Onions are low in calories and packed with vitamins, minerals and antioxidants. They also go well in a variety of dishes, salads, omelets, and guacamole. However, anyone who cooks knows that onions can make your eyes water. Why is that?
Onions are vegetables that grow underground, and there are many critters there trying to get a bite to eat, but onions have a way of protecting themselves. Sulfur in the dirt mixes with the growing onion and creates amino acid sulfoxides, which are sulfur compounds that easily convert to a gas. When an onion breaks apart, the sulfoxides and onion enzymes are released, and this leads to sulfene groups of acid. The sulfene groups of acid and onion enzymes react and create a syn-propanethial-S-oxide gas.
This gas floats up from the chopped (or bitten) onion, scaring away bugs (and making people's eyes water). It takes a lot of precise chemical reactions, and some vegetables related to onions will produce fewer tears. White, yellow and red onions all have higher concentrations of the onion enzyme needed to create syn-propanethial-S-oxide, while sweet onions, green onions and spring onions have less of the necessary enzymes.
Onions and your eyes Your eyes are sensitive, and given their responsibilities, it's good that they defend noxious gases. Your eyes react to the gas that forms, trying to flush it out with tears. Also, some people's eyes are more sensitive than others, which is why not everyone's eyes start to water when cutting onions, while others can feel the effects across the room.
Avoiding tears
There are a few different ways to avoid tears when cutting onions. You can prevent the gas from reaching your eyes by wearing goggles, but that may be a bit of an exaggeration considering you're in a kitchen, not a lab. Some people cut the onions in a bowl of water, but eye drops can also help. Unfortunately, cry-free onions are not yet available, so if you're planning to add onions to your favorite recipes, keep a few tissues handy.
