elicited the observed effects, as well as uncertainties in relating
ambient air concentrations (and associated air-related exposures) to
blood Pb levels in early childhood, as recognized in section II.A.2.b
above. We additionally recognize uncertainties associated with
conclusions drawn with regard to the nature of the epidemiological
associations with blood Pb (e.g., ISA, section 4.3.13) but note that,
based on consideration of the full body of evidence for neurocognitive
effects, the EPA has determined a causal relationship to exist between
relevant blood Pb levels and neurocognitive impacts in children (ISA,
section <IP_ADDRESS>).
    Based primarily on studies of FSIQ, the assessment of the currently
available studies, as was the case in the last review, continues to
recognize a nonlinear relationship between blood Pb levels and effects
on cognitive function, with a greater incremental effect (greater
slope) at lower relative to higher blood Pb levels within the range
thus far studied, extending from well above 10 [mu]g/dL to below 5
[mu]g/dL (ISA, section 4.3.12). This was supported by the evidence
available in the last review, including the analysis of the large
pooled international dataset comprised of blood Pb measurements and IQ
test results from seven prospective cohorts (Lanphear et al., 2005;
Rothenberg and Rothenberg, 2005; ISA, section 4.3.12). The blood Pb
measurements in this pooled dataset that were concurrent with the IQ
tests ranged from 2.5 [mu]g/dL to 33.2 [mu]g/dL.
    The study by Lanphear et al. (2005) additionally presented analyses
that stratified the dataset based on peak blood Pb levels (e.g., with
cutpoints of 7.5 [mu]g/dL and 10 [mu]g/dL peak blood Pb) and found that
the coefficients from linear models of the association for IQ with
concurrent blood Pb levels were higher in the lower peak blood Pb level
subsets than the higher groups (ISA, section 4.3.12; Lanphear et al.,
2005).\38\ In other publications, stratified analyses of several
individual cohorts also observed higher coefficients for blood Pb
relationships with measures of neurocognitive function in lower as
compared to higher blood Pb subgroups (ISA, section 4.3.12; Canfield et
al., 2003; Bellinger and Needleman, 2003; Kordas et al., 2006; Tellez-
Rojo et al., 2006). Of these subgroup analyses, those involving the
lowest mean blood Pb levels and closest to the current mean for U.S.
preschool children are listed in Table 1 of the proposal (drawn from
Table 3 of the 2008 preamble to the final rule [73 FR 67003, November
12, 2008], and Kirrane and Patel, 2014).\39\ These analyses were
important inputs for the air-related IQ loss evidence-based framework
which informed decisions on a revised standard in the last review (73
FR 67005, November 12, 2008), discussed in section II.A.1 above.
Specifically, the framework focused on the median of the four average
linear slope estimates from the studies recognized in Table 3 of the
2008 decision (73 FR 67003, November 12, 2008). As shown in Table 1 of
the proposal, the median is unchanged by

consideration of the information newly available in this review.\40\
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    \38\ As described in the PA and noted in the proposal, since the
completion of the ISA, two errors have been identified with the
pooled dataset analyzed by Lanphear et al. (2005) (Kirrane and
Patel, 2014). A recent publication and the EPA have separately
recalculated the statistics and mathematical model parameters of
Lanphear et al. (2005) using the corrected pooled dataset (see
Kirrane and Patel, 2014). While the magnitude of the loglinear and
linear regression coefficients are modified slightly based on the
corrections, the conclusions drawn from these coefficients,
including the finding of a steeper slope at lower (as compared to
higher) blood Pb concentrations, are not affected (Kirrane and
Patel, 2014).
    \39\ One of these four subgroup analyses is the analysis of the
lowest blood Pb subset of the pooled international study by Lanphear
et al. (2005). The nonlinear model developed from the full pooled
dataset is the basis of the C-R functions used in the 2007 REA, in
which risk was estimated over a large range of blood Pb levels (PA,
section <IP_ADDRESS>). Given the narrower focus of the evidence-based
framework on IQ response at the end of studied blood Pb levels
(closer to U.S. mean level), the C-R functions in Table 1 are from
linear analyses (each from separate publications) for the study
group subsets with blood Pb levels closest to mean for children in
the U.S. today.
    \40\ As the framework focused on the median of the four slopes
in Table 1, the change to the one from Lanphear et al. (2005) based
on the recalculation described above has no impact on conclusions
drawn from the framework.
---------------------------------------------------------------------------

    Several studies newly available in the current review have, in all
but one instance, also found a nonlinear blood Pb-cognitive function
relationship in nonparametric regression analyses of the cohort blood
Pb levels analyzed (ISA, section 4.3.12). These studies, however, used
statistical approaches that did not produce quantitative results for
each blood Pb group (ISA, section 4.3.12). Thus, newly available
studies have not extended the range of observation for quantitative
estimates of this relationship to lower blood Pb levels than those of
the previous review. The ISA further notes that the potential for
nonlinearity has not been examined in detail within a lower, narrower
range of blood Pb levels than those of the full cohorts thus far
studied in the currently available evidence base (ISA, section 4.3.12).
Such an observation in the last review supported the consideration of
linear slopes with regard to blood Pb levels at and below those
represented in Table 1 of the proposal. In summary, the newly available
evidence does not substantively alter our understanding of the C-R
relationship (including quantitative aspects) for neurocognitive
impact, such as IQ, with blood Pb in young children.
d. At-Risk Populations
    In this section, as elsewhere, we use the term ``at-risk
populations'' \41\ to recognize populations that have a greater
likelihood of experiencing Pb-related health effects, i.e., groups with
characteristics that contribute to an increased risk of Pb-related
health effects. These populations are also referred to as sensitive
groups (as in section I.A above). In identifying factors that increase
risk of Pb-related health effects, we have considered evidence
regarding factors contributing to increased susceptibility, generally
including physiological or intrinsic factors contributing to a greater
response for the same exposure and those contributing to increased
exposure, including that resulting from behavior leading to increased
contact with contaminated media (ISA, Chapter 5). Physiological risk
factors include both conditions contributing to a group's increased
risk of effects at a given blood Pb level and those that contribute to
blood Pb levels higher than those otherwise associated with a given Pb
exposure (e.g., ISA, sections 5.3 and 5.1, respectively).
---------------------------------------------------------------------------

    \41\ In the context of ``at-risk populations,'' the term
``population'' refers to persons having one or more qualities or
characteristics including, for example, a specific pre-existing
illness or a specific age or lifestage, with lifestage referring to
a distinguishable time frame in an individual's life characterized
by unique and relatively stable behavioral and/or physiological
characteristics that are associated with development and growth.
---------------------------------------------------------------------------

    In considering factors that increase risk by contributing to
increased exposure or to increased blood Pb levels over those otherwise
associated with a given Pb exposure, we note that the currently
available evidence continues to support a nonlinear relationship
between neurocognitive effects and blood Pb that indicates
incrementally greater impacts at lower as compared to higher blood Pb
levels (ISA, section 4.3.12), as described in section II.B.3 of the
proposal and briefly noted in section II.A.2.c above. An important
implication of this finding is that while children with higher blood Pb
levels are at greater risk of Pb-related effects than children with
lower blood Pb levels, on an incremental basis (e.g., per [mu]g/dL) the
risk is greater for children at lower blood Pb levels. This was given
particular attention in the last review of the Pb NAAQS, in which the
standard was revised with consideration of the incremental impact of
air-related Pb on young children in the U.S. and the recognition of
greater incremental impact for those children with lower absolute blood
Pb levels (73 FR 67002, November 12, 2008). Such consideration included
a focus on those C-R studies involving the lowest blood Pb levels, as
described in section II.A.1 above.
    The information newly available in this review has not appreciably
altered our previous understanding of at-risk populations for Pb in
ambient air. As in the last review, the factor most prominently
recognized to contribute to increased risk of Pb effects is childhood
(ISA, section 1.9.6). As discussed in section II.B.2 of the proposal
and briefly noted in section II.A.2.b above, while uncertainties remain
with regard to the role of Pb exposures during a particular age of life
in eliciting nervous system effects, such as cognitive function
decrements, the full evidence base continues to indicate prenatal and
early childhood lifestages as periods of increased Pb-related risk
(ISA, sections 4.3.11 and 4.3.15). Thus, in the current review, as at
the time of the last review of the Pb NAAQS, we recognize young
children as an important at-risk population, with sensitivity extending
to prenatal exposures and into childhood development.
    An additional physiological risk factor that contributes to
increased blood Pb levels is nutritional status, which can play a role
in Pb absorption from the gastrointestinal tract, with iron-, calcium-
and zinc-deficient diets contributing to increased Pb absorption and
associated blood Pb levels (ISA, sections <IP_ADDRESS>, 5.1, 5.3.10 and
5.4). Risk factors based on increased exposure include spending time in
proximity to sources of Pb to ambient air or other environmental media,
such as large active metals industries or locations of historical Pb
contamination (ISA, sections 1.9.6, 3.7.1, 5.2.5 and 5.4). Residential
factors associated with other sources of Pb exposure (e.g., leaded
paint or plumbing with Pb pipes or solder) are another exposure-related
risk factor (ISA, sections 3.7.1, 5.2.6 and 5.4). Additionally, some
races or ethnicities have been associated with higher blood Pb levels,
with differential exposure indicated in some cases as the cause (ISA,
sections 5.2.3 and 5.4).
    Lower socioeconomic status (SES) has been associated with higher Pb
exposure and higher blood Pb concentration in some study groups,
leading the ISA to conclude the evidence is suggestive for low SES as a
risk factor (ISA, sections 5.3.16, 5.2.4 and 5.4).\42\ Although the
differences in blood Pb levels, nationally, between children of lower
and higher income levels (as well as among some races or ethnicities)
have lessened, blood Pb levels continue to be higher among lower-income
children indicating higher exposure and/or greater influence of factors
independent of exposure, such as nutritional factors (ISA, sections
1.9.6, <IP_ADDRESS> and 5.4).\43\ The evidence is also suggestive of
increased risk associated with several other factors: older
adulthood,\44\ pre-

existing disease (e.g., hypertension), variants for certain genes and
increased stress (ISA, section 5.3.4).
---------------------------------------------------------------------------

    \42\ The approach used by the EPA in evaluating the evidence
regarding factors that may influence the risk of Pb-related health
effects is described in chapter 5 of the ISA.
    \43\ Although the evidence for SES continues to indicate
increased blood Pb levels in lower income children, its role with
regard to an increased health risk for the same blood Pb level is
unclear and its role generally with regard to Pb-related risk is
somewhat complicated. SES often serves as a marker term for one or a
combination of unspecified or unknown environmental or behavioral
variables. Further, it is independently associated with an adverse
impact on neurocognitive development, and a few studies have
examined SES as a potential modifier of the association of childhood
Pb exposure with cognitive function with inconsistent findings
regarding low SES as a potential risk factor.
    \44\ The ISA identifies older adulthood as a lifestage of
potentially greater risk of Pb-related health effects based
primarily on the evidence of increases in blood Pb levels during
this lifestage (ISA, sections <IP_ADDRESS>, <IP_ADDRESS>, and 5.4), as well as
observed associations of some cardiovascular and nervous system
effects with bone and blood Pb in older populations, with biological
plausibility for the role of Pb provided by experimental animal
studies (ISA, sections 4.3.5, 4.3.7 and 4.4). Exposure histories of
older adult study populations, which included younger years during
the time of leaded gasoline usage and other sources of Pb exposures
which were more prevalent in the past than today, are likely
contributors to their blood Pb levels (ISA, pp. lx-lxi; Figure 2-1
and sections 2.5.2, 3.3.5 and <IP_ADDRESS>).
---------------------------------------------------------------------------

    In summary, we recognize the sensitivity of the prenatal period and
several stages of childhood to an array of neurocognitive and
behavioral effects, and we particularly recognize young children as an
important at-risk population in light of current environmental exposure
levels. Age or lifestage was used to distinguish potential groups on
which to focus in the last review in recognition of its role in
exposure and susceptibility, and young children were the focus of the
REA in consideration of the health effects evidence regarding endpoints
of greatest public health concern and in recognition of effects on the
developing nervous system as a sentinel endpoint for public health
impacts of Pb. This identification continues to be supported by the
evidence available in the current review.
e. Potential Impacts on Public Health
    There are several potential public health impacts associated with
Pb exposure in the current U.S. population. In recognition of effects
causally related to blood Pb levels somewhat near those most recently
reported for today's population and for which the weight of the
evidence is greatest, the potential public health impacts most
prominently recognized in the ISA are population IQ impacts associated
with childhood Pb exposure and prevalence of cardiovascular effects in
adults (ISA, section 1.9.1). With regard to the latter category, as
discussed above, the full body of evidence indicates a role of long-
term cumulative exposure, with uncertainty regarding the specific
exposure circumstances contributing to the effects in the
epidemiological studies of adult populations, for whom historical Pb
exposures were likely much higher than exposures that commonly occur
today (ISA, section 4.4). There is less uncertainty regarding the
exposure patterns contributing to the blood Pb levels reported in
studies of younger populations (ISA, sections 1.9.4 and 1.10).
Accordingly, the discussion of public health implications relevant to
this review is focused predominantly on nervous system effects,
including IQ decrements, in children.
    The magnitude of a public health impact is dependent upon the type
or severity of the effect, as well as the size of populations affected.
Intelligence quotient is a well-established, widely recognized and
rigorously standardized measure of neurocognitive function, as well as
a global measure reflecting the integration of numerous processes (ISA,
section 4.3.2; 2006 CD, sections 6.2.2 and 8.4.2). In considering
population risk, the distribution of effects across members of the
population is important. For example, if Pb-related decrements are
manifested uniformly across the range of IQ scores in a population, ``a
small shift in the population mean IQ may be significant from a public
health perspective because such a shift could yield a larger proportion
of individuals functioning in the low range of the IQ distribution,
which is associated with increased risk of educational, vocational, and
social failure'' as well as a decrease in the proportion with high IQ
scores (ISA, section 1.9.1). Examples of other measures of cognitive
function negatively associated with Pb exposure include other measures
of intelligence and cognitive development and measures of other
cognitive abilities, such as learning, memory, and executive functions,
as well as academic performance and achievement (ISA, section 4.3.2).
Although some neurocognitive effects of Pb in children may be
transient, some may persist into adulthood (ISA, section 1.9.5).\45\ We
also note that deficits in neurodevelopment early in life may have
lifetime consequences as ``[n]eurodevelopmental deficits measured in
childhood may set affected children on trajectories more prone toward
lower educational attainment and financial well-being'' (ISA, section
4.3.14). Thus, population groups for which neurodevelopment is affected
by Pb exposure in early childhood are at risk of related impacts on
their success later in life.
---------------------------------------------------------------------------

    \45\ The ISA states that the ``persistence of effects appears to
depend on the duration and window of exposure as well as other
factors that may affect an individual's ability to recover from an
insult,'' with some evidence of greater recovery in children reared
in households with more optimal caregiving characteristics and low
concurrent blood Pb levels (ISA, p. 1-77; Bellinger et al., 1990).
---------------------------------------------------------------------------

    As indicated above, young children are the at-risk population that
may be most at risk of health effects associated with exposure to Pb,
and children at greatest risk from air-related Pb are those children
with highest air-related Pb exposure, which we consider to be those
living in areas of higher ambient air Pb concentrations (e.g.,
concentrations near or above the current standard). Analyses in the PA
indicate this group to be a very small subset of all young children in
the U.S. Together the analyses indicate that well below one-tenth of
one percent of the full population of children aged 5 years or younger
in the U.S. today live in areas with air Pb concentrations near or
above the current standard, with the current monitoring data indicating
the size of this population to be approximately one-hundredth of a
percent of the full population of children aged 5 or younger (PA, pp.
3-36 to 3-38, 4-25, 4-32). It is these children that were the
Administrator's focus in revising the primary standard in 2008.
3. Overview of Information on Blood Lead Relationships With Air Lead
    This section provides a brief overview of the information
summarized in section II.C of the proposal on key aspects of the
information available in this review on blood Pb as a biomarker and on
relationships of blood Pb with air Pb (80 FR 298-300, January 5, 2015).
Blood Pb is well established as a biomarker of Pb exposure and of
internal dose, with relationships between air Pb concentrations and
blood Pb concentrations informing consideration of the NAAQS for Pb
since its initial establishment in 1978. The blood Pb concentration in
childhood (particularly early childhood) can more quickly (than in
adulthood) reflect changes in total body burden (associated with the
shorter exposure history) and can also reflect changes in recent
exposures (ISA, section 3.3.5). The relationship of children's blood Pb
to recent exposure may reflect their labile bone pool, with their rapid
bone turnover in response to rapid childhood growth rates (ISA, section
3.3.5). The relatively smaller skeletal compartment of Pb in children
(particularly very young children) compared to adults is subject to
more rapid turnover. Multiple studies have demonstrated young
children's blood Pb levels to reflect Pb exposures, including exposures
to Pb in surface dust (e.g., Lanphear and Roghmann, 1997; Lanphear et
al., 1998). These and studies of child populations near sources of air
Pb emissions, such as metal smelters, have further demonstrated the
effect of airborne Pb on interior dust and on blood Pb (ISA, sections
3.4.1, 3.5.1 and 3.5.3; Hilts, 2003; Gulson et al., 2004).
    As blood Pb is an integrated marker of aggregate Pb exposure across
all pathways, the blood Pb C-R relationships described in
epidemiological studies of Pb-exposed populations do not distinguish
among different sources of Pb or pathways of

Pb exposure (e.g., inhalation, ingestion of indoor dust, ingestion of
dust containing leaded paint). Thus, our interpretation of the health
effects evidence for purposes of this review necessitates
characterization of the relationships between Pb from those sources and
pathways of interest in this review (i.e., those related to Pb emitted
into the air) and blood Pb.
    The evidence for air-to-blood relationships derives from analyses
of datasets for populations residing in areas with differing air Pb
concentrations, including datasets for circumstances in which blood Pb
levels have changed in response to changes in air Pb. The control for
variables other than air Pb that can affect blood Pb varies across
these analyses. At the conclusion of the last review in 2008, the EPA
interpreted the evidence as providing support for use (in informing the
Administrator's decision on standard level) of a range of air-to-blood
ratios \46\ ``inclusive at the upper end of estimates on the order of
1:10 and at the lower end on the order of 1:5'' (73 FR 67002, November
12, 2008). This conclusion reflected consideration of the air-to-blood
ratios presented in the 1986 CD \47\ and associated observations
regarding factors contributing to variation in such ratios, ratios
reported subsequently and ratios estimated based on modeling performed
in the REA, as well as advice from CASAC (73 FR 66973-66975, 67001-
67002, November 12, 2008). The information available in this review,
which is assessed in the ISA and largely, although not completely,
comprises studies that were available in the last review, does not
alter the primary scientific conclusions drawn in the last review
regarding the relationships between Pb in ambient air and Pb in
children's blood. The ratios summarized in the ISA in this review span
a range generally consistent with the range concluded in 2008 (ISA,
section 3.5.1).
---------------------------------------------------------------------------

    \46\ The quantitative relationship between ambient air Pb and
blood Pb, often termed a slope or ratio, describes the increase in
blood Pb (in [mu]g/dL) estimated to be associated with each unit
increase of air Pb (in [mu]g/m\3\). Ratios are presented in the form
of 1:x, with the 1 representing air Pb (in [mu]g/m\3\) and x
representing blood Pb (in [mu]g/dL). Description of ratios as higher
or lower refers to the values for x (i.e., the change in blood Pb
per unit of air Pb). Slopes are presented as simply the value of x.
    \47\ The 2006 CD did not include an assessment of then-current
evidence on air-to-blood ratios.
---------------------------------------------------------------------------

    The evidence on the quantitative relationship between air Pb and
air-related Pb in blood is now, as in the past, limited by the
circumstances (such as those related to Pb exposure) in which the data
were collected. Previous reviews have recognized the significant
variability in air-to-blood ratios for different populations exposed to
Pb through different air-related exposure pathways and at different air
and blood levels, with the 1986 CD noting that ratios derived from
studies involving the higher blood and air Pb levels pertaining to
occupationally exposed workers are generally smaller than ratios from
studies involving lower blood and air Pb levels (ISA, p. 3-132; 1986
CD, p. 11-99). Consistent with this observation, slopes in the range of
3 to 5 were estimated for child population datasets assessed in the
1986 CD (ISA, p. 3-132; 1986 CD p. 11-100; Brunekreef, 1984).
Additional studies considered in the last review and those assessed in
the ISA provide evidence of ratios above this older range (ISA, p. 3-
133). For example, a ratio of 1:6.5 to 1:7 is indicated by the study by
Hilts (2003), one of the few studies that evaluate the air Pb-blood Pb
relationship in conditions that are closer to the current state in the
U.S. (ISA, p. 3-132). We additionally note the variety of factors
identified in the ISA that may potentially affect estimates of various
ratios (including potentially coincident reductions in nonair Pb
sources during the course of the studies) and for which a lack of
complete information may preclude any adjustment of estimates to
account for their role (ISA, section 3.5).
    In summary, as at the time of the last review of the NAAQS for Pb,
the currently available evidence includes estimates of air-to-blood
ratios, both empirical and model-derived, with associated limitations
and related uncertainties. These limitations and uncertainties, which
are summarized here and also noted in the ISA, usually include
uncertainty associated with reductions in other Pb sources during the
study period. The limited amount of new information available in this
review has not appreciably altered the scientific conclusions reached
in the last review regarding relationships between Pb in ambient air
and Pb in children's blood or with regard to the range of ratios. The
currently available evidence continues to indicate ratios relevant to
the population of young children in the U.S. today, reflecting multiple
air-related pathways in addition to inhalation, to be generally
consistent with the approximate range of 1:5 to 1:10 given particular
attention in the 2008 NAAQS decision, including the ``generally central
estimate'' of 1:7 (73 FR 67002, 67004, November 12, 2008; ISA, pp. 3-
132 to 3-133).
4. Overview of Risk and Exposure Assessment Information
    This section provides a brief overview of key aspects of the risk
and exposure assessment information available in this review, which is
based primarily on the exposure and risk assessment developed in the
last review of the Pb NAAQS.\48\ This overview is drawn from the
summary presented in the proposal (80 FR 300-305, January 5, 2015). As
described in the REA Planning Document, careful consideration of the
information newly available in this review, with regard to designing
and implementing a full REA for this review, led to the conclusion that
performance of a new REA for this review was not warranted. We did not
find the information newly available in this review to provide the
means by which to develop an updated or enhanced risk model that would
substantially improve the utility of risk estimates in informing the
current Pb NAAQS review (REA Planning Document, section 2.3). Based on
its consideration of the REA Planning Document analysis, the CASAC Pb
Review Panel generally concurred with the conclusion that a new REA was
not warranted in this review (Frey, 2011b).\49\ Accordingly, the
exposure/risk information considered in this review is drawn primarily
from the 2007 REA, augmented by a limited new computation for one case
study focused on risk associated with the current standard, as
described in section II.D of the proposal and in section 3.4 and
Appendix 3A of the PA.
---------------------------------------------------------------------------

    \48\ The information in this review is based on the assessment
from the last review, described in the 2007 REA, the 2007 Staff
Paper and the 2008 notice of final decision (USEPA, 2007a; USEPA,
2007b; 73 FR 66964, November 12, 2008), as considered in the context
of the evidence newly available in this review (PA, section 3.4;
proposal, section II.D).
    \49\ In its review of the draft PA, the CASAC Pb Review Panel
reinforced its concurrence with the EPA's decision not to develop a
new REA (Frey, 2013).
---------------------------------------------------------------------------

    The focus for the risk assessment and associated estimates is on Pb
derived from sources emitting Pb to ambient air. In order to
characterize exposure and risk from these pathways, however, the
assessment also recognized the role of Pb exposure pathways unrelated
to Pb in ambient air (2007 REA, section 2.1). Sources of human Pb
exposure include current and historical air emissions sources, as well
as miscellaneous nonair sources, which can contribute to multiple
exposure media and associated pathways, such as inhalation of ambient
air, ingestion of indoor dust, outdoor soil/dust and diet or drinking
water (as recognized in section I.D above). In addition to airborne
emissions (recent or

those in the past), sources of Pb to these pathways also include old
leaded paint, including Pb mobilized indoors during renovation/repair
activities, and contaminated soils. Lead in diet and drinking water may
have air pathway-related contributions as well as contributions from
nonair sources (e.g., Pb solder on older water distribution pipes and
Pb in materials used in food processing).
    Limitations in our data and modeling tools handicapped our ability
to address the various complexities associated with exposure to ambient
air Pb and to fully separate the nonair contributions to Pb exposure
from estimates of air-related Pb exposure and risk. As a result, the
assessment included a number of simplifying assumptions in a number of
areas, and the estimates of air-related Pb risk produced are
approximate, characterized by bounds within which air-related Pb risk
is estimated to fall. The lower bound is based on a combination of
pathway-specific estimates that do not completely represent all air-
related pathways, while the upper bound is based on a combination of
pathway-specific estimates that includes pathways that are not air-
related but the separating out of which is precluded by modeling and
data limitations (PA, section 3.4).
    Key aspects of the 2007 REA, such as the exposure populations,
exposure or dose metric, health effects endpoint and risk metric were
based on consideration of the then-currently available evidence as
assessed in detail in the 2006 CD. As discussed in the REA Planning
Document (USEPA, 2011b), these selections continue to be supported by
the evidence now available in this review as described in the ISA. The
REA focused on risk to the central nervous system in childhood as the
most sensitive effect that could be quantitatively assessed, with
decrement in IQ used as the risk metric. Exposure and biokinetic
modeling was used to estimate blood Pb concentrations in children
exposed to Pb up to age 7 years.\50\ This focus reflected the evidence
for young children with regard to air-related exposure pathways and
susceptibility to Pb health impacts (e.g., ISA, sections 3.1.1, 4.3,
<IP_ADDRESS>, <IP_ADDRESS>, and 5.4). For example, the hand-to-mouth activity of
young children contributes to their Pb exposure (i.e., incidental soil
and indoor dust ingestion), and ambient air-related Pb has been shown
to contribute to Pb in outdoor soil and indoor house dust (ISA,
sections 3.1.1 and 3.4.1; 2006 CD, section 3.2.3).
---------------------------------------------------------------------------

    \50\ The pathways represented in this modeling included
childhood inhalation and ingestion pathways, as well as maternal
contributions to newborn body burden (2007 REA, Appendix H, Exhibit
H-6).
---------------------------------------------------------------------------

    The 2007 REA relied on a case study approach to provide estimates
that inform our understanding of air-related exposure and risk in
different types of air Pb exposure situations. Lead exposure and
associated risk were estimated for multiple case studies that generally
represent two types of residential population exposures to air-related
Pb: (1) Location-specific urban populations of children with a broad
range of air-related exposures, reflecting existence of urban
concentration gradients; and (2) children residing in localized areas
with air-related exposures representing air concentrations specifically
reflecting the standard level being evaluated (see PA, Table 3-6).
Thus, the two types of case studies differed with regard to the extent
to which they represented population variability in air-related Pb
exposure.
    In drawing on the 2007 REA for our purposes in this review, we
focused on two case studies, one from each of these two categories: (1)
The location-specific urban case study for Chicago and (2) the
generalized (local) urban case study (PA, Table 3-6). The generalized
(local) urban case study (also referred to as general urban case study)
was not based on a specific geographic location and reflected several
simplifying assumptions in representing exposure including uniform
ambient air Pb levels associated with the standard of interest across
the hypothetical study area and a uniform study population. Based on
the nature of the population exposures represented by the two
categories of case study, the generalized (local) urban case study
includes populations that are relatively more highly exposed by way of
air pathways to air Pb concentrations near the standard level
evaluated, compared with the populations in the location-specific urban
case. The location-specific urban case studies provided representations
of urban populations with a broad range of air-related exposures due to
spatial gradients in both ambient air Pb levels and population density.
For example, the highest air concentrations in these case studies
(i.e., those closest to the standard being assessed) were found in very
small parts of the study areas, while a large majority of the case
study populations resided in areas with much lower air concentrations.
    Air-related risk estimates for the two case studies are accompanied
by a number of uncertainties (summarized in section II.D.3 of the
proposal and described in detail in section 3.4 of the PA). Exposure
and risk modeling conducted for this analysis was complex and subject
to significant uncertainties due to limitations in the data and models,
among other aspects, as recognized at the time of the last review.\51\
The multimedia and persistent nature of Pb, the role of multiple
exposure pathways, and the contributions of nonair sources of Pb to
human exposure media all present challenges and contribute significant
additional complexity to the health risk assessment that goes far
beyond the situation for similar assessments typically performed for
other NAAQS pollutants (e.g., that focus only on the inhalation
pathway). Of particular note among the assessment limitations are
limitations in the assessment design, data and modeling tools that
handicapped us from sharply separating Pb linked to ambient air from Pb
that is not air related. The resultant, approximate, air-related risk
bounds, however, encompass estimates drawn from the air-related IQ loss
evidence-based framework, providing a rough consistency and general
support, as was the case in the last review (73 FR 67004, November 12,
2008).
---------------------------------------------------------------------------

    \51\ As summarized in section II.D.3 of the proposal, a range of
limitations and areas of uncertainty were associated with the
information available in the last review (PA, sections 3.4.4, 3.4.6
and 3.4.7), and the newly available information in this review did
not substantially reduce any of the primary sources of uncertainty
identified to have the greatest impact on risk estimates (USEPA,
2011b). Thus, the key observations regarding air-related Pb risk
modeled for the set of standard levels assessed in the 2007 REA, as
well as the risk estimates interpolated for the current standard,
are not significantly affected by the new information. Nor is our
overall characterization of uncertainty and variability associated
with those estimates (as summarized above and in sections 3.4.6 and
3.4.7 of the PA).
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B. Conclusions on the Primary Standard

    In drawing conclusions on the adequacy of the current primary Pb
standard, in view of the advances in scientific knowledge and
additional information now available, the Administrator considers the
evidence base, information and policy judgments that were the
foundation of the last review and reflects upon the body of evidence
and information newly available in this review. The Administrator has
taken into account both evidence-based and exposure- and risk-based
considerations, advice from CASAC and public comment. Evidence-based
considerations draw upon the EPA's assessment and integrated synthesis
of the scientific evidence from epidemiological studies and
experimental animal studies evaluating health effects related to
exposures to Pb,

with a focus on policy-relevant considerations as discussed in the PA.
The exposure- and risk-based considerations draw from the results of
the quantitative analyses presented in the 2007 REA (augmented as
described in the PA and summarized in section II.D of the proposal) and
consideration of those results in the PA.
    As described in section II.A.2 of the proposal, consideration of
the evidence and exposure/risk information in the PA and by the
Administrator is framed by consideration of a series of key policy-
relevant questions. Section II.B.1 below summarizes the rationale for
the Administrator's proposed decision, drawing from section II.E.4 of
the proposal. A fuller presentation of PA considerations and
conclusions, and advice from the CASAC, which were taken into account
by the Administrator, is provided in sections II.E.1 through II.E.3 of
the proposal. Advice received from CASAC in this review is briefly
summarized in section II.B.2 below, and public comments on the proposed
decision are addressed in section II.B.3. The Administrator's
conclusions in this review regarding the adequacy of the current
primary standard are described in section II.B.4.
1. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered
the assessment of the current evidence and conclusions reached in the
ISA; the currently available exposure/risk information, including
associated limitations and uncertainties; considerations and staff
conclusions and associated rationales presented in the PA; the advice
and recommendations from CASAC; and public comments that had been
offered up to that point. In reaching her proposed conclusion on the
primary standard, the Administrator first took note of the PA
discussion with regard to the complexity and associated uncertainties
involved in considering the adequacy of protection in the case of the
primary Pb standard, which differs substantially from that involved in
consideration of the primary standard in other NAAQS reviews. For the
pollutants in the other reviews, the focus is on inhalation as the
single route of exposures, which provides a relatively simpler context
than the multiple exposure pathways that are relevant to Pb.
Additionally, an important component of the evidence base for most
other NAAQS pollutants is the availability of studies that have
investigated an association between concentrations of the pollutant in
ambient air and the occurrence of health effects plausibly related to
ambient air exposure to that pollutant. Such studies of associations
with air concentrations do not figure prominently in the review of the
NAAQS for Pb. Rather, the evidence base in this review includes most
prominently epidemiological studies focused on associations of blood Pb
levels in U.S. populations with health effects plausibly related to Pb
exposures occurring by multiple pathways. Support for conclusions
regarding the plausibility for ambient air Pb to play a role in such
findings derives, in part, from studies linking Pb in ambient air with
the occurrence of health effects. However, such studies (dating from
the past or from other countries) involve ambient air Pb concentrations
many times greater than those that would meet the current standard.
Thus, in considering the adequacy of the current Pb standard, rather
than considering studies that have directly investigated current
concentrations of Pb in ambient air (including in locations where the
current standard is met) and the occurrence of health effects, we
primarily consider the evidence for, and risk estimated from, models
based upon key relationships, such as those among ambient air Pb, Pb
exposure, blood Pb and health effects. This evidence, with its
associated limitations and uncertainties, contributes to the EPA's
conclusions regarding a relationship between ambient air Pb conditions
under the current standard and health effects.
    In considering the nature and magnitude of the array of
uncertainties that are inherent in the scientific evidence and
analyses, the Administrator recognized that the current understanding
of the relationships between the presence of a pollutant in ambient air
and associated health effects is based on a broad body of information
encompassing not only more established aspects of the evidence, but
also aspects in which there may be substantial uncertainty. In her
considerations for the proposal, she took into account both the well-
established body of evidence on the health effects of Pb, which
continues to support identification of neurocognitive effects in young
children as the most sensitive endpoint associated with Pb exposure,
and of the recognition in the PA, with which the CASAC concurred, of
increased uncertainty in characterizing the relationship of effects on
IQ with blood Pb levels below those represented in the evidence base
and also in projecting the magnitude of blood Pb response to ambient
air Pb concentrations at and below the level of the current standard.
In this light, she based her proposed decision on her consideration of
the current evidence within the conceptual and quantitative context of
the air-related IQ evidence-based loss framework; the available
information and advice from CASAC regarding the public health
significance of neurocognitive effects; and the limitations and
uncertainties inherent in the evidence and its consideration within
this framework. The Administrator additionally recognized support from
the exposure/risk information, with its attendant uncertainties.
    In her consideration of the air-related IQ loss evidence-based
framework, the Administrator took note of the PA finding, with which
the CASAC concurred, that application of the air-related IQ loss
evidence-based framework, developed in the last review, continues to
provide a useful approach for considering and integrating the evidence
on relationships between Pb in ambient air and Pb in children's blood
and risks of neurocognitive effects (for which IQ loss is used as an
indicator). She additionally took note of the PA finding (described in
section II.E.1 of the proposal, and with which the CASAC concurred)
that the currently available evidence base, while somewhat expanded
since the last review, is not supportive of appreciably different
conclusions with regard to air-to-blood ratios or C-R functions for
neurocognitive decrements in young children.
    In the Administrator's consideration of the level of public health
protection provided by the current standard, she gave weight to CASAC
advice in the last review (and similar views expressed in the last
review by public health experts, such as the American Academy of
Pediatrics), which recognized a population mean IQ loss of 1 to 2
points to be of public health significance and recommended that a very
high percentage of the population be protected from such a magnitude of
IQ loss (73 FR 67000, November 12, 2008). In so doing, she additionally
noted that the EPA is aware of no new information or new commonly
accepted guidelines or criteria within the public health community for
interpreting public health significance of neurocognitive effects in
the context of a decision on adequacy of the current Pb standard, and
CASAC provided no alternate advice in this area in the current review
(PA, pp. 4-33 to 4-34). Accordingly, with the objective identified in
the CASAC advice from the 2008 review in

mind, the Administrator considered the role of the air-related IQ loss
evidence-based framework in reviewing the level of protection provided
by the current standard. In so doing, the Administrator recognized
distinctions between estimates produced by the framework, for which the
conceptual context is a subset of U.S. children, and specific
quantitative public health policy goals for air-related IQ loss for the
entire U.S. population of children. She additionally took note of the
PA conclusion on the size of the population subset that might pertain
to the situation represented by the framework (areas with elevated air
Pb concentrations equal to the standard level), as well as
uncertainties associated with the framework estimates, particularly at
successively lower standard levels. In summary, the Administrator
concluded in the proposal that the current evidence, as considered
within the conceptual and quantitative context of the evidence-based
framework, and current air monitoring information indicate that the
current standard provides protection for young children from
neurocognitive impacts, including IQ loss, consistent with advice from
CASAC regarding IQ loss of public health significance.
    The Administrator based her proposed conclusions on consideration
of the health effects evidence, including consideration of this
evidence in the context of the air-related IQ loss evidence-based
framework, and with support from the exposure/risk information,
recognizing the uncertainties attendant with both. In so doing, she
took note of the PA description of the complexities and limitations in
the evidence base associated with reaching conclusions regarding the
magnitude of risk associated with the current standard, as well as the
increasing uncertainty of risk estimates for lower air Pb
concentrations. Inherent in the Administrator's proposed conclusions
are public health policy judgments on the public health implications of
the blood Pb levels and risk estimated for air-related Pb under the
current standard, including the public health significance of the Pb
effects being considered, as well as aspects of the use of the
evidence-based framework that may be considered to contribute to the
margin of safety. These public health policy judgments include
judgments related to the appropriate degree of public health protection
that should be afforded to protect against risk of neurocognitive
effects in at-risk populations, such as IQ loss in young children, as
well as with regard to the appropriate weight to be given to differing
aspects of the evidence and the exposure/risk information, and how to
consider their associated uncertainties. Based on these considerations
and the judgments summarized here, the Administrator proposed to
conclude that the current standard provides the requisite protection of
public health with an adequate margin of safety, including protection
of at-risk populations, such as young children living near Pb emissions
sources where ambient concentrations just meet the standard.
    The Administrator's proposed conclusion that the current standard
provides the requisite protection and that a more restrictive standard
would not be requisite additionally recognized that the uncertainties
and limitations associated with many aspects of the estimated
relationship between air Pb concentrations and blood Pb levels and
associated health effects are amplified with consideration of
increasingly lower air concentrations. In reaching her proposed
conclusion, she took note of the PA conclusion, with which CASAC has
agreed, that based on the current evidence, there is appreciable
uncertainty associated with drawing conclusions regarding whether there
would be reductions in blood Pb levels and risk to public health from
alternative lower levels of the standard as compared to the level of
the current standard (PA, pp. 4-35 to 4-36; Frey, 2013b, p. 6). The
Administrator judged this uncertainty to be too great for the current
evidence and exposure/risk information to provide a basis for revising
the current standard. Thus, based on the public health policy judgments
described above, including the weight given to uncertainties in the
evidence, the Administrator proposed to conclude that the current
standard should be retained, without revision.
2. CASAC Advice in This Review
    In comments on the draft PA, the CASAC concurred with staff's
overall preliminary conclusions that it is appropriate to consider
retaining the current primary standard without revision, stating that
``the current scientific literature does not support a revision to the
Primary Lead (Pb) National Ambient Air Quality Standard (NAAQS)''
(Frey, 2013b, p. 1). The CASAC further noted that ``[a]lthough the
current review incorporates a substantial body of new scientific
literature, the new literature does not justify a revision to the
standards'' (Frey, 2013b, p. 1).
    The CASAC comments additionally indicated agreement with key
aspects of staff's consideration of the exposure/risk information and
currently available evidence in this review (Frey, 2013b, Consensus
Response to Charge Questions, p. 7).

    The use of exposure/risk information from the previous Pb NAAQS
review appears appropriate given the absence of significant new
information that could fundamentally change the interpretation of
the exposure/risk information. This interpretation is reasonable
given that information supporting the current standard is largely
unchanged since the current standard was issued.
    The CASAC agrees that the adverse impact of low levels of Pb
exposure on neurocognitive function and development in children
remains the most sensitive health endpoint, and that a primary Pb
NAAQS designed to protect against that effect will offer
satisfactory protection against the many other health impacts
associated with Pb exposure.
    The CASAC concurs with the draft PA that the scientific findings
pertaining to air-to-blood Pb ratios and the C-R relationships
between blood Pb and childhood IQ decrements that formed the basis
of the current Pb NAAQS remain valid and are consistent with current
data.

    The CASAC concurred with the appropriateness of the application of
the evidence-based framework from the last Pb NAAQS review. With regard
to the key inputs to that framework, the CASAC concluded that ``[t]he
new literature published since the previous review provides further
support for the health effect conclusions presented in that review''
and that the studies newly available in this review ``do not
fundamentally alter the uncertainties for air-to-blood ratios or C-R
functions for IQ decrements in young children'' (Frey, 2013b, Consensus
Response to Charge Questions, p. 6). The comments from the CASAC also
took note of the uncertainties that remain in this review which
contribute to the uncertainties associated with drawing conclusions
regarding air-related exposures and associated health risk at or below
the level of the current standard, stating agreement with ``the EPA
conclusion that `there is appreciable uncertainty associated with
drawing conclusions regarding whether there would be reductions in
blood Pb levels from alternative lower levels as compared to the level
of the current standard''' (Frey, 2013b, Consensus Response to Charge
Questions, p. 6).
3. Comments on the Proposed Decision
    The majority of public comments on the proposal supported the
Administrator's proposed decision to retain the current primary
standard, without revision. This group includes the National
Association of Clean Air

Agencies (NACAA), both of the state agencies that submitted comments
and nearly all of the industry organizations that submitted comments.
All of these commenters generally noted their agreement with the
rationale provided in the proposal and noted the CASAC's concurrence
with the EPA conclusion that the current evidence does not support
revision to the standard. Most also cited the EPA and CASAC statements
that information newly available in this review has not substantially
altered our previous understanding of at-risk populations, C-R
relationships or effects from exposures lower than what was previously
examined and does not call into question the adequacy of the current
standard. Some commenters stated that multimedia or multipathway
aspects of Pb make the review of the primary standard for Pb subject to
greater uncertainty than reviews of primary NAAQS for other pollutants
and/or noted greater uncertainty with consideration of lower blood Pb
and standard levels. Some also noted that EPA's task in setting NAAQS
is not to reduce risk to zero but to identify a standard that is
neither more nor less stringent than necessary. The EPA generally
agrees with these commenters and with the CASAC regarding the adequacy
of the current primary standard and the lack of support for revision of
the standard.
    Four submissions recommending revision of the standard were
received; all four advocated a tightening of the standard. These
commenters include two individuals, a secondary Pb smelting company,
and the Children's Health Protection Advisory Committee to the EPA
(CHPAC).\52\ In support of their view that the standard should be
revised, all four commenters generally stated that there is no safe
level of Pb exposure.\53\ The CHPAC submission, to which the smelting
company submission repeatedly cited, asserted that a lower standard is
needed to protect children from impacts related to neurodevelopmental
and low birthweight effects, stating that studies it cited that have
been published since the cut-off for the ISA indicate effects on
children's IQ at ``appreciably lower'' Pb exposures than those
recognized in the last review and raise concerns regarding cumulative
effects of multiple chemical exposures. These commenters additionally
cited the PA's presentation of the 2007 REA results that included lower
risk estimates for alternative more stringent standards, stating that
minority and low-income groups are more greatly impacted by Pb, and
that for these reasons the standard should be lowered. The CHPAC
submission also suggests consideration of some transient sources to
provide support for a more stringent standard. Among the reasons given
for their recommendations to substantially lower the standard level,
the individual commenters variously stated that not revising or
lowering the standard will allow increases in air Pb in locations near
some sources of Pb emissions, such as airports, and that the
persistence of Pb indicated the need for a more stringent standard.
---------------------------------------------------------------------------

    \52\ As described in its charter, the CHPAC is a policy-oriented
committee providing policy advice to EPA related to the development
of regulations, guidance and policies to address children's
environmental health, consistent with provisions of the Federal
Advisory Committee Act (http://www.epa.gov/faca/childrens-health-protection-advisory-committee-charter-september-11-2015). The role
and scope of activities for the CHPAC differs from those of the
CASAC, which is the independent scientific review committee
fulfilling the function described in the CAA of reviewing the air
quality criteria and the NAAQS for protection of public health and
welfare and making recommendations to the Administrator concerning
revisions as may be appropriate (as described in section 109(d)(2)
of the Act and summarized in section I.A above).
    \53\ In expressing this view, some commenters cited statements
by various government agencies regarding their interpretation of
children's blood Pb levels with regard to risk management decisions
based on consideration of the available information in those risk
management contexts (e.g., CDC, 2005; Cal EPA, 2007; NYDHMH, 2010).
The scientific information on health effects of Pb considered by
these agencies was also available and, to the extent relevant to
consideration of the adequacy of the NAAQS, was assessed in the
current and, in some cases, also the prior review. As discussed
below, the conclusion that a threshold level for neurocognitive
effects has not been identified was a consideration of the EPA in
the last review, and the current one.
---------------------------------------------------------------------------

    The four commenters that supported revision of the standard
suggested a wide array of alternatives. The CHPAC repeated the view it
expressed in the 2008 review that the standard should be revised to the
most stringent alternative analyzed in the 2007 REA (a potential
standard with an averaging time of one month and a level of 0.02
[micro]g/m\3\). One individual commenter expressed a preference for a
standard level of 0.0005 [micro]g/m\3\. Another individual commenter
urged revision to the lowest feasible standard, and the smelting
company recommended that EPA adopt an approach similar to a local air
quality management district's emissions standards regulation \54\ that
requires air monitoring at large Pb acid battery recycling metal
melting facilities to meet, by a future date, a 30-day average Pb
concentration of 0.1 [micro]g/m\3\, which the company indicated its
technology can address.
---------------------------------------------------------------------------

    \54\ This commenter referred to a March 2015 amendment of a
California South Coast Air Quality Management District rule on
emission standards for lead and other toxic air contaminants from
large lead-acid battery recycling facilities in that state air
quality district.
---------------------------------------------------------------------------

    We agree with commenters that a threshold level for neurocognitive
effects has not been identified in the current evidence, as stated in
section II.A.2.c above, and described in more detail in the ISA. We
additionally note that the lack of an established threshold of effects
is not uncommon among the criteria pollutant evidence bases. For
example, in past reviews of the primary standards for ozone and
particulate matter, the EPA has recognized that the available
epidemiological evidence neither supports nor refutes the existence of
thresholds at the population level, while noting uncertainties and
limitations in studies that make discerning thresholds in populations
difficult (e.g., 73 FR 16444, March 27, 2008; 71 FR 61158, October 17,
2006). The lack of a discernible threshold of exposure associated with
health effects does not of itself provide support for revision of an
existing standard or for revision to the most stringent standard one
might identify. As recognized in section I.A above, the CAA does not
require the Administrator to establish a primary national ambient air
quality standard at a zero-risk level or at background concentrations
(Lead Industries v. EPA, 647 F.2d at 1156 n.51; Mississippi v. EPA, 744
F. 3d at 1351), but rather at a level that reduces risk sufficiently so
as to protect public health with an adequate margin of safety, and the
selection of any particular approach for providing an adequate margin
of safety is a policy choice left specifically to the Administrator's
judgment (Lead Industries Association v. EPA, 647 F.2d at 1161-62;
Mississippi, 744 F. 3d at 1353). The CAA requirement in establishing a
standard is that it be set at a level of air quality that is requisite,
meaning ``sufficient, but not more than necessary'' (Whitman v.
American Trucking Ass'ns, 531 U.S. 457, 473 [2001]).
    In the setting of the current standard in 2008, a key consideration
of the Administrator was the recognition of the lack of a discernible
threshold level in the evidence with respect to neurocognitive effects
associated with Pb exposure. This recognition, which differed from the
scientific consensus at the time the previous standard was set in 1978,
led the Administrator in 2008 to depart from the threshold-based
approach used in setting the 1978 standard and to focus on
consideration of air-related Pb in the context of the air-related IQ
loss evidence-based framework (described in section II.A.1

above). In the current review of the 2008 standard, while recognizing
the continued lack of a discernible threshold of exposure associated
with neurocognitive effects, the CASAC commented regarding effects at
very low Pb levels when expressing its view that the scientific
evidence does not support revision to the Pb NAAQS. It stated that
``[a]lthough there is evidence that even very low Pb levels are related
to measurable reductions in IQ in children, the extent to which the
blood Pb levels observed in children are linked to ambient air Pb
levels below the current standard (as opposed to other sources of Pb in
the environment) has not been established'' (Frey, 2013b, Consensus
Response to Charge Questions, pp. 7-8).\55\
---------------------------------------------------------------------------

    \55\ The CASAC recognized the multimedia and legacy aspects of
Pb that, unlike the case for other criteria air pollutants,
complicate consideration of the risks of Pb concentrations in
ambient air (Frey, 2013b, p. 1).
---------------------------------------------------------------------------

    The four submissions recommending a revised standard variously cite
a number of studies as providing support for their view. Some of these
studies have been reviewed in the ISA, some were published too late to
be included in the ISA, and a few others were of a type that are not
generally included in the ISA (e.g., review articles).\56\ As discussed
in section I.C above, we have provisionally considered studies that
were not in the ISA or in previous AQCDs (``new'' studies) \57\ which
some of these commenters cite in statements about evidence of effects
at low exposures and in the presence of other pollutants. We conclude
that these studies are consistent with the scientific conclusions
reached in the ISA, including those related to blood Pb levels in
studies from which effects on IQ have been reported and related to co-
exposure with other metals. Taken in context, the information from
these studies and these findings do not materially change any of the
broad scientific conclusions of the ISA regarding the health effects
and exposure pathways of Pb in ambient air on which the Administrator
based her proposed conclusions as well as her final conclusions in this
review, as described in section II.B.4 below. We additionally note that
with regard to the inputs for the air-related IQ loss evidence-based
framework, a key aspect of the Administrator's rationale for her
proposed decision to retain the current primary standard (as described
in section II.E.4 of the proposal), none of the cited studies indicate
a steeper blood Pb-IQ slope or greater air-to-blood ratio than those
assessed in the ISA and considered in the PA and the proposal.
---------------------------------------------------------------------------

    \56\ Some studies cited by commenters are review articles or
government reviews (e.g., Henn et al., 2014; Grandjean and
Landrigan, 2014; Jakubowski, 2011; NTP, 2011), which are not
generally cited in the ISA because the ISA considers the original
studies underlying a review article, rather than a review's
interpretation of the studies. Further, in the case of government
reviews, such reports generally review the literature for specific
purposes of those government agencies (which differ from the focus
for the ISA). Many of the scientific studies reviewed in these
reports (as well as the other reviews), however, were considered
relevant to review of the lead air quality criteria (based on the
description of study selection for inclusion in the preamble to the
ISA), and thus were assessed in this review.
    \57\ These studies are listed in a memorandum to the rulemaking
docket (Kirrane, 2016).
---------------------------------------------------------------------------

    We respectfully disagree with the comment from CHPAC that studies
available since the cut-off date for the ISA contradict the PA
conclusions regarding blood Pb levels in children and effects on
cognitive function measures, such as IQ.\58\ Of the studies cited in
the comment that were published subsequent to the date for publication
in the ISA, one is an analysis that relies on data from studies that
were published prior to 2008 and assessed in the last review (Budtz-
Jorgensen et al., 2013). These data were the subject of the pooled
analysis by Lanphear et al (2005) which we assessed in both the last
and the current review. As such, this commenter-cited publication does
not present a new study of children with lower blood Pb levels; rather,
it reanalyzes existing data using a different approach for a different
purpose.\59\ The other two of the commenter-cited publications are
review articles that do not present information on specific blood Pb
levels associated with IQ effects. Thus, we do not find these
publications to be contrary to the discussion and associated
conclusions in the PA or to indicate the current standard to be
inadequate.
---------------------------------------------------------------------------

    \58\ The PA recognized the complexity associated with
considering the evidence regarding exposure levels associated with
health effects, and in particular effects on cognitive function
measures, including IQ, which the evidence base indicates to be the
most sensitive endpoint. The PA observed that the evidence available
in this review is generally consistent with that available in the
last review with regard to blood Pb levels in young children at
which such effects have been reported. Noting that blood Pb levels
are a reflection of exposure history, particularly in early
childhood, the PA concludes by extension that the currently
available evidence does not indicate Pb effects at exposure levels
appreciably lower than recognized in the last review. In so doing,
the PA continued to focus in this review (as in the last review) on
the evidence of effects in young children for which our
understanding of exposure history is less uncertain (PA, pp. 3-21 to
3-26).
    \59\ This analysis uses the data from the same studies analyzed
by Lanphear et al (2005) to extrapolate below the blood Pb
concentrations measured in the studies and estimate a 95 percent
lower confidence bound on the estimated blood Pb concentration
associated with a 1 point decrement in IQ (Budtz-Jorgensen et al.,
2013). Unlike the prior study by Lanphear et al (2005) and similar
epidemiological analyses of IQ and blood Pb, which are intended to
produce a quantitative description of the change in IQ associated
with blood Pb concentrations in the studied children, this analysis
is focused on estimating a lower bound confidence limit on the
incremental concentration in blood Pb, as compared to zero,
associated with a single point IQ decrement. Even if we were to
interpret the results of the Budtz-Jorgensen et al (2013) analysis
as providing another estimate of C-R function for IQ decrement based
on the pooled dataset from Lanphear et al (2005), we note that that
dataset is already represented among the four low blood Pb analyses
on which we focused in identifying a slope estimate for use with the
air-related IQ loss evidence-based framework, and as noted in
section II.B.3 of the proposal, revision or replacement of the
estimate for the pooled dataset has no impact on conclusions drawn
from the framework (80 FR 29295, January 5, 2015).
---------------------------------------------------------------------------

    We further disagree with the suggestion in the CHPAC submission
that the evidence related to co-exposures to other pollutants, such as
metals, provides a basis for concluding that the current standard is
not requisite. The ISA assessment of the strength of the evidence for
co-exposures to other pollutants, such as other metals, to contribute
to increased risk of a Pb-related health effects concluded the evidence
to be suggestive, ``but overall the evidence was limited'' (ISA,
sections 1.9.6 and 5.4). With regard to the articles cited by the CHPAC
that have been published subsequent to the ISA, the general conclusions
of these review articles (Henn et al., 2014; Grandjean and Landrigan,
2014) are consistent with conclusions of the ISA. As stated in the ISA,
``interactions between Pb and co-exposure with other metals were
evaluated in recent epidemiologic and toxicological studies of health
effects'' and ``[h]igh levels of other metals, such as Cd and Mn, were
observed to result in greater effects for the associations between Pb
and various health endpoints but evidence was limited due to the small
number of studies'' (ISA, p. 5-43). We note that even in raising co-
exposure as a concern, the comments recognize that the potential for
such impacts is not well understood. Further, the comments do not
explain how the limited information regarding this factor supports
their conclusion that the current standard does not provide the
requisite protection or leads to the specific revisions the comments
suggest, and we find no such support in the current evidence.
    We additionally disagree with the comment that the currently
available evidence indicates that the current standard is not
protective of effects such as low birth weight. For example, the

CHPAC cites epidemiological studies reporting associations of maternal
or cord blood Pb concentrations with reduced fetal growth (Xie et al.,
2013; Nishioka et al., 2014), stating that these studies strengthen the
association of decreased birth weight and maternal blood Pb levels.
Although we would agree that these studies present an addition to the
evidence base overall, they do not provide a basis for change in the
conclusion of the ISA, which states, ``Some well-conducted
epidemiologic studies report associations of maternal Pb biomarkers or
cord blood Pb with preterm birth and low birth weight/fetal growth;
however, the epidemiologic evidence is inconsistent overall and
findings from experimental animal studies are mixed'' (ISA, p. 1-18).
In citing these studies, in fact, the CHPAC also stated its view that
the findings of these studies are consistent with a larger study that
was assessed in the ISA; it did not explain how these studies support
its view that the current standard provides inadequate protection from
such effects, and we find no such support.
    With regard to information related to Pb impacts in minority and
low-income populations, which some comments suggested provided a basis
for a more stringent standard, we note that we have considered the
available information on such impacts, as recognized in section
II.A.2.d above and summarized more fully in section II.B.4 of the
proposal and in section 3.3 of the PA. As all of these documents have
recognized, the ISA identifies non-white populations as at-risk
populations, with this conclusion based primarily on findings of higher
blood Pb levels in black compared to white populations (ISA, section
5.4).\60\ Blood Pb levels have also been found to be higher in low SES
groups as compared to higher SES \61\ (ISA, sections 5.3.6, 5.2.4 and
5.4). However, as noted in the ISA, the number of studies examining the
relationship of SES with Pb-related health effects is limited, and the
results have differed with regard to finding increased risk with higher
or lower SES (ISA, Table 5-1, p. 5-42). The comments generally identify
impacts in minority and low income groups as a reason EPA should revise
the standard, although they provide no explanation for how the
currently available information leads to that conclusion or provides a
basis for the alternative standards the comments suggest. \62\ While
our assessment of the health effects evidence in this review concluded
there was adequate evidence for race or ethnicity (and suggestive
evidence for SES) to contribute to increased risk of Pb-related health
effects, we do not find this information to call into question the
adequacy of protection provided by the current primary standard. Nor
did the CASAC find this to be the case, based on its review of the
scientific materials in this review, including three drafts of the ISA
in which the evidence for these factors was presented. Further, to the
extent such differences may be related to exposure contributions from
air Pb and proximity to air sources,\63\ we note that children that are
exposed to air-related Pb in areas with elevated air Pb concentrations
near or equal to the level of the standard are among those that were
the focus of the 2008 decision, as recognized in sections II.A.1 and
II.A.2.e above, and are the focus of the decision described in section
II.B.4 below to retain the standard set in 2008.\64\
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    \60\ Recent data suggest that differences in blood Pb levels
between young black and white children is decreasing over time (ISA,
section 5.2.3, 5.4). Although more recent data are not available by
age group, the CDC data through 2011-2012 indicate little or no
difference between non-Hispanic blacks, Mexican Americans or all
Hispanics and non-Hispanic whites at the central tendencies of the
populations and reduced differences at the 95th percentile (CDC,
2015). Findings of some studies indicate that non-white populations
may be at greater risk of Pb-related health effects although, as
described in the ISA, this could be related to confounding by other
factors (ISA, sections 5.3.7 and 5.4).
    \61\ As with differences among groups of different races and
ethnicities, ``[t]he gap between SES groups with respect to Pb body
burden appears to be diminishing,'' although blood Pb levels
continue to be higher among lower-income children (ISA, p. 1-80,
sections 1.9.6, 5.1, <IP_ADDRESS>, 5.2.4 and 5.4), leading the ISA to
conclude that the evidence is suggestive of SES as a risk factor for
Pb-related health effects (as summarized in section II.A.2.d above).
    \62\ In making this statement, these commenters cite a 1988
study on blood Pb and early childhood scores on the BSID MDI infant
cognitive development test (Bellinger et al., 1988). The study found
that 18 and 24 month BSID MDI scores of the ``lower'' SES children
were adversely affected at lower cord blood Pb levels than were
scores of the ``higher'' SES children, finding significantly lower
scores of the lower SES children with cord blood Pb levels of 6-7
[micro]g/dL as compared to children of this SES group with cord
blood Pb levels less than 3 [micro]g/dL (Bellinger et al., 1988;
USEPA, 1990a; USEPA, 2006). As the study cohort was mostly middle to
upper-middle class, the ``lower'' SES group ``refers to [families of
SES] less than the highest SES levels and is probably in fact [of
SES levels] much closer to the median of the U.S. population than
the term suggests'' (USEPA, 1990a, p. 53). The ISA considered these
study findings in the context of considering available evidence on
this issue in the current review (ISA, section 5.3.6; Bellinger et
al., 1990). The ISA found that the available study results are
limited, have differed with regard to finding increased risk with
higher or lower SES and that ``they do not clearly indicate whether
groups with different socioeconomic status differ in Pb-related
changes for cognitive function'' (ISA, p. 5-34, Table 5-1, p. 5-42).
    \63\ As noted in section I.D above and described in more detail
in the PA and ISA, sources of Pb to which children are exposed also
include consumer goods, dust or chips of peeling Pb-containing paint
and ingestion of Pb in drinking water conveyed through Pb pipes, as
well as historically deposited Pb in urban soils (ISA, pp. pp. lxxix
to lxxx).
    \64\ Additionally, the focus of the air-related IQ loss
evidence-based framework on C-R functions observed for children with
low blood Pb levels closer to those observed in U.S. children today
reflects evidence-based conclusions from the last review, affirmed
in this review, of a steeper slope for the C-R relationship at lower
as compared to higher blood Pb levels. As noted in section II.A.2.d
above, while children with higher blood Pb levels are at greater
risk of Pb-related effects than children with lower blood Pb levels,
on an incremental basis (e.g., per [micro]g/dL) the risk is greater
for children at lower blood Pb levels. The 2008 revision of the
primary Pb standard focused on the incremental impact of air-related
Pb on young children and in so doing, recognized the greater
incremental impact for those children with lower absolute blood Pb
levels. Accordingly, the decision focused on those C-R studies
involving the lowest blood Pb levels (as summarized in II.A.1
above). Although the comment did not indicate how information that
some groups may be generally more highly exposed to Pb should be
used, we note that for the Administrator to rely on C-R functions
from analyses for higher blood Pb study groups (with a less steep
slope) would lead to consideration of a higher standard level, and
would not provide the desired protection for the sensitive group of
children with lower blood Pb levels that are exposed to air-related
Pb in areas with air Pb concentrations at the level of the standard
(73 FR 67002-07, November 12, 2008; 80 FR 311-313, January 5, 2015).
---------------------------------------------------------------------------

    With regard to consideration of the potential for risk reduction
from lower air concentrations, the PA stated that ``the uncertainties
and limitations associated with many aspects of the estimated
relationships between air Pb concentrations and blood Pb levels and
associated health effects are amplified with consideration of
increasingly lower air concentrations'' (PA, p. 4-35). Contrary to the
suggestion by the CHPAC and the smelter company, the PA did not
conclude that there would be public health benefits from a lower
standard and that such benefits were not large enough to warrant
revising the standard. Rather, the PA notes that ``[a]s recognized at
the time of the last review, exposure and risk modeling conducted for
[the REA] was complex and subject to significant uncertainties'' (PA,
p. 3-67) and recognizes ``increasing uncertainty of risk estimates''
for air Pb concentrations below those associated with the current
standard (PA, p. 4-35). The PA further stated that that ``there is
appreciable uncertainty associated with drawing conclusions regarding
whether there would be reductions in blood Pb levels and risk to public
health from alternative lower levels of the standard as compared to the
level of the current standard'' (PA, pp. 4-35 to 4-36). The CASAC
stated that it agreed with this conclusion regarding ``[t]he obvious
uncertainty'' articulated in the PA, additionally stating, as noted
above, that ``[a]lthough there is evidence that even

very low Pb levels are related to measurable reductions in IQ in
children, the extent to which the blood Pb levels observed in children
are linked to ambient air Pb levels below the current standard (as
opposed to other sources of Pb in the environment) has not been
established'' and, accordingly (as noted below), that the current
information does not provide support for lowering the primary standard
(Frey, 2013b, Consensus Response to Charge Questions, pp. 6-8). These
conclusions from the CASAC and the PA findings were among the
considerations that led to the Administrator's proposed decision
(summarized in section II.B.1 above) and her final decision in this
review, as described in section II.B.4 below, that, based on the
current scientific information, including information regarding at-risk
populations, as well as uncertainties and limitations associated with
the current information, the current primary standard provides the
requisite protection of public health with an adequate margin of
safety, including the health of at-risk populations.
    The comment regarding a potential for increases in air Pb near
sources of Pb emissions if the standard is not revised does not explain
how such a potential provides support for revising the standard. The
comment also suggests that EPA consider two alternative standard levels
well below the current standard level while providing no explanation of
why a revised standard with either of the suggested levels would be
requisite. With regard to the potential for increases in air Pb near
sources of Pb emissions if the standard is not revised, we note that
such a concern, to the extent it applies to the current standard, would
also pertain to any more stringent Pb standard except in the extreme
case in which the standard is set such that there is no location with
air quality conditions better than those that just meet the standard.
As discussed in sections II.B.1 above and II.B.4 below, the
Administrator has considered the current evidence and exposure/risk
information with regard to the potential for a revised standard to
offer additional protection, found there to be substantial uncertainty
associated with such a potential, and concluded that the current
standard is requisite. Regarding the possibility that air Pb
concentrations could increase in some locations, we additionally note
that the Clean Air Act and associated EPA permitting regulations
restrict increases in air Pb concentrations (and in other pollutants
for which there are NAAQS) in various circumstances, both in areas
already meeting the NAAQS as well as those in nonattainment (e.g., New
Source Review regulations at 40 CFR part 51, subpart I, applicable in
attainment and nonattainment areas; General Conformity regulations at
40 CFR 93.150-165, applicable in nonattainment and maintenance areas;
and, the general anti-backsliding requirements under Section 110(l) of
the Clean Air Act).
    Regarding the view expressed by some commenters that the most
restrictive standard assessed in the 2007 REA should be adopted, \65\
or that the standard level should be revised to a concentration
described in one comment as the average air Pb concentration in
pristine locations, we note the greater uncertainty in risk estimates
associated with air quality scenarios for air Pb concentrations
increasingly below those of current conditions. Additionally, the PA
described the ``increasing uncertainty recognized for air quality
scenarios involving air Pb concentrations increasingly below the
current conditions for each case study, recognizing that such
uncertainty is due in part to modeling limitations deriving from
uncertainty regarding relationships between ambient air Pb and outdoor
soil/dust Pb and indoor dust Pb'' (PA, 4-34). Further, the PA
concluded, and the CASAC agreed, that ``there is appreciable
uncertainty associated with drawing conclusions regarding whether there
would be reductions in blood Pb levels from alternative lower levels as
compared to the level of the current standard' (Frey, 2013b, Consensus
Response to Charge Questions, p. 6; PA, p.4-35 to 4-36). The CASAC
further stated that ``there is not justification for modifying the
current standard based on these data at this time'' (Frey, 2013b,
Consensus Response to Charge Questions, p. 8). In reaching her proposed
decision to retain the current standard, the Administrator took note of
the PA conclusion and associated CASAC agreement and additionally
recognized that ``the uncertainties and limitations associated with the
many aspects of the estimated relationships between air Pb
concentrations and blood Pb levels and associated health effects are
amplified with consideration of increasingly lower air concentrations''
(80 FR 313). Finally, in the proposal, as in the final decision
described in section II.B.3 below, the Administrator judges this
uncertainty to be too great for the current evidence and exposure/risk
information to provide a basis for revising the current standard. With
regard to comments recommending consideration of technological
feasibility in judging the requisiteness of the primary standard, we
note, as we have described in section I.A above, the EPA may not
consider technological feasibility or attainability in determining what
standard is requisite to protect public health with an adequate margin
of safety.
---------------------------------------------------------------------------

    \65\ The alternative more stringent primary standard suggested
by the CHPAC was the most stringent assessed in the 2007 REA and
included both a lower level and a shorter averaging time than those
for the current standard. In establishing the current standard in
2008, the EPA considered these suggestions regarding level and
averaging time, which were also made by the CHPAC at that time. The
EPA's considerations with regard to averaging time in establishing
the current standard in 2008 are summarized in section II.E.1 of the
proposal and section <IP_ADDRESS> of the PA. The comments from the CHPAC
repeat its recommendation from the last review and do not provide
any additional information or explanation in support of its view on
a revised averaging time. The EPA response to substantive comments
on averaging time in the last review from the CASAC and the public,
including the CHPAC, is described in the notice of final decision
(73 FR 66991-996, November 12, 2008).
---------------------------------------------------------------------------

    Comments on topics less directly related to consideration of the
primary standard included recommendations for addressing data gaps and
uncertainties to inform future reviews. Additionally, one comment
focused on pathways by which Pb may be further distributed in the
environment, recommending use of a ``more robust [monitoring] network
to adequately estimate children's lead exposures from transient and
other sources,'' emphasizing building demolition and Pb wheel weights.
This comment also states that the PA overlooks the contribution from
these and other sources and therefore may underestimate the number of
children exposed to Pb from transient sources. Another comment
described leaded aviation gasoline and airports as a source of Pb
emissions but did not explain how such information was relevant to the
Administrator's proposed decision that the current standard provided
the requisite protection and should be retained without revision.
    With regard to the need for research, the PA highlighted key
uncertainties associated with reviewing and establishing NAAQS for Pb
and areas for future health-related research, model development, and
data gathering. The topic areas of key uncertainties, research
questions and data gaps that were highlighted in the PA with regard to
review of the health-based primary standard overlap with many raised by
commenters. We encourage research in these areas, although we note that
research planning and priority setting are beyond the scope of this
action.
    With regard to the monitoring network in place for Pb NAAQS

surveillance, the current regulations require air monitors in areas
that are expected to or have been shown to experience or contribute to
exceedance of the standards. As described in section I.E above, this
includes requirements for monitors in areas with non-airport sources
emitting 0.5 tpy or where an airport emits 1.0 or more tpy, based on
either the most recent National Emissions Inventory or other
scientifically justifiable methods and data (40 CFR part 58, appendix
D, section 4.5). The establishment of the source-oriented monitoring
requirement reflects our conclusion that monitoring should be
presumptively required at sites near sources that have estimated Pb
emissions in exceedance of a Pb ``emissions threshold'' (73 FR 67025).
This monitoring requirement applies not only to existing industrial
sources of Pb, but also to fugitive sources of Pb (e.g., mine tailing
piles, closed industrial facilities) and airports where leaded aviation
gasoline is used. Additionally, as noted in section I.E above, to
account for other sources that may contribute to a maximum Pb
concentration in ambient air in excess of the Pb NAAQS, the monitoring
regulations also grant the EPA Regional Administrator the authority to
require additional monitoring ``where the likelihood of Pb air quality
violations is significant or where the emissions density, topography,
or population locations are complex and varied'' (40 CFR part 58,
appendix D, section 4.5(c)).
    In addition to this monitoring required for Pb NAAQS surveillance,
state or local agencies may site additional monitors and there are also
particulate matter monitoring networks that collect Pb data in specific
particle size fractions in many urban areas (40 CFR part 58, appendix
D, section 4.5). Further, as described in section I.E above,\66\
monitoring data collected at NCore sites in large population areas, in
combination with the data for all other non-source-oriented sites,
including those in urban areas, indicate air Pb concentrations well
below the Pb NAAQS (as summarized in section I.E above). Accordingly,
we believe that the current Pb monitoring requirements are consistent
with the currently available information regarding sources of Pb to the
ambient air and areas with the potential for exceedance of the Pb
standards. Further, as described below, the information available
regarding the transient sources mentioned by the commenters does not
indicate the potential for such transient sources to result in
exceedances of the NAAQS.
---------------------------------------------------------------------------

    \66\ The various air Pb monitoring networks are summarized in
section I.E above and described in more detail in section 2.2.1 of
the PA.
---------------------------------------------------------------------------

    As to the comment on the significance of building demolition or Pb
wheel weights in contributing to environmental Pb exposure pathways,
the ISA and PA considered the very limited available data pertaining to
these issues. With regard to building demolition, for which the data
are in terms of loading of dust containing Pb on alleys and sidewalks
immediately following an event, the ISA concludes that the limited data
``suggest that building demolition may be a short-term source of Pb in
the environment,'' and that ``it is unclear if demolition is related to
long-term Pb persistence in the environment'' (ISA, p. 2-21).\67\
Accordingly, we do not interpret the limited available information,
which does not include measurements of air Pb concentrations, to
indicate a potential for such occasional activities as demolition of
buildings containing leaded paint to result in air Pb concentrations
near or in exceedance of the NAAQS. \68\ With regard to the comment on
lead wheel weights, we note that the commenter states they are unaware
of studies that have assessed the impact of Pb wheel weights on
childhood blood Pb levels, as are we. The ISA examined the very limited
data on potential contribution of Pb wheel weights to Pb near roadways;
these data yield widely varying and uncertain estimates of associated
Pb releases (ISA, section <IP_ADDRESS>). Contrary to the commenter's
assertion that the PA overlooks these potential Pb exposure pathways,
the assessment and consideration of policy-relevant information in the
PA \69\ reflects these ISA findings based on consideration of the
current information for these potential transient pathways.
Specifically, the current information does not provide support for
specific estimates of exposures associated with these pathways.
Further, data for monitoring sites near roads find Pb concentrations
well below the NAAQS (e.g., ISA, Figure 2-20). Thus, we conclude that
the current information does not provide support for changes to the
current Pb monitoring regulations with regard to roadways or occasional
activities such as building demolition.
---------------------------------------------------------------------------

    \67\ Characterization of this activity by the study published
subsequent to the ISA that was cited by the CHPAC (Jacobs et al.,
2013) is consistent with findings from the limited number of studies
included in the ISA (ISA, p. 2-21).
    \68\ We note that airborne dust release from demolition of large
buildings in some areas may be regulated under various state and/or
local programs (e.g., demolition activities in some particulate
matter non-attainment or maintenance areas may be subject to
specific state implementation plan requirements on airborne dust
releases).
    \69\ Consistent with the strength and specificity of information
described in the ISA, the PA recognizes the loss of Pb wheel weights
as an additional source of Pb emissions and notes the potential for
previously deposited Pb to be resuspended into the air, without
providing detailed consideration (PA, sections <IP_ADDRESS> and <IP_ADDRESS>).
Further, the input for air-to-blood ratio in the air-related IQ loss
evidence-based framework, which the Administrator has used as a
guide in her consideration of the adequacy of the current standard,
does not restrict sources of Pb from consideration. Thus, such
ratios, which are drawn from empirical studies, would be expected to
reflect all sources contributing to children's blood Pb, including
the transient sources identified by commenters to the extent they
provide contributions (ISA, section 3.5; PA, section 3.1; 80 FR 298-
300, January 5, 2015; 73 FR 66973-66975,67004, November 12, 2008).
---------------------------------------------------------------------------

4. Administrator's Conclusions
    Having carefully considered the public comments, as discussed
above, the Administrator believes that the fundamental scientific
conclusions on the effects of Pb in ambient air reached in the ISA and
PA, and summarized in sections II.B and II.C of the proposal, remain
valid. Additionally, the Administrator believes the judgments she
reached in the proposal (section II.E.4) with regard to consideration
of the evidence and quantitative exposure/risk information remain
appropriate. Thus, as described below, the Administrator concludes that
the current primary standard provides the requisite protection of
public health with an adequate margin of safety and should be retained.
    In considering the adequacy of the current primary Pb standard, the
Administrator has carefully considered the current policy-relevant
evidence and conclusions contained in the ISA; the evaluation of this
evidence and the exposure/risk information, rationale and conclusions
presented in the PA; the advice and recommendations from the CASAC; and
public comments. In the discussion below, the Administrator gives
weight to the PA conclusions, with which the CASAC has concurred, as
summarized in section II of the proposal, and takes note of key aspects
of the rationale for those conclusions that contribute to her decision
in this review.
    As an initial matter, the Administrator recognizes the complexity
involved in considering the adequacy of protection in the case of the
primary Pb standard, which differs substantially from that involved in
consideration of the health protection provided by the primary
standards in other NAAQS reviews. For the pollutants in the other
reviews, the more limited focus solely on the inhalation pathways of
exposure is a relatively simpler context. Further, as

described in the PA and noted in section II.B.1 above, the influence of
multimedia and historical exposure on the internal biomarkers in Pb
epidemiological studies contrasts with the epidemiological studies
considered for other NAAQS pollutants which focus on generally current
concentrations of those pollutants in ambient air. While the use of an
internal biomarker strengthens conclusions regarding Pb as the causal
agent in associations observed in epidemiological studies, the
persistence of Pb and the role of multimedia and historical exposures
limit the conclusions that can be drawn regarding the particular
exposure circumstances eliciting the reported effects. Thus, as we lack
studies that can directly assess current concentrations of Pb in
ambient air (including in locations where the current standard is met)
and the occurrence of health effects, we primarily consider the
evidence for, and risk estimated from, models, based upon key
relationships, such as those among ambient air Pb, Pb exposure, blood
Pb and health effects. This information base, both with its strong,
long-established evidence of the health effects of Pb in young
children, and the associated limitations and uncertainties mentioned
here, contributes to our conclusions regarding relationships between
ambient air Pb conditions under the current standard and health
effects.
    The Administrator recognizes that in primary NAAQS reviews, our
understanding of the relationships between the presence of a pollutant
in ambient air and associated health effects is based on a broad body
of information encompassing not only more established aspects of the
evidence, but also aspects in which there may be substantial
uncertainty. In the case of this review of the primary standard for Pb,
she takes note of the increased uncertainty in characterizing the
relationship of effects on IQ with blood Pb levels below those
represented in the evidence base and in projecting the magnitude of
blood Pb response to ambient air Pb concentrations at and below the
level of the current standard. The PA recognizes this increased
uncertainty, particularly in light of the multiple factors that play a
role in such a projection (e.g., meteorology, atmospheric dispersion
and deposition, human physiology and behavior), each of which carry
attendant uncertainties. These aspects of the scientific evidence and
analyses, and the associated uncertainties, collectively contribute to
the Administrator's recognition that for Pb, as for other pollutants,
the available health effects evidence and associated information
generally reflect a continuum, consisting of levels at which scientists
generally agree that health effects are likely to occur, through lower
levels at which the likelihood and magnitude of the response become
increasingly uncertain.
    With regard to the current evidence, as summarized in the PA and
discussed in detail in the ISA, the Administrator takes note of the
well-established body of evidence on the health effects of Pb, which
has been augmented in some aspects since the last review and continues
to support identification of neurocognitive effects in young children
as the most sensitive endpoint associated with Pb exposure. For
example, while the ISA continues to recognize cardiovascular effects in
adults, in addition to neurodevelopmental effects in children, as being
associated with the lowest blood Pb levels compared to other health
effects (ISA, pp. xciii), the ISA also notes uncertainties regarding
the timing, frequency, duration and level of Pb exposures contributing
to the effects observed in adult epidemiologic studies and indicates
that higher exposures in the past (rather than lower current exposures)
may contribute to the development of health effects measured later in
life (ISA, p. lxxxviii). Given the evidence-based identification of
neurocognitive effects in young children as the most sensitive endpoint
associated with Pb exposure, the Administrator has accordingly focused
on nervous system effects in young children and particularly
neurocognitive effects. In so doing, she finds that the evidence, while
describing a broad array of health effects associated with Pb,
continues to indicate that a standard that provides protection from
neurocognitive effects in young children additionally provides
protection from other health effects of Pb, such as those reported in
adult populations.
    The Administrator takes note of the PA finding that application of
the air-related IQ loss evidence-based framework, developed in the last
review, continues to provide a useful approach for considering and
integrating the evidence on relationships between Pb in ambient air and
Pb in young children's blood and risks of neurocognitive effects (for
which IQ loss is used as an indicator). In so doing, as in the 2008
review, she notes that the framework, and the IQ loss estimates yielded
by it for specific combinations of standard level, air-to-blood ratio
and C-R function, does not provide an evidence- or risk-based bright
line that indicates a single appropriate level for the standard.
Further, the Administrator recognizes uncertainties associated with IQ
estimates produced by the framework, noting the PA conclusion that the
uncertainties increase with estimates associated with successively
lower standard levels. She additionally takes note of the PA finding
(described in section II.E.1 of the proposal) that the currently
available evidence base, while somewhat expanded since the last review,
is not appreciably expanded or supportive of appreciably different
conclusions with regard to air-to-blood ratios or C-R functions for
neurocognitive decrements in young children. The Administrator further
notes the concurrence from the CASAC on both of these points and the
lack of recommendations in public comments for a change to either of
these inputs to the evidence-based framework. Thus, she judges the
evidence base and related air-related IQ loss framework to be an
appropriate tool for informing her decision on the adequacy of the
current standard.
    In light of the continuum referenced above, the Administrator
additionally recognizes in this review, as in the 2008 review, the role
of judgment in reaching conclusions regarding Pb health effects that
are important from a public health perspective. Most specifically, the
Administrator has considered the public health significance of a
decrement of a very small number of IQ points in the at-risk population
of young children, in light of associated uncertainties. 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, as did the Administrator at the time of the
last review, 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. She recognizes, however, that in the case of
setting NAAQS, she is required to make a judgment as to what degree of
protection is requisite (neither more nor less than necessary) to
protect public health with an adequate margin of safety. As described
in the proposal with regard to considering the public health
significance of IQ loss estimates in young children, the Administrator
gives weight to the comments of the CASAC and some public commenters in
the last review which recognized a population mean IQ loss of 1 to 2
points to be of public health significance and recommended that a very
high

percentage of the U.S. population be protected from such a magnitude of
IQ loss (73 FR 67000, November 12, 2008). She additionally notes that
the CASAC did not provide a different goal in the present review. The
Administrator additionally notes that the EPA is aware of no new
information or new commonly accepted guidelines or criteria within the
public health community for interpreting public health significance of
neurocognitive effects in the context of a decision on adequacy of the
current Pb standard (PA, pp. 4-33 to 4-34), and no new information has
been identified by public commenters.
    With the objective identified by the CASAC in the 2008 review in
mind, the Administrator recognizes, as was recognized at the time of
the last review, that her judgment on the degree of protection against
IQ impacts that should be afforded by the primary standard is
particularly focused on consideration of impacts in the at-risk
population and is not addressing a specific quantitative public health
policy goal for air-related decrements in IQ that would be acceptable
or unacceptable for the entire population of children in the U.S. As in
the last review, the at-risk population to which she gives particular
attention is the small subset of U.S. children living in close
proximity to air Pb sources that contribute to elevated air Pb
concentrations that equal the level of the standard). Accordingly, she
is considering IQ impacts in this small subset of U.S. children that is
expected to experience air-related Pb exposures at the high end of the
national distribution of such exposures (as described in section II.E.4
of the proposal and summarized in section II.B.1 above), and not a
projection of the average air-related IQ loss for the entire U.S.
population of children. The evidence-based framework estimates, with
which there are associated uncertainties and limitations (as described
in section II.A.1 above), relate to this small subset of children
exposed at the level of the standard. Based on these considerations,
the Administrator judges the conceptual evidence-based framework to
continue to be appropriate for her consideration of the public health
protection afforded by the current standard. Further, she concurs with
the PA findings (summarized in section II.E.1 of the proposal and
briefly outlined in II.B.1 above) that the current evidence, as
considered within the conceptual and quantitative context of the
evidence-based framework, and current air monitoring information
indicate that the current standard would be expected to satisfy the
public health policy goal recommended by the CASAC in the last Pb NAAQS
review, from which it did not indicate a departure in the present
review.
    In the context of the Administrator's use of the framework as a
tool to inform her decision on the adequacy of the current standard,
the EPA additionally notes that the maximum, not to be exceeded, form
of the standard, in conjunction with the rolling 3-month averaging
time, is expected to result in the at-risk population of children being
exposed below the level of the standard most of the time (73 FR 67005,
November 12, 2008). In light of this and the uncertainty in the
relationship between time period of ambient level, exposure, and
occurrence of a health effect, the air-related IQ loss considered for
the current standard in applying the framework should not be
interpreted to mean that a specific level of air-related IQ loss will
occur in fact in areas where the standard is just met or that such a
loss has been determined as acceptable if it were to occur. Instead,
judgment regarding 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 the context of 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 and
uncertainties in relating those patterns to exposures and effects.
    In drawing conclusions regarding adequacy of the current standard
based on considering application of the evidence-based framework, the
Administrator further recognizes the degree to which IQ loss estimates
drawn from the air-related IQ loss evidence-based framework reflect
mean blood Pb levels that are below those represented in the currently
available evidence for young children, as described in section II.B.4
of the proposal. The Administrator views such an extension below the
lowest studied levels to be reasonable given the lack of identified
blood Pb level threshold in the current evidence base for
neurocognitive effects and the need for the NAAQS to provide a margin
of safety. She additionally takes note, however, of the PA finding that
the framework IQ loss estimates for standard levels lower than the
current standard level represent still greater extrapolations from the
current evidence base with corresponding increased uncertainty (PA,
section 3.2, pp. 4-32 to 4-33). The Administrator also gives weight to
the PA conclusion of greater uncertainty with regard to relationships
between concentrations of Pb in ambient air and air-related Pb in
children's blood, and with regard to estimates of the slope of the C-R
function of neurocognitive impacts (IQ loss) for application of the
framework to levels below the current standard, given the weaker
linkage with existing evidence as discussed in the PA (PA, sections
3.1, 3.2 and 4.2.1). Thus, consistent with the conceptual continuum
referenced above, the Administrator recognizes the increasing
uncertainty with regard to likelihood of response and magnitude of the
estimates at levels extending below the current standard.
    With respect to exposure/risk-based considerations, as in the last
review, the Administrator notes the complexity of the REA modeling
analyses and the associated limitations and uncertainties. Based on
consideration of the risk-related information for conditions just
meeting the current standard, the Administrator takes note of the
attendant uncertainties, discussed in detail in the PA (PA, sections
3.4 and 4.2.2), while finding that the quantitative risk estimates,
with a focus on those for the generalized (local) urban case study, are
roughly consistent with and generally supportive of estimates from the
air-related IQ loss evidence-based framework. She further takes note of
the PA finding of increasing uncertainty for air quality scenarios
involving air Pb concentrations increasingly below the current
conditions for each case study, due in part to modeling limitations
that derive from uncertainty regarding relationships between ambient
air Pb and outdoor soil/dust Pb and indoor dust Pb (PA, sections
<IP_ADDRESS> and 3.4.7).
    Based on the above evidence- and exposure/risk-based considerations
and with consideration of advice from CASAC and public comment, the
Administrator concludes that the current standard provides protection
for young children from neurocognitive impacts, including IQ loss, that
is consistent with advice from CASAC regarding IQ loss of public health
significance. Based on consideration of the evidence and exposure/risk
information available in this review with its attendant uncertainties
and limitations, and information that might inform public health policy
judgments, as well as advice from CASAC, including its concurrence with
the PA conclusions that revision of the primary Pb standard is not
warranted at this time, the Administrator further concludes that it is
appropriate to retain

the current standard without revision. The Administrator bases these
conclusions on consideration of the health effects evidence, including
consideration of this evidence in the context of the air-related IQ
loss evidence-based framework, and with support from the exposure/risk
information, recognizing the uncertainties attendant with both. In so
doing, she takes note of the PA description of the complexities and
limitations in the evidence base associated with reaching conclusions
regarding the magnitude of risk associated with the current standard,
as well as the increasing uncertainty of risk estimates for lower air
Pb concentrations. Inherent in the Administrator's conclusions are
public health policy judgments on the public health implications of the
blood Pb levels and risk estimated for air-related Pb under the current
standard, including the public health significance of the Pb effects
being considered, as well as aspects of the use of the evidence-based
framework that may be considered to contribute to the margin of safety
(as noted in section II.A.1 above and the 2008 decision preamble to the
final rule, 73 FR 67007, November 12, 2008). These public health policy
judgments include judgments related to the appropriate degree of public
health protection that should be afforded to protect against risk of
neurocognitive effects in at-risk populations, such as IQ loss in young
children, as well as the appropriate weight to be given to differing
aspects of the evidence and exposure/risk information, and how to
consider their associated uncertainties. Based on these considerations
and the judgments identified here, the Administrator concludes that the
current standard provides the requisite protection of public health
with an adequate margin of safety, including protection of at-risk
populations, such as, in particular, young children living near Pb
emissions sources where ambient concentrations just meet the standard.
    In reaching this conclusion with regard to the adequacy of public
health protection afforded by the existing primary standard, the
Administrator recognizes that in establishing primary standards under
the Act that are requisite to protect public health with an adequate
margin of safety, she is seeking to establish standards that are
neither more nor less stringent than necessary for this purpose. The
Act does not require that primary standards be set at a zero-risk
level, but rather at a level that avoids unacceptable risks to public
health, even if the risk is not precisely identified as to nature or
degree. The CAA requirement that primary standards provide an adequate
margin of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting, as described in section I.A above. This
requirement was also intended to provide a reasonable degree of
protection from hazards that research has not yet identified.
    In this context, the Administrator has considered conclusions drawn
in the ISA and PA with regard to interpretation of the information
concerning the broader array of health effects of Pb beyond those on
the nervous system of young children. Based on the body of evidence in
support of identification of neurocognitive effects in young children
as the most sensitive endpoint associated with Pb exposure, as noted
previously in this section and briefly summarized in section II.A.2
above, she judges that a standard providing protection from such
effects additionally provides adequate protection against the risk of
other health effects and she further concludes that consideration of
the more limited and less certain information concerning Pb exposures
associated with such other effects does not lead her to identify a need
for any greater protection.
    Further, the Administrator's conclusion that the current standard
provides the requisite protection and that a more restrictive standard
would not be requisite additionally recognizes that the uncertainties
and limitations associated with the many aspects of the estimated
relationships between air Pb concentrations and blood Pb levels and
associated health effects are amplified with consideration of
increasingly lower air concentrations. In reaching this conclusion, she
additionally takes note of the PA conclusion, with which the CASAC has
agreed, that based on the current evidence, there is appreciable
uncertainty associated with drawing conclusions regarding whether there
would be reductions in blood Pb levels and risk to public health from
alternative lower levels of the standard as compared to the level of
the current standard (PA, pp. 4-35 to 4-36; Frey, 2013b, Consensus
Response to Charge Questions, p. 6). The Administrator judges this
uncertainty to be too great for the current evidence and exposure/risk
information to provide a basis for revising the current standard. Thus,
based on the public health policy judgments described above, including
the weight given to uncertainties in the evidence, the Administrator
concludes that the current standard should be retained, without
revision.

C. Decision on the Primary Standard

    For the reasons discussed above, and taking into account
information and assessments presented in the ISA and PA, the advice
from CASAC, and consideration of public comments, the Administrator
concludes that the current primary standard for Pb is requisite to
protect public health with an adequate margin of safety, including the
health of at-risk populations, and is retaining the standard without
revision.

III. Rationale for Decision on the Secondary Standard

    This section presents the rationale for the Administrator's
decision to retain the existing secondary Pb standard, which, as
discussed more fully below, is based on a thorough review in the ISA of
the latest scientific information, generally published through
September 2011, on welfare effects associated with Pb and pertaining to
the presence of Pb in the ambient air. This decision also takes into
account (1) the PA's staff assessments of the most policy-relevant
information in the ISA and staff analyses of potential ecological
exposures and risk, upon which staff conclusions regarding appropriate
considerations in this review are based; (2) the CASAC advice and
recommendations, as reflected in discussions of drafts of the ISA and
PA at public meetings, in separate written comments, and in the CASAC's
letters to the Administrator; (3) public comments received during the
development of these documents, either in connection with CASAC
meetings or separately; and (4) public comments on the proposal.
    Section III.A provides background on the general approach for the
review of the secondary NAAQS for Pb and brief summaries of key aspects
of the current body of evidence on welfare effects associated with Pb
exposures and the exposure/risk information considered in this review.
Section III.B summarizes the basis for the proposed decision and advice
from the CASAC, addresses public comments and presents the conclusions
the Administrator has drawn from a full consideration of the
information. Section III.C summarizes the Administrator's decision on
the secondary standard.

A. Introduction

    As provided in the Act, the secondary standard is to ``specify a
level of air quality the attainment and maintenance of which in the
judgment of the

Administrator . . . is requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
the pollutant in the ambient air'' (CAA, section 109(b)(2)). The
secondary standard is not meant to protect against all known or
anticipated Pb-related effects, but rather those that are judged to be
adverse to the public welfare, and a bright-line determination of
adversity is not required in judging what is requisite (78 FR 3212,
January 15, 2013; 80 FR 65376, October 26, 2015). Thus, the level of
protection from known or anticipated adverse effects to public welfare
that is requisite for the secondary standard is a public welfare policy
judgment to be made by the Administrator. In exercising that judgment,
the Administrator seeks to establish standards that are neither more
nor less stringent than necessary for this purpose. This section
presents the rationale for the Administrator's decision to retain the
existing secondary NAAQS for Pb, without revision. The Administrator's
decision draws upon scientific information and analyses about welfare
effects, exposure and risks, as well as judgments about the range of
uncertainties that are inherent in the scientific evidence and
analyses. This approach is consistent with the requirements of the
NAAQS provisions of the Act.
    In the last review, completed in 2008, the current secondary
standard for Pb was revised substantially, consistent with the revision
to the primary standard (73 FR 66964, November 12, 2008). The 2008
decision considered the body of evidence as assessed in the 2006 CD
(USEPA, 2006a) as well as the 2007 Staff Paper assessment of the
policy-relevant information contained in the 2006 CD and the screening-
level ecological risk assessment (2006 REA; USEPA, 2007b), the advice
and recommendations of CASAC (Henderson 2007a, 2007b, 2008a, 2008b),
and public comment. At that time, the Staff Paper concluded, based on
laboratory studies and current media concentrations in a wide range of
locations, that it seemed likely that adverse effects were occurring
from ambient air-related Pb, particularly near point sources, under the
then-current standard (73 FR 67010, November 12, 2008). Given the
limited data on Pb effects in ecosystems, and associated uncertainties,
such as those with regard to factors such as the presence of multiple
metals and historic environmental burdens, the EPA also considered the
evidence of Pb effects on organisms with regard to implications for
ecosystem effects. Taking into account the available evidence and
information on media concentrations in a wide range of locations, the
Administrator concluded that there was potential for adverse effects
occurring under the then-current standard; however there were
insufficient data to provide a quantitative basis for setting a
secondary standard different from the primary (73 FR 67011, November
12, 2008). Therefore, citing a general lack of data that would indicate
the appropriate level of Pb in environmental media that may be
associated with adverse effects, as well as the comments of the CASAC
Pb panel that a significant change to current air concentrations (e.g.,
via a significant change to the standard) was likely to have
significant beneficial effects on the magnitude of Pb exposures in the
environment, the EPA revised the secondary standard substantially,
consistent with revisions made to the primary standard (73 FR 67011,
November 12, 2008).
    Building on the approach and findings in the last review, this
current review of the secondary standard considers the currently
available scientific and technical information in the context of key
policy-relevant questions. This review focuses on the consideration of
the extent to which the body of scientific evidence now available calls
into question the adequacy of the current standard. In considering the
scientific and technical information, we draw on the ecological effects
evidence presented in detail in the ISA and aspects summarized in the
PA, along with the information associated with the screening-level risk
assessment also in the PA. Thus, we have taken into account both
evidence-based and risk-based considerations pertaining to the series
of policy-relevant questions presented in the PA. These questions
generally address the extent to which we are able to characterize
effects and the likelihood of adverse effects in the environment under
the current standard. Our approach to considering this information
recognizes that the available welfare effects evidence generally
reflects laboratory-based evidence of toxicological effects on specific
organisms exposed to concentrations of Pb (ISA, section 6.5).
Additionally, it is widely recognized that environmental exposures from
atmospherically derived Pb are likely to be lower than those commonly
assessed in laboratory studies and that studies of exposures similar to
those in the environment are often accompanied by significant
confounding and modifying factors (e.g., other metals, acidification),
increasing our uncertainty about the likelihood and magnitude of
organism and ecosystem responses (ISA, Section 6.5).
1. Overview of Welfare Effects Information
    Welfare effects include, but are not limited to, ``effects on
soils, water, crops, vegetation, man-made 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 wellbeing'' (CAA, section 302(h)).
In this section, we provide an overview of the key aspects of the
current evidence of Pb-related welfare effects that is assessed in the
ISA and the 2006 CD, drawing from the summary of policy-relevant
aspects in the PA (PA, section 5.1) and section III.B of the proposed
rulemaking (80 FR 314-317, January 5, 2015).
    Lead has been demonstrated to have harmful effects on reproduction
and development, growth, and survival in many species as described in
the assessment of the evidence available in this review and consistent
with the conclusions drawn in the last review (ISA, section 1.7; 2006
CD, sections 7.1.5 and 7.2.5). A number of studies on ecological
effects of Pb are newly available in this review and are critically
assessed in the ISA as part of the full body of evidence. The full body
of currently available evidence reaffirms conclusions on the array of
effects recognized for Pb in the last review (ISA, section 1.7). In so
doing, in the context of pollutant exposures considered relevant the
ISA determines \70\ that causal \71\ or likely causal \72\
relationships exist at the individual and population level in both

freshwater and terrestrial ecosystems for Pb with effects on
reproduction and development in vertebrates and invertebrates; growth
in plants and invertebrates; and survival in vertebrates and
invertebrates (ISA, Table 1-3). With regard to saltwater ecosystems,