Deep dive by @zeynep from a sociological perspective on what we who have studied airborne transmission for a while, have been observing. nytimes.com/2021/05/07/opi… /1
There is a paradigm shift taking place, to correct misunderstanding in how respiratory infections are actually transmitted. /2
Traditional understanding of transmission routes was defined mainly by epidemiological observations--who gets sick when and where--and relied on being able to envision viruses moving between people in large droplets or on objects (fomites). /3
This led to incorrect assumptions about how viruses behave in the air. This, coupled with reluctance to call for N95s and negative pressure rooms in hospitals, produced a blind spot when it comes to transmission of viruses in aerosols. /4
Plus, hospitals usually have excellent ventilation, so it’s harder to see airborne transmission there. But in other buildings, it’s easier for viruses to accumulate in the air. /5
I suspect that inhalation of aerosols is the dominant route of transmission for SARS-CoV-2 and other respiratory viruses, but have been afraid to say so because I have been ignored or ridiculed in the past. /6
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"tracing the origins of the 5 μm threshold...ultimately revealed a conflation between various understandings and definitions of 'aerosols.' Most contemporary sources use this threshold only to explain which particles stay suspended in the air for longer times,..." /2
"yet the 5 μm distinction is clearly not based on what stays airborne but on what reaches deepest in the lungs...." /3
COVID-19 is transmitted mainly by breathing in aerosol particles carrying the virus. Two other possible routes are 1) touching a sick person or contaminated object and 2) being sprayed by large respiratory droplets. These other routes are rarer. /1
Many cases of COVID-19 have been traced to “close contacts,” and this was incorrectly interpreted to mean that large droplets were responsible for transmitting the disease. /2
We shouldn't be afraid to call SARS-CoV-2 "airborne." This is the clearest way to convey how it is transmitted. It's not waterborne, foodborne, bloodborne, or vector-borne. It's airborne. The word can still retain its special meaning in hospitals, like the word "chart." /1
A couple of updates to my 🧵from last year: I said there is no hard cutoff between droplets and aerosols. Well, there is a difference in how we are exposed: by large droplets being sprayed on us or by breathing in aerosols. /2
The associated size cut is in the range of 50-100 μm, depending on velocity of exhalation, local air flows, humidity, etc. The size cut is nowhere near the canonical 5 μm. /3
I have no doubt that infection can happen via eyes and that large droplets can land there, but aerosols are unlikely to deposit there. If I assume 1 cm jet directed at eye, it must be 6000 mph for a 1 μm particle, 300 mph for 5 μm, 73 mph for 10 μm. /1
Aerosol scientists know that it's pretty hard to collect small aerosols by impaction; need very high velocities and tight geometry. This was for Stokes number=1. Someone should check my calcs. /2
I still recommend eye protection for close contact situations to avoid large droplet spray. And stop rubbing your eyes! /3
I keep talking about HEPA filters as inserts for masks. How do you get one? I took inspiration from @SmartAirFilters (smartairfilters.com/en/blog/hepa-f…) and decided to get one and test it. My video here: TLDR: 2-ply >95% efficient. 🧵 /1
I picked up this HEPA filter from the HVAC filter aisle at the local big box store. It's pleated and comes in a frame, so you have to cut it out and pull off glue strips, as shown in the video in my first tweet. /2
How badly does the deconstruction process damage filtration efficiency? @isjinpan looked at three types of samples: 1) pristine, 2) spanning a crease, 3) under a glue strip that I pulled off. /3
Glad to see CO2 in my classroom indicates excellent ventilation. My class has been meeting online, but this is good news for our potential to meet in person. Other classes met there yesterday, and ~20 people were there when I picked it up today. More for calcs... /1
We used a mass balance approach, although we actual did the calculations in terms of volume of CO2. /2