Everything else in this domain is about making heat. This page is about not dying while you do it, and it comes first for a reason: carbon monoxide kills more people in power cuts and cold snaps than the cold does, and it kills the prepared as readily as the unprepared.
The people it kills are not careless. They are cold, tired, and reasoning that just this once, for a little while, with the window open a crack, will be fine.
Why this page comes before the stoves
Carbon monoxide has a combination of properties that no other household hazard has:
- You cannot detect it. No colour, no smell, no taste, no irritation.
- The symptoms are ones you would ignore. Headache and tiredness in a cold house in a crisis is what everybody has anyway.
- It impairs your judgement before it disables you. By the time something is wrong, the faculty you would use to work that out is already affected. People have died a few steps from an unlocked door.
- It kills you asleep, with no waking. The commonest fatal case.
- It affects the smallest and weakest first. Children, anyone pregnant, older people and pets show it earliest. A poorly pet is sometimes the first sign.
Every combustion appliance produces it. A perfectly running stove burning clean, dry wood with good airflow produces it. The question is never whether there is CO, only whether it is being carried away faster than it accumulates.
What CO actually does to you
It binds to haemoglobin, the protein that carries oxygen in your blood, and it binds roughly 200 times more readily than oxygen does. Once bound, that haemoglobin is out of service. You suffocate with your lungs working normally and plenty of oxygen in the room.
The measure is carboxyhaemoglobin, the percentage of your blood's carrying capacity taken out of action.
The row that matters most is the second one. Impaired judgement arrives before incapacity. That is why "I would notice and go outside" is not a plan: the part of you that would notice is affected before the part that would collapse.
It also clears slowly. Breathing ordinary air, it takes around 4 to 6 hours to shed half of it. That is why someone exposed overnight is still unwell the next day, and why leaving the room is a first step rather than a cure.
Recognising it
Since you cannot sense the gas, you have to recognise the pattern.
The single most useful diagnostic sign: it is flu without a fever, and it gets better when you leave the house.
If several people in a household, or the household and the pets, all feel unwell at once with headaches and tiredness and nobody has a temperature, and everyone improves after an hour outdoors and worsens again on returning, that is carbon monoxide until proven otherwise. Nothing else behaves like that.
Other things worth noticing around any burning appliance:
- Sooty or yellow-brown staining on or around the appliance.
- A lazy yellow or orange flame where it should be crisp and blue, on any gas appliance.
- Smoke that will not draw, or that rolls back into the room when you open a stove door.
- Excessive condensation on windows in the room with the appliance.
- A pilot light that keeps going out.
Your alarm is slower than you think
Fit a CO alarm certified to BS EN 50291. It is genuinely one of the highest-value items you can buy. Then understand exactly what it does, because most people assume it is a gas detector and it is not.
Placement, per BS EN 50292: between 1 metre and 3 metres from the potential source, horizontally. Not directly above the appliance. CO is close to the same density as air and moves with it, so ceiling height is fine, roughly head height is fine, and inside a cupboard or behind furniture is not.
Put one in every room with a burning appliance, and one where people sleep. In a crisis where everyone has moved into one heated room, that room needs an alarm, whatever it had before, because the whole household is now sleeping next to the only fire in the house.
Test it monthly. Note that sensors expire, typically after 7 to 10 years, and an expired alarm gives no warning that it has stopped protecting you. Write the replacement date on the casing in marker.
What never comes indoors
How much ventilation is actually enough
For an appliance genuinely designed for indoor use, the honest answer is: what the manufacturer specifies, delivered by permanent openings that cannot be closed by someone cold in the night.
That last part is where it goes wrong. Ventilation you have to remember to maintain is ventilation that will be shut at 3am.
- A cracked window is not ventilation. It provides one opening, which lets air move in and out of the same gap without exchanging the room's air. You need two openings at different heights, low for air in and high for air out, so there is a through-path.
- A permanent air brick or fixed vent is worth far more than any window, precisely because nobody can close it.
- Never block, tape over or stuff an existing air brick or vent, however draughty it feels. They were fitted because an appliance needs them. Blocking vents to keep warm is a recognised route to a fatality.
- A working flue is the primary ventilation for any solid-fuel appliance, and its condition matters more than any window. See flue sweeping in this domain.
- Extractor fans can make it worse. A powerful cooker hood or bathroom fan can pull enough air out of a sealed modern house to reverse the draught in a flue and drag combustion gases back into the room. If you run a fire and an extractor together, open a window on the far side of the house.
Modern British homes are the harder case, not the easier one. They are built airtight for efficiency, so there is far less accidental air exchange than in an older draughty house, and the same appliance is correspondingly more dangerous.
If the alarm sounds
Do not investigate. Do not open windows first. Do not look for the source.
- Get everybody out, immediately. People and animals, straight outside, into fresh air.
- Do not go back in for possessions, phones or pets you cannot reach on the way out.
- Once outside, count everybody.
- Ring 999 if anyone is unwell, drowsy, confused or unconscious. In normal times the gas emergency number is 0800 111 999 for a suspected gas or CO problem.
- Do not re-enter until the source is identified and dealt with, and the building has been thoroughly aired.
- Anyone who felt unwell needs medical attention, even if they feel fine outside. Carboxyhaemoglobin takes hours to clear, damage can be delayed, and there are treatments that work far better early. Tell whoever assesses them that you suspect CO, specifically, because it is easily mistaken for flu.
Anyone unconscious: get them out if you can do so without entering the atmosphere yourself, and start CPR outside if they are not breathing. Do not enter a space that has already dropped someone. That is how the second casualty happens, and it happens in a very high proportion of these incidents.