Short answer: cutting an onion breaks cells that normally keep enzymes and sulfur-containing compounds apart. The chemistry that follows produces the volatile irritant syn-propanethial-S-oxide. When it reaches the surface of your eyes, it triggers irritation and tear production.
The onion does not store “tear gas” ready-made
The irritant is produced after tissue damage. Enzymes begin transforming sulfur compounds, and a specialized lachrymatory-factor synthase helps form the molecule responsible for the eye-watering effect.
Why do your eyes respond with more water?
Tears are protective. Irritation activates sensory pathways around the eye and lacrimal glands increase fluid output, helping dilute and wash away the chemical.
Quick follow-ups
FAQ
Does chilling an onion help?
Cooling can slow chemical reactions and reduce volatility, although technique and airflow also matter.
Are onion tears an allergy?
Usually no. The normal reaction is chemical irritation from the volatile lachrymatory factor, not an allergic response.
Check the evidence
Sources
- Why Do Onions Make You Cry? — American Chemical Society
- Why does chopping an onion make you cry? — Library of Congress
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