Why Do Onions Make You Cry? (Your Knife Is Making It Worse)

Dr. Pippa explains the onion's chemical weapon, what high-speed cameras saw during cutting, and why a sharp knife and slow cuts may mean less spray.

Read the full article with data and references →Watch on YouTube

In This Episode

  1. 0:00 Onions are fighting back
  2. 0:15 The onion's chemical weapon
  3. 0:34 From mist to tears
  4. 0:48 What your knife is really doing
  5. 1:12 How to cut onions without crying
  6. 1:23 Cooking, and the no-tear onion
  7. 1:34 Sorry, onion

What the Evidence Does and Doesn't Show

  • Droplets, not tears. The 2025 Cornell study behind the knife advice filmed onions being cut in a lab rig. It measured droplets. It did not measure tears or the irritant in those droplets.
  • The measured effects. Droplet counts rose up to about fortyfold with a duller blade and about fourfold with faster cutting.
  • Chilling is not settled. It is often recommended, but onions held at 1 °C for 12 hours gave a larger volume of droplets in that study, and the authors call for more testing.
  • No-tear onions. Researchers have bred or engineered onions with far less of the irritant, by two different routes, though the eye tests so far have been informal.

A sharp knife only helps if you can see the board, so put it down if your eyes are watering. The full article rates each onion tip and lists every reference.

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