[The effect of mobile air filter systems on aerosol concentrations in large volume scenarios against the background of the risk of infection of COVID-19. Can classroom teaching be resumed?]

Background: As a consequence of the corona pandemic, universities nationwide had stopped classroom teaching by the start of the summer semester 2020. As part of the second lockdown, in many states schools and day care centers were also closed or reduced to a minimum. In this context the effect of room air filters has already been discussed multiple times; however, mobile devices for air filtration are currently not recommended by the German Federal Environment Agency. The following investigation shows the real effects of mobile air filters on aerosol concentrations when used in lecture theaters, canteens or school learning centers.

Methods: The effects of a mobile air purifier (DEMA-airtech, Stuttgart, Germany) were measured in three large rooms (a lecture theater, a company canteen and a learning center of a grammar school). Aerosol and carbon dioxide concentrations were determined with devices from the company Palas (Karlsruhe, Germany).

Results: All three scenarios showed a relevant and permanent decrease in aerosol concentrations through the use of air filters. The effect partly even surpassed the effectiveness of simple ventilation by opening the windows.

Conclusion: In addition to social distancing and wearing highly efficient face masks, the use air filters is recommended. This measure could enable classroom teaching to be resumed.