I read recently some tips on “How to peel onions without crying.” The list of suggestions included the claim that if you start at the root of the onion and peel up, you’ll never shed a tear. It included some downright humorous suggestions, like the claim that peeling them with a bag over your head, or wearing scuba diving gear, or peeling them with a chunk of raw potato in your mouth, prevents tears.
Chemists have identified a number of reactive and volatile sulphur compounds that cause tears when slicing an onion. Such substances are called lacrimators.
If you want to be more scientific about avoiding tears when you slice onions then there are some basic scientific principles you can apply. All of them depend on keeping those volatile components from getting into the air, and then into your eyes.
If you put the onions in a freezer just before cutting them, it decreases the vapour pressure of the volatile components, and hence fewer of the tear causing molecules get into the air. Alternatively, cutting onions under water allows the volatile components to dissolve in the water, so again they are removed and are unable to reach your eyes.
Since the tear-causing-molecules float through the air, you can always decrease their concentration near your work area by using a fan to blow most of the offending vapours away.
So next time you peel onions apply some simple science, and avoid those tears.