The Chemistry of Carbon Monoxide

It is likely that you have heard of carbon monoxide (CO), and quite likely in the context of carbon monoxide poisoning, claiming up to 20,000 lives per year globally. But what is it? Where does it come from? And what is its danger?


What it is

Time for a little bit of Chemistry. You’ve certainly heard of the other compound, carbon dioxide (CO2), a common atmospheric gas that sounds similar. The key difference is in the prefix to ‘oxide’. In CO, the prefix is mon, meaning one- there is one oxygen atom bonded to a carbon atom in the compound carbon monoxide. In carbon dioxide, di means two, and there are two oxygen atoms (also indicated by the 2 after the O in its chemical formula, CO2) bonded to the central carbon atom.

CO2 and CO
Diagram of CO2 (left) and CO (right), with carbon atoms in black and oxygen atoms in red, images sourced from Wikimedia Commons

Its physical properties include being colourless, odourless and tasteless … meaning that it is possible to walk into a room with lethal amounts of CO and not notice until you lost consciousness or experienced other effects.


Where it comes from

The most common source of CO is from burning things in areas where there is limited oxygen available. Normally when carbon-chain substances (think of oils, petrol, alcohols, etc…) burn in the presence of sufficient oxygen, the process is termed complete combustion and produces carbon dioxide (CO2) and water (H2O) by the following chemical equation.

Fuel + O2 –> CO2 + H2O

However, if there is not enough oxygen, carbon monoxide (CO) is formed through the process of incomplete combustion because each molecule of it has only one oxygen atom rather than two and therefore it requires less oxygen to form.

Fuel + O2 –> CO + H2O

This lack of oxygen commonly occurs when something is burning in an enclosed area, or if there is a large fire. In this situation, when the combustion first starts, there is plenty of oxygen and the combustion is complete. However, as the fire progresses, the amount of available oxygen decreases as it is consumed and incomplete combustion takes over as the primary combustion process.


The dangers

The dangers of CO are not from impacts on climate or pollution. Instead, CO acts as a dangerous poison on inhalation. Because of its physical and chemical similarity to oxygen (see below table), CO can bind to haemoglobin (the protein in red blood cells that transports oxygen around your body). In fact, CO has a 230 times greater affinity for binding to haemoglobin than oxygen has. This higher bonding strength is a result of how CO bonds to the iron atom in haemoglobin compared to the O2 molecule, due to the different electron arrangements and charges within the CO bond. Under normal conditions, this is a useful effect because it allows oxygen to be easily dropped off to cells rather than being retained by haemoglobin.

CO-O2 table
Table comparing some chemical and physical properties of O2 and CO. Note the similarities between the two compounds in their shape and weight, factors that mean both compounds can bond to the active site of haemoglobin (where active sites of proteins are often highly specific to only certain molecules, meaning that there must be an extremely high degree of similarity to allow a different molecule than the target molecule).

However, if your body’s oxygen carrying proteins (haemoglobin) are binding carbon monoxide, then they aren’t carrying oxygen to your important cells and tissues. But the problem gets worse than that. Because the haemoglobin-CO bond is so much stronger than the oxygen bond, CO will also displace and remove already bound O2, further depriving your body of oxygen. Additionally, the haemoglobin won’t release the CO to body tissues, meaning that it is continually bound and cannot remove CO2 buildup from tissues (as an aside, your cells create CO2 as a by-product from breaking down food compounds to release energy).

So very rapidly, your cells become severely oxygen deprived and build up toxic levels of CO2. This oxygen deprivation is known as hypoxia and if exposed to enough CO, this poisoning can kill in as little as 3 minutes- similar to drowning. Even if the person is rescued rapidly, they can sustain permanent brain and other organ damage.

Part of the danger is because the concentration of CO in the air doesn’t actually have to be very high to cause terrible damage and rapid acute poisoning (air containing less than 3% CO can be fatal within 5 minutes). This is because it binds in such great preference to oxygen … resulting in a lack of oxygen transport to cells. However, the high binding strength also means that it is very difficult to reverse the process. This means an individual can be rescued and the CO source removed, but still have so much inactive haemoglobin that they still die or sustain serious damage.

To properly treat an exposed individual, they must be removed from the CO source and then provided with extra oxygen. In this situation, there is so much extra oxygen that it overcomes the high binding strength of CO to haemoglobin and the CO can gradually become unbound and normal O2/CO2 transport can resume.

However, this process does take some time to occur and fatality can still result from oxygen deprivation. To try and counter this, some researchers are working on an antidote for CO poisoning. Their antidote involves injecting a mutated version of a different protein, neuroglobin, into the bloodstream that binds CO up to 500 times more tightly than haemoglobin.

Symptoms of CO poisoning in lower concentrations (such that it may take some time for serious effects to occur) can have side effects such as dizziness (as the brain becomes oxygen deprived), nausea, headaches and general flu-like symptoms. These concentrations can occur from household appliances, such as badly functioning heaters or stoves. Motor vehicle exhaust can also contain dangerous levels of CO if it is run in an enclosed space (ie. a garage) for a long period of time.


Anyway, that’s just your daily dose of Chemistry and some interesting information about a deadly gas. I hope you enjoyed it!


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