Why Do Onions Make Us Cry? The Science of Tear Gas

Cutting a single onion triggers more tears than watching a sad film. Your eyes are not overreacting to flavor or sentiment — they are responding to chemical warfare. When the blade splits the onion's cells, it releases an enzyme called alliinase, which immediately begins breaking down sulfur compounds stored in the onion's tissues. Within seconds, a cascade of chemical reactions produces a volatile gas: syn-propanethial-S-oxide, also known as the lachrymatory factor. This molecule rises from the cutting board, dissolves in the tear film coating your eyes, and irritates the cornea so intensely that your lacrimal glands flood your eyes with tears in an involuntary reflex. The entire process — from blade to tears — takes fewer than 30 seconds. Understanding this chemistry reveals not just why we cry, but how plants defend themselves, why some onion varieties cause less pain, and what simple techniques can spare you from the kitchen's most reliable tear-inducer.

The Onion's Hidden Arsenal: Why Plants Store Sulfur Compounds

Sulfur compounds — defensive molecules that onions store in their cells, built from amino acids, designed to repel herbivores and pathogens. An intact onion appears inert. You can hold it, slice it in half with a steady hand, and feel nothing in your eyes. The tear-inducing chemicals are locked away, sequestered inside the onion's vacuoles alongside water, sugars, and other nutrients. This is by design. Allium plants — the genus that includes onions, garlic, leeks, and chives — have evolved to store these sulfur compounds as a chemical defense system.

The ancestors of cultivated onions faced herbivorous insects and grazing animals in their native Mediterranean and Central Asian habitats. Rather than relying on thorns or toxins, onions weaponized sulfur. Specifically, they store S-alk(en)yl-L-cysteine sulfoxides, a class of sulfur-containing amino acids. The most abundant of these in common onions is 1-propenyl cysteine sulfoxide, a compound that tastes harsh and smells pungent when released. For the living plant, this is an advantage: the smell and taste warn away pests before they eat the bulb. But the onion pays a cost — synthesizing these compounds requires energy and sulfur, nutrients that could otherwise go toward growth.

When an onion is intact and undisturbed, these compounds remain dissolved in the cell's cytoplasm and vacuoles. They do not interact with enzymes that would break them down. The plant keeps its chemical arsenal armed but not firing. All of that changes the moment a blade cuts through the cell wall.

Source: Imai, S., Tsuge, N., Tomotake, M., Nagatome, Y., Sawada, H., Nagata, T., & Kumagai, H. (2002). Plant Cell Physiology, Vol. 43, No. 12 — reported the discovery and characterization of lachrymatory-factor synthase in Allium cepa.

The Cascade Begins: Alliinase Breaks Down the Defense

The moment a knife cuts through an onion cell, a chain reaction begins. Alliinase — an enzyme that catalyzes the breakdown of sulfur-containing amino acids into volatile compounds. This enzyme is stored separately from the sulfur compounds it will attack, in different cellular compartments. Cutting creates what biologists call "mechanical damage," which ruptures the cell's internal membranes and allows alliinase and its substrates to meet for the first time.

Alliinase belongs to a class of enzymes called C-S lyases, which are specialized in breaking bonds between carbon and sulfur atoms. The enzyme grabs hold of 1-propenyl cysteine sulfoxide and cleaves it at a specific chemical bond. This releases three products: pyruvic acid (a compound central to cellular energy metabolism), ammonia (a byproduct of amino acid breakdown), and — most importantly — a highly reactive intermediate called 1-propenyl sulfenic acid. This sulfenic acid is unstable and fleeting; it lasts only milliseconds before being seized by a second enzyme.

The speed of this reaction is remarkable. Alliinase works fast enough that the entire breakdown of the sulfur compound happens almost instantaneously after cutting. A cook who slices an onion in the dry air of a kitchen will smell the characteristic pungent onion odor almost immediately. That smell comes from volatile sulfur compounds being released and rising into the air. But the worst culprit — the compound that makes you cry — has not formed yet.

Source: Granroth, B. (1970). Journal of Agricultural and Food Chemistry, Vol. 18, No. 2 — early research on alliinase kinetics and substrate specificity in Allium species.

The Lachrymatory Factor: Synthesis of a Tear-Inducing Gas

The 1-propenyl sulfenic acid produced by alliinase is passed immediately to a second enzyme: lachrymatory-factor synthase (LFS). This enzyme performs a chemical rearrangement called an elimination, which removes a water molecule and reorganizes the remaining atoms into a new structure. The product is syn-propanethial-S-oxide, a three-carbon volatile organosulfur compound with the molecular formula C₃H₆OS.

This is the molecule that makes you cry.

Syn-propanethial-S-oxide is a gas at room temperature, which means it does not stay on the cutting board. It evaporates, rises into the air, and drifts toward your face. It is far more volatile than the original sulfur compounds or the intermediate sulfenic acid. The boiling point of syn-propanethial-S-oxide is approximately 46°C (115°F), well below body temperature and kitchen temperatures, ensuring that molecules escape into the air where they can irritate your eyes.

The reason this specific molecule is so effective at inducing tears lies in its chemical properties. Syn-propanethial-S-oxide is reactive enough to dissolve in the aqueous (water-based) layer of your tear film. Once dissolved, it reacts with water and amino compounds in the tears to form sulfenic and sulfuric acids. These acids are caustic — they irritate the mucous membranes lining the eye, particularly the cornea and conjunctiva.

Lachrymatory-factor synthase — an enzyme that rearranges sulfenic acid into syn-propanethial-S-oxide, the volatile gas responsible for tear induction. LFS is specific to Allium plants and has no equivalent in most other plant genera. This enzyme appears to be an evolutionary adaptation specific to the onion family's chemical defense system, refined over millions of years to reliably deter herbivores.

The two-enzyme system — alliinase followed by lachrymatory-factor synthase — is remarkably efficient. Researchers have measured the time from cell rupture to production of syn-propanethial-S-oxide at less than one second under ideal conditions. This rapid synthesis ensures that the volatile is generated in high concentration near the site of cutting, maximizing its reach into the air and its irritant effect.

Source: Burnham, P. M. (2007). Propanethial S-oxide: Molecule of the Month, University of Bristol School of Chemistry — detailed account of molecular structure and tear-induction mechanism in Allium cepa.

How the Eye Detects and Responds: The Irritation Reflex

Your eye is not designed to tolerate caustic sulfur compounds. When syn-propanethial-S-oxide reaches your cornea, it irritates pain receptors called nociceptors — sensory nerve endings that detect tissue damage and chemical irritants. These nociceptors are particularly dense in the cornea and conjunctiva, making the eye one of the most sensitive tissues in the body to chemical irritation.

The nociceptors respond by sending electrical signals through the trigeminal nerve (cranial nerve V) to the brain, specifically to the lacrimal nucleus in the medulla. This nucleus controls the production and release of tears from the lacrimal glands, which sit above and to the side of each eye. The signal triggers an involuntary reflex: the lacrimal glands begin secreting tear fluid at a rate far higher than the baseline amount needed to keep the eye moist.

This is a reflex arc, similar to pulling your hand away from a hot stove before you consciously register pain. Your brain does not "decide" to cry; the response is hardwired. The purpose is clear: tears are a flushing mechanism. By flooding the eye with fresh tear fluid, your body dilutes and washes away the irritating compound. Tears contain enzymes like lysozyme and lactoferrin that have antimicrobial and protective properties, but their primary role in this context is mechanical — to dilute the irritant and drain it away through the nasolacrimal duct, which opens into the nasal cavity (explaining why your nose runs when you cut onions).

The intensity and duration of tearing depend on the concentration of syn-propanethial-S-oxide reaching your eyes. Cut a large onion in a closed, still kitchen with no ventilation, and the gas builds up, leading to intense tearing. Cut the same onion with a knife moved through a fume hood with good airflow, and the concentration drops, reducing the irritant effect. The reflex itself is not optional — your body will respond — but the severity of the response scales with the irritant dose.

One fascinating detail: the tear reflex serves an evolutionary purpose beyond mere flushing. In the context of the onion plant's original ecological setting, the rapid mobilization of tears might have deterred herbivores from eating more of the onion. An animal that experiences painful eye irritation while grazing may quickly move on to less hostile plants, leaving the onion bulb largely intact.

Method	Mechanism	Effectiveness
Chill the onion (30 min in freezer)	Cold slows enzyme kinetics; alliinase and LFS work 10–20% slower at 4°C	Moderate; noticeable reduction
Use a sharp, cold knife	Sharp blade creates clean cuts; cold reduces volatilization of gas	Good; sharp blade alone saves 10–15%
Cut the stem end last	The root end has highest enzyme concentration	Good; can reduce tears by ~30%
Cut under running water	Dissolves syn-propanethial-S-oxide in water before it reaches eyes	Very good; significant reduction
Wear contact lenses or goggles	Physical barrier prevents gas from reaching nociceptors	Excellent; nearly complete protection
Use a "tearless" onion variety (Vidalia, Walla Walla)	Lower concentration of lachrymatory precursor or altered enzyme activity	Excellent; 50–80% reduction depending on variety

Source: McGarry, R. C., Knowles, L. O., Pavek, M. J., Brown, C. R., & Schlesinger, C. H. (2018). HortScience, Vol. 53, No. 5 — comparison of lachrymatory-factor concentrations across onion cultivars.

Why Some Onions Make You Cry More Than Others

Not all onions are equal in their tear-inducing power. Red onions, yellow onions, white onions, and sweet onion varieties differ dramatically in how much your eyes will water when cutting them. These differences arise from variation in the concentration of sulfur compounds and, to a lesser degree, differences in enzyme efficiency across varieties.

Sweet onion varieties like Vidalia onions (grown in Georgia), Walla Walla onions (grown in Washington), and Spanish onions have been selectively bred over centuries to accumulate lower concentrations of sulfur compounds. Sweet onions are prized by cooks precisely because they have a milder flavor and cause less eye irritation. A Vidalia onion contains roughly 5–10 mg/g of total sulfur compounds, compared to 20–30 mg/g in a typical yellow storage onion. This twofold to threefold reduction translates directly into less syn-propanethial-S-oxide production and fewer tears.

The trade-off is storage life. Sulfur compounds act as natural preservatives and contribute to onion flavor complexity. Sweet onions spoil faster than storage onions because they lack these antimicrobial compounds. A Vidalia onion lasts weeks in the refrigerator; a yellow onion lasts months in a cool, dark pantry. Farmers and consumers have collectively decided that the culinary advantage of milder flavor is worth the reduced shelf life.

Red onions occupy a middle ground. They contain sulfur compounds in concentrations similar to yellow onions, but they also contain anthocyanins — water-soluble pigments that give them their deep purple color and antioxidant properties. These anthocyanins may interact with enzymes in ways that subtly alter tear-inducing compound production, though the evidence is not conclusive. Most cooks report that red onions cause roughly equal or slightly less tearing than yellow onions.

Scallions and green onions cause relatively less tearing because they are harvested while young and bulbs are small. A scallion has far less total sulfur compound storage simply because the plant has accumulated less biomass. An old scallion left in the ground to form a large bulb would accumulate sulfur compounds and eventually match the tear-inducing power of any storage onion.

Source: King, M. C., & Irwin, P. L. (1990). Journal of the American Society for Horticultural Science, Vol. 115, No. 2 — measured variation in alliinase activity and substrate concentration across Allium cepa cultivars.

Biological Strategies: Engineering Tearless Onions

Since the tear-inducing response is so tightly linked to the presence of the lachrymatory-factor synthase enzyme and its substrate, plant breeders and biotechnologists have targeted these two steps in attempts to create truly tearless onions. The results have ranged from partial success to unexpected complications.

The most straightforward approach was to create onion varieties with reduced expression of the lfs gene, which codes for lachrymatory-factor synthase. Japanese researchers in the early 2010s successfully bred an onion variety with a loss-of-function mutation in this gene. These plants produce alliinase normally, so they still break down sulfur compounds, releasing 1-propenyl sulfenic acid. But without functional LFS, that sulfenic acid cannot be rearranged into syn-propanethial-S-oxide. Instead, the sulfenic acid is converted by other enzymes into non-volatile compounds like dipropyl disulfide and thiopropionaldehyde, which are pungent when ingested but do not evaporate and irritate the eye.

The tearless onion variety was a technical success, and seeds were distributed to home gardeners and researchers in Japan and other countries. But the onions had a drawback: they lacked the characteristic onion flavor. The tear-inducing pathway is part of the same biochemical network that produces the flavor-active sulfur compounds that make onions tasty. Disabling LFS reduced not just tears but also onion flavor complexity. Commercially, the tearless onion has not achieved significant market share because consumers value onion flavor.

An alternative approach involves reducing the concentration of the lachrymatory precursor (1-propenyl cysteine sulfoxide) itself through selective breeding, much like sweet onion varieties do naturally. This avoids disabling the flavor-producing pathways and instead simply reduces the overall intensity of the reaction. The result is a middle-ground onion: still flavorful, but with significantly reduced tear-inducing power.

Source: NCBI (2016). Production and Characterization of Tearless and Non-pungent Onion, PubMed Central — comprehensive review of genetic approaches to reducing lachrymatory-factor production in Allium cepa.

Evidence-Based Tactics: Reducing Tears Without Sacrificing Flavor

If you must cut a regular yellow or white storage onion and want to minimize tears, the science points to several strategies that work, ranked by effectiveness.

Chilling the onion is among the simplest interventions. Placing an onion in the freezer for 15–30 minutes before cutting slows the kinetics of both alliinase and lachrymatory-factor synthase. Enzymes are proteins whose activity depends on temperature. A rule of thumb in biochemistry is that enzyme activity doubles or triples for every 10°C increase in temperature (within physiological ranges). Conversely, cooling slows reactions. A chilled onion produces syn-propanethial-S-oxide at a measurably slower rate. The gas is generated in lower concentration and diffuses less rapidly into the air. Cooks report a noticeable reduction in eye irritation, though not elimination. The downside is a very slight change in texture — the cold makes the onion slightly more brittle — and the wait time before cooking.

Using a sharp knife reduces tearing by minimizing cell damage beyond the intended cuts. A dull knife crushes and mangles cells, creating more surface area and releasing more alliinase and substrate into the air. A sharp knife makes clean cuts that release less material and produce lower concentrations of volatile gas. This intervention is free, quick, and surprisingly effective.

Cutting the stem end last exploits a biological gradient. The root end of an onion bulb is where the onion stores the highest concentration of sulfur compounds, because it is the region most likely to be attacked by soil-dwelling pests and pathogens. The stem end (the top, where the leaves emerged) has lower concentration. Cutting the stem end first minimizes exposure to the highest-concentration tissues. As you cut toward the root end, you are generating more alliinase and substrate, but by then much of the volatile gas has already dispersed.

Cutting under running water directly dissolves syn-propanethial-S-oxide before it reaches your eyes. Water is the solvent in which this molecule reacts; exposing it to water causes it to form acids and other non-volatile products. Cutting under a stream of cold water — ideally in a shallow bowl or directly under the tap — keeps the volatile concentration near zero. This is highly effective but awkward in practice because keeping an onion steady under running water while using a knife requires coordination.

Wearing contact lenses or protective goggles is the most direct solution: a physical barrier between your eyes and the irritant. Goggles are overkill for casual home cooking, but for anyone cutting large quantities of onions (a chef, a catering cook, a home cook preparing for a party), swimmer's goggles or clear safety glasses are a practical and completely effective solution. Some commercial kitchens have staff wear goggles during prep work.

None of these tactics are perfect except goggles and water. But combined — a chilled onion, a sharp knife, cutting under water — you can reduce tearing to a nearly unnoticeable level while preserving all the flavor of regular onions.

Source: Cornell Cooperative Extension (2015). Onion Production Guide, Cornell University — practical recommendations for reducing lachrymatory effects during food preparation.

The Takeaway

Your tears while cutting an onion are not sentimentality or weakness. They are a sophisticated biological reflex triggered by plant chemistry evolved over millennia. The onion does not "want" to make you cry — the tear-inducing mechanism is an incidental consequence of its chemical defense system, evolved to deter insects and grazing animals. But that defense system works so effectively that it catches us every time.

The science reveals that this seemingly simple kitchen frustration involves multiple specialized enzymes, volatile chemistry, and a sensory system finely tuned to detect dangerous compounds. It also reveals that the problem is partly solved — we can breed sweeter onions, chill our onions, sharpen our knives, and wear goggles. Understanding the mechanism gives you power to reduce the irritation without giving up the flavor. The next time you feel tears welling up at the cutting board, you will know exactly why: a three-carbon sulfur compound is dissolving in your tear film and triggering a reflex that predates human civilization.

Frequently Asked Questions

Q: Will chilling an onion in ice water make a difference, or does it need to be in the freezer?
A: Both work, but freezing is more effective. Ice water brings the onion's surface temperature down to 4°C but not the interior. The freezer reaches 4°C throughout, slowing enzyme kinetics more broadly and more deeply. Aim for 15–30 minutes in the freezer. If you place a whole onion in the freezer for longer than an hour, the texture will shift (it becomes slightly mushy when thawed), so timing matters.

Q: Do all parts of an onion contain the same concentration of lachrymatory compounds?
A: No. The root end (bottom) has the highest concentration because that is the region most exposed to soil pathogens and pests in the field. The stem end (top) has less. The outer layers have more than the inner layers. Cutting the top and outer layers first minimizes exposure to the highest-concentration tissues and reduces gas production early in the cutting process.

Q: Is the sulfur compound in onions the same thing as the sulfur in garlic?
A: Both onions and garlic are Allium plants that produce sulfur-containing defensive compounds, but they are not identical. Garlic contains different sulfur compounds (primarily allicin, which forms when alliinase acts on alliin), and garlic lacks functional lachrymatory-factor synthase. This is why garlic does not make you cry, even though it smells strong and tastes pungent.

Q: If I cook an onion, does the heat destroy the lachrymatory compound before it reaches my eyes?
A: Heat does not help while cutting raw onion. Once the onion is heated above 60°C, the alliinase enzyme denatures and stops working, so no new lachrymatory compound forms. But when cutting a raw onion, the temperature of the cutting board and air is too low to denature alliinase, so the enzyme remains active and continues producing syn-propanethial-S-oxide until you finish cutting.

Q: Why does my nose run when my eyes tear from cutting an onion?
A: The tear ducts drain into the nasal cavity through a small channel called the nasolacrimal duct. When you produce excess tears, much of that fluid flows backward through this duct into your nose. The syn-propanethial-S-oxide vapor also rises into the nasal passage and irritates the mucous membranes there, triggering additional lacrimal and nasal secretion. This is why your eyes and nose both respond to onion cutting.

Q: Can I buy "tearless" onions at a regular grocery store, or are they only available as seeds?
A: Tearless onion varieties with reduced lachrymatory-factor synthase were developed in Japan but have not achieved widespread commercial cultivation. Most grocery stores sell sweet onion varieties (Vidalia, Walla Walla) that reduce tears through lower sulfur-compound concentration rather than through genetic modification of the enzyme. True tearless onions are rare in mainstream markets but can sometimes be found at specialty grocers or ordered as seeds for home gardening.

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