Ventilation systems can change spread of potential viral particles indoors: Study

iStock/smolaw11BY: DR. ALEXIS E. CARRINGTON and SONY SALZMAN

(NEW YORK) — A new study at the University of Minnesota College of Science and Engineering showed ventilation systems can change the spread of aerosolized and potentially virus-containing particles in common indoor areas. The study looked at results in an elevator, a supermarket and a classroom.

Although their findings are still undergoing peer review, they have caught the attention of an anxious nation gearing up to send children back to school and eager for any small hints on how to stay safe and reduce transmission indoors.

Dr. Jiarong Hong and Dr. Suo Yang, mechanical engineering associate professors at the University of Minnesota analyzed how aerosolized and potentially virus-containing particles could spread in common indoor areas using a computer simulation.

To start, they evaluated eight asymptomatic participants with COVID and calculated the particle spread from actions like talking, coughing and sneezing. They took this data and applied it into various computer simulations.

In one simulation, they created various circumstances in a standard classroom, with a teacher placed at the front and children scattered throughout the room. In other simulations, they did the same thing, but in an elevator or a grocery store. They studied various conditions, including how the spread would change if the rooms had a powerful ventilator in either the entrance or back of the room.

They found that in indoor spaces, good ventilation will filter some of the virus out of the air, but may leave more viral particles on surfaces, such as walls. Key among their findings: The aerosols spread significantly less throughout the room when the asymptomatic person was placed directly under an air vent rather than away from.

This could come into play in the classroom, with the risk of transmission theoretically lower if an unknowing asymptomatic teacher were placed directly underneath the ventilation system. Conversely, a ventilation system at the back of the room might spread potentially infectious particles throughout the room.

“If the teacher is in the front, the ventilation is in the back, the ventilation will draw the aerosol throughout the whole classroom.” says Suo Yang, one of the lead researchers. “In comparison, if we move the ventilation to the front, right above the teacher, then the ventilation will create a recirculating zone in the front, which will confine the aerosols within the front 1/3 part of the classroom.”

For the classroom case, with strong simulated ventilation, only about 10% of particles were vented out, meaning you can’t necessarily bank on a high-quality ventilation system to filter out all the potentially infectious particles in a standard classroom.

A grocery store, meanwhile, was a different story. In the simulated supermarket, 50% of the particles were vented out, according to the computer simulation, with Yang considering that the tall shelving units of a supermarket create small wind channels that more efficiently circulate air through a room.

In both of these scenarios, many of the particles ended up on surfaces, such as the floor and the walls.

“With very strong ventilation, it doesn’t mean you get aerosols out. We found the surface deposition is huge. That means the regular cleaning of the surface is important,” said Yang.

It is important to note that this study is currently under peer review. Experts caution the study has limits, and its conclusions shouldn’t be applied in a diverse set of scenarios. Every classroom, grocery store and elevator is unique, and needs to be specifically evaluated for transmission risk. Nor should the findings that some scenarios offer reduce risk mean that people should replace typical COVID-19 safety measures, such as social distancing and facial coverings.

“Modeling might be useful but we also need to understand that there are many possible scenarios that can exist for the same indoor space, so modeling all scenarios is not possible,” said Rajat Mittal, professor of mechanical engineering at Johns Hopkins University and an expert in fluid dynamics.

Nevertheless, Mittal said the results are “in line with what has been found in earlier studies.”

For Yang, one of the biggest takeaways from his research is that a good ventilation system is not a silver bullet for reducing transmission indoors.

“Normally people think ventilation will help. Our observation is, it depends on how the room is set up and where is the ventilation,” said Yang. “Sometimes it helps, sometimes it helps the spread out. The key observation is the relative location of the ventilation.”

Alexis E. Carrington, M.D., a dermatology research fellow at the University of California, Davis in Sacramento, California, and a contributor to the ABC News Medical Unit. Sony Salzman is the coordinating producer of ABC News Medical Unit.

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