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As an aerosol scientist with expertise in the niche sub-field of droplet fragmentation, I’ve been meaning to provide some commentary on this article for a while. What does it tell us about masks? Here’s what this article tells us and doesn’t tell us. 1/n https://advances.sciencemag.org/content/7/10/eabf0452
Key observation 1: droplet penetration/ fragmentation occurs at high velocity and large droplets (coughing/sneezing conditions).
Takeaway: you should not be out in public coughing and sneezing (this goes for with or without a mask) 2/
Takeaway: you should not be out in public coughing and sneezing (this goes for with or without a mask) 2/
Observation 1 is consistent with (unpublished) work we’ve been doing in my lab: fragmentation under most conditions isn’t going to happen.
100 micron droplets are more common in resp. emissions, but they are not expected to fragment until higher velocity (red line mine). 3/n
100 micron droplets are more common in resp. emissions, but they are not expected to fragment until higher velocity (red line mine). 3/n
Much of this was predictable, but had never been experimentally verified for mask material. Some discussion here using theory to predict observations:
4/n https://twitter.com/microlevitator/status/1293614220022611968
4/n https://twitter.com/microlevitator/status/1293614220022611968
In short, the vast majority of respiratory emissions are captured by the mask. Masks really do work!
5/n
5/n
Key observation 2: three-layered masks did not have droplets penetrate, even under coughing/sneezing conditions!
Takeaway: have a mask with three layers. This same recommendation has been emphasized through other work by @linseymarr @Smogdr and others. 6/n
Takeaway: have a mask with three layers. This same recommendation has been emphasized through other work by @linseymarr @Smogdr and others. 6/n
Fig. 3 of the manuscript shows how even under very high velocity (7.5 m/s) and very large droplet (600+ microns) three layers does its job.
And this would be an extreme case.
Masks work. Some just work better than others under certain conditions. 7/n
And this would be an extreme case.
Masks work. Some just work better than others under certain conditions. 7/n
Combined, what we learn is that droplet fragmentation is a rare event only likely to occur if someone sneezes or coughs while wearing a poor mask.
The advice has thus not changed: wear a good mask and stay home if sick.
Masks work.
8/8
The advice has thus not changed: wear a good mask and stay home if sick.
Masks work.
8/8
Addition: this study does not indicate poor masks will make things worse. People breathe and speak more than they cough and sneeze (and should not be out coughing, anyway). Even single-layer light fabric will capture emissions while speaking/breathing.
