Scientists finally learn exactly why cutting onions makes us cry

Every cook knows the sting that comes from chopping an onion. Your eyes water, your nose tingles, and soon you’re half crying over the cutting board.

Yet behind this everyday kitchen drama lies a fascinating story of chemistry, physics, and biology.

Scientists have now discovered how a simple slice can send pungent aerosols flying across the kitchen. Their work not only explains why onions make us cry but also how to cut them more safely.

The chemistry of tears

For years, researchers have known that onions release a compound called propanethial S-oxide. This chemical irritant is responsible for that unmistakable eye sting.

However, what remained unclear was how these sulfur-based compounds actually reach our eyes so effectively.

The new study, conducted by scientists from Cornell University, has finally revealed the physical process behind it.

By combining high-speed videography with computer modeling, they visualized the hidden burst that occurs the instant a knife breaks through an onion’s surface.

Inside an onion’s eruption

An onion may appear simple, but inside, its structure is remarkably complex.

Each layer is enclosed by thin skins at the top and bottom, forming tiny pressure chambers. When a knife presses into an onion, it compresses these internal cells.

The pressure builds until the top skin breaks, releasing a mist of fine droplets. The speed of these droplets surprised the scientists.

The team discovered that while chefs cut at about 3.3 feet per second, the droplets blast out at speeds between 16 to 131 feet 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 a professor of biological and environmental engineering in the College of Agriculture and Life Sciences.

The researchers used specialized high-speed cameras to capture these invisible sprays in slow motion, revealing the true violence of an onion’s response to a blade.

Onions spread more than tears

Dr. Zixuan Wu, first author of the study, helped quantify this rapid spray. The findings detail how these droplets form and disperse through the air.

The researchers also built a simple computer model to understand how much pressure develops inside each layer.

This combination of real footage and simulation allowed them to estimate both droplet size and launch speed with impressive precision.

The study uncovered a second, less expected danger. Beyond causing you to cry, onion sprays might help spread bacteria.

Onions have occasionally been linked to foodborne illness, and the way they eject droplets could explain why.

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

In 2024, the Centers for Disease Control and Prevention (CDC) investigated an E. coli outbreak tied to onions served at McDonald’s that led to 104 cases of illness across 14 states and one death.

Cut onions slower, cry less

The scientists calculated that onion droplets can shoot out at speeds of up to 131 feet per second. For comparison, human coughing typically releases droplets at speeds of about 66 feet per second.

That means an onion mist can move twice as fast as a cough, potentially sending bacteria farther across a kitchen.

Jung’s findings raise new concerns about kitchen hygiene, especially when onions are handled in commercial food settings.

The research does not just explain the science – it also offers practical advice. Cutting onions slowly helps reduce the pressure buildup within their layers. Using a sharp blade ensures a cleaner slice with less crushing force.

Coating the onion in a thin layer of oil before cutting can also block droplet formation by sealing the surface. These small adjustments can make a noticeable difference, protecting both eyes and health.

From cooking to chemistry

This study transforms the humble act of chopping onions into a window into fluid dynamics, cellular mechanics, and microbiology.

It reminds us that even simple ingredients carry complex behaviors waiting to be understood.

From the lab to the kitchen, these findings reveal how science can make everyday life a little clearer – and a little less tearful.

The study is published in the journal Proceedings of the National Academy of Science.

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