might consider describing a FEM in terms of PM10 that might
be acceptably applied on a site-by-site basis where an appropriate
relationship between Pb-TSP and Pb-PM10 can be developed
based on site-specific data. Alternatively, use of such an FEM might be
approved, in combination with more limited Pb-TSP monitoring, in areas
where the Pb-TSP data indicate ambient Pb levels are well below the
NAAQS level.
    These examples were intended purely for purposes of illustrating
the types of options the Agency might consider. Specific details of any
options would need to be supported by appropriate data analyses. We
solicit information and comments that would help inform such analyses
and the Agency's views on the indicator for the primary Pb NAAQS.
b. Averaging Time and Form
    The basis for the averaging time of the current standard reflects
consideration of the evidence available when the Pb NAAQS were
promulgated in 1978. At that time, the Agency had concluded that the
level of the standards, 1.5 [mu]g/m\3\, would be a ``safe ceiling for
indefinite exposure of young children'' (43 FR 46250), and that the
slightly greater possibility of elevated air Pb levels within the
quarterly averaging

period as contrasted to the monthly averaging period proposed in 1977
(43 FR 63076), was not significant for health. These conclusions were
based in part on the Agency's interpretation of the health effects
evidence as indicating that 30 [mu]g/dL was the maximum safe level of
blood Pb for an individual child.
    As discussed above, the currently available health effects evidence
\68\ indicates a variety of neurological effects, as well as immune
system and hematological effects, associated with levels below 10
[mu]g/dL as a central tendency metric of study cohorts of young
children. Further, EPA recognizes that today ``there is no level of Pb
exposure that can yet be identified, with condfidence, as clearly not
being associated with some risk of deleterious health effects'' (CD, p.
8-63). Accordingly, to the extent that air Pb contributes to variation
in blood Pb, we currently cannot identify a safe ceiling for indefinite
exposure of young children.
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    \68\ The differing evidence and associated strength of the
evidence for these different effects is described in detail in the
Criteria Document.
---------------------------------------------------------------------------

    Additionally, several aspects of the current health effects
evidence for Pb pertain to the consideration of averaging time:
     Children are exposed to ambient Pb via inhalation and
ingestion, with Pb taken into the body absorbed through the lungs and
through the gastrointestinal tract. Studies on Pb uptake, elimination
and distribution show that Pb is absorbed into peripheral tissues in
adults within a few days (USEPA 1986a; USEPA 1990b, p. IV-2).
Absorption of Pb from the gastrointestinal tract appears to be greater
and faster in children as compared to adults (CD, Section 4.2.1). Once
absorbed, it is quickly distributed from plasma to red blood cells and
throughout the body.
     Lead accumulates in the body and is only slowly removed,
with bone Pb serving as a blood Pb source for years after exposure and
as a source of fetal Pb exposure during pregnancy (CD, Sections <IP_ADDRESS>
and <IP_ADDRESS>).
     Blood Pb levels, including levels of the toxicologically
active fraction, respond quickly to increased Pb exposure, such that an
abrupt increase in Pb uptake rapidly changes blood Pb levels, with the
time to reach a new quasi-steady state with the total body burden after
such an occurrence projected to be approximately 75 to 100 days (CD, p.
4-27).
     The elimination half-life, which describes the time for
blood Pb levels to stabilize after a reduction in exposure, for the
dominant phase for blood Pb responses to changes in exposure is on the
order of 20 to 30 days for adults (CD, p. 4-25). Blood elimination
half-lives are influenced by contributions from bone. Given the tighter
coupling in children of bone stores with blood levels, children's blood
Pb is expected to respond more quickly than adults (CD, pp. 4-20 and 4-
27).
     Data from NHANES II and an analysis of the temporal
relationship between gasoline consumption data and blood lead data
generally support the inference of a prompt response of children's
blood Pb levels to changes in exposure in that children's blood Pb
levels and the number of children with elevated blood Pb levels appear
to respond to monthly variations in Pb emissions from Pb in gasoline
(EPA, 1986a, p. 11-39; Rabinowitz and Needleman, 1983; Schwartz and
Pitcher, 1989).
     The evidence with regard to sensitive neurological effects
is limited in what it indicates regarding the specific duration of
exposure associated with effect, although it indicates both the
sensitivity of the first 3 years of life and a sustained sensitivity
throughout the lifespan as the human central nervous system continues
to mature and be vulnerable to neurotoxicants (CD, Section <IP_ADDRESS>).
The animal evidence supports our understanding of periods of
development with increased vulnerability to specific types of effect
(CD, Section 5.3), and indicates a potential importance of exposures on
the order of months.
     Evidence of a differing sensitivity of the immune system
to Pb across and within different periods of life stages indicates a
potential importance of exposures as short as weeks to months duration.
For example, the animal evidence suggests that the gestation period is
the most sensitive life stage followed by early neonatal stage, and
within these life stages, critical windows of vulnerability are likely
to exist (CD, Section 5.9 and p. 5-245).
    Further, evidence described in the Criteria Document and the risk
assessment indicate that ingestion of dust can be a predominant
exposure pathway for young children to policy-relevant Pb, and that
there is a strong association between indoor dust Pb levels and
children's blood Pb levels. As stated in the Criteria Document, ``given
the large amount of time people spend indoors, exposure to Pb in dusts
and indoor air can be significant'' (CD, p. 3-27). The Criteria
Document further describes studies that evaluated the influence of dust
Pb exposure on children's blood Pb: ``Using a structural equation
model, Lanphear and Roghmann (1997) also found the exposure pathway
most influential on blood Pb was interior dust Pb loading, directly or
through its influence on hand Pb. Both soil and paint Pb influenced
interior dust Pb; with the influence of paint Pb greater than that of
soil Pb. Interior dust Pb loading also showed the strongest influence
on blood Pb in a pooled multivariate regression analysis (Lanphear et
al., 1998).'' (CD, p. 4-134).
    While some of these aspects of the health effects evidence would be
consistent with a quarterly averaging time, taken as a whole, and in
combination with information on potential response time for indoor dust
Pb levels, EPA recognizes that there is also support for consideration
of an averaging time shorter than a calendar quarter.
    When the standard was set in 1978, an analysis of ambient
measurements available at the time indicated that the distribution of
air Pb levels was such that there was little possibility that there
could be sustained periods greatly above the average value in
situations where the quarterly standard was achieved. This may have
been related to the pattern of Pb emissions at the time the standard
was set, which differed from the pattern today in that, due to
emissions from cars and trucks at that time, emissions were more
spatially distributed. In this review, based on data from 2003-2005,
the air quality analysis in Chapter 2 of the Staff Paper indicates the
presence of areas in the U.S. currently where temporal variability does
create differences between average quarterly levels and levels
sustained for shorter than quarterly periods. For example, four percent
of the monitoring sites in the three-year analysis dataset that meet
the current standard as an average over a calendar quarter exceed the
level of the current standard when considering an average for any
individual month. The same analysis indicates that this number is as
high as ten percent for some alternate lower levels.
    In further considering the appropriate form of the standard that
might accompany a shorter averaging time, EPA will take into account
analyses using air quality data for 2003-2005 that characterize maximum
quarterly average and various monthly statistics for each year across
the three year Pb-TSP dataset and also across the three year period.
The latter time period is consistent with the three calendar year
attainment period that has been adopted for the ozone and particulate
matter

NAAQS subsequent to the promulgation of the Pb NAAQS. For the three
year period, the monthly statistics derived are maximum monthly mean,
second maximum monthly mean, average of three overall highest monthly
means, and average of three annual maximum monthly means; these
statistical forms were also considered in the 1990 Staff paper.
Additionally, the maximum and 2nd maximum monthly means for each year
of the three year data set was derived, as well as the averages of
these individual year statistics.
    With regard to comparison of monthly forms with the maximum
quarterly mean, the average Pb-TSP maximum monthly mean among all 189
sites in the analysis is notably higher (nearly a factor of two) than
the average of the average maximum quarterly mean among these sites.
Further, this difference is slightly greater for source-oriented sites
than non source-oriented sites or urban sites (e.g., a factor of
approximately 1.8 as compared to one of approximately 1.6), indicating
perhaps an influence of variability in emissions. The alternate forms
of a monthly averaging time that were analyzed yield an across-site
average that is similar although slightly higher than the quarterly
average (e.g., Figure 2-8 in Chapter 2 of the Staff Paper).
    The analyses described in Chapter 2 of the Staff Paper consider
both a period of three calendar years and one of an individual calendar
year (with the form of the current standard being the maximum quarterly
mean in any one year). These analyses indicate that with regard to
either single-year or 3-year statistics for the 2003-2005 dataset, a
2nd maximum monthly mean yields very similar, although just slightly
greater, numbers of sites exceeding various alternate levels as a
maximum quarterly mean, with both yielding fewer exceedances than a
maximum monthly mean.
    In their advice to the Agency, CASAC has recommended that
consideration be given to changing from a calendar quarter to a monthly
averaging time (Henderson, 2007a, b). In making that recommendation,
CASAC emphasizes support from studies that suggest that blood Pb
concentrations respond at shorter time scales than would be captured
completely by quarterly values, as indicated by their description of
their recommendation for adoption of a monthly averaging time as ``more
protective of human health in light of the response of blood lead
concentrations that occur at sub-quarterly time scales'' (Henderson,
2007b).
    With regard to form of the standard, CASAC stated that one could
``consider having the lead standards based on the second highest
monthly average, a form that appears to correlated well with using the
maximum quarterly value'', while also indicating that ``the most
protective form would be the highest monthly average in a year.''
    The following observations support consideration of a monthly
averaging time: (1) The health evidence indicates that very short
exposures can lead to increases in blood Pb Pb levels, (2) the time
period of response of indoor dust Pb to airborne Pb can be on the order
of weeks and, (3) the health evidence indicates that adverse effects
may occur with exposures during relatively short windows of
susceptibility, such as prenatally and in developing infants. EPA also
recognizes the limited available evidence specific to the consideration
of the particular duration of sustained airborne Pb levels having the
potential to contribute to the adverse health effects identified as
most relevant to this review.
    Based on the information and air quality analyses discussed above,
EPA is requesting comment on a range of options, including the
recommendations in the Staff Paper that include changing the averaging
time to monthly, with a form of maximum or second maximum, as well as
retaining the quarterly averaging time. The EPA is also requesting
comment on, the options of changing the form to apply to a three-year
period as well as retaining a single-year period. We solicit comments
on these ranges of averaging times and forms as well as views and
related rationales that might support alternative options.
c. Level
    At this time, the Agency is interested in soliciting comment on a
wide range of possible options for consideration when making a proposed
decision on the level of the primary Pb NAAQS. These policy options
range from lowering the standard, to the levels recommended by CASAC
and the OAQPS Staff paper or lower, as well as on other alternative
levels, up to and including the current level, and the rationale upon
which such views are based.
i. Evidence-Based Considerations
    The EPA recognizes that there are several aspects to the body of
epidemiological evidence available in this review that complicate
efforts to translate the evidence into the basis for selecting an
appropriate level for an ambient air quality standard. As an initial
matter, as summarized above and discussed in greater depth in the
Criteria Document (CD, Sections 4.3 and 6.1.3), the epidemiological
evidence that associates Pb exposures with health effects uses blood Pb
as the dose metric, not ambient air concentrations. Further, for the
health effects receiving greatest emphasis in this review (neurological
effects on the developing nervous system), no threshold levels can be
discerned from the evidence. As was recognized at the time of the last
review, estimating a threshold for toxic effects of Pb on the central
nervous system entails a number of difficulties (CD, pp. 6-10 to 6-11).
The task is made still more complex by support in the evidence for a
nonlinear rather than linear relationship of blood Pb with
neurocognitive decrement, with greater risk of decrement-associated
changes in blood Pb at the lower levels of blood Pb in the exposed
population (Section 3.3.7; CD, Section 6.2.13).
    In considering how this evidence can help inform the selection of
the level of the standard, EPA will consider how the framework applied
in the establishment of the standard may be applied to the much
expanded body of evidence that is now available. This consideration
builds upon the evidence-based considerations of the adequacy of the
current standard, discussed above in Section III.C.3.a.
    As noted above, this review focuses on young children as the key
sensitive population for Pb exposures, the same population identified
in 1978. In this sensitive population, the current evidence
demonstrates the occurrence of adverse health effects, including those
on the developing nervous system, associated with blood Pb levels
extending well below 10 [mu]g/dL to 5 [mu]g/dL and possibly lower. Some
studies indicate Pb effects on intellectual attainment of young
children at blood Pb levels ranging from 2 to 8 [mu]g/dL (CD, Sections
6.2, 8.4.2 and <IP_ADDRESS>), including findings of similar Pb-related
effects in a study of a nationally representative sample of children in
which the mean blood Pb level was 1.9 [mu]g/dL (CD, pp. 6-31 to 6-32;
Lanphear et al., 2000).\69\ Further, the current evidence does not
indicate a threshold for the more sensitive health endpoints such as
adverse effects on the

developing nervous system (CD, pp. 5-71 to 5-74 and Section 6.2.13).
This differs from the Agency's inference in the 1978 rulemaking of a
threshold of 40 [mu]g/dL blood Pb for effects of Pb considered clearly
adverse to health at that time, i.e., impairment of heme synthesis and
other effects which result in anemia. Thus, the level of Pb in
children's blood associated with adverse health effect has dropped
substantially.
---------------------------------------------------------------------------

    \69\ These findings include significant associations in the
study sample subsets of children with blood Pb levels less than 10
[mu]g/dL, less than 7.5 [mu]g/dL and less than 5 [mu]g/dL. A
positive, but not statistically significant association, was
observed in the less than 2.5 [mu]g/dL subset, although the effect
estimate for this subset was largest among all the subsets. The lack
of statistical significance for this subset may be due to the
smaller sample size of this subset which would lead to lower
statistical power.
---------------------------------------------------------------------------

    As when the standard was set in 1978, EPA recognizes that there
remain today contributions to blood Pb levels from nonair sources. As
discussed above, these contributions have been reduced since 1978, with
estimates of reduction in the dietary component of 70 to 95 percent
(CD, Section 3.4). The evidence is limited with regard to the aggregate
reduction since 1978 of all nonair sources to blood Pb. However, the
available evidence and some preliminary analysis led CASAC to recommend
consideration of 1.0 to 1.4 [mu]g/dL or lower as an estimate of the
nonair component of blood Pb (Henderson, 2007a). The value of 1.4
[mu]g/dL was the mean blood Pb level derived from a simulation of
current nonair exposures using the IEUBK model (Henderson, 2007a, pp.
F-60 to F-61). These current estimates are roughly an order of
magnitude lower than the value of 12 [mu]g/dL that was used in setting
the 1978 standard.
    Regarding the relationship between air and blood, while the
evidence demonstrates that airborne Pb influences blood Pb
concentrations through a combination of inhalation and ingestion
exposure pathways, estimates of the precise quantitative relationship
(i.e., air-to-blood ratio) available in the evidence vary (USEPA,
1986a; Brunekreef, 1984) and there is uncertainty as to the values that
pertain to current exposures. Studies summarized in the 1986 Criteria
Document typically yield estimates in the range of 1:3 to 1:5, with
some as high as 1:10 or higher (USEPA, 1986a; Brunekreef, 1984).
Findings in a more recent study identified in the Criteria Document of
blood Pb response to reduced air concentrations indicate a ratio on the
order of 1:7 (CD, pp. 3-23 to 3-24; Hilts et al., 2003). A value of 1:5
has been used by the World Health Organization (2000). These ratios are
appreciably higher than the ratio of 1:2 that was used in setting the
1978 standard.
    A standard setting approach being considered is to apply the
framework relied upon in setting the standard in 1978 to the currently
available information. In applying that framework, however, EPA
recognizes that today ``there is no level of Pb exposure that can yet
be identified, with confidence, as clearly not being associated with
some risk of deleterious health effects'' (CD, p. 8-63). However, there
is increasing uncertainty with regard to the magnitude and type of
effects at levels below 5 [mu]g/dL \70\. This is in contrast to the
situation in 1978 when the Agency judged that the maximum safe blood Pb
level (geometric mean) for a population of young children was 15 [mu]g/
dL based on its conclusion that the maximum safe blood Pb level of an
individual child was 30 [mu]g/dL. \71\
---------------------------------------------------------------------------

    \70\ As stated in the Criteria Document ``Some recent studies of
Pb neurotoxicity in infants have observed effects at population
average blood-Pb levels of only 1 or 2 [mu]g/dL; and some
cardiovascular, renal, and immune outcomes have been reported at
blood-Pb levels below 5 [mu]g/dL.'' (CD, p. E-16).
    \71\ More specifically, the 1978 target of 15 [mu]g/dL was
described as the geometric mean level associated with a 99.5
percentile of 30 [mu]g/dL which the Agency described as a ``safe
level'' for an individual child (43 FR 46247-49).
---------------------------------------------------------------------------

    In illustrating the application of the 1978 framework, two blood Pb
levels are used here for illustrative purposes. A level of 2 [mu]g/dL
was used because it represents some of the lowest population levels
associated with adverse effect in the current evidence (e.g., CD, p. E-
9; Lanphear et al., 2000). In addition, a level of 5 [mu]g/dL has been
used. This level has been associated with adverse health effects with a
higher degree of certainty in the published literature, and is a level
where cognitive deficits were identified with statistical significance
(Lanphear et al., 2000).
    Using a blood Pb target of 2 [mu]g/dL as a substitute for the 1978
target of 15 [mu]g/dL for the child population geometric mean, then
subtracting 1 to 1.4 [mu]g/dL for background, yields 0.6 to 1 [mu]g/dL
as a target for the air contribution to blood Pb. Dividing the air
target by 5, consistent with currently available information on the
ratio of air Pb to blood Pb, yields a potential standard level of 0.1
to 0.2 [mu]g/m\3\. Alternatively, using the same approach substituting
5 [mu]g/dL for the child population geometric mean and subtracting 1 to
1.4 [mu]g/dL for background, yields 3.6 to 4 [mu]g/dL as a target for
the air contribution to blood Pb. Dividing the air target by 5,
consistent with currently available information on the ratio of air Pb
to blood Pb, yields a level of 0.7 to 0.8 [mu]g/m\3\. Similarly,
substitution of other blood Pb targets would result in still other
levels.
    In light of the current CDC blood Pb ``level of concern'' of 10
[mu]g/dL, some might consider a blood Pb value of 10 [mu]g/dL as a
target blood Pb value for this calculation to derive a level for the
primary standard. EPA notes, however, that the CDC does not consider
this level of concern as a safe blood Pb level or one without evidence
of adverse effects (CDC, 2005a). Rather, it is used by CDC to identify
children with elevated blood Pb levels for follow-up activities \72\ at
the individual level and to trigger communitywide prevention activities
(CDC, 2005a). The level of concern has been frequently misinterpreted
as a definitive toxicologic threshold (CDC, 2005a). As summarized in
Section III.A and above, and as described in detail in the Criteria
Document, various adverse effects have been associated with children's
blood Pb levels below 10 [mu]g/dL. For example, the Criteria Document
states that the currently available toxicologic and epidemiologic
information ``includes assessment of new evidence substantiating risks
of deleterious effects on certain health endpoints beng induced by
distinctly lower than previously demonstrated Pb exposures indexed by
blood-Pb levels extending well below 10 [mu]g/dL in children and/or
adults'' (CD, p. 8-25). Accordingly, EPA has not used a mean or an
individual target blood Pb value of 10 [mu]g/dL as the basis for an
illustrative example of deriving a standard that is intended to protect
public health with an adequate margin of safety. In recognition of
differing views on this subject, however, we solicit comment on the
appropriateness of using a mean or individual target blood Pb value of
10 [mu]g/dL as the foundation for deriving a level for the primary Pb
standard.
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    \72\ Activities such as taking an environmental history,
educating parents about Pb and conducting follow-up blood Pb
monitoring were among those suggested for children with blood Pb
levels greater than or equal to 10 [mu]g/dL (CDC, 2005a). Recently,
CDC's Advisory Committee on Childhood Lead Poisoning Prevention has
also provided information and recommendations relevant to clinical
management of children with blood Pb levels below 10 [mu]g/dL
(ACCLPP, 2007).
---------------------------------------------------------------------------

    The above examples focus on the mean target blood Pb level for the
sensitive population by way of illustrating application of the 1978
framework. The EPA solicits comment on mean target blood Pb levels as
well as other factors that would be important in applying the 1978
framework. For example, the distribution of blood Pb levels within the
sensitive population is an important aspect of the 1978 framework. When
the standard was set in 1978, the Agency stated that the population
mean, measured as the geometric mean, must be 15 [mu]g/dL in order to
ensure that 99.5 percent of children in the United States would

have a blood Pb level below 30 [mu]g/dL, which was identified as the
maximum safe blood Pb level for individual children based on the
information available at that time (43 FR 46252). Target values for the
mean of the population necessarily imply higher values for individuals
associated with the upper percentiles of the blood Pb distribution. For
example, the 2001-2002 NHANES information indicates that a geometric
mean blood level of 1.7 [mu]g/dL for children nationally, aged 1-5
years, is associated with a 95th percentile blood Pb level of 5.8
[mu]g/dL (CDC, 2005b).
    Additionally, the nonair (background) contribution to total blood
Pb is an important input to the framework and we solicit comment on the
definition and appropriate values for this parameter.\73\ In the
assessment presented in this notice, contributions attributed to
``recent air'' and to ``recent plus past air'' may include some Pb from
the historic use of Pb in paint and gasoline and other sources.
---------------------------------------------------------------------------

    \73\ As noted above, in 2001 when establishing standards for
lead-based paint hazards in most pre-1978 housing and child-occupied
facilities (66 FR 1206), the Agency grappled with the uncertainties
in what environmental levels of historic Pb in soil and dust (from
the historical use of Pb in paint and gasoline) in which specific
medium may cause blood Pb levels that are associated with adverse
effects (see Section II.C).
---------------------------------------------------------------------------

    Further, there are a range of estimates for the air-to-blood ratio
that include estimates higher than that used in 1978 when the standard
was set. We solicit comment and supporting information regarding the
air-to-blood ratio and differences in the available estimates. All of
these factors are important in applying a framework such as that used
in 1978, and we solicit comment, along with supporting information, on
all of these factors.
    Beyond the 1978 framework illustrated above, EPA recognizes a
variety of approaches can be used in translating the current evidence
to a level for the standard. With this notice, EPA solicits comment on
the 1978 standard setting framework and on alternate approaches and the
factors that are relevant to those approaches.
ii. Exposure- and Risk-Based Considerations
    To inform judgments about a range of levels for the standard that
could provide an appropriate degree of public health protection, in
addition to considering the health effects evidence, EPA will also
consider the quantitative estimates of exposure and health risks
attributable to policy-relevant Pb upon meeting specific alternative
levels of alternative Pb standards and the uncertainties in the
estimated exposures and risks, as discussed above in Section III.B. As
discussed above, the risk assessment conducted by EPA is based on
exposures that have been estimated for children of less than 7 years of
age in six case studies. The assessment estimated the risk of adverse
neurocognitive effects in terms of IQ decrements associated with total
and policy-relevant Pb exposures, including incidence of different
levels of IQ loss in three of the six case studies. In so doing, EPA is
mindful of the important uncertainties and limitations that are
associated with the exposure and risk assessments. For example, with
regard to the risk assessment important uncertainties include those
related to estimation of blood Pb concentration-response functions,
particularly for blood Pb concentrations at and below the lower end of
those represented in the epidemiological studies characterized in the
Criteria Document.
    EPA also recognizes important limitations in the design of, and
data and methods employed in, the exposure and risk analyses. For
example, the available monitoring data for Pb, relied upon for
estimating current conditions for the urban case studies is quite
limited, in that monitors are not located near some of the larger known
Pb sources, which provides the potential for underestimation of current
conditions, and there is uncertainty about the proximity of existing
monitors to other Pb sources potentially influencing exposures, such as
old urban roadways and areas where housing with Pb paint has been
demolished. All of these limitations raise uncertainty as to whether
these data adequately capture the magnitude of ambient Pb
concentrations to which the target population is currently exposed.
Additionally, EPA recognizes that there is not sufficient information
available to evaluate all relevant sensitive groups (e.g., adults with
chronic kidney disease) or all Pb-related adverse health effects (e.g.,
neurological effects other than IQ decrement, immune system effects,
adult cardiovascular or renal effects), and the scope of our analyses
was generally limited to estimating exposures and risks in six case
studies intended to illustrate a variety of Pb exposure situations
across the U.S., with three of them focused on specific areas in three
cities. As noted above, however, coordinated intensive efforts over the
last 20 years have yielded a substantial decline in blood Pb levels in
the United States. Recent NHANES data (2003-2004) yield blood lead
level estimates for children age 1 to 5 years of 1.6 [mu]g/dL (median)
and 3.9 [mu]g/dL (90th percentile). These median and 90th percentile
national-level data are lower than modeled values generated for the
three location-specific urban case studies current conditions scenarios
(see footnote 39). It is noted, however, that the urban case studies
and the NHANES study are likely to differ with regard to factors
related to Pb exposure, including ambient air levels.
    EPA also recognizes limitations in our ability to characterize the
contribution of policy-relevant Pb to total Pb exposure and Pb-related
health risk. For example, given various limitations of our modeling
tools, blood Pb levels associated with air-related exposure pathways
and current levels of Pb emitted to the air (including via
resuspension) may fall between the estimates for ``recent air'' and
those for ``recent'' plus ``past air''. However, there are limitations
associated with the indoor dust Pb models that affect our ability to
discern differences in the recent air category among different
alternate air quality scenarios and both categories may include Pb in
soil and dust from the historical use of Pb in paint.
    With these limitations in mind, EPA will consider the estimates of
IQ loss associated with policy-relevant Pb at air Pb concentrations
near those currently occurring in urban areas as illustrated by
conditions in the three cities chosen for the location-specific urban
case studies, e.g., 0.09 to 0.36 [mu]g/m\3\ as a maximum quarterly
average or 0.17 to 0.56 [mu]g/m\3\ as a maximum monthly average.
Recognizing, as described above, that estimates of IQ loss associated
with air-related exposure pathways and current levels of Pb emitted to
the air (including via resuspension) may fall between the estimates for
``recent air'' and those for ``recent'' plus ``past air'', EPA will
consider ranges reflecting those two categories. Further, as noted
above, we will focus on risk estimates derived using the LLL (log-
linear with low exposure linearization) concentration-response
function.
    The ambient air Pb related IQ loss (based on LLL function)
associated with the median IQ loss for current conditions in the three
location-specific case studies (see Tables 5-9 and 5-10 of the Risk
Assessment Report)--estimated to fall between the estimates for recent
air (0.6-0.7 points) and those for recent plus past air (2.9 points)--
appears to be of a magnitude in the range that CASAC considered to be
highly significant from a public health perspective (e.g., a

population IQ loss of 1-2 points). Comparable estimates for the current
conditions scenarios in the general urban case study are still more
significant with estimates for the general urban case study ranging
from 1.3-1.8 for recent air and 3.2-3.6 for recent plus past air. For
the primary Pb smelter case study, in which air quality exceeds the
current NAAQS, IQ loss reductions in the recent plus past air category
associated with the alternate NAAQS levels of 0.2 and 0.5 [mu]g/m\3\
ranging from 4.0 to 4.9 points IQ loss for the subarea.
    Focusing only on the recent air estimates, estimates of IQ loss
(based on the LLL function) associated with policy-relevant Pb at the
95th percentile of population total IQ loss are greater than 1 point
for all current conditions scenarios in all three urban case studies
for which the lowest air Pb concentrations are 0.09 [mu]g/m\3\ maximum
quarterly average, and 0.17 [mu]g/m\3\ maximum monthly average.
    EPA will also consider the extent to which alternative standard
levels below current conditions are estimated to reduce blood Pb levels
and associated health risk in young children (Tables 4-1 through 4-4 in
the Staff Paper), looking first to the estimates of total blood Pb. In
the general urban case study, blood Pb levels for the median of the
population associated with the lowest alternative NAAQS (0.02 [mu]g/
m\3\) are estimated to be reduced from levels in the two current
conditions scenarios by 14% (0.3 [mu]g/dL) and 24% (0.5 [mu]g/dL),
respectively. For the 95th percentile of the population, the estimated
reductions are similar in terms of percentage, but are higher in
absolute values (1.7 and 1.0 [mu]g/dL). For the three location-specific
urban case studies, median blood Pb estimates associated with the
lowest alternative standard are reduced from those associated with
current conditions by approximately 10% in the Chicago and Cleveland
study areas and 6% in the Los Angeles study area; similar percent
reductions are estimated at the 95th percentile total blood Pb. For the
localized subarea of the primary Pb smelter case study, a 65% reduction
in both median and 95th percentile blood Pb (3 and 8.1 [mu]g/dL,
respectively) is estimated for the lowest alternative NAAQS as compared
to the current NAAQS.\74\
---------------------------------------------------------------------------

    \74\ This can be compared to reductions in blood Pb, for the
primary Pb smelter case study subarea estimated to be associated
with a change in the level from the current standard to the 0.2
[mu]g/m\3\ level (either averaging time) which are approximately 45-
50% for both the median and 95th percentile values.
---------------------------------------------------------------------------

    EPA will also consider the extent to which specific levels of
alternative Pb standards reduce the estimated risks in terms of IQ loss
attributable to policy-relevant exposures to Pb (Tables 4-3 and 4-4 in
the Staff Paper). For the general urban case study, estimated
reductions in median Pb-related IQ loss associated with reduced
exposures at the lowest alternative NAAQS level (0.02 [mu]g/m\3\) were
0.5 and 0.7 points (LLL function) for the two current conditions
scenarios. Reductions at the 95th percentile were of a similar
magnitude. Among the three location-specific case study areas,
estimated reductions in median Pb-related IQ loss associated with
reduced exposures at the lowest alternate NAAQS as compared to current
conditions range from 0.4 to 0.6 points for the high-end concentration-
response function to 0.1 to 0.2 points for the low-end concentration-
response functions, with estimates for the LLL function ranging from
0.2 to 0.3 points. The reduction at the 95th percentile, based on the
LLL function, is 0.3-0.4 points. Reduced exposures associated with the
lowest alternative NAAQS in the primary Pb smelter case study subarea
as compared with the current NAAQS (which is not currently met by this
area) were more substantial, ranging from 2.8 points at the median and
3 points at the 95th percentile (based on LLL function).
    Based on estimated reductions in Pb-associated IQ loss discussed
above, EPA observes that estimates for the 95th percentile of the
population are quite similar to (for the LLL concentration-response
function) or smaller (for the high- and low-end concentration-response
functions) than those at the median for all case studies. This is
because of the nonlinear relationship between IQ decrement and blood Pb
level such that relatively smaller IQ decrement is associated with
changes in blood Pb at higher blood Pb levels.
    Reductions in air Pb concentrations from current conditions to meet
the lower alternative NAAQS (0.02 and 0.05 [mu]g/m\3\, maximum monthly
mean) are estimated to reduce the number of children having Pb-related
IQ loss greater than one point by one half to one percent in each of
the three location-specific urban case studies. More specifically,
within the three study areas this corresponds to a range of
approximately 100 to 3,000 fewer children having total IQ loss greater
than 1.0 for an alternative standard of 0.02 [mu]g/m\3\, maximum
monthly mean. Further, just meeting the lowest alternative standard in
these three study areas is estimated to reduce the number of children
having an IQ loss greater than seven points by one to two percent. This
corresponds to a range of approximately 350 (for the Cleveland study
area) up to 8,000 (for the Chicago study area) fewer children with
total Pb-related IQ loss greater than 7.0.
    As discussed above, CASAC considered a population IQ loss of 1-2
points to be highly significant from a public health perspective.
Estimates of IQ loss associated with policy-relevant Pb are of a
magnitude that appears to fall near or within this range for air
quality scenarios involving levels at or above 0.09 [mu]g/m\3\, maximum
quarterly mean, or 0.17 [mu]g/m\3\, maximum monthly mean. Estimated
reductions in risk associated with reducing air Pb concentrations from
current conditions (in the urban case studies) to the two lower
alternative levels evaluated (0.02 and 0.05 [mu]g/m\3\) appear to range
from a few tenths to just below one IQ point (for the LLL
concentration-response function) (and up to 1.5 IQ points for the
highest concentration-response function). Based on estimated changes in
risk across the population associated with the two lower alternative
levels (as compared to current conditions), reductions in the number of
children with total Pb-related IQ loss greater than 1 or greater than 7
are estimated to be on the order of hundreds to thousands of children
in the three location-specific urban case studies.
    In considering the exposure and risk information with regard to a
level for the standard, EPA notes that at the time the standard was
set, the Agency recognized a particular blood Pb level as ``safe''.
Today, current evidence does not support the recognition of a ``safe''
level. This is generally reflected in the concentration-response
functions used in the risk assessment and in CASAC recommendations on
these functions with regard to a lack of a threshold. EPA will
therefore consider a different approach in this review.
    In considering these risk estimates, EPA is mindful of CASAC's
recommendation regarding the public health significance of a population
loss of 1 to 2 IQ points, the significant implications of potential
shifts in the distribution of IQ for the exposed population, and other
unquantified Pb-related health effects. Based on these factors and the
range of estimates summarized above for IQ loss associated with policy-
relevant Pb for the current conditions scenarios of the location-
specific case studies, we recognize that some may consider reducing the
NAAQS as important from a public health perspective (from air-related
ambient Pb) relative to that afforded by the current standard.

    In considering the public health significance of IQ loss beyond
CASAC's recommendation on this issue, we note that some may consider
that any IQ loss at the population level is of potential public health
significance. That is, there is no amount of IQ loss at the population
level that is clearly recognized as being of no importance from a
public health perspective. On the other hand, we also recognize that
some may hold different views. Thus, the magnitude of IQ loss that
could be allowed by a standard that protects public health with an
adequate margin of safety is clearly a public health policy judgment to
be made by the Administrator.
    In considering the magnitudes of IQ loss estimated in our
assessment for the lowest alternative levels considered, EPA will focus
on total IQ loss and on the contribution to total IQ loss from policy-
relevant pathways. In so doing, we recognize that nonair contributions
to total Pb-related IQ loss are estimated to reach and exceed an IQ
loss of 1-2 points, and we also recognize that air Pb contributions are
generally of a much smaller magnitude. Thus, we recognize that it may
be appropriate to consider smaller estimates of IQ loss from air Pb
contributions (e.g., less than 1 point IQ loss) in identifying the
appropriate target for the policy-relevant component.
    Placing weight on incremental changes in policy-relevant Pb-related
IQ loss of less than one point IQ would lead to consideration of the
lower standard levels evaluated in the risk assessment as part of a
judgment as to what standard would protect public health with an
adequate margin of safety. EPA recognizes, however, the significant
uncertainties in the quantitative risk estimates and that uncertainty
in the estimates increases with increasing difference of the air
quality scenarios from current conditions. Thus, to the extent that
incremental exposure reductions achieved through lowering the NAAQS
might contribute to incremental reductions in children's blood Pb and
to associated reductions in health effects, consideration of NAAQS
levels below 0.1 [mu]g/m\3\ (e.g., the lower levels included in the
risk assessment of 0.02 and 0.05 [mu]g/m\3\) may be appropriate. On the
other hand, to the extent that the uncertainties and limitations in the
exposure and risk assessments are judged to be so great as to prevent
meaningful conclusions from being drawn for these low alternative
standard levels, consideration of such low levels may not be
appropriate.
    If the policy goal for the Pb NAAQS was to be defined, for example,
so as to provide protection that limited estimates of IQ loss from
policy-relevant exposures to no more than 1-2 points IQ loss at the
population-level, EPA notes that standard levels in the range of 0.1 to
0.2 [mu]g/m\3\ may achieve that goal. We also note that even with lower
levels of the standard evaluated, while the range of policy-relevant IQ
loss estimates is lower, the upper end of the range still extends up to
and in some cases above 1 point IQ loss. We note, however, appreciably
greater uncertainty associated with these estimates that increases with
increasing difference of the alternative standards from current
conditions.
    Alternatively, if the policy goal was to be defined so as to
provide somewhat greater public health protection by limiting the air-
related component of risk to somewhat less than 1 point IQ loss at the
population level, this would suggest greater consideration for
standards in the lower part of the range evaluated (0.02-0.05 [mu]g/
m\3\). Such a goal might reflect recognition that nonair sources, in
and of themselves, are estimated to contribute 1-2 points or more of IQ
loss, such that the incremental risk for policy-relevant Pb is adding
to a level of total Pb exposure that is already in a range that can be
reasonably judged to be highly significant from a public health
perspective. We note, however that considering standards in this lower
range places greater weight on the more highly uncertain risk estimates
and thus would be more precautionary in nature.
iii. CASAC Advice and Recommendations
    Beyond the evidence- and risk/exposure-based information discussed
above, EPA's consideration of the level for the NAAQS will also take
into account the advice and recommendations of CASAC, based on their
review of the Criteria Document and drafts of the Staff Paper and the
related technical support document, as well as comments from the public
on drafts of the Staff Paper and related technical support document.
Public comments pertaining to the level of the standard recommended
appreciable reductions in the level, e.g., setting it at 0.2 [mu]g/m\3\
or less.
    In their advice to the Agency during this review CASAC has
recognized the importance of both the health effects evidence and the
exposure and risk information in selecting the level for the standard
(Henderson, 2007a,b). In two separate letters, CASAC has stated that it
is the unanimous judgement of the CASAC Lead Panel that the primary
NAAQS should be ``substantially lowered'' to ``a level of about 0.2
[mu]g/m\3\ or less'', reflecting their view of the health effects
evidence (Henderson, 2007a,b). In their March 2007 letter conveying
comments on the pilot phase risk assessment, CASAC based their
recommendation as to level on consideration of the health effects
evidence they provided initial recommendations that the level should be
substantially lower, reflecting their view of the evidence itself.
    The CASAC Pb Panel also provided advice regarding how the Agency
should consider IQ loss estimates derived from the risk assessment in
selecting a level for the standard. The Panel stated that they consider
a population loss of 1-2 IQ points to be ``highly significant from a
public health perspective.'' Further they recommended that ``the
primary Pb standard should be set so as to protect 99.5% of the
population from exceeding that IQ loss.'' The Agency anticipates
further advice from CASAC with regard to level at the time of their
review of this ANPR.
iv. Policy Options
    In considering alternative levels of the primary Pb standard, EPA
will consider the health effects evidence and the exposure and risk
assessment, as well as the important uncertainties and limitations in
the evidence and the assessment results. To help inform public health
policy judgments, we specifically solicit comment on levels of IQ loss
considered to be significant from a public health perspective.
Additionally, we solicit comment on the magnitude of IQ loss associated
with exposures to ambient Pb by the pathways categorized as ``recent
air'' in the risk assessment described in this notice that are
considered to be significant from a public health perspective. We also
solicit comment on the approach of adopting a public health policy goal
of limiting policy-relevant air exposure such that the incremental
blood Pb level (and the associated resulting IQ loss) are below a
specified level (e.g., to a magnitude of 0.5 or 1 [mu]g/dL, or other
alternative values).
    The EPA takes note of the views of CASAC on these matters,
summarized above, the conclusions and recommendations in the OAQPS
Staff Paper,\75\ and the views of public commenters. We also note other
views,

including retaining the current standard level or a range of
alternative levels that includes the upper end of the alternative
standards considered in the risk assessment (i.e., 0.5 [mu]g/m\3\ as a
maximum monthly average). The EPA recognizes that there may be
differing interpretations of the available evidence, the public health
significance of various changes in population IQ loss, and various
aspects of the evidence and exposure and risk assessments, including
important uncertainties and limitations associated with the evidence
and assessments. Thus, EPA solicits comment on the range of alternative
standard levels identified above, as well as on other alternative
levels, up to and including the current level, and the rationale upon
which such views are based.
---------------------------------------------------------------------------

    \75\ The OAQPS Staff Paper recommends consideration of a range
of alternative standard levels from as high as 0.1 to 0.2 [mu]g/m\3\
down to the lower levels evaluated in the risk assessment of 0.02 to
0.05 [mu]g/m\3\.
---------------------------------------------------------------------------

IV. The Secondary Standard

    This section presents information relevant to the review of the
secondary Pb NAAQS, including information on the welfare effects
associated with Pb exposures, results of the screening-level ecological
risk assessment, and considerations related to evaluating the adequacy
of the current standard and alternative standards that might be
appropriate for the Administrator to consider.

A. Welfare Effects Information

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

turbulence and other factors govern the bioavailability of Pb in
surface waters (CD, Section 7.2.2).
    Lead exists in the aquatic environment in various forms and under
various chemical and physical parameters which determine the ability of
Pb to cause adverse effects either from dissolved Pb in the water
column or Pb in sediment. Current levels of Pb in water and sediment
also vary widely depending on the source of Pb. Conditions exist in
which adverse effects to organisms and thereby ecosystems may be
anticipated given experimental results. It is unlikely that dissolved
Pb in surface water constitutes a threat to ecosystems that are not
directly influenced by point sources. For Pb in sediment, the evidence
is less clear. It is likely that some areas with long-term historical
deposition of Pb to sediment from a variety of sources as well as areas
influenced by point sources have the potential for adverse effects to
aquatic communities. The long residence time of Pb in sediment and its
ability to be resuspended by turbulence make Pb likely to be a factor
for the foreseeable future. Criteria have been developed to indicate
concentrations of Pb in water and sediment below which no adverse
effects are expected to aquatic organisms. These values may be used to
identify areas in which there is the potential for adverse effects to
receptors based on current concentrations of Pb in water and sediment.

B. Screening Level Ecological Risk Assessment

    This section presents a brief summary of the screening-level
ecological risk assessment conducted by EPA for this review. The
assessment is described in detail in Lead Human Exposure and Health
Risk Assessments and Ecological Risk Assessment for Selected Areas,
Pilot Phase (ICF, 2006). Funding constraints have precluded performance
of a full-scale ecological risk assessment. The discussion here is
focused on the screening level assessment performed in the pilot phase
(ICF, 2006) and takes into consideration CASAC recommendations with
regard to interpretation of this assessment (Henderson, 2007a, b). The
following summary focuses on key features of the approach used in the
assessment and presents only a brief summary of the results of the
assessment. A complete presentation of results is provided in the pilot
phase Risk Assessment Report (ICF, 2006) and summarized in Chapter 6 of
the Staff Paper.
1. Design Aspects of Assessment and Associated Uncertainties
    The screening level risk assessment involved several location-
specific case studies and a national-scale surface water and sediment
screen. The case studies included areas surrounding a primary Pb
smelter and a secondary Pb smelter, as well as a location near a
nonurban roadway. An additional case study for an ecologically
vulnerable location was identified and described (ICF, 2006), but
resource constraints have precluded risk analysis for this location.
    The case study analyses were designed to estimate the potential for
ecological risks associated with exposures to Pb emitted into ambient
air. Soil, surface water, and/or sediment concentrations were estimated
from available monitoring data or modeling analysis, and then compared
to ecological screening benchmarks to assess the potential for
ecological impacts from Pb that was emitted into the air. Results of
these comparisons are not definitive estimates of risk, but rather
serve to identify those locations at which there is the greatest
likelihood for adverse effect. Similarly, the national-scale screening
assessment evaluated surface water and sediment monitoring locations
across the United States for the potential for ecological impacts
associated with atmospheric deposition of Pb. The reader is referred to
the pilot phase Risk Assessment Report (ICF, 2006) for details on the
use of this information and models in the screening assessment.
    The measures of exposure for these analyses are total Pb
concentrations in soil, dissolved Pb concentrations in fresh surface
waters (water column), and total Pb concentrations in freshwater
sediments. The hazard quotient (HQ) approach was then used to compare
Pb media concentrations with ecological screening values. The exposure
concentrations were estimated for the three case studies and the
national-scale screening analyses as described below:
     For the primary Pb smelter case study, measured
concentrations of total Pb in soil, dissolved Pb in surface waters, and
total Pb in sediment were used to develop point estimates for sampling
clusters thought to be associated with atmospheric Pb deposition,
rather than Pb associated with nonair sources, such as runoff from
waste storage piles.
     For the secondary Pb smelter case study, concentrations of
Pb in soil were estimated using fate and transport modeling based on
EPA's MPE methodology (USEPA, 1998) and data available from similar
locations.
     For the near roadway nonurban case study, measured soil
concentration data collected from two interstate sampling locations,
one with fairly high-density development (Corpus Christi, Texas) and
another with medium-density development (Atlee, Virginia), were used to
develop estimates of Pb in soils for each location.
     For the national-scale surface water and sediment
screening analyses, measurements of dissolved Pb concentrations in
surface water and total Pb in sediment for locations across the United
States were compiled from available databases (USGS, 2004). Air
emissions, surface water discharge, and land use data for the areas
surrounding these locations were assessed to identify locations where
atmospheric Pb deposition may be expected to contribute to potential
ecological impacts. The exposure assessment focused on these locations.
    The ecological screening values used in this assessment were
developed from the Eco-SSLs methodology, EPA's recommended ambient
water quality criteria, and sediment screening values developed by
MacDonald and others (2000, 2003). Soil screening values were derived
for this assessment using the Eco-SSL methodology with the toxicity
reference values for Pb (USEPA, 2005d, 2005e) and consideration of the
inputs on diet composition, food intake rates, incidental soil
ingestion, and contaminant uptake by prey (details are presented in
Section <IP_ADDRESS> and Appendix L, of ICF, 2006). Hardness-specific
surface water screening values were calculated for each site based on
EPA's recommended ambient water quality criteria for Pb (USEPA, 1984).
For sediment screening values, the assessment relied on sediment
``threshold effect concentrations'' and ``probable effect
concentrations'' developed by MacDonald et al. (2000). The methodology
for these sediment criteria is described more fully in section <IP_ADDRESS>
and Appendix M of the pilot phase Risk Assessment Report (ICF, 2006).
    The HQ is calculated as the ratio of the media concentration to the
ecotoxicity screening value, and represented by the following equation:

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

    For each case study, HQ values were calculated for each location
where either modeled or measured media concentrations were available.
Separate soil HQ values were calculated for each

ecological receptor group for which an ecotoxicity screening value has
been developed (i.e., birds, mammals, soil invertebrates, and plants).
HQ values less than 1.0 suggest that Pb concentrations in a specific
medium are unlikely to pose significant risks to ecological receptors.
HQ values greater than 1.0 indicate that the expected exposure exceeds
the ecotoxicity screening value and that there is a potential for
adverse effects.
    There are several uncertainties that apply across case studies
noted below:
     The ecological risk screen is limited to specific case
study locations and other locations for which dissolved Pb data were
available and evaluated in the national-scale surface water and
sediment screens. In identifying sites for inclusion in the assessment,
efforts were made to ensure that the Pb exposures assessed were
attributable to airborne Pb and not dominated by nonair sources.
However, there is uncertainty as to whether other sources might have
actually contributed to the Pb exposure estimates.
     A limitation to using the selected ecotoxicity screening
values is that they might not be sufficient to identify risks to some
threatened or endangered species or unusually sensitive aquatic
ecosystems (e.g., CD, p. AX7-110).
     The methods and database from which the surface water
screening values (i.e., the AWQC for Pb) were derived is somewhat
dated. New data and approaches (e.g., use of pH as indicator of
bioavailability) may now be available to estimated the aquatic toxicity
of Pb (CD, Sections AX<IP_ADDRESS> and AX<IP_ADDRESS>).
     No adjustments were made for sediment-specific
characteristics that might affect the bioavailability of Pb in
sediments in the derivation of the sediment quality criteria used for
this ecological risk screen (CD, Sections 7.2.1 and AX<IP_ADDRESS>; Appendix
M, ICF, 2006). Similarly, characteristics of soils for the case study
locations were not evaluated for measures of bioavailability.
     Although the screening value for birds used in this
analysis is based on reasonable estimates for diet composition and
assimilation efficiency parameters, it was based on a conservative
estimate of the relative bioavailability of Pb in soil and natural
diets compared with water soluble Pb added to an experimental pellet
diet (Appendix L, ICF, 2006).
2. Summary of Results
    The following is a brief summary of key observations related to the
results of the screening-level ecological risk assessment. A more
complete discussion of the results is provided in Chapter 6 of the
Staff Paper and the complete presentation of the assessment and results
is presented in the pilot phase Risk Assessment Report (ICF, 2006).
     The national-scale screen of surface water data initial
identified some 42 sample locations of which 15 were then identified as
unrelated to mining sites and having water column levels of dissolved
Pb that were greater than hardness adjusted chronic criteria for the
protection of aquatic life (with one location having a HQ of 15),
indicating a potential for adverse effect if concentrations were
persistent over chronic periods. Acute criteria were not exceeded at
any of these locations. The extent to which air emissions of Pb have
contributed to these surface water Pb concentrations is unclear.
     In the national-scale screen of sediment data associated
with the 15 surface water sites described above, threshold effect
concentration-based HQs at nine of these sites exceeded 1.0.
Additionally, HQs based on probable effect concentrations exceeded 1.0
at five of the sites, indicating probable adverse effects to sediment
dwelling organisms. Thus, sediment Pb concentrations at some sites are
high enough that there is a likelihood that they would cause adverse
effects to sediment dwelling organisms. However, the contribution of
air emissions to these concentrations is unknown.
     In the primary Pb smelter case study, all three of the
soil sampling clusters (including the ``reference areas'') had HQs that
exceeded 1.0 for birds. Samples from one cluster also had HQs greater
than 1.0 for plants and mammals. The surface water sampling clusters
all had measurements below the detection limit of 3.0 [mu]g/L. However,
three sediment sample clusters had HQs greater than 1.0. In summary,
the concentrations of Pb in soil and sediments exceed screening values
for these media indicating potential for adverse effects to terrestrial
organisms (plants, birds and mammals) and to sediment dwelling
organisms. While the contribution to these Pb concentrations from air
as compared to nonair sources is not quantified, air emissions from
this facility are substantial (see Appendix D, USEPA 2007b; ICF 2006).
     In the secondary Pb smelter case study, the soil
concentrations, developed from soil data for similar locations,
resulted in avian HQs greater than 1.0 for all distance intervals
evaluated. The scaled soil concentrations within 1 km of the facility
also showed HQs greater than 1.0 for plants, birds, and mammals. These
estimates indicate a potential for adverse effect to those receptor
groups.
     In the nonurban, near roadway case study, HQs for birds
and mammals were greater than 1.0 at all but one of the distances from
the road. Plant HQs were greater than 1.0 at the closest distance. In
summary, HQs above one were estimated for plants, birds and mammals,
indicating potential for adverse effect to these receptor groups.

C. Considerations in Review of the Standard

    This section presents an integrative synthesis of information in
the Criteria Document together with EPA analyses and evaluations. EPA
notes that the final decision on retaining or revising the current
secondary Pb standard is a public policy judgment to be made by the
Administrator. The Administrator's final decision will draw upon
scientific information and analyses about welfare effects, exposure and
risks, as well as judgments about the appropriate response to the range
of uncertainties that are inherent in the scientific evidence and
analyses.
    The NAAQS provisions of the Act require the Administrator to
establish secondary standards that, in the judgment of the
Administrator, are requisite to protect the public welfare from any
known or anticipated adverse effects associated with the presence of
the pollutant in the ambient air. In so doing, the Administrator seeks
to establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that secondary
standards be set to eliminate all risk of adverse welfare effects, but
rather at a level requisite to protect public welfare from those
effects that are judged by the Administrator to be adverse.
    The following discussion starts with background information on the
current standard (Section IV.C.1). The general approach for this
current review is summarized in Section IV.C.2. Considerations with
regard to the adequacy of the current standard are discussed in section
IV.C.3, with evidence and exposure-risk-based considerations in
subsections IV.C.3.a and b, respectively, followed by a summary of
CASAC advice and recommendations (section IV.C.3.c) and, lastly,
solicitation of comment on the broad range of policy options (section
IV.C.3.d). Considerations with regard to elements of alternative
standards are discussed in Section IV.C.4.

1. Background on the Current Standard
    The current standard was set in 1978 to be identical to the primary
standard (1.5 [mu]g Pb/m\3\, as a maximum arithmetic mean averaged over
a calendar quarter), the basis for which is summarized in Section
III.C.1. At the time the standard was set, the Agency concluded that
the primary air quality standard would adequately protect against known
and anticipated adverse effects on public welfare, as the Agency stated
that it did not have evidence that a more restrictive secondary
standard was justified. In the rationale for this conclusion, the
Agency stated that the available evidence cited in the 1977 Criteria
Document indicated that ``animals do not appear to be more susceptible
to adverse effects from lead than man, nor do adverse effects in
animals occur at lower levels of exposure than comparable effects in
humans'' (43 FR 46256). The Agency recognized that Pb may be deposited
on the leaves of plants and present a hazard to grazing animals. With
regard to plants, the Agency stated that Pb is absorbed but not
accumulated to any great extent by plants from soil, and that although
some plants may be susceptible to Pb, it is generally in a form that is
largely nonavailable to them. Further the Agency stated that there was
no evidence indicating that ambient levels of Pb result in significant
damage to manmade materials and Pb effects on visibility and climate
are minimal.
    The secondary standard was subsequently considered during the 1980s
in development of the 1986 Criteria Document (USEPA, 1986a) and the
1990 Staff Paper (USEPA, 1990). In summarizing OAQPS staff conclusions
and recommendations at that time, the 1990 Staff Paper stated that a
qualitative assessment of available field studies and animal
toxicological data suggested that ``domestic animals and wildlife are
as susceptible to the effects of lead as laboratory animals used to
investigate human lead toxicity risks.'' Further, the 1990 Staff Paper
highlighted concerns over potential ecosystem effects of Pb due to its
persistence, but concluded that pending development of a stronger
database that more accurately quantifies ecological effects of
different Pb concentrations, consideration should be given to retaining
a secondary standard at or below the level of the then-current
secondary standard of 1.5 [mu]g/m\3\.
2. Approach for Current Review
    To evaluate whether it is appropriate to consider retaining the
current secondary Pb standard, or whether consideration of revisions is
appropriate, EPA is considering an approach in this review like that
used in the Staff Paper that considers the evidence and risk analyses.
This approach recognizes that the available welfare effects evidence
generally reflects laboratory-based evidence of toxicological effects
on specific organisms exposed to concentrations of Pb at which
scientists generally agree that adverse effects are likely to occur. It
is widely recognized, however, that environmental exposures are likely
to be at lower concentrations and/or accompanied by significant
confounding factors (e.g., other metals, acidification), which
increases our uncertainty about the likelihood and magnitude of the
organism and ecosystem response.
3. Adequacy of the Current Standard
a. Evidence-Based Considerations
    In considering the welfare effects evidence with respect to the
adequacy of the current standard, EPA will consider not only the array
of evidence newly assessed in the Criteria Document but also that
assessed in the 1986 Criteria Document and summarized in the 1990 Staff
Paper. As discussed extensively in the latter two documents, there was
a significantly improved characterization of environmental effects of
Pb in the ten years after the Pb NAAQS was set. And, in the subsequent
nearly 20 years, many additional studies on Pb effects in the
environment are now available (2006 Criteria Document). Some of the
more relevant aspects of the evidence available since the standard was
set include the following:
     A more quantitative determination of the mobility,
distribution, uptake, speciation, and fluxes of atmospherically
delivered Pb in terrestrial ecosystems shows that the binding of Pb to
organic materials in the soil slows its mobility through soil and may
prevent uptake by plants (CD, Sections 7.1.2, 7.1.5, AX<IP_ADDRESS>,
AX<IP_ADDRESS>, AX<IP_ADDRESS> and AX7.1.2 ). Therefore, while atmospheric
deposition of Pb has decreased, Pb may be more persistent in some
ecosystems than others and may remain in the active zone of the soil,
where exposure may occur, for decades (CD, Sections 7.1.2, AX7.1.2 and
AX<IP_ADDRESS>).
     Plant toxicity may occur at lower levels than previously
identified as determined by data considered in development of Eco-SSLs
(CD, pp. 7-11 to 7-12, AX7-16 and Section AX<IP_ADDRESS>), although the
range of reported soil Pb effect levels is large (tens to thousands of
mg/kg soil).
     Avian and mammalian toxicity may occur at lower levels
than those previously identified, although the range of Pb effect
levels is large (<1 to >1,000 mg Pb/kg bw-day) (CD, p. 7-12, Section
AX<IP_ADDRESS>).
     There is an expanded understanding of the fate and effects
of Pb in aquatic ecosystems and of the distribution and concentrations
of Pb in surface waters throughout the United States (CD, Section
AX7.2.2).
     New methods for assessing the toxicity of metals to water
column and sediment-dwelling organisms and data collection efforts (CD,
Sections 7.2.1, 7.2.2, AX7.2.2, and AX<IP_ADDRESS>) have improved our
understanding of Pb aquatic toxicity and findings include an indication
that in some estuarine systems Pb deposited during historic usage of
leaded gasoline may remain in surface sediments for decades. (CD, p. 7-
23).
     A larger dataset of aquatic species assessed with regard
to Pb toxicity, and findings of lower effect levels for previously
untested species (CD, p. AX7-176 and Section AX<IP_ADDRESS>).
     Currently available studies have also shown effects on
community structure, function and primary productivity, although some
confounders (such as co-occurring pollutants) have not been well
addressed (CD, Section AX<IP_ADDRESS>).
     Evidence in ecological research generally indicates the
value of a critical loads approach; however, current information on Pb
critical loads is lacking for many processes and interactions involving
Pb in the environment and work is ongoing (CD, Section 7.3).
    Given the full body of current evidence, despite wide variations in
Pb concentrations in soils throughout the country, Pb concentrations
are likely in excess of concentrations expected from geologic or other
non-anthropogenic forces. In particular, the deposition of gasoline-
derived Pb into forest soils has produced a legacy of slow moving Pb
that remains bound to organic materials despite the removal of Pb from
most fuels and the resulting dramatic reductions in overall deposition
rates (CD, Section AX<IP_ADDRESS>). For areas influenced by point sources of
air Pb that meet the current standard, concentrations of Pb in soil may
exceed by many orders of magnitude the concentrations which are
considered harmful to laboratory organisms (CD, Section 3.2 and
AX<IP_ADDRESS>).
    There are several difficulties in quantifying the role of current
ambient Pb in the environment: Some Pb deposited before the standard
was

enacted is still present in soils and sediments; historic Pb from
gasoline continues to move slowly through systems as does current Pb
derived from both air and nonair sources. Additionally, the evidence of
adversity in natural systems is very sparse due in no small part to the
difficulty in determining the effects of confounding factors such as
multiple metals or factors influencing bioavailability in field
studies. However, the evidence summarized above and in Section 4.2 of
the Staff Paper and described in detail in the Criteria Document
informs our understanding of Pb in the environment today and evidence
of environmental Pb exposures of potential concern.
    Conditions exist in which Pb-associated adverse effects to aquatic
organisms and thereby ecosystems may be anticipated given experimental
results. While the evidence does not indicate that dissolved Pb in
surface water constitutes a threat to those ecosystems that are not
directly influenced by point sources, the evidence regarding Pb in
sediment is less clear (CD, Sections AX<IP_ADDRESS>.2 and AX7.2.4). It is
likely that some areas with long term historical deposition of Pb to
sediment from a variety of sources as well as areas influenced by point
sources have the potential for adverse effects to aquatic communities.
The long residence time of Pb in sediment and its ability to be
resuspended by turbulence make Pb contamination likely to be a factor
for the foreseeable future. Based on this information, the Staff Paper
concluded that the evidence suggests that the environmental levels of
Pb occurring under the current standard, set nearly thirty years ago,
may pose risk of adverse environmental effect.
b. Risk-based Considerations
    In addition to the evidence-based considerations described in the
previous section, the screening level ecological risk assessment is
informative, taking into account key limitations and uncertainties
associated with the analyses.
    The screening level risk assessment involved a comparison of
estimates of environmental media concentrations of Pb to ecological
screening levels to assess the potential for ecological impacts from Pb
that was emitted into the air. Results of these comparisons are not
considered to be definite predictors of risk, but rather serve to
identify those locations at which there is greatest likelihood for
adverse effect. Similarly, the national-scale screening assessment
evaluated the potential for ecological impacts associated with the
atmospheric deposition of Pb released into ambient air at surface water
and sediment monitoring locations across the United States.
    The ecological screening levels employed in the screening level
risk assessment for different media are drawn from different sources.
Consequently there are somewhat different limitations and uncertainties
associated with each. In general, their use here recognizes their
strength in identifying media concentrations with the potential for
adverse effect and their relative nonspecificity regarding the
magnitude of risk of adverse effect.
    As discussed in the previous section, as a result of its
persistence, Pb emitted in the past remains today in aquatic and
terrestrial ecosystems of the United States. Consideration of the
environmental risks associated with the current standard is complicated
by the environmental burden associated with air Pb concentrations that
exceeded the current standard, predominantly in the past.
    Concentrations of Pb in soil and sediments associated with the
primary Pb smelter case study exceeded screening values for those media
indicating potential for adverse effect in terrestrial organisms
(plants, birds and mammals) and in sediment dwelling organisms. While
the contribution to these Pb concentrations from air as compared to
nonair sources has not been quantified, air emissions from this
facility are substantial (see Appendix D, USEPA 2007b; ICF 2006).
Additionally, estimates of Pb concentration in soils associated with
the nonurban near roadway case study and the secondary Pb smelter case
study were also associated with HQs above 1 for plants, birds and
mammals, indicating potential for adverse effect to those receptor
groups. The industrial facility in the secondary Pb smelter case study
is much younger than the primary Pb smelter and apparently became
active less than ten years prior to the establishment of the current
standard.
    The national-scale screens, which are not focused on particular
point source locations, indicate the ubiquitous nature of Pb in aquatic
systems of the United States today. Further the magnitude of Pb
concentrations in several aquatic systems exceeded screening values. In
the case of the national-scale screen of surface water data, 15
locations were identified with water column levels of dissolved Pb that
were greater than hardness adjusted chronic criteria for the protection
of aquatic life (with one location having a HQ as high as 15),
indicating a potential for adverse effect if concentrations were
persistent over chronic periods. Further, sediment Pb concentrations at
some sites in the national-scale screen were high enough that the
likelihood that they would cause adverse effects to sediment dwelling
organisms may be considered ``probable''.
    A complicating factor in interpreting the findings for the
national-scale screening assessments is the lack of clear apportionment
of Pb contributions from air as compared to nonair sources, such as
industrial and municipal discharges. While the contribution of air
emissions to the elevated concentrations has not been quantified,
documentation of historical trends in the sediments of many water
bodies has illustrated the sizeable contribution that airborne Pb can
have on aquatic systems (e.g., Section 2.8.1). This documentation also
indicates the greatly reduced contribution in many systems as compared
to decades ago (presumably reflecting the banning of Pb-additives from
gasoline used by cars and trucks). However, the timeframe for removal
of Pb from surface sediments into deeper sediment varies across
systems, such that Pb remains available to biological organisms in some
systems for much longer than in others (Section 2.8, CD, pages AX7-141
to AX7-145).
    The case study locations included in the screening assessment, with
the exception of the primary Pb smelter site, are currently meeting the
current Pb standard, yet Pb occurs in some locations at concentrations,
particularly in soil, and aquatic sediment above the screening levels,
indicative of a potential for harm to some terrestrial and sediment
dwelling organisms. While the role of airborne Pb in determining these
Pb concentrations is unclear, the historical evidence indicates that
airborne Pb can create such concentrations in sediments and soil.
Further, environmental concentrations may be related to emissions prior
to establishment of the current standard and such concentrations appear
to indicate a potential for harm to ecological receptors today.
c. CASAC Advice and Recommendations
    In the CASAC letter transmitting advice and recommendations
pertaining to the review of the first draft Staff Paper and draft Pb
Exposure and Risk Assessments, the CASAC Pb panel provided
recommendations regarding the need for a Pb NAAQS, and the adequacy of
the current Pb NAAQS, as well as comments on the draft documents. With
regard to the need for a Pb NAAQS and adequacy of the current NAAQS,
the CASAC letter said:

    The unanimous judgment of the Lead Panel is that lead should not
be delisted as a criteria pollutant, as defined by the Clean Air
Act, for which primary (public health based) and secondary (public
welfare based) NAAQS are established, and that both the primary and
secondary NAAQS should be substantially lowered.

    Specifically with regard to the secondary NAAQS, the CASAC Pb Panel
stated that the December 2006 draft documents presented ``compelling
scientific evidence that current atmospheric Pb concentrations and
deposition--combined with a large reservoir of historically deposited
Pb in soils, sediments and surface waters--continue to cause adverse
environmental effects in aquatic and/or terrestrial ecosystems,
especially in the vicinity of large emissions sources.'' The Panel went
on to state that ``These effects persist in some cases at locations
where current airborne lead concentrations are below the level of the
current primary and secondary lead standards'' and ``Thus, from an
environmental perspective, there are convincing reasons to both retain
lead as a regulated criteria air pollutant and to lower the level of
the current secondary standard.''
    In making this recommendation, the CASAC Pb Panel also cites the
persistence of Pb in the environment, the possibility of some of the
large amount of historically deposited Pb becoming resuspended by
natural events, and the expectation that humans are not uniquely
sensitive among the many animal and plant species in the environment.
In summary, with regard to the recommended level of a revised secondary
standard, the CASAC panel stated that:

    Therefore, at a minimum, the level of the secondary Lead NAAQS
should be at least as low as the lowest-recommended primary lead
standard.

    CASAC provided further advice and recommendations on the Agency's
consideration of the secondary standard in this review in their letter
of September 2007 (Henderson, 2007b). In that letter they recognized
the role of the secondary standard in influencing the long-term
environmental burden of Pb and a need for environmental monitoring to
assess the success of the standard in this role.
d. Policy Options
    In considering the adequacy of the current secondary standard, EPA
will consider, for reasons discussed above in III.C.3.d on the primary
standard, whether it is appropriate to maintain a NAAQS for Pb or to
retain Pb on the list of criteria pollutants. We take note of the views
of CASAC, summarized above, the conclusions and recommendations in the
OAQPS Staff Paper, and the views of public commenters on these
questions. We recognize that there may be differing views on
interpreting or weighing the available information. Thus, EPA solicits
comment related to the questions of delisting and revocation.
    In further considering the adequacy of the current standard in
providing requisite protection from Pb-related adverse effects on
public welfare, EPA will focus on the body of available evidence
(briefly summarized above in Section IV.A). Depending on the
interpretation, the available data and evidence, primarily qualitative,
may suggest the potential for adverse environmental impacts under the
current standard. Given the limited data on Pb effects in ecosystems,
it is necessary to look at evidence of Pb effects on organisms and
extrapolate to ecosystem effects. Therefore, taking into account the
available evidence and current media concentrations in a wide range of
areas, EPA seeks comment on whether the evidence suggests that adverse
effects are occurring, particularly near point sources, under the
current standard. While the role of current airborne emissions is
difficult to apportion, it is conclusive that deposition of Pb from air
sources is occurring and that this ambient Pb is likely to be
persistent in the environment. Historically deposited Pb has persisted,
although location-specific dynamics of Pb in soil result in differences
in the timeframe during which Pb is retained in surface soils or
sediments where it may be available to ecological receptors (USEPA,
2007b, section 2.3.3). EPA seeks comment on the role of deposition of
Pb from current sources and the availability of this Pb to ecological
receptors.
    There is only very limited information available pertinent to
assessing whether groups of organisms which influence ecosystem
function are subject to similar effects as those in humans. The
screening-level risk information, while limited and accompanied by
various uncertainties, also suggests occurrences of environmental Pb
concentrations existing under the current standard that could have
adverse environmental effects. Environmental Pb levels today are
associated with atmospheric Pb concentrations and deposition that have
combined with a large reservoir of historically deposited Pb in
environmental media.
    The EPA takes note of the views of CASAC, summarized above, the
conclusions and recommendations in the OAQPS Staff Paper, and views of
public commenters on the adequacy of the current standard. EPA solicits
comment on the adequacy of the current standard and the rationale upon
which such views are based.
4. Elements of the Standard
    The secondary standard is defined in terms of four basic elements:
indicator, averaging time, level and form, which serve to define the
standard and must be considered collectively in evaluating the welfare
protection afforded by the standards. In considering a revision to the
current standard, EPA will consider the four elements of the standard,
the information available and advice and recommendations from CASAC
regarding how the elements might be revised to provide a secondary
standard for Pb that protects against adverse environmental effect.
    With regard to the pollutant indicator for use in a secondary NAAQS
that provides protection for public welfare from exposure to Pb, EPA
notes that Pb is a persistent pollutant to which ecological receptors
are exposed via multiple pathways. While the evidence indicates that
the environmental mobility and ecological toxicity of Pb are affected
by various characteristics of its chemical form, and the media in which
it occurs, information are insufficient to identify an indicator other
than total Pb that would provide protection against adverse
environmental effect in all ecosystems nationally.
    Lead is a cumulative pollutant with environmental effects that can
last many decades. In considering the appropriate averaging time for
such a pollutant the concept of critical loads may be useful (CD,
Section 7.3). However, information is currently insufficient for such
use in this review.
    There is a general lack of data that would indicate the appropriate
level of Pb in environmental media that may be associated with adverse
effects. The EPA notes the influence of airborne Pb on Pb in aquatic
systems and of changes in airborne Pb on aquatic systems, as
demonstrated by historical patterns in sediment cores from lakes and Pb
measurements (Section 2.8.1; CD, Section AX7.2.2; Yohn et al., 2004;
Boyle et al., 2005), 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) is likely to have significant
beneficial effects on the magnitude of Pb exposures in the environment
and Pb toxicity impacts on natural and managed terrestrial and aquatic
ecosystems in various regions of

the U.S., the Great Lakes and also U.S. territorial waters of the
Atlantic Ocean (Henderson, 2007a, Appendix E). We concur with CASAC's
conclusion that the Agency lacks the relevant data to provide a clear,
quantitative basis for setting a secondary Pb NAAQS that differs from
the primary in indicator, averaging time, level or form. Thus, EPA
solicits comment on the option of a reduction in the secondary standard
consistent with any reduction of the primary standard that would
provide increased protection against adverse environmental effect.
    Beyond the views noted above, EPA recognizes that there may be
differing interpretations of the available evidence and various aspects
of the evidence and exposure and risk information, including on the
important uncertainties and limitations associated with the evidence
and assessment. Thus, EPA solicits additional information pertaining to
and comment on the considerations described above, as well as on other
views with regard to the elements of a secondary standard for Pb, and
the rationale upon which such views are based.

V. Considerations for Ambient Monitoring

    A determination of compliance with the Pb NAAQS for any given area
is made based on ambient air monitoring data collected by State and
local monitoring agencies. This section discusses aspects of the Pb
surveillance monitoring requirements with regards to the adequacy under
the current primary Pb NAAQS as well as under options being considered
for a revised primary Pb NAAQS. These aspects include the sampling and
analysis methods, network design, sampling schedule, and data handling
methods. In addition, this section discusses the need for monitoring in
support of the secondary Pb NAAQS.

A. Sampling and Analysis Methods

    To be used in determination of compliance with the Pb NAAQS, the Pb
data must be collected and analyzed using a Federal Reference Method
(FRM), or a Federal Equivalent Method (FEM). The current FRM for Pb
sampling and analyses is based on the use of a high-volume TSP sampler
to collect the sample and the use of atomic absorption for the analysis
of Pb in the sample (40 CFR 50 Appendix G). There are 21 FEMs currently
approved for Pb-TSP (http://www.epa.gov/ttn/amtic/criteria.html). All

21 FEMs are based on the use of high-volume TSP samplers, but with a
variety of different analysis methods (e.g., XRF and ICP/MS).
    Concerns have been raised over the use of high-volume TSP samplers.
CASAC has commented that TSP samplers have poor precision, that the
upper particle cut size varies widely as a function of wind speed and
direction, and that the spatial non-homogeneity of very coarse
particles cannot be efficiently captured by a national monitoring
network (Henderson, 2007b). For these reasons, CASAC recommended
considering a revision to the Pb reference method to allow sample
collection using PM10 samplers. CASAC suggested that it may
be possible to develop a single quantitative adjustment factor from a
short period of collocated sampling at multiple sites, or a Pb-
PM10/Pb-TSP equivalency ratio could be determined on a
regional or site-specific basis.
    The EPA evaluated the precision and bias of the high-volume Pb-TSP
sampler based on data reported to AQS for collocated samplers and
results of in-field sampler flow audits and laboratory audits for Pb
(Camalier and Rice, 2007). In this evaluation, we found that the
average precision of the high-volume Pb-TSP sampler was approximately
12%, with a standard deviation of 19%, and average sampling bias (based
on flow audits) was -0.7%, with a standard deviation of 4.2%. We also
estimated the average bias for the lab analyses at -1.1% (with a
standard deviation of 5.5%) based on spiked filter audits. Total bias,
which includes bias from both sampling and laboratory analysis, was
estimated at -1.7%, with a standard deviation of 3.4%. This level of
precision and bias is comparable to the goal of the FRM and FEM for
other criteria pollutants (e.g., within 10% for PM2.5, 40
CFR 58 Appendix A). We attempted to look at the precision of low-volume
Pb-PM10 samplers based on data reported to AQS, however, we
did not have enough data (18 paired data points for one site) to make
any conclusions on the precision of this sampler.
    Evaluations of the high-volume TSP sampler have demonstrated that
the sampler's cutpoint can vary between 25 and 50 [mu]m depending on
wind speed and direction (Wedding et al., 1977, McFarland and Rodes,
1979). A study was conducted during the last Pb NAAQS review to
evaluate the effect of wind speed and direction on sampler efficiency
(Purdue, 1988). This demonstration showed that the Pb collection
efficiency of the high-volume TSP sampler ranged from 80% to 90% over a
wide range of wind speeds and directions. In comparison, a study
conducted near a primary Pb smelter indicated that the ratio of Pb-
PM10 to Pb-TSP ranged from 17% to 186% for 22 collocated
samples (Brion, 1988). We believe that the variability of the
collection efficiency of the high-volume TSP sampler does not warrant
the discontinuation of its use. However, with this notice, we are
soliciting comments on this issue.
    We analyzed data from a number of monitoring sites where collocated
Pb-TSP and Pb-PM10 data have been collected in order to
evaluate the appropriateness of using Pb-PM10 data as a
surrogate for Pb-TSP (Cavender, 2007). From this analysis it is clear
that a single relationship can not be made that would allow one to
accurately estimate Pb-TSP concentrations from Pb-PM10
measurements at all sites. However, at many locations it does appear a
strong linear relationship can be shown between Pb-TSP and Pb-PM10
concentrations. As such, it may be feasible for a monitoring agency to
develop a site-specific relationship, using conservative assumptions,
to estimate Pb-TSP based on Pb-PM10 measurements. We invite
comments on the appropriateness of using Pb-PM10 data as a
surrogate for Pb-TSP.
    While all current FRM and FEM are based on the high-volume TSP
sampler, several vendors market low-volume TSP samplers. These samplers
are identical to low-volume PM10 samplers with the exception
of the sampling head and corresponding cut size. These samplers have a
number of advantages over the high-volume TSP sampler including the
capability of sequential sampling (i.e., the ability to collect more
than one sample between operator visits). Sequential sampling would be
highly desirable if the sampling frequency is increased as part of a
change to a monthly averaging period. Currently, the FEM demonstration
requirements [40 CFR 53.33(i)] dictate that the FEM testing must be
performed with an ambient Pb-TSP concentration between 0.5 [mu]g/m\3\
to 4.0 [mu]g/m\3\. Due to the dramatic decrease in ambient Pb
concentrations, there are few (if any) areas in the country where a
vendor could be assured that the average ambient Pb-TSP concentrations
would meet the FEM demonstration requirements during the field testing
period. If the Pb NAAQS is lowered, we believe it is appropriate to
lower the FEM requirement to a level more consistent with current
ambient Pb concentrations and the lowered NAAQS to allow for continued
development and approval of Pb-TSP FEM. We invite comment on the
appropriate range of concentrations for an FEM demonstration.

    We also reviewed the method detection capabilities of the current
lab methods for the FRM and FEM to ensure that these methods had the
necessary sensitivity to accurately measure Pb-TSP at the low
concentrations considered in the Risk Assessment Report and Staff
Paper. Based on data submitted to AQS, the method detection limits for
these methods are all 0.01 [mu]g/m\3\ or less (Rice, 2007). From these
findings, we request comment on whether the current lab analysis
methods are adequate for continued use even at the lowest alternative
NAAQS levels considered in the Risk Assessment Report and Staff Paper.

B. Network Design

    The existing Pb-TSP network has decreased substantially over the
last few decades. In 1980 there were over 900 Pb-TSP sites, this number
has been reduced to approximately 200 sites. These reductions were made
because of substantially reduced ambient Pb concentrations and shifting
priorities to other criteria pollutants. Now several states have no Pb-
TSP monitors resulting in large portions of the country with no data on
current ambient Pb-TSP concentrations. In addition, many of the largest
Pb emitting sources in the country do not have nearby monitors, and
there is substantial uncertainty about ambient air Pb levels resulting
from historic Pb deposits near roadways. For these reasons, we request
comment on whether the existing Pb-TSP network may not be adequate, and
that additional monitoring sites may be needed to determine compliance
with either the current or revised Pb NAAQS.
    The minimum network design requirements are given in 40 CFR 58
Appendix D. The current network design requirements are for 2 FRM or
FEM sites in any area where Pb concentrations exceed or have exceeded
the NAAQS in the most recent 2 years. These requirements may make it
difficult to persuade state and local monitoring agencies to add
monitors in areas without existing monitors. As such, we believe that
these requirements are not adequate and should be modified (as part of
this rulemaking) to ensure monitoring is conducted in areas where NAAQS
violations may occur.
    We request comment on options for improving the coverage of the Pb
network. One option would be to adopt network requirements similar to
those recently promulgated for PM2.5 and ozone which tie the
number of required monitors to the population of the urban area and
ambient Pb concentrations (40 CFR 58 Appendix D). Under this approach,
more monitoring sites would be required in areas with larger
populations and higher Pb concentrations. This approach would result in
improved network coverage in urban areas. However, large Pb emitting
sources that are not in urban areas may still not be monitored.
    A second option would be to require one or more monitors near large
Pb emitting sources. For example, a monitor could be required at the
point near the maximum predicted concentrations for sources with a
potential Pb emission rate of 1 ton per year or more (as provided by
the most recent National Emissions Inventory, or permit data). Clearly,
some effort would be necessary to identify an appropriate emissions
threshold to ensure that all emission sources with the potential to
exceed the NAAQS are monitored without creating undue burden where
there is no potential to exceed the NAAQS. This option would ensure
coverage of the highest Pb emitting sources, but may not provide
adequate coverage in many populated areas where a combination of
smaller emissions sources and re-entrained dust may result in Pb
concentrations in excess of the NAAQS.
    A third option could be created by the combination of the first two
options discussed above: Establish a minimum number of required
monitors in urban areas based on population and ambient Pb
concentrations and require monitors near large Pb emission sources.
This option would provide good monitoring coverage in urban areas and
near Pb emissions sources. Again, care would need to be taken in
establishing an emissions threshold.
    A fourth option would be to utilize the current PM10
network if an acceptable regional or site-specific correlation of Pb-
TSP and Pb-PM10 can be made. This option would provide a
substantial increase in monitoring coverage without requiring a large
investment in new monitoring stations. The current PM10
network has been carefully established to include both rural and urban
ambient levels, though it was not designed to monitor near large Pb
emitting sources. We invite comments on these options as well as
suggestions for additional options to consider for improving the Pb
network.

C. Sampling Schedule

    The current sampling frequency requirement is for one 24-hour
sample every six days [40 CFR 58.12(b)]. For the current NAAQS, which
is based on a quarterly average, the 1-in-6 sampling schedule yields 15
samples per quarter on average with 100% completeness, or 12 samples
with 75% completeness. A change to a monthly averaging period would
result in between 4 and 6 samples per month at the current sampling
frequency. If we change the averaging time to a monthly average, we
would likely need to increase the sampling frequency as 4 samples would
not result in a statistically valid estimate of the actual air quality
for the period.
    Incomplete sampling results in increased uncertainty in the
estimate of actual ambient air quality. While some degree of
uncertainty is unavoidable due to the precision and bias inherent to
the sampling technique, it is important to understand the level of
uncertainty for each sampling option being considered and to select a
sampling frequency which achieves an acceptable level of uncertainty.
We plan to go through the Data Quality Objectives (DQO) process in
order to help us select an appropriate sampling option. The DQO process
is a series of logical steps that guides decision makers to a plan for
the resource-effective acquisition of environmental data. The DQO
process is used to establish performance and acceptance criteria, which
serve as the basis for designing a plan for collecting data of
sufficient quality and quantity to support the goals of the study (EPA,
2006e, EPA/240/B-06/001).
    We are considering several options for sampling frequency. These
options include maintaining the current 1-in-6 day sampling schedule,
increasing the sampling frequency to 1-in-3 day, or increasing the
sampling frequency to 1-in-1 day sampling (i.e., complete sampling). In
addition, we will be considering an option that relates sampling
frequency to recent ambient Pb-TSP concentrations, such that an
increased sampling frequency is required as the recent ambient Pb-TSP
concentrations approach the NAAQS level. Other options that we will be
considering include--
     Increasing sampling time duration (e.g., changing from a
24 hour sampling time duration to a 48 or 72 hour sampling time
duration).
     Allowing for compositing of samples (i.e., analyzing
sequential samples together).
     Allowing for multiple samplers at one site.
    We invite comments on the appropriateness of these sampling options
and suggestions for additional options for consideration.

D. Data Handling

    A number of data handling conventions and computations are
necessary when using ambient monitoring data to determine attainment

or non-attainment of the NAAQS. Recently, we have been codifying these
data handling conventions and computations into a separate appendix for
each NAAQS. As such, we intend to create an appendix for the
interpretation of the Pb NAAQS as part of this rule making. Specific
conventions we are considering and invite comments on at this time
include the following--
     Design values will be developed based on the most recent 3
calendar year period.
     Design values will be rounded to two significant figures
using conventional rounding methodology.
     75% of the expected number of samples is needed for a
quarter to be considered complete, or 50% for a month.
     Only one period (i.e., one month or one quarter depending
on the final form of the standard) is needed to demonstrate non-
attainment. Two periods would be needed if the NAAQS is based on the
2nd maximum.
     Three full consecutive years of complete data are needed
to re-designate an area attainment from non-attainment.
     Incomplete periods can be used to demonstrate non-
attainment, but not attainment.

E. Monitoring for the Secondary NAAQS

    Currently, the secondary NAAQS is set equal to the primary NAAQS
(1.5 [mu]g/m\3\, maximum quarterly average). We do not expect there to
be ambient air concentrations in excess of the secondary NAAQS in rural
areas that are not associated with a Pb emission source. If the
secondary standard remains equal to the primary standard at the
completion of the current review, we request comment on the option of
developing Pb surveillance monitoring requirements for the primary
NAAQS that will be sufficient to determine compliance with the
secondary NAAQS.
    While additional monitoring may not be necessary to demonstrate
compliance with the secondary NAAQS, CASAC has recommended additional
monitoring to gather information to better inform consideration of the
secondary NAAQS in the next and future reviews. Specifically, CASAC
stated that ``the EPA needs to initiate new measurement activities in
rural areas--which quantify and track changes in lead concentrations in
the ambient air, soils, deposition, surface waters, sediments and
biota, along with other information as may be needed to calculate and
apply a critical loads approach for assessing environmental lead
exposures and risks in the next review cycle'' (Henderson, 2007b).
    We currently monitor ambient Pb in PM2.5 as part of the
IMPROVE network. There are 110 formally designated IMPROVE sites
located in or near national parks and other Class I visibility areas,
virtually all of these being rural. Approximately 80 additional sites
at various urban and rural locations, requested and funded by various
parties, are also informally treated as part of the network. While we
believe it may not be appropriate to rely on either Pb-PM10
or Pb-PM2.5 monitoring to demonstrate compliance with a Pb-
TSP NAAQS, we believe the Pb-PM2.5 measurements provided by
the IMPROVE network can be used as a useful indicator to track changes
in ambient Pb concentrations and resulting Pb deposition in rural areas
that are not directly impacted by a Pb emission source. It may also be
desirable to augment the IMPROVE network with a small ``sentinel''
network of collocated Pb-TSP monitors for a period of time in order to
develop a better understanding of how Pb-PM2.5 and Pb-TSP
relate in these rural areas. Alternatively, since it is likely that at
rural locations nearly all Pb is in the less than 10 [mu]m size range,
we could analyze the PM10 mass samples (which are already
being collected) for Pb for a period of time to develop a better
understanding of how Pb-PM2.5 and Pb-PM10 relate
in these rural areas. We welcome comments on the value and
appropriateness of use of the IMPROVE Pb-PM2.5 data for
assessing trends in ambient air concentrations of Pb, and the need to
collocate a small network of Pb-TSP or Pb-PM10 monitors at
IMPROVE sites.
    The National Water-Quality Assessment (NAWQA), conducted by the
United States Geological Survey, contains data on Pb concentrations in
surface water, bed sediment, and animal tissue for more than 50 river
basins and aquifers throughout the country (CD, AX<IP_ADDRESS>). NAWQA data
are collected during long-term, cyclical investigations wherein study
units undergo intensive sampling for 3 to 4 years, followed by low-
intensity monitoring and assessment of trends every 10 years.
Similarly, the USGS is collaborating with Canadian and Mexican
government agencies on a multi-national project called ``Geochemical
Landscapes'' that has as its long-term goal a soil geochemical survey
of North America (http://minerals.cr.usgs.gov/projects/geochemical_landscapes/index.html
). The Geochemical Landscapes project has the

potential to fill the need for periodic Pb soil sampling. We note the
value of the NAWQA and Geochemical Landscapes data in the assessment of
trends in Pb concentrations in both soil and aquatic systems, and
support the continued collection of this data by the USGS.

VI. Solicitation of Comment

    With the issuance of this ANPR, the Agency is soliciting broad
public input to inform the Agency's proposed rulemaking related to the
review of the Pb NAAQS. As noted in Section I above, this ANPR, as a
consequence of the timing of the Pb NAAQS review relative to the
Agency's initiation of the new NAAQS process, summarizes information
from the OAQPS Staff Paper, and from the Agency's risk assessment and
Criteria Document. In so doing, this notice presents OAQPS staff views
on the adequacy of the current standard and on a range of policy
options for the Administrator's consideration, together with the views
of CASAC and the public as reflected in their comments on the related
documents that have been previously made available for review. The
Agency is soliciting comment on the range of views discussed above as
well as any broader range of options that members of the public feel
appropriate for the Administrator to consider. Comments are solicited
together with the rationales for the views expressed in those comments.
The Agency is also soliciting further advice from CASAC on the issues
discussed in this notice at an upcoming public meeting (announced in a
separate Federal Register notice).
    In soliciting public comment in advance of reaching proposed
decisions on whether to retain or revise the NAAQS under review, the
Agency is interested in general, specific, and technical comments on
all aspects of the rulemaking discussed in this notice and the related
documents. These aspects generally include characterization of Pb in
the ambient environment, characterization of the health effects
evidence and the assessment of human exposure and health risk,
characterization of the environmental effects evidence and
consideration of environmental exposure and risk, as well as an
assessment of the adequacy of the current primary and secondary
standards and of alternative standards for the Administrator's
consideration in reaching proposed decisions in this review of the Pb
NAAQS. We solicit broad comment on these aspects of this rulemaking,
informed by the discussion presented in this notice as well as the more
comprehensive discussion in the Criteria Document, the Staff Paper, and
related risk assessment reports.
    Several types of information pertinent to the characterization of
Pb in the

ambient environment are considered for this review. These include
characterization of sources of Pb, including source distribution within
the U.S. and associated estimates of the magnitude of air emissions.
The currently available information on the magnitude, geographic
distribution and variability of Pb levels in the ambient air is also
considered. Further, given that Pb is a multimedia pollutant,
characterization of Pb includes consideration of atmospheric deposition
and Pb in ambient soil, surface waters and sediment. Comments,
including information and views, are solicited in all of these areas as
well as any other areas related to the characterization of Pb in the
ambient environment that are relevant to this review.
    The current health effects evidence for Pb, evaluated in the
Agency's Criteria Document, encompasses a broad range of information
regarding human exposure to ambient Pb, toxicokinetics of Pb,
biological markers and models of Pb burden in humans, toxicological
effects of Pb in laboratory animals and in vitro test systems, and
epidemiologic studies of human health effects associated with Pb
exposure. In addition, based on the information in the Criteria
Documents, quantitative assessments of human exposures to Pb and
associated health risks as well as environmental exposures and related
risks have been conducted and are presented in related risk assessment
reports. We are soliciting comments, including information and views,
informed by the Criteria Document, Staff Paper, and risk assessment
reports, on characterization of the health effects evidence and
consideration of human exposure and health risk associated with Pb
exposures. Similarly, the Agency is soliciting comment on the
characterization of the environmental effects evidence and
environmental risks of Pb relevant to this review.
    With regard to the primary and secondary standards, a wide range of
views have been expressed, reflecting differing conclusions about the
scientific evidence and quantitative risk assessments and differing
public health and welfare policy judgments about appropriate standards.
These views range from asserting the need for significant strengthening
of the standards to a recommendation in public comments that the Pb
NAAQS should be revoked and/or Pb should be delisted as a criteria
pollutant. We solicit comment on these views as well as on any other
views that are thought to be appropriate for the Agency to consider,
together with rationales for the views expressed. More specifically, we
solicit comment, including views and associated rationale, informed by
the Criteria Document, Staff Paper and related risk assessment reports,
on the adequacy of the current primary and secondary standards. We also
solicit comment on the range of alternative primary and secondary
standards the Agency should consider, with a focus on the four basic
elements of the standards, including indicator, averaging time, level,
and form. Further, we are soliciting comment on the view that it is
appropriate to revoke the NAAQS for Pb or to remove Pb from the list of
criteria pollutants.
    Issues related to Pb surveillance monitoring requirements relevant
to this review are also discussed in this notice. These issues fall
into several areas, including sampling and analysis methods related to
Pb-TSP and Pb-PM10 measurements, monitoring network design,
sampling schedule, and data handling. Specific aspects of monitoring in
support of the secondary standard are also discussed. We are soliciting
comments on the issues related to Pb surveillance monitoring
requirements identified in this notice as well as on other issues
relevant to these requirements in this review.
    The Agency will consider comments received in response to this
notice in reaching proposed decisions in this rulemaking. As noted
above, the public will have an additional opportunity for comment on
the proposed rulemaking, which will further inform the Administrator's
final decisions on the Pb NAAQS.

VII. Statutory and Executive Order Reviews

Executive Order 12866: Regulatory Planning and Review

    Under Executive Order (EO) 12866 (58 FR 51735, October 4, 1993),
this action is a ``significant regulatory action.'' Accordingly, EPA
submitted this action to the Office of Management and Budget (OMB) for
review under EO 12866 and any changes made in response to OMB
recommendations have been documented in the docket for this action.

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