And while it remains unknown whether public or shared toilets are a common point of transmission of the virus, the research highlights the need during a pandemic to rethink some of the common spaces people share.
"The aerosols generated by toilets are something that we've kind of known about for a while, but many people have taken for granted," said Joshua L. Santarpia, a professor of pathology and microbiology at the University of Nebraska Medical Center who was not involved in the research. "This study adds a lot of the evidence that everyone needs in order to take better action."
Typically, the coronavirus is most at home in cells in the lungs and upper respiratory tract. But studies have found it can also dock to cell receptors in the small intestine. Patients have been reported to experience diarrhoea, nausea and vomiting among other symptoms.
And researchers have found viable virus particles in patients' faeces, as well as traces of viral RNA on toilet bowls and sinks in their hospital isolation rooms, although experiments in the lab have suggested that material may be less likely to be infectious compared with virus that is coughed out.
A computer simulation of the toilet flushing mechanism showed that when water pours into the toilet and generates a vortex, it displaces air in the bowl. These vortices move upward and the centrifugal force pushes out about 6,000 tiny droplets and even tinier aerosol particles.
Depending on the number of inlets in the toilet, flushing can force anywhere from 40 per cent to 60 per cent of the produced aerosols high above the seat.
"It's very alarming," said Ji-Xiang Wang, who studies fluid dynamics at Yangzhou University and was a co-author of the study.
It's virtually impossible to keep bathrooms sanitised all the time, and sharing a toilet may be unavoidable for family members, even when one person is sick and isolating in a separate room at home, Wang said.
As cities around the world navigate the reopening of restaurants, offices and other businesses, more and more people will also need to use public or shared restrooms. But while diners can be moved outdoors and employees spaced out, people may find it harder to practice social distancing in small bathrooms.
Aerosolised particles may still linger in single-use toilets, and bathrooms are frequently poorly ventilated spaces, which can increase the risk of exposure to infection. Users also have to consider risks from high-touch surfaces, like doorknobs and faucets.
Experience with other coronaviruses shows how quickly the faecal-oral route can lead to spread of disease. In March 2003, more than 300 people living in the Amoy Gardens apartment complex in Hong Kong got infected with the original SARS coronavirus because infectious faecal aerosols spread through faulty plumbing and ventilation systems.
While Wang acknowledged that scientists had yet to look at toilet aerosols in real-world situations involving the new coronavirus, other research has shown that viral RNA was found in shared toilet areas at one hospital in Wuhan, China.
But researchers do not know how much infectious virus is in aerosols or whether people with more severe cases of Covid-19 shed more virus than patients with milder illness, he said.
Thankfully, people can also easily prevent the spread of infections from the toilet plume.
"Close the lid first and then trigger the flushing process," Wang said, which he acknowledged isn't always possible in public bathrooms.
You should also wash your hands frequently and thoroughly, especially if you're using a shared restroom where the toilet doesn't have a lid or the flush is automatically triggered on standing up. Avoid touching your face, and keep your mask on in the bathroom, which could prevent some exposure to the coronavirus.
Wang hopes the new research will help lead to improvements in bathroom design, including increased attention to contactless dispensers for soap and paper towels, and toilets that flush only after they have been covered with a lid.
Other experts are already considering indoor ultraviolet lights and automated disinfectant sprays that will zap the coronavirus and relieve some of the pressure on keeping public toilets clean.
And Santarpia said that Wang's study could point to a way of monitoring coronavirus clusters.
"You could simply monitor samples from a shared bathroom on a daily basis," he said. "And if something were to come up positive, you could then go look at everyone who was there and who they had contact with, rather than testing everybody all the time."
Written by: Knvul Sheikh
Photographs by: Chang W. Lee
© 2020 THE NEW YORK TIMES
