How does cutting onions make you cry?

Onions eject eye-irritating chemicals that have a tear-inducing property. When you pierce the skin of an onion, you are left with stinging eyes caused by chemicals released from the onion that affect your nerves in the cornea. And it seems the cutting technique may have a role to play as well.

Onions cut by steel blades of varying sharpness and speed matter, according to a study published in Proceedings of the National Academy of Sciences.

“Using high-speed visuals, we found that an onion does not release tear-triggering fluid in one burst. Instead, the blade first punches out a layer of high-speed mist, then slower fluids snap into drops,” the authors wrote.

Blunter blades and faster cutting speeds release a larger number of droplets “causing pressure to build under the onion’s skin before the droplets were ejected. Refrigerating the onions prior to cutting produced an increased number of droplets of similar velocity, compared with the unchilled samples,” per the paper.

Using sharp knives “not only reduces tears while cutting onions but may also reduce the number of potential pathogens released while chopping other raw foods, such as poultry”.