The Tearful Truth About Onions
Jean-Philippe Guillaume | April 7, 2026
Have you ever been in the kitchen while your mom is cooking food and suddenly your eyes start watering? You turn, and there on the counter, she’s cutting onions. You rush out of the room—your eyes stinging. But why?
Onions belong to the genus Allium which contains a natural sulfuric compound that protects it from animals and pests. When an onion is cut, the cells are broken up, and these compounds mix. It’s an incredibly designed defense system for the onion.
This compound is called Syn-Propanethial-S-oxide, which evaporates quickly into the air. It then reacts with the thin layer of moisture covering your eye, forming a very mild form of sulfuric acid. For comparison, highly concentrated sulfuric acid could dissolve a human if you gave it time.
The eye reacts to this as it would any irritant: It tears up. Your body defensively tries to dilute the substance and flush it out as quickly as possible. So the stinging feeling is caused by the compound reaction to your moist eye, and then the stinging causes you to involuntarily tear up.
There are a couple ways to prevent this. First of all, you can chill your onions before cutting them. This low temperature slows down the chemical reaction, and so less of the gas is released.
Cutting the onion under running water or next to a fan helps disperse the compound, and cutting with a very sharp knife reduces the breaking in the cells. Wearing goggles does actually help, as it is a physical barrier.
The onion is such a tasty food though, that it doesn’t really matter if you tear up or not—just enjoy the end result.