Slice into an onion and you may discover that the vegetable has opinions about the situation.
Within seconds, your eyes sting.
Then they water.
Then you are chopping dinner while apparently mourning something terrible.
The onion isn’t deliberately trying to ruin your evening, obviously.
But the chemical reaction causing your tears really is part of a biological defence system.
And crucially, the substance irritating your eyes is largely created after you start cutting.
Cutting an Onion Starts a Chemical Chain Reaction
An intact onion keeps different chemical ingredients separated inside its cells.
Your knife destroys that careful arrangement.
When the cells are damaged, enzymes meet sulfur-containing compounds that had previously been kept apart.
That starts a rapid series of chemical reactions.
One particularly important enzyme is called lachrymatory factor synthase.
It helps produce the onion’s lachrymatory factor, a volatile sulfur compound known as propanethial S-oxide.
“Lachrymatory” essentially means tear-producing.
Which is refreshingly honest naming.
Because the compound is volatile, it can leave the damaged onion and travel through the air.
Unfortunately, your eyes are often directly above the chopping board.
Why Does the Gas Make Your Eyes Water?
The surface of your eyes is protected by a thin layer of tears and is packed with sensitive nerve endings designed to detect potentially harmful irritants.
When the onion’s volatile chemicals reach that surface, they trigger irritation.
Your nervous system responds by telling the tear glands to increase production.
The goal is straightforward: dilute and wash away the irritant.
So the tears are not the onion somehow drawing water directly out of your eyes.
They are your own defence system responding to the onion’s defence system.
It is basically a tiny chemical arms race happening over a chopping board.
Why Does an Onion Even Have This System?
Plants have a significant evolutionary problem.
They cannot run away.
If an animal, insect or microorganism starts eating them, their defensive options need to be built in.
Onions and their relatives use sulfur chemistry as part of that defence.
Damage the plant’s tissues and previously separated compounds can rapidly combine to create pungent and biologically active chemicals.
Those reactions contribute to the characteristic flavours and smells of onions, garlic and other members of the Allium family.
In an onion, one branch of that chemistry produces the eye-irritating lachrymatory factor.
So describing the onion as “fighting back” is slightly dramatic.
But chemically speaking, it isn’t entirely unfair.
Why Do Some Onions Make You Cry More Than Others?
Not every onion produces exactly the same experience.
Variety, growing conditions, storage and the onion’s chemical composition can all influence how pungent it is.
Your chopping technique matters too.
A blunt knife crushes and damages more cells as it forces its way through the onion.
A very sharp knife makes cleaner cuts and can reduce the amount of tissue being unnecessarily smashed apart.
It won’t turn an onion into a pacifist, but it can help.
Does Chilling an Onion Help?
It can.
Cooling the onion before cutting can slow chemical reactions and reduce the volatility of compounds escaping from it.
That means less of the irritant may reach your eyes as quickly.
Good ventilation can help for an even simpler reason: moving air carries the volatile compounds away from your face.
What about chopping onions underwater, wearing goggles or setting up elaborate kitchen contraptions involving fans?
Those can work too.
At some point, however, you do have to ask whether dinner deserves laboratory safety equipment.
Do You Eventually Become Immune?
Not exactly.
Someone who chops onions constantly may become less bothered by the sensation or learn techniques that reduce exposure, but the underlying chemistry still works.
If enough lachrymatory factor reaches the surface of your eyes, your body is still capable of responding with tears.
The Bottom Line
Onions make you cry because cutting them destroys cells and starts a chemical reaction.
That reaction produces a volatile sulfur compound called propanethial S-oxide, which reaches your eyes and irritates sensitive nerve endings.
Your tear glands then start flushing the irritant away.
So the onion is not releasing stored tear gas.
You are effectively manufacturing it by chopping the onion apart.
Which, from the onion’s point of view, is perhaps fair enough.
FAQ
What chemical in onions makes you cry?
The main tear-producing compound is propanethial S-oxide, also known as the onion lachrymatory factor.
Why doesn’t a whole onion make you cry?
The chemicals and enzymes responsible are largely kept separated inside intact cells. Cutting the onion damages those cells and allows the reaction to begin.
Does a sharp knife stop onion tears?
It may reduce them because a sharp blade can damage fewer cells than a blunt one, but it will not eliminate the chemical reaction completely.
Does putting an onion in the fridge stop it making you cry?
Chilling can reduce the effect by slowing chemical reactions and reducing how readily volatile compounds escape, although it is not guaranteed to stop tears entirely.
Further Reading
Why Does Coffee Make You Poop? It Isn’t Just the Caffeine
Why Does Spicy Food Feel Hot When It Isn’t? Your Brain Is Being Tricked
Sources
Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa :: https://pmc.ncbi.nlm.nih.gov/articles/PMC5693239/