Carbon Monoxide Warning Signs

How to recognise possible carbon monoxide exposure, what to do immediately, and how to reduce the risk in your home.

Last updated: 2026-10-19

Carbon monoxide (CO) is colourless and odourless, which is what makes it dangerous. It is produced by incomplete combustion — usually a faulty, poorly maintained or incorrectly ventilated gas appliance.

This is safety information, not a diagnosis. If you suspect CO, act immediately.

Warning signs in people

Symptoms are often described as flu-like, without a fever:

• headache
• dizziness
• nausea or vomiting
• tiredness or confusion
• shortness of breath
• chest pain

Symptoms commonly improve when away from the property and return when back inside. This pattern is a strong warning sign.

Warning signs in the appliance

• A yellow or orange flame where it should be crisp blue
• Soot or black staining around the appliance or on the wall near the flue
• A pilot light that blows out repeatedly
• Condensation on the inside of windows when the appliance is running
• A CO alarm sounding

What to do immediately

1. Stop using the appliance and turn it off if safe to do so.
2. Get everyone, including pets, out of the property into fresh air.
3. Do not use switches, naked flames, or anything that could ignite.
4. If anyone has symptoms, seek urgent medical help — call 999 and say you suspect carbon monoxide.
5. For a gas-related emergency, call the National Gas Emergency Service on 0800 111 999.
6. Do not use the appliance again until it has been checked by a Gas Safe registered engineer.

Reducing the risk

• Have gas appliances serviced annually by a Gas Safe registered engineer.
• Fit an audible CO alarm — but remember an alarm is a backstop, not a substitute for servicing.
• Keep ventilation clear; never block air bricks or vents.
• Never use a gas cooker or oven to heat a room.
• Never fit or repair a gas appliance yourself.

Getting the appliance checked

Once everyone is safe, an engineer can inspect the appliance, flue and ventilation. See our gas services page, or read about what happens at a boiler service.

Never rely on a website to diagnose a gas appliance. If in doubt, treat it as an emergency.