Carbon monoxide alarm: what to do

A carbon monoxide alarm sounding means leave the property, get everyone and any pets into fresh air, and call the Gas Emergency Service or your gas supplier's emergency number from outside. Do this before anything else. Carbon monoxide has no colour and no smell, so the alarm is the only early warning most households get, and the correct response to it is evacuation, not investigation.

Why this alarm is treated differently from a smoke alarm

A smoke alarm tells you where the danger is, because you can usually see or smell what triggered it. A carbon monoxide alarm tells you nothing about where the gas is coming from, and by the time it sounds, the air in the room may already be unsafe to breathe for long. Carbon monoxide is produced when gas, oil, wood or coal burn without enough air, most often from a boiler, a gas fire, a flue that has come loose, or a blocked chimney. The Health and Safety Executive's domestic gas safety pages set out why this gas is dangerous precisely because it gives no warning of its own, which is the reason the alarm exists at all.

The steps to take, in order

1. Leave the property. Open windows and doors on your way out if that is quick and safe, but do not stop to search for a cause.
2. Get into fresh air and take everyone in the household, including pets, with you.
3. Call the National Gas Emergency Service on 0800 111 999, open day and night, if you have mains gas. For bottled or tank LPG, call your LPG supplier's emergency line; for oil or solid fuel appliances, stop using them and contact an OFTEC or HETAS registered engineer. Give your address and say the alarm has gone off.
4. Do not go back inside until a qualified engineer or the gas emergency team has confirmed it is safe.
5. If anyone shows symptoms such as headache, dizziness, feeling sick, tiredness or confusion, get medical advice and mention possible carbon monoxide exposure: NHS 111 for mild symptoms, 999 or A&E for difficulty breathing, chest pain, confusion or loss of consciousness. The NHS page on carbon monoxide poisoning lists these symptoms in full.

What never to do when the alarm sounds

Do not open the boiler casing or any gas appliance to look for a fault. Do not light a match or a lighter to check anything, and do not operate light switches or anything electrical near the suspected source if you can avoid it on your way out. Do not silence the alarm and go back to bed, which is the single most common mistake with a low battery or an intermittent trip: either is a reason to get the alarm itself checked, not a reason to ignore the next alert. Do not assume the alarm is faulty because nobody feels unwell, since carbon monoxide can build to dangerous levels before anyone notices anything.

Once the gas emergency team gives the all clear

A visit from the gas emergency team is a safety check, not a repair, and it is a different thing from a service. If they isolate an appliance or cap a supply, that appliance stays out of use until a Gas Safe registered engineer has found and fixed the fault. The Gas Safe Register is the single legal register for engineers who may work on gas appliances in Great Britain, and checking an engineer's registration and licence card before any work begins takes a minute on their website. Our own sheet on choosing a plumber covers the checks worth making before anyone is let in to work on gas or water.

What usually turns out to be the cause

The NHS names badly installed or poorly maintained appliances, such as boilers, gas fires, cookers and heaters, as the usual source, and in practice that can mean a flue joint that has worked loose, a flue or chimney blocked by a bird's nest or debris, or an appliance short of ventilation. None of these are things a resident can safely diagnose from the symptoms alone, which is why the emergency step and the repair step are kept separate: evacuate and call first, diagnose only once a registered engineer is on site.

Alarms, batteries and the annual service

A carbon monoxide alarm needs testing regularly, using its own test button, and government guidance for rented homes recommends alarms with sealed-for-life batteries over ones with replaceable batteries. Fit one in every room with a fuel-burning appliance, following the position guidance in the box, and treat an alarm that chirps intermittently for a low battery as seriously as one that sounds fully, since both mean it may not warn you properly when it matters. An annual service of every gas appliance by a Gas Safe registered engineer is the other half of this protection, and it is covered in more detail in our sheet on the first hour of a boiler breakdown, which shares the same safety checks at the top.

What this sheet does not cover

This sheet does not cover fitting or choosing a carbon monoxide alarm model, landlord obligations for alarms in rented property, which are set out separately in government guidance, or oil and solid fuel appliances beyond the general warning above, which fall under different registers. For any suspected gas leak rather than an alarm sounding, follow the same leave-first instructions and call the National Gas Emergency Service.