Why Do Onions Make You Cry?
When you cut an onion, you break cells that release an enzyme that converts sulfur compounds into a volatile gas called syn-propanethial-S-oxide. This gas reaches your eyes, dissolves in tear fluid, and forms sulfuric acid, which irritates your eyes. Your body responds by producing tears to flush out the irritant.
Key Takeaways
- Onions evolved this chemical defense against animals eating them.
- Sweet onions cause less crying due to lower sulfur content.
- Scientist-bred 'tearless' onions exist but are not widely available.
Explanation
Onions evolved this chemical defense against animals eating them. Inside the onion cells, sulfur compounds are kept separate from the enzyme alliinase. When you cut the onion, these mix and create the irritating gas. The gas is very volatile and rises toward your face.
When the gas reaches your eyes, it dissolves in the water film covering your cornea and forms a weak sulfuric acid solution. Your nervous system detects this irritation and triggers reflex tears - the watery type designed to wash away irritants (different from emotional tears).
Ways to reduce crying: use a sharp knife (less cell damage, less gas), chill the onion (slows enzyme activity), cut under running water or a vent hood (disperses gas), wear goggles (blocks gas from reaching eyes), or cut the root end last (it has the highest concentration of enzymes).
The chemical chain reaction happens in milliseconds. When a cell wall breaks, the enzyme alliinase converts amino acid sulfoxides into sulfenic acid, which is then rearranged by a second enzyme called lachrymatory factor synthase into syn-propanethial-S-oxide—the actual tear-inducing compound. This volatile molecule has a boiling point of just 104°F, so it becomes airborne almost instantly at room temperature. Chilling an onion to 40°F in the refrigerator for 30 minutes before cutting significantly slows both enzymes, reducing gas production by up to 50%.
Not all onions are created equal when it comes to tear production. Yellow onions contain the most sulfur compounds and produce the strongest reaction. White onions are slightly milder. Sweet onions like Vidalias, Walla Wallas, and Mauis have lower concentrations of pyruvic acid (a proxy for pungency)—about 3-5 micromoles per gram compared to 8-10 in standard yellow onions. Red onions fall in the middle. Cooking eliminates the problem entirely because heat above 158°F denatures the alliinase enzyme, which is why cooked onions are sweet rather than pungent.
Things to Know
- Sweet onions cause less crying due to lower sulfur content.
- Scientist-bred 'tearless' onions exist but are not widely available.
- Some people are more sensitive than others to the irritating compounds.
- Contact lens wearers often report less tearing when cutting onions because the lenses act as a partial barrier protecting the cornea from the irritant gas.