54294 Federal Register / Vol. 76, No. 169 / Wednesday, August 31, 2011 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50, 53 and 58 

[EPA–HQ–OAR–2008–0015; FRL–9455–2] 

RIN 2060–AI43 

Review of National Ambient Air Quality 

Standards for Carbon Monoxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: This rule is being issued at 

this time as required by a court order 

governing the schedule for completion 

of this review of the air quality criteria 

and the national ambient air quality 

standards (NAAQS) for carbon 

monoxide (CO). Based on its review, the 

EPA concludes the current primary 

standards are requisite to protect public 

health with an adequate margin of 

safety, and is retaining those standards. 

After review of the air quality criteria, 

EPA further concludes that no 

secondary standard should be set for CO 

at this time. EPA is also making changes 

to the ambient air monitoring 

requirements for CO, including those 

related to network design, and is 

updating, without substantive change, 

aspects of the Federal reference method. 

DATES: This final rule is effective on 

October 31, 2011. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2008–0015. 

Incorporated into this docket is a 

separate docket established for the 2010 

Integrated Science Assessment for 

Carbon Monoxide (Docket ID No. EPA– 

HQ–ORD–2007–0925. All documents in 

these dockets are listed on the http:// 

www.regulations.gov Web site. Although 

listed in the docket index, some 

information is not publicly available, 

e.g., confidential business information 

(CBI) or other information whose 

disclosure is restricted by statute. 

Certain other material, such as 

copyrighted material, is not placed on 

the Internet and will be publicly 

available for viewing at the Public 

Reading Room. Abstracts of scientific 

studies cited in the review are also 

available on the Internet at EPA’s HERO 

Web site: http://hero.epa.gov/, by 

clicking on the box on the right side of 

the page labeled ‘‘Search HERO.’’ 

Publicly available docket materials are 

available electronically through 

www.regulations.gov or may be viewed 

at the Public Reading Room at the Air 

and Radiation Docket and Information 

Center, EPA/DC, EPA West, Room 3334, 

1301 Constitution Ave., NW., 

Washington, DC. The Public Reading 

Room is open from 8:30 a.m. to 4:30 

p.m., Monday through Friday, excluding 

legal holidays. The telephone number 

for the Public Reading Room is (202) 

566–1744 and the telephone number for 

the Air and Radiation Docket and 

Information Center is (202) 566–1742. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Deirdre Murphy, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

Mail code C504–06, U.S. Environmental 

Protection Agency, Research Triangle 

Park, NC 27711; telephone number: 

919–541–0729; fax number: 919–541– 

0237; e-mail address: 

murphy.deirdre@epa.gov. For further 

information specifically with regard to 

section IV of this notice, contact Mr. 

Nealson Watkins, Air Quality Analysis 

Division, Office of Air Quality Planning 

and Standards, Mail code C304–06, U.S. 

Environmental Protection Agency, 

Research Triangle Park, NC 27711; 

telephone number: 919–541–5522; fax 

number: 919–541–1903; e-mail address: 

watkins.nealson@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in 

this preamble: 

I. Background 

A. Legislative Requirements 

B. Related Carbon Monoxide Control 

Programs 

C. Review of the Air Quality Criteria and 

Standards for Carbon Monoxide 

D. Summary of Proposed Decisions on 

Standards for Carbon Monoxide 

E. Organization and Approach to Final 

Decisions on Standards for Carbon 

Monoxide 

II. Rationale for Decisions on the Primary 

Standards 

A. Introduction 

1. Overview of Air Quality Information 

2. Overview of Health Effects Information 

a. Carboxyhemoglobin as Biomarker of 

Exposure and Toxicity 

b. Nature of Effects and At-Risk 

Populations 

c. Cardiovascular Effects 

3. Overview of Human Exposure and Dose 

Assessment 

B. Adequacy of the Current Primary 

Standards 

1. Rationale for Proposed Decision 

2. Comments on Adequacy 

3. Conclusions Concerning Adequacy of 

the Primary Standards 

III. Consideration of a Secondary Standard 

A. Introduction 

B. Rationale for Proposed Decision 

C. Comments on Consideration of 

Secondary Standard 

D. Conclusions Concerning a Secondary 

Standard 

IV. Amendments to Ambient Monitoring 

Requirements 

A. Monitoring Methods 

1. Proposed Changes to Parts 50 and 53 

2. Public Comments 

3. Decisions on Methods 

B. Network Design 

1. Proposed Changes 

2. Public Comments 

a. Near-Road Monitoring and Collocation 

With Near-Road Nitrogen Dioxide 

Monitors 

b. Population Thresholds for Requiring 

Near-Road Carbon Monoxide Monitors 

c. Implementation Schedule 

d. Siting Criteria 

e. Area-Wide Monitoring 

f. Regional Administrator Authority 

3. Conclusions on the Network Design 

V. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act References 

I. Background 

A. Legislative Requirements 

Two sections of the Clean Air Act 

(CAA) govern the establishment and 

revision of the NAAQS. Section 108 (42 

U.S.C. 7408) directs the Administrator 

to identify and list certain air pollutants 

and then to issue air quality criteria for 

those pollutants. The Administrator is 

to list those air pollutants that in her 

‘‘judgment, cause or contribute to air 

pollution which may reasonably be 

anticipated to endanger public health or 

welfare;’’ ‘‘the presence of which in the 

ambient air results from numerous or 

diverse mobile or stationary sources;’’ 

and ‘‘for which * * * [the 

Administrator] plans to issue air quality 

criteria * * * ’’ Air quality criteria are 

intended to ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in the ambient 

air * * *’’ 42 U.S.C. 7408(b). Section 

109 (42 U.S.C. 7409) directs the 

Administrator to propose and 

promulgate ‘‘primary’’ and ‘‘secondary’’ 

NAAQS for pollutants for which air 


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1The 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). 

2Welfare effects as defined in section 302(h) (42 

U.S.C. 7602(h)) include, but are not limited to, 

‘‘effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, 

visibility and climate, damage to and deterioration 

of property, and hazards to transportation, as well 

as effects on economic values and on personal 

comfort and well-being.’’ 

3Lists 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. 

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 (DC Cir. 1980), cert. denied, 

449 U.S. 1042 (1980); American 

Petroleum Institute v. Costle, 665 F.2d 

1176, 1186 (DC Cir. 1981), cert. denied, 

455 U.S. 1034 (1982); American Farm 

Bureau Federation v. EPA, 559 F.3d 

512, 533 (DC Cir. 2009); Association of 

Battery Recyclers v. EPA, 604 F.3d 613, 

617–18 (DC Cir. 2010). Both kinds of 

uncertainties are components of the risk 

associated with pollution at levels 

below those at which human health 

effects can be said to occur with 

reasonable scientific certainty. Thus, in 

selecting primary standards that provide 

an adequate margin of safety, the 

Administrator is seeking not only to 

prevent pollution levels that have been 

demonstrated to be harmful but also to 

prevent lower pollutant levels that may 

pose an unacceptable risk of harm, even 

if the risk is not precisely identified as 

to nature or degree. The CAA does not 

require the Administrator to establish a 

primary NAAQS at a zero-risk level or 

at background concentration levels, see 

Lead Industries v. EPA, 647 F.2d at 1156 

n.51, but rather at a level that reduces 

risk sufficiently so as to protect public 

health with an adequate margin of 

safety. 

In addressing the requirement for an 

adequate margin of safety, the EPA 

considers such factors as the nature and 

severity of the health effects involved, 

the size of sensitive population(s) at 

risk, and the kind and degree of the 

uncertainties that must be addressed. 

The selection of any particular approach 

to providing an adequate margin of 

safety is a policy choice left specifically 

to the Administrator’s judgment. See 

Lead Industries Association v. EPA, 647 

F.2d at 1161–62; Whitman v. American 

Trucking Associations, 531 U.S. 457, 

495 (2001). 

In setting primary and secondary 

standards that are ‘‘requisite’’ to protect 

public health and welfare, respectively, 

as provided in section 109(b), EPA’s 

task is to establish standards that are 

neither more nor less stringent than 

necessary for these purposes. In so 

doing, EPA may not consider the costs 

of implementing the standards. See 

generally, Whitman v. American 

Trucking Associations, 531 U.S. 457, 

465–472, 475–76 (2001). Likewise, 

‘‘[a]ttainability and technological 

feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards.’’ 

American Petroleum Institute v. Costle, 

665 F. 2d at 1185. 

Section 109(d)(1) requires that ‘‘not 

later than December 31, 1980, and at 5- 

year intervals thereafter, the 

Administrator shall complete a 

thorough review of the criteria 

published under section 108 and the 

national ambient air quality standards 

* * * and shall make such revisions in 

such criteria and standards and 

promulgate such new standards as may 

be appropriate. * * *’’ Section 

109(d)(2) requires that an independent 

scientific review committee ‘‘shall 

complete a review of the criteria * * * 

and the national primary and secondary 

ambient air quality standards * * * and 

shall recommend to the Administrator 

any new * * * standards and revisions 

of existing criteria and standards as may 

be appropriate. * * *’’ Since the early 

1980’s, this independent review 

function has been performed by the 

Clean Air Scientific Advisory 

Committee (CASAC).3

B. Related Carbon Monoxide Control 

Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the Act, and related provisions, states 

are to submit, for EPA approval, state 

implementation plans (SIPs) that 

provide for the attainment and 

maintenance of such standards through 

control programs directed to sources of 

the pollutants involved. The states, in 

conjunction with EPA, also administer 

the prevention of significant 

deterioration program. See CAA 

sections 160–169. In addition, Federal 

programs provide for nationwide 

reductions in emissions of these and 

other air pollutants through the Federal 

motor vehicle and motor vehicle fuel 

control program under title II of the Act 

(CAA sections 202–250), which involves 

controls for emissions from moving 

sources and controls for the fuels used 

by these sources and new source 

performance standards for stationary 

sources under section 111. 

C. Review of the Air Quality Criteria and 

Standards for Carbon Monoxide 

EPA initially established NAAQS for 

CO on April 30, 1971. The primary 

standards were established to protect 

against the occurrence of 

carboxyhemoglobin levels in human 

blood associated with health effects of 

concern. The standards were set at 9 

parts per million (ppm), as an 8-hour 

average, and 35 ppm, as a 1-hour 

average, neither to be exceeded more 

than once per year (36 FR 8186). In the 

1971 decision, the Administrator judged 

that attainment of these standards 

would provide the requisite protection 

of public health with an adequate 

margin of safety and would also provide 

requisite protection against known and 

anticipated adverse effects on public 

welfare, and accordingly set the 

secondary (welfare-based) standards 

identical to the primary (health-based) 

standards. 

In 1985, EPA concluded its first 

periodic review of the criteria and 

standards for CO (50 FR 37484). In that 

review, EPA updated the scientific 

criteria upon which the initial CO 

standards were based through the 

publication of the 1979 Air Quality 

Criteria Document for Carbon Monoxide 

(AQCD; USEPA, 1979a) and prepared a 

Staff Paper (USEPA, 1979b), which, 

along with the 1979 AQCD, served as 

the basis for the development of the 

notice of proposed rulemaking which 

was published on August 18, 1980 (45 

FR 55066). Delays due to uncertainties 


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regarding the scientific basis for the 

final decision resulted in EPA’s 

announcing a second public comment 

period (47 FR 26407). Following 

substantial reexamination of the 

scientific data, EPA prepared an 

Addendum to the 1979 AQCD (USEPA, 

1984a) and an updated Staff Paper 

(USEPA, 1984b). Following review by 

CASAC (Lippmann, 1984), EPA 

announced its decision not to revise the 

existing primary standards and to 

revoke the secondary standard for CO 

on September 13, 1985, due to a lack of 

evidence of effects on public welfare at 

ambient concentrations (50 FR 37484). 

On August 1, 1994, EPA concluded its 

second periodic review of the criteria 

and standards for CO by deciding that 

revisions to the CO NAAQS were not 

warranted at that time (59 FR 38906). 

This decision reflected EPA’s review of 

relevant scientific information 

assembled since the last review, as 

contained in the 1991 AQCD (USEPA, 

1991) and the 1992 Staff Paper (USEPA, 

1992). Thus, the primary standards were 

retained at 9 ppm with an 8-hour 

averaging time, and 35 ppm with a 

1-hour averaging time, neither to be 

exceeded more than once per year (59 

FR 38906). 

EPA initiated the next periodic review 

in 1997 and released the final 2000 

AQCD (USEPA, 2000) in August 2000. 

After release of the AQCD, Congress 

requested that the National Research 

Council (NRC) review the impact of 

meteorology and topography on ambient 

CO concentrations in high altitude and 

extreme cold regions of the U.S. The 

NRC convened the Committee on 

Carbon Monoxide Episodes in 

Meteorological and Topographical 

Problem Areas, which focused on 

Fairbanks, Alaska, as a case-study. 

A final report, ‘‘Managing Carbon 

Monoxide Pollution in Meteorological 

and Topographical Problem Areas,’’ was 

published in 2003 (NRC, 2003) and 

offered a wide range of 

recommendations regarding 

management of CO air pollution, cold 

start emissions standards, oxygenated 

fuels, and CO monitoring. Following 

completion of the NRC report, EPA did 

not conduct rulemaking to complete the 

review. 

On September 13, 2007, EPA issued a 

call for information from the public (72 

FR 52369) requesting the submission of 

recent scientific information on 

specified topics. On January 28–29, 

2008, a workshop was held to discuss 

policy-relevant scientific and technical 

information to inform EPA’s planning 

for the CO NAAQS review (73 FR 2490). 

Following the workshop, a draft 

Integrated Review Plan (IRP) (USEPA, 

2008a) was made available in March 

2008 for public comment and was 

discussed by the CASAC via a publicly 

accessible teleconference consultation 

on April 8, 2008 (73 FR 12998; 

Henderson, 2008). EPA made the final 

IRP available in August 2008 (USEPA, 

2008b). 

In preparing the Integrated Science 

Assessment for Carbon Monoxide (ISA 

or Integrated Science Assessment), EPA 

held an authors’ teleconference in 

November 2008 with invited scientific 

experts to discuss preliminary draft 

materials prepared as part of the 

ongoing development of the CO ISA and 

its supplementary annexes. The first 

draft ISA (USEPA, 2009a) was made 

available for public review on March 12, 

2009 (74 FR 10734), and reviewed by 

CASAC at a meeting held on May 12– 

13, 2009 (74 FR 15265). A second draft 

ISA (USEPA, 2009b) was released for 

CASAC and public review on 

September 23, 2009 (74 FR 48536), and 

it was reviewed by CASAC at a meeting 

held on November 16–17, 2009 (74 FR 

54042). The final ISA was released in 

January 2010 (USEPA, 2010a). 

In May 2009, OAQPS released a draft 

planning document, the draft Scope and 

Methods Plan (USEPA, 2009c), for 

consultation with CASAC and public 

review at the CASAC meeting held on 

May 12–13, 2009. Taking into 

consideration comments on the draft 

Scope and Methods Plan from CASAC 

(Brain, 2009) and the public, OAQPS 

staff developed and released for CASAC 

review and public comment a first draft 

Risk and Exposure Assessment (REA) 

(USEPA, 2009d), which was reviewed at 

the CASAC meeting held on November 

16–17, 2009. Subsequent to that meeting 

and taking into consideration comments 

from CASAC (Brain and Samet, 2010a) 

and public comments on the first draft 

REA, a second draft REA (USEPA, 

2010d) was released for CASAC review 

and public comment in February 2010, 

and reviewed at a CASAC meeting held 

on March 22–23, 2010. Drawing from 

information in the final CO ISA and the 

second draft REA, EPA released a draft 

Policy Assessment (PA) (USEPA, 2010e) 

in early March 2010 for CASAC review 

and public comment at the same 

meeting. Taking into consideration 

comments on the second draft REA and 

the draft PA from CASAC (Brain and 

Samet, 2010b, 2010c) and the public, 

staff completed the quantitative 

assessments which are presented in the 

final REA (USEPA, 2010b). Staff 

additionally took into consideration 

those comments and the final REA 

analyses in completing the final Policy 

Assessment (USEPA, 2010c) which was 

released in October 2010. 

The proposed decision (henceforth 

‘‘proposal’’) on the review of the CO 

NAAQS was signed on January 28, 

2011, and published in the Federal 

Register on February 11, 2011. The EPA 

held a public hearing to provide direct 

opportunity for oral testimony by the 

public on the proposal. The hearing was 

held on February 28, 2011, in Arlington, 

Virginia. At this public hearing, EPA 

heard testimony from five individuals 

representing themselves or specific 

interested organizations. Transcripts 

from this hearing and written testimony 

provided at the hearing are in the docket 

for this review. Additionally, written 

comments were received from various 

commenters during the public comment 

period on the proposal. Significant 

issues raised in the public comments are 

discussed in the preamble of this final 

action. A summary of all other 

significant comments, along with EPA’s 

responses (henceforth ‘‘Response to 

Comments’’) can be found in the docket 

for this review. 

The schedule for completion of this 

review is governed by a court order 

resolving a lawsuit filed in March 2003 

by a group of plaintiffs who alleged that 

EPA had failed to perform its mandatory 

duty, under section 109(d)(1), to 

complete a review of the CO NAAQS 

within the period provided by statute. 

The court order that governs this 

review, entered by the court on 

November 14, 2008, and amended on 

August 30, 2010, provides that EPA will 

sign for publication a notice of final 

rulemaking concerning its review of the 

CO NAAQS no later than August 12, 

2011. 

Some commenters have referred to 

and discussed individual scientific 

studies on the health effects of CO that 

were not included in the ISA (USEPA, 

2010a) (‘‘’new’ studies’’). In considering 

and responding to comments for which 

such ‘‘new’’ studies were cited in 

support, EPA has provisionally 

considered the cited studies in the 

context of the findings of the ISA. 

As in prior NAAQS reviews, EPA is 

basing its decision in this review on 

studies and related information 

included in the ISA, REA and Policy 

Assessment, which have undergone 

CASAC and public review. The studies 

assessed in the ISA and Policy 

Assessment, and the integration of the 

scientific evidence presented in them, 

have undergone extensive critical 

review by EPA, CASAC, and the public. 

The rigor of that review makes these 

studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 


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NAAQS decisions can have profound 

impacts on public health and welfare, 

and NAAQS decisions should be based 

on studies that have been rigorously 

assessed in an integrative manner not 

only by EPA but also by the statutorily 

mandated independent advisory 

committee, as well as the public review 

that accompanies this process. EPA’s 

provisional consideration of these 

studies did not and could not provide 

that kind of in-depth critical review. 

This decision is consistent with EPA’s 

practice in prior NAAQS reviews and its 

interpretation of the requirements of the 

CAA. Since the 1970 amendments, the 

EPA has taken the view that NAAQS 

decisions are to be based on scientific 

studies and related information that 

have been assessed as a part of the 

pertinent air quality criteria, and has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006) (final decision on 

review of NAAQS for particulate matter) 

for a detailed discussion of this issue 

and EPA’s past practice. 

As discussed in EPA’s 1993 decision 

not to revise the NAAQS for ozone, 

‘‘new’’ studies may sometimes be of 

such significance that it is appropriate 

to delay a decision on revision of a 

NAAQS and to supplement the 

pertinent air quality criteria so the 

studies can be taken into account (58 FR 

at 13013–13014, March 9, 1993). In the 

present case, EPA’s provisional 

consideration of ‘‘new’’ studies 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health effects and exposure pathways of 

ambient CO made in the air quality 

criteria. For this reason, reopening the 

air quality criteria review would not be 

warranted even if there were time to do 

so under the court order governing the 

schedule for this rulemaking. 

Accordingly, EPA is basing the final 

decisions in this review on the studies 

and related information included in the 

CO air quality criteria that have 

undergone CASAC and public review. 

EPA will consider the ‘‘new’’ studies for 

purposes of decision-making in the next 

periodic review of the CO NAAQS, 

which EPA expects to begin soon after 

the conclusion of this review and which 

will provide the opportunity to fully 

assess these studies through a more 

rigorous review process involving EPA, 

CASAC, and the public. Further 

discussion of these ‘‘new’’ studies can 

be found in the Response to Comments 

document. 

D. Summary of Proposed Decisions on 

Standards for Carbon Monoxide 

For reasons discussed in the notice of 

proposed rulemaking, the Administrator 

proposed to retain the current primary 

CO standards. With regard to 

consideration of a secondary standard, 

the Administrator proposed to conclude 

that no secondary standards should be 

set at this time. 

E. Organization and Approach to Final 

Decisions on Standards for Carbon 

Monoxide 

This action presents the 

Administrator’s final decisions in this 

review of the CO standards. Decisions 

regarding the primary CO standards are 

addressed below in section II. 

Consideration of a secondary CO 

standard is addressed below in section 

III. Ambient monitoring methods and 

network design related to 

implementation of the CO standards are 

addressed below in section IV. A 

discussion of statutory and executive 

order reviews is provided in section V. 

Today’s final decisions are based on 

a thorough review in the Integrated 

Science Assessment of the latest 

scientific information on known and 

potential human health and welfare 

effects associated with exposure to CO 

in the environment. These final 

decisions also take into account: (1) 

Assessments in the Policy Assessment 

of the most policy-relevant information 

in the Integrated Science Assessment as 

well as quantitative exposure, dose and 

risk assessments based on that 

information presented in the Risk and 

Exposure Assessment; (2) CASAC Panel 

advice and recommendations, as 

reflected in its letters to the 

Administrator and its discussions of 

drafts of the Integrated Science 

Assessment, Risk and Exposure 

Assessment and Policy Assessment at 

public meetings; (3) public comments 

received during the development of 

these documents, either in connection 

with CASAC Panel meetings or 

separately; and (4) public comments 

received on the proposed rulemaking. 

II. Rationale for Decisions on the 

Primary Standards 

A. Introduction 

This section presents the rationale for 

the Administrator’s decision that the 

current primary standards are requisite 

to protect public health with an 

adequate margin of safety, and that they 

should be retained. In developing this 

rationale, EPA has drawn upon an 

integrative synthesis in the Integrated 

Science Assessment of the entire body 

of evidence published through mid2009 on human health effects associated 

with the presence of CO in the ambient 

air. The research studies evaluated in 

the ISA have undergone intensive 

scrutiny through multiple layers of peer 

review, with extended opportunities for 

review and comment by the CASAC 

Panel and the public. As with virtually 

any policy-relevant scientific research, 

there is uncertainty in the 

characterization of health effects 

attributable to exposure to ambient CO. 

While important uncertainties remain, 

the review of the health effects 

information has been extensive and 

deliberate. In the judgment of the 

Administrator, this intensive evaluation 

of the scientific evidence provides an 

adequate basis for regulatory decision 

making at this time. This review also 

provides important input to EPA’s 

research plan for improving our future 

understanding of the relationships 

between exposures to ambient CO and 

health effects. 

The health effects information and 

quantitative exposure/dose assessment 

were summarized in sections II.B and 

II.C of the proposal (76 FR at 8162– 

8172) and are only briefly outlined in 

sections II.A.2 and II.A.3 below. 

Responses to public comments specific 

to the material presented in sections 

II.A.1 through II.A.3 below are provided 

in the Response to Comments 

document. 

Subsequent sections of this preamble 

provide a more complete discussion of 

the Administrator’s rationale, in light of 

key issues raised in public comments, 

for concluding that the current 

standards are requisite to protect public 

health with an adequate margin of safety 

and that it is appropriate to retain the 

current primary CO standards to 

continue to provide requisite public 

health protection (section II.B). 

1. Overview of Air Quality Information 

This section briefly summarizes the 

information on CO sources, emissions, 

ambient air concentrations and aspects 

of associated exposure presented in 

section II.A of the proposal, as well as 

in section 1.3 of the Policy Assessment 

and chapter 2 of the Risk and Exposure 

Assessment. 

Carbon monoxide in ambient air is 

formed by both natural and 

anthropogenic processes. In areas of 

human activity such as urban areas, it 

is formed primarily by the incomplete 

combustion of carbon-containing fuels 

with the combustion conditions 

influencing the rate of formation. For 

example, as a result of the combustion 


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4Endogenous CO is produced from biochemical 

reactions associated with normal breakdown of 

heme proteins (ISA, section 4.5). 

5Exogenous 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). 

6For 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). 

conditions, CO emissions from large 

fossil-fueled power plants are typically 

very low because optimized fuel 

consumption conditions make boiler 

combustion highly efficient. In contrast, 

internal combustion engines used in 

many mobile sources have widely 

varying operating conditions. As a 

result, higher and more varying CO 

formation results from the operation of 

mobile sources, which continue to be a 

significant source sector for CO in 

ambient air (ISA, sections 3.4 and 3.5; 

2000 AQCD, section 7.2; REA, section 

2.2 and 3.1.3). 

Mobile sources are a substantial 

contributor to total CO emissions, 

particularly in urban areas (ISA, section 

3.5.1.3; REA, section 3.1.3). Highest 

ambient concentrations in urban areas 

occur on or near roadways, particularly 

highly travelled roadways, and decline 

somewhat steeply with distance (ISA, 

section 3.5.1.3; REA, section 3.1.3; 

Baldauf et al., 2008a,b; Zhu et al., 2002). 

For example, as described in the ISA, a 

study by Zhu et al., (2002) documented 

CO concentrations at an interstate 

freeway to be ten times as high as an 

upwind monitoring site; concentrations 

declined rapidly in the downwind 

direction to levels only approximately 

one half roadway concentrations within 

100 to 300 meters (ISA, section 3.5.1.3, 

Figure 3–29; Zhu et al., 2002). Factors 

that can influence the steepness of the 

gradient include wind direction and 

other meteorological variables, and onroad vehicle density (ISA, section 

3.5.1.3, Figures 3–29 and 3–30; Zhu et 

al., 2002; Baldauf et al., 2008a, b). These 

traffic-related ambient concentrations 

contribute to the higher short-term 

ambient CO exposures experienced near 

busy roads and particularly in vehicles, 

as described in more detail in the REA 

and PA. 

2. Overview of Health Effects 

Information 

This section summarizes information 

presented in section II.B of the proposal 

pertaining to health endpoints 

associated with the range of exposures 

considered to be most relevant to 

current ambient CO exposure levels. In 

recognition of the use of an internal 

biomarker in evaluating health risk for 

CO, the following section summarizes 

key aspects of the use of 

carboxyhemoglobin as an internal 

biomarker (section II.A.2.a). This is 

followed first by a summary of the array 

of CO-induced health effects and 

recognition of at-risk subpopulations 

(section II.A.2.b) and then by a summary 

of the evidence regarding cardiovascular 

effects (section II.A.2.c). 

a. Carboxyhemoglobin as Biomarker of 

Exposure and Toxicity 

This section briefly summarizes the 

current state of knowledge, as described 

in the Integrated Science Assessment, of 

the role of carboxyhemoglobin in 

mediating toxicity and as a biomarker of 

exposure. The section also summarizes 

the roles of endogenously produced CO 

and exposure to ambient and 

nonambient CO in influencing internal 

CO concentrations and 

carboxyhemoglobin (COHb) levels. 

At this time, as during past reviews, 

the best characterized mechanism of 

action of CO is tissue hypoxia caused by 

binding of CO to hemoglobin to form 

COHb in the blood (e.g., USEPA, 2000; 

USEPA, 1991; ISA). Increasing levels of 

COHb in the blood stream with 

subsequent decrease in oxygen 

availability for organs and tissues are of 

concern in people who have 

compromised compensatory 

mechanisms (e.g., lack of capacity to 

increase blood flow in response to 

hypoxia), such as those with preexisting 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 COinduced health effects’’ (ISA, section 

5.7.8). 

Carboxyhemoglobin is formed in the 

blood both from CO originating in the 

body (endogenous CO) 4 and from CO 

that has been inhaled into the body 

(exogenous CO).5 The amount of COHb 

that occurs in the blood depends on 

factors specific to both the physiology of 

the individual (including disease state) 

and the exposure circumstances. These 

include factors associated with an 

individual’s rate of COHb elimination 

and production of endogenous CO, as 

well as those that influence the intake 

of exogenous CO into the blood, such as 

the differences in CO concentration (and 

partial pressure) in inhaled air, exhaled 

air, and blood; duration of a person’s 

exposure to changed CO concentrations 

in air; and exertion level or inhalation 

rate (ISA, chapter 4). 

Apart from the impairment of oxygen 

delivery to tissues related to COHb 

formation, toxicological studies also 

indicate several other pathways by 

which CO acts in the body, which 

involve a wide range of molecular 

targets and internal CO concentrations 

(2000 AQCD, sections 5.6–5.9; ISA, 

section 5.1.3). The role of these 

alternative less-well-characterized 

mechanisms in CO-induced health 

effects at concentrations relevant to the 

current NAAQS, however, is not clear. 

New research based on this evidence is 

needed to further understand these 

pathways and their linkage to COinduced effects in susceptible 

populations. Accordingly, COHb level 

in blood continues to be well recognized 

and most commonly used as an 

important internal dose metric, and is 

supported by the evidence as the most 

useful indicator of CO exposure that is 

related to CO health effects of major 

concern (ISA, p. 2–4, sections 4.1, 4.2, 

5.1.1; 1991 AQCD; 2000 AQCD; 2010 

ISA). 

b. Nature of Effects and At-Risk 

Populations 

The long-standing body of evidence 

that has established many aspects of the 

biological effects of CO continues to 

contribute to our understanding of the 

health effects of ambient CO (PA, 

section 2.2.1). Inhaled CO elicits various 

health effects through binding to, and 

associated alteration of the function of, 

a number of heme-containing 

molecules, mainly hemoglobin (see e.g., 

ISA, section 4.1). The best characterized 

health effect associated with CO levels 

of concern is decreased oxygen 

availability to critical tissues and 

organs, specifically the heart, induced 

by increased COHb levels in blood (ISA, 

section 5.1.2). Consistent with this, 

medical conditions that affect the 

biological mechanisms which 

compensate for this effect (e.g., 

vasodilation and increased coronary 

blood flow with increased oxygen 

delivery to the myocardium) can 

contribute to a reduced amount of 

oxygen available to key body tissues, 

potentially affecting organ system 

function and limiting exercise capacity 

(2000 AQCD, section 7.1).6

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 


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7The 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). 

8As 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). 

9Coronary 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). 

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 shortterm CO exposures and cardiovascular 

morbidity’’ (ISA, p. 2–6, section 2.5.1). 

Using the same established framework, 

the ISA describes the evidence as 

suggestive of causal relationships 

between relevant ambient CO exposure 

and several other health effects: 

Relevant short- and long-term CO 

exposures and central nervous system 

(CNS) effects, birth outcomes and 

developmental effects following longterm exposure, respiratory morbidity 

following short-term exposure, and 

mortality following short-term exposure 

(ISA, section 2.5). However, there is 

only limited evidence for these 

relationships, and the current body of 

evidence continues to indicate 

cardiovascular effects, particularly 

effects related to the role of CO in 

limiting oxygen availability to tissues, 

as those of greatest concern at low 

exposures with relevance to ambient 

concentrations (ISA, chapter 2). The 

evidence for these effects is further 

described in section II.A.2.c below. 

As described in the proposal, the 

terms susceptibility, vulnerability, 

sensitivity, and at-risk are commonly 

employed in identifying population 

groups or life stages at relatively higher 

risk for health risk from a specific 

pollutant. In the ISA for this review, the 

term susceptibility has been used 

broadly to recognize populations that 

have a greater likelihood of 

experiencing effects related to ambient 

CO exposure, with use of the term 

susceptible populations, as used in the 

ISA, defined as follows (ISA, section 

5.7, p. 5–115): 

Populations that have a greater likelihood 

of experiencing health effects related to 

exposure to an air pollutant (e.g., CO) due to 

a variety of factors including, but not limited 

to: Genetic or developmental factors, race, 

gender, lifestage, lifestyle (e.g., smoking 

status and nutrition) or preexisting disease, 

as well as population-level factors that can 

increase an individual’s exposure to an air 

pollutant (e.g., CO) such as socioeconomic 

status [SES], which encompasses reduced 

access to health care, low educational 

attainment, residential location, and other 

factors. 

Thus, susceptible populations are at 

greater risk of CO effects and are also 

referred to as at-risk in the summary 

below. 

As described in the proposal, the 

population with pre-existing 

cardiovascular disease continues to be 

the best-characterized population at risk 

of adverse CO-induced effects, with 

CAD recognized as ‘‘the most important 

susceptibility characteristic for 

increased risk due to CO exposure’’ 

(ISA, section 2.6.1). An important factor 

determining the increased susceptibility 

of this population is their inability to 

compensate for the reduction in tissue 

oxygen levels due to an already 

compromised cardiovascular system. 

Individuals with a healthy 

cardiovascular system (i.e., with healthy 

coronary arteries) have operative 

physiologic compensatory mechanisms 

(e.g., increased blood flow and oxygen 

extraction) for CO-induced tissue 

hypoxia and are unlikely to be at 

increased risk of CO-induced effects 

(ISA, p. 2–10).7 In addition, the high 

oxygen consumption of the heart, 

together with the inability to 

compensate for tissue hypoxia, makes 

the cardiac muscle of a person suffering 

from CAD a critical target for CO. 

Thus, the current evidence continues 

to support the identification of people 

with cardiovascular disease as 

susceptible to CO-induced health effects 

(ISA, 2–12) and those having CAD as the 

population with the best-characterized 

susceptibility (ISA, sections 5.7.1.1 and 

5.7.8).8 An important susceptibility 

consideration for this population is the 

inability to compensate for CO-induced 

hypoxia since individuals with CAD 

have an already compromised 

cardiovascular system. This population 

includes those with angina pectoris 

(cardiac chest pain), those who have 

experienced a heart attack, and those 

with silent ischemia or undiagnosed 

ischemic heart disease (AHA, 2003). 

People with other cardiovascular 

diseases, particularly heart diseases, are 

also at risk of CO-induced health effects. 

Cardiovascular disease comprises 

many types of medical disorders, 

including heart disease, cerebrovascular 

disease (e.g., stroke), hypertension (high 

blood pressure), and peripheral vascular 

diseases. Heart disease, in turn, 

comprises several types of disorders, 

including ischemic heart disease 

(coronary heart disease [CHD] or CAD, 

myocardial infarction, angina), 

congestive heart failure, and 

disturbances in cardiac rhythm (2000 

AQCD, section 7.7.2.1).9 Other types of 

cardiovascular disease may also 

contribute to increased susceptibility to 

the adverse effects of low levels of CO 

(ISA, section 5.7.1.1). For example, 

evidence with regard to other types of 

cardiovascular disease such as 

congestive heart failure, arrhythmia, and 

non-specific cardiovascular disease, and 

more limited evidence for peripheral 

vascular and cerebrovascular disease, 

indicates that ‘‘the continuous nature of 

the progression of CAD and its close 

relationship with other forms of 

cardiovascular disease suggest that a 

larger population than just those 

individuals with a prior diagnosis of 

CAD may be susceptible to health 

effects from CO exposure’’ (ISA, p. 5– 

117). 

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 


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10Statistical analyses of the data from Sheps et 

al., (1987) by Bissette et al. (1986) indicate a 

significant decrease in time to onset of angina at 

4.1% COHb if subjects that did not experience 

exercise-induced angina during air exposure are 

also included in the analyses. 

11Other 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. 

12These levels and other COHb levels described 

for this study below are based on gas 

chromatography analysis unless otherwise 

specified. Matched measurements available for COoximetry (CO-Ox) and gas chromatography (GC) in 

this study indicate CO-Ox measurements of 2.65% 

(post-exercise mean) and 3.21% (post-exposure 

mean) corresponding to the GC measurement levels 

of 2.00% (post-exercise mean) to 2.38% (postexposure mean) for the lower exposure level 

assessed in this study (Allred et al., 1991). 

13The 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). 

14As 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). COoximetry 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). 

15While 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 COOx 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. 

16Another 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. 

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 COinduced effects on the cardiovascular 

system’’ (ISA, section 5.2). Additionally, 

the use of an internal dose metric, 

COHb, adds to the strength of the 

findings in these controlled exposure 

studies. As a group, these studies 

demonstrate the role of short-term CO 

exposures in increasing the 

susceptibility of people with CAD to 

incidents of exercise-associated 

myocardial ischemia. 

Among the controlled human 

exposure studies, the ISA places 

principal emphasis on the study of CAD 

patients by Allred et al. (1989a, 1989b, 

1991) 11 (which was also considered in 

the previous review) for the following 

reasons: (1) Dose-response relationships 

were observed; (2) effects were observed 

at the lowest COHb levels tested (mean 

of 2–2.4% COHb 12 following 

experimental CO exposure), with no 

evidence of a threshold; (3) objective 

measures of myocardial ischemia (STsegment depression) 13 were assessed, as 

well as the subjective measure of 

decreased time to induction of angina; 

(4) measurements were taken both by 

CO-oximetry (CO-Ox) and by gas 

chromatography (GC), which provides a 

more accurate measurement of COHb 

blood levels 14; (5) a large number of 

study subjects were used; (6) a strict 

protocol for selection of study subjects 

was employed to include only CAD 

patients with reproducible exerciseinduced 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% (postexercise), exposures to CO resulting in 

post-exercise mean COHb 

concentrations of 2.0% and 3.9% were 

observed to decrease the exercise time 

required to induce ST-segment 

depression by 5.1% (p = 0.01) and 

12.1% (p < 0.001), respectively. These 

changes were well correlated with the 

onset of exercise-induced angina, the 

time to which was shortened by 4.2% 

(p = 0.027) and 7.1% (p = 0.002), 

respectively, for the two experimental 

CO exposures (Allred et al., 1989a, 

1989b, 1991).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 shortterm 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 myocardial 


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17As noted elsewhere, the 8-hour standard is the 

controlling standard for ambient CO concentrations. 

18When 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). 

19The use of pNEM in the prior review also (1) 

relied on a limited set of activity pattern data 

(approximately 3,600 person-days), (2) used four 

broadly defined categories to estimate breathing 

rates, and (3) implemented a geodesic distance 

range methodology to approximate workplace 

commutes (Johnson et al., 1992; USEPA, 1992). 

Each of these approaches used by pNEM, while 

appropriate given the data available at that time, 

would tend to limit the ability to accurately model 

expected variability in the population exposure and 

dose distributions. 

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). 

3. Overview of Human Exposure and 

Dose Assessment 

Our consideration of the scientific 

evidence in the current review, as at the 

time of the last review, is informed by 

results from a quantitative analysis of 

estimated population exposure and 

resultant COHb levels. This analysis 

provides estimates of the percentages of 

simulated at-risk populations expected 

to experience daily maximum COHb 

levels at or above a range of benchmark 

levels under varying air quality 

scenarios (e.g., just meeting the current 

or alternative standards), as well as 

characterizations of the kind and degree 

of uncertainties inherent in such 

estimates. The benchmark COHb levels 

were identified based on consideration 

of the evidence discussed in section 

II.A.2 above. In this section, we provide 

a short overview of key aspects of the 

assessment conducted for this review. 

The assessment is summarized more 

fully in section II.C of the proposal, 

discussed in detail in the REA and 

summarized in the PA (section 2.2.2). 

The results of the analyses as they relate 

to considerations of the adequacy of the 

current standards are discussed in 

section II.B.3 below. 

As noted in the proposal notice, 

people can be exposed to CO in ambient 

air when they are outdoors and also 

when they are in indoor locations into 

which ambient (outdoor) air has 

infiltrated (ISA, sections 3.6.1 and 

3.6.5). Indoor locations may also contain 

CO from indoor sources, such as gas 

stoves and tobacco smoke. Where 

present, these indoor sources can be 

important contributors to total CO 

exposure and can contribute to much 

greater CO exposures and associated 

COHb levels than those associated with 

ambient sources (ISA, section 3.6.5.2). 

For example, indoor source-related 

exposures, such as faulty furnaces or 

other combustion appliances, have been 

estimated in the past to lead to COHb 

levels on the order of twice as high as 

short-term elevations in ambient CO 

that were more likely to be encountered 

by the general public (2000 AQCD, 

p. 7–4). Further, some exposure/dose 

assessments performed for previous 

reviews have included modeling 

simulations both without and with 

indoor (nonambient) sources (gas stoves 

and tobacco smoke) to provide context 

for the assessment of ambient CO 

exposure and dose (e.g., USEPA, 1992; 

Johnson et al., 2000), and these 

assessments have found that 

nonambient sources have a substantially 

greater impact on the highest total 

exposures and COHb levels experienced 

by the simulated population than do 

ambient sources (Johnson et al., 2000; 

REA, sections 1.2 and 6.3). While 

recognizing this potential for indoor 

sources, where present, to play a role in 

CO exposures and COHb levels, the 

exposure modeling in the current 

review (described below) did not 

include indoor CO sources in order to 

focus on the impact of ambient CO on 

population COHb levels. 

The assessment estimated ambient CO 

exposure and associated COHb levels in 

simulated at-risk populations in two 

urban study areas in Denver and Los 

Angeles, in which current ambient CO 

concentrations are below the current 

standards. Estimates were developed for 

exposures to ambient CO associated 

with current ‘‘as is’’ conditions (2006 air 

quality) and also for higher ambient CO 

concentrations associated with air 

quality conditions simulated to just 

meet the current 8-hour standard,17 as 

well as for air quality conditions 

simulated to just meet several potential 

alternative standards. Although we 

consider it unlikely that air 

concentrations in many urban areas 

across the U.S. that are currently well 

below the current standards would 

increase to just meet the 8-hour 

standard, we recognize the potential for 

CO concentrations in some areas 

currently below the standard to increase 

to just meet the standard. We 

additionally recognize that this 

simulation can provide useful 

information in evaluating the current 

standard, although we recognize the 

uncertainty associated with simulating 

this hypothetical profile of higher CO 

concentrations that just meet the current 

8-hour standard. 

The exposure and dose modeling for 

the assessment, presented in detail in 

the REA, relied on version 4.3 of EPA’s 

Air Pollutant Exposure model 

(APEX4.3), which estimates human 

exposure using a stochastic, event-based 

microenvironmental approach (REA, 

chapter 4). The review of the CO 

standards completed in 1994 relied on 

population exposure and dose estimates 

generated from the probabilistic NAAQS 

exposure model (pNEM), a model that, 

among other differences from the 

current modeling approach with 

APEX4.3, employed a cohort-based 

approach (Johnson et al., 1992; USEPA, 

1992).18 19 Each of the model 

developments since the use of pNEM in 

that review have been designed to allow 

APEX to better represent human 

behavior, human physiology, and 


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20APEX4.3 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). 

21As 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). 

22The 8 microenvironments modeled in the REA 

comprised a range of indoor and outdoor locations 

including residences as well as motor vehiclerelated locations such as inside vehicles, and public 

parking and fueling facilities, where the highest 

exposures were estimated (REA, sections 5.9 and 

6.1). 

23As 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-ofhour 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. 

24APEX4.3 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. 

microenvironmental concentrations and 

to more accurately estimate variability 

in CO exposures and COHb levels (REA, 

chapter 4).20

As used in the current assessment, 

APEX probabilistically generates a 

sample of hypothetical individuals from 

an actual population database and 

simulates each individual’s movements 

through time and space (e.g., indoors at 

home, inside vehicles) to estimate his or 

her exposure to ambient CO (REA, 

chapter 4). Based on exposure 

concentrations, minute-by-minute 

activity levels, and physiological 

characteristics of the simulated 

individuals (see REA, chapters 4 and 5), 

APEX estimates the level of COHb in the 

blood for each individual at the end of 

each hour based on a nonlinear solution 

to the Coburn-Forster-Kane equation 

(REA, section 4.4.7). 

As discussed in section II.A.2.b above, 

people with cardiovascular disease are 

the population of primary focus in this 

review, and, more specifically, coronary 

artery disease, also known as coronary 

heart disease, is the ‘‘most important 

susceptibility characteristic for 

increased risk due to CO exposure’’ 

(ISA, p. 2–11). Controlled human 

exposure studies have provided 

quantitative COHb dose-response 

information for this specific population 

with regard to effects on markers of 

myocardial ischemia. Accordingly, 

based on the current evidence with 

regard to quantitative information of 

COHb levels and association with 

specific health effects, the at-risk 

populations simulated in the 

quantitative assessment were (1) adults 

with CHD (also known as IHD or CAD), 

both diagnosed and undiagnosed, and 

(2) adults with any heart diseases, 

including undiagnosed ischemia.21

Evidence characterizing the nature of 

specific health effects of CO in other 

populations is limited and does not 

include specific COHb levels related to 

health effects in those groups. As a 

result, the quantitative assessment does 

not develop separate quantitative dose 

estimates for populations other than 

those with CHD or HD. 

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 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 

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 


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25The 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). 

percentiles of the distribution of 

population exposures and COHb dose 

levels relative to that of other portions 

of the respective distribution. When 

considering the overall quality of the 

current exposure modeling approach, 

the algorithms, and the input data used, 

alongside the identified limitations and 

uncertainties, the REA and Policy 

Assessment conclude that the 

quantitative assessment provides 

reasonable estimates of CO exposure 

and COHb dose for the simulated 

population the assessment is intended 

to represent (i.e., the population 

residing within the urban core of each 

study area). The Policy Assessment 

additionally notes the impact on the 

REA dose estimates for ambient CO 

contribution to COHb of the lack of 

nonambient sources in the model 

simulations. This aspect of the 

assessment design may contribute to 

higher estimates of the contribution of 

short-duration ambient CO exposures to 

total COHb than would result from 

simulations that include the range of 

commonly encountered CO sources 

beyond just those contributing to 

ambient air CO concentrations. 

Although the specific quantitative 

impact of this on estimates of 

population percentages discussed in 

this document is unknown, 

consideration of COHb estimates from 

the 2000 assessment indicates a 

potential for the inclusion of 

nonambient sources to appreciably 

affect absolute COHb (REA, section 6.3) 

and accordingly implies the potential, 

where present, for an impact on overall 

ambient contribution to a person’s 

COHb level. Key results of the exposure 

and dose analyses were presented in the 

Policy Assessment and summarized in 

the proposal (Tables 1 and 2 of the 

proposal). 

B. Adequacy of the Current Primary 

Standards 

In considering the evidence and 

quantitative exposure and dose 

estimates with regard to judgments on 

the adequacy afforded by the current 

standards, the final decision is largely a 

public health policy judgment. A final 

decision must draw upon scientific 

information and analyses about health 

effects and risks, as well as judgments 

about how to consider the range and 

magnitude of uncertainties that are 

inherent in the scientific evidence and 

analyses. Our approach to informing 

these judgments is based on the 

recognition that the available health 

effects evidence generally reflects a 

continuum, consisting of ambient levels 

at which scientists generally agree that 

health effects are likely to occur, 

through lower levels at which the 

likelihood and magnitude of the 

response become increasingly uncertain. 

This approach is consistent with the 

requirements of the NAAQS provisions 

of the Act and with how EPA and the 

courts have historically interpreted the 

Act. These provisions require the 

Administrator to establish primary 

standards that, in the Administrator’s 

judgment, are requisite to protect public 

health with an adequate margin of 

safety. In so doing, the Administrator 

seeks to establish standards that are 

neither more nor less stringent than 

necessary for this purpose. The Act does 

not require that primary standards be set 

at a zero-risk level, but rather at a level 

that avoids unacceptable risks to public 

health, including the health of sensitive 

groups.25

In evaluating whether it is appropriate 

to revise the current CO standards, the 

Administrator’s considerations build on 

the general approach used in the last 

review and reflect the broader body of 

evidence and information now 

available. The approach used is based 

on an integration of information on 

health effects associated with exposure 

to ambient CO; expert judgment on the 

adversity of such effects on individuals; 

and policy judgments as to when the 

standards are requisite to protect public 

health with an adequate margin of 

safety, which are informed by air quality 

and related analyses, quantitative 

exposure and risk assessments when 

possible, and qualitative assessment of 

impacts that could not be quantified. 

The Administrator has taken into 

account both evidence-based and 

quantitative exposure- and risk-based 

considerations in developing 

conclusions on the adequacy of the 

current primary CO standards. 

The Administrator’s proposed 

conclusions on the adequacy of the 

current primary standards are 

summarized below (section II.B.1), 

followed by consideration of comments 

received on the proposal (section II.B.2) 

and the Administrator’s final decision 

with regard to the adequacy of the 

current primary standards (II.B.3). 

1. Rationale for Proposed Decision 

At the time of the proposal, in 

considering the adequacy of the current 

standards, the Administrator carefully 

considered the available evidence and 

conclusions contained in the Integrated 

Science Assessment; the information, 

exposure/dose assessment, rationale and 

conclusions presented in the Policy 

Assessment; the advice and 

recommendations from CASAC; and 

public comments as of that date. In so 

doing, the Administrator noted the 

following: (1) The long-standing 

evidence base concerning effects 

associated with exposure to CO, 

including the key role played by 

hypoxia (reduced oxygen availability) 

induced by increased COHb blood 

levels, and the use of COHb as the 

bioindicator and dose metric for 

evaluating CO exposure and the 

potential for health effects; (2) the strong 

evidence of cardiovascular effects of 

short-term CO exposures including the 

evidence from controlled human 

exposure studies that demonstrate a 

reduction in time to onset of exerciseinduced markers of myocardial 

ischemia in response to increased 

COHb, and the health significance of 

responses observed at the 2% COHb 

level induced by 1-hour CO exposure, as 

compared to higher COHb levels; and 

(3) the identification of people with 

cardiovascular disease as a key 

population at risk from short-term 

ambient CO exposures. In the proposal, 

as at the time of the last review, the 

Administrator additionally considered 

and took particular note of the exposure 

and dose modeling results, recognizing 

key limitations and uncertainties, and in 

light of judgments noted above 

regarding the health significance of 

findings from the controlled human 

exposure studies, placing less weight on 

the health significance of infrequent or 

rare occurrences of COHb levels at or 

just above 2% and more weight to the 

significance of repeated such 

occurrences, as well as occurrences of 

higher COHb levels. 

The Administrator also considered 

the newly available and much-expanded 

epidemiological evidence, including the 

complexity associated with quantitative 

interpretation of these studies with 

regard to CO, particularly the few 

studies available in areas where the 

current standards are met. Further, the 

Administrator considered the advice of 

CASAC, including their overall 

agreement with the Policy Assessment 

conclusion that the current evidence 

and quantitative exposure and dose 

estimates provide support for retaining 

the current standards, their view that, in 


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light of the epidemiological studies, 

revisions to lower the standards should 

be considered and their preference for a 

lower standard, and also their advice 

regarding the complications associated 

with interpreting the epidemiological 

studies for CO. Although CASAC 

expressed a preference for a lower 

standard, CASAC also indicated that the 

current evidence provides support for 

retaining the current suite of standards 

and CASAC’s recommendations appear 

to recognize that their preference for a 

lower standard was contingent on a 

judgment as to the weight to be placed 

on the epidemiological evidence. For 

the reasons explained in the proposal, 

after full consideration of CASAC’s 

advice and the epidemiological 

evidence, as well as its associated 

uncertainties and limitations, the 

Administrator proposed to judge those 

uncertainties and limitations to be too 

great for the epidemiological evidence 

to provide a basis for revising the 

current standards. 

Taking all these considerations 

together, the Administrator proposed to 

conclude that the current suite of 

standards provides a very high degree of 

protection for the COHb levels and 

associated health effects of concern, as 

indicated by the extremely low 

estimates of occurrences, and provides 

slightly less but a still high degree of 

protection for the effects associated with 

lower COHb levels, the physiological 

significance of which is less clear. The 

Administrator additionally proposed to 

conclude that consideration of the 

epidemiological studies does not lead 

her to identify a need for any greater 

protection. Thus, the Administrator 

proposed to conclude that the current 

suite of standards provides an adequate 

margin of safety against adverse effects 

associated with short-term ambient CO 

exposures. For these and all of the 

reasons discussed above, and 

recognizing the CASAC conclusion that, 

overall, the current evidence and REA 

results provide support for retaining the 

current standards, the Administrator 

proposed to conclude that the current 

suite of primary CO standards is 

requisite to protect public health with 

an adequate margin of safety from 

effects of ambient CO. 

2. Comments on Adequacy 

In considering comments on the 

adequacy of the current standards, the 

Administrator first notes the advice and 

recommendations from CASAC. In the 

context of CASAC’s review of the 

documents prepared during the course 

of the review, CASAC sent EPA five 

letters providing advice regarding 

assessment and interpretation of the 

available scientific evidence and the 

REA for the purposes of judging the 

adequacy of the current CO standards 

(Brain and Samet, 2009; Brain and 

Samet, 2010a; Brain and Samet, 2010b; 

Brain and Samet, 2010c; Brain and 

Samet, 2010d). In conveying comments 

on the draft Policy Assessment, CASAC 

agreed with the conclusion that the 

current evidence provides support for 

retaining the current suite of standards, 

while they also expressed a preference 

for a lower standard and stated that the 

epidemiological evidence could indicate 

the occurrence of adverse health effects 

at levels of the standards (Brain and 

Samet, 2010c). With regard to the 

interpretation of epidemiological 

studies on CO, CASAC’s collective 

advice included recommendations 

regarding the weight to be placed on the 

epidemiological evidence (Brain and 

Samet, 2010c), as well as cautionary 

statements regarding interpretation of 

the epidemiological studies. Such 

statements included the observation that 

‘‘[d]istinguishing the effects of CO per se 

from the consequences of CO as a 

marker of pollution or vehicular traffic 

is a challenge, which [the ISA] needs to 

confront as thoroughly as possible’’ 

(Brain and Samet, 2009, p. 2). In another 

letter CASAC further cautioned (Brain 

and Samet, 2010d, p. 2): 

The problem of co-pollutants serving as 

potential confounders is particularly 

problematic for CO. Since exposure levels for 

CO are now low, consideration needs to be 

given to the possibility that in some 

situations CO may be a surrogate for 

exposure to a mix of pollutants generated by 

fossil fuel combustion. A better 

understanding of the possible role of copollutants 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 implementationrelated factors that are not a permissible 

basis for considering the need to revise 

the current standards, are addressed in 

the Response to Comments document. 

The public comments received on the 

proposal were divided with regard to 

support for the Agency’s proposed 

conclusion as to the adequacy of the 

current standards. All of the state and 

local environmental agencies or 

governments that provided comments 

on the standards concurred with EPA’s 

proposed conclusions as did the three 

industry commenters. All of these 

commenters generally noted their 

agreement with the rationale provided 

in the proposal, with some additionally 

citing CASAC’s recognition of support 

in the evidence for the adequacy of the 

current standards. Some of these 

commenters noted agreement with the 

weight given to the epidemiological 

studies in the proposal and also noted 

the little change in exposure/risk 

estimates since the time of the last 

review. One commenter additionally 

stated their view that the REA overstates 

the exposure and risk associated with 

the current standards. 

As described in section II.B.3 below, 

the EPA generally agrees with these 

commenters regarding the adequacy of 

the current CO standards and with 

CASAC that the evidence provides 

support for the conclusion that the 

current CO standards protect public 

health with an adequate margin of 

safety. EPA additionally has given 

consideration to CASAC’s advice 

regarding interpretation of 

epidemiological evidence for CO, 

recognizing the limitations associated 

with its use in drawing quantitative 

interpretations regarding levels of 

ambient CO related to health outcomes. 

Two submissions recommending 

revision of the standards were received 

from national environmental or public 

health organizations. Additional 

submissions recommending revision 

were received from a private consultant; 

a group of scientists, physicians, and 

others; and a group of private citizens. 

In support of their position, these 

commenters variously cited CASAC 

comments regarding emphasis to give 

epidemiological studies and CASAC’s 

stated preference for a lower standard. 

These submissions generally disagreed 

with EPA’s consideration of the 

epidemiological evidence in the 

proposal and recommended that EPA 

give greater emphasis to 

epidemiological studies of a range of 

endpoints, including developmental 

and respiratory effects, based on the 

commenters’ view that the 

epidemiological studies provided 

evidence of harm associated with 

ambient CO levels below the current 


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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). 

standards and inadequate protection for 

sensitive populations. Among these 

submissions, those that specified levels 

for revised standards recommended 

levels that were no higher than the 

lowest part of the ranges for the two 

standards that were identified for 

consideration in the Policy Assessment 

and the example options that CASAC 

suggested for inclusion in the Policy 

Assessment. Additionally, one 

commenter described the view that the 

CO standards should be revised to levels 

at or below the range of CO 

concentrations in exhaled breath of 

healthy non-smokers. 

EPA generally disagrees with these 

commenters regarding conclusions that 

can be drawn from the evidence, 

including the epidemiological studies, 

pertaining to the adequacy of the 

current CO standards. In considering the 

adequacy of the current standards, it is 

important to consider both the extent to 

which the evidence supports a causal 

relationship between ambient CO 

exposures and adverse health effects, as 

well as the extent to which there is 

evidence pertinent to such effects under 

air quality conditions in which the 

current standards are met. With regard 

to the latter point, and focusing on the 

epidemiological evidence, it is the 

studies involving air quality conditions 

in which the current standards were met 

that are most informative in evaluating 

the adequacy of the standards (PA, 

p. 2–30). We note that very few of the 

epidemiological studies observing an 

association of cardiovascular diseaserelated outcomes with short-term CO 

concentrations (or those observing 

associations for other health effects) 

were conducted in areas that met the 

current standards throughout the period 

of study, thus limiting their usefulness 

with regard to judging the adequacy of 

the current standards (PA, pp. 2–33, 

2–36). 

Further, as CASAC has cautioned, 

‘‘the problem of co-pollutants serving as 

potential confounders is particularly 

problematic for CO’’ (Brain and Samet, 

2010d). While some CO epidemiological 

studies have applied the commonly 

used statistical method, two-pollutant 

regression models, to inform 

conclusions regarding CO as the 

pollutant eliciting the effects in these 

studies, and while, in some studies, the 

CO associations remain robust after 

adjustment for another traffic 

combustion-related pollutant, such as 

PM2.5 or nitrogen dioxide (NO2) (PA, pp. 

2–36 to 2–37), the potential exists for 

there to be etiologically relevant 

pollutants that are correlated with CO 

yet absent from the analysis, 

particularly given the many pollutants 

associated with fossil fuel combustion. 

The CASAC specifically recognized this 

potential in stating that ‘‘consideration 

needs to be given to the possibility that 

in some situations CO may be a 

surrogate for exposure to a mix of 

pollutants generated by fossil fuel 

combustion’’ and ‘‘a better 

understanding of the possible role of copollutants is relevant to * * * the 

interpretation of epidemiologic studies 

on the health effects of CO’’ (Brain and 

Samet, 2010d). 

In light of these issues related to 

potential confounding by co-pollutants 

in the case of CO, uncertainty related to 

exposure error for CO is of particular 

concern in quantitatively interpreting 

the epidemiological evidence (e.g., with 

regard to ambient concentrations 

contributing to health outcomes).26 As 

noted above, CASAC cautioned the 

Agency on the importance of 

understanding the extent of exposure 

error in evaluating the epidemiological 

evidence for CO (Brain and Samet, 

2009). There are two aspects to the 

epidemiological studies in the specific 

case of CO (as contrasted with other 

pollutants such as PM and NO2) that 

may contribute exposure error in the 

studies (PA, pp. 2–34 to 2–38; 76 FR 

8177–8178). The first relates to the 

uncertainty associated with quantitative 

interpretation of the epidemiological 

study results at low ambient 

concentrations in light of the sizeable 

portion of ambient CO measurements 

that are at or below monitor method 

detection limits (MDLs). As described in 

the proposal, uncertainty related to the 

prevalence of ambient CO monitor 

concentrations at or below MDLs is a 

greater concern for the more recently 

available epidemiological studies in 

which the study areas have much 

reduced ambient CO concentrations 

compared with those in the past (PA, 

pp. 2–37 to 2–38). This complicates our 

interpretation of specific ambient CO 

concentrations associated with health 

effects (ISA, p. 3–91; Brain and Samet, 

2010d), providing us with reduced 

confidence in quantitative 

interpretations of epidemiological 

studies for CO. Additionally, as 

described in the proposal, there is 

uncertainty and potential error 

associated with exposure estimates in 

the CO epidemiological studies that 

relate to the use of area-wide or centralsite monitor CO concentrations in light 

of information about the steep gradient 

in CO concentrations with distance from 

source locations such as highlytrafficked 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 shortterm 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 doseresponse relationship over a range of 

COHb concentrations relevant to 

consideration of the NAAQS (ISA, p. 2– 

13). In considering the epidemiological 

evidence for relevant cardiovascular 

outcomes, which includes multiple 

studies reporting associations with 

ambient CO concentrations under 

conditions when the current standards 

were not met (PA, p. 2–30), the ISA 

notes that these studies are coherent 

with the findings from the controlled 

human exposure studies (ISA, p. 2–17). 

However, as summarized here, various 

aspects of the evidence complicate 

quantitative interpretation of it with 

regard to ambient concentrations that 

might be eliciting the reported health 

outcomes. 

An additional complication to our 

consideration of the CO epidemiological 

evidence is that, in contrast to the 

health effects evidence for all other 

criteria pollutants, the epidemiological 

studies for CO use a different exposure/ 

dose metric from that which is the focus 

of the broader health evidence base, and 


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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. 

28As 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. 

additional information that might be 

used to bridge this gap is lacking. In the 

case of CO, the epidemiological studies 

use air concentration as the exposure/ 

dose metric, while much of the broader 

health effects evidence for CO, and 

particularly that related to 

cardiovascular effects, demonstrates and 

focuses on an internal biomarker of CO 

exposure (COHb) which has been 

considered a critical key to CO 

toxicity.27 The strong evidence 

describing the role of COHb in CO 

toxicity is important to consider in 

interpreting the CO epidemiological 

studies and contributes to the biological 

plausibility of the ischemia-related 

health outcomes that have been 

associated with ambient CO 

concentrations. Yet, we do not have 

information on the COHb levels of 

epidemiological study subjects that we 

can evaluate in the context of the COHb 

levels eliciting health effects in the 

controlled human exposure studies. 

Further, we lack additional information 

on the CO exposures of the 

epidemiological study subjects to both 

ambient and nonambient sources of CO 

that might be used to estimate their 

COHb levels and bridge the gap between 

the two study types. Additionally the 

ISA recognizes that the changes in 

COHb that would likely be associated 

with exposure to the low ambient CO 

concentrations assessed in some of the 

epidemiological studies would be 

smaller than changes associated with 

‘‘substantially reduced [oxygen] 

delivery to tissues,’’ that might 

plausibly lead to the outcomes observed 

in those studies, with additional 

investigation needed to determine 

whether there may be another 

mechanism of action for CO that 

contributes to the observed outcomes at 

low ambient concentrations (ISA, 

p. 5–48). Thus, there are uncertainties 

associated with the epidemiological 

evidence that ‘‘complicate the 

quantitative interpretation of the 

epidemiologic findings, particularly 

regarding the biological plausibility of 

health effects occurring at COHb levels 

resulting from exposures to the ambient 

CO concentrations’’ assessed in these 

studies (ISA, p. 2–17). 

With regard to health effects other 

than cardiovascular outcomes, in 

addition to noting the complications 

cited above with regard to quantitative 

interpretation of the epidemiological 

evidence, we note that the evidence for 

these other categories of health effects is 

considered limited and only suggestive 

of a causal relationship with relevant 

exposures to CO in ambient air, or 

inadequate to infer such a relationship, 

or it supports the conclusion that such 

a relationship is not likely (see section 

II.A.2.b above). As described in the 

proposal sections II.B.2 and II.D.2.a, 

with regard to categories of health 

effects or outcomes for which the 

evidence is considered suggestive, 

evidence is lacking that might lend 

biological plausibility to 

epidemiological study results, and also 

sufficiently rule out the role of chance, 

bias and confounding in the 

epidemiological associations observed, 

for outcomes such as developmental or 

respiratory (ISA, chapters 1 and 2). 

Thus, EPA disagrees with the 

commenters’ conclusion that the 

epidemiological evidence establishes 

that a range of health effects, including 

developmental or respiratory effects, are 

occurring as a result of exposures to CO 

in ambient air at or below the current 

standards. We additionally disagree 

with commenters’ statements that imply 

EPA has inadequately considered the 

evidence with regard to protection of 

sensitive populations and to the 

protection provided by the CO 

standards. As noted in section II.A.2.b 

above, EPA’s assessment of the current 

evidence presented in the Integrated 

Science Assessment concludes that ‘‘the 

most important susceptibility 

characteristic for increased risk due to 

CO exposure is [CAD or CHD]’’ (ISA, 

p. 2–10). Accordingly, the proposal 

recognized people with cardiovascular 

disease as a key population at risk from 

short-term ambient CO exposures 

(proposal, section II.D.4). However, 

based on assessment of the evidence in 

the ISA, the proposal and other 

documents in this review also recognize 

the potential for susceptibility for 

several other populations and lifestages, 

including people with pre-existing 

diseases that may already have limited 

oxygen availability to tissues, increased 

COHb levels or increased endogenous 

CO production, older adults, and fetuses 

during critical phases of development 

(as summarized in section II.A.2.b 

above). For these groups and lifestages, 

the evidence is incomplete with regard 

to specific CO exposures or COHb levels 

that may be associated with health 

effects in these groups and the nature of 

those effects, as well as a way to relate 

the specific evidence available for the 

CAD population to the limited evidence 

for these other populations. Further, the 

currently available evidence does not 

indicate a greater susceptibility for any 

of the other populations or lifestages 

recognized as potentially at risk from 

exposure to ambient CO. In reaching a 

decision on the adequacy of the current 

standards in protecting public health in 

section II.B.3 below, however, the 

Administrator has considered EPA’s 

conclusions with regard to the effects 

likely to be causally associated with 

exposure to ambient CO and population 

groups particularly at risk, as well as 

those regarding the evidence with 

regard to the potential for other effects 

and sensitive groups, and the associated 

uncertainty. In so doing, as indicated 

below, the Administrator judges the 

current standards to provide the 

requisite protection for public health, 

including the health of sensitive 

populations, with an adequate margin of 

safety. 

3. Conclusions Concerning Adequacy of 

the Primary Standards 

Having carefully considered the 

public comments, as discussed above, 

the Administrator believes that the 

fundamental scientific conclusions on 

the effects of CO in ambient air reached 

in the Integrated Science Assessment 

and Policy Assessment, summarized in 

sections II.B and II.D of the proposal 

remain valid. Additionally, the 

Administrator believes the judgments 

she reached in the proposal (section 

II.D.4) with regard to consideration of 

the evidence and quantitative exposure/ 

dose assessments and advice from 

CASAC remain appropriate. Thus, as 

described below, the Administrator 

concludes that the current primary 

standards provide the requisite 

protection of public health with an 

adequate margin of safety and should be 

retained.28

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 


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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 shortterm CO exposures, and recognizes the 

greater significance accorded both to 

larger reductions in time to myocardial 

ischemia and to more frequent 

occurrences of myocardial ischemia. As 

at the time of the review completed in 

1994, the Administrator also takes note 

of the results for the modeling of 

exposures to ambient CO under 

conditions simulated to just meet the 

current, controlling, 8-hour standard in 

two study areas, as described in the REA 

and Policy Assessment, and the public 

health significance of those results. She 

also considers the newly available and 

much-expanded epidemiological 

evidence, including the complexity 

associated with quantitative 

interpretation of these studies, 

particularly the few studies available in 

areas where the current standards are 

met. In so doing, she notes that in 

considering the adequacy of the current 

standards, it is important to consider 

both the extent to which the evidence 

supports a causal relationship between 

ambient CO exposures and adverse 

health effects, as well as the extent to 

which there is evidence pertinent to 

such effects under air quality conditions 

in which the current standards are met. 

Further, the Administrator considers the 

advice of CASAC, including both their 

overall agreement with the Policy 

Assessment conclusion that the current 

evidence and quantitative exposure and 

dose estimates provide support for 

retaining the current standards, as well 

as their view that in light of the 

epidemiological studies, revisions to 

lower the standards should be 

considered and their preference for a 

lower standard. 

As an initial matter, the Administrator 

places weight on the long-standing 

evidence base that has established key 

aspects of CO toxicity that are relevant 

to this review as they were to the review 

completed in 1994. These aspects 

include the key role played by hypoxia 

(reduced oxygen availability) induced 

by increased COHb blood levels, the 

identification of people with 

cardiovascular disease as a key 

population at risk from short-term 

ambient CO exposures, and the use of 

COHb as the bioindicator and dose 

metric for evaluating CO exposure and 

the potential for health effects. The 

Administrator also recognizes the 

Integrated Science Assessment’s 

conclusion that a causal relationship is 

likely to exist between relevant shortterm exposures to CO and 

cardiovascular morbidity. 

In placing weight on the controlled 

human exposure studies, the 

Administrator also recognizes the 

uncertain health significance associated 

with the smaller responses to the lowest 

COHb level assessed in the study given 

primary consideration in this review 

(Allred et al., 1989a, 1989b, 1991) and 

with single occurrences of such 

responses. In the study by Allred et al. 

(1989a, 1989b, 1991), a 4–5% reduction 

in time (approximately 30 seconds) to 

the onset of exercise-induced markers of 

myocardial ischemia was associated 

with the 2% COHb level induced by 

1-hour CO exposure. In considering the 

significance of the magnitude of the 

time decrement to onset of myocardial 

ischemia observed at the 2% COHb 

level induced by short-term CO 

exposure, as well as the potential for 

myocardial ischemia to lead to more 

adverse outcomes, the EPA generally 

places less weight on the health 

significance associated with infrequent 

or rare occurrences of COHb levels at or 

just above 2% as compared to that 

associated with repeated occurrences 

and occurrences of appreciably higher 

COHb levels in response to short-term 

CO exposures. For example, at the 4% 

COHb level, the study by Allred et al., 

(1989a, 1989b, 1991) observed a 7–12% 

reduction in time to the onset of 

exercise-induced markers of myocardial 

ischemia. The Administrator places 

more weight on this greater reduction in 

time to onset of exercise-induced 

markers compared to the reduction in 

time to onset at 2% COHb. The 

Administrator also notes that at the time 

of the 1994 review, an intermediate 

level of approximately 3% COHb was 

identified as a level at which adverse 

effects had been demonstrated in 

persons with angina. Now, as at the time 

of the 1994 review, the Administrator 

primarily considers the 2% COHb level, 

resulting from 1-hour CO exposure, in 

the context of a margin of safety against 

effects of concern that have been 

associated with higher COHb levels, 

such as 3–4% COHb. 

The Administrator additionally takes 

note of the now much-expanded 

evidence base of epidemiological 

studies, including the multiple studies 

that observe positive associations 

between cardiovascular outcomes and 

short-term ambient CO concentrations 

across a range of CO concentrations, 

including conditions above as well as 

below the current NAAQS. She notes 

particularly the Integrated Science 

Assessment conclusion that the findings 

of CO-associated cardiovascular effects 

in these studies are logically coherent 

with the larger, long-standing health 

effects evidence base for CO and the 

conclusions drawn from it regarding 

cardiovascular disease-related 

susceptibility. In further considering the 

epidemiological evidence base with 

regard to the extent to which it provides 

support for conclusions regarding 

adequacy of the current standards, the 

Administrator takes note of CASAC’s 

conclusions that ‘‘[i]f the 

epidemiological evidence is given 

additional weight, the conclusion could 

be drawn that health effects are 

occurring at levels below the current 

standard, which would support the 

tightening of the current standard’’ 

(Brain and Samet, 2010c). Additionally, 

the Administrator places weight on the 

final Policy Assessment consideration of 

aspects that complicate quantitative 

interpretation of the epidemiological 

studies with regard to ambient 

concentrations that might be eliciting 

the reported health outcomes. 

For purposes of evaluating the 

adequacy of the current standards, the 

Administrator takes note of the multiple 

complicating features of the 

epidemiological evidence base, as 

described in more detail in the final 

Policy Assessment and in section 

II.D.2.a of the proposal. First, while a 

number of studies observed positive 

associations of cardiovascular diseaserelated 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 


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indoor sources of CO (Brain and Samet, 

2010c). As discussed in section II.D.2.a 

of the proposal, the interpretation of 

epidemiological studies for CO is 

further complicated because, in contrast 

to the situation for all other criteria 

pollutants, the epidemiological studies 

for CO use an exposure/dose metric (air 

concentration) that differs from the 

metric commonly used in the other key 

CO health studies (COHb). 

The Administrator notes that although 

CASAC expressed a preference for a 

lower standard, CASAC also indicated 

that the current evidence provides 

support for retaining the current suite of 

standards. CASAC’s recommendations 

appear to recognize that their preference 

for a lower standard was contingent on 

a judgment as to the weight to be placed 

on the epidemiological evidence. 

Further, as noted above and 

summarized in section II.C.2, CASAC 

has provided a range of advice regarding 

interpretation of the CO epidemiological 

studies in light of the associated 

uncertainties. Accordingly, in 

consideration of the current evidence 

with regard to conclusions to be drawn 

as to the adequacy of the current 

standards, the Administrator gives 

consideration to the full breadth of 

CASAC’s advice. 

In considering the evidence and 

quantitative exposure and dose 

estimates available in this review with 

regard to the adequacy of public health 

protection provided by the current 

primary standards, the Administrator 

recognizes that, as noted in section II.B. 

above, the final decision on such 

judgments is largely a public health 

policy judgment, which draws upon 

scientific information and analyses 

about health effects and risks, as well as 

judgments about how to consider the 

range and magnitude of uncertainties 

that are inherent in the information and 

analyses. These judgments are informed 

by the recognition that the available 

health effects evidence generally reflects 

a continuum, consisting of ambient 

levels at which scientists generally agree 

that health effects are likely to occur, 

through lower levels at which the 

likelihood and magnitude of the 

response become increasingly uncertain. 

Accordingly, the final decision requires 

judgment based on an interpretation of 

the evidence and other information that 

neither overstates nor understates the 

strength and limitations of the evidence 

and information nor the appropriate 

inferences to be drawn. As described in 

section I.A above, the Act does not 

require that primary standards be set at 

a zero-risk level; the NAAQS must be 

sufficient but not more stringent than 

necessary to protect public health, 

including the health of sensitive groups, 

with an adequate margin of safety. 

In considering the judgments to be 

made regarding adequacy of the level of 

protection provided by the current 

standards, the Administrator takes 

particular note of the findings of the 

exposure and dose assessment in light 

of considerations discussed above 

regarding the weight given to different 

COHb levels and their frequency of 

occurrence. As described in the 

proposal, the exposure and dose 

assessment results indicate that only a 

very small percentage of the at-risk 

population is estimated to experience a 

single occurrence in a year of daily 

maximum COHb at or above 3.0% 

COHb under conditions just meeting the 

current 8-hour standard in the two 

study areas evaluated, and no multiple 

occurrences are estimated. The 

Administrator also notes the results 

indicating that only a small percentage 

of the at-risk populations are estimated 

to experience a single occurrence of 2% 

COHb in a year under conditions just 

meeting the standard, and still fewer are 

estimated to experience multiple such 

occurrences. Additionally, consistent 

with findings of the assessment 

performed for the review completed in 

1994, less than 0.1% of person-days for 

the at-risk populations were estimated 

to include occurrences of COHb at or 

above 2% COHb. Taken together, the 

Administrator judges the current 

standard to provide a very high degree 

of protection for the COHb levels and 

associated health effects of concern, as 

indicated by the extremely low 

estimates of occurrences, and to provide 

slightly less but a still high degree of 

protection for the effects associated with 

lower COHb levels, the physiological 

significance of which is less clear. 

In further considering the adequacy of 

the margin of safety provided by the 

current standards, the Administrator has 

additionally considered conclusions 

drawn in the Integrated Science 

Assessment and Policy Assessment with 

regard to interpretation of the limited 

and less certain information concerning 

a relationship between exposure to 

relevant levels of ambient CO and 

health effects in other, potentially, 

susceptible groups, and with regard to 

the uncertainties concerning 

quantitative interpretation of the 

available epidemiological studies. In so 

doing, the Administrator additionally 

judges the current standards to provide 

adequate protection against the risk of 

other health effects for which the 

evidence is less certain. Further, the 

Administrator concludes that 

consideration of the epidemiological 

studies does not lead her to identify a 

need for any greater protection. For 

these and all of the reasons discussed 

above, and recognizing the CASAC 

conclusion that, overall, the current 

evidence and REA results provide 

support for retaining the current 

standards, the Administrator concludes 

that the current suite of primary CO 

standards is requisite to protect public 

health with an adequate margin of safety 

from effects of ambient CO. 

III. Consideration of a Secondary 

Standard 

As noted in section I.A. above, section 

109(b) of the Clean Air Act requires the 

Administrator to establish secondary 

standards that, in the judgment of the 

Administrator, are requisite to protect 

the public welfare from any known or 

anticipated adverse effects associated 

with the presence of the pollutant in the 

ambient air. In so doing, the 

Administrator seeks to establish 

standards that are neither more nor less 

stringent than necessary for this 

purpose. The Act does not require that 

secondary standards be set to eliminate 

all risk of adverse welfare effects, but 

rather at a level requisite to protect 

public welfare from those effects that 

are judged by the Administrator to be 

adverse. 

This section presents the rationale for 

the Administrator’s final decision not to 

set a secondary NAAQS for CO. In 

considering the current air quality 

criteria, evidence of CO-related welfare 

effects at or near ambient levels that are 

unrelated to climate has not been 

identified. Accordingly, in considering 

whether a secondary standard is 

requisite to protect the public welfare, 

the Administrator has primarily 

considered conclusions based on the 

evidence of a role for CO in effects on 

climate. Evaluation of this evidence in 

the Integrated Science Assessment and 

staff considerations in the Policy 

Assessment highlighted the limitations 

in this evidence and provided 

information indicating that this role for 

atmospheric CO is predominantly 

indirect, through its role in chemical 

reactions in the atmosphere which 

result in increased concentrations of 

pollutants with direct contributions to 

the greenhouse effect or that deplete 

stratospheric ozone. Given the 

evaluation of the evidence, as well as 

the views of CASAC, the Administrator 

concludes that no secondary standard 

should be set at this time because, as in 

the past reviews, having no standard is 

requisite to protect public welfare from 

any known or anticipated adverse 

effects from ambient CO exposures. 

In this section, we first summarize the 

evidence currently available for welfare 


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effects to inform decisions in this 

review in section III.A. Next, the 

rationale for the proposed conclusions 

is summarized in section III.B. Public 

comments and CASAC advice regarding 

consideration of a secondary standard in 

this review are summarized in section 

III.C. Lastly, the Administrator’s final 

conclusions with regard to a secondary 

standard for CO are presented in section 

III.D. 

A. Introduction 

In evaluating whether establishment 

of a secondary standard for CO is 

appropriate at this time, we adopted an 

approach in this review that builds 

upon the general approach used in the 

last review and reflects the broader body 

of evidence and information now 

available. Consideration of the evidence 

available in this review focuses on the 

following overarching question: Does 

the currently available scientific 

information provide support for 

considering the establishment of a 

secondary standard for CO? 

In considering this overarching 

question, the Policy Assessment first 

noted that the extensive literature 

search performed for the current review 

did not identify any evidence of public 

welfare effects of CO unrelated to 

climate at or near ambient levels (ISA, 

section 1.3 and p. 1–3). However, 

ambient CO has been associated with 

welfare effects related to climate (ISA, 

section 3.3). Climate-related effects of 

CO were considered for the first time in 

the 2000 AQCD and are given somewhat 

greater focus in the current ISA relative 

to the 2000 AQCD in reflection of 

comments from CASAC and increased 

attention to the role of CO in climate 

forcing (Brain and Samet, 2009; ISA, 

section 3.3). Based on the current 

evidence, the ISA concludes that ‘‘a 

causal relationship exists between 

current atmospheric concentrations of 

CO and effects on climate’’ (ISA, section 

2.2). Accordingly, the discussion in the 

Policy Assessment (summarized in the 

proposal) focuses on climate-related 

effects of CO in addressing the question 

posed above. 

The currently available information 

summarized in the ISA (ISA section, 

3.3) does not alter the current wellestablished 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 nonclimate welfare effects, including 

ecological effects and impacts to 

vegetation, there is no currently 

available scientific information that 

supports a CO secondary standard (PA, 

section 3.4). Secondly, with respect to 

climate-related effects, the Policy 

Assessment recognized the evidence of 

climate forcing effects associated with 

CO, most predominantly through its 

participation in chemical reactions in 

the atmosphere which contribute to 

increased concentrations of other more 

direct acting climate-forcing pollutants 

(ISA, sections 2.2 and 3.3). The PA also 

noted, however, that the available 

information provides no basis for 

estimating how localized changes in the 

temporal and spatial patterns of ambient 

CO likely to occur across the U.S. with 

(or without) a secondary standard 

would affect local, regional, or 

nationwide changes in climate. 

Moreover, more than half of the indirect 

forcing effect of CO is attributable to 

ozone (O3) formation, and welfarerelated effects of O3 are more 

appropriately considered in the context 

of the review of the O3 NAAQS, rather 

than in this CO NAAQS review (PA, 

section 3.4). For these reasons, the 

Policy Assessment concluded that there 

is insufficient information at this time to 

support the consideration of a 

secondary standard based on CO effects 

on climate processes (PA, section 3.4). 

B. Rationale for Proposed Decision 

In considering a secondary standard 

for CO, the proposed conclusions 

presented in the proposal were based on 

the assessment and integrative synthesis 

of the scientific evidence presented in 

the ISA, building on the evidence 

described in the 2000 AQCD, as well as 

staff consideration of this evidence in 

the Policy Assessment and CASAC 

advice. As an initial matter, the 

proposal concluded that the currently 

available scientific information with 

respect to non-climate welfare effects, 

including ecological effects and impacts 

to vegetation, does not support a CO 

secondary standard. Secondly, with 

respect to climate-related effects, the 

proposal took note of staff 

considerations in the Policy Assessment 

and concurred with staff conclusions 

that information is insufficient at this 

time to provide support for a CO 

secondary standard. Thus, based on 

consideration of the evidence, staff 

considerations in the Policy 

Assessment, as well as the views of 

CASAC, the Administrator proposed to 

conclude that no secondary standards 

should be set at this time because, as in 

the past reviews, having no standard is 

requisite to protect public welfare from 

any known or anticipated adverse 

effects from ambient CO exposures. 

C. Comments on Consideration of 

Secondary Standard 

In considering the need for a 

secondary standard, the Administrator 

first notes the advice and 

recommendations from CASAC based 

on their review of two drafts of the 

Integrated Science Assessment and of 

the draft Policy Assessment. With 

regard to consideration of a secondary 

standard for CO, CASAC noted without 

objection or disagreement the staff’s 

conclusions that there is insufficient 

information to support consideration of 

a secondary standard at this time (Brain 

and Samet, 2010c). One public comment 

generally concerning EPA’s proposed 

decision on a secondary standard is 

addressed below. Other more specific 

public comments related to 

consideration of a secondary standard 

are addressed in the Response to 

Comments document. 

One comment (joint submission from 

Center for Biological Diversity and 

others) stated that due to the global 

influence of CO on climate, EPA must 


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establish a secondary NAAQS. The 

comment provided no information as to 

what form, level, or other elements of a 

secondary standard would be 

appropriate in light of the substantial 

uncertainties and regional variation in 

the indirect effects of CO. Rather, the 

comment asserted that there is ‘‘a 

substantial body of knowledge, as 

reviewed in the ISA, regarding CO and 

climate’’ and that ‘‘uncertainty does not 

absolve the EPA of the obligation to 

protect public welfare’’ (Center for 

Biological Diversity comments at p. 9). 

As noted by the commenter, the ISA 

reviewed the body of knowledge 

regarding CO and climate. As discussed 

above, the ISA concluded that CO has 

climate-related effects, that the direct 

effects of CO are weak, that there are 

significant uncertainties concerning the 

indirect climate effects of CO, and that 

these effects appear to be highly variable 

and dependent on localized conditions. 

Further, as noted in the Policy 

Assessment, the spatial and temporal 

variation in emissions and 

concentrations of CO and the localized 

chemical interdependencies that cause 

the indirect climate effects of CO make 

it highly problematic to evaluate the 

indirect effects of CO on climate. In 

light of the fact that the climate effects 

of CO are not only uncertain but highly 

variable and dependent on local 

conditions (e.g., concentrations of other 

pollutants), EPA believes that there is 

not adequate information available to 

conclude that a secondary standard in 

the United States is requisite to protect 

public welfare. The comment points to 

the estimated global effects of CO on 

climate, but nowhere does the comment 

provide evidence that EPA’s conclusion 

regarding adequacy of the available 

information is in error. 

EPA fully appreciates that the 

NAAQS are often established on the 

frontiers of scientific knowledge, and 

EPA continually assesses scientific 

uncertainties in judging what NAAQS 

are requisite to protect public health 

and welfare. EPA is not asserting that 

the fact that there are some uncertainties 

prevents EPA from setting a standard. 

Rather, EPA has judged that, in light of 

both the significant uncertainties and 

the evidence of the direct effects being 

weak and the indirect effects being 

highly variable and dependent on local 

conditions, particularly in light of CO’s 

short lifetime, it is not possible to 

anticipate how any secondary standard 

that would limit ambient CO 

concentrations in the United States 

would in turn affect climate and thus 

any associated welfare effects. As 

additionally discussed in section III.D 

below, EPA has reviewed the available 

information and judged the absence of 

a standard as being requisite to protect 

public welfare. 

D. Conclusions Concerning a Secondary 

Standard 

The conclusions presented here are 

based on the assessment and integrative 

synthesis of the scientific evidence 

presented in the ISA, building on the 

evidence described in the 2000 AQCD, 

as well as staff consideration of this 

evidence in the Policy Assessment and 

CASAC advice, and with consideration 

of the views of public commenters on 

the need for a secondary standard. 

In considering whether the currently 

available scientific information supports 

setting a secondary standard for CO, 

EPA takes note of the ISA and Policy 

Assessment consideration of the body of 

available evidence (briefly summarized 

above in section III.A). First, EPA 

concludes that the currently available 

scientific information with respect to 

non-climate welfare effects, including 

ecological effects and impacts to 

vegetation, does not support the need 

for a CO secondary standard. Secondly, 

with respect to climate-related effects, 

the EPA takes note of the ISA’s 

conclusions that there are significant 

uncertainties concerning the indirect 

climate effects of CO, and that these 

effects appear to be highly variable and 

dependent on localized conditions as 

well as staff considerations in the Policy 

Assessment and concurs with staff 

conclusions that information is 

insufficient at this time to support the 

need for a CO secondary standard. More 

specifically, as more fully discussed in 

consideration of public comments in 

section III.C above, EPA has judged that, 

in light of both the significant 

uncertainties and the evidence of the 

direct effects of CO on climate being 

weak and the indirect effects being 

highly variable and dependent on local 

conditions, particularly in light of CO’s 

short lifetime, it is not possible to 

anticipate how any secondary standard 

that would limit ambient CO 

concentrations in the United States 

would affect climate. Consequently, 

information that might indicate the need 

for additional protection from CO 

environmental effects and on which 

basis EPA might identify a secondary 

standard for the purposes of protecting 

against CO effects on climate processes 

is not available. 

Thus, in considering the evidence, 

staff considerations in the Policy 

Assessment summarized here, as well as 

the views of CASAC and the public, 

summarized above, the Administrator 

concludes that no secondary standards 

should be set at this time because, as in 

the past reviews, having no standard is 

requisite to protect public welfare from 

any known or anticipated adverse 

effects from ambient CO exposures. 

IV. Amendments to Ambient 

Monitoring Requirements 

The EPA is finalizing changes to 

ambient air CO monitoring methods and 

the ambient monitoring network design 

requirements to support the NAAQS for 

CO discussed above in Section II. 

Because ambient CO monitoring data 

are essential to the implementation of 

the NAAQS for CO, EPA is finalizing 

minimum monitoring requirements for 

the ambient CO monitoring network. 

State, local, and Tribal monitoring 

agencies (‘‘monitoring agencies’’) collect 

ambient CO monitoring data in 

accordance with the monitoring 

requirements contained in 40 CFR parts 

50, 53, and 58. 

A. Monitoring Methods 

This section provides background and 

rationale for the amendments that EPA 

proposed to the Federal Reference 

Method (FRM) for CO and to the 

associated performance specifications 

for automated CO analyzers. It also 

discusses the public comments on those 

proposed amendments and the few 

minor changes made to them as they are 

being promulgated today. 

The use of FRMs for the collection of 

air monitoring data provides uniform, 

reproducible measurements of pollutant 

concentrations in ambient air. Federal 

equivalent methods (FEMs) allow for 

the introduction of new or alternative 

technologies for the same purpose, 

provided these methods produce 

measurements directly comparable to 

the reference methods. EPA has 

established procedures for determining 

and designating FRMs and FEMs at 40 

CFR part 53. 

For ambient air monitoring data for 

CO to be used for determining 

compliance with the CO NAAQS, such 

data must be obtained using either an 

FRM or an FEM, as defined in 40 CFR 

parts 50 and 53. All CO monitoring 

methods in use currently by state and 

local monitoring agencies are EPAdesignated FRM analyzers. No FEM 

analyzer, i.e. one using an alternative 

measurement principle, has yet been 

designated by EPA for CO. These 

continuous FRM analyzers have been 

used in monitoring networks for many 

years and provide CO monitoring data 

adequate for determining CO NAAQS 

compliance. The current list of all 

approved FRMs capable of providing 

ambient CO data for this purpose may 

be found on the EPA Web site, http:// 

www.epa.gov/ttn/amtic/files/ambient/


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criteria/reference-equivalent-methodslist.pdf. Although both the existing CO 

FRM in 40 CFR part 50 and the FRM 

and FEM designation requirements in 

part 53 remain adequate to support the 

CO NAAQS, EPA nevertheless proposed 

editorial revisions to the CO FRM and 

both technical and editorial revisions to 

part 53, as discussed below. 

1. Proposed Changes to Parts 50 and 53 

Reference methods for criteria 

pollutants are described in several 

appendices to 40 CFR part 50; the CO 

FRM is set forth in appendix C. A nondispersive infrared photometry (NDIR) 

measurement principle is formally 

prescribed as the basis for the CO FRM. 

Appendix C describes the technical 

nature of the NDIR measurement 

principle stipulated for CO FRM 

analyzers as well as two acceptable 

calibration procedures for CO FRM 

analyzers. It further requires that an 

FRM analyzer must meet specific 

performance, performance testing, and 

other requirements set forth in 40 CFR 

part 53. 

The CO FRM was first promulgated 

on April 30, 1971 (36 FR 8186), in 

conjunction with EPA’s establishment 

(originally as 42 CFR part 410) of the 

first NAAQS for six pollutants 

(including CO) as now set forth in 40 

CFR part 50. The method was amended 

in 1982 and 1983 (47 FR 54922; 48 FR 

17355) to incorporate minor updates, 

but no substantive changes in the 

fundamental NDIR measurement 

technique have been made since its 

original promulgation. 

In connection with the current review 

of the NAAQS for CO, EPA reviewed the 

existing CO FRM to determine if it was 

still adequate or if improved or more 

suitable measurement technology has 

become available to better meet current 

FRM needs as well as potential future 

FRM requirements. EPA determined 

that no new ambient CO measurement 

technique has become available that is 

superior to the NDIR technique 

specified for the current FRM, and that 

the existing FRM continues to be well 

suited for both FRM purposes and for 

use in routine CO monitoring. No 

substantive changes were needed to the 

basic NDIR FRM measurement 

principle. Several high quality FRM 

analyzer models have been available for 

many years and continue to be offered 

and supported by multiple analyzer 

manufacturers. 

However, EPA found that the existing 

CO FRM should be improved and 

updated to clarify the language of some 

provisions, to make the format match 

more closely the format of more recently 

promulgated automated FRMs, and to 

better reflect the design and improved 

performance of current, commercially 

available CO FRM analyzers. 

Accordingly, EPA proposed appropriate, 

albeit minor, changes to the FRM. 

Because these mostly editorial changes 

were quite numerous, the entire text of 

the CO FRM was revised and reproposed. 

In close association with the proposed 

editorial revision to the CO FRM 

described above, EPA also proposed to 

update the performance requirements 

for CO FRM analyzers that are contained 

in 40 CFR part 53. These requirements 

were established in the 1970’s, based 

primarily on the NDIR CO measurement 

technology available at that time. While 

the fundamental NDIR measurement 

principle, as implemented in 

commercial FRM analyzers, has 

changed little over several decades, 

FRM analyzer performance has 

improved markedly. Contemporary 

advances in digital electronics, sensor 

technology, and manufacturing 

capabilities have permitted today’s 

NDIR analyzers to exhibit substantially 

improved measurement performance, 

reliability, and operational convenience 

at modest cost. This improved 

instrument performance was not 

reflected in the previous performance 

requirements for CO FRM analyzers 

specified in 40 CFR part 53, indicating 

a need for an update to reflect that 

improved performance. 

The updated performance 

requirements that EPA proposed for CO 

analyzers make them more consistent 

with the typical performance capability 

available in contemporary FRM 

analyzers and will ensure that newly 

designated FRM analyzers will have this 

improved measurement performance. A 

review of analyzer manufacturers’ 

specifications has determined that all 

existing CO analyzer models currently 

in use in the monitoring network 

already meet the proposed new 

requirements (for the standard 

measurement range). Also in 

conjunction with this modernization of 

the analyzer performance requirements, 

EPA proposed new, more stringent 

performance requirements applicable, 

on an optional basis, to analyzers that 

feature one or more lower, more 

sensitive measurement ranges. Such 

lower ranges will support improved 

monitoring data quality in areas of low 

CO concentrations. 

These updated and new performance 

requirements are being promulgated as 

amendments to subpart B of 40 CFR part 

53, which prescribes the explicit 

procedures to be used for testing 

specified performance aspects of 

candidate FRM and FEM analyzers, 

along with the minimum performance 

requirements that such analyzers must 

meet to qualify for FRM or FEM 

designation. In particular, the new 

performance requirements appear in 

table B–1 of subpart B of 40 CFR part 

53. Although table B–1 covers candidate 

methods for sulfur dioxide (SO2), O3, 

CO, and NO2, the updates to table B–1 

that EPA is promulgating today affect 

only candidate methods for CO. 

The updated performance 

requirements apply to candidate CO 

analyzers that operate on the specified 

‘‘standard’’ measurement range (0 to 50 

ppm). This measurement range remains 

unchanged from the existing 

requirements as it appropriately 

addresses the monitoring data needed 

for assessing attainment. The 

measurement noise limit is reduced 

from 0.5 to 0.2 ppm, and the lower 

detectable limit is reduced from 1 to 0.4 

ppm. Limits for zero drift and span drift 

are lowered, respectively, from 1.0 to 

0.5 ppm, and from 2.5% to 2.0%. The 

previously existing mid-span drift limit 

requirement, tested at 20% of the upper 

range limit (URL), is withdrawn, as EPA 

has found that the mid-span drift 

requirement was unnecessary for CO 

instruments because the upper level 

span drift (tested at 80% of the URL) 

completely and more accurately 

measures analyzer span drift 

performance. 

The lag time limit is reduced from 

10 to 2 minutes, and the rise and fall 

time limits are lowered from 5 to 2 

minutes. For precision, EPA is changing 

the form of the precision limit 

specifications from an absolute measure 

(ppm) to percent (of the URL) for CO 

analyzers and setting the precision limit 

at 1 percent tested at both 20% and 80% 

of the URL. One percent is equivalent to 

the previous limit value of 0.5 ppm for 

precision for the standard (0 to 50 ppm) 

measurement range. This change in 

units from ppm to percent makes the 

requirement responsive to higher and 

lower measurement ranges (i.e., more 

demanding for lower ranges). 

The interference equivalent limit of 1 

ppm for each interferent is not changed, 

but EPA is withdrawing the previously 

existing limit requirement for the total 

of all interferents. EPA has found that 

the total interferent limit is unnecessary 

because modern CO analyzers are 

subject to only a few interferences, and 

they tend to be well controlled. 

The new performance requirements 

apply only to newly designated CO FRM 

or FEM analyzers; however, essentially 

all existing FRM analyzers in use today, 

as noted previously, already meet these 

requirements, so existing FRM analyzers 

are not required to be re-tested and reVerDate Mar<15>2010 17:35 Aug 30, 2011 Jkt 223001 PO 00000 Frm 00019 Fmt 4701 Sfmt 4700 E:\FR\FM\31AUR2.SGM 31AUR2

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designated under the new requirements. 

All currently designated FRM analyzers 

retain their original FRM designations. 

EPA also recognized that some CO 

monitoring objectives (e.g., area-wide 

monitoring away from major roads and 

rural area surveillance) require 

analyzers with lower, more sensitive 

measurement ranges than the standard 

range used for typical ambient 

monitoring. To improve data quality for 

such lower-range measurements, EPA is 

adding a separate set of performance 

requirements that apply specifically to 

lower ranges (i.e., those having a URL of 

less than 50 ppm) for CO analyzers. 

These additional, lower-range 

requirements are listed in the revised 

table B–1. A candidate analyzer that 

meets the table B–1 requirements for the 

standard measurement range (0 to 50 

ppm) can optionally have one or more 

lower ranges included in its FRM or 

FEM designation by further testing to 

show that it also meets these 

supplemental, lower-range 

requirements. 

Although no substantive changes 

were determined to be needed to the test 

procedures and associated provisions of 

subpart B for CO, the detailed language 

in many of the subpart B sections was 

in need of significant updates, 

clarifications, refinement, and (in a few 

cases) correction of minor typographical 

errors. These changes to the subpart B 

text (apart from the changes proposed 

for table B–1 discussed above) are very 

minor and almost entirely editorial in 

nature, but quite numerous. Therefore, 

EPA has revised and is re-promulgating 

the entire text of subpart B text. 

As discussed previously, table B–1, 

which sets forth the pollutant-specific 

performance limits, is being amended 

only as applicable to CO analyzers. EPA 

amended table B–1 as applicable to SO2

methods on June 22, 2010 and intends 

to amend table B–1 for O3 and NO2 later, 

if appropriate, when the associated 

NAAQS are reviewed. 

2. Public Comments 

EPA notes first that CASAC stated 

that ‘‘more sensitive and precise 

monitors need to be deployed to 

measure levels that are less than or 

equal to 1 ppm.’’ (Brain and Samet 

2010b). Comments from the public on 

the proposed revisions to CO 

monitoring methods are addressed in 

this section or in the Response to 

Comments document. Comments on the 

proposed changes to the CO monitoring 

methodology were received from only 

one member of the public, the American 

Petroleum Institute. The commenter was 

generally supportive of EPA’s efforts to 

clarify and update the regulations for 

the CO FRM and the CO analyzer 

performance requirements. In regard to 

the CO FRM (40 CFR part 50, appendix 

C), the commenter questioned EPA’s 

proposed relaxation in a flow rate 

control requirement in the dilutionmethod 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 readout device connected to the analyzer to 

monitor its reading during calibration 

should be the actual, or at least closely 

representative of the actual, data 

recording system used during field 

operation of the analyzer. EPA has 

accepted both of these suggestions, and 

appropriate changes have been 

incorporated into the changes being 

made to the CO FRM in this action. 

Another comment questioned the 

proposed withdrawal of the previous 

total interference limit requirement. In 

response to this comment, EPA reevaluated the efficacy of this limit for 

CO analyzers and again determined that 

the limit was not necessary, because the 

number of individual interferences to 

which FRM (and most potential FEM) 

CO analyzers are subject is small (only 

2 for FRMs), as listed in table B–3 of 40 

CFR part 53. Also, response to these 

interferents is typically well controlled 

in modern CO analyzers. In addition, 

the new, individual interference limit 

for the lower measurement ranges is one 

half the limit for the standard range, 

which further mitigates any need for a 

separate, total interference limit. 

The commenter questioned EPA’s 

proposed withdrawal of the previously 

existing limit requirement for span drift 

measured at 20% of the upper range 

limit (URL), contending that this limit 

was important because it is closer in 

concentration to the existing NAAQS 

than the span drift measured at 80% of 

the URL. However, the purpose of the 

span drift limit is not to directly assess 

measurement error at a particular, midscale concentration level. That purpose 

is served by the 1-point quality control 

check for CO monitors described in 

section 3.2.1 of appendix A of 40 CFR 

part 58. Rather, for the purpose of 

analyzer performance testing, the linear 

input/output functional characteristic of 

the analyzer is best described by its zero 

point and its slope, because these 

parameters are generally subject to 

change (drift) independently. Thus, zero 

drift (change in the zero point) and span 

drift (change in the slope) are tested 

separately. Zero drift is, of course, 

measured at zero concentration, and 

span drift is most accurately measured 

at a concentration near the URL. The 

span drift test at 80% URL (when the 

zero drift is within the specified 

requirement) more accurately 

determines any change in the slope 

parameter then a test at 20% URL. The 

previously specified test at 20% URL 

thus serves little, if any, purpose in 

regard to determining change in the 

slope. Therefore, EPA has concluded 

that this requirement can be withdrawn. 

Finally, the commenter was 

concerned that existing FRM analyzers 

approved under the previously existing 

performance requirements may provide 

data quality inferior to that of analyzers 

approved under the proposed new 

requirements and that older analyzers 

may be unacceptable for some 

applications that demanded higher 

performance or higher data quality. A 

‘‘tiered’’ approach was suggested to 

handle this situation. 

In proposing more stringent 

performance requirements for approval 

of new FRM and FEM analyzers, EPA 

noted that the performance of analyzers 

approved under the existing 

performance requirements was fully 

adequate for most routine compliance 

monitoring applications, and that the 

proposed new requirements were 

largely to bring the base FRM and FEM 

performance requirements up to date 

and more commensurate with the 

performance of modern commercially 

available CO analyzers. EPA further 

noted that all currently designated FRM 

analyzers already meet the proposed 

new requirements. This means that the 

quality of routine CO monitoring data 

currently being obtained is already of 

the higher level portended by the 

proposed new performance 

requirements. 

In the proposal, however, EPA did 

recognize that some special CO 

monitoring applications do require a 

higher level of performance than that 

required for routine applications. 

Therefore, EPA is promulgating 

optional, more stringent performance 

requirements for analyzers having a 

more sensitive, ‘‘lower range’’ available 

for such applications. This is, in fact, a 

‘‘tiered’’ approach. Applicants would be 

able to elect to have such lower ranges 

approved as part of their FRM or FEM 

designation. These new, special 

performance requirements will alert 

monitoring agencies that they should 


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29One 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). 

30Since 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. 

consider low-range performance of an 

analyzer for those applications that may 

require better low-level performance, 

and they can select an analyzer that has 

such a lower range approved under its 

FRM (or FEM) designation. 

3. Decisions on Methods 

As discussed above, a few relatively 

minor changes have been incorporated 

into the proposed revised CO FRM in 

appendix C of part 50, in response to 

public comments received by EPA. With 

these changes, the revised appendix C is 

being promulgated as otherwise 

proposed. Only one change has been 

made to the revision proposed for 

subpart B of part 53, to fix a 

typographical error that appeared in 

proposed table B–1 concerning reversed 

entries for the span drift limits for the 

20% and 80% URL for the CO ‘‘lower 

range’’ column. Aside from this 

correction, the revised subpart B is 

being promulgated exactly as proposed. 

B. Network Design 

This section on CO network design 

provides information on the proposed 

network design, the public comments 

received on the proposed network 

design, and the EPA’s conclusions, 

including rationale and details, on the 

final changes to the CO network design 

requirements. 

1. Proposed Changes 

The objective of an ambient 

monitoring network is to (1) provide air 

pollution data to the general public in 

a timely manner, (2) support 

compliance with ambient air quality 

standards and emissions strategy 

development, and (3) provide support 

for air pollution research (40 CFR part 

58, appendix D). The proposed CO 

network design was intended to directly 

support the NAAQS by requiring 

monitoring that provides data for use in 

the designation process and ongoing 

assessment of air quality. In particular, 

the proposed network design was 

intended to require a sufficient number 

of monitors to collect data for 

compliance purposes in the near-road 

environment, where, as noted in section 

II.A.1 above, the highest ambient CO 

concentrations generally occur, 

particularly in urban areas (ISA, section 

3.5.1.3; REA, section 3.1.3). 

The EPA proposed CO monitors to be 

required within a subset of near-road 

NO2 monitoring stations, which are 

required in 40 CFR part 58, appendix D, 

section 4.3. Per the preamble to the final 

rule for the NO2 NAAQS promulgated 

on February 9th, 2010 (75 FR 6474), 

near-road NO2 monitoring stations are 

intended to be placed in the near-road 

environment at locations of expected 

maximum 1-hour NO2 concentrations 

and are triggered for metropolitan areas 

based on Core Based Statistical Area 

(CBSA) population thresholds and a 

traffic-related threshold based on annual 

average daily traffic (AADT).29 The EPA 

proposed that CO monitors be required 

to operate in any CBSA having a 

population of 1 million or more persons, 

collocated with required near-road NO2

monitoring stations. Based upon 2009 

Census Bureau estimates and 2008 

traffic statistics maintained by the US 

Department of Transportation (US DOT) 

Federal Highways Administration 

(FHWA), the CO monitoring proposal 

was estimated to require approximately 

77 CO monitors to be collocated with 

near-road NO2 monitors within 53 

CBSAs.30

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 nearroad CO monitors be operational by 

January 1, 2013. Due to the proposed 

collocation of required CO monitors 

with required near-road NO2 monitors, 

these implementation dates were 

proposed in order to match those of the 

forthcoming near-road NO2 monitoring 

network. 

In light of the proposal to require 

near-road CO monitors be collocated 

with required near-road NO2 monitors, 

the EPA proposed that siting criteria for 

microscale CO monitors be revised to 

match those of microscale near-road 

NO2 monitors (and also microscale 

PM2.5 monitors). In particular, the EPA 

proposed that microscale CO siting 

criteria for probe height and horizontal 

spacing be changed to match those of 

near-road NO2 monitors as prescribed in 

40 CFR part 58 appendix E, sections 2, 

4(d), 6.4(a), and table E–4. Specifically, 

EPA proposed the following: (1) To 

allow microscale CO monitor inlet 

probes to be between 2 and 7 meters 

above the ground; (2) that microscale 

near-road CO monitor inlet probes be 

placed so they have an unobstructed air 

flow, where no obstacles exist at or 

above the height of the monitor probe, 

between the monitor probe and the 

outside nearest edge of the traffic lanes 

of the target road segment; and (3) that 

required near-road CO monitor inlet 

probes shall be as near as practicable to 

the outside nearest edge of the traffic 

lanes of the target road segment, but 

shall not be located at a distance greater 

than 50 meters in the horizontal from 

the outside nearest edge of the traffic 

lanes of the target road segment. 

Finally, the EPA recognized that a 

single monitoring network design may 

not always be sufficient for fulfilling 

specific or otherwise unique data needs 

or monitoring objectives for every area 

across the nation. As such, the EPA 

proposed to provide the Regional 

Administrators with the discretion to 

require monitoring above the minimum 

requirements as necessary to address 

situations where minimum monitoring 

requirements are not sufficient to meet 

monitoring objectives. 

2. Public Comments 

EPA first notes that CASAC expressed 

concern over the current monitoring 

network, stating ‘‘[m]ore extensive 

coverage may be warranted for areas 

where concentrations may be more 

elevated, such as near roadway 

locations. The Panel found that in some 

instances current networks 

underestimated carbon monoxide levels 

near roadways.’’ (Brain and Samet 

2010b). General comments from the 

public based on relevant factors that 

either support or oppose the proposed 

changes to the CO network design are 

addressed in this section. Specific 

public comments related to the network 

design, but with regard to material 

which was not specifically proposed by 

the EPA or posed for solicitation of 

comment, are addressed in the Response 

to Comments document. 

a. Near-Road Monitoring and 

Collocation With Near-Road Nitrogen 

Dioxide Monitors 

The EPA received multiple public 

comments on the overall merit of 

monitoring for CO in the near-road 

environment, the proposal that required 

CO monitors be collocated with 

required near-road NO2 monitors, and 

the number of required CO monitors 

that might be appropriate. In general, 

public health and environmental groups 

(e.g., American Lung Association [ALA], 

American Thoracic Society [ATS], 

Environmental Defense Fund [EDF]), 

some states or state environmental 

agencies or organizations (e.g. National 

Association of Clean Air Agencies 

[NACAA], Northeast States for 

Coordinated Air Use Management 

[NESCAUM], New York State 

Department of Environment 

Conservation [NYSDEC], and State of 

Wisconsin Department of Natural 


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Resources [WIDNR]), and some private 

citizen commenters provided support 

for a requirement for CO monitors in the 

near-road environment. For example, 

ALA, ATS, and EDF state that they 

‘‘* * * are pleased to see EPA take 

seriously the public health threats that 

are posed to millions of residents and 

other sensitive receptors who live near 

or work on or near highways as well as 

other high exposure areas.’’ They go on 

to note that ‘‘[near-road ambient 

monitoring] data have been sorely 

lacking from the national monitoring 

network and are long overdue.’’ Further, 

many of the commenters who were 

supportive of near-road monitoring were 

supportive of collocating CO monitors 

with near-road NO2 monitors as it 

establishes multipollutant monitoring 

within the ambient air monitoring 

network. For example, NACAA stated 

the following in their comments: ‘‘* * * 

NACAA supports EPA’s proposal to 

collocate CO near roadway monitors at 

a subset of NO2 near-roadway sites. This 

is consistent with the recommendations 

of EPA’s Clean Air Scientific Advisory 

Committee (CASAC), which urged the 

agency to develop the near roadway 

monitoring network with a 

multipollutant focus and included CO 

in its list of pollutants that should be 

measured.’’ 

Some industry commenters (e.g., 

Association of Automobile 

Manufacturers [AAM] and American 

Electric Power Service Corporation 

[AEPSC]) and a number of other states 

or state groups (e.g., Indiana Department 

of Environmental Management [IDEM], 

North Carolina Department of Air 

Quality [NCDAQ], New Mexico Air 

Quality Bureau [NMAQB], South 

Carolina Department of Health and 

Environmental Control [SCDHEC], 

Southeast Michigan Council of 

Governments [SEMCOG], and Texas 

Commission on Environmental Quality 

[TCEQ]) generally did not support the 

proposed near-road CO monitoring 

requirements. For example, IDEM stated 

that ‘‘CO measured by roadside 

monitors is not representative of 

ambient air quality everywhere in a city 

or county containing the roadway’’ and 

that ‘‘* * * roadside monitoring 

measurements represent source-specific 

data. Therefore, Indiana does not 

believe that roadside monitoring should 

apply to an ambient air quality 

standard.’’ SCDHEC stated it ‘‘* * * 

does not believe that the use of a nearroad 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 nearroad CO monitoring requirements were 

intended to ensure a network of 

adequate size and focus to provide data 

for comparison to the NAAQS, support 

health studies and model verification, 

and to fulfill Agency multipollutant 

monitoring objectives. In response to the 

comment that near-road monitoring data 

would be ‘‘source-specific’’ and may not 

be appropriately applicable to an 

ambient air standard, the Agency notes 

that monitoring for CO in the near-road 

environment (as a mobile source 

oriented measurement) is a longstanding 

agency practice, as evidenced by the 

first monitoring rule promulgated in 

1979 (44 FR 27558, May 10, 1979). That 

1979 monitoring rule included the 

requirement to monitor for ‘‘peak’’ CO 

concentrations in urban areas having 

populations of 500,000 people or more 

in locations ‘‘* * * around major traffic 

arteries and near heavily traveled streets 

in downtown areas.’’ The Agency 

believes that the use of near-road CO 

monitors as proposed is not a departure 

from the Agency’s longstanding intent 

to measure peak concentrations of CO in 

the near-road environment. Rather, the 

proposal was consistent with the 

Agency’s approach to require monitors 

for CO, and other criteria pollutants, in 

locations that likely experience peak 

ambient concentrations. The Agency 

also notes that source-oriented 

monitoring is and has long been a 

common practice in ambient monitoring 

networks, although more often 

associated with stationary sources, 

where the ambient data collected are 

used for comparison to the NAAQS. 

Data on ambient air concentrations, 

including near-road data, which may be 

most appropriately classified as on-road 

mobile source oriented, are appropriate 

to compare to the NAAQS. 

With regard to the comments asserting 

that near-road monitoring would result 

in monitoring areas of ‘‘little or no 

population’’ and thus population 

exposure is not represented, the EPA 

notes that on-road mobile sources are 

ubiquitous in urban areas and are a 

dominant component of the national CO 

emissions inventory, at nearly 60% of 

the total inventory, based on the 2008 

NEI. As such, microenvironments 

influenced by on-road mobile sources 

are important contributors to ambient 

CO exposures, particularly in urban 

areas (REA, section 2.7). Further, the 

ambient CO exposures of most concern 

are short-term. Accordingly, near-road 

monitoring is focused on characterizing 

peak or elevated ambient 

concentrations. The relevance of this 

focus for the purposes of both ensuring 

compliance with the NAAQS and 

gathering data to inform our 

consideration of ambient CO exposures 

is demonstrated by the ubiquity of onroad 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 nearroad environment would characterize 

the ambient concentrations that 

contribute to ambient CO exposure for 

a significant portion of the population 

that would otherwise not be captured. 

The AAM also commented that the 

EPA ‘‘* * * proposal to locate more 

near roadway monitors appears to be an 

attempt to find problems where none 

are likely to exist.’’ The Agency 

proposal for near-road monitors is in 

line with longstanding monitoring 

objectives to monitor for peak or 

elevated ambient pollutant 

concentrations where they may occur. 

The Agency agrees that CO is no longer 

as pervasive a problem as it was in the 

past; however, there is still a 

responsibility to appropriately 

characterize and assess ambient 

concentrations to ensure that they do 

not exceed the NAAQS. In comments on 

the first draft of the ISA, CASAC 

advised that ‘‘* * * relying only on 


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31One 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). 

EPA’s [current] fixed monitoring 

network, CO measurements may 

underestimate CO exposures for specific 

vulnerable populations such as 

individuals residing near heavily 

trafficked roads and who commute to 

work on a daily basis.’’ In comments on 

the second draft of the ISA, CASAC 

commented that ‘‘the panel expresses 

concern about the existing CO 

monitoring network, both for its 

[spatial] coverage and for its utility in 

estimating human exposure’’ and that 

‘‘CO exposures may not be adequately 

characterized for populations that may 

be exposed to higher CO levels because 

of where they live and work,’’ such as 

the near-road environment. Finally, in 

comments on the second draft of the 

REA, CASAC stated that ‘‘the approach 

for siting monitors needs greater 

consideration. More extensive coverage 

may be warranted for areas where 

concentrations may be more elevated, 

such as near-roadway locations.’’ In 

light of these comments and upon a 

review of the existing CO network, the 

Administrator concluded that the 

current CO monitoring network (circa 

2010) lacked a necessary focus. While 

some currently existing sites that were 

established in the 1970s and 1980s 

continue to monitor near-road locations 

in downtown areas or within urban 

street canyons, and a minimum number 

of area-wide monitors are currently 

required at National Core (NCore) 

multipollutant stations, few monitors 

exist that characterize the more heavily 

trafficked roads that are prevalent in the 

modern roadway network, particularly 

in our larger urban areas. The Agency’s 

proposal was intended to require a 

modest but appropriate number of CO 

monitors to characterize the near-road 

environment where peak or elevated 

ambient CO concentrations are expected 

to occur near heavily trafficked roads, as 

compared with neighborhood or urban 

background concentrations. If CO levels 

turn out to be low in these near-road 

locations, so much the better for public 

health, and monitoring networks can be 

adjusted in the future, as they have over 

time in response to an increased 

understanding of where levels of 

concern to public health are likely to 

occur. 

Although the EPA received a number 

of comments that were largely 

supportive for the proposed requirement 

of collocating CO monitors within the 

forthcoming near-road NO2 monitoring 

stations, several commenters 

encouraged the Agency to provide 

flexibility to allow for the separation of 

the newly required CO monitors from 

the near-road NO2 sites, if necessary, to 

better monitor peak near-road CO 

concentrations. In their comments 

supporting the collocation concept, 

NACAA also stated that their 

organization ‘‘* * * also encourages 

EPA to allow flexibility for state and 

local agencies to use alternative siting of 

near-roadway CO monitors on a case-bycase basis, where there is a scientific 

justification for siting the CO monitor in 

a different location from the NO2

monitor, to ensure the best possible 

measurement of near roadway CO 

concentrations.’’ Similarly, NCDAQ 

recognized that ‘‘* * * light duty 

vehicles tend to have more impact on 

CO concentrations than do heavy [duty] 

vehicles’’ and went on to surmise that 

‘‘* * * not all near-road NO2

monitoring stations will be well situated 

to measure maximum CO 

concentrations.’’ 

The Agency has expressed its intent 

to pursue the integration of monitoring 

networks and programs through the 

encouragement of multipollutant 

monitoring wherever possible, as 

evidenced by actions taken in the 2006 

monitoring rule that created the NCore 

network, the expression of the 

multipollutant paradigm in the 2008 

Ambient Air Monitoring Strategy for 

State, Local, and Tribal Air Agencies, 

and within this rulemaking process as 

part of the rationale in proposing the 

collocation of required near-road CO 

monitors with near-road NO2 monitors. 

Multipollutant monitoring is viewed as 

a means to broaden the understanding 

of air quality conditions and pollutant 

interactions, furthering the capability to 

evaluate air quality models, develop 

emission control strategies, and support 

research, including health studies. 

However, the Agency also recognizes 

that the measurement objectives of 

individual pollutants may not always 

correspond in a way that would support 

multipollutant monitoring as the most 

appropriate option in a network design. 

On the issue raised by NACAA and 

NCDAQ concerning the potential 

difference in locations of peak CO and 

NO2 concentrations in the near-road 

environment, the EPA recognizes the 

primary influence to be the different 

emission characteristics between light 

duty (LD) and heavy duty (HD) vehicles 

and vehicle operating conditions, which 

were discussed in section III.B.2 of the 

CO proposal. The public comments 

suggesting the need for flexibility in 

siting near-road CO monitors derives 

from the fact that near-road NO2 sites 

will be sited at locations where peak 

NO2 are expected to occur. Since NO2 is 

more heavily influenced by HD vehicles 

and CO is more heavily influenced by 

LD vehicles on a per vehicle basis, 

respectively, there may be cases where 

the peak CO and NO2 concentrations 

could occur along different road 

segments within the same CBSA. As a 

general observation, the EPA believes 

that this situation may have more 

likelihood of occurring in the relatively 

larger (by population) CBSAs where a 

higher number of heavily trafficked 

roads with a wider variety of fleet mix 

(e.g. HD to LD vehicle ratios) tend to 

exist versus relatively smaller CBSAs. In 

recognition of these considerations, the 

final regulation allows for flexibility in 

CO monitor placement in the near-road 

environment when justified, as 

discussed below in section IV.B.3. 

b. Population Thresholds for Requiring 

Near-Road Carbon Monoxide Monitors 

The EPA proposed that required CO 

monitors be collocated with every 

required near-road NO2 monitor in a 

CBSA with a population of 1 million or 

more persons. Due to the requirement to 

locate one CO monitor at each required 

near-road NO2 site, the proposal would 

have required two monitors in each 

CBSA having 2.5 million or more 

persons or having one or more road 

segments with Annual Average Daily 

Traffic (AADT) counts of 250,000 or 

more. The proposal would have also 

required one monitor within those 

CBSAs having 1 million or more 

persons (but fewer than 2.5 million 

persons).31 Based upon 2009 Census 

Bureau estimates and US DOT 

maintained traffic summary data, the 

proposal was estimated to require 77 

monitors within 53 CBSAs. Using recent 

2010 Census data, and US DOT 

maintained traffic summary data, the 

proposal would have required 

approximately 75 monitors within 52 

CBSAs. 

The EPA received a number of 

comments supporting different 

population thresholds by which to 

require near-road CO monitors. Those 

state agencies or state agency groups 

who generally supported required CO 

monitoring in the near-road 

environment (e.g., NACAA, NESCAUM, 

NYSDEC, and WIDNR) suggested a 

population threshold of 2.5 million by 

which near-road CO monitors should be 

required. In addition, NCDAQ, who did 

not support near-road CO monitoring, 


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suggested that if it is required, it be 

required only within CBSAs of 2.5 

million or more. The use of a population 

threshold of 2.5 million persons, versus 

1 million as proposed, would require 

approximately 42 near-road CO 

monitors within 21 CBSAs, based on 

2010 Census data. Industry commenter 

American Petroleum Institute (API) 

stated that the proposed population 

threshold of 1 million persons ‘‘* * * 

appears appropriate, but EPA should 

not require that both [near-road NO2] 

sites in the largest CBSAs host CO 

monitors.’’ API’s suggestion would 

require approximately 52 near-road CO 

monitors within 52 CBSAs. Finally, the 

public health and environmental groups 

ALA, ATS, and EDF suggested the EPA 

promulgate minimum monitoring 

requirements ‘‘* * * to encompass 

cities in smaller metro areas, including 

cities with populations of 500,000 or 

more, similar to the requirements for 

NO2 roadside monitoring.’’ ALA, ATS, 

and EDF’s suggestion would result in 

the requirement of approximately 126 

monitors within 103 CBSAs. 

As was noted in the proposal, the 

Agency believes that with the 

continuing decline of ambient CO 

levels, as summarized in the EPA’s most 

recent trends report Our Nation’s Air: 

Status and Trends Through 2008 

(http://www.epa.gov/airtrends/2010/), 

there is less likelihood for high CO 

concentrations in relatively smaller 

CBSAs (by population). Accordingly, 

the Agency proposed the requirement 

for what it believed would be a 

sufficient number of CO monitors, 

which would be collocated with 

required near-road NO2 monitors in 

CBSAs having populations of 1 million 

or more persons. The Administrator 

considered alternative population 

thresholds, including the 2.5 million 

and 500,000 person thresholds, but 

concluded that those thresholds would 

require too few or too many monitors, 

respectively, in light of existing 

information on CO emissions data, 

ambient data, and the lack of data for 

locations near highly trafficked roads. 

The rationale for the proposed 1 million 

person threshold was to require a 

modest but sufficiently sized network 

that would effectively assess near-road 

CO concentrations for comparison to the 

NAAQS and could also provide data 

from within a multipollutant framework 

to support research (which includes 

health studies), facilitate model 

verification, and assess and evaluate 

emissions control strategies. However, 

after considering public comments, the 

EPA has concluded that one monitor in 

each CBSA of 1 million or more persons 

will provide for monitoring of a wide 

range of diverse situations with regard 

to traffic volume, traffic patterns, 

roadway designs, terrain/topography, 

meteorology, climate, as well as 

surrounding land use and population 

characteristics. Accordingly, in the final 

rule EPA has modified the proposed 

requirements for CO monitors so that 

only one CO monitor is required in 

CBSAs of 1 million or more persons, as 

discussed in Section IV.B.3 below. 

c. Implementation Schedule 

The EPA received a number of 

comments on the timeline for 

implementation of any required CO 

monitoring promulgated as part of this 

rulemaking. ALA, ATS, and EDF stated 

that they ‘‘* * * support EPA’s 

requirement that CO monitors be 

installed in near-highway locations by 

July 1, 2013.’’ In light of the support 

these commenters expressed for rapid 

deployment of near road CO monitors, 

these commenters may have intended to 

support the proposed implementation 

date of January 1, 2013 instead of July 

1, 2013 as quoted. The Agency received 

a number of comments from state 

agencies, state agency organizations, 

and industry encouraging the Agency to 

extend the time by which any required 

monitoring must be implemented. For 

example, API suggested that the 

proposed date by which required nearroad CO monitors be established be 

extended to July 1, 2013, while NACAA 

and WIDNR suggested January 1, 2014. 

Several commenters suggested that 

required near-road monitors should be 

phased in over a period of time. For 

example, NACAA, stated ‘‘[i]t may be 

necessary to develop a program for 

phasing in new monitoring sites and 

reevaluate network implementation.’’ 

NACAA also pointed to comments from 

CASAC that it would be advisable to 

phase in near-road monitoring for NO2, 

because ‘‘[t]he first round of sites could 

be used to gather information on 

appropriate siting in the near roadway 

environment, near roadway gradient, 

and spatial relationships.’’ 

The EPA recognizes that states are 

already implementing newly required 

monitoring related to lead and NO2, and 

that the current financial and logistical 

burdens may make the implementation 

of new monitoring requirements 

difficult. A number of state and industry 

commenters noted the need for funding 

to accommodate a new monitoring 

requirement, and some also noted the 

financial and logistical hardships that 

many states are currently experiencing 

(e.g., IDEM, NACAA, NCDAQ, SCDHEC, 

and WIDNR). The EPA recognizes the 

significance of the financial and 

logistical burden that new monitoring 

requirements pose and the impact of 

multiple new monitoring requirements 

stemming from other recent 

rulemakings. As such, the Agency has 

taken these comments into 

consideration in the final rule with 

regard to when required CO monitors 

are to be operational, as discussed in 

Section IV.B.3 below. 

d. Siting Criteria 

The EPA received comments 

regarding the proposed revisions to 

microscale CO siting criteria. Those who 

commented (AAM, API, and NCDAQ) 

all supported having two sets of siting 

criteria that would apply to near-road 

CO monitors such as those that might be 

collocated with near-road NO2 monitors 

and to those CO monitors operating in 

downtown areas and urban street 

canyon locations, respectively. AAM 

stated that ‘‘* * * there should be two 

separate criteria for siting microscale CO 

monitors. The earlier height and 

distance guidelines are still appropriate 

for downtown areas and arterial 

highways with sidewalks, but a separate 

set of guidelines should be established 

for limited access, heavily-travelled 

expressways.’’ API commented that 

‘‘* * * the proposed CO [near-road] 

criteria are acceptable. EPA should 

create two-tiered siting criteria for 

microscale CO monitoring * * *’’ and 

that ‘‘there will be an ongoing need for 

CO monitoring in downtown, urban 

and/or street canyon[s] for healthrelated concerns as well as SIP-related 

issues.’’ Finally, NCDAQ stated that 

‘‘* * * the US EPA should maintain 

separate siting criteria for the two types 

of micro-scale CO monitoring sites 

* * *’’ noting that the current siting 

criteria intended for downtown areas 

and urban street canyon sites ‘‘* * * are 

still valid for that purpose and CO 

monitoring stations being placed for this 

purpose should still be required to meet 

these siting criteria.’’ 

The EPA agrees with the commenters 

that the existing siting criteria are still 

appropriate for any existing or future 

downtown area or urban street canyon 

CO monitoring site, and that new siting 

criteria are appropriate for CO monitors 

being collocated with near road NO2

monitors. As such, the Agency is 

finalizing siting criteria for microscale 

CO sites that include criteria for both 

downtown area/urban street canyon 

microscale sites and other near-road 

microscale CO sites, as presented below 

in Section IV.B.3. 

e. Area-Wide Monitoring 

The EPA received a number of 

comments from transportation groups, 


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32Spatial 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. 

public health and environmental 

groups, and an industry commenter 

(e.g., AAM, ALA/ATS/EDF, American 

Association of State Highway and 

Transportation Officials [AASHTO], 

New York State Department of 

Transportation [NYSDOT], Texas 

Department of Transportation [TXDOT], 

and Virginia Department of 

Transportation [VDOT]) regarding the 

fate of many of the CO monitors in the 

current network that characterize 

concentrations representative of 

neighborhood or larger spatial scales,32

known as area-wide monitors. For 

example, AASHTO commented that 

‘‘EPA appears to be proposing that CO 

monitoring sites to characterize areawide CO concentration levels at the 

neighborhood and larger spatial scales is 

no longer required. AASHTO is 

concerned that this proposal will deemphasize the need for neighborhood 

scale CO monitors.’’ AASHTO and some 

state DOTs expressed that the data for 

neighborhood scale monitors are used 

for other purposes, such as National 

Environmental Policy Act (NEPA) and 

transportation conformity, and that they 

are concerned about the potential loss of 

these types of data in the future. In 

another example, ALA/ATS/EDF stated 

that they call upon EPA to ‘‘establish a 

comprehensive roadside air pollution 

network, while retaining the current 

area-wide CO network.’’ 

The EPA notes that prior to this final 

rulemaking, the only required CO 

monitoring within 40 CFR part 58, 

appendix D was for the operation of a 

CO monitor within all NCore 

multipollutant monitoring stations. 

There are approximately 80 NCore 

stations nationwide, and by design, they 

are area-wide monitoring sites. In the 

proposal, the Agency estimated that 345 

CO monitors were operational at some 

point during 2009. A more recent 

examination of AQS data (utilizing 

EPA’s Air Explorer Web tools located at 

http://www.epa.gov/airexplorer) 

indicate that approximately 328 CO 

monitors were operational as of May 20, 

2011. These 328 active CO monitors 

include the 80 NCore monitors now in 

operation nationwide. This means that a 

significant portion of the current 

network is composed of monitors that 

are additional to those required by EPA 

as part of a national network design. It 

is critical to note that in this rulemaking 

the EPA is actually increasing the total 

number of required sites in the national 

CO monitoring network design and is 

not removing any area-wide monitoring 

requirements as AASHTO and other 

commenters suggested. Some of the 

potential for misperception on this issue 

may have arisen from the Agency’s 

stated expectation that state and local 

air monitoring agencies will likely move 

existing CO monitors into near-road 

locations to satisfy the minimum 

monitoring requirements promulgated 

in this rulemaking. Based on this final 

rule, state and local agencies would 

only move, at most, approximately 52 

monitors out of the 328 in operation 

(circa May 2011). Therefore a majority 

of CO monitors would likely continue 

operating in their existing locations. 

However, it should be noted that with 

ambient CO concentrations well below 

the NAAQS, particularly at area-wide 

sites, states may identify some areawide 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 areawide CO monitoring to meet the 

overarching monitoring objectives, 

which includes tracking long-term 

trends and to support research. In the 

proposal, the Agency did not propose 

establishing requirements for additional 

area-wide monitoring sites because: (1) 

There is the existing NCore requirement, 

and (2) there is an expectation based on 

experience that some number of nonrequired 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 shutdown. The shut-down of any State and 

Local Air Monitoring Station (SLAMS) 

monitor is allowable under certain 

conditions specified in 40 CFR 58.14 

System Modification. These conditions 

provide state and local air agencies 

multiple options by which they may 

propose, with justification, for a monitor 

to be shut down. Whatever the 

justification may be, each monitor 

proposed to be shut-down must go 

through an established process to 

receive EPA Regional Administrator 

approval for shut-down. As part of that 

process, the EPA Regional 

Administrator provides opportunity for 

public comment before making a 

decision to approve or disapprove the 

request. In conclusion, the EPA believes 

that even without requirements for areawide CO monitors additional to the 

NCore sites, some number of area-wide 

monitors will continue to operate into 

the future. EPA anticipates that 

monitors that states find useful for other 

regulatory purposes, such as NEPA, 

would be among the monitors that may 

continue to operate. The NCore sites, 

along with monitors currently operating 

in the absence of other area-wide 

monitoring requirements, will likely 

provide a sufficient set of area-wide 

monitors to meet monitoring objectives. 

The EPA also received a number of 

comments from transportation groups, 

state and local groups, and an industry 

commenter (e.g., AAM, AASHTO, 

NESCAUM, NYSDEC, NYSDOT, 

TXDOT, and VDOT) suggesting that 

required near-road CO monitors should 

be paired with an area-wide CO monitor 

within the same CBSA. For example, 

AASHTO recommended that ‘‘* * * 

EPA ensure that adequate coverage 

continues from neighborhood-scale 

monitors to estimate background 

concentration levels, and that there is at 

least one neighborhood scale monitor in 

every urbanized area that is required to 

have a near-road monitor.’’ NESCAUM 

recommended ‘‘* * * that EPA locate 

near-road CO monitors near urban 

NCore CO sites’’ (as noted above, NCore 

sites are area-wide sites by design). 

The EPA recognizes that a pairing of 

near-road CO monitors with area-wide 

CO monitors will provide information 

by which an estimate of the difference 

between near-road concentrations to 

relative background concentrations 

might be determined. As noted earlier, 

the Agency believes that the 

combination of required NCore sites and 

those area-wide monitors currently 

operating in the absence of minimum 

monitoring requirements (of which 

many will likely continue operating in 

the future) will largely fulfill the areawide component of any near-road site/ 

area-wide site pairing in an urban area. 

An analysis of NCore site locations (site 

data available from http://www.epa.gov/ 

ttn/amtic/ncore/index.html), along with 


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33The EPA notes that of the 52 CBSAs that have 

1 million or more persons, 39 CBSAs contain an 

NCore monitoring station, which includes a CO 

monitor. 

34This approach only requires one CO monitor to 

be installed in those CBSAs that have two required 

near-road NO2 monitors. 

all those area-wide CO monitors 

believed to be operating as of May 20, 

2011 (utilizing EPA’s Air Explorer Web 

tools located at http://www.epa.gov/ 

airexplorer) indicated that of the 52 

CBSAs with a population of 1 million 

persons or more, based on 2010 Census 

data,, only 4 are believed to be without 

an area-wide CO monitor.33 The EPA 

believes that, based on the 

considerations discussed above, the 

existing network will likely provide 

sufficient area-wide CO concentration 

information on which a near-road to 

area-wide data comparison could be 

based. 

f. Regional Administrator Authority 

The EPA received a number of 

comments from states and 

transportation groups (e.g., AASHTO, 

NYSDOT, TCEQ, TXDOT, and VDOT) 

on the proposal for Regional 

Administrators to have the discretion to 

require monitoring above the minimum 

requirements as necessary to address 

situations where minimum monitoring 

requirements are not sufficient to meet 

monitoring objectives. For example, 

AASHTO commented that ‘‘the 

proposed rule includes some examples 

of where additional monitors may be 

necessary. AASHTO is concerned that 

these brief examples may not be 

sufficient to ensure uniform application 

of this additional authority among the 

EPA Regions,’’ and that EPA should 

provide guidance on this so that there 

is ‘‘reasonable uniformity between EPA 

Regions in the implementation of these 

provisions.’’ TCEQ commented that it 

‘‘does not agree that this discretion is 

appropriate, particularly where EPA has 

not proposed a process by which 

Regional Administrators must consult 

with states and the public regarding 

these decisions.’’ Further TCEQ stated 

that ‘‘* * * the potential requirement 

for additional monitors when ‘minimum 

monitoring requirements are not 

sufficient to meet monitoring objectives’ 

is overly broad and should be refined to 

include objective criteria that will 

consistently applied across all EPA 

Regions.’’ 

The EPA notes that the authority of 

Regional Administrators to require 

additional monitoring above the 

minimum required is not unique to the 

CO NAAQS. For example, Regional 

Administrators have the authority to use 

their discretion to require additional 

NO2, lead, and sulfur dioxide monitors 

(40 CFR part 58 appendix D sections 

4.3.4, 4.4.3, and 4.5, respectively) and to 

work with state and local air agencies in 

designing and/or maintaining an 

appropriate ozone monitoring network 

(40 CFR part 58 appendix D section 4.1). 

The EPA believes that a nationally 

applicable network design may not 

always account for all locations in every 

area where monitors may be warranted. 

Example situations where the Regional 

Administrator authority could be 

utilized, which were provided in the 

proposal, could be for unmonitored 

locations where data or other 

information suggest that CO 

concentrations may be approaching or 

exceeding the NAAQS due to stationary 

CO sources, in downtown areas or urban 

street canyons, or in areas that are 

subject to high ground-level CO 

concentrations particularly due to or 

enhanced by topographical and 

meteorological impacts. The Agency 

cannot anticipate every example that 

may exist where the Regional 

Administrator authority might be used 

for inclusion in this preamble text. 

However, the Agency believes it is 

important for Regional Administrators 

to have the authority to address possible 

gaps in the minimally required 

monitoring network in situations such 

as those examples provided here. In 

response to public comments, the EPA 

notes that Regional Administrators 

would use their authority in 

collaboration with state agencies, 

working with stakeholders to design 

and/or maintain the most appropriate 

CO monitoring network to meet the 

needs of a given area. Finally, the 

Agency notes that any monitor required 

by the Regional Administrator (or any 

new monitor proposed by the state 

itself) is not done so with unfettered 

discretion. Any such action would be 

included in the Annual Monitoring 

Network Plan per 40 CFR 58.10, and 

this plan must be made available for 

public inspection and comment before 

any decisions are made by the EPA 

Regional Administrator. 

3. Conclusions on the Network Design 

This section provides the rationale 

and details for the final decision on 

changes to the CO monitoring network 

design and siting criteria. As discussed 

above in section IV.B.2.a, motor vehicle 

emissions are important contributors to 

ambient CO concentrations (REA, 

section 2.2), contributing nearly 60% of 

the total CO emitted nationally (per the 

2008 NEI). As a result, 

microenvironments influenced by onroad 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 nearroad environment to characterize and 

assess ambient CO concentrations 

continues to be an appropriate objective 

for the CO monitoring network. The 

EPA believes that the promulgation of 

minimum requirements for CO monitors 

in the near-road environment is 

necessary to ensure a network of 

adequate size and focus to provide data 

for comparison to the NAAQS, support 

research which includes health studies, 

allow for model verification, and fulfill 

multipollutant monitoring objectives. 

Further, considering the lack of CO 

monitors assessing higher trafficked 

roads in urban areas and CASAC’s 

advice that the Agency develop greater 

monitoring capacity for CO in near-road 

environments (Brain and Samet, 2010b), 

the Agency believes that a number of 

CO monitors should be focused in such 

locations. Highly trafficked roads are 

expected to show elevated CO 

concentrations relative to area-wide 

concentrations and to represent the 

locations where ambient CO 

concentrations may be highest in an 

area. Regarding the locations where 

required near-road CO monitors might 

be placed, the EPA proposed that they 

be collocated with a subset of near-road 

NO2 monitors. The EPA expects 

required near-road NO2 monitors (as 

prescribed in 40 CFR part 58, appendix 

D, Section 4.3) to be adjacent to highly 

trafficked roads within the CBSAs 

where they are required. Recognizing 

this and also recognizing the benefits 

associated with collocating monitors at 

the same site, the Agency is finalizing 

requirements for CO monitors that will 

leverage required near-road NO2

monitoring sites to house collocated 

near-road CO monitors to create data for 

comparison to the NAAQS, support 

research which includes health studies, 

provide data for model evaluation, and 

foster the fulfillment of multipollutant 

objectives. 

As noted in section IV.B.2.b above, 

after considering public comments, EPA 

has modified the requirements for CO 

monitors from that which was proposed 

so that only one CO monitor is required 

in each CBSA in which near-road CO 

monitoring is required.34 This approach 

reduces the total number of monitors 

that would have been required under 

the proposal from 75 monitors within 52 

CBSAs to 52 monitors within 52 CBSAs 

(based on 2010 Census data). The EPA 

believes this network design addresses 

public comments while maintaining 

monitoring in a sufficiently diverse set 


jlentini on DSK4TPTVN1PROD with RULES2
Federal Register / Vol. 76, No. 169 / Wednesday, August 31, 2011 / Rules and Regulations 54319 

of locations throughout 52 different 

urban areas around the country. By 

having monitors within 52 different 

CBSAs, this network design is expected 

to provide for monitoring in a wide 

range of diverse situations with regard 

to traffic volumes, traffic patterns, 

roadway designs, terrain/topography, 

meteorology, climate, as well as 

surrounding land use and population 

characteristics. 

The EPA is generally requiring CO 

monitors to be collocated with near-road 

NO2 monitors. However, upon 

consideration of public comments, the 

Agency is allowing flexibility for states 

to use an alternate near-road location, 

which includes downtown areas, urban 

street canyons, and other near-road 

locations. This flexibility is provided for 

a required CO monitor, on a case-bycase basis, with EPA Regional 

Administrator approval, when the state 

can provide quantitative justification 

showing the expectation of higher peak 

CO concentrations for that alternate 

location compared to a near-road NO2

location. Such requests could be based 

upon appropriate modeling, exploratory 

monitoring, or other methods, 

comparing the alternative CO location 

and the near-road NO2 location. 

In summary, based upon 2010 Census 

Bureau data this final rule will require 

approximately 52 CO monitors to be 

collocated with near-road NO2 monitors 

(or otherwise operated at an alternate, 

EPA Regional Administrator approved, 

near-road location where peak CO 

concentrations are expected) within 52 

CBSAs that have populations of 1 

million or more persons. 

Regarding the deployment and 

operation of required CO monitors, the 

Agency recognizes that many state and 

local air agencies are under financial 

and related resource duress. EPA has 

concluded that allowing additional time 

for installing CO monitors will provide 

an opportunity for state and local 

agencies to work with EPA Regions to 

identify which existing CO monitors 

may be appropriate to relocate to the 

near-road locations. In many cases, EPA 

and the state may believe it is 

appropriate to relocate monitors, 

including some of those that are 

currently operated pursuant to existing 

maintenance plans. In these cases, 

additional time may be necessary to 

allow states to revisit and possibly 

revise, in consultation with (and subject 

to the approval of) the EPA Regions, 

existing maintenance plans in a way 

that may allow certain CO monitors to 

be free for relocation, if appropriate. 

Further, if a state chooses to investigate 

whether it will request that a required 

near-road CO monitor be sited in a nearroad location other than a required nearroad NO2 site, the time allotted by the 

final rule is expected to provide states 

with adequate time to perform necessary 

analyses for submission to the Regional 

Administrator for approval. 

Furthermore, EPA has concluded that 

public comments suggesting a phased-in 

implementation, allowing for later 

stages to benefit from experience in an 

initial round of monitor installations, 

have merit. 

As a result, the EPA has chosen not 

to require the implementation of 

required CO sites by January 1, 2013 as 

was proposed. Instead, the Agency is 

finalizing a two-phased implementation 

requirement. Those CO monitors 

required within CBSAs having 2.5 

million or more persons are to be 

operational by January 1, 2015, although 

the Agency strongly encourages the 

implementation of these required 

monitors as soon as practicable. Those 

CO monitors required in CBSAs having 

1 million or more persons (and fewer 

than 2.5 million persons) are to be 

operational by January 1, 2017. EPA 

intends to review the experience of 

states with the first round of near-road 

CO monitors and the data produced by 

such monitors and consider whether 

adjustments to the network 

requirements are warranted. These 

required CO monitors shall be reflected 

in a state’s annual monitoring network 

plans due six months prior to 

installation, i.e., on July 1, 2014 or July 

1, 2016, respectively. 

Regarding siting criteria, the EPA 

received public support to adjust 

microscale CO siting criteria to match 

those of near-road NO2 monitors (and 

microscale PM2.5 monitors). The Agency 

also was urged to retain the existing 

microscale siting criteria, for explicit 

use with microscale CO sites in 

downtown areas or urban street canyon 

settings. As a result, the EPA is retaining 

the existing siting criteria for microscale 

CO monitors in downtown areas and 

urban street canyons and is finalizing 

the additional siting criteria for those 

near-road microscale CO monitors 

outside of downtown areas and urban 

street canyons to have probe height and 

horizontal spacing to match those of 

near-road NO2 monitors as prescribed in 

40 CFR part 58 appendix E, sections 2, 

4(d), 6.4(a), and table E–4. 

Specifically, the Agency is finalizing 

the following: (1) A microscale nearroad CO monitor inlet probe shall be 

between 2 and 7 meters above the 

ground; (2) a microscale CO monitor 

inlet probe in the near-road 

environment shall be placed so it has an 

unobstructed air flow, where no 

obstacles exist at or above the height of 

the monitor probe, between the monitor 

probe and the outside nearest edge of 

the traffic lanes of the target road 

segment; and (3) that CO monitors in the 

near-road environment shall have inlet 

probes as near as practicable to the 

outside nearest edge of the traffic lanes 

of the target road segment, but shall not 

be located at a distance greater than 50 

meters in the horizontal from the 

outside nearest edge of the traffic lanes 

of the target road segment. 

Further, as suggested through public 

comments, the EPA is retaining existing 

regulatory siting criteria language for 

microscale CO monitors in downtown 

areas or urban street canyon locations, 

where: (1) The inlet probe for a nearroad microscale CO monitor in a 

downtown area or urban street canyon 

shall be between 2.5 meters and 3.5 

meters above ground level; (2) the inlet 

probe for a near-road microscale CO 

monitor in a downtown area or urban 

street canyon shall be within 10 meters 

from the edge of the nearest traffic lane; 

and (3) near-road microscale CO 

monitors in street canyons are required 

to be at least 10 meters from an 

intersection. 

Finally, the EPA recognizes that a 

monitoring network design may not 

always require monitoring on a national 

scale that is sufficient in fulfilling 

specific or otherwise unique data needs 

or monitoring objectives for every area 

across the nation. Thus, the EPA is 

finalizing the provision that EPA 

Regional Administrators have the 

authority to require monitoring above 

the minimum requirements, as 

necessary, in any area, to address 

situations where the minimally required 

monitoring network is not sufficient to 

meet monitoring objectives. Example 

situations where the Regional 

Administrator Authority could be 

utilized include, but are not limited to, 

those unmonitored locations where data 

or other information suggest that CO 

concentrations may be approaching or 

exceeding the NAAQS due to stationary 

CO sources, in downtown areas or urban 

street canyons, or in areas that are 

subject to high ground-level CO 

concentrations particularly due to or 

enhanced by topographical and 

meteorological impacts. In all cases in 

which a Regional Administrator may 

consider the need for additional 

monitoring, it is expected that the 

Regional Administrators will work with 

the state or local air agencies to evaluate 

evidence or needs to determine whether 

a particular area may warrant additional 

monitoring. 


jlentini on DSK4TPTVN1PROD with RULES2
54320 Federal Register / Vol. 76, No. 169 / Wednesday, August 31, 2011 / Rules and Regulations 

V. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

Under Executive Order 12866 (58 FR 

51735, October 4, 1993), this action is a 

‘‘significant regulatory action’’ because 

it was deemed to ‘‘raise novel legal or 

policy issues.’’ Accordingly, EPA 

submitted this action to the Office of 

Management and Budget (OMB) for 

review under Executive Orders 12866 

and 13563 (76 FR 3821, January 21, 

2011) and any changes made in 

response to OMB recommendations 

have been documented in the docket for 

this action. 

B. Paperwork Reduction Act 

The information collection 

requirements in this final rule have been 

submitted for approval to the Office of 

Management and Budget (OMB) under 

the Paperwork Reduction Act, 44 U.S.C. 

3501 et seq. The information collection 

requirements are not enforceable until 

OMB approves them. The Information 

Collection Request (ICR) document 

prepared by EPA for these revisions to 

part 58 has been assigned EPA ICR 

number 0940.24. 

The information collected under 40 

CFR part 53 (e.g., test results, 

monitoring records, instruction manual, 

and other associated information) is 

needed to determine whether a 

candidate method intended for use in 

determining attainment of the NAAQS 

in 40 CFR part 50 will meet 

comparability requirements for 

designation as a FRM or FEM. We do 

not expect the number of FRM or FEM 

determinations to increase over the 

number that is currently used to 

estimate burden associated with CO 

FRM/FEM determinations provided in 

the current ICR for 40 CFR part 53 (EPA 

ICR numbers 0940.24). As such, no 

change in the burden estimate for 40 

CFR part 53 has been made as part of 

this rulemaking. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 

associated health impacts, to develop 

emissions control strategies, and to 

measure progress for the air pollution 

program. The amendments would revise 

the technical requirements for CO 

monitoring sites, require the relocation 

or siting of ambient CO air monitors, 

and the reporting of the collected 

ambient CO monitoring data to EPA’s 

Air Quality System (AQS). The annual 

average reporting burden for the 

collection under 40 CFR part 58 

(averaged over the first 3 years of this 

ICR) for a network of 311 CO monitors 

is $7,235,483. Burden is defined at 5 

CFR 1320.3(b). State, local, and Tribal 

entities are eligible for State assistance 

grants provided by the Federal 

government under the CAA which can 

be used for monitors and related 

activities. 

An agency may not conduct or 

sponsor, and a person is not required to 

respond to, a collection of information 

unless it displays a currently valid OMB 

control number. The OMB control 

numbers for EPA’s regulations in 40 

CFR are listed in 40 CFR part 9. When 

this ICR is approved by OMB, the 

Agency will publish a technical 

amendment to 40 CFR part 9 in the 

Federal Register to display the OMB 

control number for the approved 

information collection requirements 

contained in this final rule. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of today’s rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule retains existing national 

standards for allowable concentrations 

of CO in ambient air as required by 

section 109 of the CAA. See also 

American Trucking Associations v. 

EPA. 175 F. 3d at 1044–45 (NAAQS do 

not have significant impacts upon small 

entities because NAAQS themselves 

impose no regulations upon small 

entities). Similarly, the amendments to 

40 CFR part 58 address the requirements 

for States to collect information and 

report compliance with the NAAQS and 

will not impose any requirements on 

small entities. 

D. Unfunded Mandates Reform Act 

This rule does not contain a Federal 

mandate that may result in expenditures 

of $100 million or more for State, local, 

and Tribal governments, in the 

aggregate, or the private sector in any 

one year. This rule retains the existing 

national ambient air quality standards 

for carbon monoxide. The expected 

costs associated with the monitoring 

requirements are described in EPA’s ICR 

document, but those costs are expected 

to be well less than $100 million 

(adjusted for inflation) in the aggregate 

for any year. Furthermore, as indicated 

previously, in setting a NAAQS, EPA 

cannot consider the economic or 

technological feasibility of attaining 

ambient air quality standards. Thus, this 

rule is not subject to the requirements 

of sections 202 or 205 of the UMRA. 

This rule is also not subject to the 

requirements of section 203 of the 

UMRA because it imposes no 

enforceable duty on any small 

governments. 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the States, on the 

relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish and 

review NAAQS; however, CAA section 

116 preserves the rights of States to 

establish more stringent requirements if 

deemed necessary by a State. 

Furthermore, this rule does not impact 

CAA section 107 which establishes that 

the States have primary responsibility 

for implementation of the NAAQS. 

Finally, as noted in section D (above) on 

UMRA, this rule does not impose 

significant costs on State, local or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 


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Federal Register / Vol. 76, No. 169 / Wednesday, August 31, 2011 / Rules and Regulations 54321 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have Tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000). It does not have a substantial 

direct effect on one or more Indian 

Tribes, since Tribes are not obligated to 

adopt or implement any NAAQS. Thus, 

Executive Order 13175 does not apply 

to this action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

This action is not subject to EO 13045 

(62 FR 19885, April 23, 1997) because 

it is not economically significant as 

defined in EO 12866, and because the 

Agency does not believe the 

environmental health or safety risks 

addressed by this action present a 

disproportionate risk to children. This 

action’s health and risk assessments are 

described in section II.A. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ as defined in Executive 

Order 13211 (66 FR 28355 (May 22, 

2001)) because it is not likely to have a 

significant adverse effect on the supply, 

distribution, or use of energy. The rule 

concerns the review of the NAAQS for 

CO. The rule does not prescribe specific 

pollution control strategies by which 

these ambient standards will be met. 

Such strategies are developed by States 

on a case-by-case basis, and EPA cannot 

predict whether the control options 

selected by States will include 

regulations on energy suppliers, 

distributors, or users. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, section 12(d) (15 U.S.C. 272 note) 

directs EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This rulemaking involves technical 

standards with regard to ambient 

monitoring of CO. We have not 

identified any potentially applicable 

voluntary consensus standards that 

would adequately characterize ambient 

CO concentrations for the purposes of 

determining compliance with the CO 

NAAQS and none have been brought to 

our attention in comments. Therefore, 

EPA has decided to use the method 

‘‘Measurement Principle and Calibration 

Procedure for the Measurement of 

Carbon Monoxide in the Atmosphere 

(Non-Dispersive Infrared Photometry)’’ 

(40 CFR part 50, appendix C), as revised 

by this action, for the purposes of 

ambient monitoring of CO 

concentrations. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629 

(Feb. 16, 1994)) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this final 

rule will not have disproportionately 

high and adverse human health or 

environmental effects on minority or 

low-income populations because it does 

not affect the level of protection 

provided to human health or the 

environment. The action in this notice 

is to retain without revision the existing 

NAAQS for CO. Therefore this action 

will not cause increases in source 

emissions or air concentrations. 

K. Congressional Review Act 

The Congressional Review Act, 5 

U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement 

Fairness Act of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. EPA will submit a 

report containing this rule and other 

required information to the U.S. Senate, 

the U.S. House of Representatives, and 

the Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

This action is not a ‘‘major rule’’ as 

defined by 5 U.S.C. 804(2). This rule 

will be effective October 31, 2011. 

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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. 

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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. 

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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. 

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A.; Olsen L.M.; Portier C.J. (2003) 

Temperature, air pollution, and 

hospitalization for cardiovascular 

diseases among elderly people in 

Denver. Environ Health Perspect, 

111:1312–1317. 

Lippmann, M. (1984) CASAC Findings and 

Recommendations on the Scientific Basis 

for a Revised NAAQS for Carbon 

Monoxide. [Letter to W.D. Ruckelshous, 

EPA Administrator]. May 17. 

Linn W.S.; Szlachcic Y.; Gong H. Jr; Kinney 

P.L.; Berhane K.T. (2000) Air pollution 

and daily hospital admissions in 

metropolitan Los Angeles. Environ 

Health Perspect, 108:427–434. 

Mann J.K.; Tager I.B.; Lurmann F.; Segal M.; 

Quesenberry C.P. Jr; Lugg M.M.; Shan J.; 

Van den Eeden S.K. (2002) Air pollution 

and hospital admissions for ischemic 

heart disease in persons with congestive 

heart failure or arrhythmia. Environ 

Health Perspect, 110:1247–1252. 

Metzger K.B.; Tolbert P.E.; Klein M.; Peel J.L.; 

Flanders W.D.; Todd K.H.; Mulholland 

J.A.; Ryan P.B.; Frumkin H. (2004) 

Ambient air pollution and 

cardiovascular emergency department 

visits. Epidemiology, 15:46–56. 

National Research Council. (2003) Managing 

Carbon Monoxide Pollution in 

Meteorological and Topographical 

Problem Areas. The National Academies 

Press, Washington, DC. 

Sheps D.S.; Adams K.F. Jr.; Bromberg P.A.; 

Goldstein G.M.; O’Neil J.J.; Horstman D.; 

Koch G. (1987) Lack of effect of low 

levels of carboxyhemoglobin on 

cardiovascular function in patients with 

ischemic heart disease. Arch. Environ. 

Health 42:108–116. 

Symons J.M.; Wang L.; Guallar E.; Howell E.; 

Dominici F.; Schwab M.; Ange B.A.; 

Samet J.; Ondov J.; Harrison D.; Geyh A. 

(2006) A case-crossover study of fine 

particulate matter air pollution and onset 

of congestive heart failure symptom 

exacerbation leading to hospitalization. 

Am J Epidemiol, 164:421–433. 

Tolbert P.E.; Klein M.; Peel J.L.; Sarnat S.E.; 

Sarnat J.A. (2007) Multipollutant 

modeling issues in a study of ambient air 

quality and emergency department visits 

in Atlanta. J Expo Sci Environ 

Epidemiol, 17:S29–S35. 

U.S. Environmental Protection Agency. 

(1979a) Air Quality Criteria for Carbon 

Monoxide. Office of Health and 

Environmental Assessment, 

Environmental Criteria and Assessment 

Office, Research Triangle Park, NC. EPA– 

600/8–79–022. 

U.S. Environmental Protection Agency. 

(1979b) Assessment of Adverse Health 

Effects from Carbon Monoxide and 

Implications for Possible Modifications 

of the Standard. Office of Air Quality 

Planning and Standards, Research 

Triangle Park, NC. 

U.S. Environmental Protection Agency. 

(1984a) Revised Evaluation of Health 

Effects Associated with Carbon 

Monoxide Exposure: An Addendum to 

the 1979 EPA Air Quality Criteria 

Document for Carbon Monoxide. Office 

of Health and Environmental 

Assessment, Environmental Criteria and 

Assessment Office, Research Triangle 

Park, NC. EPA–600/8–83–033F. 

U.S. Environmental Protection Agency. 

(1984b) Review of the NAAQS for 

Carbon Monoxide: Reassessment of 

Scientific and Technical Information. 

Office of Air Quality Planning and 

Standards, Research Triangle Park. NC. 

EPA–450/584–904. 

U.S. Environmental Protection Agency. 

(1991) Air Quality Criteria for Carbon 

Monoxide. Office of Health and 

Environmental Assessment, 

Environmental Criteria and Assessment 

Office, Research Triangle Park, NC. EPA/ 

600/8–90/045F. Available at: http:// 

www.epa.gov/ttn/naaqs/standards/co/ 

s_co_pr.html. 

U.S. Environmental Protection Agency. 

(1992) Review of the National Ambient 

Air Quality Standards for Carbon 

Monoxide: Assessment of Scientific and 

Technical Information, OAQPS Staff 

Paper. Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA/452/R–92–004. 

U.S. Environmental Protection Agency. 

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Monoxide. National Center for 

Environmental Assessment, Office of 

Research and Development, Research 

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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. 

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Integrated Review Plan. National Center 

for Environmental Assessment and 

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Review Draft. National Center for 

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naaqs/standards/co/s_co_cr_isa.html. 

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naaqs/standards/co/s_co_cr_isa.html. 

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and Methods Plan for Health Risk and 

Exposure Assessment. Draft. Office of 

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09–004. Available at: http:// 

www.epa.gov/ttn/naaqs/standards/co/

s_co_cr_pd.html. 

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(2009d) Risk and Exposure Assessment 

to Support the Review of the Carbon 

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Quality Standards, First External Review 

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for Carbon Monoxide. National Center 

for Environmental Assessment, Research 

Triangle Park, NC. EPA/600/R–09/019F. 

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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/

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(2010c) Policy Assessment for the 

Review of the Carbon Monoxide National 

Ambient Air Quality Standards. Office of 

Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA 452/R– 

10–007. Available at: http:// 

www.epa.gov/ttn/naaqs/standards/co/

s_co_cr_pa.html. 

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(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– 

HQ–OAR–2008–0015. 

Wellenius G.A.; Bateson T.F.; Mittleman 

M.A.; Schwartz J. (2005) Particulate air 

pollution and the rate of hospitalization 

for congestive heart failure among 

medicare beneficiaries in Pittsburgh, 

Pennsylvania. Am J Epidemiol 

161:1030–1036. 

Zhu Y.; Hinds W.C.; Kim S.; Shen S.; Sioutas 

C. (2002) Study of ultrafine particles near 

a major highway with heavy-duty diesel 

traffic. Atmos Environ, 36:4323–4335. 

List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 53 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: August 12, 2011. 

Lisa P. Jackson, 

Administrator. 

For the reasons stated in the 

preamble, title 40, chapter I of the Code 

of Federal Regulations is amended as 

follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

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

■ 2. Appendix C to part 50 is revised to 

read as follows: 

Appendix C to Part 50—Measurement 

Principle and Calibration Procedure for 

the Measurement of Carbon Monoxide 

in the Atmosphere (Non-Dispersive 

Infrared Photometry) 

1.0 Applicability 

1.1 This non-dispersive infrared 

photometry (NDIR) Federal Reference 

Method (FRM) provides measurements of the 

concentration of carbon monoxide (CO) in 

ambient air for determining compliance with 

the primary and secondary National Ambient 

Air Quality Standards (NAAQS) for CO as 

specified in § 50.8 of this chapter. The 

method is applicable to continuous sampling 

and measurement of ambient CO 

concentrations suitable for determining 1- 

hour or longer average measurements. The 

method may also provide measurements of 

shorter averaging times, subject to specific 

analyzer performance limitations. Additional 

CO monitoring quality assurance procedures 

and guidance are provided in part 58, 

appendix A, of this chapter and in reference 

1 of this appendix C. 

2.0 Measurement Principle 

2.1 Measurements of CO in ambient air 

are based on automated measurement of the 

absorption of infrared radiation by CO in an 

ambient air sample drawn into an analyzer 

employing non-wavelength-dispersive, 

infrared photometry (NDIR method). Infrared 

energy from a source in the photometer is 

passed through a cell containing the air 

sample to be analyzed, and the quantitative 

absorption of energy by CO in the sample cell 

is measured by a suitable detector. The 

photometer is sensitized specifically to CO 

by employing CO gas in a filter cell in the 

optical path, which, when compared to a 

differential optical path without a CO filter 

cell, limits the measured absorption to one or 

more of the characteristic wavelengths at 

which CO strongly absorbs. However, to meet 

measurement performance requirements, 

various optical filters, reference cells, 

rotating gas filter cells, dual-beam 

configurations, moisture traps, or other 

means may also be used to further enhance 

sensitivity and stability of the photometer 

and to minimize potential measurement 

interference from water vapor, carbon 

dioxide (CO2), or other species. Also, various 

schemes may be used to provide a suitable 

zero reference for the photometer, and 

optional automatic compensation may be 

provided for the actual pressure and 

temperature of the air sample in the 

measurement cell. The measured infrared 

absorption, converted to a digital reading or 

an electrical output signal, indicates the 

measured CO concentration. 

2.2 The measurement system is calibrated 

by referencing the analyzer’s CO 

measurements to CO concentration standards 

traceable to a National Institute of Standards 

and Technology (NIST) primary standard for 

CO, as described in the associated calibration 

procedure specified in section 4 of this 

reference method. 

2.3 An analyzer implementing this 

measurement principle will be considered a 

reference method only if it has been 

designated as a reference method in 

accordance with part 53 of this chapter. 

2.4 Sampling considerations. The use of a 

particle filter in the sample inlet line of a CO 

FRM analyzer is optional and left to the 

discretion of the user unless such a filter is 

specified or recommended by the analyzer 

manufacturer in the analyzer’s associated 

operation or instruction manual. 

3.0 Interferences 

3.1 The NDIR measurement principle is 

potentially susceptible to interference from 

water vapor and CO2, which have some 

infrared absorption at wavelengths in 

common with CO and normally exist in the 

atmosphere. Various instrumental techniques 

can be used to effectively minimize these 

interferences. 


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4.0 Calibration Procedures 

4.1 Principle. Either of two methods may 

be selected for dynamic multipoint 

calibration of FRM CO analyzers, using test 

gases of accurately known CO concentrations 

obtained from one or more compressed gas 

cylinders certified as CO transfer standards: 

4.1.1 Dilution method: A single certified 

standard cylinder of CO is quantitatively 

diluted as necessary with zero air to obtain 

the various calibration concentration 

standards needed. 

4.1.2 Multiple-cylinder method: Multiple, 

individually certified standard cylinders of 

CO are used for each of the various 

calibration concentration standards needed. 

4.1.3 Additional information on 

calibration may be found in Section 12 of 

reference 1. 

4.2 Apparatus. The major components 

and typical configurations of the calibration 

systems for the two calibration methods are 

shown in Figures 1 and 2. Either system may 

be made up using common laboratory 

components, or it may be a commercially 

manufactured system. In either case, the 

principal components are as follows: 

4.2.1 CO standard gas flow control and 

measurement devices (or a combined device) 

capable of regulating and maintaining the 

standard gas flow rate constant to within ± 2 

percent and measuring the gas flow rate 

accurate to within ± 2 percent, properly 

calibrated to a NIST-traceable standard. 

4.2.2 For the dilution method (Figure 1), 

dilution air flow control and measurement 

devices (or a combined device) capable of 

regulating and maintaining the air flow rate 

constant to within ± 2 percent and measuring 

the air flow rate accurate to within ± 2 

percent, properly calibrated to a NISTtraceable standard. 

4.2.3 Standard gas pressure regulator(s) 

for the standard CO cylinder(s), suitable for 

use with a high-pressure CO gas cylinder and 

having a non-reactive diaphragm and internal 

parts and a suitable delivery pressure. 

4.2.4 Mixing chamber for the dilution 

method of an inert material and of proper 

design to provide thorough mixing of CO 

standard gas and diluent air streams. 

4.2.5 Output sampling manifold, 

constructed of an inert material and of 

sufficient diameter to ensure an insignificant 

pressure drop at the analyzer connection. 

The system must have a vent designed to 

ensure nearly atmospheric pressure at the 

analyzer connection port and to prevent 

ambient air from entering the manifold. 

4.3 Reagents 

4.3.1 CO gas concentration transfer 

standard(s) of CO in air, containing an 

appropriate concentration of CO suitable for 

the selected operating range of the analyzer 

under calibration and traceable to a NIST 

standard reference material (SRM). If the CO 

analyzer has significant sensitivity to CO2, 

the CO standard(s) should also contain 350 

to 400 ppm CO2 to replicate the typical CO2

concentration in ambient air. However, if the 

zero air dilution ratio used for the dilution 

method is not less than 100:1 and the zero 

air contains ambient levels of CO2, then the 

CO standard may be contained in nitrogen 

and need not contain CO2. 

4.3.2 For the dilution method, clean zero 

air, free of contaminants that could cause a 

detectable response on or a change in 

sensitivity of the CO analyzer. The zero air 

should contain < 0.1 ppm CO. 

4.4 Procedure Using the Dilution Method 

4.4.1 Assemble or obtain a suitable 

dynamic dilution calibration system such as 

the one shown schematically in Figure 1. 

Generally, all calibration gases including zero 

air must be introduced into the sample inlet 

of the analyzer. However, if the analyzer has 

special, approved zero and span inlets and 

automatic valves to specifically allow 

introduction of calibration standards at near 

atmospheric pressure, such inlets may be 

used for calibration in lieu of the sample 

inlet. For specific operating instructions, 

refer to the manufacturer’s manual. 

4.4.2 Ensure that there are no leaks in the 

calibration system and that all flowmeters are 

properly and accurately calibrated, under the 

conditions of use, if appropriate, against a 

reliable volume or flow rate standard such as 

a soap-bubble meter or wet-test meter 

traceable to a NIST standard. All volumetric 

flow rates should be corrected to the same 

temperature and pressure such as 298.15 K 

(25 °C) and 760 mm Hg (101 kPa), using a 

correction formula such as the following: 

Where: 

Fc = corrected flow rate (L/min at 25 °C and 

760 mm Hg), 

Fm = measured flow rate (at temperature Tm

and pressure Pm), 

Pm = measured pressure in mm Hg (absolute), 

and 

Tm = measured temperature in degrees 

Celsius. 

4.4.3 Select the operating range of the CO 

analyzer to be calibrated. Connect the 

measurement signal output of the analyzer to 

an appropriate readout instrument to allow 

the analyzer’s measurement output to be 

continuously monitored during the 

calibration. Where possible, this readout 

instrument should be the same one used to 

record routine monitoring data, or, at least, 

an instrument that is as closely 

representative of that system as feasible. 

4.4.4 Connect the inlet of the CO analyzer 

to the output-sampling manifold of the 

calibration system. 

4.4.5 Adjust the calibration system to 

deliver zero air to the output manifold. The 

total air flow must exceed the total demand 

of the analyzer(s) connected to the output 

manifold to ensure that no ambient air is 

pulled into the manifold vent. Allow the 

analyzer to sample zero air until a stable 

response is obtained. After the response has 

stabilized, adjust the analyzer zero reading. 

4.4.6 Adjust the zero air flow rate and the 

CO gas flow rate from the standard CO 

cylinder to provide a diluted CO 

concentration of approximately 80 percent of 

the measurement upper range limit (URL) of 

the operating range of the analyzer. The total 

air flow rate must exceed the total demand 

of the analyzer(s) connected to the output 

manifold to ensure that no ambient air is 

pulled into the manifold vent. The exact CO 

concentration is calculated from: 

Where: 

[CO]OUT = diluted CO concentration at the 

output manifold (ppm), 

[CO]STD = concentration of the undiluted CO 

standard (ppm), 

FCO = flow rate of the CO standard (L/min), 

and 

FD = flow rate of the dilution air (L/min). 

Sample this CO concentration until a stable 

response is obtained. Adjust the analyzer 

span control to obtain the desired analyzer 

response reading equivalent to the calculated 

standard concentration. If substantial 

adjustment of the analyzer span control is 

required, it may be necessary to recheck the 

zero and span adjustments by repeating steps 

4.4.5 and 4.4.6. Record the CO concentration 

and the analyzer’s final response. 

4.4.7 Generate several additional 

concentrations (at least three evenly spaced 

points across the remaining scale are 

suggested to verify linearity) by decreasing 

FCO or increasing FD. Be sure the total flow 

exceeds the analyzer’s total flow demand. For 

each concentration generated, calculate the 

exact CO concentration using equation (2). 

Record the concentration and the analyzer’s 

stable response for each concentration. Plot 

the analyzer responses (vertical or y-axis) 

versus the corresponding CO concentrations 

(horizontal or x-axis). Calculate the linear 

regression slope and intercept of the 

calibration curve and verify that no point 

deviates from this line by more than 2 

percent of the highest concentration tested. 

4.5 Procedure Using the MultipleCylinder Method. Use the procedure for the 

dilution method with the following changes: 

4.5.1 Use a multi-cylinder, dynamic 

calibration system such as the typical one 

shown in Figure 2. 

4.5.2 The flowmeter need not be 

accurately calibrated, provided the flow in 

the output manifold can be verified to exceed 

the analyzer’s flow demand. 

4.5.3 The various CO calibration 

concentrations required in Steps 4.4.5, 4.4.6, 

and 4.4.7 are obtained without dilution by 

selecting zero air or the appropriate certified 

standard cylinder. 

4.6 Frequency of Calibration. The 

frequency of calibration, as well as the 

number of points necessary to establish the 

calibration curve and the frequency of other 

performance checking, will vary by analyzer. 

However, the minimum frequency, 

acceptance criteria, and subsequent actions 

are specified in reference 1, appendix D, 

‘‘Measurement Quality Objectives and 

Validation Template for CO’’ (page 5 of 30). 

The user’s quality control program should 

provide guidelines for initial establishment 

of these variables and for subsequent 

alteration as operational experience is 

accumulated. Manufacturers of CO analyzers 

should include in their instruction/operation 

manuals information and guidance as to 

these variables and on other matters of 

operation, calibration, routine maintenance, 

and quality control. 


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5.0 Reference 

1. QA Handbook for Air Pollution 

Measurement Systems—Volume II. Ambient 

Air Quality Monitoring Program. U.S. EPA. 

EPA–454/B–08–003 (2008). 

BILLING CODE 6560–50–P 


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BILLING CODE 6560–50–C 

PART 53—AMBIENT AIR QUALITY 

REFERENCE AND EQUIVALENT 

METHODS 

■ 3. The authority citation for part 53 

continues to read as follows: 

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

■ 4. Subpart B of part 53 is revised to 

read as follows: 

Subpart B—Procedures for Testing 

Performance Characteristics of Automated 

Methods for SO2, CO, O3, and NO2

Sec. 

53.20 General provisions. 

53.21 Test conditions. 

53.22 Generation of test atmospheres. 

53.23 Test procedure. 

Figure B–1 to Subpart B of Part 53—Example 

Table B–1 to Subpart B of Part 53— 

Performance Limit Specifications for 

Automated Methods 

Table B–2 to Subpart B of Part 53—Test 

Atmospheres 

Table B–3 to Subpart B of Part 53— 

Interferent Test Concentration, 1 Parts 

Per Million 

Table B–4 to Subpart B of Part 53— Line 

Voltage and Room Temperature Test 

Conditions 

Table B–5 to Subpart B of Part 53—Symbols 

and Abbreviations 

Appendix A to Subpart B—Optional Forms 

for Reporting Test Results 

Subpart B—Procedures for Testing 

Performance Characteristics of 

Automated Methods for SO2, CO, O3, 

and NO2

§ 53.20 General provisions. 

(a) The test procedures given in this 

subpart shall be used to test the 

performance of candidate automated 

methods against the performance 

requirement specifications given in 

table B–1 to subpart B of part 53. A test 

analyzer representative of the candidate 

automated method must exhibit 

performance better than, or not outside, 

the specified limit or limits for each 

such performance parameter specified 

(except range) to satisfy the 

requirements of this subpart. Except as 

provided in paragraph (b) of this 

section, the measurement range of the 

candidate method must be the standard 

range specified in table B–1 to subpart 

B of part 53 to satisfy the requirements 

of this subpart. 

(b) Measurement ranges. For a 

candidate method having more than one 

selectable measurement range, one 

range must be the standard range 

specified in table B–1 to subpart B of 

part 53, and a test analyzer 

representative of the method must pass 

the tests required by this subpart while 

operated in that range. 

(i) Higher ranges. The tests may be 

repeated for one or more higher 

(broader) ranges (i.e., ranges extending 

to higher concentrations) than the 

standard range specified in table B–1 to 

subpart B of part 53, provided that the 

range does not extend to concentrations 

more than four times the upper range 

limit of the standard range specified in 

table B–1 to subpart B of part 53. For 

such higher ranges, only the tests for 

range (calibration), noise at 80% of the 

upper range limit, and lag, rise and fall 

time are required to be repeated. For the 

purpose of testing a higher range, the 

test procedure of § 53.23(e) may be 

abridged to include only those 

components needed to test lag, rise and 

fall time. 

(ii) Lower ranges. The tests may be 

repeated for one or more lower 

(narrower) ranges (i.e., ones extending 

to lower concentrations) than the 

standard range specified in table B–1 to 

subpart B of part 53. For methods for 

some pollutants, table B–1 to subpart B 

of part 53 specifies special performance 

limit requirements for lower ranges. If 

special low-range performance limit 

requirements are not specified in table 

B–1 to subpart B of part 53, then the 

performance limit requirements for the 

standard range apply. For lower ranges 

for any method, only the tests for range 

(calibration), noise at 0% of the 


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measurement range, lower detectable 

limit, (and nitric oxide interference for 

SO2 UVF methods) are required to be 

repeated, provided the tests for the 

standard range shows the applicable 

limit specifications are met for the other 

test parameters. 

(iii) If the tests are conducted and 

passed only for the specified standard 

range, any FRM or FEM determination 

with respect to the method will be 

limited to that range. If the tests are 

passed for both the specified range and 

one or more higher or lower ranges, any 

such determination will include the 

additional higher or lower range(s) as 

well as the specified standard range. 

Appropriate test data shall be submitted 

for each range sought to be included in 

a FRM or FEM method determination 

under this paragraph (b). 

(c) For each performance parameter 

(except range), the test procedure shall 

be initially repeated seven (7) times to 

yield 7 test results. Each result shall be 

compared with the corresponding 

performance limit specification in table 

B–1 to subpart B of part 53; a value 

higher than or outside the specified 

limit or limits constitutes a failure. 

These 7 results for each parameter shall 

be interpreted as follows: 

(1) Zero (0) failures: The candidate 

method passes the test for the 

performance parameter. 

(2) Three (3) or more failures: The 

candidate method fails the test for the 

performance parameter. 

(3) One (1) or two (2) failures: Repeat 

the test procedures for the performance 

parameter eight (8) additional times 

yielding a total of fifteen (15) test 

results. The combined total of 15 test 

results shall then be interpreted as 

follows: 

(i) One (1) or two (2) failures: The 

candidate method passes the test for the 

performance parameter. 

(ii) Three (3) or more failures: The 

candidate method fails the test for the 

performance parameter. 

(d) The tests for zero drift, span drift, 

lag time, rise time, fall time, and 

precision shall be carried out in a single 

integrated procedure conducted at 

various line voltages and ambient 

temperatures specified in § 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. 

§ 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 at least 25 centimeters, chart speeds 

up to 10 cm per hour, a response time 

of 1 second or less, a deadband of not 

more than 0.25 percent of full scale, and 

capability either of reading 

measurements at least 5 percent below 

zero or of offsetting the zero by at least 

5 percent. If the test analyzer does not 

have an analog signal output, or if a 

digital or other type of measurement 

data output is used for the tests, an 

alternative measurement data recording 

device (or devices) may be used for 

recording the test data, provided that 

the device is reasonably suited to the 

nature and purposes of the tests, and an 

analog representation of the analyzer 

measurements for each test can be 

plotted or otherwise generated that is 

reasonably similar to the analog 

measurement recordings that would be 

produced by a conventional chart 

recorder connected to a conventional 

analog signal output. 

(b) Calibration of the test analyzer 

shall be carried out prior to conducting 

the tests described in this subpart. The 

calibration shall be as indicated in the 

manual referred to in § 53.4(b)(3) and as 

follows: If the chart recorder or 

alternative data recorder does not have 

below zero capability, adjust either the 

controls of the test analyzer or the chart 

or data recorder to obtain a +5% offset 

zero reading on the recorder chart to 

facilitate observing negative response or 

drift. If the candidate method is not 

capable of negative response, the test 

analyzer (not the data recorder) shall be 

operated with a similar offset zero. 

Construct and submit a calibration 

curve showing a plot of recorder scale 

readings or other measurement output 

readings (vertical or y-axis) against 

pollutant concentrations presented to 

the analyzer for measurement 

(horizontal or x-axis). If applicable, a 

plot of base analog output units (volts, 

millivolts, milliamps, etc.) against 

pollutant concentrations shall also be 

obtained and submitted. All such 

calibration plots shall consist of at least 

seven (7) approximately equally spaced, 

identifiable points, including 0 and 90 

± 5 percent of the upper range limit 

(URL). 

(c) Once the test analyzer has been set 

up and calibrated and the tests started, 

manual adjustment or normal periodic 

maintenance is permitted only every 3 

days. Automatic adjustments which the 

test analyzer performs by itself are 

permitted at any time. The submitted 

records shall show clearly when any 

manual adjustment or periodic 

maintenance was made during the tests 

and describe the specific operations 

performed. 

(d) If the test analyzer should 

malfunction during any of the 

performance tests, the tests for that 

parameter shall be repeated. A detailed 

explanation of the malfunction, 

remedial action taken, and whether 

recalibration was necessary (along with 

all pertinent records and charts) shall be 

submitted. If more than one malfunction 

occurs, all performance test procedures 

for all parameters shall be repeated. 

(e) Tests for all performance 

parameters shall be completed on the 

same test analyzer; however, use of 

multiple test analyzers to accelerate 

testing is permissible for testing 

additional ranges of a multi-range 

candidate method. 

§ 53.22 Generation of test atmospheres. 

(a) Table B–2 to subpart B of part 53 

specifies preferred methods for 

generating test atmospheres and 

suggested methods of verifying their 

concentrations. Only one means of 

establishing the concentration of a test 

atmosphere is normally required, 

provided that that means is adequately 

accurate and credible. If the method of 

generation can produce accurate, 

reproducible concentrations, 

verification is optional. If the method of 

generation is not reproducible or 

reasonably quantifiable, then 

establishment of the concentration by 


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some credible verification method is 

required. 

(b) The test atmosphere delivery 

system shall be designed and 

constructed so as not to significantly 

alter the test atmosphere composition or 

concentration during the period of the 

test. The system shall be vented to 

insure that test atmospheres are 

presented to the test analyzer at very 

nearly atmospheric pressure. The 

delivery system shall be fabricated from 

borosilicate glass, FEP Teflon, or other 

material that is inert with regard to the 

gas or gases to be used. 

(c) The output of the test atmosphere 

generation system shall be sufficiently 

stable to obtain stable response readings 

from the test analyzer during the 

required tests. If a permeation device is 

used for generation of a test atmosphere, 

the device, as well as the air passing 

over it, shall be controlled to 0.1 °C. 

(d) All diluent air shall be zero air free 

of contaminants likely to react with the 

test atmospheres or cause a detectable 

response on the test analyzer. 

(e) The concentration of each test 

atmosphere used shall be quantitatively 

established and/or verified before or 

during each series of tests. Samples for 

verifying test concentrations shall be 

collected from the test atmosphere 

delivery system as close as feasible to 

the sample intake port of the test 

analyzer. 

(f) The accuracy of all flow 

measurements used to calculate test 

atmosphere concentrations shall be 

documented and referenced to a 

primary flow rate or volume standard 

(such as a spirometer, bubble meter, 

etc.). Any corrections shall be clearly 

shown. All flow measurements given in 

volume units shall be standardized to 

25 °C and 760 mm Hg. 

(g) Schematic drawings, photos, 

descriptions, and other information 

showing complete procedural details of 

the test atmosphere generation, 

verification, and delivery system shall 

be provided. All pertinent calculations 

shall be clearly indicated. 

§ 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). 

(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 to subpart B 

of part 53 shall apply to both of these 

tests. 

(ii) For an analyzer with an analog 

signal output, connect an integratingtype 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.

(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 B–1 to 

subpart B of part 53. 

(vii) Repeat steps in Paragraphs 

(b)(2)(iii) through (v) of this section 

using a pollutant test atmosphere 

concentration of 80 ± 5 percent of the 

URL instead of zero air, and let S at 80 

percent of the URL be identified as S80. 

Compare S80 to the noise limit 

specification given in table B–1 to 

subpart B of part 53. 

(viii) Both S0 and S80 must be less 

than or equal to the table B–1 to subpart 

B of part 53 noise limit specification to 

pass the test for the noise parameter. 

(c) Lower detectable limit—(1) 

Technical definition. The minimum 

pollutant concentration that produces a 

measurement or measurement output 

signal of at least twice the noise level. 

(2) Test procedure. (i) Allow sufficient 

time for the test analyzer to warm up 

and stabilize. Measure zero air and 

record the stable measurement reading 

in ppm as BZ. (See Figure B–3 in 

appendix A of this subpart.) 

(ii) Generate and measure a pollutant 

test concentration equal to the value for 

the lower detectable limit specified in 

table B–1 to subpart B of part 53. 

Note to § 53.23(c)(2): If necessary, the test 

concentration may be generated or verified at 

a higher concentration, then quantitatively 

and accurately diluted with zero air to the 

final required test concentration. 

(iii) Record the test analyzer’s stable 

measurement reading, in ppm, as BL. 

(iv) Determine the lower detectable 

limit (LDL) test result as LDL = BL ¥ BZ. 

Compare this LDL value with the noise 

level, S0, determined in § 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 to 

subpart B of part 53. In the event that 

there are substances likely to cause a 

significant interference which have not 

been specified in table B–3 to subpart B 

of part 53, these substances shall also be 

tested, in a manner similar to that for 

the specified interferents, at a 

concentration substantially higher than 

that likely to be found in the ambient 

air. The interference may be either 

positive or negative, depending on 

whether the test analyzer’s 

measurement response is increased or 

decreased by the presence of the 

interferent. Interference equivalents 

shall be determined by mixing each 

interferent, one at a time, with the 

pollutant at an interferent test 

concentration not lower than the test 

concentration specified in table B–3 to 

subpart B of part 53 (or as otherwise 

required for unlisted interferents), and 


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comparing the test analyzer’s 

measurement response to the response 

caused by the pollutant alone. Known 

gas-phase reactions that might occur 

between a listed interferent and the 

pollutant are designated by footnote 3 in 

table B–3 to subpart B of part 53. In 

these cases, the interference equivalent 

shall be determined without mixing 

with the pollutant. 

(i) Allow sufficient time for warm-up 

and stabilization of the test analyzer. 

(ii) For a candidate method using a 

prefilter or scrubber device based upon 

a chemical reaction to derive part of its 

specificity and which device requires 

periodic service or maintenance, the test 

analyzer shall be ‘‘conditioned’’ prior to 

conducting each interference test series. 

This requirement includes conditioning 

for the NO2 converter in 

chemiluminescence NO/NO2/NOX

analyzers and for the ozone scrubber in 

UV-absorption ozone analyzers. 

Conditioning is as follows: 

(A) Service or perform the indicated 

maintenance on the scrubber or prefilter 

device, as if it were due for such 

maintenance, as directed in the manual 

referred to in § 53.4(b)(3). 

(B) Before testing for each potential 

interferent, allow the test analyzer to 

sample through the prefilter or scrubber 

device a test atmosphere containing the 

interferent at a concentration not lower 

than the value specified in table B–3 to 

subpart B of part 53 (or, for unlisted 

potential interferents, at a concentration 

substantially higher than likely to be 

found in ambient air). Sampling shall be 

at the normal flow rate and shall be 

continued for 6 continuous hours prior 

to the interference test series. 

Conditioning for all applicable 

interferents prior to any of the 

interference tests is permissible. Also 

permissible is simultaneous 

conditioning with multiple interferents, 

provided no interferent reactions are 

likely to occur in the conditioning 

system. 

(iii) Generate three test atmosphere 

streams as follows: 

(A) Test atmosphere P: Pollutant test 

concentration. 

(B) Test atmosphere I: Interferent test 

concentration. 

(C) Test atmosphere Z: Zero air. 

(iv) Adjust the individual flow rates 

and the pollutant or interferent 

generators for the three test atmospheres 

as follows: 

(A) The flow rates of test atmospheres 

I and Z shall be equal. 

(B) The concentration of the pollutant 

in test atmosphere P shall be adjusted 

such that when P is mixed (diluted) 

with either test atmosphere I or Z, the 

resulting concentration of pollutant 

shall be as specified in table B–3 to 

subpart B of part 53. 

(C) The concentration of the 

interferent in test atmosphere I shall be 

adjusted such that when I is mixed 

(diluted) with test atmosphere P, the 

resulting concentration of interferent 

shall be not less than the value specified 

in table B–3 to subpart B of part 53 (or 

as otherwise required for unlisted 

potential interferents). 

(D) To minimize concentration errors 

due to flow rate differences between I 

and Z, it is recommended that, when 

possible, the flow rate of P be from 10 

to 20 times larger than the flow rates of 

I and Z. 

(v) Mix test atmospheres P and Z by 

passing the total flow of both 

atmospheres through a (passive) mixing 

component to insure complete mixing of 

the gases. 

(vi) Sample and measure the mixture 

of test atmospheres P and Z with the test 

analyzer. Allow for a stable 

measurement reading, and record the 

reading, in concentration units, as R (see 

Figure B–3). 

(vii) Mix test atmospheres P and I by 

passing the total flow of both 

atmospheres through a (passive) mixing 

component to insure complete mixing of 

the gases. 

(viii) Sample and measure this 

mixture of P and I with the test 

analyzer. Record the stable 

measurement reading, in concentration 

units, as RI. 

(ix) Calculate the interference 

equivalent (IE) test result as: 

IE = RI ¥ R. 

IE must be within the limits (inclusive) 

specified in table B–1 to subpart B of 

part 53 for each interferent tested to 

pass the interference equivalent test. 

(x) Follow steps (iii) through (ix) of 

this section, in turn, to determine the 

interference equivalent for each listed 

interferent as well as for any other 

potential interferents identified. 

(xi) For those potential interferents 

which cannot be mixed with the 

pollutant, as indicated by footnote (3) in 

table B–3 to subpart B of part 53, adjust 

the concentration of test atmosphere I to 

the specified value without being mixed 

or diluted by the pollutant test 

atmosphere. Determine IE as follows: 

(A) Sample and measure test 

atmosphere Z (zero air). Allow for a 

stable measurement reading and record 

the reading, in concentration units, as R. 

(B) Sample and measure the 

interferent test atmosphere I. If the test 

analyzer is not capable of negative 

readings, adjust the analyzer (not the 

recorder) to give an offset zero. Record 

the stable reading in concentration units 

as RI, extrapolating the calibration 

curve, if necessary, to represent negative 

readings. 

(C) Calculate IE = RI ¥ R. IE must be 

within the limits (inclusive) specified in 

table B–1 to subpart B of part 53 for 

each interferent tested to pass the 

interference equivalent test. 

(xii) Sum the absolute value of all the 

individual interference equivalent test 

results. This sum must be equal to or 

less than the total interferent limit given 

in table B–1 to subpart B of part 53 to 

pass the test. 

(e) Zero drift, span drift, lag time, rise 

time, fall time, and precision—(1) 

Technical definitions—(i) Zero drift: 

The change in measurement response to 

zero pollutant concentration over 12- 

and 24-hour periods of continuous 

unadjusted operation. 

(ii) Span drift: The percent change in 

measurement response to an up-scale 

pollutant concentration over a 24-hour 

period of continuous unadjusted 

operation. 

(iii) Lag time: The time interval 

between a step change in input 

concentration and the first observable 

corresponding change in measurement 

response. 

(iv) Rise time: The time interval 

between initial measurement response 

and 95 percent of final response after a 

step increase in input concentration. 

(v) Fall time: The time interval 

between initial measurement response 

and 95 percent of final response after a 

step decrease in input concentration. 

(vi) Precision: Variation about the 

mean of repeated measurements of the 

same pollutant concentration, expressed 

as one standard deviation. 

(2) Tests for these performance 

parameters shall be accomplished over 

a period of seven (7) or fifteen (15) test 

days. During this time, the line voltage 

supplied to the test analyzer and the 

ambient temperature surrounding the 

analyzer shall be changed from day to 

day, as required in paragraph (e)(4) of 

this section. One test result for each 

performance parameter shall be 

obtained each test day, for seven (7) or 

fifteen (15) test days, as determined 

from the test results of the first seven 

days. The tests for each test day are 

performed in a single integrated 

procedure. 

(3) The 24-hour test day may begin at 

any clock hour. The first approximately 

12 hours of each test day are required 

for testing 12-hour zero drift. Tests for 

the other parameters shall be conducted 

any time during the remaining 12 hours. 

(4) Table B–4 to subpart B of part 53 

specifies the line voltage and room 

temperature to be used for each test day. 

The applicant may elect to specify a 


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wider temperature range (minimum and 

maximum temperatures) than the range 

specified in table B–4 to subpart B of 

part 53 and to conduct these tests over 

that wider temperature range in lieu of 

the specified temperature range. If the 

test results show that all test parameters 

of this section § 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 °C. 

(5) The tests shall be conducted in 

blocks consisting of 3 test days each 

until 7 (or 15, if necessary) test results 

have been obtained. (The final block 

may contain fewer than three test days.) 

Test days need not be contiguous days, 

but during any idle time between tests 

or test days, the test analyzer must 

operate continuously and measurements 

must be recorded continuously at a low 

chart speed (or equivalent data 

recording) and included with the test 

data. If a test is interrupted by an 

occurrence other than a malfunction of 

the test analyzer, only the block during 

which the interruption occurred shall be 

repeated. 

(6) During each test block, manual 

adjustments to the electronics, gas, or 

reagent flows or periodic maintenance 

shall not be permitted. Automatic 

adjustments that the test analyzer 

performs by itself are permitted at any 

time. 

(7) At least 4 hours prior to the start 

of the first test day of each test block, 

the test analyzer may be adjusted and/ 

or serviced according to the periodic 

maintenance procedures specified in the 

manual referred to in § 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 to subpart B of part 53 

illustrating the pattern of the required 

readings.) 

(9) Test procedure. (i) Arrange to 

generate pollutant test atmospheres as 

follows. Test atmospheres A0, A20, and 

A80 shall be maintained consistent 

during the tests and reproducible from 

test day to test day. 

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 °C. 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.] 

(vi) Measure test atmosphere A80.

Allow for a stable measurement reading 

and record it as S’n, where n = 0. 

(vii) The above readings for Z’0 and 

S’0 should be taken at least four (4) 

hours prior to the beginning of test 

day 1. 

(viii) At the beginning of each test 

day, adjust the line voltage and room 

temperature to the values given in table 

B–4 to subpart B of part 53 (or to the 

corresponding alternative temperature if 

a wider temperature range is being 

tested). 

(ix) Measure test atmosphere A0

continuously for at least twelve (12) 

continuous hours during each test day. 

(x) After the 12-hour zero drift test 

(step ix) is complete, sample test 

atmosphere A0. A stable reading is not 

required. 

(xi) Measure test atmosphere A20 and 

record the stable reading (in ppm) as P1.

(See Figure B–4 in appendix A.) 

(xii) Sample test atmosphere A30; a 

stable reading is not required. 

(xiii) Measure test atmosphere A20

and record the stable reading as P2.

(xiv) Sample test atmosphere A0; a 

stable reading is not required. 

(xv) Measure test atmosphere A20 and 

record the stable reading as P3.

(xvi) Sample test atmosphere A30; a 

stable reading is not required. 

(xvii) Measure test atmosphere A20

and record the stable reading as P4.

(xviii) Sample test atmosphere A0; a 

stable reading is not required. 

(xix) Measure test atmosphere A20 and 

record the stable reading as P5.

(xx) Sample test atmosphere A30; a 

stable reading is not required. 

(xxi) Measure test atmosphere A20 and 

record the stable reading as P6.

(xxii) Measure test atmosphere A80

and record the stable reading as P7.

(xxiii) Sample test atmosphere A90; a 

stable reading is not required. 

(xxiv) Measure test atmosphere A80

and record the stable reading as P8.

Increase the chart speed to at least 10 

centimeters per hour. 

(xxv) Measure test atmosphere A0.

Record the stable reading as L1.

(xxvi) Quickly switch the test 

analyzer to measure test atmosphere A80

and mark the recorder chart to show, or 

otherwise record, the exact time when 

the switch occurred. 

(xxvii) Measure test atmosphere A80

and record the stable reading as P9.

(xxviii) Sample test atmosphere A90; a 

stable reading is not required. 

(xxix) Measure test atmosphere A80

and record the stable reading as P10.

(xxx) Measure test atmosphere A0 and 

record the stable reading as L2.

(xxxi) Measure test atmosphere A80

and record the stable reading as P11.

(xxxii) Sample test atmosphere A90; a 

stable reading is not required. 

(xxxiii) Measure test atmosphere A80

and record the stable reading as P12.

(xxxiv) Repeat steps within 

paragraphs (e)(9)(viii) through 

(e)(9)(xxxiii) of this section, each test 

day. 

(xxxv) If zero and span adjustments 

are made after the readings are taken on 


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test days 3, 6, 9, or 12, complete all 

adjustments; then measure test 

atmospheres A0 and A80. Allow for a 

stable reading on each, and record the 

readings as Z’n and S’n, respectively, 

where n = the test day number (3, 6, 9, 

or 12). These readings must be made at 

least 4 hours prior to the start of the 

next test day. 

(10) Determine the results of each 

day’s tests as follows. Mark the recorder 

chart to show readings and 

determinations. 

(i) Zero drift. (A) Determine the 12- 

hour zero drift by examining the strip 

chart pertaining to the 12-hour 

continuous zero air test. Determine the 

minimum (Cmin.) and maximum (Cmax.) 

measurement readings (in ppm) during 

this period of 12 consecutive hours, 

extrapolating the calibration curve to 

negative concentration units if 

necessary. Calculate the 12-hour zero 

drift (12ZD) as 12ZD = Cmax. ¥ Cmin. (See 

Figure B–5 in appendix A.) 

(B) Calculate the 24-hour zero drift 

(24ZD) for the n-th test day as 24ZDn = 

Zn ¥ Zn-1, or 24ZDn = Zn ¥ Z’n-1 if zero 

adjustment was made on the previous 

test day, where Zn = 1⁄2(L1+L2) for L1 and 

L2 taken on the n-th test day. 

(C) Compare 12ZD and 24ZD to the 

zero drift limit specifications in table 

B–1 to subpart B of part 53. Both 12ZD 

and 24ZD must be within the specified 

limits (inclusive) to pass the test for zero 

drift. 

(ii) Span drift. 

(A) Calculate the span drift (SD) as: 

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. 

(B) SD must be within the span drift 

limits (inclusive) specified in table B–1 

to subpart B of part 53 to pass the test 

for span drift. 

(iii) Lag time. Determine, from the 

strip chart (or alternative test data 

record), the elapsed time in minutes 

between the change in test 

concentration (or mark) made in step 

(xxvi) and the first observable (two 

times the noise level) measurement 

response. This time must be equal to or 

less than the lag time limit specified in 

table B–1 to subpart B of part 53 to pass 

the test for lag time. 

(iv) Rise time. Calculate 95 percent of 

measurement reading P9 and determine, 

from the recorder chart (or alternative 

test data record), the elapsed time 

between the first observable (two times 

noise level) measurement response and 

a response equal to 95 percent of the P9

reading. This time must be equal to or 

less than the rise time limit specified in 

table B–1 to subpart B of part 53 to pass 

the test for rise time. 

(v) Fall time. Calculate five percent of 

(P10 ¥ L2) and determine, from the strip 

chart (or alternative test record), the 

elapsed time in minutes between the 

first observable decrease in 

measurement response following 

reading P10 and a response equal to L2

+ five percent of (P10 ¥ L2). This time 

must be equal to or less than the fall 

time limit specification in table B–1 to 

subpart B of part 53 to pass the test for 

fall time. 

(vi) Precision. Calculate precision 

(both P20 and P80) for each test day as 

follows: 

(A) 

(B) 

(C) Both P20 and P80 must be equal to 

or less than the precision limits 

specified in table B–1 to subpart B of 

part 53 to pass the test for precision. 

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Figure B–1 to Subpart B of Part 53— 

Example 


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BILLING CODE 6560–50–C 

TABLE B–2 TO SUBPART B OF PART 53—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 ......... Cylinder 1 of 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 .................... Cylinder 1 of 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 gas-phase 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. 


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TABLE B–2 TO SUBPART B OF PART 53—TEST ATMOSPHERES—Continued 

Test gas Generation Verification 

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. 

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 dew-point 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.

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. 


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TABLE B–3 TO

SUBPART B OF

PART 53—INTERFERENT

TEST

CONCENTRATION,1 PARTS PER

MILLION

Pollutant Analyzer type 

Hydrochloric 

acid 

Ammonia Hydro- gen sulfide Sulfur dioxide Nitrogen dioxide Nitric oxide Carbon dioxide Ethylene Ozone Mxylene Water vapor Carbon mon- oxide Methane Ethane Naph- thalene 

SO2 ............. Ultraviolet fluorescence ................ .............. .............. 5 0.1 4 0.14 0.5 0.5 .............. .............. 0.5 0.2 20,000 .............. .............. .............. 6 0.05 SO2 ............. Flame photometric ....................... .............. .............. 0.01 4 0.14 .............. .............. 750 .............. .............. .............. 3 20,000 50 .............. .............. ..............SO2 ............. Gas chromatography ................... .............. .............. 0.1 4 0.14 .............. .............. 750 .............. .............. .............. 3 20,000 50 .............. .............. ..............SO2 ............. Spectrophotometric-wet chemical 

(pararosanaline).

0.2 0.1 0.1 4 0.14 0.5 .............. 750 .............. 0.5 .............. .............. .............. .............. .............. ..............

SO2 ............. Electrochemical ............................ 0.2 0.1 0.1 4 0.14 0.5 0.5 .............. 0.2 0.5 .............. 3 20,000 .............. .............. .............. ..............SO2 ............. Conductivity .................................. 0.2 0.1 .............. 4 0.14 0.5 .............. 750 .............. .............. .............. .............. .............. .............. .............. ..............SO2 ............. Spectrophotometric-gas phase, 

including DOAS.

.............. .............. .............. 4 0.14 0.5 .............. .............. .............. 0.5 0.2 .............. .............. .............. .............. ..............

O3 ............... Chemiluminescent ........................ .............. .............. 3 0.1 .............. .............. .............. 750 .............. 4 0.08 .............. 3 20,000 .............. .............. .............. ..............O3 ............... Electrochemical ............................ .............. 3 0.1 .............. 0.5 0.5 .............. .............. .............. 4 0.08 .............. .............. .............. .............. .............. ..............O3 ............... Spectrophotometric-wet chemical 

(potassium iodide).

.............. 3 0.1 .............. 0.5 0.5 3 0.5 .............. .............. 4 0.08 .............. .............. .............. .............. .............. ..............

O3 ............... Spectrophotometric-gas phase, 

including ultraviolet absorption and DOAS).

.............. .............. .............. 0.5 0.5 0.5 .............. .............. 4 0.08 0.02 20,000 .............. .............. .............. ..............

CO .............. Non-dispersive Infrared ................ .............. .............. .............. .............. .............. .............. 750 .............. .............. .............. 20,000 4 10 .............. .............. ..............CO .............. Gas chromatography with flame 

ionization detector.

.............. .............. .............. .............. .............. .............. .............. .............. .............. .............. 20,000 4 10 .............. 0.5 ..............

CO .............. Electrochemical ............................ .............. .............. .............. .............. .............. 0.5 .............. 0.2 .............. .............. 20,000 4 10 .............. .............. ..............CO .............. Catalytic combustion-thermal detection. .............. 0.1 .............. .............. .............. .............. 750 0.2 .............. .............. 20,000 4 10 5.0 0.5 ..............CO .............. IR fluorescence ............................ .............. .............. .............. .............. .............. .............. 750 .............. .............. .............. 20,000 4 10 .............. 0.5 ..............CO .............. Mercury replacement-UV photometric. .............. .............. .............. .............. .............. .............. .............. 0.2 .............. .............. .............. 4 10 .............. 0.5 ..............NO2 ............ Chemiluminescent ........................ .............. 3 0.1 .............. 0.5 4 0.1 0.5 .............. .............. .............. .............. 20,000 .............. .............. .............. ..............NO2 ............ Spectrophotometric-wet chemical 

(azo-dye reaction).

.............. .............. .............. 0.5 4 0.1 0.5 750 .............. 0.5 .............. .............. .............. .............. .............. ..............

NO2 ............ Electrochemical ............................ 0.2 3 0.1 .............. 0.5 4 0.1 0.5 750 .............. 0.5 .............. 20,000 50 .............. .............. ..............NO2 ............ Spectrophotometric-gas phase .... .............. 3 0.1 .............. 0.5 4 0.1 0.5 .............. .............. 0.5 .............. 20,000 50 .............. .............. ..............

1 Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value. 2Analyzer types not listed will be considered by the Administrator as special cases. 3 Do not mix with the pollutant. 4 Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value. 5 If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test. 6 If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate response for interference. 


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TABLE B–4 TO SUBPART B OF PART 53—LINE VOLTAGE AND ROOM TEMPERATURE TEST CONDITIONS

Test day Line 

voltage,1 rms 

Room 

temperature,2 °C 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. 

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 

1Voltage specified shall be controlled to ± 1 volt. 2Temperatures shall be controlled to ± 1 °C. 

Table B–5 to Subpart B of Part 53— 

Symbols and Abbreviations 

BL—Analyzer reading at the specified LDL 

test concentration for the LDL test. 

BZ—analyzer reading at 0 concentration for 

the LDL test. 

DM—Digital meter. 

Cmax—Maximum analyzer reading during the 

12ZD test period. 

Cmin—Minimum analyzer reading during the 

12ZD test period. 

i—Subscript indicating the i-th quantity in a 

series. 

IE—Interference equivalent. 

L1—First analyzer zero reading for the 24ZD 

test. 

L2—Second analyzer zero reading for the 

24ZD test. 

n—Subscript indicating the test day number. 

P—Analyzer reading for the span drift and 

precision tests. 

Pi—The i-th analyzer reading for the span 

drift and precision tests. 

P20—Precision at 20 percent of URL. 

P80—Precision at 80 percent of URL. 

ppb—Parts per billion of pollutant gas 

(usually in air), by volume. 

ppm—Parts per million of pollutant gas 

(usually in air), by volume. 

R—Analyzer reading of pollutant alone for 

the IE test. 

R1—Analyzer reading with interferent added 

for the IE test. 

ri—the i-th analyzer or DM reading for the 

noise test. 

S—Standard deviation of the noise test 

readings. 

S0—Noise value (S) measured at 0 

concentration. 

S80—Noise value (S) measured at 80 percent 

of the URL. 

Sn—Average of P7 . . . P12 for the n-th test 

day of the SD test. 

S’n—Adjusted span reading on the n-th test 

day. 

SD—Span drift 

URL—Upper range limit of the analyzer’s 

measurement range. 

Z—Average of L1 and L2 readings for the 

24ZD test. 

Zn—Average of L1 and L2 readings on the 

n-th test day for the 24ZD test. 

Z’n—Adjusted analyzer zero reading on the 

n-the test day for the 24ZD test. 

ZD—Zero drift. 

12ZD—12-hour zero drift. 

24ZD—24-hour zero drift. 

Appendix A to Subpart B of Part 53— 

Optional Forms for Reporting Test 

Results 

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BILLING CODE 6560–50–C 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ 5. The authority citation for part 58 

continues to read as follows: 

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

7611, and 7619. 

Subpart B—[Amended] 

■ 6. Section 58.10, is amended by 

adding paragraph (a)(7) to read as 

follows: 

§ 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 

(2) January 1, 2017 for other CO 

monitors. 


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■ 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 nearroad NO2 monitor, only one CO monitor is 

required to be collocated with a near-road 

NO2 monitor within that CBSA. 

(b) If a state provides quantitative evidence 

demonstrating that peak ambient CO 

concentrations would occur in a near-road 

location which meets microscale siting 

criteria in Appendix E of this part but is not 

a near-road NO2 monitoring site, then the 

EPA Regional Administrator may approve a 

request by a state to use such an alternate 

near-road location for a CO monitor in place 

of collocating a monitor at near-road NO2

monitoring site. 

4.2.2 Regional Administrator Required 

Monitoring. (a) The Regional Administrators, 

in collaboration with states, may require 

additional CO monitors above the minimum 

number of monitors required in 4.2.1 of this 

part, where the minimum monitoring 

requirements are not sufficient to meet 

monitoring objectives. The Regional 

Administrator may require, at his/her 

discretion, additional monitors in situations 

where data or other information suggest that 

CO concentrations may be approaching or 

exceeding the NAAQS. Such situations 

include, but are not limited to, (1) 

characterizing impacts on ground-level 

concentrations due to stationary CO sources, 

(2) characterizing CO concentrations in 

downtown areas or urban street canyons, and 

(3) characterizing CO concentrations in areas 

that are subject to high ground level CO 

concentrations particularly due to or 

enhanced by topographical and 

meteorological impacts. The Regional 

Administrator and the responsible State or 

local air monitoring agency shall work 

together to design and maintain the most 

appropriate CO network to address the data 

needs for an area, and include all monitors 

under this provision in the annual 

monitoring network plan. 

4.2.3 CO Monitoring Spatial Scales. (a) 

Microscale and middle scale measurements 

are the most useful site classifications for CO 

monitoring sites since most people have the 

potential for exposure on these scales. 

Carbon monoxide maxima occur primarily in 

areas near major roadways and intersections 

with high traffic density and often in areas 

with poor atmospheric ventilation. 

(1) Microscale—Microscale measurements 

typically represent areas in close proximity 

to major roadways, within street canyons, 

over sidewalks, and in some cases, point and 

area sources. Emissions on roadways result 

in high ground level CO concentrations at the 

microscale, where concentration gradients 

generally exhibit a marked decrease with 

increasing downwind distance from major 

roads, or within downtown areas including 

urban street canyons. Emissions from 

stationary point and area sources, and nonroad sources may, under certain plume 

conditions, result in high ground level 

concentrations at the microscale. 

(2) Middle scale—Middle scale 

measurements are intended to represent areas 

with dimensions from 100 meters to 0.5 

kilometer. In certain cases, middle scale 

measurements may apply to areas that have 

a total length of several kilometers, such as 

‘‘line’’ emission source areas. This type of 

emission sources areas would include air 

quality along a commercially developed 

street or shopping plaza, freeway corridors, 

parking lots and feeder streets. 

(3) Neighborhood scale—Neighborhood 

scale measurements are intended to represent 

areas with dimensions from 0.5 kilometers to 

4 kilometers. Measurements of CO in this 

category would represent conditions 

throughout some reasonably urban subregions. 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. Microscale near-road NO2 monitoring 

sites are required to have sampler inlets 

between 2 and 7 meters above ground level. 

The inlet probes for microscale carbon 

monoxide monitors that are being used to 

measure concentrations near roadways must 

be between 2 and 7 meters above ground 

level. Those inlet probes for microscale 

carbon monoxide monitors measuring 

concentrations near roadways in downtown 

areas or urban street canyons must be 

between 2.5 and 3.5 meters above ground 

level. The probe or at least 90 percent of the 

monitoring path must be at least 1 meter 

vertically or horizontally away from any 

supporting structure, walls, parapets, 

penthouses, etc., and away from dusty or 

dirty areas. If the probe or a significant 

portion of the monitoring path is located near 

the side of a building or wall, then it should 

be located on the windward side of the 

building relative to the prevailing wind 

direction during the season of highest 

concentration potential for the pollutant 

being measured. 

* * * * * 

6. * * * 

6.2 Spacing for Carbon Monoxide Probes 

and Monitoring Paths. (a) Near-road 

microscale CO monitoring sites, including 

those located in downtown areas, urban 

street canyons, and other near-road locations 

such as those adjacent to highly trafficked 

roads, are intended to provide a 

measurement of the influence of the 

immediate source on the pollution exposure 

on the adjacent area. 

(b) Microscale CO monitor inlets probes in 

downtown areas or urban street canyon 

locations shall be located a minimum 

distance of 2 meters and a maximum distance 

of 10 meters from the edge of the nearest 

traffic lane. 

(c) Microscale CO monitor inlet probes in 

downtown areas or urban street canyon 

locations shall be located at least 10 meters 

from an intersection and preferably at a 

midblock location. Midblock locations are 

preferable to intersection locations because 

intersections represent a much smaller 

portion of downtown space than do the 

streets between them. Pedestrian exposure is 

probably also greater in street canyon/ 

corridors than at intersections. 

(d) Microscale CO monitor inlet probes in 

the near-road environment, outside of 

downtown areas or urban street canyons, 

shall be as near as practicable to the outside 

nearest edge of the traffic lanes of the target 

road segment; but shall not be located at a 

distance greater than 50 meters, in the 

horizontal, from the outside nearest edge of 

the traffic lanes of the target road segment. 

(e) In determining the minimum separation 

between a neighborhood scale monitoring 

site and a specific roadway, the presumption 

is made that measurements should not be 

substantially influenced by any one roadway. 

Computations were made to determine the 

separation distance, and Table E–2 of this 

appendix provides the required minimum 

separation distance between roadways and a 

probe or 90 percent of a monitoring path. 

Probes or monitoring paths that are located 

closer to roads than this criterion allows 

should not be classified as neighborhood 

scale, since the measurements from such a 

site would closely represent the middle scale. 

Therefore, sites not meeting this criterion 

should be classified as middle scale. 

* * * * * 


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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 path 1

Horizontal and 

vertical distance 

from supporting 

structures 2 to 

probe, inlet or 

90% of monitoring 

path 1 (meters) 

Distance from 

trees to probe, 

inlet or 90% of 

monitoring path 1

(meters) 

Distance from 

roadways to 

probe, inlet or 

monitoring path 1

(meters) 

SO2 3456 ................................................. Middle (300 m) 

Neighborhood 

Urban, and Regional (1 km).

2–15 .................... > 1 ....................... > 10 ..................... N/A. 

CO 457 ..................................................... Micro [downtown 

or street canyon 

sites], micro 

[near-road 

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 345 ...................................................... Middle (300 m) 

Neighborhood, 

Urban, and Regional (1 km).

2–15 .................... > 1 ....................... > 10 ..................... See Table E–1 of 

this appendix 

for all scales. 

NO2 345 ................................................... 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) 345 .......... Neighborhood and 

Urban (1 km).

2–15 .................... > 1 ....................... > 10 ..................... See Table E–4 of 

this appendix 

for all scales. 

PM, Pb 34568 ........................................... 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. 2When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.

3Should be > 20 meters from the drip-line of tree(s) and must be 10 meters from the drip-line when the tree(s) act as an obstruction. 4 Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text). 5 Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall. 

6The 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. 7For microscale CO monitoring sites in downtown areas or street canyons (not at near-road NO2 monitoring sites), the probe must be > 10 

meters from a street intersection and preferably at a midblock location. 8 Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 

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

* * * * * 

[FR Doc. 2011–21359 Filed 8–30–11; 8:45 am] 

BILLING CODE 6560–50–P 


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