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What to do if…
a carbon monoxide alarm sounds and you feel sick or have a headache

Produced and maintained by PanicStation.org Published: Last reviewed: Editorial policy UK guide

Short answer

Treat this as possible carbon monoxide exposure: stay outside in fresh air and get urgent medical advice now. Do not go back inside until emergency services or a qualified professional says it is safe.

Do not do these things

  • Do not re-enter “just to check”, collect belongings, reset the alarm, or silence it.
  • Do not assume it is a false alarm because you cannot smell anything.
  • Do not go back inside just to open windows or doors.
  • Do not keep using, relight, or test any boiler, cooker, fire, stove, heater, or other fuel-burning appliance.
  • Do not drive yourself if you feel dizzy, confused, very sleepy, short of breath, or otherwise unwell.
  • Do not ignore symptoms that improve outside; that pattern can happen with carbon monoxide exposure.

What to do now

  1. Stay outside in fresh air. Keep everyone and any pets out of the building. Move to a neighbour’s home, an open outdoor place, or another safe nearby place away from the affected building.

  2. Call for urgent medical advice now.

    • Call 999 if anyone is finding it hard to breathe, has chest pain, has suddenly become confused, has lost consciousness, has collapsed, has had a seizure, cannot be woken, or symptoms are getting worse.
    • Otherwise, call NHS 111 and say: “Possible carbon monoxide exposure. A carbon monoxide alarm went off. We are outside and have headache or nausea.”
    • If you are told to go to A&E, do not drive yourself if you feel unwell. Ask someone else to take you, or call 999 if you need an ambulance.

  3. Report it as a gas or carbon monoxide emergency if any gas appliance could be involved. From outside, call the National Gas Emergency Service on 0800 111 999 if you are in Great Britain, and say you suspect a carbon monoxide emergency. Follow their instructions.

    • If you are in Northern Ireland, use the relevant gas emergency number for your gas network or supplier; if there is immediate danger or anyone is unwell, call 999.
    • If there is something you can shut off without re-entering, such as an outside gas meter control, do so only if it is safe and does not delay medical help.

  4. If the possible source is not gas, still treat it as urgent. Carbon monoxide can come from oil, coal, wood, solid-fuel appliances, generators, BBQs, fireplaces, or vehicle engines. Stay out, get medical advice, and arrange for the relevant fuel supplier, registered engineer, landlord, or emergency services to deal with the source only after immediate safety steps are underway.

  5. Account for everyone who was inside. Phone or shout to anyone still inside if you can do that without re-entering. Do not go back in to retrieve a person or pet; call 999 and say someone may still be inside with a carbon monoxide alarm sounding.

  6. Mention higher-risk people when you call. Say if anyone exposed is pregnant, a baby or child, older, or has a heart condition, lung condition, asthma, angina, or anaemia.

  7. Keep simple details ready for medical staff and responders.

    • when the alarm sounded, approximately,
    • who was inside and for how long,
    • what symptoms each person has,
    • whether symptoms improved after getting outside,
    • any possible sources, such as a boiler, gas cooker, solid-fuel stove, fireplace, portable heater, generator, BBQ, or vehicle engine.

  8. Do not return until the building has been made safe. If you are told to attend A&E, go. Medical staff can decide whether tests or oxygen are needed.

What can wait

  • Working out which appliance caused the problem.
  • Troubleshooting, resetting, or replacing the alarm.
  • Calling your insurer, landlord, letting agent, or repair service, unless emergency responders tell you to; medical and emergency calls come first.
  • Collecting belongings, unless emergency services say it is safe.
  • Deciding where to sleep tonight; first get medical advice and safety clearance.

Important reassurance

You have already taken the most protective first step by getting outside. Headache or nausea after a carbon monoxide alarm is enough reason to get checked and to let emergency or gas-safety professionals decide what is safe next.

Scope note

These are first steps only, to help you stay safe, get medical advice, and avoid re-entering too soon. Later decisions about repairs, appliance checks, landlord duties, insurance, or replacing alarms may need specialist help.

Important note

This is general safety information, not medical diagnosis or a substitute for professional assessment. If symptoms are severe or worsening, call 999. If you are unsure, treat the alarm as real and get urgent advice before going back inside.

Additional Resources

Closely connected guides that may be useful during the same situation or its immediate aftermath.

About this guide

This guide was produced and is maintained by PanicStation.org using its published editorial process. Official and specialist sources are checked where relevant, and AI-assisted tools may be used for drafting, organisation, and consistency checks. The site operator remains responsible for publication, revision, and removal decisions.

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