Science Shows How Flushing a Public Toilet Blasts Particles Everywhere

Here's proof that flushing a public toilet is truly disgusting.

Amanda Kooser
Freelance writer Amanda C. Kooser covers gadgets and tech news with a twist for CNET. When not wallowing in weird gear and iPad apps for cats, she can be found tinkering with her 1956 DeSoto.
Amanda Kooser
2 min read
Toilet glowing in green light as droplets of water are caught shining like someone doing a spit take out of the bowl.

Ugh. Green laser light shows the aerosols spewed out by an open-topped toilet when it's flushed.

Video screenshot by Amanda Kooser/CNET

Engineers at the University of Colorado Boulder are revealing the secret lives of public toilets. Those ubiquitous, unlidded, industrial restroom fixtures can spew out aerosolized water and waste droplets with the same gusto as a comedic actor doing a spit-take. Yeah, it's pretty gross.

Blasting aerosols are a health concern since they can spread pathogens like E. coli or noroviruses, contagious viruses that can cause diarrhea and vomiting. Those pathogens can also hang around in a toilet bowl, potentially impacting multiple users. 

We've known that unlidded toilet flushes can cough up a lot of liquid, but the engineers figured out how to visualize it in a way that will sear itself into your mind and then resurface every time you step into a public restroom. They used lasers and cameras to capture and measure the movement of droplets and aerosols. Steel yourself and give it a watch:

"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 CU Boulder engineer John Crimaldi, lead author of a study published this week in Scientific Reports. The paper is titled "Commercial toilets emit energetic and rapidly spreading aerosol plumes," which is exactly what you see in the video.

The study involved a commercial toilet like the ones often found in public restrooms in North America. The researchers used only tap water and didn't include toilet paper or solid waste.

"We had expected these aerosol particles would just sort of float up, but they came out like a rocket," Crimaldi said. The video footage resembles a rocket firing up, though the actual particle speeds were measured at 6.6 feet (2 meters) per second while reaching 4.9 feet (1.5 meters) above the toilet within eight seconds of flushing.

Three images show the progression of aerosols emitted from a flushing toilet with increasing amounts spreading through the air over about six seconds of time.
Enlarge Image
Three images show the progression of aerosols emitted from a flushing toilet with increasing amounts spreading through the air over about six seconds of time.

This progression shows a sequence of aerosols emitted by a flushing toilet.

John Crimaldi/CU Boulder/Scientific Reports

The particles made their way through the air, reached the ceiling of the lab and even spread out into the room. While larger particles can settle quickly, smaller particles tend to float for longer.

The study will surely give you inspiration to shut your toilet lid at home, which will slow but not stop the explosion of aerosols. But it shines a light on the issue of public restrooms where the loos have no lids. 

This isn't just a horror story. Good things could come from this. The researchers say the laser method could be used to test new toilet designs aimed at reducing the aerosol spew problem. It could also spur innovation in disinfection and ventilation approaches. Said Crimaldi, "Being able to see this invisible plume is a game changer."