https://www.theregister.co.uk/2017/08/03/onion_cutting_study/

We’re all familiar with the burning, eye-watering sensation felt when chopping onions, and now we know exactly why.

The US National Onion Association estimates that the average American chows through about nine kilograms (20 pounds) of onions every year – that’s a lot of tears. We know that a lachrymatory factor (LF) agent causes us to burst into tears, and this is presumably produced by the onion as a natural defense mechanism to deter us from biting into the vegetable's crunchy flesh.