Top of page

Notice
Special holiday hours in effect, December 2025 through January 2026. More information.

Everyday Mysteries

« Back to Food and Nutrition page

Question Why does chopping an onion make you cry?

Answer

Unstable chemicals.

Camp Kearny, Cal. “Anti-onion” gas mask. Between 1917-1919. Prints & Photographs Division, Library of Congress.

Onions produce the chemical irritant known as syn-Propanethial-S-oxide. It stimulates the eyes’ lachrymal glands so they release tears. Scientists used to blame the enzyme alliinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imai et al, 2002).

Maule’s seed catalogue for 1891 by Wm. Henry Maule (Firm). Henry G. Gilbert Nursery and Seed Trade Catalog Collection (on the Internet Archive), National Agricultural Library.

The process goes as follows:

  1. Lachrymatory-factor synthase is released into the air when we cut an onion.
  2. The synthase enzyme converts the amino acid sulfoxides of the onion into sulfenic acid.
  3. The unstable sulfenic acid rearranges itself into syn-Propanethial-S-oxide.
  4. Syn-Propanethial-S-oxide gets into the air and comes in contact with our eyes. The lachrymal glands become irritated and produces the tears!
Pungent red onions. Agricultural Research Service, U.S. Department of Agriculture.

Published: 11/19/2019. Last Updated: 5/27/2025. Author: Science Reference Section, Library of Congress

Have a question? Ask a science librarian