PH
Cutting an onion damages its cells and allows different compounds to mix together.
This produces a volatile substance called a lachrymatory factor, which reaches the eyes and triggers the nerves responsible for tearing. The tears are essentially the body's response to the irritating chemical.
Chilling or cutting onions in certain ways can sometimes reduce how much of the irritating compound reaches the eyes.
The tears aren't caused by the onion's smell alone—the reaction comes from chemistry released when its cells are damaged.
© American Chemical Society