Why Do Onions Make You Cry?

I can say with confidence that anyone who has been in a kitchen chopping an onion has probably found themselves fighting back tears. That feeling of your nose tingling and your eyes tearing up sometimes feels like a personal attack from the onion, but the real explanation lies in the series of chemical reactions that occur when you slice into it.

The cells in an onion contain a variety of sulfur-containing compounds, along with special enzymes. These two are usually kept separate, but when you cut or crush the onion, the cell walls break, allowing the substances to mix. Now, as you can imagine, the mixing of these two substances causes a chain of chemical reactions to occur. These reactions produce a volatile gas known as syn-propanethial-S-oxide (quite a mouthful).

This gas escapes into the air and eventually reaches your eyes, which are covered by a thin layer of moisture. The gas then dissolves in this liquid and forms a mild irritant. The eye is one of our 5 sense organs and, as expected, is sensitive, with many nerve endings present. The nerve endings detect the irritation and send signals to the brain. As a response, the brain activates the tear glands. This is the body’s natural defence system trying to protect your eyes. 

You may be wondering why onions produce this irritating chemical in the first place. The believed explanation is that it evolved as a defence mechanism in the onion, just like we have our defence mechanisms. When an animal or insect damages the plant, the released chemicals make the experience unpleasant, which discourages further attacks. However, not all onions have the same effect. The sweeter varieties often have lower levels of sulfur compounds, making them less likely to cause intense tearing. Other factors such as growing conditions, soil and climate can influence how much of these sulfur compounds are present.

If you want to avoid the tears, there are a few tricks you can try. 

1. Chill the onion before cutting it, as this slows the chemical reactions that produce the gas
2. Using a sharp knife to minimise cell damage, which reduces the mixing of the two substances and therefore reduces the amount of irritant released
3. Good ventilation to carry the gas away before it reaches your eyes
4. Wearing goggles, a face mask and gloves.

So what seems like a simple kitchen annoyance is actually an example of chemistry in everyday life. Every time you chop an onion, you are witnessing (and experiencing) chemical reactions working together in a natural defence system.

Chloe Tohme