Why Do Onions Make You Cry? The Chemistry Explained

Every cook has experienced it: the moment the knife breaks through an onion and the eyes start burning and watering within seconds. It is such a universal kitchen experience that it has become a cliché, but the chemistry behind it is genuinely interesting — and understanding it reveals why some of the popular remedies work and some are complete nonsense.

Onions are one of the most frequently used ingredients in cooking across almost every cuisine. Understanding what they do to your eyes — and how to minimize it — is practical kitchen knowledge worth having.

The Chemistry: What Actually Happens

An intact onion does not irritate your eyes. The reaction is triggered by physical damage — the knife cutting through the onion's cells. Here is the sequence of events:

Onion cells contain two types of molecules that are stored separately: an enzyme called alliinase, and sulfur-containing compounds called sulfoxides (specifically, isoalliin in the case of common onions). Under normal conditions, these molecules do not interact. The alliinase and the sulfoxides are kept in different compartments within the cell.

When you cut through the onion, the cells rupture. The separated molecules mix. The alliinase enzyme rapidly converts the sulfoxides into a family of unstable compounds, including the one most responsible for eye irritation: a volatile molecule called syn-propanethial-S-oxide, sometimes called the lachrymatory factor (from the Latin lacrima, meaning tear).

This molecule is a gas at room temperature and is extremely water-soluble. It rises from the cut surface, drifts toward your eyes, and dissolves into the moisture covering your cornea. Once dissolved, it undergoes a chemical reaction with that moisture to form a dilute sulfuric acid solution. Your eyes detect this acid and respond immediately with a reflex tearing response designed to dilute and flush the irritant away. That is the burning and watering you experience.

The whole sequence from knife cut to eye irritation takes only a few seconds, which is why even a quick, confident cut can trigger the reaction before you can move away from the cutting board.

Why Some Onions Are Worse Than Others

Not all onions produce the same level of irritation. The difference comes down to sulfur content, which varies by:

• Variety. Yellow and white onions are typically highest in the sulfur compounds that produce the lachrymatory factor. Sweet onions such as Vidalia or Walla Walla are lower in these compounds — part of why they are milder both in flavor and in eye irritation. Red onions fall somewhere in between. Shallots and spring onions are generally the mildest cutters.
• Soil sulfur levels. Onions grown in high-sulfur soils absorb more sulfur into their tissue. A high-quality yellow onion from certain growing regions will be noticeably more irritating than the same variety grown in lower-sulfur conditions. This is also connected to flavor intensity.
• Age and storage. Older onions that have been in storage for months have lower water content and more concentrated sulfur compounds. Freshly harvested onions tend to be milder.

This is also why onions become milder and sweeter when cooked. Heat denatures the alliinase enzyme and drives off the volatile sulfur compounds. Once the enzyme is deactivated, no new irritating gas can be produced, and the existing compounds break down into simpler, sweeter molecules — which is what caramelizing onions is actually exploiting.

What Actually Reduces Eye Irritation

Now that you understand the mechanism, you can evaluate which remedies are genuinely effective:

Chill the onion first (works well)

Refrigerating an onion for 30 minutes before cutting slows the enzymatic reaction. The alliinase enzyme works much less efficiently at low temperatures. You still get some gas production, but significantly less than with a room-temperature onion. This is one of the most reliable methods with no real downsides — the cold onion cuts and handles identically to a room-temperature one.

Use a sharp knife (works well)

A sharp knife cuts cleanly through onion cells. A dull knife tears and crushes them. The more cells you rupture per centimeter of cut, the more alliinase and sulfoxide molecules mix and produce gas. A sharp knife releases less gas for the same amount of onion. This is also the reason most professional cooks who cut large quantities of onions daily develop the habit of keeping their knives extremely sharp — it is not just about precision and safety. See how understanding knife technique connects to overall recipe reading ability.

Cut near running water or a fan (works partially)

The syn-propanethial-S-oxide gas is very water-soluble. Running water above and behind the cutting board intercepts rising gas before it reaches your eyes. A small fan blowing away from you toward the onion has the same effect — it redirects the gas away from your face. Neither method eliminates the gas entirely, but both meaningfully reduce exposure, especially combined with a chilled onion.

Leave the root intact until last (works well)

The root end of the onion has the highest enzyme concentration. Standard chopping technique keeps the root attached until the final step, using it as an anchor while you make your cuts. Cutting through the root early — before all your slices are made — releases a disproportionate burst of gas. Leave it intact as long as possible.

Wearing goggles (works completely, rarely practical)

Airtight swimming or kitchen goggles physically block the gas from reaching your eyes. They are the most effective single solution. In a professional kitchen handling hundreds of onions, this is sometimes used. For home cooking, the combination of a sharp knife and a chilled onion is usually sufficient.

The Myths That Don't Work

A few popular remedies have no meaningful scientific basis:

• Breathing through your mouth or holding a spoon in your mouth. No mechanism for this. The gas reacts with your eyes, not your mouth. Keeping your mouth open does not affect eye exposure.
• Lighting a candle near the cutting board. The flame does draw some gas upward via convection, but not enough to meaningfully protect your eyes at normal cutting distance. The water-running method is more effective and less of a fire risk.
• Soaking the onion in cold water before cutting. This can slightly reduce gas production at the very surface, but the bulk of the reaction happens from internal cell damage and is not significantly affected by surface moisture.

The Cooking Perspective

From a practical standpoint in the kitchen, the same sulfur compounds that make onions irritating to cut are what give them their sharp, pungent raw flavor and make them such a powerful aromatic base in cooked dishes. The mirepoix — the classic base of onion, carrot, and celery — gets its depth partly from the sulfur compounds transforming during cooking from harsh and sharp to sweet and savory.

Understanding this chemistry also helps with ingredient substitutions. If a recipe calls for raw onion and you want a milder result, chilling the onion beforehand reduces not just eye irritation but also the harshness of the raw flavor. For dishes where raw onion is prominent, letting sliced onion soak in cold water for 10 minutes removes much of the sharpness without affecting texture. For a full breakdown of how to make smart substitutions in recipes, the ingredient substitutions guide covers onion alternatives and when each works.