Why Chopping Onions Makes Us Cry

Have you ever cried like a baby while trying to chop an onion? We cry, and sometimes laugh at ourselves when we chop onions with tears streaming down our cheeks. Crying usually occurs about 30 seconds after you cut an onion open. The tearing starts to slow down after about five minutes of cutting. As hard as we try, we usually can't control the flow of tears caused by the fumes of the onions. Why does this happen?

Chopping white onions

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A Chemical Reaction

Onions contain amino acid sulfoxides that produce sulfenic acids inside the cells of the onion. When you start cutting an onion, it disrupts the cells and the normal cell enzymes mix with the sulfenic acids and it produces propanethial S-oxide. Propanethial S-oxide is a sulfur chemical that is a gas that floats through the air and into your eyes. When this gas interacts with the water in your tear film, sulfuric acid is produced. This acid is not compatible with your tears and your eyes begin to burn. Your brain says, “There is something in my eye!”

(Cooked onions won’t produce this same effect because the process of cooking the onion inactivates the enzymes needed to make propanethial-S-oxide.)

Basic Tear Production

There are two types of tear production that occur inside the eye. Basal tears, the ones that provide basic lubricant to the eye, and reflex tears, the type of tears we typically think of when we are crying. Reflex tears are produced in response to emotion and some external irritant. An external irritant, such as dust or smoke, triggers nerve endings in the cornea to communicate with the brain which turns on the lacrimal gland. The lacrimal gland, which is under the upper eyelid on the side of your temple, creates reflex tears. Your tear glands begin secreting tears to potentially dilute or wash out the offending agent.

How to Avoid Tearing Up While Cutting an Onion

  • Use a very sharp knife to chop the onion. Crushing an onion, instead of cutting it sharply, tends to release more gas. A sharp knife will allow for quick, clean slices. 
  • Chill the onion before cutting. When you refrigerate the onion before cutting, you freeze the enzymes from mixing with the sulfenic acids and the production of propanethial S-oxide is stopped. As a result, sulfuric acid is not produced when you chop the onions.
  • Wear safety goggles while chopping the onions. You may get some funny looks but it will definitely keep the acid from getting into your eyes.
  • Turn on a fan. Point the fan toward your work surface so that the fumes are drawn away from you.
  • Soak the onions in water before cutting. The gas will be absorbed into the water. Simply allow the onions to sit in a small bowl of water for about half an hour before beginning to chop them.
  • Lighting a candle by your onion chopping board is a good way to avoid tearing excessively. A candle near the chopping board will cause the fumes from the onions to become drawn towards the flame and less will reach your eyes.
  • Cut the root of the onion at the very end. The root contains more enzymes.
  • Use an electric food processor.
  • Do not touch your eyes when you are chopping onions. Make sure to wash your hands thoroughly after you are finished.

A Tear-Free Onion?

Crop & Food Research of New Zealand and House Foods Corporation of Japan have developed genetically engineered onions that do not produce the sulfur compounds that make us tear while chopping them. The onions contain onion flavor without the weepy side effects—but it's not clear when, or if, these will ever be available to consumers.

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  • Primary Care of the Anterior Segment, Second Edition, Catania, Louis J. Appleton & Lange, 1995, ISBN 00-8385-7911-6