Why onions make you cry – and how to stop it
By Dan Kauna, July 3, 2026Onions are a vital ingredient in almost every Kenyan dish, from a rich beef stew to a fresh plate of kachumbari.
But despite how often people handle them, slicing this common bulb almost always ends in unexpected tears.
Most people assume that the strong aroma triggers the crying, but it is actually a fast-acting chemical reaction that happens the exact moment your knife cuts down into the layers.
Chemistry behind your kitchen tears
When you chop an onion, the blade slices through the vegetable and breaks open its microscopic cells. This physical damage forces an enzyme called alliinase to mix with amino acid sulfoxides that are normally kept separate inside the plant tissue.
The immediate reaction creates an unstable compound, which another enzyme quickly turns into an airborne gas known as syn-propanethial-S-oxide.
A peer-reviewed study published in ACS Chemical Biology explains that this specific gas “causes familiar eye irritation associated with onion chopping.”
When this floating chemical vapour comes into contact with the moisture film protecting your eyeballs, it mixes with the water to create a mild solution of sulphuric acid. Your brain instantly flags this acid as an irritant and orders your tear glands to flush it away.
Smart kitchen hacks to stop the crying
You do not have to endure painful, watering eyes every time you prepare a meal for your family. Because this process relies entirely on chemistry, you can easily use basic kitchen physics to disrupt the reaction before the gas ever hits your face.
Chilling your onions in the refrigerator for about thirty minutes before cooking is one of the most effective tricks. Cold temperatures significantly slow down the chemical activity of the enzymes, which means far less gas escapes into the air while you work.
Your choice of knife also plays a critical role. A blunt blade crushes the delicate onion cells, spraying a large cloud of irritating mist into the room, whereas a sharp knife slices cleanly through the tissue and leaves the cell structures intact.
Lastly, positioning a small kitchen fan next to your chopping board blows the rising vapour safely away from your eyes, ensuring a completely comfortable and tear-free cooking experience.