```

[Federal Register Volume 76, Number 169 (Wednesday, August 31, 2011)]
[Rules and Regulations]
[Pages 54294-54343]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-21359]

[[Page 54293]]

Vol. 76

Wednesday,

No. 169

August 31, 2011

Part II

Environmental Protection Agency

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40 CFR Parts 50, 53 and 58

Review of National Ambient Air Quality Standards for Carbon Monoxide; 
Final Rule

Federal Register / Vol. 76 , No. 169 / Wednesday, August 31, 2011 / 
Rules and Regulations

[[Page 54294]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53 and 58

[EPA-HQ-OAR-2008-0015; FRL-9455-2]
RIN 2060-AI43

Review of National Ambient Air Quality Standards for Carbon 
Monoxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: This rule is being issued at this time as required by a court 
order governing the schedule for completion of this review of the air 
quality criteria and the national ambient air quality standards (NAAQS) 
for carbon monoxide (CO). Based on its review, the EPA concludes the 
current primary standards are requisite to protect public health with 
an adequate margin of safety, and is retaining those standards. After 
review of the air quality criteria, EPA further concludes that no 
secondary standard should be set for CO at this time. EPA is also 
making changes to the ambient air monitoring requirements for CO, 
including those related to network design, and is updating, without 
substantive change, aspects of the Federal reference method.

DATES: This final rule is effective on October 31, 2011.

ADDRESSES: EPA has established a docket for this action under Docket ID 
No. EPA-HQ-OAR-2008-0015. Incorporated into this docket is a separate 
docket established for the 2010 Integrated Science Assessment for 
Carbon Monoxide (Docket ID No. EPA-HQ-ORD-2007-0925. All documents in 
these dockets are listed on the http://www.regulations.gov Web site. 
Although listed in the docket index, some information is not publicly 
available, e.g., confidential business information (CBI) or other 
information whose disclosure is restricted by statute. Certain other 
material, such as copyrighted material, is not placed on the Internet 
and will be publicly available for viewing at the Public Reading Room. 
Abstracts of scientific studies cited in the review are also available 
on the Internet at EPA's HERO Web site: http://hero.epa.gov/, by 
clicking on the box on the right side of the page labeled ``Search 
HERO.'' Publicly available docket materials are available 
electronically through www.regulations.gov or may be viewed at the 
Public Reading Room 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.

FOR FURTHER INFORMATION CONTACT: Dr. Deirdre Murphy, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, Mail code C504-06, U.S. Environmental Protection Agency, 
Research Triangle Park, NC 27711; telephone number: 919-541-0729; fax 
number: 919-541-0237; e-mail address: murphy.deirdre@epa.gov. For 
further information specifically with regard to section IV of this 
notice, contact Mr. Nealson Watkins, Air Quality Analysis Division, 
Office of Air Quality Planning and Standards, Mail code C304-06, U.S. 
Environmental Protection Agency, Research Triangle Park, NC 27711; 
telephone number: 919-541-5522; fax number: 919-541-1903; e-mail 
address: watkins.nealson@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Legislative Requirements
    B. Related Carbon Monoxide Control Programs
    C. Review of the Air Quality Criteria and Standards for Carbon 
Monoxide
    D. Summary of Proposed Decisions on Standards for Carbon 
Monoxide
    E. Organization and Approach to Final Decisions on Standards for 
Carbon Monoxide
II. Rationale for Decisions on the Primary Standards
    A. Introduction
    1. Overview of Air Quality Information
    2. Overview of Health Effects Information
    a. Carboxyhemoglobin as Biomarker of Exposure and Toxicity
    b. Nature of Effects and At-Risk Populations
    c. Cardiovascular Effects
    3. Overview of Human Exposure and Dose Assessment
    B. Adequacy of the Current Primary Standards
    1. Rationale for Proposed Decision
    2. Comments on Adequacy
    3. Conclusions Concerning Adequacy of the Primary Standards
III. Consideration of a Secondary Standard
    A. Introduction
    B. Rationale for Proposed Decision
    C. Comments on Consideration of Secondary Standard
    D. Conclusions Concerning a Secondary Standard
IV. Amendments to Ambient Monitoring Requirements
    A. Monitoring Methods
    1. Proposed Changes to Parts 50 and 53
    2. Public Comments
    3. Decisions on Methods
    B. Network Design
    1. Proposed Changes
    2. Public Comments
    a. Near-Road Monitoring and Collocation With Near-Road Nitrogen 
Dioxide Monitors
    b. Population Thresholds for Requiring Near-Road Carbon Monoxide 
Monitors
    c. Implementation Schedule
    d. Siting Criteria
    e. Area-Wide Monitoring
    f. Regional Administrator Authority
    3. Conclusions on the Network Design
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act References

I. Background

A. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list certain air pollutants and then to 
issue air quality criteria for those pollutants. The Administrator is 
to list those air pollutants that in her ``judgment, cause or 
contribute to air pollution which may reasonably be anticipated to 
endanger public health or welfare;'' ``the presence of which in the 
ambient air results from numerous or diverse mobile or stationary 
sources;'' and ``for which * * * [the Administrator] plans to issue air 
quality criteria * * * '' Air quality criteria are intended 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 the ambient air * * *'' 42 U.S.C. 7408(b). Section 109 (42 
U.S.C. 7409) directs the Administrator to propose and promulgate 
``primary'' and ``secondary'' NAAQS for pollutants for which air

[[Page 54295]]

quality criteria are issued. Section 109(b)(1) defines a primary 
standard as one ``the attainment and maintenance of which in the 
judgment of the Administrator, based on such 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 such 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\
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    \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.''
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    The requirement that primary standards provide an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(DC Cir. 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum 
Institute v. Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 
455 U.S. 1034 (1982); American Farm Bureau Federation v. EPA, 559 F.3d 
512, 533 (DC Cir. 2009); Association of Battery Recyclers v. EPA, 604 
F.3d 613, 617-18 (DC Cir. 2010). 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 provide 
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 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.
    In addressing the requirement for an adequate margin of safety, the 
EPA considers such factors as the nature and severity of the health 
effects involved, the size of sensitive population(s) at risk, and the 
kind and degree of the uncertainties that must be addressed. The 
selection of any particular approach to providing an adequate margin of 
safety is a policy choice left specifically to the Administrator's 
judgment. See Lead Industries Association v. EPA, 647 F.2d at 1161-62; 
Whitman v. American Trucking Associations, 531 U.S. 457, 495 (2001).
    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, 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. In so doing, 
EPA may not consider the costs of implementing the standards. See 
generally, Whitman v. American Trucking Associations, 531 U.S. 457, 
465-472, 475-76 (2001). Likewise, ``[a]ttainability and technological 
feasibility are not relevant considerations in the promulgation of 
national ambient air quality standards.'' American Petroleum Institute 
v. Costle, 665 F. 2d at 1185.
    Section 109(d)(1) requires that ``not 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 * * * and shall make such 
revisions in such criteria and standards and promulgate such new 
standards as may be appropriate. * * *'' Section 109(d)(2) requires 
that an independent scientific review committee ``shall complete a 
review of the criteria * * * and the national primary and secondary 
ambient air quality standards * * * and shall recommend to the 
Administrator any new * * * standards and revisions of existing 
criteria and standards as may be appropriate. * * *'' Since the early 
1980's, this independent review function has been performed by the 
Clean Air Scientific Advisory Committee (CASAC).\3\
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    \3\ Lists of CASAC members and of members of the CASAC CO Review 
Panel are available at: http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?OpenDocument.
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B. Related Carbon Monoxide Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the Act, and related provisions, states are 
to submit, for EPA approval, state implementation plans (SIPs) 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. See CAA sections 160-169. In 
addition, Federal programs provide for nationwide reductions in 
emissions of these and other air pollutants through the Federal motor 
vehicle and motor vehicle fuel control program under title II of the 
Act (CAA sections 202-250), which involves controls for emissions from 
moving sources and controls for the fuels used by these sources and new 
source performance standards for stationary sources under section 111.

C. Review of the Air Quality Criteria and Standards for Carbon Monoxide

    EPA initially established NAAQS for CO on April 30, 1971. The 
primary standards were established to protect against the occurrence of 
carboxyhemoglobin levels in human blood associated with health effects 
of concern. The standards were set at 9 parts per million (ppm), as an 
8-hour average, and 35 ppm, as a 1-hour average, neither to be exceeded 
more than once per year (36 FR 8186). In the 1971 decision, the 
Administrator judged that attainment of these standards would provide 
the requisite protection of public health with an adequate margin of 
safety and would also provide requisite protection against known and 
anticipated adverse effects on public welfare, and accordingly set the 
secondary (welfare-based) standards identical to the primary (health-
based) standards.
    In 1985, EPA concluded its first periodic review of the criteria 
and standards for CO (50 FR 37484). In that review, EPA updated the 
scientific criteria upon which the initial CO standards were based 
through the publication of the 1979 Air Quality Criteria Document for 
Carbon Monoxide (AQCD; USEPA, 1979a) and prepared a Staff Paper (USEPA, 
1979b), which, along with the 1979 AQCD, served as the basis for the 
development of the notice of proposed rulemaking which was published on 
August 18, 1980 (45 FR 55066). Delays due to uncertainties

[[Page 54296]]

regarding the scientific basis for the final decision resulted in EPA's 
announcing a second public comment period (47 FR 26407). Following 
substantial reexamination of the scientific data, EPA prepared an 
Addendum to the 1979 AQCD (USEPA, 1984a) and an updated Staff Paper 
(USEPA, 1984b). Following review by CASAC (Lippmann, 1984), EPA 
announced its decision not to revise the existing primary standards and 
to revoke the secondary standard for CO on September 13, 1985, due to a 
lack of evidence of effects on public welfare at ambient concentrations 
(50 FR 37484).
    On August 1, 1994, EPA concluded its second periodic review of the 
criteria and standards for CO by deciding that revisions to the CO 
NAAQS were not warranted at that time (59 FR 38906). This decision 
reflected EPA's review of relevant scientific information assembled 
since the last review, as contained in the 1991 AQCD (USEPA, 1991) and 
the 1992 Staff Paper (USEPA, 1992). Thus, the primary standards were 
retained at 9 ppm with an 8-hour averaging time, and 35 ppm with a 1-
hour averaging time, neither to be exceeded more than once per year (59 
FR 38906).
    EPA initiated the next periodic review in 1997 and released the 
final 2000 AQCD (USEPA, 2000) in August 2000. After release of the 
AQCD, Congress requested that the National Research Council (NRC) 
review the impact of meteorology and topography on ambient CO 
concentrations in high altitude and extreme cold regions of the U.S. 
The NRC convened the Committee on Carbon Monoxide Episodes in 
Meteorological and Topographical Problem Areas, which focused on 
Fairbanks, Alaska, as a case-study.
    A final report, ``Managing Carbon Monoxide Pollution in 
Meteorological and Topographical Problem Areas,'' was published in 2003 
(NRC, 2003) and offered a wide range of recommendations regarding 
management of CO air pollution, cold start emissions standards, 
oxygenated fuels, and CO monitoring. Following completion of the NRC 
report, EPA did not conduct rulemaking to complete the review.
    On September 13, 2007, EPA issued a call for information from the 
public (72 FR 52369) requesting the submission of recent scientific 
information on specified topics. On January 28-29, 2008, a workshop was 
held to discuss policy-relevant scientific and technical information to 
inform EPA's planning for the CO NAAQS review (73 FR 2490). Following 
the workshop, a draft Integrated Review Plan (IRP) (USEPA, 2008a) was 
made available in March 2008 for public comment and was discussed by 
the CASAC via a publicly accessible teleconference consultation on 
April 8, 2008 (73 FR 12998; Henderson, 2008). EPA made the final IRP 
available in August 2008 (USEPA, 2008b).
    In preparing the Integrated Science Assessment for Carbon Monoxide 
(ISA or Integrated Science Assessment), EPA held an authors' 
teleconference in November 2008 with invited scientific experts to 
discuss preliminary draft materials prepared as part of the ongoing 
development of the CO ISA and its supplementary annexes. The first 
draft ISA (USEPA, 2009a) was made available for public review on March 
12, 2009 (74 FR 10734), and reviewed by CASAC at a meeting held on May 
12-13, 2009 (74 FR 15265). A second draft ISA (USEPA, 2009b) was 
released for CASAC and public review on September 23, 2009 (74 FR 
48536), and it was reviewed by CASAC at a meeting held on November 16-
17, 2009 (74 FR 54042). The final ISA was released in January 2010 
(USEPA, 2010a).
    In May 2009, OAQPS released a draft planning document, the draft 
Scope and Methods Plan (USEPA, 2009c), for consultation with CASAC and 
public review at the CASAC meeting held on May 12-13, 2009. Taking into 
consideration comments on the draft Scope and Methods Plan from CASAC 
(Brain, 2009) and the public, OAQPS staff developed and released for 
CASAC review and public comment a first draft Risk and Exposure 
Assessment (REA) (USEPA, 2009d), which was reviewed at the CASAC 
meeting held on November 16-17, 2009. Subsequent to that meeting and 
taking into consideration comments from CASAC (Brain and Samet, 2010a) 
and public comments on the first draft REA, a second draft REA (USEPA, 
2010d) was released for CASAC review and public comment in February 
2010, and reviewed at a CASAC meeting held on March 22-23, 2010. 
Drawing from information in the final CO ISA and the second draft REA, 
EPA released a draft Policy Assessment (PA) (USEPA, 2010e) in early 
March 2010 for CASAC review and public comment at the same meeting. 
Taking into consideration comments on the second draft REA and the 
draft PA from CASAC (Brain and Samet, 2010b, 2010c) and the public, 
staff completed the quantitative assessments which are presented in the 
final REA (USEPA, 2010b). Staff additionally took into consideration 
those comments and the final REA analyses in completing the final 
Policy Assessment (USEPA, 2010c) which was released in October 2010.
    The proposed decision (henceforth ``proposal'') on the review of 
the CO NAAQS was signed on January 28, 2011, and published in the 
Federal Register on February 11, 2011. The EPA held a public hearing to 
provide direct opportunity for oral testimony by the public on the 
proposal. The hearing was held on February 28, 2011, in Arlington, 
Virginia. At this public hearing, EPA heard testimony from five 
individuals representing themselves or specific interested 
organizations. Transcripts from this hearing and written testimony 
provided at the hearing are in the docket for this review. 
Additionally, written comments were received from various commenters 
during the public comment period on the proposal. Significant issues 
raised in the public comments are discussed in the preamble of this 
final action. A summary of all other significant comments, along with 
EPA's responses (henceforth ``Response to Comments'') can be found in 
the docket for this review.
    The schedule for completion of this review is governed by a court 
order resolving a lawsuit filed in March 2003 by a group of plaintiffs 
who alleged that EPA had failed to perform its mandatory duty, under 
section 109(d)(1), to complete a review of the CO NAAQS within the 
period provided by statute. The court order that governs this review, 
entered by the court on November 14, 2008, and amended on August 30, 
2010, provides that EPA will sign for publication a notice of final 
rulemaking concerning its review of the CO NAAQS no later than August 
12, 2011.
    Some commenters have referred to and discussed individual 
scientific studies on the health effects of CO that were not included 
in the ISA (USEPA, 2010a) (``'new' studies''). In considering and 
responding to comments for which such ``new'' studies were cited in 
support, EPA has provisionally considered the cited studies in the 
context of the findings of the ISA.
    As in prior NAAQS reviews, EPA is basing its decision in this 
review on studies and related information included in the ISA, REA and 
Policy Assessment, which have undergone CASAC and public review. The 
studies assessed in the ISA and Policy Assessment, and the integration 
of the scientific evidence presented in them, have undergone extensive 
critical review by EPA, CASAC, and the public. The rigor of that review 
makes these studies, and their integrative assessment, the most 
reliable source of scientific information on which to base decisions on 
the NAAQS, decisions that all parties recognize as of great import.

[[Page 54297]]

NAAQS decisions can have profound impacts on public health and welfare, 
and NAAQS decisions should be based on studies that have been 
rigorously assessed in an integrative manner not only by EPA but also 
by the statutorily mandated independent advisory committee, as well as 
the public review that accompanies this process. EPA's provisional 
consideration of these studies did not and could not provide that kind 
of in-depth critical review.
    This decision is consistent with EPA's practice in prior NAAQS 
reviews and its interpretation of the requirements of the CAA. Since 
the 1970 amendments, the EPA has taken the view that NAAQS decisions 
are to be based on scientific studies and related information that have 
been assessed as a part of the pertinent air quality criteria, and has 
consistently followed this approach. This longstanding interpretation 
was strengthened by new legislative requirements enacted in 1977, which 
added section 109(d)(2) of the Act concerning CASAC review of air 
quality criteria. See 71 FR 61144, 61148 (October 17, 2006) (final 
decision on review of NAAQS for particulate matter) for a detailed 
discussion of this issue and EPA's past practice.
    As discussed in EPA's 1993 decision not to revise the NAAQS for 
ozone, ``new'' studies may sometimes be of such significance that it is 
appropriate to delay a decision on revision of a NAAQS and to 
supplement the pertinent air quality criteria so the studies can be 
taken into account (58 FR at 13013-13014, March 9, 1993). In the 
present case, EPA's provisional consideration of ``new'' studies 
concludes that, taken in context, the ``new'' information and findings 
do not materially change any of the broad scientific conclusions 
regarding the health effects and exposure pathways of ambient CO made 
in the air quality criteria. For this reason, reopening the air quality 
criteria review would not be warranted even if there were time to do so 
under the court order governing the schedule for this rulemaking.
    Accordingly, EPA is basing the final decisions in this review on 
the studies and related information included in the CO air quality 
criteria that have undergone CASAC and public review. EPA will consider 
the ``new'' studies for purposes of decision-making in the next 
periodic review of the CO NAAQS, which EPA expects to begin soon after 
the conclusion of this review and which will provide the opportunity to 
fully assess these studies through a more rigorous review process 
involving EPA, CASAC, and the public. Further discussion of these 
``new'' studies can be found in the Response to Comments document.

D. Summary of Proposed Decisions on Standards for Carbon Monoxide

    For reasons discussed in the notice of proposed rulemaking, the 
Administrator proposed to retain the current primary CO standards. With 
regard to consideration of a secondary standard, the Administrator 
proposed to conclude that no secondary standards should be set at this 
time.

E. Organization and Approach to Final Decisions on Standards for Carbon 
Monoxide

    This action presents the Administrator's final decisions in this 
review of the CO standards. Decisions regarding the primary CO 
standards are addressed below in section II. Consideration of a 
secondary CO standard is addressed below in section III. Ambient 
monitoring methods and network design related to implementation of the 
CO standards are addressed below in section IV. A discussion of 
statutory and executive order reviews is provided in section V.
    Today's final decisions are based on a thorough review in the 
Integrated Science Assessment of the latest scientific information on 
known and potential human health and welfare effects associated with 
exposure to CO in the environment. These final decisions also take into 
account: (1) Assessments in the Policy Assessment of the most policy-
relevant information in the Integrated Science Assessment as well as 
quantitative exposure, dose and risk assessments based on that 
information presented in the Risk and Exposure Assessment; (2) CASAC 
Panel advice and recommendations, as reflected in its letters to the 
Administrator and its discussions of drafts of the Integrated Science 
Assessment, Risk and Exposure Assessment and Policy Assessment at 
public meetings; (3) public comments received during the development of 
these documents, either in connection with CASAC Panel meetings or 
separately; and (4) public comments received on the proposed 
rulemaking.

II. Rationale for Decisions on the Primary Standards

A. Introduction

    This section presents the rationale for the Administrator's 
decision that the current primary standards are requisite to protect 
public health with an adequate margin of safety, and that they should 
be retained. In developing this rationale, EPA has drawn upon an 
integrative synthesis in the Integrated Science Assessment of the 
entire body of evidence published through mid-2009 on human health 
effects associated with the presence of CO in the ambient air. The 
research studies evaluated in the ISA have undergone intensive scrutiny 
through multiple layers of peer review, with extended opportunities for 
review and comment by the CASAC Panel and the public. As with virtually 
any policy-relevant scientific research, there is uncertainty in the 
characterization of health effects attributable to exposure to ambient 
CO. While important uncertainties remain, the review of the health 
effects information has been extensive and deliberate. In the judgment 
of the Administrator, this intensive evaluation of the scientific 
evidence provides an adequate basis for regulatory decision making at 
this time. This review also provides important input to EPA's research 
plan for improving our future understanding of the relationships 
between exposures to ambient CO and health effects.
    The health effects information and quantitative exposure/dose 
assessment were summarized in sections II.B and II.C of the proposal 
(76 FR at 8162-8172) and are only briefly outlined in sections II.A.2 
and II.A.3 below. Responses to public comments specific to the material 
presented in sections II.A.1 through II.A.3 below are provided in the 
Response to Comments document.
    Subsequent sections of this preamble provide a more complete 
discussion of the Administrator's rationale, in light of key issues 
raised in public comments, for concluding that the current standards 
are requisite to protect public health with an adequate margin of 
safety and that it is appropriate to retain the current primary CO 
standards to continue to provide requisite public health protection 
(section II.B).
1. Overview of Air Quality Information
    This section briefly summarizes the information on CO sources, 
emissions, ambient air concentrations and aspects of associated 
exposure presented in section II.A of the proposal, as well as in 
section 1.3 of the Policy Assessment and chapter 2 of the Risk and 
Exposure Assessment.
    Carbon monoxide in ambient air is formed by both natural and 
anthropogenic processes. In areas of human activity such as urban 
areas, it is formed primarily by the incomplete combustion of carbon-
containing fuels with the combustion conditions influencing the rate of 
formation. For example, as a result of the combustion

[[Page 54298]]

conditions, CO emissions from large fossil-fueled power plants are 
typically very low because optimized fuel consumption conditions make 
boiler combustion highly efficient. In contrast, internal combustion 
engines used in many mobile sources have widely varying operating 
conditions. As a result, higher and more varying CO formation results 
from the operation of mobile sources, which continue to be a 
significant source sector for CO in ambient air (ISA, sections 3.4 and 
3.5; 2000 AQCD, section 7.2; REA, section 2.2 and 3.1.3).
    Mobile sources are a substantial contributor to total CO emissions, 
particularly in urban areas (ISA, section 3.5.1.3; REA, section 3.1.3). 
Highest ambient concentrations in urban areas occur on or near 
roadways, particularly highly travelled roadways, and decline somewhat 
steeply with distance (ISA, section 3.5.1.3; REA, section 3.1.3; 
Baldauf et al., 2008a,b; Zhu et al., 2002). For example, as described 
in the ISA, a study by Zhu et al., (2002) documented CO concentrations 
at an interstate freeway to be ten times as high as an upwind 
monitoring site; concentrations declined rapidly in the downwind 
direction to levels only approximately one half roadway concentrations 
within 100 to 300 meters (ISA, section 3.5.1.3, Figure 3-29; Zhu et 
al., 2002). Factors that can influence the steepness of the gradient 
include wind direction and other meteorological variables, and on-road 
vehicle density (ISA, section 3.5.1.3, Figures 3-29 and 3-30; Zhu et 
al., 2002; Baldauf et al., 2008a, b). These traffic-related ambient 
concentrations contribute to the higher short-term ambient CO exposures 
experienced near busy roads and particularly in vehicles, as described 
in more detail in the REA and PA.
2. Overview of Health Effects Information
    This section summarizes information presented in section II.B of 
the proposal pertaining to health endpoints associated with the range 
of exposures considered to be most relevant to current ambient CO 
exposure levels. In recognition of the use of an internal biomarker in 
evaluating health risk for CO, the following section summarizes key 
aspects of the use of carboxyhemoglobin as an internal biomarker 
(section II.A.2.a). This is followed first by a summary of the array of 
CO-induced health effects and recognition of at-risk subpopulations 
(section II.A.2.b) and then by a summary of the evidence regarding 
cardiovascular effects (section II.A.2.c).
a. Carboxyhemoglobin as Biomarker of Exposure and Toxicity
    This section briefly summarizes the current state of knowledge, as 
described in the Integrated Science Assessment, of the role of 
carboxyhemoglobin in mediating toxicity and as a biomarker of exposure. 
The section also summarizes the roles of endogenously produced CO and 
exposure to ambient and nonambient CO in influencing internal CO 
concentrations and carboxyhemoglobin (COHb) levels.
    At this time, as during past reviews, the best characterized 
mechanism of action of CO is tissue hypoxia caused by binding of CO to 
hemoglobin to form COHb in the blood (e.g., USEPA, 2000; USEPA, 1991; 
ISA). Increasing levels of COHb in the blood stream with subsequent 
decrease in oxygen availability for organs and tissues are of concern 
in people who have compromised compensatory mechanisms (e.g., lack of 
capacity to increase blood flow in response to hypoxia), such as those 
with pre-existing heart disease. For example, the integrative review of 
health effects of CO indicates that ``the clearest evidence indicates 
that individuals with CAD [coronary artery disease] are most 
susceptible to an increase in CO-induced health effects'' (ISA, section 
5.7.8).
    Carboxyhemoglobin is formed in the blood both from CO originating 
in the body (endogenous CO) \4\ and from CO that has been inhaled into 
the body (exogenous CO).\5\ The amount of COHb that occurs in the blood 
depends on factors specific to both the physiology of the individual 
(including disease state) and the exposure circumstances. These include 
factors associated with an individual's rate of COHb elimination and 
production of endogenous CO, as well as those that influence the intake 
of exogenous CO into the blood, such as the differences in CO 
concentration (and partial pressure) in inhaled air, exhaled air, and 
blood; duration of a person's exposure to changed CO concentrations in 
air; and exertion level or inhalation rate (ISA, chapter 4).
---------------------------------------------------------------------------

    \4\ Endogenous CO is produced from biochemical reactions 
associated with normal breakdown of heme proteins (ISA, section 
4.5).
    \5\ Exogenous CO includes CO emitted to ambient air, CO emitted 
to ambient air that has infiltrated indoors and CO that originates 
indoors from sources such as gas stoves, tobacco smoke and gas 
furnaces (ISA, section 3.6; REA, section 2.2).
---------------------------------------------------------------------------

    Apart from the impairment of oxygen delivery to tissues related to 
COHb formation, toxicological studies also indicate several other 
pathways by which CO acts in the body, which involve a wide range of 
molecular targets and internal CO concentrations (2000 AQCD, sections 
5.6-5.9; ISA, section 5.1.3). The role of these alternative less-well-
characterized mechanisms in CO-induced health effects at concentrations 
relevant to the current NAAQS, however, is not clear. New research 
based on this evidence is needed to further understand these pathways 
and their linkage to CO-induced effects in susceptible populations. 
Accordingly, COHb level in blood continues to be well recognized and 
most commonly used as an important internal dose metric, and is 
supported by the evidence as the most useful indicator of CO exposure 
that is related to CO health effects of major concern (ISA, p. 2-4, 
sections 4.1, 4.2, 5.1.1; 1991 AQCD; 2000 AQCD; 2010 ISA).
b. Nature of Effects and At-Risk Populations
    The long-standing body of evidence that has established many 
aspects of the biological effects of CO continues to contribute to our 
understanding of the health effects of ambient CO (PA, section 2.2.1). 
Inhaled CO elicits various health effects through binding to, and 
associated alteration of the function of, a number of heme-containing 
molecules, mainly hemoglobin (see e.g., ISA, section 4.1). The best 
characterized health effect associated with CO levels of concern is 
decreased oxygen availability to critical tissues and organs, 
specifically the heart, induced by increased COHb levels in blood (ISA, 
section 5.1.2). Consistent with this, medical conditions that affect 
the biological mechanisms which compensate for this effect (e.g., 
vasodilation and increased coronary blood flow with increased oxygen 
delivery to the myocardium) can contribute to a reduced amount of 
oxygen available to key body tissues, potentially affecting organ 
system function and limiting exercise capacity (2000 AQCD, section 
7.1).\6\
---------------------------------------------------------------------------

    \6\ For example, people with peripheral vascular diseases and 
heart disease patients often have markedly reduced circulatory 
capacity and reduced ability to compensate for increased circulatory 
demands during exercise and other stress (2000 AQCD, p. 7-7).
---------------------------------------------------------------------------

    This evidence newly available in this review provides additional 
detail and support to our prior understanding of CO effects and 
population susceptibility. In this review, the clearest evidence for 
ambient CO-related effects is available for cardiovascular effects. 
Using an established framework to characterize the evidence as to 
likelihood of causal relationships between exposure to ambient CO and

[[Page 54299]]

specific health effects (ISA, chapter 1), the ISA states that ``Given 
the consistent and coherent evidence from epidemiologic and human 
clinical studies, along with biological plausibility provided by CO's 
role in limiting oxygen availability, it is concluded that a causal 
relationship is likely to exist between relevant short-term CO 
exposures and cardiovascular morbidity'' (ISA, p. 2-6, section 2.5.1). 
Using the same established framework, the ISA describes the evidence as 
suggestive of causal relationships between relevant ambient CO exposure 
and several other health effects: Relevant short- and long-term CO 
exposures and central nervous system (CNS) effects, birth outcomes and 
developmental effects following long-term exposure, respiratory 
morbidity following short-term exposure, and mortality following short-
term exposure (ISA, section 2.5). However, there is only limited 
evidence for these relationships, and the current body of evidence 
continues to indicate cardiovascular effects, particularly effects 
related to the role of CO in limiting oxygen availability to tissues, 
as those of greatest concern at low exposures with relevance to ambient 
concentrations (ISA, chapter 2). The evidence for these effects is 
further described in section II.A.2.c below.
    As described in the proposal, the terms susceptibility, 
vulnerability, sensitivity, and at-risk are commonly employed in 
identifying population groups or life stages at relatively higher risk 
for health risk from a specific pollutant. In the ISA for this review, 
the term susceptibility has been used broadly to recognize populations 
that have a greater likelihood of experiencing effects related to 
ambient CO exposure, with use of the term susceptible populations, as 
used in the ISA, defined as follows (ISA, section 5.7, p. 5-115):

    Populations that have a greater likelihood of experiencing 
health effects related to exposure to an air pollutant (e.g., CO) 
due to a variety of factors including, but not limited to: Genetic 
or developmental factors, race, gender, lifestage, lifestyle (e.g., 
smoking status and nutrition) or preexisting disease, as well as 
population-level factors that can increase an individual's exposure 
to an air pollutant (e.g., CO) such as socioeconomic status [SES], 
which encompasses reduced access to health care, low educational 
attainment, residential location, and other factors.

Thus, susceptible populations are at greater risk of CO effects and are 
also referred to as at-risk in the summary below.
    As described in the proposal, the population with pre-existing 
cardiovascular disease continues to be the best-characterized 
population at risk of adverse CO-induced effects, with CAD recognized 
as ``the most important susceptibility characteristic for increased 
risk due to CO exposure'' (ISA, section 2.6.1). An important factor 
determining the increased susceptibility of this population is their 
inability to compensate for the reduction in tissue oxygen levels due 
to an already compromised cardiovascular system. Individuals with a 
healthy cardiovascular system (i.e., with healthy coronary arteries) 
have operative physiologic compensatory mechanisms (e.g., increased 
blood flow and oxygen extraction) for CO-induced tissue hypoxia and are 
unlikely to be at increased risk of CO-induced effects (ISA, p. 2-
10).\7\ In addition, the high oxygen consumption of the heart, together 
with the inability to compensate for tissue hypoxia, makes the cardiac 
muscle of a person suffering from CAD a critical target for CO.
---------------------------------------------------------------------------

    \7\ The other well-studied individuals at the time of the last 
review were healthy male adults that experienced decreased exercise 
duration at similar COHb levels during short term maximal exercise. 
This population was of lesser concern since it represented a smaller 
sensitive group, and potentially limited to individuals that would 
engage in vigorous exercise such as competing athletes (1991 AQCD, 
section 10.3.2).
---------------------------------------------------------------------------

    Thus, the current evidence continues to support the identification 
of people with cardiovascular disease as susceptible to CO-induced 
health effects (ISA, 2-12) and those having CAD as the population with 
the best-characterized susceptibility (ISA, sections 5.7.1.1 and 
5.7.8).\8\ An important susceptibility consideration for this 
population is the inability to compensate for CO-induced hypoxia since 
individuals with CAD have an already compromised cardiovascular system. 
This population includes those with angina pectoris (cardiac chest 
pain), those who have experienced a heart attack, and those with silent 
ischemia or undiagnosed ischemic heart disease (AHA, 2003). People with 
other cardiovascular diseases, particularly heart diseases, are also at 
risk of CO-induced health effects.
---------------------------------------------------------------------------

    \8\ As recognized in the ISA, ``Although the weight of evidence 
varies depending on the factor being evaluated, the clearest 
evidence indicates that individuals with CAD are most susceptible to 
an increase in CO-induced health effects'' (ISA, p. 2-12).
---------------------------------------------------------------------------

    Cardiovascular disease comprises many types of medical disorders, 
including heart disease, cerebrovascular disease (e.g., stroke), 
hypertension (high blood pressure), and peripheral vascular diseases. 
Heart disease, in turn, comprises several types of disorders, including 
ischemic heart disease (coronary heart disease [CHD] or CAD, myocardial 
infarction, angina), congestive heart failure, and disturbances in 
cardiac rhythm (2000 AQCD, section 7.7.2.1).\9\ Other types of 
cardiovascular disease may also contribute to increased susceptibility 
to the adverse effects of low levels of CO (ISA, section 5.7.1.1). For 
example, evidence with regard to other types of cardiovascular disease 
such as congestive heart failure, arrhythmia, and non-specific 
cardiovascular disease, and more limited evidence for peripheral 
vascular and cerebrovascular disease, indicates that ``the continuous 
nature of the progression of CAD and its close relationship with other 
forms of cardiovascular disease suggest that a larger population than 
just those individuals with a prior diagnosis of CAD may be susceptible 
to health effects from CO exposure'' (ISA, p. 5-117).
---------------------------------------------------------------------------

    \9\ Coronary artery disease (CAD), often also called coronary 
heart disease or ischemic heart disease, is a category of 
cardiovascular disease associated with narrowed heart arteries. 
Individuals with this disease may have myocardial ischemia, which 
occurs when the heart muscle receives insufficient oxygen delivered 
by the blood. Exercise-induced angina pectoris (chest pain) occurs 
in many of them. Among all patients with diagnosed CAD, the 
predominant type of ischemia, as identified by electrocardiogram ST 
segment depression, is asymptomatic (i.e., silent). Patients who 
experience angina typically have additional ischemic episodes that 
are asymptomatic (2000 AQCD, section 7.7.2.1). In addition to such 
chronic conditions, CAD can lead to sudden episodes, such as 
myocardial infarction (ISA, p. 5-24).
---------------------------------------------------------------------------

    As described in the proposal, several other populations are 
potentially at risk of CO-induced effects, including: Those with other 
pre-existing diseases that may already have limited oxygen 
availability, increased COHb levels or increased endogenous CO 
production, such as people with obstructive lung diseases, diabetes and 
anemia; older adults; fetuses during critical phases of development and 
young infants or newborns; those who spend a substantial time on or 
near heavily traveled roadways; visitors to high-altitude locations; 
and people ingesting medications and other substances that enhance 
endogenous or metabolic CO formation (ISA, section 2.6.1). While the 
evidence suggests a potential susceptibility of these populations, 
information characterizing susceptibility for these groups is limited. 
For example, information is lacking on specific CO exposures or COHb 
levels that may be associated with health effects in these other groups 
and the nature of those effects, as well as a way to relate the 
specific evidence

[[Page 54300]]

available for the CAD population to these other populations (PA, 
section 2.2.1).
c. Cardiovascular Effects
    Similar to the previous review, results from controlled human 
exposure studies of individuals with coronary artery disease (CAD) 
(Adams et al., 1988; Allred et al., 1989a, 1989b, 1991; Anderson et 
al., 1973; Kleinman et al., 1989, 1998; Sheps et al., 1987\10\) are the 
``most compelling evidence of CO-induced effects on the cardiovascular 
system'' (ISA, section 5.2). Additionally, the use of an internal dose 
metric, COHb, adds to the strength of the findings in these controlled 
exposure studies. As a group, these studies demonstrate the role of 
short-term CO exposures in increasing the susceptibility of people with 
CAD to incidents of exercise-associated myocardial ischemia.
---------------------------------------------------------------------------

    \10\ Statistical analyses of the data from Sheps et al., (1987) 
by Bissette et al. (1986) indicate a significant decrease in time to 
onset of angina at 4.1% COHb if subjects that did not experience 
exercise-induced angina during air exposure are also included in the 
analyses.
---------------------------------------------------------------------------

    Among the controlled human exposure studies, the ISA places 
principal emphasis on the study of CAD patients by Allred et al. 
(1989a, 1989b, 1991) \11\ (which was also considered in the previous 
review) for the following reasons: (1) Dose-response relationships were 
observed; (2) effects were observed at the lowest COHb levels tested 
(mean of 2-2.4% COHb \12\ following experimental CO exposure), with no 
evidence of a threshold; (3) objective measures of myocardial ischemia 
(ST-segment depression) \13\ were assessed, as well as the subjective 
measure of decreased time to induction of angina; (4) measurements were 
taken both by CO-oximetry (CO-Ox) and by gas chromatography (GC), which 
provides a more accurate measurement of COHb blood levels \14\; (5) a 
large number of study subjects were used; (6) a strict protocol for 
selection of study subjects was employed to include only CAD patients 
with reproducible exercise-induced angina; and (7) the study was 
conducted at multiple laboratories around the U.S. This study evaluated 
changes in time to exercise-induced onset of markers of myocardial 
ischemia resulting from two short (approximately 1-hour) CO exposures 
targeted to result in mean study subject COHb levels of 2% and 4%, 
respectively (ISA, section 5.2.4). In this study, subjects (n = 63) on 
three separate occasions underwent an initial graded exercise treadmill 
test, followed by 50 to 70-minute exposures under resting conditions to 
room air CO concentrations or CO concentrations targeted for each 
subject to achieve blood COHb levels of 2% and 4%. The exposures were 
to average CO concentrations of 0.7 ppm (room air concentration range 
0-2 ppm), 117 ppm (range 42-202 ppm) and 253 ppm (range 143-357 ppm). 
After the 50- to 70-minute exposures, subjects underwent a second 
graded exercise treadmill test, and the percent change in time to onset 
of angina and time to ST endpoint between the first and second exercise 
tests was determined. For the two CO exposures, the average post-
exposure COHb concentrations were reported as 2.4% and 4.7%, and the 
subsequent post-exercise average COHb concentrations were reported as 
2.0% and 3.9%.\15\
---------------------------------------------------------------------------

    \11\ Other controlled human exposure studies of CAD patients 
(listed in Table 2-2 of the PA, and discussed in more detail in the 
1991 and 2000 AQCDs) similarly provide evidence of reduced time to 
exercise-induced angina associated with elevated COHb resulting from 
controlled short-duration exposure to increased concentrations of 
CO.
    \12\ These levels and other COHb levels described for this study 
below are based on gas chromatography analysis unless otherwise 
specified. Matched measurements available for CO-oximetry (CO-Ox) 
and gas chromatography (GC) in this study indicate CO-Ox 
measurements of 2.65% (post-exercise mean) and 3.21% (post-exposure 
mean) corresponding to the GC measurement levels of 2.00% (post-
exercise mean) to 2.38% (post-exposure mean) for the lower exposure 
level assessed in this study (Allred et al., 1991).
    \13\ The ST-segment is a portion of the electrocardiogram, 
depression of which is an indication of insufficient oxygen supply 
to the heart muscle tissue (myocardial ischemia). Myocardial 
ischemia can result in chest pain (angina pectoris) or such 
characteristic changes in ECGs or both. In individuals with coronary 
artery disease, it tends to occur at specific levels of exercise. 
The duration of exercise required to demonstrate chest pain and/or a 
1-mm change in the ST segment of the ECG were key measurements in 
the multicenter study by Allred et al. (1989a, 1989b, 1991).
    \14\ As stated in the ISA, the gas chromatographic technique for 
measuring COHb levels ``is known to be more accurate than 
spectrophotometric measurements, particularly for samples containing 
COHb concentrations 2.5, nitrogen 
dioxide, sulfur dioxide, ozone, and CO) at fixed-site ambient monitors 
and increases in emergency department visits and hospital admissions 
for specific cardiovascular health outcomes including ischemic heart 
disease (IHD), myocardial infarction, congestive heart failure (CHF), 
and cardiovascular diseases (CVD) as a whole (Bell et al., 2009; Koken 
et al., 2003; Linn et al., 2000; Mann et al., 2002; Metzger et al., 
2004; Symons et al., 2006; Tolbert et al., 2007; Wellenius et al., 
2005). As noted by the ISA, ``[s]tudies of hospital admissions and 
[emergency department] visits for IHD provide the strongest 
[epidemiological] evidence of ambient CO being associated with adverse 
CVD outcomes'' (ISA, p. 5-40, section 5.2.3). With regard to studies 
for other measures of cardiovascular morbidity, the ISA notes that 
``[t]hough not as consistent as the IHD effects, the effects for all 
CVD hospital admissions (which include IHD admissions) and CHF hospital 
admissions also provide evidence for an association of cardiovascular 
outcomes and ambient CO concentrations'' (ISA, section 5.2.3). While 
noting the difficulty in determining the extent to which CO is 
independently associated with CVD outcomes in this group of studies as 
compared to CO as a marker for the effects of another traffic-related 
pollutant or mix of pollutants, the ISA concludes that the 
epidemiological evidence, particularly when considering the copollutant 
analyses, provides support to the clinical evidence for a direct effect 
of short-term ambient CO exposure on CVD morbidity (ISA, pp. 5-40 to 5-
41).
3. Overview of Human Exposure and Dose Assessment
    Our consideration of the scientific evidence in the current review, 
as at the time of the last review, is informed by results from a 
quantitative analysis of estimated population exposure and resultant 
COHb levels. This analysis provides estimates of the percentages of 
simulated at-risk populations expected to experience daily maximum COHb 
levels at or above a range of benchmark levels under varying air 
quality scenarios (e.g., just meeting the current or alternative 
standards), as well as characterizations of the kind and degree of 
uncertainties inherent in such estimates. The benchmark COHb levels 
were identified based on consideration of the evidence discussed in 
section II.A.2 above. In this section, we provide a short overview of 
key aspects of the assessment conducted for this review. The assessment 
is summarized more fully in section II.C of the proposal, discussed in 
detail in the REA and summarized in the PA (section 2.2.2). The results 
of the analyses as they relate to considerations of the adequacy of the 
current standards are discussed in section II.B.3 below.
    As noted in the proposal notice, people can be exposed to CO in 
ambient air when they are outdoors and also when they are in indoor 
locations into which ambient (outdoor) air has infiltrated (ISA, 
sections 3.6.1 and 3.6.5). Indoor locations may also contain CO from 
indoor sources, such as gas stoves and tobacco smoke. Where present, 
these indoor sources can be important contributors to total CO exposure 
and can contribute to much greater CO exposures and associated COHb 
levels than those associated with ambient sources (ISA, section 
3.6.5.2). For example, indoor source-related exposures, such as faulty 
furnaces or other combustion appliances, have been estimated in the 
past to lead to COHb levels on the order of twice as high as short-term 
elevations in ambient CO that were more likely to be encountered by the 
general public (2000 AQCD, p. 7-4). Further, some exposure/dose 
assessments performed for previous reviews have included modeling 
simulations both without and with indoor (nonambient) sources (gas 
stoves and tobacco smoke) to provide context for the assessment of 
ambient CO exposure and dose (e.g., USEPA, 1992; Johnson et al., 2000), 
and these assessments have found that nonambient sources have a 
substantially greater impact on the highest total exposures and COHb 
levels experienced by the simulated population than do ambient sources 
(Johnson et al., 2000; REA, sections 1.2 and 6.3). While recognizing 
this potential for indoor sources, where present, to play a role in CO 
exposures and COHb levels, the exposure modeling in the current review 
(described below) did not include indoor CO sources in order to focus 
on the impact of ambient CO on population COHb levels.
    The assessment estimated ambient CO exposure and associated COHb 
levels in simulated at-risk populations in two urban study areas in 
Denver and Los Angeles, in which current ambient CO concentrations are 
below the current standards. Estimates were developed for exposures to 
ambient CO associated with current ``as is'' conditions (2006 air 
quality) and also for higher ambient CO concentrations associated with 
air quality conditions simulated to just meet the current 8-hour 
standard,\17\ as well as for air quality conditions simulated to just 
meet several potential alternative standards. Although we consider it 
unlikely that air concentrations in many urban areas across the U.S. 
that are currently well below the current standards would increase to 
just meet the 8-hour standard, we recognize the potential for CO 
concentrations in some areas currently below the standard to increase 
to just meet the standard. We additionally recognize that this 
simulation can provide useful information in evaluating the current 
standard, although we recognize the uncertainty associated with 
simulating this hypothetical profile of higher CO concentrations that 
just meet the current 8-hour standard.
---------------------------------------------------------------------------

    \17\ As noted elsewhere, the 8-hour standard is the controlling 
standard for ambient CO concentrations.
---------------------------------------------------------------------------

    The exposure and dose modeling for the assessment, presented in 
detail in the REA, relied on version 4.3 of EPA's Air Pollutant 
Exposure model (APEX4.3), which estimates human exposure using a 
stochastic, event-based microenvironmental approach (REA, chapter 4). 
The review of the CO standards completed in 1994 relied on population 
exposure and dose estimates generated from the probabilistic NAAQS 
exposure model (pNEM), a model that, among other differences from the 
current modeling approach with APEX4.3, employed a cohort-based 
approach (Johnson et al., 1992; USEPA, 1992).18 19 Each of 
the model developments since the use of pNEM in that review have been 
designed to allow APEX to better represent human behavior, human 
physiology, and

[[Page 54302]]

microenvironmental concentrations and to more accurately estimate 
variability in CO exposures and COHb levels (REA, chapter 4).\20\
---------------------------------------------------------------------------

    \18\ When using the cohort approach, each cohort is assumed to 
contain persons with identical exposures during the specified 
exposure period. Thus, variability in exposure will be attributed to 
differences in how the cohorts are defined, not necessarily 
reflecting differences in how individuals might be exposed in a 
population. In the assessment for the review completed in 1994, a 
total of 420 cohorts were used to estimate population exposure based 
on selected demographic information (11 groups using age, gender, 
work status), residential location, work location, and presence of 
indoor gas stoves (Johnson, et al., 1992; USEPA, 1992).
    \19\ The use of pNEM in the prior review also (1) relied on a 
limited set of activity pattern data (approximately 3,600 person-
days), (2) used four broadly defined categories to estimate 
breathing rates, and (3) implemented a geodesic distance range 
methodology to approximate workplace commutes (Johnson et al., 1992; 
USEPA, 1992). Each of these approaches used by pNEM, while 
appropriate given the data available at that time, would tend to 
limit the ability to accurately model expected variability in the 
population exposure and dose distributions.
    \20\ APEX4.3 includes new algorithms to (1) simulate 
longitudinal activity sequences and exposure profiles for 
individuals, (2) estimate activity-specific minute-by-minute oxygen 
consumption and breathing rates, (3) address spatial variability in 
home and work-tract ambient concentrations for commuters, and (4) 
estimate event-based microenvironmental concentrations (PA, section 
2.2.2).
---------------------------------------------------------------------------

    As used in the current assessment, APEX probabilistically generates 
a sample of hypothetical individuals from an actual population database 
and simulates each individual's movements through time and space (e.g., 
indoors at home, inside vehicles) to estimate his or her exposure to 
ambient CO (REA, chapter 4). Based on exposure concentrations, minute-
by-minute activity levels, and physiological characteristics of the 
simulated individuals (see REA, chapters 4 and 5), APEX estimates the 
level of COHb in the blood for each individual at the end of each hour 
based on a nonlinear solution to the Coburn-Forster-Kane equation (REA, 
section 4.4.7).
    As discussed in section II.A.2.b above, people with cardiovascular 
disease are the population of primary focus in this review, and, more 
specifically, coronary artery disease, also known as coronary heart 
disease, is the ``most important susceptibility characteristic for 
increased risk due to CO exposure'' (ISA, p. 2-11). Controlled human 
exposure studies have provided quantitative COHb dose-response 
information for this specific population with regard to effects on 
markers of myocardial ischemia. Accordingly, based on the current 
evidence with regard to quantitative information of COHb levels and 
association with specific health effects, the at-risk populations 
simulated in the quantitative assessment were (1) adults with CHD (also 
known as IHD or CAD), both diagnosed and undiagnosed, and (2) adults 
with any heart diseases, including undiagnosed ischemia.\21\ Evidence 
characterizing the nature of specific health effects of CO in other 
populations is limited and does not include specific COHb levels 
related to health effects in those groups. As a result, the 
quantitative assessment does not develop separate quantitative dose 
estimates for populations other than those with CHD or HD.
---------------------------------------------------------------------------

    \21\ As described in section1.2 above, this is the same 
population group that was the focus of the CO NAAQS exposure/dose 
assessments conducted previously (e.g., USEPA, 1992; Johnson et al., 
2000).
---------------------------------------------------------------------------

    APEX simulations performed for this review focused on exposures to 
ambient CO occurring in eight microenvironments,\22\ absent any 
contribution to microenvironment concentrations from indoor 
(nonambient) CO sources. Previous assessments, that have included 
modeling simulations both with and without certain indoor sources, 
indicated that the impact of such sources can be substantial with 
regard to the portion of the at-risk population experiencing higher 
exposures and COHb levels (Johnson et al., 2000). While we are limited 
with regard to information regarding CO emissions from indoor sources 
today and how they may differ from the time of the 2000 assessment, we 
note that ambient contributions have notably declined, and indoor 
source contributions from some sources may also have declined. Thus, as 
indicated in the Policy Assessment, we have no firm basis to conclude a 
different role for indoor sources today with regard to contribution to 
population CO exposure and COHb levels.
---------------------------------------------------------------------------

    \22\ The 8 microenvironments modeled in the REA comprised a 
range of indoor and outdoor locations including residences as well 
as motor vehicle-related locations such as inside vehicles, and 
public parking and fueling facilities, where the highest exposures 
were estimated (REA, sections 5.9 and 6.1).
---------------------------------------------------------------------------

    In considering the REA dose estimates in the Policy Assessment, 
staff considered estimates of the portion of the simulated at-risk 
populations estimated to experience daily maximum end-of-hour absolute 
COHb levels above identified benchmark levels (at least once and on 
multiple occasions), as well as estimates of the percentage of 
population person-days (the only metric available from the modeling for 
the 1994 review), and also population estimates of daily maximum 
ambient contribution to end-of-hour COHb levels.\23\ In identifying 
COHb benchmark levels of interest, primary attention was given to the 
multi-laboratory study in which COHb was analyzed by the more accurate 
GC method (Allred et al., 1989a, 1989b, 1991) discussed in section 
II.A.2.c above. As summarized in the proposal, the Policy Assessment 
recognized distinctions between the REA ``baseline'' (arising from 
prior ambient exposure and endogenous CO production) and the pre-
exposure COHb levels in the controlled human exposure study (arising 
from ambient and nonambient exposure history, as well as from 
endogenous CO production), and also noted the impact of ``baseline'' 
COHb levels on COHb levels occurring in response to short ambient CO 
exposure events such as those simulated in the REA.
---------------------------------------------------------------------------

    \23\ As summarized in the proposal and described more fully in 
the REA and PA, absolute COHb refers to the REA estimates of COHb 
levels resulting from endogenously produced CO and exposure to 
ambient CO (in the absence of any nonambient sources). The 
additional REA estimates of ambient CO exposure contribution to COHb 
levels were calculated by subtracting COHb estimates obtained in the 
absence of CO exposure--i.e., that due to endogenous CO production 
alone (see REA, Appendix B.6)--from the corresponding end-of-hour 
absolute COHb estimates for each simulated individual. Thus, the REA 
reports estimates of the maximum end-of-hour ambient contributions 
across the simulated year, in addition to the maximum absolute end-
of hour COHb levels.
---------------------------------------------------------------------------

    Numerous improvements have been made over the last decade that have 
reduced the uncertainties associated with the models used to estimate 
COHb levels resulting from ambient CO exposures under different air 
quality conditions, including those associated with just meeting the 
current CO NAAQS (REA, section 4.3). This progression in exposure model 
development has led to the model currently used by the agency 
(APEX4.3), which has an enhanced capacity to estimate population CO 
exposures and more accurately predicts COHb levels in persons exposed 
to CO. Our application of APEX4.3 in this review, using updated data 
and new algorithms to estimate exposures and doses experienced by 
individuals, better represents the variability in population exposure 
and COHb dose levels than the model version used in previous CO 
assessments.\24\ However, while APEX 4.3 is greatly improved when 
compared with previously used exposure models, its application is still 
limited with regard to data to inform our understanding of spatial 
relationships in ambient CO concentrations and within microenvironments 
of particular interest. Further information regarding model 
improvements and exposure modeling uncertainties is summarized in 
section 2.2.2 of the Policy Assessment and described in detail in 
chapter 7 of the REA.
---------------------------------------------------------------------------

    \24\ APEX4.3 provides estimates for percent of population 
projected to experience a single or multiple occurrences of a daily 
maximum COHb level above the various benchmark levels, as well as 
percent of person-days.
---------------------------------------------------------------------------

    Taking into consideration improvements in the model algorithms and 
data since the last review, and having identified and characterized 
these uncertainties, the Policy Assessment concludes that the estimates 
associated with the current analysis, at a minimum, better reflect the 
full distribution of exposures and dose as compared to results from the 
1992 analysis. As noted in the Policy Assessment, however, potentially 
greater uncertainty remains in our characterization of the upper and 
lower

[[Page 54303]]

percentiles of the distribution of population exposures and COHb dose 
levels relative to that of other portions of the respective 
distribution. When considering the overall quality of the current 
exposure modeling approach, the algorithms, and the input data used, 
alongside the identified limitations and uncertainties, the REA and 
Policy Assessment conclude that the quantitative assessment provides 
reasonable estimates of CO exposure and COHb dose for the simulated 
population the assessment is intended to represent (i.e., the 
population residing within the urban core of each study area). The 
Policy Assessment additionally notes the impact on the REA dose 
estimates for ambient CO contribution to COHb of the lack of nonambient 
sources in the model simulations. This aspect of the assessment design 
may contribute to higher estimates of the contribution of short-
duration ambient CO exposures to total COHb than would result from 
simulations that include the range of commonly encountered CO sources 
beyond just those contributing to ambient air CO concentrations. 
Although the specific quantitative impact of this on estimates of 
population percentages discussed in this document is unknown, 
consideration of COHb estimates from the 2000 assessment indicates a 
potential for the inclusion of nonambient sources to appreciably affect 
absolute COHb (REA, section 6.3) and accordingly implies the potential, 
where present, for an impact on overall ambient contribution to a 
person's COHb level. Key results of the exposure and dose analyses were 
presented in the Policy Assessment and summarized in the proposal 
(Tables 1 and 2 of the proposal).

B. Adequacy of the Current Primary Standards

    In considering the evidence and quantitative exposure and dose 
estimates with regard to judgments on the adequacy afforded by the 
current standards, the final decision is largely a public health policy 
judgment. A final decision must draw upon scientific information and 
analyses about health effects and risks, as well as judgments about how 
to consider the range and magnitude of uncertainties that are inherent 
in the scientific evidence and analyses. Our approach to informing 
these judgments is based on the recognition that the available health 
effects evidence generally reflects a continuum, consisting of ambient 
levels at which scientists generally agree that health effects are 
likely to occur, through lower levels at which the likelihood and 
magnitude of the response become increasingly uncertain. This approach 
is consistent with the requirements of the NAAQS provisions of the Act 
and with how EPA and the courts have historically interpreted the Act. 
These provisions require the Administrator to establish primary 
standards that, in the Administrator's judgment, are requisite to 
protect public health with an adequate margin of safety. In so doing, 
the Administrator seeks to establish standards that are neither more 
nor less stringent than necessary for this purpose. The Act does not 
require that primary standards be set at a zero-risk level, but rather 
at a level that avoids unacceptable risks to public health, including 
the health of sensitive groups.\25\
---------------------------------------------------------------------------

    \25\ The sensitive population groups identified in a NAAQS 
review may (or may not) be comprised of low income or minority 
groups. Where low income/minority groups are among the sensitive 
groups, the rulemaking decision will be based on providing 
protection for these and other sensitive population groups. To the 
extent that low income/minority groups are not among the sensitive 
groups, a decision based on providing protection of the sensitive 
groups would be expected to provide protection for the low income/
minority groups (as well as any other less sensitive population 
groups).
---------------------------------------------------------------------------

    In evaluating whether it is appropriate to revise the current CO 
standards, the Administrator's considerations build on the general 
approach used in the last review and reflect 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 CO; expert judgment on the adversity of such 
effects on individuals; and policy judgments as to when the standards 
are 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 CO standards.
    The Administrator's proposed conclusions on the adequacy of the 
current primary standards are summarized below (section II.B.1), 
followed by consideration of comments received on the proposal (section 
II.B.2) and the Administrator's final decision with regard to the 
adequacy of the current primary standards (II.B.3).
1. Rationale for Proposed Decision
    At the time of the proposal, in considering the adequacy of the 
current standards, the Administrator carefully considered the available 
evidence and conclusions contained in the Integrated Science 
Assessment; the information, exposure/dose assessment, rationale and 
conclusions presented in the Policy Assessment; the advice and 
recommendations from CASAC; and public comments as of that date. In so 
doing, the Administrator noted the following: (1) The long-standing 
evidence base concerning effects associated with exposure to CO, 
including the key role played by hypoxia (reduced oxygen availability) 
induced by increased COHb blood levels, and the use of COHb as the 
bioindicator and dose metric for evaluating CO exposure and the 
potential for health effects; (2) the strong evidence of cardiovascular 
effects of short-term CO exposures including the evidence from 
controlled human exposure studies that demonstrate a reduction in time 
to onset of exercise-induced markers of myocardial ischemia in response 
to increased COHb, and the health significance of responses observed at 
the 2% COHb level induced by 1-hour CO exposure, as compared to higher 
COHb levels; and (3) the identification of people with cardiovascular 
disease as a key population at risk from short-term ambient CO 
exposures. In the proposal, as at the time of the last review, the 
Administrator additionally considered and took particular note of the 
exposure and dose modeling results, recognizing key limitations and 
uncertainties, and in light of judgments noted above regarding the 
health significance of findings from the controlled human exposure 
studies, placing less weight on the health significance of infrequent 
or rare occurrences of COHb levels at or just above 2% and more weight 
to the significance of repeated such occurrences, as well as 
occurrences of higher COHb levels.
    The Administrator also considered the newly available and much-
expanded epidemiological evidence, including the complexity associated 
with quantitative interpretation of these studies with regard to CO, 
particularly the few studies available in areas where the current 
standards are met. Further, the Administrator considered the advice of 
CASAC, including their overall agreement with the Policy Assessment 
conclusion that the current evidence and quantitative exposure and dose 
estimates provide support for retaining the current standards, their 
view that, in

[[Page 54304]]

light of the epidemiological studies, revisions to lower the standards 
should be considered and their preference for a lower standard, and 
also their advice regarding the complications associated with 
interpreting the epidemiological studies for CO. Although CASAC 
expressed a preference for a lower standard, CASAC also indicated that 
the current evidence provides support for retaining the current suite 
of standards and CASAC's recommendations appear to recognize that their 
preference for a lower standard was contingent on a judgment as to the 
weight to be placed on the epidemiological evidence. For the reasons 
explained in the proposal, after full consideration of CASAC's advice 
and the epidemiological evidence, as well as its associated 
uncertainties and limitations, the Administrator proposed to judge 
those uncertainties and limitations to be too great for the 
epidemiological evidence to provide a basis for revising the current 
standards.
    Taking all these considerations together, the Administrator 
proposed to conclude that the current suite of standards provides a 
very high degree of protection for the COHb levels and associated 
health effects of concern, as indicated by the extremely low estimates 
of occurrences, and provides slightly less but a still high degree of 
protection for the effects associated with lower COHb levels, the 
physiological significance of which is less clear. The Administrator 
additionally proposed to conclude that consideration of the 
epidemiological studies does not lead her to identify a need for any 
greater protection. Thus, the Administrator proposed to conclude that 
the current suite of standards provides an adequate margin of safety 
against adverse effects associated with short-term ambient CO 
exposures. For these and all of the reasons discussed above, and 
recognizing the CASAC conclusion that, overall, the current evidence 
and REA results provide support for retaining the current standards, 
the Administrator proposed to conclude that the current suite of 
primary CO standards is requisite to protect public health with an 
adequate margin of safety from effects of ambient CO.
2. Comments on Adequacy
    In considering comments on the adequacy of the current standards, 
the Administrator first notes the advice and recommendations from 
CASAC. In the context of CASAC's review of the documents prepared 
during the course of the review, CASAC sent EPA five letters providing 
advice regarding assessment and interpretation of the available 
scientific evidence and the REA for the purposes of judging the 
adequacy of the current CO standards (Brain and Samet, 2009; Brain and 
Samet, 2010a; Brain and Samet, 2010b; Brain and Samet, 2010c; Brain and 
Samet, 2010d). In conveying comments on the draft Policy Assessment, 
CASAC agreed with the conclusion that the current evidence provides 
support for retaining the current suite of standards, while they also 
expressed a preference for a lower standard and stated that the 
epidemiological evidence could indicate the occurrence of adverse 
health effects at levels of the standards (Brain and Samet, 2010c). 
With regard to the interpretation of epidemiological studies on CO, 
CASAC's collective advice included recommendations regarding the weight 
to be placed on the epidemiological evidence (Brain and Samet, 2010c), 
as well as cautionary statements regarding interpretation of the 
epidemiological studies. Such statements included the observation that 
``[d]istinguishing the effects of CO per se from the consequences of CO 
as a marker of pollution or vehicular traffic is a challenge, which 
[the ISA] needs to confront as thoroughly as possible'' (Brain and 
Samet, 2009, p. 2). In another letter CASAC further cautioned (Brain 
and Samet, 2010d, p. 2):

    The problem of co-pollutants serving as potential confounders is 
particularly problematic for CO. Since exposure levels for CO are 
now low, consideration needs to be given to the possibility that in 
some situations CO may be a surrogate for exposure to a mix of 
pollutants generated by fossil fuel combustion. A better 
understanding of the possible role of co-pollutants is relevant to 
regulation and to the design, analysis, and interpretation of 
epidemiologic studies on the health effects of CO.

CASAC additionally noted concerns regarding the spatial coverage of the 
existing CO monitoring network and the sensitivity of deployed monitors 
(Brain and Samet, 2009; Brain and Samet, 2010a; Brain and Samet, 2010b; 
Brain and Samet, 2010d). On a related note, they cautioned that 
``[u]nderstanding the extent of exposure measurement error is critical 
for evaluating epidemiological evidence'' (Brain and Samet, 2009).
    General comments from the public based on relevant factors that 
either support or oppose retention of the current primary CO standards 
are addressed in this section. Other specific public comments related 
to consideration of the adequacy of the current standards, as well as 
general comments based on implementation-related factors that are not a 
permissible basis for considering the need to revise the current 
standards, are addressed in the Response to Comments document.
    The public comments received on the proposal were divided with 
regard to support for the Agency's proposed conclusion as to the 
adequacy of the current standards. All of the state and local 
environmental agencies or governments that provided comments on the 
standards concurred with EPA's proposed conclusions as did the three 
industry commenters. All of these commenters generally noted their 
agreement with the rationale provided in the proposal, with some 
additionally citing CASAC's recognition of support in the evidence for 
the adequacy of the current standards. Some of these commenters noted 
agreement with the weight given to the epidemiological studies in the 
proposal and also noted the little change in exposure/risk estimates 
since the time of the last review. One commenter additionally stated 
their view that the REA overstates the exposure and risk associated 
with the current standards.
    As described in section II.B.3 below, the EPA generally agrees with 
these commenters regarding the adequacy of the current CO standards and 
with CASAC that the evidence provides support for the conclusion that 
the current CO standards protect public health with an adequate margin 
of safety. EPA additionally has given consideration to CASAC's advice 
regarding interpretation of epidemiological evidence for CO, 
recognizing the limitations associated with its use in drawing 
quantitative interpretations regarding levels of ambient CO related to 
health outcomes.
    Two submissions recommending revision of the standards were 
received from national environmental or public health organizations. 
Additional submissions recommending revision were received from a 
private consultant; a group of scientists, physicians, and others; and 
a group of private citizens. In support of their position, these 
commenters variously cited CASAC comments regarding emphasis to give 
epidemiological studies and CASAC's stated preference for a lower 
standard. These submissions generally disagreed with EPA's 
consideration of the epidemiological evidence in the proposal and 
recommended that EPA give greater emphasis to epidemiological studies 
of a range of endpoints, including developmental and respiratory 
effects, based on the commenters' view that the epidemiological studies 
provided evidence of harm associated with ambient CO levels below the 
current

[[Page 54305]]

standards and inadequate protection for sensitive populations. Among 
these submissions, those that specified levels for revised standards 
recommended levels that were no higher than the lowest part of the 
ranges for the two standards that were identified for consideration in 
the Policy Assessment and the example options that CASAC suggested for 
inclusion in the Policy Assessment. Additionally, one commenter 
described the view that the CO standards should be revised to levels at 
or below the range of CO concentrations in exhaled breath of healthy 
non-smokers.
    EPA generally disagrees with these commenters regarding conclusions 
that can be drawn from the evidence, including the epidemiological 
studies, pertaining to the adequacy of the current CO standards. In 
considering the adequacy of the current standards, it is important to 
consider both the extent to which the evidence supports a causal 
relationship between ambient CO exposures and adverse health effects, 
as well as the extent to which there is evidence pertinent to such 
effects under air quality conditions in which the current standards are 
met. With regard to the latter point, and focusing on the 
epidemiological evidence, it is the studies involving air quality 
conditions in which the current standards were met that are most 
informative in evaluating the adequacy of the standards (PA, p. 2-30). 
We note that very few of the epidemiological studies observing an 
association of cardiovascular disease-related outcomes with short-term 
CO concentrations (or those observing associations for other health 
effects) were conducted in areas that met the current standards 
throughout the period of study, thus limiting their usefulness with 
regard to judging the adequacy of the current standards (PA, pp. 2-33, 
2-36).
    Further, as CASAC has cautioned, ``the problem of co-pollutants 
serving as potential confounders is particularly problematic for CO'' 
(Brain and Samet, 2010d). While some CO epidemiological studies have 
applied the commonly used statistical method, two-pollutant regression 
models, to inform conclusions regarding CO as the pollutant eliciting 
the effects in these studies, and while, in some studies, the CO 
associations remain robust after adjustment for another traffic 
combustion-related pollutant, such as PM2.5 or nitrogen 
dioxide (NO2) (PA, pp. 2-36 to 2-37), the potential exists 
for there to be etiologically relevant pollutants that are correlated 
with CO yet absent from the analysis, particularly given the many 
pollutants associated with fossil fuel combustion. The CASAC 
specifically recognized this potential in stating that ``consideration 
needs to be given to the possibility that in some situations CO may be 
a surrogate for exposure to a mix of pollutants generated by fossil 
fuel combustion'' and ``a better understanding of the possible role of 
co-pollutants is relevant to * * * the interpretation of epidemiologic 
studies on the health effects of CO'' (Brain and Samet, 2010d).
    In light of these issues related to potential confounding by co-
pollutants in the case of CO, uncertainty related to exposure error for 
CO is of particular concern in quantitatively interpreting the 
epidemiological evidence (e.g., with regard to ambient concentrations 
contributing to health outcomes).\26\ As noted above, CASAC cautioned 
the Agency on the importance of understanding the extent of exposure 
error in evaluating the epidemiological evidence for CO (Brain and 
Samet, 2009). There are two aspects to the epidemiological studies in 
the specific case of CO (as contrasted with other pollutants such as PM 
and NO2) that may contribute exposure error in the studies 
(PA, pp. 2-34 to 2-38; 76 FR 8177-8178). The first relates to the 
uncertainty associated with quantitative interpretation of the 
epidemiological study results at low ambient concentrations in light of 
the sizeable portion of ambient CO measurements that are at or below 
monitor method detection limits (MDLs). As described in the proposal, 
uncertainty related to the prevalence of ambient CO monitor 
concentrations at or below MDLs is a greater concern for the more 
recently available epidemiological studies in which the study areas 
have much reduced ambient CO concentrations compared with those in the 
past (PA, pp. 2-37 to 2-38). This complicates our interpretation of 
specific ambient CO concentrations associated with health effects (ISA, 
p. 3-91; Brain and Samet, 2010d), providing us with reduced confidence 
in quantitative interpretations of epidemiological studies for CO. 
Additionally, as described in the proposal, there is uncertainty and 
potential error associated with exposure estimates in the CO 
epidemiological studies that relate to the use of area-wide or central-
site monitor CO concentrations in light of information about the steep 
gradient in CO concentrations with distance from source locations such 
as highly-trafficked roadways (ISA, section 3.5.1.3). As a result of 
differences in factors related to pollutant formation, this gradient is 
steeper for CO than for other traffic combustion-related pollutants, 
such as PM2.5 and NO2, contributing to a greater 
potential for exposure misclassification in the case of CO by the 
reliance on central site monitors in the CO epidemiological studies. 
Thus, as noted in the proposal, we recognize that the expanded body of 
epidemiological evidence available in this review includes its own set 
of uncertainties which complicates its interpretation, particularly 
with regard to ambient concentrations that may be eliciting health 
outcomes.
---------------------------------------------------------------------------

    \26\ In contrasting the strength of the epidemiological evidence 
available for the 2000 AQCD with that in the current review, the ISA 
notes that uncertainties identified in 2000 remain, including the 
ability of community fixed-site monitors to represent spatially 
variable ambient CO concentrations and personal exposures; the small 
expected increase in COHb due to ambient CO concentrations; the lack 
of biological plausibility for health effects to occur at such COHb 
levels, even in diseased individuals; and the possibility that 
ambient CO is serving as a surrogate for a mixture of combustion-
related pollutants. These uncertainties complicate the quantitative 
interpretation of the epidemiologic findings, ``particularly 
regarding the biological plausibility of health effects occurring at 
COHb levels resulting from exposures to ambient CO concentrations 
measured at AQS monitors'' (ISA, pp. 2-16 to 2-17).
---------------------------------------------------------------------------

    In our integrated assessment across all types of evidence in the 
ISA for this review, we conclude that a causal relationship is likely 
to exist for short-term exposures to ambient concentrations of CO and 
cardiovascular morbidity. In reaching this conclusion, the ISA notes 
that the most compelling evidence comes from the controlled human 
exposure studies (ISA, p. 2-5), which also document a significant dose-
response relationship over a range of COHb concentrations relevant to 
consideration of the NAAQS (ISA, p. 2-13). In considering the 
epidemiological evidence for relevant cardiovascular outcomes, which 
includes multiple studies reporting associations with ambient CO 
concentrations under conditions when the current standards were not met 
(PA, p. 2-30), the ISA notes that these studies are coherent with the 
findings from the controlled human exposure studies (ISA, p. 2-17). 
However, as summarized here, various aspects of the evidence complicate 
quantitative interpretation of it with regard to ambient concentrations 
that might be eliciting the reported health outcomes.
    An additional complication to our consideration of the CO 
epidemiological evidence is that, in contrast to the health effects 
evidence for all other criteria pollutants, the epidemiological studies 
for CO use a different exposure/dose metric from that which is the 
focus of the broader health evidence base, and

[[Page 54306]]

additional information that might be used to bridge this gap is 
lacking. In the case of CO, the epidemiological studies use air 
concentration as the exposure/dose metric, while much of the broader 
health effects evidence for CO, and particularly that related to 
cardiovascular effects, demonstrates and focuses on an internal 
biomarker of CO exposure (COHb) which has been considered a critical 
key to CO toxicity.\27\ The strong evidence describing the role of COHb 
in CO toxicity is important to consider in interpreting the CO 
epidemiological studies and contributes to the biological plausibility 
of the ischemia-related health outcomes that have been associated with 
ambient CO concentrations. Yet, we do not have information on the COHb 
levels of epidemiological study subjects that we can evaluate in the 
context of the COHb levels eliciting health effects in the controlled 
human exposure studies. Further, we lack additional information on the 
CO exposures of the epidemiological study subjects to both ambient and 
nonambient sources of CO that might be used to estimate their COHb 
levels and bridge the gap between the two study types. Additionally the 
ISA recognizes that the changes in COHb that would likely be associated 
with exposure to the low ambient CO concentrations assessed in some of 
the epidemiological studies would be smaller than changes associated 
with ``substantially reduced [oxygen] delivery to tissues,'' that might 
plausibly lead to the outcomes observed in those studies, with 
additional investigation needed to determine whether there may be 
another mechanism of action for CO that contributes to the observed 
outcomes at low ambient concentrations (ISA, p. 5-48). Thus, there are 
uncertainties associated with the epidemiological evidence that 
``complicate the quantitative interpretation of the epidemiologic 
findings, particularly regarding the biological plausibility of health 
effects occurring at COHb levels resulting from exposures to the 
ambient CO concentrations'' assessed in these studies (ISA, p. 2-17).
---------------------------------------------------------------------------

    \27\ In the case of the only other criteria pollutant for which 
the health evidence relies on an internal dose metric--lead--the 
epidemiological studies also use that metric. For lead (Pb), in 
contrast to CO, the epidemiological evidence is focused on 
associations of Pb-related health effects with measurements of Pb in 
blood, providing a direct linkage between the pollutant, via the 
internal biomarker of dose, and the health effects. Thus, for Pb, as 
compared to the case for CO, we have less uncertainty in our 
interpretations of the epidemiological studies with regard to the 
pollutant responsible for the health effects observed. For other 
criteria pollutants, including PM and NO2, air 
concentrations are used as the exposure/dose metric in both the 
epidemiological studies and the other types of health evidence. 
Thus, there is no comparable aspect in the PM or NO2 
evidence base.
---------------------------------------------------------------------------

    With regard to health effects other than cardiovascular outcomes, 
in addition to noting the complications cited above with regard to 
quantitative interpretation of the epidemiological evidence, we note 
that the evidence for these other categories of health effects is 
considered limited and only suggestive of a causal relationship with 
relevant exposures to CO in ambient air, or inadequate to infer such a 
relationship, or it supports the conclusion that such a relationship is 
not likely (see section II.A.2.b above). As described in the proposal 
sections II.B.2 and II.D.2.a, with regard to categories of health 
effects or outcomes for which the evidence is considered suggestive, 
evidence is lacking that might lend biological plausibility to 
epidemiological study results, and also sufficiently rule out the role 
of chance, bias and confounding in the epidemiological associations 
observed, for outcomes such as developmental or respiratory (ISA, 
chapters 1 and 2).
    Thus, EPA disagrees with the commenters' conclusion that the 
epidemiological evidence establishes that a range of health effects, 
including developmental or respiratory effects, are occurring as a 
result of exposures to CO in ambient air at or below the current 
standards. We additionally disagree with commenters' statements that 
imply EPA has inadequately considered the evidence with regard to 
protection of sensitive populations and to the protection provided by 
the CO standards. As noted in section II.A.2.b above, EPA's assessment 
of the current evidence presented in the Integrated Science Assessment 
concludes that ``the most important susceptibility characteristic for 
increased risk due to CO exposure is [CAD or CHD]'' (ISA, p. 2-10). 
Accordingly, the proposal recognized people with cardiovascular disease 
as a key population at risk from short-term ambient CO exposures 
(proposal, section II.D.4). However, based on assessment of the 
evidence in the ISA, the proposal and other documents in this review 
also recognize the potential for susceptibility for several other 
populations and lifestages, including people with pre-existing diseases 
that may already have limited oxygen availability to tissues, increased 
COHb levels or increased endogenous CO production, older adults, and 
fetuses during critical phases of development (as summarized in section 
II.A.2.b above). For these groups and lifestages, the evidence is 
incomplete with regard to specific CO exposures or COHb levels that may 
be associated with health effects in these groups and the nature of 
those effects, as well as a way to relate the specific evidence 
available for the CAD population to the limited evidence for these 
other populations. Further, the currently available evidence does not 
indicate a greater susceptibility for any of the other populations or 
lifestages recognized as potentially at risk from exposure to ambient 
CO. In reaching a decision on the adequacy of the current standards in 
protecting public health in section II.B.3 below, however, the 
Administrator has considered EPA's conclusions with regard to the 
effects likely to be causally associated with exposure to ambient CO 
and population groups particularly at risk, as well as those regarding 
the evidence with regard to the potential for other effects and 
sensitive groups, and the associated uncertainty. In so doing, as 
indicated below, the Administrator judges the current standards to 
provide the requisite protection for public health, including the 
health of sensitive populations, with an adequate margin of safety.
3. Conclusions Concerning Adequacy of the Primary Standards
    Having carefully considered the public comments, as discussed 
above, the Administrator believes that the fundamental scientific 
conclusions on the effects of CO in ambient air reached in the 
Integrated Science Assessment and Policy Assessment, summarized in 
sections II.B and II.D of the proposal remain valid. Additionally, the 
Administrator believes the judgments she reached in the proposal 
(section II.D.4) with regard to consideration of the evidence and 
quantitative exposure/dose assessments and advice from CASAC remain 
appropriate. Thus, as described below, the Administrator concludes that 
the current primary standards provide the requisite protection of 
public health with an adequate margin of safety and should be 
retained.\28\
---------------------------------------------------------------------------

    \28\ As explained below in section IV.A, EPA is repromulgating 
the Federal Reference Method (FRM) for CO, as set forth in Appendix 
C of 40 CFR part 50. Consistent with EPA's decision to retain the 
standards, the recodification clarifies and updates the text of the 
FRM, but does not make substantive changes to it.
---------------------------------------------------------------------------

    In considering the adequacy of the current suite of primary CO 
standards, the Administrator has carefully considered the available 
evidence and conclusions contained in the Integrated Science 
Assessment; the information, exposure/dose assessment, rationale and 
conclusions presented in the Policy

[[Page 54307]]

Assessment; the advice and recommendations from CASAC; and public 
comments. The Administrator places primary consideration on the 
evidence obtained from controlled human exposure studies that 
demonstrates a reduction in time to onset of exercise-induced markers 
of myocardial ischemia in response to increased COHb resulting from 
short-term CO exposures, and recognizes the greater significance 
accorded both to larger reductions in time to myocardial ischemia and 
to more frequent occurrences of myocardial ischemia. As at the time of 
the review completed in 1994, the Administrator also takes note of the 
results for the modeling of exposures to ambient CO under conditions 
simulated to just meet the current, controlling, 8-hour standard in two 
study areas, as described in the REA and Policy Assessment, and the 
public health significance of those results. She also considers the 
newly available and much-expanded epidemiological evidence, including 
the complexity associated with quantitative interpretation of these 
studies, particularly the few studies available in areas where the 
current standards are met. In so doing, she notes that in considering 
the adequacy of the current standards, it is important to consider both 
the extent to which the evidence supports a causal relationship between 
ambient CO exposures and adverse health effects, as well as the extent 
to which there is evidence pertinent to such effects under air quality 
conditions in which the current standards are met. Further, the 
Administrator considers the advice of CASAC, including both their 
overall agreement with the Policy Assessment conclusion that the 
current evidence and quantitative exposure and dose estimates provide 
support for retaining the current standards, as well as their view that 
in light of the epidemiological studies, revisions to lower the 
standards should be considered and their preference for a lower 
standard.
    As an initial matter, the Administrator places weight on the long-
standing evidence base that has established key aspects of CO toxicity 
that are relevant to this review as they were to the review completed 
in 1994. These aspects include the key role played by hypoxia (reduced 
oxygen availability) induced by increased COHb blood levels, the 
identification of people with cardiovascular disease as a key 
population at risk from short-term ambient CO exposures, and the use of 
COHb as the bioindicator and dose metric for evaluating CO exposure and 
the potential for health effects. The Administrator also recognizes the 
Integrated Science Assessment's conclusion that a causal relationship 
is likely to exist between relevant short-term exposures to CO and 
cardiovascular morbidity.
    In placing weight on the controlled human exposure studies, the 
Administrator also recognizes the uncertain health significance 
associated with the smaller responses to the lowest COHb level assessed 
in the study given primary consideration in this review (Allred et al., 
1989a, 1989b, 1991) and with single occurrences of such responses. In 
the study by Allred et al. (1989a, 1989b, 1991), a 4-5% reduction in 
time (approximately 30 seconds) to the onset of exercise-induced 
markers of myocardial ischemia was associated with the 2% COHb level 
induced by 1-hour CO exposure. In considering the significance of the 
magnitude of the time decrement to onset of myocardial ischemia 
observed at the 2% COHb level induced by short-term CO exposure, as 
well as the potential for myocardial ischemia to lead to more adverse 
outcomes, the EPA generally places less weight on the health 
significance associated with infrequent or rare occurrences of COHb 
levels at or just above 2% as compared to that associated with repeated 
occurrences and occurrences of appreciably higher COHb levels in 
response to short-term CO exposures. For example, at the 4% COHb level, 
the study by Allred et al., (1989a, 1989b, 1991) observed a 7-12% 
reduction in time to the onset of exercise-induced markers of 
myocardial ischemia. The Administrator places more weight on this 
greater reduction in time to onset of exercise-induced markers compared 
to the reduction in time to onset at 2% COHb. The Administrator also 
notes that at the time of the 1994 review, an intermediate level of 
approximately 3% COHb was identified as a level at which adverse 
effects had been demonstrated in persons with angina. Now, as at the 
time of the 1994 review, the Administrator primarily considers the 2% 
COHb level, resulting from 1-hour CO exposure, in the context of a 
margin of safety against effects of concern that have been associated 
with higher COHb levels, such as 3-4% COHb.
    The Administrator additionally takes note of the now much-expanded 
evidence base of epidemiological studies, including the multiple 
studies that observe positive associations between cardiovascular 
outcomes and short-term ambient CO concentrations across a range of CO 
concentrations, including conditions above as well as below the current 
NAAQS. She notes particularly the Integrated Science Assessment 
conclusion that the findings of CO-associated cardiovascular effects in 
these studies are logically coherent with the larger, long-standing 
health effects evidence base for CO and the conclusions drawn from it 
regarding cardiovascular disease-related susceptibility. In further 
considering the epidemiological evidence base with regard to the extent 
to which it provides support for conclusions regarding adequacy of the 
current standards, the Administrator takes note of CASAC's conclusions 
that ``[i]f the epidemiological evidence is given additional weight, 
the conclusion could be drawn that health effects are occurring at 
levels below the current standard, which would support the tightening 
of the current standard'' (Brain and Samet, 2010c). Additionally, the 
Administrator places weight on the final Policy Assessment 
consideration of aspects that complicate quantitative interpretation of 
the epidemiological studies with regard to ambient concentrations that 
might be eliciting the reported health outcomes.
    For purposes of evaluating the adequacy of the current standards, 
the Administrator takes note of the multiple complicating features of 
the epidemiological evidence base, as described in more detail in the 
final Policy Assessment and in section II.D.2.a of the proposal. First, 
while a number of studies observed positive associations of 
cardiovascular disease-related outcomes with short-term CO 
concentrations, very few of these studies were conducted in areas that 
met the current standards throughout the period of study. Additionally, 
in CASAC's advice regarding interpretation of the currently available 
evidence, they stated that ``[t]he problem of co-pollutants serving as 
potential confounders is particularly problematic for CO'' and that 
given the currently low ambient CO levels, there is a possibility that 
CO is acting as a surrogate for a mix of pollutants generated by fossil 
fuel combustion. The CASAC further stated that ``[a] better 
understanding of the possible role of co-pollutants is relevant to 
regulation'' (Brain and Samet, 2010d). As described in the Policy 
Assessment and summarized in section II.B.2 above, there are also 
uncertainties related to representation of ambient CO exposures given 
the steep concentration gradient near roadways, as well as the 
prevalence of measurements below the MDL across the database. The CASAC 
additionally indicated the need to consider the potential for 
confounding effects of

[[Page 54308]]

indoor sources of CO (Brain and Samet, 2010c). As discussed in section 
II.D.2.a of the proposal, the interpretation of epidemiological studies 
for CO is further complicated because, in contrast to the situation for 
all other criteria pollutants, the epidemiological studies for CO use 
an exposure/dose metric (air concentration) that differs from the 
metric commonly used in the other key CO health studies (COHb).
    The Administrator notes that although CASAC expressed a preference 
for a lower standard, CASAC also indicated that the current evidence 
provides support for retaining the current suite of standards. CASAC's 
recommendations appear to recognize that their preference for a lower 
standard was contingent on a judgment as to the weight to be placed on 
the epidemiological evidence. Further, as noted above and summarized in 
section II.C.2, CASAC has provided a range of advice regarding 
interpretation of the CO epidemiological studies in light of the 
associated uncertainties. Accordingly, in consideration of the current 
evidence with regard to conclusions to be drawn as to the adequacy of 
the current standards, the Administrator gives consideration to the 
full breadth of CASAC's advice.
    In considering the evidence and quantitative exposure and dose 
estimates available in this review with regard to the adequacy of 
public health protection provided by the current primary standards, the 
Administrator recognizes that, as noted in section II.B. above, the 
final decision on such judgments is largely a public health policy 
judgment, which draws upon scientific information and analyses about 
health effects and risks, as well as judgments about how to consider 
the range and magnitude of uncertainties that are inherent in the 
information and analyses. These judgments are informed by the 
recognition that the available health effects evidence generally 
reflects a continuum, consisting of ambient levels at which scientists 
generally agree that health effects are likely to occur, through lower 
levels at which the likelihood and magnitude of the response become 
increasingly uncertain. Accordingly, the final decision 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. As described in section I.A above, the Act does 
not require that primary standards be set at a zero-risk level; the 
NAAQS must be sufficient but not more stringent than necessary to 
protect public health, including the health of sensitive groups, with 
an adequate margin of safety.
    In considering the judgments to be made regarding adequacy of the 
level of protection provided by the current standards, the 
Administrator takes particular note of the findings of the exposure and 
dose assessment in light of considerations discussed above regarding 
the weight given to different COHb levels and their frequency of 
occurrence. As described in the proposal, the exposure and dose 
assessment results indicate that only a very small percentage of the 
at-risk population is estimated to experience a single occurrence in a 
year of daily maximum COHb at or above 3.0% COHb under conditions just 
meeting the current 8-hour standard in the two study areas evaluated, 
and no multiple occurrences are estimated. The Administrator also notes 
the results indicating that only a small percentage of the at-risk 
populations are estimated to experience a single occurrence of 2% COHb 
in a year under conditions just meeting the standard, and still fewer 
are estimated to experience multiple such occurrences. Additionally, 
consistent with findings of the assessment performed for the review 
completed in 1994, less than 0.1% of person-days for the at-risk 
populations were estimated to include occurrences of COHb at or above 
2% COHb. Taken together, the Administrator judges the current standard 
to provide a very high degree of protection for the COHb levels and 
associated health effects of concern, as indicated by the extremely low 
estimates of occurrences, and to provide slightly less but a still high 
degree of protection for the effects associated with lower COHb levels, 
the physiological significance of which is less clear.
    In further considering the adequacy of the margin of safety 
provided by the current standards, the Administrator has additionally 
considered conclusions drawn in the Integrated Science Assessment and 
Policy Assessment with regard to interpretation of the limited and less 
certain information concerning a relationship between exposure to 
relevant levels of ambient CO and health effects in other, potentially, 
susceptible groups, and with regard to the uncertainties concerning 
quantitative interpretation of the available epidemiological studies. 
In so doing, the Administrator additionally judges the current 
standards to provide adequate protection against the risk of other 
health effects for which the evidence is less certain. Further, the 
Administrator concludes that consideration of the epidemiological 
studies does not lead her to identify a need for any greater 
protection. For these and all of the reasons discussed above, and 
recognizing the CASAC conclusion that, overall, the current evidence 
and REA results provide support for retaining the current standards, 
the Administrator concludes that the current suite of primary CO 
standards is requisite to protect public health with an adequate margin 
of safety from effects of ambient CO.

III. Consideration of a Secondary Standard

    As noted in section I.A. above, section 109(b) of the Clean Air Act 
requires the Administrator to establish secondary standards that, in 
the judgment of the Administrator, are requisite to protect the public 
welfare from any known or anticipated adverse effects associated with 
the presence of the pollutant in the ambient air. In so doing, the 
Administrator seeks to establish standards that are neither more nor 
less stringent than necessary for this purpose. The Act does not 
require that secondary standards be set to eliminate all risk of 
adverse welfare effects, but rather at a level requisite to protect 
public welfare from those effects that are judged by the Administrator 
to be adverse.
    This section presents the rationale for the Administrator's final 
decision not to set a secondary NAAQS for CO. In considering the 
current air quality criteria, evidence of CO-related welfare effects at 
or near ambient levels that are unrelated to climate has not been 
identified. Accordingly, in considering whether a secondary standard is 
requisite to protect the public welfare, the Administrator has 
primarily considered conclusions based on the evidence of a role for CO 
in effects on climate. Evaluation of this evidence in the Integrated 
Science Assessment and staff considerations in the Policy Assessment 
highlighted the limitations in this evidence and provided information 
indicating that this role for atmospheric CO is predominantly indirect, 
through its role in chemical reactions in the atmosphere which result 
in increased concentrations of pollutants with direct contributions to 
the greenhouse effect or that deplete stratospheric ozone. Given the 
evaluation of the evidence, as well as the views of CASAC, the 
Administrator concludes that no secondary standard should be set at 
this time because, as in the past reviews, having no standard is 
requisite to protect public welfare from any known or anticipated 
adverse effects from ambient CO exposures.
    In this section, we first summarize the evidence currently 
available for welfare

[[Page 54309]]

effects to inform decisions in this review in section III.A. Next, the 
rationale for the proposed conclusions is summarized in section III.B. 
Public comments and CASAC advice regarding consideration of a secondary 
standard in this review are summarized in section III.C. Lastly, the 
Administrator's final conclusions with regard to a secondary standard 
for CO are presented in section III.D.

A. Introduction

    In evaluating whether establishment of a secondary standard for CO 
is appropriate at this time, we adopted an approach in this review that 
builds upon the general approach used in the last review and reflects 
the broader body of evidence and information now available. 
Consideration of the evidence available in this review focuses on the 
following overarching question: Does the currently available scientific 
information provide support for considering the establishment of a 
secondary standard for CO?
    In considering this overarching question, the Policy Assessment 
first noted that the extensive literature search performed for the 
current review did not identify any evidence of public welfare effects 
of CO unrelated to climate at or near ambient levels (ISA, section 1.3 
and p. 1-3). However, ambient CO has been associated with welfare 
effects related to climate (ISA, section 3.3). Climate-related effects 
of CO were considered for the first time in the 2000 AQCD and are given 
somewhat greater focus in the current ISA relative to the 2000 AQCD in 
reflection of comments from CASAC and increased attention to the role 
of CO in climate forcing (Brain and Samet, 2009; ISA, section 3.3). 
Based on the current evidence, the ISA concludes that ``a causal 
relationship exists between current atmospheric concentrations of CO 
and effects on climate'' (ISA, section 2.2). Accordingly, the 
discussion in the Policy Assessment (summarized in the proposal) 
focuses on climate-related effects of CO in addressing the question 
posed above.
    The currently available information summarized in the ISA (ISA 
section, 3.3) does not alter the current well-established understanding 
of the role of urban and regional CO in continental and global-scale 
chemistry, as outlined in the 2000 AQCD (PA, section 3.2). CO absorbs 
outgoing thermal infrared radiation very weakly; thus, the direct 
contribution of CO itself to climate forcing (or greenhouse warming) is 
very small (ISA, p. 3-11). Rather, the most significant effects on 
climate are indirect, resulting from CO's role as the major atmospheric 
sink for hydroxyl radicals. Through this role of CO in global 
atmospheric chemistry, CO influences the abundance of chemically 
reactive, major greenhouse gases, such as methane and ozone, that 
contribute directly to the greenhouse effect and of other gases that 
exert their effect on climate through depletion of stratospheric ozone 
(ISA, section 3.3 and p. 3-11). There is significant uncertainty 
concerning this effect, and it appears to be highly variable, with the 
ISA recognizing that climate effects of changes to emissions of a 
short-lived pollutant such as CO are very likely dependent on localized 
conditions (ISA section 3.3, pp. 3-12, 3-15, 3-16). As noted in the 
ISA, however, ``the indirect [global warming potential] values 
evaluated and summarized by [the Intergovernmental Panel on Climate 
Change] are global and cannot reflect effects of localized emissions or 
emissions changes'' (ISA at p. 3-16). Accordingly, the Policy 
Assessment stated that, as a result of the spatial and temporal 
variation in emissions and concentrations of CO and the localized 
chemical interdependencies that cause the indirect climate effects of 
CO, it is highly problematic to evaluate the indirect effects of CO on 
climate (PA, p. 3-3).
    Based upon the information and considerations summarized above, the 
Policy Assessment concluded as an initial matter that, with respect to 
non-climate welfare effects, including ecological effects and impacts 
to vegetation, there is no currently available scientific information 
that supports a CO secondary standard (PA, section 3.4). Secondly, with 
respect to climate-related effects, the Policy Assessment recognized 
the evidence of climate forcing effects associated with CO, most 
predominantly through its participation in chemical reactions in the 
atmosphere which contribute to increased concentrations of other more 
direct acting climate-forcing pollutants (ISA, sections 2.2 and 3.3). 
The PA also noted, however, that the available information provides no 
basis for estimating how localized changes in the temporal and spatial 
patterns of ambient CO likely to occur across the U.S. with (or 
without) a secondary standard would affect local, regional, or 
nationwide changes in climate. Moreover, more than half of the indirect 
forcing effect of CO is attributable to ozone (O3) 
formation, and welfare-related effects of O3 are more 
appropriately considered in the context of the review of the 
O3 NAAQS, rather than in this CO NAAQS review (PA, section 
3.4). For these reasons, the Policy Assessment concluded that there is 
insufficient information at this time to support the consideration of a 
secondary standard based on CO effects on climate processes (PA, 
section 3.4).

B. Rationale for Proposed Decision

    In considering a secondary standard for CO, the proposed 
conclusions presented in the proposal were based on the assessment and 
integrative synthesis of the scientific evidence presented in the ISA, 
building on the evidence described in the 2000 AQCD, as well as staff 
consideration of this evidence in the Policy Assessment and CASAC 
advice. As an initial matter, the proposal concluded that the currently 
available scientific information with respect to non-climate welfare 
effects, including ecological effects and impacts to vegetation, does 
not support a CO secondary standard. Secondly, with respect to climate-
related effects, the proposal took note of staff considerations in the 
Policy Assessment and concurred with staff conclusions that information 
is insufficient at this time to provide support for a CO secondary 
standard. Thus, based on consideration of the evidence, staff 
considerations in the Policy Assessment, as well as the views of CASAC, 
the Administrator proposed to conclude that no secondary standards 
should be set at this time because, as in the past reviews, having no 
standard is requisite to protect public welfare from any known or 
anticipated adverse effects from ambient CO exposures.

C. Comments on Consideration of Secondary Standard

    In considering the need for a secondary standard, the Administrator 
first notes the advice and recommendations from CASAC based on their 
review of two drafts of the Integrated Science Assessment and of the 
draft Policy Assessment. With regard to consideration of a secondary 
standard for CO, CASAC noted without objection or disagreement the 
staff's conclusions that there is insufficient information to support 
consideration of a secondary standard at this time (Brain and Samet, 
2010c). One public comment generally concerning EPA's proposed decision 
on a secondary standard is addressed below. Other more specific public 
comments related to consideration of a secondary standard are addressed 
in the Response to Comments document.
    One comment (joint submission from Center for Biological Diversity 
and others) stated that due to the global influence of CO on climate, 
EPA must

[[Page 54310]]

establish a secondary NAAQS. The comment provided no information as to 
what form, level, or other elements of a secondary standard would be 
appropriate in light of the substantial uncertainties and regional 
variation in the indirect effects of CO. Rather, the comment asserted 
that there is ``a substantial body of knowledge, as reviewed in the 
ISA, regarding CO and climate'' and that ``uncertainty does not absolve 
the EPA of the obligation to protect public welfare'' (Center for 
Biological Diversity comments at p. 9).
    As noted by the commenter, the ISA reviewed the body of knowledge 
regarding CO and climate. As discussed above, the ISA concluded that CO 
has climate-related effects, that the direct effects of CO are weak, 
that there are significant uncertainties concerning the indirect 
climate effects of CO, and that these effects appear to be highly 
variable and dependent on localized conditions. Further, as noted in 
the Policy Assessment, the spatial and temporal variation in emissions 
and concentrations of CO and the localized chemical interdependencies 
that cause the indirect climate effects of CO make it highly 
problematic to evaluate the indirect effects of CO on climate. In light 
of the fact that the climate effects of CO are not only uncertain but 
highly variable and dependent on local conditions (e.g., concentrations 
of other pollutants), EPA believes that there is not adequate 
information available to conclude that a secondary standard in the 
United States is requisite to protect public welfare. The comment 
points to the estimated global effects of CO on climate, but nowhere 
does the comment provide evidence that EPA's conclusion regarding 
adequacy of the available information is in error.
    EPA fully appreciates that the NAAQS are often established on the 
frontiers of scientific knowledge, and EPA continually assesses 
scientific uncertainties in judging what NAAQS are requisite to protect 
public health and welfare. EPA is not asserting that the fact that 
there are some uncertainties prevents EPA from setting a standard. 
Rather, EPA has judged that, in light of both the significant 
uncertainties and the evidence of the direct effects being weak and the 
indirect effects being highly variable and dependent on local 
conditions, particularly in light of CO's short lifetime, it is not 
possible to anticipate how any secondary standard that would limit 
ambient CO concentrations in the United States would in turn affect 
climate and thus any associated welfare effects. As additionally 
discussed in section III.D below, EPA has reviewed the available 
information and judged the absence of a standard as being requisite to 
protect public welfare.

D. Conclusions Concerning a Secondary Standard

    The conclusions presented here are based on the assessment and 
integrative synthesis of the scientific evidence presented in the ISA, 
building on the evidence described in the 2000 AQCD, as well as staff 
consideration of this evidence in the Policy Assessment and CASAC 
advice, and with consideration of the views of public commenters on the 
need for a secondary standard.
    In considering whether the currently available scientific 
information supports setting a secondary standard for CO, EPA takes 
note of the ISA and Policy Assessment consideration of the body of 
available evidence (briefly summarized above in section III.A). First, 
EPA concludes that the currently available scientific information with 
respect to non-climate welfare effects, including ecological effects 
and impacts to vegetation, does not support the need for a CO secondary 
standard. Secondly, with respect to climate-related effects, the EPA 
takes note of the ISA's conclusions that there are significant 
uncertainties concerning the indirect climate effects of CO, and that 
these effects appear to be highly variable and dependent on localized 
conditions as well as staff considerations in the Policy Assessment and 
concurs with staff conclusions that information is insufficient at this 
time to support the need for a CO secondary standard. More 
specifically, as more fully discussed in consideration of public 
comments in section III.C above, EPA has judged that, in light of both 
the significant uncertainties and the evidence of the direct effects of 
CO on climate being weak and the indirect effects being highly variable 
and dependent on local conditions, particularly in light of CO's short 
lifetime, it is not possible to anticipate how any secondary standard 
that would limit ambient CO concentrations in the United States would 
affect climate. Consequently, information that might indicate the need 
for additional protection from CO environmental effects and on which 
basis EPA might identify a secondary standard for the purposes of 
protecting against CO effects on climate processes is not available.
    Thus, in considering the evidence, staff considerations in the 
Policy Assessment summarized here, as well as the views of CASAC and 
the public, summarized above, the Administrator concludes that no 
secondary standards should be set at this time because, as in the past 
reviews, having no standard is requisite to protect public welfare from 
any known or anticipated adverse effects from ambient CO exposures.

 IV. Amendments to Ambient Monitoring Requirements

    The EPA is finalizing changes to ambient air CO monitoring methods 
and the ambient monitoring network design requirements to support the 
NAAQS for CO discussed above in Section II. Because ambient CO 
monitoring data are essential to the implementation of the NAAQS for 
CO, EPA is finalizing minimum monitoring requirements for the ambient 
CO monitoring network. State, local, and Tribal monitoring agencies 
(``monitoring agencies'') collect ambient CO monitoring data in 
accordance with the monitoring requirements contained in 40 CFR parts 
50, 53, and 58.

A. Monitoring Methods

    This section provides background and rationale for the amendments 
that EPA proposed to the Federal Reference Method (FRM) for CO and to 
the associated performance specifications for automated CO analyzers. 
It also discusses the public comments on those proposed amendments and 
the few minor changes made to them as they are being promulgated today.
    The use of FRMs for the collection of air monitoring data provides 
uniform, reproducible measurements of pollutant concentrations in 
ambient air. Federal equivalent methods (FEMs) allow for the 
introduction of new or alternative technologies for the same purpose, 
provided these methods produce measurements directly comparable to the 
reference methods. EPA has established procedures for determining and 
designating FRMs and FEMs at 40 CFR part 53.
    For ambient air monitoring data for CO to be used for determining 
compliance with the CO NAAQS, such data must be obtained using either 
an FRM or an FEM, as defined in 40 CFR parts 50 and 53. All CO 
monitoring methods in use currently by state and local monitoring 
agencies are EPA-designated FRM analyzers. No FEM analyzer, i.e. one 
using an alternative measurement principle, has yet been designated by 
EPA for CO. These continuous FRM analyzers have been used in monitoring 
networks for many years and provide CO monitoring data adequate for 
determining CO NAAQS compliance. The current list of all approved FRMs 
capable of providing ambient CO data for this purpose may be found on 
the EPA Web site, http://www.epa.gov/ttn/amtic/files/ambient/

[[Page 54311]]

criteria/reference-equivalent-methods-list.pdf. Although both the 
existing CO FRM in 40 CFR part 50 and the FRM and FEM designation 
requirements in part 53 remain adequate to support the CO NAAQS, EPA 
nevertheless proposed editorial revisions to the CO FRM and both 
technical and editorial revisions to part 53, as discussed below.
1. Proposed Changes to Parts 50 and 53
    Reference methods for criteria pollutants are described in several 
appendices to 40 CFR part 50; the CO FRM is set forth in appendix C. A 
non-dispersive infrared photometry (NDIR) measurement principle is 
formally prescribed as the basis for the CO FRM. Appendix C describes 
the technical nature of the NDIR measurement principle stipulated for 
CO FRM analyzers as well as two acceptable calibration procedures for 
CO FRM analyzers. It further requires that an FRM analyzer must meet 
specific performance, performance testing, and other requirements set 
forth in 40 CFR part 53.
    The CO FRM was first promulgated on April 30, 1971 (36 FR 8186), in 
conjunction with EPA's establishment (originally as 42 CFR part 410) of 
the first NAAQS for six pollutants (including CO) as now set forth in 
40 CFR part 50. The method was amended in 1982 and 1983 (47 FR 54922; 
48 FR 17355) to incorporate minor updates, but no substantive changes 
in the fundamental NDIR measurement technique have been made since its 
original promulgation.
    In connection with the current review of the NAAQS for CO, EPA 
reviewed the existing CO FRM to determine if it was still adequate or 
if improved or more suitable measurement technology has become 
available to better meet current FRM needs as well as potential future 
FRM requirements. EPA determined that no new ambient CO measurement 
technique has become available that is superior to the NDIR technique 
specified for the current FRM, and that the existing FRM continues to 
be well suited for both FRM purposes and for use in routine CO 
monitoring. No substantive changes were needed to the basic NDIR FRM 
measurement principle. Several high quality FRM analyzer models have 
been available for many years and continue to be offered and supported 
by multiple analyzer manufacturers.
    However, EPA found that the existing CO FRM should be improved and 
updated to clarify the language of some provisions, to make the format 
match more closely the format of more recently promulgated automated 
FRMs, and to better reflect the design and improved performance of 
current, commercially available CO FRM analyzers. Accordingly, EPA 
proposed appropriate, albeit minor, changes to the FRM. Because these 
mostly editorial changes were quite numerous, the entire text of the CO 
FRM was revised and re-proposed.
    In close association with the proposed editorial revision to the CO 
FRM described above, EPA also proposed to update the performance 
requirements for CO FRM analyzers that are contained in 40 CFR part 53. 
These requirements were established in the 1970's, based primarily on 
the NDIR CO measurement technology available at that time. While the 
fundamental NDIR measurement principle, as implemented in commercial 
FRM analyzers, has changed little over several decades, FRM analyzer 
performance has improved markedly. Contemporary advances in digital 
electronics, sensor technology, and manufacturing capabilities have 
permitted today's NDIR analyzers to exhibit substantially improved 
measurement performance, reliability, and operational convenience at 
modest cost. This improved instrument performance was not reflected in 
the previous performance requirements for CO FRM analyzers specified in 
40 CFR part 53, indicating a need for an update to reflect that 
improved performance.
    The updated performance requirements that EPA proposed for CO 
analyzers make them more consistent with the typical performance 
capability available in contemporary FRM analyzers and will ensure that 
newly designated FRM analyzers will have this improved measurement 
performance. A review of analyzer manufacturers' specifications has 
determined that all existing CO analyzer models currently in use in the 
monitoring network already meet the proposed new requirements (for the 
standard measurement range). Also in conjunction with this 
modernization of the analyzer performance requirements, EPA proposed 
new, more stringent performance requirements applicable, on an optional 
basis, to analyzers that feature one or more lower, more sensitive 
measurement ranges. Such lower ranges will support improved monitoring 
data quality in areas of low CO concentrations.
    These updated and new performance requirements are being 
promulgated as amendments to subpart B of 40 CFR part 53, which 
prescribes the explicit procedures to be used for testing specified 
performance aspects of candidate FRM and FEM analyzers, along with the 
minimum performance requirements that such analyzers must meet to 
qualify for FRM or FEM designation. In particular, the new performance 
requirements appear in table B-1 of subpart B of 40 CFR part 53. 
Although table B-1 covers candidate methods for sulfur dioxide 
(SO2), O3, CO, and NO2, the updates to 
table B-1 that EPA is promulgating today affect only candidate methods 
for CO.
    The updated performance requirements apply to candidate CO 
analyzers that operate on the specified ``standard'' measurement range 
(0 to 50 ppm). This measurement range remains unchanged from the 
existing requirements as it appropriately addresses the monitoring data 
needed for assessing attainment. The measurement noise limit is reduced 
from 0.5 to 0.2 ppm, and the lower detectable limit is reduced from 1 
to 0.4 ppm. Limits for zero drift and span drift are lowered, 
respectively, from 1.0 to 0.5 ppm, and from 2.5% to 2.0%. The 
previously existing mid-span drift limit requirement, tested at 20% of 
the upper range limit (URL), is withdrawn, as EPA has found that the 
mid-span drift requirement was unnecessary for CO instruments because 
the upper level span drift (tested at 80% of the URL) completely and 
more accurately measures analyzer span drift performance.
    The lag time limit is reduced from 10 to 2 minutes, and the rise 
and fall time limits are lowered from 5 to 2 minutes. For precision, 
EPA is changing the form of the precision limit specifications from an 
absolute measure (ppm) to percent (of the URL) for CO analyzers and 
setting the precision limit at 1 percent tested at both 20% and 80% of 
the URL. One percent is equivalent to the previous limit value of 0.5 
ppm for precision for the standard (0 to 50 ppm) measurement range. 
This change in units from ppm to percent makes the requirement 
responsive to higher and lower measurement ranges (i.e., more demanding 
for lower ranges).
    The interference equivalent limit of 1 ppm for each interferent is 
not changed, but EPA is withdrawing the previously existing limit 
requirement for the total of all interferents. EPA has found that the 
total interferent limit is unnecessary because modern CO analyzers are 
subject to only a few interferences, and they tend to be well 
controlled.
    The new performance requirements apply only to newly designated CO 
FRM or FEM analyzers; however, essentially all existing FRM analyzers 
in use today, as noted previously, already meet these requirements, so 
existing FRM analyzers are not required to be re-tested and re-

[[Page 54312]]

designated under the new requirements. All currently designated FRM 
analyzers retain their original FRM designations.
    EPA also recognized that some CO monitoring objectives (e.g., area-
wide monitoring away from major roads and rural area surveillance) 
require analyzers with lower, more sensitive measurement ranges than 
the standard range used for typical ambient monitoring. To improve data 
quality for such lower-range measurements, EPA is adding a separate set 
of performance requirements that apply specifically to lower ranges 
(i.e., those having a URL of less than 50 ppm) for CO analyzers. These 
additional, lower-range requirements are listed in the revised table B-
1. A candidate analyzer that meets the table B-1 requirements for the 
standard measurement range (0 to 50 ppm) can optionally have one or 
more lower ranges included in its FRM or FEM designation by further 
testing to show that it also meets these supplemental, lower-range 
requirements.
    Although no substantive changes were determined to be needed to the 
test procedures and associated provisions of subpart B for CO, the 
detailed language in many of the subpart B sections was in need of 
significant updates, clarifications, refinement, and (in a few cases) 
correction of minor typographical errors. These changes to the subpart 
B text (apart from the changes proposed for table B-1 discussed above) 
are very minor and almost entirely editorial in nature, but quite 
numerous. Therefore, EPA has revised and is re-promulgating the entire 
text of subpart B text.
    As discussed previously, table B-1, which sets forth the pollutant-
specific performance limits, is being amended only as applicable to CO 
analyzers. EPA amended table B-1 as applicable to SO2 
methods on June 22, 2010 and intends to amend table B-1 for 
O3 and NO2 later, if appropriate, when the 
associated NAAQS are reviewed.
2. Public Comments
    EPA notes first that CASAC stated that ``more sensitive and precise 
monitors need to be deployed to measure levels that are less than or 
equal to 1 ppm.'' (Brain and Samet 2010b). Comments from the public on 
the proposed revisions to CO monitoring methods are addressed in this 
section or in the Response to Comments document. Comments on the 
proposed changes to the CO monitoring methodology were received from 
only one member of the public, the American Petroleum Institute. The 
commenter was generally supportive of EPA's efforts to clarify and 
update the regulations for the CO FRM and the CO analyzer performance 
requirements. In regard to the CO FRM (40 CFR part 50, appendix C), the 
commenter questioned EPA's proposed relaxation in a flow rate control 
requirement in the dilution-method calibration procedure, from 1% to 
2%. However, EPA believes that the original 1% requirement is 
unnecessarily stringent, and that this change is appropriate and 
commensurate with the existing 2% flow rate measurement accuracy and 
with the overall calibration accuracy needed to obtain adequate data 
quality with the method.
    To further improve clarity of the FRM calibration section, the 
commenter also suggested a minor change to Equation 1 and the addition 
of language indicating that the measurement display or read-out device 
connected to the analyzer to monitor its reading during calibration 
should be the actual, or at least closely representative of the actual, 
data recording system used during field operation of the analyzer. EPA 
has accepted both of these suggestions, and appropriate changes have 
been incorporated into the changes being made to the CO FRM in this 
action.
    Another comment questioned the proposed withdrawal of the previous 
total interference limit requirement. In response to this comment, EPA 
re-evaluated the efficacy of this limit for CO analyzers and again 
determined that the limit was not necessary, because the number of 
individual interferences to which FRM (and most potential FEM) CO 
analyzers are subject is small (only 2 for FRMs), as listed in table B-
3 of 40 CFR part 53. Also, response to these interferents is typically 
well controlled in modern CO analyzers. In addition, the new, 
individual interference limit for the lower measurement ranges is one 
half the limit for the standard range, which further mitigates any need 
for a separate, total interference limit.
    The commenter questioned EPA's proposed withdrawal of the 
previously existing limit requirement for span drift measured at 20% of 
the upper range limit (URL), contending that this limit was important 
because it is closer in concentration to the existing NAAQS than the 
span drift measured at 80% of the URL. However, the purpose of the span 
drift limit is not to directly assess measurement error at a 
particular, mid-scale concentration level. That purpose is served by 
the 1-point quality control check for CO monitors described in section 
3.2.1 of appendix A of 40 CFR part 58. Rather, for the purpose of 
analyzer performance testing, the linear input/output functional 
characteristic of the analyzer is best described by its zero point and 
its slope, because these parameters are generally subject to change 
(drift) independently. Thus, zero drift (change in the zero point) and 
span drift (change in the slope) are tested separately. Zero drift is, 
of course, measured at zero concentration, and span drift is most 
accurately measured at a concentration near the URL. The span drift 
test at 80% URL (when the zero drift is within the specified 
requirement) more accurately determines any change in the slope 
parameter then a test at 20% URL. The previously specified test at 20% 
URL thus serves little, if any, purpose in regard to determining change 
in the slope. Therefore, EPA has concluded that this requirement can be 
withdrawn.
    Finally, the commenter was concerned that existing FRM analyzers 
approved under the previously existing performance requirements may 
provide data quality inferior to that of analyzers approved under the 
proposed new requirements and that older analyzers may be unacceptable 
for some applications that demanded higher performance or higher data 
quality. A ``tiered'' approach was suggested to handle this situation.
    In proposing more stringent performance requirements for approval 
of new FRM and FEM analyzers, EPA noted that the performance of 
analyzers approved under the existing performance requirements was 
fully adequate for most routine compliance monitoring applications, and 
that the proposed new requirements were largely to bring the base FRM 
and FEM performance requirements up to date and more commensurate with 
the performance of modern commercially available CO analyzers. EPA 
further noted that all currently designated FRM analyzers already meet 
the proposed new requirements. This means that the quality of routine 
CO monitoring data currently being obtained is already of the higher 
level portended by the proposed new performance requirements.
    In the proposal, however, EPA did recognize that some special CO 
monitoring applications do require a higher level of performance than 
that required for routine applications. Therefore, EPA is promulgating 
optional, more stringent performance requirements for analyzers having 
a more sensitive, ``lower range'' available for such applications. This 
is, in fact, a ``tiered'' approach. Applicants would be able to elect 
to have such lower ranges approved as part of their FRM or FEM 
designation. These new, special performance requirements will alert 
monitoring agencies that they should

[[Page 54313]]

consider low-range performance of an analyzer for those applications 
that may require better low-level performance, and they can select an 
analyzer that has such a lower range approved under its FRM (or FEM) 
designation.
3. Decisions on Methods
    As discussed above, a few relatively minor changes have been 
incorporated into the proposed revised CO FRM in appendix C of part 50, 
in response to public comments received by EPA. With these changes, the 
revised appendix C is being promulgated as otherwise proposed. Only one 
change has been made to the revision proposed for subpart B of part 53, 
to fix a typographical error that appeared in proposed table B-1 
concerning reversed entries for the span drift limits for the 20% and 
80% URL for the CO ``lower range'' column. Aside from this correction, 
the revised subpart B is being promulgated exactly as proposed.

B. Network Design

    This section on CO network design provides information on the 
proposed network design, the public comments received on the proposed 
network design, and the EPA's conclusions, including rationale and 
details, on the final changes to the CO network design requirements.
1. Proposed Changes
    The objective of an ambient monitoring network is to (1) provide 
air pollution data to the general public in a timely manner, (2) 
support compliance with ambient air quality standards and emissions 
strategy development, and (3) provide support for air pollution 
research (40 CFR part 58, appendix D). The proposed CO network design 
was intended to directly support the NAAQS by requiring monitoring that 
provides data for use in the designation process and ongoing assessment 
of air quality. In particular, the proposed network design was intended 
to require a sufficient number of monitors to collect data for 
compliance purposes in the near-road environment, where, as noted in 
section II.A.1 above, the highest ambient CO concentrations generally 
occur, particularly in urban areas (ISA, section 3.5.1.3; REA, section 
3.1.3).
    The EPA proposed CO monitors to be required within a subset of 
near-road NO2 monitoring stations, which are required in 40 
CFR part 58, appendix D, section 4.3. Per the preamble to the final 
rule for the NO2 NAAQS promulgated on February 9th, 2010 (75 
FR 6474), near-road NO2 monitoring stations are intended to 
be placed in the near-road environment at locations of expected maximum 
1-hour NO2 concentrations and are triggered for metropolitan 
areas based on Core Based Statistical Area (CBSA) population thresholds 
and a traffic-related threshold based on annual average daily traffic 
(AADT).\29\ The EPA proposed that CO monitors be required to operate in 
any CBSA having a population of 1 million or more persons, collocated 
with required near-road NO2 monitoring stations. Based upon 
2009 Census Bureau estimates and 2008 traffic statistics maintained by 
the US Department of Transportation (US DOT) Federal Highways 
Administration (FHWA), the CO monitoring proposal was estimated to 
require approximately 77 CO monitors to be collocated with near-road 
NO2 monitors within 53 CBSAs.\30\
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    \29\ One near-road NO2 monitor is required in any 
CBSA having a population of 500,000 or more persons. Two near-road 
NO2 monitors are required in any CBSA having a population 
of 2.5 million or more persons, or in any CBSA that has one or more 
road segments with an AADT count of 250,000 or more (40 CFR part 58, 
appendix D, section 4.3).
    \30\ Since the proposal, EPA has estimated that using 2010 
Census Bureau counts the proposed rule would have resulted in 
approximately 75 monitors in 52 CBSAs being required.
---------------------------------------------------------------------------

    The EPA proposed that any required near-road CO monitors shall be 
reflected in State annual monitoring network plans due in July 2012. 
Further, the Agency proposed that required near-road CO monitors be 
operational by January 1, 2013. Due to the proposed collocation of 
required CO monitors with required near-road NO2 monitors, 
these implementation dates were proposed in order to match those of the 
forthcoming near-road NO2 monitoring network.
    In light of the proposal to require near-road CO monitors be 
collocated with required near-road NO2 monitors, the EPA 
proposed that siting criteria for microscale CO monitors be revised to 
match those of microscale near-road NO2 monitors (and also 
microscale PM2.5 monitors). In particular, the EPA proposed 
that microscale CO siting criteria for probe height and horizontal 
spacing be changed to match those of near-road NO2 monitors 
as prescribed in 40 CFR part 58 appendix E, sections 2, 4(d), 6.4(a), 
and table E-4. Specifically, EPA proposed the following: (1) To allow 
microscale CO monitor inlet probes to be between 2 and 7 meters above 
the ground; (2) that microscale near-road CO monitor inlet probes be 
placed so they have an unobstructed air flow, where no obstacles exist 
at or above the height of the monitor probe, between the monitor probe 
and the outside nearest edge of the traffic lanes of the target road 
segment; and (3) that required near-road CO monitor inlet probes shall 
be as near as practicable to the outside nearest edge of the traffic 
lanes of the target road segment, but shall not be located at a 
distance greater than 50 meters in the horizontal from the outside 
nearest edge of the traffic lanes of the target road segment.
    Finally, the EPA recognized that a single monitoring network design 
may not always be sufficient for fulfilling specific or otherwise 
unique data needs or monitoring objectives for every area across the 
nation. As such, the EPA proposed to provide the Regional 
Administrators with the discretion to require monitoring above the 
minimum requirements as necessary to address situations where minimum 
monitoring requirements are not sufficient to meet monitoring 
objectives.
2. Public Comments
    EPA first notes that CASAC expressed concern over the current 
monitoring network, stating ``[m]ore extensive coverage may be 
warranted for areas where concentrations may be more elevated, such as 
near roadway locations. The Panel found that in some instances current 
networks underestimated carbon monoxide levels near roadways.'' (Brain 
and Samet 2010b). General comments from the public based on relevant 
factors that either support or oppose the proposed changes to the CO 
network design are addressed in this section. Specific public comments 
related to the network design, but with regard to material which was 
not specifically proposed by the EPA or posed for solicitation of 
comment, are addressed in the Response to Comments document.
a. Near-Road Monitoring and Collocation With Near-Road Nitrogen Dioxide 
Monitors
    The EPA received multiple public comments on the overall merit of 
monitoring for CO in the near-road environment, the proposal that 
required CO monitors be collocated with required near-road 
NO2 monitors, and the number of required CO monitors that 
might be appropriate. In general, public health and environmental 
groups (e.g., American Lung Association [ALA], American Thoracic 
Society [ATS], Environmental Defense Fund [EDF]), some states or state 
environmental agencies or organizations (e.g. National Association of 
Clean Air Agencies [NACAA], Northeast States for Coordinated Air Use 
Management [NESCAUM], New York State Department of Environment 
Conservation [NYSDEC], and State of Wisconsin Department of Natural

[[Page 54314]]

Resources [WIDNR]), and some private citizen commenters provided 
support for a requirement for CO monitors in the near-road environment. 
For example, ALA, ATS, and EDF state that they ``* * * are pleased to 
see EPA take seriously the public health threats that are posed to 
millions of residents and other sensitive receptors who live near or 
work on or near highways as well as other high exposure areas.'' They 
go on to note that ``[near-road ambient monitoring] data have been 
sorely lacking from the national monitoring network and are long 
overdue.'' Further, many of the commenters who were supportive of near-
road monitoring were supportive of collocating CO monitors with near-
road NO2 monitors as it establishes multipollutant 
monitoring within the ambient air monitoring network. For example, 
NACAA stated the following in their comments: ``* * * NACAA supports 
EPA's proposal to collocate CO near roadway monitors at a subset of 
NO2 near-roadway sites. This is consistent with the 
recommendations of EPA's Clean Air Scientific Advisory Committee 
(CASAC), which urged the agency to develop the near roadway monitoring 
network with a multipollutant focus and included CO in its list of 
pollutants that should be measured.''
    Some industry commenters (e.g., Association of Automobile 
Manufacturers [AAM] and American Electric Power Service Corporation 
[AEPSC]) and a number of other states or state groups (e.g., Indiana 
Department of Environmental Management [IDEM], North Carolina 
Department of Air Quality [NCDAQ], New Mexico Air Quality Bureau 
[NMAQB], South Carolina Department of Health and Environmental Control 
[SCDHEC], Southeast Michigan Council of Governments [SEMCOG], and Texas 
Commission on Environmental Quality [TCEQ]) generally did not support 
the proposed near-road CO monitoring requirements. For example, IDEM 
stated that ``CO measured by roadside monitors is not representative of 
ambient air quality everywhere in a city or county containing the 
roadway'' and that ``* * * roadside monitoring measurements represent 
source-specific data. Therefore, Indiana does not believe that roadside 
monitoring should apply to an ambient air quality standard.'' SCDHEC 
stated it ``* * * does not believe that the use of a near-road 
monitoring network in a state-wide ambient air monitoring network is 
the appropriate choice to protect our community's public health'' and 
that ``this monitoring method biases the monitoring effort into areas 
of little or no population while monitoring for the community 
population exposure is neglected.'' Similarly, industry commenter AAM 
stated that ``the current proposal does not include a requirement that 
the near-roadway monitors be sited in locations where there is actual 
human exposure to the ambient air for time periods corresponding to the 
1-hour or 8-hour CO NAAQS.''
    The EPA stated in the CO proposal (76 FR 8158) that the proposed 
near-road CO monitoring requirements were intended to ensure a network 
of adequate size and focus to provide data for comparison to the NAAQS, 
support health studies and model verification, and to fulfill Agency 
multipollutant monitoring objectives. In response to the comment that 
near-road monitoring data would be ``source-specific'' and may not be 
appropriately applicable to an ambient air standard, the Agency notes 
that monitoring for CO in the near-road environment (as a mobile source 
oriented measurement) is a longstanding agency practice, as evidenced 
by the first monitoring rule promulgated in 1979 (44 FR 27558, May 10, 
1979). That 1979 monitoring rule included the requirement to monitor 
for ``peak'' CO concentrations in urban areas having populations of 
500,000 people or more in locations ``* * * around major traffic 
arteries and near heavily traveled streets in downtown areas.'' The 
Agency believes that the use of near-road CO monitors as proposed is 
not a departure from the Agency's longstanding intent to measure peak 
concentrations of CO in the near-road environment. Rather, the proposal 
was consistent with the Agency's approach to require monitors for CO, 
and other criteria pollutants, in locations that likely experience peak 
ambient concentrations. The Agency also notes that source-oriented 
monitoring is and has long been a common practice in ambient monitoring 
networks, although more often associated with stationary sources, where 
the ambient data collected are used for comparison to the NAAQS. Data 
on ambient air concentrations, including near-road data, which may be 
most appropriately classified as on-road mobile source oriented, are 
appropriate to compare to the NAAQS.
    With regard to the comments asserting that near-road monitoring 
would result in monitoring areas of ``little or no population'' and 
thus population exposure is not represented, the EPA notes that on-road 
mobile sources are ubiquitous in urban areas and are a dominant 
component of the national CO emissions inventory, at nearly 60% of the 
total inventory, based on the 2008 NEI. As such, microenvironments 
influenced by on-road mobile sources are important contributors to 
ambient CO exposures, particularly in urban areas (REA, section 2.7). 
Further, the ambient CO exposures of most concern are short-term. 
Accordingly, near-road monitoring is focused on characterizing peak or 
elevated ambient concentrations. The relevance of this focus for the 
purposes of both ensuring compliance with the NAAQS and gathering data 
to inform our consideration of ambient CO exposures is demonstrated by 
the ubiquity of on-road mobile sources throughout urban areas, the time 
spent by people on or near roadways and the large number of American 
citizens living in urban areas and near roadways. As was noted in the 
ISA, the 2007 American Housing Survey (http://www.census.gov/hhes/www/housing/ahs/ahs07/ahs07.html) estimates that 17.9 million housing units 
are within 300 feet (~91 meters) of a 4-lane highway, airport, or 
railroad. Using the same survey, and considering that the average 
number of residential occupants in a housing unit is approximately 
2.25, an estimate can be made that at least 40 million American 
citizens live near 4-lane highways, airports, or railroads. Among these 
three transportation facilities, roads are the most pervasive of the 
three, suggesting that a significant number of people may live near 
major roads. Furthermore, the 2008 American Time Use Survey (http://www.bls.gov/tus/) reported that the average U.S. civilian spent over 70 
minutes traveling per day. Based on these considerations, the Agency 
has concluded that monitoring in the near-road environment would 
characterize the ambient concentrations that contribute to ambient CO 
exposure for a significant portion of the population that would 
otherwise not be captured.
    The AAM also commented that the EPA ``* * * proposal to locate more 
near roadway monitors appears to be an attempt to find problems where 
none are likely to exist.'' The Agency proposal for near-road monitors 
is in line with longstanding monitoring objectives to monitor for peak 
or elevated ambient pollutant concentrations where they may occur. The 
Agency agrees that CO is no longer as pervasive a problem as it was in 
the past; however, there is still a responsibility to appropriately 
characterize and assess ambient concentrations to ensure that they do 
not exceed the NAAQS. In comments on the first draft of the ISA, CASAC 
advised that ``* * * relying only on

[[Page 54315]]

EPA's [current] fixed monitoring network, CO measurements may 
underestimate CO exposures for specific vulnerable populations such as 
individuals residing near heavily trafficked roads and who commute to 
work on a daily basis.'' In comments on the second draft of the ISA, 
CASAC commented that ``the panel expresses concern about the existing 
CO monitoring network, both for its [spatial] coverage and for its 
utility in estimating human exposure'' and that ``CO exposures may not 
be adequately characterized for populations that may be exposed to 
higher CO levels because of where they live and work,'' such as the 
near-road environment. Finally, in comments on the second draft of the 
REA, CASAC stated that ``the approach for siting monitors needs greater 
consideration. More extensive coverage may be warranted for areas where 
concentrations may be more elevated, such as near-roadway locations.'' 
In light of these comments and upon a review of the existing CO 
network, the Administrator concluded that the current CO monitoring 
network (circa 2010) lacked a necessary focus. While some currently 
existing sites that were established in the 1970s and 1980s continue to 
monitor near-road locations in downtown areas or within urban street 
canyons, and a minimum number of area-wide monitors are currently 
required at National Core (NCore) multipollutant stations, few monitors 
exist that characterize the more heavily trafficked roads that are 
prevalent in the modern roadway network, particularly in our larger 
urban areas. The Agency's proposal was intended to require a modest but 
appropriate number of CO monitors to characterize the near-road 
environment where peak or elevated ambient CO concentrations are 
expected to occur near heavily trafficked roads, as compared with 
neighborhood or urban background concentrations. If CO levels turn out 
to be low in these near-road locations, so much the better for public 
health, and monitoring networks can be adjusted in the future, as they 
have over time in response to an increased understanding of where 
levels of concern to public health are likely to occur.
    Although the EPA received a number of comments that were largely 
supportive for the proposed requirement of collocating CO monitors 
within the forthcoming near-road NO2 monitoring stations, 
several commenters encouraged the Agency to provide flexibility to 
allow for the separation of the newly required CO monitors from the 
near-road NO2 sites, if necessary, to better monitor peak 
near-road CO concentrations. In their comments supporting the 
collocation concept, NACAA also stated that their organization ``* * * 
also encourages EPA to allow flexibility for state and local agencies 
to use alternative siting of near-roadway CO monitors on a case-by-case 
basis, where there is a scientific justification for siting the CO 
monitor in a different location from the NO2 monitor, to 
ensure the best possible measurement of near roadway CO 
concentrations.'' Similarly, NCDAQ recognized that ``* * * light duty 
vehicles tend to have more impact on CO concentrations than do heavy 
[duty] vehicles'' and went on to surmise that ``* * * not all near-road 
NO2 monitoring stations will be well situated to measure 
maximum CO concentrations.''
    The Agency has expressed its intent to pursue the integration of 
monitoring networks and programs through the encouragement of 
multipollutant monitoring wherever possible, as evidenced by actions 
taken in the 2006 monitoring rule that created the NCore network, the 
expression of the multipollutant paradigm in the 2008 Ambient Air 
Monitoring Strategy for State, Local, and Tribal Air Agencies, and 
within this rulemaking process as part of the rationale in proposing 
the collocation of required near-road CO monitors with near-road 
NO2 monitors. Multipollutant monitoring is viewed as a means 
to broaden the understanding of air quality conditions and pollutant 
interactions, furthering the capability to evaluate air quality models, 
develop emission control strategies, and support research, including 
health studies. However, the Agency also recognizes that the 
measurement objectives of individual pollutants may not always 
correspond in a way that would support multipollutant monitoring as the 
most appropriate option in a network design. On the issue raised by 
NACAA and NCDAQ concerning the potential difference in locations of 
peak CO and NO2 concentrations in the near-road environment, 
the EPA recognizes the primary influence to be the different emission 
characteristics between light duty (LD) and heavy duty (HD) vehicles 
and vehicle operating conditions, which were discussed in section 
III.B.2 of the CO proposal. The public comments suggesting the need for 
flexibility in siting near-road CO monitors derives from the fact that 
near-road NO2 sites will be sited at locations where peak 
NO2 are expected to occur. Since NO2 is more 
heavily influenced by HD vehicles and CO is more heavily influenced by 
LD vehicles on a per vehicle basis, respectively, there may be cases 
where the peak CO and NO2 concentrations could occur along 
different road segments within the same CBSA. As a general observation, 
the EPA believes that this situation may have more likelihood of 
occurring in the relatively larger (by population) CBSAs where a higher 
number of heavily trafficked roads with a wider variety of fleet mix 
(e.g. HD to LD vehicle ratios) tend to exist versus relatively smaller 
CBSAs. In recognition of these considerations, the final regulation 
allows for flexibility in CO monitor placement in the near-road 
environment when justified, as discussed below in section IV.B.3.
b. Population Thresholds for Requiring Near-Road Carbon Monoxide 
Monitors
    The EPA proposed that required CO monitors be collocated with every 
required near-road NO2 monitor in a CBSA with a population 
of 1 million or more persons. Due to the requirement to locate one CO 
monitor at each required near-road NO2 site, the proposal 
would have required two monitors in each CBSA having 2.5 million or 
more persons or having one or more road segments with Annual Average 
Daily Traffic (AADT) counts of 250,000 or more. The proposal would have 
also required one monitor within those CBSAs having 1 million or more 
persons (but fewer than 2.5 million persons).\31\ Based upon 2009 
Census Bureau estimates and US DOT maintained traffic summary data, the 
proposal was estimated to require 77 monitors within 53 CBSAs. Using 
recent 2010 Census data, and US DOT maintained traffic summary data, 
the proposal would have required approximately 75 monitors within 52 
CBSAs.
---------------------------------------------------------------------------

    \31\ One near-road NO2 monitor is required in any 
CBSA having a population of 500,000 or more persons. Two near-road 
NO2 monitors are required in CBSAs with population of 
greater than 2.5 million, or in any CBSA with a population of 
500,000 or more persons that has one or more roadway segments with 
annual average daily traffic (AADT) counts of 250,000 or more. (40 
CFR part 58, Appendix D, Section 4.3).
---------------------------------------------------------------------------

    The EPA received a number of comments supporting different 
population thresholds by which to require near-road CO monitors. Those 
state agencies or state agency groups who generally supported required 
CO monitoring in the near-road environment (e.g., NACAA, NESCAUM, 
NYSDEC, and WIDNR) suggested a population threshold of 2.5 million by 
which near-road CO monitors should be required. In addition, NCDAQ, who 
did not support near-road CO monitoring,

[[Page 54316]]

suggested that if it is required, it be required only within CBSAs of 
2.5 million or more. The use of a population threshold of 2.5 million 
persons, versus 1 million as proposed, would require approximately 42 
near-road CO monitors within 21 CBSAs, based on 2010 Census data. 
Industry commenter American Petroleum Institute (API) stated that the 
proposed population threshold of 1 million persons ``* * * appears 
appropriate, but EPA should not require that both [near-road 
NO2] sites in the largest CBSAs host CO monitors.'' API's 
suggestion would require approximately 52 near-road CO monitors within 
52 CBSAs. Finally, the public health and environmental groups ALA, ATS, 
and EDF suggested the EPA promulgate minimum monitoring requirements 
``* * * to encompass cities in smaller metro areas, including cities 
with populations of 500,000 or more, similar to the requirements for 
NO2 roadside monitoring.'' ALA, ATS, and EDF's suggestion 
would result in the requirement of approximately 126 monitors within 
103 CBSAs.
    As was noted in the proposal, the Agency believes that with the 
continuing decline of ambient CO levels, as summarized in the EPA's 
most recent trends report Our Nation's Air: Status and Trends Through 
2008 (http://www.epa.gov/airtrends/2010/), there is less likelihood for 
high CO concentrations in relatively smaller CBSAs (by population). 
Accordingly, the Agency proposed the requirement for what it believed 
would be a sufficient number of CO monitors, which would be collocated 
with required near-road NO2 monitors in CBSAs having 
populations of 1 million or more persons. The Administrator considered 
alternative population thresholds, including the 2.5 million and 
500,000 person thresholds, but concluded that those thresholds would 
require too few or too many monitors, respectively, in light of 
existing information on CO emissions data, ambient data, and the lack 
of data for locations near highly trafficked roads. The rationale for 
the proposed 1 million person threshold was to require a modest but 
sufficiently sized network that would effectively assess near-road CO 
concentrations for comparison to the NAAQS and could also provide data 
from within a multipollutant framework to support research (which 
includes health studies), facilitate model verification, and assess and 
evaluate emissions control strategies. However, after considering 
public comments, the EPA has concluded that one monitor in each CBSA of 
1 million or more persons will provide for monitoring of a wide range 
of diverse situations with regard to traffic volume, traffic patterns, 
roadway designs, terrain/topography, meteorology, climate, as well as 
surrounding land use and population characteristics. Accordingly, in 
the final rule EPA has modified the proposed requirements for CO 
monitors so that only one CO monitor is required in CBSAs of 1 million 
or more persons, as discussed in Section IV.B.3 below.
c. Implementation Schedule
    The EPA received a number of comments on the timeline for 
implementation of any required CO monitoring promulgated as part of 
this rulemaking. ALA, ATS, and EDF stated that they ``* * * support 
EPA's requirement that CO monitors be installed in near-highway 
locations by July 1, 2013.'' In light of the support these commenters 
expressed for rapid deployment of near road CO monitors, these 
commenters may have intended to support the proposed implementation 
date of January 1, 2013 instead of July 1, 2013 as quoted. The Agency 
received a number of comments from state agencies, state agency 
organizations, and industry encouraging the Agency to extend the time 
by which any required monitoring must be implemented. For example, API 
suggested that the proposed date by which required near-road CO 
monitors be established be extended to July 1, 2013, while NACAA and 
WIDNR suggested January 1, 2014. Several commenters suggested that 
required near-road monitors should be phased in over a period of time. 
For example, NACAA, stated ``[i]t may be necessary to develop a program 
for phasing in new monitoring sites and reevaluate network 
implementation.'' NACAA also pointed to comments from CASAC that it 
would be advisable to phase in near-road monitoring for NO2, 
because ``[t]he first round of sites could be used to gather 
information on appropriate siting in the near roadway environment, near 
roadway gradient, and spatial relationships.''
    The EPA recognizes that states are already implementing newly 
required monitoring related to lead and NO2, and that the 
current financial and logistical burdens may make the implementation of 
new monitoring requirements difficult. A number of state and industry 
commenters noted the need for funding to accommodate a new monitoring 
requirement, and some also noted the financial and logistical hardships 
that many states are currently experiencing (e.g., IDEM, NACAA, NCDAQ, 
SCDHEC, and WIDNR). The EPA recognizes the significance of the 
financial and logistical burden that new monitoring requirements pose 
and the impact of multiple new monitoring requirements stemming from 
other recent rulemakings. As such, the Agency has taken these comments 
into consideration in the final rule with regard to when required CO 
monitors are to be operational, as discussed in Section IV.B.3 below.
d. Siting Criteria
    The EPA received comments regarding the proposed revisions to 
microscale CO siting criteria. Those who commented (AAM, API, and 
NCDAQ) all supported having two sets of siting criteria that would 
apply to near-road CO monitors such as those that might be collocated 
with near-road NO2 monitors and to those CO monitors 
operating in downtown areas and urban street canyon locations, 
respectively. AAM stated that ``* * * there should be two separate 
criteria for siting microscale CO monitors. The earlier height and 
distance guidelines are still appropriate for downtown areas and 
arterial highways with sidewalks, but a separate set of guidelines 
should be established for limited access, heavily-travelled 
expressways.'' API commented that ``* * * the proposed CO [near-road] 
criteria are acceptable. EPA should create two-tiered siting criteria 
for microscale CO monitoring * * *'' and that ``there will be an 
ongoing need for CO monitoring in downtown, urban and/or street 
canyon[s] for health-related concerns as well as SIP-related issues.'' 
Finally, NCDAQ stated that ``* * * the US EPA should maintain separate 
siting criteria for the two types of micro-scale CO monitoring sites * 
* *'' noting that the current siting criteria intended for downtown 
areas and urban street canyon sites ``* * * are still valid for that 
purpose and CO monitoring stations being placed for this purpose should 
still be required to meet these siting criteria.''
    The EPA agrees with the commenters that the existing siting 
criteria are still appropriate for any existing or future downtown area 
or urban street canyon CO monitoring site, and that new siting criteria 
are appropriate for CO monitors being collocated with near road 
NO2 monitors. As such, the Agency is finalizing siting 
criteria for microscale CO sites that include criteria for both 
downtown area/urban street canyon microscale sites and other near-road 
microscale CO sites, as presented below in Section IV.B.3.
e. Area-Wide Monitoring
    The EPA received a number of comments from transportation groups,

[[Page 54317]]

public health and environmental groups, and an industry commenter 
(e.g., AAM, ALA/ATS/EDF, American Association of State Highway and 
Transportation Officials [AASHTO], New York State Department of 
Transportation [NYSDOT], Texas Department of Transportation [TXDOT], 
and Virginia Department of Transportation [VDOT]) regarding the fate of 
many of the CO monitors in the current network that characterize 
concentrations representative of neighborhood or larger spatial 
scales,\32\ known as area-wide monitors. For example, AASHTO commented 
that ``EPA appears to be proposing that CO monitoring sites to 
characterize area-wide CO concentration levels at the neighborhood and 
larger spatial scales is no longer required. AASHTO is concerned that 
this proposal will de-emphasize the need for neighborhood scale CO 
monitors.'' AASHTO and some state DOTs expressed that the data for 
neighborhood scale monitors are used for other purposes, such as 
National Environmental Policy Act (NEPA) and transportation conformity, 
and that they are concerned about the potential loss of these types of 
data in the future. In another example, ALA/ATS/EDF stated that they 
call upon EPA to ``establish a comprehensive roadside air pollution 
network, while retaining the current area-wide CO network.''
---------------------------------------------------------------------------

    \32\ Spatial scales are defined in 40 CFR Part 58 Appendix D, 
Section 1.2, where the scales of representativeness of most interest 
for the monitoring site types include:
    1. Microscale--Defines the concentration in air volumes 
associated with area dimensions ranging from several meters up to 
about 100 meters.
    2. Middle scale--Defines the concentration typical of areas up 
to several city blocks in size, with dimensions ranging from about 
100 meters to 0.5 kilometers.
    3. Neighborhood scale--Defines concentrations within some 
extended area of the city that has relatively uniform land use with 
dimensions in the 0.5 to 4.0 kilometers range.
    4. Urban scale--Defines concentrations within an area of city-
like dimensions, on the order of 4 to 50 kilometers. Within a city, 
the geographic placement of sources may result in there being no 
single site that can be said to represent air quality on an urban 
scale. The neighborhood and urban scales have the potential to 
overlap in applications that concern secondarily formed or 
homogeneously distributed air pollutants.
    5. Regional scale--Defines usually a rural area of reasonably 
homogeneous geography without large sources, and extends from tens 
to hundreds of kilometers.
---------------------------------------------------------------------------

    The EPA notes that prior to this final rulemaking, the only 
required CO monitoring within 40 CFR part 58, appendix D was for the 
operation of a CO monitor within all NCore multipollutant monitoring 
stations. There are approximately 80 NCore stations nationwide, and by 
design, they are area-wide monitoring sites. In the proposal, the 
Agency estimated that 345 CO monitors were operational at some point 
during 2009. A more recent examination of AQS data (utilizing EPA's Air 
Explorer Web tools located at http://www.epa.gov/airexplorer) indicate 
that approximately 328 CO monitors were operational as of May 20, 2011. 
These 328 active CO monitors include the 80 NCore monitors now in 
operation nationwide. This means that a significant portion of the 
current network is composed of monitors that are additional to those 
required by EPA as part of a national network design. It is critical to 
note that in this rulemaking the EPA is actually increasing the total 
number of required sites in the national CO monitoring network design 
and is not removing any area-wide monitoring requirements as AASHTO and 
other commenters suggested. Some of the potential for misperception on 
this issue may have arisen from the Agency's stated expectation that 
state and local air monitoring agencies will likely move existing CO 
monitors into near-road locations to satisfy the minimum monitoring 
requirements promulgated in this rulemaking. Based on this final rule, 
state and local agencies would only move, at most, approximately 52 
monitors out of the 328 in operation (circa May 2011). Therefore a 
majority of CO monitors would likely continue operating in their 
existing locations. However, it should be noted that with ambient CO 
concentrations well below the NAAQS, particularly at area-wide sites, 
states may identify some area-wide CO monitors to be no longer 
necessary. As such, the retirement of these sites may be justified, and 
their removal would save state and local resources. The EPA does 
recognize the value of maintaining some level of area-wide CO 
monitoring to meet the overarching monitoring objectives, which 
includes tracking long-term trends and to support research. In the 
proposal, the Agency did not propose establishing requirements for 
additional area-wide monitoring sites because: (1) There is the 
existing NCore requirement, and (2) there is an expectation based on 
experience that some number of non-required area-wide sites will 
continue to operate in the future without minimum monitoring 
requirements. Regarding the removal or shutdown of any individual 
ambient air pollutant monitor, the Agency notes that there is a 
publicly transparent process by which any existing CO monitor would be 
shut-down. The shut-down of any State and Local Air Monitoring Station 
(SLAMS) monitor is allowable under certain conditions specified in 40 
CFR 58.14 System Modification. These conditions provide state and local 
air agencies multiple options by which they may propose, with 
justification, for a monitor to be shut down. Whatever the 
justification may be, each monitor proposed to be shut-down must go 
through an established process to receive EPA Regional Administrator 
approval for shut-down. As part of that process, the EPA Regional 
Administrator provides opportunity for public comment before making a 
decision to approve or disapprove the request. In conclusion, the EPA 
believes that even without requirements for area-wide CO monitors 
additional to the NCore sites, some number of area-wide monitors will 
continue to operate into the future. EPA anticipates that monitors that 
states find useful for other regulatory purposes, such as NEPA, would 
be among the monitors that may continue to operate. The NCore sites, 
along with monitors currently operating in the absence of other area-
wide monitoring requirements, will likely provide a sufficient set of 
area-wide monitors to meet monitoring objectives.
    The EPA also received a number of comments from transportation 
groups, state and local groups, and an industry commenter (e.g., AAM, 
AASHTO, NESCAUM, NYSDEC, NYSDOT, TXDOT, and VDOT) suggesting that 
required near-road CO monitors should be paired with an area-wide CO 
monitor within the same CBSA. For example, AASHTO recommended that ``* 
* * EPA ensure that adequate coverage continues from neighborhood-scale 
monitors to estimate background concentration levels, and that there is 
at least one neighborhood scale monitor in every urbanized area that is 
required to have a near-road monitor.'' NESCAUM recommended ``* * * 
that EPA locate near-road CO monitors near urban NCore CO sites'' (as 
noted above, NCore sites are area-wide sites by design).
    The EPA recognizes that a pairing of near-road CO monitors with 
area-wide CO monitors will provide information by which an estimate of 
the difference between near-road concentrations to relative background 
concentrations might be determined. As noted earlier, the Agency 
believes that the combination of required NCore sites and those area-
wide monitors currently operating in the absence of minimum monitoring 
requirements (of which many will likely continue operating in the 
future) will largely fulfill the area-wide component of any near-road 
site/area-wide site pairing in an urban area. An analysis of NCore site 
locations (site data available from http://www.epa.gov/ttn/amtic/ncore/index.html), along with

[[Page 54318]]

all those area-wide CO monitors believed to be operating as of May 20, 
2011 (utilizing EPA's Air Explorer Web tools located at http://www.epa.gov/airexplorer) indicated that of the 52 CBSAs with a 
population of 1 million persons or more, based on 2010 Census data,, 
only 4 are believed to be without an area-wide CO monitor.\33\ The EPA 
believes that, based on the considerations discussed above, the 
existing network will likely provide sufficient area-wide CO 
concentration information on which a near-road to area-wide data 
comparison could be based.
---------------------------------------------------------------------------

    \33\ The EPA notes that of the 52 CBSAs that have 1 million or 
more persons, 39 CBSAs contain an NCore monitoring station, which 
includes a CO monitor.
---------------------------------------------------------------------------

f. Regional Administrator Authority
    The EPA received a number of comments from states and 
transportation groups (e.g., AASHTO, NYSDOT, TCEQ, TXDOT, and VDOT) on 
the proposal for Regional Administrators to have the discretion to 
require monitoring above the minimum requirements as necessary to 
address situations where minimum monitoring requirements are not 
sufficient to meet monitoring objectives. For example, AASHTO commented 
that ``the proposed rule includes some examples of where additional 
monitors may be necessary. AASHTO is concerned that these brief 
examples may not be sufficient to ensure uniform application of this 
additional authority among the EPA Regions,'' and that EPA should 
provide guidance on this so that there is ``reasonable uniformity 
between EPA Regions in the implementation of these provisions.'' TCEQ 
commented that it ``does not agree that this discretion is appropriate, 
particularly where EPA has not proposed a process by which Regional 
Administrators must consult with states and the public regarding these 
decisions.'' Further TCEQ stated that ``* * * the potential requirement 
for additional monitors when `minimum monitoring requirements are not 
sufficient to meet monitoring objectives' is overly broad and should be 
refined to include objective criteria that will consistently applied 
across all EPA Regions.''
    The EPA notes that the authority of Regional Administrators to 
require additional monitoring above the minimum required is not unique 
to the CO NAAQS. For example, Regional Administrators have the 
authority to use their discretion to require additional NO2, 
lead, and sulfur dioxide monitors (40 CFR part 58 appendix D sections 
4.3.4, 4.4.3, and 4.5, respectively) and to work with state and local 
air agencies in designing and/or maintaining an appropriate ozone 
monitoring network (40 CFR part 58 appendix D section 4.1). The EPA 
believes that a nationally applicable network design may not always 
account for all locations in every area where monitors may be 
warranted. Example situations where the Regional Administrator 
authority could be utilized, which were provided in the proposal, could 
be for unmonitored locations where data or other information suggest 
that CO concentrations may be approaching or exceeding the NAAQS due to 
stationary CO sources, in downtown areas or urban street canyons, or in 
areas that are subject to high ground-level CO concentrations 
particularly due to or enhanced by topographical and meteorological 
impacts. The Agency cannot anticipate every example that may exist 
where the Regional Administrator authority might be used for inclusion 
in this preamble text. However, the Agency believes it is important for 
Regional Administrators to have the authority to address possible gaps 
in the minimally required monitoring network in situations such as 
those examples provided here. In response to public comments, the EPA 
notes that Regional Administrators would use their authority in 
collaboration with state agencies, working with stakeholders to design 
and/or maintain the most appropriate CO monitoring network to meet the 
needs of a given area. Finally, the Agency notes that any monitor 
required by the Regional Administrator (or any new monitor proposed by 
the state itself) is not done so with unfettered discretion. Any such 
action would be included in the Annual Monitoring Network Plan per 40 
CFR 58.10, and this plan must be made available for public inspection 
and comment before any decisions are made by the EPA Regional 
Administrator.
3. Conclusions on the Network Design
    This section provides the rationale and details for the final 
decision on changes to the CO monitoring network design and siting 
criteria. As discussed above in section IV.B.2.a, motor vehicle 
emissions are important contributors to ambient CO concentrations (REA, 
section 2.2), contributing nearly 60% of the total CO emitted 
nationally (per the 2008 NEI). As a result, microenvironments 
influenced by on-road mobile sources are important contributors to 
ambient CO exposures, particularly in urban areas (REA, section 2.7). 
Therefore, the Administrator has concluded that monitoring in the near-
road environment to characterize and assess ambient CO concentrations 
continues to be an appropriate objective for the CO monitoring network. 
The EPA believes that the promulgation of minimum requirements for CO 
monitors in the near-road environment is necessary to ensure a network 
of adequate size and focus to provide data for comparison to the NAAQS, 
support research which includes health studies, allow for model 
verification, and fulfill multipollutant monitoring objectives. 
Further, considering the lack of CO monitors assessing higher 
trafficked roads in urban areas and CASAC's advice that the Agency 
develop greater monitoring capacity for CO in near-road environments 
(Brain and Samet, 2010b), the Agency believes that a number of CO 
monitors should be focused in such locations. Highly trafficked roads 
are expected to show elevated CO concentrations relative to area-wide 
concentrations and to represent the locations where ambient CO 
concentrations may be highest in an area. Regarding the locations where 
required near-road CO monitors might be placed, the EPA proposed that 
they be collocated with a subset of near-road NO2 monitors. 
The EPA expects required near-road NO2 monitors (as 
prescribed in 40 CFR part 58, appendix D, Section 4.3) to be adjacent 
to highly trafficked roads within the CBSAs where they are required. 
Recognizing this and also recognizing the benefits associated with 
collocating monitors at the same site, the Agency is finalizing 
requirements for CO monitors that will leverage required near-road 
NO2 monitoring sites to house collocated near-road CO 
monitors to create data for comparison to the NAAQS, support research 
which includes health studies, provide data for model evaluation, and 
foster the fulfillment of multipollutant objectives.
    As noted in section IV.B.2.b above, after considering public 
comments, EPA has modified the requirements for CO monitors from that 
which was proposed so that only one CO monitor is required in each CBSA 
in which near-road CO monitoring is required.\34\ This approach reduces 
the total number of monitors that would have been required under the 
proposal from 75 monitors within 52 CBSAs to 52 monitors within 52 
CBSAs (based on 2010 Census data). The EPA believes this network design 
addresses public comments while maintaining monitoring in a 
sufficiently diverse set

[[Page 54319]]

of locations throughout 52 different urban areas around the country. By 
having monitors within 52 different CBSAs, this network design is 
expected to provide for monitoring in a wide range of diverse 
situations with regard to traffic volumes, traffic patterns, roadway 
designs, terrain/topography, meteorology, climate, as well as 
surrounding land use and population characteristics.
---------------------------------------------------------------------------

    \34\ This approach only requires one CO monitor to be installed 
in those CBSAs that have two required near-road NO2 
monitors.
---------------------------------------------------------------------------

    The EPA is generally requiring CO monitors to be collocated with 
near-road NO2 monitors. However, upon consideration of 
public comments, the Agency is allowing flexibility for states to use 
an alternate near-road location, which includes downtown areas, urban 
street canyons, and other near-road locations. This flexibility is 
provided for a required CO monitor, on a case-by-case basis, with EPA 
Regional Administrator approval, when the state can provide 
quantitative justification showing the expectation of higher peak CO 
concentrations for that alternate location compared to a near-road 
NO2 location. Such requests could be based upon appropriate 
modeling, exploratory monitoring, or other methods, comparing the 
alternative CO location and the near-road NO2 location.
    In summary, based upon 2010 Census Bureau data this final rule will 
require approximately 52 CO monitors to be collocated with near-road 
NO2 monitors (or otherwise operated at an alternate, EPA 
Regional Administrator approved, near-road location where peak CO 
concentrations are expected) within 52 CBSAs that have populations of 1 
million or more persons.
    Regarding the deployment and operation of required CO monitors, the 
Agency recognizes that many state and local air agencies are under 
financial and related resource duress. EPA has concluded that allowing 
additional time for installing CO monitors will provide an opportunity 
for state and local agencies to work with EPA Regions to identify which 
existing CO monitors may be appropriate to relocate to the near-road 
locations. In many cases, EPA and the state may believe it is 
appropriate to relocate monitors, including some of those that are 
currently operated pursuant to existing maintenance plans. In these 
cases, additional time may be necessary to allow states to revisit and 
possibly revise, in consultation with (and subject to the approval of) 
the EPA Regions, existing maintenance plans in a way that may allow 
certain CO monitors to be free for relocation, if appropriate. Further, 
if a state chooses to investigate whether it will request that a 
required near-road CO monitor be sited in a near-road location other 
than a required near-road NO2 site, the time allotted by the 
final rule is expected to provide states with adequate time to perform 
necessary analyses for submission to the Regional Administrator for 
approval. Furthermore, EPA has concluded that public comments 
suggesting a phased-in implementation, allowing for later stages to 
benefit from experience in an initial round of monitor installations, 
have merit.
    As a result, the EPA has chosen not to require the implementation 
of required CO sites by January 1, 2013 as was proposed. Instead, the 
Agency is finalizing a two-phased implementation requirement. Those CO 
monitors required within CBSAs having 2.5 million or more persons are 
to be operational by January 1, 2015, although the Agency strongly 
encourages the implementation of these required monitors as soon as 
practicable. Those CO monitors required in CBSAs having 1 million or 
more persons (and fewer than 2.5 million persons) are to be operational 
by January 1, 2017. EPA intends to review the experience of states with 
the first round of near-road CO monitors and the data produced by such 
monitors and consider whether adjustments to the network requirements 
are warranted. These required CO monitors shall be reflected in a 
state's annual monitoring network plans due six months prior to 
installation, i.e., on July 1, 2014 or July 1, 2016, respectively.
    Regarding siting criteria, the EPA received public support to 
adjust microscale CO siting criteria to match those of near-road 
NO2 monitors (and microscale PM2.5 monitors). The 
Agency also was urged to retain the existing microscale siting 
criteria, for explicit use with microscale CO sites in downtown areas 
or urban street canyon settings. As a result, the EPA is retaining the 
existing siting criteria for microscale CO monitors in downtown areas 
and urban street canyons and is finalizing the additional siting 
criteria for those near-road microscale CO monitors outside of downtown 
areas and urban street canyons to have probe height and horizontal 
spacing to match those of near-road NO2 monitors as 
prescribed in 40 CFR part 58 appendix E, sections 2, 4(d), 6.4(a), and 
table E-4.
    Specifically, the Agency is finalizing the following: (1) A 
microscale near-road CO monitor inlet probe shall be between 2 and 7 
meters above the ground; (2) a microscale CO monitor inlet probe in the 
near-road environment shall be placed so it has an unobstructed air 
flow, where no obstacles exist at or above the height of the monitor 
probe, between the monitor probe and the outside nearest edge of the 
traffic lanes of the target road segment; and (3) that CO monitors in 
the near-road environment shall have inlet probes as near as 
practicable to the outside nearest edge of the traffic lanes of the 
target road segment, but shall not be located at a distance greater 
than 50 meters in the horizontal from the outside nearest edge of the 
traffic lanes of the target road segment.
    Further, as suggested through public comments, the EPA is retaining 
existing regulatory siting criteria language for microscale CO monitors 
in downtown areas or urban street canyon locations, where: (1) The 
inlet probe for a near-road microscale CO monitor in a downtown area or 
urban street canyon shall be between 2.5 meters and 3.5 meters above 
ground level; (2) the inlet probe for a near-road microscale CO monitor 
in a downtown area or urban street canyon shall be within 10 meters 
from the edge of the nearest traffic lane; and (3) near-road microscale 
CO monitors in street canyons are required to be at least 10 meters 
from an intersection.
    Finally, the EPA recognizes that a monitoring network design may 
not always require monitoring on a national scale that is sufficient in 
fulfilling specific or otherwise unique data needs or monitoring 
objectives for every area across the nation. Thus, the EPA is 
finalizing the provision that EPA Regional Administrators have the 
authority to require monitoring above the minimum requirements, as 
necessary, in any area, to address situations where the minimally 
required monitoring network is not sufficient to meet monitoring 
objectives. Example situations where the Regional Administrator 
Authority could be utilized include, but are not limited to, those 
unmonitored locations where data or other information suggest that CO 
concentrations may be approaching or exceeding the NAAQS due to 
stationary CO sources, in downtown areas or urban street canyons, or in 
areas that are subject to high ground-level CO concentrations 
particularly due to or enhanced by topographical and meteorological 
impacts. In all cases in which a Regional Administrator may consider 
the need for additional monitoring, it is expected that the Regional 
Administrators will work with the state or local air agencies to 
evaluate evidence or needs to determine whether a particular area may 
warrant additional monitoring.

[[Page 54320]]

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 13563: Improving Regulation and Regulatory Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), this 
action is a ``significant regulatory action'' because it was deemed to 
``raise novel legal or policy issues.'' Accordingly, EPA submitted this 
action to the Office of Management and Budget (OMB) for review under 
Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011) and any 
changes made in response to OMB recommendations have been documented in 
the docket for this action.

B. Paperwork Reduction Act

    The information collection requirements in this final 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 requirements are not enforceable until OMB 
approves them. The Information Collection Request (ICR) document 
prepared by EPA for these revisions to part 58 has been assigned EPA 
ICR number 0940.24.
    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 NAAQS in 40 CFR part 50 will 
meet comparability requirements for designation as a FRM or FEM. We do 
not expect the number of FRM or FEM determinations to increase over the 
number that is currently used to estimate burden associated with CO 
FRM/FEM determinations provided in the current ICR for 40 CFR part 53 
(EPA ICR numbers 0940.24). As such, no change in the burden estimate 
for 40 CFR part 53 has been made as part of this rulemaking.
    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 impacts, to develop emissions control 
strategies, and to measure progress for the air pollution program. The 
amendments would revise the technical requirements for CO monitoring 
sites, require the relocation or siting of ambient CO air monitors, and 
the reporting of the collected ambient CO 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 a network of 311 CO monitors is $7,235,483. 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. When this ICR is 
approved by OMB, the Agency will publish a technical amendment to 40 
CFR part 9 in the Federal Register to display the OMB control number 
for the approved information collection requirements contained in this 
final rule.

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 today's 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 final rule on small 
entities, I certify that this action will not have a significant 
economic impact on a substantial number of small entities. This final 
rule will not impose any requirements on small entities. Rather, this 
rule retains existing national standards for allowable concentrations 
of CO in ambient air as required by section 109 of the CAA. See also 
American Trucking Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do 
not have significant impacts upon small entities because NAAQS 
themselves impose no regulations upon small entities). Similarly, the 
amendments to 40 CFR part 58 address the requirements for States to 
collect information and report compliance with the NAAQS and will not 
impose any requirements on small entities.

D. Unfunded Mandates Reform Act

    This 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. 
This rule retains the existing national ambient air quality standards 
for carbon monoxide. The expected costs associated with the monitoring 
requirements are described in EPA's ICR document, but those costs are 
expected to be well less than $100 million (adjusted for inflation) 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. Thus, this rule 
is not subject to the requirements of sections 202 or 205 of the UMRA.
    This rule is also not subject to the requirements of section 203 of 
the UMRA because it imposes no enforceable duty on any small 
governments.

E. Executive Order 13132: Federalism

    This action 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 and review 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 D (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.

[[Page 54321]]

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

    This action does not have Tribal implications, as specified in 
Executive Order 13175 (65 FR 67249, November 9, 2000). 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 action.

G. Executive Order 13045: Protection of Children From Environmental 
Health and Safety Risks

    This action is not subject to EO 13045 (62 FR 19885, April 23, 
1997) because it is not economically significant as defined in EO 
12866, and because the Agency does not believe the environmental health 
or safety risks addressed by this action present a disproportionate 
risk to children. This action's health and risk assessments are 
described in section II.A.

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

    This action is not a ``significant energy action'' as defined in 
Executive Order 13211 (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 rule concerns the review of the 
NAAQS for CO. The rule does not prescribe specific pollution control 
strategies by which these ambient standards will be met. Such 
strategies are 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.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement 
Act of 1995 (NTTAA), Public Law 104- 113, section 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.
    This rulemaking involves technical standards with regard to ambient 
monitoring of CO. We have not identified any potentially applicable 
voluntary consensus standards that would adequately characterize 
ambient CO concentrations for the purposes of determining compliance 
with the CO NAAQS and none have been brought to our attention in 
comments. Therefore, EPA has decided to use the method ``Measurement 
Principle and Calibration Procedure for the Measurement of Carbon 
Monoxide in the Atmosphere (Non-Dispersive Infrared Photometry)'' (40 
CFR part 50, appendix C), as revised by this action, for the purposes 
of ambient monitoring of CO concentrations.

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 final rule will not have 
disproportionately high and adverse human health or environmental 
effects on minority or low-income populations because it does not 
affect the level of protection provided to human health or the 
environment. The action in this notice is to retain without revision 
the existing NAAQS for CO. Therefore this action will not cause 
increases in source emissions or air concentrations.

K. Congressional Review Act

    The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the 
Small Business Regulatory Enforcement Fairness Act of 1996, generally 
provides that before a rule may take effect, the agency promulgating 
the rule must submit a rule report, which includes a copy of the rule, 
to each House of the Congress and to the Comptroller General of the 
United States. EPA will submit a report containing this rule and other 
required information to the U.S. Senate, the U.S. House of 
Representatives, and the Comptroller General of the United States prior 
to publication of the rule in the Federal Register. A major rule cannot 
take effect until 60 days after it is published in the Federal 
Register. This action is not a ``major rule'' as defined by 5 U.S.C. 
804(2). This rule will be effective October 31, 2011.

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American Statistical Asssociation; pp. 104-108.
Brain, J.D. (2009) Letter from Dr. J.D. Brain to Administrator Lisa 
Jackson. Re: Consultation on EPA's Carbon Monoxide National Ambient 
Air Quality Standards: Scope and Methods Plan for Health Risk and 
Exposure Assessment. CASAC-09-012. July 14, 2009.
Brain J. and Samet J. (2009) Letter from Drs. J.D. Brain and J.M. 
Samet to Administrator Lisa Jackson. Re: Review of EPA's Integrated 
Science Assessment for Carbon Monoxide (First External Review Draft) 
EPA-CASAC-09-011. June 24, 2009.
Brain, J.D. and Samet, J.M. (2010a) Letter from Drs. J.D. Brain and 
J.M. Samet to Administrator Lisa Jackson. Re: Review of the Risk and 
Exposure Assessment to Support the Review of the Carbon Monoxide 
(CO) Primary National Ambient Air Quality Standards: First External 
Review Draft. EPA-CASAC-10-006. February 12, 2010.
Brain J.D. and Samet J.M. (2010b) Letter from Drs. J.D. Brain and 
J.M. Samet to Administrator Lisa Jackson. Re: Review of the Risk and 
Exposure Assessment to Support the Review of the Carbon Monoxide 
(CO) Primary National Ambient Air Quality Standards: Second External 
Review Draft. EPA-CASAC-10-012. May 19, 2010.
Brain J.D. and Samet J.M. (2010c) Letter from Drs. J.D. Brain and 
J.M. Samet to Administrator Lisa Jackson. Re: Review of the Policy 
Assessment for the Review of the Carbon Monoxide National Ambient 
Air Quality Standards (NAAQS): External Review Draft. EPA-CASAC-10-
013. June 8, 2010.
Brain J.D. and Samet J.M. (2010d) Letter from Drs. J.D. Brain and 
J.M. Samet to Administrator Lisa Jackson. Re: Review of Integrated 
Science Assessment for Carbon Monoxide (Second External Review 
Draft). EPA-CASAC-10-005. January 20, 2010.
Henderson R. (2008) Letter from Dr. Rogene Henderson, Chairman, 
Clean Air Scientific Advisory Committee, to Administrator Stephen 
Johnson. Re: Consultation on EPA's Draft Plan for Review of the 
Primary NAAQS for Carbon Monoxide CASAC-08-013. June 12, 2008.
Johnson T.; Capel J.; Paul R.; Wijnberg L. (1992) Estimation of 
Carbon Monoxide Exposure and associated Carboxyhemoglobin levels in 
Denver Residents Using a Probabalistic verion of NEM, prepared by 
International Technology for U.S. EPA, Office of Air Quality 
Planning and Standards, Durham, NC, Contract No. 68-D0-0062.
Johnson T.; Mihlan G.; LaPointe J.; Fletcher K.; Capel J. (2000) 
Estimation of Carbon Monoxide Exposures and Associated 
Carboxyhemoglobin Levels for Residents of Denver and Los Angeles 
Using pNEM/CO (Version 2.1). Report prepared by ICF Consulting and 
TRJ Environmental, Inc., under EPA Contract No. 68-D6-0064. U.S. 
Environmental Protection Agency, Research Triangle Park, North 
Carolina. Available at: http://www.epa.gov/ttn/fera/human_related.html. June 2000.
Kleinman M.T.; Davidson D.M.; Vandagriff R.B.; Caiozzo V.J.; 
Whittenberger J.L. (1989) Effects of short-term exposure to carbon 
monoxide in subjects with coronary artery disease. Arch. Environ. 
Health 44:361-369.
Kleinman M.T.; Leaf D.A.; Kelly E.; Caiozzo V.; Osann K.; O'Niell T. 
(1998) Urban angina in the mountains: effects of carbon monoxide and 
mild hypoxemia on subjects with chronic stable angina. Arch. 
Environ. Health 53:388-397.
Koken P.J.M.; Piver W.T.; Ye F.; Elixhauser A.; Olsen L.M.; Portier 
C.J. (2003) Temperature, air pollution, and hospitalization for 
cardiovascular diseases among elderly people in Denver. Environ 
Health Perspect, 111:1312-1317.
Lippmann, M. (1984) CASAC Findings and Recommendations on the 
Scientific Basis for a Revised NAAQS for Carbon Monoxide. [Letter to 
W.D. Ruckelshous, EPA Administrator]. May 17.
Linn W.S.; Szlachcic Y.; Gong H. Jr; Kinney P.L.; Berhane K.T. 
(2000) Air pollution and daily hospital admissions in metropolitan 
Los Angeles. Environ Health Perspect, 108:427-434.
Mann J.K.; Tager I.B.; Lurmann F.; Segal M.; Quesenberry C.P. Jr; 
Lugg M.M.; Shan J.; Van den Eeden S.K. (2002) Air pollution and 
hospital admissions for ischemic heart disease in persons with 
congestive heart failure or arrhythmia. Environ Health Perspect, 
110:1247-1252.
Metzger K.B.; Tolbert P.E.; Klein M.; Peel J.L.; Flanders W.D.; Todd 
K.H.; Mulholland J.A.; Ryan P.B.; Frumkin H. (2004) Ambient air 
pollution and cardiovascular emergency department visits. 
Epidemiology, 15:46-56.
National Research Council. (2003) Managing Carbon Monoxide Pollution 
in Meteorological and Topographical Problem Areas. The National 
Academies Press, Washington, DC.
Sheps D.S.; Adams K.F. Jr.; Bromberg P.A.; Goldstein G.M.; O'Neil 
J.J.; Horstman D.; Koch G. (1987) Lack of effect of low levels of 
carboxyhemoglobin on cardiovascular function in patients with 
ischemic heart disease. Arch. Environ. Health 42:108-116.
Symons J.M.; Wang L.; Guallar E.; Howell E.; Dominici F.; Schwab M.; 
Ange B.A.; Samet J.; Ondov J.; Harrison D.; Geyh A. (2006) A case-
crossover study of fine particulate matter air pollution and onset 
of congestive heart failure symptom exacerbation leading to 
hospitalization. Am J Epidemiol, 164:421-433.
Tolbert P.E.; Klein M.; Peel J.L.; Sarnat S.E.; Sarnat J.A. (2007) 
Multipollutant modeling issues in a study of ambient air quality and 
emergency department visits in Atlanta. J Expo Sci Environ 
Epidemiol, 17:S29-S35.
U.S. Environmental Protection Agency. (1979a) Air Quality Criteria 
for Carbon Monoxide. Office of Health and Environmental Assessment, 
Environmental Criteria and Assessment Office, Research Triangle 
Park, NC. EPA-600/8-79-022.
U.S. Environmental Protection Agency. (1979b) Assessment of Adverse 
Health Effects from Carbon Monoxide and Implications for Possible 
Modifications of the Standard. Office of Air Quality Planning and 
Standards, Research Triangle Park, NC.
U.S. Environmental Protection Agency. (1984a) Revised Evaluation of 
Health Effects Associated with Carbon Monoxide Exposure: An Addendum 
to the 1979 EPA Air Quality Criteria Document for Carbon Monoxide. 
Office of Health and Environmental Assessment, Environmental 
Criteria and Assessment Office, Research Triangle Park, NC. EPA-600/
8-83-033F.
U.S. Environmental Protection Agency. (1984b) Review of the NAAQS 
for Carbon Monoxide: Reassessment of Scientific and Technical 
Information. Office of Air Quality Planning and Standards, Research 
Triangle Park. NC. EPA-450/584-904.
U.S. Environmental Protection Agency. (1991) Air Quality Criteria 
for Carbon Monoxide. Office of Health and Environmental Assessment, 
Environmental Criteria and Assessment Office, Research Triangle 
Park, NC. EPA/600/8-90/045F. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_pr.html.
U.S. Environmental Protection Agency. (1992) Review of the National 
Ambient Air Quality Standards for Carbon Monoxide: Assessment of 
Scientific and Technical Information, OAQPS Staff Paper. Office of 
Air Quality Planning and Standards, Research Triangle Park, NC. EPA/
452/R-92-004.
U.S. Environmental Protection Agency. (2000) Air Quality Criteria 
for Carbon Monoxide. National Center for Environmental Assessment, 
Office of Research and Development, Research Triangle Park, NC. EPA/
600/P-99/001F. Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=18163.
U.S. Environmental Protection Agency. (2008a) Draft Plan for Review 
of the National Ambient Air Quality Standards for Carbon Monoxide. 
Also known as Draft Integrated Review Plan. National Center for 
Environmental Assessment and Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA-452/D-08-001. Available 
at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pd.html.
U.S. Environmental Protection Agency. (2008b) Plan for Review of the 
National Ambient Air Quality Standards for Carbon Monoxide. Also 
known as Integrated Review Plan. National Center for Environmental 
Assessment and Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/R-08-005. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pd.html.
U.S. Environmental Protection Agency. (2009a) Integrated Science 
Assessment for Carbon Monoxide, First External Review Draft. 
National Center for Environmental Assessment, Research

[[Page 54323]]

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Assessment for Carbon Monoxide, Second External Review Draft. 
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Park, NC. EPA/600/R-09/019B. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_isa.html.
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National Ambient Air Quality Standards: Scope and Methods Plan for 
Health Risk and Exposure Assessment. Draft. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-09-
004. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pd.html.
U.S. Environmental Protection Agency. (2009d) Risk and Exposure 
Assessment to Support the Review of the Carbon Monoxide Primary 
National Ambient Air Quality Standards, First External Review Draft. 
Office of Air Quality Planning and Standards, Research Triangle 
Park, NC. EPA-452/P-09-008. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_rea.html.
U.S. Environmental Protection Agency. (2010a) Integrated Science 
Assessment for Carbon Monoxide. National Center for Environmental 
Assessment, Research Triangle Park, NC. EPA/600/R-09/019F. Available 
at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_isa.html.
U.S. Environmental Protection Agency. (2010b) Quantitative Risk and 
Exposure Assessment for Carbon Monoxide--Amended. Office of Air 
Quality Planning and Standards, Research Triangle Park, NC. EPA-452/
R-10-009. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_rea.html.
U.S. Environmental Protection Agency. (2010c) Policy Assessment for 
the Review of the Carbon Monoxide National Ambient Air Quality 
Standards. Office of Air Quality Planning and Standards, Research 
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Assessment to Support the Review of the Carbon Monoxide Primary 
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Draft, U.S Environmental Protection Agency, Research Triangle Park, 
NC, report no. EPA-452/P-10-004. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_rea.html.
U.S. Environmental Protection Agency. (2010e) Policy Assessment for 
the Review of the Carbon Monoxide National Ambient Air Quality 
Standards, External Review Draft. Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA-452/P-10-005. Available 
at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pa.html.
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and Review. Memorandum to the Carbon Monoxide NAAQS Review Docket. 
EPA-HQ-OAR-2008-0015.
Wellenius G.A.; Bateson T.F.; Mittleman M.A.; Schwartz J. (2005) 
Particulate air pollution and the rate of hospitalization for 
congestive heart failure among medicare beneficiaries in Pittsburgh, 
Pennsylvania. Am J Epidemiol 161:1030-1036.
Zhu Y.; Hinds W.C.; Kim S.; Shen S.; Sioutas C. (2002) Study of 
ultrafine particles near a major highway with heavy-duty diesel 
traffic. Atmos Environ, 36:4323-4335.

List of Subjects

40 CFR Part 50

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

40 CFR Part 53

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

40 CFR Part 58

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

    Dated: August 12, 2011.
Lisa P. Jackson,
Administrator.

    For the reasons stated in the preamble, title 40, chapter I of the 
Code of Federal Regulations is amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

0
1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.

0
2. Appendix C to part 50 is revised to read as follows:

Appendix C to Part 50--Measurement Principle and Calibration Procedure 
for the Measurement of Carbon Monoxide in the Atmosphere (Non-
Dispersive Infrared Photometry)

1.0 Applicability

    1.1 This non-dispersive infrared photometry (NDIR) Federal 
Reference Method (FRM) provides measurements of the concentration of 
carbon monoxide (CO) in ambient air for determining compliance with 
the primary and secondary National Ambient Air Quality Standards 
(NAAQS) for CO as specified in Sec.  50.8 of this chapter. The 
method is applicable to continuous sampling and measurement of 
ambient CO concentrations suitable for determining 1-hour or longer 
average measurements. The method may also provide measurements of 
shorter averaging times, subject to specific analyzer performance 
limitations. Additional CO monitoring quality assurance procedures 
and guidance are provided in part 58, appendix A, of this chapter 
and in reference 1 of this appendix C.

2.0 Measurement Principle

    2.1 Measurements of CO in ambient air are based on automated 
measurement of the absorption of infrared radiation by CO in an 
ambient air sample drawn into an analyzer employing non-wavelength-
dispersive, infrared photometry (NDIR method). Infrared energy from 
a source in the photometer is passed through a cell containing the 
air sample to be analyzed, and the quantitative absorption of energy 
by CO in the sample cell is measured by a suitable detector. The 
photometer is sensitized specifically to CO by employing CO gas in a 
filter cell in the optical path, which, when compared to a 
differential optical path without a CO filter cell, limits the 
measured absorption to one or more of the characteristic wavelengths 
at which CO strongly absorbs. However, to meet measurement 
performance requirements, various optical filters, reference cells, 
rotating gas filter cells, dual-beam configurations, moisture traps, 
or other means may also be used to further enhance sensitivity and 
stability of the photometer and to minimize potential measurement 
interference from water vapor, carbon dioxide (CO2), or 
other species. Also, various schemes may be used to provide a 
suitable zero reference for the photometer, and optional automatic 
compensation may be provided for the actual pressure and temperature 
of the air sample in the measurement cell. The measured infrared 
absorption, converted to a digital reading or an electrical output 
signal, indicates the measured CO concentration.
    2.2 The measurement system is calibrated by referencing the 
analyzer's CO measurements to CO concentration standards traceable 
to a National Institute of Standards and Technology (NIST) primary 
standard for CO, as described in the associated calibration 
procedure specified in section 4 of this reference method.
    2.3 An analyzer implementing this measurement principle will be 
considered a reference method only if it has been designated as a 
reference method in accordance with part 53 of this chapter.
    2.4 Sampling considerations. The use of a particle filter in the 
sample inlet line of a CO FRM analyzer is optional and left to the 
discretion of the user unless such a filter is specified or 
recommended by the analyzer manufacturer in the analyzer's 
associated operation or instruction manual.

3.0 Interferences

    3.1 The NDIR measurement principle is potentially susceptible to 
interference from water vapor and CO2, which have some 
infrared absorption at wavelengths in common with CO and normally 
exist in the atmosphere. Various instrumental techniques can be used 
to effectively minimize these interferences.

[[Page 54324]]

4.0 Calibration Procedures

    4.1 Principle. Either of two methods may be selected for dynamic 
multipoint calibration of FRM CO analyzers, using test gases of 
accurately known CO concentrations obtained from one or more 
compressed gas cylinders certified as CO transfer standards:
    4.1.1 Dilution method: A single certified standard cylinder of 
CO is quantitatively diluted as necessary with zero air to obtain 
the various calibration concentration standards needed.
    4.1.2 Multiple-cylinder method: Multiple, individually certified 
standard cylinders of CO are used for each of the various 
calibration concentration standards needed.
    4.1.3 Additional information on calibration may be found in 
Section 12 of reference 1.
    4.2 Apparatus. The major components and typical configurations 
of the calibration systems for the two calibration methods are shown 
in Figures 1 and 2. Either system may be made up using common 
laboratory components, or it may be a commercially manufactured 
system. In either case, the principal components are as follows:
    4.2.1 CO standard gas flow control and measurement devices (or a 
combined device) capable of regulating and maintaining the standard 
gas flow rate constant to within  2 percent and 
measuring the gas flow rate accurate to within  2 
percent, properly calibrated to a NIST-traceable standard.
    4.2.2 For the dilution method (Figure 1), dilution air flow 
control and measurement devices (or a combined device) capable of 
regulating and maintaining the air flow rate constant to within 
 2 percent and measuring the air flow rate accurate to 
within  2 percent, properly calibrated to a NIST-
traceable standard.
    4.2.3 Standard gas pressure regulator(s) for the standard CO 
cylinder(s), suitable for use with a high-pressure CO gas cylinder 
and having a non-reactive diaphragm and internal parts and a 
suitable delivery pressure.
    4.2.4 Mixing chamber for the dilution method of an inert 
material and of proper design to provide thorough mixing of CO 
standard gas and diluent air streams.
    4.2.5 Output sampling manifold, constructed of an inert material 
and of sufficient diameter to ensure an insignificant pressure drop 
at the analyzer connection. The system must have a vent designed to 
ensure nearly atmospheric pressure at the analyzer connection port 
and to prevent ambient air from entering the manifold.

4.3 Reagents

    4.3.1 CO gas concentration transfer standard(s) of CO in air, 
containing an appropriate concentration of CO suitable for the 
selected operating range of the analyzer under calibration and 
traceable to a NIST standard reference material (SRM). If the CO 
analyzer has significant sensitivity to CO2, the CO 
standard(s) should also contain 350 to 400 ppm CO2 to 
replicate the typical CO2 concentration in ambient air. 
However, if the zero air dilution ratio used for the dilution method 
is not less than 100:1 and the zero air contains ambient levels of 
CO2, then the CO standard may be contained in nitrogen 
and need not contain CO2.
    4.3.2 For the dilution method, clean zero air, free of 
contaminants that could cause a detectable response on or a change 
in sensitivity of the CO analyzer. The zero air should contain min at 25 [deg]C and 760 mm Hg),
Fm = measured flow rate (at temperature Tm and pressure Pm),
Pm = measured pressure in mm Hg (absolute), and
Tm = measured temperature in degrees Celsius.

    4.4.3 Select the operating range of the CO analyzer to be 
calibrated. Connect the measurement signal output of the analyzer to 
an appropriate readout instrument to allow the analyzer's 
measurement output to be continuously monitored during the 
calibration. Where possible, this readout instrument should be the 
same one used to record routine monitoring data, or, at least, an 
instrument that is as closely representative of that system as 
feasible.
    4.4.4 Connect the inlet of the CO analyzer to the output-
sampling manifold of the calibration system.
    4.4.5 Adjust the calibration system to deliver zero air to the 
output manifold. The total air flow must exceed the total demand of 
the analyzer(s) connected to the output manifold to ensure that no 
ambient air is pulled into the manifold vent. Allow the analyzer to 
sample zero air until a stable response is obtained. After the 
response has stabilized, adjust the analyzer zero reading.
    4.4.6 Adjust the zero air flow rate and the CO gas flow rate 
from the standard CO cylinder to provide a diluted CO concentration 
of approximately 80 percent of the measurement upper range limit 
(URL) of the operating range of the analyzer. The total air flow 
rate must exceed the total demand of the analyzer(s) connected to 
the output manifold to ensure that no ambient air is pulled into the 
manifold vent. The exact CO concentration is calculated from:
[GRAPHIC] [TIFF OMITTED] TR31AU11.002

Where:

[CO]OUT = diluted CO concentration at the output manifold (ppm),
[CO]STD = concentration of the undiluted CO standard (ppm),
FCO = flow rate of the CO standard (L/min), and
FD = flow rate of the dilution air (L/min).

Sample this CO concentration until a stable response is obtained. 
Adjust the analyzer span control to obtain the desired analyzer 
response reading equivalent to the calculated standard 
concentration. If substantial adjustment of the analyzer span 
control is required, it may be necessary to recheck the zero and 
span adjustments by repeating steps 4.4.5 and 4.4.6. Record the CO 
concentration and the analyzer's final response.
    4.4.7 Generate several additional concentrations (at least three 
evenly spaced points across the remaining scale are suggested to 
verify linearity) by decreasing FCO or increasing FD. Be sure the 
total flow exceeds the analyzer's total flow demand. For each 
concentration generated, calculate the exact CO concentration using 
equation (2). Record the concentration and the analyzer's stable 
response for each concentration. Plot the analyzer responses 
(vertical or y-axis) versus the corresponding CO concentrations 
(horizontal or x-axis). Calculate the linear regression slope and 
intercept of the calibration curve and verify that no point deviates 
from this line by more than 2 percent of the highest concentration 
tested.
    4.5 Procedure Using the Multiple-Cylinder Method. Use the 
procedure for the dilution method with the following changes:
    4.5.1 Use a multi-cylinder, dynamic calibration system such as 
the typical one shown in Figure 2.
    4.5.2 The flowmeter need not be accurately calibrated, provided 
the flow in the output manifold can be verified to exceed the 
analyzer's flow demand.
    4.5.3 The various CO calibration concentrations required in 
Steps 4.4.5, 4.4.6, and 4.4.7 are obtained without dilution by 
selecting zero air or the appropriate certified standard cylinder.
    4.6 Frequency of Calibration. The frequency of calibration, as 
well as the number of points necessary to establish the calibration 
curve and the frequency of other performance checking, will vary by 
analyzer. However, the minimum frequency, acceptance criteria, and 
subsequent actions are specified in reference 1, appendix D, 
``Measurement Quality Objectives and Validation Template for CO'' 
(page 5 of 30). The user's quality control program should provide 
guidelines for initial establishment of these variables and for 
subsequent alteration as operational experience is accumulated. 
Manufacturers of CO analyzers should include in their instruction/
operation manuals information and guidance as to these variables and 
on other matters of operation, calibration, routine maintenance, and 
quality control.

[[Page 54325]]

5.0 Reference

    1. QA Handbook for Air Pollution Measurement Systems--Volume II. 
Ambient Air Quality Monitoring Program. U.S. EPA. EPA-454/B-08-003 
(2008).
BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TR31AU11.003

[[Page 54326]]

[GRAPHIC] [TIFF OMITTED] TR31AU11.004

BILLING CODE 6560-50-C

PART 53--AMBIENT AIR QUALITY REFERENCE AND EQUIVALENT METHODS

0
3. The authority citation for part 53 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.

0
4. Subpart B of part 53 is revised to read as follows:
Subpart B--Procedures for Testing Performance Characteristics of 
Automated Methods for SO2, CO, O3, and NO2
Sec.
53.20 General provisions.
53.21 Test conditions.
53.22 Generation of test atmospheres.
53.23 Test procedure.
Figure B-1 to Subpart B of Part 53--Example
Table B-1 to Subpart B of Part 53--Performance Limit Specifications 
for Automated Methods
Table B-2 to Subpart B of Part 53--Test Atmospheres
Table B-3 to Subpart B of Part 53--Interferent Test Concentration, 1 
Parts Per Million
Table B-4 to Subpart B of Part 53-- Line Voltage and Room 
Temperature Test Conditions
Table B-5 to Subpart B of Part 53--Symbols and Abbreviations
Appendix A to Subpart B--Optional Forms for Reporting Test Results

Subpart B--Procedures for Testing Performance Characteristics of 
Automated Methods for SO2, CO, O3, and NO2

Sec.  53.20  General provisions.

    (a) The test procedures given in this subpart shall be used to test 
the performance of candidate automated methods against the performance 
requirement specifications given in table B-1 to subpart B of part 53. 
A test analyzer representative of the candidate automated method must 
exhibit performance better than, or not outside, the specified limit or 
limits for each such performance parameter specified (except range) to 
satisfy the requirements of this subpart. Except as provided in 
paragraph (b) of this section, the measurement range of the candidate 
method must be the standard range specified in table B-1 to subpart B 
of part 53 to satisfy the requirements of this subpart.
    (b) Measurement ranges. For a candidate method having more than one 
selectable measurement range, one range must be the standard range 
specified in table B-1 to subpart B of part 53, and a test analyzer 
representative of the method must pass the tests required by this 
subpart while operated in that range.
    (i) Higher ranges. The tests may be repeated for one or more higher 
(broader) ranges (i.e., ranges extending to higher concentrations) than 
the standard range specified in table B-1 to subpart B of part 53, 
provided that the range does not extend to concentrations more than 
four times the upper range limit of the standard range specified in 
table B-1 to subpart B of part 53. For such higher ranges, only the 
tests for range (calibration), noise at 80% of the upper range limit, 
and lag, rise and fall time are required to be repeated. For the 
purpose of testing a higher range, the test procedure of Sec.  53.23(e) 
may be abridged to include only those components needed to test lag, 
rise and fall time.
    (ii) Lower ranges. The tests may be repeated for one or more lower 
(narrower) ranges (i.e., ones extending to lower concentrations) than 
the standard range specified in table B-1 to subpart B of part 53. For 
methods for some pollutants, table B-1 to subpart B of part 53 
specifies special performance limit requirements for lower ranges. If 
special low-range performance limit requirements are not specified in 
table B-1 to subpart B of part 53, then the performance limit 
requirements for the standard range apply. For lower ranges for any 
method, only the tests for range (calibration), noise at 0% of the

[[Page 54327]]

measurement range, lower detectable limit, (and nitric oxide 
interference for SO2 UVF methods) are required to be 
repeated, provided the tests for the standard range shows the 
applicable limit specifications are met for the other test parameters.
    (iii) If the tests are conducted and passed only for the specified 
standard range, any FRM or FEM determination with respect to the method 
will be limited to that range. If the tests are passed for both the 
specified range and one or more higher or lower ranges, any such 
determination will include the additional higher or lower range(s) as 
well as the specified standard range. Appropriate test data shall be 
submitted for each range sought to be included in a FRM or FEM method 
determination under this paragraph (b).
    (c) For each performance parameter (except range), the test 
procedure shall be initially repeated seven (7) times to yield 7 test 
results. Each result shall be compared with the corresponding 
performance limit specification in table B-1 to subpart B of part 53; a 
value higher than or outside the specified limit or limits constitutes 
a failure. These 7 results for each parameter shall be interpreted as 
follows:
    (1) Zero (0) failures: The candidate method passes the test for the 
performance parameter.
    (2) Three (3) or more failures: The candidate method fails the test 
for the performance parameter.
    (3) One (1) or two (2) failures: Repeat the test procedures for the 
performance parameter eight (8) additional times yielding a total of 
fifteen (15) test results. The combined total of 15 test results shall 
then be interpreted as follows:
    (i) One (1) or two (2) failures: The candidate method passes the 
test for the performance parameter.
    (ii) Three (3) or more failures: The candidate method fails the 
test for the performance parameter.
    (d) The tests for zero drift, span drift, lag time, rise time, fall 
time, and precision shall be carried out in a single integrated 
procedure conducted at various line voltages and ambient temperatures 
specified in Sec.  53.23(e). A temperature-controlled environmental 
test chamber large enough to contain the test analyzer is recommended 
for this test. The tests for noise, lower detectable limit, and 
interference equivalent shall be conducted at any ambient temperature 
between 20 [deg]C and 30 [deg]C, at any normal line voltage between 105 
and 125 volts, and shall be conducted such that not more than three (3) 
test results for each parameter are obtained in any 24-hour period.
    (e) If necessary, all measurement response readings to be recorded 
shall be converted to concentration units or adjusted according to the 
calibration curve constructed in accordance with Sec.  53.21(b).
    (f) All recorder chart tracings (or equivalent data plots), 
records, test data and other documentation obtained from or pertinent 
to these tests shall be identified, dated, signed by the analyst 
performing the test, and submitted.

    Note to Sec.  53.20: Suggested formats for reporting the test 
results and calculations are provided in Figures B-2, B-3, B-4, B-5, 
and B-6 in appendix A to this subpart. Symbols and abbreviations 
used in this subpart are listed in table B-5 of appendix A to this 
subpart.

Sec.  53.21  Test conditions.

    (a) Set-up and start-up of the test analyzer shall be in strict 
accordance with the operating instructions specified in the manual 
referred to in Sec.  53.4(b)(3). Allow adequate warm-up or 
stabilization time as indicated in the operating instructions before 
beginning the tests. The test procedures assume that the test analyzer 
has a conventional analog measurement signal output that is connected 
to a suitable strip chart recorder of the servo, null-balance type. 
This recorder shall have a chart width of at least 25 centimeters, 
chart speeds up to 10 cm per hour, a response time of 1 second or less, 
a deadband of not more than 0.25 percent of full scale, and capability 
either of reading measurements at least 5 percent below zero or of 
offsetting the zero by at least 5 percent. If the test analyzer does 
not have an analog signal output, or if a digital or other type of 
measurement data output is used for the tests, an alternative 
measurement data recording device (or devices) may be used for 
recording the test data, provided that the device is reasonably suited 
to the nature and purposes of the tests, and an analog representation 
of the analyzer measurements for each test can be plotted or otherwise 
generated that is reasonably similar to the analog measurement 
recordings that would be produced by a conventional chart recorder 
connected to a conventional analog signal output.
    (b) Calibration of the test analyzer shall be carried out prior to 
conducting the tests described in this subpart. The calibration shall 
be as indicated in the manual referred to in Sec.  53.4(b)(3) and as 
follows: If the chart recorder or alternative data recorder does not 
have below zero capability, adjust either the controls of the test 
analyzer or the chart or data recorder to obtain a +5% offset zero 
reading on the recorder chart to facilitate observing negative response 
or drift. If the candidate method is not capable of negative response, 
the test analyzer (not the data recorder) shall be operated with a 
similar offset zero. Construct and submit a calibration curve showing a 
plot of recorder scale readings or other measurement output readings 
(vertical or y-axis) against pollutant concentrations presented to the 
analyzer for measurement (horizontal or x-axis). If applicable, a plot 
of base analog output units (volts, millivolts, milliamps, etc.) 
against pollutant concentrations shall also be obtained and submitted. 
All such calibration plots shall consist of at least seven (7) 
approximately equally spaced, identifiable points, including 0 and 90 
 5 percent of the upper range limit (URL).
    (c) Once the test analyzer has been set up and calibrated and the 
tests started, manual adjustment or normal periodic maintenance is 
permitted only every 3 days. Automatic adjustments which the test 
analyzer performs by itself are permitted at any time. The submitted 
records shall show clearly when any manual adjustment or periodic 
maintenance was made during the tests and describe the specific 
operations performed.
    (d) If the test analyzer should malfunction during any of the 
performance tests, the tests for that parameter shall be repeated. A 
detailed explanation of the malfunction, remedial action taken, and 
whether recalibration was necessary (along with all pertinent records 
and charts) shall be submitted. If more than one malfunction occurs, 
all performance test procedures for all parameters shall be repeated.
    (e) Tests for all performance parameters shall be completed on the 
same test analyzer; however, use of multiple test analyzers to 
accelerate testing is permissible for testing additional ranges of a 
multi-range candidate method.

Sec.  53.22  Generation of test atmospheres.

    (a) Table B-2 to subpart B of part 53 specifies preferred methods 
for generating test atmospheres and suggested methods of verifying 
their concentrations. Only one means of establishing the concentration 
of a test atmosphere is normally required, provided that that means is 
adequately accurate and credible. If the method of generation can 
produce accurate, reproducible concentrations, verification is 
optional. If the method of generation is not reproducible or reasonably 
quantifiable, then establishment of the concentration by

[[Page 54328]]

some credible verification method is required.
    (b) The test atmosphere delivery system shall be designed and 
constructed so as not to significantly alter the test atmosphere 
composition or concentration during the period of the test. The system 
shall be vented to insure that test atmospheres are presented to the 
test analyzer at very nearly atmospheric pressure. The delivery system 
shall be fabricated from borosilicate glass, FEP Teflon, or other 
material that is inert with regard to the gas or gases to be used.
    (c) The output of the test atmosphere generation system shall be 
sufficiently stable to obtain stable response readings from the test 
analyzer during the required tests. If a permeation device is used for 
generation of a test atmosphere, the device, as well as the air passing 
over it, shall be controlled to 0.1 [deg]C.
    (d) All diluent air shall be zero air free of contaminants likely 
to react with the test atmospheres or cause a detectable response on 
the test analyzer.
    (e) The concentration of each test atmosphere used shall be 
quantitatively established and/or verified before or during each series 
of tests. Samples for verifying test concentrations shall be collected 
from the test atmosphere delivery system as close as feasible to the 
sample intake port of the test analyzer.
    (f) The accuracy of all flow measurements used to calculate test 
atmosphere concentrations shall be documented and referenced to a 
primary flow rate or volume standard (such as a spirometer, bubble 
meter, etc.). Any corrections shall be clearly shown. All flow 
measurements given in volume units shall be standardized to 25 [deg]C 
and 760 mm Hg.
    (g) Schematic drawings, photos, descriptions, and other information 
showing complete procedural details of the test atmosphere generation, 
verification, and delivery system shall be provided. All pertinent 
calculations shall be clearly indicated.

Sec.  53.23  Test procedures.

    (a) Range--(1) Technical definition. The nominal minimum and 
maximum concentrations that a method is capable of measuring.

    Note to Sec.  53.23(a)(1): The nominal range is given as the 
lower and upper range limits in concentration units, for example, 0-
0.5 parts per million (ppm).

    (2) Test procedure. Determine and submit a suitable calibration 
curve, as specified in Sec.  53.21(b), showing the test analyzer's 
measurement response over at least 95 percent of the required or 
indicated measurement range.

    Note to Sec.  53.23(a)(2): A single calibration curve for each 
measurement range for which an FRM or FEM designation is sought will 
normally suffice.

    (b) Noise--(1) Technical definition. Spontaneous, short duration 
deviations in measurements or measurement signal output, about the mean 
output, that are not caused by input concentration changes. Measurement 
noise is determined as the standard deviation of a series of 
measurements of a constant concentration about the mean and is 
expressed in concentration units.
    (2) Test procedure. (i) Allow sufficient time for the test analyzer 
to warm up and stabilize. Determine measurement noise at each of two 
fixed concentrations, first using zero air and then a pollutant test 
gas concentration as indicated below. The noise limit specification in 
table B-1 to subpart B of part 53 shall apply to both of these tests.
    (ii) For an analyzer with an analog signal output, connect an 
integrating-type digital meter (DM) suitable for the test analyzer's 
output and accurate to three significant digits, to determine the 
analyzer's measurement output signal.

    Note to Sec.  53.23(b)(2): Use of a chart recorder in addition 
to the DM is optional.

    (iii) Measure zero air with the test analyzer for 60 minutes. 
During this 60-minute interval, record twenty-five (25) test analyzer 
concentration measurements or DM readings at 2-minute intervals. (See 
Figure B-2 in appendix A of this subpart.)
    (iv) If applicable, convert each DM test reading to concentration 
units (ppm) or adjust the test readings (if necessary) by reference to 
the test analyzer's calibration curve as determined in Sec.  53.21(b). 
Label and record the test measurements or converted DM readings as 
r1, r2, r3 . . . ri . . . 
r25.
    (v) Calculate measurement noise as the standard deviation, S, as 
follows:
[GRAPHIC] [TIFF OMITTED] TR31AU11.005

Where i indicates the i-th test measurement or DM reading in ppm.

    (vi) Let S at 0 ppm be identified as S0; compare 
S0 to the noise limit specification given in table B-1 to 
subpart B of part 53.
    (vii) Repeat steps in Paragraphs (b)(2)(iii) through (v) of this 
section using a pollutant test atmosphere concentration of 80  5 percent of the URL instead of zero air, and let S at 80 
percent of the URL be identified as S80. Compare 
S80 to the noise limit specification given in table B-1 to 
subpart B of part 53.
    (viii) Both S0 and S80 must be less than or 
equal to the table B-1 to subpart B of part 53 noise limit 
specification to pass the test for the noise parameter.
    (c) Lower detectable limit--(1) Technical definition. The minimum 
pollutant concentration that produces a measurement or measurement 
output signal of at least twice the noise level.
    (2) Test procedure. (i) Allow sufficient time for the test analyzer 
to warm up and stabilize. Measure zero air and record the stable 
measurement reading in ppm as BZ. (See Figure B-3 in 
appendix A of this subpart.)
    (ii) Generate and measure a pollutant test concentration equal to 
the value for the lower detectable limit specified in table B-1 to 
subpart B of part 53.

    Note to Sec.  53.23(c)(2): If necessary, the test concentration 
may be generated or verified at a higher concentration, then 
quantitatively and accurately diluted with zero air to the final 
required test concentration.

    (iii) Record the test analyzer's stable measurement reading, in 
ppm, as BL.
    (iv) Determine the lower detectable limit (LDL) test result as LDL 
= BL - BZ. Compare this LDL value with the noise level, S0, 
determined in Sec.  53.23(b), for the 0 concentration test atmosphere. 
LDL must be equal to or higher than 2 x S0 to pass this 
test.
    (d) Interference equivalent--(1) Technical definition. Positive or 
negative measurement response caused by a substance other than the one 
being measured.
    (2) Test procedure. The test analyzer shall be tested for all 
substances likely to cause a detectable response. The test analyzer 
shall be challenged, in turn, with each potential interfering agent 
(interferent) specified in table B-3 to subpart B of part 53. In the 
event that there are substances likely to cause a significant 
interference which have not been specified in table B-3 to subpart B of 
part 53, these substances shall also be tested, in a manner similar to 
that for the specified interferents, at a concentration substantially 
higher than that likely to be found in the ambient air. The 
interference may be either positive or negative, depending on whether 
the test analyzer's measurement response is increased or decreased by 
the presence of the interferent. Interference equivalents shall be 
determined by mixing each interferent, one at a time, with the 
pollutant at an interferent test concentration not lower than the test 
concentration specified in table B-3 to subpart B of part 53 (or as 
otherwise required for unlisted interferents), and

[[Page 54329]]

comparing the test analyzer's measurement response to the response 
caused by the pollutant alone. Known gas-phase reactions that might 
occur between a listed interferent and the pollutant are designated by 
footnote 3 in table B-3 to subpart B of part 53. In these cases, the 
interference equivalent shall be determined without mixing with the 
pollutant.
    (i) Allow sufficient time for warm-up and stabilization of the test 
analyzer.
    (ii) For a candidate method using a prefilter or scrubber device 
based upon a chemical reaction to derive part of its specificity and 
which device requires periodic service or maintenance, the test 
analyzer shall be ``conditioned'' prior to conducting each interference 
test series. This requirement includes conditioning for the 
NO2 converter in chemiluminescence NO/NO2/
NOX analyzers and for the ozone scrubber in UV-absorption 
ozone analyzers. Conditioning is as follows:
    (A) Service or perform the indicated maintenance on the scrubber or 
prefilter device, as if it were due for such maintenance, as directed 
in the manual referred to in Sec.  53.4(b)(3).
    (B) Before testing for each potential interferent, allow the test 
analyzer to sample through the prefilter or scrubber device a test 
atmosphere containing the interferent at a concentration not lower than 
the value specified in table B-3 to subpart B of part 53 (or, for 
unlisted potential interferents, at a concentration substantially 
higher than likely to be found in ambient air). Sampling shall be at 
the normal flow rate and shall be continued for 6 continuous hours 
prior to the interference test series. Conditioning for all applicable 
interferents prior to any of the interference tests is permissible. 
Also permissible is simultaneous conditioning with multiple 
interferents, provided no interferent reactions are likely to occur in 
the conditioning system.
    (iii) Generate three test atmosphere streams as follows:
    (A) Test atmosphere P: Pollutant test concentration.
    (B) Test atmosphere I: Interferent test concentration.
    (C) Test atmosphere Z: Zero air.
    (iv) Adjust the individual flow rates and the pollutant or 
interferent generators for the three test atmospheres as follows:
    (A) The flow rates of test atmospheres I and Z shall be equal.
    (B) The concentration of the pollutant in test atmosphere P shall 
be adjusted such that when P is mixed (diluted) with either test 
atmosphere I or Z, the resulting concentration of pollutant shall be as 
specified in table B-3 to subpart B of part 53.
    (C) The concentration of the interferent in test atmosphere I shall 
be adjusted such that when I is mixed (diluted) with test atmosphere P, 
the resulting concentration of interferent shall be not less than the 
value specified in table B-3 to subpart B of part 53 (or as otherwise 
required for unlisted potential interferents).
    (D) To minimize concentration errors due to flow rate differences 
between I and Z, it is recommended that, when possible, the flow rate 
of P be from 10 to 20 times larger than the flow rates of I and Z.
    (v) Mix test atmospheres P and Z by passing the total flow of both 
atmospheres through a (passive) mixing component to insure complete 
mixing of the gases.
    (vi) Sample and measure the mixture of test atmospheres P and Z 
with the test analyzer. Allow for a stable measurement reading, and 
record the reading, in concentration units, as R (see Figure B-3).
    (vii) Mix test atmospheres P and I by passing the total flow of 
both atmospheres through a (passive) mixing component to insure 
complete mixing of the gases.
    (viii) Sample and measure this mixture of P and I with the test 
analyzer. Record the stable measurement reading, in concentration 
units, as RI.
    (ix) Calculate the interference equivalent (IE) test result as:

IE = RI - R.

IE must be within the limits (inclusive) specified in table B-1 to 
subpart B of part 53 for each interferent tested to pass the 
interference equivalent test.
    (x) Follow steps (iii) through (ix) of this section, in turn, to 
determine the interference equivalent for each listed interferent as 
well as for any other potential interferents identified.
    (xi) For those potential interferents which cannot be mixed with 
the pollutant, as indicated by footnote (3) in table B-3 to subpart B 
of part 53, adjust the concentration of test atmosphere I to the 
specified value without being mixed or diluted by the pollutant test 
atmosphere. Determine IE as follows:
    (A) Sample and measure test atmosphere Z (zero air). Allow for a 
stable measurement reading and record the reading, in concentration 
units, as R.
    (B) Sample and measure the interferent test atmosphere I. If the 
test analyzer is not capable of negative readings, adjust the analyzer 
(not the recorder) to give an offset zero. Record the stable reading in 
concentration units as RI, extrapolating the calibration curve, if 
necessary, to represent negative readings.
    (C) Calculate IE = RI - R. IE must be within the limits (inclusive) 
specified in table B-1 to subpart B of part 53 for each interferent 
tested to pass the interference equivalent test.
    (xii) Sum the absolute value of all the individual interference 
equivalent test results. This sum must be equal to or less than the 
total interferent limit given in table B-1 to subpart B of part 53 to 
pass the test.
    (e) Zero drift, span drift, lag time, rise time, fall time, and 
precision--(1) Technical definitions--(i) Zero drift: The change in 
measurement response to zero pollutant concentration over 12- and 24-
hour periods of continuous unadjusted operation.
    (ii) Span drift: The percent change in measurement response to an 
up-scale pollutant concentration over a 24-hour period of continuous 
unadjusted operation.
    (iii) Lag time: The time interval between a step change in input 
concentration and the first observable corresponding change in 
measurement response.
    (iv) Rise time: The time interval between initial measurement 
response and 95 percent of final response after a step increase in 
input concentration.
    (v) Fall time: The time interval between initial measurement 
response and 95 percent of final response after a step decrease in 
input concentration.
    (vi) Precision: Variation about the mean of repeated measurements 
of the same pollutant concentration, expressed as one standard 
deviation.
    (2) Tests for these performance parameters shall be accomplished 
over a period of seven (7) or fifteen (15) test days. During this time, 
the line voltage supplied to the test analyzer and the ambient 
temperature surrounding the analyzer shall be changed from day to day, 
as required in paragraph (e)(4) of this section. One test result for 
each performance parameter shall be obtained each test day, for seven 
(7) or fifteen (15) test days, as determined from the test results of 
the first seven days. The tests for each test day are performed in a 
single integrated procedure.
    (3) The 24-hour test day may begin at any clock hour. The first 
approximately 12 hours of each test day are required for testing 12-
hour zero drift. Tests for the other parameters shall be conducted any 
time during the remaining 12 hours.
    (4) Table B-4 to subpart B of part 53 specifies the line voltage 
and room temperature to be used for each test day. The applicant may 
elect to specify a

[[Page 54330]]

wider temperature range (minimum and maximum temperatures) than the 
range specified in table B-4 to subpart B of part 53 and to conduct 
these tests over that wider temperature range in lieu of the specified 
temperature range. If the test results show that all test parameters of 
this section Sec.  53.23(e) are passed over this wider temperature 
range, a subsequent FRM or FEM designation for the candidate method 
based in part on this test shall indicate approval for operation of the 
method over such wider temperature range. The line voltage and 
temperature shall be changed to the specified values (or to the 
alternative, wider temperature values, if applicable) at the start of 
each test day (i.e., at the start of the 12-hour zero test). Initial 
adjustments (day zero) shall be made at a line voltage of 115 volts 
(rms) and a room temperature of 25 [deg]C.
    (5) The tests shall be conducted in blocks consisting of 3 test 
days each until 7 (or 15, if necessary) test results have been 
obtained. (The final block may contain fewer than three test days.) 
Test days need not be contiguous days, but during any idle time between 
tests or test days, the test analyzer must operate continuously and 
measurements must be recorded continuously at a low chart speed (or 
equivalent data recording) and included with the test data. If a test 
is interrupted by an occurrence other than a malfunction of the test 
analyzer, only the block during which the interruption occurred shall 
be repeated.
    (6) During each test block, manual adjustments to the electronics, 
gas, or reagent flows or periodic maintenance shall not be permitted. 
Automatic adjustments that the test analyzer performs by itself are 
permitted at any time.
    (7) At least 4 hours prior to the start of the first test day of 
each test block, the test analyzer may be adjusted and/or serviced 
according to the periodic maintenance procedures specified in the 
manual referred to in Sec.  53.4(b)(3). If a new block is to 
immediately follow a previous block, such adjustments or servicing may 
be done immediately after completion of the day's tests for the last 
day of the previous block and at the voltage and temperature specified 
for that day, but only on test days 3, 6, 9, and 12.

    Note to Sec.  53.23(e)(7): If necessary, the beginning of the 
test days succeeding such maintenance or adjustment may be delayed 
as required to complete the service or adjustment operation.

    (8) All measurement response readings to be recorded shall be 
converted to concentration units or adjusted (if necessary) according 
to the calibration curve. Whenever a test atmosphere is to be measured 
but a stable reading is not required, the test atmosphere shall be 
sampled and measured long enough to cause a change in measurement 
response of at least 10% of full scale. Identify all readings and other 
pertinent data on the strip chart (or equivalent test data record). 
(See Figure B-1 to subpart B of part 53 illustrating the pattern of the 
required readings.)
    (9) Test procedure. (i) Arrange to generate pollutant test 
atmospheres as follows. Test atmospheres A0, A20, 
and A80 shall be maintained consistent during the tests and 
reproducible from test day to test day.

------------------------------------------------------------------------
                                               Pollutant concentration
              Test atmosphere                         (percent)
------------------------------------------------------------------------
A0........................................  Zero air.
A20.......................................  20  5 of the
                                             upper range limit.
A30.......................................  30  5 of the
                                             upper range limit.
A80.......................................  80  5 of the
                                             upper range limit.
A90.......................................  90  5 of the
                                             upper range limit.
------------------------------------------------------------------------

     (ii) For steps within paragraphs (e)(9)(xxv) through (e)(9)(xxxi) 
of this section, a chart speed of at least 10 centimeters per hour (or 
equivalent resolution for a digital representation) shall be used to 
clearly show changes in measurement responses. The actual chart speed, 
chart speed changes, and time checks shall be clearly marked on the 
chart.
    (iii) Test day 0. Allow sufficient time for the test analyzer to 
warm up and stabilize at a line voltage of 115 volts and a room 
temperature of 25 [deg]C. Adjust the zero baseline to 5 percent of 
chart (see Sec.  53.21(b)) and recalibrate, if necessary. No further 
adjustments shall be made to the analyzer until the end of the tests on 
the third, sixth, ninth, or twelfth test day.
    (iv) Measure test atmosphere A0 until a stable 
measurement reading is obtained and record this reading (in ppm) as 
Z'n, where n = 0 (see Figure B-4 in appendix A of this subpart).
    (v) [Reserved.]
    (vi) Measure test atmosphere A80. Allow for a stable 
measurement reading and record it as S'n, where n = 0.
    (vii) The above readings for Z'0 and S'0 
should be taken at least four (4) hours prior to the beginning of test 
day 1.
    (viii) At the beginning of each test day, adjust the line voltage 
and room temperature to the values given in table B-4 to subpart B of 
part 53 (or to the corresponding alternative temperature if a wider 
temperature range is being tested).
    (ix) Measure test atmosphere A0 continuously for at 
least twelve (12) continuous hours during each test day.
    (x) After the 12-hour zero drift test (step ix) is complete, sample 
test atmosphere A0. A stable reading is not required.
    (xi) Measure test atmosphere A20 and record the stable 
reading (in ppm) as P1. (See Figure B-4 in appendix A.)
    (xii) Sample test atmosphere A30; a stable reading is 
not required.
    (xiii) Measure test atmosphere A20 and record the stable 
reading as P2.
    (xiv) Sample test atmosphere A0; a stable reading is not 
required.
    (xv) Measure test atmosphere A20 and record the stable 
reading as P3.
    (xvi) Sample test atmosphere A30; a stable reading is 
not required.
    (xvii) Measure test atmosphere A20 and record the stable 
reading as P4.
    (xviii) Sample test atmosphere A0; a stable reading is 
not required.
    (xix) Measure test atmosphere A20 and record the stable 
reading as P5.
    (xx) Sample test atmosphere A30; a stable reading is not 
required.
    (xxi) Measure test atmosphere A20 and record the stable 
reading as P6.
    (xxii) Measure test atmosphere A80 and record the stable 
reading as P7.
    (xxiii) Sample test atmosphere A90; a stable reading is 
not required.
    (xxiv) Measure test atmosphere A80 and record the stable 
reading as P8. Increase the chart speed to at least 10 
centimeters per hour.
    (xxv) Measure test atmosphere A0. Record the stable 
reading as L1.
    (xxvi) Quickly switch the test analyzer to measure test atmosphere 
A80 and mark the recorder chart to show, or otherwise 
record, the exact time when the switch occurred.
    (xxvii) Measure test atmosphere A80 and record the 
stable reading as P9.
    (xxviii) Sample test atmosphere A90; a stable reading is 
not required.
    (xxix) Measure test atmosphere A80 and record the stable 
reading as P10.
    (xxx) Measure test atmosphere A0 and record the stable 
reading as L2.
    (xxxi) Measure test atmosphere A80 and record the stable 
reading as P11.
    (xxxii) Sample test atmosphere A90; a stable reading is 
not required.
    (xxxiii) Measure test atmosphere A80 and record the 
stable reading as P12.
    (xxxiv) Repeat steps within paragraphs (e)(9)(viii) through 
(e)(9)(xxxiii) of this section, each test day.
    (xxxv) If zero and span adjustments are made after the readings are 
taken on

[[Page 54331]]

test days 3, 6, 9, or 12, complete all adjustments; then measure test 
atmospheres A0 and A80. Allow for a stable 
reading on each, and record the readings as Z'n and S'n, respectively, 
where n = the test day number (3, 6, 9, or 12). These readings must be 
made at least 4 hours prior to the start of the next test day.
    (10) Determine the results of each day's tests as follows. Mark the 
recorder chart to show readings and determinations.
    (i) Zero drift. (A) Determine the 12-hour zero drift by examining 
the strip chart pertaining to the 12-hour continuous zero air test. 
Determine the minimum (Cmin.) and maximum (Cmax.) measurement readings 
(in ppm) during this period of 12 consecutive hours, extrapolating the 
calibration curve to negative concentration units if necessary. 
Calculate the 12-hour zero drift (12ZD) as 12ZD = Cmax. - Cmin. (See 
Figure B-5 in appendix A.)
    (B) Calculate the 24-hour zero drift (24ZD) for the n-th test day 
as 24ZDn = Zn - Zn-1, or 24ZDn = Zn - Z'n-1 if zero adjustment was made 
on the previous test day, where Zn = \1/2\(L1+L2) 
for L1 and L2 taken on the n-th test day.
    (C) Compare 12ZD and 24ZD to the zero drift limit specifications in 
table B-1 to subpart B of part 53. Both 12ZD and 24ZD must be within 
the specified limits (inclusive) to pass the test for zero drift.
    (ii) Span drift.
    (A) Calculate the span drift (SD) as:
    [GRAPHIC] [TIFF OMITTED] TR31AU11.006
    

or if a span adjustment was made on the previous test day,
[GRAPHIC] [TIFF OMITTED] TR31AU11.007

where
[GRAPHIC] [TIFF OMITTED] TR31AU11.008

n indicates the n-th test day, and i indicates the i-th measurement 
reading on the n-th test day.
    (B) SD must be within the span drift limits (inclusive) specified 
in table B-1 to subpart B of part 53 to pass the test for span drift.
    (iii) Lag time. Determine, from the strip chart (or alternative 
test data record), the elapsed time in minutes between the change in 
test concentration (or mark) made in step (xxvi) and the first 
observable (two times the noise level) measurement response. This time 
must be equal to or less than the lag time limit specified in table B-1 
to subpart B of part 53 to pass the test for lag time.
    (iv) Rise time. Calculate 95 percent of measurement reading 
P9 and determine, from the recorder chart (or alternative 
test data record), the elapsed time between the first observable (two 
times noise level) measurement response and a response equal to 95 
percent of the P9 reading. This time must be equal to or 
less than the rise time limit specified in table B-1 to subpart B of 
part 53 to pass the test for rise time.
    (v) Fall time. Calculate five percent of (P10 - 
L2) and determine, from the strip chart (or alternative test 
record), the elapsed time in minutes between the first observable 
decrease in measurement response following reading P10 and a 
response equal to L2 + five percent of (P10 - 
L2). This time must be equal to or less than the fall time 
limit specification in table B-1 to subpart B of part 53 to pass the 
test for fall time.
    (vi) Precision. Calculate precision (both P20 and 
P80) for each test day as follows:
    (A)
    [GRAPHIC] [TIFF OMITTED] TR31AU11.009
    
    (B)
    [GRAPHIC] [TIFF OMITTED] TR31AU11.010
    
    (C) Both P20 and P80 must be equal to or less 
than the precision limits specified in table B-1 to subpart B of part 
53 to pass the test for precision.

BILLING CODE 6560-50-P

[[Page 54332]]

Figure B-1 to Subpart B of Part 53--Example
[GRAPHIC] [TIFF OMITTED] TR31AU11.011

[[Page 54333]]

[GRAPHIC] [TIFF OMITTED] TR31AU11.012

BILLING CODE 6560-50-C

           Table B-2 to Subpart B of Part 53--Test Atmospheres
------------------------------------------------------------------------
          Test gas                 Generation           Verification
------------------------------------------------------------------------
Ammonia.....................  Permeation device.    Indophenol method,
                               Similar to system     reference 3.
                               described in
                               references 1 and 2.
Carbon dioxide..............  Cylinder of zero air  Use NIST-certified
                               or nitrogen           standards whenever
                               containing CO2 as     possible. If NIST
                               required to obtain    standards are not
                               the concentration     available, obtain 2
                               specified in table    standards from
                               B-3.                  independent sources
                                                     which agree within
                                                     2 percent, or
                                                     obtain one standard
                                                     and submit it to an
                                                     independent
                                                     laboratory for
                                                     analysis, which
                                                     must agree within 2
                                                     percent of the
                                                     supplier's nominal
                                                     analysis.
Carbon monoxide.............  Cylinder of zero air  Use an FRM CO
                               or nitrogen           analyzer as
                               containing CO as      described in
                               required to obtain    reference 8.
                               the concentration
                               specified in table
                               B-3.
Ethane......................  Cylinder of zero air  Gas chromatography,
                               or nitrogen           ASTM D2820,
                               containing ethane     reference 10. Use
                               as required to        NIST-traceable
                               obtain the            gaseous methane or
                               concentration         propane standards
                               specified in table    for calibration.
                               B-3.
Ethylene....................  Cylinder of pre-      Do.
                               purified nitrogen
                               containing ethylene
                               as required to
                               obtain the
                               concentration
                               specified in table
                               B-3.
Hydrogen chloride...........  Cylinder \1\ of pre-  Collect samples in
                               purified nitrogen     bubbler containing
                               containing            distilled water and
                               approximately 100     analyze by the
                               ppm of gaseous HCl.   mercuric
                               Dilute with zero      thiocyanate method,
                               air to                ASTM (D612), p. 29,
                               concentration         reference 4.
                               specified in table
                               B-3.
Hydrogen sulfide............  Permeation device     Tentative method of
                               system described in   analysis for H2S
                               references 1 and 2.   content of the
                                                     atmosphere, p. 426,
                                                     reference 5.
Methane.....................  Cylinder of zero air  Gas chromatography
                               containing methane    ASTM D2820,
                               as required to        reference 10. Use
                               obtain the            NIST-traceable
                               concentration         methane standards
                               specified in table    for calibration.
                               B-3.
Nitric oxide................  Cylinder \1\ of pre-  Gas phase titration
                               purified nitrogen     as described in
                               containing            reference 6,
                               approximately 100     section 7.1.
                               ppm NO. Dilute with
                               zero air to
                               required
                               concentration.
Nitrogen dioxide............  1. Gas phase          1. Use an FRM NO2
                               titration as          analyzer calibrated
                               described in          with a
                               reference 6.          gravimetrically
                              2. Permeation          calibrated
                               device, similar to    permeation device.
                               system described in  2. Use an FRM NO2
                               reference 6.          analyzer calibrated
                                                     by gas-phase
                                                     titration as
                                                     described in
                                                     reference 6.
Ozone.......................  Calibrated ozone      Use an FEM ozone
                               generator as          analyzer calibrated
                               described in          as described in
                               reference 9.          reference 9.

[[Page 54334]]

 
Sulfur dioxide..............  1. Permeation device  Use an SO2 FRM or
                               as described in       FEM analyzer as
                               references 1 and 2.   described in
                              2. Dynamic dilution    reference 7.
                               of a cylinder
                               containing
                               approximately 100
                               ppm SO2 as
                               described in
                               Reference 7.
Water.......................  Pass zero air         Measure relative
                               through distilled     humidity by means
                               water at a fixed      of a dew-point
                               known temperature     indicator,
                               between 20[deg] and   calibrated
                               30 [deg]C such that   electrolytic or
                               the air stream        piezo electric
                               becomes saturated.    hygrometer, or wet/
                               Dilute with zero      dry bulb
                               air to                thermometer.
                               concentration
                               specified in table
                               B-3.
Xylene......................  Cylinder of pre-      Use NIST-certified
                               purified nitrogen     standards whenever
                               containing 100 ppm    possible. If NIST
                               xylene. Dilute with   standards are not
                               zero air to           available, obtain 2
                               concentration         standards from
                               specified in table    independent sources
                               B-3.                  which agree within
                                                     2 percent, or
                                                     obtain one standard
                                                     and submit it to an
                                                     independent
                                                     laboratory for
                                                     analysis, which
                                                     must agree within 2
                                                     percent of the
                                                     supplier's nominal
                                                     analysis.
Zero air....................  1. Ambient air
                               purified by
                               appropriate
                               scrubbers or other
                               devices such that
                               it is free of
                               contaminants likely
                               to cause a
                               detectable response
                               on the analyzer.
                              2. Cylinder of
                               compressed zero air
                               certified by the
                               supplier or an
                               independent
                               laboratory to be
                               free of
                               contaminants likely
                               to cause a
                               detectable response
                               on the analyzer.
------------------------------------------------------------------------
\1\ Use stainless steel pressure regulator dedicated to the pollutant
  measured.
Reference 1. O'Keefe, A. E., and Ortaman, G. C. ``Primary Standards for
  Trace Gas Analysis,'' Anal. Chem. 38, 760 (1966).
Reference 2. Scaringelli, F. P., A. E. . Rosenberg, E*, and Bell, J. P.,
  ``Primary Standards for Trace Gas Analysis.'' Anal. Chem. 42, 871
  (1970).
Reference 3. ``Tentative Method of Analysis for Ammonia in the
  Atmosphere (Indophenol Method)'', Health Lab Sciences, vol. 10, No. 2,
  115-118, April 1973.
Reference 4. 1973 Annual Book of ASTM Standards, American Society for
  Testing and Materials, 1916 Race St., Philadelphia, PA.
Reference 5. Methods for Air Sampling and Analysis, Intersociety
  Committee, 1972, American Public Health Association, 1015.
Reference 6. 40 CFR 50 Appendix F, ``Measurement Principle and
  Calibration Principle for the Measurement of Nitrogen Dioxide in the
  Atmosphere (Gas Phase Chemiluminescence).''
Reference 7. 40 CFR 50 Appendix A-1, ``Measurement Principle and
  Calibration Procedure for the Measurement of Sulfur Dioxide in the
  Atmosphere (Ultraviolet FIuorscence).''
Reference 8. 40 CFR 50 Appendix C, ``Measurement Principle and
  Calibration Procedure for the Measurement of Carbon Monoxide in the
  Atmosphere (Non-Dispersive Infrared Photometry)''.
Reference 9. 40 CFR 50 Appendix D, ``Measurement Principle and
  Calibration Procedure for the Measurement of Ozone in the
  Atmosphere''.
Reference 10. ``Standard Test Method for C, through C5 Hydrocarbons in
  the Atmosphere by Gas Chromatography'', D 2820, 1987 Annual Book of
  Aston Standards, vol 11.03, American Society for Testing and
  Materials, 1916 Race St., Philadelphia, PA 19103.

[[Page 54335]]

                                                     Table B-3 to Subpart B of Part 53--Interferent Test Concentration,\1\ Parts per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                          Hydro-
     Pollutant         Analyzer type      chloric   Ammonia  Hydrogen   Sulfur   Nitrogen   Nitric    Carbon   Ethylene    Ozone    Mxylene    Water    Carbon    Methane   Ethane   Naphthalene
                                           acid               sulfide   dioxide   dioxide    oxide    dioxide                                  vapor   monoxide
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...............  Ultraviolet          ........  ........   \5\ 0.1  \4\ 0.14       0.5       0.5  ........  ........       0.5       0.2    20,000  ........  ........  ........    \6\ 0.05
                     fluorescence.
SO2...............  Flame photometric..  ........  ........      0.01  \4\ 0.14  ........  ........       750  ........  ........  ........       \3\        50  ........  ........  ...........
                                                                                                                                               20,000
SO2...............  Gas chromatography.  ........  ........       0.1  \4\ 0.14  ........  ........       750  ........  ........  ........       \3\        50  ........  ........  ...........
                                                                                                                                               20,000
SO2...............  Spectrophotometric-       0.2       0.1       0.1  \4\ 0.14       0.5  ........       750  ........       0.5  ........  ........  ........  ........  ........  ...........
                     wet chemical
                     (pararosanaline).
SO2...............  Electrochemical....       0.2       0.1       0.1  \4\ 0.14       0.5       0.5  ........       0.2       0.5  ........       \3\  ........  ........  ........  ...........
                                                                                                                                               20,000
SO2...............  Conductivity.......       0.2       0.1  ........  \4\ 0.14       0.5  ........       750  ........  ........  ........  ........  ........  ........  ........  ...........
SO2...............  Spectrophotometric-  ........  ........  ........  \4\ 0.14       0.5  ........  ........  ........       0.5       0.2  ........  ........  ........  ........  ...........
                     gas phase,
                     including DOAS.
O3................  Chemiluminescent...  ........  ........   \3\ 0.1  ........  ........  ........       750  ........  \4\ 0.08  ........       \3\  ........  ........  ........  ...........
                                                                                                                                               20,000
O3................  Electrochemical....  ........   \3\ 0.1  ........       0.5       0.5  ........  ........  ........  \4\ 0.08  ........  ........  ........  ........  ........  ...........
O3................  Spectrophotometric-  ........   \3\ 0.1  ........       0.5       0.5   \3\ 0.5  ........  ........  \4\ 0.08  ........  ........  ........  ........  ........  ...........
                     wet chemical
                     (potassium iodide).
O3................  Spectrophotometric-  ........  ........  ........       0.5       0.5       0.5  ........  ........  \4\ 0.08      0.02    20,000  ........  ........  ........  ...........
                     gas phase,
                     including
                     ultraviolet
                     absorption and
                     DOAS).
CO................  Non-dispersive       ........  ........  ........  ........  ........  ........       750  ........  ........  ........    20,000    \4\ 10  ........  ........  ...........
                     Infrared.
CO................  Gas chromatography   ........  ........  ........  ........  ........  ........  ........  ........  ........  ........    20,000    \4\ 10  ........       0.5  ...........
                     with flame
                     ionization
                     detector.
CO................  Electrochemical....  ........  ........  ........  ........  ........       0.5  ........       0.2  ........  ........    20,000    \4\ 10  ........  ........  ...........
CO................  Catalytic            ........       0.1  ........  ........  ........  ........       750       0.2  ........  ........    20,000    \4\ 10       5.0       0.5  ...........
                     combustion-thermal
                     detection.
CO................  IR fluorescence....  ........  ........  ........  ........  ........  ........       750  ........  ........  ........    20,000    \4\ 10  ........       0.5  ...........
CO................  Mercury replacement- ........  ........  ........  ........  ........  ........  ........       0.2  ........  ........  ........    \4\ 10  ........       0.5  ...........
                     UV photometric.
NO2...............  Chemiluminescent...  ........   \3\ 0.1  ........       0.5   \4\ 0.1       0.5  ........  ........  ........  ........    20,000  ........  ........  ........  ...........
NO2...............  Spectrophotometric-  ........  ........  ........       0.5   \4\ 0.1       0.5       750  ........       0.5  ........  ........  ........  ........  ........  ...........
                     wet chemical (azo-
                     dye reaction).
NO2...............  Electrochemical....       0.2   \3\ 0.1  ........       0.5   \4\ 0.1       0.5       750  ........       0.5  ........    20,000        50  ........  ........  ...........
NO2...............  Spectrophotometric-  ........   \3\ 0.1  ........       0.5   \4\ 0.1       0.5  ........  ........       0.5  ........    20,000        50  ........  ........  ...........
                     gas phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
\2\ Analyzer types not listed will be considered by the Administrator as special cases.
\3\ Do not mix with the pollutant.
\4\ Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
\5\ If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
\6\ If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate
  response for interference.

[[Page 54336]]

              Table B-4 to Subpart B of Part 53--Line Voltage and Room Temperature Test Conditions
----------------------------------------------------------------------------------------------------------------
                                                                   Room
                 Test day                        Line         temperature,\2\               Comments
                                            voltage,\1\ rms       [deg]C
----------------------------------------------------------------------------------------------------------------
0........................................               115                25  Initial set-up and adjustments.
1........................................               125                20  .................................
2........................................               105                20  .................................
3........................................               125                30  Adjustments and/or periodic
                                                                                maintenance permitted at end of
                                                                                tests.
4........................................               105                30  .................................
5........................................               125                20  .................................
6........................................               105                20  Adjustments and/or periodic
                                                                                maintenance permitted at end of
                                                                                tests.
7........................................               125                30  Examine test results to ascertain
                                                                                if further testing is required.
8........................................               105                30  .................................
9........................................               125                20  Adjustments and/or periodic
                                                                                maintenance permitted at end of
                                                                                tests.
10.......................................               105                20  .................................
11.......................................               125                30  .................................
12.......................................               105                30  Adjustments and/or periodic
                                                                                maintenance permitted at end of
                                                                                tests.
13.......................................               125                20  .................................
14.......................................               105                20  .................................
15.......................................               125                30  .................................
----------------------------------------------------------------------------------------------------------------
\1\ Voltage specified shall be controlled to  1 volt.
\2\ Temperatures shall be controlled to  1 [deg]C.

Table B-5 to Subpart B of Part 53--Symbols and Abbreviations

BL--Analyzer reading at the specified LDL test concentration for the 
LDL test.
BZ--analyzer reading at 0 concentration for the LDL test.
DM--Digital meter.
Cmax--Maximum analyzer reading during the 12ZD test period.
Cmin--Minimum analyzer reading during the 12ZD test period.
i--Subscript indicating the i-th quantity in a series.
IE--Interference equivalent.
L1--First analyzer zero reading for the 24ZD test.
L2--Second analyzer zero reading for the 24ZD test.
n--Subscript indicating the test day number.
P--Analyzer reading for the span drift and precision tests.
Pi--The i-th analyzer reading for the span drift and precision 
tests.
P20--Precision at 20 percent of URL.
P80--Precision at 80 percent of URL.
ppb--Parts per billion of pollutant gas (usually in air), by volume.
ppm--Parts per million of pollutant gas (usually in air), by volume.
R--Analyzer reading of pollutant alone for the IE test.
R1--Analyzer reading with interferent added for the IE 
test.
ri--the i-th analyzer or DM reading for the noise test.
S--Standard deviation of the noise test readings.
S0--Noise value (S) measured at 0 concentration.
S80--Noise value (S) measured at 80 percent of the URL.
Sn--Average of P7 . . . P12 for the n-th test 
day of the SD test.
S'n--Adjusted span reading on the n-th test day.
SD--Span drift
URL--Upper range limit of the analyzer's measurement range.
Z--Average of L1 and L2 readings for the 24ZD 
test.
Zn--Average of L1 and L2 readings on the n-th 
test day for the 24ZD test.
Z'n--Adjusted analyzer zero reading on the n-the test day for the 
24ZD test.
ZD--Zero drift.
12ZD--12-hour zero drift.
24ZD--24-hour zero drift.

Appendix A to Subpart B of Part 53--Optional Forms for Reporting Test 
Results

BILLING CODE 6560-50-P

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BILLING CODE 6560-50-C

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

0
5. The authority citation for part 58 continues to read as follows:

    Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

Subpart B--[Amended]

0
6. Section 58.10, is amended by adding paragraph (a)(7) to read as 
follows:

Sec.  58.10  Annual monitoring network plan and periodic network 
assessment.

    (a) * * *
    (7) A plan for establishing CO monitoring sites in accordance with 
the requirements of appendix D to this part shall be submitted to the 
EPA Regional Administrator. Plans for required CO monitors shall be 
submitted at least six months prior to the date such monitors must be 
established as required by section 58.13.
* * * * *

0
7. Section 58.13 is amended by adding paragraph (e) to read as follows:

Sec.  58.13  Monitoring network completion.

* * * * *
    (e) The CO monitors required under Appendix D, section 4.2 of this 
part must be physically established and operating under all of the 
requirements of this part, including the requirements of appendices A, 
C, D, and E to this part, no later than:
    (1) January 1, 2015 for CO monitors in CBSAs having 2.5 million 
persons or more; or
    (2) January 1, 2017 for other CO monitors.

[[Page 54342]]

0
8. Appendix D to Part 58 is amended by revising section 4.2 to read as 
follows:

Appendix D to Part 58--Network Design Criteria for Ambient Air Quality 
Monitoring

* * * * *

4.2 Carbon Monoxide (CO) Design Criteria

    4.2.1 General Requirements. (a) Except as provided in subsection 
(b), one CO monitor is required to operate collocated with one 
required near-road NO2 monitor, as required in Section 
4.3.2 of this part, in CBSAs having a population of 1,000,000 or 
more persons. If a CBSA has more than one required near-road 
NO2 monitor, only one CO monitor is required to be 
collocated with a near-road NO2 monitor within that CBSA.
    (b) If a state provides quantitative evidence demonstrating that 
peak ambient CO concentrations would occur in a near-road location 
which meets microscale siting criteria in Appendix E of this part 
but is not a near-road NO2 monitoring site, then the EPA 
Regional Administrator may approve a request by a state to use such 
an alternate near-road location for a CO monitor in place of 
collocating a monitor at near-road NO2 monitoring site.
    4.2.2 Regional Administrator Required Monitoring. (a) The 
Regional Administrators, in collaboration with states, may require 
additional CO monitors above the minimum number of monitors required 
in 4.2.1 of this part, where the minimum monitoring requirements are 
not sufficient to meet monitoring objectives. The Regional 
Administrator may require, at his/her discretion, additional 
monitors in situations where data or other information suggest that 
CO concentrations may be approaching or exceeding the NAAQS. Such 
situations include, but are not limited to, (1) characterizing 
impacts on ground-level concentrations due to stationary CO sources, 
(2) characterizing CO concentrations in downtown areas or urban 
street canyons, and (3) characterizing CO concentrations in areas 
that are subject to high ground level CO concentrations particularly 
due to or enhanced by topographical and meteorological impacts. The 
Regional Administrator and the responsible State or local air 
monitoring agency shall work together to design and maintain the 
most appropriate CO network to address the data needs for an area, 
and include all monitors under this provision in the annual 
monitoring network plan.
    4.2.3 CO Monitoring Spatial Scales. (a) Microscale and middle 
scale measurements are the most useful site classifications for CO 
monitoring sites since most people have the potential for exposure 
on these scales. Carbon monoxide maxima occur primarily in areas 
near major roadways and intersections with high traffic density and 
often in areas with poor atmospheric ventilation.
    (1) Microscale--Microscale measurements typically represent 
areas in close proximity to major roadways, within street canyons, 
over sidewalks, and in some cases, point and area sources. Emissions 
on roadways result in high ground level CO concentrations at the 
microscale, where concentration gradients generally exhibit a marked 
decrease with increasing downwind distance from major roads, or 
within downtown areas including urban street canyons. Emissions from 
stationary point and area sources, and non-road sources may, under 
certain plume conditions, result in high ground level concentrations 
at the microscale.
    (2) Middle scale--Middle scale measurements are intended to 
represent areas with dimensions from 100 meters to 0.5 kilometer. In 
certain cases, middle scale measurements may apply to areas that 
have a total length of several kilometers, such as ``line'' emission 
source areas. This type of emission sources areas would include air 
quality along a commercially developed street or shopping plaza, 
freeway corridors, parking lots and feeder streets.
    (3) Neighborhood scale--Neighborhood scale measurements are 
intended to represent areas with dimensions from 0.5 kilometers to 4 
kilometers. Measurements of CO in this category would represent 
conditions throughout some reasonably urban sub-regions. In some 
cases, neighborhood scale data may represent not only the immediate 
neighborhood spatial area, but also other similar such areas across 
the larger urban area. Neighborhood scale measurements provide 
relative area-wide concentration data which are useful for providing 
relative urban background concentrations, supporting health and 
scientific research, and for use in modeling.
* * * * *

0
9. Appendix E to Part 58 is amended by revising sections 2 and 6.2(a), 
6.2(b), 6.2(c), and Table E-4 to read as follows:

Appendix E to Part 58--Probe and Monitoring Path Siting Criteria for 
Ambient Air Quality Monitoring

* * * * *

2. Horizontal and Vertical Placement

    The probe or at least 80 percent of the monitoring path must be 
located between 2 and 15 meters above ground level for all 
O3 and SO2 monitoring sites, and for 
neighborhood or larger spatial scale Pb, PM10, 
PM10-2.5, PM2.5, NO2, and CO sites. 
Middle scale PM10-2.5 sites are required to have sampler 
inlets between 2 and 7 meters above ground level. Microscale Pb, 
PM10, PM10-2.5, and PM2.5 sites are 
required to have sampler inlets between 2 and 7 meters above ground 
level. Microscale near-road NO2 monitoring sites are 
required to have sampler inlets between 2 and 7 meters above ground 
level. The inlet probes for microscale carbon monoxide monitors that 
are being used to measure concentrations near roadways must be 
between 2 and 7 meters above ground level. Those inlet probes for 
microscale carbon monoxide monitors measuring concentrations near 
roadways in downtown areas or urban street canyons must be between 
2.5 and 3.5 meters above ground level. The probe or at least 90 
percent of the monitoring path must be at least 1 meter vertically 
or horizontally away from any supporting structure, walls, parapets, 
penthouses, etc., and away from dusty or dirty areas. If the probe 
or a significant portion of the monitoring path is located near the 
side of a building or wall, then it should be located on the 
windward side of the building relative to the prevailing wind 
direction during the season of highest concentration potential for 
the pollutant being measured.
* * * * *
    6. * * *
    6.2 Spacing for Carbon Monoxide Probes and Monitoring Paths. (a) 
Near-road microscale CO monitoring sites, including those located in 
downtown areas, urban street canyons, and other near-road locations 
such as those adjacent to highly trafficked roads, are intended to 
provide a measurement of the influence of the immediate source on 
the pollution exposure on the adjacent area.
    (b) Microscale CO monitor inlets probes in downtown areas or 
urban street canyon locations shall be located a minimum distance of 
2 meters and a maximum distance of 10 meters from the edge of the 
nearest traffic lane.
    (c) Microscale CO monitor inlet probes in downtown areas or 
urban street canyon locations shall be located at least 10 meters 
from an intersection and preferably at a midblock location. Midblock 
locations are preferable to intersection locations because 
intersections represent a much smaller portion of downtown space 
than do the streets between them. Pedestrian exposure is probably 
also greater in street canyon/corridors than at intersections.
    (d) Microscale CO monitor inlet probes in the near-road 
environment, outside of downtown areas or urban street canyons, 
shall be as near as practicable to the outside nearest edge of the 
traffic lanes of the target road segment; but shall not be located 
at a distance greater than 50 meters, in the horizontal, from the 
outside nearest edge of the traffic lanes of the target road 
segment.
    (e) In determining the minimum separation between a neighborhood 
scale monitoring site and a specific roadway, the presumption is 
made that measurements should not be substantially influenced by any 
one roadway. Computations were made to determine the separation 
distance, and Table E-2 of this appendix provides the required 
minimum separation distance between roadways and a probe or 90 
percent of a monitoring path. Probes or monitoring paths that are 
located closer to roads than this criterion allows should not be 
classified as neighborhood scale, since the measurements from such a 
site would closely represent the middle scale. Therefore, sites not 
meeting this criterion should be classified as middle scale.
* * * * *

[[Page 54343]]

                                Table E-4 of Appendix E to Part 58--Summary of Probe and Monitoring Path Siting Criteria
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                         Horizontal and
                                                                                       vertical distance
                                         Scale (maximum       Height from ground to     from supporting      Distance from trees       Distance from
             Pollutant                   monitoring path     probe, inlet or 80% of    structures \2\ to      to probe, inlet or     roadways to probe,
                                       length, meters) \1\     monitoring path \1\    probe, inlet or 90%     90% of monitoring     inlet or monitoring
                                                                                       of monitoring path     path \1\ (meters)      path \1\ (meters)
                                                                                          \1\ (meters)
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2 \3\ \4\ \5\ \6\................  Middle (300 m)          2-15..................  > 1..................  > 10.................  N/A.
                                      Neighborhood Urban,
                                      and Regional (1 km).
CO \4\ \5\ \7\.....................  Micro [downtown or      2.5-3.5; 2-7; 2-15....  > 1..................  > 10.................  2-10 for downtown
                                      street canyon sites],                                                                         areas or street
                                      micro [near-road                                                                              canyon microscale;
                                      sites], middle (300                                                                           50 for near-road
                                      m) and Neighborhood                                                                           microscale; see
                                      (1 km).                                                                                       Table E-2 of this
                                                                                                                                    appendix for middle
                                                                                                                                    and neighborhood
                                                                                                                                    scales.
O3 \3\ \4\ \5\.....................  Middle (300 m)          2-15..................  > 1..................  > 10.................  See Table E-1 of this
                                      Neighborhood, Urban,                                                                          appendix for all
                                      and Regional (1 km).                                                                          scales.
NO2 \3\ \4\ \5\....................  Micro (Near-road [50-   2-7 (micro); 2-15 (all  > 1..................  > 10.................  50 meters for near-
                                      300]) Middle (300m)     other scales).                                                        road microscale;
                                      Neighborhood, Urban,                                                                         See Table E-1 of this
                                      and Regional (1 km).                                                                          appendix for all
                                                                                                                                    other scales.
Ozone precursors (for PAMS) \3\ \4\  Neighborhood and Urban  2-15..................  > 1..................  > 10.................  See Table E-4 of this
 \5\.                                 (1 km).                                                                                       appendix for all
                                                                                                                                    scales.
PM, Pb 3 4 5 6 8...................  Micro: Middle,          2-7 (micro); 2-7        > 2 (all scales,       > 10 (all scales)....  2-10 (micro); see
                                      Neighborhood, Urban     (middle PM10	2.5); 2-   horizontal distance                           Figure E-1 of this
                                      and Regional.           15 (all other scales).  only).                                        appendix for all
                                                                                                                                    other scales.
--------------------------------------------------------------------------------------------------------------------------------------------------------
N/A--Not applicable.
\1\ Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and regional
  scale NO2 monitoring, and all applicable scales for monitoring SO2,O3, and O3 precursors.
\2\ When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.
\3\ Should be > 20 meters from the drip-line of tree(s) and must be 10 meters from the drip-line when the tree(s) act as an obstruction.
\4\ Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes
  above the sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text).
\5\ Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall.
\6\ The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration flues. The separation distance is
  dependent on the height of the minor source's emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur,
  ash, or lead content). This criterion is designed to avoid undue influences from minor sources.
\7\ For microscale CO monitoring sites in downtown areas or street canyons (not at near-road NO2 monitoring sites), the probe must be > 10 meters from a
  street intersection and preferably at a midblock location.
\8\ Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 meter
  apart for samplers having flow rates less than 200 liters/min to preclude airflow interference.

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[FR Doc. 2011-21359 Filed 8-30-11; 8:45 am]
BILLING CODE 6560-50-P
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