Here’s some more on that Rancourt analysis arguing that N95 and surgical masks don’t work. First, some simple physics. Rancourt says, “the filtration material itself of N95 (average pore size ~0.3−0.5 μm) does not block virion penetration.” This is simply false. It’s true that a single virus particle is smaller than the average pore size. But this isn’t relevant to the ability of the mask to block such particles, for two reasons. First, masks block particles smaller than their average pore size. Particles of the pore size, 300-500 nm in fact are blocked least efficiently by the mask:
N95 have the worst filtration efficiency for particles around 0.3”, Marr said. “If you’re smaller than that those are collected even better. It’s counter-intuitive because masks do not work like sieving out larger particles. It’s not like pasta in a colander, and small ones don’t get through.”
N95 masks actually have that name because they are 95% efficient at stopping particles in their least efficient particle size range—in this case those around 0.3 microns.
https://www.usatoday.com/story/news...too-large-stop-covid-19-particles/5343537002/
Here’s a simple way of understanding why they can block smaller particles. Think of a commercial fisherman, trawling the ocean with a net. Suppose the net’s mesh is one foot square. Fairly large fish can get through this mesh, but many won’t. Why not? They get caught in the thread or rope of the net. It’s like that for the virus, only worse, because a virus has no eyes to guide it to the opening, and no means of locomotion to move through it. It’s floating randomly about in the air, and will stick to anything it bumps into. An additional factor is that the N95 masks have an electrostatic charge which attracts the virus and makes the filter in effect even stickier. Smaller particles accelerate towards these charges faster than larger ones.
A second point is that viruses don’t float about in the air in isolation. They’re collected into aerosols or droplets, which are several orders of magnitude larger in size than a single virus particle. So while a single virus particle averages maybe 125 nm, or less than half the diameter of the N95 pore, an aerosol containing viruses will typically be 2000-3000 nm, or larger. Rancourt is well aware of that, he provides this information, but he doesn’t emphasize that this means that most aerosol particles will easily be blocked by N95 masks.
Why not? Here we get to the nub of his argument. He cites a study by Balazy et al that he says supports his claim that masks don’t work. However, Balazy et al. don’t conclude that N95 masks fail to block viral particles at all. They simply point out that sometimes 95% of the particles aren’t blocked; it might be very slightly less (more than 94%!) under some conditions. They further show that ordinary surgical masks are much less protective. One mask they examined blocked about 85% of viral particles, while another only 20% (or less, depending on the size of the particles, and the air flow rate).
What Rancourt’s entire thesis hinges on is that this doesn’t matter. He says, “if the MID [mean effective dose] is amply surpassed by the virions carried in a single aerosol particle able to evade mask-capture, then the mask is of no practical utility”. So if a mask doesn’t block
all viral particles, 100% of them, it’s useless. That seems to be the core of his claim.
In the first place, this logic only applies to use of the mask to protect the wearer, which is not why masks are being recommended. The main purpose of the mask is to protect others from the wearer. If the mask blocks only 20% of the viral particles exhaled by an infected someone—let alone 85-95%--that potentially makes a huge difference. It means the concentration of virus in the air surrounding that person is decreased by that amount. That might make all the difference in the world whether someone near that person gets infected. Even if it only takes one aerosol particle, the odds of the person breathing in that particle are decreased according to the concentration of the aerosol in the vicinity.
But second, as I pointed out upthread, Rancourt doesn’t know that a single aerosol particle is always going to be capable of infecting someone. In the first place, we don’t know how many virus copies are required for infection. A rough guess might be around one hundred, but I don’t think anyone really knows.
In the second place, we don’t know how many copies will be in an aerosol. Rancourt goes with an average virus size of .125 nm, and an aerosol particle of 2.5 nm. So the diameter of the aerosol particle is twenty times the diameter of one viral particle, or 8000 times the volume. But because smaller spheres can’t be packed into a larger sphere without gaps between them, the maximum number of virus copies that could form an aerosol of that size would probably be closer to 5000. But the aerosol also contains water, so the actual number of viral copies is likely to be much lower. Indeed, many aerosols may be composed mostly of water, with relatively few (or none, see below) viral particles attached.
Keep in mind that after a virus produces multiple copies of itself, the work it’s designed for is done. It has no way of packaging all these copies into a perfect little ball that can be exhaled out of the body in the direction of another victim. This process happens randomly. If a virus copy bumps into an aerosol, it will attach to it.
So what we need to do is measure the number of virus copies that are actually exhaled in an aerosol. One study reported that for all but one of three dozen subjects, less than 100 influenza viral copies were exhaled per cough. The median was less than 10 copies. This study also reported that copies exhaled were reduced by wearing surgical masks.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2994911/
Another study measured the number of copies exhaled through breathing. One subject exhaled 20 copies per minute. The other subjects exhaled less than three copies, and in some cases, none. This study claimed that a single active viral particle could constitute roughly an infectious dose, but also noted that only about 0.3% of the viral particles they detected were active. This same study also found that a large number of aerosol particles were exhaled, in many cases, several thousand or more per liter, but apparently most of them didn’t actually contain virus.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2442192/
In summary, masks do block many virus particles from passing through them, which means any infected person wearing one should be reducing the risk of people nearby. Rancourt’s entire thesis seems to rest on his claim that a single aerosol particle is sufficient to infect someone, but AFAIK, he provides no direct evidence of this, not even a study demonstrating how many SARS-CoV-2 copies may be present in an aerosol.
WH suggests that the pressure to open and threats of financial retaliation for those school systems that don't open is making it's own point.
The WH doesn't fund schools, of course. Mostly, my property taxes do.
I know several personally: neighbor who had quad-bypass in May, colleague of wife open heart in April, colleague of mine chemo all along, spouse of colleague dialyses all along (and scheduled for transplant this summer).
Good grief, that's ugly. It's bad enough having to have those procedures, without at the same time worrying that just by being in a hospital, your chances of getting the virus are greater.
What drives up infections much, much more is leisure (pubs, cinemas, restaurants, hotels, fitness clubs, choirs, etc.), so if you allow that, you should definitely allow reopening schools.
Couldn't agree more. That isn't an argument per se for opening schools, though, as much as for shutting down bars. And pointing to Europe, as the Administration has done, ignores the obvious fact that the case rates in the communities surrounding these schools are orders of magnitude lower than they are in the U.S.
The U.S. is in a very precarious place now. The one bright spot in the latest spike of cases was that the number of deaths was steadily declining. It didn't seem possible that could last, and it hasn't. For the last three days, the number of deaths has been at the highest level in a month. It appears that this number is going to spike, too, though it's too soon to be sure. If deaths continue to rise, perhaps to the levels we had in April, around 2000 a day, or even more, then what? What if large numbers of people refuse to stay home, and the virus continues to spread? In April, we at least were starting from a lower baseiine of active cases, and people were not yet weary of staying home.
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