Why do Onions Make you Cry?

August 15, 2015 / Food, Humans / 2 Comments /

Onions, particularly white onions, can bring a grown man to tears. They are delicious additions to meals, but such a pain (literally) to prepare. As soon as you start cutting an onion up, your eyes will start to sting, and by the time you have gotten to the end of it, your eyes will be stinging like crazy and you will probably have tears running down your face. So, here I’ll look at why onions make you cry, why red onions don’t seem to have the same effect, and what you can do to avoid onions making you cry.


Why onions make you cry

When you slice an onion you break apart the cell walls which releases all the chemicals in the onion. These chemicals will mix, and the volatile ones, will disperse into the air. Some of the chemicals released are amino acid sulfoxides and they will mix with enzymes which can convert them into a compound called propanethiol S-oxidewhich is a known lachrymatory factor, (means a ‘tear inducing’ factor). This lachrymatory factor is particularly volatile, and quickly disperses into the surroundings, and into your eye. This chemical causes tears be produced, but doesn’t cause the pain.

Propanethiol S-oxide is a particularly unstable molecule, and when it makes contact with the water in your eye it convert into sulfuric acid, which is an irritant, and causes the itching. Being an irritant, the eye will produce more tears to try and dilute it. With your eyes being teary, there will be plenty of water for the propanethiol S-oxide to react with, so the longer you are cutting the onions, the worse it will get.

So the reason that onions make you cry when you cut them, is down to a chemical called propanethiol S-oxide, which is stimulates tear production, and irritates your eyes.


Can anything be done to stop it?

There are a number of solutions for stopping onions from making you cry:

1. Use a sharp knife (not the most effective)

The sharper the knife is, the cleaner the cut, and so the fewer onion cells destroyed. This means less chemicals are released into the air, less propanethiol S-oxide is made, and so your eyes will be less irritated. Obviously, some onion cells will still be broken, and so some chemicals will be released, so this will only reduce the irritation, not stop it.

2. Cut the onion under water (effective, but awkward)

Cutting the onion under water is a bit awkward, but the water does prevent any of the volatile chemicals being released into the air. Instead, when the onion is cut open the chemicals will dissolve in the water, meaning no eye watering chemicals can reach your eyes.

3. Eye protection (effective, but you will look silly)Cutting onions with goggles on stops onions making you cry, but you can look silly!

Wearing sealed eye protection like goggles will prevent the chemicals getting to your eyes. Its not as awkward as cutting under water, and more effective than just using a sharp knife, but you might look a bit silly – so expect to be ridiculed by anyone around.

From personal experience, I’ve found that wearing contact lenses offers quite a lot of protection (and you don’t look so silly). Eventually you do feel a slight stinging, but nothing compared to without contacts in. Obviously, you need contacts to try this though.


Why don’t red onions make you cry?

Red onions don’t make you cry because they don’t contain as much of the tear inducing chemicals. I haven’t been able to find a comparison of the chemical composition of different onions, but it stands to reason that red onions don’t make you cry is because they contain less of the thing that makes you cry. It is plausible (but less likely) that red onions contain additional chemicals which mop up and neutralise the propanethiol S-oxides in the air, but my bet is that they contain less of the chemicals to create propanethiol S-oxide.



Onions make you cry because when you cut them open, you destroy their cell wall, which causes sulfoxides to mix with enzymes, which converts them into a chemical called propanethiol S-oxide. This chemical induces tears in the eye, and also gets converted into sulfuric acid when it comes into contact with the water in your eye. This acid is an irritant, and so causes stinging, and even more tears. You can reduce the by using a very sharp knife, which means less chemicals are released. Alternatively, you can cut the onion under water, which means the chemicals cannot get into the air, or possibly wear some swimming goggles, to stop the chemicals reaching your eye.

Red onions don’t make you cry because they simply contain less of the tear inducing chemicals.

Images courtesy of Susy Morris and Tizah

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When you slice onions open, you break onion cells, which releases the chemicals it contains. One of these chemicals is called  propanethiol S-oxide, which stimulates tears, and another group are called sulfoxides, which sting the eyes and cause them to water even more.