CU scientists shed light on what emerges when | CU Boulder Today

Banner image: A powerful green laser helps visualize aerosol plumes from a toilet when it’s being flushed. (Credits: Patrick Campbell/CU Boulder)

Thanks to new research from CU Boulder, 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 quickly ejected into the air when a public toilet is flushed without lid Now published in Scientific reportsis the first study to directly visualize the resulting aerosol plume and measure the speed and spread of particles within it.

These aerosolized particles are known to carry pathogens and could pose an exposure risk to public restroom users. 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 the same way again,” he said Joan Crimaldi, lead author of the study and professor of civil, environmental and architectural engineering. “By making dramatic visual images 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 flush as designed, 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 toilets and have shown that larger ones can land on surrounding surfaces, but until now, no one understood what these plumes looked like or how they particles were getting there.

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

“People know that toilets emit aerosols, but they haven’t been able to see them,” Crimaldi said. “We show that this thing is a much more energetic and rapidly spreading plume than even people in the know understood.”

The study found that these airborne particles leave quickly, 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-thousandth of a meter) can remain suspended in the air for minutes or more

Bathroom users don’t just have to worry about their own waste. Many other studies have shown that pathogens can persist in the bowl for dozens of washes, increasing the risk of potential exposure.

“The purpose of the toilet is to effectively remove waste from the bowl, but it’s also doing the opposite, which is spraying a lot of content up,” Crimaldi said. “Our lab has created a methodology that provides a basis for improving and mitigating this problem.”

It’s not a waste of time

Crimaldi directs the Laboratory of ecological fluid dynamics at CU Boulder, which specializes in using laser-based instrumentation, dyes and giant fluid tanks to study everything from how odors reach our nostrils to how chemicals move in turbulent bodies of water. The idea of ​​using lab technology to track what happens in the air after you flush a toilet was one of convenience, curiosity, and circumstance.

During a free week last June, fellow teachers Karl Linden i Mark Hernandez from the Environmental Engineering Program, and several graduate students in Crimaldi’s lab joined him to set up and run the experiment. Aaron True, the study’s second author and research associate in Crimaldi’s lab, was instrumental in running 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 into the same area. The steady laser revealed where the airborne 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 commonly seen in American public bathrooms: a lidless unit accompanied by a cylindrical flush mechanism, either manual or automatic, that rises from the rear near the wall, known as a flushmeter-style valve. The clean, new toilet was filled with only tap water.

They knew that this spur-of-the-moment experiment could be a waste of time, but instead, the research made a big impact.

“We had expected these aerosol particles to float, but they took off like a rocket,” Crimaldi said.

The energetic water particles in the air were mainly directed upwards and backwards towards the back wall, but their movement was unpredictable. The plume also rose to the ceiling of the lab and, with nowhere else to go, moved off the wall and spread forward into the room.

The experimental setup did not include solid waste or toilet paper in the bowl, and there were no stalls or people moving around. All of these real-life variables could compound the problem, Crimaldi said.

They also measured the particles in the air with 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 the particles. Smaller particles not only float in the air longer, but can escape the nose hairs and reach deeper into the lungs, making them more dangerous to human health, so it was also important to know how many particles and what size are they

While these results may be puzzling, 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 bathrooms.

“None of these improvements can be made effectively without knowing how the aerosol plume develops and how it moves,” Crimaldi said. “Being able to see that 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.