There are simple ways that you can stop from tearing up when you are around onions. The tearing up mechanism happens because when you cut into an onion, its cells rupture and releases enzymes that produce a gas called propanethial sulfoxide. When the gas reaches your eyes, it reacts with tears to produce a mild sulfuric acid. The brain signals the eyes' tear glands to produce more liquid to flush it out. The more that onions are chopped, the more your eyes tear up.
It is a good idea to chill an onion in the freezer before cutting it because cold temperatures can slow down the release of enzymes. You can also pour a tablespoon of vinegar onto the cutting board. The acid in the vinegar stops the chemical reaction of the sulfur in the onion and causes a reduction in tears. It is another good idea to put an onion under water before chopping it. The water would absorb the onion's gas.