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Citation: Al-Amin, M.; Hossain, M.T.;
Tahir, M.; Wyman, D.; Kabir, S.M.F. A
Critical Review on Reusable Face
Coverings: Mechanism, Development,
Factors, and Challenges. Textiles 2023,
3, 142–162. https://doi.org/
10.3390/textiles3010011
Academic Editor: Ivo Grabchev
Received: 25 January 2023
Revised: 23 February 2023
Accepted: 7 March 2023
Published: 9 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Review
A Critical Review on Reusable Face Coverings: Mechanism,
Development, Factors, and Challenges
Md Al-Amin 1, * , Md Tanjim Hossain 2, Muneeb Tahir 2, Diana Wyman 2and S M Fijul Kabir 1,*
1Department of Plastics Engineering, University of Massachusetts Lowell, Lowell, MA 01854, USA
2Wilson College of Textiles, NC State University, Raleigh, NC 27606, USA
*Correspondence: alamintex20@yahoo.com or md_alamin@student.uml.edu (M.A.-A.);
smfijul_kabir@uml.edu (S.M.F.K.)
Abstract:
Textile supply chain challenges due to the COVID-19 pandemic and the Russia–Ukraine
war give unique insights into how health crises and geopolitical instability could dry up supplies of
vital materials for the smooth functioning of human societies in calamitous times. Coinciding adverse
global events or future pandemics could create shortages of traditional face coverings among other
vital materials. Reusable face coverings could be a viable relief option in such situations. This review
identifies the lack of studies in the existing literature on reusable fabric face coverings available in the
market. It focuses on the development, filtration mechanisms, and factors associated with the filtration
efficiency of reusable knitted and woven fabric face coverings. The authors identified relevant papers
through the Summon database. Keeping the focus on readily available fabrics, this paper encompasses
the key aspects of reusable face coverings made of knitted and woven fabrics outlining filtration
mechanisms and requirements, development, factors affecting filtration performance, challenges, and
outcomes of clinical trials. Filtration mechanisms for reusable face coverings include interception
and impaction, diffusion, and electrostatic attraction. Face covering development includes the
identification of appropriate constituent fibers, yarn characteristics, and base fabric construction.
Factors significantly affecting the filtration performance were electrostatic charge, particle size,
porosity, layers, and finishes. Reusable face coverings offer several challenges including moisture
management, breathing resistance factors, and balancing filtration with breathability. Efficacy of
reusable face coverings in comparison to specialized non reusable masks in clinical trials has also
been reviewed and discussed. Finally, the authors identified the use of certain finishes on fabrics as a
major challenge to making reusable face coverings more effective and accessible to the public. This
paper is expected to provide communities and research stakeholders with access to critical knowledge
on the reusability of face coverings and their management during periods of global crisis.
Keywords:
COVID-19; face coverings; masks; reusable masks; woven and knitted fabrics; filtration
mechanism; filtration performance; challenges
1. Introduction
COVID-19 is part of a family of viruses known as coronaviruses. This contagious
disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [
1
].
SARS-CoV-2 is a rapidly mutating virus, and the scientific community continues to learn
more about various mutated strains of this pathogen over time. There is, however, a mature
understanding among epidemiologists that the SARS-CoV-2 pathogen relies primarily on
respiratory droplets of various sizes, including aerosols, for its transmission. There is a great
deal of discussion but no clear scientific consensus on what size of an infectious respiratory
droplet classifies as an aerosol. In this context, the World Health Organization (WHO)
has proposed a basic definition to address the ambiguity. According to WHO, droplets of
greater than 5-micron diameter classify as respiratory aerosols [
2
]. Studies have focused on
observing the particle size distribution in aerosolization due to various activities including
Textiles 2023,3, 142–162. https://doi.org/10.3390/textiles3010011 https://www.mdpi.com/journal/textiles
Textiles 2023,3143
human speech, singing, coughing, sneezing, shouting, exhaling, etc. have produced
inconsistent results [
3
–
6
]. This variation could be explained by different test methodologies
and prevalent ambient conditions for these studies. Particle size distribution has a high
degree of variability due to the interplay of factors such as the nature of the transmitted fluid
under consideration, relative humidity, the temperature at which the testing is performed,
age, health status, and gender of the test subject [
2
]. Airborne transmission of the pathogen
beyond six feet is rare and depends on special circumstances [
7
]. Large-sized droplets can
be blocked by filtration barriers with relative ease; however, more sophisticated processes
and defensive mechanisms are required for protection from the contagion-loaded aerosol.
In real-life conditions, a diverse set of variables dictate the particle size distribution in
droplets that are generated from any form of expiratory exertion [
8
]. These variables,
therefore, influence the chances of transmission among people. Against the backdrop
of these variables, it is not realistic to completely eliminate the chances of pathogenic
transfer and infection through respiration by adopting the habit of wearing a face covering.
However, the use of face coverings is still presented as one of the most significant risk-
mitigation strategies considering emergent epidemiological evidence [
9
]. In response to the
SARS-CoV-2 pandemic, the use of fabric face coverings by the public is recommended by
various governments and intergovernmental agencies to reduce transmission of the virus.
The US Center for Disease Control and Prevention (CDC) issued a public advisory to wear
face coverings in public gatherings, where social distancing was difficult to observe during
the initial days of the COVID-19 pandemic. As this global health crisis has evolved, a
voluminous body of independent research has continued to emerge that links the prevalence
of face-covering use with reduced infection rates [
10
–
12
]. Therefore, it has become vital
for global efforts to mitigate the health challenges posed by airborne or aerosol-based
contagious pathogens such as SARS-CoV-2 and understand the protection mechanism of
face coverings. [13–15]
There are three main types of face coverings available to reduce the spread or con-
traction of COVID-19. These types are fabric face coverings (woven, knitted, woven
nonwoven/knitted nonwoven hybrids), surgical masks, and professional-grade medical
respirators (N95). With limited supplies of personal protection equipment (N95s and surgi-
cal masks) for frontline medical workers amid a severe supply-side shock, CDC advised to
use available household woven or knitted fabrics to make face coverings [16].
Our globalized world is currently reeling from multiple concurrent crises including
the COVID-19 pandemic and the Russia–Ukraine conflict. These crises have spiraled fuel
prices upwards and made international shipping cost prohibitive in many cases. Likewise,
the global textile supply chain has been battered hard by simultaneous demand and supply
shocks [
17
]. These uncertain times provide an opportunity to think about how future
global health crises can be better managed via the utilization of alternative strategies
such as the employment of reusable face coverings to combat scarcity [
18
]. This review
paper focuses on readily available knit and woven fabrics used in reusable (launderable)
face coverings for daily use by the public. There is a considerable gap in the research
literature that informs scholastic understanding of the interactions between textile fibers
and the viral load carrying droplets at the micro-level. Much of the existing scholarship
about textile-based protection from pathogenic agents revolves around fabric construction
parameters, fit, and types of apparel-sourced face-covering materials. To be viable and
comfortable, face coverings need to be sufficiently breathable in addition to being efficient
at blocking particles. Knitted face coverings can be good alternatives to disposable medical
masks in critical coronavirus outbreak situations when communities are facing shortages
of more “specialized,” single-use face coverings. Knitted face covers are cost-effective,
environmentally friendly, and reusable. However, the research found that knitted fabrics
show lower droplet-blocking efficiency compared to woven fabrics even when both fabrics
have the same porosity [19].
For this review paper, the authors identified relevant papers through the Summon
database of North Carolina State University. Some additional literature was availed from the
Textiles 2023,3144
public domain due to pandemic response policies. An extensive collection of
SARS-CoV-2
literature was available online through the National Library of Medicine [
20
]. Because
of the huge volume of research available, papers that did not provide unique insight or
lend themselves to a more informed selection of fabric face covering were not included in
the review. It is likely additional research has been published between the time when this
review began and ended. In addition to academic research, guidance, and specifications
of various governmental and inter-governmental agencies were also considered. This
type of guidance is generally more visible and accessible to the public than peer-reviewed
publications, so it is important to consider how policy is related to the most current
scientific conclusions.
2. Mechanisms of Face Coverings
The filtration function provided by fabric face coverings is not necessarily based upon
sieving out larger particles that attempt to pass through to the other side of the fibrous as-
sembly, rather, it is delineated by aerodynamic, electrostatic, and molecular interactions [
21
].
Therefore, textile-based face coverings use various means to offer protection against the
virions riding atop respiratory droplets from reaching the respiratory tract. Woven or knit-
ted fabric face coverings employ the same filtration mechanisms as N95 and surgical masks
that might not be accessible to many economically vulnerable population segments around
the world [
22
], albeit at a lower efficiency [
23
]. At least four papers reviewed include useful
explanations of filtration mechanisms and how textile face-covering materials utilize these
mechanisms [
13
,
24
–
26
]. No conflict was found among these explanations, and they provide
context for data from other papers as well.
2.1. Interception and Impaction
Air curves around the fibers or yarns in textile structures; droplets are unable to follow
the air’s trajectory and collide with the textile fibers instead (Figure 1). Interception and
impaction mechanisms are relevant to fabric face coverings because viruses typically travel
on droplets that are large enough to be stopped by contact with textile fibers. Droplet
sizes of 0.1–1 micron are stopped by interception. Droplets larger than 1 micron experience
impaction [13,25,26].
Textiles 2023, 3, FOR PEER REVIEW 3
For this review paper, the authors identified relevant papers through the Summon
database of North Carolina State University. Some additional literature was availed from
the public domain due to pandemic response policies. An extensive collection of SARS-
CoV-2 literature was available online through the National Library of Medicine [20]. Be-
cause of the huge volume of research available, papers that did not provide unique insight
or lend themselves to a more informed selection of fabric face covering were not included
in the review. It is likely additional research has been published between the time when
this review began and ended. In addition to academic research, guidance, and specifica-
tions of various governmental and inter-governmental agencies were also considered.
This type of guidance is generally more visible and accessible to the public than peer-
reviewed publications, so it is important to consider how policy is related to the most
current scientific conclusions.
2. Mechanisms of Face Coverings
The filtration function provided by fabric face coverings is not necessarily based
upon sieving out larger particles that attempt to pass through to the other side of the fi-
brous assembly, rather, it is delineated by aerodynamic, electrostatic, and molecular in-
teractions [21]. Therefore, textile-based face coverings use various means to offer protec-
tion against the virions riding atop respiratory droplets from reaching the respiratory
tract. Woven or knitted fabric face coverings employ the same filtration mechanisms as
N95 and surgical masks that might not be accessible to many economically vulnerable
population segments around the world [22], albeit at a lower efficiency [23]. At least four
papers reviewed include useful explanations of filtration mechanisms and how textile
face-covering materials utilize these mechanisms [13,24–26]. No conflict was found among
these explanations, and they provide context for data from other papers as well.
2.1. Interception and Impaction
Air curves around the fibers or yarns in textile structures; droplets are unable to fol-
low the air’s trajectory and collide with the textile fibers instead (Figure 1). Interception
and impaction mechanisms are relevant to fabric face coverings because viruses typically
travel on droplets that are large enough to be stopped by contact with textile fibers. Drop-
let sizes of 0.1–1 micron are stopped by interception. Droplets larger than 1 micron expe-
rience impaction [13,25,26].
Figure 1. Filtration mechanism of face coverings [27].
Figure 1. Filtration mechanism of face coverings [27].
Textiles 2023,3145
2.2. Diffusion
At the submicron scale, pathogenic particles collide with the air molecules in a process
called diffusion. Collisions with gaseous molecules cause these particles to slam into the
fibers instead of following normal trajectories through the inter-fiber or inter-yarn voids
(Figure 1) [
25
,
26
]. Filtration by diffusion requires very fine microfibers to trap very small
particles. This mechanism is less commonly employed by woven and knitted face-covering
fabrics [13].
2.3. Electrostatic Attraction
Particles are attracted to oppositely charged fibers. While air curves around the
fibers, the particles adhere by electrostatic attraction, preventing them from reaching the
respiratory tract (Figure 1) [13,25,26].
3. Consideration for Developing Face Coverings
3.1. Moisture Management
Fabric hydrophilicity/hydrophobicity came into focus as another key factor in face-
covering comfort and effectiveness. As previously discussed, the hydrophobicity of syn-
thetic fibers can have a direct impact on filtration efficiency related to electrostatic inter-
actions. Knitted and woven fabrics that contain hydrophilic fibers have an affinity to
droplets [
19
]. More often, hydrophobicity is considered a comfort factor for wearers of
fabric face coverings. WHO (2020) [
28
] recommends a moisture-absorbing inner layer.
An absorbent inner layer moves moisture away from direct contact with the wearer’s
face. Although holding droplets may increase the droplet-blocking performance, knitted
fabrics can also hold viruses and must be decontaminated by washing. Fiber hairiness and
low porosity both have a positive effect on filtration efficiency but may negatively impact
moisture management and wearing comfort [
29
]. Increasing fabric cover due to increasing
fiber hairiness could undermine the moisture management function down to the fibrous
scale [
30
]. WHO (2020) suggests using water-resistant fabrics for the outer layer of face
coverings [
28
]. The assumption is that the layer will keep infectious droplets in the envi-
ronment from passing through the face covering and infecting the wearer. Another study
found that hydrophobic surfaces pose a greater risk because droplets remain intact, infec-
tious, and liable to spread through touch; droplets on an absorbent surface dry quickly, and
thus viruses become inactive [
31
]. Iqbal et al. developed moisture management of woolen
cloth masks using moisture-responsive wool fibers along with an electret polypropylene
non-woven layer [
32
,
33
]. They found that due to the existence of moisture-responsive
wool fibers, the developed cloth mask showed significant moisture management properties
compared to commercial 3-ply masks. Parlin et al. studied the hydrophobicity of cotton,
polyester, and silk fabrics in single and multiple layers [
34
]. They found that compared
to cotton and polyester, drops of water on silk had a significantly higher contact angle
(
cotton: 43◦
; polyester: 61
◦
; unwashed silk: 120
◦
; washed silk: 107
◦
) and lower spreading
when wetting occurred (cotton: 87 mm
2
; polyester: 25 mm
2
; unwashed silk: 12 mm
2
;
washed silk: 5 mm
2
). Cotton fiber consists of more than 95% cellulose, and the abun-
dance of OH groups in the cellulose structure is responsible for the hydrophilic behavior
of cotton [
35
]. Other tests, including resistance to aerosolized droplets, showed similar
results. Based on the hydrophobicity and lack of capillary action exhibited in the lab, the
researchers suggest silk face coverings should be studied in a clinical setting. They also
cite the advantages of their lightweight and breathability although they did not measure
breathing resistance, and the one reference cited for this claim has been called into question
by other authors [
34
]. Polypropylene is also an attractive fiber choice for the construction
of face coverings due to its moisture-wicking function [
36
], which allows polypropylene to
wick moisture away from an area of high concentration to a lower concentration area. In a
high cover factor fabric, the ability to wick moisture to prevent uncomfortable moisture
build-up is a considerable advantage. Wearing comfort is critical to ensure that wearers
Textiles 2023,3146
are not fatigued by the difficulty in breathing through the face covering. Fatigue could
promote lax attitudes towards the face covering wearing etiquette [37].
3.2. Breathing Resistance Factors
A common theme in several of the papers reviewed is the importance of balancing high
filtration efficiency with low breathing resistance [
13
]. Filtration alone is relatively easy to
achieve by adding material layers or minimizing porosity, but it is obvious that completely
stopping droplets or viruses from passing through the fabric will also significantly limit
air from being inhaled or exhaled through the material, and the same could be said for
the moisture exchange. Although product design is not addressed in this paper, it is
well-documented that air will take the path of least resistance [
38
]. In use, a fabric or
fabric assembly with high breathing resistance requires a well-fitted face covering design
to prevent breath from bypassing the material and entering/exiting through gaps at sides,
top, bottom, or even through seams.
Davies et al. conclude that based on filtration efficiency and breathability, a pillowcase
or cotton T-shirt is the most appropriate household material for an improvised face covering.
As noted above, the tea towel (and vacuum cleaner bag) provided higher filtration, but it
was deemed unsuitable due to poor breathability. It is interesting to note that the authors
recommend the T-shirt for its stretchy quality, suggesting that it would likely provide a
better fit [
39
]. Other publications, including the WHO guidance [
28
], recommend against
stretch fabrics because porosity can increase under tension. Pores of knitted fabrics are
generated on each loop and interlacing point. The breathability of the fabric is strongly
correlated with porosity. When knitted fabrics are stretched, pore sizes are also enlarged
easily, which enables water vapor and air to pass through the fabrics easily [
40
]. Therefore,
knitted fabrics are more breathable compared to woven fabrics if both fabrics are fitted on
dynamic objects using the same yarn and density [41].
In a letter to the editor regarding a paper by Konda et al., researchers analyze the
relationship between flow rate and breathing resistance [
42
]. They also note that flow
rate has direct and significant implications for particle filtration conclusions. The authors
re-analyzed the data reported by Konda et al. and attempted to replicate the results
themselves. They found an expected linear relationship between pressure drop and both
face velocity and the number of fabric layers for all fabrics tested. The original data by
Konda et al. did not show this trend. At standard face velocity (10 cm/s) and area (150 cm
2
),
the pressure drops across even a single layer of 625 threads per inch cotton fabric would
fail the breathing criteria for N95 respirators. The fabric pressure drop is 335 Pa, while
the respirator requires less than 245 Pa for exhalation and less than 343 Pa for inhalation.
A four-layer construction had a pressure drop of 1246 Pa. The letter estimates that to
create the pressure drop reported in the original paper, the face velocity would have to be
0.075 cm/s, orders of magnitude less than typically used for testing. Further, the writers
explain how very low velocities can present experimental challenges as evaporation over
the long air exchange leads to an overestimation of filtration efficiency. Rather than very
low face velocity, the Konda data could be explained by very high area, but again, it would
have to be orders of magnitude larger than normal [43].
3.3. Balancing between Filtration and Breathability
Breathing resistance is less vigorously analyzed than filtration for fabric face coverings
because most common fabrics provide sufficiently low breathing resistance. As previously
noted, balance is required, so breathing resistance becomes more critical as high filtration
is achieved. Filtration and breathability resistance tests are crucial for reusable fabric face
coverings as coverings remain in close contact to skin for a long period of time [
44
]. The tea
towel in the Davies paper that performed so well in filtration was rejected due to breathing
resistance [
39
]. Therefore, an optimum balance between requisite ventilation and filtration
function is pivotal. Aydin et. al. evaluated the performance of four knitted face coverings
prepared from household fabrics while considering a medical mask as a benchmark [
19
].
Textiles 2023,3147
Breathability (air permeability), water absorption properties, and filtration efficiency of
these face coverings were assessed to explain the relationships among breathability, porosity,
flow rate, and filtration efficiency. To investigate the performance of face coverings against
high-velocity droplets, droplets are released from 25 mm away from a nozzle to replicate
sneezing and coughing (Table 1).
Table 1. Sample details and performance parameters of face coverings reproduced from [19].
Sample No. of
Layer
Fiber
Content Weight Porosity%
Mean ±SD
n=9
Blocking Efficiency (%) at 25 mm High
Momentum Droplet) Breathability
(mm/pa-s);
Mean ±SD, n = 3
(g/m2)Minimum Medium Maximum
Medical mask -
Polypropylene
53.9 n/a 96.4 98.5 99.9 1.83 ±0.15
Used shirt 1 100% cotton 114.2 0.7 ±1 87.9 96.8 99.8 1.37 ±0.06
Used undershirt 1 100% cotton 111.5 4.5 ±1 41.1 81.9 95.2 10.7 ±0.66
Used undershirt 2 100% cotton - - 78.3 94.1 98.3 5.53 ±0.35
Used undershirt 3 100% cotton - - 96.8 98.9 99.8 3.77 ±0.06
New t-shirt 160/40%
cotton/poly 183.2 1.1 ±0.3 42 83.1 98.3 7.23 ±0.55
New t-shirt 260/40%
cotton/poly - - 94 98.1 99.6 3.87 ±0.06
New t-shirt 360/40%
cotton/poly - - - >98.1 - 2.63 ±0.06
It is found that a single layer of most knitted home fabrics can block droplets reasonably
at various velocities. Blocking efficiency is close to that of medical masks maintaining
the same or higher breathability when two or three layers are used. Droplet blocking
efficiency and breathability of the medical mask is 98.5% (minimum filtration efficiency
96.4%) and 1.83 mm/Pa s, respectively. The performance of a single-layer used knitted
shirt was evaluated first. The median blocking efficiency and breathability of the fabric
were 96.8% and 1.37 mm/Pa s, respectively. It should be noted that the minimum filtration
efficiency of this face covering is 87.9%. The face covering prepared from a 100% cotton,
new undershirt shows weaker performance compared to the face covering prepared from
a used knitted shirt. Although it has higher breathability (10.7 mm/Pa s), the median
filtration efficiency was 81.9% (minimum filtration efficiency is 41.1%). The filtration
efficiency increases significantly with the increase in the number of layers. The median
filtration efficiency of this face covering increased to 94.1 and 98.9% for two and three
layers, respectively. However, additional layers reduced the breathability of the covering
drastically. The breathability of three-layer coverings decreased to 4 mm/Pa s. A face
covering was also prepared from a new knitted T-shirt of 60/40% cotton/polyester to
investigate the effect of fiber composition. Droplet blocking efficiency and breathability
of single-layer fabric were 83.1% (minimum filtration efficiency 42%) and 7.23 mm/Pa s,
respectively. The filtration efficiency of this face covering increased to more than 98.1%,
and breathability decreased to 2.8 mm/Pa s for three layers. Knitted face coverings were
also effective against low-momentum droplets when droplets were released from 300 mm
away through a nozzle mimicking release of droplets from a nearby person while talking.
The velocity and size of droplets decreased while traveling through the air [
45
,
46
]. At lower
velocity, the median filtration efficiency of the 60/40% cotton/polyester T-shirt fabric was
94.2% (minimum filtration efficiency is 82.5%). For the same velocity, median filtration
efficiency increased to more than 94.2% for two layers of fabric. Zangmeister et al. report
that air permeability, extension, and breathing resistance are related to yarn count, fabric
mass, and weave pattern [47].
4. Development of Face Coverings
4.1. Fiber and Yarn Construction
Correlation between increasing yarn twist, increasing yarn compactness, and reduced
porosity is well established [
48
]. A study by Zhang et al. concluded that in addition to
differences in the fabric construction parameters, the inter-fiber spaces in the yarns for
polyester, polypropylene, silk, and nylon were also a factor in filtration efficiency [
26
].
Nonwoven materials take advantage of nanofibers to provide high surface area for good
Textiles 2023,3148
filtration efficiency. Most woven and knitted fabrics do not utilize such fine fibers, but
“hairiness” can increase the surface area of yarns. As noted by Zangmesiter et al., raised
fibers increase filtration efficiency [
47
]. Hairiness, i.e., fibers protruding out of the yarn’s
body, increases the possibility of successful pathogenic particle interception before these
reach the respiratory canal [
49
]. Hairiness can be created through the selection of yarn
spinning technology and pile construction in the fabric or by mechanical finishing of
the fabric surface. In contrast, it must be considered that pulling too many fibers from
the interlaced structure can create voids in the base fabric, which will reduce filtration
efficiency. Fabrics that have a high tendency to form pills or that have been subjected to
severe mechanical processes may not have a long useful life, though they still provide
longer use than disposable surgical masks [50].
4.2. Fabric Construction
4.2.1. Knit Fabric Construction
Face coverings having higher breathability show lower leakage around the sides of the
face covering as more air passes through the covering material, which is designed to block
contagion-carrying droplets [
19
]. Higher air permeability may also reduce droplet-blocking
efficiency, so it is important to maintain a proper balance between breathability and viral
particle-blocking efficiency in developing knitted face coverings [
51
,
52
]. Air permeability
of knitted face coverings depends on several factors including fabric thickness, density,
fabric structure, material types, fiber diameter, and finishing processes [
41
]. Porosity and
structure vary based on the stitch length and machine gauge while knitting [
53
]. This
means that fabric with the same construction but different weights, looseness, porosity,
or thickness could be compared to determine the effect of these properties on aerosol
filtration efficiency. Knitted fabrics deform or stretch more than woven fabric or nonwoven
medical masks. This leads to higher porosity and lower filtration efficiency against droplets.
High-speed impact of droplets causes higher deformation of knitted fabric samples while
medical mask materials do not bend much (Figure 2) [
19
]. Both transmission control
and infection prevention functions of knitted fabrics are affected by their relatively low
dimensional stability.
Textiles 2023, 3, FOR PEER REVIEW 7
Nonwoven materials take advantage of nanofibers to provide high surface area for good
filtration efficiency. Most woven and knitted fabrics do not utilize such fine fibers, but
“hairiness” can increase the surface area of yarns. As noted by Zangmesiter et al., raised
fibers increase filtration efficiency [47]. Hairiness, i.e., fibers protruding out of the yarn’s
body, increases the possibility of successful pathogenic particle interception before these
reach the respiratory canal [49]. Hairiness can be created through the selection of yarn
spinning technology and pile construction in the fabric or by mechanical finishing of the
fabric surface. In contrast, it must be considered that pulling too many fibers from the
interlaced structure can create voids in the base fabric, which will reduce filtration effi-
ciency. Fabrics that have a high tendency to form pills or that have been subjected to se-
vere mechanical processes may not have a long useful life, though they still provide longer
use than disposable surgical masks [50].
4.2. Fabric Construction
4.2.1. Knit Fabric Construction
Face coverings having higher breathability show lower leakage around the sides of
the face covering as more air passes through the covering material, which is designed to
block contagion-carrying droplets [19]. Higher air permeability may also reduce droplet-
blocking efficiency, so it is important to maintain a proper balance between breathability
and viral particle-blocking efficiency in developing knitted face coverings [51,52]. Air per-
meability of knitted face coverings depends on several factors including fabric thickness,
density, fabric structure, material types, fiber diameter, and finishing processes [41]. Po-
rosity and structure vary based on the stitch length and machine gauge while knitting
[53]. This means that fabric with the same construction but different weights, looseness,
porosity, or thickness could be compared to determine the effect of these properties on
aerosol filtration efficiency. Knitted fabrics deform or stretch more than woven fabric or
nonwoven medical masks. This leads to higher porosity and lower filtration efficiency
against droplets. High-speed impact of droplets causes higher deformation of knitted fab-
ric samples while medical mask materials do not bend much (Figure 2) [19]. Both trans-
mission control and infection prevention functions of knitted fabrics are affected by their
relatively low dimensional stability.
Figure 2. Comparative performance analysis between medical masks and knitted sample against
high-impact droplets. (A) Schematic presentation of the impact of a high-speed droplet on the med-
ical mask and knitted sample. (B) Impact response on medical masks and knitted samples [19].
Neupane and colleagues also reported a loss of filtration efficiency due to repeated
washing and drying of reusable knitted face coverings. These cycles increased the fabric
void sizes and morphed their shapes (Figure 3) [54]. Other sources warn against the use
of stretchy materials because of the tendency for voids to increase in size [13,55].
Figure 2.
Comparative performance analysis between medical masks and knitted sample against
high-impact droplets. (
A
) Schematic presentation of the impact of a high-speed droplet on the medical
mask and knitted sample. (B) Impact response on medical masks and knitted samples [19].
Neupane and colleagues also reported a loss of filtration efficiency due to repeated
washing and drying of reusable knitted face coverings. These cycles increased the fabric
void sizes and morphed their shapes (Figure 3) [
54
]. Other sources warn against the use of
stretchy materials because of the tendency for voids to increase in size [13,55].
Textiles 2023,3149
Textiles 2023, 3, FOR PEER REVIEW 8
Figure 3. Effect of washing and drying cycles on the fabric porosity. (A) bright field microscopic
images of unwashed CM9, and after (B) first, (C) second, (D) third, and (E) fourth washing and
drying cycles [54].
4.2.2. Woven Fabric Construction
Bhattacharjee et al. provide a list of factors to consider in designing an optimal woven
face covering. These include filtration efficiency, breathing resistance, filter material, wa-
ter resistance of the outer layer, high surface area, number of layers, thread count and
fineness of weave, fabric thickness, and fabric pore size. High filtration efficiency, low
breathing resistance, and the need to balance these are highlighted in several studies
[13,24]. High thread count cottons are commonly recommended as a readily available op-
tion for homemade face coverings [56]. One paper specifically recommends a density of
300–350 threads per inch [13]. Hao et al. acknowledge the potential impact of fiber diam-
eter, thickness, permeability, and fiber material on filtration performance, but use thread
count as the parameter to study the filtration potential of fabrics in their research. This
choice was confirmed by testing three pillowcase fabrics from the same manufacturer. The
pillowcases had 1000, 600, and 400 threads per inch and 55.0%, 44.6%, and 19.9% filtration
efficiency, respectively. No other fabric parameters were reported [24]. Several studies
include tea towels. Davies reported a filtration efficiency of 83.24% for a single layer of a
tea towel and 96.71% for two layers [39]. This was higher than for any other common
textile item tested and comparable to results for a nonwoven surgical mask and a vacuum
cleaner bag. Filtration testing utilized a Henderson apparatus to generate microbial aero-
sols and a downstream air sampling device to collect microorganisms that penetrated the
test material and those from a control stream with no filtration. The method measures
actual bacterial particles similar in size to influenza virus particles rather than latex or
NaCl proxies [39]. This could increase the relevance of the data, but the unique testing
protocol makes it difficult to compare results with those of other studies. Rengasamy et
al. also found towels to be one of the most effective filters among common household
textiles, though it is not clear if the towels tested were tea towels, or something closer to
bath towels [57].
In many of the papers reviewed, complete fabric descriptions are not included. It
would not be fair to assume, based on the data reported by Davies et al., that any tea towel
will provide superior filtration to fabric from a scarf or pillowcase. Without more descrip-
tive detail, it is also impossible to identify a trend that might provide guidance in optimiz-
ing fabric construction for filtration. Clase et al. reviewed 25 papers and similarly la-
mented the lack of details required for reproducibility [14]. Despite expressed skepticism
about the value of fabric face coverings for general use, the World Health Organization
[28] provides a more nuanced view of fabric construction, noting that the filtration
Figure 3.
Effect of washing and drying cycles on the fabric porosity. (
A
) bright field microscopic
images of unwashed CM9, and after (
B
) first, (
C
) second, (
D
) third, and (
E
) fourth washing and
drying cycles [54].
4.2.2. Woven Fabric Construction
Bhattacharjee et al. provide a list of factors to consider in designing an optimal woven
face covering. These include filtration efficiency, breathing resistance, filter material, water
resistance of the outer layer, high surface area, number of layers, thread count and fineness
of weave, fabric thickness, and fabric pore size. High filtration efficiency, low breathing
resistance, and the need to balance these are highlighted in several studies [
13
,
24
]. High
thread count cottons are commonly recommended as a readily available option for home-
made face coverings [
56
]. One paper specifically recommends a density of
300–350 threads
per inch [
13
]. Hao et al. acknowledge the potential impact of fiber diameter, thickness,
permeability, and fiber material on filtration performance, but use thread count as the
parameter to study the filtration potential of fabrics in their research. This choice was
confirmed by testing three pillowcase fabrics from the same manufacturer. The pillowcases
had 1000, 600, and 400 threads per inch and 55.0%, 44.6%, and 19.9% filtration efficiency,
respectively. No other fabric parameters were reported [
24
]. Several studies include tea
towels. Davies reported a filtration efficiency of 83.24% for a single layer of a tea towel
and 96.71% for two layers [
39
]. This was higher than for any other common textile item
tested and comparable to results for a nonwoven surgical mask and a vacuum cleaner
bag. Filtration testing utilized a Henderson apparatus to generate microbial aerosols and a
downstream air sampling device to collect microorganisms that penetrated the test material
and those from a control stream with no filtration. The method measures actual bacterial
particles similar in size to influenza virus particles rather than latex or NaCl proxies [
39
].
This could increase the relevance of the data, but the unique testing protocol makes it
difficult to compare results with those of other studies. Rengasamy et al. also found towels
to be one of the most effective filters among common household textiles, though it is not
clear if the towels tested were tea towels, or something closer to bath towels [57].
In many of the papers reviewed, complete fabric descriptions are not included. It
would not be fair to assume, based on the data reported by Davies et al., that any tea
towel will provide superior filtration to fabric from a scarf or pillowcase. Without more
descriptive detail, it is also impossible to identify a trend that might provide guidance in
optimizing fabric construction for filtration. Clase et al. reviewed 25 papers and similarly
lamented the lack of details required for reproducibility [
14
]. Despite expressed skepticism
about the value of fabric face coverings for general use, the World Health Organization [
28
]
provides a more nuanced view of fabric construction, noting that the filtration efficiency of
woven fabrics is also dependent on the tightness of the weave and fiber or thread diameter.
Zhao et al. found that base weight and density are not clearly related to filtration efficiency;
Textiles 2023,3150
however, they recommended that cotton woven or knit fabric for face coverings should be
of high density or used in multiple layers [
58
]. Such conflicting statements only serve to
confuse an already complicated issue. Higher thread counts can be expected to provide
higher filtration efficiency if all other factors are equal. However, all other factors are rarely
equal. Fabrics that are commercially available to the public and even to most non-vertically
integrated manufacturers will have additional variables. Research at Florida Atlantic
University more strongly emphasizes that thread count alone is not sufficient to guarantee
better filtration [
59
]; however, this conclusion is based not just on fabric construction but
on face covering design. The authors of the study cited the example of a bandana with
a high thread count performing more poorly than fitted face coverings of lower thread
count fabric.
Clase et al. [
14
] reviewed the study of Zhao et al. [
58
], where 25 available articles were
studied, and found that expected fabric details were not provided in most studies and some
recommendations were offered based on the limited data. Two layers of 600 thread count
cotton and a combination of 600-count cotton and 90-count flannel reportedly performed
well, while two layers of 80-count cotton did not. From this, the authors recommend
cotton or flannel at least 100 threads per inch with a minimum of 2 layers to get better
filtration efficiency with minimal breathing resistance [
14
]. The calculation that led to the
100 threads per inch recommendation is unclear, and neither “cotton” nor “flannel” is an
adequate description for sourcing suitable fabric. In the current environment, making such
a recommendation is dangerous not only in terms of academic reputation but also for the
health and safety of those who take the advice. The review by Clase et al. did include a
well-researched and organized table of available information from the reviewed literature.
It also concludes by returning to the theme of incomplete evidence in the discussion of
clinical studies [
14
]. A paper by Zangmeister et al. (2020) clearly explained the fallacy of
correlating cover factors directly to filtration. Cover factor expresses the area covered by
the yarns of a fabric. There are holes through the yarns as well as between them, which
leads to underestimating fabric porosity. The authors list other relevant factors as yarn
mass, fabric texture, and fabric composition [
47
]. The group specifically set out to establish
relationships among fabric parameters, filtration efficiency, and breathing resistance. They
systematically selected and characterized fabrics by fiber, thread count, weave pattern, and
mass [
47
]. The research is well-documented and organized. The authors report a ‘complex
interplay’ of fabric parameters with filtration efficiency. Counter to conventional wisdom,
the best-performing fabrics had moderate thread count and visible raised fibers [47].
5. Factors Affecting Filtration Performance
Several factors govern the performance of face coverings as pointed out in Figure 4.
Factors that are related to knitted and woven fabrics have been reviewed in the follow-
ing sections.
5.1. Electrostatic Charge
Charging of surgical masks allows them to take advantage of electrostatic interactions
between fibers and environmental particles to provide high filtration efficiency with low
breathing resistance. Household air filters use the same mechanism. It is more difficult
to impart a permanent charge on reusable, woven or knitted face-covering fabrics [
24
].
Zhao et al. studied the impact of triboelectric effect on the filtration potential of various
fiber-based assemblies [
58
]. They also observed that latex and nitrile used in fabrication of
gloves could be rubbed against polypropylene or other fibers to introduce or reintroduce
static charges. Alternatively, fabrics of fibers such as hygroscopic silk, with no electret
fibers, can be charged for a short duration by rubbing against polypropylene fabrics [
24
].
When Zhao et al. rubbed fabric samples with latex gloves to create a triboelectric charge,
cotton fabrics showed a decrease in filtration efficiency, but this was attributed to increased
pore size due to the mechanical action of rubbing [
58
]. Other samples showed improved
filtration when charged polyester and silk samples lost charge within 30 min, while nylon
Textiles 2023,3151
and polypropylene samples held a charge much longer. This paper does not extend to
practical methods of consistent and durable charging for face-covering fabrics, though the
authors suggest additional study, including the possibility of interlayer friction for charging
a multi-layer assembly [
58
]. Ghatak et al. suggested an interesting potential application of
a triboelectric nanogenerator (TENG) where they recommended a layered face-covering
design comprised of four different triboelectrically charged fabrics and an electrocution
layer (EL). Under the phenomena of TENG, human activities including breathing, speaking,
etc., generate electric charge. This charge on the electrocution layer was supposed to disrupt
the protein shell of SARS-CoV-2 virus if the virions encountered the EL [
60
]. Another study
found no significant impact of charge on filtration for the tested textile materials [
47
]. It was
reported that on washing, N95 respirators and surgical masks lose their permanent charges
from electret fibers, leading to reduction in their filtration potential. It was also noted that
fabric face coverings that do not possess permanent charges do not exhibit a major drop
in filtration efficiency after laundering [
54
]. Zhao and coworkers’ study did not explore
the effects of laundering on the polypropylene fabric face covering’s charge concentration.
It would be interesting to find if some charges were retained by the polypropylene fibers
after the laundering process. Electrostatic filtration is adversely affected by increasing
humidity levels due to poor moisture management in hydrophilic fibers [
61
]. Zhao et al.
reported that hydrophilic nylon fiber, which demonstrated charge build-up in normal
relative humidity, began losing its static charge once the humidity levels were increased
in the environment [
58
]. Hydrophobic fibers demonstrate higher levels of static charge
build-up and can retain some of that charge even in more humid conditions, which could
make them attractive candidates for use in woven and knitted face coverings. On the
other hand, hydrophilic cellulosic fibers such as cotton fibers that contain mostly cellulose
possess a low charge distribution (Figure 5) [
62
,
63
]. Zhao et al. showed that polypropylene
provided low filtration efficiency at 85% RH. Hydrophobic polypropylene fiber does not
adsorb water, which could completely replace the static charge-carrying capacity [
58
]. The
reduction in the static charge concentration was instead predominantly due to the increased
conductivity of humid air that caused leakage of charges from the fiber surface [30].
Ghatak et al. showed that relative humidity in the range of 60–90% did have a
pronounced effect on the efficacy of the electrocution mechanism. Increased relative
humidity promoted the unwanted flow of charge towards the electrocution layer (EL) [
60
].
Human breath has been shown to have a relative humidity value in the range of 65–88%
in European and Middle Eastern conditions [
64
]. Therefore, it can be argued that the
electrocution function of the EL layer will be weakened by persistent relative humidity
levels that repeatedly exhaust charges by channeling them to the EL layer.
Textiles 2023, 3, FOR PEER REVIEW 10
Figure 4. Factors affecting the development and filtration performance of reusable face coverings.
5.1. Electrostatic Charge
Charging of surgical masks allows them to take advantage of electrostatic interac-
tions between fibers and environmental particles to provide high filtration efficiency with
low breathing resistance. Household air filters use the same mechanism. It is more diffi-
cult to impart a permanent charge on reusable, woven or knitted face-covering fabrics
[24]. Zhao et al. studied the impact of triboelectric effect on the filtration potential of var-
ious fiber-based assemblies [58]. They also observed that latex and nitrile used in fabrica-
tion of gloves could be rubbed against polypropylene or other fibers to introduce or rein-
troduce static charges. Alternatively, fabrics of fibers such as hygroscopic silk, with no
electret fibers, can be charged for a short duration by rubbing against polypropylene fab-
rics [24]. When Zhao et al. rubbed fabric samples with latex gloves to create a triboelectric
charge, cotton fabrics showed a decrease in filtration efficiency, but this was attributed to
increased pore size due to the mechanical action of rubbing [58]. Other samples showed
improved filtration when charged polyester and silk samples lost charge within 30 min,
while nylon and polypropylene samples held a charge much longer. This paper does not
extend to practical methods of consistent and durable charging for face-covering fabrics,
though the authors suggest additional study, including the possibility of interlayer fric-
tion for charging a multi-layer assembly [58]. Ghatak et al. suggested an interesting po-
tential application of a triboelectric nanogenerator (TENG) where they recommended a
layered face-covering design comprised of four different triboelectrically charged fabrics
and an electrocution layer (EL). Under the phenomena of TENG, human activities includ-
ing breathing, speaking, etc., generate electric charge. This charge on the electrocution
layer was supposed to disrupt the protein shell of SARS-CoV-2 virus if the virions en-
countered the EL [60]. Another study found no significant impact of charge on filtration
for the tested textile materials [47]. It was reported that on washing, N95 respirators and
surgical masks lose their permanent charges from electret fibers, leading to reduction in
their filtration potential. It was also noted that fabric face coverings that do not possess
permanent charges do not exhibit a major drop in filtration efficiency after laundering
[54]. Zhao and coworkers’ study did not explore the effects of laundering on the polypro-
pylene fabric face covering’s charge concentration. It would be interesting to find if some
charges were retained by the polypropylene fibers after the laundering process. Electro-
static filtration is adversely affected by increasing humidity levels due to poor moisture
management in hydrophilic fibers [61]. Zhao et al. reported that hydrophilic nylon fiber,
which demonstrated charge build-up in normal relative humidity, began losing its static
charge once the humidity levels were increased in the environment [58]. Hydrophobic
fibers demonstrate higher levels of static charge build-up and can retain some of that
Figure 4. Factors affecting the development and filtration performance of reusable face coverings.
Textiles 2023,3152
Textiles 2023, 3, FOR PEER REVIEW 11
charge even in more humid conditions, which could make them attractive candidates for
use in woven and knitted face coverings. On the other hand, hydrophilic cellulosic fibers
such as cotton fibers that contain mostly cellulose possess a low charge distribution (Fig-
ure 5) [62,63]. Zhao et al. showed that polypropylene provided low filtration efficiency at
85% RH. Hydrophobic polypropylene fiber does not adsorb water, which could com-
pletely replace the static charge-carrying capacity [58]. The reduction in the static charge
concentration was instead predominantly due to the increased conductivity of humid air
that caused leakage of charges from the fiber surface [30].
Ghatak et al. showed that relative humidity in the range of 60–90% did have a pro-
nounced effect on the efficacy of the electrocution mechanism. Increased relative humidity
promoted the unwanted flow of charge towards the electrocution layer (EL) [60]. Human
breath has been shown to have a relative humidity value in the range of 65–88% in Euro-
pean and Middle Eastern conditions [64]. Therefore, it can be argued that the electrocution
function of the EL layer will be weakened by persistent relative humidity levels that re-
peatedly exhaust charges by channeling them to the EL layer.
Figure 5. Various positive and negative charge densities for textile fibers [65].
5.2. Porosity
Porosity, or lack thereof, is a major factor in interception and impaction filtration
mechanisms commonly employed by woven and knitted face coverings [66]. As interest
in fabric face coverings developed early in the COVID-19 pandemic, advice for achieving
effective filtration was based primarily on assumptions about increasing the fabric cover
factor. This represents an important transition because cotton gauze was considered suc-
cessful in reducing infection rates for plague epidemics in the first half of the 20th century
[67], though sources that report this historic context do not provide much explanation.
5.3. Layers
Multilayer face coverings show improved efficacy. One recommended three-layer
system includes a breathable middle layer with an antimicrobial finish. This system was
claimed to be effective at filtering the SARS-CoV-2 virus [68]. Two-layered fabric face cov-
erings require high yarn packing, low yarn porosity, and multilayered design to have suf-
ficient aerosol filtering efficiency. The type of fabric also determines the virus filtering
efficacy. In the laboratory setting, the efficacy against bacteriophage MS2 has been found
to be 50–70% for two-layer fabric face coverings, whereas, for three-ply surgical masks
and N95, the efficacy was 95–97% [54]. Sousa-Pinto et al. found that in terms of filtration
Figure 5. Various positive and negative charge densities for textile fibers [65].
5.2. Porosity
Porosity, or lack thereof, is a major factor in interception and impaction filtration mech-
anisms commonly employed by woven and knitted face coverings [
66
]. As interest in fabric
face coverings developed early in the COVID-19 pandemic, advice for achieving effective
filtration was based primarily on assumptions about increasing the fabric cover factor.
This represents an important transition because cotton gauze was considered successful
in reducing infection rates for plague epidemics in the first half of the 20th century [
67
],
though sources that report this historic context do not provide much explanation.
5.3. Layers
Multilayer face coverings show improved efficacy. One recommended three-layer
system includes a breathable middle layer with an antimicrobial finish. This system was
claimed to be effective at filtering the SARS-CoV-2 virus [
68
]. Two-layered fabric face
coverings require high yarn packing, low yarn porosity, and multilayered design to have
sufficient aerosol filtering efficiency. The type of fabric also determines the virus filtering
efficacy. In the laboratory setting, the efficacy against bacteriophage MS2 has been found to
be 50–70% for two-layer fabric face coverings, whereas, for three-ply surgical masks and
N95, the efficacy was 95–97% [
54
]. Sousa-Pinto et al. found that in terms of filtration and
breathability, two-layer nonwoven and knitted fabrics perform better. Nonetheless, these
performances are found to be wanting compared to the multilayered textile and surgical
masks when it comes to high levels of protection from the SARS-CoV-2 virus [69].
Hancock et al. and Mueller et al. conducted particle filtration performance test of at
least two-layer coverings made of 5–10 different hand-sewn woven fabrics loosely fitted
to the wearer’s face [
43
,
70
]. At the same time, they tested two types of standard three-
layer medical masks. The woven face coverings incorporating a nylon stocking overlayer
were also tested to see if the overlayer improved filtration capability. The results indicate
that five out of ten woven loose-fitting coverings had high particle filtration efficiency,
similar to the performance of three-layer surgical face coverings. Three-layer surgical
masks showed 75% filtration efficiency, whereas woven face coverings showed 30–60%
filtration efficiency. The authors recommended an extra layer of filter (organic cotton
batting, Pellon, or loosely woven cotton muslin) to improve the filtration efficiency of the
two-layer woven face coverings. Whiley et al. found that two-layer stretchy cotton denim
fabrics showed the highest filtration efficiency (90.90%) among woven fabrics [
71
]. Figure 6
shows the comparative graphs on the efficacy of increasing layers and flow rate. The ability
Textiles 2023,3153
of home-made (woven and knitted fabric) face covering to filter the ultrafine (0.02–0.1
µ
m)
particles at the velocity (0.5 to 25 cm/s) of coughing and their efficacy in a damp state
has been analyzed by O’Kelly et al [
23
]. The large particles are readily filtered whereas
the ultrafine particles decrease the efficiency. The filtration efficiency in a damp or wet
state after one laundering cycle reaches a maximum of 45%, which is less than in a dry
state. It was recommended that in case of an acute shortage of respirators, people can use
the homemade face covering with an extra layer as it gives approximately 90% particle
filtration efficiency.
Textiles 2023, 3, FOR PEER REVIEW 12
and breathability, two-layer nonwoven and knitted fabrics perform better. Nonetheless,
these performances are found to be wanting compared to the multilayered textile and sur-
gical masks when it comes to high levels of protection from the SARS-CoV-2 virus [69].
Hancock et al. and Mueller et al. conducted particle filtration performance test of at
least two-layer coverings made of 5–10 different hand-sewn woven fabrics loosely fitted
to the wearer’s face [43,70]. At the same time, they tested two types of standard three-
layer medical masks. The woven face coverings incorporating a nylon stocking overlayer
were also tested to see if the overlayer improved filtration capability. The results indicate
that five out of ten woven loose-fitting coverings had high particle filtration efficiency,
similar to the performance of three-layer surgical face coverings. Three-layer surgical
masks showed 75% filtration efficiency, whereas woven face coverings showed 30–60%
filtration efficiency. The authors recommended an extra layer of filter (organic cotton bat-
ting, Pellon, or loosely woven cotton muslin) to improve the filtration efficiency of the
two-layer woven face coverings. Whiley et al. found that two-layer stretchy cotton denim
fabrics showed the highest filtration efficiency (90.90%) among woven fabrics [71]. Figure
6 shows the comparative graphs on the efficacy of increasing layers and flow rate. The
ability of home-made (woven and knitted fabric) face covering to filter the ultrafine (0.02–
0.1 μm) particles at the velocity (0.5 to 25 cm/s) of coughing and their efficacy in a damp
state has been analyzed by O’Kelly et al [23]. The large particles are readily filtered
whereas the ultrafine particles decrease the efficiency. The filtration efficiency in a damp
or wet state after one laundering cycle reaches a maximum of 45%, which is less than in a
dry state. It was recommended that in case of an acute shortage of respirators, people can
use the homemade face covering with an extra layer as it gives approximately 90% particle
filtration efficiency.
Figure 6. (a). Pressure drop vs. face velocity for 1, 2, and 4 layers of 500 thread per inch (tpi) cot-
ton/polyester blend, and (b) Pressure drop at 10 cm/s face velocity for three materials [43].
Structural differences between knitted, woven, and nonwoven fabrics influence their
filtration potential and air permeability in various ways. This has been taken into consid-
eration by Varallyay et al. They examined the airflow and filtration efficiency of knitted,
woven, and nonwoven fabrics available in household and health care centers [72] that
complied with the ASTM F2299 Standard [73]. These fabrics had varying numbers of lay-
ers and were tested in both wet and dry states. Filtration efficiency was measured using a
quantitative fit testing device (TSI, Porta Count Pro Plus. Shoreview, MN) with a standard
40-nm median diameter particle generator. The airflow resistance was measured as pres-
sure drops through the fabric sample using an air compressor with a pressure gauge
(EPAuto, Model 1. Walnut, CA). The filtration efficiency was calculated according to the
Figure 6.
(
a
). Pressure drop vs. face velocity for 1, 2, and 4 layers of 500 thread per inch (tpi)
cotton/polyester blend, and (b) Pressure drop at 10 cm/s face velocity for three materials [43].
Structural differences between knitted, woven, and nonwoven fabrics influence their
filtration potential and air permeability in various ways. This has been taken into consid-
eration by Varallyay et al. They examined the airflow and filtration efficiency of knitted,
woven, and nonwoven fabrics available in household and health care centers [
72
] that
complied with the ASTM F2299 Standard [
73
]. These fabrics had varying numbers of layers
and were tested in both wet and dry states. Filtration efficiency was measured using a
quantitative fit testing device (TSI, Porta Count Pro Plus. Shoreview, MN) with a standard
40-nm median diameter particle generator. The airflow resistance was measured as pressure
drops through the fabric sample using an air compressor with a pressure gauge (EPAuto,
Model 1. Walnut, CA). The filtration efficiency was calculated according to the following
formula (Equation (1)). The normal rule of thumb is that higher filtration efficiency means
lower airflow.
Fil trati on e f f ici ency(%)=1−Filtered count
Ambient count ×100 (1)
Sample fabrics with varying base weights of 22–300 g per square meter (GSM) and
1–4 layers were selected. The effect of the layer and washing were also assessed. The results
indicate that non-elastic fabrics that are commonly found in households showed very poor
filtration and airflow efficiency. The cotton–polyester blended fabric and cotton tea towel of
higher GSM showed inferior filtration performance. Thick polyester fleece and microfiber
cleaning cloth showed better performance. Polyester-felt nonwoven fabric showed higher
performance than any other available household fabric.
5.4. Finishes
This paper focuses on the formation of fabric for general-use face coverings and will
not fully address the myriad possible finishes for these fabrics. However, a few key points
that have surfaced in recent research and recommendations are worth mentioning. To
Textiles 2023,3154
identify readily available materials for homemade face coverings, particularly very early in
the pandemic, researchers investigated not only T-shirts and towels but also everything
from coffee filters to vacuum cleaner bags. WHO warns that these items that are not
intended for clothing may contain materials that are injurious to health when breathed in
and should be avoided [
28
]. This may be due to fiber content as well as finishes. Vacuum
bags are typically made of polypropylene, the same material used for surgical masks, but
they are not designed or tested for use on the face. In commenting on the high filtration
efficiency of vacuum bags, Kahler and Hain acknowledge that these may contain harmful
fibers and unhealthy ingredients intended to kill bacteria [38].
Antimicrobial/Antiviral Finishes
Microbes include organisms that are not visible to naked eyes, such as viruses and
bacteria [
74
]. Antimicrobial finishes are imparted to a fabric surface for protection against
viruses and bacteria [
75
–
77
]. The benefits of antimicrobial/antiviral finishes are debated.
Such treatments have the potential to reduce the number of infectious particles on the face
covering, but most antimicrobials/antivirals approved for use in textiles are not tested
for the viral reduction in a face covering. In its December 2020 mask use guidance, WHO
clearly states that antimicrobial-treated fabrics should not contact mucous membranes and
should not be used for the inner layer of any face covering. Antimicrobial compounds
may cause skin irritation or penetrate through the skin in the case of direct contact with
the skin [
78
]. WHO also expresses concern about antimicrobial claims providing wearers
with an unsupported sense of security. Standard tests evaluate bacteriostatic or bactericidal
activity over the course of several hours and may be of limited value in determining the
efficacy of face coverings in the COVID-19 context, where a face covering has to contend
with viral particles [
28
]. In the case of an outbreak of airborne or aerosol-hopping bacterial
infectors, standard tests may have some utility in establishing the efficacy of a face covering.
Secondary bacterial infections of the skin due to excessive use of face coverings are another
avenue where antimicrobial finishing could be beneficial. There are some commercially
available face coverings that can retain antimicrobial properties and high filtration efficiency
without an antimicrobial finish such as M-chitosan and copper line face coverings. The M-
chitosan face covering is prepared using three layers. The first layer consists of high-density
knitted fabric with a very smooth surface. The third layer contains low-density knitted
fabric with a chitosan film. The intermediate layer works as the interlining. Chitosan
is a derivative of chitin, which is obtained by partial deacetylation of chitin in alkaline
conditions or by using enzymatic hydrolysis in the presence of chitin deacetylase [
79
].
Chitosan is a positively charged hydrophilic antimicrobial polymer, which can neutralize
negatively charged bacteria. According to the American Association of Textile Chemist and
Colorist (AATCC) test method, this chitosan-containing face covering is effective against
99.9% of bacterial growth even after 100 washes [
41
]. Commercially available anti-bacterial
copper line face coverings are prepared using polyurethane, polyester, and copper. Ionized
copper yarn is used to prepare this face covering with antibacterial properties and the
capability to eliminate viruses. It is also claimed to be skin-friendly and reduce skin
irritation. The New England Journal of Medicine recently reported that the SARS-CoV-2 virus
is eliminated within a few hours after encountering the copper surface. Ionized copper
yarn can kill 50% of bacteria in ten min and 99.5% of bacteria within one hour. The filtration
efficiency of the three-layered knitted copper line face covering is increased to 99.9% and
99.6% against viruses and particles, respectively, when an additional disposable filter is
inserted. Antibacterial properties of face coverings remain unchanged until 30 washes [
41
].
5.5. Particle Size
When the droplets hit the fabric surface with a median velocity of approximately
17.1 m/s, few droplets penetrate the fabrics and split into smaller droplets. With more
layers, fewer droplets pass through. Penetration depends on the pore size, thickness,
and velocity regardless of the impacting droplet diameter [
68
]. The most penetrating
Textiles 2023,3155
particle size (MPPS) is too small to be effectively captured by interception or impaction
and too large to be caught by diffusion or electrostatic interaction. This size has been
reported as anywhere between 0.05 and 0.5
µ
m [
13
]. Filtration test methods often target
this range as representative of a “worst case scenario.” The exact MPPS and the method
of measuring filtration for general-use face coverings are far from standardized. One of
the primary challenges of reviewing research is the heterogeneity of test conditions and
protocols, including particle size. These differences, along with frequently incomplete
characterization of tested materials make a direct comparison across studies difficult.
The French Association Française de Normalisation (AFNOR) Group [
80
] was one of
the first to publish a specification for general community use face coverings. The AFNOR
S76 guide references EN 13274-7 (CEN-2019) with particles up to 3-
µ
m. The particles
may be NaCl, or liquid paraffin oil and face coverings must exhibit at least 70% filtration
efficiency. Since the AFNOR standard, several other countries and the European Union have
published similar documents. The Belgian (NBN) and European (CEN-2020) standards also
call for at least 70% filtration of 3-
µ
m particles. The global standard published by AATCC
recommends 70% efficiency when testing with 3-
µ
m latex spheres, though this document
also notes that smaller particles could also be tested and may be more representative of
actual use conditions (AATCC) [81].
More recently, the American Society for Testing and Materials (ASTM) International
published a specification including the filtration test method commonly used to evaluate
medical respirators. The NaCl aerosol particle has an average diameter of 0.075
µ
m. The
particle size is much smaller than that used in other face covering standards, but only 20%
filtration is required [
82
]. Rengasamy et al. studied improvised fabric face coverings in 2010
in response to influenza (H5N1, H1N1) outbreaks. Fabrics from common consumer items
were tested for filtration efficiency using 0.02–1.0
µ
m particles [
57
]. The test included two
different velocities, and both polydisperse and monodisperse particles and the fabrics were
compared with surgical masks, dust masks, and an N95 respirator. Sweatshirts, T-shirts,
towels, and scarves, as well as fabric face coverings allowed at least 50% penetration of
the particles, with the exception of one poly/cotton sweatshirt with 40% penetration at
low velocity. Towels generally provide greater filtration than other apparel items [
57
]. The
paper provides no explanation for this, and the only descriptive information provided
about fabrics is fiber content. Most fabrics were cotton or a cotton/polyester blend. Fiber
content did not appear to have any direct correlation with filtration. It is likely that the
towels had the tightest construction of the fabrics tested; moreover, pile structures in such
fabrics could also be responsible for improved chances of contagion impaction. Fabrics
in the Rengasamy et al. study were selected for their availability and not optimized for
filtration. Nonetheless, the results agree with other studies that common woven and knit
textiles are less effective filters as particle size decreases [
38
,
83
]. Interestingly, two of the
three towels tested showed improved filtration between 0.3 and 1.0
µ
m. Penetration of
other samples continued to increase across the complete range of particle sizes tested [
57
].
A more recent study by Hao and colleagues found that household textiles had filtration
efficiencies below 60%. The average particle size for these tests was 0.3
µ
m. Size-dependent
filtration evaluations were also performed with particles between 0.1 and 0.6
µ
m [
24
]. Even
the top end of this range is smaller than the particle size that can be effectively filtered by
common fabrics, though the results do show an upward trend in efficiency beginning at
approximately 0.4 µm [24].
5.6. Related Effect
Kähler and Hain approached the question of face-covering filtration from the perspec-
tive of fluid dynamics [
38
]. They began with an investigation of the droplet size and spread
of cough or speech. Among the household materials tested, only high-quality vacuum bags
provided adequate filtration to protect the wearer from infectious particles in the environ-
ment. However, they also conclude that face coverings are quite effective at preventing
the spread of the wearer’s exhaled air [
38
]. This aligns with other reports [
15
,
24
,
59
] and
Textiles 2023,3156
the common messages regarding the social contract of wearing a face covering to protect
others. The fluid dynamics analysis should inform future testing and evaluation of materi-
als for the general use of face coverings. This may also explain conflicting data between
epidemiological evidence that wearing face coverings reduces transmission of SARS-CoV-2
and lab results showing most fabrics to be poor filters. Researchers at Duke University
discovered a negative effect of face coverings as a barrier to exhaled air. Their results
showed that a fleece gaiter, especially, dispersed large droplets into smaller droplets [
84
].
Smaller droplets have the potential to travel farther and remain airborne longer, increasing
the probability of contact with another person. No hypothesis was offered for why the
fleece performed differently from other fabrics, and this phenomenon is not detailed in
other papers reviewed [13,19,28–47].
6. Clinical Outcomes
There are far fewer studies of clinical outcomes than laboratory analyses. The Nanda
review covers much of the relevant work [
15
]. A publication by MacIntyre et al. found
that a randomized group using fabric face coverings had the highest rate of all infections
when compared to groups using medical masks or the wearer’s usual practice [
85
]. The
usual practice group included individuals wearing medical masks, fabric face coverings,
a combination, N95 respirators, and no covering. There was no specific no-covering
control. The authors could not conclude whether fabric face coverings simply provided less
protection than medical masks due to lower filtration efficiency or if they actually increased
the risk of infection. Possible sources of increased risk included self-contamination due
to repeated use, donning, doffing, and moisture retention [
85
]. Some of the same authors
collaborated to propose a new set of randomized trials in the community. They also
recommended properties for constructing a fabric face covering. Few references are cited
for the recommendations, and no explanation is provided. As with other papers offering
specific recommendations, there is a risk of the guidance being taken at face value. The
imperative statement, “select a fabric that is water resistant”, is one example from this
paper [
67
]. Courtney and Bax recently proposed a new perspective on the efficacy of fabric
face coverings. Rather than focusing on the ability or inability to filter potentially harmful
particles, they suggest that the increased humidity of the respiratory epithelium (cells lining
the airway) reduces the severity of illness if the wearer is exposed to infectious particles [
86
].
If true, this could steer testing from filtration to moisture management and a completely
different set of technologies. As noted in the earlier moisture management discussion, too
much humidity can lead to discomfort, so balance among properties is still required.
7. Quality Control
Chemical compliance is an important issue for fabric face coverings. Fabrics are
colored with dyes that contain many trace elements, additives, and nanoparticles (Table 2)
that can enter the human body through ingestion and inhalation [
87
]. In a recent study,
Bussan et al. found that textiles used in most of the specialized masks contained heavy
metals/trace elements that are toxic to the human body [
88
]. Blevens et al. found significant
differences between products with quality labeled and tested and therefore, argued for
strict product testing and inhalation regulation for face covering manufacturers [
89
]. As
CDC promotes the use of fabric face coverings, a cytotoxicity test (e.g., ISO-10993) could be
included as part of the quality test of fabrics before their dispatch from the manufacturing
site to ensure public health safety [90].
Textiles 2023,3157
Table 2. Harmful chemicals used in fabrics [87,91].
Category of Chemicals Used in Fabrics Restricted Chemical Compounds
Additives/Plasticizers
Flame retardants (polybrominated diphenyl ethers,
Phthalates, organophosphate esters,
hexabromocyclodecane, and Sb2O3).
Trace Elements
Metal complex dye (cobalt, copper, chromium, and
lead), pigments, mordant, antimicrobials
(nanoparticles of silver, titanium oxide, and zinc
oxide), trace metals (Ag, Al, As, B, Ba, Be, Bi, Cd, Co,
Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sb, Sc, Se, Sm,
Sn, Sr, Ti, Tl, V, and Zn)
Dyes
Aromatic Amine (azo), Quinoline and derivatives,
bisphenols (BPAs), benzothiazoles (BTHs), and
benzotriazoles (BTRs)
Nanoparticles Carbon nanotubes, nanoclays, aluminum oxide,
silicon dioxide, zinc oxide, titanium oxide, and silver.
Finishing agents Formaldehydes (anti-creasing), nanoparticles (nano
Ag), antimicrobial agents (Ag-coating)
[28,67,84–86].
8. Conclusions and Future Research Direction
A growing body of research is tackling the problem of optimizing the performance of
fabric-based face coverings recognizing the reliance of most vulnerable groups on these
coverings to protect themselves against the COVID-19 disease [
13
,
14
,
67
]. Moreover, the
impact of non-biodegradable single-use personal protection equipment, including face
coverings on the ecology of the planet, has started to be documented in various publications
now since we have spent multiple years with the COVID-19 pandemic [
92
–
94
]. Global
health and geopolitical crises and the consequent commodity super cycle have spawned
widespread supply chain disruptions. A paucity of resources has given an opportunity to
reevaluate response to future public health emergencies in times of global strife and scarcity.
Strategies to offset supply bottlenecks during pandemics include the use of immediately
available resources such as reusable fabric-based face coverings. Reuse is an important
economic and ecological advantage of fabric face coverings, but the area is not well-studied.
A better understanding of how use and laundering affect performance is needed. Complete
lifecycle analysis of both disposable and reusable face coverings would also be useful.
Other areas to explore are the human factors of selection, use, handling, and washing.
The current body of research indicates the highest degree of filtration efficiency for N95
respirators followed by surgical masks, while fabric face coverings provide comparatively
lower filtration efficiency. Despite the dearth of penetrating discussion on the effects of
key construction parameters and how these influence the utility of fabric face covering in
terms of blocking contagions, woven face coverings demonstrate better filtration efficiency
than knitted face coverings in the available literature. Knitted face coverings are, however,
more breathable. Woven face coverings are made of all types of synthetic and natural fibers
such as cotton, polyester, cotton-polyester blends, and silk with different count variations.
Among these, polypropylene is inexpensive, with excellent antimicrobial behavior, mechan-
ical strength, and chemical resistance. At least one study recommended this as an ideal
fiber for use in face coverings [44].
The spread of SARS-CoV-2 is becoming better understood but is not yet replicated
by standard laboratory tests. To evaluate the true efficacy of fabric face coverings, more
relevant methods are needed both in the field and in the lab. Nanda et al. were quite
critical of the existing literature regarding preclinical and clinical studies of face-covering
use [
85
]. They identified 10 of the 12 studies reviewed as having a high risk of bias, with
the remaining two having “some concerns.” Most of the bias arose due to deviations from
the intended intervention [44,85]
Textiles 2023,3158
Again, WHO describes the main shortfall of existing studies. The report notes that
while many materials have been studied, few fabrics or combinations have been systemati-
cally evaluated, and “there is no single design, choice of material, layering or shape among
non-medical masks that are considered optimal” [
28
]. The challenge for future studies
is similar. There is an unlimited number of fabric combinations, making it impossible to
complete an exhaustive evaluation or comparison. Nonetheless, a systematic study of
fabrics using consistent methodology could provide some practical insight of value to
manufacturers and consumers.
The scope of discussion in most of the available literature has not been expanded
to cover the effects of weave or knit types, yarn construction methods, and the selection
of fibers. In most of the publications regarding face coverings, no investigation has been
conducted to determine the impact of yarn types and fabric types on effectiveness. Very
few papers gave the details of yarn count, loop length, or fabric weight of reusable face
coverings, although these factors have a significant impact on the performance of face
coverings. Fabric weight and thickness depend on the yarn count used during fabric manu-
facturing, which has a significant impact on filtration efficiency and thermal comfort. For
meaningful research developments in the realm of estimating the filtration performances of
these face coverings, these parameters and their impact on the filtration function must be
exhaustively investigated.
Moreover, compliance in regard to the use of restricted chemicals on fabrics used
for constructing reusable face coverings has to be given utmost importance in the wider
interest of public health.
Author Contributions:
Conceptualization, M.T. and D.W.; methodology, M.A.-A.; software, M.A.-A.;
validation, M.T.H., S.M.F.K. and M.A.-A.; formal analysis, M.T.; investigation, S.M.F.K.; resources,
M.A.-A.; data curation, M.T.H.; writing—original draft preparation, M.A.-A.; writing—review and
editing, S.M.F.K.; visualization, D.W.; supervision, Md Al-Amin and M.T.H. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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