Why Does Cutting Onions Make You Cry?

Cutting onions often brings tears to the eyes, and the science behind this phenomenon is rooted in plant chemistry and human physiology. Onions, like other members of the Allium genus (which includes garlic, leeks, and shallots), absorb sulfur from the soil,

which they use to synthesize a variety of sulfur-containing compounds. When an onion is sliced or chopped, it breaks open its cells and sets off a chain of chemical reactions. The enzyme alliinase1 is released and reacts with the sulfur-containing amino acids to produce sulfenic acid2. This unstable compound quickly rearranges into syn-Propanethial-S-oxide,

a volatile gas known as the lachrymatory factor (LF)3. When this gas reaches the eyes, it reacts with the water in your tear film to form a mild sulfuric acid, which irritates the sensory nerves and triggers tear production as a defense mechanism to flush the irritant away.

Interestingly, the intensity of the eye irritation can vary depending on the onion variety, freshness, and how it’s grown. Sweet onions, such as Vidalias4, typically cause fewer tears because they contain less sulfur than more pungent varieties like yellow or red onions. Cold storage also tends to slow the enzymatic reaction,

which is why chilled onions are often recommended to reduce the tearful effects. Similarly, freshly harvested onions might not provoke as much of a reaction because the lachrymatory factor hasn’t fully developed yet, whereas onions that have been stored for some time can be more irritating due to a higher concentration of sulfur compounds. To minimize the tear-inducing effects, many home cooks employ various strategies—some grounded in science, others more anecdotal.

Chilling onions for about 30 minutes before cutting can reduce the volatility of the gas by slowing down the enzymatic reaction, as can using a very sharp knife, which causes less cell damage and thus releases fewer enzymes. Cutting under running water or near a fan can

help blow away or wash away the irritant before it reaches your eyes. Breathing through the mouth or sticking out your tongue while cutting onions is sometimes suggested and may offer minor relief, though these techniques are largely considered folk remedies with limited scientific support. Likewise, chewing gum or lighting a candle nearby might create a placebo effect,

but they haven’t been proven effective in controlled studies. Onion goggles—special eyewear that creates a seal around the eyes—do have a basis in science, as they physically block the gas from reaching your eyes. Similarly, cutting onions near a vent or an exhaust fan can help redirect the gas away from your face. Another lesser-known solution comes from biotechnology: scientists have developed genetically modified “tearless onions” by suppressing the gene responsible for producing the lachrymatory factor enzyme.

These onions, known as the “Sunion5,” have been selectively bred to reduce eye irritation significantly without sacrificing flavor, although their availability is still somewhat limited. Trivia about onions reveals their long culinary and cultural history.

Onions were revered in ancient Egypt for their medicinal properties and symbolic meaning of eternal life. In modern times, the onion’s tear-inducing quality has even inspired urban legends and dubious techniques, including cutting them underwater or placing a piece of bread in your mouth while chopping.

While some of these might marginally help, the most effective solutions are those that reduce the release or reach of the syn-Propanethial-S-oxide gas. Despite their pungent nature, onions remain a beloved ingredient, their flavor made sweeter by the effort of enduring a few tears.

Footnotes
  1. Alliinase is a sulfur-metabolizing enzyme primarily found in plants of the Allium genus, such as onions and garlic, where it plays a key role in the formation of bioactive sulfur compounds. In garlic, alliinase converts the odorless amino acid alliin into allicin, a compound responsible for garlic’s characteristic smell and many of its antimicrobial and health-related properties. In onions, alliinase acts on S-1-propenyl-L-cysteine sulfoxide to generate sulfenic acids, which are rapidly converted into syn-Propanethial-S-oxide—the volatile compound that causes eye irritation and tears when cutting onions. Alliinase is compartmentalized in intact plant tissue, separate from its sulfur-containing substrates, and only becomes active when cells are damaged, allowing the two to mix. The enzyme is sensitive to heat and acid, which is why cooking alliums usually neutralizes their pungency. Its rapid enzymatic reaction upon tissue disruption is also part of the plant’s natural defense mechanism against pests and pathogens. ↩︎
  2. Sulfenic acid is a highly reactive and short-lived organosulfur compound (general formula RSOH) that plays a crucial role as an intermediate in both biological and chemical processes, particularly in Allium plants like onions and garlic. In onions, when the enzyme alliinase breaks down sulfur-containing amino acid precursors such as S-1-propenyl-L-cysteine sulfoxide, it produces 1-propenesulfenic acid. This unstable sulfenic acid quickly rearranges into syn-Propanethial-S-oxide, the volatile lachrymatory factor responsible for eye irritation during onion chopping. Due to their instability, sulfenic acids typically do not accumulate and instead undergo rapid transformations, often forming disulfides or sulfinic acids, or acting as intermediates in antioxidant and redox signaling pathways in cells. In garlic, sulfenic acid also serves as a precursor to allicin, the compound credited with many of garlic’s antimicrobial properties. Despite their fleeting existence, sulfenic acids are key players in the chemistry of both food flavor and biological defense mechanisms. ↩︎
  3. The lachrymatory factor (LF) is the volatile sulfur compound syn-Propanethial-S-oxide that is responsible for the eye irritation and tearing experienced when chopping onions. It is produced through a complex enzymatic process: when onion cells are damaged, the enzyme alliinase converts sulfur-containing precursors (such as S-1-propenyl-L-cysteine sulfoxide) into 1-propenesulfenic acid, which is then rapidly converted by the enzyme lachrymatory-factor synthase (LFS) into syn-Propanethial-S-oxide. This compound easily vaporizes and, upon reaching the eyes, reacts with the tear film to form mild sulfuric acid, irritating sensory nerves and triggering a reflex to produce more tears to flush it out. The discovery of LFS clarified why onions, unlike garlic, produce this unique irritant, and ongoing research has even led to the development of genetically modified “tearless” onions by suppressing the production of LF. Though potent in small quantities, the lachrymatory factor is harmless and dissipates quickly, especially with heat. ↩︎
  4. Vidalias are a distinctive variety of sweet onion grown primarily in the state of Georgia, USA, known for their exceptionally mild flavor and low pungency due to their unique combination of low sulfur content and high water and sugar levels. Officially defined by the Vidalia Onion Act of 1986 and regulated by the U.S. Department of Agriculture, true Vidalias can only be cultivated in a specific 20-county region in southeastern Georgia, where the soil is unusually low in sulfur and contributes to their sweet, less tear-inducing taste. First grown commercially in the 1930s, Vidalias gained popularity through roadside stands and later became a protected geographic trademark, much like Champagne in France. Their growing season typically spans from late April through early August, and they are often eaten raw due to their delicate flavor. While genetically similar to other short-day yellow granex-type onions, it’s the specific terroir of Georgia that gives Vidalias their signature profile, making them one of the most celebrated sweet onions in the United States. ↩︎
  5. Sunions are a branded variety of tearless, sweet onions developed through decades of conventional hybridization by BASF (formerly Bayer Crop Science), designed to reduce the volatile compounds that typically cause eye irritation when cutting onions. Unlike standard onions, Sunions produce lower initial levels of sulfur-based precursors and see a further decrease in these compounds during storage, making them milder over time—an inverse trend from traditional onions, which grow sharper with age. The key to their tearless quality lies in the significant reduction of syn-Propanethial-S-oxide, the lachrymatory factor, allowing them to be sliced without triggering the body’s defensive tearing reflex. They are not genetically modified and undergo extensive sensory testing to certify both their lack of eye irritation and their sweet, mild taste. Grown mainly in the U.S. in regions like Nevada and Washington, Sunions first hit the market in 2018 and have since expanded to Europe and the UK, offering a novel solution for cooks seeking onion flavor without the tears. ↩︎
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Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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