Why do onions make you cry?

woman's eyes burning while chopping an onion

The Bottom Line

Onions are pungent, contain plentiful nutrients, and have many potential health benefits. When cut or damaged, onions can cause eye irritation and tear production due to the formation of a sulfur-containing chemicals (called “lachrymatory factor”).

close up of person chopping an onion

The Full Story

The Allium genus of plants includes approximately 500 species such as garlic, shallots, leeks, chives, scallions, and onion. Onion (Allium cepa) is one of the most popular vegetables worldwide. Eighty-five million tons of onions are grown worldwide each year, with the highest number of plants cultivated in China and India. The onion plant likely originated in central Asia but is now also grown throughout Europe and Africa as well as North America. The city of Chicago, once known for growing large amounts of onions, is named after a Native American word that described the characteristic odor of onions.

Onions are classified by their color (yellow, red, or white) and whether they are sweet or non-sweet. Regardless of their color or taste, onions are composed of mostly (89%) water. Onions also contain carbohydrates (9%), protein (1%) and vitamins (including B vitamins, vitamin C, manganese, and thiamine). Onions contain small amounts of sulfur compounds and flavonoids that are responsible for the potential health benefits and unique characteristics associated with the plant. While onions are associated with several beneficial health effects in humans, they are poisonous to some animal species. Dog and cats can develop anemia, bloody urine, and lethargy after eating onions.

Flavonoids are plant chemicals that have important nutritional and therapeutic roles in humans. Common flavonoids found in onions include anthocyanins (found in red and purple onions) and quercetin (found in yellow onions). These compounds have many reported health benefits including anticarcinogenic, anti-inflammatory, antimicrobial, and antidiabetic activities. Red onions contain the most flavonoids, followed by yellow and white onions. While storage does not appear to alter flavonoid levels in onions, frying onions decreases their flavonoid content. However, other culinary practices such as boiling, steaming, and microwaving do not significantly affect the flavonoid content of onions. 

Sulfur compounds are responsible for distinctive flavor of onions. While intact onions have little flavor or aroma, sulfur compounds that are present within the onion plant undergo chemical reactions once an onion damaged. When onions are damaged (such as when an onion is peeled, cooked, or cut), an enzyme called alliinase is released from the injured cells. The alliinase enzyme then breaks down the sulfur compounds into other chemicals including sulfenic acids, pyruvic acid, and ammonia. These chemicals contribute to the characteristic flavor profile of onions. Other factors, including soil sulfur content, growing temperature, and water supply during plant growth, can also affect the intensity of onion flavor.

One of the best-known features of onions is their ability to cause eye irritation and tearing (also known as lacrimation) due to their pungent properties. Tearing is caused by the formation of propanethial-S-oxide, also known as lachrymatory factor. The lachrymatory factor is specific to onions, is not found in garlic, and is formed when alliinase breaks down a particular sulfur compound called 1-propenylsulfenic acid. This characteristic of onions is rare: there are only four lachrymatory factors found in nature, and all are produced by plants. Besides onion, lachrymatory factor is also produced by Petriveria alliacea (a shrub that grows widely in South and Central America) and Allium siculum. 

There are many suggested tips for avoiding the pungency and tear-inducing properties of onions. These include lighting a match, wearing sealed goggles, or cutting onions while they are submerged in water. To reduce tearing when cutting onions, the National Onion Association recommends chilling onions for 30 minutes prior to cutting and leaving the root end of the onion intact as it contains the highest concentration of sulfur compounds. Additionally, since the sulfur content of the soil can affect the pungency of the vegetable, some onions are grown in low-sulfur environments. These “low-pungency” onions now account for up to a quarter of the total onions consumed in the United States each year. However, due to their relatively high water content, low pungency onions are more perishable than traditional onions. Scientists have also explored whether lachrymatory factor can be eliminated in onions through the use of genetic engineering. For now, however, there is no foolproof way to avoid crying when cutting onions.

For questions about strange or unwanted reactions to foods, contact Poison Control for expert advice. There are two ways to contact Poison Control: online at www.poison.org or by calling 1-800-222-1222. Both options are free, confidential, and available 24 hours a day.

Kelly Johnson-Arbor, MD, FACEP, FUHM, FACMT
Medical Toxicologist

Poisoned?

Call 1-800-222-1222 or

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Prevention Tips

  • Always wash your hands before, during, and after handling onions to avoid the spread of germs, as onions have been associated with Salmonella outbreaks.

  • Store onions in a cool and dry environment with plenty of ventilation.

  • Do not allow pets, including dogs and cats, to consume onions.

This Really Happened

Forty-two diabetic patients were given a sugar solution, water, insulin, or 100 grams of fresh onion slices in a clinical trial studying the effect of onion on glucose levels. As expected, the patients who were given insulin experienced a dramatic decrease in their blood sugar values. The patients who consumed fresh onion slices also had a significant reduction in their blood sugar levels, suggesting that onion has an effect on glucose control.


For More Information


References

Albishi T, John JA, Al-Khalifa AS, Shahidi F. Antioxidative phenolic constituents of skins of onion varieties and their activities. J Funct Foods. 2013;5:1191-1203.

Ascenzi P, Azzi A. Can onions be engineered for not tearing? IUBMB Life. 2003 Jan;55(1):49-50.

Bisen PS, Emerald M. Nutritional and therapeutic potential of garlic and onion (Allium sp.). Curr Nut Food Sci. 2016;12:190-199.

Brodnitz MH, Pascale JV. Thiopropanal S-oxide: a lachrymatory factor in onions. J Agric Food Chem. 1971 Mar-Apr;19(2):269-72.

Cortinovis C, Caloni F. Household Food Items Toxic to Dogs and Cats. Front Vet Sci. 2016 Mar 22;3:26.

Imai S, Tsuge N, Tomotake M, Nagatome Y, Sawada H, Nagata T, Kumagai H. Plant biochemistry: an onion enzyme that makes the eyes water. Nature. 2002 Oct 17;419(6908):685.

Kato M, Masamura N, Shono J, Okamoto D, Abe T, Imai S. Production and characterization of tearless and non-pungent onion. Sci Rep. 2016 Apr 6;6:23779.

Kianian F, Marefati N, Boskabady M, Ghasemi SZ, Boskabady MH. Pharmacological Properties of Allium cepa, Preclinical and Clinical Evidences; A Review. Iran J Pharm Res. 2021 Spring;20(2):107-134. 

Kubec R, Kim S, Musah RA. The lachrymatory principle of Petiveria alliacea. Phytochemistry. 2003 May;63(1):37-40. 

Lee SU, Lee JH, Choi SH, Lee JS, Ohnisi-Kameyama M, Kozukue N, Levin CE, Friedman M. Flavonoid content in fresh, home-processed, and light-exposed onions and in dehydrated commercial onion products. J Agric Food Chem. 2008 Sep 24;56(18):8541-8.

Taj Eldin IM, Ahmed EM, Elwahab H M A. Preliminary Study of the Clinical Hypoglycemic Effects of Allium cepa (Red Onion) in Type 1 and Type 2 Diabetic Patients. Environ Health Insights. 2010 Oct 14;4:71-7.

Tang X, Xia Z, Yu J. An experimental study of hemolysis induced by onion (Allium cepa) poisoning in dogs. J Vet Pharmacol Ther. 2008 Apr;31(2):143-9.

Upadhyay RK. Nutritional and therapeutic potential of allium vegetables. J Nutr Ther. 2017;6:18-37.

Zambrano MV, Dutta B, Mercer DG, MacLean HL, Touchie MF. Assessment of moisture content measurement methods of dried food products in small-scale operations in developing countries: a review. Trends Food Sci Technol. 2019:88:484-496.

Poisoned?

Call 1-800-222-1222 or

HELP ME online

Prevention Tips

  • Always wash your hands before, during, and after handling onions to avoid the spread of germs, as onions have been associated with Salmonella outbreaks.

  • Store onions in a cool and dry environment with plenty of ventilation.

  • Do not allow pets, including dogs and cats, to consume onions.

This Really Happened

Forty-two diabetic patients were given a sugar solution, water, insulin, or 100 grams of fresh onion slices in a clinical trial studying the effect of onion on glucose levels. As expected, the patients who were given insulin experienced a dramatic decrease in their blood sugar values. The patients who consumed fresh onion slices also had a significant reduction in their blood sugar levels, suggesting that onion has an effect on glucose control.