Cutting onions can make your eyes water!

The saying describes an everyday phenomenon: when cutting onions, people often start to cry. This is due to the substances in the onion that are released when they are chopped.

Cutting onions can make your eyes water!

Biochemical processes

Onions contain various biochemical substances stored in separate cells:

- Alliin
A sulfur-containing amino acid found in the outer cells of the onion.
- Alliinase
An enzyme found inside onion cells.

When the onion is cut, the cell structures are destroyed, causing alliin and alliinase to come into contact with each other. This chemical reaction leads to the formation of a gas called propanethial-S-oxide.

How Propanthial-S-oxide works

The resulting gas is volatile and irritating. It rises from the bulb and enters the eyes, where it irritates the conjunctiva and cornea. This irritation activates the lacrimal glands, which produce tears to flush the irritating gas out of the eyes and protect them. The tears thus serve as the body's defense mechanism against the irritation.

Additional factors that may increase tear flow:

- Sharp knife
A sharp knife cuts the cells more cleanly, releasing more alliin and alliinase.
- Sensitive eyes
Some people are more sensitive to the irritants in the onion.
- Emotions
Stress or sadness can further increase tear production.

Tips to prevent tears when cutting onions:

- Use a sharp knife
A sharp knife cuts the onion more cleanly and reduces the release of irritants.
- Cool onions in the refrigerator
Cold slows down the enzymatic reactions and reduces the release of propanthial S-oxide.
- Wear glasses
Protective glasses can prevent the gas from entering the eyes.
- Cut under running water
The water helps to flush away the released gases before they get into the eyes.
- Light a candle
It is believed that a burning candle can neutralize the irritating gases, although this has not been scientifically proven.
- Breathe through your mouth
Breathing through your mouth can reduce the amount of gas that gets into your eyes.

Interesting facts

1. Different types of onions
Some types of onions contain more of the irritating substances than others. For example, red onions are often milder than yellow onions.
2. Genetic differences
Sensitivity to onion gases may also be genetic. Some people produce more tears than others.


The saying "Cutting onions can make your eyes water!" is a humorous description of a widespread phenomenon. The tears are the body's natural protective reaction to the irritating gases released when cutting onions. However, with a few simple tricks you can effectively reduce the flow of tears.


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