Why do I get tears when I peel an onion?

–Karthik Gurumurthy

The tears are the price we pay for flavor and nutritional benefits. Onion is a rich source of nutrients (such as Vitamins B, C and G), protein, starch, and other essential compounds. The chemicals in onions are effective agents against fungal and bacterial growth; they protect against stomach, colon and skin cancers; they have anti-inflammatory, antiallergenic, antiasthmatic, and antidiabetic properties; they inhibit platelet aggregation.

The tears come from the volatile oils that help to give allium vegetables their distinctive flavors and that contains a class of organic molecules known as amino acid sulfoxides. Slicing an onion’s tissue releases enzymes called allinases, which convert these molecules to sulfenic acids. These acids, in turn, rearrange to form syn-propanethial-S-oxide, which triggers the fears. Thy also condense to form thiosulfinates, the cause of the pungent odor associated with chopping onions and often mistakenly blamed for the weepy eye. The formation of syn-propanethial-S-oxide peaks about 30 seconds after the onion is first peeled and comnpletes its cycle of chemical evolution over about five minutes.

The effects on the eye are all too familiar: a burning sensation and tears. The eye’s protective front surface, the cornea, is densely populated with sensory fibers of the ciliary nerve, a branch of the massive trigeminal nerve that brings touch, temperature, and pain sensations from the face and the front of the head to the brain. The cornea also has a smaller number of autonomic motor fibers that activate the lachrymal (tear) glands. Free nerve endings detect syn-propanethial-S-oxide on the cornea and drive activity in the ciliary nerve, which the central nervous system registers as a burning sensation. This nerve activity reflexively activates the autonomic fibers, which then carry a signal back to the eye to order the lachrymal glands to wash the irritant way.

There are several solutions to the problem of onion tears. You can heat onions before chopping to denature the enzymes. You might also try to limit contact with the vapors. Chop onions on a breezy veranda, under a steady stream of water, or mechanically in a closed container. Some say that wearing contact lenses help.