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The real reason onions make you cry

You haven’t had a bad day and you aren’t feeling particularly emotional, but something about chopping onions at your kitchen counter makes you tear up.

While not everyone cries to the same degree when cutting their onions, there's some science behind the cause of this veggie prep sensation.

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Coventry University Senior Lecturer Duane Mellor has discovered that sulphuric acid forms in our eyes as a response to a chemical the onion releases.

Sulphuric acid is dangerous, but because the chemical is so minimal, it only affects the eyes, causing them to tear up. He explained to The Conversation that onions produce propanthial s-oxide, a chemical used to ward off pests.

If you've ever wondered why onions make you well up, here's why. Photo: Getty
If you've ever wondered why onions make you well up, here's why. Photo: Getty

This volatile chemical is released when the skin of the onion is broken, evaporating and dissolving into the surface of our eyes. The chemical reaction produces a small amount of sulphuric acid, causing the eyes to tear up.

Mellor also discovered that not all onions are the same, with some making us cry more than others. According to his study, it depends on the soil. The pungency of an onion can be directly related to the soil it was planted in. If the soil had a higher amount of sulphur, the onion is more likely to make you cry. Sweeter onions are less likely to make you cry because they don’t have the compounds that produce propanthial s-oxide to irritate the tear ducts. The same goes for garlic.

Fun fact: not all onions make you cry the same amount. Photo: Getty
Fun fact: not all onions make you cry the same amount. Photo: Getty

While it’s nearly impossible to avoid tearing up when chopping onions, there are a few tricks of the trade:

  1. Using a sharp knife to cause less damage to the skin of the onions

  2. Chilling your onions before chopping them

  3. Running air conditioning while you prepare your vegetables


Mellor’s research just goes to show that onions really are nothing to cry about.

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