A Kitchen Mystery Everyone Knows
Almost everyone who has cooked has experienced it: you start chopping an onion, and within moments your eyes sting and fill with tears. It happens no matter how tough you think you are, and it can turn a simple cooking task into a teary ordeal. The reason behind this is not sadness or magic, but a fascinating piece of plant chemistry. The onion is essentially defending itself, and your eyes are caught in the crossfire.
The Onion's Hidden Defense
An onion grows in soil that contains sulfur, and the plant absorbs this sulfur and stores it inside its cells as special sulfur compounds. As long as the onion is whole and undamaged, these compounds sit harmlessly inside separate parts of the cells. The onion keeps them apart on purpose. This separation is a kind of built-in defense system, waiting to be triggered the moment something bites into or cuts the onion.
The Chemical Chain Reaction
When you slice an onion, you break open its cells, and this mixes together substances that were previously kept apart. The onion's enzymes meet the sulfur compounds, and a rapid chain of chemical reactions begins. The end result is a volatile gas, whose technical name is syn-propanethial-S-oxide. Scientists also call this gas the lachrymatory factor, meaning the tear-producing factor. Because this gas is light, it quickly drifts up into the air from the cutting board, heading straight toward your face.
Why Your Eyes React
When the gas reaches your eyes, it reacts with the natural moisture on the surface of the eye and forms a mild irritant. Your eyes are very sensitive and have nerve endings that detect this irritation immediately. In response, your brain signals the tear glands to release a flood of tears to wash the irritant away. This is the same reflex that produces tears when dust or wind bothers your eyes. There are ways to reduce the effect, such as chilling the onion before cutting, since cold slows the reactions, or using a very sharp knife, which damages fewer cells. But the tears themselves are simply your body doing its job of protecting your eyes.
Source
This article was written using information from Wikipedia.