Glossary · Faults, servicing and gas safety
What is carbon monoxide?
Carbon monoxide (CO) is a poisonous gas with no colour, taste or smell. It is produced when fuels such as gas, oil, wood or coal do not burn properly. It stops the blood carrying oxygen around the body and can kill quickly without warning, which is why faulty gas appliances and flues are dangerous.
Key points
- You cannot see, taste or smell carbon monoxide. HSE says it can kill quickly without warning.
- Gas boilers, cookers, heaters and wood, gas and coal fires can all produce it if they are faulty or poorly maintained.
- Warning signs on an appliance include yellow or orange flames, soot or staining, a pilot light that keeps blowing out and extra condensation.
- If you suspect a leak, switch off the appliance, open doors and windows, and call the National Gas Emergency Service on 0800 111 999.
- A carbon monoxide alarm is a back-up. It does not replace regular servicing by a Gas Safe registered engineer.
How carbon monoxide is made
Carbon monoxide is a colourless, odourless, tasteless and poisonous gas. It is made by the incomplete burning of carbon-based fuels, including gas, oil, wood and coal.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions The fuels themselves are safe to use. Excess carbon monoxide only appears when the fuel does not burn properly.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions
When it enters the body, it prevents the blood from bringing oxygen to cells, tissues and organs. Levels that do not kill can still cause serious harm if breathed in over a long period. In extreme cases, prolonged exposure can cause paralysis and brain damage.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions
Appliances that can produce it include gas boilers, gas cookers, gas or paraffin heaters, wood, gas and coal fires and portable generators. Using a barbecue or camping stove indoors, or running a vehicle in a closed garage, can also cause a build-up.[2]NHS, Carbon monoxide poisoning
HSE says around 7 people a year die from carbon monoxide poisoning caused by gas appliances and flues that have not been properly installed, maintained or ventilated.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions
Warning signs
There are two sets of signs to know. The first is on the appliance. The second is in your body.
| Where | Signs |
|---|---|
| On or near a gas appliance | Yellow or orange flames instead of blue, soot or yellow-brown staining, a pilot light that keeps blowing out, more condensation inside windows |
| In people | Headache, dizziness, feeling or being sick, weakness, tiredness and confusion, chest and muscle pain, shortness of breath, loss of vision |
HSE notes that fuel-effect fires and flueless appliances normally show a yellow or orange flame, so that sign does not apply to them.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions The NHS says symptoms may come and go. They may get worse when you spend time in an affected room and ease when you go outside.[2]NHS, Carbon monoxide poisoning Early symptoms can be mistaken for flu, food poisoning or tiredness.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions
What to do in an emergency
These steps are from HSE and the NHS. Do not try to repair the appliance yourself.
If you think an appliance is spilling carbon monoxide, HSE says to do the following.[1]Health and Safety Executive, Gas safety - Carbon monoxide awareness frequently asked questions
- Call the National Gas Emergency Service on 0800 111 999.
- Switch off the appliance and shut off the gas supply at the meter control valve.
- Open all doors and windows to ventilate the room.
- See your GP and say you may have been exposed.
The NHS adds that you should stop using the suspect appliance if you can, go outside, and get medical advice as soon as possible. Do not go back into the building until you have it. Call 111 if you think you have symptoms. Call 999 or go to A&E if you are finding it hard to breathe, have suddenly become confused, have lost consciousness, feel weak or have chest or muscle pain.[2]NHS, Carbon monoxide poisoning The NHS says not to drive yourself to A&E.[2]NHS, Carbon monoxide poisoning
By law it is illegal to use a gas appliance you suspect is unsafe. Turn it off and leave it alone until a Gas Safe registered engineer has checked it.[3]Health and Safety Executive, Domestic gas: frequently asked questions
Example
Karen in Halifax notices that the flame on her gas fire has turned yellow and there is brown staining above it. She also has a headache that goes when she leaves the room. She turns the fire off, opens the windows and rings the National Gas Emergency Service. She does not use the fire again until a Gas Safe registered engineer from a firm such as BASI Heating has inspected it. She tells her GP about the headaches.
What people get wrong
The most dangerous belief is that you would smell it. You would not. Carbon monoxide has no smell, so there is nothing to alert you. Act on the warning signs above rather than waiting to notice an odour.
The second mistake is thinking an alarm makes servicing optional. HSE says an alarm is one useful precaution and should not be regarded as a replacement for regular maintenance and safety checks by a Gas Safe registered engineer.[3]Health and Safety Executive, Domestic gas: frequently asked questions See our entries on the carbon monoxide alarm and the gas safety check.
The bottom line
Carbon monoxide is a silent, poisonous gas produced when fuel burns badly. Faulty or poorly maintained boilers, fires and cookers are the usual source in the home. Know the warning signs, fit an alarm and have appliances serviced every year by a Gas Safe registered engineer. In an emergency, ventilate, switch the appliance off, leave and call the National Gas Emergency Service on 0800 111 999.
Common questions
What are the symptoms of carbon monoxide poisoning?
The NHS lists a headache, dizziness, feeling or being sick, weakness, tiredness and confusion, chest and muscle pain, shortness of breath and loss of vision. Symptoms may come and go and ease when you leave the building.
What should I do if I think there is carbon monoxide in my home?
Stop using the appliance if you can, go outside and get medical advice. HSE says to call the National Gas Emergency Service on 0800 111 999, shut off the gas at the meter and open all doors and windows.
Which appliances can produce carbon monoxide?
The NHS lists gas boilers, gas cookers, gas or paraffin heaters, wood, gas and coal fires and portable generators. Barbecues and camping stoves used indoors can also cause a build-up.
How many people die from carbon monoxide each year?
HSE says that, according to its statistics, around 7 people a year die from carbon monoxide poisoning caused by gas appliances and flues that were not properly installed, maintained or ventilated.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Health and Safety Executive. Gas safety - Carbon monoxide awareness frequently asked questions. Checked 11 November 2026. ↩
- NHS. Carbon monoxide poisoning. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩