Why Do Onions Make Us Cry? The Chemistry Behind the Tears

Have you ever started chopping an onion while cooking and suddenly felt your eyes begin to sting and water uncontrollably?

Within seconds, tears start flowing, even though you’re not emotional at all.

This common kitchen experience has puzzled people for centuries. But the reason onions make us cry isn’t just a random reaction — it’s actually the result of a complex chemical defense system inside the onion.

When an onion is cut, it releases a chemical that irritates the eyes and triggers the body’s natural tear response. Scientists have studied this process in detail, revealing a fascinating interaction between plant chemistry and human biology.

Let’s explore the real science behind why onions make us cry.

What Happens Inside an Onion When You Cut It

An onion may look simple from the outside, but inside it contains a carefully organized chemical system.

When the onion is intact, different chemicals are stored in separate compartments within its cells. These compounds remain harmless as long as the onion’s cells are not damaged.

However, the moment you slice into the onion with a knife, the cells are ruptured, and these chemicals begin to mix.

This mixing triggers a series of chemical reactions that produce a volatile compound that can easily travel through the air.

Once this compound reaches your eyes, it causes irritation — and that’s when the tears begin.

The Chemical That Causes Tears

The main chemical responsible for making us cry when cutting onions is called syn-propanethial-S-oxide.

This compound is a type of lachrymatory agent, which means it causes tearing of the eyes.

Here’s how the process works:

1. When an onion is cut, enzymes inside the onion cells are released.
2. These enzymes break down sulfur-containing compounds naturally present in the onion.
3. This reaction produces syn-propanethial-S-oxide.
4. The chemical quickly evaporates and travels into the air.

Because this compound is volatile, it can easily reach your eyes while you are cutting the onion.

Why the Eyes Start Burning

When syn-propanethial-S-oxide reaches the eyes, it dissolves in the thin layer of moisture covering the eye surface.

This reaction forms mild sulfuric compounds that irritate the sensitive nerve endings in the eye.

The eyes interpret this irritation as a threat and immediately activate a protective response.

The body responds by producing reflex tears from the lacrimal glands.

These tears serve an important purpose: they help wash away the irritating chemical and protect the eyes from potential damage.

That’s why the tears often appear suddenly and in large amounts while cutting onions.

Why Onions Developed This Chemical Defense

The tear-producing chemical in onions is not meant to target humans. It is actually part of the plant’s natural defense mechanism.

Plants cannot run away from predators like animals can, so many plants develop chemical defenses to discourage insects and animals from eating them.

In onions, the release of irritating chemicals helps deter pests and animals that might try to feed on the plant.

When the onion is damaged, the chemical reaction produces compounds that can irritate the eyes and tissues of animals, making the plant less appealing as food.

This defensive strategy helps onions survive and reproduce in the wild.

Why Some Onions Make You Cry More Than Others

Not all onions produce the same level of tears.

Several factors influence how strong the tear-producing effect will be.

Onion Variety

Different types of onions contain different levels of sulfur compounds.

For example:

• yellow onions often produce stronger reactions
• white onions may be slightly milder
• sweet onions typically contain lower sulfur levels

Because sweet onions contain fewer sulfur compounds, they often cause less eye irritation when cut.

Growing Conditions

The soil where onions grow also affects their chemical composition.

Onions grown in sulfur-rich soil tend to produce more sulfur compounds, which can make them more irritating when cut.

Environmental conditions such as rainfall and soil nutrients can therefore influence how strong an onion’s tear-producing effect will be.

Freshness of the Onion

Fresh onions may release stronger chemical reactions because their cellular structures and enzymes are still highly active.

As onions age during storage, some of these reactions may become slightly weaker.

The Exact Chemistry Behind Why Onions Make Us Cry

To fully understand why onions make us cry, we need to look more closely at the chemical reactions happening inside the onion when it is cut. What seems like a simple kitchen inconvenience is actually the result of a fascinating series of biochemical processes.

The Role of Sulfur Compounds in Onions

Onions naturally contain sulfur-rich molecules that come from the soil in which they grow. As onion plants develop, they absorb sulfur from the ground and incorporate it into various organic compounds.

These sulfur-containing compounds help give onions their:

• strong flavor
• distinctive smell
• chemical defense system

When the onion remains whole, these chemicals are safely stored inside different parts of the plant cells and do not cause irritation.

However, when you cut into the onion, the cell structure is broken and the chemicals begin to interact.

The Enzyme That Starts the Reaction

Inside onion cells is an enzyme called alliinase.

Enzymes are biological molecules that speed up chemical reactions. When the onion is sliced or crushed, alliinase is released and begins reacting with the onion’s sulfur compounds.

The process works like this:

1. Onion cells are damaged by cutting.
2. The enzyme alliinase is released.
3. Alliinase reacts with sulfur-containing molecules.
4. This reaction produces a new compound known as sulfenic acid.

However, the chemical process does not stop there.

Formation of the Tear-Producing Gas

Sulfenic acids formed during the reaction are unstable. Inside the onion, another specialized enzyme called lachrymatory factor synthase converts these acids into a volatile compound.

This compound is syn-propanethial-S-oxide, the chemical responsible for making our eyes water.

Because this compound easily turns into gas, it spreads quickly through the air while you are cutting the onion.

Once the gas reaches your eyes, it dissolves in the moisture covering the eye surface and causes irritation.

Why the Eyes Produce Tears

Human eyes are extremely sensitive organs designed to protect themselves from irritants.

When the tear-producing gas from onions reaches the eyes, it activates sensory nerves located on the eye’s surface.

These nerves send a signal to the brain that something irritating is present. In response, the brain activates the lacrimal glands, which produce tears.

The purpose of these tears is to:

• dilute the irritating chemical
• flush it away from the eye surface
• protect the eye tissues

This protective reflex is why onion tears often appear quickly and intensely.

Why Refrigerating Onions Can Reduce Tears

You may have heard the common cooking tip that chilling onions before cutting them can reduce tears.

This advice actually has a scientific basis.

Lower temperatures slow down chemical reactions and reduce the rate at which volatile compounds evaporate into the air.

When onions are cold:

• enzyme activity slows
• chemical reactions occur more slowly
• the tear-producing gas spreads less rapidly

As a result, fewer irritating compounds reach the eyes.

Although refrigeration does not completely eliminate the effect, it can noticeably reduce eye irritation while cutting onions.

Why Sharp Knives Help

Another useful technique for reducing onion tears is using a sharp knife.

A dull knife crushes onion cells more aggressively, damaging a larger number of cells at once. This releases more enzymes and sulfur compounds into the air.

A sharp knife makes clean cuts, which reduces the amount of cellular damage and limits the chemical reaction.

Less damage means fewer tear-producing compounds released into the air.

Why Cutting Onions Under Running Water Works

Some people cut onions under running water or near a fan.

This works because the tear-producing gas is water soluble and easily dispersed in air.

Water can absorb some of the gas before it reaches your eyes, while airflow from a fan can carry the chemical away from your face.

Both methods reduce the concentration of the irritant that reaches the eyes.

Interesting Facts, Myths, and Ways to Stop Onion Tears

By now, it’s clear that onions make us cry because of a specific chemical reaction that produces a volatile irritant gas. But there are still several interesting facts and misconceptions about onion tears that many people don’t know.

Understanding these details can help explain why this reaction varies from person to person and why certain kitchen tricks actually work.

Why Some People Cry More Than Others When Cutting Onions

Not everyone reacts to onions the same way. Some people start tearing up almost immediately, while others can cut several onions with only mild irritation.

Several factors influence how strongly someone reacts to onions.

Sensitivity of the Eyes

Human eyes contain sensitive nerve endings that detect irritants. Some individuals naturally have more sensitive eyes, which can trigger stronger tear responses.

People who wear contact lenses sometimes experience less irritation because the lenses create a small barrier that partially protects the eye surface.

Distance From the Onion

The closer your face is to the onion while cutting, the more tear-producing gas can reach your eyes.

Professional chefs often reduce irritation simply by keeping their face slightly farther away from the cutting board.

Air Circulation

If the kitchen has poor airflow, the tear-producing gas may accumulate near the cutting area.

In contrast, good ventilation or airflow can quickly disperse the gas, reducing the amount that reaches your eyes.

Are Onion Tears Harmful?

Although the burning sensation can be uncomfortable, onion tears are not harmful to the eyes.

The body’s tear response is a natural protective mechanism designed to remove irritants from the eye surface.

Once the tear-producing compound is diluted and washed away by tears, the irritation quickly disappears.

In fact, the tear response demonstrates how efficiently the body protects one of its most sensitive organs.

The Science Behind Common Onion-Cutting Tricks

Over time, many home cooks have developed methods to reduce onion tears. Some of these techniques are supported by scientific explanations.

Refrigerating the Onion

Cold temperatures slow chemical reactions and reduce the amount of gas released.

Using a Sharp Knife

Clean cuts damage fewer cells, which reduces the chemical reaction that produces the tear-causing compound.

Cutting Near a Fan or Vent

Air movement disperses the gas before it can reach the eyes.

Cutting Under Running Water

Water can dissolve some of the tear-producing gas and prevent it from traveling through the air.

A Fascinating Plant Defense Strategy

The chemistry that causes onion tears is actually a survival strategy developed by the plant.

Plants cannot escape predators, so many species rely on chemical defenses to discourage animals from eating them.

When an onion is damaged, the chemical reaction produces irritating compounds that can affect the eyes and tissues of animals.

Although humans still eat onions despite the irritation, this defense system likely helped onions protect themselves from certain pests in nature.

The Bottom Line

Onions make us cry because of a remarkable chemical reaction that occurs when their cells are damaged.

Cutting an onion releases enzymes and sulfur compounds that form syn-propanethial-S-oxide, a volatile gas that irritates the eyes.

The body responds by producing tears to wash away the irritant and protect the eye surface.

Although this reaction can be inconvenient in the kitchen, it reveals a fascinating interaction between plant chemistry and human biology.

What seems like a simple cooking problem is actually a complex natural defense system developed by onions to protect themselves from predators.

The next time your eyes water while chopping onions, you’ll know that those tears are the result of a sophisticated chemical reaction that plants evolved over millions of years.