What Is Carbon Monoxide?
Carbon monoxide is a poisonous gas produced whenever fuel doesn't burn properly, whether that's gas, wood, coal or petrol. Recognising the carbon monoxide warning signs early matters because the gas itself gives nothing away: it has no colour, no taste and no smell.
Any home with a gas boiler, gas cooker, open fire, wood burner, gas fire or gas heater is at risk if that appliance isn't working as it should. Once carbon monoxide builds up in the blood, it starts replacing oxygen in your red blood cells, and that's when the damage begins. It’s also highly flammable.
Carbon Monoxide Warning Signs: Physical Symptoms
The physical symptoms are often the first clue something is wrong, and they're easy to write off.
Carbon monoxide symptoms feel a lot like tiredness, a hangover or flu, except there's no high temperature. Symptoms may come and go. They may get worse the longer you're in the affected room and ease once you step outside. This is an important signal to look out for.
Mild Symptoms
The earliest carbon monoxide warning signs are the ones most often mistaken for a virus, stress or overwork:
- Headache
- Dizziness
- Nausea
- Tiredness
Moderate to Severe Symptoms
- Confusion
- Shortness of breath
- Chest pain
- Loss of vision
- Loss of consciousness
If you are suffering from these moderate to severe carbon monoxide poisoning symptoms, it's a medical emergency. There may be carbon monoxide present in your home.
The more carbon monoxide you inhale, the worse your symptoms will get. You may eventually lose balance, muscle control, vision and consciousness. Even minor cases of carbon monoxide poisoning can lead to serious brain, heart and organ damage, potentially resulting in death.
Gas leaks can also cause some similar symptoms but you can normally smell mains gas, so you may become aware of a leak before experiencing any symptoms, unlike with carbon monoxide.
If you think you’ve been exposed to low levels of carbon monoxide, see your GP or call NHS 111.
If you think you’ve been exposed to high levels, go to A&E immediately but do not drive yourself. Get somebody to take you or call an ambulance.
If someone is displaying severe symptoms or has collapsed, call 999.
Symptoms in Children and Pets
Children and pets are more vulnerable to carbon monoxide poisoning and can show symptoms before anyone else in the house notices something's wrong. Unborn babies are especially at risk, because foetal blood cells take up carbon monoxide more readily than adult blood cells do, and infants and young children breathe faster, so they absorb more of the gas, more quickly.
In children, signs of carbon monoxide poisoning can include:
- headache
- dizziness
- weakness or clumsiness
- nausea and vomiting
- confusion
- chest pain
- fast or irregular heartbeat
- shortness of breath
- blurred vision
- seizures
- loss of consciousness
- in severe cases, cardiac arrest or respiratory failure
In pets, signs of carbon monoxide poisoning include:
- reluctance to go back indoors after being outside
- sudden weakness or lethargy
- rapid or laboured breathing
- vomiting
- disorientation or loss of coordination
- staggering
- seizures or collapse
- unresponsiveness
- bright red gums are sometimes reported too, though it's not a sign to rely on alone
Does Carbon Monoxide Smell?
No, carbon monoxide has no smell and no colour at all, which is exactly why it's known as the silent killer. You cannot rely on your senses to warn you it's there, however strong the leak.
That's why a working carbon monoxide alarm is essential for every home. It's the only way you'll know if you’re in danger of carbon monoxide poisoning.
Signs of a Carbon Monoxide Leak in Your Home
Signs Around Your Appliances
Appliances and flues that aren't properly vented let carbon monoxide build up, and a tightly sealed room makes that worse.
Carbon monoxide warning signs to watch for include:
- a gas cooker flame that looks lazy rather than sharp
- yellow or orange flame on gas appliances instead of blue
- sooty or yellow-brown staining around a boiler or appliance
- a pilot light that keeps going out
- an unusual amount of condensation building up on windows near an appliance
Decorative gas fires with fake coal or log effects are a normal exception, since they're designed to burn yellow for a realistic look. A boiler, cooker or heater flame should still be blue.
None of these carbon monoxide warning signs on their own means the gas is present, but they're reason enough to get your gas appliances checked by a Gas Safe registered engineer, ideally through an annual service that catches problems before they become dangerous.
Signs Your Carbon Monoxide Alarm Is Alerting
Beep patterns vary between manufacturers, so always check the sticker on your specific alarm or its manual for the exact codes. As a general guide:
- Four beeps followed by a pause is usually a sign of a carbon monoxide leak. Get everyone into fresh air immediately and call the National Gas emergency line on 0800 111 999.
- One chirp every minute usually means the battery is low and needs replacing.
- Three chirps every minute usually means the alarm has developed a fault and needs replacing.
- Five chirps every minute usually means the alarm has reached the end of its working life and needs a new unit.
If you smell gas rather than hear an alarm, that's a separate emergency, but you also need to get everyone out of the building and call the same number, the National Gas Emergency line on 0800 111 999 immediately.
Without annual gas safety inspections, houses can be ticking time bombs. Learn more about gas safety checks for homeowners and tenants here.
Signs in Neighbours or Multiple Occupants
If you and other people in the same building start feeling unwell at the same time, treat that as a serious warning sign rather than coincidence. It could be a sign of a carbon monoxide leak.
This is especially relevant in HMOs and rented properties, where one faulty appliance can affect several households at once.
What to Do If You Suspect Carbon Monoxide
If you suspect a carbon monoxide leak in a property:
- stop using any appliance you suspect is the source, such as a boiler, cooker or heater, if it's safe to do so
- get outside into fresh air straightaway
- do not go back into the property until you've had advice from a professional
- call the National Gas Emergency line on 0800 111 999
If Someone Is Showing Severe Symptoms
Call 999 immediately.
If you think you’ve been exposed to high levels, go to A&E. Do not wait and do not drive yourself. Ask someone else to drive or call an ambulance.
How to Prevent Carbon Monoxide Poisoning
Do:
Get your boiler serviced annually by a Gas Safe registered engineer.
- Fit a carbon monoxide alarm in every room with a gas, oil, coal or wood-burning appliance.
- Keep flues and chimneys clear and well maintained.
- Ventilate rooms properly when cooking with gas.
- Have heating and cooking appliances installed and maintained by a qualified, registered engineer.
Don’t:
- Don't use a barbecue, camping stove or generator indoors, in a garage, or in any other enclosed space such as a tent
- Don't ignore a pilot light that keeps going out or staining around an appliance
- Don't block or seal vents, flues or air bricks
Report Suspected Gas Theft
Illegal interference or tampering with the gas supply to steal gas can increase the risk of carbon monoxide poisoning. If you suspect energy theft, report it 100% annonymously to Stay Energy Safe on 0800 023 277 or online.
Carbon Monoxide Alarms: What to Know
Fit a carbon monoxide alarm in every room that contains a gas, oil, coal or wood-burning appliance, positioned at head height and one to three metres from the appliance itself. Test it weekly using the test button, and replace it according to the manufacturer's guidance, usually every five to ten years.
Since October 2022, landlords in England have been legally required to fit a carbon monoxide alarm in any room with a fixed combustion appliance, with the same requirements also in place in Scotland and Wales, so if you're renting and don't have one, that's worth raising with your landlord straightaway.
Learn more about gas safety checks for homeowners and tenants here.
Important: a carbon monoxide alarm will not alert you to a gas leak. If you smell gas, call the National Gas Emergency line immediately on 0800 111 999.