Given the pandemic and this topic, I had high hopes.

But, unlike the Philadelphia Phillies’ theme song  (a song Frank Sinatra sang [from the movie “A Hole in the Head”] decades ago and we half-dozen child prodigies studying in college at the age of 8 adopted as our theme song, too), this hope got deflated in a heartbeat.

Dr. Linsey Marr, along with her students or team members of the Civil Engineering Department at Virginia Tech (Jin Pan [PhD candidate], Charbel Harb, and Weinan Lang) provided a preprint of their paper, “Inward and outward effectiveness of cloth masks, a surgical mask, and a face shield”.  That would really be an interesting study, given the fact that we are all (or should be) wearing masks to keep ourselves, our loved ones, and our fellow citizenry safe.  (Note that Dr. Marr is an aerosol specialist.)

So, let’s talk about the good parts first.  They examined 11 different choices- they wanted to know how well these masks filtered out particles, protected the wearer of the mask (on a manakin), and protected the world (using a manakin to expel “breaths”).    They reported that vacuum bags, microfiber cloths, and surgical masks had filtration efficiencies of more than 50%; the rest failed even that low bar.  Except their particle size was 2 micron (μm) and up.  Who cares how well the masks work for larger micron sizes when we are dealing with a pandemic!

Marr’s group calculated that the most likely particle size to ‘rest’ in our respiratory tracts is about 2 microns.  That led to their recommendation that a tri-layer mask (outer layer- flexible fabric, with a tight weave, inner layer- designed to filter out particles, filter fabric in between [she suggested this should be similar to a vacuum bag]), which should remove 70% of the most penetrating particles- and up to 90% for some particles- if the mask fits well. (That’s why they specified a flexible mask, but we really need wire reinforcing around the nose area, to make the fit more complete.)  That choice could mean that we wouldn’t all need N95 masks.

Another change they recommended is to choose masks that tie (without the elastic ear tags).  Not only because we all are seeming to develop mask ears (from the pressure of the flexible ear loops on the backs of our ears)- but because these sorts (with flexible ear loops) of masks leave larger gaps between our faces and the masks.

And, the researchers did bring up a problem that so many fail to recognize.  Some of these tight masks have such a large pressure drop, it’s tough to use them long term.  (Especially those of us with compromised respiratory systems.  TRICK:  I use an anti-snoring device, held in place by the mask, that pumps a little bit of air into my nostrils.)

Not surprisingly (at least to me), they found those plastic face shields of no use without masks.  Sure, they protect your eyes from direct contact, but air leaks all around it.  (These devices could only work if one had an air pump that flushed the space between one’s face and the shield.)

Yet, we already knew from studies like the one by Dr. C. Makison Booth (Dr. C. Makison Booth, along with Drs. M Clayton, B Crook, and JM Gawn, all from Buxton Health and Safety Lab [UK]) that some masks effectively remove viral particles- up to a 55 fold reduction (with the average among they tested of about a 6 order reduction).

A study effected 7 years later, in the throes of a pandemic, should be seeking out more information, not less.

The first problem with this study is that SARS-CoV-2 particles can be from 0.1 to 5 microns in size.  (The latter is when they are ensconced among other respiratory droplets.)   If we are speaking or breathing normally, we can expect the aerosol size to be populated with items about 1 micron in size.  But, as you note above, the authors talked about efficiencies at 2 microns.  That’s like saying the fence I designed to keep out dogs is perfect against elephants, but not quite so good against dogs.

Then, we get to the next issue.  The authors tested the filtration efficiency of the materials using flowrates of 2.7 L/min (plus the sampler at 0.3 L/min).  That sounds great- except we breath some 7 liters of air in and out every minute.  So, why would we care what the filtration efficiency would be at volumetric flows below what these fabrics will encounter?

And, then, when checking how the mask works for inhalation and exhalation, they chose even more bizarre values.  The exhaling manakin operated at 10 L/min; the inhaling manakins at a combined ventilation rate of 15 L/min.  Except their manakins did mimic the velocity we normally breath through our mouths (range of 3.2-3.4 m/s).

So, what can we learn from this study?  Masks work.

How well?  This study didn’t add to our knowledge of how they work the way we breathe.

Next.

 

By the way, some of you may be too young to get the title reference.  The Lone Ranger- the man with the mask [not one to filter out air, though]- was called "the resourceful masked rider".