Why Don’t Onions Make Me Cry?

Whether an onion makes you cry depends on a chain of chemical, neurological, and agricultural factors, and variation in any one of them can leave your eyes perfectly dry while the person next to you is dabbing away tears. The tear-inducing culprit is a volatile gas called propanethial S-oxide, but how much of it the onion produces and how strongly your eyes react to it differ enormously from one person and one onion to the next. If you have ever wondered why you seem immune while others suffer, the explanation likely involves your own sensory wiring, the particular onion you grabbed, or both.

How an Onion Makes Most People Cry

An intact onion is harmless to your eyes. The trouble starts only when cells are ruptured by a knife. Onion cells contain sulfur-rich amino acid precursors and, in a separate compartment, an enzyme called alliinase. When cutting breaks cells open, alliinase converts those precursors into a sulfenic acid. In most onion varieties a second enzyme, lachrymatory factor synthase (LFS), immediately grabs that sulfenic acid and converts it into propanethial S-oxide, the volatile gas that drifts upward and stings your eyes.¹ For years, researchers assumed the tear gas was just a by-product of the same reaction that produces onion flavor, but the discovery of LFS in 2002 revealed it is deliberately manufactured by a dedicated enzyme. That was a significant finding because it meant the tearing and the flavor come from two separate branches of the same chemical pathway.

Once propanethial S-oxide reaches your eye, it dissolves in the thin film of moisture on the corneal surface and breaks down into sulfuric acid. That acid triggers pain receptors, specifically chemoreceptors on the cornea linked to the trigeminal nerve, and your brain responds with a reflex: tears, blinking, and the urge to turn away.² The whole thing happens fast. Real-time gas measurements of freshly cut onions show that propanethial S-oxide and its breakdown products dominate the first ten minutes after a cut, peaking quickly and then declining as other sulfur compounds take over.³ That is why the worst of the stinging tends to hit early and ease up as you keep chopping.

Why Your Eyes Might Not React

If you rarely or never cry while cutting onions, the most likely explanation is that your sensory nerves are simply less reactive to the irritant. The tearing response is a neural reflex, and reflex intensity varies from person to person. A preliminary study that measured how much people teared up while cutting onions found that the degree of crying tracked with self-reported sensitivity to nasal stinging and burning, not with how well someone could actually identify odors in a standardized smell test.⁴ In other words, people who described themselves as generally sensitive to pungent or irritating sensations cried more, while people who felt less bothered by stinging in everyday life cried less. Objective olfactory performance did not predict tearing.

The receptor that matters most is called TRPA1, an ion channel found on sensory neurons of the trigeminal nerve. TRPA1 responds to a range of pungent chemicals, and animal studies have shown that mice engineered without functional TRPA1 have blunted responses to irritants like mustard oil, as well as to cold and certain mechanical stimuli.⁵ Humans carry the same receptor, and natural variation in TRPA1 gene expression or function could easily explain why one person’s eyes stream while another’s stay dry in the same kitchen, exposed to the same onion fumes. No large-scale human genetic study has pinned down how common low-TRPA1 sensitivity is, so this remains a plausible mechanism rather than a settled statistic.

There are less exotic explanations too. Contact lens wearers often report reduced tearing, likely because the lens acts as a physical barrier between the gas and the cornea. People who wear glasses while cooking may find that the frames deflect some of the rising vapor. And if you have chronically dry eyes or reduced tear production for any reason, your reflex tearing response to irritants may be dampened simply because the machinery that produces tears is already underperforming.

The Onion Itself Varies Wildly

Even if your sensory nerves are perfectly average, the onion you happen to be cutting has a huge influence on whether you cry. Pungency and tear-inducing potential are not fixed properties of “onions” as a category. They depend on variety, growing conditions, and storage.

Sulfur is the key ingredient. The precursor molecules that eventually become propanethial S-oxide are sulfur-containing amino acids, so an onion grown in sulfur-rich soil or given sulfur fertilizer will pack more tear-inducing potential. Research on two commercial onion varieties found that sulfur fertilization increased the content of the specific precursor to the lachrymatory factor by tenfold compared to unfertilized controls, with overall pungency rising by about 35 percent.⁶ Separate experiments confirmed that both sulfur and nitrogen fertilization increased lachrymatory potency under field conditions.⁷

Sweet onion varieties like Vidalia, Walla Walla, and Maui are famous for being milder, and that reputation is earned. They are grown in low-sulfur soils and harvested at stages designed to minimize pungency. The difference between a hot yellow storage onion from sulfur-rich ground and a sweet spring onion from volcanic soil can be dramatic enough that one makes you sob and the other barely registers. If onions “never” make you cry and you tend to buy the mild sweet varieties, the onion may deserve more credit than your eyes.

Temperature and Preparation Make a Difference

Propanethial S-oxide is a volatile gas, and like most volatile compounds its evaporation rate depends on temperature. Chilling an onion slows the enzymatic reactions that produce the gas and reduces how quickly it becomes airborne. Research on fresh-cut onions stored at 0 °C showed reduced pungency compared to onions held at higher temperatures.⁸ The practical takeaway is simple: refrigerating an onion for half an hour before cutting it can meaningfully reduce how much gas reaches your eyes.

Cutting technique matters too, though the mechanism is straightforward. A sharp knife crushes fewer cells than a dull one, so less enzyme meets less precursor and less gas is produced. Slicing also damages fewer cells per unit of exposed surface area than dicing, which involves more cuts and more ruptured tissue. Running water over the cut surface rinses away some of the dissolved gas before it can go airborne. Cutting near a fan or an open window moves the vapor away from your face. These are not folklore; they all follow directly from the chemistry. Whether any of them fully prevent tearing depends on how pungent the onion is and how sensitive you are.

Ventilation is arguably the easiest intervention. Because the gas concentration peaks in the first ten minutes after cutting, even moderate airflow that redirects those initial fumes away from your face can be enough to eliminate the sting for people who are only moderately sensitive.

The Push for Truly Tearless Onions

Since the tearing and the flavor come from two different branches of the same chemical pathway, it is theoretically possible to breed or engineer an onion that does not make you cry but still tastes like an onion. Researchers in New Zealand did exactly that by using gene silencing to suppress LFS activity in six onion plants. The silencing reduced LFS enzyme activity by up to 1,544-fold, and the resulting onions produced dramatically lower levels of the lachrymatory factor when cut.⁹ With LFS suppressed, the sulfenic acid that would normally become propanethial S-oxide was instead channeled into thiosulfinates, compounds associated with flavor and potential health benefits rather than tears.

Mass spectrometry analysis of these tearless onions confirmed lower concentrations of the lachrymatory factor and much higher concentrations of the sulfenic acid condensation products, essentially verifying that the chemical pathway had been rerouted as intended.¹⁰ The result was an onion that could be cut without triggering tears, while potentially retaining or even boosting some of the flavor and bioactive compounds consumers value.

Commercially, tearless onions have been harder to bring to market than the lab results might suggest. Onions are notoriously difficult to genetically transform, and consumer wariness about genetic modification in food crops has slowed adoption. The Sunion, a conventionally bred (not genetically modified) low-pungency variety, reached U.S. grocery shelves in recent years as a milder alternative, though it is not completely tear-free. The gap between a 1,544-fold reduction in a lab and a no-tears guarantee in your kitchen is still being closed.

Why Garlic Does Not Have the Same Effect

Garlic is a close relative of the onion and also contains sulfur-rich precursor compounds, yet cutting garlic rarely provokes tears. The difference comes down to which precursor dominates. Onions are rich in a compound whose breakdown leads to propanethial S-oxide, while garlic’s primary precursor, alliin, breaks down along a different pathway that produces allicin instead. Allicin is responsible for garlic’s sharp bite and is linked to many of garlic’s purported health effects, but it is not a potent lachrymatory agent. Garlic simply lacks the specific precursor-plus-enzyme combination needed to generate the tear gas. Other allium relatives like leeks, chives, and scallions fall somewhere in between: they have some of the same precursors but in lower concentrations and often with less LFS activity, which is why they can sting a bit but rarely provoke a full crying episode.

Why the Onion Bothers Making You Cry at All

From the onion’s perspective, making your eyes water is not an accident. The biochemical pathway that produces propanethial S-oxide is part of a defense system against herbivores and microbial invaders. When an insect larva or a grazing animal breaks open onion tissue, the rapid burst of sulfuric volatiles deters further feeding.¹¹ The enzyme LFS is essentially a biological weapon, one that evolved to punish anything that damages the plant’s cells. Crystal-structure analysis of LFS has confirmed it is a distinct, dedicated enzyme rather than a minor side activity of some other protein, reinforcing the idea that this defense mechanism was important enough to warrant its own molecular machinery.

Interestingly, when you suppress LFS and redirect the pathway toward thiosulfinates, you lose the tear gas but you do not lose the antimicrobial punch entirely. Thiosulfinates have their own biological activity against fungi and bacteria. The tearless onions engineered in New Zealand still had chemical defenses; they just were not the kind that sting human eyes. Whether those alternative defenses are sufficient to protect an onion crop in real-world agricultural conditions is an open question. If tearless varieties turned out to be more vulnerable to soil-borne pathogens, that would be a practical barrier to widespread cultivation, and it is one of the concerns plant breeders are watching.

Common Misconceptions About Onion Tears

One persistent myth is that the tearing is triggered by the onion’s smell. It is not. Odor molecules and the lachrymatory factor are different compounds that happen to be released at the same time. The study of individual differences mentioned earlier found no relationship between how well someone could identify smells and how much they teared up when cutting onions.⁴ The tearing response is mediated by the trigeminal nerve’s pain pathway, not by the olfactory system. You could lose your sense of smell entirely and still cry from an onion.

Another common belief is that rinsing your eyes with water while cutting helps. It might relieve discomfort briefly, but the gas keeps arriving as long as freshly cut onion surface is exposed, so you would need to rinse continuously to keep up. Prevention strategies that reduce the gas at the source (chilling, sharp knife, ventilation) or protect the eye surface (goggles, contact lenses) are more effective than reactive rinsing.

A subtler misconception is that “getting used to” onions means your eyes have adapted. Professional chefs who handle onions daily often report less tearing over time, but whether this reflects genuine sensory adaptation, behavioral changes (better knife technique, faster work, better ventilation in commercial kitchens), or selection bias (people who are less sensitive are more comfortable in kitchen careers) has not been rigorously tested. Sensory habituation to chemical irritants does occur with repeated exposure in other contexts, so some degree of neural adaptation is plausible. But the popular framing of “building up a tolerance” overstates what the evidence supports.

When Reduced Tearing Might Signal Something Else

For most people, not crying when cutting onions is just a quirk of their sensory wiring, the particular onion they bought, or the temperature of their kitchen. But in some cases, reduced reflex tearing can be a symptom of an underlying condition worth knowing about. Reduced corneal sensation, sometimes seen in people with diabetes-related neuropathy, post-surgical changes from LASIK or cataract procedures, or certain autoimmune conditions, can blunt the tearing reflex to all irritants, not just onions. If you used to cry when cutting onions and gradually stopped over a period of months or years without any change in the onions you buy, and especially if your eyes also feel persistently dry or gritty, that shift might be worth mentioning to an eye doctor. It does not mean something is necessarily wrong. But the corneal nerves that trigger reflex tearing are the same nerves involved in maintaining healthy tear film, and losing their sensitivity can contribute to chronic dry-eye symptoms over time.