PB-244 338
GUIDES FOR SHORT-TERM EXPOSURES OF THE PUBLIC TO
fcffc POLLUTANTS. VI. GUIDE FOR CARBON MONOXIDE
National Research Council
Prepared for:
Environmental Protection Agency
March 1973
DISTRIBUTED BY:
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NAS/ACT/p-628.7
Guides for Short-Term Expos ures of the Public to Air Pollutants
VI. Guide for Carbon Monoxide
by
The Committee on Toxicology
of the
National Academy of Sciences - National Research Council
Washington, D. C.
March 1973
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BIBLIOGRAPHIC DATA
SHEET
!• Report No.
NAS/ACT/P-628.7
PB 244 338
4. Tide and Subtitle
Guides for Short-Term Exposures of the Public to Air-
Pollutants. Vi. Guide for Carbon Monoxide.
5. Report Date
March 1973
6.
7. Auttior(s)
&• Performing Organization Rept.
No- NAS/ACT/P-628.7
9. Performing Organization Name and Address
Committee on Toxicology of the National Academy of Sciences
National Research Council
2101 Constitution Avenue, N.W.
Washington, DC 20418
10. Ptoject/Task/Work Unit No.
11. Contract/Grant No.
CPA 70-57
12. Sponsoring Organization Name and Address
Environmental Protection Agency
4th and M Streets, S.W.
Washington, D.C. 20460
13. Type of Report & Period
Covered
Final
14.
15. Supplementary Notes
16. Abstracts
Recommendations are made for limits of air concentrations of carbon monoxide
to which the public may safely be exposed for short periods of time. The
scientific basis and associated literature references for the recommendations
are presented.
17. Key Words and Document Analysis. 17a. Descriptors
Air Pollution
Exposure
Sensitivity
Toxicity
Carbon monoxide
17b. Identifiers/Open-Ended Terms
Ai»- nollution effects (animals)
Aii pollution ecrects (humans)
Short-Term Public Limits (STPL's)
Public Emergency Limits (PEL's)
17c. COSATI Field/Group
18. Availability Statement
Release unlimited
FORM NTIS-35 ( 10-70)
19.. Security Class (This
Report)
UNCLASSIFIED
121. No. of Pages
20. Security Class (This
Page
UNC.l.ASS'FIF.D
USCOMM-OC 40329-P71
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Committee on Toxicology
Herbert E. Stokinger, Chairman. V. K. Rowe
Arthur B. DuBois, Vice-Chairman C. Boyd Shaffer
Bertram D. Dinman Frank G. Standaert
Seymour L. Friess James H. Sterner
Harold M. Peck Richard D. Stewart
Subcommittee on Carbon Monoxide
James H. Sterner, Chairman Clyde M. Berry
Ted A. Loomis, Reviewer Richard S. Brief
Yves Alarie Sheldon D. Murphy
Roy E. Albert
L. M. Roslinski, Staff Officer
J. H. Broome, Staff Officer
Advisory Center on Toxicology
National Academy of Sciences T National Research Council
-------
Prepared under Contract No. CPA 70-57
between the the National Academy of Sciences
and the Environment Protection Agency.
Contract Monitor:
Dr. Vaun A. Newill
Assistant for Health Effects
Office of Research and Monitoring
Environmental Protection Agency
Washington, D. C. 20460
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NOTICE
The project which is the subject of this report was approved by
the Governing Board of the National Research Council, acting in
behalf of the National Academy of Sciences. Such approval reflects
the Board's judgment that the project is of national importance and
appropriate with respect to both th'e purposes and resources of the
National Research Council.
The members of the committee selected to undertake this
project and prepare this report were chosen for recognized scholarly
competence and with due consideration for the balance of disciplines
appropriate to the project. Responsibility for the detailed aspects of
this report rests with that committee.
Each report issuing from a study committee of the National
Research Council is reviewed by an independent group of qualified
individuals according to procedures established and monitored by the
Report Review Committee of the National Academy of Sciences.
Distribution of the report is approved, by the President of the
Academy, upon satisfactory completion of the review process.
in
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INTRODUCTION
The Environmental Protection Agency has focused its initial
concerns on long-term exposures of the public to air pollutants. Although
long-term exposures generally involve exposure to low levels of air
pollutants there are occasional circumstances wherein the public may be
exposed briefly to relatively high concentrations. For example, batch-
process tec?iniques in industries may result in pulses of effluent. The
testing and launching of rockets release high concentrations of exhaust
products. Rapidly changing meteorological conditions may result in
short periods of locally high concentrations of stack effluents. Accidental
release of chemicals sometimes occurs in industrial areas or during
.transport, and may lead to public exposure.
Recognizing that these occasional peak additions to the ambient
exposures of the public do occur^ the Environmental Protection Agency
has requested the assistance of the Committee on Toxicology of the
National Academy of Sciences - National Research Council in providing
Guides for short-term exposure limits for air pollutants.
In preparing these Guides, the Committee utilized the criteria
described in the NAS-NRC document entitled "Basis for Establishing
Guides for Short-Term Exposures of the Public to Air Pollutants"
(41). Primary consideration was given to literature dealing with single
or intermittent brief exposures to the contaminant in question, in this
case carbon monoxide.
IV
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GENERAL BACKGROUND
Physical-Chemical Properties
Carbon monoxide (CO) is a colorless odorless gas possessing the
following physical properties:
Molecular weight: 28
Melting point: -207° C
Boiling point: -190° C
Density (25° C, 760 mm Hg): 1.15 g/1
At 25° C and 760 mm Hg, 1 part of CO per million parts of air
(ppm) is equivalent to 1.15 mg of CO per cubic meter of air (mg/m3).
Sources
Carbon monoxide is produced by the incomplete combustion of
.carbonaceous materials. A flame in the presence of a sufficient air
supply and at temperatures at or above the ignition temperature of the
gaseous part of the flame should produce insignificant amounts of CO.
Because complete combustion is difficult to attain, varying levels of CO
can be expected to form in most combustion processes.
The major man-made source of CO in the environment is the
exhaust of motor vehicles, accounting for approximately 60% of the total
CO emissions per year. Industrial processes, solid-waste disposal and
fuel combustion in stationary sources account for approximately 20% of
the total CO emissions while miscellaneous sources account for the
remainder of the total emissions (10).
CARBON MONOXIDE AND THE LIVING ORGANISM
Production and Distribution of Carbon Monoxide in Mammals
The first demonstration of small amounts of CO in the blood of
experimental animals and humans has been attributed to French scientists
(1). The endogenous production of CO is primarily the result of the
catabolism of the heme moiety of hemoglobin, although the metabolism of
other heme-containing proteins would also be expected to produce CO (2).
Regardless of its source (endogenous or exogenous) CO in the
organism is found chemically bound to heme proteins, primarily hemo-
globin. Myoglobin, cytochrome oxidase, cytochrome P-450, and hydro-
peroxidases are'other hemoproteins capable of reversibly binding CO
although collectively they account for only 10-15% of extra vascular CO in
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normal man (3, 4). Coburn has reported that under the conditions whereby
arterial partial pressures of oxygen (pO^) are below 40 mm Hg, CO will
shift into muscle tissue to bind with myoglobin to yield carboxymyoglobin
levels about three times greater than found at ambient arterial pO£ (4).
The full significance of the carboxymyoglobin complex is not known, and
may be important in regard to myocardial oxygen exchange (5).
Carbon monoxide binds reversibly with hemoglobin (Hb) forming
carboxyhemoglobin (Co Hb). The affinity of Hb for CO is approximately
200-250 times its affinity for oxygen. CO Hb is incapable of carrying
oxygen, so that the effect is a reduction of the tissue partial pressure of
oxygen. An additional effect is that when CO Hb is present the dissocia-
tion of the remaining oxyhemoglobin (Hb02) is altered in the direction of
impairment of release of 02 to the tissues. The decrease in oxygen-
carrying capacity of the blood together with the impaired release of
oxygen to the tis.sues results in a greater tissue oxygen deficiency than
is produced by an equivalent reduction in ambient pO? (as at altitude) or
an equivalent reduction of hemoglobin (as in anemia) (3,6).
Factors Affecting the Rate of Carboxyhemoglobin Formation
The uptake of CO in the blood is dependent on a number of
physiological parameters such as endogenous production of CO, activity
(alveolar ventilation) of the exposed individual, and duration of exposure.
Although a number of mathematical models have been reported to
describe the rate of COKb formation in persons exposed under specific
conditions (7,8) Coburn_e_t aL (56) developed a mathematical model
that'can be generally applied to consider nine variables that can influence
the rate of COHb formation. Appendix 1 contains a representation of the
"Coburn equation" applicable to exposure conditions at sea level
(760 mmllg). •
At equilibrium, when a subject is breathing atmospheric air
containing CO at concentrations < 100 ppm, the level of COHb above
background has been expressed by the following formula:
% COHb = 0.16 x [CO], the CO concentration being
expressed as ppm (8).
According to Forbes (9) the uptake of CO will vary almost in
proportion to the rate of ventilation up to minute volumes of 20 liters.
Above 20 liters the uptake will be 10 to 15% less than expected from true
proportionality. Individual variation among apparently normal, healthy,
young adults was about 25% between the fastest absorber and the slowest.
These differences, apparently due to differences in dead space and
diffusion coefficients, can be greater in pathological conditions.
- 2 -
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Goldsmith (8) reports ventilation to be six liters per minute at rest and
18 liters per^minute during light work with a somewhat less than three-
fold increase in the rate of uptake of CO observed with the three-fold
increase in ventilation during light work.
Effects on Animals (Long-Term)
Although the primary concern, for the purposes of this document,
is short-term exposures to carbon monoxide, a brief consideration of the
long-term exposure effects is important for comparative purposes.
Stupfel and Bonley (12) exposed rats at concentrations of 50 ppm
continuously for 4-1/2 days/week, 95 hr/week, for periods ranging
from 30 days to 2 years. Some of the observations made on the ani-
mals include CO_ emission, ECG, conditioned behavior, and a variety
of biochemical, hematological, bacteriological, and immunolbgical
tests. During .the first three months of exposure the following functions
were not altered: fecundity, reproduction, growth, CC>2 emission,
weight of organs, water percentages of organs, hemoglobin, serum
protein, lipids, calcium, magnesium, trarisaminases, hematological
data, bacteriological and immunological .parameters. Blood cholester-
ol levels, heart rate, ECG tracings, and avoidance conditioning appear-
ed to be slightly altered at the beginning of exposure. Mortality and
aging processes were not altered during the two-year study.
Essentially similar findings over a two-year period were recently
reported in cynomolgus monkeys (57).
Jones £t aj_. (13) exposed rats, guinea pigs, dogs, and monkeys
to 51 ppm CO for 90 days continuously and reported no significant
changes in the hematocrit or hemoglobin concentration in any of the
species. At CO concentrations of 96 and 200 ppm for 90 days, the
hematocrit and hemoglobin were significantly elevated in rats, guinea
pigs, and monkeys. Repeated 8 hr/day, 5 days/wk, 6 wks exposure of
the same species of animals to a CO concentration of 106 ppm caused a
significant increase in hemoglobin and hematocrit in rats only.
Vernot e± aU (14) exposed rhesus monkeys continuously for 100
days to a concentration of 57. 5 mg/m CO and rhesus monkeys and
beagle dogs to CO concentrations of 115, 230, 460, and 575 mg/m-* con-
tinuously for 77 to 182 days. The barometric pressure was maintained
at 260 mm Hg with an atmosphere of 68% O? and 32% N?. They observed
an increase in red blood cell counts, hemoglobin, and hematocrit that at
equilibrium appeared to be a linear function of the CO concentration. In
the dogs the red blood cell counts increased 44%, the hemoglobin 41%,
and the hematocrit 38% over controls during exposure to 575 mg/rn^ CO.
In the exposed monkeys the respective increases were 60%, 58%, and 59%.
The polycythemia was normocytic and normochromic. Total blood
v6lume increased in both species while plasma volume remained constant.
Equilibrium COHb levels in the monkeys were 12, ?l. 3, 31. 6, and 38. 5
respective to the four exposure levels. The COHb levels in the dogs were
comparable.
- 3 -
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Back (15) conducted an experiment on monkeys exposed continu-
ously to CO aj a concentration of 220 mg/m^ for 103 days under conditions
similar to those reported by Vernot (260 mm Hg, 68% 02, 32% N2). The
monkeys, however, were trained to perform discrete avoidance tasks by
both Visual and audio cues. Under these conditions COHb was reported
to plateau at 22% with increases in hematocrit and hemoglobin similar to
those observed by Vernot. Back reported no detectable performance
changes in any of the monkeys tested.
MacKenzie e± aJL (16) and Theodore et al_. (17) exposed mice, rats,
dogs, baboons, and rhesus monkeys to CO at a concentration of 460
mg/m3 for 71 days followed by exposure to a CO concentration of 575
mg/m^ for 97 days (in conjunction with and under the conditions of the
Vernot experiment). Although at the 460 mg/rn^ exposure level the COHb
level was 32% in the monkeys and 33% in the dogs and, at the 575 mg/m^
level, the COHb levels were 38% and 39% in monkeys and dogs respectively,
the only significant finding was a "marked erythrocytosis" in the experi-
mental animals. Animal survivability, growth rates, clinical chemistry,
and. pathology were not apparently different from the control animals (17).
Anatomic changes found were confined to rodents and consisted of an
i .
increase in heart and spleen weight. The authors explain this is due to
increased red blood cell volume and blood viscosity (16).
Preziosi e_t aL (18) reported ECG changes in dogs exposed to CO
concentrations of 50 and 100 ppm for 6 hr/day, 5 days/wk for 6 weeks,
or continuously for 6 weeks. At autopsy the most frequent finding was
dilation of the right heart or thinning of the myocardial wall and flattening
of the papillary muscles and trabeculae. He observed dilation of the
ventricular system of the brain, particularly of the lateral ventricles in
50% of the animals exposed continuously to CO concentrations of 50 ppm.
A similar finding was observed in the animals exposed at 100 ppm either
continuously or intermittently. Histological examination revealed
mobilization of glial cells and thinning of the white matter in the centrum
semiovale in the brain. Heart histology reportedly revealed old
scarrings in some cases and fatty degeneration of the heart muscles in
others; The blood cytology, hemoglobin, and hematocrit showed no
deviation from the control animals.
Summary of Long-Term Exposures
Animals exposed for more than 2-3 weeks to CO concentrations
greater than 100 ppm show an increase in red blood cell count, hemato-
crit, and hemoglobin. Evidently these hematologic changes are the only
effects of long-term CO exposures agreed upon by the various researchers.
While neural and cardiac lesions or changes are reported by some, others
have not reported observing such changes at similar or even higher CO
exp'osure levels.
- 4 -
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Short-Term arid Repeated Exposure Effects on Animals
Beard and Wertheim (19) exposed rats to CO concentrations of
100, 250, 500, 750, and 1,000 ppm for time periods up to 48 minutes
and reported the effects of such exposures on the performance of
Skinnerian operant behavior schedules of reinforcement. For a fixed
interval (30 sec.) reinforcement test, perceptible effects due to CO were
reported by the authors to be observed at 100 ppm CO after 11 minutes
of exposure. Increasing concentrations produced proportionately greater
effects.
Montgomery and Rubin (ZO) exposed rats to CO concentrations
ranging from Z50 ppm to 3,000 ppm for 90 minutes and then challenged
the animals with hexobarbital or zoxazolamine in order to measure the
rate, of recovery from the effects of these drugs by the CO-pre-exposed
animal. Prolongation of pharmacologic response to hexobarbital and
zoxazolamine was observed when the pre-exposure CO concentration was
1, 000 ppm and 250 ppm respectively. The authors report that these data
do not allow a conclusion to be made regarding the mechanism involved
since the duration of response to these drugs is known to be influenced
both by alteration of the microsomal drug-metabolizing system and by ..
tissue hypoxia.
Mazaleski_e_t al_. (21) exposed rats to CO at 50 ppm for 5 hr/day,
5 days/wk for 12 weeks and measured the concentrations of zinc, copper,
cobalt, iron, .and magnesium in various liver fractions prepared from
the exposed and control animals. The authors report significant variations
in all the metals, measured at 3-wk intervals; however, only the cobalt
content of the nuclear fraction of the liver of the exposed animals was
consistent and significantly lower than controls over the 12-wk exposure
period. The authors state that their results suggest that chronic exposure
to 50 ppm CO produces an effect on trace metals at the sub-cellular level ;
with a possible reduction in cellular respiration and nucleoprotein
synthesis.
Xintaras ej^al^ (22) exposed rats to concentrations of CO over the
range of 50 - 1,000 ppm for periods of 1-2 hr. The animals were condi-
tioned to respond to a light stimulus by pressing a lever. Recording
electrodes, for EEG recording from the visual cortex and superior
colliculus, were used to observe any neurological changes in the exposed
animals. No effects attributable to CO exposure were obtained in regard
to the conditioned response; however, progressive dose-related changes
were observed in the electrical recordings from the cortical and collicular
areas of the brain. Similar changes were induced by pentobarbital.
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Astrup et a 1. (?3) reported increases in the cholestrol content of
the aorta of cholesterol-fed rabbits exposed for 8 hr/day for 10 weeks to
concentrations of CO causing 20% COHb H170 ppmK t The exposed
animals had as much as five times the aortic cholesterol concentration
as was observed in the control animals. Exposing cholesterol-fed
rabbits to an atmosphere containing 10% 02 (hypoxia) for 8 weeks
resulted in aortic levels of cholesterol 3. 5 times higher than those
found in the control animals. An atmosphere of 28% 0? (hyperoxia)
produced an aortic cholesterol concentration of one-half that observed
in the control animals. The authors report that both microscopically
and macroscopically there was no difference between the arterial
lesi'ons in animals exposed to CO and the animals exposed to hypoxia.
Preziosi et al. (18) exposed dogs to CO at concentrations ranging
from I, 280 ppm to 17, 000 ppm for time periods ranging from "less than
15 minutes" to "60 minutes or more". The sequence of physiological
changes observed by the authors in dogs ha'ving 40-50% COHb was
depressed respiration, rise in cerebrospinal fluid pressure, rise in
venous pressure, a secondary rise in cerebrospinal fluid pressure, a
rapid irregular respiration, and a decrease in arterial pressure.
ElectrocarcHographic changes included depression of R wave, elevation
of ST segment, occasional increases in the T wave, deepening of the O
and S wave, and partial heart block with premature ventricular contrac-
tions "at the height of exposure". The authors exposed "control" dogs
to hypoxic conditions in order to simulate the 0£-deficient conditions of
the CO-exposed animals. These "controls" exhibited changes identical
in kind but less in degree than the CO-exposed group.
*
j Estler-
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exposed to CO at levels of 3, 300 ppm for 35 minutes at 13° C the lethality
was 32% (16/50') in the males and 0% (0/50). in the females. For a 25-
minute exposure to the same concentration of CO at 20°C the lethality
was 89% for males and 39% for females. Similar significant sex-related
susceptibility was also observed in rats exposed under similar conditions.
Castrated male mice were significantly less susceptible to CO
lethality than normal males (31% dead and 80% dead respectively), although
the castrated male deaths were still significantly greater than female
deaths (9.6%) and castrated female deaths (13%). The difference between
normal female deaths and castrated female deaths was not significant.
Effects on Humans
The effects of CO on human function, and indeed on animal function
generally, can be classified into two main categories. The first of these
is the effect of CO on the oxygen-carrying function of the blood, and any
hemodynamic changes associated with alterations of this function. The
second classification, the effects of CO on psychomotor functions, will
be considered separately for ease of discussion. It is not at this time
known whether the neurological effects of CO are entirely secondary
to 02 deprivation of the nervous system.
Permutt and Farhi (27) have discussed some of the physiological
changes that would be expected from a COITb level of approximately 9%
(expected from prolonged exposure to CO at 70 ppm). Assuming no
changes in blood flow, Hb concentration, or alveolar ventilation, a COHb
of 9% could be expected to cause a decrease in venous pO£ of 4-6 mm Hg,
which would represent a percentage change of 8-40% depending on the
magnitude of arteriovenous (A-V) 0^ differences. The authors state that
to achieve similar effects without CO the blood flow would'have to be
lowered by 1.3-37% or hemoglobin concentration reduced by a similar
percentage.
The effects of the decreased oxygen-carrying capacity of the blood
together with the relative impairment of oxygen delivery to the tissues
caused by COHb may be manifest most markedly .in the coronary circula-
tion and myocardial oxygen extraction. In the sedentary man, peripheral
tissues extract about 25% of the oxygen present in arterial blood. The
remaining 75% serves as a reserve supply. In heavy exercise the tissue
uptake can result in an increased extraction of oxygen from the perfusing
blood (from the 75% reserve) and an increase in blood flow through the
tissue (5, 27). The coronary circulation differs significantly from this
aforementioned scheme in that the myocardial tissue extracts about 75%
of the Q£ from the perfusing blood in the resting individual, leaving
essentially no reserve 02 in the blood. During times of stress, therefore,
the increased demand of the myocardium for 02 is not met by an appreciable
increase in coronary oxygen extraction from the perfusing blood (5).
- 7 -
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Ayres (5) observed that a rapid increase in COHb caused by inhala-
tion of 50,000'ppm CO in air for 30-120 seconds resulted in an increased
cardiac output in humans although inhalation of I, 000 ppm for 8-15 mins.
did not significantly alter cardiac output even though the COHb levels
produced were essentially the same.
To examine the effects of rapid COHb production of myocardial
function in 11 human patients, Ayers'jet ajL (37) placed catheters in the
ascending aorta, pulmonary artery, and proximal coronary sinus and
obtained arterial, mixed venous, and arterial coronary sinus blood prior
to exposing the patients to enough CO in air for 30-l?0 seconds to cause a
COHb of about 9% with continuous ECG monitoring. Coronary blood flow
was measured immediately before and 10 mins. after CO exposure.
Oxygen and carbon dioxide tension were measured in a similar sequence
as was also blood lactate and pyruvate. Expired breath samples were
also analyzed.
In patients with no evidence of coronary heart disease, Ayers
observed that increasing the COHb from a control level of 0. 95% to 9. 0%
over the aforementioned time periods resulted in increased coronary
blood flow, increased 02 extraction ratio by the myocardium (arterio-
coronary sinus O? difference/arterial concentration), increased oxygen
extraction, and an insignificant decrease in coronary sinus O? tension.
As indicators of anaerobic/aerobic myocardial metabolism, lactate, and
pyruvate extraction ratios did not change significantly.
In patients with coronary heart disease, an increase in COHb from
0. 66% to 8. 69% in 30 to 120 seconds did not result in a significant increase
in coronary blood flow, although O^ extraction and Q£ extraction ratio
both increased. The coronary sinus O-, tension decreased significantly
and significant decreases in the lactate-extraction ratio and the pyruvate-
extraction ratio were reported.
Ayers concludes that a potentially critical state or condition may
result from inhalation of CO in the patient with coronary heart disease
due to failure of the patient to respond to the stress by increasing coro-
nary blood flow. Because of the lack of Q£ reserve in the myocardial
circulation, a decrease in O^ extraction (to maintain physiologically
sound tissue 02 tension) must be balanced by an increase in coronary
blood flow in order to maintain the myocardial 0? requirement. A nor-
mal coronary circulation could adequately increase coronary blood flow
but a diseased coronary circulation might not be able to develop an ade-
quate coronary blood flow response.
Knelson (38) has measured the effects of relatively low levels of
CO on patients having coronary disease. Using 50 or 100 ppm CO in air
over time periods of four hours, Knelson reports that the time of onset of
- 8 -
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angina pectoris in exercising patients is significantly less in the presence i
of the CO than in ambient air. The duration of the anginal pains was
significantly longer in the CO environment after ceasing the exercise
than it was in ambient air. The COHb levels in those patients exposed to
CO were 3 to 4. 5%. The author indicates that levels of about 4% COHb
add significant stress on persons whose myocardial circulation is already
impaired.
In 1944, the theory that acute anoxia due to carbon monoxide
poisoning produces less effect on respiration, circulation, mental function,
and fine muscular movements than does acute anoxia produced by breathing
oxygen at low pressures was challenged by McFarland and co-workers (28).
Using human volunteers breathing various levels of CO through a face
mask, McFarland compared the results of visual-discrimation tests to
the results from tests performed using various levels of oxygen. The
tests were run for time periods up to 4 hours. The authors report that
effects are observed with COHb levels as low as 5% at sea level. It was
concluded that the change in oxygen tension in the blood was the deter-
mining factor in elevation of the visual threshold whether this was
brought about by loss in oxygen capacity due to CO saturation or loss in
arterial 62 tension due to altitude (28, 29).
Beard and Wertheim (19) observed the effects of various levels of
CO on human volunteers for the effects on discrimination of time inter-
vals. The authors used electronically generatedtonesof 1, 000 Hz to
present a reference signal of 1-second duration followed by a variable
signal to the subjects. The signal was varied in 18 steps between 0. 675
seconds and 1.325 seconds.
Eighteen subjects were tested in at least 15 sessions (600 trials
per session). CO was administered in concentrations of 0, 50, 100, 175,
and 250 ppm in an audiometer booth. The authors reported significant
decrements in performance at all levels of CO exposure. The time of
onset of the decrement ranged from 90 minutes with a CO exposure of
50 ppm to 20 .minutes with a CO level of 250 ppm.
In more recent reports, Beard and Grandstaff (30) found significant
dose-related performance decrements with, exposures of 64 minutes at
50 ppm CO and at proportionally shorter periods with higher CO concen-
trations (less than 20 minutes at 250 ppm CO). The experiments consisted
of estimation of the passage of 30 seconds. The exposure chamber was an
audiometer booth.
A doubling of error rate in visual function tests by human volunteers
exposed for 90 minutes to 250 ppm CO was reported by Beard (31) when
the results were compared to result's of the tests performed under control
conditions.
- 9 -
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Schulte (35) examined the effects of levels of CO producing 0 - 20%
COHb in 49 volunteers by administering 100 ppm CO in air for various
time periods. He reported no uniform physiological changes but he
demonstrated decrements in psychological tests at levels near 5% COHb.
There was a 10-fold increase in the number of errors in choice discrimi-
nation when the COHb levels reached 20%. No differentiation between
smokers and non-smokers was made.
Mikulka £t aj_. (32, 33) observed the effects of CO on the perform-
ance of a number of tests by human volunteers. Ten university students
were exposed to CO levels of 0, 50, 125, ?00 and 250 ppm for 3 hr in 12-ft
diameter transparent "domes". During test periods the windows of the
domes were covered so that the subjects could not see out, but during
rest periods the subjects were allowed to look out of the windows.
Performance of Time estimation(lO sec. ), Critical Instability Tracking
Task, and the Pensacola Ataxia Battery (balancing tasks) were measured
in the subjects. Mean COHb levels were 0. 96% for 0 ppm CO, 2. 98% for
50 ppm, 6. 64% for 125 ppm, 10. 35% for 200 ppm, and 12. 37% for 250 ppm.
No consistent effects attributable to CO were observed by the authors.
The authors attribute the differences between their findings and those
reported by Beard £t aL (30, 31) to the fact that sensory and motor depri-
vation was greater in the experiments performed by Beard.
Hosko (34), in a series of 25 experiments, examined the effects of
CO on the visual evoked response (VER) in human volunteers. CO con-
centrations of
-------
Stewart reports that no untoward symptoms were noted during,
or in the 24-liV period following, the exposures to 25, 50, and 100 ppm.
No detectable changes from control values for the clinical tests were
observed by the authors except the predictable rises in COHb during
exposure. The only psychomotor test, to indicate a significant
relationship to CO exposure was the Crawford collar and pin test. The
authors maintain that this correlation is spurious in light of the lack of
correlation in other similar tests and the lack of significance of the
paired t-tests for the pin and collar task.
Four-hour exposures to 200 ppm produced headaches by the end
of the third hour in the three subjects exposed. Clinical laboratory tests
wei-'e normal. After 1-hour exposure at 400 ppm, mild exertion produced
a 10% increase in heart rate which was observable clinically and sub-
jectively. At the end of 90 minutes a frontal headache was reported by
one of the two subjects. In a second experiment using 500 ppm, over a
2-3 hours total exposure period, two exposed subjects had mild frontal
headaches after 1 hour of exposure. After the exposure interval the
headaches were reported to intensify to "very severe, " reaching a pain
peak 3-1/2 hours post-exposure and accompanied by mild nausea. These
symptoms persisted for 7 hours. Repeating the 500-ppm experiment a
third time, Stewart observed similar symptoms in the subjects during
exposure, but the post-exposure symptoms were alleviated with hyper-
baric 0-, immediately after CO exposure.
An experiment involving the exposure of two subjects to CO at
concentrations increasing from 1 ppm to 1,000 ppm during a 2-hour
period resulted in COHb levels in excess of 30% and in frontal headaches
becoming "incapacitatingly severe" 6 hours post-exposure and still
noticeable after a night's sleep (12 hr post-exposure). Clinical tests and
ECG's were normal although visual evoked response changes were
observed at COHb levels at or in excess of 20%. These changes returned
to normal when COHb saturation fell below 15%. The results of hand
reaction time-time discrimination tests indicated an increase in reaction
time 2 hr post-exposure, but no impairment of time-estimation ability.
Performance of manual-dexterity tests decreased at the high exposure
and hand fatigue was noted by the subjects.
Horvath ej^al^ (43) observed the effects of CO on visual discrimina-
tion (vigilance) in human volunteers exposed at levels of 26 ppm (30 mg/m^)
or 111 ppm (128 (mg/m3), which caused COHb levels to average 2. 3% and
6.6%, respectively, for 135-140 minute exposure. Vigilance tests were
performed in a double-walled booth (4x4x6 1/2 feet) under "single blind"
conditions. The authors reported that there was a significant decrease
in vigilance at the 111-ppm level (6. 6% COHb), but no decrease in
vigilance when exposures were to 26 ppm (2. 3% COHb).
- 11 -
-------
Summary of Short-Term Effects on Humans
f
The question of changes in psychomotor performance with levels
of COHb between 2% and 3% has yet to be resolved.
Experiments reporting effects at levels of 2-3% COHb (19,30,31)
should be verified by independent research groups using "double blind"
studies and giving careful attention to control of parameters not easily
measured, such as motivation of the subjects and extraneous stimuli.
Interpretation of the results of such studies should focus on determining
if the observed effects are applicable generally to the public involved in
tasks such as driving a car, or if the effects are applicable only to
specialized groups of persons such as radar operators or persons
involved in critical but "non-stimulating" tasks.
Effects of CO on myocardial function in normal persons and
persons with coronary and non-coronary heart disease have been
examined by at least two experimenters. Ayers (37) found that increasing
COHb fairly rapidly to 9% caused changes in coronary blood flow and
myocardial function. Although as little as.4% COHb correlated with a
decrease in mixed venous (^ tension, significant myocardial changes
were seen in patients with elevation of COHb above 6% (5).
Knelson (38) reported decreased tolerance to exercise in angina
patients exposed to CO (50-100 ppm) for 4 hours, which resulted in
COHb levels of 3-5%. Perusal of this yet unpublished work is in order
before any conclusions can be drawn for purposes of this document.
Epidemiology
Hexter and Goldsmith (39) and Cohen_e_t aL (40) have reported
significant correlation between CO concentrations and mortality.
Hexter and Goldsmith in their study involving deaths from all
causes in Los Angeles County during a 4-year period reported a signifi-
cant regression coefficient associating excess mortality from all causes
with CO concentration. Although the logarithm of the CO concentration
used in their statistical model did not provide a direct measure of the
contribution of CO to mortality, the authors reported that comparisons
between concentrations could be made. Using the 4-year high 24-hr
average concentration (20. 2 ppm) and comparing this to the four-year
low (7. 3 ppm) 24-hr average concentration, the estimated CO contribu-
tion to mortality is reported to be 11 deaths, all other factors being equal.
Cohen £t_aL_ studied the relationship of CO and survival from myocardial
infarction in a number of patients admitted with myocardial infarcts to
35 hospitals in. the Los Angeles area. They reported that a significant
correlation could exist between fatalities and CO levels of 7. 7 to 14 ppm.
- 12 -
-------
However, the authors caution against drawing conclusions without further
studies to veri'fy their findings.
Effects on Plants
At the concentrations and for the time intervals with which this guide
is concerned, CO has not been reported to have a significant effect on
plant life.
It has been reported that 100 ppm (115 mg/m3) caused a ?.0% inhibition
of nitrogen fixation in red clover plants inoculated with Phizobium trifolii
and exposed to the CO for 1 month. Concentrations of 500 ppm (575 mg/m3)
for 1' month caused 100% inhibition.
A 4-hour exposure at 6,000 ppm (6,900 mg/m3) (No. 11 44) caused
some inhibition in nitrogen-fixing ability in Azotobacter vinelandii (45).
There is some evidence that certain soil fungi may act as a "sink"
for CO, converting the CO to CC*z (46).
Analytical Methods
The reference method for the continuous measurement of CO in the
atmosphere as recommended by the Environmental Protection Agency
is non-dispersive infrared spectrometry (47). An extensive description
of this method is contained in the reference.
An evaluation of alternative methods for CO analyses and a brief
evaluation of the parameters to be considered in using these methods is
contained in Air Quality Criteria for Carbon Monoxide (48).
Short-Term Public Limits (STPL's) and Public Emergency Limits (PEL's)
Short-term public exposures are those occurring at predictable times
and arising from single or occasionally repeated events. Where exposure
can be predicted, there is no justification for subjecting the public to any
appreciable risk (41).
"The importance of CO in the ambient air lies principally in the
ability of CO to combine with hemoglobin. " (49). "Exposures to increased
CO concentrations for relatively short periods, such as one or two hours,
are innocuous unless or until the cumulative effect is such that the blood
COHb level has been raised appreciably. " (49).
Because the absorption of CO in the blood is dependent on the dura-
tion of exposure, the concentration of CO in the inspired air, the activity
or respiratory rate of the exposed individual and a number of other
- 13 -
-------
variables, the setting of acceptable exposure limits is complicated by
the fact that all exposed persons will not necessarily be involved in the
same physcial activities. Therefore, they would be expected to absorb
CO at different rates although exposed to essentially the same level(s)
of CO for the same time periods.
For purposes of setting public limits, the Panel on Carbon Mon-
oxide and the Committee on Toxicology assumed that during an exposure
period persons exposed could be engaged in "light work" (see Appendix I)
and would therefore be expected to have a ventilatory rate greater than
sedentary persons (8, 9). Consequently, it would be expected that the
CO-absorption rate would be greater during the initial part of the expo-
sure periods although at equilibrium the COHb levels would be the same
for both resting and active individuals.
The Committee on Toxicology recommends the following STPL's
and PEL's based on the avoidance of significant myocardial changes in
persons having pre-existing coronary heart disease. It is the opinion
of the Committee that these persons represent the most susceptible
segment of the general public.
It is believed that the levels recommended below (STPL's) would
cause COHb levels of about 2% in non-smoking persons engaged in
"light work" (See Appendix I for parameters assumed). Levels of COHb
in excess of 2% may result under conditions of heavy exercise depending
on variations between humans and the extent of their physical activity.
In no instance would these levels significantly exceed 3%. The recom-
mendations, including the ten-minute levels are considered to be time-
weighted averages. Any excursions above these levels would be limited
to a factor of I. 5 and would be compensated for by an equivalent reduced
exposure during the period.
Recommended Short-Term Public Limits for CO
1 0 min 90 ppm
30 min . 35 ppm
60 min ?.5 ppm
4-5 hr/day, 3-4 days/mo I 5 ppm
PEL's
Emergency exposure limits for the public are intended for situations
in which pollutants escape in an uncontrolled manner at unpredictable
times and places as the result of accidents such as damage to transporta-
tion equipment or fire.
- 14 -
-------
Differing from the optimal conditions of the short-term public
limits, which require that there be no adverse health effects, public
emergency limits envision the possibility of some temporary discom-
fort, provided the effect is reversible and that no injury results from it.
With respect to CO exposure, it is believed that persons with myocardial
disease would be most susceptible to the effects of increased COHb.
The following PEL's are recommended as "Ceiling Limits" in
order to prevent the COHb levels from exceeding 5% in persons engaged
in "light work" and 6% in persons engaged in "heavy work". Persons at
rest would be expected to never exceed 3% COHb during exposure to the
PEL concentrations for the times given.
Recommended Public Emergency Limits
10 min . 275 ppm
30 min I 00 ppm
60 min . 60 ppm
A margin of safety may be assumed in the aforementioned PEL's
only insofar as one would not expect a person having myocardial
difficulties, such as recurring angina, to participate in "light work" for
durations of time (hat could precipitate anginal pains even in the absence
of CO.
-15 -
-------
Appendix 1
The following figures are graphic representations of the formula: (56)
CO in air (PPM) = 1316 (AC - VCQB + a (VCQB - AD))
- - 02
1 - a
where A -
M[02Hb]
DL Va
C = [COHb] + = COHb concentration (mlCO/ml blood)
at time t.
D=[COHb]Q = "background" [COHb] (mlCO /ml blood)
at time = 0.
VGO = Rate of endogenous CO production (ml/min)
/ -tA )
\ VbB / .
a = e
Vb = blood volume
P
C-02 = P0£ in capillaries (mmHg)
[(^Hb] = oxyhemoglobin cone, (ml/ml blood)
M = CO/02 affinity for Hb
DL = diffusion rate of CO through lungs (ml/min/mmHg)
PL = dry barometric pressure in lungs (mmHg)
Va = ventilation rate (ml/min)
- 16 -
-------
Assumptions ('Constants)
D = 0.0015, (0.76%)
VCO - 0. 007 ml/min
Vb = 5,500 ml
PC " °2 = 10° rnmHg
[OzHb] = 0. 2 ml/ml blood
M = 218
PL = 713 mmHg
Assumptions (Variables)
Rest Light Work Heavy Work
DL = 30. 40. 60.
Va = 6,000 18,000 30,000
Equation was solved for light work, heavy work, and rest
at COHb levels of 2%, 3%, 4%, 5%, 6%, 8%, and 10% for
times ranging from 10 minutes to 8 hours.
- 17 -
-------
a
_a
\
\
\
\
N
X
\
\
N
N^
\l
/
V
X
V
X
•
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\
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X
\
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\
V
s
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S
x
s
•>
"Co
Ac
at
We
"H
^^
\
•^.^
1
1
ncentration - Time To
hieve 2% COHb in Men
"Rest" (R), Doing "Light
>rk"(LW), and Doing
eavy Work" (HW).
X
10
20 30 40 60 80 100 200
DURATION OF EXPOSURE (MINUTES)
300 400 500
-------
1000
800
600
500
400
300
-S 200
oc
O
o
o
o
o
100
80
«
50
40
30
20
10
\
\
\
\
\
\
X
x<
V
X*
V
\
Concentration - Time To
— Achieve 3% COHb in Men
— at "Rest" (R), Doing "Ligh
_ Work" (LW), and Doing
"Heavy Work" (HW).
10
20 30 40 60 80 100 200 300 400 500
DURATION OF EXPOSURE (MINUTES)
-------
1UUU
800
600
500
400
300
?
•S 200
oc
2
2
0
< 100
uj 80
0
S 60
° 50
40
30
20
10
1
V
\
\
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s
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hieve 4% COHb in Men
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^^
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^=
BBBM-
0 20 30 40 60 80 100 200 300 400 5CX
DURATION OF EXPOSURE (MINUTES)
-------
1000
E
a
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DC
2
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cr
l~
2
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O
2
O
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Achieve 5% COHh in Men
— af'Resl" (R), Doing "Ligh
Work" (LW), and Doing
"Heavy Work" (HW).
30 40 60 80 100 200 300 400 500
DURATION OF EXPOSURE (MINUTES)
-------
1UUU
800
600
500
400
300
a •'
~ 200
cc
z
o
< 100
1-
§ 80
z
O
S 60
° 50
40
30
20
10
11
V
\
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\|
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\_
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x
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&-
x
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:x
FX
\
\
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f
s
X
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s
v
v
s
N
^
Co
A •.
Ac
at
We
ncentration - Time To
hieve 6% COHb in Men
"Rest" (R), Doing "Light
)rk"(LW), and Doing
"Heavy Work" (HW).
x
>v
X
X
X
\
^^
x^
X
*»»^^
-^
^^
X
:==
^
^W^M
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0 20 30 40 60 80 100 200 300 400 50
DURATION OF EXPOSURE (MINUTES)
-------
£
Q.
a.
g
z
LU
o
z
o
o
o
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1UUU
800
600
500
400
300
200
100
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50
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30
20
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"Rest" (R), DC
irk" (LW), and
eavy Work" ^
\
\.
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\
X,
s^__
Fime To
D in Men
jing "Light
Doing
iW).
s.
^x
^—
^^^
=-
«M^
10
20 30 40 60 80 100 200 300 400 500
DURATION OF EXPOSURE (MINUTES)
-------
£
a
_a
oc
2
O
<
or.
o
2
O
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10
20 30 40 60 80 100 200 300 400 500
DURATION OF EXPOSURE (MINUTES)
-------
E
a.
Z
z
g
<
EC
I-
uu
O
Z
O
0
O
O
IUUU
800
600
500
400
300
200
100
80
60
50
40
30
20
10
\
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Cc
A j
At
at
W(
i
ncentration - Time To
.hieve 10% COHb in Men
"Rest" (R), Doing "Light
3rk"(LW), and Doing
"Heavy Work" (HW).
N.
'/
N
X.
"^"^
\
\ ,
5tt^^,
s.
^^
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^^:
^.
X.
— ^
•IWHMMH
^^
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10
20 30 40 60 80 100 200 300 400 500
DURATION OF EXPOSURE (MINUTES)
-------
APPENDIX 2-Table 1
[col
in ppm
50-500
50
Length of exposure
77-182 days
5 hrs/day, 5 days/
week, x 12 weeks
Species
Monkeys
Dogs
Rat
Effects
Exposed continuously showed increased RBC. hemo-
globin and hcmatocrit. Total blood volume increased
in both while plasma volume remained constant.
Zinc, copper, cobalt, iron, and magnesium were seen
to be altered by CO exposure. Cobalt exhibited a mas-
sive and consistent loss from rat liver fractions.
Comments
The polycythemia was normocytic and normo-
chromic.
The consistent trace metal loss in the mitcchon-
drial fraction indicates an overall reduction of cellular
respiration and ATP production.
Ref. No.
14
21
50
3 mo-2 yrs
Rats
Exposed 95 hrs/week. No changes were noted that
could be due to CO; studies were done on fecunda-
tion, reproduction, growth, COj emission, weights
and water percentages of different organs, hemoglo-
bincmia, protidemia, lipemia, calccmia, magnesescmia,
serum transaminases, hematological data, infection &
immunization, and Guerin's grafted tumor's evolution.
It was noted that just putting the rats into an enclo-
sure (i.e., chamber-held, but no CO) affects body
growth, heart rate, ECG, and avoidance conditioning.
12
51 90 days, cent. Rats, In the continuous exposure, COHb levels were: dog,
96 " g. pigs, 51 ppm:5.7-6.2%; monkey, 51 ppm:5.3%;rat, 51
200 " dogs, ppm:5.1%;g. pig, 51 ppm:3.2%. At 96 ppm, dog:12.5%;
106 8 hrs/day, 5 days/ monkeys monkey:10.3%;rat:7.5%;g. pig:4.9%. At 200 ppm,
week X 6 weeks . dog:20.8%;monkey:20.0%;rat:16.4%;g. pig:9.4%.
At 96 and 200 ppm, there was an increase in
hcmatocrit and hemoglobin.
In the repeated exposure, the rat hemoglobin
and hematocrit were increased.
No toxic signs were noted in any of the studies.
13
100
200
Egg incubation period Chicken
Egg hatchability was 61.7%, as compared to 78.8% for
the control eggs; embryonic death appeared to be
earlier in exposed chicks.
4/30 chicks were alive at time of shell opening;
many eggs dead-in-shell showed symptoms similar to
acute CO intoxication (petcchial hemorrhage and
visceral blood clotting).
50
400
500
71 days
97 days
Monkeys,
dogs in
both
groups
COHb in monkeys:32%; in dogs:33%.
COHb in monkeys:38%; in dogs:39%.
No effects seen on survival, growth rate, clinical
chemistry. Little of significance seen on pathological
exam. There was a marked erythrocytosis. Animals
appeared able to perform learned tasks at COHb
levels of 30%.
17
191
103 days
Monkeys Developed an increased RRC, hemoglobin, and
hcmatocrit. COHb mean level was 22%. There was
no detectable change in performance.
COHb level was essentially 0%at 1 day post exposure;
other hematological changes were normal after 1 month.
15
-------
[CO]
in ppm
400
500
1.000
250
3,000
3,400
1,000
1,000
1,280-
17,000
1.500
3,000
1,600
3.000
Length of exposure
for 71 days
for 97 days
90 min
90min
90min
<90 min
4hrs
9 hrs/day X 35 days
varied
30 min
30 min
4 his
45 min
Species
Rats, mice,
baboons,
monkeys,
dogs
Rats
Rats
Rats
Rats
Mice
Mice
Dogs
Dogs
Monkeys
Rats
Effects
Heart and spleen were heavier in rats. Two rats died
with a possible contribution from circulatory failure.
No other significant pathology was seen.
Enhanced the sleeping time response to hexobarbital
in rats.
Prolonged muscle paralysis seen after zoxazolaminc.
No deaths in 15 rats.
10/10 died during exposure.
COHb was 35%; the pyruvate content of the brain was
increased and the blood glucose was decreased.
COHb was 35%; brain pyrjvate was increased; blood
glucose was decreased; brain lactate was increased; brain
phosphocreatinc was decreased.
Those dying during or shortly after exposure exhibited
extensive coagulation necrosis in the CNS or no defin-
able pathology. They consistently developed respiratory
arrest, cardiac arrest, and a rapid fall in blood pressure.
Surviving animals were examined for changes in the
CNS and heart. Changes were seen clearly only in those
animals exposed to CO in lower concentrations, with a
COHb of 40-60%. Short duration— 25% above the base line level, and
this was followed by an additional rise in CSF pres-
sure. (To 100% greater than base line.) ECG changes
consisted of depression of R wave, elevation of ST seg-
ment, occasional increases in T wave, deepening of Q
and S, and partial heart block with PVCTs. Tne severe
changes persisted to sacrifice. EEG changes of severe
depression of voltage or absence of activity was noted
in animals with CNS pathology.
There was no significant change in left ventricular 51
pressure or dP/dt, nor in arterial pOj.
COHb varied from 45.6% for animals surviving 1601- 52
1800 ppm to 62.7% for animals dying at this level..
53
crease in DNA, by 14%, and of gangliosidcs, by 8%.
There was no significant change seen in cerebrosides.
There was a distinctly red coloration to the brain, and
a marked engorgement of the superficial blood vessels.
Histologically there were no changes.
-------
[CO)
in ppm Length of exposure Species
Effects
Comments
Ref. No.
0-8.695 4 hrs
Rats, mice, Results of LCSO determinations at 0-100 psig didn't
g. pigs vary significantly within species. The COHb levels for
rats & g. pigs also showed little variation with total
pressure.
Partial pressure of O; was maintained between
140-160 mm Hg. :
25
230,000 several minutes, one G. pigs
time or 5 times
Functional and structural changes of the retina were
seen; there was inhibition of succinic acid dehydro-
gcnase, and alkaline phosphatasc; increased activity
of acid phosphatase was interpreted as a sign of en-
hancement of autolvtic orocess.
Toxic effect was long lasting.
54
10,000
10 min.
Rats
Cardiac ultrastmctural changes consisted of intracel-
lular edema, swelling of mitochondria and sarcoplasmic
reticulum, disruption and reduction.of cristae, disap-
pearance of mitochondria, appearance of lipofuscin
pigment granules & lysosomcs, increase of glycogen
granules and fat droplets.
These changes were evident 10 min after exposure
ceased, and became most prominent in 30 min to
1 hi. In 24 hrs the hearts appeared essentially
normal.
55
CA
-------
References
*
1. Sjbstrand, T. Early studies'of CO production. New York Acad.
Sciences, Annals, 174, Article 1:5-10 Get 5, 1970.
2. Forster, R. E. "Carbon monoxide production by organisms. " IN
Effects of Chronic Exposure to .Low Levels of Carbon Monoxide on
Human Health, Behavior, and Performance. A report prepared by
Committee on Effects of Atmospheric Contaminants on Human
Health and Welfare. National Academy of Sciences-National
Academy of Engineering . 1969. (p. 15)
3. Forster, R. E. "Reactions of carbon monoxide with heme proteins. '
Ibid. p. 10-13.
4. Coburn, R. F. The carbon monoxide body stores. New York Acad.
Sciences,Annals, 174, Article 1:11-22 Oct 5, 1970.
5. Ayers, S. M. , Giannelli, S. , Jr. ,. and Mueller, H. Myocardial
and systemic responses to carboxyhemoglobin. New York Acad.
Sciences, Annals, 174, Article 1:268-293 Oct 5, 1970.
6. Ganong, W. F. Review of Medical Physiology. Lange Medical
Publications, Los Altos, California. 1965 (p. 542)
7. . Peterson, J. E. and Stewart,' R. D. Absorption and elimination of
carbon monoxide by inactive young men. Arch. Environ. Health
21: 165-171 Aug 1970.
8. . Goldsmith, Jr. and L/andaw, S. A. Carbon monoxide and human
health. Science 162:1352-1359 Dec 20, 1968.
9. Forbes, W. H. Carbon monoxide uptake via the lungs. New York
Acad. Sciences, Annals, 174, Article 1:72-75 Oct 5, 1970.
10. Air Quality Criteria for Carbon Monoxide. HEW. Public Health
Service. Environmental Health Service, NAPCA Publication
No.'AP-62, Mar. 1970, p. 4-23.
11. Rodkey, F. L. , Collison, H. A. and O'Neal, J. D. Influence of
oxygen and carbon monoxide concentrations on blood carboxy-
hemoglobin saturation. Aerospace Mecl. 42:1274-1278 Dec 1971.
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