Without a doubt you’ve already asked yourself this question…..if yes this is the answer:
The cells which constitute the onion contain sulfur and a substance, more specifically an enzyme called alliinase.
This enzyme is released when the onion is cut and forms a substance that made us cry. By coming into contact with the eyes, the liquid that bathes in the corneas and is responsible for protecting the eyes alert the brain to the presence of an acid compound.
Tears are then produced for rinsing the eyes and remove the foreign body.
Consider then placing onion under a plastic cover or bag which will stop the enzymes released into the air.