Why Onions Make You Cry, and the Science-Backed Way to Stop It

Everyone has a trick. Chill the onion. Hold a spoon in your mouth. Chew gum. Light a candle. Wear swimming goggles. Most of them get passed down with total confidence and very little evidence.

Here’s the surprising part: a whole onion sitting in your veg rack doesn’t contain the chemical that makes you cry. It builds it on the spot, in a split second, the moment your knife breaks its cells. And in 2025, a team of scientists pointed high-speed cameras at onions being sliced and found that how you cut matters far more than most kitchen tricks.

The onion’s booby trap

Onions are part of the Allium family, along with garlic, leeks and chives. They’re packed with sulphur compounds, and they keep the ingredients for their defence system in separate compartments inside their cells. While the cells are intact, nothing happens.

Cut into the onion and those compartments are smashed open. The ingredients mix, and a rapid chain reaction begins:

1. An enzyme called alliinase breaks down sulphur-containing compounds (sulphoxides) in the onion.
2. That produces an unstable chemical, 1-propenesulfenic acid.
3. A second enzyme, lachrymatory factor synthase, quickly rearranges it into syn-propanethial-S-oxide.

That last one is the culprit. It’s a volatile liquid, which means it readily drifts into the air as vapour, and it’s a lachrymatory agent: a substance that triggers stinging and tears when it reaches your eyes (>Wikipedia: syn-Propanethial-S-oxide).

“Lachrymatory” comes from the Latin for tears. Tear gas belongs to the same broad group of irritants, which tells you the onion isn’t messing about.

The 2002 discovery that rewrote the textbook

For years it was assumed alliinase did the whole job by itself. Then in 2002, a team of Japanese researchers led by scientists at the food company House Foods (with colleagues from the University of Tokyo and Kyoto University) published a short paper in Nature titled “An onion enzyme that makes the eyes water” (>Imai et al., Nature, 2002).

They showed that a separate enzyme, the lachrymatory factor synthase, was needed to produce the tear-causing compound. That mattered because it suggested a possibility: an onion that lacks that enzyme might keep its flavour without making you cry.

Why your eyes react

Your eyes are covered with sensitive nerve endings. When the propanethial-S-oxide vapour lands on them, those nerves fire, your eyes sting, and your tear glands kick in to flush the irritant away. Crying over an onion is your body doing its job.

The 2025 study: it’s not just the gas, it’s the spray

Chemists had worked out what makes you cry. What nobody had properly measured was how it gets from the board to your face.

In October 2025, researchers at Cornell University published a study in the journal PNAS using high-speed cameras filming at up to 20,000 frames per second. They fitted different blades to a custom guillotine and sliced onions at different speeds (>Wu et al., PNAS, 2025).

What they found was genuinely surprising:

• An onion fires out a mist. Each layer of an onion has a tough skin on both sides. As a blade presses down, it squashes the soft tissue underneath and builds up pressure. When the blade finally breaks through, that pressure releases a burst of tiny droplets.
• The mist is fast. The droplets left the cut at an estimated 5 to 40 metres per second, far faster than the roughly 1 metre per second the team measured from professional chefs’ knives (>Cornell Chronicle).
• It travels. Droplets were thrown up to about two-thirds of a metre into the air, which is roughly the distance between a chopping board and your face.
• Blunt blades are far worse. The bluntest blades in the test produced up to 40 times more droplets than the sharpest ones. Faster cutting produced about four times more than slower cutting (>PNAS).

The reason is simple once you see it. A blunt knife squashes before it cuts, so more pressure builds up and more onion juice gets blasted out when the skin finally gives way.

One honest caveat

The study measured droplets, not tears. It didn’t test whether people cried less using sharp knives, as >Smithsonian Magazine points out. But fewer droplets carrying the irritant towards your eyes is a very sensible bet.

How to cut onions with fewer tears

Based on what the chemistry and the 2025 study show, here’s what makes sense, starting with the best-supported:

1. Sharpen your knife

This is the big one. A sharp, thin blade cuts cells cleanly instead of crushing them, which means less mist. If your knife squashes a tomato rather than slicing it, it’s too blunt for onions too. A cheap sharpener and a couple of minutes’ practice may help more than any trick on this list.

2. Slow down

Fast chopping looks impressive, but the study found faster cuts threw out roughly four times as many droplets. Gentle, steady slices are your friend.

3. Keep your face back

The droplets travelled up to about 60–70cm. Standing up straight, rather than hunching over the board, puts a bit more distance between the spray and your eyes.

4. Airflow helps

Since the irritant is a vapour, moving air away from you makes sense. Cutting near an open window or under a running cooker hood is a common tip. We couldn’t find a controlled study on it, so file it under “logical, not proven”.

5. Chilling the onion: plausible, not proven

Lots of chefs swear by a spell in the fridge before cutting. The idea is that cold slows the chemical reactions and makes the compound less likely to evaporate. That fits basic chemistry, but we didn’t find a solid study testing it on tears, so treat it as worth a try rather than a guarantee.

What about the spoon, the bread and the candle?

Holding a spoon or bread in your mouth, lighting a candle, chewing gum: we couldn’t find good evidence for any of them. If one works for you, carry on. Just don’t expect science to back you up.

Told straight

Onions make you cry because cutting them triggers a two-enzyme chain reaction that creates an irritant gas, and, as we learned in 2025, the onion physically sprays it at you. The best-supported fix isn’t a trick at all: a properly sharp knife and slow, steady cuts. Goggles work too, if you don’t mind looking ridiculous.

Sources checked October 2026. Where a tip is common advice rather than tested, we’ve said so.