Banner photo: A powerful green laser helps visualize aerosol plumes from a toilet as it is flushed. (Credit: Patrick Campbell/CU Boulder)
Thanks to new CU Boulder research, scientists are seeing 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 rapidly ejected into the air when a lidless public toilet is flushed. 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 danger to visitors to public baths. 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’s not there. But once you watch these videos, you’ll never think about toilet flushing the same way again.” John Crimaldi, lead author of the study and professor of civil, environmental, and architectural engineering. “By making exciting visuals of this process, our study can play an important role in public health messaging.”
Researchers have known for more than 60 years that when a toilet is flushed, solids and liquids fall out according to design, but small, invisible particles are also released into the air. Previous studies have used scientific tools to detect the presence of these airborne particles 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 adenoviruses—is important for mitigating exposure risks through disinfection and ventilation strategies, or improved toilet and flush design. While the virus that causes COVID-19 (SARS-CoV-2) is present in human waste, there is currently no conclusive evidence that it spreads efficiently through toilet spray.
“People knew toilets gave off spray, but they couldn’t see it,” Crimaldi said. “We show that this thing is a much more active and spreading column than even people who knew about this concept.”
The study found that these airborne particles travel rapidly, at 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) above the toilet within 8 seconds. While larger droplets tend to settle to surfaces within seconds, smaller particles (aerosols less than 5 microns, or millionths of a meter) can remain suspended in the air for minutes or longer.
It’s not just their own waste that bathroom-goers have to worry about. Several other studies have shown that pathogens can persist in a vessel for dozens of flashes, increasing the risk of potential exposure.
“The goal of a toilet is to effectively remove waste from the bowl, but it also does the opposite, which is to spray a lot of the contents to the top,” said Crimaldi. “Our lab has created a methodology that provides a basis for improving and mitigating this problem.”
Not a waste of time
Crimaldi runs a file Environmental Fluid Dynamics Lab at CU Boulder, which specializes in using laser-based devices, dyes, and giant tanks of fluid to study everything from How do odors reach our nose? of how chemicals move in turbulent bodies of water. The idea of using lab 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 Carl Linden And the Mark Hernandez from the Environmental Engineering program, and several graduate students from the Crimaldi lab joined him to set up and run the experiment. Aaron True, second author of the study and research partner in the Crimaldi lab, was instrumental in running 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 fast pulses of light over the same area. The stationary laser detected where airborne particles were located in space, while the pulsating laser could measure their speed and direction. Meanwhile, two cameras took high-resolution photos.
The toilet itself was of the same type typically seen in public restrooms in North America: a lidless unit accompanied by a cylindrical flushing mechanism—either manual or automatic—that recessed at the back close to the wall, known as a flushmeter-style valve. The new clean toilet was filled with tap water only.
They knew this sudden experiment could be a waste of time, but instead, the research created a huge boost.
“We were expecting these aerosols to kind of float, but they came out like a rocket,” Crimaldi said.
The energetic, airborne water molecules headed mostly up and back toward the back wall, but their motion was unpredictable. The shaft also rose to the ceiling of the laboratory, and with nowhere else to go, it moved outward from the wall and spread forward, into the room.
The experimental setup included 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, Crimaldi said.
They also measured airborne particles using an optical particle counter, a device that sucks in a sample of air through a small tube and shines a light on it, allowing it to count and measure particles. Not only do smaller particles float in the air longer, but they can also escape from nasal hairs and reach deeper into one’s lungs—making them more dangerous to human health—so knowing the number and size of the particles was also important.
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, in order 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 evolves and how it moves,” said Crimaldi. “Being able to see this invisible pillar is a game changer.”
Additional authors on 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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