Why do onions make you cry?

Here is the strange thing about an onion: sitting whole on the counter, it is completely harmless. You can hold it to your face and nothing happens. Yet the instant your knife breaks the skin, it begins, in seconds, to manufacture a tiny cloud of what is essentially chemical weaponry, and aim it at your eyes. That is not an accident or a nuisance quirk of vegetables. It is a trap, deliberately built, and it only springs when something starts cutting.

01 · The setupA trap that only fires when broken

The key to the whole thing is that an intact onion keeps its ingredients apart. In one place it stores a harmless sulfur-containing molecule, a precursor with the unlovely name isoalliin (chemists also call it PRENCSO). In a completely separate compartment of the cell, it keeps an enzyme called alliinase. As long as those two never meet, nothing happens. The onion is inert, patient, loaded.

Then you cut it. Your knife ruptures the cell walls and smashes the compartments together. Precursor meets enzyme, and the trap fires. This “keep the two halves apart until something attacks” design is exactly what makes it a defence: an undisturbed onion spends no energy and releases nothing, but the moment a tooth or a blade damages the tissue, the reaction ignites right at the wound.

02 · The chain reactionTwo enzymes, fired in sequence

Here’s where most people’s mental model is wrong. It isn’t one step, it’s two. Alliinase acts on isoalliin and produces an unstable intermediate called 1-propenylsulfenic acid. For decades, chemists assumed that this intermediate then simply fell apart on its own into the tear gas, no further help required.

It doesn’t. In 2002, a Japanese team led by Shinsuke Imai showed there is a second enzyme doing the crucial work. They named it lachrymatory factor synthase, or LFS, and it grabs the sulfenic acid intermediate and deliberately rearranges it into the volatile compound that stings your eyes: syn-propanethial-S-oxide. So the onion’s tear gas is not a spontaneous accident of chemistry. It is a manufactured product, built by a dedicated enzyme whose only job is to make you cry.

03 · The deliveryHow the gas reaches your eyes

The reason onions get you and, say, a cut potato doesn’t, is that syn-propanethial-S-oxide is volatile: it lifts straight off the cutting board into the air and drifts upward, conveniently, toward your face. When it lands on the wet surface of your eye, it dissolves into the tear film and reaches the sensory nerves packed into your cornea.

There it trips an irritant sensor called TRPA1, the same channel that fires for wasabi, raw mustard and actual riot-control tear gas. Your brain doesn’t interpret this as “sulfur chemistry.” It interprets it as burning pain, an attack on the eye, and it responds the only way it knows how to clear a chemical off a delicate surface: it opens the taps. The tears are not an emotion. They are a decontamination flush.

04 · The controlWhy garlic never makes you cry

Garlic is the perfect natural experiment, because it starts almost identically and ends up completely different. Crush a clove and its own alliinase fires on its own sulfur precursor, exactly as in an onion. But garlic has no LFS. There is no second enzyme waiting to rearrange the intermediate into tear gas.

So the pathway simply stops one step earlier. Instead of being converted into a lachrymatory factor, garlic’s sulfenic acids condense together into allicin, the pungent, aromatic compound behind garlic’s smell and flavour. Same opening move, missing final step, wildly different result: garlic assaults your nose, never your eyes. It’s the cleanest possible proof that the tears are LFS’s doing specifically, not just “sulfur chemistry” in general.

05 · The kitchenWhy the old tricks actually work

Once you understand the mechanism, every folk remedy suddenly makes sense, and you can see which ones are real. Chilling the onion works because chemical reactions slow down in the cold, so a fridge-cold onion builds and releases the volatile factor more sluggishly. A sharp knife works because it slices cells cleanly instead of crushing and rupturing far more of them than necessary: less damage, less gas. Cutting under running water works because the factor is water-soluble, so much of it dissolves away before it ever reaches the air.

And the most reliable trick of all is the least clever: goggles. The problem is a gas reaching your eyes, so a sealed barrier solves it outright. Swimming goggles, which form an airtight seal, are genuinely excellent for this. Ordinary glasses help a bit but leak. None of these are magic. They all just lower the dose that makes it from board to cornea.

06 · The purposeWhat the onion is actually defending against

Step back and the design looks less like a nuisance and more like armour. Onions, garlic and their Allium relatives evolved this sulfur chemistry as a chemical defence system. The lachrymatory factor and its cousins deter animals from eating the plant, and the broader family of organosulfur compounds it belongs to helps the plant fight off soil microbes and fungi. Making the irritant only on damage is the elegant part: the plant carries a loaded weapon at almost no running cost, and only pays to fire it at the exact moment and place it is attacked.

Which reframes the whole experience at your cutting board. You are not the victim of a badly designed vegetable. You are triggering a defence mechanism that evolved to punish exactly what you are doing to it: tearing it apart.

07 · The switchTearless onions, and an Ig Nobel Prize

Here’s the real payoff of finding LFS. If the tear gas made itself, there would be nothing to target. But because it is the product of one specific enzyme, you can simply remove that enzyme, and the onion loses its ability to cry-bomb you while keeping its flavour. Turn LFS down and the sulfur pathway doesn’t shut off, it diverts, flowing toward flavour compounds (thiosulfinates) instead of the irritant. Onion taste, no sting.

That is exactly how tearless onions exist. Researchers have made gene-silenced tearless onions in the lab, and conventional breeders have produced tearless varieties by selecting for low LFS, most famously Japan’s House Foods “Smile Ball,” an onion bred to be so mild and sweet you can eat it raw without a tear. And in a fittingly cheerful footnote, Imai’s team went on to win the 2013 Ig Nobel Prize in Chemistry, the award for research that first makes you laugh and then makes you think, for proving that the humble onion’s tear gas is, of all things, a made-to-order product with an off switch.