Why Onions Make You Cry

Cutting into an onion doesn't just release a smell -- it sets off a small chemical reaction that ends with a burning, tear-inducing gas reaching your eyes. Onion cells store two separate substances that stay apart as long as the cell walls are intact: an enzyme and a sulfur compound. Slicing through the onion breaks open thousands of these cells at once, letting the enzyme and the compound mix for the first time. That reaction produces a new, unstable molecule called syn-propanethial S-oxide, which quickly evaporates into the air around the cutting board. Because it's a gas, it travels easily and reaches your eyes before you even notice a smell. Once it touches the surface of your eye, it reacts with the water there to form a mild sulfuric acid, which is exactly what irritates the nerve endings and triggers the tears. The tears themselves are actually a defense mechanism, produced automatically to wash the irritating compound away. A few tricks can reduce the effect: chilling the onion slows the chemical reaction, and cutting near running water or a fan helps carry the gas away before it reaches your eyes. Interestingly, the innermost layers near the root release more of the compound than the outer layers, which is why the last part of the onion tends to be the most tear-inducing to cut.