When an onion is cut, certain (lachrymator) compounds are released causing the nerves around the eyes (lacrimal glands) to become irritated.
These compounds consist of methionine and cystine that are part of the amino acid family. (Protein is made up of several of these amino acids grouped together.)
When an onion is sliced or diced, the onion's cells release these compounds into the air. When this occurs, "enzyme" works to alter the amino acids into lachrymator compounds. This form of sulfuric acid irritates the nerves around the eyes making them tear.
How can we prevent this reaction?
Use a sharp knife to cut the onion to reduce the amount of damage to the onion cells.
Cool the onion in a refrigerator to suppress the scattering of the lachrymator.
Soak the onion in water to dissolve the amino acids. (Cutting the onions in half or quarters before soaking them is even more effective.)
Wear swimming goggles or ski goggles while cutting the onion.
Use a ventilator or fan to blow the ally l sulfide away.
hope it helps......... ^.^