High-powered lasers illuminate toilets with invisible aerosol plumes with every flush

John Crimaldi is a professor of civil, environmental, and architectural engineering at the University of Colorado, Boulder.

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Every time you flush a toilet, it releases plumes of tiny water droplets into the air around you. These drops, called aerosol plumesit 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 usually invisible. My colleagues Aaron True, Karl Linden, Mark HernandezLars Larson and Anna Pauls and I were able to use high power lasers to illuminate these plumeswhich allows us to image and measure the location and movement of aerosol plumes being dispersed from commercial bathroom cisterns in great detail.

Go up instead of down

Toilets are designed to efficiently empty the contents into the bowl through a downward movement into the drain pipe. In the wash cycle, the water comes into forceful contact with the contents inside the bowl and creates a fine spray of suspended particles in the air.

We found that a typical commercial toilet generates a strong upward jet of air with velocities exceeding 2 meters per second (6.6 feet per second), rapidly transporting these particles up to 1.5 meters (5 feet) above the cup in eight seconds after the start of the flush.

To visualize these plumes, we rigged a typical lidless commercial toilet with one flushometer style valve found throughout North America in our lab. Flowmeter 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 electrical 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 that ejected the particles from the bowl.

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

Why laser?

Scientists know it for decades that flushing toilets can release aerosol particles into the air. Despite this, experimental studies have relied heavily on devices that sample the air at fixed locations to determine the number and size of particles produced by toilets.

While these previous 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 critical to developing strategies to mitigate the formation of aerosol plumes and reduce their ability to transmit disease.

As in engineering professor whose research focuses on interactions between fluid physics and ecological or biological processes, my laboratory specializes in using lasers to determine what various things look like transported by complex fluid flows. In many cases, these things are invisible until we shine a laser on them.

An advantage of using laser light to measure fluid flows is that, unlike a physical probe, the light does not alter or disrupt the very thing you are trying to measure. Also, using lasers to make invisible things visible helps people, like visual creaturesbetter understand the complexities of the fluid environment in which they live.

Aerosols and diseases

Aerosol particles containing pathogens are important vectors of human diseases. Smaller particles that remain suspended in the air for a period of time can expose people to respiratory diseases such as flu and COVID-1 through inhalation. Larger particles that quickly settle on surfaces can spread intestinal diseases like norovirus by contact with hands and mouth.

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

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.

Limiting the spread of toilet plume

Our experimental methodology provides a basis for future work to test a range of strategies to minimize the risk of disease exposure from toilet flushing. This could include evaluating changes in aerosol plumes emanating from new toilet bowl designs or flush valves that change the length or intensity of the flush cycle.

In the meantime, there are ways to reduce human exposure to toilet plumes. An obvious strategy is to do so close the lid before washing. However, this does not completely eliminate aerosol plumes, and many toilets in public, commercial and health care settings do not have lids. Ventilation or UV disinfection The systems could also mitigate exposure to aerosol plumes in the bathroom.

This article is republished from the conversation under a Creative Commons license.