Why do we cry when we chop onions?



In the immortal words of Donkey, “ogres are like onions… they make you cry.” But unlike Shrek, who might cause you to shed fearful emotional tears, onions cause you to cry “reflex tears.” Reflex tears are produced in order to flush away potential irritants.


So, what irritant does an onion create to stimulate reflex tears? Chopping an onion produces an enzyme called lachrymatory-factor synthase. This enzyme catalyzes the conversion of compounds in the onion, called amino acid sulfoxides, into the aerosolized chemical irritant “syn-Propanethial-S-oxide.” Syn-Propanethial-S-oxide includes several sulfur compounds including sulfuric acid!


This is the same acid that is in lead acid batteries. So, don’t beat yourself up if you break down next time you chop an onion. There is no shame in tearing up when you effectively get battery juice on your eyeball.

Nicholas Norwitz