Connect combustion-air and flue faults with CO risk, then recognise when to leave and seek help.
Carbon monoxide can be produced by incomplete combustion. You cannot rely on smelling it. A blocked required air vent, an obstructed flue or an appliance fault can contribute to exposure; a familiar flame appearance does not prove that everything is safe.
Recognise possible exposure
Headache, dizziness or nausea can have many causes, but symptoms affecting several people indoors or improving outside are reasons to consider CO. Neither that pattern nor its absence is a diagnosis. Take a CO alarm seriously even when nobody feels unwell.
Put people first
Get into fresh air, stop using the suspect appliance and seek medical advice for possible poisoning. Only switch the appliance off if safe. Call 999 for serious symptoms such as breathing difficulty, sudden confusion or loss of consciousness; do not drive if affected. NHS 111 can advise on suspected poisoning without those emergency signs.
For a suspected gas-appliance incident in Great Britain, report it on 0800 111 999 from safety. Stay out until advised it is safe to return. An alarm sounding or becoming quiet does not identify the fault or establish that the installation is safe.
Read CO alarms: warning, placement and response for device limitations and placement decisions.
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