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Question of the fortnight: Why do onions make you cry?

Blogging science to life

Tue 10 August 2010, Written by: Nicole

Why do onions make you cry?

Question via twitter @Madame Moll

Thank you Madame Moll for this interesting question – as these bullying vegetables have troubled most of us at some point! To get to the root of the matter we must first look at what type of plant an onion is, its chemical make up and why it causes your body to have such a reaction.

The vegetable in question! Onions are a member of the Allium plant family, which also includes garlic, chives and leeks. This family of plants absorb the sulphur which is contained in the soil and binds it with other molecules in the plant to create amino acid sulphoxides. These chemicals are fine and completely harmless when the onion is whole, but the problem arises once you cut into it, break its cells and release its contents.

The amino acid sulphoxides form sulphenic acids and enzymes contained in the cells react with the sulphenic acids to create propanethiol s-oxide. This substance wafts up towards your eye and reacts with the water on the surface of your eye creating the chemical sulphuric acid.

This acid burns your eye and so to dilute the acid, get rid of it and the horrible burning sensation - your tear glands produce tears. Tears are produced when an irritant, such as chemicals or dust, triggers nerve endings in the cornea to communicate with the brain stem. The brain then starts the process for tear production to clean the surface of your eye and keep it free of any pesky invaders! These types of tears are called reflex tears and are slightly different from the tears you shed when something makes you happy or sad.

Tips for preventing the upsetting substances from reaching your eye include chilling the onion before it goes under the knife as the cooler it is the less of the propanethiol s-oxide will evaporate up into the air and so make its way to your eye, or chopping the onion under water as the chemicals are water soluble and so are contained in the water and can’t float up to you eye! If you don't fancy either of those options there are some special onion goggles available to buy – Happy chopping :)

If you have any other interesting or funny solutions to prevent onion tears we would love to hear about them…

You can find out even more about your amazing body when All About Us opens!

Every fortnight, we are answering your science questions about the human body. It's all part of finding out how amazing you are, in the run up to the opening of our new exhibition All About Us in February 2011.