When you cut into an onion, you rupture thousands of tiny cells that set off a rapid chain of chemical reactions, ultimately releasing a volatile sulfur gas that drifts into your eyes and triggers reflex tears. The compound responsible, called the lachrymatory factor, is not just a random byproduct of slicing through plant tissue. It is specifically manufactured by a dedicated enzyme inside the onion, part of a defense system that evolved to discourage anything that tries to damage the bulb. The story of how that gas forms, how your body reacts to it, and why some people seem to suffer more than others turns out to be more interesting than a cooking annoyance has any right to be.
What Happens Inside the Onion When You Cut It
An intact onion sits quietly on your cutting board, completely harmless. The chemistry that makes you cry only starts once you break the onion’s cells open with a knife. Onion cells store sulfur-containing amino acid precursors in one compartment and an enzyme called alliinase in another. As long as those two remain separated, nothing happens. The moment your blade tears through the cell walls, alliinase meets its sulfur-rich substrate, and the reaction kicks off almost instantly.
Alliinase chops the precursor molecule apart and produces a reactive intermediate called a sulfenic acid. In garlic, a close relative of the onion, this sulfenic acid goes on to form the pungent compounds that give garlic its smell. In onions, something different happens. A second enzyme grabs the sulfenic acid and converts it into the lachrymatory factor, a small, lightweight molecule with the chemical name (Z)-propanethial S-oxide.1PubMed Central. Enzyme That Makes You Cry-Crystal Structure of Lachrymatory Factor Synthase from Allium cepa Because this molecule is volatile, it evaporates almost as soon as it forms and floats upward as a gas. That gas is what reaches your eyes.
The whole process takes seconds. You press a knife through the onion, cell walls break, enzymes meet substrates, sulfenic acid forms, the lachrymatory factor is synthesized, and the gas is already in the air before you finish your first cut. This speed is part of what makes onion tears so hard to avoid: by the time you notice the sting, the reaction has already been running for a while.
A Surprise Discovery That Changed the Story
For decades, scientists assumed the tear-inducing gas was just a spontaneous byproduct of the alliinase reaction. The thinking was straightforward: alliinase breaks down the sulfur precursor, and the lachrymatory factor forms on its own as an unstable intermediate rearranges. No special machinery needed. This made intuitive sense, and nobody looked very hard for an alternative explanation.
That changed in 2002, when a team of Japanese researchers demonstrated that the lachrymatory factor is not formed spontaneously at all. Instead, it is specifically synthesized by a previously unknown enzyme they named lachrymatory factor synthase, or LFS.2Nature. An onion enzyme that makes the eyes water This was a genuine surprise. The onion dedicates an entire enzyme to making you cry. Without LFS, the sulfenic acid produced by alliinase goes down a different path, forming thiosulfinates, which are the flavor compounds that give onions their taste. LFS intercepts the sulfenic acid and reroutes it into tear gas.
The discovery mattered for two reasons. First, it revealed that the tear response and the flavor response are not the same reaction. They share a common starting point, the sulfenic acid, but then branch into separate pathways. That means, at least in theory, you could shut off the tear gas without destroying the flavor. Second, it opened the door to understanding the enzyme’s structure and mechanism, which researchers pursued in subsequent years by solving the crystal structure of LFS and identifying the architecture of its active site.1PubMed Central. Enzyme That Makes You Cry-Crystal Structure of Lachrymatory Factor Synthase from Allium cepa
How the Gas Actually Makes You Cry
Once the lachrymatory factor becomes airborne, it does not have to travel far. Your eyes are right there, hovering above the cutting board, and the gas dissolves easily into the thin film of moisture that coats the surface of your eye. When it reacts with that moisture, it forms a mild sulfuric acid, which is exactly as unpleasant as it sounds. Even in tiny quantities, the acid irritates the cornea.
Your cornea is one of the most densely innervated tissues in the body. Fine sensory branches of the trigeminal nerve run through it and are exquisitely sensitive to chemical irritation. When those nerve endings detect the irritant, they fire a signal back to the brainstem, which activates a reflex: your lacrimal glands ramp up tear production to flush the offending substance off the eye’s surface. Blinking speeds up simultaneously, spreading the tears across the cornea and helping to wash away the irritant. The whole sequence, from chemical contact to full-blown crying, is involuntary. You cannot will yourself not to tear up any more than you can will yourself not to flinch when something flies at your face.
The tears produced during this reflex are what eye researchers call “irritant tears” or reflex tears. They are chemically different from the tears you produce when you are sad (emotional tears) and different again from the baseline tears that keep your eyes lubricated throughout the day (basal tears). Reflex tears tend to be more watery and come in larger volume, because their job is to flush something out, not to maintain a stable tear film. That is why onion crying feels different from emotional crying: the tears stream faster, and the sensation is more stinging than sorrowful.
Why Some People Cry More Than Others
Anyone who has chopped onions alongside someone else has noticed that the experience is not evenly distributed. One person is in tears within seconds while the other barely notices. Part of this is proximity and airflow, but there is evidence that individual differences in sensory sensitivity play a genuine role.
A preliminary study examining the relationship between onion tearing severity and chemosensory function found that people who reported more intense tearing when cutting onions also tended to have better self-rated sense of smell, easier nasal breathing, and higher sensitivity to nasal stinging and burning sensations.3Laryngoscope Investigative Otolaryngology. Severity of Onion Tearing and Chemosensory Sensitivity: A Preliminary Investigation In other words, the people who cry the most may simply have more responsive trigeminal nerve systems. Their sensory hardware is tuned to a lower threshold, so the same concentration of lachrymatory factor in the air produces a bigger reflex response.
This fits with what we know about individual variation in other chemical sensitivities. Some people find chili peppers almost unbearably painful while others eat them casually; some people gag at the smell of certain cleaning products while others barely register them. The trigeminal nerve mediates many of these experiences, and its sensitivity varies meaningfully across the population. Age can play a role too, since both olfactory function and trigeminal sensitivity tend to decline with age, which may explain why older adults sometimes report less trouble with onions than they experienced earlier in life.
Contact lens wearers often report less tearing during onion cutting, and the likely reason is simple: the lens physically covers part of the cornea and reduces the surface area available for the gas to dissolve into. It is not a complete barrier, but even a partial one can blunt the reflex.
Why Do Onions Have This Defense in the First Place?
From the onion’s perspective, the lachrymatory factor is not there to ruin your dinner prep. It is a chemical defense. The bulb sits in the soil for months, vulnerable to insects, fungi, and burrowing animals that would happily eat it. When something bites into or damages the onion’s tissue, the same cell-rupture chemistry that makes you cry also releases a burst of irritating, antimicrobial sulfur compounds directly at the attacker.
The fact that onions devote a specific enzyme to producing tear gas, rather than just relying on the general sulfur chemistry that all alliums share, suggests the defense has been under strong evolutionary selection. Garlic, leeks, shallots, and chives all belong to the same genus and share the alliinase pathway, but they do not all have LFS, and they do not all make you cry. Onions have apparently faced enough pressure from their particular set of enemies that evolving a dedicated tear-gas factory provided a survival advantage. The trade-off is that some of the sulfenic acid that could have gone toward making more flavor compounds gets diverted into making tear gas instead, a point that becomes relevant when we talk about engineering tearless onions.
Practical Ways to Reduce the Tears
Given the chemistry involved, there are several approaches that actually work, and some popular ones that do not hold up as well as people think.
- Chill the onion: Cooling an onion in the refrigerator for 30 minutes or so before cutting slows down the enzymatic reactions. Enzymes work more slowly at lower temperatures, so less lachrymatory factor is produced per second. The gas is also less volatile when cold, so less of it becomes airborne.
- Use good ventilation: A fan blowing across the cutting board, an open window, or even just cutting near your stove’s range hood pushes the gas away from your face before it reaches your eyes. Since the lachrymatory factor is lighter than air and drifts upward, anything that disrupts that path helps.
- Cut under water or rinse frequently: Running water over the onion dissolves the lachrymatory factor before it becomes airborne. The downside is that water also washes away some of the flavor compounds, which share a similar chemistry. You get fewer tears at the cost of a slightly milder onion.
- Use a sharp knife: A sharp blade crushes fewer cells than a dull one. Clean cuts through the tissue rupture fewer cell walls per slice, which means less enzyme meets less substrate and less gas is produced. This does not eliminate tears, but it reduces the concentration.
- Wear goggles: Sealing your eyes off from the air is the most reliable method. Onion goggles exist as a commercial product, but any pair of tight-fitting safety goggles or even swim goggles will do the job. It looks ridiculous, but it works nearly perfectly.
Some commonly suggested tricks are less effective. Holding bread in your mouth, chewing gum, or breathing through your mouth are all based on the idea that you can somehow intercept the gas before it reaches your eyes. The problem is that the lachrymatory factor does not need to go through your nose or mouth to reach your cornea. It contacts your eyes directly from the air. Breathing through your mouth might slightly reduce nasal irritation, which can contribute to reflex tearing through a connected neural pathway, but the main route is direct eye contact with the gas, and mouth breathing does nothing to stop that.
Lighting a candle near the cutting board is another popular suggestion. The idea is that the flame burns off the sulfur compounds. In practice, a single candle flame is far too small to meaningfully reduce the concentration of gas in the air around you. If it helps at all, it is probably because the heat creates a small updraft that diverts some of the gas, acting as a weak version of the fan approach.
The Quest for a Tearless Onion
Once researchers realized that a single enzyme, LFS, was responsible for converting flavor precursors into tear gas, the obvious question was whether you could disable that enzyme and get an onion that tastes the same but does not make you cry. The answer, it turns out, is a qualified yes.
In 2008, a team in New Zealand successfully silenced the LFS gene in onion plants using a gene-silencing technique. Across six transformed plants, lachrymatory factor synthase activity dropped by up to 1,544-fold, and when the onions were wounded, they produced dramatically lower levels of the tear-inducing gas.4PubMed Central. Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile Equally interesting was what happened to the onion’s chemistry when LFS was removed from the picture. Without the enzyme diverting sulfenic acid into tear gas, the sulfur compounds were rerouted into other metabolic pathways. The overall sulfur secondary metabolite profile changed significantly, producing higher levels of certain thiosulfinates, some of which are the same compounds associated with flavor and potential health-related properties.
This did not instantly produce a supermarket-ready tearless onion. Onions are notoriously difficult to breed and transform genetically, and the 2008 work was a proof of concept rather than a commercial launch. Progress since then has been slow, partly because of the technical challenges of working with onion genetics and partly because of regulatory and consumer attitudes toward genetically modified produce in many markets.
A parallel approach has been pursued through conventional breeding. The Sunion, introduced commercially in the United States around 2018, is a sweet onion variety developed through traditional crossbreeding rather than genetic engineering. Its developers selected for onion lines that naturally produce lower levels of the lachrymatory factor. The Sunion is marketed as a “tearless” onion, though “much less tear-inducing” would be more accurate. It works well enough that it has found a niche market, though its flavor profile is mild and sweet, which is a feature for some dishes and a drawback for others.
Why Other Alliums Do Not Make You Cry
If you have ever wondered why garlic, leeks, or scallions do not produce the same dramatic tears, the answer loops back to LFS. All members of the allium family share the alliinase enzyme and produce sulfenic acids when their cells are damaged. But the lachrymatory factor synthase that converts those sulfenic acids into tear gas appears to be specific to onions. Garlic, for instance, produces allicin from its sulfenic acids, a compound with a powerful smell and antimicrobial properties but no particular tear-inducing ability. Shallots contain some LFS activity and can make your eyes sting a little, but far less than a standard yellow onion.
Even among onions, there is wide variation. Sweet onions grown in low-sulfur soils, like Vidalias from Georgia, produce less lachrymatory factor because the plant has less sulfur to work with in the first place. The enzyme is still there, but with a smaller pool of sulfur-containing precursors, less tear gas gets made. This is also why onions grown in sulfur-rich soils tend to be more pungent and more tear-inducing. The variety of onion matters, the soil it grew in matters, and the freshness matters too, since the precursor compounds can degrade somewhat during storage.
Red onions generally sit somewhere between sweet and standard yellow onions on the tear scale. Their reputation as being milder is partly deserved: many red onion varieties do produce somewhat less lachrymatory factor. But the perception of mildness also comes from the fact that red onions are more commonly eaten raw in salads, where people have already self-selected for less pungent specimens. A red onion grown in sulfur-rich soil and freshly harvested can still make you cry as readily as any yellow onion.
What Happens if You Get Lachrymatory Factor in Your Eyes Repeatedly
Professional cooks and line chefs cut onions constantly, sometimes working through fifty-pound bags in a single shift. A reasonable question is whether this repeated exposure causes any lasting damage to the eyes. The short answer is that it does not appear to. The lachrymatory factor produces a mild, transient acid on the eye’s surface, and the reflex tear response is specifically designed to wash it away before it can damage the corneal tissue. Once you step away from the onion and the gas disperses, the irritation resolves within minutes.
There is some anecdotal evidence that frequent exposure leads to a degree of desensitization, with professional chefs often reporting that onions bother them less than they used to. Whether this is a genuine physiological adaptation in the trigeminal nerve response or simply a learned behavior, like positioning the cutting board differently or working faster, is not well established. The trigeminal nerve can adapt to repeated low-level irritation in other contexts, so some real desensitization is plausible, but the effect has not been rigorously studied for onion exposure specifically.
The onion’s tear gas is not related to the eye irritation caused by stronger agents like pepper spray or industrial fumes, which involve different chemicals at much higher concentrations and can cause genuine corneal injury. Comparing them is a bit like comparing a sunburn to a chemical burn: both involve the skin, but the mechanism and severity are in completely different categories.