The tearjerker in onions is a compound called propanethial-s-oxide, which is released in a vapor when onions are cut. When the vapor comes in contact with the eye, it is converted to a form of sulfuric acid, which produces the stinging sensation and subsequent tears. Cutting onions under water or chilling them before cutting, will retard the enzymes that generate the noxious chemicals.

Some other suggestions people have mentioned for preventing tears when working with onions include:

* Keep the root on while peeling and chopping. A variation of this is to cut from the top leaving the root end intact until the last cut!
* Be in a well-ventilated place and cut fast!
* Use a good, sharp chef’s knife. It glides through the onion with ease, allowing less of the compounds that cause tears to come out.
* Wear a pair of safety goggles or a cheap, disposable painter’s mask.
* Put a slice of bread in your mouth, with half of it sticking out to “catch” the fumes.
* Put some white vinegar on the chopping board before cutting.
* Burn a candle immediately adjacent to where you’re cutting. The tear-causing gas is drawn toward the heat source.

After cutting the onions (and garlic), a suggestion that may help remove the smell from your hands is to run cool tap water over your hands while rubbing them across a stainless steel utensil, such as the bowl of a stainless steel spoon. If you have been cutting a lot of onions, this may not be as effective.

Source: University of Nebraska Extension