Why Do Onions Make You Cry? The Science Behind Tears When Cutting Onions

If you’ve ever chopped an onion while cooking, you’ve probably wondered: why do onions make you cry? It’s one of the most common eye-related questions people search online.

The reason comes down to a fascinating chemical reaction that occurs when an onion is cut.

What Happens When You Cut an Onion?

When you slice into an onion, you break open its cells. Inside those cells are enzymes and sulphur-containing compounds that are normally kept separate.

Once released, these substances react with each other to produce a gas called syn-propanethial-S-oxide.

This invisible gas quickly rises from the onion and drifts toward your eyes.

Why Your Eyes Start Watering

When the gas reaches your eyes, it reacts with the natural moisture in your tears, creating a mild irritant.

Your eyes immediately recognise this irritation and activate their natural defence mechanism: producing more tears to flush the irritant away.

So when you cry while cutting onions, your eyes are simply protecting themselves from a chemical irritant.

Why Some Onions Make You Cry More Than Others

Not all onions cause the same level of tears. Several factors influence how strong the reaction is:

• Fresh onions release more of the irritating gas

• Yellow and white onions tend to contain higher sulphur levels than sweet onions

• Warm onions release the gas more quickly than chilled ones

• Sharper knives reduce cell damage and can minimise the reaction

How to Stop Crying When Cutting Onions

If you want to reduce onion tears, a few simple tricks can help:

• Chill the onion in the fridge before cutting it

• Use a sharp knife to minimise cell damage

• Cut onions near a fan or running water

• Wear glasses or goggles to block the gas from reaching your eyes

While these tricks may not eliminate the effect completely, they can significantly reduce irritation.

Are Onion Tears Harmful to Your Eyes?

The good news is that cutting onions does not damage your eyes. The irritant created is extremely mild and the tearing response is simply your eyes cleaning themselves.

Once the gas disperses, your eyes return to normal within minutes.

A Fun Fact About Onions and Evolution

Scientists believe onions evolved this chemical reaction as a natural defence mechanism. When animals or insects try to eat the plant, the irritating gas discourages them.

Unfortunately for us, this defence still works when we chop onions in the kitchen.