Scientists find way to prevent crying while chopping onions

A new study published in the Proceedings of the National Academy of Sciences (PNAS) has revealed the precise physics behind why chopping onions makes you tear up — and why using a blunt knife makes it worse.

Titled “Droplet outbursts from onion cutting,” the study by researchers Zixuan Wu, Alireza Hooshanginejad, Weilun Wang, and Sunghwan Jung takes a closer look at what actually happens when a knife slices into an onion.

Using high-speed cameras, the team discovered that onions don’t release their tear-inducing compounds in one go. Instead, each cut triggers a two-stage process — a sharp burst of fine mist followed by slower-forming droplets. The first spray is propelled at high speed as the knife ruptures the onion’s cells, while the subsequent droplets emerge as the tissue stretches and snaps apart.

The experiments also showed that when the knife is blunt or moves too fast, both the number and energy of droplets increase dramatically — effectively sending more pungent aerosols into the air and straight to your eyes.

The researchers modelled how the onion’s tough outer skin resists fracture, forcing the softer inner layers to compress until they burst. This buildup of pressure contributes to the powerful “mist” that escapes during chopping.

The findings not only explain why a sharp, slow cut makes for a gentler experience in the kitchen but also carry broader implications. The study suggests that better blade maintenance could reduce droplet spray — potentially limiting the spread of food-borne pathogens that hitch a ride on these microscopic aerosols.

In other words, sharper knives don’t just make cooking easier — they might even make it safer, and a little less tearful.