Why Do Onions Make You Cry, and Can You Actually Stop It?

The first cut is usually harmless.

Then you make another.

A few seconds later, your eyes start stinging. You blink harder. Your vision gets watery. Before you've finished chopping, you're standing over the cutting board with tears running down your face because you had the audacity to make dinner.

It's one of those everyday things that sounds ridiculous until you think about what's actually happening.

An onion isn't simply "making you cry." Cutting it sets off a chemical reaction inside the onion, produces a volatile irritant, and sends that irritant toward your eyes. Your eyes then respond by flooding themselves with tears to get rid of it.

And yes, there are ways to reduce it.

Some work surprisingly well. Some probably deserve to stay in the kitchen folklore bin.

So what actually happens when the knife hits the onion? And which of the usual tricks are worth trying?

Let's follow the whole thing from the first slice to the tears.

The onion isn't crying. It's defending itself.

An onion is a living plant, and its chemistry has a job beyond making your stew taste good.

Inside an intact onion cell, several compounds that normally remain separated are stored in different compartments. One group includes sulfur-containing amino acid sulfoxides. Another contains enzymes, including alliinase.

As long as the cells stay intact, they're mostly minding their own business.

Then your knife arrives.

The blade ruptures the cells, mixing compounds that weren't supposed to meet. That starts a chain of reactions that eventually produces the chemical responsible for the familiar eye sting.

The compound is called syn-propanethial-S-oxide, often simply called the onion's lachrymatory factor. Scientists identified it as the volatile compound responsible for the tears.

"Lachrymatory" sounds complicated, but it basically means tear-producing.

So the onion isn't releasing some mysterious tear gas that was sitting there waiting for you.

You're creating it by cutting the onion.

That's the strange part.

What happens after you make the first cut?

Here's the chain in plain English.

Knife hits onion → cells rupture → enzymes and sulfur compounds mix → chemical reactions occur → a volatile irritant forms → the irritant reaches your eyes → your eyes react → tears appear.

The chemistry is more complicated than that, but that's the useful version.

One of the key steps involves the enzyme alliinase acting on sulfur-containing compounds in the onion. This produces sulfenic acids, which then become involved in another reaction involving an enzyme called lachrymatory factor synthase, or LFS.

That reaction produces syn-propanethial-S-oxide.

Researchers actually worked out that LFS was a missing piece of the puzzle because onions without its activity don't form the normal tear-producing compound in the same way.

So the onion's famous ability to make you cry depends on a surprisingly specific bit of plant chemistry.

And it all starts because you broke the cells.

Why does the chemical make your eyes water?

Your eyes have a pretty sensible response to irritating chemicals.

They don't know you're preparing dinner.

They just detect something irritating them.

When the onion's volatile compound reaches the surface of your eye, it activates sensory nerves associated with irritation. Your body responds by producing tears.

Those tears aren't emotional tears. They're basically your eye's rinse cycle.

The extra fluid helps dilute and wash away the irritant.

That's why the reaction can feel almost immediate. You're not slowly becoming sad because you remembered something from 2009. Your eyes are reacting to a chemical stimulus.

Research into onion tearing also points to the role of the trigeminal sensory system, which is involved in detecting irritating sensations such as burning and stinging.

And that burning sensation is exactly why your eyes can start watering before you've even finished the first few slices.

So why doesn't eating a raw onion make you cry?

This is a good question because you're putting the onion in your mouth, not your eyes.

The difference is what happens to the onion.

When you bite an onion, you also break cells and trigger chemical reactions. That's part of why raw onion has such a strong taste and smell.

But the tear-producing compound needs to reach your eyes to cause the classic onion crying response.

When you're chopping, you're releasing volatile chemicals directly into the air around your face.

Your eyes are sitting right there.

And if you're leaning over the cutting board, you're basically putting your face in the perfect position to receive the stuff.

Your mouth, meanwhile, isn't a pair of exposed, water-covered sensory organs sitting directly above the cutting board.

That's also why keeping your face farther away can help. Less of the airborne irritant reaches your eyes.

Why do some onions make you cry more than others?

You've probably noticed this.

One onion barely bothers you.

Another turns chopping into a full emotional event.

That's not necessarily your imagination.

Different onions can have different levels of pungency and different amounts of the sulfur-containing compounds involved in the reactions that produce irritating chemicals.

Growing conditions matter too. Sulfur availability in the soil can influence onion pungency, and some sweet or low-pungency onions tend to produce less of the chemistry associated with the familiar sting.

That means "onions make me cry" isn't quite as simple as saying every onion behaves identically.

The onion itself matters.

Your sensitivity matters too.

A recent study involving more than 1,000 participants found that people differed considerably in how strongly they experienced onion tearing, with stronger onion tearing associated with greater sensitivity to certain irritating sensations.

So if your friend can chop three onions without blinking while you're reaching for a towel after half of one, you may simply be more sensitive to the irritant.

Does a sharp knife actually help?

This one has a much better scientific case than some of the stranger kitchen tricks.

A sharp knife cuts through onion tissue more cleanly.

A dull knife tends to crush and deform more of the tissue before it actually cuts through it.

That matters because damaging more cells means creating more opportunities for the chemistry to happen and for irritating material to be released.

Recent research has added another interesting piece to this.

Scientists using high-speed imaging found that onion cutting can produce tiny droplets and aerosols as the onion tissue fractures. Faster cutting and blunter blades produced more droplets and greater launch energy.

That gives us a physical explanation for something cooks have noticed for years.

It's not only about the chemical reaction.

How you cut the onion also affects what gets thrown into the air.

So yes, keeping your knife sharp is a sensible way to reduce the mess and potentially reduce your exposure to irritating material.

Just don't turn "sharp knife" into "rush through the onion as fast as possible."

The same research found that faster cutting can increase droplet release, particularly with a blunt blade.

Clean cuts.

Controlled movements.

That's the better combination.

Does chilling an onion stop the tears?

This one has some science behind it too.

Cooling slows many enzyme-driven reactions.

Since onion tearing depends on a chain of enzymatic reactions, lowering the onion's temperature can slow down the chemistry that produces the irritant.

The American Chemical Society specifically recommends refrigerating onions before cutting as a way to slow the conversion and release of tear-producing compounds.

That doesn't mean a cold onion becomes chemically harmless.

It just means you're giving the reaction a harder time.

There's a catch, though.

Research into onion cutting suggests temperature can affect not only the chemistry but also the physical way droplets are released. One recent study reported that refrigerating onions produced more droplets of similar velocity in its cutting experiments, even though other evidence and kitchen guidance support chilling as a way to reduce irritation.

So I wouldn't describe chilling as a guaranteed magic trick.

It's better described as a useful method that can reduce the sting for many people, especially when combined with other measures.

If you're going to try it, cooling the onion before chopping is easy enough.

No special equipment required.

What about cutting the onion underwater?

This idea actually makes chemical sense.

If the irritating compound is being released around the onion and has to travel through the air to reach your eyes, putting the onion underwater changes the situation.

Water can capture or dissolve some of the compounds involved, reducing what escapes into the surrounding air.

The problem is practical.

You're now trying to chop an onion while it's wet and potentially slippery.

That's not a tradeoff I'd recommend taking lightly with a kitchen knife.

An experiment published as a science activity has students compare room-temperature onions, refrigerated onions, onions cut underwater, and onions cut while wearing goggles.

And real-world testing has found the same problem: underwater cutting can reduce the tears, but handling a slippery onion in a sink can make cutting awkward and less safe.

So yes, underwater cutting can work.

But that doesn't automatically make it the best method.

I'd rather use a sharp knife, some ventilation, and a sensible cutting position than turn my kitchen sink into an onion laboratory.

Do goggles really work?

This is probably the least mysterious trick on the list.

The irritant has to reach your eyes.

Goggles create a physical barrier.

That's it.

You don't need to change the onion's chemistry. You don't need to trick your tear glands. You simply stop the irritating compound from reaching the exposed surface of your eyes.

Properly sealed goggles can therefore be extremely effective.

Regular glasses aren't quite the same thing because they leave gaps around the sides.

Swimming goggles can work better because they seal around the eyes. Testing by ABC found goggles to be highly effective, although the exact setup obviously matters.

And yes, onion goggles look ridiculous.

But if the alternative is crying into your chopping board, fashion may have to take the afternoon off.

Does a fan help?

This one makes sense for a completely different reason.

You don't necessarily have to stop the onion from producing the irritant.

You can stop it from hanging around your face.

A fan or kitchen extractor can move the irritating vapour away from your eyes instead of allowing it to accumulate directly in front of you.

The Japanese Central Customs Laboratory lists ventilation as one method for reducing exposure, along with chilling, goggles, and using a sharp knife.

Position matters, though.

If your fan simply blows the onion fumes directly into your face, congratulations.

You've invented a tiny onion flamethrower.

You want airflow that carries the irritant away from your face, not toward it.

Does leaving the root intact help?

You've probably heard that you should leave the root end alone until the very end.

There's a reason this advice keeps circulating.

Some sources, including Poison Control's review of onion-tear prevention, note that the root end contains a higher concentration of sulfur compounds and recommend leaving it intact while cutting.

But this isn't a magical force field.

The onion isn't keeping all its tear-producing chemistry inside the root.

You're still cutting through the bulb and triggering reactions throughout the damaged tissue.

So if leaving the root intact helps, think of it as one small part of a larger strategy rather than the secret button that switches off onion tears.

What about chewing gum, bread, matches, or sticking out your tongue?

This is where onion folklore gets entertaining.

People have suggested chewing gum, holding bread in your mouth, lighting a match nearby, putting your tongue out, holding your breath, and plenty of other tricks.

The problem is that many of these methods don't address the main issue.

The onion is producing an airborne irritant.

The irritant reaches your eyes.

Your eyes react.

So the most sensible solutions either reduce the amount produced, slow the chemistry, move it away, or physically block it from reaching your eyes.

Chewing gum doesn't magically remove the irritant from the air.

Neither does holding a piece of bread in your mouth.

You might find one of these tricks personally useful, but that doesn't mean it has a strong mechanism behind it.

That's a useful rule for evaluating kitchen hacks generally.

Ask what the trick is actually supposed to change.

If it doesn't affect the chemistry, airflow, or your eyes, be skeptical.

Why do tears sometimes start before you feel the burn?

Because your sensory system is fast.

Your eyes don't wait around for you to consciously decide that the onion is irritating.

The sensory nerves detect the irritant and trigger a reflex response.

That's why you can go from perfectly normal to watery-eyed in what feels like seconds.

The tears are automatic.

You don't have to decide to cry.

In fact, you can be laughing while your eyes are producing tears because the two situations have completely different causes.

The onion has basically triggered your eye's emergency rinse system.

Can you stop onion tears completely?

For most people, not reliably with a normal kitchen setup.

You can reduce the exposure, sometimes dramatically.

But there's no universal trick that guarantees completely dry eyes every time.

Poison Control notes that there isn't a foolproof method for avoiding onion tears.

If you want the closest thing to a guaranteed physical solution, sealed eye protection is hard to argue with because you're directly blocking the irritant from reaching the eyes.

But for ordinary cooking, you don't need to go that far.

A sharp knife, reasonable ventilation, a chilled onion, and keeping your face out of the rising fumes can make the experience much easier.

And if you're especially sensitive, goggles are the straightforward answer.

So which onion trick actually deserves a place in your kitchen?

Here's how I'd rank the common approaches based on how directly they address the problem.

• Sharp knife: Worth doing. It reduces crushing and tissue damage, and recent research supports the idea that blade sharpness affects droplet release.
• Chill the onion: Worth trying. Lower temperature slows the enzyme-driven chemistry, although it isn't a guarantee.
• Fan or extractor: Sensible. Move the irritating material away from your face rather than letting it collect around you.
• Goggles: Extremely effective if you don't mind looking like you're about to repair a swimming pool.
• Cut underwater: Chemically reasonable, but awkward and potentially unsafe because you're handling a knife around a slippery onion.
• Leave the root until last: Reasonable as part of your cutting technique, but don't expect miracles.
• Bread, gum, matches and similar tricks: Much weaker evidence and less convincing mechanisms.

There's also another option that's often overlooked.

Buy a milder onion.

Some varieties naturally have lower pungency because of their chemistry and growing conditions. Low-pungency onions have even been developed specifically to reduce the compounds associated with the problem.

The strangest part is that the onion isn't trying to make you cry

From the onion's point of view, you're basically attacking it.

An insect bites into the bulb and breaks its cells.

You put a knife through it and break its cells.

The plant can't tell the difference in the way we do.

Its defense chemistry gets activated.

The onion's stored sulfur compounds and enzymes suddenly interact, a volatile irritant forms, and that irritant ends up in your eyes.

We just happen to be much bigger than the insect the system evolved to discourage.

And there's a lovely bit of irony here.

The same chemistry that makes a raw onion sting your eyes is part of what gives onions their characteristic flavor and aroma. Damage the cells and you don't just release the tear-producing compound. You also trigger a whole collection of reactions that change what the onion smells and tastes like.

Then heat changes the chemistry again.

That's why a raw onion and a slowly caramelized onion can taste like completely different foods.

The knife starts the chemistry.

The pan takes it somewhere else.

The easiest way to make onion chopping less miserable

You don't need a bizarre collection of kitchen tricks.

Start with a sharp knife.

Chill the onion beforehand if you're particularly sensitive.

Keep your face a reasonable distance from the cutting board.

Use ventilation that carries the fumes away from you.

And if onions still turn you into a mess every single time, wear sealed goggles.

That's really what the science comes down to.

You aren't fighting the onion. You're managing the chemistry.

The knife starts a reaction that the onion evolved to use as a defense. Your eyes detect the resulting irritant and immediately try to wash it away.

So the next time you're chopping onions and suddenly start crying, there's no mystery left.

You didn't become emotional.

You just lost a fight with plant chemistry.