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Carbon monoxide (CO) has no colour, no smell and no taste. It doesn’t irritate the eyes or throat. Yet it kills hundreds of people every year in homes with faulty boilers, blocked chimneys and generators run indoors, and it’s a leading cause of poisoning deaths in many countries. Its danger comes from a simple piece of chemistry: CO binds to the iron in haemoglobin far more tightly than oxygen does.
Where carbon monoxide comes from
Carbon monoxide is produced by incomplete combustion of carbon-containing fuels: natural gas, coal, wood, charcoal, petrol, diesel, oil and paraffin.
When there’s plenty of oxygen, fuels burn completely to carbon dioxide and water. For methane (natural gas):
CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion)
When oxygen is limited, some carbon is only partly oxidised:
2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete combustion)
(With even less oxygen, soot, which is carbon, forms too.) In CO, carbon has an oxidation state of +2, compared with +4 in CO₂ (see how to find oxidation numbers).
Common causes:
- gas boilers, fires and cookers that are poorly maintained or badly ventilated;
- blocked flues and chimneys;
- running generators, barbecues or car engines in enclosed spaces such as garages, tents or boats;
- fires in buildings (smoke contains large amounts of CO).
A properly burning gas flame is blue. A yellow or orange flame, or soot marks around an appliance, can signal incomplete combustion.
The molecule
CO has a triple bond between carbon and oxygen, and a lone pair on each atom. It’s isoelectronic with nitrogen, N₂ (see isoelectronic species). Unusually, the lone pair on carbon is the one that bonds to metals, and CO is an excellent ligand for transition metals like iron. Carbon monoxide’s density is very similar to air’s (molar mass 28 g mol⁻¹ compared with about 29 for air), so it mixes evenly through a room rather than collecting at the floor or ceiling.
How CO poisons: binding to haemoglobin
Oxygen is carried by haemoglobin, whose haem groups contain iron(II) ions that bind O₂ (see haemoglobin: how blood carries oxygen).
Carbon monoxide binds to exactly the same site on the iron, forming carboxyhaemoglobin (COHb). Two things make this deadly:
1. Extremely strong binding
Haemoglobin’s affinity for CO is about 200–250 times greater than its affinity for O₂. This is partly because CO is a better ligand for iron: it both donates electrons to the iron and accepts electron density back from the iron’s d orbitals into its own empty orbitals, making a strong bond.
As a result, even small concentrations of CO in air can tie up a large fraction of haemoglobin. As a rough guide, breathing air containing just 0.1% CO for long enough can convert about half of a person’s haemoglobin to carboxyhaemoglobin, a potentially fatal level.
(Interestingly, free haem binds CO tens of thousands of times more strongly than O₂. The protein around the haem in haemoglobin partly suppresses CO binding by forcing it to bind at a slight angle, which reduces the affinity to “only” a few hundred times that of O₂. Without this, even the small amount of CO the body produces naturally would be a problem.)
2. The remaining oxygen is held more tightly
When CO occupies one or two of haemoglobin’s four sites, it shifts the molecule towards its high-affinity (relaxed) state. The remaining haem groups then bind their oxygen more tightly and release it less readily in the tissues. The oxygen dissociation curve shifts to the left.
So CO both reduces how much oxygen the blood carries and reduces how much of that oxygen is delivered. Someone with 50% carboxyhaemoglobin is in far more danger than someone with anaemia who has half the normal amount of haemoglobin.
3. Effects inside cells
CO also binds to other haem proteins, including myoglobin in muscle (including the heart) and cytochrome c oxidase, the last enzyme of the electron transport chain, further impairing the use of oxygen in cells. The brain and heart, which need the most oxygen, are hit hardest.
Symptoms
Symptoms depend on the carboxyhaemoglobin level and the person:
| COHb (approx.) | Typical effects |
|---|---|
| Below 3% | Normal in non-smokers |
| 5–10% | Common in heavy smokers; usually no obvious symptoms |
| 10–20% | Headache, tiredness, shortness of breath on exertion |
| 20–30% | Severe headache, dizziness, nausea, confusion |
| 30–50% | Fainting, fast heart rate, collapse |
| Above 50% | Seizures, coma, death |
Early symptoms are easily mistaken for flu or food poisoning. Warning signs include symptoms that improve when away from home, several people or pets in the same building feeling ill, and symptoms that are worse when an appliance is running.
Survivors of severe poisoning can have lasting neurological problems, such as memory loss and difficulty concentrating.
Treatment
The first step is to get the person into fresh air and call emergency services.
Treatment works by displacing CO with oxygen. The binding is reversible, so increasing oxygen shifts the equilibrium (see Le Chatelier’s principle):
HbCO + O₂ ⇌ HbO₂ + CO
- Breathing ordinary air, carboxyhaemoglobin falls with a half-life of roughly 4 to 6 hours.
- Breathing 100% oxygen through a tight-fitting mask shortens this to roughly 1 to 1.5 hours.
- Hyperbaric oxygen (100% oxygen at two to three times normal pressure in a special chamber) shortens it further, to around 20–30 minutes, and may be used in severe cases.
Detecting carbon monoxide
Because people can’t sense CO, carbon monoxide alarms are essential in homes with fuel-burning appliances. Most household alarms use an electrochemical cell: CO diffusing into the sensor is oxidised to CO₂ at one electrode, producing a small current proportional to the CO concentration (see oxidation and reduction). When the concentration, averaged over time, exceeds safe limits, the alarm sounds.
Medical staff measure CO exposure with a CO-oximeter, which uses the different light absorption spectra of oxy-, deoxy- and carboxyhaemoglobin. An ordinary finger pulse oximeter can’t tell carboxyhaemoglobin from oxyhaemoglobin, so it can show falsely normal readings in CO poisoning.
Carbon monoxide in the body and in history
Surprisingly, your body makes small amounts of carbon monoxide itself, when old haem groups are broken down, and at very low levels CO acts as a signalling molecule. Smokers carry far more, because tobacco smoke contains CO, which is one reason smoking reduces fitness and harms unborn babies. Historically, coal gas (town gas), which contained a large proportion of CO, was piped into homes before natural gas replaced it, and domestic CO poisonings were far more common as a result.
Prevention
- Have fuel-burning appliances, flues and chimneys checked regularly by qualified technicians.
- Fit a CO alarm in rooms with fuel-burning appliances and near sleeping areas.
- Never use generators, barbecues or outdoor heaters indoors or in tents, and never run engines in closed garages.
- Keep vents and air bricks unblocked.
Key takeaways
- Carbon monoxide forms by incomplete combustion when fuels burn with too little oxygen.
- It binds haemoglobin’s Fe²⁺ about 200–250 times more strongly than oxygen, forming carboxyhaemoglobin.
- It also left-shifts the oxygen dissociation curve, so remaining oxygen isn’t released to tissues, and it inhibits cytochrome c oxidase.
- Treatment uses high-concentration or hyperbaric oxygen to displace CO; electrochemical alarms detect it.
- For more on how oxygen is carried, see haemoglobin, and for the element itself, see carbon.
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