Carbon monoxide is a colourless, odourless and tasteless gas existing in a miniscule concentration in the atmosphere (< 0.001%), and is a product of partial combustion. Carbon monoxide poisoning is associated with a high incidence of morbidity and mortality. Symptoms are usually non-specific and include fatigue, headache, dizziness, nausea and vomiting, cognitive impairment, and tachycardia; mimicking an influenza-like illness. Symptoms occurring in more than one person (belonging to the same family/office) simultaneously and the relief also occurring at the same point of time in them should sound the physician's mind for suspected carbon monoxide poisoning. A high index of suspicion and presence of a source of generation of carbon monoxide are the diagnostic aids which can be confirmed by blood carboxyhaemoglobin levels. Removal from the source of carbon monoxide generation, moving the person to fresh air immediately is most important. In the emergency room, oxygen therapy is the key. Immediate treatment with a high fraction of inspired oxygen and careful clinical evaluation are mandatory for effective management. Patients with a carboxyhaemoglobin level of 10% or more should always be treated and 100% oxygen for 8 hours is recommended for patients requiring artificial ventilation. Hyperbaric oxygen also holds promise in select cases specially those with a history of unconsciousness, cardiovascular instability or ischemia, persistent mental and/or neurologic deficits and probably in pregnant patients

1. CARBON MONOXIDE POISONING

2. Review Article
CARBON MONOXIDE POISONING
Anupam Prakash*, SK Agarwal** and Nirupam Prakash***
*Assistant Professor, Department of Medicine, Lady Hardinge Medical College & Smt. S.K. Hospital, New Delhi 110 001, India.
**Consultant Internal Medicine and Academic Advisor, Indraprastha Apollo Hospitals, Sarita Vihar, New Delhi 110 076, India,
*** Senior Medical Officer, Department of Posts, Lucknow, India.
Correspondence to: Dr. Anupam Prakash, Assistant Professor, Department of Medicine, Lady Hardinge Medical College &
Smt. S.K. Hospital, New Delhi 110 001, India.
Carbon monoxide is a colourless, odourless and tasteless gas existing in a miniscule concentration in the
atmosphere (< 0.001%), and is a product of partial combustion. Carbon monoxide poisoning is associated with
a high incidence of morbidity and mortality. Symptoms are usually non-specific and include fatigue, headache,
dizziness, nausea and vomiting, cognitive impairment, and tachycardia; mimicking an influenza-like illness.
Symptoms occurring in more than one person (belonging to the same family/office) simultaneously and the
relief also occurring at the same point of time in them should sound the physician’s mind for suspected carbon
monoxide poisoning. A high index of suspicion and presence of a source of generation of carbon monoxide are
the diagnostic aids which can be confirmed by blood carboxyhaemoglobin levels. Removal from the source of
carbon monoxide generation, moving the person to fresh air immediately is most important. In the emergency
room, oxygen therapy is the key. Immediate treatment with a high fraction of inspired oxygen and careful
clinical evaluation are mandatory for effective management. Patients with a carboxyhaemoglobin level of 10%
or more should always be treated and 100% oxygen for 8 hours is recommended for patients requiring artificial
ventilation. Hyperbaric oxygen also holds promise in select cases specially those with a history of
unconsciousness, cardiovascular instability or ischemia, persistent mental and/or neurologic deficits and
probably in pregnant patients.
Key words: Carbon monoxide, Asphyxiant, Poisoning, Hyperbaric oxygen.
Carbon monoxide (CO) is a colourless, odourless and
tasteless gas. It is normally present in the atmosphere in a
miniscule concentration (<0.001%). Carbon monoxide
chemically consists of one atom of carbon and oxygen.
This is in contrast to carbon dioxide which is relativley
abundant in the atmosphere (0.035%) and each molecule
of carbon dioxide consists of carbon and two oxygen
atoms. Both the gases i.e., carbon monoxide and carbon
dioxide are produced as a result of combustion of
hydrocarbon fuels and organic matter (Organic matter
refers to things made of hydrogen and carbon, which when
burnt in presence of oxygen result in formation of carbon
dioxide). It is important to note here that incomplete or
partial combustion i.e., combustion that occurs at places
which have limited oxygen in the atmosphere, results in
carbon monoxide formation which as already delineated
above has only one oxygen atom, rather than formation of
the relatively less toxic carbon dioxide. Whenever any fuel
such as gas, oil, kerosene, wood, or charcoal is burnt, these
gases are generated.
If appliances that burn fuel are maintained and used
properly, the amount of CO produced is usually not
hazardous. However, if appliances are not working
properly or are used incorrectly, dangerous levels of CO
can result. Hundreds of people die accidentally every year
from CO poisoning caused by malfunctioning or
improperly used fuel-burning appliances, or by idling
cars [1].
CO POISONING SYMPTOMS [2]
CO is an asphyxiant gas and its accumulation results
in a varied constellation of symptoms because of its
affinity for hemoglobin with which it combines forming
carboxyhemoglobin (COHb) and disrupting oxygen
transport. CO poisons by entering the lungs via breath and
displaces oxygen from the blood stream. Interruption of
the normal supply of oxygen puts at risk the functions of
the heart, brain and other vital organs. Tissues with the
highest oxygen needs — myocardium, brain, and
exercising muscle — are the first affected. Unborn
children, infants and small children, expectant mothers,
elderly people, and people with anemia or with a
history of heart or respiratory disease are especially
susceptible.
Apollo Medicine, Vol. 7, No. 1, March 2010 32

3. Review Article
angeethis in rooms whether it be palatial buildings, urban
slums or rural villages. Kitchens are also vulnerable with
the use of cooking gas ranges, ovens, wood stoves,
chulhas etc., more so in the modern apartments, wherein
there is no proper ventilation and obviously no space for
chimneys. If the flame of your gas appliances is orange,
you should get it cheked, since it indicates you are
generating more CO, although a blue flame does not
always mean you are safe. Exhausts in kitchens are a must
and should always be switched on when working in
kitchens. With increasing usage of the heating filaments
and space heaters in our country, specially in ill-ventilated
houses, chances of CO poisoning are going to increase.
Blocked furnace or blocked chimneys only add to the risk.
Idling vehicles in garages are also a potential source of
this lethal gas and the concentrations/fumes can build up
very rapidly despite the garage doors being open. A
vehicle left on in an attached garage can potentially allow
CO to seep into the house and cause poisoning.
Cases have occurred wherein the heating filaments in
houses have been inadvertently left on and the central air
conditioning was also on, the ducts of the latter acted as
conduits for the spread of lethal carbon monoxide
generated by the former resulting in deaths of all family
members of a household.
Persons, tired of sleepless nights due to power cuts,
have died while sleeping in the comfort of their
airconditioned cars parked in their garages, because of the
continuous combustion of petrol/diesel resulting in
generation of carbon monoxide which is sucked into the
inside of the car by the airconditioner fan which sends in
cool air harbouring the lethal gas. Driving slowly in heavy
traffic with the airconditioner on and the windows tightly
shut also predisposes to chances of CO poisoning. The
car’s exhaust system needs to be checked periodically.
People who use a gas oven to heat their home or using
a charcoal grill indoors, even though in a fireplace are
throwing an open invitation to “Lord Yama”. Persons who
sleep in any room with an unvented gas or kerosene space
heater are also exposing themselves to undue risk.
Generators should always be positioned in the open
verandah, though they may be covered to prevent climatic
adversities. Use of gasoline-powered engines (mowers,
weed trimmers, small engines or generators) in enclosed
spaces, again increases the risk of CO poisoning manifold.
WHEN TO SUSPECT CO POISONING?
A high index of suspicion is required [3,4]. Symptoms
occurring only in particularly closed surroundings and
improving if the person moves out of that place,
33 Apollo Medicine, Vol. 7, No. 1, March 2010
Symptoms may be non-specific and include fatigue,
headache, dizziness, nausea and vomiting, cognitive
impairment, and tachycardia, whch may mimic influenza
or viral illness. However, the entire family having similar
symptoms, specially at the same time should prompt
suspicion of CO poisoning. CO exposure shortens time to
onset of angina in exercising individuals with ischemic
heart disease and decreases exercise tolerance in those
with chronic obstructive pulmonary disease (COPD).
The formation of carboxyhaemoglobin in the
circulation depends upon the duration of exposure to CO
and its concentration in the atmosphere. Greater is the
concentration and longer the exposure, greater is the
amount of COHb formed in the circulation. The higher the
levels of COHb, greater are the symptoms and more
adverse health effects.
At levels of 2.3-4.3% COHb in blood, time to
exhaustion in exercising healthy men is reduced and at
2.9-4.5%, reduced exercise capacity is noted in patients
with angina and the duration of angina attacks is
prolonged. However, below 5% no decrements in
concentration or constituional symptoms are seen.
Between 5-17%, there is dimninution of visual perception,
manual dexterity, learning ability, deterioration in
performance of sensorimotor tasks as in driving is noted.
As levels rise to about 30%, headache, fatigue and
impaired judgement are noted. Since many of these
symptoms are similar to those of viral illnesses/flu, food
poisoning, or other illnesses, possibility of CO poisoning
could be missed easily, specially in tropical countries like
India. At 40% confusion sets in while loss of
consciousness occurs at 60%. Death occurs at 80%
concentration or even at lower concentrations if the
exposure continues for long.
The safety level for the concentration of CO in the air
is 50 parts per million (ppm). A concentration of 200 ppm
can cause slight headache within 2-3 hours; at 400 ppm,
frontal headache occurs with in 1-2 hours, becoming
widespread in 3 hours; while at 800 ppm dizziness,
nausea, convulsions occur in 45 minutes and the patient
may be unconscious in 2 hours. Average levels in homes
without gas stoves vary from 0.5 to 5 ppm. Levels near
properly adjusted gas stoves are often 5 to 15 ppm and
those near poorly adjusted stoves may be 30 ppm or
higher.
Real-life situations
Cases of CO poisoning have occurred with use of
heaters in closed rooms, use of gas stoves or oil stoves in
small unventilated or less ventilated rooms, and use of

4. Review Article
reappearing on returning back should arouse suspicion.
The surroundings could be house, garage or workplace
even.
Symptoms occurring in more than one person [5], may
be family members or at the work place, simultaneously
and the relief also occurs at the same point of time in all.
This point requires close attention and the person has
to strain his brains to answer it. Do the symptoms occur in
a closed place and does that place have even a remote
chance of having a fuel-burning appliance?
COHb levels can be measured in blood and the
diagnosis confirmed.
MEASURES TO BE TAKEN IF CO POISONING IS
SUSPECTED
Get fresh air immediately. Open all doors and
windows, turn off combustion appliances and move out in
the free air, as soon as possible.
Consult your doctor immediately and tell him that you
suspect CO poisoning, even if he does not suspect it. In an
emergency, administering oxygen is helpful since it binds
to haemoglobin forming oxyhaemoglobin displacing
carbon monoxide from carboxyhaemoglobin, and rapidly
reversing symptoms. Oxygen therapy is the key treatment
of carbon monoxide poisoning and hyperbaric oxygen has
been shown to interdict and improve the clinical outcome
in some patients specially those with a history of
unconsciousness, cardiovascular instability or ischemia
and persistent mental and/or neurologic deficits.
Apollo Medicine, Vol. 7, No. 1, March 2010 34
Immediate treatment with a high fraction of inspired
oxygen and careful clinical evaluation are mandatory for
effective management [6]. In a recent paper, it is
recommended that patients with a carboxyhaemoglobin
level of 10% or more should always be treated and 100%
oxygen for 8 hours was recommended for patients
requiring artificial ventilation. It was even recommended
that hyperbaric oxygen can be considered for pregnant
patients [7].
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3. Kao LW, Nanagas KA. Carbon monoxide poisoning. Med
Clin North Am 2005; 89: 1161-1194.
4. Kao LW, Nanagas KA. Carbon monoxide poisoning.
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5. Sadovnikoff N, Varon J, Sternbach GL. Carbon monoxide
poisoning. An occult epidemic. Postgrad Med 1992; 92:
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6. Hardy KR, Thorn SR. Pathophysiology and treatment of
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7. de Pont AC. The guideline ‘Treatment of acute carbon-monoxide
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