High-Speed Imaging Reveals How Chopping Onions Makes You Cry – and How To Avoid It

Cutting onions can be an unpleasant task for many home cooks and professional chefs alike.

Past research had already confirmed the discomfort stems from a chemical compound called propanethial S-oxide, a known eye irritant. Now, researchers have uncovered how this compound travels through the air and to irritate the eyes.

Faster than a cough

The study, published in Proceedings of the National Academy of Sciences, used high-speed cameras and computer modelling to visualise how onion layers respond to the pressure of a knife blade.

The research team, led by engineers from Cornell University, discovered that when a knife penetrates the outer skin of an onion, it releases pressure built up inside the plant cells. The onion’s internal structure, composed of stacked layers enclosed between two skins, acts like a sealed chamber. When breached by a blade, the internal pressure forces a mist of droplets into the air at speeds between 5 and 40 meters per second.

“We found out the speed of the mist coming out is much higher compared to the speed of the blade cutting through,” said Sunghwan “Sunny” Jung, the paper’s senior author and professor of biological and environmental engineering in the College of Agriculture and Life Sciences.

Measurements taken from video footage of trained chefs showed that typical cutting speeds were about 1 meter per second. In contrast, the mist ejected during slicing moved up to 40 meters per second. For comparison, a human cough can expel droplets at a maximum speed of about 20 meters per second.

Chop your onions slowly, with a sharp knife

The findings have practical implications. Since the speed of mist ejection is influenced by the blade’s impact and the resulting internal pressure, the researchers suggest that cutting onions slowly and with a well-sharpened knife may reduce droplet formation and spread. This advice could be especially relevant in kitchen environments where hygiene is essential.

“Suppose you have pathogens on the very top layer on the onion,” Jung said. “By cutting this onion these pathogens can become encapsulated in droplets where they can then spread.” 

Onions are known to carry surface pathogens and previous foodborne illness outbreaks have been linked to contaminated onions. For instance, in 2024, the US Centers for Disease Control and Prevention reported an E. coli outbreak tied to onions served at a fast-food restaurant chain. The outbreak affected more than 100 people across 14 states and resulted in one fatality.

Reference: Wu Z, Hooshanginejad A, Wang W, Hui CY, Jung S. Droplet outbursts from onion cutting. Proc Natl Acad Sci USA. 2025;122(42):e2512779122. doi: 10.1073/pnas.2512779122

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