Onions have a pungent odor, but this is not what causes our eyes to tear whenever we slice these bulbs. It is the release of syn-propanethial-S-oxide, a chemical irritant that stimulates our tear glands. When we cut an onion, an enzyme called lachrymatory-factor synthase is released into the air. This enzyme then couples with the enzyme present in the onion to produce an unstable sulfenic acid that rearranges itself to another gaseous substance, syn-propanethial-S-oxide. This gas gets into our eyes, producing sulfuric acid, and then the tears start flowing.
There are ways in getting around this problem.
- you can wear googles.
- you can chill the onion in the refrigerator before chooping it.
- you can try running tap water over the bulbs as you slice or cut the onion under water.