Localized and Whole-Room Effects of Portable Air Filtration Units on Aerosol Particle Deposition and Concentration in a Classroom Environment

In indoor environments with limited ventilation, recirculating portable air filtration (PAF) units may reduce COVID-19 infection risk via not only the direct aerosol route (i.e., inhalation) but also via an indirect aerosol route (i.e., contact with the surface where particles deposited). We systematically investigated the impact of PAF units in a mock classroom, as a supplement to background ventilation, on localized and whole-room surface deposition and particle concentration. Fluorescently tagged particles with a volumetric mean diameter near two micrometers were continuously introduced into the classroom environment via a breathing simulator with a prescibed inhalation-exhalation waveform. Deposition velocities were inferred on >50 horizontal and vertical surfaces throughout the classroom, while aerosol concentrations were spatially monitored via optical particle spectrometry. Results revealed a particle decay rate consistent with expectations based upon the reported clean air delivery rates of the PAF units. Additionally, the PAF units reduced peak concentrations by a factor of around 2.5 compared to the highest concentrations observed and led to a statistically significant reduction in deposition velocities for horizontal surfaces >2.5 m from the aerosol source. Our results not only confirm PAF units can reduce particle concentrations but also demonstrate that they may lead to reduced particle deposition throughout an indoor environment when properly positioned. Practical ImplicationsO_LIPortable air filtration units should be prioritized in classrooms as part of a multi-layed strategy to mitigate potentially infectious particle transmission by direct aerosol transmission via inhalation and indirect aerosol transmission via particle deposition to surfaces and later contact with said surfaces. C_LIO_LIWhen placing portable air filtration unit(s) within a classrom space, one should consider the airflow field within the classroom, the characteristic operational mode (heating vs. cooling) of the heating, ventilation, and air conditioning system, the predominantly occupied areas of the classroom, and interference with the regular teaching and learning activities. C_LI