Flushing a public toilet? Don’t linger, as aerosol droplets will

Flushing a toilet can generate large amounts of micro-retaining aerosols, depending on the design, water pressure, or flushing capacity of the toilet. A variety of pathogens are usually found in standing water, as well as in urine, feces and vomit. When these pathogens are widely spread through aerosolization, they can cause Ebola, Norovirus that leads to violent food poisoning, as well as COVID-19 caused by SARS-CoV-2.

Respiratory drops are the most prominent source of transmission for COVID-19, but alternative routes may exist given the discovery of small numbers of viable viruses in urine and stool samples. Public restrooms are of particular concern when broadcasting COVID-19 because they are relatively limited, have a lot of foot traffic and may not have adequate ventilation.

A team of scientists from Florida Atlantic University’s College of Engineering and Computer Science has re-challenged the physics of liquids to investigate droplets generated by flushing a toilet and urinal in a public toilet under normal ventilation conditions . To measure the droplets, they used a particle counter placed at different heights of the toilet and urinal to capture the size and number of droplets generated when flushing.

Study results, published in the journal Physics of liquids, show how public toilets can serve as hotbeds for airborne disease transmission, especially if they do not have adequate ventilation or if toilets do not have a lid or cover. Most public restrooms in the United States often do not have toilet seat covers and urinals are not covered.

For the study, researchers obtained data from three different scenarios: toilet flushing; covered toilet flush and urinal flush. They examined the data to determine the increase in aerosol concentration, the behavior of droplets of different sizes, the height of the drops, and the impact of covering the toilet. Environmental aerosol levels were measured before and after running the experiments.

“After approximately three hours of testing with more than 100 rinses, we found a substantial increase in measured aerosol levels in the environment, with the total number of droplets generated in each rinse test reaching the tens of thousands,” said Siddhartha Verma, Ph.D ., co-author and assistant professor in FAU’s Ocean and Mechanical Engineering Department. “Both the toilet and urinal produced large quantities of droplets smaller than 3 micrometers, which is a significant transmission risk if they contain infectious microorganisms. Due to their small size, these droplets can linger for a long time.”

The drops were detected at a height of up to 1.5 meters for 20 seconds or more after the start of the rinse. Researchers found a smaller number of airborne droplets when the toilet was flushed with a closed lid, although not many, suggesting that aerosol droplets escaped through small gaps between the lid and the seat.

“The significant build-up of flush-generated aerosol droplets over time suggests that the ventilation system was ineffective in removing them from the confined space, even though there was no discernible lack of airflow in the toilet,” said Masoud Jahandar Lashaki, Ph.D. , co-author and assistant professor in FAU’s Department of Civil, Environmental and Geomatics Engineering. “In the long run, these aerosols can rise with updrafts caused by the ventilation system or people moving in the toilet.”

There was a 69.5 percent increase in measured levels for particles from 0.3 to 0.5 micron, a 209 percent increase for particles from 0.5 to 1 micron, and a 50 percent increase for particles from 1 to 3 micrometer. Aside from the smallest aerosols, relatively larger aerosols also pose a risk in poorly ventilated areas, even though they experience stronger gravity. They often undergo rapid evaporation in the ambient environment and the resulting decrease in size and mass, or the eventual formation of droplet nuclei, can cause microbes to remain in suspension for several hours.

“The study suggests that incorporating adequate ventilation into the design and operation of public spaces would help prevent aerosol build-up in high-occupancy areas, such as public restrooms,” said Manhar Dhanak, Ph.D., co-author, chair from 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 whole system, as most buildings are designed to certain codes. It could just be a matter of diverting the airflow based on the layout of the toilet. “

During the 300-second sampling, the toilet and urinal were manually flushed five different times at the 30, 90, 150, 210, and 270 second mark, holding the flush handle for five consecutive seconds. The toilet was thoroughly cleaned and closed 24 hours prior to the experiments, with the ventilation system operating normally. The temperature and relative humidity in the toilet were 21 degrees Celsius (69.8 degrees Fahrenheit) and 52 percent, respectively.

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