Why Do Onions Make You Cry? The Real Reason Is More Fascinating Than You Think

You’re making dinner. Everything is going normally.

You pick up an onion, place it on the cutting board and make the first few cuts.

A minute later, your eyes start burning.

Then they water.

Then you’re blinking constantly, wiping your face, and wondering why one ordinary vegetable can make you cry like this. 🧅😭

And no, it isn’t because the onion is somehow “making you sad.”

There is a fascinating chemical reaction happening right in front of your eyes.

When you cut into an onion, you damage its cells. That releases enzymes and sulfur-containing compounds that normally stay separated inside the intact plant. Once they mix and react, they eventually produce a volatile chemical called syn-propanethial-S-oxide, commonly called the onion’s “lachrymatory factor.” When this compound reaches the surface of your eyes, it irritates them and triggers reflex tearing.

But that’s only the beginning of the story.

What Actually Happens When You Cut an Onion?

An intact onion isn’t constantly filling the air around you with an eye-burning chemical.

Most of the important chemistry is happening inside the onion’s cells.

When you slice through the onion, you’re essentially breaking open those cells.

This allows substances that were previously separated to come into contact with one another.

One important enzyme is alliinase. Once the onion tissue is damaged, this enzyme helps convert sulfur-containing precursors into other compounds, including 1-propenesulfenic acid. Another enzyme, called lachrymatory factor synthase, helps turn that intermediate into the volatile compound responsible for the familiar eye irritation.

That final compound is syn-propanethial-S-oxide.

And because it is volatile, it can leave the freshly cut onion and travel through the air.

Eventually, it reaches your eyes.

That’s when your body’s defense system kicks in.

Your Eyes Aren’t “Crying From Emotion”

This is an important distinction.

The tears you produce while chopping onions are reflex tears, not emotional tears.

Your eyes have protective mechanisms designed to respond when something irritating reaches their surface.

When the onion compound contacts the conjunctiva — the delicate tissue covering the front of the eye and inside of the eyelids — it creates an irritating sensation.

Your nervous system detects that irritation.

Then your tear glands respond.

More tears are produced.

The purpose is simple:

Get the irritant away from the surface of the eye.

So when you are standing over a cutting board with tears running down your face, your body isn’t confused.

It’s actually doing exactly what it is supposed to do.

The tears are essentially your eyes saying:

“Something irritating just got in here. Wash it away.”

Why Does It Burn Before You Start Crying?

You’ve probably noticed that onion cutting usually follows a familiar sequence.

First, you may feel a slight sting.

Then your eyes become uncomfortable.

Then they begin watering.

That’s because the onion compound is acting as an irritant before the tear response becomes obvious.

The sensory nerves around the eyes detect the irritation, and the nervous system triggers protective tearing.

This is part of a broader sensory system involving the trigeminal nerve, which helps detect irritating sensations such as burning, stinging and other chemical irritants. Recent research into onion tearing has found associations between how strongly people report onion-induced tearing and their self-reported sensitivity to irritating sensations in the nose and face.

So if one person can chop three onions without looking bothered while another person looks like they’ve just watched a heartbreaking movie, their sensory sensitivity may not be exactly the same.

Why Doesn’t Everyone Cry the Same Amount?

This is something people often notice.

You may have a family member who can chop onions completely comfortably.

Meanwhile, your eyes start watering almost immediately.

Why?

There isn’t one simple answer.

The amount of tearing can depend on several factors, including:

• How much irritating compound reaches your eyes
• How the onion is cut
• How close your face is to the onion
• Air movement in the kitchen
• The onion itself
• Individual sensitivity to irritation
• How quickly the chemical reaches your eyes

A 2026 study involving 1,001 participants found that 73% reported experiencing tears while cutting onions. The researchers also found that people who reported more onion tearing tended to report greater sensitivity to certain irritating sensations, although the objective testing results were mixed.

In other words, if onions make you cry more than the person next to you, it doesn’t necessarily mean you’re doing something wrong.

Your eyes and sensory system may simply be responding differently.

Does the Type of Onion Matter?

Yes, the chemistry of onions isn’t completely identical from one onion to another.

Different onion varieties can contain different amounts and proportions of sulfur-containing compounds and precursors involved in flavor and lachrymatory-factor production.

That’s one reason you may notice that one onion barely bothers you while another seems determined to make you miserable.

Freshness, growing conditions, variety and storage can all influence onion chemistry.

However, it’s not accurate to say that one simple characteristic — such as color alone — determines exactly how much an onion will make you cry.

The chemistry is more complicated than that.

Here’s Something Surprisingly Important: Your Knife

Your knife can influence what happens during cutting.

A dull knife doesn’t necessarily make onions “more toxic,” but cutting mechanics matter.

When you crush, squeeze or tear through onion tissue, you can cause more physical damage to the cells.

Recent research examining onion cutting found that faster cutting and blunter blades can increase the number and energy of droplets released from the onion during cutting.

A sharp knife, used safely, allows cleaner cuts.

That doesn’t magically eliminate the onion’s chemistry, because the chemical reaction still occurs when the cells are damaged.

But cleaner cutting can change how much material is mechanically thrown into the surrounding air.

So keeping your knife properly sharpened isn’t just about making cooking easier.

It can also change what happens around the cutting surface.

Does Cutting the Onion Under Water Stop the Tears?

You may have heard about all kinds of tricks.

Cut onions underwater.

Hold bread in your mouth.

Light a candle.

Wear swimming goggles.

Put the onion in the freezer.

Use a fan.

Some of these ideas are based on real principles, while others have little scientific support.

The basic goal behind the more sensible approaches is to reduce how much of the volatile irritant reaches your eyes.

For example, increasing ventilation can help move airborne compounds away from your face.

Keeping your face farther from the cutting board can also reduce direct exposure.

And using a sharp knife may reduce some of the mechanical effects associated with cutting.

But don’t assume that every viral “onion hack” online has been scientifically proven.

Some tricks may work for certain people simply because they change airflow, temperature, cutting speed or exposure.

What About Chilling the Onion?

This is another extremely common suggestion.

People often recommend putting an onion in the refrigerator before cutting it.

There is a reasonable chemical explanation for why temperature might influence the process: chemical reactions and the movement of volatile compounds can be affected by temperature.

But that doesn’t mean chilling an onion completely prevents tearing.

If it helps you, fine.

Just don’t expect a refrigerator to turn a strong onion into a completely tear-free vegetable.

And remember: if you’re preparing food, proper food handling matters more than chasing a perfect anti-tear trick.

Why Does a Fan Sometimes Help?

This one makes intuitive sense.

If the irritating compound has to travel through the air before reaching your eyes, airflow can influence where that compound goes.

A fan positioned so that it moves air away from your face may reduce how much of the irritant reaches your eyes.

But there’s an important detail:

Don’t simply blast air directly toward the onion and your face.

That could potentially push more irritating material toward you.

The goal is to improve ventilation away from your breathing and eye area, not to create a stronger stream toward your face.

Can You Become “Used to” Onion Tears?

You may have noticed that someone who cooks onions every day seems less bothered by them.

It can be tempting to assume their eyes have somehow become immune.

That’s probably too simple.

Individual sensitivity varies, and repeated exposure doesn’t necessarily mean the eye stops responding entirely.

People also develop different cutting habits.

Someone who cooks onions frequently may naturally:

• Keep their face farther away
• Cut more quickly
• Use a sharper knife
• Position themselves differently
• Improve kitchen ventilation
• Choose different onion varieties

So what looks like “immunity” may partly be experience and technique.

Is Crying From Onions Bad for Your Eyes?

For most people, onion-induced tearing is a temporary protective response.

The compound irritates the eye, and your body responds by producing tears to help clear the irritant.

Once you move away from the onion, the irritation generally settles.

That is very different from an eye injury or chemical exposure involving a strong corrosive substance.

Still, if someone experiences severe or persistent eye pain, significant redness, vision changes or symptoms that don’t settle after leaving the exposure, that’s not something to simply blame on an onion. It would be sensible to seek medical advice.

And if you wear contact lenses and find onion fumes particularly uncomfortable, remember that contact lenses can alter how the eye surface responds to irritants. If your eyes become significantly uncomfortable, follow the instructions given by your eye-care professional.

So Why Did Humans Evolve This Reaction?

Here’s the part that makes the whole thing even more interesting.

The chemicals involved in onion flavor and irritation are part of the plant’s defensive chemistry.

The onion doesn’t know you’re trying to make dinner.

When its cells are damaged, its biochemical system responds.

The resulting chemistry produces compounds that can be irritating to animals and microorganisms. Scientists have described this pathway as part of the onion’s chemical defense system.

So, in a strange way, when you’re chopping an onion, you’re triggering part of the plant’s defense mechanism.

You aren’t making the onion “angry.”

You’re simply causing enough cellular damage to activate chemistry that normally stays hidden inside the plant.

Why Does the Onion Smell Stronger After Cutting?

There’s another clue that the same process is happening.

An intact onion has a characteristic smell, but cutting it releases many volatile compounds into the surrounding air.

Those compounds contribute to the distinctive aroma you notice immediately after slicing.

Some sulfur-containing compounds are associated with the onion’s flavor and smell, while the lachrymatory factor is specifically responsible for the eye-irritating effect. The chemistry is interconnected but not identical.

That’s why the onion can simultaneously:

smell strong, taste sharp and make your eyes water.

Different compounds and reactions contribute to the overall experience.

What Is the Best Way to Reduce Onion Tears?

There isn’t a single guaranteed trick, but you can reduce exposure.

1. Use a sharp knife

A sharp knife allows controlled cutting and may reduce some of the violent crushing and droplet release associated with dull or fast cutting.

2. Improve ventilation

Use a kitchen exhaust fan or another method of moving air away from your face.

3. Keep your face farther away

You don’t need to lean directly over the cutting board.

A little extra distance can reduce direct exposure.

4. Work efficiently

The longer you’re standing directly over a freshly cut onion, the more time there is for irritating compounds to reach your eyes.

5. Try chilling the onion

If refrigeration seems to reduce irritation for you, it’s a reasonable kitchen experiment — but don’t expect it to eliminate tears completely.

6. Don’t rub your eyes

This is especially important.

If your eyes are already irritated, rubbing them can make the discomfort worse.

Instead, step away from the onion and allow your eyes to clear naturally.

If you need to rinse your eyes because of significant irritation, use clean water or an appropriate sterile eyewash rather than putting random household substances in your eyes.

The Bottom Line

So, why do onions make you cry?

Because cutting an onion damages its cells and triggers a chain of chemical reactions that produces a volatile lachrymatory compound called syn-propanethial-S-oxide.

That compound reaches your eyes, irritates the sensitive surface and activates a protective reflex.

Your tear glands respond by producing tears.

Those tears help wash the irritant away.

So the next time you’re standing in your kitchen with tears streaming down your face, remember what’s actually happening.

The onion isn’t making you emotional.

Your eyes are defending themselves.

And that ordinary onion sitting on your cutting board is carrying out a surprisingly sophisticated piece of plant chemistry — all because you decided to cut it.

One small slice starts the reaction. Your eyes finish the job. 🧅😭