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Which Face Masks are Most/Least Effective?
A Duke University study ranks them in order
By Rain Noe - August 10, 2020
With everyone wearing different styles of facemasks, Duke University physician and professor Eric Westman wondered: Are each mask-sellers' claims of efficacy true? Which masks are the most effective, and which simply provide a false sense of security?
As reported by Duke Health, Westman's colleague Martin Fischer, the chemist and physicist in charge of Duke's Advanced Light Imaging and Spectroscopy facility, devised a scientific test using a laser. Subjects spoke through a variety of masks, and the airborne droplets that made it through the mask were counted.
Types of Masks Tested (Not Ranked, Just Numbered)
1. 'Surgical' (Surgical mask, 3-layer)
2. 'Valved N95' (N95 mask with exhalation valve)
3. 'Knitted' (Knitted mask)
4. 'PolyProp' (2-layer polypropylene apron mask)
5. 'Poly/Cotton' (Cotton-polypropylene-cotton mask)
6. 'MaxAT' (1-layer Maxima AT mask)
7. 'Cotton 2' (2-layer cotton, pleated style mask)
8. 'Cotton 4' (2-layer cotton, Olson style mask)
9. 'Cotton 3' (2-layer cotton, pleated style mask)
10. 'Cotton 1' (1-layer cotton, pleated style mask)
11. 'Fleece' (Gaiter type neck fleece)
12. 'Bandana' (Double-layer bandana)
13. 'Cotton 5' (2-layer cotton, pleated style mask)
14. 'Fitted N95' (N95 mask, no exhalation valve, fitted)
'Swath' (Swath of mask material, polypropylene)
'None' (Control experiment, no mask)
Test Results
Enter a caption (optional)
The most surprising result is that wearing the fleece mask was worse than wearing no mask at all. As Fischer explains, the fleece essentially serves as an aerator on a sink faucet--it not only lets the droplets through, but breaks them up into finer droplets, allowing them to remain in the air longer.
Finally, for clarity's sake, we've arranged the images of the masks in order of efficacy.
Mask Types Ranked in Order of Efficacy:
Ranked #1
14. 'Fitted N95' (N95 mask, no exhalation valve, fitted)
Ranked #2
1. 'Surgical' (Surgical mask, 3-layer)
Ranked #3
5. 'Poly/Cotton' (Cotton-polypropylene-cotton mask)
Ranked #4
4. 'PolyProp' (2-layer polypropylene apron mask)
Ranked #5
(Not pictured) 'Swath' (Swath of mask material, polypropylene)
Ranked #6
13. 'Cotton 5' (2-layer cotton, pleated style mask)
Ranked #7
7. 'Cotton 2' (2-layer cotton, pleated style mask)
Ranked #8
2. 'Valved N95' (N95 mask with exhalation valve)
Ranked #9
8. 'Cotton 4' (2-layer cotton, Olson style mask)
Ranked #10
6. 'MaxAT' (1-layer Maxima AT mask)
Ranked #11
10. 'Cotton 1' (1-layer cotton, pleated style mask)
Ranked #12
9. 'Cotton 3' (2-layer cotton, pleated style mask)
Ranked #13
3. 'Knitted' (Knitted mask)
Ranked #14
12. 'Bandana' (Double-layer bandana)
Ranked #15
'None' (Control experiment, no mask)
Ranked #16
11. 'Fleece' (Gaiter type neck fleece)
So yeah, if you see someone in the store wearing a fleece mask, probably better to steer clear.
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Rain Noe
I’m a lapsed industrial designer. I was born in NYC and figured I’d die there, but a few years ago I abandoned New York to live on a farm in the countryside with my wife. We have six dogs.
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Which Face Masks are Most/Least Effective?
This study does not rank which masks are most/least effective. The authors clearly laid this out multiple times in the original study:
I'm not a statistician, but isn't it technically improper to rank the masks in this manner since many of the masks have mean and standard deviations so close to each other as to have no statistically significant difference? For example, the Valved N95 and Cotton 4 mask have mean values really close to each other and their standard deviations both encompass each others means, such that we can't really know based on this test design if one of them is really any better than the other due to the variability? Again, I'm not a statistician, but instead ranking them by groups of masks with similar effectiveness levels may be more accurate.
Thanks for the breakdown. I wish the study would have specified if the cotton was of the “tightly woven” kind. I recently acquired a cotopaxi 2-layer which claims to use tight weave cotton.
My bandana mask has 9 layers plus a vacuum cleaner bag material filter since I've read that that material is one of the best filter materials and a wire frame to hug to my face and keep my glasses unfogged..
Wow, the valved masks aren't as sociopathic as they seemed.
Dang it, I wear the gaiter strechy material one. It's so much more comfortable than anything else. But I guess I'm not helping anyone.