Why does slicing onions make you cry?

Whether they're sautéed, grilled, caramelized or raw, onions are a staple in many U.S. households; the average American consumes 20 pounds (9 kilograms) of onions per year. But onions' coveted flavor comes at a price: Whoever chops them may soon feel tears running from their burning eyes. But why does slicing onions make you cry?

The culprit is called the lachrymatory factor, a chemical that irritates the nerves in the cornea. When the onion is intact, a group of compounds called cysteine sulfoxides are kept separate from an enzyme called alliinase. But when you slice, dice or crush the onion, the barrier separating the compounds and enzyme is broken. The two come together, setting off a reaction: The alliinase causes the cysteine sulfoxides to become sulfenic acid.

"Sulfenic acids are not very stable," so they must change into something else, said Josie Silvaroli, an academic fellow of pharmacy education and innovation at The Ohio State University and first author of a 2017 study about the lachrymatory factor.

In an onion, there are two possibilities for the sulfenic acid. The first is that it can spontaneously condense — a reaction within itself — and become an organosulfur compound. Organosulfur compounds are what give onions their strong smell and flavor. A similar reaction happens in garlic, which is why it also has such a pungent flavor, Silvaroli said.

But the second possibility for the sulfenic acid is unique to onions and a couple of other alliums — a genus of flowering plants that produce vegetables such as onions, garlic, scallions and shallots. Another enzyme, called lachrymatory factor synthase, that's been hiding in the cell comes into play and rearranges sulfenic acid into the lachrymatory factor, known to chemists as propanethial S-oxide.

The lachrymatory factor is a volatile liquid, meaning it turns to vapor very quickly, Silvaroli said. That's how it reaches your eyes and irritates sensory nerves. "Your eyes start tearing up to get rid of the irritant," Silvaroli told Live Science.

It's likely that the organosulfur compounds that give onions both their intense flavor and the tear-inducing lachrymatory factor evolved as defense mechanisms, Silvaroli said. They are meant to stop insects, animals or parasites that might damage the onion plant.

How to cut an onion without crying

If burning tears are more than you can bear, you do have options. First, you can wear goggles or a face shield to protect your eyes. (Contact lenses also offer a barrier against the lachrymatory factor.)

The chef's chopping technique can also play a role. In a 2025 study in the journal PNAS, researchers took high-speed videos of onions getting cut to determine which practices were best.

"We found that an onion does not release tear-triggering fluid in one burst," they noted in the study. The first cut releases a "high-speed mist," which is then followed by slower fluids that "snap into drops." Chopping quickly with blunt knives released more droplets with a high launch energy — a recipe for more tears.

So the researchers advised a "practical fix: Keep knives sharp, and cut gently to spray less."

The team also found that chilled onions increased the tears; refrigerated onions produced more droplets of lachrymatory factor when cut than room-temperature onions did.

There are also efforts to create tear-free (or at least fewer-tear) onions. Cultivators of Sunions used crossbreeding techniques to make an onion they describe as "sweet and mild" as well as "tearless."

But Silvaroli is wary because the compounds responsible for the lachrymatory factor are the same flavorful organosulfur compounds that give onions their distinct taste. Reducing the cry factor, therefore, could also dull the onion's flavor. For now, Silvaroli said, "If you want that flavor, you might need to suffer through some burning."

Editor's note: This article was originally published on Jan. 17, 2022, and was updated on Oct. 17, 2025, to include information from the 2025 study in the journal PNAS.