Learn Why Onions Make You Cry

Close up of chef chopping vegetables
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Unless you've avoided cooking, you've probably cut up an onion and experienced the burning and tearing you get from the vapors it produces. When you cut an onion, you break cells, releasing their contents. A chemical process results, eventually releasing a compound that causes you to tear up when you're slicing and dicing.

Acid Effect

Amino acid sulfoxides form sulfenic acids after you slice into an onion. Enzymes that were kept separate are now free to mix with the sulfenic acids to produce ​propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.

Stop Crying

There are a few ways to stop the chemical process that causes you to cry when you cut an onion, including:

  • Cook the onion. This process inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn your eyes.
  • Wear safety goggles or run a fan. This actually prevents the vapors from the compound entering your eyes—or at least blows the compound's vapors safely away.
  • Refrigerate your onion before cutting it. Doing so slows reactions and changes the chemistry inside the onion. You can accomplish the same thing by cutting the onion under water.
  • Use stainless steel. The sulfur-containing compounds also leave a characteristic odor on your fingers. You may be able to remove or reduce some of the smell—and tears—by wiping your fingers on a stainless steel odor eater.

Other Methods

There are a few other ways to cut into or prepare an onion while avoiding the waterworks. These involve cooking prep methods, such as locating the root, removing the bulb and even slicing lengthwise—at least before you dice the vegetable.

So, take heart. With a little preparation, forethought, and understanding of basic chemistry, you can slice, dice and cook an onion and never shed a tear.