A small cut, a big reaction
It begins innocently enough: halve an onion, peel away the skin, make a few cuts. Then comes the sting. Before long, you are blinking through tears at a vegetable that never touched your eyes.
The explanation is a miniature chemical factory that starts running when the onion is damaged. In an intact bulb, its cells keep reactive ingredients largely separated. A knife breaks those cells open. Their contents mix, enzymes begin to work, and a volatile sulphur-containing compound escapes into the air. When it reaches your eyes, they respond by making tears.¹
The famous irritant has a formidable name: syn-propanethial-S-oxide. It is often called the onion's lachrymatory factor, from a word meaning tear-producing. The tears are not caused by a cloud of tiny onion pieces alone, nor by the knife somehow drawing moisture from your eyes. They are an ordinary protective response to a chemical the onion makes as it is cut.¹
What happens inside the onion
An onion stores sulphur-containing precursors in its tissues. When cutting ruptures the cells, an enzyme called alliinase acts on these compounds, producing an intermediate known as 1-propenesulphenic acid. A second enzyme, lachrymatory-factor synthase, then helps convert that intermediate into syn-propanethial-S-oxide.²
That second step was a scientific surprise. For years, researchers thought the tear-inducing substance formed spontaneously after alliinase had done its work. In 2002, a team led by Shinsuke Imai showed that a distinct enzyme was needed. This discovery made the chain reaction much clearer: damage, mixing, an initial chemical change, then a dedicated route to the lachrymatory factor.²
The compound is volatile, so some of it leaves the freshly cut onion and travels through the air. On reaching the surface of an eye, it irritates sensitive tissue. The lacrimal glands respond by releasing tears, which help wash the irritant away. It is much the same reason your eyes water when they encounter other mild airborne irritants: the body is trying to protect a delicate surface.¹
This is also why chopping garlic usually feels different, even though garlic and onions are close culinary relatives with plenty of sulphur chemistry. The particular lachrymatory factor responsible for the familiar onion tears is characteristic of onions. Similar family membership does not mean a cut clove and a cut bulb send the same chemical mixture towards your eyes.¹
The bulb is not aiming at cooks. These chemical reactions belong to the onion's broader response to tissue damage and may help deter things that would eat it. The fact that human eyes happen to be sensitive to one of the products is an unfortunate kitchen encounter, not a purpose built into the plant for making people cry.²
Why one onion can feel worse than another
Anyone who cooks regularly has met a fierce onion and a comparatively gentle one. Onions differ in variety and growing conditions, and those differences affect their sulphur chemistry and pungency. How much tissue you break and how the released material reaches your face matter too.¹
There is also more going on than an invisible vapour drifting upwards. In a 2025 experiment, researchers filmed onions being cut with high-speed imaging. A blade could launch tiny liquid droplets as it compressed and broke the layers. Blunter blades and faster cuts produced more droplets and gave them more energy. The study measured droplet release under controlled conditions; it does not provide a simple prediction of how much any particular person will cry at home.³
That distinction matters when kitchen folklore turns into confident rules. Chilling an onion is often recommended, but the cutting study found that temperature's effect on droplet release was difficult to isolate from differences between samples. Different tricks may alter chemistry, airflow or the mechanics of cutting, and the strength of evidence is not the same for all of them.³
Can we cut onions without tears?
A sharp knife is a sensible place to start. It crushes less tissue than a blunt edge and, in the controlled cutting experiment, reduced the spray of droplets. Cutting gently helped too. Moving air away from your face, such as with a kitchen extractor, can reduce the amount of irritant reaching your eyes. Close-fitting eye protection forms a more direct barrier. None of these changes guarantees a completely tear-free chopping session, but each addresses a part of the route from onion to eye.¹ ³
Could plant breeders remove the problem altogether? When researchers suppressed lachrymatory-factor synthase in experimental onion plants, the bulbs released far less tear-inducing compound after injury. Yet their sulphur chemistry changed in other ways too. A low-tear onion is therefore a real biological possibility, but removing one reaction does not necessarily leave the rest of its flavour exactly as it was.⁴
The next time an onion makes you blink, the sequence is happening on two scales at once. Inside the bulb, broken cells release ingredients for an enzyme-driven reaction. At your eye, a trace of its airborne product sets off a protective reflex. The tears are your body's simple answer to an elaborate bit of onion chemistry.