Skip to main content

Health condition · Clinically reviewed

Carbon monoxide, the silent killer you cannot see, smell or taste.

Around 40 to 50 people die from CO poisoning in the UK every year, and thousands more attend A&E. Early recognition, high-flow oxygen and a working alarm save lives.

A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE, PHE/UKHSA, RCEM and the British Hyperbaric Association.

  • 03

    Current for 2026

    Reflects modern UK guidance on high-flow oxygen, carboxyhaemoglobin testing and hyperbaric referral.

Key facts

Carbon monoxide at a glance.

What CO is, why it is so dangerous, how many people it affects in the UK, and the two interventions that matter most - high-flow oxygen and a working alarm.

  • What it is

    Poisoning by carbon monoxide (CO) - a colourless, odourless, tasteless, non-irritant gas produced by incomplete combustion of any carbon fuel.

  • Why it kills

    CO binds haemoglobin around 240 times more avidly than oxygen, forming carboxyhaemoglobin (COHb) and starving tissues of oxygen.

  • UK burden

    Around 40 to 50 deaths and roughly 4,000 A&E attendances every year in the UK, with significant under-diagnosis.

  • The silent killer

    You cannot see, smell or taste it - the earliest symptoms mimic flu, a viral illness or a hangover.

  • First-line treatment

    100 percent oxygen via a tight non-rebreathing mask at 15 litres per minute, started as soon as CO poisoning is suspected.

  • Prevention

    Annual gas appliance servicing by a Gas Safe engineer and an audible, EN 50291 kitemarked CO alarm in every room with a combustion appliance.

Why this guide matters

Under-diagnosed, entirely preventable.

CO poisoning looks like a viral illness, hides behind a normal pulse oximeter reading, and is common enough that every clinician and every household should know the pattern.

  • The gas has no warning signs

    CO is colourless, odourless, tasteless and non-irritant - the human body cannot detect it, which is why alarms are non-negotiable.

  • The symptoms mimic flu

    Headache, nausea, dizziness and confusion - clinicians and patients regularly attribute early CO poisoning to a viral illness.

  • Oxygen and alarms save lives

    Prompt high-flow oxygen shortens the half-life of COHb roughly threefold, and an EN 50291 alarm gives you the seconds you need to get out.

Where CO comes from

Common UK sources of carbon monoxide.

Anything that burns a carbon fuel can produce CO if combustion is incomplete or ventilation is poor. The list below covers most UK cases.

  • Faulty appliances

    Poorly serviced or damaged boilers, gas cookers, gas fires, wood-burning stoves, open fires and paraffin heaters.

  • Blocked flues and chimneys

    Bird nests, soot build-up and damaged flue liners force combustion products back into the room.

  • Poor ventilation

    Blocked airbricks, sealed rooms and DIY draught-proofing that traps fumes indoors.

  • Vehicle exhaust

    Cars running in an attached garage or with an exhaust leak - a classic cause of accidental and deliberate poisoning.

  • Portable generators and BBQs

    Never use a petrol generator, charcoal BBQ or camping stove indoors, in a garage, in a tent or on a boat.

  • Fires and smoke inhalation

    House fires produce large amounts of CO alongside cyanide and heat injury.

  • Methylene chloride paint stripper

    Absorbed and metabolised in the liver to CO - poorly ventilated DIY can cause delayed poisoning.

  • Houseboats, caravans and tents

    Small enclosed spaces with fuel-burning appliances are high-risk - always fit a portable CO alarm.

How the diagnosis is made

From suspicion to a confirmed COHb.

What a UK GP, paramedic or emergency clinician will normally do, in order - so you know why every step matters.

  1. 01

    Suspecting

    High index of suspicion

    Ask about heating, appliances, ventilation and whether anyone else in the household - or pets - is unwell at the same time.

  2. 02

    Suspecting

    Symptom pattern

    Look for symptoms that improve when the person is out of the house and return on going back in - a classic CO clue.

  3. 03

    Suspecting

    Pulse oximetry is unreliable

    A standard SpO2 reads carboxyhaemoglobin as oxyhaemoglobin and looks falsely normal - never use it to rule CO out.

  4. 04

    Confirming

    Blood gas with CO-oximetry

    Arterial or venous blood gas with direct CO-oximetry measures COHb percentage - the definitive test.

  5. 05

    Confirming

    Cardiac and neuro screen

    ECG for ischaemic changes, troponin and BNP, plus CT or MRI brain in severe cases - bilateral globus pallidus lesions are classic.

  6. 06

    Protecting

    Foetal monitoring in pregnancy

    Foetal haemoglobin binds CO even more avidly - CTG and obstetric review are essential in any pregnant patient.

  7. 07

    Protecting

    Source investigation

    Every confirmed case triggers a Gas Safe engineer visit, landlord notification and the local Health Protection Team.

Typical timeline: from suspicion to confirmed diagnosis in under an hour, once oxygen is running.

Symptoms

What CO poisoning actually looks like.

Non-specific early symptoms, a graded progression as COHb rises, and the delayed neuropsychiatric pattern that can appear days or weeks later.

  • Headache and flushing

    Mild exposure (COHb 10 to 20 percent) - a dull, throbbing headache with nausea and dizziness, often mistaken for flu.

  • Confusion and tachycardia

    Moderate exposure (COHb 20 to 30 percent) - severe headache, poor concentration, fast heart rate and rapid breathing.

  • Chest pain and drowsiness

    Severe exposure (COHb 30 to 40 percent) - chest pain, palpitations, weakness, drowsiness and loss of consciousness.

  • Seizures and coma

    Life-threatening exposure (COHb above 40 percent) - cardiac arrhythmias, seizures, coma and death.

  • Cherry-red skin

    A classic but uncommon and late sign - never rely on its absence to rule CO out.

  • Whole household unwell

    Multiple people or pets in the same building feeling unwell together is a major red flag for CO.

  • Winter clustering

    Cases peak in winter when heating is on, windows are shut and flues are more likely to block.

  • Delayed neuropsychiatric syndrome

    Days to weeks after exposure - parkinsonism, memory loss, personality change and focal neurological deficits.

Treatment

How CO poisoning is treated in the UK.

Remove from source, high-flow oxygen straight away, hyperbaric therapy for severe cases and in pregnancy, then a mandatory source investigation and follow-up.

  • Remove from source

    Get the person into fresh air immediately, open windows and doors, and evacuate everyone in the property including pets.

  • High-flow oxygen

    100 percent oxygen via a tight non-rebreathing mask at 15 litres per minute, or via endotracheal tube if unconscious - the mainstay of treatment.

  • Hyperbaric oxygen therapy

    100 percent oxygen at 2.5 to 3 atmospheres in a specialist chamber - considered for severe symptoms, pregnancy or COHb above 25 percent.

  • Supportive care

    IV fluids, treatment of arrhythmias, anti-seizure medication, intubation and neuroprotection as needed.

  • Obstetric management

    High-flow oxygen for longer than in adults, selective hyperbaric referral, continuous CTG and obstetric-led delivery planning.

  • Source investigation

    Mandatory - Gas Safe engineer, landlord notification, Health Protection Team, fire service and safeguarding where appropriate.

  • Follow-up and rehabilitation

    Neuropsychology, occupational therapy and rehabilitation with surveillance for delayed neuropsychiatric syndrome.

  • Prevention and public health

    Annual servicing, audible CO alarms in every room with a combustion appliance, and safe use of stoves, BBQs and generators.

For more on chamber therapy, see our guide to hyperbaric oxygen therapy.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

If you suspect CO exposure, get everyone into fresh air, call 999 or NHS 111, and ask a Gas Safe engineer to check every appliance before returning.

  • NICE CKS. Carbon monoxide poisoning.

  • UK Health Security Agency (UKHSA / PHE). Carbon monoxide: general information and toxicological overview.

  • Royal College of Emergency Medicine (RCEM). Carbon monoxide poisoning best practice guideline.

  • British Hyperbaric Association. Indications for hyperbaric oxygen therapy in acute carbon monoxide poisoning.

  • NHS. Carbon monoxide poisoning - symptoms, causes and treatment.

  • Gas Safe Register. Consumer advice on safe gas appliances and CO alarms (EN 50291).

Red flags

When CO exposure is an emergency.

Any of the following should trigger 999, immediate removal from the source and high-flow oxygen.

  • Loss of consciousness

    Any collapse or unresponsive episode after suspected CO exposure is a medical emergency - dial 999 and start high-flow oxygen.

  • Chest pain or arrhythmia

    Ischaemic chest pain, palpitations or an abnormal ECG after CO exposure indicate cardiac injury and need urgent hospital assessment.

  • Seizure

    A first seizure in the context of possible CO exposure points to severe poisoning and warrants hyperbaric referral.

  • Focal neurological deficit

    New weakness, confusion, ataxia or speech disturbance after exposure needs urgent CO-oximetry and specialist review.

  • Pregnancy

    CO crosses the placenta and foetal haemoglobin holds onto it longer - any pregnant patient with suspected CO exposure needs urgent obstetric input.

  • Whole household unwell

    Several people in the same building - or pets - sick together is a red flag for a shared environmental cause, until proven otherwise.

  • Symptoms improve away from home

    Headaches, nausea or dizziness that lift on leaving the house and return on going back in should prompt CO testing.

  • Suicidal or intentional exposure

    Deliberate CO exposure - car exhaust, charcoal burning - needs both emergency medical care and a mental health assessment.

  • Delayed neurological change

    New parkinsonism, memory or personality change in the weeks after CO exposure suggests delayed neuropsychiatric syndrome.

Prevention

Almost every case is preventable.

Four simple habits - detect, service, ventilate and recover properly - remove most of the risk from your home, your caravan and your boat.

A quiet reminder

An alarm you can hear is worth ten you cannot.

Choose EN 50291 kitemarked, audible CO alarms, test them monthly and never silence a battery warning.

  1. 01 Detect

    Fit CO alarms in every room with a fuel-burning appliance

    Choose audible, EN 50291 kitemarked alarms and test them monthly - a smoke alarm will not pick up CO.

  2. 02 Service

    Annual servicing by a Gas Safe engineer

    Every gas boiler, cooker, fire, flue and chimney should be checked once a year by a registered engineer.

  3. 03 Ventilate

    Keep vents and flues clear

    Never block airbricks or flues, and never run a petrol generator, camping stove or BBQ indoors, in a tent or in a garage.

  4. 04 Recover

    Take recovery seriously

    Even after a mild episode, follow-up matters - delayed neuropsychiatric syndrome can appear weeks later and deserves review.

Frequently asked

Everything we get asked about CO poisoning.

Quick answers on symptoms, testing, oxygen, hyperbaric therapy and prevention.

  • What is carbon monoxide poisoning?

    It is a serious form of poisoning caused by breathing in carbon monoxide (CO), a colourless, odourless, tasteless and non-irritant gas produced by the incomplete burning of any carbon-based fuel - gas, oil, wood, charcoal or petrol. CO binds haemoglobin around 240 times more strongly than oxygen, starving tissues of oxygen and injuring cells directly.

  • What are the early warning signs?

    The earliest symptoms are non-specific and often mistaken for flu, a viral illness or a hangover - headache, nausea, dizziness, tiredness and mild confusion, without a fever. Clues that point to CO include several people or pets in the same building feeling unwell together, symptoms that improve when you leave the house, and a recent problem with a boiler, fire or flue.

  • Can a normal pulse oximeter rule out CO poisoning?

    No. A standard pulse oximeter cannot tell oxyhaemoglobin from carboxyhaemoglobin and often reads falsely normal in significant CO poisoning. The definitive test is a blood gas with CO-oximetry, which measures carboxyhaemoglobin (COHb) directly - normally under 2 percent in non-smokers and up to around 10 percent in smokers.

  • What is the first treatment for CO poisoning?

    100 percent oxygen via a tight non-rebreathing mask at 15 litres per minute, started as soon as CO poisoning is suspected and continued until the patient is asymptomatic and the COHb is below 10 percent. High-flow oxygen reduces the half-life of COHb from around 5 hours on room air to roughly 90 minutes.

  • When is hyperbaric oxygen therapy used?

    Hyperbaric oxygen therapy delivers 100 percent oxygen at 2.5 to 3 atmospheres in a specialist chamber. It is considered for severe features - loss of consciousness, seizures, cardiac ischaemia, focal neurological deficit or coma - pregnancy, or COHb above 25 percent (or above 20 percent in pregnancy). UK chambers include Poole, Aberdeen, Whipps Cross, Portsmouth, Plymouth and Great Yarmouth.

  • How do I protect my household from CO?

    Have every gas or fuel-burning appliance serviced annually by a Gas Safe registered engineer, keep flues and airbricks clear, never run a petrol generator, camping stove or BBQ indoors, in a tent or in a garage, and fit an audible EN 50291 kitemarked CO alarm in every room that contains a combustion appliance. Test the alarms monthly and replace batteries in line with the manufacturer.

Related content

Keep reading.

Pulse Healthcare concierge

Send us your enquiry

A concierge service for UK private healthcare. We match you with the best vetted clinics and consultants in our network - they then contact you directly.

So we can match you to the right clinician close to you.

We reply to every enquiry within 24 hours (Mon–Fri). Confidential - your details are never shared outside our vetted consultant network.