Why Does Cutting an Onion Make You Cry?
― It's Made After the Cut
Hold a whole onion up to your nose and nothing happens — no tears. That's because the stinging compound isn't in the onion to begin with. The moment you cut it, the onion starts building a chemical from its own cells as raw material. It's a well-engineered plant trick.
Peel an onion and hold it close to your face. No tears. It smells, but it doesn't sting your eyes.
But the moment you cut into it, within seconds to a dozen seconds or so, your eyes start to sting. Something has decisively changed between before and after the cut.
Even stranger: a cooked onion doesn't make you cry. You can chop a fried onion without any trouble — and it's sweet and delicious besides.
So the compound only appears when a raw onion is cut. That's a remarkably narrow condition.
Inside an onion's cells, the raw material and the enzyme (the tool) that processes it are stored in different places. Keep them together and they'd react right away.
When a knife breaks the cells open, the material and the tool meet. The reaction runs in seconds, producing a gas that stings the eyes and rises into the air.
In short, the onion has a mechanism that only fires when it's damaged. Let's look at it step by step.
Why keep them "separate" at all?
For the onion, this setup has real advantages.
If it kept the eye-stinging compound ready-made, it would damage its own cells. And since it's a volatile substance, it would simply evaporate away if stored.
So the onion stores a harmless raw material and keeps the tool elsewhere. Only when an insect or animal bites into it and breaks the cells do the material and tool meet to form a weapon. It's safe to store day to day, yet fires instantly when needed — a clever design.
The same idea shows up widely across the plant world.
- Wasabi, mustard, daikon ― the more finely you grate them, the sharper they taste, because grinding triggers the reaction. A whole, uncut wasabi root isn't pungent at all
- Garlic ― chopping releases a strong smell. It's a similar mechanism to the onion, but the resulting compound differs, so no tears
- The smell of cut grass ― that green scent when you mow a lawn is also a compound made by the wounded plant
So the culinary wisdom that "cutting brings out the aroma" is really the kitchen exploiting a plant's defence reaction.
You're standing right in the middle of that reaction.
Why does it come out as "tears"?
The gas produced is light and spreads quickly through the air to reach your eyes. The surface of the eye is packed with nerves that sense pain and irritation.
When these nerves are stimulated, the brain concludes "something foreign has gotten in." A reflex kicks in to flush it out with tears. Blinking more often happens for the same reason.
So tears aren't so much the onion "making you cry" as your body protecting itself. Don't rub your eyes — the compound is on your hands too, and rubbing only makes it worse.
So how do you cut an onion without crying?
Once you understand the mechanism, you can pick countermeasures logically. Four things actually work.
Put it in the fridge about 30 minutes before cutting. Cold slows the enzyme down and makes the resulting compound less prone to evaporate. It suppresses both the reaction and the volatilisation.
A dull blade crushes the cells instead of cutting them. More crushed cells means more reaction. A blade that slices cleanly releases less of the compound.
The gas is no heavier than air, so it spreads out and dilutes. Run an extractor fan, point a fan at the board, don't lean in close. That alone cuts down how much reaches you.
This compound dissolves easily in water, so water absorbs it. It's harder to cut this way and the flavour can suffer, so only do it if it's practical.
Some tricks are less effective. Holding a chopstick in your mouth or holding your breath are both well-known, but it's the eyes being irritated, not the nose or mouth, so they're unlikely to help directly. Covering the eyes themselves, as with goggles, makes more logical sense.
Enzymes are proteins, so heat changes their shape and stops them working. A cooked onion doesn't make you cry because the tool itself has been broken.
And a fried onion turning sweet is down to several things happening at once: the sulphur-containing compound behind the sting breaks down into compounds that taste sweet, moisture leaves and concentrates the flavour, and the reaction that produces browning and a toasty flavour proceeds. The sweetness of caramelised onion isn't added sugar.
Something you can test in the kitchen
- Get two onions and chill just one for about 30 minutes in the fridge
- Cut the room-temperature one as usual. Count the time until your eyes start stinging
- Wash your hands, ventilate, and wait until your eyes settle
- Cut the chilled one the same way. Compare the time until it stings
- If you like, also compare a dull knife against a sharp one
The chilled one should be noticeably easier on the eyes. You can see for yourself that lower temperature slows the enzyme down. You'll be handling a knife, so take care. If your eyes sting, flush with running water without rubbing, and see an eye doctor if symptoms persist.
Summary
Cutting an onion makes you cry because the eye-stinging compound is assembled inside the cells the instant it's cut. Material and tool are stored apart and the mechanism only fires when damaged. It's a well-engineered defence the plant has evolved.
A whole onion holds nothing yet.
The knife is what triggers the reaction.
There are other examples of plants defending themselves with chemistry. How chili pepper heat acts as a "screening device that repels only mammals" is explained in this article. Why the same onion leaves a person with only tears, while it can seriously harm dogs and cats, is covered in the article on why you shouldn't give chocolate to dogs.
Want to know more? ― Terms, formulas, and links to the curriculumFrom junior-high science to open research questions, each level is labelled
- JHSCovered in junior-high science
- HighCovered in high-school "Biology Basics" / "Chemistry Basics"
- High+Covered in high-school "Biology" / "Chemistry," or in advanced/column sections of textbooks
- Univ.Not taught in high school — university-level specialist subjects (biochemistry, food science)
- ResearchNot even settled "textbook fact" at university — something researchers are actively investigating
JHSTerms for this mechanism
- Enzyme: a protein in living things that drives chemical reactions. The "tool" in this article. It's heat-sensitive and stops working when heated.
- Vacuole: a large sac inside a plant cell. The raw material is mainly stored here.
- Alliinase: the name of the enzyme found in onions and garlic. It performs the first step of breaking down the material.
- Lachrymatory factor: the name for the eye-stinging gas. Its formal name is syn-propanethial-S-oxide.
- Volatility: the tendency to become a gas at room temperature — which is how it reaches your eyes.
HighWorking it out: ranking ways to cut down on tears
Lots of onion tricks are floating around: chilling, a sharp knife, keeping your distance, ventilation. Let's line them up by calculation to see which actually helps most.
Every 10°C drop roughly halves reaction speed
| Temperature drop | Units: °C |
| Speed multiplier | ÷2 for every 10°C |
The tear-inducing compound is built by the enzyme the instant you cut. Since enzymes drive a chemical reaction, cooling slows them down.
| Room temperature | 20 °C |
| Well chilled | Take as 0 °C |
| Temperature difference | 20 − 0 ＝ 20 °C |
| How many 10°C steps | 20 ÷ 10 ＝ 2 steps |
| Speed multiplier | 2 × 2 ＝ down to 1/4 |
Down to a quarter. And chilling also cuts how much of the compound escapes into the air. A double effect.
The compound spreading out from the board gets more dilute the further away you are. It spreads in all directions through the air, so concentration falls with the square of distance.
| Usual distance | Take as 20 cm |
| Face raised to | 40 cm |
| Distance ratio | 40 ÷ 20 ＝ 2× |
| Concentration drops to | 1 ÷ (2 × 2) ＝ 1/4 |
Just raising your face 20 cm gives the same quarter as ①. It's far easier and instant, compared with chilling in the fridge.
Move back to 60 cm and you get 1 ÷ (3 × 3) ＝ 1/9. "Don't lean your face over the board" turns out to be the best-value countermeasure.
The tear-inducing compound is only made once cells break. So the amount released scales with the number of cells broken. That's exactly what cutting style changes.
| 10 mm cube dice | Take one piece's surface area as 1 |
| 2 mm fine dice | 10 ÷ 2 ＝ 5× finer |
| Total cut surface area | 5× larger |
Chopping finer really does bring on more tears — it's not your imagination. The cut surface area simply increases in step.
A sharp knife works for the same reason. A dull blade "crushes" cells rather than "cutting" them, breaking cells well beyond the cut line. Sharpening the blade means breaking fewer cells.
| Raise your face 20 cm | 1/4. Free, instant |
| Move back 60 cm | 1/9. But harder to see what you're doing |
| Chill it (near 0°C) | 1/4. Takes time |
| Don't chop too finely | Depends on the dish — not always an option |
| Run the extractor fan | Carries the compound away — stacks with the above |
The well-known "chill it" trick certainly works, but it loses to "keep your face back" on cost-effectiveness — the same quarter, for zero preparation.
And these effects multiply. Keep your face back (1/4) and chill it (another 1/4), and you get 1 ÷ (4 × 4) ＝ 1/16. It's worth remembering that these measures multiply rather than add.
※ "Halved every 10°C" is a rough rule of thumb — real enzymes vary. Diffusion also depends heavily on airflow in the room, so actual results won't match this calculation exactly. It's meant to give a sense of the ranking, not a precise figure.
HighThe reaction happens in two stages
Here's what happens inside the onion.
| ① Cell breaks | The material from the vacuole meets the enzyme from outside it |
| ② Alliinase acts | Breaks down the material into an unstable intermediate |
| ③ A second enzyme acts | Converts the intermediate into the lachrymatory factor |
| ④ Volatilises to the eyes | Reaches the eyes within seconds to a dozen seconds, causing irritation |
The key point is that ③ required a dedicated enzyme. For a long time it was thought the intermediate from ② turned into the lachrymatory factor on its own. But in 2002, a Japanese research team found that a separate enzyme was at work there too. This discovery opened the way to the "tear-free onion" discussed later.
Garlic lacks the enzyme for ③, so its intermediate follows a different path and becomes allicin instead. It has a strong smell but isn't a tear-inducing compound. The difference between onion and garlic comes down to one extra enzyme.
High+Why does heating stop it working?
Enzymes are made of protein, and their function depends on their three-dimensional shape. A reaction proceeds only when the right partner fits into a specifically shaped site (substrate specificity).
Heat breaks the weak bonds that hold this shape together, and the structure collapses (denaturation). Once that shape is lost, cooling it back down doesn't restore it — just as a hard-boiled egg never turns back into a raw one.
Slowing at low temperature is a different matter: it's simply that molecules move more slowly and meet less often, and warming it back up restores full activity. "Chilling" and "heating" affect the reaction in fundamentally different ways.
Univ.The "two-component system" defence design
Storing material and enzyme in separate compartments and letting them mix only when tissue is damaged is called a two-component chemical defence. It's a strategy seen widely across plants.
- Brassicas (wasabi, mustard, cabbage, etc.): glucosinolates and myrosinase. Mixing produces isothiocyanates — that pungency
- Allium species (onion, garlic): this article's mechanism. The raw materials are derivatives of sulphur-containing amino acids
- The smell of green leaves: lipids broken down by enzymes produce so-called leaf alcohols and leaf aldehydes
This design makes sense. The plant avoids carrying an offensive substance at all times, so it can't poison itself. It doesn't evaporate away while stored. And it can fire at high, localised concentration exactly when damage occurs. It's rather like keeping gunpowder and detonator loaded separately.
The receiving side has its own mechanism too. The lachrymatory factor is thought to activate irritant-sensing channels (such as TRPA1) on sensory nerves. This is the same pathway used by wasabi's pungent compound and by tear-gas ingredients, so it's registered as "pain," triggering a defensive reflex rather than a taste sensation.
ResearchThe "onion that doesn't make you cry" isn't finished yet
- Onions with the lachrymatory-factor enzyme suppressed have actually been made. Since the 2002 discovery, gene-silencing techniques have produced prototype onions that cut without much crying. But they haven't reached wide distribution.
- Balancing this with flavour is hard. Block the lachrymatory factor and the intermediate flows down a different path instead. That can change the onion's characteristic aroma and how sweetness develops on cooking. "No tears, but no onion flavour either" defeats the purpose. How far to suppress it while keeping only the tears down is still a work in progress.
- The link to health-related compounds is also unsettled. Some of the onion's sulphur compounds have reported effects on blood and blood vessels. How reducing the tear-inducing compound affects these isn't well understood yet.
- Why onions alone evolved to "make you cry" is itself a mystery. The closely related garlic lacks just one enzyme and makes no lachrymatory factor at all. Whether irritating the eyes was actually useful as a defence against grazing animals, or is simply a side effect of some other role, remains unclear.
Incidentally, the discovery of the enzyme behind this lachrymatory factor won the 2013 Ig Nobel Prize in Chemistry — an award for "research that makes people laugh, then think."
Links to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science: cell structure / stimulus and response / states of matter | Cell compartments, the tear reflex, volatility |
| High | Biology Basics: enzyme function / cell structure | The two-stage reaction, vacuole and cytoplasm |
| High+ | Biology: enzyme properties (denaturation and optimal temperature) | Why heat stops it working, versus low temperature |
| Univ. | Biochemistry, food science, plant physiology | Two-component defence, TRP channels, sulphur compounds |
| Research | Plant breeding, food science (unresolved) | Tear-free onions, balancing with flavour, evolutionary meaning |
| ― | Home economics | Chilling, a sharp knife, ventilation as countermeasures |
- Imai, S. et al., An onion enzyme that makes the eyes water, Nature 419, 685, 2002 (discovery of the lachrymatory-factor-synthesising enzyme).
- Eady, C. C. et al., Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile, Plant Physiology 147, 2096–2106, 2008 (tear-free onions and the resulting flavour change).
- Block, E., Garlic and Other Alliums: The Lore and the Science (a reference work on Allium chemistry).
- Halkier, B. A. & Gershenzon, J., Biology and biochemistry of glucosinolates, Annual Review of Plant Biology 57, 303–333, 2006 (the two-component defence in brassicas).
- Materials on onion composition and cultivar breeding from Japan's Ministry of Agriculture, Forestry and Fisheries (農林水産省) and various research institutions.
※ Reaction times and the size of each effect vary with cultivar, freshness, and conditions. This article describes general tendencies.
※This article is a general-audience science explainer. If your eyes are badly irritated, flush them with running water without rubbing, and see an eye doctor if symptoms persist. Please handle kitchen knives with care. The content here is intended to help explain the underlying mechanism.