• Question: Why do people cry when they're chopping onions?

    Asked by ellascientist to David, Jonathan, Ou, Pete, Sam, Amylou, Maria, Mark L, Mark U on 15 Jun 2012. This question was also asked by ciaraistall.
    • Photo: Pete Etchells

      Pete Etchells answered on 15 Jun 2012:

      It’s because onions produce a chemical when they’re cut called ‘syn-propanethial-S-oxide’ (try saying that ten times fast!). This acts as an irritant to the tear glands in our eyes, which causes them to produces tears to try and remove the substance!

    • Photo: Amy Evans

      Amy Evans answered on 20 Jun 2012:

      Hi ellascientist,

      Pete’s right, when you cut an onion, it releases a gas called Propanethiol S-oxide. When this is mixed with certain enzymes in the onion, it creates a sulfur gas. These gases then get to your eyes and create a mild acid which irritates the eyes. Whilst normally, your body would tell you to close your eyes, it isnt a good idea if you are cutting an onion so to protect your eyes as best you can, your eyes make tears.

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