Carbon monoxide (CO) is an invisible, odourless, and tasteless gas that kills around 50 people in the UK every year and causes thousands of hospitalisations. It's produced when fuels don't burn completely, often from faulty or poorly maintained gas appliances.
Do you have working CO alarms in your property?
Let's make sure you're protected.
What is carbon monoxide?
Carbon monoxide (CO) is a toxic gas produced when carbon-based fuels burn incompletely. Unlike natural gas (which has a distinctive smell added), carbon monoxide is:
- Invisible — you cannot see it
- Odourless — you cannot smell it
- Tasteless — you cannot taste it
- Non-irritating — it doesn't cause immediate discomfort
This makes CO extremely dangerous. You cannot detect it with your senses, and symptoms of poisoning can be mistaken for flu or food poisoning.
Carbon monoxide is called "the silent killer" because you cannot see, smell, or taste it. The only way to detect it is with a working CO alarm.
How does carbon monoxide kill?
When you breathe in carbon monoxide, it binds to haemoglobin in your blood — the protein that normally carries oxygen around your body. CO binds much more strongly than oxygen, preventing your blood from transporting oxygen to vital organs.
Even small amounts of CO exposure over time can cause serious harm. High concentrations can kill in minutes.
Sources of carbon monoxide
CO can be produced by any fuel-burning appliance that isn't working properly:
Common sources:
- Gas boilers and central heating systems
- Gas fires and heaters
- Gas cookers and ovens
- Solid fuel appliances (wood burners, coal fires, stoves)
- Oil-fired boilers and heaters
- Portable gas heaters
- Barbecues used indoors or in enclosed spaces
- Car engines running in attached garages
Risk factors that increase CO production:
- Poor installation of gas appliances
- Lack of maintenance or servicing
- Blocked or damaged flues and chimneys
- Inadequate ventilation
- DIY work on gas appliances
- Using appliances not designed for UK gas supply
- Using camping equipment indoors
Never use barbecues, camping stoves, or portable gas heaters indoors or in tents. Never run petrol or diesel engines in garages or enclosed spaces. These can produce lethal levels of CO in minutes.
Symptoms of carbon monoxide poisoning
CO poisoning symptoms are often mistaken for other illnesses, particularly flu, food poisoning, or viral infections.
Early symptoms (low-level exposure):
- Headache (often the first sign)
- Dizziness and nausea
- Tiredness and confusion
- Stomach pain
- Shortness of breath
- Difficulty concentrating
Severe symptoms (high-level exposure):
- Loss of consciousness
- Seizures
- Confusion and disorientation
- Loss of balance and coordination
- Chest pain
- Severe breathlessness
- Death
A key indicator of CO poisoning: symptoms improve when you leave the property and return when you come back. If multiple people in a property feel unwell at the same time, consider CO poisoning.
Long-term effects
Even if you survive CO poisoning, prolonged exposure can cause:
- Permanent brain damage
- Heart problems
- Neurological issues
- Memory loss and difficulty concentrating
- Personality changes
Family of four hospitalised after boiler fault
A family in Manchester experienced headaches, nausea, and tiredness over several days. They initially thought it was a winter bug affecting everyone.
- ✗Gas boiler had a cracked heat exchanger
- ✗Flue was partially blocked by a bird's nest
- ✗No carbon monoxide alarm in the property
- ✗Boiler hadn't been serviced for 3 years
- ✗Symptoms mistaken for seasonal illness
The father eventually called 999 after becoming confused and disoriented. All four family members required hospital treatment. The youngest child spent two days in intensive care. They were lucky to survive.
Carbon monoxide poisoning often mimics other illnesses. A working CO alarm and annual boiler servicing could have prevented this near-tragedy. The family now has multiple CO alarms and their boiler is serviced annually.
Legal requirements for CO alarms
Landlords (England)
Under the Smoke and Carbon Monoxide Alarm (England) Regulations 2015, private sector landlords must:
- Install at least one CO alarm in any room used as living accommodation that contains a fixed combustion appliance (excluding gas cookers)
- Ensure alarms are working at the start of each new tenancy
- Repair or replace alarms when notified of faults
Penalties: Local authorities can issue fines up to £5,000 for non-compliance.
Scotland, Wales, and Northern Ireland have similar but separate regulations. If you're a landlord outside England, check the specific requirements for your nation.
New builds
Building Regulations Part J require carbon monoxide alarms in:
- Rooms containing solid fuel burning appliances
- Rooms containing fixed combustion appliances (in new dwellings from October 2022)
These alarms must be mains-powered with battery backup.
Social housing
Social landlords have additional duties under the Homes (Fitness for Human Habitation) Act 2018, which includes ensuring properties are free from excess cold and have adequate heating without dangerous emissions.
Homeowners
While homeowners aren't legally required to have CO alarms (except where Building Regulations apply), it's strongly recommended if you have:
- A gas boiler or heating system
- Gas fires or appliances
- Solid fuel burning appliances (wood burners, coal fires, stoves)
- Oil-fired heating
Legal vs Recommended CO Alarm Requirements
Legal Minimum (Landlords)
- •One alarm per room with combustion appliance
- •Must be working at tenancy start
- •Replace when notified of faults
- •Covers living accommodation only
- •Battery or mains-powered acceptable
Best Practice (All Properties)
Recommended- •Alarm in every room with fuel-burning appliance
- •Alarm in every bedroom or hallway near bedrooms
- •Monthly testing by occupants
- •Annual professional appliance servicing
- •Replace alarms every 5-10 years
- •Mains-powered with battery backup preferred
Bottom line: Meeting the legal minimum is essential for landlords, but best practice offers much better protection for everyone. The cost of extra alarms (£15-40 each) is trivial compared to the risk.
Types of carbon monoxide detectors
Battery-powered alarms
Pros:
- Easy to install anywhere
- No electrical work needed
- Portable
- Typically cost £15-30
Cons:
- Batteries need replacing (usually annually)
- Can be disabled if batteries removed
- May not wake heavy sleepers
Best for: Renters, supplementary protection, properties without convenient electrical outlets
Mains-powered alarms
Pros:
- Don't need battery replacement (though backup batteries do)
- Can't be easily disabled
- Often louder than battery models
- Typically cost £30-60
Cons:
- Requires electrical installation
- Fixed location
- More expensive upfront
Best for: Landlords, permanent installations, new builds (required by Building Regulations)
Smart/connected alarms
Pros:
- Send alerts to your phone
- Can integrate with smart home systems
- Remote monitoring
- Useful for landlords or holiday homes
- Typically cost £50-100
Cons:
- More expensive
- Require WiFi connection
- May need app setup
- Battery life concerns for wireless models
Best for: Tech-savvy users, landlords managing multiple properties, second homes
What to look for when buying
- British or European standards: Look for EN 50291 certification
- Electrochemical sensor: More accurate and reliable than biomimetic sensors
- Clear display: Shows CO levels, not just an alarm
- Test button: Essential for regular testing
- End-of-life warning: Alerts you when the sensor needs replacing
- Loud alarm: At least 85 decibels
Don't buy the cheapest alarm you can find. A quality CO alarm with an electrochemical sensor costs £20-40 and could save your life. Replace the alarm when it reaches its expiry date (usually 5-10 years).
Placement and installation
Where to install CO alarms
Essential locations:
- In every room containing a fuel-burning appliance (boiler, gas fire, wood burner, etc.)
- In bedrooms or hallways near sleeping areas (CO poisoning often occurs at night)
Height guidelines:
- For rooms with fuel-burning appliances: Install at head height (1.5-2m from floor), 1-3 metres from the appliance
- For bedrooms/hallways: Can be ceiling-mounted or at head height
Avoid:
- Directly above or beside the appliance (may give false readings)
- In bathrooms, kitchens with high steam, or garages (humidity and fumes can trigger false alarms)
- Near doors, windows, or vents where draughts might affect readings
- Corners or behind furniture where air circulation is poor
Installation tips
- Read the manufacturer's instructions — specific requirements vary by model
- Test immediately after installation using the test button
- Record the installation date on the alarm or in your maintenance log
- Note the expiry date and set a reminder to replace it
- Keep the user manual for reference
CO Safety Maintenance Schedule
If your alarm has a digital display, glance at it to check it's functioning
Press the test button and ensure the alarm sounds loudly
Gas Safe registered engineer must service all gas appliances
Professional sweep for solid fuel appliances (more often if used heavily)
Check manufacturer guidance - sensors have limited lifespan
Testing and replacement
How to test your CO alarm
- Press and hold the test button (usually for 3-5 seconds)
- The alarm should sound loudly (85+ decibels)
- If it doesn't sound, try replacing the battery
- If still no sound, replace the entire alarm
Test frequency: Monthly for all alarms
When to replace batteries
- Standard alkaline batteries: Replace annually, or when the low battery warning sounds
- Sealed lithium batteries: Should last the life of the alarm (5-10 years)
- Mains-powered with battery backup: Replace backup battery according to manufacturer instructions (usually every 1-2 years)
Never ignore a low battery warning. A CO alarm without power provides no protection. Replace batteries immediately or replace the entire alarm if batteries are sealed.
When to replace the entire alarm
Replace your CO alarm:
- When it reaches the expiry date (printed on the back or base)
- If the end-of-life warning sounds
- If it fails a test and replacing the battery doesn't fix it
- If it's more than 10 years old (even if no expiry date is shown)
- After CO exposure (the alarm may be damaged)
Most CO alarms last 5-10 years. The sensor gradually degrades over time, even if the alarm hasn't been triggered.
What to do if your CO alarm sounds
Never ignore a CO alarm. Even if you feel fine, dangerous levels of carbon monoxide may be present. Always treat an alarm as real until proven otherwise.
Immediate actions
- Stop what you're doing — don't investigate or try to find the source
- Get everyone out — open doors and windows as you leave if safe to do so
- Call the emergency number — From outside, call the Gas Emergency Service (0800 111 999) or fire brigade (999)
- Go to fresh air — Move to an open area and stay there
- Don't re-enter until the property has been declared safe
If someone shows symptoms
- Call 999 and tell them you suspect CO poisoning
- Get the person into fresh air immediately
- If unconscious, follow first aid guidance (recovery position if breathing)
- Tell the ambulance crew about the CO alarm and any gas appliances
After the alarm
- Don't use gas appliances until a Gas Safe registered engineer has checked them
- Get a Gas Safety Record (CP12) showing the appliances are safe
- Consider installing additional CO alarms
- Keep windows open to ventilate until cleared
CO alarms can give false positives (caused by fumes, humidity, or malfunction), but you must always treat them as real. The risk of ignoring a genuine alarm is too great.
Maintenance of gas appliances
Preventing CO poisoning starts with maintaining fuel-burning appliances properly.
Annual gas safety checks
All gas appliances should be serviced annually by a Gas Safe registered engineer:
- Gas boilers and central heating
- Gas fires and heaters
- Gas cookers (if servicing recommended)
- Any other gas appliances
For landlords: This is a legal requirement under the Gas Safety (Installation and Use) Regulations 1998.
Signs your appliance may be producing CO
Look out for:
- Yellow or orange flames (should be mostly blue)
- Soot or staining around the appliance
- Pilot lights that frequently blow out
- Increased condensation in the room
- The appliance burns with a lazy flame or makes unusual noises
If you notice any of these signs, stop using the appliance immediately and contact a Gas Safe registered engineer. Do not wait until your next scheduled service.
Chimney and flue maintenance
For solid fuel appliances:
- Have your chimney swept at least annually (more if you use it heavily)
- Check flues and chimneys for blockages (birds' nests, debris)
- Ensure adequate ventilation when using the appliance
- Never block air vents or ventilation grilles
DIY work prohibition
Never attempt DIY work on gas appliances. It's illegal for anyone who isn't Gas Safe registered to work on gas appliances, pipes, or meters. Penalties include prosecution, fines, and imprisonment.
Carbon monoxide statistics (UK)
Understanding the scale of the problem:
- Around 50 deaths per year in England and Wales from accidental CO poisoning
- Approximately 4,000 people attend A&E each year with CO poisoning symptoms
- 200 serious injuries per year requiring hospitalization
- 40% of UK homes have no carbon monoxide alarm
- 60% of rental properties had no CO alarm before regulations (improved since 2015)
Most common scenarios:
- Faulty gas boilers (40% of cases)
- Portable gas heaters and barbecues used indoors (20%)
- Blocked flues and chimneys (15%)
- Solid fuel appliances (15%)
- Other sources including car exhaust (10%)
Peak times:
- Winter months see increased cases (more heating use)
- Post-Christmas spike (new heating appliances, blocked chimneys from heavy use)
CO Safety Cost Guide
The cost of protecting your property from carbon monoxide. Compare the small investment versus the potentially catastrophic consequences.
Annual costs include battery replacement (£5-10/year for battery alarms) and gas safety checks (£60-100/year). Alarms should be replaced every 5-10 years (£15-100 depending on type).
Frequently asked questions
No, electric appliances don't produce carbon monoxide because they don't burn fuel. You only need CO alarms if you have gas, oil, solid fuel, or other combustion appliances. However, if you have a gas cooker or any other fuel-burning appliances, you still need CO alarms.
No, carbon monoxide alarms only detect CO. They will not alert you to natural gas (methane) leaks. Natural gas has a distinctive 'rotten egg' smell added to it so you can detect leaks by smell. If you smell gas, open windows, don't use electrical switches or flames, evacuate, and call the Gas Emergency Service (0800 111 999).
Install the alarm at head height (1.5-2m from the floor), between 1-3 metres horizontally from the boiler. Don't place it directly above or beside the boiler. Follow the manufacturer's specific instructions as requirements can vary.
Most CO alarms last 5-10 years. The sensor degrades over time even if not triggered. Check the expiry date on your alarm (usually printed on the back) and replace it when it expires. Many modern alarms have an end-of-life warning that alerts you when replacement is needed.
No. CO can build up in one room while other rooms remain safe. You need a CO alarm in every room that contains a fuel-burning appliance. Best practice is also to have alarms in or near bedrooms, as CO poisoning often occurs at night when people are asleep.
Different beep patterns mean different things. A continuous alarm means CO detected - evacuate immediately. Intermittent beeps usually indicate low battery or end of life. Check the user manual. Single chirps every 30-60 seconds typically mean low battery. Replace the battery or alarm as appropriate.
Yes. In England, landlords must install a CO alarm in any room used as living accommodation that contains a fixed combustion appliance (except gas cookers). The alarm must be working at the start of each tenancy. Scotland, Wales, and Northern Ireland have similar regulations with some differences.
Key differences include sensor type (electrochemical is best), lifespan, accuracy, features (digital display, memory, smart connectivity), and build quality. Budget alarms (£15-20) provide basic protection. Mid-range (£30-50) offer better sensors and features. Premium (£50-100+) add smart home integration and remote alerts. All should meet EN 50291 standard, but quality varies.
No. Only use the test button. Cigarette smoke tests the smoke detector's ability to detect particles, not the CO sensor. CO alarms have specific electrochemical sensors that only respond to carbon monoxide. Some alarms may be damaged by smoke exposure.
Carbon monoxide has roughly the same density as air, so it mixes evenly throughout a room rather than rising or sinking. This is why CO alarm placement isn't as critical as smoke alarm placement (smoke rises), but you should still follow manufacturer guidelines for optimal detection.
Next steps
Make sure you're meeting all your gas safety obligations:
Gas Safety Certificate Guide →
Check whether you're responsible for gas safety in your property:
Concerned about carbon monoxide or want your gas appliances checked? A Gas Safe registered engineer can inspect your system, service appliances, and install CO alarms in the correct locations.
Related articles:
Useful tools:
Emergency contacts:
- Gas Emergency Service: 0800 111 999
- NHS Advice: 111
- Emergency services: 999