38914).\41\ Based on these estimates, the Administrator concluded that
``relatively few people of the cardiovascular sensitive population
group analyzed will experience COHb levels >= 2.1 percent when exposed
to CO levels in absence of indoor sources when the current standards
are attained.'' The analysis also took into account that certain indoor
sources (e.g., passive smoking, gas stove usage) contributed to total
CO exposure and EPA recognized that such sources may be of concern for
such high risk groups

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

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

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

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

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

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

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

     The associated dose-response relationship between
incremental changes in COHb and change in time to myocardial ischemia
in CAD patients indicates a 1.9% and 3.9% reduction in time to onset of
exercise-induced angina and ST-segment change, respectively, per 1%
increase in COHb concentration from average baseline COHb of 0.6%
without evidence of a measurable threshold.
    As described in section II.B.2 above, a number of epidemiological
studies of health outcome associations with ambient CO have been
conducted since the last review. These include studies that have
reported associations with different ambient CO metrics (e.g., 1-hour
and 8-hour averages, often as central-site estimates) derived from CO
measurements at fixed-site ambient monitors in selected urban areas of
the U.S. and cardiovascular endpoints other than stroke, particularly
hospitalizations and emergency department visits for specific
cardiovascular health outcomes including IHD, CHF and CVD (Bell et al.,
2009; Koken et al., 2003; Linn et al., 2000; Mann et al., 2002; Metzger
et al., 2004; Symons et al., 2006; Tolbert et al., 2007; Wellenius et
al., 2005). In general, these studies, many of which were designed to
evaluate the effects of a variety of air pollutants, including CO,
report positive associations, a number of which are statistically
significant (ISA, sections 5.2.3 and <IP_ADDRESS>). The long-standing body
of evidence for CO summarized above, including the well-characterized
role of CO in limiting oxygen availability, lends biological
plausibility to the ischemia-related health outcomes reported in the
epidemiological studies, providing coherence between these studies and
the clinical evidence of short-term exposure to CO and health effects.
Thus, although there is no new evidence

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

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

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

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

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

    Since the 1994 review, the epidemiologic and toxicological evidence
of effects on birth and developmental outcomes has expanded, although
the available evidence is still considered limited with regard to
effects on preterm birth, birth defects, decreases in birth weight,
measures of fetal growth, and infant mortality (ISA, section 5.4).
Further, while animal toxicological studies provide support and
coherence for those effects, the understanding of the mechanisms
underlying reproductive and developmental effects is still lacking
(ISA, section 5.4.1). Thus, the Policy Assessment recognizes that
although the evidence continues to ``suggest[s] that critical
developmental phases may be characterized by enhanced sensitivity to CO
exposure'' (ISA, p. 2-11), evidence is lacking for adverse
developmental or reproductive effects at CO exposure concentrations
near those associated with current levels of ambient CO (PA, pp. 2-35
to 2-36).
    As described above, the much-expanded epidemiologic database in the
current review includes studies that show associations between ambient
CO concentrations and increases in emergency room visits and
hospitalizations for disease events plausibly linked to the effects
observed in the controlled human exposure studies of CAD patients (ISA,
section 2.5.1), providing support for the ISA's conclusion regarding
coronary artery disease as the most important susceptibility
characteristic for increased health risk due to CO exposure (ISA, p. 2-
10). However, the Policy Assessment recognizes aspects of this
epidemiological evidence that complicate quantitative interpretation of
it with regard to ambient concentrations that might be eliciting the
reported health outcomes. As an initial matter, the Policy Assessment
notes the substantially fewer studies conducted in areas meeting the
current CO standards than is the case for NO2 and PM (USEPA,
2008d, 2009f). Further, the Policy Assessment recognizes complicating
aspects of the evidence that relate to conclusions regarding CO as the
pollutant eliciting the effect reported in the epidemiological studies
and to our understanding of the ambient CO and nonambient
concentrations to which study subjects demonstrating these outcomes are
exposed.
    With regard to these complications, the Policy Assessment first
considers the extent to which the use of two-pollutant regression
models, a commonly used statistical method (ISA, section 1.6.3), inform
conclusions regarding CO as the pollutant eliciting the effects in
these studies (PA, pp. 2-36 to 2-37). Although CO associations, in some
studies, are slightly attenuated in models that adjusted for other
combustion-related pollutants (e.g., PM2.5 or
NO2), they generally remain robust (ISA, Figures 5-6 and 5-
7).\50\ In considering these two-pollutant model results, however, the
Policy Assessment recognizes the potential for there to be
etiologically relevant pollutants that are correlated with CO yet
absent from the analysis. Similarly, CASAC commented that ``the problem
of co-pollutants serving as potential confounders is particularly
problematic for CO''. They stated that ``consideration needs to be
given to the possibility that in some situations CO may be a surrogate
for exposure to a mix of pollutants generated by fossil fuel
combustion'' and ``a better understanding of the possible role of co-
pollutants is relevant to * * * the interpretation of epidemiologic
studies on the health effects of CO'' (Brain and Samet, 2010d). This
issue is particularly important in the case of CO in light of
uncertainty associated with CO-related effects at low ambient
concentrations (discussed below) and in light of the sizeable portion
of ambient CO measurements that are at or below monitor detection
limits. Consequently, the extent to which multi-pollutant regression
models effectively disentangle and quantitatively interpret a CO-
specific effect distinct from that of other pollutants remains an area
of uncertainty.
---------------------------------------------------------------------------

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

    In considering ambient concentrations that may be triggering health
outcomes analyzed in the epidemiological studies, the Policy Assessment
recognizes the uncertainty introduced by exposure error. Exposure error
can occur when a surrogate is used for the actual ambient exposure
experienced by the study population (e.g., ISA, section 3.6.8). There
are two aspects to the epidemiological studies in the specific case of
CO, as contrasted with the cases of other pollutants such as
NO2 and PM, that may contribute to exposure error in the CO
studies. The first relates to the low concentrations of CO considered
in the epidemiological studies and monitor detection limits. The second
relates to the use in the epidemiological studies of area-wide or
central-site monitor CO concentrations in light of information about
the gradient in CO concentrations with distance from source locations
such as highly-trafficked roadways (ISA, section <IP_ADDRESS>).
    As discussed in the Policy Assessment, uncertainty in the
assessment of exposure to ambient CO concentrations is related to the
prevalence of ambient CO monitor concentrations at or below detection
limits, which is a greater concern for the more recently available
epidemiological studies in which the study areas have much reduced
ambient CO concentrations compared with those in the past (PA, pp. 2-37
to 2-38). For example, the ISA notes that roughly one third of the 1-
hour ambient CO measurements reported to AQS for 2005-2007 were below
the method limit of detection for the monitors analyzed (ISA, p. 3-34).
A similarly notable proportion of measurements occur below the monitor
detection limit for epidemiological study areas meeting the current
standards (e.g., Atlanta, Allegheny County) (PA, Appendix B). This
complicates our interpretation of specific ambient CO concentrations
associated with health effects (ISA, p. 3-91; Brain and Samet, 2010d).
In contrast to CO, other combustion-related criteria pollutants such as
PM2.5 and NO2 generally occur above levels of
detection, providing us with greater confidence in quantitative
interpretations of epidemiological studies for those pollutants.
    There are also differences in the spatial variability associated
with PM2.5 and NO2 concentrations as compared to
CO concentrations that add complexity

to the estimation of CO exposures in epidemiological studies. In
general, PM2.5 concentrations tend to be more spatially
homogenous across an urban area than CO concentrations. CO
concentrations in urban areas are largely driven by mobile sources,
while urban PM2.5 concentrations substantially reflect
contributions from mobile and a variety of stationary sources. The
greater spatial homogeneity in PM2.5 concentrations is due
in part to the transport and dispersion of small particles from the
multiple sources (USEPA, 2009f, sections <IP_ADDRESS> and <IP_ADDRESS>), as well
as to contributions from secondarily formed components ``produced by
the oxidation of precursor gases (e.g., sulfur dioxide and nitrogen
oxides) and reactions of acidic products with NH3 and
organic compounds'' (USEPA, 2009f, p. 3-185), which likely contribute
to spatial homogeneity. Similarly, ``because NO2 in the
ambient air is due largely to the atmospheric oxidation of NO emitted
from combustion sources (ISA, section 2.2.1), elevated NO2
concentrations can extend farther away from roadways than the primary
pollutants also emitted by on-road mobile sources'' (40 FR 6479,
February 9, 2010). In contrast to PM2.5 and NO2,
CO is not formed through common atmospheric oxidation processes, which
may contribute to the steeper CO gradient observed near roadways.
Therefore, the misclassification of exposure arising from the
utilization of central site monitors to measure PM2.5 and
NO2 exposures is likely to be smaller than is the case for
CO exposures.
    An additional complication to a comparison of our consideration of
the CO epidemiological evidence to that for other criteria pollutants
is that, in contrast to the situation for all other criteria
pollutants, the epidemiological studies for CO use a different
exposure/dose metric from that which is the focus of the broader health
evidence base, and additional information that might be used to bridge
this gap is lacking. In the case of CO, the epidemiological studies use
air concentration as the exposure/dose metric, while the broader health
effects evidence for CO demonstrates and focuses on an internal
biomarker of CO exposure (COHb) which has been considered a critical
key to CO toxicity. In the case of the only other criteria pollutant
for which the health evidence relies on an internal dose metric--lead--
the epidemiological studies also use that metric.\51\ For other
criteria pollutants, including PM and NO2, air
concentrations are used as the exposure/dose metric in both the
epidemiological studies and the other types of health evidence. Thus,
there is no comparable aspect in the PM or NO2 evidence
base. The strong evidence describing the role of COHb in CO toxicity is
important to consider in interpreting the CO epidemiological studies
and contributes to the biological plausibility of the ischemia-related
health outcomes that have been associated with ambient CO
concentrations. Yet, we do not have information on the COHb levels of
epidemiological study subjects that we can evaluate in the context of
the COHb levels eliciting health effects in the controlled human
exposure studies. Further, we lack additional information on the CO
exposures of the epidemiological study subjects to both ambient and
nonambient sources of CO that might be used to estimate their COHb
levels and bridge the gap between the two study types.
---------------------------------------------------------------------------

    \51\ In the case of lead (Pb), in contrast to that of CO, the
epidemiological evidence is focused on associations of Pb-related
health effects with measurements of Pb in blood, providing a direct
linkage between the pollutant, via the internal biomarker of dose,
and the health effects. Thus, for Pb, as compared to the case for
CO, we have less uncertainty in our interpretations of the
epidemiological studies with regard to the pollutant responsible for
the health effects observed.
---------------------------------------------------------------------------

    Additionally the ISA recognizes that the changes in COHb that would
likely be associated with exposure to the low ambient CO concentrations
assessed in some of the epidemiological studies would be smaller than
changes associated with ``substantially reduced {oxygen{time}  delivery
to tissues,'' that might plausibly lead to the outcomes observed in
those studies, with additional investigation needed to determine
whether there may be another mechanism of action for CO that
contributes to the observed outcomes at low ambient concentrations
(ISA, p. 5-48). Thus, there are uncertainties associated with the
epidemiological evidence that ``complicate the quantitative
interpretation of the epidemiologic findings, particularly regarding
the biological plausibility of health effects occurring at COHb levels
resulting from exposures to the ambient CO concentrations'' assessed in
these studies (ISA, p. 2-17).
    In summary, the Policy Assessment concludes that some important
uncertainties from the last review have been reduced, including those
associated with concerns for ambient levels of CO to pose
neurobehavioral risks as current concentrations of ambient CO are well
below those that might be expected to result in COHb levels as high as
those associated with these effects. Additionally, our exposure and
dose models have improved giving us increased confidence in their
estimates. A variety of uncertainties still remain including the
adverse nature and significance of the small changes in time to ST-
segment depression identified at the lowest COHb levels investigated,
and the magnitude of associated risk of specific health outcomes, as
well as the potential for as-yet-unidentified health effects at COHb
levels below 2%. Additionally, although the evidence base is somewhat
expanded with regard to the potential for CO effects on the developing
fetus, uncertainties remain in our understanding of the potential
influence of low, ambient CO exposures on conditions existing in the
fetus and newborn infant and on maternal-fetal relationships. We
additionally recognize that the expanded body of epidemiological
evidence includes its own set of uncertainties which complicates its
interpretation, particularly with regard to ambient concentrations that
may be eliciting health outcomes.
b. Exposure/Dose-Based Considerations
    In considering the evidence from controlled human exposure studies
to address the question regarding ambient CO concentrations associated
with health effects, we have developed estimates of COHb associated
with different air quality conditions using quantitative exposure and
dose modeling, as was done at the time of the last review. The current
estimates are presented in the REA and discussed with regard to policy-
relevant considerations in this review in the Policy Assessment (PA,
section 2.2.2). Since the last review, there have been numerous
improvements to the exposure and COHb models that we use to estimate
exposure and dose for the current review. The results of modeling using
these improved tools in the current review and associated conclusions
in the Policy Assessment are described below with regard to the
expectation for COHb levels of concern to occur in the at-risk
population under air quality conditions associated with the current CO
standards.
    In considering the results from the REA, the Policy Assessment
considered several questions including those concerning the magnitude
of COHb levels estimated in the simulated at-risk populations in
response to ambient CO exposure, as well as the extent to which such
estimates may be judged to be important from a public health
perspective.
    In addressing the questions concerning the magnitude of at-risk
population COHb levels estimated to

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

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

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

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

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

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

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

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

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

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

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

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

    In their review of the draft ISA, CASAC noted various limitations
and uncertainties associated with the evidence, particularly from the
epidemiological studies, as noted in section II.D.2.1 above. For
example, they recognized limitations in representation of population
exposure to ambient CO. Further they noted that ``[t]he problem of co-
pollutants serving as potential confounders is particularly problematic
for CO'' and that CO may be serving as a surrogate for a mixture of
pollutants generated by fossil fuel combustion (Brain and Samet, 2010d)
as well as noting uncertainty regarding the possibility for confounding
effects of indoor sources of CO (Brain and Samet, 2010c).
    In their comments on the draft PA, the CASAC CO Panel stated
overall agreement with staff's conclusion that the body of evidence and
the quantitative exposure and risk assessment provide support for
retaining or revising the current 8-hour standard. They additionally,
however, expressed a ``preference'' for a lower standard and stated
that ``[i]f the epidemiological evidence is given additional weight,
the conclusion could be drawn that health effects are occurring at
levels below the current standard, which would support the tightening
of the current standard.'' Taking this into account, the Panel further
advised that ``revisions that result in lowering the standard should be
considered'' (Brain and Samet, 2010c).
    As noted in section I.C. above, the final Policy Assessment was
completed with consideration of CASAC comments on the draft document,
as well as their comments on the second draft REA, and also public
comments. Among the revisions made in completing the final Policy
Assessment were those based on additional consideration of the
epidemiological studies in light of CASAC comments. Discussion of these
studies and the complications with regard to their quantitative
interpretation is described in section II.D.2.a above, in addition to
other evidence-based considerations described in the final Policy
Assessment, and is considered in the Administrator's proposed
conclusions below.
    The few public comments received on this review to date that have
addressed adequacy of the current standards conveyed the view that the
current standards are adequate. In support of this view, these
commenters disagreed with the REA estimates of in-vehicle exposure
concentrations and argued that little weight should be given to the
epidemiological studies.
4. Administrator's Proposed Conclusions Concerning Adequacy
    Based on the large body of evidence concerning the public health
impacts of exposure to ambient CO available in this review, the
Administrator proposes that the current primary standards provide the
requisite protection of public health with an adequate margin of safety
and should be retained.
    In considering the adequacy of the current standards, the
Administrator has carefully considered the available evidence and
conclusions contained in the Integrated Science Assessment; the
information, exposure/dose assessment, rationale and conclusions
presented in the Policy Assessment; the advice and recommendations from
CASAC; and public comments to date. In the discussion below, the
Administrator considers first the long-standing evidence base
concerning effects associated with exposure to CO, including the
controlled human exposure studies, and the health significance of
responses observed at the 2% COHb level induced by 1-hour CO exposure,
as compared to higher COHb levels. As at the time of the review
completed in 1994, the Administrator also takes note of the results for
the modeling of exposures to ambient CO under conditions simulated to
just meet the current, controlling, 8-hour standard in two study areas,
as described in the REA and Policy Assessment, and the public health
significance of those results. She also considers the newly available
and much-expanded epidemiological evidence, including the complexity
associated with quantitative interpretation of these studies,
particularly the few studies available in areas where the current
standards are met. Further, the Administrator considers the advice of
CASAC, including both their overall agreement with the Policy
Assessment conclusion that the current evidence and quantitative
exposure and dose estimates provide support for retaining the current
standard, as well as their view that in light of the epidemiological
studies, revisions to lower the standards should be considered and
their preference for a lower standard.

    As an initial matter, the Administrator takes note of the Policy
Assessment's consideration of the long-standing body of evidence for
CO, augmented in some aspects since the last review, as summarized in
the current Integrated Science Assessment. This long-standing evidence
base has established the following key aspects of CO toxicity that are
relevant to this review as they were to the review completed in 1994.
The common mechanism of CO health effects involves binding of CO to
reduced iron in heme proteins and the alteration of their function.
Hypoxia (reduced oxygen availability) induced by increased COHb blood
levels plays a key role in eliciting CO-related health effects.
Accordingly, COHb is commonly used as the bioindicator and dose metric
for evaluating CO exposure and the potential for health effects.
Further, people with cardiovascular disease are a key population at
risk from short-term ambient CO exposures.
    With regard to the evidence of health effects associated with
ambient CO exposures relevant to this review, the Administrator first
recognizes the Integrated Science Assessment's conclusion that a causal
relationship is likely to exist between relevant short-term exposures
to CO and cardiovascular morbidity. Further, as at the time of the
review completed in 1994, the Administrator takes particular note of
the evidence from controlled human exposure studies that demonstrates a
reduction in time to onset of exercise-induced markers of myocardial
ischemia in response to increased COHb resulting from short-term CO
exposures, and recognizes the greater significance accorded both to
larger reductions in time to myocardial ischemia, and to more frequent
occurrences of myocardial ischemia. The Administrator also recognizes
the uncertain health significance associated with the smaller responses
to the lowest COHb level assessed in the study given primary
consideration in this review (Allred et al., 1989a, 1989b, 1991) and
with single occurrences of such responses. In the study by Allred et
al. (1989a, 1989b, 1991), a 4-5% reduction in time (approximately 30
seconds) to the onset of exercise-induced markers of myocardial
ischemia was associated with the 2% COHb level induced by 1-hour CO
exposure. In considering the significance of the magnitude of the time
decrement to onset of myocardial ischemia observed at the 2% COHb level
induced by short-term CO exposure, as well as the potential for
myocardial ischemia to lead to more adverse outcomes, the EPA generally
places less weight on the health significance associated with
infrequent or rare occurrences of COHb levels at or just above 2% as
compared to that associated with repeated occurrences and occurrences
of appreciably higher COHb levels in response to short-term CO
exposures. For example, at the 4% COHb level, the study by Allred et
al., (1989a, 1989b, 1991) observed a 7-12% reduction in time to the
onset of exercise-induced markers of myocardial ischemia. The
Administrator places more weight on this greater reduction in time to
onset of exercise-induced markers compared to the reduction in time to
onset at 2% COHb. The Administrator also notes that at the time of the
1994 review, an intermediate level of approximately 3% COHb was
identified as a level at which adverse effects had been demonstrated in
persons with angina. Now, as at the time of the 1994 review, the
Administrator primarily considers the 2% COHb level, resulting from 1-
hour CO exposure, with regard to providing a margin of safety against
effects of concern that have been associated with higher COHb levels,
such as 3-4% COHb.
    As at the time of the last review, the Administrator additionally
considers the exposure and dose modeling results, taking note of key
limitations and uncertainties associated with the exposure and dose
assessment summarized in section II.C.2. above, and in light of
judgments above regarding the health significance of findings from the
controlled human exposure studies, placing less weight on the health
significance of infrequent or rare occurrences of COHb levels at or
just above 2% and more weight to the significance of repeated such
occurrences, as well as occurrences of higher COHb levels. Under air
quality conditions just meeting the current, controlling, 8-hour
standard, the assessment estimates that, as was the case for the
assessment conducted for the 1994 review, daily maximum COHb levels
were below 2% COHb for more than 99.9% of person-days in the study
areas evaluated. Further, under these conditions, greater than 99.9% of
the at-risk populations in the study areas evaluated would not be
expected to experience daily maximum COHb levels at or above 4% COHb,
and more than 95% and 98.6% of those populations would be expected to
avoid single or multiple occurrences, respectively, at or just above 2%
COHb.
    The Administrator additionally takes note of the now much-expanded
evidence base of epidemiological studies, including the multiple
studies that observe positive associations between cardiovascular
outcomes and short-term ambient CO concentrations across a range of CO
concentrations, including conditions above as well as below the current
NAAQS. She notes particularly the Integrated Science Assessment finding
that these studies are logically coherent with the larger, long-
standing health effects evidence base for CO and the conclusions drawn
from it regarding cardiovascular disease-related susceptibility. In
further considering the epidemiological evidence base with regard to
the extent to which it provides support for conclusions regarding
adequacy of the current standards, the Administrator takes note of
CASAC's conclusions that ``[i]f the epidemiological evidence is given
additional weight, the conclusion could be drawn that health effects
are occurring at levels below the current standard, which would support
the tightening of the current standard'' (Brain and Samet, 2010c).
Additionally, the Administrator places weight on the final Policy
Assessment consideration of aspects that complicate quantitative
interpretation of the epidemiological studies with regard to ambient
concentrations that might be eliciting the reported health outcomes.
    For purposes of evaluating the adequacy of the current standards,
there are multiple complicating features of the epidemiological
evidence base, as described in more detail in the final Policy
Assessment and in section II.D.2.a, above. First, while a number of
studies observed positive associations of cardiovascular disease-
related outcomes with short-term CO concentrations, very few of these
studies were conducted in areas that met the current standards
throughout the period of study. In addition, CASAC, in their advice
regarding interpretation of the currently available evidence commented
that ``[t]he problem of co-pollutants serving as potential confounders
is particularly problematic for CO'' and that given the currently low
ambient CO levels, there is a possibility that CO is acting as a
surrogate for a mix of pollutants generated by fossil fuel combustion.
CASAC further stated that ``[a] better understanding of the possible
role of co-pollutants is relevant to regulation'' (Brain and Samet,
2010d). As described in the Policy Assessment, there are also
uncertainties related to representation of ambient CO exposures given
the steep concentration gradient near roadways, as well as the
prevalence of measurements below the method detection limit across the
database. CASAC additionally indicated the need to consider the
potential for

confounding effects of indoor sources of CO. As discussed in section
II.D.2.a above, the interpretation of epidemiological studies for CO is
further complicated because, in contrast to the situation for all other
criteria pollutants, the epidemiological studies for CO use an
exposure/dose metric (air concentration) that differs from the metric
commonly used in the other key CO health studies (COHb).
    Although CASAC expressed a preference for a lower standard, CASAC
also indicated that the current evidence provides support for retaining
the current suite of standards. CASAC's recommendations appear to
recognize that their preference for a lower standard was contingent on
a judgment as to the weight to be placed on the epidemiological
evidence. For the reasons explained above, after full consideration of
CASAC's advice and the epidemiological evidence, as well as its
associated uncertainties and limitations, the Administrator judges
those uncertainties and limitations to be too great for the
epidemiological evidence to provide a basis for revising the current
standards.
    In considering the adequacy of the level of protection provided by
the current standards, the Administrator notes the findings of the
exposure and dose assessment in light of considerations discussed above
regarding the weight given to different COHb levels and their frequency
of occurrence. The exposure and dose assessment results indicate that
only a very small percentage of the at-risk population is estimated to
experience a single occurrence in a year of daily maximum COHb at or
above 3.0% COHb under conditions just meeting the current 8-hour
standard in the two study areas evaluated, and no multiple occurrences
are estimated. The Administrator also notes the results indicating that
only a small percentage of the at-risk populations are estimated to
experience a single occurrence of 2% COHb in a year under conditions
just meeting the standard, and still fewer estimated to experience
multiple such occurrences. Taken together, the Administrator considers
the current standard to provide a very high degree of protection for
the COHb levels and associated health effects of concern, as indicated
by the extremely low estimates of occurrences, and provides slightly
less but a still high degree of protection for the effects associated
with lower COHb levels, the physiological significance of which is less
clear. Additionally, the Administrator proposes to conclude that
consideration of the epidemiological studies does not lead her to
identify a need for any greater protection. Thus, the Administrator
proposes to conclude that the current suite of standards provides an
adequate margin of safety against adverse effects associated with
short-term ambient CO exposures. For these and all of the reasons
discussed above, and recognizing the CASAC conclusion that, overall,
the current evidence and REA results provide support for retaining the
current standard, the Administrator proposes to conclude that the
current suite of primary CO standards are requisite to protect public
health with an adequate margin of safety from effects of ambient CO.
    The Administrator also solicits comment on whether it would be
appropriate to revise the current primary standards. The Administrator
takes note that, while CASAC indicated their view that the evidence and
exposure and dose estimates provide support for retaining the current
NAAQS, they also indicated their preference for a lower standard. For
example, the CASAC CO Panel stated that giving additional weight to the
epidemiological evidence would support a tightening of the current
standard. The Administrator also takes note of the Policy Assessment
conclusions, summarized in section II.D.2.c above. Thus, in light of
views expressed by CASAC, as well as the Policy Assessment conclusions,
the Administrator additionally solicits comment on the appropriateness
of potential revisions to the form and level of the standards. Any
comments on such revisions should include an explanation of the basis
for the commenters' views.

E. Summary of Proposed Decisions on Primary Standards

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

III. Consideration of a Secondary Standard

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

A. Background and Considerations in Previous Reviews

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

B. Evidence-Based Considerations in the Policy Assessment

    To evaluate whether establishment of a secondary standard for CO is
appropriate, we adopted an approach in this review that builds upon the
general approach used in the last review and reflects the broader body
of evidence

and information now available. Considerations of the evidence available
in this review in the Policy Assessment were organized around the
following overarching question: Does the currently available scientific
information provide support for considering the establishment of a
secondary standard for CO?
    In considering this overarching question, the Policy Assessment
first noted that the extensive literature search performed for the
current review did not identify any evidence of ecological effects of
CO unrelated to climate-related effects, at or near ambient levels
(ISA, section 1.3 and p. 1-3). However, ambient CO has been associated
with welfare effects related to climate (ISA, section 3.3). Climate-
related effects of CO were considered for the first time in the 2000
AQCD. The greater focus on climate in the current ISA relative to the
2000 AQCD reflects comments from CASAC and increased attention to the
role of CO in climate forcing (Brain and Samet, 2009; ISA, section
3.3). Based on the current evidence, the ISA concludes that ``a causal
relationship exists between current atmospheric concentrations of CO
and effects on climate'' (ISA, section 2.2). Accordingly, the following
discussion focuses on climate-related effects of CO in addressing the
question posed above.
    As concluded in the Policy Assessment, recently available
information does not alter the current well-established understanding
of the role of urban and regional CO in continental and global-scale
chemistry, as outlined in the 2000 AQCD (PA, section 3.2). As
recognized in the ISA, CO is a weak direct contributor to greenhouse
warming. The most significant effects on climate result indirectly from
CO chemistry, related to the role of CO as the major atmospheric sink
for hydroxyl radicals. Increased concentrations of CO can lead to
increased concentrations of other gases whose loss processes also
involve hydroxyl radical chemistry. Some of these gases, such as
methane and ozone (O3), contribute to the greenhouse effect
directly while others deplete stratospheric O3 (ISA, section
3.3 and p. 3-11).
    Advances in modeling and measurement have improved our
understanding of the relative contribution of CO to climate forcing
(PA, section 3.2). CO contributes to climate forcing through both
direct radiative forcing (RF) of CO, estimated at 0.024 watts per
square meter (W/m\2\) by Sinha and Toumi (1996), and indirect effects
of CO on climate through methane, O3 and carbon dioxide
(Forster et al. 2007). The Intergovernmental Panel on Climate Change
estimated the combined RF for these indirect effects of CO to be ~0.2
W/m\2\ over the period 1750-2005 (Forster et al., 2007), with more than
one-half of the forcing attributed to O3 formation (ISA,
section 3.3 and p. 3-13).
    As discussed in the Policy Assessment, CO is classified as a short-
lived climate forcing agent, prompting CO emission reductions to be
considered as a possible strategy to mitigate effects of global warming
(PA, section 3.2). However, in considering the information presented in
the ISA, the Policy Assessment notes that it is highly problematic to
evaluate the indirect effects of CO on climate due to the spatial and
temporal variation in emissions and concentrations of CO and due to the
localized chemical interdependencies involving CO, methane, and
O3 (ISA section 3.3 and p. 3-12). Most climate model
simulations are based on global-scale scenarios and have a high degree
of uncertainty associated with short-lived climate forcers such as CO
(ISA, section 3.3 and p. 3-16). These models may fail to consider the
local variations in climate forcing due to emissions sources and local
meteorological patterns (ISA, section 3.3 and p. 3-16). It is possible
to compute individual contributions to RF of CO from separate emissions
sectors, although uncertainty in these estimates has not been
quantified (ISA, section 3.3, p. 3-13 and Figure 3-7).
    Uncertainties in the estimates of the indirect RF from CO are noted
in the Policy Assessment to be related to uncertainties in the chemical
interdependencies of CO and trace gases, as described above. Large
regional variations in CO concentrations also contribute to the
uncertainties in the RF from CO and other trace gases (ISA section 3.3
and p. 3-12). Although measurement of and techniques for assessing
climate forcing are improving, estimates of RF still have approximately
50% uncertainty (ISA, section 3.3, and p. 3-13).
    In summary, the Policy Assessment drew the following conclusions
based on the considerations identified above. As an initial matter,
with respect to non-climate welfare effects, including ecological
effects and impacts to vegetation, the Policy Assessment concluded that
there is no currently available scientific information that supports a
CO secondary standard (PA, section 3.4). Secondly, with respect to
climate-related effects, the Policy Assessment recognized the evidence
of climate forcing effects associated with CO (ISA, sections 2.2 and
3.3), while also noting that the available information provides no
basis for estimating how localized changes in the temporal and spatial
patterns of ambient CO likely to occur across the U.S. with (or
without) a secondary standard would affect local, regional, or
nationwide changes in climate. Moreover, more than half of the indirect
forcing effect of CO is attributable to O3 formation, and
welfare-related effects of O3 are more appropriately
considered in the context of the review of the O3 NAAQS,
rather than in this CO NAAQS review (PA, section 3.4). For these
reasons, the Policy Assessment concluded that there is insufficient
information at this time to support the consideration of a secondary
standard based on CO effects on climate processes (PA, section 3.4).

C. CASAC Advice

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

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

    In their comments on the draft Policy Assessment, CASAC took note
of the substantial evidence that CO has adverse effects on climate and
recommended that staff summarize information that is currently lacking
and would assist in consideration of a secondary standard in the future
(ISA, sections 3.2 and 3.3; Brain and Samet, 2010c).\57\ CASAC noted
without objection or disagreement the staff's conclusions that there is
insufficient information to support consideration of a secondary
standard at this time (Brain and Samet, 2010c).
---------------------------------------------------------------------------

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

D. Administrator's Proposed Conclusions Concerning a Secondary Standard

    The proposed conclusions presented here are based on the assessment
and integrative synthesis of the scientific evidence presented in the
ISA, building on the evidence described in the 2000 AQCD, as well as
staff consideration of this evidence in the Policy Assessment and CASAC
advice. In considering whether the currently available scientific
information supports setting a secondary standard for CO, EPA takes
note of the Policy Assessment consideration of the body of available
evidence (briefly summarized above in

section III.B). First, EPA concludes that the currently available
scientific information with respect to non-climate welfare effects,
including ecological effects and impacts to vegetation, does not
support a CO secondary standard. Secondly, with respect to climate-
related effects, the EPA takes note of staff considerations in the
Policy Assessment and concurs with staff conclusions that this
information is insufficient at this time to provide support for a CO
secondary standard. Thus, in considering the evidence, staff
considerations in the Policy Assessment summarized here, as well as the
views of CASAC, summarized above, the Administrator proposes to
conclude that no secondary standards should be set at this time
because, as in the past reviews, having no standard is requisite to
protect public welfare from any known or anticipated adverse effects
from ambient CO exposures.

IV. Proposed Amendments to Ambient Monitoring Requirements

    The EPA is proposing changes to the ambient air monitoring network
design requirements to support the NAAQS for CO discussed above in
section II. Because the availability of ambient CO monitoring data is
an essential element of the NAAQS implementation framework, EPA is
proposing to revise the requirements for the ambient CO monitoring
network to include a minimum set of monitors to provide data for
comparison to the NAAQS (i.e., for determining whether areas are
attaining the standards) in locations near roads where CO emissions
associated with mobile source related activity lead to increased
ambient concentrations. Under such requirements, State, local, and
Tribal monitoring agencies (``monitoring agencies'') collect ambient CO
monitoring data in accordance with the monitoring requirements
contained in 40 CFR parts 50, 53, and 58 for comparison to the NAAQS
and to meet other objectives.

A. Monitoring Methods

    Ambient air monitoring data are used for various purposes,
including determining compliance with the NAAQS. The use of reference
methods provides uniform, reproducible measurements of pollutant
concentrations in ambient air. Equivalent methods allow for the
introduction of new or alternative technologies for the same purpose,
provided these methods produce measurements directly comparable to the
reference methods. EPA has established procedures for determining and
designating reference and equivalent methods, known as Federal
Reference Methods (FRMs) and Federal Equivalent Methods (FEMs), at 40
CFR part 53.
    Ambient air monitoring data for CO must be obtained using an FRM or
an FEM, as defined in 40 CFR parts 50 and 53, for such data to be
comparable to the NAAQS for CO. All CO monitoring methods in use
currently by State and local monitoring agencies are EPA-designated FRM
analyzers (USEPA, 2010f). No FEM analyzer, i.e. one using an
alternative measurement principle, has yet been designated by EPA for
CO. These continuous FRM analyzers have been used in monitoring
networks for many years (USEPA, 2010f) and provide CO monitoring data
adequate for determining CO NAAQS compliance. The current list of all
approved FRMs capable of providing ambient CO data for this purpose may
be found on the EPA Web site, http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf. Although both
the existing CO FRM in 40 CFR part 50 and the FRM and FEM designation
requirements in part 53 remain adequate to support the CO NAAQS, EPA is
nevertheless proposing editorial revisions to the CO FRM and both
technical and editorial revisions to part 53, as discussed below.
1. Proposed Changes to Part 50, Appendix C
    Reference methods for criteria pollutants are described in several
appendices to 40 CFR part 50; the CO FRM is set forth in appendix C of
part 50. A nondispersive infrared photometry (NDIR) measurement
principle is formally prescribed as the basis for the CO FRM. Appendix
C describes the technical nature of the NDIR measurement principle
stipulated for FRM CO analyzers as well as two acceptable calibration
procedures for CO FRM analyzers. It further requires that an FRM
analyzer must meet specific performance, performance testing, and other
requirements set forth in 40 CFR part 53.
    From time to time, as pollutant measurement technology advances,
EPA assesses the FRMs in the 40 CFR part 50 FRM appendices to determine
if they are still adequate or if improved or more suitable measurement
technology has become available to better meet current FRM needs as
well as potential future FRM requirements. The CO FRM was originally
promulgated on April 30, 1971 (36 FR 8186), in conjunction with EPA's
establishment (originally as 42 CFR part 410) of the first NAAQS for
six pollutants (including CO) as now set forth in 40 CFR part 50. The
method was amended in 1982 and 1983 (47 FR 54922; 48 FR 17355) to
incorporate minor updates, but no substantive changes in the
fundamental NDIR measurement technique have been made since its
original promulgation. (Those updates included clarification that the
FRM NDIR measurement principle encompassed the specific ``gas filter
correlation'' measurement technique now used by many commercial FRM
analyzers.).
    In connection with the current review of the NAAQS for CO, EPA is
proposing to again update the existing CO FRM--with no substantive
changes--as explained in further detail below. This action is based on
the scientific view that the CO FRM, as originally established and
updated in the 1980's, is still fully adequate for FRM purposes and is
fulfilling that role well. Further, the FRM is also well suited for use
in routine CO monitoring, and several high quality FRM analyzer models
have been available for many years and continue to be offered and
supported by multiple analyzer manufacturers. Finally, EPA has
determined that no new ambient CO measurement technique has become
available that is superior to the NDIR technique specified for the
current FRM.
    While EPA believes that the current CO FRM is adequate, we also
believe that the existing CO FRM should be improved by implementing
updates to clarify the language of some provisions, to make the format
match more closely the format of more recently promulgated automated
FRMs, and to better reflect the design and improved performance of
current, commercially available CO FRM analyzers. EPA found that no
substantive changes were needed to the basic NDIR FRM measurement
principle; therefore, the proposed updates are of a very minor,
editorial nature. However, these proposed changes are numerous enough
so that EPA is proposing to re-promulgate the entire CO FRM in appendix
C of 40 CFR part 50, replacing the existing FRM language with revised
language.
2. Proposed Changes to Part 53
    In close association with the proposed editorial revision to the CO
FRM described above, EPA is also proposing to update the performance
requirements for FRM CO analyzers currently contained in 40 CFR part
53. These requirements were established in the 1970's, based primarily
on the NDIR CO measurement technology available at that time. While the
fundamental NDIR measurement principle, as implemented in commercial
FRM analyzers, has changed little over several decades,

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

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

B. Network Design

    The objectives of an ambient monitoring network include the
collection and dissemination of air pollution data to the general
public in a timely manner, to determine compliance with ambient air
quality standards and the effectiveness of emissions control
strategies, and to provide support for air pollution research (40 CFR
part 58, appendix D). This section on CO network design provides
background on the monitoring network, information on the sources of CO,
information on factors affecting CO emissions, and provides rationale
for a proposed network design intended to support the implementation of
the CO NAAQS.
1. Background
    EPA issued the first regulations for ambient air quality
surveillance, codified at 40 CFR part 58, for criteria pollutants
including CO in 1979 (44 FR 27558, May 10, 1979). These 1979
regulations established a monitoring network for CO (described in
detail in the CO Network Review and Background document [Watkins and
Thompson, 2010]) that required two CO monitors in urban areas with
500,000 or more people. The first of these two monitors was a ``peak''
concentration monitor, intended to be located in areas ``* * * around
major traffic arteries and near heavily traveled streets in downtown
areas.'' The second monitor was intended to represent a wider
geographic area, particularly at neighborhood scales ``where
concentration exposures are significant.'' The 2006 monitoring rule
(Revisions to Ambient Air Monitoring Regulations, 71 FR 61236 (October
17, 2006)) removed the minimum monitoring requirements for the ambient
CO monitoring network that were promulgated in 1979. However, the 2006
monitoring rule maintained a requirement that if there was ongoing CO
monitoring in an area, the area must have at least one monitor located
to measure maximum concentration of CO in that area. The 2006
monitoring rule also included a provision requiring the approval of the
EPA Regional Administrator before any existing CO ambient monitors
could be removed. Finally, the 2006 monitoring rule included a
requirement for CO monitors to be operated at all National Core (NCore)
multi-pollutant monitoring stations; with approximately 80 stations
projected to have been operational nationwide by January 1, 2011 to
support multi-pollutant monitoring objectives.
    An analysis of the available CO monitoring network data in the Air
Quality System (AQS) database shows that the network was comprised of
approximately 345 monitors during 2009. Information stored in AQS for
these monitors describes the most frequently stated monitor objectives
for sites in the current CO network as assessment of concentrations for
general population exposure and maximum (highest) concentrations at the
neighborhood scale.\58\ Approximately 56 of the monitors operating in
2009 were at microscale sites, a majority of which were likely sites
representing ``peak'' concentrations which were required under the
monitoring regulations originally promulgated in 1979, intended to
characterize mobile source impacts in heavily traveled downtown streets
or near major arterial roads (Watkins and Thompson, 2010). The rest of
these sites were likely being operated to meet objectives including
NAAQS comparison, to support long-term trend determination, to meet
State Implementation Plan (SIP) and maintenance plan requirements, and
to support ongoing health studies.
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    \58\ Spatial scales are defined in 40 CFR part 58 Appendix D,
Section 1.2, where the scales of representativeness of most interest
for the monitoring site types include:
    1. Microscale--Defines the concentration in air volumes
associated with area dimensions ranging from several meters up to
about 100 meters.
    2. Middle scale--Defines the concentration typical of areas up
to several city blocks in size, with dimensions ranging from about
100 meters to 0.5 kilometers.
    3. Neighborhood scale--Defines concentrations within some
extended area of the city that has relatively uniform land use with
dimensions in the 0.5 to 4.0 kilometers range.
    4. Urban scale--Defines concentrations within an area of city-
like dimensions, on the order of 4 to 50 kilometers. Within a city,
the geographic placement of sources may result in there being no
single site that can be said to represent air quality on an urban
scale. The neighborhood and urban scales have the potential to
overlap in applications that concern secondarily formed or
homogeneously distributed air pollutants.
    5. Regional scale--Defines usually a rural area of reasonably
homogeneous geography without large sources, and extends from tens
to hundreds of kilometers.
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2. On-Road Mobile Sources
    The REA for this review notes that ``motor vehicle emissions
continue to be important contributors to ambient CO concentrations''
(REA, section 2.2). Microenvironments influenced by on-road mobile
sources are important contributors to ambient CO exposures,
particularly in urban areas (REA, section 2.7), as indicated by
personal exposure studies that have generally shown that the highest
ambient CO exposure levels occur while people are in transit in motor
vehicles (ISA, section 2.3). Mobile sources are the primary
contributors to ambient CO emissions because CO is formed by incomplete
combustion of carbon-containing fossil fuels widely used in motor
vehicles (ISA, section 2.1; REA, section 3.3). Further, spark-ignition
engines (gasoline or light-duty engines) have higher CO emission rates
than diesel engines (heavy-duty engines) because they typically operate
closer to the stoichiometric air-to-fuel ratio, have

relatively short residence times at peak combustion temperatures, and
have very rapid cooling of cylinder exhaust gases (ISA, section 3.2.1).
    Ambient CO concentrations have significantly declined over the past
20 years, reflecting reductions in on-road vehicle emissions, as
described in section II.A above. Overall, based on the 2002 National
Emissions Inventory (NEI), on-road mobile sources account for
approximately 52% of total CO emissions. Based on the more recent 2005
NEI, the contributions of on-road mobile sources has now risen to
approximately 60% of the total CO emissions inventory (not counting
wildfire emissions) (http://www.epa.gov/ttn/chief/eiinformation.html).
As described in section II.A above, in some metropolitan areas in the
U.S., as much as 75% of all CO emissions result from on-road vehicle
exhaust (ISA, section 2.1).
    On-road vehicle CO emission rates vary depending on operating
conditions, such as cold-start conditions and operating speed. Under
cold start conditions, which only last for the first minutes of vehicle
operation, CO emissions are higher due to temporary ineffectiveness of
vehicle exhaust catalysts until they are heated to optimal operating
temperatures (ISA, section 3.2.1; Singer et al., 1999). Meanwhile, CO
emissions also vary based on vehicle operating speeds. Increased CO
emissions occur under conditions of high acceleration, rapid speed
fluctuations, and heavy vehicle loads (ISA, section 3.2.1). Studies
have found that CO emission rates for tested light-duty vehicles are
highest for accelerating vehicles, second highest for vehicles in
cruise, third highest for vehicles under deceleration, and fourth
highest (of four operating speed related categories) for vehicles at
idle (Frey et al., 2003). High acceleration and rapid speed
fluctuations (such as acceleration and deceleration occurring over a