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Why do eyes water when cutting onion?


Surely everyone knows that when we peel and say an onion we immediately start crying, but do you know what is the reason for this watering? When an onion is diced or sliced ​​it releases sulfenic acid and enzymes, which join to produce a gas called propanethial S-oxide. This form of sulfuric acid irritates the nerves around the eyes making them tear as a matter of fact this is a defense mechanism of the onions. Onions grow underground and are frequently under attack from critters who try to eat them, to avoid the attack the onions release this lachrymator compounds.

Here are a few tips to avoid tearing when we chop onions, because these gases are very irritants, they can turn your vision blurry and it can be dangerous if you are at the same time managing a sharp knife.

  1. Try to use a sharp knife to cut them, it helps to release less of the enzymes into the air.
  2. Chill the onions into the fridge before chopping them, this minimizes the amount of gas released into the air.
  3. Cut them in cold water.
  4. Light a match before you peel or slice the onion. The sulfur disables the compounds in onions that make your eyes water.
  5. Keep your kitchen breezy.
  6. Cut the root last, it has a higher concentration of enzymes.
  7. Use goggles to protect your eyes, you can use the swimming goggles, but there are specific goggles designed for peeling and chopping onions that can be very useful.

On the other hand, irritation is also different depending on the type of onion, Spring onions are fresh, not dried, and they're less likely to bring you to tears. The same goes for sweet onion varieties such as Vidalia. Try using spring onions during the spring and summer months, and switch to sweet onions in the fall and winter.

Remember not to touch your eyes when you are saying and chopping onions, clean your hands after you are finished. If your eyes are very irritated, flush them with cold water or use a moisturizing drop to clean and calm your eye, if the irritations persist consult with an eye specialist.

Written by:
Dr. Julia Sempere
Consultant Ophthalmologist