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

Abstract

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 four air purifiers equipped with HEPA filters in a high school classroom while regular classes were taking place. We monitored the aerosol number concentration for particles > 3 nm at two locations in the room, the aerosol size distribution in the range from 10 nm to 10 μm, PM10 and CO2 concentration. For comparison, we performed similar measurements in a neighboring classroom without purifiers. 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 (air exchange rate 5.5 h-1). The reduction was homogeneous throughout the room and for all particle sizes. The measurements are supplemented by a 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. Measurements and 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.

Airborne transmission of COVID-19 and mitigation using box fan air cleaners in a poorly ventilated classroom

“… 16 It has been recently demonstrated in a classroom with no ventilation that the usage of high-efficiency particulate air (HEPA) grade purifiers can significantly reduce the aerosol concentration level. 17 Nevertheless, the commercial purifiers used for public spaces such as classrooms and shared offices typically require a clean air delivery rate (CADR) of larger than 400 cfm 18 with price ranging from $400 to above $4000. 19 Such high costs limit the wide adoption of this mitigation approach, particularly in resource-limited indoor places, 10,14 including public schools, prisons, downmarket offices, shelters, life care centers, etc.…”

Der Effekt von mobilen Luftfiltersystemen auf die Aerosolbelastung in Großraumszenarien vor dem Hintergrund des Infektionsrisikos der COVID-19-Erkrankung. Kann die Präsenzlehre wieder aufgenommen werden?

“…Demgegenüber konnte sowohl unter Laborbedingungen [ 5 , 21 ] als auch in vivo [ 7 ] bereits nachgewiesen werden, dass Raumluftfiltersysteme durchaus einen relevanten Beitrag zur Reduktion der Aerosolkonzentration im Raum leisten können. In einer mathematischen Risikokalkulation der Infektion mit SARS-CoV‑2 erreichte die „high efficiency HEPA filtration“ eine 6‑ bis 8‑fache Risikoreduktion [ 25 ].…”

Airborne Transmission of COVID-19 and Mitigation Using Box Fan Air Cleaners in a Poorly Ventilated Classroom

“…As an alternative approach, portable air purifiers are broadly used for risk mitigation in these poorly ventilated spaces 16 . It has been recently demonstrated in a classroom with no ventilation that the usage of HEPA grade purifiers can significantly reduce the aerosol concentration level 17 . Nevertheless, the commercial purifiers used for public spaces such as classrooms and shared offices typically require a clean air delivery rate (CADR) of larger than 400 cfm 18 with price ranging from $ 400 to above $4000 19 .…”