'Unstable chemicals.

Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imani et al, 2002).

The process goes as follows:

Lachrymatory-factor synthase is released into the air when we cut an onion.
The synthase enzyme converts the sulfoxides (amino acids) 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 become irritated and produces the tears.

Many of us have cried over chopping onions. Do you know why they make your eyes water, though? Yes, yes, they have that pungent juice spraying up into your eyes. But do you know what’s in that misty juice? And don’t stop that chopping – chopping and crushing onions release health-benefiting compounds.

Let’s get straight to the point. The component in an onion that makes your eyes water is called lachrymatory factor, from Latin lacrima, or “tear.” (How appropriate is that!) Lachrymatory factor irritates the eyes and stimulate the tear glands to produce tears, much like the action of a tear gas.

Ever cut an onion and started tearing up? Most people do. It turns out when you cut an onion, it releases a gas called, ready for this, Propanethiol S-oxide. When 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. Normally, your body would signal you to close your eyes. This, of course, is not a good idea if you are cutting an onion. The next best thing your eyes can do is to make tears to protect the eyes. Rubbing your eyes is a bad idea, since your hands are likely full of the tear making onion juice. So how do you keep from tearing up? There are many theories out there. I’ve heard some have had luck burning a candle near the chopping site. (Don’t try that without an adult.) Wearing safety goggles, however, should reduce the tears every time.

Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion. Recent studies from Japan, however, proved that lachrymatory-factor synthase, (a previously undiscovered enzyme) is the culprit (Imani et al, 2002).

The process goes as follows:

Lachrymatory-factor synthase is released into the air when we cut an onion. The synthase enzyme converts the sulfoxides (amino acids) of the onion into sulfenic acid. The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.