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
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