I review a mask study published in the Journal of the American Medical Association (JAMA) from a physics perspective

Here’s the study. The authors are a couple of MDs. let’s look at their spiel.

“COVID-19 spreads primarily through respiratory droplets exhaled when infected people breathe, talk, cough, sneeze, or sing. Most of these droplets are smaller than 10 μm in diameter, often referred to as aerosols.”

“Most are smaller than 10 microns in diameter….” Ok, I have no reason to doubt this distribution.

“Exposure is greater the closer a person is to the source of exhalations.” Ummm, I think not. Bazant and Bush torpedoed this hypothesis in April 2021. Since these two docs published their research in Feb. 2021, they probably didn’t know about Bazant and Bush’s research which showed that social distancing had no impact on transmission. Just goes to show you that plausible hypotheses need checking.

“Community mask wearing substantially reduces transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 2 ways. First, masks prevent infected persons from exposing others to SARS-CoV-2 by blocking exhalation of virus-containing droplets into the air (termed source control).” No, no, no, no. This is all speculation.

Here’s a statement from the method section of the first experiment referenced in the JAMA article. “In recent laboratory experiments, multilayer cloth masks were more effective than single-layer masks, blocking as much as 50% to 70% of exhaled small droplets and particles.” One of these experiments used solid, dry particles in its aerosol. “The experimental cough aerosol was generated by nebulizing a solution of 14% KCl and 0.4% sodium fluorescein using a single-jet Collison nebulizer (BGI, Butler, NJ, USA) at 103 kPa (15 lbs./in2), passing the aerosol through a diffusion drier (Model 3062, TSI, Shoreview, MN, USA)….” The aerosol was not composed of droplets containing water. Evaporation was not a factor.

The other experiment (Ueki) was a simulation as well, whereby it was assumed that one head was the “spreader” and the other head was the “receiver.” The two heads were in an enclosed container. The spreader only exhaled and the receiver only inhaled. The distance between the heads was varied at 25, 50, and 100 cm. “Although the initial particle size exhaled was 5.5 ± 0.2 μm in mass median diameter (particle size percentages were as follows: <3 μm, 20%; 3 to 5 μm, 40%; >5 to 8 μm, 40% [3]), some of the droplets likely gradually evaporated and changed to aerosols. Therefore, both droplets and aerosols were likely present in the chamber.”

Ok, let’s see what physics has to say about droplet size and evaporation. Here’s a physics source. “Wells found that under normal air conditions, droplets smaller than 100 μm in diameter would totally dry out before falling to the ground 2 m away, i.e., the average human height. This finding allowed the establishment of the theory of droplets and droplet nuclei transmission depending on the size of the infected droplet. According to Wells (1934), droplet infection is transmitted by droplets larger than 100 μm in diameter, which rapidly settle out of the air by gravity, with the infective range being within a short distance of the source.” So small droplets (<100 microns) evaporate and large droplets (>100 microns) fall to the ground. There’s a question about “droplet nuclei” perhaps remaining in the air for hours. We aren’t sure what Wells meant, but probably it had to do with bacteria, not viruses, since bacteria are much larger than viruses. A study would have to be done to determine the distribution of droplet nuclei.

From Figure 4a of the Li paper, we see that droplets 50 microns evaporate in a couple of seconds in dry air. Droplets <5 microns would therefore evaporate in milliseconds, because their mass is on the order of a thousand times less and evaporation rate for similarly-shaped spherical objects is the same and the time to evaporate is mass-dependent. Figure 5 shows that evaporation of droplets <5 microns is virtually instantaneous. Humidity would have some impact on evaporation rate, which we see in Figure 6. Humidity doesn’t have a huge impact on very small droplets, because the surface area to mass ratio is quite large, allowing for rapid evaporation of small droplets.

Back to Ueki. The researchers are microbiologists, generally, not physicists. So they are to be excused for not realizing that most of their spreader’s exhalation would evaporate almost instantaneously. Distance between the heads wouldn’t be a factor as long as the air mixed. But the flow was unidirectional–from spreader to receiver. No walking. No air conditioning. Not exactly like a room full of people, where there is lots of spreading, what with people moving around. So there’s a big question whether the viral density of the simulation mimics real life. Perhaps the simulation shows a much worse case than real life. Oh, yeah, masks were sealed to the dummy heads with sealing tape.

So now we return to the JAMA study and we have to question their numbers of 50-70% efficiency, because the simulation studies upon which they relied were low quality.

Now let’s consider the hair salon study which the JAMA article references. “At a hair salon in which all staff and clients were required to wear a mask under local ordinance and company policy, 2 symptomatic, infected stylists attended to 139 clients and no infections were observed in the 67 clients who were reached for interviewing and testing.” The problem here is that we can’t know the cause of the symptoms. Maybe the symptoms were due to covid and maybe they were due to flu. And maybe the 67 clients were supplementing with vitamin D, which might have been more protective than masks. What would the results have been if no masks had been worn? There was no control group. Maybe the salon was exceptionally well-ventilated.

Now let’s look at the shipboard study. People who wore masks were less likely to test positive for covid, but there were major limitations.

“Second, this analysis was limited by the lack of temporal data on previous positive test results for SARS-CoV-2, which complicates interpretation of the ELISA and microneutralization assays. Third, although the date of any symptom onset was collected, information on timing, duration, and severity of individual symptoms was not collected.”

The JAMA study also relied on an ecological study, but those are low quality. Too many confounders and it’s too easy to cherry pick the data.

Now let’s consider an important study from physics on masks, wicking, and evaporation. “We show how spreading and wicking lead to water imbibition through a porous substrate, enhancing the wetted surface area and consequently promoting evaporation. These sequential dynamics offer a framework to understand the alterations in the evaporation due to porosity for the particular case of fabric materials and a clue of how face masks interact with respiratory droplets.” The result is droplet nuclei in the masks. Some of those nuclei will include bacteria, some just viruses, and some both bacteria and viruses, along with human biological material. And respiratory jets inside of masks will exert friction forces on those nuclei, breaking them down physically to small particles. We don’t know the impact yet of the nuclei because they haven’t been studied. And this is an important question in mask dynamics, yet the authors of the JAMA article failed to consider it as a possibility. A physicist would have raised the question, if asked. And there have been many physics articles about masking, so the authors ought to have been aware that they needed to consult a physicist.

So, if you are going to do a review article on masking like was done by the authors of the JAMA article, you ought to consult a physicist among others.

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