How many microbes are released when we flush the toilet? Spoiler: better lower the lid


Using bright green lasers and photographic equipment, a team of engineers conducted an experiment to reveal how tiny droplets of water, invisible to the naked eye , are rapidly ejected into the air when you flush an open public toilet. Published in Scientific Reports, it is the first study to directly visualize the resulting aerosol column and measure the velocity and diffusion of particles within it.


These aerosolized particles are known to carry pathogens and could pose an exposure risk to public toilet users. However, this vivid visualization of potential disease exposure also provides a methodology to help reduce it . “If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you watch these videos, you’ll never think of flushing the toilet the same way again,” said John Crimaldi, lead author of the study and a professor of civil, environmental and architectural engineering. “By creating visual images of this process, our study can play an important role in communicating about public health.”

Understanding the trajectories and velocities of these particles – which can carry pathogens such as E. coli, noroviruses, adenoviruses , etc. – is important for reducing the risk of exposure through disinfection and ventilation strategies or improved toilet and drain design . .

The study found that these airborne particles spread rapidly , at a speed of 2 meters per second , reaching 1.5 meters above the toilet bowl in 8 seconds . While the largest droplets tend to settle on surfaces within seconds, the smallest particles (aerosols smaller than 5 microns, or one-millionth of a metre) can remain suspended in the air for minutes or more.



Crimaldi heads CU Boulder’s Ecological Fluid Dynamics Lab, which specializes in using laser instrumentation, dyes, and giant fluid tanks to study everything from how odors reach our nostrils to how chemicals move in turbulent bodies of water.

For the study, they used two lasers: one shone continuously above the toilet, while the other sent rapid pulses of light over the same area. The constant laser revealed the position of airborne particles in space, while the pulsed laser measured their speed and direction . Meanwhile, two cameras took high-resolution images. The toilet was of the type commonly used in North American public restrooms: a lidless unit accompanied by a cylindrical flush mechanism – manual or automatic – that protrudes out the back near the wall, known as a flush valve.


“We expected these aerosol particles to be able to float, but they came out like a rocket,” Crimaldi said.


  •  Commercial toilets emit energetic and rapidly spreading aerosol plumes (


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