What chemical compound causes tearing when cutting onions?

What chemical compound causes tearing when cutting onions? Propanethial S-oxide is the volatile onion compound that irritates the eyes and triggers tears.

Cutting damages onion cells and brings previously separated substances together. Alliinase enzymes act on sulfur-containing amino acid sulfoxides, producing sulfenic acids. In onions, lachrymatory factor synthase then converts 1-propenesulfenic acid into propanethial S-oxide, also known as syn-propanethial S-oxide or the onion lachrymatory factor.

The compound is volatile, so it quickly becomes airborne and reaches the surface of the eye. It activates sensory nerves in the cornea, prompting the lacrimal glands to produce tears that dilute and flush away the irritant. This is why the effect is strongest soon after an onion is cut.

Allicin is a related sulfur compound best known from garlic and is not the final tear-causing compound in onions. “Lachrymator” is a descriptive term for an eye-irritating substance, not the compound’s specific name. Tearless onion varieties reduce production of the compound by suppressing lachrymatory factor synthase.