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Why do onions make you cry?

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Quick answer

Cutting an onion bursts its cells and lets enzymes turn stored sulfur compounds into syn-propanethial-S-oxide, a volatile irritant gas. When that gas reaches your eyes it stimulates pain-sensing nerves in the cornea, and your body responds with a reflex flow of tears to dilute and rinse the chemical away.

Onions make you cry because cutting them releases an irritating gas, syn-propanethial-S-oxide, that drifts up and stimulates pain nerves in the surface of your eyes. Your brain answers that irritation with a reflex flood of tears meant to dilute and wash the chemical away.

Step by step

  1. The knife ruptures onion cells, letting the enzyme alliinase meet the sulfur compounds that were stored separately inside them.
  2. Alliinase quickly converts those compounds into an unstable sulfenic acid inside the damaged onion tissue.
  3. Lachrymatory factor synthase rearranges the acid into syn-propanethial-S-oxide, a light gas that evaporates into the air above the board.
  4. The gas drifts upward and touches the moist surface of the eye, stimulating TRPA1 receptors on corneal nerve endings.
  5. The brain reads this as irritation and signals the tear glands, producing a stream of tears that washes the gas away.

Key things to understand

  1. Onion cells keep sulfur-containing compounds and the enzyme alliinase in separate compartments until a knife ruptures the cell walls.
  2. Once mixed, alliinase converts the sulfur compound into a reactive sulfenic acid within seconds of the cut.
  3. A second enzyme, lachrymatory factor synthase, turns that acid into syn-propanethial-S-oxide, a volatile gas that rises off the board.
  4. The gas reaches the cornea and activates TRPA1 pain receptors on the free nerve endings of the trigeminal nerve.
  5. The nerve signal triggers the lacrimal glands to release extra tears, which flush the irritant out of the eye.

Labelled diagram

  1. Cut cells

    The knife bursts cells, mixing stored sulfur compounds with the enzyme alliinase.

  2. Enzyme chain

    Two enzymes turn the sulfur compound into the volatile gas syn-propanethial-S-oxide.

  3. Rising gas

    The light irritant evaporates and drifts upward toward the face of the cook.

  4. Corneal nerves

    TRPA1 receptors on nerve endings in the cornea detect the gas as irritation.

  5. Tear glands

    Reflex signals make the lacrimal glands release tears that flush the irritant away.

From a cut onion to a teary eye

Real-world examples

  • A home cook in India slicing a large heap of onions for biryani often has to step away from the counter to wipe streaming eyes.
  • Restaurant kitchens chopping onions in bulk often use cold onions and a well-ventilated prep area to cut the irritation.
  • Swimming goggles worn while chopping stop the tears entirely because the gas never touches the eye's surface.
  • Frying onions in a pan produces a strong smell but no tears, because heat has already disabled the enzymes.

Common mistake

✗ Myth: Onion gas turns into sulfuric acid in your eyes, and that acid is what burns.

✓ Fact: This old explanation is outdated. The irritant is syn-propanethial-S-oxide itself, which triggers pain receptors on corneal nerves; the stinging comes from that nerve signal, not from acid forming in the eye.

People also ask

Why do onions make you cry but garlic doesn't?
Garlic uses the same alliinase enzyme, but it lacks lachrymatory factor synthase, the second enzyme onions use to make the tear gas. In garlic the reaction instead produces allicin, which gives the sharp smell and taste. With no syn-propanethial-S-oxide released, garlic does not trigger the eye nerves that cause reflex tears.
How to cut onions without crying?
Chill the onion for about 30 minutes, use a very sharp knife, and work near a fan or open window. Cold slows the enzymes, a sharp blade bursts fewer cells, and moving air carries the gas away. Cutting the root end last also helps, because that area holds more of the reacting compounds.
Is crying from onions bad for your eyes?
No, onion tears are a normal protective reflex and do not harm healthy eyes. The irritation fades within minutes once you stop cutting and the gas disperses. Rubbing your eyes with onion juice on your fingers can prolong the stinging, so wash your hands first, and see a doctor if irritation persists.
Why does chilling an onion reduce the tears?
Cold slows both the enzyme reactions and the evaporation of the irritant gas. A chilled onion therefore releases less syn-propanethial-S-oxide into the air while you chop, so less reaches your eyes.
Does a sharp knife really help?
Yes. A sharp blade slices cleanly through fewer cells, while a blunt one crushes and bursts many more of them. Fewer ruptured cells means less enzyme meets its sulfur compound, so less tear gas forms.
Why do cooked onions not make you cry?
Heat destroys the enzymes that build the tear-causing gas. Once onions are sautéed or boiled, alliinase and lachrymatory factor synthase stop working, and the sulfur compounds instead turn into the sweeter, milder flavour molecules of cooked onion.
Are some onions less likely to make you cry?
Yes. Sweet varieties generally contain fewer of the sulfur compounds that feed the reaction, so they release less irritant. Plant breeders have also developed onions with very low enzyme activity that produce little or no tear gas.

Part of the guide: Food & Kitchen Science →