Why carbon monoxide is so easy to miss

Carbon monoxide (CO) is often called the silent killer because it has no colour, taste or smell. It is produced when fuels such as gas, oil, wood, coal or petrol burn without enough oxygen. In a typical UK home, the main risks come from boilers, gas fires, cookers, water heaters and solid-fuel stoves, especially when an appliance is faulty, poorly maintained or the flue or chimney is blocked. Unlike a gas leak, which you might smell, CO gives you almost no warning. That is why understanding the signs, symptoms and alarm requirements matters for every household.

Warning signs around your appliances

Look and listen for changes in how your fuel-burning appliances behave. A healthy gas flame should be crisp and blue. A lazy, yellow or orange flame, or one that lifts away from the burner, can indicate incomplete combustion. Soot or black staining around an appliance, boiler or fire is another red flag. So is a pilot light that keeps blowing out, or a boiler that needs resetting repeatedly.

You may also notice more condensation than usual on windows and walls near an appliance, or a strange, stale or oily smell when a fire or boiler is running. Solid-fuel appliances can produce smoke or fumes that linger in the room. If you spot any of these signs, stop using the appliance and arrange for a Gas Safe registered engineer or qualified solid-fuel technician to inspect it.

Symptoms in people and pets

Carbon monoxide affects the body by replacing oxygen in the blood. Early symptoms are easy to mistake for flu, tiredness or a bad cold. They include headaches, dizziness, nausea, breathlessness, and feeling weak or confused. Unlike a virus, these symptoms often improve when you leave the house and return when you come back. That pattern is a powerful clue.

Pay attention to everyone in the household, including pets. Animals cannot tell you they feel unwell, but they may become drowsy, vomit, or seem unsteady. If several people in the same home develop similar symptoms at the same time, or if symptoms ease when you are away for a few hours, treat it as a possible CO incident. Children, older people, pregnant women and those with heart or lung conditions are especially vulnerable.

Alarm requirements and placement

A carbon monoxide alarm is your most important line of defence. In the UK, CO alarms should meet the British Standard BS EN 50291 and carry a British or European approval mark, such as Kitemark or CE/UKCA. Choose an audible alarm rather than a colour-change patch. Alarms have a limited lifespan, usually five to ten years, so check the expiry date and replace the unit when needed.

Fit an alarm in every room that contains a fuel-burning appliance, and in any room where you sleep. For a boiler in a cupboard or a garage, place the alarm nearby but not directly above the appliance. For a wall-mounted alarm, follow the manufacturer's instructions, but as a general rule, position it at head height, one to three metres from the appliance, and not in a dead air space such as a corner or behind furniture. Test your alarms weekly by pressing the test button, and vacuum the vents gently every few months to keep dust out.

If you rent your home, your landlord is legally required to provide a CO alarm in rooms with solid-fuel appliances, and many landlords also fit them near gas appliances. Check that yours is present, working and in date. If not, report it to your landlord or letting agent in writing.

Simple habits that reduce risk

Prevention is mostly about maintenance and ventilation. Have every gas appliance serviced annually by a Gas Safe registered engineer, and ask for a copy of the service record. Solid-fuel appliances and chimneys need sweeping at least once a year, more often if you burn wood or coal regularly. Never block vents, air bricks or flues, and keep outdoor vents clear of leaves, nests and snow.

Use appliances only for their intended purpose. Do not use a gas cooker to heat a room, and never run a barbecue, camping stove or portable generator indoors, in a garage or in a tent. If you have an open fire, make sure the chimney draws properly and that the room has enough ventilation. Installing an audible CO alarm is quick, inexpensive and gives you precious time to act.

What to do if you suspect carbon monoxide

If your alarm sounds or you notice symptoms, act immediately. Open doors and windows for fresh air. Turn off the appliance if you can do so safely, and do not use it again until it has been checked. Leave the building and take everyone with you, including pets. Call the National Gas Emergency Service on 0123456789 if you smell gas or suspect a gas leak, or contact your fuel supplier for solid-fuel or oil emergencies.

Seek medical advice promptly. Tell your GP or hospital staff that you suspect carbon monoxide exposure, even if you feel better after getting fresh air. Do not return to the property until a qualified engineer has inspected the appliance and confirmed it is safe. If you rent, inform your landlord immediately and ask for a written record of the inspection. Ask your engineer for a CO check at the next annual service.