Why Do Onions Make You Cry?

Cutting an onion ruptures its cells and mixes chemicals that were previously stored apart. Enzymes transform sulfur-containing precursors into a volatile irritant called syn-propanethial-S-oxide. When the vapor reaches the eyes, sensory nerves react and the tear glands produce fluid to dilute and remove it.

Research published in Nature identified a dedicated onion enzyme, lachrymatory-factor synthase, involved in creating the tear-producing compound, and later structural research examined that enzyme in greater detail. ^{[1] [3]}

What happens after the first cut

Inside an intact onion, enzymes and their reactants are separated by cell structures. Cutting, crushing, or grating the onion breaks those compartments and starts a rapid series of reactions:

1. Cell damage brings the enzyme alliinase into contact with sulfur-containing precursors.
2. Unstable sulfenic-acid intermediates form.
3. Lachrymatory-factor synthase converts an intermediate into syn-propanethial-S-oxide.
4. The volatile compound moves into the surrounding air.
5. Some of it reaches the eye surface and stimulates tearing.

The American Chemical Society identifies syn-propanethial-S-oxide as the principal tear-producing compound and explains that it stimulates the lacrimal system when it reaches the eyes. ^[2]

Why the onion produces these chemicals

The sulfur chemistry contributes to onion flavor and forms part of the plant’s response to tissue damage. The same broad chemical family helps create the sharp aromas and flavors associated with onions, garlic, and related plants.

The onion is not launching a calculated attack on dinner. Evolution has simply created a chemical response to damaged tissue, and your eyes happen to be collateral damage.

Why some onions seem worse

The intensity can vary with onion variety, growing conditions, sulfur availability in the soil, age and storage, how much tissue is crushed, distance from your eyes, ventilation, and individual sensitivity.

Grating and using a dull knife can damage more cells than clean slicing, potentially releasing the compounds more rapidly. A large quantity of freshly cut onion in a still room can also produce a stronger exposure than a few slices prepared near effective ventilation.

What actually helps?

Use ventilation

Work near a functioning range hood or use a fan that moves vapor away from your face. The goal is not to neutralize the compound after it reaches the eye, but to reduce how much reaches the eye at all.

A range hood that recirculates air through a filter may be less effective at removing vapor than one that exhausts outdoors, though any airflow directed away from the face can help disperse the plume.

Use a sharp knife

A sharp knife usually crushes less tissue than a dull one. This does not eliminate the reaction, but it may reduce widespread cell damage and makes the cutting task faster and more controlled.

Use ordinary knife-safety practices. A sharper knife can require less force, but it is still a sharp object rather than an onion-science accessory.

Keep your face farther away

Use a stable cutting board and avoid leaning directly over the onion. Increasing the distance and moving air away from the face reduces the concentration likely to reach the eyes.

Wear close-fitting goggles

A physical barrier is the most direct approach. Ordinary eyeglasses leave large gaps, while close-fitting kitchen or safety goggles reduce vapor reaching the eye surface.

What about chilling the onion?

Cooling can slow chemical reactions and reduce volatility, so brief refrigeration may help some cooks. It is not a guaranteed tear-free method, and long refrigeration can affect texture or aroma. Treat it as an optional exposure-reduction tactic, not a biochemical force field.

Popular methods with weak explanations

Holding bread in your mouth, placing a spoon between your teeth, or leaving a bowl of water nearby does not reliably intercept vapor moving toward your eyes.

A wet paper towel near the board might absorb a small amount of airborne material, but it does not control airflow or block the eyes. The mechanism is less direct than ventilation or goggles.

Folk methods can appear to work because onion potency, airflow, cutting time, and individual sensitivity vary from one attempt to another. A successful dinner is not the same thing as a controlled experiment, an inconvenient distinction that has also harmed much of the internet.

Why your eyes water

The compound irritates sensory nerves on the eye’s surface. The lacrimal glands respond with tears, which dilute and help wash away the irritant.

Do not rub your eyes with hands that have handled onions. Wash your hands first. Contact-lens wearers should avoid touching their lenses until their hands are thoroughly cleaned.

Persistent pain, vision changes, or symptoms after a foreign object or chemical exposure are not ordinary onion irritation and warrant appropriate medical attention.

Bottom line

Onion tears result from an enzyme-driven chemical reaction that begins when cutting ruptures the onion’s cells. Reduce exposure with ventilation, a sharp knife, greater distance, or close-fitting goggles. The onion is not angry with you, although after observing what humans do to recipes online, it would have grounds.

Limits and cautions

• Onion variety, storage, growing conditions, cutting method, ventilation, and individual sensitivity affect tear intensity.
• A sharp knife or chilled onion may reduce exposure but cannot guarantee a tear-free result.
• This article explains ordinary irritation and does not diagnose persistent eye pain or injury.

Sources reviewed

Source notes

1. An onion enzyme that makes the eyes waterNature
2. syn-Propanethial S-oxideAmerican Chemical Society
3. Enzyme That Makes You Cry: Crystal Structure of Onion Lachrymatory-Factor SynthaseAmerican Chemical Society

See something wrong or out of date? Report a correction.