Are You CO Aware?

With the onset of colder weather, many households in the UK are turning on their heating for the first time in months. Heating appliances need chimneys and flues to work safely – and these can block up over the summer months. So autumn is traditionally the period when people get poisoned by carbon monoxide (although it can happen any time of the year!)

Carbon monoxide (CO) is produced when anything containing carbon burns or smoulders. For practical purposes, this means the burning of any kind of fuel, commonly:

  • Gas
  • Coal
  • Wood/Paper/Card
  • Oil/Petrol/Diesel – (All UK cars have a ‘catalytic converter’ in the exhaust system, which converts carbon monoxide (CO) to carbon Dioxide (CO2), which is less poisonous. However, these converters need to warmed up – a cold car produces fatal amounts of CO in the exhaust)

CO is very poisonous. Exposure to as little as 300 parts per million (that’s just 0.03%) can prove fatal.


  • Dull headache – 90%
  • Dizziness/Vertogo – 50%
  • Nausea or vomiting -50%
  • Altered level of consciousness – 30%
  • Weakness -20%
  • Shortness of breath
  • Blurred vision
  • Chest Pain – ST-Depresion indicates cardiac hypoxia/toxicity
  • Cherry Red Lips – They do, but not usually until they are dead. So DON’T rely on an apparently normal level of ‘pinkness’ to rule out CO exposure – some people have tried that in the past, with eventually fatal consequences.

Testing – Pros & Cons

  • SaO2 – CO combines with haemoglobin to form carboxyhaemoglobin, which is indeed unusually bright in colour. But the only significance of this fact is that COHb confuses ‘ordinary’ pulse oximeters; they mis-read COHb as if it were oxygenated haemoglobin.
  • CO measurement – Blood Gas (art/ven/cap)/CO-oximeter/Breath-Test may be used, to measure CO levels. Normal carboxyhaemoglobin levels are:
    • Less than 1–3% for non-smokers.
    • Up to 5% in women who are pregnant, or people with anaemia.
    • Up to 10% in smokers, and up to 13% in heavy smokers.

Testing isn’t everything: Most people who have been poisoned by CO and survived it, will have normal COHb levels by the time they are tested. The dissociation half-life of COHb is about four hours when breathing ‘clean’ air. The combination of CO with blood is not ‘irreversible’, and never has been, contrary to what some textbooks say. (COHb is, however, very stable post mortem. When the patient is no longer breathing, the compound stops dissociating.)

Think – COMA test

  • C  – Companions and cohabitees; is anyone else in the house unwell?
  • O – Outdoors; do you feel better when out of the house?
  • M – Maintenance; have your heating appliances been maintained properly?
  • A – Alarm; is there a CO alarm in the house?

You might not be able to ‘prove’ CO exposure; in fact you probably won’t be able to. But if you had ask these questions, it’s unlikely that you will send patients home to carry on using the gas fires that kill them overnight.


Toxic effects appear at carboxyhaemoglobin levels of 15–20%. – A carboxyhaemoglobin level of 30% indicates severe exposure.

  • Giving 100% oxygen – NICE could’t find  guidelines that differentiate between treatment of people exposed to high levels of CO and people who are suspected to have been exposed to low levels.
    • The recommendation to give 100% oxygen in all cases of carbon monoxide poisoning until carboxyhaemoglobin is normal and the person’s symptoms of CO poisoning have resolved
    • When breathing air, the half-life of carboxyhaemoglobin is 320 minutes. This can be reduced to 80 minutes when breathing 100% oxygen
  • Performing a neurological examination – A neurological examination should be performed in all people following chronic exposure, or in people with neurological signs
    • This should include tests of fine movement and balance (fingernose movement, Romberg’s test, normal gait and heel-toe walking), a mini-mental state examination, testing of short-term memory and the ability to subtract 7, serially, from 100.
  • Further Tests (if COHb elevated & symptomatic)- Blood gas, FBC, U&E, CK, ECG

True Stories

Say you saw a husband and wife who were both feeling a bit dizzy and headachy. They live in the same house, so you tell them what we always tell people when more than one person from the same household has the same symptoms – you tell them they have a ‘virus.’

And when a mum brought her 10 and 12 year old boys to see you, because she felt really awful – headaches, dizzy, feeling sick, and the boys had actually started being sick overnight, you told her they had ‘a virus’ too – they went home, and kept warm.

Both these are true clinical scenarios, concerning real people who ae now dead, because the diagnosis of CO exposure was missed.

Further Reading 


Toxbase – for full treatment guide


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