Report on Air Quality Criteria for Carbon Monoxide (AP-62)
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Report on Air Quality Criteria for Carbon Monoxide (AP-62)
A
Report to the
United States
Environmental Protection Agency
by the
National Air Quality Criteria Advisory Committee
of the
Science Advisory Board
Science Advisory Board
U.S. Environmental Protection Agency
June 1976
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EPA NOTICE
This report has been written as a part of the activities of the
Agency Science Advisory Board, a public advisory group providing extra-
mural scientific information to the Administrator and other officials of
the Environment~l Protection Agency.
The Board is structured to provide
a balanced expert assessment of scientific matters related to problems
facing the Agency.
This report has not been reviewed for approval by the
Agency and hence its contents do not represent the views and policies of
the Environmental Protection Agency.
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Report on Air Quality Criteria for Carbon Monoxide (AP-62)
Summary Comments
Although the effects of Carbon Monoxide have been well known for
many years, great gaps exist in our knowledge of the chronic effects of
low concentration in normal persons.
Most of the studies on man have
been concerned with exposures to 50-100 ppm Carbon Monoxide for
relatively short periods of time.
Much more research is necessary to
determine the effects of long duration eXDosure to low levels of Carbon
Monoxide in air.
The National Research Council Panel on Carbon Monoxide is now revis-
ing its analysis of the available data on Carbon Monoxide.
Although the
current criteria document requires revision, it is recommended that no
change be considered by EPA until this study is completed.
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Comments on Specific Chapters
Chapter L
Introduction
Page 1-1.
Text implies that expression of concentrations in micro-
grams per cubic meter instead of parts per million by volume is a change
from some other system to the metric system~
The impression this creates
is incorrect inasmuch as ppm/v is independent of any system of units.
Chapter 2
Occurrence, Properties, and Fate of Atmospheric Carbon
Monoxide
Page 2-1.
More is known about natural sources than is implied in
Section A.
On a global emissions basis, carbon monoxide from natural sources
is estimated to be produced at a rate considerably greater than from anthro-
pogenic sources with oxidation of methane probably the main natural source.
Page 2-2.
Sections D and E should be updated and expanded.
The
probable role of photochemistry and the OH radical should be discussed in
greater detail.
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Chapter 3
Principles of Formation and Control of Carbon Monoxide
Section B
Formation of Carbon Monoxide by Combustion
The formation of carbon monoxide should be treated in greater detail.
Greater emphasis should be given to such processes as the conversion of
carbon dioxide to carbon monoxide by reduction by carbon at high tempera-
tur€s.
Trace amounts of substances capable of generating OH and H02 free
radicals will cause stoichiometric mixtures of carbon monoxide and oxygen
to explode upon ignition attesting to the speed of reaction.
Therefore,
the emphasis on the slowness of the reaction of the conversion of CO to
C02 should be q~alified.
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Chapter 4
Estimation of Carbon Monoxide Emissions
This chapter should be completely rewritten.
High levels of carbon
monoxide are most usually observed at specific locations where traffic
congestion and the local street and building configurations result in high
emissions and poor dispersion.
Around the corner or even at the next
intersection it is possible that a much less severe or no problem exists.
Therefore, the magnitude of regional, City-wide or even borough emissions
of carbon monoxide is a relatively poor indicator of problems compared to
the importance of such estimates for other pollutants.
The important calculation of carbon monoxide emissions is made on a
site-by-site basis.
One area can have equal or even much higher emissions
of carbon monoxide than another of equal size and yet have few or no
violations of standards, whereas another area with lower emissions can
have many violations.
The Long Island Expressway is a very high emissions
area because of the quantity of traffic it handles.
However,
carbon
monoxide concentrations in its immediate vicinity because of the relative
openness of the surroundings are much lower than those found at sites in
Manhattan with much lower emissions.
The calculation of emissions of carbon monoxide and other mobile
source pollutants from the exhaust of motor vehicles has made great
progress since the original criteria document was written.
The importance
of local driving patterns, local climate, percent of cold starts, vehicle
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Chapter 5
Measurement of Carbon Monoxide Concentrations in Ambient Air
Section B
Preparation of. S~rh..9~~o~oxi.de £Ci2. S~and'?_~~2
.Page 5-1.
Availability of National Bureau of Standards samples for
calibration and the importance of the composition of the metals used in
pressurized tanks should be discussed.
The performance specifications for
automated methods for carbon monoxide and the effects of known interferents
are now available and should be presented.
Newer methods for monitoring, such as electrooxidation of carbon mono-
xide at a catalytically active electrode after diffusion through a membrane,
dual isotope fluorescence, and others, should be described.
A more detailed treatment of the difficulties inherent in NDIR tech-
niques mainly as a result of interference by water vapor would be advisable.
Techniques for dealing with water vapor interference should be discussed in
some detail.
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Chapter 6
Atmospheric Carbon Monoxide Concentrations
If carbon monoxide concentrations are measured for the purpose of
determining the health effects of this pollutant upon local populatons
then this Chapter"should revolve around the siting problem associated
with monitoring.
The variations encountered with time of day and day of
week, with geographical location and mobile population throughout the
congested areas where carbon monoxide levels are the highest, make a valid
determination of the magnitude of the problem excessively difficult.
Because of sensitivity to sampling point, comparison of data from
different locations is liable to be misleading unless very detailed infor-
mation about the local configurations in each case is well documented.
The slowness with which the. human system comes to equilibrium with ambient
concentrations further complicates the situation.
Therefore, the effect
of all these variables on the usefulness and interpretation of measure-
ments must be emphasized and discussed in greater detail.
Fortunately,
the blood carboxyhemoglobin content can be used as a good indicator of
the exposure of the individuals.
Therefore, ambient measurements of
carbon monoxide should be supplemented by extensive sampling of populations
for percent COHb.
The statistical models relating averaging time to the concentrations
at various percentiles should be used with great care.
These models are
difficult to relate to carbon monoxide standards which are stated in terms
of concentrations not to be exceeded more than once per year.
Furthermore,
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these models must ~ot be used to calculate standards at different averag-
ing times from one standard set at a specific averaging time.
Thus, an
8-hour standard set at 9 ppm should not be used as a basis for calculat-
ing the I-hour standard using the statistical model because it completely
ignores the major role played by the body's rate of uptake of carbon
monoxide to form carboxyhemoglobin.
Simulation Modeling
Because of the ~ifficulties inherent in obtaining carbon monoxide
concentrations .for use in calculating accurate human exposures, the use
of simulation models has gained importance.
The pressures created by the
economic costs of controlling pollution have made investments in these
()
models a necessity.
Short term models for carbon monoxide are under
intense development.
Such models have been reported to calculate concen-
trations within twenty-five percent of measured values.
A very careful distinction must be made between the above models
which calculate concentrations at a specific site with a specific traffic
volume and speed and surrounding building configuration from models which
are applied to pollutants such as sulfur dioxide.
The latter
are much
more dependent upon meteorological conditions and treat diffusion over
much greater volumes of space.
The models discussed in the criteria
document are not the ones which should be applied to carbon monoxide.
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Chapter 7
Effects of Carbon Monoxide on Plants and Certain Microorganisms
The review is well written and factually accurate.
.The area is
essentially non-productive for research so there have been no significant
contributions t6 it in the past five years to the best of the reviewers'
knowledge.
Carbon monoxide is relatively innocuous for vegetation although
abnormal root develop~ent may be stimulated, development of pestillate
organs of the flowers may be accelerated and activities of nitrogen-fixing
bacteria may be reduced in legumes.
In tests on over 100 species, no
acute symptoms were observed at dosages lower than 100 ppm (115 mg/m3) for
7 to 23 days.
This level was never observed in any major city of the U.S.
on heavily travelled streets where average hourly levels may reach 10 to
15 ppm.
The maximum hourly level observed in many U.S. cities over an
eleven year period was 81 ppm and rarely attained more than 50 ppm in any
city.
The toxicity Jor mammals is so much more acute tha~ any standards set
to protect man or beast will protect plant life.
It should be noted that
the early literature was confounded by description of symptoms identified
to ethylene which 'in reality were due to ethylene impurities in CO gener-
ated by combustion of natural gas.
About 5000 times as much CO is required
to injure plants as ethylene'so the impurity escaped detection.
We suggest no changes be made since Chapter 7 is written in a
straightforward sty1e~
The only thing missing are studies on CO in soil.
and its effects on chlorophyll and carbon dioxide fixation at higher con-
centrations.
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Chapter 8
.Toxicological Appraisal of CO
Section A
Introduction
Appears. satisfactory.
Section B
Theoretical Considerations
The paragraph on theories to explain the sigmoid shape of the
oxyhemoglobin dissociation curve should be omitted.
It only confuses
the reader.
Measurement of Carboxyhemoglobin in Blood
Section C
This section needs updating to include new information and a more
critical evaluation of both old and new material.
One omission is
discussion of the use of the CO-oximeter indifferential spectrophotometry
(Malensant 1968) in connection with the material presented on spectro-
photometry (Amenta 1963), (Commins 1965).
Much greater emphasis needs
to be given to. gas chromatographic methods.
The work of Porter (1962)
and Collison (1968) using gas chromatography to measure CO is not
included in the original document.
A more thorough survey of the
literature is recommended.
There is urgent need to determine carboxyhemoglobin content in
populations exposed to carbon monoxide.
This will require the testing
of large numbers of the general population and of special groups whose
situation exposes them to higher concentrations of CO.
The most
practical method for making the large number of measurements needed is
the analysis of expired alveolar air.
Therefore, an in-depth review
and critique of this method is strongly indicated.
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A most helpful comparison of the methods of measuring carboxyhemo-
globin would include the sensitivity and accuracy of t';:lch [)rocedurC' at
low and high concentrations of this substance in blood.
Section D
Uptake of Carbon Monoxide by Humans
This section is satisfactory but it might be well to include some
practical examples.
For example, a person exposed to 60 Dpm CO while
driving in traffic. at the end of I hour would have only 2.5% COHb, less
than a man doing heavy work in 3.5 ppm CO for I hour.
Furthermore, the
. .
man who has a 2.5% COHb level would excrete much of this CO while work-
ing in an environment at a lower CO level, the amount depending on his
rate of work and the CO concentration in the air.
In another case, a person doing light work exposed to In ppm CO
would have only about 0.4% COHb at the end of I hour and a maximum of
1.4% by the end of 8 hours -- not 1.4% throughout the day.
No matter
how long he is exposed, his COHb level could never exceed 1.7% since
this is the equilibrium value.
Section E
Effect of Carbon Monoxide on the Central Nervous System
The original Air Quality Criteria Document placed major emphasis on
the early psychomotor studies of humans in which impairment of certain
functions was reported to occur at COHb levels of 2%.
These studies have been subject to great question because of the
methods employed and the failure to determine the COHb concentration.
Although some other studies reported some psychomotor effects at 5%
COHb, other investigators have reported no psychomotor or behavioral
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impairment with COilb levels as high as 11 - 13%.
Recent acute experiments
on dogs demonstrated that cerebral blood flow was not significantly
affected by inhalation of carbon monoxide leading to 10% HbCO and that the
oxygen consumption of the brain was not reduced at 20% RbCO, in fact, it
was slightly increased.
It is believed that cerebral vasodilation com-
pensates for the reduced oxygen tension.
Human experimentation presents a number of problems since human
subjects cannot be held for sufficiently long periods to reach an equili-
brium COHb value before testing.
Usually they have been exposed to
extremely high levels of CO for brief periods to bring their COHb up to
the desired concentration quickly.
Furthermore, the desired con centra-
tions cannot be maintained under control conditions for daysor weeks, but
only for a few hour~.
~fuen the experiments involve nsvchological and Dsychomotor tests
many more variables are added which make interpretation of the data
extremely difficult and are responsible for the widely divergent results
reported in the literature.
This section should be revised to include new data on central
nervous system effects and to reevaluate the conclusions based on the
earlier work.
Section F
Effects of Carbon Monoxide on Cardiovascular System
The recent studies on the cardiovascular system have centered on
the effects of CO on persons with preexisting coronary disease.
These
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studies should be'reviewed in revision of this chapter.
For example,
exposures of a few such persons to carbon monoxide leading to COHb at 2.9
and 4.5% resulted in a more rapid onset of anginal pain during exercise
as well as increasing the duration of the pain after the exercise.
In
men with peripheral atherosclerosis, leg pain developed more rapidly
when walking with 3% COHb.
Thus, even small increments in COHb concen-
tration appear to exacerbate symptoms in patients with ischemic heart
and arterial disease, whose 0 transport systems are marginal with respect
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to the heart and circulation.
However, an influence of CO on mortality
from arteriosclerotic heart disease (ASHD) has not been proven.
In the
most recent and best controlled study, no relation was found between
either the incidence of myocardial infarction or sudden death due to
ASHD and the average 24 hour ambient CO content (which varied from less
than one to over 9 ppm).
Furthermore, there was no differences between
HbCO among patients dying suddenly of ASHD (Kuller et al., Arch. Envir.
Med. 30: 477, October, 1975).
At the present time there is no firm
evidence to indicate that chronic exposure to low levels of CO causes
heart disease in man.
Acute exposures leading to blood levels of
10-12% HbCO in humans and 25% COHb in dogs leads to substantial increases
in coronary blood flow and increase in 02 consumption.
with coronary disease may not be able to react in this manner.
However, persons
Smokers present a special problem since they usually have 5% or
more
(even up to 15%) crnlb in their blood.
Whether the cardiac effect
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of smoking is due to CO or other chemicals is not known although some
animal experiment~suggest that CO may be the responsible factor.
Although it is "generally assumed that environmental CO exposures and
exposures due to smoking are additive this is not necessarily true.
As
pointed out in the" 'iliO report, continuous exposure of a nonsmoker to
25 ppm CO will eventually have 4% saturation.
Smokers with initial
saturation greater than 4% will excrete their excess to reach 4% equili-
brium provided they do not continue to smoke.
No matter what the
ambient air concentrations are, adequate protection of smokers cannot
be achieved.
Section H
Effects of Carbon Monoxide at High Altitude
Insufficient data are available to determine the effect of CO in
persons with other diseases such as anemia, respiratory illnesses, or
normal persons living at high altitudes.
All of these factors may alter
the normal oxygen supply to the tissues.
The problem of CO exposure
during pregnancy also requires consideration.
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~
Chapter 9
Epidemiological Appraisal
This chapter contains very little sound data.
A chronic CO poison-
ing syndrome has not been supported by the evidence presented.
Most of
the epidemiological studies were complicated by many variable factors so
that a positive relation to CO per se cannot be inferred from the data.
This chapter should be critically reviewed.
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