Putting a wet paper towel next to an onion when cutting the onion does really work to help reduce tears.
Onions release the chemical irritant when they are cut and the moisture from the wet paper towel attracts the irritant and compound and draws it away from your eyes.
The reason why onions make you cry when you cut them is because of the chemical compound called lachrymatory factor LF that is released when the onion is cut.
Onions contain a chemical compound called LF or lachrymatory factor which is a sulfur containing compound which irritates your eyes and causes your eyes to produce tears.
When you cut into the onion it releases the lachrymatory factor LF sulfur containing compound.
Also when you cut into an onion, the enzymes that are within the onion cells are activated.
Then the enzymes react with the sulfur compounds in the onion, which produces the LF or lachrymatory factor.
The lachrymatory factor compound is volatile and easily evaporates into the air.
Once it evaporates into the air the lachrymatory factor compound then comes into contact with your eyes and it also irritates the nerve endings and causes a stinging sensation and tearing or crying.
To reduce the tears from onion cutting you should use a sharp knife to minimize damaging of the onions cells.
Chilling the onions before cutting them and even soaking the onions in cold water for a few minutes or even cutting the onions under running water can also help prevent the tears or crying when cutting onions.
You can also wear some goggles to reduce the contact of the compounds to the eyes.