Flushing the toilet with the lid open could create a so-called "toilet plume," or a cloud of aerosol droplets, that contains the new coronavirus and could linger long enough to either be inhaled by the next person to enter the bathroom or land on surfaces within the room, according to a study recently published in the journal Physics of Fluids.
For the study, researchers used computational fluid dynamics to simulate the mechanism of a toilet flushing. The simulation showed that, once water comes into the toilet bowl, a vortex is formed which displaces the air in the bowl. The air then moves upward, and centrifugal force pushes around 6,000 droplets out of the bowl, alongside other smaller aerosol particles.
The simulation found that both single-inlet toilets, which push water from one port, and annular-inlet toilets, which push water from the toilet rim's edge, result in "massive upward transport of virus," with coronavirus particles reaching more than three feet above the toilet and floating in the air for over a minute. In fact, depending on how many inlets a toilet has, a flush could expel 40% to 60% of the aerosol particles within the bowl far above the seat, according to the simulation.
Although the new coronavirus has generally been found in the cells of the lungs and upper respiratory tract, some studies have found the virus can stay in cell receptors within the small intestine. Another study, published in the journal Gastroenterology in March, found large amounts of the new coronavirus within the stool of patients and concluded that viral RNA lasts within feces even after the virus is no longer in a patient's respiratory tract.
In addition, a study of air samples within two hospitals in Wuhan, China, also found that coronavirus aerosols were "higher in the toilet areas used by the patients."
Joshua Santarpia, a professor of pathology and microbiology at the University of Nebraska Medical Center, said, "The aerosols generated by toilets are something that we've kind of known about for a while, but many people have taken for granted," and "the new study adds a lot of the evidence that everyone needs in order to take better action."
However, CDC notes that it's "unclear whether the virus found in feces may be capable of causing [C]ovid-19," as "there has not been any confirmed report of the virus spreading from feces to a person."
And Charles Gerba, a microbiologist at the University of Arizona, said it hasn't yet been established whether toilet plumes can transmit respiratory viruses. "The risk is not zero, but how great a risk it is, we don't know," Gerba said. "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."
Gerba added that, while toilets shouldn't be a major concern for people hoping to avoid infection, he recommends—in the absence of firmer evidence—that people "flush and run" when using toilets that don't have lids, in addition to washing hands and using hand sanitizer after exiting the restroom.
And there is a way to prevent the potential spread of the virus via toilet plumes, according to Ji-Xiang Wang, who studies fluid dynamics at Yangzhou University and was a co-author of the study. "Close the lid first and then trigger the flushing process," he said.
Like Gerba, Wang also recommends washing your hands frequently and thoroughly, especially if you've just used a public or shared restroom and if the toilet you flushed either doesn't have a lid or flushes automatically (Sheikh, New York Times, 6/16; Brulliard/Wan, Washington Post, 6/16; Bikales, The Hill, 6/16).