An experimental study has quantified the volume of aerosol particles generated by flushing toilets in a public restroom. The real-world research suggests even with lids closed flushing toilets can increase levels of ambient aerosol particles, heightening the risk of airborne disease transmission in poorly ventilated public spaces.
In 2003, during the peak of Hong Kong’s SARS outbreak, 321 cases rapidly appeared in a single housing estate. Comprehensive investigations of the outbreak ultimately homed in on the primary case, a man visiting one of the apartment blocks.
On the day the man was visiting he had diarrhea and used a toilet in one of the apartments. Tracking subsequent cases, the investigation discovered the most likely route of viral transmission to other residents was via aerosolized particles discharged into sewage pipes.
Early in 2020, as the COVID-19 pandemic spread around the world, case studies resembling the earlier Hong Kong SARS cluster began to appear. One early cluster in a Chinese apartment building, for example, was widely analyzed and a recently published study concluded fecal aerosol transmission was possibly the cause.
It certainly is not news to suggest a large volume of micro-organisms is present in feces. But what happens to those microbes when they are struck by the water pressure of a flushing toilet?
A simulation study published last year used computational fluid dynamics to model the generation and spread of aerosol particles during toilet flushing. The researchers described their results as “alarming,” finding the flushing process creates and propels significant numbers of aerosol particles.
A new study, from a team of researchers at Florida Atlantic University, set out to quantify the volume of aerosols produced by flushing toilets in real-world conditions. To do this the researchers turned a medium-sized restroom on their university campus into an experimental laboratory.
Closed-lid toilet flushing, open-lid toilet flushing and urinal flushing were all investigated. Aerosol particles were detected for at least 20 seconds up to 5 ft (1.5 m) above the toilet after flushing.
While more particles were found to be generated by urinal and open-lid toilet flushes, the researchers still detected notable volumes of aerosols in the air after a toilet was flushed with a closed lid. As many public restrooms do not have lids on toilet seats it is suggested installing lids may help reduce aerosol dispersion but smaller particles can still escape through gaps in the lid.
Perhaps more concerning were the overall levels of ambient aerosols that were found to accumulate in the space after three hours of flush 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 co-author on the new study, Siddhartha Verma.
The researchers tracked aerosol particles smaller than three micrometers in size, which can remain suspended in the air for several hours depending on a room’s airflow. The restroom was fitted with two working air vents, however, this was not effective at dispersing the accumulating particles.
"The significant accumulation of flush-generated aerosolized droplets over time suggests that the ventilation system was not effective in removing them from the enclosed space even though there was no perceptible lack of airflow within the restroom," notes Masoud Jahandar Lashaki, another co-author the study. "Over the long-term, these aerosols could rise up with updrafts created by the ventilation system or by people moving around in the restroom.”
The new study solely focuses on the potential for biomatter to be aerosolized by a flushing toilet and does not prove infectious diseases can be spread through the air in public restrooms. Nor does it suggest this is proof COVID-19 is spread this way.
The researchers do, however, cite a number of well-reported case studies describing viral gastroenteritis, norovirus and other pathogens being spread by aerosolized particles present in public toilets. Traces of SARS-CoV-2 have also been found in fecal samples, suggesting this is a hypothetically plausible mode of transmission.
"The study suggests that incorporation of adequate ventilation in the design and operation of public spaces would help prevent aerosol accumulation in high occupancy areas such as public restrooms," explains co-author Manhar Dhanak. "The good news is that it may not always be necessary to overhaul the entire system, since most buildings are designed to certain codes. It might just be a matter of redirecting the airflow based on the restroom's layout."
The new research was published in the journal Physics of Fluids.
Source: Florida Atlantic University
