The experiment details how aerosols spread when a toilet is flushed.

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As we currently see, aerosols can carry viral particles and thus transmit infectious diseases, including influenza or COVID-19. Similarly, aerosols emitted from toilet flushes can carry pathogens found in faeces. A group of scientists are interested in how these small particles spread through the toilet.

Every time we flush the toilet, thousands of tiny droplets are released into the air. Larger droplets settle in seconds, while smaller aerosols (less than 5 microns in diameter) can remain suspended in the air. Knowing that various disease-causing agents are constantly swarming on the walls and in the water of the toilet (they can persist even after several dozen flushes!), These aerosols pose an increased risk of disease transmission.

The amount and concentration of aerosols emitted, of course, depends on the type of toilet, room ventilation, water pressure, etc. The risks are fortunately reduced (but not zero) if care is taken to lower the bowl lid before flushing. However, most public toilets do not have lids… In an effort to implement an effective preventive strategy to limit the potential exposure of users to disease, a team at the University of Colorado at Boulder set out to study the kinematics of these microparticle jets. with lasers and cameras.


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Important role in public health messages

Scientists have long known that when a toilet is flushed, tiny invisible particles are thrown into the air. But current knowledge of toilet aerosol plumes comes mainly from discrete measurements of airborne and settled particle concentrations. The present study is the first to visualize the resulting aerosol plume in real time and measure the velocity and propagation of particles within it.

“After watching these videos, you will never think about the toilet flush the same way again. By providing dramatic images of this process, our study could play an important role in public health messaging,” said John Crimaldi, who studies the interaction between fluid physics and environmental or biological processes at UC Boulder.

It is indeed important to fully understand the trajectories and velocities of these particles, which could potentially harbor pathogens such as Escherichia coli, Clostridium difficile, noroviruses, or adenoviruses, in order to implement disinfection and ventilation strategies or to improve the design of toilets and flushes.

Experience pattern and tidal intensity over time. © G. Crimaldi et al.

The team used two laser beams: one continuously aimed at and above the toilet, and the other sent fast pulses of light into the same area. The first showed where the suspended particles are, and the second allowed to measure their speed and direction. Two cameras filmed the whole process in high resolution.

Up to 1.5 meters in height in eight seconds

The toilets used in the study were of the same type as those found in public toilets in North America (and Europe): a seat without a lid, accompanied by a manual or automatic flush cylinder located at the back against the wall. The toilets were new, clean and only filled with tap water.

The results of the experiment greatly surprised its instigators: “We expected that these aerosol particles would just float up, but they flew out like a rocket,” says Crimaldi. The images showed that the lightest particles travel very quickly: they can be 1.5 meters above the toilet (or right at face level) in eight seconds. They were heading mostly up and towards the back wall, but their movement was unpredictable; they reached the ceiling of the lab before spreading to the rest of the room.

toilet particle dispersion

Photos of the aerosol plume at t = 2.8, 4.4 and 6.4 seconds after flushing the toilet. © G. Crimaldi et al.

By measuring the amount of these particles with an optical counter, the researchers realized that they could stay in the air for minutes or even longer. However, due to their tiny size, they can easily enter the respiratory tract, deep into the lungs, making them potentially dangerous. “The real problem with fine aerosols is that there’s no good way to get rid of them other than to take all that air and run it through a purification mechanism,” said Evan Inverse.Floyd, an industrial hygienist and environmental health expert at the University of Oklahoma. who did not participate in this study.

On the other hand, larger droplets tend to settle on the surface within a few seconds. But Crimaldi notes that the experience was far from real life: there was no garbage, no toilet paper in a bowl, no people moving nearby – all factors that could affect the number and dispersion of particles.

The researchers hope their research will help improve the design and operation of toilets, plumbing or even ventilation to sanitize crowded buildings such as schools, offices and hospitals, where potentially deadly pathogens can spread rapidly.

J. Crimaldi et al., Scientific Reports.

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