Why do onions make you cry?
- Fysiobasen
- Sep 18
- 3 min read
Crying when cutting onions is something most people have experienced, but few understand what actually happens in the body to trigger this reaction. Tears that stream down your face while chopping onions are a common—and often irritating—kitchen occurrence. But why does it happen, and can anything be done to prevent it? This article explores what happens when we cut onions and why it makes us cry.
What happens when cutting onions?
When you cut an onion, a chemical reaction occurs that releases certain compounds. The main culprit is a chemical known as syn-Propanethial-S-oxide. This compound forms when the onion is cut or crushed, and it is responsible for the irritating tears.
1. Chemical reaction in the onion
When cutting onions, the cell walls are broken, releasing the enzyme alliinase. This enzyme reacts with amino acids in the onion, leading to the formation of syn-Propanethial-S-oxide. This sulfur-based compound is harmless to the body but irritates the eyes upon contact.
2. How it affects the eyes
Once released, syn-Propanethial-S-oxide quickly evaporates and rises toward the eyes. The eyes have a natural defense mechanism against irritants: tear production. Tears help wash away the chemical irritants, protecting the eyes from harm.
3. Tear production as a protective response
When exposed to syn-Propanethial-S-oxide, the eyes react by producing more tears to dilute and flush out the irritant. This is why you experience watery eyes or even full-on crying when chopping onions.
How to avoid crying when cutting onions
While it is difficult to avoid onion-induced tears completely, several methods can help reduce or prevent them.
1. Cut the onion under water or rinse it first
Cutting onions under running water or rinsing them before chopping can bind syn-Propanethial-S-oxide, preventing much of it from reaching the air. This reduces eye irritation.
2. Use a sharp knife
A sharp knife slices cleanly through the onion, causing less cell damage. This results in fewer enzymes and chemicals being released into the air. Using a sharp knife can therefore reduce the amount of irritants reaching your eyes.
3. Wear protective glasses or goggles
Although it may look unusual, wearing protective glasses or swimming goggles can block syn-Propanethial-S-oxide from reaching your eyes. This is an effective option for people especially sensitive to onions.
4. Chill the onion
Cooling the onion before cutting slows down the chemical reaction that causes tears. Placing the onion in the freezer for 10–15 minutes prior to chopping can reduce irritation.
5. Improve ventilation
Good ventilation helps minimize irritant buildup in the air. Cutting onions near an open window or with a fan running can help disperse the compounds before they reach your eyes.
6. Peel the onion carefully
Some suggest peeling onions under running water or peeling them gently by hand rather than aggressively cutting, to limit the release of irritating chemicals.
Is crying from onions harmful?
Crying from onions is completely harmless. The gas released is not dangerous to the eyes, and the tears are simply a natural protective mechanism. After a short while, the tearing subsides without long-term effects. Still, reducing exposure by using the methods above can make onion cutting more comfortable.
Summary
Onion-induced tearing occurs due to a chemical reaction that produces syn-Propanethial-S-oxide, a sulfur compound that irritates the eyes. In response, the eyes produce tears to flush out the irritant. While harmless, the experience can be uncomfortable. Fortunately, methods such as using a sharp knife, chilling the onion, cutting under water, or wearing protective glasses can help reduce or prevent tears when chopping onions.
References
Mizuno Y, et al. Mechanism of lachrymatory factor production in onions and its implications for tear generation. Biochim Biophys Acta. 2005;1723(1):76–81.
Kim C, et al. Effect of different cutting techniques on the formation of tear-inducing compounds in onions. Food Chem. 2012;135(2):573–578.
Sato A, et al. Allium vegetables and their volatile sulfur compounds: a review of recent developments in understanding the biological activities. J Agric Food Chem. 2009;57(13):5744–5756.
Lamy E, et al. Onion lachrymatory factor and its effect on tear formation: biological and chemical aspects. J Agric Food Chem. 2008;56(24):11947–11952.
Bruch A, et al. The lachrymatory factor in onion (Allium cepa) and its influence on human eye irritation. J Agric Food Chem. 2006;54(7):2474–2480.
