How a cut onion turns sulfur chemistry into tears
Inside an intact onion, the ingredients for its irritating vapor are kept apart in different parts of its cells. A knife ruptures those cells, letting enzymes mix with sulfur-containing compounds. The reactions produce syn-propanethial-S-oxide, a volatile molecule—one that readily becomes airborne—that can travel from the cutting board to your eyes.
This chemistry is part of the onion’s defense system: damaged tissue releases compounds that can deter animals from eating it. The tear-inducing vapor is a by-product of the same rapid reaction that begins as soon as the cells are broken.
Why your eyes respond so quickly
The vapor dissolves in the thin, watery film covering the eye and irritates its surface. Sensory nerves detect that irritation and trigger a reflex: tear glands make extra fluid to dilute and wash away the irritant. The tears are not an emotional response; they are the eye’s ordinary protective cleanup system doing its job.
Because the vapor forms at the cut surface and disperses through the air, keeping more of it away from your face can reduce the reaction. A sharp knife can help by crushing less onion tissue, while moving air can carry the vapor away. But each fresh cut still starts the chemistry—and can send another invisible plume toward your eyes.
