Flushing toilets can launch poopy droplets, but do they spread COVID? Study adds data
Research on viruses past and some on the novel coronavirus has shown that toilets can generate microscopic aerosols with each flush — a somewhat nauseating fact when recalling visits to public restrooms.
Now, a new study by researchers at Florida Atlantic University shows that “up to tens of thousands” of droplets can be thrown up to 5 feet in the air and linger for 20 seconds or longer during each flush plume. These numbers dropped when researchers closed the toilet lid, “although not by much,” suggesting aerosolized droplets that may contain viruses can escape through gaps between the cover and seat.
Because the coronavirus has been found in urine and feces samples from infected people, public restrooms, the team of engineers suggests, could be a “hotbed for airborne disease transmission, especially if they do not have adequate ventilation” or toilet lids.
The study was published in March in the journal Physics of Fluids.
“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,” study co-author Manhar Dhanak, chair of FAU’s Department of Ocean and Mechanical Engineering, said in a statement released Tuesday.
Although the chances of contracting COVID-19 from someone else’s poop sounds worrisome, some experts agree the risk is low compared to other indoor activities such as eating in a crowded restaurant or attending a large wedding. That’s because exposure time, droplet size and environmental conditions such as humidity play a role in transmission risk.
“Our own fecal plume poses no risk to us,” Dr. John Ross, an internal medicine doctor at the Brigham and Women’s Hospital in Boston, said in the Harvard Health Blog. “The risk of bathroom exposure is largely theoretical — possible, but not proven,” especially since bathroom walls can block direct contact with other people’s potentially infected flush.
The Centers for Disease Control and Prevention says there haven’t been confirmed reports of the coronavirus spreading from feces to people, yet it remains “unclear whether the virus found in feces may be capable of causing COVID-19. However, [scientists] think this risk is low based on data from previous outbreaks of diseases caused by related coronaviruses, such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).”
What did the study find?
The researchers of the new study flushed toilets and urinals more than 100 times in a public restroom with “normal” ventilation conditions and used a device called a particle counter to measure the size and number of droplets that shot out.
During each test, the toilet and urinal were manually flushed five times at the 30-, 90-, 150-, 210- and 270-second mark. Ambient aerosol levels in the bathroom were measured before and after the study took place. The restroom was also “deep cleaned” and closed 24 hours before the experiments.
Three hours later, the team found that aerosol levels in the bathroom increased substantially. These droplets were smaller than 3 micrometers — about the thickness of three sheets of paper — meaning they can remain suspended in the air for a long time.
The largest increase (209%) in droplet levels were for those between 0.5 and 1 micrometers. For reference, the coronavirus, which would be lodged inside these water droplets, can be anywhere between 0.02 and 0.5 micrometers.
A study in Guangzhou, China, found coronavirus particles in an apartment bathroom that had long been empty on the 16th floor, which was above a restroom on the 15th floor that housed five people with COVID-19. An investigation revealed coronavirus aerosols traveled through sewage pipes after flushing and landed in the restrooms of apartments on the 25th and 27th floor, as well, where people were infected with the coronavirus.
However, studies conducted later discovered that coronavirus in poop samples from infected people were not infectious, suggesting its aerosols won’t be able to make others sick if inhaled or ingested by touching contaminated surfaces.
Still, some experts caution it’s better to be safe than sorry.
“Not all of the aerosols generated will carry the virus, of course,” Joseph Allen, director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health, wrote in a September opinion piece published in The Washington Post. “But, let’s be clear: When you flush the toilet, you’re breathing in toilet water, and whatever is in that toilet water — including viruses and bacteria.”
‘Trust your nose’
One virus that carries some significant toilet plume infection risks is the highly contagious norovirus, which most frequently causes diarrhea and vomiting.
A study was conducted on passengers of an 8-hour international flight of which eight of the 14 crew members experienced vomiting and diarrhea (likely from norovirus). Passengers who developed the illness 18 to 60 hours after getting off the plane were found to have visited the bathroom “significantly more often” than those who didn’t get sick.
The SARS virus has also been discovered in both poop and vomit.
Allen of Harvard suggests turning on exhaust fans, opening windows and closing toilet lids in restrooms to reduce risks of plume disease transmission, no matter how low the likelihood.
“Last, and perhaps most importantly, trust your nose,” he said. “Don’t go in if it smells bad. That’s your first hint that the air hasn’t been exhausted properly. Wait to let the room air out a bit.”
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