Why Do Onions Make You Cry? The Chemistry Behind the Tears

Onions make you cry because cutting one sets off a chemical reaction that produces a gas, and the gas irritates your eyes. Your eyes respond the way they respond to any irritant: they flood with tears to wash it away. The tears are not a reaction to the onion's flavor. They are a defence against something the onion released when its cells broke.

The chemistry is less mysterious than it sounds, and it explains a few things that most cooking advice gets half right. Once you know what the gas is and how it forms, the fixes make sense: a sharp knife, a chilled onion, or a few seconds of distance from the cutting board.

The Short Answer: Cutting Starts a Chain Reaction

An onion stores its sulfur chemistry in separate compartments, and cutting breaks down the walls between them. Wikipedia's article on onions describes the sequence: chopping damages cells and releases enzymes called alliinases. Those enzymes break down amino acid sulfoxides in the onion and generate sulfenic acids. One specific sulfenic acid, 1-propenesulfenic acid, is then acted on by a second enzyme, lachrymatory factor synthase, which produces the gas that causes the sting.

The gas is syn-propanethial-S-oxide. According to Wikipedia, it is released as a volatile liquid and an aerosol, and it stimulates the nerves in the eye. That is the stinging and the uncontrollable tears you feel a moment after the knife goes through.

Why the Onion Makes the Gas at All

The gas is a defence. The onion produces it when something damages its cells, which is exactly what a chewing animal or a pest does. Wikipedia describes the chain as a defence mechanism. The sulfur compounds sit in the onion's cells, waiting, and the enzymes that break them down are kept apart from them. Only when the cells are crushed do the two meet and make the irritant.

That design makes sense for a plant. The chemistry works as a deterrent, the same way a bitter leaf or a strong smell puts off a grazer. It works on people too, which is why an onion you cut for dinner feels like an attack on your eyes. The onion is not aiming at you. It is doing what it evolved to do when its cells are broken.

The same family includes garlic, leeks, chives, shallots and scallions. Wikipedia lists the genus Allium as having more than a thousand accepted species, and most of them share some version of this sulfur chemistry. Garlic's sharp smell and a leek's sting come from the same basic system, with different compounds and amounts.

Why It Hits Your Eyes and Not Your Nose

The gas is light enough to travel up to your face, and your eyes are the part of your body that reacts to it fastest. The onion makes its irritant in the air, and the gas drifts up from the board. Your nose notices the smell, but the eye nerves are where the sting registers. Your body treats the sting as a threat and sends out tears to flush it. The tears are a protective response, not a sign that the onion is doing anything dangerous to you.

That is also why an onion cut under a running tap or in a well-ventilated kitchen stings less. The gas has less time to reach your eyes before it dissipates.

Why Some Onions Are Worse Than Others

Onion strength depends on how much sulfur the plant takes up while it grows. Onions and other members of the Allium family build these sulfur compounds from the soil. More sulfur in the soil usually means more of the irritating chemistry in the bulb. That is why the same red onion can sting more in one season than in another, and why sweet onions, grown in different conditions, often feel milder.

The chemistry is also a target for plant scientists. Wikipedia notes that in 2008, the New Zealand Institute for Crop and Food Research created "no tears" onions by genetic modification to block the lachrymatory factor synthase enzyme. One study Wikipedia cites suggests consumers prefer the flavor of onions with lower levels of that enzyme, which is one reason the trait has not gone mainstream.

Why the Tears Are Not the Whole Story

The tears you feel from an onion are a reflex. Their job is to wash the irritant away, not to express feeling, and they are a different kind of tear from the ones you cry for emotional reasons. Onion tears are mostly a flush of water and salt that your eyes produce to clear the irritant. That is also why the sting fades in a minute or so once you step away from the cutting board.

What the Tears Actually Do

Once the gas reaches your eyes, the nerves send a warning and the tear glands respond. The flood of liquid does two jobs: it dilutes the irritant and it carries it off the surface of the eye. The effect is strongest in the first few seconds of cutting and fades quickly after you step back from the board.

This is also why rubbing your eyes makes the sting worse. Your fingers may have traces of the gas on them from the onion, and rubbing moves it across a sensitive surface. Wash your hands with soap and water before you touch your face.

How to Cut an Onion With Fewer Tears

The best tricks reduce how much gas is released, or how much of it reaches your eyes. These are the methods that line up with the chemistry above:

• Use a sharp knife. A dull blade crushes more cells, and crushed cells release more enzyme. A clean cut damages fewer cells, so less gas forms.
• Chill the onion first. Many cooks swear by putting the onion in the fridge or freezer for 10 to 20 minutes before cutting. Cold slows the enzyme reactions, which means less gas is produced in the first few seconds.
• Cut near air movement. A range hood, an open window or a fan pulls the gas away before it reaches your face.
• Keep the root on. Cut the root end last, so the bulb stays intact for as long as possible and you disturb fewer cells at the start.
• Wet the blade or your hands. Some people find that running the knife under water helps. The water can dissolve some of the gas before it reaches your eyes.

There is no trick that stops every tear, and some onions simply sting more. Pick the method that fits your kitchen and stop worrying about the rest.

Frequently Asked Questions

Do onions make you cry more if they are fresh?
Freshly cut onions sting most, because the enzymes have just been released and the gas is forming as you work. A whole, uncut onion does not sting at all, which is why the trouble starts only when the knife goes in.

Does the gas hurt your eyes permanently?
No. The sting is a short irritation, and it fades once you move away from the source and your tears flush it out. If you get onion juice in your eyes directly, rinse them with clean water.

Why do onions make me cry more than garlic or shallots?
Onions have a high level of the sulfur compounds that make the gas. Garlic and shallots have related chemistry, but the mix and amounts differ, so the effect varies from one allium to another.

Can you stop the tears by wearing goggles?
Some cooks use swim goggles or a fan for this, and both can work because they keep the gas off the eyes. A simple setup with air movement is usually enough.

The Bottom Line

Onion tears are a chemical alarm. Cutting breaks cell walls, enzymes make a gas, and the gas irritates your eyes. Knowing the chain lets you make smart choices: a sharp knife, a chilled bulb and a little airflow cut the sting a lot. And the next time the tears start, you know exactly what your eyes are reacting to.

Sources

• Wikipedia: Onion (the enzyme chain, syn-propanethial-S-oxide, eye irritation, "no tears" onions, storage)
• Wikipedia: Allium (the genus that onions, garlic and shallots belong to)