Why Do Onions Make You Cry? The Chemistry Behind the Tears
Whole onions have little smell. Cut one, and within seconds your eyes sting and water. The cause is chemistry that starts the moment cells are damaged.
What happens when you cut one
Poison Control explains that intact onions have little flavor or aroma. When an onion is peeled, cooked or cut, an enzyme called alliinase is released from the injured cells. It breaks down sulfur compounds into other chemicals, including sulfenic acids, pyruvic acid and ammonia, which give onions their flavor.
Tearing comes from one of those products. Alliinase breaks down a sulfur compound called 1-propenylsulfenic acid, and the result is propanethial-S-oxide, known as lachrymatory factor. It irritates the eyes and causes tears.
A second enzyme is involved
A 2002 paper in Nature reported that the tear-causing chemical is not produced spontaneously after alliinase acts, as had been presumed. A separate enzyme is involved. A later structural study in ACS Chemical Biology calls it lachrymatory factor synthase, and describes the chemical as part of the plant's defence against microbes and animals.
Not every plant does this
Poison Control says lachrymatory factor is specific to onions and not found in garlic. It notes only three lachrymatory factors are found in nature, all made by plants: onions, a shrub called Petiveria alliacea, and Allium siculum, sometimes called honey garlic.
What can reduce the tears
Poison Control lists several suggestions: lighting a match, wearing sealed goggles, or cutting onions under water. It also cites the National Onion Association advice to chill onions for 30 minutes before cutting and to leave the root end intact, because it has the highest concentration of sulfur compounds. Some "low-pungency" onions are grown in low-sulfur conditions, but they are more perishable because of higher water content. Poison Control says there is still no foolproof way to avoid crying when cutting onions.
Sources
- Poison Control, Why do onions make you cry?: https://www.poison.org/articles/why-do-onions-make-you-cry
- Nature (2002), lachrymatory factor and the enzyme that produces it: https://www.nature.com/articles/419685a
- ACS Chemical Biology, Enzyme That Makes You Cry: https://pubs.acs.org/acbcct/article/12/9/2296/1229307/Enzyme-That-Makes-You-Cry-Crystal-Structure-of
Facts checked: October 8, 2026.