If slicing an onion reliably brings tears, your knife might be to blame. New research suggests that a sharper blade and a slower hand don’t just feel better—they dramatically cut down the eye-stinging spray that onions launch into the air. The finding turns a familiar kitchen frustration into a miniature physics lesson with implications well beyond dinner.
Why onions make us cry
Onions aren’t out to get you, but their chemistry is primed to defend the bulb. When you pierce an onion, you rupture pressurized cells and set off a cascade that creates propanethial S-oxide, a volatile compound that binds to sensory nerves in the eyes. The result is the unmistakable sting that sends tears streaming.
Scientists have been untangling this chemistry for decades. In 2002, Japanese researcher Shinsuke Imai and colleagues reported the discovery of an enzyme called lachrymatory-factor synthase, which the onion uses specifically to make the tear-inducing irritant. That work helped separate the tear trigger from other sulfur reactions responsible for much of an onion’s flavor. Still, the physical mechanics of how an onion’s juices become airborne—and how far they travel—remained murky.
What the new study found
In work reported in the Proceedings of the National Academy of Sciences, researchers tracked onion droplets in flight using high-speed cameras and carefully controlled cuts. The team tested blades of varying sharpness and measured force and speed as a knife or a miniature guillotine sliced through the layers. The goal: capture, frame by frame, how the onion’s internal pressure turns a cut into a fine mist.
The images revealed a simple rule with striking consequences. Dull blades require more force, crush more cells and build more pressure before the skin gives way. Quick chops with a blunt knife produced a spray of tiny liquid beads that shot upward—sometimes nearly 40 meters, high enough to put a plume at head height and beyond. Gentle, unhurried cuts with sharp blades ruptured fewer cells and kept that spray much lower, below eye level.
“This is something everybody’s dealing with,” said Navid Hooshanginejad, a physicist who investigated the problem while at Cornell University and whose comments were reported by Science News. “Now we can also explain and understand it better fundamentally.”
Hooshanginejad’s team approached the problem like fluid dynamicists, showing that onion juice behaves a bit like water under tension that’s suddenly released. The sharper the blade and the slower the slice, the less energy is injected into the system and the fewer tiny droplets are propelled into the air.
“It’s just like the idea of a water balloon,” said Jim Wilking, a physicist at Mayo Clinic in Rochester, Minnesota, who was not involved in the research, as reported by Science News. “If you pop a water balloon by using a needle, you don’t need to apply much pressure. But if you come in with your finger, you’re essentially pressurizing the elastic shell in order to pop it.”
That extra pressure, the researchers found, doesn’t just fling droplets higher—it also helps them break apart in flight, increasing the number of airborne particles that can reach your eyes.
From the lab to the cutting board
The takeaway for home cooks is refreshingly practical: you don’t need special goggles or gimmicks if you can control two variables—sharpness and speed. A sharp knife slices cleanly, minimizing crushed cells and curbing the formation and ejection of the onion’s irritant-laden mist. Slowing down reduces turbulence and pressure spikes that turn juice into aerosol.
- Keep your chef’s knife sharp. A few passes on a honing steel before use and regular sharpening sessions make a noticeable difference.
- Ease up on speed and let the blade do the work. Smooth, deliberate strokes reduce cell crushing and spray.
- Work with ventilation. A range hood or small fan that moves air away from your face can carry any lingering plume off your cutting board.
- Chill, don’t freeze. Briefly refrigerating onions can slow reactions and evaporation without making the bulb dangerously hard or the knife prone to slipping.
- Leave the root end intact until the final cuts. This helps hold layers together and minimizes exposed cell surface area early in the process.
These small adjustments are about mechanics, not magic. They align your technique with what the physics shows: less forceful, cleaner cuts generate fewer airborne droplets, which means fewer irritants reaching your eyes.
Does gentler cutting change flavor?
Flavor in onions emerges when sulfur-rich compounds meet enzymes after cells rupture. The more you macerate an onion—say, with a dull blade or a rough chop—the more pungency you can unleash in the moment. Cutting cleanly and calmly likely reduces some of that immediate harshness in raw onions, which is one reason finely shaved onions can taste sweeter than ragged dice.
But cooking reshapes those compounds, turning sharp sulfur notes into complex sweetness and savor as the onion softens and browns. In most hot preparations, the difference between a careful and a careless cut will matter far less by the time the onion hits the pan. The new study focused on droplet formation and flight, not taste, so any flavor effects remain an informed extrapolation grounded in known chemistry rather than a measured outcome.
Longer term, the line between tears and taste may be disentangled further. The 2002 discovery of lachrymatory-factor synthase showed that the tear trigger sits on a distinct biochemical path from many flavor compounds. That insight enabled breeders and food scientists to imagine onions that retain their character without the eye-watering punch—a reminder that better understanding often opens both scientific and culinary doors.
Why onion spray matters beyond dinner
At first glance, an onion’s mist seems trivial. Yet the study’s core contribution—how pressurized fluids break into airborne droplets under stress—echoes across biology. The same physics governs how many other fluids, from plant saps to bodily secretions, aerosolize and disperse. By mapping how droplet size and speed change with the force and sharpness of a rupture, researchers gain clues to how particles travel, settle and interact with their surroundings.
That’s why the work has drawn interest from physicists studying aerosols and pathogen dispersal. Understanding how micro-droplets form and fly can help refine models of how particles move in air, how long they linger and what might limit their reach. The onion, in this sense, becomes a safe, repeatable stand-in for more complex—and far less appetizing—systems.
