Why Do Your Eyes Water When You Slice Onions?
Onions contain strong-smelling oil. When the onion is peeled and cut, this oil escapes into the air in the form of vapor (gas). The onion chemicals in the vapor irritate the sensitive lining of your eyelids and nose. That’s when the waterworks start. The eyes automatically blink and produce a flow of tears that washes away the irritation.
It turns out when you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide. When mixed with certain enzymes in the onion, it creates a sulfur gas. These gases then get to your eyes and create a mild acid which irritates the eyes.
Normally, your body would signal you to close your eyes. This, of course, is not a good idea if you are cutting an onion. The next best thing your eyes can do is to make tears to protect the eyes. Rubbing your eyes is a bad idea, since your hands are likely full of the tear making onion juice.
But the onion chemicals that bring tears to your eyes dissolve in water. By holding the smelly vegetable underwater when you peel and cut it, you can help protect your eyes from the stinging vapor.