Re: Why does peeling onions make people cry and also make their eyes sting?

Thanks for your question! I have also been wondering about onion-induced tears, and was happy for an excuse to find out the answer!

I looked in the Merck Index, which lists information about a very wide variety of chemical compounds. Onion oil contains allylpropyl bisulfide, s-(1-propenyl) cysteine sulfoxide, and 1-propenyl sulfenic acid which is thought to be the lacrimator, or tear-inducer, in onions. Because the stinging, burning sensation abates when the onion-slicer moves away from the onion, I suspect that 1-propenyl sulfenic acid is simply an irritant, not an allergen. An irritant (like an acid, or a pesky kid on the bus) can usually be rinsed off, or moved away from, and the symptoms will quickly lessen. With an allergen (like poison oak, or pollen) usually the unpleasant symptoms persist, or even worsen, after the person discontinues contact with the substance.

You ask if all people cry when they peel onions. I asked around, and some people seem to be more sensitive to onion oil than others. Also, it depends on the type of onion. Vidalia onions, for example, are not nearly so tear inspiring (or strong-tasting) as yellow onions.

There are many methods proposed to prevent tears while chopping onions. I can recommend http://www.exploratorium.edu/awards/experiments/onion.html, which lists suggestions which range from holding matches in your mouth while chopping, to chopping onions under a ventilation hood.

Erin Cram
Grad Student, UC Berkeley

ps If you try the matches, don't light them.