Why Do Onions Make Us Cry?

by Mozhy Hosseini-Ashrafi

The onion is a household staple vegetable used in a large number of savoury dishes to enhance flavour. However, despite the fact it is easily available, cheap and very flexible in the way it can be used, onions are renowned for making people cry when they are cut open. Many have asked the age old question, why? And can we stop them from making us cry?

Many old wives tales circulate about the best way to cut onions, ranging from wearing goggles to keeping a spoon in your mouth! Whilst these methods have varying success rates, it is difficult to come up with the perfect solution when we don’t know the science behind it.

We can assume that onions use their tear-jerking properties as a defence mechanism against predators that might, for example, cut them up into tiny pieces.

Onions contain amino acid sulfoxides, which form sulfenic acids when sliced. These enzymes are free to mix with the sulfenic acids to produce ​propanethial S-oxide, a volatile sulfur compound gas which wafts upward and into your eyes. This gas reacts with tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritation away, and before we know it, we are reduced to a weeping mess!

Although it’s not quite rocket science, the best way to prevent tears when cutting onions, is to open a window! Or if there are no windows, a fan will do. This will allow the gases from the onion to get blown away and prevent it from going in your eyes. Another way you can prevent crying when cutting onions, (wait for it:) is wearing safety goggles! This is another efficient way to cook with onions, incase you were struggling.

I hope this has been a useful and educational lecture on onions.