Everyday explained: Why does cutting onions bring tears to our eyes?

Cutting onions usually makes our eyes water, but does not really damage it. Here is why that happens.

“If you cut me, I won't cry, but I will make your eyes water. What am I?” Almost everyone knows the answer to this riddle: an onion. But why is it that cutting an onion makes our eyes well up with tears? Here is the explanation.

Why does cutting onions make us cry?

Onions are bulbs that mature underground. They are particularly good at absorbing sulfur, which they use to synthesise several amino acids.

When we cut an onion, we break open numerous cells whose contents come spilling out. It results in the previously separated enzymes meeting the sulfur-rich amino acids and forming unstable sulfenic acids, which then rearrange into a chemical known as syn-propanethial-S-oxide.

The final chemical is a volatile compound, which vaporises, floats up from the chopping board and reaches our eyes, creating a burning sensation. As per Healthline, propanethial S-oxide is a lachrymatory agent, meaning that it generates tears when it touches the eye.

The chemical turns into sulfuric acid when it touches the water layer that covers and protects our eyeballs. To protect the eyes from being damaged, the brain quickly triggers a tear response to rinse the irritant away. This is what causes the red and teary eyes while cutting an onion.

The irritant is part of the onion’s defence mechanism. If a person is allergic or more sensitive to onions, they can display symptoms such as hives, itching and tingling.

The irritants released by the onion while being cut are the vegetable's defence mechanism. (Pexel)

Home remedies for onion tears

While it may sting, the irritants from onions are not actually harmful to the eyes. And not all onions irritate the eyes at the same level. Sweet onions have less sulphur and thus result in fewer tears.

There are a number of home remedies that are commonly used to prevent the eyes from welling up while cutting onions. One of the most promising is to refrigerate the onion before cutting, which slows down the enzymes inside the vegetable.

Steps that one can take to reduce irritation and tears include:

• Putting distance or a barrier between oneself and the raw onion one is cutting.
• Rinsing the eyes with cool, clean water.
• Placing a cool compress or cool cucumbers over the eyes to help reduce irritation.
• Using lubricating eye drops to flush out the eyes.
• Using other sore eye remedies, such as cotton balls soaked with aloe vera gel and water or sterile castor oil eye drops.