Covid aerosol can remain in public washrooms much longer than open spaces

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Covid-19, known to spread through inhalation of virus-laden aerosols, can persist in public washrooms 10 times longer than other open spaces, finds an alarming study led by researchers at the Indian Institutes of Technology-Bombay, pressing the need for proper ventilation of indoor spaces. Covid-19 virus rides inside tiny microscopic droplets or aerosols ejected from our mouths when we speak, shout, sing, cough, or sneeze. It then floats within the air, where it can be inhaled by and transmitted to other people. But in indoor spaces, it tends to linger more in dead zones like shared washrooms, corners of a room or around furniture increasing the potential for transmission of infection.

In a new study, published in the journal Physics of Fluids, the team at IIT-Bombay explored how airflow can mitigate the transmission of Covid-19 indoors by taking cue from airflow around aircraft and engines. The researchers discovered that the chances of infection are significantly higher in a dead zone.

"Surprisingly, they can be near a door or window, or right next to where an air conditioner is blowing in the air. You might expect these to be safe zones, but they are not," said Krishnendu Sinha, Professor of aerospace engineering at the IIT-Bombay, in a statement.

In washrooms -- present within offices, restaurants, schools, airplanes, trains, and other public spaces -- the use of water was found as a major source of aerosol, and computer simulations of airflow within a public washroom showed infectious aerosols in dead zones can linger up to 10 times longer than the rest of the room, the researchers said.

"Computer simulations show that air flows in circuitous routes, like a vortex," added Vivek Kumar, a co-author.

"Ideally, air should be continuously removed from every part of the room and replaced by fresh air. This isn't easy to do when air is trapped in a dead zone," he explained.

Currently, ventilation design is often based on air changes per hour. While these design calculations assume fresh air reaches every corner of the room uniformly, computer simulations and experiments within a real washroom show this does not occur, Sinha said.

"Air changes per hour is not the same for all parts of the room. It can be 10 times lower for dead zones. To design ventilation systems to be more effective against the virus, we need to place ducts and fans based on the air circulation within the room. Blindly increasing the volume of air through existing ducts will not solve the problem," he suggested.