WHY IS IT SO? Why Do Onions Make Us Cry?

• Bilkul Online | Sunday Special

You are not sad.

You are not emotional.

And there is certainly no heartbreak involved.

You are simply cutting an onion!

Yet within seconds, your eyes begin to sting, tears roll down your cheeks and you start wondering what exactly this innocent-looking vegetable has against you.

So… why is it so?

The onion is fighting back!

The surprising answer is that an onion does not actually contain the chemical that makes you cry.

It creates it when you cut it.

An onion is made up of millions of tiny cells. Inside those cells are different substances that normally remain separated. When your knife cuts through the onion, those cells are damaged and their contents mix.

That starts a chain of chemical reactions.

One of the enzymes involved is called alliinase. It helps turn sulfur-containing compounds inside the onion into other substances. Another onion enzyme, called lachrymatory-factor synthase, helps produce the real troublemaker: a volatile chemical called syn-propanethial-S-oxide, commonly known as the onion’s lachrymatory factor.

And this chemical does exactly what its name suggests.

It makes you cry.

What happens to your eyes?

The newly formed chemical is volatile, so it escapes into the air and reaches your eyes.

Your eyes detect the irritant and immediately go into defence mode.

The nerves in the eye signal the tear glands to produce more tears. The tears help wash away and dilute the irritant.

So those tears aren’t tears of sadness.

They are your eyes’ emergency cleaning system.

But why would an onion do this?

This is the really interesting part.

The onion isn’t deliberately trying to make you cry.

It is trying to protect itself.

Onions grow underground, where they can be attacked by insects and other organisms. Their sulfur-based chemistry acts as a natural defence mechanism when the plant tissue is damaged. In other words, the chemical that makes us reach for a handkerchief is part of the onion’s built-in protection system.

So when you cut an onion, you are essentially triggering its chemical alarm system.

And scientists actually found the culprit!

For a long time, scientists knew that cutting onions produced the irritating chemical, but the complete mechanism was not understood.

In 2002, Japanese researchers identified the previously unknown enzyme lachrymatory-factor synthase, showing that it plays a specific role in producing the tear-inducing compound. Later research helped explain the enzyme’s molecular mechanism in much greater detail.

Can you stop the tears?

You can certainly reduce them.

A sharp knife can help because it causes less crushing of the onion tissue. Chilling the onion before cutting can also reduce tearing, and leaving the root end intact is another commonly suggested kitchen trick.

And scientists have even developed tearless onions by altering the biochemical pathway responsible for producing the lachrymatory factor.

But perhaps we should leave the onion with one small victory.

After all, it may make us cry for a few minutes…

but we still put it in almost everything!

WHY IS IT SO!

Because cutting an onion triggers a chemical defence mechanism that produces an airborne irritant.

The onion isn’t making you cry because it is sad.
It is making you cry because it is fighting back!

(Every week, we take a closer look at something we experience, see or wonder about in everyday life—and uncover the simple science, logic or story behind it. Because sometimes, the most interesting questions are the ones we rarely stop to ask)