In “Why Do Onions Make You Cry” by the American Chemical Society video series Reactions, the narrator explains why onions and other allium plants have such a strong odor and why onions, in particular, cause tears when cut.
Onions are a vegetable that belong to the plant genus allium. Along with 400 other variations that include garlic and chives. Sulfur in the earth is absorbed by these vegetables to form amino acids sulfoxides, sulfur compounds that readily turn into a gas. When you cut into an onion, you break open loads of individual onion cells. These ruptured ells release special enzymes as well as amino acid sulfoxides, which form sulfenic acid. The sulfenic acid and onion enzymes react to form syn-propanethial s-oxide. This tough to pronounce gas floats up from the chopped onions and eventually reaches your eyes.