Why does cutting an onion make you cry?

Holding a whole onion doesn't hurt your eyes. That's because inside the onion's cells, sulfur-containing compounds and the substances that transform them are kept in separate compartments. When knife strokes burst the cell walls, the two meet, several reaction steps happen in an instant, and at the end a light gas that floats upward is produced.

When this gas touches your eyes, nerves on the eye's surface sense a sting, and your body floods them with tears to wash it away. The sting is strongest about 30 seconds after cutting and then eases a little, because the gas is newly made with each cut and then disperses. In 2002, a study published in the journal Nature reported that a separate substance handles this final step.

Some say that holding a slice of bread in your mouth or lighting a candle nearby will stop the tears. But the gas enters through your eyes, not your mouth, so biting bread can't block it. The claim that a candle burns the gas away also lacks solid evidence, and any effect is likely just from the air moving a little.

Some people who wear contact lenses feel they cry less. Part of the eye's surface is covered, so less of it is exposed to the gas. Swimming goggles will definitely block it, though they're a bit of a hassle to wear in the kitchen.

What actually helps is making less gas and keeping it from rising. If you put an onion in the fridge for about 30 minutes before cutting, the reaction slows and less gas spreads. Cutting with a sharp knife crushes fewer cells, so less of the ingredients mix.

The root end holds a lot of these compounds, so it's best to cut it last. Turning on the range hood or letting air flow toward a window to carry the gas away from your face also works. Soaking cut onion in cold water right away takes out some of the sharpness, but leave it too long and the sweetness and aroma drain away too.