Flush and run! Viruses like COVID-19 can linger for hours in public restrooms

BOCA RATON, Fla. — Is there anything more disgusting than feeling yourself be hit by toilet water in a public restroom? As horrifying as that may be, a new study is providing even more reason to flush and run. Researchers from Florida Atlantic University have discovered that flushing any toilet ejects large numbers of microbe-containing droplets — or aerosols. Even more alarming, these particles can carry viruses and even COVID-19 which linger for hours after flushing.

Respiratory droplets are the most common way coronavirus spreads from person to person. However, researchers say there are other ways the illness can travel, including through contaminated water. Previous studies have revealed the presence of the virus in urine and stool samples, making public bathrooms a serious health risk during the pandemic.

A team from FAU’s College of Engineering and Computer Science investigated these aerosols coming from flushing a toilet or urinal under normal ventilation conditions. To measure their buildup, scientists used a particle counter set at various heights over the public toilets and urinals. The counter then captured the size and number of droplets coming from flushes over several seconds.

Researchers say their results point to public restrooms easily becoming “hotbeds for airborne disease transmission” without proper ventilation. They also find the effect is even worse when toilets don’t have a lid, something very common in U.S. restrooms.

Even closing the lid doesn’t really help

Researchers studied these facilities in three different scenarios, flushing toilets, flushing covered toilets, and flushing urinals. Each time, they examined the increase in aerosol concentration, droplet sizes, and height of the water ejection after flushing.

The team manually flushed the toilets five times over a 300-second period, holding down the lever for five seconds. Researchers note the restroom was deep cleaned and closed 24 hours before the study. The ventilation system maintained a temperature 69.8 degrees Fahrenheit with 52 percent humidity.

“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, Ph.D., from FAU’s Department of Ocean and Mechanical Engineering in a media release.

“Both the toilet and urinal generated large quantities of droplets smaller than 3 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.”

Results show droplets reached heights of up to five feet for at least 20 seconds after flushing. Even when researchers closed the lid, the number of particles escaping only dropped slightly. The FAU team suggests aerosols likely escape through the gaps between the seat and cover.

“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,” adds Masoud Jahandar Lashaki, Ph.D., from FAU’s Department of Civil, Environmental and Geomatics Engineering. “Over the long-term, these aerosols could rise up with updrafts created by the ventilation system or by people moving around in the restroom.”

How long can viruses live in flush droplets?

While it may make your skin crawl when a big water droplet hits you after flushing, the study finds there are plenty of tinier ones you’re likely not seeing — or feeling. The report reveals a 69.5 percent increase in particles between 0.3 to 0.5 micrometers. Flushing also produced a massive 209 percent increase in particles between 0.5 to 1 micrometer in size. Among larger droplets, flushing created a 50 percent increase in particles between one and three micrometers.

Scientists warn that even large aerosols pose to risk to human health in poorly ventilated bathrooms. This is despite the fact that larger droplets generally succumb to gravity much faster than smaller aerosols.

The study finds large droplets undergo rapid evaporation in the relatively neutral environment of a bathroom. This results in them shrinking in size and forming a droplet nuclei. Such a droplet can keep carrying potentially dangerous microbes for several hours.

“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,” says Manhar Dhanak, Ph.D., chair of FAU’s Department of Ocean and Mechanical Engineering and professor and director of SeaTech. “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.”

“Aerosolized droplets play a central role in the transmission of various infectious diseases including COVID-19, and this latest research by our team of scientists provides additional evidence to support the risk of infection transmission in confined and poorly ventilated spaces,” concludes Stella Batalama, Ph.D., dean of the College of Engineering and Computer Science.

The study appears in the journal Physics of Fluids.

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