Data-informed Fabric Masks and Disposable Masks
After a year and a half of sewing masks and struggling to decide which
fabrics are likely to provide enough filtration... I paid for data.
After reading dozens of papers... I knew how to interpret the data.
Seeing kids spill out of schools... I learned the difference between
theory and practice of mask fit.
As part of MakerMask, I learned from
colleagues. And then, in an amazing collaboration of academics and
citizen scientists, I was thrilled to help mentor undergraduates and
analyze data for a publication.
Here I share my insights with you.
Each blue box is a link
Masking in our community:
Infographic: The spectrum of masks
Masks perform really quite differently. This summarizes the differences.
Infographic: 5um particles, filtration, and Covid
85% of Covid viral load is in particles ≤5um, which is a range where
fabric filtration varies. This infographic shows filtration for a KN95,
a medical mask, a non-medical mask, and some fabric masks for particles
in this key size range.
Infographic: Mask fit
Mask fit matters a lot. Air that goes around a mask is not filtered by
the mask. This inforgraphic shows data from a study of inward filtration
and offers guidance on how to assess mask fit.
Infographic: Masks for source control may not protect
One intermittently-masked individual infected half a classroom with Covid. All the others were masked. This illustrates how masks intended for source control may not offer personal protection.
Infographic: How does source control work?
How can my mask protect you more than me?
An infographic boiling down evidence that we exhale wet particles large enough
to be well-filtered by many fabrics (good source control), but once they dry and
shrink and waft to another person they are too small for most fabrics to
filter (poor personal protection).
PoweCom size reduction
I find the PoweCom KN95s to be the most comfortable mask for me... but a smidge big.
This is my trick to reduce the size.
Infographic: KN95 counterfeits vs fabric
Counterfeit KN95s with sub-standard filtration are a real concern. This
infographic shows measurements from a study in which even most counterfeit
KN95s outperformed two-layer cotton filtration.
Infographic: Recorders in MVWSD
The present plan for playing recorders includes indoor and outdoor play
options, depending on weather. The indoor playing plan provides no source
control, and poorly fitting fabric masks which (I expect) provide very
little personal protection, a scenario overall equivalent to an unmasked
On sewing fabric masks:
- Woven cotton is ok but not great. Cotton masks help reduce
transmission and so are effective for public health. But if you want to
protect yourself, incorporate a better fabric.
- Non-woven polypropylene and non-woven polyester and flannel are
better than cotton. For the same level of breathability, they provide
better filtration. Two layers of a medium-weight interfacing will hugely
improve your mask's filtration with little impact on breathability,
but the stiffer construction may require adjusting your sewing pattern.
- Washing is fine. Ten machine wash/dry cycles with a laundry
bag don't affect filtration.
- Particle size is a complex topic without which we can't
properly interpret fabric filtration data nor public health recommendations.
Last updated 10 February 2023
© Anna Mitros
Publication: Filtration and breathability of nonwoven fabrics
used in washable masks
I was priviledged to do some of the data analysis and co-author
Thomas Bement did a fantastic undergraduate project in
Steve Rogak's lab at UBC (Univ. of British Columbia), and David
Downey collected subsequent data.
MakerMask.org helped advise the design of the study.
MakerMask post: Recommendation for a fabric combination better than cotton
In this MakerMask post, I discuss the data and
science behind the recommendation for
two layers of ~65 gsm (gram/m2) spunbond nonwoven polypropylene (NWPP) plus one layer of ~130 gsm woven cotton.
Filtration was tested against 0.5μm-12μm
particles at Colorado State University.
Post: Summary of CTT data for sewists
An explanation of measurements of
filtration of submicron NaCl and of Bacterial
Filtration Efficiency, and what fabrics I had tested,
stated as plainly and simply as I can.
Post: Summary of CTT data for scientists
Deeper details and mathematical analysis.
Sewing my mask pattern
Sewists have asked how to sew the mask featured in the
On particle sizes
Particles >10μm are filtered well by most fabrics, resulting in clear
reduction of community transmission with mask use regardless of
type of mask. However, 1μm to 10μm particles also can be
infectious, and so excellent protection for the mask wearer necessitates
adequate filtration in this range as well.
Particle sizes used for filtration testing vary across
standards and studies.
Microscope photos of various fabrics used for masks.
Single layer data is not enough, since the first layer in a stack-up
seems to filter with higher efficacy than subsequent layers.
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