
comments on alternative levels of a primary Pb-TSP standard within
ranges from above 0.30 [mu]g/m \3\ up to 0.50 [mu]g/m \3\ and below
0.10 [mu]g/m \3\. In addition, the Administrator solicited comments on
when, if ever, it would be appropriate to set a NAAQS for Pb at a level
of zero.
    The Administrator's consideration of alternative levels of the
primary Pb-TSP standard built on his proposed conclusion, discussed
above in section II.B.1, that the overall body of evidence indicates
that the current Pb standard is not requisite to protect public health
with an adequate margin of safety and that the standard should be
revised to provide increased public health protection, especially for
members of at-risk groups, notably including children, against an array
of adverse health effects. These effects include IQ loss, decrements in
other neurocognitive functions, other neurological effects and immune
system effects, as well as cardiovascular and renal effects in adults,
with IQ loss the health outcome quantified in the risk assessment. In
reaching a proposed decision about the level of the Pb primary
standard, the Administrator considered: The evidence-based
considerations from the Criteria Document, Staff Paper, and ANPR, and
those based on the air-related IQ loss evidence-based framework
discussed in the proposal; the results of the exposure and risk
assessments summarized in section II.A.3 above and in the Staff Paper,
giving weight to the exposure and risk assessments as judged
appropriate; CASAC advice and recommendations, as reflected in
discussions of the Criteria Document, Staff Paper, and ANPR at public
meetings, in separate written comments, and in CASAC's letters to the
Administrator; EPA staff recommendations; and public comments received
during the development of these documents, either in connection with
CASAC meetings or separately. In considering what standard is requisite
to protect public health with an adequate margin of safety, the
Administrator noted at the time of proposal that he was mindful that
this choice requires judgment based on an interpretation of the
evidence and other information that neither overstates nor understates
the strength and limitations of the evidence and information nor the
appropriate inferences to be drawn.
    In reaching a proposed decision on a range of levels for a revised
standard, as in reaching a proposed decision on the adequacy of the
current standard, the Administrator primarily considered the evidence
in the context of the air-related IQ loss evidence-based framework as
described in the proposal (section II.E.3.a.ii). The air-related IQ
loss evidence-based framework considered by the Administrator in the
proposal focuses on the contribution of air-related Pb to the
neurocognitive effect of IQ loss in children, with a public health goal
of identifying the appropriate ambient air level of Pb to protect
exposed children from health effects that are considered adverse, and
are associated with their exposure to air-related Pb. In this air-
related IQ loss evidence-based framework, the Agency drew from the
entire body of evidence as a basis for concluding that there are causal
associations between air-related Pb exposures and IQ loss in children.
Building on recommendations from CASAC to consider the body of evidence
in a more quantitative manner, the framework additionally draws more
quantitatively from the evidence by combining air-to-blood ratios with
evidence-based C-R functions from the epidemiological studies to
quantify the association between air Pb concentrations and air-related
population mean IQ loss in exposed children. This framework was also
premised on a public health goal of selecting a proposed standard level
that would prevent air-related IQ loss (and related effects) of a
magnitude judged by the Administrator to be of concern in populations
of children exposed to the level of the standard. The framework
explicitly links a public health goal regarding IQ loss with two key
parameters--a C-R function for population IQ response associated with
blood Pb level and an air-to-blood ratio.
    As a general matter, in considering this evidence-based framework,
the Administrator recognized that in the case of Pb there are several
aspects to the body of epidemiological evidence that add complexity to
the selection of an appropriate level for the primary standard. As
discussed above, these complexities include evidence based on blood Pb
as the dose metric, multimedia exposure pathways for both air-related
and nonair-related Pb, and the absence of any discernible threshold
levels in the health effects evidence. Further, the Administrator
recognized that there are a number of important uncertainties and
limitations inherent in the available health effects evidence and
related information, including uncertainties in the evidence of
associations between total blood Pb and neurocognitive effects in
children, especially at the lowest blood Pb levels evaluated in such
studies, as well as uncertainties in key parameters used in the
evidence-based framework, including C-R functions and air-to-blood
ratios. In addition, the Administrator recognized that there are
currently no commonly accepted guidelines or criteria within the public
health community that would provide a clear basis for reaching a
judgment as to the appropriate degree of public health protection that
should be afforded to neurocognitive effects in sensitive populations,
such as IQ loss in children.
    Based on the discussion of the key parameters used in the
framework, as discussed in the proposal, the Administrator concluded
that, in considering alternative standard levels below the level of the
current standard, it was appropriate to take into account two sets of
C-R functions (described in section II.E.3.a.ii of the proposal),
recognizing uncertainties in the related evidence. In the proposal, the
first set of C-R functions was described as reflecting the evidence
indicative of steeper slopes in relationships between blood Pb and IQ
in children, and the second set of C-R functions as reflecting
relationships with shallower slopes between blood Pb and IQ in
children.\73\ In addition, the Administrator concluded that it was
appropriate to consider various air-to-blood ratios within a range of
values considered to be generally supported by the available evidence,
again recognizing the uncertainties in the relevant evidence.\74\
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    \73\ As described in section II.E.3.a.ii of the proposal, the
first set focused on C-R functions from analyses involving
population mean concurrent blood Pb levels of approximately 3 [mu]g/
dL (closer to current mean blood Pb levels in U.S. children). The
second set (CD, pp. 8-78 to 8-80) considered functions descriptive
of the C-R relationship from a larger set of studies that include
population mean blood Pb levels ranging from a mean of 3.3 up to a
median of 9.7 [mu]g/dL (see Table 1).
    \74\ In considering alternative levels for the standard within
the air-related IQ loss framework, the Agency focused on estimates
using an air-to-blood ratio of 1:5 and also provided IQ loss
estimates using higher and lower estimates (i.e., 1:3 and 1:7).

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

    With regard to making a public health policy judgment as to the
appropriate level of protection against air-related IQ loss and related
effects, the Administrator first noted that ideally air-related (as
well as other) exposures to environmental Pb would be reduced to the
point that no IQ impact in children would occur. The Administrator
recognized, however, that in the case of setting a NAAQS, he is
required to make a judgment as to what degree of protection is
requisite to protect public health with an adequate margin of safety.
The NAAQS must be sufficient but not more stringent than necessary to
achieve that result, and does not require a zero-risk standard.
Considering the advice of CASAC and public comments on this issue,
notably including the comments of the American Academy of Pediatrics
(AAP, 2008), the Administrator proposed to conclude that an air-related
population mean IQ loss within the range of 1 to 2 points could be
significant from a public health perspective, and that a standard level
should be selected to provide protection from air-related population
mean IQ loss in excess of this range.
    In reaching his proposed decision, the Administrator considered the
application of this air-related IQ loss framework with this target
degree of protection in mind, drawing from the information presented in
Table 7 of the proposal (section II.E.3.a.ii) which addresses a broad
range of standard levels. In so doing, the Administrator considered
estimates associated with both sets of C-R functions and the range of
air-to-blood ratios identified in the proposal, and noted those that
would limit the estimated degree of impact on population mean IQ loss
from air-related Pb to the proposed range of protection.
    Taking these considerations into account, and based on the full
range of information presented in Table 7 of the proposal on estimates
of air-related IQ loss in children over a broad range of alternative
standard levels, the Administrator concluded that it was appropriate to
propose a range of standard levels, and that a range of levels from
0.10 to 0.30 [mu]g/m3 would be consistent with the target
for protection from air-related IQ loss in children identified in the
proposal. In recognition of the uncertainties in the key parameters of
air-to-blood ratio and C-R functions, the Administrator stated that the
selection of a standard level from within this range was conditional on
judgments as to the most appropriate parameter values to use in the
context of this evidence-based framework. He noted that placing more
weight on the use of a C-R function with a relatively steeper slope
would tend to support a standard level in the lower part of the
proposed range, while placing more weight on a C-R function with a
shallower slope would tend to support a level in the upper part of the
proposed range. Similarly, placing more weight on a higher air-to-blood
ratio would tend to support a standard level in the lower part of the
proposed range, whereas placing more weight on a lower ratio would tend
to support a level in the upper part of the range. In soliciting
comment on a standard level within this proposed range, the
Administrator specifically solicited comment on the appropriate values
to use for these key parameters in the context of this evidence-based
framework.
    The Administrator also considered the results of the exposure and
risk assessments conducted for this review to provide some further
perspective on the potential magnitude of air-related IQ loss.\75\ The
Administrator found these quantitative assessments to provide a useful
perspective on the risk from air-related Pb. However, in light of the
important uncertainties and limitations associated with these
assessments, as discussed in sections II.A.3 above and section II.E.3.b
of the proposal, for purposes of evaluating potential new standards,
the Administrator placed less weight on the risk estimates than on the
evidence-based assessments. Nonetheless, the Administrator found the
risk estimates to be roughly consistent with and generally supportive
of the evidence-based air-related IQ loss estimates discussed in
section II.E.3.b of the proposal, lending support to the proposed range
based on this evidence-based framework.
---------------------------------------------------------------------------

    \75\ In considering the risk estimates in light of IQ loss
estimates based on the air-related IQ loss evidence-based framework
in the proposal, the Agency focused on risk estimates for the
general urban and primary Pb smelter subarea case studies as these
case studies generally represent population exposures for more
highly air-pathway exposed children residing in small neighborhoods
or localized residential areas with air concentrations nearer the
standard level being evaluated, as compared to, the location-
specific case studies in which populations have a broader range of
air-related exposures including many well below the standard level
being evaluated.
---------------------------------------------------------------------------

    In the proposal, the Administrator noted his view that the above
considerations, taken together, provided no evidence- or risk-based
bright line that indicates a single appropriate level. Instead, he
noted, there is a collection of scientific evidence and judgments and
other information, including information about the uncertainties
inherent in many relevant factors, which needs to be considered
together in making this public health policy judgment and in selecting
a standard level from a range of reasonable values. Based on
consideration of the entire body of evidence and information available
at the time of proposal, as well as the recommendations of CASAC and
public comments, the Administrator proposed that a standard level
within the range of 0.10 to 0.30 [mu]g/m3 would be requisite
to protect public health, including the health of sensitive groups,
with an adequate margin of safety. He also recognized that selection of
a level from within this range was conditional on judgments as to what
C-R function and what air-to-blood ratio are most appropriate to use
within the context of the air-related IQ loss framework. The
Administrator noted that this proposed range encompasses the specific
level of 0.20 [mu]g/m3, the upper end of the range
recommended by CASAC and by many public commenters on the ANPR. The
Administrator provisionally concluded that a standard level selected
from within this range would reduce the risk of a variety of health
effects associated with exposure to Pb, including effects indicated in
the epidemiological studies at low blood Pb levels, particularly
including neurological effects in children, and cardiovascular and
renal effects in adults.
    The proposal noted that there is no bright line clearly directing
the choice of level within this reasonable range, and therefore the
choice of what is appropriate, considering the strengths and
limitations of the evidence, and the appropriate inferences to be drawn
from the evidence and the exposure and risk assessments, is a public
health policy judgment. To further inform this judgment, the
Administrator solicited comment on the air-related IQ loss evidence-
based framework considered by the Agency and on appropriate parameter
values to be considered in the application of this framework. More
specifically, we solicited comment on the appropriate C-R function and
air-to-blood ratio to be used in the context of the air-related IQ loss
framework. The Administrator also solicited comment on the degree of
impact of air-related Pb on IQ loss and other related neurocognitive
effects in children considered to be significant from a public health
perspective, and on the use of this framework as a basis for selecting
a standard level.
    The Administrator further noted that the evidence-based framework,
with the inputs illustrated at the time of

proposal, indicated that for standard levels above 0.30 [mu]g/
m3 up to 0.50 [mu]g/m3, the estimated degree of
impact on population mean IQ loss from air-related Pb would range from
approximately 2 points to 5 points or more with the use of the first
set of C-R functions and the full range of air-to-blood ratios
considered, and would extend from somewhere within the proposed range
of 1 to 2 points IQ loss to above that range when using the second set
of C-R functions and the full range of air-to-blood ratios considered.
The Administrator proposed to conclude in light of his consideration of
the evidence in the framework discussed above that the magnitude of
air-related Pb effects at the higher blood Pb levels that would be
allowed by standards above 0.30 up to 0.50 [mu]g/m3 would be
greater than what is requisite to protect public health with an
adequate margin of safety.
    In addition, the Administrator noted that for standard levels below
0.10 [mu]g/m3, the estimated degree of impact on population
mean IQ loss from air-related Pb would generally be somewhat to well
below the proposed range of 1 to 2 points air-related population mean
IQ loss regardless of which set of C-R functions or which air-to-blood
ratio within the range of ratios considered are used. The Administrator
proposed to conclude that the degree of public health protection that
standards below 0.10 [mu]g/m3 would likely afford would be
greater than what is requisite to protect public health with an
adequate margin of safety.
    Having reached these proposed decisions based on the interpretation
of the evidence, the evidence-based frameworks, the exposure/risk
assessment, and the public health policy judgments described above, the
Administrator recognized that other interpretations, frameworks,
assessments, and judgments are possible. There are also potential
alternative views as to the range of values for relevant parameters
(e.g., C-R function, air-to-blood ratio) in the evidence-based
framework that might be considered supportable and the relative weight
that might appropriately be placed on any specific value for these
parameters within such ranges. In addition, the Administrator
recognized that there may be other views as to the appropriate degree
of public health protection that should be afforded in terms of air-
related population mean IQ loss in children that would provide support
for alternative standard levels different from the proposed range.
Further, there may be other views as to the appropriate weight and
interpretation to give to the exposure/risk assessment conducted for
this review. Consistent with the goal of soliciting comment on a wide
array of issues, the Administrator solicited comment on these and other
issues.
    In the proposal, the Administrator also recognized that Pb can be
considered a non-threshold pollutant \76\ and that, as discussed in
section I.B above, the CAA does not require that NAAQS be established
at a zero-risk level, but rather at a level that reduces risk
sufficiently so as to protect public health with an adequate margin of
safety. However, expecting that, as time goes on, future scientific
studies will continue to enhance our understanding of Pb, and that such
studies might lead to a situation where there is very little if any
remaining uncertainty about human health impacts from even extremely
low levels of Pb in the ambient air, the Administrator recognized that
there is the potential in the future for fundamental questions to arise
as to how the Agency could continue to reconcile such evidence with the
statutory provision calling for the NAAQS to be set at a level that is
requisite to protect public health with an adequate margin of safety.
In light of such considerations, EPA solicited comment on when, if
ever, it would be appropriate to set a NAAQS for Pb at a level of zero.
---------------------------------------------------------------------------

    \76\ Similarly, in the most recent reviews of the NAAQS for
ozone and PM, EPA recognized that the available epidemiological
evidence neither supports nor refutes the existence of thresholds at
the population level, while noting uncertainties and limitations in
studies that make discerning thresholds in populations difficult
(e.g., 73 FR 16444, March 27, 2008; 71 FR 61158, October 17, 2006).
---------------------------------------------------------------------------

b. Comments on Level
    In this section we discuss advice and recommendations received from
CASAC and the public on the proposed range of levels for the primary Pb
standard with a Pb-TSP indicator,\77\ including comments on specific
levels and ranges appropriate for the standard, comments pertaining to
the use of the evidence-based framework and inputs to the framework,
and comments related to the risk assessment. More detailed responses to
some of the public comments on level described below, as well as
responses to other comments related to level not discussed here, are
provided in the Response to Comments document.
---------------------------------------------------------------------------

    \77\ Some commenters provided recommendations with regard to a
level for a Pb-PM10-based standard. While these comments
are instructive on that issue, the Administrator has decided to
retain the current indicator of Pb-TSP, and therefore they do not
need to be addressed here.
---------------------------------------------------------------------------

(i) General Comments on Range of Levels
    In considering comments received on the proposal related to the
standard level, EPA first notes the general advice provided by CASAC
concerning the proposal in a July 2008 letter to the Administrator
(Henderson, 2008b). In that letter, CASAC emphasized their unanimous
recommendation (initially stated in their March 2007 letter) regarding
``the need to substantially lower the level'' of the primary Pb
standard such that the upper bound should be ``no higher than 0.2
[mu]g/m\3\'' (emphasis in originals).
    The vast majority of public comments that addressed a level for the
standard recommended standard levels below, or no higher than 0.2
[mu]g/m\3\. Many of these commenters noted the advice of CASAC and
recommended that EPA follow this advice. Specific rationales provided
by this large group of commenters included various considerations, such
as recognition that the current evidence indicates Pb effects at much
lower exposure levels than when the current standard was set and in
multiple systems (e.g., neurological effects in children,
cardiovascular and renal effects in adults), and does not indicate a
threshold; impacts associated with some neurological effects can
persist into adulthood; and there is now evidence of a greater air-to-
blood ratio than was considered when the standard was set. Many of
these commenters recommended a specific level or range of levels for
the standard that was equal to or below 0.2 [mu]g/m\3\. In recommending
levels below 0.2 [mu]g/m\3\, some of these stated that CASAC's
recommendation for an upper bound of 0.2 [mu]g/m\3\ should not be read
to imply that CASAC supported a standard level of 0.2 [mu]g/m\3\ if
that level did not account for CASAC's other specific recommendations
on the framework and its inputs. Some commenters' specific
recommendations for level (including a standard level of 0.15 [mu]g/
m\3\) were based on consideration of the air-related IQ loss evidence-
based framework and their application of it using their recommended
parameter inputs and public health policy goal. The specific
recommendations on application of the framework are discussed
separately below. Some commenters (including EPA's Children's Health
Protection Advisory Committee, NESCAUM, several States and Tribes, and
several environmental or public health organizations) specified levels
below 0.2 [mu]g/m\3\ as necessary to protect public health with an
adequate margin of safety, with some of these additionally

stating that in assuring this level of protection, EPA must take into
account susceptible or vulnerable subgroups. In discussing these
subgroups, some commenters noted factors such as nutritional
deficiencies as contributing to susceptibility and identified minority
and low-income children as a sensitive subpopulation for Pb exposures.
Some of these commenters recommended much lower levels, such as 0.02
[mu]g/m\3\, based on their views as to the level needed to protect
public health with an adequate margin of safety in light of their
interpretation of the advice of CASAC and EPA Staff and the evidence,
including the lack of identifiable threshold. Some of these commenters
recommending much lower levels expressed the view that the standard
should be as protective as possible.
    A second, much smaller, group of comments (including some industry
comments and some state agency comments), recommended levels for the
standard that are higher than 0.2 [mu]g/m\3\. Among this group, some
commenters provide little or no health-based rationale for their
comment. Other commenters, in recommending various levels above 0.2
[mu]g/m\3\, generally state that there is no benefit to be gained by
setting a lower level for the standard. In support of this general
conclusion, the commenters variously stated that there is substantial
uncertainty associated with the slope of the blood Pb-IQ loss
concentration-response function at lower blood Pb levels, such that EPA
should not rely on estimates that indicate a steeper slope at lower
blood Pb levels; that the risk assessment results for total risk at
alternative standard levels indicate no benefit to be achieved from a
standard level below 0.5 [mu]g/m\3\; that levels derived from the
evidence-based framework need upward adjustment for use with an
averaging time less than a year and that IQ loss estimates derived from
the evidence-based framework presented in the proposal for levels from
0.10 to 0.50 [mu]g/m\3\ do not differ much (e.g., from 2 to 4.1 points
IQ loss [steeper slopes] and from 1.1 to 2.2 points IQ loss [shallower
slope] for the two sets of C-R functions).
    For the range of reasons summarized in section II.C.3.a above, and
the reasons described more fully in section II.C.3.c below, EPA does
not believe that a level for the standard above 0.2 [mu]g/m\3\ would
protect public health with an adequate margin of safety. Rather, EPA
concludes that such a level for the standard would not be protective of
public health with an adequate margin of safety. Further, EPA disagrees
with the industry comment that levels identified using the evidence-
based framework should be adjusted upward; this and other specific
aspects of comments summarized above are discussed further in the
Response to Comments document.
(ii) Use of Air-related IQ Loss Evidence-based Framework
    As noted above, EPA received advice and recommendations from CASAC
and comments from the public with regard to application of the air-
related IQ loss evidence-based framework in the selection of a level
for the primary standard. In the discussion that follows, we first
describe CASAC advice and public comments on the appropriate degree of
public health protection that should be afforded to at-risk populations
in terms of IQ loss in children as estimated by this framework, We then
describe CASAC advice and public comments on the specific parameters of
C-R function and air-to-blood ratio.
    In their July 2008 advice to the Agency on the proposal notice,
CASAC characterized the target degree of protection proposed for use
with the air-related IQ loss framework to be inadequate (Henderson,
2008a). As basis for this characterization, they repeat the advice they
conveyed with their March 2007 letter, that they considered that ``a
population loss of 1-2 IQ points is highly significant from a public
health perspective'' and that ``the primary lead standard should be set
so as to protect 99.5% of the population from exceeding that IQ loss''
(emphasis in original). They further emphasized their view that an IQ
loss of 1-2 points should be ``prevented in all but a small percentile
of the population--and certainly not accepted as a reasonable change in
mean IQ scores across the entire population'' (emphasis in original).
    Recommendations from several commenters, including the American
Academy of Pediatrics, and state health agencies that commented on this
issue, are in general agreement with the view emphasized by CASAC that
air-related IQ loss of a specific magnitude, such as on the order of 1
or 2 points, should be prevented in a very high percentage (e.g.,
99.5%) of the population.
    EPA generally agrees with CASAC and the commenters that emphasize
that the NAAQS should prevent air-related IQ loss of a significant
magnitude in all but a small percentile of the population. However, it
is important to note that in selecting a target degree of public health
protection from air-related IQ loss in children for the purposes of
this review, EPA is addressing this issue more specifically in the
context of this evidence-based framework. In so doing, EPA is not
determining a specific quantitative public health policy goal in terms
of an air-related IQ loss that is acceptable or unacceptable in the
U.S. population per se, but instead is determining what magnitude of
estimated air-related IQ loss should be used in conjunction with the
specific air-related IQ loss evidence-based framework being applied in
this review, recognizing the uncertainties and limitations in this
framework. As discussed later, the estimated air-related IQ loss
resulting from the application of this evidence-based framework should
not be viewed as a bright line estimate of expected IQ loss in the
population that would or would not occur. Nonetheless, these results
provide a useful guide for the Administrator to use in making the
basically qualitative public health policy judgment about the risk to
public health that could reasonably be expected to result from exposure
to the ambient air quality patterns that would be allowed by varying
levels of the standard, in light of the averaging time, form, and
indicator specified above.
    In that context, it is important to recognize that the air-related
IQ loss framework provides estimates for the mean of a subset of the
population. It is an estimate for a subset of children that are assumed
to be exposed to the level of the standard. The framework in effect
focuses on the sensitive subpopulation that is the group of children
living near sources and more likely to be exposed at the level of the
standard. The evidence-based framework estimates a mean air-related IQ
loss for this subpopulation of children; it does not estimate a mean
for all U.S. children.
    EPA is unable to quantify the percentile of the U.S. population of
children that corresponds to the mean of this sensitive subpopulation.
Nor is EPA confident in its ability to develop quantified estimates of
air-related IQ loss for higher percentiles than the mean of this
subpopulation. EPA expects that the mean of this subpopulation
represents a high, but not quantifiable, percentile of the U.S.
population of children. As a result, EPA expects that a standard based
on consideration of this framework would provide the same or greater
protection from estimated air-related IQ loss for a high, albeit
unquantifiable, percentage of the entire population of U.S. children.
    One industry association commenter noted agreement with EPA's focus
on population mean (or median) for the framework, and the statement of
greater confidence in estimates for air-related (as contrasted with
total Pb-related) IQ loss at a central point in the distribution

than at an upper percentile. This commenter also stated the view that
there is likely little difference in air-related IQ loss between the
mean and the upper percentiles of the exposed population, based on
their interpretation of EPA risk estimates for the location-specific
urban case studies. While EPA disagrees with the commenter's view and
interpretation of the risk estimates from these case studies (as seen
by differences in median and 95th percentile estimates presented in
section 5.3.2 of the Risk Assessment Report), EPA agrees that there is
a much higher level of confidence in estimates of air-related IQ loss
for the mean as compared to that for an upper percentile, consistent
with the Agency's recognition of such limitations in the blood Pb
estimates from the risk assessment, due to limitations in the available
data (as noted in section II.C.h of the proposal).
(iii) Air-to-Blood Ratio
    Regarding the air-to-blood ratio, CASAC, in their July 2008 advice
to the Agency on the proposal, objected to constraining the range of
ratios used with the framework to the range from 1:3 to 1:7 (Henderson,
2008a). In so doing, they noted that the Staff Paper concluded that
while ``there is uncertainty and variability in the absolute value of
an air-to-blood relationship, the current evidence indicates a notably
greater ratio [than the value of 1:2 used in 1978] * * * e.g., on the
order of 1:3 to 1:10'' (USEPA, 2007, p. 5-17). With regard to the range
of 1:3 to 1:7 emphasized in the proposal, CASAC stated that the lower
end of the range (1:3) ``reflects the much higher air and blood levels
encountered decades ago'' while ``the upper end of the range (1:7)
fails to account for the higher ratios expected at lower current and
future air and blood Pb levels, especially when multiple air-related
lead exposure pathways are considered.'' With particular recognition of
the analysis of declining blood Pb levels documented by NHANES that
reflected declines in air Pb levels associated with declining use of
leaded gasoline over the same period and from which CASAC notes a ratio
on the order of 1:10 (Schwartz and Pitcher, 1989, as cited in
Henderson, 2007a), CASAC recommended that EPA consider an air-to-blood
ratio ``closer to 1:9 to 1:10 as being most reflective of current
conditions'' (Henderson, 2008b).
    Similar to the advice from CASAC, many commenters, including EPA's
Children's Health Protection Advisory Committee, NESCAUM and Michigan
Department of Environmental Quality recommended that EPA consider
ratios higher than the upper end of the range used in the proposal
(1:7), such as values on the order of 1:9 or 1:10 or somewhat higher
and rejected the lower ratios used in the proposal as being
inappropriate for application to today's children. In support of this
recommendation, commenters cite ratios resulting from the study noted
by CASAC (Schwartz and Pitcher, 1989), as well as others by Hayes et
al. (1994) and Brunekreef et al. (1983), and also air-to-blood ratio
estimates from the exposure/risk assessment.
    EPA agrees with CASAC and these commenters that an upper end air-
to-blood ratio of 1:7 does not give appropriate weight to the air-to-
blood ratios derived from or reported by the studies by Schwartz and
Pitcher (1989) and Brunekreef et al. (1983) \78\ and on ratios derived
from the risk assessment results, which extend higher than the range
identified in the proposal for consideration with the framework.
Accordingly, EPA agrees that the range of air-to-blood estimates
appropriate for consideration in using the air-related IQ loss
evidence-based framework should extend up to ratios greater than the
1:7 ratio presented as an upper end in the proposal, such that the
evidence-based framework should also consider values on the order of
1:10.
---------------------------------------------------------------------------

    \78\ EPA agrees that the study by Hayes et al. (1994), cited by
CASAC and commenters, presents an air-to-blood ratio greater 1:10,
but notes that we are not relying on this study in our decision as
it has not been reviewed as part of the Criteria Document or Staff
Paper (as described in Section I.C).
---------------------------------------------------------------------------

    Alternatively, two industry commenters supported the range
presented in the proposal of 1:3 to 1:7.\79\ These two and another
industry commenter asserted that higher air-to-blood ratios are not
supported by the evidence. Specifically, one commenter disagrees with
CASAC's interpretation of the Schwartz and Pitcher (1989) study with
regard to air-to-blood ratio, stating that the study indicates a
potential ratio of 1:7.8, rather than 1:9 or 1:10 as stated by CASAC,
and that there is a weak association between air Pb associated with
leaded gasoline usage and blood Pb, making the Schwartz and Pitcher
study inappropriate to consider. EPA considers both the CASAC approach
and the alternate approach presented by the commenter to generally
represent conceptually sound strategies for translating the
relationship between gasoline usage and blood Pb (provided in the
Schwartz and Pitcher, 1989 study) to air-to-blood Pb ratios. In
addition, EPA notes that these approaches support both the commenters
ratio of approximately 1:8 and the CASAC recommendation for EPA to use
an estimate ``closer to 1:9 to 1:10''. Further, EPA disagrees with the
commenter's view that the association between gasoline-related air Pb
and blood Pb is weak. On the contrary, the body of evidence regarding
this relationship is robust (e.g., USEPA, 1986a, sections 11.3.6 and
11.6). As stated in the 1986 Criteria Document, ``there is strong
evidence that changes in gasoline lead produce large changes in blood
lead'' (USEPA, 1986a, p. 11-187). Further, EPA notes that the analysis
by Hayes et al. (1994), cited by the commenter as basis for their view
regarding leaded gasoline, recognizes the role of leaded gasoline
combustion in affecting blood Pb levels through pathways other than the
inhalation pathway (e.g., via dust, soil and food pathways).\80\
---------------------------------------------------------------------------

    \79\ A ratio of 1:5 was recommended by one of these commenters
(Doe Run Resources Corp.).
    \80\ See previous footnote regarding Hayes et al. (1994).
---------------------------------------------------------------------------

    Additionally, two commenters stated that the ``higher ratios'' have
been generated inappropriately, citing ratios reported by Brunekreef
(1984) or those derived from NHANES data (e.g., Schwartz and Pitcher,
1989 or Hayes et al., 1994) as inappropriately including blood Pb not
associated with air Pb concentrations in the derivation of the air-to-
blood ratio. Last, two of the three industry commenters suggested that
some of the air-to-blood ratios derived from the risk assessment are
overstated as a result of the methodology employed.
    EPA generally disagrees with these commenters' assertions that
nonair sources of blood Pb are a source of bias in studies indicating
ratios above 1:7 that were identified in the proposal, and emphasized
by CASAC and by other commenters, as described above. For example, in
section II.B.1.c of the proposal, the proposal noted ratios of 1:8.5
(Brunekreef et al., 1983; Brunekreef, 1984), as well as a ratio of
approximately 1:10 (presented by CASAC in consideration of Schwartz and
Pitcher, 1989). In reporting these ratios, authors of these studies
described how consideration was given or what adjustments were made for
other sources of blood Pb, providing strength to their conclusion that
the reported air-to-blood ratio reflects air Pb contributions, with
little contribution from nonair sources. In addition, the study by
Hilts (2003) includes an analysis that provides control for potential
confounders, including

alternate sources of Pb exposure, through study design (i.e., by
following a similar group of children located within the same study
area over a period of time). As discussed in section II.A.2.a above,
the study authors report a ratio of 1:6 from this study and additional
analysis of the data by EPA for the initial time period of the study
resulted in a ratio of 1:7.
    With regard to air-to-blood ratios derived from the risk
assessment, while EPA recognizes uncertainties in these estimates,
particularly those extending substantially above 1:10 (as described in
the Risk Assessment Report and section II.C of the proposal), EPA
disagrees with commenters' conclusions that they do not provide support
for estimates on the order of 1:10.
    In summary, while EPA agrees with the industry commenters that a
ratio of 1:5 or 1:7.8 is supportable for use in the evidence-based
framework, as noted above, EPA interprets the current evidence as
providing support for use of a higher range than that described in the
proposal that is inclusive at the upper end of estimates on the order
of 1:10 and at the lower end on the order of 1:5. Further, EPA agrees
with CASAC that the lower end of the range in the proposal, an air-to-
blood ratio of 1:3, is not supported by the evidence for application to
the current population of U.S. children, in light of the multiple air-
related exposure pathways by which children are exposed, in addition to
inhalation of ambient air, and of today's much lower air and blood Pb
levels. Taking these factors into consideration, we conclude that the
air-related IQ loss evidence-based framework should consider air-to-
blood ratios of 1:10 at the upper end and 1:5 at the lower end.
(iv) Concentration--Response Functions
    Regarding the appropriate C-R functions to consider with the
evidence-based framework, CASAC, in their July 2008 advice to the
Agency on the proposal notice (Henderson, 2008a), objected to EPA's
consideration of C-R functions based on analyses of populations
``exhibiting much higher blood Pb levels than is appropriate for
current U.S. populations'' (emphasis in original). They note that the
second set of C-R functions, while including some drawn from analyses
of U.S. children with mean blood Pb levels below 4 [mu]g/dL, also
includes studies with mean or median blood Pb levels ranging up to 9.7
[mu]g/dL. Further, they emphasize that we are concerned ``with current
blood Pb levels in the setting of a health-protective NAAQS, not with
blood Pb levels of the past'' (emphasis in original). In conclusion,
they state that ``the selection of C-R function should be based on
determining which studies indicate slopes that best reflect the
current, lower blood Pb levels for children in the U.S.--which, in this
instance, are those studies from which steeper slopes are drawn''
(emphasis in original) (Henderson, 2008a).
    A number of commenters (including EPA's Children's Health
Protection Advisory Committee, NESCAUM and some state agencies) made
recommendations with regard to C-R functions that were similar to those
of CASAC. These commenters recommended consideration of C-R functions
with slopes appreciably steeper than the median value representing the
second set of functions in the proposal, giving greater weight to
steeper slopes drawn from analyses involving children with lower blood
Pb levels, closer to those of children in the U.S. today. Some of these
commenters (e.g., NESCAUM) additionally suggested alternate approaches
to identify a slope estimate relevant to today's blood Pb levels,
considering lower blood Pb level studies across both sets of functions
presented in the proposal, and to avoid placing inappropriate weight on
a single highest value.
    Based on the evidence described in detail in the Criteria Document
and briefly summarized in section II.A.2.c above, EPA agrees with CASAC
and these commenters that, given the nonlinearity of the blood Pb-IQ
loss relationship (steeper slope at lower blood Pb levels), the C-R
functions appropriate to use with the air-related IQ loss framework are
those drawn from analyses of children with blood Pb levels closest to
those of children in the U.S. today. As a result of this nonlinear
relationship, a given increase in blood lead levels (e.g., 1 [mu]g/dl
of Pb) is expected to cause a greater incremental increase in adverse
neurocognitive effects for a population of children with lower blood Pb
levels than would be expected to occur in a population of children with
higher blood Pb levels. Thus, estimates of C-R functions drawn from
analyses of children with blood Pb levels that are more comparable to
blood Pb levels in today's U.S. children are likely to better represent
the relationship between health effects and blood Pb levels that would
apply for children in the U.S. now and in the future, as compared to
estimates derived from analyses of children with higher blood lead
levels. As discussed in section II.A.2.a.ii above, blood Pb levels in
U.S. children have declined dramatically over the past thirty years.
The geometric mean blood Pb level for U.S. children aged five years and
below, reported for NHANES in 2003-04 (the most recent years for which
such an estimate is available), is 1.8 [mu]g/dL and the 5th and 95th
percentiles are 0.7 [mu]g/dL and 5.1 [mu]g/dL, respectively (Axelrad,
2008a, 2008b). The mean blood Pb levels in all of the analyses from
which C-R functions were drawn and described in the proposal (presented
in Table 1 of section II.A.2.c above) are higher than this U.S. mean
and some are substantially higher.
    In consideration of the advice from CASAC and comments from the
public, we have further considered the analyses presented in Table 1 of
section II.A.2.c above from which quantitative relationships between IQ
loss and blood Pb levels are described in the proposal (section
II.B.2.b) for the purpose of focusing on those analyses that are based
on blood Pb levels that best reflect today's population of children in
the U.S. Given the evidence of nonlinearity and of steeper slopes at
lower blood Pb levels (summarized in section II.A.2.c above), a focus
on children with appreciably higher blood Pb levels could not be
expected to identify a slope estimate that would be reasonably
representative for today's population of children. More specifically,
in applying the evidence-based framework, we are focused on a
subpopulation of U.S. children, those living near air sources and more
likely to be exposed at the level of the standard. While the air-
related Pb in the blood of this subpopulation is expected to be greater
than that for the general population given their greater air-related Pb
exposure, we do not have information on the mean total blood Pb level
(or, more specifically, the nonair component) for this subpopulation.
However, even if we were to assume, as an extreme hypothetical example,
that the mean for the general population of U.S. children included zero
contribution from air-related sources, and added that to our estimate
of air-related Pb for this subpopulation, the result would still be
below the lowest mean blood Pb level among the set of quantitative C-R
analyses.\81\ Thus, our goal in considering these quantitative analyses
was to identify C-R analyses with mean blood Pb levels closest to those
of today's U.S. children, including the at-risk subpopulation.\82\
---------------------------------------------------------------------------

    \81\ Using the ratio of 1:7 identified above as central within
the reasonable range of air-to-blood ratios, the estimate of air-
related blood Pb associated with a standard level of 0.15 [mu]g/m\3\
would be approximately 1 [mu]g/dL. Adding this to the mean total
blood Pb level for the U.S. population would yield a mean total
blood Pb estimate of 2.8 [mu]g/dL.
    \82\ As noted above, we also recognize that blood Pb levels are
expected to further decline in response to this and other public
health protection actions, including those described above in
section I.D.

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

    Among the analyses presented in the proposal (Table 1), we note
that six study groups from four different studies have blood Pb levels
appreciably closer to the mean blood Pb levels in today's young
children. Mean blood Pb levels for these study groups range from 2.9 to
4.3 [mu]g/dL, while mean blood Pb levels for the other three study
groups considered in the proposal range from 7.4 up to 9.7 [mu]g/dL.
Further, among the six slopes from analyses with blood Pb levels
closest to today's blood Pb levels, four come from two studies, with
these two studies each providing two analyses of differing blood Pb
levels. Focusing on the single analysis from each of the four studies
that has a mean blood Pb level closest to today's mean for U.S.
children yields four slopes ranging from -1.56 to -2.94, with a median
of -1.75 IQ points per [mu]g/dL (Table 3). Consistent with the evidence
for nonlinearity in the C-R relationship, the slopes for the C-R
functions from these four analyses are steeper than the slopes for the
other higher blood Pb level analyses. In considering the C-R functions
from these four analyses with the air-related IQ loss framework in
section II.C.3.c below, we have placed greater weight on the median of
the group, giving less weight to the minimum or maximum values,
recognizing the uncertainty in determining the C-R relationship.

 Table 3--Summary of Quantitative Relationships of IQ and Blood Pb for Analyses With Blood Pb Levels Closest to
                                       Those of Children in the U.S. Today
----------------------------------------------------------------------------------------------------------------
                  Blood Pb levels ([mu]g/dL)                                                      Average linear
---------------------------------------------------------------                                    slope \A\ (IQ
                                                  Range (min-            Study/analysis             points per
                Geometric mean                       max)                                            [mu]g/dL)
----------------------------------------------------------------------------------------------------------------
2.9...........................................         0.8-4.9  Tellez-Rojo et al. 2006, <5                -1.71
                                                                 subgroup.
3.24..........................................         0.9-7.4  Lanphear et al. 2005 \B\, <7.5             -2.94
                                                                 peak subgroup.
3.32..........................................         0.5-8.4  Canfield et al. 2003 \B\, <10              -1.79
                                                                 peak subgroup.
3.8...........................................           1-9.3  Bellinger and Needleman 2003               -1.56
                                                                 \B\, <10 peak subgroup.
                                               -----------------------------------------------------------------
    Median value..............................  ..............  ................................           -1.75
----------------------------------------------------------------------------------------------------------------
\A\ Average linear slope estimates here are for relationship between IQ and concurrent blood Pb levels except
  for Bellinger & Needleman for which study reports relationship for 10-year-old IQ with 24-month blood Pb
  levels.
\B\ The Lanphear et al. (2005) pooled International study includes blood Pb data from the Rochester and Boston
  cohorts, although for different ages (6 and 5 years, respectively) than the ages analyzed in Canfield et al.
  (2003) and Bellinger and Needleman (2003).

    Some commenters representing a business or industry association
recommended that EPA rely on the median estimate from the second set of
C-R functions presented in the proposal. As their basis for this view,
these commenters made several points. For example, they stated that the
extent and magnitude of nonlinearity in the IQ-blood Pb C-R
relationship is ``highly uncertain,'' and as part of their rationale
for this statement they cited studies by Jusko et al. (2007) and Surkan
et al. (2007) as not providing support for a nonlinear C-R function.
Other statements made by these commenters in support of their view are
that the maximum slope in the first set is an ``outlier,'' that the
second set reflects a greater number of studies and subjects than the
first set, and that simply being closer to the blood Pb levels of
today's children does not provide a better estimate than the median of
the second set, with some noting that the second set is inclusive of
some analyses with blood Pb levels similar to those in first set.
    EPA disagrees with these commenters' view that a focus on analyses
of children with blood Pb levels closer to today's children is not an
important criterion for selecting a C-R function for use with the IQ
loss framework. On the contrary, as stated above, EPA agrees with CASAC
that this is an essential criterion for this analysis. While EPA
recognizes uncertainty in the quantitative characterization of the
nonlinearity in the blood Pb-IQ loss relationship, the weight of the
current evidence (described in detail in the Criteria Document)
supports our conclusion that the blood Pb-IQ loss relationship is
nonlinear, with steeper slopes at lower blood Pb levels. While EPA
agrees there are a greater number of studies and subjects in the second
set, the nonlinearity of the relationship at issue means that a focus
on C-R functions from the studies in that set involving children with
appreciably higher blood Pb levels could not be expected to identify a
slope estimate that would be reasonably representative for today's
population of children. In reviewing the available studies with this
important criterion in mind, as described above, we have identified
four different studies from which C-R functions can be drawn, and in
considering these functions in the context of the air-related IQ loss
framework, have focused on the median estimate for the group,
consequently avoiding focus on a single estimate that may be unduly
influenced by one single analysis.
    With regard to the ``new'' studies cited by commenters above, EPA
notes that we are not relying on them in this review for the reasons
stated above in section I.C. After provisional consideration of these
studies cited by commenters (discussed further in the Response to
Comments document), EPA has determined that the more recent cited
studies provide only limited information with regard to the shape of
the C-R curve and, in light of other recent provisionally considered
studies and those studies reviewed in the Criteria Document, do not
materially change EPA's conclusion regarding nonlinearity that is well
founded in the evidence described in the Criteria Document.
(v) Role of Risk Assessment
    Some commenters recommended that the Administrator place greater
weight on the risk estimates derived in the quantitative risk
assessment, with some (e.g., the Association of Battery Recyclers)
concluding that these estimates supported a level for the standard
above the proposed range and some (e.g., NRDC and Missouri Coalition
for the Environment) concluding that they supported a level at the
lower end or below the proposed range. For the reasons identified in
the

proposal and noted in section II.C.3.c below, the Administrator has
placed primary weight on the air-related IQ loss evidence-based
framework in his decision with regard to level, and less weight on risk
estimates from the quantitative risk assessment. At the same time, as
stated in section II.C.3.c below, he finds those estimates to be
roughly consistent with and generally supportive of the estimates from
the evidence-based framework.
c. Conclusions on Level
    Having carefully considered the public comments on the appropriate
level of the Pb standard, as discussed above, the Administrator
believes the fundamental scientific conclusions on the effects of Pb
reached in the Criteria Document and Staff Paper, briefly summarized
above in sections II.A.1 and II.A.2 and discussed more fully in
sections II.A and II.B of the proposal, remain valid. In considering
the level at which the primary Pb standard should be set, as in
reaching a final decision on the need for revision of the current
standard, the Administrator considers the entire body of evidence and
information, in an integrated fashion, giving appropriate weight to
each part of that body of evidence and information. In that context the
Administrator continues to place primary consideration on the body of
scientific evidence available in this review on the health effects
associated with Pb exposure. In so doing, the Administrator primarily
focuses on the air-related IQ loss evidence-based framework summarized
in section II.C.3.a above and described in the proposal, recognizing
that it provides useful guidance for making the public health policy
judgment on the degree of protection from risk to public health that is
sufficient but not more than necessary.
    As described in section II.E.3.d of the proposal and recognized in
section II.C.3.a above, the air-related IQ loss framework is used to
inform the selection of a standard level that would protect against
air-related IQ loss (and related effects) of a magnitude judged by the
Administrator to be of concern in subpopulations of children exposed to
the level of the standard, taking into consideration uncertainties
inherent in such estimates. This framework calls for identifying a
target degree of protection in terms of an air-related IQ loss for such
subpopulations of children (discussed further below), as well as two
other parameters also relevant to this framework--a C-R function for
population IQ response associated with blood Pb level and an air-to-
blood ratio.
    With regard to estimates for air-to-blood ratio, the Administrator
has further considered the evidence regarding air-to-blood
relationships described in section II.A.2.a.iii above in light of
advice from CASAC and comments from the public as described in section
II.C.2.b above. Accordingly, he recognizes that the evidence includes
support for ratios greater than 1:7 (the upper end of the range focused
on in the proposal), including estimates ranging from 1:8 to 1:10. He
also recognizes that the estimates developed from the quantitative
exposure and risk assessments also include values greater than 1:7,
including values ranging up to 1:10 and some higher. Additionally, as
noted in section II.A.2.a.iii above, the evidence as a whole also
indicates that variation in the value of the ratios appears to relate
to the extent to which the range of air-related pathways are included
and the magnitude of the air and blood Pb levels assessed, such that
higher ratios appear to be associated with more complete assessments of
air-related pathways and lower air and blood Pb levels. Taking all of
these considerations into account, the Administrator concludes that the
reasonable range of air-to-blood estimates to use in the air-related IQ
loss framework includes ratios of 1:5 up to ratios on the order of
1:10. He does not consider lower ratios to be representative of the
full range of air-related pathways and the ratios expected at today's
air and blood Pb levels. The Administrator also concludes that it is
appropriate to focus on 1:7 as a generally central value within this
range.
    With regard to C-R functions, the Administrator has further
considered the evidence regarding quantitative relationships between IQ
loss and blood Pb levels described in section II.A.2.c above, in light
of advice from CASAC and comments from the public as described in
section II.C.3.b above. He recognizes the evidence of nonlinearity and
of steeper slopes at lower blood Pb levels (summarized in section
II.A.2.c above), and as a result, he believes it is appropriate to
focus on those analyses that are based on blood Pb levels that most
closely reflect today's population of children in the U.S., recognizing
that the evidence does not include analyses involving mean blood Pb
levels as low as the mean blood Pb level for today's children. He notes
that, as described in section II.C.3.b above, a review of the evidence
with this focus in mind has identified four analyses that have a mean
blood Pb level closest to today's mean for U.S. children and that yield
four slopes ranging from -1.56 to -2.94, with a median of -1.75 IQ
points per [mu]g/dL (Table 3). The Administrator concludes that it is
appropriate to consider this set of C-R functions for use in the air-
related IQ loss evidence based framework, as this set of C-R functions
best represents the evidence pertinent to children in the U.S. today.
In addition, the Administrator determines that it is appropriate to
give more weight to the central estimate for this set of functions,
which is the median of the set of functions, and not to rely on any one
function.
    As noted in the proposal, in considering this evidence-based
framework, the Administrator recognizes that there are currently no
commonly accepted guidelines or criteria within the public health
community that would provide a clear basis for reaching a judgment as
to the appropriate degree of public health protection that should be
afforded to protect against risk of neurocognitive effects in sensitive
populations, such as IQ loss in children. With regard to making a
public health policy judgment as to the appropriate protection against
risk of air-related IQ loss and related effects, the Administrator
believes that ideally air-related (as well as other) exposures to
environmental Pb would be reduced to the point that no IQ impact in
children would occur. The Administrator recognizes, however, that in
the case of setting a NAAQS, he is required to make a judgment as to
what degree of protection is requisite to protect public health with an
adequate margin of safety.
    The Administrator generally agrees with CASAC and the commenters
who emphasize that the NAAQS should prevent air-related IQ loss of a
significant magnitude in all but a small percentile of the population.
However, as discussed above in section II.C.3.b, it is important to
note that in selecting a target degree of public health protection that
should be afforded to at-risk populations of children in terms of air-
related IQ loss as estimated by the evidence-based framework being
applied in this review, the Administrator is not determining a specific
quantitative public health policy goal for air-related IQ loss that
would be acceptable or unacceptable for the entire population of
children in the United States. Instead, he is determining what
magnitude of estimated air-related IQ loss should be used in
conjunction with this specific framework, in light of the uncertainties
in the framework and the limitations in using the framework.

    In that context, the air-related IQ loss framework provides
estimates for the mean air-related IQ loss of a subset of the
population of U.S. children, and there are uncertainties associated
with those estimates. It provides estimates for that subset of children
likely to be exposed to the level of the standard, which is generally
expected to be the subpopulation of children living near sources who
are likely to be most highly exposed. In providing estimates of the
mean air-related IQ loss for this subpopulation of children, the
framework does not provide estimates of the mean air-related IQ loss
for all U.S. children. The Administrator recognizes, as discussed
above, that EPA is unable to quantify the percentile of the U.S.
population of children that corresponds to the mean of this sensitive
subpopulation, nor can EPA confidently develop quantified estimates for
upper percentiles for this subpopulation. EPA expects that the mean of
this subpopulation represents a high, but not quantifiable, percentile
of the U.S. population of children. As a result, the Administrator
expects that a standard based on consideration of this framework would
provide the same or greater protection from estimated air-related IQ
loss for a high, albeit unquantifiable, percentage of the entire
population of U.S. children.\83\
---------------------------------------------------------------------------

    \83\ Further, in determining what level of estimated IQ loss
should be used for evaluating the results obtained from this
specific evidence-based framework, the Administrator is not
determining that such an IQ loss is appropriate for use in other
contexts.
---------------------------------------------------------------------------

    In addition, EPA expects that the selection of a maximum, not to be
exceeded, form in conjunction with a rolling 3-month averaging time
over a three-year span, discussed in section II.C.2. above, will have
the effect that the at-risk subpopulation of children will be exposed
below the level of the standard most of the time. In light of this and
the significant uncertainty in the relationship between time period of
ambient level, exposure, and occurrence of a health effect, the choice
of an air-related IQ loss to focus on in applying the framework should
not be seen as a decision that a specific level of air-related IQ loss
will occur in fact in areas where the revised standard is just met or
that such a loss has been determined as acceptable if it were to occur.
Instead, the choice of such an air-related IQ loss is one of the
judgments that need to be made in using the evidence-based framework to
provide useful guidance in making the public health policy judgment on
the degree of protection from risk to public health that is sufficient
but not more than necessary, taking into consideration the patterns of
air quality that would likely occur upon just meeting the standard as
revised in this rulemaking.
    In considering the appropriate air-related IQ loss to accompany
application of the framework, the Administrator has considered the
advice of CASAC and public comments on this issue, discussed above in
section II.C.3.b. The Administrator recognizes that comments on the
proposal have highlighted the ambiguity in using an air-related IQ loss
for the framework that is phrased in terms of a range. For example, if
a range of 1-2 points IQ loss is selected, it is unclear whether the
intent is to limit points of air-related IQ loss to below 1, below 2,
or below some level in between. For clarity, it is more useful to use a
specific level as compared to a range. In addition, recognizing the
uncertainties inherent in evaluating the health impact of an IQ loss
across a population, as well as the uncertainties in the inputs to the
framework, the Administrator believes it is appropriate to use a whole
number for the air-related IQ loss level.
    In consideration of comments from CASAC and the public and in
recognition of the uncertainties in the health effects evidence and
related information, as well as the role of a selected air-related IQ
loss in the application of the framework, the Administrator concludes
that an air-related IQ loss of 2 points should be used in conjunction
with the evidence-based framework in selecting an appropriate level for
the standard. Given the uncertainties in the inputs to the framework,
the uncertainties in the relationship between ambient levels, exposure
period, and occurrence of health effects, and the focus of the
framework on the sensitive subpopulation of more highly exposed
children, a standard level selected using this air-related IQ loss, in
combination with the selected averaging time and form, would
significantly reduce and limit for a high percentage of U.S. children
the risk of experiencing an air-related IQ loss of that magnitude.
    With this specific air-related IQ loss in mind, the Administrator
considered the application of this framework to a broad range of
standard levels, using estimates for the two key parameters--air-to-
blood ratio and C-R function--that are appropriate for use within the
framework, as shown in Table 4 below. In so doing, the Administrator
recognized that, relying on the median of the four C-R functions from
analyses with blood Pb levels closest to those of today's children, a
standard level in the lower half of the proposed range (0.10-0.20
[mu]g/m\3\) would limit the estimated mean IQ loss from air-related Pb
to below 2 points, depending on the choice of air-to-blood ratio within
the range from 1:5 to 1:10.
    As noted above, however, the Administrator does not believe it is
appropriate to consider only a single air-to-blood ratio. Using the
air-to-blood ratio of 1:7, a generally central estimate within the well
supported range of estimates, the estimates of air-related IQ loss are
below a 2-point IQ loss for standard levels of 0.15 [mu]g/m\3\ and
lower. At a level of 0.15 [mu]g/m\3\, the Administrator recognizes that
use of a 1:10 ratio produces an estimate greater than 2 IQ points and
use of a 1:5 ratio produces a lower IQ loss estimate. Given the
uncertainties and limitations in the air-related IQ loss framework, the
Administrator views it as appropriate to place primary weight on the
results from this central estimate rather than estimates derived using
air-to-blood-ratios either higher or lower than this ratio.

 Table 4--Estimates of Air-Related Mean IQ Loss for the Subpopulation of
              Children Exposed at the Level of the Standard
------------------------------------------------------------------------
                         Air-related mean IQ loss (points) for the
                     subpopulation of children exposed at level of the
                                          standard
                  ------------------------------------------------------
                     IQ loss estimate is based on median slope of 4 C-R
 Potential level     functions with blood Pb levels closer to those of
   for standard       today's U.S. children (range shown for estimates
   ([mu]g/m\3\)           based on lowest and highest of 4 slopes)
                  ------------------------------------------------------
                                     Air-to-blood ratio
                  ------------------------------------------------------
                          1:10               1:7               1:5
------------------------------------------------------------------------
           0.50               >5 *               >5 *     4.4 (3.9-7.4)

           0.40                         4.9 (4.4-8.2)     3.5 (3.1-5.9)
           0.30      5.3 (4.7-8.8)      3.7 (3.3-6.2)     2.6 (2.3-4.4)
           0.25      4.4 (3.9-7.4)      3.1 (2.7-5.1)     2.2 (2.0-3.7)
           0.20      3.5 (3.1-5.9)      2.5 (2.2-4.1)     1.8 (1.6-2.9)
           0.15      2.6 (2.3-4.4)      1.8 (1.6-3.1)     1.3 (1.2-2.2)
           0.10      1.8 (1.6-2.9)      1.2 (1.1-2.1)     0.9 (0.8-1.5)
           0.05      0.9 (0.8-1.5)      0.6 (0.5-1.0)     0.4 (0.4-0.7)
           0.02      0.4 (0.3-0.6)      0.2 (0.2-0.4)     0.2 (0.2-0.3)
------------------------------------------------------------------------
* For these combinations of standard levels and air-to-blood ratios, the
  appropriateness of the C-R function applied in this table becomes
  increasingly uncertain such that no greater precision than ``>5'' for
  the IQ loss estimate is warranted.

    The Administrator has also considered the results of the exposure
and risk assessments conducted for this review to provide some further
perspective on the potential magnitude of risk of air-related IQ loss.
The Administrator finds that these quantitative assessments provide a
useful perspective on the risk from air-related Pb. However, in light
of the important uncertainties and limitations associated with these
assessments, as summarized in section II.A.3 above and discussed in
sections II.C and II.E.3.b of the proposal, for purposes of evaluating
potential standard levels, the Administrator places less weight on the
risk estimates than on the evidence-based assessment. Nonetheless, the
Administrator finds that the risk estimates are roughly consistent with
and generally supportive of the evidence-based air-related IQ loss
estimates summarized above.\84\
---------------------------------------------------------------------------

    \84\ For example, in considering a standard level of 0.2 [mu]g/
m\3\, we note that the risk assessment provides estimates falling
within the range of 1.2 to 3.2 points IQ loss for the general urban
case study and <3.7 for the primary Pb smelter subarea. These
estimates are inclusive of the range of estimates for the 0.20
standard level presented in Table 4 based on the median C-R slope
applied in the air-related IQ loss framework. As noted in section
II.A.3.a above, these case studies, based on the nature of the
population exposures represented by them, relate more closely to the
air-related IQ loss evidence-based framework than other case studies
assessed.
---------------------------------------------------------------------------

    In the Administrator's view, the above considerations, taken
together, provide no evidence-or risk-based bright line that indicates
a single appropriate level. Instead, there is a collection of
scientific evidence and other information, including information about
the uncertainties inherent in many relevant factors, which needs to be
considered together in making the public health policy judgment to
select the appropriate standard level from a range of reasonable
values. In addition, the results of the evidence-based framework are
seen as a useful guide in determining whether the risks to public
health from exposure to ambient levels of Pb in the air, in the context
of a specified averaging time and form, provide a degree of protection
from risk with an adequate margin of safety that is sufficient but not
more than necessary.
    Based on consideration of the entire body of evidence and
information available at this time, as well as the recommendations of
CASAC and public comments, the Administrator has decided that a level
for the primary Pb standard of 0.15 [mu]g/m\3\, in combination with the
specified choice of indicator, averaging time, and form, is requisite
to protect public health, including the health of sensitive groups,
with an adequate margin of safety. The Administrator notes that this
level is within the range recommended by CASAC, the Staff Paper, and by
the vast majority of commenters. The Administrator concludes that a
standard with a level of 0.15 [mu]g/m\3\ will reduce the risk of a
variety of health effects associated with exposure to Pb, including
effects indicated in the epidemiological studies at low blood Pb
levels, particularly including neurological effects in children, and
the potential for cardiovascular and renal effects in adults.
    The Administrator notes that the evidence-based framework indicates
that for standard levels above 0.15 [mu]g/m\3\, the estimated mean air-
related IQ loss in the subpopulation of children exposed at the level
of the standard would range in almost all cases from above 2 points to
5 points or more with the range of air-to-blood ratios considered. He
concludes, in light of his consideration of all of the evidence,
including the framework discussed above, that the protection from air-
related Pb effects at the higher blood Pb levels that would be allowed
by standards above 0.15 [mu]g/m\3\ would not be sufficient to protect
public health with an adequate margin of safety.
    In addition, the Administrator notes that for standard levels below
0.15 [mu]g/m\3\, the estimated mean IQ loss from air-related Pb in the
subpopulation of children exposed at the level of the standard would
generally be somewhat to well below 2 IQ points regardless of which
air-to-blood ratio within the range of ratios considered was used. The
Administrator concludes in light of all of the evidence, including the
evidence-based framework, that the degree of public health protection
that standards below 0.15 [mu]g/m\3\ would likely afford would be
greater than what is necessary to protect public health with an
adequate margin of safety.
    The Administrator also recognizes that several commenters expressed
concern that the proposal did not adequately address the need for the
standard to be set with an adequate margin of safety. As noted above,
in section I, the requirement that primary standards include an
adequate margin of safety was intended to address uncertainties
associated with inconclusive scientific and technical information
available at the time of standard setting. It was also intended to
provide a reasonable degree of protection against hazards that research
has not yet identified. Both kinds of uncertainties are components of
the risk associated with pollution at levels below those at which human
health effects can be said to occur with reasonable scientific
certainty. Thus, in

selecting a primary standard that includes an adequate margin of
safety, the Administrator is seeking not only to prevent pollutant
levels that have been demonstrated to be harmful but also to prevent
lower pollutant levels that may pose an unacceptable risk of harm, even
if the risk is not precisely identified as to nature or degree.
    Nothing in the Clean Air Act, however, requires the Administrator
to identify a primary standard that would be protective against
demonstrated harms, and then identify an additional ``margin of
safety'' which results in further lowering of the standard. Rather, the
Administrator's past practice has been to take margin of safety
considerations into account in making decisions about setting the
primary standard, including in determining its level, averaging time,
form and indicator, recognizing that protection with an adequate margin
of safety needs to be sufficient but not more than necessary.
    Consistent with past practice, the Administrator has taken the need
to provide for an adequate margin of safety into account as an integral
part of his decision-making on the appropriate level, averaging time,
form, and indicator of the standard. As discussed above, the
consideration of health effects caused by different ambient air
concentrations of Pb is extremely complex and necessarily involves
judgments about uncertainties with regard to the relationships between
air concentrations, exposures, and health effects. In light of these
uncertainties, the Administrator has taken into account the need for an
adequate margin of safety in making decisions on each of the elements
of the standards. Consideration of the need for an adequate margin of
safety is reflected in the following elements: selection of TSP as the
indicator and the rejection of the use of PM10 scaling
factors; selection of a maximum, not to be exceeded form, in
conjunction with a 3-month averaging time that employs a rolling
average, with the requirement that each month in the 3-month period be
weighted equally (rather than being averaged by individual data) and
that a 3-year span be used for comparison to the standard; and, the use
of a range of inputs for the evidence-based framework, that includes a
focus on higher air-to-blood ratios than the lowest ratio considered to
be supportable, and steeper rather than shallower C-R functions, and
the consideration of these inputs in selection of 0.15 [mu]g/m\3\ as
the level of the standard. The Administrator concludes based on his
review of all of the evidence (including the evidence-based framework)
that when taken as a whole the standard selected today, including the
indicator, averaging time, form, and level, will be sufficient but not
more than necessary to protect public health, including the health of
sensitive subpopulations, with an adequate margin of safety.
    Thus, after carefully taking the above comments and considerations
into account, and fully considering the scientific and policy views of
the CASAC, the Administrator has decided to revise the level of the
primary Pb standard to 0.15 [mu]g/m\3\. In the Administrator's
judgment, based on the currently available evidence, a standard set at
this level and using the specified indicator, averaging time, and form
would be requisite to protect public health with an adequate margin of
safety. The Administrator judges that such a standard would protect,
with an adequate margin of safety, the health of children and other at-
risk populations against an array of adverse health effects, most
notably including neurological effects, particularly neurobehavioral
and neurocognitive effects, in children. A standard set at this level
provides a very significant increase in protection compared to the
current standard. The Administrator believes that a standard set at
0.15 [mu]g/m\3\ would be sufficient to protect public health with an
adequate margin of safety, and believes that a lower standard would be
more than what is necessary to provide this degree of protection. This
judgment by the Administrator appropriately considers the requirement
for a standard that is neither more nor less stringent than necessary
for this purpose and recognizes that the CAA does not require that
primary standards be set at a zero-risk level, but rather at a level
that reduces risk sufficiently so as to protect public health with an
adequate margin of safety.

D. Final Decision on the Primary Lead Standard

    For the reasons discussed above, and taking into account
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and the public
comments, the Administrator is revising the various elements of the
standard to provide increased protection for children and other at-risk
populations against an array of adverse health effects, most notably
including neurological effects in children, including neurocognitive
and neurobehavioral effects. Specifically, the Administrator has
decided to revise the level of the primary standard to a level of 0.15
[mu]g/m\3\, in conjunction with retaining the current indicator of Pb-
TSP. The Administrator has also decided to revise the form and
averaging time of the standard to a maximum (not to be exceeded)
rolling 3-month average evaluated over a 3-year period.
    Corresponding revisions to data handling conventions, including
allowance for the use of Pb-PM10 data in certain
circumstances, and the treatment of exceptional events are specified in
revisions to Appendix R, as discussed in section IV below.
Corresponding revisions to aspects of the ambient air monitoring and
reporting requirements for Pb are discussed in section V below,
including sampling and analysis methods (e.g., a new Federal reference
method for monitoring Pb in PM10, quality assurance
requirements), network design, sampling schedule, data reporting, and
other miscellaneous requirements.

III. Secondary Lead Standard

A. Introduction

    The NAAQS provisions of the Act require the Administrator to
establish secondary standards that, in the judgment of the
Administrator, are requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
the pollutant in the ambient air. 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 secondary
standards be set to eliminate all risk of adverse welfare effects, but
rather at a level requisite to protect public welfare from those
effects that are judged by the Administrator to be adverse.
    This section presents the rationale for the Administrator's final
decision to revise the existing secondary NAAQS. In considering the
currently available evidence on Pb-related welfare effects, there is
much information linking Pb to potentially adverse effects on organisms
and ecosystems. However, given the evaluation of this information in
the Criteria Document and Staff Paper which highlighted the substantial
limitations in the evidence, especially the lack of evidence linking
various effects to specific levels of ambient Pb, the Administrator
concludes that the available evidence supports revising the secondary
standard but does not provide a sufficient basis for establishing a
secondary standard for Pb that is different from the primary standard.

1. Overview of Welfare Effects Evidence
    A secondary NAAQS addresses welfare effects and ``effects on
welfare'' include, but are not limited to, effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and on
personal comfort and well-being. CAA section 302(h). A qualitative
assessment of welfare effects evidence related to ambient Pb is
summarized in this section, drawing from the Criteria Document, Chapter
6 of the Staff Paper and from the Proposed Rule. The presentation here
summarizes several key aspects of the welfare evidence for Pb. Lead is
persistent in the environment and accumulates in soils, aquatic systems
(including sediments), and some biological tissues of plants, animals
and other organisms, thereby providing long-term, multi-pathway
exposures to organisms and ecosystems. Additionally, EPA recognizes
that there have been a number of uses of Pb, especially as an
ingredient in automobile fuel but also in other products such as paint,
lead-acid batteries, and some pesticides, which have significantly
contributed to widespread increases in Pb concentrations in the
environment, a portion of which remains today (e.g., CD, Chapters 2 and
3).
    Ecosystems near smelters, mines and other industrial sources of Pb
have demonstrated a wide variety of adverse effects including decreases
in species diversity, loss of vegetation, changes to community
composition, decreased growth of vegetation, and increased number of
invasive species. These sources may have multiple pathways for
discharging Pb to ecosystems, and apportioning effects between air-
related pathways and other pathways (e.g., discharges to water) in such
cases is difficult. Likewise, apportioning these effects between Pb and
other stressors is complicated because these point sources also emit a
wide variety of other heavy metals and sulfur dioxide which may cause
toxic effects. There are no field studies which have investigated
effects of Pb additions alone but some studies near large point sources
of Pb have found significantly reduced species composition and altered
community structures. While these effects are significant, they are
spatially limited: The majority of contamination occurs within 20 to 50
km of the emission source (CD, section AX<IP_ADDRESS>).
    By far, the majority of air-related Pb found in terrestrial
ecosystems was deposited in the past during the use of Pb additives in
gasoline. Many sites receiving Pb predominantly through such long-range
transport of gasoline-derived small particles have accumulated large
amounts of Pb in soils (CD, p. AX7-98). There is little evidence that
terrestrial sites exposed as a result of this long range transport of
Pb have experienced significant effects on ecosystem structure or
function (CD, section AX<IP_ADDRESS> and p. AX7-98). Strong complexation of
Pb by soil organic matter may explain why few ecological effects have
been observed (CD, p. AX7-98). Studies have shown decreasing levels of
Pb in vegetation which seems to correlate with decreases in atmospheric
deposition of Pb resulting from the removal of Pb additives to gasoline
(CD, section AX <IP_ADDRESS>).
    Terrestrial ecosystems remain primarily sinks for Pb but amounts
retained in various soil layers vary based on forest type, climate, and
litter cycling (CD, section 7.1). Once in the soil, the migration and
distribution of Pb is controlled by a multitude of factors including
pH, precipitation, litter composition, and other factors which govern
the rate at which Pb is bound to organic materials in the soil (CD,
section 2.3.5).
    Like most metals the solubility of Pb is increased at lower pH.
However, the reduction of pH may in turn decrease the solubility of
dissolved organic material (DOM). Given the close association between
Pb mobility and complexation with DOM, a reduced pH does not
necessarily lead to increased movement of Pb through terrestrial
systems and into surface waters. In areas with moderately acidic soil
(i.e., pH of 4.5 to 5.5) and abundant DOM, there is no appreciable
increase in the movement of Pb into surface waters compared to those
areas with neutral soils (i.e., pH of approximately 7.0). This appears
to support the theory that the movement of Pb in soils is limited by
the solubilization and transport of DOM. In sandy soils without
abundant DOM, moderate acidification appears likely to increase outputs
of Pb to surface waters (CD, section AX <IP_ADDRESS>).
    Lead exists in the environment in various forms which vary widely
in their ability to cause adverse effects on ecosystems and organisms.
Current levels of Pb in soil also vary widely depending on the source
of Pb but in all ecosystems Pb concentrations exceed natural background
levels. The deposition of gasoline-derived Pb into forest soils has
produced a legacy of slow moving Pb that remains bound to organic
materials despite the removal of Pb from most fuels and the resulting
dramatic reductions in overall deposition rates. For areas influenced
by point sources of air Pb, concentrations of Pb in soil may exceed by
many orders of magnitude the concentrations which are considered
harmful to laboratory organisms. Adverse effects associated with Pb
include neurological, physiological and behavioral effects which may
influence ecosystem structure and functioning. Ecological soil
screening levels (Eco-SSLs) have been developed for Superfund site
characterizations to indicate concentrations of Pb in soils below which
no adverse effects are expected to plants, soil invertebrates, birds
and mammals. Values like these may be used to identify areas in which
there is the potential for adverse effects to any or all of these
receptors based on current concentrations of Pb in soils.
    Atmospheric Pb enters aquatic ecosystems primarily through the
erosion and runoff of soils containing Pb and deposition (wet and dry).
While overall deposition rates of atmospheric Pb have decreased
dramatically since the removal of Pb additives from gasoline, Pb
continues to accumulate and may be re-exposed in sediments and water
bodies throughout the United States (CD, section 2.3.6).
    Several physical and chemical factors govern the fate and
bioavailability of Pb in aquatic systems. A significant portion of Pb
remains bound to suspended particulate matter in the water column and
eventually settles into the substrate. Species, pH, salinity,
temperature, turbulence and other factors govern the bioavailability of
Pb in surface waters (CD, section 7.2.2).
    Lead exists in the aquatic environment in various forms and under
various chemical and physical parameters which determine the ability of
Pb to cause adverse effects either from dissolved Pb in the water
column or Pb in sediment. Current levels of Pb in water and sediment
also vary widely depending on the source of Pb. Conditions exist in
which adverse effects to organisms and thereby ecosystems may be
anticipated given experimental results. It is unlikely that dissolved
Pb in surface water constitutes a threat to ecosystems that are not
directly influenced by point sources. For Pb in sediment, the evidence
is less clear. It is likely that some areas with long term historical
deposition of Pb to sediment from a variety of sources as well as areas
influenced by point sources have the potential for adverse effects to
aquatic communities. The long residence time of Pb in sediment and its
ability to be

resuspended by turbulence make Pb likely to be a factor for the
foreseeable future. Criteria have been developed to indicate
concentrations of Pb in water and sediment below which no adverse
effects are expected to aquatic organisms. These values may be used to
identify areas in which there is the potential for adverse effects to
receptors based on current concentrations of Pb in water and sediment.
2. Overview of Screening Level Ecological Risk Assessment
    This section presents a brief summary of the screening-level
ecological risk assessment conducted by EPA for this review. The
assessment is described in detail in Lead Human Exposure and Health
Risk Assessments and Ecological Risk Assessment for Selected Areas,
Pilot Phase (ICF, 2006). Various limitations have precluded performance
of a full-scale ecological risk assessment. The discussion here is
focused on the screening level assessment performed in the pilot phase
(ICF, 2006) and takes into consideration CASAC recommendations with
regard to interpretation of this assessment (Henderson, 2007a, b). The
following summary focuses on key features of the approach used in the
assessment and presents only a brief summary of the results of the
assessment.
    A screening level risk assessment was performed to estimate the
potential for ecological risks associated with exposures to Pb emitted
into ambient air. A case study approach was used which included areas
surrounding a primary Pb smelter and a secondary Pb smelter, as well as
a location near a nonurban roadway. Soil, surface water, and/or
sediment concentrations were estimated for each of the three initial
case studies from available monitoring data or modeling analysis, and
then compared to ecological screening benchmarks to assess the
potential for ecological impacts from Pb that was emitted into the air.
A national-scale screening assessment was also used to evaluate surface
water and sediment monitoring locations across the United States for
the potential for ecological impacts associated with atmospheric
deposition of Pb. An additional case study was identified to look at
gasoline derived Pb effects on an ecologically vulnerable ecosystem but
various limitations precluded any analyses.
    The ecological screening values used in this assessment to estimate
the potential for ecological risk were developed from the Eco-SSLs
methodology, EPA's recommended ambient water quality criteria, and
sediment screening values developed by MacDonald and others (2000,
2003). Soil screening values were derived for this assessment using the
Eco-SSL methodology with the toxicity reference values for Pb (USEPA,
2005d, 2005e) and consideration of the inputs on diet composition, food
intake rates, incidental soil ingestion, and contaminant uptake by prey
(details are presented in section <IP_ADDRESS> and Appendix L, of ICF,
2006). Hardness specific surface water screening values were calculated
for each site based on EPA's recommended ambient water quality criteria
for Pb (USEPA, 1984). For sediment screening values, the assessment
relied on sediment ``threshold effect concentrations'' and ``probable
effect concentrations'' developed by MacDonald et al. (2000). The
methodology for these sediment criteria is described fully in section
<IP_ADDRESS> and Appendix M of the pilot phase Risk Assessment Report (ICF,
2006).
    A Hazard Quotient (HQ) was calculated for various receptors to
determine the potential for risk to that receptor. The HQ is calculated
as the ratio of the media concentration to the ecotoxicity screening
value, and represented by the following equation:

HQ = (estimated Pb media concentration) / (ecotoxicity screening
value)

    For each case study, HQ values were calculated for each location
where either modeled or measured media concentrations were available.
Separate soil HQ values were calculated for each ecological receptor
group for which an ecotoxicity screening value has been developed
(i.e., birds, mammals, soil invertebrates, and plants). HQ values less
than 1.0 suggest that Pb concentrations in a specific medium are
unlikely to pose significant risks to ecological receptors. HQ values
greater than 1.0 indicate that the expected exposure exceeds the
ecotoxicity screening value and that there is a potential for adverse
effects.
    There are several uncertainties that apply across case studies
noted below:
     The ecological risk screen is limited to specific case
study locations and other locations for which Pb data were available.
Efforts were made to ensure that the Pb exposures assessed were
attributable to airborne Pb and not dominated by nonair sources.
However, there is uncertainty as to whether other sources might have
actually contributed to the Pb exposure estimates.
     A limitation to using the selected ecotoxicity screening
values is that they might not be sufficient to identify risks to some
threatened or endangered species or unusually sensitive aquatic
ecosystems (e.g., CD, p. AX7-110).
     The methods and database from which the surface water
screening values (i.e., the AWQC for Pb) were derived is somewhat
dated. New data and approaches (e.g., use of pH as indicator of
bioavailability) may now be available to estimated the aquatic toxicity
of Pb (CD, sections X<IP_ADDRESS> and AX<IP_ADDRESS>).
     No adjustments were made for sediment-specific
characteristics that might affect the bioavailability of Pb in
sediments in the derivation of the sediment quality criteria used for
this ecological risk screen (CD, sections 7.2.1 and AX<IP_ADDRESS>; Appendix
M, ICF, 2006). Similarly, characteristics of soils for the case study
locations were not evaluated for measures of bioavailability.
     Although the screening value for birds used in this
analysis is based on reasonable estimates for diet composition and
assimilation efficiency parameters, it was based on a conservative
estimate of the relative bioavailability of Pb in soil and natural
diets compared with water soluble Pb added to an experimental pellet
diet (Appendix L, ICF, 2006).
    The following is a brief summary of key observations related to the
results of the screening-level ecological risk assessment. A complete
discussion of the results is provided in Chapter 6 of the Staff Paper
and the complete presentation of the assessment and results is
presented in the pilot phase Risk Assessment Report (ICF, 2006).
    For the case studies, the concentrations of Pb in soil and
sediments in various locations exceeded screening values for these
media indicating potential for adverse effects to terrestrial organisms
(plants, birds and mammals) and to sediment dwelling organisms. While
it was not possible to dissect the contributions of air Pb emissions
from other sources, it is likely that, at least for the primary
smelter, that the air contribution is significant. For the other case
studies, the contributions of current air emissions to the Pb burden,
is less clear.
    The national-scale screen of surface water data initially
identified 15 areas for which water column levels of dissolved Pb were
greater than hardness adjusted chronic criteria for the protection of
aquatic life indicating a potential for adverse effect if
concentrations were persistent over chronic periods. Acute criteria
were not exceeded at any of these locations. The extent to which air
emissions of Pb have contributed to these surface water Pb
concentrations is unclear. In the national-scale screen of sediment
data associated with the 15 surface water sites described above,
threshold effect

concentration-based HQs at nine of these sites exceeded 1.0.
Additionally, HQs based on probable effect concentrations exceeded 1.0
at five of the sites, indicating probable adverse effects to sediment
dwelling organisms. Thus, sediment Pb concentrations at some sites are
high enough that there is a likelihood that they would cause adverse
effects to sediment dwelling organisms. However, the contribution of
air emissions to these concentrations is unknown.

B. Conclusions on the Secondary Lead Standard

1. Basis for the Proposed Decision
    The current standard was set in 1978 to be identical to the primary
standard (1.5 [mu]g Pb/m\3\, as a maximum arithmetic mean averaged over
a calendar quarter), the basis for which is summarized in section
II.C.1. At the time the standard was set, the Agency concluded that the
primary air quality standard would adequately protect against known and
anticipated adverse effects on public welfare, as the Agency stated
that it did not have evidence that a more restrictive secondary
standard was justified. In the rationale for this conclusion, the
Agency stated that the available evidence cited in the 1977 Criteria
Document indicated that ``animals do not appear to be more susceptible
to adverse effects from lead than man, nor do adverse effects in
animals occur at lower levels of exposure than comparable effects in
humans'' (43 FR 46256). The Agency recognized that Pb may be deposited
on the leaves of plants and present a hazard to grazing animals. With
regard to plants, the Agency stated that Pb is absorbed but not
accumulated to any great extent by plants from soil, and that although
some plants may be susceptible to Pb, it is generally in a form that is
largely unavailable to them. Further the Agency stated that there was
no evidence indicating that ambient levels of Pb result in significant
damage to manmade materials and Pb effects on visibility and climate
are minimal.
    The secondary standard was subsequently considered during the 1980s
in development of the 1986 Criteria Document (USEPA, 1986a) and the
1990 Staff Paper (USEPA, 1990b). In summarizing OAQPS staff conclusions
and recommendations at that time, the 1990 Staff Paper stated that a
qualitative assessment of available field studies and animal
toxicological data suggested that ``domestic animals and wildlife are
as susceptible to the effects of lead as laboratory animals used to
investigate human lead toxicity risks.'' Further, the 1990 Staff Paper
highlighted concerns over potential ecosystem effects of Pb due to its
persistence, but concluded that pending development of a stronger
database that more accurately quantifies ecological effects of
different Pb concentrations, consideration should be given to retaining
a secondary standard at or below the level of the then-current
secondary standard of 1.5 [mu]g/m\3\.
    Given the full body of current evidence, despite wide variations in
Pb concentrations in soils throughout the country, Pb concentrations
are in excess of concentrations expected from geologic or other non-
anthropogenic forces. There are several difficulties in quantifying the
role of recent air emissions of Pb in the environment: Some Pb
deposited before the standard was enacted is still present in soils and
sediments; historic Pb from gasoline continues to move slowly through
systems as does current Pb derived from both air and nonair sources.
Additionally, the evidence of adversity in natural systems is limited
due in no small part to the difficulty in determining the effects of
confounding factors such as multiple metals or factors influencing
bioavailability in field studies.
    The evidence summarized above, in the Proposed Rule, in section 4.2
of the Staff Paper, and described in detail in the Criteria Document,
informs our understanding of Pb in the environment today and evidence
of environmental Pb exposures of potential concern. For areas
influenced by point sources of air Pb that meet the current standard,
concentrations of Pb in soil may exceed by many orders of magnitude the
concentrations which are considered harmful to laboratory organisms
(CD, sections 3.2 and AX<IP_ADDRESS>). In addition, conditions exist in
which Pb associated adverse effects to aquatic organisms and thereby
ecosystems may be anticipated given experimental results. While the
evidence does not indicate that dissolved Pb in surface water
constitutes a threat to those ecosystems that are not directly
influenced by point sources, the evidence regarding Pb in sediment is
less clear (CD, sections AX<IP_ADDRESS>.2 and AX7.2.4). It is likely that
some areas with long term historical deposition of Pb to sediment from
a variety of sources as well as areas influenced by point sources have
the potential for adverse effects to aquatic communities. The Staff
Paper concluded, based on laboratory studies and current media
concentrations in a wide range of areas, that it seems likely that
adverse effects are occurring, particularly near point sources, under
the current standard. The long residence time of Pb in sediment and its
ability to be resuspended by turbulence make Pb contamination likely to
be a factor for the foreseeable future. Based on this information, the
Staff Paper concluded that the evidence suggests that the environmental
levels of Pb occurring under the current standard, set nearly thirty
years ago, may pose risk of adverse environmental effect.
    In addition to the evidence-based considerations described in the
previous section, the screening level ecological risk assessment is
informative, taking into account key limitations and uncertainties
associated with the analyses. As discussed in the previous section, as
a result of its persistence, Pb emitted in the past remains today in
aquatic and terrestrial ecosystems of the United States. Consideration
of the environmental risks associated with the current standard is
complicated by the environmental burden associated with air Pb
concentrations that exceeded the current standard, predominantly in the
past. Concentrations of Pb in soil and sediments associated with the
case studies exceeded screening values for those media, indicating
potential for adverse effect in terrestrial organisms (plants, birds,
and mammals) and in sediment dwelling organisms. While the contribution
to these Pb concentrations from air as compared to nonair sources has
not been quantified, air emissions from the primary smelting facility
at least are substantial (Appendix D, USEPA 2007b; ICF 2006).
    The national-scale screens, which are not focused on particular
point source locations, indicate the ubiquitous nature of Pb in aquatic
systems of the United States today. Further, the magnitude of surface
water Pb concentrations in several aquatic systems exceeded screening
values and sediment Pb concentrations at some sites in the national-
scale screen were high enough that the likelihood that they would cause
adverse effects to sediment dwelling organisms may be considered
``probable''. A complicating factor in interpreting the findings for
the national-scale screening assessments is the lack of clear
apportionment of Pb contributions from air as compared to nonair
sources, such as industrial and municipal discharges. While the
contribution of air emissions to the elevated concentrations has not
been quantified, documentation of historical trends in the sediments of
many water bodies has illustrated the sizeable contribution that
airborne Pb can have on aquatic systems (e.g., Staff Paper, section
2.8.1). This documentation also indicates the greatly reduced
contribution in many systems as compared to decades ago (presumably

reflecting the phase-out of Pb-additives from gasoline used by cars and
trucks). However, the timeframe for removal of Pb from surface
sediments into deeper sediment varies across systems, such that Pb
remains available to biological organisms in some systems for much
longer than in others (Staff Paper, section 2.8; CD, pp. AX7-141 to
AX7-145).
    The case study locations included in the screening assessment, with
the exception of the primary Pb smelter site, are currently meeting the
current Pb standard, yet Pb occurs in soil and aquatic sediment in some
locations at concentrations indicative of a potential for harm to some
terrestrial and sediment dwelling organisms. While the role of airborne
Pb in determining these Pb concentrations is unclear, the historical
evidence indicates that airborne Pb can create such concentrations in
sediments and soil.
    Based on its review of the Staff Paper, CASAC advised the
Administrator that ``The Lead Panel unanimously affirms its earlier
judgments that, as with the primary (public-health based) Lead NAAQS,
the secondary (public-welfare based) standard for lead also needs to be
substantially lowered * * * Therefore at a minimum, the level of the
secondary Lead NAAQS should be at least as low as the level of the
recommended primary lead standard.'' (Henderson, 2008a). CASAC also
recognized that EPA lacked data to provide a clear quantitative basis
for setting a secondary standard that differed from the primary
standard. (Henderson 2007a, 2008a).
    In considering the adequacy of the current standard in providing
protection from Pb-related adverse effects on public welfare, the
Administrator considered in the proposal the body of available evidence
(briefly summarized above in section III.). The proposal indicated that
depending on the interpretation, the available data and evidence,
primarily qualitative, suggests that there was the potential for
adverse environmental impacts under the current standard. Given the
limited data on Pb effects in ecosystems, it is necessary to look at
evidence of Pb effects on organisms and extrapolate to ecosystem
effects. Therefore, taking into account the available evidence and
current media concentrations in a wide range of areas, the
Administrator concluded in the proposal that there is potential for
adverse effects occurring under the current standard, although there
are insufficient data to provide a quantitative basis for setting a
secondary standard different than the primary. While the role of
current airborne emissions is difficult to apportion, deposition of Pb
from air sources is occurring and this ambient Pb is likely to be
persistent in the environment similarly to that of historically
deposited Pb which has persisted, although location specific dynamics
of Pb in soil result in differences in the timeframe during which Pb is
retained in surface soils or sediments where it may be available to
ecological receptors (USEPA, 2007b, section 2.3.3).
    Based on these considerations, and taking into account the
observations, analyses, and recommendations discussed above, the
Administrator proposed to revise the current secondary Pb standard by
making it identical in all respects to the proposed primary Pb standard
(described in section II.D above).
2. Comments on the Proposed Secondary Standard
    EPA notes that CASAC, in their July 2008 letter, did not provide
comments on the discussion and proposal regarding the secondary
standard. Commenters who expressed an opinion on the proposed revision
to the secondary standard, including a number of national
organizations, individual States, Tribal associations, and local
organizations, and combined comments from various environmental groups
supported the position that the secondary Pb standard should be revised
to the level of the primary standard. Some commenters recommended that
the secondary standard be no less stringent than the primary, one
commenter recommended that the standard be no more stringent than the
primary, and some commenters recommended that the secondary standard be
identical to the primary. One commenter concurred with the Agency's
finding, consistent with CASAC's prior advice, that the current
scientific knowledge was lacking and that further research was
necessary to quantitatively inform an appropriate secondary standard.
For the reasons discussed above and in the proposal, we agree with
commenters that the secondary standard should be at this time set equal
to the primary in indicator, level, form and averaging time and that
more research is needed to further inform the development of a
secondary Pb standard.
3. Administrator's Conclusions
    In considering the adequacy of the current secondary standard in
providing requisite protection from Pb-related adverse effects on
public welfare, the Administrator has considered the body of available
evidence (briefly summarized above and in the proposal). The screening-
level risk assessment, while limited and accompanied by various
uncertainties, suggests occurrences of environmental Pb concentrations
existing under the current standard that could have adverse
environmental effects in terrestrial organisms (plants, birds and
mammals) and in sediment dwelling organisms. Environmental Pb levels
today are associated with atmospheric Pb concentrations and deposition
that have combined with a large reservoir of historically deposited Pb
in environmental media.
    In considering this evidence, as well as the views of CASAC,
summarized above, the Staff Paper and associated support documents, and
views of public commenters on the adequacy of the current standard, the
Administrator concurs with CASAC's recommendation that the secondary
standard should be substantially revised and concludes that given the
current state of evidence, the current secondary standard for Pb is not
requisite to protect public welfare from known or anticipated adverse
effects.

C. Final Decision on the Secondary Lead Standard

    The secondary standard is defined in terms of four basic elements:
Indicator, averaging time, level and form, which serve to define the
standard and must be considered collectively in evaluating the welfare
protection afforded by the standards. With regard to the pollutant
indicator for use in a secondary NAAQS, EPA notes that Pb is a
persistent pollutant to which ecological receptors are exposed via
multiple pathways. While the evidence indicates that the environmental
mobility and ecological toxicity of Pb are affected by various