The crying question
Why onion makes you cry
Cutting an onion triggers a remarkably specific response: the eyes water, sometimes severely, while the nose is unaffected and the person cutting feels no pain from the compound causing the tearing. This is not a general irritant response — it is a highly targeted chemical mechanism that operates specifically on the eye's lacrimal glands. Understanding it reveals why certain cutting techniques reduce tearing and why cooking onions eliminates the effect entirely.
The Science
What compound causes onion tearing and how is it produced?
When onion cells are damaged by cutting, two compartmentalised compounds mix: alliinase enzyme and its substrate isoalliin. The alliinase converts isoalliin into propanethial S-oxide — a lachrymatory factor (tear-inducing compound). This volatile molecule evaporates rapidly from the cut surface and contacts the lacrimal glands in the eyes, stimulating them to produce tears as a flushing response. The entire reaction from cutting to tearing takes less than 30 seconds. Heat above 60°C destroys alliinase — which is why cooked onions don't cause tearing, and why briefly microwaving an onion before cutting reduces the effect.
35 second read
How to Reduce Onion Tearing
Techniques that actually work and those that don't
- Chill the onion: cold temperature slows the alliinase enzyme activity and reduces propanethial S-oxide production. Refrigerate 30 minutes before cutting.
- Sharp knife: a sharp knife causes less cell damage than a dull knife — fewer damaged cells means less alliinase-isoalliin mixing and less propanethial S-oxide produced.
- Cut near ventilation: a fan or open window disperses the volatile compound before it reaches eye level.
- Brief microwave (10–15 seconds): partially deactivates alliinase in the outer layers — reduces but doesn't eliminate tearing.
- Goggles: prevents the compound from contacting the eyes. Effective but impractical for most.
- Bread in mouth / candle nearby: no reliable evidence these work.