Why Do Onions Make You Cry?

It's the bane of every cook--from the home kitchen to the fanciest restaurant. Peel an onion and you're likely to cry. Your eyes will sting. Tears will run down your face. Ouchies!

Why? It's the onion's way of protecting itself.

Low in calories and packed with vitamins, minerals and antioxidants, onions are a perfect accompaniment in salads, omelets, soups, stir-fry and guacamole. That's the good news. The bad news is that they also contain a kind of gas that human eyes don't like.

Robert H. Rosa Jr., M.D., an ophthalmologist and professor of surgery and medical physiology at the Texas A&M College of Medicine explains why onions make you cry: It's a very complicated chemical process.

Onions are vegetables that grow underground, and beneath the surface there are a lot of critters who are trying to grab a bite to eat--so onions have a way to guard against this. Sulfur in the dirt mixes with the growing onion and creates amino acid sulfoxides, which are sulfur compounds that readily turn into a gas. When an onion breaks apart, the sulfoxides and onion enzymes are released, and this creates sulfenic acid. The sulfenic acid and onion enzymes react and create syn-propanethial-S-oxide--a tough-to-pronounce gas.

This gas floats up from the chopped (or bitten) onion and deters critters and causes humans to shed tears. It takes a lot of precise chemical reactions, and some vegetables related to onions will also produce fewer tears. White, yellow and red onions all have a higher concentration of the onion enzyme necessary to create syn-propanethial-S-oxide, while sweet onions, green onions and scallions have fewer of the necessary enzymes.

"Your eyes have a set of nerves that detect anything that's potentially harmful to your eyes," Rosa explains. "Your eyes react to the gas that is formed, and your eyes try to flush it out with tears."

Luckily, the gases that are produced from chopping onions are more nuisance than harm. "Chopping onions can cause some burning and irritation and tears," Rosa said. "Other than that, it's pretty safe on your eyes. It's a temporary sensation with no known long-term effects, nor will it worsen any other conditions, like pink eye."

Also, some people may have more sensitive eyes than others, which is why not everyone will tear up when they chop onions but why some may feel the effects on the other side of the room.

How can you avoid tears while chopping onions? Here are some tips:

  • Wear protective goggles. (OK, that may be a bit excessive.)
  • Cut the onion in a bowl of water.
  • Freeze the onion.
  • Put a piece of bread between your front teeth while you're cutting the onion.
  • Use comfort eye drops to lubricate or rinse your eyes, as well as dilute the gas exposure.

--From the Editors at Netscape

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