🧅 Why do onions make you cry, and how to avoid it?

Cutting an onion almost always causes the same reaction: the eyes sting and tears appear quickly. This well-known phenomenon has a precise chemical origin.

Onions produce irritating molecules intended to defend against external aggressions. When the bulb is damaged, these substances are released and trigger the tearful reaction, as explained by the Library of Congress in a scientific analysis dedicated to this phenomenon.


When an onion is intact, the different chemical compounds remain separated in the cells of the bulb. But when the onion is cut with a knife, these cells are broken. Their content then mixes, which triggers a series of chemical reactions. Enzymes come into contact with sulfur molecules naturally present in the onion, a typical reaction of plants in the Allium genus.

One of these reactions produces a very volatile substance called syn-propanethial-S-oxide, the true tear-inducing compound responsible for tears. This gas forms when the sulfur molecules of the onion are transformed by several enzymes. Work on food chemistry shows that this molecule is specifically produced during the rupture of the bulb's cells, as described by an analysis published by Chemistry World.

Once formed, this compound diffuses rapidly into the air around the cut onion. Because it is very volatile, it easily reaches the eyes of the person preparing the vegetable. When it comes into contact with the moist surface of the eye, it causes irritation. This chemical interaction triggers a nerve signal that activates the tear reflex.

Faced with this irritation, the tear glands start producing more tears. Their role is to dilute and eliminate the irritating substance present on the surface of the eye. Tears therefore serve to rinse the cornea and evacuate the chemical compound responsible for the burning sensation. This protective reflex is part of the eye's normal defense mechanisms.

Not all onion varieties cause the same reaction. Sweeter onions generally contain fewer sulfur compounds. Their irritating effect is therefore weaker, because the amount of tear gas produced is less. The concentration of these molecules can also vary depending on the cultivated variety and growing conditions.

Some culinary tricks rely directly on these chemical mechanisms. For example, chilling the onion or using a very sharp knife can limit the production of irritating gas by reducing cell destruction. Similarly, cutting the onion under a stream of air or near a water source can disperse the gas before it reaches the eyes.