Why do our eyes water when we chop up onions?

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Onions emit a sulphur-containing gas when they are chopped. The gas irritates the cells of the eye, which react by producing tears that are intended to clear away the substance.

In ordinary onions, the gas originates because the enzyme alliinase is liberated to react with a sulphur-containing amino acid, producing a sulphonic acid. Another enzyme subsequently converts the sulphonic acid into lachrymatory PTSO (propanethial-S-oxide).

Onions have evolved the ability to secrete PTSO as a survival mechanism. Onion bulbs grow underground, where they would be easy prey for small burrowing animals if they lacked  any self-defence strategies.

Three ways to avoid onion tears

1. Use a tight diving mask or swimming goggles.

2. Peel the onions. Place them in the freezer for 30 minutes, and then chop them.

3. Hold a slice of bread in front of your face with your teeth (it will absorb the gas).

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