I've already posted one link above about the filtering properties of various materials that was done by a company that tests N95 masks using the same machine used for qualit control of those masks. Here a couple more links with regards to testing various materials filtering properties. One from Wake Forest Baptist Health (testing done by Manufacturing Development Center of WF Institute of Regenerative Medicine) which was a summary article of their results and one from the American Chemical Society with a whole detailed run down of their study and I copied the sections on their observations and conclusions.
From the article:
What the test team found was that the masks’ effectiveness varied widely. The best homemade masks achieved 79% filtration as compared to surgical masks (62% to 65%) and N95 masks (97%). But other homemade masks tested performed significantly worse, sometimes demonstrating as little as 1% filtration, Segal said.
The best-performing design was constructed of two layers of high-quality, heavyweight “quilter’s cotton” with a thread count of 180 or more, and those with especially tight weave and thicker thread such as batiks. A double-layer mask with a simple cotton outer layer and an inner layer of flannel also performed well, he said.
The inferior performers consisted of single-layer masks or double-layer designs of lower quality, lightweight cotton.
“As important as this information is for hospitals, it is also important for people who want to make masks for their own use,” Segal said. “We don’t want people to think that just any piece of cloth is good enough and have a false sense of security.”
https://newsroom.wakehealth.edu/New...oth-Used-in-Homemade-Masks-Makes-a-Difference
The American Chemical Society study: (gory details in the link but here's some summary)
We highlight a few observations from our studies for cloth mask design:
Fabric with tight weaves and low porosity, such as those found in cotton sheets with high thread count, are preferable. For instance, a 600 TPI cotton performed better than an 80 TPI cotton. Fabrics that are porous should be avoided.
Materials such as natural silk, a chiffon weave (we tested a 90% polyester–10% Spandex fabric), and flannel (we tested a 65% cotton–35% polyester blend) can likely provide good electrostatic filtering of particles. We found that four layers of silk (as maybe the case for a wrapped scarf) provided good protection across the 10 nm to 6 μm range of particulates.
Combining layers to form hybrid masks, leveraging mechanical and electrostatic filtering may be an effective approach. This could include high thread count cotton combined with two layers of natural silk or chiffon, for instance. A quilt consisting of two layers of cotton sandwiching a cotton−polyester batting also worked well. In all of these cases, the filtration efficiency was >80% for <300 nm and >90% for >300 nm sized particles.
The filtration properties noted in (i) through (iii) pertain to the intrinsic properties of the mask material and do not take into account the effect of air leaks that arise due to improper “fit” of a mask on the user’s face. It is critically important that cloth mask designs also take into account the quality of this “fit” to minimize leakage of air between the mask and the contours of the face, while still allowing the exhaled air to be vented effectively. Such leakage can significantly reduce mask effectiveness and are a reason why properly worn N95 masks and masks with elastomeric fittings work so well.
In conclusion, we have measured the filtration efficiencies of various commonly available fabrics for use as cloth masks in filtering particles in the significant (for aerosol-based virus transmission) size range of ∼10 nm to ∼6 μm and have presented filtration efficiency data as a function of aerosol particle size. We find that cotton, natural silk, and chiffon can provide good protection, typically above 50% in the entire 10 nm to 6.0 μm range, provided they have a tight weave. Higher threads per inch cotton with tighter weaves resulted in better filtration efficiencies. For instance, a 600 TPI cotton sheet can provide average filtration efficiencies of 79 ± 23% (in the 10 nm to 300 nm range) and 98.4 ± 0.2% (in the 300 nm to 6 μm range). A cotton quilt with batting provides 96 ± 2% (10 nm to 300 nm) and 96.1 ± 0.3% (300 nm to 6 μm). Likely the highly tangled fibrous nature of the batting aids in the superior performance at small particle sizes. Materials such as silk and chiffon are particularly effective (considering their sheerness) at excluding particles in the nanoscale regime (<∼100 nm), likely due to electrostatic effects that result in charge transfer with nanoscale aerosol particles. A four-layer silk (used, for instance, as a scarf) was surprisingly effective with an average efficiency of >85% across the 10 nm −6 μm particle size range. As a result, we found that hybrid combinations of cloths such as high threads-per-inch cotton along with silk, chiffon, or flannel can provide broad filtration coverage across both the nanoscale (<300 nm) and micron scale
(300 nm to 6 μm) range, likely due to the combined effects of electrostatic and physical filtering. Finally, it is important to note that openings and gaps (such as those between the mask edge and the facial contours) can degrade the performance. Our findings indicate that leakages around the mask area can degrade efficiencies by ∼50% or more, pointing out the importance of “fit”. Opportunities for future studies include cloth mask design for better “fit” and the role of factors such as humidity (arising from exhalation) and the role of repeated use and washing of cloth masks. In summary, we find that the use of cloth masks can potentially provide significant protection against the transmission of particles in the aerosol size range.
https://pubs.acs.org/doi/10.1021/acsnano.0c03252
