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Why do onions make you cry, and how can you stop it?

Ever wondered why chopping onions turns your eyes into waterfalls? Discover the chain reaction that starts with cell damage and ends in a full-blown tear reflex[1].

  • Why Do Onions Make You Cry? Enzymes, Treatments & More
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Step 1: Slicing an onion ruptures its cells, releasing enzymes that mix with stored sulfur compounds. This reaction produces propanethial S-oxide, the volatile gas that makes your eyes water[1][3].

  • Infographic on the chemistry of onions. As onions are sliced, they release a class of enzymes, allinases, which break down amino acid sulfoxides. A specific compound produced during this process is 1-propenesulfenic acid. 1-propenesulfenic acid is rearranged by another enzyme, called lachrymatory factor synthase, to produce syn-propanethial-S-oxide. Production of this gas peaks 30 seconds after mechanical damage to the onion, and it stimulates sensory neurons in the eye causing a stinging sensat
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Step 2: When this gas reaches your eyes, it reacts with the tear film to form a mild acid. This irritates your corneal nerves and triggers the tear reflex as your body tries to flush the irritant out[3].

  • Onion gas causing eyes to burn
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Prevention Tip #1: Wearing goggles is the most effective method—it physically blocks the irritant from reaching your eyes. A fan blowing the vapors away also works wonders by dispersing the gas before it strikes you[4][7]

  • A pair of black and green rimmed glasses lined with a layer of foam sits on a cutting board next to chopped onion
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Prevention Tip #2: Using a very sharp knife minimizes cell damage, reducing the amount of gas released. Pre-chilling your onions slows enzymatic reactions, further cutting down on the tear-inducing effect[10][9]

  • The Best Chef’s Knife, Tested and Reviewed
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Which method surprised you most? Reply with your onion-cutting hacks and share how you keep those tears at bay!

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