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Why Do We Cry When Chopping Onions?

Updated on March 8, 2020
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The writer is a creative content strategist who loves to write about global entertainment and issues concerning Africa.

Chopping Onions Can Make You Cry
Chopping Onions Can Make You Cry

Ever wondered why you cry like a baby whenever you chop onions? Read on to find out.

About 170 nations across the world engage in onion farming and about 9.2 million acres of the vegetable are harvested every year globally. Onions are believed to be low in calories and rich in vitamin C, dietary fiber, folic acid, calcium, antioxidants and iron. Additionally, they are an excellent addition to salads, egg, chicken, soup, guacamole, and beef recipes.

However, anyone who has ever entered a kitchen knows that cutting onions usually leads to some form of crying. It all starts with a stinging feeling, then the tears start to stream down the face.

I have always wondered why that happens. Therefore, I did some research and here’s what I discovered.

Onions are used in numerous recipes
Onions are used in numerous recipes

It’s All Thanks to Syn-Propanethial-S-oxide

Onions grow beneath the ground surface where they are exposed to plenty of critters that want to eat them. However, onions know how to guard themselves against the underground critters.

Growing onions pick up sulfur from the surrounding soil, leading to the formation of amino acids sulfoxides (compounds of sulfur). The sulfur compounds readily transform into gaseous form. When an onion is bitten or chopped, its enzymes and the sulfoxides are emitted, and create something called sulfenic acid. After that, the enzymes of the onion and the acid react to form syn-propanethial-S-oxide, which is a gas.

The syn-propanethial-S-oxide floats up into the surrounding space and deters critters or causes people to cry.

Onions that are white, yellow or red have a higher concentration of the enzymes needed to form the syn-propanethial-S-oxide gas. Green onions and scallions usually have fewer amounts of the enzymes.

The Impact of Onions on Human Eyes

Human eyes are sensitive by nature, and considering their role, it’s essential for them to have strong defenses against toxic fumes. The eyes have certain nerves that sense anything that can potentially harm the sense of sight.

Fortunately, syn-propanethial-S-oxide is a harmless gas that poses no danger to your eyes. You just tear up, and that’s all.

Chopping onions brings some temporary burning, irritation and tearing that is a nuisance, but completely harmless. It has no long-term effects, and does not worsen any existing conditions, such as pink eye.

Furthermore, some persons are more sensitive to onions than others. Not everyone tears up when cutting onions, while some people may be affected by the gas when on the opposite side of a room.

How Can You Cut Onions Without Crying?

If crying while chopping onions bothers you, there are some measures you can take to minimize the effect of syn-propanethial-S-oxide.

Cold onions do not release syn-propanethial-S-oxide as easily as warm ones. That means you can avoid the weeps by cooling your onions 10-15 minutes before you start chopping them. For more efficiency, just store your onions in the fridge from the start.

Additionally, you can prevent the onion gas from penetrating your eyes by facing a fan towards your cutting station to blow away the irritant gas.

You can also prevent an onion attack by using a freshly sharpened knife and chopping fast and cleanly. That is a trick used by professional chefs, and it helps to limit the amount of syn-propanethial-S-oxide released.

If you want to go for a more intense measure, wear a pair of goggles. They will help to protect your eyes from the irritant gas. But, wearing goggles might be an excessive measure, because you’re cooking and not performing a scientific experiment.


Onions are one of the most beloved vegetables, because they add a tantalizing taste and aroma to all sorts of recipes. However, they also bring some tears when you chop them, because of syn-propanethial-S-oxide. The gas is a nuisance, but it cannot cause any harm to the eyes or make existing conditions worse.


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