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uk Health & medical scares carbon monoxide alarm going off • co detector sounded • outside after co alarm • headache after alarm • nausea after alarm • feeling sick after boiler • suspected carbon monoxide exposure • gas appliance fumes • symptoms improve outdoors • dizzy after being indoors • possible co poisoning • carbon monoxide leak at home • woke up with headache indoors • co alarm beeping • partner or kids still inside • pet still inside • tenant gas safety emergency • boiler flue problem • gas cooker carbon monoxide • heater carbon monoxide

What to do if…
a carbon monoxide alarm goes off and you are now outside with headache or nausea

Short answer

Treat this as possible carbon monoxide (CO) exposure: stay outside in fresh air and get urgent medical advice. Do not go back inside until you’ve been told it’s safe.

Do not do these things

  • Do not re-enter “just to check” or to reset/silence the alarm.
  • Do not assume it’s a false alarm because you “can’t smell anything”.
  • Do not open windows/doors if it requires going back in or delays getting help.
  • Do not keep using or relight any appliance (boiler, cooker, fire, heater).
  • Do not drive yourself if you feel dizzy, confused, very sleepy, or unwell.
  • Do not ignore symptoms that improve outside — that pattern can happen with CO exposure.

What to do now

  1. Stay outside, away from doors/windows. Keep everyone (and pets) out. Move to a neighbour’s property, your car (parked outside), or another safe place nearby.
  2. Call for urgent medical advice now.
    • Call 999 if anyone has severe symptoms (trouble breathing, chest pain, collapse/fainting, seizures, marked confusion, cannot be woken, or symptoms are getting worse).
    • Otherwise, call NHS 111 and say: “Possible carbon monoxide exposure. CO alarm went off. Now outside with headache/nausea.”
    • If you’re told to go to A&E, don’t drive yourself if you feel unwell — ask someone to take you, or call 999 and request an ambulance.
  3. Report it as a gas/CO emergency if there’s any chance it’s from gas appliances. From outside, call the National Gas Emergency Service: 0800 111 999 and say you suspect a carbon monoxide emergency. Follow their instructions.
    • If there is something you can safely shut off without re-entering (for example, an external appliance switch or an outside gas meter control), do so only if it does not put you back at risk or delay getting help.
  4. Account for everyone who was inside. If someone is still inside and it’s safe to do so without re-entering, shout/phone them. Do not go back in to retrieve them—call 999 and tell them someone may still be inside with a CO alarm sounding.
  5. Tell responders about higher-risk people. If anyone exposed is pregnant, a baby/child, older, or has heart/lung conditions, say so when you call—this can change how urgently they want you assessed.
  6. If you need hospital assessment, take these details with you (or text them to yourself now):
    • when the alarm sounded (approximate time),
    • who was inside and for how long,
    • your symptoms and when they started,
    • any fuel-burning sources (boiler, gas cooker, solid-fuel stove, fireplace, portable heater).
  7. Do not return until the building is declared safe. If you’re told to attend A&E/ED, go—CO exposure is checked with tests and may need oxygen treatment.

What can wait

  • Figuring out which appliance caused it or trying to troubleshoot the alarm.
  • Calling your insurer or arranging repairs (after immediate safety and medical checks).
  • Collecting belongings (keys/meds) unless emergency services tell you it’s safe to do so.
  • Deciding whether to stay elsewhere tonight—first get safety clearance and medical advice.

Important reassurance

Headache and nausea after a CO alarm is a strong “take it seriously” signal, and you’ve already done the most protective thing by getting outside. Getting checked promptly is the safest next step and helps prevent delayed complications.

Scope note

These are first steps to keep you safe and get you assessed. Later steps (appliance checks, landlord/engineer follow-up, alarm replacement) can be done after the immediate risk is controlled.

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’re unsure, treat it as real and get urgent advice before going back inside.

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