You can chop carrots, slice potatoes, and dice tomatoes without a problem. But the moment you cut into an onion, your eyes start burning, tears begin rolling down your cheeks, and suddenly, preparing dinner feels like an emotional experience. What makes this ordinary kitchen ingredient so different?
The answer lies in a remarkable chemical defence system hidden inside every onion. When you slice through its layers, you trigger a series of reactions that produce an invisible, irritating gas. Within seconds, that gas reaches your eyes and activates the body’s natural protective response.
But why do onions contain these chemicals in the first place? Why do some onions make you cry more than others? And can you really prevent onion tears by chilling the onion, chewing gum, or holding a piece of bread in your mouth?
The science behind onion tears is more fascinating than you might expect.
What Happens When You Cut an Onion?
An onion may look like a simple collection of tightly packed layers, but its cells contain an elaborate chemical defence system. While the onion remains intact, important components of this system are kept apart within its tissues. Cutting, crushing, or chewing damages the cells and allows substances that were previously separated to come into contact.
Onions contain sulfur-bearing compounds known as S-alk(en)yl-L-cysteine sulfoxides. These compounds are important precursors to many of the characteristic flavours and aromas associated with onions. When the cells are damaged, an enzyme called alliinase acts on these compounds, producing a range of reaction products.
One of the products formed during this process is a compound called 1-propenesulfenic acid. An enzyme known as lachrymatory factor synthase converts this intermediate into syn-propanethial-S-oxide, the substance responsible for making your eyes water.
This compound is volatile, meaning it can readily enter the air. As it moves away from the freshly cut onion, some of it reaches the surface of your eyes and causes irritation.
The entire process begins almost immediately after the onion’s tissues are damaged. That is why your eyes may start watering before you have finished chopping.
What Is the Chemical That Makes You Cry?
The main chemical responsible for onion tears is syn-propanethial-S-oxide, commonly called the onion lachrymatory factor. The term lachrymatory comes from a word associated with tears and describes a substance that causes the eyes to water.
This compound is produced through a sequence of enzyme-driven reactions rather than being stored in large quantities inside an intact onion. Its formation is part of the onion’s response to tissue damage.
Once released, the volatile compound can reach the eyes and stimulate sensory nerve endings associated with irritation. The nervous system responds by increasing tear production, helping dilute and remove the irritating substance from the eye’s surface.
This is an important distinction. Onion tears are not an emotional reaction. They are a protective reflex designed to keep the eyes comfortable and defend their sensitive tissues from irritating chemicals.
The familiar burning sensation and watery eyes are therefore signs that your body’s protective mechanisms are working.
Why Do Onions Have a Chemical Defence System?
Onions produce sulfur-containing compounds for reasons that extend beyond making human cooks uncomfortable. These substances are part of the plant’s chemistry and contribute to the characteristic flavour, smell, and defensive responses of onions and related plants.
In an intact onion, the chemical precursors and enzymes are separated. When an animal, insect, or other source of damage breaks the tissues, chemical reactions begin producing pungent and potentially irritating substances.
These reactions can help discourage some organisms from eating or damaging the plant. Similar enzyme-activated chemical defence systems occur in other plants, although the compounds and mechanisms differ between species.
Onions belong to the genus Allium, which also includes garlic, leeks, shallots, and chives. Members of this group are known for their distinctive sulfur chemistry, but they do not all produce the same compounds in the same proportions.
The sharp smell released when an onion is chopped is therefore closely connected to the chemistry that helps give the plant its distinctive biological and culinary characteristics.
Why Do Some Onions Make You Cry More Than Others?
If you have ever chopped several types of onions, you may have noticed that some produce an intense burning sensation while others cause very little irritation. This difference is influenced by onion variety, chemical composition, growing conditions, storage, and preparation methods.
Onion varieties differ in their concentrations of sulfur-containing flavour precursors and in the activity of enzymes involved in producing volatile compounds. These differences affect both pungency and the amount of lachrymatory factor that can form when the onion is damaged.
Growing conditions also matter. Sulfur availability in the soil can influence the development of sulfur compounds in onion bulbs, although the relationship between soil conditions and the final intensity of eye irritation is not always straightforward.
Storage conditions and the onion’s physiological state may also affect its chemistry. As a result, two onions that look similar can produce noticeably different experiences when chopped.
Sweet onions are often milder than pungent storage onions, but sweetness alone does not determine how much an onion will make you cry. The important factors include the balance of flavour compounds, enzyme activity, and the amount of irritating substance released during cutting.
Why Don’t Cooked Onions Make You Cry as Much?
Raw onions are particularly effective at producing tears because their enzymes remain active and can rapidly react with sulfur-containing compounds when the cells are damaged. Cooking changes this chemistry.
Heat can denature enzymes, altering their structure and reducing their ability to catalyse chemical reactions. As an onion cooks, many of the compounds responsible for its sharp raw flavour also react, evaporate, or undergo other chemical changes.
This helps explain why slowly cooked onions develop a sweeter, mellower flavour compared with freshly chopped raw onions. Their harsh pungency decreases as the chemical composition changes.
However, cooking onions can still release irritating vapours, particularly during the early stages when freshly cut onion tissues are heated. The amount of irritation depends on the preparation method, temperature, ventilation, and individual sensitivity.
The transformation from sharp raw onion to soft, sweet cooked onion is a useful example of how heat changes both the chemistry and sensory qualities of food.
Does Refrigerating an Onion Stop the Tears?
Chilling an onion before cutting is one of the more plausible methods for reducing eye irritation. Lower temperatures can slow enzyme-catalysed reactions and reduce the rate at which volatile substances escape into the air.
When a chilled onion is cut, these effects may reduce the amount of irritating vapour reaching your eyes during preparation. However, refrigeration does not completely prevent the formation of the lachrymatory factor.
The effectiveness of chilling depends on factors such as the onion’s initial pungency, how long it has been cooled, the cutting method, and the surrounding temperature.
For practical cooking, placing an onion in the refrigerator for a short period before chopping may help reduce discomfort. It is best to treat this as a potentially useful technique rather than a guaranteed solution.
There is also no need to freeze an onion solid simply to avoid tears. Freezing and thawing can damage its texture, which may be undesirable when preparing dishes that require crisp raw onion.
Does Cutting an Onion Under Water Work?
Cutting onions under water is another commonly suggested method for preventing tears. The idea is that water can help contain or absorb some of the irritating compounds before they spread through the surrounding air.
Because the lachrymatory factor must travel from the onion to your eyes, limiting its movement through the air can reduce exposure. Water may therefore provide a physical barrier during cutting.
However, chopping onions while they are submerged is inconvenient and can be unsafe. Wet surfaces, slippery ingredients, and reduced visibility make knife control more difficult.
A safer approach is to use good kitchen ventilation and a stable cutting board. There is little reason to introduce additional knife-handling risks when simpler methods are available.
Does Chewing Gum Prevent Onion Tears?
Chewing gum while chopping onions is a popular kitchen trick, but there is little convincing scientific evidence that it reliably prevents the irritation caused by onion vapours.
Some explanations suggest that chewing encourages breathing through the mouth, reducing exposure through the nose. However, the lachrymatory factor can travel directly through the air to the eyes. Changing the way you breathe does not create a protective barrier around them.
Other suggestions include holding bread in the mouth, placing a spoon between the teeth, or keeping a matchstick nearby. These methods are frequently shared as traditional kitchen advice, but they lack a well-established scientific basis.
If a particular trick appears to work, the result may depend on differences in the onions being cut, room ventilation, cutting speed, or personal sensitivity.
For dependable results, it makes more sense to focus on reducing the release of irritating compounds and limiting their contact with your eyes.
How to Cut Onions Without Crying
Although no ordinary kitchen method guarantees completely tear-free chopping, several practical techniques can reduce irritation.
Use a Sharp Knife
A sharp knife cuts through onion tissues more cleanly than a dull blade. A dull knife can crush more cells as it passes through the onion, potentially increasing the release of compounds involved in the lachrymatory reaction.
Using a properly sharpened knife also improves cutting control, making it easier to work efficiently and safely.
Improve Kitchen Ventilation
Good ventilation can help carry irritating vapours away from your face. A properly positioned extractor fan may reduce the concentration of airborne onion compounds around the cutting area.
The direction of airflow matters. A fan that blows vapours toward your face could make the irritation worse rather than better.
Chill the Onion Before Cutting
Cooling the onion may slow some chemical reactions and reduce the release of volatile compounds. This can make chopping more comfortable, particularly when working with pungent varieties.
Wear Protective Goggles
Protective eyewear provides a physical barrier between the eyes and airborne irritants. Well-fitting goggles can be especially helpful for people who are highly sensitive to onions or regularly prepare large quantities.
Ordinary glasses may provide limited protection because vapours can still reach the eyes from the sides.
Reduce Unnecessary Cutting and Crushing
Prepare the onion efficiently and avoid repeatedly crushing or squeezing its tissues. Every additional cut damages more cells and creates opportunities for the chemical reactions responsible for pungency.
These methods are more consistent with the known chemistry of onions than popular tricks involving chewing gum or bread.
Why Do Onions Make Your Eyes Burn but Garlic Usually Doesn’t?
Garlic and onions belong to the same plant genus, yet their chemical responses to tissue damage differ considerably.
Garlic contains sulfur-bearing precursors that react when its cells are crushed or cut. One important reaction produces allicin, a compound associated with garlic’s characteristic sharp aroma and flavour.
Onions, by contrast, can produce substantial amounts of syn-propanethial-S-oxide through the activity of lachrymatory factor synthase. This particular compound is especially important in causing the familiar eye-watering response.
Garlic can still produce strong aromas and irritating substances, but its typical chemical profile differs from that of common onions. This helps explain why crushing garlic usually does not cause the same intense tearing experienced when chopping a pungent onion.
The difference illustrates how closely related plants can develop distinct chemical pathways that influence their flavour and behaviour in the kitchen.
Can Onion Tears Damage Your Eyes?
For most people, the irritation caused by ordinary onion chopping is temporary. The eyes respond by producing tears that help dilute and remove the irritating substance.
Symptoms usually improve after exposure decreases. Moving away from the cutting area and allowing fresh air to circulate can help reduce discomfort.
However, people with sensitive eyes or certain eye conditions may experience greater irritation. Rubbing the eyes can also make discomfort worse, particularly if onion juice has been transferred to the fingers.
If onion juice enters the eyes directly, rinse them gently with clean, lukewarm water. Persistent pain, significant redness, or changes in vision should be assessed by a healthcare professional.
The temporary tearing caused by normal onion preparation is generally different from the effects of direct exposure to concentrated onion juice.
The Surprising Role of Onion Chemistry in Cooking
The chemistry that makes onions irritating is also central to their culinary value. When onion cells are damaged, enzyme-driven reactions generate compounds that contribute to the vegetable’s characteristic pungency and flavour.
The way an onion is cut can therefore influence the intensity of its flavour. Finely chopping, crushing, or grating an onion damages more tissue than leaving it in large pieces, potentially creating a stronger initial release of reactive compounds.
Cooking then transforms many of these substances. Gentle heating softens the onion and reduces its sharpness, while prolonged cooking develops deeper flavours through chemical reactions involving sugars, amino acids, and other components.
This is one reason onions appear in so many culinary traditions. They can provide sharpness when raw, sweetness when slowly cooked, and savoury depth when browned.
From Indian curries and French onion soup to Italian sauces and Middle Eastern rice dishes, onions contribute flavours that change dramatically depending on how they are prepared.
The same chemistry responsible for tears also helps make onions one of the most versatile ingredients in the kitchen.
Frequently Asked Questions About Onion Tears
Why do onions make you cry?
Cutting onions damages their cells and activates chemical reactions that produce syn-propanethial-S-oxide, a volatile compound that irritates the eyes. Tear production is the body’s protective response to this irritation.
Which onion makes you cry the most?
The amount of irritation varies with onion variety, growing conditions, storage, and chemical composition. More pungent onions often cause stronger reactions than milder varieties, but there is no single type that always produces the most tears.
Does putting an onion in the fridge stop tears?
Chilling may reduce irritation by slowing chemical reactions and limiting the release of volatile compounds. However, it does not guarantee that an onion will become completely tear-free.
Why don’t garlic and onions affect the eyes in the same way?
Garlic and onions produce different mixtures of sulfur-containing compounds when damaged. Onions generate a particular lachrymatory factor that is strongly associated with eye irritation, while garlic’s chemistry is more closely associated with compounds such as allicin.
Can you become immune to onion tears?
People vary in their sensitivity to onion vapours, but there is no reliable evidence that regular onion chopping makes someone permanently immune to the lachrymatory factor.
Is it harmful to cry while cutting onions?
Temporary tearing during ordinary onion preparation is generally harmless for most people. However, persistent irritation, severe pain, or vision changes should not be ignored.
The Real Reason Onions Make You Cry
Onions make you cry because cutting them activates a chemical defence system that produces an irritating, volatile compound. Once that compound reaches your eyes, your nervous system triggers tear production to help protect their delicate surfaces.
What appears to be a simple kitchen inconvenience is actually the result of a carefully coordinated series of chemical reactions involving sulfur-containing molecules and specialized enzymes.
Fortunately, understanding the science also makes the problem easier to manage. A sharp knife, good ventilation, a chilled onion, and protective goggles can all help reduce discomfort.
So the next time an onion brings tears to your eyes, remember that it is not simply making you cry. You are witnessing a remarkable example of plant chemistry in action, right on your cutting board.
Sources and Further Reading
- Imai, S., et al. (2002). “An onion enzyme that makes the eyes water.” Nature, 419, 685. A landmark study identifying the enzyme responsible for producing the onion lachrymatory factor.
- Block, Eric (2010). Garlic and Other Alliums: The Lore and the Science. Royal Society of Chemistry. A detailed scientific examination of sulfur chemistry in onions, garlic, and related plants.
- McGee, Harold (2004). On Food and Cooking: The Science and Lore of the Kitchen. Scribner. Explains the chemistry and culinary behaviour of vegetables, including onions.
- Encyclopaedia Britannica. Entries on onions and the genus Allium, providing botanical and historical context.
Editorial note: Common onion-cutting tricks vary in the quality of evidence supporting them. Methods with plausible chemical or physical explanations have been distinguished from claims that remain scientifically unproven.
