Don’t linger after you flush: Public toilets may spread COVID-19 aerosols, study finds

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EDMONTON, Alberta (CTV Network) — Though a public washroom isn’t typically the kind of place you want to spend a lot of time in, new research exploring COVID-19 transmission suggests you may not want to linger after flushing a public toilet.

A team of scientists from Florida Atlantic University’s College of Engineering and Computer Science conducted a series of tests investigating the spread of microbe-containing aerosol droplets generated from flushing a toilet or a urinal in a public restroom.

Using a particle counter to measure the size and number of droplets generated upon flushing, researchers found that the droplets were detected at heights of up to five feet (1.5 metres) for 20 seconds or longer after flushing.

Worse yet, researchers detected a smaller number of droplets in the air even when the toilet was flushed with a closed lid, suggesting that aerosol droplets can escape through small gaps between the cover and the seat.

“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,” study co-author Siddhartha Verma said in a press release issued Tuesday.

“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. Due to their small size, these droplets can remain suspended for a long time.”

Though COVID-19 is typically spread through respiratory droplets produced when an infected person coughs, breathes or sneezes on another person, the study suggests that alternative routes of transmission – such as toilet flushing – may be possible given the discovery of small numbers of viruses in urine and stool samples.

Previous research and several wastewater monitoring programs across Canada have found traces of SARS-CoV-2, the virus that causes COVID-19, in wastewater. Flushing a toilet can generate large quantities of those microbe-containing aerosols depending on the design, water pressure, and flushing power of the toilet.

When combined with heavy foot traffic, confined space, and a lack of adequate ventilation, researchers say this could make public restrooms a “hotbed” for virus transmission.

“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,” co-author Masoud Jahandar Lashaki, an assistant professor in FAU’s Department of Civil, Environmental and Geomatics Engineering, said in the press release.

“Over the long-term, these aerosols could rise up with updrafts created by the ventilation system or by people moving around in the restroom.”

During the sampling, the toilet and urinal were flushed manually five different times at the 30-, 90-, 150-, 210-, and 270-second mark, holding the flushing handle down for five seconds at a time. The restroom was deep cleaned and closed for 24 hours prior to the experiments, with the ventilation system operating normally.

However, researchers did not analyze the aerosol droplets found in the restroom. Therefore, it is unclear whether COVID-19 aerosols were found in the sampling.

This isn’t the first study to warn of the potential transmission of COVID-19 via flushing.

A study published in June 2020 found that aerosol particles can shoot from a toilet bowl at a velocity of up to five metres per second, and even after the initial flush, particles that escaped the toilet bowl will continue to spread through the air.

The study noted that in the case of an annular flushing toilet, “the velocity [of the spray] will be even higher when a toilet is used frequently, such as in the case of a family toilet during busy times or a public toilet in a densely populated area.”