More face mask science

I'm emotionally exhausted by the pandemic and all the crappy news.  I've posted links to useful Covid-19 information when I come across them on microblogging platforms like IG and Twitter.  It's just easier than blogging.  But, there is also value to putting it on a blog, with more context and explanations.

I've written about masks in The mask/no mask dilemma back on March 18, when some US government messaging was that masks were not effective for the lay public. A lot of people have written about that bad messaging and I'm not going to belabor it here.  Professor Zeynep Tufecki does it as well as anyone.

Anyway, I also posted some cool visual proof of the efficacy of masks in This is why we wear masks. Those are Schlieren images and show breath vapor, not the motion of aerosols and droplets, which will stay even closer than the images shown.

Almost any type of cloth face covering will catch droplets. Most multi-layered ones will catch the majority of the aerosols as well. Masks hold your air closer to your face than without masks.

As Dr Lucy Jones says, "don't share your air."

In my first post, I (and many readers) didn't know what they meant by "Cotton Mix" but were impressed by its filtration efficacy.

After reading this subsequent article, I learned that it was Cotton/Poly blend. Polyester, and any fabric that has static cling, has electrostatic properties that attract and hold particles. That's why your furnace filter is made of polyester fabric. In fact, some people were buying furnace filters and cutting them up to insert inside their DIY masks.

Read Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. I excerpted some figures and a table from the paper. Even though it is published in a scientific journal, it's written so they lay public can understand it. It is freely available and I hope you download and read it. It's good.

We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 and="" nm="">300 nm, respectively, the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. 

This figure shows that filters can trap particles by either mechanical filtration (tight weave) or by electrostatic attraction.

Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies.

Notice that Cotton Quilt fabric (medium weave 120 threads per inch) performs better overall than the 600 threads per inch Cotton except at larger particle sizes?  Looser weave Cotton (cheaper quilting cotton at 80 threads per inch) does a lousy job.

Filtration efficiencies of the hybrids (such as cotton–silk, cotton–chiffon, cotton–flannel) was >80% (for particles <300 and="" nm="">90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration.

The hybrids do a better job than pure Cotton. Some even outperform surgical masks, which were never designed to catch smaller particles.  (They were designed to catch larger droplets exhaled by health care workers to protect the patients.)

Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of “fit” and leakage, while allowing the exhaled air to vent efficiently.

The biggest factor is fit. Isn't that the #1 reason why we sew?  We can customize the fit.

We also sew for improved comfort and face masks are no exception.  If the mask fits and is comfortable, we can wear them for as long as we need.

Pay attention to the pressure drop.  It doesn't matter if the weave and fit are tight, but it's too hot and uncomfortable to wear.  I know that I can wear a real surgical mask while riding my bike or for a long-haul airplane flight.  So I want a combo that has a similar pressure drop of 2.5.  I'll sew something with a good enough filtration efficiency, with the same comfort rating.

Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.

Notice that, in every case, more layers offers more protection. 

If you read up on Brownian motion, you learn that smaller particles move faster and encounter more obstacles than larger particles.  So your fabric or filter material doesn't have to be tight enough to mechanically trap the smallest aerosols.  A filter can alter the particle trajectory so that it comes at a slant instead of perpendicular to the filter.  That makes the particle more likely to be caught by subsequent layers. 

That's the magic behind multiple layers.

Knit fabric structure kind of has those layer properties, which is why just one layer of t-shirt jersey can be effective.  Aerosol expert, Professor Linsey Marr, Tweeted that she wears a knit gaiter-style mask single layer when running outside and then doubles it up (by rolling the top over her nose and mouth) if anyone is nearby.

I've come to the realization that I need a wardrobe of masks for different situations.  I'll make some knit gaiter ones for outdoor exercise.  I'll make some fitted woven ones for my essential trips to indoor locales.

It's time to bust out all my cotton/poly or cotton/silk mixes for fitted masks and some high-performance poly knits for gaiters.

I wear a mask to protect you. You wear a mask to protect me. We can sew our own and make it a fashion statement.

Finally, read Masks offer much more protection against coronavirus than many think. Even if the mask is not 100% efficient at filtering out aerosols and droplets containing Coronavirus, it reduces the dose of exposure. So, even if we do get sick, the illness is milder.

"Perfect is the enemy of good," means that waiting for perfection to act is a bad strategy.  We can always strive to do better, learn more.  But we already know enough and have the tools to reduce our collective risk by a huge amount.  Let's all be good enough and defeat this pandemic.