When a toilet is flushed, many small droplets — including pathogens from the toilet water — are released into the air. American scientists visualized this effect for the first time. “It’s more shocking than expected.”
“When you see these pictures, you will never look at a toilet that flushes the same way again,” said John Credialdi, a physicist at the University of Colorado Boulder in the US. He talks about photos he took with his colleagues and published them last week in Commerce Scientific reports. They reveal something invisible to the naked eye: the cloud of water droplets that ooze from the toilet bowl after flushing.
It’s an effect that scientists have known about for decades. Although toilets were developed to promote hygiene, they also have less fresh aspects. Upon rinsing, the stool disappears from the bowl, but the spilled water also causes the contents to travel into the immediate vicinity in the opposite direction.
This carries risks, because toilet water contains pathogens that originate from feces. These droplets can bind into very small droplets – the so-called aerosols – which visitors to the toilet can then inhale. Especially in public toilets, this can pose a number of health risks.
Like a rocket
However, it was not known exactly how such a cloud would behave. The only knowledge so far has come from simulations and indirect measurements. Reason enough for Criminaldi and his colleagues to install a toilet in their lab last summer. With a special measurement setup — including two lasers that shine on the droplets and cameras that record the position — the researchers were able to visualize the motion of the “passing cloud.”
These movements turned out to be stronger than expected. “We thought the aerosols were just going to float up, but they took off like a rocket,” Credialdi says. Within eight seconds, the drops had reached five feet above the toilet, after which the cloud finally expanded to the ceiling. Where larger droplets fell to the ground or a nearby surface within seconds, small aerosols also appeared to float in the air for minutes. These droplets are particularly dangerous, as they can penetrate deep into the lungs after inhalation.
“A shocking insight,” says Daniel Bohn, a physicist at the University of Amsterdam. He was not involved in the study, but uses similar techniques to investigate the behavior of fluids in his lab. “The images show more aerosols than I expected. This is an important public health note. The images illustrate the problem.”
So awareness was one of the goals, Criminaldi points out. “By creating exciting visuals of this process, our study can play an important role in public health communications.” By the way, the search yielded more than just special images: The team also determined the size and speed of the droplets, and the way they moved. This information could be valuable in improving toilets and hygiene measures, says Criminaldo. “You can’t do this effectively without knowing how the aerosol cloud is evolving and moving.”
The researchers confirm that there was only water in the toilet during the experiment. The presence of feces and toilet paper, as well as people walking around, can exacerbate cloud spread. In addition, the cleaning method is also important: the team used a so-called flush meter, which is common in public toilets in the United States. Unlike the usual tank in the Netherlands, this method uses water pressure (not gravity).
“Maybe the high water pressure makes the problem more serious,” says Boone. “However, even with a ‘Dutch’ toilet, or with low water pressure, it is likely that many small droplets will still be released.”
According to Boone, the mantra is: Always close the lid while cleaning. “An easy but effective solution. There may still be some aerosol slip under the lid, but it will be minimal.”
Corona in the toilet?
Can you get sick from aerosols in the toilet? This question arose regularly during the Corona pandemic in particular. After all, it is known that particles of the virus remain in the feces of infected people – it is not for nothing that nowadays wastewater is used to monitor the spread of corona.
At the moment, it is not clear whether “toilet spray” can also spread Corona. “This chance seems small,” says Daniel Boone. “Although it’s hard to know for sure: we rarely get to know exactly where people are infected.”
Corona is not the only danger. Bacteria and other viruses can stay in the toilet for a long time – sometimes dozens of flushes. In public toilets in particular, there is a chance that pathogens, which originate from the ‘big letter’ of previous toilet users, can be inhaled via aerosols.
Bonn: “Pathogens from faeces, such as Escherichia coli, are not soft. For the sake of general hygiene, it is therefore good to take into account the aerosol cloud, corona pandemic or not.”
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