CU scientists shed light on what happens when you flush

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

Thanks to new research from CU Boulder, scientists are seeing the impact of 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 public toilet without a cover is flushed. Now published in Scientific reportsthis is the first study to directly visualize the resulting aerosol plume and measure the velocity and propagation of particles within it.

These aerosolized particles are known to carry pathogens and could pose an exposure risk for patrons of public restrooms. 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 see these videos, you’ll never think of a toilet flush the same way again,” said John Crimeldi, the study’s lead author and professor of civil, environmental and architectural engineering. “By creating dramatic visual images of this process, our study can play an important role in public health messaging.”

Researchers have known for over 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 airborne particles above flushing toilets and have shown that larger ones can land on surrounding surfaces, but so far , no one understood what these plumes looked like or how the particles got there.

Understanding the trajectories and velocities of these particles – which can carry pathogens such as E. coli, C. difficile, noroviruses and adenoviruses – is important for mitigating the risk of exposure through disinfection and ventilation strategies. , or improved design of toilets and flushes. Although the virus that causes COVID-19 (SARS-CoV-2) is present in human waste, there is currently no conclusive evidence that it is effectively spread through toilet aerosols.

“People knew the toilets were emitting aerosols, but they couldn’t see them,” Crimeldi said. “We’re showing this thing to be a much more energetic, fast-spreading plume than even people who were aware of it realized.”

The study found that these airborne particles shoot out rapidly, at speeds of 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) above the toilet in 8 seconds. While larger droplets tend to settle on surfaces within seconds, smaller particles (aerosols less 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 restroom customers have to worry about. Numerous other studies have shown that pathogens can persist in the toilet bowl for dozens of hot flashes, increasing the risk of potential exposure.

“The purpose of the toilet is to effectively remove waste from the bowl, but it’s also the opposite, which sprays a lot of the contents upwards,” Crimeldi said. “Our lab has created a methodology that provides a foundation for improving and mitigating this issue.”

Not a waste of time

Crimeldi directs the Ecological Fluid Dynamics Laboratory at CU Boulder, which specializes in using laser-based instruments, dyes, and giant fluid reservoirs to study everything from how odors reach our nostrils to how chemicals move through turbulent bodies of water. The idea of ​​using lab technology to track what happens in the air after the toilet is flushed was a matter of convenience, curiosity and circumstance.

During a free week last June, fellow professors Karl Linden and Mark Hernandez from the environmental engineering program, along with several graduate students from Crimaldi’s lab joined him to set up and run the experiment. Aaron True, the study’s second author and a research associate in Crimeldi’s lab, was instrumental in performing and recording the laser measurements for the study.

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

The toilet itself was of the same type commonly seen in North American public restrooms: a lidless unit accompanied by a cylindrical flushing mechanism, either manual or automatic, which stands at the back close to the wall, known as a flush type valve. The brand new and clean toilets were only filled with tap water.

They knew this impulsive experiment might be a waste of time, but instead the research caused a stir.

“We expected these aerosol particles to float somehow, but they came out like a rocket,” Crimeldi said.

The airborne energetic water particles were mostly moving up and back toward the back wall, but their movement was unpredictable. The plume also rose to the ceiling of the lab and, having nowhere to go, moved out 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-life variables could all make the problem worse, Crimeldi said.

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

Although these results may be disconcerting, the study provides plumbing and public health experts with a consistent way to test improved plumbing design and disinfection and ventilation strategies to reduce the risk of exposure to agents. pathogens in public toilets.

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

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