• Question: What chemicals in an onion makes you cry?

    Asked by InumidunA on 17 Mar 2025.
    • Photo: Alice Carstairs

      Alice Carstairs answered on 17 Mar 2025:

      I don’t know what the name of the chemical is, but its acidic which is why it makes your eyes water. When we cut the onion it’s breaking the cells open which contain the chemical. The bottom end of the onion has more of it in, so I always cut that off last!

    • Photo: Iain Brownlee

      Iain Brownlee answered on 3 Apr 2025:

      The chemical’s name is propanethial-S-oxide which reacts with the water at the surface of your eye to make an acid that results in the stinging sensation as Alice explains below. The eye tries to clear this by increasing fluid release, hence the tears.

      Perhaps most interestingly, release of these compounds is an example of how plants have developed ways to defend themselves from being eaten.

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