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Cloth filtration experiment
3 February 2021
Goals
What fabrics?
The sewing community continues to sew cloth masks, and people who
sew continue to ask each other about the optimal fabric combination.
The goal of this study is to provide some data to help answer that
question.
Individual protection:
Many people who buy or sew cloth masks expect those masks to
protect the person wearing the mask.
This is in contrast to what some experts believe.
Lindsey Marr put the expert view quite clearly:
"N95 respirators and cloth masks serve different purposes.... An N95 must be able to protect an individual... on the other hand, the overall goal of wearing cloth masks... is to reduce community transmission."
(Marr 2020)
In contrast, a 70-something year old relative who cares for her 104
year old mother believes she's being "very careful"
when she wears a two-layer cotton mask to the
grocery store. I see similar sentiments in FaceBook sewing groups,
a belief that cloth masks are quite protective for the wearer.
My goal in this study is to inform the sewist and mask
wearer what level of protection they may expect as individuals.
Washing:
Washing is not addressed in most studies. Cloth masks tend to be
washed, dried, and reused many times. Most studies of cloth masks
evaluate new fabrics which have not been washed or dried. I
will include some samples of fabrics washed and dried 10 times
in a standard washer and dryer on the default wash and dry cycles
(LG front-loaders, dryer is electric).
Fabric selections
Many prior studies
of filtration effectiveness of cloth provide
inadequate details of the tested fabrics to allow people who sew
to purchase identical fabrics. The following include links to the
manufacturer websites for most of the fabrics I plan to test:
Microscope photos of these (and other) fabrics are at:
ofb.net/~ania/fabrics/
Particle size
Many studies of cloth masks use submicron particle
sizes to test filtration by cloth fabrics and cloth masks, consistent
with the 0.3um particle size standard for N95 masks.
However,
Marr 2020
notes that NaCl and proteins are usually part of the viral droplet,
so even dried out particles floating the the air could be expected
to be much larger than a single virus. However, Marr does not
quantify what the overall particle size range may be.
Nelson labs offers several filtration tests, which are also available through
CTT Group:
- NaCl: Sodium Chloride Aerosol Challenge
is intended as a NIOSH pre-qualification test, so presumably uses
0.3um NaCl particles.
- BFE and VFE:
Bacterial Filtration Efficiency (BFE)
uses Staphylococcus aureus aerosolized to a
mean particle size (MPS) of 3.0 ± 0.3 um.
Viral Filtration Efficiency (VFE)
uses bacteriophage phiX174 aerosolized to a
mean particle size (MPS) of 3.0 ± 0.3 um.
So... Test with 0.3um or 3.0um particles?
- Covid perspective: What particle size dominates Covid transmission seems to be an open question. While large particles can hold more virus, large particles tend to be generated in the oral cavity which is not where Covid seems to occur. Covid is a lower respiratory illness, primarily, and small (1um or so) particles are generated there. A 0.3um particle is unlikely to contain enough virus to constitute an infectious dose. Probably 1um to 10um particles are of key concern... but we don't know.
- Fabric masks perspective: Cloth masks block 5um and larger particles quite well. They block 0.3um particles quite poorly. That threshold for where cloth masks start to be effective falls right smack in that range where we don't know exactly which particle size to fret about.
Sample List
Following is my list of samples for NaCl submicron filtration and 3.0um BFE
(Bacterial Filtration Efficiency).
Total of 9 samples for NaCl, 4 for BFE.
Sample ID |
Test @0.3um |
Test @3.0um |
Outer layer |
Middle layer |
Face layer |
Washed? |
Description |
3a
| Yes
| -
| Kona
| P40
| Flannel
| No
| My most common construction.
|
3b
| Yes
| -
| Yes
|
3c
| -
| Yes
| No
|
4a
| Yes
| -
| Sateen
| OlyFun
| Flannel
| No
| My present best guess at the best combination
|
4c
| -
| Yes
| No
|
5a
| Yes
| -
| Kona
| -
| Kona
| No
| Community minimum: This is common in the sewing community.
|
5b
| Yes
| -
| Yes
|
5c
| -
| Yes
| No
|
6a
| Yes
| -
| Kona
| OlyFun
| Kona
| No
| Non-woven polypropylene is believed by some
sewists to be superior to non-woven polyester for filtration.
This sample will
allow a comparison, with 6a vs 7a comparing polypropylene
vs polyester.
Also, while some studies show non-woven polypropylene as
filtering well, I haven't seen assessment of filtration
after washing.
|
6b
| Yes
| -
| Yes
|
7a
| Yes
| -
| Kona
| 930
| Kona
| No
| A standard construction in the sewing community.
|
7c
| -
| Yes
| No
|
8a
| Yes
| -
| Sateen
| 930
| Sateen
| No
| Kona cotton has ~130um gaps visible in
microscope photographs
while this sateen is a similar weight, thinner yarns, and
without such gaps.
7a vs 8a will allow comparison of Kona (quilting woven cotton)
vs sateen (a woven cotton with fewer gaps between yarns)
|
Results
I see several things in this data:
- Best guess: Rewardingly, my "best guess" combination
sateen/OlyFun/flannel indeed performed best.
I've sewn masks with similar materials and found them acceptably
breathable, and happily look forward to sewing a mask with my
best guess.
- Washing: Washing and drying on the default cycles
10 times did not affect filtration. The dashed lines in the
plot show washed samples, and they filtered nearly identically
to their unwashed counterparts.
The fabrics included in the washed samples are:
Kona woven cotton, OlyFun non-woven polypropylene,
Robert Kaufman cotton flannel, Pellon P40 non-woven
polyester/viscose, which is a fairly broad range of fabrics
used by home sewists.
- Polyester vs polypropylene: Non-woven polypropylene
(OlyFun) filters better than non-woven polyester (Pellon 930).
We see this when
we compare Kona/OlyFun/Kona vs Kona/930/Kona.
- Filter layer: A third filter layer is worthwhile.
For 600um and larger particles, two layers of Kona cotton woven
performed significantly worst.
Filtration at most penetrating particle size |
Sample ID |
Result @0.3um |
Layers |
Washed? |
3a
| 23.6%@200nm
| Kona
| P40
| Flannel
| No
|
3b
| 26.2%@200nm
| Yes
|
3c
| -
| No
|
4a
| 45.4%@300nm
| Sateen
| OlyFun
| Flannel
| No
|
4c
| -
| No
|
5a
| 13%@300nm
| Kona
| -
| Kona
| No
|
5b
| 7.9%@40nm
| Yes
|
5c
| -
| No
|
6a
| 32.4%@40nm
| Kona
| OlyFun
| Kona
| No
|
6b
| 33.3%@200nm
| Yes
|
7a
| 14.5%@40nm
| Kona
| 930
| Kona
| No
|
7c
| -
| No
|
8a
| 24%@200nm
| Sateen
| 930
| Sateen
| No
|
Timeline
- 9 February, 2021 (Tuesday): Samples mailed to CTT Group.
- 6 to 10 business days expected for shipping.
- 2-3 weeks expected for testing by CTT Group.
- 23 February, 2021: Samples received by CTT.
- 1 March, 2021: Results received from CTT for 0.3um samples
Last updated 11 February 2021
© Anna Mitros
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