Carbon monoxide: the signs and what to do

You cannot see it or smell it. An alarm can.

The answer in 2 lines. Carbon monoxide (CO) is invisible and has no smell. It comes from appliances that burn gas, oil, coal or wood when they are faulty or poorly ventilated. If you suspect it: open doors and windows, turn off the appliance if you can, get outside, and call the gas emergency line on 0800 111 999. If anyone feels unwell, call 999 or go to A&E and say you think it is CO. To prevent it: have appliances serviced every year by a Gas Safe registered engineer, never block vents, and fit a CO alarm.

If you suspect carbon monoxide

Open doors and windows straight away, and turn off the fuel-burning appliance if you can.
Get everyone outside into fresh air.
Call the gas emergency line on 0800 111 999. If anyone is unwell, call 999. Do not use the appliance again until it has been checked.

Stop: danger

Someone drowsy, confused, collapsed or breathless: get them into fresh air and call 999.
Never use a barbecue, generator or patio heater indoors, in a tent, garage or caravan.
Never block air vents or flues to keep the heat in.
A boiler or fire that has never been serviced is a risk.

Recognise the signs

The early signs can feel like flu or food poisoning: headache, dizziness, feeling sick, tiredness and confusion, stomach pain, shortness of breath.

A big clue: several people (or pets) in the same home feel unwell at the same time.
Symptoms get better when you go out, and come back when you return home.
Signs on the appliance: a lazy yellow or orange flame instead of a crisp blue one, black marks or soot around it, a pilot light that keeps going out, more condensation than usual.

Prevention every day

Have gas boilers, fires and cookers serviced every year by a Gas Safe registered engineer (check their ID card).
Have chimneys swept regularly if you burn wood or coal.
Fit an audible CO alarm in rooms with a fuel-burning appliance, and test it like a smoke alarm.
Keep vents, flues and air bricks clear.

If you rent

Your landlord must have every gas appliance and flue they provide checked by a Gas Safe registered engineer every year, and give you a copy of the gas safety record.
In England, private and social landlords must fit a CO alarm in any room with a fixed combustion appliance (except gas cookers), and repair or replace it once you report a fault.
Report any problem to your landlord in writing.

What not to do

Ignoring a CO alarm because "nobody feels ill".
Letting someone who is not Gas Safe registered work on gas appliances.
Running a car or generator in an attached garage.

Good to know

In Northern Ireland the gas emergency number is different: see nidirect's list of utility emergency contacts. In Scotland and Wales the number is the same as in England, 0800 111 999.
If you rent in Scotland or Wales, the alarm rules are set by their own laws.
When you go to hospital or call 999, say you think it may be carbon monoxide: a simple test can confirm it.

In Leeds, Kwame and his partner both wake up with a headache that fades during the day at work. Their new CO alarm goes off one evening. They open the windows, turn off the boiler, go outside and call 0800 111 999. The engineer finds a blocked flue and makes the boiler safe.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 18 October 2026, for England. Confidence: high for the symptoms, the emergency steps and the 0800 111 999 number (England, Scotland, Wales); high for the landlord rules in England. Official statistics on CO poisoning are not quoted here. This guide describes common cases, not a diagnosis.