Lasers catch aerosols that explode out of flushing toilets

Lasers catch aerosols that explode out of flushing toilets

This article originally appeared on The Conversation.

Every time you flush a toilet it releases tiny water droplets into your environment. These droplets, aerosol plumps ,, can spread pathogens from human waste to people using public restrooms.

The fact that aerosol plumes are invisible has hindered scientific understanding and public awareness. My colleagues Aaron True, Karl Linden, Mark Hernandez, Lars Larson and Anna Pauls and I were able to use high-power lasers to illuminate these plumes, enabling us to image and measure the location and motion of spreading aerosol plumes from flushing commercial toilets in vivid detail.

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Going up instead of down

Toilets can be used to empty the contents of the bowl by moving down into the drain pipe. The flush cycle is when water makes contact with the bowl contents and creates a fine spray suspended in air.

We found that a typical commercial toilet produces a strong upward jet, with velocities exceedingly high at 6.6 feet per sec (2 meters per sec), quickly carrying these particles upto 5 feet (1.5m) above the bowl within eight second of the flush.

To visualize these plumes, we set up a typical lidless commercial toilet with a flushometer-style valve found throughout North America in our lab. Flushometer valves direct water into the bowl using pressure instead of gravity. Special optics were used to create a thin, vertical beam of laser light that illuminated the area from the top of bowl to ceiling. After flushing the toilet with an electrical trigger, the aerosol particles scattered enough laser light to make it visible. We can then use cameras to image this plume.

We expected to see these particles but were still astonished by the force of the jet that ejected them from the bowl.

A related study used a computational model of an idealized toilet to predict the formation of aerosol plumes, with an upward transport of particles at speeds above the bowl approaching 3.3 feet per second (1 meter per second), which is about half of what we observed with a real toilet.

Why lasers?

Scientists have known for decades that flushing toilets can release aerosol particles into the air. However, experimental studies have largely relied on devices that sampled the air at fixed locations to determine the number and size of particles toilets produce.

While these earlier methods can confirm the existence of aerosols they do not provide much information about the physics behind the plumes, such as how they spread, what they look like and how fast they move. This information is crucial to develop strategies to reduce the formation of aerosols and their ability to transmit disease.

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As an engineering professor whose research focuses on interactions between fluid physics and ecological or biological processes, my laboratory specializes in using lasers to determine how various things are transported by complex fluid flows. These things are often invisible until we use lasers to illuminate them.

The advantage of using laser light for fluid flow measurement is that unlike a probe, the light doesn’t alter or disrupt the object you are measuring. Furthermore, using lasers to make invisible things visible helps people, as visual creatures, better understand complexities in the fluid environment they live in.

Aerosols and disease

Aerosol particles containing pathogens are important human disease vectors. Smaller particles that remain suspended in air for a period of time can expose people to respiratory diseases like influenza and COVID-19 through inhalation. Larger particles that settle quickly on surfaces can spread intestinal diseases like norovirus through contact with the hands and mouth.

Toilet bowl water contaminated by feces can have pathogen concentrations that persist after dozens of flushes. But it is still an open question as to whether toilet aerosol plumes present a transmission risk.

While we were able to visually and quantitatively describe how aerosol plumes move, disperse and move, our research does not address how toilet plumes transmit diseases. This remains an ongoing area of research.

Limiting toilet plume spread

Our experimental methodology provides a foundation to future work to evaluate a variety of strategies to reduce the risk of getting ill from flushing toilets. This could include testing aerosol plumes from new toilet bowl designs and flush valves that alter the length or intensity of the flush cycle.

There are many ways to reduce human exposure. An obvious strategy is to close the lid prior to flushing. This does not eliminate all aerosol plumes. Many toilets in public, commercial, and health care settings don’t have lids. Ventilation or UV disinfection systems could also mitigate exposure to aerosol plumes in the bathroom.

John Crimaldi is funded by the National Science Foundation, National Institutes of Health and the US Army DEVCOM Chemical Biological Center.

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