Characterization of Exhaled Particle Deposition and Ventilation in an Indoor Setting

Exhalation of infectious micrometer-sized particles has been strongly implicated in respiratory infection spread. An important fundamental question is then the fate of infectious exhaled particles in indoor spaces, i.e., whether they will remain suspended in an aerosol until ventilation leads to their clearance or whether they will deposit, and if so, on what surfaces in an indoor space. We investigated the interplay between deposition and ventilation using model experiments with a breathing simulator manikin in an office environment. The breathing simulator utilized physiologically correct exhalation and inhalation breathing waveforms as well as an anatomically correct manikin. The simulator output fluorescein-doped particles with a volume distribution spanning the 1-3 um range. The office environment was a 344 m3 room equipped with desks. Four different test conditions were created by changing the simulator location and via different air change rates and MERV ratings of filters in the HVAC system. We found that the rate of ventilation exceeds the rate of deposition on all surfaces (quantified by Stanton numbers, which were below unity) with several important exceptions: (1) surfaces close to (within 2 m) the simulator;and (2) non-passive surface exteriors (return grilles and diffusers). A detectable decrease in Stanton number with distance suggests that the room environment cannot be approximated as truly well-mixed. The finding of enhanced deposition on non-passive surfaces at all distances from the room highlights that infectious particles may preferentially deposit on such surfaces in indoor spaces. Finally, while our results highlight particular surfaces with enhanced deposition, our results confirm the importance of ventilation in a room as a means to reduce infectious aerosol particle concentrations, as in large part the clearance for particles appears to occur by ventilation.

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 aerosol 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 2 μm were continuously introduced into the classroom environment via a breathing simulator with a prescribed 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 that PAF units can reduce particle concentrations but also demonstrate that they may lead to reduced particle deposition throughout an indoor environment when properly positioned with respect to the location of the particle source(s) within the room (e.g., where the largest group of students sit) and the predominant air distribution profile of the room.