How a Cut Onion Makes an Irritant
Inside an intact onion, sulfur-containing compounds and the enzymes that can act on them are kept apart in different cell compartments. A knife ruptures those cells, letting the ingredients meet. An enzyme helps turn one sulfur compound into a short-lived intermediate; another enzyme converts it into syn-propanethial-S-oxide, the volatile chemical commonly called the onion’s lachrymatory factor.
“Volatile” means it readily enters the air. The irritant drifts from the cut onion toward your face, so the reaction can reach your eyes even though the onion never touches them.
Why the Eyes Water
The vapor dissolves in the thin layer of moisture covering the eye and irritates sensory nerves there. Those nerves trigger a reflex: tear glands produce extra fluid to dilute and wash away the irritant. The burning sensation and watering are therefore parts of the same protective response—not simply a reaction to the onion’s smell.
The key event happens at the cut surface, where damaged cells bring the ingredients together. Each fresh slice creates more opportunity for the reaction, sending more irritant into the air and giving the eyes more to rinse away.
