Why Onions Make You Cry

Almost everyone knows the feeling. You start to cut an onion, and a few seconds later your eyes begin to burn and fill with tears. You are not sad, of course. The onion is simply defending itself. It grows underground, where insects and animals can easily eat it, so over a long time it has developed a chemical trick to keep them away.

Inside a whole onion, two things are kept apart. One is a group of sulphur compounds. The other is an enzyme, a substance that causes a chemical change. As long as the onion is whole, the two never meet. When you cut it, you break open thousands of tiny cells, and the sulphur and the enzyme mix together. The enzyme changes the sulphur into a gas that rises into the air. You can think of it as the onion's own tear gas.

When the gas reaches your eyes, it touches the nerves on the surface. Your eyes read it as a sign of danger, a bit like smoke, and your brain tells them to make tears to wash it away. The gas also irritates those nerves directly, and this is what makes your eyes sting. That is why the burning feels worse when you hold your face close to the chopping board.

Cooks have several tricks to reduce the effect. Many cooks put the onion in the fridge for a while before cutting it, because cold slows the reaction, though tests do not agree on how much this helps. A sharp knife is better than a blunt one, because it damages fewer cells and lets less gas escape. Cutting near an open window or a fan blows the gas away from your face, and some people even wear goggles. Scientists have also made onions that produce much less of the gas. In one experiment, they switched off the gene for the enzyme that makes it. Some tearless onions now sold in shops were made in other ways, such as careful breeding over many years. So the next time you cry over the kitchen board, remember that it is just chemistry doing its job.