Carbon Monoxide: Symptoms, Warning Signs and What to Do

What carbon monoxide actually is

Carbon monoxide — CO — is a poisonous gas produced when a fuel such as gas, oil, coal or wood burns without enough air. It has no smell, no colour and no taste, which is exactly what makes it dangerous: your senses give you no warning at all. Breathed in, it stops your blood carrying oxygen around your body, and at high concentrations it can be fatal.

Any fuel-burning appliance can produce it when something is wrong — a boiler with a cracked heat exchanger, a blocked or leaking flue, a gas fire starved of ventilation. An appliance burning cleanly produces very little; a faulty one can fill a room while looking, to the eye, completely normal. That is why the rest of this guide is about the indirect evidence: what the appliance shows you, what your body tells you, and what an alarm is for.

Warning signs on the appliance

Because you cannot sense the gas itself, watch the appliance instead. Four signs are worth taking seriously:

• A lazy yellow or orange flame where there should be a crisp blue one.
• Soot or dark staining on or around the appliance, or scorch marks near the casing.
• A pilot light that keeps going out for no obvious reason.
• More condensation than usual on the windows of the room the appliance is in.

None of these proves carbon monoxide is present — a decorative gas fire is designed to burn with yellow flames, for example — but on a boiler, cooker or conventional fire they all say the same thing: the burn is not right. Stop using the appliance and have it checked by a Gas Safe registered engineer.

Symptoms of carbon monoxide poisoning

The early symptoms are easy to mistake for a virus or plain tiredness:

• Headache — the most common symptom
• Dizziness and feeling or being sick
• Tiredness, confusion and difficulty thinking clearly
• Breathlessness
• At higher levels, collapse and loss of consciousness

It feels like flu, with one telling difference: carbon monoxide does not give you a temperature. Two more clues point away from a bug. Symptoms tend to ease when you are away from home and come back when you return, and everyone in the household — pets included — tends to be affected at the same time. If that pattern sounds familiar, do not wait for it to get worse. Follow the steps below and get medical advice the same day.

If your alarm sounds or you suspect poisoning

1. Stop using your fuel-burning appliances and open doors and windows on your way out.
2. Get everyone out into fresh air — and stay out.
3. Call the National Gas Emergency Service free on 0800 111 999. The line is open 24 hours a day.
4. Get medical help: see a doctor or call 111 if anyone feels unwell, and call 999 if anyone has collapsed or lost consciousness.
5. Do not use the appliance again until a Gas Safe registered engineer has inspected it and confirmed it is safe.

Do not stay inside to hunt for the source, and do not assume the alarm is faulty. Treat every activation as real.

Carbon monoxide alarms: cheap, and not optional

A working alarm is the only reliable way to detect the gas. Look for an audible alarm certified to BS EN 50291 — decent ones cost less than £20. Fit one in every room with a fuel-burning appliance, follow the fitting instructions on the box, and press the test button regularly. If you are not sure where yours should go, ask: we position CO alarms for customers free on any visit.

For landlords this is a legal duty, not advice. Since October 2022, landlords in England must fit a carbon monoxide alarm in any room used as living accommodation that contains a fixed combustion appliance — gas cookers are the one exclusion — and must repair or replace any alarm reported as faulty.

A real example from Edgbaston

We put a combustion analyser on every repair for a reason, and one job from our Edgbaston work is the clearest example we can give. The callout was for an ignition lockout — on paper, a routine repair. As we wrote it up at the time: "An Ideal Logic L2 fault in Edgbaston turned out to be more than a lockout: combustion readings showed dangerous carbon monoxide levels. We made the boiler safe immediately, traced the fault to the gas valve and injector, fitted replacements and fully recommissioned it — exactly why a proper analyser matters on every repair."

The customer's complaint was a boiler that would not stay lit. The carbon monoxide only showed up on the analyser — which is precisely the point.

Prevention: servicing, not luck

Almost every carbon monoxide problem is caught the same way: during planned maintenance, with proper combustion readings, before anyone was harmed. An annual boiler service is the single most effective protection there is — a service is where cracked heat exchangers, blocked flues and incomplete combustion get caught, and ours starts at £95. For landlords, the annual landlord gas safety certificate (CP12) covers the legal side from £90, with every gas appliance and flue checked.

To be clear about the boundary: an alarm is your last line of defence, not a substitute for maintenance — and a service is not a substitute for an alarm. You want both.

Written by Saeed Iqbal, the engineer who leads Synergy Gas & Plumbing (Gas Safe registered 594385). Last reviewed August 2026.