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Apologies in advance: After reading this story and clicking on the accompanying multimedia horror show, you may never use a public restroom again. 

At least not until somebody invents an impermeable toilet lid with an unbreakable seal.

You see, intrepid and iron-stomached researchers from the University of Colorado Boulder are using laser light and green dye to show the cloud of aerosols that erupts when you flush a high-powered public toilet. 

It’s almost beautiful. But it’s definitely not pretty. 

A laser-illuminated aerosol plume ejects from a commercial toilet during the first 8 seconds of the flush cycle, illustrating the energetic flapping of the chaotic jet emanating from the bowl, as well as the rapid spread of the aerosol plume. (University of Colorado Boulder)

Toilet plumes emit a cloud at once putrid and pathogenic. The CU researchers are not just gunning for special effects jobs on the next “Saw” franchise installment, though it’s a good audition tape. They hope the alarming images will help nudge public health officials and seasoned plumbers to design better ventilation, pipes and water flow to contain the noxious clouds.

Our jaws dropped. Honestly, we all kind of started laughing. You’ve got to be kidding me — that’s what happens when you flush the toilet?

— John Crimaldi, CU professor

The toilet-launched particles exit the bowl atmosphere at a rate of 6.6 feet per second, reaching just under 5 feet high in 8 seconds. The smallest droplets, one-millionth of a meter in diameter, can stay in the air for long minutes. 

Particulates containing the COVID virus as well as other infectious diseases such as C. diff have been shown to linger on surfaces for days. 

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’re never going to think about a toilet flush the same way again,” said John Crimaldi, a CU professor of civil, environmental, architectural and horror-show engineering, who co-authored the study. “By making dramatic visual images of this process, our study can play an important role in public health messaging.”

A toilet flushing under natural lighting compared to under a laser, showing that the aerosol plume is normally completely invisible. (University of Colorado Boulder)

Crimaldi’s paper is entitled “Commercial toilets emit energetic and rapidly spreading aerosol plumes.” (And you’re welcome for multiple possibilities for the name of your nephew’s emerging punk metal band.) 

Part of the ingenuity in the research involved new combinations of precision camera equipment and green laser light. 

“People have known that toilets emit aerosols, but they haven’t been able to see them,” Crimaldi said. “We had expected these aerosol particles would just sort of float up, but they came out like a rocket.” 

A graphic showing how CU Boulder researchers set up their experiment to study flushing public toilets.
A schematic from the University of Colorado Boulder researchers’ paper published in “Scientific Reports,” showing how they shot lasers, filmed, and computer-graphed the timing of aerosolized particles. (University of Colorado Boulder)

CU’s news office helpfully points out that it’s not just your own business you have to worry about. “Many other studies have shown that pathogens can persist in the bowl for dozens of flushes, increasing potential exposure risk,” a news release says.

Other things you did not ask and probably shouldn’t: The researchers used water alone in the toilet bowls, declining to … um … complicate the research with objects commonly associated with restroom culture. 

In case images alone are not enough to reflect the undeniable science of the lingering public toilet threat, we’ll leave you with these words published in this week’s edition of “Scientific Reports”:

“The flow is surprisingly energetic and chaotic, and exhibits strong jet-like behavior.” 

(Left) The toilet and laser are in the background as Dr. Aaron True runs an experiment as he watches image acquisition computers. When the toilet is flushed, you can see the plume from the toilet in the background, and simultaneously you can see the live image data of the plume appearing on Aaron’s screen. (Right) The larger field of view shows how far the plume spreads across the laboratory during the flush cycle. (University of Colorado Boulder)

Crimaldi says the researchers working on the study all knew from past science that droplets spread throughout bathrooms. But seeing was disbelieving. 

“Our jaws dropped,” he said. “Honestly, we all kind of started laughing. You’ve got to be kidding me — that’s what happens when you flush the toilet? I don’t think anybody really expected to see that much energy coming out.”


None of the scientists involved are public health experts, Crimaldi said, and they don’t want to risk bad health advice on how to handle the plumes. Most public bathrooms don’t even have lids, just a seat. The problem right now is in things the public can’t control. 

“There’s opportunities here to rethink the way toilets function,” Crimaldi said. 

Keep the COVID masks on during bathroom breaks? Crimaldi is fully comfortable with that tip. 

“I’m certainly going on the record saying that’s a great idea,” he said. 

Michael Booth is The Sun’s environment writer, and co-author of The Sun’s weekly climate and health newsletter The Temperature. He and John Ingold host the weekly Sun-Up podcast on The Temperature topics every Thursday. He is co-author with...