Measurement of Size-specific Filtration Efficiency and Pressure Drop through Face Masks
Creators: Seo, Solbee
Publisher: name: University of Alberta Library
Year: 2023

<p>It is common for face masks to be used in conditions that mask manufacturers did not intend, such as multiple donning of masks during supply shortage, but the mask performance in varying usage conditions are typically not regulated and assumed beneficial without the quantification of the actual mask performance. The objective of this thesis was to examine the effects of the following two common usage conditions which may influence parameters that are critical to filtration: 1) moist heat incubation decontamination of respirators for reuse and 2) lower face velocity from school-aged children breathing through woven and nonwoven masks. The results were supported by measuring size-specific filtration efficiency (FE) and pressure drops of different masks, using a custom experiment set-up with an Electrical Low Pressure Impactor.<br>
When mask supply was limited due to high demand, various decontamination methods were used for reuse of masks, and moist heat incubation was deemed one of the least damaging methods. The first part of the thesis exposed two different respirator brands to multiple cycles of high temperature and humidity to emulate repeated moist heat incubation. The moist heat incubation method was found negligibly damaging to a certified N95 but significantly decreased the other respirator’s FE, while both respirators’ pressure drops were not affected. Scanning electron microscopy images of the respirators confirmed minimal physical degradation on both respirators after the decontamination. The shift of the most penetrating particle size from 0.3 μm to 0.5 μm suggested that the electrostatic effect may have been reduced and consequently decreased FE.<br>
Mask performance when used by younger populations has been presumed to be similar to adults, however, children’s vulnerability to a wider range of particle sizes and lower face velocity compelled the quantifications of children’s masks at a face velocity representative of children.<br>
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When lower face velocity expected for school-aged children was used, the study confirmed the single fiber filtration theory in which lower face velocity increases FE and decreases pressure drop in general, but it did not yield a statistically significant increase in FE for woven masks beyond 0.05 μm.<br>
The results from this thesis demonstrate that a more inclusive FE test with test particle sizes ranging from nanometers to microns as well as different flow rates can reveal the unforeseen flaws of modifications to masks. The decrease in FE from repetitive moist heat incubation decontaminations was quantified, and the range of particle sizes in which the decrease occurred suggested the potential decay of the electrostatic filtration effect. Furthermore, compared to nonwoven masks, woven masks lacked protection against submicron-sized particles at both face velocities representative of adults and school-aged children. Coupled with future undertakings including examination of mask fit and intrinsic mask material properties, the results from this thesis and similar future studies can assist in developing masks resilient to changes in external factors like face velocity and user environment.</p>