Food & Chemistry

Why Do We Cry When Cutting Onions?

Onions do not make you cry because they smell strong. Cutting them creates a gas that reacts with moisture in your eyes and triggers tears.

The short answer

When you cut an onion, you break open its cells. Two chemicals that were stored separately in those cells mix together and react to form a volatile gas called syn-propanethial S-oxide. This gas floats up from the cutting board and reaches your eyes within seconds. When it contacts the moisture on your eye's surface, it reacts with water to form a dilute sulfuric acid. Your eye's nerve endings detect this irritant immediately and signal the lacrimal glands to produce tears to dilute and flush it away. The burning and watering you feel is your eye doing exactly what it is supposed to do when something harmful lands on it.

Person wiping tears from their eyes while cutting onions on a kitchen counter

The culprit gas

Syn-propanethial S-oxide, a volatile sulfur compound released when onion cells are ruptured

What it becomes in your eye

The gas reacts with eye moisture to form a dilute sulfuric acid, which irritates nerve endings

Why onions make this gas

It evolved as a defense mechanism against insects and animals that try to eat the onion

Best prevention

Sharp knife, cold onion, and good ventilation reduce how much gas reaches your eyes

Visual answer

From Cut Onion to Tears: The Chemistry in Four Steps

How cutting an onion triggers a chemical reaction that ends in watering eyes.

Knife ruptures onion cells

Cutting breaks open cells that contain two separately stored chemicals: sulfenic acids and an enzyme called alliinase. When the cells break, these mix for the first time.

Chemical reaction produces the gas

Alliinase rapidly converts sulfenic acids into syn-propanethial S-oxide, a volatile gas that immediately begins evaporating and rising from the cut surface.

Gas reaches the eyes

The gas is lighter than air and disperses upward. Within seconds it contacts the thin moisture layer covering the eye's surface.

Acid forms, tears flush it out

The gas reacts with water in the eye to form dilute sulfuric acid. Nerve endings signal the lacrimal glands, which flood the eye with tears to dilute and wash away the irritant.

Diagram showing onion cell rupture, enzyme reaction, gas formation, eye contact, and tear response

Real reason

Onions Store a Chemical Weapon in Two Separate Compartments

An intact onion is completely harmless to your eyes. The onion keeps sulfenic acid precursors in one part of the cell and the enzyme alliinase in another. As long as the cell walls are intact, the two never meet. The moment a knife, teeth, or any mechanical force breaks the cell, the compartments rupture and the contents mix. The enzyme works fast, converting the sulfenic acids into syn-propanethial S-oxide in under a second.

This is a defense mechanism. Onions evolved this chemical reaction to deter insects and other animals from eating them. The release is triggered specifically by damage, so the plant only produces the irritant when it is under attack. The same general strategy is used by many plants: store the components of a chemical weapon separately and let them mix only when the plant is damaged.

The gas is water-soluble, which is why it reacts so readily with the moisture on your eye surface. It is also why chilling an onion before cutting helps. Colder temperatures slow the enzymatic reaction, reducing the speed at which the gas is produced and giving it less time to volatilize and reach your eyes. Cutting under water eliminates the problem almost entirely, because the gas dissolves into the water before it can travel.

Myth vs reality

Myth vs Reality

What people think

Onions just happen to have a chemical that makes eyes water

It is not accidental. The lachrymatory response, a term that literally means tear-inducing, is a defense mechanism the onion plant evolved over millions of years. The two-compartment system exists precisely because the weapon is only useful when the plant is being attacked.

What actually happens

Onions evolved the two-compartment system as a deliberate defense

The chemical weapon only forms when the cell is broken. Storing the components separately means the onion is harmless until something eats or cuts it. At that point the reaction triggers instantly, deterring the attacker.

Prevention methods

Ways to Reduce Crying When Cutting Onions

Use a sharp knife

A sharp blade crushes fewer cells than a dull one, releasing less gas per cut. One of the most practical methods.

Chill the onion first

Cold temperatures slow the alliinase enzyme, reducing how fast the gas is produced after cells break.

Cut near a running fan or open window

Ventilation disperses the gas away from your face before it can reach your eyes.

Cut under water or near a running tap

Water dissolves the gas on contact, preventing it from ever becoming airborne. Very effective but awkward for large cuts.

Wear goggles

Creates a physical barrier. Completely effective and mostly used by professional kitchen workers handling large volumes.

Tiny note

A lachrymase-free onion was created in a lab

In 2008, researchers in New Zealand used RNA interference to silence the gene that produces the lachrymase enzyme in onions. The resulting onions retained their flavor compounds but did not produce syn-propanethial S-oxide when cut. The modified onions were being developed for commercial cultivation, targeting people who cook with onions frequently. As of the early 2020s, they had not yet reached mass market shelves.

Quick answers

Common questions

Why do some onions make you cry more than others?

Different onion varieties contain different concentrations of sulfenic acid precursors. Yellow onions tend to produce more of the irritant gas than sweet onions like Vidalia, which have lower sulfur compound levels due to lower-sulfur soil. Red and white onions fall in between.

Does holding bread in your mouth while cutting onions actually work?

The idea is that bread absorbs the gas before it reaches your eyes. There is limited evidence it works reliably. The gas disperses quickly and does not all travel through your mouth. Ventilation or chilling the onion is more consistently effective.

Why do the tears from cutting onions feel different from emotional tears?

They are chemically different. Reflex tears from irritants like onion gas are thinner and watery, designed to dilute and flush. Emotional tears contain more proteins, lipids, and hormones. Both are produced by the lacrimal glands, but in response to different signals.

Does cooking an onion destroy the chemical that causes tears?

Yes. Heat breaks down syn-propanethial S-oxide and deactivates the alliinase enzyme. Cooked onions do not cause tearing. Microwaving an onion briefly before cutting is a quick way to deactivate the enzyme without fully cooking the onion.

Why do some people cry much more than others from onions?

Individual sensitivity varies. The density of irritant-detecting nerve endings on the eye surface, tear production rate, and even contact lens use can all affect how strongly someone reacts. People who wear contact lenses often report less reaction because the lenses provide a partial barrier.

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