Carbon monoxide (CO) is a gas you can't see, smell or taste. Most CO alarms only tell you something when they sound. This guide explains what CO levels in ppm mean in a home, where CO comes from, the signs to look out for, and where to put a CO alarm or monitor.
What is carbon monoxide?
Carbon monoxide is a poisonous gas made when fuels such as gas, oil, coal and wood don't burn fully. The Health and Safety Executive describes it as colourless, odourless and tasteless, which is why you can't rely on your senses to notice it.
In a home, CO usually comes from an appliance that burns fuel and isn't working or venting properly: a boiler, gas fire, cooker, wood burner or open fire.
Carbon monoxide levels in ppm: what's normal and what's high?
CO is measured in parts per million (ppm): how many parts of carbon monoxide there are in a million parts of air. A reading of 10 ppm means 10 parts CO for every million parts of air.
What's normal in a home?
Outdoor air contains a tiny amount of CO, around 0.1 ppm, according to the UK Health Security Agency (UKHSA). Indoors, levels are usually low too, but they rise when fuel is burned. UKHSA notes that studies of UK kitchens have measured weekly averages of roughly 0.3 to 2.4 ppm, with higher short-term peaks.
What's a safe level of carbon monoxide?
There isn't one single "safe" number, because what matters is both how high the level is and how long you're breathing it. The World Health Organization (WHO) sets indoor air guidelines based on time:
| Time spent breathing it | WHO indoor guideline | Roughly in ppm |
|---|---|---|
| 15 minutes | 100 mg/m³ | 87 ppm |
| 1 hour | 35 mg/m³ | 31 ppm |
| 8 hours | 10 mg/m³ | 9 ppm |
| 24 hours | 7 mg/m³ | 6 ppm |
Converted from mg/m³ at 25 °C. UKHSA uses the same 8-hour guideline (10 mg/m³, about 8.6 ppm).
So a level that's within the guideline for a few minutes can be above it if it carries on all day or overnight. That's the main reason it helps to see the number, not just wait for a sound.
CO levels chart: what the HALDEN colours mean
The HALDEN monitor shows the CO level from 0 to 999 ppm and changes colour as it rises:
| Reading | Colour | What the monitor does |
|---|---|---|
| 0–50 | Green | Normal range. The green light pulses gently and there's no sound. You can still see the exact number. |
| 51–100 | Yellow | A yellow warning shows on screen and the 85 dB alert sounds. |
| 101–150 | Orange | An orange warning shows on screen, the light flashes and the alert sounds. |
| 150+ | Red | A red warning shows on screen. The alert can't be silenced and the screen stays on. |
See the number, any time you look
HALDEN plugs into a standard UK socket and shows CO in ppm, plus temperature and humidity, on a 2-inch colour screen. No app or Wi-Fi.
Shop the monitorWhere carbon monoxide comes from at home
The NHS says household appliances used for heating and cooking can produce CO if they're not installed properly, are faulty or are poorly maintained. Common sources include:
- Gas boilers, including combi boilers in kitchens and utility rooms
- Gas fires and gas heaters
- Gas cookers and hobs
- Wood burners, log burners and open fires, especially with a blocked chimney or flue
- Oil and solid fuel boilers and range cookers
- Barbecues, generators and engines used in or near enclosed spaces, such as a garage
Blocked flues, chimneys and air vents can all stop fumes from escaping properly.
Warning signs and symptoms
Signs on your appliances
The Health and Safety Executive lists these signs that a gas appliance may not be working properly:
- Yellow or orange flames instead of crisp blue ones
- Soot or yellow-brown staining on or around the appliance
- A pilot light that keeps going out
- More condensation than usual on the inside of windows
If you notice any of these, stop using the appliance and have it checked by a Gas Safe registered engineer.
Symptoms of carbon monoxide
According to the NHS, symptoms can include a headache, dizziness, feeling or being sick, feeling weak, tiredness and confusion, chest and muscle pain, and shortness of breath. They can be easy to mistake for flu or tiredness, and may ease when you leave the room or the house. The NHS page on carbon monoxide poisoning has full guidance.
A number you can check is easier to act on than a feeling you can't place.
Why a carbon monoxide alarm with a display helps
A standard CO alarm without a screen has two states: quiet, or sounding. That's useful, but it doesn't tell you what's happening in between.
A CO alarm or monitor with a digital ppm display shows the actual level in the room. That means you can:
- See low readings that are below the level where an alert sounds, for example after the heating comes on or while you're cooking
- Spot patterns, such as a reading that creeps up whenever one particular appliance is running
- Check at a glance as you walk past, without pressing anything
- Show an engineer exactly what you saw and when
HALDEN also shows temperature and humidity on the same screen, which is handy for spotting cold or damp rooms through the winter.
Where to put a carbon monoxide alarm or monitor
The NHS recommends a CO alarm in each room with a fuel-burning appliance. When choosing the spot:
- Same room as the appliance. Put it in the room with the boiler, gas fire, cooker or wood burner.
- Not too close. Keep it 1 to 3 metres from the appliance, so it isn't reacting to the burst of fumes when the appliance first lights.
- Away from draughts. Avoid spots next to windows, doors, extractor fans and air vents, which can dilute the reading.
- Away from direct heat and steam. Not directly above a cooker, kettle or sink.
- Out in the open. Not inside a cupboard or behind curtains or furniture, where air can't reach it.
With a plug-in monitor like HALDEN, that means choosing a wall socket in the right room that meets these points. If the only free socket is mounted upside down, hold the bell button to flip the display so it reads the right way up.
Rooms people often forget
- Utility rooms, where many boilers are fitted
- Living rooms with a gas fire, open fire or log burner
- Bedrooms, so you can check the level at night
- Garages and workshops with heaters or engines
- Holiday lets and annexes, where guests won't know the appliances
How many carbon monoxide alarms do you need?
Count the rooms with a fuel-burning appliance, then add any bedrooms you'd like to cover. For most homes that's two to four: typically the kitchen or utility room with the boiler, plus a living room with a fire or wood burner.
That's why HALDEN comes in packs of 1, 2, 4 and 8. The more you buy, the lower the price for each one, down to £29.50 each in an 8-pack.
Keeping carbon monoxide down all year
- Service appliances every year. Have boilers, fires, stoves and water heaters serviced annually by a qualified, registered engineer. Gas work must be done by a Gas Safe registered engineer.
- Sweep chimneys and check flues. Have chimneys swept and flues inspected at least once a year if you use them.
- Keep vents clear. Don't block air vents, airbricks or flues, even to stop draughts.
- Keep fuel-burning equipment outside. Barbecues, generators and engines belong outdoors, away from windows and doors.
- Test your monitor monthly. On HALDEN, short press the top button. The alert sounds and the light flashes.
Common questions
What is a safe level of carbon monoxide in a home?
There's no single safe number, because time matters as much as level. WHO's indoor guidelines are about 9 ppm over 8 hours and about 31 ppm over 1 hour. Most homes read close to 0 ppm most of the time.
What ppm of carbon monoxide is dangerous?
Risk rises with both the level and how long you're exposed. Short spikes are more of a concern the higher they go, and lower levels become a concern if they carry on for hours. That's why seeing the reading, not just hearing an alert, is useful.
Can you smell carbon monoxide?
No. CO has no smell, colour or taste. A smell of gas is a different problem: natural gas has an odour added so you can notice leaks, but CO doesn't.
What's the difference between a CO alarm and a CO monitor?
Both sound an alert if CO rises. A monitor with a display also shows you the level in ppm all the time, so you can see low readings and changes before anything sounds.
Do I need a carbon monoxide alarm with a combi boiler?
Yes. A combi boiler burns gas, so it's a possible source of CO. Put one in the room where the boiler is fitted.
Where should a CO alarm go with a log burner?
In the same room as the log burner, 1 to 3 metres away from it, out of direct heat and away from draughts.
How many CO alarms do I need?
One in each room with a fuel-burning appliance. Many homes need two to four.
Know what's happening at home
The HALDEN carbon monoxide monitor shows CO in ppm, temperature and humidity on one colour screen. Free tracked UK delivery, 30-day returns and a lifetime warranty.
Choose your packSources: NHS: Carbon monoxide poisoning · Health and Safety Executive: Carbon monoxide · UKHSA: Carbon monoxide toxicological overview · WHO Housing and Health Guidelines · Energy UK: Carbon monoxide. This guide is general information about carbon monoxide levels and doesn't replace advice from a qualified engineer or medical professional.