New research has revealed the large amounts of minuscule aerosol particles that can be thrown up by a flushed toilet; these droplets have huge potential for carrying bacteria and disease.
We know that pathogens lurk in stagnant water, urine, feces and vomit – the usual inhabitants of public loos. Computer models have previously shown that the act of flushing can send germs a few meters into the surrounding air.
Those nasties could include Ebola, the food poisoning bug norovirus, and even COVID-19, suggest the researchers in this latest study, which tested the spread of aerosolized particles from public toilet flushes.
The research team set up a particle counter placed at various heights next to both a toilet and a urinal in a public restroom. Ambient aerosol levels were measured before and after the experiments.
"After about three hours of tests involving more than 100 flushes, we found a substantial increase in the measured aerosol levels in the ambient environment with the total number of droplets generated in each flushing test ranging up to the tens of thousands," says Siddhartha Verma, a professor in mechanical engineering from Florida Atlantic University.
Aerosols rose as high as 109 centimeters (3.6 feet) above toilet bowls and as high as 69 centimeters (2.3 feet) above urinals during the course of the experiments, and could hang around for as long as 20 seconds. That's in line with studies we've seen previously.
The researchers report a 69.5 percent increase in particles sized 0.3 to 0.5 micrometers, a 209 percent increase for particles sized 0.5 to 1 micrometers, and a 50 percent increase for particles sized 1 to 3 micrometers after flushing.
Several factors influence just how many aerosols are whipped up by toilet flushing – the water pressure in the toilet, the design of the bowl, and the power of the flush itself can all have an effect, according to the new study. Keeping the toilet lid closed (if it has one) can help, though not much as the aerosols may still escape through the gaps, the researchers say.
"Both the toilet and urinal generated large quantities of droplets smaller than three micrometers in size, posing a significant transmission risk if they contain infectious microorganisms," says Verma. "Due to their small size, these droplets can remain suspended for a long time."
In the coronavirus pandemic era there's a particular cause for concern – public restrooms are often small, poorly ventilated, and busy with people, a combination that we know leads to increased risks of SARS-CoV-2 transmission indoors. While there are no confirmed cases of COVID-19 caught from a toilet, the theoretical risk is there.
Improved ventilation could certainly help, the team behind the new research says, with this study carried out in a restroom described as having normal standards of ventilation – and that's something that building owners can look into.
Whatever the state of the airflow in the next public toilet you visit, our advice would be to get out of there as soon as you can – your chances of catching something will be much lower. We're not sure why anyone would be hanging around anyway... but you have been warned.
The research has been published in Physics of Fluids.