Document ID: EPA-HQ-OAR-2008-0015-0122
Agency: epa
Document Type: Proposed Rule
Title: National Ambient Air Quality Standards for Carbon Monoxide
Posted Date: 2011-02-11T05:00Z

[Federal Register Volume 76, Number 29 (Friday, February 11, 2011)]
[Proposed Rules]
[Pages 8158-8220]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-2404]

[[Page 8157]]

Vol. 76

Friday,

No. 29

February 11, 2011

Part VI

Environmental Protection Agency

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40 CFR Parts 50, 53 and 58

National Ambient Air Quality Standards for Carbon Monoxide; Proposed 
Rule

  Federal Register / Vol. 76, No. 29 / Friday, February 11, 2011 / 
Proposed Rules  

[[Page 8158]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53 and 58

[EPA-HQ-OAR-2008-0015; FRL-9261-4; 2060-AI43]

National Ambient Air Quality Standards for Carbon Monoxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: Based on its review of the air quality criteria and the 
national ambient air quality standards (NAAQS) for carbon monoxide 
(CO), EPA is proposing to retain the current standards. EPA is also 
proposing changes to the ambient air monitoring requirements for CO 
including those related to network design.

DATES: Comments must be received on or before April 12, 2011.
    Public Hearings: If, by February 18, 2011, EPA receives a request 
from a member of the public to speak at a public hearing concerning the 
proposed regulation, we will hold a public hearing on February 28, 2011 
in Arlington, Virginia.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2008-0015 by one of the following methods:
     http://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     E-mail: a-and-r-Docket@epa.gov.
     Fax: 202-566-9744.
     Mail: Docket No. EPA-HQ-OAR-2008-0015, Environmental 
Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW., 
Washington, DC 20460. Please include a total of two copies.
     Hand Delivery: Docket No. EPA-HQ-OAR-2008-0015, 
Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution 
Ave., NW., Washington, DC. Such deliveries are only accepted during the 
Docket's normal hours of operation, and special arrangements should be 
made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2008-0015. EPA's policy is that all comments received will be included 
in the public docket without change and may be made available online at 
http://www.regulations.gov, including any personal information 
provided, unless the comment includes information claimed to be 
Confidential Business Information (CBI) or other information whose 
disclosure is restricted by statute. Do not submit information that you 
consider to be CBI or otherwise protected through http://www.regulations.gov or e-mail. The http://www.regulations.gov Web site 
is an ``anonymous access'' system, which means EPA will not know your 
identity or contact information unless you provide it in the body of 
your comment. If you send an e-mail comment directly to EPA without 
going through www.regulations.gov your e-mail address will be 
automatically captured and included as part of the comment that is 
placed in the public docket and made available on the Internet. If you 
submit an electronic comment, EPA recommends that you include your name 
and other contact information in the body of your comment and with any 
disk or CD-ROM you submit. If EPA cannot read your comment due to 
technical difficulties and cannot contact you for clarification, EPA 
may not be able to consider your comment. Electronic files should avoid 
the use of special characters, any form of encryption, and be free of 
any defects or viruses. For additional information about EPA's public 
docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
    Public Hearing. If a public hearing is held, it will be held at the 
U.S. Environmental Protection Agency Conference Center, First Floor 
Conference Center South, One Potomac Yard, 2777 S. Crystal Drive, 
Arlington, VA 22202. All visitors will need to go through security and 
present a valid photo identification, such as a driver's license. To 
request a public hearing or information pertaining to a public hearing, 
contact Ms. Jan King, Health and Environmental Impacts Division, Office 
of Air Quality Planning and Standards (C504-02), Environmental 
Protection Agency, Research Triangle Park, North Carolina 27711; 
telephone number (919) 541- 5665; fax number (919) 541-2664; e-mail 
address: king.jan@epa.gov. See the SUPPLEMENTARY INFORMATION for 
further information about a possible public hearing.
    Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some 
information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in http://www.regulations.gov or in hard copy at the Air and Radiation 
Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 
Constitution Ave., NW., Washington, DC. The Public Reading Room is open 
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal 
holidays. The telephone number for the Public Reading Room is (202) 
566-1744 and the telephone number for the Air and Radiation Docket and 
Information Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Deirdre Murphy, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail code C504-06, 
Research Triangle Park, NC 27711; telephone: 919-541-0729; fax: 919-
541-0237; e-mail: 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, U.S. Environmental Protection Agency, Mail code 
C304-06, Research Triangle Park, NC 27711; telephone: 919-541-5522; 
fax: 919-541-1903; e-mail: watkins.nealson@epa.gov. To request a public 
hearing or information pertaining to a public hearing, contact Ms. Jan 
King, Health and Environmental Impacts Division, Office of Air Quality 
Planning and Standards (C504-02), Environmental Protection Agency, 
Research Triangle Park, North Carolina 27711; telephone number (919) 
541- 5665; fax number (919) 541-2664; e-mail address: king.jan@epa.gov.

SUPPLEMENTARY INFORMATION:

General Information

What should I consider as I prepare my comments for EPA?

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

[[Page 8159]]

     Identify the rulemaking by docket number and other 
identifying information (subject heading, Federal Register date and 
page number).
     Follow directions--the agency may ask you to respond to 
specific questions or organize comments by referencing a Code of 
Federal Regulations (CFR) part or section number.
     Explain why you agree or disagree, suggest alternatives, 
and substitute language for your requested changes.
     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of the documents that are relevant to this rulemaking are 
available through EPA's Office of Air Quality Planning and Standards 
(OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/co/s_co_index.html. These documents 
include the Plan for Review of the National Ambient Air Quality 
Standards for Carbon Monoxide (Integrated Review Plan or IRP, USEPA, 
2008), available at http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pd.html, the Integrated Science Assessment for Carbon Monoxide 
(USEPA, 2010a), available at http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_isa.html, the Quantitative Risk and Exposure Assessment for 
Carbon Monoxide--Amended (USEPA, 2010b), available at http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_rea.html, and the Policy 
Assessment for the Review of the Carbon Monoxide National Ambient Air 
Quality Standards (USEPA, 2010c), available at http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pa.html. These and other related 
documents are also available for inspection and copying in the EPA 
docket identified above.

How can I find information about a possible public hearing?

    To request a public hearing or information pertaining to a public 
hearing on this document, contact Ms. Jan King, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards (C504-02), Environmental Protection Agency, Research Triangle 
Park, North Carolina 27711; telephone number (919) 541- 5665; fax 
number (919) 541-2664; e-mail address: king.jan@epa.gov. If a request 
for a public hearing is received by February 18, 2011, information 
about the hearing will be posted prior to the hearing on EPA's Web site 
for carbon monoxide regulatory actions at http://www.epa.gov/airquality/urbanair/co/.

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
II. Rationale for Proposed Decisions on the Primary Standards
    A. Air Quality Information
    1. Anthropogenic Sources and Emissions of Carbon Monoxide
    2. Ambient Concentrations
    B. Health Effects Information
    1. Carboxyhemoglobin as Biomarker and Mechanism of Toxicity
    2. Nature of Effects
    3. At-Risk Populations
    4. Potential Impacts on Public Health
    C. Human Exposure and Dose Assessment
    1. Summary of Design Aspects
    2. Key Limitations and Uncertainties
    D. Conclusions on Adequacy of the Current Standards
    1. Approach
    2. Evidence-Based and Exposure/Dose-Based Considerations in the 
Policy Assessment
    3. CASAC Advice
    4. Administrator's Proposed Conclusions Concerning Adequacy
    E. Summary of Proposed Decisions on Primary Standards
III. Consideration of a Secondary Standard
    A. Background and Considerations in Previous Reviews
    B. Evidence-Based Considerations in the Policy Assessment
    C. CASAC Advice
    D. Administrator's Proposed Conclusions Concerning a Secondary 
Standard
IV. Proposed Amendments to Ambient Monitoring Requirements
    A. Monitoring Methods
    1. Proposed Changes to Part 50, Appendix C
    2. Proposed Changes to Part 53
    3. Implications for Air Monitoring Networks
    B. Network Design
    1. Background
    2. On-Road Mobile Sources
    3. Near-Road Environment
    4. Urban Downtown Areas and Urban Street Canyons
    5. Meteorological and Topographical Influences
    6. Proposed Changes
    7. Microscale Carbon Monoxide Monitor Siting Criteria
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and 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 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 ``air pollutant[s]'' that in her 
``judgment, cause or contribute to air pollution which may reasonably 
be anticipated to endanger public health or welfare'' and satisfy two 
other criteria, including ``whose presence * * * in the ambient air 
results from numerous or diverse mobile or stationary sources'' and to 
issue air quality criteria for those that are listed. Air quality 
criteria are 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 * * * .''
    Section 109 (42 U.S.C. 7409) directs the Administrator to propose 
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for 
which air 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

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associated with the presence of such air 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 include an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (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). Both kinds of uncertainties are components of the 
risk associated with pollution at levels below those at which human 
health effects can be said to occur with reasonable scientific 
certainty. Thus, in selecting primary standards that include an 
adequate margin of safety, the Administrator is seeking not only to 
prevent pollution levels that have been demonstrated to be harmful but 
also to prevent lower pollution levels that may pose an unacceptable 
risk of harm, even if the risk is not precisely identified as to nature 
or degree. The CAA does not require the Administrator to establish a 
primary NAAQS at a zero-risk level or at background concentration 
levels, see Lead Industries Association v. EPA, 647 F.2d at 1156 n. 51, 
but rather at a level that reduces risk sufficiently so as to protect 
public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, EPA 
considers such factors as the nature and severity of the health effects 
involved, the size of the population(s) at risk, and the kind and 
degree of the uncertainties that must be addressed. The selection of 
any particular approach to providing an adequate margin of safety is a 
policy choice left specifically to the Administrator's judgment. Lead 
Industries Association v. EPA, 647 F.2d at 1161-62; Whitman v. American 
Trucking Associations, 531 U.S. 457, 495 (2001).
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. Whitman v. American Trucking 
Associations, 531 U.S. 457, 473. In establishing ``requisite'' primary 
and secondary standards, EPA may not consider the costs of implementing 
the standards. Id. at 471.
    Section 109(d)(1) of the CAA requires that ``[n]ot later than 
December 31, 1980, and at 5-year intervals thereafter, the 
Administrator shall complete a thorough review of the criteria 
published under section 108 and the national ambient air quality 
standards * * * 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. * * 
*'' This independent review function is performed by the Clean Air 
Scientific Advisory Committee (CASAC).

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; new 
source performance standards under section 111; and title IV of the Act 
(CAA sections 402-416), which specifically provides for major 
reductions in CO emissions.

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 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 standard 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 the final 2000 
AQCD (U.S. EPA, 2000) was released in August 2000. After release of the 
AQCD, Congress requested that the National Research Council (NRC) 
review the

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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. A workshop was held on January 28-29, 
2008 (73 FR 2490) to discuss policy-relevant scientific and technical 
information to inform EPA's planning for the CO NAAQS review. 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 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 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, notices of proposed 
and final rulemaking concerning its review of the CO NAAQS no later 
than January 28, 2011 and August 12, 2011, respectively.
    This action presents the Administrator's proposed decisions on the 
current CO standards. Throughout this preamble a number of conclusions, 
findings, and determinations proposed by the Administrator are noted. 
Although they identify the reasoning that supports this proposal, they 
are not intended to be final or conclusive in nature. The EPA invites 
general, specific, and technical comments on all issues involved with 
this proposal, including all such proposed judgments, conclusions, 
findings, and determinations.

II. Rationale for Proposed Decisions on the Primary Standards

    This section presents the rationale for the Administrator's 
proposed decision to retain the existing CO primary standards.\3\ As 
discussed more fully below, this rationale is based on a thorough 
review, in the Integrated Science Assessment, of the latest scientific 
information, published through mid-2009, on human health effects 
associated with the presence of CO in the ambient air. This proposal 
also takes into account: (1) Staff assessments of the most policy-
relevant information in the ISA and staff analyses of air quality, 
human exposure and health risks presented in the REA and the Policy 
Assessment, upon which staff conclusions regarding appropriate 
considerations in this review are based; (2) CASAC advice and 
recommendations, as reflected in discussions of drafts of the ISA, REA 
and PA at public meetings, in separate written comments, and in CASAC's 
letters to the Administrator; and (3) public comments received during 
the development of these documents, either in connection with CASAC 
meetings or separately.
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    \3\ As explained below in section IV.A, EPA is proposing to 
repromulgate the Federal reference method for CO, as set forth in 
Appendix C of 40 CFR part 50. Consistent with EPA's proposed 
decision to retain the standards, the recodification clarifies and 
updates the text of the FRM, but does not make substantive changes 
to it.
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    In presenting the rationale and its foundations, this section 
begins with a summary of current air quality information in section 
II.A. Section II.B summarizes the body of evidence supporting this 
rationale, including key health endpoints associated with exposure to 
ambient CO. This rationale also draws upon the results of the 
quantitative exposure and risk assessments, discussed below in section 
II.C. Evidence- and exposure/dose-based considerations that form the 
basis for the Administrator's proposed decisions on the adequacy of the 
current standard are discussed in section II.D.2.a and II.D.2.b, 
respectively. CASAC advice is summarized in section II.D.3. The 
Administrator's proposed conclusions are presented in section II.D.4.

A. Air Quality Information

    This section provides a general overview of the current air quality 
conditions to provide context for this consideration of the current 
standards for carbon monoxide. A more comprehensive discussion of air 
quality information is provided in the ISA (ISA, sections 3.2 and 3.4) 
and summarized in the Policy Assessment, and a more detailed discussion 
of aspects particularly relevant to the exposure assessment is provided 
in the REA (REA, chapter 3).

[[Page 8162]]

1. Anthropogenic Sources and Emissions of Carbon Monoxide
    Carbon monoxide in ambient air is formed primarily by the 
incomplete combustion of carbon-containing fuels and by photochemical 
reactions in the atmosphere. As a result of the combustion 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. 
Therefore, higher and more varying CO formation results from the 
operation of these mobile sources (ISA, section 3.2). As with previous 
reviews of the CO NAAQS, mobile sources continue to be a significant 
source sector for CO in ambient air, as indicated by national emissions 
estimates from on-road vehicles, which accounted for approximately half 
of the total CO emissions by individual source sectors in 2002 (ISA, 
Figure 3-1).\4\ National-scale anthropogenic CO emissions have 
decreased by approximately 45% between 1990 and 2005, with nearly all 
of this national-scale reduction coming from reductions in on-road 
vehicle emissions (ISA, Figure 3-2; PA, Figure 1-1; 2005 NEI \5\). The 
role of mobile source emissions is evident in the spatial and temporal 
patterns of ambient CO concentrations, which are heavily influenced by 
the patterns associated with mobile source emissions (ISA, chapter 3). 
In some metropolitan areas of the U.S., due to their greater motor 
vehicle density relative to rural areas, on-road mobile source 
contribution to all ambient CO emissions was estimated to be as high as 
approximately 75%, based on the 2002 National Emissions Inventory (ISA, 
p. 3-2). However, the mobile source contribution can vary widely in 
specific areas. As an example, 2002 NEI estimates of on-road mobile 
source emissions in urban Denver County, Colorado are about 74% of 
total CO emissions and emissions from all mobile sources (on-road and 
non-road combined) are estimated to contribute about 98% (ISA, section 
3.2.1). In contrast, 2002 NEI estimates of on-road CO emissions were 
just 20% of the total for rural Garfield County, Colorado\6\ (ISA, 
chapter 3, Figure 3-6).
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    \4\ EPA compiles CO emissions estimates for the U.S. in the 
National Emissions Inventory (NEI). Estimates come from various 
sources and different data sources use different data collection 
methods, most of which are based on engineering calculations and 
estimates rather than measurements. Although these estimates are 
generated using well-established approaches, uncertainties are 
inherent in the emission factors and models used to represent 
sources for which emissions have not been directly measured. 
Uncertainties vary by source category, season and region (ISA, 
section 3.2.1). At the time of the ISA development, the 2002 NEI was 
providing the most recent publicly available CO emissions estimates 
for the U.S. that meet EPA's data quality assurance objectives. Such 
estimates are now available from the 2005 NEI.
    \5\ The emissions trends information in this statement is drawn 
from recently available 2005 National Emissions Inventory estimates 
(http://www.epa.gov/ttn/chief/net/2005inventory.html, Tier 
Summaries) and 1990 and other estimates, available at http://www.epa.gov/ttn/chief/net/critsummary.html Figure 3-2 from the ISA 
provides estimates through 2002.
    \6\ The 2002 National Emissions Inventory estimate for on-road 
emissions in Garfield County is 20,000 tons, and the total emissions 
from all sources is estimated to be 98,831 (99K) tons. Thus, in this 
example the on-road vehicles accounts for 20.2% of the total 
emissions (ISA, section 3, figure 3-6). In contrast, the 2002 Denver 
County on-road emissions account for 74% of the total for the county 
which is estimated at approximately 180,000 tons.
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2. Ambient Concentrations
    As described in section II.A.1 above, mobile source emissions are 
major contributors to CO emissions in urban areas, with corresponding 
influence on ambient CO concentrations and associated concentration 
gradients, with highest ambient concentrations occurring on or nearest 
roadways, particularly highly travelled roadways, and lowest 
concentrations in more distant locations (ISA, section 3.5.1.3; REA, 
section 3.1.3). For example, as described in the ISA CO concentrations 
measured within 20 meters of an interstate highway can range from 2 to 
10 times greater than CO concentrations measured as far as 300 meters 
from a major road, possibly influenced by wind direction 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). Additionally, the role of motor 
vehicles in influencing ambient concentrations contributes to the 
occurrence of diurnal variation in concentrations reflecting rush hour 
patterns (ISA, 3.5.2.2; REA, p. 3-8). The influence of motor vehicle 
emissions on ambient concentrations contributes to the important role 
of in-vehicle microenvironments in influencing short-term ambient CO 
exposures, as described in more detail in the REA and summarized in 
sections II.C.1 and II.D.2 below.
    In 2009, approximately 350 ambient monitoring stations across the 
U.S. reported continuous hourly averages of CO concentrations to EPA's 
Air Quality System.\7\ For the most recent period for which air quality 
status relative to the CO NAAQS has been analyzed (2009), all areas of 
the U.S. meet both CO NAAQS.\8\ As of September 27, 2010, there are no 
areas designated as nonattainment for the CO NAAQS (75 FR 59090). Since 
2005, one area (Jefferson County, Alabama) has failed to meet the 8-
hour standard during some periods. Large CO emissions sources in this 
area are associated with an integrated iron and steel facility. As 
described in section 1.3.3 of the Policy Assessment, 2009 
concentrations of CO at most currently operating monitors are well 
below the current standards, with just a few locations having 
concentrations near the controlling 8-hour standard of 9 ppm as a 
second maximum 8-hour average.\9\ Of the counties with monitoring sites 
in 2009, sites in 3 counties reported second maximum 8-hour average 
concentrations at or above 6.4 ppm (PA, Figure 1-2).
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    \7\ http://www.epa.gov/ttn/airs/airsaqs/.
    \8\ The air quality status in areas monitored relative to the CO 
NAAQS is provided at http://www.epa.gov/air/airtrends/values.html.
    \9\ As the form of the CO 8-hour standard is not-to-be-exceeded 
more than once per year, the second highest 8-hour average in a year 
is the design value for this standard. Based on the current rounding 
convention, the standard is met if the CO concentrations over a year 
result in a design value at or below 9.4 ppm. Additional information 
is available at http://www.epa.gov/airtrends/values.html.
---------------------------------------------------------------------------

    The current levels of ambient CO across the U.S. reflect the steady 
declines in ambient concentrations that have occurred over the past 
several years. Both the second highest 1-hour and 8-hour concentrations 
have significantly declined since the last review. At the set of sites 
across the U.S. that have been continuously monitored since 1990 the 
average second highest 8-hour and 1-hour concentrations have declined 
by nearly 70% (PA, section 1.3.3).

B. Health Effects Information

1. Carboxyhemoglobin as Biomarker and Mechanism of Toxicity
    As discussed in the Integrated Science Assessment, in this review, 
as in the past (e.g., USEPA, 2000; USEPA, 1991), the best characterized 
mechanism of action of CO is tissue hypoxia caused by binding of CO to 
hemoglobin to form carboxyhemoglobin (COHb). Accordingly, COHb level in 
blood continues to be well recognized as an important internal dose 
metric and the one most commonly used in evaluating CO exposure and the 
potential for health effects (ISA, p. 2-4, sections 4.1, 4.2, 5.1.1; 
1991 AQCD, 2000 AQCD, 2010 ISA).
    Increasing levels of COHb with subsequent decrease in oxygen 
availability for organs and tissues are of

[[Page 8163]]

concern in people with pre-existing heart disease who have compromised 
compensatory mechanisms (e.g., lack of capacity to increase blood flow 
in response to increased CO). The integrative review of health effects 
of CO indicates that ``the clearest evidence indicates that individuals 
with [coronary artery disease] are most susceptible to an increase in 
CO-induced health effects'' (ISA, section 5.7.8) and the evidence 
continues to support levels of COHb in the blood as the most useful 
indicator of CO exposure that is related to the health effects of CO of 
major concern.
    Carboxyhemoglobin occurs in the blood due to endogenous CO 
production from biochemical reactions associated with normal breakdown 
of heme proteins, as well as in response to inhaled (exogenous) CO 
exposures (ISA, section 4.5). The production of endogenous CO and 
levels of endogenous COHb vary with several physiological 
characteristics (e.g., slower COHb elimination with increasing age), as 
well as some disease states, which can lead to higher endogenous levels 
in some individuals (ISA, section 4.5). The amount of COHb formed in 
response to exogenous CO is dependent on the CO concentration and 
duration of exposure, exercise (which increases the amount of air 
removed and replaced per unit of time for gas exchange), the pulmonary 
diffusing capacity for CO, ambient pressure, health status, and the 
specific metabolism of the exposed individual (ISA, chapter 4; 2000 
AQCD, chapter 5). The formation of COHb is a reversible process, but 
the high affinity of CO for hemoglobin, which affects the elimination 
half-time for COHb, can lead to increased COHb levels in some 
circumstances.
    As discussed in the REA, exposure to CO in ambient air can occur 
outdoors as well as through infiltration of ambient air into indoor 
locations (REA, section 2.3). Additionally, indoor sources such as gas 
stoves and tobacco smoke can, where present, be important contributors 
to total CO exposure and can result in much greater CO exposures and 
associated COHb levels than those associated with ambient sources (ISA, 
section 3.6.5.2).\10\ 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 those short-term exposures to ambient CO considered more likely 
to be encountered by the general public (2000 AQCD, p. 7-4). Further, 
some assessments performed for previous reviews have included modeling 
simulations both without and with indoor sources (gas stoves and 
tobacco smoke) to provide context for the assessment of ambient CO 
exposure and dose (e.g., U.S. EPA, 1992; Johnson et al., 2000), and 
these assessments have found that nonambient sources have a 
substantially greater impact on the highest total exposures experienced 
by the simulated population than do ambient sources (Johnson et al., 
2000; REA, sections 1.2 and 6.3).\11\. However, the focus of this REA, 
conducted to inform the current review of the CO NAAQS, is on sources 
of ambient CO. While recognizing this information regarding the 
potential for indoor sources, where present, to play a role in CO 
exposures and COHb levels, the exposure modeling in the current review 
(described in section II.C below) did not include indoor CO sources in 
order to focus on the impact of ambient CO sources on population COHb 
levels.
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    \10\ A significant source of nonambient CO long recognized as 
contributing to elevated COHb levels is tobacco smoking (e.g., ISA, 
Figure 4-12). Further, baseline COHb levels in active smokers have 
been estimated to range from 3 to 8% for one- to two-pack-per-day 
smokers. As a result of their higher baseline COHb levels, smokers 
may exhale more CO into the air than they inhale from the ambient 
environment when not smoking. Tobacco smoking can also contribute to 
increased CO exposures and associated COHb levels in nonsmokers 
(2000 AQCD, p. 7-4).
    \11\ As has been recognized in previous CO NAAQS reviews, such 
sources cannot be effectively mitigated by setting more stringent 
ambient air quality standards (59 FR 38914).
---------------------------------------------------------------------------

    Apart from the impaired oxygen delivery to tissues related to COHb 
formation, the evidence also indicates alternative mechanisms of CO-
induced effects independent of limited oxygen availability (2000 AQCD, 
section 5.9; ISA, section 5.1.3). These mechanisms are primarily 
associated with CO's ability to bind heme-containing proteins other 
than hemoglobin and myoglobin, and involve a wide range of molecular 
targets and CO concentrations, as described in the 2000 AQCD (USEPA, 
2000, section 5.6) and in the ISA (ISA, section 5.1.3). Older 
toxicological studies demonstrated that exposure to high concentrations 
of CO resulted in altered functions of heme proteins other than 
myoglobin and hemoglobin, potentially interfering with basic cell and 
molecular processes and leading to dysfunction and/or disease. More 
recent toxicological in vitro and in vivo studies have provided 
evidence of alteration of nitric oxide signaling, inhibition of 
cytochrome C oxidase, heme loss from protein, disruption of iron 
homeostasis and alteration of cellular reduction-oxidation status (ISA, 
section 5.1.3.2). The ISA notes that these mechanisms may be 
interrelated. The evidence for these alternative mechanisms and the 
role they may play in CO-induced health effects at concentrations 
relevant to the current NAAQS is not clear.
    As noted in the ISA, ``CO may be responsible for a continuum of 
effects from cell signaling to adaptive responses to cellular injury, 
depending on intracellular concentrations of CO, heme proteins and 
molecules which modulate CO binding to heme proteins'' (ISA, section 
5.1.3.3). However, as noted in the Policy Assessment, new research 
based on this evidence for pathways other than those related to 
impaired oxygen delivery to tissues is needed to further understand 
these pathways and their linkage to CO-induced effects in susceptible 
populations. Thus, the evidence indicates that COHb continues to be the 
most useful and well-supported indicator of CO exposures and the best 
biomarker to characterize the potential for health effects associated 
with exposures to ambient CO at this time (PA, section 2.2.1).
2. Nature of Effects
    As observed in the Policy Assessment, 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). Binding to heme proteins and the 
alteration of their function is the common mechanism underlying 
biological responses to CO. Upon inhalation, CO diffuses through the 
respiratory zone (alveoli) to the blood where it binds to hemoglobin, 
forming COHb. Accordingly, 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 hypoxia (reduced oxygen availability) induced by 
increased COHb levels in blood and decreased oxygen availability to 
critical tissues and organs, specifically the heart (ISA, section 
5.1.2). Consistent with this, medical conditions that affect the 
biological mechanisms to 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

[[Page 8164]]

function and limiting exercise capacity (2000 AQCD, section 7.1).\12\
---------------------------------------------------------------------------

    \12\ 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).
---------------------------------------------------------------------------

    The body of health effects evidence for CO has grown considerably 
since the review completed in 1994 with the addition of numerous 
epidemiological and toxicological studies (ISA; 2000 AQCD). This 
evidence provides additional detail and support to our prior 
understanding of CO effects and population susceptibility. Most 
notably, the current evidence includes much expanded epidemiological 
evidence that is consistent with previous conclusions regarding 
cardiovascular disease-related susceptibility (ISA, section 5.7; 2000 
AQCD, section 7.7). 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 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 \13\ short-term CO exposures and cardiovascular 
morbidity'' (ISA, p. 2-6, section 2.5.1). Additionally, as mentioned 
above, the ISA judges the evidence to be suggestive of causal 
relationships between relevant short- and long-term CO exposures and 
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, Table 2-1).
---------------------------------------------------------------------------

    \13\ Relevant CO exposures are defined in the ISA as ``generally 
within one or two orders of magnitude of ambient CO concentrations'' 
(ISA, section 2.5).
---------------------------------------------------------------------------

    Similar to the previous review, results from controlled human 
exposure studies of individuals with coronary artery disease (CAD) \14\ 
(Adams et al., 1988; Allred et al., 1989a, 1989b, 1991; Anderson et 
al., 1973; Kleinman et al., 1989, 1998; Sheps et al., 1987 \15\) 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. Toxicological studies described in the current 
review provide evidence of CO effects on the cardiovascular system, 
including electrocardiographic effects of 1-hour exposures to 35 ppm CO 
in a rat strain developed as an animal model of cardiac susceptibility 
(ISA, section 5.2.5.3).
---------------------------------------------------------------------------

    \14\ 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 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).
    \15\ 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) \16\ (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 \17\ following experimental CO exposure), with no 
evidence of a threshold; (3) objective measures of myocardial ischemia 
(ST-segment depression) \18\ 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 \19\; (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%.\20\
---------------------------------------------------------------------------

    \16\ 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.
    \17\ These levels and other COHb levels described for this study 
below are based on GC 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).
    \18\ 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).
    \19\ 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 < 5%'' (ISA, p. 5-41). CO-oximetry is a 
spectrophotometric method commonly used to rapidly provide 
approximate concentrations of COHb during controlled exposures (ISA, 
p. 5-41). At the low concentrations of COHb (<5%) more relevant to 
ambient CO exposures, co-oximeters are reported to overestimate COHb 
levels compared to GC measurements, while at higher concentrations, 
this method is reported to produce underestimates (ISA, p.4-18).
    \20\ While the COHb blood level for each subject during the 
exercise tests was intermediate between the post-exposure and 
subsequent post-exercise measurements (e.g., mean 2.4-2.0% and 4.7-
3.9%), the study authors noted that the measurements at the end of 
the exercise test represented the COHb concentrations at the 
approximate time of onset of myocardial ischemia as indicated by 
angina and ST segment changes. The corresponding ranges of CO-Ox 
measurements for the two exposures were 2.7-3.2% and 4.7-5.6%. In 
this document, we refer to the GC-measured mean of 2.0% or 2.0-2.4% 
for the COHb levels resulting from the lower experimental CO 
exposure.

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[[Page 8165]]

    Across all subjects, the mean time to angina onset for control 
(``room'' air) exposures was approximately 8.5 minutes, and the mean 
time to ST endpoint was approximately 9.5 minutes (Allred et al., 
1989b). Relative to room-air exposure that resulted in a mean COHb 
level of 0.6% (post-exercise), exposure to CO resulting in post-
exercise mean COHb concentrations of 2.0% and 3.9% were observed to 
decrease the exercise time required to induce ST-segment depression by 
5.1% (p=0.01) and 12.1% (p<0.001), respectively. These changes were 
well correlated with the onset of exercise-induced angina, the time to 
which was shortened by 4.2% (p=0.027) and 7.1% (p=0.002), respectively, 
for the two experimental CO exposures (Allred et al., 1989a, 1989b, 
1991).\21\ As at the time of the last review, while ST-segment 
depression is recognized as an indicator of myocardial ischemia, the 
exact physiological significance of the observed changes among those 
with CAD is unclear (ISA, p. 5-48).
---------------------------------------------------------------------------

    \21\ Another indicator measured in the study was the combination 
of heart rate and systolic blood pressure which provides a clinical 
index of the work of the heart and myocardial oxygen consumption, 
since heart rate and blood pressure are major determinants of 
myocardial oxygen consumption (Allred et al., 1991). A decrease in 
oxygen to the myocardium would be expected to be paralleled by 
ischemia at lower heart rate and systolic blood pressure. This heart 
rate-systolic blood pressure indicator at the time to ST-endpoint 
was decreased by 4.4% at the 3.9% COHb dose level and by a 
nonstatistically-significant, smaller amount at the 2.0% COHb dose 
level.
---------------------------------------------------------------------------

    No controlled human exposure studies have been specifically 
designed to evaluate the effect of controlled short-term exposures to 
CO resulting in COHb levels lower than a study mean of 2% (ISA, section 
5.2.6). However, an important finding of the multi-laboratory study was 
the dose-response relationship observed between COHb and the markers of 
myocardial ischemia, with effects observed at the lowest increases in 
COHb tested, without evidence of a measurable threshold effect. As 
reported by the authors, the results comparing ``the effects of 
increasing COHb from baseline levels (0.6%) to 2 and 3.9% COHb showed 
that each produced further changes in objective ECG measures of 
ischemia'' implying that ``small increments in COHb could adversely 
affect myocardial function and produce ischemia'' (Allred et al., 
1989b, 1991).
    The epidemiological evidence has expanded considerably since the 
last review including numerous additional studies that are coherent 
with the evidence on markers of myocardial ischemia from controlled 
human exposure studies of CAD patients (ISA, section 2.7). The most 
recent set of epidemiological studies in the U.S. have evaluated the 
associations between ambient concentrations of multiple pollutants 
(i.e. fine particles or PM2.5, 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 (MI), 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). 
Findings of positive associations for these outcomes with metrics of 
ambient CO concentrations are coherent with the evidence from 
controlled human exposure studies of myocardial ischemia-related 
effects resulting from elevated CO exposures (ISA, section 2.5.1; ISA, 
Figure 2-1). In these studies, the ambient CO concentration averaging 
time for which health outcomes were analyzed varied from 1 hour to 24 
hours, with the air quality metrics based on either a selected central-
site monitor for the area or an average for multiple monitors in the 
area of interest. The study areas for which positive associations of 
these metrics were reported with IHD, MI and CVD outcomes include: the 
Atlanta, Georgia metropolitan statistical area; the greater Los 
Angeles, California area; and a group of 126 urban counties. Together 
the individual study periods spanned the years from 1988 through 2005. 
The risk estimates from these studies indicate statistically 
significant positive associations were observed with ambient CO 
concentrations based on air quality for the day of hospital admission 
or based on the average of the selected ambient CO concentration metric 
across that day and 2 or 3 days previous (ISA, Figures 5-2 and 5-5). 
Many of the studies for these outcomes include same day or next day lag 
periods, which, as noted in the ISA ``are consistent with the proposed 
mechanism and biological plausibility of these CVD outcomes'' (ISA, p. 
5-40).\22\
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    \22\ Of the studies for which risk estimates are based on multi-
day averages (the Atlanta studies and the California study by Mann 
et al., 2002), the California study by Mann et al., (2002) also 
observed a significant positive association with same day CO 
concentration.
---------------------------------------------------------------------------

    Additionally, there are U.S. studies reporting associations with 
hospital admissions for CHF, a condition that affects an individual's 
ability to compensate for reduced oxygen availability. These include 
one in southern California which reported a significant association for 
ambient CO with hospital admissions for CHF (Linn et al., 2000), as 
well as studies in Allegheny County (Pittsburgh) for 1987-1999 study 
period (Wellenius et al., 2005), and Denver for the months of July-
August during 1993-1997 (Koken et al., 2003; ISA, pp. 5-31 to 5-33). 
Risk estimates for all three of these studies are based on the 24-hour 
CO concentration, with the California and Allegheny County studies' 
association with same-day air quality, while the association shown for 
the Denver study was with ambient CO concentration three days prior to 
health outcome (PA, Table 2-1).
    As noted by the ISA, ``[s]tudies of hospital admissions and ED 
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).
    As discussed in detail in the ISA, additional epidemiological 
studies have evaluated associations of ambient CO with other 
cardiovascular effects since the last review. For example, preliminary 
evidence of a link between exposure to CO and alteration of blood 
markers of coagulation and inflammation in individuals with CAD or CVD 
has been provided by a few well conducted and informative studies (ISA, 
Table 5-6; Delfino et al., 2008; Liao et al., 2005). As noted by the 
ISA, however, further studies are warranted to investigate the role of 
these markers in prothrombotic events and their possible contribution 
to the pathophysiology of CO-induced aggravation of ischemic heart 
disease

[[Page 8166]]

(ISA, section 5.2.1.8). Other epidemiological studies (including field 
and panel studies) also provide some evidence of a link between CO 
exposure and heart rate and heart rate variability (ISA, section 
5.2.1.1). With regard to the two of three studies reporting a positive 
association with heart rate, the ISA concluded that ``further research 
is warranted'' to corroborate the results, while the larger number of 
studies for heart rate variability parameters is characterized as 
having mixed associations (ISA, p. 5-15). Additionally, of the two 
studies of electrocardiogram changes indicative of ischemic events 
(ISA, section 5.2.1.2), one found no association and, in the other 
study, the association with CO did not remain statistically significant 
in multipollutant models, unlike the association with black carbon in 
that study (ISA, p. 5-16). A limited number of epidemiological studies 
(Bell et al., 2009; Linn et al., 2000) have investigated hospital 
admissions for stroke (including both hemorrhagic and ischemic forms) 
and generally report small or no associations with ambient CO 
concentrations (ISA, section 5.2.1.9, Table 5-8 and Figure 5-3).
    At the time of the last review, there was evidence for effects 
other than cardiovascular morbidity, including neurological, 
respiratory and developmental effects. Evidence for these effects 
includes the following.
     With regard to neurological effects, acute exposures to CO 
have long been known to induce CNS effects such as those observed with 
CO poisoning, although limited and equivocal evidence available at the 
time of the last review included indications of some neurobehavioral 
effects to result from CO exposures resulting in a range of 5-20% COHb 
(2000 AQCD, section 6.3.2). No additional clinical or epidemiological 
studies are now available that investigated such effects of CO at 
ambient levels (ISA, section 5.3).
     With regard to potential effects of CO on birth outcomes 
and developmental effects, the potential vulnerability of the fetus and 
very young infant to CO was recognized during the 1994 review and in 
the 2000 AQCD. The CO-specific evidence available, however, included 
limited epidemiological analyses focused primarily on very high CO 
exposures associated with maternal smoking, and animal studies 
involving very high CO exposures (USEPA, 1992; 2000 AQCD). The 2000 
AQCD concluded that typical ambient CO levels were unlikely to cause 
increased fetal risk (2000 AQCD, p. 6-44). The current review includes 
additional epidemiological and animal toxicological studies. The 
currently available evidence includes limited but suggestive 
epidemiologic evidence for a CO-induced effect on preterm-birth, birth 
defects, decrease in birth weight, other measures of fetal growth, and 
infant mortality (ISA, section 5.4.3). The available animal 
toxicological studies provide some support and coherence for these 
birth and developmental outcomes at higher than ambient exposures,\23\ 
although a clear understanding of the mechanisms underlying potential 
reproductive and developmental effects is still lacking (ISA, section 
2.5.3).
---------------------------------------------------------------------------

    \23\ The lowest exposures eliciting an effect in the animal 
studies were exposures of 22 hours per day over about 14 prenatal 
days at a concentration of 12 ppm (ISA, Table 5-17).
---------------------------------------------------------------------------

     With regard to respiratory effects, the 2000 AQCD 
concluded it unlikely that CO has direct effects on lung tissue, except 
at extremely high concentrations (2000 AQCD, p. 6-45). There is 
currently limited, suggestive evidence of an association between short-
term exposure to CO and respiratory-related outcomes. Only preliminary 
evidence is available, however, regarding a mechanism that could 
provide plausibility for CO-induced effects (ISA, section 5.5.5.1).
    Thus, while there is some additional evidence on neurological, 
respiratory and developmental effects, it remains limited.
    In summary, rather than altering conclusions from the previous 
review, the current evidence provides continued support and some 
additional strength to the previous conclusions regarding the health 
effects associated with exposure to CO and continues to indicate 
cardiovascular effects, particularly effects related to the role of CO 
in limiting oxygen availability, as those of greatest concern at low 
exposures.
3. At-Risk Populations
    In identifying population groups or life stages at greatest risk 
for health risk from a specific pollutant, the terms susceptibility, 
vulnerability, sensitivity, and at-risk are commonly employed. The 
definition for these terms sometimes varies, but in most instances 
``susceptibility'' refers to biological or intrinsic factors (e.g., 
lifestage, gender) while ``vulnerability'' refers to nonbiological or 
extrinsic factors (e.g., visiting a high-altitude location, medication 
use). Additionally, in some cases, the terms ``at-risk'' and sensitive 
have been used to encompass both of these concepts. At times, however, 
factors of ``susceptibility'' and ``vulnerability'' are intertwined and 
are difficult to distinguish. 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, such that use of the term susceptible populations in the ISA 
is 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 corresponding discussion in the 
REA and Policy Assessment and the summary below.
    The current evidence, while much expanded in a number of ways, 
continues to support the conclusions from the previous review regarding 
susceptible populations for exposure to ambient CO. In the AQCD for the 
review completed in 1994 and in the 2000 AQCD, the evidence best 
supported the identification of patients with CAD as a population at 
increased risk from low levels of CO (USEPA, 1992; 2000 AQCD). Other 
groups were also recognized as potentially susceptible in the 2000 AQCD 
based on consideration of the clinical evidence and theoretical work, 
as well as laboratory animal research (2000 AQCD, p. 7-6). These 
include fetuses and young infants; pregnant women; the elderly, 
especially those with compromised cardiovascular function; people with 
conditions affecting oxygen absorption, blood flow, oxygen carrying 
capacity or transport; people using drugs with central nervous system 
depressant properties or exposed to chemical substances that increase 
endogenous formation of CO; and people who have not adapted to high 
altitude and are exposed to a combination of high altitude and CO. For 
these potentially susceptible groups, little empirical evidence was 
available by which to specify health effects associated with ambient or 
near-ambient CO exposures (2000 AQCD, p. 7-6).
    As summarized in the Policy Assessment, based on the evidence from 
controlled human exposure studies also considered in the last review, 
and the

[[Page 8167]]

now much-expanded epidemiological evidence base which is coherent with 
the evidence from these studies, the population with pre-existing 
cardiovascular disease associated with limitation in oxygen 
availability 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 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 hypoxia and are unlikely to 
be at increased risk of CO-induced effects (ISA, p. 2-10).\24\ In 
addition, the high oxygen consumption of the heart, together with the 
inability to compensate for the hypoxic effects of CO, make the cardiac 
muscle of a person suffering with CAD a critical target for the hypoxic 
effects of CO.
---------------------------------------------------------------------------

    \24\ 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).
---------------------------------------------------------------------------

    In the Integrated Science Assessment for the current review, 
recognition of susceptibility of the population with pre-existing 
cardiovascular disease, such as CAD, is supported by the expanded 
epidemiological database, which includes a number of studies reporting 
significant increases in hospital admissions for IHD, angina and MI in 
relation to CO exposures (ISA, section 2.7). Further support is 
provided by epidemiologic studies (Mann et al., 2002; and Peel et al., 
2007) of increased hospital admissions and emergency department visits 
for IHD among individuals with secondary diagnoses for other 
cardiovascular outcomes including arrhythmia and congestive heart 
failure (ISA, section 5.7), and toxicological studies reporting altered 
cardiac outcomes in animal models of cardiovascular disease (ISA, 
section 5.2.1.9).
    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 (CHD or CAD, myocardial infarction, angina), 
congestive heart failure, and disturbances in cardiac rhythm (2000 
AQCD, section 7.7.2.1). Types of cardiovascular disease other than 
those discussed above may also contribute to increased susceptibility 
to the adverse effects of low levels of CO (ISA, section 5.7.1.1). For 
example, some evidence with regard to other types of cardiovascular 
disease such as congestive heart failure, arrhythmia, and non-specific 
cardiovascular disease, although more limited 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).
    Although there were little experimental data available at the time 
of the last review to adequately characterize specific health effects 
of CO at ambient levels for other potentially at-risk populations, 
several other populations were identified as being potentially more at 
risk of CO-induced effects due to a number of factors. These factors 
include pre-existing diseases that could inherently decrease oxygen 
availability to tissues, lifestage vulnerabilities (e.g., fetuses, 
young infants or newborns, the elderly), gender, lifestyle, medications 
or alterations in the physical environment (e.g., increased altitude). 
This is consistent with the ISA conclusions in the current review which 
recognize other populations that may be potentially susceptible to the 
effects of CO as including: Those with other pre-existing diseases that 
may have already limited oxygen availability or increased COHb 
production or levels, 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). In recognizing the potential susceptibility of these 
populations, the Policy Assessment also noted the lack of information 
on specific 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 available for the CAD population to these 
other populations (PA, section 2.2.1).
    The current evidence continues to support the identification of 
people with cardiovascular disease as having susceptibility to CO-
induced health effects (ISA, 2-12), with those having CAD as the 
population with the best characterized susceptibility to CO-induced 
health effects (ISA, sections 5.7.1.1 and 5.7.8).\25\ 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. Included in this susceptible 
population are those with angina pectoris (cardiac chest pain), those 
who have experienced a heart attack, and those with silent ischemia or 
undiagnosed IHD (AHA, 2003). People with other cardiovascular diseases, 
particularly heart diseases, are also at risk of CO-induced health 
effects. We also recognize other populations potentially susceptible to 
CO-induced effects, most particularly those with other pre-existing 
diseases that cause limited oxygen availability, increased COHb levels, 
or increased endogenous CO production, such as people with obstructive 
lung diseases, diabetes and anemia; however, information characterizing 
susceptibility for this population is limited.
---------------------------------------------------------------------------

    \25\ 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).
---------------------------------------------------------------------------

4. Potential Impacts on Public Health
    In light of the evidence described above with regard to factors 
contributing to greater susceptibility to health effects of ambient CO, 
this section, drawing from the Integrated Science Assessment and 
discussion in the Policy Assessment, discusses the health significance 
of the effects occurring with the lowest relevant (short-term) 
exposures to ambient CO and the size of the at-risk populations in the 
U.S. These considerations are important elements in the 
characterization of potential public health impacts associated with 
exposure to ambient CO.
    We first consider the effects identified by the evidence at the 
lowest studied short-term exposures. As discussed in section II.B.2 
above, the study by Allred et al., (1989a, 1989b, 1991) indicates that 
increases in blood COHb in response to 1-hour CO exposures

[[Page 8168]]

produce evidence of myocardial ischemia in CAD patients with 
reproducible exercise-induced angina. At a study group average COHb 
level of 2-2.4%, the statistically significant reduction in the time to 
exercise-induced markers of myocardial ischemia in CAD patients was 4-
5% on average (approximately 30 seconds), with larger reductions 
observed at the higher studied COHb level. In discussing public health 
implications of the observed responses, the study authors noted that 
the responses observed at the studied COHb levels were similar to those 
considered clinically significant when evaluating medications to treat 
angina from coronary artery disease (Allred et al., 1989a, 1991). The 
independent review panel for the study further noted that frequent 
encounters in ``everyday life'' with increased COHb levels on the order 
of those tested in the study might be expected to limit activity and 
affect quality of life (Allred et al., 1989b, pp. 38, 92-94; 1991 AQCD, 
p. 10-35).
    In the review completed in 1994, the body of evidence that 
demonstrated cardiovascular effects in CAD patients exposed to CO was 
given primary consideration, with the Administrator judging that 
``cardiovascular effects, as measured by decreased time to onset of 
angina pain and by decreased time to onset of significant ST-segment 
depression, are the health effects of greatest concern, which clearly 
have been associated with CO exposures at levels observed in the 
ambient air'' (59 FR 38913). Additionally, as discussed in section 
II.B.2 above, a dose-response relationship has been documented for COHb 
resulting from brief, elevated CO exposures in persons with pre-
existing CAD, with no evidence of threshold (59 FR 38910; ISA, section 
5.2.4; Allred et al., 1989a, 1989b, 1991).
    In the 1994 review decision (as discussed in section II.D.1.a 
below), less significance was ascribed to the effects at the lower COHb 
level assessed in the Allred et al., study (1989a, 1989b, 1991), which 
were described to be of less certain clinical importance, than effects 
reported from short-term CO exposure studies that assessed higher COHb 
levels (59 FR 38913-38914). In the current review of the evidence, the 
ISA describes the physiological significance of the changes at the 
lowest tested dose level (e.g., 2% COHb from Allred et al., 1989b) as 
unclear, additionally noting that variability in severity of disease 
among individuals with CAD is likely to influence the critical level of 
COHb which leads to adverse cardiovascular effects (ISA, p. 2-6).
    In considering potential public health impacts of CO in ambient 
air, we also consider the size of the at-risk populations. The 
population with CAD is well recognized as susceptible to increased risk 
of CO-induced health effects (ISA, sections 5.7.1.1 and 5.7.8). The 
2007 estimate from the National Health Interview Survey (NHIS) 
performed by the U.S. Centers for Disease Control of the size of the 
U.S. population with coronary heart disease, angina pectoris (cardiac 
chest pain) or who have experienced a heart attack (ISA, Table 5-26) is 
13.7 million people (ISA, pp. 5-117). Further, there are estimated to 
be three to four million additional people with silent ischemia or 
undiagnosed IHD (AHA, 2003). In combination, this represents a large 
population that is more susceptible to ambient CO exposure when 
compared to the general population (ISA, section 5.7).
    In addition to the population with diagnosed and undiagnosed CAD, 
the ISA notes the size of the larger population of people with all 
types of heart disease (HD), which may also be at increased risk of CO-
induced health effects (ISA, section 2.6.1). Within this broader group, 
implications of CO exposures are more significant for those persons for 
whom their disease state affects their ability to compensate for the 
hypoxia-related effects of CO (ISA, section 4.4.4). The NHIS estimates 
for 2007 indicate there is a total of approximately 25 million people 
with heart disease of any type (ISA, Table 5-26).
    Other populations potentially susceptible to the effects of CO 
include people with chronic obstructive pulmonary disease, diabetes and 
anemia, as well as older adults and fetuses during critical phases of 
development (as discussed in section II.B.3 above). In considering 
potential impacts on such populations, we recognize that the evidence 
is limited or lacking with regard to effects of CO at ambient levels, 
and associated exposures and COHb levels, while providing no indication 
of susceptibility to ambient CO greater than that of CHD and HD 
populations.

C. Human Exposure and Dose Assessment

    Our consideration of the scientific evidence in the current review, 
as at the time of the last review (summarized in section II.D.1 below), 
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). The benchmark COHb 
levels were identified based on consideration of the evidence discussed 
in section II.B above. The following subsections summarize the design 
and methods of the quantitative assessment (section II.C.1) and the 
important uncertainties associated with these analyses (section 
II.C.2). The results of the analyses, as they relate to considerations 
of the adequacy of the current standards, are discussed in section 
II.D.2 below.
1. Summary of Design Aspects
    In this section, we provide a summary of key aspects of the 
assessment conducted for this review, including the study areas and air 
quality scenarios investigated, modeling tools used, at-risk 
populations simulated, and COHb benchmark levels of interest. The 
assessment is described in detail in the REA and summarized in the PA 
(section 2.2.2).
    The assessment estimated 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. We selected these areas because: (1) Areas of both 
cities have been included in prior CO NAAQS exposure assessments and 
thus serve as an important connection with past assessments; (2) 
historically, they have generally had the highest ambient CO 
concentrations among urban areas in the U.S.; and (3) Denver is at high 
altitude and represents an important risk scenario due to the potential 
increased susceptibility to CO exposure associated with high altitudes. 
In addition, of 10 urban areas across the continental U.S. selected for 
detailed air quality analysis in the ISA and having ambient monitors 
meeting a 75% completeness criterion, the two study area locations were 
ranked first (Los Angeles) and second (Denver) regarding the percentage 
of elderly population within 5, 10, and 15 km of monitor locations, and 
ranked first (Los Angeles) and fifth (Denver) regarding number of 1- 
and 8-hour daily maximum CO concentration measurements (ISA, section 
3.5.1.1).
    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

[[Page 8169]]

meet the current 8-hour standard,\26\ as well as for air quality 
conditions simulated to just meet several 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. Accordingly, we simulated conditions 
of increased CO concentrations that just meet the current 8-hour 
standard in the two study areas. In so doing, we recognize the 
uncertainty associated with simulating this hypothetical profile of 
higher CO concentrations that just meet the current 8-hour standard. We 
note, however, that an analysis of the ratios of 1-hour to 8-hour 
design value metrics based on 2009 ambient CO concentrations in U.S. 
locations indicates that the relationships between design values for 
the two study areas under the air quality conditions simulated to just 
meet the current 8-hour standard fall well within the 2009 national 
distribution of such ratios (Policy Assessment, section 2.2.2).\27\
---------------------------------------------------------------------------

    \26\ As noted elsewhere, the 8-hour standard is the controlling 
standard for ambient CO concentrations.
    \27\ More specifically, the ratio of the 1-hour design value to 
the 8-hour design value for the Los Angeles study area corresponds 
to approximately the 25th percentile of U.S. counties in 2009 and 
the ratio for the Denver study area corresponds to approximately the 
75th percentile of U.S. counties in 2009. Under ``as is'' conditions 
the ratios for these two study areas correspond to approximately the 
40th percentile of the 2009 national distribution (Policy 
Assessment, section 2.2.2).
---------------------------------------------------------------------------

    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). 
This model has a history of application, evaluation, and progressive 
model development in estimating human exposure and dose for several 
NAAQS reviews, including CO, ozone (O3), nitrogen dioxide 
(NO2), and sulfur dioxide (SO2). As described in 
section II.D.1 below, 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; U.S. EPA, 
1992).28 29 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 microenvironmental 
concentrations and to more accurately estimate variability in CO 
exposures and COHb levels (REA, chapter 4).\30\
---------------------------------------------------------------------------

    \28\ 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).
    \29\ 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; 
U.S. EPA, 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.
    \30\ 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). The individual's movements are simulated 
based on data available from recent activity pattern surveys (CHAD \31\ 
now has about 34,000 person-days of data) and the most recent U.S. 
census data on population demographics and home-to-workplace commutes. 
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). These results 
across each simulated individual were then summarized in the REA and 
discussed in the Policy Assessment in terms of the percent of the 
simulated at-risk populations expected to experience one or more 
occurrences of daily maximum end-of-hour COHb levels of interest.
---------------------------------------------------------------------------

    \31\ CHAD is EPA's Comprehensive Human Activity Database which 
provides input data for APEX model simulations (REA, sections 4.3 
and 4.4).
---------------------------------------------------------------------------

    As discussed in section II.B above, people with cardiovascular 
disease are the population of primary focus in this review, and more 
specifically, as described in the ISA, 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 ischemic heart disease [IHD] or CAD), both diagnosed and 
undiagnosed, and (2) adults with any heart diseases, including 
undiagnosed ischemia.\32\ 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.
---------------------------------------------------------------------------

    \32\ As described in section 1.2 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).
---------------------------------------------------------------------------

    In representing the two at-risk populations and their activity 
patterns, individuals were simulated based on age and gender 
distributions for CHD and HD populations. These distributions were 
developed by augmenting the prevalence estimates provided by the 
National Health Interview Survey for adults with CAD and adults with 
heart diseases of any type (HD) with estimates of undiagnosed ischemia 
(as described in section 5.5.1 of the REA). The undiagnosed ischemia 
estimates were developed based on two assumptions: (1) There are 3.5 
million persons in U.S. with undiagnosed IHD (AHA, 2003) and (2) 
persons with undiagnosed IHD are distributed within the population in 
the same manner as persons with diagnosed IHD (REA, section 5.5.1).
    APEX simulations performed for this review focused on exposures to 
ambient

[[Page 8170]]

CO occurring in eight microenvironments,\33\ absent any contribution to 
microenvironment concentrations from indoor (nonambient) CO sources. As 
noted in section II.B.1 above, however, where present, indoor sources, 
including gas stoves, attached garages and tobacco smoke, can also be 
important contributors to total CO exposure (ISA, sections 3.6.1 and 
3.6.5). 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.
---------------------------------------------------------------------------

    \33\ 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).
---------------------------------------------------------------------------

    The REA developed COHb estimates for the simulated at-risk 
populations with attention to both COHb in absolute terms and in terms 
of the contribution to absolute levels associated with ambient CO 
exposures. 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.
    As discussed in the Policy Assessment (section 2.2.2), the absence 
of indoor (nonambient) sources in the REA simulations is expected to 
result in simulated individuals with somewhat higher estimates of the 
contribution of short-duration increases in ambient CO exposure to COHb 
levels (ambient contribution) than would be expected for individuals in 
situations where the presence of nonambient sources contributes to 
higher baseline COHb levels (i.e., COHb prior to a short-duration 
exposure event). The amount by which the ambient contribution estimates 
might differ is influenced by the magnitude of nonambient-source 
exposures and associated baseline COHb levels. One reason for this is 
that in the presence of indoor sources, baseline COHb levels will be 
higher for a given population group than COHb levels for that group 
arising solely from endogenous CO in the absence of any exposure, which 
is the ``baseline'' for the REA estimates of ambient contribution to 
COHb (REA, appendix B.6).\34\ As CO uptake depends in part on the 
amount of CO already present in the blood (and the blood-air CO 
concentration gradient), in general, a higher baseline COHb, with all 
other variables unchanged, will lead to relatively lesser uptake of CO 
from short-duration exposures (ISA, section 4.3; AQCD, section 5.2). 
Additionally, as is indicated by the REA estimates, the attainment of a 
particular dose level is driven largely by short-term (and often high 
concentration) exposure events. This is because of the relatively rapid 
uptake of CO into a person's blood, as demonstrated by the pattern in 
the REA time-series of ambient concentrations, microenvironmental 
exposures, and COHb levels (REA, Appendix B, Figure B-2). For example 
the time lag for response of an individual's COHb levels to variable 
ambient CO (and hence exposure) concentrations may be only a few hours 
(e.g., REA, Figure B-2).
---------------------------------------------------------------------------

    \34\ As they result only from endogenous CO formation, the REA 
``baseline'' COHb levels would also be expected to be, and generally 
are, lower than the initial, pre-exposure, COHb levels of subjects 
in the controlled exposure studies. REA estimates of endogenously 
formed COHb averaged about 0.3% across the simulated populations, 
with slightly higher levels in the higher altitude Denver study area 
(REA, pp. B-21 to B-22). Levels in the Denver study population 
ranged from 0.1 to 1.1% COHb, with an average of 0.31%, while levels 
for Los Angeles ranged from 0.1 to 0.7% with an average of 0.27% 
COHb. Initial, pre-exposure COHb levels in the subjects of the 
Allred et al. study (1989b), which reflect the subjects pre-study 
exposure history as well as endogenous CO formation, ranged from 0.2 
to 1.1%, averaging about 0.6% COHb.
---------------------------------------------------------------------------

    In considering the REA dose estimates in the Policy Assessment, as 
described in section II.D.2 below, 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. 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.B.2 above. The REA identified a series of 
benchmark levels for considering estimates of absolute COHb: 1.5%, 
2.0%, 2.5% and 3% COHb (REA, section 2.6). This range includes the 
range of COHb levels identified as levels of concern in the review 
completed in 1994 (2.0 to 2.9%) and the level given particular focus 
(2.1%) at that time, as described in section 2.1.1 above (USEPA, 1992; 
59 FR 48914). Selection of this range of benchmark levels is based on 
consideration of the evidence from controlled human exposure studies of 
subjects with CAD (discussed in section 2.2.1 above), with the lower 
end of the range extending below the lowest mean COHb level resulting 
from controlled exposure to CO in the clinical evidence (e.g., 2.0% 
post-exercise in Allred et al., 1989b). The extension of this range 
reflects a number of considerations, including: (1) Comments from the 
CASAC CO panel on the draft Scope and Methods Plan (Brain, 2009); (2) 
consideration of the uncertainties regarding the actual COHb levels 
experienced in the controlled human exposure studies; (3) that these 
studies did not include individuals with most severe cardiovascular 
disease;\35\ (4) the lack of studies that have evaluated effects of 
experimentally controlled short-term CO exposures resulting in mean 
COHb levels below 2.0-2.4%; and (5) the lack of evidence of a threshold 
at the increased COHb levels evaluated. We note that CASAC comments on 
the first draft REA recommended the addition of a benchmark at 1.0% 
COHb and results are presented for this COHb level in the REA. Given 
that this level overlaps with the upper part of the range of endogenous 
levels in healthy individuals as characterized in the ISA (ISA, p. 2-
6), and is within the upper

[[Page 8171]]

part of the range of baseline COHb levels in the study by Allred et al 
(1989b, Appendix B), however, we considered that it may not be 
appropriate to place weight on it as a benchmark level and accordingly 
have not focused on interpreting absolute COHb estimates at and below 
this level in the discussion below. Additionally we note the REA 
estimates indicating that, in the absence of CO exposure, approximately 
0.5% to 2% of the simulated at-risk populations in the two study areas 
were estimated to experience a single daily maximum end-of-hour COHb 
level, arising solely from endogenous CO production, at or above 1% 
(REA, Appendix B, Figure B-3).
---------------------------------------------------------------------------

    \35\ Although the CAD patients evaluated in the controlled human 
exposure study by Allred et al. (1989a, 1989b, 1991) are not 
necessarily representative of the most sensitive population, the 
level of disease in these individuals ranged from moderate to 
severe, with the majority either having a history of myocardial 
infarction or having >=70% occlusion of one or more of the coronary 
arteries (ISA, p. 5-43).
---------------------------------------------------------------------------

    The Policy Assessment also considered the evidence from controlled 
human exposure studies in interpreting the REA estimates of maximum 
ambient exposure contributions to end-of-hour COHb levels (described in 
sections 4.4.7 and 5.10.3 of the REA). As discussed above, the study by 
Allred et al (1989a, 1989b, 1991) observed reduced time to exercise-
induced angina and ST-segment change in groups of subjects with pre-
existing CAD for which controlled CO exposures increased their COHb 
levels by on average 1.4-1.8% and 3.2-4.0% COHb from initial COHb 
levels of on average 0.6% COHb (ISA, section 5.2.4; Allred et al., 
1989a, 1989b, 1991). The study reported a dose-response relationship in 
terms of time reduction per 1% increase in COHb concentration based on 
analysis of the full data set across both exposure groups. For purposes 
of the discussion in this document, we have presented the percentage of 
the simulated at-risk populations estimated to experience maximum 
ambient contribution to end-of-hour COHb levels above and below a range 
of levels extending from 1.4 to 2.0%. As noted above, 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 as discussed above.
2. Key Limitations and Uncertainties
    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.\36\ 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 remaining exposure modeling uncertainties are 
summarized in section 2.2.2 of the Policy Assessment and described in 
detail in chapter 7 of the REA.
---------------------------------------------------------------------------

    \36\ 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.
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    The uncertainties associated with the quantitative estimates of 
exposure and dose were considered using a generally qualitative 
approach intended to identify and compare the relative impact that 
important sources of uncertainty may have on the estimated potential 
health effect endpoints (i.e., estimates of the maximum end-of-hour 
COHb levels in the simulated at-risk population). The approach used was 
developed using World Health Organization (WHO) guidelines on 
conducting a qualitative uncertainty characterization (WHO, 2008) and 
was also applied in the most recent NO2 (USEPA, 2008c) and 
SO2 NAAQS reviews (USEPA, 2009e). A qualitative approach was 
employed given the extremely limited data available to inform 
probabilistic uncertainty analyses. The qualitative approach used 
varies from that of WHO (2008) in that a greater focus of the 
characterization performed was placed on evaluating the direction and 
the magnitude of the uncertainty; that is, qualitatively rating how the 
source of uncertainty, in the presence of alternative information, may 
affect the estimated exposures and health risk results. Additionally, 
consistent with the WHO (2008) guidance, the REA discusses the 
uncertainty in the knowledge base (e.g., the accuracy of the data used, 
acknowledgement of data gaps) and decisions made where possible (e.g., 
selection of particular model forms), though qualitative ratings were 
assigned only to uncertainty regarding the knowledge base.
    Sixteen separate sources of uncertainty associated with four main 
components of the assessment were identified. By comparing judgments 
made regarding the magnitude and direction of influence that the 
identified sources have on estimated exposure concentrations and dose 
levels and the existing uncertainties in the knowledge base, seven 
sources of uncertainty (i.e., the spatial and temporal representation 
of ambient monitoring data, historical data used in representing 
alternative air quality scenarios, activity pattern database, 
longitudinal profile algorithm, microenvironmental algorithm and input 
data, and physiological factors) were identified as the most important 
areas of uncertainty in this assessment (PA, section 2.2.2). Taking 
into consideration improvements in the model algorithms and data since 
the last review, and having identified and characterized these 
uncertainties here, 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 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 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

[[Page 8172]]

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.

D. Conclusions on Adequacy of the Current Standards

    The initial issue to be addressed in the current review of the 
primary CO standards is whether, in view of the advances in scientific 
knowledge and additional information now available, the existing 
standards should be retained or revised. In evaluating whether it is 
appropriate to retain or revise the current standards, the 
Administrator builds upon the last review and reflects the broader body 
of evidence and information now available. 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. Evidence-based considerations include the 
assessment of evidence from controlled human exposure, toxicological 
and epidemiological studies evaluating short- or long-term exposures to 
CO, with supporting evidence related to dosimetry and potential mode of 
action, as well as the integration of evidence across each of these 
disciplines, and with a focus on policy-relevant considerations as 
discussed in the PA. The exposure/dose-based considerations draw from 
the results of the quantitative analyses presented in the REA and 
summarized in section II.C above, and consideration of those results in 
the PA. More specifically, estimates of the magnitude of ambient CO-
related exposures and associated COHb levels associated with just 
meeting the current primary CO NAAQS have been considered. Together the 
evidence-based and risk-based considerations have informed the 
Administrator's proposed conclusions related to the adequacy of the 
current CO standards in light of the currently available scientific 
evidence.
1. Approach
    In considering the evidence and quantitative exposure and dose 
estimates with regard to judgments on the adequacy afforded by the 
current standards, we note that 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, discussed more 
fully below, 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.\37\
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    \37\ 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).
---------------------------------------------------------------------------

    The following subsections include background information on the 
approach used in the previous review of the CO standards (section 
II.D.1.a) and also a description of the approach for the current review 
(section II.D.1.b).
a. Previous Reviews
    The current primary standards for CO are 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. These standards were initially set 
in 1971 to protect against the occurrence of carboxyhemoglobin (COHb) 
levels that may be associated with effects of concern (36 FR 8186). 
Reviews of these standards in the 1980s and early 1990s identified 
additional evidence regarding ambient CO, CO exposures, COHb levels, 
and associated health effects (USEPA, 1984a, 1984b; USEPA, 1991; USEPA, 
1992; McClellan, 1991, 1992). Assessment of the evidence in those 
reviews, completed in 1985 and 1994, led the EPA to retain the existing 
primary standards without revision (50 FR 37484, 59 FR 38906).
    The 1994 decision to retain the primary standards without revision 
was based on the evidence published through 1990 and reviewed in the 
1991 AQCD (USEPA, 1991), the 1992 Staff Paper assessment of the policy-
relevant information contained in the AQCD and the quantitative 
exposure assessment (USEPA, 1992), and the advice and recommendations 
of CASAC (McClellan 1991, 1992). At that time, as at the time of the 
first NAAQS review (50 FR 37484), COHb levels in blood were recognized 
as providing the most useful estimate of exogenous CO exposures and 
serving as the best biomarker of CO toxicity for ambient-level 
exposures to CO (59 FR 38909). Consequently, COHb levels were used as 
the indicator of health effects in the identification of health effect 
levels of concern for CO (59 FR 38909).
    In reviewing the standards in 1994 the Administrator first 
recognized the need to determine the COHb levels of concern ``taking 
into account a large and diverse health effects database.'' The more 
uncertain and less quantifiable evidence was taken into account to 
identify the lower end of this range to provide an adequate margin of 
safety for effects of clear concern. To consider ambient CO 
concentrations likely to result in COHb levels of concern, a model 
solution to the Coburn-Forster-Kane (CFK) differential equation was 
employed in the analysis of CO exposures expected to occur under air 
quality scenarios related to just meeting the current 8-hour CO NAAQS, 
the controlling standard (USEPA, 1992).\38\ Key considerations in this 
approach are described below.
---------------------------------------------------------------------------

    \38\ Air quality analyses of CO levels in the U.S. consistently 
demonstrate that meeting the 8-hour standard results in 1-hour 
maximum concentrations well below the corresponding 1-hour standard.
---------------------------------------------------------------------------

    The assessment of the science that was presented in the 1991 AQCD 
(USEPA, 1991) indicated that CO is associated with effects in the 
cardiovascular system, central nervous system (CNS), and the developing 
fetus. Additionally, factors recognized as having the potential to 
alter the effects

[[Page 8173]]

of CO included exposures to other pollutants, some drugs and some 
environmental factors, such as altitude. Cardiovascular effects of CO, 
as measured by decreased time to onset of angina and to onset of 
significant electrocardiogram (ECG) ST-segment depression were judged 
by the Administrator to be ``the health effects of greater concern, 
which clearly had been associated with CO exposures at levels observed 
in ambient air'' (59 FR 38913).
    Based on the consistent findings of response in patients with 
coronary artery disease across the controlled human exposure evidence 
(Adams et al., 1988; Allred et al., 1989a, 1989b, 1991; Anderson et 
al., 1973; Kleinman et al., 1989, 1998; Sheps et al., 1987 \39\) and 
discussions of adverse health consequences in the 1991 AQCD and the 
1992 Staff Paper,\40\ at the CASAC meetings and in the July 1991 CASAC 
letter, the Administrator concluded that ``CO exposures resulting in 
COHb levels of 2.9-3.0 percent (CO-Ox) or higher in persons with heart 
disease have the potential to increase the risk of decreased time to 
onset of angina pain and ST-segment depression'' (59 FR 38913). While 
EPA and CASAC recognized the existence of a range of views among health 
professionals on the clinical significance of these responses, CASAC 
noted that the dominant view was that they should be considered 
``adverse or harbinger of adverse effect'' (McClellan, 1991) and EPA 
recognized that it was ``important that standards be set to 
appropriately reduce the risk of ambient exposures which produce COHb 
levels that could induce such potentially adverse effects'' (59 FR 
38913).
---------------------------------------------------------------------------

    \39\ See footnote 15 above.
    \40\ Based on consideration of the key studies, including those 
two that investigated more than a single target COHb level, 
discussions in the 1991 AQCD and with CASAC, the 1992 Staff Paper 
recommended that ``2.9-3.0% COHb (CO-Ox), representing an increase 
above initial COHb of 1.5 to 2.2% COHb, be considered a level of 
potential adversity for individuals at risk'' (59 FR 38911; USEPA, 
1992; USEPA, 1991, pp. 1-11 to 1-12; Allred et al., 1989a, 1989b, 
1991; Anderson et al., 1973).
---------------------------------------------------------------------------

    In further considering additional results from the controlled human 
exposure evidence, such as the results from Allred et al. (1989a, 
1989b) at 2.0% COHb (using GC measurement) induced by short 
(approximately 1-hour) CO exposure, as well as other aspects of the 
available evidence and uncertainties regarding modeling estimates of 
COHb formation and human exposure to COHb levels in the population 
associated with attainment of a given CO NAAQS, the Administrator 
recognized the need to extend the range of COHb levels for 
consideration in evaluating whether the current CO standards provide an 
adequate margin of safety to those falling between 2.0 to 2.9% COHb (59 
FR 38913). Factors considered in recognizing this margin of safety 
included the following (59 FR 38913).
     Uncertainty regarding the clinical importance of 
cardiovascular effects associated with exposures to CO that resulted in 
COHb levels of 2 to 3 percent. Although recognizing the possibility 
that there is no threshold for these effects even at lower COHb levels, 
the clinical importance of cardiovascular effects associated with short 
(approximately 1-hour) exposures to CO resulting in COHb levels as low 
as 2.0% COHb by GC (Allred et al., 1989a,b) was described as ``less 
certain'' than effects noted for exposures contributing to higher COHb 
(CO-Ox) levels (59 FR 38913).
     Findings of short-term reduction in maximal work capacity 
measured in trained athletes exposed to CO at levels resulting in COHb 
levels of 2.3 to 7 percent.
     The potential that the most sensitive individuals have not 
been studied, the limited information regarding the effects of ambient 
CO in the developing fetus, and concern about visitors to high 
altitudes, individuals with anemia or respiratory disease, or the 
elderly.
     Potential for short term peak CO exposures to be 
responsible for impairments (impairment of visual perception, 
sensorimotor performance, vigilance or other CNS effects) which could 
be a matter of concern for complex activities such as driving a car, 
although these effects had not been demonstrated to be caused by CO 
concentrations in ambient air.
     Concern based on limited evidence for individuals exposed 
to CO concurrently with drugs (e.g., alcohol), during heat stress, or 
co-exposure to other pollutants.
     Uncertainties, described as ``large,'' that remained 
regarding modeling COHb formation and estimating human exposure to CO 
which could lead to overestimation of COHb levels in the population 
associated with attainment of a given CO NAAQS.
     Uncertainty associated with COHb measurements made using 
CO-Ox which may not reflect COHb levels in angina patients studied, 
thereby creating uncertainty in establishing a lowest effects level for 
CO.
    Based on these considerations of the evidence, the Administrator 
identified a range of COHb levels for considering margin of safety, 
extending from 2.9% COHb (representing an increase of 1.5% above 
baseline when using CO-Ox measurements) at the upper end down to 2% at 
the lower end (59 FR 38913), and also concluded that ``evaluation of 
the adequacy of the current standard should focus on reducing the 
number of individuals with cardiovascular disease from being exposed to 
CO levels in the ambient air that would result in COHb levels of 2.1 
percent'' (59 FR 38914). She additionally concluded that standards that 
``protect against COHb levels at the lower end of the range should 
provide an adequate margin of safety against effects of uncertain 
occurrence, as well as those of clear concern that have been associated 
with COHb levels in the upper-end of the range'' (59 FR 38914).
    To estimate CO exposures and resulting COHb levels that might be 
expected under air quality conditions that just met the current 
standards, an analysis of exposure and associated internal dose in 
terms of COHb levels in the population of interest in the city of 
Denver, Colorado was performed (59 FR 38906; USEPA, 1992). That 
analysis indicated that if the 9 ppm 8-hour standard were just met, the 
proportion of the nonsmoking population with cardiovascular disease 
experiencing a daily maximum 8-hour exposure at or above 9 ppm for 8 
hours decreased by an order of magnitude or more as compared to the 
proportion under then-existing CO levels, down to less than 0.1 percent 
of the total person-days in that population. Further, upon meeting the 
8-hour standard, EPA estimated that less than 0.1% of the nonsmoking 
cardiovascular-disease population would experience a COHb level greater 
than or equal to 2.1% and a smaller percentage of the at-risk 
population was estimated to exceed higher COHb levels (59 FR 
38914).\41\ Based on these estimates, the Administrator concluded that 
``relatively few people of the cardiovascular sensitive population 
group analyzed will experience COHb levels >= 2.1 percent when exposed 
to CO levels in absence of indoor sources when the current standards 
are attained.'' The analysis also took into account that certain indoor 
sources (e.g., passive smoking, gas stove usage) contributed to total 
CO exposure and EPA recognized that such sources may be of concern for 
such high risk groups

[[Page 8174]]

as individuals with cardiovascular disease, pregnant women, and their 
unborn children but concluded that ``the contribution of indoor sources 
cannot be effectively mitigated by ambient air quality standards'' (59 
FR 38914).
---------------------------------------------------------------------------

    \41\ In the 1992 assessment, the person-days (number of persons 
multiplied by the number of days per year exposed) and person-hours 
(number of persons multiplied by the number of hours per year 
exposed) were the reported exposure metrics. Upon meeting the 8-hour 
standard, it was estimated that less than 0.1% of the total person-
days simulated for the nonsmoking cardiovascular-disease population 
were associated with a maximum COHb level greater than or equal to 
2.1% (USEPA, 1992; Johnson et al., 1992).
---------------------------------------------------------------------------

    Based on consideration of the evidence and the quantitative results 
of the exposure assessment, the Administrator concluded that revisions 
of the current primary standards for CO were not appropriate at that 
time (59 FR 38914). The Administrator additionally concluded that both 
averaging times for the primary standards, 1 hour and 8 hours, be 
retained. The 1-hour and 8-hour averaging times were first chosen when 
EPA promulgated the primary NAAQS for CO in 1971. The selection of the 
8-hour averaging time was based on the following: (a) Most individuals' 
COHb levels appeared to approach equilibrium after 8 hours of exposure, 
(b) the 8-hour time period corresponded to the blocks of time when 
people were often exposed in a particular location or activity (e.g., 
working or sleeping), and (c) judgment that this provided a good 
indicator for tracking continuous exposures during any 24-hour period. 
The 1-hour averaging time was selected as better representing a time 
period of interest to short-term CO exposure and providing protection 
from effects which might be encountered from very short duration peak 
exposures in the urban environment (59 FR 38914).
b. Current Review
    To evaluate whether it is appropriate to consider retaining the 
current primary CO standards, or whether consideration of revisions is 
appropriate, 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. As summarized above, the 
Administrator's decisions in the previous review were 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 a public health policy judgment as to what standard is 
requisite to protect public health with an adequate margin of safety, 
which were informed by air quality and related analyses, quantitative 
exposure and risk assessments when possible, and qualitative assessment 
of impacts that could not be quantified. Similarly, in this review, as 
described in the Policy Assessment, we draw on the current evidence and 
quantitative assessments of exposure pertaining to the public health 
risk of ambient CO. In considering the scientific and technical 
information, here as in the Policy Assessment, we consider both the 
information available at the time of the last review and information 
newly available since the last review, including the current ISA and 
the 2000 AQCD (USEPA, 2010a; USEPA, 2000), as well as current and 
preceding quantitative exposure/dose assessments (USEPA 2010b; Johnson 
et al., 2000; USEPA 1992).
    As described earlier, at this time as at the time of the last 
review, the best characterized health effect associated with CO levels 
of concern is hypoxia (reduced oxygen availability) induced by 
increased COHb levels in blood (ISA, section 5.1.2). Accordingly, CO 
exposure is of particular concern for those with impaired 
cardiovascular systems, and the most compelling evidence of 
cardiovascular effects is that from a series of controlled human 
exposure studies among exercising individuals with CAD (ISA, sections 
5.2.4 and 5.2.6). Additionally available in this review are a number of 
epidemiological studies that investigated the association of 
cardiovascular disease-related health outcomes with concentrations of 
CO at ambient monitors. To inform our review of the ambient standards, 
we performed a quantitative exposure and dose modeling analysis that 
estimated COHb levels associated with different air quality conditions 
in simulated at-risk populations in two U.S. cities, as described in 
detail in the REA and summarized in the Policy Assessment (PA, section 
2.2.2). Thus, in developing conclusions with regard to the CO NAAQS, 
EPA has taken into account both evidence-based and exposure/dose-based 
considerations.
    The approach to reaching a decision on the adequacy of the current 
primary standards is framed by consideration of the following series of 
key policy-relevant questions.
     Does the currently available scientific evidence- and 
exposure/dose/risk-based information, as reflected in the ISA and REA, 
support or call into question the adequacy of the protection afforded 
by the current CO standards?
     Does the current evidence alter our conclusions from the 
previous review regarding the health effects associated with exposure 
to CO?
     Does the current evidence continue to support a focus on 
COHb levels as the most useful indicator of CO exposures and the best 
biomarker to characterize potential for health effects associated with 
exposures to ambient CO? Or does the current evidence provide support 
for a focus on alternate dose indicators to characterize potential for 
health effects?
     Does the current evidence alter our understanding of 
populations that are particularly susceptible to CO exposures? Is there 
new evidence that suggest additional susceptible populations that 
should be given increased focus in this review?
     Does the current evidence alter our conclusions from the 
previous review regarding the levels of CO in ambient air associated 
with health effects?
     To what extent have important uncertainties identified in 
the last review been reduced and/or have new uncertainties emerged?
    The following sections describe the assessment of these issues in 
the Policy Assessment, the advice received from CASAC, as well as the 
comments received from various parties, and then presents the 
Administrator's proposed conclusions regarding the adequacy of the 
current primary standards.
2. Evidence-Based and Exposure/Dose-Based Considerations in the Policy 
Assessment
    The Policy Assessment (chapter 2) considers the evidence presented 
in the Integrated Science Assessment, and preceding AQCDs, as discussed 
above in section II.B as a basis for evaluating the adequacy of the 
current CO standards, recognizing that important uncertainties remain. 
The Policy Assessment concludes that the combined consideration of the 
body of evidence and the results from the quantitative exposure and 
dose assessment provide support for standards at least as protective as 
the current suite of standards to provide appropriate public health 
protection for susceptible populations, including most particularly 
individuals with cardiovascular disease, against effects of CO in 
exacerbating conditions of reduced oxygen availability to the heart 
(PA, section 2.4). More specifically, the Policy Assessment concludes 
that the combined consideration of the evidence and quantitative 
estimates from the REA may be viewed as providing support for either 
retaining or revising the current suite of standards (PA, p. 2-59). 
CASAC stated agreement with this conclusion, while additionally 
expressing a ``preference'' for revisions to a lower standard. Members 
of the public who provided comments on the draft Policy Assessment 
supported retaining the current standard without revision. The specific 
considerations on which the Policy Assessment conclusions are based are 
described in the subsections below.

[[Page 8175]]

a. Evidence-Based Considerations
    In considering the evidence available for the current review of the 
CO NAAQS, the Policy Assessment discussed whether or not, or the extent 
to which, the current evidence alters conclusions reached in the 
previous review regarding levels of CO in ambient air associated with 
health effects and associated judgments on adequacy of the current 
standards. With this discussion, the Policy Assessment also considered 
the extent to which important uncertainties identified in the last 
review have been reduced or new uncertainties have emerged.
    As an initial matter, the Policy Assessment recognized that at the 
time of the last review, EPA's conclusions regarding the adequacy of 
the existing CO standards were drawn from the combined consideration of 
the evidence of COHb levels for which cardiovascular effects of concern 
had been reported and the results of an exposure and dose modeling 
assessment (59 FR 38906). As described in more detail above, the key 
effects judged to be associated with CO exposures resulting from 
concentrations observed in ambient air were cardiovascular effects, as 
measured by decreased time to onset of exercise-induced angina and to 
onset of ECG ST-segment depression (59 FR 38913). As at the time of the 
last review, the Policy Assessment noted that the evidence available in 
this review includes multiple studies that document decreases in time 
to onset of exercise-induced angina (a symptom of myocardial ischemia) 
in multiple studies at post-exposure COHb levels ranging from 2.9 to 
5.9% (CO-Ox), which represent incremental increases of approximately 
1.4-4.4% COHb from baseline (CO-Ox) (PA, Table 2-2; Adams et al., 1988; 
Allred et al., 1989a, 1989b, 1991; Anderson et al., 1973; Kleinman et 
al., 1989, 1998 \42\; Sheps et al., 1987 \43\). The study results from 
Allred et al. (1989a, 1989b, 1991) also provide evidence for these 
effects in terms of COHb measurements using gas 
chromatography.44 45 Evidence also available at the time of 
the last review of effects in other clinical study groups includes 
effects in subjects with cardiac arrhythmias and effects on exercise 
duration and maximal aerobic capacity in healthy adults. Among the 
studies of myocardial ischemia indicators in patients with CAD, none 
provide evidence of a measurable threshold at the lowest experimental 
CO exposures and associated COHb levels assessed (e.g., mean of 2.0-
2.4% COHb, GC) which resulted in average increases in COHb of about 
1.5% over pre-exposure baseline (Anderson et al., 1973; Kleinman et 
al., 1989; Allred et al. 1989a, 1989b, 1991).\46\ Allred et al. (1989a, 
1989b, 1991) further reported a dose-response relationship between the 
increased COHb levels and the response of the assessed indicators of 
myocardial ischemia (Allred et al., 1989a, 1989b, 1991). While this 
evidence informs our conclusions regarding COHb levels associated with 
health effects, the CO exposure concentrations employed in the studies 
to achieve these COHb levels were substantially above ambient 
concentrations. Thus, an exposure and dose assessment was performed to 
consider the COHb levels that might be attained as a result of 
exposures to ambient CO allowed under the current NAAQS, as described 
in section II.C above.
---------------------------------------------------------------------------

    \42\ One new study of this type is available since the 1994 
review. This study, which focused on a target COHb level of 3.9% 
COHb (CO-Ox) and is discussed in the 2000 AQCD is generally 
consistent with the previously available studies (2000 AQCD, section 
6.2.2; Kleinman et al., 1998).
    \43\ See footnote 15 above.
    \44\ Gas chromatography is generally recognized to be the more 
accurate method for COHb levels below 5% (ISA, section 5.2.4).
    \45\ In the lower CO exposure group, the post-exposure mean COHb 
was 3.21% by CO-Ox and 2.38% by GC, while the post-exercise mean 
COHb was 2.65% by CO-Ox and 2.00% by GC (Allred et al., 1989a, 
1989b, 1991).
    \46\ The studies by Anderson et al. (1973) and Kleinman et al. 
(1989) did not use GC to measure COHb levels, and reported reduced 
exercise duration due to increased chest pain at CO exposures 
resulting in 2.8-3.0% COHb (CO-Ox). The COHb levels assessed in 
these two studies represented increase in average COHb levels over 
baseline of 1.4% and 1.6% COHb.
---------------------------------------------------------------------------

    Since the time of the last review, there have been no new 
controlled human exposure studies specifically designed to evaluate the 
effects of CO exposure in susceptible populations at study mean COHb 
levels at or below 2% COHb. Thus, similar to the last review, the 
multilaboratory study by Allred et al. (1989a, 1989b, 1991) continues 
to be the study that has evaluated cardiovascular effects of concern 
(i.e., reduced time to exercise-induced myocardial ischemia as 
indicated by ECG ST-segment changes and angina) at the lowest tested 
COHb levels (ISA, section 2.7). This study is also of particular 
importance in this review because it is considered the most rigorous 
and well designed study, presenting the most sensitive analysis methods 
(GC used in addition to CO-Ox) to quantify COHb blood levels. Key 
findings from that study with regard to levels of CO associated with 
health effects, as discussed in section II.B.2 above, include the 
following:
     Short (50-70 minute) exposure to increased CO 
concentrations that resulted in increases in COHb to mean levels of 
2.0% and 3.9% (post-exercise) from mean a baseline level of 0.6% 
significantly reduced exercise time required to induce markers of 
myocardial ischemia in CAD patients. For the more objective marker of 
ST-segment change, the lower exposure reduced the time to onset by 5.1% 
(approximately one half minute) and the higher exposure reduced the 
time to onset by 12.1%.\47\
---------------------------------------------------------------------------

    \47\ Across all subjects, the mean time to angina onset for 
baseline or control (``clean'' air) exposures was approximately 8.5 
minutes, and the mean time to ST endpoint was approximately 9.5 
minutes, with the ``time to onset'' reductions of the two exposure 
levels being approximately one half and one minute, respectively for 
ST-segment change, and slightly less and slightly more than one half 
minute, respectively, for angina (Allred et al., 1989b).
---------------------------------------------------------------------------

     The associated dose-response relationship between 
incremental changes in COHb and change in time to myocardial ischemia 
in CAD patients indicates a 1.9% and 3.9% reduction in time to onset of 
exercise-induced angina and ST-segment change, respectively, per 1% 
increase in COHb concentration from average baseline COHb of 0.6% 
without evidence of a measurable threshold.
    As described in section II.B.2 above, a number of epidemiological 
studies of health outcome associations with ambient CO have been 
conducted since the last review. These include studies that have 
reported associations with different ambient CO metrics (e.g., 1-hour 
and 8-hour averages, often as central-site estimates) derived from CO 
measurements at fixed-site ambient monitors in selected urban areas of 
the U.S. and cardiovascular endpoints other than stroke, particularly 
hospitalizations and emergency department visits for specific 
cardiovascular health outcomes including IHD, CHF and CVD (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). In general, these studies, many of which were designed to 
evaluate the effects of a variety of air pollutants, including CO, 
report positive associations, a number of which are statistically 
significant (ISA, sections 5.2.3 and 5.2.1.9). The long-standing body 
of evidence for CO summarized above, including the well-characterized 
role of CO in limiting oxygen availability, lends biological 
plausibility to the ischemia-related health outcomes reported in the 
epidemiological studies, providing coherence between these studies and 
the clinical evidence of short-term exposure to CO and health effects. 
Thus, although there is no new evidence

[[Page 8176]]

regarding the effects of short-term controlled CO exposures that result 
in lower COHb levels, the evidence is much expanded with regard to 
epidemiological \48\ analyses of ambient monitor concentrations, which 
observed associations between specific and overall cardiovascular-
related outcomes and ambient CO measurements.
---------------------------------------------------------------------------

    \48\ Few epidemiological studies that had investigated the 
relationship between CO exposure and ischemic heart disease were 
available at the time of the last completed review (1991 AQCD, 
section 10.3.3).
---------------------------------------------------------------------------

    The Policy Assessment considered the combined evidence base for CO 
cardiovascular effects in the context of a conceptual model of the 
pathway from CO exposures to the occurrence of these effects (as 
described in section 2.2.1 of the PA). In this context, the Policy 
Assessment noted differences between the controlled human exposure and 
epidemiological studies, described above, with regard to the elements 
along this pathway that have been investigated in those studies. The 
controlled human exposure studies document relationships between 
directly measured controlled short-term CO exposures and specific 
levels of an internal dose metric, COHb, which elicited specific 
myocardial ischemia-related responses in CAD patients. These studies 
inform our interpretation of the associations we observed in the 
epidemiological studies. The epidemiological studies reported 
associations between CO levels measured at fixed-site monitors and 
emergency department visits and/or hospital admissions for IHD and 
other cardiovascular disease-related outcomes that are plausibly 
related to the effects on physiological indicators of myocardial 
ischemia (e.g., ST-segment changes) demonstrated in the controlled 
human exposure studies, providing coherence between the two sets of 
findings (ISA, p. 5-48). With regard to extending our understanding of 
effects occurring below levels of CO evaluated in the controlled human 
exposure studies, however, the epidemiological evidence for CO is 
somewhat limited. The epidemiological evidence lacks measurements of 
COHb or personal exposure concentrations that would facilitate 
integration with the controlled human exposure study data. Furthermore, 
the epidemiological evidence base for IHD outcomes or CVD outcomes as a 
whole includes a number of studies involving conditions in which the 
current standard was not met. Though these studies are informative to 
consideration of the relationship of health effects to the full range 
of ambient CO concentrations, the Policy Assessment indicated that they 
are less useful to informing our conclusions regarding adequacy of the 
current standards.
    As discussed in the Policy Assessment, the smaller set of 
epidemiological studies, under conditions where the current standards 
were met, is considered to better inform our assessment of the adequacy 
of the standards or conditions of lower ambient concentrations. Among 
the few studies conducted during conditions in which the current 
standards were always met, however, the studies reporting statistical 
significance for IHD or all CVD outcomes are limited to a single study 
area (i.e. Atlanta). When the analyses reporting significance for 
association with CHF outcomes are also considered, a second study area 
is identified (Allegheny County, PA) in which the current standard is 
met throughout the study period. The analyses for both areas involve 
the use of central site monitor locations or area-wide average 
concentrations, which given the significant concentration gradients of 
CO in urban areas (ISA, section 3.6.8.2), complicates our ability to 
draw conclusions from them regarding ambient CO concentrations of 
concern. Therefore, the Policy Assessment primarily focused 
consideration of the epidemiological studies on the extent to which 
this evidence is consistent with and generally supportive of 
conclusions drawn from the combined consideration of the controlled 
human exposure evidence with estimates from the exposure and dose 
assessment, as discussed below. The Policy Assessment indicated that, 
as in the previous review, the integration of the controlled human 
exposure evidence with the exposure and dose estimates will be most 
important to informing conclusions regarding ambient CO concentrations 
of public health concern.
    With regard to areas of uncertainty, the Policy Assessment 
recognized that some important uncertainties have been reduced since 
the time of the last review, some still remain and others, associated 
with newly available evidence, have been identified. This range of 
uncertainties identified at the time of the last review (59 FR 38913, 
USEPA, 1992), as well as any newly identified uncertainties were 
considered in the Policy Assessment as discussed below (PA, section 
2.2.1).
    The CO-induced effects considered of concern at the time of the 
last review were reduced time to exercise-induced angina and ST-segment 
depression in patients suffering from coronary artery disease as a 
result of increases in COHb associated with short CO exposures. These 
effects had been well documented in multiple studies, and it was 
recognized that the majority of cardiologists at the time believed that 
recurrent exercise-induced angina was associated with substantial risk 
of precipitating myocardial infarction, fatal arrhythmia, or slight but 
cumulative myocardial damage (USEPA, 1992, p. 22; 59 FR 38911; Basan, 
1990; 1991 AQCD). As at the time of the last review, although ST-
segment depression is a recognized indicator of myocardial ischemia, 
the exact physiological significance of the observed changes among 
individuals with CAD is unclear (ISA, p. 5-48).
    In interpreting the study results at the time of the last review, 
EPA recognized uncertainty in the COHb measurements made using CO-Ox 
and associated uncertainty in establishing a lowest effects level for 
CO (USEPA, 1992, p. 31). A then-recent multicenter study (Allred et 
al., 1989a, 1989b, 1991) was of great importance at that time for 
reasons identified above. Similarly, the Science and Policy Assessments 
place primary emphasis on the findings from this study in the current 
review of the evidence related to cardiovascular effects associated 
with CO exposure, recognizing the superior quality of the study, both 
in terms of the rigorous study design as well as the sensitivity of the 
analytical methods used in determining COHb concentrations (ISA, 
section 2.7). No additional controlled human exposure studies are 
available that evaluate responses to lower COHb levels in the 
cardiovascular-disease population, and uncertainties still remain in 
determining specific and quantitative relationships between the CO-
induced effects in these studies and the increased risk of specific 
health outcomes. Further, with regard to then-unidentified effects at 
lower COHb levels, no studies have identified other effects on the CAD 
population or on other populations at lower exposures (ISA, sections 
5.2.2).
    The last review recognized uncertainty with regard to the potential 
for short-term CO exposures to contribute to CNS effects which might 
affect an individual's performance of complex activities such as 
driving a car or to contribute to other effects of concern. It was 
concluded, however, that the focus of the review on cardiovascular 
effects associated with COHb levels below 5% also provided adequate 
protection against potential

[[Page 8177]]

adverse neurobehavioral effects.\49\ No new controlled human exposure 
studies have evaluated CNS or behavioral effects of exposure to CO 
(ISA, section 5.3.1). However, given the drastic reduction in CO 
ambient concentrations, the Policy Assessment concludes that occurrence 
of these effects in response to ambient CO would be expected to be rare 
within the current population. Thus, the Policy Assessment concludes 
that uncertainty with regard to the potential for such effects to be 
associated with current ambient CO exposures is reduced (PA, p. 2-35).
---------------------------------------------------------------------------

    \49\ The evidence available at the time of the last review was 
based on a series of studies conducted from the mid 1960's through 
the early 1990's, with inconsistent findings of neurological effects 
at exposures to CO resulting in COHb levels ranging from 5-20% (1991 
AQCD).
---------------------------------------------------------------------------

    Since the 1994 review, the epidemiologic and toxicological evidence 
of effects on birth and developmental outcomes has expanded, although 
the available evidence is still considered limited with regard to 
effects on preterm birth, birth defects, decreases in birth weight, 
measures of fetal growth, and infant mortality (ISA, section 5.4). 
Further, while animal toxicological studies provide support and 
coherence for those effects, the understanding of the mechanisms 
underlying reproductive and developmental effects is still lacking 
(ISA, section 5.4.1). Thus, the Policy Assessment recognizes that 
although the evidence continues to ``suggest[s] that critical 
developmental phases may be characterized by enhanced sensitivity to CO 
exposure'' (ISA, p. 2-11), evidence is lacking for adverse 
developmental or reproductive effects at CO exposure concentrations 
near those associated with current levels of ambient CO (PA, pp. 2-35 
to 2-36).
    As described above, the much-expanded epidemiologic database in the 
current review includes studies that show associations between ambient 
CO concentrations and increases in emergency room visits and 
hospitalizations for disease events plausibly linked to the effects 
observed in the controlled human exposure studies of CAD patients (ISA, 
section 2.5.1), providing support for the ISA's conclusion regarding 
coronary artery disease as the most important susceptibility 
characteristic for increased health risk due to CO exposure (ISA, p. 2-
10). However, the Policy Assessment recognizes aspects of this 
epidemiological evidence that complicate quantitative interpretation of 
it with regard to ambient concentrations that might be eliciting the 
reported health outcomes. As an initial matter, the Policy Assessment 
notes the substantially fewer studies conducted in areas meeting the 
current CO standards than is the case for NO2 and PM (USEPA, 
2008d, 2009f). Further, the Policy Assessment recognizes complicating 
aspects of the evidence that relate to conclusions regarding CO as the 
pollutant eliciting the effect reported in the epidemiological studies 
and to our understanding of the ambient CO and nonambient 
concentrations to which study subjects demonstrating these outcomes are 
exposed.
    With regard to these complications, the Policy Assessment first 
considers the extent to which the use of two-pollutant regression 
models, a commonly used statistical method (ISA, section 1.6.3), inform 
conclusions regarding CO as the pollutant eliciting the effects in 
these studies (PA, pp. 2-36 to 2-37). Although CO associations, in some 
studies, are slightly attenuated in models that adjusted for other 
combustion-related pollutants (e.g., PM2.5 or 
NO2), they generally remain robust (ISA, Figures 5-6 and 5-
7).\50\ In considering these two-pollutant model results, however, the 
Policy Assessment recognizes the potential for there to be 
etiologically relevant pollutants that are correlated with CO yet 
absent from the analysis. Similarly, CASAC commented that ``the problem 
of co-pollutants serving as potential confounders is particularly 
problematic for CO''. They stated 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). This 
issue is particularly important in the case of CO in light of 
uncertainty associated with CO-related effects at low ambient 
concentrations (discussed below) and in light of the sizeable portion 
of ambient CO measurements that are at or below monitor detection 
limits. Consequently, the extent to which multi-pollutant regression 
models effectively disentangle and quantitatively interpret a CO-
specific effect distinct from that of other pollutants remains an area 
of uncertainty.
---------------------------------------------------------------------------

    \50\ In interpreting the epidemiological evidence for 
cardiovascular morbidity the ISA notes that it ``is difficult to 
determine from this group of studies the extent to which CO is 
independently associated with CVD outcomes or if CO is a marker for 
the effects of another traffic-related pollutant or mix of 
pollutants. On-road vehicle exhaust emissions are a nearly 
ubiquitous source of combustion pollutant mixtures that include CO 
and can be an important contributor to CO in near-road locations. 
Although this complicates the efforts to disentangle specific CO-
related health effects, the evidence indicates that CO associations 
generally remain robust in copollutant models and supports a direct 
effect of short-term ambient CO exposure on CVD morbidity.'' (ISA, 
pp. 5-40 to 5-41).
---------------------------------------------------------------------------

    In considering ambient concentrations that may be triggering health 
outcomes analyzed in the epidemiological studies, the Policy Assessment 
recognizes the uncertainty introduced by exposure error. Exposure error 
can occur when a surrogate is used for the actual ambient exposure 
experienced by the study population (e.g., ISA, section 3.6.8). There 
are two aspects to the epidemiological studies in the specific case of 
CO, as contrasted with the cases of other pollutants such as 
NO2 and PM, that may contribute to exposure error in the CO 
studies. The first relates to the low concentrations of CO considered 
in the epidemiological studies and monitor detection limits. The second 
relates to the use in the epidemiological studies of area-wide or 
central-site monitor CO concentrations in light of information about 
the gradient in CO concentrations with distance from source locations 
such as highly-trafficked roadways (ISA, section 3.5.1.3).
    As discussed in the Policy Assessment, uncertainty in the 
assessment of exposure to ambient CO concentrations is related to the 
prevalence of ambient CO monitor concentrations at or below detection 
limits, which 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). For example, the ISA notes that roughly one third of the 1-
hour ambient CO measurements reported to AQS for 2005-2007 were below 
the method limit of detection for the monitors analyzed (ISA, p. 3-34). 
A similarly notable proportion of measurements occur below the monitor 
detection limit for epidemiological study areas meeting the current 
standards (e.g., Atlanta, Allegheny County) (PA, Appendix B). This 
complicates our interpretation of specific ambient CO concentrations 
associated with health effects (ISA, p. 3-91; Brain and Samet, 2010d). 
In contrast to CO, other combustion-related criteria pollutants such as 
PM2.5 and NO2 generally occur above levels of 
detection, providing us with greater confidence in quantitative 
interpretations of epidemiological studies for those pollutants.
    There are also differences in the spatial variability associated 
with PM2.5 and NO2 concentrations as compared to 
CO concentrations that add complexity

[[Page 8178]]

to the estimation of CO exposures in epidemiological studies. In 
general, PM2.5 concentrations tend to be more spatially 
homogenous across an urban area than CO concentrations. CO 
concentrations in urban areas are largely driven by mobile sources, 
while urban PM2.5 concentrations substantially reflect 
contributions from mobile and a variety of stationary sources. The 
greater spatial homogeneity in PM2.5 concentrations is due 
in part to the transport and dispersion of small particles from the 
multiple sources (USEPA, 2009f, sections 3.5.1.2 and 3.9.1.3), as well 
as to contributions from secondarily formed components ``produced by 
the oxidation of precursor gases (e.g., sulfur dioxide and nitrogen 
oxides) and reactions of acidic products with NH3 and 
organic compounds'' (USEPA, 2009f, p. 3-185), which likely contribute 
to spatial homogeneity. Similarly, ``because NO2 in the 
ambient air is due largely to the atmospheric oxidation of NO emitted 
from combustion sources (ISA, section 2.2.1), elevated NO2 
concentrations can extend farther away from roadways than the primary 
pollutants also emitted by on-road mobile sources'' (40 FR 6479, 
February 9, 2010). In contrast to PM2.5 and NO2, 
CO is not formed through common atmospheric oxidation processes, which 
may contribute to the steeper CO gradient observed near roadways. 
Therefore, the misclassification of exposure arising from the 
utilization of central site monitors to measure PM2.5 and 
NO2 exposures is likely to be smaller than is the case for 
CO exposures.
    An additional complication to a comparison of our consideration of 
the CO epidemiological evidence to that for other criteria pollutants 
is that, in contrast to the situation 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 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 the broader health 
effects evidence for CO demonstrates and focuses on an internal 
biomarker of CO exposure (COHb) which has been considered a critical 
key to CO toxicity. 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.\51\ 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. 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.
---------------------------------------------------------------------------

    \51\ In the case of lead (Pb), in contrast to that of 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.
---------------------------------------------------------------------------

    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{time}  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).
    In summary, the Policy Assessment concludes that some important 
uncertainties from the last review have been reduced, including those 
associated with concerns for ambient levels of CO to pose 
neurobehavioral risks as current concentrations of ambient CO are well 
below those that might be expected to result in COHb levels as high as 
those associated with these effects. Additionally, our exposure and 
dose models have improved giving us increased confidence in their 
estimates. A variety of uncertainties still remain including the 
adverse nature and significance of the small changes in time to ST-
segment depression identified at the lowest COHb levels investigated, 
and the magnitude of associated risk of specific health outcomes, as 
well as the potential for as-yet-unidentified health effects at COHb 
levels below 2%. Additionally, although the evidence base is somewhat 
expanded with regard to the potential for CO effects on the developing 
fetus, uncertainties remain in our understanding of the potential 
influence of low, ambient CO exposures on conditions existing in the 
fetus and newborn infant and on maternal-fetal relationships. We 
additionally recognize that the expanded body of epidemiological 
evidence includes its own set of uncertainties which complicates its 
interpretation, particularly with regard to ambient concentrations that 
may be eliciting health outcomes.
b. Exposure/Dose-Based Considerations
    In considering the evidence from controlled human exposure studies 
to address the question regarding ambient CO concentrations associated 
with health effects, we have developed estimates of COHb associated 
with different air quality conditions using quantitative exposure and 
dose modeling, as was done at the time of the last review. The current 
estimates are presented in the REA and discussed with regard to policy-
relevant considerations in this review in the Policy Assessment (PA, 
section 2.2.2). Since the last review, there have been numerous 
improvements to the exposure and COHb models that we use to estimate 
exposure and dose for the current review. The results of modeling using 
these improved tools in the current review and associated conclusions 
in the Policy Assessment are described below with regard to the 
expectation for COHb levels of concern to occur in the at-risk 
population under air quality conditions associated with the current CO 
standards.
    In considering the results from the REA, the Policy Assessment 
considered several questions including those concerning the magnitude 
of COHb levels estimated in the simulated at-risk populations in 
response to ambient CO exposure, as well as the extent to which such 
estimates may be judged to be important from a public health 
perspective.
    In addressing the questions concerning the magnitude of at-risk 
population COHb levels estimated to

[[Page 8179]]

occur in areas simulated to just meet the current, controlling, 8-hour 
standard and what portion of the at-risk population is estimated to 
experience maximum COHb levels above levels of potential health 
concern, the Policy Assessment first noted the context for the 
population COHb estimates provided by the REA simulations of exposure 
to ambient CO (REA, section 6.2). As in the last review, the Policy 
Assessment recognized that indoor sources of CO can be important 
determinants of population exposures to CO and to population 
distributions of daily maximum COHb levels, and that for some portions 
of the population, these sources may dominate CO exposures and related 
maximum COHb levels. The Policy Assessment additionally took note of 
the conclusions drawn in the previous review that the contribution of 
indoor sources to individual exposures and associated COHb levels 
cannot be effectively mitigated by ambient air quality standards (e.g., 
59 FR 38914) and so focused on COHb levels resulting from ambient CO 
exposures. In so doing, however, the Policy Assessment also recognized 
as noted in section II.C above, that simulations focused solely on 
exposures associated with ambient CO may overestimate the response of 
COHb levels to short-duration ambient exposures (the ambient 
contribution) as pre-exposure baseline COHb levels will necessarily not 
reflect the contribution of both nonambient and ambient sources. 
Additionally, these simulations may underestimate COHb levels that 
would occur in situations with appreciable nonambient exposure.
    As recognized in the Policy Assessment and described in detail in 
the REA, estimates for exposure concentrations indicated that highest 
ambient CO exposures occurred in in-vehicle microenvironments, with 
next highest exposures in microenvironments where running vehicles 
congregate such as parking areas and fueling stations, (REA, section 
6.1).
    In considering the REA estimates for current or ``as is'' air 
quality conditions and conditions simulated to just meet the current 8-
hour standard, the Policy Assessment particularly focused on the extent 
to which the current standards provide protection to the simulated at-
risk population from COHb levels of potential concern, by comparing the 
estimated levels in the population to the benchmarks described above. 
As described above, the REA presents two sets of COHb estimates: the 
first set of absolute estimates reflect the impact of ambient CO 
exposures in the absence of exposure to nonambient CO, but in the 
presence of endogenous CO production, while the second set are 
estimates of the portion of absolute COHb estimated to occur in 
response to the simulated ambient CO exposures, i.e., after subtraction 
of COHb resulting from endogenous CO production (REA, sections 4.4.7 
and 5.10.3). In describing the REA results, the Policy Assessment draws 
from exposure and dose estimates for both the HD and CHD populations 
(REA, section 6.2), recognizing that, in terms of percentages of 
persons exposed and experiencing daily maximum end-of-hour COHb at or 
above specific levels, the results are similar for the two simulated 
at-risk populations (HD and CHD). We note that, in terms of absolute 
numbers of persons, the results differ due to differences in the size 
of the two populations.
    The Policy Assessment first considered the absolute COHb results 
with regard to the percentage of simulated populations experiencing at 
least one day with an end-of hour COHb level above selected benchmarks 
(Table 1 includes these results for the HD populations). Another 
dimension of the analysis, presented in Table 2 (for the CHD 
populations),\52\ is the percentage of simulated populations 
experiencing multiple days in the simulated year with an end-of-hour 
COHb level above the same benchmarks. These two dimensions of the dose 
estimates are combined in the metric, person-days, which is presented 
in Tables 6-15, 6-16, 6-18 and 6-19 of the REA. The metric, person-
days, was the focus of exposure/dose considerations in the last review 
for which a previous version of the exposure/dose model was used (59 FR 
38914; USEPA, 1992).\53\ The person-days metric, which summarizes 
occurrences across the number of persons in the at-risk population 
multiplied by the number of days in the year, is a common cumulative 
measure of population exposure/dose that simultaneously takes into 
account both the number of people affected and the numbers of times 
each is affected.
---------------------------------------------------------------------------

    \52\ As described in the REA, the analyses providing results for 
Table 2 were only performed for the CHD populations, and so are not 
available for the larger HD population, although as mentioned above 
the results in terms of percentage are expected to be similar.
    \53\ As described in section II.C. above, pNEM, the model used 
in the last review, employed a cohort-based approach from which 
person-days were the exposure and dose metrics (USEPA, 1992; Johnson 
et al., 1992).
---------------------------------------------------------------------------

    As expected, given that current ambient concentrations in the two 
study areas are well below the CO standards, the absolute COHb 
estimates under current air quality conditions are appreciably lower 
than the corresponding estimates for conditions of higher ambient CO 
concentrations in which the current 8-hour standard is just met (Table 
1). Under ``as is'' (2006) conditions in the two study areas, no person 
in the simulated at-risk populations is estimated to experience any 
days in the year with end-of-hour COHb concentrations at or above 3% 
COHb, and less than 0.1% of the simulated at-risk populations are 
estimated to experience at least one end-of-hour COHb concentration at 
or above 2% (Table 1).
    Under conditions with higher ambient CO concentrations simulated to 
just meet the current 8-hour standard, the portion of the simulated at-
risk populations estimated to experience daily maximum end-of-hour COHb 
levels at or above benchmarks is greater in both study areas, with 
somewhat higher percentages for the Denver study area population (Table 
1). In both study areas, nonetheless, less than 1% of the simulated at-
risk populations is estimated to experience a single day with a maximum 
end-of hour COHb level at or above 3% (Table 1) and no person is 
estimated to experience more than one such day in a year (Table 2). 
Further, less than 0.1% of either simulated population in either study 
area is estimated to experience a single day with maximum end-of-hour 
COHb at or above 4%. A difference between the study areas is more 
evident for lower benchmarks, with less than 5% of the simulated at-
risk population in the Denver study area and less than 1% of the 
corresponding population in the Los Angeles study area estimated to 
experience any days with a maximum end-of-hour COHb level at or above 
2% (Table 1). Appreciably smaller percentages of the simulated at-risk 
population were estimated to experience more than one day with such 
levels (Table 2). For example, less than 1.5% of the population is 
estimated to experience more than one day in a year with a maximum COHb 
level at or above 2.0%, and less than 0.1% are estimated to experience 
six or more such days in a year. Additionally, consistent with the 
findings of the assessment performed for the review completed in 1994, 
less than 0.1% of person-days for the simulated at-risk populations 
were estimated to have end-of-hour COHb levels at or above 2% COHb 
(REA, Tables 6-18 and 6-19).

[[Page 8180]]

  Table 1--Portion of Simulated HD Populations With at Least One Daily Maximum End-of-Hour COHb Level (Absolute) at or Above Indicated Levels Under Air
                                 Quality Conditions Simulated to Just Meet the Current Standard and ``as is'' Conditions
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                 Percentage (%) of simulated HD population \A\
                                                     ---------------------------------------------------------------------------------------------------
                                                      Just meeting current 8-hour standard  (8-hr DV =             ``As is'' (2006) conditions
      Daily maximum end-of-hour COHb (absolute)                           9.4 ppm)                     -------------------------------------------------
                                                     --------------------------------------------------  Los Angeles (8-hr DV =   Denver (8-hr DV = 3.1
                                                       Los Angeles (1-hr DV =   Denver (1-hr DV = 16.2  5.6 ppm) (1-hr DV = 8.2    ppm) (1-hr DV = 4.6
                                                             11.8 ppm)                   ppm)                     ppm)                     ppm)
--------------------------------------------------------------------------------------------------------------------------------------------------------
>= 4.0%.............................................                        0                \B\ < 0.1                        0                        0
>= 3.0%.............................................                \B\ < 0.1                      0.3
>= 2.5%.............................................                \B\ < 0.1                      0.9
>= 2.0%.............................................                      0.6                      4.5                \B\ < 0.1                \B\ < 0.1
>= 1.5%.............................................                      5.0                     24.5                      1.6                      1.2
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Drawn from Tables 6-15 through 6-19 of the REA.
\B\ <0.1 is used to represent nonzero estimates below 0.1%.
Abbreviations: hr = hour, DV = Design Value.

Table 2--Portion of Simulated CHD Population With Multiple Days of Maximum End-of-Hour COHb Levels (Absolute) at or Above the Indicated Levels Under Air
                                 Quality Conditions Simulated To Just Meet the Current Standard and ``as is'' Conditions
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                            Percentage (%) of simulated CHD population \A\
                                             -----------------------------------------------------------------------------------------------------------
                                               Just meeting current 8-hour standard (8-hr DV = 9.4               ``As is'' (2006) conditions
                                                                      ppm)                         -----------------------------------------------------
                                             ------------------------------------------------------   Los Angeles (8-hr DV =     Denver (8-hr DV = 3.1
  Maximum end-of-hour COHb level (absolute)     Los Angeles (1-hr DV =     Denver (1-hr DV = 16.2    5.6 ppm) (1-hr DV = 8.2    ppm) (1-hr DV = 4.6 ppm)
                                                      11.8 ppm)                     ppm)                       ppm)           --------------------------
                                             ---------------------------------------------------------------------------------
                                                >= 2     >= 4     >= 6     >= 2     >= 4     >= 6     >= 2     >= 4     >= 6     >= 2     >= 4     >= 6
                                                days     days     days     days     days     days     days     days     days     days     days     days
--------------------------------------------------------------------------------------------------------------------------------------------------------
>= 3.0%.....................................        0        0        0        0        0        0        0        0        0        0        0        0
>= 2.5%.....................................    \B\ <        0        0    \B\ <        0        0        0        0        0        0        0        0
                                                  0.1                        0.1
>= 2.0%.....................................      0.2    \B\ <    \B\ <      1.4      0.2    \B\ <        0        0        0    \B\ <    \B\ <    \B\ <
                                                           0.1      0.1                        0.1                                 0.1      0.1      0.1
>= 1.5%.....................................      2.2      0.7      0.5     11.2      5.0      3.3      0.5      0.2      0.1      0.7      0.5      0.4
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ These estimates are drawn mainly from Figures 6-5 and 6-6 of the REA and represent the percentage of persons experiencing greater than or equal to
  2, 4, or 6 days with a maximum end-of-hour COHb (absolute) at or above the selected level.
\B\ <0.1 is used to represent nonzero estimates below 0.1%.

    As described above, the REA also presented estimates of the portion 
of the absolute COHb levels occurring in response to the simulated 
ambient CO exposures (i.e., that not derived from endogenous CO 
production). The REA refers to these estimates as the ambient CO 
contribution to (absolute) COHb. As observed with the absolute COHb 
estimates under conditions just meeting the standard, the results for 
the Denver study area included larger percentages of the population 
above specific COHb ambient contribution levels than those for the Los 
Angeles study area, reflecting the study area difference in 1-hour peak 
concentrations. Although estimates of population percentages for 
multiple occurrences are not available for the ambient contribution 
estimates, it is expected that similar to those for absolute COHb, they 
would be appreciably lower than those shown here for at least one 
occurrence. Additionally, as mentioned above, somewhat lower ambient 
contribution estimates might be expected if other (nonambient) CO 
sources were present in the simulations.
    In considering the estimates of population occurrences of daily 
maximum COHb levels for REA simulations under conditions just meeting 
the current 8-hour standard (presented in Tables 1 and 2 above), the 
Policy Assessment notes that an important contributing factor to the 
higher percentages estimated for the Denver study area population is 
the occurrence of higher 1-hour peak ambient CO concentrations and 
consequent higher CO exposures than occur in the corresponding Los 
Angeles study area simulation (REA, section 6.1.2, Tables 6-7 and 6-
10). The difference in the peak 1-hour ambient concentrations is 
illustrated by the higher 1-hour design value for Denver as compared to 
Los Angeles (16.2 ppm versus 11.8 ppm), as noted in Tables 1 and 2. 
This difference, particularly at the upper percentiles of the air 
quality distribution, is likely driving the higher population 
percentages estimated to experience higher 1-hour and 8-hour exposures 
in the Denver study area as compared to Los Angeles (REA, Tables 6-7 
and 6-10).\54\ The situation is largely reversed under ``as is'' 
conditions, where the Los Angeles study area has generally higher 1-
hour and 8-hour ambient CO concentrations as illustrated by the design 
values for as is conditions in Tables 1 and 2 above (as well as Tables 
3-1 to 3-6, 5-14 and 5-16 of the REA), and Los Angeles also has higher 
percentages of people estimated to be exposed to the higher exposure 
concentrations (REA, Tables 6-1 and 6-4). Thus, the Policy Assessment 
recognizes the impact on daily maximum COHb levels of 1-hour

[[Page 8181]]

ambient concentrations separate from the impact of 8-hour average 
concentrations, and takes note of this in considering the REA results 
with regard to the adequacy of the 1-hour standard. The Policy 
Assessment concludes that, taken together, the REA results indicate 
occurrences of COHb levels above the benchmarks considered here that 
are associated with 1-hour ambient concentrations that are not 
controlled by the current suite of standards (PA, section 2.2.2).
---------------------------------------------------------------------------

    \54\ Other factors that contribute less to differences in COHb 
estimates between the two study areas include altitude, which 
slightly enhances endogenous CO and COHb formation and can enhance 
COHb formation induced by CO exposure under resting conditions (ISA, 
p. 4-19), and design aspects of the study areas with regard to 
spatial variation in monitor CO concentrations and population 
density near these monitors (REA, section 7.2.2.1).
---------------------------------------------------------------------------

    In considering the public health implications of the quantitative 
dose estimates, the Policy Assessment considered the daily maximum end-
of-hour levels estimated in the REA for conditions just meeting the 
current suite of standards in light of the effects identified by the 
evidence at the COHb benchmark levels considered. For example, as a 
result of ambient CO exposures occurring under air quality conditions 
adjusted to just meet the current 8-hour standard, the REA estimates 
that 0.6 percent of the Los Angeles and 4.5 percent of the Denver study 
at-risk populations may experience an occurrence of a daily maximum 
end-of-hour COHb level at or above 2% COHb, the low end of the range of 
average COHb levels experienced by the lower controlled exposure group 
in the study by Allred et al. (1989a, 1989b, 1991), while 0.2 and 1.4 
percent, respectively, of the simulated at-risk populations are 
estimated to experience more than one such occurrence. Additionally, 
less than 0.1 percent of the simulated populations in either study area 
are estimated to experience a COHb level similar to the higher 
controlled exposure group (4% COHb). As discussed in II.B.4 above, the 
Policy Assessment recognized the magnitude of the ``time to onset'' 
reductions observed in the study by Allred et al. (1989a, 1989b, 1991), 
the similarity of the study responses to responses considered 
clinically significant when evaluating medications to treat angina from 
coronary artery disease, and conclusions reached by the independent 
review panel for the study regarding the expectation that frequent 
encounters in ``everyday life'' with increased COHb levels on the order 
of those tested in the study might limit activity and affect quality of 
life (Allred et al., 1989b, pp. 38, 92-94; 1991 AQCD, p. 10-35), as 
well as considerations in the review completed in 1994 and assessment 
of the study findings in the current ISA.
    In considering public health implications of the REA estimates, the 
Policy Assessment also considered the size of the at-risk populations 
simulated as described in section II.B.4 above, recognizing that the 
U.S. population with coronary heart disease, angina pectoris (cardiac 
chest pain) or who have experienced a heart attack in combination with 
those with silent or undiagnosed ischemia comprises a large population 
represented by the REA analyses and for which the COHb benchmarks 
described above (based on studies of CAD patients) are relevant, that 
is, more susceptible to ambient CO exposure when compared to the 
general population (ISA, section 5.7). The Policy Assessment also 
recognized that the REA also simulated ambient CO exposures for the 
larger HD population, which may also be at increased risk of CO-induced 
health effects (ISA, section 2.6.1), while noting that within this 
broader group, implications of CO exposures are more significant for 
those persons for whom their disease state affects their ability to 
compensate for the hypoxia-related effects of CO (ISA, section 4.4.4).
    In summary, the Policy Assessment, while noting the substantial 
size of the population of individuals with CHD or other heart diseases 
in the U.S., recognized that the REA results for conditions just 
meeting the current standards indicate a very small portion of this 
population that might be expected to experience more than one 
occurrence of COHb above 2%, with less than 0.1% of this population 
expected to experience such a level on as many as six days in a year or 
a single occurrence as high as 4%, and 0% of the population expected to 
experience more than one occurrence above 4% COHb. In light of the 
implications of the health evidence discussed in section II.B.4 and 
summarized above, the Policy Assessment concluded that the public 
health significance of these REA results and conclusions regarding the 
extent to which they are important from a public health perspective 
depends in part on public health policy judgments about the public 
health significance of effects at the COHb benchmark levels considered 
and judgments about the level of public health protection with an 
adequate margin of safety.
c. Summary
    With regard to the different elements of the current standards, the 
Policy Assessment concludes that it is appropriate to continue to use 
measurements of CO in accordance with Federal reference methods as the 
indicator to address effects associated with exposure to ambient CO, 
and that it is appropriate to continue to retain standards with 
averaging times of 1 and 8 hours. With regard to form and level for 
these standards, the Policy Assessment concludes that the information 
available in this review supports consideration of either retaining the 
current suite of standards or revising one or both standards.
    The Policy Assessment concludes that the extent to which the 
current standards are judged to be adequate depends on a variety of 
factors inclusive of science policy judgments and public health policy 
judgments. These factors include public health policy judgments 
concerning the appropriate COHb benchmark levels on which to place 
weight, as well as judgments on the public health significance of the 
effects that have been observed at the lowest levels evaluated, 
particularly with regard to relatively rare occurrences. The factors 
relevant to judging the adequacy of the standards also include 
consideration of the uncertainty associated with interpretation of the 
epidemiological evidence as providing information on ambient CO as 
distinct from information on the mixture of pollutants associated with 
traffic, and, given this uncertainty, the weight to place on 
interpretations of ambient CO concentrations for the few 
epidemiological studies available for air quality conditions that did 
not exceed the current standards. And, lastly these factors include the 
interpretation of, and decisions as to the weight to place on, the 
results of the exposure assessment for the two areas studied relative 
to each other and to results from past assessments, recognizing the 
implementation of an improved modeling approach and new input data, as 
well as distinctions between the REA simulations and resulting COHb 
estimates and the response of COHb levels to experimental CO exposure 
as recorded in the controlled human exposure studies.
    The Policy Assessment conclusions with regard to the adequacy of 
the current standards are drawn from both the evidence and from the 
exposure and dose assessment, taking into consideration related 
information, limitations and uncertainties recognized above. The 
combined consideration of the body of evidence and the quantitative 
exposure and dose estimates are concluded to provide support for a 
suite of standards at least as protective as the current suite. 
Further, the Policy Assessment recognizes that conclusions regarding 
the adequacy of the current standards depend in part on public health 
policy judgments identified above and judgments about the level of 
public health protection with an adequate margin of safety.

[[Page 8182]]

    The Policy Assessment additionally notes the influence that hourly 
ambient CO concentrations well below the current 1-hour standard may 
have on ambient CO exposures and resultant COHb levels under conditions 
just meeting the 8-hour standard, as indicated by the REA results. The 
REA results are concluded to indicate the potential for the current 
controlling 8-hour standard to allow the occurrence of 1-hour ambient 
concentrations that contribute to population estimates of daily maximum 
COHb levels, that depending on public health judgments in the areas 
identified above, may be considered to call into question the adequacy 
of the 1-hour standard and support consideration of revisions of that 
standard in order to reduce the likelihood of such occurrences in areas 
just meeting the 8-hour standard. Thus, the Policy Assessment concludes 
that the combined consideration of the evidence and quantitative 
estimates may be viewed as providing support for either retaining or 
revising the current suite of standards.
    The Policy Assessment conclusion that it is appropriate to consider 
retaining the current suite of standards without revision is based on 
consideration of the health effects evidence in combination with the 
results of the REA (PA, sections 2.2.1, 2.2.2, 2.3.2 and 2.3.3) and 
what may be considered reasonable judgments on the public health 
implications of the COHb levels estimated to occur under the current 
standard, the public health significance of the CO effects being 
considered, the weight to be given to findings in the epidemiological 
studies in locations where the current standards are met, and advice 
from CASAC. Such a conclusion takes into account the long-standing body 
of evidence that supports our understanding of the role of COHb in 
eliciting effects in susceptible populations, most specifically the 
evidence for those with cardiovascular disease, and gives particular 
weight to findings of controlled exposure studies of CAD patients in 
which sensitive indicators of myocardial ischemia were associated with 
COHb levels resulting from short-duration, high-concentration CO 
exposures. This conclusion also takes into account uncertainties 
associated with the differing circumstances of ambient air CO exposures 
from the CO exposures in the controlled human exposure studies, as well 
as the unclear public health significance of the size of effects at the 
lowest studied exposures. As in the last review, this conclusion gives 
more weight to the significance of the effects observed in these 
studies at somewhat higher COHb levels. Additionally, this conclusion 
takes into account judgments in interpreting the public health 
implications of the REA estimates of COHb associated with ambient 
exposures based on the application of our current exposure modeling 
tools, and the size of the at-risk populations estimated to be 
protected from experiencing daily maximum COHb levels of potential 
concern by the current standard. Further, this conclusion considers the 
uncertainties in quantitative interpretations associated with the 
epidemiological studies to be too great for reliance on information 
from the few studies where the current standards were met as a basis 
for selection of alternative standards.
    In addition to considering retaining the current suite of standards 
without revision, the Policy Assessment also concludes that it is 
reasonable to consider revising the 1-hour standard downward to provide 
protection from infrequent short-duration peak ambient concentrations 
that may not be adequately provided by the current standards. While the 
quantitative analyses for this review focused predominantly on the 
controlling, 8-hour standard, the analyses have indicated the 
influential role of elevated 1-hour concentrations in contributing to 
daily maximum COHb levels over benchmark levels. In addition to the REA 
results, the Policy Assessment notes the health effects evidence from 
1-hour controlled exposures, which indicates the effects in susceptible 
groups from such short duration exposures. The Policy Assessment 
interpreted the evidence and REA estimates to indicate support for 
consideration of a range of 1-hour standard levels which would address 
the potential for the current 8-hour standard, as the controlling 
standard, to ``average away'' high short-duration exposures that may 
contribute to exposures of concern. Consequently, in considering 
alternative standard levels, the Policy Assessment focuses on the 1-
hour standard as providing the most direct approach for controlling the 
likelihood of such occurrences.
    With regard to a revision of the 1-hour standard, the Policy 
Assessment identified a range of 1-hour standard levels from 15 to 5 
ppm as being an appropriate range for consideration. These levels are 
in terms of a 99th percentile daily maximum form, averaged over three 
years, which the Policy Assessment considers to provide increased 
regulatory stability over the current form. The Policy Assessment 
additionally takes note of CASAC's preference for a revision to the 
standards to provide greater protection and observes that the range of 
1-hour standard levels discussed is also the range that the CASAC CO 
Panel suggested was appropriate for consideration.
    The Policy Assessment indicates that the upper part of the range of 
1-hour standard levels for consideration (11-15 ppm) was identified 
based on the objective of providing generally equivalent protection, 
nationally, to that provided by current 8-hour standard and potentially 
providing increased protection in some areas, such as those with 
relatively higher 1-hour peaks that are allowed by the current 8-hour 
standard. This part of the range is estimated to generally correspond 
to 1-hour CO levels occurring under conditions just meeting the current 
8-hour standard based on current relationships between 1-hour and 8-
hour average concentrations at current U.S. monitoring locations (PA, 
Appendix C). The Policy Assessment states that selection of a 1-hour 
standard within this upper part of the range would be expected to allow 
for a somewhat similar pattern of ambient CO concentrations as the 
current, controlling 8-hour standard, although with explicit and 
independent control against shorter-duration peak concentrations which 
may contribute to daily maximum COHb levels in those exposed. 
Consideration of 1-hour standard levels in this part of the range would 
take into account the factors recognized with regard to the option of 
retaining the current standards. But it would give greater weight to 
the importance of limiting 1-hour concentrations that are not 
controlled by the current 8-hour standard but that may contribute to 
exceedances of relevant COHb benchmark levels.
    The Policy Assessment also concluded that, based on the evidence 
and REA estimates and alternative judgments regarding appropriate 
population targets for maximum COHb levels induced by ambient CO 
exposures, it may be appropriate to consider standard levels that 
provide additional protection than that afforded by the current 
standards against the occurrence of short-duration peak ambient CO 
exposures and associated COHb levels. With this policy objective in 
mind, the Policy Assessment also described a rationale for 
consideration of 1-hour standard levels of 9-10 ppm, which comprise the 
middle part of the range of 1-hour standard levels suggested for 
consideration (PA, section 2.3.5). Additionally, the Policy

[[Page 8183]]

Assessment identified 1-hour standard levels of 5-8 ppm, in the lower 
part of the range for consideration in light of alternative judgments 
with regard to the evidence and REA, including the weight to place on 
public health significance of smaller changes in COHb and the small 
number of epidemiological studies in areas meeting the current 
standards (PA, section 2.3.5).
    In considering the relative strength of the evidence supporting 
each of the 3 parts of the range, the Policy Assessment concludes that 
the upper part of the range is most strongly supported, both with 
regard to judgments concerning adversity and quantitative 
interpretation of the epidemiological studies with regard to ambient 
concentrations that may elicit effects. For the lower parts of the 
range, the Policy Assessment concludes that support provided by the 
available information is more limited, especially for the lowest part 
of the range.
    In conjunction with consideration of a revised 1-hour standard, the 
Policy Assessment, also concludes it is appropriate to consider 
retaining a standard with an 8-hour averaging time, recognizing that, 
as when it was established, the 8-hour standard continues to provide 
protection from multiple-hour ambient CO exposures which may contribute 
to elevated COHb levels and associated effects. In conjunction with 
consideration of a revised 1-hour standard, the Policy Assessment 
additionally describes revision to the 8-hour standard form that may be 
appropriate to consider to potentially provide greater regulatory 
stability, with adjustment to level to provide generally equivalent 
protection as the current 8-hour standard or as a revised 1-hour 
standard level (PA, section 2.3.5). The range of 8-hour levels 
identified in the Policy Assessment is inclusive of the range of levels 
included in the example policy option suggested by CASAC.
3. CASAC Advice
    In our consideration of the adequacy of the current standards, in 
addition to the evidence- and exposure/dose-based information discussed 
above, we have also considered the advice and recommendations of CASAC, 
based on their review of the ISA, the REA, and the draft Policy 
Assessment, as well as comments from the public on drafts of these 
documents.\55\ In these reviews, CASAC has provided an array of advice, 
both with regard to interpreting the scientific evidence and 
quantitative exposure/dose assessment, as well as with regard to 
consideration of the adequacy of the current standards (Brain and 
Samet, 2009, 2010a, 2010b, 2010c, 2010d).
---------------------------------------------------------------------------

    \55\ All written comments submitted to the Agency thus far in 
this review are available in the docket for this rulemaking, as are 
transcripts of the public meetings held in conjunction with CASAC's 
review of the draft PA, of drafts of the REA, and of drafts of the 
ISA.
---------------------------------------------------------------------------

    In their review of the draft ISA, CASAC noted various limitations 
and uncertainties associated with the evidence, particularly from the 
epidemiological studies, as noted in section II.D.2.1 above. For 
example, they recognized limitations in representation of population 
exposure to ambient CO. Further they noted that ``[t]he problem of co-
pollutants serving as potential confounders is particularly problematic 
for CO'' and that CO may be serving as a surrogate for a mixture of 
pollutants generated by fossil fuel combustion (Brain and Samet, 2010d) 
as well as noting uncertainty regarding the possibility for confounding 
effects of indoor sources of CO (Brain and Samet, 2010c).
    In their comments on the draft PA, the CASAC CO Panel stated 
overall agreement with staff's conclusion that the body of evidence and 
the quantitative exposure and risk assessment provide support for 
retaining or revising the current 8-hour standard. They additionally, 
however, expressed a ``preference'' for a lower standard and stated 
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.'' Taking this into account, the Panel further 
advised that ``revisions that result in lowering the standard should be 
considered'' (Brain and Samet, 2010c).
    As noted in section I.C. above, the final Policy Assessment was 
completed with consideration of CASAC comments on the draft document, 
as well as their comments on the second draft REA, and also public 
comments. Among the revisions made in completing the final Policy 
Assessment were those based on additional consideration of the 
epidemiological studies in light of CASAC comments. Discussion of these 
studies and the complications with regard to their quantitative 
interpretation is described in section II.D.2.a above, in addition to 
other evidence-based considerations described in the final Policy 
Assessment, and is considered in the Administrator's proposed 
conclusions below.
    The few public comments received on this review to date that have 
addressed adequacy of the current standards conveyed the view that the 
current standards are adequate. In support of this view, these 
commenters disagreed with the REA estimates of in-vehicle exposure 
concentrations and argued that little weight should be given to the 
epidemiological studies.
4. Administrator's Proposed Conclusions Concerning Adequacy
    Based on the large body of evidence concerning the public health 
impacts of exposure to ambient CO available in this review, the 
Administrator proposes that the current primary standards provide the 
requisite protection of public health with an adequate margin of safety 
and should be retained.
    In considering the adequacy of the current 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 Assessment; the advice and recommendations from 
CASAC; and public comments to date. In the discussion below, the 
Administrator considers first the long-standing evidence base 
concerning effects associated with exposure to CO, including the 
controlled human exposure studies, and the health significance of 
responses observed at the 2% COHb level induced by 1-hour CO exposure, 
as compared to higher COHb levels. 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. 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 
standard, 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.

[[Page 8184]]

    As an initial matter, the Administrator takes note of the Policy 
Assessment's consideration of the long-standing body of evidence for 
CO, augmented in some aspects since the last review, as summarized in 
the current Integrated Science Assessment. This long-standing evidence 
base has established the following key aspects of CO toxicity that are 
relevant to this review as they were to the review completed in 1994. 
The common mechanism of CO health effects involves binding of CO to 
reduced iron in heme proteins and the alteration of their function. 
Hypoxia (reduced oxygen availability) induced by increased COHb blood 
levels plays a key role in eliciting CO-related health effects. 
Accordingly, COHb is commonly used as the bioindicator and dose metric 
for evaluating CO exposure and the potential for health effects. 
Further, people with cardiovascular disease are a key population at 
risk from short-term ambient CO exposures.
    With regard to the evidence of health effects associated with 
ambient CO exposures relevant to this review, the Administrator first 
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. Further, as at the time of the 
review completed in 1994, the Administrator takes particular note of 
the evidence 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. 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, with regard to providing a margin of safety against 
effects of concern that have been associated with higher COHb levels, 
such as 3-4% COHb.
    As at the time of the last review, the Administrator additionally 
considers the exposure and dose modeling results, taking note of key 
limitations and uncertainties associated with the exposure and dose 
assessment summarized in section II.C.2. above, and in light of 
judgments 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. Under air 
quality conditions just meeting the current, controlling, 8-hour 
standard, the assessment estimates that, as was the case for the 
assessment conducted for the 1994 review, daily maximum COHb levels 
were below 2% COHb for more than 99.9% of person-days in the study 
areas evaluated. Further, under these conditions, greater than 99.9% of 
the at-risk populations in the study areas evaluated would not be 
expected to experience daily maximum COHb levels at or above 4% COHb, 
and more than 95% and 98.6% of those populations would be expected to 
avoid single or multiple occurrences, respectively, at or just above 2% 
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 finding 
that 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, 
there are 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, above. 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. In addition, CASAC, in their advice 
regarding interpretation of the currently available evidence commented 
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. 
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, 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 method detection limit across the 
database. CASAC additionally indicated the need to consider the 
potential for

[[Page 8185]]

confounding effects of indoor sources of CO. As discussed in section 
II.D.2.a above, 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).
    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. For the reasons explained above, after full consideration of 
CASAC's advice and the epidemiological evidence, as well as its 
associated uncertainties and limitations, the Administrator judges 
those uncertainties and limitations to be too great for the 
epidemiological evidence to provide a basis for revising the current 
standards.
    In considering the adequacy of the level of protection provided by 
the current standards, the Administrator notes 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. 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 estimated to experience 
multiple such occurrences. Taken together, the Administrator considers 
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 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. Additionally, the Administrator proposes to conclude that 
consideration of the epidemiological studies does not lead her to 
identify a need for any greater protection. Thus, the Administrator 
proposes 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 standard, the Administrator proposes to conclude that the 
current suite of primary CO standards are requisite to protect public 
health with an adequate margin of safety from effects of ambient CO.
    The Administrator also solicits comment on whether it would be 
appropriate to revise the current primary standards. The Administrator 
takes note that, while CASAC indicated their view that the evidence and 
exposure and dose estimates provide support for retaining the current 
NAAQS, they also indicated their preference for a lower standard. For 
example, the CASAC CO Panel stated that giving additional weight to the 
epidemiological evidence would support a tightening of the current 
standard. The Administrator also takes note of the Policy Assessment 
conclusions, summarized in section II.D.2.c above. Thus, in light of 
views expressed by CASAC, as well as the Policy Assessment conclusions, 
the Administrator additionally solicits comment on the appropriateness 
of potential revisions to the form and level of the standards. Any 
comments on such revisions should include an explanation of the basis 
for the commenters' views.

E. Summary of Proposed Decisions on Primary Standards

    For the reasons discussed above, and taking into account 
information and assessments presented in the Integrated Science 
Assessment and Policy Assessment, the advice and recommendations of 
CASAC, and the public comments to date, the Administrator proposes to 
retain the existing suite of primary CO standards. Additionally, the 
Administrator solicits comment on the appropriateness of revisions to 
the form and level of the standards.

III. Consideration of a Secondary Standard

    This section focuses on the key policy-relevant issues related to 
the review of public welfare-related effects of CO. Under section 
109(b) of the Clean Air Act, a secondary standard is to be established 
at a level ``requisite to protect the public welfare from any known or 
anticipated adverse effects associated with the presence of the 
pollutant in ambient air.'' Section 302(h) of the Act defines effects 
on welfare in part as ``effects on soils, water, crops, vegetation, 
man-made materials, animals, weather, visibility, and climate.'' We 
first summarize the history of EPA's consideration of secondary 
standards for CO in section III.A. In section III.B, we then discuss 
the evidence currently available for welfare effects to inform 
decisions in this review as to whether, and if so how, to establish 
secondary standards for CO based on public welfare considerations as 
presented in the Policy Assessment. Advice from CASAC is summarized in 
section III.C. Lastly, the Administrator's proposed conclusions are 
presented in section III.D.

A. Background and Considerations in Previous Reviews

    With the establishment of the first NAAQS for CO in 1971, secondary 
standards were set identical to the primary standards. CO was not shown 
to produce detrimental effects on certain higher plants at levels below 
100 ppm. The only significant welfare effect identified for CO levels 
possibly approaching those in ambient air was inhibition of nitrogen 
fixation by microorganisms in the root nodules of legumes associated 
with CO levels of 100 ppm for one month (U.S. DHEW, 1970). In the first 
review of the CO NAAQS, which was completed in 1985, the threshold 
level for plant effects was recognized to occur well above ambient CO 
levels, such that vegetation damage as a result of CO in ambient air 
was concluded to be very unlikely (50 FR 37494). As a result, EPA 
concluded that the evidence did not support maintaining a secondary 
standard for CO, as welfare-related effects had not been documented to 
occur at ambient concentrations (50 FR 37494). Based on that 
conclusion, EPA revoked the secondary standard. In the most recent 
review of CO, which was completed in 1994, EPA again concluded there 
was insufficient evidence of welfare effects occurring at or near 
ambient levels to support setting a secondary NAAQS (59 FR 38906). That 
review did not consider climate-related effects.

B. Evidence-Based Considerations in the Policy Assessment

    To evaluate whether establishment of a secondary standard for CO is 
appropriate, 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

[[Page 8186]]

and information now available. Considerations of the evidence available 
in this review in the Policy Assessment were organized around 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 ecological effects of 
CO unrelated to climate-related effects, 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. The greater focus on climate in the current ISA relative to the 
2000 AQCD reflects 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 following 
discussion focuses on climate-related effects of CO in addressing the 
question posed above.
    As concluded in the Policy Assessment, recently available 
information 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). As 
recognized in the ISA, CO is a weak direct contributor to greenhouse 
warming. The most significant effects on climate result indirectly from 
CO chemistry, related to the role of CO as the major atmospheric sink 
for hydroxyl radicals. Increased concentrations of CO can lead to 
increased concentrations of other gases whose loss processes also 
involve hydroxyl radical chemistry. Some of these gases, such as 
methane and ozone (O3), contribute to the greenhouse effect 
directly while others deplete stratospheric O3 (ISA, section 
3.3 and p. 3-11).
    Advances in modeling and measurement have improved our 
understanding of the relative contribution of CO to climate forcing 
(PA, section 3.2). CO contributes to climate forcing through both 
direct radiative forcing (RF) of CO, estimated at 0.024 watts per 
square meter (W/m\2\) by Sinha and Toumi (1996), and indirect effects 
of CO on climate through methane, O3 and carbon dioxide 
(Forster et al. 2007). The Intergovernmental Panel on Climate Change 
estimated the combined RF for these indirect effects of CO to be ~0.2 
W/m\2\ over the period 1750-2005 (Forster et al., 2007), with more than 
one-half of the forcing attributed to O3 formation (ISA, 
section 3.3 and p. 3-13).
    As discussed in the Policy Assessment, CO is classified as a short-
lived climate forcing agent, prompting CO emission reductions to be 
considered as a possible strategy to mitigate effects of global warming 
(PA, section 3.2). However, in considering the information presented in 
the ISA, the Policy Assessment notes that it is highly problematic to 
evaluate the indirect effects of CO on climate due to the spatial and 
temporal variation in emissions and concentrations of CO and due to the 
localized chemical interdependencies involving CO, methane, and 
O3 (ISA section 3.3 and p. 3-12). Most climate model 
simulations are based on global-scale scenarios and have a high degree 
of uncertainty associated with short-lived climate forcers such as CO 
(ISA, section 3.3 and p. 3-16). These models may fail to consider the 
local variations in climate forcing due to emissions sources and local 
meteorological patterns (ISA, section 3.3 and p. 3-16). It is possible 
to compute individual contributions to RF of CO from separate emissions 
sectors, although uncertainty in these estimates has not been 
quantified (ISA, section 3.3, p. 3-13 and Figure 3-7).
    Uncertainties in the estimates of the indirect RF from CO are noted 
in the Policy Assessment to be related to uncertainties in the chemical 
interdependencies of CO and trace gases, as described above. Large 
regional variations in CO concentrations also contribute to the 
uncertainties in the RF from CO and other trace gases (ISA section 3.3 
and p. 3-12). Although measurement of and techniques for assessing 
climate forcing are improving, estimates of RF still have approximately 
50% uncertainty (ISA, section 3.3, and p. 3-13).
    In summary, the Policy Assessment drew the following conclusions 
based on the considerations identified above. As an initial matter, 
with respect to non-climate welfare effects, including ecological 
effects and impacts to vegetation, the Policy Assessment concluded that 
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 (ISA, sections 2.2 and 
3.3), while also noting 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 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).

C. CASAC Advice

    In consideration of a secondary standard, in addition to the 
evidence discussed above, EPA has also considered the advice and 
recommendations of CASAC, based on their review of the ISA, and the 
draft Policy Assessment.\56\
---------------------------------------------------------------------------

    \56\ Thus far in this review, no public comments have been 
received regarding the secondary standard.
---------------------------------------------------------------------------

    In their comments on the draft Policy Assessment, CASAC took note 
of the substantial evidence that CO has adverse effects on climate and 
recommended that staff summarize information that is currently lacking 
and would assist in consideration of a secondary standard in the future 
(ISA, sections 3.2 and 3.3; Brain and Samet, 2010c).\57\ 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).
---------------------------------------------------------------------------

    \57\ This recommendation is addressed in section 3.5 of the 
Policy Assessment.
---------------------------------------------------------------------------

D. Administrator's Proposed Conclusions Concerning a Secondary Standard

    The proposed 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. In considering whether the currently available scientific 
information supports setting a secondary standard for CO, EPA takes 
note of the Policy Assessment consideration of the body of available 
evidence (briefly summarized above in

[[Page 8187]]

section III.B). 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 a CO secondary standard. Secondly, with respect to climate-
related effects, the EPA takes note of staff considerations in the 
Policy Assessment and concurs with staff conclusions that this 
information is insufficient at this time to provide support for a CO 
secondary standard. Thus, in considering the evidence, staff 
considerations in the Policy Assessment summarized here, as well as the 
views of CASAC, summarized above, the Administrator proposes 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.

IV. Proposed Amendments to Ambient Monitoring Requirements

    The EPA is proposing changes to the ambient air monitoring network 
design requirements to support the NAAQS for CO discussed above in 
section II. Because the availability of ambient CO monitoring data is 
an essential element of the NAAQS implementation framework, EPA is 
proposing to revise the requirements for the ambient CO monitoring 
network to include a minimum set of monitors to provide data for 
comparison to the NAAQS (i.e., for determining whether areas are 
attaining the standards) in locations near roads where CO emissions 
associated with mobile source related activity lead to increased 
ambient concentrations. Under such requirements, 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 for comparison to the NAAQS 
and to meet other objectives.

A. Monitoring Methods

    Ambient air monitoring data are used for various purposes, 
including determining compliance with the NAAQS. The use of reference 
methods provides uniform, reproducible measurements of pollutant 
concentrations in ambient air. Equivalent methods 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 reference and equivalent methods, known as Federal 
Reference Methods (FRMs) and Federal Equivalent Methods (FEMs), at 40 
CFR part 53.
    Ambient air monitoring data for CO must be obtained using an FRM or 
an FEM, as defined in 40 CFR parts 50 and 53, for such data to be 
comparable to the NAAQS for CO. All CO monitoring methods in use 
currently by State and local monitoring agencies are EPA-designated FRM 
analyzers (USEPA, 2010f). 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 (USEPA, 2010f) 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/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 is 
nevertheless proposing editorial revisions to the CO FRM and both 
technical and editorial revisions to part 53, as discussed below.
1. Proposed Changes to Part 50, Appendix C
    Reference methods for criteria pollutants are described in several 
appendices to 40 CFR part 50; the CO FRM is set forth in appendix C of 
part 50. A nondispersive 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 FRM CO 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.
    From time to time, as pollutant measurement technology advances, 
EPA assesses the FRMs in the 40 CFR part 50 FRM appendices to determine 
if they are 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. The CO FRM was originally 
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. (Those updates included clarification that the 
FRM NDIR measurement principle encompassed the specific ``gas filter 
correlation'' measurement technique now used by many commercial FRM 
analyzers.).
    In connection with the current review of the NAAQS for CO, EPA is 
proposing to again update the existing CO FRM--with no substantive 
changes--as explained in further detail below. This action is based on 
the scientific view that the CO FRM, as originally established and 
updated in the 1980's, is still fully adequate for FRM purposes and is 
fulfilling that role well. Further, the FRM is also well suited for use 
in routine CO monitoring, and several high quality FRM analyzer models 
have been available for many years and continue to be offered and 
supported by multiple analyzer manufacturers. Finally, EPA has 
determined that no new ambient CO measurement technique has become 
available that is superior to the NDIR technique specified for the 
current FRM.
    While EPA believes that the current CO FRM is adequate, we also 
believe that the existing CO FRM should be improved by implementing 
updates 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. EPA found that no 
substantive changes were needed to the basic NDIR FRM measurement 
principle; therefore, the proposed updates are of a very minor, 
editorial nature. However, these proposed changes are numerous enough 
so that EPA is proposing to re-promulgate the entire CO FRM in appendix 
C of 40 CFR part 50, replacing the existing FRM language with revised 
language.
2. Proposed Changes to Part 53
    In close association with the proposed editorial revision to the CO 
FRM described above, EPA is also proposing to update the performance 
requirements for FRM CO analyzers currently 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,

[[Page 8188]]

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 is not reflected in the current 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 part 53 
performance requirements would also apply to candidate FEM CO 
analyzers, if any new, alternative CO measurement technology should be 
developed.
    As noted previously, the performance of FRM analyzers designated 
under the presently specified performance requirements of Part 53 is 
fully adequate for current monitoring needs. 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). 
Upgrading the analyzer performance requirements to be more consistent 
with the typical performance capability available in contemporary FRM 
analyzers would ensure that newly designated FRM analyzers will have 
this improved measurement performance. Therefore, EPA believes that the 
Part 53 requirements should be updated to be at least commensurate with 
this typical level of CO analyzer performance. In addition, this 
modernization also provides for optional, new performance requirements 
applicable to lower, more sensitive measurement ranges that would 
support improved monitoring data quality in areas of low CO 
concentrations. Accordingly, EPA is proposing to amend the performance 
requirements applicable to CO FRMs (and any new FEMs) set forth in 
subpart B of 40 CFR part 53, as described in the following discussion.
    Subpart B of 40 CFR part 53 prescribes explicit test 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. These 
performance requirements are specified in Table B-1 of subpart B. 
Although Table B-1 covers candidate methods for SO2, 
O3, CO, and NO2, the updates to Table B-1 that 
EPA is now proposing would be applicable only to candidate methods for 
CO.
    Some updated performance requirements are being proposed for 
candidate CO analyzers that operate on the specified ``standard'' 
measurement range (0 to 50 ppm). This measurement range would remain 
unchanged from the existing requirements as it appropriately addresses 
the monitoring data needed for assessing attainment. However, based on 
EPA's review of the performance of currently available CO FRM analyzers 
(USEPA, 2010g), EPA is proposing revised performance requirements for 
CO analyzers in Table B-1, as follows. The measurement noise limit 
would be reduced from 0.5 to 0.2 ppm, and the lower detectable limit 
would be reduced from 1 to 0.4 ppm. Zero drift would be reduced from 
1.0 to 0.5 ppm, and span drift would be lowered from 2.5% to 2.0%. The 
existing mid-span drift requirement, tested at 20% of the upper range 
limit (URL), would be withdrawn. EPA has found that the mid-span drift 
requirement is unnecessary for CO instruments because the upper level 
span drift (tested at 80% of the URL) completely and much more 
accurately defines analyzer span drift performance.
    EPA proposes to change the lag time allowed from 10 to 2 minutes, 
and the rise and fall times from 5 to 2 minutes. For precision, EPA 
proposes to change the form of the precision limit specifications from 
an absolute measure (ppm) to percent (of the URL) for CO analyzers and 
to set the limit at 1 percent for both 20% and 80% of the URL. One 
percent is equivalent to the existing limit value of 0.5 ppm for 
precision for the standard (50 ppm) measurement range. This change in 
units from ppm to percent will make 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 
would not be changed, but EPA proposes to withdraw the existing limit 
requirement for the total of all interferents. EPA has found that the 
total interferent limit is redundant with the individual interferent 
limit for modern CO analyzers.
    These proposed new performance requirements would apply only to 
newly designated CO FRM or FEM analyzers. Essentially all existing FRM 
analyzers in use today, as noted previously, are providing CO 
monitoring data of adequate quality and fulfill the proposed 
requirements. Thus, existing FRM analyzers would not be required to be 
re-tested and re-designated under the proposed new requirements. All 
currently designated FRM analyzers would retain their original FRM 
designations.
    EPA recognizes 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. Part 53 (40 CFR 
53.20(b)) allows an FRM or FEM designation to include lower ranges. To 
make such lower-range measurements more meaningful, EPA is proposing a 
separate set of performance requirements that would apply specifically 
to lower ranges (i.e., those having a URL of less than 50 ppm) for CO 
analyzers. The proposed additional, lower-range requirements are listed 
in the proposed revised Table B-1. A candidate analyzer that meets the 
Table B-1 requirements for the standard measurement range (0 to 50 ppm) 
could optionally have one or more lower ranges included in its FRM or 
FEM designation by further testing to show that it also meets these 
proposed supplemental, lower-range requirements.
    Although no substantive changes have been 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 is in need of 
significant updates, clarifications, refinement, and (in a few cases) 
correction of minor typographical errors. EPA believes that these 
provisions should be amended at this time in its on-going, pollutant-
by-pollutant effort to bring the entire content of subpart B fully up 
to date.
    The proposed 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, with no changes to the substance of the 
requirements. However, because these small changes are quite numerous, 
EPA believes that it is expedient and advantageous to propose 
replacement of the subpart B text, in its entirety, with the modified 
text. As discussed previously, Table B-1, which sets forth the 
pollutant-specific performance limits and was recently amended as 
applicable primarily to SO2 analyzers, would be amended at 
this time only as necessary and applicable to CO analyzers. EPA intends 
to amend Table B-1 for the remaining pollutant methods (O3 
and NO2) later, at such time as each of those pollutants--
along with its associated FRM in part 50--is addressed specifically.

[[Page 8189]]

3. Implications for Air Monitoring Networks
    As noted previously, existing CO FRM analyzers (no CO FEMs are 
presently available) are currently providing monitoring data that are 
adequate for the current CO NAAQS. Although EPA is proposing to re-
promulgate the entire CO FRM, the changes are minor, with no 
substantive changes being proposed. Thus, this action would have 
little, if any, effect on existing air monitoring networks. Similarly, 
EPA is proposing revisions to subpart B of part 53, which specifies the 
testing and performance requirements for FRM and FEM analyzers. Again, 
the changes are minor, with the exception of the CO analyzer 
performance requirements in Table B-1, which EPA is proposing to make 
more consistent with modern CO analyzers representative of monitors 
used in the current CO monitoring network. These new requirements would 
be used for designation of new CO FRM and FEM analyzers. Existing EPA-
designated FRMs would be unaffected by the proposed changes and would 
continue to be designated. As most commercially available CO FRM 
analyzers already meet the proposed new performance requirements, the 
cost of new CO analyzers that would meet the proposed new performance 
requirements would not be increased by the proposed new requirements. 
Therefore, there would be no immediate impact on monitoring agencies or 
on their CO monitoring networks due to the proposed amendments to the 
CO FRM and the associated new performance requirements proposed for 
subpart B.
    In the longer term, the proposed new performance requirements would 
ensure that CO network monitors, going forward, would maintain their 
improved performance. Monitoring agencies would benefit by having 
greater confidence in their CO monitoring data quality, particularly at 
the lower ambient levels prevalent in most areas. Further, the 
assurance of increased CO data quality in years to come will provide 
better databases to support future reviews of the CO NAAQS.

B. Network Design

    The objectives of an ambient monitoring network include the 
collection and dissemination of air pollution data to the general 
public in a timely manner, to determine compliance with ambient air 
quality standards and the effectiveness of emissions control 
strategies, and to provide support for air pollution research (40 CFR 
part 58, appendix D). This section on CO network design provides 
background on the monitoring network, information on the sources of CO, 
information on factors affecting CO emissions, and provides rationale 
for a proposed network design intended to support the implementation of 
the CO NAAQS.
1. Background
    EPA issued the first regulations for ambient air quality 
surveillance, codified at 40 CFR part 58, for criteria pollutants 
including CO in 1979 (44 FR 27558, May 10, 1979). These 1979 
regulations established a monitoring network for CO (described in 
detail in the CO Network Review and Background document [Watkins and 
Thompson, 2010]) that required two CO monitors in urban areas with 
500,000 or more people. The first of these two monitors was a ``peak'' 
concentration monitor, intended to be located in areas ``* * * around 
major traffic arteries and near heavily traveled streets in downtown 
areas.'' The second monitor was intended to represent a wider 
geographic area, particularly at neighborhood scales ``where 
concentration exposures are significant.'' The 2006 monitoring rule 
(Revisions to Ambient Air Monitoring Regulations, 71 FR 61236 (October 
17, 2006)) removed the minimum monitoring requirements for the ambient 
CO monitoring network that were promulgated in 1979. However, the 2006 
monitoring rule maintained a requirement that if there was ongoing CO 
monitoring in an area, the area must have at least one monitor located 
to measure maximum concentration of CO in that area. The 2006 
monitoring rule also included a provision requiring the approval of the 
EPA Regional Administrator before any existing CO ambient monitors 
could be removed. Finally, the 2006 monitoring rule included a 
requirement for CO monitors to be operated at all National Core (NCore) 
multi-pollutant monitoring stations; with approximately 80 stations 
projected to have been operational nationwide by January 1, 2011 to 
support multi-pollutant monitoring objectives.
    An analysis of the available CO monitoring network data in the Air 
Quality System (AQS) database shows that the network was comprised of 
approximately 345 monitors during 2009. Information stored in AQS for 
these monitors describes the most frequently stated monitor objectives 
for sites in the current CO network as assessment of concentrations for 
general population exposure and maximum (highest) concentrations at the 
neighborhood scale.\58\ Approximately 56 of the monitors operating in 
2009 were at microscale sites, a majority of which were likely sites 
representing ``peak'' concentrations which were required under the 
monitoring regulations originally promulgated in 1979, intended to 
characterize mobile source impacts in heavily traveled downtown streets 
or near major arterial roads (Watkins and Thompson, 2010). The rest of 
these sites were likely being operated to meet objectives including 
NAAQS comparison, to support long-term trend determination, to meet 
State Implementation Plan (SIP) and maintenance plan requirements, and 
to support ongoing health studies.
---------------------------------------------------------------------------

    \58\ 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.
---------------------------------------------------------------------------

2. On-Road Mobile Sources
    The REA for this review notes that ``motor vehicle emissions 
continue to be important contributors to ambient CO concentrations'' 
(REA, section 2.2). Microenvironments influenced by on-road mobile 
sources are important contributors to ambient CO exposures, 
particularly in urban areas (REA, section 2.7), as indicated by 
personal exposure studies that have generally shown that the highest 
ambient CO exposure levels occur while people are in transit in motor 
vehicles (ISA, section 2.3). Mobile sources are the primary 
contributors to ambient CO emissions because CO is formed by incomplete 
combustion of carbon-containing fossil fuels widely used in motor 
vehicles (ISA, section 2.1; REA, section 3.3). Further, spark-ignition 
engines (gasoline or light-duty engines) have higher CO emission rates 
than diesel engines (heavy-duty engines) because they typically operate 
closer to the stoichiometric air-to-fuel ratio, have

[[Page 8190]]

relatively short residence times at peak combustion temperatures, and 
have very rapid cooling of cylinder exhaust gases (ISA, section 3.2.1).
    Ambient CO concentrations have significantly declined over the past 
20 years, reflecting reductions in on-road vehicle emissions, as 
described in section II.A above. Overall, based on the 2002 National 
Emissions Inventory (NEI), on-road mobile sources account for 
approximately 52% of total CO emissions. Based on the more recent 2005 
NEI, the contributions of on-road mobile sources has now risen to 
approximately 60% of the total CO emissions inventory (not counting 
wildfire emissions) (http://www.epa.gov/ttn/chief/eiinformation.html). 
As described in section II.A above, in some metropolitan areas in the 
U.S., as much as 75% of all CO emissions result from on-road vehicle 
exhaust (ISA, section 2.1).
    On-road vehicle CO emission rates vary depending on operating 
conditions, such as cold-start conditions and operating speed. Under 
cold start conditions, which only last for the first minutes of vehicle 
operation, CO emissions are higher due to temporary ineffectiveness of 
vehicle exhaust catalysts until they are heated to optimal operating 
temperatures (ISA, section 3.2.1; Singer et al., 1999). Meanwhile, CO 
emissions also vary based on vehicle operating speeds. Increased CO 
emissions occur under conditions of high acceleration, rapid speed 
fluctuations, and heavy vehicle loads (ISA, section 3.2.1). Studies 
have found that CO emission rates for tested light-duty vehicles are 
highest for accelerating vehicles, second highest for vehicles in 
cruise, third highest for vehicles under deceleration, and fourth 
highest (of four operating speed related categories) for vehicles at 
idle (Frey et al., 2003). High acceleration and rapid speed 
fluctuations (such as acceleration and deceleration occurring over a 
short time period) can be associated with congested, stop-and-go 
traffic conditions.
3. Near-Road Environment
    Information in the ISA and other peer-reviewed literature suggest 
that concentrations of mobile source pollutants, such as CO, typically 
display peak concentrations on or immediately adjacent to roads, 
typically producing a gradient in pollutant concentrations where 
concentrations decrease with increasing distance from roads (ISA, 
section 2.3; ISA, section 3.5.1.3; Baldauf et al., 2008; Clements et 
al., 2009; Karner et al., 2010; Zhou and Levy, 2008; Zhu et al., 2002). 
CO is emitted by on-road mobile sources, and is not secondarily formed 
in the near-road environment like NO2 (which is both 
primarily emitted and secondarily formed in the near-road environment). 
As a result, the near-road gradient for CO can be quite steep, where 
concentrations rapidly decay with increasing distance away from the 
road when compared to other mobile source pollutants such as 
NO2. Karner et al. (2010), synthesized findings from 41 
near-road pollutant monitoring studies ranging from 1978 through June 
2008 to advance the understanding of on-road mobile source pollutant 
dispersion. They performed two regression analyses, one being a local 
regression of background normalized concentrations on distance, and the 
second being a local regression of edge [of road] normalized 
concentrations on distance. These analyses found CO to have the highest 
approximate edge-of-road peaks, as much as 21 times background 
concentrations, of all pollutants analyzed, and also showed CO to have 
one of the fastest decay rates with increasing distance from the road, 
showing as much as a 90 percent drop in concentration 150 meters from 
the edge of the road. A key reason in the difference in decay rate with 
increasing distance from roads between CO and NO2 is due to 
how the two pollutants are introduced into the near-road environment. 
CO is a primary emission from motor vehicle fuel combustion, while 
NO2 is both emitted as a primary emission and secondarily 
formed in the near-road environment. The Integrated Science Assessment 
for Oxides of Nitrogen--Health Criteria (NOX ISA; USEPA, 
2008d) notes that the direct emission of NO2 from mobile 
sources is estimated to be only a few percent of the total 
NOX emissions for light duty gasoline vehicles, and from 
less than 10 percent up to 70 percent of the total NOX 
emission from heavy duty diesel vehicles, depending on the engine, the 
use of emission control technologies such as catalyzed diesel 
particulate filters (CDPFs), and mode of vehicle operation. Although 
much of the NOX emissions are initially in the form of NO, 
the rate of conversion of NO to NO2 is generally a rapid 
process (i.e., on the order of a minute) (NOX ISA, section 
2.2.2). Thus, more of the NO2 in the near-road environment 
is a result of secondary formation than from primary emissions, while 
CO is almost exclusively a result of direct emissions from tailpipes.
    Overall, the literature suggests that CO concentrations generally 
return to near-background levels within a few hundred meters from the 
road (Karner et al., 2010; Zhou and Levy, 2007). The actual 
concentrations of CO, and other mobile source pollutants such as 
NOX and particulate matter, that occur in the near-road 
environment, and the rate of decay of those pollutant concentrations 
with increasing distance from the road, are dependent on a number of 
variables including traffic volume, traffic fleet mix, roadway type, 
roadway design, surrounding features, topography (or terrain), and 
meteorology (Baldauf et al., 2009; Baldauf et al., 2008; Clements et 
al., 2009; Hagler et al., 2010; Heist et al., 2009). EPA notes that 
these factors were taken into account in the requirements for the near-
road NO2 monitoring network, promulgated in February 2010 
(75 FR 6474), which required near-road NO2 sites to be 
selected with consideration given to traffic volume (via use of Annual 
Average Daily Traffic [AADT] counts), fleet mix, congestion patterns, 
roadway design, terrain, and meteorology.
4. Urban Downtown Areas and Urban Street Canyons
    As noted above in section IV.B.2, increased CO emissions occur 
under operating conditions of high acceleration, rapid speed 
fluctuations (such as acceleration and deceleration occurring over a 
short time period), and increased vehicle loads (ISA, section 3.2.1). 
High acceleration and rapid speed fluctuations can be associated with 
congested traffic conditions, such as stop-and-go traffic, which can 
occur on heavily trafficked roads such as highways, freeways, and along 
major arterial roads, and also along roads with multiple intersections 
in relatively close proximity to each other. Thus, elevated CO 
concentrations, relative to surrounding background concentrations, can 
occur not only along heavily trafficked roads but also may be found in 
urban downtown areas, where a relatively higher number of roads exist 
in an area (high density of roads per unit area) and a relatively 
higher density of roadway intersections exist in an area (high roadway 
intersection per unit area), which can lead to increased occurrences of 
vehicles operating under modes of high acceleration and/or rapid speed 
fluctuations. Even though streets in urban downtown areas may not 
individually carry as much traffic as larger highways, freeways, or 
major arterials, the impact of many relatively smaller streets in close 
proximity carrying traffic experiencing periods of high acceleration 
and/or rapid speed fluctuations, or congested traffic, may collectively 
contribute to elevated CO concentrations in that downtown area.

[[Page 8191]]

    In addition to traffic undergoing periods of high acceleration and/
or rapid speed fluctuations or experiencing general traffic congestion, 
urban downtown areas often have a number of relatively tall buildings, 
typically in close proximity to each other. Such configurations of tall 
buildings in relatively close proximity often create urban features 
called urban canyons or urban street canyons. Although the term urban 
canyon, or urban street canyon, is not formally defined, it can 
generally be described as an urban feature, resembling a natural canyon 
\59\, where streets or roads exist within dense blocks of relatively 
tall buildings. These urban features are of interest because, as noted 
in the ISA, recent research by Kaur and Nieuwenhuijsen (2009), and 
Carlaw et al. (2007), suggest CO concentrations are related to traffic 
volume and fleet mix in the urban street canyon environment, which can 
influence potential exposures. EPA has had monitoring requirements in 
the past that characterized concentrations of CO in heavily trafficked 
downtown streets, i.e. ``urban street canyons,'' (Watkins and Thompson, 
2010), and notes such locations may have still have relevance going 
forward.
---------------------------------------------------------------------------

    \59\ A natural canyon may be defined as a ``deep narrow valley 
with steep sides'' (http://www.merriam-webster.com/dictionary/canyon).
---------------------------------------------------------------------------

5. Meteorological and Topographical Influences
    In 2003, the National Research Council (NRC) of the National 
Academies published a document titled Managing Carbon Monoxide 
Pollution in Meteorological and Topographical Problem Areas. This 
report noted how drastically ambient CO concentrations had dropped 
across the country from the 1970s through the early 2000s, and that 
some of the remaining areas of the country that continued to have 
relatively high concentrations tended to have meteorological and 
topographical characteristics that exacerbate pollution. In particular, 
meteorological impacts can concentrate pollutant build-up in an area 
due to atmospheric inversions and cold temperatures. Atmospheric 
inversions essentially prevent pollutant emissions in an area from 
dispersing through vertical mixing. As explained by the NRC (NRC, 
2003), the extent to which air mixes vertically depends on how the air 
temperature changes with altitude. Warm air is less dense than cold air 
and thus more buoyant, allowing surface air to mix upward as relatively 
warmer air rises in the atmosphere. However, if the vertical 
temperature profile is such that temperatures decrease more slowly than 
normal, or increase with height, vertical mixing is inhibited. 
Inversions can be caused by several different specific phenomena, 
including surface based cooling (for example, due to snow on the 
ground), due to high altitudes, and sometimes due to warm air advection 
at higher altitudes.
    The topographical impacts that can lead to pollutant build-up in an 
area are typically due to physical terrain features that may aid in 
trapping pollution in an area and/or contribute to meteorological 
related inversions. An example of topographical impacts might be an 
urban area within a valley, or surrounded on several sides by mountain 
ranges. In such a case, pollutant dispersion is inhibited in the 
horizontal, with terrain features effectively preventing mixing or 
transport of pollution from a given area. Further, in some cases both 
meteorological and topographical impacts can combine to exacerbate 
pollutant build-up, such as in an area partially surrounded by high 
terrain which is also subject to inversions.
    Although there is available information on what can cause increased 
potential for air pollutant build-up due to meteorological and 
topographical impacts, there are no easily defined or applied criteria 
that could be implemented nationally by which all such locations could 
be identified. Identification of such locations would require a case-
by-case approach, where localized and detailed information on terrain 
and meteorology would be needed, plus an understanding of the types and 
amounts of emission sources in or around any particular area.
6. Proposed Changes
    Although EPA is proposing to retain the current 8-hour and 1-hour 
CO NAAQS, as discussed above in section II, the Agency is proposing to 
revise the requirements for the ambient CO monitoring network to 
include a minimum set of monitors to collect data for comparison to the 
NAAQS in near-roadway locations where CO emissions associated with 
mobile source related activity lead to increased ambient 
concentrations. The current network of CO monitors, beyond those at 
NCore sites, consists of monitors that were established to meet the 
1979 monitoring rule requirements or which were placed by State and 
local air monitoring agencies to meet their own needs or objectives. 
These additional monitors in the current network are being operated 
without being required under EPA monitoring network regulations and as 
a result, they do not reflect a national monitoring network design. In 
CASAC comments on the second draft REA, the CASAC panel, aware of the 
current CO monitoring network configuration, commented on the need to 
reconsider CO monitoring network designs, stating that `` * * * the 
approach for siting [CO] monitors needs greater consideration. More 
extensive coverage may be warranted for areas where concentrations may 
be more elevated, such as near roadway locations'' (Brain and Samet, 
2010b). Since there is a strong relationship between CO exposures and 
mobile source activity, as described in the ISA and REA and summarized 
in sections II.D.2 and IV.B.2 above, primarily in the near-road 
environment, EPA believes that some CO monitors should be located near 
on-road mobile source activity, where ambient concentrations are 
expected to be more elevated, as noted by CASAC.
    Accordingly, EPA is proposing to require locating ambient CO 
monitors which would produce data for comparison to both the 8-hour and 
1-hour NAAQS at a subset of near-road NO2 monitoring 
stations, which are required under the Primary National Ambient Air 
Quality Standards for Nitrogen Dioxide; Final Rule (75 FR 6474), 
codified at 40 CFR part 58, appendix D. This requirement would support 
the objective of characterizing ambient conditions at highly trafficked 
near-road locations where elevated CO concentrations (relative to 
surrounding background concentrations) are expected to occur.
    The EPA is not proposing to require dedicated CO monitoring sites 
to characterize area-wide concentrations representing neighborhood and 
larger spatial scales. Based on a recent review of the current CO 
monitoring network (Watkins and Thompson, 2010), EPA believes that the 
required NCore sites and many of the existing monitoring sites in the 
network provide data representative of neighborhood and larger spatial 
scales. These monitors are useful in providing relative background 
concentrations that, when compared to near-road CO monitors, could aid 
in the quantification of the near-road gradient of CO in a given urban 
area. Between the required NCore sites, and an expectation based on 
experience that some number of non-required area-wide sites will 
continue to operate in the future, we do not believe it is necessary to 
propose a specific area-wide monitoring requirement in this rulemaking.
    EPA believes that the proposed network design which places CO 
monitors at a subset of near-road NO2

[[Page 8192]]

monitoring stations, as described in detail in the following sections, 
will require a relatively modest amount of new resources by State and 
local air agencies. Recalling that there were approximately 345 CO 
monitors operating in 2009, which were largely discretionary monitors 
not operated pursuant to Federal network design requirements, the 
Agency believes that a large majority of State and local air agencies 
could meet the proposed minimum monitoring requirements by relocating 
an existing CO monitor to a near-road NO2 monitoring 
station. In some of these cases, the EPA believes that the relocation 
of a CO monitor from an existing stand-alone site to a multi-pollutant 
near-road NO2 site may also result in additional operational 
cost savings as, in some areas, the total number of ambient monitoring 
sites for which operational support is needed could be reduced.
    The EPA believes that the proposed requirement for placing CO 
monitors at some of the forthcoming near-road NO2 monitoring 
stations would provide an important benefit by facilitating the 
implementation of a more targeted ambient CO monitoring network that 
provides data for comparison to the NAAQS, and is considerably smaller 
than the CO network currently in operation. EPA notes that under the 
current regulation, the current CO network is subject to a potentially 
significant reduction in size (as detailed in Watkins and Thompson, 
2010) since non-required CO monitoring stations can be shut down upon 
State request, an evaluation of historical data to evaluate 
concentrations relative to the NAAQS (per 40 CFR 58.14), and EPA 
Regional Administrator approval. The occurrence of such a reduction, 
however, would lack the focus and direction needed to ensure retention 
of a network with the surveillance aspects essential to supporting the 
implementation of the CO NAAQS. In addition to ensuring that an 
effective, modestly sized network shall operate in the future, other 
benefits of the proposed approach of co-locating required CO monitors 
at required near-road NO2 monitoring stations include: 
ongoing comparison of data to the NAAQS (for assessing attainment), 
providing data that can support health studies, providing data that can 
be used in verification of modeling results, and supporting the 
implementation of the Agency's multi-pollutant monitoring 
objectives.\60\
---------------------------------------------------------------------------

    \60\ The EPA's strategy encouraging multi-pollutant monitoring 
is presented most recently in the Ambient Air Monitoring Strategy 
for State, Local, and Tribal Air Agencies document published 
December 2008 (http://www.epa.gov/ttn/amtic/files/ambient/monitorstrat/AAMS%20for%20SLTs%20%20-%20FINAL%20Dec%202008.pdf).
---------------------------------------------------------------------------

a. Monitoring for Carbon Monoxide at Required Near-Road Nitrogen 
Dioxide Monitoring Stations
    Traffic volume on urban area roads is much greater than in the more 
rural areas of the country, as was noted in the preamble to the final 
rule to the NO2 NAAQS (75 FR 6474). The U.S. Department of 
Transportation Federal Highway Administration's Status of the Nation's 
Highways, Bridges, and Transit: 2008 Conditions and Performance 
document (http://www.fhwa.dot.gov/policy/2008cpr/es.htm#c2b) states 
that ``while urban mileage constitutes only 25.8 percent of total 
(U.S.) mileage, these roads carried 66.3 percent of the 3 trillion 
vehicles miles travelled (VMT) in the United States in 2006.'' The 
document also states that urban interstate highways made up only 0.8 
percent of total (U.S.) mileage but carried 16.3 percent of total VMT.
    The EPA notes that the 2007 American Housing Survey (http://www.census.gov/hhes/www/housing/ahs/ahs07/ahs07.html) estimates that 
over 20 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, it is estimated that at least 45 
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, and as recognized in 
section II.D.2.b, the exposure and dose assessment for this review 
found in-vehicle microenvironments to be those with the highest ambient 
CO exposures. Additionally, as described in the ISA, PA and the REA, 
higher concentrations are reported at locations immediately near or on 
roadways as compared to monitors somewhat removed from the roadways 
(ISA, section 3.6; PA, section 2.2.1; REA, section 2.7). These 
locations capture ambient concentrations that contribute to ambient 
exposure concentrations occurring in vehicles. Accordingly, EPA 
believes that air pollution monitors near major roads will provide 
information pertaining to a significant component of ambient CO 
exposure for a large portion of the population that would otherwise not 
be available.
    The EPA recognizes the information mentioned above regarding the 
dominant role of mobile sources in the national CO emission inventory 
(discussed in section IV.B.2 above), findings of the substantial near-
road concentration gradient, with elevated CO concentrations in the 
near-road environment compared to relative background concentrations 
(discussed in section IV.B.3 above), and the importance of on-road 
mobile sources as contributors to ambient CO exposures particularly in 
urban areas (REA, section 2.7). We also note that (as referenced above) 
CASAC indicated that additional monitoring near roadways may be 
warranted, and further stated ``the Panel found in some instances 
current networks underestimated carbon monoxide levels near roadways. 
Such underestimation is a critical issue * * *'' (Brain and Samet, 
2010b). In light of this information, and the fact that we generally 
expect the increased levels of ambient CO (and the greatest exposure to 
ambient CO) to occur near-roadways, EPA has determined that it is 
appropriate to propose requiring CO monitoring near heavily trafficked 
roads in urban areas.
    EPA additionally notes that near-road NO2 monitoring 
sites will be placed near highly trafficked roads in urban areas, where 
elevated CO concentrations due to on-road mobile sources are known to 
occur, and that CASAC has recommended that EPA establish a near-road 
monitoring network that would include sites with both NO2 
and CO monitors (Russell and Samet, 2010). Accordingly, the EPA is 
proposing to require CO monitors that will provide data for comparison 
to the NAAQS to operate at a subset of required near-road 
NO2 monitoring stations, which are required in 40 CFR part 
58, appendix D. Specifically, the EPA is proposing that CO monitors be 
required in any required near-road NO2 monitoring station in 
a core based statistical area (CBSA) with a population of 1,000,000 or 
more persons. Based on 2009 U.S. Census estimates (http://www.census.gov) and Federal Highway Administration data (http://www.fhwa.dot.gov/policyinformation/tables/02.cfm) applied to near-road 
NO2 network design requirements (noted above), there would 
be approximately 77 CO monitoring sites required within near-road 
NO2 monitoring stations within 53 CBSAs (including San Juan, 
PR).\61\
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    \61\ The near-road NO2 monitoring stations, which are 
proposed to house required CO monitors, shall be selected per 
considerations spelled out in 40 CFR part 58, Appendix D, section 
4.3.2(a)(1), which prescribes site selection by ranking all road 
segments in a CBSA by AADT and then identifying a location or 
locations adjacent to those highest ranked road segments, 
considering fleet mix, roadway design, congestion patterns, terrain, 
and meteorology.

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[[Page 8193]]

    In this proposal, EPA concludes that, given the strong relationship 
between CO exposures and mobile source activity, placing CO monitors at 
near-road NO2 monitoring sites (which will be near highly 
trafficked roads in urban areas) is needed to fulfill the ambient CO 
monitoring objectives identified in section IV.B above. While having 
two monitors within CBSAs of 500,000 or more persons was the historical 
monitoring requirement (discussed in detail in Watkins and Thompson, 
2010), with declining ambient levels we believe there is less 
likelihood for high CO concentrations in relatively smaller (in 
population) CBSAs. Accordingly, we believe that proposing to require CO 
monitoring only in near-road NO2 monitoring stations in 
CBSAs of 1,000,000 or more persons is a reasonable approach that 
results in a sufficient number of CO monitors near highly trafficked 
roads in urban areas to provide data for supporting the NAAQS, for use 
in health studies, for model validation, and to support multi-pollutant 
monitoring objectives. The EPA solicits comment upon the proposed 
requirement to require CO monitors to operate within a subset of 
required near-road NO2 monitoring stations, specifically 
those in CBSAs with 1,000,000 or more persons. The EPA solicits comment 
on using alternative population thresholds within which CO monitors 
might be required to operate in near-road NO2 monitoring 
stations, e.g. CBSAs with 750,000 or 500,000 or more persons (which 
would require approximately 92 and 126 monitors, respectively), in 
light of the proposal to retain the existing CO NAAQS. Finally, the EPA 
also solicits comment on the merits of having any minimum near-road 
monitoring requirements for the CO monitoring network.
b. Regional Administrator Authority
    The EPA is proposing to include a provision allowing the 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 presented above in section IV.B.1. The EPA recognizes that 
minimum monitoring requirements may not always result in a network 
sufficient to fulfill one or more data needs or monitoring objectives 
for a particular area. An example of when an EPA Regional Administrator 
might require an additional monitor above the minimum requirements is 
to address a situation where data or other information suggest that a 
stationary CO source may be contributing to ground level concentrations 
that are approaching or exceeding the NAAQS. A second example of where 
an EPA Regional Administrator might require additional monitoring is in 
otherwise unmonitored urban downtown areas or urban street canyons (as 
discussed above in section IV.B.4), where data or other information 
suggest CO concentrations may be approaching or exceeding the NAAQS. A 
third example of where an EPA Regional Administrator might require 
additional monitoring is in unmonitored areas that are subject to high 
ground level CO concentrations particularly due to or enhanced by 
topographical and meteorological impacts, as discussed in section 
IV.B.5 above. In all cases, the Regional Administrator and the 
responsible State or local air monitoring agency should work together 
to design and/or maintain the most appropriate CO network to service 
monitoring objectives and any particular variety of data needs for an 
area.
c. Required Network Implementation
    EPA proposes that state and, when appropriate, local air monitoring 
agencies provide a plan for deploying required CO monitors by July 1, 
2012. We also propose that the ambient CO monitoring network be 
physically established no later than January 1, 2013. These dates 
correspond with the implementation schedule of the required near-road 
NO2 sites, which are the same locations at which CO monitors 
have been proposed to be placed. EPA solicits comment on these proposed 
implementation dates.
7. Microscale Carbon Monoxide Monitor Siting Criteria
    Carbon monoxide monitors that are proposed to operate at near-road 
NO2 sites would likely be classified as microscale-type 
sites, per the general definition of microscale sites in 40 CFR part 
58, appendix D, section 1.2. Such CO monitors would be paired with 
NO2 monitors required to have inlet probe heights between 2 
and 7 meters, and be placed within 50 meters of a target road segment. 
However, when the original minimum monitoring requirements for CO were 
introduced in the 1979 monitoring rule (44 FR 27571), the siting 
criteria codified for microscale CO sites was specifically intended to 
account for the installation of a near-road site in street canyon or 
street corridor locations. The specific siting criteria for microscale 
CO sites, currently located at 40 CFR part 58, appendix E, section 6.2, 
and listed in Table E-4 of appendix E, state that ``the inlet probes 
for microscale carbon monoxide monitors that are being used to measure 
concentrations near roadways must be between 2.5 and 3.5 meters above 
ground level.'' Likewise, criteria currently located at 40 CFR part 58, 
appendix E, section 6.2, and listed in Table E-4 of appendix E state 
that microscale CO monitors are to be between 2 and 10 meters from the 
edge of the nearest traffic lane. These siting criteria, originally 
developed in 1979, were for use primarily in the urban downtown and 
urban street canyon environment. In that type of urban environment, 
such specific and relatively tight siting criteria were, and still are, 
appropriate since there is often little space within which ambient air 
monitoring inlets can be accommodated due to the typical dense 
configuration of buildings. However, outside of the urban downtown and 
urban street canyon environment, such criteria may be less applicable, 
considering site placement logistics and site safety for monitoring 
near the major highways, freeways, interstates, and major arterials 
that carry so much of today's urban traffic volume.
    As noted above, the intent of existing microscale CO siting 
criteria reflects the historical intent of monitoring in urban downtown 
areas and urban street canyons. Since EPA is proposing that CO monitors 
be required to operate at a subset of near-road NO2 sites to 
characterize roadway pollutant concentrations the majority of which are 
not anticipated to be in urban street canyons, EPA has revisited the 
appropriateness of the existing microscale CO siting requirement, 
particularly for near-road sites that exist outside of the downtown 
urban areas and urban street canyons. EPA consulted on this issue with 
the CASAC Ambient Air Monitoring and Methods Subcommittee (CASAC-AAMMS) 
in September, 2010. Specifically, EPA requested feedback on whether it 
would be appropriate to revise existing microscale CO siting criteria 
to match those of near-road NO2 monitors and microscale 
PM2.5 monitors. In their response to EPA, the CASAC-AAMMS 
recommended ``that sampling criteria for CO and other monitors at sites 
installed to monitor [at] near-road NO2 [sites] match those 
for NO2.'' The CASAC-

[[Page 8194]]

AAMMS also noted that ``sampling configurations of existing microscale 
CO monitors should be assessed in terms of their own sampling 
objectives, and need not necessarily conform to those of near-road 
NO2 monitors'' (Russell and Samet, 2010).
    Based in part on the CASAC-AAMMS comments above, EPA believes that 
it is appropriate to revise the existing siting criteria for microscale 
CO monitors to encompass both the current criteria, which are still 
appropriate when monitoring in the urban downtown and/or urban street 
canyon environment, as well as the criteria for near-road 
NO2 sites. Therefore, EPA is proposing that microscale CO 
siting criteria for probe height and horizontal spacing be changed to 
match those of near-road NO2 sites as prescribed in 40 CFR 
part 58 appendix E, sections 2, 4(d), 6.4(a), and Table E-4. 
Specifically, EPA proposes to allow microscale CO monitor inlet probes 
to be between 2 and 7 meters above the ground; that 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 that the CO monitor inlet probe 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.
    These proposed siting criteria encompass, or bracket, the current 
allowable vertical and horizontal spacing criteria for microscale CO 
sites, which will allow current microscale CO sites to continue to meet 
siting criteria. EPA believes the proposed revision to the microscale 
CO siting criteria presented above will allow States to meet siting 
criteria while co-locating required microscale CO monitors with 
required near-road NO2 monitors near heavily trafficked 
roads outside of urban downtown areas and urban street canyons. EPA 
solicits comment upon the revised CO siting requirements proposed 
above. The Agency also solicits comment upon whether it should create 
two distinct sets of siting criteria for microscale CO monitoring. One 
set of siting criteria would be those proposed above, while the second 
set would be the current siting criteria, but directed specifically to 
apply to existing or new microscale CO monitoring sites located in 
downtown urban areas and urban street canyons.

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and 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 Order 12866 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 0940.23.
    The information collected under 40 CFR part 53 (e.g., test results, 
monitoring records, instruction manual, and other associated 
information) is needed to determine whether a candidate method intended 
for use in determining attainment of the National Ambient Air Quality 
Standards (NAAQS) in 40 CFR part 50 will meet comparability 
requirements for designation as a Federal reference method (FRM) or 
Federal equivalent method (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 0940.23). 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.
    To comment on the Agency's need for this information, the accuracy 
of the provided burden estimates, and any suggested methods for 
minimizing respondent burden, EPA has established a public docket for 
this rule, which includes this ICR, under Docket ID number EPA-HQ-OAR-
2008-0015. Submit any comments related to the ICR to EPA and OMB. See 
ADDRESSES section at the beginning of this notice for where to submit 
comments to EPA. Send comments to OMB at the Office of Information and 
Regulatory Affairs, Office of Management and Budget, 725 17th Street, 
NW, Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is 
required to make a decision concerning the ICR between 30 and 60 days 
after February 11, 2011, a comment to OMB is best assured of having its 
full effect if OMB receives it March 14, 2011. The final rule will 
respond to any OMB or public comments on the information collection 
requirements contained in this proposal.

C. Regulatory Flexibility Act

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

[[Page 8195]]

organization that is any not-for-profit enterprise which is 
independently owned and operated and is not dominant in its field.
    After considering the economic impacts of this proposed rule on 
small entities, I certify that this action will not have a significant 
economic impact on a substantial number of small entities. This 
proposed rule will not impose any requirements on small entities. 
Rather, this rule proposes to retain 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 proposed 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. We continue to be interested in the potential impacts of the 
proposed rule on small entities and welcome comments on issues related 
to such impacts.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public 
Law 104-4, establishes requirements for Federal agencies to assess the 
effects of their regulatory actions on State, local, and Tribal 
governments and the private sector. Unless otherwise prohibited by law, 
under section 202 of the UMRA, EPA generally must prepare a written 
statement, including a cost-benefit analysis, for proposed and final 
rules with ``Federal mandates'' that may result in expenditures to 
State, local, and Tribal governments, in the aggregate, or to the 
private sector, of $100 million or more in any one year (adjusted for 
inflation). Before promulgating an EPA rule for which a written 
statement is required under section 202, section 205 of the UMRA 
generally requires EPA to identify and consider a reasonable number of 
regulatory alternatives and to adopt the least costly, most cost-
effective or least burdensome alternative that achieves the objectives 
of the rule. The provisions of section 205 do not apply when they are 
inconsistent with applicable law. Moreover, section 205 allows EPA to 
adopt an alternative other than the least costly, most cost-effective 
or least burdensome alternative if the Administrator publishes with the 
final rule an explanation why that alternative was not adopted. Before 
EPA establishes any regulatory requirements that may significantly or 
uniquely affect small governments, including Tribal governments, it 
must have developed under section 203 of the UMRA a small government 
agency plan. The plan must provide for notifying potentially affected 
small governments, enabling officials of affected small governments to 
have meaningful and timely input in the development of EPA regulatory 
proposals with significant Federal intergovernmental mandates, and 
informing, educating, and advising small governments on compliance with 
the regulatory requirements.
    This action is not subject to the requirements of sections 202 and 
205 of the UMRA. EPA has determined that this proposed rule does not 
contain a Federal mandate that may result in expenditures of $100 
million or more for State, local, and Tribal governments, in the 
aggregate, or the private sector in any one year (adjusted for 
inflation). This rule proposes to retain 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.
    EPA has determined that this proposed rule contains no regulatory 
requirements that might significantly or uniquely affect small 
governments because it imposes no enforceable duty on any small 
governments. Therefore, this rule is not subject to the requirements of 
section 203 of the UMRA.

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 proposed 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.
    However, as also noted in section D (above) on UMRA, EPA recognizes 
that States will have a substantial interest in this rule, including 
the proposed air quality surveillance requirements of 40 CFR part 58. 
Therefore, in the spirit of Executive Order 13132, and consistent with 
EPA policy to promote communications between EPA and State and local 
governments, EPA specifically solicits comment on this proposed rule 
from State and local officials.

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

    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 rule.

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 sections II.C and II.D.2.b.
    The public is invited to submit comments or identify peer-reviewed 
studies and data that assess effects of early life exposures to CO.

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

[[Page 8196]]

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 proposed 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.
    EPA welcomes comments on this aspect of the proposed rule, and 
specifically invites the public to identify potentially applicable 
voluntary consensus standards and to explain why such standards should 
be used in the regulation.

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

    Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes 
Federal executive policy on environmental justice. Its main provision 
directs Federal agencies, to the greatest extent practicable and 
permitted by law, to make environmental justice part of their mission 
by identifying and addressing, as appropriate, disproportionately high 
and adverse human health or environmental effects of their programs, 
policies, and activities on minority populations and low-income 
populations in the United States.
    EPA has determined that this proposed rule will not have 
disproportionately high and adverse human health or environmental 
effects on minority or low-income populations because it does not 
affect the level of protection provided to human health or the 
environment. The action proposed 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.

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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. (2008c) Risk and Exposure 
Assessment to Support the Review of the NO2 Primary 
National Ambient Air Quality Standard. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-08-
008a.
U.S. Environmental Protection Agency. (2008d) Integrated Science 
Assessment for Oxides of Nitrogen--Health Criteria (Final Report). 
EPA/600/R-08/071.
U.S. Environmental Protection Agency. (2009a) Integrated Science 
Assessment for Carbon Monoxide, First External Review Draft. 
National Center for Environmental Assessment, Research Triangle 
Park, NC. EPA/600/R-00/019. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_isa.html
U.S. Environmental Protection Agency. (2009b) Integrated Science 
Assessment for Carbon Monoxide, Second External Review Draft. 
National Center for Environmental Assessment, Research Triangle 
Park, NC. EPA/600/R-09/019B. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_isa.html
U.S. Environmental Protection Agency. (2009c) Carbon Monoxide 
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. (2009e) Risk and Exposure 
Assessment to Support the Review of the SO2 Primary National Ambient 
Air Quality Standard. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/R-09-007. August 2009. Available 
at http://www.epa.gov/ttn/naaqs/standards/so2/data/200908SO2REAFinalReport.pdf.
U.S. Environmental Protection Agency. (2009f) Integrated Science 
Assessment for Particulate Matter (Final Report). National Center 
for Environmental Assessment, Research Triangle Park, NC. EPA/600/R-
08/139F.
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 
Triangle Park, NC. EPA 452/R-10-007. Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pa.html
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Assessment to Support the Review of the Carbon Monoxide Primary 
National Ambient Air Quality Standards, Second External Review 
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
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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
U.S. Environmental Protection Agency. (2010f) Analyzer Use in U.S. 
Monitoring Networks. Spreadsheet of air monitoring method 
utilization in U.S. monitoring networks by year. Office of Air 
Quality Planning and Standards.
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Performance Data. Spreadsheet of performance data for existing FRM 
analyzers. Office of Research and Development.
Watkins N. and Thompson R. (2010) CO Monitoring Network Background 
and Review. Memorandum to the Carbon Monoxide NAAQS Review Docket. 
EPA-HQ-OAR-2008-0015.
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Particulate air pollution and the rate of hospitalization for 
congestive heart failure among Medicare beneficiaries in Pittsburgh, 
Pennsylvania. Am J Epidemiol 161:1030-1036.
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document on characterizing and communicating uncertainty in exposure 
assessment. Available at: http://www.who.int/ipcs/methods/harmonization/areas/exposure/en/.
Zanobetti A. and Schwartz J. (2001) Are diabetics more susceptible 
to the health effects of airborne particles? Am J Respir. Crit. Care 
Med. 164:831-833.
Zhou, Y and Levy J.I. (2007) Factors influencing the spatial extent 
of mobile source air pollution impacts: A meta-analysis. BMC Public 
Health, 7:89.
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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: January 28, 2011.
Lisa P. Jackson,
Administrator.

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

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

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

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

    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

[[Page 8199]]

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.

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 < 0.1 ppm CO.
    4.4 Procedure Using the Dilution Method.
    4.4.1 Assemble or obtain a suitable dynamic dilution calibration 
system such as the one shown schematically in Figure 1. Generally, all 
calibration gases including zero air must be introduced into the sample 
inlet of the analyzer. However, if the analyzer has special, approved 
zero and span inlets and automatic valves to specifically allow 
introduction of calibration standards at near atmospheric pressure, 
such inlets may be used for calibration in lieu of the sample inlet. 
For specific operating instructions, refer to the manufacturer's 
manual.
    4.4.2 Ensure that there are no leaks in the calibration system and 
that all flowmeters are properly and accurately calibrated, under the 
conditions of use, if appropriate, against a reliable volume or flow 
rate standard such as a soap-bubble meter or wet-test meter traceable 
to a NIST standard. All volumetric flow rates should be corrected to 
the same temperature and pressure such as 298.15 K (25 [deg]C) and 760 
mm Hg (101 kPa), using a correction formula such as the following:

[[Page 8200]]

[GRAPHIC] [TIFF OMITTED] TP11FE11.154

Where:

Fc = corrected flow rate (L/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.
    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] TP11FE11.155

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.

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

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[GRAPHIC] [TIFF OMITTED] TP11FE11.138

[[Page 8202]]

[GRAPHIC] [TIFF OMITTED] TP11FE11.139

BILLING CODE 6560-50-C

PART 53--AMBIENT AIR QUALITY REFERENCE AND EQUIVALENT METHODS

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

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

    4. Subpart B of Part 53 is revised to read as follows:
Subpart B--Procedures for Testing Performance Characteristics of 
Automated Methods for SO[bdi2], CO, O[bdi3], and NO[bdi2]
Sec.
53.20 General provisions.
53.21 Test conditions.
53.22 Generation of test atmospheres.
53.23 Test procedure.
Appendix A to Subpart B--Optional Forms for Reporting Test Results

Subpart B--Procedures for Testing Performance Characteristics of 
Automated Methods for SO[bdi2], CO, O[bdi3], and NO[bdi2]

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

[[Page 8203]]

table B-1. 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 
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, and a test analyzer representative of the 
method must pass the tests required by this subpart while operated in 
that range.
    (1) 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, 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. 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.
    (2) 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. For methods for some 
pollutants, table B-1 specifies special performance limit requirements 
for lower ranges. If special low-range performance limit requirements 
are not specified in table B-1, 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 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.
    (3) 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; 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.

                                            Table B-1--Performance Limit Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                          SO2                           CO
                                                                 --------------------    O3    --------------------    NO2
         Performance parameter                   Units 1                      Lower     (Std.               Lower     (Std.      Definitions and test
                                                                    Std.     range 2   range)     Std.     range 2   range)           procedures
                                                                   range 3      3                range 3      3
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range..............................  ppm.....................     0-0.5     < 0.5     0-0.5      0-50      < 50     0-0.5  Sec. 53.23(a).
2. Noise..............................  ppm.....................     0.001    0.0005     0.005       0.2       0.1     0.005  Sec. 53.23(b).
3. Lower detectable limit.............  ppm.....................     0.002     0.001     0.010       0.4       0.2     0.010  Sec. 53.23(c).
4. Interference equivalent:
    Each interferent..................  ppm.....................        minus>    minus>    minus>    minus>    minus>
                                                                     0.005     0.005      0.02       1.0       0.5      0.02
    Total, all interferents...........  ppm.....................  ........  ........      0.06  ........  ........      0.04  Sec. 53.23(d).
5. Zero drift, 12 and 24 hour.........  ppm.....................        minus>    minus>    minus>    minus>    minus>
                                                                     0.004     0.002      0.02       0.5       0.3      0.02
6. Span drift, 24 hour:
    20% of upper range limit..........  Percent.................  ........        minus>              minus>    minus>
                                                                                 3.0      20.0                 2.0      20.0
    80% of upper range limit..........  Percent.................                  minus>    minus>              minus>
                                                                       3.0                 5.0       2.0                 5.0
7. Lag time...........................  Minutes.................         2         2        20       2.0       2.0        20  Sec. 53.23(e).
8. Rise time..........................  Minutes.................         2         2        15       2.0       2.0        15  Sec. 53.23(e).
9. Fall time..........................  Minutes.................         2         2        15       2.0       2.0        15  Sec. 53.23(e).
10. Precision:
    20% of upper range limit..........  ppm.....................  ........  ........     0.010  ........  ........     0.020  Sec. 53.23(e).
                                        Percent.................         2         2  ........       1.0       1.0  ........  Sec. 53.23(e).

[[Page 8204]]

 
    80% of upper range limit..........  ppm.....................  ........  ........     0.010  ........  ........     0.030  Sec. 53.23(e).
                                        Percent.................         2         2  ........       1.0       1.0  ........  Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
1 To convert from parts per million (ppm) to [mu]g/m3 at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
  Percent means percent of the upper measurement range limit.
2 Tests for interference equivalent and lag time do not need to be repeated for any lower range provided the test for the standard range shows that the
  lower range specification (if applicable) is met for each of these test parameters.
3 For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
  suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
  within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
  calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
  designation.
4 For nitric oxide interference for the SO2 UVF method, interference equivalent is  0.0003 ppm for the lower range.
 

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 a 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 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 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

[[Page 8205]]

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.

                                           Table B-2--Test Atmospheres
----------------------------------------------------------------------------------------------------------------
              Test gas                             Generation                           Verification
----------------------------------------------------------------------------------------------------------------
Ammonia............................  Permeation device. Similar to system   Indophenol method, reference 3.
                                      described in references 1 and 2.
Carbon dioxide.....................  Cylinder of zero air or nitrogen       Use NIST-certified standards
                                      containing CO2 as required to obtain   whenever possible. If NIST
                                      the concentration specified in table   standards are not available, obtain
                                      B-3.                                   2 standards from 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 or nitrogen       Use an FRM CO analyzer as described
                                      containing CO as required to obtain    in reference 8.
                                      the concentration specified in table
                                      B-3.
Ethane.............................  Cylinder of zero air or nitrogen       Gas chromatography, ASTM D2820,
                                      containing ethane as required to       reference 10. Use NIST-traceable
                                      obtain the concentration specified     gaseous methane or propane
                                      in table B-3.                          standards for calibration.
Ethylene...........................  Cylinder of pre-purified nitrogen      Do.
                                      containing ethylene as required to
                                      obtain the concentration specified
                                      in table B-3.
Hydrogen chloride..................  Cylinder \1\ of pre-purified nitrogen  Collect samples in bubbler
                                      containing approximately 100 ppm of    containing distilled water and
                                      gaseous HCl. Dilute with zero air to   analyze by the mercuric thiocyanate
                                      concentration specified in table B-3.  method, ASTM (D612), p. 29,
                                                                             reference 4.
Hydrogen sulfide...................  Permeation device system described in  Tentative method of analysis for H2S
                                      references 1 and 2.                    content of the atmosphere, p. 426,
                                                                             reference 5.
Methane............................  Cylinder of zero air containing        Gas chromatography ASTM D2820,
                                      methane as required to obtain the      reference 10. Use NIST-traceable
                                      concentration specified in table B-3.  methane standards for calibration.
Nitric oxide.......................  Cylinder \1\ of pre-purified nitrogen  Gas phase titration as described in
                                      containing approximately 100 ppm NO.   reference 6, section 7.1.
                                      Dilute with zero air to required
                                      concentration.
Nitrogen dioxide...................  1. Gas phase titration as described    1. Use an FRM NO2 analyzer
                                      in reference 6.                        calibrated with a gravimetrically
                                                                             calibrated permeation device.
                                     2. Permeation device, similar to       2. Use an FRM NO2 analyzer
                                      system described in reference 6.       calibrated by gas-phase titration
                                                                             as described in reference 6.
Ozone..............................  Calibrated ozone generator as          Use an FEM ozone analyzer calibrated
                                      described in reference 9.              as described in reference 9.
Sulfur dioxide.....................  1. Permeation device as described in   Use an SO2 FRM or FEM analyzer as
                                      references 1 and 2.                    described in reference 7.
                                     2. Dynamic dilution of a cylinder      ....................................
                                      containing approximately 100 ppm SO2
                                      as described in Reference 7.
Water..............................  Pass zero air through distilled water  Measure relative humidity by means
                                      at a fixed known temperature between   of a dew-point indicator,
                                      20[deg] and 30[deg] C such that the    calibrated electrolytic or piezo
                                      air stream becomes saturated. Dilute   electric hygrometer, or wet/dry
                                      with zero air to concentration         bulb thermometer.
                                      specified in table B-3.
Xylene.............................  Cylinder of pre-purified nitrogen      Use NIST-certified standards
                                      containing 100 ppm xylene. Dilute      whenever possible. If NIST
                                      with zero air to concentration         standards are not available, obtain
                                      specified in table B-3.                2 standards from 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.
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 Fluorscence).''
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)''.

[[Page 8206]]

 
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.

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 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] TP11FE11.140

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.
    (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 of 
this subpart.
    (viii) Both S0 and S80 must be less than or 
equal to the table B-1 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.

    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. In the event that there are 
substances likely to cause a significant interference which have not 
been specified in table B-3, 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 (or as otherwise 
required for unlisted interferents), and 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. 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 (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

[[Page 8207]]

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.
    (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 (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 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, 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 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 pass the test.
BILLING CODE 6560-50-P

[[Page 8208]]

[GRAPHIC] [TIFF OMITTED] TP11FE11.141

[[Page 8209]]

[GRAPHIC] [TIFF OMITTED] TP11FE11.142

BILLING CODE 6560-50-C
    (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

[[Page 8210]]

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 of this section specifies the line voltage and room 
temperature to be used for each test day. The applicant may elect to 
specify a wider temperature range (minimum and maximum temperatures) 
than the range specified in table B-4 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 illustrating the pattern of the required readings.)

                                              Table B-4--Line Voltage and Room Temperature Test Conditions
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                       Line           Room
             Test day              voltage,\1\  temperature,\2\                                          Comments
                                       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.

[[Page 8211]]

BILLING CODE 6560-60-P
[GRAPHIC] [TIFF OMITTED] TP11FE11.143

BILLING CODE 6560-60-C
    (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.

------------------------------------------------------------------------
         Test  atmosphere             Pollutant concentration (percent)
------------------------------------------------------------------------
A0................................  Zero air.
A20...............................  205 of the upper range
                                     limit.
A30...............................  305 of the upper range
                                     limit.
A80...............................  805 of the upper range
                                     limit.
A90...............................  905 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

[[Page 8212]]

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 of this subpart 
(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 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. 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] TP11FE11.144
    

or if a span adjustment was made on the previous test day,
[GRAPHIC] [TIFF OMITTED] TP11FE11.145

where
[GRAPHIC] [TIFF OMITTED] TP11FE11.146

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 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 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 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 pass the test for fall time.
    (vi) Precision. Calculate precision (both P20 and 
P80) for each test day as follows:
    (A)
    [GRAPHIC] [TIFF OMITTED] TP11FE11.147
    
    (B)
    [GRAPHIC] [TIFF OMITTED] TP11FE11.148
    
    (C) Both P20 and P80 must be equal to or less 
than the precision limits specified in table B-1 to pass the test for 
precision.

                  Table B-5--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.

[[Page 8213]]

 
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.
RI...............................  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-th 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

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BILLING CODE 6560-50-C

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

    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]

    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 
Administrator by July 1, 2012. The plan shall provide for all required 
monitoring stations to be operational by January 1, 2013.
* * * * *
    7. Section 58.13 is amended by adding paragraph (e) to read as 
follows:

Sec.  58.13  Monitoring network completion.

* * * * *
    (e) The network of CO monitors must be physically established no 
later than January 1, 2013, and at that time, must be operating under 
all of the requirements of this part, including the requirements of 
appendices A, C, D, and E to this part.
    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.

[[Page 8219]]

    4.2.1 General Requirements. (a) One CO monitor is required to 
operate co-located with any 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. Continued operation of 
existing, but non-required SLAMS CO sites using an FRM or FEM is 
required until discontinuation is approved by the EPA Regional 
Administrator, per section Sec.  58.14 of this part.
    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 urban 
downtown areas or urban street canyons, and (3) characterizing CO 
concentrations in areas that are subject to high ground level CO 
concentrations particularly due or enhanced by topographical and 
meteorological impacts.
    (b) The Regional Administrator and the responsible State or 
local air monitoring agency should work together to design and/or 
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 
from 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 urban 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.
* * * * *
    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 ozone and 
sulfur dioxide monitoring sites, and for neighborhood or larger 
spatial scale Pb, PM10, PM10-2.5, 
PM2.5, NO2, and carbon monoxide 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. 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 or urban street canyon CO monitoring microscale sites are 
intended to provide a measurement of the influence of the immediate 
source on the pollution exposure on the adjacent area. In order to 
provide some reasonable consistency and comparability in the air 
quality data from microscale sites, the CO monitor probe 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.
    (b) Downtown urban area or urban street canyon (microscale) CO 
monitor inlet probes must 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.
    (c) 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.
* * * * *

                                Table E-4 of Appendix E to Part 58--Summary of Probe and Monitoring Path Siting Criteria
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                       Horizontal and
                                                                                   vertical distance from  Distance from trees to      Distance from
                                        Scale (maximum      Height from ground to   supporting structures  probe, inlet or 90% of    roadways to probe,
             Pollutant                 monitoring path     probe, inlet or 80% of  \2\ to probe, inlet or    monitoring path \1\    inlet or monitoring
                                       length, meters)       monitoring path \1\   90% of monitoring path         (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).

[[Page 8220]]

 
CO 4,5,7..........................  Micro, middle (300 m)  2-7: 2-15               >1                      >10                     2-10 for downtown
                                    Neighborhood (1 km)..                                                                           urban area or street
                                                                                                                                    canyon microscale;
                                                                                                                                    <=50 for near-road
                                                                                                                                    microscale; see
                                                                                                                                    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]).                 other scales)                                                           road microscale;
                                    Middle (300m)........                                                                          See Table E-1 of this
                                    Neighborhood, Urban,                                                                            appendix for all
                                     and Regional (1 km).                                                                           other scales.
Ozone precursors (for PAMS) 3,4,5.  Neighborhood and       2-15                    >1                      >10                     See Table E-4 of this
                                     Urban (1 km).                                                                                  appendix for all
                                                                                                                                    scales.
PM,Pb 3,4,5,6,8...................  Micro: Middle,         2-7 (micro);            >2 (all scales,         >10 (all scales)        2-10 (micro); see
                                     Neighborhood, Urban   2-7 (middle PM10	2.5);   horizontal distance                             Figure E-1 of this
                                     and Regional.         2-15 (all other          only)                                           appendix for all
                                                            scales)                                                                 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.

* * * * *
[FR Doc. 2011-2404 Filed 2-10-11; 8:45 am]
BILLING CODE 6560-50-P