To stop this from happening, onions are equipped with a defense mechanism designed to protect them as they grow.
Onions spew enzymes and sulfenic acid when their skin is broken. These compounds combine to produce Propanethial S-oxide, an irritating gas.
Propanethial S-oxide is a lachrymatory (tear stimulating) agent, meaning that it generates tears when it touches or surrounds the eye. Propanethial S-oxide turns into sulfuric acid (also contained in other allicins like onions)when it touches the water layer that covers and protects your eyeballs.
But just like onions, your eyes are also equipped with a defense mechanism meant to protect them from harm. When the nerves in the eye detect a lachrymatory agent, they generate tears to flush it out.
Some people are more likely to cry when cutting onions than others. If you’re sensitive or allergic to onions or other alliums, your symptoms may be more severe.
Other uses