Why does chopping onions always make us cry?

We’ve all experienced the dreaded crying tears that appear as soon as we start chopping onions. Rubbing our eyes whilst doing it only makes matters worse. But have you ever wondered why it happens?
Let’s look at the scientific reason behind it.
While chopping onions, cells inside the onion are broken and the gas that comes out forms sulfenic acid. The acid gets mixed with enzymes and forms a gas called propanethiol S-oxide. When this gas reaches our eyes, it gets mixed with water and forms sulphuric acid.

Sulphuric acid makes our eyes sting, which results in more tears and the water in our eyes produces more acid thus more irritation. Once you start cooking onions it deactivates the enzymes and slowly our eyes become normal.

The process goes like this:

– Lachrymatory-factor synthase is released into the air when we chop onions.
– The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid.
– The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.
– Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes.

The lachrymal glands in our eyes become irritated and produces tears.