Carbon monoxide: why it is so dangerous

In complete combustion each carbon atom joins with two oxygen atoms to form carbon dioxide (CO₂). If oxygen is lacking, or the flame is cooled, for example by a cold cooking pot or poor air supply, the reaction stops halfway: carbon monoxide (CO), with only one oxygen atom, is formed. In an enclosed space a fire also gradually uses up the oxygen, so combustion becomes more and more incomplete.

CO is about as heavy as air and therefore spreads through the whole room rather than collecting specifically at the floor or ceiling. It is colourless, odourless and tasteless. The smell you notice with smoke or exhaust comes from other substances; an appliance or a charcoal barbecue can give off a lot of CO without any noticeable smell.

Oxygen is carried in the blood by the red pigment haemoglobin. CO binds to exactly the same spot, and many times more strongly than oxygen – of the order of around 200 times. That is why even small amounts of CO in the air are enough to occupy a large share of the transport sites over time. The CO only comes off again slowly.

The organs that need the most oxygen suffer first: the brain and heart. The first signs are headache, dizziness, nausea, tiredness and drowsiness. Because they resemble flu or an upset stomach, they are often misinterpreted. An important clue: several people or pets are affected at the same time, and it gets better outside in the fresh air.

Another danger is that CO clouds thinking. Those affected become drowsy and confused and may no longer recognise the danger in time. This is especially treacherous during sleep, when the signs go unnoticed altogether.

CO poisonings become more frequent during power cuts and cold snaps, when people improvise heating or cooking: with a charcoal barbecue in the flat, an unflued gas heater or a generator in the basement or garage. Even with a window or garage door open this is dangerous, because the gas can build up faster than it escapes.

When camping, a stove or a smouldering barbecue in a closed tent is a known danger. In a car buried in snow with the engine running, exhaust can enter the cabin if the exhaust pipe is blocked by snow. At home, faulty or poorly maintained stoves, boilers and fireplaces can release CO.

The most important rule: never run barbecues, charcoal, gas stoves, unflued heaters or generators in living spaces, basements, garages, tents or vehicles – not even with a window open. Have fireplaces and heating appliances checked regularly by professionals.

In rooms with fuel-burning appliances, a CO alarm installed and tested according to the manufacturer's instructions protects you. A smoke alarm does not detect CO; they are two different devices. If CO is suspected – the alarm sounds or several people have a headache, dizziness or nausea – get everyone into fresh air immediately, do not go back in, and call the local emergency number (112 in the EU). Only return once the fire service or professionals have checked the room.

Carbon monoxide and carbon dioxide are often confused because the names sound similar. We breathe out carbon dioxide (CO₂) ourselves all the time; in large amounts it displaces oxygen, but in normal amounts it is harmless. Carbon monoxide (CO), on the other hand, is toxic even in small amounts because it blocks oxygen transport in the blood.

A common misconception is that you can recognise CO by the smoke. Smoke only shows that something is burning incompletely – but CO can form without any visible smoke at all, for example from glowing charcoal or a gas appliance. You cannot rely on your own senses here; a working CO alarm is far more reliable.