Cutting onions releases a gas called syn-Propanethial-S-oxide, which stimulates the eyes’ tear glands and causes them to produce tears. This gas is released when the onion is cut or sliced, and it reacts with the moisture in the air to form sulfuric acid. When this acid reaches the eyes, it causes a stinging sensation, and the tear glands produce tears to flush the irritant out of the eyes.
The tear glands are located in the upper eyelid and produce tears that flow down and across the eye. When we cut an onion, the tear glands are stimulated, and the tears produced help to wash away the irritant gas and reduce the discomfort caused by the sulfuric acid.
To minimize the amount of gas released, it is recommended to chill the onion in the refrigerator for a while before cutting it, or to cut it under running water or while wearing goggles. Some people also find that chewing gum while cutting onions can help reduce tearing.