Thursday, April 23, 2020

COVID-19 Prophylaxes: Face Mask Material, Filtration

In my previous post, 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 masks. I correct that shortcoming, and then some, in this post.

Clearly, some masks are better than others. I'll begin with the not-quite-a-party line. The N95 masks are the best, because they stop 95% of something or other that may or may not begin with the letter N. The N95 masks are generally cup-shaped, and they even have "N95" printed right smack dab on the nose. N95 masks, though, need to be properly fitted to be N95-worthy. Unfortunately, Mosts folks don't fit them properly, so the so-called N95 masks are actually N_something_less_than_95 masks, but that's neither as catchy nor as comforting.

Next apparently, in my under-informed opinion, are the so called surgical masks. They're the blue, pleated, nearly flat, paper (or paperish) masks so commonly seen among the people of the 13 countries that have such low infection rates. Good enough for a surgeon, I say, good enough for thee. Not according to the CDC, though. I attach their summary below. Click to enlarge.


The CDC helpfully advises that a surgical mask "Does NOT provide the wearer with a reliable level of protection from inhaling smaller airborne particles and is not considered respiratory protection."

The CDC should probably not share their wisdom with the non-N95-face-mask-wearing folks of Taiwan, since Taiwan's infection rate (as of 13 April 2020) was 17 per million while ours, here in the good ol' US of A, was a hundred times that, at 1,773 per million.

Instead, the CDC might want to consider several observational studies that show that surgical face masks provide the same level of protection against pathogens as do the N_not_quite_95 masks.

The earlier of the two studies was reported way, way, way back in 2009 in JAMA (Journal of the American Medical Association) as Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers. The key points of that study read:
"Context: Data about the effectiveness of the surgical mask compared with the N95 respirator for protecting health care workers against influenza are sparse. Given the likelihood that N95 respirators will be in short supply during a pandemic and not available in many countries, knowing the effectiveness of the surgical mask is of public health importance. 
"Results: Between September 23, 2008, and December 8, 2008, 478 nurses were assessed for eligibility and 446 nurses were enrolled and randomly assigned the intervention; 225 were allocated to receive surgical masks and 221 to N95 respirators. […] 
"Conclusion: Among nurses in Ontario tertiary care hospitals, use of a surgical mask compared with an N95 respirator resulted in noninferior rates of laboratory-confirmed influenza."
Allow me to translate the somewhat muddled portion. The surgical masks were "noninferior," meaning that they were "just as good."

Ten years later on, in 2019, the same exact Journal published the results of another, significantly larger study, under the nearly identical, noninferior title of "N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel." The key points of that study read:
"Question:  Is the use of N95 respirators or medical masks more effective in preventing influenza infection among outpatient health care personnel in close contact with patients with suspected respiratory illness? 
"Findings:  In this pragmatic, cluster randomized clinical trial involving 2862 health care personnel, there was no significant difference in the incidence of laboratory-confirmed influenza among health care personnel with the use of N95 respirators (8.2%) vs medical masks (7.2%). 
"Meaning:  As worn by health care personnel in this trial, use of N95 respirators, compared with medical masks, in the outpatient setting resulted in no significant difference in the rates of laboratory-confirmed influenza."
Imagine that!  Still no difference. The N95 masks were noninferior to the surgical masks!

So what about home-made masks? Given how much as the CDC is troubled by even surgical masks, I assume that the thought of an uninformed citizenry sporting about in home-made masks would cause the Center for Disease Controllers to suffer severe functional dyspepsia. Nonetheless, I have found two studies that actually tested various materials to see which, if any, might work for the construction of homemade masks. I combine the filtration results of those two studies in this post. In the next post, we will consider the rather important issue of being able to breath through the various materials. In the post after that, the third in this august sub-series within an august series, we will consider the issue of sealing the masks to the face.

The first study was reported online in a 22 May 2013 National Institute of Health technical paper, "Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?"

The second study is more recent, less formal, and way more entertaining. It is presented in a blog post, not a technical paper. That august post is entitled The Ultimate Guide to Homemade Face Masks for Coronavirus." It is the fine work of the fine folks at Smart Air. Their mission statement follows.
"Smart Air is a social enterprise and B-corp that combats air pollution by delivering cost-effective air purifiers and providing open-source data on air pollution.”
The emphasis is mine. In my opinion, those folks are providing the most timely, most informative, most useful, most clear, and most entertaining information about face masks, bar none. You can do a lot worse than reading through their entire list of blog posts about the coronavirus.

The Smart Air folks focused their studies on particle diameters near 0.3 micron, which is three one-tenths of a millionth of a meter, which is really really really small, clearly smaller than a breadbox, and approximately three times the the size of the COVID-19 virus.

The NIH folks focused on two microorganisms, one of them being the Bacillus atrophaeus, a black stained bacterium frequently used in biomedical testing, having a diameter ten times that of the coronavirus. Their second microorganism was the Bacteriophage MS2, a single-stranded RNA virus (as is the coronavirus) but only 1/5 as large as the coronavirus. I'll provide the NIH results for both particle sizes below, sorting the items by decreasing order of filtering efficiency, winners near the top, losers near the bottom.

Surgical Mask:  96% for 1 micron,  88% for .02 micron
Dish towel / tea towel (double layer):  97%,  N/A
Vacuum cleaner bag:  94%,  86%
Dish towel / tea towel:  83%,  72%
Cotton mix:  75%,  70%
T-shirt, 100% Cotton (double layer):  70%,  N/A
T-shirt, 100% Cotton:  69%,  51%
Pillow case, antimicrobial:  66%,  67%
Scarf:  62%,  49%
Pillow case:  61%,  47%
Linen:  60%,  62%
Silk:  58%,  44%

Now for the lengthier, more innovative, more relevant Smart Air test results for 0.3 micron particles

3M N95 mask:  96%
HEPA filter:  83%
Surgical mask:  75%
Coffee filter, HERO:  62%
Coffee filter, CHEMEX:  49%
Nylon, 40D:  49%
Canvas, 1.1 mm thick:  49%
Dish towel / tea towel:  48%
Paper towel, brown, hand drying:  33%
Paper towel, kitchen (double layer):  33%
Canvas, 0.75 mm thick: 31%
Denim, 10 oz, 0.7 mm thick: 29%
Bed sheet, 100% cotton, 120 thread: 24%
Paper towel, kitchen:  23%
Bed sheet, 100% cotton, 80 thread:  21%
Canvas, 0.45 mm thick:  19%
Paper towel, Scott's blue shop towel:  19%
T-shirt, 100% cotton (double layer): 15%
Bra pad, muslin + sponge:  14%
Velvet synthetic fiber:  13%
Nylon, 70D:  12%
Polypropylene bag, non-woven:  11%
T-shirt, quick-dry, 95% polyester + 5% spandex:  7%
Bed sheet, synthetic brocade:  7%
Cleaning cloth, floor, disposable:  7%
Cloth, dusting:  7%
Scarf, wool, 100% cashmere:  6%
T-shirt, 100% cotton:  3%
Scarf, light, 100% ramie:  3%
Neck warmer / snood, 100% microfiber polyester:  2%
Bandana, 100% cotton:  2%

Once again, I congratulate the fine folks at Smart Air for doing all the testing and making the results available to all at no charge.

The results are quite interesting, as far as they go. Without considering one's ability to breath through something like a vacuum cleaner bag or silk tent material, it's a wee bit early to make even an under-informed recommendation. For that, hang tight until the next post.

[Mea culpa note in reduced font size: I initially published this post on 23 April 2020. On 24 April, I discovered that I had posted the Smart Air test results for 1 micron particles rather than for 0.3 micron particles. The filtration percentages were substantially worse and the order ranking order was somewhat different. I published this corrected version on 24 April.]

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