Why do your Eyes Water when Slicing an Onion?

When chopping an onion you will often find that no matter what you do your eyes water? It is like a chemical fog has enveloped your whole face and the tears start to stream. Whilst cutting an onion recently my daughter was convinced that I was crying. I tried to explain to her that it was the onion making me cry – but how does this happen?

Onions contains sulphur containing amino acids. When we start to slice up an onion, one of these chemical compounds, S-1-propenylcysteine-sulphoxide, is decomposed to form the eye watering and debilitating propanthial S-oxide. This is what irritates your eye when slicing onions.

Onions get the sulphur they need from the soil when they grow. Slicing the onion is effectively breaking the cells of the onion and creating propanthial S-oxide in a gaseous state.

When your brain recognises an irritant in the eye it triggers something called the lacrimal glands.

These glands go into overdrive to try and wash out and dilute the irritant in your eye.

So what can we do to try and slow down the chemical reaction that produces propanthial S-oxide, or even better, stop the gaseous irritant making it to our eyes in the first place?

Well I have to say I can’t vouch for most of these, but here are a few methods that I have found with some quick research and a few “old wives tales”.

  • Cut the onion under water (don’t get in the bath!)
  • Put the onion in the fridge for about 30 minutes before cutting it. The colder temperature will help slow down the chemical reactions while you slice the onion
  • Have a fan blowing across you and your face while chopping the onion. This is thought to help stop the gaseous irritant from making it into your eyes
  • Light a candle and have it as close as possible to your cutting surface when chopping up that onion
  • Cut the onion with a spoon in your mouth *[link]
  • And finally, and the inspiration for the cover image on this post, wear a tight fitting pair of swimming goggles while cutting the onion!

Image Credits: [credit – Pantsjustneedpockets], [credit – Tetkoren].