PAllen
Science Advisor
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On the other hand, I am highly skeptical of surface transmission. Someone sneezes near a surface and it gets contaminated. Virus starts degrading at a steady rate. Someone touches the surface getting, say, 1/10 the viral load in the air near the surface at the time of the sneeze. Then, they scratch the outside of their nose, inhaling at the same time, say another tenfold reduction in viral load. Thus, compared to inhalation of aerosols near where someone recently sneezed, you have 100 fold reduction in viral load for the multistep touch transmission. Note, no amount of viral load on your hands will matter unless you touch your nose or mouth (possibly, but unverified, eyes) before washing your hands or waiting for the virus to degrade. Epidemiologists have so far documented no case whatsoever of coronovirus transmission primarily by surface touching. And of spread by droplet/aerosol inhalation, indoor cases outnumber outdoors by 50-1.
In my view, the best evidence supports the following:
1) indoors, the most critical thing is near universal mask wearing because aerosols can last an hour or more and distance is minimal protection. Evidence: detailed analysis of restaurant transmission, where adjacent tables as well as tables 20+ feet away connected by air flow patterns were the ones affected. Distance is the next most important factor. Mask wearing has to be near universal because the weaknesses of non ND95 masks are minimized if all parties wear them (at least covering nose and mouth).*
2) Outdoors, mask wearing is largely irrelevant unless people are a few feet or less from each other for many minutes, or (equivalently) there are a very large number of close contacts in a relatively short span of time.
Inhalation of droplets or aerosols is the only confirmed mode of transmission.
* Let's think about some basic physics of a mask that at least reasonably covers nose and mouth, but is not high tech. On inhale, negative pressure seals the edges reasonably well, on exhale (or sneeze) they open. With both wearing masks, these effects compensate to reduce viral load inhaled. Note, I have verified by my own experiments with chilled slides that all masks other than professionally fitted ND95 leak significantly on the edges on exhale, putting a lie to the oft quoted claim that they protect others more than you. However, epidemiologists are now more and more agreeing with the idea that for indoors protection when others are not wearing masks, the only adequate protection is an ND95 mask, irrespective of distance; and that otherwise, best bet for indoors is universal mask wearing as long as coverage is from bridge of nose to below chin (and distance doesn't much matter indoors, because of how far aerosols can travel before they dissipate).
In my view, the best evidence supports the following:
1) indoors, the most critical thing is near universal mask wearing because aerosols can last an hour or more and distance is minimal protection. Evidence: detailed analysis of restaurant transmission, where adjacent tables as well as tables 20+ feet away connected by air flow patterns were the ones affected. Distance is the next most important factor. Mask wearing has to be near universal because the weaknesses of non ND95 masks are minimized if all parties wear them (at least covering nose and mouth).*
2) Outdoors, mask wearing is largely irrelevant unless people are a few feet or less from each other for many minutes, or (equivalently) there are a very large number of close contacts in a relatively short span of time.
Inhalation of droplets or aerosols is the only confirmed mode of transmission.
* Let's think about some basic physics of a mask that at least reasonably covers nose and mouth, but is not high tech. On inhale, negative pressure seals the edges reasonably well, on exhale (or sneeze) they open. With both wearing masks, these effects compensate to reduce viral load inhaled. Note, I have verified by my own experiments with chilled slides that all masks other than professionally fitted ND95 leak significantly on the edges on exhale, putting a lie to the oft quoted claim that they protect others more than you. However, epidemiologists are now more and more agreeing with the idea that for indoors protection when others are not wearing masks, the only adequate protection is an ND95 mask, irrespective of distance; and that otherwise, best bet for indoors is universal mask wearing as long as coverage is from bridge of nose to below chin (and distance doesn't much matter indoors, because of how far aerosols can travel before they dissipate).
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