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Why do onions make you cry?

Ever ponder why onions cause so many people to cry? This lesson gives two explanations.

The first is a matter of survival, while the second concerns the specific chemical reactants involved in this process.

close up of sliced red onion, intended as a background image

The first is a matter of survival, while the second concerns the specific chemical reactants involved in this process.

What Makes Us Cry?

What do bittersweet movies, sad songs, and onions have in common? They make us cry, of course!  But why? As for movies and songs, you’ll have to ask the screenwriters and songwriters, but this article will explain why onions make you cry!

Young People Watching Drama Movie

It has to do with our ability to survive and different chemical substances that affect how our eyes and brains function.

Defensive Mechanisms

We can answer the question, “Why do onions make people cry?” in two different ways. One perspective is to view it from a broad perspective. Why do onions do this to us, generally speaking?   What really causes the tears to start flowing?

Generally speaking, it may be that onions make people cry as a defensive mechanism.  Some plants have really horrible smell, while others have very sharp thorns. All of those might serve as the plants’ defense mechanisms.


They may be trying to ward off anyone or anything that may want to eat them, like herbivores. Since they are gleefully eaten everywhere in the world, onions are obviously doing a bad job when it comes to humans.

The Chemistry

Therefore, the simple explanation for why onions make people cry is a matter of survival; in other words, the whole perspective. Regarding our magnifying glass, we must consider the scientific basis of what actually makes us cry. That is to say, the small but important details behind the defensive mechanism.

A lavender colored or purple allium flower in full bloom with honeybee bee. violet colored alium

Onions belong to the plant genus Allium, to which garlic (among other plants) also belong.  Onions take up sulfur, a chemical element, from the earth as they grow.

Onions use sulfur to create something known as amino acid sulfoxides. These are compounds that are important precursors for the distinctive flavors and aromas of Allium vegetables.

Cut onion

As soon as you cut, break, or crush an onion, you break apart its cells.  When cells are damaged, they produce a variety of enzymes, which are protein molecules that quicken biochemical reactions. Alliinase is the name of one of these enzymes.

It converts a specific amino acid sulfoxide, called S-1-propenyl-L-cysteine sulfoxide, into a sulfenic acid called 1-propenyl sulfenic acid. Sulfenic acid is a sulfur-based acid compound that is produce from sulfur that plants absorb from the ground.

This acid is unstable and is quickly converted by an enzyme called lachrymatoryfactor synthase into syn-propanethial-S-oxide, a tear-inducing and volatile sulfur compound.

The latter chemical spreads as a vapor into the air, where it comes into contact with our eyes. A small portion of it reacts with the water present in our protective tear film to generate sulfuric acid once it comes into contact with our eyes. This acid is extremely strong.

close up view of young woman green eye looking at camera

Our eyes have incredibly sensitive receptors that can sense changes in the physical and chemical environment.  The nerves associated with these receptors send signals to the brain that a chemical irritant is annoying our precious eyes! 

Face of unhappy female in tears from close up in darkness

Our lacrimal (tear) glands receive signals from the brain, which interprets them as a burning or stinging sensation and tells them to start creating tears to wipe the chemical irritants away.


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