Is it possible to cut or chop onions in the kitchen without stinging eyes and looking as if you just watched the saddest movie ever? Before we get to that answer, it's important to know why we tear up when cutting raw onions in the first place. What is this irritant? Are you reacting to the odor? The answer to the latter question is "no," and the irritant responsible is amino acid sulfoxides.
Amino acid what? Sulfoxides. You see, onions are part of the Allium family, which includes more than 700 different types of flowers and vegetables, of which onions, shallots, and garlic are the most well-known. When growing underground, the onions absorb the sulfur from the Earth's soil.
When we cut into the vegetable's tissue, the pressure of the knife releases enzymes into the air along with the onion's amino acid sulfoxides. Once in the air, these molecules — previously separated by a cell membrane — react and convert into sulfenic acid which then undergoes one more transformation into syn-Propanethial S-oxide, a volatile sulfur compound that when wafted toward the eyes, triggers tears as we naturally attempt to rid of it.
In fact, these tears are technically called "reflex tears," a kind of basal tears that rise when summoned by the eyes to fight off irritants, such as onion vapors or dust. When we cut, chop, or peel an onion, we also start the chemical process that results in odorous thiosulfinates, responsible for the onion's strong, pungent smell. These simultaneous reactions for the eyes and olfactory system have led people to reason that the two are somehow one in the same. But they're not. It's definitely the sulfur compound that causes the tear reaction, not the odor.
So now that we know why our eyes water when chopping onions, is there a way to keep them from doing so? The answer to that is a resounding "Yes."
The most effective way (other than to buy them pre-chopped) is to be like you're going for the gold and wear swimming goggles to prevent the onion's vapors from reaching the eyes. The syn-Propanethial S-oxide is strongest about 30 seconds after the chemical process starts with the first cut (chop or peel). It lasts about five minutes total, which is why raw onions in a salad or sandwiches are perfectly fine and delicious — they're way past the time of causing tears.
However, if you don't have swimming goggles handy, you could try some other methods such as chilling the onion before cutting it, which will make the enzymes less reactive. On the flip side, you could try heating it to the same effect. Lighting a candle is a way to go, as is sticking a piece of bread in your mouth. And since you may wash the onion(s) anyway, cutting under running water, which will wash the sulfuric fumes away, is a logical, practical option, too.