**ENVIRONMENTAL PROTECTION AGENCY**

**40 CFR Parts 50, 53 and 58**

**\[EPA-HQ-OAR-2006-0735; FRL- \]**

**RIN**

**National Ambient Air Quality Standards for Lead**

**AGENCY:** Environmental Protection Agency (EPA).

**ACTION:** Proposed rule.

**SUMMARY:** Based on its review of the air quality criteria and
national ambient air quality standards (NAAQS) for lead (Pb), EPA
proposes to make revisions to the primary and secondary NAAQS for Pb to
provide requisite protection of public health and welfare, respectively,
and to make corresponding revisions in data handling procedures and
ambient air monitoring and reporting requirements for Pb.

With regard to the primary standard for Pb, EPA proposes to revise
various elements of the standard to provide increased protection for
children and other at-risk populations against an array of adverse
health effects, most notably including effects on the developing nervous
system. With regard to the level and indicator of the standard, EPA
proposes to revise the level of the standard to a level within the range
of 0.10 to 0.30 µg/m^3^ in conjunction with retaining the current
indicator of Pb in total suspended particles (Pb-TSP) but with allowance
for the use of Pb-PM~10~ data. EPA also solicits comment on alternative
levels beyond the proposed range up to 0.50 µg/m^3^ and down through
0.02 µg/m^3^. With regard to the averaging time and form of the
standard, EPA proposes two options: (1) to retain the current averaging
time of a calendar quarter and the current not-to-be-exceeded form,
revised so as to apply across a 3-year span, and (2) to revise the
averaging time to a calendar month and the form to the second-highest
monthly average across a 3-year span. EPA also solicits comment on
revising the indicator to Pb-PM~10~ and on associated ranges of levels
that correspond to the ranges of levels associated with the proposed
Pb-TSP indicator identified above. EPA also invites comment on whether
it would ever be appropriate to set a NAAQS for Pb at a level of zero.

With regard to the secondary standard for Pb, EPA proposes to make the
secondary standard identical in all respects to the proposed primary
standard.

EPA is proposing corresponding changes to data handling procedures for
the proposed Pb standards, including the treatment of exceptional
events. EPA is also proposing changes to the ambient air monitoring and
reporting requirements for Pb related to sampling and analysis methods
(including quality assurance requirements and a new Federal reference
method for monitoring Pb in PM~10~), network design (including
population-based and source-based monitoring requirements), sampling
schedule, and data reporting. Finally, EPA is providing guidance on its
proposed approach for implementing the proposed revised primary and
secondary standards for Pb.

Consistent with the terms of a court order, by September 2008 the
Administrator will sign a notice of final rulemaking for publication in
the Federal Register.

**DATES:** Comments must be received by **\[insert date 60 days after
date of publication in the [Federal Register]{.underline}\]**.

*Public Hearings:* EPA intends to hold public hearings on this proposed
rule in June 2008 in St. Louis, Missouri and Baltimore, Maryland. These
will be announced in a separate Federal Register notice that provides
details, including specific times and addresses, for these hearings.

**ADDRESSES:** Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2006-0735 by one of the following methods:

-   [www.regulations.gov](http://www.regulations.gov/): Follow the
    on-line instructions for submitting comments.

-   Email: <a-and-r-Docket@epa.gov>.

-   Fax: 202-566-9744.

-   Mail: Docket No. EPA-HQ-OAR-2006-0735, Environmental Protection
    Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW., Washington,
    DC 20460. Please include a total of two copies.

-   Hand Delivery: Docket No. EPA-HQ-OAR-2006-0735, Environmental
    Protection Agency, EPA West, Room 3334, 1301 Constitution Ave., NW,
    Washington, DC. Such deliveries are only accepted during the
    Docket's normal hours of operation, and special arrangements should
    be made for deliveries of boxed information.

*Instructions:* Direct your comments to Docket ID No.
EPA-HQ-OAR-2006-0735. The EPA's policy is that all comments received
will be included in the public docket without change and may be made
available online at [www.regulations.gov](http://www.regulations.gov/),
including any personal information provided, unless the comment includes
information claimed to be Confidential Business Information (CBI) or
other information whose disclosure is restricted by statute. Do not
submit information that you consider to be CBI or otherwise protected
through [www.regulations.gov](http://www.regulations.gov/) or email. The
[www.regulations.gov](http://www.regulations.gov/) website is an
"anonymous access" system, which means EPA will not know your identity
or contact information unless you provide it in the body of your
comment. If you send an email comment directly to EPA without going
through [www.regulations.gov](http://www.regulations.gov/), your email
address will be automatically captured and included as part of the
comment that is placed in the public docket and made available on the
Internet. If you submit an electronic comment, EPA recommends that you
include your name and other contact information in the body of your
comment and with any disk or CD-ROM you submit. If EPA cannot read your
comment due to technical difficulties and cannot contact you for
clarification, EPA may not be able to consider your comment. Electronic
files should avoid the use of special characters, any form of
encryption, and be free of any defects or viruses. For additional
information about EPA's public docket, visit the EPA Docket Center
homepage at <http://www.epa.gov/epahome/dockets.htm>.

*Docket:* All documents in the docket are listed in the
[www.regulations.gov](http://www.regulations.gov/) index. Although
listed in the index, some information is not publicly available, e.g.,
CBI or other information whose disclosure is restricted by statute.
Certain other material, such as copyrighted material, will be publicly
available only in hard copy. Publicly available docket materials are
available either electronically in
[www.regulations.gov](http://www.regulations.gov/) or in hard copy at
the Air and Radiation Docket and Information Center, EPA/DC, EPA West,
Room 3334, 1301 Constitution Ave., NW, Washington, DC. The Public
Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday,
excluding legal holidays. The telephone number for the Public Reading
Room is (202) 566-1744 and the telephone number for the Air and
Radiation Docket and Information Center is (202) 566-1742.

{*Public Hearings:* EPA intends to hold public hearings on this proposed
rule in June 2008 in St. Louis, Missouri and Baltimore, Maryland. These
will be announced in a separate Federal Register notice that will
provide details, including specific times and addresses, for these
hearings.}

**FOR FURTHER INFORMATION CONTACT:** For further information in general
or specifically with regard to sections I through III or VII, contact
Dr. Deirdre Murphy, Health and Environmental Impacts Division, Office of
Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Mail code C504-06, Research Triangle Park, NC 27711; telephone:
919-541-0729; fax: 919-541-0237; email: <Murphy.deirdre@epa.gov>. With
regard to Section IV, contact Mr. Mark Schmidt, Air Quality Analysis
Division, Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Mail code C304-04, Research Triangle
Park, NC 27711; telephone: 919-541-2416; fax: 919-541-1903; email:
<Schmidt.mark@epa.gov>. With regard to Section V, contact Mr. Kevin
Cavender, Air Quality Analysis Division, Office of Air Quality Planning
and Standards, U.S. Environmental Protection Agency, Mail code C304-06,
Research Triangle Park, NC 27711; telephone: 919-541-2364; fax:
919-541-1903; email: <Cavender.kevin@epa.gov>. With regard to Section
VI, contact Mr. Larry Wallace, Air Quality Policy Division, Office of
Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Mail code C539-01, Research Triangle Park, NC 27711; telephone:
919-541-0906; fax: 919-541-0824; email: <Wallace.larry@epa.gov>.

**SUPPLEMENTARY INFORMATION:**

**General Information**

[What Should I Consider as I Prepare My Comments for EPA?]{.underline}

1\. *Submitting CBI.* Do not submit this information to EPA through
[www.regulations.gov](http://www.regulations.gov/) or email. Clearly
mark the part or all of the information that you claim to be CBI. For
CBI information in a disk or CD ROM that you mail to EPA, mark the
outside of the disk or CD ROM as CBI and then identify electronically
within the disk or CD ROM the specific information that is claimed as
CBI. In addition to one complete version of the comment that includes
information claimed as CBI, a copy of the comment that does not contain
the information claimed as CBI must be submitted for inclusion in the
public docket. Information so marked will not be disclosed except in
accordance with procedures set forth in 40 CFR part 2.

2\. *Tips for Preparing Your Comments.* When submitting comments,
remember to:

-   Identify the rulemaking by docket number and other identifying
    information (subject heading, [Federal Register]{.underline} date
    and page number).

-   Follow directions -- the agency may ask you to respond to specific
    questions or organize comments by referencing a Code of Federal
    Regulations (CFR) part or section number.

-   Explain why you agree or disagree, suggest alternatives, and
    substitute language for your requested changes.

-   Describe any assumptions and provide any technical information
    and/or data that you used.

-   If you estimate potential costs or burdens, explain how you arrived
    at your estimate in sufficient detail to allow for it to be
    reproduced.

-   Provide specific examples to illustrate your concerns, and suggest
    alternatives.

-   Explain your views as clearly as possible, avoiding the use of
    profanity or personal threats.

-   Make sure to submit your comments by the comment period deadline
    identified.

[Availability of Related Information]{.underline}

A number of documents relevant to this rulemaking, including the advance
notice of proposed rulemaking (72 FR 71488), the *Air Quality Criteria
for Lead* (Criteria Document) (USEPA, 2006a), the Staff Paper, related
risk assessment reports, and other related technical documents are
available on EPA=s Office of Air Quality Planning and Standards (OAQPS)
Technology Transfer Network (TTN) web site at
<http://www.epa.gov/ttn/naaqs/standards/pb/s_pb_index.html>. These and
other related documents are also available for inspection and copying in
the EPA docket identified above.

**Table of Contents**

The following topics are discussed in this preamble:

I. Background

> A. Legislative Requirements
>
> B. History of Lead NAAQS Reviews
>
> C. Current Related Lead Control Programs
>
> D. Current Lead NAAQS Review

II\. Rationale for Proposed Decision on the Primary Standard

> A. Multimedia, Multipathway Considerations and Background
>
> 1\. Atmospheric Emissions and Distribution of Lead
>
> 2\. Air-related Human Exposure Pathways
>
> 3\. Nonair-related and Air-related Background Human Exposure Pathways
>
> 4\. Contributions to Children's Lead Exposures
>
> B. Health Effects Information
>
> 1\. Blood Lead
>
> a\. Internal Disposition of Lead
>
> b\. Use of Blood Lead as Dose Metric
>
> c\. Air-to-blood Relationships
>
> 2\. Nature of Effects
>
> a\. Broad Array of Effects
>
> b\. Neurological Effects in Children
>
> 3\. Lead-Related Impacts on Public Health
>
> a\. At-risk Subpopulations
>
> b\. Potential Public Health Impacts
>
> 4\. Key Observations
>
> C. Human Exposure and Health Risk Assessments
>
> 1\. Overview of Risk Assessment from Last Review
>
> 2\. Design Aspects of Exposure and Risk Assessments
>
> a\. CASAC Advice
>
> b\. Health Endpoint, Risk Metric and Concentration-response Functions
>
> c\. Case Study Approach
>
> d\. Air Quality Scenarios
>
> e\. Categorization of Policy-relevant Exposure Pathways
>
> f\. Analytical Steps
>
> g\. Generating Multiple Sets of Risk Results
>
> h\. Key Limitations and Uncertainties
>
> 3\. Summary of Results and Key Observations
>
> a\. Blood Pb Estimates
>
> b\. IQ Loss Estimates

D. Conclusions on Adequacy of the Current Primary Standard

> 1\. Background
>
> a\. The Current Standard
>
> b\. Policy Options Considered in the Last Review
>
> 2\. Considerations in the Current Review
>
> a\. Evidence-based Considerations
>
> b\. Exposure- and Risk-based Considerations
>
> 3 CASAC Advice and Recommendations
>
> 4\. Administrator's Proposed Conclusions Concerning Adequacy

E. Conclusions on the Elements of the Standard

> 1\. Indicator
>
> 2\. Averaging Time and Form
>
> 3\. Level for a Pb NAAQS with Pb-TSP Indicator
>
> a\. Evidence-based Considerations
>
> b\. Exposure- and Risk-based Considerations
>
> c\. CASAC Advice and Recommendations
>
> d\. Administrator's Proposed Conclusion Concerning Level
>
> 4\. Level for a Pb NAAQS with Pb-PM~10~ Indicator
>
> a\. Considerations with Regard to Particles not Captured by PM~10~
>
> b\. CASAC Advice
>
> c\. Alternative Levels

F. Proposed Decision on the Primary Standard

III\. Rationale for Proposed Decision on the Secondary Standard

> A. Welfare Effects Information
>
> B. Screening Level Ecological Risk Assessment
>
> 1\. Design Aspects of the Assessment and Associated Uncertainties
>
> 2\. Summary of Results
>
> C. The Secondary Standard
>
> 1\. Background on the Current Standard
>
> 2\. Approach for Current Review
>
> 3\. Conclusions on Adequacy of the Current Standard
>
> a\. Evidence-based Considerations
>
> b\. Risk-based Considerations
>
> c\. CASAC Advice and Recommendations
>
> d\. Administrator's Proposed Conclusions on Adequacy of Current
> Standard
>
> 4\. Conclusions and Proposed Decision on the Elements of the Secondary
> Standard

IV\. Proposed Appendix R on Interpretation of the NAAQS for Lead and
Proposed Revisions to the Exceptional Events Rule

A. Background

B. Proposed Appendix R

C. Proposed Revisions to Exceptional Events Information Submission
Schedule

V. Proposed Amendments to Ambient Monitoring Requirements

> A. Sampling and Analysis Methods
>
> 1\. Background
>
> 2\. Proposed Changes
>
> a\. Pb-TSP Sampling Method
>
> b\. Pb-PM~10~ Sampling Method
>
> c\. Analysis Method
>
> d\. FEM Criteria
>
> e\. Quality Assurance
>
> B. Network Design
>
> 1\. Background
>
> 2\. Proposed Changes
>
> C. Sampling Schedule
>
> 1\. Background
>
> 2\. Proposed Changes
>
> D. Monitoring for the Secondary NAAQS
>
> 1\. Background
>
> 2\. Proposed Changes
>
> E. Other Monitoring Regulation Changes
>
> 1\. Reporting of Average Pressure and Temperature
>
> 2\. Special Purpose Monitoring Exemption

VI\. Implementing a Revised Lead NAAQS

A.  Designations for the Lead NAAQS

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1.  Potential Schedule for Designations of A Revised Lead NAAQS

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B.  Lead Nonattainment Area Boundaries

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1.  County-Based Boundaries

2.  MSA-Based Boundaries

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C.  Classifications

D.  Section 110(a)(2) Lead NAAQS Infrastructure Requirements

E.  Attainment Dates

F.  Attainment Planning Requirements

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1.  Schedule for Attaining A Revised Pb NAAQS

2.  RACM for Lead Nonattainment Areas

3.  Demonstration of Attainment for Lead Nonattainment Areas

4.  Reasonable Further Progress (RFP)

5.  Contingency Measures

6.  Nonattainment New Source Review (NSR) and Prevention of Significant
    Deterioration (PSD) Requirements

7.  Emissions Inventories

8.  Modeling

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G.  General Conformity

> H. Transition from the Current NAAQS to a Revised NAAQS for Lead

VII\. Statutory and Executive Order Reviews

References

I. Background

A. Legislative Requirements

Two sections of the Clean Air Act (Act) govern the establishment and
revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list each air pollutant that "in his
judgment, cause or contribute to air pollution which may reasonably be
anticipated to endanger public health and welfare" and whose "presence .
. . in the ambient air results from numerous or diverse mobile or
stationary sources" and to issue air quality criteria for those that are
listed. Air quality criteria are to "accurately reflect the latest
scientific knowledge useful in indicating the kind and extent of all
identifiable effects on public health or welfare which may be expected
from the presence of \[a\] pollutant in ambient air . . .". Section 109
(42 U.S.C. 7409) directs the Administrator to propose and promulgate
"primary" and "secondary" NAAQS for pollutants listed under section 108.
Section 109(b)(1) defines a primary standard as one "the attainment and
maintenance of which in the judgment of the Administrator, based on
\[air quality\] criteria and allowing an adequate margin of safety, are
requisite to protect the public health."[^1] A secondary standard, as
defined in Section 109(b)(2), must "specify a level of air quality the
attainment and maintenance of which, in the judgment of the
Administrator, based on criteria, is requisite to protect the public
welfare from any known or anticipated adverse effects associated with
the presence of \[the\] pollutant in the ambient air."[^2]

The requirement that primary standards include an adequate margin of
safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable degree
of protection against hazards that research has not yet identified.
*Lead Industries Association v. EPA*, 647 F.2d 1130, 1154 (D.C. Cir
1980), *cert. denied*, 449 U.S. 1042 (1980); *American Petroleum
Institute v. Costle*, 665 F.2d 1176, 1186 (D.C. Cir. 1981), *cert.
denied*, 455 U.S. 1034 (1982). Both kinds of uncertainties are
components of the risk associated with pollution at levels below those
at which human health effects can be said to occur with reasonable
scientific certainty. Thus, in selecting primary standards that include
an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration
levels, see *Lead Industries Association v.* EPA, 647 F.2d at 1156 n.
51, but rather at a level that reduces risk sufficiently so as to
protect public health with an adequate margin of safety.

The selection of any particular approach to providing an adequate margin
of safety is a policy choice left specifically to the Administrator's
judgment. *Lead Industries Association v. EPA*, 647 F.2d at 1161-62. In
addressing the requirement for an adequate margin of safety, EPA
considers such factors as the nature and severity of the health effects
involved, the size of the population(s) at risk, and the kind and degree
of the uncertainties that must be addressed.

In setting standards that are "requisite" to protect public health and
welfare, as provided in section 109(b), EPA's task is to establish
standards that are neither more nor less stringent than necessary for
these purposes. *Whitman v. America Trucking Associations*, 531 U.S.
457, 473. Further the Supreme Court ruled that "\[t\]he text of §
109(b), interpreted in its statutory and historical context and with
appreciation for its importance to the CAA as a whole, unambiguously
bars cost considerations from the NAAQS-setting process. . ." *Id*. at
472.[^3] Section 109(d)(1) of the Act requires that "\[n\]ot later than
December 31, 1980, and at 5-year intervals thereafter, the Administrator
shall complete a thorough review of the criteria published under section
108 and the national ambient air quality standards promulgated under
this section and shall make such revisions in such criteria and
standards and promulgate such new standards as may be appropriate in
accordance with section 108 and subsection (b) of this section." Section
109(d)(2)(A) requires that \"The Administrator shall appoint an
independent scientific review committee composed of seven members
including at least one member of the National Academy of Sciences, one
physician, and one person representing State air pollution control
agencies.\" Section 109(d)(2)(B) requires that, \"\[n\]ot later than
January 1, 1980, and at five-year intervals thereafter, the committee
referred to in subparagraph (A) shall complete a review of the criteria
published under section 108 and the national primary and secondary
ambient air quality standards promulgated under this section and shall
recommend to the Administrator any new national ambient air quality
standards and revisions of existing criteria and standards as may be
appropriate under section 108 and subsection (b) of this section.\"
Since the early 1980\'s, this independent review function has been
performed by the Clean Air Scientific Advisory Committee (CASAC) of
EPA's Science Advisory Board.

B. History of Lead NAAQS Reviews

On October 5, 1978 EPA promulgated primary and secondary NAAQS for Pb
under section 109 of the Act (43 FR 46246). Both primary and secondary
standards were set at a level of 1.5 micrograms per cubic meter
(μg/m^3^), measured as Pb in total suspended particulate matter
(Pb-TSP), not to be exceeded by the maximum arithmetic mean
concentration averaged over a calendar quarter. This standard was based
on the 1977 *Air Quality Criteria for Lead* (USEPA, 1977).

A review of the Pb standards was initiated in the mid-1980s. The
scientific assessment for that review is described in the 1986 *Air
Quality Criteria for Lead* (USEPA, 1986a), the associated Addendum
(USEPA, 1986b) and the 1990 Supplement (USEPA, 1990a). As part of the
review, the Agency designed and performed human exposure and health risk
analyses (USEPA, 1989), the results of which were presented in a 1990
Staff Paper (USEPA, 1990b). Based on the scientific assessment and the
human exposure and health risk analyses, the 1990 Staff Paper presented
options for the Pb NAAQS level in the range of 0.5 to 1.5 µg/m^3^, and
suggested the second highest monthly average in three years for the form
and averaging time of the standard (USEPA, 1990b). After consideration
of the documents developed during the review and the significantly
changed circumstances since Pb was listed in 1976, the Agency did not
propose any revisions to the 1978 Pb NAAQS. In a parallel effort, the
Agency developed the broad, multi-program, multimedia, integrated *U.S.*
*Strategy for Reducing Lead Exposure* (USEPA, 1991). As part of
implementing this strategy, the Agency focused efforts primarily on
regulatory and remedial clean-up actions aimed at reducing Pb exposures
from a variety of nonair sources judged to pose more extensive public
health risks to U.S. populations, as well as on actions to reduce Pb
emissions to air.

C. Current Related Lead Control Programs

States are primarily responsible for ensuring attainment and maintenance
of national ambient air quality standards once EPA has established them.
Under section 110 of the Act (42 U.S.C. 7410) and related provisions,
States are to submit, for EPA approval, State implementation plans
(SIP's) that provide for the attainment and maintenance of such
standards through control programs directed to sources of the pollutants
involved. The States, in conjunction with EPA, also administer the
prevention of significant deterioration program (42 U.S.C. 7470--7479)
for these pollutants. In addition, Federal programs provide for
nationwide reductions in emissions of these and other air pollutants
through the Federal Motor Vehicle Control Program under Title II of the
Act (42 U.S.C. 7521--7574), which involves controls for automobile,
truck, bus, motorcycle, nonroad engine, and aircraft emissions; the new
source performance standards under section 111 of the Act (42 U.S.C.
7411); and the national emission standards for hazardous air pollutants
under section 112 of the Act (42 U.S.C. 7412).

As Pb is a multimedia pollutant, a broad range of Federal programs
beyond those that focus on air pollution control provide for nationwide
reductions in environmental releases and human exposures. In addition,
the Centers for Disease Control and Prevention (CDC) programs provide
for the tracking of children's blood Pb levels nationally and provide
guidance on levels at which medical and environmental case management
activities should be implemented (CDC, 2005a; ACCLPP, 2007).[^4] In
1991, the Secretary of the Health and Human Services (HHS) characterized
Pb poisoning as the "number one environmental threat to the health of
children in the United States" (Alliance to End Childhood Lead
Poisoning. 1991). In 1997, President Clinton created, by Executive Order
13045, the President's Task Force on Environmental Health Risks and
Safety Risks to Children in response to increased awareness that
children face disproportionate risks from environmental health and
safety hazards (62 FR 19885).[^5] By Executive Orders issued in October
2001 and April 2003, President Bush extended the work for the Task Force
for an additional three and a half years beyond its original charter (66
FR 52013 and 68 FR 19931). The Task Force set a Federal goal of
eliminating childhood Pb poisoning by the year 2010 and reducing Pb
poisoning in children was the Task Force's top priority.

Federal abatement programs provide for the reduction in human exposures
and environmental releases from in-place materials containing Pb (e.g.,
Pb-based paint, urban soil and dust and contaminated waste sites).
Federal regulations on disposal of Pb-based paint waste help facilitate
the removal of Pb-based paint from residences.[^6] Further, in 1991, EPA
lowered the maximum levels of Pb permitted in public water systems from
50 parts per billion (ppb) to 15 ppb (56 FR 26460).

Federal programs to reduce exposure to Pb in paint, dust and soil are
specified under the comprehensive federal regulatory framework developed
under the Residential Lead-Based Paint Hazard Reduction Act (Title X).
Under Title X and Title IV of the Toxic Substances Control Act, EPA has
established regulations and associated programs in the following five
categories: (1) Training and certification requirements for persons
engaged in lead-based paint activities; accreditation of training
providers; authorization of State and Tribal lead-based paint programs;
and work practice standards for the safe, reliable, and effective
identification and elimination of lead-based paint hazards; (2) Ensuring
that, for most housing constructed before 1978, lead-based paint
information flows from sellers to purchasers, from landlords to tenants,
and from renovators to owners and occupants; (3) Establishing standards
for identifying dangerous levels of Pb in paint, dust and soil; (4)
Providing grant funding to establish and maintain State and Tribal
lead-based paint programs, and to address childhood lead poisoning in
the highest-risk communities; and (5) Providing information on Pb
hazards to the public, including steps that people can take to protect
themselves and their families from lead-based paint hazards.

Under Title IV of TSCA, EPA established standards identifying hazardous
levels of lead in residential paint, dust, and soil in 2001. This
regulation supports the implementation of other regulations which deal
with worker training and certification, Pb hazard disclosure in real
estate transactions, Pb hazard evaluation and control in Federally-owned
housing prior to sale and housing receiving Federal assistance, and U.S.
Department of Housing and Urban Development grants to local
jurisdictions to perform Pb hazard control. The TSCA Title IV term
\"lead-based paint hazard\" implemented through this regulation
identifies lead-based paint and all residential lead-containing dust and
soil regardless of the source of Pb, which, due to their condition and
location, would result in adverse human health effects. One of the
underlying principles of Title X is to move the focus of public and
private decision makers away from the mere presence of lead-based paint,
to the presence of lead-based paint hazards, for which more substantive
action should be undertaken to control exposures, especially to young
children. In addition the success of the program will rely on the
voluntary participation of states and tribes as well as counties and
cities to implement the programs and on property owners to follow the
standards and EPA\'s recommendations. If EPA were to set unreasonable
standards (e.g., standards that would recommend removal of all Pb from
paint, dust and soil), States and Tribes may choose to opt out of the
Title X Pb program and property owners may choose to ignore EPA\'s
advice believing it lacks credibility and practical value. Consequently,
EPA needed to develop standards that would not waste resources by
chasing risks of negligible importance and that would be accepted by
States, Tribes, local governments and property owners. In addition, a
separate regulation establishes, among other things, under authority of
TSCA section 402, residential Pb dust cleanup levels and amendments to
dust and soil sampling requirements (66 FR 1206).

On March 31, 2008, the Agency issued a new rule (Lead: Renovation,
Repair and Painting Program) to protect children from lead-based paint
hazards. This rule applies to renovators and maintenance professionals
who perform renovation, repair, or painting in housing, child-care
facilities and schools built prior to 1978. It requires that contractors
and maintenance professionals be certified; that their employees be
trained; and that they follow protective work practice standards. These
standards prohibit certain dangerous practices, such as open flame
burning or torching of lead-based paint. The required work practices
also include posting warning signs, restricting occupants from work
areas, containing work areas to prevent dust and debris from spreading,
conducting a thorough cleanup, and verifying that cleanup was effective.
The rule will be fully effective by April 2010. States and tribes may
become authorized to implement this rule, and the rule contains
procedures for the authorization of states, territories, and tribes to
administer and enforce these standards and regulations in lieu of a
federal program.

Programs associated with the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA or Superfund) and Resource
Conservation Recovery Act (RCRA) also implement abatement programs,
reducing exposures to Pb and other pollutants. For example, EPA
determines and implements protective levels for Pb in soil at Superfund
sites and RCRA corrective action facilities. Federal programs, including
those implementing RCRA, provide for management of hazardous substances
in hazardous and municipal solid waste.[^7] For example, Federal
regulations concerning batteries in municipal solid waste facilitate the
collection and recycling or proper disposal of batteries containing
Pb.[^8] Similarly, Federal programs provide for the reduction in
environmental releases of hazardous substances such as Pb in the
management of wastewater (<http://www.epa.gov/owm/>).

A variety of federal nonregulatory programs also provide for reduced
environmental release of Pb containing materials through more general
encouragement of pollution prevention, promotion of reuse and recycling,
reduction of priority and toxic chemicals in products and waste, and
conservation of energy and materials. These include the Resource
Conservation Challenge
(<http://www.epa.gov/epaoswer/osw/conserve/index.htm>), the National
Waste Minimization Program
(<http://www.epa.gov/epaoswer/hazwaste/minimize/leadtire.htm>), "Plug in
to eCycling" (a partnership between EPA and consumer electronics
manufacturers and retailers;
<http://www.epa.gov/epaoswer/hazwaste/recycle/electron/crt.htm#crts>),
and activities to reduce the practice of backyard trash burning
(<http://www.epa.gov/msw/backyard/pubs.htm>).

Efforts such as those programs described above have been successful in
that blood Pb levels in all segments of the population have dropped
significantly from levels observed around 1990. In particular, blood Pb
levels for the general population of children 1 to 5 years of age have
dropped to a median level of 1.6 μg/dL and a level of 3.9 μg/dL for the
90^th^ percentile child in the 2003-2004 National Health and Nutrition
Examination Survey (NHANES) as compared to median and 90^th^ percentile
levels in 1988-1991 of 3.5 μg/dL and 9.4 μg/dL, respectively
(http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm).
These levels (median and 90^th^ percentile) for the general population
of young children[^9] are at the low end of the historic range of blood
Pb levels for general population of children aged 1-5 years. However, as
discussed in Section II.B.1.b, levels have been found to vary among
children of different socioeconomic status and other demographic
characteristics (CD, p. 4-21) and racial/ethnic and income disparities
in blood Pb levels in children persist. The decline in blood Pb levels
in the United States has resulted from coordinated, intensive efforts at
the national, state and local levels. The Agency has continued to
grapple with soil and dust Pb levels from the historical use of Pb in
paint and gasoline and other sources.

EPA's research program, with other Federal agencies, defines, encourages
and conducts research needed to locate and assess serious risks and to
develop methods and tools to characterize and help reduce risks. For
example, EPA's Integrated Exposure Uptake Biokinetic Model for Lead in
Children (IEUBK model) for Pb in children and the Adult Lead Methodology
are widely used and accepted as tools that provide guidance in
evaluating site specific data. More recently, in recognition of the need
for a single model that predicts Pb concentrations in tissues for
children and adults, EPA is developing the All Ages Lead Model (AALM) to
provide researchers and risk assessors with a pharmacokinetic model
capable of estimating blood, tissue, and bone concentrations of Pb based
on estimates of exposure over the lifetime of the individual. EPA
research activities on substances including Pb focus on better
characterizing aspects of health and environmental effects, exposure and
control or management of environmental releases (see
<http://www.epa.gov/ord/researchaccomplishments/index.html>).

D. Current Lead NAAQS Review

EPA initiated the current review of the air quality criteria for Pb on
November 9, 2004 with a general call for information (69 FR 64926). A
project work plan (USEPA, 2005a) for the preparation of the Criteria
Document was released in January 2005 for CASAC and public review. EPA
held a series of workshops in August 2005, inviting recognized
scientific experts to discuss initial draft materials that dealt with
various lead-related issues being addressed in the Pb air quality
criteria document. The first draft of the Criteria Document (USEPA,
2005b) was released for CASAC and public review in December 2005 and
discussed at a CASAC meeting held on February 28-March 1, 2006.

A second draft Criteria Document (USEPA, 2006b) was released for CASAC
and public review in May 2006, and discussed at the CASAC meeting on
June 28, 2006. A subsequent draft of *Chapter 7 - Integrative Synthesis*
(Chapter 8 in the final Criteria Document), released on July 31, 2006,
was discussed at an August 15, 2006 CASAC teleconference. The final
Criteria Document was released on September 30, 2006 (USEPA, 2006a;
cited throughout this preamble as CD). While the Criteria Document
focuses on new scientific information available since the last review,
it integrates that information with scientific criteria from previous
reviews.

In February 2006, EPA released the *Plan for Review of the National
Ambient Air Quality Standards for Lead* (USEPA 2006c) that described
Agency plans and a timeline for reviewing the air quality criteria,
developing human exposure and risk assessments and an ecological risk
assessment, preparing a policy assessment, and developing the proposed
and final rulemakings.

In May 2006, EPA released for CASAC and public review a draft *Analysis
Plan for Human Health and Ecological Risk Assessment for the Review of
the Lead National Ambient Air Quality Standards* (USEPA, 2006d), which
was discussed at a June 29, 2006 CASAC meeting (Henderson, 2006). The
May 2006 assessment plan discussed two assessment phases: a pilot phase
and a full-scale phase. The pilot phase of both the human health and
ecological risk assessments was presented in the draft *Lead Human
Exposure and Health Risk Assessments and Ecological Risk Assessment for
Selected Areas* (ICF, 2006; henceforth referred to as the first draft
Risk Assessment Report) which was released for CASAC and public review
in December 2006. The first draft Staff Paper, also released in December
2006, discussed the pilot assessments and the most policy-relevant
science from the Criteria Document. These documents were reviewed by
CASAC and the public at a public meeting on February 6-7, 2007
(Henderson, 2007a).

Subsequent to that meeting, EPA conducted full-scale human exposure and
health risk assessments, although no further work was done on the
ecological assessment due to resource limitations. A second draft Risk
Assessment Report (USEPA, 2007a), containing the full-scale human
exposure and health risk assessments, was released in July 2007 for
review by CASAC at a meeting held on August 28-29, 2007. Taking into
consideration CASAC comments (Henderson, 2007b) and public comments on
that document, we conducted additional human exposure and health risk
assessments. A final Risk Assessment Report (USEPA, 2007b) and final
Staff Paper (USEPA, 2007c) were released on November 1, 2007.

The final Staff Paper presents OAQPS staff's evaluation of the public
health and welfare policy implications of the key studies and scientific
information contained in the Criteria Document and presents and
interprets results from the quantitative risk/exposure analyses
conducted for this review. Further, the Staff Paper presents OAQPS staff
recommendations on a range of policy options for the Administrator to
consider concerning whether, and if so how, to revise the primary and
secondary Pb NAAQS. Such an evaluation of policy implications is
intended to help "bridge the gap" between the scientific assessment
contained in the Criteria Document and the judgments required of the EPA
Administrator in determining whether it is appropriate to retain or
revise the NAAQS for Pb. In evaluating the adequacy of the current
standard and a range of alternatives, the Staff Paper considered the
available scientific evidence and quantitative risk-based analyses,
together with related limitations and uncertainties, and focused on the
information that is most pertinent to evaluating the basic elements of
national ambient air quality standards: indicator[^10], averaging time,
form[^11], and level. These elements, which together serve to define
each standard, must be considered collectively in evaluating the public
health and welfare protection afforded by the Pb standards. The
information, conclusions, and OAQPS staff recommendations presented in
the Staff Paper were informed by comments and advice received from CASAC
in its reviews of the earlier draft Staff Paper and drafts of related
risk/exposure assessment reports, as well as comments on these earlier
draft documents submitted by public commenters.

Subsequent to completion of the Staff Paper, EPA issued an advance
notice of proposed rulemaking (ANPR) that was signed by the
Administrator on December 5, 2007 (72 FR 71488-71544). The ANPR is one
of the key features of the new NAAQS review process that EPA has
instituted over the past two years to help to improve the efficiency of
the process the Agency uses in reviewing the NAAQS while ensuring that
the Agency's decisions are informed by the best available science and
broad participation among experts in the scientific community and the
public. The new process (described at
[www.epa.gov/ttn/naaqs/](http://www.epa.gov/ttn/naaqs/).) is being
incorporated into the various ongoing NAAQS reviews being conducted by
the Agency, including the current review of the Pb NAAQS.

A public meeting of the CASAC was held on December 12-13, 2007 to
provide advice and recommendations to the Administrator based on its
review of the ANPR and the previously released final Staff Paper and
Risk Assessment Report. Information about this meeting was published in
the Federal Register on November 20, 2007 (72 FR 65335-65336),
transcripts of the meeting are in the Docket for this review and CASAC's
letter to the Administrator (Henderson, 2008) is available on the EPA
web site ([www.epa.gov/sab](http://www.epa.gov/sab)).

A public comment period for the ANPR extended from December 17, 2007
through January 16, 2008 and comments received are in the Docket for
this review. Comments were received from nearly 4000 private citizens,
13 state and local agencies, one federal agency, 3 regional or national
associations of government agencies or officials, 15 nongovernmental
environmental or public health organizations (including one submission
on behalf of a coalition of 23 organizations) and 5 industries or
industry organizations. Although the Agency has not developed formal
responses to comments received on the ANPR, these comments have been
considered in the development of this notice and are generally described
in subsequent sections on proposed conclusions with regard to the
adequacy of the standards and with regard to the Administrator's
proposed decisions on revisions to the standards.

The schedule for completion of this review is governed by a judicial
order in *Missouri Coalition for the Environment, v. EPA* (No.
4:04CV00660 ERW, Sept. 14, 2005). The order governing this review,
entered by the court on September 14, 2005, specifies that EPA sign, for
publication, notices of proposed and final rulemaking concerning its
review of the Pb NAAQS no later than May 1, 2008 and September 1, 2008,
respectively.

This action presents the Administrator's proposed decisions on the
review of the current primary and secondary Pb standards. Throughout
this preamble a number of judgments, conclusions, findings and
determinations proposed by the Administrator are noted. While they
identify the reasoning that supports this proposal, they are not
intended to be final or conclusive in nature. The EPA invites general,
specific, and/or technical comments on all issues involved with this
proposal, including all such proposed judgments, conclusions, findings,
and determinations.

II\. Rationale for Proposed Decision on the Primary Standard

This section presents the rationale for the Administrator's proposed
decision that the current primary standard is not requisite to protect
public health with an adequate margin of safety, and that the existing
Pb primary standard should be revised. With regard to the primary
standard for Pb, EPA is proposing options for the revision of the
various elements of the standard to provide increased protection for
children and other at-risk populations against an array of adverse
health effects, most notably including effects on the developing nervous
system. With regard to the indicator and level of the standard, EPA
proposes the following options: (1) to revise the level of the standard
to a level within the range of 0.10 to 0.30 µg/m^3^ in conjunction with
retaining the current indicator of Pb in total suspended particles (TSP)
but with allowance for the use of Pb-PM~10~ data, and (2) to revise the
indicator to Pb in PM~10~ in conjunction with a level within the range
of from somewhat below 0.10 to 0.20 µg/m^3^. With regard to the form and
averaging time of the standard, under either indicator/level option, EPA
proposes the following options: (1) to retain the current averaging time
of a calendar quarter and the current not-to-be-exceeded form, revised
so as to apply across a 3-year span, and (2) to revise the averaging
time to a calendar month and the form to be the second-highest monthly
average across a 3-year span.

As discussed more fully below, this proposal is based on a thorough
review, in the Criteria Document, of the latest scientific information
on human health effects associated with the presence of Pb in the
ambient air. This proposal also takes into account: (1) staff
assessments of the most policy-relevant information in the Criteria
Document and staff analyses of air quality, human exposure, and health
risks presented in the Staff Paper, upon which staff recommendations for
revisions to the primary Pb standard are based; (2) CASAC advice and
recommendations, as reflected in discussions of the ANPR and drafts of
the Criteria Document and Staff Paper at public meetings, in separate
written comments, and in CASAC's letters to the Administrator; and (3)
public comments received during the development of these documents,
either in connection with CASAC meetings or separately.

In developing this proposal, EPA has drawn upon an integrative synthesis
of the entire body of evidence, published through late 2006, on human
health effects associated with Pb exposure. Some 6000 newly available
studies were considered in this review. As discussed below in section
II.B, this body of evidence addresses a broad range of health endpoints
associated with exposure to Pb (EPA, 2006a, chapter 8), and includes
hundreds of epidemiologic studies conducted in the U.S., Canada, and
many countries around the world since the time of the last review (EPA,
2006a, chapter 6). In considering this evidence, EPA focuses on those
health endpoints that have been demonstrated to be caused by exposure to
Pb. This proposal also draws upon the results of the quantitative
exposure and risk assessments, discussed below in section II.C.
Evidence- and exposure/risk-based considerations that form the basis for
the Administrator's proposed decisions on the adequacy of the current
standard and on the elements of the proposed alternative standards are
discussed below in section II.D.2 and II.D.3, respectively.

A. Multimedia, Multipathway Considerations and Background

1\. Atmospheric Emissions and Distribution of Lead

Lead is emitted into the air from many sources encompassing a wide
variety of source types (Staff Paper, Section 2.2). Further, once
deposited out of the air, Pb can subsequently be resuspended into the
air (CD, pp. 2-62 to 2-66). There are over 100 categories of sources of
Pb emissions included in the EPA's 2002 National Emissions Inventory
(NEI)[^12], the top five of which include: mobile sources[^13];
industrial, commercial, institutional and process boilers; utility
boilers; iron and steel foundries; and primary Pb smelting (Staff Paper
Section 2.2). Further, there are some 13,000 industrial, commercial or
institutional point sources in the 2002 NEI, each with one or more
processes that emit Pb to the atmosphere. Among these sources, more than
one thousand are estimated to emit at least a tenth of a ton of Pb per
year (Staff Paper, Section 2.2.3).

Lead emitted to the air is predominantly in particulate form, with the
particles occurring in many sizes. Once emitted, Pb particles can be
transported long or short distances depending on their size, which
influences the amount of time spent in aerosol phase. In general, larger
particles tend to deposit more quickly, within shorter distances from
emissions points, while smaller particles will remain in aerosol phase
and travel longer distances before depositing. Additionally, once
deposited, Pb particles can be resuspended back into the air and undergo
a second dispersal. Thus, the atmospheric transport processes of Pb
contribute to its broad dispersal, with larger particles generally
occurring closer to the point of emission than smaller particles (CD,
Section 2.3.1 and p. 3-3).

Airborne concentrations of Pb in total suspended particulate matter
(Pb-TSP) in the United States have fallen substantially since the
current Pb NAAQS was set in 1978.[^14] Despite this decline, there have
still been a small number of areas, associated with large stationary
sources of Pb, that have not met the NAAQS over the past few years. The
average maximum quarterly mean concentration for the time period
2003-2005 among source-oriented monitoring sites in the U.S. is 0.48
µg/m^3^, while the corresponding average for non source-oriented sites
is 0.03 µg/m^3^.[^15] Among all monitoring sites for this time period,
the average and median maximum quarterly mean levels are 0.17 µg/m^3^
and 0.03 µg/m^3^, respectively. Coincident with the historical trend in
reduction in Pb levels, however, there has also been a substantial
reduction in number of Pb-TSP monitoring sites. A review of the
network's coverage of the highest Pb emitting sources (as identified in
the 2002 NEI) indicated that many of the highest Pb emitting sources in
the 2002 NEI do not have nearby Pb-TSP monitors. This finding of a lack
of monitors near large sources indicates that we are likely currently
underestimating the extent of occurrences of relatively higher Pb
concentrations (Staff Paper, Section 2.3.2).

2\. Air-related Human Exposure Pathways

As when the standard was set in 1978, we recognize that exposure to air
Pb can occur directly by inhalation, or indirectly by ingestion of
Pb-contaminated food, water or nonfood materials including dust and soil
(43 FR 46247). This occurs as Pb emitted into the ambient air is
distributed to other environmental media and can contribute to human
exposures via indoor and outdoor dusts, outdoor soil, food and drinking
water, as well as inhalation of air (CD, pp. 3-1 to 3-2). Accordingly,
people are exposed to Pb emitted into ambient air by both inhalation and
ingestion pathways. In general, air-related pathways include those
pathways where Pb passes through ambient air on its path from a source
to human exposure. EPA considers risks to public health from exposure to
Pb that was emitted into the air as relevant to our consideration of the
primary standard. Therefore , we consider these air-related pathways to
be policy-relevant in this review. Air-related Pb exposure pathways
include: inhalation of airborne Pb (that may include Pb emitted into the
air and deposited and then resuspended); and ingestion of Pb that, once
airborne, has made its way into indoor dust, outdoor dust or soil,
dietary items (e.g., crops and livestock), and drinking water (e.g., CD,
Figure 3-1).

Ambient air Pb contributes to Pb in indoor dust through transport of Pb
suspended in ambient air that is then deposited indoors and through
transport of Pb that has deposited outdoors from ambient air and is
transported indoors in ways other than through ambient air (CD, Section
3.2.3; Adgate et al., 1998). For example, infiltration of ambient air
into buildings brings airborne Pb indoors where deposition of particles
contributes to Pb in dust on indoor surfaces (CD, p. 3-28; Caravanos et
al., 2006a). Indoor dust may be ingested (e.g., via hand-to-mouth
activity by children; CD, p. 8-12) or may be resuspended through
household activities and inhaled (CD, p. 8-12). Ambient air Pb can also
deposit onto outdoor surfaces (including surface soil) with which humans
may come into contact (CD, Section 2.3.2; Farfel et al., 2003; Caravanos
et al., 2006a,b). Human contact with this deposited Pb may result in
incidental ingestion from this exposure pathway and may also result in
some of this Pb being carried indoors (e.g., on clothes and shoes)
adding to indoor dust Pb (CD, p. 3-28; von Lindern et al., 2003a, b).
Additionally, Pb from ambient air that deposits on outdoor surfaces may
also be resuspended and carried indoors in the air where it can be
inhaled. Thus, indoor dust receives air-related Pb directly from ambient
air coming indoors and also more indirectly, after deposition from
ambient air onto outdoor surfaces.

As mentioned above, humans may contact Pb in dust on outdoor surfaces,
including surface soil and other materials, that has deposited from
ambient air (CD, Section 3.2; Caravanos et al., 2006a; Mielke et al.,
1991; Roess et al 1980). Human exposure to this deposited Pb can occur
through incidental ingestion, and, when the deposited Pb is resuspended,
by inhalation. Atmospheric deposition of Pb also contributes to Pb in
vegetation, both as a result of contact with above ground portions of
the plant and through contributions to soil and transport of Pb into
roots (CD, pp. 7-9 and AXZ7-39; USEPA, 1986a, Sections 6.5.3 and
7.2.2.2.1). Livestock may subsequently be exposed to Pb in vegetation
(e.g., grasses and silage) and in surface soils via incidental ingestion
of soil while grazing (USEPA 1986a, Section 7.2.2.2.2). Atmospheric
deposition is estimated to comprise a significant proportion of Pb in
food (CD, p. 3-48; Flegel et al 1990; Juberg et al., 1997; Dudka and
Miller, 1999). Atmospheric deposition outdoors also contributes to Pb in
surface waters, although given the widespread use of settling or
filtration in drinking water treatment, air-related Pb is generally a
small component of Pb in treated drinking water (CD, Section 2.3.2 and
p. 3-33).

Air-related exposure pathways are affected by changes to air quality,
including changes in concentrations of Pb in air and/or changes in
atmospheric deposition of Pb. Further, because of its persistence in the
environment, Pb deposited from the air may contribute to human and
ecological exposures for years into the future (CD, pp. 3-18 to 3-19,
pp. 3-23 to 2-24). Thus, because of the roles in human exposure pathways
of both air concentration and air deposition, and of the persistence of
Pb, once deposited, some pathways respond more quickly to changes in air
quality than others. Pathways most directly involving Pb in ambient air
and exchanges of ambient air with indoor air respond more quickly while
pathways involving exposure to Pb deposited from ambient air into the
environment generally respond more slowly (CD, pp. 3-18 to 3-19).

Exposure pathways tied most directly to ambient air, and that
consequently have the potential to respond relatively more quickly to
changes in air Pb, include inhalation of ambient air, and ingestion of
Pb in indoor dust directly contaminated with Pb from ambient air. [^16]
Lead from ambient air contaminates indoor dust directly when outdoor air
comes inside (through open doors or windows, for example) and Pb in that
air deposits to indoor surfaces (Caravanos et al., 2006a; CD, p. 8-22).
This includes Pb that was previously deposited outdoors and is then
resuspended and transported indoors. Lead in dust on outdoor surfaces
also responds to air deposition (Caravanos et al., 2006). Pathways in
which the air quality impact is reflected over a somewhat longer time
frame generally are associated with outdoor atmospheric deposition, and
include ingestion pathways such as the following: (1) ingestion of Pb in
outdoor soil; (2) ingestion of Pb in indoor dust indirectly contaminated
with Pb from the outdoor air (e.g., "tracking in" of Pb deposited to
outdoor surface soil, as compared to ambient air transport of
resuspended outdoor soil); (3) ingestion of Pb in diet that is
attributable to deposited air Pb, and; (4) ingestion of Pb in drinking
water that is attributable to deposited air Pb (e.g., Pb entering water
bodies used for drinking supply).

3\. Nonair-related and Air-related Background Human Exposure Pathways

As when the standard was set in 1978, there continue to be multiple
sources of exposure, both air-related and others (nonair-related). Human
exposure pathways that are not air-related are those in which Pb does
not pass through ambient air. These pathways as well as air-related
human exposure pathways that involve natural sources of Pb to air are
considered policy-relevant background in this review. In the context of
NAAQS for other criteria pollutants which are not multimedia in nature,
such as ozone, the term policy-relevant background is used to
distinguish anthropogenic air emissions from naturally occurring
non-anthropogenic emissions to separate pollution levels that can be
controlled by U.S. regulations from levels that are generally
uncontrollable by the United States (USEPA, 2007d). In the case of Pb,
however, due to the multimedia, multipathway nature of human exposures
to Pb, policy-relevant background is defined more broadly to include not
only the "quite low" levels of naturally occurring Pb emissions into the
air from non-anthropogenic sources such as volcanoes, sea salt, and
windborne soil particles from areas free of anthropogenic activity (see
below), but also Pb from nonair sources. These are collectively referred
to as "policy-relevant background".

The pathways of human exposure to Pb that are not air-related include
ingestion of Pb from indoor Pb paint[^17], Pb in diet as a result of
inadvertent additions during food processing, and Pb in drinking water
attributable to Pb in distribution systems (CD, Chapter 3). Other less
prevalent, potential pathways of Pb exposure that are not air-related
include ingestion of some calcium supplements or of food contaminated
during storage in some Pb glazed glassware, and hand-to-mouth contact
with some imported vinyl miniblinds or with some hair dyes containing Pb
acetate, as well as some cosmetics and folk remedies (CD, pp. 3-50 to
3-51).

Some amount of Pb in the air derives from background sources, such as
volcanoes, sea salt, and windborne soil particles from areas free of
anthropogenic activity (CD, Section 2.2.1). The impact of these sources
on current air concentrations is expected to be quite low (relative to
current concentrations) and has been estimated to fall within the range
from 0.00002 µg/m3 and 0.00007 µg/m^3^ based on mass balance
calculations for global emissions (CD, Section 3.1 and USEPA 1986,
Section 7.2.1.1.3). The midpoint in this range, 0.00005 µg/m^3^, has
been used in the past to represent the contribution of naturally
occurring air Pb to total human exposure (USEPA 1986, Section
7.2.1.1.3). The data available to derive such an estimate are limited
and such a value might be expected to vary geographically with the
natural distribution of Pb. Comparing this to reported air Pb
measurements is complicated by limitations of the common analytical
methods and by inconsistent reporting practices. This value is one half
the lowest reported nonzero value in AQS. Little information is
available regarding anthropogenic sources of airborne Pb located outside
of North America, which would also be considered policy-relevant
background. In considering contributions from policy-relevant background
to human exposures and associated health effects, however, any credible
estimate of policy-relevant background in air is likely insignificant in
comparison to the contributions from exposures to nonair media.

4\. Contributions to Children's Lead Exposures

As when the standard was set in 1978, EPA recognizes that there remain
today contributions to blood Pb levels from nonair sources. The relative
contribution of Pb in different exposure media to human exposure varies,
particularly for different age groups. For example, some studies have
found that dietary intake of Pb may be a predominant source of Pb
exposure among adults, greater than consumption of water and beverages
or inhalation (CD, p. 3-43).[^18] For young children, however, ingestion
of indoor dust can be a significant Pb exposure pathway, such that dust
ingested via hand-to-mouth activity can be a more important source of Pb
exposure than inhalation, although indoor dust can also be resuspended
through household activities and pose an inhalation risk as well (CD, p.
3-27 to 3-28; Melnyk et al 2000).[^19]

Estimating contributions from nonair sources are complicated by the
existence of multiple and varied air-related pathways (as described in
section II.A.2 above), as well as the persistent nature of Pb. For
example, Pb that is a soil or dust contaminant today may have been
airborne yesterday or many years ago. The studies currently available
and reviewed in the Criteria Document that evaluate the multiple
pathways of Pb exposure, when considering exposure contributions from
outdoor dust/soil, do not usually distinguish between outdoor soil/dust
Pb resulting from historical emissions and outdoor soil/dust Pb
resulting from recent emissions. Further, while indoor dust Pb has been
identified as being a predominant contributor to children's blood Pb,
available studies do not generally distinguish the different pathways
(air-related and other) contributing to indoor dust Pb. The exposure
assessment for children performed for this review has employed available
data and methods to develop estimates intended to inform a
characterization of these pathways (as described in section II.C below).

Relative contributions to a child's total Pb exposure from air-related
exposure pathways (such as those identified in the sections above)
compared to other (nonair-related) Pb exposures depends on many factors
including ambient air concentrations and air deposition in the area
where the child resides (as well as in the area from which the child's
food derives), access to other sources of Pb exposure such as Pb paint,
tap water affected by plumbing containing Pb and access to Pb-tainted
products. Studies indicate that in the absence of paint-related
exposures, Pb from other sources such as stationary sources of Pb
emissions may dominate a child's Pb exposures (CD, section 3.2). In
other cases, such as children living in older housing with peeling paint
or where renovations have occurred, the dominant source may be lead
paint used in the house in the past (CD, pp. 3-50 and 3-51). Depending
on Pb levels in a home's tap water, drinking water can sometimes be a
significant source (CD, section 3.3). And in still other cases, there
may be more of a mixture of contributions from multiple sources, with no
one source dominating (CD, Chapter 3).

As recognized in sections B.1.1 and II.B.3.a, blood Pb levels are the
commonly used index of exposure for Pb and they reflect external sources
of exposure, behavioral characteristics and physiological factors. Lead
derived from differing sources or taken into the body as a result of
differing exposure pathways (e.g., air- as compared to nonair-related),
is not easily distinguished. As mentioned above, complications to
consideration of estimates of air-related or conversely, nonair, blood
Pb levels are the roles of air Pb in human exposure pathways and the
persistence of Pb in the environment. As described in section II.A.2,
air-related pathways (those in which Pb passes through the air on its
path from source to human exposure) are varied, including inhalation and
ingestion, indoor dust, outdoor dust/soil and diet, Pb suspended in and
deposited from air, and encompassing a range of time frames from more
immediate to less so. Estimates of blood Pb levels associated with
air-related exposure pathways or only with nonair exposure pathways will
vary depending on how completely the air-related pathways are
characterized.

Consistent with reductions in air Pb concentrations (as described in
section II.A.1 above) which contribute to blood Pb, nonair contributions
have also been reduced. For example, the use of Pb paint in new houses
has declined substantially over the 20th century, such that according to
the *National Survey of Lead and Allergens in Housing* (USHUD, 2002) an
estimated 24% of U.S. housing constructed between 1960 and 1978; 69% of
the housing constructed between 1940 and 1959; and 87% of the pre-1940
housing contains lead-based paint. Additionally, Pb contributions to
diet have been reported to have declined significantly since 1978,
perhaps as much as 70% or more between then and 1990 (WHO, 1995) and the
2006 Criteria Document identifies a drop in dietary Pb intake by 2 to 5
year olds of 96% between the early 1980s and mid 1990s (CD, Section 3.4
and p. 8-14).[^20] These reductions are generally attributed to
reductions in gasoline-related airborne Pb as well as the reduction in
use of Pb solder in canning food products (CD, Section 3.4).[^21] There
have also been reductions in tap water Pb levels (CD, section 3.3 and
pp. 8-13 to 8-14). Contamination from the distribution/plumbing system
appears to remain the predominant source of Pb in the drinking water
(CD, section 3.3 and pp. 8-013 to 8-14).

The availability of estimates of blood Pb levels resulting only from
air-related sources and exposures or only from those unrelated to air is
limited and, given the discussion above, would be expected to vary for
different populations. In their advice to the Agency on levels for the
standard, the CASAC Pb Panel explored several approaches to deriving a
level, one of which required an estimate of the nonair component of
blood Pb for the average young child. They recommended consideration of
1.0 to 1.4 ug/dL or lower for such an estimate for the average nonair
blood Pb level for young children (Henderson, 2007a, p. D-1). This range
was developed with consideration of simulations of the integrated
exposure and uptake biokinetic (IEUBK) model for lead for which the
exposure concentration inputs included zero air concentration and
concentrations for soil and dust of 50 ppm and 35 ppm, respectively
(Henderson, 2007a, p. F-60).[^22] [^23]

As is evident from the prior discussion, the many different exposure
pathways contributing to children's blood Pb levels complicate our
consideration of the available data with regard to characterization of
levels particular to specific pathways, air-related or otherwise.

B. Health Effects Information

Number range CHAPTERThe following summary focuses on health endpoints
associated with the range of exposures considered to be most relevant to
current exposure levels and makes note of several key aspects of the
health evidence for Pb. First (as described in Section II.A, above),
because exposure to atmospheric Pb particles occurs not only via direct
inhalation of airborne particles, but also via ingestion of deposited
ambient Pb, the exposure considered is multimedia and multipathway in
nature, occurring via both the inhalation and ingestion routes. Second,
the exposure index or dose metric most commonly used and associated with
health effects information is an internal biomarker (i.e., blood Pb).
Additionally, the exposure duration of interest (i.e., that influencing
internal dose pertinent to health effects of interest) may span months
to potentially years, as does the time scale of the environmental
processes influencing Pb deposition and fate. Lastly, the nature of the
evidence for the health effects of greatest interest for this review,
neurological effects in young children, are epidemiological data
substantiated by toxicological data that provide biological plausibility
and insights on mechanisms of action (CD, sections 5.3, 6.2 and 8.4.2).

In recognition of the multi-pathway aspects of Pb, and the use of an
internal exposure metric in health risk assessment, the next section
describes the internal disposition or distribution of Pb, and the use of
blood Pb as an internal exposure or dose metric. This is followed by a
discussion of the nature of Pb-induced health effects that emphasizes
those with the strongest evidence. Potential impacts of Pb exposures on
public health, including recognition of potentially susceptible or
vulnerable subpopulations, are then discussed. Finally, key observations
about Pb-related health effects are summarized.

1\. Blood Lead

The health effects of Pb are remote from the portals of entry to the
body (i.e., the respiratory system and gastrointestinal tract).
Consequently, the internal disposition and distribution of Pb in the
blood is an integral aspect of the relationship between exposure and
effect. Additionally, the focus on blood Pb as the dose metric in
consideration of the Pb health effects evidence, while reducing our
uncertainty with regard to causality, leads to an additional
consideration with regard to contribution of air-related sources and
exposure pathways to blood Pb.

a\. Internal Disposition of Lead

This section briefly summarizes the current state of knowledge of Pb
disposition pertaining to both inhalation and ingestion routes of
exposure as described in the Criteria Document.

Inhaled Pb particles deposit in the different regions of the respiratory
tract as a function of particle size (CD, pp. 4-3 to 4-4). Lead
associated with smaller particles, which are predominantly deposited in
the pulmonary region, may, depending on solubility, be absorbed into the
general circulation or transported to the gastrointestinal tract (CD,
pp. 4-3). Lead associated with larger particles, which are predominantly
deposited in the head and conducting airways (e.g., nasal pharyngeal and
tracheobronchial regions of respiratory tract), may be transported into
the esophagus and swallowed, thus making its way to the gastrointestinal
tract (CD, pp. 4-3 to 4-4), where it may be absorbed into the blood
stream. Thus, Pb can reach the gastrointestinal tract either directly
through the ingestion route or indirectly following inhalation.

Once in the blood stream, where approximately 99% of the Pb associates
with red blood cells, the Pb is quickly distributed throughout the body
(e.g., within days) with the bone serving as a large, long-term storage
compartment, and soft tissues (e.g., kidney, liver, brain, etc) serving
as smaller compartments, in which Pb may be more mobile (CD, sections
4.3.1.4 and 8.3.1.). Additionally, the epidemiologic evidence indicates
that Pb freely crosses the placenta resulting in continued fetal
exposure throughout pregnancy, and that exposure increases during the
later half of pregnancy (CD, section 6.6.2).

During childhood development, bone represents approximately 70% of a
child's body burden of Pb, and this accumulation continues through
adulthood, when more than 90% of the total Pb body burden is stored in
the bone (CD, section 4.2.2). Accordingly, levels of Pb in bone are
indicative of a person's long-term, cumulative exposure to Pb. In
contrast, blood Pb levels are usually indicative of recent exposures.
Depending on exposure dynamics, however, blood Pb may -- through its
interaction with bone - be indicative of past exposure or of cumulative
body burden (CD, section 4.3.1.5).

Throughout life, Pb in the body is exchanged between blood and bone, and
between blood and soft tissues (CD, section 4.3.2), with variation in
these exchanges reflecting "duration and intensity of the exposure, age
and various physiological variables" (CD, p. 4-1). Past exposures that
contribute Pb to the bone, consequently, may influence current levels of
Pb in blood. Where past exposures were elevated in comparison to recent
exposures, this influence may complicate interpretations with regard to
recent exposure (CD, sections 4.3.1.4 to 4.3.1.6). That is, higher blood
Pb concentrations may be indicative of higher cumulative exposures or of
a recent elevation in exposure (CD, pp. 4-34 and 4-133).

In several recent studies investigating the relationship between Pb
exposure and blood Pb in children (e.g., Lanphear and Roghmann 1997;
Lanphear et al., 1998), blood Pb levels have been shown to reflect Pb
exposures, with particular influence associated with exposures to Pb in
surface dust. Further, as stated in the Criteria Document "these and
other studies of populations near active sources of air emissions (e.g.,
smelters, etc.), substantiate the effect of airborne Pb and resuspended
soil Pb on interior dust and blood Pb" (CD, p. 8-22).

b\. Use of Blood Lead as Dose Metric

Blood Pb levels are extensively used as an index or biomarker of
exposure by national and international health agencies, as well as in
epidemiological (CD, sections 4.3.1.3 and 8.3.2) and toxicological
studies of Pb health effects and dose-response relationships (CD,
Chapter 5). The prevalence of the use of blood Pb as an exposure index
or biomarker is related to both the ease of blood sample collection (CD,
p. 4-19; Section 4.3.1) and by findings of association with a variety of
health effects (CD, Section 8.3.2). For example, the U.S. Centers for
Disease Control and Prevention (CDC), and its predecessor agencies, have
for many years used blood Pb level as a metric for identifying children
at risk of adverse health effects and for specifying particular public
health recommendations (CDC, 1991; CDC, 2005a). In 1978, when the
current Pb NAAQS was established, the CDC recognized a blood Pb level of
30 μg/dL as a level warranting individual intervention (CDC, 1991). In
1985, the CDC recognized a level of 25 μg/dL for individual child
intervention, and in 1991, they recognized a level of 15 μg/dL for
individual intervention and a level of 10 μg/dL for implementing
community-wide prevention activities (CDC, 1991; CDCa, 2005). In 2005,
with consideration of a review of the evidence by their advisory
committee, CDC revised their statement on Preventing Lead Poisoning in
Young Children, specifically recognizing the evidence of adverse health
effects in children with blood Pb levels below 10 μg/dL and the data
demonstrating that no "safe" threshold for blood Pb had been identified,
and emphasizing the importance of preventative measures (CDC, 2005a,
ACCLPP, 2007).[^24]

Since 1976, the CDC has been monitoring blood Pb levels nationally
through the National Health and Nutrition Examination Survey (NHANES).
This survey has documented the dramatic decline in mean blood Pb levels
in the U.S. population that has occurred since the 1970s and that
coincides with regulations regarding leaded fuels, leaded paint, and
Pb-containing plumbing materials that have reduced Pb exposure among the
general population (CD, Sections 4.3.1.3 and 8.3.3; Schwemberger et al.,
2005). The Criteria Document summarizes related information as follows
(CD, p. E-6).

In the United States, decreases in mobile sources of Pb, resulting from
the phasedown of Pb additives created a 98% decline in emissions from
1970 to 2003. NHANES data show a consequent parallel decline in blood-Pb
levels in children aged 1 to 5 years from a geometric mean of \~15 μg/dL
in 1976-1980 to \~1-2 μg/dL in the 2000-2004 period.

While levels in the U.S. general population, including geometric mean
levels in children aged 1-5, have declined significantly, mean levels
have been found to vary among children of different socioeconomic status
(SES) and other demographic characteristics (CD, p. 4-21). For example,
while the 2001-2004 median blood level for children aged 1-5 of all
races and ethnic groups is 1.6 μg/dL, the median for the subset living
below the poverty level is 2.3 μg/dL and 90th percentile values for
these two groups are 4.0 μg/dL and 5.4 μg/dL, respectively. Similarly,
the 2001-2004 median blood level for black, non-hispanic children aged
1-5 is 2.5 μg/dL, while the median level for the subset of that group
living below the poverty level is 2.9 μg/dL and the median level for the
subset living in a household with income more than 200% of the poverty
level is 1.9 μg/dL. Associated 90th percentile values for 2001-2004 are
6.4 μg/dL (for black, non-hispanic children aged 1-5), 7.7 μg/dL (for
the subset of that group living below the poverty level) and 4.1 μg/dL
(for the subset living in a household with income more than 200% of the
poverty level). [^25]

Bone measurements, as a result of the generally slower Pb turnover in
bone, are recognized as providing a better measure of cumulative Pb
exposure (CD, Section 8.3.2). The bone pool of Pb in children, however,
is thought to be much more labile than that in adults due to the more
rapid turnover of bone mineral as a result of growth (CD, p. 4-27). As a
result, changes in blood Pb concentration in children more closely
parallel changes in total body burden (CD, pp. 4-20 and 4-27). This is
in contrast to adults, whose bone has accumulated decades of Pb
exposures (with past exposures often greater than current ones), and for
whom the bone may be a significant source long after exposure has ended
(CD, Section 4.3.2.5).

c\. Air-to-blood Relationships

As described in Section II.A, Pb in ambient air contributes to Pb in
blood by multiple pathways, with the pertinent exposure routes including
both inhalation and ingestion (CD, Sections 3.1.3.2, 4.2 and 4.4; Hilts
et al., 2003). The quantitative relationship between ambient air Pb and
blood Pb, which is often termed a slope or ratio, describes the increase
in blood Pb (in µg/dL) per unit of air Pb (in µg/m^3^).[^26]

The evidence on this quantitative relationship, is now, as in the past,
limited by the circumstances in which the data are collected. These
estimates are generally developed from studies of populations in various
Pb exposure circumstances. As recognized by the 1986 Criteria Document,
estimates that included air-related ingestion pathways in addition to
the inhalation pathway are "necessarily higher" (in terms of blood Pb
response) than those estimates based on inhalation alone (USEPA 1986a,
p. 11-106). Thus, the extent to which studies account for the full set
of air-related exposure pathways affects the magnitude of the resultant
air-to-blood estimates, such that fewer pathways included as
"air-related" yields lower ratios. Ratios derived from studies focused
only on inhalation pathways (e.g., chamber studies, occupational
studies) have generally been on the order of 1:2 or lower, while ratios
derived from studies including more air-related pathways were generally
higher (USEPA, 1986a, p. 11-106). Additionally, ratios derived from
studies involving higher blood and air Pb levels are smaller than ratios
from studies involving lower blood and air Pb levels (USEPA, 1986a. p.
11-99).

The 1986 Criteria Document reports estimates of air-to-blood ratios,
based on air contributions from inhalation and some ingestion exposure
pathways, that generally ranged between 1:3 to 1:5, with some higher
(USEPA 1986a, pp. 11-99 to 11-100 and 11-106; Brunekreef, 1984).[^27]
More recently, a study of changes in children's blood Pb levels
associated with reduced Pb emissions and associated air concentrations
near a Pb smelter in Canada indicates a ratio on the order of 1:7 (CD,
pp. 3-23 to 3-24; Hilts et al., 2003).[^28] In their advice to the
Agency, CASAC identified values of 1:5 as used by the World Health
Organization (2000) and 1:10 as supported by an empirical analysis of
changes in air Pb and changes in blood Pb between 1976 and the time when
the phase-out of Pb from gasoline was completed (Henderson, 2007a).
[^29]

While there is uncertainty in the absolute value of the air-to-blood
relationship, the evidence to inform our understanding of how blood Pb
levels respond to Pb in the air through the accumulated impact of
exposure via the multiple air-related exposure pathways is much expanded
from when the current standard was set. The currently available evidence
indicates a notably greater[^30] ratio, than the 1978 1:2 relationship.
This evidence includes ratios generally ranging from 1:3 to 1:5, with
some higher, including some on the order of 1:10, based on consideration
of multiple air-related pathways and not just inhalation. The evidence
also indicates that variation in the value of the ratios appears to
relate to the extent to which the range of air-related pathways are
included and the magnitude of the air and blood Pb levels assessed. For
example, ratios tend to be higher when air-related pathways are more
completely included and when the magnitude of Pb in air and blood is
lower.

Further, in the exposure assessment for this review summarized in
Section II.C, current modeling tools and information on children's
activity patterns, behavior and physiology (e.g., CD, Section 4.4) were
used to estimate blood Pb levels associated with multimedia and
multipathway Pb exposure. The results from this assessment, with
consideration of the context in which they were derived (e.g., the
extent to which the range of air-related pathways were simulated), are
also informative to our understanding of air-to-blood ratios. As
summarized in Section II.C.3.a, the ratio estimates developed from the
assessment results generally range from 1:4 -- 1:6 (from general urban
case study), with some up to 1:10 and higher (from primary Pb smelter
case study for the range of standard levels assessed). Because of
limitations in the assessment, these estimates, however, reflect only a
subset of air-related pathways, with the lower ones generally including
those pathways with the potential to respond most rapidly to changes in
air Pb (inhalation of ambient air, and ingestion of Pb in indoor dust
directly contaminated with Pb from ambient air). Those lower estimates
do not reflect other air-related pathways, in most of which the air
quality impact is reflected over a somewhat longer time frame and is
also generally associated with outdoor atmospheric deposition (as
described in Section II.A.2 above). Accordingly, given our understanding
of the influence of the completeness of the pathway coverage on the
magnitude of the ratio, the resultant ratios might have been somewhat
higher than 1:4 to 1:6 if the assessment had simulated the relationships
between ambient air and blood Pb levels *via* the remaining air-related
pathways. In summary, the current evidence and estimates based on the
exposure assessment, based on consideration of both inhalation and
ingestion exposure pathways, indicate ratios greater than the 1978 1:2
estimate, and generally ranging from 1:3 to 1:5 and higher, with some on
the order of 1:10.

2\. Nature of Effects

a\. Broad Array of Effects

Lead has been demonstrated to exert "a broad array of deleterious
effects on multiple organ systems via widely diverse mechanisms of
action" (CD, p. 8-24 and Section 8.4.1). This array of health effects
includes effects on heme biosynthesis and related functions;
neurological development and function; reproduction and physical
development; kidney function; cardiovascular function; and immune
function. The weight of evidence varies across this array of effects and
is comprehensively described in the Criteria Document. There is also
some evidence of Pb carcinogenicity, primarily from animal studies,
together with limited human evidence of suggestive associations (CD,
Sections 5.6.2, 6.7, and 8.4.10). [^31]

This review is focused on those effects most pertinent to ambient
exposures, which given the reductions in ambient Pb levels over the past
30 years, are generally those associated with individual blood Pb levels
in children and adults in the range of 10 μg/dL and lower. Tables 8-5
and 8-6 in the Criteria Document highlight the key such effects observed
in children and adults, respectively (CD, pp. 8-60 to 8-62). The effects
include neurological, hematological and immune effects for children, and
hematological, cardiovascular and renal effects for adults. As evident
from the discussions in Chapters 5, 6 and 8 of the Criteria Document,
"neurotoxic effects in children and cardiovascular effects in adults are
among those best substantiated as occurring at blood Pb concentrations
as low as 5 to 10 μg/dL (or possibly lower); and these categories are
currently clearly of greatest public health concern" (CD, p. 8-60). The
toxicological and epidemiological information available since the time
of the last review "includes assessment of new evidence substantiating
risks of deleterious effects on certain health endpoints being induced
by distinctly lower than previously demonstrated Pb exposures indexed by
blood Pb levels extending well below 10 μg/dL in children and/or adults"
(CD, p. 8-25). Some health effects associated with individual blood Pb
levels extend below 5 ug/dL, and some studies have observed these
effects at the lowest blood levels considered.

With regard to population mean levels, the Criteria Document points to
studies reporting "Pb effects on the intellectual attainment of
preschool and school age children at population mean concurrent blood-Pb
levels ranging down to as low as 2 to 8 μg/dL" (CD, p. E\--9).

We note that many studies over the past decade have, in investigating
effects at lower blood Pb levels, utilized the CDC advisory level for
individual children (10 µg/dL) as a benchmark for assessment, and this
is reflected in the numerous references in the Criteria Document to 10
µg/dL. Individual study conclusions stated with regard to effects
observed below 10 µg/dL are usually referring to individual blood Pb
levels. In fact, many such study groups have been restricted to
individual blood Pb levels below 10 µg/dL. We note that the mean blood
Pb level for any such group will necessarily be well below 10 µg/dL.

Threshold levels, in terms of blood Pb levels in individual children,
for these effects cannot be discerned from the currently available
studies. With regard to a threshold blood Pb level, the Criteria
Document states "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). As discussed in the
Criteria Document," a threshold for Pb neurotoxic effects may exist at
levels distinctly lower than the lowest exposures examined in these
epidemiologic studies" (CD, p. 8-67).

In summary, the Agency has identified neurological, hematological and
immune effects in children and neurological, hematological,
cardiovascular and renal effects in adults as the "most important"
effects observed at blood Pb levels near or below 10 μg/dL and further
considers neurological effects in children and cardiovascular effects in
adults to be categories of effects that "are currently clearly of
greatest public health concern" (CD, pp. 8-60 to 8-62). Neurological
effects in children are discussed further below.

b\. Neurological Effects in Children

Among the wide variety of health endpoints associated with Pb exposures,
there is general consensus that the developing nervous system in young
children is among, if not, the most sensitive. As described in the
Criteria Document, neurotoxic effects in children and cardiovascular
effects in adults are categories of effects that are "currently clearly
of greatest public health concern" (CD, p. 8-60).[^32] While also
recognizing the occurrence of adult cardiovascular effects at somewhat
similarly low blood Pb levels, neurological effects in children are
considered to be the sentinel effects in this review and are the focus
of the quantitative risk assessment conducted for this review (discussed
below in section III.C).

The nervous system has long been recognized as a target of Pb toxicity,
with the developing nervous system affected at lower exposures than the
mature system (CD, Sections 5.3, 6.2.1, 6.2.2, and 8.4). While blood Pb
levels in U.S. children ages one to five years have decreased notably
since the late 1970s, newer studies have investigated and reported
associations of effects on the neurodevelopment of children with these
more recent blood Pb levels (CD, Chapter 6). Functional manifestations
of Pb neurotoxicity during childhood include sensory, motor, cognitive
and behavioral impacts. Numerous epidemiological studies have reported
neurocognitive, neurobehavioral, sensory, and motor function effects in
children with blood Pb levels below 10 μg/dL (CD, Sections 6.2 and
8.4).[^33] As discussed in the Criteria Document, "extensive
experimental laboratory animal evidence has been generated that (a)
substantiates well the plausibility of the epidemiologic findings
observed in human children and adults and (b) expands our understanding
of likely mechanisms underlying the neurotoxic effects" (CD, p. 8-25;
Section 5.3).

The evidence for neurotoxic effects in children is a robust combination
of epidemiological and toxicological evidence (CD, Sections 5.3, 6.2 and
8.5). The epidemiological evidence is supported by animal studies that
substantiate the biological plausibility of the associations, and
provides an understanding of mechanisms of action for the effects (CD,
Section 8.4.2).

Cognitive effects associated with Pb exposures that have been observed
in epidemiological studies have included decrements in intelligence test
results, such as the widely used IQ score, and in academic achievement
as assessed by various standardized tests as well as by class ranking
and graduation rates (CD, Section 6.2.16 and pp 8-29 to 8-30). As noted
in the Criteria Document with regard to the latter, "Associations
between Pb exposure and academic achievement observed in the above-noted
studies were significant even after adjusting for IQ, suggesting that
Pb-sensitive neuropsychological processing and learning factors not
reflected by global intelligence indices might contribute to reduced
performance on academic tasks" (CD, pp 8-29 to 8-30).

Other cognitive effects observed in studies of children have included
effects on attention, executive functions, language, memory, learning
and visuospatial processing (CD, Sections 5.3.5, 6.2.5 and 8.4.2.1),
with attention and executive function effects associated with Pb
exposures indexed by blood Pb levels below 10 μg/dL (CD, Section 6.2.5
and pp. 8-30 to 8-31). The evidence for the role of Pb in this suite of
effects includes experimental animal findings (discussed in CD, Section
8.4.2.1; p. 8-31), which provide strong biological plausibility of Pb
effects on learning ability, memory and attention (CD, Section 5.3.5),
as well as associated mechanistic findings. With regard to persistence
of effects the Criteria Document states the following (CD, p. 8-67):

Persistence or apparent "irreversibility" of effects can result from two
different scenarios: (1) organic damage has occurred without adequate
repair or compensatory offsets, or (2) exposure somehow persists. As Pb
exposure can also derive from endogenous sources (e.g., bone), a
performance deficit that remains detectable after external exposure has
ended, rather than indicating irreversibility, could reflect ongoing
toxicity due to Pb remaining at the critical target organ or Pb
deposited at the organ post-exposure as the result of redistribution of
Pb among body pools. The persistence of effect appears to depend on the
duration of exposure as well as other factors that may affect an
individual's ability to recover from an insult. The likelihood of
reversibility also seems to be related, at least for the adverse effects
observed in certain organ systems, to both the age-at-exposure and the
age-at-assessment.

The evidence with regard to persistence of Pb-induced deficits observed
in animal and epidemiological studies is described in discussion of
those studies in the Criteria Document (CD, Sections 5.3.5, 6.2.11, and
8.5.2). It is additionally important to note that there may be long-term
consequences of such deficits over a lifetime. Poor academic skills and
achievement can have \"enduring and important effects on objective
parameters of success in real life,\" as well as increased risk of
antisocial and delinquent behavior (CD, Section 6.2.16).

As discussed in the Criteria Document, while there is no direct animal
test parallel to human IQ tests, "in animals a wide variety of tests
that assess attention, learning, and memory suggest that Pb exposure {of
animals} results in a global deficit in functioning, just as it is
indicated by decrements in IQ scores in children" (CD, p. 8-27). The
animal and epidemiological evidence for this endpoint are consistent and
complementary (CD, p. 8-44). As stated in the Criteria Document (p.
8-44):

Findings from numerous experimental studies of rats and of nonhuman
primates, as discussed in Chapter 5, parallel the observed human
neurocognitive deficits and the processes responsible for them. Learning
and other higher order cognitive processes show the greatest
similarities in Pb-induced deficits between humans and experimental
animals. Deficits in cognition are due to the combined and overlapping
effects of Pb-induced perseveration, inability to inhibit responding,
inability to adapt to changing behavioral requirements, aversion to
delays, and distractibility. Higher level neurocognitive functions are
affected in both animals and humans at very low exposure levels (\<10
μg/dL), more so than simple cognitive functions.

Epidemiologic studies of Pb and child development have demonstrated
inverse associations between blood Pb concentrations and children's IQ
and other cognitive-related outcomes at successively lower Pb exposure
levels over the past 30 years (CD, p. 6-64). This is supported by
multiple studies performed over the past 15 years (as discussed in the
CD, Section 6.2.13). For example, the overall weight of the available
evidence, described in the Criteria Document, provides clear
substantiation of neurocognitive decrements being associated in young
children with mean blood Pb levels in the range of 5 to 10 μg/dL, and
some analyses indicate Pb effects on intellectual attainment of young
children for which population mean blood Pb levels in the analysis
ranged from 2 to 8 μg/dL (CD, Sections 6.2, 8.4.2 and 8.4.2.6). That is,
while blood Pb levels in U.S. children ages one to five years have
decreased notably since the late 1970s, newer studies have investigated
and reported associations of effects on the neurodevelopment of children
with these more recent blood Pb levels (CD, Chapter 6).

The evidence described in the Criteria Document with regard to the
effect on children's cognitive function of blood Pb levels at the lower
concentration range includes the international pooled analysis by
Lanphear and others (2005), studies of individual cohorts such as the
Rochester, Boston, and Mexico City cohorts (Canfield et al., 2003a;
Canfield et al., 2003b; Bellinger and Needleman, 2003; Tellez-Rojo et
al., 2006), the study of African-American inner-city children from
Detroit (Chiodo et al., 2004), the cross-sectional study of young
children in three German cities (Walkowiak et al., 1998) and the
cross-sectional analysis of a nationally representative sample from the
NHANES III, conducted from 1988-1994 (Lanphear et al., 2000). These
studies included analyses of association of neurocognitive decrement in
cohorts of children with mean blood Pb levels below 5 µg/dL (Canfield et
al., 2003a; Canfield, 2008; Chiodo et al., 2004, Chiodo, 2008; Walkowiak
et al., 1998; Tellez-Rojo et al 2006; Tellez-Rojo, 2008; Lanphear et al
2000; Auinger, 2008).

The epidemiological studies that have investigated blood Pb effects on
IQ (as discussed in the CD, Section 6.2.3) have considered a variety of
specific blood Pb metrics, including: 1) blood concentration
"concurrent" with the response assessment (e.g., at the time of IQ
testing), 2) average blood concentration over the "lifetime" of the
child at the time of response assessment (e.g., average of measurements
taken over child's first 6 or 7 years), 3) peak blood concentration
during a particular age range, and 4) early childhood blood
concentration (e.g., the mean of measurements between 6 and 24 months
age). With regard to the latter two, the Criteria Document has noted
that age has been observed to strongly predict the period of peak
exposure (around 18-27 months when there is maximum hand-to-mouth
activity). Further, this maximum exposure period coincides with a period
of time in which major events are occurring in central nervous system
(CNS) development. Accordingly, the belief that the first few years of
life are a critical window of vulnerability is evident particularly in
the earlier literature (CD, p. 6-60). However, more recent analyses have
found even stronger associations between blood Pb at school age and IQ
at school age (i.e., concurrent blood Pb), indicating the important role
that is continued to be played by Pb exposures later in life. In fact,
concurrent and lifetime averaged measurements were stronger predictors
of adverse neurobehavioral effects (better than the peak or 24 month
metrics) in all but one of the prospective cohort studies (CD, pp. 6-61
to 6-62). While all four specific blood Pb metrics were correlated with
IQ in the international pooled analysis by Lanphear and others (2005),
the concurrent blood Pb level exhibited the strongest relationship with
intellectual deficits (CD, p. 6-29).

The Criteria Document presentation on toxicological evidence also
recognizes neurological effects elicited by exposures subsequent to
earliest childhood (CD, sections 5.3.5 and 5.3.7). For example, research
with monkeys has indicated that while exposure only during infancy may
elicit a response, exposures (with similar blood Pb levels) that only
occurred post-infancy also elicit responses. Further, in the monkey
research, exposures limited to post-infancy resulted in a greater
response than exposures limited to infancy (Rice and Gilbert, 1990;
Rice, 1992).

A study by Chen and others (2005) involving 622 children has attempted
to directly address the question regarding periods of enhanced
susceptibility to Pb effects (CD, pp. 6-62 to 6-64). The authors found
that the concurrent blood Pb association with IQ was always stronger
than that for 24-month blood Pb. As children aged, the relationship with
concurrent blood Pb grew stronger while that with 24-month blood Pb grew
weaker. Further, in models including both prior blood Pb (at 24-months
age) and concurrent blood Pb (at 7-years age), concurrent blood Pb was
always more predictive of IQ. In fact, concurrent blood Pb explained
most of Pb-related variation in IQ such that prior blood Pb (at
24-months age) was rendered nonsignificant and nearly null.[^34] The
effect estimate for concurrent blood Pb was robust and remained
significant, little changed from its value without adjustment for
24-month blood Pb level. The Criteria Document concluded the following
regarding the results of this study (CD, pp. 6-63 to 6-64).

These results support the idea that Pb exposure continues to be toxic to
children as they reach school age, and do not lend support to the
interpretation that all the damage is done by the time the child reaches
2 to 3 years of age. These findings also imply that cross-sectional
associations seen in children, such as the study recently conducted by
Lanphear et al. (2000) using data from NHANES III, should not be
dismissed. Chen et al. (2005) concluded that if concurrent blood Pb
remains important until school age for optimum cognitive development,
and if 6 and 7 year olds are as or more sensitive to Pb effects than 2
year olds, then the difficulties in preventing Pb exposure are magnified
but the potential benefits of prevention are greater.

In addition to findings of association with neurocognitive decrement
(including IQ) at study group mean blood Pb levels well below 10 μg/dL,
the current evidence includes multiple studies that have examined the
quantitative relationship between IQ and blood Pb level in children with
blood Pb concentrations below 10 μg/dL (e.g., CD, Table 6-1). This has
been done in studies where all or the majority of study subjects had
blood Pb levels below 10 μg/dL and also where an analysis was performed
on a subset of children whose blood Pb levels have never exceeded 10
μg/dL. For studies not restricted to blood Pb levels below 10 μg/dL, EPA
has estimated the average slope of change in IQ with change in blood Pb
between the 10^th^ percentile blood Pb level and 10 μg/dL The resultant
group of reported and estimated average linear slopes for IQ change with
blood Pb levels no higher than 10 μg/dL range from -0.4 to -1.8 IQ
points per μg/dL blood Pb (CD, Tables 6-1 and 8-7), with a median of
-0.9 IQ points per μg/dL blood Pb (CD, p. 8-80).

Further, the evidence indicates that the slope for Pb effects on IQ is
steeper at lower blood Pb levels (CD, section 6.2.13). As stated in the
CD, "the most compelling evidence for effects at blood Pb levels \<10
μg/dL, as well as a nonlinear relationship between blood Pb levels and
IQ, comes from the international pooled analysis of seven prospective
cohort studies (n=1,333) by Lanphear et al. (2005)" (CD, pp. 6-67 and
8-37 and section 6.2.3.1.11). Using concurrent blood Pb level as the
exposure metric and IQ as the response from the pooled dataset of seven
international studies, Lanphear and others (2005) employed mathematical
models of various forms, including linear, cubic spline, log-linear, and
piece-wise linear, in their investigation of the blood Pb
concentration-response relationship (CD, p. 6-29; Lanphear et al.,
2005). They observed that the shape of the concentration-response
relationship is nonlinear and the log-linear model provides a better fit
over the full range of blood Pb measurements than a linear one (CD, p.
6-29 and pp. 6-67 to 6-70; Lanphear et al., 2005). In addition, they
found that no individual study among the seven was responsible for the
estimated nonlinear relationship between Pb and deficits in IQ (CD p.
6-30). Others have also analyzed the same dataset and similarly
concluded that, across the range of the dataset's blood Pb levels, a
log-linear relationship was a significantly better fit than the linear
relationship (p=0.009) with little evidence of residual confounding from
included model variables (CD, Section 6.2.13; Rothenberg and Rothenberg,
2005).

The impact of the nonlinear slope is illustrated by the log-linear
model-based estimates of IQ decrements for similar changes in blood Pb
level at different absolute values of blood Pb level (Lanphear et al.,
2005). These estimates of IQ decrement are 3.9 (with 95% confidence
interval, CI, of 2.4-5.3), 1.9 (95% CI, 1.2-2.6) and 1.1 (95% CI,
0.7-1.5), for increases in concurrent blood Pb from 2.4 to 10 μg/dL, 10
to 20 μg/dL, and 20 to 30 μg/dL, respectively (Lanphear et al., 2005).
For an increase in concurrent blood Pb levels from \<1 to 10 ug/dL, the
log-linear model estimates a decline of 6.2 points in full scale IQ
which is comparable to the 7.4 point decrement in IQ for an increase in
lifetime mean blood Pb levels up to 10 ug/dL observed in the Rochester
study (CD, pp 6-30 to 6-31).

A nonlinear blood Pb concentration-response relationship is also
suggested by several other analyses that have observed that each μg/dL
increase in blood Pb may have a greater effect on IQ at lower blood Pb
levels (e.g., below 10 μg/dL) than at higher levels (CD, pp. 8-63 to
8-64; Figure 8-7). While this may at first seem at odds with certain
fundamental toxicological concepts, a number of examples of non- or
supralinear dose-response relationships exist in toxicology (CD, pp.
6-76 and 8-38 to 8-39). With regard to the effects of Pb on
neurodevelopmental outcome such as IQ, the CD suggests that initial
neurodevelopmental effects at lower Pb levels may be disrupting very
different biological mechanisms (e.g., early developmental processes in
the central nervous system) than more severe effects of high exposures
that result in symptomatic Pb poisoning and frank mental retardation
(CD, p. 6-76).

The Criteria Document describes this issue with regard to Pb as follows
(CD, p. 8-39).

In the case of Pb, this nonlinear dose-effect relationship occurs in the
pattern of glutamate release (Section 5.3.2), in the capacity for long
term potentiation (LTP; Section 5.3.3), and in conditioned operant
responses (Section 5.3.5). The 1986 Lead AQCD also reported U-shaped
dose-effect relationships for maze performance, discrimination learning,
auditory evoked potential, and locomotor activity. Davis and Svendsgaard
(1990) reviewed U-shaped dose-response curves and their implications for
Pb risk assessment. An important implication is the uncertainty created
in identification of thresholds and "no-observed-effect-levels" (NOELS).
As a nonlinear relationship is observed between IQ and low blood Pb
levels in humans, as well as in new toxicologic studies wherein
neurotransmitter release and LTP show this same relationship, it is
plausible that these nonlinear cognitive outcomes may be due, in part,
to nonlinear mechanisms underlying these observed Pb neurotoxic effects.

More specifically, various findings within the toxicological evidence
presented in the Criteria Document provides biologic plausibility for a
steeper IQ loss at low blood levels, with a potential explanation being
that the predominant mechanism at very low blood-Pb levels is rapidly
saturated and that a different, less-rapidly-saturated process, becomes
predominant at blood-Pb levels greater than 10 μg/dL. [^35]

In comparing across the individual epidemiological studies and the
international pooled analysis, it is observed that at higher blood Pb
levels, the slopes (for change in IQ with blood Pb) derived for
log-linear and linear models are almost identical, and for studies with
lower blood Pb levels, the slopes appear to be steeper than those
observed in studies involving higher blood Pb levels (CD, p. 8-78,
Figure 8-7). Among this set of epidemiological studies (CD, Tables 6-1
and 8-7), the steepest slopes for change in IQ with change in blood Pb
level are those derived for the subsets of children in the Rochester and
Boston cohorts for which peak blood Pb levels were \<10 μg/dL; these
slopes are -1.8 (for concurrent blood Pb influence on IQ) and -1.6 (for
24-month blood Pb influence on IQ), respectively. The mean blood Pb
levels for children in these subsets of the Rochester and Boston cohorts
are 3.32 and 3.8 μg/dL, respectively (Canfield, 2008; Bellinger, 2008).
Other studies with analyses involving similarly low blood Pb levels
(e.g., mean levels below 4 μg/dL) also had slopes greater than 1.5.
These include the subset of 24-month old children in the Mexican cohort
for which the mean concurrent blood Pb level was 2.9 μg/dL (slope of
-1.71) and the subset of 6-10 year old children whose peak blood Pb
levels never exceeded 7.5 μg/dL, and for which the mean concurrent blood
Pb level was 3.24 μg/dL (slope of -2.94) (Tellez-Rojo et al 2006, 2008;
Lanphear et al 2005; Hornung 2008). Thus, from the subset analyses
involving distributions of individual blood Pb levels closer to the
distribution for today's children nationally the slopes range from -1.71
to -2.94 IQ points per μg/dL of concurrent blood Pb.

3\. Lead-Related Impacts on Public Health

In addition to the advances in our knowledge and understanding of Pb
health effects at lower exposures (e.g., using blood Pb as the index),
there has been some change with regard to the U.S. population Pb burden
since the time of the last Pb NAAQS review. For example, the geometric
mean blood Pb level for U.S. children aged 1-5, as estimated by the U.S.
Centers for Disease Control, declined from 2.7 μg/dL (95% CI: 2.5-3.0)
in the 1991-1994 survey period to 1.7 μg/dL (95% CI: 1.55-1.87) in the
2001-2002 survey period (CD, Section 4.3.1.3) and 1.8 μg/dL in the
2003-2004 survey period (Axelrad, 2008).[^36] Blood Pb levels have also
declined in the U.S. adult population over this time period (CD, Section
4.3.1.3).[^37] As noted in the Criteria Document, "blood-Pb levels have
been declining at differential rates for various general subpopulations,
as a function of income, race, and certain other demographic indicators
such as age of housing" (CD, p. 8-21). For example, the 2001-2004 90^th^
percentile blood Pb level for children aged 1-5 of all races and ethnic
groups is 4.0 μg/dL, while the corresponding level for the subset of
children living below the poverty level is 5.4 μg/dL, and that level for
black, non-hispanic children living below the poverty level is 7.7 μg/dL
(<http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm> -
then click on \"Download a universal spreadsheet file of the Body
Burdens data tables\"). Further, the 95^th^ percentile blood Pb level in
the 2003-2004 NHANES for children aged 1-5 of all races and ethnic
groups is 5.1 μg/dL, while the corresponding level for the subset of
children living below the poverty level is 6.6 μg/dL, and that level for
black, non-hispanic children living below the poverty level is 10.5
μg/dL (Axelrad, 2008).

a\. At-risk Subpopulations

Potentially at-risk subpopulations include those with increased
susceptibility (i.e., physiological factors contributing to a greater
response for the same exposure) and those with increased exposure
(including that resulting from behavior leading to increased contact
with contaminated media) (USEPA 1986a, p. 1-154). A behavioral factor of
great impact on Pb exposure is the incidence of hand-to-mouth activity
that is prevalent in very young children (CD, Section 4.4.3).
Physiological factors include both conditions contributing to a
subgroup's increased risk of effects at a given blood Pb level, and
those that contribute to blood Pb levels higher than those otherwise
associated with a given Pb exposure (CD, Section 8.5.3). These factors
include nutritional status (e.g., iron deficiency, calcium intake), as
well as genetic and other factors (CD, chapter 4 and sections 3.4, 5.3.7
and 8.5.3).

We also considered evidence pertaining to vulnerability to
pollution-related effects which additionally encompasses situations of
elevated exposure, such as residing in older housing with Pb-containing
paint or near sources of ambient Pb, as well as socioeconomic factors,
such as reduced access to health care or low socioeconomic status (SES)
(USEPA, 2003, 2005c) that can contribute to increased risk of adverse
health effects from Pb. With regard to elevated exposures in particular
socioeconomic and minority subpopulations, we observe notably higher
blood Pb levels in children in poverty and in black, non-hispanic
children compared to those for more economically well-off children and
white children, in general (as recognized in section II.B.1.b above).

Three particular physiological factors contributing to increased risk of
Pb effects at a given blood Pb level are recognized in the Criteria
Document (e.g., CD, Section 8.5.3): age, health status, and genetic
composition. With regard to age, the susceptibility of young children to
the neurodevelopmental effects of Pb is well recognized (e.g., CD,
Sections 5.3, 6.2, 8.4, 8.5, 8.6.2), although the specific ages of
vulnerability have not been established (CD, pp 6-60 to 6-64). Early
childhood may also be a time of increased susceptibility for Pb
immunotoxicity (CD, Sections 5.9.10, 6.8.3 and 8.4.6). Further early
life exposures have been associated with increased risk of
cardiovascular effects in humans later in life (CD, p. 8-74). Early life
exposures have also been associated with increased risk, in animals, of
neurodegenerative effects later in life (CD, p. 8-74).[^38] Health
status is another physiological factor in that subpopulations with
pre-existing health conditions may be more susceptible (as compared to
the general population) for particular Pb-associated effects, with this
being most clear for renal and cardiovascular outcomes. For example,
African Americans as a group, have a higher frequency of hypertension
than the general population or other ethnic groups (NCHS, 2005), and as
a result may face a greater risk of adverse health impact from
Pb-associated cardiovascular effects. A third physiological factor
relates to genetic polymorphisms. That is, subpopulations defined by
particular genetic polymorphisms (e.g., presence of the δ-aminolevulinic
acid dehydratase-2 \[ALAD-2\] allele) have also been recognized as
sensitive to Pb toxicity, which may be due to increased susceptibility
to the same internal dose and/or to increased internal dose associated
with same exposure (CD, p. 8-71, Sections 6.3.5, 6.4.7.3 and 6.3.6).

Childhood is well recognized as a time of increased susceptibility, and
as summarized in section II.B.2.b above and described in more detail in
the Criteria Document, a large body of epidemiological evidence
describes neurological effects on children at low blood Pb levels. The
toxicological evidence further helps inform an understanding of specific
periods of development with increased vulnerability to specific types of
neurological effect (CD, Section 5.3). Additionally, the toxicological
evidence of a differing sensitivity of the immune system to Pb across
and within different periods of life stages indicates the potential
importance of exposures of duration as short as weeks to months. For
example, the animal studies suggest that the gestation period is the
most sensitive life stage followed by early neonatal stage, and that
within these life stages, critical windows of vulnerability are likely
to exist (CD, Section 5.9 and p. 5-245).

In summary, there are a variety of ways in which Pb exposed populations
might be characterized and stratified for consideration of public health
impacts. Age or lifestage was used to distinguish potential groups on
which to focus the quantitative risk assessment because of its influence
on exposure and susceptibility. Young children were selected as the
priority population for the risk assessment in consideration of the
health effects evidence regarding endpoints of greatest public health
concern. The Criteria Document recognizes, however, other population
subgroups as described above may also be at risk of Pb-related health
effects of public health concern.

b\. Potential Public Health Impacts

As discussed in the Criteria Document, there are potential public health
implications of low-level Pb exposure, indexed by blood Pb levels,
associated with several health endpoints identified in the Criteria
Document (CD, Section 8.6).[^39] These include potential impacts on
population IQ, which is the focus of the quantitative risk assessment
conducted for this review, as well as heart disease and chronic kidney
disease, which are not included in the quantitative risk assessment (CD,
Sections 8.6, 8.6.2, 8.6.3 and 8.6.4). It is noted that there is greater
uncertainty associated with effects at the lower levels of blood Pb, and
that there are differing weights of evidence across the effects
observed.[^40] With regard to potential implications of Pb effects on
IQ, the Criteria Document recognizes the "critical" distinction between
population and individual risk, noting that a "point estimate indicating
a modest mean change on a health index at the individual level can have
substantial implications at the population level" (CD, p. 8-77).[^41] A
downward shift in the mean IQ value is associated with both substantial
decreases in percentages achieving very high scores and substantial
increases in the percentage of individuals achieving very low scores
(CD, p. 8-81). [^42] For an individual functioning in the low IQ range
due to the influence of developmental risk factors other than Pb, a
Pb-associated IQ decline of several points might be sufficient to drop
that individual into the range associated with increased risk of
educational, vocational, and social handicap (CD, p. 8-77).

The magnitude of a public health impact is dependent upon the size of
population affected and type or severity of the effect. As summarized
above, there are several population groups that may be susceptible or
vulnerable to effects associated with exposure to Pb, including young
children, particularly those in families of low SES (CD, p. E-15), as
well as individuals with hypertension, diabetes, and chronic renal
insufficiency (CD, p. 8-72). Although comprehensive estimates of the
size of these groups residing in proximity to sources of ambient Pb have
not been developed, total estimates of these population subpopulations
within the U.S. are substantial (as noted in Table 3-3 of the Staff
Paper).[^43]

With regard to estimates of the size of potentially vulnerable
subpopulations living in areas of increased exposure related to ambient
Pb, the information is still more limited. The limited information
available on air and surface soil concentrations of Pb indicates
elevated concentrations near stationary sources as compared with areas
remote from such sources (CD, Sections 3.2.2 and 3.8). Air quality
analyses (presented in Chapter 2 of the Staff Paper) indicate
dramatically higher Pb concentrations at monitors near sources as
compared with those more remote. As described in Section 2.3.2.1 of the
Staff Paper, however, since the 1980s the number of Pb monitors has been
significantly reduced by states (with EPA guidance that monitorings well
below the current NAAQS could be shut down) and a lack of monitors near
some large sources may lead to underestimates of the extent of
occurrences of relatively higher Pb concentrations. The significant
limitations of our monitoring and emissions information constrain our
efforts to characterize the size of at-risk populations in areas
influenced by sources of ambient Pb. For example, the limited size and
spatial coverage of the current Pb monitoring network constrains our
ability to characterize current levels of airborne Pb in the U.S.
Further, the available information on emissions and locations of sources
indicates that the network is inconsistent in its coverage of the
largest sources identified in the 2002 National Emissions Inventory
(NEI), with monitors within a mile of only 2 of 26 facilities in the
2002 NEI with emissions greater than 5 tons per year (tpy).
Additionally, there are various uncertainties and limitations associated
with source information in the NEI.

In recognition of the significant limitations associated with the
currently available information on Pb emissions and airborne
concentrations in the U.S. and the associated exposure of potentially
at-risk populations, Chapter 2 of the Staff Paper summarizes the
information in several different ways. For example, analyses of the
current monitoring network indicated the numbers of monitoring sites
that would exceed alternate standard levels, taking into consideration
different statistical forms. These analyses are also summarized with
regard to population size in counties home to those monitoring sites (as
presented in Appendix 5.A of the Staff Paper). Information for the
monitors and from the NEI indicates a range of source sizes in proximity
to monitors at which various levels of Pb are reported. Together this
information suggests that there is variety in the magnitude of Pb
emissions from sources that could influence air Pb concentrations.
Identifying specific emissions levels of sources expected to result in
air Pb concentrations of interest, however, would be informed by a
comprehensive analysis using detailed source characterization
information, which was not feasible within the time and data constraints
of this review. Instead, we have developed a summary of the emissions
and demographic information for Pb sources that includes estimates of
the numbers of people residing in counties in which the aggregate Pb
emissions from NEI sources is greater than or equal to 0.1 tpy or in
counties in which the aggregate Pb emissions is greater than or equal to
0.1 tpy per 1000 square miles (as presented in Tables 3-4 and 3-5,
respectively, in the Staff Paper).

Additionally, the potential for historically deposited Pb near roadways
to contribute to increased risks of Pb exposure and associated risk to
populations residing nearby is suggested in the Criteria Document.
Although estimates of the number of individuals, including children,
living within close proximity to roadways specifically recognized for
this potential have not been developed, these numbers may be
substantial.[^44]

4\. Key Observations

The following key observations are based on the available health effects
evidence and the evaluation and interpretation of that evidence in the
Criteria Document.

-   Lead exposures occur both by inhalation and by ingestion (CD,
    Chapter 3). 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).

-   Children, in general and especially those of low SES, are at
    increased risk for Pb exposure and Pb-induced adverse health
    effects. This is due to several factors, including enhanced exposure
    to Pb via ingestion of soil Pb and/or dust Pb due to normal
    childhood hand-to-mouth activity (CD, p. E-15, Chapter 3 and Section
    6.2.1).

-   Once inhaled or ingested, Pb is distributed by the blood, with
    long-term storage accumulation in the bone. Bone Pb levels provide a
    strong measure of cumulative exposure which has been associated with
    many of the effects summarized below, although difficulty of sample
    collection has precluded widespread use in epidemiological studies
    to date (CD, Chapter 4).

-   Blood levels of Pb are well accepted as an index of exposure (or
    exposure metric) for which associations with the key effects (see
    below) have been observed. In general, associations with blood Pb
    are most robust for those effects for which past exposure history
    poses less of a complicating factor, i.e., for effects during
    childhood (CD, Section 4.3).

-   Both epidemiological and toxicologic studies have shown that
    environmentally relevant levels of Pb affect many different organ
    systems (CD, p. E-8). With regard to the most important such effects
    observed in children and adults, the Criteria Document states
    (CD, p. 8-60) that "neurotoxic effects in children and
    cardiovascular effects in adults are among those best substantiated
    as occurring at blood-Pb concentrations as low as 5 to 10 μg/dL (or
    possibly lower); and these categories of effects are currently
    clearly of greatest public health concern. Other newly demonstrated
    immune and renal system effects among general population groups are
    also emerging as low-level Pb-exposure effects of potential public
    health concern."

-   Many associations of health effects with Pb exposure have been found
    at levels of blood Pb that are currently relevant for the U.S.
    population, with individual children having blood Pb levels of 5-10
    μg/dL and lower, being at risk for neurological effects (as
    described in the subsequent bullet). Supportive evidence from
    toxicological studies provides biological plausibility for the
    observed effects. (CD, Chapters 5, 6 and 8)

-   Pb exposure is associated with a variety of neurological effects in
    children, notably intellectual attainment and school performance.
    Both qualitative and quantitative evidence, with further support
    from animal research, indicates a robust and consistent effect of Pb
    exposure on neurocognitive ability at mean concurrent blood Pb
    levels in the range of 5 to 10 μg/dL. Specific epidemiological
    analyses have further indicated association with neurocognitive
    effects in analyses restricted to children with individual blood Pb
    levels below 5-10 μg/dL, and for which group mean levels are lower.
    A recent analysis of a nationally representative U.S. sample
    suggested Pb effects on intellectual attainment of young children at
    population mean concurrent blood Pb levels ranging down slightly
    below 2 μg/dL. (CD, Sections 5.3, 6.2, 8.4.2 and 6.10; Auinger,
    2008)

-   Deficits in cognitive skills may have long-term consequences over a
    lifetime. Poor academic skills and achievement can have enduring and
    important effects on objective parameters of success in real life as
    well as increased risk of antisocial and delinquent behavior. (CD,
    Sections 6.1 and 8.4.2)

-   The current epidemiological evidence indicates a steeper slope of
    the blood Pb concentration-response relationship at lower blood Pb
    levels, particularly those below 10 μg/dL (CD, Sections 6.2.13 and
    8.6).

-   At mean blood Pb levels, in children, on the order of 10 μg/dL, and
    somewhat lower, associations have been found with effects to the
    immune system, including altered macrophage activation, increased
    IgE levels and associated increased risk for autoimmunity and asthma
    (CD, Sections 5.9, 6.8, and 8.4.6).

-   In adults, with regard to cardiovascular outcomes, the Criteria
    Document included the following summary (CD, p. E-10).

Epidemiological studies have consistently demonstrated associations
between Pb exposure and enhanced risk of deleterious cardiovascular
outcomes, including increased blood pressure and incidence of
hypertension.[^45] A meta-analysis of numerous studies estimates that a
doubling of blood-Pb level (e.g., from 5 to 10 μg/dL) is associated with
\~1.0 mm Hg increase in systolic blood pressure and \~0.6 mm Hg increase
in diastolic pressure. Studies have also found that cumulative past Pb
exposure (e.g., bone Pb) may be as important, if not more, than present
Pb exposure in assessing cardiovascular effects. The evidence for an
association of Pb with cardiovascular morbidity and mortality is limited
but supportive.

Studies of nationally representative U.S. samples observed associations
between blood Pb levels and increased systolic blood pressure at
population mean blood Pb levels less than 5 μg/dL, particularly among
African Americans (CD, Section 6.5.2). With regard to gender
differences, the Criteria Document states the following (CD, p. 6-154).

Although females often show lower Pb coefficients than males, and Blacks
higher Pb coefficients than Whites, where these differences have been
formally tested, they are usually not statistically significant. The
tendencies may well arise in the differential Pb exposure in these
strata, lower in women than in men, higher in Blacks than in Whites. The
same sex and race differential is found with blood pressure.

Animal evidence provides confirmation of Pb effects on cardiovascular
functions. (CD, Sections 5.5, 6.5, 8.4.3 and 8.6.3)

-   Renal effects, evidenced by reduced renal filtration, have also been
    associated with Pb exposures indexed by bone Pb levels and also with
    mean blood Pb levels in the range of 5 to 10 μg/dL in the general
    adult population, with the potential adverse impact of such effects
    being enhanced for susceptible subpopulations including those with
    diabetes, hypertension, and chronic renal insufficiency (CD,
    Sections 6.4, 8.4.5, and 8.6.4). The full significance of this
    effect is unclear, given that other evidence of more marked signs of
    renal dysfunction have not been detected at blood Pb levels below
    30-40 μg/dL in large studies of occupationally-exposed Pb workers
    (CD, pp. 6-270 and 8-50).[^46]

-   Other Pb associated effects in adults occurring at or just above 10
    μg/dL include hematological (e.g., impact on heme synthesis pathway)
    and neurological effects, with animal evidence providing support of
    Pb effects on these systems and evidence regarding mechanism of
    action. (CD, Sections 5.2, 5.3, 6.3 and 6.9.2)

C. Human Exposure and Health Risk Assessments

This section presents a brief summary of the human exposure and health
risk assessments conducted by EPA for this review. The complete
full-scale assessment, which includes specific analyses conducted to
address CASAC comments and advice on an earlier draft assessment, is
presented in the final Risk Assessment Report (USEPA, 2007b).

The focus of this Pb NAAQS risk assessment is on characterizing risk
resulting from exposure to policy-relevant Pb (i.e., exposure to Pb that
has passed through ambient air on its path from source to human exposure
-- as described in section II.A.2). The design and implementation of
this assessment needed to address significant limitations and complexity
that go far beyond the situation for similar assessments typically
performed for other criteria pollutants. Not only was the risk
assessment constrained by the timeframe allowed for this review in the
context of breadth of information to address, it was also constrained by
significant limitations in data and modeling tools for the assessment,
as discussed further in Section B2e below. Furthermore, the multimedia
and persistent nature of Pb, and the role of multiple exposure pathways
(discussed in section II.A), add significant complexity to the
assessment as compared to other assessments that focus only on the
inhalation pathway.

The remainder of this overview of the human health risk assessment is
organized as follows. An overview of the human health risk assessment
completed in the last review of the Pb NAAQS in 1990 (USEPA, 1990a) is
presented first. Next, design aspects of the current risk assessment are
presented, including: (a) CASAC advice regarding the design of the risk
assessment, (b) description of health endpoints and associated risk
metrics modeled, including the concentration-response functions used,
(c) overview of the case study approach employed, (d) description of air
quality scenarios modeled, (e) explanation of air-related versus
background classification of risk results in the context of this
analysis, (f) overview of analytical (modeling) steps completed for the
risk assessment and (g) description of the multiple sets of risk results
generated for the analysis. Then, key sources of uncertainty associated
with the analysis are presented. And finally, a summary of exposure and
risk estimates and key observations is presented.

1\. Overview of Risk Assessment from Last Review

The risk assessment conducted in support of the last review used a case
study approach to compare air quality scenarios in terms of their impact
on the percentage of modeled populations that exceeded specific blood Pb
levels chosen with consideration of the health effects evidence at that
time (USEPA, 1990b; USEPA, 1989). The case studies in that analysis,
however, focused exclusively on Pb smelters including two secondary and
one primary smelter and did not consider exposures in a more general
urban context. The analysis focused on children (birth through 7 years
of age) and middle-aged men. The assessment evaluated impacts of
alternate NAAQS on numbers of children and men with blood Pb levels
above levels of concern based on health effects evidence at that time.
The primary difference between the risk assessment approach used in the
current analysis and the assessment completed in 1990 involves the risk
metric employed. Rather than estimating the percentage of study
populations with exposures above blood Pb levels of interest as was done
in the last review (i.e., 10, 12 and 15 μg/dL), the current analysis
estimates changes in health risk, specifically IQ loss, associated with
Pb exposure for child populations at each of the case study locations
with that estimated IQ loss further differentiated between air-related
and background Pb exposure categories.

2\. Design Aspects of Exposure and Risk Assessments

This section provides an overview of key elements of the assessment
design, inputs, and methods, and includes identification of key
uncertainties and limitations.

a\. CASAC Advice

The CASAC conducted a consultation on the draft analysis plan for the
risk assessment (USEPA, 2006c) in June, 2006 (Henderson, 2006). Some key
comments provided by CASAC members on the plan included: (1) placing a
higher priority on modeling the child IQ metric than the adult endpoints
(e.g., cardiovascular effects), (2) recognizing the importance of indoor
dust loading by Pb contained in outdoor air as a factor in Pb-related
exposure and risk for sources considered in this analysis, and (3)
concurring with use of the IEUBK biokinetic blood Pb model. Taking these
comments into account, a pilot phase assessment was conducted to test
the risk assessment methodology being developed for the subsequent
full-scale assessment. The pilot phase assessment is described in the
first draft Staff Paper and accompanying technical report (ICF 2006),
which was discussed by the CASAC Pb panel on February 6-7 (Henderson,
2007a).

Results from the pilot assessment, together with comments received from
CASAC and the public, informed the design of the full-scale analysis.
The full-scale analysis included a substitution of a more generalized
urban case study for the location-specific near-roadway case study
evaluated in the pilot. In addition, a number of changes were made in
the exposure and risk assessment approaches, including the development
of a new indoor dust Pb model focused specifically on urban residential
locations and specification of additional IQ loss concentration-response
(C-R) functions to provide greater coverage for potential impacts at
lower exposure levels.

The draft full-scale assessment was presented in the July 2007 draft
risk assessment report (USEPA, 2007a) that was released for public
comment and provided to CASAC for review. In their review of the July
draft risk assessment report, the CASAC Pb Panel made several
recommendations for additional exposure and health risk analyses
(Henderson, 2007b). These included a recommendation that the general
urban case study be augmented by the inclusion of risk analyses in
specific urban areas of the U.S. In this regard, they specifically
stated the following (Henderson, 2007b, p. 3)

... the CASAC strongly believes that it is important that EPA staff make
estimates of exposure that will have national implications for, and
relevance to, urban areas; and that, significantly, the case studies of
both primary lead (Pb) smelter sites as well as secondary smelter sites,
while relevant to a few atypical locations, do not meet the needs of
supporting a Lead NAAQS. The Agency should also undertake case studies
of several urban areas with varying lead exposure concentrations, based
on the prototypic urban risk assessment that OAQPS produced in the 2nd
Draft Lead Human Exposure and Health Risk Assessments. In order to
estimate the magnitude of risk, the Agency should estimate exposures and
convert these exposures to estimates of blood levels and IQ loss for
children living in specific urban areas.

Hence, EPA included additional case studies in the risk assessment
focused on characterizing risk for residential populations in three
specific urban locations. Further, CASAC recommended using a
concentration-response function with a change in slope near 7.5 μg/dL.
Accordingly, EPA included such an additional concentration-response
function in the risk assessment. Results from the initial full-scale
analyses, along with comments from CASAC, such as those described here,
and the public resulted in a final version of the full-scale assessments
which is briefly summarized here and presented in greater detail in the
Risk Assessment Report and associated appendices (USEPA, 2007b).

In their review of the final risk assessment, CASAC expressed strong
support, stating as follows (Henderson, 2008a, p. 4):

The Final Risk Assessment report captures the breadth of issues related
to assessing the potential public health risk associated with lead
exposures; it competently documents the universe of knowledge and
interpretations of the literature on lead toxicity, exposures, blood
lead modeling and approaches for conducting risk assessments for lead.

b\. Health Endpoint, Risk Metric and Concentration-response Functions

The health endpoint on which the quantitative health risk assessment
focuses is developmental neurotoxicity in children, with IQ decrement
(or loss) as the risk metric. Among the wide variety of health endpoints
associated with Pb exposures, there is general consensus that the
developing nervous system in young children is the most sensitive and
that neurobehavioral effects (specifically neurocognitive deficits),
including IQ decrements, appear to occur at lower blood levels than
previously believed (i.e., at levels \<10 μg/dL). The selection of
children's IQ for the quantitative risk assessment reflects
consideration of the evidence presented in the Criteria Document as well
as advice received from CASAC (Henderson, 2006, 2007a).

Given the evidence described in detail in the Criteria Document
(Chapters 6 and 8), and in consideration of CASAC recommendations
(Henderson, 2006, 2007a, 2007b), the risk assessment for this review
relies on the functions presented by Lanphear and others (2005) that
relate absolute IQ as a function of concurrent blood Pb or of the log of
concurrent blood Pb, and lifetime average blood Pb, respectively. As
discussed in the Criteria Document (CD, p. 8-63 to 8-64), the slope of
the concentration-response relationship described by these functions is
greater at the lower blood Pb levels (e.g., less than 10 μg/dL). As
discussed in the Criteria Document and summarized in section II.B.2,
threshold blood Pb levels for these effects cannot be discerned from the
currently available epidemiological studies, and the evidence in the
animal Pb neurotoxicity literature does not define a threshold for any
of the toxic mechanisms of Pb (CD, Sections 5.3.7 and 6.2).

In applying relationships observed with the international pooled
analysis by Lanphear and others (2005) to the risk assessment, which
includes blood Pb levels below the range represented by the pooled
analysis, several alternative blood Pb concentration-response models
were considered in recognition of a reduced confidence in our ability to
characterize the quantitative blood Pb concentration-response
relationship at the lowest blood Pb levels represented in the recent
epidemiological studies. The functions considered and employed in the
initial risk analyses for this review include the following.

-   Log-linear function with low-exposure linearization, for both
    concurrent and lifetime average blood metrics, applies the nonlinear
    relationship down to the blood Pb concentration representing the
    lower bound of blood Pb levels for that blood metric in the pooled
    analysis and applies the slope of the tangent at that point to blood
    Pb concentrations estimated in the risk assessment to fall below
    that level.

-   Log-linear function with cutpoint, for both concurrent and lifetime
    average blood metrics, also applies the nonlinear relationship at
    blood Pb concentrations above the lower bound of blood Pb
    concentrations in the pooled analysis dataset for that blood metric,
    but then applies zero risk to all lower blood Pb concentrations
    estimated in the risk assessment.

In the additional risk analyses performed subsequent to the August 2007
CASAC public meeting, the two functions listed above and the following
two functions were employed (details on the forms of these functions as
applied in this risk assessment are described in Section 5.3.1 of the
Risk Assessment Report).

-   Population stratified dual linear function for concurrent blood Pb,
    derived from the pooled dataset stratified at peak blood Pb of 10
    μg/dL and

-   Population stratified dual linear function for concurrent blood Pb,
    derived from the pooled dataset stratified at 7.5 μg/dL peak blood
    Pb.

In interpreting risk estimates derived using the various functions,
consideration should be given to the uncertainties with regard to the
precision of the coefficients used for each analysis. The coefficients
for the log-linear model from Lanphear et al. (2005) had undergone a
careful development process, including sensitivity analyses, using all
available data from 1,333 children. The shape of the exposure-response
relationship was first assessed through tests of linearity, then by
evaluating the restricted cubic spline model. After determining that the
log-linear model provided a good fit to the data, covariates to adjust
for potential confounding were included in the log-linear model with
careful consideration of the stability of the parameter estimates. After
the multiple regression models were developed, regression diagnostics
were employed to ascertain whether the Pb coefficients were affected by
collinearity or influential observations. To further investigate the
stability of the model, a random-effects model (with sites random) was
applied to evaluate the results and also the effect of omitting one of
the seven cohorts on the Pb coefficient. In the various sensitivity
analyses performed, the coefficient from the log-linear model was found
to be robust and stable. The log-linear model, however, is not
biologically plausible at the very lowest blood Pb concentrations as
they approach zero; therefore, in the first two functions the log-linear
model is applied down to a cutpoint (of 1 μg/dL for the concurrent blood
Pb metric), selected based on the low end of the blood Pb levels in the
pooled dataset, followed by a linearization or an assumption of zero
risk at levels below that point.

In contrast, the coefficients from the two analyses using the population
stratified dual linear function with stratification at 7.5 μg/dL and 10
μg/dL, peak blood Pb, have not undergone as careful development. These
analyses were primarily done to compare the lead-associated decrement at
lower blood Pb concentrations and higher blood Pb concentrations. For
these analyses, the study population was stratified at the specified
peak blood Pb level and separate linear models were fitted to the
concurrent blood Pb data for the children in the two study population
subgroups.[^47] While these analyses are quite suitable for the purpose
of investigating whether the slope at lower concentration levels is
greater compared to higher concentration levels, use of such
coefficients as the primary C-R function in a risk analysis such as this
may be inappropriate. Further, only 103 children had maximal blood Pb
levels less than 7.5 μg/dL and 244 children had maximal blood Pb levels
less than 10 μg/dL. While these children may better represent current
blood Pb levels, not fitting a single model using all available data may
lead to bias. Slob et al. (2005) noted that the usual argument for not
considering data from the high dose range is that different biological
mechanisms may play a role at higher doses compared to lower doses.
However, this does not mean a single curve across the entire exposure
range cannot describe the relationship. The fitted curve merely assumes
that the underlying dose-response follows a smooth curve over the whole
dose range. If biological mechanisms change when going from lower to
higher doses, this change will result in a gradually changing slope of
the dose-response. The major strength of the Lanphear et al. (2005)
study was the large sample size and the pooled analysis of data from
seven different cohorts. In the case of the study population subgroup
with peak blood Pb below 7.5 μg/dL, less than 10% of the available data
is used in the analysis, with more than half of the data coming from one
cohort (Rochester) and the six other cohorts contributing zero to 13
children to the analysis. Such an analysis consequently does not make
full use of the strength of the pooled study by Lanphear and others
(2005).

In consideration of the preceding discussion, greater confidence is
placed in the log-linear model form compared to the dual-linear
stratified models for purposes of the risk assessment described in this
notice. Further, in considering risk estimates derived from the four
core functions (log-linear function with low-exposure linearization,
log-linear function with cutpoint, dual linear function, stratified at
7.5 µg/dL peak blood Pb, and dual linear function, stratified at 10
µg/dL peak blood Pb), greatest confidence is assigned to risk estimates
derived using the log-linear function with low-exposure linearization
since this function (a) is a nonlinear function that describes greater
response per unit blood Pb at lower blood Pb levels consistent with
multiple studies identified in the discussion above, (b) is based on
fitting a function to the entire pooled dataset (and hence uses all of
the data in describing response across the range of exposures), (c) is
supported by sensitivity analyses showing the model coefficients to be
robust, and (d) provides an approach for predicting IQ loss at the
lowest exposures simulated in the assessment (consistent with the lack
of evidence for a threshold). Note, however, that risk estimates
generated using the other three concentration-response functions are
also presented to provide perspective on the impact of uncertainty in
this key modeling step. We additionally note that the CASAC Pb Panel
recommended that C-R function derived from the pooled dataset stratified
at 7.5 µg/dL, peak blood Pb, be given weight in this analysis
(Henderson, 2008).

c\. Case Study Approach

For the risk assessment described in this notice, a case study approach
was employed as described in Sections 2.2 (and subsections) and 5.1.3 of
the Risk Assessment Report (USEPA, 2007b). In summarizing the assessment
in this notice, we have focused on five[^48] case studies that generally
represent two types of population exposures: 1) more highly air-pathway
exposed children residing in small neighborhoods or localized
residential areas with air concentrations somewhat near the standard
level being evaluated, and 2) urban populations with a broader range of
air-related exposures. These five case studies are:

-   [A general urban case study:]{.underline} This case study is not
    based on a specific geographic location and reflects several
    simplifying assumptions used in representing exposure including
    uniform ambient air Pb levels associated with the standard of
    interest across the hypothetical study area and a uniform study
    population. This case study characterizes risk for a localized part
    of an urban area at different standard levels, but based on national
    average estimates of the relationships between the different
    standard form assessed and ambient air exposure concentrations.
    Thus, while this provides characterization of risk to children that
    are relatively more highly air pathway exposed, this case study is
    not considered to represent a high-end scenario with regard to the
    characterization of ambient air Pb levels and associated risk.[^49]

-   [A primary Pb smelter case study:]{.underline}[^50] This case study
    estimates risk for children living in an area currently not in
    attainment with the current NAAQS that is impacted by Pb emissions
    from a primary Pb smelter. Results described here are those for the
    area within 1.5 km of the facility (the "subarea") where airborne Pb
    concentrations are closest to the current standard. As such, this
    case study characterizes risk for a specific more highly exposed
    population and also provides insights on risk to child populations
    living in areas near large sources of Pb emissions.

-   [Three location-specific urban case studies]{.underline}: These
    urban case studies focus on specific urban areas (Cleveland, Chicago
    and Los Angeles) to provide representations of the distribution of
    ambient air-related risk in specific densely populated urban
    locations. These case studies represent areas with specific
    population distributions and that experience a broader range of
    air-related exposures due both to potential spatial gradients in
    ambient air Pb levels and population density. A large majority of
    the population in these case studies resides in areas with much
    lower air concentrations than those in the very small subareas of
    these case studies with the highest concentrations. Ambient air Pb
    concentrations are characterized using source-oriented and other
    Pb-TSP monitors in these cities, while location-specific US Census
    demographic data are used to characterize the spatial distribution
    of residential child populations in these study areas.

These different case studies generally represent two types of population
exposures. The general urban and primary Pb smelter subarea provide
estimates of risk for more highly air-pathway exposed children residing
in small neighborhoods or localized residential areas with air
concentrations somewhat near the standard level being evaluated. By
contrast, the three location-specific urban case studies included in the
analysis provide risk estimates for an urban population with a broader
range of air-related exposures. In fact, for the location-specific urban
case studies, the majority of the modeled populations experience ambient
air Pb levels significantly lower than the standard level being
evaluated, with only a small population experiencing ambient air Pb
levels at or near the standard. [^51]

In considering risk results generated for the location-specific urban
case studies, we note that, given the wide range of monitored Pb levels
in urban areas, combined with the relatively limited monitoring network
characterizing ambient levels in the urban setting, it is not possible
to determine where these case studies fall within the distribution of
ambient air-related risk in U.S. cities.

d\. Air Quality Scenarios

Air quality scenarios assessed include (a) a current conditions scenario
for the location-specific urban case studies, the general urban case
study and the secondary Pb smelter case study, (b) a current NAAQS
scenario for the location-specific urban case studies, the general urban
case study and the primary Pb smelter case study, and (c) a range of
alternative NAAQS scenarios for all case studies. The alternative NAAQS
scenarios include levels of 0.5, 0.2, 0.05, and 0.02 µg/m^3^, with a
monthly averaging time, as well as a level of 0.2 µg/m^3^ scenario using
a quarterly averaging time.[^52]

The current NAAQS scenario for the urban case studies assumes ambient
air Pb concentrations higher than those currently occurring in nearly
all urban areas nationally.[^53] While it is extremely unlikely that Pb
concentrations in urban areas would rise to meet the current NAAQS and
there are limitations and uncertainties associated with the roll-up
procedure used for the location-specific urban case studies (as
described in Section III.B.2.g below), this scenario was included to
provide perspective on potential risks associated with just meeting the
current NAAQS relative to current conditions. When evaluating these
results it is important to keep these limitations and uncertainties in
mind.

Current conditions for the three location-specific urban case studies in
terms of maximum quarterly average air Pb concentrations are 0.09, 0.14
and 0.36 µg/m^3^ for the study areas in Los Angeles, Chicago and
Cleveland, respectively. In terms of maximum monthly average the values
are 0.17 μg/m^3^, 0.31 μg/m^3^ and 0.56 μg/m^3^ for the study areas in
Los Angeles, Chicago and Cleveland, respectively.

Details of the assessment scenarios, including a description of the
derivation of Pb concentrations for air and other media are presented in
Sections 2.3 (and subsections) and Section 5.1.1 of the Risk Assessment
Report (USEPA, 2007b).

e\. Categorization of Policy-relevant Exposure Pathways

As discussed in Section IIA, this review focuses on air-related exposure
pathways (i.e., those pathways where Pb passes through ambient air on
its path from source to human exposure). These include both inhalation
of ambient air Pb (including both Pb emitted directly into ambient air
as well as resuspended Pb); and ingestion of Pb that, once airborne, has
made its way into indoor dust, outdoor dust or soil, dietary items
(e.g., crops and livestock), and drinking water. Because of the
nonlinear response of blood Pb to exposure (simulated in the IEUBK blood
Pb model) and also the nonlinearity reflected in the C-R functions for
estimation of IQ loss, this assessment first estimates total blood Pb
and risk (air- and nonair-related), and then separates out those
estimates of blood Pb and associated risk associated with the pathways
of interest in this review.

To separate out risk for the pathways of interest in this review, we
split the estimates of total (all-pathway) blood Pb and IQ loss into
background and two air-related categories (referred to as "recent air"
and "past air"). However, significant limitations in our modeling tools
and data resulted in an inability to parse specific risk estimates into
specific pathways, such that we have approximated estimates for the
air-related and background categories.

Those Pb exposure pathways identified in section II.A.2 as being tied
most directly to ambient air, which consequently have the potential to
respond relatively more quickly to changes in air Pb (inhalation and
ingestion of indoor dust loaded directly from ambient air Pb) were
placed into the \"recent air\" category. The other air-related Pb
exposure pathways, associated with atmospheric deposition, were placed
into the "past air" category. These include ingestion of Pb in outdoor
dust/soil and ingestion of the portion of Pb in indoor dust that after
deposition from ambient air outdoors is carried indoors with humans (as
described in section II.A.2 above).

Thus, total blood Pb and IQ loss estimates were apportioned into the
following pathways or pathway combinations:

-   Inhalation of ambient air Pb (i.e., "recent air" Pb): This is
    derived using the blood Pb estimate resulting from Pb exposure
    limited to the inhalation pathway (and includes inhalation of Pb in
    ambient air from all sources contributing to the ambient air
    concentration estimate, including potentially resuspension).

-   Ingestion of "recent air" indoor dust Pb: This is derived using the
    blood Pb estimate resulting from Pb exposure limited to ingestion of
    the Pb in indoor dust that is predicted in this assessment from
    infiltration of ambient air indoors and subsequent deposition.[^54]

-   Ingestion of "other" indoor dust Pb (considered part of \"past air\"
    exposure): This is derived using the blood Pb estimate resulting
    from Pb exposure limited to ingestion of the Pb in indoor dust that
    is not predicted from infiltration of ambient air indoors and
    subsequent deposition.[^55] This is interpreted to represent indoor
    paint, outdoor soil/dust, and additional sources of Pb to indoor
    dust including historical air (as discussed in the Risk Assessment
    Report, Section 2.4.3). As the intercept in regression dust models
    will be inclusive of error associated with the model coefficients,
    this category also includes some representation of dust Pb
    associated with current ambient air concentrations (described in
    previous bullet). For the primary Pb smelter case study, estimates
    for this pathway are not separated from estimates for the pathway
    described above due to uncertainty regarding this categorization
    with the model used for this case study (Risk Assessment Report,
    Section 3.1.4.2). This pathway is included in the "past air"
    category.

-   Ingestion of outdoor soil/dust Pb: This is derived using the blood
    Pb estimate resulting from Pb exposure limited to ingestion of
    outdoor soil/dust Pb. This pathway is included in the "past air"
    category (and could include contamination from historic Pb emissions
    from automobiles and Pb paint).

-   Ingestion of drinking water Pb: This is derived using the blood Pb
    estimate resulting from Pb exposure limited to ingestion of drinking
    water Pb. This pathway is included in the policy-relevant background
    category.

-   Ingestion of dietary Pb: This is derived using the blood Pb estimate
    resulting from Pb exposure limited to ingestion of dietary Pb. This
    pathway is included in the policy-relevant background category.

As noted above, significant limitations in our modeling tools and data
resulted in an inability to parse risk estimates for specific pathways,
such that we approximated estimates for the air-related and background
categories. Of note in this regard is the apportionment of background
(nonair) pathways. For example, while conceptually indoo Pb paint
contributions to indoor dust Pb would be considered background and
included in the "background" category for this assessment, due to
technical limitations related to indoor dust Pb modeling, ultimately,
dust from Pb paint was included as part of \"other\" indoor dust Pb
(i.e., as part of past air exposure). The inclusion of indoor lead Pb as
a component of \"other\" indoor air (and consequently as a component of
the "past air" category) represents a source of potential high bias in
our prediction of exposure and risk associated with the "past air"
category because conceptually, exposure to indoor paint Pb is considered
part of background exposure. Further, Pb in ambient air does contribute
to the exposure pathways included in the "background" category (drinking
water and diet), and is likely a substantial contribution to diet (CD,
p. 3-48). But we could not separate the air contribution from the nonair
contributions, and the total contribution from both the drinking water
and diet pathways are categorized as "background" in this assessment. As
a result, our "background" risk estimate includes some air-related risk.

Further, we note that in simulating reductions in exposure associated
with reducing ambient air Pb levels through alternative NAAQS (and
increases in exposure if the current NAAQS was reached in certain case
studies) only the exposure pathways categorized as "recent air"
(inhalation and ingestion of that portion of indoor dust associated with
outdoor ambient air) were varied with changes in air concentration. The
assessment did not simulate decreases in "past air" exposure pathways
(e.g., reductions in outdoor soil Pb levels following reduction in
ambient air Pb levels and a subsequent decrease in exposure through
incidental soil ingestion and the contribution of outdoor soil to indoor
dust). These exposures were held constant across all air quality
scenarios. In comparing total risk estimates between alternate NAAQS
scenarios, this aspect of the analysis will tend to underestimate the
reductions in risk associated with alternative NAAQS. However, this does
not mean that overall risk has been underestimated. The net effect of
all sources of uncertainty or bias in the analysis, which may also tend
to under- or overestimate risk, could not be quantified. Interpretation
of risk estimates is discussed more fully in section II.C.3.b.

In summary, because of limitations in the assessment design, data and
modeling tools, our risk estimates for the "past air" category include
both risks that are truly air-related and potentially, some background
risk. Because we could not sharply separate Pb linked to ambient air
from Pb that is background, some of the three categories of risk are
underestimated and others overestimated. On balance, we believe this
limitation leads to a slight overestimate of the risks in the "past air"
category. At the same time, as discussed above, the \"recent air\"
category does not fully represent the risk associated with all
air-related pathways. Thus, we consider the risk attributable to
air-related exposure pathways to be bounded on the low end by the risk
estimated for the "recent air" category and on the upper end by the risk
estimated for the "recent air" plus "past air" categories.

f\. Analytical Steps

The risk assessment includes four analytical steps, briefly described
below and presented in detail in Sections 2.4.4, 3.1, 3.2, 4.1, and 5.1
of the Risk Assessment Report (USEPA, 2007b).

-   *Characterization of Pb in ambient air*: The characterization of
    outdoor ambient air Pb levels uses different approaches depending on
    the case study (as explained in more detail below): (a)
    source-oriented and non-source oriented monitors are assumed to
    represent different exposure zones in the city-specific case
    studies, (b) a single exposure level is assumed, based on monitoring
    data for various cities, for the general urban case study, and (c)
    ambient levels are estimated using air dispersion modeling based on
    Pb emissions from a particular facility in the point source case
    studies.

-   *Characterization of outdoor soil/dust and indoor dust Pb
    concentrations*: Outdoor soil Pb levels are estimated using
    empirical data and fate and transport modeling. Indoor dust Pb
    levels are predicted using a combination of (a) regression-based
    models that relate indoor dust to ambient air Pb and outdoor soil
    Pb, and (b) mechanistic models.[^56]

-   *Characterization of blood Pb levels*: Blood Pb levels for each
    exposure zone are derived from central-tendency blood Pb
    concentrations estimated using the Integrated Exposure and Uptake
    Biokinetic (IEUBK) model, and concurrent or lifetime average blood
    Pb is estimated from these outputs as described in Section 3.2.1.1
    of the Risk Assessment Report (USEPA, 2007b). For the point source
    and location-specific urban case studies, a probabilistic exposure
    model is used to generate population distributions of blood Pb
    concentrations based on: (a) the central tendency blood Pb levels
    for each exposure zone, (b) demographic data for the distribution of
    children (less than 7 years of age) across exposure zones in a study
    area, and (c) a geometric standard deviation (GSD) intended to
    characterize interindividual variability in blood Pb (e.g.,
    reflecting differences in behavior and biokinetics related to Pb).
    For the general urban case study, as demographic data for a specific
    location are not considered, the GSD is applied directly to the
    central tendency blood Pb level to estimate a population
    distribution of blood Pb levels. Additional detail on the methods
    used to model population blood Pb levels is presented in Sections
    3.2.2 and 5.2.2.3 of the Risk Assessment Report (USEPA, 2007b).

-   *Risk characterization (estimating IQ loss)*: Concurrent or lifetime
    average blood Pb estimates for each simulated child in each case
    study population are converted into total Pb-related IQ loss
    estimates using the concentration-response functions described
    above.

We have also used the results of exposure modeling to estimate
air-to-blood ratios for two of the case studies (the general urban and
primary Pb smelter case studies). Specifically, we compared the change
in ambient air Pb between adjacent NAAQS levels with the associated
reduction in concurrent blood Pb levels (for the median population
percentile) to derive air-to-blood ratios. As they relate air
concentrations[^57] input to the first analytical step to blood Pb
estimates output from the third analytical step, they may be viewed as a
collapsed alternate to the three steps for the exposure pathways
directly linked to air concentrations in this assessment. The values for
these ratios are affected by design aspects of the risk assessment, most
notably those identified here:

-   Because they are derived from differences in blood Pb estimates
    between air quality scenarios and the only pathways varied with air
    quality scenarios are ambient air and indoor dust (as described in
    section II.C.2.e above), the exposure pathways reflected in the
    ratios are generally the "recent air" pathways (described in section
    II.C.2.e above), which include inhalation of ambient air and
    ingestion of indoor dust loaded by infiltration of ambient air.
    Ratios for the primary Pb smelter case study subarea may
    additionally reflect some contributions to indoor dust from other
    ambient air-related pathways (e.g., "tracking in" of soil containing
    ambient air Pb), yet still not all air-related pathways. Thus, the
    air-to-blood ratios derived for both case studies are lower than
    they would be if they reflected all air-related pathways.

-   The blood Pb estimates used in this calculation are for the
    "concurrent" metric (i.e., concentrations during the 7th year of
    life). Accordingly, the resultant air-to-blood ratios are lower than
    they would be if based on blood Pb estimates for the 2nd year of
    life (e.g., peak) or estimates averaged over the exposure period.

Key limitations and uncertainties associated with the application of
these specific analytical steps are summarized in Section III.B.2.k
below.

g\. Generating Multiple Sets of Risk Results

In the initial analyses for the full-scale assessment (USEPA, 2007a),
EPA implemented multiple modeling approaches for each case study
scenario in an effort to characterize the potential impact on exposure
and risk estimates of uncertainty associated with the limitations in the
tools, data and methods available for this risk assessment and with key
analytical steps in the modeling approach. These multiple modeling
approaches are described in Section 2.4.6.2 of the final Risk Assessment
Report (USEPA, 2007b). In consideration of comments provided by CASAC
(Henderson, 2007b) on these analyses regarding which modeling approach
they felt had greater scientific support, a pared down set of modeling
combinations was identified as the core approach for the subsequent
analyses. The core modeling approach includes the following key
elements:

-   ambient air Pb estimates (based on monitors or modeling and
    proportional rollbacks, as described below),

-   background exposure from food and water (as described above),

-   the hybrid indoor dust model specifically developed for urban
    residential applications (which predicts Pb in indoor dust as a
    function of ambient air Pb and nonair contribution),

-   the IEUBK blood Pb model (which predicts blood Pb in young children
    exposed to Pb from multiple exposure pathways),

-   the concurrent blood Pb metric,

-   a GSD for concurrent blood Pb of 2.1 to characterize interindividual
    variability in blood Pb levels for a given ambient level for the
    urban case studies, and

-   four different functions relating concurrent blood Pb to IQ loss,
    including two log-linear models (one with a cutpoint and one with
    low-exposure linearization) and two dual-linear models with
    stratification, one stratified at 7.5 µg/dL peak blood Pb and the
    other at 10 µg/dL peak blood Pb.

For each case study, the core modeling approach employs a single set of
modeling elements to estimate exposure and the four different
concentration-response functions referenced above to derive four sets of
risk results from the single set of exposure estimates. The spread of
estimates resulting from application of all four functions captures much
of the uncertainty associated model choice in this analytical step.
Among these four functions, EPA has greater confidence in estimates
derived using the log-linear with low-exposure linearization
concentration-response function as discussed above.

In addition to employing multiple concentration-response functions, the
assessment includes various sensitivity analyses to characterize the
potential impact of uncertainty in other key analysis steps on exposure
and risk estimates. The sensitivity analyses and uncertainty
characterization completed for the risk analysis are described in
Sections 3.5, 4.3, 5.2.5 and 5.3.3 of the Risk Assessment Report (USEPA,
2007b).

h\. Key Limitations and Uncertainties

As recognized above, EPA has made simplifying assumptions in several
areas of this assessment due to the limited data, models, and time
available. These assumptions and related limitations and uncertainties
are described in the Risk Assessment Report (USEPA, 2007b). Key
assumptions, limitations and uncertainties are briefly identified below,
with emphasis on those sources of uncertainty considered most critical
in interpreting risk results. In the presentation below, limitations
(and associated uncertainty) are listed, beginning with those regarding
design of the assessment or case studies, followed by those regarding
estimation of Pb concentrations in ambient air indoor dust, outdoor
soil/dust, and blood, and lastly regarding estimation of Pb-related IQ
Loss.

-   *Temporal aspects*: Exposure modeling uses a 7 year exposure period
    for each simulated child, during which time, media concentrations
    remain fixed (at levels associated with the ambient air Pb level
    being modeled) and the child remains at the same residence, while
    exposure factors and physiological parameters are adjusted to match
    the age of the child. These aspects are a simplification of
    population exposures that contributes some uncertainty to our
    exposure and risk estimates.

-   *General urban case study*: This case study is not based on a
    specific location and is instead intended to represent a smaller
    neighborhood experiencing ambient air Pb levels at or near the
    standard of interest. Consequently, it assumes (a) a single exposure
    zone within which all media concentrations of Pb are assumed to be
    spatially uniform and (b) a uniformly distributed population of
    unspecified size. While these assumptions are reasonable in the
    context of evaluating risk for a smaller subpopulation located close
    to a monitor reporting values at or near the standard of interest,
    there is significant uncertainty associated with extrapolating these
    risks to a specific urban location, particularly if that urban
    location is relatively large, given that larger urban areas are
    expected to have increasingly varied patterns of ambient air Pb
    levels and population density.

-   *Location-specific urban case studies*: The Pb-TSP monitoring
    network is currently quite limited and consequently, the number of
    monitors available to represent air concentrations in these case
    studies is limited, ranged from six for Cleveland to 11 for Chicago.
    Accordingly, our estimates of the magnitude of and spatial variation
    of air Pb concentrations are subject to uncertainty associated with
    the limited monitoring data and method used in extrapolating from
    those data to characterize an ambient air Pb level surface for these
    modeled urban areas. Details on the approach used to derive ambient
    air Pb surfaces for the urban case studies based on monitoring data
    are presented in Section 5.1.3 of the Risk Assessment Report (USEPA,
    2007b). As recognized in Section, III.B.2.a, the analyses for these
    case studies were developed in response to CASAC recommendations on
    the July 2007 draft Risk Assessment (Henderson, 2007b).
    Subsequently, the CASAC has reviewed the approach used in conducting
    the final draft of the full-scale risk assessment, including the
    inclusion of the location-specific urban case studies and expressed
    broad support for the technical approach used (Henderson, 2008).

-   *Current NAAQS air quality scenarios*: For the location-specific
    urban case studies, proportional roll-up procedures were used to
    adjust ambient air Pb concentrations up to just meet the current
    NAAQS (a detailed discussion is provided in Sections 2.3.1 and
    5.2.2.1 of the Risk Assessment Report, USEPA, 2007b). This procedure
    was used to provide insights into the degree of risk which could be
    associated with ambient air Pb levels at or near the current
    standard in urban areas. EPA recognizes that it is extremely
    unlikely that Pb concentrations would rise to just meet the current
    NAAQS in urban areas nationwide and that there is substantial
    uncertainty with our simulation of such conditions. For the primary
    Pb smelter case study, where current conditions exceed the current
    NAAQS, attainment of the current NAAQS was simulated using air
    quality modeling, emissions and source parameters used in developing
    the 2007 proposed revision to the State Implementation Plan for the
    area (described in Section 3.1.1.2 of the Risk Assessment Report
    (USEPA, 2007b)).

-   *Alternative NAAQS air quality scenarios*: In all case studies,
    proportional roll-down procedures were used to adjust ambient air Pb
    concentrations downward to attain alternative NAAQS (described in
    Sections 2.3.1 and 5.2.2.1 of the Risk Assessment Report, USEPA,
    2007b). There is significant uncertainty in simulating conditions
    associated with the implementation of emissions reduction actions to
    meet a lower standard.

-   *Estimates of outdoor soil/dust Pb concentrations*: Outdoor soil Pb
    concentration for both the urban case studies and the primary Pb
    smelter case study are based on empirical data (as described in
    Section 3.1.3 of the Risk Assessment). To the extent that these data
    are from areas containing older structures, the impact of Pb paint
    weathered from older structures on soil Pb levels will be reflected
    in these empirical estimates. In the case of the urban case studies,
    a mean value from a sample of houses built between 1940 and 1998 was
    used to represent soil Pb levels (as described in Section 3.1.3.1 of
    the Risk Assessment). In the case of the primary Pb smelter case
    study subarea, site-specific data are used. As there has been
    remediation of soil in this subarea, the measurements do not reflect
    historical air quality. Additionally, studies since remediation have
    reported increasing soil Pb levels indicating that soil
    concentrations are still responding to current air quality, and
    consequently underestimate eventual steady state conditions for the
    current air quality. In all case studies, the same outdoor soil/dust
    Pb concentrations (based on these datasets) are used for all air
    quality scenarios (i.e., the potential longer-term impact of
    reductions in ambient air Pb on outdoor soil/dust Pb levels and
    associated impacts on indoor dust Pb have not be simulated). In
    areas where air concentrations have been greater in the past,
    however, implementation of a reduced NAAQS might be expected to
    yield reduced soil Pb levels over the long term. As described in
    Section 2.3.3 of the Risk Assessment Report (USEPA, 2007b), however,
    there is potentially significant uncertainty associated with this
    conclusion, particularly with regard to implications for areas in
    which a Pb source may locate where one of comparable size had not
    been previously. Additionally, it is possible that control measures
    implemented to meet alternative NAAQS may result in changes to soil
    Pb concentrations; these are not reflected in the assessment.

-   *Estimates of indoor dust Pb concentrations for the urban case
    studies (application of the hybrid model)*: The hybrid
    mechanistic-empirical model for estimating indoor dust Pb for the
    urban case studies (as described in Section 3.1.4.1 of the Risk
    Assessment Report, USEPA, 2007b) utilizes a mechanistic model to
    simulate the exchange of outdoor ambient air Pb indoors and
    subsequent deposition (and buildup) of Pb on indoor surfaces, which
    relies on a number of empirical measurements for parameterization
    (e.g., infiltration rates, deposition velocities, cleaning
    frequencies and efficiencies). There is considerable uncertainty
    associated with these parameter estimates. In addition, there is
    uncertainty associated with the partitioning of total indoor dust Pb
    estimates between the infiltration-related ("recent air") component
    and other contributions ("other" as described in section II.C.2.e).

-   *Estimates of indoor dust Pb concentrations for the primary Pb
    smelter case study (application of the site-specific regression
    model):* There is uncertainty associated with the site-specific
    regression model applied in the remediation zone (as described in
    Section 3.1.4.2 of the Risk Assessment Report), and relatively
    greater uncertainty associated with its application to air quality
    scenarios that simulate notably lower air Pb levels (as is typically
    the case when applying regression-based models beyond the bounds of
    the datasets used in their derivation). The log-log form of the
    regression model prevents the ready identification of an intercept
    term handicapping us in partitioning estimates of air-related indoor
    dust (and consequently exposure and risk estimates) between "recent
    air" and "other" components. In addition, limitations in the
    model-derived air estimates used in deriving the regression model
    prevented effective consideration for the role of ambient air Pb
    related to resuspension in influencing indoor dust Pb levels.

-   *Characterizing interindividual variability using a GSD*: There is
    uncertainty associated with the GSD specified for each case study
    (as described in Sections 3.2.3 and 5.2.2.3 of the Risk Assessment
    Report). Two factors are described here as contributors to that
    uncertainty. Interindividual variability in blood Pb levels for any
    study population (as described by the GSD) will reflect, to a
    certain extent, spatial variation in media concentrations, including
    outdoor ambient air Pb levels and indoor dust Pb levels, as well as
    differences in physiological response to Pb exposure. For each case
    study, there is significant uncertainty in the specification of
    spatial variability in ambient air Pb levels and associated indoor
    dust Pb levels, as noted above. In addition, there are a limited
    number of datasets for different types of residential child
    populations from which a GSD can be derived (e.g., NHANES
    datasets[^58] for more heterogeneous populations and individual
    study datasets for likely more homogeneous populations near specific
    industrial Pb sources). This uncertainty associated with the GSDs
    introduces significant uncertainty in exposure and risk estimates
    for the 95^th^ population percentile.

-   *Exposure pathway apportionment for higher percentile blood Pb level
    and IQ loss estimates:* Apportionment of blood Pb levels for higher
    population percentiles is assumed to be the same as that estimated
    using the central tendency estimate of blood Pb in an exposure zone.
    This introduces significant uncertainty into projections of pathway
    apportionment for higher population percentiles of blood Pb and IQ
    loss. In reality, pathway apportionment may differ in higher
    exposure percentiles. For example, paint and/or drinking water
    exposures may increase in importance, with air-related contributions
    decreasing as an overall percentage of blood Pb levels and
    associated risk. Because of this uncertainty related to pathway
    apportionment, as mentioned earlier, greater confidence is placed in
    estimates of total Pb exposure and risk in evaluating the impact of
    the current NAAQS and alternative NAAQS relative to current
    conditions.

-   *Relating blood Pb levels to IQ loss:* Specification of the
    quantitative relationship between blood Pb level and IQ loss is
    subject to significant uncertainty at lower blood Pb levels (e.g.,
    below 5 µg/dL concurrent blood Pb). As discussed earlier, there are
    limitations in the datasets and concentration-response analyses
    available for characterizing the concentration-response relationship
    at these lower blood Pb levels. For example, the pooled
    international dataset analyzed by Lanphear and others (2005)
    includes relatively few children with blood Pb levels below 5 µg/dL
    and no children with levels below 1 µg/dL. In recognition of the
    uncertainty in specifying a quantitative concentration-response
    relationship at such levels, our core modeling approach involves the
    application of four different functions to generate a range of risk
    estimates (as described in Section 4.2.6 and Section 5.3.1 of the
    Risk Assessment Report, USEPA, 2007b). The difference in absolute IQ
    loss estimates for the four concentration-response functions for a
    given case study/air quality scenario combination is typically close
    to a factor of 3. Estimates of differences in IQ loss between air
    quality scenarios (in terms of percent), however, are more similar
    across the four functions, although the function producing higher
    overall risk estimates (the dual linear function, stratified at 7.5
    µg/dL, peak blood Pb) also produces larger absolute reductions in IQ
    loss compared with the other three functions.

3\. Summary of Estimates and Key Observations

This section presents blood Pb and IQ loss estimates generated in the
exposure and risk assessments. Blood Pb estimates (and air-to-blood Pb
ratios) are presented first, followed by IQ loss estimates.

a\. Blood Pb Estimates

This section presents a summary of blood Pb modeling results for
concurrent blood Pb drawn from the more detailed presentation in the
Staff Paper and the Risk Assessment Report (USEPA, 2007a, 2007b, 2007c).

Blood Pb level estimates for the current conditions air quality
scenarios for these cases studies differ somewhat from the national
values associated with recent NHANES information. For example, median
blood Pb levels for the current conditions scenario for the urban case
studies are somewhat larger than the national median from the NHANES
data for 2003-2004. Specifically, values for the three location-specific
urban case studies range from 1.7 to 1.8 μg/dL with the general urban
case study having a value of 1.9 μg/dL (current-conditions mean)
(presented in Risk Assessment Report, Volume I, Table 5-5), while the
median value from NHANES (2003-2004) is 1.6 μg/dL
(http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm).
Additionally, NHANES values for the 90^th^ percentile (for 2003-2004)
were identified and these values can be compared against 90^th^
percentile estimates generated for the urban case studies (see Risk
Assessment Report, Appendix O, Section O.3.2 for the location-specific
urban case study and Appendix N, Section N.2.1.2 for the general urban
case study). The 90^th^ percentile blood Pb levels for the current
conditions scenario, for the three location-specific urban case studies
range from 4.5 to 4.6 μg/dL, while the estimate for the general urban
case study is 5.0 μg/dL. These 90^th^ percentile values for the case
study populations are larger than the 90^th^ percentile value of 3.9
μg/dL reported by NHANES for all children in 2003-2004. It is noted that
ambient air levels reflected in the urban case studies are likely to
differ from those underlying the NHANES data.[^59]

Table 1 presents total blood Pb estimates for alternative standards,
focusing on the median in the assessed population, and associated
estimates for the air-related percentage of total blood Pb (i.e.,
bounded on the low end by the "recent air" contributions and on the high
end by the "recent" plus "past air" contribution to total Pb exposure).

Generally, 95^th^ percentile blood Pb estimates across air quality
scenarios for all case studies (not shown here) are 2-3 times higher
than the median estimates in Table 1. For example, 95^th^ percentile
estimates of total blood Pb for the current NAAQS scenario are 10.6
μg/dL for the general urban case study, 12.3 μg/dL for the primary Pb
smelter subarea, and 7.4 to 10.2 μg/dL for the three location-specific
urban case studies (Staff Paper, Table 4-2). While the estimates
indicate similar fractions of total blood Pb that is air-related between
the 95^th^ percentile and median, there is greater uncertainty in
pathway apportionment among air-related and other sources for higher
percentiles, including the 95^th^ percentile.

**Table 1. Summary of median blood Pb estimates for concurrent blood Pb
(total).**

+------------------+--------+---------+---------+---------+----------+
| **NAAQS Level    | *      |         |         |         |          |
| Simulated**      | *Total |         |         |         |          |
|                  | blood  |         |         |         |          |
| **(μg/m^3^ max   | Pb     |         |         |         |          |
| monthly, except  | (μg    |         |         |         |          |
| as noted         | /dL)** |         |         |         |          |
| below)**         |        |         |         |         |          |
|                  | **     |         |         |         |          |
|                  | (air-r |         |         |         |          |
|                  | elated |         |         |         |          |
|                  | perce  |         |         |         |          |
|                  | ntage) |         |         |         |          |
|                  | ^A^**  |         |         |         |          |
+==================+========+=========+=========+=========+==========+
|                  | **G    | **      | **Loc   |         |          |
|                  | eneral | Primary | ation-s |         |          |
|                  | urban  | Pb      | pecific |         |          |
|                  | case   | smelter | urban   |         |          |
|                  | s      | (s      | case    |         |          |
|                  | tudy** | ubarea) | st      |         |          |
|                  |        | case    | udies** |         |          |
|                  |        | stu     |         |         |          |
|                  |        | dy^B^** |         |         |          |
+------------------+--------+---------+---------+---------+----------+
|                  |        |         | **Clev  | **Ch    | **Los    |
|                  |        |         | eland** | icago** | A        |
|                  |        |         |         |         | ngeles** |
|                  |        |         | (0.56   | (0.31   |          |
|                  |        |         | μ       | μ       | (0.17    |
|                  |        |         | g/m^3^) | g/m^3^) | μg/m^3^) |
+------------------+--------+---------+---------+---------+----------+
| **1.5 max        | 3.1    | 4.6     | 2.1^D^  | 3.0^D^  | 2.6^D^   |
| quarterly^C^**   |        |         |         |         |          |
+------------------+--------+---------+---------+---------+----------+
|                  | (61 to | (up to  | (57 to  | (63 to  | (50 to   |
|                  | 84%)   | 87%)    | 86%)    | 83%)    | 81%)     |
+------------------+--------+---------+---------+---------+----------+
| **0.50**         | 2.2    | 3.2     | 1.8     | ^E^     | ^E^      |
+------------------+--------+---------+---------+---------+----------+
|                  | (41 to | (up to  | (39 to  |         |          |
|                  | 73%)   | 81%)    | 72%)    |         |          |
+------------------+--------+---------+---------+---------+----------+
| **0.20**         | 1.9    | 2.3     | 1.7     | 1.8     | 1.7 ^F^  |
+------------------+--------+---------+---------+---------+----------+
|                  | (26 to | (up to  | (6 to   | (17 to  | (18 to   |
|                  | 74%)   | 78%)    | 65%)    | 67%)    | 71%)     |
+------------------+--------+---------+---------+---------+----------+
| **0.05**         | 1.7    | 1.7     | 1.6     | 1.6     | 1.6      |
+------------------+--------+---------+---------+---------+----------+
|                  | (12 to | (up to  | (1 to   | (6 to   | (13 to   |
|                  | 65%)   | 65%)    | 63%)    | 69%)    | 69%)     |
+------------------+--------+---------+---------+---------+----------+
| **0.02**         | 1.6    | 1.6     | 1.6     | 1.6     | 1.6      |
+------------------+--------+---------+---------+---------+----------+
|                  | (6 to  | (up to  | (1 to   | (1 to   | (6 to    |
|                  | 69%)   | 69%)    | 63%)    | 63%)    | 63%)     |
+------------------+--------+---------+---------+---------+----------+
| A - Blood Pb     |        |         |         |         |          |
| estimates are    |        |         |         |         |          |
| rounded to one   |        |         |         |         |          |
| decimal place.   |        |         |         |         |          |
| Air-related      |        |         |         |         |          |
| percentage is    |        |         |         |         |          |
| bracketed by     |        |         |         |         |          |
| "recent air"     |        |         |         |         |          |
| (lower bound of  |        |         |         |         |          |
| presented range) |        |         |         |         |          |
| and "recent"     |        |         |         |         |          |
| plus "past air"  |        |         |         |         |          |
| (upper bound of  |        |         |         |         |          |
| presented        |        |         |         |         |          |
| range).          |        |         |         |         |          |
|                  |        |         |         |         |          |
| B -- In the case |        |         |         |         |          |
| of the primary   |        |         |         |         |          |
| Pb smelter case  |        |         |         |         |          |
| study, only      |        |         |         |         |          |
| recent plus past |        |         |         |         |          |
| air estimates    |        |         |         |         |          |
| are available.   |        |         |         |         |          |
|                  |        |         |         |         |          |
| C -- Roughly     |        |         |         |         |          |
| 0.7 - 1.0        |        |         |         |         |          |
| μg/m^3^ maximum  |        |         |         |         |          |
| monthly mean,    |        |         |         |         |          |
| across the urban |        |         |         |         |          |
| case studies.    |        |         |         |         |          |
|                  |        |         |         |         |          |
| D- A "roll-up"   |        |         |         |         |          |
| was performed so |        |         |         |         |          |
| that highest     |        |         |         |         |          |
| site in study    |        |         |         |         |          |
| are just met     |        |         |         |         |          |
| this level.      |        |         |         |         |          |
|                  |        |         |         |         |          |
| E - A "roll-up"  |        |         |         |         |          |
| to this level    |        |         |         |         |          |
| was not          |        |         |         |         |          |
| performed.       |        |         |         |         |          |
|                  |        |         |         |         |          |
| F -- A "roll-up" |        |         |         |         |          |
| to this level    |        |         |         |         |          |
| was not          |        |         |         |         |          |
| performed; these |        |         |         |         |          |
| estimates are    |        |         |         |         |          |
| based on current |        |         |         |         |          |
| conditions in    |        |         |         |         |          |
| this area.       |        |         |         |         |          |
+------------------+--------+---------+---------+---------+----------+

As described in section II.C.2.f, the risk assessment also developed
estimates for air-to-blood ratios. These ratios reflect a subset of
air-related pathways related to inhalation and ingestion of indoor dust;
inclusion of the remaining pathways would be expected to yield higher
ratios. Additionally, these ratios are based on blood Pb estimates for
the 7^th^ year of exposure (concurrent blood Pb) which are lower than
blood Pb estimates at younger ages (and than the lifetime-averaged blood
Pb metric). Ratios based on such other blood Pb estimates would be
higher (e.g., by a factor of approximately 1.3 for ratios based on the
lifetime --averaged blood Pb metric).[^60]

-   For the general urban case study, estimates of air-to-blood ratios
    ranged from 1:2 to 1:9, with the majority of the estimates ranging
    from 1:4 to 1:6.[^61] As noted in Section II.C.2.f, because the risk
    assessment only reflects the impact of reductions on recent
    air-related pathways in predicting changes in indoor dust Pb for
    urban case studies, these ratios are lower than they would be if
    they had also reflected potential reductions in other air-related
    pathways (e.g., changes in outdoor surface soil/dust Pb levels and
    diet with changes in ambient air Pb levels).

-   For the primary Pb smelter, estimates of air-to-blood ratios ranged
    from 1:10 and higher.[^62] One reason for these estimates being
    higher than those for the urban case study is that the dust Pb model
    used may reflect somewhat ambient air-related pathways other than
    that of ambient air infiltrating a home (as described in Section
    II.C.2.f above).

b\. IQ Loss Estimates

The risk assessment estimated IQ loss associated with both total Pb
exposure and air-related Pb exposure. This section summarizes findings
in relation to air-related Pb exposure, since this is the category of
risk results considered most relevant to the review in considering
whether the current NAAQS and potential alternative NAAQS provide
protection of public health with an adequate margin of safety.[^63] In
considering this presentation, we note that IQ loss associated with
air-related exposure for each NAAQS scenario is bounded by *recent-air*
on the low-end and *recent plus past air* on the high-end (as described
in section II.C.2.e above). In considering differences in these risk
estimates (or in the total risk estimates presented in the final Risk
Assessment Report) for alternative NAAQS, we note that these comparisons
underestimate the true impacts of the alternate NAAQS and accordingly,
the benefit to public health that would result from lower NAAQS levels.
This is due to our inability to simulate in this assessment reductions
in several outdoor air deposition-related pathways (e.g., diet,
ingestion of outdoor surface soil). The magnitude of this
underestimation is unknown.

As with the discussion of blood Pb results, the IQ loss estimates are
summarized here according to air quality scenario and case study
category (Table 2). In presenting these results, we have focused this
presentation on estimates for the median in each case study population
of children because of the greater confidence associated with estimates
for the median as compared to those for 95^th^ percentile.[^64]
Generally, 95^th^ percentile IQ loss estimates for all case studies are
80 to 100% higher than the median results in Table 2. The fraction of
total IQ loss that is air-related for the 95^th^ percentile is generally
similar to that for the median (for a particular combination of case
study and air quality scenario).

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

**Table 2. Summary of risk attributable to air-related Pb exposure.**

+--------------------+--------+---------+---------+---------+-------+
| **NAAQS Level      | **     |         |         |         |       |
| Simulated**        | Median |         |         |         |       |
|                    | air-r  |         |         |         |       |
| (μg/m**^3^** max   | elated |         |         |         |       |
| monthly,           | IQ     |         |         |         |       |
|                    | loss** |         |         |         |       |
| except as noted    | ^A^    |         |         |         |       |
| below)             |        |         |         |         |       |
+--------------------+--------+---------+---------+---------+-------+
|                    | **G    | **      | **Loc   |         |       |
|                    | eneral | Primary | ation-s |         |       |
|                    | urban  | Pb      | pecific |         |       |
|                    | case   | smelter | urban   |         |       |
|                    | s      | (s      | case    |         |       |
|                    | tudy** | ubarea) | st      |         |       |
|                    |        | case    | udies** |         |       |
|                    |        | study   |         |         |       |
|                    |        | ^B^**   |         |         |       |
+--------------------+--------+---------+---------+---------+-------+
|                    |        |         | **Clev  | **Ch    | **Los |
|                    |        |         | eland** | icago** | Ange  |
|                    |        |         |         |         | les** |
|                    |        |         | (0.56   | (0.31   |       |
|                    |        |         | μ       | μ       | (0.17 |
|                    |        |         | g/m^3^) | g/m^3^) | μg/   |
|                    |        |         |         |         | m^3^) |
+--------------------+--------+---------+---------+---------+-------+
| **1.5 max          | *      | **\<    | **2.8 - | **3.4 - | **    |
| quarterly^C^**     | *3.5 - | 6**     | 3.9     | 4.7     | 2.7 - |
|                    | 4.8**  |         | ^D^**   | ^D^**   | 4.2   |
|                    |        |         |         |         | ^D^** |
+--------------------+--------+---------+---------+---------+-------+
|                    | (1.5 - | \       | (0.6 -  | (1.4 -  | (     |
|                    | 7.7)   | <(3.2 - | 4.6)    | 7.4)    | 1.1 - |
|                    |        | 9.4)    |         |         | 6.2)  |
+--------------------+--------+---------+---------+---------+-------+
| **0.5**            | *      | **\<    | **0.6 - | ^E^     | ^E^   |
|                    | *1.9 - | 4.5**   | 2.9**   |         |       |
|                    | 3.6**  |         |         |         |       |
+--------------------+--------+---------+---------+---------+-------+
|                    | (0.7 - | \       | (0.2 -  |         |       |
|                    | 4.8)   | <(2.1 - | 3.9)    |         |       |
|                    |        | 7.7)    |         |         |       |
+--------------------+--------+---------+---------+---------+-------+
| **0.2**            | *      | **\<    | **0.6 - | **0.6 - | **    |
|                    | *1.2 - | 3.7**   | 2.8**   | 2.9**   | 0.7 - |
|                    | 3.2**  |         |         |         | 2.9   |
|                    |        |         |         |         | ^F^** |
+--------------------+--------+---------+---------+---------+-------+
|                    | (0.4 - | \       | (0.1 -  | (0.3 -  | (     |
|                    | 4.0)   | <(1.2 - | 3.2)    | 3.6)    | 0.2 - |
|                    |        | 5.1)    |         |         | 3.5)  |
+--------------------+--------+---------+---------+---------+-------+
| **0.05**           | *      | **\<    | **0.1 - | **0.2 - | **    |
|                    | *0.5 - | 2.8**   | 2.6**   | 2.6**   | 0.3 - |
|                    | 2.8**  |         |         |         | 2.7** |
+--------------------+--------+---------+---------+---------+-------+
|                    | (0.2 - | \       | (       | (0.1 -  | (     |
|                    | 3.3)   | <(0.9 - | \<0.1 - | 3.2)    | 0.1 - |
|                    |        | 3.4)    | 3.1)    |         | 3.2)  |
+--------------------+--------+---------+---------+---------+-------+
| **0.02**           | *      | **\<    | **      | **0.1 - | **    |
|                    | *0.3 - | 2.9**   | \<0.1 - | 2.6**   | 0.1 - |
|                    | 2.6**  |         | 2.6**   |         | 2.6** |
+--------------------+--------+---------+---------+---------+-------+
|                    | (0.1 - | \       | (       | (       | (\<   |
|                    | 3.1)   | <(0.9 - | \<0.1 - | \<0.1 - | 0.1 - |
|                    |        | 3.3)    | 3.0)    | 3.1)    | 3.1)  |
+--------------------+--------+---------+---------+---------+-------+
| A - Air-related    |        |         |         |         |       |
| risk is bracketed  |        |         |         |         |       |
| by "recent air"    |        |         |         |         |       |
| (lower bound of    |        |         |         |         |       |
| presented range)   |        |         |         |         |       |
| and "recent" plus  |        |         |         |         |       |
| "past air" (upper  |        |         |         |         |       |
| bound of presented |        |         |         |         |       |
| range). While      |        |         |         |         |       |
| differences        |        |         |         |         |       |
| between standard   |        |         |         |         |       |
| levels are better  |        |         |         |         |       |
| distinguished by   |        |         |         |         |       |
| differences in the |        |         |         |         |       |
| "recent" plus      |        |         |         |         |       |
| "past air"         |        |         |         |         |       |
| estimates (upper   |        |         |         |         |       |
| bounds shown       |        |         |         |         |       |
| here), these       |        |         |         |         |       |
| differences are    |        |         |         |         |       |
| inherently         |        |         |         |         |       |
| underestimates.    |        |         |         |         |       |
|                    |        |         |         |         |       |
| Boldface values    |        |         |         |         |       |
| are estimates      |        |         |         |         |       |
| generated using    |        |         |         |         |       |
| the log-linear     |        |         |         |         |       |
| with low-exposure  |        |         |         |         |       |
| linearization      |        |         |         |         |       |
| function. Values   |        |         |         |         |       |
| in parentheses     |        |         |         |         |       |
| reflect the range  |        |         |         |         |       |
| of estimates       |        |         |         |         |       |
| associated with    |        |         |         |         |       |
| all four           |        |         |         |         |       |
| conc               |        |         |         |         |       |
| entration-response |        |         |         |         |       |
| functions.         |        |         |         |         |       |
|                    |        |         |         |         |       |
| B -- In the case   |        |         |         |         |       |
| of the primary Pb  |        |         |         |         |       |
| smelter case study |        |         |         |         |       |
| , only recent plus |        |         |         |         |       |
| past air estimates |        |         |         |         |       |
| are available.     |        |         |         |         |       |
|                    |        |         |         |         |       |
| C -- Roughly 0.7 - |        |         |         |         |       |
| 1.0 μg/m^3^        |        |         |         |         |       |
| maximum monthly    |        |         |         |         |       |
| mean, across the   |        |         |         |         |       |
| urban case studies |        |         |         |         |       |
|                    |        |         |         |         |       |
| D - A \"roll-up\"  |        |         |         |         |       |
| was performed so   |        |         |         |         |       |
| that highest site  |        |         |         |         |       |
| in study area just |        |         |         |         |       |
| met this level.    |        |         |         |         |       |
|                    |        |         |         |         |       |
| E -- A "roll-up"   |        |         |         |         |       |
| to this level was  |        |         |         |         |       |
| not performed.     |        |         |         |         |       |
|                    |        |         |         |         |       |
| F -- A "roll-up"   |        |         |         |         |       |
| to this level was  |        |         |         |         |       |
| not performed;     |        |         |         |         |       |
| these estimates    |        |         |         |         |       |
| are based on       |        |         |         |         |       |
| current conditions |        |         |         |         |       |
| in this area.      |        |         |         |         |       |
+--------------------+--------+---------+---------+---------+-------+

Key observations regarding the median estimates of air-related risk for
the current NAAQS and alternative standards presented in Table 2
include:

-   For the current NAAQS scenario, in all five case studies,
    air-related risk exceeds 2 points IQ loss at the median and the
    upper bound of air-related risk is near or above 4 points IQ loss.

-   Alternate standards provide substantial reduction in air-related
    risk across the full set of alternative NAAQS considered in this
    analysis (i.e., 0.5 to 0.02 μg/m**^3^** max monthly).

-   In the general urban case study, the lower bound of air-related risk
    falls below 2 points IQ loss for an alternative NAAQS of 0.5
    μg/m**^3^** max monthly, and below 1 point IQ loss somewhere between
    an alternative NAAQS of 0.2 and 0.05 μg/m**^3^** max monthly.

-   The upper-bound of air-related risk for the primary Pb smelter is
    generally higher than that for the general urban case study, likely
    due to the difference in indoor dust models used for the two case
    studies. The indoor dust Pb model used for the primary Pb smelter
    considered more completely, the impact of outdoor ambient air Pb on
    indoor dust (compared to the hybrid indoor dust Pb model used in the
    urban case studies). Specifically, the regression model used for the
    primary Pb smelter included consideration for longer-term
    relationships between outdoor ambient air and indoor dust (e.g.,
    changes in outdoor soil and subsequent tracking in of soil Pb).

-   As noted above (section II.C.2.c), the three location-specific urban
    case studies provide risk estimates for populations with a broader
    range of air-related exposures. Accordingly, because of the
    population distribution in these three case studies, the air-related
    risk is smaller for them than for the other case studies,
    particularly at the population median. Further, the majority of the
    population in each case study resides in areas with ambient air Pb
    levels well below each standard level assessed, particularly for
    levels above 0.05 μg/m**^3^** max monthly. Consequently, risk
    estimates indicate little response to alternative standard levels
    above 0.05 μg/m**^3^** max monthly.

In addition to the results described above, we also estimated the number
of children associated with different magnitudes of IQ loss resulting
from total Pb exposure under different ambient air quality scenarios
(i.e., IQ loss incidence estimates) (as described in Section 5.3.1.2 of
the Risk Assessment Report). These incidence estimates were only
generated for the location-specific urban case studies, since these have
larger enumerated study populations. This metric illustrates the overall
number of children within a given urban case study location projected to
experience various levels of IQ los due to Pb exposure and how that
distribution of incidence changes with alternate standard levels

D. Conclusions on Adequacy of the Current Primary Standard

The initial issue to be addressed in the current review of the primary
Pb standard is whether, in view of the advances in scientific knowledge
and additional information, the existing standard should be retained or
revised. In evaluating whether it is appropriate to retain or revise the
current standard, the Administrator builds on the general approach used
in the initial setting of the standard, as well as that used in the last
review, and reflects the broader body of evidence and information now
available.

The approach used is based on an integration of information on health
effects associated with exposure to ambient Pb; expert judgment on the
adversity of such effects on individuals; and policy judgments as to
when the standard is requisite to protect public health with an adequate
margin of safety, which are informed by air quality and related
analyses, quantitative exposure and risk assessments when possible, and
qualitative assessment of impacts that could not be quantified.

The Administrator has taken into account both evidence-based and
quantitative exposure- and risk-based considerations in developing
conclusions on the adequacy of the current primary Pb standard.
Evidence-based considerations include the assessment of evidence for a
variety of health endpoints from epidemiological, animal toxicological,
and controlled human exposure studies. For those endpoints based on
epidemiological studies, greater weight has been placed on associations
with health endpoints that are causal or likely causal based on an
integrative synthesis of the entire body of evidence, including not only
all available epidemiological evidence but also evidence from animal
toxicological and controlled human exposure studies. Consideration of
quantitative exposure- and risk-based information draws from the results
of the exposure and risk assessments described above. More specifically,
estimates of the magnitude of Pb-related exposures and risks associated
with air quality levels associated with just meeting the current primary
Pb NAAQS have been considered.

In this review, a series of general questions frames the approach to
reaching a decision on the adequacy of the current standard, such as the
following: (1) to what extent does newly available information reinforce
or call into question evidence of associations of Pb exposures with
effects identified when the standard was set?; (2) to what extent has
evidence of new effects or at-risk populations become available since
the time the standard was set?; (3) to what extent have important
uncertainties identified when the standard was set been reduced and have
new uncertainties emerged?; and (4) to what extent does newly available
information reinforce or call into question any of the basic elements of
the current standard?

The question of whether the available evidence supports consideration of
a standard that is more protective than the current standard includes
consideration of: (1) whether there is evidence that associations,
especially likely causal associations, extend to ambient Pb
concentration levels that are as low as or lower than had previously
been observed, and the important uncertainties associated with that
evidence; (2) the extent to which exposures of potential concern and
health risks are estimated to occur in areas upon meeting the current
standard and the important uncertainties associated with the estimated
exposures and risks; and (3) the extent to which the Pb-related health
effects indicated by the evidence and the exposure and risk assessments
are considered important from a public health perspective, taking into
account the nature and severity of the health effects, the size of the
at-risk populations, and the kind and degree of the uncertainties
associated with these considerations.

The following discussion starts with background information on the
current standard (section II.D.1), including both the basis for
derivation of the current standard and considerations and conclusions
from the 1990 Staff Paper (USEPA, 1990b). This is followed by a
discussion of the Agency's approach in this review for evaluating the
adequacy of the current standard, in section II.D.2, including both
evidence-based and exposure/risk-based considerations (sections II.D.2.a
and b, respectively). CASAC advice and recommendations concerning
adequacy of the current standard are summarized in section II.D.3.
Lastly, the Administrator's proposed conclusions with regard to the
adequacy of the current standard are presented in section II.D.4.

1\. Background

a\. The Current Standard

The current primary standard is set at a level of 1.5 μg/m^3^, measured
as Pb-TSP, not to be exceeded by the maximum arithmetic mean
concentration averaged over a calendar quarter. The standard was set in
1978 to provide protection to the public, especially children as the
particularly sensitive population subgroup, against Pb-induced adverse
health effects (43 FR 46246). In setting the standard, EPA relied on
conclusions regarding sources of exposure, air-related exposure
pathways, variability and susceptibility of young children, the most
sensitive health endpoints, blood Pb level thresholds for various health
effects and the stability and distributional characteristics of Pb (both
in the human body and in the environment) (43 FR 46247). The specific
basis for selecting each of the elements of the standard is described
below.

i\. Level

EPA's objective in selecting the level of the current standard was "to
estimate the concentration of Pb in the air to which all groups within
the general population can be exposed for protracted periods without an
unacceptable risk to health" (43 FR 46252). As stated in the notice of
final rulemaking, "This estimate was based on EPA's judgment in four key
areas:

\(1\) Determining the 'sensitive population' as that group within the
general population which has the lowest threshold for adverse effects or
greatest potential for exposure. EPA concludes that young children, aged
1 to 5, are the sensitive population.

\(2\) Determining the safe level of total lead exposure for the
sensitive population, indicated by the concentration of lead in the
blood. EPA concludes that the maximum safe level of blood lead for an
individual child is 30 μg Pb/dl and that population blood lead, measured
as the geometric mean, must be 15 μg Pb/dl in order to place 99.5
percent of children in the United States below 30 μg Pb/dl.

\(3\) Attributing the contribution to blood lead from nonair pollution
sources. EPA concludes that 12 μg Pb/dl of population blood lead for
children should be attributed to nonair exposure.

\(4\) Determining the air lead level which is consistent with
maintaining the mean population blood lead level at 15 μg Pb/dl \[the
maximum safe mean level\]. Taking into account exposure from other
sources (12 μg Pb/dl), EPA has designed the standard to limit air
contribution after achieving the standard to 3 μg Pb/dl. On the basis of
an estimated relationship of air lead to blood lead of 1 to 2, EPA
concludes that the ambient air standard should be 1.5 μg Pb/m^3^." (43
FR 46252)

EPA's judgments in these key areas, as well as margin of safety
considerations, are discussed below.

The assessment of the science that was presented in the 1977 Criteria
Document (USEPA, 1977), indicated young children, aged 1 to 5, as the
population group at particular risk from Pb exposure. Children were
recognized to have a greater physiological sensitivity than adults to
the effects of Pb and a greater exposure. In identifying young children
as the sensitive population, EPA also recognized the occurrence of
subgroups with enhanced risk due to genetic factors, dietary
deficiencies or residence in urban areas. Yet information was not
available to estimate a threshold for adverse effects for these
subgroups separate from that of all young children. Additionally, EPA
recognized both a concern regarding potential risk to pregnant women and
fetuses, and a lack of information to establish that these subgroups are
more at risk than young children. Accordingly, young children, aged 1 to
5, were identified as the group which has the lowest threshold for
adverse effects of greatest potential for exposure (i.e., the sensitive
population) (43 FR 46252).

In identifying the maximum safe exposure, EPA relied upon the
measurement of Pb in blood (43 FR 46252-46253). The physiological effect
of Pb that had been identified as occurring at the lowest blood Pb level
was inhibition of an enzyme integral to the pathway by which heme (the
oxygen carrying protein of human blood) is synthesized, i.e.,
delta-aminolevulinic acid dehydratase (δ-ALAD). The 1977 Criteria
Document reported a threshold for inhibition of this enzyme in children
at 10 μg Pb/dL. The 1977 Criteria Document also reported a threshold of
15-20 μg/dL for elevation of erythrocyte protoporphyrin (EP), which is
an indication of some disruption of the heme synthesis pathway. EPA
concluded that this effect on the heme synthesis pathway (indicated by
EP) was potentially adverse. EPA further described a range of blood
levels associated with a progression in detrimental impact on the heme
synthesis pathway. At the low end of the range (15-20 μg/dL), the
initial detection of EP associated with blood Pb was not concluded to be
associated with a significant risk to health. The upper end of the range
(40 μg/dL), the threshold associated with clear evidence of heme
synthesis impairment and other effects contributing to clinical symptoms
of anemia, was regarded by EPA as clearly adverse to health. EPA also
noted that for some children with blood Pb levels just above those for
these effects (e.g., 50 μg/dL), there was risk for additional adverse
effects (e.g., nervous system deficits). Additionally, in the Agency's
statement of factors on which the conclusion as to the maximum safe
blood Pb level for an individual child was based, EPA stated that the
maximum safe blood level should "no higher than the blood Pb range
characterized as undue exposure by the Center for Disease Control of the
Public Health Service, as endorsed by the American Academy of
Pediatrics, because of elevation of erythrocyte protoporphyrin (above 30
μg Pb/dL)".[^65]

Having identified the maximum safe blood level in individual children,
EPA next made a public health policy judgment regarding the target mean
blood level for the U.S. population of young children (43 FR
46252-46253). With this judgment, EPA identified a target of 99.5
percent of this population to be brought below the maximum safe blood Pb
level. This judgment was based on consideration of the size of the
sensitive subpopulation, and the recognition that there are special
high-risk groups of children within the general population. The
population statistics available at the time (the 1970 U.S. Census)
indicated a total of 20 million children younger than 5 years of age,
with 15 million residing in urban areas and 5 million in center cities
where Pb exposure was thought likely to be "high". Concern about these
high-risk groups influenced EPA's determination of 99.5 percent,
deterring EPA from selecting a population percentage lower than 99.5 (43
FR 46253). EPA then used standard statistical techniques to calculate
the population mean blood Pb level that would place 99.5 percent of the
population below the maximum safe level. Based on the then available
data, EPA concluded that blood Pb levels in the population of U.S.
children were normally distributed with a GSD of 1.3. Based on standard
statistical techniques, EPA determined that a thus described population
in which 99.5 percent of the population has blood Pb levels below 30
μg/dL would have a geometric mean blood level of 15 μg/dL. EPA described
15 μg/dL as \"the maximum safe blood lead level (geometric mean) for a
population of young children\" (43 FR 46247)

When setting the current NAAQS, EPA recognized that the air standard
needed to take into account the contribution to blood Pb levels from Pb
sources unrelated to air pollution. Consequently, the calculation of the
current NAAQS included the subtraction of Pb contributed to blood Pb
from nonair sources, from the estimate of a safe mean population blood
Pb level. Without this subtraction, EPA recognized that the combined
exposure to Pb from air and nonair sources would result in a blood Pb
concentration exceeding the safe level (43 FR 46253). In developing an
estimate of this nonair contribution, EPA recognized the lack of
detailed or widespread information about the relative contribution of
various sources to children's blood Pb levels, such that an estimate
could only be made by inference from other empirical or theoretical
studies, often involving adults. Additionally, EPA recognized the
expectation that the contribution to blood Pb levels from nonair sources
would vary widely, was probably not in constant proportion to air Pb
contribution, and in some cases may alone exceed the target mean
population blood Pb level (43 FR 46253-46254). The amount of blood Pb
attributed to nonair sources was selected based primarily on findings in
studies of blood Pb levels in areas where air Pb levels were low
relative to other locations in U.S. The air Pb levels in these areas
ranged from 0.1 to 0.7 μg/m^3^. The average of the reported blood Pb
levels for children of various ages in these areas was on the order of
12 μg/dL. Thus, 12 μg/dL was identified as the nonair contribution, and
subtracted from the population mean target level of 15 μg/dL to yield a
value of 3 μg/dL as the limit on the air contribution to blood Pb.

In determining the air Pb level consistent with an air contribution of 3
μg Pb/dL, EPA reviewed studies assessed in the 1977 Criteria Document
that reported changes in blood Pb with different air Pb levels. These
studies included a study of children exposed to Pb from a primary Pb
smelter, controlled exposures of adult men to Pb in fine particulate
matter, and a personal exposure study involving several male cohorts
exposed to Pb in a large urban area in the early 1970s (43 FR
46254).[^66] Using all three studies, EPA calculated an average slope or
ratio over the entire range of data. That value was 1.95 (rounded to 2
μg/dL blood Pb concentration to 1 μg/m^3^ air Pb concentration), and is
recognized to fall within the range of values reported in the 1977
Criteria Document. On the basis of this 2-to-1 relationship, EPA
concluded that the ambient air standard should be 1.5 μg Pb/m^3^ (43 FR
46254).

In consideration of the appropriate margin of safety during the
development of the current NAAQS, EPA identified the following factors:
(1) the 1977 Criteria Document reported multiple biological effects of
Pb in practically all cell types, tissues and organ systems, of which
the significance for health had not yet been fully studied; (2) no
beneficial effects of Pb at then current environmental levels were
recognized; (3) data were incomplete as to the extent to which children
are indirectly exposed to air Pb that has moved to other environmental
media, such as water, soil and dirt, and food; (4) Pb is chemically
persistent and with continued uncontrolled emissions would continue to
accumulate in human tissue and the environment; and (5) the possibility
that exposure associated with blood Pb levels previously considered safe
might influence neurological development and learning abilities of the
young child (43 FR 46255). Recognizing that estimating an appropriate
margin of safety for the air Pb standard was complicated by the multiple
sources and media involved in Pb exposure, EPA chose to use margin of
safety considerations principally in establishing a maximum safe blood
Pb level for individual children (30 μg Pb/dL) and in determining the
percentage of children to be placed below this maximum level (about 99.5
percent). Additionally, in establishing other factors used in
calculating the standard, EPA used margin of safety considerations in
the sense of making careful judgment based on available data, but these
judgments were not considered to be at the precautionary extreme of the
range of data available at the time (43 FR 46251).

EPA further recognized that, because of the variability between
individuals in a population experiencing a given level of Pb exposure,
it was considered impossible to provide the same margin of safety for
all members in the sensitive population or to define the margin of
safety in the standard as a simple percentage. EPA believed that the
factors it used in designing the standards provided an adequate margin
of safety for a large proportion of the sensitive population. The Agency
did not believe that the margin was excessively large or on the other
hand that the air standard could protect everyone from elevated blood Pb
levels (43 FR 46251).

ii\. Averaging Time, Form, and Indicator

The averaging time for the current standard is a calendar quarter. In
the decision for this aspect of the standard, the Agency also considered
a monthly averaging period, but concluded that "a requirement for the
averaging of air quality data over calendar quarter will improve the
validity of air quality data gathered without a significant reduction in
the protectiveness of the standards." As described in the notice for
this decision (43 FR 46250), this conclusion was based on several
points, including the following:

-   An analysis of ambient measurements available at the time indicated
    that the distribution of air Pb levels was such that there was
    little possibility that there could be sustained periods greatly
    above the average value in situations where the quarterly standard
    was achieved.

-   A recognition that the monitoring network may not actually represent
    the exposure situation for young children, such that it seemed
    likely that elevated air Pb levels when occurring would be close to
    Pb air pollution sources where young children would typically not
    encounter them for the full 24-hour period reported by the monitor.

-   Medical evidence available at the time indicated that blood Pb
    levels re-equilibrate slowly to changes in air exposure, a finding
    that would serve to dampen the impact of short-term period of
    exposure to elevated air Pb.

-   Direct exposure to air is only one of several routes of total
    exposure, thus lessening the impact of a change in air Pb on blood
    Pb levels.

The statistical form of the current standard is a not-to-be-exceeded or
maximum value. EPA set the standard as a ceiling value with the
conclusion that this air level would be safe for indefinite exposure for
young children (43 FR 46250).

The indicator is total airborne Pb collected by a high volume sampler
(43 FR 46258). EPA's selection of Pb-TSP as the indicator for the
standard was based on explicit recognition both of the significance of
ingestion as an exposure pathway for Pb that had deposited from the air
and of the potential for Pb deposited from the air to become
re-suspended in respirable size particles in the air and available for
human inhalation exposure. As stated in the final rule, "a significant
component of exposure can be ingestion of materials contaminated by
deposition of lead from the air," and that, "in addition to the indirect
route of ingestion and absorption from the gastrointestinal tract,
non-respirable Pb in the environment may, at some point become
respirable through weathering or mechanical action" (43 FR 46251).

b\. Policy Options Considered in the Last Review

During the 1980s, EPA initiated a review of the air quality criteria and
NAAQS for Pb. CASAC and the public were fully involved in this review,
which led to the publication of a criteria document with associated
addendum and a supplement (USEPA, 1986a, 1986b, 1990a), an exposure
analysis methods document (USEPA, 1989) and a staff paper (USEPA,
1990b).

Total emissions to air were estimated to have dropped by 94 percent
between 1978 and 1987, with the vast majority of it attributed to the
reduction of Pb in gasoline. Accordingly, the focus of the last review
was on areas near stationary sources of Pb emissions. Although such
sources were not considered to have made a significant contribution (as
compared to Pb in gasoline) to the overall Pb pollution across large,
urban or regional areas, Pb emissions from such sources were considered
to have the potential for a significant impact on a local scale. Air Pb
concentrations, and especially soil and dust Pb concentrations had been
associated with elevated levels of Pb absorption in children and adults
in numerous Pb point source community studies. Exceedances of the
current NAAQS were found at that time only in the vicinity of nonferrous
smelters or other point sources of Pb.

In summarizing and interpreting the health evidence presented in the
1986 Criteria Document and associated documents, the 1990 Staff Paper
described the collective impact on children of the effects at blood Pb
levels above 15 μg/dL as representing a clear pattern of adverse effects
worthy of avoiding. This is in contrast to EPA's identification of 30
μg/dL as a safe blood Pb level for individual children when the NAAQS
was set in 1978. The Staff Paper further stated that at blood Pb levels
of 10-15 μg/dL, there was a convergence of evidence of Pb-induced
interference with a diverse set of physiological functions and
processes, particularly evident in several independent studies showing
impaired neurobehavioral function and development. Further, the
available data did not indicate a clear threshold in this blood Pb
range. Rather, it suggested a continuum of health risks down to the
lowest levels measured.[^67]

For the purposes of comparing the relative protectiveness of alternative
Pb NAAQS, the staff conducted analyses to estimate the percentages of
children with blood Pb levels above 10 μg/dL and above 15 μg/dL for
several air quality scenarios developed for a small set of stationary
source exposure case studies. The results of the analyses of child
populations living near two Pb smelters indicated that substantial
reductions in Pb exposure could be achieved through just meeting the
current Pb NAAQS. According to the best estimate analyses, over 99.5% of
children living in areas significantly affected by the smelters would
have blood Pb levels below 15 μg/dL if the current standard was
achieved. Progressive changes in this number were estimated for the
alternative monthly Pb NAAQS levels evaluated in those analyses, which
ranged from 1.5 μg/m^3^ to 0.5 μg/m^3^.

In light of the health effects evidence available at the time, the 1990
Staff Paper presented air quality, exposure, and risk analyses, and
other policy considerations, as well as the following staff conclusions
with regard to the primary Pb NAAQS (USEPA, 1990b, pp. xii to xiv):

\(1\) "The range of standards ... should be from 0.5 to 1.5 μg/m^3^."

\(2\) "A monthly averaging period would better capture short-term
increases in lead exposure and would more fully protect children's
health than the current quarterly average."

\(3\) "The most appropriate form of the standard appears to be the
second highest monthly averages *{sic}* in a 3-year span. This form
would be nearly as stringent as a form that does not permit any
exceedances and allows for discounting of one "bad" month in 3 years
which may be caused, for example, by unusual meteorology."

\(4\) "With a revision to a monthly averaging time more frequent
sampling is needed, except in areas, like roadways remote from lead
point sources, where the standard is not expected to be violated. In
those situations, the current 1-in-6 day sampling schedule would
sufficiently reflect air quality and trends."

\(5\) "Because exposure to atmospheric lead particles occurs not only
via direct inhalation, but via ingestion of deposited particles as well,
especially among young children, the hi-volume sampler provides a
reasonable indicator for determining compliance with a monthly standard
and should be retained as the instrument to monitor compliance with the
lead NAAQS until more refined instruments can be developed."

Based on its review of a draft Staff Paper, which contained the above
recommendations, the CASAC strongly recommended to the Administrator
that EPA should actively pursue a public health goal of minimizing the
Pb content of blood to the extent possible, and that the Pb NAAQS is an
important component of a multimedia strategy for achieving that goal
(CASAC, 1990, p. 4). In noting the range of levels recommended by staff,
CASAC recommended consideration of a revised standard that incorporates
a "wide margin of safety, because of the risk posed by Pb exposures,
particularly to the very young whose developing nervous system may be
compromised by even low level exposures" (id., p. 3). More specifically,
CASAC judged that a standard within the range of 1.0 to 1.5 μg/m^3^
would have "relatively little, if any, margin of safety;" that greater
consideration should be given to a standard set below 1.0 μg/m^3^; and,
to provide perspective in setting the standard, it would be appropriate
to consider the distribution of blood Pb levels associated with meeting
a monthly standard of 0.25 μg/m^3^, a level below the range considered
by staff (id.).

After consideration of the documents developed during the review, EPA
chose not to propose revision of the NAAQS for Pb. During the same time
period, the Agency published and embarked on the implementation of a
broad, multi-program, multi-media, integrated national strategy to
reduce Pb exposures (USEPA, 1991). As discussed above in section I.C.,
as part of implementing this integrated Pb strategy, the Agency focused
efforts primarily on regulatory and remedial clean-up actions aimed at
reducing Pb exposures from a variety of non-air sources judged to pose
more extensive public health risks to U.S. populations, as well as on
actions to reduce Pb emissions to air, particularly near stationary
sources.

2\. Considerations in the Current Review

a\. Evidence-based Considerations

In considering the broad array of health effects evidence assessed in
the Criteria Document with respect to the adequacy of the current
standard, the discussion here, like that in the Staff Paper and ANPR,
focuses on those health endpoints associated with the Pb exposure and
blood levels most pertinent to ambient exposures. In so doing, EPA gives
particular weight to evidence available today that differs from that
available at the time the standard was set with regard to its support of
the current standard.

First, with regard to the sensitive population, the susceptibility of
young children to the effects of Pb is well recognized, in addition to
more recent recognition of effects of chronic exposure to low level Pb
with advancing age (CD, Sections 5.3.7 and pp. 8-73 to 8-75). The
prenatal period and early childhood are periods of increased
susceptibility to Pb exposures, with evidence of adverse effects on the
developing nervous system that generally appear to persist into later
childhood and adolescence (CD, Section 6.2). [^68] Thus, while the
sensitivity of the elderly and other particular subgroups is recognized,
as at the time the standard was set, young children continue to be
recognized as the key sensitive population for Pb exposures.

With regard to the exposure levels at which adverse health effects
occur, the current evidence demonstrates the occurrence of adverse
health effects at appreciably lower blood Pb levels than those
demonstrated by the evidence at the time the standard was set, at which
time the Agency identified 30 µg/dL as the maximum safe blood Pb level
for individual children and 15 µg/dL as the maximum safe geometric mean
blood Pb level for a population of children (as described in section
II.D.1.a above). This change in the evidence since the time the standard
was set is reflected in changes made by the CDC in their advisory level
for Pb in children's blood, and changes they have made in their
characterization of that level. Although CDC recognized a level of 30
μg/dL blood Pb as warranting individual intervention in 1978 when the Pb
NAAQS was set, in 2005 they recognized the evidence of adverse health
effects in children with blood Pb levels below 10 μg/dL and the data
demonstrating that no "safe" threshold for blood Pb had been identified
(CDC, 1991; CDC, 2005).

As summarized in section II.B above, the Criteria Document describes
current evidence regarding the occurrence of a variety of health
effects, including neurological effects in children associated with
blood Pb levels extending well below 10 µg/dL (CD, Sections 6.2, 8.4 and
8.5).[^69] For example, some studies indicate Pb effects on intellectual
attainment of young children for which population mean blood Pb levels
in the analysis ranged from approximately 2 to 8 μg/dL (CD, Sections
6.2, 8.4.2 and 8.4.2.6). With regard to the evidence of neurological
effects at these low levels, EPA notes, in particular, the international
pooled analysis by Lanphear and others (2005), studies of individual
cohorts such as the Rochester, Boston, and Mexico City cohorts (Canfield
et al., 2003a; Canfield et al., 2003b; Bellinger and Needleman, 2003;
Tellez-Rojo et al., 2006), the study of African-American inner-city
children from Detroit (Chiodo et al., 2004), the cross-sectional study
of young children in three German cities (Walkowiak et al., 1998) and
the cross-sectional analysis of a nationally representative sample from
the NHANES III, conducted from 1988-1994 (Lanphear et al., 2000). In the
NHANES study the mean blood Pb for the full study group was 1.9 µg/dL
and the mean blood Pb level in the lowest blood Pb subgroup with which a
statistically significant association with effect was found (individual
blood Pb values \<5 µg/dL) was 1.7 µg/dL (CD, pp. 6-31 to 6-32; Lanphear
et al., 2000; Auinger, 2008).[^70] We also note that 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), with the Criteria Document noting that any
such threshold may exist "at levels distinctly lower than the lowest
exposures examined in these epidemiological studies" (CD, p. 8-67).

In considering the adequacy of the current standard, we have first
considered the evidence in the context of the framework used to
determine the standard in 1978, as adapted to reflect the current
evidence. In so doing, we recognize that the health effects evidence
with regard to characterization of a threshold for adverse effects has
changed since the standard was set in 1978, as have the Agency's views
on the characterization of a safe blood Pb level. As described in
section II.D.1.a, parameters for this framework include estimates for
average nonair blood Pb level, and air-to-blood ratio, as well as a
maximum safe individual and/or geometric mean blood Pb level. For this
last parameter, we have for the purposes of this evaluation considered
the lowest population mean blood Pb levels with which effects have been
associated in the evidence.

As when the standard was set in 1978, there remain today contributions
to blood Pb levels from nonair sources. In 1978, the Agency estimated
the average blood Pb level for young children associated with nonair
sources to be 12 µg/dL (as described in section II.D.1.a). However,
consistent with reductions since that time in air Pb concentrations[^71]
which contribute to blood Pb, nonair contributions have also been
reduced (as described in section II.A.4 above), although the current
evidence is limited with regard to estimates of the aggregate reduction
since 1978 of all nonair sources to blood Pb and with regard to an
estimate of current nonair blood Pb levels (discussed in sections
II.A.4). In recognition of temporal reductions in nonair sources
discussed in section II.A.4 and in the context of estimates pertinent to
an application of the 1978 framework, the CASAC Pb Panel recommended
consideration of 1.0-1.4 µg/dL or lower as an estimate of the nonair
component of blood Pb pertinent to average blood Pb levels (as described
in section II.A.4 above; Henderson, 2007b).

As in 1978, the evidence demonstrates that Pb in ambient air contributes
to Pb in blood, with the pertinent exposure routes including both
inhalation and ingestion (CD, Sections 3.1.3.2, 4.2 and 4.4). In 1978,
the evidence indicated a quantitative relationship between ambient air
Pb and blood Pb in terms of an air-to-blood ratio that ranged from 1:1
to 1:2 (USEPA, 1977). In setting the standard, the Agency relied on a
ratio of 1:2, i.e., 2 µg/dL blood Pb per 1 µg/m^3^ air Pb (as described
in section II.D.1.a above). As discussed in section II.B.1.c, the
current evidence includes ratios generally ranging from 1:3 to1:5, with
some on the order of 1:10, based on consideration of both inhalation and
ingestion exposure pathways. Estimates derived from the exposure
assessment (section II.C.3.a) appear to generally range from 1:4 to 1:6
and higher. While there is uncertainty and variability in the absolute
value of an air-to-blood relationship, the current evidence and
estimates based on the exposure assessment indicate ratios greater than
the 1978 1:2 estimate, and generally ranging from 1:3 to 1:5 and higher,
with some on the order of 1:10.

In summary, the current evidence and estimates based on the exposure
assessment, based on consideration of both inhalation and ingestion
exposure pathways, indicate ratios greater than the 1978 1:2 estimate,
and generally ranging from 1:3 to 1:5 and higher, with some on the order
of 1:10.

Based on the information described above, the Staff Paper concluded that
young children remain the sensitive population of primary focus in this
review, there is now no recognized safe level of Pb in children's blood,
and studies appear to show adverse effects at population mean concurrent
blood Pb levels as low as approximately 2 µg/dL (CD, pp. 6-31 to 6-32;
Lanphear et al., 2000). Further, while the nonair contribution to blood
Pb has declined, perhaps to a range of 1.0-1.4 µg/dL, the air-to-blood
ratio appears to be higher at today's lower blood Pb levels than the
estimates at the time the standard was set, with current estimates on
the order of 1:3 to 1:5 and perhaps up to 1:10. Adapting the framework
employed in setting the standard in 1978, the Staff Paper concluded that
the more recently available evidence suggests a level for the standard
that is lower by an order of magnitude or more.

Since completion of the Staff Paper, the Agency has further considered
the evidence with regard to adequacy of the current standard using an
approach other than the adapted 1978 framework. This alternative
evidence-based framework shifts focus from identifying an appropriate
target population mean blood lead level and instead focuses on the
effects of air related Pb on neurocognitive functions. This framework
builds on a recommendation by the CASAC Pb Panel to consider the
evidence in a more quantitative manner, and is discussed in more detail
in section II.E.3.a, concerning the level of the standard.

In this alternate framework, we have drawn from the entire body of
evidence as a basis for concluding that there are causal associations
between air-related Pb exposures and population IQ loss. We have also
drawn more quantitatively from the evidence by using evidence-based C-R
functions to quantify the association between air Pb concentrations and
air-related population mean IQ loss. Thus, this framework more fully
considers the evidence with regard to the concentration-response
relationship for the effect of Pb on IQ, and it also draws from
estimates for air-to-blood ratios. []{.mark}

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

Applying the estimates of air-to-blood ratios ranging from 1:3 to 1:5
(as used above), a population of children exposed at the current level
of the standard might be expected to result in an average air-related
blood Pb level above 4 µg/dL[^72] Multiplying these blood Pb levels by
the slope estimate, identified above, for blood Pb levels extending up
to 10 µg/dL (-0.9 IQ points per µg/dL), would imply an average
air-related IQ loss for such a group of children on the order of 4 or
more IQ points.

b\. Exposure- and Risk-based Considerations

As discussed above in section II.C, we have estimated exposures and
health risks associated with air quality that just meets the current
standard to help inform judgments about whether or not the current
standard provides adequate protection of public health, taking into
account key uncertainties associated with the estimated exposures and
risks (summarized above in section II.C and more fully in the Risk
Assessment Report).

As discussed above, young children are the sensitive population of
primary focus in this review. The exposure and risk assessment estimates
Pb exposure for children (less than 7 years of age), and associated risk
of neurocognitive effects in terms of IQ loss. In addition to the risks
(IQ loss) that were quantitatively estimated, EPA recognizes that there
may be long-term adverse consequences of such deficits over a lifetime,
and there are other, unquantified adverse neurocognitive effects that
may occur at similarly low exposures which might additionally contribute
to reduced academic performance, which may have adverse consequences
over a lifetime (CD, pp. 8-29 to 8-30). Other impacts at low levels of
childhood exposure that were not quantified in the risk assessment
include: other neurological effects (sensory, motor, cognitive and
behavioral), immune system effects (including some related to allergic
responses and asthma), and early effects related to anemia.
Additionally, as noted in section II.B.2, other health effects evidence
demonstrates associations between Pb exposure and adverse health effects
in adults (e.g., cardiovascular and renal effects).

As noted in the Criteria Document, a modest change in the population
mean of a health index, that is quantified for each individual, can have
substantial implications at the population level (CD, p. 8-77, Sections
8.6.1 and 8.6.2; Bellinger, 2004; Needleman et al., 1982; Weiss, 1988;
Weiss, 1990)). For example, for an individual functioning in the low
range of IQ due to the influence of risk factors other than Pb, a
Pb-associated IQ loss of a few points might be sufficient to drop that
individual into the range associated with increased risk of educational,
vocational, and social handicap (CD, p. 8-77), while such a decline
might create less significant impacts for the individual near the mean
of the population. Further, given a somewhat uniform manifestation of
Pb-related decrements across the range of IQ scores in a population, a
downward shift in the mean IQ value is associated not only with a
substantial increase in the percentage of individuals achieving very low
scores, but also with substantial decreases in percentages achieving
very high scores (CD, p. 8-81). The CASAC Pb Panel has advised on this
point that "a population loss of 1-2 IQ points is highly significant
from a public health perspective" (Henderson, 2007a, p. 6).

In considering exposure and risk estimates with regard to adequacy of
the current standard, EPA has focused on IQ loss for air-related
exposure pathways. As described in section II.C.2.e above, limitations
in our data and modeling tools have resulted in an inability to develop
specific estimates such that we have approximated estimates for the
air-related pathways, bounded on the low end by exposure/risk estimated
for the "recent air" category and on the upper end by the exposure/risk
estimated for the "recent air" plus "past air" categories. Thus, the
following discussion presents air-related IQ loss estimates in terms of
upper and lower bounds. In addition, as noted above (section II.C.3.b),
this discussion focuses predominantly on risk estimates derived using
the log-linear with low-exposure linearization (LLL) C-R function, with
the range associated with the other three functions used in the
assessment also being noted. Further, air-related risk estimates are
presented for the median and for an upper percentile (i.e., the 95^th^
percentile of the population assessed).

EPA and CASAC recognize uncertainties in the risk estimates in the tails
of the distribution and consequently the 95^th^ percentile is reported
as the estimate of the high end of the risk distribution (Henderson,
2007b, p. 3). In so doing, however, EPA notes that it is important to
consider that there are individuals in the population expected to have
higher risk, particularly in light of the risk management objectives for
the current standard which was set in 1978 to protect the 99.5^th^
percentile. Further, we note an increased uncertainty in our estimates
of air-related risk for the upper percentiles, such as the 95^th^
percentile, due to limitations in the data and tools available to us to
estimate pathway contributions to blood Pb and associated risk for
individuals at the upper ends of the distribution.

In order to consider exposure and risk associated with the current
standard, EPA developed estimates for a case study based on air quality
projected to just meet the standard in a location of the country where
air concentrations currently do not meet the current standard (the
primary Pb smelter case study). Estimates of median air-related IQ loss
associated with just meeting the current NAAQS in the primary Pb smelter
case study subarea range from 3.2 points to 9.4 points IQ loss ("recent
air" plus "past air" category of Pb exposures) for the range of C-R
functions (Table 2). This estimate (recent air plus past air) for the
subarea based on the LLL C-R function is 6.0 points IQ loss for the
median and 8.0 points IQ loss for the 95^th^ percentile, with which we
note a greater uncertainty than for the median estimate (as discussed
above).[^73] Modeling limitations have affected our ability to derive
lower bound estimates for this case study.

Additionally, we developed estimates of blood Pb and associated IQ loss
associated with the current standard for the urban case studies. We note
that we consider it extremely unlikely that air concentrations in urban
areas across the U.S. that are currently well below the current standard
would increase to just meet the standard. However, we recognize the
potential for air Pb concentrations in some areas currently well below
the standard to increase to just meet the standard by way of, for
example, expansion of existing sources (e.g., facilities operating as
secondary smelters may exercise previously used capabilities as primary
smelters) or by the congregation of multiple Pb sources in adjacent
locations. We have simulated this scenario (increased Pb concentrations
to just meet the current standard) in a general urban case study and
three location-specific urban case studies. For the location-specific
urban case studies, we note substantial uncertainty in simulating how
the profile of Pb concentrations might change in the hypothetical case
where concentrations increase to just meet the current standard.

Turning first to the estimates of total blood Pb for the current NAAQS
scenario simulated for the general urban case study, which is a
simplified representation of a location within an urban area (described
in section II.C.2.h above), median estimates of air-related IQ loss
range from 1.5 to 7.7 points (across all four C-R functions), with an
estimate based on the LLL function bounded at the low end by 3.4 points
and at the high end by 4.8 points (Table 2). At the 95th percentile for
total IQ loss (LLL estimate), IQ loss associated with air-related Pb is
estimated to fall somewhere between 5.5 and 7.6 points (Staff Paper,
Table 4-6).

In considering the estimates for the three location-specific urban case
studies, we first note the extent to which exposures associated with
increased air Pb concentrations that simulate just meeting the current
standard are estimated to increase blood Pb levels in young children.
The magnitude of this for the median total blood Pb ranges from 0.3
µg/dL (an increase of 20 percent) in the case of the Cleveland study
area (where the highest monitor is estimated to be approximately one
fourth of the current NAAQS), up to approximately 1 µg/dL (an increase
of 50 to 70%) for the Chicago and Los Angeles study areas, where the
highest monitor is estimated to be at or below one tenth of the current
NAAQS (Table 1). Median estimates of air-related risk for these case
studies range from 0.6 points IQ loss (recent air estimate using low-end
C-R function) to 7.4 points IQ loss (recent plus past air estimate using
the high-end C-R function). The corresponding estimates based on the LLL
C-R function range from 2.7 points (lowest location-specific recent air
estimate) to 4.7 points IQ loss (highest location-specific recent plus
past air estimate). The comparable estimates of air-related risk for
children at the 95^th^ percentile in these three case studies range from
2.6 to 7.6 points IQ loss for the LLL C-R function (Staff paper, Table
4-6), although we note increased uncertainty in the magnitude of these
95^th^ percentile air-related estimates.

Another way in which the risk assessment results might be considered is
by comparing current NAAQS scenario estimates to current conditions,
although in so doing, it is important to recognize that, as stated below
and described in section II.C., this will underestimate air-related
impacts associated with the NAAQS. In making such a comparison of
estimates for the three location-specific urban case studies, the
estimated difference in total Pb-related IQ loss for the median child is
about 0.5 to 1.4 points using the LLL C-R function and a similar
magnitude of difference is estimated for the 95^th^ percentile. The
corresponding comparison for the general urban case study indicates the
current NAAQS scenario median total Pb-related IQ loss is 1.1 to 1.3
points higher than the two current conditions scenarios. As described in
section II.C, such comparisons are underestimates of air-related impacts
brought about as a result of increased air Pb concentrations, and
consequently they are inherently underestimates of the true impact of an
increased NAAQS level on public health.

In considering the exposure/risk information with regard to adequacy of
the current standard, the Staff Paper first considered the estimates
described above, particularly those associated with air-related risk.
[^74] The Staff Paper described these estimates, as indicating levels of
IQ loss associated with air-related risk that may reasonably be judged
to be highly significant from a public health perspective.

The Staff Paper also describes a different risk metric that estimated
differences in the numbers of children with different amounts of
Pb-related IQ loss between air quality scenarios for current conditions
and for the current NAAQS in the three location-specific urban case
studies. For example, estimates of the additional number of children
with IQ loss greater than one point (based on the LLL C-R function) in
these three study areas, for the current NAAQS scenario as compared to
current conditions, range from 100 to 6,000 across the three locations.
The corresponding estimates for the additional number of children with
IQ loss greater than seven points, for the current NAAQS as compared to
current conditions, range from 600 to 35,000. These latter values for
the change in incidence of children with greater than seven points
Pb-related IQ loss represent 5 to 17 percent of the children (aged less
than 7 years of age) in these study areas. This increase corresponds to
approximately a doubling in the number of children with this magnitude
of Pb-related IQ loss in the study area most affected. The Staff Paper
concluded that these estimates indicate the potential for significant
numbers of children to be negatively affected if air Pb concentrations
increased to levels just meeting the current standard.

Beyond the findings related to quantified risks, the Staff Paper
recognized the potential for other, unquantified adverse effects that
may occur at similarly low exposures. In summary, the Staff Paper
concluded that taken together, the quantified IQ effects associated with
the current NAAQS and other, nonquantified effects are important from a
public health perspective, indicating a need for consideration of
revision of the standard to provide an appreciable increase in public
health protection.

3\. CASAC Advice and Recommendations and Public Comment

CASAC's recommendations in this review builds upon the CASAC
recommendations during the 1990 review, which also advised on
consideration of more health protective NAAQS. In CASAC's review of the
1990 Staff Paper, as discussed in Section II.D.1.b, they generally
recommended consideration of levels below 1.0 µg/m^3^, specifically
recommended analyses of a standard set at 0.25 µg/m^3^, and also
recommended a revision to a monthly averaging time (CASAC, 1990).

In its letter to the Administrator subsequent to consideration of the
ANPR, the final Staff Paper and the final Risk Assessment Report, the
CASAC Pb Panel unanimously and fully supported "Agency staff's
scientific analyses in recommending the need to substantially lower the
level of the primary (public-health based) Lead NAAQS, to an upper bound
of no higher than 0.2 µg/m^3^ with a monthly averaging time" (Henderson,
2008, p. 1). This recommendation is consistent with their
recommendations conveyed in two earlier letters in the course of this
review (Henderson, 2007a, 2007b). Further, in their advice to the Agency
over the course of this review, CASAC has provided rationale for their
conclusions that has included their statement that the current Pb NAAQS
"are totally inadequate for assuring the necessary decreases of lead
exposures in sensitive U.S. populations below those current health
hazard markers identified by a wealth of new epidemiological,
experimental and mechanistic studies", and stated that "Consequently, it
is the CASAC Lead Review Panel's considered judgment that the NAAQS for
Lead must be decreased to fully-protect both the health of children and
adult populations" (Henderson, 2007a, p. 5). CASAC drew support for
their recommendation from the current evidence, described in the
Criteria Document, of health effects occurring at dramatically lower
blood Pb levels than those indicated by the evidence available when the
standard was set and of a recognition of effects that extend beyond
children to adults.

The Agency has also received comments from the public on drafts of the
Staff Paper and related technical support document, as well as on the
ANPR[^75]. Public comments received to date that have addressed adequacy
of the current standard overwhelmingly concluded that the current
standard is inadequate and should be substantially revised, in many
cases suggesting specific reductions to a level at or below 0.2 µg/m^3^.
Two comments were received from specific industries expressing the view
that the current standard might need little or no adjustment. One
comment received early in the review stated that current conditions
justified revocation of the standard.

4\. Administrator's Proposed Conclusions Concerning Adequacy

Based on the large body of evidence concerning the public health impacts
of Pb, including significant new evidence concerning effects at blood Pb
concentrations substantially below those identified when the current
standard was set, the Administrator proposes that the current standard
does not protect public health with an adequate margin of safety and
should be revised to provide additional public health protection.

In considering the adequacy of the current standard, the Administrator
has carefully considered the conclusions contained in the Criteria
Document, the rationale and recommendations contained in the Staff
Paper, the advice and recommendations from the CASAC, and public
comments received on the ANPR and other documents to date.

The Administrator notes that the body of available evidence, summarized
above in section III.B and discussed in the Criteria Document, is
substantially expanded from that available when the current standard was
set three decades ago. The Criteria Document presents evidence of the
occurrence of health effects at appreciably lower blood Pb levels than
those demonstrated by the evidence at the time the standard was set.
Subsequent to the setting of the standard, the Pb NAAQS criteria review
during the 1980s and the current review have provided (a) expanded and
strengthened evidence of still lower Pb exposure levels associated with
slowed physical and neurobehavioral development, lower IQ, impaired
learning, and other indicators of adverse neurological impacts; and (b)
other effects of Pb on cardiovascular function, immune system
components, calcium and vitamin D metabolism and other health endpoints
(discussed fully in the Criteria Document).

The Administrator notes particularly the robust evidence of neurotoxic
effects of Pb exposure in children, both with regard to epidemiological
and toxicological studies. While blood Pb levels in U.S. children have
decreased notably since the late 1970s, newer studies have investigated
and reported associations of effects on the neurodevelopment of children
with these more recent blood Pb levels. The toxicological evidence
includes extensive experimental laboratory animal evidence that
substantiates well the plausibility of the epidemiologic findings
observed in human children and expands our understanding of likely
mechanisms underlying the neurotoxic effects. Further, the Administrator
notes the current evidence of a steeper dose-response relationship at
these lower blood Pb levels than at higher blood Pb levels, indicating
the potential for greater incremental impact, associated with exposure
at these lower levels.

In addition to the evidence of health effects occurring at significantly
lower blood Pb levels, the Administrator recognizes that the current
health effects evidence together with findings from the exposure and
risk assessments (summarized above in section III.B), like the
information available at the time the standard was set, supports our
finding that air-related Pb exposure pathways contribute to blood Pb
levels in young children, by inhalation and ingestion. Furthermore, the
Administrator takes note of the information that suggests that the
air-to-blood ratiorelationship (i.e., the quantitative relationship
between air concentrations and blood concentrations), is now likely
larger, and may be much larger, when air inhalation and ingestion are
considered, than that estimated when the standard was set.

Based on evidence discussed above, the Administrator first considered
the evidence in the context of an adaptation of the 1978 framework,
recognizing that the health effects evidence with regard to
characterization of a threshold for adverse effects has changed
dramatically since the standard was set in 1978. Applying this
framework, the Administrator concludes that the more recently available
evidence indicates that protecting public health with an adequate margin
of safety requires a level for the standard that is substantially lower
than the current level.

The Administrator has further considered the evidence in the context of
the alternate evidence-based framework described above. In applying this
framework to the current evidence, the Administrator concludes that the
air-related IQ loss indicated to be associated with the current standard
is large from a public health perspective and provides further support
for concluding that the current standard does not protect public health
with an adequate margin of safety. Together these two different
considerations of the current evidence indicate the need for a level for
the standard that is substantially lower to provide increased public
health protection, especially for at-risk groups, including most notably
children, against an array of effects, most importantly including
effects on the developing nervous system

The Admininstrator has also considered the results of the exposure and
risk assessments conducted for this review, to provide some further
perspective on the potential magnitude of air-related IQ loss. In areas
projected to just meet the current standard, the quantitative estimates
of IQ loss associated with air-related Pb indicate risk of a magnitude
that in the Administrator's judgment is significant from a public health
perspective. Further, although the current monitoring data indicate few
areas with airborne Pb near or just exceeding the current standard, the
Administrator recognizes significant limitations with the current
monitoring network and thus the potential that the prevalence of such
levels of Pb concentrations may be underestimated by currently available
data.

The Administrator believes that the air-related blood Pb and IQ loss
estimates discussed in the Staff Paper and Risk Assessment Report, and
summarized above, as well as the air-related IQ loss discussed in the
alternative evidence-based framework, are important from a public health
perspective and are indicative of potential risks to susceptible and
vulnerable groups. In reaching this proposed judgment, the Administrator
considered the following factors: (1) the estimates of blood Pb and IQ
loss for children from air-related Pb exposures associated with the
current standard, (2) the estimates of numbers of children with
different amounts of increased Pb-related IQ loss associated with the
current standard, (3) the variability within and among areas in both the
exposure and risk estimates, (4) the uncertainties in these estimates,
and (5) the recognition that there is a broader array of Pb-related
adverse health outcomes for which risk estimates could not be quantified
and that the scope of the assessment was limited to a sample of case
studies and to some but not all at-risk populations, leading to an
incomplete estimation of public health impacts associated with Pb
exposures across the country.[^76] In addition to the evidence-based and
risk-based conclusions described above, the Administrator also notes
that it was the unanimous conclusion of the CASAC Panel that EPA needed
to "substantially lower" the level of the primary Pb NAAQS to fully
protect the health of children and adult populations (Henderson, 2007a,
2007b, 2008).

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

E. Conclusions on the Elements of the Standard

The four elements of the standard -- indicator, averaging time, form,
and level -- serve to define the standard and must be considered
collectively in evaluating the health and welfare protection afforded by
the standard. In considering revisions to the current primary Pb
standard, as discussed in the following sections, EPA considers each of
the four elements of the standard as to how they might be revised to
provide a primary standard for Pb that is requisite to protect public
health with an adequate margin of safety. Considerations and proposed
conclusions on indicator are discussed in section II.E.1, and on
averaging time and form in section II.E.2. Considerations and proposed
conclusions on a level for a Pb NAAQS with a Pb-TSP indicator are
discussed in section II.E.3 and considerations on a level for a Pb NAAQS
with a Pb-PM~10~ indicator are discussed in section II.E.4.

1\. Indicator

The indicator for the current standard is Pb-TSP (as describied in
section II.D.1.a above).[^77] When the standard was set in 1978, the
Agency proposed Pb-TSP as the indicator, but considered identifying Pb
in particulate matter less than or equal to 10 µm in diameter
(Pb-PM~10~) as the indicator. EPA had received comments expressing
concern that because only a fraction of airborne particulate matter is
respirable, an air standard based on total air Pb is unnecessarily
stringent. The Agency responded that while it agreed that some Pb
particles are too small or too large to be deposited in the respiratory
system, a significant component of exposures can be ingestion of
materials contaminated by deposition of Pb from the air. In addition to
the route of ingestion and absorption from the gastrointestinal tract,
nonrespirable Pb in the environment may, at some point, become
respirable through weathering or mechanical action. EPA concluded that
total airborne Pb, both respirable and nonrespirable fractions, should
be addressed by the air standard (43 FR 46251). The federal reference
method (FRM) for Pb-TSP specifies the use of the high-volume FRM sampler
for TSP.

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

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

In the current review, the Staff Paper evaluated the evidence with
regard to the indicator for a revised primary standard. This evaluation
included consideration of the basis for using Pb-TSP as the current
indicator, information regarding the sampling methodology for the
current indicator, and CASAC advice with regard to indicator (described
below). Based on this evaluation, the Staff Paper recommended retaining
Pb-TSP as the indicator for the primary standard. The Staff Paper also
recommended activities intended to encourage collection and development
of datasets that will improve our understanding of national and
site-specific relationships between Pb-PM~10~ (collected by low-volume
sampler) and Pb-TSP to support a more informed consideration of
indicator during the next review. The Staff Paper suggested that such
activities might include describing a federal equivalence method (FEM)
in terms of PM~10~ and allowing its use for a TSP-based standard in
certain situations, such as where sufficient data are available to
adequately demonstrate a relationship between Pb-TSP and Pb-PM~10~ or,
in combination with more limited Pb-TSP monitoring, in areas where
Pb-TSP data indicate Pb levels well below the NAAQS level.

The ANPR further identified issues and options associated with
consideration of the potential use of Pb-PM~10~ data for judging
attainment or nonattainment with a Pb-TSP NAAQS. These issues included
the impact of controlling Pb-PM~10~ for sources predominantly emitting
Pb in particles larger than those captured by PM~10~ monitors[^78]
(i.e., ultra-coarse)[^79], and the options included potential
application of Pb-PM~10~ FRM/FEMs at sites with established
relationships between Pb-TSP and Pb-PM~10~, and use of Pb-PM~10~ data,
with adjustment, as a surrogate for Pb-TSP data. The ANPR broadly
solicited comment in these areas.

In the current review, both the CASAC Pb Panel and members of the CASAC
Ambient Air Monitoring Subcommittee (AAMS) have recommended that EPA
consider a change in the indicator to PM~10~, utilizing low-volume
PM~10~ sampling (Henderson, 2007a, 2007b, 2008; Russell, 2008)[^80]. In
their January 2008 letter, the CASAC Lead Panel unanimously recommended
that EPA revise the Pb NAAQS indicator to rely on low-volume PM~10~
sampling (Henderson, 2008). They indicated support for their
recommendation in a range of areas. First, they noted poor precision in
high-volume TSP sampling, wide variation in the upper particle size-cut
as a function of wind speed and direction, and greater difficulties in
capturing the spatial non-homogeneity of ultra-coarse particles with a
national monitoring network. They stated that the low-volume PM~10~
collection method is a much more accurate and precise collection method,
and would provide a more representative characterization on a large
spatial scale of monitored particles which remain airborne longer, thus
providing a characterization that is more broadly representative of
exposures over large spatial scales. They also note the automated
sequential sampling capability of low-volume PM~10~ monitors that would
be particularly useful if the averaging time is revised (i.e., to a
monthly averaging time, as recommended by CASAC), which, in CASAC's view
would necessitate an increased monitoring frequency. Further, they noted
the potential for utilization of the more widespread PM~10~ sampling
network (Henderson, 2007a, 2007b, 2008). In their advice, CASAC also
stated that they "recognize the importance of coarse dust contributions
to total Pb ingestion and acknowledge that TSP sampling is likely to
capture additional very coarse particles which are excluded by PM10
samplers" (Henderson 2007b). They suggested that an adjustment of the
NAAQS level would accommodate the loss of these ultra-coarse Pb
particles, and that development of such a quantitative adjustment might
appropriately be based on concurrent Pb-PM~10~ and Pb-TSP sampling
data[^81] (Henderson, 2007a, 2007b, 2008).

The Agency received comments on the discussion of the indicator in the
ANPR from several state and local agencies and national/regional air
pollution control organizations, as well as a national environmental
organization. These public comments were somewhat mixed. Most of these
commenters recommended maintaining Pb-TSP as the indicator to ensure
that Pb emitted in larger particles is not overlooked by the Pb NAAQS.
Some of those comments and others suggested keeping TSP as the indicator
but revising the FRM to a low-volume TSP method[^82] and considering
tighter sampling height criteria to reduce variability[^83]. Others, in
considering a potential PM~10~-based indicator or the use of PM~10~ data
as a surrogate for Pb-TSP, noted the need for characterization of the
relationship between Pb-PM~10~ and Pb-TSP, which varies with proximity
to some sources. One state agency and a national organization of
regulatory air agencies expressed clear support for revising the
indicator to Pb-PM~10~, predominantly citing advantages associated with
improved technology and efficiency in data collection.

In considering these issues concerning the appropriate indicator, EPA
takes note of previous Agency conclusions that the health evidence
indicates that Pb in all particle size fractions, not just respirable
Pb, contributes to Pb in blood and to associated health effects.
Further, the evidence and exposure/risk estimates in the current review
indicate that ingestion pathways dominate air-related exposure. Lead is
unlike other criteria pollutants, where inhalation of the airborne
pollutant is the key contributor to exposure. For Pb it is the quantity
of Pb in ambient particles with the potential to deposit indoors or
outdoors, thereby leading to a role in ingestion pathways, that is the
key contributor to air-related exposure. As recognized by the Agency in
setting the standard, and as noted by CASAC in their advice during this
review, these particles include ultra-coarse particles. Thus, choosing
the appropriate indicator requires consideration of the impact of the
indicator on protection from both the inhalation and ingestion pathways
of exposure.

As discussed in section V.A., the Agency recognizes the body of evidence
indicating that the high-volume Pb-TSP sampling methodology contributes
to imprecision in resultant Pb measurements due to variability in the
size of particles captured and thus, in the mass of Pb measured. For
example, the measured values from a high-volume TSP sampler may differ
substantially, depending on wind speed and direction, for the same
actual ambient concentration of Pb-TSP.[^84] Variability is most
substantial in samples with a large portion of Pb particles greater than
10 microns, such as those samples collected near sources with emissions
of ultra-coarse particles. The result is a clear risk of error from
underestimating the ambient level of total Pb in the air, especially in
areas near sources of ultra-coarse particles, by underestimating the
amount of the ultra-coarse particles. There is also the potential for
overestimation of individual sampling period measurements associated
with high wind events.[^85]

The low-volume PM~10~ sampling methodology does not exhibit such
variability[^86] due both to increased precision of the monitor and
decreased spatial variation of Pb-PM~10~ concentrations. As a result,
greater precision is associated with sample measurements for Pb
collected using the PM~10~ sampling methodology. The result is a lower
risk of error of underestimating the ambient Pb in the PM~10~ size class
than there is risk of error of underestimating the ambient Pb in the TSP
size class. At the same time, there is a clear error in not including
the Pb in particles greater than PM~10~ that also contributes to the
health risks posed by air-related Pb, especially in areas influenced by
sources of ultra-coarse particles. There are also concerns over whether
implementation of a NAAQS based on a Pb-PM~10~ indicator will result in
effective control strategies for ultra-coarse Pb-containing particles.
In evaluating these two indicators, the differences in the nature and
degree of these sources of error between Pb-TSP and Pb-PM~10~ need to be
considered and weighed, to determine the appropriate way to protect the
public from exposure to air-related Pb.

As noted above, EPA is concerned about the total mass of all Pb
particles emitted into the air and subsequently inhaled or ingested.
Measurements of Pb-TSP have the potential to represent a greater
fraction of the particles of concern from a public health perspective,
than measurements of Pb-PM~10~, but limitations with regard to the
sampler mean that these data are less precise, and generally biased in
the direction of under-representation of true total air Pb. EPA
recognizes substantial variability in the high-volume Pb-TSP method,
meaning there is a risk of not consistently identifying sites that fail
to achieve the standard, both across sites and across time periods for
the same site.

Alternatively, using low-volume Pb-PM~10~ as the indicator would allow
the use of a technology that has better precision. In addition, since
Pb-PM~10~ concentrations have less spatial variability, such monitoring
data may be representative of Pb-PM~10~ air quality conditions over a
larger geographic area (and larger populations) than would Pb-TSP
measurements. The larger scale of representation for Pb-PM~10~ would
mean that reported measurements of this indicator, and hence designation
outcomes, would be less sensitive to exact monitor siting than with
Pb-TSP as the indicator.[^87] The result is a clear reduction in the
main sources of error described above for Pb-TSP (the variability in
measurements). However, there would be a different source of error, in
that Pb in particles not captured by PM~10~ samplers would not be
directly included. The fraction of Pb collected with a TSP sampler not
collected by a PM~10~ sampler varies depending on proximity to sources
of ultra-coarse Pb particles and the size mix of the particles they emit
(as well as the sampling variability inherent in the method discussed
above). This means that this error is of most concern in locations in
closer proximity to such sources, which may also be locations with some
of the higher ambient air levels. This would need to be addressed in
determining the appropriate level for a standard based on a Pb-PM~10~
indicator.

Accordingly, we believe it is reasonable to consider continued use of a
Pb-TSP indicator as one option, focusing on the fact that it
specifically includes the ultra-coarse Pb particles in the air that are
of concern and need to be addressed in protecting public health from
air-related exposures. Nonetheless, we also recognize the value of
considering a move to a Pb-PM~10~ indicator which would allow for
greater precision and more consistent temporal and spatial
representativeness in Pb measurements by using low-volume Pb-PM~10~
samplers as recommended by CASAC. EPA must weigh the relative benefits
of these two approaches for achieving the goals of the NAAQS. The option
of moving to a Pb-PM~10~ indicator is considered below. Before turning
to the option of a Pb-PM~10~ indicator, however, we summarize our
consideration of what types of monitoring may be appropriate for
implementing a NAAQS based on a Pb-TSP indicator, because practical
monitoring issues are relevant to how well each of the two approaches to
the indicator may achieve the goals of the NAAQS. Monitoring topics are
discussed in more detail in section V of this preamble.

In considering the option of retaining Pb-TSP as the indicator, EPA
recognizes that high-volume FRM TSP samplers would continue to be used
at many monitoring sites operated by State and local agencies. In
addition, it is possible that one or more low-volume TSP monitors would
be approved as FEM, under the provisions of 40 CFR 53, Ambient Air
Monitoring Reference and Equivalent Methods. EPA believes, along with
some commenters as noted above, that low-volume Pb-TSP sampling would
have important advantages over high-volume Pb-TSP sampling.[^88] To
facilitate the ability of monitor vendors and monitoring agencies to
gain FEM status for low-volume Pb-TSP monitors, EPA is proposing certain
revisions to the side-by-side equivalent testing requirements in 40 CFR
53 regarding the ambient Pb concentrations required during testing so
that testing is more practical for a monitor vendor to conduct, as
described in more detail in section V below. We note that 40 CFR 53.7,
Testing of Methods at the Initiative of the Administrator, allows EPA
itself to conduct the required equivalence testing for a method and then
determine whether the requirements for equivalence are met. It would
also be possible for EPA to promulgate amendments to 40 CFR 50
establishing one or more particular designs of a low-volume sampler as a
Pb-TSP FRM, or to establish performance specifications that would
facilitate the approval of low-volume samplers as FRM on a performance
basis rather than a design basis; this could be done as a replacement
for the high-volume TSP and Pb-TSP FRM or as an alternative TSP and/or
Pb-TSP FRM. Either path to FRM status would avoid the need for the
side-by-side testing, prescribed by 40 CFR 53, of low-volume samplers to
demonstrate equivalence to the high-volume FRM sampler, although some
amount and type of new testing in the field or in a wind tunnel may be
appropriate before such changes should be made. EPA invites comments on
the low-volume TSP sampler concept.

Within the option of continued use of a Pb-TSP indicator, EPA recognizes
that some State, local, or tribal monitoring agencies, or other
organizations, for the sake of the advantages noted above, may wish to
deploy low-volume Pb-PM~10~ samplers rather than Pb-TSP samplers. In
anticipation of this, therefore, we have also considered an approach
within the option of retaining Pb-TSP as the indicator that would allow
the use of Pb-PM~10~ data (when and if low-volume Pb-PM~10~ samplers
have been approved by EPA as either FRM or FEM), with adjustment(s), for
monitoring for compliance with the Pb-TSP NAAQS. This approach would
have five components: (1) the establishment of a FRM specification for
low-volume Pb-PM~10~ monitoring including both a PM~10~ sampler
specification and a reference chemical analysis method for determination
of Pb in the collected particulate matter; (2) the establishment of a
path to FEM designation for Pb-PM~10~ monitoring methods that differ
from the FRM in either the sampler or the analytical method; (3)
flexibility for monitoring agencies to deploy low-volume Pb-PM~10~
monitors anywhere that the revised Pb monitoring requirements to help
implement the revised NAAQS would nominally require a Pb-TSP monitor;
(4) a rule that, whenever and wherever Pb-TSP data from a monitoring
site is sufficient for determining whether or not the Pb-TSP standard
has been exceeded, any collocated Pb-PM~10~ data from that site will not
be considered, and (5) specific steps for applying an adjustment to
low-volume Pb-PM~10~ data for purposes of making comparisons to the
level of the NAAQS specified in terms of Pb-TSP. The first four
components are discussed in depth in sections IV and V. Because the
issue of adjustment to low-volume Pb-PM~10~ data is linked closely to
considerations of the advantages of one indicator option versus another,
it is discussed here.

In identifying the appropriate adjustment(s) to be made to Pb-PM~10~
data for purposes of making comparisons to the level of the NAAQS
specified in terms of Pb-TSP, we recognize the importance to protecting
public health of taking into account the ultra-coarse particles that are
not included in Pb-PM~10~ measurement. It is appropriate to adjust or
scale Pb-PM~10~ data upwards before comparison to a Pb-TSP NAAQS if they
were collected in an area that can be expected to have ultra-coarse
particles present.

Pb-PM~10~/Pb-TSP relationships vary from site to site and time to time,
and have a systematic variation with distance from emissions sources
emitting particles larger than would be captured by Pb-PM~10~ as a
result of the faster deposition of the ultra-coarse particles. The exact
size mix of particles at the point(s) of emissions release and the
height of the release point(s) also affect the relationship.
Accordingly, one approach might rely on different adjustments for
Pb-PM~10~ data collected near Pb sources[^89] versus data collected not
near Pb sources. Another approach would further subdivide the Pb source
category, thus relying on different adjustments for sites near sources
with significant emissions of ultra-coarse particles versus sites near
sources for which those emissions are less significant or for which we
have less information indicating the significance of those emissions.
Either of these two approaches might also be combined with one-time
studies to establish site-specific relationships, such that the final
rule could provide default scaling factors but also allow them to be
superceded by locally determined factors based on local study data.

In considering what default scaling factor(s) could be included in the
final rule for use of Pb-PM~10~ data in conjunction with a Pb-TSP
indicator, EPA has identified and analyzed available collocated
Pb-PM~10~ and Pb-TSP data from various monitoring sites across the U.S.
(Schmidt, 2008). This analysis considered both source-oriented and
nonsource-oriented sites. In this analysis, EPA identified three
monitoring sites with such collocated data as source-oriented. One of
these sites is near a Pb smelter; Pb emissions from smelters typically
contain both ultra-coarse particles from material handling and
resuspension of contaminated dust, and fine and coarse particles from
the high temperature smelting operation itself. The other two
source-oriented sites include one located somewhat near a battery
manufacturer, and one located near an automobile plant. The dataset for
the smelter site was collected in 1988 and indicates an average Pb-TSP
concentration of about 2.5 µg/m^3^. The data for the battery
manufacturer site were collected in the mid-1990s and indicate an
average Pb-TSP concentration of about 0.09 µg/m^3^; data for the third
site, collected within the past 5 years, indicate an average Pb-TSP
concentration at that site of about 0.03 µg/m^3^. One-day ratios between
Pb-TSP and Pb-PM~10~ concentrations varied somewhat within the data for
each site, but the ratio of the concentration averages were 2.0 for the
smelter site, 1.6 at the site near the battery manufacturer and 1.1 at
the site located near an automotive plant.

Collectively, these three monitoring sites suggest that available
source-oriented site-specific adjustment (or scaling) factors to be
applied to Pb-PM~10~ concentrations before comparison to a Pb-TSP NAAQS,
might vary between 1.1 and 2.0 or greater. EPA notes in selecting a
default factor for this category it would be appropriate to consider
default adjustment factors from within the mid to upper part of this
range rather than the lower end to avoid underestimating the appropriate
scaling factor for a large proportion of the source-oriented sites. On
this basis, EPA proposes a default factor or factors selected from
within the range of approximately 1.6 and 2.0 would be appropriate for
all source-oriented monitoring sites, while providing States the option
of developing a site-specific factor through analysis of paired
monitoring data. We are proposing not to make the selection of the
scaling factor depend on a characterization of the size mix of the Pb
source, because reliable information on the mix of coarse and
ultra-coarse particles may often be unavailable. EPA invites comment on
the selection of a single or multiple default factors for
source-oriented sites from within this range. For example, EPA could
select a default factor that is at or near the upper end of the range,
2.0, to avoid the risk of underprotection in situations in which there
is as high or nearly as high a proportion of ultra-coarse Pb as at the
smelter site, or we might discount the smelter data set on the basis
that the 1988 data set does not reasonably represent any likely current
or future situation. Another concept would be to specify a test with
three possible outcomes: (1) if application of the 2.0 scaling factor
nevertheless results in a finding of non-exceedance then the scaling
factor will be 2.0; (2) if the application of the 1.0 factor
nevertheless results in a finding of exceedance then the scaling factor
will be 1.0; and (3) if neither of these is the case then the Pb-PM~10~
may not be compared to the standard at all. The latter outcome would
mean that an area might be initially unclassifiable as to attainment or
that a nonattainment area might be unable to demonstrate that it has
later reached attainment, until a local scaling factor were developed,
concentrations either increased or decreased to make the situation
unambiguous, or sufficient Pb-TSP data are available. EPA also invites
comment on other sets of paired data from near Pb sources of which we
may be unaware, and comment on other approaches of selecting a default
factor for the final rule based on paired data, including approaches
that might use more than one default factor for source-oriented
monitoring sites with the selection of the factor for a given monitoring
site depending on the characteristics of the nearby sources, the ambient
concentration of Pb-PM~10~, or other factors.

EPA has also considered what default scaling factor(s) should be
established for monitoring sites which, as far as is known, are not
influenced by nearby emission sources. We have reviewed paired data from
20 monitoring sites that appear to fit this description (Schmidt, 2008).
Average Pb-TSP concentrations at nearly all these sites were near to or
below the lowest concentration on which comments are invited as to the
NAAQS level (0.02 µg/m^3^, as discussed in section II.E.3). Judging from
ratios at these 20 sites, it appears that site-specific factors
generally range from 1.0 to 1.4 (with the factors for three sites
ranging from 1.8 to 1.9), and the ratios may be influenced by
measurement variability in both samplers as well as by actual air
concentrations. Given the low ambient concentrations, the value of a
default scaling factor selected within this range would have little
effect on the NAAQS compliance determination at such site unless the
level of the NAAQS were set near the lower end of the range on which
comments are sought. EPA invites comment on the selection of a default
scaling factor from within the range of 1.0 to 1.4 and also solicits
comment on selection of a default scaling factor from within the broader
range of 1.0 to 1.9.[^90]

As a possible second option, taking into consideration the advice of the
CASAC Pb Panel and members of the AAMS, EPA has also considered
potential revision of the indicator to Pb-PM~10~. In so doing, we
recognize several potential important benefits of such a revision, as
well as the need to reflect such a revision in the selection of level of
the standard. [^91] We recognize that the low volume PM~10~ sampler
provides better precision and size selection characteristics which would
make the associated data more comparable across sites.

In considering a potential revision of the indicator to Pb-PM~10~, we
recognize that an important issue is whether regulating concentrations
of Pb-PM~10~ will lead to appropriate controls on all particle size Pb
emissions from sources. For example, it would be of concern if a NAAQS
based on a Pb-PM~10~ indicator resulted in emissions control decisions
at sources with a large percentage of Pb in the size range not
substantially captured by PM~10~ sampling (e.g., fugitive dust emissions
from Pb smelters) that were different than the emission control
decisions that would be made if the NAAQS was based on Pb-TSP. In that
case, a PM~10~-based NAAQS might not yield emissions changes by some Pb
sources which, under a Pb-TSP indicator, would have contributed to NAAQS
exceedances and subsequent emissions changes. Alternatively, while
collocated Pb-TSP and Pb-PM~10~ data are lacking for a broad range of
source types, there are likely many sources (e.g., high temperature
combustion processes) for which virtually all of the emitted particles
represented in a Pb-TSP measurement would be captured by a Pb-PM~10~
measurement. Further, there are likely other source types with a range
of particle sizes extending beyond Pb-PM~10~, for which controls adopted
to meet a Pb-PM~10~ requirement would also achieve a proportional
reduction in ultra-coarse particles. In these situations, one would not
expect any difference in emissions control decisions whether the NAAQS
is Pb-PM~10~-based or Pb-TSP-based.

EPA proposes that it would be appropriate to select Pb-PM~10~ as the
indicator for the revised Pb NAAQS if EPA determined that the health
protection benefits from the practical monitoring advantages of the
low-volume Pb-PM~10~ sampling approach outweigh the health risks from
the above described uncertainty regarding protection from ultra-coarse
Pb in all situations.[^92] In this case, low-volume Pb-PM~10~ samplers
would become the required approach to Pb monitoring at required
monitoring sites and wherever else NAAQS-oriented Pb monitoring is
undertaken. Nonetheless EPA notes that retaining Pb-TSP monitors at some
relatively small subset of the Pb-PM~10~ monitoring sites would be
beneficial for purposes of scientific understanding of both ambient
conditions and the performance of the two types of measurement systems,
and EPA would encourage this to be done.

For reasons discussed here, and taking into account information and
assessments presented in the Criteria Document, Staff Paper and ANPR,
the advice and recommendations of CASAC and of members of the CASAC
AAMS, and public comments to date, the Administrator concludes it is
appropriate to propose to retain the current indicator of Pb-TSP,
measured by the current FRM or by an FEM approved under the proposed
revisions to 40 CFR 53, but with expansion of the measurements accepted
for determining attainment or nonattainment of the Pb NAAQS to provide
an allowance for use of Pb-PM~10~ data with scaling factors as described
above and more specifically below in section IV. In consideration of all
of the issues discussed above, the Administrator concludes it is also
appropriate to invite comment on a second option, a revision of the
current indicator to Pb-PM~10~, measured by the new low-volume Pb-PM~10~
FRM specified in the proposed appendix Q to 40 CFR 50 or by a FEM
approved under the proposed revisions to 40 CFR 53, with an adjustment
of the NAAQS level (as discussed in section II.E.4 below) in recognition
of the importance for public health of those ultra-coarse dust
contributions not captured by PM~10~ samplers. The Administrator
solicits comment on all of the issues discussed above, and specifically
with regard to the potential for a Pb-PM~10~ indicator to influence
implementation of controls in ways that would lead to less control
associated with larger particles than might be achieved with a
Pb-TSP-based NAAQS, taking into account the variability noted above for
TSP sampling.

2\. Averaging Time and Form

The statistical form of the current standard is a not-to-be-exceeded or
maximum value, averaged over a calendar quarter. This might also be
described as requiring that no average air Pb concentration representing
a time period of duration as long as calendar quarter (or longer) may
exceed the level of the standard. As noted in section II.D.1.a, EPA set
the standard in 1978 as a ceiling value with the conclusion that this
air level would be safe for indefinite exposure for young children (43
FR 46250).

The basis for selection of the current standard's averaging time of
calendar quarter reflects consideration of the evidence available when
the Pb NAAQS were promulgated in 1978. At that time, the Agency had
concluded that the level of the standard, 1.5 µg/m^3^, would be a "safe
ceiling for indefinite exposure of young children" (43 FR 46250), and
that the slightly greater possibility of elevated air Pb levels for
shorter periods within the quarterly averaging period as contrasted to
the monthly averaging period proposed in 1977 (43 FR 63076), was not
significant for health. These conclusions were based in part on the
Agency's interpretation of the health effects evidence as indicating
that 30 µg/dL was the maximum safe level of blood Pb for an individual
child.

With regard to averaging time, after consideration of the evidence
available at that time, the 1990 Staff Paper concluded that "\[a\]
monthly averaging period would better capture short-term increases in
lead exposure and would more fully protect children's health than the
current quarterly average" (USEPA, 1990b). The 1990 Staff Paper further
concluded that "\[t\]he most appropriate form of the standard appears to
be the second highest monthly average in a 3-year span. This form would
be nearly as stringent as a form that does not permit any exceedances
and allows for discounting of one 'bad' month in 3 years which may be
caused, for example, by unusual meteorology." In their review of the
1990 Staff Paper, the CASAC Pb Panel concurred with the staff
recommendation to express the lead NAAQS as a monthly standard not to be
exceeded more than once in three years.

As summarized in section II.B above and discussed in detail in the
Criteria Document, the currently available health effects evidence[^93]
indicates a wider variety of neurological effects, as well as immune
system and hematological effects, associated with substantially lower
blood Pb levels in children than were recognized when the standard was
set in 1978. Further, the health effects evidence with regard to
characterization of a threshold for adverse effects has changed since
the standard was set in 1978, as have the Agency's views on the
characterization of a safe blood Pb level.[^94] In consideration of
averaging time for the Pb NAAQS, we note the following aspects of the
current health effects evidence.

-   Children are exposed to ambient Pb via inhalation and ingestion,
    with Pb that is taken into the body absorbed through the lungs and
    through the gastrointestinal tract. Studies on Pb uptake,
    elimination, and distribution show that Pb is absorbed into
    peripheral tissues in adults within a few days (USEPA 1986a; USEPA
    1990b, p. IV-2). Absorption of Pb from the gastrointestinal tract
    appears to be greater and faster in children as compared to adults
    (CD, Section 4.2.1). Once absorbed, it is quickly distributed from
    plasma to red blood cells and throughout the body.

-   Lead accumulates in the body and is only slowly removed, with bone
    Pb serving as a blood Pb source for years after exposure and as a
    source of fetal Pb exposure during pregnancy (CD, Sections 4.3.1.4
    and 4.3.1.5).

-   Blood Pb levels, including levels of the toxicologically active
    fraction, respond quickly to increased Pb exposure, such that an
    abrupt increase in Pb uptake rapidly changes blood Pb levels. The
    associated time to reach a new quasi-steady state with the total
    body burden after such an occurrence is projected to be
    approximately 75 to 100 days (CD, p. 4-27).

-   The elimination half-life, which describes the time for blood Pb
    levels to stabilize after a reduction in exposure, for the dominant
    phase for blood Pb responses to changes in exposure is on the order
    of 20 to 30 days for adults (CD, p. 4-25). Blood elimination
    half-lives are influenced by contributions from bone. Given the
    tighter coupling in children of bone stores with blood levels,
    children's blood Pb is expected to respond more quickly than adults
    (CD, pp. 4-20 and 4-27).

-   Data from NHANES II and an analysis of the temporal relationship
    between gasoline consumption data and blood lead data generally
    support the inference of a prompt response of children's blood Pb
    levels to changes in exposure. Children's blood Pb levels and the
    number of children with elevated blood Pb levels appear to respond
    to monthly variations in Pb emissions from Pb in gasoline (EPA,
    1986a, p. 11-39; Rabinowitz and Needleman, 1983; Schwartz and
    Pitcher, 1989).

-   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
    8.4.2.7). The animal evidence supports our understanding of periods
    of development with increased vulnerability to specific types of
    effect (CD, Section 5.3), and indicates a potential importance of
    exposures on the order of months.

-   Evidence of a differing sensitivity of the immune system to Pb
    across and within different periods of life stages indicates a
    potential importance of exposures as short as weeks to months
    duration. For example, the animal evidence suggests that the
    gestation period is the most sensitive life stage followed by early
    neonatal stage, and within these life stages, critical windows of
    vulnerability are likely to exist (CD, Section 5.9 and p. 5-245).

Evidence described in the Criteria Document and the risk assessment
indicate that ingestion of dust can be a predominant exposure pathway
for young children to air-related Pb, and that there is a strong
association between indoor dust Pb levels and children's blood Pb
levels. As stated in the Criteria Document, "given the large amount of
time people spend indoors, exposure to Pb in dusts and indoor air can be
significant" (CD, p. 3-27). The Criteria Document further describes
studies that evaluated the influence of dust Pb exposure on children's
blood Pb: "Using a structural equation model, Lanphear and Roghmann
(1997) also found the exposure pathway most influential on blood Pb was
interior dust Pb loading, directly or through its influence on hand Pb.
Both soil and paint Pb influenced interior dust Pb; with the influence
of paint Pb greater than that of soil Pb. Interior dust Pb loading also
showed the strongest influence on blood Pb in a pooled multivariate
regression analysis (Lanphear et al., 1998)." (CD, p. 4-134). Further, a
recent study of dustfall near an open window in New York City indicates
the potential for a relatively rapid response of indoor dust Pb loading
to ambient airborne Pb, on the order of weeks (CD, p. 3-28; Caravanos et
al., 2006a).

We note that the health effects evidence identifies varying length
durations in exposure that may be relevant and important. In light of
uncertainties in aspects such as response times of children's exposure
to airborne Pb, we recognize, as in the past, that this evidence
provides a basis for consideration of both calendar quarter and calendar
month as averaging times.

In considering averaging time and form, EPA has combined the current
quarterly averaging time with the current not-to-be exceeded (maximum)
form and has also combined a monthly averaging time with a second
maximum form, so as to provide an appropriate degree of year-to-year
stability that a maximum monthly form would not afford. We also note
that, as discussed below, the second maximum monthly form provides a
roughly comparable degree of protection on a broad national scale.

In this consideration of averaging time and form, EPA has taken into
account analyses using air quality data for 2003-2005 that are presented
in the Staff Paper (chapter 2). These analyses consider both a period of
three calendar years and a period of one calendar year (with the form of
the current standard being the maximum quarterly mean). These analyses
indicate that, with regard to either single-year or 3-year statistics
for the 2003-2005 dataset, a second maximum monthly mean yields very
similar, although just slightly greater, numbers of sites exceeding
various alternative levels as a maximum quarterly mean, with both
yielding fewer exceedances than a maximum monthly mean. That is, these
two averaging time and form combinations resulted in roughly the same
number of areas that would not attain a standard at any given level on a
broad national scale, suggesting roughly comparable public health
protection. However, the relative protection provided by these two forms
may differ from area to area. For example, some of the areas meeting a
maximum quarterly mean standard over the 2003-2005 period at a given
level did not meet a second maximum monthly mean standard at the same
level because there were at least two months with high monthly
concentrations which were averaged with a lower concentration month in
the same quarter. On the other hand, theoretically it is possible for an
area to meet a given standard level with a second maximum monthly mean
averaging time and form and not meet it for a maximum quarterly mean
(e.g., the second highest monthly average may be below the standard
level while the quarterly average may exceed it). Moreover, control
programs to reduce quarterly mean concentrations may not have the same
protective effect as control programs aimed at reducing concentrations
in every individual month. Given the limited scope of the current
monitoring network which lacks monitors near many significant Pb sources
and uncertainty about Pb source emissions and possible controls, it is
difficult to more quantitatively compare the protectiveness of the
quarterly mean versus the second maximum monthly mean approaches.

In their advice to the Agency in this review, CASAC has recommended that
consideration be given to changing from a calendar quarter to a monthly
averaging time (Henderson, 2007a, 2007b, 2008). In making that
recommendation, CASAC emphasizes support from studies that suggest that
blood Pb concentrations respond at shorter time scales than would be
captured completely by quarterly values, as indicated by their
description of their recommendation for adoption of a monthly averaging
time as "more protective of human health in light of the response of
blood lead concentrations that occur at sub-quarterly time scales"
(Henderson, 2007a). With regard to form of the standard, CASAC has
stated that one could "consider having the lead standards based on the
second highest monthly average, a form that appears to correlate well
with using the maximum quarterly value", while also indicating that "the
most protective form would be the highest monthly average in a year"
(Henderson, 2007a).

Among the public comments the Agency received on the discussion of
averaging time and form in the ANPR, the majority concurred with the
CASAC recommendation for a revision of the averaging time to a calendar
month.

The 1990 Staff Paper and the Staff Paper for this review both
recommended that the Administrator consider specifying, in the form of
the NAAQS, that compliance with the NAAQS will be evaluated over a
3-year period. The Administrator has considered this recommendation and
is proposing to adopt it. In the 3-year approach, a monitor would be
considered to be in violation of the NAAQS as of a certain date if in
any of the three previous calendar years with sufficiently complete data
(as explained in detail in section IV below), the value of the selected
form of the indicator (e.g., second maximum monthly average or maximum
quarterly average) exceeded the level of the NAAQS. A monitor, initially
or after once having violated the NAAQS, would not be considered to have
attained the NAAQS until three years have passed without the form and
level of the standard being violated. Many types of Pb sources have
variable emissions from day-to-day and year-to-year due to market
conditions for their products and/or weather variations that can affect
the generation of fugitive dust from contaminated roadways and grounds.
In addition, variations in wind patterns from year to year can cause a
near-source Pb monitor to be exposed to high concentrations on more days
in one year than in another, even if source emissions are constant,
especially if it operates on only some days. Thus, it is possible for a
monitor to indicate a violation of a hypothetical form and level in one
period but not in another, even if no permanent controls have been
applied at nearby source(s). Analysis of historical Pb air concentration
data has confirmed that this pattern of fluctuating monitoring results
can happen at the levels and forms being proposed. It would potentially
reduce the public health protection afforded by the standard if areas
fluctuated in and out of formal nonattainment status so frequently that
states do not have opportunity and incentive to identify sources in need
of more emission control and to require those controls to be put in
place. The 3-year approach would help ensure that areas initially found
to be violating the NAAQS have effectively controlled the contributing
lead emissions before being redesignated to attainment/maintenance.

In considering averaging time and form for the standard, the
Administrator has considered the information summarized above (described
in more detail in Criteria Document and Staff Paper), as well as the
advice from CASAC and public comments. The Administrator recognizes that
there is support in the evidence for a monthly averaging time consistent
with the following observations: (1) the health evidence indicates that
very short exposures can lead to increases in blood Pb levels, (2) the
time period of response of indoor dust Pb to airborne Pb can be on the
order of weeks and, (3) the health evidence indicates that adverse
effects may occur with exposures during relatively short windows of
susceptibility, such as prenatally and in developing infants. The
Administrator also recognizes limitations and uncertainties in the
evidence including the limited available evidence specific to the
consideration of the particular duration of sustained airborne Pb levels
having the potential to contribute to the adverse health effects
identified as most relevant to this review, as well as variability in
the response time of indoor dust Pb loading to ambient airborne Pb.
Based on these considerations and the air quality analyses summarized
above, the Administrator concludes that this information provides
support for an averaging time no longer than a calendar quarter.
Further, the Administrator recognizes that if substantial weight is
given to the evidence of even shorter times for response of dust Pb,
blood Pb, and associated effects to airborne Pb, a monthly averaging
time may be appropriate. Accordingly, the Administrator is proposing two
options with regard to the form and averaging time for the standard, and
with both he proposes changing the duration of the time period evaluated
in considering attainment to a 3-year period. One option is to retain
the current not-to-be-exceeded form with an averaging time of a calendar
quarter, such that the form would be maximum quarterly average across a
3-year span. The second option is to revise the averaging time to a
calendar month and the form to be the second highest monthly average
across a 3-year span.

3\. Level for a Pb NAAQS with a Pb-TSP Indicator

With regard to level of the standard, for a standard using a Pb-TSP
indicator, we first discuss evidence-based and exposure/risk-based
considerations, including considerations and conclusions of the Staff
Paper, in sections II.E.3.a and II.E.3.b below. This is followed by a
summary of CASAC advice and recommendations and public comments (section
II.E.3.c) and the Administrator's proposed conclusions (section
II.E.3.d). In addition, we discuss considerations and the
Administrator's proposed conclusions with regard to a level of the
standard using a Pb-PM~10~ indicator in section II.E.4 below.

a\. Evidence-based Considerations

As a general matter, EPA recognizes that in the case of Pb there are
several aspects to the body of epidemiological evidence that add
complexity to the selection of an appropriate level for the primary
standard. As summarized above and discussed in greater depth in the
Criteria Document (CD, Sections 4.3 and 6.1.3), the epidemiological
evidence that associates Pb exposures with health effects generally
focuses on blood Pb for the dose metric.[^95] In addition, exposure to
Pb comes from various media, only some of which are air-related. This
presents a more complex situation than does evidence of associations
between occurrences of health effects and ambient air concentrations of
an air pollutant, such as is the case for particulate matter and ozone.
Further, for the health effects receiving greatest emphasis in this
review (neurological effects 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 this
context EPA notes that the health effects evidence most useful in
determining the appropriate level of the NAAQS is this large body of
epidemiological studies. Unlike the recent review of the NAAQS for
ozone, there are no clinical studies useful for informing a
determination of the appropriate level for a standard.[^96] The
discussion below therefore focuses on the epidemiological studies,
recognizing and taking into consideration the complexity and resulting
uncertainty in using this body of evidence to determine the appropriate
level for the NAAQS.

In considering the evidence with regard to selection of the level of the
standard, we have considered the same evidence-based frameworks that
informed our consideration of the adequacy of the current standard, as
discussed above in section II.D.2.a. That is, we have considered how to
apply the much expanded body of evidence that is now available to (1)
the framework applied in the establishment of the standard in 1978,
building on the approach discussed in the Staff Paper and ANPR (i.e.,
the adapted 1978 framework), and to (2) an alternative evidence-based
framework that builds on a recommendation by the CASAC Pb Panel (i.e.,
air-related IQ loss framework). Both of these evidence-based approaches
are discussed below in considering the appropriate standard levels to
propose.

As noted in section II.D.2.a above, this review focuses on young
children as the key sensitive population for Pb exposures, the same
population identified in 1978. In this sensitive population, the current
evidence demonstrates the occurrence of health effects, including
neurological effects, associated with blood Pb levels extending well
below 10 µg/dL (CD, sections 6.2, 8.4 and 8.5). As further described in
section II.D.2.a above, some studies indicate Pb effects on intellectual
attainment of young children for which population mean blood Pb levels
in the analysis ranged from approximately 2 to 8 μg/dL (CD, Sections
6.2, 8.4.2 and 8.4.2.6). Further, as noted above, the current evidence
does not indicate a threshold for the more sensitive health endpoints
such as effects on the developing nervous system (CD, pp. 5-71 to 5-74
and Section 6.2.13).[^97]

As when the standard was set in 1978, there remain today contributions
to blood Pb levels from nonair sources. As discussed above (section
II.D.2), current evidence is limited with regard to estimates of the
aggregate reduction since 1978 of all nonair sources to blood Pb and
with regard to an estimate of current nonair blood Pb levels (discussed
in sections II.A.4) In recognition of temporal reductions in nonair
sources discussed in section II.A.4 and in the context of estimates
pertinent to an application of the 1978 framework, the CASAC Pb Panel
recommended consideration of 1.0 to 1.4 μg/dL or lower as an estimate of
the nonair component of blood Pb pertinent to average blood Pb levels
(as described in section II.A.4 above; Henderson, 2007a).

As discussed in section II.D.2.a, while there is uncertainty and
variability in the absolute value of an air-to-blood relationship, the
current evidence and estimates based on the exposure assessment, based
on consideration of both inhalation and ingestion exposure pathways,
indicate ratios greater than the 1978 1:2 estimate, and generally
ranging from 1:3 to 1:5, and higher, with some on the order of 1:10. In
considerations here, we have described the value of 1:5 as falling
somewhat central within the range supported by the evidence.

i\. Adapted 1978 Evidence-based Framework

Recommendations in the Staff Paper on standard levels were based upon an
approach that built upon and adapted the general approach used by EPA in
setting the standard in 1978. In adapting this approach to the currently
available information, the Staff Paper recognized the more extensive and
stronger body of evidence now available on a broader range of health
effects associated with exposure to Pb. For example, EPA recognizes that
today "there is no level of Pb exposure that can yet be identified, with
confidence, as clearly not being associated with some risk of
deleterious health effects" (CD, p. 8-63). This is in contrast to the
situation in 1978 when the Agency judged that the maximum safe
individual and geometric mean blood Pb levels for a population of young
children were 30 µg/dL and 15 µg/dL, respectively.[^98] In the current
review, the Criteria Document has described the substantiation of
neurotoxic effects in children and cardiovascular outcomes in adults at
blood Pb levels below 5 μg/dL (CD, sections 6.2, 6.4, 6.5 and 6.8),
including analyses of Pb-associated neurcognitive effects in children
for which population mean blood Pb levels in the analysis ranged from 2
to 8 μg/dL (CD, sections 6.2, 8.4.2 and 8.4.2.6).

In the Staff Paper application of an adapted 1978 framework, the focus
shifted away from identifying a safe blood Pb level for an individual
child (and then determining an ambient air level that would keep a very
high percentage of children at or below that safe level), because
information was no longer available to identify such a level. Rather,
the Staff Paper approach focused on identifying an appropriate
population mean blood Pb level, and then identifying an ambient air
level that would keep the mean blood Pb levels of children exposed at
that air level below the target population mean blood Pb level. This
Staff Paper approach builds on the evidence base now available, which
includes a large body of epidemiological evidence in which associations
have been observed in many study populations of children between blood
lead levels and IQ or other neurocognitive effects.

Based on the review of the evidence, the Staff Paper approach
substituted a level of 2 µg/dL for the target population geometric mean
blood Pb of 15 µg/dL used in 1978. In the absence of a demonstrated safe
level, at either an individual or a population level, the Staff Paper
used 2 µg/dL as representative of the lowest population mean level for
which there is evidence of a statistically significant association
between blood lead levels and health effects (e.g., CD, p. E-9; Lanphear
et al., 2000). This mean level was not chosen as a level that was
demonstrated as a threshold or a "safe" level, because at this time the
body of evidence does not demonstrate the existence of any such specific
level.

This approach does not evaluate the magnitude or degree of health
effects occurring across the population at that mean blood lead level.
In this adaptation of the 1978 approach the focus is solely on the
existence of a relationship between blood lead levels and neurocognitive
effects. The approach takes as the appropriate public health goal the
identification of an ambient air lead level that can be expected to keep
the mean blood lead level of an exposed population of children at or
below the lowest level at which a statistically significant association
has been demonstrated between blood lead level and neurocognitive
effects.[^99]

Starting with a target population geometric mean blood lead level of 2
µg/dL for the population of exposed children, then subtracting 1 to 1.4
µg/dL for the nonair component of blood Pb, yields 0.6 to 1 µg/dL as a
target for the geometric mean air contribution to blood Pb. The Staff
Paper divided the air-related target by 5, an air-to-blood ratio
somewhat central within the range of air Pb to blood Pb ratios generally
supported by the currently available evidence. This resulted in a
potential standard level of 0.1 to 0.2 µg/m^3^.

The Staff Paper conclusions on level for the primary Pb standard built
on staff 's conclusion that the overall body of evidence clearly calls
into question the adequacy of the current standard with regard to health
protection afforded to at-risk populations. Based on consideration of
the health effects evidence, as described above, the Staff Paper
concluded that it is reasonable to consider a range for the level of the
standard, for which the upper part is represented by 0.1 to 0.2 μg/m^3^.

In analyses subsequent to the Staff Paper, the Agency has further
considered the evidence with regard to its use in an adapted 1978
framework. In so doing, we have evaluated in greater detail the
appropriate values to use for the three central elements in this adapted
1978 framework - the target population geometric mean blood lead level,
the contribution from non-air-related sources, and the air-to-blood
ratio for air-related sources of Pb. We have more completely considered
the group of studies described in the Criteria Document in which an
association of blood Pb with neurocognitive decrement (e.g., full-scale
IQ, academic performance) was assessed, potential confounders were
considered,[^100] and the study population mean blood Pb level was near
or below 10 µg/dL. These studies are cited in Figure 1 along with the
study population mean blood Pb levels. Where not reported in the
publications, the mean blood Pb concentrations for subgroup analyses
have been drawn from personal communications with the authors, which are
available in the docket for this rulemaking (Hornung, 2008; Canfield,
2008: Chiodo, 2008; Tellez-Rojo 2008; Auinger, 2008). As seen in Figure
1, these studies demonstrate a strong pattern of statistical
significance across the studies, giving strength to our conclusion
regarding the findings of association with blood Pb and providing
greater confidence in findings for the lowest blood Pb levels evaluated.

In this additional consideration of the evidence within an adapted 1978
framework, we place primary emphasis on the evidence of blood Pb
associations with neurocognitive decrement and the point at which
uncertainty in the association substantially increases in identifying a
substitute for the "maximum safe" population mean blood Pb level
identified in 1978. From the studies presented in Figure 1, we identify
1.7 µg/dL as the lowest population mean blood Pb level for which a
statistically significant association with effects has been
demonstrated. This is the population mean blood Pb for the analysis by
Lanphear and others (2000) was limited to children with blood Pb levels
below 5 µg/dL (Lanphear et al., 2000; Auinger, 2008). The study from
which this subgroup mean is extracted is a large cross-sectional study
(more than 4000 subjects in both the full study and the subgroup
analysis) that reports consistently statistically significant
associations in subgroup analyses down to and including the subgroup
with individual blood Pb levels below 5 µg/dL (Lanphear et al., 2000).

![](media/image1.wmf){width="7.8597222222222225in"
height="5.477777777777778in"}**Figure 1. Neurocognitive Decrement
Associations in Analyses of Concurrent Blood Pb Distributions with
Population Mean Concurrent Blood Lead Levels Near or Below 10 ug/dL.**

In addition to a target population mean blood Pb level, this framework
is also based on specifying (1) an air-to-blood ratio, for the
air-related contribution to the target population mean blood Pb level
and (2) a nonair-related contribution to this population mean blood Pb
level. In selecting appropriate values for each of these parameters, we
note that the air-to-blood ratio and nonair blood Pb levels must be
considered jointly, since, as recognized in section II.A.4, the
magnitude of each of these parameters is affected by how completely the
air-related pathways are accounted for.[^101] In consideration of this
issue, we have identified a range of 1:3 to 1:7, supported by the
evidence, as being appropriate to use in consideration with a level of
nonair blood Pb of 1 to 1.4 µg/dL.

The estimates of each of these two parameters that we consider most
appropriate joint estimates are approximately central within the ranges
discussed above -- that is, an air-to-blood ratio of 1:5 (within a range
of 1:3 to 1:7) coupled with a nonair BLL of 1.2 µg/dL (within a range of
1 to 1.4 µg/dL). Alternatively, we consider the joint use of the lower
end of each of these ranges (i.e., 1:3 and 1 µg/dL) to evaluate with
regard to the highest standard level based on a consideration of
parameter values that are still plausible, taking into consideration the
uncertainties in such estimates. Values outside of the ranges identified
here (above and below) for either of these parameters have not been
considered in our assessment.

In this extended adaptation of the 1978 framework, using a blood Pb
target population mean of 1.7 µg/dL as the target population geometric
mean blood Pb level for the child population, then subtracting 1.2 µg/dL
for background, yields 0.5 µg/dL as a target for the populaton mean 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 ambient air standard level of 0.10 µg/m^3^.
Consideration of estimates falling at the low end of the supportable
range for nonair blood Pb and air-to-blood ratios (1.0 µg/dL and 1:3,
respectively) would yield a potential standard level of approximately
0.20 µg/m^3^.

ii\. Alternative Air-related IQ Loss Evidence-based Framework

As mentioned above, in analyses subsequent to the Staff Paper, the
Agency has further considered the evidence with regard to an approach
other than the adapted 1978 framework. As discussed above, the 1978
framework was premised on an evidentiary basis that clearly identified
an adverse health effect and a health-based policy judgment that
identified a level that would be safe for an individual child with
respect to this adverse health effect. The adaptation to the 1978
framework applies this framework to a situation where there is no longer
an evidentiary basis to determine a safe level for individual children.
In addition, this approach does not address explicitly what magnitude of
effect should be considered adverse. Given these two limitations, EPA
considered a different evidence-based framework that shifts focus from
identifying an appropriate target mean blood lead level and instead
focuses on the effects of air related Pb on neurocognitive functions.

This alternative framework focuses on the contribution of air-related Pb
to neurocognitive effects, with a public health goal of identifying the
appropriate ambient air level of Pb to protect exposed children from
health effects that are considered adverse, and are associated with
their exposure to air-related Pb. This framework does not focus on
overall blood lead levels or on nonair contribution to blood lead
levels. While this avoids some of the limitations noted above with the
adapted 1978 approach, EPA recognizes that looking at air-related Pb in
isolation could be considered a limitation for this alternative
framework. The different limitations of each of these frameworks derive
from the limitations in the underlying body of evidence available for
this review.

In this alternate evidence-based framework, we have drawn from the
entire body of evidence as a basis for concluding that there are causal
associations between air-related Pb exposures and population IQ loss. We
have drawn more quantitatively from the evidence by combining
air-to-blood ratios with evidence-based C-R functions from the
epidemiological studies to quantify the association between air Pb
concentrations and air-related population mean IQ loss. This air-related
IQ loss framework focuses on selecting a standard that would prevent
air-related IQ loss (and related effects) of a magnitude judged by the
Administrator to be of concern in populations of children exposed to the
level of the standard, taking into consideration such factors as the
uncertainties inherent in such estimates. In addition to this judgment
by the Administrator, this framework is also based on specifying an
air-to-blood ratio (also used in the adapted 1978 framework) and a C-R
function(s) for population IQ response associated with blood Pb level.

In considering the evidence with regard to C-R functions, and in
recognition of the finding in the evidence of a steeper slope at lower
blood Pb levels (i.e., the nonlinear relationship), we have identified
two sets of C-R functions (discussed more fully above in section
II.B.2.b). The first set focuses on C-R functions particular to lower
blood Pb levels that are more representative of, although still somewhat
higher, than the distribution of blood Pb levels in the current U.S.
population of children. The second set considers functions descriptive
of the C-R relationship over a larger range of blood Pb levels that
extend up to 10 ug/dL.

As discussed above in section II.B.2.b, the C-R functions from analyses
involving the lower mean blood Pb levels, that are more representative
of current mean blood Pb levels in U.S. children, provide slopes of IQ
loss with increasing blood Pb that range from -1.71 to -2.94 IQ points
per µg/dL blood Pb. These include C-R functions recommended for
consideration by CASAC, in light of the current blood Pb levels of U.S.
children (Henderson, 2008), and also the C-R function given greatest
weight in the risk assessment (discussed above in section II.C.2.b), the
loglinear function with low-exposure linearization (the LLL function).
The lower-end function in this range is from the analysis by Tellez-Rojo
and others (2006) of very young children with blood Pb levels below 5
µg/dL, with a group mean blood Pb level of 2.9 µg/dL. The higher-end
function in this range is from the analysis by Lanphear and others
(2005) of young children whose blood Pb levels never exceeded 7.5 µg/dL,
with a group mean blood Pb level of 3.24 µg/dL. The LLL function falls
within the range of the other two functions at lower blood Pb levels,
with an average slope of -2.29 across blood Pb levels extending below 2
µg/dL.

The second set of C-R functions discussed in section II.B.2.b is drawn
from a larger group of studies, although these studies include groups of
children with higher blood Pb levels (CD, pp. 8-78 to 8-80) that are
less representative of the distribution of blood Pb levels in the
current U.S. population of children. This second set of C-R functions is
represented by a median of -0.9 IQ points per µg/dL blood Pb (CD, p.
8-80).

In applying this alternative evidence-based framework, as with the
adapted 1978 framework, we recognize uncertainty in our estimates for
the two input parameters (air-to-blood ratio and C-R function slope).
Accordingly, we have considered combinations of parameter estimates that
are potentially supportable within this framework in associating various
standard levels with the estimated magnitudes of air-related mean IQ
loss that would likely be prevented by keeping exposed populations below
such standard levels. With regard to the C-R functions we have drawn
estimates from both sets of functions. For the first set of C-R
functions (for lower blood Pb levels), we have relied on the upper and
lower-end values to provide a range at lower blood Pb levels, and have
focused on the LLL function at somewhat higher blood Pb levels (e.g.,
above the current mean for U.S. children). From the second set of C-R
functions, we have relied on the median estimate across the range of
blood Pb levels considered. For air-to-blood ratios, we have focused on
the somewhat central estimate of 1:5 as above, and also provided
estimates using higher and lower estimates within the range supported by
the evidence. These estimates are discussed and presented in a staff
memo (Murphy, 2008), available in the docket.

We turn first to the alternative standard levels identified by the first
evidence-based framework above (0.10 to 0.20 µg/m^3^). Using the
approximately central air-to-blood ratio of 1:5 with the range of slopes
from the first set of C-R functions (for lower blood Pb levels)
indicates an air-related mean IQ loss estimate of 0.9 to 1.5 points for
a population of children exposed at the standard level of 0.10 µg/m^3^.
Similarly, the air-related mean IQ loss estimate for a standard level of
0.20 µg/m^3^ is 1.7 to 2.9 points. Alternatively, using the air-to-blood
ratio of 1:5 and the slope from the second set of C-R functions (from
blood Pb levels extending up to 10 µg/dL) in the calculation indicates
an air-related mean IQ loss of 0.5 points for a population of children
exposed at the standard level of 0.10 µg/m^3^; the corresponding
air-related mean IQ loss estimate for a standard level of 0.20 µg/m^3^
is 0.9 points.

As mentioned above, we recognize uncertainty in the air-to-blood values,
and have accordingly also considered estimates for air-to-blood ratio
that are lower and higher than the 1:5 value used above. Accordingly, we
note that using a lower air-to-blood ratio, such as 1:3 (low end of
range from evidence) generally results in lower air-related IQ loss
estimates (approximately 40% lower than those using a ratio of 1:5).
Similarly, use of a higher air-to-blood ratio, such as 1:7, yields
higher air-related mean IQ loss estimates (approximately 40% higher than
those using a ratio of 1:5).

In addition to considering standard levels indicated by the adapted 1978
framework above, we have also considered higher standard levels, up to
the highest alternative level included in the risk assessment (e.g.,
0.30 to 0.50 µg/m^3^). Using the 1:5 air-to-blood ratio with first set
of C-R functions indicates an air-related mean IQ loss estimate of
approximately 3 points for a population of children exposed at the
standard level of 0.30 µg/m^3^. The corresponding air-related mean IQ
loss estimate for a standard level of 0.50 µg/m^3^ is approximately 4
points. Alternatively, using the slope from the second set of C-R
functions indicates an air-related mean IQ loss estimate of 1.4 points
for a population of children exposed at the standard level of 0.30
µg/m^3^, with the corresponding air-related mean IQ loss estimate for a
standard level of 0.50 µg/m^3^ being 2.3 points IQ loss. Using the 1:3
air-to-blood ratio with the first set of C-R functions indicates an
air-related mean IQ loss estimate of 1.5 to 2.6 points for a population
of children exposed at the standard level of 0.30 µg/m^3^. The
corresponding air-related mean IQ loss estimate for a standard level of
0.50 µg/m^3^ is approximately 3 points. Alternatively, using the 1:3
air-to-blood ratio and the slope for the second set of C-R functions
indicates an air-related mean IQ loss estimate of 0.8 points for a
population of children exposed at the standard level of 0.30 µg/m^3^,
and an air-related mean IQ loss estimate of 1.4 points for a population
of children exposed at the standard level of 0.50 µg/m^3^.

Further, we have also considered lower standard levels, down to the
lowest alternative levels included in the risk assessment (e.g., 0.05 to
0.02 µg/m^3^). For example, across both sets of C-R functions and the
range of air-to-blood ratios considered above (1:3 to 1:7), a standard
level of 0.05 µg/m^3^ indicates an air-related mean IQ loss of
approximately 0.1 to 1 point. The estimates are approximately 50% lower
at the standard level of 0.02 µg/m^3^.

b\. Exposure- and Risk-based Considerations

To inform judgments about a range of levels for the standard that could
provide an appropriate degree of public health protection, in addition
to considering the health effects evidence, EPA also considered the
quantitative estimates of exposure and health risks attributable to
air-related Pb upon meeting specific alternative levels of alternative
Pb standards and the uncertainties in the estimated exposures and risks,
as discussed above in Section III.B. As discussed above, the risk
assessment conducted by EPA is based on exposures that have been
estimated for children of less than 7 years of age in several case
studies. The assessment estimated the risk of adverse neurocognitive
effects in terms of IQ loss associated with total and air-related Pb
exposures, including incidence of different magnitudes of IQ loss in the
three location-specific case studies. In so doing, EPA is mindful of the
important uncertainties and limitations that are associated with the
exposure and risk assessments. For example, with regard to the risk
assessment important uncertainties include those related to estimation
of blood Pb C-R functions, particularly for blood Pb concentrations at
and below the lower end of those represented in the epidemiological
studies characterized in the Criteria Document.

EPA also recognizes important limitations in the design of, and data and
methods employed in, the exposure and risk analyses. For example, the
available monitoring data for Pb relied upon for estimating current
conditions for the urban case studies is quite limited, in that monitors
are not located near many of the larger known Pb sources, which results
in potential underestimation of current conditions, and there is
uncertainty about the proximity of existing monitors to other Pb sources
potentially influencing exposures, such as old urban roadways and areas
where housing with Pb paint has been demolished or has undergone
extensive exterior renovation. All of these limitations raise
uncertainty as to whether these data adequately capture the magnitude of
ambient Pb concentrations to which the target population is currently
exposed. Additionally, EPA recognizes that there is not sufficient
information available to evaluate all relevant sensitive groups (e.g.,
adults with chronic kidney disease) or all Pb-related health effects
(e.g., neurological effects other than IQ loss, immune system effects,
adult cardiovascular or renal effects), and the scope of our analyses
was generally limited to estimating exposures and risks in case studies
intended to illustrate a variety of Pb exposure situations across the
U.S., with three of them focused on specific areas in three cities. As
noted above, however, coordinated intensive efforts over the last 20
years have yielded a substantial decline in blood Pb levels in the
United States. Recent NHANES data (2003-2004) yield blood lead level
estimates for children age 1 to 5 years of 1.6 μg/dL (median) and 3.9
μg/dL (90th percentile). These median and 90th percentile national-level
data are lower than modeled values generated for the three
location-specific urban case studies current conditions scenarios
(described in section II.C.3.a above). 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.

As described in section II.C.2.e, we also recognize limitations in our
ability to characterize the contribution of air-related Pb to total Pb
exposure and Pb-related health risk. As a result, we have approximated
estimates for the air-related pathways, bounded on the low end by
exposure/risk estimated for the "recent air" category and on the upper
end by the exposure/risk estimated for the "recent air" plus "past air"
categories.[^102]

We generally focus in this discussion on risk estimates derived using
the LLL (log-linear with low exposure linearization) C-R function.

In considering the results of the risk assessment, we note that the risk
estimates are roughly consistent with and generally supportive of the
evidence-based mean air-related IQ loss estimates described above
(section II.E.3.a). For example, at a standard level of 0.20 µg/m^3^,
the evidence-based approach indicates estimates of mean air-related IQ
loss ranging from less than 1 to approximately 3 points IQ loss, while
the median air-related risk estimates for this level in the general
urban case study are represented by a lower bound near 1 point IQ loss
and an upper bound near 3 points IQ loss. The corresponding upper bound
air-related IQ loss estimate for the primary Pb smelter case study
subarea is 3.7 points. Alternatively, at a standard level of 0.50
µg/m^3^, the evidence-based approach indicates estimates of mean
air-related IQ loss ranging from approximately 1.5 points to greater
than 4 points, while the median air-related risk estimates for this
level for the general urban case study are represented by a lower bound
near 2 points IQ loss and an upper bound just below 4 points IQ loss
(section II.C.3.b). The corresponding upper bound air-related IQ loss
estimate for the primary Pb smelter case study subarea is 4.5 points.

As mentioned above (section II.E.3.a), the Staff Paper conclusions on
level for the primary Pb standard built on staff 's conclusion that the
overall body of evidence clearly calls into question the adequacy of the
current standard with regard to health protection afforded to at-risk
populations. Drawing from both consideration of the evidence and
consideration of the quantitative risk and exposure information
(described in section II.E.3.b), staff concluded that the available
information provides strong support for consideration of a range of
standard levels that are appreciably below the level of the current
standard in order to provide increased public health protection for
these populations, with support for this conclusion. With regard to the
risk estimates, the Staff paper recognized that, to the extent one
places weight on risk estimates for the lower standard levels, those
estimates may suggest consideration of a range of levels that extend
down to the lowest levels assessed in the risk assessment, 0.02 to 0.05
µg/m^3^. In summary, the Staff Paper concluded that a level for the
standard set in the upper part of the staff recommended range (0.1-0.2
µg/m3, particularly with a monthly averaging time) is well supported by
the evidence and also supported by estimates of risk associated with
air-related Pb that overlap with the range of IQ loss that may
reasonably be judged to be highly significant from a public health
perspective, and is judged to be so by CASAC. Further, the Staff Paper
concluded that a standard set in the lower part of the range would be
more precautionary and would place weight on the more highly uncertain
range of estimates from the risk assessment.

c\. CASAC Advice and Recommendations and Public Comments

Beyond the evidence- and risk/exposure-based information discussed
above, EPA's consideration of the level for the TSP-based standard also
takes into account the advice and recommendations of CASAC, based on
their review of the Criteria Document, the Staff Paper and the related
technical support document, and the ANPR, as well as comments from the
public on drafts of the Staff Paper and related technical support
document and the ANPR.

In their advice to the Agency during this review CASAC has recognized
the importance of both the health effects evidence and the exposure and
risk information in selecting the level for the standard (Henderson,
2007a, 2007b, 2008). In two separate letters sent prior to publication
of the ANPR, CASAC stated that it is the unanimous judgment of the CASAC
Lead Panel that the primary NAAQS should be "substantially lowered" to
"a level of about 0.2 µg/m^3^ or less," reflecting their view of the
health effects evidence (Henderson, 2007a,b). In their most recent
letter, reflecting their review of the ANPR and Staff Paper, the Panel
reiterated their earlier judgment, stating that "*\[t\]he Committee
unanimously and fully supports Agency staff's scientific analyses in
recommending the need to substantially lower the level of the primary
(public-health based) Lead NAAQS, to an upper bound of no higher than
0.2* *µg/m^3^ with a monthly averaging time*."

The CASAC Pb Panel also provided advice regarding how the Agency should
consider IQ loss estimates derived from the risk assessment in selecting
a level for the standard (Henderson, 2007a). The Panel stated that they
consider a population loss of 1-2 IQ points to be "highly significant
from a public health perspective". 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."

Among the many public comments the Agency has received in this review
regarding the level of the standard, the overwhelmingly majority
recommended appreciable reductions in the level, e.g., setting it at 0.2
µg/m^3^ or less, while only a few recommended that the Agency make no or
only a modest adjustment. Among the comments recommending appreciable
reduction, many noted the importance of considering exposures and risks
to vulnerable and susceptible populations. Some recognized that blood Pb
levels are disproportionately elevated among children living in poverty,
and recommended more explicit consideration of issues of environmental
justice. And some comments also noted the need for the standard to
provide an adequate margin of safety, indicating that such a need might
provide support for consideration of much lower levels. The American
Academy of Pediatrics recommended that EPA set the level at 0.2 or
lower, and also recommended that EPA consider the approach developed by
the State of California Environmental Protection Agency (Cal-EPA) for
the purposes of school site assessment, which has at its goal prevention
of a rise in blood Pb level that Cal-EPA has predicted to be associated
with an incremental increase estimated to decrease IQ by 1 point.

d\. Administrator's Proposed Conclusion Concerning Level

For the reasons discussed below, and taking into account information and
assessments presented in the Criteria Document and Staff Paper, the
advice and recommendations of CASAC, and the public comments to date,
the Administrator proposes to revise the existing primary Pb standard.
Specifically, the Administrator proposes to revise the level of the
primary Pb standard, defined in terms of the current Pb-TSP indicator,
to within a range from 0.10 to 0.30 µg/m^3^.

Further, in recognition of alternative views of the science, the
exposure and risk assessments and the uncertainties inherent in these
assessments, and the appropriate public health policy responses based on
the currently available information, the Administrator also solicits
comments on whether to proceed instead with alternative levels of a
primary Pb-TSP standard within ranges from above 0.30 µg/m^3^ to 0.50
µg/m^3^ and below 0.10 µg/m^3^ down through the lowest level analyzed in
this review (0.02 µg/m^3^). Based on the comments received and the
accompanying rationales, the Administrator may adopt other standards
within the range of the alternative levels identified above in lieu of
the standards he is proposing today. In addition, as discussed below,
the Administrator also solicits comments on the concept of setting the
standard at a level of zero.

The Administrator's consideration of alternative levels of the primary
Pb-TSP standard builds on his proposed conclusion, discussed above in
section II.D.4, that the overall body of evidence indicates that the
current Pb standard is not requisite to protect public health with an
adequate margin of safety because it does not provide sufficient
protection and that the standard should be revised to provide increased
public health protection, especially for members of at-risk groups,
notably including children, against an array of adverse health effects.
These effects range from IQ loss, a health outcome that could be
quantified in the risk assessment, to health outcomes that could not be
directly estimated, including decrements in other neurocognitive
functions and other neurocognitive functions, ther neurological effects,
immune system effects and early effects related to anemia, as well as
cardiovascular and renal effects in adults. In reaching a proposed
decision about the level of the Pb primary standard, the Administrator
has considered: the evidence-based considerations from the Criteria
Document and the Staff Paper and those discussed above ; the results of
the exposure and risk assessments discussed above and in the Staff
Paper, giving weight to the exposure and risk assessments as judged
appropriate; CASAC advice and recommendations, as reflected in
discussions of the Criteria Document and Staff Paper and the ANPR at
public meetings, in separate written comments, and in CASAC's letters to
the Administrator; EPA staff recommendations; and public comments
received during the development of these documents, either in connection
with CASAC meetings or separately. In considering what standard is
requisite to protect public health with an adequate margin of safety,
the Administrator is mindful that this choice requires judgment based on
an interpretation of the evidence and other information that neither
overstates nor understates the strength and limitations of the evidence
and information nor the appropriate inferences to be drawn.

In reaching a proposed decision on a range of levels for a revised
standard, as in reaching a proposed decision on the adequacy of the
current standard, the Administrator considered the evidence in the
context of two evidence-based frameworks, as described above in section
II.E.3.a. As a general matter, in considering these evidence-based
frameworks, the Administrator recognizes that in the case of Pb there
are several aspects to the body of epidemiological evidence that add
complexity to the selection of an appropriate level for the primary
standard. As discussed above, these complexities include evidence based
on blood Pb as the dose metric, exposure pathways that are both
air-related and nonair-related, and the absence of any discernible
threshold levels in the health effects evidence. Further, the
Administrator recognizes that there are a number of important
uncertainties and limitations inherent in the available health effects
evidence and related information, including uncertainties in the
evidence of associations between total blood Pb and neurocognitive
effects in children, especially at the lowest blood Pb levels evaluated
in such studies, as well as uncertainties in key parameters used in
these evidence-based frameworks, such as air-to-blood ratio,
contribution to total blood Pb from nonair sources, and C-R functions.
In addition, the Administrator recognizes that there are currently no
commonly accepted guidelines or criteria within the public health
community that would provide a clear basis for reaching a judgment as to
the appropriate degree of public health protection that should be
afforded to neurocognitive effects such as IQ loss in children.

Looking first to the adapted 1978 framework discussed above, the
Administrator notes that this approach takes as the appropriate public
health goal the identification of an ambient air lead level that can be
expected to keep the mean blood lead level of an exposed population of
children at or below the lowest level at which a statistically
significant association has been demonstrated between blood lead level
and neurocognitive effects. That is, it is based on identifying the
point below which the evidence is judged too uncertain to use as a basis
for the standard. Based on the available evidence, as discussed above in
section II.E.3.a, the Administrator recognizes that the lowest
population mean blood Pb level for which a statistically significant
association with effects has been demonstrated is 1.7 µg/dL, from the
Lanphear et al. (2000) study, and concludes that this level is an
appropriate target population mean blood Pb level for the child
population to use in the context of this evidence-based framework. The
Administrator also concludes that it is appropriate to use values for
the other relevant parameters in this framework -- an air-to-blood ratio
and a nonair blood Pb level -- that are approximately central within the
ranges of these parameters that are generally supported by the available
evidence (i.e., an air-to-blood ratio of 1:5 and a nonair blood Pb level
of 1.2 µg/dL). Applying the adapted 1978 framework with this set of
parameters, as described above in section II.E.3.a, yields a standard
level of approximately 0.10 µg/m^3^. In recognition of uncertainties is
these parameters, the Administrator further concludes that it is also
appropriate to consider plausible values for these parameters that are
at the lower end of the ranges that are generally supportable. Applying
the adapted 1978 framework with this alternative set of parameters
(i.e., an air-to-blood ratio of 1:3 and a nonair blood Pb level of 1.0
µg/dL), as described above in section II.E.3.a, yields a standard level
of approximately 0.20 µg/m^3^. The Administrator judges that this
evidence-based adapted 1978 framework is an appropriate basis for
reaching a proposed decision on the standard level, and proposes to
conclude that based on this framework a standard within the range of
0.10 to 0.20 µg/m^3^ would be requisite to protect public health with an
adequate margin of safety.

The Administrator also considered the alternative evidence-based
framework to further inform his proposed decision on the standard level.
The Administrator notes that this alternative framework shifts the focus
from identifying an appropriate target mean blood Pb level (atwhich
statistically significant health effects have been found), and instead
focuses on quantitative relationships between air-related Pb and
neurocognitive effects (e.g., IQ loss) in children, building on
recommendations from CASAC to consider the body of evidence in a more
quantitative manner. The Administrator recognizes that this framework is
another approach for identifying the appropriate ambient air level of Pb
that will adequately protect children from health effects that are
considered adverse and that are associated with their exposure to
air-related Pb. More specifically, this framework is premised on a
public health goal of selecting a standard level that would prevent
air-related IQ loss (and related effects) of a magnitude judged by the
Administrator to be of concern in populations of children exposed to the
level of the standard, taking into consideration uncertainties inherent
in such estimates and providing an adequate margin of safety. In
addition to this public health policy judgment regarding IQ loss, the
Administrator notes that two other parameters are relevant to this
framework -- an air-to-blood ratio, as in the adapted 1978 framework,
and a C-R function for population IQ response associated with blood Pb
level. Based on the discussion of these parameters in section II.E.3.a
above, the Administrator concludes that it is appropriate to use the
first set of C-R functions reflecting lower blood Pb levels that are
more representative of, although still somewhat higher, than the
distribution of blood Pb levels in the current U.S. population of
children; and, reflecting uncertainties in this parameter, that it is
also appropriate to consider the second set of C-R functions based on a
larger range of blood Pb levels that is less representative of the
current distribution. In addition, the Administrator concludes that is
appropriate to use an air-to-blood ratio that is approximately central
within the range generally supported by the available evidence (i.e.,
1:5), as well as to also consider ratios that are at the lower and upper
ends of that range (i.e., 1:3 and 1:7) reflecting uncertainties in this
parameter.

With regard to making a public health policy judgment as to the
appropriate level of protection against air-related IQ loss in
populations of children (and related effects), the Administrator first
notes that ideally air-related (as well as other) exposures to
environmental Pb would be reduced to the point that no IQ impact in
children would occur. The Administrator recognizes, however, that in the
case of setting a NAAQS, he is required to make a judgment as to what
degree of protection is requisite to protect public health with an
adequate margin of safety. The NAAQS must be sufficient but not more
stringent than necessary to achieve that result, and does not require a
zero-risk standard. Considering the advice of CASAC and public comments
on this issue, notably including the comments of the American Academy of
Pediatrics, the Administrator proposes to conclude that an air-related
population mean IQ loss within the range of 1 to 2 points could be
significant from a public health perspective, and that a standard level
should be selected to avoid this degree of impact from exposure to
air-related Pb.

In considering the application of this air-related Pb IQ loss framework,
the Administrator first focused on the use of the first set of C-R
functions in conjunction with the approximately central air-to-blood
ratio, which indicates that a standard level of 0.10 µg/m^3^ would limit
the estimated degree of impact on population mean IQ loss from
air-related Pb to no more than 1.5 points, the mid-point of the proposed
range of protection. The Administrator notes that a standard set at this
level (0.10 µg/m^3^) would limit the estimated degree of air-related
impact on population mean IQ loss to a range from less than 1 point to
around 2 points, using the full range of plausible air-to-blood ratios
together with the first set of C-R functions. In recognition of
uncertainties in the C-R functions, the Administrator also focused on
the use of the second set of C-R functions in conjunction with the
approximately central air-to-blood ratio, which indicates that a
standard level of 0.30 µg/m^3^ would also limit the estimated degree of
impact on population mean IQ loss from air-related Pb to just below 1.5
points. In also considering the full range of plausible air-to-blood
ratios together with the second set of C-R functions, the Administrator
notes that a standard set at this level (0.30 µg/m^3^) would also limit
this estimated degree of air-related impact on population mean IQ loss
to less than 1 point at the lower end and to less than 2 points at the
upper end. The Administrator judges that this alternative evidence-based
air-related Pb IQ loss framework is also an appropriate basis for
reaching a proposed decision on the standard level, and proposes to
conclude that based on this framework, in conjunction with a proposed
judgment to limit air-related population mean IQ loss to within a range
of 1 to 2 points, a standard within the range of 0.10 to 0.30 µg/m^3^
would be requisite to protect public health with an adequate margin of
safety.

The Administrator has also considered the results of the exposure and
risk assessments conducted for this review to provide some further
perspective on the potential magnitude of air-related IQ loss. The
Administrator finds that quantitative assessments provide a useful
perspective on the risk from air-related Pb. However, in light of the
important uncertainties and limitations associated with these
assessments, as discussed above in sections II.C and II.E.3.b, for
purposes of evaluating potential new standards, the Administrator places
less weight on the risk estimates than on the evidence-based
assessments. Nonetheless, the Administrator finds that the risk
estimates are roughly consistent with the evidence-based air-related IQ
loss estimates described above, as discussed above in section II.E.3.b.
This lends support to the proposed range based on this evidence-based
framework.

In the Administrator's view, the above considerations, taken together,
provide no evidence- or risk-based bright line that indicates a single
appropriate level. Instead, there is a collection of scientific evidence
and judgments and other information, including information about the
uncertainties inherent in many relevant factors, that needs to be
considered together in making this public health policy judgment, and
selecting a standard level from a range of reasonable values. Based on
consideration of the entire body of evidence and information available
at this time, as well as the recommendations of CASAC and public
comments, the Administrator proposes that a standard within the range of
0.10 to 0.30 µg/m^3^ would be requisite to protect public health,
including the health of sensitive groups, with an adequate margin of
safety. The Administrator notes that this range encompasses the specific
level of 0.20 µg/m^3^ and the upper end of the range recommended by
CASAC and by many public commenters. A standard level within this range
(0.10 to 0.30 µg/m^3^) would reduce the risk of a variety of health
effects associated with exposure to Pb, including effects indicated in
the epidemiological studies at low blood Pb levels, particularly
including neurological effects in children, and cardiovascular and renal
effects in adults.

Because there is no bright line clearly directing the choice of level
within this reasonable range, the choice of what is appropriate,
considering the strengths and limitations of the evidence, and the
appropriate inferences to be drawn from the evidence and the exposure
and risk assessments, is a public health policy judgment. To further
inform this judgment, the Administrator solicits comment on the two
evidence-based frameworks considered by the Agency and appropriate
parameter values to be considered in the application of those
frameworks. The Administrator also solicits comment on the degree of
impact of air-related Pb on IQ loss and other related neurocognitive
effects in children considered to be significant from a public health
perspective, and on the use of this metric as a basis for selecting a
standard level.

In considering the available evidence as discussed above, the
Administrator judges that a standard level above 0.30 µg/m^3^ would not
be appropriate because such a level would not provide sufficient public
health protection. In reaching this judgment, the Administrator notes
that the adapted 1978 framework does not provide a rationale for any
standard level above 0.20 µg/m^3^ when the Lanphear et al. (2000) study
is used to provide a target population mean blood Pb level. The
Administrator also notes that the alternative air-related IQ loss
framework indicates that for standard levels above 0.30 µg/m^3^ up to
0.50 µg/m^3^, the estimated degree of impact on population mean IQ loss
from air-related Pb would range from above 2 points to 5 points or more
with the use of the first set of C-R functions and the full range of
supportable air-to-blood ratios, and would extend from somewhere within
the proposed range of 1 to 2 points IQ loss to above that range when
using the second set of C-R functions and the full range of supportable
air-to-blood ratios. The Administrator proposes to conclude that there
is sufficient certainty in the existence and magnitude of air-related Pb
effects at the higher blood Pb levels that would be allowed by standards
at these levels, and the degree of public health protection that such
levels would likely forego, to judge that levels above 0.30 µg/m^3^
would be higher than what is requisite to protect public health,
including the health of at-risk groups, with an adequate margin of
safety.

In addition, the Administrator also judges that a standard level below
0.10 µg/m^3^ would not be appropriate because such a level would provide
a degree of public health protection beyond that which is necessary. In
reaching this judgment, the Administrator notes that the adapted 1978
framework would indicate such levels only with the use of values for the
relevant parameters that are at the highest end of the ranges that are
generally supportable. The Administrator also notes that the alternative
air-related IQ loss framework indicates that for standard levels below
0.10 µg/m^3^, including the levels of 0.05 and 0.02 µg/m^3^ that were
included in the Agency's assessment, the estimated degree of impact on
population mean IQ loss from air-related Pb would generally be below,
ranging to well below, the proposed range of 1 to 2 points air-related
population mean IQ loss even when using the first set of C-R functions
and the full range of supportable air-to-blood ratios. The Administrator
proposes to conclude that the greater uncertainty in the existence and
magnitude of air-related Pb effects at these levels, and the lack of a
clear basis for concluding that the degree of public health protection
that such levels would likely afford in warranted, provide the basis for
judging that levels below 0.10 µg/m^3^ would be lower than what is
requisite to protect public health, including the health of at-risk
groups, with an adequate margin of safety.

For the reasons discussed above, the Administrator proposes to revise
the level of the primary Pb standard, defined in terms of the current
Pb-TSP indicator, to within the range of 0.10 to 0.30 µg/m^3^.

Having reached this decision based on the interpretation of the
evidence, the evidence-based frameworks, and the public health policy
judgments described above, the Administrator recognizes that other
interpretations, frameworks, and judgments are possible. For example,
recognizing the complexity and limitations of the epidemiological
evidence, and the potential for confounding in any specific study, the
use of the Lanphear et al. (2000) study as the basis for an appropriate
target population mean blood Pb level to use in the context of the
adapted 1978 evidence-based framework could be judged to place undue
weight on this single study. There are also potential alternative views
as to the range of values for relevant parameters (e.g., air-to-blood
ratio, nonair Pb, C-R function) in each of the evidence-based frameworks
that might be considered supportable and the relative weight that might
appropriately be placed on any specific value for these parameters
within such ranges. In addition, the Administrator recognizes that there
may be other views as to the appropriate degree of public health
protection that should be afforded in terms of air-related population
mean IQ loss in children that would provide support for alternative
standard levels different from the proposed range. Consistent with the
goal of soliciting comment on a wide array of issues, the Administrator
solicits comment on these and other issues.

More specifically, the Administrator solicits comment on alternative
levels of a primary Pb-TSP standard within the range of above 0.30
µg/m^3^ to 0.50 µg/m^3^. In considering the air-related IQ loss
framework, a standard level as high as 0.50 µg/m^3^ would still limit
the estimated degree of impact on population mean IQ loss from
air-related Pb to no more than 1.5 points, the mid-point of the proposed
range of protection, in the case when the second set of C-R functions is
used in conjunction with the lowest air-to-blood ratio within the range
characterized by EPA as the supportable range. Comment is solicited on
levels within this range and the associated rationale for selecting such
a level in terms of the appropriate weight to place on relevant
parameter values that may extend to values outside of EPA's range of
supportable values, or alternative frameworks that might provide a
perspective on such a range.

In addition, the Administrator solicits comment on alternative levels
within the range of below 0.10 µg/m^3^ down through the lowest level
analyzed in this review (0.02 µg/m^3^). In considering either of the
evidence-based frameworks discussed above, a standard level within this
range would likely provide a degree of protection in terms of
air-related population mean IQ loss that is greater than the proposed
range based on the use of any of the relevant parameter values within
the ranges characterized by EPA as supportable. Comment is solicited on
levels within this range and the associated rationale for selecting such
a level in terms of the appropriate weight to place on relevant
parameter values that may extend to values outside of EPA's range of
supportable values, or alternative public health policy judgments as to
the degree of protection that is warranted, or the appropriate weight to
place on the results of the risk assessment.

More broadly, as discussed above, the Administrator recognizes that Pb,
like NAAQS pollutants such as PM and ozone, can be considered a
non-threshold pollutant. At this time no threshold has been identified
below which we are scientifically confident that there is no risk of
harm. Currently there is no scientific certainty as to whether or not
there is such a threshold, as the scientific evidence does not resolve
that question. Given the current state of scientific evidence, we
recognize that there is no level below which we can say with scientific
confidence that there is no risk of harm from exposure to ambient air
related lead.

As discussed above, EPA is proposing a range of levels for the primary
Pb NAAQS, with the range extending down to 0.10 µg/m^3^. This range
reflects the Administrator's proposed conclusion that lower levels would
be more than necessary to protect public health with an adequate margin
of safety. This proposed conclusion is based in large part on EPA's
evaluation of the evidence, recognizing important uncertainties in the
scientific evidence and related assessments, and reflects the proposed
public heath policy judgment of the Administrator on theses issues. As
discussed above, these uncertainties stem in part from the complexities
of determining the health impact of air-related Pb given the multi-media
exposure pathways for exposure to lead. The major areas of uncertainty
include the appropriate air-to-blood ratio; the apportionment of Pb
between air-related and nonair Pb; the increasing uncertainty at lower
blood Pb levels as to the existence, nature, and degree of health
effects; and the uncertainty over the public health significance of
smaller and smaller impacts on IQ or other similar neurocognitive
metrics from exposure to air-related Pb. In recognition of such
uncertainties, EPA is also soliciting comment on a lower range of
standard levels down through 0.02 µg/m^3^, the lowest level evaluated in
the Agency's assessments.

In so doing, EPA fully recognizes that a standard set at the lowest
proposed level of 0.10 µg/m^3^, or any non-zero level, would not be a
risk-free standard. As in numerous prior NAAQS reviews, EPA believes
that section 109 of the CAA does not require that EPA set a risk-free
standard. Instead, EPA is to recognize and take risk into account, and
set a standard that is requisite to protect public health with an
adequate margin of safety based on the currently available information.
This calls for a public health policy judgment informed by many factors,
most notably the nature and severity of the health effects at issue, the
size of the population(s) at risk, and the kind and degree of
uncertainties involved. After considering all of these factors in this
review, the Administrator's proposed judgment is that a standard set
below 0.10 µg/m^3^ would not satisfy this statutory directive.

The Administrator recognizes that the current state of the scientific
evidence clearly indicates that health effects from Pb occur at much
lower blood Pb levels than we understood in the past, and that the
appropriate level for ambient air Pb is much lower than we thought in
the past. Further the Administrator expects that, as time goes by,
future scientific studies will continue to enhance our understanding of
Pb, and anticipates that such studies might lead to a situation where
there is very little if any remaining uncertainty about human health
impacts from even extremely low levels of Pb in the ambient air. This
has the potential to raise fundamental questions in the future as to how
the Agency can continue to reconcile such evidence with the statutory
provision calling for the NAAQS to be set at a level that is requisite
to protect public health with an adequate margin of safety. Faced, in
the future, with scientific evidence that could reasonably be
interpreted as demonstrating that any ambient Pb level above zero
contributes to adverse health effects in at-risk populations, some might
conclude that the only standard requisite to protect public health with
an adequate margin of safety would be a standard set at zero. While
EPA's proposed conclusions on the current scientific evidence and an
appropriate standard based on that evidence clearly differ from such a
view, EPA nonetheless believes that inviting comment in this review on
the views described above and the issues raised by such circumstances is
appropriate.

More specifically, EPA invites comment on whether it would ever be
appropriate to set a NAAQS for Pb at a level of zero. Comments on this
question might address issues such as: the level of scientific certainty
that would be needed to support such a decision; the level of harm, e.g.
severity of health effect and size of affected population, that would be
needed to support such a decision; and whether there are normative or
quantitative criteria that could be applied in deciding whether or not
to set a standard at zero. EPA invites comment on the broad range of
scientific, policy, and legal issues associated with deciding whether a
level of zero would be appropriate.

4\. Level for a Pb NAAQS with a Pb-PM~10~ Indicator

EPA is requesting comment on the option of revising the indicator for
the Pb NAAQS from Pb-TSP to Pb-PM~10~, based on low-volume sample
collection as discussed in section II.E.1and section V.A. In this
section, we discuss considerations important to selection of a level for
such a Pb-PM~10~-based standard (section II.E.4.a) and CASAC's advice
and public comments on this issue (section II.E.4.b). A range of
alternative levels for a Pb NAAQS with a Pb-PM~10~ indicator under
consideration and on which EPA is soliciting comment is presented in
II.E.4.c.

a.  Considerations with Regard to Particles not Captured by PM~10~

In the course of deciding to propose the Pb-TSP indicator approach as
described in section II.E.1 above, EPA has noted the important role of
both respirable and non-respirable Pb particles in air-related Pb
exposure of concern and the lesser capture of these particles by PM~10~
samplers compared to TSP samplers. We recognize that the health evidence
indicates that Pb in all particle size fractions, not just respirable
Pb, contributes to Pb in blood and to associated health effects.
Further, the quantity of Pb in ambient particles with the potential to
deposit (indoors and outdoors, leading to a role in ingestion pathways)
is a key contributor to air-related exposure, and these particles
include ultra-coarse mode particles that are not captured by PM~10~
samplers (as discussed in section II.E.1 above). In recognition of these
considerations, both of the indicator options discussed in this notice
recognize the need for an adjustment, either when considering the
proposed optional use of Pb-PM~10~ data for comparison with a
Pb-TSP-based NAAQS, or when considering a level for a NAAQS based on the
option of a Pb-PM~10~ indicator.

With regard to the proposed option that retains Pb-TSP as the indicator
and allows the use of Pb-PM~10~ data, collected by low-volume sampling
(as described in section II.E.1and section V.A), section II.E.1
identifies two ranges from which scaling factors could be chosen that
would be applied to the Pb-PM~10~ measurements to derive surrogate
Pb-TSP concentrations for use in making comparisons to the NAAQS. In
recognition of the influence of proximity to sources on the relationship
between Pb-TSP and Pb-PM~10~ measurements for source types with a high
fraction of ultra-coarse particles containing Pb, different scaling
factors are proposed for source-oriented monitoring sites and
nonsource-oriented monitoring sites, with the proposed ranges for these
factors being 1.6 to 2.0 and 1.0 to 1.4[^103], respectively (as
described in section II.E.1). The proposed ranges have been developed
based on analyses of the available collocated Pb-TSP and Pb-PM~10~ data
(Schmidt, 2008), and recognition of variability and uncertainty inherent
in this data set. The proposed data handling requirements for use of
Pb-PM~10~ data with a Pb-TSP standard additionally provide for the
collection and consideration of site-specific paired collocation data,
which may lead to alternative site-specific scaling factors (section
IV.B).

In considering the magnitude of a potential adjustment to derive a
standard level for a Pb-PM~10~-based NAAQS on which we are soliciting
comment, we note the inherent variability in the TSP sampling
methodology which will contribute variability to relationships derived
between Pb-PM~10~ and Pb-TSP data. We also note the influence on such
relationships of proximity to sources of Pb particles that would not be
captured by PM~10~ samplers. This latter influence is evident in the
difference between the two ranges of scaling factors proposed in section
IV. The data supporting the range for source-oriented scaling factors
proposed in section IV, as discussed in Schmidt (2008), indicate the
potential, in areas influenced by some types of sources (e.g., Pb
smelters), for PM~10~ samplers to capture as little as approximately 50%
of the Pb that is measured with Pb-TSP monitors. At sites not known to
be near Pb sources, PM~10~ samplers in some cases capture as much as
100% of the Pb that is measured with Pb-TSP monitors (Schmidt, 2008).
EPA recognizes that the concern over uncertainty in the TSP sampling
methodology carries over to concern about uncertainty in the scaling
factors as well. If EPA adopts a PM~10~ indicator, the approach of using
two adjustment factors representing source-oriented and non
source-oriented sites and the approach of site-specific adjustment
factors would not be used in setting a standard level, recognizing that
the concern over uncertainty in the TSP sampling methodology carries
over to concern about uncertainty in the scaling factors as well. The
full complexity of the site-to-site variability in the ambient mix of
ultra-coarse particles would have to be simplified into a single
adjustment factor.[^104]

We are also aware of the limitations of the dataset available on which
to base these decisions, including those related to the quantity of
collocated measurements and particularly the very limited number of
source-influenced monitors for which such measurements are available,
and the correspondingly limited number of types of sources represented.
Moreover, the available collocation measurements suggesting the
above-referenced 50% figure in a source-influenced location are from
conditions, in which ambient concentrations were just above the current
standard level and well above the proposed range of levels

If the contributing emissions sources had been controlled so that local
concentrations were within or near this range, it is unclear whether the
relationship between Pb-PM~10~ and Pb-TSP data would have been different
or not. The Pb-TSP concentrations at sites in the dataset analyzed that
were not known to be source-influenced were well below the proposed
range of the standard, leaving uncertainty about typical proportions of
ultra-coarse particles in nonsource areas with Pb-TSP concentrations
near the proposed range of levels.

b\. CASAC Advice

As noted above, CASAC has described the use of an adjustment of the
NAAQS level to accommodate the loss of the ultra-coarse Pb particles
that are important contributions to Pb exposure but that are excluded by
PM~10~ samplers (section II.E.1). For example, in discussion of the
recommendation for the Agency to revise the Pb NAAQS indicator to
Pb-PM~10~ (using low-volume samplers) in their February 2007 letter, the
CASAC Pb Panel stated that "Presumably a downward scaling of the level
of the Lead NAAQS could accommodate the loss of very large coarse-mode
lead particles..." (Henderson, 2007a). With regard to the magnitude of
such scaling, CASAC has recognized the usefulness of some "short period
of concurrent PM~10~ and TSP lead sampling" to "help develop
site-specific scaling factors at sites with highest concentrations"
(Henderson, 2007a) and also indicated an expectation that, in general,
Pb-PM~10~ will represent a large fraction of, and be highly correlated
with TSP Pb (Henderson, 2007b). In their most recent letter, the Panel
stated generally that "it would be well within EPA's range of
discretionary options to accept a slight loss of ultra-coarse lead at
some monitoring sites by selecting an appropriately conservative level
for the revised Pb NAAQS" (Henderson, 2008). In summary, while the CASAC
recognized the need for an adjustment to the level for a Pb-PM~10~-based
NAAQS, they did not provide a quantitative value, but did note interest
in sites with highest concentrations and a view that the overall
health-related benefits from moving to a PM~10~-based standard could
outweigh a small loss in protection from exposure to ultra-coarse
particles in some areas.

The Agency received few public comments with regard to a standard level
for a revised indicator of Pb-PM~10~. Of these, some generally agreed
with CASAC that an adjustment to the level was appropriate, recognizing
the difference in the two sampling methods. Some were concerned that the
current data may not support the derivation of a single scaling factor
that would provide requisite protection for some communities near some
large point source emitters of dust.

c\. Alternative Levels

As noted in section II.E.1 above, EPA's consideration of a Pb-PM~10~
indicator is accompanied by consideration of an adjustment of the NAAQS
level in recognition of the importance for public health of those
ultra-coarse dust contributions not captured by PM~10~ samplers.

In considering the appropriate level for a standard for which the
indicator is Pb-PM~10~, EPA recognizes the importance of all particle
size fractions and the dominant role of the ingestion pathway in
contributing to human exposures to air-related Pb. We also recognize
that the proportion of Pb captured by TSP monitors that is not captured
by PM~10~ monitors will vary, not only in reflection of the inherent
greater variability of the TSP sampler (as compared to the PM~10~
sampler), but also based on proximity to sources emitting ultra-coarse
Pb particles. An appreciably lower proportion of the Pb captured by TSP
monitors will be captured by PM~10~ monitors in areas near such sources
(e.g., Pb smelters). However, we are also aware of the limitations with
regard to the available Pb monitoring data on which to base a decision
with regard to an adjustment that appropriately recognizes these
considerations. Additionally, we take note of advice from CASAC and
public comments with regard to considerations for a level to accompany a
Pb-PM~10~ indicator. Based on these and other considerations summarized
above (II.E.4.a and II.E.1), including the data indicating the
proportion of Pb-TSP that may not be captured by PM~10~ samplers in some
source-oriented locations, EPA concludes that a level for a NAAQS with a
Pb-PM~10~ indicator should be notably lower than the level for a NAAQS
with a Pb-TSP indicator.

In considering a level for a NAAQS with a Pb-PM~10~ indicator, EPA notes
that a Pb-TSP to Pb-PM~10~ ratio of 1.5 is consistent with the ratios
observed relatively recently at the battery manufacturer site where
Pb-TSP concentrations were about 0.10 µg/m^3^. Application of this ratio
to a level of 0.30 µg/m^3^ (the upper end of the range proposed in
section II.E.3 for a Pb-TSP-based NAAQS) results in a potential level
for a Pb-PM~10~ indicator of 0.20 µg/m^3^. Similarly, application of a
factor of 2, a value similar to the Pb-TSP to Pb-PM~10~ ratio observed
at the smelter site, yields a potential Pb-PM~10~ level of 0.15 µg/m^3^.
Applying these two different factors to a level of 0.10 µg/m^3^ for a
Pb-TSP indicator would yield potential levels for a Pb-PM~10~ NAAQS
somewhat below 0.10 µg/m^3^. Thus, EPA requests comment on setting the
level of the primary standard to a level within a range from 0.20
µg/m^3^ to somewhat below 0.10 µg/m^3^ if a new Pb-PM~10~ indicator is
adopted. The level of 0.20 µg/m^3^ for a Pb-PM~10~ indicator may be
appropriate if the Administrator determines that a Pb-TSP value of 0.30
µg/m^3^ would be the level requisite to protect public health with an
adequate margin of safety under a Pb-TSP standard, but concludes that
the net public health related advantages of Pb-PM~10~ monitors warrant
making a Pb-PM~10~ standard preferred. A level somewhat below 0.10
µg/m^3^ for a Pb-PM~10~ indicator may be appropriate if the
Administrator determines that a Pb-TSP value of 0.10 µg/m^3^ would be
the level requisite to protect public health with an adequate margin of
safety under a Pb-TSP standard, recognizing that, while there are places
where all Pb particles captured by a Pb-TSP monitor are also captured by
a Pb-PM~10~ monitor, there will also be areas where this is not the
case.

In recognition of alternative views of the science, the exposure and
risk assessments and the uncertainties inherent in these assessments,
uncertainties in the appropriate adjustment for a Pb-PM~10~ based level
(due to the very limited collocated dataset with which to evaluate
relationships between Pb-TSP and Pb-PM~10~), and the appropriate policy
responses based on the currently available information, the EPA also
solicits comment on whether to proceed instead with alternative levels
of a primary Pb-PM~10~ standard within the ranges of above 0.20 µg/m^3^
to 0.50 µg/m^3^ and below 0.10 µg/m^3^ down through 0.02 µg/m^3^, the
lowest level evaluated in the risk assessment, if a Pb-PM~10~ indicator
were to be adopted. Based on the comments received and the accompanying
rationales, EPA may adopt standards within the broad range of
alternative levels identified above.

F. Proposed Decision on the Primary Standard

For the reasons discussed above, and taking into account information and
assessments presented in the Criteria Document and Staff Paper, the
advice and recommendations of CASAC, and the public comments to date,
the Administrator is proposing options for the revision of the various
elements of the standard to provide increased protection for children
and other at-risk populations against an array of adverse health
effects, most notably including effects on the developing nervous
system. Specifically, with regard to the indicator and level of the
standard, the Administrator proposes to revise the level of the standard
to a level within the range of 0.10 to 0.30 µg/m^3^ in conjunction with
retaining the current indicator of Pb-TSP but with allowance for the use
of Pb-PM~10~ data. With regard to the form and averaging time of the
standard, the Administrator proposes two options: (1) to retain the
current averaging time of a calendar quarter and the current
not-to-be-exceeded form, revised so as to apply across a 3-year span,
and (2) to revise the averaging time to a calendar month and the form to
be the second-highest monthly average across a 3-year span.

Corresponding revisions to data handling conventions and the schedule
for States to request exclusion of ambient Pb concentration data
affected by exceptional events are specified in proposed revisions to
Appendix R, as discussed in section IV below. Corresponding revisions to
aspects of the ambient air monitoring and reporting requirements for Pb
are discussed in section V below, including sampling and analysis
methods (e.g., a new Federal reference method for monitoring Pb in
PM~10~; quality assurance requirements), network design, sampling
schedule, data reporting, and other miscellaneous requirements.

In recognition of alternative views of the science and the exposure and
risk assessments, the uncertainties inherent in this information, and
the appropriate policy responses based on the currently available
information, the Administrator also solicits comments on other options.
More specifically, the Administrator solicits comment on revising the
indicator to Pb-PM~10~ in conjunction with a level within the range of
from somewhat below 0.10 to 0.20 µg/m^3^. For either indicator option,
the Administrator also solicits comment on whether to proceed instead
with alternative levels beyond the ranges identified above up to 0.50
µg/m^3^ and down through 0.02 µg/m^3^. In addition, the Administrator
invites comment on whether it would ever be appropriate to set a NAAQS
for Pb at a level of zero. Based on the comments received and the
accompanying rationales, the Administrator may adopt other standards
within the range of the alternative levels identified above in lieu of
the standards he is proposing today.

**III. Rationale for Proposed Decision on the Secondary Standard**

This section presents the rationale for the Administrator's proposed
decision to revise the existing secondary NAAQS. In considering the
currently available evidence on Pb-related welfare effects, the Staff
Paper notes that there is much information linking Pb to potentially
adverse effects on organisms and ecosystems. However, given the
evaluation of this information in the Criteria Document and Staff Paper
which highlighted the substantial limitations in the evidence,
especially the lack of evidence linking various effects to specific
levels of ambient Pb, the Administrator concludes that the available
evidence supports revising the secondary standard but does not provide a
sufficient basis for establishing a distinct secondary standard for
Pb.*Number range CHAPTER*

*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. These sources may have multiple pathways for discharging Pb to
ecosystems, and apportioning effects between air-related pathways and
other pathways (e.g. discharges to water) in such cases is difficult.
Likewise, apportioning these effects between Pb and other stressors is
complicated because these point sources also emit a wide variety of
other heavy metals and sulfur dioxide which may cause toxic effects.
There are no field studies which have investigated effects of Pb
additions alone but some studies near large point sources of Pb have
found significantly reduced species composition and altered community
structures. While these effects are significant, they are spatially
limited: the majority of contamination occurs within 20 to 50 km of the
emission source (CD, AX7.1.4.2).

By far, the majority of air-related Pb found in terrestrial ecosystems
was deposited in the past during the use of Pb additives in gasoline.
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, pl 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, AX7.1.4.2, 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 7.1.4.2).

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
7.1.4.1).

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 7.1.3.1 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 7.1.3.3 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 AX7.2.1.2 and AX7.2.1.3).

-   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 AX7.2.1.4; 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 3.0µg/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
    (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. The Secondary Standard

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 III.C.1). The general approach for this
current review is summarized in section III.C.2. Considerations and
conclusions with regard to the adequacy of the current standard are
discussed in section III.C.3, with evidence and exposure- risk-based
considerations in sections III.C.3.a and b, respectively, followed by a
summary of CASAC advice and recommendations (section III.C.3.c) and the
Administrator's proposed conclusions (section III.C.3.d).
Considerations, conclusions and the Administrator's proposed decision
with regard to elements of the secondary standard are discussed in
section III.C.4.

1\. Background on the Current Standard

The current standard was set in 1978 to be identical to the primary
standard (1.5 μg Pb/m^3^, as a maximum arithmetic mean averaged over a
calendar quarter), the basis for which is summarized in Section II.C.1.
At the time the standard was set, the Agency concluded that the primary
air quality standard would adequately protect against known and
anticipated adverse effects on public welfare, as the Agency stated that
it did not have evidence that a more restrictive secondary standard was
justified. In the rationale for this conclusion, the Agency stated that
the available evidence cited in the 1977 Criteria Document indicated
that "animals do not appear to be more susceptible to adverse effects
from lead than man, nor do adverse effects in animals occur at lower
levels of exposure than comparable effects in humans" (43 FR 46256). The
Agency recognized that Pb may be deposited on the leaves of plants and
present a hazard to grazing animals. With regard to plants, the Agency
stated that Pb is absorbed but not accumulated to any great extent by
plants from soil, and that although some plants may be susceptible to
Pb, it is generally in a form that is largely 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 µg/m^3^.

2\. Approach for Current Review

In evaluating whether it is appropriate to retain the current secondary
Pb standard, or whether revision is appropriate, the Administrator has
considered the evidence and risk analyses presented in the Criteria
Document, the Staff Paper, the ANPR and the associated technical support
documents, \[together with the associated uncertainties\] and CASAC
advice and public comment on these documents. The Staff Paper and ANPR
recognize 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\. Conclusions on Adequacy of the Current Standard

a\. Evidence--based Considerations

In considering the welfare effects evidence with respect to the adequacy
of the current standard, the Administrator considers 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, AX7.1.4.1,
    AX7.1.4.2, AX7.1.4.3 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 AX7.1.4.3).

-   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 AX7.1.3.2), 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
    AX7.1.3.3).

-   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 AX7.2.2.2) 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 AX7.2.4.3).

-   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 AX7.1.4.2).

-   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 AX7.1.4.3). 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
AX7.1.2.3).

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 AX7.2.2.2.2 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 (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., Staff Paper, section 2.8.1). This documentation
also indicates the greatly reduced contribution in many systems as
compared to decades ago (presumably reflecting the 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 (Staff Paper,
section 2.8; CD, pp. AX7-141 to AX7-145).

The case study locations included in the screening assessment, with the
exception of the primary Pb smelter site, are currently meeting the
current Pb standard, yet Pb occurs in 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 ANPR and final Staff Paper and 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 documents. With regard to the revision of the primary
and secondary NAAQS, this CASAC letter (Henderson 2008) said:

The Committee unanimously and fully supports Agency staff's scientific
analyses in recommending the need to substantially lower the level of
the primary (public-health based) Lead NAAQS, to an upper bound of no
higher than 0.2 μg/m3 with a monthly averaging time. The CASAC is also
unanimous in its recommendation that the secondary (public-welfare
based) standard for lead needs to be substantially lowered to a level at
least as low as the recommended primary NAAQS for Lead.

In earlier comments on the December 2006 draft documents, the CASAC Pb
Panel concluded they 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."
(Henderson, 2007a)

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.

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.

Similarly, in CASAC's advice on the ANPR and final Staff Paper they
concluded:

\[I\]t is critical that the secondary Lead NAAQS be set at a
sufficiently-stringent level so as to ensure that there is no reversal
of the current downward trend in lead concentrations in the environment.
Therefore, at a minimum, the level of the secondary Lead NAAQS should be
at least as low as the level of the recommended primary lead standard.
Moreover, the Agency needs to give greater priority to the monitoring of
environmental lead in the ambient air.

d\. Administrator's Proposed Conclusions on Adequacy of Current Standard

In considering the adequacy of the current standard in providing
requisite protection from Pb-related adverse effects on public welfare,
the Administrator has considered the body of available evidence (briefly
summarized above in Section III.A). Depending on the interpretation, the
available data and evidence, primarily qualitative, suggests the
potential for adverse environmental impacts under the current standard.
Given the limited data on Pb effects in ecosystems, it is necessary to
look at evidence of Pb effects on organisms and extrapolate to ecosystem
effects. Therefore, taking into account the available evidence and
current media concentrations in a wide range of areas, the Administrator
concludes that 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).

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.

In considering this evidence, as well as the views of CASAC, summarized
above, the Staff Paper and associated support documents, and views of
public commenters on the adequacy of the current standard, the
Administrator proposes to conclude that the current secondary standard
for Pb is not requisite to protect public welfare from known or
anticipated adverse effects.

4\. Conclusions and Proposed Decision on the Elements of the Secondary
Standard

The secondary standard is defined in terms of four basic elements:
indicator, averaging time, level and form, which serve to define the
standard and must be considered collectively in evaluating the welfare
protection afforded by the standards.

With regard to the pollutant indicator for use in a secondary NAAQS 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 is insufficient to identify an indicator other than
total Pb that would provide protection against adverse environmental
effect in all ecosystems nationally. Thus, the same concerns regarding
the relative advantages of TSP and PM10 as the basis for the indicator
apply here as for the primary standard.

Lead is a cumulative pollutant with environmental effects that can last
many decades. In considering the appropriate averaging time for a
secondary standard 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. The Administrator concurs 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.

Based on these considerations, and taking into account the observations,
analyses, and recommendations discussed above, the Administrator
proposes to revise the current secondary Pb standard by making it
identical in all respects to the proposed primary Pb standard (described
in section II.D.4 above).

IV\. Proposed Appendix R -- Interpretation of the NAAQS for Lead and
Proposed Revisions to the Exceptional Events Rule

The EPA is proposing to add Appendix R, Interpretation of the National
Ambient Air Quality Standards for Pb, to 40 CFR part 50 in order to
provide data handling procedures for the proposed Pb standard. The
proposed Appendix R would detail the computations necessary for
determining when the proposed Pb NAAQS is met. The proposed appendix
also would address data reporting; sampling frequency and data
completeness considerations; the use of scaled Pb-PM~10~ data as a
surrogate for Pb-TSP data (or vice versa), including associated scaling
instructions; and rounding conventions. Although the Administrator is
proposing one indicator and inviting comment on another, and proposing
several possible combinations of different averaging times, forms, and
levels, for simplicity the proposed data handling appendix text only
directly addresses one combination: a Pb-TSP indicator with an option
for using scaled Pb- PM~10~ data for NAAQS comparisons, an averaging
time of monthly, a second maximum (over three years) form, and a level
of 0.20 µg/m^3^. The proposed appendix text indicates in brackets the
change that would be needed if the level of the standard is set at 0.10
or 0.30 µg/m^3^ rather than at 0.20 µg/m^3^. A decision to adopt
Pb-PM~10~ as the indicator, to adopt a different indicator, averaging
time, and/or form, or not to make use of surrogate data would require
other differences in the text of the appendix; the proposed differences
in the appendix text to accommodate such difference are described below,
after the explanation of the proposed version of the appendix.

The EPA is also proposing Pb-specific changes to the deadlines, in 40
CFR 50.14, by which States must flag ambient air data that they believe
has been affected by exceptional events and initial descriptions of
those events, and the deadlines by which States must submit detailed
justifications to support the exclusion of that data from EPA
determinations of attainment or nonattainment with the NAAQS. The
deadlines now contained in 40 CFR 50.14 are generic, and are not
appropriate for Pb given the anticipated schedule for the designations
of areas under the proposed Pb NAAQS.

A. Background

The principal purpose of a data interpretation guideline is to provide
the practical details on how to make a comparison between multi-day,
possibly multi-monitor, and (in the unique instance of this proposed
Pb-NAAQS) possibly multi-parameter (i.e., Pb-TSP and/or Pb- PM~10~)
ambient air concentration data to the level of the NAAQS, so that
determinations of compliance and violation are as objective as possible.
Data interpretation guidelines also provide criteria for determining
whether there are sufficient data to make a NAAQS level comparison at
all. When data are insufficient, for example because of failure to
collect valid ambient data on enough days in enough months (because of
operator error or events beyond the control of the operator), then no
determination of current compliance or violation is possible.

The regulatory language for the current Pb NAAQS, originally adopted in
1977, contains no data interpretation instructions. Because of that, the
EPA has issued various guidance documents and memoranda relevant to the
topic. This situation contrasts with the situations for ozone, PM~2.5~,
and PM~10~ for which there are detailed data interpretation appendices
in 40 CFR 50 based on years of experience. EPA has used its experience
drafting and applying these other data interpretation appendices to
develop the proposed text for appendix R.

An exceptional event is an event that affects air quality, is not
reasonably controllable or preventable, is an event caused by human
activity that is unlikely to recur at a particular location or a natural
event, and is determined by the Administrator in accordance with 40 CFR
50.14 to be an exceptional event. Air quality data affected by an
exceptional event in certain specified ways may be excluded from
consideration when EPA makes a determination that an area is meeting or
violating the associated NAAQS, subject to EPA review and concurrence.
Section 50.14 contains both substantive criteria that an event and the
associated air concentration data must meet in order to be excluded, and
process steps and deadlines for a State to submit specified information
to EPA. The key deadlines are that a State must initially notify EPA
that data have been affected by an event and provide an initial
description of the event by July 1 of the year after the data are
collected, and that the State must submit the full justification for
exclusion within 3 years after the quarter in which the data were
collected. However, if a regulatory decision based on the data, for
example, a designation action is anticipated, the schedule is
foreshortened and all information must be submitted to EPA no later than
a year before the decision is to be made. This schedule presents
problems when a NAAQS has been recently revised, as discussed below.

The Staff Paper did not address data interpretation details, and
although the ANPR discussed data handling to a limited extent, there
have been only limited comment by CASAC or the public to date (other
than comments on the related issues of form and indicator for the
standard, including scaling factor issues). Similarly, no comments were
received on exceptional event issues..

B. Proposed Appendix R

1\. Proposed Text for a Standard Based on Pb-TSP, Second Maximum Monthly
Mean (over Three Years), with Scaling of Pb-PM~10~ Data as Surrogate for
Pb-TSP Data

The purpose of a data interpretation rule for the Pb NAAQS is to give
effect to the form, level, averaging time, and indicator specified in
the proposed regulatory text at 40 CFR 50.16, anticipating and resolving
in advance various future situations that could occur. The proposed
Appendix R, like the existing NAAQS interpretation appendices for ozone,
PM~2.5~, and PM~10~, addresses the possible situation of there being
less than 100% complete data available, which is an issue in common
across NAAQS pollutants. It also addresses several issues which are
specific to the proposed Pb NAAQS, as described below.

With regard to data completeness, the proposed Appendix follows past EPA
practice for other NAAQS pollutants by requiring that in general at
least 75% of the monitoring data that should have resulted from
following the planned monitoring schedule in a period must be available
for the key air quality statistic from that period to be considered
valid. For the combination of NAAQS parameters addressed in the proposed
text, the key air quality statistic is the mean concentration in an
individual month, and so the 75% requirement is applied for that time
period. With the proposed required sampling schedule of one day in three
under a monthly mean form for the standard (section V), typically there
will be 10 required sampling days so a monthly mean would be considered
valid if there were data available for at least 8 of those days.[^105]
EPA invites comment on this proposed 75% requirement, recognizing that
for the current NAAQS based on a quarterly mean concentration form with
a required one-day-in-six schedule, the current EPA policy is
effectively that there be at least 11 days of data in a quarterly mean.

The proposed rule text for Pb data interpretation, like the
corresponding existing rule for PM~2.5~, has two provisions that help a
monitoring agency guard against a month ending up with data completeness
below 75%. First, there is a provision to allow data from secondary,
collocated samplers to substitute for data from a primary monitor on a
day when the primary monitor for some reason fails to deliver valid
data. There is also a provision which would allow a monitoring agency to
make up a sampling day on which no valid data were collected, and to
count the make-up sampling data in the assessment of data completeness.
To help insure that sampling days are well distributed across the month
and that a make-up day will generally fall within generally the same
source emissions and meteorological regimes as the missed sampling days,
a number of specific restrictions are proposed on the number of make-up
days per month and on how soon after the missed scheduled sampling day
they must occur. These restrictions are stated in the proposed rule
text, and are adapted from current practice for PM~2.5~ with adaptations
to fit the monthly form of the proposed Pb standard.

A monthly mean Pb concentration for Pb-TSP would be calculated from all
available daily mean concentrations within that calendar month,
including successfully completed sampling days, allowed make-up sampling
days, and any other sampling days actually completed successfully by the
primary monitor or by secondary monitors if there is no data from a
primary monitor. These other sampling days would not be used in
calculating data completeness, however; this follows the example of the
current requirements for PM~2.5~ data interpretation.

Recognizing that even allowing for make-up samples, there may be months
with fewer than 75% complete data, the proposed text provides for two
diagnostic tests which are intended to identify those cases with
completeness less than 75% in which it nevertheless is very likely, if
not virtually certain, that the monthly mean concentration would have
been observed to be either above or below the level of the NAAQS if
monitoring data had been complete. One test, to be applied if the mean
of the incomplete data is above the NAAQS level, substitutes low
hypothetical concentrations for as much of the missing data as needed to
meet the 75% requirement; if the resulting mean is still above the NAAQS
level, then the NAAQS level is considered to have been exceeded for the
month. The hypothetical low values would be set equal to the lowest
concentration observed in the same month over the 3-year period being
evaluated, in effect giving the benefit of the doubt as to the actual
concentrations on the days with missing data. If the monthly mean
nevertheless is above the NAAQS, it is virtually certain that the mean
of complete data would also have been above the NAAQS. The other test,
to be applied if the mean of the incomplete data is below the NAAQS
level, works similarly except that at most 50% of the scheduled data can
be missing and all missing data is substituted with the highest value
observed in the same month over the 3-year period, with the same
rationale. Data substitution tests similar to these are currently used
for ozone and PM~2.5~. It should be noted that one outcome of applying
the substitution tests proposed for Pb is that a month with incomplete
data may be determined to not have a valid monthly mean and to be
unusable in making NAAQS exceedance determinations for that monthly time
period. In turn, this may make it impossible to make a determiniation of
compliance or violation for the 3-year period, depending on the
completeness and levels of the concentration data from the other months.

EPA invites comment on also incorporating into the final rule two other
possible tests that could allow a NAAQS exceedance determination to be
made on the basis of monthly data that is not at least 75% complete. EPA
may incorporate a version of either of both of these additional tests
into the final rule. The first additional test would allow use of the
monthly mean based on data that is between 50% and 75% complete if that
monthly mean were below some percentage (for example, 50%) the NAAQS, on
the rationale that if the available daily values (typically there would
be 5 values in a month with 50% complete data) have a mean below some
sufficiently low limit, day-to-day variability at the site must be small
and the actual concentrations on the days with missing data are very
unlikely to have been high enough to make the true monthly mean exceed
the NAAQS level.

The second additional test would be more statistically rigorous, yet
will allow compliance determinations to be made on some smaller data
sets by considering uncertainty bounds. The test would use the available
data to create a two-sided statistical confidence interval around the
calculated monthly mean concentration. A reduced minimum completeness
percentage such as 50% would still be applied to ensure that there are
enough sampling days that they could not all be from within a very short
period of time. As expected, the uncertainty range about the monthly
mean would increase as the number of samples decreases, and as there is
more variability in the data that were collected (more high
concentrations days mixed with low concentration days). If the
prescribed two-sided confidence interval is entirely above the level of
the NAAQS, then the NAAQS would be deemed to have been exceeded in that
month. Note that the calculated monthly mean in this situation would
also have been above the NAAQS level. If the confidence interval is
entirely below the level of the NAAQS, then the NAAQS would be deemed to
have not been exceeded in that month. EPA invites comment on the
statistical assumptions that should be considered to create a confidence
interval from the available data, for example the assumed distribution
of the underlying ambient data and how the confidence intervals should
be constructed. For example, the confidence interval could be
constructed based on the concept of a "finite population correction
factor," where means based on data with between 50 and 75% completeness
would have an associated uncertainty range.[^106] Any data that is at
least 75% complete could be considered "complete" and would have no
confidence interval. This approach would make the general completeness
test and this statistical test yield the same result for a month with at
least 75% completeness. EPA notes that such a statistical confidence
interval approach is not presently used in data interpretation for any
other NAAQS, but no other NAAQS involves the combination of an averaging
period as short as a month with a sampling schedule as infrequent as one
day in three.

EPA invites comment on whether the proposed data completeness provisions
taken together provide a good balance between avoiding situations in
which no determination of attainment or nonattainment can be made until
more data are collected during another calendar year, and avoiding
erroneous determinations caused by reliance on small sample sizes
affected by data variability. EPA also plans to explore this question
prior to the final rule, by analyzing hypothetical cases reflecting the
variability seen in historical monitoring data, and may make adjustments
to the proposed provisions for the final rule.[^107]

The proposed rule text would require that only a minimum of two valid
monthly means be available over the 3-year period in order to determine
that a site has violated the NAAQS, since if the NAAQS has been observed
to be exceeded twice the concentrations in the other months would be
irrelevant to a finding of NAAQS violation. Valid monthly means would be
required for all 36 possible months in the 3-year period in order to
make a finding that the NAAQS has been met. An exception is allowed if
there are 35 valid monthly means and none of them exceed the NAAQS,
because in that case it is irrelevant whether the one month with
incomplete data experienced an exceedance or not.

The proposed text of Appendix R has provisions to implement the proposal
that scaled Pb-PM~10~ data be treated as surrogate Pb-TSP data. These
provisions are somewhat complex, to be able to address various possible
situations without ambiguity. These situations arise from the
possibility that both Pb-TSP and Pb-PM~10~ monitoring might take place
at a single site, with differences from day to day within the 3-year
period as to which samplers were operating and yielded valid data for
the day. The proposed approach is to consider all Pb-TSP and Pb-PM~10~
data that have been collected and submitted by the monitoring agency,
i.e., once Pb-PM~10~ data have been collected and submitted the
monitoring agency could not choose to have them ignored.[^108] However,
where and when both types of data exist, the Pb-TSP data would be given
first consideration. Specifically the proposed approach is to treat as
separate questions whether the Pb-TSP monitor and the Pb-PM~10~ monitor
have produced a valid monthly mean concentration, taking into account
the provisions for make-up samples and data substitution from secondary
monitors, but not mixing Pb-TSP and Pb-PM~10~ data within the month. If
valid monthly means for both Pb-TSP and Pb-PM~10~ have been achieved,
i.e., the main or a supplemental data completeness test has been passed,
the Pb-TSP data takes precedence and the Pb-PM~10~ data for that month
are ignored. However, across the 3-year period, monthly means for Pb-TSP
and scaled Pb-PM~10~ can be considered together in determining whether
more than one monthly mean of TSP has exceeded the level of the NAAQS.
This allows for the possibility that a monitoring agency may have
switched from one type of monitoring to the other during the 3 years, or
that it has been more successful in getting complete TSP data in some
months than in others.

The proposed Appendix R addresses the issue of scaling factors for
Pb-PM~10~ data. The scaling factor is the number that multiplies
Pb-PM~10~ data to get a surrogate for Pb-TSP data. Because TSP samplers
collect a broader range of particle sizes than PM~10~ samplers, the
scaling factor logically can not be less than 1.00. EPA is proposing two
ranges of default scaling factors. The first range is 1.6 to 2.0 and
would apply to Pb-PM~10~ data collected at source-oriented monitoring
sites. The other range is 1.0 to 1.4[^109] and would apply to Pb-PM~10~
data collected at monitoring sites that are not source-oriented. These
ranges have been developed based on historical data from sites where the
two types of monitors were operated on the same days, as explained in
section III.E.1. EPA will consider comments on these ranges and their
basis in historical data, and for the final rule the Administrator
proposes to select a single default scaling factor for each type of
monitoring site. Because there would be different default scaling
factors for the two monitoring types, it would be necessary for each
monitoring agency to determine and designate, subject to EPA review,
whether each site is in fact source-oriented and to document that
determination in the Annual Monitoring Plan required by 40 CFR 58.10
(see section V for more information on the requirement for this plan and
for designating sites as source-oriented or not). The proposal also
would allow States to develop customized scaling factors for one or more
of their own monitoring sites, based on at least a year of measurements
of both types. EPA invites comment on the detailed criteria for
developing such local scaling factors, given in section 2(b) of the
proposed Appendix.

The existing FRM for Pb-TSP, Appendix G of 40 CFR 50, contains
procedures for calculating Pb concentration data in micrograms per cubic
meter at standard conditions of temperature and pressure (STP). The
proposed FRM for low-volume Pb-PM~10~, Appendix Q of 40 CFR 50, requires
reporting of concentration data at local conditions of temperature and
pressure, for reasons explained in section V. For consistency going
forward, we are proposing that for monitoring conducted on or after
January 1, 2009, Pb-TSP data should be reported at local conditions of
temperature and pressure also. The first deadline for such reporting
will be about June 30, 2009 (to be exact, 90 days from March 31, 2009)
so monitoring agencies will have ample lead time to change their
reporting procedures. However, EPA believes is would be an unnecessary
burden to require monitoring agencies to re-submit pre-January 1, 2009
Pb-TSP data corrected to local conditions, given that the adjustment
would in most cases be small. The proposed Appendix R would provide that
Pb-TSP data reported in STP is to be compared directly to the level of
the standard with no adjustment for the difference in reporting forms,
but gives the monitoring agency the option of re-submitting the data
corrected to local conditions. EPA invites comment on this approach.

Both FRM rules require reporting with three decimal places. When monthly
means are calculated, they are to be rounded to two decimal places for
purposes of comparing to the level of the NAAQS, which is expressed to
two decimal places.

2\. Differences to Address Other Combinations of NAAQS Elements

If a quarterly maximum mean form is adopted for the final standard, the
final Appendix R would refer to quarterly means where it now refers to
monthly means, and language allowing one exceedance in the 3-year period
would be removed. The design value would be defined as the maximum
quarterly mean concentration in the 3-year period. To be determined to
violate the standard, at least one valid quarterly mean in the 3-year
period would be required. To be determined to meet the standard, 12
valid quarterly means in the 3-year period would be required. The text
of the requirements for data completeness would otherwise remain as
proposed. Conforming changes would be made to the equations.

If Pb-TSP is adopted as the indicator for the final standard but use of
Pb-PM~10~ data is not allowed as a surrogate, the final text of Appendix
R will be simplified by removing all mention of the scaling/surrogate
concept, the designation of sites as source-oriented or not
source-oriented, and the detailed instructions in the proposed text
dealing with the selection of one type of data in preference to the
other. Conforming changes would be made to the definitions section.
Equation 2 would be omitted.

As explained in section VI on implementation, if Pb-PM~10~ is adopted as
the indicator for the final standard, many areas may not have sufficient
Pb-PM~10~ data to allow a determination of compliance or violation with
the Pb standard within the two or three years allowed under the Clean
Air Act for initial designations. We have considered an approach that
would allow the use of Pb-TSP data, with adjustment(s), for comparing
ambient concentrations of Pb to a Pb-PM10 NAAQS for the sole purpose of
making initial designations. The scaling issues, relevant data, and
possible approaches are similar to those described in section II.E.1. We
invite comment on this possible approach. If it is adopted in the final
rule, references to the two types of data would be reversed from the way
they appear in the proposed text of Appendix R. Scaling factors would be
1.0 or less. EPA invites comment on the selection of appropriate default
scaling factors for this situation. Language would also be added
restricting the use of scaled Pb-TSP data to determinations made for
purposes of designations within three years of promulgation of the
revised standard. This generally means that scaling factors would be
used officially only on 2007-2009 and possibly on earlier data, because
Pb-PM10 monitoring is proposed to be required to begin by January 1,
2010. Because of this limited time period of use, realistically it would
not be possible to do collocation testing to develop local scaling
factors, so the associated provisions of Appendix R would be omitted.

C. Proposed Revisions to Exceptional Events Information Submission
Schedule

As explained above, 40 CFR 50.14 contains generic deadlines for a State
to submit to EPA specified information about exceptional events and
associated air concentration data. A State must initially notify EPA
that data has been affected by an event by July 1 of the year after the
data are collected; this is done by flagging the data in AQS. The State
must also provide an initial description of the event by July 1. Also,
the State must submit the full justification for exclusion within 3
years after the quarter in which the data were collected; however, if a
regulatory decision based on the data (for example, a designation
action) is anticipated, the schedule for the full justification is
foreshortened and all information must be submitted to EPA no later than
a year before the decision is to be made.

These generic deadlines are suitable for the period after initial
designations have been made under a NAAQS, when the decision that may
depend on data exclusion is a redesignation from attainment to
nonattainment or from nonattainment to attainment. However, these
deadlines present problems with respect to initial designations under a
revised NAAQS. One problem is that some of the deadlines, especially the
deadlines for flagging data, can have already passed for some relevant
data by the time the revised NAAQS is promulgated. However, until the
level and form of the NAAQS have been promulgated a State does not know
whether the criteria for excluding data (which are tied to the level and
form of the NAAQS) were met on a given day, so the only way a State can
be sure to have flagged all data of concern and possible eligibility for
exclusion by the deadline is to flag far more data than will eventually
be eligible for exclusion. Another problem is that some of the data that
may be used for final designations may not be collected and submitted to
EPA until later than one year before the final designation decision,
making it impossible to flag that data one year before the decision.
When Section 50.14 was revised to add these deadlines in March 2007, EPA
was mindful that designations were needed under the recently revised
PM~2.5~ NAAQS, and so exceptions to the generic deadline were included
for PM~2.5~ only.

For the specific case of Pb, EPA anticipates that designations under the
revised NAAQS may be made in September 2010 based on 2007-2009 data and
thus will depend in part on data collected as late as December 2009. See
Section VI below for more detailed discussion of the designation
schedule and what data EPA proposes to use. There is no way for a State
to flag and submit documentation regarding events that happen in
October, November, and December 2009 by one year before designation
decisions are made in September 2010.

The proposed revisions to 40 CFR 50.14 involve only changes in
submission dates for information regarding claimed exceptional events
affecting Pb data. We propose to extend the generic deadline for
flagging data (and providing a brief initial description of the event)
of July 1 of the year following the data collection, to July 1, 2009 for
data collected in 2005-2007. The extension includes 2005 and 2006 data
because Governors' designation recommendations will consider 2006-2008
data, and possibly EPA will consider 2006-2008 or 2005-2007 data if
complete data for 2007-2009 are not available at the time of final
designations. EPA does not intend to use data prior to 2005 in making Pb
designation decisions. The generic deadline would continue to apply to
data from 2008, and would thus also be July 1, 2009. This would allow a
State time following the September 2008 promulgation of the revised Pb
NAAQS to consider what data it wishes to flag and to submit those flags.
The Governor of that State would be required to submit designation
recommendations to EPA in September 2009, and would therefore know what
data have been flagged when formulating those recommendations. For data
collected in 2009, we propose to move the generic deadline of July 1,
2010 for data flagging to May 1, 2010 (which is also the applicable
deadline for certifying 2009 data in AQS as being complete and accurate
to the best knowledge of the responsible monitoring agency head). This
would give a State less time, but EPA believes still sufficient time, to
decide what 2009 data to flag, and would allow EPA to have access to the
flags in time for EPA to develop its own proposed and final plans for
designations.

Finally, EPA proposes to make the deadline for submission of detailed
justifications for exclusion of data collected in 2008 or earlier be
September 1, 2009. This allows more time than in the generic schedule
for data collected in January to June 30, 2006, as detailed
demonstrations for these data would normally have to be submitted by
March 31, 2009 and June 30, 2009, respectively. Although EPA generally
does not anticipate data from 2006 being used in final Pb designations,
we do not want to foreclose that possibility at this time so it is
appropriate to allow the State more time following promulgation of the
revised standard to develop its justifications for that data. For data
collected between June 30, 2006 and December 31, 2008, the new deadline
will be sooner than allowed by the generic deadline, but consistent with
the current requirement that justifications be submitted no later than
one year before a regulatory decision is to be made. For data collected
in 2009, EPA proposes to make the deadline for submission of
justifications be May 1, 2010. This is less than a year before the
designation decisions would be made, but we believe it is a good
compromise between giving a State a reasonable period to prepare the
justifications and EPA a reasonable period to consider the information
submitted by the State.

EPA invites comment on these proposed changes in exceptional event
deadlines. Noting that the above discussion and the proposed changes
presume that designations will be made in September 2010, we also invite
comment on whether and how 40 CFR 50.14 should be further amended to
make appropriate provision for the possibility that designations might
not be made until September 2011, three years after the promulgation of
a revised Pb NAAQS.

**V. Proposed Amendments to Ambient Monitoring and Reporting
Requirements**

We are proposing several changes to the ambient air monitoring and
reporting requirements for Pb. Ambient Pb monitoring data are used to
determine whether an area is in violation of the Pb NAAQS. Ambient data
are collected and reported by State, local, and Tribal monitoring
agencies ("monitoring agencies") according to the monitoring
requirements contained in 40 CFR parts 50, 53, and 58. This section
explains aspects of the existing Pb monitoring and reporting
requirements as background and discusses the changes we are proposing to
support the changes being proposed in the Pb NAAQS and other options for
the NAAQS on which EPA is inviting comments, discussed above in section
II.E. These aspects include the sampling and analysis methods (including
quality assurance requirements), network design, sampling schedule, data
reporting, and other miscellaneous requirements.

A. Sampling and Analysis Methods

We are proposing changes to the sampling and analysis methods for the Pb
monitoring network. Specifically, we are proposing a new Federal
Reference Method (FRM) for Pb in PM~10~ (Pb-PM~10~) and revised Federal
Equivalent Method (FEM) criteria. We are maintaining the current FRM for
Pb in TSP (Pb-TSP) and lowering the Pb concentration range required
during Pb-TSP and Pb-PM~10~ candidate FEM comparability testing. The
following sections provide background, rationale, and details for the
proposed changes to the sampling and analysis methods.

1\. Background

Lead monitoring data must be collected and analyzed using FRM or FEM
methods in order to be comparable to the NAAQS. The current FRM for Pb
sampling and analysis is based on the use of a high-volume TSP FRM
sampler to collect the particulate matter sample and the use of atomic
absorption (AA) spectrometry for the analysis of Pb in a nitric acid
extract of the filter sample (40 CFR 50 Appendix G). There are 21 FEMs
currently approved for Pb-TSP[^110]. All 21 FEMs are based on the use of
high-volume TSP samplers and a variety of approved equivalent analysis
methods.[^111]

Concerns have been raised over the use of the high-volume TSP samplers
to collect samples for subsequent Pb analysis. It is known that the
high-volume TSP sampler's particulate matter capture efficiency varies
as a function of wind speed and wind direction due to the
non-symmetrical inlet design and the lack of an integral particle
separator. Early evaluations of the high-volume TSP sampler demonstrated
that the sampler's 50% collection efficiency cutpoint can vary between
25 and 50 µm depending on wind speed and direction (Wedding et al.,
1977, McFarland and Rodes, 1979). More recently, 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 study showed that
despite the effect of wind speed and wind direction on the sampler's
collection efficiency for larger particles, for particle distributions
typical of those near industrial sources the overall Pb collection
efficiency of the high-volume TSP sampler ranged from 80% to 90% over a
wide range of wind speeds and directions.

CASAC commented in the context of their review of the Staff Paper that
TSP samplers have poor precision, that the upper particle cut size of
TSP samplers 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, 2007a,
Henderson, 2008). For these reasons, CASAC recommended considering a
revision to the Pb reference method to allow sample collection using
low-volume PM~10~ samplers.[^112]

As part of preparing the ANPR for this rulemaking, we performed and
reported in the ANPR the results of an analysis of the precision and
bias of the high-volume TSP sampler based on Pb-TSP data reported to AQS
for collocated samplers and the results of in-field sampler flow audits
and laboratory audits for lead (Camalier and Rice, 2007). 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%. The average bias
for the lab analyses of Pb-spiked audit strips was -1.1% with a standard
deviation of 5.5%. Total bias, which includes bias from both sampling
and laboratory analysis, was estimated at -1.7% with a standard
deviation of 3.4%. These findings are specific for the times and sites
of the sampling, including the nature and total quantity of TSP and
Pb-TSP that prevailed during the sampling, and may not be indicative of
the TSP FRM performance in other places. Also, we did not investigate to
determine whether the physical arrangement of the collocated samplers
was such as to provide a good test of sensitivity to wind speed and wind
direction.[^113] However, we note that at face value these bias and
precision results are not greatly different than has historically been
considered acceptable for other criteria pollutants.

The CASAC and some public comments on the ANPR again stressed concerns
with the use of the high-volume TSP sampler and a strong interest in
moving to a low-volume Pb-PM~10~ sampler. The CASAC reiterated the
disadvantages of retaining TSP and of utilizing it as the "gold
standard" against which new and better technologies are compared
(Henderson 2008). On March 25, 2008, the AAMM subcommittee of CASAC and
EPA staff conducted a consultation by conference call, at which the
subcommittee members confirmed and elaborated on the views CASAC
expressed in their comments on the ANPR. Public comments were also
generally supportive of moving away from the current high-volume PM
sampling technology and moving toward modern, sequential, low-volume PM
monitors, especially if sampling frequencies are increased. On the other
hand, several monitoring agencies cautioned against moving to Pb-PM~10~
as the indicator because it would miss much of the Pb in the atmosphere
especially near Pb sources.

CASAC recommended that Pb-PM~10~ be measured with low-cost,
multi-element analysis methods with improved detection limits (e.g.,
x-ray fluorescence, XRF) for measuring concentrations typical of today's
ambient air. One public commenter suggested that the MDL be
significantly reduced to enable measurement of average Pb levels of 0.08
µg/m^3^ or below.

The current post-sampling FRM analysis method for Pb-TSP is atomic
absorption (AA) spectrometry. A typical or nominal lower detectable
limit (LDL) for Pb, for high-volume sample collection followed by AA
analysis, stated in the FRM regulation in Appendix G to Part 50 for
informational purposes only, is 0.07 µg/m^3^. This value was calculated
by doubling the between-laboratory standard deviation obtained for the
lowest measurable lead concentration (Long 1979). This value can be
considered a conservative (i.e., upper bound) estimate of the
sensitivity for the AA method currently used by air monitoring
laboratories, as evidence by the fact that data obtained from AQS
includes reported locally determined MDL values for the AA FRM that are
well below 0.07 µg/m^3^ (typically 0.01 μg/m^3^ or below).

One estimate of the method detection limit (MDL) for AA analysis of a
low-volume sample of either Pb-PM~10~ or Pb-TSP, taking into account the
nominal LDL of 0.07 µg/m^3^ (or 140 µg/L), and the smaller sample
volume, extraction volume, and filter size for low-volume sampling, is
about 0.12 µg/m^3^ (see Table 1). Assuming an LDL of 0.01 μg/m^3^ for
TSP sampling, the MDL for low-volume sample would be about 0.02 μg/m^3^.
Other Pb-TSP FEM analysis methods currently used with the high-volume
sampling method, such as XRF, inductively coupled plasma mass
spectrometry (ICP/MS) and graphite furnace atomic absorption (GFAA) are
more sensitive than AA analysis, and are clearly sensitive enough to
support low-volume sampling and a reduced NAAQS level.

2\. Proposed Changes

As discussed in Section II.E.3 of this preamble, after considering the
CASAC and public comments on monitoring issues, we are proposing to
retain Pb-TSP, as measured by the FRM method specified in 40 CFR 50
Appendix G (which cross references Appendix B, the specification of the
TSP FRM) as the indicator for the Pb standard, and to invite comment on
a second option which would instead make Pb-PM~10~ measured by a
low-volume monitor the indicator. We further propose that monitoring
agencies should be given the option to use adjusted or scaled low-volume
Pb-PM~10~ monitoring data as a surrogate for Pb-TSP data. Details on how
this option would work are discussed in the data handling section of
this preamble (Section IV). Also, in Section II.E.3 we are inviting
comment on whether, if low-volume Pb-PM~10~ is selected as the
indicator, Pb-TSP data with an adjustment should be useable as a
surrogate for Pb-PM~10~ data for the specific purpose of initial
designations under the revised standard. In this section, we discuss the
Pb-TSP and Pb-PM~10~ sampling and analysis issues themselves and propose
approaches for these issues, as these issues are relevant to the use of
data from each method directly or as surrogates for the other.

a\. TSP Sampling Method

If the final standard is based on Pb-TSP we believe it is appropriate to
continue to allow, although perhaps not to encourage, the use of the
current high-volume FRM for measuring Pb-TSP. The selection of Pb-TSP as
the NAAQS indicator would depend on a conclusion that the precision,
bias, and MDL (discussed above) of the TSP sampler is adequate for
continued use in the Pb monitoring network, including a conclusion that
although the TSP sampler's size selection performance is affected by
wind speed and wind direction, we do not believe that this effect is so
significant as to prevent the continued use of this sampler in the Pb
network. EPA proposes to make several minor clarifying changes in
Appendix G to correct long-standing errors in reference citations. We
are not proposing any other substantive changes to Appendix G

However, we also believe that low-volume Pb-TSP samplers might be
superior to high-volume TSP samplers. Presently, a low-volume TSP
sampler cannot obtain FRM status, because the FRM is specified in design
terms that preclude designation of a low-volume sampler as a FRM. A
low-volume Pb-TSP monitoring system (including an analytical method for
Pb) can in principle be designated as a FEM Pb-TSP monitor, if
side-by-side testing is performed as prescribed by 40 CFR 53.33. We are
proposing amendments to this CFR section, described below, to make such
testing more practical and to clarify that both high-volume and
low-volume TSP methods may use this route to FEM status. Note that the
terms of the revised FEM procedures can also be used to obtain FEM
status for Pb-PM~10~ samplers.

Appendix G currently specifies the reporting of concentrations corrected
to standard temperature and pressure conditions. We propose to not
change this, even though we are proposing (below) that concentrations be
corrected to local temperature and pressure when Pb measurements are
made using the low-volume Pb-PM~10~ FRM (see below for our rationale).
Keeping the current requirement will maintain consistency between data
already collected and reported and data collected in the future. Rather
than changing Appendix G, we have chosen to include a provision in the
newly proposed data handling Appendix R that discusses how Pb-TSP data
would be corrected to local conditions.

b\. PM~10~ Sampling Method

If the final standard is based on Pb-PM~10~, or if the final rule for a
standard based on Pb-TSP includes an option to monitor Pb-PM~10~ instead
of Pb-TSP, we will need to promulgate both an FRM for measuring
Pb-PM~10~ and an appropriate set of FEM criteria. Accordingly, we are
proposing new FRM and FEM criteria for measuring Pb-PM~10~. The proposed
FRM for Pb-PM~10~ can be broken down into two parts: (1) the sampling
method (i.e., the procedures and apparatus used for collecting PM~10~ on
a filter) and (2) the analysis method (i.e., the procedures and
apparatus used to analyze the collected particulate matter for Pb
content).

Currently, the FRM specification for PM~10~ monitoring, Appendix J to 40
CFR Part 50, is based on a performance test and does not specify whether
a sampler is high-volume or low-volume. Early commercialized samplers
were high-volume, but more recently a number of low-volume PM~10~
samplers have received FRM approvals. To be certain that Pb-PM~10~
monitoring is conducted with low-volume samplers without specifying the
use of particular sampler brands or models, it is necessary to establish
a new FRM specification for low-volume PM~10~ samplers. There is a
recently promulgated FRM for particulate matter with aerodynamic
diameter between 2.5 and 10 microns (PM~10-2.5~) (Appendix O to 40 CFR
Part 50) that is based on a pair of low-volume samplers for PM~2.5~ and
PM~10~ to provide a PM~10-2.5~ concentration by difference. We are
proposing to create a FRM for Pb-PM~10~ sampling by cross-referencing to
the specification for the PM~10~ sampler in this paired FRM (referred to
as the PM~10C~ sampler, where the "C" refers to the use of this PM~10~
sampler as part of a pair for measuring coarse PM). We are proposing to
use the low-volume PM~10C~ sampler for the FRM for Pb-PM~10~. rather
than the existing PM~10~ FRM specified by Appendix J, or several
reasons. Appendix J to Part 50 has resulted in the designation of both
high-volume and low-volume PM~10~ samplers as FRM for PM~10~. We believe
high-volume PM~10~ sampling should not be used to measure Pb- PM~10~
under a revised Pb standard. A low-volume PM~10C~ FRM sampler must meet
more demanding performance criteria than is required for PM~10~ samplers
in general in Appendix J. We note the current availability of samplers
that meet these more demanding performance criteria (already in use for
PM~2.5~ and PM~10-2.5~ sampling) that are equipped with sequential
sampling capabilities (i.e., the ability to schedule multiple samples
between operator visits, which is desirable if the proposed sampling
frequency requirements are increased to support a monthly averaging form
of Pb NAAQS). The geometry of commercial high-volume PM~10~ samplers
makes sequential sampling with a single sampler impossible. The
low-volume sampler also precisely maintains a constant sample flow rate
corrected to actual conditions by actively sensing changes in
temperature and pressure and regulating sampling flow rate. Use of a
low-volume sampler for the Pb-PM~10~ FRM would also provide network
efficiencies and operational consistencies with the samplers that are in
widespread use for the PM~2.5~ FRM network, and that are seeing growing
use in the PM~10~ and PM~10-2.5~ networks. Finally, the use of a
low-volume sampler is consistent with the comments and recommendations
from CASAC and members of CASAC\'s Ambient Air Monitoring Subcommittee
(Henderson 2007a, Henderson 2008, Russell 2008).

Low-volume Pb-PM~10~ sampler and the data systems that they connect to
can be configured to report concentrations corrected to standard
conditions of temperature and pressure or based on local conditions of
temperature and pressure. We are proposing that the FRM specification
for samplers used to collect Pb data report concentrations based on
local conditions, for a few reasons. The actual concentration of Pb in
the atmosphere is a better indicator of the potential for deposition
than the concentration based on standard pressure and temperature. In
addition, there are practical advantages to moving to local conditions
since both PM~2.5~ and PM~10-2.5~ are also based on local conditions.

c\. Analysis Method

There are several potential analysis methods for a Pb-PM~10~ FRM. Atomic
absorption (AA) is the analysis method for the current Pb-TSP FRM. In
addition, there are several other analysis methods (e.g., XRF, ICP/MS)
approved as FEMs for the measurement of Pb-TSP. Table 1 summarizes the
estimated MDLs for the analysis methods considered in developing the
proposed FRM for Pb-PM~10~. The estimated MDLs are based on published
instrument detection limits and LDLs, which typically take into account
only instrument signal-to-noise ratios and laboratory-related
variability but not variability related to sample collection and
handling. It is important to note that the MDLs in Table 1 are estimates
and these values will vary as a function of the specific instrument
used, detector age, instrument signal-to-noise level, etc. and
therefore, MDLs must be determined for the specific instrument used.

+----------------------------+-------------------+---------------------+
| **Table 1. Summary of      |                   |                     |
| Candidate Analysis Method  |                   |                     |
| Detection Limits for a     |                   |                     |
| Pb-PM~10~ FRM or FEM with  |                   |                     |
| Low-volume sample          |                   |                     |
| collection**               |                   |                     |
+----------------------------+-------------------+---------------------+
| **Analysis Method**        | **Estimated DLs   | **Estimated MDL^b^  |
|                            | ^a^**             | (µg/m^3^)**         |
+----------------------------+-------------------+---------------------+
| Atomic Absorption (AA)     | 0.07 µg/m^3^ ^c^  | 0.12^f^             |
+----------------------------+-------------------+---------------------+
|                            | 0.01 µg/m^3\ d^   | 0.02 ^f^            |
+----------------------------+-------------------+---------------------+
| X-Ray Fluorescence (XRF)   | 1.5 ng/cm^2\ e^   | 0.001^g^            |
+----------------------------+-------------------+---------------------+
| Graphite Furnace Atomic    | 0.05 µg/L ^h^     | 0.00004 ^f^         |
| Absorption (GFAA)          |                   |                     |
+----------------------------+-------------------+---------------------+
| Inductively Coupled        | 0.08 µg/L ^e^     | 0.00006 ^f^         |
| Plasma/Mass Spectrometry   |                   |                     |
| (ICP/MS)                   |                   |                     |
+----------------------------+-------------------+---------------------+
| ^a^ Detection limits (DLs) |                   |                     |
| found in available         |                   |                     |
| literature as provided in  |                   |                     |
| footnotes below.           |                   |                     |
|                            |                   |                     |
| ^b^ Estimated MDLs         |                   |                     |
| determined using estimated |                   |                     |
| DL, extraction volume, and |                   |                     |
| sample volume as noted in  |                   |                     |
| footnotes provided.        |                   |                     |
|                            |                   |                     |
| ^c^ The lower detectable   |                   |                     |
| limit (LDL) for Pb-TSP     |                   |                     |
| taken from Appendix G to   |                   |                     |
| Part 50 based on 2400m^3^  |                   |                     |
| sample volume, 0.10L       |                   |                     |
| extraction volume, and 12  |                   |                     |
| strips per filter.         |                   |                     |
|                            |                   |                     |
| ^d^ Based on MDLs reported |                   |                     |
| in AQS.                    |                   |                     |
|                            |                   |                     |
| ^e^ DL expressed as        |                   |                     |
| nanogram per square        |                   |                     |
| centimeter of filter       |                   |                     |
| surface is taken from the  |                   |                     |
| Compendium of Methods for  |                   |                     |
| the Determination of       |                   |                     |
| Inorganic Compounds in     |                   |                     |
| Ambient Air (Reference 2)  |                   |                     |
|                            |                   |                     |
| ^f^ Based on 46.2-mm       |                   |                     |
| filter extraction volume   |                   |                     |
| of 0.020 L and sample      |                   |                     |
| volume of 24 m^3^ of air.  |                   |                     |
|                            |                   |                     |
| ^g^ Based on 46.2-mm       |                   |                     |
| filter area of 11.86 cm^2^ |                   |                     |
| and sample volume of 24    |                   |                     |
| m^3^ of air.               |                   |                     |
|                            |                   |                     |
| ^h^ Taken from the Perkin  |                   |                     |
| Elmer Guide to Atomic      |                   |                     |
| Spectroscopy Techniques    |                   |                     |
| and Applications (Perkin   |                   |                     |
| Elmer, 2000)               |                   |                     |
+----------------------------+-------------------+---------------------+

One disadvantage of the low-volume sampler is that the total mass of the
PM~10~ sample collected is significantly lower than that of the
high-volume sampler due to the lower volume of air sampled (24 m^3^ per
24 hours for the low-volume sampler versus. over 1500 m^3^ per 24 hours
for a high-volume sampler). The lower mass of sample collected results
in higher MDLs for any given analysis method when coupled with the
low-volume sampler. As can be seen in Table 1, even assuming the smaller
LDL reported to AQS for recent sampling, the estimated MDL for atomic
absorption (the current FRM analysis method for Pb-TSP when coupled with
low-volume sampling) is the highest (least sensitive) of all potential
methods for use as an FRM/FEM method for Pb-PM~10~.

AA, GFAA, and ICP/MS are destructive methods and require solvent
extractions that possibly involve the use of strong acids to adequately
extract Pb from the collected PM for analysis. The specific extraction
solutions and methods are selected and optimized in order to meet the
required extraction efficiency for a measurement program. Both methods
are destructive, meaning that the sample collected on the filter is
destroyed during analysis. These methods also have higher analysis costs
relative to XRF.

While GFAA and ICP/MS have more than adequate MDLs to support a reduced
NAAQS level, we believe that the XRF analysis method has several
advantages which make it a desirable analysis method to specify as the
FRM. XRF does not require sample preparation or extraction with acids
prior to analysis. It is a non-destructive method; therefore, the sample
is not destroyed during analysis and can be archived for future analysis
or re-analysis if needed. XRF analysis is a cost-effective approach that
could be used at the option of the monitoring agency to simultaneously
analyze for many additional metals (e.g., arsenic, antimony, and iron)
which may be useful in source apportionment. XRF is also the method used
for the urban PM~2.5~ speciation monitoring networks and for the mostly
rural visibility monitoring program in Class I visibility areas, and is
being considered for the PM~10-2.5~ coarse speciation monitoring network
that will be implemented by monitoring agencies as part of the NCore
multi-pollutant network. The XRF analysis method should have acceptable
precision, bias, and MDL for use as the FRM for Pb-PM~10~ when coupled
with the low-volume PM~10~ sampler. Finally, CASAC recommended the use
of XRF as a low-cost and sensitive analysis method for the FRM
(Henderson 2007a, Henderson 2008). For these reasons, we are proposing
to base the analysis method for the proposed Pb-PM~10~ FRM on XRF.

d\. FEM Criteria

The FEM criteria provide for approval of candidate methods that employ
an alternative analysis method for Pb, an alternative sampler, or both.

The proposed Pb-PM~10~ FRM is based on the low-volume PM~10c~ sampler
and XRF analysis. Under the proposed revisions to 40 CFR 53.33,
Pb-PM~10~ data from any candidate FEM using an alternative sampler would
be compared to side-by-side data from the low-volume PM~10c~ FRM
sampler. An FEM candidate using only an alternative analysis method
would be evaluated by collecting paired filters from paired low-volume
PM~10c~ FRM samplers, and analyzing one filter of each pair with XRF and
the other filter with the candidate method.

As mentioned above, there are other analysis methods commonly used which
are also expected to meet the precision, bias, and MDLs necessary to be
used in the Pb surveillance monitoring network (e.g., GFAA and ICP/MS).
These analysis methods would be compared to the proposed XRF method and
would be approvable as FEMs through the performance testing requirements
outlined in regulation Section 53.33 of 40 CFR Part 53, Subpart C.
Several of these requirements need revisions for consistency with a
potentially lowered Pb NAAQS and for the potential addition of a
Pb-PM~10~ FRM. The following paragraphs describe the aspects of the FEM
criteria that we are proposing to revise.

The current FEM requirements state that the ambient Pb concentration
range at which the FEM comparability testing must be conducted to be
valid is 0.5 to 4.0 μg/m^3^. Currently there are few locations in the
United States where FEM testing can be conducted with assurance that the
ambient concentrations during the time of the testing would exceed 0.5
μg/m^3^. In addition, the Agency is proposing to lower the Pb NAAQS
level to between 0.10 and 0.30 µg/m^3^. As such, we are proposing to
revise the Pb concentration requirements for candidate FEM testing to a
range of 30% of the NAAQS to 250% of the NAAQS in µg/m^3^. For example,
if the level of the Pb NAAQS is finalized at 0.20 µg/m^3^, the ambient
concentrations that would be required for FEM testing would have to
range between 0.06 µg/m^3^ to 0.50 µg/m^3^. The requirements were
changed from actual concentration values to percentages of the NAAQS to
allow the FEM text to remain appropriate if subsequent changes to NAAQS
levels occur in the future.

The current FEM requirements state that the maximum precision and
accuracy for candidate analytical methods must be 15% and 5%
respectively. No changes are proposed for these requirements. Based on
the results for the current high-volume Pb-TSP precision and bias
(Camalier and Rice, 2007), these requirement seem reasonable for the
proposed FEM requirements. The current FEM does not have a requirement
for a maximum MDL. In order to ensure that candidate analytical methods
have adequate sensitivity or MDLs, we are proposing to add a requirement
that as part of the testing of a candidate FEM, the applicant must
demonstrate that the MDL of the method is less than 1% of the level of
Pb NAAQS. We believe this MDL requirement will ensure that FEM methods
will have enough sensitivity to detect Pb concentrations much less than
the proposed NAAQS level, but will not unnecessarily restrict methods
which could be used to provide data sufficient for the purpose of
determining compliance with the NAAQS. Subsequent users of a previously
approved FEM would not be required to demonstrate the MDL of the method
as implemented in their laboratories, but EPA plans to encourage them to
do so periodically as a good quality assurance practice.

The existing FEM requirements require that audit samples (the known
concentration or reference samples provided on request by EPA used to
verify the accuracy with which a laboratory conducts the FRM analytical
procedure before it may begin comparing the FRM to the candidate FEM) be
analyzed at levels that are equal to 100, 300, and 750 µg per spiked
filter strip (equivalent to 0.5, 1.5, and 3.75 µg/m^3^ of sampled air).
We are proposing to revise the levels of the audit concentrations to
percentages (30%, 100% and 250%) of the Pb NAAQS to provide for reduced
audit concentrations for a lowered NAAQS. These percentages are roughly
equivalent to the percentages of the current NAAQS level (1.5 µg/m^3^)
used to set the spiked filter strip audit concentrations provided above
in the original FEM regulation.

The existing FEM requirements are based on the high-volume TSP sampler,
and as such, refer to ¾ x 8-inch glass fiber strips. In order to also
accommodate the use of low-volume sample filters, we are proposing to
add references to 46.2-mm sample filters where appropriate. Pairs of
these filters will be collected by a pair of FRM samplers, so that there
is no need to cut the 46.2 mm filters into to parts before analysis.

e\. Quality Assurance

Modifications are needed to the quality assurance (QA) requirements for
Pb in 40 CFR Part 58 Appendix A paragraph 3.3.4 in order to accommodate
Pb-PM~10~ monitoring. Paragraph 3.3.4 specifies requirements for annual
flow rate audits for TSP samplers used in Pb monitoring and Pb strip
audits for laboratories performing analysis of TSP filters for Pb. Other
QA requirements specified in section 3.3.1 for all TSP samplers are also
applicable to Pb-TSP samplers. As part of the overall Pb NAAQS review,
it is appropriate to revise these requirements to consolidate all the QA
requirements for Pb monitoring in paragraph 3.3.4, to add provisions
specific for Pb-PM~10~ measurements and to eliminate cross references to
the general TSP provisions. The following paragraphs detail the QA
requirements we are proposing to change.

The collocation requirement for all TSP samplers (paragraph 3.3.1)
applies to TSP samplers used for Pb-TSP monitoring. These requirements
are the same for PM~10~ (paragraph 3.3.1); as such, no changes are
needed to accommodate low-volume Pb-PM~10~. However, to clarify that
this requirement also applies to Pb monitoring we are proposing to add a
reference to this requirement in paragraph 3.3.4.

The sampler flow rate verifications requirement (paragraph 3.3.2) for
low-volume PM~10~ and for TSP are at different intervals. While this
appears appropriate and no change is needed, to clarify that this
requirement also applies to Pb monitoring we are proposing to add a
reference to this requirement in paragraph 3.3.4.

Paragraph 3.3.4.1 has an error in the text that suggests an annual flow
rate audit for Pb, but then includes reference in the text to
semi-annual audits. The correct flow rate audit frequency is
semi-annual. We are proposing to correct this error. Also, we are
proposing to change the references to the Pb FRM to include the proposed
Pb-PM~10~ FRM.

Paragraph 3.3.4.2 discusses the audit procedures for the lead analysis
method. This section assumes the use of a high-volume TSP sampler, and
we are proposing edits to account for the proposed Pb-PM~10~ FRM. In
addition, the audit concentration ranges will not be appropriate if the
NAAQS is lowered. We are proposing to lower the audit ranges for Pb-TSP
from the current range of 0.5-1.5 µg/m^3^ to a range from 30-100% of the
proposed Pb NAAQS level for the low concentration audit and from 3.0-5.0
µg/m^3^ to 200-300% of the proposed NAAQS for the higher concentration
audit standard. The requirements would also be changed from specific
concentration value-based ranges to ranges based on the percentages of
the NAAQS to allow these QA requirements to remain appropriate if
changes to NAAQS levels occur during future reviews.

Unlike the PM~2.5~ and PM~10-2.5~ Performance Evaluation Program (PEP),
the existing QA program requirements for Pb monitoring do not include a
requirement for the collection of data appropriate for making an
independent estimate of the overall sampling and analysis bias. We are
proposing to require one PEP-like audit at one site within each primary
quality assurance organization (PQAO) once per year. We are also
proposing that, for each quarter, one half of a collocated sample pair
from one site within each PQAO be sent to an independent laboratory for
analysis. The independent measurement on one filter from each pair would
be compared to the monitoring agency's regular laboratory's measurement
on the other filter of the pair, to allow estimation of any bias in the
regular laboratory's measurements. EPA believes that the combination of
the PEP data and the independent collocation data will be enough to
provide a reasonable assessment of overall bias and data comparability
on a PQAO basis over the designation period. As currently is the case
for PEP auditing of PM~2.5~ and PM~10-2.5~ monitoring sites, it would be
the responsibility of each State to ensure that Pb PEP testing and
collocation testing as described here is performed as required. EPA
plans to consult with monitoring agencies after completion of this
rulemaking as to whether a centrally run program managed by EPA and
funded with State and Tribal Assistant Grant funds would be a more
efficient and preferred alternative than individual State-managed
programs.

B.  Network Design

As a result of this Pb NAAQS review and the proposed tightening of the
standards, EPA recognizes that the current network design requirements
are inadequate to assess compliance and determine the extent of all the
areas that may violate the revised NAAQS. As such, we are proposing new
network design requirements for the Pb NAAQS surveillance network. The
following sections provide background, rationale and details for the
proposed changes to the Pb network design requirements.

1\. Background

The once large Pb surveillance network of FRM samplers for Pb-TSP has
decreased substantially over the last few decades. In 1980 there were
over 900 Pb surveillance sites. This number has been reduced to
approximately 200 sites today. These reductions were made because of
substantially reduced ambient Pb concentrations causing monitoring
agencies to shift priorities to other criteria pollutants including
PM~2.5~ and ozone which were believed to pose a greater health risk. As
a result of these reductions, many states currently have no ambient air
Pb monitors resulting in large portions of the country with no data on
current ambient Pb air concentrations. In addition, many of the largest
Pb emitting sources in the country do not have nearby ambient Pb air
monitors.

There is also a smaller network, the National Air Toxics Trends Stations
network, of 23 monitoring sites measuring Pb-PM~10~. Some of these use a
high-volume PM~10~ sampler to collect the particulate matter and some
use a low-volume PM~10~ sampler. Most are in urban areas.

The current network design requirements for Pb monitoring are given in
40 CFR 58 Appendix D section 4.5. The current minimum network design
requirements are for two Federal Reference Method (FRM) or Federal
Equivalent Method (FEM) sites in any area where Pb concentrations exceed
or have exceeded the NAAQS in the most recent two years. These current
minimum monitoring requirements cannot be relied upon to cause
monitoring agencies to fill the existing gaps in the current network and
will make it difficult to develop the necessary network to properly
evaluate ambient air concentrations during the designation process,
especially if the NAAQS is finalized at a significantly lower level than
the current standard.

For these reasons, EPA indicated in the Advanced Notice of Proposed
Rulemaking (ANPR, 72 FR 71488) that the existing Pb NAAQS surveillance
network may not be adequate for a lowered Pb NAAQS, and that if the
NAAQS is substantially lowered as proposed additional monitoring sites
would be needed to provide estimates of ambient Pb air concentrations
near Pb emission sources and for characterizing ambient air
concentrations in large urban areas. Comments received from CASAC and
other public commenters on the ANPR stated that the Pb surveillance
network should be expanded in order to provide better coverage of Pb
emission sources and to better understand population exposures to Pb
from ambient air. After considering these comments and evaluating the
existing network, EPA is proposing changes to the network as described
below.

2\. Proposed changes

We are proposing to modify the existing network design requirements for
the Pb surveillance monitoring network to achieve better understanding
of ambient Pb air concentrations near Pb emission sources and to provide
better information on population exposure to Pb in large urban areas.
The following paragraphs provide the rationale and details for the
proposed changes.

The primary objective of the Pb monitoring network is to provide data on
the ambient Pb air concentrations in areas where there is the potential
for a violation of the NAAQS. Ambient Pb concentrations have dropped
dramatically in most urban areas due to the elimination of Pb in
gasoline. However, based on our analysis of the ambient Pb data,
relatively large sources of Pb continue to have the potential to cause
ambient air concentrations in excess of the proposed NAAQS (EPA, 2007c).
Furthermore, it appears, based on the limited network still operating,
that violations of the proposed range for the revised NAAQS levels are
likely to exist only near such sources of Pb emissions, with lower
levels of Pb away from such sources. Accordingly, we are proposing to
require monitoring near Pb emission sources such as Pb smelters,
metallurgical operations, battery manufacturing, and other source
categories that emit Pb. By implementing the NAAQS through a
source-oriented monitoring network, Pb concentrations will be kept below
the NAAQS level for those living near these sources and for those living
farther away.

The 2002 National Emissions Inventory (NEI) lists over 13,000 thousand
sources of Pb, with emission rates from as low as 1 pound to nearly 60
tons per year (according to the NEI 90% of lead sources emit less than
0.1 tpy). It is not practical to conduct monitoring at every Pb emission
source, nor is it likely that very small Pb emission sources will cause
ambient concentrations to exceed the proposed NAAQS. Therefore, it is
appropriate to limit the source oriented monitoring requirement to
emission sources that may have the potential to result in ambient air
concentrations in excess of the proposed NAAQS.

We are proposing that monitoring be presumptively required at sources
that have Pb emissions (as identified in the latest NEI or other
scientifically justifiable methods and data) that exceed a Pb "emissions
threshold." This monitoring requirement would apply not only to existing
industrial sources of lead, but also to fugitive sources of lead (e.g.,
mine tailing piles, closed industrial facilities) and airports where
leaded aviation gas is used. In this context, the emissions threshold is
the Pb emission rate for a source that may reasonably be expected to
result in ambient air concentrations in excess of the proposed Pb NAAQS.
We conducted an analysis to estimate the appropriate emission threshold
(Cavender 2008b). Using the "central tendency" from this study, we
propose that the emission threshold be set in the range of 200 kg-600 kg
per year total Pb emissions (including point, area, and fugitive
emissions and including Pb in all sizes of PM). We are proposing a range
for the emission threshold since we are proposing a range for the level
of the standard. If the final NAAQS is set at 0.10 μg/m^3^, we would set
the emission threshold at 200 kg per year. Conversely, if the final
NAAQS is set at 0.30 μg/m^3^, we would set the emission threshold at 600
kg per year. Emission thresholds for other NAAQS levels would be set
proportionally.

We recognize that a number of factors influence the actual impact a
source of Pb has on ambient Pb concentrations (e.g., local meteorology,
emission release characteristics, and terrain). As such, we are also
proposing to allow monitoring agencies to petition the EPA Regional
Administrator to waive this requirement for a source that emits less
than 1 ton per year where it can be shown (by demonstrating actual
emissions are less than the threshold, through modeling, historical
monitoring data, or other means) a source will not cause ambient air
concentrations to exceed 50% of the NAAQS during a three year period. We
are proposing that for facilities identified as emitting more than 1 tpy
in the NEI, a waiver is possible only by demonstrating that actual
emissions are less than the emissions threshold. By requiring every
source actually emitting more than 1 tpy to be monitored, we will avoid
the possibility that faulty or uncertain modeling demonstrations or past
monitoring programs would be the basis for not monitoring sources that
are the most likely to cause NAAQS violations.

We seek comments on the appropriateness of requiring monitoring near Pb
emissions sources and the proposed emission rate threshold. We also seek
comments on the appropriateness of allowing monitoring agencies to seek
waivers from this requirement and the upper emission threshold level at
which waivers should no longer be allowed.

The required source-oriented monitors shall be located at sites of
maximum impact and will be classified primarily as microscale monitors
representative of small hot-spot areas adjacent or nearly adjacent to
facility fence-lines. In some cases, source-oriented monitors may be
representative of somewhat bigger areas due to the orientation of
sources with respect to areas with locations appropriate for ambient
monitoring. In these cases, the source-oriented monitors may be
classified as middle-scale, but should still represent the locations
where maximum Pb concentrations around a facility are expected to occur,
consistent with applicable siting regulations and the outputs of
quantitative tools (e.g., dispersion modeling) used to determine maximum
impacts.

We are proposing to require a small network of non-source oriented
monitors in urban areas in addition to the source oriented monitors
discussed above, in order to gather information on the general
population exposure to Pb in ambient air. While it is expected that
these non-source oriented monitors will show lower concentrations than
source oriented monitors, data from these non-source oriented monitors
will be helpful in understanding the risk posed by Pb to the general
population. Data from these monitors will also be useful in determining
impacts on Pb concentrations from mobile sources (e.g., re-entrained
roadway dust), construction and demolition projects; and in determining
the spatial variation in Pb concentrations between areas that are source
impacted and non-source impacted. Such data on spatial variations within
an urban area could assist with the determination of non-attainment
boundaries.

We are proposing to require one non-source oriented monitor in each Core
Base Statistical Area (CBSA, as defined by the Office of Management and
Budget[^114]) with a population of 1,000,000 people or more as
determined in the most recent census estimates. Based on the most
current census estimates, 50 CBSAs would be required to have non-source
oriented population monitors. We request comments on the appropriateness
of requiring non-source oriented monitors and the proposed population
threshold of 1,000,000 people for this requirement. []{.mark}

Lead concentrations near roadways are not well understood at this time.
The Pb critieria document discussed data for the South Coast Air Quality
Management District where a modeling effort suggested that Pb deposited
during the days when leaded gasoline was used could be a significant
portion of their ambient Pb inventory. However, this work was conducted
in an area of the country where quarterly average Pb-TSP concentrations
are considerably less than 0.1 µg/m^3^. We analyzed ambient air Pb
concentrations near a number of large roadways (Cavender 2008). Based on
this analysis it appears unlikely that roadways will result in ambient
Pb air concentrations in excess of the lowest Pb NAAQS level being
proposed in this action. In addition, the CASAC AAMM subcommittee agreed
that a separate monitoring requirement for roadways was unnecessary
based on the results of this analysis. As such, we are not proposing to
require Pb monitoring near roadways. We do, however, propose to allow
monitoring near roadways to satisfy the requirements of the non-source
oriented monitoring requirement discussed above. For example, a
monitoring agency could place a monitor in a CBSA with a population
greater than one million and locate that monitor nearly adjacent to a
major roadway in a populated area. That monitor would satisfy the
non-source oriented requirements while also gathering data on possible
roadway exposure. We request comments on the need for monitoring near
roadways and the appropriateness of allowing near roadway monitoring to
be used to satisfy the requirement for non-source oriented monitoring.

Monitoring agencies would need to install new Pb monitoring sites as a
result of these proposed revisions to the Pb monitoring requirements. We
are proposing to allow states until January 1, 2010 (16 months after the
court-ordered deadline for promulgation of the final Pb NAAQS revision)
to install and begin operation of any newly required Pb monitoring
sites. We are also proposing to allow monitoring agencies one year
following the release of updates to the NEI or an update to the census
to add new monitors if these updates would trigger new monitoring
requirements. Monitoring agencies would be required to identify and
propose new Pb monitoring sites as part of their annual network plan
required under 40 CFR 58.10. The annual network plan due on July 1, 2009
must contain the plan for the Pb monitoring that will start by January
1, 2010

The type of monitor that must be used at these required monitoring sites
will depend on whether for a final revised NAAQS based on Pb-TSP scaled
monitoring data for Pb-PM~10~ may be used as a surrogate. If cross-use
of data is permitted, then either type of monitor could be used at a
required monitoring site. EPA intends to encourage a relatively small
number of sites to operate both types of monitors. The proposed Appendix
R (see section IV) explains how data would be selected for purposes of
NAAQS compliance determinations if both types of monitors operate in the
same month or quarter. One approach on which EPA is seeking comment
would be to change the Pb indicator to Pb-PM~10~ and allow the use of
Pb-TSP data only for the purpose of initial designations. If this
approach is adopted, a Pb-TSP monitor could not be used in lieu of a
Pb-PM~10~ sampler at a required monitoring site after the area
containing the monitoring site had received its initial designation (see
section VI for an explanation of the anticipated designation schedule).

If the final Pb standard is based on Pb-TSP, the July 1, 2009 monitoring
plan would be required to designate which Pb-PM~10~ monitoring sites if
any are source-oriented, so that this designation can be available for
public comment and can be reviewed by the EPA Regional Administrator.
This site designation information is needed to determine scaling factors
for the Pb concentration data from these Pb-PM~10~ monitoring sites (see
section IV). Sites that are counted towards meeting the required number
of source-oriented monitoring sites should of course be designated as
source-oriented. It may be appropriate to designate other sites as
source-oriented also. Because sources may come and go, or be newly
discovered, the revised 40 CFR 58.10 requires the monitoring agency to
consider whether revisions in site designations are needed as part of
the preparation of each year's monitoring plan.

C.  *Sampling Schedule*

We are proposing to increase the sampling frequency if the final Pb
NAAQS is based on a monthly averaging form. Specifically, we are
proposing to increase the sampling frequency to require one 24-hour
sample taken every 3 days (referred to as "1 in 3 day sampling") if the
final Pb NAAQS is based on a monthly average. The remainder of this
section provides background, rationale, and details for the proposed
changes to the Pb sampling frequency.

1\. Background

The current required sampling frequency requirement for Pb is one
24-hour sample every six days (40 CFR 58.12(b)). For the current form of
the NAAQS that is based on a quarterly average, the 1-in-6 day 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 with 100% completeness, or between 3 and
5 samples with 75% completeness.

In the ANPR, we indicated that if we changed the averaging time to a
monthly average, we would need to consider increasing the required
sampling frequency from 1-in-6 days since 3 to5 samples would likely not
result in a reasonably confident estimate of the actual air quality for
the period. We suggested several alternatives which included increasing
the sampling frequency to 1-in-3 day, or increasing the sampling
frequency to 1-in-1 day sampling (i.e., every day sampling). In
addition, we suggested 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 concentration
approaches the NAAQS level. In addition, we sought comments on several
practices that would help to reduce the burden associated with more
frequent sampling including:

-   Increasing sampling time duration (e.g., changing from a 24-hour
    sampling time duration to a 48-hour or 72-hour sampling time
    duration),

-   Allowing for compositing of samples (i.e., extracting and analyzing
    several sequential samples together), and

-   Allowing for multiple samplers at one site.

In CASAC's comments on the ANPR, they recommended increasing the
sampling frequency to 1-in-3 day sampling, or higher. They discouraged
increasing the sample duration and the allowance for compositing of
samples, as these practices would reduce the ability to use the samples
in source apportionment techniques that may be useful in identifying
what sources contributed to the ambient air Pb concentrations.

2\. Proposed Changes

We propose increasing the sampling frequency to 1-in-3 day sampling if
we change the form of the revised NAAQS to a monthly average in the
final rule. A 1-in-3 day sampling frequency would yield 10 samples per
month on average at 100% completeness. At 75% completeness, a 1-in-3 day
sampling frequency would yield 6 or 7 samples per month at a minimum.

We recognize that at concentrations considerably below the level of the
NAAQS there is less potential to misclassify an area due to the error
resulting from less than complete sampling. We believe it is appropriate
to allow for less frequent sampling in areas with low ambient air Pb
concentrations relative to the level of the NAAQS. As such, we are
proposing to allow monitoring agencies to request a reduction in the
sampling frequency to 1-in-6 day sampling if the most recent 3-year
design value is less than 70% of the NAAQS.

We request comment on the proposed change to 1-in-3 day sampling and the
proposed option to reduce sampling to 1-in-6 day sampling in areas with
low ambient Pb concentrations. We also seek comments on the need to
increase sampling frequency further to 1-in-1 day sampling in areas with
ambient air Pb concentrations near the level of the final NAAQS.

We are currently assessing how different sampling schedules could affect
the confidence in the estimate of a mean monthly Pb concentration as
part of developing Data Quality Objectives (DQOs) for Pb monitoring.
This assessment will include evaluating temporal variability at current
Pb monitoring sites (both Pb-TSP and Pb-PM~10~) in order to provide
uncertainty estimates associated with various sampling frequency
scenarios. We will evaluate 1-in-1 day, 1-in-3 day, and 1-in-6 day
sampling frequencies, at varying degrees of completion between 50% and
100%, and for each we plan to estimate the margin of error about a mean
monthly estimate, focusing on sites assumed to be close to the proposed
NAAQS. Based upon this assessment, expected to be complete in June of
2008, we will be able to better understand the uncertainties around a
monthly estimate. We will use this better understanding and information
provided in public comment to choose the final sampling frequency
requirements.

D. Monitoring for the Secondary NAAQS

We are not proposing additional monitoring requirements for the
secondary NAAQS because the proposed monitoring requirements for the
primary NAAQS will be sufficient to demonstrate compliance with the
secondary NAAQS. The remainder of this section provides background and
rationale on our decision to not propose additional monitoring
requirements for the secondary NAAQS.

1\. Background

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 PM~2.5~ (Pb-PM~2.5~) as part of the
Interagency Monitoring of Protected Visual Environments (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 IMPROVE network. While we believe it
is not appropriate to rely on Pb-PM~2.5~ monitoring to demonstrate
compliance with a Pb-TSP NAAQS, we believe the Pb-PM~2.5~ measurements
provided by the IMPROVE network can be used as a useful indicator to
temporal and spatial patterns in ambient Pb concentrations and resulting
Pb deposition in rural areas that are not directly impacted by a nearby
Pb emission source. In the ANPR, we suggested it might 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-PM~2.5~ and Pb-TSP relate in these rural
areas. Alternatively, since it is likely that at rural locations nearly
all ambient Pb is in the less than 10 µm size range, we suggested it
might be possible to analyze the MPROVE PM~10~ mass samples (which are
already being collected) for Pb for a period of time to develop a better
understanding of how Pb-PM~2.5~ and Pb-PM~10~ relate in these rural
areas.

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, AX7.2.2.2). 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.

2\. Proposed Changes

As discussed in Section III of this preamble, we are proposing to set
the secondary NAAQS equal to the primary NAAQS. Based on our analysis of
the existing ambient Pb monitoring data (EPA 2007c), we do not expect
there to be ambient air concentrations in excess of the proposed
secondary NAAQS in rural areas that are not associated with a Pb
emission source. As noted earlier in this section, we are proposing Pb
surveillance monitoring requirements for Pb sources to demonstrate
compliance with the primary NAAQS that will also be sufficient to
determine compliance with the secondary NAAQS.

The Pb-PM~2.5~ data collected as part of the IMPROVE program provides
useful information on Pb concentrations in rural areas that can be used
to track trends in ambient air Pb concentrations in rural areas and
important ecosystems. These data are available through the VIEWS web
portal (http://vista.cira.colostate.edu/views/) and are also reported to
AQS. While collection of a limited amount of collocated Pb-TSP or
Pb-PM~10~ would be useful in understanding the relationship between
Pb-PM~2.5~ and Pb-TSP (or Pb-PM~10~) in rural areas, we do not believe
it is appropriate to establish a regulatory requirement for the
collection of these data. Rather, we believe it is more appropriate to
work with the monitoring agencies responsible for IMPROVE monitoring to
encourage the collection of a limited amount of collocated Pb data from
PM~10~ or TSP samplers. We seek comments on our decision to not require
additional monitoring requirements for the proposed secondary Pb NAAQS.

F.  Other Monitoring Regulation Changes

We are proposing to make a two minor changes to various aspects of the
Pb monitoring regulations to make them consistent with the proposed
NAAQS. The remainder of this section discusses the proposed changes.

1\. Reporting of Average Pressure and Temperature

The high-volume FRM for Pb-TSP monitoring is based on standard pressure
and temperature (25 degrees C, and 760 mmHg). We are not proposing to
change this. As discussed in Section II.E of this preamble, we are
proposing to adopt a new FRM for low-volume Pb-PM~10~ monitoring with
concentration reporting based on local temperature and pressure. We are
proposing to specify reporting based on local temperature and pressure
because the actual concentration of Pb in the atmosphere is a better
indicator of the potential for deposition than the concentration based
on standard pressure and temperature. In addition, there are practical
advantages to moving to local conditions since both PM~2.5~ and
PM~10-2.5~ are also based on local conditions. We are proposing to
revise 40 CFR Section 58.16(a) to add a requirement that the monitoring
agency report the average pressure and temperature during the time of
sampling for both Pb-TSP monitoring and Pb-PM~10~ monitoring, consistent
with the requirements for such reporting contained in the PM~2.5~ and
PM~10-2.5~ FRMs. For low-volume Pb-PM~10~ monitors, this requirement is
easily met because the monitors incorporate temperature and pressure
sensors and the monitor software makes reporting these parameters
automatic. High-volume TSP samplers do not incorporate these sensors, so
more effort may be needed to report the data. We note that
sampler-generated average daily temperature and pressure are already
required to be reported to AQS from filter-based PM~2.5~ FRM/FEM
samplers, and that the current submission of these data would fulfill
the temperature and pressure reporting requirements for any Pb-TSP
sampling at the same site. Relevant measurements could also be obtained
from nearby National Weather System (NWS) monitoring sites, nearby
low-volume PM~2.5~ or PM~10~ samplers, and other nearby meteorological
measurements that undergo routine quality control checks and quality
assurance; relying on one of these sources would mean that a separate
data submission action would be needed to associate the data with the
Pb-TSP monitoring site. The reporting of average pressure and
temperature data would support the ability to investigate data quality
and other data analysis questions that may be arise with regard to the
Pb-TSP or Pb-PM~10~ monitors.

We seek comment on the requirement to report the average temperature and
pressure recorded during Pb measurements and the usefulness of such data
in supporting data analysis purposes.

2\. Special Purpose Monitoring Exemption

According to 40 CFR Section 58.20(e) "If an SPM using an FRM, FEM, or
ARM is discontinued within 24 months of start-up, the Administrator will
not designate an area as nonattainment for the CO, SO~2~, NO~2~, Pb, or
24-hour PM~10~ NAAQS solely on the basis of data from the SPM. Such data
are eligible for use in determinations of whether a nonattainment area
has attained one of these NAAQS." When this provision was added in the
October 2006 revisions to the ambient monitoring regulations, we stated
that the basis for finalizing a prohibition on the use of SPM data to
designate an area as nonattainment for Pb (as well as CO, SO~2~, NO~2~,
and PM~10~) was EPA's discretion to not make a finding of nonattainment
even though a SPM indicated a violation of the relevant NAAQS (see 71 FR
61252). We stated that even though the NAAQS for these pollutants have
forms that allow a nonattainment finding based on less than 24 months of
data, EPA does not have a mandatory duty to make nonattainment
redesignations until such time as the NAAQS are revised. Since EPA is
proposing to revise the Pb NAAQS, and the form of the proposed NAAQS
would allow a nonattainment finding to be based on only 1 or 2 years of
data, and such a NAAQS revision must be followed by a mandatory round of
designations, we are proposing to revise 40 CFR Section 58.20(e) by
removing the specific reference to Pb in the rule language.

V. Implementation Considerations

This section explains EPA's proposed approach for implementing the lead
primary and secondary standards based on the structure outlined in the
CAA, existing rules, existing guidance, and in some cases proposed
revised guidance. We intend the preamble to the final rule revising the
Pb NAAQS to provide EPA's final implementation guidance, possibly
supplemented by other guidance action(s) to be completed by September 1,
2008.

The CAA assigns important roles to EPA, states, and Tribal governments
in implementing NAAQS. States have the primary responsibility for
developing and implementing State Implementation Plans (SIPs) that
contain state measures necessary to achieve the air quality standards in
each area. EPA provides assistance to states and Tribes by providing
technical tools, assistance, and guidance, including information on the
potential control measures.

A SIP is the compilation of regulations and control programs that a
state uses to carry out its responsibilities under the CAA, including
the attainment, maintenance, and enforcement of the NAAQS. States use
the SIP development process to identify the emission sources that
contribute to the nonattainment problem in a particular area, and to
select the emission reduction measures most appropriate for the
particular area in question. Under the CAA, SIPs must ensure that areas
reach attainment as expeditiously as practicable. This section of the
proposal discusses the specific CAA requirements that must be addressed
when implementing any new or revised Pb NAAQS.

Currently only two areas in the United States are designated as
nonattainment and eleven areas are designated as maintenance areas for
the current Pb NAAQS. If the Pb NAAQS is lowered to the range proposed,
it is likely (based on a review of the current air quality monitoring
data) that additional areas would be designated as nonattainment. States
determined to have lead nonattainment areas would be required to submit
SIPs that identify and implement specific air pollution control measures
to reduce the ambient concentrations of lead to meet the NAAQS.

The EPA's analysis of the available Pb monitoring data suggests that a
large majority of recent exceedances of Pb levels in the range of 0.10
to 0.30 µg/m3 have occurred in locations with active or retired
industrial sources of Pb. Accordingly, if this pattern also prevails for
concentrations observed from new monitoring sites, many states may be
able to attain the revised NAAQS by implementing air pollution control
measures on lead emitting industrial sources only. These controls could
include measures such as fabric filter particulate control measures and
industrial fugitive control measures applied in plant buildings and on
plant grounds. However, it may become necessary in some areas to also
implement controls on non-industrial sources. Based on these
considerations, EPA believes that some of the regulations and guidance
being used to implement the current Pb NAAQS is still appropriate to
implement any of the options being proposed in this rulemaking for a new
or revised Pb NAAQS.

The regulations and guidance for implementing the current NAAQS for Pb
are mainly provided in the following documents: (1) "State
Implementation Plans; General Preamble for the Implementation of Title I
of the Clean Air Act Amendments of 1990, 57 FR 13549, April 16, 1992",
(2) "State Implementation Plans for Lead Nonattainment Areas; Addendum
to the General Preamble for the Implementation of Title I of the Clean
Air Act Amendments of 1990" and regulations at 40 CFR 51.117 . The
aforementioned documents address requirements such as designating areas,
setting nonattainment area boundaries, promulgating area
classifications, nonattainment area SIP requirements such as Reasonably
Available Control Measures (RACM), Reasonably Available Control
Technology (RACT), New Source Review (NSR), Prevention of Significant
Deterioration (PSD), and emissions inventory requirements. We have
summarized the most relevant information from these documents below for
your convenience. The EPA believes that there is sufficient guidance and
regulations to fully implement the proposed revised Pb NAAQS, although
EPA may review and revise or update as necessary, policies, guidance,
and regulations for implementing the Pb NAAQS in the future. The EPA
solicits comment on whether additional guidance is necessary for
implementation of the revised Pb NAAQS.

A. Designations for the Lead NAAQS

After EPA establishes or revises a NAAQS, the CAA requires EPA and the
states to begin taking steps to ensure that the new or revised NAAQS are
met. The first step is to identify areas of the country that do not meet
the new or revised NAAQS. The CAA defines EPA's authority to designate
areas that do not meet a new or revised NAAQS. Section 107(d)(1)
provides that "By such date as the Administrator may reasonably require,
but not later than 1 year after promulgation of a new or revised NAAQS
for any pollutant under section 109, the Governor of each state shall
.....submit to the Administrator a list of all areas (or portions
thereof) in the state" that designates those areas as nonattainment,
attainment, or unclassifiable. Section 107(d)(1)(B)(i) further provides,
"Upon promulgation or revision of a NAAQS, the Administrator shall
promulgate the designations of all areas (or portions thereof) ....as
expeditiously as practicable, but in no case later than 2 years from the
date of promulgation. Such period may be extended for up to one year in
the event the Administrator has insufficient information to promulgate
the designations."

The term "promulgation" has been interpreted by the courts to be
signature and dissemination of a rule.[^115] As noted above, the CAA
requires EPA to establish a deadline for the states' submission of the
designation recommendations, but under the CAA, it can be no later than
one year after promulgation of the rule. Therefore, Governors of states
will be required to submit their designation recommendations to EPA no
later than September 2009. This notice may be given in the preamble to
the final rule revising the Pb NAAQS, or in a guidance memorandum issued
at the same time as that final rule. The designation of areas for any
new or revised NAAQS for lead must occur no later than September 2010,
although that date may be extended by up to one year under the CAA (or
no later than September 2011) if EPA has insufficient information to
promulgate the designations. By no later than 120 days prior to
promulgating final designations, EPA is required to notify states or
Tribes of any intended modifications to their boundaries as EPA may deem
necessary. States and Tribes then have an opportunity to comment on
EPA's tentative decision. Whether or not a state or a Tribe provides a
recommendation, EPA must promulgate the designation that it deems
appropriate.

1\. Potential Schedule for Designations of a Revised Lead NAAQS

As stated previously, section 107(d)(1)(B)(i) provides EPA the authority
to promulgate designations for all areas of the country for any new or
revised NAAQS, as expeditiously as practicable, but in no case later
than 3 years from the date of promulgation of the new or revised NAAQS.
If the Administrator revises the Pb NAAQS by retaining Pb in TSP as the
indicator, but lowering the level of the standard, EPA anticipates being
able to designate areas for the Pb NAAQS within the two year period
provided by the CAA following the promulgation of a new or revised
NAAQS. The existing monitoring network for Pb in TSP (Pb-TSP), possibly
supplemented by additional appropriate monitors being deployed after
this proposal, will allow many parts of the country to be designated
either attainment or nonattainment. The EPA recognizes that some areas
will have to be designated as unclassifiable or
attainment/unclassifiable because of the lack of a sufficient record of
Pb-TSP monitoring data (and also a lack of sufficient data on Pb in
PM-10 as a surrogate for Pb-TSP data).[^116] The EPA would address the
latter areas once sufficient monitoring data has been collected for
each. This would result in designations occurring no later than
September 2010.

Under the scenario described above, states would be required to submit
designation recommendations by no later than September 2009 (i.e., one
year following promulgation of any new or revised NAAQS). Governors
should be able to consider ambient data collected through the end of
2008 and part way through 2009 when formulating their recommendations.
Whether or not a state or Tribe provides a recommendation, EPA must
promulgate the designation that it deems appropriate. As noted above,
following the presumptive statutory timeline, designations for any new
or revised Pb NAAQS would then be required to occur no later than
September 2010. As stated previously, by no later than 120 days prior to
promulgating designations, EPA is required to notify states or Tribes of
any intended modifications to their boundaries as EPA may deem
necessary. This would occur in May 2010. At that point, EPA will have
access to data from 2009 which state monitoring officials have certified
is complete and accurate, since the deadline for such certification is
April 30, 2010. EPA plans to consider data for calendar years 2007-2009
in formulating its proposed revisions, if any, to the designations
proposed by Governors. States and Tribes will then have an opportunity
to provide a demonstration as to why the proposed modification indicated
by EPA is inappropriate. The EPA is soliciting comments on the proposed
time line for designations of any new or revised NAAQS based on Pb-TSP
monitoring as described above.

If the revised NAAQS is based on Pb-PM~10~ as the indicator, the time
line for designations may need to be different, in that currently there
are very few monitoring sites in operation for this indicator.[^117] In
terms of this option, new Pb-PM~10~ monitors would be required to be
installed by January 1, 2010. Under the 2 year default timeline, none of
the data from these new monitors would be available. As such, it may be
appropriate for EPA to make use of the additional year to complete the
designations for all or some areas. If EPA requests an additional year
to complete the designations, designations would be due by May 2011, and
monitoring data for the year 2010 would be available for most of the new
monitoring sites. Based on the form of the proposed NAAQS (either
maximum quarter or 2^nd^ maximum monthly), EPA would be able to
designate those areas that exceeded the NAAQS during 2010 as
nonattainment, however, EPA would not have the three full years of
monitoring data necessary to designate areas as attainment. As such,
many areas would need to be designated as unclassifiable during the
initial designations. EPA would designate the remaining areas when
sufficient data is available. EPA invites comment on the time line issue
posed by the possible use of Pb-PM~10~ as the indicator in a revised Pb
NAAQS.

B. Lead Nonattainment Area Boundaries

As stated previously, the process for designating areas following the
promulgation of a new or revised NAAQS is contained in section 107(d)(1)
of the CAA. This section of the CAA provides each state Governor an
opportunity to recommend designations of attainment, nonattainment, or
unclassifiable for each area in the state. Section 107(d)(1) of the CAA
also directs the state to provide the appropriate boundaries to EPA for
each area of the state, and provides that EPA may make modifications to
the boundaries submitted by the state as it deems necessary. A lead
nonattainment area must consist of that area that does not meet (or
contributes to ambient air quality in a nearby area that does not meet)
the Pb NAAQS. Thus, a key factor in setting boundaries for nonattainment
areas is determining the geographic extent of nearby source areas
contributing to the nonattainment problem. Therefore, for each monitor
or group of monitors that exceed a standard, nonattainment boundaries
must be set that include a sufficiently large enough area to include
both the area judged to be violating the standard as well as the source
areas that are determined to be contributing to these violations.

Historically, Pb NAAQS violations have been the result of lead emissions
from large stationary sources and mobile sources that burn lead based
fuels. In some locations, a limited number of area sources have also
contributed to violations. Since lead was phased out of motor vehicle
gasoline, mobile sources are no longer a significant source of
violations of the current Pb NAAQS. At the current standard level, EPA
expects stationary sources to be the primary contributor to violations
of the NAAQS. At the lower standard levels contemplated in this
proposal, it is possible that fugitive dust emissions from area sources
containing deposited lead will also contribute to violations of any new
or revised Pb NAAQS. The location and dispersion characteristics of
these sources of ambient lead concentrations are important factors in
determining nonattainment area boundaries. The EPA is proposing that the
county boundary be the presumptive boundary for lead nonattainment
areas. However, we would also like to take comment on whether
Metropolitan Statistical Area (MSA) boundaries should be the presumptive
boundaries for lead nonattainment areas.

The first policy option is to presumptively define the boundaries for
designating areas by using the perimeter of the county boundaries
related to the air quality monitors which recorded violations of the
standard. This presumption was used in defining the boundaries for the
current Pb NAAQS. The second policy option is to define the boundaries
for the Pb NAAQS by using the OMB Metropolitan Statistical Area (MSA).
In either policy option, an area-specific nine-factor analysis (based on
factors identified in section 107(d)(1) of the CAA) could lead EPA to
depart from the presumptive boundary. The nine factors are identified
below.

1\. County-Based Boundaries

In the policy option being proposed by EPA for comment is that lead
nonattainment boundaries would be presumptively defined by the perimeter
of the county in which the ambient lead monitor(s) recording a violation
of the NAAQS is located, unless area-specific information indicates that
some other boundary is more appropriate. In addition, if the relevant
air quality monitor measuring a violation(s) is located near another
county, then EPA would presume that the contributing county should also
be designated as nonattainment for the Pb NAAQS. In some instances, a
boundary other than the county perimeter, which addresses areas impacted
by specific sources of lead, may also be appropriate. Also, under this
option Governors may chose alternatively to recommend the lead
nonattainment boundaries by using any one, or a combination of the
following techniques, the results of which EPA would consider when
making a decision as to whether and how to modify the Governors'
recommendations: (1) Qualitative analysis, (2) spatial interpolation of
air quality monitoring data, or (3) air quality simulation by dispersion
modeling. These techniques are more fully described in "Procedures for
Estimating Probability of Nonattainment of a PM-10 NAAQS Using Total
Suspended Particulate or PM-10 Data," December 1986. The EPA recommends
that the state submit a defensible rationale for the boundaries
recommended at the same time that the Governor submits recommendations
for the designation for an area so that EPA may fully evaluate the
recommendation.

EPA would consider state, local, and Tribal recommendations of
nonattainment area boundaries that deviate from county boundaries based
on various factors. These factors are discussed below. Consideration of
these factors may warrant a nonattainment area that has additions and/or
deletions relative to the county within which the violating monitor(s)
is located.

The factors for determining nonattainment area boundaries for the Pb
NAAQS under this option would closely resemble the factors identified in
recent EPA guidance for the 1997 8-hour ozone and the PM-2.5 NAAQS
nonattainment area boundaries. EPA proposes to apply these factors in
evaluating boundary modifications for both states and Tribes. For this
particular option, EPA would consider the following factors in assessing
whether to exclude portions of a county and whether to include
additional nearby areas outside the county as part of the designated
nonattainment area:

• Emissions in areas potentially included versus excluded from the
nonattainment area,

• Air quality in potentially included versus excluded areas,

• Population density and degree of urbanization including commercial
development in included versus excluded areas,

• Traffic and commuting patterns,

• Expected growth (including extent, pattern and rate of growth),

• Meteorology (weather/transport patterns),

• Geography/topography (mountain ranges or other air basin boundaries),

• Jurisdictional boundaries (e.g., counties, air districts,
Reservations, etc.),

• Level of control of emission sources

Analyses of these factors may suggest nonattainment boundaries that are
either larger or smaller than the county. A demonstration supporting the
designation of boundaries that are less than the full county must show
both that violation(s) are not occurring in the excluded portions of the
county and that the excluded portions are not source areas that
contribute to the observed violations. Under this option, a state or
Tribal government submittal that only addresses whether violations are
occurring throughout the county will not suffice as a justification for
designating a nonattainment area smaller than the county. States and
Tribes are encouraged to justify such recommendations by addressing all
of the factors identified above. Recommendations to designate a
nonattainment area larger than the county should also be based on an
analysis of these factors. EPA will consider these factors in evaluating
state and Tribal recommendations and assessing whether any modifications
are appropriate. EPA solicits comments on the use of these factors, and
other approaches, for adjusting county boundaries in designating
nonattainment areas.

2\. MSA-Based Boundaries

The EPA is also taking comment on whether lead nonattainment boundaries
should be presumptively defined by the perimeter of the Census Based
Statistical Area (CBSA) within which a violating monitor(s) is located.
The Metropolitan Area, as delineated by the Office of Management and
Budget (OMB), provides a presumptive definition of the populated area
associated with a core urban area. Accordingly, EPA is proposing in this
policy option that the Metropolitan Area would provide the presumptive
definition of the source area that contributes to a lead nonattainment
problem. This presumption would take the view that, in the absence of
evidence to the contrary, violations of the Pb NAAQS in urban areas may
be presumed attributable, at least in part, to contributions from large
sources of lead emissions distributed throughout the Metropolitan Area.
The EPA anticipates that OMB will publish revised Metropolitan Area
lists in 2008.

As in the first policy option, EPA would consider state, local, and
Tribal recommendations of nonattainment area boundaries that deviate
from MSA boundaries related to this particular option for defining
boundaries for the Pb NAAQS based on various factors. These factors
would be the same as discussed above.

As stated previously, EPA is proposing that the county boundaries be
used as the presumptive boundaries for any new or revised Pb NAAQS, but
is also requesting comments the MSA boundaries being used as the
presumptive boundaries for any new or revised Pb NAAQS. []{.mark}

C. Classifications

Section 172(a)(1)(A) of the CAA authorizes EPA to classify areas
designated as nonattainment for the purposes of applying an attainment
date pursuant to section 172(a)(2), or for other reasons. In determining
the appropriate classification, EPA may consider such factors as the
severity of the nonattainment problem and the availability and
feasibility of pollution control measures (see section 172(a)(1)(A) of
the CAA). The EPA may classify lead nonattainment areas, but is not
required to do so.

While section 172(a)(1)(A) provides a mechanism to classify
nonattainment areas, section 172(a)(2)(D) provides that the attainment
date extensions described in section 172(a)(2)(A) do not apply to
nonatainment areas having specific attainment dates that are addressed
under other provisions of the part D of the CAA. Section 192(a), of part
D, specifically provides an attainment date for areas designated as
nonattainment for the Pb NAAQS. Therefore, EPA has legal authority to
classify lead nonattainment areas, but the 5 year attainment date under
section 192(a) cannot be extended pursuant to section 172(a)(2)(D). The
EPA is proposing not to establish classifications within the 5 year
interval for attaining any new or revised NAAQS. This approach is
consistent with EPA's previous classification decision in the 1992
General Preamble (See 57 FR 13549, April 16, 1992).

D. Section 110(a)(2) Lead NAAQS Infrastructure Requirements

Under section 110(a)(1) and (2) of the CAA, all states are required to
submit plans to provide for the implementation, maintenance, and
enforcement of any new or revised NAAQS. Section 110(a)(1) and (2)
require states to address basic program elements, including requirements
for emissions inventories, monitoring, and modeling, among other things.
States are required to submit SIPs to EPA demonstrating these basic
program elements within 3 years of the promulgation of any new or
revised NAAQS. Subsections (A) through (M), of section 110(a)(2), set
forth the elements that a state's program must contain in their SIP. The
list below identifies the required program elements contained in section
110(a)(2).[^118] The list of section 110(a)(2) NAAQS implementation
requirements are the following:

-   Emissions limits and other control measures: Section 110(a)(2)(A).

-   Ambient air quality monitoring/data system: Section 110(a)(2)(B)
    requires SIPs to provide for setting up and operating ambient air
    quality monitors, collecting and analyzing data and making these
    data available to EPA upon request.

-   Program for enforcement of control measures: Section 110(a)(2)(C)
    requires SIPs to include a program providing for enforcement of
    measures and regulation of new/modified (permitted) sources.

-   Interstate transport: Section 110(a)(2)(D) requires SIPs to include
    provisions prohibiting any source or other type of emissions
    activity in one State from contributing significantly to
    nonattainment in another State or from interfering with measures
    required to prevent significant deterioration of air quality or to
    protect visibility.

-   Adequate resources: Section 110(a)(2)(E) requires States to provide
    adequate funding, personnel and legal authority for implementation
    of their SIPs.

-   Stationary source monitoring system: Section 110(a)(2)(F) requires
    States to establish a system to monitor emissions from stationary
    sources and to submit periodic emissions reports to EPA.

-   Emergency power: Section 110(a)(2)(G) requires States to provide for
    authority to implement the emergency episode provisions in their
    SIPs.

-   Provisions for SIP revision due to NAAQS changes or findings of
    inadequacies: Section 110(a)(2)(H) requires States to revise their
    SIPs in response to changes in the NAAQS, availability of improved
    methods for attaining the NAAQS, or in response to an EPA finding
    that the SIP is inadequate.

-   Provisions for meeting part D: Section 110(a)(2)(I) requires areas
    designated as nonattainment to meet the applicable nonattainment
    planning requirements of part D, title I of the CAA.

-   Section 121 consultation with local and Federal government
    officials: Section 110(a)(2)(J) requires States to meet applicable
    local and Federal government consultation requirements of section
    121.

-   Section 127 public notification of NAAQS exceedances: Section
    110(a)(2)(J) requires States to meet applicable requirements of
    section 127 relating to public notification of violating NAAQS.

-   PSD and visibility protection: Section 110(a)(2)(J) also requires
    States to meet applicable requirements of title I part C related to
    prevention of significant deterioration and visibility protection.

-   Air quality modeling/data: Section 110(a)(2)(K) requires that SIPs
    provide for performing air quality modeling for predicting effects
    on air quality of emissions from any NAAQS pollutant and submission
    of data to EPA upon request.

-   Permitting fees: Section 110(a)(2)(L) requires the SIP to include
    requirements for each major stationary source to pay permitting fees
    to cover the cost of reviewing, approving, implementing and
    enforcing a permit.

Consultation/participation by affected local government: Section
110(a)(2)(M) requires States to provide for consultation and
participation by local political subdivisions affected by the SIP.

E. Attainment Dates

Generally, the date by which an area is required to attain the Pb NAAQS
is determined by the effective date of the nonattainment designation for
the area. For areas designated nonattainment for any new or revised
primary Pb NAAQS, SIPs must provide for attainment of the NAAQS as
expeditiously as practicable, but no later than 5 years from the date of
the nonattainment designation for the area (see section 192(a) of the
CAA). So, for example, if final designations are effective in Fall 2010,
then nonattainment areas must plan to attain the NAAQS by no later than
Fall 2015.

F. Attainment Planning Requirements

Any state containing an area designated as nonattainment with respect to
the Pb NAAQS must develop for submission, a SIP meeting the requirements
of part D, Title I, of the CAA, providing for attainment (see sections
191(a) and 192(a) of the CAA). As indicated in section 191(a) all
components of the lead part D SIP must be submitted within 18 months of
an areas nonattainment designation. So, for example, if final
designations are effective in Fall 2010, the part D SIPs must be
submitted by Spring 2012. Additional specific plan requirements for lead
nonattainment areas are outlined in 40 CFR 51.117.

The general part D nonattainment plan requirements are set forth in
section 172 of the CAA. Section 172(c) specifies that SIPs submitted to
meet the part D requirements must, among other things, include
Reasonably Available Control Measures (RACM) (which includes Reasonably
Available Control Technology (RACT)), provide for Reasonable Further
Progress (RFP), include an emissions inventory, require permits for the
construction and operation of major new or modified stationary sources
(see also section 173), contain contingency measures, and meet the
applicable provisions of section 110(a)(2) of the CAA related to the
general implementation of a new or revised NAAQS. The EPA has provided
guidance on implementing the above provisions for the current NAAQS and
believes that this guidance can also be used for the implementation of
any new or revised Pb NAAQS \[See "State Implementation Plans; General
Preamble for the Implementation of Title I of the Clean Air Act
Amendments of 1990, 57 FR 13549, April 16, 1992", (2) "State
Implementation Plans for Lead Nonattainment Areas; Addendum to the
General Preamble for the Implementation of Title I of the Clean Air Act
Amendments of 1990" and regulations at 40 CFR 51.117\]. It is important
to note that lead nonattainment SIPs must meet all of the requirements
related to part D of the CAA, including those specified in section
172(c), even if EPA does not provide guidance for each provision (e.g.
applicable provisions of section 110(a)(2)).

1\. Schedule for Attaining Any New or Revised Pb NAAQS

Section 192(a) provides that the attainment date for a nonattainment
area must be Aas expeditiously as practicable, but in no case later than
5 years from the date of designation for the area.@ If designations are
promulgated for any new or revised Pb NAAQS in September 2010, the
initial attainment date for nonattainment areas for any new or revised
NAAQS would be as expeditiously as practicable, but no later than
September 2015. For an area with an attainment date of September 2015,
EPA would determine whether it had attained the Pb NAAQS by evaluating
air quality monitoring data from the 1, 2, or 3 previous calendar years
(i.e., 2012, 2013, and 2014) as available. The proposed alternative
forms of the NAAQS, maximum quarterly average concentration over three
years and second maximum monthly concentration over three years, would
both allow a nonattainment determination based on less than three years
of data, if the monitoring data in a more limited time period includes a
quarterly average above the level of the NAAQS or if it includes two
monthly averages above the level of the NAAQS. In such a case, EPA
intends to designate the affected area nonattainment even though less
than three years of data are available. EPA would designate an area
attainment only if three calendar years of data indicate the absence of
a violation. As stated above, EPA anticipates that some areas will have
to be designated unclassifiable. EPA would revisit their designation
once sufficient data for an attainment or nonattainment designation are
available.

Table 2 below is an overview of this proposed timeline for designations,
SIP submittal, and attainment dates under this proposed schedule.

**Table 2. Proposed Timeline for a Possible 2008 New or Revised Pb
NAAQS**

  ----------------------------------- -----------------------------------
  Promulgation date of standard       September 2008

  Monitoring data used for State      2007-2009
  recommendations                     

  State recommendations to EPA        September 2009

  Final designations signature        September 2010

  Effective date of designations      Fall 2010

  Infrastructure SIPs due             September 2011

  Nonattainment SIPs due              Spring 2012 (i.e., 18 months after
                                      designation)

  Attainment date                     Up to Fall 2015 (based on 2012-2014
                                      data)
  ----------------------------------- -----------------------------------

> 2\. RACM for Lead Nonattainment Areas

Lead nonattainment area SIPs must contain RACM (including RACT) that
addresses sources of ambient lead concentrations. In general, as stated
previously, EPA believes that lead nonattainment area issues are usually
attributed to emissions from stationary sources, but some emissions may
also be attributed to smaller area sources. As a general rule, the
stationary sources in lead nonattainment areas tend to emit a relatively
large amount of particulate matter containing lead. In EPA's 2002
National Emissions Inventory (NEI), there were 29 stationary sources in
the country with lead emissions over 5 tons per year, and 239 sources
over 1 ton of lead emissions per year.

At primary lead smelters, for example, the process of reducing
concentrated ore to lead involves a series of steps, some of which are
completed outside of buildings, or inside of buildings which are not
totally enclosed. Over a period of time, emissions from these sources
have been deposited in neighboring communities (e.g., on roadways,
parking lots, yards, and off-plant property). This historically
deposited lead, when disturbed, may be re-entrained into the ambient air
and re-entrained fugitive lead bearing dust may contribute to violations
of the Pb NAAQS in the affected area. There are also potential sources
of lead that are under federal control. As a part of the Regulatory
Impact Analysis for this rule, the EPA is reviewing the impact of these
and other sources of lead emissions to assess their impact on any new or
revised Pb NAAQS. States must also meet the requirements outlined in 40
CFR 51.117(a) related to control strategy demonstrations.

The first step in addressing RACM for lead is identifying potential
control measures for sources of lead in the nonattainment area. A
suggested starting point for specifying RACM in lead nonattainment area
SIPs is outlined in appendix 1, of the guidance entitled "State
Implementation Plans for Lead Nonattainment Areas; Addendum to the
General Preamble for the Implementation of Title I of the Clean Air Act
Amendments of 1990, 58 FR 67752, December 22, 1993." If a state receives
substantive public comments that demonstrate through appropriate
documentation, that additional control measures may be reasonably
available in a particular circumstance for an area, those measures
should be added to the list of available measures for consideration in
that particular area.

While EPA does not presume that these control measures are reasonably
available in all areas, a reasoned justification for rejection of any
available control measure should be prepared. If it can be shown that
measures or groups of measures are unreasonable because emissions from
the affected sources are insignificant, those measures may be excluded
from further consideration as they would not be representative of RACM
for an area. The resulting control measures should then be evaluated for
reasonableness, considering their technological feasibility and the cost
of control in the area for which the SIP applies. In the case of public
sector sources and control measures, this evaluation should consider the
impact and reasonableness of the measures on the municipal, or other
governmental entity that must assume the responsibility for their
implementation. It is important to note that a state should consider the
feasibility of implementing measures in part when full implementation
would be infeasible. A reasoned justification for partial or full
rejection of any available control measure, including those considered
or presented during the state's public hearing process, should be
prepared. The justification should contain an explanation, with
appropriate documentation, as to why each rejected control measure is
deemed infeasible or otherwise unreasonable for implementation.

Economic feasibility considers the cost of reducing emissions and the
difference between the cost of the emissions reduction approach at the
particular source in question and the costs of emissions reduction
approaches that have been implemented at other similar sources. Absent
other indications, EPA presumes that it is reasonable for similar
sources to bear similar costs of emissions reduction. Economic
feasibility for RACT purposes is largely determined by evidence that
other sources in a source category have in fact applied the control
technology or process change in question. EPA also encourages the
development of innovative measures not previously employed which may
also be technically and economically feasible.

The capital costs, annualized costs, and cost effectiveness of an
emissions reduction technology should be considered in determining
whether a potential control measure is reasonable for an area or state.
One available reference for calculating costs is the EPA Air Pollution
Control Cost Manual,[^119] which describes the procedures EPA uses for
determining these costs for stationary sources. The above costs should
be determined for all technologically feasible emission reduction
options. States may give substantial weight to cost effectiveness in
evaluating the economic feasibility of an emission reduction technology.
The cost effectiveness of a technology is its annualized cost (\$/year)
divided by the emissions reduced (i.e., tons/year) which yields a cost
per amount of emission reduction (\$/ton). Cost effectiveness provides a
value for each emission reduction option that is comparable with other
options and other facilities. With respect to a given pollutant, a
measure is likely to be reasonable if it has a cost per ton similar to
other measures previously employed for that pollutant. In addition, a
measure is likely to be reasonable from a cost effectiveness standpoint
if it has a cost per ton similar to that of other measures needed to
achieve expeditious attainment in the area within the CAA's time frames.

The fact that a measure has been adopted or is in the process of being
adopted by other states is an indicator (though not a definitive one)
that the measure may be technically and economically feasible for
another state. We anticipate that states may decide upon RACT and RACM
controls that differ from state to state, based on the state's
determination of the most effective strategies given the relevant
mixture of sources and potential controls in the relevant nonattainment
areas, and differences in difficulty of attaining expeditiously.
Nevertheless, states should consider and address RACT and RACM measures
developed for other areas or other states as part of a well reasoned
RACT and RACM analysis. The EPA's own evaluation of SIPs for compliance
with the RACT and RACM requirements will include comparison of measures
considered or adopted by other states.

In considering what level of control is reasonable, EPA is not proposing
a specific dollar per ton cost threshold for RACT. Areas with more
serious air quality problems typically will need to obtain greater
levels of emissions reductions from local sources than areas with less
serious problems, and it would be expected that their residents could
realize greater public health benefits from attaining the standard. For
these reasons, we believe that it will be reasonable and appropriate for
areas with more serious air quality problems and higher design values to
impose emission reduction requirements with generally higher costs per
ton of reduced emissions than the cost of emissions reductions in areas
with lower design values. In addition, where essential reductions are
more difficult to achieve (e.g., because many sources are already
controlled), the cost per ton of control may necessarily be higher.

The EPA believes that in determining appropriate emission control
levels, the state should consider the collective public health benefits
that can be realized in the area due to projected improvements in air
quality. Because EPA believes that RACT requirements will be met where
the state demonstrates timely attainment, and areas with more severe air
quality problems typically will need to adopt more stringent controls,
RACT level controls in such areas will require controls at higher cost
effectiveness levels (\$/ton) than areas with less severe air quality
problems.

In identifying the range of costs per ton that are reasonable,
information on benefits per ton of emission reduction can be useful as
one factor to consider. The Pb NAAQS RIA will provide information on the
estimated benefits per ton of reducing Pb emissions from various
emissions sources. It should be noted that such benefits estimates are
subject to significant uncertainty, and that benefits per ton vary in
different areas. Nonetheless this information could be used in a way
that recognizes these uncertainties. If a per ton cost of implementing a
measure is significantly less than the anticipated benefits per ton,
this would be an indicator that the cost per ton is reasonable. If a
source contends that a source-specific RACT level should be established
because it cannot afford the technology that appears to be RACT for
other sources in its source category, the source should support its
claim by providing detailed and verified information regarding the
impact of imposing RACT on:

\-- fixed and variable production costs (\$/unit),

\-- product supply and demand elasticity,

\-- product prices (cost absorption vs. cost pass-through),

\-- expected costs incurred by competitors,

\-- company profits, and

\-- employment costs.

The technical guidance that discusses in detail the suggested initial
measures for RACM and RACT for lead are contained in a document entitled
"Fugitive Dust Background Document and Technical Information Document
for Best Available Control Measures" (EPA-450/2-92-004, September 1992).
This document serves to provide an example for states on how to analyze
control costs for a given area.

Once the process of determining RACM for an area is completed, the
individual measures should then be converted into a legally enforceable
vehicle (e.g., a regulation or permit program) (see section 172(c)(6)
and section 110(a)(2)(A) of the CAA). The regulations or other measures
submitted should meet EPA's criteria regarding the enforceability of
SIPs and SIP revisions. These criteria were stated in a September 23,
1987 memorandum (with attachments) from J. Craig Potter, Assistant
Administrator for Air and Radiation; Thomas L. Adams, Jr. Assistant
Administrator for Enforcement and Compliance Monitoring; and S. Blake,
General Counsel, Office of the General Counsel; entitled "Review of
State Implementation Plans and Revisions of Enforceability and Legal
Sufficiency." As stated in this memorandum, SIPs and SIP revisions that
fail to satisfy the enforceability criteria should not be forwarded for
approval. If they are submitted, they will be disapproved if, in EPA's
judgment, they fail to satisfy applicable statutory and regulatory
requirements.

The EPA's historic definition of RACT is the lowest emissions limitation
that a particular source is capable of meeting by the application of
control technology that is reasonably available considering
technological and economic feasibility.[^120] RACT applies to the
"existing sources" of lead including stack emissions, industrial process
fugitive emissions, and industrial fugitive dust emissions (e.g.,
on-site haul roads, unpaved staging areas at the facility, etc) (see
section 172(c)(1)). For the current Pb NAAQS, EPA is proposing to
recommend that stationary sources which actually emit a total of 5 tons
per year of lead or lead compounds, measured as elemental lead, be the
minimum starting point for RACT analysis. Generally, EPA recommends that
available control technology be applied to those existing sources in the
nonattainment area that are reasonable to control in light of the
attainment needs of the area and the feasibility of such controls. Thus,
for the current Pb NAAQS, a state's control technology analysis may need
to include sources which actually emit less than 5 tons per year of lead
or lead compounds in the area, or other sources in the area that are
reasonable to control, in light of the attainment needs and feasibility
of control for the area. Given the proposal for promulgating a new or
revised Pb NAAQS significantly lower than the current standard, and
which may affect sources lower than the current 5 tpy threshold, EPA
requests comments on a RACT threshold that is between 0.3 and 5 tpy of
lead emissions.

3\. Demonstration of Attainment for Lead Nonattainment Areas

The SIPs for lead nonattainment areas should provide for the
implementation of control measures for point and area stationary sources
and mobile sources of lead emissions which demonstrate attainment of the
Pb NAAQS as expeditiously as practicable, but no later than the
applicable statutory attainment date for the area (See also 40 CFr
51.117(a) for additional control strategy requirements). Therefore, if a
state adopts less than all available measures in an area but
demonstrates, adequately, that reasonable further progress (RFP), and
attainment of the Pb NAAQS are assured, and application of all such
available measures would not result in attainment any faster, then a
plan which requires implementation of less than all technologically and
economically available measures may be approved (see 44 FR 20375 (April
4, 1979) and 56 FR 5460 (February 11, 1991)). The EPA believes that it
would be unreasonable to require that a plan which demonstrates
attainment include all technologically and economically available
control measures even though such measures would not expedite
attainment. Thus, for some sources in areas which demonstrate
attainment, it is possible that some available control measures may not
be "reasonably" available because their implementation would not
expedite attainment.

4\. Reasonable Further Progress (RFP)

Part D SIPs must provide for RFP (see section 172(c)(2) of the CAA).
Section 171 of the CAA defines RFP as "such annual incremental
reductions in emissions of the relevant air pollution as are required by
part D, or may reasonably be required by the Administrator for the
purpose of ensuring attainment of the applicable NAAQS by the applicable
attainment date." Historically, for some pollutants, RFP has been met by
showing annual incremental emission reductions generally sufficient to
maintain linear progress toward attainment by the applicable attainment
date. Requiring linear emission reduction progress to maintain RFP may
be appropriate where:

-   Pollutants are emitted by numerous and diverse sources;

-   The relationship between any individual source and the overall air
    quality is not explicitly quantified;

-   There is a chemical transformation involved; and

-   The emission control system utilized (e.g., at major point sources)
    will result in swift and significant emission reductions.

The EPA believes that it may not be reasonable to require linear
reductions in emissions in SIPs for lead nonattainment areas because the
air quality problem is not usually due to a vast inventory of sources.
However, this is not to suggest that generally it would be unreasonable
for EPA to require annual incremental reductions in emissions in lead
nonattainment areas. RFP for lead nonattainment areas should be met, at
least in part, by "adherence to an ambitious compliance schedule" which
is expected to periodically yield significant emission reductions, and
as appropriate, linear progress.[^121] The EPA recommends that SIPs for
lead nonattainment areas provide a detailed schedule for compliance of
RACM (including RACT) in the areas and accurately indicate the
corresponding annual emission reductions to be achieved. In reviewing
the SIP, EPA believes that it is appropriate to expect early
implementation of less technology-intensive control measures (e.g.,
controlling fugitive dust emissions at the stationary source, as well as
required controls on area sources) while phasing in the more
technology-intensive control measures, such as those involving the
installation of new hardware. Finally, it should be noted that failure
to implement the SIP provisions required to meet annual incremental
reductions in emissions (i.e., RFP) in a particular area could result in
the application of sanctions as described in sections 110(m) and 179(b)
of the CAA (pursuant to a finding under section 179(a)(4)), and the
implementation of contingency measures required by section 172(c)(9) of
the CAA.

5\. Contingency Measures

Section 172(c)(9) of the CAA defines contingency measures as measures in
a SIP that are to be implemented if an area fails to achieve and
maintain RFP, or fails to attain the NAAQS by the applicable attainment
date. Contingency measures must be designed to become effective without
further action by the state or the Administrator, upon determination by
EPA that the area has failed to achieve or maintain reasonable further
progress, or attain the Pb NAAQS by the applicable statutory attainment
date. Contingency measures should consist of available control measures
that are not already included in the primary control strategy for the
affected area.

Contingency measures are important for lead nonattainment areas, which
is generally due to emissions from stationary sources, for several
reasons. First, process and fugitive emissions from these stationary
sources, and the possible re-entrainment of historically deposited
emissions, have historically been difficult to quantify. Therefore, the
analytical tools for determining the relationship between reductions in
emissions, and resulting air quality improvements, can be subject to
some uncertainties. Second, emission estimates and attainment analysis
can be influenced by overly-optimistic assumptions about fugitive
emission control efficiency.

Examples of contingency measures for controlling area fugitive emissions
may include stabilizing additional storage piles etc. Examples of
contingency measures for processed-related fugitive emissions include
increasing the enclosure of buildings, increasing air flow in hoods,
increasing operation and maintenance procedures, etc. Examples for
contingency measures for stack sources include reducing hours of
operation, changing the feed material to lower lead content, and
reducing the occurrence of malfunctions by increasing operation and
maintenance procedures, etc.

Section 172(c)(9) provides that contingency measures should be included
in the SIP for a lead nonattainment area and shall "take effect without
further action by the state or the Administrator." The EPA interprets
this requirement to mean that no further rulemaking actions by the
state, or EPA, would be needed to implement the contingency measures
(see generally 57 FR 12512 and 13543-13544). The EPA recognizes that
certain actions, such as the notification of sources, modification of
permits, etc., may be needed before a measure could be implemented.
However, states must show that their contingency measures can be
implemented with minimal further action on their part and with no
additional rulemaking actions such as public hearings or legislative
review. After EPA determines that a lead nonattainment area has failed
to maintain RFP or timely attain the Pb NAAQS, EPA generally expects all
actions needed to affect full implementation of the measures to occur
within 60 days after EPA notifies the state of such failure. The state
should ensure that the measures are fully implemented as expeditiously
as practicable after the requirement takes effect.

6\. Nonattainment New Source Review (NSR) and Prevention of Significant
Deterioration (PSD) Requirements

The PSD and nonattainment NSR programs contained in parts C and D of
title I of the CAA govern preconstruction review and permitting programs
for any new or modified major stationary sources of air pollutants
regulated under the CAA as well as any precursors to the formation of
that pollutant when identified for regulation by the Administrator. EPA
rules addressing these regulations can be found at 40 CFR 51.165,
51.166, 52.21, 52.24, and part 51, appendix S.

Areas designated as nonattainment for the Pb NAAQS must submit SIPs that
address the requirements of nonattainment area NSR. Specifically,
section 172(c)(5) of the CAA requires that States which have areas
designated as nonattainment for the Pb NAAQS must submit, as a part of
the nonattainment area SIP, provisions requiring permits for the
construction and operation of new or modified stationary sources
anywhere in the nonattainment area, in accordance with the permit
requirements pursuant to section 173 of the CAA.

Stationary sources that emit lead are currently subject to regulation
under existing requirements for the preconstruction review and approval
of new and modified stationary sources. The existing requirements,
referred to collectively as the New Source Review (NSR) program, require
any major and minor stationary sources of any air pollutant for which
there is a NAAQS to undergo review and approval prior to the
commencement of construction.[^122] The NSR program is composed of three
different permit programs:

-   Prevention of Significant Deterioration (PSD);

-   Nonattainment NSR (NA NSR); and,

-   Minor NSR.

The PSD program and nonattainment NSR programs, contained in parts C and
D, respectively, of Title I of the CAA, are often referred to as the
major NSR program because these programs regulate only major sources.

The PSD program applies when a major source, that is located in an area
that is designated as attainment or unclassifiable for any criteria
pollutant, is constructed, or undergoes a major modification.[^123] The
NA NSR program applies when a major source that is located in an area
that is designated as nonattainment for any criteria pollutant is
constructed or undergoes a major modification. The minor NSR program
addresses both major and minor sources that undergoes construction or
modification activities that do not qualify as major, and it applies
regardless of the designation of the area in which a source is located.

The national regulations that apply to each of these programs are
located in the CFR as shown below:

  ----------------------- -----------------------------------------------
                          Applications

  PSD                     40 CFR 52.21, 40 CFR 51.166, 40 CFR 51.165(b)

  NA NSR                  40 CFR 52.24, 40 CFR 51.165, 40 CFR part 51,
                          Appendix S

  Minor NSR               40 CFR 51.160-164
  ----------------------- -----------------------------------------------

The PSD requirements include but are not limited to the following:

-   Installation of Best Available Control Technology (BACT)

-   Air Quality monitoring and modeling analyses to ensure that a
    project's emissions will not cause or contribute to a violation of
    any NAAQS or maximum allowable pollutant increase (PSD increment),

-   Notification of Federal Land Manager of nearby Class I areas, and

-   Public comment on permit.

> Nonattainment NSR requirements include but are not limited to:

-   Installation of Lowest Achievable Emissions Rate (LAER) control
    technology.

-   Offsetting new emissions with creditable emissions reductions.

-   A certification that all major sources owned and operated in the
    state by the same owner are in compliance with all applicable
    requirements under the CAA.

-   An alternative citing analysis demonstrating that the benefits of
    proposed source significantly outweigh the environmental and social
    costs imposed as a result of its location, construction, or
    modification.

-   Public comment on the permit.

Minor NSR programs must meet the statutory requirements in section
110(a)(2)(C) of the CAA which requires "\* \* \* regulation of the
modification and construction of any stationary source \* \* \* as
necessary to assure that the \[NAAQS\] are achieved."

Areas which are newly designated as nonattainment for the Pb NAAQS as a
result of any changes made to the NAAQS will be required to adopt the NA
NSR program to address major sources of lead where the program does not
currently exist for the Pb NAAQS. Prior to adoption of the SIP revision
addressing NSR for lead nonattainment areas, the requirements of 40 CFR
part 51, Appendix S will apply.

7\. Emissions Inventories

States must develop and periodically update a comprehensive, accurate,
current inventory of actual emissions affecting ambient lead
concentrations. The emissions inventory is used by states and EPA to
determine the nature and extent of the specific control strategy
necessary to help bring an area into attainment of the NAAQS. Emissions
inventories should be based on measured emissions or documented
emissions factors. Generally, the more comprehensive and accurate the
inventory, the more effective the evaluation of possible control
measures can be for the affected area (see section 172(c)(3) of the
CAA).

Pursuant to its authority under section 110 of Title I of the CAA, EPA
has long required states to submit emission inventories containing
information regarding the emissions of criteria pollutants as well as
their precursors. The EPA codified these requirements in 40 CFR part 51,
subpart Q in 1979 and amended them in 1987. The 1990 Clean Air Act
Amendments (CAAA) revised many of the provisions of the CAA related to
attainment of the NAAQS. These revisions established new emission
inventory requirements applicable to certain areas that were designated
as nonattainment for certain pollutants. In the case of lead, the
emission inventory provisions are in the general provisions pursuant to
section 173(c)(3) of the CAA.

In June 2002, EPA promulgated the Consolidated Emissions Reporting Rule
(CERR)(67 FR 39602, June 10, 2002). The CERR consolidates the various
emissions reporting requirements that already exist into one place in
the CFR, and establishes new requirements for the state wide reporting
of area source and mobile source emissions. States should follow the
requirements under the CERR as well as any new or revised guidance
related to emissions inventories for criteria pollutants. The CERR
establishes two types of required emissions inventories: (1) Annual
inventories, and (2) 3- year cycle inventories. The annual inventory
requirement is limited to reporting statewide emissions data from the
larger point sources. For the 3-year cycle inventory, states will need
to report data from all of their point sources plus all of the area and
mobile sources on a statewide basis. States should estimate mobile
source emissions by using the latest emissions models and planning
assumptions available at the time the SIP is developed. Information and
guidance on the latest emissions models is available at
http://www.epa.gov/otaq/stateresources/transconf/policy.htm#models and
at http://[www.epa.gov/otaq/models](http://www.epa.gov/otaq/models).htm.

By merging emissions information from relevant point sources, area
sources and mobile sources into a comprehensive emission inventory,
state, local and tribal agencies may do the following:

-   set a baseline for SIP development.

-   measure their progress in reducing emissions.

-   answer the public=s request for information.

The EPA uses the data submitted by the states to develop the National
Emission Inventory (NEI). The NEI is used by EPA to show national
emission trends, as modeling input for analysis of potential
regulations, and other purposes.

Most importantly, states need these inventories to help in the
development of control strategies and demonstrations to attain the Pb
NAAQS. While the CERR sets forth requirements for data elements, EPA
guidance complements these requirements and indicates how the data
should be prepared for SIP submissions. Our regulations at 40 CFR
51.117(e) require states to include in the inventory all point sources
that emit 5 or more tons of lead emissions per year. If EPA decides to
tighten the Pb NAAQS we will also consider lowering the threshold for
RACT. In this proposed rulemaking we will be taking comment on whether
the threshold for RACT should be somewhere in the range of 0.3-5 tpy.

The SIP inventory must be approved by EPA as a SIP element and is
subject to public hearing requirements, whereas the CERR is not. Because
of the regulatory significance of the SIP inventory, EPA will need more
documentation on how the SIP inventory was developed by the State as
opposed to the documentation required for the CERR inventory. In
addition, the geographic area encompassed by some aspects of the SIP
submission inventory will be different from the statewide area covered
by the CERR emissions inventory.

The EPA has proposed the Air Emissions Reporting Rule (AERR) at 71 FR 69
(Jan. 3, 2006). When finalized, the AERR would update the CERR reporting
requirements by consolidating and harmonizing new emissions reporting
requirements with pre-existing sets of reporting requirements under the
Clean Air Interstate Rule (CAIR) and the NOx SIP Call. At this time, EPA
expects to finalize the AERR rulemaking in the Fall of calendar year
2008. The AERR is expected to be a means by which the Agency will
implement additional data reporting requirements for the Pb NAAQS SIP
emission inventories.

8\. Modeling

The lead SIP regulations found at 40 CFR 51.117 require states to employ
atmospheric dispersion modeling for the demonstration of attainment for
areas in the vicinity of point sources listed in 40 CFR 51.117(a)(1). To
complete the necessary dispersion modeling, meteorological, and other
data are necessary. Dispersion modeling should follow the procedures
outlined in EPA's latest guidance document entitled "Guideline on Air
Quality Models". This guideline indicates the types and historical
records for data necessary for modeling demonstrations (e.g., on-site
meteorological stations are used, 12 months of data are required in
order to demonstrate attainment for the affected area).

G. General Conformity

Section 176(c) of the CAA, as amended (42 U.S.C. 7401 et seq.), requires
that all Federal actions conform to an applicable implementation plan
developed pursuant to section 110 and part D of the CAA. Section 176(c)
of the CAA requires EPA to promulgate criteria and procedures for
demonstrating and assuring conformity of Federal actions to a SIP. For
the purpose of summarizing the general conformity rule, it can be viewed
as containing three major parts: applicability, procedure, and analysis.
These are briefly described below.

The general conformity rule covers direct and indirect emissions of
criteria pollutants or their precursors that are caused by a Federal
action, are reasonably foreseeable, and can practicably be controlled by
the Federal agency through its continuing program responsibility. The
general conformity rule generally applies to Federal actions except; (1)
Actions covered by the transportation conformity rule; (2) Actions with
respect to associated emissions below specified de minimis levels; and
(3) Certain other actions that are exempt or presumed to conform.

The general conformity rule also establishes procedural requirements.
Federal agencies must make their conformity determinations available for
public review. Notice of draft and final general conformity
determinations must be provided directly to air quality regulatory
agencies and to the public by publication in a local newspaper.

The general conformity determination examines the impacts of direct and
indirect emissions related to Federal actions. The general conformity
rule provides several options to satisfy air quality criteria and
requires the Federal action to also meet any applicable SIP requirements
and emissions milestones. Each Federal agency must determine that any
actions covered by the general conformity rule conform to the applicable
SIP before the action is taken. The criteria and procedures for
conformity apply only in nonattainment and maintenance areas with
respect to the criteria pollutants under the CAA:[^124] carbon monoxide
(CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter
(PM-2.5 and PM-10), and sulfur dioxide (SO2). The general conformity
rule establishes procedural requirements for Federal agencies for
actions related to all NAAQS pollutants, both nonattainment and
maintenance areas and will apply one year following the promulgation of
designations for any new or revised Pb NAAQS.[^125]

H. Transition from the Current NAAQS to a Revised NAAQS for Lead

EPA is proposing to revise the level of the Pb NAAQS significantly, as
well as changing the indicator and averaging time. The EPA believes that
Congress's intent, as evidenced by section 110(l), 193, and section
172(e) of the CAA, was to ensure that continuous progress, in terms of
public health protection, takes place in transitioning from a current
NAAQS for a pollutant to a new or revised NAAQS. Therefore, in this
section, EPA is proposing that the existing NAAQS will be revoked one
year following the promulgation of designations for any new NAAQS,
except that the existing NAAQS will not be revoked for any current
nonattainment area until the affected area submits, and EPA approves, an
attainment demonstration which addresses the attainment of the new Pb
NAAQS.

The NAAQS will be revoked for all other areas one year after
designations for any new or revised Pb NAAQS. The CAA contains a number
of provisions that indicate Congress's intent to not allow states to
alter or remove provisions from implementation plans if the plan
revision would jeopardize the air quality protection being provided by
the plan. For example, section 110(l) provides that EPA may not approve
a SIP revision if it interferes with any applicable requirement
concerning attainment and RFP, or any other applicable requirement under
the CAA. In addition section 193 of the CAA prohibits the modification
of a control, or a control requirement, in effect or required to be
adopted as of November 15, 1990 (i.e., following the promulgation of the
Clean Air Act Amendments (CAAA) of 1990), unless such a modification
would ensure equivalent or greater emissions reductions. One other
provision of the CAA provides additional insight into Congress's intent
related to the need to continue progress towards meeting air quality
standards during periods of transition from one standard to another.
Section 172(e) of the CAA, related to future modifications of a
standard, applies when EPA promulgates a new or revised NAAQS and makes
it less stringent than the previous NAAQS. This provision of the CAA
specifies that in such circumstances, states may not relax control
obligations that apply in nonattainment area SIPs, or avoid adopting
those controls that have not yet been adopted as required.

Because it is EPA's belief that Congress did not intend to permit states
to remove control measures when EPA revises a standard until the new or
revised standard is implemented, we believe that controls that are
required under the current Pb NAAQS, or that are currently in place
under the current Pb NAAQS, should remain in place until designations
are promulgated for any new or revised standard. As a result, EPA is
proposing that the current Pb NAAQS should stay in place for at least
one year following the effective date of designations for any new or
revised NAAQS before being revoked. Pursuant to CAA section 110(l), any
proposed SIP revision being considered by EPA after the effective date
of the revised Pb NAAQs would be evaluated for its potential to
interfere with attainment or maintenance of the new standard. Unlike the
transition from the 1-hour ozone standard to the 8-hour ozone standard,
EPA believes that any area attaining the revised Pb NAAQS would also
attain the existing Pb NAAQS, and thus reviewing proposed SIP revisions
for interference with the new standard will be sufficient to prevent
backsliding. Consequently, in light of nature of the proposed revision
of the Pb NAAQS, and the small number of nonattainment areas, EPA
believes that retaining the current standard for a limited period of
time will adequately serve the anti-backsliding goals of the CAA. The
EPA requests comment on this proposed option for transition to any new
or revised Pb NAAQS that may be promulgated.

**VI. Statutory and Executive Order Reviews**

*A. Executive Order 12866: Regulatory Planning and Review*

Under section 3(f)(1) of Executive Order (EO) 12866 (58 FR 51735,
October 4, 1993), this action is an "economically significant regulatory
action" because it is likely to have an annual effect on the economy of
\$100 million or more. 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 (EPA-HQ-OAR-2006-0735). In addition, EPA
prepared a Regulatory Impact Analysis (RIA) of the potential costs and
benefits associated with this action. A copy of the analysis is
available in the RIA docket (EPA-HQ-OAR-2008-0253) and the analysis is
briefly summarized here. The RIA estimates the costs and monetized human
health and welfare benefits of attaining four alternative Pb NAAQS
nationwide. Specifically, the RIA examines the alternatives of 0.30
μg/m^3^, 0.20 μg/m^3^, 0.10 μg/m^3^ and 0.05 μg/m^3^. The RIA contains
illustrative analyses that consider a limited number of emissions
control scenarios that States and Regional Planning Organizations might
implement to achieve these alternative Pb NAAQS. However, the CAA and
judicial decisions make clear that the economic and technical
feasibility of attaining ambient standards are not to be considered in
setting or revising NAAQS, although such factors may be considered in
the development of State plans to implement the standards. Accordingly,
although an RIA has been prepared, the results of the RIA have not been
considered in issuing this proposed rule.

*B. Paperwork Reduction Act*

The information collection requirements in this proposed rule have been
submitted for approval to the Office of Management and Budget (OMB)
under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The
Information Collection Request (ICR) document prepared by EPA has been
assigned EPA ICR number **0940.21.**

The monitoring, recordkeeping, and reporting requirements in 40 CFR
parts 53 and 58 are specifically authorized by sections 100, 301(a), and
319 of the Clean Air Act. All information submitted to EPA pursuant to
the monitoring, recordkeeping, and reporting requirements for which a
claim of confidentiality is made is safeguarded according to Agency
policies in 40 CFR part 2, subpart B.

The information collected under 40 CFR part 53 (e.g, test results,
monitoring records, instruction manual, and other associated
information) is needed to determine whether a candidate method intended
for use in determining attainment of the National Ambient Air Quality
Standards (NAAQS) in 40 CFR part 50 will meet the design, performance,
and/or comparability requirements for designation as a Federal reference
method (FRM) or Federal equivalent method (FEM). This proposed rule adds
requirements for Pb-PM~10~ FRM and FEM determinations. The incremental
annual reporting and recordkeeping burden for the collection of
information under 40 CFR part 53 is estimated to increase by X labor
hours and \$Y cost/year. The capital/startup costs for test equipment
and qualifying tests are estimated at \$Y.

The information collected and reported under 40 CFR part 58 is needed to
determine compliance with the NAAQS, to characterize air quality and
associated health and ecosystem impacts, to develop emissions control
strategies, and to measure progress for the air pollution program. The
proposed amendments would revise the technical requirements for Pb
monitoring sites, require the siting and operation of additional Pb
ambient air monitors, and the reporting of the collected ambient Pb
monitoring data to EPA's Air Quality System (AQS). The annual average
reporting burden for the collection under 40 CFR part 58 (averaged over
the first **3** years of this ICR) for {**Y#}** respondents is estimated
to increase by a total of {X} labor hours be year with an increase of
{\$Y} per year. Burden is defined at 5 CFR 1320.3(b). State, local, and
Tribal entities are eligible for State assistance grants provided by the
Federal government under the CAA which can be used for monitors and
related activities.

An agency may not conduct or sponsor, and a person is not required to
respond to, a collection of information unless it displays a currently
valid OMB control number. The OMB control numbers for EPA\'s regulations
in 40 CFR are listed in 40 CFR part 9.

To comment on the Agency\'s need for this information, the accuracy of
the provided burden estimates, and any suggested methods for minimizing
respondent burden, EPA has established a public docket for this rule,
which includes this ICR, under Docket ID number EPA-HQ-OAR-2006-0735.
Submit any comments related to the ICR to EPA and OMB. See ADDRESSES
section at the beginning of this notice for where to submit comments to
EPA. Send comments to OMB at the Office of Information and Regulatory
Affairs, Office of Management and Budget, 725 17th Street, NW,
Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is
required to make a decision concerning the ICR between 30 and 60 days
after **\[Insert date of publication in the Federal Register\]**, a
comment to OMB is best assured of having its full effect if OMB receives
it by **\[Insert date 30 days after publication in the Federal
Register.\]**. The final rule will respond to any OMB or public comments
on the information collection requirements contained in this proposal.

*C. Regulatory Flexibility Act*

The Regulatory Flexibility Act (RFA) generally requires an agency to
prepare a regulatory flexibility analysis of any rule subject to notice
and comment rulemaking requirements under the Administrative Procedure
Act or any other statute unless the agency certifies that the rule will
not have a significant economic impact on a substantial number of small
entities. Small entities include small businesses, small organizations,
and small governmental jurisdictions.

For purposes of assessing the impacts of this rule on small entities,
small entity is defined as: (1) a small business that is a small
industrial entity as defined by the Small Business Administration's
(SBA) regulations at 13 CFR 121.201; (2) a small governmental
jurisdiction that is a government of a city, county, town, school
district or special district with a population of less than 50,000; and
(3) a small organization that is any not-for-profit enterprise which is
independently owned and operated and is not dominant in its field.

After considering the economic impacts of this proposed rule on small
entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This proposed
rule will not impose any requirements on small entities. Rather, this
rule establishes national standards for allowable concentrations of Pb
in ambient air as required by section 109 of the CAA. *American Trucking
Ass'ns v. EPA,* 175 F. 3d 1027, 1044-45 (D.C.cir. 1999) (NAAQS do not
have significant impacts upon small entities because NAAQS themselves
impose no regulations upon small entities). We continue to be interested
in the potential impacts of the proposed rule on small entities and
welcome comments on issues related to such impacts.

*D. Unfunded Mandates Reform Act*

Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law
104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and Tribal
governments and the private sector. Unless otherwise prohibited by law,
under section 202 of the UMRA, EPA generally must prepare a written
statement, including a cost-benefit analysis, for proposed and final
rules with "Federal mandates" that may result in expenditures to State,
local, and Tribal governments, in the aggregate, or to the private
sector, of \$100 million or more in any one year. Before promulgating an
EPA rule for which a written statement is required under section 202,
section 205 of the UMRA generally requires EPA to identify and consider
a reasonable number of regulatory alternatives and to adopt the least
costly, most cost-effective or least burdensome alternative that
achieves the objectives of the rule. The provisions of section 205 do
not apply when they are inconsistent with applicable law. Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes any regulatory
requirements that may significantly or uniquely affect small
governments, including Tribal governments, it must have developed under
section 203 of the UMRA a small government agency plan. The plan must
provide for notifying potentially affected small governments, enabling
officials of affected small governments to have meaningful and timely
input in the development of EPA regulatory proposals with significant
Federal intergovernmental mandates, and informing, educating, and
advising small governments on compliance with the regulatory
requirements.

The revisions to the Pb NAAQS impose no enforceable duty on any State,
local or Tribal governments or the private sector. EPA has determined
that this proposed rule does not contain a Federal mandate that may
result in expenditures of \$100 million or more for State, local, and
tribal governments, in the aggregate, or the private sector in any one
year. Therefore, this action is not subject to the requirements of
sections 202 and 205 of the UMRA. States may be required by this rule to
satisfy increased monitoring requirements, but the costs of implementing
those requirements are not expected to exceed \$100 million in the
aggregate for any year. Furthermore, as indicated previously, in setting
a NAAQS EPA cannot consider the economic or technological feasibility of
attaining ambient air quality standards. Because the Clean Air Act
prohibits EPA from considering the types of estimates and assessments
described in section 202 when setting the NAAQS, the UMRA does not
require EPA to prepare a written statement under section 202 for the
revisions to the Pb NAAQS.

With regard to implementation guidance, the CAA imposes the obligation
for States to submit SIPs to implement the Pb NAAQS. In this proposed
rule, EPA is merely providing an interpretation of those requirements.
However, even if this rule did establish an independent obligation for
States to submit SIPs, it is questionable whether an obligation to
submit a SIP revision would constitute a Federal mandate in any case.
The obligation for a State to submit a SIP that arises out of section
110 and section 191 of the CAA is not legally enforceable by a court of
law, and at most is a condition for continued receipt of highway funds.
Therefore, it is possible to view an action requiring such a submittal
as not creating any enforceable duty within the meaning of 2 U.S.C. 658
for purposes of the UMRA. Even if it did, the duty could be viewed as
falling within the exception for a condition of Federal assistance under
2 U.S.C. 658.

EPA has determined that the proposed rule contains no regulatory
requirements that might significantly or uniquely affect small
governments. Therefore, this rule is not subject to the requirements of
section 203 of the UMRA.

E. Executive Order 13132: Federalism

Executive Order 13132, entitled "Federalism" (64 FR 43255, August 10,
1999), requires EPA to develop an accountable process to ensure
"meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications."
"Policies that have federalism implications" is defined in the Executive
Order to include regulations that have "substantial direct effects on
the States, on the relationship between the national government and the
States, or on the distribution of power and responsibilities among the
various levels of government."

This proposed rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government, as
specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding the
establishment and implementation of air quality improvement programs as
codified in the CAA. Under section 109 of the CAA, EPA is mandated to
establish NAAQS; however, CAA section 116 preserves the rights of States
to establish more stringent requirements if deemed necessary by a State.
Furthermore, this rule does not impact CAA section 107 which establishes
that the States have primary responsibility for implementation of the
NAAQS. Finally, as noted in section E (above) on UMRA, this rule does
not impose significant costs on State, local, or Tribal governments or
the private sector. Thus, Executive Order 13132 does not apply to this
rule.

However, EPA recognizes that States will have a substantial interest in
this rule and any corresponding revisions to associated air quality
surveillance requirements, 40 CFR part 58. Therefore, in the spirit of
Executive Order 13132, and consistent with EPA policy to promote
communications between EPA and State and local governments, EPA
specifically solicits comment on this proposed rule from State and local
officials.

*F. Executive Order 13175: Consultation and Coordination with Indian
Tribal Governments*

Executive Order 13175, entitled "Consultation and Coordination with
Indian Tribal Governments" (65 FR 67249, November 9, 2000), requires EPA
to develop an accountable process to ensure "meaningful and timely input
by tribal officials in the development of regulatory policies that have
tribal implications." This proposed rule does not have Tribal
implications, as specified in Executive Order 13175. It does not have a
substantial direct effect on one or more Indian Tribes, since Tribes are
not obligated to adopt or implement any NAAQS. Thus, Executive Order
13175 does not apply to this rule.

Although Executive Order 13175 does not apply to this rule, EPA
contacted Tribal environmental professionals during the development of
this rule. EPA staff participated in the regularly scheduled Tribal Air
call sponsored by the National Tribal Air Association during the spring
of 2008 as the proposal was under development. EPA specifically solicits
additional comment on this proposed rule from Tribal officials.

*G. Executive Order 13045: Protection of Children from Environmental
Health & Safety Risks*

Executive Order 13045, "Protection of Children from Environmental Health
Risks and Safety Risks" (62 FR 19885, April 23, 1997) applies to any
rule that: (1) is determined to be "economically significant" as defined
under Executive Order 12866, and (2) concerns an environmental health or
safety risk that EPA has reason to believe may have a disproportionate
effect on children. If the regulatory action meets both criteria, the
Agency must evaluate the environmental health or safety effects of the
planned rule on children, and explain why the planned regulation is
preferable to other potentially effective and reasonably feasible
alternatives considered by the Agency.

This proposed rule is subject to Executive Order 13045 because it is an
economically significant regulatory action as defined by Executive Order
12866, and we believe that the environmental health risk addressed by
this action may have a disproportionate effect on children. The proposed
rule will establish uniform national ambient air quality standards for
Pb; these standards are designed to protect public health with an
adequate margin of safety, as required by CAA section 109. However, the
protection offered by these standards may be especially important for
children because neurological effects in children are among if not the
the most sensitive health endpoints for Pb exposure. Because children
are considered a sensitive population, we have carefully evaluated the
environmental health effects of exposure to Pb pollution among children.
These effects and the size of the population affected are summarized in
chapters 6 and 8 of the Criteria Document and sections 3.3 and 3.4 of
the Staff Paper, and the results of our evaluation of the effects of Pb
pollution on children are discussed in sections II.B and II.C of this
preamble.

*H. Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution or Use*

This proposed rule is not a "significant energy action" as defined in
Executive Order 13211, "Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use" (66 FR 28355
(May 22, 2001)) because it is not likely to have a significant adverse
effect on the supply, distribution, or use of energy. The purpose of
this rule is to establish revised NAAQS for Pb. The rule does not
prescribe specific control strategies by which these ambient standards
will be met. Such strategies will be developed by States on a
case-by-case basis, and EPA cannot predict whether the control options
selected by States will include regulations on energy suppliers,
distributors, or users. Thus, EPA concludes that this rule is not likely
to have any adverse energy effects and does not constitute a significant
energy action as defined in Executive Order 13211.

*I. National Technology Transfer and Advancement Act*

Section 12(d) of the National Technology Transfer and Advancement Act of
1995 (NTTAA), Public Law No. 104-113, §12(d) (15 U.S.C. 272 note)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical. Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies. The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.

The proposed amendments include revisions to methods used for the
measurement of Pb in ambient air. These measurements would be used to
demonstrate compliance with the proposed revised Pb NAAQS. In developing
this proposed rule, EPA conducted a search for technical standards that
might be used to meet the need for measurements of Pb in ambient air.
One potential method was identified by this search (ISO 9855:1993),
however, this method was not adopted because the method does not provide
for the detection limits necessary to adequately characterize ambient Pb
concentrations for the purpose of determining compliance with the
proposed revisions to the Pb NAAQS. As such, as discussed in section
VI.A of this preamble, EPA is proposing a new FRM and FEM criteria for
the measurement of Pb in ambient air. EPA welcomes comments on the
proposed measurement methods, and specifically invites the public to
identify potentially applicable voluntary consensus standards and to
explain why such standards should be used in the regulation.

*J. Executive Order 12898: Federal Actions to Address Environmental
Justice in Minority Populations and Low-Income Populations*

Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes Federal
executive policy on environmental justice. Its main provision directs
Federal agencies, to the greatest extent practicable and permitted by
law, to make environmental justice part of their mission by identifying
and addressing, as appropriate, disproportionately high and adverse
human health or environmental effects of their programs, policies, and
activities on minority populations and low-income populations in the
United States.

EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or
low-income population. The proposed rule will establish uniform national
standards for Pb in ambient air.

EPA is continuing to assess the impact of Pb air pollution on minority
and low- income populations. The results of this assessment will be
presented in a technical memo entitled **\[INSERT TITLE, FULL
REFERENCE\]** which will be available in the docket by the date of
publication of this proposed rule in the Federal Register. As a result
of this analysis, EPA has determined that this proposed rule will not
have disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or
low-income population.

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of EPA Lead Reference Method\"

1\. Guide to Atomic Spectroscopy Techniques and Applications: AA, GFAA,
ICP, ICP-MS; PerkinElmer instruments, Norwalk, CT; 2000
(www.perkinelmer.com)

2\. Inorganic Compendium Method IO-3.3; Determination of Metals in
Ambient Particulate Matter Using X-Ray Fluorescence (XRF) Spectroscopy;
U.S. Environmental Protection Agency, Cincinnati, OH 45268.
EPA/625/R-96/010a. June 1999.

**List of Subjects in 40 CFR Part 50**

Environmental protection, Air pollution control, Carbon monoxide, Lead,
Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Dated:

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Stephen L. Johnson,

Administrator.

For the reasons stated in the preamble, title 40, chapter I of the code
of Federal regulations is proposed to be amended as follows:

*{See separate files}*

[^1]: The legislative history of section 109 indicates that a primary
    standard is to be set at "the maximum permissible ambient air level
    . . . which will protect the health of any \[sensitive\] group of
    the population," and that for this purpose "reference should be made
    to a representative sample of persons comprising the sensitive group
    rather than to a single person in such a group." S. Rep. No.
    91-1196, 91st Cong., 2d Sess. 10 (1970)

[^2]: Welfare effects as defined in section 302(h) (42 U.S.C. 7602(h))
    include, but are not limited to, "effects on soils, water, crops,
    vegetation, man-made materials, animals, wildlife, weather,
    visibility and climate, damage to and deterioration of property, and
    hazards to transportation, as well as effects on economic values and
    on personal comfort and well-being."

[^3]: In considering whether the CAA allowed for economic considerations
    to play a role in the promulgation of the NAAQS, the Surpreme Court
    rejected arguments that because many more factors than air pollution
    might affect public health, EPA should consider compliance costs
    that produce health losses in setting the NAAQS. 531 U.S. at 466.
    Thus, EPA may not take into account possible public health impacts
    from the economic cost of implementation. *Id.*

[^4]: As described in Section III below the CDC stated in 2005 that no
    "safe" threshold for blood Pb levels in young children has been
    identified (CDC, 2005a).

[^5]: Co-chaired by the Secretary of the HHS and the Administrator of
    the EPA, the Task Force consisted of representatives from 16 Federal
    departments and agencies.

[^6]: See "Criteria for Classification of Solid Waste Disposal
    Facilities and Practices and Criteria for Municipal Solid Waste
    Landfills: Disposal of Residential Lead-Based Paint Waste; Final
    Rule" EPA-HQ-RCRA-2001-0017.

[^7]: See, e.g., "Hazardous Waste Management System; Identification and
    Listing of Hazardous Waste: Inorganic Chemical Manufacturing Wastes;
    Land Disposal Restrictions for Newly Identified Wastes and CERCLA
    Hazardous Substance Designation and Reportable Quantities; Final
    Rule",
    http://www.epa.gov/epaoswer/hazwaste/state/revision/frs/fr195.pdf
    and http://www.epa.gov/epaoswer/hazwaste/ldr/basic.htm.

[^8]: See, e.g., "Implementation of the Mercury-Containing and
    Rechargeable Battery Management Act"
    http://www.epa.gov/epaoswer/hazwaste/recycle/battery.pdf and
    "Municipal Solid Waste Generation, Recycling, and Disposal in the
    United States: Facts and Figures for 2005"
    http://www.epa.gov/epaoswer/osw/conserve/resources/msw-2005.pdf.

[^9]: The 95^th^ percentile value for the 2003-2004 NHANES is 5.1 μg/dL
    (Axelrad, 2008).

[^10]: The "indicator" of a standard defines the chemical species or
    mixture that is to be measured in determining whether an area
    attains the standard.

[^11]: The "form" of a standard defines the air quality statistic that
    is to be compared to the level of the standard in determining
    whether an area attains the standard.

[^12]: As noted in the Staff Paper, quantitative estimates of emissions
    associated with resuspension of soil-bound Pb particles and
    contaminated road dust are not included in the 2002 NEI.

[^13]: The emissions estimates identified as mobile sources in the
    current NEI are currently limited to combustion of leaded aviation
    gas in piston-engine aircraft. Lead emissions estimates for other
    mobile source emissions of Pb (e.g., brake wear, tire wear, loss of
    Pb wheel weights and others) are not included in the current NEI.

[^14]: Air Pb concentrations nationally are estimated to have declined
    more than 90% since the early 1980s, in locations not known to be
    directly influenced by stationary sources (Staff Paper, pp. 2-22 to
    2-23).

[^15]: Details with regard to the data set and analyses supporting the
    values provided here are presented in Section 2.3.2 of the Staff
    Paper.

[^16]: We note that in the risk assessment, we only assessed alternate
    standard impacts on the subset of air-related pathways that respond
    relatively quickly to changes in air Pb.

[^17]: Weathering of outdoor Pb paint may also contribute to soil Pb
    levels adjacent to the house.

[^18]: Some recent exposure studies have evaluated the relative
    importance of diet to other routes of Pb exposure. In reports from
    the NHEXAS, Pb concentrations measured in households throughout the
    Midwest were significantly higher in solid food compared to
    beverages and tap water (Clayton et al., 1999; Thomas et al., 1999).
    However, beverages appeared to be the dominant dietary pathway for
    Pb according to the statistical analysis (Clayton et al., 1999),
    possibly indicating greater bodily absorption of Pb from liquid
    sources (Thomas et al., 1999). Dietary intakes of Pb were greater
    than those calculated for intake from home tap water or inhalation
    on a μg/day basis (Thomas et al., 1999). The NHEXAS study in Arizona
    showed that, for adults, ingestion was a more important Pb exposure
    route than inhalation (O'Rourke et al., 1999). (CD, p. 3-43)

[^19]: For example, the Criteria Document states the following: "Given
    the large amount of time people spend indoors, exposure to Pb in
    dusts and indoor air can be significant. For children, dust ingested
    via hand-to-mouth activity is often a more important source of Pb
    exposure than inhalation. Dust can be resuspended through household
    activities, thereby posing an inhalation risk as well. House dust Pb
    can derive both from Pb-based paint and from other sources outside
    the home. The latter include Pb-contaminated airborne particles from
    currently operating industrial facilities or resuspended soil
    particles contaminated by deposition of airborne Pb from past
    emissions." (CD, p. E-6)

[^20]: Additionally, the 1977 Criteria Document included a dietary Pb
    intake estimate for the general population of 100 to 350 µg Pb/day
    (USEPA 1977, p. 1-2) and the 2006 Criteria Document cites recent
    studies indicating a dietary intake ranging from 2 to 10 µg Pb/day
    (CD, Section 3.4 and p. 8-14).

[^21]: Sources of Pb in food were identified in the 1986 Criteria
    Document as including air-related sources, metals used in processing
    raw foodstuffs, solder used in packaging and water used in cooking
    (1986a, section 3.1.2).

[^22]: The soil and dust levels are described as \"typical geochemical
    non-air input levels for dust and soil\" (Henderson, 2007a, p.
    F-60). The values used for these levels in this simulation fall
    within the range of 1 to 200 ppm described in the Criteria Document
    for soil not influenced by sources (CD, p. 3-18).

[^23]: The other IEUBK inputs (e.g., exposure and biokinetic factors)
    were those used in the IEUBK modeling for the risk assessment in
    this review (Henderson, 2007a, p. F-60).

[^24]: ^?^ With the 2005 statement, CDC identified a variety of reasons,
    reflecting both scientific and practical considerations, for not
    lowering the 1991 level of concern, including a lack of effective
    clinical or public health interventions to reliably and consistently
    reduce blood Pb levels that are already below 10 μg/dL, the lack of
    a demonstrated threshold for adverse effects, and concerns for
    deflecting resources from children with higher blood Pb levels (CDC,
    2005a). CDC's Advisory Committee on Childhood Lead Poisoning
    Prevention recently provided recommendations regarding interpreting
    and managing blood Pb levels below 10 μg/dL in children and reducing
    childhood exposures to Pb (ACCLPP, 2007).

[^25]: This information is available at:
    hhttp://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm
    (click on \"Download a universal spreadsheet file of the Body
    Burdens data tables\").

[^26]: Ratios are presented in the form of 1:x, with the 1 representing
    air Pb (in µg/m^3^) and x representing blood Pb (in µg/dL).
    Description of ratios as higher or lower refers to the values for x
    (i.e., the change in blood Pb per unit of air Pb). Slopes are
    presented as simply the value of x.

[^27]: For example, adjusted ratios from Brunekreef (1984, Table 1)
    ranged up to 1:8.5 and unadjusted ratios extended above 1:10.

[^28]: Public comments submitted to the Agency have cited support for
    estimates of 1:4, 1:6 and 1:7.

[^29]: The CASAC Panel stated \"The Schwartz and Picher analysis showed
    that in 1978, the midpoint of the National Health and Nutrition
    Examination Survey (NHANES) II, gasoline Pb was responsible for 9.1
    μg/dL of blood Pb in children. Their estimate is based on their
    coefficient of 2.14 μg/dL per 100 metric tons (MT) per day of
    gasoline use, and usage of 426 MT/day in 1976. Between 1976 and when
    the phase-out of Pb from gasoline was completed, air Pb
    concentrations in U.S. cities fell a little less than 1 μg/m^3^
    (24). These two facts imply a ratio of 9-10 μg/dL per μg/m^3^
    reduction in air Pb, taking all pathways into account.\" (Henderson,
    2007 a, page D-2 to D-3).

[^30]: Throughout this notice, characterization of ratios as higher or
    lower is based on consideration of the second value in the ratio
    (i.e., with regard to blood Pb response), such that a ratio of 1:3
    is described as greater than one of 1:2.

[^31]: Lead has been classified as a probable human carcinogen by the
    International Agency for Research on Cancer, based mainly on
    sufficient animal evidence, and as reasonably anticipated to be a
    human carcinogen by the U.S. National Toxicology Program (CD,
    Section 6.7.2). U.S. EPA considers Pb a probable carcinogen
    (<http://www.epa.gov/iris/subst/0277.htm>; CD, p. 6-195) .

[^32]: The Criteria Document states "neurotoxic effects in children and
    cardiovascular effects in adults are among those best substantiated
    as occurring at blood-Pb concentrations as low as 5 to 10 ug/dL (or
    possibly lower); and these categories of effects are currently
    clearly of greatest public health concern (CD, p. 8-60)"

[^33]: Further, neurological effects in general include behavioral
    effects, such as delinquent behavior (CD, sections 6.2.6 and
    8.4.2.2), sensory effects, such as those related to hearing and
    vision (CD, sections 6.2.7 and 8.4.2.3), and deficits in neuromotor
    function (CD, p. 8-36).

[^34]: As summarized in section II.B.1 above and discussed in more
    detail in the Criteria Document, blood Pb levels at any point in
    time are influenced by current as well as past exposures, e.g.,
    through exchanges between blood and bone.

[^35]: The toxicological evidence presented in the Criteria Document of
    biphasic dose-effect relationships includes: suppression of
    stimulated hippocampal glutamate release at low exposure levels and
    induction of glutamate exocytosis at higher exposure levels (CD,
    Section 5.3.2); downregulation of NMDA receptors at low blood Pb
    levels and upregulation at higher levels (CD, section 5.3.2); Pb
    causes elevated induction threshold and diminished magnitude of
    long-term potentiation at low exposures , but not at higher
    exposures (CD, section 5.3.3); and low-level Pb exposures increase
    fixed-interval response rates and high-level Pb exposures decrease
    fixed interval response rates in learning deficit testing in rats
    (CD, section 5.3.5). Additional *in vitro* evidence includes Pb
    stimulation of PKC activity at picomolar concentrations and
    inhibition of PKC activity at nano- and micro-molar concentrations
    (CD, section 5.3.2).

[^36]: These levels are in contrast to the geometric mean blood Pb level
    of 14.9 μg/dL reported for U.S. children (aged 6 months to 5 years)
    in 1976-1980 (CD, Section 4.3.1.3).

[^37]: For example, NHANES data for older adults (60 years of age and
    older) indicate a decline in overall population geometric mean blood
    Pb level from 3.4 μg/dL in 1991-1994 to 2.2 μg/dL in 1999-2002; the
    trend for adults between 20 and 60 years of age is similar to that
    for children 1 to 5 years of age
    (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5420a5.htm).

[^38]: Specifically, among young adults who lived as children in an area
    heavily polluted by a smelter and whose current Pb exposure was low,
    higher bone Pb levels were associated with higher systolic and
    diastolic blood pressure (CD, p. 8-74). In adult rats, greater early
    exposures to Pb are associated with increased levels of amyloid
    protein precursor, a marker of risk for neurodegenerative disease
    (CD, p. 8-74).

[^39]: The differing evidence and associated strength of the evidence
    for these different effects is described in detail in the Criteria
    Document.

[^40]: As is described in Section II.C.2.a, CASAC, in their comments on
    the analysis plan for the risk assessment described in this notice,
    placed higher priority on modeling the child IQ metric than the
    adult endpoints (e.g., cardiovascular effects).

[^41]: Similarly, "although an increase of a few mmHg in blood pressure
    might not be of concern for an individual's well-being, the same
    increase in the population mean might be associated with substantial
    increases in the percentages of individuals with values that are
    sufficiently extreme that they exceed the criteria used to diagnose
    hypertension" (CD, p. 8-77).

[^42]: For example, for a population mean IQ of 100 (and standard
    deviation of 15), 2.3% of the population would score above 130, but
    a shift of the population to a mean of 95 results in only 0.99% of
    the population scoring above 130 (CD, pp. 8-81 to 8-82).

[^43]: For example, approximately 4.8 million children live in poverty,
    while the estimates of numbers of adults with hypertension, diabetes
    or chronic kidney disease are on the order of 20 to 50 million (see
    Table 3-3 of Staff Paper).

[^44]: For example, the 2005 American Housing Survey, conducted by the
    U.S. Census Bureau indicates that some 14 million (or approximately
    13% of) housing units are \"within 300 feet of a 4-or-more-lane
    roadway, railroad or airport\" (U.S. Census Bureau, 2006).
    Additionally, estimates developed for Colorado, Georgia and New York
    indicate that approximately 15-30% of the populations in those
    states reside within 75 meters of a major roadway (i.e., a "Limited
    Access Highway", "Highway", "Major Road" or "Ramp", as defined by
    the U.S. Census Feature Class Codes) (ICF, 2005).

[^45]: The Criteria Document states that "While several studies have
    demonstrated a positive correlation between blood pressure and blood
    Pb concentration, others have failed to show such association when
    controlling for confounding factors such as tobacco smoking,
    exercise, body weight, alcohol consumption, and socioeconomic
    status. Thus, the studies that have employed blood Pb level as an
    index of exposure have shown a relatively weak association with
    blood pressure. In contrast, the majority of the more recent studies
    employing bone Pb level have found a strong association between
    long-term Pb exposure and arterial pressure (Chapter 6). Since the
    residence time of Pb in the blood is relatively short but very long
    in the bone, the latter observations have provided rather compelling
    evidence for a positive relationship between Pb exposure and a
    subsequent rise in arterial pressure" (CD, pp. 5-102 to 5-103).
    Further, in consideration of the meta-analysis also described here,
    the Criteria Document stated that "The meta-analysis provides strong
    evidence for an association between increased blood Pb and increased
    blood pressure over a wide range of populations" (CD, p. 6-130) and
    "the meta-analyses results suggest that studies not detecting an
    effect may be due to small sample sizes or other factors affecting
    precision of estimation of the exposure effect relationship" (CD, p.
    6-133).

[^46]: In the general population, both cumulative and circulating Pb has
    been found to be associated with longitudinal decline in renal
    functions. In the large NHANES III study, alterations in urinary
    creatinine excretion rate (one indicator of possible renal
    dysfunction) was observed in hypertensives at a mean blood Pb of
    only 4.2 μg/dL. These results provide suggestive evidence that the
    kidney may well be a target organ for effects from Pb in adults at
    current U.S. environmental exposure levels. The magnitude of the
    effect of Pb on renal function ranged from 0.2 to -1.8 mL/min change
    in creatinine clearance per 1.0 μg/dL increase in blood Pb in
    general population studies. However, the full significance of this
    effect is unclear, given that other evidence of more marked signs of
    renal dysfunction have not been detected at blood Pb levels below
    30-40 μg/dL among thousands of occupationally-exposed Pb workers
    that have been studied. (CD, p. 6-270)

[^47]: Neither fit of the model nor other sensitivity analyses were
    conducted (or reported) for these coefficients.

[^48]: A sixth case study (the secondary Pb smelter case study) is also
    described in the Risk Assessment Report. However, as discussed in
    Section 4.3.1 of that document (USEPA, 2007b), significant
    limitations in the approaches employed for this case study have
    contributed to large uncertainties in the corresponding estimates.

[^49]: [In representing the different forms of each standard level
    assessed (maximum monthly or maximum quarterly) as annual air
    concentrations]{.mark} for input to the blood Pb model for this case
    study, however, we relied on averages of these relationships for
    large urban areas nationally. As the averages are higher than the
    medians, localized areas near more than half the urban monitoring
    locations would have higher exposures and associated risks than
    those reported for this case study. Further, we note that exposure
    concentrations would be twice those used here if the 25^th^
    percentile values for these relationships had been used in place of
    the averages. For this reason, this case study should not be
    interpreted as representing a high-end scenario with regard to the
    characterization of ambient air Pb levels and associated risk.

[^50]: See Section III.B.2.a for a summary of CASAC's comment with
    regard to the primary and secondary Pb smelter case studies.

[^51]: Based on the nature of the population exposures represented by
    the two categories of case study, the first category (the general
    urban and primary Pb smelter case studies) relates more closely to
    the second evidence-based framework (see Sections II.D.2.a and
    II.E.3.a) with regard to estimates of air-related IQ loss. As
    mentioned above these case studies, as compared to the other
    category of case studies, include populations that are relatively
    more highly air pathway exposed to air Pb concentrations somewhat
    near the standard level evaluated.

[^52]: For further discussion of the air quality scenarios and averaging
    times included in the risk assessment, see section 2.3.1 of the Risk
    Assessment Report (USEPA, 2007b)

[^53]: This scenario was simulated for the location-specific urban case
    studies using a proportional roll-up procedure. For the general
    urban case study, the maximum quarterly average ambient air
    concentration was set equal to the current NAAQS.

[^54]: Recent air indoor dust Pb was estimated using the mechanistic
    component of the hybrid blood Pb model (see Section 3.1.4 of the
    Risk Assessment Report). For the primary Pb smelter case study,
    estimates for this pathway are not separated from estimates for the
    pathway described in the subsequent bullet due to uncertainty
    regarding this categorization with the model used for this case
    study (Section 3.1.4.2 of the Risk Assessment Report).

[^55]: \"Other\" indoor dust Pb is estimated using the intercept in the
    dust models plus that predicted by the outdoor soil concentration
    coefficient (for models that include soil Pb as a predictor of
    indoor dust Pb) (Section 3.1.4 of the Risk Assessment Report).

[^56]: Indoor dust Pb modeling for the urban case studies is based on a
    hybrid mechanistic-empirical model which considers the direct impact
    of Pb in ambient air on indoor dust Pb (i.e., which models the
    infiltration of ambient air indoors and subsequent deposition of Pb
    to indoor surfaces). This modeling does not consider other ambient
    air-related contributions to indoor dust, such as "tracking in" of
    outdoor soil Pb. By contrast, indoor dust Pb modeling for the
    primary Pb smelter case study subarea uses a site-specific
    regression model which relates average dust Pb values (based on a
    recent multi-year datast) to annual average air Pb concentrations
    (based on air dispersion modeling). In this way, modeling for the
    primary Pb smelter subarea may reflect some contributions to indoor
    dust Pb that relate to longer term impacts of ambient air (e.g.,
    "tracking in" of outdoor soil), as well as contributions from
    infiltration of ambient air. Additional detail on the methods used
    in characterizing Pb concentrations in outdoor soil and indoor dust
    are presented in Sections 3.1.3 and 3.1.4 of the Risk Assessment,
    respectively. Data, methods and assumptions here used in
    characterizing Pb concentrations in these exposure media may differ
    from those in other analyses that serve different purposes.

[^57]: Because the IEUBK blood Pb model runs with an annual time step,
    the air concentrations input to the "recent air" pathways modeling
    steps were in terms of annual average air concentration.

[^58]: The GSD for the urban case studies, in the risk assessment
    described in this notice, was derived using NHANES data for the
    years 1999-2000.

[^59]: [For example, the maximum quarterly mean Pb concentrations in the
    location-specific case studies ranged from 0.09-0.36 μg/m^3^, while
    the median value for this statistic for all sites in the 2003-05
    national dataset 0.03 μg/m^3^ (USEPA, 2007a, appendix A).]{.mark}

[^60]: Air-to-blood Pb ratios calculated based on peak blood Pb levels
    (e.g., at approximately 2 years of age) would be even larger than
    the ratios based on the lifetime-average blood Pb metric results
    discussed here.

[^61]: The ratios increase as the level of the alternate standard
    decreases. This reflects nonlinearity in the Pb response predicted
    by the hybrid dust model, which is greater on a per-unit basis for
    lower ambient air Pb levels.

[^62]: As with such estimates for the urban case study, ratios are
    higher at lower ambient air Pb levels, reflecting the nonlinearity
    of the dust Pb response with air concentration.

[^63]: The detailed results are provided in the Risk Assessment Report
    (USEPA, 2007b).

[^64]: A complete presentation of risk estimates is available in the
    final Risk Assessment Report, including a presentation of estimates
    for the 95th percentile in Table 5-10 of that report.

[^65]: The CDC subsequently revised their advisory level for children's
    blood Pb to 25 μg/dL in 1985, and to 10 μg/dL 1991. In 2005, with
    consideration of a review of the evidence by their advisory
    committee, CDC revised their statement on Preventing Lead Poisoning
    in Young Children, specifically recognizing the evidence of adverse
    health effects in children with blood Pb levels below 10 μg/dL and
    the data demonstrating that no "safe" threshold for blood Pb in
    children had been identified, and emphasizing the importance of
    preventative measures (CDC, 2005a). Recently, CDC's Advisory
    Committee on Childhood Lead Poisoning Prevention noted the 2005 CDC
    statements and reported on a review of the clinical interpretation
    and management of blood Pb levels below 10 μg/dL (ACCLPP, 2007).
    More details on this level are provided in Section II.B.1.

[^66]: Mean blood Pb levels in the adult study groups ranged above 10
    μg/dL to approximately 30 μg/dL and in the child groups they ranged
    from approximately 20 μg/dL up to 65 μg/dL (USEPA, 1986a, section
    11.4.1).

[^67]: In 1991, the CDC reduced their advisory level for children's
    blood Pb from 25 μg/dL to 10 μg/dL.

[^68]: For example, the following statement is made in the Criteria
    Document "Negative Pb impacts on neurocognitive ability and other
    neurobehavioral outcomes are robust in most recent studies even
    after adjustment for numerous potentially confounding factors
    (including quality of care giving, parental intelligence, and
    socioeconomic status). These effects generally appear to persist
    into adolescence and young adulthood." (CD, p.E-9)

[^69]: For context, it is noted that the 2001-2004 median blood level
    for children aged 1-5 of all races and ethnic groups is 1.6 μg/dL,
    the median for the subset living below the poverty level is 2.3
    μg/dL and 90th percentile values for these two groups are 4.0 μg/dL
    and 5.4 μg/dL, respectively. Similarly, the 2001-2004 median blood
    level for black, non-hispanic children aged 1-5 is 2.5 μg/dL, while
    the median level for the subset of that group living below the
    poverty level is 2.9 μg/dL and the median level for the subset
    living in a household with income more than 200% of the poverty
    level is 1.9 μg/dL. Associated 90th percentile values for 2001-2004
    are 6.4 μg/dL (for black, non-hispanic children aged 1-5), 7.7 μg/dL
    (for the subset of that group living below the poverty level) and
    4.1 μg/dL (for the subset living in a household with income more
    than 200% of the poverty level).
    (http://www.epa.gov/envirohealth/children/body_burdens/b1-table.htm -
    then click on \"Download a universal spreadsheet file of the Body
    Burdens data tables\").

[^70]: These findings include significant associations in the study
    sample subsets of children with blood Pb levels less than 10 µg/dL,
    less than 7.5 µg/dL, and less than 5 µg/dL. The mean blood Pb level
    in the third subset was 1.7 µg/dL (Auinger, 2008). A positive, but
    not statistically significant association, was observed in the less
    than 2.5 μg/dL subset (mean blood Pb of 1.2 µg/dL \[Auinger,
    2008\]), although the effect estimate for this subset was largest
    among all the subsets (Lanphear et al., 2000). The lack of
    statistical significance for this subset may be due to the smaller
    sample size of this subset which would lead to lower statistical
    power.

[^71]: Air Pb concentrations nationally are estimated to have declined
    more than 90% since the early 1980s, in locations not known to be
    directly influenced by stationary sources (Staff Paper, pp. 2-22 to
    2-23).

[^72]: This is based on the calculation in which 1.5 µg/m^3^ is
    multiplied by a ratio of 3 µg blood Pb per 1 µg/m^3^ air Pb to yield
    an air-related blood Pb estimates of 4.5 µg/dL; using a 1:5 ratio
    yields an estimate of 7.5 µg/dL. As with the 1978 framework, the
    context for use of the air-to-blood ratio here is a population being
    exposed at the level of the standard.

[^73]: We note that while we have termed risk estimates drived for the
    sum of "recent air" plus "past air" exposure pathways as "upper
    bound" estimates of air-related risk, the primary Pb smelter subarea
    is an area where soil has been remediated and thus does not reflect
    any historical deposition. Further, soil Pb concentrations in this
    area are not stable and may be increasing, seeming to indicate
    ongoing response to current atmospheric depositon in the area. Thus,
    for this case study, the "recent air" plus "past air" estimates are
    less of an "upper bound" for air-related risk than in other case
    studies where historical Pb deposition may have some representation
    in the "past air" soil ingestion pathway.

[^74]: As recognized in section III.B.2.d above, to simulate air
    concentrations associated with the current NAAQS, a proportional
    roll-up of concentrations from those for current conditions was
    performed for the location-specific urban case studies. This was not
    necessary for the primary Pb smelter case study in which air
    concentrations currently exceed the current standard, nor for the
    general urban case study.

[^75]: All written comments submitted to the Agency are available in the
    docket for this rulemaking, are transcripts of the public meetings
    held in conjunction with CASAC's review of the Staff Paper, the Risk
    Assessment Report, the Criteria Document and the ANPR.

[^76]: While recognizing that there are significant uncertainties
    associated with the risk estimates from the case studies, EPA places
    an appropriate weight on the risk assessment results for purposes of
    evaluating the adequacy of the current standard, given the strength
    of the evidence of the existence of effects at blood Pb levels
    associated with exposures at the level of the current standard, the
    magnitude of the IQ losses that are estimated, and the consistency
    of these IQ losses with the estimates of IQ loss derived from the
    alternative evidence-based framework The weight to place on the risk
    assessment results for purposes of evaluating alterative levels of
    the standard is discussed later in the discussion on the level of
    the standard.

[^77]: The current standard specifies the measurement of airborne Pb
    with a high-volume TSP federal reference method (FRM) sampler with
    atomic absorption spectrometry of a nitric acid extract from the
    filter for Pb, or with an approved equivalent method

[^78]: For simplicity, the discussion here and below speaks as if PM~10~
    samplers have a sharp size cut-off. In reality, they have a size
    selection behavior in which 50% of particles 10 microns in size are
    captured, with a progressively higher capture rate for smaller
    particles and a progressively lower capture rate for larger
    particles. The ideal capture efficiency curve for PM~10~ samplers
    specifies that particles above 15 microns not be captured at all,
    although real samplers may capture a very small percentage of
    particles above 15 microns. TSP samplers have 50% capture points in
    the range of 25 to 50 microns, which is broad enough to include
    virtually all particles capable of being transported any significant
    distance from their source except under extreme wind events. As
    explained below, the capture efficiency of a high-volume TSP sampler
    for any given size particle is affected by wind speed and wind
    direction.

[^79]: In this notice, we use "ultra-coarse" to refer to particles
    collected by a TSP sampler but not by a PM~10~ sampler (we note that
    CASAC has variously also referred to these particles as "very
    coarse" or "larger coarse-mode" particles), "fine" to refer to
    particles collected by a PM~2.5~ sampler, and "coarse" to refer to
    particles collected by a PM~10~ sampler but not by a PM2.5 sampler,
    recognizing that there will be some overlap in the particle sizes in
    the three types of collected material.

[^80]: "Low-volume PM~10~ sampling" refers to sampling using any of a
    number of monitor models that draw 16.67 liters/minute (1.000 m^3^
    /hour) of air through the filter, in contrast to "high-volume"
    sampling of either TSP or PM~10~ in which the monitor draws 1100 to
    1700 liters/minute (66 to 102 m^3^ /hour). All commercial TSP FRM
    samplers at this time are high-volume samplers; both high-volume and
    low-volume PM~10~ FRM samplers are available. Low-volume sampling is
    the more recently introduced method. Low-volume and high-volume
    samplers differ in many other ways also, including filter size,
    accuracy of the flow control, and degree of computerization.

[^81]: In their advice, CASAC recognized the potential for site-to-site
    variability in the relationship between Pb-TSP and Pb-PM~10~
    (Henderson, 2007a, 2007b).

[^82]: The Pb-TSP FRM specification, 40 CFR 50 appendix G, currently
    explicitly requires the use of the high-volume TSP FRM sampler which
    is required by appendix B for the mass of TSP. Therefore it would
    require amendments to 40 CFR 50 appendix B and/or G (or a new
    dedicated appendix) to establish a low-volume TSP sampler as the
    only FRM, or as an alternative FRM, for TSP and/or Pb-TSP
    measurement. A number of researchers have utilized both self-built
    and commercially available low-volume TSP samplers in ambient air
    studies. Typically, these samplers are identical to low-volume
    PM~10~ FRM samplers with the exception that their inlets and other
    size separation devices (or lack thereof) are aimed at collecting
    TSP. EPA is not aware of any rigorous evaluation of the performance
    of these available, non-designated low-volume TSP samplers or their
    equivalence to the TSP FRM. No one has applied to date for
    designation of a low-volume TSP sampler as a FEM, either for TSP
    measurement per se or for purposes of Pb-TSP measurement.

[^83]: Currently, probe heights for Pb-TSP and PM~10~ sampling are
    allowed to be between 2 and 15 meters above ground level for
    neighborhood-scale monitoring sites (those intended to represent
    concentrations over a relatively large area around the site) and
    between 2 and 7 meters for microscale sites. Near very low-height
    sources of TSP, including fugitive dust sources at ground level,
    concentrations of TSP, especially the concentrations of particles
    larger than 10 microns, can vary substantially across this height
    range with higher concentrations closer to the ground; near-ground
    concentrations can also vary more in time than concentrations higher
    up.

[^84]: As noted in section V, the collection efficiency of particles
    larger than approximately 10 microns in a high-volume TSP FRM
    sampler varies with wind speed due to aerodynamic effects, with a
    lower collection efficiency under high winds. The collection
    efficiency also varies with wind direction due to the
    non-cylindrical shape of the TSP sampler inlet. These
    characteristics tend in the direction of reporting less than the
    true TSP concentration.

[^85]: We note that it is possible for high winds to blow Pb particles
    onto a high-volume TSP sampler's filter after the end of its 24-hour
    collection period before the filter is retrieved, causing the
    reported concentration for the 24-hour period to be higher than the
    actual 24-hour concentration.

[^86]: Low-volume PM~10~ samplers are equipped with an omni-directional
    (cylindrical) inlet, which reduces the effect of wind direction, and
    a sharp particle separator which excludes most of the particles
    greater than 10-15 microns in diameter whose collection efficiency
    is most sensitive to wind speed. Also, in low-volume samplers, the
    filter is protected from post-sampling contamination.

[^87]: The larger scale would also make comparisons between two or more
    monitoring sites more indicative of the true comparison between the
    areas surrounding the monitoring sites, with regard to the Pb
    captured by Pb-PM~10~ monitors, which could be informative in
    studies of Pb uptake and health effects in populations

[^88]: Low-volume Pb-TSP samplers could be assembled by making low-cost
    parts substitution to either low-volume PM~10~ or low-volume PM~2.5~
    samplers; some models would have the same sequential sampling
    ability as CASAC has noted for low-volume Pb-PM~10~ samplers;
    sensitivity to wind direction would be eliminated; and their flow
    control and data processing and reporting abilities would be
    substantially better than high-volume Pb-TSP samplers. Low-volume
    Pb-TSP sampling data would have the same geographic variability as
    high-volume Pb-TSP sampling data, however. The size-specific capture
    efficiency curves of currently available commercial low-volume
    sampling systems are not well characterized, nor their sensitivity
    to wind speed. EPA therefore recognizes some uncertainty about their
    equivalence to high-volume samplers in terms of the capture of
    ultra-coarse particles.

[^89]: As noted below, there are likely many sources for which virtually
    all of the emitted particles represented in a Pb-TSP measurement
    would be captured by a Pb-PM~10~ measurement (i.e., the emitted
    particles include virtually no ultra-coarse particles). A near Pb
    source adustment factor would apply to all Pb sources except those
    identified as not emitting ultra-coarse particles.

[^90]: Schmidt (2008) describes these data, as well as those for the 3
    source-oriented sites described above.

[^91]: EPA recognizes and has specifically considered that such a
    decision would affect the selection of the level of the standard,
    recognizing that it is the combination of indicator and level (with
    averaging and time and form) that determine the degree of protection
    afforded by the standard. Section II.E.4 further considers the
    impact of adoption of a Pb-PM10 indicator on the selection of a
    level for the standard.

[^92]: There may be monitoring-related issues in implementing a Pb
    standard based on Pb-PM~10~ as the indicator related to the time it
    would take to deploy and collect data with new Pb-PM~10~ monitors
    and the schedule for designations. This is discussed in section VI
    below along with other implementation issues related to the Pb
    NAAQS.

[^93]: The differing evidence and associated strength of the evidence
    for these different effects is described in detail in the Criteria
    Document.

[^94]: For example, EPA recognizes today that "there is no level of Pb
    exposure that can yet be identified, with confidence, as clearly not
    being associated with some risk of deleterious health effects" (CD,
    p. 8-63).

[^95]: Among the studies of Pb health effects, in which blood Pb level
    is generally used as an index of exposure, the sources of exposure
    vary and are inclusive of air-related sources of Pb such as smelters
    (e.g., CD, chapter 6).

[^96]: See, e.g., 72 FR 37878-9 (July 11, 2007) (Ozone NAAQS Notice of
    Proposed Rulemaking).

[^97]: This differs from the Agency's recognition in the 1978 rulemaking
    of a threshold of 40 µg/dL blood Pb for an individual child for
    effects of Pb considered clearly adverse to health at that time,
    i.e., impairment of heme synthesis and other effects which result in
    anemia.

[^98]: More specifically, when the standard was set in 1978, the Agency
    stated that the population mean, measured as the geometric mean,
    must be 15 µg/dL in order to ensure that 99.5 percent of children in
    the United States would have a blood Pb level below 30 µg/dL, which
    was identified as the maximum safe blood Pb level for individual
    children based on the information available at that time (43 FR
    46247-46252).

[^99]: There are some similarities between this approach and the
    approach employed in determining the levels for the daily and annual
    PM standards in the latest PM review, where EPA determined an
    ambient PM level based on the ambient levels in the epidemiology
    studies that found statistically significant associations between
    changes in ambient PM levels and changes in occurrences of health
    effects. See 71 FR 61144 (October 17, 2006). However, there are
    several important differences in this adaptation to the 1978
    approach for lead. For example, the health effects evaluated in the
    PM epidemiological studies were clearly adverse health effects,
    ranging from hospital admissions to premature mortality. In
    addition, the studies looked directly at the association between
    ambient level and occurrences of health effects. Here the
    epidemiology studies look at the association between blood lead
    level and neurocognitive effect, and there is an additional step to
    link the blood lead level to air-related lead. In addition, at a
    population level there is a less clear delineation of when the
    neurocognitive effect is adverse to public health. This is discussed
    below in this section with respect to the impact on public health of
    a shift in the mean IQ of a population of children.

[^100]: In such studies described here, an adjustment was performed for
    potential confounders (e.g., parental IQ and HOME score) or for
    surrogates (e.g., parental education and SES factors).

[^101]: As discussed in section II.A and B, how many and which of the
    various air-related pathways are included in any estimate of
    "nonair" blood Pb level affects how much of total blood Pb is
    considered "nonair". Fewer pathways recognized as "air-related"
    corresponds to more pathways being categorized as nonair and
    consequently yields a higher proportion of total BLL that is
    categorized as nonair. In contrast, recognizing more pathways (and
    their associated blood Pb contributions) as air-related accordingly
    yields a lower proportion of total BLL that is estimated to be
    nonair-related. This consideration also applies to air-to-blood
    ratios, such that recognition of more (*vs* fewer) pathways as
    air-related yields higher air-to-blood ratios. Accordingly, when
    both parameters are used within the same framework, such as the 1978
    framework, attention is needed to ensure they are generally
    compatible in this regard.

[^102]: As noted in section II.C.2.e above, the recent air category does
    not include a variety of air-related categories (including some
    associated with air deposition to outdoor surfaces and diet) and
    both the recent air and past air categories may include some Pb in
    soil or dust from the historical use of Pb in paint.

[^103]: EPA is additionally soliciting comment on a broader range of 1.0
    to 1.9 for nonsource-oriented sites as discussed in section II.E.1.

[^104]: As discussed below i[n section VI]{.mark}, however, EPA is
    soliciting comment on the potential use of Pb-TSP data for initial
    designations for Pb-PM~10~ standard and whether the associated use
    of scaling factors would be appropriate.

[^105]: Fewer days could be required, and fewer needed for the monthly
    average to be valid, in months where the sampling schedule does not
    to call for sampling on the first two days of the month, and for
    sites approved for only one-day-in-six sampling because they have a
    history of recording concentrations well below the level of the
    NAAQS. See Section V for more detail on required sampling schedules.

[^106]: See, for example, the explanation of the finite population
    correction factor approach at
    grants.nih.gov/grants/funding/modular/eval/Sample_MGAP.doc. Another
    useful reference is "Sampling: Design and Analysis", Lohr, Sharon
    L., Brooks/Cole Publishing Co., Pacific Grove, CA, 1999.

[^107]: This exploration will be somewhat similar to, but likely more
    simplistic in light of the more limited available data, the work EPA
    did on data quality objectives for the PM~2.5~ monitoring network.

[^108]: Section 3(a) of the proposed Appendix R has a more detailed
    statement of what ambient data will be considered when determining
    compliance with the NAAQS than is given in other data interpretation
    appendices to 40 CFR 50. EPA invites comment on this codification of
    current practice. One new feature is a provision for the use of data
    collected before the promulgation of the proposed changes and
    additions to the FRM/FEM criteria, to make it clear that these
    changes and additions are in effect retroactive. FRM/FEM revisions
    and new FRM/FEM designations have not always been treated as
    retroactive but in the case of the revised Pb NAAQS EPA wishes to
    maximize the available data for making designations.

[^109]: EPA is also soliciting comment on a broader range of 1.0 to 1.9
    for nonsource-oriented sites as discussed in section II.E.1.

[^110]: For a list of currently approved FRM/FEMs for Pb-TSP refer to:
    www.epa.gov/ttn/amtic/criteria.html

[^111]: The 21 distinct approved FEMs represent less than 21
    fundamentally different analysis methods, as some differ in only in
    minor aspects.

[^112]: PM~10~ can be measured with either a "low-volume" or a
    "high-volume" sampler. CASAC specifically recommended the low-volume
    sampler, for reasons explained here and in section II.E.1.

[^113]: If the collocated TSP samplers were always oriented in the same
    direction, they would be exposed to the same wind speed and wind
    direction, and the appearance of good precision between them would
    not necessarily be indicative of the sensitivity of Pb-TSP
    measurements to wind speed and wind direction.

[^114]: For the complete definition of CBSA refer to:
    ww.census.gov/population/www/estimates/aboutmetro.html

[^115]: American Petroleum Institute v. Costle, 609 F.2d 20 (D.C. Cir.
    1979)

[^116]: The EPA would address such areas once sufficient monitoring data
    has been collected for each.

[^117]: As discussed in Section IV of this notice, EPA is soliciting
    comment on the use of Pb-TSP monitoring data, with or without a
    scaling factor, as a surrogate for Pb-PM~10~ data where Pb-PM~10~
    data are not available, particularly for initial designations.

[^118]: Two elements identified in section 110(a)(2) are not listed
    below because, as EPA interprets the CAA, SIPs incorporating any
    necessary local nonattainment area controls would not be due within
    3 years, but rather are due at the time the nonattainment area
    planning requirements are due. The elements are: (1) Emission limits
    and other control measures, section 110(a)(2)(A), and (2) Provisions
    for meeting part D, section 110(a)(2)(I), which requires areas
    designated as nonattainment to meet the applicable nonattainment
    planning requirements of part D, title I of the CAA.

[^119]: [EPA Air Pollution Control Cost Manual - Sixth
    Edition]{.underline} (EPA 452/B-02-001), EPA Office of Air Quality
    Planning and Standards, Research Triangle Park, NC, Jan 2002.

[^120]: See for example, 44 FR 53762 (September 17, 1979) and footnote 3
    of that notice. Note that EPA's emissions trading policy statement
    has clarified that the RACT requirement may be satisfied by
    achieving "RACT equivalent" emission reductions in the aggregate
    from the full set of existing stationary sources in the area. See
    also EPA's economic incentive proposal which reflects the Agency's
    policy guidance with respect to emissions trading, 58 FR 11110,
    February 23, 1993.

[^121]: As previously stated most of the lead nonattainment problems are
    caused by point sources. For this reason EPA believes that the RFP
    for Pb should parallel the RFP policy for SO2 (see General Preamble,
    57 FR 13545, April 16, 1992)

[^122]: The terms "major" and "minor" define the size of a stationary
    source, for applicability purposes, in terms of an annual emissions
    rate (tons per year, tpy) for a pollutant. Generally, a minor source
    is any source that is not "major." "Major" is defined by the
    applicable regulations---PSD or nonattainment NSR.

[^123]: In addition, the PSD program applies to most non-criteria
    regulated pollutants.

[^124]: Criteria pollutants are those pollutants for which EPA has
    established a NAAQS under section 109 of the CAA.

[^125]: Transportation conformity is required under CAA section 176(c)
    (42 U.S.C. 7506(c) to ensure that federally supported highway and
    transit project activities are consistent with ("conform to") the
    purpose of the SIP. Transportation conformity applies to areas that
    are designated nonattainment, and those areas redesignated to
    attainment after 1990 ("maintenance areas" with plans developed
    under CAA section 175A) for transportation-related criteria
    pollutants. In light of the elimination of Pb additives from
    gasoline transportation conformity does not apply to the Pb NAAQS.