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EO12866_CO-NAAQS-RIN2060-AI43-NPR-PreambleAndRegText_20110127.docx

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53 and 58

[EPA-HQ-OAR-2008-0015; FRL-xxxxx]

2060-AI43

Primary National Ambient Air Quality Standards for Carbon Monoxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule 

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 [insert date 60 days after date of 

publication in the Federal Register]. 

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-OAR2008-0015 by one of the following methods:

www.regulations.gov: Follow the on-line instructions for submitting comments.

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

Fax: 202-566-9744
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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 www.regulations.gov, including any 

personal information provided, unless the comment includes information claimed to be 

Confidential Business Information (CBI) or other information whose disclosure is 

restricted by statute. Do not submit information that you consider to be CBI or otherwise 

protected through www.regulations.gov or e-mail. The www.regulations.gov website is 

an ―anonymous access‖ system, which means EPA will not know your identity or contact 

information unless you provide it in the body of your comment. If you send an 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 
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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; email 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 www.regulations.gov index. 

Although listed in the index, some information is not publicly available, e.g., CBI or 

other information whose disclosure is restricted by statute. Certain other material, such 

as copyrighted material, will be publicly available only in hard copy. Publicly available 

docket materials are available either electronically in www.regulations.gov 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.
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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; email: 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; email: 

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; email 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 

www.regulations.gov or email. Clearly mark the part or all of the information that you 

claim to be CBI. For CBI information in a disk or CD ROM that you mail to EPA, mark 

the outside of the disk or CD ROM as CBI and then identify electronically within the disk 

or CD ROM the specific information that is claimed as CBI. In addition to one complete 

version of the comment that includes information claimed as CBI, a copy of the comment 

that does not contain the information claimed as CBI must be submitted for inclusion in 
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the public docket. Information so marked will not be disclosed except in accordance with 

procedures set forth in 40 CFR part 2.

2. Tips for Preparing Your Comments. When submitting comments, remember 

to:

Identify the rulemaking by docket number and other identifying information 

(subject heading, Federal Register 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
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(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; email 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
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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
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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 

 

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 
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secondary standard, as defined in section 109(b)(2), must ―specify a level of air quality 

the attainment and maintenance of which, in the judgment of the Administrator, based on 

such criteria, is requisite to protect the public welfare from any known or anticipated 

adverse effects associated with the presence of such air pollutant in the ambient air.‖ 2

The requirement that primary standards include an adequate margin of safety was 

intended to address uncertainties associated with inconclusive scientific and technical 

information available at the time of standard setting. It was also intended to provide a 

reasonable degree of protection against hazards that research has not yet identified. Lead 

Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980), cert. denied, 449 

U.S. 1042 (1980); American Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. 

Cir. 1981), cert. denied, 455 U.S. 1034 (1982). Both kinds of uncertainties are 

components of the risk associated with pollution at levels below those at which human

health effects can be said to occur with reasonable scientific certainty. Thus, in selecting 

primary standards that include an adequate margin of safety, the Administrator is seeking 

not only to prevent pollution levels that have been demonstrated to be harmful but also to 

prevent lower 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 

 

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)].

2Welfare 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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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).
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B. Related Carbon Monoxide Control Programs

States are primarily responsible for ensuring attainment and maintenance of 

ambient air quality standards once EPA has established them. Under section 110 of the 

Act, and related provisions, States are to submit, for EPA approval, State implementation 

plans (SIPs) that provide for the attainment and maintenance of such standards through 

control programs directed to sources of the pollutants involved. The States, in 

conjunction with EPA, also administer the prevention of significant deterioration 

program. See CAA sections 160-169. In addition, Federal programs provide for 

nationwide reductions in emissions of these and other air pollutants through the Federal 

motor vehicle and motor vehicle fuel control program under title II of the Act,(CAA 

sections 202- 250) which involves controls for emissions from moving sources and 

controls for the fuels used by these sources; 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 
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accordingly set the secondary (welfare-based) standards identical to the primary (healthbased) 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).
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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 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 
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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 
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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 

 

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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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.

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, 
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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).

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 fossilfueled 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 NEI5). 

 

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 
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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, Colorado6(ISA, chapter 3, Figure 3-6).

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 

 

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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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 

 

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.
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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).

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 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 
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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 

 

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 
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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., US 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.

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 

 

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

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 

 

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). 
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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 relevant13 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). 

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., 

198715) 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 

 

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).

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 exerciseinduced angina during air exposure are also included in the analyses.
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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). 

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% COHb17 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 

 

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).
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(GC), which provides a more accurate measurement of COHb blood levels19; 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

 

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 
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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 exerciseinduced 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). 

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 multilaboratory 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 

 

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. 

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.
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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, 
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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 

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 

 

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.
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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 (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 
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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 
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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). 

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 

 

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).
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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 highaltitude 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 
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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 now much-expanded 
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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.

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 

 

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). 
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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, 
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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 COinduced 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 

 

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).
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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.

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 shortterm 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 

produce evidence of myocardial ischemia in CAD patients with reproducible exerciseinduced 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 
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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 
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(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 hypoxiarelated 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 
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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).
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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 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 

 

26 As noted elsewhere, the 8-hour standard is the controlling standard for ambient CO 

concentrations.
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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

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 

 

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).
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et al., 1992; US 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

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 

(CHAD31 now has about 34,000 person-days of data) and the most recent US 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 

 

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; US 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). 

31 CHAD is EPA‘s Comprehensive Human Activity Database which provides input data for APEX 

model simulations (REA, sections 4.3 and 4.4).
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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. 

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 doseresponse 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. 

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 

 

32 As described in section1.2 above, this is the same population group that was the focus of the CO 

NAAQS exposure/dose assessments conducted previously (e.g., USEPA, 1992; Johnson et al., 2000). 
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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 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.

The REA developed COHb estimates for the simulated at-risk populations with 

 

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).
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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 
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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 shortduration 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 shortterm (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).

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 

 

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 prestudy exposure history as well as endogenous CO formation, ranged from 0.2 to 1.1%, averaging about 

0.6% COHb.
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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 disease35; (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 

 

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).
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upper 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).

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 
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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 
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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 result 

from simulations that include the range of commonly encountered CO sources beyond 

just those contributing to ambient air CO concentrations. Although the specific 
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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. 
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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

 

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 
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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 

 

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).
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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.

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 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., 

 

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.
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1989a, 1989b, 1991; Anderson et al., 1973; Kleinman et al., 1989, 1998; Sheps et al., 

198739) 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).

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 

 

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-0x), 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). 
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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.
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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 
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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 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).

Based on consideration of the evidence and the quantitative results of the 

 

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). 
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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 
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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-
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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/riskbased 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?
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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. 
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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; 
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Kleinman et al., 1989, 199842; Sheps et al., 198743). 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 

 

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.
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allowed under the current NAAQS, as described in section II.C above.

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% (postexercise) 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

 

The associated dose-response relationship between incremental changes in 

 

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). 
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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 regarding the effects of short-term controlled CO exposures that result in lower 
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COHb levels, the evidence is much expanded with regard to epidemiological48 analyses 

of ambient monitor concentrations, which observed associations between specific and 

overall cardiovascular-related outcomes and ambient CO measurements. 

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 

 

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).
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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 
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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 exerciseinduced 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 
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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 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 

 

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).
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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).

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 
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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 

 

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).
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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. 

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 
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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 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, 
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―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. 

 

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.
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The strong evidence describing the role of COHb in CO toxicity is important to consider 

in interpreting the CO epidemiological studies and contributes to the biological 

plausibility of the ischemia-related health outcomes that have been associated with 

ambient CO concentrations. Yet, we do not have information on the COHb levels of 

epidemiological study subjects that we can evaluate in the context of the COHb levels 

eliciting health effects in the controlled human exposure studies. Further, we lack 

additional information on the CO exposures of the epidemiological study subjects to both 

ambient and nonambient sources of CO that might be used to estimate their COHb levels 

and bridge the gap between the two study types. 

Additionally the ISA recognizes that the changes in COHb that would likely be 

associated with exposure to the low ambient CO concentrations assessed in some of the 

epidemiological studies would be smaller than changes associated with ―substantially 

reduced {oxygen} delivery to tissues,‖ that might plausibly lead to the outcomes 

observed in those studies, with additional investigation needed to determine whether 

there may be another mechanism of action for CO that contributes to the observed 

outcomes at low ambient concentrations (ISA, p. 5-48). Thus, there are uncertainties 

associated with the epidemiological evidence that ―complicate the quantitative 

interpretation of the epidemiologic findings, particularly regarding the biological 

plausibility of health effects occurring at COHb levels resulting from exposures to the 

ambient CO concentrations‖ assessed in these studies (ISA, p. 2-17).

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 
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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 
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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 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 
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associated with ambient CO may overestimate the response of COHb levels to shortduration 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 
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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.

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 

 

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). 
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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 atrisk 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 atrisk 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 
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maximum COHb level at or above 2.0%, and less than 0.1% are estimated to experience 

six or more days 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).

Table 1. Portion of simulated HD populations with at least one daily maximum endof-hour COHb level (absolute) at or above indicated levels under air quality 

conditions simulated to just meet the current standard and ―as is‖ conditions. 

Daily 

Maximum 

End-of-hour 

COHb

(Absolute)

Percentage (%) of Simulated HD PopulationA

Just Meeting Current 8-hour Standard

(8-hr DV=9.4 ppm) “As is” (2006) Conditions

Los Angeles

(1-hr DV = 11.8 ppm)

Denver

(1-hr DV = 16.2 ppm)

Los Angeles

(8-hr DV = 5.6 ppm)

(1-hr DV = 8.2 ppm)

Denver

(8-hr DV = 3.1 ppm)

(1-hr DV = 4.6 ppm)

> 4.0% 0 <0.1B

> 3.0% < 0.1B 0.3 0 0

> 2.5% < 0.1B 0.9

> 2.0% 0.6 4.5 < 0.1

B < 0.1B

> 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 endof-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. 

Maximum 

End-ofhour 

COHb 

Level

(Absolute)

Percentage (%) of Simulated CHD PopulationA

Just Meeting Current 8-hour Standard

(8-hr DV=9.4 ppm) “As is” (2006) Conditions

Los Angeles

(1-hr DV = 11.8 ppm)

Denver

(1-hr DV = 16.2 ppm)

Los Angeles

(8-hr DV = 5.6 ppm)

(1-hr DV = 8.2 ppm)

Denver

(8-hr DV = 3.1 ppm)

(1-hr DV = 4.6 ppm)

>2 

days

>4 

days

>6 

days

>2 

days

>4 

days

>6 

days

>2 

days

>4 

days

>6 

days

>2

days

>4 

days

>6 

days
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≥ 3.0% 0 0 0 0 0 0 0 0 0 0 0 0

≥ 2.5% < 0.1 B 0 0 < 0.1B 0 0 0 0 0 0 0 0

≥ 2.0% 0.2 < 0.1B < 0.1B 1.4 0.2 < 0.1 B 0 0 0 < 0.1B < 0.1B < 0.1B

≥ 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 
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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 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).

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 

 

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).
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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 
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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 
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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 
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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. 

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 
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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 
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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 atrisk 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. 
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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 
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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 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.
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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). 

In their review of the draft ISA, CASAC noted various limitations and 

 

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.
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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 
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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 
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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.

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 
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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-
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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 
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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 
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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 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 
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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 
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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 
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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 
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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 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 
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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/m2) 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/m2over 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 
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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 
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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

 

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 

 

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

standard.

57 This recommendation is addressed in section 3.5 of the Policy Assessment.
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insufficient information to support consideration of a secondary standard at this time 

(Brain and Samet, 2010c). 

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 

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 climaterelated 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 
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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 
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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 website, 

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 
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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 
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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, 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 
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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 
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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 is 

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 
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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 B1. 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 
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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.

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. 
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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 
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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 scale58. Approximately 56 of the monitors operating in 2009 were at 

 

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:
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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 longterm trend determination, to meet State Implementation Plan (SIP) and maintenance plan 

requirements, and to support ongoing health studies.

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 

 

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.
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engines) because they typically operate closer to the stoichiometric air-to-fuel ratio, have 

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 onroad 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 
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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 
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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 nearbackground 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 
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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. 
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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 canyon59, 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. 

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 

 

59 A natural canyon may be defined as a ―deep narrow valley with steep sides‖ 

(http://www.merriam-webster.com/dictionary/canyon).
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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-
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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 
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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 monitoring stations, as described in detail in the following 

sections, will require a relatively modest amount of new resources by state and local air 
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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 colocating required CO monitors at required near-road NO2 monitoring stations include: 
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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 

objectives60. 

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 US 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 (US) 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 

(US) 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 

 

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). 
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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 
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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. 

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 

 

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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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 
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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-
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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 
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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 (CASACAAMMS) 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 CASACAAMMS 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 
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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.
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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 
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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, 
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Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is required to 

make a decision concerning the ICR between 30 and 60 days after [insert date of 

publication in the Federal Register], a comment to OMB is best assured of having its 

full effect if OMB receives it [insert date 30 days after publication in the Federal 

Register]. The final rule will respond to any OMB or public comments on the 

information collection requirements contained in this proposal.

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a 

regulatory flexibility analysis of any rule subject to notice and comment rulemaking 

requirements under the Administrative Procedure Act or any other statute unless the 

agency certifies that the rule will not have a significant economic impact on a substantial 

number of small entities. Small entities include small businesses, small organizations, 

and small governmental jurisdictions.

For purposes of assessing the impacts of 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 

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

Governments

This action does not have tribal implications, as specified in Executive Order 

13175 (65FR 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 
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a case-by-case basis, and EPA cannot predict whether the control options selected by 

States will include regulations on energy suppliers, distributors, or users.

I. National Technology Transfer and Advancement Act

Section 12(d) of the National Technology Transfer and Advancement Act of 1995 

(NTTAA), Public Law 104– 113, section 12(d) (15 U.S.C. 272 note) directs EPA to use 

voluntary consensus standards in its regulatory activities unless to do so would be 

inconsistent with applicable law or otherwise impractical. Voluntary consensus standards 

are technical standards (e.g., materials specifications, test methods, sampling procedures, 

and business practices) that are developed or adopted by voluntary consensus standards 

bodies. The NTTAA directs EPA to provide Congress, through OMB, explanations when 

the Agency decides not to use available and applicable voluntary consensus standards.

This 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 
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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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U.S. Environmental Protection Agency. (1979a) Air Quality Criteria for Carbon 
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Monoxide. Office of Health and Environmental Assessment, Environmental Criteria and 

Assessment Office, Research Triangle Park, NC. EPA-600/8-79-022.

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Standard. Office of Air Quality Planning and Standards, Research Triangle Park, NC.

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Assessment, Environmental Criteria and Assessment Office, Research Triangle Park, NC. 

EPA-600/8-83-033F

U.S. Environmental Protection Agency. (1984b) Review of the NAAQS for 

Carbon Monoxide: Reassessment of Scientific and Technical Information. Office of Air 

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http://www.epa.gov/ttn/naaqs/standards/co/s_co_pr.html

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Performance Data. Spreadsheet of performance data for existing FRM analyzers. Office 

of Research and Development.

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Review. Memorandum to the Carbon Monoxide NAAQS Review Docket. EPA-HQOAR-2008-0015.

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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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particles near a major highway with heavy-duty diesel traffic. Atmos Environ, 36: 4323-

4335.
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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
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For the reasons stated in the preamble, title 40, chapter I of the Code of Federal 

Regulations is proposed to be amended as follows:

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 § 50.8 of this chapter. The method is 

applicable to continuous sampling and measurement of ambient CO concentrations 

suitable for determining 1-hour or longer average measurements. The method may also 

provide measurements of shorter averaging times, subject to specific analyzer 

performance limitations. Additional CO monitoring quality assurance procedures and 

guidance are provided in part 58, appendix A, of this chapter and in reference 1 of this 

appendix C.

2.0 Measurement Principle

2.1 Measurements of CO in ambient air are based on automated measurement of the 

absorption of infrared radiation by CO in an ambient air sample drawn into an analyzer 

employing non-wavelength-dispersive, infrared photometry (NDIR method). Infrared 
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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.
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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.
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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.
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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 
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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 °C) and 760 mm Hg (101 kPa), using a 

correction formula such as the following:

760( 273.15)

298.15

m

m

c m

T

P

F F

(1)

where:

Fc = corrected flow rate (L/min at 25 oC 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 
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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:

D CO

STD CO

OUT F F

CO F

CO [ ][ ]

(2)

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 
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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.
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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).
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167

Figure 1. Dilution method for calibration of CO analyzers.

Flow 

controller Flowmeter

Flow

controller Flowmeter

Mixing

chamber

Zero

air

Extra outlets capped

Vent

To inlet of analyzer

CO

std. Output

manifold

FC

FD
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168

Figure 2. Multiple cylinder method for calibration of CO analyzers.

Flow

controller Flowmeter

Extra outlets capped

when not in use

Vent

To inlet of analyzer

under calibration

CO

std.

Output

manifold

CO

std.

CO

std.

Zero

air

CO

std.
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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 

SO2, CO, O3, and NO2

Sec.

53.20 General Provisions

53.21 Test Conditions

53.22 Generation of Test Atmospheres

53.22 Test Procedure

Appendix A to Subpart B – Optional Forms For Reporting Test Results

Subpart B – Procedures for Testing Performance Characteristics of Automated Methods for 

SO2, CO, O3, and NO2

§ 53.20 General provisions.

(a) The test procedures given in this subpart shall be used to test the performance of candidate 

automated methods against the performance requirement specifications given in table B-1. 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.

(i) Higher ranges. The tests may be repeated for one or more higher (broader) ranges (i.e., ranges 

extending to higher concentrations) than the standard range specified in table B-1, provided that the 

range does not extend to concentrations more than four times the upper range limit of the standard 
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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 §53.23(e) may be abridged to include only those 

components needed to test lag, rise and fall time.

(ii) Lower ranges. The tests may be repeated for one or more lower (narrower) ranges (i.e., ones 

extending to lower concentrations) than the standard range specified in table B-1. 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.

(iii) If the tests are conducted and passed only for the specified standard range, any FRM or FEM 

determination with respect to the method will be limited to that range. If the tests are passed for both 

the specified range and one or more higher or lower ranges, any such determination will include the 

additional higher or lower range(s) as well as the specified standard range. Appropriate test data shall 

be submitted for each range sought to be included in a FRM or FEM method determination under this 

paragraph (b).

(c) For each performance parameter (except range), the test procedure shall be initially repeated 

seven (7) times to yield 7 test results. Each result shall be compared with the corresponding 

performance limit specification in table B-1; 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.
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(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 § 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 °C and 30 °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 

§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 § 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.
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TABLE B-1—PERFORMANCE LIMIT SPECIFICATIONS FOR AUTOMATED METHODS

Performance parameter Units1

SO2 O3

(Std.

range)

CO NO2 

(Std.

range)

Definitions and test 

procedures Std. 

range3

Lower 

range2,3

Std. 

range3

Lower 

range2,3

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

 Total, all interferents

ppm

ppm

±0.005

---

4

±0.005

---

±0.02

 0.06

±1.0

 ---

±0.5

 

±0.02

 0.04

Sec. 53.23(d).

Sec. 53.23(d).

5. Zero drift, 12 and 24 hour ppm ±0.004 ±0.002 ±0.02 ±0.5 ±0.3 ±0.02 Sec. 53.23(e).

6. Span drift, 24 hour 20% 

of upper range limit 80% of 

upper range limit

Percent 

Percent ---±3.0

---±3.0 ±20.0

 ±5.0

---

±2.0

---±2.0 ±20.0

±5.0 Sec. 53.23(e)

Sec. 53.23(e).

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

 80 % of upper range limit

 

ppm

Percent

ppm

Percent

---

2

---

2

---

2

---

2

0.010

---

0.010

---

---

1.0

---

1.0

---

 1.0

---

 1.0

0.020

---

0.030

---

Sec. 53.23(e).

Sec. 53.23(e).

Sec. 53.23(e).

Sec. 53.23(e).

1. To convert from parts per million (ppm) to μg/m3at 25 °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.
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

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§ 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 § 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 § 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, 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 174 of 207

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.

§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. 
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(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 

oC.

(d) All diluent air shall be zero air free of contaminants likely to react with the test atmospheres or 

cause a detectable response on the test analyzer.

(e) The concentration of each test atmosphere used shall be quantitatively established and/or 

verified before or during each series of tests. Samples for verifying test concentrations shall be 

collected from the test atmosphere delivery system as close as feasible to the sample intake port of the 

test analyzer.

(f) The accuracy of all flow measurements used to calculate test atmosphere concentrations shall be 

documented and referenced to a primary flow rate or volume standard (such as a spirometer, bubble 

meter, etc.). Any corrections shall be clearly shown. All flow measurements given in volume units 

shall be standardized to 25 oC. 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.
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TABLE B-2—TEST ATMOSPHERES

Test gas Generation Verification

Ammonia Permeation device. Similar to system described 

in references 1 and 2

Indophenol method, reference 3.

Carbon 

dioxide

Cylinder of zero air or nitrogen containing CO2 as 

required to obtain the concentration specified in 

table B-3.

Use NIST-certified standards whenever 

possible. If NIST standards are not available, 

obtain 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 containing CO as 

required to obtain the concentration specified in 

table B-3.

Use an FRM CO analyzer as described in 

reference 8.

Ethane Cylinder of zero air or nitrogen containing ethane 

as required to obtain the concentration specified in 

table B-3.

Gas chromatography, ASTM D2820, reference 

10. Use NIST-traceable gaseous methane or 

propane standards for calibration.

Ethylene Cylinder of pre-purified nitrogen containing 

ethylene as required to obtain the concentration 

specified in table B-3.

Do. 

Hydrogen 

chloride

Cylinder1of pre-purified nitrogen containing 

approximately 100 ppm of gaseous HCl. Dilute with 

zero air to concentration specified in table B-3

Collect samples in bubbler containing distilled 

water and analyze by the mercuric thiocyanate 

method, ASTM (D612), p. 29, reference 4.

Hydrogen 

sulfide

Permeation device system described in references 

1 and 2.

Tentative method of analysis for H2S content of 

the atmosphere, p. 426, reference 5.

Methane Cylinder of zero air containing methane as required 

to obtain the concentration specified in table B-3.

Gas chromatography ASTM D2820, reference 

10. Use NIST-traceable methane standards for 

calibration.

Nitric oxide Cylinder1of pre-purified nitrogen containing 

approximately 100 ppm NO. Dilute with zero air to 

required concentration.

Gas phase titration as described in reference 6, 

section 7.1

Nitrogen 

dioxide

1. Gas phase titration as described in reference 6.

2. Permeation device, similar to system described 

in reference 6.

1. Use an FRM NO2 analyzer calibrated with a 

gravimetrically calibrated permeation device.

2. Use an FRM NO2 analyzer calibrated by gasphase titration as described in reference 6.

Ozone Calibrated ozone generator as described in 

reference 9.

Use an FEM ozone analyzer calibrated as 

described in reference 9.

Sulfur 

dioxide

1. Permeation device as described in references 1 

and 2.

2. Dynamic dilution of a cylinder containing 

approximately 100 ppm SO2 as described in 

Reference 7.

Use an SO2 FRM or FEM analyzer as described 

in reference 7.
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TABLE B-2—TEST ATMOSPHERES (CONTINUED)

1 Use stainless steel pressure regulator dedicated to the pollutant measured.

Reference 1. O'Keefe, A. E., and Ortaman, G. C. "Primary Standards for Trace Gas Analysis," Anal. Chem. 38, 760 (1966).

Reference 2. Scaringelli, F. P., A. E. . Rosenberg, E*, and Bell, J. P., "Primary Standards for Trace Gas Analysis." Anal. 

Chem. 42, 871 (1970).

Reference 3. "Tentative Method of Analysis for Ammonia in the Atmosphere (Indophenol Method)", Health Lab Sciences, 

vol. 10, No. 2, 115-118, April 1973.

Reference 4. 1973 Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race St., 

Philadelphia, PA.

Reference 5. Methods for Air Sampling and Analysis, Intersociety Committee, 1972, American Public Health Association, 

1015.

Reference 6. 40 CFR 50 Appendix F, "Measurement Principle and Calibration Principle for the Measurement of Nitrogen 

Dioxide in the Atmosphere (Gas Phase Chemiluminescence)."

Reference 7. 40 CFR 50 Appendix A-1, "Measurement Principle and Calibration Procedure for the Measurement of Sulfur 

Dioxide in the Atmosphere (Ultraviolet FIuorscence)."

Reference 8. 40 CFR 50 Appendix C, "Measurement Principle and Calibration Procedure for the Measurement of Carbon 

Monoxide in the Atmosphere" (Non-Dispersive Infrared Photometry)".

Reference 9. 40 CFR 50 Appendix D, "Measurement Principle and Calibration Procedure for the Measurement of Ozone in 

the Atmosphere".

Reference 10. "Standard Test Method for C, through C5 Hydrocarbons in the Atmosphere by Gas Chromatography", D 

2820, 1987 Annual Book of Aston Standards, vol 11.03, American Society for Testing and Materials, 1916 Race St., 

Philadelphia, PA 19103.

§ 53.23 Test procedures.

(a) Range—(1) Technical definition. The nominal minimum and maximum concentrations that a 

method is capable of measuring.

NOTE TO § 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).

Test gas Generation Verification

Water Pass zero air through distilled water at a fixed known 

temperature between 20° and 30° C such that the air 

stream becomes saturated. Dilute with zero air to 

concentration specified in table B-3.

Measure relative humidity by means of a dewpoint indicator, calibrated electrolytic or piezo 

electric hygrometer, or wet/dry bulb 

thermometer.

Xylene Cylinder of pre-purified nitrogen containing 100 ppm 

xylene. Dilute with zero air to concentration specified 

in table B-3.

Use NIST-certified standards whenever 

possible. If NIST standards are not available, 

obtain 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.
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(2) Test procedure. Determine and submit a suitable calibration curve, as specified in § 53.21(b), 

showing the test analyzer's measurement response over at least 95 percent of the required or indicated 

measurement range.

NOTE TO § 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 § 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 § 53.21(b). 

Label and record the test measurements or converted DM readings as r1, r2, r3 ... ri ... r25.
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(v) Calculate measurement noise as the standard deviation, S, as follows:

2

25

1

25

1

2

25

1

24

1

i

i

i

i S r r 

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 B1.

(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 § 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.
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(iv) Determine the lower detectable limit (LDL) test result as LDL = BL − BZ. Compare this LDL

value with the noise level, S0, determined in § 53.23(b), for the 0 concentration test atmosphere. LDL

must be equal to or higher than 2 × 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:
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Page 181 of 207

(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 § 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 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).
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Page 182 of 207

(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 
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Page 183 of 207

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.
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TABLE B-3—INTERFERENT TEST CONCENTRATION,

1PARTS PER MILLION

Pollutant Analyzer Type

Hydrochloric

acid

Ammonia

Hydrogen

sulfide

Sulfur

dioxide

Nitrogen

dioxide

Nitric

oxide

Carbon

dioxide

Ethylene Ozone

Mxylene Water

vapor

Carbon

monoxide

Methane

Ethane Naphthalene

SO2

Ultraviolet 

fluorescence 0.15

0.1440.5 0.5 0.5 0.2 20,000 0.056

SO2 Flame photometric 0.01 0.144750 20,000350

SO2 Gas chromatography 0.1 0.144750 20,000350

SO2

Spectrophotometricwet chemical 

(pararosanaline)

0.2 0.1 0.1 0.1440.5 750 0.5

SO2 Electrochemical 0.2 0.1 0.1 0.1440.5 0.5 0.2 0.5 20,0003

SO2 Conductivity 0.2 0.1 0.1440.5 750

SO2

Spectrophotometricgas phase, including 

DOAS

0.1440.5 0.5 0.2

O3 Chemiluminescent 0.13750 0.08420,0003

O3 Electrochemical 0.130.5 0.5 0.084

O3

Spectrophotometricwet chemical 

(potassium iodide)

0.130.5 0.5 0.530.084

O3

Spectrophotometricgas phase, including 

ultraviolet absorption 

and DOAS)

0.5 0.5 0.5 0.0840.02 20,000

CO Non-dispersive 

Infrared 750 20,000 104

CO

Gas chromatography 

with flame ionization 

detector

20,000 1040.5

CO Electrochemical 0.5 0.2 20,000 104

CO Catalytic combustionthermal detection 0.1 750 0.2 20,000 1045.0 0.5

CO IR fluorescence 750 20,000 1040.5

CO

Mercury 

replacement-UV 

photometric

0.2 1040.5
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TABLE B-3—INTERFERANT TEST CONCENTRATION,

1

PARTS PER MILLION (CONTINUED)

Pollutant Analyzer Type

Hydrochloric

acid

Ammonia

Hydrogen

sulfide

Sulfur

dioxide

Nitrogen

dioxide

Nitric

oxide

Carbon

dioxide

Ethylene Ozone MxyleneWater

vapor

Carbon

monoxide

Methane

Ethane

Naphthalene

NO2 Chemiluminescent 0.130.5 0.140.5 20,000

NO2

Spectrophotometricwet chemical (azodye reaction)

0.5 0.140.5 750 0.5

NO2 Electrochemical 0.2 0.130.5 0.140.5 750 0.5 20,000 50

NO2

Spectrophotometricgas phase 0.13

0.5 0.140.5 0.5 20,000 50

1. Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value.

2. Analyzer types not listed will be considered by the Administrator as special cases.

3. Do not mix with the pollutant.

4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value.

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the 

scrubber, and evaluate response for interference.
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Page 186 of 207

(e) Zero drift, span drift, lag time, rise time, fall time, and precision—(1) Technical 

definitions—(i) Zero drift: The change in measurement response to zero pollutant 

concentration over 12- and 24-hour periods of continuous unadjusted operation.

(ii) Span drift: The percent change in measurement response to an up-scale pollutant 

concentration over a 24-hour period of continuous unadjusted operation.

(iii) Lag time: The time interval between a step change in input concentration and the 

first observable corresponding change in measurement response.

(iv) Rise time: The time interval between initial measurement response and 95 percent 

of final response after a step increase in input concentration.

(v) Fall time: The time interval between initial measurement response and 95 percent 

of final response after a step decrease in input concentration.

 (vi) Precision: Variation about the mean of repeated measurements of the same 

pollutant concentration, expressed as one standard deviation.

(2) Tests for these performance parameters shall be accomplished over a period of 

seven (7) or fifteen (15) test days. During this time, the line voltage supplied to the test 

analyzer and the ambient temperature surrounding the analyzer shall be changed from 

day to day, as required in paragraph(e)(4) of this section. One test result for each 

performance parameter shall be obtained each test day, for seven (7) or fifteen (15) test 

days, as determined from the test results of the first seven days. The tests for each test 

day are performed in a single integrated procedure.

(3) The 24-hour test day may begin at any clock hour. The first approximately 12 

hours of each test day are required for testing 12-hour zero drift. Tests for the other 

parameters shall be conducted any time during the remaining 12 hours.
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(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 §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 oC.

(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.
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(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 § 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 § 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

Test 

day

Line voltage,1

rms

Room temperature,2

oC Comments

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.
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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 oC.
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

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Figure B-1. Example showing the nature of the tracing obtained 

during the test sequence for 24-hour drift, lag time, 

rise time, fall time, and precision. The time scale has 

been greatly compressed.

(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 ....................... 20±5 of the upper range limit.

A30 ....................... 30±5 of the upper range limit.

A80 ....................... 80±5 of the upper range limit.

A90 ....................... 90±5 of the upper range limit.

(ii) For steps within paragraphs (e)(9)(xxv) through (e)(9)(xxxi) of this section, a 

chart speed of at least 10 centimeters per hour (or equivalent resolution for a digital 

representation) shall be used to clearly show changes in measurement responses. The 

actual chart speed, chart speed changes, and time checks shall be clearly marked on the 

chart.

(iii) Test day 0. Allow sufficient time for the test analyzer to warm up and stabilize at 

a line voltage of 115 volts and a room temperature of 25 oC. Adjust the zero baseline to 5 

percent of chart (see § 53.21(b)) and recalibrate, if necessary. No further adjustments 

shall be made to the analyzer until the end of the tests on the third, sixth, ninth, or twelfth 

test day.

(iv) Measure test atmosphere A0 until a stable measurement reading is obtained and 

record this reading (in ppm) as Z’n, where n = 0 (see Figure B-4 in appendix A of this 

subpart).

(v) [Reserved.] 
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(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.
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(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.
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(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:

100%

1

1

n

n n

n

S

S S

SD

or if a span adjustment was made on the previous test day,

100%

1

1

n

n n

n

S

S S

SD

where

12

6 7

1

i

Sn Pi

,
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 194 of 207

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)

100%

6

1

5

1 1

2

6

1

6

1

2

20

i

i

i

Pi P

URL

P
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(B)

100%

6

1

5

1 1

2

12

7

12

7

2

80

i

i

i

Pi P

URL

P

(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.

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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NOISE TEST DATA

Applicant ______________________________________ Date ____________________

Analyzer ______________________________________ Pollutant _________________

Range _____________________ Test No. _________________

READING

NUMBER (i) TIME

0% of URL 80% of URL

DM 

READING ri

, ppm

DM 

READING ri

, ppm

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

STD. DEVIATION S0 = S80 =

Figure B-2. Form for noise test data (see §53.23(b)).
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LDL and INTERFERENCE TEST DATA

Applicant ____________________________________________ Date ____________________

Analyzer _____________________________________________ Pollutant _________________

TEST

PARAMETER

READING or 

CALCULATION

TEST NUMBER

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

LOWER 

DETECTABLE LIMIT

BZ

BL

LDL = BL ‒ BZ

INTERFERENCE

EQUIVALENT

1

R1

RI1

IE = RI1 – R1

2

R2

RI2

IE = RI2 – R2

3*

R3

RI3

IE = RI3 – R3

4*

R4

RI4

IE = RI4 – R4

5*

R5

RI5

IE = RI5 – R5

TOTAL*

n

i

i

IE

1

*If required.

Figure B-3. Form for test data and calculations for lower detectable limit (LDL) and interference equivalent (IE)

(see § 53.23(c) and (d)).
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DRIFT AND PRECISION TEST DATA

Applicant ____________________________________________ Date ____________________

Analyzer _____________________________________________ Pollutant _________________

ANALYZER READING, ppm

TEST DAY 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

DATE

P1

P2

P3

P4

P5

P6

P7

P8

P9

P10

P11

P12

12

6 7

1

i

Sn Pi

L1

L2

Z’n

S’n

Cmax

Cmin

Figure B-4. Form for drift and precision test data (see § 53.23(e)).
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CALCULATION OF ZERO DRIFT, SPAN DRIFT, AND PRECISION

Applicant ____________________________________________ Date ____________________

Analyzer _____________________________________________ Pollutant _________________

TEST

PARAMETER CALCULATION

TEST DAY (n)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ZERO 

DRIFT

12 HOUR 12ZD = Cmax – Cmin

24

HOUR

Z = (L1 + L2)/2

24ZD = Zn – Zn-1

24ZD = Z’n – Z’n-1

SPAN

DRIFT

24

HOUR

12

6 7

1

i

Sn Pi

100%

1

1

n

n n

n

S

S S

SD 100%

'

'

1

1

n

n n

n

S

S S

SD

PRECISION

20%

URL (P20)

P20 = STANDARD

DEVIATION of (P1...P6)

80%

URL (P80)

P80 = STANDARD

DEVIATION of (P7...P12)

Figure B-5. Form for calculating zero drift, span drift, and precision (§ 53.23(e)).
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TEST DATA SUMMARY

 Applicant ___________________________________________ Analyst 

_____________________________________________

 Analyzer ____________________________________________ Pollutant 

____________________________________________

 Range _______________________ Other information _____________________________________ 

Test dates _________________

Performance

Parameter

Table 

B-1 

Spec.

Test Number (first set) Test Number (second set) Number 

of 

Failures

Pass

or 

Fail 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Noise,

ppm

0% 

URL

80% 

URL

LDL (> 2 x 0% 

noise)

Interference

Equivlent,

ppm

IE1

IE2

IE3

IE4

IE5

IE6

Total

Zero

Drift,

ppm

12 hr

24 hr.

Span 

Drift, %

80% 

URL

Lag Time, min

Rise Time, min

Fall Time, min

Precision,

percent

20% 

URL

80% 

URL

Figure B-6. Form for reporting a summary of the test results (see § 53.23).
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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:

§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:

§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:
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Appendix D to Part 58—Network Design Criteria for Ambient Air Quality Monitoring

* * * * *

4.2 Carbon Monoxide (CO) Design Criteria.

4.2.1 General Requirements. (a) 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 §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.
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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 
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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 
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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.

* * * * *
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Table E-4 of Appendix E to Part 58. Summary of Probe and Monitoring Path Siting Criteria

Pollutant Scale (maximum 

monitoring path 

length, meters)

Height from 

ground to 

probe, inlet or 

80% of 

monitoring 

path1

Horizontal and 

vertical distance 

from supporting 

structures2to 

probe, inlet or 

90% of 

monitoring path1

(meters)

Distance from 

trees to probe, 

inlet or 90% of 

monitoring 

path1(meters)

Distance from 

roadways to probe, 

inlet or monitoring 

path1(meters)

SO2

3,4,5,6 Middle

(300 m)

Neighborhood

Urban, and 

Regional

(1 km)

2-15 > 1 > 10 N/A

CO 4,5,7 Micro, middle

(300 m), 

Neighborhood (1 

km)

2-7: 2-15 > 1 > 10 2-10 for downtown 

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)

Neighborhood, 

Urban, and 

Regional (1 km) 

2-15 > 1 > 10 See Table E-1 of 

this appendix for 

all scales.

NO2 

3,4,5 Micro (Near-road 

[50-300]) 

Middle(300m)

Neighborhood, 

Urban, and 

Regional (1 km)

2-7 (micro);

2-15(all other 

scales)

> 1 > 10 < 50 meters for 

near-road 

microscale;

See Table E-1 of 

this appendix for 

all other scales
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 207 of 207

Ozone 

precursors 

(for 

PAMS)3,4,5

Neighborhood and 

Urban (1 km)

2-15 > 1 > 10 See Table E-4 of 

this appendix for 

all scales.

PM,Pb 3,4,5,6,8 Micro: Middle,

Neighborhood,

Urban and

Regional

2-7 (micro);

2-7 (middle 

PM10-2.5);

2-15 (all other 

scales)

> 2 (all scales, 

horizontal 

distance only)

> 10 (all scales) 2-10 (micro); see 

Figure E-1 of this 

appendix for all 

other scales.

N/A--Not applicable.

1 Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and regional scale 

NO2 monitoring, and all applicable scales for monitoring SO2,O3, and O3 precursors.

2 When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.

3

Should be >20 meters from the drip-line of tree(s) and must be 10 meters from the drip-line when the tree(s) act as an obstruction.

4 Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the 

sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text).

5 Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall.

6 The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration flues. The separation distance is dependent on the 

height of the minor source's emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur, ash, or lead content). This 

criterion is designed to avoid undue influences from minor sources.

7

For microscale CO monitoring sites in downtown areas or street canyons (not at near-road NO2 monitoring sites), the probe must be >10 meters from a street 

intersection and preferably at a midblock location.

8 Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 meter apart for 

samplers having flow rates less than 200 liters/min to preclude airflow interference.

* * * * *