Environmental Medicine

Carbon monoxide poisoning

Carbon monoxide binds haemoglobin far more avidly than oxygen, so oxygen carriage collapses while the arterial oxygen tension and the pulse oximeter both stay deceptively normal.

In a nutshell

Carbon monoxide binds haemoglobin far more avidly than oxygen, so oxygen carriage collapses while the pulse oximeter reading and arterial oxygen tension both stay normal. Give the highest possible oxygen concentration, a non-rebreathing mask at 15 litres/minute, before any carboxyhaemoglobin (COHb) result.

Classic presentation

A family and their dog all develop headache, nausea and dizziness at home during a cold snap, and all feel better once they are outside.

Key points

Haemoglobin binds carbon monoxide with 245 times the affinity it has for oxygen, and 80 to 90% of absorbed gas ends up as carboxyhaemoglobin.
Half-time of elimination: 320 minutes on air, 80 minutes on 100% oxygen, 23 minutes hyperbaric. That ratio is the entire argument for the mask.
Baseline carboxyhaemoglobin is 1 to 2% in non-smokers and 5 to 10% in smokers, so ask about smoking before calling a level high.
Symptom frequencies: headache 90%, nausea and vomiting 50%, vertigo 50%, altered consciousness 30%, weakness 20%. Nothing on that list is specific.
COMA screen, from the UK Health Security Agency algorithm: Cohabitees affected, Outdoors better, Maintenance of appliances, Alarm fitted.
NHS England does not routinely commission hyperbaric oxygen for carbon monoxide poisoning. Cochrane found no effect on neurological signs at four to six weeks.
In pregnancy the fetal half-life is up to 4 to 5 times the mother's and fetal levels run 10 to 15% higher than hers.
Cardiac damage during the poisoning raises mortality for the following 10 years, so a raised troponin changes follow-up, not just the admission.

First-line investigation

Carboxyhaemoglobin by co-oximetry, a blood-gas method reporting each haemoglobin species separately, on a heparinised arterial or venous sample. 30% or more means severe exposure.

Management

Oxygen before anything else

Move to fresh air. Trauma or reservoir mask with non-rebreathing bag at 10 to 15 litres/minute, or nasal high-flow cannulae up to 60 litres/minute.^2,4
Start it irrespective of the oxygen saturation and before any carboxyhaemoglobin result. Aim for the highest achievable saturation, not 94 to 98%.^5,4,2

Measure what the oximeter cannot

Heparinised arterial or venous sample for carboxyhaemoglobin by co-oximetry. Venous is acceptable. Record time since leaving the source, oxygen already given, and smoking status.^2,6
Interpret against baseline: 1 to 2% in a non-smoker, 5 to 10% in a smoker. 30% or more means severe exposure; below that excludes nothing.¹
12-lead ECG and cardiac monitoring in everyone needing assessment. Blood gas, lactate, capillary glucose, pupil size, troponin if indicated. Pregnancy test where relevant.^2,1

Resuscitate, and look for the co-toxin

ABCDE (airway, breathing, circulation, disability, exposure). Intubate if consciousness or ventilation fails. Treat seizures, hypotension, acidosis, pulmonary oedema, rhabdomyolysis and acute kidney injury.²
After a house fire, assume cyanide poisoning and smoke inhalation as well, and look for trauma and burns.^2,9

Poisons service, and the honest position on hyperbaric oxygen

Phone the National Poisons Information Service on 0344 892 0111. Its TOXBASE database needs professional registration and sits behind a login.^9,6
NHS England does not routinely commission hyperbaric oxygen for carbon monoxide poisoning, in the 2018 policy and in the specification effective October 2025.^3,10
Trials are small and conflicting: Cochrane odds ratio 0.78 (95% confidence interval 0.54 to 1.12) for neurological signs at four to six weeks.³

Pregnancy

Maternal carboxyhaemoglobin understates fetal exposure: the fetal half-life is up to 4 to 5 times the mother's and fetal levels run 10 to 15% higher.¹
Discuss every pregnant patient with the poisons service, which sets the oxygen duration, and arrange fetal growth and wellbeing monitoring.^8,9

Stop the exposure, then safety-net

Continue oxygen until carboxyhaemoglobin returns to 1 to 2% or poisoning is excluded. On 100% oxygen the half-time is 80 minutes, so this takes hours.^2,1
Do not discharge without a warning not to use the suspect appliances. National Gas Emergency Service 0800 111 999. Audible alarm to BS EN 50291.^6,7
Warn about memory and language problems, personality change, parkinsonism, chorea, apraxia and psychosis, up to 40 days later. Arrange GP follow-up.¹

Exam traps

A normal SpO2 (oxygen saturation on a pulse oximeter) does not exclude it: the device cannot tell carboxyhaemoglobin from oxyhaemoglobin. A pulse CO-oximeter is different.
A normal arterial oxygen tension is expected and is not reassuring. It measures oxygen dissolved in plasma, not oxygen carried on haemoglobin.
The 94 to 98% target does not apply here, and neither does 88 to 92%. This is the one place you deliberately over-oxygenate.
Carboxyhaemoglobin does not predict severity or outcome, and falls once the patient leaves the source or gets oxygen. Treat the patient, not the number.
Cherry-red skin is rarely seen, and the UK Health Security Agency toxicological overview does not list it among the reported features at all.
Delayed neuropsychiatric features can appear up to 40 days later, so an apparent full recovery in the emergency department is not the end of it.
In a house fire, assume cyanide poisoning and smoke inhalation alongside the carbon monoxide.

Illustrations

Pulse CO-oximetry in suspected carbon monoxide poisoningA pulse CO-oximeter with a finger probe measures carboxyhaemoglobin separately from oxygen saturation; conventional pulse oximetry cannot reliably distinguish carboxyhaemoglobin from oxyhaemoglobin and can appear falsely reassuring.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.