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Even good ventilation removes only 10% of coronavirus particles from classrooms, study finds

Even with good ventilation, only a small fraction of the novel coronavirus may be filtered out from a room, a new study suggests.

Researchers have been looking at how the virus spreads indoors and how aerosol particles, which are expelled though breathing and talking, flow through three settings: classrooms, elevators and supermarkets.

In one classroom, ventilation filtered out just 10 percent of virus particles and left the rest stuck on the walls.

The team, from the University of Minnesota College of Science and Engineering, says the findings can help businesses and schools learn what precautions to take to reduce the risk of spreading COVID-19 when they reopen.  

‘In general, this is the first quantitative risk assessment of the spatial variation of risks in indoor environments,’ co-lead author Dr Jiarong Hong, an associate professor of mechanical engineering, said in a press release.

‘You see a lot of people talking about what the risks are of staying in confined spaces, but nobody gives a quantitative number. 

‘I think the major contribution we’ve made is combining very accurate measurements and computational fluid dynamics simulation to provide a very quantitative estimate of the risks.’ 

For the study, published on the pre-print site, the team looked at airborne transmission of the virus via aerosols.

Using measures from eight asymptomatic people with confirmed coronavirus cases, they modeled the flow through three indoor space: an elevator, a classroom and a supermarket. 

Next, they looked at different levels of ventilation and different levels of social distancing among the occupants of a room.

This included side-by-side in an elevator, next to each other in a supermarket aisle and rows of desks in a classroom.

Results showed good ventilation could filter some of the virus out of the air, but virus particles would still be left behind on surfaces.

For example, in the classroom simulation with an asymptomatic teacher consistently talking for 50 minutes, only 10 percent of the aerosols were filtered out. 

Most of the particles ended up stuck on the walls.

‘Because this is very strong ventilation, we thought it would ventilate out a lot of aerosols, but  10 percent is really a small number,’ said co-author Dr Suo Yang, an assistant professor in mechanical engineering, in a press release.

‘The ventilation forms several circulation zones called vortexes, and the aerosols keep rotating in this vortex. When they collide with the wall, they attach to the wall. But, because they are basically trapped in this vortex, and it’s very hard for them to reach the vent and actually go out.’ 

Additionally, in classrooms, the team mapped out ‘hot spots’ where the aerosols tended to congregate.

If the teacher was placed directly under a vent, and at least every other row of desks was removed, it would limit the spread and make the ‘hot spot’ only at the front of the classroom.

In an elevator, however, in the scenario in which an asymptomatic person stood opposite to the door and spoke to someone else for one minute, there were very few ‘hot spots’ due to the short duration of time.  

During the final scenario, in which an asymptomatic shopper spends 30 minutes in a grocery store, only between 30 to 50 percent of aerosols were filtered out.

The team says stores would have to ventilate from several locations so the cashier doesn’t end up in a ‘hot spot.’

‘After our work goes out, I think more people will ask for help because I think many businesses reopening will have this need – movie theaters, drama theaters, any place with large gatherings,’ Yang said. 

‘If you do a good job, if you have good ventilation at the right location, and if you scatter the seating of the audience properly, it could be much safer.’ 

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