Updated 28 July 2026 · Written by Daniel Wright
Cutting an onion ruptures its cells, which release enzymes that form a volatile sulfur gas. That gas drifts up, reacts with the moisture in your eyes to make a mild acid, and stings, making you tear up. Chilling the onion, using a sharp knife, and cutting near airflow all reduce it.
Onions release a sulfur gas when cut that irritates your eyes. Fix: chill the onion, use a very sharp knife, and cut near a fan or open window.
The Science
Onion cells store sulfur compounds and enzymes separately. Cutting breaks the cells and mixes them, producing a volatile gas called syn-propanethial-S-oxide. It rises to your eyes, dissolves in the tears’ moisture to form a weak sulfuric acid, and triggers stinging and reflex tears.
How to Chop Onions Tear-Free
- Chill the onion in the fridge for 15 minutes to slow the gas release.
- Use a very sharp knife, which ruptures fewer cells than a dull one.
- Cut near a fan, open window or extractor to blow the gas away.
- Leave the root end intact until last, as it holds the most compounds.
- Wet the onion or cut it under a running tap to wash gas away.
Frequently Asked Questions
Does chilling an onion really stop tears?
Yes, to a degree. Cold slows the enzyme reaction that produces the irritating gas, so a chilled onion releases less of it while you cut.
Why does a sharp knife help with onions?
A sharp blade slices cleanly and ruptures fewer cells, so less of the tear-inducing gas is released than with a dull knife that crushes the onion.
