COVID-19, influenza, RSV, chickenpox, and numerous other diseases are commonly transmitted by respiratory aerosols. Densely occupied classrooms pose an increased risk of spreading such airborne pathogens, especially when air quality is inadequate. Frequent and intensive aeration can significantly reduce this risk, but entails reduced comfort and energy losses in winter. Especially in manually ventilated classrooms, mobile air purifiers lend themselves as complementary tools for reducing aerosol exposure, as they are able to remove aerosols and thus also pathogens from indoor air.
In order to be able to comprehensively and scientifically assess the current situation in classrooms, in autumn 2022 the school administration of the city of Adliswil commissioned a study of indoor air quality and the suitability of mobile air purifiers for use in classrooms. The study was conducted as a collaboration of the Swiss Center for Occupational and Environmental Health (SCOEH) and the non-profit association #ProtectTheKids (Switzerland).
The investigations were carried out in the buildings of the Kopfholz elementary school. Kopfholz 1 is a renovated old building with window airing, Kopfholz 2 is a new building with mechanical ventilation that meets standards for low energy consumption. Up to three mobile air purifiers were installed in each classroom. The companies Electrolux, Philips and Stadler Form kindly provided a large number of units. The air purifiers were evaluated for cleaning performance and noise generation while operating in the real environment. Aerosol reduction rates were determined using a test aerosol of saline crystals and sensors distributed throughout the room, and noise levels and frequencies were measured with an audio and acoustics analyzer. In addition, the noise levels of the air purifier configurations that teachers considered as acceptable upper limits were determined. CO2 measurements were used to assess indoor air quality and to determine the air change rates achieved by manual and/or mechanical ventilation.
The air purifiers evaluated improve the mixing of indoor air by directing cleaned air upwards. Tested as individual units at maximum speed, they showed air change rates of two to three eACH (equivalent Air Changes per Hour) in the 240 m3 classrooms of Kopfholz 1, corresponding to a CADR (Clean Air Delivery Rate) of around 480 to 720 m3/h.
The noise level at a distance of two meters was between 45 and 55 dB(A). The teachers preferred lower operating speeds associated with noise levels between 32 and 42 dB(A). By using multiple units in parallel, high air purification rates of 3 to 5 eACH were achieved at these lower speeds, at noise levels compatible with a quiet office environment.
Air quality was very different in the two buildings: in Kopfholz 2, CO2 was mostly in the good to very good range below 1000 ppm with air change rates of 2.3 to 2.9 ACH. In the manually ventilated old building Kopfholz 1, the statistics obtained from continuously recorded CO2 data over a total teaching time of 684 hours showed a clear need for action: CO2 levels were frequently in the hygienically unacceptable range above 2000 ppm and 61 % of the time in the learning-impairing range above 1000 ppm. The aeration strategies observed in classrooms ventilated only by opening windows were inefficient in most cases: The strategy “ventilate infrequently, but particularly strongly” was significantly less efficient than “ventilate frequently and strongly” due to the poor temporal distribution of air changes. Even with an ideal strategy, indoor air quality was often moderate, and only 20 % of the time were CO2 levels in the ideal range for cognitive performance below 800 ppm. When refurbishing old buildings with mechanical ventilation systems, decentralized and centralized solutions should be compared.
Air purifiers are designed to remove aerosols from indoor air; they thus complement measures for better ventilation, which reduce both CO2 and respiratory aerosol levels by supplying outside air. In typical situations with aeration through windows, mean air change rates in the range of 0.3 to 2.5 ACH were measured, so that an air purifier performance of 3 eACH resulted in a reduction of the viral dose by at least 50 %. In some situations, the purifiers even reduced the dose by 90 %. Combined airing and filtering was particularly effective in reducing aerosol pollution compared to opening windows alone. The mobile air purifiers could be used flexibly and integrated easily into everyday school life by configuring them to maintain a quiet learning environment.
However, in order to successfully implement an additional layer of protection to curb the spread of disease, whether by cleaning indoor air or by any other measure, it is essential to raise awareness among all stakeholders that accepting repeat infections – even asymptomatic ones – is a risky and unsustainable way to gain or maintain immunity.