I always wondered why cutting up raw onions makes me cry. Why does it make me so sad?
How Stuff Works provides a good explanation:
When you slice through an onion, you break open a number of onion cells. Some of these cells have enzymes inside of them, and when they are sliced open, the enzymes escape. The enzymes then decompose some of the other substances that have escaped from sliced cells. Some of these substances, amino acid sulfoxides, form sulfenic acids, which then quickly rearrange themselves into a volatile gas.
The gas reaches your eyes and reacts with the water that keeps them moist. This changes the chemical’s form again, producing, among other things, a mild sulfuric acid, which irritates the eyes. The nerve endings in your eyes are very sensitive and so they pick up on this irritation (this is why our eyes sting when we slice onions). The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid so the eyes are protected.
It is these same sulfur compounds that form the nice aroma when onions are being cooked.
There are various methods for preventing eye irritation while slicing onions. Among them are to stand as far away from the onion as your arms will allow, and breathe through your mouth (which reduces the airflow through the nose and into the eyes). Cooking the onion before you slice it will minimize the release of gas because the change in temperature alters the compounds in the onion. You may also slice the onion under water; however, you will wash some of the aroma out. Another way to avoid irritation is by not cutting off the root of the onion, or by doing it last, as the root of the onion has a higher concentration of enzymes.
Some suggest that placing a slice of lemon under your top lip, holding a sugar cube between your teeth, or holding a piece of bread (a quarter slice) between your lips will absorb the gas before it reaches your eyes. This makes me think of those “hiccup cures.”
Then again, you could risk looking silly and wear goggles. My personal preference: omit the onions.