I have been following reports on masks, face shields, and social distancing with interest. The analysis is mostly fluid mechanics and filtration. My specialty is fluid mechanics and I have conducted research on automotive air filtration. I have served on the SAE Air Filter Test Code Committee and been an expert witness on air filter testing. To start with, the symptomatic who are coughing and sneezing are producing droplets that are about 5 microns. The asymptomatic who are expelling droplets while breathing, speaking, and singing are expelling droplets which are around 1 micron. The 6 foot social distancing rule is based on very old research about how far 5 micron droplets travel before falling to the floor. I got concerned when I learned about the 1 micron droplets because of a rule of thumb used for measurements using optics. Laser Doppler Anemometry and Particle Imaging Velocimetry measure turbulent air flow using what are called ''seed particles" to reflect laser light. One really measures particle velocity, but makes sure that the particles are small enough to move with the air. The rule of thumb is that 1 micron particles follow laboratory air flows very well. Thus for a face shield, the gross 5 micron particles from those with symptoms sneezing impact on the shield, but the 1 micron particles of the asymptomatic move with the air that is sucked in or out by breathing behind the shield. The small particles just flow around the corners of the shield. These aerosol particles can remain airborne and travel through buildings. Shields only are effective for the larger droplets. When Sen. Daschle received his envelope of anthrax powder, the particles were about 1 micron. That indicated that it was sent by someone who knew what they were doing. These droplets of mucus surrounding virus particles change size as a function of humidity as they evaporate, etc. The importance of asymptomatic transmission has been becoming more recognized, but there still are questions about how long the aerosols remain viable. On masks, there are different types of N95 masks. The basic standard is that they filter particles which are 0.3 microns and larger at 95% efficiency. They capture both the 5 micron and 1 micron droplets well The N95 masks work very well for medical purposes except for the ones which have a bypass valve making it easier to breathe out. These let out the virus you are expelling. Surgical masks and homemade masks also work, but not as well. They do a good job on larger particles, but are not as good on the small ones, though they still are useful, even with filtration efficiencies of 40 and 50%. There is a pretty good, very comprehensive report on masks. It does, however, give more credence than deserved to the study done at Duke which indicated gaiters were worse than nothing. A story about this in the Washington Post generated lots of publicity this past week. I read the report carefully and they were not even doing standard efficiency measurements, ratioing downstream to upstream measurements. They just measured downstream and compared to measurements without a mask. One has to be careful, because there are a lot of non-peer-reviewed reports coming out from those who are novices at filtration. It is easy to mess up, for the filtration efficiency can be a strong function of the velocity through the filtration media. If one can’t measure flow rate well, one can’t measure filter performance accurately. http://built-envi.com/what-kind-of-mask-should-i-be-wearing-to-protect-against-covid-19/
Prof. Linsey Marr of VirginiaTech has been writing and been interviewed a lot on these topics. I have talked withher and she is pretty good. Frank Frank W. Chambers2 Sabroso Pl.Santa Fe, NM 87508Home: (505) 466-1942Cell: (405) 614-4353
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