Post #924: Mask test published in JAMA

Posted on December 22, 2020

A recent article in the Journal of the American Medical Association (JAMA) performed a sophisticated test of several face masks.  The results had a few surprises.  I thought I might take the opportunity to repeat a few key results, and then, in a separate posting, maybe compile all the presumably legitimate masks tests that have been published.  You don’t want to make too much out of any one test, but this one appears to be about as realistic and accurate as you are likely to find.

Obviously, if you want the best protection, use some variant on an N95 respirator.  I only recently found those once again for sale to the general public through legitimate retail channels (Post #918).  I’ve now bought and tried the masks in Post #918, and while they are odd-looking and awkward to put on, they appear to be as-advertised US-made duck-bill N95 respirators, not for medical use.

But if you can’t or won’t use an N95 respirator:

Source:  JAMA.

The first big surprise in the JAMA article was that close-fitting two-layer nylon masks with ear loops did quite well, after they were washed.  They filtered out almost 80% of fine aerosols (on the same test where an N95 respirator would filter out 95%).

This same test showed that single-layer cloth masks, non-woven polypropylene masks, polyester masks, and cotton masks were vastly inferior to the double-layer nylon mask.

Source:  JAMA.  The second big surprise (for me) is that a simple modification of a standard cheap medical procedure mask greatly improved filtration effectiveness.  When “tied and tucked” this way, these masks filtered more than 60% of fine aerosols.  (Versus under 40% off-the-shelf).  I think the clear implication here is that if you’re still wearing one of those cheap disposable procedure masks, you should do yourself a favor and take 30 seconds to perform the tied-and-tucked modification.  That tied-and-tucked method is shown in this YouTube video.

Various other methods for improving the “fit” of those masks also produced improved filtration, but none was as simple as the “tied and tucked” approach.

Source:  JAMA.  A third surprise is that, when property worn, a cotton bandana will trap half of aerosols.  The catch here is that I have yet to see any bandana user actually wear one like this.  Note the very large bandana, doubled over, fully covers the face and is tucked into the shirt.  That’s how they got that 50% filtration.  By contrast, every guy (and it’s always a guy) I’ve seen with a bandana basically has an open flap of cloth over his face.  You aren’t going to get 50% filtration when the air your are breathing flows around (rather than than through) your cloth face covering.

In any case, I thought this was a good article, and is very accessible if you skip the text and just go to the figures.  I’m adding in the table of results below, by reference.  I also want to give a proper citation for the article.  It is:

Clapp PW, Sickbert-Bennett EE, Samet JM, et al. Evaluation of Cloth Masks and Modified Procedure Masks as Personal Protective Equipment for the Public During the COVID-19 Pandemic. JAMA Intern Med. Published online December 10, 2020. doi:10.1001/jamainternmed.2020.8168

The main table of results should appear below, from the JAMA URL.  “FFE” is shorthand for the test under which an N95 respirator should score 95% or above to pass.  Read down the %FFE column to see which masks filter best.  Ignore the (SD) column — that shows how much variation there was in the measured efficiency across multiple tests.