Why Do Onions Make You Cry? The Chemistry and Science Explained

Why Do Onions Make You Cry? The Chemistry and Science Explained

Author: Nohil Kodiyatar

ORCID: 0000-0001-8430-1641

ResearchGate: Nohil Kodiyatar

LinkedIn: Nohil Kodiyatar

Amazon Author Page: Nohil Kodiyatar

Keywords: Why do onions make you cry, onion chemistry, lachrymatory factor, onion cutting tips, sulfur compounds, eye irritation, plant defense mechanisms, food science.

Abstract

Cutting an onion can quickly make your eyes sting and produce tears. This reaction is not caused by emotion but by a chemical defense mechanism inside the plant. When onion cells are damaged, enzymes trigger reactions that produce a volatile irritant called syn-propanethial-S-oxide. When this chemical reaches the eyes, it stimulates tear production. Research published in 2025 also investigated how knife sharpness and cutting speed influence the droplets released during chopping (Imai et al., 2002; Wu et al., 2025).

Why Do Onions Make You Cry?

An intact onion contains sulfur-containing compounds stored inside its cells. When you cut the onion, you damage those cells and allow previously separated substances to interact.

An enzyme called alliinase helps initiate reactions involving sulfur compounds. Another enzyme, lachrymatory-factor synthase, contributes to the formation of syn-propanethial-S-oxide, the volatile chemical responsible for irritating your eyes (Imai et al., 2002).

This chemical travels through the air and reaches the moist surface of your eyes. Sensory nerves detect the irritant, prompting the tear-producing glands to release additional fluid.

The tears help dilute and remove the irritating substance.

Interestingly, the onion did not evolve this chemical reaction to make humans cry. It is part of the plant's chemical defense system, which can help protect damaged tissue against potential threats.

Why Do Some Onions Make You Cry More Than Others?

Not every onion produces the same level of irritation.

The amount of tear-inducing compounds can vary with onion variety, growing conditions, and the concentrations of relevant sulfur compounds and enzymes.

Some sweet onion varieties tend to be less pungent and may produce less irritation than stronger varieties. However, sweetness alone does not guarantee that an onion will be tear-free.

Scientists have also investigated whether onion chemistry could be modified to reduce tear production while preserving desirable flavor characteristics.

How Can You Cut Onions Without Crying?

You cannot always prevent irritation completely, but several practical methods can help reduce it.

1. Use a sharp knife. A sharp blade cuts onion tissue more cleanly and can reduce the force needed to penetrate its layers.

2. Cut slowly. Research published in Proceedings of the National Academy of Sciences found that faster cutting and blunter blades increased the number and energy of droplets ejected during onion cutting under the experimental conditions studied (Wu et al., 2025).

3. Improve ventilation. Use an extractor fan or position yourself so that airflow carries the irritant away from your face.

4. Chill the onion before cutting. Cooling can slow some enzyme-driven reactions and may reduce irritation, although it does not eliminate it.

5. Consider eye protection. Properly fitting protective goggles can help prevent the airborne irritant from reaching the eyes.

Among these options, using a sharp knife and cutting gently has a particularly useful scientific rationale.

What Did Scientists Discover About Onion Cutting?

In 2025, researchers investigated the physical process that releases droplets when an onion is cut. Using high-speed imaging and mechanical analysis, they observed an initial burst of droplets followed by slower fluid fragmentation.

The study found that blade sharpness and cutting speed influenced the number and energy of the droplets produced. This helps explain why a blunt knife can cause more forceful spraying than a sharp one (Wu et al., 2025).

The research focused on the mechanics of droplet release, not simply the chemical reaction responsible for eye irritation.

It also has a broader food-safety implication: droplets released from contaminated produce may help spread microorganisms around a preparation area. Good food hygiene remains important whenever vegetables are handled.

Conclusion

Onions make us cry because damaged cells release substances that react to form a volatile chemical irritant. When this compound reaches the eyes, the body's protective response produces tears.

The science also explains why cutting technique matters. A sharp knife, slower cutting, and good ventilation can reduce exposure to irritating material.

A simple kitchen task reveals a fascinating combination of plant chemistry, enzyme activity, fluid mechanics, and human physiology.

References

Imai, S., Tsuge, N., Tomotake, M., Nagatome, Y., Sawada, H., Nagata, T., & Kumagai, H. (2002). An onion enzyme that makes the eyes water. Nature, 419, 685. https://doi.org/10.1038/419685a

Wu, Z., Hooshanginejad, A., Wang, W., Hui, C.-Y., & Jung, S. (2025). Droplet outbursts from onion cutting. Proceedings of the National Academy of Sciences, 122(42), Article e2512779122. https://doi.org/10.1073/pnas.2512779122

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