Why Do Onions Make You Cry?

What happens inside an onion when you cut it?

An onion does not contain its tear-causing chemical until it is damaged.^[4] Inside each onion cell, enzymes are sealed away in a small compartment called a vacuole.^[3] A knife or a bite breaks these compartments open, and the enzymes mix with other chemicals in the cell, setting off a rapid chain of reactions.^[3]

Onions hold many sulfur-containing molecules, and the starting point for the irritant is an amino acid called S-1-propenyl-L-cysteine sulfoxide.^[4] When cells break, the enzyme alliinase is released and reacts with it, forming a mixture of products that lead to the onion's flavor compounds, including 1-propenyl sulfenic acid.^[4] A second enzyme, lachrymatory-factor synthase, then turns that sulfenic acid into propanethial S-oxide.^[2][4] Scientists call this compound the onion's lachrymatory factor, or LF, and it is responsible for both the tears and the sharp taste of fresh onions.^[2]

Chemist Eric Block, who has written a book on onions and their relatives, calls the irritant syn-propanethial-S-oxide.^[3] Its molecules are tiny and light, so they float up into the air and reach the eyes of the person doing the cutting.^[3]

How does onion gas make your eyes water?

According to a "Molecule of the Month" explainer on the University of Bristol School of Chemistry's website, propanethial S-oxide is a lachrymator: an irritant that makes the eyes fill with tears without damaging them.^[4] When it touches the surface of the eye, the cornea, the nervous system detects it and signals the tear glands to respond.^[4] The tears that follow dilute the irritant.^[4] Science News describes the compound as binding to sensory nerves in the eyes and causing a stinging feeling.^[6]

The irritant does not only drift up as a gas; it also flies out in tiny droplets.^[6] In October 2025, Cornell University researchers reported in the Proceedings of the National Academy of Sciences how this spray forms.^[5][6] Each layer of an onion is wrapped in skin on its top and bottom.^[5] A knife pressing down crushes the cells beneath, and their juices push against the skin.^[6] When the blade breaks through, the pent-up pressure forces out a mist of tiny droplets at an estimated 5 to 40 meters per second, far faster than the roughly 1 meter per second at which professional chefs were filmed slicing.^[5] This spray can rise up to two-thirds of a meter into the air.^[5]

Why would a plant do this? Block says the onion is a bulb living in the ground among animals looking for a snack, so it has evolved a chemical defense system.^[3] The Bristol explainer likewise says the compound is thought to protect the plant from herbivores.^[4]

What did the 2002 Nature study discover?

The irritant was long considered a by-product of the same reactions that make the onion's flavor compounds.^[1] In October 2002, Shinsuke Imai and colleagues at the Japanese company House Foods reported in Nature that onions use a separate enzyme to make it, which they named lachrymatory-factor synthase.^[1] They came across it by accident while trying to make the irritant from onion compounds and alliinase taken from garlic, a close relative of the onion that does not bring on tears.^[1]

The finding mattered because the new enzyme plays no part in making the onion's flavor compounds, so switching it off could, in principle, give an onion that keeps its taste but causes no tears.^[1] Food scientist Kirk Parkin cautioned that some irritant might still form on its own, and Block warned that an onion stripped of this defense might be more open to attack by insects and microorganisms.^[1]

Why do some onions make you cry less?

How much an onion stings depends on how much lachrymatory factor it produces.^[2][7] The University of Georgia's onion-testing laboratory explains that LF causes the tearing and is also a major source of an onion's heat.^[7] The lab measures an onion's pungency as its pyruvic acid content, because pyruvic acid forms alongside LF.^[7] The levels depend on the variety and on growing conditions such as high temperatures, drought and the sulfur available to the plants.^[7] Parkin noted in 2002 that onions grown on low-sulfur soils already cause fewer tears, though their flavor is weaker.^[1]

Breeders have also produced onions designed not to sting.^[2][8] In a 2016 paper in Scientific Reports, House Foods researchers described onions grown from seeds exposed to a neon-ion beam, a method whose products are not considered genetically modified.^[2] The two best lines made about 7.5 times less LF than normal onions because they produced very little alliinase, and 20 tasters found that they caused no tears.^[2]

In the United States, Bayer spent 30 years cross-breeding an onion called the Sunion, selecting the least pungent bulbs in each generation.^[8] Freshly picked, it can bring on tears like any yellow onion, but while most onions get stronger in storage, it becomes milder.^[8]

Do chilling and a sharp knife really help?

Block says chilling onions in the refrigerator before chopping helps because the irritating compounds are less volatile when cold.^[3] He also suggests cutting in a stream of air blowing away from the face, so the molecules are carried off before they reach the eyes.^[3] The Bristol explainer notes that opinion on how well such home methods work is mixed.^[4]

A sharp knife works differently: it does less damage to the cells.^[4] In the Cornell experiments, blunt blades needed more force to break the skin and squashed more cells, while unhurried cuts with sharper blades kept the spray from reaching eye level.^[6] The researchers' advice is to cut onions slowly with a sharpened blade.^[5] They add that limiting the spray could also help stop bacteria on an onion from spreading around a kitchen.^[5]