CU scientists shine a light on what comes up when you flush | CU Boulder today

CU scientists shine a light on what comes up when you flush | CU Boulder today

Banner image: A powerful green laser helps visualize a toilet’s aerosol plumes as it is flushed. (Credit: Patrick Campbell/CU Boulder)

Thanks to new CU Boulder research, scientists are seeing the impact of flushing the toilet 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 public toilet is flushed without a lid. Now published in Scientific Reportsit is the first study to directly visualize the resulting aerosol plume and measure the speed and dispersion of particles within it.

These aerosol particles are known to carry pathogens and may pose an exposure risk to visitors to public restrooms. However, this vivid visualization of potential exposure to disease also provides a method to help reduce it.

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’ll never think of flushing a toilet the same way again,” he said John Criminaldi, lead author of the study and professor of civil, environmental and structural engineering. “By creating dramatic visual images of this process, our research can play an important role in public health news coverage.”

Researchers have known for more than 60 years that when a toilet is flushed, solids and liquids go down as intended, but tiny, invisible particles are also released into the air. Previous studies have used scientific instruments to detect the presence of these particles in the air above flushed 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 transport pathogens such as E. coli, C. difficile, noroviruses and adenoviruses – is important to reduce exposure risk through disinfection and ventilation strategies, or improved toilet and flush design. Although 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 aerosols.

“People knew that toilets emit aerosols, but they haven’t been able to see them,” says Criminaldi. “We’re showing that this thing is a much more energetic and fast-spreading plume than even the people who knew about it understood.”

The study found that these airborne particles shoot out rapidly, at speeds of 2 meters per second, reaching 1.5 meters above the toilet within 8 seconds. While the largest droplets tend to settle on surfaces within seconds, the smaller particles (aerosols less than 5 microns or one millionth of a meter) can linger in the air for minutes or longer.

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

“The purpose of the toilet is to effectively remove waste from the bowl, but it also does the opposite, which is to squirt a lot of contents up,” says Criminaldi. “Our lab has developed a methodology that provides a foundation for improving and mitigating this problem.”

Aaron True, postdoctoral researcher (left) and John Criminaldi
A powerful green laser helps to visualize a toilet's aerosol plumes

Above: Aaron True, postdoctoral researcher (left) and John Criminaldi pose for a photo with the equipment. Below: A powerful green laser helps visualize a toilet’s aerosol plumes as it is flushed. (Credit: Patrick Campbell/CU Boulder)

No time wasted

Criminaldi runs the Ecological Fluid Dynamics Lab at CU Boulder, which specializes in using laser-based instruments, dyes, and giant fluid tanks to study everything from how smells reach our nostrils how chemicals move in turbulent bodies of water. The idea of ​​using the lab’s technology to track what happens in the air after a toilet is flushed was one of convenience, curiosity and circumstance.

Fellow professors during a week off last June Charles Linden and Mark Hernandez of the Environmental Engineering Program, and several graduate students from Crimeldi’s lab joined him to set up and run the experiment. Aaron True, second author of the study and a research associate in Crimeldi’s lab, was instrumental in performing and recording the laser-based measurements for the study.

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

The toilet itself was of the same kind commonly seen in public restrooms in North America: a lidless unit accompanied by a cylindrical flushing mechanism—manual or automatic—protruding from the back near the wall, known as a flush-meter-like valve. The brand new, clean toilet was only filled with tap water.

They knew this spontaneous experiment could be a waste of time, but instead the research made a big impression.

“We expected these aerosol particles to just float up, but they came out like a rocket,” said Criminaldi.

The energetic airborne water particles usually moved up and back to the back wall, but their movement was unpredictable. The plume also rose to the ceiling of the lab, and with nowhere else to go, it moved out from the wall and spread forward 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-life variables can all exacerbate the problem, Criminaldi said.

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

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

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

Additional authors of this publication are: Aaron True, Karl Linden, Mark Hernandez, Lars Larson and Anna Pauls of the Department of Civil, Environmental and Architectural Engineering.



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