For some people, slicing a raw onion is no big deal, but for others, it causes a stinging reaction that results in tears and mild discomfort.
What’s to blame for this teary reaction? Enzymes in the onion that release a pungent gas when you slice into it, and when the gas comes into contact with your eyes, it forms sulfuric acid, which is responsible for that telltale stinging sensation. "The more pungent the onion is, the more likely it will make you tear up," says Irwin Goldman, PhD, department chair and professor of horticulture at the University of Wisconsin-Madison.
That means that yellow onions popular in cooking are the biggest culprits, and sweet, mild Vidalia onions are the least likely to trigger tears. Luckily, onions are the only type of vegetable that cause this crying reaction, because of their unique sulfur compounds.
So why do onions make you well up, but don't seem to make your partner weepy at all? Dr. Goldman says it's probably due to the individual chemistry of your eyes: Some people have little or no reaction to sulfuric acid, while others have a stronger sensitivity. And while some people find that wearing contact lenses reduces their onion-related tears, others may find it makes no difference.