Carbon Monoxide Symptoms — Know the Signs Before It's Too Late

Carbon monoxide (CO) is produced when fuels such as gas, oil, coal, and wood burn without enough oxygen. It is sometimes called the "silent killer" — you cannot see it, smell it, or taste it, and by the time you realise something is wrong, you may already be too impaired to act. Understanding the symptoms and knowing what to do could genuinely save your life.

Symptoms of Carbon Monoxide Poisoning

CO symptoms are often mistaken for flu, food poisoning, or a migraine. The key difference: flu comes with a raised temperature, CO poisoning does not. The classic symptoms are:

At high concentrations, CO causes unconsciousness and death within minutes. At lower, chronic exposures, symptoms can persist for days or weeks, often being mistaken for a persistent viral illness.

The Key Warning Sign — Symptoms That Improve Outside

The most important indicator that CO — rather than illness — is the cause is this: symptoms improve significantly when you leave the building and return when you come back inside. If everyone in the household (including pets) seems unwell at the same time, and if the symptoms ease when you spend time outdoors, take this very seriously.

Pets — particularly birds — are often affected before humans because they are smaller and more sensitive to CO. If your pets appear unusually ill or lethargic, do not dismiss it.

Warning Signs in Your Home

As well as physical symptoms, look out for these signs that a gas appliance may be producing CO:

• Yellow or orange flame on a gas boiler or fire instead of the normal crisp blue flame
• Black, sooty marks on or above gas appliances — a sign of incomplete combustion
• Pilot light keeps going out on a gas fire or boiler
• More condensation than usual on windows, particularly near appliances
• Appliances seem to be working harder than normal or making unusual sounds

What Causes Carbon Monoxide in the Home?

Any fuel-burning appliance can produce carbon monoxide if it malfunctions, is not properly maintained, or is used in a poorly ventilated space. The most common sources in UK homes are:

• Gas boilers — especially older models that have not been serviced regularly
• Gas fires and wood-burning stoves — blocked flues or chimneys are a common cause
• Gas cookers and hobs — using them for heating a room is particularly dangerous
• Portable generators or barbecues used indoors or in garages
• Blocked flues or chimneys — preventing combustion gases from escaping outside
• Faulty or incorrectly installed appliances — work done by an unregistered person is a leading cause

An annual boiler service includes a combustion analysis that checks whether the boiler is burning gas cleanly and efficiently. This is one of the most important reasons to have your boiler serviced every year.

Carbon Monoxide Alarms — Essential, Not Optional

A CO alarm is the only reliable way to detect carbon monoxide before you experience symptoms. They cost from around £20 and should be fitted:

• In every room where there is a gas, oil, or solid fuel appliance
• Near the boiler (but not directly above it)
• In bedrooms if you have a gas fire, gas boiler, or flue running through a bedroom wall
• At breathing height — around head height on a wall or on a shelf

Since December 2022, CO alarms are a legal requirement in all rooms with a fixed combustion appliance in rented properties in England. Even if you own your home, fitting one is one of the lowest-cost, highest-impact safety measures you can take.

Replace CO alarms according to the manufacturer's instructions — most have a 5-10 year lifespan. Press the test button monthly to check the alarm is working.

What to Do If Your CO Alarm Sounds

1. Get everyone out of the building immediately — do not stop to collect belongings
2. Leave the door open behind you to ventilate the property as you leave
3. Call 999 if anyone is displaying symptoms of CO poisoning
4. Call 0800 111 999 (National Gas Emergency, free 24/7) to report the alarm
5. Do not re-enter until emergency services say it is safe
6. Seek medical attention even if you feel fine — CO poisoning can have delayed effects

Frequently Asked Questions