Why Do Onions Make You Cry? The Chemistry of Tears and How to Stop Them

It is one of the most common little dramas in the kitchen: the knife goes into an onion and within seconds your eyes start stinging, then streaming. This is no accident. The onion is activating a chemical defence system it evolved to protect itself.

Why do onions have a weapon like this?

Onions grow underground, where they are vulnerable to animals, insects and microbes that want to eat them. For a plant that cannot run away, the best defence is a chemical one: when something bites into an onion, the irritating compounds released deter it. When we cut an onion with a knife, the same alarm goes off.

The chemistry of tears, step by step

An intact onion contains no tear gas. The raw materials and the enzymes that turn them into gas are ready and waiting — just kept in separate compartments. When you cut it:

1. Cells are torn open. The knife ruptures cell walls and the separated ingredients mix.
2. The enzyme alliinase gets to work. It breaks down the onion’s sulphur-containing amino acid derivatives (sulphoxides) into unstable compounds called sulphenic acids.
3. The tear enzyme does its job. An enzyme discovered by Japanese researchers in 2002, lachrymatory factor synthase (LFS), rapidly converts that acid into syn-propanethial-S-oxide.
4. The gas escapes into the air. This compound is highly volatile and rises from the chopping board to your eyes within seconds.

Before this discovery, scientists thought the tear gas formed spontaneously as a by-product of alliinase. The study by Imai and colleagues, published in Nature, showed that it actually depends on a separate enzyme.

Why do your eyes water?

When the gas reaches the moist film covering the front of your eye, it stimulates sensory nerves in the surface of the eye. These nerves carry receptors called TRPA1, “chemical danger” sensors that also react to the pungent compounds in mustard and wasabi.

Your nervous system reads the signal as a threat and triggers a protective reflex: the tear glands release a flood of tears to dilute and wash the irritant away. Tears, in other words, are the eye cleaning itself.

Science-backed ways to chop onions without crying

Knowing how the gas forms also explains how to fight it:

Cutting an onion under water also reduces the gas in the air but is impractical. Briefly microwaving it can deactivate the enzymes, although this also changes the onion’s texture and flavour.

Why do onions turn sweet when cooked?

Raw onion’s sharpness and tear-jerking power come from the sulphur compounds formed at the moment of cutting. Heat both destroys the enzymes and drives off most of those compounds, leaving the onion’s natural sugars. Slowly cooked onions develop a sweet, deep flavour as those sugars caramelise and undergo Maillard reactions.

Is a tearless onion possible?

Yes. In Japan, onions with very low LFS activity were developed through conventional breeding. Elsewhere, “tearless” varieties that produce fewer sulphur compounds over time have also reached supermarkets. These onions tend to taste sweeter and milder, though some cooks say they miss the punch of a classic onion.

In short

You cry when cutting onions because of a clever chemical trap the onion uses to defend itself. Enzymes that mix at the moment of cutting produce a volatile gas, the gas triggers danger sensors in your eyes, and your eyes rinse themselves with tears. A cold onion, a sharp knife and good ventilation will disarm most of that trap.