Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS‐CoV‐2

The airborne transmission of SARS‐CoV‐2 through virus‐containing aerosol particles has been established as an important pathway for Covid‐19 infection. Suitable measures to prevent such infections are imperative, especially in situations when a high number of persons convene in closed rooms. Here we tested the efficiency and practicability of operating air purifiers in a high school classroom while regular classes were taking place. Four air purifiers equipped with HEPA filters were installed in a classroom. We monitored the total aerosol number concentration for particles > 3 nm at two locations in the room (uCPC), the aerosol size distribution in the range from 10 nm to 10 μm (SMPS and OPS), aerosol mass (PM10 derived from OPS) as well as the CO2 concentration in the room. For comparison, we performed similar measurements in parallel by operating a uCPC and an OPS in a neighboring class room without air purifiers. The measurements show that in times when classes were conducted with windows and door closed, the aerosol concentration was reduced by more than 90 % within less than 30 minutes when running the purifiers with a volume flow of 1027 m3/h in the classroom with a volume of 186 m3. The reduction was homogeneous throughout the room, as well as homogeneous for all particle sizes. The measurements are supplemented by a basic calculation estimating the maximum concentration levels of virus‐containing aerosol from a highly contagious person speaking in a closed room with and without air purifiers. The measurements and the calculation demonstrate that air purifiers represent a well suited measure to reduce the risks of airborne transmission of SARS‐CoV‐2 substantially. Staying for two hours in a closed room with a super infective person, we estimate that the inhaled dose is reduced by a factor of six when using air purifiers with a total air exchange rate of 5.7 h-1. Nevertheless, in closed rooms with a high person density frequent ventilation supplying fresh air is necessary to keep the CO2 concentration levels below the maximum permissible values.