Why Do Onions Make You Cry?

An uncut onion doesn’t make you cry. You can hold one for an hour. Nothing happens.

The moment you cut it, your eyes are watering.

This is not coincidence. The onion is doing something deliberate. And the mechanism — once you see it — is a beautifully designed chemical weapon that only arms itself on impact.

The Compartment System

An onion’s cells are carefully organized around a chemistry problem: sulfenic acid precursors in the cytoplasm, and an enzyme called allinase in the cell vacuoles. These two things are stored separately, like the two chambers of a binary munition.

As long as the cell walls are intact, they never mix. The onion sits in your kitchen, harmless.

When you cut through it, you rupture thousands of cells simultaneously. The contents mix. Allinase contacts the sulfenic acid precursors and catalyzes a rapid conversion — producing a volatile sulfur compound called syn-propanethial-S-oxide.

This compound evaporates instantly. It diffuses upward as a gas toward your eyes.

What Happens at the Eye

Syn-propanethial-S-oxide — also called the lachrymatory factor — contacts the surface of your cornea and the conjunctival lining of your eye. The cornea is densely innervated by the trigeminal nerve, which has free nerve endings sensitive to chemical irritants.

Your eyes have TRPA1 receptors (the same channel activated by wasabi and tear gas) that respond to reactive chemicals. Syn-propanethial-S-oxide is reactive. The trigeminal nerve fires. The signal reaches your brainstem’s lacrimation center, which controls the lacrimal glands.

The lacrimal glands produce tears. Lots of them — reflex tears, which are chemically different from emotional tears. They’re designed to flush the eye surface.

The whole sequence — cutting onion to eyes watering — takes about 30 seconds.

Why the Onion Built This

Onions evolved this system as a defense mechanism.

A bulb is a stored energy reserve — months of carbohydrate built up for the plant’s next growing season. Animals, insects, bacteria, and fungi all want to eat it. The onion needs to deter them.

The challenge: if you produce a constant irritant, you’d be poisoning the soil around you and possibly yourself. So the onion evolved a triggered release — the irritant only forms when the cell is breached. The defense is only active when there’s an attacker.

This is one of the more elegant examples of plant chemical defense: the compound is synthesized on demand, from non-irritating precursors, at the moment of attack. The plant does not carry a loaded weapon — it carries the components to build one.

Many plants use versions of this strategy. Garlic produces allicin from alliin through allinase (same enzyme family) when crushed. Horseradish produces allyl isothiocyanate (the wasabi compound, also a TRPA1 activator) when the cells are ruptured. The biochemistry varies; the design principle is the same.

The Second Enzyme Discovery

For most of the 20th century, the lachrymatory factor was thought to be a simple byproduct of allinase acting on the sulfenic acid precursors. The chemistry was understood in broad strokes.

In 2002, a team of Japanese scientists led by Shinsuke Imai made a more precise discovery: a second, separate enzyme — lachrymatory factor synthase (LFS) — that specifically converts the intermediate sulfenic acid compound into syn-propanethial-S-oxide. Without LFS, the intermediate goes down a different pathway entirely, forming flavor compounds but no lachrymatory factor.

This was the key. The tear-producing compound wasn’t just a side effect of the allinase reaction — it had its own dedicated enzyme.

In 2008, scientists at the Crop & Food Research institute in New Zealand used RNA interference to silence the LFS gene in onions. The result: transgenic onions that produced normal sulfur flavor compounds but no lachrymatory factor. You could cut them without tearing up.

These tearless onions exist. They’re not widely grown yet — partly because the commercial seed supply takes decades to shift, partly because some people suspect the tearful process contributes to flavor (it doesn’t, significantly — the flavor compounds and the lachrymatory factor use different pathways).

But the technology is proven. The tear-factor can be surgically removed from the chemistry while leaving the taste intact.

The Workarounds, Explained

Understanding the mechanism explains every folk remedy:

Chill the onion first. Allinase and LFS are enzymes — enzymatic reactions are temperature-dependent. Cold slows the reaction, reducing the amount of lachrymatory factor produced per second. You still produce some, but the concentration in the air around you is lower.

Cut under water. Syn-propanethial-S-oxide is highly water-soluble. It dissolves into the water before becoming airborne. Your eyes see nothing. This works very well; it’s just annoying.

Use a sharp knife. A sharp knife ruptures fewer cells per cut — cleaner cuts mean less total cell damage and less enzyme release. A dull knife tears and crushes, maximizing the mixing.

Cut near ventilation. The gas disperses. Keep it moving away from your face. A kitchen fan over the cutting board helps more than most people expect.

Goggles. This works perfectly and nobody uses it because it looks ridiculous.

Bread in your mouth. This doesn’t work chemically. What happens is the bread absorbs some gas near your mouth and nose — reducing (slightly) the ambient concentration. The effect is small. Probably placebo for most people.

The Tears Are Real, The Emotion Is Not

It bears noting: reflex tears from onions are not the same tears as emotional tears.

Emotional tears contain higher concentrations of prolactin, ACTH, and enkephalin — hormones associated with stress response. They appear to serve a social signaling function and possibly a physiological release function.

Reflex tears (from onions, or from wind, or from bright lights) are primarily a flushing mechanism — largely water and electrolytes, designed to dilute and remove the irritant.

Same glands, different chemistry, different triggers. You’re not sad about the onion. You’re executing a corneal self-defense protocol.

An onion is a precision weapon that only arms itself when attacked. It carries its components in separate compartments, deploys a dedicated enzyme to synthesize the irritant on demand, and produces a volatile gas specifically sized to reach the eyes of the animal that disturbed it.

The fact that you can now engineer onions without this system is a minor miracle of molecular biology.

The fact that nobody has done it at scale yet is just capitalism.