Why Onions Make You Cry: The Chemistry of Cutting an Onion

Every cook knows the sting of a freshly cut onion, and the tears arrive faster than the chopping. The irritant is not a random acid. It is a small, volatile sulfur compound that the onion makes only after its cells are damaged. Following the reaction step by step explains both why the sting happens and why some kitchen habits help.

Onions store chemistry in separate compartments

An intact onion keeps its chemistry apart. Its cells hold sulfur-containing amino acid derivatives, mainly sulfoxides such as isoalliin, in one compartment, while an enzyme called alliinase sits in another. As long as the cell walls stay intact, the two never meet, and the onion stays mild.

Cutting ruptures the cells. The enzyme and its substrates mix, and a rapid chain of reactions begins. Crushing the onion breaks more cells than slicing does, which is why a dull knife or a food processor can produce a stronger effect.

The enzyme reaction

Alliinase converts the sulfoxide into an unstable sulfenic acid. That intermediate rearranges quickly into a compound called propanethial S-oxide, or in fuller form syn-propanethial-S-oxide, with the formula C3H6OS. Chemists call this molecule the lachrymatory factor, from the Latin word for tears. The enzyme also releases pyruvate and ammonia as byproducts.

The lachrymatory factor is small and volatile. Its molecules escape from the cut surface into the air and drift toward the eyes. The whole process takes seconds, which is why the sting appears almost immediately after the knife makes its first cut.

Why it reaches your eyes

The compound is detected by sensory nerve endings in the cornea. Those nerves respond to irritating chemicals and trigger the tear reflex, which sends fluid over the eye to dilute and wash away the irritant. The sensation is protective, even though it is unpleasant. The eye is doing exactly what it evolved to do.

A common claim holds that onions release sulfuric acid when cut, and that the acid causes the sting. That idea is a myth. The irritant is an organic sulfur compound, not a strong acid, and the tearing comes from the reflex that responds to its vapour.

Table: cutting methods and their effects

Several kitchen habits reduce the sting by slowing the release or the spread of the lachrymatory factor. The table lists common methods and the reasoning behind each.

Method	Mechanism	Effect on tears
Sharp knife	Breaks fewer cells	Less enzyme mixing
Chilled onion	Lowers vapour pressure	Slower release
Cutting near running water	Dilutes and carries vapour away	Less reaches the eyes
Good ventilation or fan	Moves vapour away from face	Lower local concentration

The methods work for different reasons, so combining them usually helps more than relying on one alone. None of them removes the lachrymatory factor completely, because the reaction still happens wherever cells break.

Why chilling helps

Chilling works through physical chemistry. Lower temperature reduces the vapour pressure of volatile compounds, so fewer molecules escape into the air per second. The enzyme also works more slowly in the cold, which slows the initial conversion. Chilling the onion in a refrigerator before cutting is a simple way to lower both rates together.

Why some onions sting more than others

Onion varieties differ in how much sulfur precursor they contain. Pungent storage onions tend to hold higher levels of sulfoxides, which means more lachrymatory factor can form when they are cut. Sweet onions are usually grown in conditions and varieties that produce lower sulfur levels, so they release less of the irritant. Growing conditions, soil sulfur and storage time also change the balance.

This variation explains why the same cutting method feels different with different onions. The chemistry is consistent, but the starting amount of precursor is not.

Heat changes the flavour

Cooking breaks down the lachrymatory factor and the other volatile sulfur compounds. Many of the products that form during heating are sweeter and less sharp. This is why onions sauteed slowly taste mellow, while raw onions taste biting. The sulfur chemistry does not disappear, but its character changes.

The same family of sulfur compounds also contributes to the characteristic flavour of garlic, leeks and other alliums. Learning the chemistry of one onion helps explain much of the flavour of the whole group.

Why the sting fades

The irritation usually fades once the vapour disperses. The lachrymatory factor is volatile, so it leaves the cut surface and mixes into the surrounding air. Once it drifts away from the eyes and the tear film has washed over the cornea, the nerve signal weakens. Fresh cuts keep producing vapour, which is why the stinging returns each time you slice into a new section of onion.

This pattern also explains why a second cook in the same room can be affected even without touching the onion. The vapour spreads through the air, and the eyes respond to any sufficient concentration nearby. Opening a window or switching on an extractor fan lowers the concentration in the room and reduces the effect for everyone present.

Chemists have also studied the chemistry of the reaction itself. The enzyme acts quickly, and the intermediate rearranges within seconds, so the vapour forms almost as soon as the cell walls break. The timing matches the experience of the sting arriving almost at once, while the tears build over the following moments.

Practical kitchen habits

Put these ideas together and the routine becomes straightforward. Chill the onion first and use the sharpest knife available. Work near running water or in front of a fan if the sting is still strong. Keep the cut surfaces small and avoid crushing when you can.

Key takeaways

Cutting an onion ruptures cells and lets an enzyme called alliinase convert sulfoxides into propanethial S-oxide, the lachrymatory factor. That small volatile molecule reaches the eyes and triggers the tear reflex. The effect is protective, and it is not caused by sulfuric acid.

Chilling, sharp knives, ventilation and water all reduce the release or the spread of the irritant. Sweet onions contain less sulfur precursor, and cooking converts the sharp compounds into milder flavours.

Understanding the sequence of reactions turns a frustrating kitchen moment into a clear example of enzyme chemistry at work. The next time you reach for a knife, you will know exactly which molecule is doing the stinging and which habit will reduce it.