Finally! Scientists May Have Found Out Why We Cry When Cutting an Onion—And the Simple Secret to Stopping the Tears
You go to chop an onion to toss in your marinara or guac … only to end up with tears streaming down your face. What gives?! Even when you know onions trigger waterworks, the tears are still somehow surprising. How can an ordinary vegetable make you all weepy? All you wanted was a little flavor, and now you’ve got mascara running down your face and concerned family members asking if everything’s OK.
As you (carefully) wipe away your tears, you can’t help but wonder why this is happening and if there’s any way to avoid it. Well, you can guess where we’re going with this: Researchers at Cornell University recently discovered a new reason we cry from onions—and a way to prevent it. I reached out to one of the researchers, as well as a neuroscientist, to get the details. Keep reading to find out everything you need to know before you head back into the kitchen.
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What are the tear-inducing compounds in onions?
When you slice or dice an onion, you break open the onion’s cells. The problem? You’re releasing sulfur compounds in the process. “Breaking open the cells allows certain compounds [called syn-propanethial-S-oxide molecules] to form, which become a gas,” says Alicia Walf, PhD, assistant professor of cognitive science at Rensselaer Polytechnic Institute in Troy, New York. The gas then gets in your eyes and makes you miserable.
However, this type of “crying” is distinct from what you normally experience when the waterworks start. “The crying from chopping onions is not the same as from negative emotions,” Walf notes. “It’s a protective response rather than an emotional one.”
How do onions make us cry, exactly?
Once that gas gets in your eyes, it “likes to combine with water, such as the fluid your eyes are bathed in to keep them moist,” Walf says. “When the gas combines with the lubricating tears in your eyes, a sulfuric acid is formed, which irritates your eye and causes pain.”
This uncomfortable situation kicks your eyes into full-on protection mode. How do your eyes protect themselves? You guessed it: by pumping out tears! “Your eyes reflexively tear up to wash away what is irritating to them,” Walf says. “In this case, that’s the sulfuric acid, but a similar reaction occurs with any irritant, such as an eyelash or smoke.”
But that’s not the whole story. As it turns out, there’s another scientific reason for the tears, according to a brand-new 2025 study published in Proceedings of the National Academy of Science. “We found in our study another important factor: The droplets splash upward into the air when the onion tissue is compressed,” says Sunghwan (Sunny) Jung, PhD, a professor of biological and environmental engineering, the director of graduate studies at Cornell’s College of Agriculture and Life Sciences, and one of the co-authors of the study. “As a knife compresses the tough outer skin of the onion, the underlying soft mesophyll is squeezed. When the skin ruptures, the compressed liquid creates droplets that can splash high into the air.” This makes them more likely to land in your eyes.
Do certain factors make the problem even worse?
Yep! It’s not just how sensitive your eyes are on any given day or how fresh (or, um, not fresh) the onions are. It’s all about the knife you’re using. The study found that blunt knives cause more droplets to form, according to the recent study. “The knife sharpness matters,” says Jung. “A sharp knife reduces tissue compression and droplet spray.” Translation: Sharpen those knives for a less-weepy onion interaction!
That said, not everyone “cries” while slicing and dicing onions. “Some of us have a lower threshold to engage the defense mechanism,” Walf explains. “We are more sensitive to the effects of the irritant.”
And not all onions are equal offenders. “Onions vary with how much of these compounds they have,” Walf adds. “And there are usually more of them in the section of onion near the root.” Noted!
What’s the other unexpected problem with cutting an onion?
This will also give you something to cry about: Cutting onions could spread bacteria around your kitchen, since they sometimes contain E. coli and salmonella. In fact, in 2024, a major E. coli outbreak that hospitalized 34 people and killed one person was tied to onions from McDonald’s, according to the CDC.
And using a blunt knife is an additional culprit that causes more bacteria to fly around your kitchen. After all, if those droplets are contaminated, you’ll have more of them when you use a blunt knife.
This is a pretty big discovery—and an important one to remember when chopping onions. “Droplets can carry pathogenic entities like bacteria or fungi, so understanding this finding helps reduce contamination risk,” Jung says.
Is there a way to minimize the tears?
Make sure you’re using the sharpest knife in the drawer! “Our findings show that the radius of curvature should be less than 7 micro meters,” Jung says. But he acknowledges that the measurement won’t mean much to most of us. So the takeaway is to just remember to sharpen your knives.
Does anything else work to reduce tears?
Unfortunately, all those old-time hacks you may have heard—like rinsing the onion in cold water before cutting it—don’t actually hack it. The Cornell scientists tested a few, including running onions under cold water and chilling them in the refrigerator, but these strategies didn’t reduce tear-inducing compounds or droplet spray, Jung says.
While not a hack, having a physical barrier does make a difference. For instance, some people with contacts don’t tear up as much, Walf says. “You can also use a barrier method like safety glasses,” she adds, “to protect your eyes from having contact with the irritating gas in the first place.”
But if you really want to keep the waterworks away, sharpen before you slice.
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Sources:
- Sunghwan Jung, PhD, professor of biological and environmental engineering and director of graduate studies at Cornell’s College of Agriculture and Life Sciences in Ithaca, New York; email interview; Nov. 16, 2025
- Alicia Walf, PhD, assistant professor of cognitive science at Rensselaer Polytechnic Institute in Troy, New York; email interview, Nov. 17, 2025
- Proceedings of the National Academy of Science: “Droplet outbursts from onion cutting”
- ACS Chemical Biology: “Enzyme That Makes You Cry—Crystal Structure of Lachrymatory Factor Synthase from Allium cepa”
- CDC: “E. coli Outbreak Linked to Onions Served at McDonald’s”
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