(SciTechDaily) Thanks to new research, scientists see the impact of flushing the toilet in a whole new light—and now, the world can as well.Engineers ran an experiment to reveal how tiny water droplets, invisible to the naked eye, are rapidly ejected into the air when a lid-less, public restroom toilet is flushed.
The team of scientists, from the University of Colorado Boulder (CU Boulder), used bright green lasers and camera equipment to capture the stunning videos. It is the first study to directly visualize the resulting aerosol plume and measure the speed and spread of particles within it. It was published on December 8 in the journal Scientific Reports.
These aerosolized particles are known to transport pathogens and could pose an exposure risk to public bathroom patrons. However, this vivid visualization of potential exposure to disease also provides a methodology to help reduce it.
A powerful green laser helps visualize the aerosol plumes from a toilet while it’s being flushed. Credit: John Crimaldi
“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’re never going to think about a toilet flush the same way again,” said John Crimaldi, lead author on 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 over 60 years that when a toilet is flushed, solids and liquids go down as designed, 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 flushed toilets and shown that larger ones can land on surrounding surfaces, but until now, no one understood what these plumes looked like or how the particles got there.
On the left, nothing is visible to the naked eye. On the right, a powerful green laser helps visualize the aerosol plumes from a toilet while it’s being flushed. Credit: John Crimaldi
Understanding the trajectories and velocities of these particles—which can transport pathogens such as E. coli, C. difficile, noroviruses, and adenoviruses—is important for mitigating exposure risk 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 not currently conclusive evidence that it spreads efficiently through toilet aerosols.
“People have known that toilets emit aerosols, but they haven’t been able to see them,” said Crimaldi. “We show that this thing is a much more energetic and rapidly spreading plume than even the people who knew about this understood.”
A powerful green laser helps visualize the aerosol plumes from a toilet while it’s being flushed. Credit: Patrick Campbell / University of Colorado Boulder
The study found that these airborne particles shoot out quickly, at speeds of 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) above the toilet within 8 seconds. While the largest droplets tend to settle onto surfaces within seconds, the 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 only their own waste that bathroom patrons have to worry about. Many other studies have shown that pathogens can persist in the bowl for dozens of flushes, increasing potential exposure risk.
“The goal of the toilet is to effectively remove waste from the bowl, but it’s also doing the opposite, which is spraying a lot of contents upwards,” said Crimaldi. “Our lab has created a methodology that provides a foundation for improving and mitigating this problem.”
A powerful green laser helps visualize the aerosol plumes from a toilet while it’s being flushed. Credit: John Crimaldi
Not a waste of time
Crimaldi runs the Ecological Fluid Dynamics Lab 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 to use the lab’s 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 Karl Linden and Mark Hernandez of the Environmental Engineering Program, and several graduate students from Crimaldi’s lab joined him to set up and run the experiment.
They used two lasers: One shone continuously on and above the toilet, while the other sent out fast pulses of light over the same area. The constant laser revealed where in space the airborne particles were, while the pulsing laser could measure their speed and direction. Meanwhile, two cameras took high-resolution images.
