A powerful green laser helps visualize the aerosol plumes from a toilet when it’s being flushed. (Photo by Patrick Campbell/University of Colorado)

Just like washing your hands and cleaning the bathroom regularly, flushing with the lid closed is a habit that has been addressed from time immemorial. If like most people, however, you’ve refused to heed these instructions, you might want to brace yourself because you are about to learn just how much invisible plume you’ve been slinging around. 

Through a series of experiments, scientists at the University of Colorado at Boulder have demonstrated that flushing with the lid open after doing your business exposes you to droplets of pee, poop, and disease-causing organisms that spurt violently into the air due to the toilet’s turbulent flow. These “plumes” can end up settling on your towels, your soap, and even worse, your toothbrush!

According to Professor John Crimaldi, lead author and professor of civil, environmental, and architectural engineering, “the goal of the toilet is to effectively remove waste from the bowl, but it’s also doing the opposite which is spraying a lot of content upwards.”


To visually demonstrate the droplet’s locations, motions, and speed, the study’s researchers developed a nifty little setup involving strategically placed scientific cameras and two green lasers around a brand new, typical commercial toilet with a cylindrical flushing system. 

While one laser illuminated the area above the toilet, the other laser sent rapid pulses of light over the same area, leaving the camera to take high-resolution images of exactly where the airborne particles were in space. 


The unique setup revealed that aerosol particles released upon flushing are unpredictable, spreading pretty much everywhere and anywhere. The particles headed upwards and then back toward the rear wall. They rose, hitting the lab’s ceiling, then moved out from the wall spreading into the entire room. 

“We had expected these aerosol particles would just sort of float up, but they came out like a rocket,” notes Crimaldi.

A technique called ‘particle image velocimetry’ used to measure the particle’s speed showed that they shot up at speeds of 6.6 feet per second, reaching heights of almost 5 feet above the toilet in just eight seconds. Larger particles tend to settle on nearby surfaces after a short while, that includes people if they hang around long enough, while the smaller ones just linger in the air for many minutes.

While the study’s authors stuck with tap water for the study, they noted that the results could have been much, much worse if a regular old toilet with actual waste and toilet paper had been used. 


Most people might be familiar with the dangers posed by lidless toilets, but this proper visual representation of just how bad things can get should serve as a wake-up call and perhaps inspire a teeny tiny bit of fear in those who still choose to ignore them.  

Countless infectious agents call human poop home, from E. coli to C. difficile, to even the SARS CoV-2 virus. And although there isn’t enough conclusive evidence to show diseases can be spread through these aerosols, their findings still highlight the need to always shut the lid before you flush.

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