A chemical booby trap
Why Does Chopping Onions Make You Cry?
A cut onion mixes two harmless chemicals into an irritating gas that reaches your eyes.
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Step 1 of 5
A whole onion cannot hurt you.
Hold one to your face and nothing happens. The two ingredients are stored in separate compartments inside the cells, kept safely apart.
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Step 2 of 5
Cutting is what mixes them.
The blade breaks cell walls, an enzyme meets a sulphur compound, and the reaction makes a small, light molecule that drifts upward as gas.
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Step 3 of 5
Your eye treats it as an attack.
The gas dissolves in the wet film on your eye and forms a mild acid. Nerves in the surface detect it and demand an immediate rinse.
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Step 4 of 5
This is a defence aimed at animals, not you.
The gas discourages things that dig up and chew bulbs. Onions are not defending themselves against cooks — we are collateral damage.
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Step 5 of 5
Cold and water both help.
A chilled onion releases the gas more slowly, and a sharp blade tears fewer cells. Cutting near running water gives the gas somewhere else to dissolve.
Onion varieties bred to be sweet and mild are lower in the sulphur compound, so they sting less as well as tasting milder.
The short version
Cutting an onion mixes two stored chemicals into a rising gas that turns mildly acidic on your wet eye, so your eye floods to rinse it.
Try it yourself
Chop half an onion straight from the fridge and half after an hour on the counter. The warm half is far worse.
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