Onions make you cry because of a gas.

🤯 Verified Fact · Food

Wacky WorldFood

Short answer: Cutting an onion breaks its cells and starts an enzyme-driven reaction that releases a volatile sulfur compound called syn-propanethial S-oxide. When the vapour reaches the eyes, sensory nerves trigger tears to dilute and remove the irritant.

What happens when the knife cuts?

An intact onion stores enzymes and sulfur-containing precursor molecules in separate cellular compartments. Slicing ruptures those compartments. The enzyme alliinase acts on amino-acid sulfoxides and produces sulfenic acids; another enzyme, lachrymatory-factor synthase, rearranges one of those products into the tear-causing compound.

Is it really a gas?

People commonly call it a gas, but “volatile vapour” is more precise. Syn-propanethial S-oxide readily enters the air above a cut onion. It then dissolves in the moisture covering the eye and stimulates sensory nerves. The lacrimal glands respond by releasing tears.

Why would an onion make this chemical?

The pathway is part of the plant’s chemical defence. Damaged tissue releases pungent sulfur compounds that can discourage animals and interfere with microbes. Humans experience the same defence system as aroma, sharp flavour and eye irritation.

How can tears be reduced?

A sharp knife crushes fewer cells than a blunt one. Chilling the onion slows the chemical reactions and evaporation. Good ventilation carries vapour away from the face, while an extractor fan can pull it away from the cutting board. Protective goggles work because they form a physical barrier.

Cutting under running water is sometimes suggested, but it can make the onion and knife harder to control. Safe, stable technique matters more than a dramatic kitchen trick. Moving the face slightly farther from the board also reduces the concentration reaching the eyes.

What does not cause the tears?

The culprit is not ordinary sulfur dioxide or an “onion acid” splashing upward. Tears are also not evidence of an allergy in most people. They are a normal protective reflex caused by a specific, short-lived sulfur compound reaching the eye surface.

The chemistry in five steps

The knife breaks onion cells.
Previously separated enzymes and precursors mix.
Alliinase helps form sulfenic-acid intermediates.
Lachrymatory-factor synthase produces syn-propanethial S-oxide.
The vapour reaches the eyes and tears flush the irritant.

Evidence and limits

Onion varieties differ in precursor levels, freshness and growing conditions, so they do not all cause equal irritation. Personal sensitivity and cutting method also matter. The American Chemical Society identifies syn-propanethial S-oxide as the principal lachrymatory factor, while structural research explains the enzyme responsible for producing it.