Researchers at Cornell used high‑speed imaging and sensitive sensors to show that sharper knives and slower, gentler cuts substantially reduce the aerosol mist produced when onions are chopped. The team identified two release modes—an immediate, high‑speed ejection and a slower seep—and found dull blades create more and faster droplets that can travel up to 40 m/s (144 km/h; 89 mph). Chilling onions did not reduce mist and can sometimes worsen it. The study also highlights food‑safety risks because droplets can encapsulate and spread surface pathogens.
Cut Onion Tears: Sharper Knives and Gentler Cuts Slash Tear‑Causing Mist, Study Finds
Researchers at Cornell used high‑speed imaging and sensitive sensors to show that sharper knives and slower, gentler cuts substantially reduce the aerosol mist produced when onions are chopped. The team identified two release modes—an immediate, high‑speed ejection and a slower seep—and found dull blades create more and faster droplets that can travel up to 40 m/s (144 km/h; 89 mph). Chilling onions did not reduce mist and can sometimes worsen it. The study also highlights food‑safety risks because droplets can encapsulate and spread surface pathogens.
00:15, 05.11.2025Science
Cut Onion Tears: Sharper Knives and Gentler Cuts Slash Tear‑Causing Mist
When onions are sliced they release a volatile compound—propanethial S‑oxide—that irritates the eyes and causes tearing. New experiments at Cornell University reveal a simple, practical way to reduce that tear‑causing aerosol: use sharp blades and make slower, gentler cuts.
How the study was done
Biomechanist Zixuan Wu and colleagues used a mini guillotine, a high‑resolution camera and sensitive droplet detectors to track the aerosol produced during cutting. They systematically varied knife sharpness, cutting speed and applied force to identify what controls droplet formation and ejection.
Key findings
Two distinct release modes: the team observed an immediate, high‑speed ejection of aerosolized droplets when an onion layer was breached, followed by a slower seep of fluids between layers.
Blade sharpness matters: dull knives generated substantially more droplets and more forceful sprays. Because blunt blades require greater force to penetrate the onion’s layers, internal pressure builds and energetic cuts with a dull blade can propel droplets farther.
Splash speed is surprising: the initial droplet ejection can reach up to 40 m/s (≈144 km/h or 89 mph), making these high‑speed droplets the primary hazard for the eyes.
Chilling myths debunked: contrary to common belief, starting temperature had no clear benefit; in some tests chilling the onion made aerosol release worse.
"We found out the speed of the mist coming out is much higher compared to the speed of the blade cutting through," says physicist Sunghwan Jung.
Implications
Using sharper knives and gentler, slower cuts keeps most droplet mist below eye level and reduces eye irritation. The findings also carry food‑safety implications: pathogens present on an onion’s surface can be encapsulated into droplets during cutting and spread, which helps explain how contaminated onions have been linked to outbreaks.
The research, led by Wu and Jung, is published in PNAS.