Why do onions sting your eyes when you chop them?

A whole onion is quiet. A knife opens its partitioned interior, and substances that were kept apart meet to make a sharp gas. As a Japanese team led by Imai showed in 2002, that gas is not formed only by chance; it takes one more step. The name is long, but it rises over the board like nose-stinging steam.

Leaning close over the cutting board exposes you to more of it. Your eyes can water even without liquid splashing in. Stinging hands and stinging eyes begin at the same cut surface. Peeling the skin alone stays calm; the vapor really comes alive once you slice the flesh.

When that vapor dissolves in the thin moisture layer on your eyes, they feel a sting. The eyes make more tears to wash the irritation away. It feels similar to tears from spicy soup. Britannica describes the same chain: once the gas reaches the eyes and burns, the brain signals tears to rinse it out.

A common misconception is that juice must splash into your eyes. Vapor alone is enough. A little water on the eyes can ease the sting a bit, but residue on your hands needs soap. Water alone does not clear what stays on the hands. Wash your hands first after chopping.

An onion from the fridge is a bit milder. Vapor rises more slowly from a cold interior. Wetting the knife or opening a window reduces what collects in front of your face. A side fan that blows the gas away from you also helps.

Cultivars bred to release less of the sharp vapor are sold too. Folk tips like lighting a match or chewing bread are often described as weak. A little while after chopping finishes, the eye sting fades. The root side often feels sharper than the leaf side, so on a rushed day start from the leaf end. Goggles keep more of the gas off your eyes and feel much kinder.