Toilets spit out invisible aerosols every time you flush

Make you think twice about flushing while seated … here is an analysis by John Crimaldi, professor at the University of Colorado Boulder.

ANALYSIS – This is the action that concludes any visit to the toilet: you activate the flush and this one, by filling the bowl with a crash, releases plumes of tiny droplets of water into the ambient air without your knowledge. Droplets that may seem innocuous… But these aerosols (fine particles, solid or liquid, suspended in the air) can in fact spread pathogens – depending, among other things, on what was in the bowl.< /p>

In public toilets, this means that people passing through will potentially be exposed to contagious diseases (or bring their share of pathogens)…

However, while numerous studies have established that most contamination involve transfer of bacteria and other pathogens to the mouth via the hands, equivalent research on the risks associated with aerosols is lacking when it has been known for decades that toilet flushes can release particles into the air.

Scientific understanding of the spread of these aerosol plumes – and public awareness of their existence – has been hampered by the fact that they are invisible. We have fixed this issue.

My colleagues Aaron True, Karl Linden, Mark Hernandez, Lars Larson, Anna Pauls and I used high power lasers to illuminate them. A spotlight that allowed us to image and measure the location and movement of aerosol plumes spreading from commercial flush toilets in stunning detail.

This video compares the visibility of an aerosol plume after a flush without and with lasers in a lab.

From models to concrete results

Toilets are designed to efficiently empty the contents of the bowl by a downward movement of water to the drain pipe. The water from the activated flush therefore comes into violent contact with the said content to repel it… which in reaction creates a diffuse projection of particles which will remain suspended in the air.

We have found that a typical commercial toilet causes a strong upward chaotic air projection at speeds exceeding 2 meters per second. Within eight seconds of the start of the flush, the particles generated (particularly from our faeces, etc.) are transported up to 1.5 meters above the bowl.

Water is flowing forcefully into the toilet bowl during a flush cycle. SouthHamsian/Wikimedia, CC BY-NC-SA

To visualize these plumes, we installed a typical coverless commercial toilet in our lab, fitted with a flush-type valve found throughout North America. The valves present use pressure instead of gravity to direct water into the bowl. In order not to create any disturbance by our presence, we have installed a mechanism to trigger the flush electrically from a distance.

We used special optics to create a thin vertical “sheet” of laser light to illuminate the area from the top of the bowl to the ceiling. As the aerosol particles scatter enough laser light to become visible with this type of device, we were able to use cameras to visualize the plume they would form.

Even though we expected to see these particles , we were surprised by the force of the jet ejecting them from the bowl.

A related study had used a computational model of an idealized toilet to predict the formation of aerosol plumes. But, with upward particle transport at above-the-bowl velocities close to one meter per second, it was only about half of what we observed with a real toilet.

Why lasers?

To date, experimental studies have largely relied on devices that sample the air at fixed locations to determine the number and size of particles produced by the toilet.

While these approaches could indicate the presence of aerosols, they provided little information about the physics of the plumes generated: what they look like, how they spread, and how fast they move. This information is essential for developing strategies to mitigate their formation and reduce their ability to transmit disease. So the question is not trivial…

This video shows Aaron True monitoring live image data of a flushing plume on a computer screen.

As an engineering professor, my 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 streams. In many cases, these elements are invisible until we illuminate them with lasers.

One advantage of using laser light to measure fluid flows is that unlike a physical probe, the light does not alter or disturb the thing you are trying to measure. Also, using lasers to make invisible things visible helps us, who rely so heavily on our visual system, better understand the complexities of the fluid environment in which we move.

Aerosols and diseases

Aerosol particles containing pathogens are important vectors of human disease, mainly in two ways:

– Small particles that remain airborne for a period of time can expose people through inhalation to respiratory illnesses – like the flu and Covid-19 to be in the news.

– Larger particles that settle quickly on surfaces can spread intestinal diseases (diarrhea, vomiting, etc. caused by norovirus in particular) through contact with the hands and mouth.

Toilet water contaminated with feces can have concentrations of pathogens that persist after dozens of flushes. But the complementary question of whether toilet aerosols pose a strong risk of transmission remains open.

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: this is an area of ​​ongoing research.

Limit the spread of the potentially contaminated plume

Our experimental methodology and results (quantification of toilet plumes and associated flow velocities), however, provide a basis for future work to test strategies that will minimize the risk of exposure to diseases spread by toilet flushing. toilet water. This could include assessing changes in aerosol plumes emanating from new designs of specially designed toilet bowls or flush valves that change the length or intensity of the flush cycle.

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In the meantime, there are already ways to reduce our exposure to these invisible, charged plumes… An obvious strategy is to close the lid before flushing. However, this does not completely eliminate aerosol fumes. Additionally, many restrooms in public, commercial, and healthcare facilities are not equipped with covers.

Ventilation or UV disinfection systems could also mitigate exposure to aerosol plumes. Our data could also be useful in helping to disinfect the pathogens they contain.

John Crimeldi, Professor of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder

This article is republished from The Conversation under a Creative Commons license. Read the original article.