It turns out that onions are adept at absorbing sulfur. Once chopped, cell contents are released, including sulfur-rich amino acids. An enzyme called alliinase aims to convert sulfur-rich amino acids into sulfenic acids, extremely strong and unstable acids. Synthase, another enzyme, manipulates this sulfenic acid to make a lachrymatory agent called syn-propanethial S-oxide, which is responsible for the tear reaction. This is because the agent can easily evaporate into the air and irritate the eyes.
Nerves in the eyes detect the agent. Tear secretion follows to rinse the irritant away to protect the eyes. So, in fact, the tear reaction is just a defense mechanism triggered by the brain; they are different from emotional tears and are called reflex tears when exposed to irritants. But crying isn’t the only reaction people experience. Those that are more sensitive to onions or other alliums may experience severe reactions such as itching or hives.
There are also many types of onions, ranging in particular from those that cause less or more tears. For instance, sweeter onions like green onions contain less sulfur, and therefore cause less tears. Enzymes are not able to do their job as much with the lack of sulfur.
With this, there are several ways to minimize the tear reaction, such as aiming to stop the enzyme activity, as crazy as that sounds. Enzymes have certain environmental conditions in which they can thrive or regress. Knowing their conditions helps to develop a distinct protocol before chopping them.
For example, temperature plays a considerable role in enzyme activity. Since enzymes generally work best in warm temperatures, cooling the onions in a freezer is a good way to slow them down. It's important to chop the onions right after they've cooled, because as the environment heats up, the enzymes thrive as well.
But you can also take simpler measures like simply wear goggles to prevent the agent from reaching and irritating your eyes. So the next time you hop into the kitchen to chop some onions, remember that you don't have to endure the backlash of tears or whatever reaction you end up experiencing and there are indeed ways to minimize them!