Friday, April 24, 2020

COVID-19 Prophylaxes: Face Mask Material, Breathability

In my first post of this mini-series within a series, I explained that the Prophylactic Index for widespread use of face masks (for those of us north of the tropics) is 12.7. I recommended that reasonable face mask requirements be part of any plan to relax social distancing rules. I said nothing about the efficacy of various types of face mask materials. I corrected that shortcoming, and then some, in my previous post.

While that previous post provided you with filtration information on nearly three dozen candidate materials for DIY face masks, it provided no insight into how easy of difficult it might be to breath through them.  I correct that shortcoming herein, and I make additional under-informed recommendations.

The fine folks at both the NIH and Smart Air tested the ease or difficulty of moving air through the various face mask materials that they tested for filtration efficiency. The NIH folks measured pressure drop across the material. The Smart Air folks measured the fan power required for a give flow rate through the material, and they provided their results as breathability scores ranging from 1 to 6.

Since most people don't relate particularly well to pressure drop, or have six fingers, I've converted both the NIH and Smart Air scores to a breathability score of 0 to 10, with the surgical mask at 5 and the most easy-to-breath-through material at 10. I provide a composite list of the results below, breathability winners on top.

Silk:  breathability score = 10
Pillowcase:  7
Scarf, 100% Cashmere:  6
Scarf, 100% Ramie:  6
T-shirt, 100% cotton: 6
Linen:  6
Cloth, floor cleaning, disposable, 3M: 6
Synthetic fiber, velvet: 6
T-shirt: quick-dry, 95% Polyester + 5% Spandex:  6
Bed sheet, brocade:  6
Scarf, wool, 100% merino: 6
HEPA Filter: 6
Polypropylene bag, non-woven: 6
Bandana, 100% cotton: 6
Neck warmer / snood, 100% microfiber polyester: 6
Cloth, dusting: 6
Paper towel, Scott’s blue shop towel: 6
Canvas, 0.45 mm thick:  6
Surgical mask:  5
T-shirt, 100% cotton (double layer): 5
Pillowcase (double layer):  5
Bra pad, muslin + sponge: 4
Nylon, 70D: 4
Dish towel / tea towel:  3
Pillowcase, antimicrobial:  3
Paper towel, brown, hand drying: 3
Bed sheet, 100% cotton, 120 thread:  3
Bed sheet, 100% cotton, 80 thread: 3
Denim, 10 oz, 0.7 mm thick:  3
3M N95 mask: 3
Coffee filter, CHEMEX: 2
Canvas, 0.75mm:  2
Nylon, 40D: 1
Coffee filter, HERO: 1
Canvas, 1.1 mm thick:  1
Dish towel, tea towel (double layer):  0
Vacuum cleaner bag:  0

So ...

All that you have to do to decide whether or not you should make your own masks, and settle on what material you should use (assuming you decide to go rogue), is look at two different sources of test data, for three different particle micron sizes, and somehow weigh those data against the breathability numbers provided above.

Or ...

You could just continue reading this august post, the one in which I have scored each of the candidate materials with another of my made up numbers, this time the soon to be famous Face Mask Index, or FMI. The Face Mask Index is merely the breathability score multiplied by the material filtration efficiency for 0.3 micron particles. (As an informative but boring aside, I had to adjust the NIH efficiencies for 1.0 and 0.02 micron particles based on 0.3 micron particle results for similar materials from the Smart Air data.)

The FMI results for each of the face mask designs / materials are presented below, in descending order of FMI. The winners are on the top; the losers are at the bottom. The list contains a few surprising results, but it is mostly populated with disappointing results.

Here we go.

HEPA Filter: FMI = 5.0
Surgical mask:  3.8
3M N95 mask: 2.9
T-shirt, 100% cotton (double layer):  1.5
Dish towel / tea towel:  1.4
Pillowcase:  1.4
Silk:  1.3
Linen:  1.2
Canvas, 0.45 mm thick:  1.1
Paper towel, Scott’s blue shop towel: 1.1
Pillowcase (double layer):  1.0
Coffee Filter, CHEMEX: 1.0
T-shirt, 100% cotton (double layer):  0.8
Synthetic fiber, velvet: 0.8
Pillowcase, antimicrobial:  0.7
Polypropylene bag, non-woven: 0.7
Bed sheet, 100% cotton, 120 thread:  0.7
Bed sheet, 100% cotton, 80 thread: 0.6
Canvas, 0.75mm:  0.6
Denim, 10 oz, 0.7 mm thick:  0.6
Bra pad, muslin + sponge: 0.6
Nylon, 70D: 0.5
Cloth, floor cleaning, disposable, 3M: 0.4
Cloth, dusting: 0.4
Bed sheet, brocade:  0.4
Scarf, wool, 100% Merino: 0.4
Scarf, 100% Cashmere:  0.4
T-shirt, quick-dry, 95% Polyester + 5% Spandex:  0.4
Scarf, 100% ramie:  0.2
T-shirt, 100% cotton:  0.2
Paper towel, brown, hand drying: 1.0
Bandana, 100% cotton: 0.1
Neck warmer / snood, 100% Microfiber Polyester: 0.1
Nylon, 40D: 0
Dish towel, tea towel (double layer):  0
Coffee filter, HERO: 0
Canvas, 1.1 mm thick:  0
Vacuum cleaner bag:  0

From a story-telling perspective, the most discouraging result is that I will not be recommending that people try to save themselves from the coronavirus by breathing through vacuum cleaner bags.

The most surprising result is the sequence of the top three items in the list. The HEPA filter wins because it has the best combination of filtration efficiency at 0.3 microns (83%) and breathability score (6). Though the N95 masks have the best filtration efficiency (96%), they get clobbered by their breathability score (3).

The disappointing result is that there are no great alternatives to the top three masks. All but the top three have such low filtration efficiencies that their breathability scores can't save them.

The most frightening result is that bandana score so low. I've seen many sites that mention bandanas as a viable face covering, and I fear those sites are doing more harm than good. Those poor people who take such bandana advice at face value might just as well wear their home made face mask around their neck, or put it in their back pocket. Either way, they will be taking it in the neck.

The bottom line is that you can't really do much better than just buying the reusable surgical masks, presuming you can get them. They have, after all, been working great for those 13 countries that began wide-spread use of face masks prior to 1 March 2020.

Not everyone will be able to acquire disposable surgical face masks, though. Given my ongoing efforts to free some of those wrongfully convicted, I'm particularly sensitive to those people we have behind bars, in close quarters, almost entirely reliant on others for potentially life-saving decisions regarding matters such as "which face mask, if any, should I wear?' In the one case consuming most of my wrongful conviction time and energy, I'm informed that the prisoners have been provided face masks, and that those face masks are seemingly made out of the same material as their bed sheets. That's pretty scary.

For those people who, for any reason, cannot get hold of N95 or surgical face masks, I recommend they somehow construct their own face mask using some combination of materials from the prioritized list below, using as much thickness as their breathing permits.

#1  HEPA filters
#2  coffee filters
#3  canvas
#4  paper towels
#5  denim
#6  bed sheets
#7  t-shirts

And my sincere best wishes to all those people in such dire straits.

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