Face Mask Science 3, August 2020 edition

Do you have face mask confusion?  Do conflicting headlines give you headaches?

I sort of feel like I've read enough to know that I should wear one whenever I am indoors in any place that is not my own home.  I should also carry one whenever I am outdoors and be ready to put it on whenever anyone is nearby.

There is no way to reduce risk 100%, but we can reduce it to pretty low by:

• Social distancing in the sense that we minimize going out where we could encounter other people--especially in poorly-ventilated indoor places
• Social distancing in the sense of putting as much space (and air) between you and others outside your household when you do go out
• Universal mask wearing, especially indoors
• Good hygiene (cleaning surfaces, washing hands, not touching face, etc)

The even better news is emerging evidence from multiple countries and situations/studies that show that universal masking reduces the severity of Covid-19 illness if you do catch it. The idea is that viral load matters.  The higher the viral load, the more severe the illness. 

• Try to avoid situations where you can be exposed to the virus.  
• Assume that everyone is contagious (even though usually, it's less than 1%, often much less than 1% of the population.) 
• Then minimize your exposure.

The shaped 2-layer cotton masks I sew are fine for essential shopping trips and the occasional doctor visit.  The problem is finding masks that are comfortable enough to wear when being active (walking, biking, gardening.)

I made some gaiter-style masks with poly/lycra knit sewn into a tube, with a ponytail port at each end.  They were comfortable enough to wear for exercise, but a recent article said that gaiter masks might be worse than nothing.  That made no sense.

Fortunately, Professor Brent Stephens wrote 6000 words about what we know and don't know about masks, different materials, and Covid-19. What kind of mask should I be wearing to protect against COVID-19?

He has some thoughts on Gaiter-Gate, which I need not replicate here. Go read his analysis on why that gaiter measurement (with no control) is highly suspect.  Other scientists pointed out that the measurement that showed a "fleece gaiter" was worse than a bare face at containing droplets might have been measuring fleece fiber shedding instead of droplets.

Professor Linsey Marr wrote how she runs with a gaiter and doubles it up when anyone is nearby.  She posted a presentation on the blocking efficiency of neck gaiters.  The bottom line is that the thin kind that I sew blocks 50-90% of particle sizes that matter when worn in a single layer.  When doubled over, they block 90% over the entire range.

Lab setup:


The data:

The airbrush and spray bottle produce large droplets that, in the absence of a mask or neck gaiter, land on glass slides attached to the opposite manikin’s face. When either neck gaiter was affixed to the spraying manikin, no droplets were observed on the opposite manikin. Thus, the neck gaiters are 100% efficient at blocking these droplets from reaching the opposite manikin’s face.

I know that I sound like a broken record, but the best mask is the one that you will keep on.

Anyway, I am heartened from all of this advice from scientific experts.  It makes Covid-19 less scary.  I have enough to worry about with the elections and the census. 

In other news, this pandemic has made me read up on indoor ventilation and reminded me of sick building syndrome.  I was the canary in the goldmine for that and remember being treated like I was crazy.  One doctor even told me that he ordinarily sends patients who present with my symptoms to psychiatrists but he didn't think that was my problem; after all, I don't dislike technology and chemicals if I decided to major in it.  I'm assembling a blog post with very interesting things I learned.  I even ordered test equipment for my home. 

The mask series.

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.

This is why we wear masks

We have a preponderance of evidence that Covid-19 is spread by aerosols and large droplets expelled through our mouths when we talk and through our mouth and nose when we sneeze or cough.  We also have plenty of evidence that (nearly) universal mask-wearing in public is extremely effective in Reducing transmission of SARS-CoV-2.  The linked article by Prof. Kim Prather et al is written extremely clearly in plain English and I highly recommend reading it.

I'm embedding two videos because they are helpful to understand social distancing and wearing masks.  But, it's important to keep in mind that they are showing hot air instead of droplet and aerosol motion, which does not spread as far.

Schlieren or shadowgraph imagery shows changes of thermal patterns in the air. We expel heated air, which will mix with ambient air and might waft upwards (hot air rises.)  Large droplets are heavier and will fall downwards. Aerosols might hang around a few minutes to hours, but eventually fall.

With these caveats, these images nevertheless demonstrate the effectiveness of wearing face masks that cover the nose and mouth.

First, this charming* video of NIST scientist, Matthew E. Staymates, working from home. Note how his breath stays much closer to his head when he wears a mask vs when he doesn't. This is why mask wearing, along with social distancing, dramatically reduces transmission risk.  Remember, this is showing hot air; virus-containing droplets will most likely by caught in any type of mask as long as it covers the mouth and nose.



I particularly like that he wears different styles of cloth masks sewn by NIST colleagues, who did socially-distanced handoffs outside his home.  All types of masks work as long as they are comfortable enough for you to actually wear them over both your nose and mouth.

This older video demonstrates how much air you exchange with others just in normal face-to-face conversation.  It starts with normal nose breathing, then mouth breathing, talking, coughing, conversation, etc.



I'm cautious and minimize my shopping trips.  When I go into stores, I use hand sanitizer, then put on my mask and then enter.  I don't touch the mask until I get back outside.  This protects me.  It protects you.

Give people without masks a very wide berth, 2 meters or more, and minimize your exposure to them.  If someone takes off their mask to talk in a store, run away.  They've contaminated everything within droplet distance.  If they talk loudly, that can be far greater than 2 m.   If a store allows unmasked people to enter, assume all surfaces are contaminated.  (This creates so much more work and risk for retail workers.)

If all staff and shoppers are wearing masks, I would not stress about breaking the 2 m bubble for a short time. Remember, it's also the viral load in a short amount of time that is dangerous.  If you get a tiny bit of viral load, and you have a healthy immune system, your body would destroy the viruses before they can get a toe-hold in your respiratory tract.

When I see people wear masks under their nose, I give them wider berth, but don't say anything to them about it.  They tried, but maybe they are too uncomfortable.  Or maybe they are conspiracy theorists who don't understand that oxygen and carbon dioxide pass through the masks just fine.  Either way, I don't want to engage with them.  A mask that covers their mouth would still contain droplets expelled by talking unless they sneeze or cough.

For those of us in low-risk settings (not health care workers or people who work around others all day) the best mask is the one you will actually wear.  If you try to make it too efficient at filtering, it can be difficult to breath through, or get soggy from your exhaled breath.  If you take off your mask when indoors (other than in your own home,) then you lost protection.

I walk and bike outdoors without a mask, but carry one in case it gets crowded.  I'm coming around to the idea that perfect is the enemy of good and I may sew lower-coverage masks that don't extend as far across my cheeks for outdoor exercise and gardening.  The best mask is the one that you will keep on.

* Maybe I'm biased because I got my PhD at a NIST lab.

The mask/no mask dilemma

During tree pollen season, I often wear a surgical mask outdoors to minimize allergies.  In February, I tried to restock ahead of the season. I also heard about a very bad flu virus that would make wearing a mask in crowded places prudent.  Too late, I realized that, not only were the stores near me sold out, they were not going to get any more in the foreseeable future.

I read intriguing posts on the internets about homemade masks, but wasn't sure if they were nothing more than dangerous placebos.

Please read Zeynep Tufekci's Why Telling People They Don’t Need Masks Backfired (NYT paywall)

First, she is right about the messaging.  Lying to the public, even for altruistic reasons, is still counterproductive.  I prefer to be treated like an adult and told that there are people who cannot stay home and need the masks more than I do.

Surgical masks, though not foolproof, offer meaningful protection for both the wearer and the people they are in contact with.

This doesn't help me now when stores don't have anything to buy and my doctor is saving masks for her own use and for her most critically ill patients (or caregivers.)  Professor Tufekci helpfully added a link to a paper about the effectiveness of different materials you can use to sew your own face masks.

I'm about to crank out some masks for my daily walks to protect me from pollen and stray human contact. I'm also making some for people I know who are essential workers and risking their lives to keep our society going.

This table is helpful, but takes a little interpretation. The first 2 columns are virus reductions over wearing no mask with surgical masks and home-made masks with a variety of materials. The third column is the pressure drop across the fabric.

That pressure drop is really important. If a mask is too hot, and you take it off, you are in danger again.  So you want to find that sweet spot of comfortable to wear for hours, and good filtration.  My normal go-to mask is the 3M micropore one.  I can wear it comfortably for long haul flights, taking it off only to eat and drink.

Two layers of a tea towel (a thicker fabric,) is as effective as a surgical mask, but it will be as comfortable as wearing a vacuum cleaner bag.

@pdxsquared showed pictures of her home-made ear loop masks.  She also shared per pattern.  One commentator said that she was irresponsible for posting it--2 layers of quilting cotton isn't real protection.  Well, the peer-reviewed lab-tested experiment showed that 2 layers of a pillowcase (percale, similar to quilting cotton) is 62% effective vs the 96% effectiveness of a real surgical mask, and has a similar comfort rating.  You should absolutely minimize contact with other people right now.  But, if you have to go out to work or get supplies or exercise, wearing DIY masks is definitely helpful.

I'm intrigued by the 75% effectiveness of "cotton mix" but can't figure out what that is.  I may try some cotton/nylon/spandex shirting I was saving for a special project someday.  That day is today.  I'm also going to use fun quilting and shirting fabrics.  Right now, I need more joy and fun.

BTW, I saw an old DIY emergency face mask pattern that suggested using 3 layers of cotton, laid at right angles.  This makes sense because fabrics typically have higher thread count (and smaller holes) in one direction.  Perhaps I should use cotton mix with a layer of silk scrap in the middle?

ASIDE:
If you are in a good financial place, spread the wealth.

I told our housecleaner that, because my husband continues to get paid, so will she.  She cannot work from home so I'm just mailing the check to her house.  Her husband is heroically driving city buses to get essential workers to their jobs--and doing it without protective equipment.  I'm sending a bunch of masks to them along with the check.

We're paying students to weed our yard (while staying socially distant.)  I'm making masks for them, too.  I know it helps with pollen and mold.

Show me what you are making and how you are coping.  We're anxious, but know that we are luckier than most.  I'm in no danger of running out of fabric.  ;-)

UPDATE:
Physician/sewing blogger Kaddidlehopper has some good tips and a free pattern: https://katiekadiddlehopper.blogspot.com/2020/03/fabric-surgical-style-mask-free-pattern.html

ElleC left a link in the comments on how Dr Dr Chen Xiaoting improvises surgical masks with disposable tissues or TP encased in washable/reusable cotton.
https://mustsharenews.com/cloth-face-mask/