When you slice an onion, you cut through the tiny plant cells within it, releasing their content. When this happen amino acids within onion cells oxidise to form sulfenic acid. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide. This is a volatile sulfur compound that wafts upward toward your eyes and reacts with the moisture in your eyes to form sulfuric acid. The sulfuric acid burns your eyeballs, stimulating your eyes to release more tears to wash the irritant away. The best way to avoid this is to :
- Refrigerate the onion at least 30 minutes before cutting. By cooling the onion, you will slow the enzyme/sulfoxides reaction rate, minimizing the propanethiol S-oxide production.
- Chop the onion under cold water/running water. The propanethiol S-oxide is water soluble, so can be simply washed away before getting in the air.
- Turn on a fan, blowing across the onion and away from you.
Source : Chemistry.about.com
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