Improving air quality in classrooms – Overview

What is the air quality in schools with and without mechanical ventilation? To what extent can aerosol pollution be reduced by using mobile air purifiers? – We(1)(2) investigated these questions for the City of Adliswil in Switzerland during the pilot study “Verbesserung der Luftqualität in Adliswiler Klassenzimmern” (Improving air quality in classrooms of Adliswil).

The results of the pilot study in a nutshell:

CO2 levels in the mechanically ventilated building were mostly in the good to excellent range below 1000 ppm, with calculated air change rates of 2.3 to 2.9 ACH (Air Changes per Hour).

In the manually ventilated building, air quality was often poor to hygienically unacceptable, depending on the window opening strategy chosen by the teacher. In half of the classrooms, CO2 levels above 2000 ppm, which are hazardous to health and unacceptable at workplaces according to the State Secretariat for Economic Affairs SECO, were measured on a daily basis. In whole-class format instruction, CO2 levels were in the learning-impairing range above 1000 ppm 61 % of the time, above 1200 ppm 41 % of the time, above 1400 ppm 26 % of the time, and in the unacceptable range above 2000 ppm 5 % of the time. Even the teacher with the most efficient ventilation strategy was unable to match the air quality of a mechanically ventilated classroom.

Mobile HEPA (High Efficiency Particolate Air) air purifiers are effective at reducing aerosol pollution in classrooms, especially in school buildings without mechanical ventilation. Running two to three units per classroom at a medium to low speed allowed for a higher total CADR (Clean Air Delivery Rate) than with just one unit at maximum speed – at a lower noise level, compatible with a quiet work environment.

Combined window airing and filtration was much more efficient at reducing aerosol concentrations than window airing alone. In typical situations, window airing led to average air change rates ranging from as low as 0.3 to 2.5 ACH. Adding air purifiers that were configured to provide 3 eACH (equivalent Air Changes per Hour) resulted in an additional reduction of aerosol concentration (and thus viral dose in case a contagious person was present) by at least 50 %, in some situations by up to 90 %.

(1) Michael Riediker, Dr. sc. nat. ETH, Director SCOEH
(2) Fredy Neeser, Dr. sc. techn. ETH, Scientific consultation #ProtectTheKids (Switzerland)

Further action of the school’s Board of Administration

On 15.09.2023, the public school of the City of Adliswil published the report on the pilot study “Verbesserung der Luftqualität in Adliswiler Klassenzimmern” (Improving air quality in classrooms of Adliswil) along with an assessment of the school’s Board of Administration.

In view of the excessively low average air change rates in classrooms ventilated only by opening windows, which are associated with frequently very high CO2 levels and do not provide adequate protection against respiratory aerosols, the Adliswil school administration seeks to refurbish all school buildings that are not yet mechanically ventilated with state-of-the-art mechanical ventilation systems as a medium-term solution, while at the same time improving energy efficiency.

Until the older school buildings are refurbished with mechanical ventilation, the school’s Board of Administration seeks to improve guidance for manually ventilating classrooms.

Excerpts from the report

Report on the pilot study (in German)