Who’s Cutting Onions: A Review on Onions and Reflex Tears

Have you ever wondered why you cry when you cut onions? You slice and dice onions only to quickly feel a sharp, stinging sensation in your eyes while tears cloud your vision. You try to squint and tilt your head back to lessen the effect but the tears just keep flowing. The next time, you try to shield your eyes with safety glasses. Unfortunately, you still end up bawling.

Many people believe that the onion’s juices are triggering the tears ducts by landing in your eyes. This is a common misconception. Tears that you shed when cutting onions are actually a reflex when the eyes are exposed to airborne irritants.

Onions, also known as allium cepa, are bulb-shaped vegetables in the allium group. The allium vegetable group includes vegetables such as onions, leeks, chives, and garlic. Onion bulbs grow underground where they absorb their nutrients from the soil. More specifically, onions absorb lots of sulfur because they use sulfur in many amino acids. An important sulfur-containing amino acid is called S-1-propenyl-L-cysteine sulfoxide.

When an onion is cut, its cells become damaged and it releases an enzyme called alliinase. The release of alliinase enables a reaction between water and S-1-propenyl-L-cysteine sulfoxide to produce multiple sulfurous compounds, one being 1-propenyl sulfenic acid. The enzyme lachrymatory-factor synthase then converts 1-propenyl sulfenic acid into propanethial S-oxide.

Propanethial S-oxide is an airborne sulfur-containing organic chemical that is similar to that of tear gas. Because it is an airborne chemical, it is difficult to block propanethial S-oxide from reaching the eyes. When this compound touches the protective water layer of the eyeball, it reacts with the water to produce sulfuric acid. Sulfuric acid can be severely harmful, causing burning sensations in the eyes so the eyes also have a defense mechanism. Nerves within the eyes detect the sulfuric acid as a threat and generate tears to flush out the chemicals. This is why propanethial S-oxide is a lachrymatory agent. A lachrymatory agent is a chemical compound that stimulates the nerves in the eyes to generate tears.

Fun Fact: Lacrima is the Latin word for tears.

Onions don’t only use this mechanism to make us cry when we cut them. Because they grow underground, onions are basically wide open for any predators that snack on them such as voles and mice. To combat this, onions possess a defense mechanism that protects them from these hungry animals. 

Onions and their chemical products can actually have many beneficial effects.

Propanethial S-oxide can actually be used to kill bacteria and fungi, making it a useful antibiotic. Its medicinal effects were already discovered centuries ago when onion juice was used to treat gunshot wounds in the American Civil War. Onions also contain antioxidants that help prevent and slow cell damage. Other sulfurous compounds are said to be able to prevent cancer and fight asthma, cardiovascular diseases, and diabetes. These sulfurous compounds and sulfur-containing amino acids come from the soil they grow in. This means that some onions contain less sulfur than others. But, the only way for sulfur to be in the soil is through sulfuric acid that is deposited by acid rain. Acid rain is precipitation caused by human activity such as coal burning, which is known to harm the environment. Because many people around the world are now implementing ways to prevent environmental damage and to be eco-friendly, it is likely that onions will begin to contain less and less sulfur.

The next time you try to avoid crying when cutting onions, try to use something that is airtight such as swimming goggles. While you might look ridiculous for about thirty seconds, at least you won’t be bawling your eyes out for a vegetable. I hope you learned something new! Keep a lookout for the next post!

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References

Arney, K. (2018 Nov. 23). Propanethial-S-oxide: how chopping onions makes you cry. Royal Society of Chemistry. Retrieved June 17, 2023, from https://www.chemistryworld.com/podcasts/propanethial-s-oxide-how-chopping-onions-makes-you-cry/3009790.article

Bjarnadottir, A. (2023 Feb. 13). Onions 101: Nutrition Facts and Health Effects. healthline. Retrieved June 17, 2023, from https://www.healthline.com/nutrition/foods/onions

Petruzzello, M. (2016, June 15). Why Do Onions Make You Cry?. Britannica. Retrieved June 17, 2023, from https://www.britannica.com/story/why-do-onions-make-you-cry

Whelan, C. (2020 Sep. 22). Why Do Onions Make You Cry?. healthline. Retrieved June 17, 2023, from https://www.healthline.com/health/why-do-onions-make-you-cry