Since the start of the novel coronavirus pandemic, scientists and doctors have been working around the clock to better understand the virus and how it spreads. While the current consensus is that its primary mode of transmission is via droplets that infected people expel by sneezing, coughing or talking, some results point to another possible route.
Studies have found viral matter in the faeces of patients who have tested positive for COVID-19. This raises the possibility that the virus may not only make the respiratory system its home but also survive in the gastrointestinal tract. Further, symptoms like nausea, diarrhoea and vomiting have been identified in patients. So it was just a matter of time before someone asked if toilets could be transmission hotspots for COVID-19.
This is a multi-part question plus it’s loaded. For example, we don’t know the efficacy of virus particles attached to the faecal or gastrointestinal lining nor do we know how long contagions can survive in faecal matter.
But assuming viral content is produced during toilet-based activities, can a basic task like flushing exacerbate the problem?
In a study published last week, a team of Chinese researchers reported the results of some careful flushing experiments. Flushing is inherently a fluid dynamics problem thanks to the copious amounts of water and complicated flows involved. Add to this the presence of air and viral particles, and flushing becomes a hard problem at that – one that researchers in the field call a multi-phase flow problem.
Studying such flows is exceedingly difficult in the laboratory because it requires a level of sophistication and control that can be infeasible to achieve during a pandemic. Thankfully, in fluid dynamics, experiments can be mimicked faithfully on a computer as well.
To simulate a flush, the researchers used the Navier-Stokes equations, which are the Swiss army knife equations of fluid mechanics. First they constructed a model of a toilet bowl of specific make and dimensions on the computer – and then broke it down into a 3D grid. A computer algorithm uses the equations to determine the flow’s properties at each point on the grid.
Some physical details are particularly important such that the equations will have to be modified to account for them. For example, the incoming clean water flows in turbulent fashion, and the subsequent mixing of faecal matter and water is turbulent as well — both conditions that matter to fluid-dynamicists. The motion of viral particles is also influenced by the motion of air and water around it, and needs to be carefully tracked.
Once the researchers made all these modifications, they began the experiment with the press of a button. It’s pretty much like flushing a toilet by turning a knob except the events don’t happen in real-time and the results are prettier than the real deal.
When you flush, water enters from a tank and mixes with the contents of the toilet bowl even as it flows down the drain pipe. The vigorous sloshing of liquid also perturbs the air above it. This generates complicated airflow patterns and currents that refuse to die down even as most of the water exits the toilet bowl.
The researchers’ simulations showed that the speed and forces generated by the airflow are enough to expel viral particles from the bowl upwards, from where they can diffuse and spread. This upward spurt of air containing viral particles from a toilet bowl is called a toilet plume — like ash-laden gas rising from a volcano.
The role of toilets and toilet plumes in spreading infectious diseases is a subject of great interest. A specific outbreak of SARS in 2003 is believed to have been helped along by airborne viral particles originating from infected faecal matter. Although flushing wasn’t implicated in the outbreak, it’s not inconceivable that flushing could be a source of infected bio-aerosols, considering toilet bowls are a cesspool of viral matter leftover by infected individuals.
As Charles Gerba, a microbiologist and expert in toilet-aided transmission of infectious diseases, told The Washington Post, “The risk is not zero, but how great a risk it is, we don’t know…”
Computational studies combining fluid dynamics and transport of viral particles offer an idea of this risk.
Such studies also force us to rethink toilet design, even as lockdown restrictions are being eased around the world. Public health officials should assess the use of public toilets, especially if they’re poorly ventilated. The Chinese study offers one easy fix: simply close the lid before flushing. If there isn’t a lid, Gerba says to “flush and run”.
This said, it’s not easy to interpret the studies’ findings and fixes for Indian settings. Indian-style toilets don’t have lids and have different structure and flushing design. These differences in turn could affect the fluids and airflow in significant ways. Understanding the generation and transport of bio-aerosols due to flushing events in an Indian-style toilet requires a separate study, perhaps by Indian researchers.
For now, most experts agree that there is minimal risk of COVID-19 transmission due to flushing. But when it comes to infectious diseases like those caused by the novel coronavirus, it’s good to exercise caution. At the least, good flushing habits could contribute to improving public hygiene. So close the lid before you flush. It’s an easy preventive measure, not just against the novel coronavirus but any potentially infectious matter in the poop.
Ronak Gupta is pursuing a PhD in fluid mechanics at the University of British Columbia, Vancouver.