Why Do Onions Make You Cry?
Quick answer
Cutting an onion damages its cells, and enzymes then build a volatile sulfur compound called syn-propanethial-S-oxide, the lachrymatory factor. When it reaches your eye, it activates corneal nerves and tears follow. Sharp blades and slow cuts make fewer droplets in lab tests, but tears were not measured, and most other tips remain unproven.
In this article
Why Do Onions Make You Cry?
Onions make you cry because cutting them starts a chemical reaction that produces an eye irritant. Once its cells are damaged, enzymes build a sulfur compound that your eyes answer with tears.
That compound is called the lachrymatory factor, or LF. "Lachrymatory" just means tear-causing. Chemists write its name in more than one way: you will see syn-propanethial-S-oxide in a Scientific American explainer[4] and (Z)-propanethial S-oxide in a 2017 paper on the enzyme that makes it.[1]
How Does Cutting an Onion Release the Tear Chemical?
The reaction runs in two enzyme steps, and damage is what sets it off.
First, when onion tissue is damaged, an enzyme called alliinase cuts apart sulfur-containing molecules called S-alk(en)yl cysteine sulfoxides. That produces a sulfenic acid.[1] This step also helps create the onion's familiar flavor, so an intact onion has little flavor or aroma.[9]
Second, a separate enzyme, lachrymatory factor synthase (LFS), rearranges that sulfenic acid into LF.[1] Scientists once assumed LF formed on its own after alliinase did its job. A short 2002 paper in Nature reported that a dedicated enzyme was required.[2]
LF is volatile, which means it readily drifts into the air.[1] A 1999 explainer by University of Delaware researcher Thomas Scott says its effects peak about 30 seconds after the damage and that the chemical changes finish over about five minutes.[4] That timing comes from an expert's short summary, so treat it as a rough figure.
What Does the Eye Actually Do?
The cornea, the clear front of your eye, has sensory nerves. The authors of a 2025 study in PNAS sum up the onion case this way: when propanethial S-oxide reaches the eye, it activates ciliary nerves in the cornea and causes tears.[3] Scott's explainer adds the next step. Free nerve endings on the cornea detect the compound, the brain registers burning, and a reflex tells the tear glands to flush it away.[4]
A 2025 review describes corneal pain-sensing nerves as critical for starting blinking and tearing reflexes, and lists several TRP channels, a family of sensor proteins, in the human cornea.[5] That review does not discuss onions, so it cannot tell us which sensor LF triggers.
In a survey of 1,001 adults at a museum in Dresden, 73% said their eyes water when cutting onions, with 37% answering "partly true" and 36% "absolutely true." The higher-tearing group also rated their noses as a little more sensitive to stinging and burning.[6] The authors call this preliminary. It relied largely on self-report and could not show why people differ.
Which Onion Tips Have Evidence?
The best recent test is a Cornell study that filmed onions being cut by a blade in a free-fall rig. It measured droplets, not tears, and did not measure LF in those droplets.[3]
| Tip | What we found | Status |
|---|---|---|
| Use a sharper blade | Droplet count rose up to about fortyfold as blade edge width went from 0.91 to 13.3 micrometers.[3] | Measured for droplets, not tears |
| Cut more slowly | Droplet count rose about fourfold as cutting speed went from 0.44 to 2.03 meters per second.[3] | Measured for droplets, not tears |
| Chill the onion | Often recommended,[9] but onions held at 1 °C for 12 hours gave a noticeably larger volume of droplets, with no significant change in droplet speed.[3] | Disputed; the volume difference needs more testing |
| Cut under water | Water solubility makes it plausible, but the Curious Clinicians podcast notes no rigorous study.[12] | Untested in what we read |
| Wear sealed goggles | Goggles are presumed to block vapor.[12] | Logical, but we found no controlled test |
| Keep the root end on | The National Onion Association says this end holds the most sulfur compounds.[9] | Advice; we found no test |
| Light a match, hold bread | These circulate as tricks,[9] and a chemistry explainer says opinion is mixed and many have been discounted.[11] | Folklore |
On chilling, the two sides measure different things. One explainer suggests cold might reduce the gas's volatility.[10] The Cornell team looked at droplets instead, and said further controlled experiments are needed.[3] A cold onion could plausibly release less vapor and more spray at once. Nobody in the sources we read has settled that.
The fastest droplets left the onion at up to 40 meters per second,[3] and Cornell reports aerosols rising up to two-thirds of a meter.[13]
Safety note: A sharp knife is safer only when you can see what you are doing. If your eyes are watering too much to see the board, put the knife down first. Skip the lit match trick around a cutting board and kitchen towels. We ran no tests of our own for this article.
Can You Grow an Onion That Doesn't Make You Cry?
Researchers have made onions with far less LF, by two different routes.
In 2008, a team silenced the LFS gene with RNA interference. Enzyme activity in the bulb fell by up to 1,544-fold, and LF in bulbs fell 6.4- to 28.2-fold depending on the line. The only eye test was informal: the authors held crushed tissue near their own eyelashes, and normal tissue stung while the low-LF tissue did not.[7]
In 2016, another team irradiated seeds with neon ions, which the authors say is not considered transgenic. Their mutant lines made very little alliinase, and LF was about 7.5-fold lower than normal. Twenty panelists chewed the fresh bulbs and reported no tear-inducing sensation, but those data were not shown and tears from the eyes were not measured.[8]
Milder onions already exist through ordinary growing. Poison Control reports that low-pungency onions grown in low-sulfur conditions make up about a quarter of U.S. onion consumption each year.[9]
Myths vs Facts
| Myth | Fact |
|---|---|
| Alliinase alone makes the irritant. | A separate enzyme, LFS, makes LF from the sulfenic acid.[1] |
| Chilling is proven to stop tears. | No source we read shows that, and one experiment saw more droplets from chilled onions.[3] |
| It is settled that sulfuric acid forms in your eye. | Sources disagree. One explainer says sulfuric acid forms,[11] another says only "an acid,"[10] and the Curious Clinicians podcast calls LF a direct corneal irritant.[12] |
What We Still Can't Tell You
We cannot tell you how many fewer tears a sharper knife will save you. The Cornell numbers describe droplets from a lab rig with a steel blade, and the authors did not measure tears or LF.[3] They do describe their results as support for the old belief about sharp knives, but that link remains an inference.
We also do not know how much of the sting comes from gas compared with droplets, or which nerve sensor LF activates. We found no study measuring acid formation in a human eye, though we cannot say none exists. We could not read one reported small 2004 contact lens study ourselves, and know it only through a summary saying wearers felt irritation later.[12]
As Poison Control puts it, there is no foolproof way to avoid crying when cutting onions.[9]
Frequently Asked Questions
What chemical in onions makes you cry?
It is a volatile sulfur compound called the lachrymatory factor, also written as syn-propanethial-S-oxide. It forms only after onion tissue is damaged, through two enzyme steps involving alliinase and lachrymatory factor synthase.
Does chilling an onion stop the tears?
That is not established. Chilling is widely recommended, but a 2025 lab study saw a noticeably larger volume of droplets from onions held at 1 °C for 12 hours. That study did not measure tears, and its authors say more experiments are needed.
Does a sharp knife really help?
It reduces spray in lab tests. Droplet counts rose up to about fortyfold from the sharpest to the bluntest blade tested. The researchers measured droplets rather than tears, so we cannot say how many fewer tears you would get.
Are there onions that don't make you cry?
Researchers have made onions with far less lachrymatory factor, through gene silencing in one study and seed irradiation in another. Milder low-pungency onions are also grown in low-sulfur conditions, but we have not confirmed that any variety is free of tears.
References
- Silvaroli JA, Pleshinger MJ, Banerjee S, Kiser PD, Golczak M. Enzyme That Makes You Cry: Crystal Structure of Lachrymatory Factor Synthase from Allium cepa. ACS Chemical Biology (2017). Accessed Nov 1, 2026.Supports: On tissue damage, alliinase cleaves S-alk(en)yl cysteine sulfoxides to a sulfenic acid; LFS converts it into volatile (Z)-propanethial S-oxide, which causes eye irritation.
- Imai S, Tsuge N, Tomotake M, Nagatome Y, Sawada H, Nagata T, Kumagai H. An onion enzyme that makes the eyes water. Nature (2002). Accessed Nov 1, 2026.Supports: The tear factor was presumed to form spontaneously after alliinase acted, but a separate enzyme, LFS, is required. Seen only in a search excerpt.
- Wu Z, Hooshanginejad A, Wang W, Hui CY, Jung S. Droplet outbursts from onion cutting. Proceedings of the National Academy of Sciences (2025). Accessed Nov 1, 2026.Supports: Droplet count rose about fourfold with cutting speed (0.44 to 2.03 m/s) and up to fortyfold with blade edge width (0.91 to 13.3 micrometers); droplets reached up to 40 m/s; chilled onions (1 °C, 12 h) gave a noticeably larger droplet volume. Tears and LF were not measured.
- Scott T. What is the chemical process that causes my eyes to tear when I peel an onion?. Scientific American (1999). Accessed Nov 1, 2026.Supports: Effects of syn-propanethial-S-oxide peak about 30 seconds after damage and finish changing over about five minutes; corneal free nerve endings detect it and a reflex activates the tear glands.
- Park R, Spritz S, Zeng AY, et al.. Corneal Sensory Receptors and Pharmacological Therapies to Modulate Ocular Pain. International Journal of Molecular Sciences (2025). Accessed Nov 1, 2026.Supports: Corneal nociceptors play a critical role in initiating blinking and tearing reflexes; several TRP channels are expressed in the human cornea. The review does not discuss onions.
- Mahmut MK, Herz RS, Bormann A, Weise S, Pang LH, Soncini A, Hummel T. Severity of Onion Tearing and Chemosensory Sensitivity: A Preliminary Investigation. Laryngoscope Investigative Otolaryngology (2026). Accessed Nov 1, 2026.Supports: Of 1,001 museum visitors, 73% reported tearing when cutting onions (37% partly true, 36% absolutely true); the higher-tearing group rated stinging and burning sensitivity slightly higher.
- Eady CC, Kamoi T, Kato M, Porter NG, Davis S, Shaw M, Kamoi A, Imai S. Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile. Plant Physiology (2008). Accessed Nov 1, 2026.Supports: RNA interference cut LFS activity in bulbs up to 1,544-fold and bulb LF 6.4- to 28.2-fold; the only eye test was an informal one by the authors.
- Kato M, Masamura N, Shono J, Okamoto D, Abe T, Imai S. Production and characterization of tearless and non-pungent onion. Scientific Reports (2016). Accessed Nov 1, 2026.Supports: Neon-ion irradiated seeds gave non-transgenic mutants with very low alliinase and about 7.5-fold lower LF; 20 panelists chewing the bulbs reported no tear-inducing sensation (data not shown).
- Johnson-Arbor K. Why do onions make you cry?. Poison Control (National Capital Poison Center). Accessed Nov 1, 2026.Supports: Intact onions have little flavor; lists chilling and root-end advice from the National Onion Association and tricks such as matches; low-pungency onions are about a quarter of U.S. consumption; no foolproof way to avoid crying.
- Arney K. Why chopping onions can turn on the waterworks. Royal Society of Chemistry (RSC Education) (2019). Accessed Nov 1, 2026.Supports: Onions make propanethial-S-oxide only when cells are damaged; on the eye it reacts with water to create an acid; cooling might reduce the gas's volatility.
- Burnham PM. Molecule of the Month: Thiopropanal S-oxide. University of Bristol School of Chemistry (Molecule of the Month) (2007). Accessed Nov 1, 2026.Supports: States that a small amount of the compound forms sulfuric acid in the eye; lists tips such as holding bread and says opinion is mixed and many have been discounted.
- Cooper AZ, Abrams HR, Breu AC. Episode 47: Something to Cry About. The Curious Clinicians Podcast (2022). Accessed Nov 1, 2026.Supports: LF is thought to be a direct corneal irritant; home tips have not been rigorously studied; goggles would presumably block it; summarizes a small 2004 contact lens study reporting delayed irritation.
- Ramanujan K. 'Mist opportunity' reveals how onions make cooks cry. Cornell Chronicle (2025). Accessed Nov 1, 2026.Supports: Onion cutting ejects aerosols up to two-thirds of a meter into the air.
Dr. Pippa is a fictional animated character who guides you through the science. She is not a real doctor or researcher.
