Carbon monoxide poisoning

Carbon monoxide (CO) is an odourless, colourless, non-irritant gas. Find out where it comes from

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Carbon monoxide (CO) is an odourless, colourless, non-irritant gas. It is the most common cause of fatal poisoning in Britain today. It causes the accidental deaths of up to 50 people each year in the UK and up to 200 non-fatal poisonings.

These figures could be just the tip of the iceberg because poisoning by carbon monoxide is almost certainly underdetected. There are two main reasons for this.

Firstly, there is generally little awareness of carbon monoxide poisoning among the general public and the medical profession.

Secondly, the signs and symptoms associated with carbon monoxide poisoning are not easy to diagnose as they often mimic many other conditions.

To overcome this deadly killer requires improved awareness among the public of the risks and dangers of carbon monoxide poisoning and increased vigilance on the part of healthcare professionals in its detection.

Children, pregnant women, babies, and individuals with a heart or lung condition are those at most risk – but CO poisoning can affect anyone.

Where does carbon monoxide come from?




When the waste products of combustion are not effectively removed – for example because of blocked flues and chimneys – poisonous gas mixtures may re-enter the room.

This problem is not just associated with older or poorer homes; it can also affect the occupants of newer homes with gas central heating. Exhaust fumes from cars is another obvious source.

Domestic sources of carbon monoxide include:

  • domestic heating systems
  • blocked flues/chimneys
  • inadequate ventilation in living areas
  • inadequate ventilation in adjoining car garages
  • leakage from faulty appliances and chimneys/flues.

    How is carbon monoxide formed?

    When any fire burns, in an enclosed room, the amount of oxygen available gradually decreases. At the same time the amount of carbon dioxide increases.

    As the amounts of these two gases change, this increasingly causes the combustion process to alter from one of complete combustion to one of incomplete combustion. This results in the release of increasing amounts of CO.

    This highlights an important issue. Even with perfectly designed and maintained heating appliances (or any kind of combustion device), they too will eventually begin producing dangerous amounts of CO if used in confined and poorly ventilated areas.

    Having poorly operating appliances, only makes the problem worse more quickly. Maintaining appliances and ensuring sufficient fresh air is available are two easy ways of avoiding potentially lethal scenarios.

    How does carbon monoxide cause poisoning?

    To explain this aspect, we need to explain how the body uses oxygen from the air.

    Oxygen is transported around the body via the red blood cells. Specifically, oxygen binds to a substance within the red blood cells called haemoglobin, which is also responsible for their red colour.

    Haemoglobin takes up oxygen as blood passes through the lungs, and at the same time carbon dioxide, produced by the body's metabolism, is released from the blood into the exhaled breath.

    The combination of oxygen with haemoglobin is called oxyhaemoglobin and this 'oxygenated' blood is carried away from the lungs through the bloodstream to all the tissues of the body.

    Carbon monoxide can also bind to haemoglobin but does so about 240 times more tightly than oxygen, forming a compound called carboxyhaemoglobin.

    This means that if both carbon monoxide and oxygen are inhaled, carbon monoxide will preferentially bind to haemoglobin. This reduces the amount of haemoglobin available to bind to oxygen, so the body and tissues become starved of oxygen.

    Carboxyhaemoglobin also has direct effects on the blood vessels of the body – causing them to become 'leaky'. This is seen especially in the brain, causing the brain to swell, leading to unconsciousness and neurological damage.

    What are the symptoms of carbon monoxide poisoning?

    Symptoms Adult Child Infant
    General Dizziness, fatigue, weakness Not feeling well
    Neurological Headache, drowsiness, disorientation, fits Headache, drowsiness, fits, uncoordinated movement
    Stomach/intestine Nausea, vomiting, stomach pains Vomiting, stomach pains, anorexia, diarrhoea Loss of appetite
    Heart Chest pain, wheeziness, palpitations, hyperventilation Hyperventilation


    Often the onset of symptoms is gradual, occurring without the individual or doctor being fully aware of what is happening. Coupled with this is the fact that the severity of the poisoning depends on:

    • how much carbon monoxide is actually present in the environment
    • the duration you are exposed to carbon monoxide
    • the general state of health


      • headache (90 per cent)
      • nausea and vomiting (50 per cent)
      • vertigo (50 per cent)
      • altering states of consciousness (30 per cent)
      • weakness (20 per cent).

        The likely symptoms in adults, children and infants are shown in Table 1.

        How is carbon monoxide poisoning diagnosed?

        Individuals can either be exposed to high levels of carbon monoxide over a relatively short period of time (acute exposure) or to lower levels of exposure over a longer period of time (chronic exposure).

        Acute exposure is easier to diagnose as the symptoms are more pronounced, but it is the more common chronic exposure symptoms that are more subtle and difficult to tell apart from other conditions. Where whole families are affected by suspected 'food poisoning' this has been known to be due to carbon monoxide exposure.

        Where such symptoms are reported repeatedly, domestic carbon monoxide poisoning should be suspected. Clues that point towards a problem within the home include:

        • more than one family member being affected
        • symptoms appear or get worse when gas appliances are in use
        • symptoms are worse in the winter when gas boilers/heaters are in use
        • symptoms improve when family members are not at home, but recur on their return.

          Important information can also be obtained by inspecting gas-operated heating appliances within the home. Relevant points include:

          • black soot marks on gas fire burners or on walls near cookers, boilers, gas fires
          • a yellow gas flame colour, rather than the blue colour it should be.

            How is carbon monoxide poisoning treated?

            The first step is to move the affected individual(s) away from further exposure to the carbon monoxide source.

            Their signs and symptoms will then determine what happens next.

            If the individual is only mildly affected they should seek medical attention, but they may not need to be admitted to hospital. All other exposed individuals will require hospital treatment.

            Administering 100 per cent oxygen, via a tightly fitting mask with an inflated face-seal, is the first treatment.

            A high concentration of oxygen in the air being breathed will speed up the formation of oxyhaemoglobin to replace carboxyhaemoglobin.

            The severity of the CO exposure can be checked by measuring the amount of carbon monoxide in the air breathed out by the individual or by taking a blood sample and measuring the carboxyhaemoglobin levels, and taking these laboratory tests along with the clinical signs and symptoms present in the affected person.

            If exposure is deemed to be significant and signs indicate nerve damage, then 'hyperbaric' oxygen therapy may be considered.

            This involves placing the individual in a sealed pressure chamber, similar to those used in treating decompression sickness in divers, and exposing the person to oxygen at high pressure.

            Using this technique, there's greater penetration of oxygen at tissue level, and oxygen displaces carboxyhaemoglobin from the red cells more quickly.

            As a guide to who should be offered hyperbaric oxygen therapy, the following indications have been recommended:

            • loss of consciousness
            • neurological signs (other than a headache)
            • abnormal heart rhythm or lack of blood to the heart
            • women who are pregnant.

              It should be noted that this treatment is still considered controversial.

              Although it speeds the reduction of carboxyhaemoglobin in the blood back to normal levels, symptom reduction may not be seen at the same time.

              Prevention is always better than cure

              The best course of action is to take steps that prevent carbon monoxide becoming a problem in the first place.

              • Raise the general awareness of the risks associated with carbon monoxide by communicating the relevant information to friends, family and work colleagues.
              • Be aware of the sources of carbon monoxide especially in domestic properties where a number of appliances could be responsible.
              • Get appliances professionally installed and regularly serviced to ensure they work efficiently and safely.
              • Ensure adequate ventilation for all fuel burning appliances.
              • Install monitoring devices for the early detection of excess carbon monoxide.
              • Recognise the early signs and symptoms of carbon monoxide poisoning, particularly when more than one family or work member is affected, and seek medical advice promptly.

                How do you measure carbon monoxide levels?

                Carbon monoxide levels can be measured either in the environment or in the blood.

                The latter is usually performed in a hospital setting to check how much carbon monoxide (in the form of carboxyhaemoglobin) there is present in the blood.

                There are also ways by which carbon monoxide levels can be monitored in the home or office.

                Carbon monoxide monitors

                Carbon monoxide detectors are available from most local hardware and DIY stores. They can provide an audible high-pitched alarm when high levels of carbon monoxide are detected or provide an alarm plus a digital display of the concentration of carbon monoxide detected in units of 'parts per million' (ppm).

                Three types of carbon monoxide detectors are available.

                Chem-optical (gel cell) technology

                Chem-optical technology (or gel cell or biomimetic technology) alarms use a type of sensor that simulates haemoglobin in the blood.

                Electrochemical alarm

                Electrochemical alarms work by converting the carbon monoxide electrochemically to carbon dioxide, which generates an electrical current that is taken as a measure of the gas concentration. Electrochemical alarms are usually powered by a battery lasting about five years.

                Semiconductor technology

                These alarms use semiconductors or tin dioxide technology to detect carbon monoxide levels. Unlike the alarms above, semiconductor detector alarms do not require any replacement sensors.

                The British Standards Institute (BSI) is a national standards body, responsible for ensuring products meet certain agreed standards of safety. BSI standard 50291:2001 is the current one for monitors that detect carbon monoxide at levels well before they become dangerous for humans. It replaces BS EN 7860: 1996.

                What to do if the alarm sounds

                It is essential to read the instruction manual accompanying the detector because it provides important information on where to place it, how to use it and what to do if the alarm goes off.

                It will also contain important information about the levels of carbon monoxide detected and the risks associated with varying levels.

                The manual should be placed somewhere (ideally near the detector), so it can be accessed quickly in the event of an emergency.

                The following are some general points to bear in mind.

                • If your alarm goes off and you have a detector that displays the amount of carbon monoxide detected, make a mental note of what the reading states. Pick up the instruction manual but take it outside the house to read it.
                • Check whether you or any other family member is affected by any of the signs and symptoms of carbon monoxide poisoning (headache, dizziness, nausea, fatigue). If carbon monoxide poisoning symptoms are suspected, everyone should vacate the house and call for medical assistance. Dial 999 for an ambulance if necessary.
                • If no one has any symptoms of poisoning, promptly turn off all gas or other fuel burning appliances. Ventilate the whole house by opening all windows and doors.
                • Contact a professional appliance specialist, eg the Gas Safe Register, to check your appliances.

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                  1. Department of Health, From the Chief Medical Officer and Chief Nursing Officer - Carbon monoxide: The Forgotten Killer. September 1998.

                  2. Henry JA. Carbon monoxide. Journal of Accident and Emergency Medicine 1999; 16: 91-92.

                  3. Spedding R et al. Carbon monoxide poisoning. Update 1999: 568-571.

                  4 Walker E and Hay A. Carbon monoxide poisoning. BMJ 1999; 319: 1082-1083.

                  5 Tibbles PM et al. Hyperbaric oxygen therapy. New England Journal of Medicine. 1996; 334: 1642-1648.

                  6 Weaver L K. Hyperbaric oxygen in carbon monoxide poisoning. BMJ 1999;319; 1083-1084.

                  Based on a text by Dr Dan Rutherford, GP

                  Last updated 01.09.2016

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