Washington D.C.: Ever cut an onion and started tearing up? Most people do. In Reactions’ latest video, we get to the bottom of this teary phenomenon and reveal exactly what chemical mechanisms trigger it.
According to the video, the answer to why onions make you cry begins underground. Sulphur in the earth is absorbed by onions to form amino acid sulfoxies. Sulphur compounds then turn into gas.
When you cut open an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes.
This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away. The video also features a few chemistry-backed tips you can try at home to stop the tears before they start.