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Abstract
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for the novel coronavirus disease 2019 (COVID-19), has caused a global pandemic on a scale not seen for over a century. Increasing evidence suggests that respiratory droplets and aerosols are likely the most common route of transmission for SARS-CoV-2. Since the virus can be spread by presymptomatic and asymptomatic individuals, universal face masking has been recommended as a straightforward and low-cost strategy to mitigate virus transmission. Numerous governments and public health agencies around the world have advocated for or mandated the wearing of masks in public settings, especially in situations where social distancing is not possible. However, the efficacy of wearing a mask remains controversial. This interdisciplinary review summarizes the current, state-of-the-art understanding of mask usage against COVID-19. It covers three main aspects of mask usage amid the pandemic: quality standards for various face masks and their fundamental filtration mechanisms, empirical methods for quantitatively determining mask integrity and particle filtration efficiency, and decontamination methods that allow for the reuse of traditionally disposable N95 and surgical masks. The focus is given to the fundamental physicochemical and engineering sciences behind each aspect covered in this review, providing novel insights into the current understanding of mask usage to curb COVID-19 spread.
Keywords: COVID-19; Decontamination; Filtration; Mask; Particle; SARS-CoV-2.
Copyright © 2021 Elsevier B.V. All rights reserved.
Conflict of interest statement
Declaration of Competing Interest The authors declare no competing financial interests.
Figures
Respiratory droplets and aerosols expelled during a sneeze or cough. (a) Droplet formation by a sneeze. The large millimeter-sized, semi-ballistic droplets (green) settle with gravity, while the multiphase cloud/puff comprised of small aerosols (red) propagates over a longer distance. Adapted with permission from [9]. Copyright 2020 Elsevier. (b) A picture of multiphase turbulent gas clouds formed by a human sneeze, obtained with a high-speed camera. It shows that clusters of droplets/aerosols travelled 7–8 m, i.e., up to 26 ft. Adapted with permission from ref. [15]. Copyright 2020 JAMA. (c) Time-dependent path of an emulated, uncovered, heavy cough jet. The jet is composed of vaporized droplets and aerosols, visualized with a green laser. It can be seen that the jet travels up to 12 ft within 53 s. Adapted with permission from [16]. Copyright 2020 American Institute of Physics.
Epidemiological impact of mask-wearing on COVID-19 transmission rate, measured by the effective reproduction number (Rt). (a) Illustration of Rt for two representative cases, Rt = 1 and Rt = 4, respectively. Adapted with permission from ref. [43]. Copyright 2020 JAMA. (b) Rt decreases with reported mask use. The plot is stratified by quartiles of the percentage of individuals who reported that they were “very likely” to wear a mask with family or friends and to the grocery store. Adapted with permission from ref. [45]. Copyright 2021 Lancet. (c) Effect of public mask wearing on Rt from an initial basic reproduction number R0 = 2.4. The blue area is what is needed to slow the spread of COVID-19. Each black line represents a specific disease transmission level with Rt indicated. Adapted with permission from ref. [19]. Copyright 2021 PNAS. (d) Combined social distancing and mask wearing has the highest impact on reducing Rt. The horizontal dashed line was placed at Rt = 0.8 for community transmission control. Adapted with permission from ref. [45]. Copyright 2021 Lancet.
Schematics of various face masks. (a) An elastomeric respirator, equipped with a replaceable cartridge or filter, which is designed to be reusable. (b) A particle filtering respirator, commonly known as an N95 mask, which is designed to be disposable. (c) A surgical mask, also known as a medical, procedure, or dental mask, which is designed to be disposable. (d) A cloth mask, or a cloth face covering, which is not standardized or regulated. Adapted with permission from ref. [46]. Copyright 2020 United States Food and Drug Administration (FDA).
Effect of face masks on the propagation of respiratory droplets and aerosols. (a) Time-dependent path of an emulated cough jet, through a folded cotton handkerchief mask constructed following the recommendation by the United States Surgeon General. (b) Time-dependent path of an emulated cough jet, through a homemade cloth mask stitched with two-layers of cotton quilting fabric of 70 threads per inch. In comparison to Fig. 1, it can be seen that both face masks tested significantly impeded the propagation of respiratory droplets and aerosols expelled from the emulated cough. The cloth mask (b) was able to limit the forward motion of the cough jet to within 3 in. from the mouth. Leakage of droplets and aerosols from the cloth mask occurred mostly from the gap between the nose and the mask along the top edge, indicating the importance of mask fit. Adapted with permission from ref. [16]. Copyright 2020 American Institute of Physics.
Filtration mechanisms of masks. (a) The filtration mechanisms of masks generally involve mechanical (MF) and electrostatic filtrations (EF). Adapted with permission from ref. [60]. Copyright 2020 American Chemical Society. (b) Gravity sedimentation, whereby a large particle falls and adheres to a fiber cross-section. The grey particle (top) falls onto the filter and adheres via van der Waals attraction, indicating successful filtration, shown in red. (c) Inertial impaction, whereby a large particle with large inertia travels linearly, eventually coming into contact with a fiber. The grey particle (left) does not follow the streamline around the fiber and is unable to avoid it, adhering and becoming filtered. (d) Interception, whereby a small particle is led to adhere to a fiber by motion along the streamline. (e) Diffusion, whereby Brownian motion of a small particle results in contact and adhesion to a fiber. (f) Electrostatic attraction, whereby a charged particle is trapped on a fiber of opposite charge through electrostatic attraction. Particles shown in b-f were not drawn to scale. Adapted with permission from ref. [59]. Copyright 2021 KeAi Communications Co.
General design of an experimental setup for quantitatively determining the particle filtration efficiency (PFE) of a mask or fabric material. The mask is fixed in the cross-section of an airtight test tube. Airflow is generated by downstream suction and the flowrate is measured with a velocity meter. Aerosols are generated upstream with an aerosol generator, and the aerosol concentrations before and after passing through the mask or the fabric sample are measured using a particle counter. Breathability is determined by measuring the pressure difference across the mask using a differential pressure meter. Modified with permission from ref. [72]. Copyright 2020 BMJ Publishing Group Ltd.
Scanning electron microscopy (SEM) images of various mask filter materials. (a) Meltblown (non-woven) polypropylene (PP) from a N95 mask. (b) Meltblown (non-woven) PP from a surgical mask. (c) Spunbonded (non-woven) PP. (d) Cotton (woven) from a pillow cover. (e) Cotton (knit) from a t-shirt. (f) Cotton (knit) from a sweater. (g) Polyester (knit) from a toddler wrap. (h) Silk (woven) from a napkin. (i) Nylon (woven) from exercise pants. Adapted with permission from ref. [78]. Copyright 2020 American Chemical Society.
Comparison of the PFE of N95, surgical masks, and several cloth mask materials. (a) The PFEs of an N95 mask (black), the N95 base fabric (orange), a surgical mask (pink), and a cotton/polyester (65%/35%) twill blend (blue), as a function of the particle mobility diameter (Dm). This comparison suggests that the PFE ranks in the order of N95 mask > N95 base fabric > > Surgical mask > Cloth mask. It also shows that the most penetrating particle size (MPPS) of the surgical mask and cloth mask is in the range of 200–300 nm. Adapted with permission from ref. [63]. Copyright 2020 American Chemical Society. (b) The PFEs of cotton fabrics with different thread counts, including a cotton quilt consisting of two 120 TPI cotton sheets enclosing a ~ 0.5 cm thick cotton batting (red), 600 TPI cotton (blue), and 80 TPI quilted cotton (green). (c) The PFEs of different fabric materials, including one layer of natural silk (red), four layers of natural silk (green), one layer of flannel (cotton/polyester, blue), and one layer of chiffon (polyester/Spandex, purple). Adapted with permission from ref. [60]. Copyright 2020 American Chemical Society.
Nano-enabled self-cleaning, antimicrobial, antiviral, and reusable masks. (a) N95 mask impregnated with copper oxide nanoparticles. This mask consists of four layers (a1). The two external spunbond polypropylene layers contain 2.2% copper oxide nanoparticles, and one internal meltblown polypropylene (PP) layer contains 2% copper oxide nanoparticles. Scanning electron microscopy (SEM) micrographs show the ultrastructures of nanoparticle-containing exterior and interior layers (a2), along with X-ray photoelectron spectroscopy (XPS) analysis (a3). Adapted with permission from ref. [135]. Copyright 2010 PLoS ONE. (b) Laser-induced graphene surgical mask. (b1) Illustration of laser-induced forward transfer (LIFT) for roll-to-roll production of graphene-coated masks. (b2) SEM images of the graphene-coated nonwoven fiber. (b3) Illustration of the self-cleaning capacity of the mask, with the demonstrated high water contact angle on the mask. (b4) Photothermal sterilization of the mask. Surface temperature, measured with an infrared camera, increased to an average of 91.61 °C after exposure to sunlight for 5 min. Adapted with permission from ref. [141]. Copyright 2020 American Chemical Society. (c) Photoactive antiviral mask (PAM). (c1) Illustration of the self-cleaning and the antiviral mechanisms of the mask through photothermal and photocatalytic effects in response to solar irradiation. (c2) SEM images of hybrid shellac‑copper nanoparticles coated on nonwoven polypropylene fibers of a surgical mask. (c3) Confocal microscopy images that showed inactivation (represented by red color) of E. coli after 5 min exposure to solar irradiation. (c4) Disruption of virus-like particles by the PAM. Adapted with permission from ref. [143]. Copyright 2020 American Chemical Society.
Comparison of the performance and the impact of four decontamination methods, i.e., 70% ethanol, 70 °C dry heat, UVGI (UVC), and vaporized hydrogen peroxide (VHP), on N95 masks, after each decontamination round and two hours of wear, for three consecutive decontamination and wear sessions. (a) The inactivation rate of SARS-CoV-2 on N95 filter fabric, quantified with the TCID50, in comparison to the TCID50 of stainless steel. Both VHP and ethanol yielded extremely rapid inactivation on N95 masks, in comparison to UV and heat treatments. (b) Fit factors of N95 masks, quantified with QNFT, after each decontamination round. After the third decontamination round, only VHP- and UV-treated masks maintained an acceptable level of performance. (c) The overall performance of decontamination methods against SARS-CoV-2, after 1, 2, and 3 decontamination rounds, was determined by the virus kill rate, post-treatment mask integrity, and the fit factor. The ethanol treatment is shown in green, heat in orange, UV in purple, VHP in blue, and non-treatment control in grey. VHP treatment displayed the best combination of rapid inactivation of SARS-CoV-2 and preservation of N95 respirator integrity after all three decontamination cycles. Adapted with permission from ref. [152]. Copyright 2021 United States Centers for Disease Control and Prevention (CDC).
References
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- World Health Organization, Coronavirus Disease (COVID-19) Pandemic https://www.who.int/emergencies/diseases/novel-coronavirus-2019 (accessed 2021-4-26)
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