SCIENCE & NATURE

Why do onions make you cry?

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Onions make you cry because they release a chemical gas called syn-propanethial-S-oxide when their cells are cut open. This gas irritates your eyes and triggers tear production as a protective response.

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Chemical compoundSyn-propanethial-S-oxide (a sulfur-based gas)
When it's releasedWhen onion cells are damaged by cutting or chopping
Why tears formThe gas irritates the cornea of your eye, triggering tear ducts to produce tears
Raw onions vs cookedRaw onions produce the gas; cooking breaks down the compound before it's released
DurationThe irritation stops once you finish cutting and the gas dissipates

How onions release the chemical

Onions contain amino acids and enzymes that don't interact under normal conditions. When you cut or chop an onion, you break open its cells. This damage allows the enzyme alliinase to mix with the amino acids, which creates the syn-propanethial-S-oxide gas as a byproduct.

How the gas affects your eyes

The gas released from onions travels through the air and reaches your eyes. It irritates the cornea, which is the clear protective layer covering your eye. Your eye responds to this irritation by activating tear glands to produce tears. These tears are meant to wash away the irritating substance and protect your eye.

Why onions evolved to do this

Scientists believe onions developed this chemical defense mechanism to protect themselves in nature. The strong-smelling gas and tears discourage animals from eating the onion plant. This is a survival strategy for the plant, similar to how other plants have thorns or bitter tastes.

Ways to reduce tears when cutting onions

You can minimize tears by chilling onions in the refrigerator before cutting, which slows down the gas release. Cutting the onion under running water or near a fan can carry the gas away from your face. Using a sharp knife creates fewer damaged cells and releases less gas than a dull knife. Some people also light a candle nearby, as the flame can help break down the irritating compound.

Sources

  1. sciencefocus.com (sciencefocus.com)
  2. nasa.gov (nasa.gov)
  3. britannica.com (britannica.com)