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. 2020 May 26;14(5):6339-6347.
doi: 10.1021/acsnano.0c03252. Epub 2020 Apr 24.

Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks

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Free PMC article

Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks

Abhiteja Konda et al. ACS Nano. .
Free PMC article

Abstract

The emergence of a pandemic affecting the respiratory system can result in a significant demand for face masks. This includes the use of cloth masks by large sections of the public, as can be seen during the current global spread of COVID-19. However, there is limited knowledge available on the performance of various commonly available fabrics used in cloth masks. Importantly, there is a need to evaluate filtration efficiencies as a function of aerosol particulate sizes in the 10 nm to 10 μm range, which is particularly relevant for respiratory virus transmission. We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 nm and >300 nm, respectively, the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. Filtration efficiencies of the hybrids (such as cotton-silk, cotton-chiffon, cotton-flannel) was >80% (for particles <300 nm) and >90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration. Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies. Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of "fit" and leakage, while allowing the exhaled air to vent efficiently. Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.

Keywords: COVID-19; SARS-CoV-2; aerosols; cloth masks; face masks; personal protection; respiratory protection.

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic of the experimental setup. A polydisperse NaCl aerosol is introduced into the mixing chamber, where it is mixed and passed through the material being tested (“test specimen”). The test specimen is held in place using a clamp for a better seal. The aerosol is sampled before (upstream, Cu) and after (downstream, Cd) it passes through the specimen. The pressure difference is measured using a manometer, and the aerosol flow velocity is measured using a velocity meter. We use two circular holes with a diameter of 0.635 cm to simulate the effect of gaps on the filtration efficiency. The sampled aerosols are analyzed using particle analyzers (OPS and Nanoscan), and the resultant particle concentrations are used to determine filter efficiencies.
Figure 2
Figure 2
Particle concentration as a function of particle size at a flow rate of 1.2 CFM. Plots showing the particle concentration (in arbitrary units) upstream and downstream through a single layer of natural silk for particle sizes <300 nm (a,c) and between 300 nm and 6 μm (b,d). Each bin shows the particle concentration for at least six trials. The particle concentrations in panels (b) and (d) are given in log scale for better representation of the data. The y-axis scales are the same for panels "a" and "c"; and for panels "b" and "d".
Figure 3
Figure 3
Filtration efficiency of individual fabrics at a flow rate of 1.2 CFM (without gap). (a) Plot showing the filtration efficiencies of a cotton quilt consisting of two 120 threads per inch (TPI) cotton sheets enclosing a ∼0.5 cm thick cotton batting, 80 TPI quilters cotton (Q Cotton 80 TPI), and a 600 TPI cotton (cotton 600 TPI). (b) Plot showing the filtration efficiencies of one layer of natural silk (Silk-1L), four layers of natural silk (Silk-4L), one layer of flannel, and one layer of chiffon. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.
Figure 4
Figure 4
Filtration efficiency of hybrid fabrics at a flow rate of 1.2 CFM. (a) Plot showing the filtration efficiencies without gap for an N95 respirator and a combination of different fabrics: 1 layer of 600 threads per inch (TPI) cotton and 2 layers of silk (cotton/silk), 1 layer of 600 TPI cotton and 2 layers of chiffon (cotton/chiffon), and 1 layer of 600 TPI cotton and 1 layer of flannel (cotton/flannel). (b) Plot showing the filtration efficiencies of a surgical mask and cotton/silk with (dashed) and without a gap (solid). The gap used is ∼1% of the active mask surface area. The error bars on the <300 nm measurements are higher, particularly for samples with high filtration efficiencies because of the small number of particles generated in this size range, the relatively poorer counting efficiency of the detector at <300 nm particle size, and the very small counts downstream of the sample. The sizes of the error bars for some of the data points (>300 nm) are smaller than the symbol size and hence not clearly visible.

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