Dossier on the Carbon Monoxide


Carbon monoxide is a colorless and odorless gas. Its density is close to that of air. Its presence is the result of incomplete combustion, and regardless of the fuel used: wood, butane, coal, gasoline, oil, natural gas, oil, propane. It spreads very quickly in the environment. It acts as a highly toxic poison gas, absorbed in a few minutes by the body, binds to haemoglobin.

  • 0.1% (1000 ppm) of CO in the air kills in an hour,
  • 1% (10,000 ppm) of CO in the air kills in 15 minutes,
  • Between 10 and 40 minutes at a concentration of 100 ppm,

What is carbon monoxide?


Carbon monoxide (CO) is the most polluting gas produced in the world (by volume) compared to all other polluting gases combined. In addition it is a particularly insidious gas: it is odorless and colorless and hence undetectable. We know its deadly effects when in confined areas. Formed during the combustion of carbonaceous substances (oil, coal, wood) it is very toxic when its concentration reaches a certain threshold.

CO penetrates the circulatory system via the lungs and is absorbed by the haemoglobin (red corpuscles) 200 times more rapidly than oxygen. As the CO accumulates in the blood, the body is more and more deprived of oxygen. Long term exposure to a weak concentration of CO may lead to permanent brain or heart damage. A headache may be a first symptom.

CO intoxications: How do they occur?


Carbon monoxide is inhaled from the air and absorbed by the lungs while breathing. Its toxic characteristics are caused when it is combined with haemoglobin, a protein which normally vehicles oxygen in the blood. This combination leads to the development of a relatively stable compound called carboxyhaemoglobin (COHb) which prevents the haemoglobin from conveying the oxygen to the tissues. Because there is a far greater affinity between haemoglobin and CO than with oxygen, when you breathe air containing CO, it will infiltrate the haemoglobin first, thus taking the place of the oxygen. CO can also combine with other proteins such as myoglobin (present in muscle tissue) and certain cytochromes.

The development of carboxyhaemoglobin is can be stopped either by eliminating the CO from the respiratory system by placing the person in a sound environment or by making him breathe oxygen possibly under high pressure (hyperbaric therapy).

In the majority of cases incidents result from:


  • Faulty evacuation of combustible substances (blocked or under-sized chimney flue),
  • Absence of ventilation in the room where the appliance is located (blocked air vents, draught-proof room),
  • Bad maintenance of heaters and hot water boilers as well as fireplaces, stoves, cookers, any supply heating,
  • Bad condition of appliances
  • Ill-use of certain appliances (for example continual use of supply heating, electric generator).
  • incompatibility of different installations in the same dwelling

When accidents occur an accumulation of errors and other factors mentioned above are often to blame.

Carbon monoxide source


All types of appliances, whatever the type of combustible used, are a source of carbon monoxide which varies in quantity according to the type of combustible and the quality of combustion :

  • Wood, coal, gas or oil-fired boilers,
  • Water boilers and bath heaters,
  • Fireplaces, stoves, supply heating,
  • Mobile booster heaters,
  • Wood, coal or gas cooker
  • Car engines in garages,
  • Petrol or fuel powered electric generators,
  • Any ‘makeshift’ device such as braziers

Under-privileged families run a greater risk of CO intoxication because their heaters may be in poor condition. The maintenance of their heaters and of their ventilation systems may be neglected because of lack of money. There is frequently more than one victim, usually a whole family of parents and children.

Cases of mass intoxication occur regularly in large shops, restaurants, churches, or ice-skating rinks causing the death of several victims of all ages.

Specific climatic conditions, low atmospheric pressure and fog for example, or exceptional conditions like very bad weather and severe cold entail greater risks especially because of widespread use of makeshift heating systems to make up for a deterioration of the usual heating system used (electric generator, petroleum stove, brazier.).

In the case of the 1999 December storm in France, the risks were increased: chimney flues or extractors were ripped out & a greater use of electric generators inside premises (although they should normally be used outside) because of power cuts.

Certain regions are particularly vulnerable; for example in the north of France when it is foggy, there are a great number of simultaneous intoxications sometimes justifying emergency measures.
Source: Directorate general of health

CO intoxication: time and threshold of exposure


As soon as the level of CO exceeds the threshold of 50 ppm, precautions must be taken especially for vulnerable persons: children, pregnant women, persons suffering from anaemia, heart conditions, respiratory problems, asthma, chronic bronchitis or emphysema.

Up until now surveillance of the level of CO in the air indoors has been limited to work places. There is no agreed limit concerning levels of CO in ordinary buildings. However the World Health Organization has proposed guidelines using levels of concentration of CO in the air with levels of carboxyhaemoglobin in the blood taking into account the length of exposure and the level of alveolar ventilation. The equation which is most commonly used is the Coburn-Forster-Kane equation.

In view of the well known toxic effects of CO on our health and the link with the level of carboxyhaemoglobin (COHb), the World Health Organisation considers that a level of COHb amounting to 2.5% was the maximum accepted level for the safety of the population. Guidelines have thus been established ensuring that the limit of 2.5% should not be exceeded even if the person is taking moderate or slight physical exercise.


Guidelines fixed by the W.H.O:

  • 100 mg/m3 (87 ppm) during a period of 15 minutes,
  • 60 mg/m3 (52 ppm) during a period of 30 minutes,
  • 30 mg/m3 (26 ppm) during a period of 1 hour,
  • 10 mg/m3 (9 ppm) during a period of 8 hours.

CO intoxication: elimination of gas


The elimination of CO from the body depends on the amount of oxygen in the air and the atmospheric pressure.The curve below shows the amount of time it takes for levels of COHb to drop in ambient air, in pure oxygen and in a hyperbaric chamber.

Degassing of carboxyhemoglobin is usually characterized by the decay rate as follows:

Vol. of oxygen Time taken to eliminate half the quantity (in minutes)
21% (oxygen in ambient air) 240 - 300
80 % 80 - 100
100 % 50 - 70
100 % (at 3 atm.) 20 - 25

Elimination of gas is, just as intoxication by CO, a particularly slow process especially under normal conditions: normal atmospheric pressure and 21% volume of oxygen in the air. Under these conditions, it takes 4h30 for the concentration of COHb to be halved. The process may be speeded up by increasing the concentration of oxygen or, even better, by placing the victim in a hyperbaric chamber with more than 2000 mm of mercury during 12 to 30 minutes. This process increases the level of oxygen in the blood and speeds up the separation of CO and haemoglobin.

CO poisoning: symptoms


There are two different types of intoxication :

  • Acute intoxication which calls for immediate action on behalf of emergency services and with symptoms of dizziness, fainting, muscular impotence, even a coma or death.
  • Chronic intoxication, which causes headaches, nausea, mental confusion. It is difficult to detect and may cause cardiac and respiratory problems in the long run. This type of intoxication has recently been suspected of impairing the normal brain development of children and particularly their intellectual abilities.

The clinical signs are not specific: headaches, dizziness, faintness, nausea, dyspnoea, problems with eye sight, sense of smell and taste, difficulties with sleep, memorisation and concentration, pains in the chest, abdomen and muscles may be observed to varying degrees. CO has been described as a "great imitator" because CO intoxications often produce erroneous diagnoses of influenza, gastro-enteritis or other mild disorders. Moreover CO intoxication is frequently revealed through the discovery of a latent heart or cerebral condition. The fact that ready made explanations can be given to explain the observed symptoms doubtlessly leads to an underestimation of the number of CO intoxications of patients with "known" vascular problems.

Three particular clinical conditions ought to be pointed out: pregnant women (high risk for the foetus), children under the age of two (unusual behaviour may occur, for example screaming), elderly persons (unspecific symptoms could be wrongfully ascribed to old age).

Taken individually, symptoms may not therefore confirm intoxication. It seems unreasonable to ask for a report of suspected cases merely on a clinical basis as it would doubtlessly entail a large number of pointless investigations. However the clinical signs should lead the doctor to suspect CO intoxication and to look for other signs in order to confirm CO intoxication. In all similar situations the air exhaled by the patient should be tested with a carboxymeter (the carboxymeter is a specifically used to detect carbon monoxide).

Useful information


CO Intoxications: the figures


Each year in France 6000 persons are victims of an intoxication due to carbon monoxide and eventually 300 of them die. Anybody can be a victim of this type of intoxication which often occurs when carrying out daily activities, such as turning on the heater or hot water. Acute intoxication calls for urgent action.

The chronic type of intoxication is much more difficult to detect. It is therefore essential that everyone, medical professionals, social workers, heating specialists or retailers, but also each individual should be vigilant in detecting this type of intoxication which is still not widely known.

Accidents vary enormously depending on the social and geographical backgrounds. Thus in the Pas de Calais region in northern France alone, carbon monoxide causes 40 deaths and 1000 hospitalizations a year.

CO intoxications: means of monitoring


A measuring device such as the CO-meter enables the user to:

  • Identify a heater or stove which doesn’t draw well enough,
  • Discover downdrafts,
  • Locating a leak in a flue
  • Identify the faulty evacuation of the exhaust fumes of an electric generator,
  • Identify bad ventilation in a garage.


The use of the device can provide other types of information: tobacco is an important source of CO. In certain public places, like badly ventilated offices, it is sometimes possible to identify levels of up to 30 ppm. Similarly in car traffic, particularly in tunnels or underground car parks, increased levels of CO can be found. A device which allows continuous monitoring of CO levels such as the coach, will moreover monitor appliances which are potential sources of CO in the home.