A new study by Chinese researchers at Yangzhou University shows how fluid dynamics can predict if flushing a toilet with the lid open could lead to the transmission of a virus. The study titled, “Can a toilet promote virus transmission? From a fluid dynamics perspective,” was published in the latest issue of the journal Physics of Fluids.
What was the study about?
The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused the COVID-19 pandemic raging across the world, affecting almost all nations. As of today, it has infected more than 8.38 million individuals and taken the lives of over 448,000. It has led to a significant public health crisis, with tens of thousands needing critical care support and ventilation. The economic losses from this pandemic have also been staggering as millions are forced to stay home to break the chain of transmission of this highly infectious virus.
Novel Coronavirus SARS-CoV-2 Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
The researchers wrote that this virus has the capacity of “fecal-oral transmission.” This means that the virus could be transmitted from one to another via water contaminated with fecal matter or sewage. One of the methods of this form of transmission is via unsafe toilet use. The researchers wrote, “It is clear from daily experience that flushing a toilet generates strong turbulence within the bowl.” They tried to answer the pertinent question, “Will this flushing-induced turbulent flow expel aerosol particles containing viruses out of the bowl?” in this paper.
What was done?
For this study, the team of researchers utilized the principles of fluid dynamics to explore and see the flow of fluid during the flushing of toilets and how it could generate aerosol particles. They used the Volume of Fluid (VOF) model to simulate two processes of flushing that are commonly employed – “single-inlet flushing and annular flushing.” They utilized a “VOF–discrete phase model (DPM) method,” to assess the movement of the aerosols generated on either type of flushing.
What was found?
The results were “alarming,” said the researchers. Their simulated processes showed that there was a considerable movement of aerosol particles during flushing. These trajectories spread the possibly infected aerosols all around the toilet, they noted. They write that 40 to 60 percent of the particles or viruses present in the aerosols reached the toilet seat. This could lead to a vast spread of the virus.
When the two methods of flushing were compared, they noted that there were both similarities and differences. In both types of flushing, there was a creation of airflow vortices within the toilet bowl. There was a generation of centrifugal forces that led to high airflow speeds. The researchers wrote, “it is reasonable to assume that the high-speed airflow will expel aerosol particles from the bowl to regions high in the air above the toilet, allowing viruses to spread indoors causing risks to human health.”
Coming to the differences between single inlet and annular flushing, they wrote that the latter leads to “stronger turbulence with a higher Y-component velocity.” Thus the energy impact within the bowl is higher in the first stage. The team explains that in the annular flushing model, there is a supply of water from two opposite ports, and thus the two streams of water collide in the center of the bowl. This leads to the “high-speed upward flow” of the aerosols. The team wrote, “It is statistically estimated that nearly 60% of the total aerosol particles (including those escaping from outlet 1) rise above the toilet seat. This is 33.3% larger than that in the case of single-inlet flushing.”
Conclusions and implications
The researchers wrote that since there was a considerable risk of spread of virus-containing aerosols around the toilet during flushing, safer designing of toilets are needed to avoid such spread during the pandemic. From this study, the authors found that the upward velocity of the aerosols generated could be as high as 5 meters per second, and the height of these particles could reach 106.5 cm from the floor. The aerosols continued to climb even after flushing was over for 35 to 70 seconds, they wrote. The team concluded, “The data analysis indicates that given the same amount of water and the same gravitational potential energy, annular flushing causes more virus spread.”
The researchers recommended certain measures to stop the spread of the virus via toilets. They also encouraged the toilet manufacturers to produce better-designed toilets where the lids automatically close before flushing and can be cleaned before and after flushing. Some of the recommendations for users were as follows;
- Keeping the lid down before flushing to prevent viral transmission.
- Cleaning the toilet seat before using it to remove the viral articles that may have settled on it during the last flush.
- Washing hands thoroughly after flushing because the viral particles could be present on the door handle and flush button.