Carbon Monoxide NAAQS

Science and Background Briefing 

for Final Rulemaking

for OMB

July 28, 2011

Point of Contact: Lydia Wegman

Health and Environmental Impacts Division

OAQPS
Summary

• Key Issues in Review

– Standards: Adequacy of current CO NAAQS

– Monitoring: Need for revisions to network

• Final Rulemaking

– Standards: Retain current CO NAAQS, without revision, as proposed

– Monitoring: Revise network to require near-road monitors, as proposed

• Provide additional flexibility in monitor placement from what was proposed

• Extend implementation date later than date proposed 

2
Timeline of Review

Major Milestones Completion Date

Workshop on Key Policy-Relevant Issues √ January 2008 

Integrated Review Plan Final √ August 2008

Integrated Science Assessment Final √ January 29, 2010

Risk/Exposure Assessment Plan √ April 2009

3

Risk/Exposure Assessment Plan

Final

√ April 2009

√ May 28, 2010

Policy Assessment Draft

Final

√ March 2, 2010

√ Oct 22, 2010

Rulemaking

Proposed Rule Signed

(publication date)

Jan 28, 2011

(Feb 11, 2011)

Public Comment Period (60 days) Feb 11 - April 12, 2011

OMB Review -- NFR July 18-Aug 11

Final Rule Signed Aug 12, 2011

Underlined dates indicate court-ordered deadlines
• Current standards - 9 ppm, 8-hour average and 35 ppm, 1-hour average 

– Not to be exceeded more than once per year

• Standards set in1971, reviews completed in 1985 and 1994

– No revisions made to primary standards

Background 

4

– No revisions made to primary standards

– Secondary standards revoked in 1985

• Scientific evidence for secondary standards still inadequate today

• Late 1990s – third review begun but not completed

– Air quality criteria document completed in 2000

– Review halted for Congress-requested National Research Council review of 

factors affecting ambient CO concentrations in high altitude/cold regions 

(completed 2003)

• 2007 – Began current review in response to deadline litigation
Background - Health Effects Evidence

• Internal biomarker is important element in evaluating toxicity

– In the body, CO competes with oxygen in binding to hemoglobin to form 

carboxyhemoglobin (COHb); formation of COHb can reduce blood oxygen levels

– This use of a biomarker is similar to lead (focus on blood lead levels), and different 

from other criteria pollutants (focus on inhaled ambient concentrations)

• CO’s role in lowering blood oxygen levels and reducing oxygen availability 

to tissues (hypoxia) can affect functioning of heart, brain and other organ 

systems

– People with coronary heart disease identified as population with greatest 

susceptibility to short-term CO exposure

5
Background - Health Effects Evidence 

(continued)

• Key for assessing short-term CO effects is controlled human exposure 

studies (1980s) of coronary heart disease patients with exercise-induced 

angina

– Increases Increases in COHb result in reduced result in reduced time to exercise time to exercise-induced induced angina (indicative of (indicative of 

hypoxia)

• Interpretation of epidemiological evidence (based on ambient concentrations) is 

more complicated for CO compared to other criteria pollutants

– Studies for cardiovascular morbidity are logically coherent with larger, long-standing 

health effects evidence base and supportive of ISA conclusion of likely causality

– Limitations and uncertainties complicate interpretation, particularly with regard to 

estimating ambient concentrations that may be triggering health outcomes

• Quantitative risk and exposure assessment (REA)

– Estimates of portion of at-risk population (in two study areas) expected to experience one 

or more occurrences of COHb levels at or above levels associated with responses in 

controlled human exposure studies

6
Controlled Human Exposure Studies:

• Multiple studies of coronary artery disease patients with exercise-induced angina

Background - Health Effects Evidence 

(continued) 

7

• Multiple studies of coronary artery disease patients with exercise-induced angina

• These studies document physiological effects associated with changes in COHb

levels induced by short-term CO exposures

• Key study (multicenter): ~1-hour exposures to CO concentrations targeted to 

achieve 2% and 4% COHb caused statistically significant reductions in time to 

exercise-induced indicators of myocardial ischemia 

– Angina

– Specific change in electrocardiogram, well-established objective measure

– Pooled data analysis found statistically significant dose-response relationships, with no 

evidence of threshold
Proposal - Standards

• Proposal

– Retain current primary standards (9 ppm, 8-hour average and 35 ppm, 1-hour average)

– Solicited comment on appropriateness of revision of form and level, as discussed in 

Policy Assessment and by CASAC

– Evidence remains insufficient to support a secondary CO standard

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• Rationale

– Focus on levels of internal biomarker (COHb) associated with short-term ambient CO 

exposures

– Provide protection from repeated occurrences of COHb levels above 2% and from 

appreciably higher COHb levels associated with health effects of concern in people 

with cardiovascular disease

• Individual or infrequent occurrences of COHb levels near 2% (lowest in human exposure 

studies) were judged of uncertain health significance

– Judge uncertainties and limitations of epidemiological evidence too great to provide a 

basis for revising the current standards

• “Administrator proposes to conclude that consideration of epidemiological studies does not 

lead her to identify a need for any greater protection”
Standards – CASAC Advice

• CASAC did not provide comments on proposal; had provided range of 

comments on draft Policy Assessment and other draft documents

– Agreed with staff’s conclusion that the body of evidence and exposure/dose 

assessment provide support for retaining or revising the current 8-hour standard

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assessment provide support for retaining or revising the current 8-hour standard

– Expressed “preference” for lower standard, in light of epidemiological studies, 

and suggested “example options” focused on lower part of range identified by 

Staff in Policy Assessment

– Stated that, depending on weight given epidemiological evidence, conclusion 

could be drawn that health effects are occurring at levels below current NAAQS 

– Recognized difficulty in interpreting epidemiological studies

• Co-pollutants serving as potential confounders is particularly problematic for CO; a better 

understanding of the possible role of co-pollutants is relevant to regulation; CO may be a 

surrogate for a mix of pollutants
Overview – Who Commented?

• Public health and environmental organizations

– American Lung Assoc., American Thoracic Soc. and Environmental Defense Fund (jointly)

– Physicians for Social Responsibility (signatories include 27 groups and ~930 individuals)

– Center for Biological Diversity and four others, including 2 of litigants (jointly) 

– Safe Air for Everyone

• Air agencies

– 2 national/regional organizations (NACAA, NESCAUM)

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– 7 state agencies (Indiana, New Mexico, New York, North Carolina, South Carolina, Texas, Wisconsin)

• Transportation agencies

– 1 national organization (AASHTO)

– 4 state agencies (New York, Texas, Virginia, Washington)

• Local governments (Southeast Michigan Council of Governments)

• Industries/Industry organizations

– Assoc of Automobile Manufacturers, American Petroleum Institute and American Electric Power Service Corp. 

• ~130 individuals (121 associated w. same petition)

– Notably Albert Donnay, private consultant (also significant contributor to some public health organization 

comments above)

*Complete listing of groups commenting listed in attachment
Standards – Public Comments

• Public health/environmental organizations conclude current standards are 

not requisite, recommend revision

– Cite epidemiological studies, and CASAC advice regarding weight on 

epidemiological studies and preference for lower standard 

• 5 State air agencies (IN, NM, NC, TX, WI), VA DOT, Southeast Michigan 

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COG and all industry commenters support proposed retention

– Air agencies cite rationale in proposal and CASAC recognition of support in 

evidence for current standards

– Industry: agree that proposal gives appropriate weight to epidemiological 

studies, note little change in risk estimates since last review, claim 

exposure/dose assessment overstates risk
Standards – Public Comments (continued)

• NESCAUM indicates support for NAAQS at least as protective as 

current

– Supports CASAC preference for lower standards but notes the 

epidemiological studies do not indicate a specific level

• Donnay: ISA and other review documents do not reflect latest 

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scientific knowledge useful in identifying ambient CO effects

• Center for Biological Diversity (et al) states that a secondary 

standard should be set “at a level sufficient to attenuate the 

climate impacts”

NESCAUM=Northeast States for Coordinated Air Use Management
Proposal - Monitoring

• Proposal

– Require near-road network

• Co-locate with subset of NO2 monitors (multipollutant concept) in cities with > 1 million 

population (~77 monitors in 53 CBSAs)

• Schedule and siting criteria synchronized with NO2

• Solicited comment on near-road network, population threshold, and siting criteria

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• Solicited comment on near-road network, population threshold, and siting criteria

– Regional Administrator authority

• Proposed and solicited comment on RA authority to require additional monitors anywhere on 

case-by-case basis

– Technical updates to analytical measurement method and performance testing 

requirements

• Rationale

– Mobile sources are responsible for large majority of CO emissions

– On-road and near-road environment is where peak CO concentrations are expected to 

occur 

– Co-locating with near-road NO2network provides for multipollutant monitoring and 

leveraging of resources
Monitoring - Public Comments

• Near-road monitoring requirement

– Some states and health/environmental groups support near-road monitoring & multipollutant concept

• NACAA, NESCAUM, NYSDEC, ALA/ATS/EDF

– Other states and Other states and industry do not support near industry do not support near-road monitoring monitoring

• IN, NM, SC, TX, Southeast Michigan Council of Gov., AAM, and AEPSC

• Details of near-road monitoring

– All states who support near-road monitoring want population threshold for requirement 

changed from 1 million to 2.5 million

– ALA/ATS/EDF want population threshold for required monitors at 500,000

– NACAA & NC want states to have freedom to have required near-road CO monitors NOT be 

co-located with NO2sites, if states choose

– Many commenters are concerned over losing area-wide monitoring sites, including 

transportation groups, industry, and public health/environmental groups

• AASHTO, NYSDOT, TXDOT, VDOT, AAM, ALA/ATF/EDF

– States want full funding

14
Monitoring - Public Comments (continued)

• Other monitoring network comments

– Safe Air for Everyone wants monitoring Air for Everyone wants monitoring in/near in/near agricultural agricultural burn areas

– Some states and transportation groups want required near-road monitors paired 

with corresponding area-wide monitoring

• NESCAUM, AASHTO, NYSDOT, TXDOT, VDOT

– Transportation groups & Texas have concern over RA Authority to require 

monitoring above the minimum requirements

• Monitoring methods comments

– API provided comments on measurement methods update

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Summary

• Key Issues in Review

– Standards: Adequacy of current CO NAAQS

– Monitoring: Need for revisions to network

• Final Rulemaking

– Standards: Retain current CO NAAQS, as proposed

– Monitoring: Revise network to require near-road monitors, as 

proposed

• Provide additional flexibility in monitor placement from proposed

– Allow states to choose whether a required near-road CO monitor is to be 

co-located with a near-road NO2 monitor

• Extend implementation date from proposed date

– Implementation deadline of January 1, 2015

16
Attachments

– Complete listing of public commenters

17

– Additional background

• Monitoring network

• CO concentrations

• Health effects evidence

• Risk/Exposure assessment
Public Commenters on Proposal

National/Regional Air Organizations

- Northeast States for Coordinated Air Use Management 

(NESCAUM)

- National Association of Clean Air Agencies (NACAA)

State Environmental/Air Agencies

– Indiana Department of Environmental Management 

(INDEM)

Medical, Public Health, and Environmental Groups

– American Thoracic Society (ATS), American Lung 

Association (ALA), Environmental Defense Fund (EDF)

– Safe Air for Everyone

– Center for Biological Diversity (CfBD), Communities for a 

Better Environment*, WildEarth Guardians, Coalition for a 

Safe Environment*, Alaska Center for the Environment 

– Physicians for Social Responsibility (PSR)* (additional 

signatories included 27 organizations and ~930 

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(INDEM)

– New Mexico Air Quality Bureau (NMAQB)

– New York Department of Environmental Conservation 

(NYDEC)

– North Carolina Division of Air Quality (NCDAQ)

– South Carolina Department of Health and Environmental 

Control (SCDHEC)

– Texas Council on Environmental Quality (TXCEQ)

– Wisconsin Department of Natural Resources (WIDNR)

Transportation Associations/ State Agencies

– American Association of State Highway and Transportation 

Officials (NAASHTO)

– Texas Dept Transportation

– Virginia Dept Transportation

– Washington Dept Transportation

Local Governments

– Southeast Michigan Council of Governments

– Physicians for Social Responsibility (PSR)* (additional 

signatories included 27 organizations and ~930 

individuals)

Industry Commenters

– Alliance of Automobile Manufacturers

– American Petroleum Institute

– American Electric Power Service Corporation 

Private Citizens, includes:

– 121 associated with email petition

– 3 with specific technical knowledge, 1 of whom was major 

contributor to PSR comments and email petition

– Several law class students

* plaintiff in litigation
Monitoring: Current CO Network

• 345 CO monitors operated during 2009.

• Minimum monitoring requirements were removed in the 2006 monitoring rule, except for CO 

measurements at National Core (NCore) monitoring sites.

– Old requirements called for one ‘peak’ concentration site (near a high traffic road or in an urban core) and one 

area/community wide monitor in any urban area over 500,000 population.

• We have reviewed AQS metadata to characterize monitor objectives and spatial scales.

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• We have reviewed AQS metadata to characterize monitor objectives and spatial scales.

– Population exposure is the dominant monitoring objective.

– The neighborhood spatial scale is the most common scale of representativeness.

– Near-road and urban canyon monitors would appropriately be classified as microscale sites.

19

Monitor Objectives

*one site can have multiple objectives

Population Exposure ~47%

Highest Concentration ~22%

General Background ~5%

Regional Transport ~2%

Source Oriented ~1%

Spatial Scales 

(of representativeness)

Micro (~1 to 100m) ~16%

Middle (100 to 500m) ~10%

Neighborhood (500m – 4km) ~43%

Urban (4km – 50km) ~5%

Regional (50km +) ~2%
Background: Ambient CO - Monitoring Stations (2009)

There were 345 CO monitors 

operating during the year 2009.

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Monitoring: CASAC Feedback on Network

 In the CASAC CO Committee letter on the exposure/dose assessment, they 

noted: “More extensive [monitoring] coverage may be warranted for 

areas...such as near roadway locations.”

21

 We met with the CASAC Ambient Air Monitoring Methods (AAMM) 

Subcommittee on September 29th and 30th regarding near-road monitoring.

 We provided charge questions asking about where CO monitoring would be 

appropriate.

 Key message – In their general discussions and final letter, CASAC 

indicated there is value in monitoring for CO at the future near-road NO2

sites (as well as giving some consideration to urban downtown/canyon 

areas).
Background: Ambient CO - Emissions

• Nationally and in urban areas, generally, mobile sources dominate emissions

– ~45% reduction in national-scale anthropogenic CO emissions between 1990 and 

2005, due primarily to reductions in on-road vehicle emissions 

– In some individual areas, stationary source emissions important

22
Background: Ambient CO - Concentration Trend (8-hour)

16

CO Air Quality, 1990 - 2009

(Based on Annual 2nd Maximum 8-hour average)

National Trend Based on 187 sites

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0

2

4

6

8

10

12

14

1

9

9

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1

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1

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9

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1

9

9

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1

9

9

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1

9

9

8

1

9

9

9

2

0

0

0

2

0

0

1

2

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2

2

0

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3

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2

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9

C

o

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c

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ntratio

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p

m)

National Standard 9 ppm

90% of sites have concentrations below this line

10% of sites have concentrations below this line

Average among all sites
Background: Ambient CO - Concentration Trend (1-hour) 

35

CO Air Quality, 1990 - 2009

(Based on Annual 2nd Maximum 1-hour average)

National Trend Based on 187 sites

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0

5

10

15

20

25

30

1

9

9

0

1

9

9

1

1

9

9

2

1

9

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3

1

9

9

4

1

9

9

5

1

9

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6

1

9

9

7

1

9

9

8

1

9

9

9

2

0

0

0

2

0

0

1

2

0

0

2

2

0

0

3

2

0

0

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2

0

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2

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C

o

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p

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National Standard 35 ppm

90% of sites have concentrations below this line

10% of sites have concentrations below this line

Average among all sites
Current Ambient Concentrations (1-hour)

• Second maximum 1-hour average concentrations (current form of standard) of CO at monitors nationally, 2009

25

ppm 0.0-4.4 4.5-9.4 9.4-35.4 >35.4

No. of Counties 180 17 7 0

curr1hr09 0 - 4.4 4.5 - 9.4 >= 9.4
Current Ambient Concentrations (8-hour)

• Second maximum 8-hour average concentrations (current form of standard) of CO at monitors nationally, 2009

26

curr8h09 0.0 0 - 2.4 2.5 - 6.4 >= 6.4 ppm -2.4 2.5-6.4 6.4-9.4 >9.4

No. of Counties 176 25 3 0
Controlled human exposure studies

• Multiple studies of coronary artery disease patients with exercise-induced angina

• These studies document physiological effects associated with changes in COHb

levels induced by short-term CO exposures

Health Effects Evidence:

Considerations in Current Review

27

levels induced by short-term CO exposures

• Key study: ~1-hour exposures to CO concentrations targeted to achieve 2% and 

4% COHb caused statistically significant reductions in time to exercise-induced 

indicators of myocardial ischemia 

– Angina

• 4% COHb group - on average 7% time reduction (~40 seconds) 

• 2% COHb group - on average 4% time reduction (~20 seconds) 

– Change in electrocardiogram [ECG], which is well-established objective measure

• 4% COHb group - on average 12% time reduction (~70 seconds) 

• 2% COHb group - on average 5% time reduction (~30 seconds) 

– Pooled data analysis found statistically significant dose-response relationships, with no 

evidence of threshold
Controlled human exposure studies ... adversity of effects

• No established guidelines for judging adversity of different responses

Health Effects Evidence:

Considerations in Current Review (continued)

28

• 2010 Science Assessment (ISA) notes that physiological significance of small 

changes in time-to-occurrence (in key study) at lowest COHb level studied (e.g., 

2%) is not clear 

• Previous review recognized range of professional judgments, described effects at 

lowest studied COHb level as “potentially adverse”, and considered them in 

evaluating adequate margin of safety (1994 final rule)

• The key study (1989, 1991) indicates a continuum of effects on indicators of 

myocardial ischemia ranging from 4% COHb down to at least 2% COHb. 

– Study authors considered these effects to be similar to those considered clinically significant when 

evaluating medications to treat angina from coronary artery disease.
Epidemiological studies

• Much expanded database since 1994

Health Effects Evidence: 

Considerations in Current Review (continued)

29

• Much expanded database since 1994

• Multiple studies observe positive associations between cardiovascular outcomes 

and short-term ambient CO concentrations across a range of CO concentrations

– Studies of cardiovascular disease hospitalizations consistently report such associations

• Although only 4 studies (3 with statistical significance) in U.S. locations where current 

standards met 

• Logical coherence between epidemiological study findings and larger, long-standing 

health effects evidence base

– Study findings are coherent with effects observed in human exposure studies of 

susceptible individuals and supportive to ISA conclusion of likely causality for 

cardiovascular morbidity
Epidemiological studies ... quantitative interpretation

• Quantitative interpretation is complicated for CO compared to other criteria 

pollutants, affecting estimation of ambient concentrations that may be triggering 

Health Effects Evidence: 

Considerations in Current Review (continued)

30

pollutants, affecting estimation of ambient concentrations that may be triggering 

health outcomes

– Substantially fewer CO studies in areas meeting current standards than for NOx and PM

– CASAC:

• “the problem of co-pollutants serving as potential confounders is particularly problematic for CO”

• “CO may be a surrogate for mix of pollutants related to fossil fuel combustion”

– Reduced confidence in representation of ambient CO exposures

• Use of central-site ambient concentrations in light of near-roadway gradient (many times steeper 

than NOx)

• Prevalence of CO measurements near/below method detection levels

– For CO, unlike for all other criteria pollutants, the epidemiological studies use a different 

exposure/dose metric (air concentration) than that used in other key CO studies (COHb)

• Additional information that might be used to bridge this gap is lacking
• To assess extent to which ambient CO exposures may result in at-risk populations 

experiencing COHb levels associated with effects in controlled human exposure studies, 

we use exposure/dose modeling

– In the last review, for conditions just meeting the 8-hour standard, the Administrator noted:

Health Effects Evidence: 

Interpretation of Exposure/Dose Estimates

31

– In the last review, for conditions just meeting the 8-hour standard, the Administrator noted:

• “Relatively few” of at-risk population estimated to experience COHb >2.1% in absence of indoor 

sources when current standards just met

– A daily maximum > 2.1% COHb was estimated to occur for less than 0.1% of person-days

• Standards protecting against ambient CO levels resulting in COHb >2.1% “should provide an 

adequate margin of safety against effects of uncertain occurrence, as well as those of clear 

concern”

– In considering current exposure/dose modeling, we focus primarily on occurrence and 

frequency of COHb levels > 2% 

• 2% is lowest level assessed in human exposure studies

• Exposure/dose assessment (REA) also presents estimates for higher and lower COHb levels
Exposure/Dose: Overview of Current Assessment

• Two study areas: Denver and Los Angeles

• Two at-risk populations: Coronary heart disease and all 

heart disease adults

• Several air quality scenarios: “as is” (2006), just meeting 

Denver

32

• Several air quality scenarios: “as is” (2006), just meeting 

current 8-hour standard and alternatives

– In Denver, the 1-hour level (current form) associated with just meeting 

the current 8-hour standard was 16.2 ppm.

– In Los Angeles, the 1-hour level (current form) associated with just 

meeting the current 8-hour standard was 11.8 ppm.

• Focus on population exposure and dose estimates

– Percent of population at/above/below COHb levels of interest (or 

with multiple such occurrences), and percent of population 

person-days (metric in last review)

Los Angeles