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Informing Homemade Emergency Facemask Design: The Ability of Common Fabrics to
Filter Ultrafine Particles
AUTHORS
Eugenia O’Kelly, Department of Engineering, University of Cambridge – currently in
San Francisco, USA
Orchid ID: 0000-0002-4748-3957
Sophia Pirog, Department of Medical Social Sciences, Northwestern University –
Chicago, USA
Orchid ID: 0000-0003-3422-4304
James Ward, Department of Engineering, University of Cambridge – Cambridge, UK
Orchid ID: 0000-0002-0362-4711
P. John Clarkson, Department of Engineering, University of Cambridge – Cambridge,
UK
Orchid ID: 0000-0001-8018-7706
CORRESPONDING AUTHOR ADDRESS
Eugenia O’Kelly
999 Green St, Apt 1505
San Francisco, California
94133
USA
Phone: 1-415-359-0092
Fax: 1-415-520-6460
E-mail: eo339@cam.ac.uk
WORD COUNT
2,026
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ABSTRACT
Objectives:
To examine the ability of fabrics which might be used to create homemade face masks to filter
out ultrafine (smaller than 1μm in diameter) particles.
Method:
Twenty commonly available fabrics and materials were evaluated for their ability to reduce air
concentrations of ultrafine particles. Further assessment was made on the filtration ability of
select fabrics while damp and of fabric combinations which might be used to construct
homemade masks.
Results:
Single fabric layers blocked a range of ultrafine particles. When fabrics were layered,
significantly more ultrafine particles were filtered. Several fabric combinations were successful
in removing similar amounts of ultrafine particles when compared to an N95 mask and surgical
mask.
Conclusions:
The current coronavirus pandemic has left many communities without access to commercial
facemasks. Our findings suggest that face masks made from layered common fabric can help
filter ultrafine particles and provide some protection for the wearer when commercial facemasks
are unavailable.
KEYWORDS
SARS-CoV-2, Coronavirus, Infection Control, Respiratory Infections, Facemask,
Public Health, Infectious Disease, PPE
STRENGHTS AND LIMITATIONS OF THIS STUDY
● Tested a large number of potential facemask materials
● Tested ability of materials to filter virus-sized particles dry and while damp
● Did not discriminate between pathogenic and non-pathogenic particles
● Breathing resistance was estimated based on qualitative feedback
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Informing Homemade Emergency Facemask Design: The Ability of Common Fabrics to
Filter Ultrafine Particles
INTRODUCTION
The current SARS-CoV-2 outbreak has left many communities without sufficient quantities of
face masks for the protection of medical staff, let alone sufficient quantities of masks for the
general population’s use[1]. Despite this severe shortage, many areas have begun requiring the
use of facemasks for individuals who leave a green zone.
Homemade face masks have now become a necessity for many to both meet the demands that
cannot be met by supply chains and/or to provide more affordable options. Although widespread
online resources are available to help home sewers and makers create masks, scientific guidance
on the most suitable materials is currently limited.
Though not as effective as surgical masks or respirators, homemade face masks have been shown
to provide benefit in filtering viral and bacterial particles[2-4]. In addition, homemade face
masks are likely to confer similar non-filtration benefits as commercial masks, such as
encouraging social distancing and discouraging hand contact with the nose and mouth.
Furthermore, even partial protection is likely to reduce overall pathogen exposure.
Scant evidence is available on how effective common fabrics are in filtering pathogens, nor
whether the homemade masks sold online and provided to hospitals and the community are able
to offer adequate protection. Little research has been done regarding the best materials to use for
those seeking to create face masks at home. In addition, past studies have tested only a limited
set of similar materials, namely t-shirts, sweatshirts, scarves, and tea towels. These results do not
provide adequate guidance on the full scope of materials currently used for homemade mask
construction.
This study aims to address the paucity of information regarding materials for face mask
construction by evaluating the efficiency of twenty widely available fabrics and materials,
particularly those available to the general public in filtering particles smaller than 0.1 μm (100
nm). Both individual materials and material combinations were tested with the goal of
increasing particle filtration of homemade masks. In addition, materials which could be washed
and dried in very hot water were preferred for their efficacy ameliorating the risk of infection in
two particular situations: (1) infection due to the reusing of masks, and (2) reduction of filtration
efficacy due to moisture buildup.
Traditional in-hospital masks are intended to be used only once; however the CDC is currently
encouraging individuals to reuse masks if possible[5]. This increases the risk of infection if the
user comes in contact with the outside of a contaminated mask or if the mask material becomes
too damp to be optimally effective. To reduce this inherent risk, we chose washable materials
which could withstand hot water washing and/or hot cycle drying. In addition, as normal
respiration generates moisture which can reduce the filtration efficiency of face masks, a
selection of materials were tested in both damp and dry states to assess their changes in
efficiency.
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In conclusion, the results of this study may also inform emergency mask creation in response to
environmental emergencies where ultrafine particle levels are high, such as from smoke or smog.
Repeated face mask shortages during the California wildfires over the past few years have
illustrated the recurring need for scientific data to guide the construction of homemade face
masks when commercial supply chains are unable to meet demand.
METHODS
This study was conducted in response to the rapidly growing SARS-CoV-2 outbreak. As such,
priority was given to developing a test apparatus which could be constructed and provide usable
results in a short amount of time.
Preference was given to materials which are widely available and not likely to become
unavailable during the SARS-CoV-2 outbreak. Additional preference was given to materials
which could be cleaned in a home washing machine and/or dryer at its hottest setting. All
materials were washed and dried before testing. This caused significant shrinkage of the wool
felt but did not hinder its efficiency, which had been pre-tested. The top-performing materials
were subjected to five additional tests when damp. Dampness was achieved by applying 7
milliliters of filtered water to a 2” square section of the material.
Testing Apparatus
Tests were conducted as described by Hutten[6]. An airtight apparatus allowed simultaneous
testing of unfiltered and filtered air. A 1” diameter tube provided access to two ultrafine particle
counters (P-Trak model 8525) which measured concentrations of particles 0.1 μm and smaller.
The tube held a 1” diameter sample of the filter material. Readings were taken 1.5” in front of
and behind the filter medium. Airflow was controlled through suction, which pulled air through
the filter medium at a rate of about 16.5 m/s.
Calculating Filtration Efficiency
Hutten’s formula was used to assess filtration efficiency (FE).
𝐹𝐸 = $ 𝑈𝑝𝑠𝑡𝑟𝑒𝑎𝑚$𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒$𝐶𝑜𝑢𝑛𝑡 −𝐷𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚$𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒$𝐶𝑜𝑢𝑛𝑡 ×100
𝑈𝑝𝑠𝑡𝑟𝑒𝑎𝑚$𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒$𝐶𝑜𝑢𝑛𝑡
For each material or material combination, ten sets of readings were collected. Readings were
collected using two P-Trak Ultrafine Particle Counters, Model 8525. Each reading was collected
as a 10-second average of ultrafine air particle concentrations.
Interpreting Filtration efficiency
The flow rate of air used in this study may represent the velocity of air expelled during human
coughing[7]. As the velocity was significantly higher than in previous studies, filtration
efficiency was expected to be lower. Numbers in this experiment should be interpreted as low
baselines, representing material performance at high levels of stress rather than normal
respiratory rates.
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Filtration efficiency was expected to be lower than viral filtration studies, as particles larger than
0.1 μm were not measured. Many viruses are carried on droplets which are significantly larger
than 0.1 μm and may, due to their size, be more easily filtered.
Material Resistance
To estimate the breathing resistance of each material and thus their suitability for use in a face
mask, two members of the team held sections of each fabric tightly over their mouth and inhaled
through their mouth. Each fabric was scored on a 0-3 scale where 3 represented a great difficulty
in drawing breath, 2 represented that there was noticeable resistance but breath could be drawn, 1
represented some limitation but relative ease of breathing, and 0 represented no noticeable
hindrance. Combining and layering fabric was not found to significantly increase the breathing
difficulty. All face mask fabric combinations scored 1 or 2.
Note on Study Design
It should be noted that, due to the limitations imposed by this outbreak, this study was done with
available materials. Data from this study should be treated as preliminary and used to inform
decisions about filtration media only in relation to existing studies which assess viral filtration
through the collection of viral cultures.
All effort was made to ensure the quality of the study design and accuracy of the equipment
used. Ten samples were taken for each material from at least two different sections of the fabric
to ensure accurate representation. Zero readings were taken on the particle testers regularly to
ensure proper functioning.
RESULTS
Materials
All materials blocked some ultrafine particles (see Figure 1). HEPA vacuum bags from
Kenmore blocked the most ultrafine particles, with the N95 mask from 3M blocking the second
greatest percentage of particles. Other materials, such as the denim jeans and windbreaker
blocked a high proportion of ultrafine particles but were very difficult to breathe through (see
Figure 2) and are thus ill-suited for face mask construction. These materials may be suited to a
loose fitting face mask which protects from splashes. When taking into account breathing
resistance and filtration efficiency, the most suitable fabrics for face mask construction were
thickly felted wool, quilting cotton, and cotton flannel. A single sock held flat also compared
well with the above and, when pressed tight against the nose and mouth, is a good emergency
substitute for a mask.
Repurposing HEPA filters holds great promise for emergency facemasks; however, great care
should be taken that the materials within the filter do not pose dangers to those making or
wearing the face mask. While the Kenmore’s single-use HEPA vacuum bag material showed the
greatest ability to filter ultrafine particles, the layers fell apart when the material was cut,
exposing inner layers of the fabric. The reusable, washable HEPA bags had a construction more
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suitable to creating emergency facemasks as the material held together well and did not expose
inner fibers.
The filtration efficiencies of select materials were tested when damp (see Figure 2). Only minor
differences in filtration efficiency were noted for quilting cotton, cotton flannel, and craft felt.
Denim showed a significant decrease in efficiency while the HEPA single-use vacuum bags
showed an increase in efficiency when damp.
Nonwoven Fusible Interfacing
Nonwoven fusible interfacing, the kind used for stiffening collars and other areas in garments,
was able to significantly improve the ability of the fabrics to filter ultrafine particles without
increasing breathing resistance. Of particular note, we found that brand was important. HTC
lightweight interfacing was more effective than Heat-n-Bond lightweight interfacing. Applying
two layers of the Heat-n-Bond achieved similar improvements to filtration efficiency as the HTC
brand. Wonder Under, a double sided, heavyweight fusible interfacing for constructing bags and
craft projects. showed similar filtration ability to the HTC brand but may be too stiff to be
suitable for face mask construction.
Material Combinations
When layered to create potential face mask configurations, common fabrics were able to achieve
much higher levels of ultrafine particle filtration (see Figure 1). Some material combinations
were able to filter out higher percentages of ultrafine particles than the surgical or N95 mask
tested, although this should not be taken to mean they provide higher levels of protection from
viruses. All fabric combinations scored between a 2 and 3 on the breathing resistance test,
indicating they were more difficult to breathe through than an N95 mask.
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Figure 1: The filtration efficiency of tested fabrics and fabric combinations with error bars
showing 95% confidence.
Dry
Damp
Fabric
Fabric Weight
grams/meter2
Fiber
Composition
Ease of Breathing
Through Material
Mean % FE
SD
Mean % FE
SD
3M N95 Mask
N/A
N/A
1
52.47
2.222
45.68
1.247
Surgical Mask
N/A
N/A
2
47.46
1.087
42.73
1.664
Disposable HEPA Vacuum Bags (Kenmore)
N/A
N/A
2
60.86
0.761
71.93
4.407
Windbreaker
2.87
100% Polyester
3
47.12
1.332
45.55
3.535
Jeans Denim
10.74
100% Cotton
3
45.94
2.176
30.69
5.314
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Washable Vacuum Bag HEPA
N/A
N/A
2
43.64
1.852
44.97
2.267
Thick felted wool
10.2
100% Merino
Wool
0
35.87
0.502
Cotton, Heavyweight Woven
4.3
100% Cotton
2
35.77
2.707
Folded Sock
N/A
Cotton, Lycra
2
35.36
1.146
Quilting Cotton
4.4
100% Cotton
1
34.54
2.047
31.88
1.406
Two Sided Minky Fabric
7.61
N/A
1
34.17
0.716
Shirting Cotton
7.2
100% Cotton
1
33.59
2.097
Cotton, Lightweight Woven
2.5
100% Cotton
0
30.20
1.499
Cotton Quilt Batting
3.28
100% Cotton
0
29.81
1.270
Cotton Flannel
4.8
100% Cotton
1
28.50
1.529
30.14
1.196
Craft Felt
4.74
Acrylic,
Polyester
0
27.72
0.748
100% Nylon Woven
1.53
100% Nylon
3
27.61
1.303
T-Shirt, Heavyweight
5.51
100% Cotton
1
25.21
0.471
Cotton Jersey Knit
6.37
100% Cotton
0
24.56
4.800
Lycra
5.25
82% Nylon,
18% Spandex
0
21.60
1.477
Fusible Interfacing
N/A
N/A
0
15.00
1.672
T-Shirt, Lightweight
3.15
100% Cotton
0
10.50
1.293
Figure 2: Chart of materials weight, composition, breathing resistance, mean FE, standard
deviation of FE, and, where available, FE when damp.
CONCLUSIONS
Our data suggests that, in times of severe supply shortage, common fabrics can be layered to
create face masks which protect wearers high percentages of ultrafine particles. It should not be
inferred that these layered fabrics can protect wearers from more viral particles than N95 masks
or surgical masks as our study did not discriminate between viral particles and other ultrafine
particles. The difference between ultrafine particle filtration of the surgical masks, t-shirt fabric,
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and a woven cotton tested in this study and the viral filtration of the surgical mask, t-shirt, and
mixed woven cotton seen in Davies et al.’s study were proportionally similar2. This suggests
viral filtration might be proportionally similarly for other fabrics tested here but further research
is needed to confirm.
It is suggested homemade face masks should not be used in place of other protective measures
such as self-isolation or social distancing during this coronavirus pandemic. Rather, our results
suggest homemade face masks may be a viable protective measure for those who cannot remain
isolated and cannot obtain commercial face masks.
Repurposing material for homemade face masks comes with its own risks. Particular
consideration should be given to respiratory hazards which may arise from the material used to
construct a homemade facemask. For example, concern has been expressed that certain HEPA
vacuum bags include fibers which, if inhaled, can cause lung injury. Fabrics which shed lint
may also lead to lung damage if worn regularly. For this reason, we would caution those
needing to create homemade face masks to ensure all material is safe, nontoxic, thoroughly
prewashed, and lint-free. Fabrics which readily shed fibers may not be suited for face mask
construction. The risks associated with such materials are an important area of further study, as
large numbers of people are currently creating, wearing, distributing, and selling homemade
facemasks. Further research should also evaluate the ability of these materials and material
combinations to filter specific viruses, pollutants, and other harmful airborne particles.
Additional research on homemade facemask fit and fit testing is also critical at this time.
It is our hope that this study can assist home sewers and makers to create the best facemask
possible when standardized commercial personal protective equipment is unavailable. Our study
shows face masks can be created from common fabrics to provide wearers with significant
protection from ultrafine particles. Until further research can establish the safety and viral
filtration of fabric face masks, we advise the use of approved respiratory protection whenever
possible and the use of homemade face masks only when these products are unavailable.
REFERENCES
1 Davies, A., Thompson, K. A., Giri, K., et al. Testing the efficacy of homemade masks: would they
protect in an influenza pandemic? Disaster Med Public Health Prep 2013:7(4),413–418.
https://doi.org/10.1017/dmp.2013.43
2 Ha, K. O. The Global Mask Shortage May Get Much Worse. Retrieved March 16, Bloomberg
2020:from https://www.bloomberg.com/news/articles/2020-03-10/the-global-mask-shortage-may-be-
about-to-get-much-worse
3 Hutten, I. M. Handbook of Nonwoven Filter Media (2nd ed.). Butterworth-Heinemann. 2015:
https://doi.org/https://doi.org/10.1016/C2011-0-05753-8
4 National Centers for Disease Control and Prevention. Strategies for Optimizing the Supply of
Facemasks. 2010:Retrieved from https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/face-
masks.html
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author/funder, who has granted medRxiv a license to display the preprint in perpetuity. is the(which was not peer-reviewed) The copyright holder for this preprint .https://doi.org/10.1101/2020.04.14.20065375doi: medRxiv preprint
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5 Rengasamy, S., Eimer, B., & Shaffer, R. E. Simple respiratory protection - Evaluation of the filtration
performance of cloth masks and common fabric materials against 20-1000 nm size particles. Annals of
Occupational Hygiene 2010:54(7),789–798. https://doi.org/10.1093/annhyg/meq044
6 van der Sande, M., Teunis, P., & Sabel, R. (2008). Professional and Home-Made Face Masks Reduce
Exposure to Respiratory Infections among the general population. PLoS One 2008:3(7),3–8.
https://doi.org/10.1371/journal.pone.0002618
7 Zhu, S. W., Kato, S., & Yang, J. H. Study on transport characteristics of saliva droplets produced by
coughing in a calm indoor environment. Building and Environment 2006:41(12),1691–1702.
https://doi.org/10.1016/j.buildenv.2005.06.024
~ ~ ~
AUTHOR’S CONTRIBUTIONS
Eugenia O’Kelly
Conceived of the study, developed study methodology, obtained study materials and testing
apparatus, collected study data, wrote manuscript
Sophia Pirog
Obtained study materials, analyzed data and performed calculations, designed graphs, edited
manuscript
James Ward
Developed study methodology, reviewed data, edited manuscript, supervised study
John Clarkson
Developed study methodology, reviewed data, edited manuscript, supervised study
AWKNOWLEDGMENTS
The authors would like to thank Corinne E. O’Kelly for supporting this research.
CONFLICT OF INTEREST / COMPETING INTERESTS
There are no conflicts of interests/competing interests for any of the paper’s contributing authors.
DATA STATEMENT
Data from this study is freely available under a CC BY license on Cambridge University’s
Apollo open data repository.
Link: https://doi.org/10.17863/CAM.51390
Citation: O’Kelly, E., The Ability of Common Fabrics to Filter Ultrafine Particles [Data file].
Cambridge University: Cambridge, United Kingdom; 2020.https://doi.org/10.17863/CAM.51390
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FUNDING
This research received no specific grant from any funding agency in the public, commercial or
not-for-profit sectors.
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