Why do onions make you cry? Eating them is fine, but for some reason, tears spring to the eyes within a few yards of a sliced-open onion.

The ancient Romans believed that onion juice improved vision. They even brought cut onions near the face to clean their eyes or rubbed their eyes with the onion juice to heal poor vision. Obviously, tears formed and a stinging sensation was felt in the eyes as a result.

While that sounds fun and all, is there any truth to the idea that cut onions can help the eyes?

It all comes down to the chemical reactions that occur in damaged onions. When an onion is cut, its cells are broken and the enzyme alliinase is released into the air. Alliinase converts compounds in the onion into sulfenic acid which immediately rearranges into sulfur gas with the help of another enzyme. This resulting volatile sulfur gas is the chemical that can reach the eyes and mix with the water in the tears that are always present on the surface of the eye. This sulfur gas, by the way, helps create the lovely smell of onions.

Eventually, small quantities of sulfuric acid are produced, which the eye definitely does not appreciate. The acid irritates the lachrymal glands, producing a stinging sensation. Interestingly, cooked onions do not cause this reaction because cooking onions destroys the enzymes that convert the onion chemicals into the acidic compounds.

Perhaps it would be better to cut onions like this…

But were the ancient Romans onto something? Possibly, but not in the way they were expecting. While crying protects the eye, it does not heal any pre-existing damage. The protection results from producing tears, which are the immediate reaction to an eye irritant, in case you didn't know.

Tears work by washing away or diluting the chemicals before they can harm the eye. The tears also have an enzyme called lysozyme which kills bacteria. Tears therefore protect the eyes from microorganisms and chemicals alike. Though the stinging is not pleasant and the blurry vision is inconvenient, the result is a protective buffer of tears around the eyeball. So, perhaps the Romans were right to an extent: it turns out that crying from cutting an onion is pretty beneficial, but not a cure for any and all eye damage.