Carbon Monoxide poisoning


In UK approximately 1800 deaths a year

  • 45% are self-poisoning and suicide
  • 33% are domestic heater accidents
  • 20% are related to fire and smoke inhalation

In Ireland circa 6 deaths a year, many more poisoned

  • Natural gas CO poisonings account for a small minority of all cases

Carbon monoxide can be dissolved in plasma without being attached to the haemoglobin molecule. It is not the COHb that is important it is the carbon monoxide that binds to other haem proteins in the mitochondria of the brain and heart cells that is very important which leads to a reduction in cellular respiration and a lack of ATP and resultant consequences.

Although carbon monoxide has a very high affinity for haemoglobin it reacts with the molecule slowly. This means that carboxyhaemoglobin levels are not a reliable indicator to the presence of carbon monoxide poisoning. Increased levels of carboxyhaemoglobin can only indicate that exposure has occurred. However, levels of in excess of 40% normally indicate a requirement to treat and levels of greater than 65% are usually considered fatal without therapy. The purpose of hyperbaric oxygen therapy is to get rid of the CO by mass action and to treat the hypoxic cells etc.

The flow diagram gives some guidelines to the management of carbon monoxide in the Emergency Department. The normal level of carboxyhaemoglobin is <1%. Smokers may typically have levels of 5 - 8%. An inner-city jogger may have levels of 12%. Symptoms start at 18 - 20%.

Potential hyperbaric treatment

The following categories should be considered for transfer to a hyperbaric unit:

  • Patients with initial COHb levels higher than 25%
  • Any history of loss of consciousness
  • Any neurological signs (including cerebellar signs)(you must assess the patient walking)
  • Ischaemic ECG changes
  • Metabolic acidosis
  • All pregnant patients
  • All children

The main stay of assessment of severity is clinical observation

COHb levels are no substitute and are not indicated unless there are significant symptoms.


Symtoms Signs
Headache 90% Cherry red skin (rare)
Nausea & vomiting 50% Neurological signs (may be subtle)
Vertigo 50% Check - finger nose, Rhomberg's, heel toe gait
Altered consciousness 30% Check - Mini mental exam, short term memory
Subjective weakness 20%
  • Onset often insidious
  • Beware if more than one person in a household has any similar symptoms
  • Acute exposure leads to collapse, chronic exposure similar to influenza or food poisoning. Ref 1
  • Many present with unexplained headaches, chest pains and muscular weakness. Or GI upset (sickness, diarrhoea and abdo pain)
  • Occasionally "postural" dizziness and general lethargy


Content by Dr Íomhar O' Sullivan 23/06/2000.   Reviewed by Dr ÍOS 03/11/2004, 13/05/2007. Last review Dr ÍOS 21/06/21