Why onions make you cry

Transcript

When onion cells are cut, alliinase and lachrymatory-factor synthase help create syn-propanethial S-oxide, the volatile lachrymatory factor that escapes into the air.

That vapor dissolves in the tear film, forms a mild acid, and irritates corneal sensory nerve endings described as trigeminal pain receptors or nociceptors.

The trigeminal signal then tells the brain to drive the lacrimal glands, flooding the eye with tears to dilute and wash away the irritant, and the effect is usually weaker with low-sulfur onions, freshness, a sharp knife, chilling, good airflow, the root end saved for last, and snug goggles rather than ordinary glasses.