What makes onions bring tears to your eyes?

When George Washington felt a tickle in his throat or an impending sniffle, he would often indulge in a strong onion-based snack before bed. Legend has it that the Founding Father ate cooked onions to fend off colds. His homeopathic approach made sense nutritionally—white onions are packed with vitamin C, B1, and B6, along with potassium, phosphorus, and fiber. Though onions may not be a favorite for everyone, they can help reduce bad cholesterol, ease hypertension, and even minimize blood clotting [source: McNamee].

On the other hand, onions do have a temporary (but unpleasant) effect on those who dare to slice through their various colored skins—white, yellow, or red. Within moments of cutting into an onion, the sharp aroma reaches our noses, and our eyes start to water. For some, it's just a mild eye irritation; for others, it's an all-out cry fest.

Unless you're a chef with a freshly broken heart, the tears you shed while chopping onions aren't due to emotion. This leaves two other types of tears: basal and reflexive. Basal tears are the ones that stay in our eyes and eyelids to keep them lubricated, which means we’re left with reflex tears. The lachrymal glands, located above the eyelids, control tear production. When reflex crying occurs, an external irritant like smoke or dust triggers nerve endings in the cornea, which sends a signal to the brain stem. The brain registers the irritation, prompting the lachrymal glands to produce tears to flush out the invader.

If we're chopping onions from a distance, what causes this tearful reaction? The answer lies in the soil. Onions belong to the Allium plant family, which also includes garlic, chives, leeks, and about 400 other related plants. These vegetables absorb sulfur from the earth, which then forms volatile organic compounds called amino acid sulfoxides. These sulfoxides are the real tear-inducers when onions are cut.

Onion's Tear Jerker

As we slice through an onion, it releases enzymes called lachrymatory-factor synthase. These enzymes trigger a chemical reaction that ultimately results in you crying at the kitchen counter. They interact with sulfoxides, turning them into sulfenic acids [source: Scott]. Sulfenic acids are unstable and quickly transform into a compound known as syn-propanethial-S-oxide [source: Library of Congress].

When syn-propanethial-S-oxide (a troublesome mix of sulfuric acid, sulfur dioxide, and hydrogen sulfide) enters the air around us and reaches our eyes, it triggers the reflexive tear reaction explained earlier [source: Norton]. Nerve endings in the cornea detect the syn-propanethial-S-oxide as a potential irritant, prompting the brain stem to alert the lachrymal glands, causing tears to flow.

For those who dread mascara running and tears staining their faces, researchers in New Zealand may have found hope. By isolating and blocking the lachrymatory enzyme, they’ve developed a tear-free onion. The secret to growing the genetically modified Supasweet onion? Low-sulfur soil [source: Highfield].

Onion enthusiasts might have noticed that different types of onions cause varying levels of tears. For example, Georgia's Vidalia onion and other sweet varieties, harvested in spring and summer, don’t make us cry as much as the sharp, tart onions harvested in the fall and winter. The higher sugar and water content in sweet onions helps reduce the irritating enzymes.

When your recipe calls for yellow Spanish onions or other pungent varieties, there are a few tricks you can try to avoid crying while cutting them:

But don’t let the tears get you down: Despite all this crying, there is some good news. Allegedly, the more you cook with onions and endure their sting, the less they will affect you over time [source: Hillman]. Just keep some lemons on hand to wash away the strong onion scent from your hands.