Shop Talk from the Archives
Aug. 28, 2020
Masks, masks, masks. Everywhere we turn the talk is all about masks.
Last weekend, my mom asked me if my mask provided enough protection. And in Monday’s Zoom faculty meeting, my colleagues discussed the pros and cons of gaiters, bandanas, N95s, and cotton face coverings.
I must confess that here at The Evidence Workshop we don’t get out much. And when we do, we keep our distance. So, we hadn’t been paying much attention to the evolving literature on masks. But we were curious. So, we pulled the recent paper from Fischer et al. (Sci Adv. Aug. 2020) that compared respiratory droplet transmission in 14 different types of face coverings.
We discovered what my mom and it seems the rest of the world already knew. N95, surgical, cotton, and cotton/poly masks block droplets better than bandanas and gaiters.
Here are a few thoughts on the Fischer paper that shed light on evidence in general.
Where does the Fischer mask paper sit on the evidence hierarchy?
It sits on the bottom rung as a form of mechanism-based reasoning. Indeed, the authors acknowledge this point when they refer to their work as a proof of principle study.
Recall that folks are studying face masks because we all want to know how well they reduce the spread of disease, especially COVID-19. While other groups have looked explicitly at disease spread (Chu et al. 2020, Leung et al. 2020), the Fischer study did not. It measured how many respiratory droplets (think, drops of spit) were emitted when participants recited the phrase, “stay healthy people.” They found that gaiter-type neck fleeces increased the number of droplets emitted. It seems that the gaiter splits the larger droplets into many smaller droplets. Think, gentle mist vs. harsh spray. All the other masks reduced droplet count, as compared to no mask at all. Values ranged from 50% reduction (bandana) to 99.9% reduction (professionally fitted N95). Most non-N95 masks reduced spit spray by at least 60%.
Getting back to evidence hierarchies, this study falls under mechanism-based reasoning because it does NOT demonstrate DIRECTLY that cotton masks (for example) reduce disease spread better than bandanas. Instead, we must reason it out. If cotton masks block more spit and if COVID-19 is transmitted by spit, then it’s plausible that cotton masks are better than bandanas for reducing disease transmission.
Is it a problem that the study uses mechanism-based reasoning?
No. All scientific inquiry starts somewhere, and mechanism-based studies are good starting points. Often, they can be completed more quickly than other study types, and they provide evidence for biological plausibility. Indeed, Fischer and her team point out that it is plausible that some masks reduce disease transmission better than others because some masks block spit better than others. We also must understand the limits of mechanism-based conclusions. The Fischer paper cannot (and does not) conclude that some masks reduce the spread of COVID-19 better than others. As we continue to study mask use among non-medical personnel, new evidence will likely move up the evidence hierarchy and get us closer to the truth of the matter. Stay tuned.
How can I learn more?
Read the full manuscript at https://advances.sciencemag.org/content/early/2020/08/07/sciadv.abd3083.
Emma P. Fischer, Martin C. Fischer, David Grass, Isaac Henrion, Warren S. Warren, and Eric Westman. Low-cost measurement of facemask efficacy for filtering expelled droplets during speech. Sci. Adv. 10.1126/sciadv.abd3083 (2020).
Read about masks and disease transmission.
D. K. Chu, E. A. Akl, S. Duda, K. Solo, S. Yaacoub, H. J. Schünemann, COVID-19 Systematic Urgent Review Group Effort (SURGE) study authors, Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: A systematic review and meta-analysis. Lancet 395, 1973–1987 (2020). doi:10.1016/S0140-6736(20)31142-9 Medline.
N. H. L. Leung, D. K. W. Chu, E. Y. C. Shiu, K.-H. Chan, J. J. McDevitt, B. J. P. Hau, H.-L. Yen, Y. Li, D. K. M. Ip, J. S. M. Peiris, W.-H. Seto, G. M. Leung, D. K. Milton, B. J. Cowling, Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat. Med. 26, 676–680 (2020). doi:10.1038/s41591-020-0843-2 Medline.
Look at the existing body of literature.
Go to www.pubmed.gov and enter "COVID-19" [Supplementary Concept] AND (MASKS [MeSH] OR Respiratory Protective Devices [MeSH]) in the dialog box. You’ll get over 219 hits since the start of 2020, which speaks to the pace at which the evidence is growing.