Why do people flush after using urinals

Corona: urinal danger zone?

Often disgusting - but also contagious? It has long been known that flushing toilets releases a whole cloud of aerosols. Now a model simulation shows that urinals are even larger aerosol spinners: their droplets are faster and are thrown higher than with the toilet. The researchers therefore advise wearing mouth and nose protection in public toilets and especially when using urinals.

Public toilets are not exactly a feel-good place per se - and rightly so. Because despite regular cleaning, door handles, taps and other objects are often teeming with germs. According to a study, airport toilets could therefore be downright hotspots for the transmission of multi-resistant bacteria.

Toilet flushing as an aerosol spinner

But the danger is not just lurking on the surfaces: with every flush, turbulence in the toilet bowl releases entire clouds of aerosols. In the worst case, they can also harbor pathogens - for example, if the predecessor did not remove correctly. In the course of the corona pandemic, the question arises whether a transmission of SARS-CoV-2 through public toilets is possible or likely.

A few weeks ago, the first studies showed that infection through the aerosols generated during rinsing cannot be ruled out. "What's even worse is that two Covid-19 cases in Beijing are said to have been infected in a public toilet - which in principle proves that such places represent at least a potential danger," say Ji-Xiang Wang of Yangzhou University and his colleagues .

Recently, researchers also isolated SARS-CoV-2 virus particles from the urine of Covid-19 patients. "That means that urine-based transmission could well represent a route of infection," said Wang and his team.

Urinal aerosols in a physics model

But how acute is the danger? To find out, the scientists studied the spread of aerosol in urinals - the facilities most used by men in public toilets. "Similar to flushing the toilet, flushing in the urinal leads to interactions between liquid and air," explain Wang and his team. How far the droplets fly and how high has hardly been investigated so far.

For their study, the researchers simulated the interactions between water and air in a physical model. “We used a method from computer-aided fluid dynamics with which we were able to model the movements of particles during rinsing,” explains Wang's colleague Xiangdong Liu.

"Alarming upward current"

The result: the aerosol cloud created when flushing in the urinal spreads more than expected. As the model revealed, strong eddies form at the bottom of the basin, creating an ascending suction. This pulls up droplets of urine and water with it. "Our results show that the turbulence creates an alarming upward current," say the researchers.

As a result, more than 57 percent of the aerosol droplets could get out of the urinal when flushing - and this at high speed: According to the simulation, the particles rise at a brief maximum speed of up to 4.6 meters per second. You can reach the height of your hands or even your face in seconds.

“The aerosols in the urinal have a much greater tendency to climb than in a toilet - and they are much faster,” explains Liu.

Mask on in public toilets

According to the researchers, public urinals could therefore be a potential source of infection for Covid-19. “Our work shows that flushing the urinal encourages the spread of bacteria and viruses in the room,” says Liu. And SARS-CoV-2 is known to be spread via droplets and aerosols.

He and his colleagues therefore recommend wearing a mask in public toilets. "In addition, urgent measures should be taken to minimize the diffusion of aerosols when flushing," the researchers said. (Physics of Fluids, 2020; doi: 10.1063 / 5.0021450)

Source: American Institute of Physics

20th August 2020

- Nadja Podbregar