High school students in a social distanced classroom. RichLegg/Getty Images

In a classroom model of coronavirus spread, the back corners of the room were safest from aerosol particles.

The combination of open windows and air conditioning removed about 69% of aerosol particles from the classroom.

Glass shields installed at desks, when combined with distancing, also helped reduce particle transmission.

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In a simulation of coronavirus spread in a socially distanced classroom, the back corners of the room were the safest spots.

Researchers at the University of New Mexico simulated the path of small and large particles in a computer model of a classroom setting, published today in Physics of Fluids. They found that the distribution of aerosol particles — airborne viral particles smaller than one micron, which can float further than six feet — was not uniform.

As such, where students sit relative to air-conditioning outlets matters.

Students seated in corners also benefit from not being surrounded by all sides, Khaled Talaat, co-author of the study, told Business Insider. Those seated in the middle of the classroom, on the other hand, were most likely to transmit particles to others, so Talaat recommended getting rid of the middle seat entirely.

The study also found that opening windows, in addition to air conditioning, increased the amount of particles cleared from the room. Additionally, installing desk shields helped divert the path of any remaining particles.

Up to 50% of smaller particles exited the classroom via the air conditioning, while larger particles deposited on the ground, on desks, and on surfaces throughout the room.

“I’d recommend that people sanitize their hands, even without coming into contact with other people’s belongings, ” Khaled Talaat, co-author of the study, told Business Insider. “Particles can travel from one person to another person’s belongings, like to your desk and even to your t-shirt, so by just touching yourself, you can be putting yourself at risk.”

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Aerosols re-concentrate near AC outlets

The air conditioning configuration modeled in the study — which is standard for an average classroom, Talaat said — had inlets located in the corners and center and outlets around the edge of the room. The corners were the safest spots because they were farthest from the outlets, where aerosol-sized particles are found at higher concentrations.

“Initially, when somebody exhales aerosols, this aerosol is sort of concentrated,” Talaat said. “It then disperses in the room and the concentration decreases. But, it re-concentrates again at the outlets of the air conditioning.”

Many air conditioning systems recycle the air they take in, so if the filtering system isn’t top-notch, some particles will be re-expelled into the room via AC outlets. You can tell the difference between AC inlets and outlets based on whether air is flowing in or out, and schools should have them clearly labeled, Talaat said.

The back corners were slightly safer than front corners because people typically exhale aerosols facing forward, sending more particles towards the front of the room. Still, the differences between front and back corners was very small, Talaat said.

Opening windows increases the amount of particles that leave the classroom by 40%

Open windows allow some particles to leave the classroom entirely, instead of exiting via the air conditioning system.

Without the windows open, anywhere from 24 to 50% of particles leave the classroom through the air conditioning. Opening the windows increases the fraction of particles that exit the classroom by about 40%, Talaat said, clearing a total of about 69% of aerosolized particles from the classroom in their model.

“The challenge with opening windows is that obviously there are some cold cities right where it’s not really a feasible measure to open the windows,” Talaat said. 

If you can’t open the windows, good ventilation via air conditioning is especially important. Siddartha Verma, an assistant professor of fluid mechanics at Florida Atlantic University who was not affiliated with the study, also stressed the importance of ventilation.

“It is essential to remove aerosolized droplets as quickly as possible,” Verma wrote in an email to Business Insider. “The aerosolized droplets accumulate over time (even from just talking and breathing), and can stay suspended for several hours, which also poses a hazard to classes that may follow in the same room.”

Glass shields can redirect airflow

As for the particles not cleared by AC and open windows, glass shields can help redirect their path to avoid transmitting virus particles between students.

“They don’t stop the particles directly, but what we see is that they affect the local airflow field near the source and change the trajectory of the particles,” Talaat said.

The glass screens employed in the model were about 70 centimeters, or 27.5 inches high, so they would reach about a foot above a student’s head when seated. They extended across the full width of the desk.

The effectiveness of shields depends on the student’s positioning relative to the air conditioning source, but overall, the combination of screens and distancing reduced aerosol transmission by about 92% — good enough that Talaat recommended glass screens be installed in classrooms this year.

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