Don’t Cry!

There’s no need to cry over spilled milk, but what about chopped onions?

You can thank a chemical combination of enzymes and sulfur for the tears that well up while you make dinner. Onions use sulfur to make a mixture of amino acids and enzymes during the growing process. The acids and enzymes are separated and stored in different regions of the onion’s cells, which are called vacuoles. While the onion remains whole, the amino acids and enzymes in the onion’s cells remain separated.

Once you cut into the onion, however, everything mixes together. When the two substances are combined, they form a chemical known as syn-Propanethial-S-oxide, or lachrymatory factor (LF). LF is an irritant that’s easily vaporized when it reacts with the air.

LF isn’t strong enough to affect tougher parts of your body such as your skin, but it can irritate more sensitive regions. As the vapors waft up toward your face, your eyes will begin to sting. Your body — sensing the irritant — will release a torrent of tears in an attempt to wash the chemicals from your eyes. Luckily, LF can’t do any serious damage, even in high quantities.

Producing LF is the onion’s way offending against anything that may want to eat it. As soon as an animal bites into the root, its eyes start to burn and it’s reminded to stay away from onions. Unfortunately for onions, humans are persistent.