Why Do Onions Make Us Cry, But Not Other Vegetables?

Ever wondered why chopping onions brings on the waterworks, while carrots, potatoes, or lettuce leave your eyes dry? If you’ve ever teared up over a cutting board, you’re not alone—and there’s real chemistry behind this kitchen mystery. Let’s explore why onions make us cry, why other vegetables don’t, and what you can do to keep the tears at bay.

The Culprit: A Special Chemical Reaction in Onions

Onions are unique among vegetables because they contain a powerful defense mechanism. When you slice into an onion, you break open its cells, allowing two previously separated substances to mix: an enzyme called alliinase and sulfur-containing compounds known as amino acid sulfoxides.

This triggers a chain reaction:

• The enzyme alliinase converts the amino acid sulfoxides into sulfenic acid.
• Another enzyme steps in, transforming sulfenic acid into syn-propanethial-S-oxide—a volatile, sulfur-containing gas.

This gas quickly wafts up toward your eyes, where it reacts with the water in your tear film to form a mild sulfuric acid. Your eyes, sensing irritation and potential harm, respond by producing tears to flush the irritant away.

Why Don’t Other Vegetables Make Us Cry?

Most vegetables don’t have the same combination of enzymes and sulfur compounds as onions. While other alliums (like garlic, leeks, and shallots) can cause some irritation, onions—especially yellow, white, and red varieties—contain higher concentrations of the necessary chemicals to create syn-propanethial-S-oxide.

Vegetables outside the allium family, such as carrots, potatoes, or celery, simply lack both the precursor compounds and the enzymes needed for this reaction. That’s why you can chop them all day without shedding a tear.

Why Do Onions Have This Defense Mechanism?

Onions grow underground, making them easy targets for hungry animals and insects. Over time, they evolved this chemical defense to deter predators. The irritating gas is a natural way for onions to protect themselves from being eaten while they’re still developing beneath the soil.

Are All Onions Equally Tear-Inducing?

Not all onions are created equal when it comes to making you cry:

• Yellow, white, and red onions: Highest in the enzymes and sulfur compounds that create the tear-inducing gas.
• Sweet onions, green onions, and scallions: Contain much lower levels of these chemicals, so they’re less likely to cause tears.
• Spring onions: Tend to be milder and less likely to make you cry.

How to Stop Onion Tears: Kitchen Hacks

If you want to chop onions without the tears, try these proven tips:

• Chill the onion: Place it in the fridge or freezer for 30 minutes before cutting. Cold temperatures slow down the chemical reaction.
• Cut under water: Slicing onions in a bowl of water or under running water can help trap the gas.
• Use a sharp knife: A sharp blade causes less cell damage, releasing fewer irritants.
• Wear goggles: It might look silly, but airtight goggles can block the gas from reaching your eyes.
• Try a slice of bread: Holding a piece of bread in your mouth while chopping can absorb some of the gas.

The Future: Tearless Onions?

Scientists are working on developing onions that don’t produce syn-propanethial-S-oxide, either through selective breeding or genetic modification. Some “tearless” onions, like Sunions, are already available in certain markets, offering the flavor of onions without the tears.

Key Takeaways: Why Onions Make Us Cry

• Onions release syn-propanethial-S-oxide, a gas that irritates the eyes and triggers tears.
• This reaction is unique to onions and closely related alliums due to their specific chemical makeup.
• The tear-inducing gas is a defense mechanism to protect onions from underground predators.
• Most other vegetables lack the necessary enzymes and sulfur compounds, so they don’t make us cry.
• You can minimize tears with simple kitchen tricks, and tearless onions may be the future.