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Add this to our list of worries in these anxious times: coronavirus-containing clouds that waft into the air when a toilet is flushed.

Scientists who simulated toilet water and air flows say in a new research paper that aerosol droplets forced upward by a flush appear to spread wide enough and linger long enough to be inhaled. The novel coronavirus has been found in the feces of covid-19 patients, but it remains unknown whether such clouds could contain enough virus to infect a person. The authors say the possibility of that mode of transmission calls for action in the midst of a pandemic — first and foremost, by closing the lid.

“Flushing will lift the virus up from the toilet bowl,” co-author Ji-Xiang Wang, who researches fluids at Yangzhou University in Yangzhou, China, said in an email. Bathroom users “need to close the lid first and then trigger the flushing process,” Wang said, and wash their hands thoroughly if closure isn’t possible.

Toilets and modern sanitation systems have been a huge boon to public health and life expectancy since the 19th century. Even so, people have long been leery of germs in bathrooms, and that wariness has only increased during the pandemic. But experts say most of us are focusing on the wrong aspect.

For all our paranoia about the surface of toilet seats — the tissue paper we oh-so-carefully lay down, the thin covers often offered in public stalls — germ transmission from skin contact is a relatively small health risk compared with what happens after you flush. That’s when bits of fecal matter swish around so violently that they can be propelled into the air, become aerosolized and then settle on the surroundings.

Experts call it the “toilet plume.”

The potential for airborne transmission of infectious disease via sewage has been the subject of research for more than a century, and the toilet plume’s role has been scrutinized since the 1950s. Scientists who have seeded toilet bowls with bacteria and viruses have found contamination of seats, flush handles, bathroom floors and nearby surfaces. This is one reason we are told to wash our hands after visiting the john.

Even though public bathrooms are known to contribute to the spread of viruses that transmit via ingestion, such as the noroviruses that haunt cruise ships, their role in the transmission of respiratory viruses has not been established, said Charles P. Gerba, a microbiologist at the University of Arizona.

“The risk is not zero, but how great a risk it is, we don’t know,” Gerba, who has studied the intersection of toilets and infectious disease for 45 years, said of the potential for flushing to spread the coronavirus. “The big unknown is how much virus is infectious in the toilet when you flush it … and how much virus does it take to cause an infection.”

A study published in March in the journal Gastroenterology found significant amounts of coronavirus in the stool of patients and determined that viral RNA lasted in feces even after the virus cleared from the patients’ respiratory tracts. Another study in the journal Lancet found coronavirus in feces up to a month after the illness had passed. Scientists around the world are now studying sewage to track the spread of the virus.

Researchers say the virus’s presence in excrement and the gastrointestinal tract raises the prospect of transmission via toilets, because many covid-19 patients experience diarrhea or vomiting. A study of air samples in two hospitals in Wuhan, China, found that although coronavirus aerosols in isolation wards and ventilated patient rooms was very low, “it was higher in the toilet areas used by the patients.”

The Centers for Disease Control and Prevention says it remains “unclear whether the virus found in feces may be capable of causing covid-19,” and “there has not been any confirmed report of the virus spreading from feces to a person.” For now, the CDC characterizes the risk as low based on observations from previous outbreaks of other coronaviruses, such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

Wang decided to use computer models to simulate toilet plumes while isolating at home, per Chinese government orders, and thinking about how a fluids researcher “could contribute to the global fight against the virus.”

The resulting study was published Tuesday in the journal Physics of Fluids. It found that flushing of both single-inlet toilets, which push water into the bowl from one port, and annular-inlet toilets, which pour water into the bowl from the rim’s surrounding edge with even greater energy, results in “massive upward transport of virus.” Particles can reach heights of more than three feet and float in the air for more than a minute, it found.

The paper recommends not just lid-closing and hand-washing but urges manufacturers to produce toilets that close and self-clean automatically. It also suggests that toilet-users wipe down the seat.

Gerba, however, said seats shouldn’t be a major concern. Research has found that public and household toilet seats are typically the cleanest surface in restrooms, he said, probably because so many people already wipe them off before using them. Also, he said of SARS-CoV-2, the virus that causes covid-19: “I don’t think it’s butt-borne, so I don’t think you have to worry.”

Gerba has been studying coronavirus transmission for two decades, and in 2003 he was sent by the World Health Organization to Hong Kong to investigate the role of toilet flushing in a SARS outbreak at an apartment complex called Amoy Gardens. Investigators suspected cases were driven in part by the apartment’s sanitary system, because the outbreak seemed to begin after an index patient suffering from diarrhea used a toilet in the apartment complex. Gerba said improper ventilation in the building, not flushing, seemed to be the culprit.

But Gerba does not rule out the possibility that the novel coronavirus might be spread by flushing. He said he once did an experiment, never published, in which he placed six cages holding mice at different heights in a small bathroom and then flushed a mouse pneumonia known only to transmit by inhalation. More than a quarter of the mice were infected, though none of those that had been placed at six feet above toilet level, he said.

In the absence of clear evidence, Gerba said, his advice remains the same: “Flush and run” when using a public toilet without a lid. Wash hands well post-flush and use hand sanitizer after leaving the restroom. Choose well-ventilated bathrooms if possible, and “don’t hang around the restroom” in any case.

But most important, Gerba said, is closing the lid before flushing. He’s been doing it for decades, even at home.

“I have my toothbrush too close to the toilet,” Gerba said. “I don’t want to brush my teeth with what’s in the toilet.”