Abstract
In the wake of the COVID-19 pandemic, an increased risk of infection by virus-containing aerosols indoors is assumed. Especially in schools, the duration of stay is long and the number of people in the rooms is large, increasing the risk of infection. This problem particularly affects schools without pre-installed ventilation systems that are equipped with filters and/or operate with fresh air. Here, the aerosol concentration is reduced by natural ventilation. In this context, we are investigating the effect of large mobile air purifiers (AP) with HEPA filters on particle concentration and their suitability for classroom use in a primary school in Germany. The three tested APs differ significantly in their air outlet characteristics. Measurements of the number of particles, the particle size distribution, and the CO2 concentration were carried out in the classroom with students (April/May 2021) and with an aerosol generator without students. In this regard, the use of APs leads to a substantial reduction of aerosol particles in the considered particle size range of 0.178–17.78 µm. At the same time, the three APs are found to have differences in their particle decay rate, noise level, and flow velocity. In addition to the measurements, the effect of various influencing parameters on the potential inhaled particle dose was investigated using a calculation model. The parameters considered include the duration of stay, particle concentration in exhaled air, respiratory flow rate, virus lifetime, ventilation interval, ventilation efficiency, AP volumetric flow, as well as room size. Based on the resulting effect diagrams, significant recommendations can be derived for reducing the risk of infection from virus-laden aerosols. Finally, the measurements were compared to computational fluid dynamics (CFD) modeling, as such tools can aid the optimal placement and configuration of APs and can be used to study the effect of the spread of aerosols from a source in the classroom.
Highlights
In the course of the SARS-CoV-2 pandemic, schools and other public institutions in Germany, as well as other countries, were closed because the risk of SARS-CoV-2 infection is considered significantly higher indoors than outdoors [1]
The particle size distribution is measured in the range of 0.178–17.78 μm
In the measurement series without students, where an aerosol generator was used as a particle source, the decay rates of the tested air purifiers (AP) are evaluated and compared with the case without AP
Summary
In the course of the SARS-CoV-2 pandemic, schools and other public institutions in Germany, as well as other countries, were closed because the risk of SARS-CoV-2 infection is considered significantly higher indoors than outdoors [1]. The large floor-standing units are characterized by the fact that the discharged air flows out horizontally at a height of more than 2 m or is directed towards the ceiling at a lower discharge height This generates a flow along the ceiling, which leads to a large-area distribution of the filtered air, reducing the aerosol concentration in the room as homogeneously as possible. The aim is to determine whether even a single AP can homogeneously and significantly reduce the aerosol concentration of the investigated particle size range of 0.178–17.78 μm in the entire room without negatively influencing the teaching process, e.g., through a high noise level. The room air is first passed through a prefilter, whereby the corresponding filter area varies in size between the units (see Table A1 in the Appendix A) This prefilter is intended to separate coarser particles and protect the main filter against rapid clogging.
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