Cooking can be a relaxing, meditative act—that is, until your eyes start to sting unbearably.
Cutting an onion often leads to an involuntary stream of tears, but a new scientific discovery has revealed exactly how the pungent compounds launch into the air—and how you can stop them.
Researchers at Cornell University found that the key to avoiding tears is to use a sharpened blade and cut slowly, or to coat the onion in oil before cutting.
This advice comes from a study, published in the Proceedings of the National Academies of Science, which investigated the mechanics of how tear-inducing chemicals are ejected from the vegetable.
The Two-Stage Tear Cascade
While past research had already identified the irritant in onions—known to chemists as propanethial S-oxide—this study is the first to explain how the sulfur compound reaches a cook’s eyes.
The team used high-speed cameras and a computer model to determine what happens when a knife slices into an onion.
They found that pressing a knife against the onion layers pressurizes the cells inside. Since each layer is encased in a top and bottom skin, when the knife penetrates the top skin, “the pent-up pressure within the layer forces out a mist of tiny droplets.”
The study provided a more detailed mechanical view, showing that “the blade first punches out a layer of high-speed mist, then slower fluids snap into drops.” This is referred to as a two-stage process.
The speed of the expelled mist was a surprise to the researchers. The droplets were launched at estimated rates of 11–89 miles per hour—”much higher compared to the speed of the blade cutting through,” noted paper author and engineer Sunghwan “Sunny” Jung in a statement.
Professional chefs, whose cuts were measured, slice at a rate of only one meter per second.
Blunter Knives Launch Pathogens Faster
The research also found that the method of cutting directly impacts the amount of spray released.
“When the blade is blunter and faster, droplet number and launch energy increase sharply,” the researchers wrote in their paper. This, they added, confirms a “culinary intuition” and suggests a practical fix: “Keep knives sharp and cut gently to spray less.”
The findings have implications beyond simply reducing tears. Professor Jung noted that limiting the spray could help prevent the spread of bacteria in a kitchen, as onions have been known to harbor pathogens.
In fact, the speed of the onion mist is a serious concern for food safety. Jung noted that the mists can eject from the onion’s skin at up to twice the speed of particles when we cough; this means that a contaminated onion can spray pathogens surprisingly far.
“Suppose you have pathogens on the very top layer on the onion,” Dr. Jung said. “By cutting this onion, these pathogens can become encapsulated in droplets where they can then spread.”
The research therefore highlights the importance of blade sharpening to enhance overall food safety.
Do you have a tip on a science story that Newsweek should be covering? Do you have a question about GLP-1s? Let us know via [email protected].
Reference
Wu, Z., Hooshanginejad, A., Wang, W., Hui, C.-Y., & Jung, S. (2025). Droplet outbursts from onion cutting. Proceedings of the National Academy of Sciences, 122(42). https://doi.org/10.1073/pnas.2512779122