CU researchers shed light on what happens when you float

Banner photo: A powerful green laser helps visualize aerosol plumes from a toilet as it flushes. (Credit: Patrick Campbell/CU Boulder)

Thanks to new CU Boulder research, scientists see the effect of toilet flushing in a whole new light – and now the world can too.

Using bright green lasers and camera equipment, a team of CU Boulder engineers conducted an experiment to reveal how tiny water droplets, invisible to the naked eye, are quickly flushed into the air when a public toilet is flushed without a lid. It is now published in Scientific reportsIt is the first study to directly visualize the resulting aerosol column and measure the velocity and diffusion of particles within it.

These volatile particles are known to transmit pathogens and can pose a hazard to visitors to public baths. However, this vivid visualization of potential exposure to disease also provides a method to reduce it.

“If it’s something you can’t see, it’s easy to pretend it’s not there. But once you’ve seen these videos, you’ll never think about flushing the toilet the same way again.” John Criminaldi, lead author of the study and professor of civil, environmental and structural engineering. “By creating exciting images of this process, our research can play an important role in public health messages.”

Scientists have known for more than 60 years that when a toilet is flushed, solids and liquids fall out, but small, invisible particles are also released into the air. Previous studies have used scientific tools to detect the presence of these particles in the air above flushing toilets and have shown that larger particles can land on surrounding surfaces, but until now no one understood what these plumes looked like or how the particles got there .

Understanding the trajectories and velocities of these particles – which can transmit pathogens such as Escherichia coli, Clostridium difficile, norovirus and adenovirus – is important for reducing exposure risks through disinfection and ventilation strategies or improved toilet and flushing design. Although the virus that causes COVID-19 (SARS-CoV-2) is found in human waste, there is currently no conclusive evidence that it is effectively spread through toilet spray.

“People knew that toilets release spray, but they couldn’t see it,” Criminaldi said. “We’re showing that this thing is a much more active and widespread column than even people who knew about this concept.”

The study found that these airborne particles travel quickly at a speed of 2 meters per second and reach 1.5 meters above the toilet within 8 seconds. While larger droplets tend to settle on surfaces within seconds, smaller particles (aerosols smaller than 5 microns or one millionth of a meter) can remain suspended in the air for minutes or longer.

It’s not just their own waste that bathroom visitors have to worry about. Several other studies have shown that pathogens can remain in a vessel for dozens of flashes, increasing the risk of potential exposure.

“The purpose of a toilet is to effectively remove waste from the bowl, but it also does the opposite, which is to push out a large part of the contents,” says Criminaldi. “Our laboratory has developed a method that provides a basis for improving and alleviating this problem.”

Above: Aaron True, postdoctoral researcher (left) and John Criminaldi posed with the device. Below: A powerful green laser helps visualize aerosol plumes from a toilet as it flushes. (Credit: Patrick Campbell/CU Boulder)

No time wasted

Criminaldi runs an Environmental Fluid Dynamics Lab at CU Boulder, which specializes in using laser-based devices, dyes and giant liquid tanks to study everything from How do our noses smell? of how chemicals move in turbulent water masses. The idea of ​​using laboratory technology to track what happens in the air after a toilet is flushed was one of convenience, curiosity and circumstance.

During a free week last June, fellow professors Karel Linden En de Mark Hernández from the environmental engineering program and several graduate students from the Criminaldi lab joined him to set up and run the experiment. Aaron True, co-author of the study and research partner in the Criminaldi laboratory, was instrumental in performing and recording the laser-based measurements for the study.

They used two lasers: one that shone continuously over and above the toilet, and the other that sent rapid pulses of light over the same area. The stationary laser detected where floating particles were in space, while the pulsed laser could measure their speed and direction. Meanwhile, two cameras took high-resolution images.

The toilet itself was of the same type typically seen in public toilets in North America: a lidless unit accompanied by a cylindrical flushing mechanism—manual or automatic—recessed at the back close to the wall, known as a flush meter-like called valve. The new clean toilet was only filled with tap water.

They knew that this sudden experiment could be a waste of time, but instead the research gave a huge boost.

“We expected these aerosols to float, but they came out like a rocket,” Criminaldi said.

The energetic airborne water molecules normally moved up and back to the back wall, but their movement was unpredictable. The shaft also rose to the ceiling of the laboratory, and with nowhere else to go, it moved out from the wall and spread out into the room.

The experimental setup contained no solid waste or toilet paper in the bowl, and there were no stalls or people moving around. These real-world variables can all exacerbate the problem, Criminaldi said.

They also measured particles in the air using an optical particle counter, a device that sucks a sample of air through a tube and shines a light on it so it can count and measure particles. Smaller particles not only float longer in the air, but they can also escape the nasal hairs and penetrate deeper into the lungs, making them more dangerous to human health. So it was also important to know the number and size of the particles.

While these findings may be alarming, the study provides plumbing and public health experts with a consistent way to test improved plumbing design, disinfection, and ventilation strategies to reduce the risk of exposure to pathogens in public restrooms.

“None of these improvements can be made effectively without knowing how the aerosol column develops and how it moves,” Criminaldi said. “Being able to see this invisible pillar is a game changer.”

Additional authors of this publication include: Aaron True, Carl Linden, Mark Hernandez, Lars Larsson and Anna Pauls from the Department of Civil, Environmental and Architectural Engineering.

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