I think you're missing the point. Gupta didn't say masks were ineffective. She said they should be limited to those at risk. If she believed masks are ineffective, why would she recommend that? At the same time, her recommendation, again, is backwards. It's the people less at risk who need to be wearing them.
Good grief, look at the opening question in that Bhattacharya interview you posted. The woman points out that some public official tested positive despite wearing a mask. Making a statement like this is basically spreading false information, implying that a mask is supposed to protect a wearer, rather than others. What really matters is that most people at that event did not wear masks--video confirms that.
And no, I wouldn't be emphasizing Gupta's authority. Again and again, I have made my points by citing studies, regardless of who published them. I've never singled out anyone as someone who ought to be listened to, I rarely mention any names at all. I simply show studies.
It depends on what you call effective against viruses. There are dozens and dozens of studies showing that masks do block viral particles/aerosols. The evidence is beyond debate.
What anti-mask advocates are arguing is that there is no compelling evidence that masks reduce transmission rates or disease incidence. But these are very difficult studies to run, because the goal is not to show that masks reduce disease for the wearer--an individual effect, the kind that most health scientists are accustomed to demonstrating (and most of the studies Rancourt cites are of this kind)--but for others in contact with the wearer, a social effect. Citing situations where many people wearing masks hasn't reduced case levels doesn't necessarily prove anything, because there are too many other factors that can't be controlled.
To take just one example, mask mandates are usually put into place when cases are rising, which in turn frequently result when economies open up and people stop taking simple precautions. The result is that masks are being worn under conditions that promote transmission more than when the masks were not being worn. In fact, many people feel that if they wear a mask, they can go out in public more than they would otherwise, and pay less attention to social distancing.
With regards to Rancourt in particular, yes, I responded to that, but I later looked more closely into his claims, and didn't post on that, as I recall. The fundamental flaw in Rancourt's argument is his assumption that a single air-borne particle can result in infection, and even the best masks will not block every single particle. What is his evidence for that? He cites a single source that argues--not on the basis of evidence, but on theory--that the size of certain air-borne particles are large enough to contain several thousand viruses, and that this is a large enough number to ensure infection. Even this cited author admits this is purely a theoretical calculation, that a sphere of a certain radius could contain several thousand viruses, if they were all packed together.
But that isn't how viral transmission works. This is how a physicist like Rancourt thinks it works, but it isn't how virologists and epidemiologists know how it works. The newly replicated viruses in an infected person don't have a program that enables them to pack themselves tightly into a compact sphere. The actual events involve particles getting into saliva, and it's droplets or aerosols of saliva that transmit the virus. Thus the key factor is, what is the concentration of virus in saliva, and how many individual viruses in an aerosol does that indicate? If you use the actual data on viral concentrations in saliva, it turns out that most aerosol particles will contain no virus at all. An air-borne particle has to be fairly large before it has a significant probability of carrying virus, and even then, probably only a few dozen at most, which are likely not enough to cause infection. Much larger particles can contain larger numbers of virus, but then these particles quickly settle to the ground because of their size.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7275719/
So Rancourt is incorrect that a single air-borne particle is even likely, let alone certain, to transmit the virus. It almost always takes many particles, and this is why masks can be effective. If they block a significant proportion of exhaled particles, they reduce the likelihood of infection of other people.
some studies of viral concentration in saliva:
https://www.thelancet.com/pdfs/journals/laninf/PIIS1473-3099(20)30196-1.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7107974/
https://www.thelancet.com/pdfs/journals/laninf/PIIS1473-3099(20)30196-1.pdf
https://pubmed.ncbi.nlm.nih.gov/32235945/
studies of droplet size:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7126899/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6382806/
J Aerosol Sci. 40(2): 122–133
Here's a table I constructed from the data of multiple studies:
| Particle size (um) | Virions in ave. load (7 x 106) | Virions in peak load (7 x 108) | Infectious dose (peak particles) | Settling time (100 cm) |
| 1 | < 0.1 | .01 | 10,000 or more | 6 hr |
| 2 | < 0.1 | 0.37 | 100 - 1000 | 1.5 hr |
| 5 | < 0.1 | 5.6 | 10 – 100 | 15 min. |
| 10 | 0.46 | 46 | 1 – 10 | 5 min. |
The particle size is after dehydration, which takes place in milliseconds after saliva is exhaled into the air. Dehydration results in a reduction of the particle’s diameter to roughly 20% of its original value, which means less than 1% of its original volume. I have shown estimates (number of viruses per particle) for average viral loads (column 2) in infected people, about 7 million per cc, and the highest loads recorded (column 3), about 100 times more concentrated. Even for the latter, the average number of viruses per particle of 10 um--the largest that might remain suspended in the air for several minutes--is only about 50. One or a very small number of these particles might be capable of infecting someone. But this is an extreme case, involving unusually high viral loads--and even for individuals like these, these levels would only occur in a narrow window of time. And other studies, shown in the list above, show that particles this size are not among the most numerous typically exhaled.
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