Why do onions make you cry?
The answer lies within the mixing and alteration of chemicals within the onion and water on the surface of your eye.
Onion cells have two, one contains the enzymes called alliinases and the other sulphur containing amino acids. These two chemicals mix on slicing as the compartment walls are broken. On mixing a sulphenic acid called 1-propenesulphenic acid is formed. Another enzyme (lachrymatory factor synthase) rearranges the sulfenic acid to form propanethial s-oxide, which is a gas. Thus by diffusion the molecule comes into contact with the surface of your eye - which has a film over it containing a cocktail of compounds one of which is water. This causes three compounds to be formed: hydrogen sulphide, propanol and sulphuric acid. The sulphuric acid is of a high enough concentration too cause irritation meaning the eye rapidly produces tears to try and dilute the acid further.
Methods of prevention include too cover your eyes as this acts as a physical barrier preventing the gas from coming into contact with the eye or by slightly opening the mouth meaning you breath in through you mouth thus the gas is sucked into your throat rather than coming into contact with your eyes.
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