How it works
The compound reaches the eye surface and activates sensory nerves. Tears dilute and wash away the irritant, which is why the response continues while the vapor is present.
What people often miss
A sharp knife reduces cell damage, and chilling the onion can slow the reactions. Good ventilation helps more reliably than many kitchen myths.
Why this question comes up
The reaction is harmless for most people but dramatic enough to inspire countless folk remedies.
Key points
- Cutting an onion breaks cells and starts chemical reactions that release a volatile sulfur compound, which irritates the eyes and triggers tears.
- The compound reaches the eye surface and activates sensory nerves. Tears dilute and wash away the irritant, which is why the response continues while the vapor is present.
- A sharp knife reduces cell damage, and chilling the onion can slow the reactions. Good ventilation helps more reliably than many kitchen myths.
Bottom line
Cutting an onion breaks cells and starts chemical reactions that release a volatile sulfur compound, which irritates the eyes and triggers tears.
Source and verification
This Answer was checked against American Chemical Society: Onion Chemistry. Follow the source for the full official explanation and any later updates.