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Why do onions burn my eyes?

Dr. Michelle Calder-Cardwell is the owner and lead optometrist at
Urban Optiques Vision & Eyewear in Northville, MI.

Q. I love to cook, but every time I have to chop onions, my eyes water and burn. Why do onions burn my eyes?

A. When you cut into an onion, you're actually breaking open microscopic cells filled with enzymes that turn into volatile gasses when they escape. This causes a chemical reaction and creates a lachrymatory agent, similar to the chemicals used in tear gas. When the fumes reach the almond-shaped glands in the corner of your eyes, your eyes begin to produce tears in an attempt to dilute and flush the chemical from your eyes.

It can be an irritating and even painful process to slice and dice onions, so here are some ideas you can try without getting weepy:

  • Use a sharp knife to cut the onion—you'll release less of the enzymes into the air.
  • Cut onions in cold water.
  • Cut the root last—it has a higher concentration of enzymes.
  • Chill or freeze onions to minimize the amount of gas released into the air.
  • Light a match before you peel or slice the onion. The sulfur disables the compounds in onions that make your eyes water.
  • Use a small manual or electric food chopper or food processor.
  • Wear kitchen goggles to protect your eyes.

Remember, don't touch your eyes when you chop onions. And make sure to wash your hands thoroughly after you're finished. The good news is that typically the burning sensation you experience when cutting onions only lasts a little while, but if your eyes are especially irritated, flush them with cool water or a soothing eye rinse. If your symptoms don't disappear within a few hours, see your VSP eye doctor right away.