Why Onions Make You Cry

Cutting this vegetable releases a gas that triggers tears, but the process is more complex.

Cutting an onion makes you cry because it releases a gas that reaches your eyes and makes them produce tears to wash it away.

When you slice the layers of an onion, you damage its cells. These damaged cells release a chemical called syn-propanethial-S-oxide into the air around you. This specific gas is what causes the stinging sensation in your eyes when you chop the vegetable.

Once this gas enters your eye, it dissolves in the tear fluid that is already there. The chemical reaction changes the liquid into sulfuric acid, which is a weak form of this strong irritant. Your eye immediately tries to protect itself from the mild burn caused by this sudden change in chemistry.

The tear ducts then work overtime to flush out the irritant, and this results in a sudden burst of tears. You might notice that cutting an onion indoors makes your eyes water much faster than doing it outside where the wind blows the gas away.

You can try cutting an onion under a running tap, as the water vapor will neutralize the gas before it reaches your eyes. Another useful trick is to chill the onion in a bag for an hour, since cold temperatures slow down the release of the irritating gas.

If you cut a large onion quickly, your eyes will likely water more than if you take your time. The gas travels through the air faster when there is a strong draft, so opening a window helps prevent the tears from forming in the first place.