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EO12866_CO-NAAQS-RIN2060-AI43-NFR-PreambleAndRegText_20110811.docx

6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53 and 58

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

RIN 2060-AI43

Review of National Ambient Air Quality Standards for Carbon Monoxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final Rule.

SUMMARY: This rule is being issued at this time as required by a court order 

governing the schedule for completion of this review of the air quality criteria and the 

national ambient air quality standards (NAAQS) for carbon monoxide (CO). Based on its 

review, the EPA concludes the current primary standards are requisite to protect public 

health with an adequate margin of safety, and is retaining those standards. After review 

of the air quality criteria, EPA further concludes that no secondary standard should be set 

for CO at this time. EPA is also making changes to the ambient air monitoring 

requirements for CO, including those related to network design, and is updating, without

substantive change, aspects of the federal reference method. 

DATES: This final rule is effective on [insert date 60 days after date of publication in 

the FEDERAL REGISTER]. 

ADDRESSES: EPA has established a docket for this action under Docket ID No. EPAHQ-OAR-2008-0015. Incorporated into this docket is a separate docket established for 
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the 2010 Integrated Science Assessment for Carbon Monoxide (Docket ID No. EPA-HQORD-2007-0925. All documents in these dockets are listed on the www.regulations.gov

website. Although listed in the docket index, some information is not publicly available, 

e.g., confidential business information (CBI) or other information whose disclosure is 

restricted by statute. Certain other material, such as copyrighted material, is not placed 

on the Internet and will be publicly available for viewing at the Public Reading Room. 

Abstracts of scientific studies cited in the review are also available on the Internet at 

EPA’s HERO Web site: http://hero.epa.gov/, by clicking on the box on the right side of 

the page labeled “Search HERO.” Publicly available docket materials are available 

electronically through www.regulations.gov or may be viewed at the Public Reading 

Room at the Air and Radiation Docket and Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., NW, Washington, DC. The Public Reading Room 

is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. 

The telephone number for the Public Reading Room is (202) 566-1744 and the telephone 

number for the Air and Radiation Docket and Information Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Deirdre Murphy, Health and 

Environmental Impacts Division, Office of Air Quality Planning and Standards, Mail 

code C504-06, U.S. Environmental Protection Agency, Research Triangle Park, NC 

27711; telephone number: 919-541-0729; fax number: 919-541-0237; e-mail address: 

murphy.deirdre@epa.gov. For further information specifically with regard to section IV

of this notice, contact Mr. Nealson Watkins, Air Quality Analysis Division, Office of Air 

Quality Planning and Standards, Mail code C304-06, U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711; telephone number: 919-541-5522; fax
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number: 919-541-1903; email address: watkins.nealson@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in this preamble:

I. Background

A. Legislative Requirements

B. Related Carbon Monoxide Control Programs

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

D. Summary of Proposed Decisions on Standards for Carbon Monoxide

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

Monoxide

II. Rationale for Decisions on the Primary Standards

A. Introduction

1. Overview of Air Quality Information

2. Overview of Health Effects Information

a. Carboxyhemoglobin as Biomarker of Exposure and Toxicity

b. Nature of Effects and At-Risk Populations

c. Cardiovascular Effects

3. Overview of Human Exposure and Dose Assessment 

B. Adequacy of the Current Primary Standards

1. Rationale for Proposed Decision

2. Comments on Adequacy

3. Conclusions Concerning Adequacy of the Primary Standards

III. Consideration of a Secondary Standard

A. Introduction

B. Rationale for Proposed Decision

C. Comments on Consideration of Secondary Standard

D. Conclusions Concerning a Secondary Standard

IV. Amendments to Ambient Monitoring Requirements

A. Monitoring Methods

1. Proposed Changes to Parts 50 and 53

2. Public Comments

3. Decisions on Methods

B. Network Design

1. Proposed Changes

2. Public Comments 

a. Near-road Monitoring and Collocation with Near-road Nitrogen

Dioxide Monitors 

b. Population Thresholds for Requiring Near-road Carbon 

Monoxide Monitors

c. Implementation Schedule

d. Siting Criteria
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e. Area-Wide Monitoring

f. Regional Administrator Authority

3. Conclusions on the Network Design

V. Statutory and Executive Order Reviews

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

Order 13563: Improving Regulation and Regulatory Review 

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination with Indian 

Tribal Governments

G. Executive Order 13045: Protection of Children from Environmental 

Health and Safety Risks

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, 

Distribution or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions to Address Environmental 

Justice in Minority Populations and Low-Income Populations

K. Congressional Review Act

References

I. Background

A. Legislative Requirements

Two sections of the Clean Air Act (CAA) govern the establishment and revision 

of the NAAQS. Section 108 (42 U.S.C. section 7408) directs the Administrator to 

identify and list certain air pollutants and then to issue air quality criteria for those 

pollutants. The Administrator is to list those air pollutants that in her “judgment, cause or 

contribute to air pollution which may reasonably be anticipated to endanger public health 

or welfare;” “the presence of which in the ambient air results from numerous or diverse 

mobile or stationary sources;” and “for which . . . [the Administrator] plans to issue air 

quality criteria...” Air quality criteria are intended to “accurately reflect the latest 

scientific knowledge useful in indicating the kind and extent of all identifiable effects on 

public health or welfare which may be expected from the presence of [a] pollutant in the 
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ambient air . . .” 42 U.S.C. § 7408(b). Section 109 (42 U.S.C. 7409) directs the 

Administrator to propose and promulgate “primary” and “secondary” NAAQS for 

pollutants for which air quality criteria are issued. Section 109(b)(1) defines a primary

standard as one “the attainment and maintenance of which in the judgment of the 

Administrator, based on such criteria and allowing an adequate margin of safety, are 

requisite to protect the public health.”1 A secondary standard, as defined in section 

109(b)(2), must “specify a level of air quality the attainment and maintenance of which, 

in the judgment of the Administrator, based on such criteria, is requisite to protect the 

public welfare from any known or anticipated adverse effects associated with the 

presence of [the] pollutant in the ambient air.”2

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

intended to address uncertainties associated with inconclusive scientific and technical 

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

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

Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (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); American Farm Bureau Federation 

v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 2009); Association of Battery Recyclers v. EPA, 

604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are components of 

 

1 The legislative history of section 109 indicates that a primary standard is to be set at “the 

maximum permissible ambient air level . . . which will protect the health of any [sensitive] group of the 

population,” and that for this purpose “reference should be made to a representative sample of persons 

comprising the sensitive group rather than to a single person in such a group” S. Rep. No. 91-1196, 91st 

Cong., 2d Sess. 10 (1970).

2 Welfare effects as defined in section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, 

“effects on soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and 

climate, damage to and deterioration of property, and hazards to transportation, as well as effects on 

economic values and on personal comfort and well-being.”
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the risk associated with pollution at levels below those at which human health effects can 

be said to occur with reasonable scientific certainty. Thus, in selecting primary standards 

that provide an adequate margin of safety, the Administrator is seeking not only to 

prevent pollution levels that have been demonstrated to be harmful but also to prevent 

lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not 

precisely identified as to nature or degree. The CAA does not require the Administrator 

to establish a primary NAAQS at a zero-risk level or at background concentration levels, 

see Lead Industries v. EPA, 647 F.2d at 1156 n.51, but rather at a level that reduces risk 

sufficiently so as to protect public health with an adequate margin of safety.

In addressing the requirement for an adequate margin of safety, the EPA 

considers such factors as the nature and severity of the health effects involved, the size of 

sensitive population(s) at risk, and the kind and degree of the uncertainties that must be 

addressed. The selection of any particular approach to providing an adequate margin of 

safety is a policy choice left specifically to the Administrator’s judgment. See Lead 

Industries Association v. EPA, 647 F.2d at 1161-62; Whitman v. American Trucking 

Associations, 531 U.S. 457, 495 (2001).

In setting primary and secondary standards that are “requisite” to protect public 

health and welfare, respectively, as provided in section 109(b), EPA’s task is to establish 

standards that are neither more nor less stringent than necessary for these purposes. In so 

doing, EPA may not consider the costs of implementing the standards. See generally, 

Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). 

Likewise, “[a]ttainability and technological feasibility are not relevant considerations in 

the promulgation of national ambient air quality standards.” American Petroleum 
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Institute v. Costle, 665 F. 2d at 1185.

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year 

intervals thereafter, the Administrator shall complete a thorough review of the criteria 

published under section 108 and the national ambient air quality standards . . . and shall 

make such revisions in such criteria and standards and promulgate such new standards as 

may be appropriate . . . .” Section 109(d)(2) requires that an independent scientific 

review committee “shall complete a review of the criteria . . . and the national primary

and secondary ambient air quality standards . . . and shall recommend to the 

Administrator any new . . . standards and revisions of existing criteria and standards as 

may be appropriate . . . .” Since the early 1980's, this independent review function has 

been performed by the Clean Air Scientific Advisory Committee (CASAC).3

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 

 

3 Lists of CASAC members and of members of the CASAC CO Review Panel are available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?OpenDocument.
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controls for the fuels used by these sources and new source performance standards for 

stationary sources under section 111. 

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

EPA initially established NAAQS for CO on April 30, 1971. The primary

standards were established to protect against the occurrence of carboxyhemoglobin levels 

in human blood associated with health effects of concern. The standards were set at 9 

parts per million (ppm), as an 8-hour average, and 35 ppm, as a 1-hour average, neither to 

be exceeded more than once per year (36 FR 8186). In the 1971 decision, the 

Administrator judged that attainment of these standards would provide the requisite 

protection of public health with an adequate margin of safety and would also provide 

requisite protection against known and anticipated adverse effects on public welfare, and 

accordingly set the secondary (welfare-based) standards identical to the primary (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, 
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1984b). Following review by CASAC (Lippmann, 1984), EPA announced its decision 

not to revise the existing primary standards and to revoke the secondary standard for CO 

on September 13, 1985, due to a lack of evidence of effects on public welfare at ambient 

concentrations (50 FR 37484). 

On August 1, 1994, EPA concluded its second periodic review of the criteria and 

standards for CO by deciding that revisions to the CO NAAQS were not warranted at that 

time (59 FR 38906). This decision reflected EPA's review of relevant scientific 

information assembled since the last review, as contained in the 1991 AQCD (USEPA, 

1991) and the 1992 Staff Paper (USEPA, 1992). Thus, the primary standards were 

retained at 9 ppm with an 8-hour averaging time, and 35 ppm with a 1-hour averaging 

time, neither to be exceeded more than once per year (59 FR 38906).

EPA initiated the next periodic review in 1997 and released the final 2000 AQCD 

(U.S. EPA, 2000) in August 2000. After release of the AQCD, Congress requested that 

the National Research Council (NRC) review the impact of meteorology and topography

on ambient CO concentrations in high altitude and extreme cold regions of the U.S. The 

NRC convened the Committee on Carbon Monoxide Episodes in Meteorological and 

Topographical Problem Areas, which focused on Fairbanks, Alaska, as a case-study. 

A final report, “Managing Carbon Monoxide Pollution in Meteorological and 

Topographical Problem Areas,” was published in 2003 (NRC, 2003) and offered a wide 

range of recommendations regarding management of CO air pollution, cold start 

emissions standards, oxygenated fuels, and CO monitoring. Following completion of the 

NRC report, EPA did not conduct rulemaking to complete the review.

On September 13, 2007, EPA issued a call for information from the public (72 FR 
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52369) requesting the submission of recent scientific information on specified topics. On 

January 28–29, 2008, a workshop was held to discuss policy-relevant scientific and 

technical information to inform EPA’s planning for the CO NAAQS review (73 FR 

2490). Following the workshop, a draft Integrated Review Plan (IRP) (USEPA, 2008a) 

was made available in March 2008 for public comment and was discussed by the CASAC 

via a publicly accessible teleconference consultation on April 8, 2008 (73 FR 12998; 

Henderson, 2008). EPA made the final IRP available in August 2008 (USEPA, 2008b).

In preparing the Integrated Science Assessment for Carbon Monoxide (ISA or 

Integrated Science Assessment), EPA held an authors’ teleconference in November 2008 

with invited scientific experts to discuss preliminary draft materials prepared as part of 

the ongoing development of the CO ISA and its supplementary annexes. The first draft 

ISA (USEPA, 2009a) was made available for public review on March 12, 2009 (74 FR 

10734), and reviewed by CASAC at a meeting held on May 12-13, 2009 (74 FR 15265). 

A second draft ISA (USEPA, 2009b) was released for CASAC and public review on 

September 23, 2009 (74 FR 48536), and it was reviewed by CASAC at a meeting held on 

November 16-17, 2009 (74 FR 54042). The final ISA was released in January 2010 

(USEPA, 2010a).

In May 2009, OAQPS released a draft planning document, the draft Scope and 

Methods Plan (USEPA, 2009c), for consultation with CASAC and public review at the 

CASAC meeting held on May 12-13, 2009. Taking into consideration comments on the 

draft Scope and Methods Plan from CASAC (Brain, 2009) and the public, OAQPS staff 

developed and released for CASAC review and public comment a first draft Risk and 

Exposure Assessment (REA) (USEPA, 2009d), which was reviewed at the CASAC 
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meeting held on November 16-17, 2009. Subsequent to that meeting and taking into 

consideration comments from CASAC (Brain and Samet, 2010a) and public comments 

on the first draft REA, a second draft REA (USEPA, 2010d) was released for CASAC 

review and public comment in February 2010, and reviewed at a CASAC meeting held 

on March 22-23, 2010. Drawing from information in the final CO ISA and the second 

draft REA, EPA released a draft Policy Assessment (PA) (USEPA, 2010e) in early 

March 2010 for CASAC review and public comment at the same meeting. Taking into 

consideration comments on the second draft REA and the draft PA from CASAC (Brain 

and Samet, 2010b, 2010c) and the public, staff completed the quantitative assessments 

which are presented in the final REA (USEPA, 2010b). Staff additionally took into 

consideration those comments and the final REA analyses in completing the final Policy

Assessment (USEPA, 2010c) which was released in October 2010.

The proposed decision (henceforth “proposal”) on the review of the CO NAAQS 

was signed on January 28, 2011, and published in the Federal Register on February 11, 

2011. The EPA held a public hearing to provide direct opportunity for oral testimony by

the public on the proposal. The hearing was held on February 28, 2011, in Arlington, 

Virginia. At this public hearing, EPA heard testimony from five individuals representing 

themselves or specific interested organizations. Transcripts from this hearing and written 

testimony provided at the hearing are in the docket for this review. Additionally, written 

comments were received from various commenters during the public comment period on 

the proposal. Significant issues raised in the public comments are discussed in the 

preamble of this final action. A summary of all other significant comments, along with 

EPA’s responses (henceforth “Response to Comments”) can be found in the docket for 
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this review.

The schedule for completion of this review is governed by a court order resolving 

a lawsuit filed in March 2003 by a group of plaintiffs who alleged that EPA had failed to 

perform its mandatory duty, under section 109(d)(1), to complete a review of the CO 

NAAQS within the period provided by statute. The court order that governs this review, 

entered by the court on November 14, 2008, and amended on August 30, 2010, provides 

that EPA will sign for publication a notice of final rulemaking concerning its review of 

the CO NAAQS no later than August 12, 2011.

Some commenters have referred to and discussed individual scientific studies on 

the health effects of CO that were not included in the ISA (EPA, 2010a) (“'new' studies”). 

In considering and responding to comments for which such “new” studies were cited in 

support, EPA has provisionally considered the cited studies in the context of the findings 

of the ISA. 

As in prior NAAQS reviews, EPA is basing its decision in this review on studies 

and related information included in the ISA, REA and Policy Assessment, which have 

undergone CASAC and public review. The studies assessed in the ISA and Policy

Assessment, and the integration of the scientific evidence presented in them, have 

undergone extensive critical review by EPA, CASAC, and the public. The rigor of that 

review makes these studies, and their integrative assessment, the most reliable source of 

scientific information on which to base decisions on the NAAQS, decisions that all 

parties recognize as of great import. NAAQS decisions can have profound impacts on 

public health and welfare, and NAAQS decisions should be based on studies that have 

been rigorously assessed in an integrative manner not only by EPA but also by the 
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statutorily mandated independent advisory committee, as well as the public review that 

accompanies this process. EPA’s provisional consideration of these studies did not and 

could not provide that kind of in-depth critical review. 

This decision is consistent with EPA’s practice in prior NAAQS reviews and its 

interpretation of the requirements of the CAA. Since the 1970 amendments, the EPA has 

taken the view that NAAQS decisions are to be based on scientific studies and related 

information that have been assessed as a part of the pertinent air quality criteria, and has 

consistently followed this approach. This longstanding interpretation was strengthened 

by new legislative requirements enacted in 1977, which added section 109(d)(2) of the 

Act concerning CASAC review of air quality criteria. See 71 FR 61144, 61148 (October 

17, 2006) (final decision on review of PM NAAQS) for a detailed discussion of this issue 

and EPA’s past practice. 

As discussed in EPA’s 1993 decision not to revise the NAAQS for ozone, ‘‘new’’ 

studies may sometimes be of such significance that it is appropriate to delay a decision on 

revision of a NAAQS and to supplement the pertinent air quality criteria so the studies 

can be taken into account (58 FR at 13013–13014, March 9, 1993). In the present case, 

EPA’s provisional consideration of ‘‘new’’ studies concludes that, taken in context, the 

‘‘new’’ information and findings do not materially change any of the broad scientific 

conclusions regarding the health effects and exposure pathways of ambient CO made in 

the air quality criteria. For this reason, reopening the air quality criteria review would not 

be warranted even if there were time to do so under the court order governing the 

schedule for this rulemaking. 

Accordingly, EPA is basing the final decisions in this review on the studies and 
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related information included in the CO air quality criteria that have undergone CASAC 

and public review. EPA will consider the “new” studies for purposes of decision-making 

in the next periodic review of the CO NAAQS, which EPA expects to begin soon after 

the conclusion of this review and which will provide the opportunity to fully assess these 

studies through a more rigorous review process involving EPA, CASAC, and the public. 

Further discussion of these ‘‘new’’ studies can be found in the Response to Comments 

document.

D. Summary of Proposed Decisions on Standards for Carbon Monoxide 

For reasons discussed in the notice of proposed rulemaking, the Administrator 

proposed to retain the current primary CO standards. With regard to consideration of a 

secondary standard, the Administrator proposed to conclude that no secondary standards 

should be set at this time.

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

Monoxide

This action presents the Administrator’s final decisions in this review of the CO 

standards. Decisions regarding the primary CO standards are addressed below in section 

II. Consideration of a secondary CO standard is addressed below in section III. Ambient 

monitoring methods and network design related to implementation of the CO standards 

are addressed below in section IV. A discussion of statutory and executive order reviews 

is provided in section V.

Today’s final decisions are based on a thorough review in the Integrated Science 

Assessment of the latest scientific information on known and potential human health and 

welfare effects associated with exposure to CO in the environment. These final decisions 
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also take into account: (1) assessments in the Policy Assessment of the most policyrelevant information in the Integrated Science Assessment as well as quantitative 

exposure, dose and risk assessments based on that information presented in the Risk and 

Exposure Assessment; (2) CASAC Panel advice and recommendations, as reflected in its 

letters to the Administrator and its discussions of drafts of the Integrated Science 

Assessment, Risk and Exposure Assessment and Policy Assessment at public meetings; 

(3) public comments received during the development of these documents, either in 

connection with CASAC Panel meetings or separately; and (4) public comments received 

on the proposed rulemaking.

II. Rationale for Decisions on the Primary Standards

A. Introduction

This section presents the rationale for the Administrator’s decision that the current 

primary standards are requisite to protect public health with an adequate margin of safety, 

and that they should be retained. In developing this rationale, EPA has drawn upon an 

integrative synthesis in the Integrated Science Assessment of the entire body of evidence 

published through mid-2009 on human health effects associated with the presence of CO 

in the ambient air. The research studies evaluated in the ISA have undergone intensive 

scrutiny through multiple layers of peer review, with extended opportunities for review 

and comment by the CASAC Panel and the public. As with virtually any policy-relevant 

scientific research, there is uncertainty in the characterization of health effects 

attributable to exposure to ambient CO. While important uncertainties remain, the review 

of the health effects information has been extensive and deliberate. In the judgment of 

the Administrator, this intensive evaluation of the scientific evidence provides an 
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adequate basis for regulatory decision making at this time. This review also provides 

important input to EPA’s research plan for improving our future understanding of the 

relationships between exposures to ambient CO and health effects.

The health effects information and quantitative exposure/dose assessment were 

summarized in sections II.B and II.C of the proposal (76 FR at 8162–8172) and are only 

briefly outlined in sections II.A.2 and II.A.3 below. Responses to public comments 

specific to the material presented in sections II.A.1 through II.A.3 below are provided in 

the Response to Comments document.

Subsequent sections of this preamble provide a more complete discussion of the 

Administrator’s rationale, in light of key issues raised in public comments, for concluding 

that the current standards are requisite to protect public health with an adequate margin of 

safety and that it is appropriate to retain the current primary CO standards to continue to 

provide requisite public health protection (section II.B). 

1. Overview of Air Quality Information

This section briefly summarizes the information on CO sources, emissions, 

ambient air concentrations and aspects of associated exposure presented in section II.A of 

the proposal, as well as in section 1.3 of the Policy Assessment and chapter 2 of the Risk 

and Exposure Assessment. 

Carbon monoxide in ambient air is formed by both natural and anthropogenic 

processes. In areas of human activity such as urban areas, it is formed primarily by the 

incomplete combustion of carbon-containing fuels with the combustion conditions 

influencing the rate of formation. For example, as a result of the combustion conditions, 

CO emissions from large fossil-fueled power plants are typically very low because 
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optimized fuel consumption conditions make boiler combustion highly efficient. In 

contrast, internal combustion engines used in many mobile sources have widely varying 

operating conditions. As a result, higher and more varying CO formation results from the 

operation of mobile sources, which continue to be a significant source sector for CO in 

ambient air (ISA, sections 3.4 and 3.5; 2000 AQCD, section 7.2; REA, section 2.2 and 

3.1.3). 

Mobile sources are a substantial contributor to total CO emissions, particularly in 

urban areas (ISA, section 3.5.1.3; REA, section 3.1.3). Highest ambient concentrations in 

urban areas occur on or near roadways, particularly highly travelled roadways, and 

decline somewhat steeply with distance (ISA, section 3.5.1.3; REA, section 3.1.3; 

Baldauf et al., 2008a,b, Zhu et al., 2002). For example, as described in the ISA, a study

by Zhu et al (2002) documented CO concentrations at an interstate freeway to be ten 

times as high as an upwind monitoring site; concentrations declined rapidly in the 

downwind direction to levels only approximately one half roadway concentrations within 

100 to 300 meters (ISA, section 3.5.1.3, Figure 3-29 Zhu et al., 2002). Factors that can 

influence the steepness of the gradient include wind direction and other meteorological 

variables, and on-road vehicle density (ISA, section 3.5.1.3, Figures 3-29 and 3-30; Zhu 

et al., 2002; Baldauf et al., 2008a,b). These traffic-related ambient concentrations 

contribute to the higher short-term ambient CO exposures experienced near busy roads 

and particularly in vehicles, as described in more detail in the REA and PA.

2. Overview of Health Effects Information

This section summarizes information presented in section II.B of the proposal 

pertaining to health endpoints associated with the range of exposures considered to be 
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most relevant to current ambient CO exposure levels. In recognition of the use of an 

internal biomarker in evaluating health risk for CO, the following section summarizes 

key aspects of the use of carboxyhemoglobin as an internal biomarker (section II.A.2.a). 

This is followed first by a summary of the array of CO-induced health effects and 

recognition of at-risk subpopulations (section II.A.2.b) and then by a summary of the 

evidence regarding cardiovascular effects (section II.A.2.c).

a. Carboxyhemoglobin as Biomarker of Exposure and Toxicity

This section briefly summarizes the current state of knowledge, as described in 

the Integrated Science Assessment, of the role of carboxyhemoglobin in mediating

toxicity and as a biomarker of exposure. The section also summarizes the roles of 

endogenously produced CO and exposure to ambient and nonambient CO in influencing

internal CO concentrations and carboxyhemoglobin (COHb) levels.

At this time, as during past reviews, the best characterized mechanism of action of 

CO is tissue hypoxia caused by binding of CO to hemoglobin to form COHb in the blood 

(e.g., USEPA, 2000; USEPA, 1991; ISA). Increasing levels of COHb in the blood stream 

with subsequent decrease in oxygen availability for organs and tissues are of concern in 

people who have compromised compensatory mechanisms (e.g., lack of capacity to 

increase blood flow in response to hypoxia), such as those with pre-existing heart disease. 

For example, the integrative review of health effects of CO indicates that “the clearest 

evidence indicates that individuals with CAD [coronary artery disease] are most 

susceptible to an increase in CO-induced health effects” (ISA, section 5.7.8). 
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Carboxyhemoglobin is formed in the blood both from CO originating in the body

(endogenous CO) 4and from CO that has been inhaled into the body (exogenous CO).5 

The amount of COHb that occurs in the blood depends on factors specific to both the 

physiology of the individual (including disease state) and the exposure circumstances. 

These include factors associated with an individual’s rate of COHb elimination and 

production of endogenous CO, as well as those that influence the intake of exogenous CO 

into the blood, such as the differences in CO concentration (and partial pressure) in 

inhaled air, exhaled air, and blood; duration of a person’s exposure to changed CO 

concentrations in air; and exertion level or inhalation rate (ISA, chapter 4).

Apart from the impairment of oxygen delivery to tissues related to COHb 

formation, toxicological studies also indicate several other pathways by which CO acts in

the body, which involve a wide range of molecular targets and internal CO concentrations 

(2000 AQCD, sections 5.6-5.9; ISA, section 5.1.3). The role of these alternative lesswell-characterized mechanisms in CO-induced health effects at concentrations relevant to 

the current NAAQS, however, is not clear. New research based on this evidence is 

needed to further understand these pathways and their linkage to CO-induced effects in 

susceptible populations. Accordingly, COHb level in blood continues to be well 

recognized and most commonly used as an important internal dose metric, and is 

supported by the evidence as the most useful indicator of CO exposure that is related to 

CO health effects of major concern (ISA, p. 2-4, sections 4.1, 4.2, 5.1.1; 1991 AQCD, 

2000 AQCD, 2010 ISA).

 

4 Endogenous CO is produced from biochemical reactions associated with normal breakdown of 

heme proteins (ISA, section 4.5).

5 Exogenous CO includes CO emitted to ambient air, CO emitted to ambient air that has infiltrated 

indoors and CO that originates indoors from sources such as gas stoves, tobacco smoke and gas furnaces 

(ISA, section 3.6; REA, section 2.2).
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b. Nature of Effects and At-Risk Populations

The long-standing body of evidence that has established many aspects of the 

biological effects of CO continues to contribute to our understanding of the health effects 

of ambient CO (PA, section 2.2.1). Inhaled CO elicits various health effects through 

binding to, and associated alteration of the function of, a number of heme-containing 

molecules, mainly hemoglobin (see e.g., ISA, section 4.1). The best characterized health 

effect associated with CO levels of concern is decreased oxygen availability to critical 

tissues and organs, specifically the heart, induced by increased COHb levels in blood 

(ISA, section 5.1.2). Consistent with this, medical conditions that affect the biological

mechanisms which compensate for this effect (e.g., vasodilation and increased coronary

blood flow with increased oxygen delivery to the myocardium) can contribute to a 

reduced amount of oxygen available to key body tissues, potentially affecting organ 

system function and limiting exercise capacity (2000 AQCD, section 7.1).6 

This evidence newly available in this review provides additional detail and 

support to our prior understanding of CO effects and population susceptibility. In this 

review, the clearest evidence for ambient CO-related effects is available for 

cardiovascular effects. Using an established framework to characterize the evidence as to 

likelihood of causal relationships between exposure to ambient CO and specific health 

effects (ISA, chapter 1), the ISA states that "Given the consistent and coherent evidence 

from epidemiologic and human clinical studies, along with biological plausibility

provided by CO’s role in limiting oxygen availability, it is concluded that a causal 

relationship is likely to exist between relevant short-term CO exposures and 

 

6

For example, people with peripheral vascular diseases and heart disease patients often have 

markedly reduced circulatory capacity and reduced ability to compensate for increased circulatory demands 

during exercise and other stress (2000 AQCD, p. 7-7). 
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cardiovascular morbidity" (ISA, p. 2-6, section 2.5.1). Using the same established 

framework, the ISA describes the evidence as suggestive of causal relationships between 

relevant ambient CO exposure and several other health effects: relevant short- and longterm CO exposures and central nervous system (CNS) effects, birth outcomes and 

developmental effects following long-term exposure, respiratory morbidity following 

short-term exposure, and mortality following short-term exposure (ISA, section 2.5). 

However, there is only limited evidence for these relationships, and the current body of 

evidence continues to indicate cardiovascular effects, particularly effects related to the 

role of CO in limiting oxygen availability to tissues, as those of greatest concern at low 

exposures with relevance to ambient concentrations (ISA, chapter 2). The evidence for 

these effects is further described in section II.A.2.c below.

As described in the proposal, the terms susceptibility, vulnerability, sensitivity, 

and at-risk are commonly employed in identifying population groups or life stages at 

relatively higher risk for health risk from a specific pollutant. In the ISA for this review, 

the term susceptibility has been used broadly to recognize populations that have a greater 

likelihood of experiencing effects related to ambient CO exposure, with use of the term 

susceptible populations, as used in the ISA, defined as follows (ISA, section 5.7, p. 5-

115): 

Populations that have a greater likelihood of experiencing health effects 

related to exposure to an air pollutant (e.g., CO) due to a variety of 

factors including, but not limited to: genetic or developmental factors, 

race, gender, lifestage, lifestyle (e.g., smoking status and nutrition) or 

preexisting disease, as well as population-level factors that can increase 
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an individual's exposure to an air pollutant (e.g., CO) such as 

socioeconomic status [SES], which encompasses reduced access to health 

care, low educational attainment, residential location, and other factors.

Thus, susceptible populations are at greater risk of CO effects and are also referred to as 

at-risk in the summary below. 

As described in the proposal, the population with pre-existing cardiovascular 

disease continues to be the best-characterized population at risk of adverse CO-induced 

effects, with CAD recognized as “the most important susceptibility characteristic for 

increased risk due to CO exposure” (ISA, section 2.6.1). An important factor 

determining the increased susceptibility of this population is their inability to compensate 

for the reduction in tissue oxygen levels due to an already compromised cardiovascular 

system. Individuals with a healthy cardiovascular system (i.e., with healthy coronary

arteries) have operative physiologic compensatory mechanisms (e.g., increased blood 

flow and oxygen extraction) for CO-induced tissue hypoxia and are unlikely to be at 

increased risk of CO-induced effects (ISA, p. 2-10).7 In addition, the high oxygen 

consumption of the heart, together with the inability to compensate for tissue hypoxia, 

makes the cardiac muscle of a person suffering from CAD a critical target for CO. 

Thus, the current evidence continues to support the identification of people with 

cardiovascular disease as susceptible to CO-induced health effects (ISA, 2-12) and those 

having CAD as the population with the best-characterized susceptibility (ISA, sections 

 

7 The other well-studied individuals at the time of the last review were healthy male adults that 

experienced decreased exercise duration at similar COHb levels during short term maximal exercise. This

population was of lesser concern since it represented a smaller sensitive group, and potentially limited to 

individuals that would engage in vigorous exercise such as competing athletes (1991 AQCD, section 

10.3.2). 
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5.7.1.1 and 5.7.8).8 An important susceptibility consideration for this population is the 

inability to compensate for CO-induced hypoxia since individuals with CAD have an 

already compromised cardiovascular system. This population includes those with angina 

pectoris (cardiac chest pain), those who have experienced a heart attack, and those with 

silent ischemia or undiagnosed ischemic heart disease (AHA, 2003). People with other 

cardiovascular diseases, particularly heart diseases, are also at risk of CO-induced health 

effects. 

Cardiovascular disease comprises many types of medical disorders, including 

heart disease, cerebrovascular disease (e.g., stroke), hypertension (high blood pressure), 

and peripheral vascular diseases. Heart disease, in turn, comprises several types of 

disorders, including ischemic heart disease (coronary heart disease [CHD] or CAD, 

myocardial infarction, angina), congestive heart failure, and disturbances in cardiac 

rhythm (2000 AQCD, section 7.7.2.1).9 Other types of cardiovascular disease may also 

contribute to increased susceptibility to the adverse effects of low levels of CO (ISA, 

section 5.7.1.1). For example, evidence with regard to other types of cardiovascular 

disease such as congestive heart failure, arrhythmia, and non-specific cardiovascular 

disease, and more limited evidence for peripheral vascular and cerebrovascular disease, 

indicates that “the continuous nature of the progression of CAD and its close relationship 

 

8 As recognized in the ISA, “Although the weight of evidence varies depending on the factor being 

evaluated, the clearest evidence indicates that individuals with CAD are most susceptible to an increase in 

CO-induced health effects” (ISA, p. 2-12).

9 Coronary artery disease (CAD), often also called coronary heart disease or ischemic heart 

disease, is a category of cardiovascular disease associated with narrowed heart arteries. Individuals with 

this disease may have myocardial ischemia, which occurs when the heart muscle receives insufficient 

oxygen delivered by the blood. Exercise-induced angina pectoris (chest pain) occurs in many of them. 

Among all patients with diagnosed CAD, the predominant type of ischemia, as identified by 

electrocardiogram ST segment depression, is asymptomatic (i.e., silent). Patients who experience angina 

typically have additional ischemic episodes that are asymptomatic (2000 AQCD, section 7.7.2.1). In 

addition to such chronic conditions, CAD can lead to sudden episodes, such as myocardial infarction (ISA, 

p. 5-24).
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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).

As described in the proposal, several other populations are potentially at risk of 

CO-induced effects, including: those with other pre-existing diseases that may already 

have limited oxygen availability, increased COHb levels or increased endogenous CO 

production, such as people with obstructive lung diseases, diabetes and anemia; older 

adults; fetuses during critical phases of development and young infants or newborns; 

those who spend a substantial time on or near heavily traveled roadways; visitors to highaltitude locations; and people ingesting medications and other substances that enhance 

endogenous or metabolic CO formation (ISA, section 2.6.1). While the evidence 

suggests a potential susceptibility of these populations, information characterizing 

susceptibility for these groups is limited. For example, information is lacking on specific 

CO exposures or COHb levels that may be associated with health effects in these other 

groups and the nature of those effects, as well as a way to relate the specific evidence 

available for the CAD population to these other populations (PA, section 2.2.1). 

c. Cardiovascular Effects

Similar to the previous review, results from controlled human exposure studies of 

individuals with coronary artery disease (CAD) (Adams et al., 1988; Allred et al., 1989a, 

1989b, 1991; Anderson et al., 1973; Kleinman et al., 1989, 1998; Sheps et al., 198710) are 

the “most compelling evidence of CO-induced effects on the cardiovascular system” 

(ISA, section 5.2). Additionally, the use of an internal dose metric, COHb, adds to the 

 

10 Statistical analyses of the data from Sheps et al., (1987) by Bissette et al. (1986) indicate a 

significant decrease in time to onset of angina at 4.1% COHb if subjects that did not experience exerciseinduced angina during air exposure are also included in the analyses.
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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. 

Among the controlled human exposure studies, the ISA places principal emphasis 

on the study of CAD patients by Allred et al. (1989a, 1989b, 1991)11 (which was also 

considered in the previous review) for the following reasons: 1) dose-response 

relationships were observed; 2) effects were observed at the lowest COHb levels tested 

(mean of 2-2.4% COHb12 following experimental CO exposure), with no evidence of a 

threshold; 3) objective measures of myocardial ischemia (ST-segment depression) 13 were 

assessed, as well as the subjective measure of decreased time to induction of angina; 4) 

measurements were taken both by CO-oximetry (CO-Ox) and by gas chromatography 

(GC), which provides a more accurate measurement of COHb blood levels14; 5) a large 

number of study subjects were used; 6) a strict protocol for selection of study subjects 

 

11 Other controlled human exposure studies of CAD patients (listed in Table 2-2 of the PA, and 

discussed in more detail in the 1991 and 2000 AQCDs) similarly provide evidence of reduced time to 

exercise-induced angina associated with elevated COHb resulting from controlled short-duration exposure 

to increased concentrations of CO.

12 These levels and other COHb levels described for this study below are based on gas 

chromatography analysis unless otherwise specified. Matched measurements available for CO-oximetry 

(CO-Ox) and gas chromatography (GC) in this study indicate CO-Ox measurements of 2.65% (postexercise mean) and 3.21% (post-exposure mean) corresponding to the GC measurement levels of 2.00% 

(post-exercise mean) to 2.38% (post-exposure mean) for the lower exposure level assessed in this study

(Allred et al., 1991).

13

 The ST-segment is a portion of the electrocardiogram, depression of which is an indication of 

insufficient oxygen supply to the heart muscle tissue (myocardial ischemia). Myocardial ischemia can 

result in chest pain (angina pectoris) or such characteristic changes in ECGs or both. In individuals with 

coronary artery disease, it tends to occur at specific levels of exercise. The duration of exercise required to 

demonstrate chest pain and/or a 1-mm change in the ST segment of the ECG were key measurements in the 

multicenter study by Allred et al. (1989a, 1989b, 1991).

14 As stated in the ISA, the gas chromatographic technique for measuring COHb levels “is known 

to be more accurate than spectrophotometric measurements, particularly for samples containing COHb 

concentrations < 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).
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was employed to include only CAD patients with reproducible exercise-induced angina; 

and 7) the study was conducted at multiple laboratories around the U.S. This study

evaluated changes in time to exercise-induced onset of markers of myocardial ischemia 

resulting from two short (approximately 1-hour) CO exposures targeted to result in mean 

study subject COHb levels of 2% and 4%, respectively (ISA, section 5.2.4). In this 

study, subjects (n=63) on three separate occasions underwent an initial graded exercise 

treadmill test, followed by 50 to 70-minute exposures under resting conditions to room 

air CO concentrations or CO concentrations targeted for each subject to achieve blood 

COHb levels of 2% and 4%. The exposures were to average CO concentrations of 0.7 

ppm (room air concentration range 0-2 ppm), 117 ppm (range 42-202 ppm) and 253 ppm 

(range 143-357 ppm). After the 50- to 70-minute exposures, subjects underwent a second 

graded exercise treadmill test, and the percent change in time to onset of angina and time 

to ST endpoint between the first and second exercise tests was determined. For the two 

CO exposures, the average post-exposure COHb concentrations were reported as 2.4% 

and 4.7%, and the subsequent post-exercise average COHb concentrations were reported 

as 2.0% and 3.9%.15

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), exposures to CO resulting in 

 

15 While the COHb blood level for each subject during the exercise tests was intermediate between 

the post-exposure and subsequent post-exercise measurements (e.g., mean 2.4-2.0% and 4.7-3.9%), the 

study authors noted that the measurements at the end of the exercise test represented the COHb 

concentrations at the approximate time of onset of myocardial ischemia as indicated by angina and ST 

segment changes. The corresponding ranges of CO-Ox measurements for the two exposures were 2.7-3.2% 

and 4.7-5.6%. In this document, we refer to the GC-measured mean of 2.0% or 2.0-2.4% for the COHb 

levels resulting from the lower experimental CO exposure. 
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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).16 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 

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 

 

16 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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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, congestive heart failure (CHF), and cardiovascular diseases (CVD) as a whole 

(Bell et al., 2009; Koken et al., 2003; Linn et al., 2000; Mann et al., 2002; Metzger et al., 

2004; Symons et al., 2006; Tolbert et al., 2007; Wellenius et al., 2005). As noted by the 

ISA, “[s]tudies of hospital admissions and [emergency department] visits for IHD 

provide the strongest [epidemiological] evidence of ambient CO being associated with 

adverse CVD outcomes” (ISA, p. 5-40, section 5.2.3). With regard to studies for other 

measures of cardiovascular morbidity, the ISA notes that “[t]hough not as consistent as 

the IHD effects, the effects for all CVD hospital admissions (which include IHD 

admissions) and CHF hospital admissions also provide evidence for an association of 

cardiovascular outcomes and ambient CO concentrations” (ISA, section 5.2.3). While 

noting the difficulty in determining the extent to which CO is independently associated 

with CVD outcomes in this group of studies as compared to CO as a marker for the 

effects of another traffic-related pollutant or mix of pollutants, the ISA concludes that the 

epidemiological evidence, particularly when considering the copollutant analyses, 

provides support to the clinical evidence for a direct effect of short-term ambient CO 

exposure on CVD morbidity (ISA, pp. 5-40 to 5-41).
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3. Overview of Human Exposure and Dose Assessment 

Our consideration of the scientific evidence in the current review, as at the time of 

the last review, is informed by results from a quantitative analysis of estimated 

population exposure and resultant COHb levels. This analysis provides estimates of the 

percentages of simulated at-risk populations expected to experience daily maximum 

COHb levels at or above a range of benchmark levels under varying air quality scenarios 

(e.g., just meeting the current or alternative standards), as well as characterizations of the 

kind and degree of uncertainties inherent in such estimates. The benchmark COHb levels 

were identified based on consideration of the evidence discussed in section II.A.2 above. 

In this section, we provide a short overview of key aspects of the assessment conducted 

for this review. The assessment is summarized more fully in section II.C of the proposal, 

discussed in detail in the REA and summarized in the PA (section 2.2.2). The results of 

the analyses as they relate to considerations of the adequacy of the current standards are 

discussed in section II.B.3 below.

As noted in the proposal notice, people can be exposed to CO in ambient air when 

they are outdoors and also when they are in indoor locations into which ambient 

(outdoor) air has infiltrated (ISA, sections 3.6.1 and 3.6.5). Indoor locations may also 

contain CO from indoor sources, such as gas stoves and tobacco smoke. Where present, 

these indoor sources can be important contributors to total CO exposure and can 

contribute to much greater CO exposures and associated COHb levels than those 

associated with ambient sources (ISA, section 3.6.5.2). For example, indoor sourcerelated exposures, such as faulty furnaces or other combustion appliances, have been 

estimated in the past to lead to COHb levels on the order of twice as high as short-term 
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elevations in ambient CO that were more likely to be encountered by the general public 

(2000 AQCD, p. 7-4). Further, some exposure/dose assessments performed for previous 

reviews have included modeling simulations both without and with indoor (nonambient) 

sources (gas stoves and tobacco smoke) to provide context for the assessment of ambient 

CO exposure and dose (e.g., 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 and COHb levels experienced by the simulated population than do 

ambient sources (Johnson et al., 2000; REA, sections 1.2 and 6.3). While recognizing 

this potential for indoor sources, where present, to play a role in CO exposures and 

COHb levels, the exposure modeling in the current review (described below) did not 

include indoor CO sources in order to focus on the impact of ambient CO on population 

COHb levels. 

The assessment estimated ambient CO exposure and associated COHb levels in 

simulated at-risk populations in two urban study areas in Denver and Los Angeles, in 

which current ambient CO concentrations are below the current standards. Estimates 

were developed for exposures to ambient CO associated with current “as is” conditions 

(2006 air quality) and also for higher ambient CO concentrations associated with air 

quality conditions simulated to just meet the current 8-hour standard,17 as well as for air 

quality conditions simulated to just meet several potential alternative standards. 

Although we consider it unlikely that air concentrations in many urban areas across the 

U.S. that are currently well below the current standards would increase to just meet the 8-

hour standard, we recognize the potential for CO concentrations in some areas currently 

 

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

concentrations.
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below the standard to increase to just meet the standard. We additionally recognize that 

this simulation can provide useful information in evaluating the current standard, 

although we recognize the uncertainty associated with simulating this hypothetical profile 

of higher CO concentrations that just meet the current 8-hour standard.

The exposure and dose modeling for the assessment, presented in detail in the 

REA, relied on version 4.3 of EPA’s Air Pollutant Exposure model (APEX4.3), which 

estimates human exposure using a stochastic, event-based microenvironmental approach 

(REA, chapter 4). The review of the CO standards completed in 1994 relied on 

population exposure and dose estimates generated from the probabilistic NAAQS 

exposure model (pNEM), a model that, among other differences from the current 

modeling approach with APEX4.3, employed a cohort-based approach (Johnson et al., 

1992; US EPA, 1992).

18,19

 Each of the model developments since the use of pNEM in 

that review have been designed to allow APEX to better represent human behavior, 

human physiology, and microenvironmental concentrations and to more accurately

estimate variability in CO exposures and COHb levels (REA, chapter 4).20

 

18 When using the cohort approach, each cohort is assumed to contain persons with identical 

exposures during the specified exposure period. Thus, variability in exposure will be attributed to 

differences in how the cohorts are defined, not necessarily reflecting differences in how individuals might 

be exposed in a population. In the assessment for the review completed in 1994, a total of 420 cohorts 

were used to estimate population exposure based on selected demographic information (11 groups using 

age, gender, work status), residential location, work location, and presence of indoor gas stoves (Johnson, 

et al., 1992; USEPA, 1992). 

19 The use of pNEM in the prior review also 1) relied on a limited set of activity pattern data 

(approximately 3,600 person-days), 2) used four broadly defined categories to estimate breathing rates, and 

3) implemented a geodesic distance range methodology to approximate workplace commutes (Johnson et 

al., 1992; 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.

20 APEX4.3 includes new algorithms to 1) simulate longitudinal activity sequences and exposure 

profiles for individuals, 2) estimate activity-specific minute-by-minute oxygen consumption and breathing 

rates, 3) address spatial variability in home and work-tract ambient concentrations for commuters, and 4) 

estimate event-based microenvironmental concentrations (PA, section 2.2.2). 
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As used in the current assessment, APEX probabilistically generates a sample of 

hypothetical individuals from an actual population database and simulates each 

individual’s movements through time and space (e.g., indoors at home, inside vehicles) to 

estimate his or her exposure to ambient CO (REA, chapter 4). Based on exposure 

concentrations, minute-by-minute activity levels, and physiological characteristics of the 

simulated individuals (see REA, chapters 4 and 5), APEX estimates the level of COHb in 

the blood for each individual at the end of each hour based on a nonlinear solution to the 

Coburn-Forster-Kane equation (REA, section 4.4.7). 

As discussed in section II.A.2.b above, people with cardiovascular disease are the 

population of primary focus in this review, and, more specifically, coronary artery

disease, also known as coronary heart disease, is the “most important susceptibility

characteristic for increased risk due to CO exposure” (ISA, p. 2-11). Controlled human 

exposure studies have provided quantitative COHb dose-response information for this 

specific population with regard to effects on markers of myocardial ischemia. 

Accordingly, based on the current evidence with regard to quantitative information of 

COHb levels and association with specific health effects, the at-risk populations 

simulated in the quantitative assessment were (1) adults with CHD (also known as IHD 

or CAD), both diagnosed and undiagnosed, and (2) adults with any heart diseases, 

including undiagnosed ischemia.21 Evidence characterizing the nature of specific health 

effects of CO in other populations is limited and does not include specific COHb levels 

related to health effects in those groups. As a result, the quantitative assessment does not 

develop separate quantitative dose estimates for populations other than those with CHD 

 

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

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

APEX simulations performed for this review focused on exposures to ambient CO 

occurring in eight microenvironments,22 absent any contribution to microenvironment 

concentrations from indoor (nonambient) CO sources. Previous assessments, that have 

included modeling simulations both with and without certain indoor sources, indicated 

that the impact of such sources can be substantial with regard to the portion of the at-risk 

population experiencing higher exposures and COHb levels (Johnson et al., 2000). While 

we are limited with regard to information regarding CO emissions from indoor sources 

today and how they may differ from the time of the 2000 assessment, we note that 

ambient contributions have notably declined, and indoor source contributions from some 

sources may also have declined. Thus, as indicated in the Policy Assessment, we have no 

firm basis to conclude a different role for indoor sources today with regard to 

contribution to population CO exposure and COHb levels.

In considering the REA dose estimates in the Policy Assessment, staff considered 

estimates of the portion of the simulated at-risk populations estimated to experience daily 

maximum end-of-hour absolute COHb levels above identified benchmark levels (at least 

once and on multiple occasions), as well as estimates of the percentage of population 

person-days (the only metric available from the modeling for the 1994 review), and also 

population estimates of daily maximum ambient contribution to end-of-hour COHb 

 

22 The 8 microenvironments modeled in the REA comprised a range of indoor and outdoor 

locations including residences as well as motor vehicle-related locations such as inside vehicles, and public 

parking and fueling facilities, where the highest exposures were estimated (REA, sections 5.9 and 6.1).
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levels.23 In identifying COHb benchmark levels of interest, primary attention was given 

to the multi-laboratory study in which COHb was analyzed by the more accurate GC 

method (Allred et al., 1989a, 1989b, 1991) discussed in section II.A.2.c above. As 

summarized in the proposal, the Policy Assessment recognized distinctions between the 

REA “baseline” (arising from prior ambient exposure and endogenous CO production) 

and the pre-exposure COHb levels in the controlled human exposure study (arising from 

ambient and nonambient exposure history, as well as from endogenous CO production), 

and also noted the impact of “baseline” COHb levels on COHb levels occurring in 

response to short ambient CO exposure events such as those simulated in the REA.

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 

 

23 As summarized in the proposal and described more fully in the REA and PA, absolute COHb 

refers to the REA estimates of COHb levels resulting from endogenously produced CO and exposure to 

ambient CO (in the absence of any nonambient sources). The additional REA estimates of ambient CO 

exposure contribution to COHb levels were calculated by subtracting COHb estimates obtained in the 

absence of CO exposure - i.e., that due to endogenous CO production alone (see REA, Appendix B.6) -

from the corresponding end-of-hour absolute COHb estimates for each simulated individual. Thus, the 

REA reports estimates of the maximum end-of-hour ambient contributions across the simulated year, in 

addition to the maximum absolute end-of hour COHb levels. 
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assessments.24 However, while APEX 4.3 is greatly improved when compared with 

previously used exposure models, its application is still limited with regard to data to 

inform our understanding of spatial relationships in ambient CO concentrations and 

within microenvironments of particular interest. Further information regarding model 

improvements and exposure modeling uncertainties is summarized in section 2.2.2 of the 

Policy Assessment and described in detail in chapter 7 of the REA.

Taking into consideration improvements in the model algorithms and data since 

the last review, and having identified and characterized these uncertainties, the Policy

Assessment concludes that the estimates associated with the current analysis, at a 

minimum, better reflect the full distribution of exposures and dose as compared to results 

from the 1992 analysis. As noted in the Policy Assessment, however, potentially greater 

uncertainty remains in our characterization of the upper and lower percentiles of the 

distribution of population exposures and COHb dose levels relative to that of other 

portions of the respective distribution. When considering the overall quality of the 

current exposure modeling approach, the algorithms, and the input data used, alongside 

the identified limitations and uncertainties, the REA and Policy Assessment conclude that 

the quantitative assessment provides reasonable estimates of CO exposure and COHb 

dose for the simulated population the assessment is intended to represent (i.e., the 

population residing within the urban core of each study area). The Policy Assessment 

additionally notes the impact on the REA dose estimates for ambient CO contribution to 

COHb of the lack of nonambient sources in the model simulations. This aspect of the 

assessment design may contribute to higher estimates of the contribution of short-

 

24 APEX4.3 provides estimates for percent of population projected to experience a single or 

multiple occurrences of a daily maximum COHb level above the various benchmark levels, as well as 

percent of person-days.
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duration ambient CO exposures to total COHb than would result from simulations that 

include the range of commonly encountered CO sources beyond just those contributing to 

ambient air CO concentrations. Although the specific quantitative impact of this on 

estimates of population percentages discussed in this document is unknown, 

consideration of COHb estimates from the 2000 assessment indicates a potential for the 

inclusion of nonambient sources to appreciably affect absolute COHb (REA, section 6.3) 

and accordingly implies the potential, where present, for an impact on overall ambient 

contribution to a person’s COHb level. Key results of the exposure and dose analyses 

were presented in the Policy Assessment and summarized in the proposal (Tables 1 and 2 

of the proposal). 

B. Adequacy of the Current Primary Standards

In considering the evidence and quantitative exposure and dose estimates with 

regard to judgments on the adequacy afforded by the current standards, the final decision 

is largely a public health policy judgment. A final decision must draw upon scientific 

information and analyses about health effects and risks, as well as judgments about how 

to consider the range and magnitude of uncertainties that are inherent in the scientific 

evidence and analyses. Our approach to informing these judgments is based on the 

recognition that the available health effects evidence generally reflects a continuum, 

consisting of ambient levels at which scientists generally agree that health effects are 

likely to occur, through lower levels at which the likelihood and magnitude of the 

response become increasingly uncertain. This approach is consistent with the 

requirements of the NAAQS provisions of the Act and with how EPA and the courts have 

historically interpreted the Act. These provisions require the Administrator to establish 
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primary standards that, in the Administrator’s judgment, are requisite to protect public 

health with an adequate margin of safety. In so doing, the Administrator seeks to 

establish standards that are neither more nor less stringent than necessary for this 

purpose. The Act does not require that primary standards be set at a zero-risk level, but 

rather at a level that avoids unacceptable risks to public health, including the health of 

sensitive groups.25

In evaluating whether it is appropriate to revise the current CO standards, the 

Administrator’s considerations build on the general approach used in the last review and 

reflect the broader body of evidence and information now available. The approach used 

is based on an integration of information on health effects associated with exposure to 

ambient CO; expert judgment on the adversity of such effects on individuals; and policy

judgments as to when the standards are requisite to protect public health with an adequate 

margin of safety, which are informed by air quality and related analyses, quantitative 

exposure and risk assessments when possible, and qualitative assessment of impacts that 

could not be quantified. The Administrator has taken into account both evidence-based 

and quantitative exposure- and risk-based considerations in developing conclusions on 

the adequacy of the current primary CO standards.

The Administrator’s proposed conclusions on the adequacy of the current primary

standards are summarized below (section II.B.1), followed by consideration of comments 

 

25 The sensitive population groups identified in a NAAQS review may (or may not) be comprised 

of low income or minority groups. Where low income/minority groups are among the sensitive groups, the 

rulemaking decision will be based on providing protection for these and other sensitive population groups. 

To the extent that low income/minority groups are not among the sensitive groups, a decision based on 

providing protection of the sensitive groups would be expected to provide protection for the low

income/minority groups (as well as any other less sensitive population groups).
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received on the proposal (section II.B.2) and the Administrator’s final decision with 

regard to the adequacy of the current primary standards (II.B.3).

1. Rationale for Proposed Decision

At the time of the proposal, in considering the adequacy of the current standards, 

the Administrator carefully considered the available evidence and conclusions contained 

in the Integrated Science Assessment; the information, exposure/dose assessment, 

rationale and conclusions presented in the Policy Assessment; the advice and 

recommendations from CASAC; and public comments as of that date. In so doing, the 

Administrator noted the following: (1) the long-standing evidence base concerning 

effects associated with exposure to CO, including the key role played by hypoxia 

(reduced oxygen availability) induced by increased COHb blood levels, and the use of 

COHb as the bioindicator and dose metric for evaluating CO exposure and the potential 

for health effects; (2) the strong evidence of cardiovascular effects of short-term CO 

exposures including the evidence from controlled human exposure studies that 

demonstrate a reduction in time to onset of exercise-induced markers of myocardial 

ischemia in response to increased COHb, and the health significance of responses 

observed at the 2% COHb level induced by 1-hour CO exposure, as compared to higher 

COHb levels; and (3) the identification of people with cardiovascular disease as a key

population at risk from short-term ambient CO exposures. In the proposal, as at the time 

of the last review, the Administrator additionally considered and took particular note of 

the exposure and dose modeling results, recognizing key limitations and uncertainties, 

and in light of judgments noted above regarding the health significance of findings from 

the controlled human exposure studies, placing less weight on the health significance of 
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infrequent or rare occurrences of COHb levels at or just above 2% and more weight to the 

significance of repeated such occurrences, as well as occurrences of higher COHb levels. 

The Administrator also considered the newly available and much-expanded 

epidemiological evidence, including the complexity associated with quantitative 

interpretation of these studies with regard to CO, particularly the few studies available in 

areas where the current standards are met. Further, the Administrator considered the 

advice of CASAC, including their overall agreement with the Policy Assessment 

conclusion that the current evidence and quantitative exposure and dose estimates 

provide support for retaining the current standards, their view that, in light of the 

epidemiological studies, revisions to lower the standards should be considered and their 

preference for a lower standard, and also their advice regarding the complications

associated with interpreting the epidemiological studies for CO. Although CASAC 

expressed a preference for a lower standard, CASAC also indicated that the current 

evidence provides support for retaining the current suite of standards and CASAC’s 

recommendations appear to recognize that their preference for a lower standard was 

contingent on a judgment as to the weight to be placed on the epidemiological evidence. 

For the reasons explained in the proposal, after full consideration of CASAC’s advice and 

the epidemiological evidence, as well as its associated uncertainties and limitations, the 

Administrator proposed to judge those uncertainties and limitations to be too great for the 

epidemiological evidence to provide a basis for revising the current standards. 

Taking all these considerations together, the Administrator proposed to conclude 

that the current suite of standards provides a very high degree of protection for the COHb 

levels and associated health effects of concern, as indicated by the extremely low 
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estimates of occurrences, and provides slightly less but a still high degree of protection 

for the effects associated with lower COHb levels, the physiological significance of 

which is less clear. The Administrator additionally proposed to conclude that 

consideration of the epidemiological studies does not lead her to identify a need for any 

greater protection. Thus, the Administrator proposed to conclude that the current suite of 

standards provides an adequate margin of safety against adverse effects associated with 

short-term ambient CO exposures. For these and all of the reasons discussed above, and 

recognizing the CASAC conclusion that, overall, the current evidence and REA results 

provide support for retaining the current standards, the Administrator proposed to 

conclude that the current suite of primary CO standards is requisite to protect public 

health with an adequate margin of safety from effects of ambient CO.

2. Comments on Adequacy

In considering comments on the adequacy of the current standards, the 

Administrator first notes the advice and recommendations from CASAC. In the context 

of CASAC’s review of the documents prepared during the course of the review, CASAC

sent EPA five letters providing advice regarding assessment and interpretation of the 

available scientific evidence and the REA for the purposes of judging the adequacy of the 

current CO standards (Brain and Samet, 2009; Brain and Samet, 2010a; Brain and Samet, 

2010b; Brain and Samet, 2010c; Brain and Samet, 2010d). In conveying comments on 

the draft Policy Assessment, CASAC agreed with the conclusion that the current 

evidence provides support for retaining the current suite of standards, while they also 

expressed a preference for a lower standard and stated that the epidemiological evidence 

could indicate the occurrence of adverse health effects at levels of the standards (Brain 
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and Samet, 2010c). With regard to the interpretation of epidemiological studies on CO, 

CASAC’s collective advice included recommendations regarding the weight to be placed 

on the epidemiological evidence (Brain and Samet, 2010c), as well as cautionary

statements regarding interpretation of the epidemiological studies. Such statements 

included the observation that “[d]istinguishing the effects of CO per se from the 

consequences of CO as a marker of pollution or vehicular traffic is a challenge, which 

[the ISA] needs to confront as thoroughly as possible” (Brain and Samet, 2009, p. 2). In 

another letter CASAC further cautioned (Brain and Samet, 2010d, p. 2):

The problem of co-pollutants serving as potential confounders is 

particularly problematic for CO. Since exposure levels for CO are now 

low, consideration needs to be given to the possibility that in some 

situations CO may be a surrogate for exposure to a mix of pollutants 

generated by fossil fuel combustion. A better understanding of the possible 

role of co-pollutants is relevant to regulation and to the design, analysis, 

and interpretation of epidemiologic studies on the health effects of CO. 

CASAC additionally noted concerns regarding the spatial coverage of the existing CO 

monitoring network and the sensitivity of deployed monitors (Brain and Samet, 2009; 

Brain and Samet, 2010a; Brain and Samet, 2010b; Brain and Samet, 2010d). On a related 

note, they cautioned that “[u]nderstanding the extent of exposure measurement error is 

critical for evaluating epidemiological evidence” (Brain and Samet, 2009). 

General comments from the public based on relevant factors that either support or

oppose retention of the current primary CO standards are addressed in this section. Other 

specific public comments related to consideration of the adequacy of the current 

standards, as well as general comments based on implementation-related factors that are 

not a permissible basis for considering the need to revise the current standards, are 

addressed in the Response to Comments document.
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The public comments received on the proposal were divided with regard to 

support for the Agency’s proposed conclusion as to the adequacy of the current standards. 

All of the state and local environmental agencies or governments that provided comments 

on the standards concurred with EPA’s proposed conclusions as did the three industry

commenters. All of these commenters generally noted their agreement with the rationale 

provided in the proposal, with some additionally citing CASAC’s recognition of support 

in the evidence for the adequacy of the current standards. Some of these commenters 

noted agreement with the weight given to the epidemiological studies in the proposal and 

also noted the little change in exposure/risk estimates since the time of the last review. 

One commenter additionally stated their view that the REA overstates the exposure and 

risk associated with the current standards.

As described in section II.B.3 below, the EPA generally agrees with these 

commenters regarding the adequacy of the current CO standards and with CASAC that 

the evidence provides support for the conclusion that the current CO standards protect 

public health with an adequate margin of safety. EPA additionally has given 

consideration to CASAC’s advice regarding interpretation of epidemiological evidence 

for CO, recognizing the limitations associated with its use in drawing quantitative 

interpretations regarding levels of ambient CO related to health outcomes.

Two submissions recommending revision of the standards were received from 

national environmental or public health organizations. Additional submissions 

recommending revision were received from a private consultant; a group of scientists, 

physicians, and others; and a group of private citizens. In support of their position, these 

commenters variously cited CASAC comments regarding emphasis to give 
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epidemiological studies and CASAC’s stated preference for a lower standard. These 

submissions generally disagreed with EPA’s consideration of the epidemiological 

evidence in the proposal and recommended that EPA give greater emphasis to 

epidemiological studies of a range of endpoints, including developmental and respiratory

effects, based on the commenters’ view that the epidemiological studies provided 

evidence of harm associated with ambient CO levels below the current standards and 

inadequate protection for sensitive populations. Among these submissions, those that 

specified levels for revised standards recommended levels that were no higher than the 

lowest part of the ranges for the two standards that were identified for consideration in 

the Policy Assessment and the example options that CASAC suggested for inclusion in 

the Policy Assessment. Additionally, one commenter described the view that the CO 

standards should be revised to levels at or below the range of CO concentrations in 

exhaled breath of healthy non-smokers.

EPA generally disagrees with these commenters regarding conclusions that can be 

drawn from the evidence, including the epidemiological studies, pertaining to the 

adequacy of the current CO standards. In considering the adequacy of the current 

standards, it is important to consider both the extent to which the evidence supports a 

causal relationship between ambient CO exposures and adverse health effects, as well as

the extent to which there is evidence pertinent to such effects under air quality conditions 

in which the current standards are met. With regard to the latter point, and focusing on

the epidemiological evidence, it is the studies involving air quality conditions in which 

the current standards were met that are most informative in evaluating the adequacy of 

the standards (PA, p. 2-30). We note that very few of the epidemiological studies 
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observing an association of cardiovascular disease-related outcomes with short-term CO 

concentrations (or those observing associations for other health effects) were conducted 

in areas that met the current standards throughout the period of study, thus limiting their 

usefulness with regard to judging the adequacy of the current standards (PA, p. 2-36). 

Further, as CASAC has cautioned, “the problem of co-pollutants serving as 

potential confounders is particularly problematic for CO” (Brain and Samet, 2010d). 

While some CO epidemiological studies have applied the commonly used statistical 

method, two-pollutant regression models, to inform conclusions regarding CO as the 

pollutant eliciting the effects in these studies, and while, in some studies, the CO 

associations remain robust after adjustment for another traffic combustion-related 

pollutant, such as PM2.5 or NO2 (PA, pp. 2-36 to 2-37), the potential exists for there to be 

etiologically relevant pollutants that are correlated with CO yet absent from the analysis, 

particularly given the many pollutants associated with fossil fuel combustion. The 

CASAC specifically recognized this potential in stating that “consideration needs to be 

given to the possibility that in some situations CO may be a surrogate for exposure to a 

mix of pollutants generated by fossil fuel combustion” and “a better understanding of the 

possible role of co-pollutants is relevant to ... the interpretation of epidemiologic studies 

on the health effects of CO” (Brain and Samet, 2010d). 

In light of these issues related to potential confounding by co-pollutants in the 

case of CO, uncertainty related to exposure error for CO is of particular concern in 

quantitatively interpreting the epidemiological evidence (e.g., with regard to ambient 
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concentrations contributing to health outcomes).

26

 As noted above, CASAC cautioned 

the Agency on the importance of understanding the extent of exposure error in evaluating 

the epidemiological evidence for CO (Brain and Samet, 2009). There are two aspects to 

the epidemiological studies in the specific case of CO (as contrasted with other pollutants

such as PM and NO2) that may contribute exposure error in the studies (PA, pp.2-34 to 2-

38; 76 FR 8177-8178). The first relates to the uncertainty associated with quantitative 

interpretation of the epidemiological study results at low ambient concentrations in light 

of the sizeable portion of ambient CO measurements that are at or below monitor method 

detection limits (MDLs). As described in the proposal, uncertainty related to the 

prevalence of ambient CO monitor concentrations at or below MDLs is a greater concern 

for the more recently available epidemiological studies in which the study areas have 

much reduced ambient CO concentrations compared with those in the past (PA, pp. 2-37 

to 2-38). This complicates our interpretation of specific ambient CO concentrations 

associated with health effects (ISA, p. 3-91; Brain and Samet, 2010d), providing us with 

reduced confidence in quantitative interpretations of epidemiological studies for CO. 

Additionally, as described in the proposal, there is uncertainty and potential error 

associated with exposure estimates in the CO epidemiological studies that relate to the 

use of area-wide or central-site monitor CO concentrations in light of information about 

the steep gradient in CO concentrations with distance from source locations such as 

 

26 In contrasting the strength of the epidemiological evidence available for the 2000 AQCD with 

that in the current review, the ISA notes that uncertainties identified in 2000 remain, including the ability 

of community fixed-site monitors to represent spatially variable ambient CO concentrations and personal 

exposures; the small expected increase in COHb due to ambient CO concentrations; the lack of biological 

plausibility for health effects to occur at such COHb levels, even in diseased individuals; and the possibility 

that ambient CO is serving as a surrogate for a mixture of combustion-related pollutants. These 

uncertainties complicate the quantitative interpretation of the epidemiologic findings, “particularly 

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

ambient CO concentrations measured at AQS monitors” (ISA, pp. 2-16 to 2-17).
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highly-trafficked roadways (ISA, section 3.5.1.3). As a result of differences in factors 

related to pollutant formation, this gradient is steeper for CO than for other traffic 

combustion-related pollutants, such as PM2.5 and NO2, contributing to a greater potential 

for exposure misclassification in the case of CO by the reliance on central site monitors 

in the CO epidemiological studies. Thus, as noted in the proposal, we recognize that the 

expanded body of epidemiological evidence available in this review includes its own set 

of uncertainties which complicates its interpretation, particularly with regard to ambient 

concentrations that may be eliciting health outcomes.

In our integrated assessment across all types of evidence in the ISA for this 

review, we conclude that a causal relationship is likely to exist for short-term exposures 

to ambient concentrations of CO and cardiovascular morbidity. In reaching this 

conclusion, the ISA notes that the most compelling evidence comes from the controlled 

human exposure studies (ISA, p. 2-5), which also document a significant dose-response 

relationship over a range of COHb concentrations relevant to consideration of the 

NAAQS (ISA, p. 2-13). In considering the epidemiological evidence for relevant 

cardiovascular outcomes, which includes multiple studies reporting associations with 

ambient CO concentrations under conditions when the current standards were not met 

(PA, p. 2-30), the ISA notes that these studies are coherent with the findings from the 

controlled human exposure studies (ISA, p. 2-17). However, as summarized here, 

various aspects of the evidence complicate quantitative interpretation of it with regard to 

ambient concentrations that might be eliciting the reported health outcomes. 

An additional complication to our consideration of the CO epidemiological 

evidence is that, in contrast to the health effects evidence for all other criteria pollutants, 
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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 much of the broader health effects 

evidence for CO, and particularly that related to cardiovascular effects, demonstrates and 

focuses on an internal biomarker of CO exposure (COHb) which has been considered a 

critical key to CO toxicity.

27

 The strong evidence describing the role of COHb in CO 

toxicity is important to consider in interpreting the CO epidemiological studies and 

contributes to the biological plausibility of the ischemia-related health outcomes that 

have been associated with ambient CO concentrations. Yet, we do not have information 

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

of the COHb levels eliciting health effects in the controlled human exposure studies. 

Further, we lack additional information on the CO exposures of the epidemiological 

study subjects to both ambient and nonambient sources of CO that might be used to 

estimate their COHb levels and bridge the gap between the two study types. Additionally

the ISA recognizes that the changes in COHb that would likely be associated with 

exposure to the low ambient CO concentrations assessed in some of the epidemiological 

studies would be smaller than changes associated with “substantially reduced [oxygen]

delivery to tissues,” that might plausibly lead to the outcomes observed in those studies, 

 

27 In the case of the only other criteria pollutant for which the health evidence relies on an internal 

dose metric – lead - the epidemiological studies also use that metric. For lead (Pb), in contrast to CO, the 

epidemiological evidence is focused on associations of Pb-related health effects with measurements of Pb 

in blood, providing a direct linkage between the pollutant, via the internal biomarker of dose, and the health 

effects. Thus, for Pb, as compared to the case for CO, we have less uncertainty in our interpretations of the 

epidemiological studies with regard to the pollutant responsible for the health effects observed. For other 

criteria pollutants, including PM and NO2, air concentrations are used as the exposure/dose metric in both 

the epidemiological studies and the other types of health evidence. Thus, there is no comparable aspect in 

the PM or NO2 evidence base. 
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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).

With regard to health effects other than cardiovascular outcomes, in addition to 

noting the complications cited above with regard to quantitative interpretation of the 

epidemiological evidence, we note that the evidence for these other categories of health 

effects is considered limited and only suggestive of a causal relationship with relevant 

exposures to CO in ambient air, or inadequate to infer such a relationship, or it supports 

the conclusion that such a relationship is not likely (see section II.A.2.b above). As 

described in the proposal sections II.B.2 and II.D.2.a, with regard to categories of health 

effects or outcomes for which the evidence is considered suggestive, evidence is lacking 

that might lend biological plausibility to epidemiological study results, and also 

sufficiently rule out the role of chance, bias and confounding in the epidemiological 

associations observed, for outcomes such as developmental or respiratory (ISA, chapters 

1 and 2). 

Thus, EPA disagrees with the commenters’ conclusion that the epidemiological 

evidence establishes that a range of health effects, including developmental or respiratory

effects, are occurring as a result of exposures to CO in ambient air at or below the current 

standards. We additionally disagree with commenters’ statements that imply EPA has 
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inadequately considered the evidence with regard to protection of sensitive populations 

and to the protection provided by the CO standards. As noted in section II.A.2.b above, 

EPA’s assessment of the current evidence presented in the Integrated Science Assessment 

concludes that “the most important susceptibility characteristic for increased risk due to 

CO exposure is [CAD or CHD]” (ISA, p. 2-10). Accordingly, the proposal recognized 

people with cardiovascular disease as a key population at risk from short-term ambient 

CO exposures (proposal, section II.D.4). However, based on assessment of the evidence 

in the ISA, the proposal and other documents in this review also recognize the potential 

for susceptibility for several other populations and lifestages, including people with preexisting diseases that may already have limited oxygen availability to tissues, increased 

COHb levels or increased endogenous CO production, older adults, and fetuses during 

critical phases of development (as summarized in section II.A.2.b above). For these 

groups and lifestages, the evidence is incomplete with regard to specific CO exposures or 

COHb levels that may be associated with health effects in these groups and the nature of 

those effects, as well as a way to relate the specific evidence available for the CAD 

population to the limited evidence for these other populations. Further, the currently 

available evidence does not indicate a greater susceptibility for any of the other 

populations or lifestages recognized as potentially at risk from exposure to ambient CO. 

In reaching a decision on the adequacy of the current standards in protecting public health 

in section II.B.3 below, however, the Administrator has considered EPA’s conclusions 

with regard to the effects likely to be causally associated with exposure to ambient CO 

and population groups particularly at risk, as well as those regarding the evidence with 

regard to the potential for other effects and sensitive groups, and the associated
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uncertainty. In so doing, as indicated below, the Administrator judges the current 

standards to provide the requisite protection for public health, including the health of

sensitive populations, with an adequate margin of safety.

3. Conclusions Concerning Adequacy of the Primary Standards

Having carefully considered the public comments, as discussed above, the 

Administrator believes that the fundamental scientific conclusions on the effects of CO in 

ambient air reached in the Integrated Science Assessment and Policy Assessment,

summarized in sections II.B and II.D of the proposal remain valid. Additionally, the 

Administrator believes the judgments she reached in the proposal (section II.D.4) with 

regard to consideration of the evidence and quantitative exposure/dose assessments and 

advice from CASAC remain appropriate. Thus, as described below, the Administrator 

concludes that the current primary standards provide the requisite protection of public 

health with an adequate margin of safety and should be retained.28

In considering the adequacy of the current suite of primary CO standards, the 

Administrator has carefully considered the available evidence and conclusions contained 

in the Integrated Science Assessment; the information, exposure/dose assessment, 

rationale and conclusions presented in the Policy Assessment; the advice and 

recommendations from CASAC; and public comments. The Administrator places 

primary consideration on the evidence obtained from controlled human exposure studies 

that demonstrates a reduction in time to onset of exercise-induced markers of myocardial 

ischemia in response to increased COHb resulting from short-term CO exposures, and 

 

28 As explained below in section IV.A, EPA is repromulgating the Federal Reference Method 

(FRM) for CO, as set forth in Appendix C of 40 CFR part 50. Consistent with EPA’s decision to retain the 

standards, the recodification clarifies and updates the text of the FRM, but does not make substantive 

changes to it.
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recognizes the greater significance accorded both to larger reductions in time to 

myocardial ischemia and to more frequent occurrences of myocardial ischemia. As at the 

time of the review completed in 1994, the Administrator also takes note of the results for 

the modeling of exposures to ambient CO under conditions simulated to just meet the 

current, controlling, 8-hour standard in two study areas, as described in the REA and 

Policy Assessment, and the public health significance of those results. She also considers 

the newly available and much-expanded epidemiological evidence, including the 

complexity associated with quantitative interpretation of these studies, particularly the 

few studies available in areas where the current standards are met. In so doing, she notes 

that in considering the adequacy of the current standards, it is important to consider both 

the extent to which the evidence supports a causal relationship between ambient CO 

exposures and adverse health effects, as well as the extent to which there is evidence 

pertinent to such effects under air quality conditions in which the current standards are 

met. Further, the Administrator considers the advice of CASAC, including both their 

overall agreement with the Policy Assessment conclusion that the current evidence and 

quantitative exposure and dose estimates provide support for retaining the current 

standards, as well as their view that in light of the epidemiological studies, revisions to 

lower the standards should be considered and their preference for a lower standard.

As an initial matter, the Administrator places weight on the long-standing 

evidence base that has established key aspects of CO toxicity that are relevant to this 

review as they were to the review completed in 1994. These aspects include the key role 

played by hypoxia (reduced oxygen availability) induced by increased COHb blood 

levels, the identification of people with cardiovascular disease as a key population at risk 
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from short-term ambient CO exposures, and the use of COHb as the bioindicator and 

dose metric for evaluating CO exposure and the potential for health effects. The 

Administrator also recognizes the Integrated Science Assessment’s conclusion that a 

causal relationship is likely to exist between relevant short-term exposures to CO and 

cardiovascular morbidity. 

In placing weight on the controlled human exposure studies, the Administrator 

also recognizes the uncertain health significance associated with the smaller responses to 

the lowest COHb level assessed in the study given primary consideration in this review 

(Allred et al., 1989a, 1989b, 1991) and with single occurrences of such responses. In the 

study by Allred et al. (1989a, 1989b, 1991), a 4-5% reduction in time (approximately 30 

seconds) to the onset of exercise-induced markers of myocardial ischemia was associated 

with the 2% COHb level induced by 1-hour CO exposure. In considering the 

significance of the magnitude of the time decrement to onset of myocardial ischemia 

observed at the 2% COHb level induced by short-term CO exposure, as well as the 

potential for myocardial ischemia to lead to more adverse outcomes, the EPA generally 

places less weight on the health significance associated with infrequent or rare 

occurrences of COHb levels at or just above 2% as compared to that associated with 

repeated occurrences and occurrences of appreciably higher COHb levels in response to 

short-term CO exposures. For example, at the 4% COHb level, the study by Allred et al., 

(1989a, 1989b, 1991) observed a 7-12% reduction in time to the onset of exerciseinduced markers of myocardial ischemia. The Administrator places more weight on this 

greater reduction in time to onset of exercise-induced markers compared to the reduction 

in time to onset at 2% COHb. The Administrator also notes that at the time of the 1994 
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review, an intermediate level of approximately 3% COHb was identified as a level at 

which adverse effects had been demonstrated in persons with angina. Now, as at the time 

of the 1994 review, the Administrator primarily considers the 2% COHb level, resulting 

from 1-hour CO exposure, in the context of a margin of safety against effects of concern 

that have been associated with higher COHb levels, such as 3-4% COHb. 

The Administrator additionally takes note of the now much-expanded evidence 

base of epidemiological studies, including the multiple studies that observe positive 

associations between cardiovascular outcomes and short-term ambient CO concentrations 

across a range of CO concentrations, including conditions above as well as below the 

current NAAQS. She notes particularly the Integrated Science Assessment conclusion 

that the findings of CO-associated cardiovascular effects in these studies are logically

coherent with the larger, long-standing health effects evidence base for CO and the 

conclusions drawn from it regarding cardiovascular disease-related susceptibility. In 

further considering the epidemiological evidence base with regard to the extent to which 

it provides support for conclusions regarding adequacy of the current standards, the 

Administrator takes note of CASAC’s conclusions that “[i]f the epidemiological evidence 

is given additional weight, the conclusion could be drawn that health effects are occurring 

at levels below the current standard, which would support the tightening of the current 

standard” (Brain and Samet, 2010c). Additionally, the Administrator places weight on 

the final Policy Assessment consideration of aspects that complicate quantitative 

interpretation of the epidemiological studies with regard to ambient concentrations that 

might be eliciting the reported health outcomes. 
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For purposes of evaluating the adequacy of the current standards, the 

Administrator takes note of the multiple complicating features of the epidemiological 

evidence base, as described in more detail in the final Policy Assessment and in section 

II.D.2.a of the proposal. First, while a number of studies observed positive associations 

of cardiovascular disease-related outcomes with short-term CO concentrations, very few 

of these studies were conducted in areas that met the current standards throughout the 

period of study. Additionally, in CASAC’s advice regarding interpretation of the 

currently available evidence, they stated that “[t]he problem of co-pollutants serving as 

potential confounders is particularly problematic for CO” and that given the currently low 

ambient CO levels, there is a possibility that CO is acting as a surrogate for a mix of 

pollutants generated by fossil fuel combustion. The CASAC further stated that “[a] better 

understanding of the possible role of co-pollutants is relevant to regulation” (Brain and 

Samet, 2010d). As described in the Policy Assessment and summarized in section II.B.2 

above, there are also uncertainties related to representation of ambient CO exposures 

given the steep concentration gradient near roadways, as well as the prevalence of 

measurements below the MDL across the database. The CASAC additionally indicated 

the need to consider the potential for confounding effects of indoor sources of CO (Brain 

and Samet, 2010c). As discussed in section II.D.2.a of the proposal, the interpretation of 

epidemiological studies for CO is further complicated because, in contrast to the situation 

for all other criteria pollutants, the epidemiological studies for CO use an exposure/dose 

metric (air concentration) that differs from the metric commonly used in the other key

CO health studies (COHb). 
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The Administrator notes that although CASAC expressed a preference for a lower 

standard, CASAC also indicated that the current evidence provides support for retaining 

the current suite of standards. CASAC’s recommendations appear to recognize that their 

preference for a lower standard was contingent on a judgment as to the weight to be 

placed on the epidemiological evidence. Further, as noted above and summarized in 

section II.C.2, CASAC has provided a range of advice regarding interpretation of the CO 

epidemiological studies in light of the associated uncertainties. Accordingly, in 

consideration of the current evidence with regard to conclusions to be drawn as to the 

adequacy of the current standards, the Administrator gives consideration to the full 

breadth of CASAC’s advice.

In considering the evidence and quantitative exposure and dose estimates 

available in this review with regard to the adequacy of public health protection provided 

by the current primary standards, the Administrator recognizes that, as noted in section 

II.B. above, the final decision on such judgments is largely a public health policy 

judgment, which draws upon scientific information and analyses about health effects and 

risks, as well as judgments about how to consider the range and magnitude of 

uncertainties that are inherent in the information and analyses. These judgments are 

informed by the recognition that the available health effects evidence generally reflects a 

continuum, consisting of ambient levels at which scientists generally agree that health 

effects are likely to occur, through lower levels at which the likelihood and magnitude of 

the response become increasingly uncertain. Accordingly, the final decision requires 

judgment based on an interpretation of the evidence and other information that neither 

overstates nor understates the strength and limitations of the evidence and information 
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nor the appropriate inferences to be drawn. As described in section I.A above, the Act 

does not require that primary standards be set at a zero-risk level; the NAAQS must be 

sufficient but not more stringent than necessary to protect public health, including the 

health of sensitive groups, with an adequate margin of safety. 

In considering the judgments to be made regarding adequacy of the level of 

protection provided by the current standards, the Administrator takes particular note of 

the findings of the exposure and dose assessment in light of considerations discussed 

above regarding the weight given to different COHb levels and their frequency of 

occurrence. As described in the proposal, the exposure and dose assessment results 

indicate that only a very small percentage of the at-risk population is estimated to 

experience a single occurrence in a year of daily maximum COHb at or above 3.0% 

COHb under conditions just meeting the current 8-hour standard in the two study areas 

evaluated, and no multiple occurrences are estimated. The Administrator also notes the 

results indicating that only a small percentage of the at-risk populations are estimated to 

experience a single occurrence of 2% COHb in a year under conditions just meeting the 

standard, and still fewer are estimated to experience multiple such occurrences. 

Additionally, consistent with findings of the assessment performed for the review 

completed in 1994, less than 0.1% of person-days for the at-risk populations were 

estimated to include occurrences of COHb at or above 2% COHb. Taken together, the 

Administrator judges the current standard to provide a very high degree of protection for 

the COHb levels and associated health effects of concern, as indicated by the extremely

low estimates of occurrences, and to provide slightly less but a still high degree of 
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protection for the effects associated with lower COHb levels, the physiological 

significance of which is less clear. 

In further considering the adequacy of the margin of safety provided by the 

current standards, the Administrator has additionally considered conclusions drawn in the 

Integrated Science Assessment and Policy Assessment with regard to interpretation of the 

limited and less certain information concerning a relationship between exposure to 

relevant levels of ambient CO and health effects in other, potentially, susceptible groups, 

and with regard to the uncertainties concerning quantitative interpretation of the available 

epidemiological studies. In so doing, the Administrator additionally judges the current 

standards to provide adequate protection against the risk of other health effects for which 

the evidence is less certain. Further, the Administrator concludes that consideration of 

the epidemiological studies does not lead her to identify a need for any greater protection. 

For these and all of the reasons discussed above, and recognizing the CASAC conclusion 

that, overall, the current evidence and REA results provide support for retaining the 

current standards, the Administrator concludes that the current suite of primary CO 

standards is requisite to protect public health with an adequate margin of safety from 

effects of ambient CO.

III. Consideration of a Secondary Standard

As noted in section I.A. above, section 109(b) of the Clean Air Act requires the 

Administrator to establish secondary standards that, in the judgment of the Administrator, 

are requisite to protect the public welfare from any known or anticipated adverse effects 

associated with the presence of the pollutant in the ambient air. In so doing, the 

Administrator seeks to establish standards that are neither more nor less stringent than 
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necessary for this purpose. The Act does not require that secondary standards be set to 

eliminate all risk of adverse welfare effects, but rather at a level requisite to protect public 

welfare from those effects that are judged by the Administrator to be adverse. 

This section presents the rationale for the Administrator’s final decision not to set 

a secondary NAAQS for CO. In considering the current air quality criteria, evidence of

CO-related welfare effects at or near ambient levels that are unrelated to climate has not 

been identified. Accordingly, in considering whether a secondary standard is requisite to 

protect the public welfare, the Administrator has primarily considered conclusions based 

on the evidence of a role for CO in effects on climate. Evaluation of this evidence in the

Integrated Science Assessment and staff considerations in the Policy Assessment 

highlighted the limitations in this evidence and provided information indicating that this 

role for atmospheric CO is predominantly indirect, through its role in chemical reactions 

in the atmosphere which result in increased concentrations of pollutants with direct 

contributions to the greenhouse effect or that deplete stratospheric ozone. Given the 

evaluation of the evidence, as well as the views of CASAC, the Administrator concludes 

that no secondary standard should be set at this time because, as in the past reviews, 

having no standard is requisite to protect public welfare from any known or anticipated 

adverse effects from ambient CO exposures.

In this section, we first summarize the evidence currently available for welfare 

effects to inform decisions in this review in section III.A. Next, the rationale for the 

proposed conclusions is summarized in section III.B. Public comments and CASAC 

advice regarding consideration of a secondary standard in this review are summarized in 
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section III.C. Lastly, the Administrator’s final conclusions with regard to a secondary

standard for CO are presented in section III.D.

A. Introduction

In evaluating whether establishment of a secondary standard for CO is appropriate

at this time, we adopted an approach in this review that builds upon the general approach 

used in the last review and reflects the broader body of evidence and information now 

available. Consideration of the evidence available in this review focuses on the following 

overarching question: Does the currently available scientific information provide support 

for considering the establishment of a secondary standard for CO? 

In considering this overarching question, the Policy Assessment first noted that 

the extensive literature search performed for the current review did not identify any 

evidence of public welfare effects of CO unrelated to climate at or near ambient levels 

(ISA, section 1.3 and p. 1-3). However, ambient CO has been associated with welfare 

effects related to climate (ISA, section 3.3). Climate-related effects of CO were 

considered for the first time in the 2000 AQCD and are given somewhat greater focus in 

the current ISA relative to the 2000 AQCD in reflection of comments from CASAC and 

increased attention to the role of CO in climate forcing (Brain and Samet, 2009; ISA, 

section 3.3). Based on the current evidence, the ISA concludes that “a causal relationship 

exists between current atmospheric concentrations of CO and effects on climate” (ISA, 

section 2.2). Accordingly, the discussion in the Policy Assessment (summarized in the 

proposal) focuses on climate-related effects of CO in addressing the question posed 

above.
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The currently available information summarized in the ISA (ISA section, 3.3) 

does not alter the current well-established understanding of the role of urban and regional 

CO in continental and global-scale chemistry, as outlined in the 2000 AQCD (PA, section 

3.2). CO absorbs outgoing thermal infrared radiation very weakly; thus, the direct 

contribution of CO itself to climate forcing (or greenhouse warming) is very small (ISA, 

p. 3-11). Rather, the most significant effects on climate are indirect, resulting from CO’s 

role as the major atmospheric sink for hydroxyl radicals. Through this role of CO in 

global atmospheric chemistry, CO influences the abundance of chemically reactive, 

major greenhouse gases, such as methane and ozone, that contribute directly to the 

greenhouse effect and of other gases that exert their effect on climate through depletion 

of stratospheric ozone (ISA, section 3.3 and p. 3-11). There is significant uncertainty 

concerning this effect, and it appears to be highly variable, with the ISA recognizing that 

climate effects of changes to emissions of a short-lived pollutant such as CO are very

likely dependent on localized conditions (ISA section 3.3, pp. 3-12, 3-15, 3-16). As 

noted in the ISA, however, “the indirect [global warming potential] values evaluated and 

summarized by [the Intergovernmental Panel on Climate Change] are global and cannot 

reflect effects of localized emissions or emissions changes” (ISA at p. 3-16). 

Accordingly, the Policy Assessment stated that, as a result of the spatial and temporal 

variation in emissions and concentrations of CO and the localized chemical

interdependencies that cause the indirect climate effects of CO, it is highly problematic to 

evaluate the indirect effects of CO on climate (PA, p. 3-3).

Based upon the information and considerations summarized above, the Policy

Assessment concluded as an initial matter that, with respect to non-climate welfare 
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effects, including ecological effects and impacts to vegetation, there is no currently 

available scientific information that supports a CO secondary standard (PA, section 3.4). 

Secondly, with respect to climate-related effects, the Policy Assessment recognized the 

evidence of climate forcing effects associated with CO, most predominantly through its 

participation in chemical reactions in the atmosphere which contribute to increased 

concentrations of other more direct acting climate-forcing pollutants (ISA, sections 2.2 

and 3.3). The PA also noted, however, that the available information provides no basis 

for estimating how localized changes in the temporal and spatial patterns of ambient CO 

likely to occur across the U.S. with (or without) a secondary standard would affect local, 

regional, or nationwide changes in climate. Moreover, more than half of the indirect 

forcing effect of CO is attributable to O3 formation, and welfare-related effects of O3 are 

more appropriately considered in the context of the review of the O3 NAAQS, rather than 

in this CO NAAQS review (PA, section 3.4). For these reasons, the Policy Assessment 

concluded that there is insufficient information at this time to support the consideration of 

a secondary standard based on CO effects on climate processes (PA, section 3.4). 

B. Rationale for Proposed Decision

In considering a secondary standard for CO, the proposed conclusions presented 

in the proposal were based on the assessment and integrative synthesis of the scientific 

evidence presented in the ISA, building on the evidence described in the 2000 AQCD, as 

well as staff consideration of this evidence in the Policy Assessment and CASAC advice. 

As an initial matter, the proposal concluded that the currently available scientific 

information with respect to non-climate welfare effects, including ecological effects and 

impacts to vegetation, does not support a CO secondary standard. Secondly, with respect 
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to climate-related effects, the proposal took note of staff considerations in the Policy

Assessment and concurred with staff conclusions that information is insufficient at this 

time to provide support for a CO secondary standard. Thus, based on consideration of 

the evidence, staff considerations in the Policy Assessment, as well as the views of 

CASAC, the Administrator proposed to conclude that no secondary standards should be 

set at this time because, as in the past reviews, having no standard is requisite to protect 

public welfare from any known or anticipated adverse effects from ambient CO 

exposures.

C. Comments on Consideration of Secondary Standard

In considering the need for a secondary standard, the Administrator first notes the 

advice and recommendations from CASAC based on their review of two drafts of the 

Integrated Science Assessment and of the draft Policy Assessment. With regard to 

consideration of a secondary standard for CO, CASAC noted without objection or 

disagreement the staff’s conclusions that there is insufficient information to support 

consideration of a secondary standard at this time (Brain and Samet, 2010c). One public 

comment generally concerning EPA’s proposed decision on a secondary standard is 

addressed below. Other more specific public comments related to consideration of a 

secondary standard are addressed in the Response to Comments document. 

One comment (joint submission from Center for Biological Diversity and others) 

stated that due to the global influence of CO on climate, EPA must establish a secondary

NAAQS. The comment provided no information as to what form, level, or other 

elements of a secondary standard would be appropriate in light of the substantial 

uncertainties and regional variation in the indirect effects of CO. Rather, the comment 
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asserted that there is “a substantial body of knowledge, as reviewed in the ISA, regarding 

CO and climate” and that “uncertainty does not absolve the EPA of the obligation to 

protect public welfare” (Center for Biological Diversity comments at p. 9).

As noted by the commenter, the ISA reviewed the body of knowledge regarding 

CO and climate. As discussed above, the ISA concluded that CO has climate-related 

effects, that the direct effects of CO are weak, that there are significant uncertainties 

concerning the indirect climate effects of CO, and that these effects appear to be highly 

variable and dependent on localized conditions. Further, as noted in the Policy

Assessment, the spatial and temporal variation in emissions and concentrations of CO 

and the localized chemical interdependencies that cause the indirect climate effects of CO

make it highly problematic to evaluate the indirect effects of CO on climate. In light of 

the fact that the climate effects of CO are not only uncertain but highly variable and 

dependent on local conditions (e.g., concentrations of other pollutants), EPA believes that 

there is not adequate information available to conclude that a secondary standard in the 

United States is requisite to protect public welfare. The comment points to the estimated 

global effects of CO on climate, but nowhere does the comment provide evidence that 

EPA’s conclusion regarding adequacy of the available information is in error. 

EPA fully appreciates that the NAAQS are often established on the frontiers of 

scientific knowledge, and EPA continually assesses scientific uncertainties in judging 

what NAAQS are requisite to protect public health and welfare. EPA is not asserting that 

the fact that there are some uncertainties prevents EPA from setting a standard. Rather, 

EPA has judged that, in light of both the significant uncertainties and the evidence of the

direct effects being weak and the indirect effects being highly variable and dependent on 
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local conditions, particularly in light of CO’s short lifetime, it is not possible to anticipate 

how any secondary standard that would limit ambient CO concentrations in the United 

States would in turn affect climate and thus any associated welfare effects. As 

additionally discussed in section III.D below, EPA has reviewed the available 

information and judged the absence of a standard as being requisite to protect public 

welfare. 

D. Conclusions Concerning a Secondary Standard

The conclusions presented here are based on the assessment and integrative 

synthesis of the scientific evidence presented in the ISA, building on the evidence 

described in the 2000 AQCD, as well as staff consideration of this evidence in the Policy

Assessment and CASAC advice, and with consideration of the views of public 

commenters on the need for a secondary standard. 

In considering whether the currently available scientific information supports 

setting a secondary standard for CO, EPA takes note of the ISA and Policy Assessment 

consideration of the body of available evidence (briefly summarized above in section 

III.A). First, EPA concludes that the currently available scientific information with 

respect to non-climate welfare effects, including ecological effects and impacts to 

vegetation, does not support the need for a CO secondary standard. Secondly, with 

respect to climate-related effects, the EPA takes note of the ISA’s conclusions that there 

are significant uncertainties concerning the indirect climate effects of CO, and that these 

effects appear to be highly variable and dependent on localized conditions as well as staff 

considerations in the Policy Assessment and concurs with staff conclusions that 

information is insufficient at this time to support the need for a CO secondary standard. 
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More specifically, as more fully discussed in consideration of public comments in section 

III.C above, EPA has judged that, in light of both the significant uncertainties and the

evidence of the direct effects of CO on climate being weak and the indirect effects being 

highly variable and dependent on local conditions, particularly in light of CO’s short 

lifetime, it is not possible to anticipate how any secondary standard that would limit 

ambient CO concentrations in the United States would affect climate. Consequently, 

information that might indicate the need for additional protection from CO environmental 

effects and on which basis EPA might identify a secondary standard for the purposes of 

protecting against CO effects on climate processes is not available. 

Thus, in considering the evidence, staff considerations in the Policy Assessment 

summarized here, as well as the views of CASAC and the public, summarized above, the 

Administrator concludes that no secondary standards should be set at this time because, 

as in the past reviews, having no standard is requisite to protect public welfare from any 

known or anticipated adverse effects from ambient CO exposures.

IV. Amendments to Ambient Monitoring Requirements

The EPA is finalizing changes to ambient air CO monitoring methods and the 

ambient monitoring network design requirements to support the NAAQS for CO 

discussed above in Section II. Because ambient CO monitoring data are essential to the 

implementation of the NAAQS for CO, EPA is finalizing minimum monitoring 

requirements for the ambient CO monitoring network. State, local, and tribal monitoring 

agencies (“monitoring agencies”) collect ambient CO monitoring data in accordance with 

the monitoring requirements contained in 40 CFR parts 50, 53, and 58.

A. Monitoring Methods
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This section provides background and rationale for the amendments that EPA 

proposed to the Federal Reference Method (FRM) for CO and to the associated 

performance specifications for automated CO analyzers. It also discusses the public 

comments on those proposed amendments and the few minor changes made to them as 

they are being promulgated today.

The use of FRMs for the collection of air monitoring data provides uniform, 

reproducible measurements of pollutant concentrations in ambient air. Federal equivalent 

methods (FEMs) allow for the introduction of new or alternative technologies for the 

same purpose, provided these methods produce measurements directly comparable to the 

reference methods. EPA has established procedures for determining and designating 

FRMs and FEMs at 40 CFR part 53.

For ambient air monitoring data for CO to be used for determining compliance 

with the CO NAAQS, such data must be obtained using either an FRM or an FEM, as 

defined in 40 CFR parts 50 and 53. All CO monitoring methods in use currently by state 

and local monitoring agencies are EPA-designated FRM analyzers. No FEM analyzer, 

i.e. one using an alternative measurement principle, has yet been designated by EPA for 

CO. These continuous FRM analyzers have been used in monitoring networks for many 

years and provide CO monitoring data adequate for determining CO NAAQS 

compliance. The current list of all approved FRMs capable of providing ambient CO 

data for this purpose may be found on the EPA 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 
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nevertheless proposed editorial revisions to the CO FRM and both technical and editorial 

revisions to part 53, as discussed below.

1. Proposed Changes to Parts 50 and 53

Reference methods for criteria pollutants are described in several appendices to 

40 CFR part 50; the CO FRM is set forth in appendix C. A non-dispersive infrared 

photometry (NDIR) measurement principle is formally prescribed as the basis for the CO 

FRM. Appendix C describes the technical nature of the NDIR measurement principle 

stipulated for CO FRM analyzers as well as two acceptable calibration procedures for CO 

FRM analyzers. It further requires that an FRM analyzer must meet specific 

performance, performance testing, and other requirements set forth in 40 CFR part 53.

The CO FRM was first promulgated on April 30, 1971 (36 FR 8186), in 

conjunction with EPA’s establishment (originally as 42 CFR part 410) of the first 

NAAQS for six pollutants (including CO) as now set forth in 40 CFR part 50. The 

method was amended in 1982 and 1983 (47 FR 54922; 48 FR 17355) to incorporate 

minor updates, but no substantive changes in the fundamental NDIR measurement 

technique have been made since its original promulgation. 

In connection with the current review of the NAAQS for CO, EPA reviewed the 

existing CO FRM to determine if it was still adequate or if improved or more suitable 

measurement technology has become available to better meet current FRM needs as well 

as potential future FRM requirements. EPA determined that no new ambient CO 

measurement technique has become available that is superior to the NDIR technique 

specified for the current FRM, and that the existing FRM continues to be well suited for 

both FRM purposes and for use in routine CO monitoring. No substantive changes were 
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needed to the basic NDIR FRM measurement principle. Several high quality FRM 

analyzer models have been available for many years and continue to be offered and 

supported by multiple analyzer manufacturers.

However, EPA found that the existing CO FRM should be improved and updated 

to clarify the language of some provisions, to make the format match more closely the 

format of more recently promulgated automated FRMs, and to better reflect the design 

and improved performance of current, commercially available CO FRM analyzers. 

Accordingly, EPA proposed appropriate, albeit minor, changes to the FRM. Because 

these mostly editorial changes were quite numerous, the entire text of the CO FRM was 

revised and re-proposed.

In close association with the proposed editorial revision to the CO FRM described 

above, EPA also proposed to update the performance requirements for CO FRM 

analyzers that are contained in 40 CFR part 53. These requirements were established in 

the 1970’s, based primarily on the NDIR CO measurement technology available at that 

time. While the fundamental NDIR measurement principle, as implemented in 

commercial FRM analyzers, has changed little over several decades, FRM analyzer 

performance has improved markedly. Contemporary advances in digital electronics, 

sensor technology, and manufacturing capabilities have permitted today’s NDIR 

analyzers to exhibit substantially improved measurement performance, reliability, and 

operational convenience at modest cost. This improved instrument performance was not 

reflected in the previous performance requirements for CO FRM analyzers specified in 

40 CFR part 53, indicating a need for an update to reflect that improved performance. 
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The updated performance requirements that EPA proposed for CO analyzers 

make them more consistent with the typical performance capability available in 

contemporary FRM analyzers and will ensure that newly designated FRM analyzers will 

have this improved measurement performance. A review of analyzer manufacturers’ 

specifications has determined that all existing CO analyzer models currently in use in the 

monitoring network already meet the proposed new requirements (for the standard 

measurement range). Also in conjunction with this modernization of the analyzer 

performance requirements, EPA proposed new, more stringent performance requirements 

applicable, on an optional basis, to analyzers that feature one or more lower, more 

sensitive measurement ranges. Such lower ranges will support improved monitoring data 

quality in areas of low CO concentrations.

 These updated and new performance requirements are being promulgated as 

amendments to subpart B of 40 CFR part 53, which prescribes the explicit procedures to 

be used for testing specified performance aspects of candidate FRM and FEM analyzers, 

along with the minimum performance requirements that such analyzers must meet to 

qualify for FRM or FEM designation. In particular, the new performance requirements 

appear in table B-1 of subpart B of 40 CFR part 53. Although table B-1 covers candidate 

methods for sulfur dioxide (SO2), O3, CO, and nitrogen dioxide (NO2), the updates to 

table B-1 that EPA is promulgating today affect only candidate methods for CO.

The updated performance requirements apply to candidate CO analyzers that 

operate on the specified “standard” measurement range (0 to 50 ppm). This measurement 

range remains unchanged from the existing requirements as it appropriately addresses the 

monitoring data needed for assessing attainment. The measurement noise limit is 
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reduced from 0.5 to 0.2 ppm, and the lower detectable limit is reduced from 1 to 0.4 ppm. 

Limits for zero drift and span drift are lowered, respectively, from 1.0 to 0.5 ppm, and 

from 2.5% to 2.0%. The previously existing mid-span drift limit requirement, tested at 

20% of the upper range limit (URL), is withdrawn, as EPA has found that the mid-span 

drift requirement was unnecessary for CO instruments because the upper level span drift 

(tested at 80% of the URL) completely and more accurately measures analyzer span drift 

performance.

The lag time limit is reduced from 10 to 2 minutes, and the rise and fall time 

limits are lowered from 5 to 2 minutes. For precision, EPA is changing the form of the 

precision limit specifications from an absolute measure (ppm) to percent (of the URL) for 

CO analyzers and setting the precision limit at 1 percent tested at both 20% and 80% of 

the URL. One percent is equivalent to the previous limit value of 0.5 ppm for precision 

for the standard (0 to 50 ppm) measurement range. This change in units from ppm to 

percent makes the requirement responsive to higher and lower measurement ranges (i.e., 

more demanding for lower ranges).

The interference equivalent limit of 1 ppm for each interferent is not changed, but 

EPA is withdrawing the previously existing limit requirement for the total of all 

interferents. EPA has found that the total interferent limit is unnecessary because modern 

CO analyzers are subject to only a few interferences, and they tend to be well controlled. 

The new performance requirements apply only to newly designated CO FRM or FEM 

analyzers; however, essentially all existing FRM analyzers in use today, as noted 

previously, already meet these requirements, so existing FRM analyzers are not required 
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to be re-tested and re-designated under the new requirements. All currently designated 

FRM analyzers retain their original FRM designations.

EPA also recognized that some CO monitoring objectives (e.g., area-wide 

monitoring away from major roads and rural area surveillance) require analyzers with 

lower, more sensitive measurement ranges than the standard range used for typical 

ambient monitoring. To improve data quality for such lower-range measurements, EPA 

is adding a separate set of performance requirements that apply specifically to lower 

ranges (i.e., those having a URL of less than 50 ppm) for CO analyzers. These 

additional, lower-range requirements are listed in the revised table B-1. A candidate 

analyzer that meets the table B-1 requirements for the standard measurement range (0 to 

50 ppm) can optionally have one or more lower ranges included in its FRM or FEM 

designation by further testing to show that it also meets these supplemental, lower-range 

requirements.

 Although no substantive changes were determined to be needed to the test 

procedures and associated provisions of subpart B for CO, the detailed language in many 

of the subpart B sections was in need of significant updates, clarifications, refinement, 

and (in a few cases) correction of minor typographical errors. These changes to the 

subpart B text (apart from the changes proposed for table B-1 discussed above) are very

minor and almost entirely editorial in nature, but quite numerous. Therefore, EPA has 

revised and is re-promulgating the entire text of subpart B text.

As discussed previously, table B-1, which sets forth the pollutant-specific 

performance limits, is being amended only as applicable to CO analyzers. EPA amended 
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table B-1 as applicable to SO2 methods on June 22, 2010 and intends to amend table B-1 

for O3 and NO2 later, if appropriate, when the associated NAAQS are reviewed.

2. Public Comments

EPA notes first that CASAC stated that “more sensitive and precise monitors need 

to be deployed to measure levels that are less than or equal to 1 ppm.” (Brain and Samet 

2010b). Comments from the public on the proposed revisions to CO monitoring methods 

are addressed in this section or in the Response to Comments document. Comments on 

the proposed changes to the CO monitoring methodology were received from only one 

member of the public, the American Petroleum Institute. The commenter was generally 

supportive of EPA’s efforts to clarify and update the regulations for the CO FRM and the 

CO analyzer performance requirements. In regard to the CO FRM (40 CFR part 50, 

appendix C), the commenter questioned EPA’s proposed relaxation in a flow rate control 

requirement in the dilution-method calibration procedure, from 1% to 2%. However, 

EPA believes that the original 1% requirement is unnecessarily stringent, and that this 

change is appropriate and commensurate with the existing 2% flow rate measurement 

accuracy and with the overall calibration accuracy needed to obtain adequate data quality 

with the method.

To further improve clarity of the FRM calibration section, the commenter also 

suggested a minor change to Equation 1 and the addition of language indicating that the 

measurement display or read-out device connected to the analyzer to monitor its reading 

during calibration should be the actual, or at least closely representative of the actual, 

data recording system used during field operation of the analyzer. EPA has accepted 
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both of these suggestions, and appropriate changes have been incorporated into the 

changes being made to the CO FRM in this action.

Another comment questioned the proposed withdrawal of the previous total 

interference limit requirement. In response to this comment, EPA re-evaluated the 

efficacy of this limit for CO analyzers and again determined that the limit was not 

necessary, because the number of individual interferences to which FRM (and most 

potential FEM) CO analyzers are subject is small (only 2 for FRMs), as listed in table B3 of 40 CFR part 53. Also, response to these interferents is typically well controlled in 

modern CO analyzers. In addition, the new, individual interference limit for the lower 

measurement ranges is one half the limit for the standard range, which further mitigates 

any need for a separate, total interference limit.

The commenter questioned EPA’s proposed withdrawal of the previously existing 

limit requirement for span drift measured at 20% of the upper range limit (URL), 

contending that this limit was important because it is closer in concentration to the 

existing NAAQS than the span drift measured at 80% of the URL. However, the purpose 

of the span drift limit is not to directly assess measurement error at a particular, mid-scale 

concentration level. That purpose is served by the 1-point quality control check for CO 

monitors described in section 3.2.1 of appendix A of 40 CFR part 58. Rather, for the 

purpose of analyzer performance testing, the linear input/output functional characteristic 

of the analyzer is best described by its zero point and its slope, because these parameters 

are generally subject to change (drift) independently. Thus, zero drift (change in the zero 

point) and span drift (change in the slope) are tested separately. Zero drift is, of course, 

measured at zero concentration, and span drift is most accurately measured at a 
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concentration near the URL. The span drift test at 80% URL (when the zero drift is 

within the specified requirement) more accurately determines any change in the slope 

parameter then a test at 20% URL. The previously specified test at 20% URL thus serves 

little, if any, purpose in regard to determining change in the slope. Therefore, EPA has 

concluded that this requirement can be withdrawn.

Finally, the commenter was concerned that existing FRM analyzers approved 

under the previously existing performance requirements may provide data quality inferior 

to that of analyzers approved under the proposed new requirements and that older 

analyzers may be unacceptable for some applications that demanded higher performance 

or higher data quality. A “tiered” approach was suggested to handle this situation.

In proposing more stringent performance requirements for approval of new FRM 

and FEM analyzers, EPA noted that the performance of analyzers approved under the 

existing performance requirements was fully adequate for most routine compliance 

monitoring applications, and that the proposed new requirements were largely to bring 

the base FRM and FEM performance requirements up to date and more commensurate 

with the performance of modern commercially available CO analyzers. EPA further 

noted that all currently designated FRM analyzers already meet the proposed new 

requirements. This means that the quality of routine CO monitoring data currently being 

obtained is already of the higher level portended by the proposed new performance 

requirements.

In the proposal, however, EPA did recognize that some special CO monitoring 

applications do require a higher level of performance than that required for routine 

applications. Therefore, EPA is promulgating optional, more stringent performance 
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requirements for analyzers having a more sensitive, “lower range” available for such 

applications. This is, in fact, a “tiered” approach. Applicants would be able to elect to 

have such lower ranges approved as part of their FRM or FEM designation. These new, 

special performance requirements will alert monitoring agencies that they should 

consider low-range performance of an analyzer for those applications that may require 

better low-level performance, and they can select an analyzer that has such a lower range 

approved under its FRM (or FEM) designation. 

3. Decisions on Methods

As discussed above, a few relatively minor changes have been incorporated into 

the proposed revised CO FRM in appendix C of part 50, in response to public comments 

received by EPA. With these changes, the revised appendix C is being promulgated as 

otherwise proposed. Only one change has been made to the revision proposed for subpart 

B of part 53, to fix a typographical error that appeared in proposed table B-1 concerning 

reversed entries for the span drift limits for the 20% and 80% URL for the CO “lower 

range” column. Aside from this correction, the revised subpart B is being promulgated 

exactly as proposed. 

B. Network Design

This section on CO network design provides information on the proposed network 

design, the public comments received on the proposed network design, and the EPA’s 

conclusions, including rationale and details, on the final changes to the CO network 

design requirements. 

1. Proposed Changes
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The objective of an ambient monitoring network is to 1) provide air pollution data 

to the general public in a timely manner, 2) support compliance with ambient air quality

standards and emissions strategy development, and 3) provide support for air pollution 

research (40 CFR part 58, appendix D). The proposed CO network design was intended 

to directly support the NAAQS by requiring monitoring that provides data for use in the 

designation process and ongoing assessment of air quality. In particular, the proposed 

network design was intended to require a sufficient number of monitors to collect data for 

compliance purposes in the near-road environment, where, as noted in section II.A.1 

above, the highest ambient CO concentrations generally occur, particularly in urban areas 

(ISA, section 3.5.1.3; REA, section 3.1.3). 

The EPA proposed CO monitoring to be required within a subset of near-road 

NO2 monitoring stations, which are required in 40 CFR part 58, appendix D, section 4.3. 

Per the preamble to the final rule for the NO2 NAAQS promulgated on February 9th, 2010 

(75 FR 6474), near-road NO2 monitoring stations are intended to be placed in the nearroad environment at locations of expected maximum 1-hour NO2 concentrations and are 

triggered for metropolitan areas based on Core Based Statistical Area (CBSA) population 

thresholds and a traffic-related threshold based on annual average daily traffic (AADT)29. 

The EPA proposed that CO monitors be required to operate in any CBSA having a 

population of 1 million or more persons, collocated with required near-road NO2

monitoring stations. Based upon 2009 Census Bureau estimates and 2008 traffic 

statistics maintained by the US Department of Transportation (US DOT) Federal 

 

29 One near-road NO2 monitor is required in any CBSA having a population of 500,000 or more 

persons. Two near-road NO2 monitors are required in any CBSA having a population of 2.5 million or 

more persons, or in any CBSA that has one or more road segments with an AADT count of 250,000 or 

more (40 CFR Part 58, Appendix D, Section 4.3).
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Highways Administration (FHWA), the CO monitoring proposal was estimated to require 

approximately 77 CO monitors to be collocated with near-road NO2 monitors within 53 

CBSAs30. 

The EPA proposed that any required near-road CO monitors shall be reflected in 

State annual monitoring network plans due in July 2012. Further, the Agency proposed 

that required near-road CO monitors be operational by January 1, 2013. Due to the 

proposed collocation of required CO monitors with required near-road NO2 monitors, 

these implementation dates were proposed in order to match those of the forthcoming 

near-road NO2 monitoring network. 

In light of the proposal to require near-road CO monitors be collocated with 

required near-road NO2 monitors, the EPA proposed that siting criteria for microscale CO 

monitors be revised to match those of microscale near-road NO2 monitors (and also 

microscale PM2.5 monitors). In particular, the EPA proposed that microscale CO siting 

criteria for probe height and horizontal spacing be changed to match those of near-road 

NO2 monitors as prescribed in 40 CFR part 58 appendix E, sections 2, 4(d), 6.4(a), and 

table E-4. Specifically, EPA proposed the following: 1) to allow microscale CO monitor 

inlet probes to be between 2 and 7 meters above the ground; 2) that microscale near-road 

CO monitor inlet probes be placed so they have an unobstructed air flow, where no 

obstacles exist at or above the height of the monitor probe, between the monitor probe 

and the outside nearest edge of the traffic lanes of the target road segment; and 3) that 

required near-road CO monitor inlet probes shall be as near as practicable to the outside 

nearest edge of the traffic lanes of the target road segment, but shall not be located at a 

 

30 Since the proposal, EPA has estimated that using 2010 Census Bureau counts the proposed rule 

would have resulted in approximately 75 monitors in 52 CBSAs being required.
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distance greater than 50 meters in the horizontal from the outside nearest edge of the 

traffic lanes of the target road segment.

Finally, the EPA recognized that a single monitoring network design may not 

always be sufficient for fulfilling specific or otherwise unique data needs or monitoring 

objectives for every area across the nation. As such, the EPA proposed to provide the 

Regional Administrators with the discretion to require monitoring above the minimum 

requirements as necessary to address situations where minimum monitoring requirements 

are not sufficient to meet monitoring objectives.

2. Public Comments

EPA first notes that CASAC expressed concern over the current monitoring 

network, stating “[m]ore extensive coverage may be warranted for areas where 

concentrations may be more elevated, such as near roadway locations. The Panel found 

that in some instances current networks underestimated carbon monoxide levels near 

roadways.” (Brain and Samet 2010b). General comments from the public based on 

relevant factors that either support or oppose the proposed changes to the CO network 

design are addressed in this section. Specific public comments related to the network 

design, but with regard to material which was not specifically proposed by the EPA or 

posed for solicitation of comment, are addressed in the Response to Comments 

document.

a. Near-road Monitoring and Collocation with Near-road Nitrogen Dioxide 

Monitors

The EPA received multiple public comments on the overall merit of monitoring 

for CO in the near-road environment, the proposal that required CO monitors be 
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collocated with required near-road NO2 monitors, and the number of required CO 

monitors that might be appropriate. In general, public health and environmental groups 

(e.g., American Lung Association [ALA], American Thoracic Society [ATS], 

Environmental Defense Fund [EDF]), some states or state environmental agencies or

organizations (e.g. National Association of Clean Air Agencies [NACAA], Northeast 

States for Coordinated Air Use Management [NESCAUM], New York State Department 

of Environment Conservation [NYSDEC], and State of Wisconsin Department of Natural 

Resources [WIDNR]), and some private citizen commenters provided support for a 

requirement for CO monitors in the near-road environment. For example, ALA, ATS, 

and EDF state that they “...are pleased to see EPA take seriously the public health threats 

that are posed to millions of residents and other sensitive receptors who live near or work 

on or near highways as well as other high exposure areas.” They go on to note that 

“[near-road ambient monitoring] data have been sorely lacking from the national 

monitoring network and are long overdue.” Further, many of the commenters who were 

supportive of near-road monitoring were supportive of collocating CO monitors with 

near-road NO2 monitors as it establishes multipollutant monitoring within the ambient air 

monitoring network. For example, NACAA stated the following in their comments: 

“...NACAA supports EPA’s proposal to collocate CO near roadway monitors at a subset 

of NO2 near-roadway sites. This is consistent with the recommendations of EPA’s Clean 

Air Scientific Advisory Committee (CASAC), which urged the agency to develop the 

near roadway monitoring network with a multipollutant focus and included CO in its list 

of pollutants that should be measured.” 
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Some industry commenters (e.g., Association of Automobile Manufacturers 

[AAM] and American Electric Power Service Corporation [AEPSC]) and a number of 

other states or state groups (e.g., Indiana Department of Environmental Management 

[IDEM], North Carolina Department of Air Quality [NCDAQ], New Mexico Air Quality

Bureau [NMAQB], South Carolina Department of Health and Environmental Control 

[SCDHEC], Southeast Michigan Council of Governments [SEMCOG], and Texas 

Commission on Environmental Quality [TCEQ]) generally did not support the proposed 

near-road CO monitoring requirements. For example, IDEM stated that “CO measured 

by roadside monitors is not representative of ambient air quality everywhere in a city or 

county containing the roadway” and that “... roadside monitoring measurements 

represent source-specific data. Therefore, Indiana does not believe that roadside 

monitoring should apply to an ambient air quality standard.” SCDHEC stated it “... does 

not believe that the use of a near-road monitoring network in a state-wide ambient air 

monitoring network is the appropriate choice to protect our community’s public health” 

and that “this monitoring method biases the monitoring effort into areas of little or no 

population while monitoring for the community population exposure is neglected.” 

Similarly, industry commenter AAM stated that “the current proposal does not include a 

requirement that the near-roadway monitors be sited in locations where there is actual 

human exposure to the ambient air for time periods corresponding to the 1-hour or 8-hour 

CO NAAQS.” 

The EPA stated in the CO proposal (76 FR 8158) that the proposed near-road CO 

monitoring requirements were intended to ensure a network of adequate size and focus to 

provide data for comparison to the NAAQS, support health studies and model 
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verification, and to fulfill Agency multipollutant monitoring objectives. In response to 

the comment that near-road monitoring data would be “source-specific” and may not be 

appropriately applicable to an ambient air standard, the Agency notes that monitoring for 

CO in the near-road environment (as a mobile source oriented measurement) is a 

longstanding agency practice, as evidenced by the first monitoring rule promulgated in 

1979 (44 FR 27558, May 10, 1979). That 1979 monitoring rule included the requirement 

to monitor for “peak” CO concentrations in urban areas having populations of 500,000 

people or more in locations “...around major traffic arteries and near heavily traveled 

streets in downtown areas.” The Agency believes that the use of near-road CO monitors 

as proposed is not a departure from the Agency’s longstanding intent to measure peak 

concentrations of CO in the near-road environment. Rather, the proposal was consistent 

with the Agency’s approach to require monitors for CO, and other criteria pollutants, in 

locations that likely experience peak ambient concentrations. The Agency also notes that 

source-oriented monitoring is and has long been a common practice in ambient 

monitoring networks, although more often associated with stationary sources, where the 

ambient data collected are used for comparison to the NAAQS. Data on ambient air 

concentrations, including near-road data, which may be most appropriately classified as 

on-road mobile source oriented, are appropriate to compare to the NAAQS. 

With regard to the comments asserting that near-road monitoring would result in 

monitoring areas of “little or no population” and thus population exposure is not 

represented, the EPA notes that on-road mobile sources are ubiquitous in urban areas and 

are a dominant component of the national CO emissions inventory, at nearly 60% of the 

total inventory, based on the 2008 NEI. As such, microenvironments influenced by on-
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road mobile sources are important contributors to ambient CO exposures, particularly in 

urban areas (REA, section 2.7). Further, the ambient CO exposures of most concern are 

short-term. Accordingly, near-road monitoring is focused on characterizing peak or 

elevated ambient concentrations. The relevance of this focus for the purposes of both 

ensuring compliance with the NAAQS and gathering data to inform our consideration of 

ambient CO exposures is demonstrated by the ubiquity of on-road mobile sources 

throughout urban areas, the time spent by people on or near roadways and the large 

number of American citizens living in urban areas and near roadways. As was noted in 

the ISA, the 2007 American Housing Survey

(http://www.census.gov/hhes/www/housing/ahs/ahs07/ahs07.html) estimates that 17.9 

million housing units are within 300 feet (~91 meters) of a 4-lane highway, airport, or 

railroad. Using the same survey, and considering that the average number of residential 

occupants in a housing unit is approximately 2.25, an estimate can be made that at least 

40 million American citizens live near 4-lane highways, airports, or railroads. Among 

these three transportation facilities, roads are the most pervasive of the three, suggesting 

that a significant number of people may live near major roads. Furthermore, the 2008 

American Time Use Survey (http://www.bls.gov/tus/) reported that the average U.S. 

civilian spent over 70 minutes traveling per day. Based on these considerations, the 

Agency has concluded that monitoring in the near-road environment would characterize 

the ambient concentrations that contribute to ambient CO exposure for a significant 

portion of the population that would otherwise not be captured. 

The AAM also commented that the EPA “...proposal to locate more near 

roadway monitors appears to be an attempt to find problems where none are likely to 
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exist.” The Agency proposal for near-road monitors is in line with longstanding 

monitoring objectives to monitor for peak or elevated ambient pollutant concentrations 

where they may occur. The Agency agrees that CO is no longer as pervasive a problem 

as it was in the past; however, there is still a responsibility to appropriately characterize 

and assess ambient concentrations to ensure that they do not exceed the NAAQS. In 

comments on the first draft of the ISA, CASAC advised that “...relying only on EPA’s 

[current] fixed monitoring network, CO measurements may underestimate CO exposures 

for specific vulnerable populations such as individuals residing near heavily trafficked 

roads and who commute to work on a daily basis.” In comments on the second draft of 

the ISA, CASAC commented that “the panel expresses concern about the existing CO 

monitoring network, both for its [spatial] coverage and for its utility in estimating human 

exposure” and that “CO exposures may not be adequately characterized for populations 

that may be exposed to higher CO levels because of where they live and work,” such as 

the near-road environment. Finally, in comments on the second draft of the REA, 

CASAC stated that “the approach for siting monitors needs greater consideration. More 

extensive coverage may be warranted for areas where concentrations may be more 

elevated, such as near-roadway locations.” In light of these comments and upon a review 

of the existing CO network, the Administrator concluded that the current CO monitoring 

network (circa 2010) lacked a necessary focus. While some currently existing sites that 

were established in the 1970s and 1980s continue to monitor near-road locations in 

downtown areas or within urban street canyons, and a minimum number of area-wide 

monitors are currently required at National Core (NCore) multipollutant stations, few 

monitors exist that characterize the more heavily trafficked roads that are prevalent in the 
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modern roadway network, particularly in our larger urban areas. The Agency’s proposal 

was intended to require a modest but appropriate number of CO monitors to characterize 

the near-road environment where peak or elevated ambient CO concentrations are 

expected to occur near heavily trafficked roads, as compared with neighborhood or urban 

background concentrations. If CO levels turn out to be low in these near-road locations, 

so much the better for public health, and monitoring networks can be adjusted in the 

future, as they have over time in response to an increased understanding of where levels 

of concern to public health are likely to occur.

Although the EPA received a number of comments that were largely supportive 

for the proposed requirement of collocating CO monitors within the forthcoming nearroad NO2 monitoring stations, several commenters encouraged the Agency to provide 

flexibility to allow for the separation of the newly required CO monitors from the nearroad NO2 sites, if necessary, to better monitor peak near-road CO concentrations. In their 

comments supporting the collocation concept, NACAA also stated that their organization 

“...also encourages EPA to allow flexibility for state and local agencies to use alternative 

siting of near-roadway CO monitors on a case-by-case basis, where there is a scientific 

justification for siting the CO monitor in a different location from the NO2 monitor, to 

ensure the best possible measurement of near roadway CO concentrations.” Similarly, 

NCDAQ recognized that “...light duty vehicles tend to have more impact on CO 

concentrations than do heavy [duty] vehicles” and went on to surmise that “...not all 

near-road NO2 monitoring stations will be well situated to measure maximum CO 

concentrations.” 
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The Agency has expressed its intent to pursue the integration of monitoring 

networks and programs through the encouragement of multipollutant monitoring 

wherever possible, as evidenced by actions taken in the 2006 monitoring rule that created 

the NCore network, the expression of the multipollutant paradigm in the 2008 Ambient 

Air Monitoring Strategy for State, Local, and Tribal Air Agencies, and within this 

rulemaking process as part of the rationale in proposing the collocation of required nearroad CO monitors with near-road NO2 monitors. Multipollutant monitoring is viewed as 

a means to broaden the understanding of air quality conditions and pollutant interactions, 

furthering the capability to evaluate air quality models, develop emission control 

strategies, and support research, including health studies. However, the Agency also 

recognizes that the measurement objectives of individual pollutants may not always 

correspond in a way that would support multipollutant monitoring as the most appropriate 

option in a network design. On the issue raised by NACAA and NCDAQ on the 

potential difference in locations of peak CO and NO2 concentrations in the near-road 

environment, the EPA recognizes the primary influence to be the different emission 

characteristics between light duty (LD) and heavy duty (HD) vehicles and vehicle 

operating conditions, which were discussed in section III.B.2 of the CO proposal. The 

public comments suggesting the need for flexibility in siting near-road CO monitors 

derives from the fact that near-road NO2 sites will be sited at locations where peak NO2

are expected to occur. Since NO2 is more heavily influenced by HD vehicles and CO is 

more heavily influenced by LD vehicles on a per vehicle basis, respectively, there may be 

cases where the peak CO and NO2 concentrations could occur along different road 

segments within the same CBSA. As a general observation, the EPA believes that this 
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situation may have more likelihood of occurring in the relatively larger (by population) 

CBSAs where a higher number of heavily trafficked roads with a wider variety of fleet 

mix (e.g. HD to LD vehicle ratios) tend to exist versus relatively smaller CBSAs. In 

recognition of these considerations, the final regulation allows for flexibility in CO 

monitor placement in the near-road environment when justified, as discussed below in 

section IV.B.3.

b. Population Thresholds for Requiring Near-road Carbon Monoxide Monitors

The EPA proposed that required CO monitors be collocated with every required 

near-road NO2 monitor in a CBSA with a population of 1 million or more persons. Due 

to the requirement to locate one CO monitor at each required near-road NO2 site, the 

proposal would have required two monitors in CBSAs having 2.5 million or more 

persons or having one or more road segments with Annual Average Daily Traffic 

(AADT) counts of 250,000 or more. The proposal would have also required one monitor 

within those CBSAs having 1 million or more persons (but fewer than 2.5 million 

persons).31 Based upon 2009 Census Bureau estimates and US DOT maintained traffic

summary data, the proposal was estimated to require 77 monitors within 53 CBSAs. 

Using recent 2010 Census data, and US DOT maintained traffic summary data, the 

proposal would have required approximately 75 monitors within 52 CBSAs. 

The EPA received a number of comments supporting different population 

thresholds by which to require near-road CO monitors. Those states or state agencies or 

state agency groups who generally supported required CO monitoring in the near-road 

 

31 One near-road NO2 monitor is required in any CBSA having a population of 500,000 or more 

persons. Two near-road NO2 monitors are required in CBSAs with population of greater than 2.5 million, 

or in any CBSA with a population of 500,000 or more persons that has one or more roadway segments with 

annual average daily traffic (AADT) counts of 250,000 or more. (40 CFR Part 58, Appendix D, Section 

4.3).
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environment (e.g., NACAA, NESCAUM, NYSDEC, and WIDNR) suggested a 

population threshold of 2.5 million by which near-road CO monitors should be required. 

In addition, NCDAQ, who did not support near-road CO monitoring, suggested that if it 

is required, it be required only within CBSAs of 2.5 million or more. The use of a 

population threshold of 2.5 million persons, versus 1 million as proposed, would require 

approximately 42 near-road CO monitors within 21 CBSAs, based on 2010 Census data. 

Industry commenter American Petroleum Institute (API) stated that the proposed 

population threshold of 1 million persons “...appears appropriate, but EPA should not 

require that both [near-road NO2] sites in the largest CBSAs host CO monitors.” API’s 

suggestion would require approximately 52 near-road CO monitors within 52 CBSAs. 

Finally, the public health and environmental groups ALA, ATS, and EDF suggested the 

EPA promulgate minimum monitoring requirements “...to encompass cities in smaller 

metro areas, including cities with populations of 500,000 or more, similar to the 

requirements for NO2 roadside monitoring.” ALA, ATS, and EDF’s suggestion would 

result in the requirement of approximately 126 monitors within 103 CBSAs. 

As was noted in the proposal, the Agency believes that with the continuing 

decline of ambient CO levels, as summarized in the EPA’s most recent trends report Our 

Nation’s Air: Status and Trends Through 2008 (http://www.epa.gov/airtrends/2010/), the 

Agency believes that there is less likelihood for high CO concentrations in relatively 

smaller CBSAs (by population). Accordingly, the Agency proposed the requirement for 

what it believes would be a sufficient number of CO monitors, which would be 

collocated with required near-road NO2 monitors, in CBSAs having populations of 1 

million or more persons. The Administrator considered alternative population thresholds, 
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including the 2.5 million and 500,000 person thresholds, but concluded that those 

thresholds would require too few or too many monitors, respectively, in light of existing 

information on CO emissions data, ambient data, and the lack of data for locations near 

highly trafficked roads. The rationale for this proposed 1 million person threshold was to 

require a modest but sufficiently sized network that would effectively assess near-road 

CO concentrations for comparison to the NAAQS and could also provide data from 

within a multipollutant framework to support research (which includes health studies), 

facilitate model verification, and assess and evaluate emissions control strategies. 

However, after considering public comments, the EPA has concluded that one monitor in 

each CBSA of 1 million or more persons will provide for monitoring of a wide range of 

diverse situations with regard to traffic volume, traffic patterns, roadway designs, 

terrain/topography, meteorology, climate, as well as surrounding land use and population 

characteristics. Accordingly, in the final rule EPA has modified the proposed 

requirements for CO monitors so that only one CO monitor is required in CBSAs of 1 

million or more persons, as discussed in Section IV.B.3 below. 

c. Implementation Schedule

The EPA received a number of comments on the timeline for implementation of 

any required CO monitoring promulgated as part of this rulemaking. ALA, ATS, and 

EDF stated that they “...support EPA’s requirement that CO monitors be installed in 

near-highway locations by July 1, 2013.” In light of the support these commenters 

expressed for rapid deployment of near road CO monitors, these commenters may have 

intended to support the proposed implementation date of January 1, 2013 instead of July

1, 2013 as quoted. The Agency received a number of comments from state agencies, 
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state agency organizations, and industry encouraging the Agency to extend the time by

which any required monitoring must be implemented. For example, API suggested that 

the proposed date by which required near-road CO monitors be established be extended 

to July 1, 2013, while NACAA and WIDNR suggested January 1, 2014. Several 

commenters suggested that required near-road monitors should be phased in over a period 

of time. For example, NACAA, stated “[i]t may be necessary to develop a program for 

phasing in new monitoring sites and reevaluate network implementation.” NACAA also 

pointed to comments from CASAC that it would be advisable to phase in near-road 

monitoring for NO2, because “[t]he first round of sites could be used to gather 

information on appropriate siting in the near roadway environment, near roadway

gradient, and spatial relationships.” 

The EPA recognizes that states are already implementing newly required 

monitoring related to lead and NO2, and that the current financial and logistical burdens 

may make the implementation of new monitoring requirements difficult. A number of 

state and industry commenters noted the need for funding to accommodate a new 

monitoring requirement, and some also noted the financial and logistical hardships that 

many states are currently experiencing (e.g., IDEM, NACAA, NCDAQ, SCDHEC, and 

WIDNR). The EPA recognizes the significance of the financial and logistical burden that 

new monitoring requirements pose and the impact of multiple new monitoring 

requirements stemming from other recent rulemakings. As such, the Agency has taken 

these comments into consideration in the final rule with regard to when required CO 

monitors are to be operational, as discussed in Section IV.B.3 below.
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d. Siting Criteria

The EPA received comments regarding the proposed revisions to microscale CO 

siting criteria. Those who commented (AAM, API, and NCDAQ) all supported having 

two sets of siting criteria that would apply to near-road CO monitors such as those that 

might be collocated with near-road NO2 monitors and to those CO monitors operating in 

downtown areas and urban street canyon locations, respectively. AAM stated that 

“...there should be two separate criteria for siting microscale CO monitors. The earlier 

height and distance guidelines are still appropriate for downtown areas and arterial 

highways with sidewalks, but a separate set of guidelines should be established for 

limited access, heavily-travelled expressways.” API commented that “...the proposed 

CO [near-road] criteria are acceptable. EPA should create two-tiered siting criteria for 

microscale CO monitoring...” and that “there will be an ongoing need for CO monitoring 

in downtown, urban and/or street canyon[s] for health-related concerns as well as SIPrelated issues.” Finally, NCDAQ stated that “...the US EPA should maintain separate 

siting criteria for the two types of micro-scale CO monitoring sites...” noting that the 

current siting criteria intended for downtown areas and urban street canyon sites “...are 

still valid for that purpose and CO monitoring stations being placed for this purpose 

should still be required to meet these siting criteria.”

The EPA agrees with the commenters that the existing siting criteria are still 

appropriate for any existing or future downtown area or urban street canyon CO 

monitoring site, and that new siting criteria are appropriate for CO monitors being 

collocated with near road NO2 monitors. As such, the Agency is finalizing siting criteria 

for microscale CO sites that include criteria for both downtown area/urban street canyon 
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microscale sites and other near-road microscale CO sites, as presented below in Section 

IV.B.3.

e. Area-wide Monitoring

The EPA received a number of comments from transportation groups, public 

health and environmental groups, and an industry commenter (e.g., AAM, 

ALA/ATS/EDF, American Association of State Highway and Transportation Officials 

[AASHTO], New York State Department of Transportation [NYSDOT], Texas 

Department of Transportation [TXDOT], and Virginia Department of Transportation 

[VDOT]) regarding the fate of many of the CO monitors in the current network that 

characterize concentrations representative of neighborhood or larger spatial scales32, 

known as area-wide monitors. For example, AASHTO commented that “EPA appears to 

be proposing that CO monitoring sites to characterize area-wide CO concentration levels 

at the neighborhood and larger spatial scales is no longer required. AASHTO is 

concerned that this proposal will de-emphasize the need for neighborhood scale CO 

monitors.” AASHTO and some state DOTs expressed that the data for neighborhood 

scale monitors are used for other purposes, such as National Environmental Policy Act 

 

32 Spatial scales are defined in 40 CFR Part 58 Appendix D, Section 1.2, where the scales of 

representativeness of most interest for the monitoring site types include:

1. Microscale – Defines the concentration in air volumes associated with area dimensions ranging from 

several meters up to about 100 meters.

2. Middle scale – Defines the concentration typical of areas up to several city blocks in size, with 

dimensions ranging from about 100 meters to 0.5 kilometers.

3. Neighborhood scale – Defines concentrations within some extended area of the city that has relatively 

uniform land use with dimensions in the 0.5 to 4.0 kilometers range. 

4. Urban scale – Defines concentrations within an area of city-like dimensions, on the order of 4 to 50 

kilometers. Within a city, the geographic placement of sources may result in there being no single site 

that can be said to represent air quality on an urban scale. The neighborhood and urban scales have the 

potential to overlap in applications that concern secondarily formed or homogeneously distributed air 

pollutants.

5. Regional scale – Defines usually a rural area of reasonably homogeneous geography without large 

sources, and extends from tens to hundreds of kilometers.
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(NEPA) and transportation conformity, and that they are concerned about the potential 

loss of these types of data in the future. In another example, ALA/ATS/EDF stated that 

they call upon EPA to “establish a comprehensive roadside air pollution network, while 

retaining the current area-wide CO network.” 

The EPA notes that prior to this final rulemaking, the only required CO 

monitoring within 40 CFR part 58, appendix D was for the operation of a CO monitor 

within all NCore multipollutant monitoring stations. There are approximately 80 NCore 

stations nationwide, and by design, they are area-wide monitoring sites. In the proposal, 

the Agency estimated that 345 CO monitors were operational at some point during 2009. 

A more recent examination of AQS data (utilizing EPA’s Air Explorer web tools located 

at www.epa.gov/airexplorer) indicate that approximately 328 CO monitors were 

operational as of May 20, 2011. These 328 active CO monitors include the 80 NCore 

monitors now in operation nationwide. This means that a significant portion of the 

current network is composed of monitors that are additional to those required by EPA as 

part of a national network design. It is critical to note that in this rulemaking the EPA is 

actually increasing the total number of required sites in the national CO monitoring 

network design and is not removing any area-wide monitoring requirements as AASHTO 

and other commenters suggested. Some of the potential for misperception on this issue 

may have arisen from the Agency’s stated expectation that state and local air monitoring 

agencies will likely move existing CO monitors into near-road locations to satisfy the 

minimum monitoring requirements promulgated in this rulemaking. Based on this final 

rule, state and local agencies would only move, at most, approximately 52 monitors out 

of the 328 in operation (circa May 2011). Therefore a majority of CO monitors would 
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likely continue operating in their existing locations. However, it should be noted that 

with ambient CO concentrations well below the NAAQS, particularly at area-wide sites, 

states may identify some area-wide CO monitors to be no longer necessary. As such, the 

retirement of these sites may be justified, and their removal would save state and local 

resources. The EPA does recognize the value of maintaining some level of area-wide CO 

monitoring to meet the overarching monitoring objectives, which includes tracking longterm trends and to supporting research. In the proposal, the Agency did not propose 

establishing requirements for additional area-wide monitoring sites because: 1) there is 

the existing NCore requirement, and 2) there is an expectation based on experience that 

some number of non-required area-wide sites will continue to operate in the future 

without minimum monitoring requirements. Regarding the removal or shutdown of any

individual ambient air pollutant monitor, the Agency notes that there is a publicly

transparent process by which any existing CO monitor would be shut-down. The shutdown of any State and Local Air Monitoring Station (SLAMS) monitor is allowable 

under certain conditions specified in 40 CFR 58.14 System Modification. These 

conditions provide state and local air agencies multiple options by which they may

propose, with justification, for a monitor to be shut down. Whatever the justification may

be, each monitor proposed to be shut-down must go through an established process to 

receive EPA Regional Administrator approval for shut-down. As part of that process, the 

EPA Regional Administrator provides opportunity for public comment before making a 

decision to approve or disapprove the request. In conclusion, the EPA believes that even 

without requirements for area-wide CO monitors additional to the NCore sites, some 

number of area-wide monitors will continue to operate into the future. EPA anticipates 
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that monitors that states find useful for other regulatory purposes, such as NEPA, would 

be among the monitors that may continue to operate. The NCore sites, along with 

monitors currently operating in the absence of other area-wide monitoring requirements, 

will likely provide a sufficient set of area-wide monitors to meet monitoring objectives. 

The EPA also received a number of comments from transportation groups, state 

and local groups, and an industry commenter (e.g., AAM, AASHTO, NESCAUM, 

NYSDEC, NYSDOT, TXDOT, and VDOT) suggesting that required near-road CO 

monitors should be paired with an area-wide CO monitor within the same CBSA. For 

example, AASHTO recommended that “...EPA ensure that adequate coverage continues 

from neighborhood-scale monitors to estimate background concentration levels, and that 

there is at least one neighborhood scale monitor in every urbanized area that is required 

to have a near-road monitor.” NESCAUM recommended “...that EPA locate near-road 

CO monitors near urban NCore CO sites” (as noted above, NCore sites are area-wide 

sites by design).

The EPA recognizes that a pairing of near-road CO monitors with area-wide CO 

monitors will provide information by which an estimate of the difference between nearroad concentrations to relative background concentrations might be determined. As 

noted earlier, the Agency believes that the combination of required NCore sites and those 

area-wide monitors currently operating in the absence of minimum monitoring 

requirements (of which many will likely continue operating in the future) will largely 

fulfill the area-wide component of any near-road site/area-wide site pairing in an urban 

area. An analysis of NCore site locations (site data available from 

/www.epa.gov/ttn/amtic/ncore/index.html), along with all those area-wide CO monitors 
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believed to be operating as of May 20, 2011 (utilizing EPA’s Air Explorer web tools 

located at www.epa.gov/airexplorer) indicated that of the 52 CBSAs with a population of 

1 million persons or more, based on 2010 Census data, , only 4 are believed to be without 

an area-wide CO monitor33. The EPA believes that, based on the considerations 

discussed above, the existing network will likely provide sufficient area-wide CO 

concentration information on which a near-road to area-wide data comparison could be 

based. 

f. Regional Administrator Authority

The EPA received a number of comments from states and transportation groups 

(e.g., AASHTO, NYSDOT, TCEQ, TXDOT, and VDOT) on the proposal for Regional 

Administrators to have the discretion to require monitoring above the minimum 

requirements as necessary to address situations where minimum monitoring requirements 

are not sufficient to meet monitoring objectives. For example, AASHTO commented that 

“the proposed rule includes some examples of where additional monitors may be 

necessary. AASHTO is concerned that these brief examples may not be sufficient to 

ensure uniform application of this additional authority among the EPA Regions,” and that 

EPA should provide guidance on this so that there is “reasonable uniformity between 

EPA Regions in the implementation of these provisions.” TCEQ commented that it 

“does not agree that this discretion is appropriate, particularly where EPA has not 

proposed a process by which Regional Administrators must consult with states and the 

public regarding these decisions.” Further TCEQ stated that “...the potential requirement 

for additional monitors when ‘minimum monitoring requirements are not sufficient to 

 

33 The EPA notes that of the 53 CBSAs that have 1 million or more persons, 39 CBSAs contain an 

NCore monitoring station, which includes a CO monitor.
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meet monitoring objectives’ is overly broad and should be refined to include objective 

criteria that will consistently applied across all EPA Regions.”

The EPA notes that the authority of Regional Administrators to require additional 

monitoring above the minimum required is not unique to the CO NAAQS. For example, 

Regional Administrators have the authority to use their discretion to require additional 

NO2, lead, and sulfur dioxide monitors (40 CFR part 58 appendix D sections 4.3.4, 4.4.3, 

and 4.5, respectively) and to work with state and local air agencies in designing and/or 

maintaining an appropriate ozone monitoring network (40 CFR part 58 appendix D 

section 4.1). The EPA believes that a nationally applicable network design may not 

always account for all locations in every area where monitors may be warranted. 

Example situations where the Regional Administrator authority could be utilized, which 

were provided in the proposal, could be for unmonitored locations where data or other 

information suggest that CO concentrations may be approaching or exceeding the 

NAAQS due to stationary CO sources, in downtown areas or urban street canyons, or in 

areas that are subject to high ground-level CO concentrations particularly due to or 

enhanced by topographical and meteorological impacts. The Agency cannot anticipate 

every example that may exist where the Regional Administrator authority might be used 

for inclusion in this preamble text. However, the Agency believes it is important for 

Regional Administrators to have the authority to address possible gaps in the minimally 

required monitoring network in situations such as those examples provided here. In 

response to public comments, the EPA notes that Regional Administrators would use 

their authority in collaboration with state agencies, working with stakeholders to design 

and/or maintain the most appropriate CO monitoring network to meet the needs of a 
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given area. Finally, the Agency notes that any monitor required by the Regional 

Administrator (or any new monitor proposed by the state itself) is not done so with 

unfettered discretion. Any such action would be included in the Annual Monitoring 

Network Plan per 40 CFR §58.10, and this plan must be made available for public 

inspection and comment before any decisions are made by the EPA Regional 

Administrator. 

3. Conclusions on the Network Design 

This section provides the rationale and details for the final decision on changes to 

the CO monitoring network design and siting criteria. As discussed above in section 

IV.B.2.a, motor vehicle emissions are important contributors to ambient CO 

concentrations (REA, section 2.2), contributing nearly 60% of the total CO emitted 

nationally (per the 2008 NEI). As a result, microenvironments influenced by on-road 

mobile sources are important contributors to ambient CO exposures, particularly in urban 

areas (REA, section 2.7). Therefore, the Administrator has concluded that monitoring in 

the near-road environment to characterize and assess ambient CO concentrations 

continues to be an appropriate objective for the CO monitoring network. The EPA 

believes that the promulgation of minimum requirements for CO monitors in the nearroad environment is necessary to ensure a network of adequate size and focus to provide 

data for comparison to the NAAQS, support research which includes health studies, allow 

for model verification, and fulfill multipollutant monitoring objectives. Further, 

considering the lack of CO monitors assessing higher trafficked roads in urban areas and 

CASAC’s advice that the Agency develop greater monitoring capacity for CO in nearroad environments (Brain and Samet, 2010b), the Agency believes that a number of CO 
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monitors should be focused in such locations. Highly trafficked roads are expected to 

show elevated CO concentrations relative to area-wide concentrations and to represent 

the locations where ambient CO concentrations may be highest in an area. Regarding the 

locations where required near-road CO monitors might be placed, the EPA proposed that 

they be collocated with a subset of near-road NO2 monitors. The EPA expects required 

near-road NO2 monitors (as prescribed in 40 CFR part 58, appendix D, Section 4.3) to be 

adjacent to highly trafficked roads within the CBSAs where they are required. 

Recognizing this and also recognizing the benefits associated with collocating monitors 

at the same site, the Agency is finalizing requirements for near-road CO monitors that 

will leverage required near-road NO2 monitoring sites to house collocated near-road CO 

monitors to create data for comparison to the NAAQS, support research which includes 

health studies, provide data for model evaluation, and foster the fulfillment of 

multipollutant objectives. 

As noted above, after considering public comments, EPA has modified the 

requirements for CO monitors from that which was proposed so that only one CO 

monitor is required in each CBSA in which near-road CO monitoring is required.34 This 

approach reduces the total number of monitors that would have been required under the 

proposal from 75 monitors within 52 CBSAs to 52 monitors within 52 CBSAs (based on 

2010 Census data). The EPA believes this network design addresses public comments 

while maintaining monitoring in a sufficiently diverse set of locations throughout 52 

different urban areas around the country. By having monitors within 52 different 

CBSAs, this network design is expected to provide for monitoring in a wide range of 

 

34

 This approach only requires one CO monitor to be installed in those CBSAs that have two 

required near-road NO2 monitors.
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diverse situations with regard to traffic volumes, traffic patterns, roadway designs, 

terrain/topography, meteorology, climate, as well as surrounding land use and population 

characteristics. The EPA is generally requiring CO monitors to be collocated with nearroad NO2 monitors. However, upon consideration of public comments, the Agency is 

allowing flexibility for states to use an alternate near-road location, which includes 

downtown areas, urban street canyons, and other near-road locations, for a required CO 

monitor on a case-by-case basis, with EPA Regional Administrator approval, when the 

state can provide quantitative justification showing the expectation of peak CO 

concentrations for that alternate location compared to a near-road NO2 location. Such 

requests could be based upon appropriate modeling, exploratory monitoring, or other 

methods, comparing the alternative CO location and the near-road NO2 location. 

In summary, based upon 2010 Census Bureau data this final rule for CO 

monitoring will require approximately 52 CO monitors to be collocated with near-road 

NO2 monitors (or otherwise operated at an alternate, EPA Regional Administrator 

approved, near-road location where peak CO concentrations are expected) within 52 

CBSAs that have populations of 1 million or more persons. 

Regarding the deployment and operation of required CO monitors, the Agency

recognizes that many state and local air agencies are under financial and related resource 

duress. EPA has concluded that allowing additional time for installing CO monitors will 

provide an opportunity for state and local agencies to work with EPA Regions to identify

which existing CO monitors may be appropriate to relocate to the near-road locations. In 

many cases, EPA and the state may believe it is appropriate to relocate monitors, 

including some of those that are currently operated pursuant to existing maintenance 
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plans. In these cases, additional time may be necessary to allow states to revisit and 

possibly revise, in consultation with (and subject to the approval of) the EPA Regions, 

existing maintenance plans in a way that may allow certain CO monitors to be free for 

relocation, if appropriate. Further, if a state chooses to investigate whether it will request 

that a required near-road CO monitor be sited in a near-road location other than a 

required near-road NO2 site, the time allotted will provide states with some time to 

perform necessary analyses for submission to the Regional Administrator for approval. 

Furthermore, EPA has concluded that public comments suggesting a phased-in 

implementation, allowing for later stages to benefit from experience in an initial round of 

monitor installations, have merit. 

As a result, the EPA has chosen not to require the implementation of required CO 

sites by January 1, 2013 as was proposed. Instead, the Agency is finalizing a two-phased 

implementation requirement. Those CO monitors required within CBSAs having 2.5 

million or more persons are to be operational by January 1, 2015, although the Agency

strongly encourages the implementation of these required monitors as soon as practicable. 

Those CO monitors required in CBSAs having 1 million or more persons (and fewer than 

2.5 million persons) are to be operational by January 1, 2017. EPA intends to review the 

experience of states with the first round of near-road CO monitors and the data produced 

by such monitors and consider whether adjustments to the network requirements are 

warranted. These required CO monitors shall be reflected in a state’s annual monitoring 

network plans due six months prior to installation, i.e., on July 1, 2014 or July 1, 2016, 

respectively. 
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Regarding siting criteria, the EPA received public support to adjust microscale 

CO siting criteria to match those of near-road NO2 monitors (and microscale PM2.5

monitors). The Agency also was urged to retain the existing microscale siting criteria, for 

explicit use with microscale CO sites in downtown areas or urban street canyon settings. 

As a result, the EPA is retaining the existing siting criteria for microscale CO monitors in 

downtown areas and urban street canyons and is finalizing the additional siting criteria 

for those near-road microscale CO monitors outside of downtown areas and urban street 

canyons to have probe height and horizontal spacing to match those of near-road NO2

monitors as prescribed in 40 CFR part 58 appendix E, sections 2, 4(d), 6.4(a), and table 

E-4. 

Specifically, the Agency is finalizing the following: 1) a microscale near-road CO 

monitor inlet probe shall be between 2 and 7 meters above the ground; 2) a microscale 

CO monitor inlet probes in the near-road environment shall be placed so it has an 

unobstructed air flow, where no obstacles exist at or above the height of the monitor 

probe, between the monitor probe and the outside nearest edge of the traffic lanes of the 

target road segment; and 3) that CO monitors in the near-road environment shall have 

inlet probes as near as practicable to the outside nearest edge of the traffic lanes of the 

target road segment, but shall not be located at a distance greater than 50 meters in the 

horizontal from the outside nearest edge of the traffic lanes of the target road segment. 

Further, as suggested through public comments, the EPA is retaining existing 

regulatory siting criteria language for microscale CO monitors in downtown areas or 

urban street canyon locations, where: 1) the inlet probe for a near-road microscale CO 

monitor in a downtown area or urban street canyon shall be between 2.5 meters and 3.5 
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meters above ground level; 2) the inlet probe for a near-road microscale CO monitor in a 

downtown area or urban street canyon shall be within 10 meters from the edge of the 

nearest traffic lane; and 3) near-road microscale CO monitors in street canyons are 

required to be at least 10 meters from an intersection.

Finally, the EPA recognizes that a monitoring network design may not always 

require monitoring on a national scale that is sufficient in fulfilling specific or otherwise 

unique data needs or monitoring objectives for every area across the nation. Thus, the 

EPA is finalizing the provision that EPA Regional Administrators have the authority to 

require monitoring above the minimum requirements, as necessary, in any area, to 

address situations where the minimally required monitoring network is not sufficient to 

meet monitoring objectives. Example situations where the Regional Administrator 

Authority could be utilized include, but are not limited to, those unmonitored locations 

where data or other information suggest that CO concentrations may be approaching or 

exceeding the NAAQS due to stationary CO sources, in downtown areas or urban street 

canyons, or in areas that are subject to high ground-level CO concentrations particularly 

due to or enhanced by topographical and meteorological impacts. In all cases in which a 

Regional Administrator may consider the need for additional monitoring, it is expected 

that the Regional Administrators will work with the state or local air agencies to evaluate 

evidence or needs to determine whether a particular area may warrant additional 

monitoring.

V. Statutory and Executive Order Reviews

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

13563: Improving Regulation and Regulatory Review 
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Under Executive Order 12866 (58 FR 51735, October 4, 1993), this action is a 

"significant regulatory action” because it was deemed to “raise novel legal or policy

issues.” Accordingly, EPA submitted this action to the Office of Management and 

Budget (OMB) for review under Executive Orders 12866 and 13563 (76 FR 3821, 

January 21, 2011) and any changes made in response to OMB recommendations have 

been documented in the docket for this action.

B. Paperwork Reduction Act 

The information collection requirements in this final rule have been submitted for 

approval to the Office of Management and Budget (OMB) under the Paperwork 

Reduction Act, 44 U.S.C. 3501 et seq. The information collection requirements are not 

enforceable until OMB approves them. The Information Collection Request (ICR) 

document prepared by EPA for these revisions to part 58 has been assigned EPA ICR 

number 0940.24. 

The information collected under 40 CFR part 53 (e.g., test results, monitoring 

records, instruction manual, and other associated information) is needed to determine 

whether a candidate method intended for use in determining attainment of the NAAQS in 

40 CFR part 50 will meet comparability requirements for designation as a FRM or FEM. 

We do not expect the number of FRM or FEM determinations to increase over the 

number that is currently used to estimate burden associated with CO FRM/FEM 

determinations provided in the current ICR for 40 CFR part 53 (EPA ICR numbers 

0940.24). As such, no change in the burden estimate for 40 CFR part 53 has been made 

as part of this rulemaking. 
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The information collected and reported under 40 CFR part 58 is needed to 

determine compliance with the NAAQS, to characterize air quality and associated health 

impacts, to develop emissions control strategies, and to measure progress for the air 

pollution program. The amendments would revise the technical requirements for CO 

monitoring sites, require the relocation or siting of ambient CO air monitors, and the 

reporting of the collected ambient CO monitoring data to EPA’s Air Quality System 

(AQS). The annual average reporting burden for the collection under 40 CFR part 58 

(averaged over the first 3 years of this ICR) for a network of 311 CO monitors is 

$7,235,483. Burden is defined at 5 CFR 1320.3(b). State, local, and Tribal entities are 

eligible for State assistance grants provided by the Federal government under the CAA 

which can be used for monitors and related activities. 

An agency may not conduct or sponsor, and a person is not required to respond to, 

a collection of information unless it displays a currently valid OMB control number. The 

OMB control numbers for EPA’s regulations in 40 CFR are listed in 40 CFR part 9. 

When this ICR is approved by OMB, the Agency will publish a technical amendment to 

40 CFR part 9 in the Federal Register to display the OMB control number for the 

approved information collection requirements contained in this final rule.

C. Regulatory Flexibility Act

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

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

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

agency certifies that the rule will not have a significant economic impact on a substantial 
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number of small entities. Small entities include small businesses, small organizations, 

and small governmental jurisdictions.

For purposes of assessing the impacts of today’s rule on small entities, small 

entity is defined as: (1) a small business that is a small industrial entity as defined by the 

Small Business Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small 

governmental jurisdiction that is a government of a city, county, town, school district or 

special district with a population of less than 50,000; and (3) a small organization that is 

any not-for-profit enterprise which is independently owned and operated and is not 

dominant in its field.

After considering the economic impacts of this final rule on small entities, I 

certify that this action will not have a significant economic impact on a substantial 

number of small entities. This final rule will not impose any requirements on small 

entities. Rather, this rule retains existing national standards for allowable concentrations 

of CO in ambient air as required by section 109 of the CAA. See also American 

Trucking Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do not have significant 

impacts upon small entities because NAAQS themselves impose no regulations upon 

small entities). Similarly, the amendments to 40 CFR part 58 address the requirements 

for States to collect information and report compliance with the NAAQS and will not 

impose any requirements on small entities. 

D. Unfunded Mandates Reform Act

This rule does not contain a Federal mandate that may result in expenditures of 

$100 million or more for State, local, and Tribal governments, in the aggregate, or the 

private sector in any one year. This rule retains the existing national ambient air quality
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standards for carbon monoxide. The expected costs associated with the monitoring 

requirements are described in EPA’s ICR document, but those costs are expected to be 

well less than $100 million (adjusted for inflation) in the aggregate for any year. 

Furthermore, as indicated previously, in setting a NAAQS, EPA cannot consider the 

economic or technological feasibility of attaining ambient air quality standards. Thus, this 

rule is not subject to the requirements of sections 202 or 205 of the UMRA.

This rule is also not subject to the requirements of section 203 of the UMRA 

because it imposes no enforceable duty on any small governments.

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial 

direct effects on the States, on the relationship between the national government and the 

States, or on the distribution of power and responsibilities among the various levels of 

government, as specified in Executive Order 13132. The rule does not alter the 

relationship between the Federal government and the States regarding the establishment 

and implementation of air quality improvement programs as codified in the CAA. Under 

section 109 of the CAA, EPA is mandated to establish and review NAAQS; however, 

CAA section 116 preserves the rights of States to establish more stringent requirements if 

deemed necessary by a State. Furthermore, this rule does not impact CAA section 107 

which establishes that the States have primary responsibility for implementation of the 

NAAQS. Finally, as noted in section D (above) on UMRA, this rule does not impose 

significant costs on State, local or Tribal governments or the private sector. Thus, 

Executive Order 13132 does not apply to this rule.
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Page 107 of 166

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 action.

G. Executive Order 13045: Protection of Children from Environmental Health and 

Safety Risks

This action is not subject to EO 13045 (62 FR 19885, April 23, 1997) because it is 

not economically significant as defined in EO 12866, and because the Agency does not 

believe the environmental health or safety risks addressed by this action present a 

disproportionate risk to children. This action’s health and risk assessments are described 

in section II.A.

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, 

Distribution or Use

This action is not a “significant energy action” as defined in Executive Order 

13211, (66 FR 28355 (May 22, 2001)) because it is not likely to have a significant 

adverse effect on the supply, distribution, or use of energy. The rule concerns the review 

of the NAAQS for CO. The rule does not prescribe specific pollution control strategies 

by which these ambient standards will be met. Such strategies are developed by States on 

a case-by-case basis, and EPA cannot predict whether the control options selected by 

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

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

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

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

inconsistent with applicable law or otherwise impractical. Voluntary consensus standards 

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

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

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

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

This rulemaking involves technical standards with regard to ambient monitoring 

of CO. We have not identified any potentially applicable voluntary consensus standards 

that would adequately characterize ambient CO concentrations for the purposes of 

determining compliance with the CO NAAQS and none have been brought to our 

attention in comments. Therefore, EPA has decided to use the method “Measurement 

Principle and Calibration Procedure for the Measurement of Carbon Monoxide in the 

Atmosphere (Non-Dispersive Infrared Photometry)” (40 CFR Part 50, appendix C), as 

revised by this action, for the purposes of ambient monitoring of CO concentrations.

J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations

Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes federal 

executive policy on environmental justice. Its main provision directs federal agencies, to 

the greatest extent practicable and permitted by law, to make environmental justice part 

of their mission by identifying and addressing, as appropriate, disproportionately high 
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Page 109 of 166

and adverse human health or environmental effects of their programs, policies, and 

activities on minority populations and low-income populations in the United States. 

EPA has determined that this final rule will not have disproportionately high and 

adverse human health or environmental effects on minority or low-income populations 

because it does not affect the level of protection provided to human health or the 

environment. The action in this notice is to retain without revision the existing NAAQS 

for CO. Therefore this action will not cause increases in source emissions or air 

concentrations.

K. Congressional Review Act

The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement Fairness Act of 1996, generally provides that before a 

rule may take effect, the agency promulgating the rule must submit a rule report, which 

includes a copy of the rule, to each House of the Congress and to the Comptroller 

General of the United States. EPA will submit a report containing this rule and other 

required information to the U.S. Senate, the U.S. House of Representatives, and the 

Comptroller General of the United States prior to publication of the rule in the Federal 

Register. A major rule cannot take effect until 60 days after it is published in the Federal 

Register. This action is not a “major rule” as defined by 5 U.S.C. 804(2). This rule will 

be effective [insert date 60 days after publication in the Federal Register].
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Washington, DC: American Statistical Asssociation; pp. 104-108.

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Consultation on EPA’s Carbon Monoxide National Ambient Air Quality Standards: 

Scope and Methods Plan for Health Risk and Exposure Assessment. CASAC-09-012. 

July 14, 2009.
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Brain J. and Samet J. (2009) Letter from Drs. J.D. Brain and J.M. Samet to 

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Carbon Monoxide (First External Review Draft) EPA-CASAC-09-011.June 24, 2009.

Brain, J.D. and Samet, J.M. (2010a) Letter from Drs. J.D. Brain and J.M. Samet 

to Administrator Lisa Jackson. Re: Review of the Risk and Exposure Assessment to 

Support the Review of the Carbon Monoxide (CO) Primary National Ambient Air 

Quality Standards: First External Review Draft. EPA-CASAC-10-006. February 12, 

2010.

Brain J.D. and Samet J.M. (2010b) Letter from Drs. J.D. Brain and J.M. Samet to 

Administrator Lisa Jackson. Re: Review of the Risk and Exposure Assessment to 

Support the Review of the Carbon Monoxide (CO) Primary National Ambient Air 

Quality Standards: Second External Review Draft. EPA-CASAC-10-012. May 19, 2010.

Brain J.D. and Samet J.M. (2010c) Letter from Drs. J.D. Brain and J.M. Samet to 

Administrator Lisa Jackson. Re: Review of the Policy Assessment for the Review of the 

Carbon Monoxide National Ambient Air Quality Standards (NAAQS): External Review 

Draft. EPA-CASAC-10-013. June 8, 2010.

Brain J.D. and Samet J.M. (2010d) Letter from Drs. J.D. Brain and J.M. Samet to 

Administrator Lisa Jackson. Re: Review of Integrated Science Assessment for Carbon 

Monoxide (Second External Review Draft). EPA-CASAC-10-005. January 20, 2010.

Henderson R. (2008) Letter from Dr. Rogene Henderson, Chairman, Clean Air 

Scientific Advisory Committee, to Administrator Stephen Johnson. Re: Consultation on 

EPA’s Draft Plan for Review of the Primary NAAQS for Carbon Monoxide CASAC-08-

013. June 12, 2008.

Johnson T.; Capel J.; Paul R.; Wijnberg L. (1992) Estimation of Carbon 

Monoxide Exposure and associated Carboxyhemoglobin levels in Denver Residents 

Using a Probabalistic verion of NEM, prepared by International Technology for U.S. 

EPA, Office of Air Quality Planning and Standards, Durham, NC, Contract No. 68-D0-

0062.

Johnson T.; Mihlan G.; LaPointe J.; Fletcher K.; Capel J. (2000) Estimation of 

Carbon Monoxide Exposures and Associated Carboxyhemoglobin Levels for Residents 

of Denver and Los Angeles Using pNEM/CO (Version 2.1). Report prepared by ICF 

Consulting and TRJ Environmental, Inc., under EPA Contract No. 68-D6-0064. U.S. 

Environmental Protection Agency, Research Triangle Park, North Carolina. Available at: 

http://www.epa.gov/ttn/fera/human_related.html. June 2000.

Kleinman M.T.; Davidson D.M.; Vandagriff R.B.; Caiozzo V.J.; Whittenberger 

J.L. (1989) Effects of short-term exposure to carbon monoxide in subjects with coronary

artery disease. Arch. Environ. Health 44:361-369.

Kleinman M.T.; Leaf D.A.; Kelly E.; Caiozzo V.; Osann K.; O'Niell T. (1998) 

Urban angina in the mountains: effects of carbon monoxide and mild hypoxemia on 

subjects with chronic stable angina. Arch. Environ.Health 53:388-397.

Koken P.J.M.; Piver W.T.; Ye F.; Elixhauser A.; Olsen L.M.; Portier C.J. (2003) 

Temperature, air pollution, and hospitalization for cardiovascular diseases among elderly 
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people in Denver. Environ Health Perspect, 111:1312-1317.

Lippmann, M. (1984) CASAC Findings and Recommendations on the Scientific 

Basis for a Revised NAAQS for Carbon Monoxide. [Letter to W.D. Ruckelshous, EPA 

Administrator]. May 17.

Linn W.S.; Szlachcic Y.; Gong H. Jr; Kinney P.L.; Berhane K.T. (2000) Air 

pollution and daily hospital admissions in metropolitan Los Angeles. Environ Health 

Perspect, 108:427-434. 

Mann J.K.; Tager I.B.; Lurmann F.; Segal M.; Quesenberry C.P. Jr; Lugg M.M.; 

Shan J.; Van den Eeden S.K. (2002) Air pollution and hospital admissions for ischemic 

heart disease in persons with congestive heart failure or arrhythmia. Environ Health 

Perspect, 110:1247-1252. 

Metzger K.B.; Tolbert P.E.; Klein M.; Peel J.L.; Flanders W.D.; Todd K.H.; 

Mulholland J.A.; Ryan P.B.; Frumkin H. (2004) Ambient air pollution and cardiovascular 

emergency department visits. Epidemiology, 15:46-56.

National Research Council. (2003) Managing Carbon Monoxide Pollution in 

Meteorological and Topographical Problem Areas. The National Academies Press, 

Washington, D.C. 

Sheps D.S.; Adams K.F. Jr.; Bromberg P.A.; Goldstein G.M.; O'Neil J.J.; 

Horstman D.; Koch G. (1987) Lack of effect of low levels of carboxyhemoglobin on 

cardiovascular function in patients with ischemic heart disease. Arch. Environ. Health 

42:108-116.

Symons J.M.; Wang L.; Guallar E.; Howell E.; Dominici F.; Schwab M.; Ange 

B.A.; Samet J.; Ondov J.; Harrison D.; Geyh A. (2006) A case-crossover study of fine 

particulate matter air pollution and onset of congestive heart failure symptom 

exacerbation leading to hospitalization. Am J Epidemiol, 164:421-433.

Tolbert P.E.; Klein M.; Peel J.L.; Sarnat S.E.; Sarnat J.A. (2007) Multipollutant 

modeling issues in a study of ambient air quality and emergency department visits in 

Atlanta. J Expo Sci Environ Epidemiol, 17:S29-S35.

U.S. Environmental Protection Agency. (1979a) Air Quality Criteria for Carbon 

Monoxide. Office of Health and Environmental Assessment, Environmental Criteria and 

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

U.S. Environmental Protection Agency. (1979b) Assessment of Adverse Health 

Effects from Carbon Monoxide and Implications for Possible Modifications of the 

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

U.S. Environmental Protection Agency. (1984a) Revised Evaluation of Health 

Effects Associated with Carbon Monoxide Exposure: An Addendum to the 1979 EPA Air 

Quality Criteria Document for Carbon Monoxide. Office of Health and Environmental 

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

EPA-600/8-83-033F.

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

Carbon Monoxide: Reassessment of Scientific and Technical Information. Office of Air 
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Quality Planning and Standards, Research Triangle Park. NC. EPA-450/584-904.

U.S. Environmental Protection Agency. (1991) Air Quality Criteria for Carbon 

Monoxide. Office of Health and Environmental Assessment, Environmental Criteria and 

Assessment Office, Research Triangle Park, NC. EPA/600/8-90/045F. Available at: 

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

U.S. Environmental Protection Agency. (1992) Review of the National Ambient 

Air Quality Standards for Carbon Monoxide: Assessment of Scientific and Technical 

Information, OAQPS Staff Paper. Office of Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA/452/R-92-004.

U.S. Environmental Protection Agency. (2000) Air Quality Criteria for Carbon 

Monoxide. National Center for Environmental Assessment, Office of Research and 

Development, Research Triangle Park, NC. EPA/600/P-99/001F. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=18163.

U.S. Environmental Protection Agency. (2008a) Draft Plan for Review of the 

National Ambient Air Quality Standards for Carbon Monoxide. Also known as Draft 

Integrated Review Plan. National Center for Environmental Assessment and Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. EPA-452/D-08-001. 

Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pd.html.

U.S. Environmental Protection Agency. (2008b) Plan for Review of the National 

Ambient Air Quality Standards for Carbon Monoxide. Also known as Integrated Review 

Plan. National Center for Environmental Assessment and Office of Air Quality Planning 

and Standards, Research Triangle Park, NC. EPA-452/R-08-005. Available at: 

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

U.S. Environmental Protection Agency. (2009a) Integrated Science Assessment 

for Carbon Monoxide, First External Review Draft. National Center for Environmental 

Assessment, Research Triangle Park, NC. EPA/600/R-00/019. Available at: 

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

U.S. Environmental Protection Agency. (2009b) Integrated Science Assessment 

for Carbon Monoxide, Second External Review Draft. National Center for Environmental 

Assessment, Research Triangle Park, NC. EPA/600/R-09/019B. Available at: 

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

U.S. Environmental Protection Agency. (2009c) Carbon Monoxide National 

Ambient Air Quality Standards: Scope and Methods Plan for Health Risk and Exposure 

Assessment. Draft. Office of Air Quality Planning and Standards, Research Triangle 

Park, NC. EPA-452/R-09-004. Available at: 

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

U.S. Environmental Protection Agency. (2009d) Risk and Exposure Assessment 

to Support the Review of the Carbon Monoxide Primary National Ambient Air Quality

Standards, First External Review Draft. Office of Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA-452/P-09-008. Available at: 

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

U.S. Environmental Protection Agency. (2010a) Integrated Science Assessment 
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for Carbon Monoxide. National Center for Environmental Assessment, Research Triangle 

Park, NC. EPA/600/R-09/019F. Available at: 

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

U.S. Environmental Protection Agency. (2010b) Quantitative Risk and Exposure 

Assessment for Carbon Monoxide - Amended. Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. EPA-452/R-10-009. Available at: 

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

U.S. Environmental Protection Agency. (2010c) Policy Assessment for the 

Review of the Carbon Monoxide National Ambient Air Quality Standards. Office of Air 

Quality Planning and Standards, Research Triangle Park, NC. EPA 452/R-10-007. 

Available at: http://www.epa.gov/ttn/naaqs/standards/co/s_co_cr_pa.html.

U.S. Environmental Protection Agency. (2010d) Risk and Exposure Assessment 

to Support the Review of the Carbon Monoxide Primary National Ambient Air Quality

Standards, Second External Review Draft, U.S Environmental Protection Agency, 

Research Triangle Park, NC, report no. EPA-452/P-10-004. Available at: 

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

U.S. Environmental Protection Agency. (2010e) Policy Assessment for the 

Review of the Carbon Monoxide National Ambient Air Quality Standards, External 

Review Draft. Office of Air Quality Planning and Standards, Research Triangle Park, 

NC. EPA-452/P-10-005. Available at: 

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

Watkins N. and Thompson R. (2010) CO Monitoring Network Background and 

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

Wellenius G.A.; Bateson T.F.; Mittleman M.A.; Schwartz J. (2005) Particulate air 

pollution and the rate of hospitalization for congestive heart failure among medicare 

beneficiaries in Pittsburgh, Pennsylvania. Am J Epidemiol 161:1030-1036. 

Zhu Y.; Hinds W.C.; Kim S.; Shen S.; Sioutas C. (2002) Study of ultrafine 

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

4335.
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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: August 12, 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 amended as follows:

PART 50 – NATIONAL PRIMARY AND SECONDARYAMBIENT AIR 

QUALITY STANDARDS

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

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

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

Appendix C to Part 50—Measurement Principle and Calibration Procedure for the 

Measurement of Carbon Monoxide in the Atmosphere (Non-Dispersive Infrared 

Photometry)

1.0 Applicability

1.1 This non-dispersive infrared photometry (NDIR) Federal Reference Method 

(FRM) provides measurements of the concentration of carbon monoxide (CO) in ambient 

air for determining compliance with the primary and secondary National Ambient Air 

Quality Standards (NAAQS) for CO as specified in § 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
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2.1 Measurements of CO in ambient air are based on automated measurement of the 

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

employing non-wavelength-dispersive, infrared photometry (NDIR method). Infrared 

energy from a source in the photometer is passed through a cell containing the air sample 

to be analyzed, and the quantitative absorption of energy by CO in the sample cell is 

measured by a suitable detector. The photometer is sensitized specifically to CO by

employing CO gas in a filter cell in the optical path, which, when compared to a 

differential optical path without a CO filter cell, limits the measured absorption to one or 

more of the characteristic wavelengths at which CO strongly absorbs. However, to meet 

measurement performance requirements, various optical filters, reference cells, rotating 

gas filter cells, dual-beam configurations, moisture traps, or other means may also be 

used to further enhance sensitivity and stability of the photometer and to minimize 

potential measurement interference from water vapor, carbon dioxide (CO2), or other 

species. Also, various schemes may be used to provide a suitable zero reference for the 

photometer, and optional automatic compensation may be provided for the actual 

pressure and temperature of the air sample in the measurement cell. The measured 

infrared absorption, converted to a digital reading or an electrical output signal, indicates 

the measured CO concentration.

2.2 The measurement system is calibrated by referencing the analyzer’s CO 

measurements to CO concentration standards traceable to a National Institute of 

Standards and Technology (NIST) primary standard for CO, as described in the 

associated calibration procedure specified in section 4 of this reference method.
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2.3 An analyzer implementing this measurement principle will be considered a 

reference method only if it has been designated as a reference method in accordance with 

part 53 of this chapter.

2.4 Sampling considerations. The use of a particle filter in the sample inlet line of a 

CO FRM analyzer is optional and left to the discretion of the user unless such a filter is 

specified or recommended by the analyzer manufacturer in the analyzer’s associated 

operation or instruction manual.

3.0 Interferences

3.1 The NDIR measurement principle is potentially susceptible to interference from 

water vapor and CO2, which have some infrared absorption at wavelengths in common 

with CO and normally exist in the atmosphere. Various instrumental techniques can be 

used to effectively minimize these interferences.

4.0 Calibration Procedures

4.1 Principle. Either of two methods may be selected for dynamic multipoint 

calibration of FRM CO analyzers, using test gases of accurately known CO 

concentrations obtained from one or more compressed gas cylinders certified as CO 

transfer standards:

4.1.1 Dilution method: A single certified standard cylinder of CO is quantitatively

diluted as necessary with zero air to obtain the various calibration concentration standards 

needed.
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4.1.2 Multiple-cylinder method: Multiple, individually certified standard cylinders of 

CO are used for each of the various calibration concentration standards needed.

4.1.3 Additional information on calibration may be found in Section 12 of reference 1.

4.2 Apparatus. The major components and typical configurations of the calibration 

systems for the two calibration methods are shown in Figures 1 and 2. Either system may

be made up using common laboratory components, or it may be a commercially

manufactured system. In either case, the principal components are as follows:

4.2.1 CO standard gas flow control and measurement devices (or a combined device) 

capable of regulating and maintaining the standard gas flow rate constant to within ±2 

percent and measuring the gas flow rate accurate to within ±2 percent, properly calibrated 

to a NIST-traceable standard.

4.2.2 For the dilution method (Figure 1), dilution air flow control and measurement 

devices (or a combined device) capable of regulating and maintaining the air flow rate 

constant to within ±2 percent and measuring the air flow rate accurate to within ±2 

percent, properly calibrated to a NIST-traceable standard.

4.2.3 Standard gas pressure regulator(s) for the standard CO cylinder(s), suitable for 

use with a high-pressure CO gas cylinder and having a non-reactive diaphragm and 

internal parts and a suitable delivery pressure.

4.2.4 Mixing chamber for the dilution method, of an inert material and of proper 
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design to provide thorough mixing of CO standard gas and diluent air streams.

4.2.5 Output sampling manifold, constructed of an inert material and of sufficient 

diameter to ensure an insignificant pressure drop at the analyzer connection. The system 

must have a vent designed to ensure nearly atmospheric pressure at the analyzer 

connection port and to prevent ambient air from entering the manifold.

4.3 Reagents.

4.3.1 CO gas concentration transfer standard(s) of CO in air, containing an 

appropriate concentration of CO suitable for the selected operating range of the analyzer 

under calibration and traceable to a NIST standard reference material (SRM). If the CO 

analyzer has significant sensitivity to CO2, the CO standard(s) should also contain 350 to 

400 ppm CO2 to replicate the typical CO2 concentration in ambient air. However, if the 

zero air dilution ratio used for the dilution method is not less than 100:1 and the zero air 

contains ambient levels of CO2, then the CO standard may be contained in nitrogen and 

need not contain CO2.

4.3.2 For the dilution method, clean zero air, free of contaminants that could cause a 

detectable response on or a change in sensitivity of the CO analyzer. The zero air should 

contain <0.1 ppm CO.

4.4 Procedure Using the Dilution Method.

4.4.1 Assemble or obtain a suitable dynamic dilution calibration system such as the 

one shown schematically in Figure 1. Generally, all calibration gases including zero air 

must be introduced into the sample inlet of the analyzer. However, if the analyzer has 
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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:

(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. Connect the 

measurement signal output of the analyzer to an appropriate readout instrument to allow 

the analyzer’s measurement output to be continuously monitored during the calibration. 

Where possible, this readout instrument should be the same one used to record routine 

monitoring data, or, at least, an instrument that is as closely representative of that system 

as feasible.

760( 273.15)

298.15







m

m

c m

T

P

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

Page 122 of 166

4.4.4 Connect the inlet of the CO analyzer to the output-sampling manifold of the 

calibration system.

4.4.5 Adjust the calibration system to deliver zero air to the output manifold. The 

total air flow must exceed the total demand of the analyzer(s) connected to the output 

manifold to ensure that no ambient air is pulled into the manifold vent. Allow the 

analyzer to sample zero air until a stable response is obtained. After the response has 

stabilized, adjust the analyzer zero reading. 

4.4.6 Adjust the zero air flow rate and the CO gas flow rate from the standard CO 

cylinder to provide a diluted CO concentration of approximately 80 percent of the 

measurement upper range limit (URL) of the operating range of the analyzer. The total 

air flow rate must exceed the total demand of the analyzer(s) connected to the output 

manifold to ensure that no ambient air is pulled into the manifold vent. The exact CO 

concentration is calculated from:

(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.

D CO

STD CO

OUT F F

CO F

CO







[ ]

[ ]
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Page 123 of 166

4.4.7 Generate several additional concentrations (at least three evenly spaced points 

across the remaining scale are suggested to verify linearity) by decreasing FCO or 

increasing FD. Be sure the total flow exceeds the analyzer’s total flow demand. For each 

concentration generated, calculate the exact CO concentration using equation (2). Record 

the concentration and the analyzer’s stable response for each concentration. Plot the 

analyzer responses (vertical or y-axis) versus the corresponding CO concentrations 

(horizontal or x-axis). Calculate the linear regression slope and intercept of the 

calibration curve and verify that no point deviates from this line by more than 2 percent 

of the highest concentration tested.

4.5 Procedure Using the Multiple-Cylinder Method. Use the procedure for the 

dilution method with the following changes:

4.5.1 Use a multi-cylinder, dynamic calibration system such as the typical one shown 

in Figure 2.

4.5.2 The flowmeter need not be accurately calibrated, provided the flow in the output 

manifold can be verified to exceed the analyzer’s flow demand.

4.5.3 The various CO calibration concentrations required in Steps 4.4.5, 4.4.6, and 

4.4.7 are obtained without dilution by selecting zero air or the appropriate certified 

standard cylinder.

4.6 Frequency of Calibration. The frequency of calibration, as well as the number of 

points necessary to establish the calibration curve and the frequency of other performance 

checking, will vary by analyzer. However, the minimum frequency, acceptance criteria, 

and subsequent actions are specified in reference 1, appendix D, “Measurement Quality

Objectives and Validation Template for CO” (page 5 of 30). The user’s quality control 

program should provide guidelines for initial establishment of these variables and for 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 124 of 166

subsequent alteration as operational experience is accumulated. Manufacturers of CO 

analyzers should include in their instruction/operation manuals information and guidance 

as to these variables and on other matters of operation, calibration, routine maintenance, 

and quality control.

5.0 Reference

1. QA Handbook for Air Pollution Measurement Systems—Volume II. Ambient Air Quality 

Monitoring Program. U.S. EPA. EPA–454/B–08–003 (2008).
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

125

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

126

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

Page 127 of 166

PART 53 – AMBIENT AIR QUALITY REFERENCE AND EQUIVALENT METHODS

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

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

4. Subpart B of part 53 is revised to read as follows:

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

SO2, CO, O3, and NO2

Sec.

53.20 General Provisions

53.21 Test Conditions

53.22 Generation of Test Atmospheres

53.23 Test Procedure

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

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

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

Page 128 of 166

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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Page 129 of 166

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

Page 130 of 166

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***

Page 131 of 166

§ 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 132 of 166

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

Page 133 of 166

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

Page 134 of 166

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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Page 135 of 166

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

Page 136 of 166

(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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Page 137 of 166

(v) Calculate measurement noise as the standard deviation, S, as follows:

 

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.





























     

2

25

1

25

1

2

25

1

24

1

i

i

i

i

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

Page 138 of 166

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

Page 139 of 166

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

Page 140 of 166

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

Page 141 of 166

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

Page 142 of 166

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

Page 143 of 166

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

Page 144 of 166

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

Page 145 of 166

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

Page 146 of 166

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

Page 147 of 166

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***

Page 148 of 166

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

Page 149 of 166

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

Page 150 of 166

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

Page 151 of 166

(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:

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

where

,

n indicates the n-th test day, and i indicates the i-th measurement reading on the n-th test 

day.

100%

1

1 









n

n n

n

S

S S

SD

100%

1

1 



 







n

n n

n

S

S S

SD





12

6 7

1

i

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

Page 152 of 166

(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)

(B)

100%

6

1

5

1 1

2

6

1

6

1

2

20 





























     i

i

i

Pi P

URL

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

Page 153 of 166

(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

100%

6

1

5

1 1

2

12

7

12

7

2

80 





























     i

i

i

Pi P

URL

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

Page 154 of 166

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

Page 155 of 166

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*

*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)).

DRIFT AND PRECISION TEST DATA



n

i

i

IE

1
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

Page 156 of 166

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

L1

L2

Z’n

S’n

Cmax

Cmin

Figure B-4. Form for drift and precision test data (see § 53.23(e)).





12

6 7

1

i

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

Page 157 of 166

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

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)).





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

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

Page 159 of 166

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 EPA Regional Administrator. Plans for required 

CO monitors shall be submitted at least six months prior to the date such monitors must be 

established as required by section 58.13.

* * * * *

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

§58.13 Monitoring network completion.

* * * * *

(e) The CO monitors required under Appendix D, section 4.2 of this part must be 

physically established and operating under all of the requirements of this part, including the 

requirements of appendices A, C, D, and E to this part no later than:

1) January 1, 2015 for CO monitors in CBSAs having 2.5 million persons or more; or
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review***

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2) January 1, 2017 for other CO monitors. 

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

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

* * * * *

4.2 Carbon Monoxide (CO) Design Criteria.

4.2.1 General Requirements. (a) Except as provided in subsection (b), one CO monitor is 

required to operate collocated with one required near-road NO2 monitor, as required in Section 

4.3.2 of this part, in CBSAs having a population of 1,000,000 or more persons. If a CBSA has 

more than one required near-road NO2 monitor, only one CO monitor is required to be collocated 

with a near-road NO2 monitor within that CBSA. 

(b) If a state provides quantitative evidence demonstrating that peak ambient CO concentrations 

would occur in a near-road location which meets microscale siting criteria in Appendix E of this 

part but is not a near-road NO2 monitoring site, then the EPA Regional Administrator may

approve a request by a state to use such an alternate near-road location for a CO monitor in place 

of collocating a monitor at near-road NO2 monitoring site. 

4.2.2 Regional Administrator Required Monitoring. (a) The Regional Administrators, in 

collaboration with states, may require additional CO monitors above the minimum number of 

monitors required in 4.2.1 of this part, where the minimum monitoring requirements are not 

sufficient to meet monitoring objectives. The Regional Administrator may require, at his/her 

discretion, additional monitors in situations where data or other information suggest that CO 

concentrations may be approaching or exceeding the NAAQS. Such situations include, but are 
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not limited to, 1) characterizing impacts on ground-level concentrations due to stationary CO 

sources, 2) characterizing CO concentrations in downtown areas or urban street canyons, and 3) 

characterizing CO concentrations in areas that are subject to high ground level CO 

concentrations particularly due or enhanced by topographical and meteorological impacts. The 

Regional Administrator and the responsible State or local air monitoring agency shall work 

together to design and maintain the most appropriate CO network to address the data needs for 

an area, and include all monitors under this provision in the annual monitoring network plan.

4.2.3 CO Monitoring Spatial Scales. (a) Microscale and middle scale measurements are 

the most useful site classifications for CO monitoring sites since most people have the potential 

for exposure on these scales. Carbon monoxide maxima occur primarily in areas near major 

roadways and intersections with high traffic density and often in areas with poor atmospheric 

ventilation.

(1) Microscale —Microscale measurements typically represent areas in close proximity

to major roadways, within street canyons, over sidewalks, and in some cases, point and area 

sources. Emissions on roadways result in high ground level CO concentrations at the microscale, 

where concentration gradients generally exhibit a marked decrease with increasing downwind 

distance from major roads, or within downtown areas including urban street canyons. Emissions 

from stationary point and area sources, and non-road sources may, under certain plume 

conditions, result in high ground level concentrations at the microscale. 

(2) Middle scale —Middle scale measurements are intended to represent areas with 

dimensions from 100 meters to 0.5 kilometer. In certain cases, middle scale measurements may 

apply to areas that have a total length of several kilometers, such as “line” emission source areas. 
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This type of emission sources areas would include air quality along a commercially developed 

street or shopping plaza, freeway corridors, parking lots and feeder streets.

(3) Neighborhood scale – Neighborhood scale measurements are intended to represent 

areas with dimensions from 0.5 kilometers to 4 kilometers. Measurements of CO in this 

category would represent conditions throughout some reasonably urban sub-regions. In some 

cases, neighborhood scale data may represent not only the immediate neighborhood spatial area, 

but also other similar such areas across the larger urban area. Neighborhood scale measurements 

provide relative area-wide concentration data which are useful for providing relative urban 

background concentrations, supporting health and scientific research, and for use in modeling. 

* * * * *

9. Appendix E to Part 58 is amended by revising sections 2 and 6.2(a), 6.2(b), 6.2(c), and Table 

E-4 to read as follows:

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

Quality Monitoring

* * * * *

2. Horizontal and Vertical Placement

The probe or at least 80 percent of the monitoring path must be located between 2 and 15 meters 

above ground level for all O3 and SO2 monitoring sites, and for neighborhood or larger spatial 

scale Pb, PM10, PM10–2.5, PM2.5, NO2, and CO sites. Middle scale PM10–2.5 sites are required to 

have sampler inlets between 2 and 7 meters above ground level. Microscale Pb, PM10, PM10–2.5,

and PM2.5 sites are required to have sampler inlets between 2 and 7 meters above ground level. 
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Microscale near-road NO2 monitoring sites are required to have sampler inlets between 2 and 7 

meters above ground level. The inlet probes for microscale carbon monoxide monitors that are 

being used to measure concentrations near roadways must be between 2 and 7 meters above 

ground level. Those inlet probes for microscale carbon monoxide monitors measuring 

concentrations near roadways in downtown areas or urban street canyons must be between 2.5 

and 3.5 meters above ground level. The probe or at least 90 percent of the monitoring path must 

be at least 1 meter vertically or horizontally away from any supporting structure, walls, parapets, 

penthouses, etc., and away from dusty or dirty areas. If the probe or a significant portion of the 

monitoring path is located near the side of a building or wall, then it should be located on the 

windward side of the building relative to the prevailing wind direction during the season of 

highest concentration potential for the pollutant being measured.

* * * * *

6. * * * 

6.2 Spacing for Carbon Monoxide Probes and Monitoring Paths. (a) Near-road 

microscale CO monitoring sites, including those located in downtown areas, urban street 

canyons, and other near-road locations such as those adjacent to highly trafficked roads, are

intended to provide a measurement of the influence of the immediate source on the pollution 

exposure on the adjacent area. 

(b) Microscale CO monitor inlets probes in downtown areas or urban street canyon locations 

shall be located a minimum distance of 2 meters and a maximum distance of 10 meters from the 

edge of the nearest traffic lane.

(c) Microscale CO monitor inlet probes in downtown areas or urban street canyon locations shall

be located at least 10 meters from an intersection and preferably at a midblock location. 
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Midblock locations are preferable to intersection locations because intersections represent a 

much smaller portion of downtown space than do the streets between them. Pedestrian exposure 

is probably also greater in street canyon/corridors than at intersections. 

(d) Microscale CO monitor inlet probes in the near-road environment, outside of downtown areas 

or urban street canyons, shall be as near as practicable to the outside nearest edge of the traffic 

lanes of the target road segment; but shall not be located at a distance greater than 50 meters, in 

the horizontal, from the outside nearest edge of the traffic lanes of the target road segment.

(e) In determining the minimum separation between a neighborhood scale monitoring site and a 

specific roadway, the presumption is made that measurements should not be substantially

influenced by any one roadway. Computations were made to determine the separation distance, 

and Table E–2 of this appendix provides the required minimum separation distance between 

roadways and a probe or 90 percent of a monitoring path. Probes or monitoring paths that are 

located closer to roads than this criterion allows should not be classified as neighborhood scale, 

since the measurements from such a site would closely represent the middle scale. Therefore, 

sites not meeting this criterion should be classified as middle scale.

* * * * *
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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)1

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 [downtown 

or street canyon 

sites], micro [nearroad sites], middle

(300 m) and 

Neighborhood (1 

km)

2.5-3.5; 2-7; 2-

15 

> 1 > 10 2-10 for downtown 

areas 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

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 
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Page 166 of 166

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.

* * * * *