As onions are sliced, cells are broken, allowing enzymes called alliinases to break down sulfides and generate sulfenic acids (amino acid sulfoxides). Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.

Supplying ample water to the reaction prevents the gas from reaching the eyes. Eye irritation can, therefore, be avoided by cutting onions under running water or submerged in a basin of water. Rinsing the onion and leaving it wet while chopping may also be effective. Another way to avoid irritation is by not cutting off the root of the onion. Chilling or freezing onions prevents the enzymes from activating, limiting the amount of gas generated. Using a sharp blade to chop onions will limit the cell damage and the release of enzymes that drive the irritation response.

The volume of sulphenic acids released, and the irritation effect, differs among Allium species.

A firm in Toronto, Canada, attempted to utilize this property of onions in the manufacture of a form of tear gas for civilian use. It was marketed in 1991 but was unsuccessful as it had an effective shelf life of only three months.[citation needed]

The characteristic odour of onions can be removed with lemon.

Onions are from the same family as garlic, leeks and shallots and provide a range of nutritional benefits. Onions are low in calories and contain vitamin B, vitamin C and small amounts of minerals.

In addition, onions contain the antioxidant quercetin, flavonoids and several other micronutrients which are proven to reduce blood clotting and raise healthy cholesterol (high-density-lipoproteins).