John Crimeldi is a professor of civil, environmental and architectural engineering at the University of Colorado at Boulder.
Each time you flush the toilet, it releases plumes of tiny water droplets into the air around you. These droplets, called aerosol plumes, can spread pathogens from human waste and expose people in public restrooms to contagious diseases.
Scientific understanding of the spread of aerosol plumes – and public awareness of their existence – has been hampered by the fact that they are normally invisible. My colleagues Aaron True, Karl Linden, Mark Hernandez, Lars Larson and Anna Pauls and I were able to use high powered lasers to illuminate these plumes, allowing us to image and measure location and movement of spreading aerosol plumes from commercial toilet flushes in vivid detail.
Go up instead of down
Toilets are designed to efficiently empty the contents inside the bowl by a downward motion in the drain pipe. In the flush cycle, the water comes into forceful contact with the contents inside the bowl and creates a fine spray of airborne particles.
We have found that a typical commercial toilet generates a powerful upward jet of air with velocities in excess of 6.6 feet per second (2 meters per second), rapidly transporting these particles up to 5 feet (1.5 meters) above the toilet bowl within eight seconds of the start of the flush.
To visualize these plumes, we installed in our lab a typical coverless commercial toilet with a flush-type valve found throughout North America. Flushometer valves use pressure instead of gravity to direct water into the bowl. We used special optics to create a thin vertical sheet of laser light that illuminated the region from the top of the bowl to the ceiling. After flushing the toilet with a remote electric trigger, the aerosol particles scatter enough laser light to become visible, allowing us to use cameras to image the particle plume.
Although we expected to see these particles, we were still surprised by the force of the jet ejecting the particles from the bowl.
A related study used a computer model of an idealized toilet to predict the formation of aerosol plumes, with upward transport of particles at velocities above the toilet bowl approaching 3.3 feet per second (1 meter per second ), which is about half of what we observed. with a real toilet.
Scientists have known for decades that toilet flushing can release aerosol particles into the air. However, experimental studies have largely relied on devices that sample the air at fixed locations to determine the number and size of particles produced by toilets.
While these earlier approaches can confirm the presence of aerosols, they provide little information about the physics of plumes: what they look like, how they spread, and how fast they move. This information is essential for developing strategies to mitigate the formation of aerosol plumes and reduce their ability to transmit disease.
As an engineering professor whose research focuses on the interactions between fluid physics and ecological or biological processes, my lab specializes in using lasers to determine how various things are transported by complex fluid flows. In many cases, these things are invisible until we illuminate them with lasers.
One advantage of using laser light to measure fluid flow rates is that unlike a physical probe, the light does not alter or disturb the very thing you are trying to measure. Additionally, using lasers to make invisible things visible helps people, as visual creatures, better understand the complexities of the fluid environment in which they live.
Aerosols and diseases
Aerosol particles containing pathogens are important vectors of human disease. Smaller particles that remain airborne for a period of time can expose people to respiratory illnesses like influenza and COVID-19 through inhalation. Larger particles that settle quickly on surfaces can spread intestinal diseases like norovirus through contact with hands and mouth.
Toilet bowl water contaminated with feces can have concentrations of pathogens that persist after dozens of flushes. But the question remains open whether toilet aerosol plumes pose a transmission risk.
Although we were able to visually and quantitatively describe how aerosol plumes move and disperse, our work does not directly address how toilet plumes transmit disease, and this remains an ongoing aspect of research.
Limit the spread of the toilet plume
Our experimental methodology provides a foundation for future work aimed at testing a range of strategies to minimize the risk of exposure to disease from flushing toilets. This could include assessing changes to aerosol plumes emanating from new designs of toilet bowls or flush valves that alter the duration or intensity of the flush cycle.
Meanwhile, there are ways to reduce human exposure to toilet plumes. An obvious strategy is to close the lid before rinsing. However, this does not completely eliminate aerosol plumes, and many toilets in public, commercial, and healthcare facilities are uncovered. Ventilation or UV disinfection systems could also mitigate exposure to aerosol plumes in the bathroom.
This article is republished from The conversation under Creative Commons license.
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