Carbon monoxide (CO) is a poisonous gas that can be produced when fuel-burning appliances do not operate correctly. It is particularly dangerous because you cannot see, smell or taste it.
That means a household cannot rely on its senses to detect carbon monoxide. Knowing the warning signs around gas appliances, recognising possible symptoms and having the right safety measures in place are therefore essential.
With Gas Safety Week running from 14–20 September 2026, this is a timely reminder for UK households to check that their gas appliances and carbon monoxide protection are being properly looked after.
What Is Carbon Monoxide?
Carbon monoxide is produced when carbon-based fuels do not burn completely. Gas, oil, wood and coal appliances can all produce CO if combustion is not occurring correctly.
A properly installed appliance should have the right air supply for combustion and discharge its combustion products safely through an appropriate flue or chimney. Faulty appliances, poor installation, inadequate ventilation, blocked flues or a lack of proper maintenance can increase the risk.
Carbon monoxide is dangerous because it reduces the blood's ability to carry oxygen around the body. High levels can cause serious illness and can be fatal.
Carbon Monoxide Warning Signs at Home
You cannot see carbon monoxide itself, but you may notice signs that a gas appliance is not operating correctly.
Yellow or orange flame
Where an appliance is designed to produce a blue flame, an unexpected yellow or orange flame can indicate incomplete combustion.
It does not automatically mean carbon monoxide is present, and some appliances are specifically designed to have different flame colours. The important point is a change from the appliance's normal appearance.
Don't attempt to adjust or repair the appliance yourself. Have it checked by a Gas Safe registered engineer.
Soot or staining
Black soot or yellow-brown staining around a gas appliance should not simply be cleaned away and forgotten.
It can indicate a problem with combustion or the removal of combustion products. HSE identifies soot and staining as warning signs that warrant attention.
Unusual condensation
Condensation is common in UK homes, so it does not automatically indicate a gas-safety problem.
However, unusual or increased condensation inside windows, particularly in a room containing a gas appliance, can be a warning sign.
This is one of the signs that homeowners can easily overlook. Gas Safety Week research has previously found that many people did not recognise excess condensation as a possible indicator of an unsafe gas appliance.
A pilot light that keeps going out
If a gas appliance has a pilot light that repeatedly goes out, don't simply keep relighting it.
A recurring flame problem can indicate that the appliance is not operating correctly and should be professionally checked.
What Are the Symptoms of Carbon Monoxide Exposure?
The symptoms of CO exposure can be difficult to recognise because they may resemble flu, food poisoning or ordinary tiredness.
Possible symptoms include:
- headache or dizziness;
- nausea or vomiting;
- unusual tiredness or weakness;
- breathlessness;
- chest or stomach pain;
- confusion or drowsiness;
- loss of consciousness in serious cases.
The pattern of symptoms can also be significant.
If someone feels unwell at home but improves after going outside, or if several people in the household develop similar symptoms, carbon monoxide exposure should be considered.
Symptoms alone cannot confirm CO poisoning. If exposure is suspected, seek medical advice and explain that carbon monoxide may be involved.
Carbon Monoxide Is Not the Same as a Gas Leak
It is important not to confuse these two hazards.
A natural gas leak involves escaping gas from the supply or an appliance. Domestic natural gas is odorised, so a leak may produce the distinctive smell associated with gas.
Carbon monoxide is different. It is a product of incomplete combustion and is colourless, odourless and tasteless.
Therefore, not smelling gas does not mean that there is no carbon monoxide risk.
If you smell gas, follow the appropriate gas-emergency procedure. If you suspect carbon monoxide, take action even when there is no smell.
What Can Cause Carbon Monoxide Problems?
A carbon monoxide risk is not necessarily caused by an old boiler.
Potential causes include:
- a faulty gas appliance;
- incorrect installation;
- inadequate ventilation;
- a blocked or defective flue or chimney;
- poor maintenance;
- combustion problems.
Ventilation and flues deserve particular attention. An appliance needs sufficient air for safe combustion, while the flue needs to remove combustion products effectively.
This is why the fact that a boiler or other appliance is still producing heat does not prove that it is operating safely.
Why You Need a Carbon Monoxide Alarm
A CO alarm provides an important additional layer of protection because it can detect a hazard that people cannot sense themselves.
HSE recommends suitable CO alarms as a precaution, but an alarm does not replace professional servicing, maintenance or safety checks.
When buying one, choose an alarm complying with BS EN 50291 and follow the manufacturer's instructions for installation, testing and maintenance.
Think of a CO alarm as a safety net, not a substitute for maintaining the appliance that could produce the gas.
What to Do If You Suspect Carbon Monoxide
If you think carbon monoxide may be present:
- Stop using the suspected appliance if it is safe to do so.
- Get everyone into fresh air.
- Open doors and windows if this can be done safely.
- Do not use the appliance again until it has been checked.
- Call the National Gas Emergency Service on 0800 111 999 if a gas appliance or gas supply is involved.
- Seek medical advice if anyone has symptoms of possible CO exposure.
If someone is seriously unwell, unconscious or having difficulty breathing, call 999.
How Regular Gas Safety Checks Help
Prevention is better than relying on warning signs after something has gone wrong.
HSE advises gas consumers to have gas appliances checked for safety at least every 12 months by a Gas Safe registered engineer. Regular professional checks can identify combustion, ventilation, flue and appliance problems that may not be obvious during everyday use. (HSE)
For homeowners, this advice should not be confused with the specific legal requirements that apply to landlords and their CP12 Gas Safety Records. Those responsibilities will be covered separately in the forthcoming landlord guide.
Keep Your Home Gas Safe
Carbon monoxide safety starts with understanding that there may be no obvious warning.
Know what your gas appliances normally look and sound like. Don't ignore an unexpected flame colour, soot or staining, unusual condensation or a pilot light that repeatedly goes out.
Keep a suitable CO alarm in place, make sure ventilation and flues are not obstructed, and arrange regular professional safety checks.
Gas Safety Week is also a good opportunity to share these warning signs with family members, particularly older relatives or anyone who may not recognise the signs of an unsafe appliance.
If you are concerned about a gas appliance or need professional gas safety assistance, Maintracts' Gas Safe registered engineers can inspect gas appliances and provide appropriate safety documentation where required.
Call Maintracts on 020 8682 2244 to discuss your gas safety requirements.