Why do onions make you cry?
Cutting an onion sets off a two-enzyme chain reaction that releases a gas, syn-propanethial-S-oxide. When it reaches your eyes, it irritates them, and your tear glands try to wash it away.
Quick Answer
When you cut an onion, you break open its cells and mix chemicals that are normally kept apart. An enzyme called alliinase turns the onion’s sulfur compounds into a sulfenic acid, and a second enzyme, lachrymatory-factor synthase, converts that into a volatile gas called syn-propanethial-S-oxide. The gas drifts up to your face and irritates nerve endings in your eyes, and your tear glands respond by flushing it away. Scientists in Japan identified that second enzyme in 2002, overturning the idea that the tear gas simply formed as a side effect of the first reaction. To cry less, the National Onion Association suggests chilling onions for 30 minutes before cutting and leaving the root end intact, since it holds the highest concentration of sulfur compounds.
By the Numbers
Key Facts
- The culprit
- A gas called syn-propanethial-S-oxide
- How it forms
- Cutting mixes the onion’s enzymes with its sulfur compounds
- Key discovery
- The tear-making enzyme, identified by Japanese scientists in 2002
- Fewer tears
- Chill onions for 30 minutes and leave the root end on
From knife to tears
- You cut the onion
Broken cells release an enzyme, alliinase, that’s normally kept apart from the onion’s sulfur compounds.
- Sulfur compounds break down
Alliinase turns those compounds into a sulfenic acid.
- A second enzyme makes the gas
Lachrymatory-factor synthase converts the acid into syn-propanethial-S-oxide.
- The gas reaches your eyes
It drifts up and irritates nerve endings on the surface of the eye.
- Your eyes flush it out
Tear glands release tears to wash the irritant away.
A chemical reaction you set off
An intact onion keeps its enzymes and its sulfur compounds in separate parts of its cells. A knife breaks those walls, the chemicals mix, and within moments the reaction produces the gas that makes your eyes sting.
The 2002 discovery
For years, scientists thought the tear gas formed on its own as part of the alliinase reaction. In 2002, researchers in Japan reported in Nature that a separate enzyme, which they named lachrymatory-factor synthase, is what makes it.
Onions that don’t make you cry
Knowing which enzyme is responsible gave breeders a target. In 2016, researchers in Japan reported onions bred to be tearless, publishing their results in the journal Scientific Reports.
Myth vs. Fact
MythIt’s the onion juice that stings your eyes.
FactIt’s a gas, released when the onion is cut, that floats up to your eyes.
MythThe tear gas is just a side effect of the onion’s other chemistry.
FactA dedicated enzyme makes it, as Japanese researchers showed in 2002.
The Bottom Line
Onions make you cry because cutting them triggers a two-enzyme chain reaction that releases an irritating gas. Chilling the onion first and leaving the root on can cut down the tears.
Sources & Further Reading
- Library of Congress — Why does chopping an onion make you cry?
- Nature — An onion enzyme that makes the eyes water (2002)
- Royal Society of Chemistry — Propanethial-S-oxide
- Poison Control — Why do onions make you cry?
- Scientific Reports — Production and characterization of tearless and non-pungent onion (2016)