Why do onions make you cry?

Cutting releases a defence chemistry

An intact onion keeps enzymes and sulfur-containing precursors in separate cell compartments. A knife breaks those cells, allowing reactions that produce a volatile lachrymatory factor. The compound travels through the air toward the eyes.

This chemistry is part of the plant’s defence system. It is activated by physical damage, which is why chopping creates a much stronger effect than simply looking at an uncut onion.

The eye turns irritation into tears

The irritant dissolves in the thin tear film covering the cornea and stimulates sensory nerves. The brain interprets the signal as a threat to the eye, increasing tear production and blinking. Tears dilute the compound and help wash it away.

The burning sensation and the tears are related but not identical: nerves signal irritation, while the lacrimal glands provide the protective response.

Why methods only partly work

Chilling an onion can slow the enzymes, and a sharp knife can reduce crushing. Ventilation can carry vapour away from the face. These methods change reaction speed or exposure; none makes every onion chemically harmless.

Different onion varieties contain different precursor levels, so people can experience very different reactions in the same kitchen.

Why distance and airflow help

The irritant must travel from the cut surface to the eye. A fan, open window or greater distance can lower the concentration reaching the tear film. These methods do not remove the chemical reaction inside the onion; they reduce exposure after the compound has formed.