Filtering Efficiency and Design Properties of Medical- and Non-Medical-Grade Face Masks: A Multiscale Modeling Approach

Abstract

Approved medical face masks have been shown to prevent the spread of respiratory droplets associated with coronavirus transmission in specific settings. The primary goal of this study was to develop a new strategy to assess the filtering and transmissibility properties of medical- and non-medical-grade face masks. In this study, we designed and assessed the filtering efficiency of particles through six different masks with a diverse set of fabrics, textures (woven and non-woven), fiber diameters, and porosity. The filtering and transmissibility properties of face mask layers individually and in combination have been assessed using mathematical analyses and new experimental data. The latter provided velocity profiles and filtration efficiencies for which the data were shown to be predictable. The filtration efficacy and pressure drop across each fabric have been tested using an aerosol particle spray and scanning electron microscopy. To assess clinical significance, the temperature and humidity of the masks were tested on a group of healthy volunteers spanning various age ranges (9–79 years old), utilizing an embedded temperature sensor disc. Also, a mask filter model was developed using fluid dynamic simulations (Solidworks Flow) to evaluate the aerodynamic dispersion of respiratory droplets. Overall, the FFP2 and FFP3 masks demonstrated the highest filtration efficiencies, each exceeding 90%, a feature of multi-layered masks that is consistent with simulations demonstrating higher filtering efficiencies for small particles (<5 µm). The velocity and temperature simulations of all six masks revealed a low air velocity (~1 m/s) inside the mask and a temperature variation of approximately 3 °C during the breathing cycle.

Design and Evaluation of Face Mask Filtration: Mechanisms, Formulas, and Fluid Dynamics Simulations

“…where E D , E R , and E I are the filtration efficiencies due to diffusion, inertia, and interception, respectively. Instead, in our previous study [9], we developed a new method to calculate the filtration efficiency based on the air flow velocity used to test the facemasks. In particular, the filtration efficiency F, expressed as a weighted percentage, was calculated as the difference between the initial (v 0 ) and final velocity (v 1 ) divided by the initial velocity, as shown below.…”

“…In particular, the filtration efficiency F, expressed as a weighted percentage, was calculated as the difference between the initial (v 0 ) and final velocity (v 1 ) divided by the initial velocity, as shown below. Validation data on filtration efficiency have been included in our recent study [9], where comparisons are made between experimentally obtained filtration efficiency data and those obtained through simulations using our developed formula. The similarity of the results confirmed the reliability of our equation below.…”

“…Based on the air flow models discussed above, Solidworks 2022 (v. SP 2.0, Dassault Systems, Paris, France) was used to draw all the designs shown in Figure 1 below. Figure 1a shows the design and related dimensions of the air duct mask container used to perform the tests illustrated in our previous paper [9]. The air supply is located at the top of the duct, and the pitot tubes, positioned at 7.5 cm on either side, were used to measure the air flow velocity.…”

“…Face masks have different filtration efficiency based on fiber size, morphology (random or ordered structure), pore size, and material weave. Furthermore, air flow velocity and breathing have an impact on filtration efficiency [7][8][9][10]. Therefore, researchers are investigating the fabrication of face masks with high filtration efficiency to decrease the spread of airborne diseases.…”

“…Approved masks can have up to six layers (FFP3) constructed in this manner, but normally fewer layers are employed. A range of masks with these properties were investigated [5,9]. This model has the advantage of a mathematical analysis of the average central air flow velocity in connection with filtration efficiency determination.…”