But it is one of the most essential ingredients for good food, so I rarely let my eyes stop crying.
When we cut open an onion, we allow an enzyme called alliinases to react and break down amino acid to generate sulphenic acid. At this point the chemical still remains on the cutting board-- so how does it get to our eyes?
The natural reaction to the Propanethiol S-oxide invasion is to shut your eyes. This, of course, is not a good idea if you are cutting an onion. Rubbing your eyes is a bad idea, since your hands are likely full of the tear-making onion juice, and by rubbing your eyes you are actually transferring the sulphenic acid to your eyes, in addition to the sulfuric acid induced by the Propanethiol S-oxide gas.
Otherwise, I'll stick to wearing goggles.
http://www.sciencebob.com/questions/q-onion_tears.php http://chemistry.about.com/od/chemistryfaqs/f/onionscry.htm http://web.fccj.org/~ethall/h2so4/h2so4.htm