Why Do Onions Make You Cry, and How to Cut with Fewer Tears

An onion sitting whole on the counter is odorless and harmless to your eyes, and you can carry it across the kitchen, peel it, and set it on the board without feeling a thing. The sting only arrives once the knife goes in. Ask why onions make you cry, and the answer starts in that one moment. It turns out to be a chemistry problem, which is good news, because every trick your grandmother swore by is really aimed at one part of that reaction.

Nothing happens until you cut

Nothing about an intact onion is set up to make you cry, and the irritant does not exist yet, which means something has to assemble it first. What is it that the knife wakes up?

Cutting an onion releases enzymes that form a volatile sulfur compound irritating to the eyes.¹

The two words worth holding on to are enzymes and volatile. Enzymes only get to work once the cells are broken open, which is why a whole onion sits there silent while a chopped one does not, and volatile means the product never stays politely on the board but floats up toward your face. Hold those two ideas together and the fixes almost suggest themselves. An onion is also not the only kitchen bulb that keeps its chemistry locked up until something breaks it, and garlic runs a strikingly similar play.

Crushing garlic activates an enzyme that produces allicin, the source of its pungent aroma.²

In both cases neither the flavor nor the sting is sitting in the whole vegetable waiting for you; damage wakes an enzyme, and the enzyme builds the compound on the spot, right where the knife went in. That leaves you two angles of attack. Slow the reaction down so it makes less of the irritant, or keep whatever it does make from ever reaching your eyes. The steps below split neatly along those two lines.

Cut with fewer tears

None of the steps below stops the reaction outright, but each one either slows it down or keeps the volatile compound away from your face. There is no single fix, which is why there are several.

• Use a sharp knife. A dull one crushes its way through, while a sharp one opens fewer cells and so releases less of the irritant.
• Chill the onion in the refrigerator before cutting, since a cold onion gives off its volatiles more slowly; parking it there while you prep everything else costs you nothing.
• Cut near a running range hood or an open window so a draft carries the compound away from your eyes.
• Leave the root end intact until last, cutting toward it rather than through it at the start. It feels awkward the first time and stops feeling that way quickly.
• Rinse the onion and the board partway through to wash off some of what has already formed. You can wait until your eyes start to sting and still catch up.
• Wear glasses or goggles if you cut a lot of onions at once, keeping the vapor off your eyes directly, so you can watch the onions turn golden in the pan afterward instead of blinking through it.