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1/ This video on a classroom filled with plexiglass barriers is making the rounds. As the poster notes, it’s not a great student experience to be stuck in one of these. But there’s another problem here – this is a counterproductive solution to combat transmission of SARS-CoV-2. https://twitter.com/KatBrezler/status/1360324184891662341
2/ If this disease were only spread by droplets that fell to the ground within 2 metres, this would be a perfectly fine solution to limit transmission. Unfortunately, we unequivocally know that aerosols are important with this disease.
3/ Aerosols (some containing virus if emitted by a carrier) are small enough to float around and build up in indoor places with poor ventilation. Aerosols are important over both short distances (i.e. within 2 metres) and more moderate distances (i.e. at the other end of a room).
4/ They’ll tend to spread out throughout a space, depending on what the local air currents are doing. An extremely useful tool to help reduce the risk of transmission from aerosols is to improve ventilation (see recent guidance by PHAC: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/guidance-documents/guide-indoor-ventilation-covid-19-pandemic.html).
5/ The NHL removed the glass behind benches to increase ventilation ( https://www.espn.com/nhl/story/_/id/30833068/nhl-removing-glass-adding-more-social-distancing-efforts-combat-covid-19-spread-sources-say). Mechanistically, the glass behind the bench creates a dead-zone where aerosol is not efficiently removed. Players would be exposed continually, but removing the glass helps avoid this.
6/ The solution shown in the original video (a bunch of small plexiglass cubicles dotted throughout a classroom) creates dozens of dead-zones where aerosol can persist for longer than it would have if the plexiglass wasn’t there to begin with.
7/ These dead-zones can be sort of imagined as recirculating eddies of air, like what occurs when you have air moving past a plate: https://upload.wikimedia.org/wikipedia/commons/c/ce/Flow_plate_perpendicular.svg.
8/ It’s not really as simple for real-world geometries like these plexiglass cubicles in a classroom filled with students, but the idea of recirculation behind blunt bodies (and trapping of aerosols within) is a very well understood phenomenon in fluid mechanics.
9/ As such, it’s hard to predict exactly how the introduction of these plexiglass barriers will influence ventilation in a classroom (it will be room and situation dependent). But they will undoubtably create dead zones.
10/ Dead zones = bad, because you aren’t removing aerosol that can contain virus from the space. Anyone who is unlucky enough to be sitting in a dead zone will be exposed far more than if these plexiglass barriers were not present.
11/ A better solution includes mandatory masking (source control and PPE) together with improved ventilation and additional filtration with portable HEPA-type cleaners (maybe even reduced class sizes). Here's a good resource on that: https://schools.forhealth.org
12/ $5 million could get you several very nice no-nonsense HEPA cleaners that would be far more productive than these plexiglass barriers in this situation.
13 extra/ Note, I do think there are some places where plexiglass barriers are useful, like in the checkout lines at box stores . But widespread use of them is a reflection of the droplet dogma that we know isn't accurate.
